Rubbers: Materials and Processing Technology
RUBBERS MATERIALS: INTERDUCTION
The technology of rubber began with the natural product known as natural
rubber (NR). Historically, rubber (NR) as a material was known to
and used by man as early as the sixth century, as excavations subsequent
to the discovery of America have revealed. The early reported uses of
NR were limited to such items as playing balls and waterproof fabrics or
garments, People of Europe became familiar with this natural product
and its properties by the end of the eighteenth century. From its popular
application as eraser of pencil and ink marks developed in Europe in the
middle of the eighteenth century, the name “rubber” was coined to it.
Earlier, the natural product was known by the term “Caoutchouc” which,
however, is now reserved in the English language to denote the pure rubber
hydrocarbon.
The process technology of making waterproof objects based on rubber-
coated fabrics passed into an advanced phase with the discovery of
coal tar naphtha as a good solvent for rubber by Charles Macintosh. This
led to the development of the “sandwich” process for the so-called double
texture fabric, imparting much improvements in the life and performance
of the waterproof garments. But the inherent drawback in the susceptibility
of rubber to changes of temperature (becoming soft and sticky
in warm wheather, and hard and stiff in cold weather) still remained unsolved,
thus limiting expansion and diversification of its use. The difficulty
was finally overcome through the discovery of vulcanization of
rubber using sulphur by Charles Goodyear in 1839 in USA. The process
was also developed in London by Thomas Hancock at about the same
time who applied for the first patent on vulcanization or curing of rubber
in 1843. The discovery of vulcanization, which in effect is a cross-linking
process, literally infused a revolution in the rubber industry and a
whole range of consumer and industrial rubber products soon became
available in the market.
NATURAL RUBBER (NR)
Natural Rubber Plantation
There was a time till the middle of the nineteenth century when raw
rubbers (NR) came almost entirely from the equatorial forest in the Amazon
valley in South America. Rubber randomly tapped and collected from
natural forests is variable in quality and is commonly known as wild rubber.
Under economic, technical and other compulsions, and for a regular,
uninterrupted supply of rubber, comprehensives plans for cultivation of
rubber trees from seeds of Hevea brasiliensis in the equatorial climatic
zones of South and South-East Asia began in the later part of the nineteenth
century. Present day NR or the Hevea rubber is almost entirely
obtained from the plantation industries.
Tapping of Rubber Latex
The rubber plant produces a milk-white latex that contains the natural
rubber hydrocarbon in a fine emulsion form in an aqueous serum.
After a thin shaving of bark of the Hevea tree has been cut, the latex that
comes out is allowed to flow into a cup through a spout that is stuck into
the bark below the bottom end of the cut. A little of sodium sulphite
solution put into the empty cup before tapping helps prevent some darkening
or discolouration of the latex which may otherwise develop as a
consequence of an enzymatic reaction in the latex involving its phenolic
constituents producing the dark coloured pigment melanin.
Chlorinated Rubber
Natural rubber can be readily halogenated. Only chlorination has
been commercially developed. Direct chlorination with chlorine results
in both addition and substitution reactions and HC1 is evolved as a
byproduct. For only additive chlorination, maximum attainable chlorine
content would be about 51%, while for commercial products of good
stability, the range of chlorine content is 60-68%, and they are resinous
in character. It is generally believed that good degree of cyclization also
takes place on chlorination. Rubber, cut into small pieces is dissolved in
carbon tetra-chloride in presence of a small amount of benzoyl peroxide
which acts as a depolymerizing agent and lowers the solution viscosity.
A relatively uniform chlorinalion is achieved by spraying the solution at
the top of a chlorinating tower in which a stream of chlorine, let in at the
bottom, is allowed to ascend. The droplets of the chlorinated product
collect at the bottom. The collected solution from the bottom is degassed
to remove excess chlorine and then sprayed into a steam chamber where
the admitted steam causes rapid volatilization of CCl4. CCl4 is then recovered
and reused; the chlorinated rubber collected as a wet mass from
the bottom is washed, dried, milled if necessary and stored. Chlorinated
rubber is resistant to many chemicals. It even resists concentrated nitric
acid. It is, however, soluble in a wide range of solvents and it is used in
the formulation of many paints, lacquors, adhesives and printing inks.
Because of high chlorine content, chlorinated rubber has prominent flame
retardant characteristics.
Rubber can also be modified into a resinous product by
hydrochlorination. The rubber hydrochloride may be prepared directly
from stabilized latex using hydrochloric acid gas. It is, however, better
obtained by hydrochlorination with gaseous HCl using rubber in benzene
solution. Films made from the solution of the hydrochloride to which
plasticizers and stabilizers have been added are used for making laminates
with paper or films of other plastics such as cellulose acetate, etc.,
for use as heat sealable packages for dry food, cosmetics, shampoo, etc.
The hydrochloride also finds use in the formulation of adhesives and
bonding agents.
Polyacrylic Rubber or Acrylate Rubber (ACR)
Poly(ethyl acrylate) is soft and rubbery in nature. Copolymers of
ethyl acrylate (95%) and 2-chloroethyl acrylate or 2-chloroethyl vinyl
ether (cure site monomer) have been commercially developed and the
products are known as polyacrylic rubber or acrylate rubber. These rubbers
are suitably cured using aliphatic linear diamines and polyamines.
Cross-linking apparently occurs by HC1 elimination and intermolecular
link up through the diamines or via ester hydrolysis and establishment of
intermolecular amide linkages. Small amount of sulphur is used as an
anti-aging additive. The cured rubber is particularly useful as hoses, seals
and gaskets. Reinforcing carbon blacks are used as fillers. For pale shades,
siliceous fillers are used. The rubber has good resistance to oils and to
ozone and it may be used over a wide temperature zone (—40 to nearly +
200°C). The uncured polyacrylic rubbers are soluble in ketones, esters
and alcohol-ester mixtures. The cured polyacrylic rubbers are better than
nitrile rubbers in heat and oil resistance. Their resistance to ozone attack
and to sunlight and weathering are good. A terpolymer (AEM) of methyl
acrylate, ethylene and a cure site monomer, known in the trade by the
name Vamac, is of more recent development.
Fluorocarbon Rubber (FKM)
Fluorocarbon rubbers or elastomers are copolymers of vinylidene
fluoride and chlorotrifluoroethylene (50 : 50 or 30 : 70 ratio). Better products
are obtained by copolymerization of vinylidene fluoride and
bexafluoropropylene (“Viton” elastomers from Du Pont). They are usually
cured with amine type curatives in presence of a metallic oxide (litharge
or calcined magnesia). Curing is apparently effected by the elimination
of hydrogen fluoride.
The fluorocarbon (copolymer) elastomers are prepared by batch or
continuous process following the emulsion polymerization technique. The
latex-obtained is coagulated by hydrochloric acid and the polymer is
washed and dried. The Vitons are normally soluble in lower ketones and
the doughs formed are suitable for spreading over glass cloth to produce
coated fabrics useful as oil seals and gaskets with a long service life at
high temperatures (  100 h at nearly 400°C, > 5000 h at 200°C).
The fluorocarbon rubbers exhibit excellent resistance to oils, lubricants,
hydrocarbon solvents, mineral acids and chemicals and to heat; in
these respects, they are superior to almost all other commercial rubbers.
The fluorocarbon rubbers can be suitably compounded to give vulcanizates
of tensile strength of about 200 kg/cm2 and elongation at break in the
range of 200-300%. They are flame resistant and they exhibit outstanding
resistance to oxygen and ozone attack. Their good low temperature
flexibility makes them advantageously useful at low temperatures up to
— 30°C. The application of fluorocarbon rubbers is limited to only special
or unusual service conditions where other rubbers are altogether unsuitable
and where their high cost is not a hindrance. The applications
include seals, gaskets and diaphragms, fire-resistant and protective clothing
from coated fabrics, and wire and cable insulation.
THERMOPLASTIC ELASTOMERS (TPE)
Development of thermoplastic elastomers (TPE) has narrowed the
basic difference between the processing of thermoplastics and elastomers
or rubbers for many non-tyre products. The TPEs make useful products
for which the tensile and set properties are not much critical, such as in
many automotive parts, footwear, cables, sealants and adhesives, hoses,
coated fabrics, tubings and sheetings. Their light weight, flexibility, impact
resistance and weathering resistance make them useful in many of
these applications.
The thermoplastic elastomers are processed like reusable and
reprocessable thermoplastics and under service conditions, they behave
like vulcanized rubbers. The useful mechanical and elastic properties of
conventional vulcanized rubbers are attributed to the chemical cross-links
established between the rubber chain molecules during vulcanization to
produce a space network structure. In the TPEs, the network is basically
formed through thermally labile physical cohesive forces between specific
segments of different polymer chains but not really through intermolecular
chemical linkages. At elevated and processing temperatures,
the thermolabile physical bonds weaken and finally break up, permitting
flow under shear and thus enabling them to be moulded or formed like a
conventional thermoplastic material. At and around ambient temperatures,
the TPEs exist in two phases; the soft rubbery phase forms the
continuous matrix in which the hard resinous phase remains dispersed in
discrete domains. The volume fraction of the rubbery matrix is usually
higher than that of the hard thermoplastic domains which materially act
as cross-links and as stiffening fillers or points of reinforcements.
