The C. P. H a l l C o m p a n y , A k r o n , O h i o
i J N COMPARISON to rub-
J
ber, GR-S gives a vulcan-~ ¡zed product which is more resistant to deterioration from oxidation than is the natural product (8, 9). How
ever, the changes resulting from the effect of heat on GR-S are rapid and are often evident during the course of vulcan
ization. Under normal service conditions, the vulcanizate may
be subjected to heat of external origin or to that which arises from hysteresis effects. This is particularly true of GR-S inner tubes and tires, and is probably one of several factors contributing to premature failure.
The effect of heat aging on the physical properties of GR-S vulcanizates has been reported, and the magnitude of such changes has been compared to similar natural rubber compounds (7, 9). In general, the effect of heat is a trend toward a shorter, more brittle stock, showing an increase in modulus and durometer hardness and a decrease in elongation, tear resistance, and tensile strength. The rate of deterioration for many of these physical properties is approximately doubled for an 18° F. (10° C.) rise in aging temperature. In contrast, natural rubber vulcanizates, upon continued heating, usually become softer and exhibit de
polymerization.
During heat aging, the deterioration of natural rubber may be considered to be the combined effect of depolymerization, oxidation, and aftervulcanization. Other conditions being equal, the magnitude of the deterioration produced is dependent on the amount of sulfur present and its state in the vulcanizate, the type of accelerator, and the effectiveness of the added anti
oxidant. In many cases a combination of two types of anti
oxidants provides a protection which is greater than the additive effect of the components.
It is not unlikely that GR-S vulcanizates, when subjected to air-oven aging, undergo deterioration resulting from a combina
tion of several effects, i f such is the case, it might be expected that the amount and type of the curative would exert a consider
able effect on heat aging. The addition of an antioxidant of a type different from that already present in the GR-S may show some value over a single-component antioxidant.
Up to the present time, much of the art of the compounding of GR-S has of necessity been confined to the adaptation of com
pounding materials normally used with natural rubber. The purpose of this investigation is to determine the effect of some of these materials on the stability of GR-S during heat of vulcaniza
tion, as well as the stability of the vulcanizate to heat aging as indicated by air-oven tests.
TESTING A N D HEAT A G IN G
Data were obtained on three separate batches of GR-S, sup
plied by Rubber Reserve Company during June, August, and September, 1943. Comparison of physical properties was made only on stocks compounded from the same batch of GR-S, cured, and tested at the same time.
A mixture of G R - S and E P C black, in the absence of curatives, stiffens w hen subjected to h e a t Tests have been made on a G R - S tread c o m p o u n d to determine whether various natural rubber com p o u n d in g materials m ay reduce the deleterious effects of heat. Persistent accelerators contri
bute to an increasing stiffness with co rresp ond ing decrease in elonga tiq n, and loss in tear resistance with increasing time of cure and o n air-oven aging. A nonpersistent accelerator and the a d dition of d ip h e n y l ethylene diam ine a id in red ucing these effects. In a G R - S vulcanízate containing a nonpersistent accelerator, the am ount o f a d d e d sulfur has a marked effect o n the rate o f change in p hysical properties on air-oven a g in g; 5 parts of zinc o x id e y ie ld a more stable vulcanízate than lesser amounts; natural rubber antioxidants show varyin g degrees of effectiveness in m aintaining elongation after air-oven aging.
The following physical characteristics were determined at room temperature: stress strain (f), Shore durometer hardness type A (4), crescent tear resistance using die A (S). Air-oven tests were carried out at 250° F. and Geer oven tests (2) at 158° F. All tests were confined to a GR-S tread-type compound.
The nonpersistent accelerator used was lead (phenyl amino- ethyl) dimethyl dithiocarbamate (11). It is referred to simply as lead dithiocarbamate type.
T o illustrate the instability to heat of unvulcanized GR-S, a mixture was compounded and treated as follows: GR-S, 100 parts; EPC black, 50; softener, 5.
The stock was sheeted smooth and free from air bubbles on the back roll of a 12-inch laboratory mill and then heated in a vulcanizing press for 60 minutes at 287° F. (142° C.). Sufficient stiffening had taken place to permit the removal of the stock from the mold. Dumb bell strips were died out and aged in the air oven for 1, 3, 5, and 10 days at 250° F. (121° C.). The change in physical properties with time of aging is shown in Figure 1.
