Low Temperature Characteristics of Elastomers S. D. GEH;\I_1;V.D. E. TVOODFOKD. i U D C. S. W1LICISSOS. J R . The (,oorlyenr ‘f‘irc & K8tbbcr Contpnii?. i k r o n . Ohio T h e low temperature stiffeuinp of elastomers f‘reqiiciitiy limits their usefulness. 4 new laboratory t e s t for niw-uring their stiffness a t low temperatures is tlescrihecl. strips of t h e SIOCLS to be tested are mounted arouiid ‘I c-) lindrical rach in a iertical, cylindrical insulated chamber. ‘I’hc teniperature in t h e chamber i s controlled b 3 c-ooling the babe elternally with d r j ice and by a nioderatr regulated floic of precooled air through dry ice i n the bottoni of the chamber. This s> stem giles stable teniperatiirc. H hich arc easilj controlled. The chamber ran be rotated to n t t a t h the samples i n succession, by i n e a n ~of projecting top gripa, to a suitablj mounted toriion irr. The stiffness i i measured 11) t h e angle of twist of the saniplr when tlic torsion heaS ii rotated 180‘. The relati\e ~ n o d u l ifor i ~ an? temperature is calculated a* the ratio of the niodiilui a t this temperature to t h a t a t 25’ C . Plots of angle of t w i i t again91 temperature show a rather sharp heal. a t the lox\ temperature end of t h e curie. This determiner a somewhat subjectile “freezing point.” Curies are gi\en to illustrate t h e wide iariety of 10w temperaturr stiffening
c.harat.teri.tic*- f o r elastomer-. In unplastic-i7cd s t o c 1., t he c,heniic.al tomposition of the monomers is the tlon~in‘iting f a c t o r for these properties for Farious ajnthetic. rubbers. The stiffness of elastomers which are capaf1le of‘ c~r?.tallil;ation upon stretching, such as IIekea, neoprene, and 15iity1 rubher. depends not only on temperature but a160 o n time of exposure. To study these effects. the forcgoing apparatus w a s u s e d in a cold room. \ rather long inclurtion period orciirs during H hich the stiffness is e..entiall? conitant. I t then increases and e\cntudll\ reatlie. ’I larger constant \ d u e . Seieral months ma? be required to c~onipletethebe changes. X-ray examination of ffeiea and Birt’l pro\ed t h a t the increased stiffness onlong e\po.nre ita3 due to crystallization. No change w a - obieried i t i the stiffneci of GR-3 oier t h e period of 2.5 month% h t -30” C,. Keduttion in the speed of retraction is :I c-ritic-nl nieabiire of the deterioration of high elasticity a t 10% trniperaturei. I t gi>er a wide differentiation a t nioderately l o w temperatures between Butyl rubber and €Ic\t-a or (,Et-$. whereas a -10% modulus test does not.
