literature Cited
might well be explained from the presence of trace quanrities of free radical scavengers, which effcctively reduce the free radical concentration of the black and thus interfere seriously with normal pelletization.
Conclusion
Carbon black particles undergo chemical bonding through a free radical mechanism upon densification. This process is the cause of the irreversible changes found in the blacks upon densification. While structural bonds create more anisometric particles by the formation of particle chains and networks, bonds formed upon densification are randomly directed and cause the formation of more isometric particle agglomerates.
(1) Bennett, J. E., Ingram, D. J. E., Tapley, J. G, J . Chem. Phys. 23, 215 (1955). (2) Donnet. J. B.: Henrich, G., Riess, G., Reu. Ge'n. Cuoutchou' 38, 1803 (1961). 13) Gessler. A. M.. Rubber Aee 86. 1017 (1960). i 4 j Krausej G., Collins, R. E., RLbber &odd 139, 219 (1958). (5) Mrozowski, S., U. S. Patent 2,682,686 (1954). (6) Pauling, L., "The Nature of the Chemical Bond," Cornel1 University Press, Ithaca, N. Y . , 1948. (7) Uebersfeld, J., Etienne, A , , Combrisson, J., Nulure 174, 614 (1954). (8) Voet, A,, J . Phys. Chem. 61, 301 (1957). (9) Voet, A., Rubber World 146, 77 (1962). (10) Voet, A., Teter, A. C., Am. I n k Maker 38 (A), 44 (1960). (11) Voet, A., LYhitten, W.N., Jr., Rubber Age 86, 811 (1960).
RECEIVED for review May 4, 1962 ACCEPTEDJuly 2, 1962 Division of Rubber Chemistry, ACS, Boston, Mass., April 1962.
RAPID CURING OF URETHANE ELASTOMERS S. W
.
U R S,' Naugatuck Chemical, Division o j U . S. Rubber Co., Naugatuck, Conn.
Long curing time (30 minutes or more) of urethane elastomers has been a bottleneck in production. This obstacle has been overcome in a new urethane elastomer, based on an adipate polyester and chainextended to contain urea linkages. The new elastomer cures in 2 to 3 minutes a t 350" to 400" F., developing full physical properties. Data demonstrate the completeness of cure in stocks with carbon black as well as silica fillers. Stress-strain properties, swelling characteristics, and high temperature aging properties of the elastomer are given. Properties of the new elastomer are compared with those of an elastomer, also based on an adipate polyester, which does not contain urea linkages or cure as rapidly.
elastomers ordinarily require long periods of curing time extending to more than 30 minutes. This is a serious drawback in the fast commercial production of molded goods, particularly when plastics equipment such as injection molding and extrusion machines is used. Therefore, curing certain millable urethane elastomers a t elevated temperatures for short periods of time was explored. after appropriately modifying the formulation. RETHANE
Millable Urethane Elastomers
Polyurethane elastomers investigated were based on reaction products ( 7 ) of a polyester, such as polyethylene propylene adipate and polyethylene butylene adipate, and a n organic diisocyanate such as toluene diisocyanate and diphenylmethane diisocyanate to which either a small amount of water or a diprimary diamine was added to form urea linkages along the backbone of the polymer molecule. The resulting product is a polyurea-urethane elastomer. The amounts of polyester, diisocyanate, water, and diamine used in the reaction were adjusted to leave no residual reactive functional groups in the polymer. Preparation of Polyurea-Urethane Gum Polymer
In a Baker-Perkins mixer were placed 2000 grams (1 mole) of dehydrated polyethylene propylene adipate (OH number 55.0,acid number 1.0) and heated to 100" C,; 300 grams (1.2 mole) of diphenylmethane diisocJianate were added and the mixture was allowed to react for one hour. To the fluid mixture 23.6 grams (0.2mole) of anhydrous hexamethylenediamine were added and the mixing was continued for another minutes, solid gum polymer M,as obtained M-hich was readily removed from the mixer, This polymer was designed to contain one urea group per 6792 molecular units. Present address, Polymer Research Department, Olin Mathieson Chemical Corp., New Haven, Conn.
