Polymeric films

Abstract: Polymeric films, especially for packaging, consist of a relatively thick layer of a polyolefin having thereon a relatively thin layer of a blend of a polymer which has a low heat seal strength to polyvinylidene chloride and a copolymer containing units derived from an alkene and an unsaturated mono-basic carboxylic acid or an ester thereof. The relatively thick layer is preferably of polypropylene, and the relatively thin layer is preferably a blend of polyethylene with a copolymer containing at least 74 percent by weight of units derived from ethylene and not more than 36 percent by weight of units derived from acrylic acid or an ester thereof. The relatively thin layer has good anti-block with the relatively thick layer and to other layers such as PVdC applied thereto, and the relatively thin layers have good heat seal strengths to themselves and such other layers.

Description: This invention concerns polymeric films, for example for use in packaging. It is known to impart various surface characteristics to polymeric films depending on the intended end use of the films. Thus films used in high speed packaging machinery are often treated with anti-block agent to prevent them blocking to themselves during the packaging process. Heat-sealable surfaces are provided to enable polymeric films to be heat sealed to themselves to form packages, and various coatings are provided to impart printability or to aid lamination to other layers, for example using overlacquers of polyvinylidene chloride (PVdC). In practice, anti-block agents tend to reduce the efficiency for example of heat seal layers. EP-A-0065898 describes films with good interply bond strength consisting of a first layer consisting of a terpolymer of ethylene, an alkyl acrylate and maleic anhydride, bonded to second layer consisting of low density polyethylene, polypropylene, polybutene, a polyamide, polystyrene or polyvinyl chloride. EP-A-0269325 describes multilayer films for the cook-in of food products. The films concerned include an outer layer of a polyamide which can optionally be blended with polyethylene or copolymers of alpha-olefins with alpha, beta-ethylenically unsaturated carboxylic acids, the preferred amounts of polyamide in such blends being from 60 to 100%. According to the present invention there is provided a polyolefin film comprising a relatively thick layer of a polyolefin having thereon a relatively thin layer compared with the thickness of the relatively thick layer of a blend of a copolyamide which has a low heat seal strength to polyvinylidene chloride, said heat seal strength being less than 50g/25cm as measured at ambient temperature after contact for 1 second at 115°C under a load of 9.5x10⁶Pa, and a copolymer containing units derived from an alkene and an unsaturated mono-basic carboxylic acid or ester thereof, the copolyamide being present in the relatively thin layer in an amount of from 5 to 30 weight percent of the relatively thin layer. The surface of the relatively thin layers of films of the present invention have shown good anti-block to the surfaces of the relatively thick layer and to other layers applied to the relatively thick layer, eg polyvinylidene chloride coatings, and also good heat seal strengths to themselves and to such coatings. the relatively thin layers also have shown good priming for polyvinylidene chloride. The copolymer of an alkene and an unsaturated mono-basic carboxylic acid or an ester thereof preferably contains units derived from at least one of ethylene, propylene and but-1-ene, optionally with a higher homologue thereof, eg hex-1-ene or oct-1-ene. The unsaturated mono-basic carboxylic acid is preferably acrylic acid, it being particularly preferred that the copolymer contains units derived from an acrylic acid ester. Especially preferred are ethylene/acrylic ester copolymers. The copolymer can consist of units derived from one or more alkenes and an unsaturated carboxylic acid or an ester thereof, or it can additionally contain units derived from at least one unsaturated dicarboxylic acid or acid anhydride, preferably maleic anhydride. Preferred copolymers for use in the present invention will usually contain at least 65 percent by weight of units derived from an alkene, eg ethylene, the amount of units derived from the unsaturated acid and/or ester thereof being not greater than 35 percent by weight of the copolymer. The amount of unsaturated acid and/or ester thereof is more preferably from 10 to 20, especially from 14 to 18, and advantageously about 17 percent by weight of the copolymer. Preferred terpolymers contain at least 65 percent by weight of units derived from an alkene, for example at least 74 percent by weight of units derived from ethylene, with the unsaturated acid and/or ester content being not greater than 29 percent of the terpolymer, and especially not more than 20 percent by weight of units derived from the unsaturated acid or an ester thereof. The