RUBBER COMPOUNDING AND PROCESSING TECHNOLOGY
Introduction
No rubber is considered technically useful if its molecules are not
cross-linked by a process known as curing or vulcanization. The process
of vulcanization is usually associated with two chemical processes taking
place simultaneously, cross-linking and chain degradation, though at
widely different rates. For natural rubber and many synthetic rubbers,
particularly the diene rubbers, the curing agent most commonly used is
sulphur. But sulphur curing takes place at technically viable rates only at
a high temperature (> 140°C), and heating with sulphur alone leads to
optimum curing after nearly 8 h at 140°C using a fairly high dose of
sulphur (8-10 phr). Use of metal oxides, such as those of zinc, calcium,
magnesium, lead, etc., brings about some advantages with respect to time
of curing and improvements in physical properties without much reduction
in the sulphur dose. Aniline is considered as the first organic accelerator
tried in an attempt to quicken the curing process. Some of the more
efficient and less toxic modern organic accelerators of rubber vulcanization
are aniline derivatives. Sulphur dose has been substantially lowered
with the advent of organic accelerators.
Extensive studies and experimentation have revealed that neither
sulphur nor application of heat is indispensable for effecting cross-linking
and associated changes in physical properties of rubbers. Peroxides,
metal oxides, amines, amine derivatives and oximes have been found to
bring about curing of selected rubbers quite effectively. High-energy
radiations can bring about effective curing; but high-energy radiation
curing has not been developed into a commercial process of even limited
acceptability. Selenium and tellurium can substitute sulphur either totally
or partly to effect satisfactory curing of diene rubbers. Sulphur
monochloride can bring about room temperature or cold curing of diene
rubbers.
Improvements in physical and mechanical properties that can be
achieved through vulcanization using sulphur/accelerator systems only
are rather limited. The needs of imparting colour, stability, resistance to
tearing and abrasion, flexibility etc., and of improving processibility and
mechanical properties necessitate incorporation of a host of additives or
compounding ingredients in the rubber by what is commonly known as
the mixing or compounding process.
Rubber Vulcanization
VULCANIZATION AND ITS EFFECTS
Chemical crosslinks between macromolecules may occur in polymerization
or in fabricating articles, resulting in polymers with network structures.
The reaction which occurs during fabrication is known as “vulcanization”
in the rubber industry, and “curing” or “hardening” in the
plastics industry. Crosslinks in rubber are formed by the reaction with a
suitable “vulcanizing agent”, usually sulphur.
A typical crosslinking reaction is the formation of short chains of
sulphur atoms linking linear molecules in rubbers during vulcanization.
Lightly-vulcanized rubber shows good elastic properties, whereas vulcanization
to the maximum extent possible will lead to a full network
structure, and a hard and rigid material. Thermosetting resins may be
crosslinked by the addition of a curing agent, such as hexamethylene
tetramine and/or heat. Typical examples are epoxy resins, polyester resin
and poly-urethane foams. Owing to the greater restriction on the mobility
of the macro-molecular chains as a result of crosslinking, the modulus
and glass transition temperature increases in both hardened resins and
sulphur crosslinked rubber.
Charles Goodyear discovered sulphur vulcanization of rubber in 1839
and developed many new applications for rubber in industry.
Crosslinking of rubber occurs by a chemical process which is initiated
at favourable reaction sites by means of some form of energy input.
Double bonds in diene polymers or reaction sites left by abstraction of
hydrogen or halogen atoms are favourable reaction sites. These are coupled
by carbon-carbon crosslinks or by bridges formed with curatives
such as sulphur, acrylates, phenolics or triazines. Crosslinking through
sulphur atoms is quite common in biological molecules, e.g. disulphide
(cystine) bridges in polypeptides.
There are four principal changes which are brought about by vulcanization.
1. Rubber is converted from essentially a plastic substance of
very low strength to an elastic material of considerable strength
and resilience.
2. The physical properties, such as tensile strength and other
properties shown in Fig. 1 undergo a profound change as
vulcanization progresses.
3. The physical properties of vulcanizates are maintained over a
much wider temperature range than in the case of unvulcanized
rubber.
4. The crosslinked polymer (vulcanizate) only swells in liquids
which normally dissolve the uncrosslinked polymer.
Rubber Reclaiming
In the mid-1800s Charles Goodyear developed a technique for vulcanizing
rubber. He blended natural rubber with sulfur and placed it on his
wood stove, where the rubber cured into a water-impermeable sheet.
Because of the shortage of natural rubber at that time, a rubber-reclaiming
process, based on steam pressure to devulcanize used rubber, was
developed in 1858. The cured ground rubber was subjected to steam pressure
for 48 hours. Today, high cost silicone rubber polymers are reclaimed
in much the same way, although most synthetic polymers require more
complicated techniques. Rubber recycling has been extended to the recovery
of rubber in asphalt, scrap rubber as fuel, rubber pyrolysis, tire
splitting, and others. However, the discovery of plastics and oil-extended
rubbers and the drop in crude oil prices have led to a reduction in rubber
recycling except for expensive polymers, such as silicones (qv) and
fluorocarbons. Pyrolysis of scrap rubber is expensive and markets are
limited. It is more expensive to prepare and burn scrap rubber for fuel
than to burn natural gas, fuel oil, or coal. High fuel costs and petroleum
scarcity in Europe and other parts of the eastern hemisphere have resulted
in the use of recycled rubber as fuel and made it more economical
than in the United States. However, with today’s technological advances,
recycled rubber will be used more and more in the United States, but with
regulations for handling and disposing of worn tires. Crude oil costs have
dropped from about $189/m3 ($30/bbl) to $88/m3 ($14/bbl) over the last
few years, making it seemingly unprofitable to recycle rubber, especially
tires. However, tires discarded in landfills tend to float on top; mosquito
breeding and illegal tire disposal are causing problems which could be
alleviated by recycling. Approximately 70% of the scrap rubber, primarily
as tires, is discarded in landfills. Private landfills may charge up to S3
per tire, and disposal costs at municipal landfills are ca $0.30-0.60 per
tire. These costs encourage illegal disposal of tires. As the tire piles grow,
rubber recovery should become more economical. Companies entering
into multimillion dollar energy recovery projects will be encouraged by
an adequate supply of old tires.
Some states have restricted the disposal of tires in landfills, e.g., Washington,
Minnesota, and Oregon. Other states are studying legislation and
New Jersey is considering a surcharge. Iowa, Massachusetts, and Michigan
are studying tire-disposal problems, and Minnesota and Washington
have imposed a surcharge. Some communities refuse old tires while others
require tire splitting or shredding to prevent floating problems in
landfills. Fires resulting from the storage of tires have led to regulations
governing stockpiles. Mosquito infestation caused by illegal disposal of
tires has prompted Saginaw, Mich., to purchase a shredder for landfill
disposal.
Advances in the technology of shredding tires, reclaiming rubber,
retread equipment, energy-related projects, and pyrolysis are helping to
solve the disposal problem. Technical and marketing professionals associated
with the recycling and disposal industries are paying more attention
to tire-recycling and energy-generating projects.
SCRAP RUBBER AS FUEL SOURCE
The use of scrap rubber for fuel offers the best alternative for reusing
rubber, as fuel costs increase and tire disposal problems become more
serious. Tires in the form of 2.50-cm chips are most economical because
shredding is not expensive. Tires contain more than 90% organic materials
and have a heat value of ca 32.6 MJ/kg (ca 14,000 Btu/lb), compared
with coal values of 18.6-27.9 MJ/kg (ca 8,000-12,000 Btu lb). A cyclonic,
rotary-hearth boiler fired with whole tires was operated by the
Goodyear Tire and Rubber Co. from 1975 to 1977. It was designed to
burn 1400 kg/h and generated 11,300 kg of steam per hour.
In the Lucas tire-burning furnace, tires are conveyed into an airtight
chamber and then onto the outer rim of a rotating hearth. The chamber
prevents flashback fires and limits air leaks. An air-velocity head of 5.1
cm provides the turbulence necessary for combustion. Residues from the
burning tires form a char that increases combustion heat loss and tends to
clog furnace grates, but the carbon black content reduces slagging problems.
Improper combustion at the ash-removal area of the furnace may
prevent the burning of the carbon black. The Lucas-Goodyear furnace
was shut down because of mechanical problems and failure to comply
with Michigan’s air pollution emission standards.
Shredded tire chips have been burned in stoker-fired boilers. Uniroyal
fired a 15% mixture of tire chips with coal and both General Motors and
B. F. Goodrich have burned a 10% tire-chip mixture with coal. Tiregrinding
size-reduction problems and delivery costs have stymied projects
based on combined tire and coal fuel. The Lucas furnace was developed
to burn tires without size reduction. Transportation of tire scrap can cost
$0.05/kg, exclusive of grinding costs. Thus tire-fired boilers are limited
to areas with ample scrap-tire supplies, e.g., large cities or tire manufacturers.
The cost of burning one metric ton of tires per hour in an incinerator
was ca $0.20—0.40 per tire in 1974, which increased to $0.35-0.70
per tire in 1987.
The Oxford Energy Company uses a technology developed by
Gummi-Mayer Company (FRG) to incinerate tires and produce electricity.
The technology will be used in the Modesto Project near Westley,
Calif. The facility generates 14.4 MW of electricity and cost $38 × 106.
Construction began in December 1985 and was completed in August 1987.
The project is designed to generate electricity by incinerating whole waste
tires on reciprocating stoker grates located beneath two water wall boilers.
