GB2063277A - Composition for Production of Cold Drawn Film - Google Patents

Abstract

A polymer composition useful for production of a stretched, shrinkable wrapping film by cold-drawing comprises a homogeneous blend of: (a) at least one polymer selected from low-density polyethylene and copolymers of ethylene with vinyl ester monomers, unsaturated aliphatic monocarboxylic acids and alkyl esters of said acids which are all copolymerizable with ethylene, and (b) an elastomeric random copolymer of ethylene with propylene or butene-1 having a density of not more than 0.91 g/cm<3>. h

Description

SPECIFICATION Composition for Production of Cold Drawn Film In our copending patent application no.

39283/78 we describe and claim a cold drawn film having a tensile strength of not less than 5.0 kg/mm2 and a haze of not more than 4.0%, which film comprises a homogeneous blend of components (A), (B) and (C) in the combinations (A)+(B), (B)+(C) or (A)+(B)+(C), wherein (A) is at least one polymer selected from low-density polyethylene and copolymers of ethylene with vinyl ester monomers, unsaturated aliphatic monocarboxylic acids and alkyl esters of said carboxylic acids which are all copolymerizable with ethylene, (B) is an ethylene-aolefin copolymer elastomer having a density of not more than 0.91 g/cm3, and (C) is at least one polymer selected from crystalline polypropylene, high-density polyethylene or crystalline polybutene-1.

The said copending application also describes and claims a method for producing the above cold drawn film. The present invention provides certain compositions which may be used to produce the film which is the subject of patent application no.

39283/78.

The present invention provides a composition comprising a homogeneous blend of components (a)+(b); wherein (a) is at least one polymer selected from lowdensity polyethylene and copolymers of ethylene with vinyl ester monomers, unsaturated aliphatic monocarboxylic acids and alkyl esters of said acids which are all copolymerizable with ethylene, and (b) is an elastomeric random copolymer of ethylene with propylene or butene-1 having a density of not more than 0.91 g/cm3.

The manufacture of a cold drawn film from this composition is described in detail in patent application no. 39283/78.

The polymer (a) to be used in the composition of the present invention is selected from LDPE (low density polyethylene) and copolymers of ethylene with vinyl ester monomers, unsaturated aliphatic monocarboxylic acids and alkyl esters of said monocarboxylic acids. LDPE possesses a density of not more than 0.935 g/cm3, preferably not more than 0.925 g/cm3, and a melt index [determined in accordance with ASTM D-1238 (1900C)] of from 0.2 to 10, preferably from 0.1 to 5. Examples of the copolymers satisfying the requirement include EVA (ethylene-vinyl acetate copolymer), ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethyleneethyl acrylate copolymer, ethylene-methyl methacrylate copolymer, and the like.In any of these copolymers, the amount of a monomer contained besides ethylene is desired to be in the range of from 3 to 30% by weight, preferably from 3 to 25% by weight.

If the amount of the monomer is less than 3% by weight, the cold stretchability of a film produced from the composition is somewhat inferior when the stretching is carried out on the film in its uncrosslinked form. Also the film's strength, thermal shrinkability and sealing property are not sufficient. If the amount exceeds 30% by weight, the tubular sheet has inferior processibility and the opposed surfaces of film undergo the phenomenon of mutual blocking to render the film handling difficult, and the mixing property of the components to make up the composition, the strength of film and the optical properties, etc. are impaired. As described above, the melt index of the polymer is in the range of from 0.2 to 10, preferably from 0.3 to 5.If the melt index is less than 0.2, the mixing property of the components of the composition and the extrudability of the resulting blend are not satisfactory. If the melt index exceeds 10, the blend fails to provide sufficient strength and the extruded sheet tends to sustain rupture readily at the time of stretching. These drawbacks are suffered also when the sheet is subjected to the treatment for crosslinking.

Of the various possible copolymers usable as the component (a) in the composition of this invention, the most desirable is ethylene-vinyl acetate copolymer. In the case of a low-density polyethylene, the blend is desired to be subjected to a treatment for crosslinking.

