Method of producing a polymeric composite film

IS 2 139 589 T3

DESCRIPTION

Polymeric film.

This invention relates to a polymeric film, and in particular to a composite polymeric film.

It is known that polymeric films often have poor handling properties that can cause difficulties in winding the films to form high-quality reels, as well as a problematic passage through the different processing stations, such as cutting equipment. in strips.

In GB Patent No. 1465973 there are disclosed polyether film composites comprising a homopolyether layer and a copolyether layer. Copolyesters can be used as heat sealable layers. European Patent N^° 35835 discloses a similar polyether film composite comprising in the copolyester layer a filler having an average particle size greater than the thickness of the layer. The filler particles protrude through the copolyether layer, providing a film with good handling properties. However, the aforementioned film may suffer from reduced heat sealability due to the presence of filler particles having a particle size greater than the thickness of the heat sealable layer. In addition, the aforementioned handling property is achieved only when certain relationships of thickness of the heat-sealable layer / size of the filler particles are given, so that any variation that is required in the thickness of the heat-sealable layer (for example in a different commercial application) requires a modification of the size of the filler particles. This situation can result in a range of different loads being required for different applications. A relatively high charge concentration may be required in the copolyester film in order to obtain the required handling properties, which can result in an unacceptable decrease in ioptic transparency and an increase in film haze.

We have now developed a composite film that significantly reduces or eliminates at least one or more of the above-mentioned problems.

Consequently, the present invention provides a composite film comprising a substrate layer of oriented polymeric material having on at least one face thereof a heat-sealable layer having a thickness of at least 0.5 μm and capable of forming a bonding by heat sealing without softening or melting the substrate layer, the heat sealing layer comprising a particulate additive, comprising the exposed face of the heat sealable layer mine of 100 surface projections per mm² produced by the particulate additive, the surface projections having an average peak height within the range of 5 to 400 nm measured from the average level of the surface of the heat-sealable layer.

The invention also provides a method for producing a composite film as described in claim 1, which method comprises the steps of forming a substrate layer of polymeric material having a heat-sealable layer on at least one side thereof, and applying a heat-sealable layer. particulate additive, in the dry state or as a dispersion in an aqueous or organic solvent, on the exposed side of the heat-sealable layer, in such a way that the particulate additive binds to and / or penetrates the heat-sealable layer, the exposed face of the heat-sealable layer comprising 100 surface projections per mm² produced by the particulate additive, the surface projections having an average peak height within the range of 5 to 400 nm measured from the average level of the surface of the heat-sealable layer.

The substrate of a polymeric film compound according to the invention can be formed from any synthetic film-forming polymeric material. Suitable thermoplastic materials include a homopolymer or copolymer of a 1-olefin, such as ethylene, propylene and but-1ene, a polyamide, a polycarbonate and, particularly, a synthetic linear polyether that can be obtained by condensing one or more acids. dicarboxylics or their lower alkyl diesters (with up to 6 carbon atoms), and p. ex. terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, succinic acid, sebiacic acid, adipic acid, azelaic acid, 4,4'-diphenyldicarboxylic acid or 1-hexahydrophthalene hydroxylic acid , 2-bis-p-carboxyphenoxyethane (optionally with a monocarboxylic acid such as pivialic acid), with one or more glycols, and particularly aliphatic glycols, such as p. ex. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, and 1,4-cyclohexanedimethanol. A polyethylene terephthalate film is particularly preferred, and especially such a film which has been biaxially oriented by sequential stretching in two mutually perpendicular directions, typically at a temperature within the temperature range of 70 to 125^°, and which has preferably been thermoset, typically at a temperature of 150 to 250^°, for example as described in British patent 838708.

The substrate may also comprise a polyaryl ether or a thio-analog thereof, and in particular a polyaryl etherketone, polyaryl ether sulfone, poraryl ether etherketone or polyaryl ether ether sulfone, or a copolymer or thio-anailog thereof. Examples of these polymers are described in EP-A-1879, EP-A-184458 and US-A-4008203. The substrate may comprise a poly (arylenesulfide), and particularly poly-pphenylene sulfide or copolymers thereof. Mixtures of these polymers can also be used.