The structure of such a polyurea-urethane elastomer is simply written as :
-o[
.) (CO-(CH~)~-CO-O-(CH~)~-O
1
)po-NH-R-
NH mCO-NH-(CH2)sNH where 1 = 9 to 12, m = 4 to 6, and n = 2 to 3. This is a block copolymer containing urea groups far in excess of those present in straight polyurethane elastomers, where a few urea groups may be accidentally present due to traces of moisture in the polyester. Properties of the polymer are shown in Table I. The gum polymer, by itself in the unvulcanized state, possessed no good mechanical properties. had cold flow, and was readily soluble in many solvents. The gum polymer could be vulcanized by curing with a peroxide. O n compounding with peroxides such as dicumyl peroxide (DiCup) and 2,5-dimethyl-2:5-di-tert-butylperoxyhexane (Varox, Vanderbilt Co.) a process- and storage-stable compound was obtained. Like conventional rubber stock, the urethane gum could be compounded, stored, and processed without scorching. Compounded stocks are ordinarily cured in 30 to 6 0 minutes to yield vulcanizates of high mechanical properties '1, but these stocks cured completely in 3 minutes at 350' F. Straight polyurethane elastomer did not cure as rapidly in 3 minutes at 305' F. (Figure 1). as evidenced by the poor quality of the cures and the difficulty in stripping the vulcanizate from the mold after curing for only 3 minutes. O n the other hand, if the straight polyurethane was cured a t 305' F. for 45 minutes, the stress-strain properties of the elastomer were identical with those of polyurea-urethane elastomer cured for brief periods a t elevated temprratures. VOL. 1
NO. 3 S E P T E M B E R 1 9 6 2
199
Unlike cast polyurethanes ( 3 ) ,which are cured by the reaction of excess isocyanate groups with hydroxyl or amino groups, the gum polyurethanes did not require postcuring ; vulcanization was completed in the mold. However, the gum polyurethanes required reinforcement with fillers such as carbon black, silica, and clay to develop the desired modulus in the vulcanizate. Peroxide Curing Agents
E l o n g a t i o n i o/o)
Figure 1. Polyurea-urethane vs. regular polyurethane elastomers Cure: 3 minutes at 350' F. 20 parts HAF black and 4 parts DiCup 40C
Several peroxides ( 2 ) and hydroperoxides were investigated in curing the polyurea-urethane gum stocks. Two peroxidesdicumyl peroxide and 2,5-dimethyl-2,5-di-tert-butylperoxyhexane-gave the best cures with optimum mechanical properties. Stocks were normally cured in 30 to 60 minutes at 305' F. Temperatures below 290' F. yielded highly undercured stocks, presumably because of production of an insufficient number of free radicals to initiate cross linking. High Temperature Cures
Recipes of the polyurea-urethane elastomer, containing either dicumyl peroxide or Varox, were cured in 45 minutes a t 305' F.. 3 minutes at 350' F., or 2 minutes a t 400' F., to yield vulcanizates with good modulus and high mechanical properties. High temperature cures for short periods contributed to higher break elongations in the vulcanizate (Table 111). This is further reflected in the tensile product ( 4 ) , a useful term which gives the actual stress in the elastomer a t its breaking point based on its cross section. The term is calculated as follows :
1
4000
.-
I
2 3000
-
c
:I
c
2000
Tensile product = tensile strength
VI
( io -I- 1
Elastomers, cured for short periods of time at elevated temperatures, should be useful in the preparation of Spandex thread, because of higher break elongation.
IO00
I
I
I
I
200 400 Elongotion
Figure 2.
I
1
600
Effect of Peroxide Concentration
( * l o )
Lse of increased concentrations of peroxide in the recipe a t 20 parts of added HAF carbon black resulted in increased load-bearing characteristics of the elastomer, as seen in Figures 2 . 3, and 4. With peroxide constant at 4 parts per 100 parts of gum, the modulus varied in direct proportion to the black content
Effect of DiCup
Cure: 3 minutes at 350' F. 20 parts HAF black
4000
-
-
.i 3000 ci
vl 3000 LL
c
1
VI
2
*
2000
VI
v)
-
t I
IO00 -
01 0
I
I
I
I
200 400 Elongotion (o/o)
Figure 3.
I
I
600
Effect of DiCup
Cure: 400' F. 20 parts HAF black
200
l&EC
)
P R O D U C T 'RESEARCH A N D D E V E L O P M E N T
'"""I u 400
200
OO
E I a nga tion
Figure 4.
