THE INDIA RUBBER WORLD 



[June 1, 1919. 



of 1 to 9 was produced only by a load just short of that required 

 to produce rupture or break. 



Table II. 



, Per Cent Accelerator on Rubber. > 



Accelerator 5 10 15 

 Used. Mixture. I'er Cent. Per Cent. Per Cent. Per Cent. 



I First late.v 100 100 100 100 



Heavy Zinc oxide 100 92 83 71 



calcined \ Heavy calcined 



maenesia. I magnesia S 10 15 



* [Sulphur 5 5 5 5 



I First latex 100 100 100 



Lii-ht J Zinc oxide 90 80 70 



mannesia. 1 Light magnesia. ... 5 10 IS 



L Sulphur 5 5 S 



I First latex 100 100 100 



J Zinc oxide 87 75 62 



Lime. JLime 5 10 IS 



[Sulphur 5 S S 



Table III. 



Time in Tensile Elon- 



Minutes Strength gation 



Per for at at 



Cent Technical Break, Break, 



Accelerator Accel- Cure at G. per Per Sulphur 



Used. erator. 298° F. Sq. Mm. Cent. Cofficient. 



C^jntrol 120 1,331 725 3.075 



Heavy I 5 90 1,553 700 2.586 



calcined i 10 75 1,627 7Z5 1.723 



magncjia. L 13 "tS 1.''02 675 



^. ^. (5 90 1,322 700 2.780 



Light . J 10 45 1,875 750 2.184 



magnesia. (^ 15 ^q 1350 775 



( 5 90 1,294 800 1.990 

 Lime. ■: 10 45 1,565 



,512 



750 



In ihe second experiment where much larger amounts of in- 

 organic accelerators were employed, it was desired that the 

 eflect of the accelerator as a filler should be minimized to the 

 greatest possible extent. This was accomplished by employing 

 mixtures which contained zinc oxide in such an excess that from 

 5 to 15 per cent of an inorganic accelerator could be included 

 in the mixture, by replacement of a similar volume of zinc oxide, 

 without decreasing the effect, or function, of the latter substance 

 (Table II). The sulphur content of the various mixtures was 

 also cut down from 11 to 5 per cent, calculated upon the rubber. 

 In this instance, and unlike the preceding experiments, the mi.x- 

 tures were vulcanized to maximum physical properties and their 

 respective sulphur coefficients determined at this point. Portions 

 of each of the mixtures were vulcanized in a plateii press at SO 

 pounds steam pressure (298 degrees F.) over a wide range of 

 times and the correct cure determined as the point of coincident 

 maximum tensile strength and percentage elongation (technical 

 cure). The sulphur coefficient of each mixture when vulcanized 

 to this degree was then determined (Column 6, Table III). 



The results obtained show that, for all three accelerators 

 used, the best physical properties were obtained with about ten 

 per cent of each in the mixture. The effects produced by light 

 magnesia, heavy calcined magnesia, and lime ranked in the order 

 named. These differences, however, were small enough to be 

 accounted for in the value of each of these substances as a 

 filling material. However, it is evident that the value of these 

 accelerators as filling materials is of limited extent, because, 

 when present in larger amount (IS per cent), in each case the 

 vulcanized mixtures showed inferior physical properties (Col- 

 umns 4 and S, Table III). Moreover, the sulphur coefficients of 

 the various mixtures were found not to reflect, or be a measure 

 of, their physical properties. With both varieties of magnesia, 

 the mixtures which contained 10 per cent of these substances 

 were found to have lower sulphur coefficients than the mixtures 

 which contained but 5 per cent, and the latter had lower coeffi- 

 cients than the control which was vulcanized without the assist- 

 ance of an accelerator. On the other hand, the results obtained 

 with lime were remarkable in that with 5 per cent of this sub- 

 stance, a much lower sulphur coefficient was obtained than in the 

 case of the control, while with 10 per cent, contrary to the results 

 obtained with magnesia, the sulphur coefficient was increased 

 almost to that of the control. 



