IMERIC PO FILMSFIELD OF THE INVENTIONThe present invention relates to polymeric films and more particularly to polyolefin films having good antiblock or antiblock characteristics.
BACKGROUND OF THE INVENTIONPolyolefin films exhibit the undesirable property of blocking, ie, self-adhesion when brought into contact with each other, unless additives are present in the films to reduce this effect. Until now, both organic and inorganic anti-blocking additives have been proposed for this purpose. Examples of the inorganic antiblocking additives proposed hitherto include silicates and aluminum silicates, and examples of the organic antiblocking additives proposed hitherto include polyamides, polyalkyl methacrylates and particulate silicone resins. When used as anti-blocking additives, the selected materials are incorporated as parti-Ref.22888These particles usually have an average particle size from 0.5 to 8 Ji m, so that the particles extend beyond the general level of the outer surface of the films. This is very usually achieved by incorporating the anti-blocking 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 anti-blocking additive is present, becomes thinner and this causes the particles of the additive are projected beyond the surface of the thinned polymer layer. The typical amounts proposed hitherto of the anti-blocking additives used to achieve satisfactory anti-blocking properties for polyolefin films are in the range of from 500 to 6000 ppm based on the anti-blocking additive which is present in a layer of the outer film which usually has a thickness of up to 5 Um after stretching. It has been a conventional practice in the art to use a unique anti-blocking additive to achieve the desired levels of anti-blocking. However, it has been proposed in US Patent No. 5077129 to use mixtures of polymerized polyisobutyl methacrylate and polymethyl methacrylate as an anti-blocking additive.for polypropylene films oriented at least monoaxially. More recently, it has been proposed in EP 0615839-A and in EP 0618070-A to use mixtures of antiblock additives of different average particle sizes in an attempt to impart improved anti-blocking properties on the oriented polyolefin films. The average particle sizes proposed in Patent EP 0615839-A and EP 0618070-A are from 3 to 5 μm and from 1 to 2.5 μm.
DETAILED DESCRIPTION OF THE INVENTIONAccording to the present invention there is provided a multilayer oriented polyolefin film comprising a base layer containing polypropylene and at least one outer layer, the outer layer includes a mixture of a first anti-blocking additive having an average particle size from 3 to 6 juum and a second antiblock additive having an average particle size from 2 to 4 JJ.m, with the proviso that when the first additive is present in an amount from 3 to 5 U, the second anti-blocking additive has a particle size greater than 2.5 u.
The films according to the present invention have shown particularly good antiblocking properties, and indeed have been found in synergistic effect in the sliding of the films according to the invention with respect to the metal, compared with films using the same amount either of the first anti-blocking additive or of the second anti-blocking additive alone. Furthermore, this synergistic effect has been observed both with the sliding of the untreated film with respect to the metal, and with the sliding of the treated film with a corona discharge with respect to the metal. The average particle size of the first anti-blocking additive must be from 3 to 6 JLLm, preferably from 4.5 to 5 lm. The average particle size of the second antiblock additive should be from 2 to 4 i_, m, preferably from 2.5 to 3.5 i m. Although the range of the average particle sizes for the first and second anti-lock additives are superimposed, they should differ, preferably at least 1.5 μm, and more preferably from 2 to 2.5 μm. In general it is preferred that the difference in the average particle size of the antiblock additives be larger rather than smaller. However, when the difference increases, the propertiesOptical films tend to get worse, particularly their darkening. However, when the difference in the average particle size between the first and second antiblock additives is diminished, the synergistic improvement in anti-blocking properties of the films decreases. Furthermore, when the average particle size of the second anti-blocking additive is decreased, although the effect of the presence of the two anti-blocking additives on the optical properties of the films tends to decrease, the contribution of the second anti-blocking additive to the anti-blocking characteristics of the films tend to decrease, with the result that the synergistic effect of using a combination of antiblock additives having different average particle sizes tends to decrease. The weight ratio of the first anti-blocking additive to the second anti-lock additive in any of the outer layers of the films according to the invention is preferably in the range from 90:30 to 10:70, and more preferably from 75:40. until 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, to achieve a satisfactory level of anti-blocking properties.