Suitable substrate materials consisting of thermosetting resins include adicioin-polymerization resins such as acrylic resins, vinyls, bis-maleimides and unsaturated polyesters, formaldehyde condensate resins, such as condensates with urea, melamine or phenols, cyanate resins, polyyesters functionalized, polyamides or polyimides.

The polymeric film substrate for pro2

The production of a composite film according to the invention may be uniaxially oriented, but is preferably biaxially oriented by stretching in two mutually perpendicular directions in the plane of the film to achieve a satisfactory combination of mechanical and physical properties. Simultaneous biaxial orientation can be carried out by extruding a thermoplastic polymeric tube that is then rapidly cooled, reheated, and then expanded by internal gas pressure to produce transverse orientation, and extracted at a speed that caused orientation. longitudinal. Sequential stretching can be carried out in a stenter process by extruding the thermoplastic substrate material as flat extruded material which is then stretched first in one direction and then in the other mutually perpendicular direction. In general, it is preferred to draw first in the longitudinal direction, that is, in the direction of advance through the film stretching machine, and then in the transverse direction. A stretched substrate film can be, and preferably is, dimensionally stabilized by thermosetting under dimensional fixation at a temperature above its glass transition temperature.

It is desirable that the polymeric film substrate of the present invention be optically transparent, preferably having a wide angle haze for a 12 μm thick film of <3.5%, more preferably <1.5%, and particularly <0, 5%, measured according to ASTM D 1003-61.

The heat-sealable layer should be capable of forming a heat-sealing bond with itself or with the substrate, or preferably both, by heating to soften the polymeric material of the heat-sealable layer and apply pressure without softening or melting the polymeric material of the layer. substrate layer.

A heat-sealable layer suitably comprises a polyester resin, and in particular a copoly ester resin derived from one or more dibasic aromatic carboxylic acids such as terephthalic acid, isophthalic acid and hexahydroterephthaolic acid, and one or more glycols, such as ethylene glycol, such as ethylene glycol. and neopentyl glycol. Topical copolyesters that provide satisfactory heat-sealing properties are those of ethylene terephthalate and ethylene isophthalate, especially in the molar ratios of 50 to 90 mole% of ethylene terephthalate, and correspondingly, 50 to 10 mole% of isophthalate. of ethylene. Preferred copolyesters comprise 65 to 85 mole% ethylene terephthalate and 35 to 15 mole% ethylene isophthalate, and especially a copolyether of about 82 mole% ethylene terephthalate and about 18 mole% of ethylene isophthalate.

The formation of a heat-sealable layer on the substrate layer can be carried out by conventional techniques, and for example by molding the polymer on a preformed substrate layer. Conveniently, however, the formation of a composite sheet (substrate layer and heat-sealable layer) is carried out by coextrusion, either through simultaneous coextrusion of the respective film-forming layers through independent holes of a multi-hole matrix, and based on joining the layers still fused, or, preferably, based on coextrusion in a single conduit, in which the melt streams of the respective polyomers are first joined within a conduit leading to a die manifold, and are then extruded together through the die orifice under conditions of laminar flow without intermixing, to thereby producing a composite sheet.

A coextruded sheet is stretched to effect molecular orientation of the substrate, and is preferably thermoset. Generally, the conditions applied to stretch the substrate layer caused partial crystallization of the heat sealable polymer, and therefore it is preferred to carry out thermosetting under dimensional fixation at a selected temperature to develop the desired morphology of the heat sealable layer. Furthermore, by carrying out the thermosetting at a temperature below the crystalline melting temperature of the heat sealable polymer and allowing or causing the compound to cool, the heat sealable polymer would remain essentially crystalline. However, by effecting the thermosetting at a temperature higher than the crystalline melting temperature of the heat-sealable polymer, the latter was rendered essentially amorphous. The thermosetting of a composite sheet comprising a polyester substrate and a heat-sealable copoly-ester layer is conveniently carried out at a temperature within a temperature range from 175 to 200 ° C to obtain a proactively crystalline heat-sealable layer, or from 200 to 250 ^ C to obtain a heat-sealable, proactively amorphous layer. A proactively amorphous heat sealable layer is preferred.

