Thermoplastic vulcanizates and process for making the same

Details Thermoplastic vulcanizates and process for making the same Thermoplastic vulcanizates and process for making the same

2000-05-24

2002-08-20

Short, Patricia A. (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S106000, C524S291000, C524S342000, C524S400000, C524S432000, C525S133000, C525S134000, C525S139000, C525S141000, C525S456000

Reexamination Certificate

active

06437030

ABSTRACT:

TECHNICAL FIELD
The present invention is generally directed toward thermoplastic vulcanizates and processes for making the same. More particularly, the thermoplastic vulcanizates of this invention include a rubber and a thermoplastic polymer. The rubber is advantageously cured by using a phenolic resin curative in the presence of a catalyst system formed by combining a metal halide and a metal carboxylate.
BACKGROUND OF THE INVENTION
Phenolic resins, which are also referred to as resole resins, are known crosslinking agents for unsaturated elastomers. For example, halogenated phenolic resins have been employed to crosslink butyl rubber as described in U.S. Pat. No. 2,972,600. This crosslinking reaction is typically carried out in the presence of a catalyst, such as zinc oxide, as set forth in U.S. Pat. No. 3,093,613. Where unhalogenated phenolic resins are employed, the crosslinking reaction may occur in the presence of a halogenated elastomer as taught by U.S. Pat. No. 3,887,756. Alternatively, unhalogenated phenolic resins may be used in the presence of a metal halide as a catalyst. The use of metal halides, however, has presented solubility and corrosion problems. One solution to this problem, which is taught by U.S. Pat. No. 3,287,440, is to form the metal halide in situ by reacting a halogen donor and a metal compound that is capable of reacting with the halogen donor at elevated temperatures.
Phenolic resins have also been employed to cure the rubber component of thermoplastic vulcanizates as set forth in U.S. Pat. No. 4,311,628. Thermoplastic vulcanizates are technologically useful compositions of matter that often include small, finely-divided particles of rubber that are well dispersed within a thermoplastic matrix. These compositions are very useful because they have many of the properties of an elastomer and are processable as thermoplastics. Typically, these compositions are formed by dynamic vulcanization whereby the elastomer is crosslinked within a blend that includes the elastomer and a thermoplastic polymer while both polymers are undergoing molten-state mixing or masticating.
Generally, the rubber of useful thermoplastic vulcanizates is cured to an extent that only less than about 15 percent of the rubber is extractable. To achieve this extent of cure when a phenolic resin is employed as the curative, it is known that the vulcanizable composition must contain at least about eight milliequivalents of halide per 100 grams of rubber. The source of halide can be from the phenolic resin, the rubber, or the addition of a halogen donor such as a metal halide. Below this amount of halide, however, the rate of cure decreases by about ten-fold and the elastomeric properties of the vulcanizate diminish as evidenced by a higher oil swell, an increased compression set, and an increased modulus.
The problem, however, is that the thermoplastic vulcanizates that are cured in the presence of at least about eight or more milliequivalents of halide per 100 grams of rubber attract and absorb atmospheric moisture. This moisture may impact processing, physical properties, end-product performance, and surface appearance. As a result, moisture pick-up control is very important for successful processing. In fact, effectively controlling the moisture is necessary to attain good surface finishes and high quality finished parts. Therefore, these thermoplastic vulcanizates must be dried prior to processing.
Accordingly, while the use of phenolic resin curatives provides useful thermoplastic vulcanizates that have many desirable properties, there is a need to improve the hygroscopic properties of these thermoplastic vulcanizates.
SUMMARY OF INVENTION
In general the present invention provides a process for forming a thermoplastic vulcanizate compromising the steps of dynamically vulcanizing a rubber within a blend that comprises the rubber and a thermoplastic polymer, where said step of vulcanizing is carried out by using a phenolic resin in the presence of a catalyst system formed by combining a metal halide and a metal carboxylate.
The present invention also includes a thermoplastic vulcanizate formed by a process comprising the steps of dynamically vulcanizing a rubber within a blend that comprises the rubber and a thermoplastic polymer, where said step of vulcanizing is carried out by using a phenolic resin in the presence of a catalyst system formed by combining a metal halide and a metal carboxylate.
Still further, the present invention provides a thermoplastic vulcanizate comprising cured rubber blended with a thermoplastic polymer, where said cured rubber is formed by dynamically curing a rubber by using a phenolic resin curative in the presence of a catalyst system formed by combining a metal halide and a metal carboxylate, where the thermoplastic vulcanizate contains less than seven milliequivalents of halide per 100 grams of rubber, and where said rubber is cured to an extent that less than 15 weight percent of the rubber is extractable from the thermoplastic vulcanizate.
Advantageously, the use of the curative system of the present invention provides useful thermoplastic vulcanizates that are less hygroscopic than Ethermoplastic vulcanizates that are prepared with conventional phenolic resin curing systems. It has been found that the cure system employed in this invention allows for reduced halide and therefore reduces the hygroscopicity of the resulting thermoplastic vulcanizates without an appreciable compromise to the properties of the thermoplastic vulcanizate. Furthermore, the reduction in the amount of halide employed advantageously reduces mold fouling and corrosion.
PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION
This invention is directed toward thermoplastic vulcanizates and processes for making the same. The thermoplastic vulcanizates of this invention include a cured rubber and thermoplastic polymer where the rubber is cured by using a phenolic resin curative in the presence of a catalyst system that is formed by combining a metal halide and a metal carboxylate. These thermoplastic vulcanizates are preferably prepared by dynamically vulcanizing at least one phenolic resin-curable rubber within a blend that includes the rubber and at least one thermoplastic polymer. The thermoplastic vulcanizates of this invention may further include optional fillers, extender oils, antioxidants, and other additives that are commonly used in rubber compositions. In a preferred embodiment, the thermoplastic vulcanizates include a heat stabilizer.
The phenolic resin-curable rubber may include any rubber or mixture thereof that is capable of being crosslinked with a phenolic resin. Reference to a rubber may include mixtures of more than one rubber. Useful rubbers typically contain some degree of unsaturation in their polymeric main chain. Some non-limiting examples of these rubbers include elastomeric copolymers, butyl rubber, natural rubber, styrene-butadiene copolymer rubber, butadiene rubber, acrylonitrile rubber, halogenated rubber such as brominated and chlorinated isobutylene-isoprene copolymer rubber, butadiene-styrene-vinyl pyridine rubber, urethane rubber, polyisoprene rubber, epichlolorohydrine terpolymer rubber, and polychloroprene. The preferred rubbers are elastomeric copolymers and butyl rubber.
As used within this specification, the term elastomeric copolymer refers to rubbery copolymers polymerized from ethylene, at least one &agr;-olefin monomer, and at least one diene monomer. The &agr;-olefins may include, but are not limited to, propylene, butene-1, hexene-1, 4-methyl-1 pentene, octene-1, decene-1, or combinations thereof. The preferred a-olefins are propylene, hexene-1, octene-1 or combinations thereof. The diene monomers may include, but are not limited to, 5-ethylidene-2-norbornene; 1,4-hexadiene; 5-methylene-2-norbornene; 1,6-octadiene; 5-methyl-1,4-hexadiene; 3,7-dimethyl-1,6-octadiene; 1,3-cyclopentadiene; 1,4-cyclohexadiene; dicyclopentadiene; 5-vinyl-2-norbornene and the like, or a combination thereof. The preferred diene monomers are 5-ethylidene-2-norb

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