I
E.
E. GRUBER
and 0. C. KEPLINGER
The General Tire & Rubber Co., Akron, Ohio
Cure of Polyurethane Elastomers with Peroxides Peroxide cures can overcome many of the problems associated with polyurethane rubbers and permit a wider range of formulations
POLYURETHANES
cured with excess isocyanate have several deficiencies, particularly in dynamic properties. In commercial processes, they are often difficult to handle. Peroxides were explored as replacements for the isocyanate usually used to cure these elastomers.
yields a process- and storage-stable compound which shows a simple and sharp dependence of final cure on temperature. Like conventional rubber stocks, this compound can be compounded, stored, and processed without scorching. Similar polyurethane gums compounded with excess isocyanate ( 5 ) can only approach this scorch performance (Figure 2 ) and are definitely inferior to the peroxide stocks in storage stability. Further, peroxide-cured stocks reach maximum physical properties right after removal from the mold.
Millable Polyurethanes
The gum chosen for study was based on the product resulting from approximately equimolar quantities of hydroxyterminated 80 to 20 ethylene propylene adipate polyester and methylene di-pphenylene diisocyanate (MDI). Because of the great increase in chain growth as the equimolar point is achieved ( 3 ) , the reagents must be metered precisely (Figure 1). However, if properly made, the resultant gum has almost unlimited storage life. A gum of this type compounded with a peroxide such as dicumyl peroxide,
Isocyanate Cures
The behavior of a typical cast urethane system ( 7 , 4) exemplifies the postcuring phenomenon required by all isocyanate-cured stocks including millable stocks cured with excess isocyanate (Figure 3). An increase in molecular weight aftef the initial gelation reaction
occurs only if the specimen has been exposed to air. I t is now well established that this is a reaction of the excess isocyanate with moisture as it diffuses into the polyurethane ( 2 ) . Earlier workers postulated the formation of an amine and then a urea Lvith concomitant chain extension from the reaction of isocyanates and moisture. Branches and cross links were thought to result from the ultimate reaction of residual isocyanate at the urea hydrogen. It is surprising. then. to find that during the oven cure the loss in weight of the product (which corresponds to the carbon dioxide evolved in the first step) often exceeded the amount calculated for the conversion to the urea form of all the residual isocyanate known to be present at the time of molding (Table I). There appears to be a shortage of isocyanate groups for cross linking. It is also disturbing to find that even at advanced stages of the reaction, the
p 1.0
-
(I)
INTERNAL N C O
CURE
(Mol)
i
I
I
6o
/
t
(2) EXTERNAL NCO CURE WITH PAPI
I\
lo
t
( POLYMETHYLENE
I
POLYPHENK ISOCYANATE
PEROXIDE CURE
(3)
, t , l , , j , I < , , , l , ,
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IO
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,
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I
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,
20
,
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30
MINUTES MOLE S EXCESS
no
?-Ixo, +
OCN
NCO
-
a
o
Q
c
E
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Figure 1. Influence of isocyanate to hydroxyl ratio on intrinsic viscosity Reagents must b e metered precisely
Figure 2.
Scorch at
250' F.
Stocks compounded with excess isocyanate a r e inferior to peroxide stocks in storage stability Compounds with "internal NCO" mixed from gum polyurethane terminated with isocyanate groups. Compounds with "external NCO" derived from gum terminated b y hydroxyl groups (deficient in isocyanate). Polyisocyanate for curing incorporated during milling step
VOL. 51, NO. 2
0
FEBRUARY 1959
151
soor X
8
9
D1cUMYL
/ //
I
-
200
METHYL CUMYL I-ISOBUTOXY-I-CUMYL PEROXYETHAN T-BUTYL PERBENZOATE
100
T-BUTYL HYDROPEROX1DE
I
I
100
0.1
I N , ~
Figure 3.
,
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I
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1
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I
1
I
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,
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150
Figure 4. Moduli of polyurethane gum stocks cured with peroxides
200
100 HOURS
140
130
CURE 6 0 M I N . AT 'C.
I
GROWTH IN A CLOSED MOLD AT liO°C. I
I20
110
Molecular growth of cast polyurethane
Ketone peroxides and bisazoisobutyronitrile degraded the Recipe: 100 PEPA/0.97 MDI gum; 2 Acrawax-C; 3 peroxide
polymer
Molecular weight increased only after exposure to air
T-BUTYL-
co
v
CUMYL
A .
3 _I
g
IO00
z
* 0 0 m
ob
HYDF(OPEROXI0E
Figure oxides
5.
Moduli of black-loaded polyurethane cured with per-
/
Dicumyl peroxide caused the largest increase in modulus Recipe: 100 PEPA/0.97 MDI; 2 Acrawax-C; 20 black; 3 peroxide
product in time can be dissolved in dimethyl formamide to make viscosity measurements. This, too, indicates either a very low level of cross linking or very easily broken cross-linked structures.
