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Rubber Engineering |
Types of
Synthetic Rubber :
Neoprene
One of the first successful synthetic rubbers resulting from Carothers's
research was neoprene, which is the polymer of the monomer chloroprene,
chemical formula CH2:C(Cl)CH:CH2. The raw materials of
chloroprene are acetylene and hydrochloric acid. Developed in 1931, neoprene
has high resistance to heat and such chemicals as oils and gasoline.
Neoprene is used in hose for conveying gasoline and as an insulating
material for cables and in machinery.
Butadiene Or Buna Rubbers
In 1935 German chemists developed the first of a group
of synthetic rubbers called Buna, which is produced by copolymerization—that
is, the polymerization of two monomers, called comonomers. The name
Buna is derived from the initial letters of
butadiene, used as one of the comonomers,
and natrium (sodium), which was used as a catalyst. One of these
products, Buna-N, uses
acrylonitrile (CH2:CH(CN))
as the other comonomer. Acrylonitrile is produced from cyanide. Buna-N is
valuable for uses requiring resistance to the action of oils or abrasion.
During World War II
a Buna-type rubber called GR-S (Government Rubber-Styrene)
was designated as the general-purpose rubber for the U.S. war effort. The
basic rubber produced by the present-day U.S. synthetic-rubber industry, GR-S
is a copolymer of butadiene and styrene. The various grades of GR-S are
classified in two categories, regular and cold, depending on the
temperatures of copolymerization. Cold GR-S types, which exhibit superior
properties, are prepared at 5° C (41° F); regular GR-S types are prepared at
temperatures of 50° C (122° F). Cold GR-S is used to make longer-wearing
tires for automobiles and trucks.
Styrene-Butadiene
Styrene-Butadiene
(SBR) is an elastomeric copolymer consisting of styrene and
butadiene. It has good abrasion resistance and good aging stability. SBR is
stable in: mineral oils, fats, aliphatic, aromatic and chlorinated
hydrocarbons.
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Possible
temperature range: approx. -40 to +100 °C (-40 to +212 °F)
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Chemical
Type: styrene-butadiene (copolymer)
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Trade names
(common): GRS, Buna S (SBR)
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Elongation
(%): 450-500
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Useful
temperature range: -60 to 120 degrees Celsius (-75 to 250 degrees
Fahrenheit)
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Major
application characteristics: good physical properties; excellent abrasion
resistance; not oil, ozone, or weather resistant; electrical properties
good, but not outstanding
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Typical
applications: pneumatic tires and tubes; heels and soles; gaskets
Butyl Rubber
Butyl rubber,
produced initially in 1940, is prepared by copolymerization of isobutylene
with butadiene or isoprene. It is plastic and can be compounded like natural
rubber, but is difficult to vulcanize. Although butyl rubber is not as
resilient as natural rubber and other synthetics, it is extremely resistant
to oxidation and the action of corrosive chemicals. Because of its low
permeability to gas, butyl rubber is used widely for inner tubes in
automobile tires.
polyisobutylene (C4H8)—is a synthetic rubber, a
homopolymer of 2-methyl-1-propene. Polyisobutylene is produced by
polymerization of about 98% of isobutylene with about 2% of isoprene.
Structurally, polyisobutylene resembles polypropylene, having two methyl
groups substituted on every other carbon atom. It has excellent
impermeability, and the long polyisobutylene segments of its polymer chains
give it good flex properties. Polyisobutylene is a colorless to light yellow
elastic semi-solid or viscous substance. It is generally odorless and
tasteless, though it may exhibit a slight characteristic odor.
Usage as a
fuel and lubricant additive
Polyisobutylene (in the form of polyisobutylene
succinimide) has interesting properties when used as an additive in
lubricating oils and motor fuels. Polyisobutylene added in small amounts to
the lubricating oils used in machining results in a significant reduction in
the generation of oil mist and thus reduces the operator's inhalation of oil
mist. It is also used to clean up waterborne oil spills as part of the
commercial product Elastol. When added to crude oil it increases the oil's
viscoelasticity when pulled; causing the oil to resist breakup when it is
vacuumed from the surface of the water.
As a fuel
additive, polyisobutylene has detergent properties. When added to diesel
fuel, it resists fouling of fuel injectors, leading to reduced hydrocarbon
and particulate emissions. Polyisobutylene is manufactured by BASF (as
well as its competitors) and blended with other detergents and additives to
make a "detergent package" that is blended into gasoline and diesel fuel to
resist buildup of deposits and engine knock. Because fuel additive formulas
are closely held trade secrets, it is impossible to know which additives any
particular brand of gasoline may contain.
