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Chemical
Composition and Structure of Elastomers
The term 'elastomer' (from "elastic polymer") refers to any member of a class of
polymeric substances that possess the quality of elasticity, i.e., the ability
to regain shape after deformation. Elastomers are the base material for
all rubber products, both natural and synthetic, and for many adhesives.
Polymers are chemical compounds whose molecules consist of several thousand
smaller molecules, called monomers, that are linked together to form long
chains. In elastic polymers these chains are highly flexible, disordered, and
intertwined. In chemical terms, elastomers are extremely viscous fluids, lacking
the rigidity of a glass or the ordered arrangement of a crystal. When stretched,
the molecules are pulled into alignment and often take on aspects of a
crystalline arrangement, but upon release they return spontaneously to their
naturally disordered, entangled state. This return to natural disorder
distinguishes elastomers from plastic polymers, which are normally glassy or
crystalline and therefore retain much of the shape to which they are deformed.
Most elastomers are hydrocarbons; i.e., they are composed principally of
carbon and hydrogen and their compounds. Some occur naturally—e.g.,
polyisoprene, which is formed in the latex of the rubber tree and is processed
into natural rubber. Most elastomers, however, are produced synthetically
from derivatives of petroleum and natural gas. Monomers such as isoprene,
butadiene, and butylene are subjected to various polymerization reactions in
which they are built up into large molecules. In many cases other chemical
elements or compounds are incorporated into the polymer in order to modify basic
properties—e.g., chlorine in polychloroprene (neoprene) and sulfur in
polyalkylene polysulfide (Thiokol), which contribute to the oil-resistance of
these rubbers. Properties can also be modified by producing elastomers as
copolymers, i.e., polymers made up of more than one type of monomer. Examples
include nitrile rubber (an acrylonitrile-butadiene copolymer) and butyl rubber
(a copolymer of isobutylene and isoprene). In another method, some elastomers
are blended with various plastic polymers such as polypropylene or polystyrene;
the resultant materials, known as thermoplastic elastomers, or TPR's
(thermoplastic rubber), retain the resilience of rubber but, unlike others, can
be remolded and reprocessed upon the application of heat (a property important
in recycling).
In order to be made into useful rubber products, elastomeric materials must be
subjected to various modifications. These include: strengthening of the material
by cross-linking the polymer chains (for instance, by sulfur atoms in the
process known as vulcanization); further strengthening by fillers such as carbon
black; and treatment with chemicals that provide resistance to weathering and
chemical attack. For fabrication into adhesives, elastomers are often dissolved
in organic solvents and treated with various other additives to improve their
application, adhesion, and durability.
The elastomer with the longest history of use is natural rubber, which is made from the milky sap, or latex, of the Hevea and other trees. Natural rubber is still an important industrial polymer, but it now competes with a number of synthetic elastomers, such as styrene-butadiene rubber and polybutadiene, which are derived from by-products of petroleum and natural gas. More than half of all rubber produced goes into automobile tires; the rest goes into mechanical parts such as mountings, gaskets, belts, and hoses, as well as consumer products such as shoes, clothing, furniture, and toys.
Polymers and elasticity
A polymeric molecule consists of several thousand chemical repeating units, or monomers, linked together by covalent bonds. The assemblage of linked units is often referred to as the “chain,” and the atoms between which the chemical bonding takes place are said to make up the “backbone” of the chain.
In most cases polymers are made up of carbon backbones—that is, chains of carbon (C) atoms linked together by single (C−C) or double (C=C) bonds. In theory, carbon chains are highly flexible, because rotation around carbon-carbon single bonds allows the molecules to take up many different configurations. In practice, however, many polymers are rather stiff and inflexible.
All polymers are glassy below a characteristic glass transition temperature (Tg), which ranges from as low as −125° C (−195° F) for an extremely flexible molecule such as polydimethyl siloxane (silicone rubber) to extremely high temperatures for stiff, bulky molecules.
