BIOLOGICAL DETERIORATION OF PAINTS AND PAINT FILMS
Description of the Problem
Micro and macro organisms can destroy both the decorative properties and durabilities of paint films. Bio deterioration of paint falls into two general categories: enzymatic degradation of protein and cellulosic thickeners that produces an irreversible viscosity loss in latex emulsion paints while still in the container, and microbial disfigurement and deterioration of both water thinned and solvent thinned paint films. Enzymes catalyzing the degradation of protein and cellulosic thickeners may be introduced into the paint through contaminated raw materials, storage tanks, and other equipment or be released by bacteria (and less frequently, yeast) growing actively in the liquid paint.
The susceptibility of paint films to attack by micro organisms is determined in part by the chemical nature of the nonvolatile binder, the choice of pigmentation, and the pigment volume concentration. To a much greater degree however, the susceptibility or resistance of a paint film to biological attack is determined by the presence and concentration of antimicrobial agents.
Microorganisms Associated with Paint
Microorganisms associated with paints and paint films have been well established. Pseudomonas aeruginosa is the predominant bacterium isolated from spoiled latex emulsion paints in the container. A much greater number of fungi and bacteria are found on and within paint films, although again one fungus and one bacterium predominate. Goll and Coffey were the first to observe and report the wide spread growth of Pulluloria pullulans. In isolation studies of oil and alkyd paint films at six wide spread geographical locations, Rothwell confirmed the predominance of P. pullulans but noted the close resemblance of, and predominance in, certain geographical areas of Cladesporium sp. Other fungi frequently isolated included Alternaria dianthicola and Phoma pigmentivora. The same studies indicated the consistent presence of bacteria within the paint film and at the paint wood interface. Flavobacterium marinum was by far the predominant bacterium isolated. Despite the differences in the chemical nature of latex emulsion binders. Drescher isolated essentially the same micro organisms from latex emulsion paint films exposed at the same location.
The microflora of interior paint films in breweries, dairies, canneries, and other food processing plants was reported by Krumperman and included many fungi rarely found on exterior paint films. Prominent among these are Aspergillus species and Penicillium species. Clado sporium species and P. pullulans were found to a lesser extent. His investigations again indicated the frequent occurrence of the bacillus F. marinum.
Growth Structures of Fungi
Fungi are present on the surface of paint films in two forms. They may be present as thread like structures, technically referred to as mycelia or as clusters of spherical, usually black colored, spores. These two different appearances of fungi have been popularly labeled the trees and fruit of fungi. In actuality, they represent the two different growth forms in the life cycle of the fungi. The mycelial structures are observed when the fungi are actively growing and reproducing. Spore clusters are found when conditions for growth and reproduction are less favorable. Spores are more resistant to environmental changes and antimicrobial agents than the mycelial forms.
The mycelial growth structure of fungi is recognized easily by its thread like form. Spores and spore clusters are frequently difficult to differentiate from soil or soot particles, and examination with a magnifying lens or microscope is frequently necessary for positive identification by even the skilled microbiologist. Photomicrographs of the two different forms of fungal growth. Whenever doubt exists to whether surface disfigurement is fungus or dirt, culturing the deposit on tryptone glucose extract agar, potato dextrose agar, or other suitable culture media will provide the final answer.
Chemical Methods for Fungal Identification
The protein nature of fungi permits the use of chemical identifications. Treating disfigured paint films with a sodium hypochlorite bleach solution containing 5 percent of sodium hypochlorite in water is generally employed. The bleach solution is applied to a relatively small area of the paint film when this disfigurement is representative of that observed in the overall surface. It is allowed to remain approximately 1 min at which time the treated area is flushed with water and blotted with absorbent paper.
Bleaching indicates that the disfigurement is fungus. The test has its limitations and thus should be interpreted with some degree of caution. Insect eggs or fecal material will bleach since both are composed of protein. The test should be limited to white or lightcolored paint since on deeper colored paint films, the bleaching of fungal growth may be insignificant compared to that of the paint. Moreover, a heavy chalk face interferes with the test and areas discolored by metal may give false results.
Chemical agents, used to control or prevent the deterio rating effect of microorganisms, are referred to as biostats if they do not kill microorganisms but prevent their reproduction and as biocides if they kill. Such agents used in paint films fall into two distinct categories that include inorganic pigments and organic additives. Zinc oxide and barium metaborate are examples of the first category, and phenylmercury compounds and chlorinated phenols are examples of the second category. Some of the more frequently employed antimicrobial agents used in paint are listed in Table 1. Most of the microbistats and microbicides used in paint films effectively control fungi and bacteria by interfering with their metabolic functions.
Bacterial Resistance of Liquid Paints
Resistance of emulsion paints in the container to attack by bacteria can be determined in accordance with ASTM Method D 2574, Resistance of Emulsion Paints in the Container to Attack by Microorganisms. This test predicts the package stability of water thinned latex emulsion paints as related to bacterial growth in the paint and degradation of protein and cellulose thickening agents.
The test consists of two parts. The paint under test is first cultured on tryptone glucose agar to determine if living bacteria are present. A negative result indicates the absence of bacteria but not necessarily resistance to attack. To determine if the test paint can withstand bacterial attack, a specimen of spoiled paint containing Pseudomonas aeruginosa is introduced into the test paint and the latter is incubated at room temperature for a period of six weeks. At intervals of 24, 48, and 72 h, and at one week intervals for the remainder of the test period, the inoculated test paint is streaked on tryptone glucose extract agar slants. The test paint is reported to be resistant to bacterial attack if no living organisms can be recovered through six weeks of incubation. Conversely, the paint is reported to be not resistant to bacterial attack if living bacteria are recovered at anytime during the incubation period. The principal difference in the ASTM test and previously employed tests of this type is the use of spoiled paint as an inoculum, rather than aqueous suspensions of bacteria removed from laboratory growth medium. By employing paint containing P. aeruginosa, already adapted to a paint environment, the shock of a drastic environmental change is eliminated. Repeated inoculations may be necessary to obtain a spoiled paint for use as an inoculum, but, once prepared, it can be maintained indefinitely.
Measuring the Fungal Resistance of Paint Films
The inability to duplicate the use environments of exterior and interior paint films has made it difficult to develop suitable accelerated tests for the evaluation of their fungal resistance. Most laboratory tests have been based on the widely used agar plate method or modifications of it. Simply described, the agar plate test consists of placing a painted substrate on a bed of agar, inoculating the system with the test organism and observing growth during a prescribed incubation period.
ERDL Method The ERDL (Engineer Research and Development Laboratories) method, which is the agar plate test most frequently referred to in specifications for paints utilized by agencies of the United States government, employs sucrose, mineral salts, agar medium, and Aspergillus oryzae as the inoculating organism. The agar medium is prepared according to the recipe shown in Table 3. The pH of the medium may be adjusted to 5.5 to 6.5 with 0.1 N hydrochloric acid (HCl) or sodium hydroxide (NaOH). The medium is sterilized in an autoclave for 15 min at 15 psi and 121 C. Approximately 30 ml is poured into sterile petri dishes and allowed to harden.
The inoculum is prepared by adding 10 ml of sterile water containing 0.005 percent nontoxic wetting agent such as Tween 80 to a tubed subculture of A. oryzae. The mixture of spores and mycelia are removed by gently stroking the agar surface with a sterile camels hair brush. The aqueous suspension is removed and diluted with sterile water to 100 ml.
Using a sterile pipet, 1.0 to 1.5 ml of the diluted spore mycelial inoculum is distributed over the painted surface and surrounding agar surface. Duplicate plates should be prepared. The inoculated agar plates are incubated for 7 days at 28 to 30 C and 90 percent relative humidity. At the end of the incubation period, the specimens are examined at 1 and approximately 18 magnification. Fungal growth on the agar surface or on the sides of the painted filter paper is ignored, and such specimens are considered to pass the test.
Nuodex Method In order to improve its accuracy, the ERDL test was modified by the Nuodex Laboratories as follows: Pullularia pullulans replaced Aspergillus oryzae because it is the fungus most frequently isolated from exterior house paints. Malt extract agar replaced the sucrose mineral salts agar because, in it, P. pullulans exhibits growth forms that are typically observed on exterior paints rather than yeast like forms that it exhibits when grown on the sucrose mineral salts agar.
Hutchinson Method The Hutchinson method is similar to the ERDL agar plate test but employs glass string rather than filter paper as the paint substrate, a liquid broth culture media containing no carbon source and a mixed spore suspension of Aspergillus niger, Aspergillus flavus, and Penicillium leterium. The glass string is dipped into the test paint which is then allowed to dry for 48 h. Then the string is dipped into the spore suspension for 1.5 min. One inch sections are the placed on the agar surface.
Proposed ASTM Environmental Chamber Test Subcommittee 28 of ASTM Committee D 1 has developed a tentative method for measuring the resistance of interior paint films to fungus attack. This test reportedly provides more accurate results by virtue of removing the artificial aspects of previously described laboratory method. Test paints are applied to either white pine or gypsum board panels measuring 3 by 4 by 0.5 in. The specimens are then conditioned at 75 F and 50 percent relative humidity for 4 days after application of the last coat before being placed in the test chamber. The chamber may be any cabinet capable of maintaining a relative humidity of 95 to 100 percent and a temperature of 90 F and large enough to accommodate test specimens, a water bath, and a soil bed that serves as an inoculum source. The soil bed is constructed of a stainless steel or plastic tray with a monel mesh bottom (16 mesh). The soil employed is a good quality, greenhouse grade potting soil containing 25 percent peat moss. The pH of the soil is maintained between 5.5 and 7.6. The soil is inoculated with spore, mycelium suspensions of Pullalaria pulluluns, Aspergillus niger, and Penicillium sp. prepared from 10 to 14 day old agar slants. At least 14 days should be allowed for the fungi to sporolate prior to beginning any tests.
WEATHERING TESTS NATURAL WEATHERING
The final test of a paint is its performance under actual conditions of use. For exterior paints, this means on the walls of buildings, railway cars, highway vehicles, ships, and the like. Such tests are expensive and time consuming. Hence, there has developed the practice of conducting tests on a small scale. These screening tests allow studies of the effects of many variables to be made in a fraction of the time and at a fraction of the cost of full scale tests. For final judgement, full scale tests must usually be made.
Many variables enter into the testing of paints on a small scale, and it is doubtful if small scale tests can usually be the basis for positive statements about the performance of paints on large structures. Under the practical conditions existing during the painting of exterior surfaces, the effects of weathering may not always agree with those that occur in small scale tests. The differences may arise, not from the compositions of the paints, but rather from the technique of application and schedule of maintenance, or from other factors such as differences of temperature and moisture content between buildings and test panels, particularly when wood is concerned.
Weathering tests are necessarily long time undertakings, requiring very careful planning and preparation. It should not be attempted unless it is possible to make it the major duty of at least one adequately trained man. Evaluation of weathering tests may be more informative if certain laboratory tests are made during the exposure period. The trend of changes in properties such as distensibility, adhesion, and porosity may be used to predict the probable usefulness of a paint.
Effect of Climate
The type and rate of failure of a paint film varies different combinations of climatic conditions. Hence, the climate of the test site should be representative, geographically, climatically, and in atmospheric contamination, of that of the location in which the paint is to be used (Table 1).
The sun is an important factor in the degradation of paint films. It raises the temperature and thus increases the rates of chemical reactions with oxygen or with gaseous contaminants that may be present in the atmosphere or between ingredients of the paint itself. The actinic radiation of the sun catalyzes many of the reactions. Fluctuations of temperature, caused by the day night cycle and by clouds, impart physical stress (expansion and contraction), resulting in gross cracking at one end of the scale and microscopic cracking at the other end. The latter may manifest itself as adherent dust (chalk). Sudden severe drops in temperature have been known to pop paint from galvanized metal.
Water is one of paints worst enemies. It causes blisters and peeling, and promotes the growth of mold on the paint. In the form of dew it is more harmful than rain. Dew forms within cracks and makes intimate contact with the paint film. Water as rain often flows across cracks without entering them. By remaining in contact with the film, dew may promote reactions with dissolved contaminants. Rain may wash these contaminants away and thus minimize the reactions. Tests started on arid, sunny mountain tops did not start to chalk until they were brought down to sea level.
The simplest type of rack is one to which the specimens are fastened by nails or screws to horizontal stringers, or are held in place flaps or in grooves. Slots and grooves, if wide enough, serve to protect a portion of the surface, thus allowing changes of appearance to be readily noted. A hinged flap over the top of the specimens is probably better because less dirt accumulates. The advantage of this construction is the ease with which the specimens may be removed for careful inspection in the laboratory. A simple rack of this type is described in ASTM Recommended Practice D 1006, Conducting Exterior Exposure Tests on Wood.
A type of rack that simulates actual wall construction of a wood frame house. There appears to be little advantage to this type, since under the conditions of the test there is little or no condensation of moisture within the stud space. However, for other reasons, one might select it or a similar type.
The lower specimens on an ordinary rack are subject to contamination by runoff water from the higher. To eliminate this disturbing factor, racks in which each row of horizontal specimens is offset have been designed.
Angle of Exposure
To use more of the suns energy, it is common to tilt the racks toward the sun, a compromise angle of 45 deg being the usual practice. Walker calculated the relative amounts of energy received by specimens oriented vertically at 45 deg and at an angle equal to the latitude of the exposure site. Inspection of the data in Table 2 shows that the intensity at 45 deg is from 1.35 to 2.44 times that received at 90 deg at several different latitudes in the United States.
Estimates derived from actual exposure tests range from 2 to 3 (Table 3). Many authorities hold that exposure at 45 deg cannot be accepted as accelerating all reactions occurring in paint films equally. In some films, chalking may be accelerated in others, cracking. In other words, changing the angle from vertical to 45 deg is equivalent to conducting the test in a different climate, and the effects of climate are not always predictable.