"
Figure 6. Dependence of stress-strain properties on black loading
IO
TENSILE
300 % M O D U L U S
I
I . , , , I ,
,
IO 20 30
20 30 40 5 0 60
MINUTES
CURE
AT
40 50 60
298OF.
Modulus and tensile strength varied directly with black content a t constant peroxide Recipe 100 PEPA/0.97 gum; 2 Acrawax-C; 3 dicumyl peroxide; FEF black shown
Peroxide Curing Agents
Cure of a polyurethane gum stock by diacyl peroxides, peroxyesters, a peroxyacetal, dialkyl peroxides, diaralkyl peroxides, and hydroperoxides was demonstrated (Figure 4). Ketone peroxides and bisazoisobutyronitrile degraded the polymer. Dicumyl peroxide, the most effective curing agent, was followed by tert-butyl cumyl peroxide, cymyl cumyl peroxide, and methyl cumyl peroxide. Similar in curing action was l-isobutoxy1-cumyl peroxyethane. Benzoyl peroxide and lauroyl peroxide were more active at lower temperatures, while tert-butyl perbenzoate showed an intermediate behavior. Di-tert-butyl peroxide was much less active than the cumyl peroxides but similar in its thermal response. Among the hydroperoxides,
152
Table 1.
Cure of Cast Polyurethanes
Comparison of weight losses with potential carbon dioxide evolution shows shortage of isocyanate groups for cross linking
NCO/ Active Isocyanate N-1,5b N-1,5b MDP
Polyester 80/20 PEPAC PEAd 80/20 PEPA"
'
HZ 1.20 1.20 1.12 1.18 1.34
Excess Isocyanate, Equiv./ 100 Grams 0.0235 0.024 0.18
0.027 0.46
Wt. Loss, Gram/100 Grams 0bsd . 60 hr. in air Water Calcd.' cure poach 0.58 0.305 0.32 0.273 0.420 0.312 0.440 0.234 0.308 0.350 0.321 0.460 0.600 0.52 Too soft
H O H -NCO adipate.
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
II I
+ -N-C-N--.
Polyethylene adipate.
Naphthalene 1,5-diisocyanate. Polyethglenepropylene Methylene di-p-phenylene diisocyanate.
e
P O L Y U R E T H A N E ELASTOMER CURE tert-butyl hydroperoxide alone contributed to tensile or modulus enhancement in a polyurethane stock. Cumene hydroperoxide, p-methane hydroperoxide, and tert-butyl isopropylbenzene hydroperoxide were presumed to exert a quasi curing action from reduced extensibility and increased recovery from tensile set by stocks in which they had been incorporated. Methyl ethyl ketone peroxide, dicyclohexanone peroxide, and methyl amyl ketone peroxide degraded the polymer. Similar experiments were conducted with 20 parts of added black (Figure 5). Carbon black increased the modulus and tensile strength of all stocks cured with dialkyl or aralkyl peroxides or tert-butyl hydroperoxide. Dicumyl peroxide again caused the largest increase in modulus. Best of the noncumyl peroxides was 2.2bis(tert-butoxy) butane. Stocks containing diacyl peroxides, ketone peroxides, or bisazoisobutyronitrile degraded in the presence of carbon black and stock mixed with tert-butyl perbenzoate cured poorly. Some degradation was evident in all polyurethane stocks at temperatures above 160' C. Good cures were obtained with dicumyl peroxide in 45 minutes at 154' C. With peroxide constant a t 3 parts per 100 parts of gum, modulus and tensile strength were found to vary in direct proportion to the black content (Figure 6). Tear strength increased but so also did the heat build-up in the Goodrich flexometer test (Figure 7). When carbon black was held constant, modulus and hardness increased in direct proportion to the peroxide content of the stock (Figure 8). Compression set and heat build-up during the Goodrich test were reduced by high loadings of peroxide (Figures 8, 9). In this work, the optimum level of peroxide is defined by a 12% maximum permissible set in the Goodrich flexometer at 100' C. and by a minimum requirement of 50,000 flexes in the DeMattia apparatus at room temperature. For this stock, 1.5 to 1.75 parts of dicumyl peroxide provided optimum cure. I n this range, the stock would have a 300% modulus of from 1800 to 2200 p.s.i., a tensile of about 4500 p s i , elongation of 470%, hardness of 55 (Shore A), rebound of 68'%, and a compression set value below 20%.
Figure 7.
and hysteresis
1
With constant Deroxide, tear strength increased but so did heat build-up
-
cured with diisocyanate, and a millable gum cured by peroxide were prepared from an 80 to 20 poly(ethy1ene propylene) adipate with a molecular weight of 1940 and an acid number below 1 using methylene di p - phenylene diisocyanate (MDI) (Table 11). In spite of equivalent stress-strain perform-
201
10
