September 1, 1921 



THE INDIA RUBBER WORLD 



895 



other cases, in which the vulcanizing properties are wholly 

 normal. For instance, the ordinary changing over to a new tap- 

 ping surface is mostly accompanied by a yellow color in the 

 crepe without any change in its rate of cure or other properties. 

 This is an interesting physiological fact as it shows that the 

 yellow substance in the latex is locally fixed in the tree, in 

 contrast to the accelerators of vulcanization which seem to 

 travel easily through the tree and are present in the same pro- 

 portion over the whole tapping area. 



Besides the properties about which a few details have here 

 been given there are certainly others that are of importance in 

 manufacture, but about which ■ little is known. A systematic 

 study of such points can be arranged only when more is known 

 about the necessary methods of testing and about the ends that 

 should be attained. 



REJ\.GENTS FOR VULCANIZATION 



An invention has recently been patented' for the vulcanization 

 of natural or synthetic rubber by the employment of special re- 

 agents to insure the following advantages: (1) the diminution 

 or elimination of the quantity of sulphur which otherwise al- 

 ways remains in a free state in vulcanized rubber, with the con- 

 sequent elimination of the efflorescence on the articles produced ; 

 (2) the elimination of all immediate or subsequent over-vul- 

 canization ; (3) an improvement in the mechanical qualities and 

 durability of the product. 



The reagents employed in the process are salts of bivalent or 

 trivalcnt metals of the N., monoalkyl, or N.N., dialkyl, or N.N. 

 alkylcne-dithio-carbamic acids or other mono or bisubstituted 

 dithio-carbamic acids, having the general formulas where R' 

 can be an alkylic or other monovalent radical or chain, R" an 

 alk>'lenic radical or other organic bivalent radical with open or 

 closed chain, etc., and Me" a bivalent metal such as magnesium, 

 zinc, cadmium, mercury, etc., or an inorganic bivalent radical 

 such as uranyl and the like. The letters N. and N.N. are used 



f /XR'.-j 



c=s I 

 _ \s ].. 



as above to indicate that the alkyl or other radicals are bound 

 to the nitrogen atom and not to the sulphur atom. Correspond- 

 ing formulas may be adopted for the salts of trivalent metals 

 such as aluminum, iron, chromium, antimony, etc. 



The process is carried out by adding to mixings of rubber and 

 sulphur suitable quantities of the reagents named. The employ- 

 ment of the salts allows the rubber to vulcanize with the use 

 of a quantity of sulphur less than two per cent of the rubber 

 present, a quantity which may be reduced to 0.2 per cent, where- 

 as by all other known processes of hot vulcanization it is nec- 

 essary to employ five per cent. The rubber thus treated shows 

 after vulcanization no efflorescence due to free sulphur and can- 

 not be overcured even by prolonged reheating. 



/NHR'] 

 C=S I 



L\S J = 



Me" or 



Me" or 



■ /NR" 

 C=S 



\s 



Me" 



1 G. Bnini, British patent No. 140.387. 



SUVB RUBBER > 



The uniformity of rubber from matured coagulum (slab rub- 

 ber) has been ascertained experimentally to be not more uniform 

 in rate of cure than crepe, but the reverse. Whether or not air 

 has access during maturation has little or no influence on the rate 

 of cure of matured rubber. 



In each of two series of matured sheets of increasing thickness 

 the rate of cure is observed first to fall with increasing thickness 

 and then to rise. The irregular results obtained are not unex- 

 pected when we remember that during maturation the coagulum 

 provides a medium for the growth of every kind of micro- 

 organism, at the same time and in competition with one another. 

 As each organism will probably produce different products in 

 different proportions, and as chance circumstances will largely 



determine which of these organisms will flourish and which will 

 not, we can only expect the formation of a variable product. 



We know that the changes taking place during maturation at 

 the exjwsed surface of the coagulum are different from those 

 taking place in the interior of the mass. The surface becomes coated 

 with an alkaline slime, which does not form when the air is ex- 

 cluded as in the anaerobic coagulation process. The substances 

 which accelerate vulcanization have alkaline properties, conse- 

 quently we should conclude that the aerobic change must result in 

 a faster curing rubber. Previous negative or irregular results 

 may be due to the relatively small surface of the coagulum exposed 

 to the air. The surface is soft and slimy, and much of it would 

 be washed away during creping. Hence the exposed surface, 

 although containing accelerating substances in relatively large 

 proportions, might not have much effect on the rate of cure of 

 the whole mass when washed, creped and dried. 



We may conclude that aerobic influences account for the forma- 

 tion of at least part of the accelerating substances. As, how- 

 ever, beyond a certain thickness, an increase in rate of cure takes 

 place, it appears that accelerating substances are also formed under 

 anaerobic conditions. 



^H. P. Stevens. Bulletin of the Rubbers Growers' Association, 2, 1920, 

 6877; ibid 1. 1919, 42-4; ibid 2, 1920, 212-4. 



METHOD OF AVOIDING RESINIFICATION 



The inconvenience of the resinification and decomposition of 

 india rubber, balata, gutta percha or compositions containing any 

 one of these substances, when vulcanized in hot air has hitherto 

 been obviated by incorporating litharge, or its derivatives, in the 

 rubber, notably in the case of rubber shoes or rubber-coated 

 fabrics. This plan, hovi'ever, by reason of the temperature and 

 the presence of sulphur, causes the formation of lead sulphide 

 which imparts a black color to the rubber. 



If litharge, etc., is replaced by reducing substances it is pos- 

 sible to avoid resinification and retain the colors imparted to the 

 rubber mixing by organic coloring matters. This method allows 

 manufacturing objects of all colors, by the hot-air vulcanization 

 process. The reducing agents used should be easily oxidizable, 

 in order that they may be acted upon by atmospheric oxygen 

 during vulcanization before the rubber itself is acted upon. The 

 presence of the reducing agent in neutral or alkaline rubber com- 

 pounds facilitates the formation of nascent colloidal sulphur, as 

 well as its vulcanizing action upon the rubber. Their protective 

 action is more effective, the more intimate the mixture, and de- 

 pends also upon the extent to which they are able to effect the 

 fixation of free oxygen, and the stability of fixation of this 

 oxygen in the molecule of the reducer. 



Examples of suitable reducing agents are : hydroquinone, 

 pyrogallol, the tannins and their derivatives, paramido-phenol, gly- 

 cine, trio.xymcthylcne and other derivatives of formaldehyde, 

 hydro-sulphite, such as those of magnesium, calcium, zinc or soda 

 and its derivatives, the neutral sulphites, etc. The reducing agent 

 should not, in general, exceed ten per cent of the amount of rub- 

 ber present. A perceptible effect is obtained by the use of two 

 per cent of tannin or hydroquinone, from two to five per cent be- 

 ing sufficient for the hot-air vulcanization, at 130 degrees C. for 

 two hours, of a white composition containing 30 to 50 per cent 

 of rubber, white coloring matters and sulphur. — Andre Heil- 

 bronner, 74 Boulevard Malesherbes, Paris, France. British patent 

 No. 142,083. 



LEAD PIGMENTS' 



In addition to leaded zinc oxide and zinc-lead oxide, two 

 other lead pigments are made directly from ore, namely, 

 sublimed white lead and sublimed blue lead. Sublimed white 

 lead consists of lead sulphate 75 per cent, lead oxide 20 per 

 cent, and zinc oxide five per cent. Subliined blue lead con- 



