State of Cure of Neoprene Vulcanizates Measurement by the T-50 Test D. B. FORMAS ASD R. R. RADCLIFF E. I. du Pont de Neniours 8r Company, Inc., Wilmington, Del. T h e state of cure of neoprene vulcanizates can be determined by measuring retraction temperatures with the T-50 test apparatus. Low temperatures of retraction are indicative of high states of cure. The test has been found useful for evaluating the effects of accelerators, plasticizers, fillers, and changes in the curing conditions of neoprene compositions. Finally, an estimation of the tendency of neoprene vulcanizates to harden at 1.67' C. can be made from the retraction data-the T-50 values.
G
EKERALLY the physical properties of elastomer vulcani-
zates are considered to depend on their states of cure. Although many tests for measuring the state of cure have been developed, qone are completely satisfactory. Some of them fail to distinguish between the physical or chemical effects of compounding ingredients, others are too laborious to be of practical value. The T-50 test developed by Gibbons, Gerke, and Tingey (5) for natural rubber vulcanizates eliminates some of these difficulties and further reduces the number of tests required for production control. The T-50 test is based on the retraction of a stretched and frozen specimen with an increase in temperature. The temperature a t which the sample retracts to 50% of its initial frozen elongation is termed the T-50 value. From a physical standpoint the test is sensitive, precise, and casily performed. From a chemical standpoint the origina! workers (6) as well as Vila (8) showed that for any specific natural rubber composition the T-50 value correlates with the amount of combined sulfur. Since the state of cure of a rubber vulcanizate is frequently discussed in terms of combined sulfur, the T-50 test offers a correlation of the state of cure with conlbined sulfur without the necessity for involved chemical analyses. In the case of neoprene vulcanizates, no correlation betLveeIi state of cure and a chemical property is known. For this reason it might be expected that the T-50 test viould not be applicahli: to determinations of the state of cure of neoprene vulcanizates. Reports by the Naugatuck Cheniical Division of the U. S.Rubber Cqmpany ( 7 ) and by Cohan and Steinberg ( 2 ) indicate tha; the T-50 test probably is not adapt,able to measurements of the state of cure of all vulcanized elastomers. Only one report on the use of a T-50 type test with neoprene compounds is known. This \vas made by Yerzley and Fraeer (9) who described a retractiontemperature test as one of several which could be used for estlmating the freeze resistance of elastomer vulcanizates. In the case of natural rubber vulcanizates, extensive retraction occurs within very narrow temperature limits near the T-50 point. The,precision of the measurement of this point is therefore high. An analysis of the work of Terzley and Fraser (8, showed that this is not true for neoprene vulcanixates, and thar measurements of the T-50 point only are not so precise or significant as they are with natural rubber. For this work, thcrefore, the T-50 test was extended to include temperature nieisurements, not only at the point of 50% retraction, but also a t other 10%
intervals betv-een 10 and 90% retractions. Smooth curves that are often 8-shaped are obtained by plotting the per cent retractions as ordinates and the temperatures as abscissas. These curves shorn the greatest sensitivity between 30 and 70y0retraciions; therefore, in comparing neoprene compounds for changes in their state of cure, it is more satisfactory to consider the complete retraction-temperature curves rather than the single T-50 values. Changes in the slopes and the positions of the curves are indicative of changes in the state of cure. The T-50 values have been found to be reproducible.with an accuracy of * 1O C. and have been selected as a reference point. The T-50 values on the retraction curves are suitable for controlling the manufacture of neoprene products as well as for expediting laboratory investigations. Hence, they are applicable t o neoprene compounding in much the same way that the conventional T-50 measurements are applicable to the compounding of natural rubber. Although the work described is restricted to tests on Neoprene Type GRSI, supplementary tests have shown t'hat this test is equally a p plicable to other types-for cxample, Seoprene Type F R and Scoprene Type CG. TEST PROCEDURES
Except as described here, the test procedures conformed to eatablished A.8.T.M. methods (I). The specimens were mounted in a special T-50 type holder, fitted with a graduated scale for measuring retractions at 10% intervals. As the. holding apparatus was designed to hold samples a t given elongations, the teste ryere run on specimens stretched 170, 270, and 340%. Accordingly, the results are based on these elongations rather than 50% of the breaking elongations as recommcnded by Gibbons (6). -1fter mounting, the samples were stretched and then conditioned io minutes in an alcohol bath a t 20 O C. The temperature w u then lowered a t a rate of 2' C. per minute by a circulating acetonecarbon dioxide system until - 40' C. was reached. The samples were heldat -40" for 10 minutes, and the temperature was then .oviered 1'C. per minute until -70" C. was reached. The samples were held a t -70"for 10 minutes and then released for retraction. Finally the temperature was raised 1' per minute, and readings w r e taken at each 10% interval of retraction through 90%. Table I gives the neoprene compositions described in this paper, and their physical tests are shown in Table IT. T-50 VALUES AYD STATE OF CURE
Several of the more irnporxant factors affecting the state of cure neoprene vulcanizates are considered in the following parsgraphs. The relations between these factors and T-50values are discussed. For convenience, cross lines have been drawn on the graphs a t the point where the T-50 value is 0" C. EFFECTOF CURISG TINE ASD TEMPERATURE. The fundamental differences in the retraction temperatures of vulcanizates having varying states oi cure are illustrated in Figure 1. The retraction curvcs shift to the left, and specific T-valucs become -3:
1048
October, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
1049
FIGURE 2 EFFECT OF ACCELERATION COMPOUNDS: B AND 8-3 CURE 10 MIN. AT 153OC
COMPOUND B
70x NO c 14b
8 60I
2
0 a
50-
U I-
W
40-
I
-30
-20
I
90
-
80
-
70
-
-10 0 TEMPERATURE
I
I
IO
-20
20
OC.
