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THE INDIA RUBBER WORLD 



[July 1, 1913. 



Problems in Vacuum Drying. 



By J. P. Devine. 



A PAPER READ AT THE THIRD IXTKR.N ATIO.N AL lU'BnER COXFERENXE, HEED IX XEW YORK, 1912. 



IX the selection of a topic for discussion 1 rallier reluctantly, and 

 yet from necessity, selected one that is intimately identified 

 with the business in which I am engaged, and my remarks 

 may at times seem somewhat personal. Among so many who have 

 devoted their time, ability and energy to solving the many intri- 

 cate problems in the cultivation and production of crude rub- 

 ber, as well as to improving and refining methods and pro- 

 cesses of manufacture of the ever increasing number of articles 

 — the product of rubber, I can assure you tluit my limitations 

 are not an unknown quantity. 



My introduction to and connection witli tlic rubber industry 

 was made about ten years ago when I sought to interest manu- 

 facturers in this country in a new method of drying rubber. I 

 found, however, that the manufacturer was disposed to be quite 

 satisfied with the then existing method and did not liesitate to 

 forcibly express his views and opinions as to any improvement 

 in the drying process by shortening the time or otherwise, ex- 

 cept by sacrificing the quality of the rubber and thereby 

 jeopardizing the standards that years had taken to establish. 



I had been convinced that the Passburg Vacuum Drying Ap- 

 paratus, originated and designed by Emil Passburg and Will- 

 iam Strohn, of Berlin, Germany, possessed unusual merit for 

 economically and thoroughly removing moisture from materials 

 and particularly where it is desired that the final traces should 

 be removed ; as is necessary in rubber and rubber compounding 

 materials, I was ready to and did back my judgment liy my 

 reputation and resources for its introduction in this country. 

 Many installations had at that time been inade in Gerinany 

 fur dfying rubber as well as other materials, and the results 

 from its use, as well as its advantages, were highly satisfactory. 

 Notwithstanding the discouragements of my early efforts, I made 

 investigation as to methods of drying rubber and learned that 

 little, if any, advancement or improvement had been made up 

 to that time, and this confirmed my opinion of the value of the 

 vacuum process, and I have maintained ever since its intro- 

 duction that the vacuum drying process and apparatus introduced 

 in Europe by Messrs. Passburg and Strohn, and in this country 

 by myself, would be and has since proven of inestimable bene- 

 fit to the rubber indu.stry. 



For years rubber was dried by any tncans convenient at hand 

 that would not represent an expenditure of capital, the boiler 

 room or a rootn adjacent thereto serving the purpose, until the 

 effects of the elements upon the rubber began to receive at- 

 tention and study. And then, the improvements consisted largely 

 in the construction of drying lofts, the racks on which the 

 sheeted rubber was hung and the distribution of heat by coils, 

 or by circulation of hot air by means of fans and blowers ; 

 little regard, however, being given to temperature or length of 

 time of the drying proces.s. Until the introduction of the vacuum 

 drying apparatus this primitive method was still in use. and 

 occasionally an advocate is still found, wdio asserts that the hot 

 air method is necessary for the proper curing of some particular 

 grade of rubber. The fallacy of such assertions is proved by 

 the use of the vacuum apparatus drying every grade of crude 

 rubber, and I shall endeavor to make clear to you the claims I 

 make for the superior quality of rubber dried by a properly 

 designed and constructed vacuum apparatus. 



While it is true, considerable thought was given to improving 

 processes for drying rubber, there were no striking departures 

 from the antiquated method of using hot air as the heating 

 medium. The dust and dirt that would settle upon the rubber 

 were the least of the evils; the construction of. special drying 



rooms from which direct sunlight was excluded, and provisions 

 to eliminate dust and dirt, and the regulation of temperatures 

 for various grades of rubber, as well as the attempt to dry the 

 air before being admitted into the drying room, all contributed 

 to avoid the deterioration of the rubber by such means ; but the 

 value of these improvements was doubtful as they only tended 

 to reduce the effect of high temperatures with a consequent 

 prolongation of the drying period. The fact is that the two 

 insidious enemies of rubber are heat and oxygen and these ele- 

 ments arc, and always will be present, and necessarily so, in 

 any system of hot air drying. They are deteriorating agents 

 and their elimination is most essential for the proper drying of 

 rubber. Their elimination by the vacuum apparatus has proven 

 the superiority of the vacuum-dried rubber in the processes of 

 its manufacture. 



Another and serious objection to the hot air system of drying 

 rubber is, that rubber as it coines from the washing machine, 

 contains a very large proportion of mechanically bound moisture ; 

 while this is readily given ofif in the hot air drying room, its 

 expulsion causes a contraction of the rubber, which, witli the 

 oxidation constantly taking place, causes a hardening of the 

 surface that prevents the elimination of the last moisture within 

 the rubber, except by a very prolonged drying period, during 

 which time the rubber is further subjected to oxidation and 

 not unlikely to excessive heat. Unless the last traces of moisture 

 are eliminated, "blowing" is sure to result during the following 

 stages of its manufacture. 



We still hear occasionally about "curing" rubber ; but in 

 reality this is simply the removal of the final traces of moisture ; 

 as stated, under atmospheric conditions, this can only be ac- 

 complished by a prolonged drying period, while under vacuum 

 the rubber is thoroughly dried in a very short time, and in 

 practice rubber is immediately worked up after removal from the 

 vacuum dryer. 



The deteriorating agents — o.xygen and e.xcessive heat — can be 

 eliminated only by the vacuum process and apparatus. This 

 process and apparatus alone afford the proper conditions to dry 

 rubber rapidl}-, uniformly and thoroughly at a low temperature 

 and without oxidation, independent of climatic conditions. 



It must be liorne in mind that under atmospheric conditions 

 a rapid boiling can only take place at 100 degs. C. or 212 degs. F. 

 and that as the temperature decreases, the drying titue is ex- 

 tended ; while under vacuum the boiling point is greatly de- 

 creased and increasingly so as the barometric reading is ap- 

 proached. To illustrate, under a vacuum of 29 in. water boils 

 at 25 degs. C. or 77 degs. F. Rubber dried in the vacuum cham- 

 ber, while the first free water is being removed, will not need 

 to be heated, practically, above the boiling point of water at 

 that particular vacuum. As the moisture is evaporated from 

 the rubber, naturally the temperature of the rubber being dried 

 tends to increase ; to prevent any overheating the supply of 

 the heating medium — steam or hot water — is regulated accord- 

 ingly and entirely shut off before the final drying; the last traces 

 of moisture are therefore drawn off by the latent heat in the 

 dryer accelerated by the high vacuum. Because seemingly high 

 teiTiperatures are used at the beginning of the drying process 

 to expedite evaporation, the erroneous impression is sometimes 

 formed that the rubber is overheated in the vacuuiu chamber ; 

 but in a properly constructed vacuum chamber with its auxil- 

 iaries — condenser and pump — properly balanced, the application 

 of well-knowm physical laws absolutely prevents any overheating, 

 if only reasonable care is taken in its operation. 



