5 , !,, I e*,, < +"'df"''
ROSS LMORRIS, ARTHUR E. BARRETT, AND RICHARD E. HARMON Rubber Laboratory, Navy Yard, Man Idand, Calil.
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THEODORE A. WERKENTHIN Bureau of Ships, Navy Dep.r*nent, Waihington, D. C.
WMeompounding a naturaHubbu with GRS in the proportion Po to SO, it i i &a geow to u u 3 par(, d rullw rathu than ~ e u m t p ~ r&n of chou Wled Inblending wid^ GR-S are, in order d prdance, Hevu, G y m g i n , domestic d d n a t d guayule, Maxiun daalmted guayule. Twenty p u u n t b b d i of thew mpccth rvbbvr in a GRS $to& am much bckir kfon rulunimUm than the sormponding 100 per m t G RS hock. At bnt cure and overcure the blends cantdming 3 par(, iulfur ganerally auwn the 100 per cent GRS stocla conthing either P or 3 NIIWin the folloMng ~IPCC& ultimate elongation at 82' and at 200' F., (ur ruistance at 82' and at 900" F., tenslle strangth at 200" F. The blendi .n abaut equ* to tk.100 per cent OR-S in the followh~g mweubr m.sh.niul ' & c ~ . m , ultllmk elongation a f t u o m aging, kmlb h n B h e n ktd at 8P' F. It ahould k noted cht cbm cenclu~lon,~DDIV onlv to blendr of GRS wlth McUrd rubban whid hwa the.&m* c~ackrirtlcru t h o r tmkd her..
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Mexican resinifemus and deresinated guayules, domestic reinifemua and deresinated guayule, chilte, go!denmd, and C ~ ~ p t o s t b E&. Anslytical and other data ooncerning these rubber8 M given in Table I. The rubbers varied widely in purity; Hewn and goldenrod were the heat, and oh& Nbber WM by fat the womt. The Mexican deminated guayule rubber wea exceptional in that ita content of scatone and bemm inenlublea wns oonsiderably lower than the insoluble conhta of the other Buaynb Nbbrs. REWES AND PRWARAnON OF SOCKS
Two recipea went employed in this study. The first wntained the usual 2 parte of d u r ; the second contained 3 parta. The recipes of the control and experimental stocks are given in Table 11. The experimental were similar to the control stocks except that the respective natural ruhbem went substituted for 20 per cant of the GR-S. The accelerationin both recipes WM a d j d so that the control stocks ranched a beat cura in 25 minutea at a87'F.s~judgedbytbetenailepmduct.
LUB known to be deficient in building tack. Borne improvement is obtained by compounding witb large proportions of certain coal tar or reainous aofteana, bnt these materids ara detrimental to many p b ~ c a l m~ertiesof the GR-8 vuloaniaatea. The resilienee or mecbeni. ._ 2.9 0.1 &3 e5ciency of the VulcaniMVtes is always impaired by includ~n 22.9 7.1 of lsrge qwtities of softeners. Any decreaae in d e n c e is 1.1 1.8 particular* undesirable for applicationa entailing rapid &xing, such M tires and drive belts, beesuee GR-8 vulcani9ate-aam inherently poorer in this respect tbsn ~imilarH e m vulcaniaatea. 18.4 S.4 0.8. De* ki,S.unU, W. The anmver to this problem 888rrm to be found in the ohserva2.4 6.9 U. 8. Dspt. hi. Wnu. , CdS. tion by Carlton and Rainbold (1) that substitution of.Heves rubber for ZJ-'26 per cent of the rubber in GR-8 formulatiom which contsm little or no mftener pmducea fair tack, improves 0 t h p m i n g cbaracteriski~~, and actually enhancas tensile pmperties, tear resistance, and d e n m of GR-S v u l h a t e s . The benefita derived from blending Heveg rubber with GR-S ara m outstanding that, in view of the shortage of Hevea, wnaidemtion should be given to blending other natural rubbers with G W . An investigation in this direction might help answer the question ea to bow far this country should go in producing natural rubha. It 888rrm that the prcduction of synthetic rubbern baa gone forward with such &idea tbst adv0cate-aof the In formulating the CBTCBSB stocks, consideration WM given to domestic natural rubber induatry ara having difficulty in justifythe critical mlation which ex& hetween bysteraeis and brittleing expamion of gm*, goldend, and Cryptoategisproduonefa in GR8 stooks. Vila (8) showed that the bigher the sulhv tion. Howevar, if them natural rubbers BDDomplish the w e contsnt, the lower the hyeteresis and the peatar the brittlenem. improvements as HaVaa Nbber W b s n blended with GR-S,this Low hystereSia in, of 001~88,an advantage in mcaen stools, hut factwould be a h n g argument for inoresaing their production. brittleness is not. It WM expected that the presence of natural The purpoes of the work reported berein wea to determine rubber in the stock containing 8 psrte of sulfur would M i t e the Me& of mbstituting esoh of the folIowingrubbersfor 20 per the brittlene&aengendered by the bighm sulfur, bnt would sw1 mt of the rubbe, in G R 8 umnm&c& formulations: Hem, maintain the beneficial &ect of the higher sulfur on hyatemis. 60
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1944
For t h e preparation of the carcass stocks, two master batches were used which contained 80 parts of GR-S and all of the compounding ingredients, including the respective sulfur concentrations. The master batches were mixed on a cold 40-inch mill after preliminary breakdown of the GR-S by thirty passes through the cold tight mill. The remaining 20 parts of rubber, either GR-8 or natural, were added in the final mixing on a 20inch mill warmed to 158" F. The stocks were allowed to rest overnight, and were then calendered to 0.09 inch gage. All samples for curing were prepared from the calendered sheets.
