580 



THE INDIA RUBBER WORLD 



[July 1, 1917. 



What the Rubber Chemists x4re Doing. 



CATALYTIC ACCELERATION OF VULCANIZATION. 



IN the "Journal of the Society of Chemical Industry," April 30, 

 1917, S. J. Peachey discusses organic accelerations with special 

 reference to para-nitrosodimethylaniline discovered and patent- 

 ed by him as an accelerator of the vulcanizing process. [English 

 patent N'o. 4,263 (1914).] 



Extracts from the original paper and discussion follow : 



Generally speaking, the addition of 0.3 to 0.5 per cent of 

 nitroso-base to any mi.xing of good qualily is sufficient to reduce 

 the time of vulcanization from one-quarter to one-third of that 

 normally required. The nitroso-base is now being employed 

 successfully under the trade name of "Accelerenc." 



/>-Kitrosodimethylaniline is a green crystalline powder of 

 melting point 85 degrees C. It is usually marketed in a moist 

 condition (about 30 per cent of water), this adding considerably 

 to its stability and rendering it safer to store in bulk. Before 

 use it may conveniently be dried by exposure to warm air, or 

 preferably in a vacuum dryer. It should be incorporated with 

 the rubber towards the end of the mixing operation, as there 

 is otherwise some danger of a partial vulcanization taking place 

 on the rolls. 



The time required for curing the mixing may be roughly cal- 

 culated on the assumption that the use of 0.5 per cent (on the 

 weight of the mixing) of nitroso-base reduces the time of cure 

 to one third of the normal. In the case of red mixings con- 

 taining antimony sulphide as the vulcanizing agent without added 

 sulphur, the accelerator fails to develop its full effect. In such 

 cases it becomes necessary to introduce a certain quantity of 

 sulphur, mainly as pigment. The presence ot litharge in any 

 quantity tends to diminish the effectiveness of the organic ac- 

 celerator. The presence of a small quantity of magnesia aug- 

 ments very considerably the accelerating power of the nitroso- 

 base, and by using the two substances in suitable proportions 

 it is pos.sible to prepare high-class mixings which will vulcanize 

 perfectly in about one-eighth of the normal time, or about ten 

 minutes. 



It is to be noted that the maximum accelerating power of p- 

 nitrosodimethylaniline is only fully manifested when new rub- 

 ber is used ; with reclaimed rubber or rubber waste the results 

 obtained are not very satisfactory. 



One of the most valuable applications of the new accelerator 

 is in the manufacture of vulcanite. A pure vulcanite mixing of 

 100 parts rubber and 40 parts sulphur normally requires about 

 six hours to cure. Such a mixing, by the addition of 0.6 to 0.75 

 per cent of nitroso-base, can be perfectly cured in two hours. 



The nitroso-base effectively obviates "blooming" or "sulphuring- 

 ■up." In such case there is little or no reduction in the time of 

 vulcanization owing to the diminished proportion of sulphur 

 present, the power of the catalyst being expended in bringing 

 about practically complete combination between the molecules of 

 the rubber and the sulphur. 



It is fairly generally accepted that vulcanization by Good- 

 year's process involves the following changes: 



1. Depolymerization by heat of the complex rubber molecule. 



2. Adsorption of sulphur by the rubber colloid. 



3. Combination of the depolymerized rubber with sulphur. 



4. Repolymerization to complex molecules as a result of (3). 

 The final product— the vulcanized rubber— is regarded as being 



a solid (or semi-solid) solution of the compound of rubber and 

 sulphur in unchanged rubber. 



The newly discovered organic accelerators differ from the 

 so-called sulphur carriers in that an exceedingly small quantity 

 of material is effective in bringing about a very powerful ac- 

 celeration, evidently of a catalytic nature. The main character- 

 istics of catalytic actions are that the catalyst is usually present 



in relatively small concentration and that it does not start a 

 reaction, but only accelerates a change which can proceed by 

 itself. Taking /i-nitrosodimethylaniline as an example, its pres- 

 ence in one two-hundredth of the mass of the reacting sub- 

 stances is sufficient to treble the speed of the reaction. The in- 

 ternal heat theory cannot possibly apply here. The accelerating 

 effect of these nitroso-bases is in some manner bound up with 

 the presence of a nitroso group directly linked up with the 

 nucleus. 



It is important to note that the catalytic action of />-nitroso- 

 dimcthylaniline appears to differ entirely in mechanism from 

 that of piperidine and of the other bases covered by Bayer's 

 patents. The latter appear to act by virtue of their powerful 

 basic character. /i-Xitrosodimethylaniline is a very feeble base 

 with a dissociation constant far below that quoted in Bayer's 

 patent. The accelerators discovered by the author constitute a 

 new class and appear to owe their accelerating power to the 

 presence of the nitroso group. 



The author in his experiments found that certain organic 

 compounds possess the property of retarding vulcanization, ap- 

 parently acting as anti-catalysts. Phenylhydrazine is an example. 

 Other substances behave similarly, notably glucose and methy- 

 lene blue. 



In reply to discussion of his paper the author further ex- 

 plained his researches on accelerators in part as follows : 



Not much progress has been made as regards the possibility 

 of detecting the nitroso-base in the vulcanized rubber, but the 

 acetone extract of such a rubber is always so lightly colored 

 that in the absence of anything else which would yield a dark 

 color, that alone would suggest the use of the accelerator. 



If rubber were heated with nitroso-base a tarry mass was 

 formed ; there was no evidence of vulcanization. 



With regard to the prejudicial effect of /i-nitrosodimethylaniline 

 upon the health of workers using it, it was largely a question 

 of the impurities originally present in the nitroso-base and the 

 pure material is not so objectionable as the crude. 



Regarding the stability of vulcanized rubber, samples have 

 been kept for two years or more. Except in the case of a few 

 which were not more than 0.1 millimeter thick there has been 

 no deterioration. 



The strength tests carried out with material vulcanized in 

 conjunction with the accelerator gave better results than those 

 obtained with material normally vulcanized as nearly as pos- 

 sible in the correct time to give the same cure. There appeared 

 to be a uniform slight increase in strength as the result of using 

 the accelerator, but the experiments must be confirmed on a very 

 large number of samples. 



PLANTATION FACTORY PRACTICE IN RELATION TO VITLCANIZATION 



TESTS. 



Bulletin No. 27 of the Ceylon Department of .Agriculture, en- 

 titled "Vulcanization Tests." details the results of investiga- 

 tions at the Imperial Institute on samples of plantation Hevea 

 rubber prepared by L. E. Campbell, B.Sc, the rubber research 

 chemist. 



For comparison the experiments were conducted in duplicate 

 with latex from 7-year trees and also from trees 16 to 20 years 

 old. The 88 specimens dealt with were prepared with the object 

 of studying the effect of (1), the smoking of sheet rubber; (2), 

 the addition of alkaline creosote solution to the latex before 

 coagulation; (3), the Brazilian, Wickham and Byrne processes; 

 (4), the rolling up of plain sheet and smoked sheet rubber, both 

 dry and wet, with and without tension; (5), the conversion of 

 crepe rubber into wet and dry block, and (6), the separation of 

 the rubber from the latex in successive portions. 



