ANALYTICAL CHEMISTRY
322 (18) India Rubber World, “Compounding Ingredients in Rubber,” 2nd ed., 1947. (19) Jarrijon, A., Rev. g S n . caoutchouc. 20, 155-7, 177-82 (1943); translated in Rubber Chem. and Technol., 19, 1061-76 (1946). (20) Koch, H. P., J . Chem. Soc., 92,401-8 (1949). (21) Lee, J. H., IND.ENG.CHEM.,ANAL.ED.,18,659-61 (1946). (22) Lothian, G. F., “Absorption Spectrophotometry,” London, England, Hilger and Watts, Ltd., Hilger Div., 1949. (23) Maclean, hl. E., Jencks, P. J., and Acree, S. F., J . Research Natl. Bur. Standards, 34,271-80 (1945); R P 1643. (24) Mandell, J., and Knox, E. C., Office of Rubber Reserve, from Natl. Bur. Standards, Washlngton, D. C., Rept. SP-T-301 (1948). (25) hIeehan, E. J., J . Polynzer Sci., 1, 175-82 (1946); Rubber Chem. and Technol., 19, 1077-84 (1946).
(26) (27) (28) (29)
(30)
Reconstruction Finance Coi p., Office of Rubber Reserve, “Specifications for Government Synthetic Rubbers,” effective Jan. 1,1946. Rogers, S. S., ed., ”Vanderbilt Rubber Handbook,” 9th ed., pp. 514-18, XewYork, R. T. l‘anderbilt Co.. 1948. Schimada, J . SOC.Rubber Ind. Jopan, 5 , 4 2 0 (1932). Tunnicliff, D. D., Brattain, R. R., and Zumwalt, L. R., ; I N ~ L . CHEM.,21, 8 9 G 4 (1949). Tuunicliff, D. D., Rasmussen, R. S . , and Morse, M. L., Ibid., 21, 895-900 (1949).
Vandenbelt, J. hl., Forsyth, J.. and Garrett, A., IND. ENG. CREM.,ANAL.ED.,17, 235-7 (1945). (32) Vaughn, R. T., and Stearn, A . E., A N ~ L CHEM., . 21, 1361-3 (31)
(1949).
RECEIVED September 28, 1950.
Retraction Test for Serviceability of Elastomers at low Temper atures 0. H. SMITH, W. A. HERMONAT, H. E. HAXO, A N D A . W. JIEYER United States Rubber Co.,Passaic, ,V. J .
*
W
Elastomer vulcanizates progressively stiffen as the temperature is lowered. Additional stiffening, due to crystallization, may occur as exposure to low ternperatures is prolonged. The available methods of testing the low temperature flexibility of rubber and rubberlike materials do not reveal the losses in flexibility caused by crystallization except by using prolonged storage at low temperatures. A retraction test employing large deformations, which greatly increases the rate of crystallization, has been developed. This test rapidly gives a temperature index correlating with the stiffness of elastomer vulcani-
zates after storage at low temperatures, and can be used to measure the merit for low temperature applications of both crystallizable and noncrystallizable elastomers. This test in conjunction with conventional (room temperature) tests has been used successfully to study the low- temperature performance of Hevea, GR-S, Paracril, and polybutadiene vulcanizates along with vulcanizates of many experimental elastomers. Correlation of results with cold compression set and hardness after low temperature storage has been excellent and substantiates the usefulness of the test.
