ZH of the several available oil-reSiating synthetic ubbers has a unique combinstion of inherent p r o p
Perbunan Properties
and Compounding R...A...Moll'. . - - R. ,-
M. Hoi
r
md D. J. Buckley'
L ties. By compounding methods t h w properties my& modified to suit the rqukementu of a range of applicatiow Compoundins modifications am,however, always limited by @e basic charactmisties of the polymer. For a wide vari@y of UBBB, therefore, no single rubberlike material can pmSjd0 the optimum balance of critical vulcanized compound pigp d e s . Indeed, the best a d d selection of one for a particular purpose will in any case involve on one or more speci6c qualities. The broad principles of compounding for one type of oilresisting synthetic rubber, Perbunan, have been daaoribed by several writers (4-7,s1J). The more importantLiterature references are l i d by Wood (16). The purpose of thin paper in to point out mme ~ y ~ latitude pasaible t t h e off applicati a d o r y ~ p polymer. It in further *dependence of certain p r o w e a of butadi polp-th respect to variations in polymer corn-
Procealng operations, the two outatandingproblerms m aonnection a h t e handline of Perbunan are the rela-
In factory pro
Err0 Laboratories, Standard Oil Dewlopmant Company, Elizabeth, N.
+
Thniques for processing conventional rubber factow viewed. In most cases proper compounding i s a compromise between optimum vulcaniz quality and ease of fabrication. Thus, Perbung may be chemically plasticized by hot Banburying with xyl* mercaptan a t moderate sacrifice in some cured p r o p erties. Most of the common rubber accelerators of vulcanization are effective in Perbunan. Where certain special characteristics are required, particularly low hysteresis and optimum high-temperature properties, acceleration i s specific. Major variations in Perbunan compound types are effected by changes in pigment loadings and softeners Data on several representative softeners and pigments are presented. Some properties of mixed pigments are shown. Perbunan may be compounded for applications involving contact with the various petroleum products used in fuel, lubrication, and hydraulic systems. Low-temperature properties of various Perbunan compounds are described. Cured with a thiuram, it has excellent compression set characteristics at
1910 c.
It i s possible to vary the Perbunan type, for instance, either low-temperature flexibility or resistance to solvent swelling may be improved at the expense of the other property.
(13).
e standpoint of vulcanizate quality, the beat for plasticidng Perbunan comprises exed milling on tightset cold rolls of dSerentiaI ape&. tory practice it is seldom deemed economical to prolong mastication for the period of time required to obtain optimum plasticity. Most dS5cultiea encountered in the aubuequent operations of extrusion and calendering are a result of thin reluctance or inability to spend time and equipment capacity on premastication of the polymer. Williams and Smith (14)claimed the we of aromatic mercaptan to give a more plastic rubber product. Bryan and Habgood (1) patented a process for plasticbhg aynthetic rubberme material^ in which butadiene, or, a homolog of butadiene, in the main ingredient. This method involves incorporating therein one or more plsstieiserS of the c h which comprises aromatic mercaptans of the bensene and naphthalene series. These inveatigstora cautioned Sgsinst allowing the stock containing the plesticizing agent to become hot. In the test data cited, worded temperatures of the 6nal mix were of the order of 74-77' C. Ofthe chemical plasticisem for Perbunan investigated in this laboratory, xylyl mercaptan appean, promising for p m duction use. The plastioimtion reaotion was studied with controlled variatiom of xylyl mercaptan content, temperature, and time of Banbury mastication. In these experiments a size B laboratory Banbury mixer was employed. Plasticitg measurements were made with a Williams plastMneter at SOo C. using a 10-kg. weight, and also with a Mooney v i s cometer operated at 100' C. The plasticized ~ampleswere allowed to rest 24 hours before te8ting. Figure 1 illustrates mme deck of xylyl rdenxtptan concentration and temperature of mastication on the plasticity of Perbunan. It in apparent, particularly at the h i g h mastication temperature, that plasticity tendsto increase with
E4-C
.
I
..
