Polythiol Accelerated Radiation Crosslinking of Olefinically Unsaturated Polymers

Radiation curing of polymers is well known in the art. Though difficulties have been encountered in radiation curing or vulcanizing of polymers, improved results have been obtained by the use of additives. For example, in the case of polyethylene, it has been demonstrated that the addition of unsaturated polyfunctional monomers will produce comparable cross-link densities at reduced dose levels, in contrast to the radiation of ordinary polyethylene.

It is well known that polymers such as polyisobutylene are effectively destroyed when exposed to ionizing radiation. See for example, "Calorimetric Study of Radiation Crosslinking of Polyisobutylene In The Presence of Monomer Additives," Dokl. Akad, Nauk SSSR (Phys. Chem.) 193 (4), 855-857 (1970). The addition of small amounts of polyfunctional monomers such as p-divinylbenzene causes effective crosslinking of polyisobutylene as does the addition of acrylonitrile to polyisobutylene blends. Allyl acrylate has also been found to cause crosslinking of polyisobutylene under ionizing radiation conditions, see for example Polymer Letters, 2, pages 819-821 (1964). Although crosslinking does occur, degradation of the polyisobutylene results even in the presence of the allyl acrylates or allyl methacrylates at higher dose levels, e.g., above about 0.8 megarads.

Butyl rubber being substantially polyisobutylene also, not surprisingly, has been found to degrade in the presence of ionizing radiation.

The use of dithiols in the conventional curing of polymers is well known in the art. For example, the heat curing of a cellulose methacrylate derivative with simple dithiols is disclosed in British patent 588,018. Sulfur vulcanization of styrene butadiene is accelerated by the addition of bis-mercaptophenyl diphenyl oxide (see, for example, U.S. Pat. No. 3,326,822). Chlorinated butyl rubber has been vulcanized with dithiols. See, for example Hodges, Rubber Plastics Weekly 141 pages 666-668 (1964) wherein glycol dimercapto acetate is recommended because of its low odor.

The use of zinc oxide and stearic acid in the thermal vulcanization of polymers, and elastomers in particular, is well established in the prior art. See, for example, "Vulcanization of Elastomers", edited by G. Alliger and J. J. Sjothun, esp. pages 146-150 (1964, Reinhold Publ. Corp.). However, their unique capability for further improving curing using ionizing radiation when in combination with a hydrocarbon polythiol has not been previously described.

Elastomers have been prepared from long-chain dithiols with long-chain diolefins using as the crosslinking agent triolefin or long-chain trithiols; see, for example, Klotz et al., I and EC Product Research and Development 7, pages 165-169 September (1968).

In the present invention, it was surprisingly found that butyl type polymers containing allylic chlorine or bromine are radiation crosslinked rather than degraded in the presence of polythiols. In such cures, polythiols are unexpectedly more effective than dithiols. In contrast to the clean dithiol cure via addition of polydiene elastomers, the curing method of the present allylically halogenated elastomers, in the presence of polythiols, involves hydrogen halide elimination to a major extent.

It has also been surprisingly found that curing by ionizing radiation of olefinically unsaturated polymers is further enhanced by the combined presence of the components described herein; a hydrocarbon polythiol, a bivalent metal oxide and an aliphatic acid.

In a process for the curing of a polymer having a number average of at least 50,000, using ionizing radiation, wherein said polymer is an olefinically unsaturated polymer having at least 15 mole % unsaturation the improvement which comprises incorporating with the polymer, (a) about 0.5 to 6 parts per hundred by weight of polymer of a cure enhancer comprising a hydrocarbon polythiol having a number average molecular weight of about 150 to about 3,000 and having at least three thiol groups, (b) about 1.0 to 10.0 parts per hundred by weight of polymer of a bivalent metal oxide, and (c) about 0.5 to 5 parts per hundred by weight of polymer of an aliphatic acid with about 12 to 18 carbon atoms. The preferred polymers of this invention are elastomeric polymers; suitable are, for example, styrene-butadiene (SBR) and polybutadiene rubber. The preferred polythiol accelerators are hydrocarbon polythiols, particularly aliphatic thioether polythiols, having on the average more than three thiol groups per molecule. The preferred bivalent metal oxide is zinc oxide and the preferred aliphatic acid is stearic acid, however, these latter two materials may be replaced by their reaction product, e.g., zinc stearate, with adjustment of concentration so as to make available the bivalent metal in an amount approximately equivalent to that of the individually added component.