HEN the temperature is lowered, the flexibility of a polymer decreases as the second-order transition temperature is
shows the holder with the samples inserted; three of the samples are unstretched and three are stretched ready for insertion in the cooling bath. Wire leads (piano wire), attached t o the samples by means of hooks, pass through binding posts which permit the samples to be anchored at any elongation. Strings attached to the ends of the wire leads pass over small pulleys at the top of the instrument. The free ends of the string are attached to small counterweights. A scale graduated in 0.1 inch is inserted behind the leads. Attached t o the leads are disk-shaped indicators to enable the length of the sample to be read. The over-all view in Figure 1 ehows the a paratus standing in an unsilvered Dewar flask which is containeain a wooden frame. This frame, which was built to act as a convenient stand for the apparatus and additional insulation, is filled with glass wool and held in place with a sheet of polyethylene. A window in the frame permits the reading of a totally immersed thermometer. The Dewar flask contains a stirrer and a heating element connected to the house current through an autotransformer to maintain a proper heating rate. The procedure is based upon the background material described in the following sections. A 2-inch T-50 sample (60-gage) of the vulcanizate under test is placed in the hooks, stretched 250y0 (from 2 t o 7 inches), and locked in the stretched position by turning the thumb nut on the binding post. The rack containing the stretched samples is placed in a methanol bath, which had been cooled to -70” C. by dipping into it dry ice contained in a cylindrical wire cage. The stretched samples are conditioned for 10 minutes. The thumb nuts are released, allowing the samples to retract freely. The temperature of the bath is then raised 1 C. per minute by means of the heating coil. The length of each sample is measured at 2 “ intervals. The bath is agitated throughout the test.
approached. The decrease in flexibility is caused by increased internal viscosity. This phenomenon is called the retarded elastic effect or viscoelastic effect. Additional decreased flexibility may be caused by first-order transition effects (crystallization) in polymers having a structure of sufficient regularity. When usefulness of elastomers a t low temperatures is evaluated, both effects should be measured whenever possible. .4retraction test has been developed which rapidly gives a temperature index that correlates with the ultimate stiffness of elastomer vulcanizates at low temperatures. Ultimate stiffness includes increased modulus due t o viscoelastic and first-order transition effects if present. The large deformation employed in this test (250% elongation) causes rapid appearance of the stiffening due t o crystallization in polymers having a regular structure. The test is based upon the T-50 test ( 4 ) . Various similar pieces of apparatus have been designed which yield the eame type of data. Terzley et al. (8) applied such a test t o neoprene and stated that it was inadequate. More recently, Svetlik ( 7 )applied a technique to the study of elastomers using only 50% test elongation. The test described in this paper uses 250% initial elongation and differs from other tests also in the method of analyzing the data. APPARATUS AND TESTING METHOD
The retraction tester consists of an apparatus that permits the measurement of the elongation of 2-inch T-50 samples ( 1 ) (60gage) at all times during a run. The front view in Figure 1
The temperatures a t which the sample retracts 10, 30, 50, and 70% of the original elongation are called TRIO, TR30,TR50, and TR70, respectively. These oslues give a n adequate picture of
VOLUME 23, NO. 2, FEBRUARY 1 9 5 1
the low temperature behavior. Retraction values (TRIO, TR30, TR50, TR70) are computed from the data by the following formula:
(
%retraction = 100 1 where
L.
LT - Lo - __ L. - L o )
= over-all length of sample in stretched condition a t LT = length a t observed temperature, and Lo =
start of test,
length in unstretehed condition. For example, when LT is 6.5 inches a t temperature T and the sample was stretched from 2 to 7 inches at the start of the test, the per cent retraction equals
100 (1
6.5 - 2 - 7.0 -) - 2
323
The TRlO vsluc indicates the low tcmperature ment of tho elastomer prior t o low temperature storage. In this respect, this value is similar to the TI,(5) value obtained when using the torsion modulus test. This measurement is influenced by viscoelastic effects and very little by crystallisation. Because crystallization does influence low temperature properties greatly, especially after low temperature storage, additional criteria such 8.9 TR70 must be used to o h t a n a complete picture of the low temperature behavior of elastomers.
or 10%
The temperature at which this occurs is the TRIO value, TYPICAL DATA
Figure 2 contains typical retraction curves of Hevea and GR-S 10 vulcenim,tes containing 50 parts of carbon black, accelerators, and 2 parts of sulfur. GR-S 10 does not crystallize and thereby yields a smooth retraction curve. However, Hevea has a strong tendency t o crystallize, which causes an irregular retraction curve. The retraction values were obtained from these curves and recorded in Table I. In the remainder of the paper retraction curves are not given; only the retraction values are presented-that is, the TRIO, TR30, TR50, and TR70 wlups.
Figure
1.