I
I
I
I
-10 0 TEMPERATURE
I
I
IO
2c
OC.
I
90-
FIGURE 3 EFFECTS OF CARBON BLACKS COMPOUNDS: A ,B ,C CURE: 30' AT 142%
80
-
FIGURE 4 EFFECTS OF PLASTICIZERS COMPOUNDS: G,C-2,C-3 CURE: 40 MIN. AT 153%
70 -
* a p6 0 I
z
0 I-
o 50 < a
a
U
I-
i-
a
W
w
COMPOUNDS ETHYL PHOSPHATE 30 PTS. TRICRESYL PHOSPHAT E
ASRF GARBON BLACK o M T CARBON BLACK
I TEMPERATURE
"C.
I
-30
I
I
-20 -10 TEMPERATURE
I
I
0
IO
OC.
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
1050 TABLE
6
n
A io0
.,.
."0 .. ... ...
4.0
',
B-2 100 0..5 2.0
28 8
IS'S
28 8
1.0
1.0
2.0
...
... ... ...
., ,,
,, ,
.
. ,. ,.
...
..
...
. . ., , ,
,, ,
5.0
c
n-3
0 5 2.0
B-i 100
io0 0.5 2.0
. .
.
, ,
3.0
.
,
5 0
, 4:
...
,
100 0.5 2.0 ,4;
ion0
5 2.0
5$:
28.8
5.0
,
5 0
T.\BLEI I. PIIYSICAL PI~OPCRT OFI E ~~ . 5 O P R E S E GR-31 CO.\IPOUSDS
No. A B
Time Min.' 30 60 15 30 60 120 10
B-1 B-2 n-3
C
c-1 c-2 c-3
60 60 60 10
30 40 10 120 40 40
Cure Temp., C. 142 108 142 142 142 142 153 153 153 I53 I53 142 153 163 153 153 153
Stress a t 30056, Lb.;Sq. In.
Ts,Lb./
300 250 850 975 1175 1375 1200 1350 1875 2250 1625
3975 750 2525 2450 2650 2623 2900 26x1 2400 2550 2i00
1025
825
1850 1650 1700 1950
700
;;:
!;
383 680 730
66 36 40
650 925 1750 325 400
Sq. In.
1550
1575
Elongagation,
%
800
880 740 66.5 510 780 54.5 350
310 545
:
,
5.0
Phen).I-a-naphthylan~ine DOTG salt of dicatechol'borate.