It is possible that the changes on heating are not entirely due to the instability of the polymer, but may be due in part to action of the carbon black on GR-S or to the presence of small amounts of sulfur or sulfur compounds in the polymer (5) and in the softener. Undoubtedly the carbon black contributes much to tensile strength and modulus. In any case, the strik
ing point is that such a mixture, common to a large number of GR-S type compounds, undergoes a high degree of change even in the absence of added curatives. This instability possibly persists in the vulcanizate and causes much of the characteristic hardening of GR-S compounds on heating.
PERSISTENT A N D NONPERSISTENT ACCELERATORS
Ef f e c t o f Ad d e d An t i o x i d a n t. With regard to the gen
eral pattern of the change in physical properties with time of cure, accelerators are of two types (6, 9). The persistent accelerators, of which mercaptobenzothiazole is typical, are characterized by a relatively slow development of physical properties and a comparatively greater change in properties with time of cure.
Derivatives of mercaptobenzothiazole, sulfamines, and mer- captothiazolines follow patterns similar to mercaptobenzothia
zole (9). In contrast, the nonpersistent accelerators (dithiocar
bamate types) show a more rapid development of physical properties and better maintenance of such properties on extended time of cure.
720
August, 1944 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 721 Accumulated evidence indicates that, other things being equal,
hysteresis and heat build-up are largely dependent on the state of cure. Thus, the use of a persistent type of accelerator, high amounts of accelerator, high amounts of sulfur, and longer periods of cure, all contribute to a high state of cure and minimum heat build-up. This, however, is accomplished at the expense of other physical characteristics which are in many cases equally as im
portant. Where the state of cure is essentially the same, no great difference in heat build-up exists due either to the type of accelerator or to the amount of sulfur in the compound.
Figure 1.
3 5
DAYS IN A IR OVEN AT 2 5 0 'f.
C h an ge in P h ysic al Properties after A i r - O v e n A g i n g
The effect of added diphenyl ethylene diamine, an antioxidant used in natural rubber, was investigated with a number of per
sistent and nonpersistent accelerators. Typical results obtained with one of each type of accelerator are reported. The base compound used was: GR-S, 100; EPC black, 50; zinc oxide, 5;
softener, 5; sulfur, 1.6; accelerator and antioxidant as indicated.
The stocks were cured over a range of time at 280° F. (138° C.).
Original physical properties are shown in Table I.
At this sulfur level the persistent accelerator is less efficient and the resulting vulcanízate does not reach a state of cure as high as that attained in the
compound containing the nonpersistent type.
V
This is evident in lower moduli and excessive
ly high ultimate elongations. In the compound containing the persistent type accelerator, di
phenyl ethylene diamine appears to have a retard
ing effect on the increase in modulus and decrease in elongation with extended time of cure. This effect is more apparent with the nonpersistent type accelerator. The modulus remains constant at a lower level after a maximum value has de
veloped. Ultimate elongation remains higher throughout a broader curing range.
The 60- and 30-minute cures for the compounds containing the persistent and nonpersistent type accelerators, respectively, were aged for 6, 12, 18, 24, 30, and 48 hours in the air oven at 250° F.
The type of accelerator has a marked effect on the rate of change in elongation with time of aging (Figure 2). It is significant that, in maintaining relatively high percentages of the original elonga
tion, the nonpersistent accelerator-diphenyl
ethylene diamine combination is effective throughout aging periods two to three times longer than the persistent type of accelerator alone.
On prolonged aging at 250° F., differences in the maintenance of elongation, due to different types of accelerator, become pro
gressively less. The protective effect of diphenyl ethylene di
amine, however, is still evident after 48 hours.
Ef f e c t i n WPB Tr e a d St o c k. The effect of the nonpersist
ent accelerator-diphenyl ethylene diamine combination above on the original physical properties and on the maintenance of these properties after air-oven aging was investigated in a practical WPB tread type compound (10):
Stock A B C D E
G R -S 100 100 100 100 100
Stearic acid 1 1 1 1 1
Zinc oxide 3 3 3 3 3
M P C black 43 43 43 43 43
Bardol 4.6 4 .6 4 .5 4 .5 4 .6
Cumar M H 2*/i 2 .0 2 .0 2 .0 2 .0 2 .0
Sulfur 1 .6 1.75 2 .5 1 .6 1.8
Accelerator, 1 . 6 + ... . , 0 . 1 ° 1.26 1 .2 « 1 . 2d 1.04
Diphenyl ethylene diamine 2 .0 2 .0
a M ercaptobeniothia*ole-D P G mixture (persistent type).
