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INDUSTRIAL A N D ENGINEERING CHEMISTRY
marked a difference between oils in their ability to swell the rubber as in their property of breaking down the rubber itself to form a clear solution. Hence, the rate of disintegration gives us more nearly the measure of relative depolymerization effects of the various softeners. By the swelling method it was also found that accelerators materially hasten the swelling of the rubber and that the addition of ?both accelerator and sulfur causes a further increase over either used alone. The retarding effect of four reinforcing pigments-zinc oxide, clay, glue, and carbon black-in a stock was also investigated, with the result that it was found that equal volumes had practically the same effect when the coefficient was approximately the same. Fine pigments apparently retard the swelling of the rubber.
EFFECTOF MIXTURES As oils are frequently used as mixtures, the effect of mixed oils was tried by the disintegrating method. I n general, if both oils had even slight depolymerizing action the time required was nearly an average of the time taken by each of the ingredients separately. For instance, vaseline and cottonseed oil required 4 hrs. 30 min. and 34 hrs., respectively, to dissolve the rubber, while a 50/50 mixture of the two required approximately 20 hrs. On the other hand, oils having no action on the rubber, such as linseed or castor, almost totally retarded the action of other oils with which they were mixed. A mixture of castor oil and paraffin was heated for 40 hrs. without causing more than a slight swelling of the rubber. Another peculiar result was that nothing was found which would even slightly retard a sulfur and terpene mixture. A mixture of 70 parts castor oil, 30 parts pine oil, and 4 g. sulfur required only 2 hrs. to secure a clear solution. In previous work on rubber soles it was observed that when ground, cured scrap had been incorporated in the stock, ;this scrap was extremely hard to dissolve. This was thought t o be due to the higher state of cure of this rubber as it had gone through two vulcanizations. To test this theory a number of tests were run by the solvating method. In every
Vol. 15, No. 3
case, except in a long overcure on a low sulfur stock after all sulfur had been combined, this theory proved to be true. However, in the case of the overcured stocks, an increase in the amount dissolved in the rubber was observed. This increase took place shortly after a weakening of the stock was observed, judging from the stress-strain curves. At this point the stock had evidently passed the equilibrium point where the rate of depolymerization was equal to the rate of polymerization, and was then suffering a depolymerization alone because practically all the sulfur had been combined. The state of aggregation was then becoming more nearly that of the lower cures. The essential difference between the aggregates of low cure and of this overcure was that more sulfur is combined with each aggregate combined in the overcure. An interesting conclusion can be made from this last experiment-the higher the state of cure the slower the rate. of depolymerization becomes. This means that the greatest depolymerizing action takes place in the raw stock or a t the lower states of cure. Whether or not this depolymerization is beneficial remains to be seen. From physical tests alone the evidence is strongly against undue depolymerization, as the more rapid the cure the higher the tensile which can be obtained. Thus, the thiocarbamates give us extraordinary tensiles, such as 3000 lbs. (with a 5-min. cure) per sq. in., in nearly pure gum stocks, while a suliurrubber mix requiring 3 hrs. to obtain an approximateIy optimum cure will give us only 1500 lbs. per sq. in. I n the first case less than 11/2 per cent of sulfur combines with the rubber, while in the Iatter more than 3 per cent combines. Howevei, as the aging is of supreme importance it remains to be proved whether or not the property is benefited. It remains to be proved that we must strive to keep depolymerization down to a minimum or plan to secure a certain amount in order to obtain good aging. The authors maintain that until depolymerization is thoroughly understood and measured and until the effect on aging is determined, very little progress can be made toward making rubber compounding a science instead of the meaningless mass of data which it is a t present. Tests are now being made with little or no consideration of the greatest of variables-depolymerization.
Improvement in Chemical Exports Considering the low index figure for prices in the chemical group, the export trade of the United States in chemicals and allied products for the eleven months, January to November, 1922, is most encouraging. For the first time since the postwar depression began there is an increase over the corresponding total of the preceding year. Inasmuch as the aggregate exports of all classes of American merchandise continued to fall below those of a year ago, this recovery in the foreign trade in chemicals, although small, has unusual significance. American chemicals and allied products exported in the eleven months of 1922 had a total value of $97,215,547, compared with $96,267,165 in the corresponding period cf 1921, a gain .of 1 per cent. (If naval stores, gums, and resins are included, an increase of 7 per cent is shown ) Deductions based on such a varying element as price are of necessity unreliable. Were it possible to procure the actual weight of all American chemicals shipped to foreign countries (some classes of which are recorded by value only), it is not improbable that the figures would show, not the falling off that values have a t times indicated, but a continued and healthy growth in this comparatively young industry. Although there still exists a loss of 4 per cent in the value of genera1 chemicals sent to foreign countries, gains occurred in medicinal and pharmaceutical preparations (13 per cent), fertilizers and fertilizer materials (4 per cent in quantity and 3 per cent in value), explosives (90 per cent in quantity and 46
per cent in value), and perfumery, cosmetics, and toilet preparations (33 per centuin value). Among the heavy chemicals, total foreign shipments of which fell in value from $49,625,532 in 1921.to $47,668,525 in 1922, the largest increases as t o quantity were in borax, 296 per cent, which rose from 3,658,059 lbs. (value $246,658) in 1921 t o 14,501,189 lbs. (value $755,612) in 1922; caustic soda, 238 per cent-from 40,460,561 lbs. ($1,627,334) to 136,900,479 lbs. ($4,934,944); chloride of lime, or bleaching powder, 149 per cent-from 14,892,143lbs. ($409,106) to 36,972,472 lbs. ($610,684);potassium chlorate, 75 per cent-from 297,002 Ibs. ($40,961) to 522,416 lbs. ($46,876); acetate of lime, 61 per cent-from 15,750,798 lbs. ($345,379) to 25,385,756 lbs. ($522,700); and copper sulfate, 54 per cent-from 3,153,278 lbs. ($198,260) t o 4,858,331 lbs. ($234,772). Some of the other American heavy-chemical sales abroad which have shown marked improvement during this period were calcium carbide, glycerol, sodium silicate, sodium bicarbonate, washing powder, and crude tar. On the other hand, benzene decreased only 4 per cent in quantity but 19 per cent in value-from 88,277,375 Ibs. ($2,888,126) in 1921 to 64,630,735 lbs. ($2,353!136) in 1922; formaldehyde, from $312,407 t o $195,961; sulfuric acid, from 12,155,349 Ibs. ($306,068) to 11,938,403 lbs. ($188,057); and soda ash, from 32,699,844 lbs. ($800,378) to 26,617,260 lbs. ($615,733). Salsoda increased in quantity but fell in value.
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