Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1997-05-29
2001-03-27
Mullis, Jeffrey C. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S145000, C525S191000, C525S196000, C525S232000, C525S240000, C428S523000
Reexamination Certificate
active
06207754
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
1. Field of the Invention
This invention relates to thermoplastic olefin compositions of a low modulus which are suitable for fabrication into flexible skins and liners for applications which heretofore have been serviced by skins and liners produced essentially only from plasticized polyvinyl chloride resin compositions.
2. Background of the Invention
Thermoplastic elastomers (TPEs) are an important class of polymeric composition which are particularly useful in producing durable components through conventional extrusion or injection molding processes. Typically a TPE is a blend of thermoplastic polymer and a cured elastomer (rubber). Articles may be produced from a TPE that have a behavior similar to a cured elastomer but the composition has the advantage, compared to a rubber (elastomer) resin, that the TPE undergoes plastic flow above the softening point of the thermoplastic polymer component of the blend. This permits TPEs to be used in component fabrication through common polymer processing techniques, such as injection molding techniques to produce finished articles having resilient rubber-like properties without the need for a vulcanizing cure of the finished article. This provides TPEs with an advantage compared to conventional curable elastomers because conventional curable elastomers are tacky, do not undergo plastic flow at elevated temperatures and therefore cannot be fabricated into finished article forms by an extrusion or injection molding technique.
The components for a thermoplastic elastomer (TPE) blend may be formed as a reactor blend of a thermoplastic polymer and an uncured elastomer—with the thermoplastic polymer and the elastomer being simultaneously formed by different catalysts in a single reactor vessel—or the respective thermoplastic polymer and elastomer components for the blend may be separately prepared and then melt blended, generally by a high shear mixing technique. The elastomeric component of a TPE may be precured or cured in situ by a curing agent added during its melt blending with the thermoplastic polymer component. When the elastomer component of a TPE is cured during blending with the thermoplastic polymer component, the TPE may also be referred to as an “alloy” and/or as a “dynamically vulcanized alloy.”
When both the thermoplastic polymer component and the elastomer component are composed of olefin monomeric units, the resulting TPE is often referred to as a thermoplastic olefin (TPO). Thermoplastic olefin elastomer compositions (TPOs) are a class of TPEs based predominately or wholly on olefin polymers. A typical TPO is a melt blend or reactor blend of a polyolefin plastic, typically a propylene polymer, with an olefin copolymer elastomer (OCE), typically an ethylene-propylene rubber (EPM) or an ethylene-propylene-diene rubber (EPDM). The polyolefin plastic imparts to the TPO the temperature resistance and rigidity typical of that thermoplastic resin while the olefin copolymer elastomer imparts flexibility, resilience and toughness to the TPO. For example, a propylene homopolymer or random copolymer having at least 95 wt. % propylene content with an alpha-olefin comonomer content no greater than about 5 wt. % is a thermoplastic polymer which when blended by reactor or melt compounding with an ethylene-propylene rubber (EPM) or an ethylene-propylene-diene (EPDM) rubber results in a composition that would properly be called a thermoplastic olefin. Wherein such EPM or EPDM comprises not more than about 20 wt. % of this propylene polymer blend, this TPO composition is typically referred to as an impact modified polypropylene (im-PP).
For many purposes, such as enhanced weatherability, low temperature impact strength and reduced material cost, a TPO form of TWE composition may be and is preferred, provided that the TPO composition can be formulated to have a set of properties which will meet the service needs for its intended end use application. For example, TPOs that are impact modified polypropylenes are particularly well suited for producing resilient structures, such as body parts for automotive applications like bumper covers, air dams, and other trim parts, etc. The capability of such TPOs to be injection molded makes them particularly attractive for high volume production necessary in automotive body part applications. However, for other end use applications, such as for production of skin and/or liner articles—such as dash board and interior door panel skin surface layers in the automotive industry or as geomembranes and/or reinforced roof membrane liners—impact modified polypropylenes (im-PP) heretofore known have been found wanting in their properties, and have not been adopted for applications such as skins and/or liners. Chief among the deficiencies of the heretofore known im-PP compositions that has forestalled their adoption for use as skins/liners has been the stiffness-flexible softness-conformability properties of skins and liners formed of such compositions, as indicated by the 1% secant modulus property of the base composition (as measured per ASTM D-790 on injection molded test specimens). Generally, TPEs, and particularly TPOs in the nature of impact modified polypropylenes, have a 1% secant modulus (hereinafter “secant modulus”) significantly exceeding 30,000 psi (200 MPa). Further, the potential expedient of increasing the content of the EPM or EPDM component of a polypropylene TPO blend to levels greater than about 20 wt. % to further reduce its secant modulus is not viable in practice since higher elastomer contents tend to render the blend to be too tacky for convenient processing and can even lead to a phase inversion of the blend that detracts from the other physical properties of an im-PP.
Many service applications require a skin and/or liner to be very compliant or flexible in order to finally conform or fashion the skin or liner about the contours of that substrate to which it must be affixed to form the finished article. Hence, many applications require a skin or layer, the resin base of which is both resilient while being flexible and compliant. This property requirement for such a skin or liner translates to a secant modulus not exceeding and generally significantly less than about 30,000 psi (200 MPa) for the base resin from which the skin or liner is to be fabricated. And this low secant modulus must be accomplished without sacrificing the other properties that would make the resin seem suitable or desirable for production of a skin/liner such as automotive door skins and instrument panel skins. In the fabrication process these skins need to be embossed with desirable surface patterns, and subsequently formed into the final contours by various processes such as low pressure molding or vacuum forming. Grain retention of the embossed patterns on such skins during fabrication is therefore essential. During thermoforming, the heated skin needs to stretch rapidly during deep drawing while maintaining the embossed grain patterns. To date, plasticized polyvinyl chloride compositions have been essentially the only resin that has been found to meet this low secant modulus service requirement while also retaining its other properties required for service as a skin/liner.
The use of plasticized polyvinyl chloride (PVC) for service as a flexible skin or liner is not without its disadvantages. First, the plasticizer required to impart compoundability to the PVC resin so that it may in the first instance be fabricated into the form of a skin/liner—a high surface area article form—tends over time to migrate to the surface of the skin/liner and emits therefrom as an odor (i.e., the “new car” smell for example) which may or may not be objectionable to a user of the finished article. The continued loss over service lifetime of plasticizer from the PVC layer promotes hairline cracks, which in time can lead to the eventual failure of the PVC skin. By comparison to olefin based resins, a PVC resin is of 30-40% greater density; meaning that to fabricate a sk
Exxon Chemical Patents Inc.
Mullis Jeffrey C.
Prodnuk Stephen D.
Runnyan Charles E.
Schneider John E.
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