& Sulfamine type (persistent).
c Aldehyde-amine thiaiole derivative (persistent).
d Lead dithiocarbam ate type accelerator (nonpersistent type).
The compounds were cured over a considerable range of time at 280° F. Values for all cures before and after air-oven aging for 24 hours at 250° F. are reported (Figure 3).
' With regard to the change in properties with time of cure, stocks A, B, and C undergo what has been called “ marching modulus” , whereas D and E are remarkably flat curing. On aging, the nonpersistent accelerated stockB show better mainte
nance of tensile strength, and the increase in modulus is less than half of that shown by the persistent accelerators.
The average values over the cure range of 45 to 120 minutes for ultimate elongation, tear resistance, and durometer hardness for all stocks are nearly alike (Figure 4). On aging, stocks D and E are outstandingly superior in the maintenance of these properties.
The degree of improvement in resistance to heat aging seems to be of the order of that reported for dinitrobenzene vulcanizates (S).
Juve and Garvey (5) showed that much of the continued stiff
ening of a persistent type accelerated stock with increasing time of cure is due to the availability of a considerable amount of free sulfur. The nonpersistent accelerators combine sulfur at a more rapid rate and to a comparatively greater extent; thus sub
stantially no free sulfur is available after optimum cure is reached.
PERSISTEN T A C CELERA TO R N O N - P E R S IS T E N T ACCELERATO R
18 2 4 3 0 H O U R S AT 2 5 0 *F .
Figure 2. C hange In Elongation with Tim e of A i r - O v e n A g i n g
72 2 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 8
It is believed that these differences in the amount of free sulfur in the vulcanizate contribute largely to the differences in the initial rate of deterio
ration on air-oven aging. Diphenyl ethylene diamine appears to have an activating effect on vulcanizate to air-oven aging (8). The accelera
tors used, however, were of the persistent type.
The following stocks were compounded to de
termine the effect of the amount of added sulfur with a nonpersistent accelerator and to show the effect of diphenyl ethylene diamine at the lower sulfur levels: accelerator. The lower-sulfur stocks provide improved maintenance of tensile strength and elongation on aging. After 3 days in the air oven at 250° F. the elongation of the stock containing 2 parts sulfur is approximately half that of the stock containing 1 part sulfur.
Diphenyl ethylene diamine shows additional improvement at both sulfur levels.
The above stocks were aged for 1, 3, and 6 markedly less severe. With regard to elongation, approximately 6 weeks at 158° F. are required to produce changes of the order of those obtained
M ercaptobensothiazole 1.6 Lead Dithiocarbamate ________ + P P G 0.1________ ___________ Type 1.2
Lb. per sq. in. Lb. per sq. in.
Min. at Modulus Tensile Elonga Modulus Tensile elonga
280° F. at 300% strength tion at 300% strength tion
15 85 615 960 350 1445 700
30 460 2025 700 1110 2285 470
45 750 2575 620 1320 2330 430
60 970 2545 560 1420 2320 410
00 1185 2505 510 1430 2250 400
120 1275 2255 480 1490 2245 390
D P G 0.1 + Diphenyl Ethylene Lead Dithiocarbamate T ype 1.2 Diamine 2___________ 4- Diphenyl Ethylene Diamine 2 Lb. per sq. in. Lb. per sq. in.
Modulus Tensile elonga Modulus Tensile elonga
at 3 00% strength tion at 300% strength tion
245 1135 810 690 2265 600
August, 1944 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 723
ORIGINAL ELONGATION
517 520 532
ORIGINAL TEAR
272 2 6 4 284
Durom-eter
Hard-nesa
ELONGATION
ORIGINAL HARDNESS
59 59 59 Table III. Effect of V a rio u s A n tlo x ld a n ts with Nonpersistent A c c e le ra tio n
(Average of 30-, 45-, 00-, 90-, and 120-minuto cures at 287° F.) rtriwinal Elougation Retained after Elonga- Air-Oven Aging at 2 5 0 -F., % Added Antioxidant tion, % 24 hr. 48 hr.
