Nonpolar elastomers

Uses Cover coat bonding agent tor EPDM, nonpolar elastomer Features General purpose bonds a wide variety of rubbers to metals and other substrates... 

Nitrile rubber vulcanizates have a considerably higher electrical conductivity than do those of nonpolar elastomers and are, thus, not generally used in parts tbat require low electrical conductivities. 

Polyurethane adhesives also are suitable for bonding nonpolar elastomers, for example, natural rubber, styrene-butadiene rubber, or ethylene-propylene terpol-ymers, after chemical pretreatment of the surface. 

Paraffinic process oiis are chemically more compatible with nonpolar elastomers (such as EPDM-based compounds) than naphthenic or aromatic oils. 

The petroleum oils are used primarily with nonpolar elastomers and are either naphthenic, paraffinic, or aromatic, depending upon elastomer compatibility, volatility, and expected product service conditions. Vegetable oils are typically from castor or tall oils. Vulcanized vegetable oil products have the unique ability to promote flow imder shear but to resist high... 

This group of high volume nonpolar elastomers is typically plasticized with soybean and rapeseed oil factices and the full range of hydrocarbon process oils. Only when a special lubricating effect is wanted is a synthetic plasticizer used. To illustrate, the following two examples (a) 10 phr DOA added to an EPDM Arctic CB radio coil cord where it acted as an extreme low temp (LT) flexibilizer (b) a typical LT glycol ester (TEG-2EH) used at 8 phr in BR to achieve both processing and LT enhancements. In both examples, the incompatible plasticizer acted as a lubricant. 

Two gas chromatograms showing the effect of polarity of the stationary phase on the separation efficiency for three substances of increasing polarity toluene, pyridine, and benzaldehyde. (a) Separation on silicone SE-30, a nonpolar phase, and (b) separation on elastomer OV-351, a more polar phase. Note the greatly changed absolute and relative retention times the more polar pyridine and benzaldehyde are affected most by the move to a more polar stationary phase. 

All these elastomers, especially poly(ethylene- (9-butylene) and poly(ethylene- (9-propylene), are nonpolar. The corresponding block copolymers can thus be compounded with hydrocarbon-based extending oils, but do not have much oil resistance. Conversely, block copolymers with polar polyester or polyether elastomer segments have Htde affinity for such hydrocarbon oils and so have better oil resistance. 

Schmalz H., Abetz V., Lange R., and Soliman, M. New thermoplastic elastomers by incorporation of nonpolar soft segments in pbt-based copolyesters, Macromolecules, 34, 775, 2001. 

Speckhard T.A. and Cooper S.L., Ultimate tensile properties of segmented pol3furethane elastomers Factors leading to reduced properties for pol3mrethane based on nonpolar soft segments. Rubber Chem. TechnoL, 59, 405, 1986. 

Today it is claimed that the surface fluorination of polymers using F2 gas mixtures enhances a wide range of properties, e.g., low permeability to nonpolar liquids4 improved permselectivity,5-6 excellent wettability and adhesion,7 low friction coefficient (especially for elastomers),8 and chemical inertness.9 Obviously, these properties depend on the chemical composition ofthe fluorinated layer, which in turn is determined by the chemical structure ofthe base polymer, the composition of the F2 gas mixture, and the fluorination parameters. 

Nonpolar molecules such as heptane and PE are attracted to each other by weak London or dispersion forces that result from induced dipole-dipole interactions. The temporary or transient dipoles are due to instantaneous fluctuations in the electron cloud density. The energy range of these forces is fairly constant and about 8 kJ/mol. This force is independent of temperature and is the major force between chains in largely nonpolar polymers, for example, those in classical elastomers and soft plastics such as PE. 

Tables 13.4 and 13.5 contain a summary of typical stability values for a number of polymers and elastomers against typical chemical agents. As expected, condensation polymers generally exhibit good stability to nonpolar liquids while they are generally only (relatively) moderately or unstable toward polar agents and acids and bases. This is because of the polarity of the connective condensation linkages within the polymer backbone. By comparison, vinyl type of polymers exhibit moderate to good stability toward both polar and...

A fluoroelastomer manufactured by Du Pont called Kalrez, has mechanical properties and resistance to oxidants which are similar to those of Viton. In contrast with Viton, Kalrez has good resistance to polar molecules such as amines, ethers, ketones, and esters. Kalrez is unique among the elastomers in its tolerance to both polar and nonpolar solvents. The cost of O-rings made from Kalrez is very high, but for certain critical applications this cost can be justified because of the outstanding range of solvent tolerance. 

Several general requirements must be met for a polymer to be elastic—that is, to stretch under the application of force but return to its original shape when the force is released. The polymer should be predominantly amorphous so that its Tg is below room temperature. The individual molecules of an amorphous polymer are not in fully extended, anti conformations instead, they have random, coiled conformations. When a force that pulls on opposite ends of the molecules is applied, the molecules assume an anti conformation about more bonds and thus they become longer that is, they stretch. Although the stretching tends to arrange the molecules in extended, zigzag conformations that are favorable for crystallization, the overall shapes of the elastomer molecules are such that crystallization does not readily occur. Furthermore, most elastomers Eire nonpolar, so only weak attractive forces exist between chains. Therefore, when the force is removed, the molecules tend to return to their initial random conformations because these random shapes are favored by entropy (disorder). 

Ionomers are copolymers in which a small portion of the repeat units have ionic pendant groups on usually a nonpolar backbone. The ionic groups tend to separate themselves into domains similar to the polystyrene segments in the SBS rubber because they are insoluble in the nonpolar polymer chains. Therefore, these ionic clusters serve as cross-links up to temperatures where they tend to disassociate. Most commercial grades of ionic elastomers are based on ethylene and propylene monomers. 

A variation of the sequential monomer addition technique described in Section 9.2.6(i) is used to make styrene-diene-styrene iriblock thermoplastic rubbers. Styrene is polymerized first, using butyl lithium initiator in a nonpolar solvent. Then, a mixture of styrene and the diene is added to the living polystyryl macroanion. The diene will polymerize first, because styrene anions initiate diene polymerization much faster than the reverse process. After the diene monomer is consumed, polystyrene forms the third block. The combination of Li initiation and a nonpolar solvent produces a high cis-1,4 content in the central polydiene block, as required for thermoplastic elastomer behavior. 

Early fundamental studies of gas transport in polymers were almost entirely confined to hydrocarbon materials above their glass transition temperatures. The essentially nonpolar structures of the elastomers led to a number of reasonably successful attempts to correlate gas transport parameters with various physical characteristics of the gases and the polymers. These have been summarized and discussed in a number of papers In addition to studies with hydrocarbon elastomers a few studies of other amorphous polymers above their glass transition temperatures have dealt with polyvinyl acetate silicones and fluorocarbon polymers Recent studies have also dealt with poly(methyl aciylate) poly-(vinyl methyl ether) and poly(vinyl methyl ketone) With these more... 

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