FILM BLENDS OF A POLYETHYLENE AND A POLYSTYRENE

Abstract: A film comprising a uniform blend of a major proportion of a polyethylene and a minor proportion of a polystyrene as a deadfold food wrap material that has paper-tear charac­ teristics.

Description: FILM BLEND6 OF A POLYETHXLENE AND A POLYSTYRENE Most thermoplastic film compositions are characterized by having a resistance to folding or creasing at normal ambient temperatures. Two materials that are commonly employed in the wrapping of comestibles, such as candy, are glassine paper and waxed paper. Glassine paper is known as a thin dense paper, supposedly highly resistant to the passage of air and oils. However, in terms of today's present technology, glassine paper is not as good a barrier to air and oleaginous material as the food industry would like to see. In addition glassine paper is made by a comparatively expensive process. Waxed paper also is a comparatively expensive material that does have limited use in the food wrapping industry, particularly with the advent of microwave ovens. However, waxed paper is heat sensitive and does not provide an adequate barrier to air. It would be highly desirable to have an inexpensive plastic film substitute for glassine paper and waxed paper which has some of the desirable physical characteristics of these materials but to a greater degree. This invention provides a film comprising a uniform blend of a major proportion of a polyethylene and a minor proportion of a polystyrene. It is preferred that the polyethylene be a member selected from the group consisting of a high density polyethylene, a low density polyethylene, copolymers of ethylene and a C3-C10 alpha-olefin and any mixture thereof. It is also preferred that the. polystyrene be a member of the group consisting of general purpose crystal polystyrene, poly(p-methylstyrene) and mixtures thereof. It is preferred that 65-90% by weight of the blend be polyethylene and from 10-35% by weight be polystyrene. The above percentages are based on the total weight of the film. The term "polyethylene" is employed generically and relates to all polyethylenes which, when blended with the defined polystyrene and formed into a film, will yield a film having the above-identified paper-like qualities of being deadfoldable and having paper-like tear characteristics. The term "dead-fold" refers to that property which will permit the film to be folded upon itself, creased with light pressure, and have the film retain an extremely acute angle at the fold point. Having the "paper-like tear" characteristics means that the film can be torn fairly easily, in any direction, without the impediment of any film orientation. In contrast thereto polymer films not having dead foldable characteristics, such as polyethylene, tend to revert to the unfolded condition. A rounding at the creaseline progresses after the crease pressure is terminated. The polyethylenes contemplated herein include low density polyethylene, commonly known as LDPE, high density polyethylene, commonly known as HDPE, and copolymers of ethylene and a C3-0 alpha-olefin commonly known as LLDPE. As employed herein LDPE relates to polyethylene that has a density up to 0.939, preferably 0.910 to 0.925 grams/cm3 . The HDPE contemplated herein will have a density ranging from 0.940 to 0.970 and higher. It is to be understood that both the LDPE and HDPE can be homopolymers or copolymers so long as the general characteristics of polyethylene are retained. This usually will permit the polymerization of ethylene with up to 5 wt. % of other canonomers, such as vinyl acetate. These modified polyethylenes, as they are called, are not so strongly influenced by the comonomer that they lose their polyethylene characteristics. The LLDPE is a special form of a copolymer of ethylene and an alpha olefin such as butene-l, hexene-l, or octene-l. These linear low density ethylene copolymers are commercially available materials having a density range of 0.918-0.938 gm/cm and are manufactured by low pressure processes employing organometallic catalysts. These materials usually contain from 1 to 5 mole percent of C3-C10 alpha-olefin hydrocarbon copolymerized with ethylene. Manufacturing processes for linear low density polyethylenes are disclosed in U.S. Patent Nos. 4,076,698 and 4,205,021. The polystyrene contemplated, as indicated above, is crystal polystyrene, poly(p-methylstyrene) and mixtures thereof. Crystal polystyrene is a clear