In explanation of the results with magnesia, we have_ con- 

 sistently found that mixtures vulcanized quickly to maximum 

 physical properties with the assistance of accelerators invariably 

 show lower sulphur coefficients than similar mixtures also vul- 

 canized to maximum physical properties, but without the assist- 

 ance of an accelerator. Frequently, much Ijigher physical values 

 are developed by those mixtures which contain accelerators. The 

 same is true in lesser extent when a short period of vulcaniza- 

 tion is effected by the use of higher temperatures. It is at least 



indicated that the time required to effect the cure of a given 

 mixture is reflected both in its sulphur coefficient and physical 

 properties. 



CONCLUSIONS. 



The physical properties of vulcanized rubber mixtures are more 

 fully expressed in terms of the tensile strength and elongation 

 at break than by the load required to effect an extension of 

 1 to 9. 



When used in small amount magnesia is less active in accel- 

 erating vulcanization than certain organic accelerators, and it 

 does not impart to the mixtures the physical improvement char- 

 acteristic of the latter substances. 



With mixtures which contain even small amounts of either 

 inorganic or organic accelerators, no direct relationship exists 

 between the sulphur coefficient and the state of cure as measured 

 by the physical properties of the mixture. 



When mixtures are vulcanized quickly, with the assistance of 

 inorganic accelerators, the correct state of cure, as reflected by 

 their physical properties, is obtained at abnormally low sulphur 

 coefficients. 



COLLOIDS AND RUBBER. 



At a recent meeting of the Society of Chemical Industry at 

 the University of Birmingham, Dr. D. T. Twiss spoke on the 

 properties of the colloid state as exhibited by rubber. The fol- 

 lowing outline is condensed from the report published by "The 

 India-Rubber Journal," January 4, 1919, page 6. 



Rubber is so completely and typically colloidal that it is 

 difficult to decide what details should be selected in order to 

 give briefly a general indication of its colloidal character. 



Natural rubber made its first appearance in a condition which 

 might be described as doubly colloidal. Rubber latex is a milky 

 fluid containing minute globules of a colloid, probably rubber 

 itself in a state of colloidal suspension in an aqueous fluid or 

 serum. These rubber globules are microscopic in size and show 

 a distinct Brownian movement. Rubber latex is a negative 

 suspensoid and the precipitation of rubber from Hevea latex ex- 

 hibits analogies to the precipitation of such substances as clay 

 or arsenious sulphide from colloidal aqueous, suspension. The 

 precipitation of negative suspensoids such as these is greatly 

 accelerated by the addition of acids. Most of the rubber pro- 

 duced to-day is separated from its latex by the addition of 

 small quantities of acetic acid. 



Alkalies increase the stability of negative suspensoids, including 

 rubber latex. Also the presence of an additional colloidal or 

 emulsoid substance can increase the stability of the suspensoid. 



Masticated raw rubber immersed in a solvent slowly absorbs 

 the latter and swells enormously, finally yielding a colloidal so- 

 lution of high viscosity. Some solvents, such as carbon disul- 

 phide, chloroform and benzene, yield almost transparent clear 

 solutions, while others such as shale naphtha, petroleum ether 

 and ordinary ether, yield solutions of milky appearance. Al- 

 though rubber latex is closely comparable with the ordinary sus- 

 pensoid colloids, rubber itself is an emulsoid. Emulsoids are, 

 as a rule, more viscous than suspensoids, and the propor- 

 tion of dispersed substance to medium is often higher. 



Raw rubber may be considered as a fairly extreme case in 

 which the rubber hydrocarbon is in a fine state of dispersion 

 throughout a medium, probably consisting in part of the protein 

 matter from the latex. As rubber freed from protein matter 

 still retains its typical consistency, the emulsoid state must be 

 attributed mainly to the presence of rubber in at least two forms 

 of different molecular weight, or of different molecular condition, 

 the rubber thus supplying not only its disperse phase, but also 

 its own dispersion medium. Other peculiarities in the behavior 

 of rubber confirm this lack of uniformity. 



One of the greatest obstacles in the way of the production of 

 synthetic rubber is the colloidal nature of the material. The prob- 

 lem is not merely to produce a substance of known molecular 

 weight and structure, because the required material is of un- 

 determined molecular magnitude and less desirable in a pure 

 condition than when containing so-called impurities. 