\
i=
noticeable in either calendering or extruding operations carried out on a laboratory scale. Perbunan plasticized for 10 minutes at a Banbunr temneratwe of 166' C. in the DEZ+ enm of 0.90 part of xylyim&ptan gave smoother calendered ah&, with less dimensional ehanae than a similar Perbunan stock which was conventiod~masticatedon an open laboratory mill. The behavior of these two compounds in the laboratory extruder was 8imilar. When compounds are prepared from Perbunan plaeticised with 0.90 part of xylyl mercaptan in a Banbury mixer at 166' C., a moderate e a c r i h in vulcanized properties is enWed. The data in Table I illustrate the d e c t of this type of plssticisstion on product quality. It is indicatad that those small resulting chsnges in vuloanid properties are ~ u 8 ceptible to impruvement by adjustment of the sulfur c o centration of the comwund. The freeze k t a n m of Perbunan compounde conknhg polymer plastioised with xylyl mercaptan, in the manner outlined. in not materiallv different
150
W A
A
Fi ure 1. Effect of Concentration of Xylyl dercaptan on Plasticity of Perbunan, Banbury-Masticated for 10 Minutes at 165' C. mercaptan concentration. Practical considerations, however, Buggeet the UBB of about 0.9 part xylyl mercaptan in conjunction with a stock temperatwe of appmximately 165' C. [This corresponds to 2.5 pa& RPA No. 3, which was used in this work. RPA No. 3 contains 36.6 per ceht xylyl mercaptan and the balance inert hydrocarbon. The manufacturer's bulletin @)should be consultad for safe handling practices.] Perbunan masticated under these conditions is more olastic than that broken down under routine omduction condiions on 8 &inch mill, and is equivalent in p k i c i t y to that obtained on a cold, tight laboratory mill under ideal conditions. The time of Banbury operation dunng the plasticiring of Perbunan in the preeenoe of 0.W part xylyl mercaptan at a Banbury temperature of 165' C. was found to be not critical within the range of 5 to 20 minutes. However, in Figure 2 the data suggest that &cation under these conditions for periods in exm of 20 minutes may tend to c a m the Perbunan to be less plastic. Although the Williams r e c ~ v e r ydata for tbia chemically plasticid Perbunan are, in general, higher than those ohtained on mill-mticated Perbunan, no adverse d e e t iq
Figure Q. Effect of Banbury Time at 165' C. on the Plasticity of Perbunan Containing 0.90 Part xylyl Mercaptan
Laboratory data have thua indicated that Perbunan may be rapidly, &ectively, and economically plastiakl by chemical meana in those ~ 8 8 8 8where BOLIE s s d m in vulcsnired propertiea can be tolerated. T h i ~is a promising field of invethigation in which little work has been reported. Exprience with tbia technique on a factory scale is limitad at this time. In any attempta to project the labomtory experience described to production scale, full consideration should be given the p d b l e toxicity of matariala handled. Accclcreton of Vulcanization
(fa
T a t &ts
It has been shown that representatives of all common classes of rubber acmlerator~may be nsed for promoting the Perbunan vulcanization reaction. Bensothiasyl M d e done, or in combination with amall amounta of a gnanidine, hes beem mwt widely ueed for general-purpose acceleration. Many other combinations are eqnally mitable for most applications. However, w h certain specisl- c ' '08 are required, particularly low hptmeeh and optimum resistance to detmioration by heat, aceeleration is sped%. E m c r ox ACCELEBATION ON HYSTEZESIS PBO~TIES. Five Perbunan compounds w n t a i i vsrious aadelerstors, b g e t h e r w i t h f ! Q m 'e ~and hystsresis data, are given in Table 11. The ditiereemces in bystex& properties due to
******
l2E6
INDUSTRIAL A N D ENGINEERING CHEMISTRY
choice of accelerator are more clearly indicated here by heut build-up during flexing in the Goodrich Flexometer than by the rebound resilience data obtained using a constant angle of pendulum release. The temperature rise is shown in Figure 3 aa a function of the time of flexing under relatively severe conditions of load and strokefor one cure of each of the five compounds. Bantocure (condensation product of mercaptobenaothiazole and cyclohexylamine) is shown to be superior to benzothiazyl disulfide, while Bafex (dinitrophenyl dimethylthiocarbamylsulfide) gave the lowest heat build-up of the three accelerators. Activation of bemothiseyl disulfide by diphenylgusnidine, or of Safex by bemothiasyl h a d e , yields a marked improvement in the height and dope of the curves of temperature rise w. time of flexing. The accelerator combination of 1 part Safex plus 0.25 part benaothiil disulfide in conjunction with 2.5 parte sulfur is therefore shown to be one type of acceleration producing vulcanisatea of specilically low hysteresis. The tensile 8trenpth, elongation,and modulus data indicate that superior hysteresis characteristics are obtainable only with very tight c u m and a consequent tendency toward "shortnees" or brittlenw. Any degree of compromise between optimum mechanical &ciency and maximum tensile product is therefore available. E m m OF AC~LEFATION ON HEATREmmmm. When compounds of Perbunan are subjected to prolonged exposure to elevated temperaturea in the presence of air, a gradual hardeningoccm, which is accompanied by lose of rrxtensibility and tensile strength. The rate at which this type of degradation proceeds is controllable to some extent by choice of pigment loading and softener (13). Major improvements in reeietance to this deterioration phenomenon, however, are accomplished by vulcani5ation without added elementary
r
0
I
m
***
Vol. 34. No. 11
I 20
I
so
TIHE OF REXDK. IN UNJTLS, CWORW FLLEXOYLTLR
Figure 3. Effect of Accelerator Type on Hysteresis Properties of Perbunan
sulfur.