Thus, not all polymers have the necessary internal flexibility to be extensible and highly elastic. In order to have these properties, polymers must have little internal hindrance to the random motion of their monomer subunits (in other words, they must not be glassy), and they must not spontaneously crystallize (at least at normal temperatures). On release from being extended, they must be able to return spontaneously to a disordered state by random motions of their repeating units as a result of rotations around the carbon-carbon bond. Polymers that can do so are called elastomers. All others are termed plastics or resins.
Four common elastomers are cis-polyisoprene (natural rubber, NR), cis-polybutadiene (butadiene rubber, BR), styrene-butadiene rubber (SBR), and ethylene-propylene monomer (EPM). SBR is a mixed polymer, or copolymer, consisting of two different monomer units, styrene and butadiene, arranged randomly along the molecular chain. EPM also consists of a random arrangement of two monomers—in this case, ethylene and propylene. In SBR and EPM, close packing and crystallinity of the monomer units are prevented by their irregular arrangement along each molecule. In the regular polymers NR and BR, crystallinity is prevented by rather low crystal melting temperatures of about 25° and 5° C (approximately 75° and 40° F), respectively. In addition, the glass transition temperatures of all these polymers are quite low, well below room temperature, so that all of them are soft, highly flexible, and elastic.
Chemical interlinking: from elastomers to rubbery solids
Vulcanization
The molecular behavior outlined above is sufficient to give polymers the properties of extensibility and elasticity, but in many cases the properties of elastomers must be modified in order to turn them into useful rubbery materials. The necessity for such modification was first demonstrated by natural rubber when it began to be produced commercially in the 18th century. Rubber was soon found to have two serious disadvantages: it becomes soft and sticky when warm, because it is really a viscous liquid, and it becomes hard when cold, because it crystallizes slowly below about 5° C (40° F). These disadvantages were overcome in 1839 by the discovery of vulcanization by the American inventor Charles Goodyear. Goodyear found that a mixture of rubber with some white lead and about 8 percent by weight of sulfur was transformed, on heating, to an elastic solid that remained elastic and resilient at high temperatures and yet stayed soft at low temperatures. It is now known that sulfur reacts with unsaturated hydrocarbon elastomers. One of the consequences is that a few sulfur interlinks (−Sn−) are formed between the polymer molecules, making a loose molecular network, as shown in Figure 1. The original elastomeric liquid is thus converted into a solid that will not flow even when warm because the molecules are now permanently tied together. Moreover, addition of a small amount of sulfur in various forms makes the rubber molecules sufficiently irregular that crystallization (and, hence, hardening at low temperatures) is greatly impeded. The
linking process is often called curing or, more commonly, vulcanization (after Vulcan, the Roman god of fire). More accurately, the phenomenon is referred to as cross-linking or interlinking, because this is the essential chemical reaction.
All long, flexible polymer molecules naturally become entangled, like spaghetti. Although all such molecules will disentangle and flow under stress, their physical entanglements will act as temporary “interlinks,” especially when the molecules are long and slow-moving. It is therefore difficult at first sight to distinguish a covalently interlinked elastomer from one that is merely tangled or (as is described below) one that is held together by strong intermolecular associations. One means of distinguishing is to test whether the polymer dissolves in a compatible solvent or merely swells without dissolving. Covalently interlinked molecules do not dissolve. Interlinking is therefore necessary for good solvent resistance or for use at high temperatures.
Free-radical interlinking
Interlinking can be carried out with reagents other than sulfur—for example, by free-radical reactions that do not require the presence of C=C bonds. Free radicals are formed by irradiation with ultraviolet light, by electron-beam or nuclear radiation, or by the decomposition of unstable additives. In each case a hydrogen atom is torn away from the elastomer molecule, leaving a highly reactive carbon atom (the radical) that will couple with another carbon radical to create a stable C−C bond, interlinking different molecules. Even polyethylene and other fully saturated polymers can be interlinked by a free-radical process. However, for the curing of rubbery materials, sulfur is usually still the reagent of choice. Using accelerators and activators, the vulcanization reaction can be modified in various desirable ways, and sulfur interlinking also yields products of higher strength.