Compound
temperature-retraction curves of neopren.: stocks containing tributoxy ethyl phosc-i c-z c-3 phate and tricresyl phosphate wit'h a con100 100 100 trol stock containing no plasticizer. The 0.5 0,i 0.5 2.0 2.0 2 o temperature-retraction curve for the tri5$; 5f: cresyl phosphate compound indicates that ., .. .. .. .. .. .. .. this plasticizer has a definite activating 1.0 ... ,.. effect. The temperature-retraction curve for the tributoxy ethyl phosphate com,. . . 30.0 , , 30,0 , . O 5.0 5.0 pound has an abnormal shape, and the same phenomenon has been observed with several different plasticizers. As judged by the higher end of the curve-that is, 25-90%. - retractionstributoxy ethyl phosphate produces a mild retarding effect The lower end of the curve-below 25% retraction-crosseu the curve of the control stock. This is believed to be cause0 by physical effects which are somewhat independent of the state of cure. Unusually low T-10 values (temperature of 10% retraction were observed in some stocks containing plasticizers which were added to improve freeze resistance (4). These low T-10 values can be correlated with such freeze resistance tests as brittle points and hardening during short exposures to very low temperatures. Plasticizers that have produced the best freeze resistance have also produced the lowest T-10 values. Since tributoxy ethy: phosphate is known to be an excellent plasticizer for improving
1. FDRLICLAR OF ~ E O P R F . S E GI?-.\[ COVPOL-XDS
Compound No. Neoprene GR-,\I Stearic acid Neoaone A n Extra-light-calcined 'lagnesia RIT carbon black SRF black , Permnluxb Sulfur Tributoxy ethyl phosphate Tricresd phosphate Zinc oGide Ii
Vol. 38, No. 10
Shore Hardness 46 55 55 56 56 60 56 60 66 68 61 56
lower as the time or temperature of curc increases. Accordingly, : retraction temperatures decrease as the st,ate of 0curc is increased. In general, the greatest differences between two or more retract'ion curves lie between 30 and 70% recoveries. EFFECTOF ACCELERATION.The curves of Figure 2 show the increase in the state of cure of a neoprene c,omposition caused by the addition of an accelerator such as the diortliotolylguanidine salt of dicatechol borate (Permalux). A further example of the use of temperature retraction curves as a method of evaluating acceleration is brought out by comparing the curves of Figures 1 and 2. The T-50 value of the stock containing 1% Permalux, cured 10 minutes a t 153' C. (Figure 2) is approximately equal to the T-50 value of the control stock containing no Permalux, cured 120 minutes a t 142' C. (Figure 1). This indicates that these vulcanizates have essentially t,he.same state of cure; the stress-strain data of Table I1 confirm this by showing that the properties of these vulcanizates are nearly equal. EFFECTSOF LOADING.Figure 3 show great differences betlveen the retraction curvcs of & gum vulcanizate and those of ,two carbon black vulcanizates. -4lthough the magnesia-zinc oxide ratio of compound A differs from those of compounds B and C, unpublished data obtained in this laboratory have s1lon-11that these differences would not affect the relations between these compounds. Therefore, measurement of T-values may he an excellent method for determining the effects of different fillers on the rate of cure of Xeoprene GR-31. EFFECTS OF PLASTICIZERS. Stress-strain propertirs, compression set tests, resilience data, etc., do not provide a clear distinction between the physical and chemical effects of plasticizers. For example, there is little difference in the stress-atrain proper-I ties of compounds C-2 and C-3 (Table 11) $ 5 which, respectively, contain equal quantities of 5: tributoxy ethyl phosphate and tricresyl phosphate. The T-50 test appears t o be a method Iof measuring the chemical effects of added -5' Ib plastirizcr. Figure 4 compares the complete
:
st;6"
, ,
,
35 4
cn W
/
t
a 10I
5-
1
1
-10
I
-5
I
I
I
0
5
IO
T-50 VALUES
OC
DAYS REQUIRED FOR A TEN POINT HARDNESS INCREASE AT A GIVEN T - 5 0 VALUE
*---, X
210
o;
40 50 $0 DAYS AT 1.67°C. (35OF.)
70
alo
gb
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
October, 1946
OF TABLE111. EFFECT
FIGURE 7 EFFECT OF SULFUR COMPOUNDS: B, B-I, E-2 C U R E i 6 0 MIN. AT 153%
80
Plasticizer
I 2
PL.4STICIZCRS OS STATE OF C U R E
T-5! Value,
Effect on Cure
C.
Amount0
Cured 30 XIinutes a t 142' C. Control (compound C) Circo process oil 15 0 Dioctyl sehacate 15 +4 Flexol 3GOb 15 Flexol4GOC 15 Flexol DOPd 15 +O. 5
..
......
+r
Accelerates Accelerates Neutral Neutral Accelerates
$5
70t 3
1051
Cured 40 Minutes a t 153' C.
I
..
Control (compound C) Circo process oil 15 Circo process oil 30 Dibutvl sebacate 15 Dicapiyl phthalate 30 Tributoxv ethyl phosphate 30 Flexol 3G0 30 a Plasticizer added t o control composition. b Triethylene glj-col d i - k t h y l h e x o a t e . 0 Polvethvlene glycol di-2-ethylhexoate, d Di-3-etLylhex3.1 phthalate.