TABLE 11. RECIPESFOR STOCKS GR-9
Natural rubber (as indicated) Semireinforcing furnace black (Pelletex) Zinc oxide Refined coal tar softener (Bardol) Cyclohexyl benzothiaeyl sulfonamide (Sentocure) Sulfur
Normal Sulfur Control Blend 100.0 8 0 . 0 ... 20.0
50.0 5.0 5.0
50.0 5.0
1.0 2.0
1.0 2.0
5.0
High Sulfur Control Blend 100.0 80.0 ,. 20.0 50.0 5.0 5.0
50.0 5.0 5.0
0.75 3.0
0.75 3.0
CHARACTERISTICS OF RAW BLENDS. The improvement in smoothness of the calendered sheets due to the presence of the natural rubbers was definite. The raw stocks were rated for tackiness by hand test. The procedure was as follows: Strips of the stocks were cut from sheets immediately after calendering and were rolled down on Holland cloth. After standing overnight, the Holland cloth was pulled off the rubbers, and the tackiness of a protected surface was determined by pressing the surface against itself with the fingers and then attempting to pull it apart. The following results of the tackiness test are the consensus of two observers: RUBBER IN BLEND None Hevea Mexican resinferous guayule Mexioan deresinated guayule Domestic resiniferous guayule Domestic deresinated guayule Chilte Goldenrod Cryptostegia NATURAL
TACK Poor Good Good Good Good Good Fair
Poor Good
The poor tack exhibited by the stock containing goldenrod rubber was surprising inasmuch as the goldenrod rubber itself was very soft and tacky. CURING. Samples were cured a t 287" F. in a hydraulic press. The samples for tensile and tear tests were 6 X 6 X 0.08 inch slabs. Thosexor the hardness and rebound tests were 1 X 1 X 2 inch blocks, Those for the flexometer tests were 1 X 2l/4 X 23/4 inch blocks. Due allowance was made for the size of the hardness and flexometer samples by curing them 15 and 20 minutes longer, respectively, than the corresponding slabs. All samples were conditioned for a t least 2 days at 82" F. before testing. TESTING.The War Production Board (3) listed the following tests in order of importance for evaluating GR-S carcass stocks: mechanical efficiency, ultimate elongation a t overcure, ultimate elongation at high temperature, ultimate elongation after oven aging, modulus, Shore hardness, tear resistance, and tensile strength. Therefore the following tests were peiformed on the stocks: 1. Mechanical efficiency: ( a ) rebound with Goodyear Healy pendulum, using 15" angle of fall; ( b ) temperature rise over room temperature after one hour in Goodrich flexometer, using 0.25inch stroke under load of 118 pounds per square inch at frequency at 1800 cycles per minute. 2. Tensile properties: (a) tensile strength, ultimate elongation, and modulus at 82" F.; ( b ) same tests at 200" F.; (c) same tests a t 82" F. after aging for 96 hours a t 212' F. in a Geer oven.
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3. Hardness: ( a ) Shore (30-second) and A. S. T. M. hardness a t 82" F.; ( 6 ) same tests a t 82" F. after aging for 96 hours a t 212" F. in a Geer oven. 4. Tear resistance: ( a ) tear resistance a t 82' F. using crescent-shaped specimens; ( b ) same test a t 200" F.