Molecular branching
Some rubbery solids are made by simultaneous polymerization and interlinking. If during polymerization each unit can add more than one other unit, then as the molecule increases in size it will branch out with many arms that will divide and interlink to create a densely cross-linked solid. The length of molecule between interlinks is small in this case, sometimes only a few carbon atoms long. Such materials are hard and inflexible; epoxy resins are an example. However, if molecular branching is made less frequent, then soft, rubbery materials will be produced. Rubbery products can be made in this way by casting—that is, by using low-viscosity liquid precursors with reactive end-groups. Examples are castable polyurethanes and silicones.
Intermolecular Association: thermoplastic elastomers
Hydrogen bonding
Other rubbery materials consist of elastomers having strong intermolecular associations but no real chemical interlinks. Examples are molecules containing a few hydrogen-bonding groups. If the associations between the molecules are strong enough to prevent flow under moderate stresses, such materials can serve as practical rubbery solids. Also, because the weak interlinks give way at high temperatures, allowing the material to take on a new shape in response to pressure, they can be reprocessed and reused. For this reason these rubbery materials are called thermoplastic elastomers.
Block copolymers
Another type of intermolecular association is shown by thermoplastic block copolymers, where each molecule consists of long sequences, or blocks, of one unit followed by long sequences of another. Because different polymers are generally incompatible (i.e., do not dissolve into one another), blocks of the same type tend to aggregate and separate into small “domains.” This type of material can be exemplified by styrene-butadiene-styrene (SBS), a “tri-block” copolymer composed of butadiene repeating units in the centre portion of the chain and styrene units at the ends. Polystyrene and polybutadiene are incompatible, so that the polystyrene end-groups associate together to form domains of glassy polystyrene in a sea of elastic polybutadiene. The polybutadiene center portions thus form a connected elastomeric network held together by rigid domains of polystyrene end-blocks, which are relatively stable up to the glass transition temperature of polystyrene (about 100° C, or 212° F). Thus, the material is a rubbery solid at normal temperatures, even though there are no chemical bonds interlinking the molecules. Above the Tg of polystyrene the aggregates can be sheared apart, and the material can be reprocessed and remolded.
Polymer blends
Yet another kind of thermoplastic elastomer is made by blending a specific
elastomer with a specific plastic material, forming what has come to be commonly
called TPR's (thermoplastic rubber). One brand, Santoprene
® consists of a mixture of approximately 60 parts ethylene-propylene-diene monomer copolymer (EPDM) with 40 parts polypropylene. A hydrocarbon oil, compatible with EPDM, and interlinking reagents for EPDM are also added. Because the polymers are molecularly incompatible, they form a fine, heterogeneous blend, the individual materials remaining as small, separate regions. During mixing, the EPDM portion becomes chemically interlinked to create a rubbery solid that can be molded (and remolded) at high temperatures, when the polypropylene component becomes soft and fluid. There is some uncertainty about the exact mechanism of elasticity in this material, because the polypropylene component appears to form continuous strands and should therefore make the mixture hard, not rubbery. Polymer blends are finding increasing use as elastomers because processing is simple and because they can be recycled.
Heterochain polymers
Polysiloxanes (silicones)
Polysiloxanes are polymers whose backbones consist of alternating atoms of silicon and oxygen. Although organic substituents are attached to the silicon atoms, lack of carbon in the backbones of the chains makes polysiloxanes into unusual “inorganic” polymers. They can exist as elastomers, greases, resins, liquids, and adhesives. Their great inertness, resistance to water and oxidation, and stability at high and low temperatures have led to a wide range of commercial applications.
Siloxanes were first characterized as macromolecules by the English chemist Frederic Stanley Kipping in 1927. Because Kipping thought that the structure of the repeating unit was essentially that of a ketone (that is, the polymer chains formed by silicon atoms, with oxygen atoms attached by double bonds), he incorrectly called them silicones, a name that has persisted. In 1943 Eugene George Rochow at the General Electric Company Laboratories in Schenectady, N.Y., U.S., prepared silicones by the hydrolysis of dialkyldimethoxysilane—a ring-opening process that he patented in 1945 and that remains the basis of modern polymerization methods.