The boilers produce 55,000 kg of steam per hour and power a single
General Electric turbine generator. The facility is being constructed next
to the Filbin Tire Collection Agency, Inc., which administers one of the
largest stockpiles of waste tires in the United States (ca 300 × 103 t); the
stockpile increases annually by 27 t. Oxford Energy is also involved in
the construction of a New Hampshire tire incineration project, generating
15 MW of electricity; facility is expected to be completed in late
1988 at a cost of $35 × 106. The company has received air quality and
waste management permits from the State of New Hampshire. The firm
also plans to build a 22-MW electricity-generating facility in northeastern
Connecticut; construction is expected to be completed in 1989 at a
cost of $50 × 106. The company has projects to incorporate waste produced
from manufacturing facilities along with waste tires, thus providing
energy to manufacturing facilities as well as local utilities. Approximately
20 × 106 tires are stockpiled at two sites in New England.
These sites, together with the Filbin stockpile, contain at least 480 × 103
t of tires (53 × 106 tires).
CRYOGENIC PULVERIZING AND MECHANICAL TIRE
SHREDDING
Tires must be pulverized or shredded before they can be reclaimed
by devulcanization or used in asphalt and other recycling processes. The
tires are mechanically ground, sometimes using cryogenic freezing or
solvent-swelling techniques to enhance grinding efficiency. In one process,
a polar solvent is used to swell the rubber, followed by shearing to
reduce particle size. Ground tire-crumb rubber is commonly referred to
as rubber reclaim, even though the rubber has not been devulcanized.
Cryogenics in conjunction with mechanical action has been used to
make crumb rubber. Nitrogen cools the rubber below the glass-transition
temperature, and the brittle rubber is pulverized in a grinding mill. A
small cryogenic system can be installed at the site to integrate the scraprubber
crumb into the compound mixing process. Cryogenic-grinding
costs are $0.20-0.40/kg, depending on the desired particle size and the
type of rubber; harder rubber is easier to grind. Ground-rubber scrap can
be devulcanized, pyrolyzed, or recycled directly into the rubber compound.
Ground rubber is also added to plastics.
Air Products and Chemicals, Inc., Allentown, Pa., developed a cryogenic
process for grinding scrap tires in the mid-1960s. Liquid nitrogen
freezes the rubber to facilitate shredding. Midwest Elastomers installed a
cryogenic system in 1980 to powder tire peels in its Wapakoneta, Ohio,
plant. The equipment and materials for cryogenic processing are expensive
and have slowed expansion. Cryogenic grinding requires recycling
close to an air-processing facility to make the use of liquid nitrogen economical.
Air Products has a marketing and development relationship with
Brown & Ferris Industries to develop waste-to-energy and alternative
fuel projects.
The tires are mechanically ground with a two-roll, grooved-rubber
mill. The two-mill rolls turn at a ratio of ca 1:3, providing the shearing
action necessary to rip the tire apart. The rubber chunks are screened and
the larger material is recycled until the desired size is reached. Bead wire
is removed by hand or with magnets. For most applications, e.g.,
devulcanization or pyrolysis, crumb-rubber particles smaller than 1.19
mm (16 mesh) are desired, and several milling steps are required. Tire
fiber is removed in intermediate operations with hammer mills, reel beaters,
and air tables that blow a steady stream of air across the rubber
Latex and Foam Rubber
INTRODUCTION
Rubber latex is a term used to cover a range of colloids having macro
molecular substances as the dispersed phase and water as the dispersion
medium. Most important latex used in the rubber industry is natural rubber
(NR) latex. This is an aqueous dispersion of cis 1,4 polyisoprene in
water containing dissolved serum substances. NR latex is harvested from
Hevea brasilienesis trees by inflicting controlled wounds on the bark of
the trunk of the tree.
Under inorganic salts seven different types of ions are identified.
Similarly, under each of the organic ingredients a number of compounds
belonging to that group are characterised. In synthetic lattices also, a
number of ingredients other than the polymer and water are present.
PRODUCTS FROM LATEX
There are eight important steps in the manufacture of products from
latex –
1. Selection and quality control of raw materials
2. Preparation of raw materials
3. Compounding and design of the mix
4. Maturation
5. Processing/shaping
6. Vulcanisation
7. Testing for grading
8. Packing and marketing.
There are different types of products manufactured from latex. Some
of the steps are common in all products but differences exist in others.
The similarities and differences in the various steps for different types of
products are discussed below briefly:
Selection of Raw Materials
Following are the raw materials used in the latex industry —
Latex containing the polymer
Stabilisers and viscosity modifiers
Activator
Accelerator
Curing agent
Antioxidant
Special additives, if any
Gelling/coagulation system.
The raw material selected for product manufacture must be of good
quality and conforming to specifications prescribed for this by reputed
institutions like the BIS (ISI). In the case of natural rubber latex, there are
few processors who can supply high ammonia-preserved concentrated
natural rubber latex conforming to IS 5430. It may be noted that the specifications
for this HA latex have been formulated by the combined effort
of experts from rubber plantations and the goods manufacturing industry.
It is desirable for the industry to insist that the suppliers should deliver
of HA latices conforming to IS 5430.
In foam rubber there are two important processes in vogue commercially.
One is called the Dunlop process and the other called Tulalay process.
In India, most factories use the Dunlop process. In Dunlop process,
foaming is carried out in two stages. In the first stage latex is whipped up
with foam promoters and stabilisers till the desired expansion is obtained.
Thereafter the whipping rate is reduced and the foam is clarified or refined.
Gelling agents are added at the end of the foam clarifying stage.
The density of foam rubber produced depends on froth height. The level
in which latex is whipped should be calibrated properly for specific gravity
for various froath heights.
The materials selected for mould construction for foam production
must be light, cheap, should have high thermal capacity and good thermal
conductivity: A mould designed from copper free cast aluminum
meets these requirements. It is generally reported that in the cost of production
of foam rubber around 5% of the cost is on account of contribution
from mould cost. The tensile and cure properties of the hot wet gel
are appreciably lower than those of the dry finished products. So if care is
not exercised in stripping the foam from mould, it may as well be damaged.
For enabling easy and quick stripping, the mould surface should be
kept clean and polished. Mould release agent has to be applied on surface
of mould after each operation. Examples of mould release agents are
aqueous or alcoholic solutions of polyethylene glycol of fairly high molecular
weight such as carbo wax-4000, carboxymethyl cellulose or silicone
fluids.
In Tullalay process, the expansion of latex compound is achieved by
liberation of gases like oxygen liberated by catalytic decomposition of
hydrogen peroxide.
Latex Thread
In the production of latex rubber thread the matured latex has to be
carefully processed to avoid defects. The compounded matured latex is
de-aerated and filtered before it is allowed to pass through capillaries for
injection into an acid bath. The nozzle of the capillary is immersed in
coagulant. The nozzle depth in coagulant bath has direct relation to the
thread diameter. The rate of extrusion is 30-40 feet per minute. The orifice
of the nozzle is usually round and the size of the bore varies from 0.5
to 1 mm. Coagulant normally used is 15-40% acetic acid. Generally,
weaker solutions of acid are used for large diameter thread and strong
solutions for higher count threads. For the production of high count thread
alcohol is the most suitable coagulant. Between extruding nozzle and
winding, the thread is usually subjected to some degree of stretching which
may be upto 200%. Stretching is necessary to facilitate washing, to reduce
permanent set and to increase modulus of the thread.
Vulcanisation
Latex products are generally vulcanised at low temperatures and pressure
when compared to dry rubber products. It is desirable to wash, leach
and dry the latex products before subjecting them to vulcanisation temperature.
Vulcanisation of some products is carried out in hot air. Some
others are vulcanised in open steam in autoclaves.
Construction of Moulds
The moulds for foam rubber are usually made from cast aluminium
because of the lightness, durability and good heat conductivity of the
metal. When cushions and mattresses are made, cylindrical cores are attached
to the lids which give the characteristic cavity structure of the
back side of the foam. These cavities help to minimise the actual thickness
of the product to about 1 inch to ensure uniform curing throughout
the mass of the product. The cavities also assist in great saving of the
latex compound (of the order of 40%) without affecting the comfort. In
designing moulds, allowance should be given to shrinkage of the product
which amounts to roughly 8-10% linear.
Mould lubricants like polyethylene glycol and alkyl sulphate solutions
are applied to the mould surface to facilitate easy stripping of the
product after vulcanisation.
Curing
Mould foam products are normally cured by open steam (at 100°C)
which is more efficient and quicker than hot air curing. Thin sections of
foam, e.g., as used in carpet underlays, can be suitably cured by
hot air.
Polybutadiene and Polyisoprene
INTRODUCTION
Polyisoprene is a major component of manufacture rubber. It is also made
synthetically and forms are stereospecific cis 1-4, and trans-1-4-
polyisoprene. Both can be produced synthetically by the effect of heat
and pressure on isoprene in the presence of stereospecific catalyst. Natural
rubber is cis-1-4. Synthetic cis-1-4 is sometimes called synthetic natural
rubber. Trans-l, 4-polyisoprene resembles gutta percha. Polyisoprene
is the thermoplastic until mixed with sulphur and vulcanised. It supports
combustion and is non toxic
Polybutadiene is a synthetic thermoplastic polymer made by polymerising
1,3 butadiene with a stereospecific organometallic catalyst (butyl
lithium) though other catalysts such as titanium tetrachloride and aluminium
iodide may be used. The cis-isomer, which is similar to natural
rubber, is used in the tread due to its abrasion and crack resistance and
low heat built up. Large quantities are also used as blend in SBR rubber.