Follow the Sun Racks
In order to use the suns energy more effectively, Gardner suggested that racks be built in the manner of equatorial telescopes so that they would face the sun at all hours of the day. A simple demonstration model was built by supporting the rack on pivots in a yoke to allow the angle of inclination to be varied. The yoke in turn was mounted on a post on which it could be turned continuously to face the sun. The angle of inclination was changed manually at regular intervals during the day. The rack was rotated mechanically by water power.
A far simpler system is described by Daiger. Experience has demonstrated that 45 deg exposures in Florida accelerate chalk fade of automotive and industrial finishes without seriously distorting the relative performance in other respects. Exposures at the horizontal and at 5 deg to obtain even greater acceleration have been gaining favor. However, on an annual basis, either angle has a drawback. Tests started in summer chalk at a faster rate than those at 45 deg, but tests started in winter chalk at a slower rate. The solution to the dilemma is to change the angle at intervals so that the specimen is never more than 5 deg from perpendicular to the sun at noon.
The trend to a more effective use of the suns energy was continued by Caryl and Helmick by using an automatic equatorial mount. But even this ambitious step did not satisfy them. The machine was redesigned to use mirrors (up to ten) to increase the suns radiation onto the specimens. The new machine was christened EMMAQUA for Equatorial Mount with Mirrors for Acceleration plus AQUA (water). In this machine the specimens are located on the underside of a cross member at the top of the machine, the target area, 6 ft by 6 in. The mirrors, opposite the target, face the sun and reflect its energy back to the specimens. The mirrors are bright rolled aluminum sheet with Alzak finish and reflect about 85 percent of the visible radiation and about 70 to 80 percent of the ultraviolet. With ten mirrors, the specimens receive about eight times the radiation received by a simple equatorial mount and ten times that received by a 45 deg exposure. A strong current of air keeps the surface temperature in the range prevailing in the 45 deg exposure.
Present, practice is to operate the machines only on sunny days from 7:30 am to 4:30 pm in the summer and from 8:15 am to 3:45 pm in the winter.
A comparative study of four types of house paint and of six automotive finishes showed that 14 weeks in the EMMAQUA machine correlated very well with 3 years exposure at 45 deg in Phoenix, Ariz.
On the other hand, as a result of tests on pigments, Papillo concluded that The EMMAQUA cannot be used in an absolute way for prediction of service life or for quantitative prediction of the relative performance of two pigmentations, using 5 deg South Florida exposure as standard. The unit has been found very reliable, however, in providing qualitative information regarding relative weatherability of coatings. It is considered useful as a time saving adjunct to screening programs in the development of new pigments for high fastness coatings.
Application of Paints
Specimens for exterior exposure should be painted out of doors in suitable painting weather. The exception might be when tests are designed to study the effect of adverse weather for painting on the performance of the paint. It is permissible to apply the paint indoors provided that the specimens are removed immediately to the outside for drying. Alternately, an open shed or a canvas shelter might be used. If both applied and dried indoors, undercoats may remain uncured and checks may form in the top coats. On the other hand, films may not cure properly if the painting is done out of doors during cold or damp weather, or in an industrial area contaminated with acidic gases. If, during winter, specimens must be prepared indoors for shipment to remote test sites, it is advisable to cure them in well lighted rooms or cabinets through which outside air, suitably warmed, is passed. In hot, sunny summer weather, it may be advisable to attach the panels temporarily to the shady side of the test rack during the application and drying stages, subsequently removing the panels to their permanent location.
When the purpose of the test is to compare commercial paints, it may be appropriate to apply them at what seems to be their natural spreading rates. When the purpose is to study variation in composition, the paint should be applied at suitable predetermined spreading rates.
Unless the paint chemist is careful, he may find that he applies paint to small specimens at a greater spreading rate (less paint) than does the experienced painter. For this reason, many laboratories find it desirable to employ painters for this work.
Panels of factory applied paints should be inserted in the production line or on a specimen of the finished product, or a specimen cut from the finished product should be taken for test.
Tests on Wood
Wood test panels and their selection should receive careful consideration. Extensive tests made in eleven different parts of the United States showed that the species of wood has a very pronounced influence on the durability of the coating (Table 4). Moreover, within a given species, paint holding properties are influenced by density, grain, and grade. Boards of average density, edge grain, and select grades hold paint better than boards of high density, flat grain, and grades containing numerous knots and pitch streaks.
Plywood for general paint tests should be the exterior type in which water resistant glue has been used. Hard board should be of the exterior grade and tempered.
Large specimens not nailed or screwed to a rack might well be reinforced across the back with wood or channel iron cleats to prevent warping.
The paint technologist must bear in mind that lumber is rarely chosen primarily for its painting characteristics. To the user of lumber other properties, such as cost, availability and working properties are also important. Therefore, if time and space are limited, testing procedure should include a poor paint holding species such as southern pine, a good paint holding species such as western red cedar, and perhaps an intermediate, species such as white pine. If paints being compared differ only slightly in ordinary performance, the use of poor paint holding species may be vital to a proper evaluation of the paint. These principles and examples are the subject matter of ASTM Standard D 358, Wood to be used as Panels in Weathering Tests of Paints and Varnishes.
Number of Specimens
In addition to standard panels all tests should include a standard reference paint. The best way to include both features is by the use of matched specimens. According to this procedure, the reference paint is applied to a portion of the panel that receives the new or competitive paint. It is convenient to apply the reference paint to the midsection and the other paints to the end sections. This makes it possible to obtain evidence of paint performance otherwise obtainable only by preparing many more separate specimens. In any event, duplicate specimens are necessary. To be statistically sound five specimens should be tested.
Tests on Iron and Steel
One of the earliest tests of paint on iron and steel was started in 1908 at Atlantic City, N.J., under the auspices of ASTM. Several hundred 18 gage panels, 18 by 36 in., were used. One objective was to find the relationship between the Thompson laboratory test and outdoor weathering. The results are summarized in Table 6.
In some respects, weathering tests on metal, such as iron and steel, require more attention to details than do tests on wood. Rust and mill scale vary in nature and amount. Pretreatments are common. Contamination by fingerprints must be considered.
Shapes have a great effect of weathering. Angles and curves form pockets that trap water, shield adjoining surfaces from the sun, or present them more directly to the sun. Compared to wood, a larger percentage of outdoor metal surface is oriented at angles other than vertical. Thus, to develop the complete picture of the performance of paint on metal, flat specimens should be exposed horizontally and at 45 deg, as well as vertically structural shapes should be included.
SPECIFIC PRODUCTS TESTS ON VARNISHES
Tests described in this chapter apply 10 oleoresinous and catalytic cured varnishes, such as exterior, interior, floor, and rubbing varnishes to nonoxidizing types, formerly known as spirit varnishes, such as cellulosic, vinylic, and acrylic lacquers shellac and floor seater. Most of the tests are listed in ASTM Methods of Testing D 154, Varnishes and D 333 Clear Lacquers and Lacquer Enamels. Many appear in both documents.
TESTS ON LIQUID VARNISH
To determine the presence or absence and to describe the nature of undesirable solid matter or nonmiscible liquid in clear liquids varnishes and lacquers, among others is the purpose of ASTM Method D 2090, Clarity and Cleanness of Paint Liquids. Various terms have become established in the coatings field to describe the nature of the foreign matter:
Foreign matter is anything visibly unrelated to the origin of the material.
Sediment is any solid, such as foots, grain, or gum that can settle or be centrifuged from the liquid.
Skins are partially solid layers of material, usually formed from the liquid itself.
Turbid describes the presence of non settling, suspended matter in a concentration high enough to reduce clarity to translucency.
Haze describes the presence of nonsettling, suspended matter in a concentration not high enough to reduce transparency to translucency.
Clear describes a complete lack of visible nonuniformity when viewed in thick layers in bottles or test tubes in strong transmitted light.
Clean describes a complete lack of any visible nonuniformity when viewed in thin films.
Examination should be made under at least 50 ft candles. It is convenient to use the specimen prepared for the determination of viscosity by the bubble method. Tilt the tube just slightly from the horizontal so that the bubble moves slowly and permits observation in the moving liquid of fine particles that might otherwise escape detection. It may be helpful to charge a second tube with the liquid, to allow both tubes to stand for 24 hand note any sediment to shake one tube thoroughly and, after the bubbles have broken, to compare the appearance of the tubes (any difference indicates haze or worse). Drain one tube, replace the stopper, and let stand for 15 min or until flow is complete and a thin film protected from dust remains. Strong transmitted or reflected light may reveal particles that otherwise escape detection. A liquid may appear clean in a thick film but not clean in a thin film. For a discussion of temperature and some other factors, the reader should consult the original method.
The color of liquid varnish is only an indication of the color of the dry film. The initial color may bleach or other color develops, depending upon the conditions of exposure.
If the intensity of the color is appreciably greater than water white, comparison with Gardner Color Standards is recommended. Paler colors may be in the range of the platinum cobalt standards.
The effort required to apply a varnish is related closely to its viscosity. For application by brush it is in the range of 1 to 2 stokes for application by spray it is somewhat lower for application by roller it is higher. Lithographic varnishes and vehicles for paint may have viscosities as high as 100 stokes.
Gardner Holdt bubble tubes are used widely for determining the viscosity of oleoresinous varnishes. The Ford cup is used for nontransparent varnishes. For precise determinations needed in research, capillary viscometers are often used.
Viscosity Control during Manufacture
Bodying reactions may continue for several days after a varnish has been thinned. If the extent of the bodying can be predicted, thinning and storing can be done with confidence. A method for doing this follows. The viscosity of an aged batch of the varnish is determined over a convenient temperature range, say 77 to 130 F, and a temperature/viscosity curve is constructed. The viscosities of several batches are determined at catch temperatures. The viscosity at 77 F of each varnish is estimated by drawing curves, parallel to the first one, from the catch temperature to 77 F. The average increase due to aging is thus obtained, and a new curve, the standard for future batches, is constructed.
For several reasons a laboratory determination of nonvolatile content may not agree with the actual content. At the elevated temperature of the determination, reactions of oleoresinous varnish with oxygen from the air may proceed in directions different from those at the ambient temperatures of drying. Reacting resinous constituents of catalytic cured varnishes may eliminate water or may add moisture from the air. Cellulosic lacquers may lose plasticizer. Several methods for the determination are available.
The nonvolatile content by volume is recognized as a factor in film thickness.
Two general methods, A and B, are described in ASTM Method D 1644, Nonvolatile Content of Varnishes. Method A tends to give higher values, especially for strongly oxidizing types. For varnishes containing highly volatile thinner, Method B is not recommended because of the potential danger at its higher temperature.
Resin solutions are essentially a type of varnish. They usually contain more solids than do varnishes. Consequently, they are more viscous and tend to trap solvent. Ways to avoid the difficulty are given next.
Oil Addition Method This method is essentially the same as Method A, previously mentioned. The difference is the addition of 0.5 to 1.0 g of medium body soybean oil to the dish as a part of the tare weight. The oil helps to keep the specimen open during the heating. As a check on loss of added oil, a blank may be run.
Thin Film Methods These are to be found in ASTM Method D 1259, Nonvolatile Content of Resin Solutions. There are two modifications: A, for nonheat reactive resins, such as ester gum and alkyd B, for heat reactive resins, such as formaldehyde reaction products with urea, melamine, and phenol, and for resins that release solvents slowly such as epoxy resins. The only practical difference is the duration of heating. A unique feature is the very thin film that minimizes retention of the solvent.
A sheet of aluminum or tin foil, 6 by 12 by 0.0015 to 0.0020 inch. is weighed. One end is placed, shiny side up, on a sheet of plate glass and rolled smooth, if necessary. The sandwich is opened and placed on the tray shown in Fig. 1. and the tray is placed in an oven (gravity or forced ventilation) at 105 C for 30 min. The specimen is then removed from the oven, the sandwich is closed, and the determination is completed in the usual way.
The procedure is the same as for Method A except that the specimen is heated in a forced ventilation oven for 2 h.
Vacuum Method According to this method, the solution is diluted with a high boiling liquid, such as dibutyl phthalate, and heated under vacuum, with agitation, to distill the original solvent in which the resin was dissolved. As shown in Fig. 3, two flasks are rocked about an axis passing through the bottoms. The flasks hold 50 ml and are connected to the vacuum, a manometer, and to two solvent traps cooled in a mixture of Cellosolv and dry ice. Between the flasks and the manometer is a needle valve to control the pressure.
In each of the two flasks are placed six steel balls to provide bubble forming surfaces, and 10 ml of dibutylphthalate. The flasks are weighed and from 2 to 3 ml of resin solution are added to each, and they are again weighed to obtain the amounts of the specimens. The flasks are clamped in position and lowered into the bath, which is kept at 100 c. With the needle valve open, the pump is started and the rocker arm set in motion. The valve is closed at a rate that causes moderate boiling and establishes full vacuum in 5 min. Distillation is continued for 45 min at a bath temperature of 100 C, or for 30 min at 110 C. Some specimens may require other temperatures and heating periods. At the end of the period, air is admitted to the flasks, the pump is stopped, the flasks are detached and allowed to cool, are wiped clean and weighed, and the percentage of nonvolatile matter is computed. A blank is run on the dibutylphthalate. If it loses more than 5 mg, the supply is sparged with dry air for 48 h. An accuracy of 0.2 to 0.3 percent is claimed.
Precipitation Methods for Cellulosic Lacquers In these methods the solids are precipitated with a nonsolvent, the volatile matter is evaporated on a steam bath, and the nonvolatile is dried and weighed. Two variations are practiced.
Method A is suitable for cellulose nitrate base solutions and lacquers that contain no toluene soluble ingredients.
Method B is suitable for high viscosity lacquers. From 4 to 6 g, weighed to the nearest milligram, of the lacquer is transferred to a tared 70 mm aluminum drying dish containing a glass stirring rod, diluted with 100 ml. of acetone, and stirred until solution is complete. The solids are now precipitated by adding dropwise with vigorous stirring, 10 ml. of distilled water. The dish is evaporated to dryness on a steam bath and finally dried at 100 to 105 C for 1 h, cooled in a desiccator, and weighed.