E
turc characteri5tics of rubbc~i,. II’ I tic use oi the rubber involven circunxtancc~swhich differ widely from the test conditions, such a s rapid cieiormations, large deformations, er long exposure to ion. tenipcratures, irhich conditions are frequently encountered in servicc. the modulus and brittleness tests as ordinarily carricti out are inadequate. To secure a correct perspective, it is then necessary to modify the text or to set up special test conditions, in order to conform more closely to the service conditions. This ~ o r k in , addition to giving results from the torsion modulus test, is concerned ~ i t some h aspects of the low temperature behavior for rapid deformations, free rrtraction. and long esposures to lox t~inpri’irtui
LASTOAIEKS characteristically stiffeii and s u f f ~ ri n i p a b nient of high elasticity at lon temperaturw. The practical limitations which this imposes on their respective fields of w e has led t o the development of many laboratory procedures for testing and evaluating low temperature properties. A comparative discu’ssion of the various published methods was undertaken by Liska (9) and Mullins ( I O ) . A broad line of demarkation exists between t\vo types of tests, brittleness and modulus tests: thc. theoretical relations for xhich have been elucidated by Boycr and Spencer (3). I t appears that the most generally applicable type of test is one which gives a measure of the stiffness over a range of temperatures low enough to include the transition rcgion through whieh t,he long-chain molecules lose the ability to unkink or uncoil, so that the rubber becomes essentially an ordinary solid. Such modulus determinations have been made in a wide variety of \ w y ~ using either stretching, bending, or torsional deformations. This results have much in common as they arc ohtairiid for relatively small slow deformations, ivhich are more fundamcntal conditiolis than the type of deformation. This paper d i i in securing convenient, torsion apparatus, which ha. I ~ c ~ uwful and illusnittrs t h c i ~ t w l t ’ :ohtninc~tl. the difficultier in trying to i>v:iluatclo\v temperature stiffming by a‘sinyle viiluc. of tliv ri.lativc iiicduiii>. He suggested reporting the temperatuies at whirli thc relatives modulus (ratio of 1nodulu.s at l o x tcnipc’rature t o tlic modulus itt 20” C.) is tn-o and ten. I t seciiis prefcrahlc to rcport the entiw modulus curve ivhenevtar possible. so that tlie rubber compound;. can he compared in the tcnipcrature ~ n g of e interest for ~ a c h particular application. For some purposes it may he sufficient to report the relative iiiodulue at a given temperwturc or at two ternperatures bounding the range of interest. Although a simple modulus or stiffness trbst in ssufficiciit in a general way for many purposes and is a useful guidc for compound and polymer devclopment, in many cases it leaves largc, gaps in the information n-hich is desirable for understanding low tempera-
LOW TE\lPERA‘r‘URL.: DEFORMATIOZIS
ticity explains the tcmpt:ratui (’ The, kiiii~rirthC’t t o these rcwlt.$ Ius measurements as o i , t l i u:ii,ily iiiudt~and Xvhirh are disC.1 icrc show increased stiffncw a t lo\vc,t tvniperst urw. The apparent contradiction is duc to thix diffcreiit i~sperinicntalprocedures used. Thc theory applirs to equilihriuin conditions which are rcadily obtained by stretchitig the ruhbcr and changing the tc,niperaturc. In the usual type of iiirasureinent, on thc other hand, the rubher is brought to the low temperature and then deformed. I n another alternative the rubber may be deformed at the higher temperature and cooled so that it shows the normal Joule effect, but subsequent small deformations a t low temperature will indicate an increase in the modulus ovcr that measured at the high temperature.
1108
[ j
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1941
1109
".
The theoretical esplanation of these effects lies in the large influence of the time factor for lo^ temperature deformations. T h e time for a rubber deformation to occur depends upon molecular relasation times which, in simple theory, depend esponentially on the temperature. For the usual type of measurement this effect more than counteracts the relatively small linear depi'ndence on absolute temperature esplained by the kinetic theory.
t
Figure 2.
+
P Figure 1.