The trans-isomer resembles gutta percha and has limited utility, liquid
polybutadiene, which is sodium catalysed has speciality uses as a coating
resin. It is cured with organic peroxides. Combustible liquid form is probably
toxic by ingestion and inhalation, as well as skin irritant. The different
polymer structures of polybutadiene are given in Fig. 6.
POLYMER STRUCTURE
Polyisoprene
The polymerisation of isoprene and butadiene are examples of addition
polymerisation in which the repeating structural unit within the poly
mer backbone has the same molecular weight as the entering monomer
unit.
With isoprene, the building of this polymer backbone can occur in
several ways, depending upon where the addition occurs. The polymerisation
addition process can be a reaction involving the 1, 2-, 3, 4- or 1, 4-
positions of isoprene to give the structures shown in Figure 1.
In case of 1,2- and 3, 4- addition, an asymmetric carbon is formed
that can have either an R or an S (“d” or “1”) configuration. In general,
equal numbers of R and S configurations are produced during an addition
polymerisation, which results in no net optical activity for the polymer.
The disposition of the R and S configurations along the polymer
backbone, however, results in diastereomeric isomerism. Although many
combinations of sequences are possible, only three arrangements are commonly
considered in polymers. These diastereomeric isomers are referred
to as isotactic, syndiotactic, and atactic.
Gel and Branching
The structure and behaviour of polyisoprene is further complicated
by the possibility of branching and gel. The degree of gel and branching
is dependent upon the source of the cis-1, 4-polyisoprene. For Al-Ti catalysed
cis, 4-polyisoprene, the gel content is 5-25% most of which is termed
“loose” gel since it breaks down readily upon mastication. The gel content
in natural rubber is usually greater than that in the Al-Ti catalysed
polymer. This gel is also a “loose” gel. Lithium polymerised cis-l, 4-
polyisoprene usually contains no gel. Both natural rubber and Al-Ti-catalysed
cis-1,4-polyisoprene also contain large quantities of microgel.
The determination of the nature and degree of branching in natural
rubber and Al-Ti catalysed cis-1, 4-polyisoprene are complicated by the
presence of the gel and microgel. Removal of the gel/microgel fraction
from the soluble portion can have a marked effect on the resulting physical
properties such as viscosity, molecular weight, and branching.
Polybutadiene
The configurations of polybutadiene are cIS-, trans-, and vinyl. Since
either or both of the double bonds in butadiene can be involved in the
polymerisation mechanism, the resulting polymer may have a variety of
configurations. These result from the fact that the spatial arrangement of
the methylene groups in the polybutadiene backbone allow for geometric
isomerism to occur along the polymer chain. The different polymer structures
of polybutadiene are given in Figure 6.
Participation of both double bonds in the polymerisation process gives
rise to a 1, 4-addition, which can be either cis-1,4- or trans-l,4-, depending
upon the disposition of groups about the polymer double bond. Participation
of only one double bond results in a vinyl, or 1,2-addition, that
can have three possible structures just as 1,2- and 3,4- polyisoprene have
isotactic, syndiotactic, and atactic.
Reclaimed Rubber
INTRODUCTION
Reclaimed rubber is the product resulting when waste vulcanised scrap
rubber is treated to produce a plastic material which can be easily processed,
compounded and vulcanised with or without the addition of either
natural or synthetic rubbers. It is recognised that the vulcanisation process
is not truly reversible; however, an accepted definition for
devulcanisation is that it is a change in vulcanised rubber which results in
a decreased resistance to deformation at ordinary temperatures.
Reclaimed rubber is manufactured by suitable treatment to old and
worn out tyres, tubes and other used rubber articles with certain chemical
agents. A substantial devulcanisation or regeneration is effected to the
rubber compound in this process whereby its original plasticity is regained.
In short it may be stated that reclaiming is essentially a
depolymerisation process where the combined sulphur is not removed.
The reclaimed rubber is used in the manufacture of rubber goods, with or
without admixture of natural or synthetic rubber.
Recycled rubber can be more generally described as any sort of rubber
waste that has been converted into an economically useful form such
as reclaimed rubber, ground rubber, reprocessed synthetic rubber, and
die-cut punched parts.
One method of recycling some scrap rubber is to grind it as fine as
possible and work it into new rubber as an elastomeric filler. This was the
first method of reclaiming and is suitable only for compounding carriage
springs, which were fairly large barrel-shaped molded articles but not
suitable for footwear products.
In order to make a high-quality reclaim, the fibre must be removed
from the rubber scrap by soaking the rubber in water, and then taking a
small knife and starting the rubber from the cloth and stripping it off. But
this was not an effective way to produce large quantities of products.
TYPES OF RECLAIM
A variety of grades of reclaimed rubber is offered today, but mention
is made here of only the important ones.
Whole Tyre Reclaim
Whole tyre reclaim is the one produced in the largest quantity. Firstquality
reclaim made from whole tyres contains about 45% rubber hydrocarbon
by weight. The remaining 55% consists of valuable carbon
black, a little mineral filler, and softeners, all of which are substantially
unchanged by the reclaim manufacturing operation, and may be considered
to function as virgin materials. The manufacturing Minimum Staining Reclaim
Minimum staining reclaim can replace the conventional whole tyre
material when occasion demands. As implied, it has a much lower tendency
to stain, by either migration or contact, than conventional reclaim.
The reduction in staining characteristics is achieved by the use of activated-
carbon non-staining oils and by selecting tyres containing a higher
proportion of natural to synthetic rubber.
Drab and Coloured Reclaims
As the names imply, drab and coloured reclaims are made from nonblack
scrap. The digester process is usually employed and, when fabric is
present, a small addition of caustic is made in order to destroy it. The
period of heat treatment is usually several hours at 195°C.
Butyl Reclaim
Whereas reclaimed rubbers have been successfully produced from
scrap CR, NBR,SI, and other speciality rubbers, the only one of substantial
commercial importance is butyl reclaim. The starting material for this
is butyl inner tubes. A modified digester process is adopted, every precaution
being taken to avoid contamination by NR or SBR, because of
their adverse effect on the curing characteristics of the butyl. Extensive
control tests are necessary to ensure that the curing properties are satisfactory.
The nerve of butyl reclaim is much reduced compared with that
of the original polymer. Because of this, compounds containing butyl
reclaim will mix, calender, and extrude faster and more smoothly than
similar compounds based on virgin rubber.
Reclaimed Rubber
By the application of heat and chemical agents to ground vulcanised
waste rubber, a substantial regeneration of the rubber compound to its
original plastic state is effected, yielding a product known as ‘reclaim’ or
reclaimed rubber, capable of being processed, compounded, and
revulcanised. The process is essentially one of depolymerisation. Reclaimed
rubber has become widely accepted as a raw material which possesses
processing and economic characteristics that are of great value in
the compounding of natural and synthetic rubber stocks.
There are four principal reclaiming processes in use today, of which
the digester and reclaimator processes are important.
The raw material for reclaiming is scrap rubber in a wide variety of
forms, but tyres, as is to be expected, form the major quantity.
The first stage, in all processes, are the cracking and grinding of the
scrap rubber to reduce it to a crumb passing through a 20 to 30 mesh
screen.
Digester Process
At one time, most reclaim was made using the digester process. A
digester is essentially a steam-jacketed, agitator-equipped autoclave,
mounted either horizontally or vertically. This is a wet process in which
the coarsely ground scrap is submerged in a solution of water and reclaiming
agents. These agents may include many types of light and/or
heavy oils, naval stores, pine tar and coal tar pitches, and chemical
peptisers.
Until the advent of synthetic rubber, the digesting solution also included
caustic soda to remove free sulphur and to act as a defibring agent.
In fact, the process was generally referred to as the alkali digester
method.
The ground waste is loaded into a digester along with water, reclaiming
oils, and other additives, such as activated black (for minimum stain
ing grades). The digester is a cylindrical jacketed pressure vessel fitted
with a horizontal agitator, and steam can be supplied to both interior and
jacket, thus enabling a uniform temperature to be maintained throughout
the mass. The contents of the digester are then heated to about 190°C and
maintained at this temperature for some 4-10 hours with continuous agitation.
The digester is then ‘blown down’, and the contents deposited on
to a conveyor. Any necessary adjustments to the specific gravity and plasticity
by addition of plasticiser, carbon black, or fillers are carried out in
a ribbon blender, and the stock is then automatically conveyed to extruders
for straining, refining, and leafing on to a drum from which it is removed
in slabs.
Reclaimator Process
The reclaimator process is the only commercially successful continuous
technique for devulcanising tire scrap; all the others are batch
processes. Tyres are ground, the metal and fibre are mechanically separated,
then the rubber is further ground to a fine particle size. This fineground
rubber and the various reclaiming agents are all metered into a
blending system and conveyed to the reclaimator.
The reclaimator is a special type of screw-extrusion machine. It is
jacketed to provide for several zones of controlled temperature using either
hot oil or cooling water; in addition, the clearances between the screw
and the chamber wall are close and adjustable. The object is to subject
the rubber to a controlled amount of high heat and pressure in a continuously
moving environment. The residence time of the rubber in the machine
is less than 5 minutes. During this period, the rubber undergoes
devulcanisation. After the softened rubber is discharged from the head of
the machine, it is cooled and further processed in refining mills just as is
done in other reclaiming methods.
It can be shown that ground vulcanised rubber heated in a temperature
range of 120-200°C undergoes a rapid initial increase in plasticity,
and, on continued heating, passes through an inversion point and rehardens
unit, after prolonged heating, a further but slower increase in plasticity is
attained. It follows, therefore, that three points of equal plasticity occur
in this cycle.