Architectural paints treated in this chapter include solvent thinned and water thinned and exterior and interior types. Cement base paint is treated separately, even though it contains water.
The tests are treated in the following order: Liquid Paint Properties Application and Film Formation and Film Properties. All tests may not be required for each paint. Selection of tests must be guided by experience and the requirements in each case, and be subject to agreement between buyer and seller.
Conditions Affecting Use of Paint
Substrate may be lumber, wood product, hardboard, concrete, brick, metal, or even plastic.
Quality of the substrate will depend on knots and grain in lumber ratio of cement to aggregate porosity of brick, cinder block, and concrete alkalinity of concrete and mortar or previously applied paint.
Type and quality of priming coats.
Weather during and after application of paint.
Orientation, such as that of soffits, fascia boards, porch rails, lumber adjoining masonry, and vertical siding.
Environment, such as sunny or shady side of structure, proximity of other structures, trees and shrubs.
Character of the structure, such as presence of structural defects or defect caused by neglect.
Substrate may be wood, hardboard, wallboard and joint cement system, plaster, metal, or previously applied paint.
Quality and condition of the substrate, such as porosity, smoothness, and color. For topcoats, quality of primer and time between priming and top coating.
Atmospheric conditions, such as temperature and relative humidity during application.
Condition in Container
Condition in the container covers a number of characteristics, such as the presence of curds, agglomerates, gel bodies, seeds, putrefaction, and gas, all of which are objectionable under any condition. Characteristics, such as settling and syneresis, are objectionable if excessive and if the paint cannot be restored to satisfactory condition. Coarse particles, abnormal viscosity, loss of drying, and color drift are acceptable, if within specification limits.
Examining and reporting the condition in the container and the storage characteristics of latex paint requires special attention because of the possibility of decomposition of the paint. In addition to the immediate examination, as described next, of the contents of an unopened, original container, another unopened, original container is weighed and set aside for a specified period of time and temperature. (Note Storage for I month at 125 F simulates some of the effects of storage for 6 to 12 months at 77 F. However it should be recognized that storage at 125 F may not simply accelerate changes that occur at 77 F for example, the growth of some putrefying bacteria is inhibited.)
Qualities 1, 2, 3, and 6 are rated as Absent, Negligible, Considerable, or Severe. Qualities 4 and 5 are rated in the usual manner.
Solvent thinned paint that contains oxidizing filmogen is subject to the formation of an insoluble skin on its surface when air (oxygen) has access to a partially filled container. The tendency to skin is measured.
The character and extent of settling may be determined. The tendency of the pigment to settle naturally is observed usually by setting aside a completely filled container for an agreed upon period, usually six months. Accelerated tests are also described in the same section.
Curds, Agglomerates, and the Like
After any skins have been removed, and, if the pigment has settled, uniformity has been restored, the paint is examined for curds, agglomerates, and the like as it flows from the paddle or spatula.
Generally, in order to produce a film of good appearance, a paint should be free from coarse particles. The glossier the film, the more important is this requirement. An exception is texture paint, which depends in part, on the presence of coarse particles for its decorative effect.
Fineness of Dispersion
This property is a measure of oversize particles, not to be confused with coarse particles. Enamels and high gloss paints should be processed to a high degree of dispersion.
Density (weight per gallon) is a check on the theoretical weight per gallon and on the uniformity of manufacturing. It is not a measure of quality.
This property bears no direct relation to the quality of a paint. However, it is information necessary for classifying solvent thinned paint for shipment by common carrier.
This is a measure of the stability of a solvent thinned paint when thinned to the desired consistency. The recommended thinner should mix readily with a minimum of stirring or shaking. According to FTMS Method 4203, the thinned paint is, allowed to stand for 4 h and is then inspected for curdling or other precipitation or separation into layers. If there is doubt about the condition, some of the material is flowed, without agitation, onto a glass panel. Any of the phenomena mentioned above is then readily observable.
Consistency (Viscosity, Rheological Properties)
The principal reasons for determining consistency are to check the uniformity of manufacturing and to estimate the working properties of the product. Examples of the latter are the Krebs Stormer viscometer to measure the brushing property of a paint and the Ford cup to measure the spraying property of a lacquer. However, the relationships between rheological properties and application (working) properties and leveling properties of architectural paints is not yet known well enough to persuade many technologists to depend on the former to describe the latter. Hence, direct determination of these properties has been practiced.
These are descriptive of the paints response to manipulation by a brush, spray gun, roller, or other means of application. Subjective evaluation with a minimum of instrumentation is common.
Federal Test Method Standard No. 141 (FTMS). good quality wall brush, a 2 by 2 ft coldrolled steel or an aluminum panel, or a 2 by 4 ft gypsum wallboard panel that has been primed with a standard primer. On the wallboard, the paint is applied in sections with the usual back and forth motion. The lay off strokes are applied at a right angle to the lay on strokes. Subsequent sections are always worked toward the wet edge of the section last painted. The effort required to apply the paint and the flowing quality are noted. After the film is dry, it is examined for lap marks, brush marks, and variations in gloss.
Instruments for direct measurement of brushing properties have been proposed but are not used very widely. Methods for calculating brushability from rheological data are also available.
Wet Edge Time
This property is important for paints applied by brush. It is the length of time that a film remains fluid enough to allow the next lap to be merged into the overlap without visible imperfections. It is evaluated usually at the end of a specified period of drying.
The paint is applied to one end of a 1 by 2 ft metal panel, and the film is laid off crosswise, ending along the unpainted half. The panel is placed in a vertical position with the painted end uppermost. Shortly before the specified wet edge time, the painting of the second half is started at the remote edge so that overlapping of the first edge occurs at the end of the specified time. If required, the overlap is cross brushed, and the second half, is laid off parallel to the first half of the specimen. When the paint is dry, the overlap is examined for imperfections, such as film continuity, leveling, gloss, color, etc.
The use of an appropriate gun and a steel panel not less than 4 by 8 inch. The material is reduced as specified in the product specification. During the spraying the gun is held perpendicularly to the panel and is moved in a straight line across the face of the panel. For quick drying material, the spraying distance is 6 to 8 inch for slower drying materials, 8 to 10 inch.
The wet film is examined for running, sagging, and fogging. The dry film is examined for dust, floating, mottling, bubbles, wrinkles, streaks, pinholes, craters, blush, bloom, and silking.
Weigh the loaded roller and roll the paint on the Morest charts evenly, being careful do not exceed the limits of the chart. Allow the weight of the roller to spread the paint uniformly. Finish by rolling in the direction that produces the smoothest film. Reweight the roller to find the weight of the paint used. If this is not within 10 percent of the desired amount, repeat the test until a check is obtained, or until it is determined that the amount is impractical.
Roller Spatter Test The tendency of a paint to spatter when applied with a roller may be determined by the following test devised by T. M. Keenan of the David Litter Laboratories. The spatter is caught on a plastic sheet (black for light, colors, white for dark colors) mounted on the handle of the roller. The sheet is easily mounted on the handle by cutting a slit from a long edge to a hole in the middle and securing with 2 faucet washers. The sheet is 2 in from the roller, and the short dimension is parallel to the roller.
Absorption and Holdout
These properties may be confusing in that they may refer (a) to the substrate or (b) to the coating that is being applied or (c) to the fast coat that has been applied. Usage (b) is preferred by most technologists.
Strong absorption is a necessity for adhesion or paint to a chalky or rusty substrate. On the other hand, weak absorption is desired if a glossy paint or enamel is to exhibit uniform gloss when applied to porous primers or undercoaters. Penetration and holdout are other names for weak absorption and strong absorption, respectively.
A subjective measurement of primer absorption may be made according to FTMS Method 6261, Primer Absorption, and Topcoat Holdout.
Vehicle Migration Test
A roughly quantitative measure of absorption may be obtained by applying the paint to an absorbent surface, such as filter paper.
FTMS Method 4421, Absorption Test, directs that a frictiontop cover for a half pint can be completely filled with the paint and covered with a Whatman No. 12 filter paper (12.5 cm is a convenient size) and allowed to remain for 3 h. The average distance of vehicle migration from the edge of the cover is recorded as the absorption (penetration). Blotting paper may be also used.
An indirect method for measuring the degree to which a coat of paint will holdout a subsequent coat depends upon penetration of the coat by a special staining agent. The specimen to be tested is applied to a nonporous surface, and the reflectance the dry film is measured. A special ink like compound is applied, the excess is removed, and the reflectance measured again. The difference between the two reflectance determinations is a measure of the porosity of the paint film.
In one form or another this test has been practiced for many years. It is now being proposed for adoption by ASTM.
The substrate for the test is a white plastic or a white cardboard sheet, firmly held on a vacuum plate. The paint is applied with a blade spreader (width, 6 inch clearance, 0.012 in.), allowed to dry for 48 h and its reflectance then measured. After 5 min the specimen is suspended from one end and the excess of staining agent is removed with the aid of petroleum spirits from a squirt bottle and a camel hair brush. The specimen, still suspended, is allowed to dry for 3 h, and its reflectance is determined again.
Freeze Thaw Stability
Freezing may adversely affect the consistency and homogeneity of water thinned paint. Two ASTM methods exist for determining the extent of the damage a general method, D 2243, Freeze Thaw Resistance of Latex and Emulsion Paints, and a specific method, D 2337, Freeze Thaw Stability of Multicolor Lacquer.
Two 1 pt cans are charged with two thirds of a pint of the paint. The KrebsStormer viscosity of one specimen (control) is determined. This specimen is then set aside and maintained at 25 C for 168 h. The second specimen (test) is conditioned in a chamber at 9.4 C (15 F) for 168 h. At the end of the period, both specimens are allowed to come to thermal equilibrium at 25 C (requires about 5 h). After one additional hour, and before being stirred, both specimens are examined for settling, gelation, or other abnormalities. They are then stirred, and their viscosities are determined as described before. Immediately thereafter, and again after 48 h, films are brushed onto hiding power charts. Twenty four hours later the films are examined for differences between the test and control films for differences in hiding power, sheen, or other property.
Drying oils include the more or less unsaturated glycerides of long chain fatty acids. All except fish oil are of vegetable origin. Examination of the oils is mainly for quality, although adulteration as low as 5 percent may be sometimes detected. Most of the tests are chemical. A few are based on absorption in the ultraviolet portion of the spectrum. However, the most promising tools for better methods are probably infrared absorption and chromatographic separations
Briefs on the Common Drying and Semidrying Oils
Cacahuananche Oil Also known as Mexican oiticica oil, this oil is obtained from the nuts of the tree Licania arborea. So far as the usual laboratory tests are concerned, this oil and Brazilian oiticica oil are pretty much alike. The raw oil becomes lard like on aging but may be permanently liquefied by heat. The raw and slightly heat treated oil wrinkles as it dries, similarly to oiticica and tung oils.
Castor Oil This oil is obtained from the seed of Ricinus communis. Its principal characteristics are light color, relatively high specific gravity and viscosity, and its solubility in alcohol. It differs from other oils in that its composition is mostly hydroxy fatty acids. It is essentially a nondrying oil, but it may be converted to a drying oil by chemical dehydration by which a hydroxy group and an adjacent hydrogen atom are removed as water to form a drying oil fatty acid ester with two double bonds, one of them being conjugated. This dehydration yields what commonly is known as dehydrated castor oil. in its original undehydrated form, castor oil is well known for its use in resins and as a plasticizer for cellulose ester lacquers.
Chia Oil This oil is obtained from the seed of chia plants, the best known being Salvia hispanica. The most important habitat is Mexico. A prominent characteristic of the oil is its high surface tension, which causes it to crawl. Cooking at 500 F for a short time destroys this property.
Corn Oil This oil is obtained from the kernels of Indian corn, maize, Zea mays. It is semidrying, lying between cottonseed and soybean oils.
Cottonseed Oil This oil from the plant Gossypium malvaceae, is essentially semi drying. As oil it is used rarely in paint.
Fish Oils These oils are obtained from the bodies of many different species of marine fish, the most important ones being menhaden (Alosa menhaden), pilchard (Clupea pitchardis), and the sardine (Clupea sardinis). The menhaden is found in the Atlantic Ocean, while the pilchard and sardine are found in the Pacific Ocean. In addition to glycerides of stearic and the lesser unsaturated fatty acids, fish oils contain glycerides of clupanodonic acid, which appears to contain four double bonds. The iodine value varies over a wide range, approximately 130 to 190. The tendency of fish oil films to yellow considerably is due to the presence of highly unsaturated groups in the molecule.
Hempseed Oil This is a semidrying oil obtained from the plant, Cannabis sativa, usually classed with soybean, poppy seed, sunflower, and walnut oils. Its use in paints is sometimes reported.
Linseed Oil This best known and most widely used oil in the paint industry is characterized by its relatively short drying time. Its high degree of unsaturation, to which its good dry characteristics can be partially ascribed, is due to the presence of large percentages of linolenic and linoleic triglycerides. Many years ago the oil was obtained from seed by mechanical pressure including both hydraulic presses and later expellers. In recent years the more modern solvent extraction is used. Oils thus obtained show lower percentages of impurities and better overall quality. Linseed oil responds very readily to a variety of refining techniques and is used in the paint industry both as a drying oil and as an ingredient in a very array of modified resins of many varieties.
Lumbang Oil This oil, also called candlenut oil, is obtained from the nuts of the tree Aleurites molucanna. Although a product of an Aleurites tree, it contains no elaeostearin. It dries somewhat better than soybean oil.
Oiticlca Oil This oil is obtained Iron the nuts of Licana rigida. It is similar to tung oil in that it has a high specific gravity a high refractive index, and similar gel time when heated. The principal fatty acid, licanic, contains three conjugated double bonds and a keto group. The oil supplements the supply of tung oil.
Perilla Oil This oil is obtained from the seed of the perilla plant, a native of the Orient. The most important plants are probably the P. ocymoides L. and P. nankinensis D. Like chia oil, raw perilla oil exhibits the property of crawling, which is decreased by cooking at 500 F for 15 min or more. It has the highest iodine value of all known vegetable oils except Chia.