The tests were conducted on the best cure and the 50 per cent overcure of each stock. The best cures were selcctcd from rangeof-cure data on the basis of maximum tensile product. The best cures both of control stocks and of Mexican resiniferous and goldenrod stocks containing 2 parts of sulfur were 25 minutes at 287" F. The best cures of all other stocks were 20 minutas a t 287" F. Thus, the natural rubbers had a beneficial effect on the curing rate of GR-S. PHYSICAL PROPERTIES
The results of the tests are presented in Figures 1 to 8. The designations B, 0, 2S, and 35 signify best cure, 50 per cent overcure, 2 parts sulfur, and 3 parts sulfur, respectively. hlR, MD, DR, and D D are abbreviations for Mexican resiniferous, Mexican deresinated, domestic resiniferous, and domestic deresinated, respectively. MECHANICAL EFFICIENCY. Figure 1 shows that the rebounds of most of the stocks increased as the time of cure or the sulfur content increased. The rebounds of the Hevea and D D guayule stocks were slightly better than those of the control. The rebounds of the M D guayule, goldenrod, and Cryptostegia were about the Same as those of the control. The rebounds of the stocks containing rubbers with high resin contents-namely, M R guayule, DR guayule, and chilte-were below those of the control. The results in the Goodrich flexometer (Figure 2) show that the temperature rise was always less when the time of cure was lengthened or when the sulfur content was increased. These trends conform with the rebound data. The Hevea stocks were the only ones that produced consistently less temperature rise than the controls. For the normal sulfur stocks it was found that the MD guayule, DD guayule, chilte, goldenrod, and Cryptostegia samples exhibited somewhat higher temperature rises than the control, whereas the M R guayule and D R guayule stocks exhibited much higher temperature rises than the control. I n the case of the higher sulfur stocks, the increase in sulfur benefited many of the specimens more than the control. The temperature rises of the Hevea, D D guayule, and Cryptostegia stocks were about the same as those of the control. The temperature rises of the other samples, except the M R and the DR guayule etocks, were not far above those of the control. The temperature rises of the PrlR and D R guayule stocks wcre 15-20' F. above those of the controls.
TABLE 111. SHORE HARDSEW (30-SECOND) Natura Rubber in Blend None Hevea Mexican resiniferoua guayule Mexican deresinated guayule Domestic resiniferous guayule Domestic deresinated guayule Chilte Goldenrod Cryptostegia
OF
BESTCURES
Sulfur Content 2 parts 3 parts 52 54 53 57 49 54 62 56 50 53 52 55 49 52 51 53 52 55
ULTIMATEELONQATION. The ultimate elongations of the stocks at 82 " and at 200 F. are presented in Figure 3. Thc se data show clearly the beneficial effect of the natural rubbers on ultimate elongation,. particularly at 200 F. The resiniferous guayule stocks showed best elongation at 82" F. and very good elongation at 200" F. When tested a t 200" F., all of the stocks containing natural rubbers, except the M R guayule stock, suf-
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Vol. 36, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
fered a greater drop in elongation due to overcure than the respective controls. The ultimate elongations of the stocks after oven aging are given in Figure 4. Oven aging seemed to have leveled off the elongations so that none differed to any marked extent from the corresponding elongations of the controls. MODULUS. The moduli a t 300 per cent elongation of the stocks are plotted in Figure 5. The Hevea, MD guayule, and DD guayule stocks had approximately the same moduli as the respective controls. The other stocks showed somewhat lower moduli than the respective controls. HARDNESS.The A. S. T. M. hardnesses of the stocks before and after oven aging are plotted in Figure 6. These data are given in preference to Shore hardnesses because they are considered more precise. The Shore hardnesses of the best cures are given in Table 111; it is evident from the data that these stocks are somewhat too hard to be truly representative of tire carcass stocks. Nevertheless, it is believed that the directional relations disclosed by this work will be entirely applicable to the compounding of GR-S carcsss stocks. The plotted data show that hardness increased in every case with increasing sulfur. There wtts a tendency for the stocks which contained natural rubber to be softer than the corresponding control before oven aging. They were all softer than the corresponding control after oven aging. TEARRESISTANCE.The tear resistances of the stocks a t 82" and 200" F. are presented in Figure 7. These data show that blending with the natural rubbers augmented the tear resistances of the GR-S stocks at both temperatures, but particularly a t the higher temperature. I n fact, the enhancement in tear resistance at 200' F. was the most benefioial effect resulting from blending with natural rubbers. The tear resistances of the Mends generally underwent a greater decrease due to overcure than the tear resistances of the
Figure41 to 4 ----f
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200
2,TEMPERATURE RISE IN GOODRICH FLEXOMETER
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CONTROL
IAFTER
HEVEA
OVEN AGING FOR 96 HOURS
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MR GUAYULE
MD GUAYULE
DR GUAYULE
DD GUAYULE
4. ULTIMATE
CHILTE
GOLDENROD
ELONCAT ION
EFFECT OF AGING
CRYPTOSTEGIA
INDUSTRIAL AND ENGINEERING CHEMISTRY
Muarp. 1944
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I 7. TEAR
RESISTANCE- EFFECT
Or
TELPERAltJRe-0
TESTED
AT
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