Table IRubber 100, sulfur 7.5, zinc oxide 5, cured at 115°C. with various proportions of piperidinium pentamethylenedithiocarbamate ,-(a) 0.25 part P. P.-s ^-(6) 0.33 part P. P.-> ) 0.50 part P. P.-.,-(d) 0.66 part P. P.-.
ALTHOUGH much has been done on accelerators, in such a vast field there is always room for additional work, particularly if, as Whitby predicts, ultra-accelerators will be the main class used in course of time. This investigation was started with the following points in view: (1) to prepare several thiuramdisulfides and related compounds; (2) to note their relative behavior when used as accelerators; and, if possible, deduce some explanation for the mode of action of the class or for the behavior of some particular compound, or gain additional evidence for any existing theories. The relative inactivity of thiurams derived from primary amines has been mentioned but no explanation published. Evidence is presented to show that decomposition of the compound into hydrogen sulfide and a mustard oil takes place so quickly at the temperatures used that there is not time for the generation of much active accelerator. It was found that tetramethylthiurammonosulfide plus sulfur equivalent to the difference in sulfur content between it and the disulfide gave a cure in a rubber-zinc oxide mix practically equivalent to that given by the disulfide alone. This indicates that zinc dimethyldithiocarbamate may be the active accelerating agent.
AS INTIMATED in a previous report of the Committee presented at the St. Louis meeting in April, 1928, a temporary procedure was adopted in order to ascertain whether or not the five laboratories represented on the Committee could obtain reasonably comparable stress-strain relationships using the same batch of rubber. A complete report is appended in which the procedure is outlined and the results of each laboratory are given in considerable detail. After careful deliberations the Committee has concluded that the testing of raw rubber is not in a very satisfactory state. It therefore makes the following recommendations: (1) The testing of raw rubber should be made the subject of thorough investigations. (2) The work should be undertaken by a Physical Testing Committee, preferably under the jurisdiction of the Rubber Division of the American Chemical Society.
From the results obtained it is concluded that the boiling water hydrolyzes some of the esters in the acetone extract. In the case of direct extraction of the rubber with water, hydrolysis of esters must take place in the rubber, forming water-soluble acids; or else, although the esters are practically insoluble in the water, it extracts a little ester each time the extraction thimble empties (Bailey-Walker apparatus), carrying the ester down to be hydrolyzed in the boiling water below. Temperature and time of heating play a prominent part. The temperature must be close to the boiling point and the acidity increases progressively with time of heating. The fact that temperature is so important in producing this effect accounts for the erratic values obtained by direct water extraction. On the electric hot plates used water did not always boil steadily, and that contained in the thimble might vary considerably in temperature, much of the time not being hot enough to hydrolyze or extract any ester. Therefore, to avoid any difficulty in the determination of water-soluble acids in rubber, extract first with acetone, then digest this extract with water on a boiling water bath until no increase in acidity of the water extract is obtained. This is a part of the procedure developed at the Netherlands Government Institute by Van Rossem and Dekker.
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