Poppyseed Oil This oil is obtained from the plant Papaver somniferum and other Papaver species. It is semidrying and has been used as a medium for artists colors. Like soybean oil and most of the semi drying oils, its films are resistant to yellowing.
Rapeseed Oil This oil is obtained from Brassica rapa and other species. That from the B. campestris is called ravison oil. The terms colza and ruben have been also applied to rapeseed oil. In addition to palmitic and stearic acids, rapeseed oil contains considerable quantities of saturated acids with 20, 22, and 24 carbon atoms. The oil has very poor drying properties but finds considerable use as a plasticizer for nitrocellulose lacquers.
Safflower Oil This oil is obtained from the seed of Carthamus tinctorius, a native of India. It is now readily available from seed grown in the United States. Its drying characteristics lie between those of linseed and soybean oils. One of its main advantages for paint and varnishes is its extremely low after yellowing. This is due to its very low linolenic acid content.
Soybean Oil This is a semidrying oil obtained from the plant Soja hispida, a native of Asia, but also grown extensively throughout the world. When refined it finds wide use as a component in both exterior and interior paints. Its widest use is in the preparation of alkyds.
Sunflower Oil This semidrying oil from the plant Helianthus annus has recently become commercially important in the coatings industry. Blight resistant strains suitable for growing in the United States have been developed. Its fatty acid content is quite similar to that of safflower oil.
Tung Oil Tung is the common name for oil obtained from Aleurites fordii and Aleurites montana. It is also known as wood oil, Chinese wood oil. It is characterized by relatively high viscosity, specific gravity, and refractive index. It dries and polymerizes under heat very rapidly. Its fatty acids are mainly eleostearic, which contains three conjugated double bonds. Its greatest use is in exterior varnish and in alkyds vehicles for exterior paints where water resistance is of prime importance.
A device for taking samples is known as an oil thief. For taking samples from casks, drums, and the like, it may be a suitable length of glass tubing, constricted at both ends so that it may be used as a pipet.
If it is impossible or impractical to thoroughly mix the nonhomogeneous contents of a horizontal cylindrical tank, such as a tank car, a more elaborate device is required in order to get a representative sample. Two such devices are the Bacon Cargille Bomb and the Curtin Zone Sampler, Fig. 2. These permit sampling at any level in the tank. The glass construction of the Curtin Zone Sampler permits the user to check the level where stratification or sedimentation exists. Samples are drawn from the bottom by lowering the thief with a line until it strikes the bottom, when the plunger valve opens automatically, permitting the material to enter. Withdrawal automatically closes the valve. Samples at other depths may be taken by the use of a separate line for manual operation of the valve.
Method for Dark Oils
A number of variants have been proposed for use when the dark color of the oil obscures the color of the indicator. One scheme masks the phenolphthalein indicator by using a solution of 1.6 g of phenolphthalein and 2.7 g of methylene blue in 500 ml of denatured ethanol, the pH being adjusted with alkali solution so that the greenish blue color is faintly tinged with purple. The color change at the end point is from green to purple. Another scheme is to use 100 ml of ether as the solvent. Still another is to add water, salt, and carbon tetrachloride to create a two phase system, in which the indicator enters the supper aqueous layer where it can be seen more readily.
This method is of value in determining the pH of oils and varnishes, for it probably is this factor rather than the total amount of free acid that is responsible for some undesirable effects, such as livering. It must be remembered that dissociation of acids in organic media may be quite different from that in aqueous media. Nevertheless, in any specific solvent, for example alcohol benzene, it should be possible to arrange various acids in the order of their activities. Caldwell and decreasing strength in alcoholbenzene for some typical acid: sulfuric, benzoic, stearic, mixed linseed fatty, linolenic.
Method for Films
Numerous studies have been made of the composition of drying oils during cooking and during the early stages of drying, but, beyond determination of carbon, hydrogen, oxygen, and peroxides, not enough have been made of the chemical nature of aged oil films One piece of the jigsaw puzzle has been supplied by Frilette in his method of determining acid values of dry films and relating them to alkali and water resistance. Films are spread on glass plates with a doctor blade, and the dry films are removed with a razor blade. From 30 to 40 mg of film is transferred to a 25 ml glass stoppered conical flask. To the flask there is now added 5 ml of a 1:1 blend of ethanol and benzene, and 0.5 ml of a 0.01 percent ethanolic solution of Victoria Blue B as indicator (phenolphthalein is destroyed by peroxides in the film). The extract is titrated with 0.01 N NaOH solutions from a Koch microburet. The first end points fade rather rapidly. As the final end point is approached, the flask is warmed in a hot water bath in order to expel carbon dioxide. The true end point should persist for 10 min, and the entire titration may require about I h.
The saponification value of an oil is a measure of the molecular weight of its fatty acids. It is not related to the identity of individual oils. It is not changed appreciably by polymerization, but does increase with oxidation. It is expressed as the number of mg of potassium hydroxide that reacts with 1 g of oil. The value is useful for certain computations involving the use of the oil, such as the manufacture of alkyd resins.
In the determination, the oil is saponified with an excess of alkali, and the excess is determined by back titration with standard acid. Two blanks are also tritrated with the acid.
This is Method D 1962, Saponification Value of Drying Oils, Fatty Acids, and Polymerized Fatty Acids.
To a conical flask (250 to 300 ml) is transferred an amount of oil, usually about 2.0 g, weighed to the nearest milligram, such that the back titration ranges from 45 to 55 percent of the blank. To this flask, and to one or two additional flasks to be carried through as blanks, is added 25 ml of alcoholic KOH solution. A condenser loop is placed in the neck of each flask, and the flasks are heated for 1 h on a steam bath to saponify the oil. The flasks are cooled, and the contents are titrated with 0.5 N sulfuric acid (H2SO4) or hydrochloric acid (HCL), using phenolphthalein as indicator.
A potentiometric method for the saponification value of highly colored oils has been developed. It is time consuming but has led to a double indicator method in which no blank is required. The first indicator accounts for the excess alkali.
Double Indicator Method
The oil is saponified in the regular manner and allowed to cool. Seventeen drops (0.3 ml) of 1 percent alcoholic phenol phthalein indicator are added. The saponified oil is titrated with 0.5 N HCI until the pink color is discharged. The volume of acid need not be noted. The buret is refilled 3 drops (0.2 ml) of 0.010 M bromophenol blue and 10 ml of benzene are added to the flask and the titration is continued to a green end point. This titration represents the difference between the blank and the sample in the usual method.
The solution becomes yellow shortly before the end point is reached. Then as the fatty acids are extracted by the benzene, the blue color returns.
The benzene also extracts coloring matter (usually yellow) of the oil. Agitation produces an emulsion of the yellow benzene solution in the blue aqueous solution, which may appear green momentarily, but the emulsion breaks quickly when agitation is stopped, and the actual color of the aqueous phase can be observed.
The method fails with oils having fatty acids of low molecular weight because such acids give lower pHs than the usual fatty acids.
The unsaponifiable matter in an oil is a measure of the materials that are not converted to water soluble soaps under the conditions of the test. A small amount of unsaponifiable matter is characteristic of all natural oils, varying with the conditions surrounding the extraction and refining. Within the limits of the individual oil specifications, the amount of unsaponifiable matter is no measure of the quality or identity of the oil. An excessive amount of unsaponifiable matter indicates contamination with nonglyceride matter, such as mineral oil, hydrocarbon resins, etc.
The determination consists in saponifying the oil with alkali and extracting the unsaponifiable matter with petroleum ether. An extraction cylinder, glass stoppered, approximately 35 by 300 mm, with graduation marks at 40, 80, and 130 ml is convenient.
About 5 g of sample, weighed to 0.01 g is saponified with alcoholic KOH solution. The resulting soap solution is transferred to an extraction cylinder and extracted seven times with 50 ml of petroleum ether. The extract is transferred to a tared beaker and evaporated to dryness and constant weight. Unsaponifiable oil from adulterated drying oils may be volatile and as a consequence may evaporate on long heating. Therefore, during the evaporation, the ether fumes should be removed with a current of dry air, and the heating should be discontinued as soon as the odor of ether is gone. The residue in the beaker includes any fatty acid that may have formed by hydrolysis of the soap. To correct for this, dissolve the residue, after weighing, in 50 ml of warm alcohol previously made neutral to phenol: phthalien, and titrate with 0.02 N NaOH to the same neutral point. For further details the original method should be consulted.
The amount and nature of the unsaturation in fats and oils indicates their drying properties and rate of heat polymerization. The amount is expressed as the iodine value centigrams of iodine absorbed per gram of oil (percentage by weight). The iodine value is a fairly satisfactory measure of the relative rate of drying and heat polymerizing among oils of the same type however, both properties are affected by the kind of fatty acids and their distribution. Hence, iodine values are not particularly useful for comparing oils of different types. The measurement of unsaturation is an alternative to the determination of the individual acids for identifying natural oils, as each oil has its own range of unsaturation values. Infrared spectophotometry and gas liquid chromatography are particularly useful for such determinations.
Wijs Iodine Value
This method has largely superseded the Hanus and other methods that tend to give high results. It is particularly applicable to oils having no conjugated double bonds, such as linseed, soy, and safflower. Precision and accuracy are reasonably satisfactory. When applied to oils containing conjugated double bonds the results are only relative and do not measure total unsaturation. However, the results are reproducible and serve as a basis for comparison. The Rosenmund Kuhnhenn method is recommended for measurement of total unsaturation.
driers and metallic soaps
Metallic soaps are compounds of alkaline earth metals or heavy metals and monobasic carboxylic acids of 7 to 22 carbon atoms. It is usually convenient to include resinates (usually from rosin) and naphthenates in a discussion of metallic soaps. Their water insolubility differentiates metallic soaps from ordinary soaps. Their solubility or solvation in organic solvents accounts for their use in paints. Commercial metallic soaps are made and used in solid, paste, and liquid forms. The form depends on the metal and its amount, the nature of the organic acid, and the presence or absence of solvents or additives during manufacture. Metals of low atomic weight usually form soaps of high melting points. Long, straight chain, or saturated fatty acids form soaps of higher melting points than do short or branched chains, or unsaturated acids. Soaps made by precipitation are likely to be light fluffy powders. Soaps made by fusion are hard dense solids. Liquid and paste forms are solutions or suspensions in petroleum or other solvents. It is customary to divide metallic soaps into two functional groups: (1) paint driers and (2) modifiers of consistency, gloss, or other properties. The first function is possessed by soaps of lead, cobalt, manganese, iron, and to some extent, by calcium and zinc. The metals found in the second group include zinc, calcium, magnesium, barium, and aluminum.
Liquid driers are evaluated by both physical tests and by chemical analysis. On the other hand concentrated driers are evaluated mainly by their metal content. Metallic soaps are evaluated mainly by using them in formulations and noting how well they fulfill the function for which they are used. See Table 1.
Physical Tests on Driers
Unless otherwise noted, the tests in this section are described in ASTM Method D 564, Testing Liquid Driers, or are specified in ASTM Standard Specification D 600, Liquid Paint Driers.
If visual inspection discloses suspended matter, the amount may be determined by filtering an appropriate amount, say 1 to 5 g, washing with turpentine or petroleum spirits, and drying to constant weight at 49 C.
The color of a solution of the drier in linseed oil is of more interest than that of the drier itself. Comparison is made with Gardner standards dichromate sulfuric acid, or other standards.
The drier is mixed with raw linseed oil and any cloudiness or other separation, immediately after mixing, and after 1, 2, 3, and 24 h, is noted.
A blend of the drier with raw linseed oil (1 volume +19 volume) is flowed onto a clean glass plate. The plate is placed in a vertical position, and the wet to touch time is determined. This is the time elapsed when the oil does not stick to the finger, or the surface is not marred when the finger is lightly drawn across it. It should be noted that this test is somewhat more severe than the usual set to touch point.
Carrier coined the name Aridyne for the unit of drying power. As a standard with which all liquid driers could be compared, he suggested one containing 6.4 oz of oil soluble lead per gallon, equivalent to about 6 percent of lead by weight, and approximately the amount contained in commercial driers. This standard would be 100 proof. However, since commercial driers usually contain other metals in addition to the lead, a 100 proof drier is relatively weak. For practical purposes, a 200 proof or a 300 proof solution is recommended. If the standard contains other metals, the strength is designated in this style: 100 proof 10:1 lead manganese, meaning 6.4 oz of lead and 0.64 oz of manganese per gallon. For driers that contain no lead, a 100 proof product would have the drying power of a 100 proof lead drier.
The Tag Closed Cup is used.
Nonvolatile Matter A 1.5 g sample is heated for 3 h at 105 to 110 C in a tared shallow dish, cooled, and weighed. This is essentially the same as Method A for varnish.
Use any convenient method, such as the Weight Per Gallon Cup. Specific Gravity Balance or a Hydrometer.
The Gardner Bubble Method is recommended.
After 7 days standing, the drier is examined for gelling, clotting, or other form of precipitation.
Classical methods for determining the metallic content of driers are still used, but methods based on flame spectroscopy and on chelate titrations using EDTA (ethylene diaminetetraacetic acid) are rapidly replacing the older procedures. One important advantage is that there is no need to remove organic matter.
Among methods for removing organic matter, when necessary, are ashing, wet oxidation, extraction with mineral acids, and conversion to insoluble oxalates.
Metal Separation by Ashing
This is one of the oldest methods for destroying organic matter to prepare a sample for inorganic analysis.
Heat just to ignition and continue to heat as required to maintain slow burning of the organic matter. When no more flame issues from the crucible, continue to heat, over a flame or in a muffle, to red heat for an hour or so. Dissolve the ash in a minimum amount of nitric acid, and proceed as directed in the appropriate section next.
Metal Separation by Wet Oxidation This method is especially suitable for determining lead as sulfate and is applicable to mixed driers containing lead, manganese, and cobalt.
