October 1, 1920.] 



THE INDIA RUBBER WORLD 



27 



What the Rubber Chemists Are Doing 



INVESTIGATIONS ON DIFFERENT COAGULANTS 



THE FOLLOWING SUMMARY of results is taken from the paper 

 by Dr. O. de Vries, publislitd in Archief I'oor dc Rubbcr- 

 ctiUuur, May, 1920. 



SULPHURIC ACID 



Sulphuric acid has been used on several estates during the war. 

 It is a strong coagulant which partly or wholly replaces the 

 anti-oxidant, sodium bisulphite. Small quantities used for coag- 

 ulation do not give large deviations' in properties of the rubber, 

 but by the use of large quantities the rate of cure and viscosity 

 of the rubber are markedly decreased and more rapidly than 

 when corresponding amounts of acetic acid are used. Uni- 

 formity in rate of cure of the product is more difficult to pre- 

 serve when using sulphuric acid as coagulant because variations 

 in the amount used may always occur in practice. 



Many manufacturers do not like the use of sulphuric acid, 

 as they fear a harmful action of traces left in the rubber. Aging 

 experiments have not shown this effect. The properties of 

 the raw or vulcanized product, on aging, changed in exactly the 

 same way as after acetic acid coagulation. The slow rate of 

 cure caused by sulphuric acid changed more rapidly than with 

 acetic, and the abnormal values recovered to normal ones, as 

 if some retarding substance had gradually disappeared. 



ALUM 



Alum as a coagulant is very largely used by native planters. 

 Coagulation proceeds best with undiluted latex, in which three 

 to four grams per liter would be sufficient, while eight to 12 

 grams give rapid coagulation. Small quantities of alum cause 

 a marked decrease in rate of cure and viscosity, while large 

 doses, such as are often used to obtain rapid coagulation, give 

 abnormally slow-curing rubber with a low viscosity. The tensile 

 strength shows no marked deterioration, no greater than might 

 be expected from the much longer time of cure. The slope of 

 the stress-strain curve decreases somewhat by larger doses. 

 ACETIC ACID 



Acetic acid obtained by wood distillation has the drawback 

 that the tarry substances are difficult to remove completely, so 

 that crepe cannot be prepared with it and the color of the sheet 

 becomes too dark. Acetic acid prepared by fermentation of 

 alcohol is cheap and a good coagulant. Commercial acetic acid, 

 if cheap enough, is preferred. The crude acid gives rubber 

 of practically the same properties as the pure commercial acid. 



FERMENTED COCONUT WATER 



The juice from coconuts, on fermentation, gives an acid that 

 proved satisfactory as a coagulant. Transport of this dilute 

 acid fluid is too expensive, so that it can be used only on 

 estates growing both rubber and coconuts. Large series of coag- 

 ulation experiinents proved that these acids gave rubber of 

 exactly the same properties as commercial acetic acid, which 

 are uniform over just as long periods. 



ACID COFFEE JUJCE 



The acid juice obtained when fresh red coffee berries are 

 allowed to ferment for some days in water was tried as a 

 coagulant, but as it has a dark red color, crepe cannot be pre- 

 pared with it, and sheet also takes a darker red than usual. 

 The properties of the rubber from some experiments seemed 

 to be injured somewhat. This coagulant would be available 

 only during the few months of the coffee harvest and is not 

 likely to gain any importance except in emergencies. 



CRATER LAKE ACID WATER 



Acid water from a crater lake in the volcano Idjen in East 

 Java was tried as a coagulant. It has a strong coagulating 



power, containing alum, sulphuric and hydrochloric acids. Its 

 composition varies with the depth from which it is taken and, 

 as might be expected, it proved injurious to the properties of 

 the rubber, especially when used in large quantities. Aging 

 tests on ruHbtr coagulated by this acid crater water showed 

 marked deterioration in tensile strength and viscosity. 



ALCOHOL 

 Alcohol and denatured spirits were tried as coagulants. Coag- 

 ulation is quite different from ordinary acid coagulation, as it 

 is instantaneous. Wlierc alcohol mixes with the latex it forms 

 a clot and no further coagulation of the remaining latex occurs. 

 Only strong alcohol and undiluted latex can be used, else 

 the quantities of alcohol become much too large. The cost of 

 this coagulant is ten times that of acetic acid even when using 

 denatured spirits free from duty. .-Mcohol has some advantages 

 for trial coagulations and experimental purposes, as it gives 

 rapid and complete coagulation. Coagulation by alcohol was 

 found to leave the rate of cure unchanged, while the slope 

 of the stress-strain curve was always steeper and the viscosity 

 mostly lower. The tensile strength remains unchanged or be- 

 comes somewhat less (by denatured spirit). 



FORMIC ACID 

 Considering the price and coagulating power, formic acid 

 might be a good substitute for acetic acid, but it seems to be 

 irregular in composition, perhaps sometimes containing for- 

 maldehyde, and gives irregular results, notably sometimes a 

 marked decrease in rate of cure. 



LACTIC ACID 



Lactic acid has no importance as a coagulant in practice, 

 as it is too expensive. Since this acid is found in most fer- 

 mentation processes and plays a role in spontaneous coagula- 

 tion, and in different fermented saps, such as coconut water, 

 some experiments were made with it. The properties of the 

 rubber were found to be unaltered, except for a sinall decrease 

 in rate of cure. 



HYDROCHLORIC ACID 



An experiment with liydrochloric acid showed that a strong 

 dose retarded the cure very much, although somewhat less than 

 an equivalent dose of sulphuric acid, while viscosity and tensile 

 strength deteriorated markedly and the slope became some- 

 what better. .Xfter two years the sample became tacky and 

 could not be vulcanized, while the viscosity decreased to a very 

 low figure. The samples prepared with strong doses of acetic 

 and sulphuric acid remained unchanged in appearance, though 

 the viscositv decreased. 



BLACKENING OF RED RUBBEJl TUBE:Si 



The black spots which sometimes appear on the surface of 

 rubber goods, and also the blackening of the interior during 

 vulcanization, are most likely due to the interference of iron. This 

 may be obviated to a great extent by taking special precautions 

 to prevent the direct contact with either condensed water con- 

 taining traces of iron in solution or suspension, or the sulphiding 

 and subsequent reducing action of the iron surface of the mandrel 

 acting on the red antimony in the red rubber goods. Probably 

 the actual blackening is not caused directly by the formation of 

 ferrous sulphide, but rather that the iron, in conjunction with the 

 water vapor present, tends to act in such a way as to form a tem- 

 porary solution of a small part of the antimony. This results in 

 the ultimate reprecipitation of the antimony sulphide in the black 

 variety on the part affected. 



"From The Rubber Age, London. June, 1920, page 152. 