I
~
~
L
S LIGLTPCUVLL ~ L
ua~a VI,
vn-a
rO and
Hevea
The TR70 value indicates the low temperature merit of the elastomer after a long period of low temperature storage. This measurement is influenced by both viscoelasticeffects and crystallization, thereby giving a measure of ultimate stiffness. Hevea has a TR70 value of -5.0' C. and GR-S 10 has a TR70 value of -28.7" C. This indicates that upon storage a t low temperatures Hevea would eventually become less flexible than GR-S 10, especially under statio stress. Thus, higher cold compression set w.m found for Hevea than for GR-S 10.
Table I. Retraction Values of GR-S 10 and Hevea from Curves i n Figtire 1 GR-6 10 C. Hevea, "C.
TRIO -45 4 -54 2
TR30 -40 3
TR5O -35 6
-29 G
-11
Lift.
Right.
Sample holder Over-dl view
c
Effect of I3longation on Retraction Values. The sample is > * ,. LO , purposely given a large uerormaoon induce rapid crystallization upon cooling. The retraction values of the Hevea and GR-S 10 vulcanizates previously described were measured a t various testing elongations. It was found that increasing the testing elongation of Hevea (above 100%) caused a sharp rise in the TR70 value, which begins to level off at 200% elongation (see Figure 3). This rise is caused by the presenoe of crystalliaation during the test. Near maximum effect is reached a t 250% testing elongation. Increasing the testing elongation of GR-S 10 c&usesthe TR70 value to decrease shasply until 200% is reached; thereafter larger deformations cause little further decrease. The TRlO values are influenced only slightly by changes in testing elongation. Because little change in the retraction values occurs on increasing the elongation above 25OYO,this elongation was adopted as
.
Figure 1. Retraction Apparatus
1
TR70 7
-28 -5
ANALYTICAL CHEMISTRY
324 standard. Nany experimental vulcanizates break in the apparatus when greater elongations are used. Using 250% elongatmion is desirable from that, st,andpoint also. Testing Hevea Gum Stocks. The usc of 250;; testing elongation has been found adequate in every caw except Hevea gum vulcanizates. Using the standard testing procedure of 2507, testing elongation for Hevea vulcanizates containing less than 40 parts of carbon black will give misleading results. In Figure 4 are data showing the effect of both testing elongation and concentration of carbon black o n TRTO. Using 2507, testing elongation for a pure gum Hevea gave a TR70 of -56.8" C., Trhereas a testing elongation of 400yo gave a TRiO of -1.8" C. Therefort., elongations of 400% should be used for Hevea gum stocks.
20
I
I
0
1
1
i n the cold box and it subsequent run was made. The TRiO determined by this procedure and the normal procedure is conipared in Tablc 11. If storage a t Ion- temperature has increased the cryst~allizationin a sample, it,s TR70 should be a t a higher tcniperature. Only small changes due t,o storage were observed. The magnitudt, of the change in TR70 due to storage is close to the accuracy of the test. Therefore, these changes are not considered significant, but they are indicative of the high dcgree of crystallinity obtained in this test.
-
1
HEVEA GR-S
10
0
E s t e r plasticizer 10 EPC Black
u
Zinc Oxlde
J
Paraflux S t e a r l c Acid
4
-20
Varlable 5.0 2.5 0.7
I-
2 3
-
2.0
P W
-40
-60
20
0
so
YO
PARTS CARBOW BLACK
Figure 4 .
-60
0
IO0
200
300
Yo0
Effect of Carbon Black on TR70 of Hevea Vulcanizates
T E S T I N 6 ELON6ATlOM
Figure 3.
Effect of Testing Elongation upon Retraction Values
Effect of Low Temperature Storage upon TR70. Sornially, compounded Hevea crystallizes sloidy. Whvn the stor:ige temperature is varied froni the optiniuni, -25" C., the crystallizatiori may appear more slowly. The same effect occurs in GR-S elastomers having low styrene content M hen polymerized a t low temperatures; the optimum for such elastonicis is closc to -45' C. It is also kno.i\n that the application of stress t o a sample increases the rate of crystallization. Thr result of thest, tendmcies depends upon the selected storage temperature arid degree of stress imposed upon the sample. Undclr the condition. of this test crystnllization appears rapidly.