An appropriate amount of drier is transferred to a 400 ml beaker and heated on a hot plate until the solvent is removed. About 5 ml of nitric acid (HNO3) and 40 to 50 ml of dilute sulfuric acid (H2SO4) are added, and the system is evaporated to dense white fumes, the evaporation being continued to a volume of about 5 ml. If the solution darkens, a few drops of HNO3 are added from time to time until it remains colorless after being heated to white fumes. After the solution is cool, a few milliliters of 30 percent hydrogen peroxide (H2O2) are added, the solution is boiled for a minute, and carefully diluted to about 100 ml, and digested on a hot plate to ensure solution of anhydrous sulfates.
Metal Separation as Acetate
The sample of drier is dissolved in chloroform or ether, glacial acetic acid is added, and the system is refluxed.
Metal Separation as Oxalate
An appropriate amount of drier is transferred to a 500 ml conical flash and dissolved in 75 ml of alcohol acetone, warmed slightly, if necessary, to aid solution. Ten ml of a 10 percent solution of oxalic acid in alcohol is added, and the system is refluxed for about 1 h. The precipitate is filtered and washed with alcohol toluene.
Determination of Lead
If only lead is to be determined, the metal is isolated by wet oxidation and precipitated as lead sulfate (PbSO4). After standing for an hour, the precipitate is collected on a tared Gooch crucible, washed with 0.5 percent sulfuric acid and then with alcohol, ignited at 500 to 600 C for 15 min, and weighed as PbSO4.
Determination of Manganese
The heavy metals and calcium are extracted by a slight modification of Subsection 188.8.131.52. The modification consists of working with a 10 g sample in a 200 mI tall form beaker, preliminary evaporation of the thinner, and extraction of the organic acids with beeswax instead of petroleum spirits. One hundred milliliters of HCI (1 volume concentrated acid + 4 volume water) and 5 g of beeswax are added to the nonvolatile portion of the drier and heated to near boiling, with occasional stirrring for 1 h. The system is cooled and filtered, the cake of wax is washed, and the washings are added to the filtrate. The filtrate is made ammoniacal, and the metals are precipitated with ammonium monosulfide. The sulfides are collected on a filter, and the filtrate is reserved for the determination of calcium, if desired.
Hot HNO3 (1 volume acid, concentrated, 3 volume water) is poured through the filter, and the filter is washed with hot water. Five milliliters of H2SO4 are added to the filtrate, and the system is treated beginning with the evaporation to dense white fumes. If desired the determination of lead is completed. The filtrate is reserved for the step in the following paragraph.
An aliquot of the filtrate is boiled to remove the alcohol, cooled in ice water to 15 C or lower, and treated with about 2 g of sodium bismuthate for at least 10 min to oxidize the manganese to permanganate.
Determination of Cobalt
A 5 g sample is fused in a large porcelain crucible with 5 to 10 g of potassium pyro sulfate (K2S3O7). The fused mass is extracted with 50 to 100 ml of dilute H2SO4 and filtered through fine paper to remove the insoluble sulfates, carbon, etc., and is then diluted to 500 ml in a volumetric flask. To an aliquot containing approximately 50 mg of cobalt is added 10 drops of 30 percent hydrogen peroxide, 1 or 2 drops of phenolphthalein indicator, and enough 0.5 N sodium hydroxide (NaOH) solution to precipitate the cobalt completely and to show an alkaline reaction.
Determination of Zinc
Use a solution in HCl of the ash obtained or an aliquot of the extract obtained. The sample should contain about 0.1 g of zinc. Manganese, cobalt, nickel, and iron interfere and must be removed.
Determination of Iron
Use an aliquot of hydrochloric acid extract corresponding to about 0.1 to 0.2 g of iron. Proceed as directed beginning with reduction with stannous chloride.
Chelometric Determination of Drier Metals
EDTA (ethylenedinitrilotetracetate, according to American Chemical Society (ACS) nomenclature, but commonly called ethylenediaminetetracetate) combines (chelates) strongly with metals used in driers and metallic soaps. However, it is the disodium salt rather than the free acid that is used in chelometric titrations. The compound is also known by trade names, such as Komplexon and Versene.
By conducting the reaction in a basic solution, the hydrogen ion is removed, and the reaction becomes quantitative. What makes the method so attractive in drier analysis is that extractions or combustions are eliminated in many cases.
In addition to the resins traditionally known as natural resins, this chapter includes tests on rosin and lac. It does not include bitumens.
Also included are tests for the physical properties of synthetic resins the chemical analysis of which appears in the Chapter.
The traditional natural resins are exudations of trees and may be classified according to origin as fossil, semifossil, or recent or according to use, as oil soluble or spirit soluble (Table l).
According to most investigators, varnish resins consist largely of resin acids and neutral substances of unknown composition, designated resenes, with small proportions of volatile compounds, ash, and impurities. The absence of esters, ethers, anhydrides, and lactones (except for rosin) has been suggested.
Identification of Natural Resins
Very few tests for specific natural resins are available, and none is entirely satisfactory, except perhaps the Liebermann Storch and the Halphen Hicks tests for rosin. When the natural resins were much more important in varnish technology than they are today, their identification was based mainly on odor, color, hardness, and solubility. Today, the availability of many more solvents should make solubility tests more useful.
According to Brauer reagents based on phosphomolybdic and phosphotungstic acids are useful in detecting natural resins and in identifying some of them.
To detect resins in linseed oil, a small sample is covered with ether, several drops of a freshly prepared concentrated solution of phosphomolybdic acid are added, and a few drops of ammonia water. Upon shakling, all resins that were examined gave a typical blue color, which, in some cases, turned towards green. Linseed oil, itself, gave only a faint green.
A reagent said to differentiate among resins is made by shaking 0.1 to 0.2 g of powdered ammonium molybdate with 5 ml of concentrated sulfuric acid. Addition of a few milliliters of resin solution produces characteristic colors. Rosin gives a Prussian blue color, and addition of ammonia converts the solution to a gelatinous mass.
Resins containing coniferin or related compounds give a cherry red color with phloroglucinol.
Stock examined resins by capillary analysis, which today would be called paper chromatography. Strips of filter paper are immersed partially in resins solutions for periods up to 24 h, withdrawn, and allowed to dry. Adsorption pictures appear on the strips, showing strata of different colors, extent, intensity, opacity, etc. Important variables include type of paper, strength of solution, depth and time of immersion, size of vessel, temperature, and relative humidity. Forty eight reproductions in black and white of the results of tests on single resins and mixtures, with full descriptions are given.
Fonrobert examined the method and concluded that it enables one to distinguish between groups of resins but does not reveal differences within a group.
Kostka examined various natural and artificial ambers under ultraviolet radiation from a mercury in quartz lamp transmitting radiation of wavelength 440 to 280 nm. Natural amber is strongly fluorescent, giving a. greenish light, but sometimes more bluish or yellowish. Nontransparent specimens appear to be bathed in white light with bluish or greenish tints. Artificial ambers vary phenol formaldehyde plastics do not fluoresce, urea formaldehyde plastics emit blue, casein derivatives bluish to bluish white, celluloid and cellon emit bright blue.
Wolff found that a combination of fluorescence and capillary analysis gave more information than fluorescence alone, as zones of fluorescence are often more distinct in one case than in the other.
Identification of Rosin
Liebermann Storch Test Also known in Europe as the Storch Morawski test, this might be considered to be the classical test for rosin and its derivatives. Typical directions are those of ASTM Method D 1542, Qualitative Tests for Rosin in Varnishes.
The specimen is dissolved in boiling acetic anhydride. To 1 or 2 ml of the cooled solution in a white porcelain dish is added drop of sulfuric acid (sp gr 1.43 prepared by mixing 34.7 ml of 1.84 acid and 35.7 ml of water). Rosin gives a fugitive violet color lasting for a second or two, Stoppel emphasizes the use of small samples (5 to 8 drops of varnish, for example), boiling for several minutes, and acid of proper concentration. The color produced by ester gum is less blue than by rosin. Elsner observed that copal and sandarac tend to give the same color that turpentine in varnish may interfere.
Michel Modification of L S Test Dissolve 0.1 to 0.05 g of sample in 3 ml of chloroform add 5 ml of sulfuric acid (sp gr 1.56 to 1.58) and shake thoroughly. After the chloroform layer becomes clear, add acetic anhydride drop by drop. If the merest trace of rosin is present, the chloroform layer becomes purple. By repeated vigorous shaking, the sulfuric acid layer dissolves the coloring matter and becomes carmine to purplish red. The amount of sample should be regulated in order to avoid a color that is too intense.
Halphen Hicks Test This test, along with the Liebermann Storch, appears in ASTM Method D 1542. Two reagents are needed.
Dissolve a small quantity of the sample in 1 to 2 ml of the phenol reagent. Fill a cavity of a spot plate with the solution so that some of the solution spreads beyond the cavity. Immediately in an adjacent cavity transfer about 1 ml of the bromine reagent so that bromine vapors will spread over the other solution. It may be helpful to cover both solutions with a watch glass or to move the bromine vapors with a gentle current of air. A fugitive violet color indicates the presence of rosin.
Martni Test In a test tube, slowly heat from 0.2 to 1.0 g of resin with 2 to 3 g of calcium oxide and pass the fumes over several drops on nitric acid (sp gr 1.4). A reddish violet color indicates rosin. The color changes to green and then to blue. It has been stated that lac reacts similarly.
Donath Test This method is claimed to be approximately quantitative for rosin in beeswax, ceresin, etc., substances that give no color with nitric acid (spgr 1.32 to 1.33). The sample is added to 5 times its volume of the acid. The mixture is boiled for 1 min, then diluted with an equal quantity of water, and treated with an excess of ammonia. Rosin produces a red color. However, according to Stock the method is indicative only, and not conclusive when other resins are present.
Identification of Lac
Lac may be recognized by the odor developed upon heating. Unbleached lac may be also identified by the dark purple color of its alkaline solution. Natural resins do not give this color, but some synthetic ones may.
According to Bhattacharya unbleached lac is the only common natural or synthetic resin which is in any degree soluble in aqueous alkali bisulfite solutions. A warm 10 percent solution of the bisulfite will dissolve up to 50 percent of its weight of lac. The solubility of lac decreases with age and its degree of polymerization.
Commercial Grades of Natural Resins
These are based on color, amount of impurities, and size. Usually the amount of impurities increases with decrease in size.
This parameter is not particularly useful for characterizing or identifying resins. The index of synthetic resins is somewhat higher than that of natural resins Table 4. For low melting point resins or for solutions, the Abbe refractometer may be used, but immersion methods using the microscope are probably more useful. An indirect method is to determine the indexes of a series of solutions of increasing concentrations, to plot the indexes against the concentrations, and to extrapolate to 100 percent concentration.
Resins do not exhibit sharply defined melting points as do crystalline organic compounds. As the temperature rises resins gradually soften and become less brittle and less viscous. In a sense, determining softening point is determining viscosity under arbitrary conditions. For results to be comparable, procedures must be rigorously defined. Dimensions of apparatus, the immediate recent history of the specimen, and rate of heating must be standardized.
Capillary Tube Method
This method has been used. The temperature at which the resin begins to darken and coalesce is the arbitrary softening point the temperature at which it loses its powdery appearance and becomes completely transparent is the melting point. Examples of some softening and melting points obtained in this way are given in Table 5.
Ring and Ball Method
This is ASTM Method E 28, Softening Point by Ring and Ball Apparatus, developed in ASTM Committee D 17 on Naval Stores.
Derivatives of cellulose used in paint and related materials include the inorganic ester, the nitrate the organic esters, the acetate, acetate propionate, and acetate butyrate the ethers, methyl and ethyl, and some of their derivatives, such as the hydroxy ethyl, the hydroxypropyl ethyl, and the carboxymethyl.
The tests in this chapter are for quality and uniformity.
Soluble cellulose nitrate, also known as nitrocellulose, is a white amorphous powder or cotton like solid. It is always handled dampened with at least 20 percent of water, or 20 to 25 percent of an alcohol. For some uses, toluene is the dampening liquid. In the dampened condition, cellulose nitrate presents no unusual hazard. Dry cellulose nitrate, if ignited by fire, spark, or static electricity, burns very rapidly. It must never be stored.
ASTM Specifications and Methods of Testing D 301, for Soluble Cellulose Nitrate, covers appearance, ash content, nitrogen content, stability, viscosity, solubility, and appearance of solution, film formation, and toluene dilution ratio. It also includes instructions for drying the dampened material needed for some of the tests.
The producer, of necessity, makes all of the tests routinely. Rarely, if ever, is it necessary for the coatings manufacturer to make any tests other than viscosity, solubility and appearance of solution, film formation, and toluene dilution ratio.
Drying Cellulose Nitrate: This is a necessary preliminary operation for most tests, as the results are based on the dry weight of the cellulose nitrate. Also, most formulations, in effect, are based on dry weight.
Only the amount necessary for immediate testing should be dried. Excess material and specimens left after testing should be wet with water and destroyed by burning on a safe burning ground.
Larger amounts may be dried by passing warm (60 to 65 C) compressed air through the material for about 1 h. Figure 1 shows equipment suitable for this purpose.
If the cellulose nitrate is dampened with alcohol, it is best to dilute with a small amount of water before the drying.
This test is not often made by the coatings manufacturer. It is the viscosity of a specific solution of the cellulose nitrate and is the key to the viscosity of coatings made with the specific grade of cellulose nitrate. The standard method is described in ASTM Method D 1343, Viscosity of Cellulose Derivatives by the Ball Drop Method.
Solutions are prepared according to one of the formulas in Table 1 with the dried cellulose nitrate dried. The material dries faster if it is first wet with alcohol and toluene and the mixture allowed to stand for a few minutes before the ethyl acetate is added. Solution is completed by tumbling or shaking and is brought to 25 C for the test.
Solubility and Appearance of Solution
This test is a check on the possible presence of impurities that might discolor the solution, or impart haze, grain, or flock, to it. The cellulose nitrate is dissolved according to Formulas A, B, or C (Table 1), and the solution is compared with a fresh solution of the reference standard, similarly prepared. The comparison is made in small vials on the basis of color, turbidity, grain, and flock.
