POLYMERIC FILMS

Abstract: Oriented polyolefin multilayer film having a base layer containing polypropylene and at least one outer layer, the outer layer including a mixture of a first antiblock additive having a mean particle size of from 3 to 6 mu m and a second antiblock additive having a mean particle size of from 2 to 4 mu m, with the proviso that when the first additive is present in an amount of from 3 to 5 mu m, the second antiblock additive has a particle size of greater than 2.5 mu m. Such films have particularly good antiblock characteristics compared with those having either of the particular types of antiblock agent on their own.

Description: POLYMERIC FILMS This invention concerns polymeric films and more particularly polyolefin films having good antiblock characteristics. Polyolefin films exhibit the undesirable property of blocking, that is self-adhesion when brought into contact with each other, unless additives are present in the films to reduce this effect. Both inorganic and organic antiblock additives have been proposed hitherto for the purpose. Examples of hitherto proposed inorganic antiblock additives include silica and aluminium silicates, and examples of hitherto proposed organic antiblock additives include polyamides, polyalkyl methacrylates and particulate silicone resins. When used as antiblock additives, the selected materials are incorporated as particles, usually with a mean particle size of from 0.5 to 8 μ , so that the particles extend beyond the general level of the outer surface of the films. This is most usually achieved by incorporating the antiblock additive into the polyolefin used to form the outer layers of the films, and then stretching the films so that the outer layer in which the antiblock additive is present thins and causes the additive particles to project beyond the surface of the thinned polymer layer. Typical hitherto proposed amounts of antiblock additive used to achieve satisfactory antiblock properties for polyolefin films are in the range of from 500 to 6000 ppm based on the antiblock additive being present in an outer film layer usually having a thickness of up to 5 μm after stretching. It has been conventional practice in the art to use a single antiblock additive to achieve desired levels of antiblock. However, it has been proposed in US 5077129 to use mixtures of polymerized polyisobutyl methacrylate and polymethyl methacrylate as an antiblock additive for at least monoaxially oriented polypropylene films. More recently, it has been proposed in EP 0615839-A and EP 0618070-A to use mixtures of antiblock additives of differing mean particle sizes in an attempt to impart improved antiblock properties on oriented polyolefin films. The mean particle sizes proposed in EP 0615839-A and EP 0618070-A are from 3 to 5 μm and from 1 to 2.5 μm. According to the present invention there is provided oriented polyolefin multilayer film comprising a base layer containing polypropylene and at least one outer layer, the outer layer including a mixture of a first antiblock additive having a mean particle size of from 3 to 6 μm and a second antiblock additive having a mean particle size of from 2 to 4 μm, with the proviso that when the first additive is present in an amount of from 3 to 5 μm, the second antiblock additive has a particle size of greater than 2.5 μm. Films in accordance with the present invention have shown particularly good antiblock properties, and indeed a synergistic effect has been found in the slip of films in accordance with the invention to metal compared with films using the same amount of either the first antiblock additive or the second antiblock additive alone. Furthermore, this synergistic effect has been observed both with untreated film-to-metal slip and with corona discharge treated film-to-metal slip. The mean particle size of the first antiblock additive should be from 3 to 6 urn, preferably from 4.5 to 5 μm. The mean particle size of the second antiblock additive should be from 2 to 4 μm, preferably from 2.5 to 3.5 μm. Although the range of mean particle sizes for the first and second antiblock additives overlap, they should differ, preferably by at least 1.5 μm, and more preferably from 2 to 2.5 μm. It is generally preferred that the difference in mean particle size of the antiblock additives is larger rather than smaller. However, as the difference increases, the optical properties of the films tend to worsen, particularly their haze. However, as the difference in mean particle size between the first and second antiblock additives is decreased, the synergistic improvement in the antiblock properties of the films decreases. Furthermore, as the mean particle size of the second antiblock additive is decreased, although the effect of the presence of the two antiblock additives on the optical properties of the films tends to decrease, the contribution of the second antiblock additive to the antiblock characteristics of the films tends to decrease, with the result that the synergistic effect of using a combination of antiblock additives having different mean particle sizes tends to decrease. The weight ratio of the first antiblock additive to the second antiblock additive in any one outer layer of films in accordance with the invention is preferably in the range of from 90:30 to 10:70, and more preferably from 75:40 to 25:60. The combined weight of the