None 385 7 1 .7 62.6
Diphenyl ethylene diamine 454 8 4.1 71.1
apm-Di-d-naphthyl-p-phenylene
diamine 417 7 8.9 69.3
Di-o-tolyl ethylene diamine 437 7 5 .9 67.1
Ketone-amine reaction produot 425 7 3.9 65.6
Ketone-diarylam ine 430 6 9.3 61.2
Polymerized
trimethyldihydro-quinoline 420 7 6 .0 6 7.8
Diphenyl propylene diamine 446 8 4.7 7 5.5
< ^ DUR0METER HARDNESS
1 Ü --- 1---T ---1 AVERAGE OF 45 60 90 120 CURES AT 280°F
2 4 H o u rs at 2 5 0 ° F.
Ef f e c t o f Am o u n t o f Zi n c Ox i d e a n d Ad d i t i o n o f An t i
o x i d a n t. Zinc oxide in amounts as low as 1% based on GR-S is
sufficient to activate substantially the nonpersistent lead dithio- carbamate accelerator above. In order to determine the effect on heat aging produced by amounts of zinc oxide considerably higher than that required for activation and to show the effect of diphenyl ethylene diamine at each zinc oxide level, a series of stocks was compounded. The base compound used was: GR-S, 100; EPC black, 40; softener, 5.0; sulfur, 1.5; lead dithiocar- bamate type accelerator, 1.25; zinc oxide and diphenyl ethylene diamine as indicated. These compounds were cured over a range of time at 280° F. and were tested both before and after aging 24 hours in the air oven at 250° F. The average physical properties are shown in Table II.
The higher amounts of zinc oxide result in a more stable vul
canízate with improved maintenance of tensile strength and ultimate elongation, and less change in modulus on aging. How
ever, there appears to be no significant difference in the aged modulus values at all three zinc oxide levels. The amount of zinc oxide has no appreciable effect on tear resistance or on the maintenance of tear resistance.
The addition of diphenyl ethylene diamine at the lower zinc oxide level results in less change in modulus on aging. This effect is minimized as the amount of zinc oxide is increased to 5 parts;
however, the aged modulus values are considerably lower At
AVERAGE O F 3 0 ! 4 5 , '6 0 ' CURES AT 2 8 7 “F. AVERAGE OF 30;45', 60' CURES AT 287 “ E
3 0 0 0
3 0 0 0
T E N S IL E STPENGTH TEN SILE STRENGTH2000 2000
1000 1000
MODULUS AT 3 0 0 MODULUS A
0 1 3 6 0 1 3 6 0 1 3 6 W EEKS AT 158° F.
ELONGATION
ELONGATON
0 1 3 6 0 1 3 6 0 1 3 6 W EEKS AT !58°F.
Figur« 5. Effect of A m o u n t o f Sulfur and A d d it io n of A n tio x id a n t
724 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 8 each zinc oxide level there is improved maintenance of elongation
and the degree of improvement is of a higher order at the higher zinc oxide level. Diphenyl ethylene diamine has no effect on the maintenance of tensile strength; the maintenance of tear resist
ance is, however, somewhat improved.
Ef f e c t o f Va r i o u s An t i o x i d a n t s. T o determine whether materials other than diphenyl ethylene diamine improved the resistance of GR-S vulcanizates to air-oven aging, a number of organic compounds having antioxidant properties in natural rubber were investigated. The base compound used was:
GR-S, 100; EPC black, 50; zinc oxide, 5; sulfur, 1.5; and lead dithiocarbamate type accelerator, 1.2. Table III shows the per
centage elongation retained after 24 and 48 hours of air-oven ag
ing at 250° F. when 2 parts of these materials are added to the antioxidant already present in the GR-S.
The results show that these materials have varying degrees of effectiveness in maintaining elongation in this particular base compound. At higher sulfur levels and in compounds accelerated with persistent type accelerators, there is a tendency for the pro
tective effect obtained with added antioxidant to be minimized or completely overshadowed by the comparatively more rapid rate