Heat sealable layers can be provided on one side or on both sides of the substrate layer. The film composites may have a total thickness in the range of 10 to 500 µm, and the heat sealable layer or each of the heat sealable layers preferably constitutes 1 to 30% of the total thickness of the composite. The heat sealable layers preferably have a thickness of up to 50 µm, more preferably up to 10 µm, and especially, up to 5 µm.

The required handling properties are achieved when the heat sealable layer comprises more than 100 and preferably less than 5000 peaks of surface projections per mm.², more preferably from 400 to 2000, particularly from 600 to 1500, and especially from 700 to 900 protrusion peaks per mm². The average height of a protrusion peak measured from the average level of the surface of the heat sealable layer is 5 to 400 nm, more preferably 10 to 200 nm, particularly 15 to 100 nm, and especially 20 to 60 nm. It is preferred that more than 50%, par3

In particular, more than 70%, and especially more than 90% of the peaks of the projections have a height within the aforementioned preferred average height ranges. The shape of the protrusion peaks can also play an important role in the handling properties of the resulting film. The median ridge peak preferably has a shape (the shape meaning the height of a ridge peak (as defined above) divided by the width of the base of the peak measured at the median surface level of the heat-sealable layer) at 0.8 x 10^-3, most preferably within the range of 1.0 x 10^-3 at 100 x 10^-3, located especially within the range of values ​​that goes from 1.5 x10^-3 a20x10^-3, and located particularly within the range of values ​​that goes from 2.0 x 10^-3a4,0x10^-3.

The number, size and shape of the protrusion peaks can be determined by any relevant surface analysis technique. Methods that are performed without contact, such as interferometry, using for example a Topo-3D Profiler (manufactured by Wyko Corporation) are preferred, and are described herein in the test methods listed below.

The particulate additive intended for inclusion in the heat-sealable layer must be chemically inert in relation to the polymeric material of the heat-sealable layer and to the materials from which it is produced or with any other additives present in the heat-sealable layer, and it preferably comprises inorganic particles that comprise a material or a mixture of materials that make up the group formed by natural or synthetic salice, glass beads, calcium borate, calcium carbonate, magnesium carbonate, barium sulfate, calcium silicate, calcium phosphate, aluminum trihydrate, aluminosilicates including the hydrated and calcined forms thereof, and titanium dioxide. Other suitable particulate additives include polymeric polymeric particles that melt at a temperature higher than the highest temperatures used in the production of the film compound and / or are immiscible with the heat sealable polymer. The preferred particulate additive comprises salty particles, and in particular comprises a type of salice commercially referred to as smoked or pyrogenic salice. Smoked salice can be formed by reacting silicon tetrachloride in an oxygen flame to form individual spherical particles of silicon dioxide. The aforementioned particles grow by collision and coalescence to form larger particles, i.e. primary particles. As the particles cool and begin to solidify, but continue to collide, they adhere but do not coalesce, forming solid aggregates that in turn continue to collide to form conglomerates or agglomerates.

The particulate filler that is suitable to be used in the heat sealable layer of a composite film according to the present invention is preferably of practically circular cross section regardless of the observation point that is selected. Conveniently, an individual charge particle has a shape ratio d1: d2 (where d1 and d2, respectively, are the maximum and minimum dimensions of the particle) ranging from 1: 1 to 1: 0.5, and preferably 1: 1 to 1: 0.8.

The mean size of the particles (which means the mean diameter of the particles) of the particulate filler, and in particular of the salice particles, that are incorporated into the heat-sealable layer is conveniently located within a range of sizes that goes from 0.01 to 0.09 µm, preferably 0.02 to 0.08 µm, and especially 0.03 to 0.06 µm.

