THE INDIA RUBBER WORLD 



719 



is provided with 6 outlets connected with a steam trap (3) that 

 is provided with inlets and controlling valves. The hose to be 

 tested is cut into lengths that will just fit between the connec- 

 tions on the headers, the bottom connections being made with 

 unions. .Steam at specilied pressure passes into the header (1) 



and thence through the hose to the header (,2) from wliich the 

 condensation is carried to the steam trap. The hose under test 

 should be protected by a frame of metal or wood or of heavy 

 plate glass so that in case a piece breaks during the test, 

 the operator will not be injured. 



Notes on Accelerators. 



By Henry P. Stevens, MA., PhD., F.I.C. 



ALTHOUGH the mineral accelerators such as magnesia and 

 litharge have been used for many years, the term accel- 

 erator was seldom heard until the introduction of the or- 

 ganic accelerators or vulcanizing catalysts. As the latter term 

 implied, these substances are believed to owe their efficiency to 

 their catalytic effect as intermediary in the reaction between 

 the caoutchouc molecule and the sulphur. This view is rather 

 supported by the recent discovery of Peachey that vulcanization 

 is effected by allowing sulphur dioxide and hydrogen sulphide 

 gases to react in the presence of the rubber. The reaction takes 

 place spontaneously at room temperature, showing the greatly 

 increased reactivity of sulphur in the nascent (atonic?) state. 

 Catalysts are supposed by some to owe their efficiency to the 

 formation of intermediate products. In the case of accelerators, 

 the intermediate product would be formed with the sulphur, 

 which would then decompose, liberating the sulphur again and 

 reforming the accelerator. The liberated sulphur would not 

 be in the ordinary molecular state at the moment of liberation 

 and might therefore b« more reactive as in the case of the 

 nascent sulphur liberated by the reaction between sulphur diox- 

 ide and hydrogen sulphide. It does not follow, however, that 

 ivhen an element is liberated in the course of a chemical reac- 

 tion it is necessarily in a more reactive form. Sulphur liberated 

 by reactions other than that described does not appear to vul- 

 canize rubber. In such cases, either the element is not par- 

 ticularly reactive at the moment of liberation or the conditions 

 are such that ordinary molecular sulphur is formed before the 

 active form has had an opportunity to react with the caoutchouc 

 or rubber present. It may be necessary that the catalyst should 

 be in intimate contact (to use crude expression) with the 

 caoutchouc molecules or aggregates. In Peachey's reaction we 

 have a gas (sulphur dioxide) which is soluble in rubber to an 

 appreciable extent. We may assume that the gas is in intimate 

 contact with the caoutchouc aggregates, possibly adsorbed on 

 the particles of which the colloid is composed. Under these 

 circumstances vulcanization takes place when the sulphur is 

 liberated and is so far complete that an ordinary degree of vul- 

 canization can be obtained with hardly any free sulphur pro- 

 duced, that is to say, practically the whole of the sulphur reacts 

 with the rubber at the moment it is liberated. 



The organic accelerators are substances which easily mix with 

 rubber, in fact they are probably soluble to a certain extent in 

 raw rubber and consequently fulfil the above suggested condi- 

 tion of intimate contact in the event of their combining with 

 sulphur and consequently liberating this sulphur in a more reac- 

 tive form. If this theory is correct the liberation of the sulphur 

 and consequent vulcanization of the rubber may be regarded as 

 taking place instantaneously. On the other hand the formation 

 of the intermediate substance by combination between accelerator 

 and molecular sulphur may take an appreciable time period. If 

 this be so the efficiency of an accelerator will primarily depend 

 on the rate of reaction between it and molecular sulphur, pro- 

 vided that the substance formed can be dissolved or adsorbed to 

 a sufficient extent by the caoutchouc with which it reacts. The 

 reaction between the accelerator and the sulphur has been dis- 

 cussed by Bedford and Scott (The India Rubber World, Janu- 

 ary, 1920. page 207), who show that the efficiency of some ac- 



celerators is probably dependent upon the intermediate forma- 

 tion of thiourea derivatives as they react with sulphur at vul- 

 canization temperatures. Similarly it has been suggested that 

 the efficiency of isnitroso dimethyl aniline depends on its re- 

 duction and the formation of an amino derivative which actually 

 functions as the accelerator. This, however, is not supported 

 by the relative accelerating power of para nitroso phenol and 

 para amido phenol. The former is an accelerator, the latter has 

 no appreciable accelerating effect. In some cases it may be that 

 the intermediate product is not so readily formed and requires a 

 temperature higher than the ordinary vulcanizing temperature if it 

 is to be produced in appreciable quantity. At any rate it is claimed 

 that distinct advantages result from the use of the reaction product 

 of the accelerator and sulphur over a mixture of the two. As 

 examples, methylene-aniline and methylene-diphenyl diamine have 

 been quoted. (British patent No. 130,857. The Indi.-v Rubber 

 World, November 1, 1919, page 80.) The main curing effect pro- 

 duced has been ascribed to the formation of their carbanilides. 

 It is obvious that the action of accelerators may in many in- 

 stances be very complicated and there is a large field open for 

 investigation. My own work carried out years ago (1911 and 

 1912) showed that without a nitrogenous constituent rubber could 

 hardly be got to vulcaniw at all. Nature fortunately provided 

 the rubber manufacturer not only with the raw material but also 

 vnth the catalyzer necessary to effect vulcanization. 



The action of inorganic accelerators differs in some respects 

 from the organic accelerators. It is commonly stated that the 

 latter are more efficient and this impression arises from the rela- 

 tively small quantity of inorganic accelerators employed in rubber 

 compounding, quantities such as H to 1 per cent on the weight 

 of the rubber being commonly employed. It is obvious, however, 

 that larger quantities would detract from the physical properties 

 of the rubber, for, apart from their accelerating value, they are 

 merely com.pounding ingredients without strength or elasticity. 

 Moreover, the price would restrict the quantity employed. On 

 the other hand, some of the mineral accelerators are generally 

 regarded as exerting a beneficial effect apart from their accelerat- 

 ing action. The magnesias, carbonate and calcined, are examples. 

 Large quantities of the light carbonate are used for the apparent 

 toughening effect produced, apart from the small accelerating 

 action which this mineral possesses. The calcined magnesia has 

 very considerable accelerating power and is comparable with 

 many efficient organic accelerators, particularly when used in 

 quite small amounts — up to say H Per cent. Kratz and Flower 

 are of opinion that its accelerating power is only indirect. That is 

 to say, it does not accelerate vulcanization by direct action on the 

 rubber but merely liberates and renders more active the nitrog- 

 enous accelerator present in the rubber. Some of the organic 

 accelerators are extraordinarily efficient— for example, dimethyl- 

 ammonium-dimcthyl-dithiocarbamate (the addition product of di- 

 methylaminc and carbon bisulphide), which, according to Cranor, 

 is rendered still more active in a rubber mix containing a small 

 quantity of zinc oxide. One-half per cent of this accelerator en- 

 ables satisfactory vulcanization to be effected in three or four 

 minutes instead of 50 minutes. It was also found that spon- - 

 taneous vulcanization of this stock takes place in the cold and 

 the compound is very fairly vulcanized after one to two months' 