This invention relates to a method of radiation crosslinking of olefinically unsaturated polymers, particularly elastomers such as halogenated butyl rubber, polybutadiene and styrene butadiene rubber (SBR), which comprises incorporating into the elastomer composition polythiol accelerator and/or crosslinking agent, a bivalent metal oxide and an aliphatic acid. In particular, this invention relates to the improvement in radiation vulcanization of unsaturated elastomers by polythiols having an average of at least three thiol groups and normally liquid at room temperature having a molecular weight of at least 150 and preferably less than 3,000, plus a bivalent metal oxide and an aliphatic acid with about 12 to 18 carbon atoms. More preferably the bivalent metal oxide is zinc oxide and the acid is stearic acid.

The improved radiation curing process of this invention is applicable to any olefinically unsaturated polymer which preferably has a molecular weight in excess of 1,500, more preferably in excess of 10,000, and most preferably in excess of 50,000. These polymers contain (1) at least 15 mole % unsaturation, or (2) at least 0.5 mole % halogen in the allylic position, e.g. chlorine, bromine. Elastomers having a number average molecular weight (M.sub.n) of at least 50,000 are preferred. Both types of functionality may be present in the same polymer. For example, halogenated conjugated dienes, polymers and copolymers, e.g. polychloroprene, contain both the requisite levels of unsaturation and the allylic halogen.

As it is generally defined, the allylic halogen in the present elastomers may be bound to a primary, secondary or tertiary allylic carbon, i.e. a carbon next to an olefinically unsaturated carbon: ##STR1##

In most halogen-containing elastomers having olefinic substitution, secondary and tertiary allylic halogen structures are more prevalent than primary structures. This is exemplified by polychloroprene wherein the predominant reactive crosslinking unit is the tertiary allylic chlorine from 1,2 polymerization of chloroprene. ##STR2##

The secondary and tertiary allylic halide groups may occur in the main polymer chain, e.g. in the case of Chlorobutyl most of the chlorine is in unit I and some of it is in unit II. ##STR3##

Surprisingly, when conventional butyl rubber is halogenated to Chlorobutyl or Bromobutyl, it can be readily cured by the method of the present invention.

The term "butyl rubber" as employed in the specification and claims is intended to include copolymers made from the polymerization of reactant mixtures having therein about 70 to about 99.5 percent by weight of an isoolefin which has about 4 to 7 carbon atoms, e.g., isobutylene, and about 0.5 to 30 percent by weight of a conjugated multiolefin having about 4 to about 14 carbon atoms, e.g., isoprene, piperylene, cyclopentadiene, etc. The resulting copolymer contains 85 to 99.8 percent by weight of combined isoolefin, and about 0.5 to about 15 percent of combined multiolefin. Butyl rubber generally has a Staudinger molecular weight of about 20,000 to about 500,000, preferably about 25,000 to about 400,000, especially about 100,000 to about 400,000, Wijs Iodine Number of about 0.5 to about 50, preferably about 1 to about 15.

The preparation of butyl rubber is described in U.S. Pat. No. 2,356,128 which is incorporated herein by reference.

For the purposes of this invention, the butyl rubber may have incorporated therein about 0.2 to about 10 mole percent of combined multiolefin, preferably about 0.5 to about 6 percent, more preferably about 1 to about 4 percent, e.g., 2 percent. Illustrative of such a butyl rubber is Exxon Butyl 268 (Exxon Chemical Company) having viscosity average molecular weight of about 450,000, a mole percent unsaturation of about 1.5% and a Mooney viscosity of about 55.degree. at 260.degree. F.