amphorous polymer which exhibits high stiffness and good dimensional stability. The commercial grades of crystal polystyrene offer a wide range in melt flow index with high heat, high molecular weight grades of around 2g/10 minutes (ASTM Condition G) and other grades as high as 20g/10 minutes. The poly(p-methylstyrene) (PPMS) preferably contains less than 40% meta-methylstyrene and is substantially free of ortho-methylstyrene. The preferred PPMS is more particularly defined in U.S. Patent No. 4,347,344. Set forth below is an exemplary method of forming a blown tubular film of the present invention. Prior to extrusion, the components of the desired blend are mixed to form a substantially homogeneous resin mixture. This may be accomplished, for example, by masticating the components in a roll mill or in a similar polymer blending apparatus such as a Banbury mill. The film is then extruded into a film using a standard tubular extruder. Conventional additives such as anti-oxidants, coloring pigments and anti-static agents may be incorporated in minor amounts, such as less than 2% by weight, in the blend during or after the blending operation. Films of various thicknesses may be manufactured utilizing the novel resin compositions of the present invention. The gauge may generally vary from 0.1 mil up to 10 mils and preferably from 0.5 mil to 2 mils. In the following examples, films having nominal thicknesses of 1.5 mils were prepared. The extrusion system included a 5.1 cm (2 inch) circular die having a 50 mil gap. An approximately 2:1 blow up ratio was employed to yield a film tube having a 15.9 cm (6 1/4 inch) lay flat dimension. The LLDPE employed is identified as GP-2 and is a copolymer of ethylene with about 4 mole percent butene-l. It has a melt flow index of 9 grams /10 minutes and a density of 0.920 gm/cm3 . The poly (p-methylstyrene), POMS, employed had a melt flow index of 9 grams/l0 minutes. The polystyrene, PS 1240, employed had a melt flow index of 9 grams/l0 minutes. The following table illustrates examples of film blends within the scope of the invention. TABLE A-1 A-2 A-3 A-4 A-5 B-1 B-2 B-3 B-4 GP-2 100 90 85 80 70 90 85 80 70 PPMS -- 10 15 20 30 -- -- -- - PS 1240 -- -- -- -- -- 10 15 20 30 1% Sec. Modulus MD 2.48 3.39 -- -- 10.3 6.32 -- -- 13.6 x 10-4 psi TD 2.62 2.97 -- -- 4.2 2.73 -- -- 5.37 Break Strength MD 5.18 2.43 -- -- 3.45 2.29 -- -- 4.14 psi x 10-3 TD 3.91 1.76 -- -- 1.47 2.45 -- -- 1.95 Yield Strength MD 1.45 1.47 -- -- 3.17 2.28 -- -- 3.87 psi x 10-3 TD 1.42 1.22 -- -- 1.42 1.65 -- -- 1.79 %Elongation at MD 749 598 -- -- 59 541 -- -- 126 Break TD 847 586 -- -- 186 648 -- -- 543 Elmendorf Tear MD 74 109 98 50 (1) 103 112 58 (1) g/mil TD 269 158 72 22 (1) 141 25 23 (1) Spencer Impact Strength g/mil 135 252 214 244 (1) Less than 30 g/mil Employing LLDPE as representative of a polyethylene resin useful in formulating the subject blends, the data of the foregoing Table show physical characteristics which render the product an excellent substitute for glassine paper and waxed paper. Neither pigments nor fillers are necessary in order for the film to achieve its dead-fold and paper-tear characteristics. Further, no solvent treatment or surface modification is necessary to bring about these characteristics. The film of the present invention has a high degree of opacity and an attractive pearl essence appearance. As a food or candy wapper it easily retains a fold or a twist imparted during packaging.

Claims: CLAIMS 1. A film comprising a uniform blend of a major proportion of a polyethylene and a minor proportion of a polystyrene, the polyethylene being present in 65-90 parts by weight and the polystyrene being present in 10-35 parts by weight. 2. The film of Claim 1 wherein the polyethylene is a member selected from the group consisting of a high density polyethylene, a low density polyethylene, copolymers of ethylene and a C3-C10 alpha oleo in and any mixtures thereof; and the polystyrene is a member of the group consisting of crystal polystyrene, poly(p-methylstyrene) and mixtures thereof. 3. The film of Claims 1 or 2 wherein the polyethylene is a copolymer of ethylene and 1 to 5 mole percent of a C3-C10 alpha olef in. 4. The film of any preceding claim having a thickness of 0.01 to 10 mils.

Inventors: CANTERINO, PETER JOHN, SHAW, RICHARD GREGG
Applicants: MOBIL OIL CORPORATION
Application year: 1984
Main classification: C08L23
IPC codes: C08L23, C08L7, C08L25, C08L21, C08L101, C08J5