first and second antiblock additives is preferably at least 1000 ppm based on the weight of the outer layer, in order to achieve a satisfactory level of antiblock properties. However, it is generally preferred not to use a combined weight of more than 8000 ppm of antiblock additives, based on the outer layer, in order not to have too serious an effect on the optical properties of the films. Although in accordance with the present invention two antiblock additives are used having different mean particle sizes, the material from which these antiblock additives are made can be the same or different. In general, the material of which the antiblock additives are made can be selected from antiblock additives known in the polyolefin film art. Examples of antiblock agents which can be used in accordance with the present invention include silicates, silica, glass or ceramic microspheres, polyamides, cross- linked silicones, and cross-linked polyalkyl acrylates and methacrylates. A major proportion of the two antiblock agents is preferably substantially spherical. The polymer forming the outer layer in which the combination of antiblock additives is present is preferably a polyolefin. The polyolefin can be a homopolymer derived from a single olefin or it can be a co- or terpolymer derived from two or more olefins, for example from two or more of ethylene, propylene, butene-1 and higher homologues thereof, e.g. octene-1. Examples of polyolefins which can be used include polyethylenes, e.g. linear low density polyethylene, low density polyethylene, medium density polyethylene, high density polyethylene, polypropylene, polybutene-1, ethylene/propylene copolymers, ethylene/butene-1 copolymers, and ethylene/propylene/butene-1 terpolymers, and mixtures of two or more thereof. The base layer of films of the present invention is preferably of a propylene homopolymer, usually having a high content of isotactic polypropylene, usually in excess of 95%. The base layer can, however, consist of or contain a copolymer containing units derived from propylene and one or more further alkene, for example ethylene and/or butene- 1. Films in accordance with the present invention can consist merely of a base layer and an outer layer. However, it is usually preferred to include one or more further layer. For example, a second outer layer can be present on the opposite side of the base layer from the above specified outer layer. The second outer layer can be of the same or substantially the same composition as the other outer layer. Selection of the material used for the second outer layer can therefore be made according to the intended end use of the films. Films in accordance with the present invention can include one or more intermediate layers between the base layer and the outer layer containing the mixture of the first and second antiblock additives. Furthermore, films in accordance with the present invention can include one or more intermediate layers between the base layer and a second outer layer, when present, irrespective of whether there is an intermediate layer between the base layer and the outer layer containing the mixture of antiblock additives. Thus the present invention includes films having not only the two specified layers, but films having three, four, five and more layers. Any of the respective layers of films of the present invention can contain antistatic agents, migratory slip agents, antioxidants, or catalyst scavengers. If desired, one or more of the layers of films of the present invention can include a pigment, for example titanium dioxide, and/or a material which initiates microvoids within the layer in which such a material is present. The void initiating material can be organic or inorganic, examples of materials which can be used for the purpose include polyamides, polyesters, e.g. polyethylene terephthalate or polybutylene terephthalate , and chalk. Films in accordance with the present invention can include other additives within one or more of the various layers, for example resins, e.g. hydrogenated hydrocarbon resins and hydrogenated rosin. Films in accordance with the present invention can be prepared by known methods. They should, however, be oriented in at least one direction, and preferably be biaxially oriented. It is therefore generally preferred to produce films in accordance with the present invention by coextruding melts of the polymers containing the various additives required for the final film and then stretching the coextrudate either in one direction or in two directions, the latter being either simultaneously or sequentially. If sequential stretching is effected, the monoaxially stretched coextrudate can be subjected to treatment, for example coating with a coating material, before being stretched in a direction perpendicular to the original direction of stretching. Films in accordance with the present invention can be subjected to a variety of known treatment processes following at least monoaxial stretching, for example they can be subjected to a treatment process which increases the surface energy of one or other surface of the films, e.g. corona discharge treatment or flame treatment. Films in accordance with the present invention can be in any of a variety of forms known in the art for polyolefin films, for example they can be in a form suitable for vertical or horizontal form-fill seal end