RADIATION-CURED CASTABLE ELASTOMERIC POLYURETHANES AND POLYTHIOETHERS M O R T O N J .
P.
A N D
A .
K L O T Z ' ,
K I S P E R S K Y , ELI
R .
1.
T .
E.
P H I L L I P S ,
C A R L E T O N ,
C .
W .
V Y T A U T A S
S A L T O N S T A L L , G R A K A U S K A S ,
M I S H U C K
Aerojet General Corporation, Azusa, Calif. Elastomeric polyurethane castings were prepared by ""Co irradiation of liquid unsaturated polyurethanes. At 7.5 x 10' rads per hour, no comonomer was required. The best elastomers were produced from tolylene diisocyanate, polypropylene glycol, and glyceryl monolinoleate. The tensile strength, the crosslink density, and G( crosslinking) were roughly inversely proportional to the concentration of unsaturated diol. The efficiency of the crosslinking reaction increased as the concentration of the crosslinking agent decreased. G( crosslinking) ranged from 64 to 145, consistent w i t h a vinyl polymerization mechanism of short chain length. The crosslink yield, moles of crosslinks per mole of unsaturated diol, ranged from 0.24 to 1.37; the latter figure suggests that both double bonds of the linoleate chain entered into the reaction. Strong elastomers were prepared by the reaction of long-chain dithiols with long-chain diolefins, using triolefins or long-chain trithiols as crosslinking agents. Free-radical initiators gave erratic behavior, and ultraviolet irradiation could not be used because of opacity, but y-radiation from a "CO source gave excellent results. Swelling ratios, sol fractions, and tensile strengths were measured as a function of radiation dose. The reaction of a diolefin w i t h a dithiol followed second-order kinetics, with an initial G value for the formation of thioether bonds of 4560 per 100 e.v. The G value decreases w i t h increasing extent of reaction. When the reaction is 95% complete, G(thioether bonds) is of the same order of magnitude as -G( C-H), so that decomposition becomes a competing reaction.
THEobjective of this work was to explore the potential advantages of y-radiation for curing, or crosslinking, castable elastomers. Among the expected advantages (over catalyzed thermal curing) were: better control of the curing rate by control of the radiation intensity, since the rate of production of radicals is only slightly dependent on temperature; longer pot life, by elimination of chemical catalysts; reduced shrinkage, by curing a t room temperature or lower; production of new types of elastomers not preparable by other methods-e.g., because soluble catalysts with suitable reactivities are not available, because of side reactions with the catalyst, or because of opacity to ultraviolet light; and improved aging stability, by elimination of catalyst residues which can cause depolymerization. Not all of the above advantages were demonstrated, but two particularly interesting types of elastomers were made-polyurethanes and polythioethers. Polyurethanes
Experimental. In preliminary work, a 1000-curie boCo radiation source with a dose rate of 1.2 x 10' rads per hour was used. I n later work, a 10,000-curie 6"Co source was used, with dose rates (depending on sample position) of 7 to 9 x 10' rads per hour. Dose rates were determined from the rate of oxidation of ferrous sulfate solutions ' Present address, Aerojet-General Corporation, Sacramento, Calif. 96813
(Weiss et al., 1956). Samples were irradiated in 1-inch diameter test tubes, except for mechanical-property specimens, which were irradiated in dumbbell-shaped molds. Preliminary Investigations. Preliminary experiments were conducted to determine, first, the feasibility of crosslinking unsaturated polyurethanes with y-radiation, and second, the types of prepolymers and comonomers required to produce elastomeric products. Briefly, elastomeric castings were prepared by @'Co irradiation of liquid unsaturated polyurethanes, which were prepared from 15 unsaturated diols, two diisocyanates, and two polymeric saturated diols. At 1.2 x 10' rads per hour, vinyl comonomers were required to achieve crosslinking. T o increase the probability of obtaining a crosslinked product with any of the unsaturated diols, three comonomers-methyl acrylate, styrene, and acrylonitrile-were used because they cover a wide range of Alfrey-Price Q values (Alfrey et al., 1952). All of the compositions gave crosslinked polymers, but the most promising elastomer was prepared from tolylene diisocyanate, polypropylene glycol, and glyceryl monolinoleate, with acrylonitrile as the most effective crosslinking agent. All further work was devoted to this system. The Glyceryl Monolinoleate-Tolylene DiisocyanatePolypropylene Glycol System. Subsequent work was done in a 10,000-curie source a t dose rates of 7 to 9 x 10' rads per hour to increase the rate of reaction. The first experiments showed that the prepolymer alone, without VOL. 7 NO. 3 S E P T E M B E R 1 9 6 8
