Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2002-05-22
2003-11-25
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S081000, C521S098000, C521S140000
Reexamination Certificate
active
06653360
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a thermoplastic foam, and more particularly to a flexible foamed thermoplastic article formed from a foamed polyolefin.
BACKGROUND OF THE INVENTION
Polyolefin foam products are used in numerous fields, including thermal and electrical insulation, cushioning and packaging. For example, pipe insulation has been produced by extruding foamed polyethylene in a hollow tubular configuration, cutting the tubular material to lengths, and longitudinally slitting the tube to allow for placing the tube around a pipe. Foamed polyethylene is also used as an electrical insulation or dielectric in various cable constructions, such as coaxial cables, for example. Polyethylene foam is also commonly used as a protective cushion in packaging, or as a cushion material in furniture.
Polyethylene foam has highly desirable attributes including low cost, chemical inertness, and clean electrical performance. However, the relative stiffness of the polymer makes it unsuited for certain applications where flexibility, resilience, bending performance, or resistance to high temperatures are important. Although the high temperature resistance can be improved by cross-linking, the resulting cross-linked foam is not recyclable and the peroxide initiators used for cross-linking are not environmentally friendly.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a foam having improved physical properties. A more particular object of the present invention is to provide a foamed article which is thermoplastic and which has excellent flexibility, resilience, and temperature resistance.
The flexible thermoplastic article of the present invention is formed from a blend comprising from about 2 to about 15 weight percent of a styrene-elastomer block copolymer; from about 60 to about 90 weight percent polyolefin; from about 4 to about 15 weight percent plasticizer oil, and optionally from about 0 to about 3 weight percent of at least one additive. The additives are typically present in a concentration of from 0.05 to about 3 weigh percent and can include those additives conventionally used in polyolefin foam compositions, including antioxidants and stabilizers, processing aids, etc. The styrene-elastomer block copolymer is preferably a tri-block copolymer structure which includes styrene end-blocks and a mid-block of a saturated olefin elastomer. A particularly suitable styrene elastomer block copolymer is a styrene-ethylene/butylene-styrene (SEBS) block copolymer. In one embodiment of the present invention, the styrene-ethylene/butylene-styrene block copolymer has a styrene/rubber ratio of from 30 to 70 with an Average Molecular Weight (M
n
) in the range of 50,000 to 300,000. The polyolefin is preferably predominately a polyethylene and desirably comprises from 85 to 100 weight percent polyethylene and from 0 to 15 weight percent polypropylene, more preferably from 95 to 100 weight percent polyethylene and from 0 to 5 weight percent polypropylene. Particularly suitable for inclusion in the foam composition of the present invention are those polyolefins which are produced with a single-site catalyst and are generally referred to in the industry as metallocene polyethylene. The polyolefin component can also include a blend of linear low density polyethylene and high density polyethylene. The plasticizer oil preferably has a molecular weight of from 200 to 1,000, and most desirably around 400.
In accordance with one aspect of the present invention, the foam article may be in the form of a hollow tubular pipe insulation. In another aspect, the foam article can serve as the dielectric in a coaxial cable.
In a further aspect of the present invention, there is provided a composition for producing a flexible thermoplastic foam. The composition consists essentially of a) from about 2 to about 15 percent by weight styrene-elastomer block copolymer; b) from about 60 to about 90 percent by weight polyolefin; c) from about 4 to about 15 percent by weight plasticizer oil;d) at least one blowing agent in an amount sufficient to produce a foam structure; and e) from about 0 to about 3 percent by weight of at least one additive. The blowing agent may be selected from the group consisting of fluorinated aliphatic hydrocarbons, aliphatic hydrocarbons, or chemical blowing agents. When the blowing agent is a chemical blowing agent, it may comprise at least one member selected from the group consisting of azodicarbonamide, sodium bicarbonate, sodium carbonate, ammonium carbonate, and citric acid.
DETAILED DESCRIPTION OF THE INVENTION
Flexible thermoplastic articles in accordance with the present invention are produced with conventional extrusion and foaming equipment. The ingredients are blended together thoroughly, typically in the barrel of an extruder, mixed with a chemical or physical blowing agent or combinations thereof, and extruded from a die of a suitable shape and configuration for the article which is being manufactured. Alternatively, the flexible foamed article can be produced by other processes, such as injection molding.
The styrene-elastomer block copolymer is preferably a tri-block copolymer structure which includes styrene end-blocks and a mid-block of a saturated olefin elastomer. Typically, in these tri-block copolymer structures, the saturated olefin elastomer mid-block may comprise butadiene, isoprene, ethylene-butylene, or ethylene-propylene. Particularly preferred for the present invention is a styrene-ethylene/butylene-styrene (SEBS) block polymer, and most desirably a SEBS block copolymer which is predominately a linear tri-block copolymer structure. The preferred SEBS block copolymer has a styrene to rubber (elastomer) ratio of from 30 to 70 with an Average Molecular Weight (M
n
) in the range of 50,000 to 300,000, most desirably about 200,000.
Linear tri-block SEBS copolymers of this type are manufactured and sold by Kraton Polymers as the KRATON G series of thermoplastic rubber and by Kuraray America, Inc. as the SEPTON® series of thermoplastic rubbers. Preferably, the block copolymer has a styrene content of from 25 to 40 weight percent, and most desirably from about 30 to 35 weight percent; a specific gravity of 0.89 to 0.93 percent, most desirably about 0.90 to 0.92 percent. The preferred SEBS block copolymer does not melt, and therefore cannot be characterized by its melt viscosity or melt index. However, when blended with polyolefin and plasticizer oil, the block copolymer is melt processable.
The polyolefins which are useful in the present invention are homo or copolymers of olefins such as polyethylene, polypropylene, polybutylene, etc. and blends thereof. The polyolefin used herein is preferably predominately a polyethylene homopolymer or copolymer, preferably a low density polyethylene homopolymer and preferably comprises from 85 to 100 percent by weight of the polyethylene, which may be blended with up to 15 weight percent of other polyolefins, such as polypropylene, most preferably from 0 to 5 weight percent polypropylene. A particularly preferred polyethylene is a metallocene polyethylene. Suitable commercially available polyethylene resins include the Affinity series of resins from Dow Chemical Company.
The polyethylene may be blended with small amounts of other polyolefins, such as polypropylene up to about 15 percent by weight of the total polyolefin composition. The addition of a small amount of a polypropylene can be used to alter the melt flow rate of the final composition. The addition of polypropylene also improves the thermal and weather resistance of the foam composition. Preferably, the blend has a melt flow rate (measured at 190° C. with a 2.56 kg weight) of from about 5 to 35, more preferably from about 5 to 12.
The plasticizer oil may suitably be a mineral oil or other paraffinic hydrocarbon oil, preferably one having a molecular weight from about 200 to 1,000.
The foamable composition may also contain additives of the type and at the concentration conventionally used in the manufacture of polyolef
Alston & Bird LLP
Foelak Morton
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