Data on tensile, elongation, and hardness,before and aftm expoaure for 50 hours in an air oven at 121' C., for a tetramethylthiuram disulfidecureandalsoabensothiaayldisulfid~ sulfur cure are given in Table 111. These data indicate that the tetramethylthiuramdisdlide cured Perbunan has superior aging characteristics, especially in respect to retention of Compression set d u e s , extensibility and original hard~~w. obtained at the conventional teathi3 temperature of 70" C., show the two compomds to be approximately equivalent. However, when this teat is repeated at 121' C., the tetramethylthiuram disulfide cured compound hss a compression eet which is lees than half of that obtained on the bansothiazyl disulfide accelerated stock. Thus, retention at an elevated tempmature of the rubberlike p r o p r t k of extensibility and abfity to recover after deformation has been considerably enhanced by choice of accelerator. Table
II. Elled Or Accelerator Type
on Hy8tmais 01 Perbunan
formula Peb100,sine oxide S . b r i o wid 0.5. a.&inforoiu 46. dibutyl phthhta 20. d u r 2.6. wder*tor Y .horn
B-
blwk
fur-
.. 1% '. .. .. .. .. 1 ..k
0?46
eo
66
1.26
1.26
0.26 1.00
"
::
T r t dnt.
S6 66.4 110
20.4
ea
60.7 60.4 B0.S 66
8.1
ea 76 14.7 16.2
e4 60.8
s¶
1.1
Pigment Lordinm
Major altmations in Perbunan compound types, with resped to most properties, are effected by changes in pigment loadinga and softenem. S i c e the vsrious carbon blacks are by far the most d u l pigmenta for Perbunan, a clear understanding of the effecta of varying concentrations of these blacks is of fundamental importance. For most purposea a medium-partidW channel black, E semireinforcing furnace black, a soft thermaldecompcsition black, or some combinstion of theae materials may be used to beat advantage. In the three upper graph of Figure 4 the effecta on tensile, hardnw, elongation, tear, and compression set, of varying concentrations of these three carbon hlsck types are shown. The data are for t y p i d 1.6part sulfur (benmthiawl disul5de accelerated) vulcani5at.a with curing time and accelerator conmtxation approximately adjusted for maximum tensile strmgth. The softener neto facilitate factory pmoeesing wae omitted to accentuate the effect of these carbon blackson the physical propertiesunderinvestigation. In general, the thermaldemmpositi6n type black is used where high volume loadinga are required to reduce cost or volume change in contact with solvents. Compound8 oontaining high loadings of such blacks are extensible, rubberlike, and not PoLDeaSivelyhard, but are characterized by rather poor tear resistance. Channel black is applicable only in medium concentrstions because of ita marked stifiening effect. Me-
N&#
1942
***
INDUSTRIAL AND ENGINEERING CHEMISTRY
* * ** * *
1287
1286
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INDUSTRIAL AND ENGINEERING CHEMISTRY
***
Vol. 34, No. 11
lhas of rubberlike properties with decrease in temperature has been measured by various methods (8,10,1.9,16). Theae techniqueshave ' (B-o form* Peb100. M o oxide 5 . ~ 6 mid 0 1, .omMorOjng furbJML 75, be-thiawl di.uIKde 1. mlfur 1 .5. .oftanor. 