amount of units derived from the dicarboxylic acid or acid anhydride is preferably not more than 6, more preferably from 2 to 4, and advantageously about 3, percent by weight of the terpolymer. The unsaturated acid ester can be one or more esters of the acid with a lower (C₁₋₄) alkanol, ie methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl or t-butyl esters. The typical heat seal strength copolyamides (31g/25cm) should be compared with the heat seal strength of over 350g/25cm for a typical ethylene/acrylic ester/maleic anhydride terpolymer which in accordance with the invention can be used in the layer of a blend having good seal strength and acceptable anti-block properties. The amount of copolyamide present in the blend forming the relatively thin layer is from 5 to 30 weigh percent of the relatively thin layer, and preferably from 5 to 15 weight percent. Polyolefin films of the present invention can consist of the two specified layers only. However, the relatively thick layer can have a second relatively thin layer thereon so that both sides of the relatively thick layer have a relatively thin layer thereon. If a second relatively thin layer of a blend is not present, the relatively thick layer of polyolefin, for example polypropylene, is preferably treated to improve print adhesion or to provide adhesion for a primer coating for a subsequently applied polyvinylidene chloride overlacquer. A preferred treatment is to provide the relatively thick layer of polyolefin with a layer of a propylene/ethylene copolymer, the propylene/ethylene copolymer preferably containing from 2 to 6, and more preferably about 4 percent of units derived from ethylene. This layer is preferably treated to provide print adhesion or adhesion for a primer coating for a subsequently applied PVdC overlacquer. The blend layer can also be treated to improve its sealability to PVdC. The relatively thick layer can be a clear polyolefin or it can be voided, for example by the inclusion of an organic or inorganic voiding agent in the polyolefin. The relatively thick layer will usually be from 15 to 70 micrometers thick. However, the relatively thin layer or layers of the blend or the optional propylene/ethylene copolymer will usually be less than 5 microns thick. When the relatively thick layer is voided and has only one relatively thin layer of the blend thereon, it is preferred that the relatively thick layer has a layer of clear olefin homopolymer thereon. For example, a relatively thick layer of voided propylene homopolymer preferably has a relatively thin layer of the blend on one side and a layer of non-voided propylene homopolymer on the other side. Such non-voided layers, for example from 1.5 to 5.5 micrometers thick, can provide films of the present invention with a particularly glossy appearance. Furthermore, it is possible to provide such non-voided layers with coatings, for example of an acrylic polymer. Films of the present invention are preferably produced by coextruding a layer of the polyolefin with a layer of the blend, and optionally with the other layers, as desired. Thereafter the extruded web of two or more polymer layers will usually be stretched biaxially to effect orientation, and when a voiding agent, eg an organic or inorganic particulate material, is present, to effect voiding of the layer containing the voiding agent. The stretching is conveniently effected sequentially, orientation being effected in the direction of extrusion, for example using heated rollers, and thereafter in the transverse direction using a stenter. This also allows coating materials to be applied to the film before the final orientation step, for example the acrylic coating applied to films having a glossy layer of non-voided homopolymer on a voided relatively thick layer referred to above. If desired, particulates can be added to the thin layer to provide the films with known properties, eg improved slip. Examples of materials which can be used include silica and aluminum silicate. After orientation, the films are preferably subjected to a surface treatment to improve print adhesion and/or heat sealability, for example using corona discharge or flame treatment. This treatment can be applied to either or both sides of the film, depending on the properties required for the film. It is, however, generally preferred to subject the blend to such a treatment. The following Examples are given by way of illustration only. All parts are by weight unless otherwise stated. Examples A series of polymeric films was produced by first coextruding a propylene homopolymer core layer with a layer of a propylene/ethylene copolymer (4% by weight of units derived from ethylene) on one side and a layer of various blends of polymers containing 1000ppm of 5 micrometres particle size silica on the other side, and then biaxially stretching the web. The webs were each stretched by 4.5:1 in the machine direction using hot rollers, and by 10:1 in the transverse direction