( '10 )
Effect of Varox
Cure: 2 minutes at 400" F. 20 parts HAF black
(Figure 5). Crescent tear reached a maximum a t 40 parts of black and then decreased. For the polyurea-urethane elastomer, 4 parts of DiCup 40C (dicumyl peroxide, 40%,, dispersed on inert calcium carbonate) was found to be the optimum concentration, the curing conditions being 45 minutes at 305' F., 3 minutes a t 350' F., or 2 minutes a t 400' F. In this range, the vulcanizate: containing 20 parts of HAF black, possessed a 3oOy0 modulus of 2200 to 2900 p.s.i.. tensile strength of 4200 to 4400 p s i , break elongation of 400 to 47oYob, Shore A hardness of 65, rebound of SO%, and compression set value of 2070.
Properties of Polyurea-Urethane Elastomer
Table 1.
Appearance Odor Specific gravity Mooney viscosity, ML4/212 Mooney scorch at 250' F. Scorch time, min. Cure rate, min. Storage stability Solubility
Table II.
Straight Polyurethane Elastomer
A straight polyurethane elastomer was prepared by reacting polyethylene propylene adipate and diphenylmethane diisocyanate, with precautions to eliminate all moisture contamination during the reaction so as to avoid urea formation. This polymer readily underwent reinforcement by carbon black and silica fillers and cured with dicumyl peroxide, yielding \ulcanizates with good mechanical properties. However, when subjected to rapid curing a t elevated temperatures. the vulcanizate did not develop modulus as rapidly as in the long cure a t 305' F. (Figure 1). I t appears that the presence of a substantial number of urea groups on the elastomer backbone enables the elastomer to undergo faster vulcanization.
Pale yellow Faint, characteristic 1 15 50 =t10
F.
25 35 Excellent Soluble in MEK, THF, DMF, and pyridine ~~
Mechanical Properties of Polyurea-Urethane Elastomer
Recipe Polymer, parts by weight DiCup 40C,a parts by weight HAF black, parts by weight Stearic acid, parts by weight Cure time, min. Cure temp., F.
100
4 20 0.25 45 305
Properties
300y0 modulus, p.s.i. Tensile. u.s.i. Elongatibn, yc Tear strength, lb./inch (ASTM die C ) Shore A hardness a
2900 4200 400 500 65
Dicumyl peroxide, 4076,on culcium carbonate.
Heat Aging
The polyurea-urethane elastomer resisted degradation under conditions of aging a t 250' and 300" F. and retained a high percentage of mechanical properties (Table IV). The elastomer also exhibited good low temperature properties (Table V.)
Table 111.
Effect of Curing Temperatures on Tensile Product 35 M i n . at ,305' F.
Tensile stren th, p.s.i. Elongation, Tensile product, p.s.i.
&!
4200 400 21 X I O 3
3 Man. at 350' F.
2 Min. at 400" F.
41 40 4300 470 450 23 6 X lo3 23.65 X IO3
Solvent Resistance
The poylurea-urethane elastomer exhibited high resistance to aging in oils (Table V I ) , fuels (Table V I I ) , and perchloroethylene (Table V I I I ) . Resistance of the elastomer to perchloroethylene is of considerable significance, if it is to be used in Spandex thread, which may be dry cleaned. Vulcanization Process
Diisocyanate- and diamine-cured, cast polyurea-urethane elastomers contain predominantly secondary cross links attributable to hydrogen bonding. These are readily broken under the influence of high temperatures and also highly
3000
Table IV.
Heat Aging of Polyurea-Urethane Elastomer
7cRetention Aged 70 hr. at 250" F.
Property
300y0 modulus Tensile Elongation Shore A hardness
94.1 105 100
O
-7 5
F.
Temp. of retraction. ' C. -33.4 -23.7 -11 .o
TRIO TR30
TRK
Gehman modulus,
O
C.
-25.5 -33.1 -39.5
TP
T5
6600 C r e s c e n t Teor
PIS
IO
20
I
I
30 40 50 60 Pts. H A F B l o c k
I
70
Ti00
Table VI.
4 0 0 0-
0-1
60.0 73.0 122 95.4
Table V. l o w Temperature Properties of Elastomer (Elastomer contains 20 parts HAF black)
Bell brittle point,
r
90.4
Aged 70 hr. at 300" F.
H A f Black
I
60
Figure 5. Effect of black on modulus and tear of polyureaurethane elastomer
Oil Resistance of Polyurea-Urethane Elastomer 3 Min. Curing Conditions at 350' F.