hIechanica1 RIodel
The situation can be better understood by reference to the niechanical model shown in Figure 1, which idealizes the rubber structure a s a system of linear springs and dashpots. T h e mathematical solution for this model (2, 8)gives the deformation due to a conctant load a t time t after load application as
+ k2
D ( t ) = ___ [ 1 + kl
2(1 - e - : ) ]
where k l , kp = spring stiffnesses q = viscosity of dashpot The instantJncous deformation, t = 0, is gilyrl the first term terln, \,.l1ich has a lirlliting value sem , represents the contribution of high elaaticity to the deformation. T o extend the mechanical analogy to rubber, the relasation time T is considered to be a function of temperature, according to the equation of Equation 1. ~h~ cured by making t =
Parts of Torsional Stiffness Tester
as the relasation times. I t is not to be cxpected that very good quantitative agreement would be secured with such a simple theory. It is k n o w i that a n infinite distribution of relasation times is probably involved in rubber deformations. Furthermore, the elastic and viscous elements may not be linear, and there may be structural effects. I t does not seem obvious that the structure achieved in respect to molecular orieni.ation, even after a long period of time in a specimen which is cooled and stretched, \vould be identical n-ith that in the same specimen which was stretched and then cooled. LOW' TEMPERATURE TORSION T E S T
Equipment for measuring torsional stiffness at low teniperatures is shown in Figures 2 and 3. T h e test piwes used are small rubber strips died out from tensile test sheets. T h e dimensions are 1.625 inches long, 0.125 inch wide, and 0.079 inch thick. Five of these are faatened vertically in the Micarta grips of the cylindrical rack illustrated at the right in Figure 2. T h e t o p and bottom disks of the rack are also Micarta. The (center post and the external freelv turnine studs are brass. The insulated chamber used is shown" a t the lkft of Figurr 2. It is made of a n inner aluminum tube 6.5 inches long and 3 inches in inside diameter, and a n outer concentric tube of cadmium-plated steel, 4.5 inches in outside diameter. ~h~ space betneen the tubes is filled Iyith insulating material held in place by Llicarta rings pressed be-
71
\v11ere
u
= energy of activation k = Boltzmann's constant ( 1 )
Equations 1 and 3 show t h a t longer periods of time will be required to attain a given deformation a t lower temperatures. For t = a, the deformation is P j k , , and is independent of 7 . The kinetic theory deals n-ith the relatively sinal1 dependence of k l on temperature-that, is, eyuilibriuni deformations obtaincd theoretically at t = m . T h e time factor, lion.evcr., is or' pwdominant importance for most 1oi.r- temperaturc measurements as carried out in practice because of the effect of the term containing T . T h e deformation a t a finit,e esperimental time, D ( t ) , is less than D(m ) . This is interpreted as an increase in modulus. This idealized model emphasizes the importance of time as a factor in low temperature measurements for rubber, when t h e experimental time of measurement is of the same order of magnitude
Figure 3.
.4ssembled Torsional Stiffness Tester
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 39, No. 9
LARGE TORSION W I R E
TREAD
Figure 1. Typical Plot of Twist
1's.
STOCK f
Temperature
160-
I40
-
-
$9-\ \ \
120 -
v)
ru
20 100-
2
L
h
60H E V E A GUM STOCK E C U R E : 70MIN./27S0F.
604020 -
BY Z ELONCATION -60" '
I GO
-40
I -30
I
-20
I
1
-10-C. 0
Figure 5. Effect of Time Interla1 on Experimental Curves tween the tube. at top and bottom. .I Micarta disk haviiig a 0.25-inch hole in the center is pressed in the bottom of the inner 1 I I I -20 tube. -40 -20 0 20 40'C The torsion head arid stand are shuwri a t the right in Figure 2 The base includes provision for introducing a sJreaii? of, air in Figure 6. Effect of Elongation on the bottom of the insulated chamher. The torsion wire is s t r i ~ l Relative 3Iodulus of 0.011-inch diamcter and 2.38 inchrs long. -qpointer is provided n.ith a movahlr angular scale graduated in degrees for convenient. exact adjustment of thc HEVEA CUM STOCK E / P I o point. CURE: 7 0 n r ~ / Z 7 $ ~ C R - S TREADSTOCK A Figure 3 shoa b the, equipinelit as assembled foi use. Thestandisset inashalloir, insulated pan, which also L: contains a flat spiral of copper tubing for precooling the air introduced thiough the hole in the bottom oi the APPLIED TOR P UE R E L €AS ED AFTER EACH READING, insulated (ahamber. The air passes over acetone in SAMPLE RETURNING TO the bottle as shoirn, to Z E R O TWIST prevent frost formation. The pan is filled v ith lumps of dry ice moistened with act'tone. Dry ice is also uwd in the bottom of the \TORSION HEAD HELD insulated chamber. Manual AT 90' regulation of the air flow T E M P E R A TU R E VARIED provides temperatures in < the test chamber down to P -90 C. i f necessary. The I I 1 I I E l temperature can be deter-60 -40 -20 0 20'C. mined either by R t hermom.'I -20 -40 - i o 0 io 40-c eter or thermocouples. Tht. Figure i. Experimental Test of \lagriitude latter have the advantagt. Figure 8. Effert of Cure on StitToes* of \lodtilus Chanpes
-
2
INDU STR IA L AND ENG I NEER IN G CHEM I STA
September 1947
1111
CR-S TREAD i T O C K A
CR-S STOCK A - SO PARTS BLACK ,, B -40 . II
''? -60
Figure 9.