Halogenated butyl rubber is commercially available and may be prepared by halogenating butyl rubber in a solution containing about 1 to about 60 percent by weight of butyl rubber in a substantially inert C.sub.5 -C.sub.8 hydrocarbon solvent, such as pentane, hexane, heptane, etc., and contacting this butyl rubber cement with a halogen gas for a period of about 0.5 to about 25 minutes, whereby halogenated butyl rubber and a hydrogen halide are formed; the copolymer containing up to 1 halogen atom per double bond in the copolymer.

The preparation of halogenated butyl rubber is old in the art, see for example U.S. Pat. No. 3,099,644 which is incorporated herein by reference. This invention is not intended to be limited in any way by the manner in which butyl rubber is halogenated, and both chlorinated and brominated butyl rubber are suitable for use in this invention.

Illustrative of halogenated butyl rubbers is Exxon Butyl HT-10-66, a chlorinated butyl rubber containing about 1.3 weight percent chlorine having about 1.7 mole percent unsaturation and a viscosity average molecular weight of about 357,000.

Styrene butadiene rubber (SBR) is well known to the art. Typically these polymers contain about 5 to about 40 weight percent of styrene and have a Wijs Iodine Number of about 300 to about 5,000 and a number average molecular weight of about 30,000 to about 600,000, preferably about 100,000 to about 150,000. The polymer may be prepared either by the bulk polymerization of styrene and butadiene or polymerization of the rubber in a latex form by emulsion polymerization, a suspension polymerization. This invention is in no way intended to be limited by the method of preparation of the styrene-butadiene rubber.

Polybutadiene is well known in the art and may be prepared in various ways including butyl lithium catalysis of butadiene. Illustrative examples of polybutadiene are: Firestone Diene 55 having 45% 1,4 cis enchainment, 45% 1,4 trans enchainment and 10% 1,2 vinyl pendant groups; Phillips' Cis 4 having 92% 1,4 cis, 2% 1,2 adduct of butadiene and some 1,4 trans enchainment.

Other elastomers suitable for use in the practice of this invention include polychloroprene and acrylonitrile-butadiene copolymers.

The elastomers suitable for use in the practice of this invention preferably have a M.sub.n of at least 50,000, more preferably at least 100,000, most preferably at least 120,000.

Any polythiol having at least two preferably three thiol groups which has a molecular weight of at least 150, may be used in the practice of this invention, assuming that it has no groups other than SH which are sensitive to radiation. Although the polythiol structure may include other atoms besides carbon, hydrogen, and sulfur; e.g., nitrogen, or oxygen in the form of amides, ethers, esters, etc., the preferred polythiols are hydrocarbon polythiol oxy and thioethers. The preferred polythiols have a molecular weight of about 150 to about 3,000 and have at least 2, preferably about 3 to about 5 thiol groups per molecule; more preferably, about 200 to about 1,000: most preferably about 250 to about 400. Hydrocarbon thioether polythiols which are most preferred in general contain one, two, three or more thioether groups. The number of thioether groups is about 1 to 100, preferably 1 to 25, more preferably 1 to 100, most preferably 1 to 5. The number of thioether groups is limited by economic considerations since these polythiols are used on an equivalent thiol basis. These cure enhancers are incorporated in the polymer at about 0.5 to about 6 parts per hundred (phr) by weight of the rubber, more preferably about 1 to about 3 phr.

Useful dithiols are hydrocarbon dithiols such as dodecane dithiol, dipentene dimercaptan, vinyl cyclohexene dimercaptan. Such dithiols are generally prepared by reacting a large excess of hydrogen sulfide with a diolefin or acetylene.

More preferred dithiols are thioether dithiols such as those derived by the reaction of excess hydrogen sulfide or dithiols with diunsaturates, such as ethylene diacrylate and methylacetylene.

Even more preferred are trithiols such as hydrocarbon trithiols exemplified by triallyl cyanurate trimercaptan, triacryloyl triazine trimercaptan, cyclodecatriene trimercaptan, trivinyl cyclohexane trimercaptan, vinyl butyl acetylene trimercaptan. Such trithiols are prepared by reacting a very large excess of hydrogen sulfide with a triunsaturated compound.