Solutions of the sample and reference standard, prepared and diluted with equal volumes of n butyl acetate are poured side by side on a clean glass plate and allowed to dry in a nearly vertical position. When dry, the sample film is rated against the reference film on the basis of undissolved particles, gloss, and flow.
Toluene Dilution Ratio
This is a simplified version of the method. The solution contains 12.1 g of the dry cellulose nitrate in 87.8 g of n butyl acetate. Fifty milliliters of the solution is titrated with toluene to the first permanent separation of cellulose nitrate. No adjustment of concentration is made.
Cellulose Nitrate Base Solutions
These are prepared by dispersing various kinds and concentrations of soluble cellulose nitrate in various solvent blends. Since the compositions of the solutions vary widely, the limits desired for a specific type must be agreed upon by the interested parties. Suitable methods of test appear in ASTM Method D 365, Testing Soluble Nitrocellulose Solutions. The following tests are specified, and coatings manufacturers usually make all four.
Three methods for this parameter are specified. The one to be used depends on whether the viscosity, determined by ASTM Method D 1343 is: (1) from 3 to 500 s, (2) less than 3 s, or (3) more than 500s.
Proceed as directed in Method A for varnish. Method B for varnish is potentially dangerous because of the higher temperature used.
A method that avoids oven hazard and also the possibility of entrapping solvent in the nonvolatile residue precipitates the cellulose nitrate with toluene (xylene or highflash naphtha, if high boiling solvents are present in the base solution), evaporates the solvent in a steam bath, and finally dries the precipitate in an oven at 105 to 110 C. An incidental feature is handling the specimen in a collapsible tube. About 20 g of the base solution is loaded into a collapsible tube (available in drug stores). From 4 to 6 g, weighed to the nearest milligram, is transferred to a tared 100 ml beaker containing a glass stirring rod. Without delay, from a buret, 5 ml of toluene for each gram of solution is added slowly, with vigorous stirring, to the base solution. Too rapid addition may precipitate the cellulose nitrate as lumps. The beaker is now suspended in a steam bath until the solvent has evaporated (20 to 30 min). Water on the outside of the beaker is wiped off, and the specimen is dried at 100 to 105 C for 1 h, or to constant weight, cooled in a desiccator, and weighed.
The depth of color is matched against platinum cobalt or caramel standards depending on which standards include the specimen for the description of the standards.
Cellulose Acetate is a white, tasteless, odorless, fluffy powder. Unlike cellulose nitrate its flammability is low, and its handling presents no unusual hazard.
ASTM Method D 871, Testing Cellulose Acetate, covers color and haze, combined acetyl or acetic acid content, free acidity, heat stability, hydroxyl content, intrinsic viscosity, moisture content, primary hydroxyl content, sulfur or sulfate content, and viscosity. The coatings manufacturer usually restricts his testing to viscosity, color and haze, and solubility and appearance of solution.
This is the ball drop viscosity of a solution of the dry cellulose acetate in a solvent and at a concentration agreed upon by the interested parties. Suitable formulas are listed in Table 3.
Color and Haze
Ratings for color and haze of cellulose acetate solutions are made by comparison with liquid standards in the light box shown in Fig. 2. The box is 17 in. high, 14 in. wide and 13 in. deep. On the front is an enclosed shelf for the specimen and the color and haze standards.
The color standards are solutions of platinum and cobalt. The haze standards are suspensions of fullers earth in dilute hydrochloric acid solution containing from 10 to 400 ppm. The specimen to be rated is dissolved in the specified amount and kind of solvent in the same kind of bottle used for the color and haze standards French square bottles, 16 oz, with screw caps. Suggested formulas are listed in Table 3.
The specimen to be rated is placed on the front of the shelf, and behind it is placed a similar bottle containing water. The selected haze standard, freshly shaken, is placed beside the specimen with the color standard behind it. The standards are changed as needed until the optimum match has been found. Ratings for both color and haze are reported in parts per million.
Solubility and Appearance of Solution
Cellulose Acetate Butyrate and Cellulose Acetate Propionate
These mixed esters of cellulose resemble cellulose acetate in appearance, but they have more flexibility and better solubility, and are compatible with more resins and plasticizers than are the straight esters. ASTM Method D 817, Testing Cellulose Acetate Propionate and Cellulose Acetate Butyrate, contains the following tests acetyl, and propionyl or butyryl contents acetyl content, apparent free acidity ash color and haze heat stability hydroxyl content primary hydroxyl content moisture content
Ethylcellulose is a white, odorless, tasteless, nontoxic granular solid. ASTM Method D 914 specifies tests for moisture content, ash content, chloride content, ethoxy content, and viscosity. Only the viscosity is determined routinely by coatings manufacturers.
Any acceptable method may be used, although ASTM Method D 445, Viscosity of Transparent and Opaque Liquids is recommended when there is need for relatively high precision. The determination is made on a solution prepared according to one of the formulas in Table 4.
Methylcellulose is a white, or slightly yellow, odorless, tasteless solid, in the form of powder or granules. It is available in alkali soluble or water soluble type. ASTM Method D 1347, Testing Methylcellulose, specifies the following tests moisture, ash, chlorides, alkalinity, iron, heavy metals, methoxy, viscosity, pH, solids, and density. Only viscosity is determined routinely by the coatings manufacturer.
Viscosity of Alkali Soluble Methylcellulose
This is determined in the same way as for the water soluble type, except that 1 N sodium hydroxide instead of hot water is the solvent.
This material is a white or pale yellow solid, available as powder or granules. Unlike straight carboxymethylcellulose, it is soluble in both hot and cold water. Many grades, based on etherification, viscosity, purity, and other characteristics, are available. ASTM Method D 1439, Testing Sodium Carboxy methylcellulose, specifies the following tests: moisture, degree of etherification, viscosity, purity, sodium glycollate, and sodium chloride. Only viscosity needs to be determined routinely by the coatings manufacturer.
This is an empirical method for the viscosity of solutions of sodium Carboxymethylcellulose in the range of 10 to 10,000 cp at 25 C. Hence, the results do not agree necessarily with those obtained on other types of viscometers.
The concentration to be used should be agreed upon by the interested parties. It should be such that the viscosity falls within the range of the test. The determinations are run on the calculated dry basis. The Brookfield viscometer, Model LVF or equal, has been selected for the test. The spindle and speeds given in Table 5 are recommended.
Hydroxyethylcellulose is a white, odorless, tasteless solid, in the form of powder or granules. ASTM Method D 2364, Testing Hydroxyethylcellulose, contains only three tests, namely, moisture, ash, and viscosity. Of these, the coatings manufacturer is usually concerned only with viscosity. The method is the same except that the volume of the solution is 250 g, and stirring at 1500 rpm is permitted in the preparation.
Several thousand high boiling solvents that impart permanent flexibility to otherwise rigid plastics have been created in recent decades to supplement to relative few available in the twenties. The appearance of new types of plastics and their adaptation to new uses requiring flexibility has prompted this search. The utility of a plasticizer is judged by the performance characteristics of the resin or plastic to which it has been added. This indirect test on the plasticizer implies that its properties are uniform. In fact, producers place great emphasis on quality, and the properties determined and methods used are of equal importance to his customer.
Both physical and chemical tests are required by the manufacturer to meet his commercial grade specifications, and by the user to ensure that the plasticizer meets his requirements as a raw material. It is the purpose of this chapter to suggest basic properties and methods for their determination. It is further intended to suggest means for isolation, identification, and semiquantitative measurement of plasticizers present in lacquers and in the dried film after application to a substrate. It will be obvious that many of these methods apply equally to flexible plastics.
The scope of this chapter precludes detailed description of the methods involved, but the reader may choose from among the references such tests as he may need.
The complexity of the potential problems involved will be apparent in Table 1. This table lists representative types and classes of plasticizers, and major basic types of resins or plastics, which together are classed as lacquer type coatings.
Physical and Chemical Test Methods
Acidity in a plasticizer may be due to improper refining techniques, instability in storage, or contamination. A suitable procedure is ASTM Method D 1613, Acidity in Volatile Solvents and Chemical Intermediates Used in Paint, Varnish, Lacquer, and Related Products. The sample is mixed with an equal volume of alcohol (ethyl or isopropyl), and titrated with aqueous sodium or potassium hydroxide to the phenol phthalein end point. The test results may be expressed as percent by weight as acetic acid, acid number (milligrams potassium hydroxide consumed per gram of sample) or, if the acid used in preparing the ester is known, as percent of that acid.
The presence or absence of color is an indication of the degree of refinement or cleanliness of the shipping or storage container. Plaslicizers in general are essentially colorless, but polymeric plasticizers may have the appearance of a light molasses. The usual method is comparison with platinum cobalt standard solutions.
The plasticizer must be completely miscible with the resin or plastic component(s) after drying of the lacquer on the substrate. Test by adding plasticizer to the lacquer solution in an amount equal to the base resin. If the dried film remaining on a glass plate after evaporation of the solvent is not clear and transparent, repeat the test with reduced amounts of plasticizer until a transparent film is obtained. Oily or solid exudates should not be present on the surface of the film. This is a go no go measure of compatibility and is convenient for initial screening of the plasticizer. In selecting a plasticizer for use in a lacquer, it is well to remember that permanence of the mixture on the substrate may be influenced by the conditions of ultimate exposure, including temperature, effect of light (UV), humidity, and other components in the formulation.
This test is applied normally to hydrocarbon solvents and is a visual estimate of the presence of free and combined sulfur. Some types of plasticizers, that is, sulfonic acid derivatives, should be evaluated for degree of discoloration.
This property may be affected by improper refining techniques, impurities inherent in the sample, or contamination. Atmospheric distillations are made according to ASTM Method D 1078, Distillation Range of Volatile Organic Liquids, or ASTM Method D 86, Distillation of Petroleum Products. The high temperatures involved may cause decomposition, and more significant values may be obtained by conducting distillations under vacuum as low as 5 mm. This is a critical property.
Trace quantities of impurities often impart electrical conductance to otherwise high resistance plasticizers. The electrical insulating qualities are measured by d c resistivity and by the power factor. The procedure for the former is to be found in ASTM Method D 257, Electrical Resistance of Insulating Materials for the latter in ASTM Method D 150, A C Loss Characteristics and Dielectric Constant (Permittivity) of Solid Electrical Insulating Materials.
Since most of the large volume plasticizers are esters, this test may be used to estimate purity. The remaining portion of the sample usually is the alcohol associated with the original reaction to produce the ester. The preferred test is described in ASTM Method D 1617, Ester Value of Lacquer Solvents and Thinners. The sample is saponified with an excess of 0.5 N potassium hydroxide (KOH) in a pressure bottle immersed in a bath of boiling water. The excess alkali is titrated with standard sulfuric acid, and the percentage of ester is computed from the following equation.
ASTM Method D 92, Flash and Fire Point by Cleveland Open Cup, is commonly used. Test results are influenced by improper refining or by contamination with low boiling material.
The Abbe refractometer is used, but the Pulfrich refractometer is also satisfactory. For details, see ASTM Method D 1218, Refractive Index and Refractive Dispersion of Hydrocarbon Liquids, or directions accompanying the instrument being used. This is a very precise test and may be used as an identifying test and to indicate contamination.
Odor may be influenced by improper refining or by contamination. The usual test is ASTM Method D 1296. Since plasticizers are relatively nonvolatile, odor is noted after the saturated filter paper strip has drained for 5 min.
Representative samples are a prerequisite for the evaluation of plasticizers. If familiarity with the plasticizer permits, ASTM Method D 1045, Sampling and Testing Plasticizers, may be used. Otherwise, the more elaborate ASTM Recommended Practice E 300, Sampling Industrial Chemicals, should be used.
The term solidification point, rather than freezing point, distinguishes industrial grade material from the high purity material otherwise inferred. The temperature at which solidification occurs relates in part to retention of solubility in, and flexibility of, the dry lacquer film. ASTM Method D 1493, Solidification Point of Industrial Organic Chemicals, should be used. Since temperatures as low as 70 C will be encountered, ASTM E 1, Specifications for ASTM Thermometers, should be consulted for thermometers to use. For such low temperatures a denatured alcohol dry ice bath, or equivalent, will be required.
This test provides a means of identification, where used in conjunction with other tests, but is affected by impurities. The hydrometer or the pyconometer methods may be used.
This test is a measure of the flow characteristics of the plasticizer at various temperatures. The Brookfield viscometer is preferred to efflux or other rotational types because of its ready adaptability to all temperatures or viscosities likely to be encountered.
Plasticizers in general may absorb small amounts of water, and this could have an adverse effect on lacquers containing hydrocarbon solvents both in the liquid form and on the dry film. The recommended method is ASTM Method D 1364, Water in Volatile Solvents, Fischer Reagent Titration Method.
Isolation of Plasticizer
If the sample is a plasticizer, preliminary preparation is not necessary. A laquer, however, should be dried on an amalgamated plate. The film thus prepared as well as scrapings from an already dry lacquer coating should be extracted with hot ethyl ether in a Soxhlet apparatus to isolate the plasticizer. The conditions of ASTM Method D 494, Acetone Extraction of Phenolic Molded or Laminated Products, are convenient, substituting ether for acetone and conti nuing the extraction for 6 h. After the ether has been evaporated the specific tests are applied to the residual plasticizer. If the isolated plasticizer is hazy, mix with several milliliters of ethanol and filter. This treatment removes polymers that may have been soluble in the ether.
The sample is fused with metallic sodium for the detection of the elements nitrogen, chlorine, sulfur, and phosphorus. To a clean, dry. 6 in. test tube supported near the open end in a vertical position with a clamp and iron stand, add a 3 mm cube of freshly cut metallic sodium. Heat the bottom of the tube until a layer of sodium vapor 1 cm deep is formed. Add directly to the vapor 2 to 3 drops of liquid sample. An equivalent amount of lacquer scrapings or dry film may be treated in the same manner. Remove the flame immediately. When the tube is cold, break off the end with the sodium in a mortar. Add several milliliters of alcohol to destroy unreacted sodium, then add 20 ml of distilled water and grind coarsely. Transfer to a beaker, bring to a boil, and filter. The filtrate should be colorless.