Other Specialty
Rubbers
Many other types of synthetic rubber have been developed for purposes
requiring specific properties. One such specialty rubber, called Koroseal,
is a polymer of vinyl chloride (CH2:CHCl). Vinyl chloride
polymers are heat-, electricity-, and corrosion-resistant and are unaffected
by exposure to light or by long storage. Koroseal cannot be vulcanized, but,
when not subjected to high temperatures, it is more resistant to abrasion
than natural rubber or leather.
Another specialty
rubber is Thiokol, produced by copolymerization of ethylene dichloride (CHCl:CHCl),
and sodium tetrasulfide (Na2S4). This type, which
can be compounded and vulcanized like natural rubber, is resistant to the
action of oils and to organic solvents used for lacquers, and is useful
for electrical insulation because it does not deteriorate when exposed to
electrical discharge and light.
Many other
types of synthetic rubber are produced in the United States, mostly by
methods similar to those described above. Certain changes in the process
or the polymerization recipes have succeeded in improving quality as
well as reducing production costs. In one outstanding development,
petroleum oil was used as an additive; it lowered the cost by conserving
a substantial amount of synthetic-rubber stock. Tires made from such
oil-extended rubber are very durable. Other important advances include
the development of synthetic foam rubber, used mainly for upholstery,
mattresses, and pillows; and cellular-crepe rubber, used by the shoe
industry.
Polybutadiene
Polybutadiene is a synthetic rubber that has a
high resistance to wear and is used especially in the manufacture of
tires. It exhibits a recovery of 80% after stress is applied, a value only
exceeded by elastin and resilin. Polybutadiene is a polymer formed from
the polymerization of the monomer 1,3-butadiene.
Polymerization of butadiene
1,3-butadiene is an organic compound which is a rather simple conjugated
diene hydrocarbon; the chemical structure is shown as a reactant in the
diagram below. A hydrocarbon diene molecule has two C=C double bonds (i.
e. between two sets of carbon atoms). Polybutadiene can be formed from
many 1,3-butadiene monomers undergoing free radical polymerization to make
a much longer polymer chain molecule.
A chain
propagating step in this chemical reaction involves a free radical near
the end of a growing polymer chain forming a covalent bond with the #1
carbon in a 1,3-butadiene monomer molecule being added, resulting in a
polymer chain intermediate with a substituted allyl free radical at the
end of the chain. This allyl free radical, formed from the butadiene just
added, can further bond to another monomer molecule at either the #2 or #4
carbons of the previous butadiene monomer. Most of the time, the new
monomer bonds to the #4 or terminal carbon of the previous butadiene,
resulting in a 1,4-addition of the previous butadiene unit. In a
1,4-addition, the two double bonds of the previous butadiene unit are
turned into single bonds and a new double bond is formed between the #2
and #3 carbons. This new double bond may have either a cis
or a trans configuration. A smaller fraction of the time
(perhaps 20%), the new monomer bonds to the #2 carbon of the previous
butadiene, resulting in a 1,2-addition of the previous butadiene unit. The
double bond between the #1 and #2 carbons turns into a single bond in the
previous butadiene unit, and the double bond between the #3 and #4 carbons
remains intact in a short vinyl side group available for branching or
cross-linking. Cis or trans configurations are not
applicable in 1,2-additions of butadiene. See the following reaction
diagram for examples of 1,2- and 1,4-addition in a polybutadiene chain.
The
trans double bonds formed during polymerization allow the polymer
chain to stay rather straight, allowing sections of polymer chains to line
up against each other and effectively form microcrystalline regions in the
material. The cis double bonds cause a bend in the polymer chain,
preventing polymer chains from lining up and forming crystalline regions
and resulting in larger regions of amorphous polymer. It has been found
that a substantial percentage of cis double bond configurations in
the polymer will result in a material with flexible elastomer
(rubber-like) qualities. In free radical polymerization, both cis
and trans double bonds will form in percentages which depend on
temperature. There are different catalysts available which can result in
polymerization either in the cis or the trans
configurations.
1,3-butadiene can be copolymerized with other types of monomers such as
styrene and acrylonitrile to form rubbers or plastics with various
qualities. These copolymers are commonly graft copolymers, meaning that
there are sections of polymer of one kind of monomer forming the main
chains and grafts made of another type of monomer forming branches and
cross-links bonded to the main chains. This way a copolymer material can
be made with good stiffness, hardness, and toughness.
Each time a
free radical hits an ethene molecule a new longer free radical is formed
(this is shown by the propagation section of fig2). This will continue
producing larger and free radicals until two free radicals collide
producing the final molecule as no new free radicals are formed (known as
termination shown in fig2). Because termination is a random process
poly(ethene) will be made of chains of various lengths
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Rubber
Engineering
