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
2000-11-06
2002-09-24
Nutter, Nathan M. (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...
C525S192000, C525S197000, C525S198000, C525S209000, C525S221000, C525S222000, C525S240000, C525S241000
Reexamination Certificate
active
06455637
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to polymeric compositions and their uses, and more particularly to crosslinked compositions of silane-modified polyolefins blended with polypropylene homopolymers and/or copolymers, and their uses as heat shrinkable coating and insulating materials, and as wire and cable insulation materials, but not necessarily restricted thereto.
BACKGROUND OF THE INVENTION
Polypropylenes are ideally suited to the preparation of coatings and insulations designed for use at operating temperatures in excess of those that can be withstood by other polyolefins, for example, polyethylene, which exhibit lower softening and melting temperatures. Other attractive features are their high rigidity and toughness, low cost and relatively low density. Applications for these coatings and insulations would include polymeric insulation for electrical wires and cables, and heat-shrinkable protective sleeves for high-temperature, transmission pipelines, or applications requiring greater toughness and rigidity than is afforded by polyethylene-based systems. For example, heat-shrinkable sleeves used for the corrosion protection of high temperature pipeline joints are, required to maintain dimensional stability and integrity at the operating temperature of the pipeline. Hence it is necessary to use a material, such as polypropylene, with a softening temperature or melting point high enough to prevent creeping or sagging of the sleeve from the pipe at the continuous operating temperature of the pipeline. Also, in order to maximise heat-resistance and physical properties, it is necessary to impart some thermoset characteristic to the material. This is done by crosslinking the polymer to some required degree. Crosslinking is also necessary for the production of heat-shrinkable articles to impart controlled shrinkage characteristics. The aim of this invention is to provide a means of preparing crosslinked, predominantly polypropylene-based materials, which can be used in the applications described, but not necessarily restricted thereto.
Polymers in which the predominant chain units comprise an alpha olefin, such as polypropylenes, are known to preferentially depolymerise when exposed to free radicals required to effect crosslinkng. Hence, unlike similar materials, namely polyolefins such as polyethylenes and copolymers of polyethylene, it is not possible to crosslink polypropylene-based materials to satisfactory levels, as is required, for example, in the production of wire and cable insulations, and heat-shrinkable articles, such as tubing, sheet, and moulded shapes, by using standard free-radical methods of crosslinking, such as electron beam irradiation or peroxide initiated crosslinking.
It is also a well known process to produce crosslinked polyolefins, and articles made therefrom, using a process for grafting a vinyl silane onto an olefin homopolymer or copolymer such as is described in U.S. Pat. No. 3,646,155. Alternatively, the vinyl silane may be copolymerised directly with olefin monomers as described in U.S. Pat. No. 4,413,066, for example. Since these methods requires a free-radical generator to initiate the grafting reaction, polypropylene is also unsuited to this method of crosslinking. Hence, it is therefore necessary to resort to alternative methods to crosslink polypropylene.
SUMMARY OF THE INVENTION
The present invention overcomes the above-discussed problems of the prior art by providing moisture crosslinkable compositions of silane-modified polyolefins, typically of polyethylenes and copolymers of polyethylene, blended with non silane-modified polypropylenes, with or without an additional material added as a compatibilising agent for the polyolefin and polypropylene.
In the method of the present invention, one or more polypropylene homopolymners, or copolymers of polypropylene with an olefin other than polypropylene, are blended with one or more polyolefins, preferably polyethylenes or copolymers of polyethylene, previously reacted with silane to create silane-grafted polyolefins. Alternatively, the silane may be incorporated during polymerisation of the polyolefin to produce silane-olefin copolymers. Polyolefins reacted with silane by either of these methods are referred to in this document as silane-modified polyolefins.
U.S. Pat. No. 5,929,129 describes a process of blending a silane-grafted polyolefin with a non silane-grafted polypropylene, but this is limited to materials required for the manufacture of foamed articles, said materials being unsuitable for the manufacture of the preferred articles of this invention due to an insufficient level of crosslinking and blend mixture-compatibility.
Suitable polyolefins in this invention would include those materials known in the industry as low density polyethylene, high density polyethylene, linear low density polyethylene; copolymers of polyethylene, including those based on ethylene-butene, ethylene-hexene, ethylene-octene, ethylene-vinyl-acetate, ethylene-methyl-acrylate, ethylene-ethyl-acrylate, ethylene-butyl-acrylate, and similar materials; and ethylene-propylene or ethylene-propylene diene elastomers; and, in particular those of the above prepared using so-called metallocene catalysts.
Alternatively, one or more additional materials may be incorporated to act as compatibilising agents for the silane-modified polyolefin and the polypropylene. Such materials would include the polypropylenes and polyolefin materials described above; polyolefins modified with reactive functional groups, such as acrylic acids, methacrylic acids, acrylates, methacrylates and anhydrides; and block copolymers, such as styrene-butadiene, styrene-butadiene-styrene, styrene-ethylene/propylene and styrene-ethylene/butylene-styrene.
The blended composition is then formed into the desired article by melt processing techniques such as extrusion and moulding, including multi-layer processing, for example co-extrusion of the blend with another material to form discrete but intimately bonded layers. The article thus formed is cross-linked in the presence of a silanol condensation catalyst under suitable conditions of heat and moisture, the catalyst being either blended with the composition during melt processing or added subsequently by coating the formed article, for example. The crosslinking thus performed also helps to stabilise the physical structure of the mixture of polypropylene with the polyolefin contained in the silane-modified polyolefin composition.
Accordingly, in one aspect, the present invention provides a heat shrinkable coating material of a crosslinked composition comprising a silane-modified, preferably silane-grafted, polyolefin and non silane-modified polypropylene, said material being formed by a process comprising: (a) reacting a polyolefin with a silane to form a silane-grafted polyolefin or silane-olefin copolymer; (b) blending the silane-modified polyolefin produced in (a) with the polypropylene homopolymer or copolymer and a silanol condensation catalyst; (c) forming the coating or insulating material by melt processing the mixture formed in step (b); and (d) crosslinking the coating or insulating material by exposing it to combined heat and moisture.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The crosslinking of polymers, in particular polyolefins, by the combined process of chemical grafting of silane molecules onto the polymer to form a silane-grafted resin, followed by catalysed hydrolysis and condensation of the silane, is a well known and established process, such as is described in U.S. Pat. No. 3,646,155, which is incorporated herein by reference in its entirety.
The preferred polyolefins for use in the compositions of the present invention include polyethylene or co-polymers of ethylene prepared by polymerising ethylene with one or more of an unsaturated olefin monomer having from 3 to 20 carbon atoms, preferably propylene, butylene, hexene or octene, a substituted olefin such as vinyl acetate, methyl acrylate, ethyl acrylate or butyl acrylate, or a diene monomer, such as ethylidene
Heydrich Marcus P.
Jackson Peter
Steele Robert Edward
Tailor Dilip K.
Tathgur Amarjit
Nutter Nathan M.
Ridout & Maybee LLP
ShawCor Ltd.
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