-40
-20
0
20
40'C.
Effect of Cure o n Relative Cold Hardening
Figure 10. Effect of Carbon Black Relative Cold Hardening
011
sonie subjectivity in the deterniiiiatioii iiiust be ackno\dedged. There may also lie some dependence upon the apparatus and testing procedure. Severtheless, the curv~"sin Figure 4 show that the freezing point is s h i f t d only about 2 " C. when the constant of the torsion wirr is changed by a factor of about four. Figure, 5 shows a very sniall effect on the frerzing point when tht, time Cor taking the tn-ist readings aftrr application of the torque is in the onds. The stiffening point dc+~nrdbelon. is a1.w insensitive in this range, sirice values for the 5-, lo-, and 20-second intervals arc', respectively, -48.7", -50.5', and -52' c'. -kt a n y rate, the freezing temperature sc'rv~~st h r purposix of marking the start of the transition rcgion for t h k torsioil test. It does not seeiii to have any correlation with britth points n-hich have been reported for stocks similar to thost. tcsted hew. Frwzing points for Hevea and GR-S tread stocks are actually fouiid to he in reverse order from riaporttd brittle temprraturw (.9), n.1iic-h give GR-S as having a lolvcr brittk point than Hvwa. This 1ai.l; of correlation is not surpriiiiig i n v i t 3 n . oi NEOPRENE G N CHEMICL'M N-Y
BUT
CHEMICUM N.3
TREID STOCKS
~
-60
Figure 11.
Cold Hardening of Natural . Rubber
Figure 12.
I
-40
-20
0
20
40'C
Cold Hardening of \ a r i o u ~ Elastomers
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1112
I
I 40
4s
I
I
PARTS OFSTYREN? Figure 13.
I
2s
Effert of Styrene Content on Stiffening and Freezing Points of GR-S
The numerator represents the torque a p p l i d to thtb test piec,t:. Tht? torsion head is turned 180", but the applied torque is reduced by the twist of the test piece The factor of proportionality in Equdtion 4 involves both the torsion constant of ihe wire and the gcomrytry of the test piece. Since only relative values of the niodulus at different temperatures are of interest in this work, arid since a geometrical factor calculated from the theor>-of elasticity for small deformations would be of doubtful applicability, these facto1.s may be omitted. Values of the twist at various teniperatures are taken from tnist-temperature curves such as those illustrated in Figure 4. Corresponding values of the fraction (180"tn.ist/twist are read from a table. The ielative modulus can then lie readily computed and plotted as a function of temperat i t i ~ i ~ . The principal compounds used are given in Table I. RESULTS OF MODULUS TESTS
Figures 6 t o 14 exhibit tlie type of information which may be secured n i t h the torsion modulus test described. Since the relative modulus curves plotted in the figures are dtxrived from smooth csperiniental twist curves such as those in Figure 4, which s h o ~ v litti(. scattering of the points from the curves, there would be no objcct in entering points on the relative modulus curves. Figiirti 6 illustrates the small effect of superposed elongations 011 the test results. Any accidental variaiion in the tension of the test piwe when installed for regular test purposes is negligible. 600 1