Also highly preferred are polythiols containing more than three thiol groups such as a low molecular weight polybutadiene polymercaptan.

Most preferred are thioether trithiols, tetrathiols and pentathiols. Such compounds are prepared, by the addition (preferably in an anti-Markovnikov manner) of excess hydrogen sulfide and excess dithiols to trienes and polyenes. Especially preferred species are derived by the addition of H.sub.2 S and C.sub.2 to C.sub.14 alkylene dithiols to C.sub.9 to C.sub.20 trienes, especially trivinyl compounds, for example, triterminal trienes such as trivinyl cyclohexane. Such adducts have 3 to 5 thiol groups on the average. The preparation of such polythiols and polythioether polythiols is described in U.S. Pat. No. 3,625,925 by Oswald et al.

Illustrative examples of the polythioether polythiols are the reaction product of trivinylcyclohexane and 1,3-propanedithiol, which has the general formula ##STR4## trimethylol propane tris-thiolglycoate, pentaerythritol tetra-3-mercaptopropionate, pentaerythritol tris-3-mercaptopropionate, the reaction product of cyclodedecatriene and 1,3-propanedithiol, which is a mixture of thioether thiols having the general formula ##STR5## such as trimethylol propane tris-mercaptoacetate, the reaction product of H.sub.2 S and/or dithiols with polyepoxides. See U.S. Pat. No. 3,625,925 incorporated herein by reference for other suitable polythiols and their method of preparation.

Bivalent metal oxides suitable for use in the present invention include zinc oxide, cadmium oxide, litharge (PbO) and red lead (Pb.sub.3 O.sub.4). Of the materials available commercially, zinc oxide is the most preferred for reasons of economics and effectiveness. These materials are used effectively at a level of about 1.0 to 10.0 parts per hundred by weight of polymer, preferably about 2.0 to 8.0 parts, for example 5.0 parts.

Aliphatic acids with 12 to 18 carbon atoms are suitable for use in this invention. Such acids include stearic, oleic, lauric, palmitic and linoleic. To the extent that they are also comprised of acids with 12 to 18 carbon atoms, the vegetable derived fatty acids can also be used. e.g., soybean, cotton seed, linseed, coconut and corn; stearic acid is preferred, especially in combination with zinc oxide. When used, the aliphatic acid is effective at a level of about 0.5 to 5 parts per hundred by weight of polymer, preferably about 0.5 to 3.0 parts, e.g., 1 part.

In contrast to adding the bivalent metal oxide and aliphatic acid independently to the composition, an acceptable alternative is to add the reaction product of these two materials, the metal salt of the acid. Under such cirumstances the metal would be selected from the group consisting of zinc, lead and cadmium, e.g., zinc stearate, zinc laurate, lead oleate, and lead stearate. In order that an effective amount of the metal in the metal salt of the fatty acid be present in the composition, the level of said metal salt is typically in the range of about 4.0 to 40.0 parts per hundred by weight of polymer, preferably about 8.0 to 24.0 parts, e.g., 16.0 parts. Adjustments to the desired level are readily made by factoring in the molecular weight of metal and fatty acid residue in the particular salt of interest.

The term "radiation" as used in the specification and claims includes light of different wavelengths, .alpha.- and .beta.-irradiation and heat irradiation.

The preferred "ionizing radiation" as used in the specification and claims is intended to include gamma radiation and electron beam (.beta.-ray) irradiation.

The Co.sup.60 gamma radiation source referred to in the Examples of this specification is a 2,900 Curie source (1.2 Mev). Electron beam irradiation was produced from a Dynamitron. For a description of this equipment, see Harmer and Ballentine, "Radiation Processing", Chemical Engineering, pages 98-116 (Apr. 19, 1971). Although gamma radiation has greater penetrating power than electron beams, this characteristics was not a factor in these experiments since the test samples were 100 mil sheets or less. Where significant differences occur between the results from Co.sup.60 and electron beam irradiation, it is attributable to differences in dose rate, the former at 0.01 megarad/minute and the latter at 30 megarad/minute.

