Guayule Rubber in Tires and Tubes Service Tests in Which the Rubber Was Exclusively Guayule J. H. DOERING, T h e Firestone Tire and R u b b e r Company, Akron, Ohio
T
Tires and tubes have been made in which the would affect Processing of the country upon o u t s i d e rubber used was exclusively guayule, These stock a n d t h e q u a l i t y a n d serviceability of t h e p r o d u c t sources for its s u p p l y were of the 4.50 X 21 size and were tested in adversely. of rubber is well known. At the Florida ouer a period of 2 years. These tires present time, with no PROPERTIES O F STOCKS failed at mileages between 8500 and 10,500 betions, with rubber prices a t their TESTED p r e s e n t low l e v e l , a n d with cause of tread wear. The inner tubes gave satisfactory service throughout the duration of the I n view of t h e e c o n o m i c nothing to hinder the steady inimportance of guayule rubber, flow of Hevea rubber from the test. and a t the request of Edison, Middle East and other foreign so u r c e 8 , t h e r e would appear The Problems connected With the development it w a s d e c i d e d several years ago to test t i r e s a n d t u b e s , and processing of the compounds are discussed, to be little need f o r c o n c e r n and the formulas are given. the rubber of which consisted Over t h i s dependence. Howsolely of this American-grown ever, it is not inconceivable that t h e i m p o r t a t i o n of rubber product. These tires a n d tubes were made in the 4.50 x 21 size and were tested on a might be cut off or greatly reduced. It was the possibility of such a situation that led Edison, Model A Ford service car operated by the Edison Botanic in the last few years of his life, to make an extensive search Research Corporation at Fort Myers, Fla. The guayule for rubber-bearing plants and shrubs which, in addition to the selected was one of the regular brands being supplied to the guayule shrub, could be grown within our borders. While trade and contained from 18 to 20 per cent of resins. It was considerable progress has been made with this important early realized that in the development of the stocks the project, it has not reached the point where rubber in commer- problems of processing would have to be considered as much, cia1 quantities has been produced. Therefore, for rubber that if not more, than those of quality, because of the high resin can be grown within our border we have a t present but one content of the rubber used. For the tread and side-wall commercial source-the guayule shrub. The development stocks, a formula that was being used in regular production, of the guayule rubber industry, originally dependent on the specifying 41 parts gas black per 100 pounds of rubber and wild Mexican shrub but, within the last decade and in the di-o-tolylguanidine (D. 0. T. G.) as the accelerator, was southwest, placed on a plantation basis employing efficient selected as the base stock. The oils ordinarily used for harvesting and extracting methods, is a creditable achieve- processing purposes were omitted, and the ratio of sulfur to ment ( 1 , 2 , 3 , 5 ) . 100 parts rubber was increased from 3.0 to 4.25, in order to There is no doubt that in an emergency guayule rubber obtain the hardness of the cured stock believed to be neceswould receive first consideration, not only because it can be sary. A comparison of the flexing results on the guayule produced commercially, but because it is the product of an stock and the base stock showed the former to be inferior, industry that needs only the encouragement of higher prices however. I n order to improve its flexing behavior, the gas than now exist for rubber to produce guayule far greater in black content was reduced from 24.0 to 20.5 volumes per 100 amount and better in quality than that supplied in the past. volumes of rubber. It was realized that this move would There would be a period of several years in which the supply probably reduce tread wear resistance and make the stock of guayule would be limited to that amount which was stored softer and more difficult to process, but it was felt that an in warehouses and in plants that had already reached the improvement in flex-cracking resistance was needed. Furproper age for harvesting. Increased amounts aould not be ther improvement in this respect mas made by a change in expected until new plantings became sufficientlyold. Spence antioxidant. The final formula for test was: (5) has stated that this would take a t least two years. I n the 100 Phenyl-#-naphthylamine 2.1 meantime, it is probable that the rubber requirements of the Sulfur 3.5 D O.T.G. 0.7 country could be met from the rubber in storage, the reZinc oxide 4.5 Gas black 29.6 stricted amount that could be imported, reclaimed ribber, and The modulus and tensile characteristics of this stock (A) synthetic rubber. us. those of a production tread stock (B) used for comparison The chief drawback to the extensive use of guayule in the (cured a t 298" F.) are as follows: past has been its high resin content. This handicap can be Cure, min.: 25 35 45 55 65 overconic by complete deresinification, as mas shown by MODULUS AT 400 PER C E N T E L O N Q ~ ~ 1 0 x 4 Spence (4, G), who made rubber compounds from completely A 690 790 850 880 810 deresinified guayule rubber that had tensile properties apB 760 1040 1230 1380 1480 proaching those of compounds made from Hevea rubber. T E N S I L E S T R E N Q T H AT B R E A K " A 2680 2750 2600 2450 2690 The deresinification process is costly, however, and would be B 2590 3300 3650 3450 3500 practical only as an emergency measure. a I n pounds per square inch. Despite its high resin content, guayule rubber has found rather wide application. It has been used in limited amounts While the modulus and tensile strength values of the guayule in tire stocks, the amounts being governed by price, by the stock (A) are l o m r than those of the production stock (B), service requirements of the product, and by the ability of the rate of cure is somewhat faster. the production departments to handle the stock. However, Although the guayule tread stocks were soft, they were it is obvious that the use of guayule beyond certain limits handled without much difficulty. Some care had to be HE dependence of this
