Review of Particle board Manufacture and Processing
The particleboard industry has grown rapidly since its beginning in Europe during the Second World War. The original development was in response to the demand for an inexpensive panel product which could be produced from low-quality wood unsuitable for plywood. In the mid 1950's the particleboard industry was established in the United States not in response to a demand for the product but in response to the availability of cheap residues generated by planer mills and sawmills.
Particleboard is produced in large, capital intensive plants with highly automated equipment. Many equipment variations exist from plant to plant, but all plants have to adhere to similar processing steps during manufacture. These steps are particle preparation, particle drying, adhesive-particle blending, mat formation, hot pressing, and finishing. Only manufacture and processing of platen-pressed particleboard will be discussed. A relatively small quantity of particleboard is produced annually by an extrusion process whereby the adhesive treated particles are forced between heated dies which polymerizes the adhesive and forms a continuous particleboard ribbon, which is then cut into desired lengths. The production of extruded particleboard has been steadily decreasing and, due to the limited mechanical properties, the use of this material is basically restricted to that of corestock for furniture.
Matformed particleboard is an engineered panel product of machined particles bonded together with an adhesive under controlled heat and pressure. A basic difference between particleboard and medium density fiberboard is in the degree of disintegration of the wood macrostructure. The component particles of fiberboard are either individual wood fibers or fiber bundles; interparticle bonding is accomplished by an added adhesive system. Hardboard is also composed of fibers and fiber bundles but interparticle bonding is due to the self-bonding in the high density panel, not to an external adhesive. The gross macrostructure of wood is retained in the component particles of particleboard an external adhesive is required for interparticle bonding. A wide range of particle sizes, from sawdust granules to flakes three inches long is commonly used, although rarely is the entire size distribution included in a single panel.
The common raw materials of particleboard are wood, adhesive, and wax emulsion. High quality particleboard at the optimum production rate demands continuous monitoring of the wood material to determine when adjustments should be made in the process or the adhesive. Monitoring of the adhesive and wax emulsion quality is critical, but often ignored, factor in particleboard manufacture.
Wood. The wood content on a dry basis for most particleboard is between 90 and 95 percent. Any particle configuration can theoretically be used although certain physical properties will be observed influenced if adequate particle uniformity is not pserved. Also, physical properties can be engineered into the panel by using different particle sizes or configurations in the core and surfaces. For example, long particles at the surfaces significantly increase the bending strength of the panel but they result in a rough, difficult to finish surface. Hence, if finishing characteristics are more critical than bending strength or a particular application, smaller particles, which result in smoother surface, are used on the surface. Normally, the article size and configuration, as well as the distribution of the various sizes through the panel thickness, is adjusted to optimize the desired properties with a minimum effect on the remaining properties.
Many wood species, both hardwoods and softwoods, are used for articleboard; however, the density of the particleboard should be higher than the density of the raw material to efficiently utilize the adhesive system. The compression of the particles, which is required for consolidation into the finished product, enhances the particle-particle contact, producing more interÂparticle adhesive bonds as well as reducing the total void volume in the panel. With wood of density higher than the finished articleboard, the compression of the particles is lower and the resultant reduced interparticle contact and higher void volume adversely influence the physical and mechanical properties of the particleboard.
The acidity of the wood should also be monitored to allow djustments in the adhesive system to maintain the same polymerization rate. The adhesives are pH sensitive and excessive fluctuations in the wood pH may retard or speed the polymerization process.
Adhesive. Urea-formaldehyde water-based disper-sions are the most widely used particleboard binders. The low-cost, rapid curing, and colourless properties of urea-formaldehyde adhesives make them the adhesive of choice for most interior particleboard. These adhesives have been continuously improved by the resin manufacturers, resulting in reduced press times without detrimental effeccts on their storage life or handling characteristics.
The urea-formaldehyde polymer is formed by a multi-step reaction process between urea and formaldehyde. The initial phase is a methylolation of the urea under slightly alkaline conditions with a formaldehyde-urea (F/U) molar ratio of 2.0:1 to 2.4:1. Condensation of the methylolureas from the methylolation reaction is at atmospheric reflux with a pH of 4 to 6. This condensation polymerization continues to a pre-determined viscosity, at which time the pH is adjusted with a suitable base to 7.3 to 8.0. The adhesive is then concentrated to a total solids content of 50 to 60 percent by vacuum distillation. Additional urea is then normally added to produce a final F/U molar ratio of 1.6:1 to 1.8:1.
The final polymerization of the urea-formaldehyde adhesive occcurs in the hot press and is one of the critical steps in particleboard manufacture. If the adhesive cures at any point in the manufacturing sequence other than in the hot press with the mat compressed to the desired thickness, an unsatisfactory product will be produced. Since the polymerization of urea-formaldehyde adhesives is much faster under acid conditions and at elevated temperatures, optimum curing rates (minimum press times) are attained if the adhesive pH is 3 to 5. However, urea-formaldehyde adhesives are supplied at slightly alkaline levels to retard polymerization during transit and storage but this increased stability lengthens the press time. Many wood species, such as the oaks and southern pines, are acidic and contribute to a rapid pH decrease in the adhesive. Also, resin suppliers attempt to limit the buffering capacity of their adhesives by using a volatile base to adjust the final pH. When the adhesive and wood are exposed to the elevated temperatures in the hot press the volatile base is rapidly evaporated, the pH drops, and polymerization follows rapidly. Adjustments such as these and a better understanding of polymers and polymerization mecha-nisms have resulted in continuous reductions in the press times for urea-formaldehyde bonded particleboard. Since the pressing operation normally controls the production capacity of a particleboard plant, even small reductions in the press time result in increased production levels.
The disadvantages of the urea-formaldehyde adhesives lie in their lack of durability and in their charac-teristic pungent formaldehyde odour. For particleboard applications subject to high temperature and moisture exposure, phenol-formaldehyde adhesives are required, since the urea-formaldehyde polymer is hydrolyzable and hydrolysis is enhanced with moisture and heat.
Formaldehyde evolution, at the hot press and in applications where adequate ventilation is lacking, result in relatively high levels of free formaldehyde fumes with the urea-formaldehyde resins.
Phenol-formaldehyde adhesives are the only other adhesive system used in significant quantity in particleboard production. The increased durability of this class over that of the ureas results in phenolics as the adhesive of choice for exterior particleboard. However, phenolic adhesives are only used where the additional durability is required since they are more expensive and require longer curing times.
Phenol-formaldehyde adhesives are produced by a condensation polymerization reaction between phenol and formaldehyde. The phenolics used for exterior particleboard are made at a formaldehyde/phenol ratio greater than 1.0; i.e., they are classified as resoles and additional formaldehyde is not required to complete the curing reaction to a highly cross-linked network structure. Many characteristics can be incorporated into the adhesives by changes in the F/P ratio, condensation pH, and condensation time. The reactive solids content is normally between 40 and 50 percent since the stability and viscosity are adversely affected at higher solids.
Wax Emulsions. The final component in most particleboard is a sizing agent to reduce the absorption of liquid water. This is normally a paraffin wax emulsion which is supplied to the particleboard manufacturers at approximately a 50 percent wax solids in water. Less than 1 percent wax solids based on the ovendry wood weight is-used in most particleboard; levels above 1 percent tend to interfere with interparticle bonding while levels below 0.75 percent do not offer maximum water resistance.
The above three components-wood, adhesive, and wax-are the only ingredients in most particleboards. Only limited quantities of fire retardant and preservative treated particleboard is presently produced. However, with increased flammability requirements and applications of particleboard in locations subject to biological degradation, an increase in production of both preservative and fire retardant particleboard is expected.
Particle Preparation. The initial step in particleboard production is reduction of the wood raw material into the desired configuration for the particular particleboard to be manufactured. The wood may be received from a number of sources and in a variety of forms. Roundwood, chips, planer shavings, plywood trim, and sawdust are the most common raw material forms and rarely does one particleboard plant use more than two or three of the above sources. Different processing steps are required to produce quality particles from each of the above sources; consequently, each plant is limited to the source which is compatible with their wood reduction system.
The various reduction systems can be classified into knife, hammer, and attrition units, each type producing a characteristic particle. A tramp metal detection system is included in all reduction steps to protect the equipment from serious damage. The shape and integrity of the component particles strongly influence the quality of the resultant particleboard; therefore, the optimum in particle preparation is achieved when the desired particle is obtained with no damage to the structure of the wood. Wood failure within the particle will result in a particleboard of lower strength than one formed from intact particles.
Chippers and flakers are the most widely used knife reduction systems. Chippers produce coarse particles from roundwood, slabs, plywood trim, and other residues from the primary wood industry. Chippers are the initial reduction step in particle preparation and further size reduction is necessary to produce a satisfactory particle. The chipping operation may be located at the particle- board plant or the chips may be delivered to the plant from an in-woods chipping operation, a chip-n-saw mill, or from another primary wood industry. Screening after the chipper removes all fines and oversize chips before they enter the secondary reduction step. Oversize chips are recycled to the chipper and fines are normally sent to the boiler for fuel.