20) involved determination of either the degree of DBP SCT TBEP DE8 DEGP DVR XM retention at various low temperatures of a critiw ~ p ~ t y r e m " 01y (70' C.. 64%. cal property such as modulus or the brittle I d ) 137-7 162-12 141-13 142-10 188-81 169-10 19743 temperature at which the sample f& to survive a sudden shock. Many acceptance speciBo 60 60 46 e4 60 Bo fications have recently required that strips of 2170 u80 2250 a870 2240 1140 aux, 3w 640 10 3m 880 370 MI0 compound be bent through a 180' anglearound a 0.375-iich diameter rod at a specified tem1410 2154 192O 1980 l4Kl lSa0 1sa6 Bo 58 M ea 57 58 63 perature without breaking. As a general criterion of low-temperature performance, this 100 100 100 81.7 95.5 100 86.8 method appears inadequate. Many compounds 38.2 41.1 34.8 69.3 37.5 37.8 43.4 may be bent without breaking, in a manner 18 15 9 10 16 17 11 similar to tire bending of a metal strip, at temperatures below that at which their rubberlike charactaristica are lost. The bardness values 19.7 a2.e 18.0 18.1 18.1 17.3 20.4 given in this paper were obtained on a mcdiiied asphalt penetrometer, employing an increased weight and a needle with an hemisphericaltip. 1.6 0.8 9.8 0.6 7.7 2.3 8.1 For convenience in interpreting results, pene5.7 1.6 10.6 1.6 11.0 5.1 10.1 trations were d b r a t e d against Shore dnrometer readinps, and are reported as such. Curves of hardness vu. temperature for each of the Beven test compounds are given in Figure 6. The dabs show that dibutyl aebacab, tributoxy ethyl phosphate, and dibutyl phthalate (in that order) are most dective in prevenfmg From the standpoint of degree of plasticization imparted hardening at low temperatures. Diethylene glycol phthalate to the unvulcanized stock, the softeners dibutyl phthalate, and soft coal tar impart poor f m e reaiatance. It is notable tributoxy ethyl phwphate, and dibutyl Bebacate are outthat the compound containing diethylene glycol phthalate standing, as shown by the william^ plasticity data. Msactnally has poorer freeze resbtance than the one containing terials of this type are therefore generally to be preferred, no softener. except where softeners with lesa favorable processing charE ~ COQTCONTACT WITH PETROLIWM Oms. Perbunan ia acteristics impart certain highly desirable properties to the baing used for hase connections, gaskets, and miscellaneous vulcanized compound. molded parta which are in continuousor intermittent contact The strea&etrSjn characterktica of all compounds in this with petroleum oils. The volume change of a rubber in oil is series are similar, except that those contsining Duraplex C-50 LV 100 per cent, dibutyl aebmate, and soft coal tar of importance in two connections. If the rubber swells exmxively, it will m l y be weakened and the construction have somewhat lower moduli of elasticity. Likewise comdesign of the part may be seriously distorted. On the other pression set values indicate the approximate similarity of hand if, because of extraction of compound ingredients. it these compounds, although the stock containing chemically premasticated Perbunan charaoteristically has a slightly higher Bet. Thus the compounds studied are approximately I similar in physical properties at ordinary temperatures. The tendency of a Perbnnan compound to become stitI and lose its rubberlike properties with decrease in temperature is strongly in9uenced by softener type and concentration. The Toblc IV. Repnxn*Hve S0Renn.n in e Pabunan Formula
E
,a
10
b3
90
,m
110
IW
1-
L
*NILINI POINT, OFCREcs C.
Figure 5.
Effect of Softener Type on Freeze Resistance of Perbunan Compounds
Figure 6. EKect of Softener Type on Volume Change of Perbunan Compounds in Oils of Vwying Aniline Points
.
INDUSTRIAL
N c m n h r , 1942
Figure 7.