CAUTiON Perform all the above under a hood using a face shield and avoid contact with water until after the alcohol treatment.
Sulfur To 5 ml of filtrate add 2 ml of a 10 percent solution of solution hydroxide (NaOH) containing 2 to 3 drops of a 10 percent solution of lead acetate. A black precipitate of lead sulfide will form if sulfur is present.
Nitrogen Boil for 1 min, 2 ml of tiltrate, 5 drops of a 10 percent solution of NaOH, and 5 drops of 10 percent ferrous sulphate solution. Cool, and add 10 percent solution of hydrochloric acid (HCl), drop by drop, until the solution is acid and the precipitate of ferrous hydroxide has dissolved. Avoid excess acid. A blue or green color or blue precipitate indicates presence of nitrogen.
Chloride Acidify 5 ml of filtrate with several drops dilutes sulfuric acid (H2SO4) and boil for several min if sulfur or nitrogen is present. Cool and acidify with nitric acid (HNO3) and add several drops of 10 percent silver nitrate (AgNO3) solution. A whitish precipitate indicates presence of chlorine.
Phosphorus Boil 5 ml of filtrate with 3 ml of concentrated HNO3 for 1 min. Cool and add twice the volume of 10 percent ammonium molybdate solution. Heat to about 60 C and set aside to cool. A yellow precipitate indicates the presence of phosphorus.
Phthalates Add about 0.05 g of resorcinol and 0.05 g of phenol to separate 6 in. test tubes and to each add 2 to 3 drops of the isolated plasticizer and a drop of concentrated H2SO4 then heat several minutes in an oil bath at 160 C. Cool and add 2 ml of distilled water and 2 ml of 10 percent NaOH and stir. If phthalates are present, the tube with resorcinol will show a pronounced green fluorescence, and the tube with phenol will be red. Sebacates and ricinoleates will give a faint greenish fluorescence.
Phenols Destructively distill 1 to 2 drops of the isolated plasticizer in a 5 in. test tube and collect the vapors in a second tube containing several mililiters of distilled water. Shake and filter. To a portion of the liquid add 1 drop of Miltons reagent (prepare by dissolving 1 part mercury in 2 parts concentrated HNO3 (weight, weight) and dilute with 2 volumes of distilled water. Use the supernatant liquid for the test and heat gently. A reddish coloration develops if phenols are present. The test may be confirmed by adding to a separate portion of this liquid several crystals of 2, 6 dibromoquinonechloroimide. Shake and add 1 drop of 10 percent NaOH. A blue streak in the liquid or blue on the edges of the undissolved crystals of reagent indicates phenols. A positive test indicates the presence of tricresyl phosphate or other phenolic plasticizer.
The quantitative measurement of plasticizers at the present time is limited to those characterizing components discussed in the qualitative tests. Methods for the estimation of these components are given next.
The quantitative results obtained are calculated back to structures of known formulas.
Sulfur A suitable procedure is included in ASTM Method D 817, Cellulose Acetate Propionate and Cellulose Acetate Butyrate. Reactions involving perchloric acid are hazardous, and suitable precautions must be observed.
Nitrogen is determined by the Kjeldahl method as found in ASTM Method E 258, Total Nitrogen in Organic Materials by Modified Kjeldahl Method, or in ASTM Method D 1013, Total Nitrogen in Resins and Plastics. The sample is digested in a mixture of concentrated sulfuric acid, potassium sulfate, and mercuric oxide. The organic material is oxidized, and the nitrogen is converted into ammonium sulfate. Sodium sulfide is added to the digested mixture to precipitate the mercury after which the solution is made alkaline with strong sodium hydroxide solution and the ammonia that is liberated is distilled into a measured volume of standard acid. The excess acid is titrated with standard sodium hydroxide solution.
Chlorine The Thompson Oakdale method appears to be very satisfactory for this determination. The version in ASTM Method D 1156, Total Chlorine in Poly (Vinyl Chloride) Polymers and Co polymers Used for Surface Coatings, may be followed.
The sample is decomposed in a special glass apparatus by stepwise treatment with H2SO4, potassium persulfate (K2S2O8), and potassium permanganate (KMnO4). Chloride is converted to free chlorine. The chlorine is absorbed in a sodium arsenite solution. This solution is acidified with HNO3 and treated with AgNO3 to precipitate the chlorine as silver chloride.
Phosphorus: Phosphorus may be determined as directed in ASTM Method D 1091, Phosphorus in Lubricating Oils and Additives. Two methods, photometric and gravimetric, are given, but the latter is preferred.
Organic matter is destroyed and the phosphorus is converted to phosphate ion by oxidation with sulfuric acid, nitric acid, and hydrogen peroxide. The phosphate ion is then separated from interfering metals by precipitation as ammonium molybdophos phate in nitric acid solution. The solution is made ammoniacal and the phosphorus is precipitated as magnesium ammonium phosphate, ignited, and weighed as magnesium pyrophosphate.
Oxirane ASTM Method D 1652, Epoxy Content of Epoxy Resins, is available for this determination.
The sample is dissolved in a suitable solvent, and the resulting solution is titrated directly with a standard solution of hydrogen bromide in glacial acetic acid. The hydrogen bromide reacts stoichiometrically with epoxy groups to form bromohydrins therefore, the quantity of acid consumed is a measure of the epoxy content.
Identifications by Refractive Index and Density
Seymour describes a method of classifying plasticizers by plotting refractive index against density. He then narrows the classification by plotting refractivity index against density. By comparing boiling point against these data he arrives at satisfactory identifications in most instances.
Forziatti has tabulated the fluorescent colors of a number of plasticizers as indicated in Table 2. To perform the test, place a drop of plasticizer on a filter paper and expose to black light (UV at 3650 A). Perform this test in a dark room.
Definition and Requirements
A solvent may be defined as a liquid that is used to bring a solid or semisolid material into a liquid form. The ability to dissolve a material is the distinguishing characteristic of a solvent, the primary performance property. Of nearly equal importance in almost every coatings application is evaporation rate. Direct measurement of these performance properties is not always feasible or convenient. Other tests, therefore, are used to estimate them. There are other important properties, and tests to measure them, that are not concerned directly with basic performance. These include safety of handling, uniformity, purity, composition, and compliance with air pollution laws.
A solvent reduces the coating viscosity to the level required for application. The required viscosity, as well as the required evaporation rate, varies with the end use and the method of application. Evaporation of the solvent is always the first and sometimes the only mechanism in drying of the coating. It is the first stage in the drying of coatings made with reactive resins that dry by oxidation or catalytic or heat curing. It is the only mechanism in the drying of lacquer type coatings.
Resins that dry by oxidation or heat curing are very often soluble in hydrocarbon solvents made from petroleum or coal tar. In cases where other types of solvents are used, the possible participation of the solvent in the reaction must be considered. For lacquer type resins, which dry by solvent evaporation, there are three solvent constituent types: active, latent, and diluent. An active solvent is a true solvent for the resin. A latent solvent alone will not dissolve the resin but becomes a solvent or has a synergistic effect when blended with an active solvent. A diluent has no solvency for the resin but is tolerated by it in blends and is added to reduce cost and sometimes vehicle viscosity.
A solvent does not remain permanently in a coating film, but its effects are apparent in the film. Leveling and sagging are apparent for the life of a coating, but they can be controlled by proper choice of solvents. In a paint, the solvent initially imparts the proper flow and usually is a factor in the thickness of the film that can be applied. As the solvent evaporates, the viscosity of the film increases, and this, in turn, affects the appearance of the film.
Solvency manifests itself in two ways: (a) by the miscibility of a solvent and a resin and (A) by the efficiency of a solvent in reducing resin viscosity. Miscibility is usually detected visually. Lack of miscibility creates layers of liquid that do not blend a cloudy solution, or a solid precipitate. The best method for choosing solvents that are miscible with resins is to use solubility parameters.
Solubility Parameter System
A scientific system has been developed for selecting the solvent or solvent combination for a given coating requirement. This is based on solubility parameter, which is a numerical constant characteristic for each solvent and film forming material. The small Greek letter delta is used to designate these values which are single numbers for solvents, whereas it is more convenient to designate a range of values for resins or polymers.
If the value for a liquid lies within the range designated for the film former, that liquid will be a solvent for the film former. Thus, a simple matching of numbers is all that is required to predict solubility. It is necessary, however, to introduce one other factor in order to make accurate predictions, namely, Hydrogen bonding.
Solvents may be grouped into three classes according to their hydrogen bonding characteristics:
Class I Poorly Hydrogen Bonded Solvents: includes aliphatic, aromatic, chlorinated, and nitro hydrocarbons.
Class II Moderately Hydrogen Bonded Solvents: includes esters, ketones, ethers, and ether alcohols.
Class III Strongly Hydrogen Bonded Solvents: includes alcohols, amines, and acids.
It is customary to specify the solubility parameter ranges for the film formers in each of these classes. As may be seen in Table 1, individual resins may be soluble in different numerical ranges in any one or more of the hydrogen bonding classes.
Values for Solvents Solubility parameters can be calculated in a variety of ways. A simple method uses the latent heat of vaporization and density. Hoy examined solvent parameters based on vapor pressure. Values have been already calculated for most solvents, and tables are available listing them in alphabetical, numerical, and boiling point orders. An abridged set of values for common solvents appears in Table 2.
Values for Film Formers Since it is usually impossible to volatilize a polymer, values for film formers must be obtained indirectly. A convenient method for determining ranges is the following:
A gram or two of solid polymer is placed in a test tube, and an approximate amount of a selected solvent is added such that the final solution would have about the correct solids content for the expected commercial use, for example, 50 percent for alkyds, 20 percent for vinyls, etc. The exact amount is often unimportant except for poor solvents it should be kept in mind that polymers are usually miscible in concentrated solutions, although they may form two phases in dilute solution. The mixture may be warmed and stirred to speed up solution, but it should be cooled and observed at room temperature. The resulting mixture should be single phase, clear and free from gel particles or cloudiness or else the polymer is judged insoluble. The solvents to be used are selected from the Solvent Spectra, Table 3.
Here a group of solvents has been especially selected so that the values increase by reasonably constant steps within each H bonded class. The object of using this solvent spectrum is to establish a solubility parameter range for a polymer rather than a single valued number. This has the advantage of automatically showing the allowable difference that can be tolerated between the absolute values of the polymer and solvent. In carrying out the procedure it is convenient to select the first trials about one third and two thirds of the way down any one column for example, in the poorly H bonded group toluene and nitroethane would be chosen. If the polymer is soluble in both, there is no need to try intermediate solvents because experience (as well as theory) has shown that the polymer will be soluble in every case instead the solvents at the ends of the spectrum should be tried next. If the polymer was soluble in one but not both of the initial trials, the third trial should be about halfway between the two. By successive choices sets of two adjacent solvents will be found, one of which dissolves the polymer and one of which does not. The parameter values of the solvents which do dissolve the polymer mark the ends of the range. The procedure is then repeated for the other two H bonded classes. Some values for typical resins may be found in Table l, and a more extensive compilation in.
Values for Mixed Solvents The scientifically correct method for calculating the value for a mixture of solvents is discussed in Ref 1, but for most purposes the average value of the components based on percent composition by volume is sufficiently accurate. The same is also true in a general way for hydrogen bonding, that is, a mixture of toluene (poorly hydrogen bonded) and ethanol (strongly hydrogen bonded) will tend to behave like Cellosolve (moderately hydrogen bonded). In critical cases where the average value of a solvent mixture is near one end of the range for a film former, the mixture may not be as good as a single solvent. Where the components of a mixture are nonsolvents by themselves, it is advisable to have 5 to 10 percent of additional true solvent present.
How to Use Solubility Parameter Data Tables l and 2 can be used to select solvents for the film formers listed. For example, Acryloid B 44, which has a poorly hydrogen bonded range of 8.9 to 11.9, would be soluble in the following typical solvents listed in Table 2: acetonitrile (11.9), acrylonitrile (10.5), but not in apcothinner (7.8), etc. and in the moderately hydrogen bonded range of 8.5 to 13.3 in acetone (10.0), n amyl acetate (8.5) but not sec amyl acetate (8.3), etc. A zero value for Acryloid B 44 in the strongly hydrogen bonded column indicates that it is not soluble in any strongly bonded solvent.
Obviously other considerations such as cost, availability, odor, toxicity, volatility, etc., will determine the final solvent selection, but using solubility parameter tables greatly narrows the choice down to those that will indeed be solvents.
Diluents are likely to be nonsolvents chosen because of their low cost. The amount of diluent nonsolvent permissible can be estimated by calculating the average value of a mixture with true solvent that will still lie within the mid 80 percent of a given film former range. The amount of diluent can often be increased by using a latent solvent which should be selected near the end of the solubility parameter range opposite from the diluent so that the average values fall near the midpoint of the range.
One of the most important aspects of solvent choice is viscosity. Solubility parameter has no direct relationship to viscosity except that 6 values near the extreme ends of film former ranges may produce high viscosities. The primary factor effecting viscosity control is the viscosity of the solvent itself. Low viscosity solvents will produce low viscosity solutions and vice versa. This factor may be correlated with solubility parameter by constructing a chart such as Fig. 1 where viscosity of solvents is plotted against 6, Other data such as volatility may be included by showing shaded circles. If it is desired to lower (or raise) the viscosity of a given solution, determine the value for the solvent present and then replace it with a solvent selected from the chart which has a lower (or higher) viscosity.