165
added crosslinking comonomer, could be crosslinked by y -radiation. A crosslinked elastomer of good appearance
was obtained after 16 hours of irradiation (about 1.3 x 10' rads). A liquid prepolymer of polypropylene glycol and tolylene diisocyanate alone showed only a slight increase in viscosity after 19 hours of irradiation, indicating that the curing takes place primarily through the unsaturated groups in the prepolymer rather than through crosslinking by hydrogen abstraction from the polypropylene glycol or tolylene diisocyanate moieties. Virtually unlimited pot life was demonstrated with one sample of polymer which remained castable after two weeks a t ambient temperature and then cured normally on irradiation. The advantage of radiation curing for this polymer was proved by the failure of thermal cures of the same catalyzed formulation: samples containing from 0.05 to 0 . 5 5 methyl ethyl ketone peroxide remained liquid after 48 hours a t 43°C. and 24 hours a t 82°C. The relationship between the total radiation dose and the properties of the resulting elastomers was investigated to estimate the minimum dose required for curing. Table I shows the effects of 3, 6, and 24 hours of irradiation in the 10,000-curie source on the hardness and solubility. While insoluble products were obtained in as little as 3 hours (2.8 x 10' rads), the hardness continued to increase throughout the 24-hour period, although the samples remained elastomeric. The mechanical properties of this type of polymer can be varied over a wide range by changing the ratio of saturated to unsaturated diol and/ or changing the NCO to OH equivalents ratio. Tensile strengths a t break ranging from 83 to 103 p.s.i. were typical, with elongations a t break in the range of 90 to 130%. G Values and Crosslink Yields. The crosslink densities of four formulations were determined from stressrelaxation data (Flory, 1953). The results, shown in Table 11, parallel the tensile strengths. The G values for crosslinking, calculated from these crosslink densities, are shown in Table 11. These G values indicate that the efficiency of the crosslinking reaction increases as the concentration of the crosslinking agent, glyceryl monolinoleate, is decreased. This may be because less crosslinker is consumed in intramolecular (ring-forming) reactions as its concentration is decreased, but this is mere conjecture a t present. The only similar studies reported in the literature have been with maleic acid polyesters. Charlesby et al. (1958) calculated G (crosslinking) = 200 from viscosity measurements and concluded that the mechanism of crosslinking resembled a free-radical addition polymerization of short chain length. Intuitively, one would expect such a mechanism to be even more probable in the present case, in view of the known reluctance of maleates to homopolymerize. Table I. Effect of Radiation Dose on Properties of an Unsaturated Polyurethane"
Total Dose, Radiation
Shore A , Hardness
0 2.8 x lob 5.6 x lo6 2.2 x 10' a Formulation, ut. 7 ;: 14.70 dimethylformamide at 90" C.
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Solubility~
7 Soluble 10 Insoluble 13 Insoluble 18 Insoluble GML, 13.20 T D I , 72.10 PPG. ' I n
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Table II. Mechanical Properties, G Values, and Crosslink Yields
Composition, wt. % GML TDI PPG PPG:GML Molar Ratio Tensile Stress at 50% elongation, p.s.i. Crosslink Density" (from stress relaxation data), moles of chains/cc. x 10' Crosslink Density,b moles of crosslinksicc. x 10' Total dose, 100 e.v./cc. x 10" G(cross1inking) = crosslinks/ 100 e.v. Moles of GML/cc. x 10' Crosslink yields, moles of crosslinks per mole of GML
14.47 14.56 70.97 1:l
8.12 12.25 79.63 2:l
5.64 11.35 83.01 3:l
4.32 10.88 84.80 4:l
49.1
41.9
62.1
77.3
1.5
1.3
1.9
2.5
1.00
0.87
1.27
1.67
8.1
8.1
6.9
6.9
74 4.09
64 2.98
111 1.59
145 1.22
0.24
0.29
0.80
1.37
"Calculations based on an assumed density of 1.00 g./cc. for all polymers. 'Number of crosslinks = 213 number of chains, since each crosslink connects 3 chains and each chain is counted twice.
