THE INDIA RUBBER WORLD 



219 



to cuijtaiii ,Tii.\ results similar to those they have obtained in 

 respect ot (1) the large increases in weight on storage, (2) the 

 formation of a large aqueous extract in the samples, and (3) 

 the conversion of the sulphur to a large extent into aqueous 

 soluble compounds. 



PRACTICAL SYNTHETIC RUBBER. 



The practical manufacture of synthetic rubber in Germany 

 under war conditions is described in "Gummi-Zcituug," July 11, 

 1919, page 750. The most available source was found to be 

 calcium carbide, from which methyl rubber was produced from 

 acetone in accordance with the following procedure : 



SYNTHESIS OF RUBBER. 



The acetylene gas produced from calcium carbide and water 

 changes in the pressure of contact bodies by the addition of 

 water into acetaldehyde. This is oxidized with acid into acetic 

 acid, which is separated into acetone by elimination of carbonic 

 acid by blowing over a contact substance. This liquid is diluted 

 wilh lienzol and under proper conditions is reduced in the 

 presence of aluminum scrap into pinacone, a crystalline substance 

 of 42 degrees C. melting point and 170 degrees C. boiling point. 



After reduction the mixture is diluted with water and distilled. 

 Any undecomposed acetone-benzol mixture is first removed and 

 later parted by separate distillation. Only on further distillation 

 are these converted into oils, boiling at a higher temperature, 

 most of which contain pinacone. These higher boiling oils are 

 treated with water, whereupon pinaconehydrate appears as a 

 white crystalline product. This is then treated with acid salts 

 on free acid to obtain dimethyl butadiene having a boiling point 

 of 70 degrees C. and is freed by fractional distillation from the 

 by-product pinacoline, a mixed ketone boiling at 106 to 120 

 degrees C. The dimethyl butadiene is placed in vessels where 

 by the addition of a small quantity of a catalyzer it is polymerized 

 into methyl rubber. The total amount of methyl rubber obtainable 

 from acetone is about 40 per cent. 



COMMERCIAL VARIETIES. 



Methyl rubber was produced by the Farbenfabriken, Elberfeld, 

 in two qualities, one designated as "H" and the other as "W." 

 The H variety results from cold polymerization (about 35 degrees 

 C. ) and is adapted to the manufacture of hard rubber, while the 

 W variety results from warm polymerization (about 70 degrees 

 C.) and is suitable for making soft rubber goods. 



PROPERTIES AND DEFECTS. 



Both sorts of synthetic rubber require a long time for polymeri- 

 zation, the H three and one-half months and the W as long as 

 five months. Neither form is stable but oxidizes readily in the 

 air as soon as formed; consequently a preservative agent is 

 required to prevent this. At first, aldehyde ammonia was used, 

 next piperidine, followed by experiments with other agents. At 

 the same time a vulcanization acceleraator is incorporated. 



On account of their instability these rubbers are shipped in 

 tightly closed zinc-lined iron containers. 



H SYNTHETIC RUBBER. 



More of the H grade is used than of the W. It is yellowish in 

 color and very dry and stiff because the masses are heavily pressed 

 together, expelling the air from the interior to secure protection 

 against oxidation. The characteristic properties of extensibility 

 and toughness which mark natural rubber are present in much 

 less degree in methyl rubber. The latter crumbles at first on the 

 mill almost completely and reunites into a sheet only after pro- 

 longed milling. 



The milled rubber becomes capable of taking up compounding 

 ingredients only after an hour of milling. Likewise, long milling 

 is found necessary to eliminate porosity and secure solidity in 

 molded work. The milling should take place only on moderately 

 warm mills. Under hot milling. H rubber becomes sticky. In 

 order to increase its capacity to absorb filling materials it is 

 found advisable to wash the rubber thoroughly. It then becomes 



necessary to dry it completely at low temperature to avoid oxida- 

 tion and conversion of the rubber into a dark sticky mass in which 

 condition it is not suited to vulcanization and produces only 

 porous products. Washed and dried H rubber is even softer and 

 more elastic after vulcanization than products made from the 

 same rubber unwashed. 



When heated in the air to its melting point, H rubber oxidizes 

 and the rubber molecule breaks down, as inferred from the fact 

 that when similarly heated at considerable temperatures in inert 

 gases such as nitrogen, carbonic dioxide, and sulphuretted hydro- 

 gen this effect is not observed. 



To increase the ability of the rubber to absorb compounding 

 ingredients the Farbenfabriken, Elberfeld, recommends the addi- 

 tion of a certain percentage of their ER solution or from five 

 to ten per cent of pinacoline. The addition of a small amount 

 of reclaimed rubber permits the more ready absorption of large 

 quantities of mineral ingredients. 



To increase the elasticity of synthetic rubber a number of 

 minerals are recommended such as diphenylamine, diamethyl- 

 aniline and toluidine. 



The chief accelerator employed is the preparation known as 

 Vulcacit. This material also acts powerfully in the acceleration of 

 the cure of natural rubber. 



The W variety of methyl rubber is much darker than the H 

 grade. It is reddish brown in color. In structure it is net-like 

 and is strongly suggestive of natural rubber. On the mill rolls 

 it is tough and would readily be mistaken for natural rubber. 

 It differs, however, by reason of its internal stickiness. On its 

 torn edges it presents fine silk-like threads. While both grades 

 are suitable for mold work the W grade is not so readily mold- 

 able as the H grade. In the case of plied up articles extra 

 pressure must be used during the vulcanization to insure adhesion 

 of the component parts. 



NATURAL AND ARTIFICIAL RUBBER. 



The investigation of A. Tschirch on the constitution of na- 

 tural and artificial rubber has been published in "Schxccitzcr 

 Chemikcr Zeitschrift," 1919, pages 153-156. In abstract his 

 results are as follows : 



The term rubber covers a variety of raw materials. Those 

 of African origin furnish colloidal solutions with chloroform, 

 whereas Hevca and Manihot rubbers are only slightly soluble, 

 though they swell up considerably in that solvent. The term 

 "synthetic" rubber is incorrect because the synthesis is confined 

 to a single constituent which the author designates as "caout- 

 chougutta," a mixture of hydrocarbons in various forms and in 

 varying quantity m one and the same kind of rubber. 



The substance known as synthetic rubber is not identical with 

 the caoutchougutta of the natural article. Natural rubber is 

 by no means the unaltered sap of rubber plants. It contains 

 protoiactoretin, and undergoes alteration shortly before it exudes 

 into the air, further changes ensuing during coagulation and 

 smoking. Protoiactoretin is a polymer of isoprene, and is not 

 the same in all rubber plants. Commercial rubber is regarded 

 by the author as a semi-manufactured article rather than a 

 natural product. 



According to the author, synthetic rubber is analogous to 

 Para rubber, the artificial caoutchougutta being derived from 

 aliphatic hydrocarbons (mythylated butanes). The varying 

 solubility of rubbers probably may be due to the structural con- 

 ditions of the colloid substance. Rubber occupies an intermediate 

 position between the emulsoids and the dispersoids, and is hetero- 

 genous, containing at least two structural phases, a solid col- 

 loidal solution. Under the microscope Para rubber that has 

 been extracted with chloroform is seen to consist of a fine 

 network of minute straight and bent rods, forming the turges- 

 cent component, which, however, in the case of extracted syn- 

 thetic isoprene rubber, is irregular and not reticulated. 