The olefinically unsaturated polymers of this invention may be sulfur vulcanized subsequent to radiation curing. The radiation precure has process advantages where handling of shapes on sheets is necessary prior to vulcanization. Sulfur vulcanization may be carried out using free sulfur and accelerators or in the absence of free sulfur using sulfur donors. The term "sulfur vulcanized" means vulcanization using either free sulfur or sulfur donors. Both methods of sulfur vulcanization are well known to the art and are discussed in detail in Chapters 2 and 3 of Vulcanization and Vulcanizing Agents, W. Hofmann, Palmerton Publishing Col, New York (1967) incorporated herein by reference.

Illustrative examples of accelerators which may be used in the sulfur cure of polymers of this invention are dithiocarbamates, xanthates, thiurams, thiazoles, aldehyde-amine accelerators and basic accelerators such as guanidine accelerators. Specific examples of these accelerators are zinc dimethyl dithiocarbamate, zinc diethyl dithiocarbamate, ammonium-N'-pentamethylene dithiocarbamate, tellurium diethyl dithiocarbamate, sodium isopropyl xanthate, zinc butyl xanthate, tetramethyl thiuram monosulfide, tetraethyl thiuram disulfide, dimethyl diphenyl thiuram disulfide, 2-mercapto-benzothiazole, etc.

The advantages of the instant invention may be more readily appreciated by reference to the following examples.

A polythioether polythiol, formed by the reaction product of trivinylcyclohexane and 1,3-propane dithiol was used as the vulcanizing crosslinking agent. This compound has the general formula ##STR6## The calculated molecular weight of the polythiol is 912, but the actual molecular weight determined by vapor pressure osmometry was 818. This product was designated E2323 III. It contains 0.0049 mole equivalents of SH per gram.

The polythiol was compounded with SBR and natural rubber (NR) according to the formulations shown in Table I.

It was observed in other radiation curing experiments that the presence of zinc oxide provided an added accelerating effect to that of the polythiol. Thus, the control compounds in both elastomer blends contain this ingredient. Sample compounds were molded at 175.degree. F. for five minutes to form a 0.025 inch film protected with Mylar; prior to radiation all samples were soluble in cyclohexane. Samples were then exposed to electron beam radiation at 30 megarads per minute for 2.5, 5, 10, and 15 MR. Network properties were determined by volume swelling in cyclohexane and converted to the parameters of Mc and XL/cc, wherein Mc is Molecular weight between crosslinks and XL/cc is crosslinks per cc. The results are shown in Table II.

The effect of the polythiol on the radiation induced crosslinking of SBR is extremely great at the levels used. This can be noted especially at the lower levels of radiation dose wherein % volume increase, % soluble and Mc are all markedly lower, and crosslinked densities are manyfold greater. This is evident also at higher dose levels. The emulsion polymerization of butadiene would be random in nature with considerable 1,2 addition resulting in very reactive pendant vinyl groups. The effect with natural rubber is much less pronounced but still evident on the basis of swelling derived network parameters. Generated crosslink densities are increased 2 to 3 fold at the higher dose levels of 10 to 15 MR while displaying evidences of network formation at the 5 MR level. The control composition 4 has not formed a desirable network at this point. This lower reactivity or response to the polythiol is a reflection of the substituted internally unsaturated isoprenoid structure of cis-polyisoprene.

Chlorinated butyl rubber under radiation tends to quickly reach a crosslinking limit where further radiation begins to promote chain scission. It is also dose rate sensitive so that as the rate of energy absorption is increased, reaction selectivity is shifted from crosslinking to chain scission. In contrast to more highly unsaturated general purpose elastomers, a mild radiation precure of a compounded formulation detracts from a subsequent thermal curing process. The radiationinduced crosslinking of chlorinated butyl rubber can be greatly enhanced by small amounts of polythioether polythiols as illustrated by the following experiments.

