384 



THE INDIA RUBBER WORLD 



[SePI EMBER I, 1903. 



THE BLOOMING OF RED RUBBER GOODS.* 



IN the manufacture of red rubber goods it is particularly 

 desirable, besides possessing the other qualities required 

 in rubber products, that they shall be free from liability 

 to "bloom." The blooming of rubber goods is most an- 

 noying to the manufacturer; it cannot be foreseen, but appears 

 generally only after the goods have gone into consumption, 

 and yet he has to suffer the consequences — inconvenient dis- 

 putes with the customer and, usually, the return of the goods. 

 No one desires red rubber goods with a whitish tinge, and in 

 many instances they are entirely useless. It is natural, there- 

 fore, that rubber chemists should have devoted their energies 

 to this subject, and various explanations as to the causes of 

 blooming have appeared. 



If we consider, in the first place, what this bloom consists 

 of, we shall find that it is not produced exclusively by the sul- 

 phur of vulcanization, though this may be the cause in a ma- 

 jority of cases. In the bloomed out mass, which generally is 

 crystalline, and often dusty and meally, other substances be- 

 sides sulphur are found, such as the carbonates of zinc, calcium, 

 and magnesium, and also resins and oils, even if, as a rule, they 

 exist in small percentages only. Of course the bloom does not 

 appear on red rubber goods alone, but also on goods made 

 of white and grey compounds. This defect on the last two 

 named is not so apparent, and is objected to only when the 

 appearance is too pronounced ; while in black goods — it is im- 

 material whether carbons or lead compounds have been used 

 — it is rarely found. The existence of antimonoxids in the 

 bloom, as is often supposed, must be excluded, because they 

 could be traced only to the golden sulphurets, which would 

 produce a decomposition during vulcanization, causing the 

 rubber to turn brown or black. 



Pure sulphur bloom appears only on goods in which sulphur 

 is used in connection with rubber which is free from resin ; 

 that is, the best grades of rubber. But in the use of inferior 

 rubber, containing a considerable percentage of resins, gener- 

 ally small portions of these are segregated and sweated out in 

 vulcanization. This is even more true of the solid greases which 

 form part of some of the substitutes used in rubber mixtures. 

 On giving close attention to the appearance of the bloom on 

 rubber goods, we find that it returns persistently, even after 

 having been removed with carbon disulphid or other solvents, 

 or even after being ground off. If the latter process be resorted 

 to, it must not be done too deep, because of the singular fact 

 that rubber goods blooming on the exterior do not show that 

 condition below the surface. When cut with a knife the cut 

 surface will remain clean. 



As already mentioned, the bloom caused by sulphur (and 

 also paraffin) is crystalline in its composition ; it looks exactly 

 as if rubber had been treated to a coat of solution of sul- 

 phur in carbon disulphid and the latter had evaporated. The 

 bloom, therefore, would appear to be nothing more than the 

 segregation of sulphur from any solution — for instance, resin 

 existing in rubber and made fluid by the heat of vulcanization. 

 This theory virtually is admitted by those scientists whose ex- 

 planation of blooming is that it is caused by faulty methods of 

 vulcanization, by which the sulphur is set free. 



This explanation would be plausible enough did it suffice for 

 all the concurrent features. Not only is the segregation of sul- 

 phur wholly external, recurring repeatedly after removal, but it 



* Translated from the Gummi-Zeitung (Dresden). 



is most prevalent in soft elastic rubbers — the best grades — de- 

 creasing proportionately with the inferiority and hardness of 

 the rubber, which then contains a larger percentage of rubber 

 and might easily be overloaded with it. Blooming never oc- 

 curs in hard rubber (vulcanite). Further, rubber blooms only 

 when exposed to the air, and not in water or other fluids, or in 

 water containing superoxid of hydrogen, which certainly would 

 cause blooming if oxidation played any part in it. It manifests 

 itself alike under high and low temperatures ; what degree en- 

 hances or retards it has not been established. The chemical 

 actions and causes producing bloom are still a doubtful quan- 

 tity, and the most that can be done is to take particular notice 

 of all the various changes and thus try to overcome the objec- 

 tionable feature. 



The methods to this end vary in the case of different observ- 

 ers. Naturally vulcanization receives first attention, since in 

 that process many have been led to believe that the key to the 

 situation is to be found. We are convinced that the heating of 

 rubber to a high temperature is the primary cause of bloom- 

 ing, but under what condition it occurs has not yet, in spite of 

 the most minute observation and registering, been determined. 

 It has been pointed out repeatedly in the technical press that 

 correct proportions of sulphur and golden sulphuret would pre- 

 vent blooming, but what the particular proportion is, or how it 

 is to be ascertained and on what fundamental principle, remains 

 to be stated. Every rubber expert, after having established to 

 his own satisfaction a fixed theory to overcome this defect, 

 suddenly discovers that his led rubber blooms, and he has to 

 keep on experimenting. 



Next, attention is given to the golden sulphuret, and its 

 composition and influence on red rubber, and many experi- 

 ments in practice and theory have been made in this connec- 

 tion. The easy decomposition of many biands of golden sul- 

 phuret has been thought to have an importance bearing upon 

 the matter. It is pointed out that the sulphurets, treated with 

 caibon disulphid, which was intended to dissolve the free sul- 

 phur only, often dissolve the chemically bound sulphur, and 

 change into the compound Sb, S4. This decomposition does 

 not take place in all golden sulphurets, and by the use of an 

 article free from this condition, blooming, so far as it may be 

 attributed to the sulphurets, may be prevented. The golden 

 sulphur of antimony is then tested by extraction by means of 

 carbon disulphid in a reflux cooler, at which operation no 

 more than about 4 or 5 per cent, of sulphur should be ex- 

 tracted. This naturally only refers to so-called chemically 

 pure golden sulphurets. Sulphurets containing a larger pro- 

 portion of free sulphur, will show an increased loss in weight, 

 for which reason, in order to obtain accurate results, the anti- 

 mony must also be considered. 



The theory is often stated that the bloom on rubber con- 

 sists partly of antimonoxid. This is impossible, for if anti- 

 monoxids are present in noticeable quantities, or should form 

 during vulcanization, the partial or total decomposition into 

 black antlmontrisulphid would be inevitable; the color of the 

 rubber would then become, instead of bright red, a red brown 

 or brown to black. 



It is possible for free sulphur to segregate in crystalline 

 form, from oils and resins, as well as from gold sulphurets. 

 Here also vulcanization is without any material influence, the 

 amalgamation of these substances occurring at higher temper- 



