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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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proteins, fatty acids, resins and inorganic materials (salts) are found
in natural rubber. Polyisoprene can be also created synthetically,
producing what is sometimes referred to as "synthetic natural
rubber", but the synthetic and natural routes are completely different.
CH
3
O
HC CH
H2C CH
H2C
O
P
2
O
HC C
O
P
O O
CH
CH
O
CH
3
HC C
H2CCH
3
2
2
CH
HC C
2
CH
3
H2C CH
HC C
2
CH
H2C
3
Chemical structure of cis-polyisoprene, the main constituent
of natural rubber. Synthetic cis-polyisoprene and natural cispolyisoprene are derived from different precursors. Some natural
rubber sources called gutta-percha are composed of trans-1,4polyisoprene, a structural isomer that has similar, but not identical,
properties.
Natural rubber is an elastomer and a thermoplastic. Once the
rubber is vulcanized, it will turn into a thermoset. The final
properties of a rubber item depend not just on the polymer, but also
on modifiers and fillers like carbon black, factice, whiting, and a host
of others.
Current sources
Close to 21 million tons of rubber were produced in 2005, of
which approximately 42% was natural. Today, Asia is the main
source of natural rubber, accounting for about 94% of output in 2005.
The three largest producing countries, Thailand, Indonesia and
Malaysia, together account for around 72% of all natural rubber
production. Natural rubber is not cultivated widely in its native
continent of South America due to the existence of South American
leaf blight, and other natural predators of the rubber tree.
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n
Cultivation
Rubber latex is extracted from rubber trees. The economic
life period of rubber trees in plantations is around 32 years – up to 7
years of immature phase and about 25 years of productive phase.
The soil requirement of the plant is generally well-drained
weathered soil consisting of laterite, lateritic types, sedimentary
types, nonlateritic red or alluvial soils.
The climatic conditions for optimum growth of rubber trees
consist of:
Rainfall of around 250 cm evenly distributed without any
marked dry season and with at least 100 rainy days per year.
Temperature range of about 20°C to 34°C with a monthly
mean of 25°C to 28°C.
High atmospheric humidity of around 80%.
Bright sunshine amounting to about 2000 hours per year at
the rate of 6 hours per day throughout the year.
Absence of strong winds.
Many high-yielding clones have been developed for
commercial planting. These clones yield more than 2000 kg of dry
rubber per hectare per year, when grown under ideal conditions and
ideal field.
Collection
The trees will drip latex for about four hours, stopping as
latex coagulates naturally on the tapping cut, thus blocking the latex
tubes in the bark. Tappers usually rest and have a meal after finishing
their tapping work, then start collecting the latex at about midday.
Some trees will continue to drip after the collection and this leads to
a small amount of cup lump which is collected at the next tapping.
The latex that coagulates on the cut is also collected as tree lace. Tree
lace and cup lump together account for 10–20% of the dry rubber
produced.
The latex will coagulate in cup if kept for long. The latex has
to be collected before coagulation. The collected latex is transferred
in to coagulation tanks for the preparation of dry rubber or
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transferred into air-tight containers with sieving for ammoniation.
Ammoniation is necessary to preserve the latex in colloidal state for
long.
Latex is generally processed into either latex concentrate for
manufacture of dipped goods or it can be coagulated under
controlled, clean conditions using formic acid. The coagulated latex
can then be processed into the higher grade technically specified
block rubbers such as SVR 3L or SVR CV or used to produce
Ribbed Smoke Sheet grades.
Naturally coagulated rubber (cup lump) is used in the
manufacture of TSR10 and TSR20 grade rubbers. The processing of
the rubber for these grades is a size reduction and cleaning process to
remove contamination and prepare the material for the final stage of
drying. The dried material is then baled and palletized for storage
and shipment in various methods of transportation.
Transportation
Natural rubber latex is shipped from factories in South-West
Asia, South America and North Africa to destinations around the
world. As cost of natural rubber has risen significantly, the shipping
methods which offer the lowest cost per unit are preferred.
