August 1, 1921 



THE INDIA RUBBER WORLD 



817 



What the Rubber Chemists Are Doing 



ESTATE RUBBERi 



INFLUENCE OF HEAT 



OX dry rubber, heat is harmful only when the temperature 

 rises high. The effect is stronger in proportion as the air 

 has more access to the rubber and as more oxidation can 

 take place. Short heating only produces a marked deterioration 

 at 85 degrees C. (185 degrees F.) or higher. For more prolonged 

 •drying the limit seems to be somewhat lower, but in any case 50 to 

 55 degrees C. (112 to 133 degrees F.) is a safe temperature limit 

 for drying-houses; when this limit is passed, a weak and sticky 

 exterior gives warning long before the inner properties, especially 

 the properties after vulcanization, are noticeably affected. The vis- 

 •cosity seems to be somewhat lowered by heating above 60 degrees 

 C. (140 degrees F.), and only decreases markedly— at least after 

 short heating — when the temperature reaches the neighborhood 

 of 80 degrees C. (176 degrees F.). The rate of cure and slope 

 are unchanged. It is only when the rubber is very strongly 

 heated that a small decrease in the rate of cure is found. The 

 tensile strength also decreases only at higher temperatures, prob- 

 ably in connection with oxidation or decomposition phenomena. 



FAULTS AND DETERIORATION IN CREPE RUBBER 



Faults in crepe rubber may be many and of various kinds. 

 .'Vccidental admixtures caused by some oversight or negligence 

 are of no general importance, although their causes are often 

 difficult to trace. 



SPOTS AND STREAKS 



Several kinds of spots on crepe may be caused during prepara- 

 tion, such as the following. 



Lumps of preformed coagulum may be seen later as yellowish 

 or violet-brown spots in the crepe. Their quantity, however, is 

 generally small, and when they are mixed up with the rest during 

 compounding, no decrease in properties after vulcanization is 

 generally found. 



Violet coloration of the coagulum by oxidation on the surface 

 which is in contact with the air may cause dark-violet or blue- 

 black spots and streaks. This discoloration is caused by the oxi- 

 dation of some non-rubber substances, and the rubber itself is not 

 attacked, .\lthough the small amounts of oxidation products 

 cause a very sticky discoloration or streaking of the product, the 

 harm to the rubber is negligible. 



Some of the most common spots on crepe are formed during 

 drying, namely, when drying is too slow during periods of wet 

 weather, or when the crepe is not rolled thin enough. Orange', 

 pink, violet, blue and dark-purple spots may develop, most in- 

 tensively in the thickest parts of the crepe. Similar spots may be 

 formed if the dry, clean crepe becomes moist after its despatch. For 

 instance, from rain or sea water on the chests, or from storing on a 

 moist cement floor, and also from packing-chests which are not 

 dry enough. These spots are caused by micro-organisms, and 

 as long as this exposure lasts only a short time, the exterior of 

 the rubljer may be largely damaged by the discolored spots, but 

 the properties after vulcanization, and even the viscosity, are not 

 harmed. 



In certain cases the effect may become serious, and the proper- 

 ties may be very badly injured. The nature of the decomposition 

 that causes such deterioration, and the organisms producing it, 

 as distinct from those that cause only an innocent though very 

 striking discoloration, are not yet known. 



TACKINESS 



Stickiness and tackiness may develop from very different 

 causes, of which the principal are: (1) stickiness caused by too 

 liigh temperature during drying; (2) stickiness caused by heat- 



ing during milling; (3) tackiness caused by infection from soil or 

 water; (4) tackiness caused by copper compounds; (5) tackiness 

 caused by sunlight. 



Concerning the chemical changes accompanying tackiness, rela- 

 tively few data are available, and the different types of stickiness 

 have not been separately studied and distinguished. 



FAULTS IN SHEET RUBBER 



Trouble from faults in sheet rubber is certainly not less than 

 with crepe. One great advantage of smoked sheet is that the 

 brown smoke-color covers and makes invisible many differences 

 in shade which may give so varying an exterior to unsmoked 

 sheet. Even the violet discoloration, caused by oxidation on 

 the surface of the coagulum, becomes practically invisible when 

 the sheets are thoroughly smoked. Against this advantage in 

 smoked sheet this form of rubber has two distinct disadvantages 

 which tip the balance in favor of crepe rubber, as far as trouble 

 with the exterior goes. The first is, that the form of sheet 

 rubber is of importance and that every deformation remains 

 visible, while with crepe this can be restored by recreping. The 

 second disadvantage of sheet rubber is formed by the larger con- 

 tent of serum substances which cause such faults as greasiness, 

 mouldiness and rustiness. Tackiness may occur, of course, in 

 sheet rubber as well as in crepe. Spots are less important than 

 in crepe. Still, colored patches caused, for instance, by Bacterium 

 prodigiosum may occur on sheets, and spots caused by fungi may 

 also be found. 



MOULDINESS 



Perhaps the most serious defect in sheet rubber is the great sus- 

 ceptibility to mouldiness of the output of many estates. In former 

 years this defect did not trouble the planters so much, as the 

 rubber was sent away from the estate before any mouldiness 

 could develop, and at the ports the rubber was regularly and rapidly 

 handled and shipped. During the war, when shipment was held 

 up, this trouble made itself felt in the producing countries, and 

 has perhaps formed one of the most important complaints. 



It may be assumed that the majority of cases of mouldiness is 

 caused by chests getting wet during transport. If the water 

 actually reaches the rubber, it is absorbed by the sheets, which then 

 show large, white, opaque patches, like partly dried sheets, and 

 with mould developing in many places, a most unsightly lot of 

 rubber is formed, though the real harm done is generally not very 

 great. It does not seem possible to make sheet rubber wholly 

 immune to fungi, as indeed is the case also for crepe. 



GREASINESS 



By greasiness is meant a somewhat sticky and moist feel of 

 the sheet. In a moist atmosphere such sheet may absorb so much 

 moisture that little drops are found on the surface, as if the sheet 

 had sweated. It would be best to confine the term greasiness to 

 a hydroscopicity of the sheets caused by serum substances which 

 attract moisture from the atmosphere. This kind of greasiness is 

 generally coupled with a strong tendency to mouldiness, and is 

 prevented by soaking the sheets in water, after milling, so that 

 the easily-soluble hydroscopic serum-substances are extracted. 

 These two defects, greasiness and mouldiness, do not always go 

 hand in hand. 



The details of the origin and the cause of greasiness arc not yet 

 known. For estate practice the most important point is that 

 greasiness may be prevented by the same means as that applied for 

 reducing the susceptibility to mouldiness, namely, soaking the 

 freshly-rolled sheets in water. Even dry sheets that show greasi- 



* "Estate Rubber. Its Preparation and Testing.' 

 1920. Abstract from Chapter 12. 



By Dr. O. de Vries, 



