Gerard Kraus

Interactions of Elastomers and Reinforcing Fillers

Abstract Use of reinforcing fillers in elastomers ranks as one of the two most important processes in rubber technology—only vulcanization can be considered to surpass it in its universal applicati...

Degree of Cure in Filler-Reinforced Vulcanizates by the Swelling Method

Abstract The Flory-Rehner theory of swelling allows an estimate of the number of crosslinks in gum vulcanizates. In reinforced rubbers the apparent degree of crosslinking is raised by some fillers, particularly carbon blacks. It has been shown experimentally that, for vulcanizates of a given polymer at a fixed filler loading and type, most physical properties are, in the first approximation, simple functions of the apparent number of network chains per unit volume, regardless of the possible origin of the linkages. Aside from controlling the number of apparent linkages, sulfur and accelerator levels as well as cure time have relatively minor, secondary effects on vulcanizate properties. Comparison of swelling data of gum and black stocks shows that the apparent contribution of the carbon black to crosslinking increases with the level of cure. Softeners and extender oils cause a reduction in the number of linkages, presumably by reducing the efficiency of sulfur utilization. It has been demonstrated that t...

Molecular Weight Effects in Adsorption of Rubbers on Carbon Black

Abstract The molecular weight dependence of the adsorption of polybutadiene on carbon black from a poor solvent, n-heptane, and bulk, i.e., the phenomenon of “bound rubber”, was investigated. For narrow distribution polymers the adsorption is proportional to Mn, where n = 0.14 for adsorption from n-heptane solution; n = 0.5 for adsorption from bulk. Anomalously low solution adsorption was observed for polymers of very high molecular weight (> 500,000). This is ascribed to a sieve effect by aggregates of carbon black particles which cannot be penetrated by the large molecular coils. In high structure blacks, which pack more loosely, and in large particle blacks, which form larger interstices between particles, onset of anomalous adsorption is shifted toward higher molecular weights.

The Viscosity of Carboxy-Terminated Polybutadienes

Abstract Newtonian viscosities have been determined for carboxy-terminated polybutadienes varying in microstructure, molecular weight and degree of carboxy-functionality. The latter was varied by total or partial methylation of initially di- or mono-functional polymers. The viscosity of non-functional (methylated) polybutadienes is shown to be proportional to the 3.4 power of the weight average chain length, irrespective of microstructure, provided the comparison is made at equal (T−Tg). The temperature dependence of viscosity is well represented by the Vogel equation, with constants given by Berry and Fox for polybutadiene of 8% vinyl unsaturation, again independent of microstructure. The presence of carboxy end groups increases the viscosity as a result of an association equilibrium with raises the effective molecular weight of the polymer. The absolute magnitude of the temperature coefficient of viscosity increases with carboxy-functionality, as required by the postulated association equilibrium. Theor...

Odd Electrons in Rubber Reinforcing Carbon Blacks

Abstract The number of unpaired electrons in several rubber-reinforcing blacks has been determined by quantitative electron spin resonance assay. The odd electron concentrations are of the order of 1019 to 1020 spins/gram. These concentrations are consistent with the negative (diamagnetic) net magnetic susceptibility of the blacks. Oxygen and other paramagnetic substances, even in extremely minute quantities, exert a powerful influence on the electron spin resonance observed. Their effect is to broaden the resonance line and this reduces considerably the signal intensity. In extreme cases the intensity may be reduced to the noise level ; in less severe instances the broadening may lead to erroneously low spin assays. Carbon blacks differ in their susceptibility toward line broadening effects. Evidence is presented for a correlation between the odd electron concentration of carbon blacks and the modulus they impart to rubber, suggesting a combination reaction between the carbon black radicals and polymeric...

Chlorination of Natural and Synthetic Polyisoprenes

Abstract The extent of additive and substitutive chlorination was determined for emulsion polyisoprene, Hevea and gutta-percha chlorinated to different chlorine contents in darkness and in a nitrogen atmosphere. The maximum amount of additive chlorine that can be introduced into the polymer is 1.21 atoms of chlorine per isoprene unit for synthetic emulsion polyisoprene, 1.14 for Hevea. The chlorination involves a cyclization reaction by pairs, leading to isolation of reactive units. The loss in unsaturation due to cyclization has been calculated statistically by the method of Flory and Wall, and has been shown to be consistent with the total amount of additive chlorine in the fully chlorinated product. Cyclization takes place in the early stages of the chlorination simultaneously with the initial substitution. The theoretical limit of cyclization calculated statistically is 86.5 per cent of all isoprene units cyclized for natural rubber and 72 per cent for polyisoprene containing 10 per cent vinyl side ch...

Behavior ofcis-Polybutadiene during Mastication

Abstract Cold mastication of natural rubber in presence of oxygen leads to scission by a shear mechanism with subsequent stabilization of the free radicals by oxygen, while at high temperatures oxidative scission becomes the dominant mechanism. Studies of the behavior of cis-polybutadiene reveal that similar reactions are effective, but occur at much reduced rates. Shear degradation is particularly difficult to achieve in typical cis-polybutadienes. This is related to the molecular weight distribution of these polymers which usually lack the very high molecular species most subject to shear-induced degradation, in agreement with the theory of F. Bueche. It is shown that when such a high molecular weight fraction is supplied, shear breakdown may be realized. All cis-polybutadienes undergo oxidative breakdown at high temperatures, the scission rate becoming appreciable above 140° C. A number of oxidation catalysts have been found which markedly increase the scission rate without leading to a correspondingly...