The most common siloxane polymer, polydimethylsiloxane, is formed when the chlorine atoms of the monomer, dichlorodimethylsilane (Cl2Si[CH3]2), are replaced by hyroxyl (OH) groups by hydrolysis. The resultant unstable compound, silanol (Cl2Si[OH]2), condenses in step-growth fashion to form the polymer, with concomitant loss of water. Some cyclic products are also formed, and these are
purified by distillation and converted to polysiloxane by ring-opening polymerization. The repeating unit of polydimethylsiloxane has the following structure:
Siloxane molecules rotate freely around the Si−O bond, so that, even with vinyl, methyl, or phenyl groups attached to the silicon atoms, the molecule is highly flexible. In addition, the Si−O bond is highly heat-resistant and is not readily attacked by oxygen or ozone. As a result, silicone rubbers are remarkably stable, and they have the lowest glass transition temperature and the highest permeability to gases of any elastomer. On the other hand, the Si−O bond is susceptible to hydrolysis and attack by acids and bases, and the rubber vulcanizates are relatively weak and readily swollen by hydrocarbon oils.
Non-vulcanized, low-molecular-weight polysiloxanes make excellent lubricants and hydraulic fluids and are known as silicone oils. Vulcanized silicone rubber is prepared in two principal forms: (1) as low-molecular-weight liquid room-temperature-vulcanizing (RTV) polymers that are interlinked at room temperature after being cast or molded into a desired shape or (2) as heat-curable, high-temperature-vulcanizing (HTV) elastomers of higher viscosity that are mixed and processed like other elastomers. RTV elastomers are usually interlinked using reactive vinyl end-groups, whereas HTV materials are usually interlinked by means of peroxides. Silicone rubber is used mainly in O-rings, heat-resistant seals, caulks and gaskets, electrical insulators, flexible molds, and (owing to its chemical inertness) surgical implants.
Carbon-chain polymers
Fluorinated polymers
Fluoroelastomers (Viton™, other trade names)
A number of fluorinated polymers or copolymers having elastomeric properties are produced that incorporate the monomers vinylidene fluoride (CH2=CF2), hexafluoropropylene (CF2=CFCF3), and chlorotrifluoroethylene (CF2=CFCl) in addition to tetrafluoroethylene. These elastomers have outstanding resistance to oxygen, ozone, heat, and swelling by oils, chlorinated solvents, and fuels. With service temperatures up to 250° C (480° F), they are the elastomers of choice for use in industrial and aerospace equipment subjected to severe conditions. However, they have a relatively high density, are
swollen by ketones and ethers, are attacked by steam, and become glassy at temperatures not far below room temperature. Also, their low reactivity makes interlinking the polymer chains a long and complex process. Principal applications are as temperature-resistant O-rings, seals, and gaskets.
Vinyl copolymers
Nitrile rubber (nitrile-butadiene rubber, NBR, Buna-n)
Like SBR, nitrile rubber is a product of synthetic rubber research during and between the two world wars. Buna N, a group of acrylonitrile-butadiene copolymers, was patented in the United States in 1934 by IG Farben chemists Erich Konrad and Eduard Tschunkur. Produced in the United States during World War II as GR-N (Government Rubber-Nitrile), it has become valued for its outstanding resistance to oil.
NBR is prepared in emulsion processes using free-radical initiators. The amount of acrylonitrile present in the copolymer varies from 15 to 50 percent. With increasing acrylonitrile content the rubber shows higher strength, greater resistance to swelling by hydrocarbon oils, and lower permeability to gases—although the glass transition temperature is also raised, with the result that the rubber is less flexible at lower temperatures. The main uses of NBR are in fuel hoses, gaskets, rollers, and other products in which oil resistance is required. It is also employed in textiles, where its application to woven and non-woven fabrics improves the finish and waterproofing properties.
A hydrogenated version, abbreviated as HNBR, is also highly resistant to thermal and oxidative deterioration and remains flexible at lower temperatures.