Rubbers: Materials and Processing Technology
RUBBERS MATERIALS: INTERDUCTION
The technology of rubber began with the natural product known as natural
rubber (NR). Historically, rubber (NR) as a material was known to
and used by man as early as the sixth century, as excavations subsequent
to the discovery of America have revealed. The early reported uses of
NR were limited to such items as playing balls and waterproof fabrics or
garments, People of Europe became familiar with this natural product
and its properties by the end of the eighteenth century. From its popular
application as eraser of pencil and ink marks developed in Europe in the
middle of the eighteenth century, the name “rubber” was coined to it.
Earlier, the natural product was known by the term “Caoutchouc” which,
however, is now reserved in the English language to denote the pure rubber
hydrocarbon.
The process technology of making waterproof objects based on rubber-
coated fabrics passed into an advanced phase with the discovery of
coal tar naphtha as a good solvent for rubber by Charles Macintosh. This
led to the development of the “sandwich” process for the so-called double
texture fabric, imparting much improvements in the life and performance
of the waterproof garments. But the inherent drawback in the susceptibility
of rubber to changes of temperature (becoming soft and sticky
in warm wheather, and hard and stiff in cold weather) still remained unsolved,
thus limiting expansion and diversification of its use. The difficulty
was finally overcome through the discovery of vulcanization of
rubber using sulphur by Charles Goodyear in 1839 in USA. The process
was also developed in London by Thomas Hancock at about the same
time who applied for the first patent on vulcanization or curing of rubber
in 1843. The discovery of vulcanization, which in effect is a cross-linking
process, literally infused a revolution in the rubber industry and a
whole range of consumer and industrial rubber products soon became
available in the market.
NATURAL RUBBER (NR)
Natural Rubber Plantation
There was a time till the middle of the nineteenth century when raw
rubbers (NR) came almost entirely from the equatorial forest in the Amazon
valley in South America. Rubber randomly tapped and collected from
natural forests is variable in quality and is commonly known as wild rubber.
Under economic, technical and other compulsions, and for a regular,
uninterrupted supply of rubber, comprehensives plans for cultivation of
rubber trees from seeds of Hevea brasiliensis in the equatorial climatic
zones of South and South-East Asia began in the later part of the nineteenth
century. Present day NR or the Hevea rubber is almost entirely
obtained from the plantation industries.
Tapping of Rubber Latex
The rubber plant produces a milk-white latex that contains the natural
rubber hydrocarbon in a fine emulsion form in an aqueous serum.
After a thin shaving of bark of the Hevea tree has been cut, the latex that
comes out is allowed to flow into a cup through a spout that is stuck into
the bark below the bottom end of the cut. A little of sodium sulphite
solution put into the empty cup before tapping helps prevent some darkening
or discolouration of the latex which may otherwise develop as a
consequence of an enzymatic reaction in the latex involving its phenolic
constituents producing the dark coloured pigment melanin.
Chlorinated Rubber
Natural rubber can be readily halogenated. Only chlorination has
been commercially developed. Direct chlorination with chlorine results
in both addition and substitution reactions and HC1 is evolved as a
byproduct. For only additive chlorination, maximum attainable chlorine
content would be about 51%, while for commercial products of good
stability, the range of chlorine content is 60-68%, and they are resinous
in character. It is generally believed that good degree of cyclization also
takes place on chlorination. Rubber, cut into small pieces is dissolved in
carbon tetra-chloride in presence of a small amount of benzoyl peroxide
which acts as a depolymerizing agent and lowers the solution viscosity.
A relatively uniform chlorinalion is achieved by spraying the solution at
the top of a chlorinating tower in which a stream of chlorine, let in at the
bottom, is allowed to ascend. The droplets of the chlorinated product
collect at the bottom. The collected solution from the bottom is degassed
to remove excess chlorine and then sprayed into a steam chamber where
the admitted steam causes rapid volatilization of CCl4. CCl4 is then recovered
and reused; the chlorinated rubber collected as a wet mass from
the bottom is washed, dried, milled if necessary and stored. Chlorinated
rubber is resistant to many chemicals. It even resists concentrated nitric
acid. It is, however, soluble in a wide range of solvents and it is used in
the formulation of many paints, lacquors, adhesives and printing inks.
Because of high chlorine content, chlorinated rubber has prominent flame
retardant characteristics.
Rubber can also be modified into a resinous product by
hydrochlorination. The rubber hydrochloride may be prepared directly
from stabilized latex using hydrochloric acid gas. It is, however, better
obtained by hydrochlorination with gaseous HCl using rubber in benzene
solution. Films made from the solution of the hydrochloride to which
plasticizers and stabilizers have been added are used for making laminates
with paper or films of other plastics such as cellulose acetate, etc.,
for use as heat sealable packages for dry food, cosmetics, shampoo, etc.
The hydrochloride also finds use in the formulation of adhesives and
bonding agents.
Polyacrylic Rubber or Acrylate Rubber (ACR)
Poly(ethyl acrylate) is soft and rubbery in nature. Copolymers of
ethyl acrylate (95%) and 2-chloroethyl acrylate or 2-chloroethyl vinyl
ether (cure site monomer) have been commercially developed and the
products are known as polyacrylic rubber or acrylate rubber. These rubbers
are suitably cured using aliphatic linear diamines and polyamines.
Cross-linking apparently occurs by HC1 elimination and intermolecular
link up through the diamines or via ester hydrolysis and establishment of
intermolecular amide linkages. Small amount of sulphur is used as an
anti-aging additive. The cured rubber is particularly useful as hoses, seals
and gaskets. Reinforcing carbon blacks are used as fillers. For pale shades,
siliceous fillers are used. The rubber has good resistance to oils and to
ozone and it may be used over a wide temperature zone (—40 to nearly +
200°C). The uncured polyacrylic rubbers are soluble in ketones, esters
and alcohol-ester mixtures. The cured polyacrylic rubbers are better than
nitrile rubbers in heat and oil resistance. Their resistance to ozone attack
and to sunlight and weathering are good. A terpolymer (AEM) of methyl
acrylate, ethylene and a cure site monomer, known in the trade by the
name Vamac, is of more recent development.
Fluorocarbon Rubber (FKM)
Fluorocarbon rubbers or elastomers are copolymers of vinylidene
fluoride and chlorotrifluoroethylene (50 : 50 or 30 : 70 ratio). Better products
are obtained by copolymerization of vinylidene fluoride and
bexafluoropropylene (“Viton” elastomers from Du Pont). They are usually
cured with amine type curatives in presence of a metallic oxide (litharge
or calcined magnesia). Curing is apparently effected by the elimination
of hydrogen fluoride.
The fluorocarbon (copolymer) elastomers are prepared by batch or
continuous process following the emulsion polymerization technique. The
latex-obtained is coagulated by hydrochloric acid and the polymer is
washed and dried. The Vitons are normally soluble in lower ketones and
the doughs formed are suitable for spreading over glass cloth to produce
coated fabrics useful as oil seals and gaskets with a long service life at
high temperatures (  100 h at nearly 400°C, > 5000 h at 200°C).
The fluorocarbon rubbers exhibit excellent resistance to oils, lubricants,
hydrocarbon solvents, mineral acids and chemicals and to heat; in
these respects, they are superior to almost all other commercial rubbers.
The fluorocarbon rubbers can be suitably compounded to give vulcanizates
of tensile strength of about 200 kg/cm2 and elongation at break in the
range of 200-300%. They are flame resistant and they exhibit outstanding
resistance to oxygen and ozone attack. Their good low temperature
flexibility makes them advantageously useful at low temperatures up to
— 30°C. The application of fluorocarbon rubbers is limited to only special
or unusual service conditions where other rubbers are altogether unsuitable
and where their high cost is not a hindrance. The applications
include seals, gaskets and diaphragms, fire-resistant and protective clothing
from coated fabrics, and wire and cable insulation.
THERMOPLASTIC ELASTOMERS (TPE)
Development of thermoplastic elastomers (TPE) has narrowed the
basic difference between the processing of thermoplastics and elastomers
or rubbers for many non-tyre products. The TPEs make useful products
for which the tensile and set properties are not much critical, such as in
many automotive parts, footwear, cables, sealants and adhesives, hoses,
coated fabrics, tubings and sheetings. Their light weight, flexibility, impact
resistance and weathering resistance make them useful in many of
these applications.
The thermoplastic elastomers are processed like reusable and
reprocessable thermoplastics and under service conditions, they behave
like vulcanized rubbers. The useful mechanical and elastic properties of
conventional vulcanized rubbers are attributed to the chemical cross-links
established between the rubber chain molecules during vulcanization to
produce a space network structure. In the TPEs, the network is basically
formed through thermally labile physical cohesive forces between specific
segments of different polymer chains but not really through intermolecular
chemical linkages. At elevated and processing temperatures,
the thermolabile physical bonds weaken and finally break up, permitting
flow under shear and thus enabling them to be moulded or formed like a
conventional thermoplastic material. At and around ambient temperatures,
the TPEs exist in two phases; the soft rubbery phase forms the
continuous matrix in which the hard resinous phase remains dispersed in
discrete domains. The volume fraction of the rubbery matrix is usually
higher than that of the hard thermoplastic domains which materially act
as cross-links and as stiffening fillers or points of reinforcements.