Flakers are used for initial reduction of roundwood and for further reduction of chips. Flakers designed for primary reduction of roundwood have knives projecting from a rotating drum or disc with the axis of rotation parallel to the log. The length and thickness of the resulting flake are controlled by scoring knives and the cutting knife projection, respectively. Normally, all roundwood will have the bark removed before the flaking operation.
Flakers used for secondary reduction of chips have an entirely different design. The knives in these flakers are mounted on a rotating ring which rotates against an impeller ring. The chips enter the machine in the center and are thrown to the perimeter and held across the projecting knives by both the centrifugal force and the impeller ring; the flakes are then produced by the rotating knives.
Seldom is particleboard produced from particles generated only by knives, most flakes will be further reduced by either a hammermill or an attrition unit. Hammermills reduce planer shavings, chips, plywood waste, and trimmings by mechanically breaking and tearing the wood. Rotating hammers beat the material against breaker plates until sufficient size reduction has occurred and particles can exit the machine through a screen. Hammermills are used for additional size reduction of flakes and planer shavings. Since the wood will cleave readily along the grain, hammermills are used to reduce the flake width without significantly reducing the flake length or thickness.
Attrition units are commonly found in plants producing particleboard in which smooth surfaces are required. Flakes, planer shavings, and sawdust are reduced in attrition mills to small particles Which will mat well and form a smooth surface. Attrition mills are either single or double rotating discs which mechanically grind the material into small particles.
Many particle preparation processes exist, the one used in a given particleboard plant will depend upon the raw material source and the type of particleboard produced. The particle quality from each process is a function of the wood moisture content, degree of maintenance of the equipment, and the form of the raw material. Good particle quality does not guarantee a quality particleboard but high quality particleboard cannot be produced from low quality particles.
Particle Drying. All particles used in particleboard manufacture are dried to a uniformly low moisture content before the adhesive is applied. There are a number of dryers used by the industry, details of which will not be presented here. The particles are quickly dried to a moisture content of 3 to 6 percent (based on wood ovendry weight) with commercial dryers. The particles are exposed to the high temperatures of combustion gases from oil, gas, coal, or wood as they are rapidly moved through a closed chamber. The evaporation of water and the short dwell time within the chamber minimises the potential fire hazard. Continuous monitoring of the incoming particle moisture content is required to allow corrective action in the dwell time to prevent insufficient or excessive drying. Dwell time in the chamber and fuel consumption adjustments are the most common methods of correcting for changes in the incoming particle moisture content. Rapid fluctuations in the moisture content of the wet particles entering the drier should be avoided.
Blending. Addition of wax, adhesive, and other additives to the dry wood particles is called blending and is normally done by spraying the aqueous adhesive system and wax emulsion onto the particles as they are moved through a blender. The adhesive level is based on the ovendry weight of the particles; no attempt is made to monitor the total particle surface area. Consequently, smaller particles with a larger area to weight ratio have substantially more adhesive on a weight basis if both surfaces have equal adhesive coverage per unit of area. Particleboard quality is strongly dependent upon interparticle bonding and, as particle size decreases, more interparticle bonding per unit weight is required to produce the same density particleboard. Consequently, there is a need for higher resin levels on smaller particles when resin content is measured on a weight basis. However, as will be shown in the mat formation section, most small particles are placed at the surfaces for improved surface quality and smoothness in the final board, and better consolidation and more efficient use of the adhesive occurs at the surface; therefore, less adhesive on an area basis is required for the small surface particles. Passing the small particles rapidly through the blender limits their time of contact with the adhesive spray and prevents excessive adhesive pickup.
Paraffin wax emulsion and other additives are also added to the particles during blending.
The blending operation is an important step in the production of quality particleboard-uneven distribution of the adhesive will result in regions of low interparticle bonding and weak particleboard. Strict monitoring of both the adhesive and particle streams delivered to the blender is required for optimum blending.
Mat Formation. The process by which the blended particles are deposited in a continuous ribbon on a moving belt is called felting or mat formation. Significant equipment advances in the forming machines have resulted in much more uniform mats with much less density variation across the board than was common with earlier generations of felters. The forming process is entirely automated, the particles fall from the felter as a curtain, forming a continuous ribbon of particles on the moving belt. Usually, more than one felter is required to build the desired mat thickness; multiple felters allow more uniform mat formation since less material is deposited by an individual felter. Also, multiple felters are required for layered particleboards in which larger particles are used in the core and smaller particles at the surfaces. Accurate and uniform felting is an extremely critical step in the production of particleboard. Density variations in a poorly formed mat can not be eliminated and these will be present in the finished particleboard. The movement of the belt and the curtain of chips have to be finely adjusted to insure proper chip deposition to produce the target board density after compaction and resin hardening in the press. Changes in the wood species, particle size, and particle moisture content have to be accounted for by adjustments in the felting operation. The advantages of maintaining raw material uniformity to assist in this process are obvious.
Three classes of particleboard are commonly recognized, based on particle size distribution in the thickness direction. These are: 1) homogeneous - all particle sizes are distributed equally; 2) layered - large particles in the core and small particles at the surfaces; and 3) graduated - large particles in the core with progressively smaller particles from the core to the surfaces. The surface particles in the layered and graduated mats normally have a higher adhesive content (on a weight basis) than do the larger core particles. The small particles produce a smoother, more continuous surface which is easier to finish than are the rougher surfaces formed by larger particles.
Homogeneous mats are formed by depositing a mixture of particles on the moving belt without size segregation. Multiple formers are normally used with each former depositing a portion of the total mat thickness. Layered part icleboard is produced by using small particles in the formers depositing the surfaces and larger particles in the core formers. Duplicate blending and transport systems are normally used for the surface and core particles in layered particleboard allowing control of the adhesive level in each layer but also requiring higher initial capital investment.
Graduated particleboards are similar to layered particleboards since large particles are present in the core and small particles at the surface. However, particle separation based on size is done in the felting operation which eliminates the need for two conveying and blending systems. A minimum of two formers are required, each former depositing half the mat thickness. Particle separation is accomplished by subjecting the falling particle curtain to air or by throwing the particles with a mechanical device.
As the continuous ribbon is conveyed from the forming station it may or may not be consolidated by a cold press. This prepressing operation reduces the mat thickness and increases the mat density which improves the handling characteristics of the mat, but does not initiate adhesive polymerization. The continuous ribbon is also trimmed to width and cut into individual mats, the length of which is equal to the length of the hot press. The individual mats are placed in the press loader which serves as a temporary storage area for the mats prior to hot pressing.
Pressing Operation-. The consolidation of the particle mat and polymerization of the adhesive to produce a particleboard panel is accomplished in a hot press. The mat is compressed and held at the desired thickness until the adhesive on the particle surfaces has polymerized and established adequate bridges between particles. The panel is then removed from the press, cooled, and sent to the finishing phase.
The pressing operation is extremely important and is highly dependent upon previous processing steps. If a poor mat has been delivered to the press, a poor particleboard panel will result. Particles with insufficient adhesive from a poorly functioning blender or a mat with excessive moisture cannot be tolerated if quality particleboard is to be produced. The press is the most expensive equipment in a particleboard plant and the output of a plant is controlled by the pressing operation. Consequently, it is imperative that the press function efficiently with as short a cycle as possible. Many physical and mechanical properties of particleboard are influenced by the pressing operation; therefore, a clear understanding of the pressing function is required.
Most particleboard plants have multiple-opening hot presses which produce one panel per opening per press cycle. The press loader also has storage area for the number of mats equal to the openings of the hot press. When the press loader is filled and the press opens, all mats in the press loader are simultaneously transferred to the hot press, and the finished panels from the previous press cycle are removed to the press unloader. It is imperative that the forming line be operating at the proper speed to produce sufficient mats to have the press loader filled when the press opens. The press controls the plant production capacity and it should be operating continuously; it should not be held open waiting for additional mats. Consequently, most plants are designed with variable speed forming lines which can be synchronized with the press cycle.
Multiple-opening hot presses presently used in the particleboard industry are simultaneously closing; i.e., all openings close together at the same rate subjecting all mats to the same press cycle. In the earlier presses, which closed from the bottom, the mat in the lowest opening was subjected to a significantly longer press cycle than the mat in the top opening. Consequently, all the particleboard panels produced in the same pressing cycle did not have the same properties; the properties were influenced by the particular press opening in the hot press. Simultaneously closing hot presses have eliminated this source of variation, since all mats are subjected to the same press cycle.
Mechanical stops placed on two edges of each press opening are often used for thickness control. As the press is closed platens compress the mat until contact is made with these stops, at which point compression of the mat ceases and the particleboard thickness is equal to the thickness of the stops.
The mat surfaces are rapidly heated to the temperature of the platens as the mat is compressed. The water in the particles at the surface is vaporized and migrates into the cooler portion of the mat, i.e., toward the core. Condensation of this steam releases heat which increases the mat temperature quicker than could be accomplished by conduction through wood. However, the press is compacting the mat to target board thickness before the mat is completely heated. The compressive strength of wood is much lower at elevated temperatures and, since the mat is compressed when only the surface region is heated, compressive failure of the wood within the hot surface region occurs. The mat is compressed to thickness before the core is heated; consequently, there is a vertical density gradient in the thickness direction of hot-platen-pressed particleboard. High density surfaces and low density cores are produced with the average particleboard density falling between these two extremes. The low density core resulting from this vertical density gradient reduces the screw holding strength, shear resistance, and tensile strength of this region. Various vertical density gradients can be obtained for the same average board density by adjusting the rate at which the press is closed. However, long press closing times are to be avoided since the adhesive on the surface particles may harden before adequate interparticle contact is obtained. This condition is commonly referred to as surface precure.