The polythiol used is the reaction product of cyclododecatriene and 1,3 propane dithiol, designated E1255 VII. The polythiol is a mixture of the following compounds: ##STR7## The molecular weight of the mixture by Vapor Pressure Osmometry was found to be 578. For purposes of compounding, the functionality per unit weight was based on VPO molecular weight of 578 or approximately 0.005 SH/gm. See U.S. Pat. No. 3,625,925 incorporated herein by reference for methods of preparing this polythiol. The glycol dimercapto acetate has a functionality of 0.0095 SH/gm. Table III shows blends prepared for a comparison of E1255 VII and the glycol dimercapto acetate in the radiation crosslinking of chlorinated butyl rubber.

The chlorinated butyl rubber used has approximately 0.032 equivalent Cl functionality per 100 grams. Two grams of E1255 VII polythiol is the equivalent to the addition of 0.01 SH groups per 100 grams of chlorinated butyl rubber. One hundred grams of chlorinated butyl rubber contain 0.032 allylic chlorine groups.

The compounds of Table III were molded as 0.025 inch at 200.degree. F. for 5 minutes between mylar film and exposed to Cobalt 60 radiation for various time periods. After exposure, the film was subjected to solvent swelling analysis in a cyclohexane to determine the extent of crosslinking. The results are tabulated in Table IV.

A comparison of columns 1 and 2 shows that zinc oxide has a very mild activating effect upon the radiationinduced crosslinking of chlorinated butyl rubber. The presence of glycol dimercapto acetate provides little or no additional benefit (column 3). On the other hand, the addition of E1255 VII polythiol (column 4) provides a marked enhancement or radiation-induced crosslinking. By any criterion: lower volume swelling, soluble polymer, molecular weight Mc or crosslinked density (XL/cc), the polythioether polythiol has a marked accelerating effect. On the basis of crosslinks formed, it provides about a threefold increase over the control of column 2 and an increase of about 21/2 times over the dimercaptan of column 3.

In order to study the concentration effect of E1255 VII polythiol of Example 2 on radiation curing, samples were prepared in the manner of Example 2 at various polythiol concentrations. The samples were molded at 175.degree. F. for 5 minutes. The formulations are shown in Table V.

The results of Cobalt 60 exposure are tabulated in Table VI.

There is a progressive crosslinking response to the chlorinated butyl rubber system toward radiation as the amount of polythiol is increased. Volume swelling, soluble polymer and Mc between crosslinks all decrease at any given megarad dose level. The generated crosslinked density is increased about 3 to 4 times over the control column 1 at the highest concentration of polythiol.

The effects of polythioether polythiol E1255 VII on the electron beam irradiation response of chlorinated butyl rubber was evaluated by repeating the experiments of Example 3, substituting electron beam irradiation for Cobalt 60 radiation. The results are shown in Table VII.

It is evident that the presence of the polythioether polythiol promotes gelation and retards chain scission.

A comparison of various polythiols was made using the polythiols shown in Table VIII.

In addition to the polythiols of Table VIII, the Thiokol liquid polysulfide dithiol rubber LP3 was also evaluated. This rubber had a specification molecular weight of 1,000 and a wt. % mercaptan range of 5.9 to about 7.7 (average 6.8). This is equivalent to 0.002 SH groups per gram compared to .005 SH groups per gram of E1255 VII. Samples were prepared in the manner of Exaple 4, the polythiols being incorporated in the compositions at equivalent SH functionality. The compositions and the effect of Cobalt 60 radiation on these compositions is shown in Table IX.

it is evident that the presence of polythioether polythiols increase the crosslinking response of chlorinated butyl rubber three- to fourfold. The tri- and tetrathiols are superior to the dithiol represented by E1263 VIII. The tetra functional E1221 VIII as shown in columns 4 and 5 is especially noteworthy because of its ability to promote the crosslinking response at quite low concentration levels. The commercial Thiokol rubber has only a mildly activating effect under Cobalt 60 radiation which appears to be further reduced under the higher dose rates of the electron beam.

