Dynamically partially cured thermoplastic blend of monoolefin copolymer rubber and polyolefin plastic

Abstract

Thermoplastic dynamically partially cured blends of monoolefin copolymer rubber (such as EPM or EPDM rubber) with a polyolefin resin (such as polypropylene, polyethylene) exhibit good physical properties and are reprocessable. The dynamic partial cure of the blend is effected by heating the blend with a curative such as a peroxide while shearing the blend. The blends are elastomeric and are used to make molded or extruded articles which have good physical properties without requiring vulcanization.

Having thus described my invention, what I claim and desire to protect by Letters Patent is: 
     
       1. A method of making a thermoplstic blend comprising mixing monoolefin copolymer rubber, polyolefin plastic and a curative therefor, .[.and.]. masticating and shearing said materials at curing temperature to produce a partial cure insufficient to render the blend unprocessable, .Iadd.and substantially exhausting the action of the curative so that the thus partially cured blend has little or no tendency for further advancement of cure to take place subsequently, .Iaddend.the amount of the said monoolefin copolymer rubber being from 50 to 90 parts by weight and the amount of the said polyolefin plastic being correspondingly from 50 to 10 parts by weight, the said monoolefin copolymer rubber being a copolymer of at least two different monoolefins, one of which is ethylene and the other or others being alpha-monoolefins of the formula CH 2  ═CHR where R is an alkyl radical having 1 to 12 carbon atoms, with or without at least one copolymerizable polyene, the resulting partially cured blend having a gel content of from 60 to 93% by weight measured in cyclohexane at 73° F., the said blend being repeatedly reprocessable, and having the characteristics of a thermoplastic elastomer, whereby shaped articles having good physical properties may be made therefrom without requiring vulcanization. 
     
     
       2. A method as in claim 1 in which the said curative is an organic peroxide. 
     
     
       3. A method as in claim 2 in which the partially cured blend has a percent swell of from 2% to 15% measured in cyclohexane at 73° F. 
     
     
       4. A method as in claim 2 in which the said monoolefin copolymer rubber is ethylene-propylene-non-conjugated diene terpolymer, and the percent swell of the partially cured blend is from 2% to 10%, measured in cyclohexane at 73° F. 
     
     
       5. A method of making a thermoplastic elastomeric blend comprising dynamically working and shearing from 60 to 80 parts by weight of a rubbery copolymer of at least two different alpha-monoolefins, one of which is ethylene, with at least one copolymerizable polyene, in admixture with correspondingly 40 to 20 parts by weight of a polyolefin resin selected from the group consisting of polyethylene and polypropylene, at a temperature of from 160° F. to 550° F. in the presence of a curative for from 3 to 20 minutes to produce a gel content in the blend of from 60 to 93% measured in cyclohexane at 73° F., the partial cure thus imparted to the blend being insufficient to render the blend unprocessable.Iadd., and substantially exhausting the action of the curative so that the thus partially cured blend has little or no tendency for further advancement of cure to take place subsequently.Iaddend.. 
     
     
       6. A method of making a processable, thermoplastic elastomer comprising masticating at a temperature of from 270° to 450° F. for a period of 3-20 minutes a blend of 50 to 90 parts by weight of an ethylene-propylene-nonconjugated diene copolymer rubber with correspondingly from 50 to 10 parts by weight of polyethylene or polypropylene, in the presence of insufficient peroxide curative to cure the blend to the extent that it becomes unprocessable but sufficient to produce a gel content in the thus-processed mixture of from 60 to 93% by weight measured in cyclohexane at 73° F., said gel content being at least 10% greater than the gel content of the mixture similarly processed in the absence of said curative.Iadd., the action of the peroxide being substantially exhausted so that the thus partially cured blend has little or no tendency for further advancement of cure to take place subsequently.Iaddend.. 
     
     
       7. A method as in claim 6 in which, after masticating the said materials to produce the said gel content, there is added to the mixture a free-radical scavenging agent in amount sufficient to inactivate any residual peroxide curative. 
     
     
       8. A method of making a thermoplastic elastomer capable of being shaped into articles displaying low permanent set without curing the articles, comprising semicuring a blend of from 50 to 90 parts by weight of ethylene-propylene-nonconjugated diene terpolymer rubber, and correspondingly from 50 to 10 parts by weight, of polyethylene resin or polypropylene resin, said semicure being effected dynamically by masticating said blend with insufficient curative to cure said blend to the extent that it becomes unprocessable and incapable of forming a coherent sheet on a mill but sufficient to increase the gel content by at least 20% by weight, the final gel content being from 60 to 93% by weight, measured in cyclohexane at 73° F., .Iadd.the semi-curing being continued until the curative is substantially exhausted so that the semicured blend has little or no tendency for further advancement of cure to take place subsequently, .Iaddend.the resulting dynamically semicured blend being a processable material. 
     
     
       9. A method as in claim 8 in which the said curative is an organic peroxide and the said resin is polypropylene. 
     
     
       10. A method as in claim 8 in which the semicure is carried out at a temperature at least as high as the activation temperature of the peroxide curative, for a time equal to at least four half-lives for the curative. 
     
     
       11. A method as in claim 10 in which a free-radical scavenger is added at the conclusion of the semicure. 
     
     
       12. A method of making a .Iadd.shaped .Iaddend.thermoplastic elastomer comprising dynamically mixing and shearing in an internal mixer at a temperature of 270°-450° F. from 50 to 90 parts by weight of ethylene-propylene-dicyclopentadiene terpolymer and correspondingly from 50 to 10 parts by weight of polypropylene resin, in the presence of insufficient peroxide curvative to cure the mixture to the extent that it becomes unprocessable but sufficient to increase the gel content of the mix by at least 20% by weight and to decrease the percent swell of the mix by at least 10%, the final gel content being from 60 to 93% by weight, measured in cyclohexane at 73° F., .Iadd.the curing action of the peroxide being substantially exhausted so that thus partially cured blend has little or no tendency for further advancement of cure to take place subsequently, .Iaddend.the thus semicured mixture being a thermoplastic processable material capable of being shaped into elastomeric articles having low permanent set without vulcanization.Iadd., and thereafter shaping the material into such an article without vulcanization, the resulting thermoplastic article being reprocessable.Iaddend.. 
     
     
       13. A method as in claim 12 in which as a result of the semicure the performance factor of the mixture is at least 20% higher than that of an otherwise similar mixture not semicured, wherein the performance factor, P.F., is given by the expression: ##EQU2##