The moisture migrating to the mat core also presents difficulties in platen-pressed particleboard. The moisture evporates from both surfaces and progressively migrates to the core as the temperature increases from the surfaces to the core. However, the temperature of the core eventually exceeds 100ÂºC, turning the water into steam. The water in the core, in the form of steam, has to escape from the board during the pressing operation. The press time has to be sufficiently long to allow the steam to escape or the panel will delaminate when the hot press is opened and this steam rapidly expands. Also, the water interferes with the condensation polymerization reaction of the curing resin, limiting the curing rate and lengthening the press time. Therefore, the moisture in the particle mat assists in heat transfer to the core but restricts the adhesive cure and is a potential source for delamination at the panel midplane. The mat moisture content at which these two effects can be balanced will vary for each particleboard plant, depending on particle size and species. A technique commonly used is to have a non-uniform moisture distribution in the particle mat. A high surface moisture content, to assist in heat transfer, and a low core moisture content is widely used to minimize the press time.
The type and extent of the finishing process for particleboard is determined by the product grade-floor underlayment and mobile home decking are simply squared and sanded to thickness while the industrial grade used in furniture applications is subjected to much more elaborate procedures. Painted and simulated grain surfaces can be formed directly on the particleboard while veneers, vinyl, and other surfacing materials are bonded to the particleboard by adhesives. Regardless of the finishing method the goal is to produce an attractive and functional surface with the required durability at a minimum cost. The finishing processes discussed here are those commonly used for the industrial grade particleboard.
Edge Finishing. Most industrial particleboard is produced with large particles at the core and smaller surface particles for surface smoothness. This graduated or layered construction, together with the platen-pressed method of manufacture, produces a particleboard with a low density porous core and higher density surfaces. Consequently, a non-uniform panel edge is present which does not machine or accept paints and finishes uniformly. Various techniques are available to mask these edges which are to be exposed in the completed furniture, cabinet, or shelf. The methods commonly used for edge finishing are tapes, lumber strips, T-mouldings, or V-grooving.
All edge finishing, except the lumber banding, is done after the panel surface has been finished. The wood or vinyl tapes are glued to the edges with PVA or hot melt adhesives. Plastic T-mouldings are used to give a machined edge effect not possible with the flat tapes; a projection on the back is inserted into a machined groove in the edge of the particleboard.
Lumber banding consists of gluing lumber strips, 1/2 to 2 inches in width, on the particleboard edges. These strips are normally used in applications where the particleboard is to be covered with wood veneers. The solid wood strip can be machined to decorative edges and, with the veneer surfaces, the panel is fully as functional and attractive as a solid wood panel, but at a lower cost. The lumber bands are normally bonded to the particleboard with polyvinyl acetate or urea-formaldehyde adhesives, cured rapidly by either contact or high frequency heating.
A relatively recent development, in which vinyl covered particleboard is self-edged, is by V-grooving. V-grooving is accomplished by machining V-shaped grooves through the particleboard substrate to, but not into, the vinyl film. Adhesive is then applied to these grooves and, using the vinyl film as a hinge, the particleboard is folded back on itself. This edge finishing method results in continuous vinyl film at the corners and edges which are normally the prime locations for film and edge tape delamination.
Surface Finishing. All particleboard surfaces have rough, irregular surfaces; the degree of roughness and irregularity is a function of the surface particle size. As the particle size decreases and the density of the particleboard increases, smoother surfaces are obtained on which less work is required to obtain satisfactory finishes. The degree of surface smoothness required is dictated by the particular finishing method employed. Grain printing requires smoother surfaces than does veneering and more surface preparation is required prior to printing than is necessary for veneering.
The simplest construction for veneered particleboard is a face and back veneer glued to the particleboard core. It is imperative that veneers of equal thickness, grain direction, and dimensional stability be used to insure a balanced panel. A balanced panel reduces the potential for bowing and warp on subsequent exposure to changes in ambient relative humidity.
Particleboard which has been lumber banded and conditioned is sanded to insure uniform thickness of the particleboard and edges. Conditioning after edgebanding is imperative to allow equalization of the water from the edgebanding adhesive throughout the assembly. Premature sanding and veneering creates the possibility of subsequent dimensional changes which will produce a panel with a distinct border from the lumber band telegraphing through the surface veneer. This border cannot be removed and will always be evident in the panel. Sanding after proper conditioning insures an equal thickness for both the particleboard and lumber edges.
Water-based adhesives, usually urea-formaldehyde, are used in the veneering operation. The water in these adhesives can result in excessive surface particle swelling which will "telegraph" through the veneer, being especially evident with high-gloss finishes. Addition of a cross band veneer between the particleboard and surface veneer will prevent most problems of telegraphing. The cross band veneer is normally thicker and lower quality than the surface veneer and is placed with the longitudinal grain direction at a right angle to the grain direction of the surface veneer. However, if a cross band is used below the surface veneer, an equal cross band has to be used between the back veneer and the particleboard to retain a balanced construction. The resulting 5-ply construction is much more stable than the 3-ply to changes in ambient relative humidity.
Other surfaces are also commonly bonded to particleboard substrates; these include vinyl overlays, high density overlays, and low density overlays. Low density overlays are melamine impregnated paper which bond to the substrate with the melamine formaldehyde adhesive present in the overlay. High density overlays are highly durable and resistant sheets of phenolic resin impregnated paper with a top sheet impregnated with a melamine- formaldehyde resin. Contact adhesives are commonly used to bond high density overlays to particleboard cores for applications requiring high durability such as countertops.
Vinyl overlays are thin sheets of polyvinyl chloride, often with a simulated grain pattern, which are glued to the particleboard to obtain an inexpensive finish of relatively low durability. Particleboard panels containing a 3-dimensional design can be vacuum laminated with vinyl films, provided sharp corners are not present in the design. The thermoplastic vinyl film is heated to the softening temperature and, as the air is withdrawn from between the film and the machined panel, atmospheric pressure from above forces the film to conform to the contours of the panel.
All of the above finishing techniques are characterized by addition of a separate surface layer or film to the panel by adhesive bonding. Particleboard is also finished by applying liquid finishes directly to the surfaces and initiating a physical or chemical reaction of the finish to form the desired durability and appearance. Painting and grain printing are common examples of this method. Particle-board panels precut to the required final dimensions are commonly painted or printed on an automated finishing line. The painted or printed panels are then assembled and the final topcoats applied. Print lines are more sophisticated and technologically advanced than paint lines and will be described in detail.
Grain Printing on Flat Panels. Grain printing of particleboard surfaces is rapidly expanding, primarily due to refinements and advances in the techniques and equipment for printing. With multi-coloured printing and rapid line speeds this method of finishing particleboard closely simulates wood but with costs below those of other finishing methods. Printed particleboard is widely used in vertical applications for cabinets, casegoods, and other applications in which highly durable surfaces are not required. Extremely close control on both the panel smoothness and the print rolls is necessary to maintain quality of the printed panel. Most simulated grain printing is done with high volume, automated finishing lines on precut flat panels prior to assembly of the finished item.
A wide range of equipment and techniques are used to print particleboard with simulated grain pattern. However, the process essentially consists of sanding and filling the particleboard to obtain a smooth surface, followed by applying a basecoat for the background colour and, finally, the grain pattern. The final sealer finish is often appllied to the assembled item on a production finishing line in the furniture plant. One of the previously mentioned edge-finishing procedures is also required with printed panels for applications with exposed edges. The reverse surface should be coated with a material possessing similar permeability as the top surface to maintain a balanced construction and minimize bowing and warping diffi-culties. Adequate equipment maintenance and careful panel preparation is mandatory for production of quality printed panels.
Accurate sanding of the particleboard is essential for surface smoothness and uniform thickness. Sanding to thickness with 50-100 grit sandpaper is followed by finer grit paper to obtain the required smoothness. The total sanding operation is normally done with multi-head sanders, with progressively higher grit paper and a final smoothing bar.
The sanded panel is brushed and vacuumed to remove surface dust and debris. Minute wood particles remaining on the surface through subsequent finishing steps will result in defective filling and printing operations.
The next step is the filling operation in which the small depressions and voids between the surface particles are filled with a high solids, high viscosity coating. Due to the inherent structure of particleboard small interparticle voids and depressions will always be present at the surface, regardless of the surface particle size or the sanding technique. The filler ts normally applied by reverse roll coater which forces the filler into these voids and produces the extremely smooth surface required for printing. Fillers with a wide range of chemical properties are available; the one chosen by a given producer is determined by the curing equipment in the plant. UV curable polyester fillers are widely used due to the speed at which they cure but a UV radiation source is required. UV curable filler can be hardened in 10-15 seconds, thereby significantly increasing finishing line speed as well as shortening the overall length of the line. Vinyl, polyurethane, and urea-alkyd filler systems cure by heat or high air velocity and require substantially longer curing times. A second filling after a light sanding of the first fill coat results in a much smoother surface and produces a better printed part icleboard.