The polythioether polythiols of this invention will promote the radiation crosslinking of chlorinated butyl rubber in the absence of zinc oxide and stearic acid as demonstrated by the results in Table X.

Although zinc oxide and stearic acid are not essential for radiation cure, they enhance the effect of the polythiols on radiation cure.

A series of experiments were conducted to determine whether other smaller molecular structures containing active hydrogen species would enhance the radiation crosslinking response of chlorinated butyl rubber. Included in these studies were mono-, di- and trithiols as shown in Table XI.

Functionalities per gram are shown adjacent to each additive and the amounts added would be comparable to the functionality of 2 parts of E1255 VII added to 100 parts of chlorinated butyl rubber. These obnoxious and sometimes toxic chemicals were added to chlorinated butyl rubber, zinc oxide, stearic acid masterbatch by solution mixing. Solutions were made in n-pentane and solid films recovered by room temperature evaporation in a hood followed by vacuum drying at room temperature. All films prior to irradiation with CO.sup.60 source were soluble in cyclohexane. Crosslinking densities of the networks were obtained as a function of dose levels and are shown below in Table XII.

A comparison of the data in Table XII shows that the monothiol of column 2 has a retarding effect upon radiation-induced crosslinking over the control column 1. The dithiol has a very modest activating effect, as do the phosphine and to a lesser extent, the silane as shown in columns 3, 5 and 6. The trithiol reflecting some of the activity of the polythioether polythiols has a more pronounced activating effect, although it does not appear to progress as radiant energy absorbed is increased. On a functionality basis, it can be considered inferior to the several types of polythioether polythiols evaluated.

Prior experiences have shown that radiation precure of chlorinated butyl rubber subsequently compounded with ingredients that promote chemical thermal curing, generally detracted from the subsequent thermal cure.

Table XIV shows the effect of polythiol E1255 VII on thermal cure after a precure by electron beam exposure.

These results show that at 1 pt. per 100 of polythiol there is a subtractive effect on the thermal cure; however, at higher levels the effect is additive.

Various polythiols were compared for their effectiveness with E1221 VIII polythiol. The three thiols tested in these experiments are as follows:

These two thiol containing ester structures were compared with a polythioether polythiol of similar molecular weight and functionality per gram. 98-3 E1221 III from trivinylcyclohexane and H.sub.2 S. The structure based upon an experimental molecular weight of 500 indicates a single degree of coupling to provide on an average of monothioether polythiol. ##STR13## Formulations were prepared and exposed to electron beam irradiation. The results are shown in Table XV.

With column 1 as the control, the greater effectiveness of polythiol 98-3 can be observed in columns 6 and 7, as compared to 2 through 5. At the 5 MR exposure level, the ester structures (98-1; 98-2) on the basis of generated crosslinks (XL/cc) provide a response only marginally better than the control, while 98-3 provides an increase in crosslinking response of 5 to 6 times. This advantage is maintained at the higher dose levels.

Under Co.sup.60 gamma radiation, at 0.01 MR/minute, the ester structures 98-1, 98-2 displayed a three-fold advantage in crosslinking response when compared to the control with 98-3 providing a further advantage.

Conventional butyl rubber will degrade under gamma ray irradiation through main chain scission while in contrast a chlorinated butyl rubber possesses considerable crosslinking potential. The addition of the polythiols of this invention do not change the response of unhalogenated butyl rubber to radiation exposure. The samples irradiated are described in Table XVI.

After irradiation, the polymers were still completely soluble in cyclohexane whereas the halogenated butyl rubbers cured under similar circumstances showed improved crosslink density.

Thiol structures that are not polythioether polythiols also have utility as coagents capable of accelerating the radiation-induced crosslinking of chlorinated butyl rubber as was demonstrated in Example 9. The experiments of Example 9 were repeated using Cobalt 60 irradiation instead of electron beam irradiation. These results are shown in Table XVII.