Figuw 7 illuzirat(~s:in c.xpc~rinicritalvc.i,ific,atiori I J ~the thew. \\7heri tho torquc is a p p l i d I.W(> tlroppctl, a small tli~c*rt~:rs(~ iii ~ I N :ippar~~iit . niodulus is \\-11e11, on til(\ ott1c.r 1 1 3 i o r q i w i: applitd aftcr I > ii: coolcd, a large, iiirrc *tiftiic-- i* olis(>rved:it lo\vt'r tc~nipc~rn Figure 8 ic :I p t u i of t l J ~ ~ i ( J l stiffri l~1 i m t tenipcrature i01, a sc'i,i!~ of c u m oi GIservice. idus test. Figure 19 compares the effect of temperature on the tiynamic niodulus for several synthetic polymers and for Hevea. ACKNOWLEDGMENT I t ia possilile that the ordinary type of static modulus test might be generally satkt'actory for rating polymer5 in the rorrcrt The authors Tvish t o tixprcsri their thanks tci Thc, (hjixlytsar Tire 1rt11.rfor low teniperaturr dynamic service, hut any specific valuc & Rubber Company and I,. B. Sebrell for pcrmissinri t o puhli~h t i i r h r , modulus thus deterniined would depart rather iviticlly this work, t o H. J. Osterhof for his encouragrment, it1 thii n-~~rli, t ' t ~ i n i the dynamic valucss, even at moderately low teniperaturrs. m d to G. H. Gates and J. H. Ficlding for furnishing t h e coni( )ne of the most sensitive nieahures for the effert of 1 o ~trniprrapounds uscd. This invcrtigation \vas carried out under tho q ~ i n tirw on elasticity is the> spwd of t'rre retraction whrn the sample .;orship of thr Office* of Kuliher Rcwrvc, Keronstruction F'inarict, is released after stret chirig. The velocity of rrt raction ilt,prnds Corporation, in rnnnc~rticinwith t h p governnirnt's synthetic r i t t i iioth upon the modulur and the internal friction. T h r appaiwt her program. :iicrdulus is higher at lower trniperat ures, as has been esplaiiiod, LITERATURE CITED t u t the deprndence of this increased modulus on increased int i ~ r rial friction i? indicated 111-rrdured velocities of rr,traction. Eyuip.Ueksiiidror, A . P.. a n d Laeurkin, E'. S.,.I. Trr-h. /'h!js. (L.S. S.K.1 9, 1249 (1939); R u b b e r C h ~ m Tech.. . 13,X i i f i 119401. iticiit for measuring the speed of free retraction has heen alread>Bilmes. L., ,I. ,Sei. Instruments. 22, 16 (1945). (lesrribcd in detail (11). The apparatus n a s provided with a 1oTv Royer. It. F.,a n d Spencer. R. S.,"Advaiirw in C'olloid Sri'eniprrature jacket (Figure 20). The jacket co eiive." 1-01,2 , p . 1. New York. Ititerscic~iic,e Pul)li$hri.. . Inr., qitudinal compartments, one containing a pan of 1946. Cotiant. 1;. H.. arid Li-ka. J. K., J . A p p l i r d I'hj/s., 15. 767 ~ h t test . piece. -1ir is circulated over the dry ice and througk thi, (,1!>441. sample compartment, bj- m w i s l i t a m a l l variahlcqxwl fan. Forinan. I).B.. I r u . Esu. (:HEM., 36, 73s i(1944). T h r tixnipprature is controlled 1)y manual adjuRtment, of t l i t , fail Gehnian, S.D.. Wnnrlfnrd, D. E., and Starnhaugh. I{, H . . f 0 ; d . . i p r d Suitable baffle$ to control t h r air f l o in ~ the samplt.corii33, 1032 (1941). Kemp. A. R . , Maliii. +'. R., and Winspear. (i. G.,Z h j d . , 35,4XR irartment ensure constariry (it' teniprraturc, along thc strc.1chid (19431. wiiplt' to about +=2'F. Leaderman. H.. "Elastic and Creep Propertie3 O i F i l ~ i i i e n t ~ ~ u s Figures 21 and 22 coniparr a p c w l of rvtraction rtwltsk for Butyl. Xnterials." p . 56. Washington. D . C., 'rhr Tcxtilt, Fniindatkvca, and GR-S tread stocks at room temprrature, 25"(',, and .tion. 1943. Liska, J. Vi., I s u . Exi.. ( ' H E M . , 36,40 ( 1 9 4 4 ~ . ieces w r r i~oolrd,stretched, and heltl foi. 1 Miillin>, L.. T i m i s . I n s t . RiihbFr I d . . 