Thermoplastic silicone elastomers formed from polyester resins

Details Thermoplastic silicone elastomers formed from polyester resins Thermoplastic silicone elastomers formed from polyester resins

2000-12-04

2002-07-09

Dawson, Robert (Department: 1712)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C524S086000, C524S089000, C524S095000, C524S292000, C524S478000, C524S588000, C524S862000, C528S015000, C528S031000, C528S032000

Reexamination Certificate

active

06417293

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a thermoplastic elastomer composition wherein a silicone gum and a stabilizer are dispersed in a polyester resin and the silicone gum is dynamically vulcanized in the resulting mixture.
BACKGROUND OF THE INVENTION
Thermoplastic elastomers (TPEs) are polymeric materials which possess both plastic and rubbery properties. They have elastomeric mechanical properties but, unlike conventional thermoset rubbers, they can be re-processed at elevated temperatures. This re-processability is a major advantage of TPEs over chemically crosslinked rubbers since it allows recycling of fabricated parts and results in a considerable reduction of scrap.
In general, two main types of thermoplastic elastomers are known. Block copolymer thermoplastic elastomers contain “hard” plastic segments which have a melting point or glass transition temperature above ambient as well as “soft” polymeric segments which have a glass transition or melt point considerably below room temperature. In these systems, the hard segments aggregate to form distinct microphases and act as physical crosslinks for the soft phase, thereby imparting a rubbery character at room temperature. At elevated temperatures, the hard segments melt or soften and allow the copolymer to flow and to be processed like an ordinary thermoplastic resin.
Alternatively, a thermoplastic elastomer referred to as a simple blend, or physical blend, can be obtained by uniformly mixing an elastomeric component with a thermoplastic resin. When the elastomeric component is also cross-linked during mixing, a thermoplastic elastomer known in the art as a thermoplastic vulcanizate (TPV) results. Since the crosslinked elastomeric phase of a TPV is insoluble and non-flowable at elevated temperature, TPVs generally exhibit improved oil and solvent resistance as well as reduced compression set relative to the simple blends.
Typically, a TPV is formed by a process known as dynamic vulcanization, wherein the elastomer and the thermoplastic matrix are mixed and the elastomer is cured with the aid of a crosslinking agent and/or catalyst during the mixing process. A number of such TPVs are known in the art, including some wherein the crosslinked elastomeric component can be a silicone polymer while the thermoplastic component is an organic, non-silicone polymer (i.e., a thermoplastic silicone vulcanizate or TPSiV). In such a material, the elastomeric component can be cured by various mechanisms including radical, condensation and hydrosilylation method, but each method has its limitations.
Arkles, in U.S. Pat. No. 4,500,688, discloses semi-interpenetrating networks (semi-IPNs) wherein a vinyl-containing silicone fluid having a viscosity of 500 to 100,000 cS is dispersed in a conventional thermoplastic resin. Arkles only illustrates these IPNs at relatively low levels of silicone. The vinyl-containing silicone is vulcanized in the thermoplastic during melt mixing according to a chain extension or crosslinking mechanism which employs a silicon hydride-containing silicone component. Typical thermoplastics mentioned include polyesters, polyurethanes, styrenics, polyacetals and polycarbonates. This disclosure is expanded by Arkles in U.S. Pat. No. 4,714,739 to include the use of hybrid silicones which contain unsaturated groups and are prepared by reacting a hydride-containing silicone with an organic polymer having unsaturated functionality. Although Arkles discloses a silicone fluid content ranging from 1 to 40 weight percent (1 to 60% in the case of the '739 patent), there is no suggestion of any criticality as to these proportions or to the specific nature of the organic resin.
Crosby et al., in U.S. Pat. No. 4,695,602, teach composites wherein a silicone semi-IPN vulcanized via a hydrosilation reaction is dispersed in a fiber-reinforced thermoplastic resin having a high flexural modulus. The silicones employed are of the type taught by Arkles, cited supra, and the composites are said to exhibit improved shrinkage and warpage characteristics relative to systems which omit the IPN.
Ward et al., in U.S. Pat. No. 4,831,07 1, disclose a method for improving the melt integrity and strength of a high modulus thermoplastic resin to provide smooth-surfaced, high tolerance profiles when the modified resin is melt-drawn. As in the case of the disclosures to Arkles et al., cited supra, a silicone mixture is cured via a hydrosilation reaction after being dispersed in the resin to form a semi-IPN, after which the resulting composition is extruded and melt-drawn.
U.S. Pat. No. 6,013,715 to Gornowicz et al. teaches the preparation of TPSiV elastomers wherein a silicone gum (or filled silicone gum) is dispersed in either a polyolefin or a poly(butylene terephthalate) resin and the gum is subsequently dynamically vulcanized therein via a hydrosilation cure system. The resulting elastomers exhibit an ultimate elongation at break of at least 25% and have significantly improved mechanical properties over the corresponding simple blends of resin and silicone gum in which the gum is not cured (i.e., physical blends). This is, of course, of great commercial significance since the vulcanization procedure, and the cure agents required therefor, add to both the complexity as well as the expense of the preparation and vulcanization would be avoided in many applications if essentially identical mechanical properties could be obtained without its employ. However, this patent specifically teaches that poly(ethylene terephthalate) resin, as well as other thermoplastic resins, could not be modified according to the disclosed method.
Although the above publications disclose the preparation of compositions using various thermoplastic resins as the matrix and a dispersed phase consisting of a silicone oil or elastomer which is dynamically vulcanized therein, neither these references, nor any art known to applicants, teach the preparation of TPSiV elastomers based on polyester resins having superior tensile and elongation properties, as disclosed herein.
SUMMARY OF THE INVENTION
It has now been discovered that TPSiV elastomers of the type described in above cited U.S. Pat. No. 6,013,715 can be prepared from various polyester resins, including poly(ethylene terephthalate). As in the case of the teachings of U.S. Pat. No. 6,013,715, the elastomers disclosed herein generally also have good appearance and have a tensile strength and/or elongation at least 25% greater than that of the corresponding simple (physical) blend wherein the diorganopolysiloxane is not cured. However, it has been surprisingly found that such properties are significantly enhanced when a minor portion of a stabilizer is incorporated in the formulation, this resulting in a TPSiV having an elongation of at least 30%. Furthermore, unlike the teachings of Arkles, cited supra, and others, the silicone component which is dispersed in the thermoplastic resin, and dynamically cured therein, must include a high molecular weight gum, rather than a low viscosity silicone fluid, the latter resulting in compositions having poor uniformity. Surprisingly, polyesters having a softening point greater than about 300° C. could not be modified according to the present invention to prepare TPSiVs having the required 30% elongation.
The present invention, therefore, relates to a method for preparing a thermoplastic elastomer, said method comprising:
(I) mixing
(A) a thermoplastic resin comprising more than 50 percent by volume of a polyester resin other than poly(butylene terephthalate), said thermoplastic resin having a softening point of 23° C. to 300° C.,
(B) a silicone elastomer comprising
(B′) 100 parts by weight of a diorganopolysiloxane gum having a plasticity of at least 30 and having an average of at least 2 alkenyl groups in its molecule and, optionally,
(B″) up to 200 parts by weight of a reinforcing filler, the weight ratio of said silicone elastomer (B) to said thermoplastic resin (A) being greater than 35:65 to 85:15,
(C) 0.01 to 5 parts by weight of a stabilizer for

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