A N D ENQINBFRING CHEMISTRY
* **** *
Effect of Softener Type on Volume Change, Tensile Strength Retention, and Compounds in Gdsoline
e ofTerbundn
*
'*
ahrinLB markedly in volume, &et sasls will be rendered inetrectusl and other di5cdtiea encounted. It in therefom generally conceded that most compounds should show alight or no volume ahrinkage under d c e conditions. The degree of pwitive swelling permitted vsriea with the application. Campounding with 8 4 amounts of less oil-raktant synthetic rubbers, natural rubber, or reclaimed rubber o m s i o d y has been employed aa an expedient to ob& a positive swelling characteristic in high-aniline-point oils. Csrman and cc-workem (8) showed that the logarithm of the volume increase of a synthetic rubber compound in an inverse function of the aniline point of the oil in which it is i m m d . The following four test oils were selected for this work: Aniline Point.
c.
Lubricaliu oil A Lub-ting oil B mdmulio oil c R-&ID
-vmmit-
(Isybolt
UniwraJ .%C.
125
loo
ea
150
110 88
Te-r
tun. 0
c.
98.9
60 95
'
98.9 87.8 87.8
The p r o w oil, which because of ita low aniline point haa been much specilied 88 a test liquid for synthetic rubber, is included for refersme. Figure 6 shows volume change ws. aniline point of the im.memion media for each of the seven test compounds. The compounds divide into t6ree groups. Those containing the practicaUy insoluble diethylene glycol phthalate and Duraples c50 LV 100 per cent, together with compound XM containing no softener, show the greatest volume change and mainkin positive swdlhg in all o h . The partidy soluble soft eosl tar compound SCT in inkmediate in this respect and swells positively only in hydraulic oil C and process oil D. The soluble softenem, dibutyl phthalata, dibutyl sebacate, and tribubxy ethyl phosphate are extracted, and compounds of them show positive volume change only in the low aniline point process oil D. At an aniline point of 110-C.and above, the sotualswell of the polymer, aa shown by the softener-free mmpound XM,is needy mro. Petroleum products in contact with Perbunan psrte may leach 8 conaiderable fraction of softener from the compound. The fuel or oil in then contaminated by the extracted softener.
Also, P!vaumiw properties of the
so+ camrate, II -pund
1286)
I
modify speci60 obplge may
result. The most dective f erakting softene according to the experience of this aboratory, are ext ble to Borne degw in petroleum oils. The& &eed for a good freere depressant which in insoluble in petroleum hgdrocarbons. Until such an ideal materisl in available, the synthetic rubber compounder must depend principally on the inherent freeze resietance of the seleated polymer. He will therefore choose the best synthetic in this respect, the other properties of which are comktmt with his requimnents. E m m OF CONTACT v l ~ GASOLINES. a Perbunan is being employed for many applications, such aa hose tubes, &e&, and carburetor dhpbragms, which are usesin contact with petroleum fuels. The etrect of the fuel on important compound criteria such 88 volume change, t e d e retention, and flexiiity at low temperature, in largely dependent on the composition of the fuel with reap& to aromatic content. In this study three gaer~linw,containing, mpectively, by volume 5,24, and 43 per cent mixed aromatics, were employed as teat fuels. They were made by blending with E base naphtha the required pmportions of the following aromatic mixture: 1 part benzene, 4 pmta toluene, and 3 park xylene. The mount of material, cbie5y softeners, extractable by gwline from the seven test compounds in shown in Table IV. Reeults indicate that only the softener diethylene glycol phthalate is substantially insoluble in both gaeolmea. Duraplex c-50 LV 100 per cent and soft coal tar are only slightly extracted by the 5 per cent aromatic gasoline, and are less than half removed by gasoline containing 43 per cent are matios. It is of interest that Dumplea CSO LV 100 per cent is eaaily soluble in any gasoline prior to heat conversion during vulcanisation. All other softenem were largely extracted by the low aroinatic content fuel and almost completely withdrawn by the high aromatic content gasoline. Cornpowid XM, containing no softemer, may be conaidered a control aa an indication of the small part of each extract deriving fmm stesric acid and other compounding materials exclusive of softener. Figure 7 (left) shows volume increase for the seven Perbunan compounds m. aromatic content of the immersion media. The volume change is almost a linear function of the
$"
r.38
******
INDUSTRIAL AND ENGINEERING CHEMISTRY
