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THE INDIA RUBBER WORLD 



(Makih 1, 1919. 



What the Rubber Chemists Are Doing. 



COMPARATIVE METHODS FOR DETERMINING THE STATE 

 OF CURE OF RUBBER. 



A VALUABLE paper by Henry P. Stevens on comparative 

 methods for dclcrmining the state of cure of rubber, 

 appeared in the "Journal of the Society of Chemical 

 Industry," August 31, 1918, page 280t. Tabulated data of 

 physical tests and curves are given, showing the relationship of 

 the coefficient of vulcanization to breaking load and time of 

 cure for ordinary crepe and smoked sheet. 



The state of cure or degree of vulcanization of rubber may 

 be formulated in reference to (1) the percentage of combined 

 sulphur calculated on the rubber present, which is the coeffi- 

 cient of vulcanization; (2) the physical properties of the vul- 

 canizate, particularly the load supported per unit cross-sectional 

 area at a given elongation or z'ice versa. The former method is 

 independent of the age and external conditions of the vul- 

 canizate, while the latter is dependent on these conditions. It 

 is therefore necessary to make a careful comparative study of 

 the coefficients and the corresponding physical properties under 

 varying conditions before the latter can be taken as a measure 

 of the condition or state of cure. I have already shown that 

 the physical properties are dependent on the age of the vul- 

 canized specimen,' so that comparable results are only obtain- 

 able when the specimens are tested at a fixed period subsequent 

 to vulcanization. The present results show that the tem- 

 perature also has a considerable influence on the physical prop- 

 erties, and uniform conditions must be observed in order to 

 obtain comparable figures. 



Summary. 



The position of the stress-strain curves is appreciably in- 

 fluenced by the period elapsing between vulcanizing and testing 

 the rubber and by the temperature. Hence, the results are only 

 comparable when these conditions are kept constant. Results 

 obtained in summer are not comparable with those obtained in 

 winter, nor those obtained in the tropics with those obtained 

 in Europe. 



The position of the stress-strain curves is mfluenced by the 

 type of rubber, that is, whether crepe or sheet, probably owing 

 to a variation in the proportion of non-caoutchouc ingredients. 



The coefficient of vulcanization is independent of the above 

 and other conditions and is therefore a more reliable index of 

 the rate of cure. In any case, if the stress-strain curves are to 

 be taken as an index of the state of cure, it is essential that 

 these curves be obtained under standard conditions. 



Of particular interest is the greater curvature of the graphs 

 for crepe from matured coagulum than of those for ordinary 

 crepe and sheet. This may be attributed to the larger propor- 

 tion of accelerating base contained in the former. 



To insure as great a degree of uniformity as possible, all 

 speciments should be tested as soon as possible after vulcaniza- 

 tion, for instance, the next day; and between vulcanizing and 

 testing, the specimens should be kept as nearly as possible at 

 30 degrees C. 



VULCANIZATION RESEARCHES. 



B. J. Eaton summarizes the vulcanization researches of the 

 chemical laboratory of the Agricultural Department of the 

 Federated Malay States for the first half of 1918, as follows: 



The principal investigation has been on the efTect of different 

 alum salts used as coagulants. The writer has previously shown 

 that common potash alum, when used as a coagulant, has a 

 very deleterious effect on rubber, especially in regard to its 

 effect on the rate of cure. The subject is of considerable im- 

 portance since, owing to the rise in priceof acetic acid, the use 

 of alum by Asiatic small holders has increased considerably, 

 and the result, with the comparatively large amount of rubber 

 from such holding now being manufactured, may give a bad 

 name to plantation Para rubber. American manufacturers, who 

 have been large buyers of rubber from such sources on the 

 Singapore market, have previously drawn attention to the sub- 

 ject. 



The present investigation was carried out to ascertain whether 



nal of the Society of Chemical Industry," 1916, 



the different alum sails had a similar effect and the result of 

 the investigation has shown that such is the case. The eflfect of 

 the following alum salts was investigated: potash alum (pure), 

 commercial potash alum, soda alum, ammonia alum and 

 aluminum sulphate. The use of alum salts generally as co- 

 agulants should therefore be discouraged. 



The effect of alum as a retarding agent in vulcanization, and 

 of some other substances, namely, mineral acids, has not yet 

 been investigated on a scientific basis with a view to ascertain- 

 ing whether the effect is due to the bactericidal or anti- 

 enzyme action of these substances or their effect as direct 

 negative catalysts in vulcanization. Experiments with certain 

 other reagents, described below, indicate that these substances 

 behave directly as negative catalysts or retarding agents in 

 vulcanization. 



Effect of Certain Chemicals When Added to Finished Dry 

 Rubber. 



When used as coagulants the various salts or acids may re- 

 tard vulcanization by virtue of their bactericidal action on the 

 organisms or enzymes which bring about the changes in raw 

 rubber during the maturation period of six to seven days. By 

 adding these chemicals, however, to the finished rubber, any 

 effect must be due to a neutralization of the effect of the natural 

 accelerator or to a direct negative effect of the chemical added. 



The following substances have so far been tested in this man- 

 ner: starch (as a neutral organic adjunct), boracic acid, tan- 

 nic acid, molybdic acid and phosphotungstic acid used as pre- 

 cipitants of proteins, amines and basic nitrogenous compounds. 



All of these substances, with the exception of starch, when 

 added to the extent of one to two per cent of the rubber- 

 sulphur mixing, had a very marked effect in retarding vulcani- 

 zation, both in the case of "slab" crepe and crepe samples. Gen- 

 erally the larger amount had a greater effect than the smaller, 

 and the effect appears to be specific, that is to say, the vul- 

 canization of the "slab" crepe is retarded to a certain extent, 

 but is not as slow as the slow-curing crepe which had been 

 treated similarly. Rubbers having different rates of cure due 

 to different amounts of the natural accelerators present, are 

 not all brought to the same degree of slowness in the 

 vulcanization. 



The following method is that of Andre Dubosc in "Le 

 Caoutchouc et la Gutta-Percha," November 15, 1918, page 9646. 



The value of a reclaimed rubber depends on the amount of 

 caoutchouc it contains in free or uncombined condition. The 

 following method of analysis requires certain precautions and 

 considerable care, but gives very exact results. 



Apparatus for Chlgrhydration. 

 The sample for analysis reduced to 120-mesh fineness which 

 may be done easily after swelling it in a mixture of the tetra- 

 chloride and sulphide of carbon and drying at 60 degrees C. 

 before sifting. Ten grams of the powdered sample is weighed 

 for chlorhydration. This is placed in a flask provided with 

 two tubulures, one of which is connected to a source of cold, 

 dry, hydrochloric-acid gas, and the other to an absorption flask 

 containing a solution of caustic soda or milk of lime and con- 

 nected to a vacuum pump to facilitate the passage of the gas 

 through the sample. Cork stoppers boiled in paraffine are used 

 to close the tubulures and the inlet and outlet gas tubes pass 

 loosely through them to permit rotating the flask for the pur- 

 pose of exposing fresh surfaces of the powdered rubber to 

 the action of the gas. The lower part of the gas flask is 

 cooled in water or a freezing mixture to prevent overheating 

 the mass. A double-walled container or any water-sprinkling 

 arrangement may be used for cooling. The temperature of the 

 powder is not allowed to exceed 30 to 35 degrees C. 



