Polymeric films

Abstract: Biaxially oriented polymeric films consisting of a core layer of voided polypropylene with a layer of non-voided polypropylene homopolymer on one surface and a layer of a heat sealable polymer on the non-voided layer, the other surface of the core layer having at least one non-voided polymeric layer thereon. Such films have shown good puncture resistance combined with good gloss for the heat sealable polymer layer on the non-voided homopolymer layer.

Description: This invention concerns polymeric films, especially polypropylene films, having puncture resistance. It has been proposed hitherto in Research Disclosure 226030A to impart puncture resistance to polymeric films by the inclusion of a layer consisting of a blend of polypropylene with a low density polyethylene. Although such layers impart some degree of puncture resistance, depending upon their relative thickness compared with that of the film itself, they have the disadvantage of reducing the gloss of the films. EP 0180087-A describes biaxially oriented films with a voided propylene homopolymer core layer next to two non-voided propylene homopolymer layers, and two cover layers which are heat sealable. The two non-voided layers have a thickness of 10 µm according to the examples. EP 0312226-A describes biaxially oriented films with a voided polypropylene homopolymer core layer next to a non-voided polypropylene homopolymer layer, and two outer heat sealable layers of ethylene/propylene copolymers. EP 0221690-A describes biaxially oriented films with a voided propylene homopolymer core layer next to two non-voided propylene homopolymer layers, one of the non-voided layers being covered by a heat sealable coating. The advantage of these films is said to be in relation to their sealability and surface properties such as blocking resistance and low coefficient of friction combined with low light and oxygen transmission. According to the present invention there is provided a polymeric film comprising five coextruded layers which have been biaxially oriented together, the film having a core layer of voided polypropylene homopolymer with a layer of substantially non-voided polypropylene homopolymer on both sides thereof, and an outer layer of a heat sealable polymer on each of the layers of substantially non-voided polypropylene homopolymer so that the film is heat sealable, the layers of substantially non-voided polypropylene homopolymer each having a thickness of from 1 to 5µm. Films of the present invention have shown good puncture resistance combined with good gloss, for the heat seal layers. The core layer can be voided with any of the hitherto proposed voiding agents which can be organic or inorganic. Examples of organic voiding agents which can be used include polyamides, for example nylons, and polyesters, for example polyethylene terephthalate and polybutylene terephthalate. Examples of inorganic voiding agents which can be used include chalk. The use of voiding agents with polypropylene is well know in the art, as are the amounts of voiding agents which can be used and the particle size necessary to induce voiding when orientation is effected. The amount of voiding agent will usually represent from 5 to 15 percent by weight of the core layer. If desired, the core layer can contain other materials which do not induce voiding, for example pigments, eg titanium dioxide, and/or substances which impart surface properties to the outer surface of the films, eg slip agents. The amount of such other materials when present will usually be up to 5 percent of the core layer, and they will usually be present in an amount of at least 1 percent by weight of the core layer. On each surface of the core layer of films of the present invention is a substantially non-voided layer of a propylene homopolymer. Such layers can include substances which impart surface properties to the outer surface of the films. However the pigment in one of these layers should be present in an amount such that it should not induce voiding to anything other than a minimal degree. In order to obtain good gloss, it is generally necessary for the non-voided polypropylene homopolymer layer not to contain other polyolefinic materials. The outer layers of films of the present invention should be heat sealable. The sealable layers are preferably of a copolymer containing units derived from propylene and at least one other alpha-olefin, eg ethylene and/or but-1-ene, but other heat sealable polymer layers can be used, for example layers of polypropylene homopolymers. Copolymers of propylene with ethylene, for example containing about 4 percent of units derived from ethylene, can conveniently be used. If desired the heat seal layers can include other materials, for example a pigment, eg in amounts up to 20 percent by weight of the heat seal layer. Either or both outer surfaces of films of the present invention are preferably treated to improve their surface energy, eg by flame or corona discharge treatment, for example to improve the adhesion of the films to metal layers and/or inks and/or other coating materials, eg other films which may be laminated thereto. Films of the present invention produced by coextrusion of the respective layers followed by biaxial stretching. The biaxial stretching is preferably effected sequentially. Various thicknesses of films can be produced, but they are conventionally from 30 to 60µm thick for packaging applications. The thickness of the various layers other than the intermediate layers of non-voided polypropylene homopolymer, which are from 1 to 5µm thick, can also be selected within wide limits, it generally being preferred that the core layer be from 15 to 35µm thick. The outer, heat seal layers will generally be of a conventional thickness for heat sealing, e.g. from 1 to 2µm. The following Examples are given by way of illustration only. Example 1 A five layer polymer web was produced by coextruding a core layer of a polypropylene homopolymer containing 9 percent by weight of chalk having a mean particle diameter of 5µm and 3 percent by weight of titanium dioxide having a mean particle diameter of less than 0.2µm, with a layer of polypropylene homopolymer on each surface and a layer of a propylene/ethylene copolymer (4 percent by weight of units derived from ethylene) on each homopolymer layer. The web was then heated by passing it over heated rollers at 120°C and stretched 4.5 times in the direction of extrusion. Thereafter, it was stretched 10 times in the transverse direction at a temperature of 156°C. The stretched web had a core layer thickness of 30 microns, with on each surface a polypropylene homopolymer layer 5 microns thick and containing no chalk, the stretched heat seal layers each being 1 micron thick. The film had a density of 0.66 g/cm 3 , an optical density of 0.61 as measured by Tobias TCX Densitometer, and a gloss of 50 at 20° as measured by ASTM D523(85). The film furthermore had a stiffness of 0.82 in the machine direction, and 1.96 in the transverse direction. The puncture resistance of the film as measured using an Instron Tensile Tester with a 1mm probe was 0.99, and with a 3mm probe was 5.05. This compared favourably with a three layer film produced in substantially the same manner but without the non-voided homopolymer layers which had 1mm probe puncture resistance of 0.50 and a 3mm probe puncture resistance of 2.00. The gloss value for this three layer film was 10 at 20° as measured by ASTM D523(85). Example 2 (comparison) A three-layer polymer web was produced using the technique of Example 1, the two polypropylene homopolymer layers on each surface of the core being omitted. The overall thickness of the biaxially stretched film was 40 microns, the heat seal layers each being 1.5 microns thick. The film had a puncture resistance with a 1mm probe of 0.43 and with a 3mm probe of 1.75 using the measuring technique referred to in Example 1. Example 3 (comparison) A three-layer film was produced using the techniques used in Example 2, except that one heat seal layer had a thickness of 2.5 microns. The other heat seal layer had a thickness of 1.5 microns, and the overall thickness of the film was 40 microns. In addition, an aqueous acrylic coating composition was applied to the film after stretching it in the longitudinal direction but before stretching it in the transverse direction. The acrylic coating after the film had been stretched in the transverse direction was about 1 micron thick. The puncture resistance of the film using a 1mm probe was 0.47 and with a 3mm probe was 1.77 using the measuring technique referred to in Example 1

