GB2060658A

GB2060658A - Composition for production of cold drawn film

Info

Publication number: GB2060658A
Application number: GB8100325A
Authority: GB
Original assignee: Asahi Dow Ltd
Current assignee: Asahi Dow Ltd
Priority date: 1979-05-10
Filing date: 1980-04-22
Publication date: 1981-05-07
Also published as: GB2060658B

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, (b) an ethylene- alpha -olefin copolymer elastomer having a density of not more than 0.91 g/cm<3> and (c) at least one polymer selected from crystalline polypropylene and crystalline polybutene-1, the amounts of the components being such as to satisfy in terms of weight ratio the expressions 0.90 >/= b (a+b) >/= 0.05 and 2.0 >/= c (a+b) >/= 0.05.

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-n-olefin 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) + (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 acids which are all copolymerizable with ethylene, (b) is an ethylene-z-olefin copolymer elastomer having a density of not more than 0.91 g/cm3 and (c) is at least one polymer selected from crystalline polypropylene'and crystalline polybutene-1, the amounts of the components being such as to satisfy in terms of weight ratio the expressions 0.90 > b/(a + b) > 0.05 and 2.0 > c/(a + b) > 0.05.

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

The polymer (o) 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)]ffrom 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, ethylene-ethyl acrylate copolymer, ethylenemethyl 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 seaiing 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 thermoplastic elastomer comprising a copolymer of ethylene with at least one a-olefin as the component (b) is a non-rigid copolymer of ethylene with at least one ct-olefin selected from a-olefins having from 3 to 1 2 carbon atoms. 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. Examples of the a-olefin suitable for this purpose include propylene, butene-1, hexene-1, heptene-1, 4-methyl-i -pentene, octene-1, etc. Of these a-olefins, preferable are propylene and butene-1.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 moi%, 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 X-ray.

The component (b) is particularly desired to be a copolymer of ethylene with propylene or butene1, 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, therefore, 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.

The polymer (c) is at least one selected from crystalline PP (polypropylene) and high molecular PB1 (polybutene-1) which each possess relatively high rigidity and relatively high degree of crystallinity.

The polymer has relatively high rigidity and desirably a Vicat softening point of not less than 1 000C.

The crystalline PP which is one of the members of the group from which the component (c) of the cornposition of this invention is selected is a crystalline PP with high isotacticity usually available on the market. It is desired to be a homo-polymer of propylene or any of the copolymers of propylene with not more than 10 mol% of ethylene, 1 -butene or some other a-olefin. It may be a mixture of these copolymers.

The polybutene-1 is desired to be a crystalline homo-polymer of more than 90 mol% of butene-1 with other monomer. Unlike a liquid-to-waxy low molecular polymer, these polymers are desired to possess a melt index in the range of from 0.2 to 10 for the same reason as mentioned above. Of the possible members of the aforementioned group, it is desirable to use chiefly the crystalline polypropylene.

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

The mixing ratio of these components in terms of weight ratio is such as to satisfy 0.05 I b/(a + b) < 0.90 and 0.05 < = c/(a + b) < = 2.0, and more desirably to satisfy: 0.07 < b/(a + b) < 0.70 and 0.07 < c/(a + b) < 1.0, and preferably to satisfy: 0.10 < b/(a + b) 6 0.50 and 0.10 c/(a + b) 1.0.

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 processibility, 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.

In the combination of (a) + (b) + (c), if the amount of the resin of the component (c) used in thecomposition is smaller than 5 parts by weight, the blend exhibits insufficient stretchabiiity and the extruded sheet tends to sustain punctures and regain its original dimension and does not easily produce a film of uniform thickness possessing the outstanding properties mentioned above and tends to give inferior finish to the package. The film, when produced in a particularly small thickness, fails to provide sufficient modulus. Consequently, the film has poor dimensional stability and, therefore, tends to undergo deterioration by aging similarly to the plasticized PVC film, with the result that the heat resistance, heat seal strength, heat seal temperature range and finish of package are all adversely affected.

If the amount is greater than 200 parts by weight, the blend exhibits inferior stretchability and tends to sustain puncture and the film suffers from insufficient optical properties, uniformity of wall thickness and low-temperature shrinkability. The component (c) serves to improve not only modulus but also seal properties such as, for example, thermal properties including heat resistance particularly in the higher portion of the allowable temperature range.

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 a specific high-energy ray 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 100 C) under specific stretching conditions, giving rise to a film of outstanding properties.

Now, the composition produced by using the components in the preferred mixing ratio will be described. Generally, the crystalline PP component (c) is hardly crosslinked even when it is subjected to a treatment with a high-energy ray and it offers rather insufficient compatibility with EVA to be used as another component (a).