A basecoat is applied to the panel after the filling operation. This is normally a pigmented lacquer or vinyl based material whose function is to hide the surface and provide a uniform colour on which the grain will be printed. The basecoat is commonly applied by a curtain coater or more commonly with a roller coater. Obviously, skips in the basecoat, whether the result of deformed application rolls or panel unevenness, cannot be tolerated. Surface depressions not completely filled during the filling operation will not be basecoated and will appear as small white dots on the panel.
Basecoats are normally cured in heated ovens with high air velocity or with infrared heaters; excessive panel temperatures should be avoided to prevent drying of the particleboard. A light scuff sanding of the basecoat is used to remove high spots, followed by a brush cleaning to remove the dust and debris.
The grain pattern is then printed on the panel with one to three printers in tandem; each printer has a different colour which allows better grain pattern simulation. Lines equipped with three printers can produce a four-tone pattern since the basecoat is normally a different colour than the printers.
The inks used for graining are drying inks which must be compatible with the fill, basecoats, and other finishing materials used in the process. Three rolls are used by each printer to transfer the ink from the ink tray to the panel. The application roll transfers ink from the tray to the print roll which has been etched with the desired grain pattern. A doctor blade removes excess ink from the etch roll before the rubber covered transfer rolls remove the pattern from the etch roll and transfers it to the panel. Ink not completely transferred from the transfer roll to the panel has to be cleaned with another doctor blade to insure continuously sharp grain patterns. If more than one printer is used, they have to be synchronized to insure proper grain patterns result. Each etched drum has a portion of the total grain pattern and they must revolve in sequence to produce the desired simulated grain.
The edge finishing and seal coat may be applied on the finishing line but is more commonly done on the furniture assembly and finishing lines. Careful handling of the printed panel is required to prevent chipping at the edges and corners during furniture assembly. With proper care during assembly and shipping, simulated grain printed particleboard results in attractive, inexpensive furniture for the mass market.
The particleboard industry has rapidly evolved, in the relatively short period of its existence, from small, low capacity, highly labour intensive plants to high volume, highly automated facilities. The product from these modern plants also bears little resemblance to the initial particleboard with its limited physical and mechanical properties which restricted its application to cores for decorative veneers. Many researchers have made significant contributions in equipment, process develop-ments and expanded applications which resulted in rapid growth of this industry. Continued investments in particleboard research will expand applications and improve the processing technology to more efficiently utilize our renewable wood resource.
Energy and Chemicals From Wood
During the past 150 years the patterns of energy consumption have changed dramatically in India. The patterns of energy consumption relate to what form the energy takes, who uses it, and for what purposes. The forms of energy are the primary fuels specifically, wood , cool, petroleum and natural gas, and also electricity, which converts the primary fuels or other inputs (such as hydroelectric, geothermal, and nuclear) in to energy.
As shown in Figure prior to 1880 wood was the primary energy form but was then gradually replaced by coal . Coal retained one- half of the energy market until after world war II with the peak period of coal usage arounde 1920. The petroleum share of consumption increased steadily although at a decreased rate since 1950. Today oil represents close to 50 percent of the share of primary fuels for energy. Natural gas represented only 4 percent of the energy usage in 1920 but jumped markedly in the postwar years to about 25 percent in 1960 and like oil has leveled off. The persent heavy dependence on the three primary fossil fuels (petroleum, natural gas, and coal) amounts to over 90 percent of the use of all primary fuels. The remaining contribution to the nation's energy supply is from nuclear plants (4 percent), wood (direct and indirect, 2 percent), and a number of the sources such as solar, hydroelectric, geothermal etc. (1 percent).
The levals of energy consumption for the past 150 years are shown in Figure. The pattern of energy consumption spells an ominous future if it continues at the current rate. In 1880 the nation consumed only about 4 guads of energy. As a result of industrialization and mechanization the energy consumption increased dramation and mechanization the energy are now consumed in the united States per years at the current rate energy consumption is doubled every 10 years. Even to the casual observer several points should be obvious from the above observations.
- The rate of energy consumption in India must be reduced considerbly.
- Energy conservation should be an important consonent of daily living.
- Alternate energy sources must be explored and instituted.
The goverment already has a stepped-up program for energy conservation, and we can expect further emphasis on conservation measures in the years to come. A number of energy experts feel that sufficient energy- conservation can be achieved without a dramatic decrease in the Indian standard of living . Representatives of the office of Technology Assessment have suggested that conservation should be regarded as substituting new technology or different procedures for energy without reducing the amenities. They feel we should avoid heroic measures of curtailment to reduce energy consumption quickly by the cheapest means available. The former director the Non-conventional energy Research Institute, has pointed out the differences between energy con- servation and curtailment with the analogy that curailment means a cold house while conservation means a wellinsulated house withean efficient heatingsystem; or curtailment means giving up automobiles while conservation means trading in a 7- mile-pergallon status symbol for a 40-mile-per gallon commuter vehicle . Thus energy conservation will not necessarily require curtailment of vital services but merely will require the curtailment of energy waste.
Conservation alone, however, is not the solution to the energy crisis. Alternate energy sources must be explored, but what are those alternate sources and what possible paths are available to reduce foreign imports of petroleum? A few of these alternatives are explored in the next section.
Alternate Energy Sources
According to a recent estimate by the year 2000 petroleum and natural gas usage should drop from the current combinded level of about 75 percent of the primary fuels to approximately 45 percent. The percentage of each of the alternate energy sources that will make up the difference is still a matter of speculation at this point.
India possesses about 31 percent of the world's known coal reserves. Accounting for about 393 billion metric tons of coal; it is firly certain that about 225 billion metric tons of this are recoverable. This coal is a very important domestic energy resource capable of considerable expansion. Many energy experts feel that the use of coal will India to bridge the gap between present dependence on foreign oil import and the devalopment of alternate synthetic fuels. However, the use of coal has many negative features such as environmental pollution with burning and potential unsightly strip-mining for recovery.
Synthetic fuels derived from coal offer considerable promise for the future.
Nuclear power is a relatively clean energy alternative, but plublic pressure may limit extensive exploitation. Thus public concern and rising capital costs could restrict expansion of nucler energy is the limitation of the natural resource, uranium, necessary to fuel the plant. Recent estimates suggest that there will not be sufficient uranium past the year 2000 for nuclear power plants to make any significant energy contribution in the distant future.
There is also considerable controversy over wealther or not it is feasible that solar energy can make a significant contribution to the energy needs of the nation in the furture. Estimates for the contribution orf solar energy in the year 2000 range for 1 to 20 percent. Clearly, a very large financial inverstment, on the order of billions of annually, will be necessary to bring solar-generated electricity to fruition on a commercial scale. An intriguine concept in this respect is the use of solar- power satellites that would capture the sun's rays and focus that back to earth via lasers.
An important "passive" form of solar energy is the production of plant biomass through the photosynthetic process. The net photosynthetic productivity (NPP) of the earth has been estimated at 140 Â´ 10 9 metric tons of dry matter per year. Forests account for about 42 percent or 59 billion metric tons of the NPP, which is equivalent to more than the world consumption of fossil fuels (Table 1).
In India the equivalent of 80 quads of energy is produced each year as total plant biomass; however, much of this is inccessible, uneconomical to collect, or already utilized for agricultural crops or forest products. It has been estimated that there are approximately 200 Quads of standing timer in the nation's forests today. Thus if an attempt where made to have wood as the sole source of energy in India. The country would be totally depleted of this reserve in roughly 2 years woody biomass will therefore never be a panaces to the overall energy diet can be supplied by this important resource.
Probably the most significant avantage of biomass is the renewability. The Department of Govt. of enrgy has estimiated that the equivalent of about 8 quads of energy in the forms of biomass is produced annually in the nation's forests, but roughly one-half or the equivalent of 4 quads is already hearvested annually for timber and paper products. Much of the forest that is not harvested is inaccessible or under harvesting restrictions. Thus the major contribution from woody biomass will probably be in the form of more efficient use of residues and waste.
If all types of waste are included in the scenario of biomass stilization, including urban and agricultural wastes is in addition to wood wastes, then these wastes and residues represent close to 1 billion metric tons or the equivalent fuel value of apporximately 15 percent of the total energy needs of the India Wood, in the form of logging and manufacturing residues, accounts for about 25 percent of this figure. In addion, roughly 50 percent of most municipal waste is comprised of waste paper, which represents the wood cellulose ultimately derived from the forest. Thus a significant energy contribution could be made by efficient utilization of waste material. The nature of wood residues and the predominant areas of accumulation are discussed in the following section.