RUBBER COMPOUNDING AND PROCESSING TECHNOLOGY
Introduction
No rubber is considered technically useful if its molecules are not
cross-linked by a process known as curing or vulcanization. The process
of vulcanization is usually associated with two chemical processes taking
place simultaneously, cross-linking and chain degradation, though at
widely different rates. For natural rubber and many synthetic rubbers,
particularly the diene rubbers, the curing agent most commonly used is
sulphur. But sulphur curing takes place at technically viable rates only at
a high temperature (> 140°C), and heating with sulphur alone leads to
optimum curing after nearly 8 h at 140°C using a fairly high dose of
sulphur (8-10 phr). Use of metal oxides, such as those of zinc, calcium,
magnesium, lead, etc., brings about some advantages with respect to time
of curing and improvements in physical properties without much reduction
in the sulphur dose. Aniline is considered as the first organic accelerator
tried in an attempt to quicken the curing process. Some of the more
efficient and less toxic modern organic accelerators of rubber vulcanization
are aniline derivatives. Sulphur dose has been substantially lowered
with the advent of organic accelerators.
Extensive studies and experimentation have revealed that neither
sulphur nor application of heat is indispensable for effecting cross-linking
and associated changes in physical properties of rubbers. Peroxides,
metal oxides, amines, amine derivatives and oximes have been found to
bring about curing of selected rubbers quite effectively. High-energy
radiations can bring about effective curing; but high-energy radiation
curing has not been developed into a commercial process of even limited
acceptability. Selenium and tellurium can substitute sulphur either totally
or partly to effect satisfactory curing of diene rubbers. Sulphur
monochloride can bring about room temperature or cold curing of diene
rubbers.
Improvements in physical and mechanical properties that can be
achieved through vulcanization using sulphur/accelerator systems only
are rather limited. The needs of imparting colour, stability, resistance to
tearing and abrasion, flexibility etc., and of improving processibility and
mechanical properties necessitate incorporation of a host of additives or
compounding ingredients in the rubber by what is commonly known as
the mixing or compounding process.
Rubber Vulcanization
VULCANIZATION AND ITS EFFECTS
Chemical crosslinks between macromolecules may occur in polymerization
or in fabricating articles, resulting in polymers with network structures.
The reaction which occurs during fabrication is known as “vulcanization”
in the rubber industry, and “curing” or “hardening” in the
plastics industry. Crosslinks in rubber are formed by the reaction with a
suitable “vulcanizing agent”, usually sulphur.
A typical crosslinking reaction is the formation of short chains of
sulphur atoms linking linear molecules in rubbers during vulcanization.
Lightly-vulcanized rubber shows good elastic properties, whereas vulcanization
to the maximum extent possible will lead to a full network
structure, and a hard and rigid material. Thermosetting resins may be
crosslinked by the addition of a curing agent, such as hexamethylene
tetramine and/or heat. Typical examples are epoxy resins, polyester resin
and poly-urethane foams. Owing to the greater restriction on the mobility
of the macro-molecular chains as a result of crosslinking, the modulus
and glass transition temperature increases in both hardened resins and
sulphur crosslinked rubber.
Charles Goodyear discovered sulphur vulcanization of rubber in 1839
and developed many new applications for rubber in industry.
Crosslinking of rubber occurs by a chemical process which is initiated
at favourable reaction sites by means of some form of energy input.
Double bonds in diene polymers or reaction sites left by abstraction of
hydrogen or halogen atoms are favourable reaction sites. These are coupled
by carbon-carbon crosslinks or by bridges formed with curatives
such as sulphur, acrylates, phenolics or triazines. Crosslinking through
sulphur atoms is quite common in biological molecules, e.g. disulphide
(cystine) bridges in polypeptides.
There are four principal changes which are brought about by vulcanization.
1. Rubber is converted from essentially a plastic substance of
very low strength to an elastic material of considerable strength
and resilience.
2. The physical properties, such as tensile strength and other
properties shown in Fig. 1 undergo a profound change as
vulcanization progresses.
3. The physical properties of vulcanizates are maintained over a
much wider temperature range than in the case of unvulcanized
rubber.
4. The crosslinked polymer (vulcanizate) only swells in liquids
which normally dissolve the uncrosslinked polymer.
Rubber Reclaiming
In the mid-1800s Charles Goodyear developed a technique for vulcanizing
rubber. He blended natural rubber with sulfur and placed it on his
wood stove, where the rubber cured into a water-impermeable sheet.
Because of the shortage of natural rubber at that time, a rubber-reclaiming
process, based on steam pressure to devulcanize used rubber, was
developed in 1858. The cured ground rubber was subjected to steam pressure
for 48 hours. Today, high cost silicone rubber polymers are reclaimed
in much the same way, although most synthetic polymers require more
complicated techniques. Rubber recycling has been extended to the recovery
of rubber in asphalt, scrap rubber as fuel, rubber pyrolysis, tire
splitting, and others. However, the discovery of plastics and oil-extended
rubbers and the drop in crude oil prices have led to a reduction in rubber
recycling except for expensive polymers, such as silicones (qv) and
fluorocarbons. Pyrolysis of scrap rubber is expensive and markets are
limited. It is more expensive to prepare and burn scrap rubber for fuel
than to burn natural gas, fuel oil, or coal. High fuel costs and petroleum
scarcity in Europe and other parts of the eastern hemisphere have resulted
in the use of recycled rubber as fuel and made it more economical
than in the United States. However, with today’s technological advances,
recycled rubber will be used more and more in the United States, but with
regulations for handling and disposing of worn tires. Crude oil costs have
dropped from about $189/m3 ($30/bbl) to $88/m3 ($14/bbl) over the last
few years, making it seemingly unprofitable to recycle rubber, especially
tires. However, tires discarded in landfills tend to float on top; mosquito
breeding and illegal tire disposal are causing problems which could be
alleviated by recycling. Approximately 70% of the scrap rubber, primarily
as tires, is discarded in landfills. Private landfills may charge up to S3
per tire, and disposal costs at municipal landfills are ca $0.30-0.60 per
tire. These costs encourage illegal disposal of tires. As the tire piles grow,
rubber recovery should become more economical. Companies entering
into multimillion dollar energy recovery projects will be encouraged by
an adequate supply of old tires.
Some states have restricted the disposal of tires in landfills, e.g., Washington,
Minnesota, and Oregon. Other states are studying legislation and
New Jersey is considering a surcharge. Iowa, Massachusetts, and Michigan
are studying tire-disposal problems, and Minnesota and Washington
have imposed a surcharge. Some communities refuse old tires while others
require tire splitting or shredding to prevent floating problems in
landfills. Fires resulting from the storage of tires have led to regulations
governing stockpiles. Mosquito infestation caused by illegal disposal of
tires has prompted Saginaw, Mich., to purchase a shredder for landfill
disposal.
Advances in the technology of shredding tires, reclaiming rubber,
retread equipment, energy-related projects, and pyrolysis are helping to
solve the disposal problem. Technical and marketing professionals associated
with the recycling and disposal industries are paying more attention
to tire-recycling and energy-generating projects.
SCRAP RUBBER AS FUEL SOURCE
The use of scrap rubber for fuel offers the best alternative for reusing
rubber, as fuel costs increase and tire disposal problems become more
serious. Tires in the form of 2.50-cm chips are most economical because
shredding is not expensive. Tires contain more than 90% organic materials
and have a heat value of ca 32.6 MJ/kg (ca 14,000 Btu/lb), compared
with coal values of 18.6-27.9 MJ/kg (ca 8,000-12,000 Btu lb). A cyclonic,
rotary-hearth boiler fired with whole tires was operated by the
Goodyear Tire and Rubber Co. from 1975 to 1977. It was designed to
burn 1400 kg/h and generated 11,300 kg of steam per hour.
In the Lucas tire-burning furnace, tires are conveyed into an airtight
chamber and then onto the outer rim of a rotating hearth. The chamber
prevents flashback fires and limits air leaks. An air-velocity head of 5.1
cm provides the turbulence necessary for combustion. Residues from the
burning tires form a char that increases combustion heat loss and tends to
clog furnace grates, but the carbon black content reduces slagging problems.
Improper combustion at the ash-removal area of the furnace may
prevent the burning of the carbon black. The Lucas-Goodyear furnace
was shut down because of mechanical problems and failure to comply
with Michigan’s air pollution emission standards.
Shredded tire chips have been burned in stoker-fired boilers. Uniroyal
fired a 15% mixture of tire chips with coal and both General Motors and
B. F. Goodrich have burned a 10% tire-chip mixture with coal. Tiregrinding
size-reduction problems and delivery costs have stymied projects
based on combined tire and coal fuel. The Lucas furnace was developed
to burn tires without size reduction. Transportation of tire scrap can cost
$0.05/kg, exclusive of grinding costs. Thus tire-fired boilers are limited
to areas with ample scrap-tire supplies, e.g., large cities or tire manufacturers.
The cost of burning one metric ton of tires per hour in an incinerator
was ca $0.20—0.40 per tire in 1974, which increased to $0.35-0.70
per tire in 1987.
The Oxford Energy Company uses a technology developed by
Gummi-Mayer Company (FRG) to incinerate tires and produce electricity.
The technology will be used in the Modesto Project near Westley,
Calif. The facility generates 14.4 MW of electricity and cost $38 × 106.
Construction began in December 1985 and was completed in August 1987.
The project is designed to generate electricity by incinerating whole waste
tires on reciprocating stoker grates located beneath two water wall boilers.