uses. They can also be in a form suitable for producing labels. The present invention also includes thermally shrinkable films. Films in accordance with the present invention can be of a wide variety of thicknesses, and the various layers can also be of a variety of thicknesses. The outer layer containing the mixture of antiblock additives should, however, usually be of a thickness such that following orientation the particles of the antiblock additives are disposed relative to the film surface to enable their antiblocking properties to become manifest. The following Examples are given by way of illustration only. Examples 1-8 A series of three layer films was produced by first coextruding a polymer web through a slot die from respective melts for a core layer of propylene homopolymer containing 500 ppm of a bis-ethoxylated amine antistatic agent and on either side of the core layer an outer layer of a propylene/ethylene copolymer (4 percent by weight of units derived from ethylene) containing different amounts of either or both of two different antiblock additives. The amount of the two additives used in each case are shown in the accompanying Table. The first antiblock additive was a cross linked silicone with a mean particle size of 4.5 μm (Tospearl 145 - ex GE Silicones), and the second antiblock additive was microspheres with a mean particle size of 3 μm (Zeeospheres - ex Zeelan Industries Inc - used as a masterbatch (ABVT 18SCN - ex Schulman) ) . The web was cooled on a water-cooled chill roll, and the cooled web was stretched 4.5 times in the direction of extrusion by passing it over rolls rotating at differing peripheral speeds at a temperature of 130°C and then 10 times in the direction perpendicular thereto using a stenter oven at a temperature of 165°C. The resulting biaxially oriented film was heat set, one surface of the film was thereafter subjected to corona discharge treatment to increase its surface energy to 42 dynes, and the film was wound up. Twenty four hours after manufacture, the film-to-metal-slip of each of the films was then measured using a Davenport coefficient of friction tester with a 700g sled, both for the corona discharge treated surface of each of the films and for the surface of the films which had not been so treated. The results are shown in the accompanying Table. Table Example Antiblock Additives Film/metal dynamic slip First (ppm) Second (pp ) Untreated Treated 3500 3.1 2.1 3000 500 3.5 3.34 2500 1000 3.55 2.71 2000 1500 4.0 3.8 5 1500 2000 3.39 3.03 6 1000 2500 3.33 2.38 7 500 3000 3.02 1.97 8 0 3500 3.2 2.2

Claims: Claims 1. Oriented polyolefin multilayer film comprising a base layer containing polypropylene and at least one outer layer, the outer layer including a mixture of a first antiblock additive having a mean particle size of from 3 to 6 μm and a second antiblock additive having a mean particle size of from 2 to 4 μm, with the proviso that when the first additive is present in an amount of from 3 to 5 μm, the second antiblock additive has a particle size of greater than 2.5 μm. 2. Film according to claim 1, wherein the first antiblock additive has a mean particle size of from 4.5 to 5 μπi. 3. Film according to either of the preceding claims, wherein the second antiblock additive has a mean particle size of from 2.5 to 3.5 μm. 4. Film according to any of the preceding claims, wherein the difference in mean particle size between the first and second antiblock additives is at least 1.5 μm. 5. Film according to claim 4, wherein the difference in mean particle size between the firt and second antiblock additives is from 2 to 2.5 μm. 6. Film according to any of the preceding claims wherein the weight ratio of the first to the second antiblock additives is from 90:30 to 10:70. 7. Film according to claim 6, wherein the weight ratio is from 75:40 to 25:60. 8. Film according to any of the preceding claims, wherein the combined weight of first and second antiblock additives is at least 1000 ppm based on the weight of the outer layer. 9. Film according to any of the preceding claims, wherein the combined weight of first and second antiblock additives is not more than 8000 ppm based on the weight of the outer layer. 10. Film according to any of the preceding claims, wherein the material of the first and second antiblock additives is the same. 11. Film according to any of claims 1 to 9, wherein the material of the first and second antiblock additives is different. 12. Film according to any of the preceding claims, wherein at least one of the first and second antiblock additives comprises a major proportion of substantially spherical particles. 13. Film according to any of the preceding claims, wherein at least one of the first and second antiblock additives comprises a silicate, silica, glass or ceramic microspheres, a polyamide, a cross-linked silicone, or a cross-linked polyalkyl aerylate or methacrylate. 14. Film according to any of the preceding claims, wherein the outer layer containing the first and second antiblock additives comprises a co- or terpolymer containing units derived from two or more of ethylene, propylene, butene-1 or a higher alkene-1.

Inventors: ALDER PAUL THOMAS, ALDER, PAUL, THOMAS
Applicants: ALDER PAUL THOMAS, COURTAULDS FILMS HOLDINGS, ALDER, PAUL, THOMAS, COURTAULDS FILMS (HOLDINGS) LIMITED
Application year: 1995
Main classification: B32B27
IPC codes: B32B27