ammatic content, and the slopes of the curve8 for the several compounds are similar. Chmpounds containing the higbly extractable dibutyl phthalate, dibutyl sebamte, and tributoxy ethyl phosphate show a relatively low change in volume w would be expected. It is notable that compounds containing the softeners Duraplex and diethylene glycol in gasoline even higher than phthalate show volume inthat of compound XM containing no extractable softener. This has not bean fuUy paplained but is l o g i d y due either to swelling of conversion producte of these relatively k d u b l e softeners or to an increase in resistance to swelling imparted to the polymer by treatment with the mercaptan. S i c e the latter has not been found true in earlier work, the former is prasUmeed. Figure 7 (center) shows tensile retention after immeraion in fuels plotted against aromatic content of the gaaolinea. The tensile specimens were broken, and no drying period ww allowed after *moval from the teat liquid, according to A. 9. T. M. method D-471-4OT. They are, however, reported on the basis of original gage in accordance with the recommendations of the 8.A. E.-A. S. T. M. committea on synthetic rubbers (11). The relation between tensile strength retention and aromatic content of the gasoline is apparent, but there is considerable deviation from straight lines and the curves do not have similar dopes. Generalizations are acult, and it i~indicated that tensile retention b no relation to extractahility of the stocks by the fuels over the aromatic range investigated. There is, however, a tendency for lesa change in tensile retention with i n c w i n g aromatics content in the case of compounds containing the lass soluble qftanm diethylene glycol phthdate and Duraplex cdo LV 100 per cent. Figure 7 (right) shows hsrdnees data on the seven comd at -30" C. after immeraion in the three pounds m gwolines for 4 days at mom temperature, plotted ag& the aromatic content of the gwolines. The hardness determinations were made on specimens while immersed in the gasoline in a manner similar to that already descriW for dry 8peoimens. The data indicate that at the tempratwe of investigation, --30° C., the effectiveneas of the absorbed gasoline w a freeze depressant is a function of the aromaticity of the gasoline. The d i f I m c e s in hardnass at -30" C. due to softener type are considerable in 5 and 24 per cent aromatic content fuels but are greatly minimized in 43 per cent aromatics. Those compounds containing diethylene glycol phthalate and soft coal tar, w well w compound XM, are the
** *
Vol. 34, No. 11
least flexible at -30" C. when immersed in gwline containing up to 24 per cent aromatic& Most r n b h l k e at -30' C. after extended immersion in 6 and 24 per cent aromatic content gsaolines are compounds containing tributoxy ethyl phoephate, dibutyl sebacate, and dibutyl phthalate. It is then apparent that, for compounds to be used in contact with fuels, softener selection will be detemined by the principal properties sought. In d c a w t&a must be made under simulated service conditions. Thus, tributoxy ethyl phosphate is shown to he gemrally applicable where good flexibility, dry and in contaat with fuels, over a range of low temperatures is required. Where low extractability in ammatic fuels in combmation with moderately good f m a e renktmce is desired, heaecOnvertible minE of the o h rep resented by Duraplex C E O LV 100 per cent are attractive.
Figure 9. Correlation of Acrylonitrile Content of Buna N Polyrnen with Volume Change in Gasoline and Freeze Resistance
Bunr
N Types
The Buns N r u k , copolymers of butadiene and acrylonitrile, are 88 a class characteri5edby good phyeical properties in combination with a high order of resistance to swelling by petroleum producte. The preaent Perbunan, one member of this Buna N polymer family,contains 74 per cent butadiene and 26 per cent acrylonitrile. Apart from variations in polymer qualities introduced by the conditionsof polperi58tion, the inherent properties of compounded Mllosnisstee of the Buna N types are governed by nitrile content.
Some &ects of variations in polymer cornpition are here shown by teat data on MllcsnisateS of Buna N types ranging from 26 to 46 per cent acrylonitrile. A formula containing 50 parts Bemireinforcing black, with normal sulfur and aceelera. tor concentrations but no added softener, was used in this work. In Figure 8 (left) the volume change in gasoline of these comwnnds. containinE various nitrile content wlvmers. is shown as a function f the aromaticity of t h i &&on medium. It is evident that an incraase in nitrile content of the polymer is reflected in a corresponding improvement in resistance to swelling in each gasoline. S l o p of the m e s of volume incraase us. aromaticity for the several compounds are approximately aimilar. Improvement in resistance to solvent swelling with incTBQBednitrile content is not obtained, however, without corresponding impairment of flexibility d low temperaturas. In Figure 8 (canter) curves of converted Shore hardness ue. temperature for the same compound series show th&he high nitrile-content polymera yield vulcanisatea which rapidly approach brittlenass with decreased a Shorehardnass the 48per cent nitrile content compound of 90 at 0' C., whereae the compound containing 26 nitrile polymer attaina the same hardness at -25'
Measurements A
I
on Synthetic
*
_. +
Rubbers
'
Lawrence A. Wood, Norman Bekkadahl, and Frank L. Roth Vational Bureau of Standards, Washinfion, 0.C.