Volume swell, 70 hr. at 250" F. ASTM oil 1 ASTM oil 3
VOL. 1
NO. 3
-3.1 +4.5
SEPTEMBER 1962
201
Table VII.
Fuel Resistance of Polyurea-Urethane Elastomer 3 Min.
Curing Conditions
at 350" F.
Volume swell, 7 days at 85 F., % ' Fuel A Fuel B
-0.45 -l-15.0
Table VIII. Perchloroethylene Resistance of Polyurea Urethane Elastomer
Elastomer
literature Cited
PolyureaUrethane
Nitrile Rubber, 32Y0 A C N
f21.8
f37
88 72 80
...
Volume swell, aged 14 days at 85" F., 7 0 % ," retention 30070 modulus Tensi1e Elongation
polar solvents such as dimethylformamide and pyridine. In the case of peroxide-cured polyurea-urethane elastomers, primary cross links are formed by the coupling of free radicals formed on the a-methylene carbon a t o m on the adipic acid residue of the elastomer molecule Primary cross-link density (5) has been calculated in this type of elastomers to be 1.65 X 2.13 X and 2.72 X lop4 mole per cc., when the amount of dicumyl peroxide (40%) is 4, 5, and 6 parts, respectively, per 100 parts of the gum.
(1 Bayer, O., Angcw. Chcm. 59, 257-72 (1947); 62, 57-66 (1950). (21 Gruber, E. E., Keplinger, 0. C., IND.END.CHEM.51, 151 (1959). (3) Saunders, J. H., Rubber Chem. Technol. 33, 1259 (1960). (4) Urs, S. V., unpublished results. (5) Weisfeld, L. B., et al., J. Polymer Sci. 56, 455-63 (1962). RECEIVED for review May 4, 1962 ACCEPTEDJuly 2, 1962
50 87
Division of Rubber Chemistry, ACS, Boston, Mass., April 1962.
COMPARISON OF ANTIOXIDANT ACTIVITY OF VARIOUS BUTYLATED ARALKYLATED CRESOLS R . B . S P A C H T , C . W . W A D E L I N , W . S. H O L L I N G S H E A D , A N D D . C . W I L L S Research Division, Goodyear Tire and Rubber Co., Akron 16, Ohio
The present study was undertaken to evaluate a series of butylated aralkylated creso!s as antioxidants. An unexpected relationship was found between the 6-tert-butyl-2-aralkyl derivatives of p-cresol. In this series the antioxidant activity increases as one goes from dimethylbenzyl to methylbenzyl to benzyl. This suggests that a phenol can be overhindered for maximum antioxidant activity.
IN
past 15 years much work has been reported on the use of alkylated phenols as nondiscoloring antioxidants (8, 70-72, 74, 7g)' It was apparent that hindered phenols were the best antioxidants and, in general, other things being equal, the more complete the hindrance the better the antioxidant properties. THE
Table 1.
9 0H p H,C-$-()-j-o c
Ha
i;
H
p a C Ha
Ha
S - o i - 0
Ha
MBPC
!$o Hs
H
i; Ha
BPC
c
0
in a natural gum vulcanizate, The five materials studied are listed in Table 1. ~ 1 are 1 cresol derivatives and all contain a 6-tert-butyl group. They differ in the position of the methyl group, and in the position and nature of the aralkyl group, which may be benzyl, methylbenzyl, or dimethylbenzyl. Code letters used throughout the paper are given below each structure .
H.c-E-@o
I
DMBPC
'
In the present work, five phenolic antioxidants were evaluated as stabilizers for various polymers and as antioxidants
H
Ha
C Ha
C
202
Antioxidants Studied
Scope
MBMC
MBQC
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Experimental
With one exception the compounds were synthesized by reacting the cresol being used with benzyl chloride, styrene, or a-methylstyrene according to well known procedures (76). The monoaralkyl cresol was separated by vacuum distillation and then butylated with isobutylene in the presence of HzS04 as a catalyst. The final compounds were separated by vacuum distillation and purified by several recrystallizations from petroleum ether at acetone-dry ice bath temperatures. The effectiveness of these compounds as polymer stabilizers was measured by oxygen absorption. A volumetric oxygen absorption method was used to determine quantitatively the amount of oxygen absorbed (75). I n the case of the stereopolymers the tests were run a t 90' C., and for SBR polymers the tests were run at 80' C. Cements of all polymers containing