The sizes of the particles can be measured by means of an electron microscope, Coulter counter or analysis by sedimentation, and the average size of the particles can be determined based on graphically recording a cumulative distribution curve that represents the percentage of particles located below the chosen particle sizes.

In the case of spherical particulate additives, the mean particle size is determined as the diameter of the particles. However, many particulate additives, and especially inorganic particles, are non-spherical in shape, and for the purposes of the present disclosure their average size is determined as the size of the particles at their largest dimension.

In a preferred embodiment of the invention, the primary or individual filler particles aggregate to form conglomerates or agglomerates comprising a plurality of primary particles. The aggregation process of the primary filler particles can take place during the synthesis of the filler itself and / or during the film-forming process. The values ​​of the mean particle sizes contained herein refer to the size of the non-agglomerated primary particles . It is preferred that the average particle size of the filler agglomerates, such as the agglomerates of primary salice particles, in the heat sealable layer of the final composite film product is within the range of 0.05 to 0, 45 µm, preferably 0.08 to 0.4 µm, and especially 0.1 to 0.35 µm.

The particulate additive can be added to the heat-sealable polymer prior to extrusion thereof, so that the particulate additive is indeed evenly distributed throughout the entire layer, preferably being present in the layer in an amount of up to 10%, and more preferably up to 5% by weight based on the weight of the heat sealable polymeric material. Adequate handling properties of the film can generally be provided by amounts of the particulate additive of up to 3% by weight and especially 0.5 to 2.0% by weight of the polymeric material.

The presence of the particulate additive distributed throughout the heat sealable layer can have a detrimental effect on the transparency of the composite film, and in a preferred embodiment of the invention the particulate additive is applied.

ES 2 139 589 T3 exposed to the face of the heat sealable layer, preferably in a liquid medium such as a dispersion in an organic solvent or in organic solvents, and preferably as an aqueous dispersion. The liquid medium can optionally include a plasticizer for the heat sealable layer. Examples of compounds that may be suitable as plasticizers for heat sealable polyether layers include benzaldehyde, benzylalcohol, methyl salicylate, odichlorobenzene, dimethyl phthalate, diethyl oxalate, diethyl succinate, tetrachloroethane, phenylphenol, 1-phenylethanol, and diloromethane. Alternatively, the particulate additive can be applied to the heat-sealable layer in the dry state as a gas cloud, by electrostatic deposition, or from a fluidized bed optionally with electrostatic assistance. When applied in the dry state, the particulate additive can, if desired, be spread over the heat-sealable layer to obtain a reasonably uniform distribution over the entire surface.

If desired, slight pressure can be applied to the particulate additive layer to push the particles into the heat sealable layer. Excess particulate additive that has not bonded to or penetrated the heat sealable layer can be removed from the surface of the heat sealable layer. ex. by inverting the composite film, by dispersing the particles with a jet of air, or by removing the particles by brushing or washing. The composite film can be allowed to cool in air, or it can be subjected to rapid cooling to complete the bonding of the particles to the heat-sealable layer, and the cooling or rapid cooling operation can be carried out before or after the excess particles have been removed. from the surface of the film.

The particulate additive can be applied to an already oriented film. However, the application of the particulate additive is preferably carried out before or during the stretching operation.

In particular, it is preferred that the particulate additive medium is applied to the film substrate between the two stages of a biaxial (longitudinal and transverse) stretching operation. Such a stretching and coating sequence is especially preferred for the production of a heat-sealable coating copolyester layer bonded to a linear polyether film substrate, such as a polyethylene terephthalate substrate, which is preferably first stretched in the longitudinal direction through a series of rotating rollers, is coated and is then stretched transversely in a stenter furnace, preferably then thermosetting is carried out. During the thermosetting process, the film is preferably heated to a temperature above the softening point of the heat-sealable layer, so that the particulate additive penetrates the polymer layer. The particulate additive preferably penetrates just below the surface layer of the polymer, thereby being fully immersed within the layer but confined to the surface region of the heat sealable layer. More than 90% and preferably more than 95% of the tin particles preferably within the top 1 µm thickness and preferably within the top 0.5 µm thickness of the heat sealable layer. The result of this is that mostly the particles within the layer produce a surface overhang which preferably has an average peak height of 5 nm.