The crosslink yield, defined as the moles of crosslinks per mole of GML, is shown in the last line of Table 11. The crosslink yield increases with increasing crosslinking efficiency, G; moreover, since the yield for Formulation 0 is over one-half, some of the linoleate groups apparently enter into more than one crosslink, probably by utilization of both double bonds. Polythioethers
The addition of a thiol across the double bond of an olefin was first investigated by Posner (1905). Braun and Hahn (1926), upon distilling allyl mercaptan, obtained an undistillable residue, presumably a polythioether. The first utilization of this type of reaction for the preparation of polymers was by Vaughn and Rust (1942) who used H2S as the "dithiol" with diallyl or diallyl ether. Coffman (1944) used organic dithiols and diolefins. The most extensive study of this type of polymerization has been carried out by Marvel and coworkers, the results of which are summarized by Davis and Fettes (1962). All of the earlier work used free-radical initiators, or more successfully, ultraviolet irradiation. I n these laboratories, systems were found which gave attractive elastomers with free-radical initiators, but with a wide variety of initiators the control was poor: either little or no curing occurred, or the reaction was so rapid that the pot life was short. However, pot lives of several days were obtained in the absence of catalysts. Ultraviolet radiation was not useful because many of the systems were opaque. With y-radiation from a buCosource, the curing was rapid and the resulting products were strong and elastomeric. The mechanical properties of the elastomers were readily controlled by the use of trithiols. or triolefins as crosslinking agents. The advantages expected from the production of elastomers through this reaction, in addition to those already cited as general advantages for radiation curing, include: (1) Low shrinkage after gelation, because gelation should occur a t a high extent of reaction. Kinetically, the course of the reaction is similar to that of a conventional poly-
condensation reaction (Davis and Fettes, 1962). (2) Low polymerization exotherms: 14 kcal. per equivalent (Walling and Helmreich, 1959), compared with about 20 kcal. for vinyl polymerization and about 52 kcal. for urethane formation. (3) High efficiency of utilization of radiation (G values), since the polymerization should occur by a chain reaction. (4) Good thermal stability because of the absence of catalysts. ( 5 ) I n contrast t o an ionic polycondensation reaction, an increased pot life may be obtained by the addition of a free-radical inhibitor. Experimental. Irradiation conditions were as described above for polyurethanes. The bis- and tris(mercapt0acetates) were prepared by acid-catalyzed esterification of diols or triols with mercaptoacetic acid. The bis- and tris(ally1 carbonates) were prepared by base-catalyzed reaction of diols or triols with allyl chloroformate. Formulation Studies. The formulations of the most important of the polythioethers studied are given in Table 111, and a glossary of abbreviations is given in Table IV. In the initial experiments, Formulation AS was irradiated a t a dose rate of 9.7 x 10’ rads per hour for periods of 40 minutes, 3 hours, and 22 hours. After 40 minutes, crosslinking had occurred, but the sample did not appear to be fully cured. After 3 hours, the elastomer was firmer and possessed good elongation and tensile strength. No significant difference was observed between the 3-hour and 22-hour samples. Figure 1 shows that the swelling ratio and sol fraction of polythioether Formulation CS decreased with an increasing total dose up to 2 megarads (165 minutes in a 10,000-curie source). N o further changes occurred upon exposure to 8 times this dose (16.4 megarads). The residual sol fraction presumably results from impurities in the monomers, which leads to a low-molecular-weight polymer of insufficient functionality to be incorporated into the gel structure. The tensile properties of Formulation CS, cured for 1.5 hours, are compared in Table V with the same formulation cured for 6.0 hours (both a t a dose rate of 7.4 x 10’ rads per hour). The sample cured for 6.0 hours has a higher modulus and a higher tensile strength than that cured for a shorter period. The over-all shrinkage of Formulation CS was found t o be approximately 1.6‘5, about one half the shrinkage observed in these laboratories for typical elastomeric poly-
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Table IV. Glossary of Abreviations of Monomers
PTME PPG
Polytetramethylene ether diol: R[OICH2CHzCHzCH20),H]2 Polypropylene ether diol: R[Of CH,CH*O);H]2
I
PPG-EO-10
CH3 PPG extended with 10% ethylene oxide: R[Of CHCHrO)
