Mbber crosslinked

In sulphur vulcanisation of mbber, crosslinking is considered to be effected by sulphur atoms, which are therefore termed sulphur bridges. See Vulcanisation. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

A much more heavily crosslinked material can be obtained by increasing the amount of sulfur in the mixture, so that it represents about a third of the mass of the product. Heating such a mixture of raw mbber and sulfur at 150 °C until reaction is complete gives a hard, thermoset material that is not at all elastic. This material is called ebonite and is used to make car battery cases. 

An integrally monlded composite article comprised of (I) a nonfoam layer formed from a thermoplastic elastomer powder composition (A) and (II) a foam layer formed from a foamable composition comprised of (i) (B) a thermoplastic synthetic resin powder, and (C) a heat decomposable foaming agent and (D) a liquid coating agent, wherein the thermoplastic elastomer powder (A) is comprised of a composition of an ethylene-alpha-olefm copolymer mbber and a polyolefin resin or thermoplastic elastomer powder comprised of a partially crosslinked composition of an ethylene-alpha-olefm copolymer mbber and a polyolefin resin, the thermoplastic elastomer powder having a complex dynamic viscosity at 250 deg C and a freqnency of 1 radian/sec of not more than 1.5x1,000,000 poise and having a Newtonian viscosity index n, calculated by a specific formula. 

Automobile tires are the largest-volume mbber material manufactured, and they are formed from natural or synthetic monomers with sulfur compounds added to crosslink the polymer within the tire mold. Tires also contain large amounts (typically 35%) of carbon black particles, which are made by hydrocarbon combustion in excess fuel, as we have considered previously in Chapters 9 and 10. 

The modem theory of mbber-like elasticity theory suggests that there are two types of elastically active network chains which contribute to the overall equilibrium rubbery modulus, G (1) chains attached to the network by chemical crosslinks, G and (2) chains attached by physical crosslinks or entangelements, G . That is,... 

This chapter contains information on additives such as crosslinking agents, processing aids, retarders and stearates. While these compounds may not be as widely used as plasticizers or antioxidants, they play an important role in plastic and mbber processing. 

Make a Mooney-Rivlin plot and determine the constants C and 2- Obtain an estimate of the degree of crosslinking of the mbber band. (Density of mbber band = 1.09 g/cm. ) Answer ... 

Elastin, a stmctural protein with mbber-like elastic properties. It is the main component of the elastic yellow connective tissue occurring, e.g., in the lungs and aorta. The amount of elastin is rather low in the inelastic white connective tissue of tendons. Elastin consists of 850-870 aa with a high content of Gly (27%), Ala (23%), Val (17%), and Pro (12%). It forms a three-dimensional network of fibers crosslinked by desmosine, lysinonor-leucine, and isodesmosine. It has been reported that elastin has an unanticipated regulatory function during arterial development, controlling the proliferation of smooth muscle and stabilizing arterial structure [L. Robert, W. Hornebeck (Eds.), Elastin and Elastases, Volume 1, CRC Press, Boca Raton, EL, 1989 D. R. Eyre et al, Annu. Rev. Biochem. 1984, 53, 717 D. Y. Li et al., Nature 1998, 393, 276]. 

At 70 °C, both systems are above Tg and behave hke rubbery" materials. No permanent strain exists after fracture. At temperatures above Tg, viscous flow is hindered due to crosslinking. However, chain segments remain flexible and the adhesive can withstand high strains. Although these strains are not as high as in a mbber, they can no longer be considered small . Hence, a hyperelastic material model must be used. The second system (Fig. 33.2, right) already shows hyperelastic properties at 23 °C due to the low strain rate. 

Peroxides are vulcanizing agents for elastomers, which contain no sites for attack by other types of vulcanizing agents. They are useful for ethylene-propylene rubber (EPR), ethylene-vinyl acetate copolymers (EAM), certain millable urethane rubbers, and silicone mbbers. They generally are not useful for vulcanizing butyl rubber (poly[isobutylene-co-isoprene]) because of a tendency toward chain scission, rather than crosslinking, when the polymer is subjected to the action of peroxide. 

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