April 1, 1921 



THE INDIA RUBBER WORLD 



495 



What the Rubber Chemists Are Doing 



ACTION OF HEAT AND LIGHT ON VULCANIZED RUBBER' 



AMONG THE DESTRUCTIVE ACEN-ciES Causing deterioration of vul- 

 caiuzcd rubber are heat, light, air and oils. Of these, heat 

 and light arc probably the most serious and are so frequently 

 spoken of together that many have come to believe that their action 

 is essentially the same, whereas this is not so and the favorite ex- 

 planations, "oxidation" and "depolymerization," fail as applied 

 alike to the deteriorating action of either heat or light. 



The reaction between unvulcanized rubber and sulphur is gen- 

 erally recognized to be a chemical one involving the addition of 

 sulphur at the double bond; therefore the speed of the reaction 

 varies with a change in temperature. The familiar phenomena 

 of "burning on the mill" and hardening or "burning" of com- 

 pounded rubber during storage, especially when piled up while 

 yet quite warm from milling, illustrate this point. In the presence 

 of certain accelerators, such as the dithiocarbamates, partial vul- 

 canization may occur at room temperatures in 24 to 48 hours. 

 These well-known facts show that it is not necessary to reach 

 the ordinary vulcanizing range, 275 to 300 degrees F., to effect 

 the union of rubber and sulphur. 



It is apparent that vulcanization is the chemical addition of 

 sulphur to rubber, the speed of the reaction depending upon the 

 temperature, and the nature of the catalyst present. When only 

 rubber and sulphur are present, the speed of reaction at ordinary 

 temperatures is practically zero, and with a few exceptions this 

 is also true with most accelerators. Vulcanization is therefore a 

 cycle of three steps; (1) zero speed of reaction in the raw 

 stocks; (2) high speed of reaction during vulcanization; (3) a 

 return to zero speed of reaction after vulcanization. To effect 

 proper vulcanization, one must find the temperature at which the 

 quickest cure can be obtained consistent with safe manufacturing 

 practice, and slop the process when the desired point is reached. 

 In curing molded articles, this is done by quick removal from the 

 molds ; in such articles as tires, by drenching them with cold 

 water. 



After properly vulcanized rubber products have left the manu- 

 facturers' hands, there is danger that they may be ruined by ex- 

 posure to elevated temperatures. .As an example, the custom of 

 carrying uncovered spare tires on the rear of automobiles is poor 

 practice, because the heat from the exhaust strikes the lower 

 part of such tires, raising the temperature locally and inducing 

 after-vulcanization. 



The principal physical change in vulcanized rubber, when ex- 

 posed to sunlight, i^ "cracking" or "checking," which lowers the 

 tensile property. This action is largely dependent on the com- 

 position of the mixing. Certain unpublished tests made by the 

 author throw light on this point, .•\bout 40 samples of inner tubes 

 were exposed to direct sunlight. Of these, 25 were red and the 

 balance gray. With two or three exceptions, all of the red tubes 

 showed very rapid checking, whereas the gray tubes show-ed a 

 much higher average degree of resistance. This property is not 

 inherent in the color, for later one of the red tubes which had 

 shown a high resistance to checking was tested against a new 

 sample of gray tube, an<l the latter showed serious checking after 

 only four to six hours' exposure, whereas the red tube was in 

 good condition after several weeks' exposure. These tests were 

 performed outdoors in winter, and the temperature seldom rose 

 over 50 degrees F., so it may be assumed that the changes were 

 due to sunlight only. If the action of heat and light were the 

 same, the chemical and physical properties would vary alike, 

 differences, if any, being in amount and not in kind. 



Heat produces an after-vulcanization, which lowers the tensile 

 properties. Sunlight also reduces the tensile properties, so that 



we must look farther for differences in their behavior. In 

 studying the chemical deterioration, we may use the method of 

 change in solubility of the rubber substance in such solvents as 

 acetone, alcohol, chloroform, etc. In the study of the effect of 

 heat and light on balloon fabrics", there was little increase in the 

 amount of acetone-soluble material after heating in the dark for 

 28 days at 70 degrees C, although the physical appearance of the 

 samples showed that a marked change had occurred. Some of the 

 fabrics tested were so hard as to crack when bent. The same 

 fabrics, exposed to weathering, showed an increase in acetone 

 extract as the rubber substance decomposed. Some fabrics in- 

 creased from an initial extract of 2 to 20 degrees, after IS to 30 

 days' exposure, and all showed an increase in extract in time. 

 The same phenomena have been observed in other tests on bal- 

 loon fabrics. As long as the rubber retains its original tensile 

 properties, there is very little change in the acetone extract, but 

 after a certain point, which represents the end of the useful life 

 of the material, a sharp break occurs in the solution curve, marked 

 by a rapid increase in the percentage of acetone extract. The 

 same behavior was noted in the natural aging of rubber and 

 reported to the Rubber Section of the American Chemical Society 

 at its meeting in New York, September, 1916'. The samples un- 

 der observation were rubber bands. A very rapid increase in 

 acetone extract was noted after the bands began to be hard and 

 brittle. These samples were stored in a cool, dark place, at a 

 temperature not exceeding 25 degrees C. 



Probably this increase in acetone extract is a true case of 

 oxidation, the reaction being accelerated by sunlight. There is 

 evidence that the speed of this reaction is increased tremendously 

 by the presence of any appreciable quantity of oxidized rubber. 

 It has been noted repeatedly that there is little change in solubility 

 during the early stages of deterioration, but when begun the rate 

 shows a marked increase. This is true irrespective of the time 

 required for tiie sample to go through the so-called "early stage 

 of deterioration " Rubber bands containing reclaimed rubber 

 showed this break in the curve at a much earlier point than the 

 bands which contained only new rubber. 



Another important difference between the action of heat and 

 sunlight is that heat acts throughout the entire mass while sun- 

 light exerts, at first, essentially a surface change, although this 

 rapidly travels to the interior. In the deterioration due to sun- 

 light, the increase in the percentage of acetone extract is in- 

 fluenced by the thickness of the test pieces. Very thin ones such 

 as balloon fabrics show much more rapid increase than thicker 

 test pieces. The difference in the action of heat on thin and 

 thick test pieces is much less marked, showing that the reaction 

 occurs throughout the mass, and such differences as do occur 

 are easily explainable on the basis of the heat conductivity of 

 the rubber. 



^By Tohn B. Tuttle. Published by courtesy of the American Chemical 

 Society. Read at the meeting of the Rubber Division of the American 

 Chemical Society in ChicaKO, September, 1920. 



"Third Annual Report, National Advisory Committee for Aeronautics. 



•The India Rubber World, December 1, 1916, page 129. 



LITHOPONE' 



The manufacture of lithopone is divided into three steps : The 

 production of a pure barium sulphide solution ; the preparation of 

 a pure zinc sulphate solution ; the manufacture of lithopone from 

 these two solutions. 



BARIUM SULPHIDE SOLUTION 



The most important raw material is barytes. This is now 

 obtained chiefly from Georgia, because the ore deposits there 



iPaper read hy Donald Ross, chief chemist Krebs Pigment & Chemical 

 Co., before the Delaware Section of the American Chemical Society. 