Thcs rvtraction values for wv(~i.aIrtnnclard elastomers are cotitainrd in Table 111. The TRlO vu1ur.s indicate that Hevea and Butyl will be the most flexible niatcrials at low temperatures whon no storage is encountered. The ordpr of merit as described by THIO would be polybutsdiene (41' F.), GR-I, Hevea, GR-S 10, GR-S(-11 F.), neoprene, and Paracril. However, if each material is stored s t low temperatures to allow crystallization to take place., the flexibility of polyhutadic~nc(41 O F.), Hevea, and neoprenc Ivould be greatly reduced and the order of merit would bc>mine: GR-S10, GR-S (41" I?.)) GR-I, polyhutadiene (41' F.1, l'mxrril, Hcvca, and iitx)prciic as indicutcd by the TR70 valuw. LOW T&\II'EHiTURE
FLEXIBILITY OF B U T i D I E N E - S T Y R E N E POLYMERS
The TR70 of several polybutadienc vulcanizates made at vanous temprraturc,s has been measured. Table I V contains the lable 11. Effect of Storage at -55" C. upon TRiO of Viilcanizates of Elastomers Polymor
Herea
GR-S 10 B/S, 90/10, 41' P. Polybutadiene, 41' 1;. Polybutadiene, 77 'I.'.
Time of Storage, IIoiir-
Storage Temgeratiiri.. C.
0 71 0 73 0 73 0 73 0
- J_ J_
72
- 2,5 - 22
-5;
__
-,,1
01
I
I
I
13.0
1
-
'I'R70. O C . -5.4 -4.0 -29.8 -29.7
-2.0
-30.4 -29.1 -17.0 -15.0 -34 1 -32.4
w
u
Inno
urn
-1.0
Y f C C I
0
w
c
n u
c
w
W
Q
z w
It is believed that 250% testing elongation creates a condition which induces nearly maximum crystallization during the short time of the test. This was verified by storing the retraction samples a t 250% elongation a t -55' C. Without warming them, the samples yere plunged t o -70" C. in the retraction bath placed
o-6C < 0
E? 40
80
I20
POLYMERIZATION TEMPERATURE.
160
cL Lo
O F .
Figure 5 . Effect of Polymerization Temperature on TRSO and Extent of Cr>-stallizationfor Polybutadiene
V O L U M E 23, N O . 2, F E B R U A R Y 1 9 5 1
325
Table 111. Retraction I-alues of Standard Elastomers ( " C.)
Riibber
TRlO
-48.4 -45.1 -54.3 Polybutadiene (41' I:.) -59.9 Paracril 2GXS90 -27.5 Hevea -54.2 -40.4 Neoprene Compounded with 50 parts of black GR-S 10 GR-S (41' F'.) GR-I
Table 1V.
a b C
d
TR30
TR5O
TR7O
-40.3 -40.2 -41.8 -41.3 -22.5 -29.6 -35.2
-35.5 -34.8 -31.9 -29.3 -17.8 -11.1 -24.6
-28.7 -27.8 -23.7 -21.8 -12.0 -5.0 +3.G
using standard curing methods.
Type Formula
Po1y mer EPC channel black Zinc oxide Paraflux Stearic acid Sulfur MBTb DPGC Cure 45 minutes a t 292' I:. Except where otherwise specified. Mercaptobenzothiazole. Diphenylguanidine. Varied to equalize rate of cure.
100 50 5 5 1.5 2.0a 1.5 0.4d
vu1caiiiz:ition dihtailh I t M a i found that decreasing thr temperature of polymerization caused the TR70 t o rise (pee Figurcl 5). Dccirasing thc temperature of polymerization from 122" to 0 " F. raised the TRiOfroni -45 4'to -8.9"C. Thischitngc can be cxplniiicd by understanding the accompanying structural changes Hart a i d Meyer ( 5 ) found by infrared absorption studies that the teniprrature of polymerization increased the structuial regularity, as predicted through x-ray analysis bv Beu et al. ( 2 ) and through dilatometer xork by Lucas (6). Some dilatomcter data obtained at the General Laboratories are presented iii Figure 5 . Tt may be assumed that the decrease in volume :iccompanyiiig low temperature storage is due to crystallization and this decrease is a nieasure of the crystallization. I t can bc casilv seen that increasing crystallization due to low teniperatuie poI\mcriz:itioii raises the retraction value TR70
IO1
I
I
I
I
I
L
10
20
30
PERCENT STYRENE
Figure 6.
r -40 0
1 2
4
6
PARTS SULFUR
Figure 7. Effect of Siilfiir on TK70 of Butadiene-Styrene Copolymers >lade at 41 a F.