Ethylene-propylene copolymers (EP, EPR, EPDM)
There are two major types of ethylene-propylene copolymers with elastomeric properties: those made with the two monomers alone and those made with small amounts (approximately 5 percent) of a diene—usually ethylidene norbornene or 1,4-hexadiene. Both copolymers are prepared in solution using Ziegler-Natta catalysts. The former are known as EPM (ethylene-propylene monomer) and the latter as EPDM (ethylene-propylene-diene monomer). The copolymers contain approximately 60 percent by weight ethylene. A pronounced advantage of EPDM is that the residual carbon-carbon double bond (i.e., the double bond that remains after polymerization) is attached to the polymer chain rather than being made part of it. Carbon-carbon double bonds are quite reactive. For example, ozone in the atmosphere adds quickly to a double bond to form an unstable product that spontaneously decomposes. Regular diene polymers, such as natural rubber or styrene-butadiene rubber, have many double bonds in the main chain, so that, when one double bond is attacked, the entire molecule is broken. EPDM, with the double bonds located in the side groups, is much less susceptible to degradation by weathering and sunlight, because any breaking of the double bonds by ozonolysis, thermal deterioration, or oxidation leaves the main chains intact. In addition, some crystallinity appears to be induced by stretching, so that even without fillers vulcanized ethylene-propylene copolymers are quite strong. However, like other hydrocarbon elastomers, the ethylene-propylene copolymers are swollen and weakened by hydrocarbon oils.
The principal uses of EPM are in automobile parts and as an impact modifier for polypropylene. EPDM is employed in flexible seals for automobiles, wire and cable insulation, weather stripping, tire sidewalls, hoses, and roofing film.
EPDM is also mixed with polypropylene to make a thermoplastic elastomer. These polymer blends, which usually contain 30 to 40 mole percent polypropylene, are rubbery solids, though they are not nearly as springy and elastic as covalently interlinked elastomers. However, owing to the thermoplastic properties of polypropylene, they can be processed and reprocessed, and they are resistant to oxidation, ozone attack, and weathering. They are therefore used in such low-severity applications as shoes, flexible covers, and sealing strips. The trademarked product Santoprene, produced by Advanced Elastomer Systems, L.P., is an example.
Some block copolymers of ethylene and propylene, called polyallomers, are marketed. Unlike EPM and EPDM, which have a relatively amorphous morphology, the polyallomers are crystalline and exhibit properties of high-impact plastics.
Diene polymers
Polychloroprene (chloroprene rubber, CR, Neoprene)
Polychloroprene is the polymer name for the synthetic rubber known as neoprene (a proprietary trade name of DuPont that has become generic). One of the first successful synthetic elastomers, neoprene was first prepared in 1931 by Arnold Collins, a chemist in Wallace Hume Carothers' research group at DuPont, while he was investigating by-products of divinylacetylene. It is a good general-purpose rubber, but it is limited to special-properties applications because of its high cost.
Polychloroprene is prepared by emulsion polymerization of chloroprene, or 2-chlorobutadiene, which is obtained by the chlorination of butadiene or isoprene.
This polymer tends to crystallize and harden slowly at temperatures below about 10° C (50° F). It also crystallizes on stretching, so that cured components are strong even without fillers. Because the double bond between the carbon atoms is shielded by the pendant atoms and CH2 groups, the molecular interlinking necessary for producing a cured rubber is usually effected through the chlorine atom. The presence of chlorine in the molecular structure causes this elastomer to resist swelling by hydrocarbon oils, to have greater resistance to oxidation and ozone attack, and to possess a measure of flame resistance. Principal applications are in products such as hoses, belts, springs, flexible mounts, and gaskets where resistance to oil, heat, flame, and abrasion are required.
References
- Encyclopedia Britannica, "Industrial Polymers, Major" © Encyclopedia Britannica, Inc.
- Encyclopedia Britannica, "Elastomer"
- Encyclopedia Britannica, "Elastomer - Production of Elastomers" © Encyclopedia Britannica, Inc.
- "Rubber" - Encarta® 98 Desk Encyclopedia © 1996-97 Microsoft Corporation.
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Elastomer Chemistry
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