Although the thioether polythiol provides the greatest degree of radiation cure enhancement (based upon generated crosslinks per cc) on an equivalent functionality basis, the polythiols with ester structures were also effective as accelerating coagents. At the lowest megarad dose of 5.1, the E1221 additive enhanced generated crosslinks up to sixfold while the other additives registered a threefold improvement. The difference may be associated with coagent solubility in the hydrocarbon polymer medium. These data also illustrate that the sample E1221 additive would be one of the preferred structures from the standpoint effectiveness per unit weight.

The radiation cure of polybutadiene is well known in the art. Two typical polybutadienes were radiation cured using the method of this invention by preparing compositions which were molded into test samples of 0.025 thick films by press molding at 175.degree. F. for five minutes between Mylar film. All samples were hydrocarbon-soluble prior to radiation treatment. Exposures were made under a 30 MR/min. electron beam accelerator for various dose levels. The results are shown in Table XVIII.

Columns 1 and 4 are the respective polymer controls with no additives, columns 2 and 5 have just the 3% of added E1262 VII and columns 3 and 6 are further supported with zinc oxide and stearic acid. The accelerating effect for either elastomer by the PTEPT is marked with further enhancement by zinc oxide, stearic acid on the basis of any network parameter. This is shown by a reduction in volume swelling and soluble polymer as well as by a smaller molecular weight between effective crosslinks. Crosslink density is increased many fold, as much as nine times for the Diene 55 at 5 MR exposure. The Diene 55 is more responsive presumably due to its larger amount of vinyl pendant groups. Data of this nature reveal the tremendous potential for the precuring of polydiene elastomers at markedly reduced radiation dose levels.

The chemistry of thioetherpolythiol acceleration of Chlorobutyl radiation curing was followed using a pilot plant prepared sample of the trivinylcyclohexane -- H.sub.2 S reaction product. This material, designated 5188, possessed the following analytical specifications.

Thiol sulfur -- 21.0% by wt.

Total sulfur -- 29.3% by wt.

Molecular Wt. -- 413 by vapor pressure osmometry Thiol sulfur is thus 71.7% of the total sulfur, and the difference between these sulfur analyses attests to the thioether structure of the polythiol.

The polythiol accelerator, 5188, was mixed with Chlorobutyl, zinc oxide and stearic acid at the two concentration levels shown in Table XIX. The acceleration of radiation cure, exposed to a 1.4 Mev electron beam, was compared to an elastomer system with no polythiol. The course and chemistry of the crosslinking reaction was followed by four parameters.

1. Reduction of soluble polymer.

2. Increase in crosslink concentration.

3. Reduction of reactive elastomer chlorine.

4. Increase in the combined thiol sulfur in the crosslinked network.

In order to obtain crosslinked samples free of inorganic residues (ZnO and ZnCl.sub.2) as well as unreacted polythiol, an elaborate extraction process was performed to obtain transparent crosslinked films.

(a) Crosslinked films were digested in a solution of 1/3 naphthenic acid and 2/3 normal hexane at a refluxing temperature of 74.degree. C. for 10 hours to remove zinc oxide as soluble zinc naphthenate.

(b) Extracted samples with chloroform to remove ZnCl.sub.2 and uncombined polythiol.

(c) Final extraction with fresh hexane to displace CHCl.sub.3.

(d) Dry to constant weight under high vacuum at 60.degree.-70.degree. C.

The levels of crosslinking desired can be varied by changes in the polythiol concentration. If a radiation process was designed to give only an initial precure to a rubber composition (for example to reduce viscous flow) lower levels of polythiol and reduced radiation exposures (down to 1-2 megarads) could be employed. To obtain highest levels of crosslinking, comparable to the best thermal cure formulations, the molar thiol content of the polythiol should equal the halogen functionality of the Chlorobutyl and up to 15 megarads of radiation energy could be used.

Examination of the data show that the presence of the polythiol markedly enhances the state of radiation crosslinking over the control. These increases in crosslink density and reductions in soluble polymer are accompanied by greater amounts of chlorine removal (for a given radiation dose level) and the presence of increasing amounts of combined thiol sulfur in the network. The polythiol is thus contributing to the network formation, probably via addition to double bonds as well as accelerating hydrogen chloride removal as chloride salts during the process.