60 1
E50U
a
......
+3 -3 -15 f2.5 - 13
Accelerates Accelerates Neutral Accelerates Retards Acceleratee
+s -1
I-
z40a t 1.67" C. Sulfur vulcanization, which prevents the crystallization of Seoprene GR->I ( 3 ) ,has a pronounced effect on T-50 values The effwts of 1 and 2% added sulfur are showninFigure 7 which compares compounds B, B-1, and B-2. At a given cure (60 minutes a t 153" C.) the T-50 value decreases from $3" to -9" C. as the sulfur content is increased from 0 to 2%. Availnble data are insufficient to show whether the combined sulfur in neoprene vulcanizates can be estimated from T-50 measurements.
30 -
* N O SULFUR 20 -
A
2% SULFUR
EFFECTS OF T E S T ELONGATION
-20
i
L
IO
2(
I
I
-IO
0
TEMPERATURE
freeze resistance (0, we can expect that its compounds would show lover than normal temDeratures for lower retractions. Based upon T-50 values, Table I11 classifies several other plasticizers with respect to their accelerating or retarding tendencies.
r '
90t 80
1
I
I
l
I
I
I
I
I
FIGURE 8 ORIGINAL STRAIN VS. RETRACTION R A T E S COMPOUND C-l
.i7
STATE OF CURE AND CRY STALLIZATIOK
I'he time consuming teste required to measure hardness or modulus increases caused by the crystallization of neoprene vulcanizates a t moderately low temperatures (4, 6) can be partially eliminated by determining the T-50 temperature. Since both T-50 temperatures and crystallization tendencies are dependent upon state of cure, it was assumed that vulcanizates l a v i n g certain T-50 values mould not crystallize. It has been observed that neoprene compositions containing filler and no plasticizer have little or no tendency t o crystallize if the -1'-50 temperature is belorr - 5 " C. Figures 5 and 6 and Table I V show the relation between T-50 values and increases in hnrtincss duringlungexposurez
Actual T-50 values of specific neoprene compositions, like those of natural rubber vulcanizates, are dependent upon the elongation applied to the test specimen. Table V s h o w the differences in the T-50 values of compound C-1 when tn-o cures, 10 and 120
OC.
ORIGINAL STRAIN
340 %
@I
z 0
5 50-
s I-
40-
301 I
1
-25-20
I
-10
I
I
0
IO
I
l
20 -25 -20
TEMPERATURE OC.
I
I
I
I
-10
0
IO
I!
I N D U S T R I A L AND E N G I N E E R I N G C H E M I S T R Y
1052
~~
TABLEIF-.
RELATIOX BETTEES T-50
\-.%LUES
ASD
Vol. 38, No. 10
~
STIFFESISG
Increase in Hardness a t 1.07’ C. Cure a t (35’ F.) Specinien 1.53” C., T-5! Value, 4 10 36 50 124 s0.a llin. C., d a y s dah-s d a y s d a y s d a y s 1 20 -3 0 3 3 6.~ 2 1 60 -5 5 0 1 2 3 5 2 20 +5 1 25 34 33 35 60 +I 0 6 27 31 34 3b 20 +5 1 9 29 31 33 GO +3 0 4 21 28 31 4c 20 0 0 3 6 11 17 60 -3 5 1 3 3 4 6 3 20 0 0 3 4 6 1 9 (j0 -9 0 2 1 G ‘0 -4 S 0 3 2 ; : 00 -9 2 0 1 3 3 7 20 33 8 38 40 39 GO 23 2 38 35 36 0 6 +1 28 14 8 . 20 f 60 -4 1 4 5 10 ‘3 20 33 +8 12 33 36 3s 60 7 26 33 T 7 35 32 10 ”0 0 26 32 +5 33 29 00 0 10 34 33 30 +-i 11 20 --J 0 2 1 3 00 -i 0 2 ‘ 1 2 4 a Some of t h e compositions listed are in addition t o those i n Table I, ani! were used in a s t u d y of crystallization conducted in this laboratory; nor,t: contained plasticizers. b Same a s compound B , Table I. Same as compound B-1, Table I. d Same as comDound B-2. Table I.