In contrast, EVA when treated with a high-energy ray undergoes the reaction of crosslinking more easily than the ordinary low-density polyethylene. The elastomer of the copolymer of a-olefin (b) exhibits rather high compatibility with both polypropylene and EVA and induces the reaction of crosslinking as readily as EVA. Consequently, the synergistic effect brought about from the proper dispersion of the three components in the composition is coupled with the synergistic effect which issues from the action of the high-energy ray. 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 physical 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 excel those possessed by the plasticized PVC film and PP film which have heretofore been rated to be the best films.

In the present invention, 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.

Compositions in accordance with this invention are illustrated in the following Examples.

EXAMPLE 1 To 100 weight parts of a composition composed of 82% by weight of EVA (a) (vinyl acetate unit content 10% by weight, melt index 1.0) and 18% by weight of ethylene-a-olefin copolymer (bl) (a- olefin is propylene, said copolymer comprises 1 5 mol% of propylene and 4% by weight of ethylidene norbornene) having a melt index of 0.45, a Vicat softening point of not more than 400C and a density of 0.88 g/cm3, were mixed 1 8 weight parts of a crystalline PP (c1) having a melt index of 1.0, density of 0.88 g/cm3 and Vicat softening point of 1 460C. The composition was extruded, at a maximum temperature of the cylinder part of 2500C, from an annular die 1 50 mm in diameter having a slit of 1.5 mm provided with a mixing head type screw 65 mm in diameter and a ratio (L/D) of 37. The extruded product was quenched at about 10 cm distance from the lip of the die by water. Thus, there was obtained a raw tubular film 1 00 mm in diameter, 200 u in thickness, deviation of + 1.8% in thickness.

The said composition had a Vicat softening point of 750C. When the obtained raw tubular film was treated with an electron beam of 500 KV energy at a dose of 5 M rad at normal temperature a gel% of 3% by weight in boiling xylene and a melt index of 0.07 was obtained.

The working up of a cold drawn film from the irradiated and non-irradiated raw film is described in Example 1 of GB 2007685.

EXAMPLE 2 EVA (a2) (vinyl acetate unit content; 15% by weight, melt index; 0.7) and ethylene-a-olefin copolymer elastomer (b2) (a-olefin is propylene and 20 mol% of a-olefin unit are present, melt index: 0.25, density: 0.88 g/cm3, and Vicat softening point: less than 400 C) and crystalline PP (c2) (ethylene unit content; 5% by weight, melt index: 0.6, Vicat softening point: 1200 C, density: 0.87 g/cm3) were mixed in the ratios shown in Table 1 and extruded as in Example 1 to obtain a raw tubular film (1 50 L1 in thickness and deviation of + 1.8% in thickness).

TABLE 1

Composition 3 | 4 5 6 7 8 a2 90 70 50 80 60 40 b2 10 30 50 20 40 60 C2 10 30 30 70 100 70 Vicat softening point of the mixed composition (C) 1 64 68 62 88 93 84 (The amounts of the components are in parts by weight) The production of cold drawn film from the raw film is described in Example 2 of GB 2007685.

EXAMPLE 3 EVA (a3) (melt index: 0.7, vinyl acetate unit content: 13 weight%), ethylene-a-olefin copolymer elastomer (b3) (melt index: 0.25, Vicat softening point: less than 500 C, density: 0.88 g/cm3, cr-olefin is butene-1 and a-olefin unit content: 20 mol%) and crystalline PP (c2) (mentioned above) were extruded in the mixing ratios and the conditions shown in Table 2 as described in Example 1.

TABLE 2

Composition 9 10 11 12 13 14 a3 90 70 50 80 60 40 b3 10 30 50 20 40 60 C2 10 30 30 70 100 70 Dose (Mrad) 3 5 10 7.5 7.5 5 Gel (%) 1.5 2.5 20 12 8 1.0 Ml 0.1 007 less 0.08 O.OS 0.15 than 0.05 (The amounts of the components are in parts by weight) The production of stretched film is described in Example of GB 2007685.

EXAMPLE 4 Raw tubular films 200,z in thickness and having a deviation of +2.0% in thickness were manufactured using the polymer compositions and mixing ratios shown in Table 3 according to the process described in Example 1. The production of stretched film is described in Example 4 of GB 2007685.

