Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-07-24
2002-10-15
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C264S210500, C264S210700, C242S614000, C525S105000, C525S106000, C526S279000, C528S017000, C528S018000
Reexamination Certificate
active
06465547
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to polymeric compositions and their uses, and more particularly to crosslinked compositions of silane-modified blends of polyolefins, more specifically polyolefins such as polyethylene with homopolymers and/or copolymers of propylene and other higher olefins, 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
Polyolefins derived from propylene and other higher olefins 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, or do not retain suitable physical properties at higher 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 for applications requiring greater heat distortion resistance, 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, toughness 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 higher than the continuous operating temperature of the pipeline to prevent creeping or sagging of the sleeve from the pipe at this temperature.
Also, in order to maximise heat-resistance, hot deformation resistance, and physical properties, such as is required for high temperature electrical insulations, 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 propylene or higher olefins, such as butene, are known to preferentially depolymerise when exposed to free radicals required to effect crosslinking. Hence, unlike similar materials, namely polyolefins such as polyethylene and copolymers of polyethylene, it is not possible to directly crosslink these 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, by 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. However, since these methods require a free-radical generator to initiate the grafting reaction, the polypropylene or higher polyolefin by itself is unsuited to this method of crosslinking. Hence, it is therefore necessary to resort to alternative strategies to create crosslinked compositions consisting predominantly of polypropylene or higher polyolefins of the type that preferentially degrade in the presence of free-radical generators.
SUMMARY OF THE INVENTION
The present invention overcomes the above-discussed problems of the prior art by providing moisture crosslinkable blends of predominantly polypropylene, or higher polyolefins such as polybutene or polymethylpentene, with silane-modified polyolefins; more specifically of silane-modified blends of polyethylenes or polyethylene copolymers with polypropylene copolymers or homopolymers, 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 homopolymers, or copolymers of polypropylene with an olefin other than polypropylene, are blended with one or more polyolefins other than polypropylene (hereinafter referred to as polyolefins), preferably polyethylenes or copolymers of polyethylene, the blend being than grafted with a suitable silane to produce the desired silane-modified polypropylene containing composition.
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 those of the above prepared using so-called metallocene catalysts.
In addition, one or more additional materials may be incorporated to act as compatibilising agents for the polyolefin and polypropylene blend. Such materials would include the polypropylenes, higher polyolefins, and polyolefin materials described above, including those 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. These compatibilising agents may be incorporated prior to the silane grafting reaction, but may also be added to the silane-modified composition during subsequent melt processing.
The silane-modified blend 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 stabilises the physical structure of the blend of silane-modified polypropylene and polyolefin through the formation of an interpenetrating network.
Accordingly, in one aspect, the present invention provides a heat shrinkable coating material of a crosslinked composition consisting of a silane-modified polypropylene polyolefin blend, said material being formed by a process comprising: (a) reacting a pre-blended mixture of polypropylene and polyolefin resins with an appropriate silane and silane-grafting initiator to produce a silane-modified polypropylene—polyolefin composition; (b) forming the coating or insulating material by melt processing the composition produced in step (a) with a silanol condensation catalyst; and (c) crosslinking the formed coating or insulating material by exposing it to a combined regimen of 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.
According to the present invention, a blended mixture is formed from a polyolefin which primarily crosslinks in the presence of free radicals; and a poly
Heydrich Marcus P.
Jackson Peter
Steele Robert Edward
Tailor Dilip K.
Tathgur Amarjit
Dawson Robert
DeLio & Peterson LLC
Shawcor Ltd.
Zimmer Marc S
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