However, it is generally preferred not to use a combined weight of more than 8000 ppm of the anti-blocking additives, based on the outer layer, so as not to have too serious an effect on the optical properties of the films. Although two antiblock additives having different average particle sizes are used according to the present invention, the material from which these anti-blocking additives are made can be the same or different. In general, the material from which the antiblocking additives are made can be selected from the anti-blocking additives known in the art of polyolefin films. Examples of antiblock agents which may be used in accordance with the present invention include silicates, silica, glass or ceramic microspheres, polyamides, crosslinked silicones, and crosslinked polyalkyl acrylates and methacrylates. A greater proportion of the two antiblock agents is preferably substantially spherical. The polymer forming the outer layer in which the combination of the antiblock additives is present, is preferably a polyolefin. The polyolefin can be a homopolymer derived from a single olefin or the same can be a co- or terpolymer derived from one or more olefins, for example from two or more ofthe olefins of ethylene, propylene, butene-1 and higher homologs thereof, for example octene-1. Examples of the polyolefins which may be used include polyethylenes, for example linear low density polyethylene, low density polyethylene, intermediate density polyethylene, high density polyethylene, polypropylene, polybutene-1, ethylene / propylene copolymers, copolymers of ethylene / butene-1, and ethylene / propylene / butene-1 terpolymers, and mixtures of two or more thereof. The base layer of the films of the present invention is preferably of a propylene homopolymer, which usually has a high content of isotactic polypropylene, usually in excess of 95%. The base coat, however, may consist of or contain a copolymer containing units derived from propylene and one or more additional alkenes, for example ethylene and / or butene-1. The films according to the present invention can consist only of a base layer and an outer layer. However, it is usually preferred to include one or more additional layers. For example, a second outer layer may be present on the opposite side of the base layer from the outer layer specified above. The second outer layer may be of thesame composition or substantially the same composition as the other outer layer. The selection of the material used for the second outer layer can therefore be made in accordance with the proposed final use of the films. The films according to the present invention may include one or more intermediate layers between the base layer and the outer layer containing the mixture of the first and second antiblock additives. further, the films according to the present invention may include one or more intermediate layers between the base layer and a second outer layer, when present, without considering whether there is an intermediate layer between the base layer and the outer layer containing the mixture of anti-blocking additives. Accordingly, the present invention includes films having not only the two specified layers, but films having three, four, five or more layers. Any of the respective layers of the films of the present invention may contain antistatic agents, migratory slip agents, antioxidants, or scavengers or catalyst cleaners. If desired, one or more of the layers of the films of the present invention may include a pigment, for example titanium dioxide, and / or a material which initiates microvoids or microvoids withinof the layer in which such material is present. The material that initiates the gap or vacuum, can be organic or inorganic, examples of the materials that can be used for this purpose include polyamides, polyesters, for example polyethylene terephthalate or polybutylene terephthalate, and chalk. The films according to the present invention can include other additives within one or more of the various layers, for example resins, for example hydrogenated hydrocarbon resins and hydrogenated rosin. The films according to the present invention can be prepared by known methods. They, however, must be oriented in at least one direction, and preferably should be oriented biaxially. Therefore, it is generally preferred to produce films according to the present invention by coextruding molten materials from the polymers containing the various additives required for the final film and then stretching the co-extruded material either in one direction or in both directions, the latter being carried out simultaneously or subsequently. If the sequential stretching is carried out, the coextruded monoaxially stretched material can be subjected to treatment, for example coating with a materialcoating, before it is stretched in a direction perpendicular to the original stretch direction. The films according to the present invention can be subjected to a variety of known treatment processes following at least the monoaxial stretch, for example they can be subjected to a treatment process which increases the surface energy of a or another surface of the films, for example the treatment with a corona arc discharge or the treatment with a flame. The films according to the present invention may be any of a variety of forms known in the art for polyolefin films, for example they may be in a form suitable for final sealing uses with vertical or horizontal filling . They can also be in a suitable form to produce labels. The present invention also includes films that thermally contract. The films according to the present invention may be of a wide variety of thicknesses, and the various layers may also be of a variety of thicknesses. The outer layer that contains the mixtureof anti-blocking additives, however, should usually be such that following the orientation of the particles of the anti-blocking additives is positioned in a manner related to the surface of the film to enable its anti-blocking properties to become apparent. The following Examples are given by way of illustration only.
Examples 1-8A series of three layer films was first produced by coextruding a continuous roll of polymer through a slotted die from the respective molten materials for a core layer of the propylene homopolymer containing 500 ppm of an amine antistatic agent. bis-ethoxylated and on either side of the core layer an outer layer of a propylene / ethylene copolymer (4 weight percent of units derived from ethylene) containing different amounts of either or both of the two different antiblock additives. The quantity of the two additives used in each case are shown in the attached Table. The first anti-blocking additive was a siliconecrosslinked with an average particle size of 4.5 m (Silicones Tospearl 145 - ex GE), and the second anti-blocking additive were microspheres with an average particle size of 3 ¿a (Zeeospheres - ex Zeelan Industries Inc. - used as a master batch ( ABVT 18SCN - ex Schulman)). The continuous roll is cooled on a cooling roller cooled with water, and the cooled continuous roll is stretched 4.5 times in the direction of the extrusion by passing it over the rollers which rotate at different peripheral speeds at a temperature of 130 ° C and then 10 times in the direction perpendicular to it using a stenter oven at a temperature of 165 ° C. The resulting biaxially oriented film hardens with heating, a surface of the film is thereafter subjected to treatment with a corona arc discharge to increase its surface energy to 42 dynes, and the film is wound. Twenty-four hours after manufacture, the sliding of the film with respect to the metal of each of the films was then measured using a Davenport coefficient of friction tester with a towed target of 700 g, both for the surface treated with a discharge of crown arch of eachof the films as for the surface of the films which have not been treated like that. The results are shown in the following table.
TableIt is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects to which it is applied.refers. Having described the invention as above, property is claimed as contained in the following