asoline is shown in Fi
ernod has been developed for preparing specimens of synthetic rubber in a suitable+ precise measurements of the ity. The rubber IS outgassed in a vacuum chamber and, while still under vacuum, is molded into a *et about 1/16 inch thick. Specimens weighinrabout 1 gram each are cut from this *eet and are employed for the measurement of the density by the method of hydrostatic weighings. The values obtained with different specimens from the same sample rarely differ from one another by more than 0.05 per cent. Values dre given for the mean densities obtained from six specimens made from single samples of each of eighteen of the most important commercial varieties of synthetic rubber now made in the United States.
of hardnass at -20' C. us. &maticity deceases with increased nitrile content. Then, even in contact with high aromatic content gasoline, the low-temperat&exibity of very high nitrile Buna N types is relatively poor. Thus,these data indicate that, for the Buns. N types,)Che properties of &stance to solvent swelling and resistance to hardening with decreased temperature are inherently interdependent functions of the acrylonitrile content of the polymer. This is most clearly shown here by the c w e s of volume change and freeze resistance us. polymer composition (Figure 9). Acknowledsment
The writers are indebted to many of their associates for assistance in the preparation of thia paper. Literature Cited b a n . V., and Habgmd. B. J. (to Imperial Chemiod Induatries). Brit. Patant 498.302 (Jan. 2. 1939). Carman, F. A., Powers, P. 0.. and Robinaon, H. A., h. ENQ. Cmu.. 32, 1-72 (1940). Du Pont de Nemoura, E. I.. & G., ha.. Rubber Chem. Div., Supplement to Rewrt 2W (Nov., 1938). mebasttai, c. A, ~ndiaI M ~ 102, NO. 6. a3-s (1940). Ibid., 102, No. 6, 37-9 (1940). Ibid., 103. No. 1. 46-8 (1940). Koch. Albert. IND.ENa. CHIu.,32,464-7 (1940). Kwh. E. A., Kautwhuk, 16, No, 12, 161-0 (1940). Lightborn, I. E., Bubbw Age (N. Y.),47,19-21(1940). MoCortney. W. J., and Hendriak. J. V.. IND.E m . Cmx., 33.
wm.
679-81 (1941).
S.A. E. Journd, 50, No. 0.20-2 (1942). 8elher. M. L., Winspear, G. G.. and Kemp, A. R.. IND.ENa. CHIu.. 54, 16740 (1942). Stsom Eiatributora Ino.. "Perbunan. Gmooundinn - and Prw-
eming-. 1942. (14) Williams. Ira. snd Bmith, C. C. (to du Pont de Nemoura & Co.). U.8. Patant 2,064,680 (Dee. 16.1930). (16) Wmd, L. A.. Nstl. Bur. Standards, Circ. 0127 (1940). (16) Yersley, F. L..and F-. D. F.. h. &a. 34. a a w (1842).
c-..
OR many purposas the density of a material need be known with an accuracy of only a few per cent. For many materiale the density of one sample may mer from that of another by this amount, and no useful purpoee may be served by making more precise measuremente on individual samples. In natural rubber the densities of Merent samples have been shown, in a compilation (2) of twenty-one values, to lie with two exceptions between 0.905 and 0.919 gram per cc. at 25" C. The variations probably represent real merences in the samples and not accidental errom of observation. Since one can seldom know exactly the origin and subsequent treatment of a sample of natural rubber, there is little d u e in i n c r d g the precision of mwurement. Synthetic rubbers, on the other band, can be regarded as ususlly produced under conditions which are much better controlled and known. It is logical, then, to measure the density with greater precision and to hope to be able to ascribe significance to its variations from one sample to another. The present work is concerned with the development of a
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