The boilers produce 55,000 kg of steam per hour and power a single
General Electric turbine generator. The facility is being constructed next
to the Filbin Tire Collection Agency, Inc., which administers one of the
largest stockpiles of waste tires in the United States (ca 300 × 103 t); the
stockpile increases annually by 27 t. Oxford Energy is also involved in
the construction of a New Hampshire tire incineration project, generating
15 MW of electricity; facility is expected to be completed in late
1988 at a cost of $35 × 106. The company has received air quality and
waste management permits from the State of New Hampshire. The firm
also plans to build a 22-MW electricity-generating facility in northeastern
Connecticut; construction is expected to be completed in 1989 at a
cost of $50 × 106. The company has projects to incorporate waste produced
from manufacturing facilities along with waste tires, thus providing
energy to manufacturing facilities as well as local utilities. Approximately
20 × 106 tires are stockpiled at two sites in New England.
These sites, together with the Filbin stockpile, contain at least 480 × 103
t of tires (53 × 106 tires).
CRYOGENIC PULVERIZING AND MECHANICAL TIRE
SHREDDING
Tires must be pulverized or shredded before they can be reclaimed
by devulcanization or used in asphalt and other recycling processes. The
tires are mechanically ground, sometimes using cryogenic freezing or
solvent-swelling techniques to enhance grinding efficiency. In one process,
a polar solvent is used to swell the rubber, followed by shearing to
reduce particle size. Ground tire-crumb rubber is commonly referred to
as rubber reclaim, even though the rubber has not been devulcanized.
Cryogenics in conjunction with mechanical action has been used to
make crumb rubber. Nitrogen cools the rubber below the glass-transition
temperature, and the brittle rubber is pulverized in a grinding mill. A
small cryogenic system can be installed at the site to integrate the scraprubber
crumb into the compound mixing process. Cryogenic-grinding
costs are $0.20-0.40/kg, depending on the desired particle size and the
type of rubber; harder rubber is easier to grind. Ground-rubber scrap can
be devulcanized, pyrolyzed, or recycled directly into the rubber compound.
Ground rubber is also added to plastics.
Air Products and Chemicals, Inc., Allentown, Pa., developed a cryogenic
process for grinding scrap tires in the mid-1960s. Liquid nitrogen
freezes the rubber to facilitate shredding. Midwest Elastomers installed a
cryogenic system in 1980 to powder tire peels in its Wapakoneta, Ohio,
plant. The equipment and materials for cryogenic processing are expensive
and have slowed expansion. Cryogenic grinding requires recycling
close to an air-processing facility to make the use of liquid nitrogen economical.
Air Products has a marketing and development relationship with
Brown & Ferris Industries to develop waste-to-energy and alternative
fuel projects.
The tires are mechanically ground with a two-roll, grooved-rubber
mill. The two-mill rolls turn at a ratio of ca 1:3, providing the shearing
action necessary to rip the tire apart. The rubber chunks are screened and
the larger material is recycled until the desired size is reached. Bead wire
is removed by hand or with magnets. For most applications, e.g.,
devulcanization or pyrolysis, crumb-rubber particles smaller than 1.19
mm (16 mesh) are desired, and several milling steps are required. Tire
fiber is removed in intermediate operations with hammer mills, reel beaters,
and air tables that blow a steady stream of air across the rubber
Latex and Foam Rubber
INTRODUCTION
Rubber latex is a term used to cover a range of colloids having macro
molecular substances as the dispersed phase and water as the dispersion
medium. Most important latex used in the rubber industry is natural rubber
(NR) latex. This is an aqueous dispersion of cis 1,4 polyisoprene in
water containing dissolved serum substances. NR latex is harvested from
Hevea brasilienesis trees by inflicting controlled wounds on the bark of
the trunk of the tree.
Under inorganic salts seven different types of ions are identified.
Similarly, under each of the organic ingredients a number of compounds
belonging to that group are characterised. In synthetic lattices also, a
number of ingredients other than the polymer and water are present.
PRODUCTS FROM LATEX
There are eight important steps in the manufacture of products from
latex –
1. Selection and quality control of raw materials
2. Preparation of raw materials
3. Compounding and design of the mix
4. Maturation
5. Processing/shaping
6. Vulcanisation
7. Testing for grading
8. Packing and marketing.
There are different types of products manufactured from latex. Some
of the steps are common in all products but differences exist in others.
The similarities and differences in the various steps for different types of
products are discussed below briefly:
Selection of Raw Materials
Following are the raw materials used in the latex industry —
Latex containing the polymer
Stabilisers and viscosity modifiers
Activator
Accelerator
Curing agent
Antioxidant
Special additives, if any
Gelling/coagulation system.
The raw material selected for product manufacture must be of good
quality and conforming to specifications prescribed for this by reputed
institutions like the BIS (ISI). In the case of natural rubber latex, there are
few processors who can supply high ammonia-preserved concentrated
natural rubber latex conforming to IS 5430. It may be noted that the specifications
for this HA latex have been formulated by the combined effort
of experts from rubber plantations and the goods manufacturing industry.
It is desirable for the industry to insist that the suppliers should deliver
of HA latices conforming to IS 5430.
In foam rubber there are two important processes in vogue commercially.
One is called the Dunlop process and the other called Tulalay process.
In India, most factories use the Dunlop process. In Dunlop process,
foaming is carried out in two stages. In the first stage latex is whipped up
with foam promoters and stabilisers till the desired expansion is obtained.
Thereafter the whipping rate is reduced and the foam is clarified or refined.
Gelling agents are added at the end of the foam clarifying stage.
The density of foam rubber produced depends on froth height. The level
in which latex is whipped should be calibrated properly for specific gravity
for various froath heights.
The materials selected for mould construction for foam production
must be light, cheap, should have high thermal capacity and good thermal
conductivity: A mould designed from copper free cast aluminum
meets these requirements. It is generally reported that in the cost of production
of foam rubber around 5% of the cost is on account of contribution
from mould cost. The tensile and cure properties of the hot wet gel
are appreciably lower than those of the dry finished products. So if care is
not exercised in stripping the foam from mould, it may as well be damaged.
For enabling easy and quick stripping, the mould surface should be
kept clean and polished. Mould release agent has to be applied on surface
of mould after each operation. Examples of mould release agents are
aqueous or alcoholic solutions of polyethylene glycol of fairly high molecular
weight such as carbo wax-4000, carboxymethyl cellulose or silicone
fluids.
In Tullalay process, the expansion of latex compound is achieved by
liberation of gases like oxygen liberated by catalytic decomposition of
hydrogen peroxide.
Latex Thread
In the production of latex rubber thread the matured latex has to be
carefully processed to avoid defects. The compounded matured latex is
de-aerated and filtered before it is allowed to pass through capillaries for
injection into an acid bath. The nozzle of the capillary is immersed in
coagulant. The nozzle depth in coagulant bath has direct relation to the
thread diameter. The rate of extrusion is 30-40 feet per minute. The orifice
of the nozzle is usually round and the size of the bore varies from 0.5
to 1 mm. Coagulant normally used is 15-40% acetic acid. Generally,
weaker solutions of acid are used for large diameter thread and strong
solutions for higher count threads. For the production of high count thread
alcohol is the most suitable coagulant. Between extruding nozzle and
winding, the thread is usually subjected to some degree of stretching which
may be upto 200%. Stretching is necessary to facilitate washing, to reduce
permanent set and to increase modulus of the thread.
Vulcanisation
Latex products are generally vulcanised at low temperatures and pressure
when compared to dry rubber products. It is desirable to wash, leach
and dry the latex products before subjecting them to vulcanisation temperature.
Vulcanisation of some products is carried out in hot air. Some
others are vulcanised in open steam in autoclaves.
Construction of Moulds
The moulds for foam rubber are usually made from cast aluminium
because of the lightness, durability and good heat conductivity of the
metal. When cushions and mattresses are made, cylindrical cores are attached
to the lids which give the characteristic cavity structure of the
back side of the foam. These cavities help to minimise the actual thickness
of the product to about 1 inch to ensure uniform curing throughout
the mass of the product. The cavities also assist in great saving of the
latex compound (of the order of 40%) without affecting the comfort. In
designing moulds, allowance should be given to shrinkage of the product
which amounts to roughly 8-10% linear.
Mould lubricants like polyethylene glycol and alkyl sulphate solutions
are applied to the mould surface to facilitate easy stripping of the
product after vulcanisation.
Curing
Mould foam products are normally cured by open steam (at 100°C)
which is more efficient and quicker than hot air curing. Thin sections of
foam, e.g., as used in carpet underlays, can be suitably cured by
hot air.
Polybutadiene and Polyisoprene
INTRODUCTION
Polyisoprene is a major component of manufacture rubber. It is also made
synthetically and forms are stereospecific cis 1-4, and trans-1-4-
polyisoprene. Both can be produced synthetically by the effect of heat
and pressure on isoprene in the presence of stereospecific catalyst. Natural
rubber is cis-1-4. Synthetic cis-1-4 is sometimes called synthetic natural
rubber. Trans-l, 4-polyisoprene resembles gutta percha. Polyisoprene
is the thermoplastic until mixed with sulphur and vulcanised. It supports
combustion and is non toxic
Polybutadiene is a synthetic thermoplastic polymer made by polymerising
1,3 butadiene with a stereospecific organometallic catalyst (butyl
lithium) though other catalysts such as titanium tetrachloride and aluminium
iodide may be used. The cis-isomer, which is similar to natural
rubber, is used in the tread due to its abrasion and crack resistance and
low heat built up. Large quantities are also used as blend in SBR rubber.
