Thermoplastic elastomer and process for preparation

Abstract: Thermoplastic elastomers in which low molecular weight ethylene/propylene/nonconjugated diene rubber ("liquid" EPDM rubber) replaces part of the high molecular weight EPDM rubber used in conventional blends exhibit better tension set properties than conventional blends in which liquid EPDM rubber is not used. A masterbatch containing liquid EPDM rubber and phenolic curing resin which is added to a mixture of polyolefin, high molecular weight EPDM rubber, and other ingredients during mixing results in a thermoplastic elastomer having better mechanical properties as well as better tension set properties than blends of conventional materials.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention. The present invention relates to elastomers generally and, more particularly, but not by way of limitation, to a novel process for producing a thermoplastic elastomer having improved physical properties. 2. Background Art. Previously known processes to produce thermoplastic elastomers have involved dynamically vulcanizing a mixture of polyolefin and high molecular weight EPDM rubber. Such processes are described, for example, in US Patent No. 4,130,535, issued December 19, 1978, to Coran et al, and US Patent No. 4,311,628, issued January 19, 1982, to Abdou-Sabet et al. The general process is to mix the polyolefin and EPDM rubber at temperatures sufficient to soften or melt the polyolefin and then to add a curative which is preferably a phenolic curing resin. A cure accelerator is added and mixing is continued until curing is complete. Mixing equipment employed may be any conventional masticating equipment, such as Banbury mixers, Brabender mixers, or certain mixing extruders. The thermoplastic elastomers thus produced exhibit relatively good oil resistance and physical properties compared to partially vulcanized and sulfur or peroxide cured compositions. Although the above processes produce improved thermoplastic elastomers, the physical properties thereof are still somewhat deficient, due, primarily to the poor distribution of phenolic curing resin during the mixing/curing step. Distribution of phenolic curing resin in the blends of EPDM rubbers and polyolefin resin during dynamic vulcanization affects the final properties. Therefore, thorough dispersal of the curing resin before the cure accelerator is added is necessary to have highly desirable characteristics. However, since the phenolic resins have low melting temperatures and very low viscosities after melting, they are difficult to distribute into the highly viscous rubber matrix, in the short time available, to prevent any unwanted reaction such as self-polymerization. Accordingly, an improved method of distributing phenolic curing resin in the blends is needed to improve elastic properties as well as improved mechanical properties. EP-A-246745 describes a curable composition comprising a blend of (A) an elastomer having a molecular weight of at least 40,000; and (B) an ethylene/alpha-olefin/non-conjugated polyene terpolymer having a molecular weight of from 1000 to 15,000 such that the viscosity of the blend of components (A) and (B) is at least about 5% less than the viscosity of component (A) alone; and (C) a curative. EP-A-0256724 describes a thermoplastic elastomer composition prepared by partially crosslinking a composition of an extremely low density ethylene copolymer prepared by copolymerizing ethylene and an alpha-olefin in the presence of a specific catalyst, a propylene polymer and an ethylene/alpha-olefin copolymer rubber. Accordingly, it is a principal object of the present invention to provide a method for producing thermoplastic elastomers which provides for improved distribution of phenolic curing resin in mixtures of polyolefin and high molecular weight EPDM rubber. Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description. SUMMARY OF THE INVENTION It has been discovered that thermoplastic elastomer compositions in which low molecular weight ethylene/propylene/nonconjugated diene rubber ("liquid" EPDM rubber) replaces part of the high molecular weight EPDM rubber used in otherwise conventional blends exhibit better tension set properties than conventional blends in which liquid EPDM rubber is not used. Furthermore, it has been discovered also that a masterbatch containing liquid EPDM rubber and phenolic curing resin which is added to a mixture of polyolefin, high molecular weight EPDM rubber, and other ingredients during mixing results in a thermoplastic elastomer