Depending on the destination, warehouse availability, transportation
conditions, some methods are more suitable to certain buyers than
others. In international trade, latex rubber is mostly shipped in 20
foot ocean containers. Inside the ocean container, various types of
smaller containers are used by factories to store latex rubber.
Vulcanization
Natural rubber is often vulcanized, a process by which the
rubber is heated and sulfur, peroxide or bisphenol are added to
improve resistance and elasticity, and to prevent it from perishing.
The development of vulcanization is most closely associated with
Charles Goodyear in 1839. Carbon black is often used as an additive
to rubber to improve its strength, especially in vehicle tires.
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Properties
Rubber exhibits unique physical and chemical properties.
Rubber's stress-strain behavior exhibits the Mullins effect, the Payne
effect, and is often modeled as hyperelastic. Rubber strain
crystallizes.
Owing to the presence of a double bond in each repeat unit,
natural rubber is susceptible to vulcanisation and sensitive to ozone
cracking.
There are two main solvents for rubber: turpentine and
naphtha (petroleum). An ammonia solution can be used to prevent
the coagulation of raw latex while it is being transported from its
collection site.
In most of its useful forms it has a large stretch ratio, high
resilience, and is extremely waterproof.
Elasticity
In its relaxed state, rubber consists of long, coiled-up chains.
When rubber is stretched, the chains are taut. Their kinetic energy is
released as heat. The entropy decreases during elongation but
increases during relaxation. This change in entropy related to the
changes in degrees of freedom. Relaxation of a stretched rubber band
is thus driven by an increase in entropy, and the force experienced is
a result of the thermal energy of the material being converted to
kinetic energy. Rubber relaxation is endothermic, and for this reason
the force exerted by a stretched piece of rubber increases with
temperature. The material undergoes adiabatic cooling during
contraction. Stretching reduces the "space" available to each section
of chain.
Vulcanization of rubber creates disulfide bonds between
chains, so it limits the degrees of freedom. The result is that the
chains tighten more quickly for a given strain, thereby increasing the
elastic force constant and making rubber harder and less extensible.
When cooled below the glass transition temperature, the
quasi-fluid chain segments "freeze" into fixed geometries and the
rubber abruptly loses its elastic properties, although the process is
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reversible. This property it shared by most elastomers. At very low
temperatures, rubber is rather brittle. This critical temperature is the
reason winter tires use a softer version of rubber than normal tires.
Application
Natural rubber is used extensively in many applications and
products, either alone or in combination with other materials. The
use of rubber is widespread, ranging from household to industrial
products, entering the production stream at the intermediate stage or
as final products. Tires and tubes are the largest consumers of rubber.
The remaining 44% are taken up by the general rubber goods (GRG)
sector, which includes all products except tires and tubes.
Manufacturing
Other significant uses of rubber are door and window
profiles, hoses, belts, matting, flooring and dampeners (antivibration
mounts) for the automotive industry. Gloves (medical, household and
industrial) and toy balloons are also large consumers of rubber,
although the type of rubber used is that of the concentrated latex.
Significant tonnage of rubber is used as adhesives in many
manufacturing industries and products, although the two most
noticeable are the paper and the carpet industries. Rubber is also
commonly used to make rubber bands and pencil erasers. Many
aircraft tires and inner tubes are still made of natural rubber due to
the high cost of certification for aircraft use of synthetic
replacements.
Textile applications
Rubber produced as a fiber sometimes called elastic, has
significant value for use in the textile industry because of its
excellent elongation and recovery properties. For these purposes,
manufactured rubber fiber is made as either an extruded round fiber
or rectangular fibers that are cut into strips from extruded film.