The trans-isomer resembles gutta percha and has limited utility, liquid
polybutadiene, which is sodium catalysed has speciality uses as a coating
resin. It is cured with organic peroxides. Combustible liquid form is probably
toxic by ingestion and inhalation, as well as skin irritant. The different
polymer structures of polybutadiene are given in Fig. 6.
POLYMER STRUCTURE
Polyisoprene
The polymerisation of isoprene and butadiene are examples of addition
polymerisation in which the repeating structural unit within the poly
mer backbone has the same molecular weight as the entering monomer
unit.
With isoprene, the building of this polymer backbone can occur in
several ways, depending upon where the addition occurs. The polymerisation
addition process can be a reaction involving the 1, 2-, 3, 4- or 1, 4-
positions of isoprene to give the structures shown in Figure 1.
In case of 1,2- and 3, 4- addition, an asymmetric carbon is formed
that can have either an R or an S (“d” or “1”) configuration. In general,
equal numbers of R and S configurations are produced during an addition
polymerisation, which results in no net optical activity for the polymer.
The disposition of the R and S configurations along the polymer
backbone, however, results in diastereomeric isomerism. Although many
combinations of sequences are possible, only three arrangements are commonly
considered in polymers. These diastereomeric isomers are referred
to as isotactic, syndiotactic, and atactic.
Gel and Branching
The structure and behaviour of polyisoprene is further complicated
by the possibility of branching and gel. The degree of gel and branching
is dependent upon the source of the cis-1, 4-polyisoprene. For Al-Ti catalysed
cis, 4-polyisoprene, the gel content is 5-25% most of which is termed
“loose” gel since it breaks down readily upon mastication. The gel content
in natural rubber is usually greater than that in the Al-Ti catalysed
polymer. This gel is also a “loose” gel. Lithium polymerised cis-l, 4-
polyisoprene usually contains no gel. Both natural rubber and Al-Ti-catalysed
cis-1,4-polyisoprene also contain large quantities of microgel.
The determination of the nature and degree of branching in natural
rubber and Al-Ti catalysed cis-1, 4-polyisoprene are complicated by the
presence of the gel and microgel. Removal of the gel/microgel fraction
from the soluble portion can have a marked effect on the resulting physical
properties such as viscosity, molecular weight, and branching.
Polybutadiene
The configurations of polybutadiene are cIS-, trans-, and vinyl. Since
either or both of the double bonds in butadiene can be involved in the
polymerisation mechanism, the resulting polymer may have a variety of
configurations. These result from the fact that the spatial arrangement of
the methylene groups in the polybutadiene backbone allow for geometric
isomerism to occur along the polymer chain. The different polymer structures
of polybutadiene are given in Figure 6.
Participation of both double bonds in the polymerisation process gives
rise to a 1, 4-addition, which can be either cis-1,4- or trans-l,4-, depending
upon the disposition of groups about the polymer double bond. Participation
of only one double bond results in a vinyl, or 1,2-addition, that
can have three possible structures just as 1,2- and 3,4- polyisoprene have
isotactic, syndiotactic, and atactic.
Reclaimed Rubber
INTRODUCTION
Reclaimed rubber is the product resulting when waste vulcanised scrap
rubber is treated to produce a plastic material which can be easily processed,
compounded and vulcanised with or without the addition of either
natural or synthetic rubbers. It is recognised that the vulcanisation process
is not truly reversible; however, an accepted definition for
devulcanisation is that it is a change in vulcanised rubber which results in
a decreased resistance to deformation at ordinary temperatures.
Reclaimed rubber is manufactured by suitable treatment to old and
worn out tyres, tubes and other used rubber articles with certain chemical
agents. A substantial devulcanisation or regeneration is effected to the
rubber compound in this process whereby its original plasticity is regained.
In short it may be stated that reclaiming is essentially a
depolymerisation process where the combined sulphur is not removed.
The reclaimed rubber is used in the manufacture of rubber goods, with or
without admixture of natural or synthetic rubber.
Recycled rubber can be more generally described as any sort of rubber
waste that has been converted into an economically useful form such
as reclaimed rubber, ground rubber, reprocessed synthetic rubber, and
die-cut punched parts.
One method of recycling some scrap rubber is to grind it as fine as
possible and work it into new rubber as an elastomeric filler. This was the
first method of reclaiming and is suitable only for compounding carriage
springs, which were fairly large barrel-shaped molded articles but not
suitable for footwear products.
In order to make a high-quality reclaim, the fibre must be removed
from the rubber scrap by soaking the rubber in water, and then taking a
small knife and starting the rubber from the cloth and stripping it off. But
this was not an effective way to produce large quantities of products.
TYPES OF RECLAIM
A variety of grades of reclaimed rubber is offered today, but mention
is made here of only the important ones.
Whole Tyre Reclaim
Whole tyre reclaim is the one produced in the largest quantity. Firstquality
reclaim made from whole tyres contains about 45% rubber hydrocarbon
by weight. The remaining 55% consists of valuable carbon
black, a little mineral filler, and softeners, all of which are substantially
unchanged by the reclaim manufacturing operation, and may be considered
to function as virgin materials. The manufacturing Minimum Staining Reclaim
Minimum staining reclaim can replace the conventional whole tyre
material when occasion demands. As implied, it has a much lower tendency
to stain, by either migration or contact, than conventional reclaim.
The reduction in staining characteristics is achieved by the use of activated-
carbon non-staining oils and by selecting tyres containing a higher
proportion of natural to synthetic rubber.
Drab and Coloured Reclaims
As the names imply, drab and coloured reclaims are made from nonblack
scrap. The digester process is usually employed and, when fabric is
present, a small addition of caustic is made in order to destroy it. The
period of heat treatment is usually several hours at 195°C.
Butyl Reclaim
Whereas reclaimed rubbers have been successfully produced from
scrap CR, NBR,SI, and other speciality rubbers, the only one of substantial
commercial importance is butyl reclaim. The starting material for this
is butyl inner tubes. A modified digester process is adopted, every precaution
being taken to avoid contamination by NR or SBR, because of
their adverse effect on the curing characteristics of the butyl. Extensive
control tests are necessary to ensure that the curing properties are satisfactory.
The nerve of butyl reclaim is much reduced compared with that
of the original polymer. Because of this, compounds containing butyl
reclaim will mix, calender, and extrude faster and more smoothly than
similar compounds based on virgin rubber.
Reclaimed Rubber
By the application of heat and chemical agents to ground vulcanised
waste rubber, a substantial regeneration of the rubber compound to its
original plastic state is effected, yielding a product known as ‘reclaim’ or
reclaimed rubber, capable of being processed, compounded, and
revulcanised. The process is essentially one of depolymerisation. Reclaimed
rubber has become widely accepted as a raw material which possesses
processing and economic characteristics that are of great value in
the compounding of natural and synthetic rubber stocks.
There are four principal reclaiming processes in use today, of which
the digester and reclaimator processes are important.
The raw material for reclaiming is scrap rubber in a wide variety of
forms, but tyres, as is to be expected, form the major quantity.
The first stage, in all processes, are the cracking and grinding of the
scrap rubber to reduce it to a crumb passing through a 20 to 30 mesh
screen.
Digester Process
At one time, most reclaim was made using the digester process. A
digester is essentially a steam-jacketed, agitator-equipped autoclave,
mounted either horizontally or vertically. This is a wet process in which
the coarsely ground scrap is submerged in a solution of water and reclaiming
agents. These agents may include many types of light and/or
heavy oils, naval stores, pine tar and coal tar pitches, and chemical
peptisers.
Until the advent of synthetic rubber, the digesting solution also included
caustic soda to remove free sulphur and to act as a defibring agent.
In fact, the process was generally referred to as the alkali digester
method.
The ground waste is loaded into a digester along with water, reclaiming
oils, and other additives, such as activated black (for minimum stain
ing grades). The digester is a cylindrical jacketed pressure vessel fitted
with a horizontal agitator, and steam can be supplied to both interior and
jacket, thus enabling a uniform temperature to be maintained throughout
the mass. The contents of the digester are then heated to about 190°C and
maintained at this temperature for some 4-10 hours with continuous agitation.
The digester is then ‘blown down’, and the contents deposited on
to a conveyor. Any necessary adjustments to the specific gravity and plasticity
by addition of plasticiser, carbon black, or fillers are carried out in
a ribbon blender, and the stock is then automatically conveyed to extruders
for straining, refining, and leafing on to a drum from which it is removed
in slabs.
Reclaimator Process
The reclaimator process is the only commercially successful continuous
technique for devulcanising tire scrap; all the others are batch
processes. Tyres are ground, the metal and fibre are mechanically separated,
then the rubber is further ground to a fine particle size. This fineground
rubber and the various reclaiming agents are all metered into a
blending system and conveyed to the reclaimator.
The reclaimator is a special type of screw-extrusion machine. It is
jacketed to provide for several zones of controlled temperature using either
hot oil or cooling water; in addition, the clearances between the screw
and the chamber wall are close and adjustable. The object is to subject
the rubber to a controlled amount of high heat and pressure in a continuously
moving environment. The residence time of the rubber in the machine
is less than 5 minutes. During this period, the rubber undergoes
devulcanisation. After the softened rubber is discharged from the head of
the machine, it is cooled and further processed in refining mills just as is
done in other reclaiming methods.
It can be shown that ground vulcanised rubber heated in a temperature
range of 120-200°C undergoes a rapid initial increase in plasticity,
and, on continued heating, passes through an inversion point and rehardens
unit, after prolonged heating, a further but slower increase in plasticity is
attained. It follows, therefore, that three points of equal plasticity occur
in this cycle.