1
0
0
polybutadiene. These apparent discrepancies can be explained by determining the crystallization in these polymers. These results are also presented in Figure 6. The addition of 15 parts of styrene as a comonomer reduces the crystallization to a very low quantity for 41 O F. copolymers. Here the decrease in volume wit'h storage at -45" C. is almost negligible. Reducing crystallization by introducing a coniononier, styrene, improves the TR70 despite the rise in second-order trnrisition temperature which accompanies this procedure. Usually ultimate stiffness is difficult to inertsure using ordinary test methods, because increasiiig the comonomer not only dccreases the amount of crystallization but a l r o circreases the rate a t Tvhich it appears. This effect is esaggercttcd wht,n the polymclr is in the compounded condition. The cure ill these polymers ~ U J duces large effects upon their low tenipcr:iturc: characteristics.
Effect of Styrene on TKSO and Extent of Crj-stallization
Copolpmerizatiori of butadieiic~with comoiioniei~haviiig high second-order transition tmiperatures generally results in polymers having poorer low temperature properties. Figure 6 contains a plot of TR70 against the styrene content in vulcanizatcxs of butadiene-styrene copolymers made at 41 F. The compouiitiing formula is presented in Table IT. Figurc 6 sholw that increasing the styrene content to 15 or 20 parts actually improves the ultimate flexibility of the vulcanizates a t low temperatures. Even a polymer containing 29y0 styrene hap R lo\z-er TRiO thar!
Figure 7 contains the TRTO values of vulcaiiizates of butadicnestyrene copolymers made at 41 ' F. n-ith various states of cure. Thc compounding formula is contained in Tuble IV. A sharp iniprovmierit is noted in the low tcmpcwturc properties, as irtdicated hy TR70 values of polybutadiene with increasing cureHo~vever,increasing cure for GR-S (29% styrene) made at 41 ' F. produces vulcanizat,es of poorer low tempcraturc flexibility. Increasing cure reduces crystallixabion, t8herc+yimproving the TRiO of polybutadiene, and also reduces flexil)iiity on-ing t o tho increased number of sulfur cross links. Becawc~there is no crystdlization in GR-S made a t 41 O F., increasing cure produces poorer TR70 valucs. Thc plot of TR70 versus cure will show a niiriimum for a polymer having an intermediate styrene coritcnt. The mininiuni TR70 indicates the state of cure at which crystallizat,ion has been arrested. A polynier containing 10 parts of styrene requires a higher cure than t,he polynirlr c:ontainiiiK 15 parts of styrene t o ohtain this minimum. CORRELATION O F T R I O w w t I T~~~
It was Etated above that the TRlO value \vas :I function of the viscoelastic effect but not of crystallizatioii. The TRlO can therefore be uscd as a figure of merit for Ion- temperature flexibilit>- of crystalline elastoniers for dynitmic applications, or for noncrystalline elastomers under all low temperature condit.ions. This figure of merit will be siniilar to the Tl0 value obtained whcn testing low temperature flczihility with a torsion modulus appcira tus .
326
ANALYTICAL CHEMISTRY
The TRlO and 2'10 values were measured for vulcanizates of butadiene-isoprene, butadiene-styrene, butadiene-acrylonitrile, and butadiene-isoprene-acrylonitrile. Because exact knowledge of the composition of these elastomers is not essential to the understanding of the test, this information has not been included. Table V contains compounding data. Formula A was used for butadiene-isoprene and butadiene-styrene, whereas Formula B was used exclusivcly for polymers containing acrylonitrile. In Figure 8 is a plot of the TRlO value versus the Tlovalue of these vulcanizates. The points form a straight line. The order of low temperature merit of the polymers given by the TRlO is therefore similar to that given by 2'10. This correlation of TRlO with Tlo is more general than is indicated in Figure 8. In addition to the polymers mentioned above, Hevea, polyisoprene, terpolymers of butadiene and isoprene with styrene, aryi acrylates, and vinylpyridine also have the same correlation. Variation of sulfur from 1t o 4 parts, addition of plasticizers (ester or hydrocarbon), and changes in carbon black content do not alter the relationship. It is believed, therefore, that the TRlO can be used reliably as a measure of the low temperature flexibility of elastomer vulcanizates where low temperature storage is not encountered and for noncrystalline vulcanizates.
