Rubber reinforcement of silicas with different surface alkylation have been evaluated by studies of NH3-modified swelling, swelling volume fraction, stress-strain tests, and scanning electron microscopy. Effects of silica alkylation on filler dispersion has been explained on the basis of surface energy and acid-base interactions. Bound rubber reductions have been explained according to acid-base interactions and/or physical entanglements. Hysteresis losses have been interpreted based on the reductions of polymer-filler and filler-filler interactions. The results show that the extent of reinforcement in NR and SBR vulcanizates is reduced when the silicas are alkylated, especially in the case of the hexadecyl group. However, in NBR vulcanizates the alkylation seems to have less influence on the reinforcement characteristics of the silica.
The physical characteristics of silica pigments covering a wide range of properties were determined and related by regression analysis to the static and dynamic mechanical properties of SBR vulcanizates reinforced with these silicas. Three morphological parameters were found to determine the vulcanizates' characteristics: the external surface area by CTAB adsorption, the structural index, and the DBP absorption number. The adherence between silica and elastomer was studied by means of swelling in solvents, by determination of the volume increase on deformation, and by scanning and transmission electron micrography. It was found that dewetting, the formation of vacuoles on deformation, is a most important factor, limiting reinforcement by silica at loadings above 40 phr. Equally, at such loadings the pH of the silica has an important influence : the higher the acidity, the less the adherence between pigment particle and elastomer. Silane coupling agents not only increase adherence, but also improve the degree of dispersion of the silica, which is frequently characterized by the presence of large, partly unwetted particle clumps under conventional conditions of incorporation, thus severely limiting the strength properties of the vulcanizates. The curing system employed and the accelerators used are also of paramount significance for adherence.
Generally, to determine the fibre-matrix interfacial properties in fibre reinforced plastics, it is necessary to know the tensile strength of the fibre at very short lengths, for which direct measurements are not possible. Accordingly, in this study, the determination of the tensile strength of high strength carbon fibres and their gauge length dependence are analysed by means of the Weibull model. The influence of the estimator chosen and of the sample size on the calculated value of the tensile strength of the fibre are first determined. Secondly, the accuracy of the three-and the two-parameter Weibull distributions is examined. Finally, it is shown that the most appropriate extrapolation at short length is performed by means of a linear logarithmic dependence on gauge length of the tensile strength. This method seems to be valid for untreated as well as for surface-treated high strength carbon fibres.
The low electrical resistance of vulcanizates loaded with conductive carbon blacks in a given particle size range is cause by the low density of the particles, high structure, or both. In addition, the smaller the particle size of the black, the lower the resistivity. The same factors also cause high modulus and, in liquid dispersions, high viscosity. It is shown that acetylene black owes its lower resistivity in vulcanizates to its high structure. Vulcan XC 72 owes its even lower resistivity to both high structure and low particle density, while EC Black owes its lowest resisitivity to an extremely low particle density caused by the presence of a large number of hollow-shell particles, as indicated by electron micrography. It was shown that the resistivity of vulcanizates loaded with EC Black is practically unaffected by deformation up to 150%, in contrast to other conductive blacks at comparable resistivities. It has been observed that increasing the degree of loading leads to a lower limit of resistivity, about equal for all of the blacks. The various phenomena could be explained by the concept that the resistivity of carbon black loaded vulcanizates resides in the gaps between particles or aggregates and is determined by a process of electron tunneling.
SYNOPSIST h e interactions between rubbers and silicas were studied by means of bound rubber determination, vulcanizate swelling procedure, and tensile retraction measurement. The mobilities of rubber chain segments in mixture with silicas were investigated by thermomechanical and dynamic mechanical analyzers. T h e results show that when the untreated silica was incorporated into rubbers, the strong interaction of rubber with the silica apparently constrains the segmental motions of the rubber molecules, resulting in the decrease in the dynamic loss peak and the thermal expansion coefficient, and the increase in the glass transition temperature. The alkylations on silica surfaces weaken t h e interfacial interaction between rubbers and silicas, and the mobilities of the rubber molecules become less restricted. Furthermore, proton spin-spin relaxation time (T2) measurements by the pulsed NMR on the insoluble bound rubber, indicate that there are two relaxing regions: a n immobile (or a tight) region and a relatively free (or a loose) one. T h e types of the rubbers and the activities of the silica surfaces have influences on the relaxation process of the two bound rubber components. 0
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