2;
minutes a t 153” C., were tested a t elongations of 170, 270, and 340%. The over-all differences in the retraction curves are shown in Figure 8. This analysis indicates that it should not be necessary to test neoprene compositions a t a given percentage of their ultimate elongations. Relations between test specimen-
curea t 1.53’ C . Xin. 10
120
L l t i i n s t e Eiongation Re7 s i l t & from Following Test Elongation: 170% 270% 340% 23.tC ii) ;s 51.1 48 ;t 77 1 97.2
T-50 Values. 170% 270%
+9
0
C. 340%
+lo.: 0
+5
-7
:~rcsimilar for each test e:(Jngation used. Therofore, in using the _ . 1-50 test for control or development work, any given elongation should be satisfactory i ~ testing r most rulcanizates. LITER.ATLRE CITED (I)
Am. SOC.Testiiig \I Lteriak, Standards, Procedure D599-40T,
p. 66 (1944). 12) Cohan, L. S.,an([ .Sri?in’xrg, If.,ISD. ENG.CHEM.,A N A L . ED., 16, 562 (1944;. (3) Forman, D . B., I):)P$J!:LCo., Rubber Chemicals Div., Re&. BL-196 (1945j. (4) Forman, D . B., ISO. ESG. CHEX.,36, 738 (1944). ( 5 ) Gibbons, W. A . . Gerke. R. H., and Tingey, H. C., IND. ENO, CHEM.,-1st~. E D , 5, 297 (1933). 6 ) Liska, J. W., I s u . E r c CHEW.,36, 40 (1944). (7) Naugatuck Chemical Dix i-mn of U. S. Rubber Co., Rept. N-40-19 (Jan. 27, 1942). ( 8 ) Vila, G. R.. India Rubber K o r l d , 101, KO.2, 34 (1939). ‘0) Yerzley, F. L., and r r a i e r , D . F.,ISD. EXG. CHEM.,34, 332 (1942). I’RESLXTEDbefore the Dii-1-;?:. of r h o AJIF.RICASC H E M I C A :s,cL
,: Rutiticr I.
Chemistry a t the lO9ttl 3Ieetiog sl-,.\ylxntic C i t y , E,J.
Role of Carbon in Oxidation of GR-S Vulcanizates HCGH WINN, J. REID SHELTON, AND DAVID TCRSBULL Case School of Applied Science, Cleveland, Ohio Carbon black is shown to be a catalyst for the oxidation of GR-S vulcanizates. The incqeased rate of oxygen absorption with higher loading is shown to be caused by the increase in carbon surface area. Furthermore, the increased oxygen absorption is showrn to result in a correspondingly greater deterioration of physical properties. The relative activity of a unit of surface varies with the type of carbon, and furnace carbons are shown to be only 0.55 as active as channel blacks in promoting oxidation. A theoretically derived equation is presented which correlates oxygen absorption with surface area and loading.
0
XYGEK was shoiyn t o be a major factor in the deterioration of GR-S tread stocks (3,6 ) , and a quantitative study of the rate of oxygen absorption by GR-S vulcanizates ivas described in the preceding paper of this series (4). Since reinforcing carbons are essential in most GR-S stocks for developing adequate physical properties, it seemed desirable to study the role of carbon in the oxidation. Little information is available with respect to the effect of carbon upon the oxidation of natural rubber. Ruby and Depew (2)observed that a carbon black stock did not age as well as a tread stock loaded with zinc oxide. They state that, although the cause is uncertain, it appears that “carbon black acts as a catalyst and oxidizes the soft rubber to a hard rubber”. Vogt ( 5 ) compared the rate of oxygen absorption with gain in weight
for a tire stock with variou- pigments. He found that the “gas black” stock absorbed the most oxygen and gave the poorest aged properties, whereas . ~ h c r m a t o m i ccarbon” was intermediate between it and the inert fillei -. EXPERIJIENTAL PROCEDURE
The volumetric method (of measuring oxygen absorption, in which the sample is held a t constant temperature and oxygen pressure, was used throughout this study. The absorption is reported in terms of the volume of oxygen (calculated to 25“ C. st 760 mm.) taken up by the jample during various time intervals. The apparatus and experimental details were described completely in a previous publication ( d ) , in which it was shown that the actual reaction rate can he accurately determined by this method and that the effect oi cure is negligible. The method is subject to less variation t h a n aging studies based on physical property measurements, and the rate of oxidation is not, masked hy the effect of variati0r.s in compounding upon rha.rigc:s in physical properties. The following base forn,ai:it iun was employed: P a r t s by Weight GR-S. Bardol F a t acid Zinc oxide Sulfur Santooure Carbon black
100.0 5.0 1.5 5.0 2.0 1.2 Variable