Table V. Type Formula A no Polymer 100 EPC black 50 .5 Zinc oxide Paraflux ... 1.5 Stearic acid Sulfur 1-2 MBT; ... DPG ... hlBTSe 1.5 Cure 45 minutes at 292' F. a Used exclusively for polymers Containing acrylonitrile. b hlercaptobenzothiaaole. C Diphenylguanidine. d Varied t o equalize cure. 8 hqercaptobenzothiazyl disulfide.
TR70 AS A MEASURE OF COMPRESSION SET
Both TRTO and compression set (measured according to S a v y Department Specifications 33-R-9) were measured on a series of experimental stocks, which contained 15 parts of ester plasticizer and 40 parts of Philblack 0. It was found that the TR70 could be used as a index of compression set, because stocks having equal TR70 values have equal compression set values. Natura!ly, a3 the TRTO values decrpase the compression set values also decrease.
-3c h
v)
a A
a
0
-
Compression Set us. TR70
Butadiene-styrene copolymers 0 Perbunan Hevea (50 parts channel black) 0 Hevea (gum)
TR70 has also been correlated with hardening in cold storage (Figures 11 and 12). Both tread stocks containing 50 parts of easy processing carbon black and 2 parts of sulfur and gasket stocks containing 15 parts of ester plasticizer, 40 parts of Phi!black 0, and 0.7 parts of sulfur were included in this correlation.
5(
0
0
t-
20
CORRELATION OF TR70 WITH HARDENING IN COLD STORAGE
v)
Y
0
-uc
t
2 LL + 0
Figure 9.
- 20
TR70, "C.
caused by crystallization, which takes place less rapidly a t -45 "C. than a t -36" C. Under the conditions of the compression set test, apparently maximum crystallization was not obtained at -45' C. Hevea stocks therefore, when stored a t -45' C. for 96 hours, yield compression set values that are too low. If the storage time of tho compression set test were increased sufficiently, the compression set values would eventually rise to the value indicated by its TR70 in Figure 10.
-2(
!2
-40
-60
-6(
. I 00 v)
p:
-7(
- 50
-60 TRIO,
OC.
s
-40
-30
(RETRACTION)
Figure 8. TI0 us. TRlO of Butadiene-Styrene, Butadiene-Isoprene, and Butadiene-Acrylonitrile Vulcanizates
80
W
t I-
60
W v)
z
- . 0
40
v)u - 0
W Y )
If the TR70 of these stocks is plotted against their compression set (at -35" and -45' C.) smooth curves are developed (see Figures 9 and lo). The retraction test can now be used to estimate the compression set of vulcanizates. This function is important, because the retraction test can be performed in 45 minutes. Storage (48t o 96 hours) is necessary when the compression set values are measured. At -35" C., but not at -45' C., the compression set values of Hevea stocks lie on the curve. This apparent discrepancy is
a t 0
I
20
1
ol0 4
0 -60
-40
- 20
0
TR70, " c . Figure 10. Compression Set us. TR70 0 Butadiene-Btyrene copolymers
0 Perbunan
4 Hevea (50 parte channel black) 0 Hevea (gum)
20
V O L U M E 23, NO. 2, F E B R U A R Y 1 9 5 1
J 100
I
'
327
4 - 9
u
YI f
0
LD W I
I4
I
-%
n e
N
E
'
l-
O
O
I
-
7
7
e (0
>. 4 n
8 0
-
N
60
a
I-
Y
W
I-
w
I
I
0
a
0: 0
0
n a
a
n
TR70, "C. Figure 11. Hardness after Storage at -45' TR7O
TR70, " C . C. vs.
Figure 12.