Extensions of the Solubility Parameter Concept Many other examples illustrating the use of solubility parameter are given plasticizers may be chosen by considering them as nonvolatile solvents. Solvent resistance may be obtained by selecting a film former from Table 1 which shows a range as far removed as possible from the value of the solvent which must be resisted. Solvents for organosols should be formulated just outside the range of solubility. Swelling of applicator rolls, gaskets, printing blankets, etc., can be handled by using Table 3 to determine the values at which maximum swelling occurs, then formulating paints or inks with solvents differing as much as possible from those values.Compatibility of two or more film formers can be assured if they are selected such that the midpoints of the ranges do not differ by more than one unit.
Viscosity reduction is determined by measuring the viscosities of solutions of different concentrations of a given resin in the solvent and plotting the viscosity versus the resin concentration. This type of plot is shown in Fig. 2. Different solvents, even though they may be miscible with the resin, will give curves with different slopes. At high resin concentrations, solution viscosity will depend upon the solvency of the solvent and the solubility of the resin. At low resin concentrations, the solution viscosity is proportional to the solvent viscosity. An extreme case is shown in Fig. 3. The data show viscosities of 50 percent solutions of a medium oil alkyd resin in blends of VM&P naphtha with isobutyl alcohol and the same VM&P naphtha with n butyl acetate. Isobutyl alcohol has a high viscosity but is a strong solvent for alkyd resins. The resin solution in isobutyl alcohol has a relatively high viscosity. As the VM&P naphtha percentage in the solvent is increased, the viscosity drops even though the true solvency is decreasing. This is because the viscosity of the solvent portion is decreasing. As the percentage of the VM&P naphtha is increased above 50, the viscosity of the solution increases because the solvent is becoming weaker, that is, its ability to solvate resin molecules is decreasing. The n butyl acetate has a relatively low viscosity, and viscosities of the resin solutions increase as the VM&P naphtha is added to the nbutyl acetate.
Viscosities of a medium oil alkyd and a bodied linseed oil in toluene and in iso octane are shown in Fig. 4. The linseed oil is very soluble, and the difference in solvency between the toluene and isooctane are less important than in solutions of the medium oil alkyd. Also, the viscosities of the solutions remain low at a higher linseed oil concentration.
Viscosities of resin solutions can be measured precisely by ASTM Method D 445, Viscosity of Transparent and Opaque Liquids. ASTM Method D 1545, Viscosity of Transparent Liquids by Bubble Time Method is a simpler, less precise, but more widely used method for determining viscosity reduction, and for viscosities of resin solutions.
Aniline point is one of several methods for estimating solvency that are based on a correlation with some observed phenomenon. It is used only for petroleum thinners having aromatic hydrocarbon contents of less than about 50 percent. The aniline point is the lowest temperature at which equal volumes of aniline and the thinner will mix and give a clear solution technically this is known as the critical solution temperature. A low value indicates high solvent power and vice versa. The test is run by mixing 10ml of thinner with 10ml of aniline in a jacketed test tube. The solution is stirred continuously during the test. If the mixture is initially cloudy, it is warmed until it becomes clear. If the mixture is initially clear, it is cooled until it becomes cloudy. The aniline point is the temperature at which transition occurs. Mixed aniline point is a test for estimating the solvent power of high aromatic petroleum solvents. It is similar to an aniline point, except that the sample is mixed with an equal volume of n heptane before testing this blend is then tested with an equal volume of aniline. The final test mixture thus contains 5 ml of the sample, 5 ml of n heptane, and 10 ml of aniline. The modified procedure is necessary because aromatic solvents and aniline will form clear, homogeneous mixtures to temperatures as low as the freezing point of aniline. The n heptane raises the cloud point of the mixture and permits the estimation of the relative solvent power of aromatic solvents. Again, a low value indicates high solvent power and vice versa. A disadvantage of aniline point and mixed aniline point is that the two scales are not continuous. It is, therefore, difficult to compare solvencies of high and low aromatic content materials.
ASTM Method D 611, Test for Aniline Point and Mixed Aniline Point of Petroleum Products and Hydrocarbon Solvents, and ASTM Method D 1012, Aniline Point and Mixed Aniline Point of Hydro carbon Solvents, describe similar and suitable methods for determining the aniline points of petroleum solvents. The experimental equipment. It is sometimes a temptation, particularly since the advent of air pollution laws, to use aniline point to estimate the solvency of blends of hydrocarbons with other types of solvents. This should not be done because aniline point has no systematic correlation with solvency for materials other than hydrocarbons.
Kauri Butanol Value
Kauri butanol value is an alternate to aniline point for estimating solvency of a hydrocarbon thinner. It has the advantage that it is a continuous scale ranging from a low of about 26 for odorless mineral spirits to 105 for toluene. The procedure is described in ASTM Method D 1133, Kauri Butanol Value of Hydrocarbon Solvents. The kauri butanol value of a solvent is the volume of solvent in milliliters required to produce a specified degree of turbidity when added to 20 g of a standard solution of kauri resin in normal butyl alcohol.
The 20 g of standard kauri resin solution is weighed into an Erlenmeyer flask and placed in a water bath. It is titrated with the solvent being tested until the sharp outlines of 10 point print on a sheet placed under the water bath and observed through the liquid are obscured or blurred but not illegible. The turbidity is caused by precipitation of the resin.
Kauri butanol value, as aniline point, is not suitable for evaluating any solvent other than a hydrocarbon.
Dilution ratio is important in formulating lacquer solvents. ASTM Method D 1720, Dilution Ratio in Cellulose Nitrate Solutions for Active Solvents, Hydrocarbon Diluents, and Cellulose Nitrates, describes the procedures. The ratio of hydrocarbon diluent to butyl acetate that will be tolerated by a solution of 8 g of nitrocellulose in a total of 100 ml of solvent and diluent gives a measure of the suitability of the diluent for use in lacquer solvent formulations.
Similarly, the ratio of toluene as the standard diluent to an oxygenated solvent under the same conditions gives a measure of the suitability of the oxygenated solvent portion as a lacquer solvent. Variations in the cellulose nitrate can be explored using butyl acetate and toluene as standard solvents.
The objective in formulating a lacquer solvent is to produce a lacquer with a low viscosity and a low cost. It is, therefore, desirable to use the highest percentage of diluent that can be tolerated by the nitrocellulose and that will give the desired performance. The dilution ratio test provides a tool for achieving this goal.
The procedure is to dissolve the nitrocellulose in the true solvent and add the diluent by titration. The end point occurs when resin precipitates or gelation appears. Additional true solvent is added, and the titration is continued. Data are plotted to determine the ratio of diluent to true solvent at exactly 8 g of cellulose nitrate per 100 ml of volatile matter.
Many resins are soluble at high concentrations in a solvent but precipitate when diluted below a critical concentration. Often this concentration is within the range of practical formulations. Thus, it is important to know the dilution limits of resins.
To determine dilution limit, a known weight of resin is dissolved in the solvent. Solvent is added until precipitation occurs. Toward the end of the determination, cloudiness will occur, and solvent should be added in small increments. The end point is reached when the cloudiness becomes persistent. Dilution limit is expressed as the percent by weight of solids at the critical concentration. These determinations should be made at a standard temperature.
Evaporation rate of a solvent is second only to solvency in its importance in the coatings industry. Solvent evaporation controls the setting time of all coatings and drying time of lacquer type coatings. The solvent must remain in the film long enough to allow flow sufficient to produce satisfactory adhesion, gloss, and leveling it must evaporate fast enough to prevent sagging and inadequate film thickness. There are few paint properties not affected by flow and thus by solvent evaporation.
The relationship between evaporation rate and solvency is always critical with blends of different molecular types. Constituents rarely evaporate at the same rate therefore, the composition and resulting solvency change as the blend evaporates. Film properties can vary widely because of this phenomenon.
Solvent evaporation rate is not an absolute value in practical situations because it depends upon environmental conditions. Temperature, air movement, the presence of a solute, surface area, and sometimes humidity are factors that affect the evaporation of a single solvent. Most evaporation rate data, therefore, are relative. One solvent is compared against another under the same conditions.
Vapor pressure is the fundamental property controlling evaporation rate. If all solvents were pure compounds and environmental conditions could be neglected, evaporations rates would be proportional to vapor pressures. Vapor pressure is the pressure exerted by the molecules of vapor in equilibrium with liquid, which in turn is a measure of the escaping tendency of the molecules.
Vapor pressure is not often used to describe the evaporation rates of solvents. This may be partly because vapor pressures are difficult and tedious to measure precisely. One basic technique involves a differential manometer. One leg of the manometer is exposed to saturated vapor, while the other is evacuated. Extreme care must be taken that no air is present. Another basic technique is to control the pressure and measure the boiling temperature. At this boiling temperature, the vapor pressure is equal to the applied external pressure. A third method is to bubble dry gas through the liquid in such a way that it becomes saturated with vapor. Then the gas stream is analyzed, and the partial pressure of the compound in the gas is the vapor pressure.
With solvent blends or petroleum thinners, vapor pressure cannot be used directly because the composition and, therefore, the vapor pressure changes as the solvent evaporate.
Vapor pressure varies markedly with temperature, as shown by the data in Fig. 6. These data show also that the rate of change of vapor pressure with temperature is different for different molecular types, For all materials, the boiling point is defined as the temperature at which the vapor pressure equals 760 mm of mercury or atmospheric pressure.
Evaporation Rates by Electrobalance
The Chevron Research Company Evapograph is essentially a recording balance A 6 in.2 piece of blotter card backed with aluminum foil is suspended by a fine wire from a strain gage. The solvent to be tested is dispensed onto the blotter card with a hypodermic syringe. During this step, a Petri dish containing the same solvent is placed so that the liquid level is approximately ¼ in. below the blotter. The blotter is, therefore, in the vapor space above the liquid reservoir. The sides of the Petri dish keep the air stream from flowing across the specimen. Under these conditions, essentially no evaporation occurs during the charging step. The recorder is adjusted so that the pen is on the baseline of the chart. The test is started by lowering the Petri dish so that air flows across the specimen. Air enters through a 2 by 4 in. bundle of 3 mm inside diameter glass tubes to create laminar flow. The volume is 20 liters per minute. The temperature of the evaporation chamber is controlled at 80 F. Relative humidity can be varied from almost zero to almost 100 percent. As the specimen evaporates, its weight is recorded on the chart as a function of time. Typical data are shown in Fig. 7, where weight percent evaporated is plotted as a function of time. Precise, repeatable measurements can be made over a wide range of values. Hexane requires about 6 min for total evaporation kerosine requires about three days. Evaporation of solvents from resin solutions can be also studied if they are not too viscous. Resin retards the evaporation of a solvent. Typical data for a solvent evaporating from an alkyd resin are shown in Fig. 8
The Shell thin film evaporometer is also a recording electrobalance for measuring the evaporation rate of solvents. A filter paper, 9 cm in diameter, is suspended in the evaporation chamber from an electronic optical weight sensing device. The sensing device and evaporation compartment are encased in a second cabinet which is insulated to assist in maintaining uniform temperature. Before charging a sample, the recording pen is adjusted to the baseline. Sample is added from an hypodermic syringe and distributed over the entire area of the filter paper. Sample size is 0.70 ml, which should be added within a period of 10 s. Some evaporation may occur during this time. However, it is significant only for fast evaporating solvents and in any case can be compen sated for by extrapolating the recorder chart after the run is completed. Temperature in the evaporation chamber is controlled at 77 F relative humidity is maintained at less than 5 percent, and air flows through the chamber at a rate of 21 liters per minute. Data are usually reported as time in seconds at 10 percent weight increments through the evaporation cycle. The Shell thin film evaporometer is available commercially.
Butyl Acetate Evaporation Standard
It has become common practice to use the evaporation rate of n butyl acetate as a reference standard. This compound is arbitrarily assigned a value of 1. 0 or 100, depending upon the scale being used. Those materials evaporating faster than butyl acetate have larger evaporation rate values those solvents evaporating slower than butyl acetate have lower numerical values. This comparison procedure is used with a variety of evaporation rate methods. When an electro balance is used for the evaporation measurement, the time for 90 percent of the sample to evaporate is used frequently as the reference point. Sometimes the specimen and butyl acetate are simply evaporated side by side from evaporating dishes. Use of a reference standard compensates for differences in procedure or environmental factors. Some values are given in Table 4 in comparison with that for n butyl acetate.
Historical Evaporation Rate Methods
Early evaporation rate determinations were simple. A known quantity of solvent was put in a dish or spread on a piece of filter paper and the loss of weight obtained at regular intervals. Weighings were made on an ordinary analytical balance or on special balances. Various types of dishes have been used including friction top can lids and Petri dishes. Bridgeman suggested pans with bottoms flat on the outside but dished on the inside. Rubek and Dahl used a small metal tripod to hold the paper flat against the bottom of a can lid.
Liquid chromatography, which is described is particularly useful for determining the aromatic contents of hydrocarbon thinners and solvents. These analyses are performed usually with silica gel in a glass tube. The sample is displaced through the tube with an alcohol and separates into molecular types as it migrates downward through the column.
ASTM Method D 1319, Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption, uses the equipment shown in Fig. 14. A capillary column, a separator section, and a portion of the reservoir section are filled with 100 200 mesh silica gel. A fluorescent dye is either added to the liquid, or a section of dyed gel is included in the charger section. Approximately 0.75 ml of specimen is introduced at the top of the column. The reservoir is filled with isopropyl alcohol. Air pressure is applied to force the liquid down the tube. The alcohol, being the most strongly adsorbed, pushes the specimen ahead of it. Molecular types in the specimen are separated as the liquid migrates through the tube. Paraffins and naphthenes are the least strongly adsorbed and travel farthest down the capillary column. They are followed in order by olefins and aromatics. Portions of the fluorescent dye make the zones visible under ultraviolet light. Paraffins and naphthenes are colorless but visible because the adsorbent is wet. Olefins fluoresce a chartreuse color. Aromatics fluoresce a violet color. When all of the specimen is in the capillary section, lengths of the sections are measured and are proportional to the volume percentages of the hydrocarbon types.
A modification by Ellis and LeTourneau extends this method to the determination of the total oxygenated portion of lacquer thinners. The modification involves an additional dye component and substituting n butyl amine for the isopropyl alcohol to displace the sample down the column.