exhibiting better mechanical properties as well as better tension set properties than blends of conventional materials. Tension set is an important measure of elasticity. Because of its liquid state even at room temperature, due to the low molecular weight of liquid EPDM rubber, the phenolic curing resin can be readily mixed with liquid EPDM rubber at a temperature above the melting point of the curing resin but lower than the activation temperature of the resin. The resulting "masterbatch" once made has certain advantages, such as easy handling and also precise feeding of phenolic curing resin in the compositions which results in consistent product quality. DESCRIPTION OF THE PREFERRED EMBODIMENTS Generally, the final compositions of the thermoplastic elastomer produced by the present invention in terms of total EPDM rubber are conventional and may comprise a blend of from 14 to 80 parts by weight of high molecular weight EPDM rubber per 100 parts by weight of polyolefin resin and total final rubber content and from 1 to 30 parts by weight of liquid EPDM rubber per 100 parts by weight of polyolefin resin and total final rubber content and from 85 to 15 parts by weight of thermoplastic crystalline polyolefin resin per 100 parts by weight of polyolefin resin and total final rubber content. The invention is especially advantageous for preparing compositions comprising from 23 to 70 parts by weight of high molecular weight EPDM rubber per 100 parts by weight of polypropylene resin and total final rubber content, from 2 to 15 parts by weight of liquid EPDM rubber per 100 parts by weight of polypropylene resin and total final rubber content, and from 75 to 20 parts by weight of polypropylene resin per 100 parts by weight of polypropylene resin and total final rubber content. The thermoplastic elastomer compositions of the invention may be prepared by blending a mixture of high molecular weight EPDM rubber, liquid EPDM rubber, softened or molten polyolefin resin, and a masterbatch containing phenolic curing resin and liquid EPDM rubber at a temperature which is high enough to maintain the molten mixture and promote curing by dynamic vulcanization using conventional mixing equipment such as Banbury mixer, Brabender mixer, or various kinds of extruders. The mixing is continued until the curing is completed. Table I below is presented to illustrate comparative physical properties and is not intended to be a limitation on the full range of compositions of EPDM/polyolefin thermoplastic elastomers, the invention being applicable to the full range of known compositions set forth above. The thermoplastic elastomer compositions comprising high molecular weight EPDM rubber, polypropylene, and liquid EPDM rubber were prepared by dynamic vulcanization. The compositions of the elastomers and measured properties thereof are set forth in Table 1. A Brabender mixer was used for mixing the components at 80 rpm rotor speed with a set temperature of 180 degrees Centigrade. To prepare the compositions (except Stocks 3, 6 and 9), high molecular weight EPDM rubber and liquid EPDM rubber (if used) were first mixed for two minutes, then polypropylene was added and mixing was continued for four additional minutes. Phenolic curing resin was then added and mixing continued for an additional one minute. Then, zinc oxide as a cure accelerator was added and mixing was continued for five additional minutes, at the end of which time curing was completed. To prepare Stocks 3, 6 and 9, which used a masterbatch containing liquid EPDM and phenolic curing resin, high molecular weight EPDM rubber and liquid EPDM rubber were first mixed for two minutes, then polypropylene was added and mixing continued for an additional four minutes. The masterbatch containing liquid EPDM rubber and phenolic curing resin was then added and mixing continued for an additional one minute. Then, zinc oxide was added and mixing was continued for five additional minutes, at the end of which time curing was completed. Samples of the cured stocks were compression molded at 210 degrees Centigrade and then cooled to room temperature under pressure before removal. Stocks 1, 4 and 7 are controls containing no liquid EPDM rubber. Stocks 2, 5 and 8 are controls containing liquid EPDM rubber, but do not use masterbatch. It is clear that the use of liquid EPDM rubber improves tension set despite the slight decrease of other physical properties. Liquid EPDM rubber improved tension set regardless of the ratio of PP/EPDM. Stocks 3, 6 and 9, which used masterbatch containing