Crystallization of rubber

In principle any of the tests could be used to study crystallization of rubbers by conditioning the test pieces for much longer times than normal, but in practice the favored method is change in hardness, one reason being that unvulcanized materials can be tested. Because crystallization is more rapid in the strained state, a particular type of compression set test has also been standardized for rubbers. 

The ability of a polymer to crystallize is determined by temperature and this has to be between Tg and T. The rate of crystallization is also governed by temperature. This can be illustrated by the rate of crystallization of natural rubber in which the maximum rate of crystallization is at — 30 °C, thus explaining the crystallization of rubber bands in freezers (Figure 1.6). The size of the crystals (i.e. spherulite size) can be altered by nucleation. This is an important aspect in fibre science. 

The strain or the stretching crystallization of rubbers is another important property to be considered in sealing applications. The stretching crystallization of various rubbers is given Table 5.4. 

M.E Bukhina in Crystallization of Rubbers, Khimiya, Moscow, Russia, 1973, 233. 

There arc other criticisms of the Avrami method here. Negahban points out that modeling of crystallization kinetics using Avrami-type equations is incompatible with the entropy production inequahty (i.e., Clausius Duhem inequahty) that is basic to die termination of crystallization in real situations. Much of his work has concentrated on the crystallization of rubber (hterature results) with all its ancillary effects/property-wise material functions that arc amenable to simulation, hr the same marmer, the kinetic theory of crystallization, based upon concepts of molecular chain-folding and it s related parameters,at the same time recognizing some of the shortcomings involved. An attempt has been made to keep... 

White crystals, m.p. 114" C. Manufactured by reacting aniline with excess ethanoic acid or ethanoic anhydride. Chief use is in the manufacture of dye intermediates such as p-nitro-acetanilide, p-nitroaniline and p-phenylene-diamine, in the manufacture of rubber, and as a peroxide stabilizer. 

Fit securely to the lower end of the condenser (as a receiver) a Buchner flask, the side-tube carrying a piece of rubber tubing which falls well below the level of the bench. Steam-distil the ethereal mixture for about 30 minutes discard the distillate, which contains the ether, possibly a trace of unchanged ethyl benzoate, and also any biphenyl, CeHs CgHs, which has been formed. The residue in the flask contains the triphenyl carbinol, which solidifies when the liquid is cooled. Filter this residual product at the pump, wash the triphenyl-carbinol thoroughly with water, drain, and then dry by pressing between several layers of thick drying-paper. Yield of crude dry product, 8 g. The triphenyl-carbinol can be recrystallised from methylated spirit (yield, 6 g.), or, if quite dry, from benzene, and so obtained as colourless crystals, m.p. 162. ... 

Insoluble Sulfur. In natural mbber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of mbber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

The stored strain energy can also be determined for the general case of multiaxial stresses [1] and lattices of varying crystal structure and anisotropy. The latter could be important at interfaces where mode mixing can occur, or for fracture of rubber, where f/ is a function of the three stretch rations 1], A2 and A3, for example, via the Mooney-Rivlin equation, or suitable finite deformation strain energy functional. 

Many authors studying the formation of ECC from melts and solutions suggested that preliminary unfolding and extension of macromolecules occurs. Keller and Maehin25 have shown that in all known cases (including such extreme variants as the crystallization of natural rubber under extension and a polyethylene melt under flow) the same initial process of linear nucleation occurs and fibrillar structures is formed by the macromolecu-lar chains oriented parallel to the fibrillar axes27. ... 

Livigni, R. A. Hargis, I. G. Aggarwal, S. L. "Structure and Properties of Rubbers for Tires and New Developments for Crystallizing Butadiene Rubbers" Paper presented at Int l. Rubber Conf., Kiev, U.S.S.R., 1978. 

The principles were refined by Meyer in a second paper (70). In it he proposed that the micelles occurred at regular intervals. He also included an explanation of the elasticity of rubber based on the assumption that the molecular chains tended to roll together in knots in unstretched rubber, but line up when stretched. This explanation was especially elucidating since it agreed well with Katz s discovery (53) that amorphous rubber crystallizes when stretched. 

When Herman Mark first evaluated the crystal structure of rubber (with E. A. Hauser) and cellulose (with J. R. Katz) in 1924 and 1925, it was generally accepted that these materials were low molecular weight or monomeric. The unusual properties of these substances, now known to be related to high molecular weight, were then attributed to aggolomeration or "association" of the low molecular weight precursors. A common explanation for the associations were secondary forces such as Johannes Thiele s partial valences. 

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