21, 217 (19451. . Tnslit~rtioiiof thtl curves s h o w a startling Stainhaugh. R . B.. Ilohiiw. AI.. arid Gehtrim, S.T),,./. . 4 p / ~ / i ~ d ~lrficiericvin t.lasticity for Butyl rulihrr at the lon-er tempt'raturr Phys., 15, 740 (1944). :+s c.iimpai,td t o Hwea and C;K-S, This n-ould n w e r t)r suapectecl Wonrl, I,. 1..xiid i i o t h . E'. L., l b i d . , 15, 710. 7 x 1 (l!l441; \vere plotted in Figure 12. from iiindulus measurenimt r sucli Thew Butyl seems to he atrout o n a par with the other t l v o in thit ! i i . r ~ i . t ! ~ i i c * tof ~ its w l > t w r l i k ( t i.liauic,tw at I i i i v t i ~ i i i i ) i ~ ~ a t i iThta i~~~~.
:ttiII
I
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Effect of Nitrogen-Containing Compounds on Drying of Paints 'l'he etfect of free anlilies o n t h e t t r j i n p of oleoresiiioiib p a i n t s w a s s t u d i e d , arid the d a t a i n d i c a t e t h a t t h e s e a m i n e s are e q u a l t o the complex coordinated r o b a l t aniirie* in a c c e l e r a t i n g the d r y i n g r a t e arid m a i n t a i n i n g the drying t i m e stability of t h e p a i n t s tested. It w a s f o u n d t h a t r e r t a i n h e t e r o c j clic, pol>cyrlic. p o l y a m i n e s c o n t a i n i n g tertiary n i t r o g e n atoms preatl? accelerate the d r j i n g rate of oleoresinous p a i n t s , H hereas a l i p h a t i c and other aromatic. a m i n e s either retard or ha>e l i t t l e effert on drjing r a t e . D a t a are presented w h i c h i n d i c a t e that there is little, if a n y , c o o r d i n a t i o n of t h e free a m i n e w i t h t h e cobalt m e t a l a t o m o f the n a p h t h e n a t e d r i e r a f t e r the t w o ha\e been mixed t o g e t h e r in the p a i u t .
T
HE study of the diying of pigriiented oil syuteni- ha\
twtsri
the subject of many research investigations. One phase of Such studies has involved the problem of so-called drier adsorption. I t has been frequrntlv observed that paints containing certain
pig!nr.iits exhihit 103s of tlryirig upo~iaging (1, 2 ) . Of particdar notice arc thosf, paints which contain carlion h1ac.k or t itanium iliosidc pigments. This arrion has h t ~ x n tit trit)iitd, i ~ ~ ~ r h a p ~ cwoneou.qly, to adsorption of the mcLtallir driw hy I h f b I~iynic~nt (1, 2, 6 ) . The llontrral Pain( and \.arnish Production ('luti madta an vxtrnsive study of the phenomenon of drier adsorption (31. The adsorption of simple and complex cobalt ion> on titanium clioxitle was studied hy Sicholiion (6). The lattcr o1)srrvc.r suggwttd (.$) that loss of drying may he due to changes in the polymrrization processes. This investigator also reported a study ( 5 ) in whieh coordinated cobalt amine oleates have bpen conipared wit ti simpk oleates of equal cobalt metal concentration as drying catalysts. According to this rrport ( 5 ) , "cobalt compounds when fully cyrdinated and oxidized t o the cobaltic state are extremrly stable to reduction. Thus, by preparing such a cobaltic compound and using this material as a drier in a paint, one n-ould expect practically no catalytic action by this material since t,he metal atom, heing fully coordinated, would be incapable of coordinating with