The component (b) is an elastomer and a random copolymer of ethylene with propylene or butene-1, and this copolymer may, when necessary, incorporate therein a small amount of a compound of the diene structure in the form of a copolymer. It is typically a thermoplastic elastomer in the form of a random copolymer obtained by the polymerization using a catalyst of the system produced with a vanadium compound and an organic aluminum compound. The elastomer possesses a melt index of from 0.1 to 10, preferably from 0.2 to 6.

As occasion demands, this elastomer may be further copolymerized with a small amount of a hydrocarbon of the polyene structure such as, for example, 1 4-hexadiene, ethylidene norbornene, etc. In any of these copolymers, the ethylene content is desired to fall in the range of from 20 to 90 mol%, more desirably from 40 to 90 mol%, preferably from 65 to 88 mol%.

These copolymers are of such nature that the density is not more than 0.91 g/cm3, the Vicat softening point as determined in accordance with ASTM D-1 525 (value under 1 kg of load) is not more than 800 C, preferably not more than 700C and the crystallinity in the rubbery zone generally ranges from substantial amorphousness to low partial crystallinity of the order of not more than 30% of crystallinity degree determined with Xray.

The composition of the present invention comprises a combination of components (a)+(b).

The mixing ratio of these components in terms of weight ratio is such as to satisfy 0.05~b,/(a+b)~0.90 and more desirably to satisfy: 0.07 < b/(a+b)~0.70 and preferably to satisfy: 0.10~b/(a+b} < 0.50 If the amount of the non-rigid component (b) is less than the allowable lower limit indicated above, the blend fails to manifest the expected synergistic effect and, therefore, suffers from inferior processability, lowered film strength and impaired optical properties and low-temperature shrinkability. if the amount is more than the allowable upper limit, a tubular sheet produced from the blend is degraded in film-forming property and stretchability and becomes so soft as to entail the phenomenon of film-to-film blocking and the produced film exhibits insufficient heat resistance, sealability, strength and optical properties.

As described above, this invention permits the quenched tubular sheet which has been produced from the composition obtained by using the specific components in their respective specific amounts to be cold stretched with ample stability in the manner to be described hereinafter. If the tubular sheet is further treated with high-energy radiation so as to have the gel content (insolubles in boiling xylene) or the melt index brought into a specific range, the components in the composition produce a synergistic effect such as to manifest the desired cold stretchability (at temperatures in the range of from 20 to 1 000C) under specific stretching conditions, giving rise to a film of outstanding properties.

EVA when treated with high-energy radiation undergoes the reaction of crosslinking more easily than the ordinary low-density polyethylene. The elastomer of the copolymer (b) exhibits high compatibility with EVA and induces the reaction of crosslinking as readily as EVA. Consequently, the synergistic effect brought about from the proper dispersion of the components in the composition is coupled with the synergistic effect which issues from the action of the high-energy radiation. The combination of these synergistic effects is believed to result in the production of a film wherein there is formed a specific, molecularly heterogeneous crosslinked matrix.

The treatment with high-energy radiation, accordingly, improves notably the stable cold stretchability of the tubular sheet and the film's heat resistance and heat seal strength, enhances the thermal shrinkability and strength of the film at low temperatures, represses possible degradation in optical properties and physcial properties after thermal shrinkage (such as optical properties, seal strength and mechanical strength) and expands the range of packaging temperatures. Thus, the properties possessed by the film which is produced from the crosslinked tubular sheet far excei those possessed by the plasticized PVC film and PP film which have heretofore been rated to be the best films.

The composition of this invention may be effectively used when it is mixed with some other composition insofar as the amount of the additive composition does not impair the stretchability and various other properties of a film which is produced from the composition.

The compositions of this invention are illustrated in the following Examples.