Claims: A polymeric film comprising five coextruded layers which have been biaxially oriented together, the film having a core layer of voided polypropylene homopolymer with a layer of substantially non-voided polypropylene homopolymer on both sides thereof, and an outer layer of a heat sealable polymer on each of the layers of substantially non-voided polypropylene homopolymer so that the film is heat sealable, the layers of substantially non-voided polypropylene homopolymer each having a thickness of from 1 to 5µm. A film according to claim 1, wherein the core layer includes a pigment which does not induce voiding. A film according to claim 2, wherein the pigment comprises titanium dioxide. A film according to claim 2 or claim 3, wherein the pigment is present in the core layer in an amount of up to 5 percent of the weight of the core layer. A film according to any of the preceding claims, wherein at least one outer layer of the film contains a pigment. A film according to any of the preceding claims, wherein the heat sealable layers are of a propylene/ethylene copolymer containing not more than 10 percent by weight of units derived from ethylene, or are of a propylene homopolymer. A film according to any of the preceding claims, wherein at least one outer surface of the film has been treated to increase its surface energy. A film according to any of the preceding claims, wherein at least one outer surface thereof has had a metal coating applied thereto.

Inventors: FOULKES JONATHAN PAUL, WILLIAMSON DAVID, FOULKES, JONATHAN PAUL, WILLIAMSON, DAVID
Applicants: HOECHST TRESPAPHAN GMBH
Application year: 1992
Main classification: B32B27
IPC codes: B32B27