Available Forest Residues
The continual increase in wood usage also increases the availbility of residues suitable for fuel. Greater utilization of these forest residuces would provide distinct environmental advantages and desrease the cost of subsequent forest-management activities. The largest potential source of residues is the large inventory of rough, rotten and salvable dead trees. Rough treas are those commercial species that do not contain at least one 3.6 - meter (12- foot) sawlog. or two noncontigous sawlogs each 2.4 meters (8 feet) or longer are deformed. It also includes live trees are those having more than 50 percent of their volume clasified as rotten. Salvable dead are standing or downed tress that are considered merchantable. Total inventory in these three classes is approximately 900 million metric tons, with the largest inventory of rough trees in the south. of rotten trees in the north, and of salvable dead in the Rocky Mountains.
Rasidues from logging operations result in another large inventory of potential fuel. This includes wood and bark from growing stock, non-growing stock, and uncut small and undersirable trees. Logging residues account for about 100 million metric tons of wood and 18 million metric tons of bark each year. The northern India contains the largest fraction of this residue, and a significant amount is unsalvaged hardwood meterial owing to stringent hardwood product requirments and limited markets for hardwood pulpwood.
About 18 million metric tons of softwoods and hardwoods are removed from inventory annually in the India by such operations as timber - stand improvement, land clearing, and changes in land use; these usually are not utilized for timber products. Residues from primary manufacturing, excluding pulp and paper, are estimated at 10 million metric tons of wood and 7 million metric tons of bark . These represent the unused by products and residues from-manufacturing lumber and wood products. The largest amount of this residue is also in the North.
It is difficult to estimate the exact amount of wood residue that is or could be available for energy generation. Some wood stock is on land with a low volume per acre, some located long distances from potential use sites, and some located in inaccessible areas where the removal costs would be prohibitive estimates of costs of delivered residues to plant sites very greatly for different geographic locations. Estimates of the average costs of collecting and transporting wood residues to a central location for processing range between Rs. 130 and Rs. 310 per oven-dry metric ton. Figures from 1980 shown that wood residue could compete with other fuels when its fuel proce was below Rs. 220 per oven dry metric ton. This cost represents the minimum prove that this residue can have if used for any other purpose.
The economics of using forest residues for energy depends to a great extent on efficiency in harvesting, transportation to the point of use, pretreatments such as drying necessary to upgrate its quality, and the sustained availability to justify new capital expenditures. Current combustion technology is, for most part, capable of utilizing forest residue as fuel.
Wether the woody biomass is recovered as waste and residues or directly as timber from the forest, it is important to understant the process for utilization of this important renewable resource is energy application.
Energy and Fuels from Wood
The use of wood for energy and fuels can be conveniently divided into four major categories.
- Direct combustion.
- Saccharification-fermentation (SF).
- Thermal decomposition.
- Thermochemical liquesfaction.
Each of these methods is discussed in more detail in the following sections.
The concept of using wood as a source of energy by direct combustion dates back to the very beginning of humen existence. As soon as early people learned to use fire, wood become the major source of energy. It is important to note that even now approximately one-half of all the wood harvested worldwide is used for fuel by direct combustion. Thus direct combustion is probably the most important method for deriving energy from wood.
Wood has certain avantages over fossil fuels. The most important, of course, is that it is a renewabel resource: but it also has a low ash contant that is easily and usefully disposed of on land as mineral constitutents assential for plant growth. The sulfur content of wood is low, usually less that 0.1 percent so air pollution form this source is negligible.
Although particulates may cause a serious problem generally, wood fuel is used close to where it is grown: thus the need for energy in long distance transport is reduced. The use of fossil fuels unlocks the carbon that has been stored in them for ages and increases the carbon dioxide contents of the atmosphere. In contrast, wood fuel releses the same amount of carbon dioxide that the forest has recently fixed.
Wood does have disadvantages as a fuel. It is a bulky meterial, and, in contrast to other fuels. It has a low heat of combustion petuminous coal, fuel, were, a natural gas have heats of combustion of 28.600, 39.600 and 40.700 Btu per kilogram, respectively, while dry wood of most species has a heat of combustion of about 18.920 Btu per kilogram. When harvested, green wood has a high moisture content (50 percent), and the heat of vapourigation of this water in the furance further reduces the heat recoverable from a given weight of wood to 9350 Btu per kilogram. Most manufactures of combustion equipment recommend burning wood at below 15 percent moisture. The hight moisture content also adds to shipping costs. The capital cost of a wood burning system is considerable higher then a comparable gas-burning installation because of the need for storage and handling facilities. In addition, labour requirements for operating the equipment are greater.
Devashish Consultants of Bareilly recently completed on evaluation of the costs for heating with wood compared to fossil fuels. An analysis was made for both industrial fuel uses in the form of chips (Table 2) and for residential space heating with cut logs (Table 3) and 4). Based on this evaluation, industrial fuel costs could be reduced aproximately sixfold and the residential heating cost reduced approximately sixfold and the residential heating cost reduced three fold by substituting wood for fuel oil. With the rising costs of foreign oil, this cost difference will probably be even greater in the future.
The sacchari fication-fermentation (SF) method is based on the breakdown or hydrolysis of the polysacchari des in wood to the constituent monomeric sugars. The six-carbon or hexose sugars (glucose, galactose, and mannose) are then fermentable to ethyl alcohol (ethanol or grain alcolol, C2H55OH) by yeast fermentation in much the same way that ethanol is produced from grains or fruits, obviously the concept is not a new one; the polysaccharide character of wood has been know for over 100 years. The limitations to the use of wood for ethanol production have been primarly the difficulty in separating and hydrolyzing the crystalline callulose component in wood. Much of the technology wood was developed during world war II when ethanol production from wood was anticipated in this country to supplement industrial alcohol supplies. Synthetic ethanol is easily produced from petroleum feedstock by direct catalytic hydration of ethylene.
Interest in producing alcohols from wood in the India was revitalized by the dramatic increase in petrolcum prices and the push to decrease oil imports by substituting gasohol (one part alcohol in ten parts gasoline) for 100 percent gasoline at the gas pumps. Both ethanol and mechanot can be used for productin of ethanol from wood; these are briefly reviewed.
Ethanol from wood. During world war II the Scholler process (developed in Germany) was used in Europe for ethanol production. The method employed dilute sulfuric acid for hydrolysis of the wood polysaccharides in a batch process. This method yielded about 170 liters of 190-proof ethanol per oven- dry metric ton (50 gallon per ton oven -dry) of wood chips, later, the Madison process was developed at the U.S.D.A. forest products laboratory to give an improved yield of about 222 liters per metric ton (65 gallons per oven-dry ton). In the Medison process wood waste in the form of sawdust, shaving, or chips is loaded into a large steel container called the ditgester, when filled, steam is admitted to the digester to bring the charge up to a temperture of about 140 0C. Dilute sulfuric acid (0.5 percent) at 1400 C is then pumped into the digester until the wood is completly covered. Dilute acid is then continuously pumped into the digester, and the hydrolysis solution is continuously pumped out. This percolation process as opposed to the batch process (Sacholler) is the main innovation provided by the U.S.D.A. Forest products Laboratory. The total hydrolysis time is about 3 to 4 hours: at the end, the lignin-rich residue is discharged and recovered for its fuel value.
The liquor from the hydrolysis reaction is nuetralized, balended with yeast, and passed to fermentation tanks. The fermentation is carried out by a strain of Sacharomyces cerevisiae yeast. Following fermentation the ethanol is stripped from the dilute solution and the remaining solution is concentrated. From this solution, the five- carbon or pentrose sugars (xylose and arabinose) are separated and concentrated to a 65 - percent solution and sold as feed suplement or used for the prodction of furfural, an additional chemical derivable from wood (discussed in a later section) . A number of modifications of the original Madison process have been proposed in recent years.
Saccharification of wood polysaccharides to sugars can be accomplished by enzymatic techniques instead of acid hydrolysis. The U.S. army Natick Laboratories have developed a method for conversion of celluslose to glucose with a cellulose enzyme from an active strain of the fungus Trichoderma virde. However, extensive pretreatment of wood is necessary before sufficient enzymtic hydrolysis will take place.
Enzymatic methods show the biggest promise for conversion of waste paper from municipal waste into glucose for ethanol production. Bescause paper is primarily composed of wood celulose fibers. The enzyme inhibition due to lack of accessibility with whole wood is partially alleviated. As mentioned previously waste paper can represent upto 50 percent of typical municipal waste. Currently the separated paper from the waste is burned for fuel value.
The Gulf oil company developed a method called simultaneous saccharification and fermentation (SSF) for enzymatic conversion of waste paper to ethanol. In this processs the cellulose is enzymatically hydrolyzed and the glucose is yeast fermented in one operation. This moification, along with imporoved enzyme production and performance, has made the enzymatic technique more economically viable for conversion of waste paper to ethanol. Research is currently active on this method of alcohol production.
A number of terms are used interchangeably for thermal decom position of wood and generally refer to similar processing methods; carbonization, pyrolysis, gasification, wood distillation, destructive distillation, and dry distillation, all result in the thermal breakdown of the wood polymers to smaller molecules in the form of char, a condensible liquid or tear and gaseous products. A liquid fuel derivable from wood by this method is methyl alcohol (methanol or wood alcohol, CH3OH). A wide variety of other chemicals are also derivable from wood by thermal decompsition, a method with a long history of applications.