Example I 20 parts by weight of an ethylene--olefin copolymer elastomer (melt index: 0.25, Vicat softening point: less than 500C, density 0.88 g/cm3, a-olefin: butene-1, olefin unit content 20 mol%) were mixed with 80 parts by weight of EVA (vinyl acetate unit content: 15% by weight, melt index: 0.7). The composition was extruded, at the maximum temperature of the cylinder part of 2500C, from an annular die 150 mm in diameter having a slit of 1.5 mm provided with a mixing head type screw 65 mm in diameter and a ratio (yod) of 37. The extruded film was quenched with water about 10 cm from the lip of the die. A raw tubular film 200 in thickness and with a deviation in thickness of +2.0% was obtained.

The production of a stretched film is described in Example 4 (Run No. 23) of GB 2007685.

Example 2 15 parts by weight of ethylene-a-olefin copolymer elastomer (a-olefin: propylene, said copolymer contains 15 5 mol % propylene and 4% by weight ethylidene norbornene, melt index: 0.45, density: 0.88 g/cm3, Vicat softening point: not more than 400C) were mixed with 85 parts by weight of EVA (vinyl acetate unit content: 10% by weight, melt index: 1.0).

The composition was plasticized and kneaded, at the maximum temperature of the cylinder part of 2600 C, by a mixing head type screw 45 mm in diameter (yod=44), and pelletized.

The composition was extruded through an extruded 45 mm in diameter (L/D=37) fitted with a T-type die which had a slit 1 mm in thickness and 40 cm in width. While extruding a liquid additive was injected into the rear part of the cylinder under pressure. The melted polymer composition extruded from the die was introduced into water to form a raw tubular film 100 in thickness.

The raw film was treated with 7 Mrad of highenergy radiation so that it had a gel content of 33% and a melt index of less than 0.05.

The production of a stretched film is described in Example 10 (Run No. 28) of GB 2007685.

Claims (12)

Claims

1. A composition comprising a homogeneous blend of components (a)+(b); wherein (a) is at least one polymer selected from lowdensity polyethylene and copolymers of ethylene with vinyl ester monomers, unsaturated aliphatic monocarboxylic acids and alkyl esters of said acids which are all copolymerizable with ethylene, and (b) is a random copolymer elastomer of ethylene with propylene or butene-1 having a density of not more than 0.91 g/cm3.

2. A composition according to Claim 1, wherein the amounts of the components are such as to satisfy in terms of weight ratio the expression O.90~b/(a+b)~0.05

3. A composition according to Claim 1 or 2, wherein the component (a) is at least one copolymer selected from ethylene-vinyl acetate, ethylene-acrylic acid, ethylene-methacrylic acid, ethylene-ethyl acrylate, and ethylene-methyl methacrylate copolymers.

4. A composition according to Claim 1 or 2, wherein the component (a) is an ethylene-vinyl acetate copolymer.

5. A composition according to Claim 4, wherein the ethylene-vinyl acetate copolymer has a vinyl acetate unit content in the range of from 3 to 30% by weight.

6. A composition according to any one of Claims 1 to 5, wherein the ethylene-a-olefin copolymer as the component (b) has an ethylene unit content of not more than 90 mol % and not less than 20 mol%.

7. A composition according to any one of Claims 1 to 5, wherein the ethylene-a-olefin copolymer as the component (b) has an ethylene unit content of not more than 90 mol% and not less than 40 mol%.

8. A composition according to any one of Claims 1 to 7, wherein the component (b) has copolymerized polyene units in addition to the main components derived from ethylene and aolefin.

9. A composition according to Claim 8, wherein the polyene contains not more than 5 mol % of a non-conjugate diene selected from hexadiene and norbornene derivatives.

10. A composition according to any one of Claims 1 to 9, wherein the components are in amounts such as to satisfy the expressions 0.70~b/(a+b)~0.07 in terms of weight ratio.

11. A composition according to Claim 1 substantially as described in either of the Examples.

12. A composition according to any one of Claims 1 to 11 in the form of an unstretched film.