The particulate additive composition can be applied to the heat sealable layer as a dispersion or aqueous solution in an organic solvent by any suitable conventional coating technique such as dip coating, globular coating, reverse roll coating or slot coating. The particulate additive is preferably applied to the heat-sealable layer as a dispersion, preferably in aqueous media, at a concentration of 2 to 15%, and more preferably 4 to 8%.

In a preferred embodiment of the invention, a composite film has high optical transparency and low haze, preferably having a large-angle haze, measured according to ASTM D 1003-61, of <8%, more preferably <6%, particularly <5% and especially <3% for a 15 µm thick film. The aforementioned optical characteristics can be adequately achieved by making little or no particulate additive present in the substrate. The substrate may contain relatively small amounts of filler material, for example due to the normal practice of using make-up film in the film-making process. The optical properties of the composite film can be further improved by not including any additional particulate additive in the polymer of the heat-sealable layer before the formation of the film, that is to say in such a way that practically all the particulate additive that is present in The heat-sealable layer comes from the application of the particulate additive as a coating on its surface.

However, in an alternative embodiment of the invention the composite film is opaque, said opaque film being defined as a film exhibiting a Density ratio of O^í Optics by Transmission (Sakura Densitometer; PDA 65 type; transmission mode) / film thickness (in mm) from 7.5 to 17.5, and particularly from 12.0 to 15.0. The opacity of the composite film is preferably achieved by providing an opaque substrate layer. The substrate layer is conveniently rendered opaque by incorporating an effective amount of an opacifying agent into the synthetic polymer. However, in a preferred embodiment of the invention the opaque substrate layer is a void layer, ie it comprises a cellular structure containing at least a proportion of discrete closed cells. It is therefore preferred to incorporate into the substrate polymer an effective amount of an agent that is capable of generating an opaque void structure of the substrate layer. Suitable voiding agents that also

ES 2 139 589 T3 conferring opacity include an incompatible resin filler, a particulate inorganic filler, or a mixture of two or more such fillers.

"Incompatible resin" means a resin that does not melt or is practically immiscible with the substrate polymer at the highest temperature that occurs during extrusion and manufacture of the layer. Such resins include polyamides and olephane polymers, and particularly a homopolymer or copolymer of a mono-alpha-olefin containing up to 6 carbon atoms in its molecule, for incorporation into polyesters, or polyesters of the class described above for incorporation. in polyolefin films.

Particulate inorganic fillers that are suitable for generating an opaque substrate layer with voids include conventional inorganic pigments and fillers, and particularly metal or metalloid oxides, such as alumina, salice, and titania, and alkali metal salts such as carbonates and sulfates. calcium and barium. Barium sulfate is a particularly preferred filler that also functions as a voiding agent.

Non-void-forming particulate inorganic fillers can also be added to the film-forming polymeric substrate layer.

Suitable void-forming and / or non-void-forming fillers may be homogeneous and may consist essentially of a single filler material or compound, such as titanium diaoxide or barium sulfate alone. Alternatively, at least a proportion of the filler can be heterogeneous, with the primary filler being associated with an additional modifying component. For example, the primary filler particle can be treated with a surface modifier such as a pigment, soap, surface coupling agent, or other modifier to increase or alter the degree to which the filler is compatible with the substrate polymer.

The production of a substrate layer having satisfactory degrees of opacity, void formation and whiteness requires that the filler be finely divided, and the mean particle size thereof is conveniently 0.1 to 10 μm, provided when the actual particle size of 99.9% of the particles does not exceed 30 μm. Preferably, the filler has an average particle size of 0.1 to 10 µm, and particularly preferably 0.2 to 0.75 µm. Decreasing the size of the particles improves the gloss of the substrate.