liquid EPDM rubber and phenolic curing resin, also clearly show that not only tension set but also other physical properties are improved. The masterbatch used comprised 40 weight percent liquid EPDM rubber and 60 weight percent phenolic curing resin and was prepared by mixing with low heat. Sufficient liquid EPDM rubber must be added to provide a fairly viscous mixture, while still maintaining a relatively high concentration of phenolic curing resin. The preferred range of liquid EPDM rubber concentrations is from 20 to 60 weight percent. While liquid EPDM rubber having a molecular weight of 8,000 was used in the examples, a satisfactory range of molecular weights for the liquid EPDM rubber is from 500 to 20,000, and preferably from 2,500 to 10,000. Mixing torque before completion of mixing was also measured. The final torque data indicate that addition of liquid EPDM rubber reduces shear viscosity as well as mixing energy. All physical properties were measured according to standard ASTM procedures. The properties of the thermoplastic elastomers produced by the present invention may be modified by adding other ingredients which are conventional in the compounding of EPDM rubber. Such ingredients include particulate fillers, clays, colored pigment, zinc oxide, stearic acid, reinforcing agent, stabilizers, antioxidants, flame retardants, processing aids, tackifiers, plasticizers, wax, and extender oils. The liquid EPDM used was Trilen® furnished by Uniroyal Chemical Company, Naugatuck, Connecticut. The polyolefin used was polypropylene furnished by Himont of Italy under the name Profax®. The phenolic curing resin used was SP-1055® furnished by Schenectady Chemical Company, Schenectady, New York. It will thus be seen that the objects set forth above, among those elucidated in, or made apparent from, the preceding description, are efficiently attained. It will be understood that the above specific examples are intended to be illustrative only and that the invention is not limited to the specific conditions, materials, or concentrations given therein, but encompasses the full range of effective conditions and concentrations which may be used in practicing the invention.

Claims: A method for producing a thermoplastic elastomer comprising dynamically vulcanizing a mixture containing EPDM rubber and polyolefin resin in the presence of a phenolic curing resin characterised by said elastomer having: (1) from 14 to 80 parts by weight of high molecular weight EPDM rubber per 100 parts by weight of polyolefin and total final rubber content, (2) from 1 to 30 parts by weight of liquid EPDM rubber per 100 parts by weight of polyolefin and total final rubber content, and (3) from 85 to 15 parts by weight polyolefin resin, and in that the method comprises: (a) mixing together a phenolic curing resin of sufficient quantity to cure the rubber/polyolefin mixture to a thermoplastic elastomer and a portion of said liquid EPDM rubber to form a curing masterbatch; (b) mixing together said high molecular weight EPDM rubber and the balance of said liquid EPDM rubber; (c) adding said polyolefin resin to (b); (d) mixing the mixture of (c); (e) adding said masterbatch to (d); (f) mixing the mixture of (e); (g) adding a sufficient quantity of a cure accelerator to (f); and (h) dynamically vulcanizing the mixture of (g) until a thermoplastic elastomer is formed. A method as defined in Claim 1, wherein: (1) said high molecular weight EPDM rubber is present in an amount of from 23 to 70 parts by weight of polyolefin and total final rubber content, (2) said liquid EPDM rubber is present in an amount of from 2 to 15 parts by weight of polyolefin and total final rubber content, and (3) said polyolefin is present in an amount of from 75 to 20 parts by weight of polyolefin and total final rubber content. A method as defined in either claim 1 or claim 2, wherein said liquid EPDM rubber has a molecular weight in the range of from 500 to 20,000. A method as defined in claim 3, wherein said liquid EPDM rubber has a molecular weight in the range of from 2,500 to 10,000. A method as defined in any one of claims 1 to 4, wherein said masterbatch comprises from 20 to 60% by weight of liquid EPDM rubber and from 40 to 80% by weight of phenolic curing resin. A method as defined in any one of claims 1 to 5, wherein said polyolefin is polypropylene.

Inventors: KIM MYUNG HWAN, LEE BYUNG HYUNG, KIM, MYUNG HWAN, LEE, BYUNG HYUNG
Applicants: YUKONG LTD, YUKONG LIMITED
Application year: 1992
Main classification: C08L23
IPC codes: C08L23, C08L61, C08J3