Because of its low dye acceptance, feel and appearance, the rubber
fiber is either covered by yarn of another fiber or directly woven with
other yarns into the fabric. In the early 1900s, for example, rubber
yarns were used in foundation garments. While rubber is still used in
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textile manufacturing, its low tenacity limits its use in lightweight
n
garments because latex lacks resistance to oxidizing agents and is
damaged by aging, sunlight, oil, and perspiration. Seeking a way to
address these shortcomings, the textile industry has turned to
neoprene (chloroprene polymer), a type of synthetic rubber as well as
another more commonly used elastomer fiber, spandex (also known
as elastane), because of their superiority to rubber in both strength
and durability.
3.2 Polyisoprene (Synthetic Natural Rubber)
C C
2
H2CCH
CH
3
H
Synthetic polyisoprene (IR) represents one of the important classes
of polymers produced in a solution medium. The successful
development of a stereospecific catalyst system has permitted the
production of a synthetic analog of natural rubber with improved
uniformity and processing.
History
Modern synthetic polyisoprene is designed to be similar to
natural rubber in structure and properties. The polymer chains in the
early synthetics contained mixtures of all possible molecular
configurations joined together in a random fashion. Specifically, they
lacked the very high cis-1,4 structure of the natural rubber backbone
that gives it the ability to undergo strain crystallization. In the mid
1950s, researchers discovered and developed new types of catalyst
systems that could selectively join together monomer units in a wellordered fashion. The “stereospecific” catalysts allowed realization of
a nearly pure cis-1,4 structure, and in doing so, the production of a
synthetic natural rubber. Initial commercialization of a stereoregular,
low cis-1,4 IR (90% to 92%) was produced with an alkyl lithium
catalyst (Li-IR). Next was a Ziegler-Natta (titanium-aluminum)
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catalyzed IR (Ti-IR) with a cis-1,4 content of 98.5%, finally allowing
n
the benefits of strain-crystallization to be realized.
Chemistry
Many chemical structures (microstructures) are possible and
that different catalyst systems result in specific microstructures with
different physical properties.
The polymerization of isoprene monomer can proceed in a
1,2- (a), 3,4- (b), and cis-1,4- (c) or trans-1,4- (d) mode.
C C
2
H2CCH
CH
3
H
CH
C C
2
CH
CH
2
3
n
HC CH
CH C
CH
H
n
H2C
CH CH
C
CH
3
n
3
2
3
a b c d
The structural content of natural rubber, Ti-IR and Li-IR are
given in Table 6.
Table 6.
Microstructure of cis-1,4-Polyisoprene Rubber (%)
NR Ti-IR Li-IR
Microstructure
Cis-1,4 100 98.5 90.0
Trans-1,4 0 1.0 5.0
3,4 0 0.5 5.0
Purity
Rubber content 94 >99.0 >99.0
Stress crystallization in cis-1,4 IR leads to important physical
properties such as green strength, tear strength, and gum tensile
strength. For Li-IR, while x-ray diffraction patterns have indicated
some crystallinity in stretched specimens, no crystallinity is seen in
the unstretched state. Ti-IR and NR both undergo crystallization in
the unstretched state at low temperatures (the maximum rate of
crystallization occurs at –25°C), but the rate is greatest for NR. Both
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undergo crystallization at room temperature on stretching, with NR
stress crystallizing at a lower elongation than Ti-IR.
Properties
Typical raw polymer and vulcanized properties of
polyisoprene are similar to values obtained for natural rubber.
Natural rubber and synthetic polyisoprene both exhibit good inherent
tack, high compounded gum tensile, good hysteresis, and good hot
tensile properties. The very specific nature of synthetic polyisoprene
provides a number of factors that differentiate it from natural rubber.
There is minimal variance in physical properties lot to lot.
Polymerization conditions are narrowly controlled to assure that the
polymer is highly specific chemically. There is a low level of nonpolymer constituents as compared to natural rubber.
Synthetic polyisoprene is easy in processability. The end
results are time and power savings as well as increased throughput.
In addition, synthetic polyisoprene exhibits greater compatibility
than natural rubber in blends with solution SBR and EPDM.