takeii to cool the treads tiioroug1:liIy before "himking" t,Iioin and n o t to pile them one on another during storage. In tlie case of the ply stock, processing was more of a problem and became tlie main iasno in develoliing tile stock. Oils and softeners were again omitted, except for a sriiall amount of stearic acid for activation of the accelerator, since the resins in t i l e guayule were more than aniple to meet tlic requirements for processing. Starbing vitli a nonpiarrient,ed st,ock, which was found to be mucli too soft am1 sticky for processing purposes, several pigments known to exert a drying
effect, U~OLI tlie stock were tried; they were added in sufficiently large amounts to reduce the stickiness to a point wliere calendering and subiequent liandling of tlic fabric were gossihle. One of tlie thermal-decompositioti carbon blacks, when u s d in the proportionof 10 to I5 parts per 100parts by weight of guayule, was sufficient to prodnce a st,ock that could be handled. At the same time, a slight recnforceincnt in niodulus and tensile properties was obtained. The stock selected for test had tlie following formula: G"*YUk
Sulfur Zinc oxide
100 3.9
3.0
Tlieinmldeeoiupn. blsck
Sleario acid
~lleiosptohenaotliinaoie
12.0
100 4.4
stearic mid
\leioapto~Jsnr"tllia.oie
6
n-Addophenol Antioxidmta Aooelwrstor Ab
1.0 1.0 0.6 0.1
Tiiermnl-deoompi.>,blnok 8.5 Amelerator B e Pent m-toluylenediamine. Condensstion product oi aoetaldldebyda. butyraldehyde, and aniline. TeLrametl~yltliiurnin monoaulfide.
92.5 per cent 8henyl-~-"*~l,thuiamine7.5 per
Tlie pamidopbei~olwas added in order to obtain stiffness in the uncured stock, and the antioxidant to improve flexchecking resistance and to stiffen t,lie uncured stock further. The nioduliis and tensile charact,eristios of the stock, cured at 307" F., nre as follows:
1.2
A gui~yulecushion stock was also developed and used in the breaker and cushion of the tire. It liad tlie following formula: Zinc oxide
100 4.0 3.5
0.45
Tire modulus and tensile characteristics of tliis st,oek wbeti cirred n t 270"F. are shown as follow: zioili.Li-x * . r o iii.oau.,TloN ~ ~ ~ 'reNniL.srileniirw Ir H,.*;..~ cure,,axaT,, :in 15 60 75 :io 4 5 ~ 60 7s l.i,./ri,.ia. :mi 1IO 5:m BIU lY50 2300 2360 2060
Guayule suiiur
sucti n stock difficult,. I'iginents known to have a drying effect were tried, but tlie results in all cases were that the stock became mushy rather than firm. I n no case nms a stock developed which was entirely satisfactory from a fac,tory standpoint. Ry special Iimdling during preparation and by reducing tlie period of storage between extruding and curing to an absolut,e minimum, it was possible to obtain molded tubes wlricli were uniionn in gage and free from molding defects such as tliin spots, pockmarks, and heavy rind, A few lubes were made from calendered stock, and cured on R mandrel, but the difficnlties encountered due to calendering blisters, which sliowed up as thin spots in the finished tube, due to "pull down" wlien removing the calendered stock from tbe liner in which it was stored, and due to the sticking of the cured stock to the wrapping tape, m i l i t a t e d against this method or processing. I'erbaps the worst handicap in connection wiili tlie use of guayule rubber in tube stocks ~ - 1 1 the s presence of hark and dirt. I n the cxse oi t.lie calendered stock these gave rise to piiiliolrs and rapid seepage of air from inflated riir(4 tules. I n the case of the extruded st.ock, the dirt lodged itself in the orifice of the tube maaliine, cansing a slit or scratch in the stock l&g extruded, and it \vas necessary to scrap this stock. The removal of the dirt, as well as the rrduction oi the resin content, would have to be accomplished before guayule rubber could be considered for use in manufacturing tube9 . The dirt, could undoubtedly be removed by the nso of modern straining equipment, such as is used in the cleaning of reclaim. Several stocks were tested in service, and tlre one which proved best had thefollowingformula:
1.6
0.03
59.0
Tlie tires containing these stocks were cured for 60 niiniit,es at. 29980 I?. The development of a guayule tube stock was almost, 011tirely a proposition oi producing a stock sufficientlyfirm and dry to be calendered witliont idisters or witbout sulxcqnent, sticking i o tbe liners in which it was stored, or producing a stocrlc with sufficient body so that aiter it was extruded it, (;auld pass tlirougli tlie various subsequent manufaeburing operations without "thinning out." The large arnonnt of rosin cont,ained in guayule rubber made the development of
Cure, / I / i / / . Lb./eci,in.
\looa',"r *'F600% CLosunllos '1~v.Nnrl.aSTleeura , r 13,