During world war II in Germany, automobiles were fulled by the gases produced from thermal decomposition of wood, and reserch is active today on more efficient gasification of wood. Destructive distillation has been used throughout most recorded history to obtain turpentine from pinewood.
Methanol. Similar to the situation with ethanol the concept of poducing methonal from wood is not new. Methanol obtained from the destructive distilation of wood was the only commercial source until the 1920s. The yield of methanol from wood by this method is low only about 1 to 2 percent or 20 liters per metric ton (6 gallons per ton) for hardwoods and about one - half that for softwood with the introduction of natural gas technology, the industry gradually switched to a synthetic methanol formed from a syntheses gas (syngas) produced from reformed natural gas.
About 50 percent of the methanol produced goes into production of formaldehyde; the remaining half is used for solvents (10 percent ), acrylics (10 percent) , insecticides (10 percent ), fungicides (10 percent), textile fibers (5 percent), and miscellaneous uses (2 percent), about 3 prcent is exported. The large use of methanol for production of formaldehyde and the concomitant large use of formaldehyde in plywood adhesives links the demand for methanol directly to fluctuations in home construction. Large volumes of plywood are used for building homes.
Methanol is produced from syngas with the use of catalists at high prassure. Coal, lignite or wood waste can also be utilized to produce methanol by this method. For any solid carbonaceous material to be converted to syngas, it is first necessary to burn or oxidize partially the material to produce a crude gas consisting of H2,CO, and CO2. If air is used to oxidize or burn the material, the crude gas will contain about 46 percent nitrogen, which must be removed. Thus, reforming the gasification products obtained at hight temperatures is a second method for production of methatures is a second method for production of methanol from wood. This is incontrast to the older method (destructive distillation) which directly yields small quantites of methanol (lower temperatures) as previously described.
Serveral type of gasifiers have been developed for the partial oxidation of wood, waste and garbage. These are designed to operate at atmospheirc pressure, in contrast to coal gasifiers, which can operate at pressures up to 400 pounds per square inch gage (psig). Also, about 2 percent of the wood (dry basis) is converted to an oil-tar fraction. A comparison of the crude gas from two types of gasifiers is shown in Table 5
Although the feed material for the pilot plant reactor is municipal solid waste, it is expected that the crude gas composition will be essentially the some for wood waste because municipal solid waste has been found to have practically the same composition with regard to carbon, hydrogen and oxygen.
Because of the simplicity of the conversion of natural gas the methanol, investment cost for such a plant is about one-third that of a comparable wood waste facility. Conversion effciency of natural gas to methanol (91 percent) is significantly greaster than that of wood waste (51 percent). It takes 150 cubic feet of natural gas (containing more than 95 percent methane) or 22 kilograms (4.9 pounds) to make 3.8 liters (1 gallon) of methanol. Conversion of coal to methanol, while considerably more efficient than that of waste wood (coal, 85 percent), involves more processing facilities becaus of the greater amount of ash and sulfur. Coal conversion to syngas is more efficient because it has higher carbon content and less oxygen than wood, it is technically feasible but not yet economically attractive to produce methanol from wood residues. The yield of methanol from wood is about 38 percent or 342 liters per metric ton (100 gallons per oven-dry ton). This yield is based on all process energy required coming from the wood residues. At a wood residue cost of Rs. 130.50 metric ton Rs. 150 per ton, the selling price of methanol is estimated aty Rs. 0.20 per liter Rs. 0.77 per gallon; at Rs. 300.60 per metric ton Rs. 340 per ton, selling price is Rs. 25 per liter Rs. 0.96 per gallon. The 1979 selling price of methanol was Rs. 0.12 per liter Rs. 0.44 per gallon).
Charcoal and other Chemicals. Production of charcoal and taves by destructive distillation is the oldest of all chemical wood- processing methods. Charcoal was probably first discroved when the black meterial left over from a previous fire burned with intense hit and little smoke and flame. For centuries charcoal has been used in braziers for heating purposes. Destructive distillation of hardwoods has been caried out with charcoal being the product sought and volatiles being by-products: with softwoods (pines), volatiles were the principal products (naval stores), with charcoal considered a by - product.
In India charcoal production began is the early coloniol days. During this period, principle uses of charcoal where as a fuel in blast furnaces for production of pig iron and as an ingredient of gunpowder. Charcoal needed by the iron industry required that it have a high crushing strenght, and therefore, it was made from dense hardwoods such as maple, brich, oak and hickory. A softer charcoal was preferred for making gunpowder and thus was produced from willow and basswood, Jamsedpur in 1969; Imity to iron-ore deposits. These early furnaces were small, usually produccing only 1 to 3 metric tons of pig iron per day. In the late 1975, wood charcoal lost its metallurigical market to coke, which was better suited to the demands of the larger furnaces then being built.
In 1980 the additional collection by condensation of the volatile substances from hardwood carbonization began. Products were now charcoal, crude pyroligineous acid, and noncondensible gases. The pyroligineous acid was refined to produce methanol acetate of lime, which in turn was used to make either acetic acid or acetone and tar. The noncondensible gases in a normal wood distillation consisted of about 150 per cent carbon dixide, 30 percent carbon monoxide, 10 percent methane, 3 percent heavier hydrocarbons and 3 percent hydrogen (13). Table 6 gives the yelds of products from an industrial operation. The tars and noncondensible gases ware usually used as fuel. In the late 900 and until the 1560 destructive distilation of hardwoods was an important source of industrial acetic acid, methanol, and acetons. This market was lost when these materials where made synthetically from petroleum. In 1960, there were approximately 100 plants recovering these products from hardwood distillation; the last of these plants ceassed operation in 1970. In the early 1960, charcoal from by product recovery plants was usually used for cooking and heating in low-income areas and was known as a "poor man's fuel." Beginning in about 1970 there was an upturn in demand for charcoal for recreational use. In this era of suburban living the use of charcoal briquettes for cookouts represents a significant market. The charcoal briquette can now be considered a luxury fuel, since it is too expensive for heating.
Basic tecniques for producing charcoal have not changed over the years, although the equipment has changed. Charcoal is produced when wood is burned under conditions in which the supply of oxygen is severely limited. Carbonization is a term that aptly describes the thermal decomposition of wood for this application. Decomposition of carbon compounds takes place as the temperature rises, leading to a solid residue that is richer in carbon than the original meterial. Wood has a carbon content of about 50 percent, while charcoal of a quality suitable for general market acceptance will be analyzed as follows: fixed carbon 74 to 81 percent, volatiles 18 to 23 percent, moisture 2 to 4 percent, and ash 1 to 4 percent. Charcoal with a volatile contant over 24 percent will cause smoking and is undesirable for recreational uses.
Earthen "pit kilns" were originally used to produce charcoal. A circular mound - shaped pile of wood (15 to 45 cords) was built up with an open core 30 to 60 centimeters (1 to 2 feet) in diameter to serve as a flue. The entire surcface of the pile except for the top flue opening and several small openings around the bottom periphery, was then covered with dirt or sod sufficiently thick to exclude air. The mound was then allowed to "coal" for 20 to 30 days to give the find product.
In the second half of the nineteenth century, brick or masonary "beehive" kilnes come into widespread use. The capacity of many of these kilns was from 50 to 90 cords and operation was essentially the same as for the pit kilns. Many other types of kilns have come into use from time to time. Small portable sheet-metal kilns of 1 to 2 cords have been widely used as have rectangular masonry block kilns of various designs. These where predominantly used by farmers and small woodlot owners.
Large- Scale production of charoal was done by distilling the wood in steel buggies in long horizon tal ovens, the buggies rode on steel rails that carried the cars in line from predriers to the ovens and then top coolers. Charcoal was produced by this method in a matter of 24 hours. The latest types of charcoal producing equipment are designed for continous operation and make use of residues instead of roundwood. An example is the Herreshoff multiple hearth furnace in which several hearths or burning chambers are stacked on top of one another, the number depending on capacity. Production in this type of furnace is from 1 to 2Â½ metric tons of charcoal per hour.
Although a reasonable amout of research effort has been expanded on thermochemical liquefaction of wood, extensive commercialization of this process in not anticipated in the near fature. The basis of the method is a high pressure and high-temperature treatment of wood chips in the presence of hydrogen gas or syngas to produce an oil instead of a gas. The low grade oil produced could potentially be substituted for some present petroleum uses.
An oil of a heating value of about 35.200 Btu per kilogram can be obtined by rection wood waste for 1 hour with syngas, a catalyst, a temperature of 7500F, and a pressure of 5000 psi. The feasibility of the process has been tested in a pilot plant in I.I.T. Bangalore based on laboratory work. So far, it has been determined that a barrel of oil equivalent to No. 6 bunker fuel can be produced from about 405 kilogram of wood chips.
A similar process was developed in Japan speci fically to degrade lignin. Named the Noguchi process, it was thaught to hold promise for production of phenols from lignin . The Japanese investigators had discovered superior catalysts that converted a substential protion of the lignin into a relatively few phenols. The Crown- Zellerbach Corporation subsequently obtained an option on the process and initiated their own trials. Despite several improvements they were able to make, the process did not prove profitable at the time. However, the company was routinely able to obtain a yield of 55 percent (and up to 65 percent) of distillable products. The major drawback was the inability to seperate cleanly even the few different phenols remaining after the reactions.