Synthetic polyisoprene’s uniformity is a factor where the desire for
consistent quality is paramount, as is increasingly the case in many
industries with an emphasis on precise dimensional control in
processing. Because of the lower raw polymer viscosity of synthetic
polyisoprene, part or the entire breakdown step normally used for
natural rubber should be eliminated.
Synthetic polyisoprene compounds at the same plasticity of
natural rubber will have less die swell because of having less nerve.
Also, at the same plasticity, the synthetic polymer will have
significantly faster extrusion rates. Synthetic polyisoprene
compounds can be adapted for curing in any conventional molding
operation whether it is compression, transfer, or injection. Synthetic
polyisoprene is especially well suited for injection molded
compounds. Because of its uniform cure rate, exact time/temperature
press cycles can be established with assurance that all pieces will be
uniformly cured. In addition, the Mooney of synthetic polyisoprene
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reduces injection pressures and times with a resultant increase in
output.
Production
Before entering the reactors, the solvent, catalyst and isoprene
monomer must be free of chemical impurities, moisture and air - all
of which are catalyst poisons. The purified streams first enter a chain
of reactors in series into which the catalyst is injected, and the
polymerization begins.
After the desired extent of polymerization has been attained, a
short stop or catalyst deactivator is added to the cement so no further
linkage of monomer or polymer takes place. A non-staining
antioxidant is then added to protect the polymer during finishing and
storage.
In the next step, the cement mixture is put through a stripping
operation whereby the solvent is recovered and the polymer cement
converted to a crumb by hot water and steam. The crumb slurry is
processed through extruders to remove water before it is cooled,
baled, packaged and placed in storage ready for shipment.
Applications
Currently synthetic polyisoprene is being used in a wide
variety of industries in applications requiring low water swell, high
gum tensile strength, good resilience, high hot tensile, and good tack.
Gum compounds based on synthetic polyisoprene are being used in
rubber bands, cut thread, baby bottle nipples, and extruded hose.
Black loaded compounds find use in tires, motor mounts, pipe
gaskets, shock absorber bushings and many other molded and
mechanical goods. Mineral filled systems find applications in
footwear, sponge, and sporting goods. In addition, recent concerns
about allergic reactions to proteins present in natural rubber have
prompted increased usage of the more pure synthetic polyisoprene in
some applications. Consumption of synthetic polyisoprene stabilized
in the early 1990’s as polyisoprene’s availability was limited by
manufacturing capacity and monomer availability. Recent increases
in capacity, concerns about the stability of the price of natural rubber,
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and the mandate to move away from natural rubber in certain
applications provide avenues for future growth in the industry.
Trade names
Polyisoprene suppliers provide many grades for different
applications. Synthetic IR trade names and manufacturers include
Natsyn from Goodyear Chemical; Isogrip from Karbochem; SKI
from Nizhnekamskneftekhim, Inc.; SynthezKauchuk SKI and
Nipol™ IR from Zeon Corporation.
Other synthetic polyisoprene suppliers include JSR
Corporation, Togliatti (Togliattisyntezkauchuk Co) and Sibur
Holdings.
3.3 Polybutadiene
CH2CH CH CH
2
n
Polybutadiene (BR) is a synthetic rubber that is a polymer formed
from the polymerization process of the monomer 1,3-butadiene.
History
The Russian chemist Sergei Vasilyevich Lebedev was the
first to polymerize butadiene in 1910. In 1926 he invented a process
for manufacturing butadiene from ethanol, and in 1928, developed a
method for producing polybutadiene using sodium as a catalyst.
The government of the Soviet Union strived to use
polybutadiene as an alternative to natural rubber and built the first
pilot plant in 1930, using ethanol produced from potatoes. The
experiment was a success and in 1936 the Soviet Union built the
world's first polybutadiene plant in which the butadiene was obtained
from petroleum. By 1940, the Soviet Union was by far the largest
producer of polybutadiene with 50,000 tons per year.
Following Lebedev's work, other industrialized countries
such as Germany and the United States developed polybutadiene and
SBR as an alternative to natural rubber.
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