Crosslinked, heat shrinkable polypropylene compositions

Details Crosslinked, heat shrinkable polypropylene compositions Crosslinked, heat shrinkable polypropylene compositions

2000-11-06

2003-05-27

Mullis, Jeffrey (Department: 1711)

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

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C525S070000, C525S198000, C525S240000, C525S195000, C525S191000, C525S197000

Reexamination Certificate

active

06569915

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to polymeric compositions and their uses, and more particularly to crosslinked compositions of polypropylene with ethylene-propylene elastomers, and their uses as coating and insulating materials, particularly those that are heat-shrinkable, but not necessarily restricted thereto.
BACKGROUND TO 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 such as, 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, heat-shrinkable, corrosion protection sleeves for high-temperature transmission pipeline joints, heat-shrinkable tubing or shapes for electrical insulation and mechanical protection, or in 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 thermal stability 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 in order to impart controlled shrinkage characteristics. The aim of this invention is to provide a means of preparing crosslinked, predominantly polypropylene-based, heat-shrinkable compositions, 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 or degrade when exposed to free radicals required to effect crosslinking. 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 heat-shrinkable articles such as tubing, sheet, and moulded shapes, by using standard free-radical methods of crosslinking, such as electron beam irradiation, gamma irradiation, or peroxide-initiated crosslinking.
Work described in U.S. Pat. Nos. 3,717,559 and 4,424,293, for example, show that certain polypropylenes with the addition of acrylate crosslinking promoters can be crosslinked by irradiation to satisfactory levels for the production of polypropylene foam. However, the elastic strength and elongation of these materials at temperatures above the melting point have been found to be wholly insufficient to impart the high temperature resistance and controlled recovery characteristics required for the satisfactory production and performance of the heat-shrinkable products described above, and to confer the resistance to deformation and mechanical failure at elevated temperatures of electrical insulation, and similar, products. Hence it is necessary to resort to alternative methods to provide the necessary crosslinking of polypropylenes.
SUMMARY OF THE INVENTION
The present invention overcomes the above discussed problems of the prior art by providing a means whereby a predominantly polypropylene-based composition can be crosslinked by irradiation to the required level for the production of heat-shrinkable articles and functional high temperature insulation products by blending the polypropylene with a polymer that is highly sensitive to crosslinking by irradiation.
Hence, on crosslinking such a polymer blend, the radiation-sensitive component will preferably crosslink before the polypropylene can depolymerise to any great extent, and thereby form what may be termed as an interpenetrating crosslinked network with the predominantly uncrosslinked component. The crosslinking also acts to stabilise the blend through compatibilisation of the two relatively immiscible components by inducing a chemical interaction at the interface of the two components. As a result, the blend exhibits the properties of a crosslinked system whilst retaining the high temperature performance, stability and toughness of a predominantly polypropylene-based, semi-crystalline material. The crosslinked network allows the material to be heated close to or above the softening point without melting, such that it may be stretched a predetermined amount without rupture, and then frozen in the stretched state. Subsequent heating of the crosslinked, stretched material near or above the softening point will cause it to recover to approximately its original, unstretched dimensions.
In this invention, an ethylene-propylene elastomer, namely an ethylene-propylene copolymer (EPM) or, more preferably, an ethylene-propylene-diene terpolymer (EPDM), and, most preferably, an ethylene-propylene-diene terpolymer polymerised using metallocene catalysts (herein designated mEPDM), for example, the Nordel IP EPDM materials developed by DuPont Dow Elastomers L.L.C. using their INSITE® constrained-geometry catalyst technology, or a blend thereof, provides the necessary crosslinking sensitivity for blends with polypropylene.
The preferred mEPDM terpolymers, are prepared by copolymerising propylene with additional comonomers, specifically ethylene and a diene monomer usually chosen from 5-ethylidene-2-norbornene, dicyclopentadiene, or 1,4-hexadiene, using a highly stereospecific, single-site, constrained geometry, or so-called metallocene, catalyst. They differ substantially from existing EPDM materials produced using standard Ziegler Natta coordination catalysts in that it is possible to more accurately control the quantity and position of the comonomers within the polymer structure to provide a more precise molecular weight distribution and a more regular molecular architecture, resulting in higher crystallinity, for example, and superior material properties. More importantly with respect to the current invention, it is possible to adjust the comonomer levels for optimum sensitivity of the mEPDM to crosslinking by electron beam irradiation.
Suitable polypropylenes in this invention would include those materials commonly known in industry as polypropylene homopolymers, or polypropylene copolymers, the latter typically being copolymers of propylene and ethylene. Additionally, said polypropylene homopolymers include polypropylenes modified with reactive functional groups, such as acrylic acids, methacrylic acids, acrylates, methacrylates and anhydrides.
Alternatively, one or more additional materials may be incorporated to act as compatibilising or modifying agents for the ethylene-propylene elastomer and the polypropylene. Such materials would include the polypropylenes, EPM, EPDM and mEPDM materials described above; other ethylene-propylene elastomers; polyethylenes and copolymers of polyethylene, including those known in the industry as low density polyethylene, high density polyethylene, linear low density polyethylene, and 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 particularly those prepared using metallocene catalysts; 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.
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