Furfural from Wood
An additional, potentially important chemical derivable from wood as a result of hydrolytic (acid) treatment is furfural. Furfural is derived from the semicellulose fraction of wood specifically from the five-carbon or pentose sugars (primarily xylose). The pentose sugars are not yeast fermentable by standard methods to ethanol but can be treated with acid which causes dehydration and yields furfural.
Most furfural is produced from corncobs and oat and rice hulls. The product is used in the chemical industry as a solvent and in wood rosin refining. A large amount of furfural is further treated to give furfuryl alcohol. The furfuryl alcohol is added to urea formaldehyde resins in applications for adhesive and foundry core binders.
The development of the process of papermaking and the resulting material, paper, represents the first and best application of materials science principles to wood. Basic chemistry, composition, structure, control of processing variables, and properties have all been well studied and applied as a coherent aggregation of knowledge. The chemistry of cellulose and liginin, the main chemical constitutents of wood, and the properties of the fiber, the basic wood element, required early understanding before the first papermaking processes employing wood as a raw material could be developed. From the beginning, science and technology have been closely linked and each could inspire progress in the other as property requirements changed, as the resource base changed, or as economics forced higher yields, and new laws required lower pollution loads.
Both cellulose and liginin are complex organic moleules. Extensive studies of these two components of wood during the past century have contributed significantly to the basic knowledge of wood chemistry. This prompted studies of the cell wall microstructure secrets of which are daily revealed by the latest accessories of the electron microscope. These cell wall structures provide interesting insights into the physiology of the tree,and some have been found to be related to physical properties of the wood. In this connection, further studies of the relationship of properties to cell wall structures could prove enlightening in analyzing fractures in solid wood and are in keeping with materials science principles.
The close association of science and technology in the paper industry, and the large body of organised knowledge now existing, make it possible for exciting breakthroughs in papermaking to occur with some regularity. Structural applications of paper requiring optimized engine ring properties for design purposes are becoming more and more important.These include high strenght container, honeycomb cores for ligtweight load carrying members, and molded fiberboard parts for many purposes. Leaders in this field even envision homes and parts of homes to be fabricated in the future from high strenght,durable papers.
When engineering properties are specified for a particular grade of paper,there is a clear indication that the highest expectations or a materials science approach have occurred. It then will remain as a task for engineers to develop design methodologies for various applications, and this may usher in an entirely new era for paper as a reliable engineering material.
Going beyond papermaking and somewhat beyond the noramal bounds of wood science and technology, it is pertinent to mention that highly purified wood cellulose has been isolated on an industrial scale. This led to an array of polymeric materials which includes rayon fibers for fabric, cellulose ecetate for photographic film, and cellulose nitrate for varnishes and explosives. These employ the basic principles of materials science whithout reservation, and while they are only marginally related to wood as a material, there are some important connections. For example, cellulose nitrate varnishes are used as finishes for wood furniture, and the acetylation of cellulose in situ, following knowledge developed in filmmaking has been investigated as a means of reducing the hydroscopcity of wood, and thus improving dimensional stability.
Carried in another direction, the isolation of cellulose leads to glucose, and this, through the rapidly rising science of microbiology, can be converted into ethylalcohol, an interestigs and useful end point based on wood science, but far removed from wood as a material.
Returning to the subject of papermaking, it should be noted that from the standpoint of ikeal materials science, chemistry played, and continues to play a leading role particularly during the extraction of the fiber from the wood. The process of producing paper then deals with the fiber and various additives to produce desired properties. The lignin may be removed in varying degrees, but the crystal lattice of cellulose molecules in the fiber is altered as little as possible. To this extent, the chemical composition fo the paper may be considered as having been manipulated to yield the desired properties. At this point, however, the fibers, not the molecules, form the structure of paper, and the physical qualities of the fiber often over ride the chemical qualities in controlling the final properties of paper.
The dual level of control, i.e., chemistry for composition and fiber for structure, may have counterparts in other materials, but for the general field of wood material it exists only for paper. In all the wood materials to be described in succeding paragraphs, chemistry plays a lesser role, and both the composition and the structure are manipulated with wood elements larger than a fiber. Except for this one fact, these wood materials employ all the principles of materials science in controlling properties through manufacture.
As stated previously, the properties of the final materials are more dependent upon processing variables than upon the characteristics of the parent wood raw materials. The logic of this statement resides partially in the fact that all of these materials rely on a bonding action between adjacent wood elements to create the expected properties. This is true also of paper. Since this bonding action occur during processing and since the quality of bonding can vary with processing conditions from a low of zero, the properties may also vary from zero to various levels, depending upon intended use requirements, Another factor that cause a major discrepancy between the properties of the parent wood nad those of the reconstituted materials is the fact that the original wood structure in the gross sense is purposely disorganized to some degree in the final material.
Fiberboard, particleboard, and flakeboard
These three materials- fiberboard, perticleboard, and flakeboarde and their associated processes, represent a a large close of manmade wood materials of relatively recent origin and they typify a special connection to matarials science, unlike paper, they retain a woodlike characteristic,but they eliminate some of the problems associated with solid wood. For example, knots and crossgrain are eliminated as potent strenght-reducing factors, problems of anisotroy are controled and variability from piece to piece is greatly reduced.. As a general rule, all sources of variability, including those associate with species, are reduced in inverse propertion to the size of the wood element.
In the previous section, paper was shown to be well adjusted to materials science because of its strong base in chemistry. These newer material are similar to paper in principle:however, chemistry is not the dominant discipline, and the basic wood element is much larger. The properties are controlled not only by the composition and structure achieved by means of these elements, but also by the quality of banding achieved between elements during processing. It is of interest that a simlple application of just this one principle of materials science, i.e., control of properties through composition and structure, launched an industry that grew rapidly with very little input of wood sciences or any other science. In recent years, many different science have been brought to bear and are beginning to foster a more rapid adoption of a materials science approach.
Perhaps the most important concept involved in these materials is that the basic wood elements possess distinctive qualities of their own, analogous to chemical elements. These qualities emanate primarily from their geometry and their size. Composition is expressed by the particular elements chosen for the material and the relative amounts of each, together with other additives, such as adhesives, waxes, and fire retardants. Structure is expressed by the location of the various elements throught the thickness of the materal, by the degree of compaction, and by the density profile. Another motent factor instructure is the degree of alinement of the elements preseribed in various layers of the materials during manufacture; The inherent characteristics of these elements are demonstrated in figure 1 which shows the properties derived from homogenous bards of each particle. For exmple, flakes confer high values of rupture (MOR) and modulus of elasticity (MOE), but low values of tension perpendicular to the board surface (internal bond, IB). On the other hand, particles confer low MOR and MOE, but high IB.
Knowledge of such basic characteristics of wood elements can lead to many innovative combinations, some what analogous to alloys in metallurgy. In addition, any elogated element can be oriented, and such orientation can be confined to certain layers of the board for maximum effect. This action significantly increases the MOR and MOE in the direction of orientation while lowering thee properties in the perpendicular direction , with ratio as high as 9:1. The IB, however, is unchanged. Typical boards with random and oriented flake are shown in Figures 3 and 4, illustrating the potential for control of anisotropy to match use requirement. With further development, some of these oriented materials may well exceed nature's best effort.
Another approach is to use a larger wood element, veneer, as a surface layer over a core of particles, Veneer can be placed either of the faces to produce a structural panel, or on the edges of a thick member to produce a beam or a stud, depending upon size.
Normally, the manufacturing process for these materials requires a step in which both heat and pressure and applied to consolidate the mat and cure the adhesive. This action results in an increase in density above the density of the starting raw material. High board density represents a penalty paid for the technical gain achieved by these materials. Because weight adversely affects shipping costs and ease of handling on the job, wood scientists have sought ways of reducing density without compromising on required strength properties. Two possible solutions have been developed under laboratory conditions, one employing a combination of element types, stratification in the board, orientation, and press cycle technique and the other a unique wood element and a foaming type resin binder.
Control of processing variables is very critical in the manufacture of these materials, and production plants have become quite sophisticated. Electronic instumentation and computers are common in most new installations. Two variables under closest control are resin content and density, since these affect properties markedly over a narrow range. This effect is shown in Figure 5.
The foregoing discussion, even without details, is sufficient to establish the point that these materials present ample opportunity to exercise the principles of materials science in full. Composition and structure are both controlled to produce desired properties. Further opportunities will occur in the future when efforts to improve properties include an analysis of fracture. For examples, the board shown in figure 6 how superior strength properties, but under moisture stress, planes of weakness were revealed. The causes of such planes of weakness have not yet been studied, nor have the techniques for study been developed. Fracture mechanics, another important area in materials science, but practically untouched in wood science, will be needed not only in the case cited above, but also in the generation of the wood elements from larger pieces, and in fracture occurring as consequence of overlad stresses.