(parts by weight)

15 17 19 20 21 a2 EVA 80 75 80 a3 EVA 80 a4 EVA 80 (melt index: 1.5, vinyl acetate unit content: 20% by weight) b3 Ethylene-α-olefin elastomer 20 20 20 25 b4 Ethylene-α-olefin copolymer (EPM) 20 (a-olefin is propylene, a-olefin unit content: 42 mol %, density: 0.87 g/cm3, Vicat softening point: less than 30C) Crystalline pp 18 30 10 25 25 (Melt index: 0.8, ethylene unit content: 6% by weight) C3 Polybutene-1 30 (Melt index: 0.4, density: 0.913 g/cm3) c6 Polybutene-1 70 (Melt index: 1.0, density: 0.908 g/cm3, mod. ethylene unit content: 8% by weight) Dose (Mrad) 5 5 Gel /O 4 == 5 M I 0.08 0009

EXAMPLE 5 70 weight parts of LDPE (aS) (melt index; 0.3, density; 0.917 g/cm3), 30 weight parts of ethylene a-olefine copolymer (b1) (referred to above) and 1 5 weight parts of crystalline pp (c,) were extruded according to the process of Example 1 to obtain a raw tubular film. As this film was apt to break in stretching, the film was cross-linked by high energy radiation of 10 Mrad so as to have a melt index of less than 0.05 and contain about 40% by weight of gel.

Production of stretched film is described in Example 6 of GB 2007685.

EXAMPLE 6 Raw films were obtained as in Example 1, using ethylene-ethyl acrylate copolymer (a6) (ethyl acrylate unit content: 10 weight %, melt index: 2.5) or ethylene-methyl methacrylate copolymer (a7) (methyl methacrylate unit content: 1 5 weight %, melt index: 2.0) instead of EVA. The gel% was 12 and 1 5 weight % respectively by irradiating with high energy ray of 7.5 Mrad.

The production of stretched film is described in Example 7 of GB 2007685.

EXAMPLE 7 Compositions Nos. 27 and 30 composed of the components as shown in Table 4 were plasticized and kneaded, at the maximum temperature of the cylinder part of 2600C, by a mixing head type screw -45 mm in diameter (L/D=44) and pelletized. Composition No. 31 was manufactured by kneading PP (c2) and EPM (b4) in a Banbury mixer to make a master batch which was diluted with component (a,). The compositions were extruded through an extruder-45 mm in diameter (L/D-37) fitted with 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 cylinder under pressure. The melted polymer compositions extruded from the die were introduced into water to form raw tubular films 100 y in thickness. The raw films Nos. 30. 31 were treated with high energy radiation.

The production of stretched film is described in Example 10 of GB 2007685.

TABLE 4

Composition 27 30 31 a1 85 80 85 b4 15 b, 15 20 . Cr C2 - 20 80 20 Dose (Mrad) 10 5 Gel % 8 20 Ml I 0.07 less than 0.05

Claims

1. A composition comprising a homogeneous blend of components (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 acids which are all copolymerizable with ethylene, (b) is an ethylene-c-olefin copolymer elastomer havng a density of not more than 0.91 g/cm3 and (c) is at least one polymer selected from crystalline polypropylene and crystalline polybutene-1.

the amounts of the components being such as to satisfy in terms of weight ratio the expressions 0.90 > b/(a + b) > 0.05 and 2.0 > c/(a + b) > 0.05.

2. A composition according to Claim 1 , 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.

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

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

5. A composition according to any one of Claims 1 to 4, 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%.

6. A composition according to any one of Claims 1 to 4, 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 mop%.

7. A composition according to any one of Claims 1 to 6, wherein the elastomer as the component (b) is a non-rigid copolymer having a Vicat softening point of not more than 80"C and a degree of crystallization of not more than 30%, wherein the a-olefin component contained therein is at least one cr-olefin having from 3 to 12 carbon atoms.

8. A composition according to any one of Claims 1 to 7, wherein the elastomer as the component (b) is a random copolymer wherein the a-olefin component is propylene or butene-1.

9. A composition according to any one of Claims 1 to 8, wherein the ethylene-a-olefin copolymer as the component (b) has copolymerized polyene units in addition to the main components derived from ethylene and a-olefin.

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

11. A composition according to any one of Claims 1 to 10, wherein the component (c) is a rigid polymer having a Vicat softening point of not less than 100 C.

12. A composition according to any one of Claims 1 to 11, wherein the component (c) is a crystalline polypropylene or a crystalline copolymer of propylene with ethylene in an amount of not more than 10 mol% or a mixture thereof.

13. A composition according to any one of Claims 1 to 12, wherein the components are in amounts such as to satisfy the expressions 0.70 > = b/(a + b) > 0.07 and 1.0 1 c/(a + b) > 0.10 in terms of weight ratio.

14. A composition according to Claim 1 substantially as described in any one of the Examples.

15. A composition according to any one of Claims 1 to 14 in the form of an unstretched film.