X-Ray Study of Reactions Involving Accelerators. Orientation of Crystalline Phases on Stretching of Rubber Stocks

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Abstract
About a year ago, in the course of a systematic survey of the x-ray diffraction patterns of a number of routine rubber samples with different accelerators present, certain similarities were noted in the diffraction effects from the accelerators in these stocks. Close examination of these effects showed that they were identical. The stocks contained the accelerators tetramethylthiuram disulfide, tetramethylthiuram monosulfide and zinc dimethyldithiocarbamate, which are chemically similar (Table I); the first is the disulfide, the second, the monosulfide, and the third, the zinc salt. Thus, the diffraction patterns indicate that, on curing, the disulfide and the monosulfide in the presence of sulfur react with zinc oxide to form the zinc salt. This is not a new concept. The zinc salts have been extracted with solvents from stocks similar to the above, and it has been supposed that such reactions occur. However, the extraction method leaves some doubt as to whether the salt was originally present in the rubber matrix or whether it was formed in the solvent on extraction. Every crystalline material has its own characteristic x-ray diffraction pattern, which is different from that of every other crystalline material, just as the fingerprints of each person are different from those of every other person. Thus, with x-ray “fingerprint”patterns it is possible to identify positively the presence of these salts in the rubber matrix and show that they must have been formed in the rubber mix on curing. Further, it was noted that this crystalline salt is strongly oriented in the rubber matrix on stretching. It seemed advisable, then, to investigate other similar reactions by the x-ray method and to investigate the significance of the strong orientation of these reaction products in the rubber matrix.

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