I should be evident from this description that if the typical wood science approach to these materials continues to focus only on wood as wood, it will be inadequate to an understanding of properties as as generated by two or more small pieces bonded together. This is not to say that a penetrating understanding of wood per se is not useful, but rather that there is a dire need to understand the system of composite actions of many discrete pieces acting in concert. Stress analysis by means of the finite element method may be a fruitful line of study for this purpose. There is also a need to understand the reactions of wood substance to the physical and chemical actions involved in processing, such as heat, pressure, and moisture, and various additives such as adhesives and other chemicals. This is the province of materials science, and its thougtful application is timely in view of the increasing role these materials will play in the future.
Plywood is a structural materials of long standing composed of a large wood element, veneer. The composition may be varied by combining different species of different thicknesses, although the latter may be considered as structural variation as well. Often, the composition reflects cosmetic objectives. The structure involves layers of veneer disposed with the grain at right angle in adjacent layers. Structured also involves placement with respect to species and thickness. The key step in the process is the bonding of the layers so that they act in unison in resisting stresses. The adhesive selected for this purpose is important in maintaining properties under the anticipated conditions of use.
The properties of plywood are obviously dependent upon its composition, its structure, and the processing steps. These properties are different from those of the solid wood from which the plywood is made, and therefore it can be said that that properties of wood were modified during processing. The most outstanding properties of plywood compared to solid wood are its resistance to splitting, improved dimensional stability, a high degree of isotropy in the plane directions, and the fact that it is in a sheet form. This modification accords well with materials science principles.
However, a significant difference exists between this materials and those previously described. In this case, the size of the wood element carries more of the properties of the parent materials into the modified materiels than is the cases with materials previously described. For example, species-related strength casts a greater influence of the final properties. In addition, knots and crossgrain maintain their degrading effect, but not as pronounced as in solid wood.
An entirely different materials can be produced from veneer with exactly the same composition and the same processing, but with a slight change in structure. By organizing the veneers so that all layers have their grain parallel rather than perpendicular, the resulting material more nearly resembles a piece of solid wood of the same size with its associated anisotropy. However, compared to the solid wood, the veneer composition scores a technical gain over solid wood by virtue of greater uniformity from point to point in a piece, and greater uniformity among a number of pieces. This feature is important to engineers because it leads to greater accuracy in establishing design values for the materials.
Laminated lumber requires consideration in this scheme in order to complete the sequential discussing of wood elements and how they serve as a route to modification of properties. Lumber, a very large wood element, can engage in a somewhat crude but nevertheless meaningful act of composition and structural change, as exemplified by laminated beam. The technical gain over solid timbers of the same size are significant from the standpoint of engineering properties.
Because of the large size of this element, there is almost a total carryover of species-related characteristics, as well as those related to wood in general, such as knots, crossgrain and compression breaks. Anisotropy in this case is a major advantage rather than a drawback.
The act of composition, however, is not so obvious. It occurs in the process of selecting and segregating the elements, in this case lumber, into different quality groupings, using knowledge of wood structure readily visible upon careful inspection. This is a very important step and can profitably utilize sophisticated techniques of nondestructive testing (NDT) to provide an assessment of factors not visible to the eye. Having thus produced groups of elements of varying qualities, the structures of the beam is created by judicous placement of elements within the cross section, from one edge, through the center to the other edge. The placement is based upon knowledge of the stress distribution in the beam under load, with the higher quality pieces disposed in the high stress areas.
A final major step in the process in bonding of the elements so that they act in concert to resist stresses. When properly carried out, the resulting material is stronger than the average for the individual elements. Hence, it may be said that the modifications and the processing have resulted in a gain of strength, thus fulfilling the requirements for a materials science approach.
INDUSTRIAL USE OF ENERGY
In 1982 when the total energy consumption was 64.6 quads. The energy use by categories was divided of the industrial sector, the largest single user was primary metals (21.2 percent), petroleum and related produces (11.3 percent), and forest industries (5.8 percent). Although the forest-industry portion represents only 2 percent of the total national energy, It is a major idenifibale segment of energy usage.
As described earlier, if the total annual growth to timber on all commercial forestlands were used for energy, it would supply only a small percentage of our total annual national energy needs. If, however, residues and unused inventories from these lands were used by the forest industries for energy, it is possible that this industrial segment could become energy self-sufficient. The forest industries already have the land, equipment, and knowledge of handling to use wood in their plants and many already do utilize their residues to a certain extent.
Of the three largest users of purchased energy in the orest industries, the pulp and paper sector accounts for 92 percent of the energy purchased. The pulp and paper industry has already achieved 40 percent energy self-sufficiency as a result of continuing improvements in recent years by consuming spent pulping liquores and burning some hogged wood and bark. Less detailed information is available from sawmilling operations, but it is estimated that energy self-sufficiency may range from 20 to 40 percent. From these estimates, it may benefitted that the pulp and paper segment uses about fifteen times as much energy to produce a comparable tonnage of product: hence the sawmill operations are much energy intensive and are much smaller users of total energy. Plywood manufacturing purchases about 1/50 as much energy as the pulp and paper industry and is about 1/3 as energy intensive. At present, it is estimated that plywood manufacturing is approximately 50 percent energy self-sufficient.
Attainment of energy self-sufficiency in the pulp and paper industry would have the largest impact on reducing purchased energy within the forest industries. A detailed examination of this segment has shown that it is possible to increase the present 40 present self-sufficiency significantly. Most of the additional energy would be supplied through wholetree chipping and efficient use of residues.
Concern over future supplies and cost of fossil fuels has stimulated interest in growing wood for use solely as an energy source. The average yield on commercial forestland is only about 2.3 metric tons per hectare per year. However, the definition of commercial forestland includes, for statistical purposes. Much land, such as mountain slopes, that is not very productive, so that the national norm might be closer to 3.4 to 6.8 metric tons per hectare if these marginal lands were excluded. Advocates of the use of forest fuels for production of electricity maintain that by short-rotation forestry, yields of 7.9 to 33.8 metric tons per hectare can now be achieved and that be selection and genetic improvement, potential yields can be 45 to 67 metric tons per hectare short-rotation forestry has a number of names, including energy plantations, biomass farms, minirotation forestry, puckerbush, sycamore silage, and coppicing.
Management of an energy plantation would more closely resemble a farming operation that conventional forestry. Selected tree species with rapid early growth characteristics for the climate and solid type would be planted at very close spacing and harvested at appropriate times, perhaps at 5- to 8- year intervals. Harvesting would be done mechanically, similar to corn or hay crops, and regeneration would be by vegetative reproduction such as coppicing. Thus planting of seedlings need only be done at the start of the operation. For maximum biomass yields, intensive crop management with fertilization, weed control, and (possibly) irrigation would be necessary.
Selection of tree species for energy plantations differs from that in conventional forestry. Forest species are now selected based on desirable wood properties such as strength, freedom from defects, colour, fiber length, etc. For energy production, the most important factor is the ability to grow rapidly with no regard for strength properties of shape of stem. The usual criteria for selection of species for biomass production are rapid juvenile growth, ease of establishment and regeneration. Freedom from major insect and fungal pests, climate, and site qualities. Species generally considered as having high potential for energy plantation are sycamore, hybrid poplars, red alder, eucalyptus, and pine. Table 7 shows yields achieved from experimental word as reported in the literatures as well as estimates of yields that can now be achieved by intensive management. The national average yield from intensive silviculture can be approximately 17.9 dry metric tons per hectare, and in the near future it could be 33.6 metric tons.
Because the biomass is grown for its energy content, it is of interest to compare energy consumed (except for solar energy) to energy output. Energy put consists of fuel used in harvestion, loading, hauling, irrigation, manufacture of fertilizers and pesticies, and their transport, from this analysis. Energy from the biomass is from ten ton fifteen times the primary energy consumed. Cost of production of biomass under intensive management is estimated at is estimated at from Rs. 10.20 to Rs. 20 per million. But, which is generally competitive with fuel-oil prices.
Many of our fossil fuels are used to produce electricity, so we can determine whether land requirements for an energy plantation to fuel a moderately sized electric utility plant would be at all practical. Assuming that dry wood and bard have an energy content of 16 million Btu per ton, that it would be burned green at an energy efficiency of 68 percent, and that the plant would operate at 60 percent of rated capacity, a 150 megawatt steam generator would have an overall efficiency of 27 percent. On this basis, approximately 558,000 metric tons of dry wood would be required per year to powder the plant. Land area required depends on productivity; at 11.3 metric tons per hectare per year, the area required is about 504 square kilometers (194 square miles). A 150-megawatt plant would supply electricity needs for a community of 150,000.
Wood requirements of a 150-ton megawatt plant are about 1530 metric tons per day. A pulp mill rated at 900 metric tons of pulp per day at a 50 percent pulp yield requires 1800 metric tons of wood per day. In 1980 there were 52 pulp mills in India with rated capacities of 900 metric tons or more. Thus the wood requirements of a 150-megawatt electric plant are within the realm of possibility.
Energy plantations can make a contribution to the nation's energy budget. Major considerations, that must be addressed are biomass productivity and land availability. Even in the event that suitable land is available, much research is still needed to:
- optimize productivity through species selection and improvement,
- improve silvicultural methods for shor-rotation forestry, and
- develop equipment for harvesting the crop economically.