using a stenter. The resulting films were corona discharge treated on both faces and then wound up. The films were 25 micrometres thick with a core 23.4 microns thick and outer layers each 0.8 micrometres thick. The heat seal strengths of each film were then measured for the surfaces of the films consisting of the blend of polymers, the seal strengths to PVdC and to themselves being measured at 115°C using a dwell time of 1 second under a load of 9.5 x 10⁶ Pa. The results are given in the accompanying Table in units of g/25cm. Anti-block was evaluated for each film by contacting the polymer blend surface of 6.3 cm samples of the film with a PVdC layer on a sample of a polymer film of the same size, and between paper sheets of the same size, the film sample being heated for 6 hours at 42°C while under a load of 9.5 kg. After cooling the samples for 10 minutes, anti-block was evaluated on the following scale by the ease with which the two samples of film could be separated:- 1. no blocking - samples fall apart from each other; 2. light pressure required to separate the samples; 3. medium pressure required to separate the samples; 4. heavy pressure required to separate the samples; 5. samples inseparable by touch. Acceptable antiblock is considered to be not more than 3 - 4, i.e. medium to high pressure is required to separate the samples. Example Tested Polymer Surface Self Heat Seal PVdC Heat Seal Antiblock 1 90% Terpolymer A/10% Copolyamide C 280 220 2-3 - HDPE (density 0.952g/cm³) 2 - LDPE (density 0.93g/cm³) 5 - LLDPE (density 0.924g/cm³) 6 - Copolyamide C 31 - Polypropylene D 2 - Terpolymer A 350+ - Copolymer B 350+ Notes Terpolymer A - 91% ethylene, 6% ethylene acrylate, 3% maleic anhydride Copolymer B - 83% ethylene, 17% ethyl acetate Copolyamide C - Density 1.09g/cm³, melting point 160°C Polypropylene D - Melt flow index of 3 at 230°C under ASTM D1238/73 (condition L)

Claims: A polyolefin film comprising a relatively thick layer of a polyolefin having thereon a relatively thin layer compared with the thickness of the relatively thick layer of a blend of a copolyamide which has a low heat seal strength to polyvinylidene chloride, said heat seal strength being less than 50g/25cm as measured at ambient temperature after contact for 1 second at 115°C under a load of 9.5x10⁶Pa, and a copolymer containing units derived from an alkene and an unsaturated mono-basic carboxylic acid or ester thereof, the copolyamide being present in the relatively thin layer in an amount of from 5 to 30 weight percent of the relatively thin layer. A film according to claim 1, wherein the said alkene is ethylene, propylene or but-1-ene. A film according to claim 1 or claim 2, wherein the unsaturated mono-basic carboxylic acid is acrylic acid. A film according to any of the preceding claims, wherein the said copolymer is an ethylene/acrylic ester polymer. A film according to any of the preceding claims, wherein the layer of relatively thick polyolefin is of polypropylene. A film according to any of the preceding claims, wherein the said copolymer is a copolymer containing not more than 35 percent by weight of units derived from the unsaturated carboxylic acid or ester thereof. A film according to claim 6, wherein the said copolymer contains from 10 to 20 percent by weight of units derived from an acrylic ester. A film according to any of the preceding claims, wherein the said copolymer also contains units derived from an unsaturated dicarboxylic acid or anhydride thereof. A film according to claim 8, wherein the unsaturated dicarboxylic acid anhydride is maleic anhydride. A film according to claim 8 or claim 9, wherein the said copolymer contains at least 74 percent by weight of units derived from ethylene. A film according to claim 10, wherein the said copolymer contains not more than 20 percent by weight of units derived from the unsaturated acid or ester thereof. A film according to claims 9 to 11, wherein the said copolymer contains not more than 6 percent by weight of units derived from maleic anhydride. A film according to claim 12, wherein the said copolymer contains from 2 to 4 percent by weight of units derived from maleic anhydride. A film according to any of the preceding claims, wherein the blend has been treated to improve its sealability to polyvinylidene chloride. A film according to any of the preceding claims, wherein the relatively thick layer has a relatively thin layer of the said blend on one side, and the other side has been treated to improve print adhesion or has been provided with a polyvinylidene chloride overlacquer. A film according to claim 16, wherein the said other side has a propylene/ethylene copolymer layer thereon. A film according to any of the preceding claims, wherein the relatively thick layer is clear.

Inventors: CHRISTOPHERSON ROY, CHRISTOPHERSON, ROY
Applicants: COURTAULDS FILMS HOLDINGS, COURTAULDS FILMS (HOLDINGS) LIMITED
Application year: 1989
Main classification: B32B27
IPC codes: B32B27