0 Butadiene-isoprene-styrene
0 Butadiene-isoprene-styrene co- and terpolymers
00-
Hevea (50 parts channel black) 0 Hevea (gum)
Hevea (50 parts channel black) 0 Hevea (gum)
Most of the stocks were based on butadiene-styrene copolymers polymerized a t 41' and 122" F., respectively, and contained various proportions of styrene. Tread stocks based on Hevea, butadiene-isoprene copolymers, butadiene-isoprene-styrene terpolymer, and a gasket stock containing no black based on Hevea were included. Durometers were measured a t intervals over 12-day periods a t -35" and -45' C., respectively. Hardness of all the stocks with the esception of the Hevea stocks correlated fairly well with TR70 at both -35" and -45' C. Hardness of the Hevea stocks did not correlate with TR70 a t either temperature. The smoked sheet tread compound attained approximately equivalent hardness a t -35" and -45' C. The smoked sheet gasket compound was about as hard as the tread compound when stored for 12 days a t -35" C., but when stored for a similar period a t -45" C. the gasket compound hardened very little whereas the tread compound became even harder than a t -35' C. The difference between the correlation of the Hevea compound nith TR70 a t -35' C. in the hardness test and the compression set test discussed previously can be explained by assuming less crystallization in the hardness test due t o the absence of strain \vhich increases tendency to crystallize.
Hardness after Storage at -35' TR70
C.
us.
and terpolymers
ACKKOW LEDGMENT
The authors wish l o express their thanks to P. R. Van Buskirk and A. S. Weisser for their assistance. This work was carried out under the sponsorship of the Chcmicals and Plastics Section, Research and Development Branch, Office of the Quartermaster General. LITER4TURE CITED
(1) Am. Soc. Testing Materials, A.S.T.51. Designation D 599401'. (2) Beu, K. E . , Reynolds, W.B., Fryling, C. F., and Slurry, H. L., J . Polymer Sci., 3, 465 (1948). (3) Gehman, S. D., Woodford, D. E., and Wilkinson, C. S., Jr., Ind. Eng. Chem., 39,1108 (1947). (4) Gibbons, W. A., Gerke, R. H., and Tingey, H. C., IND. ENQ. CHEX.,ANAL.E D . ,5 , 2 7 9 (1933). (5) Hart, E. J., and Meyer, A. JT.,J . Am. Chem. Sac., 71, 1980 (1949).
(6) Lucas, V. E., Johnson, P. H., Wakefield, L. B., and Johnson, B. L., Ind. Eng. Chem.,41, 1629 (1949). (7) Svetlik, J. F., private communication through A.S.T.M. (8) Yereley, F. L., and Fraser, D. F., Ind. Eng. Chem., 34, 332 (1942). RECEIVEDOctober 16, 19.50. Presented before the Division of Rubber Chemistry, AWERICAV C H E M I C ~SOCIETY, L International Meeting, Cleveland, Ohio, Oatober 11 to 13, 1950. Contribution 109 from General Laboratories, United States Rubber Co.
Determination of Trigonelline in Goff ee R . G. MOORES
AND
DOROTHY >I. GRENINGER, General Foods Corp., Hoboken, S. J .
Trigonelline represents about 5% of the soluble solids in coffee beverage. It contributes to coffee flavor and aroma, and may have important physiological properties. A n easy and reliable method for determining trigonelline was needed in the study of coffee composition and flavor development. In the new method, trigonelline is purified by chromatography on a solid adsorbent column, and is measured by ultraviolet absorption spectrophotometry. This procedure is more reliable and convenient than earlier methods. The method will be useful in further studies on the composition and processing of coffee, as well as in physiological studies involving closely related compounds such as nicotinic acid.
T
HE presence of trigonelline in green coffee was first reported by Polstorff in 1909 ( 2 1 ) , and later verified by Gorter ( 6 ) . I t is the methylbetaine of nicotinic acid with the ~tructural formula: H I
I
CH, Trigonelline represents about 5% of the water-soluble portion of roasted coffee. It has a bitter taste about one fourth that of caffeine. Trigonelline may be considered the source of pyridine found in coffee aroma. Hughes and Smith (9) found pyridine