It is preferred that none of the filler particles incorporated into the substrate layer have an actual particle size of more than 30 µm. Particles that exceed such a size can be removed by sieving processes that are known in the art. However, the sieving operations do not always give a totally satisfactory result with respect to the removal of all the particles that exceed a chosen size. Therefore, in practice the size of 99.9% of the particles should not exceed 30 µm. Most preferably, the size of 99.9% of the particles should not exceed 20 µm.

The incorporation of the opacifying / voiding agent into the polymer of the substrate layer can be carried out by conventional techniques, and for example by mixing with the monomeric reagents from which the polymer is derived, or by dry mixing with the polymer in granular or chip form before forming a film based on said materials.

The amount of filler, particularly barium sulfate, incorporated into the polymer of the substrate layer should suitably be not less than 5% and not more than 50% by weight, based on the weight of polymer. Particularly satisfactory levels of opacity and gloss are achieved when the filler concentration is about 8 to 30%, and especially 15 to 20% by weight, based on the weight of the polymer in the substrate layer.

Other additives may optionally be incorporated into the substrate layer, generally in relatively small amounts. For example, Chinese clay can be incorporated in amounts of up to 25% to ensure void formation, optical color brighteners in amounts of up to 1500 parts per million to contribute to whiteness, and coloring matters in amounts of up to 10 parts per million. millet to modify the color, the specified concentrations being by weight, based on the weight of the substrate polymer.

In a film composite typically having a heat-sealable layer thickness within the range of 2 to 4 μm, 8% by weight of aqueous dispersion of salice particles having a coating is applied to the heat-sealable layer. an average size of the primary particles within the range of 30 to 50 nm, to create a surface concentration of protrusion peaks that protrude from the surface of the secondary layer within the range of 700 to 900 per mm², said peaks having an average peak height of 20 to 60 nm. Such a surface provides excellent handling properties and heat sealing properties in film composites comprising a thermoset, biaxially oriented polyethylene terephthalate substrate layer and a 70 to 85 mol% ethylene terephthalate / 30 a copolyester heat sealable layer. 15 mol% of ethylene isophthalate, said properties being represented by a static friction coefficient for the heat-sealable layer tested against itself, situated within the range of values ​​from 0.50 to 0.65 and by a heat-sealing resistance measured by sealing the layer. heat-sealable with itself located within the range of values ​​that goes from 200 to 1500 Nm^-1.

The film composites according to the present invention are suitable to be heat sealed with themselves or with the surfaces of other films such as polyethylene terephthalate films using conventional heat sealing apparatus and conditions, the seal being formed by heating the film composite.

ES 2 139 589 T3 up to a temperature at which the essentially amorphous heat-sealable layer is softened and bonded to the receiving surface. Heat sealable layers comprising a 70 to 85 mole% ethylene terephthalate / 30 to 15 mole% ethylene isophthalate copolyester are particularly suitable for heat sealing. A composite film according to the invention preferably has a heat-sealing resistance measured by sealing the heat-sealable layer with itself of> 200, more preferably> 300 and particularly> 400 Nm^-1.

The film compounds according to the invention also have acceptable handling properties, preferably exhibiting a coefficient of static friction of <1.0, more preferably <0.8, and particularly <0.65.

Modifying the surface of the substrate and / or the heat-sealable layer (s), e.g. ex. By flame treatment, ion bombardment, electron beam treatment, ultraviolet light treatment or preferably by corona discharge, it can improve the adhesion of any subsequently applied layers, such as e.g. ex. metallized layers, but may not be essential to provide satisfactory adhesion.

The preferred corona discharge treatment can be carried out in air at atmospheric pressure with conventional equipment using a high frequency, high voltage generator preferably having a power output of 1 to 20 kW with a potential of 1 to 100 kV. Discharge is conveniently carried out by passing the film over a dielectric support roll at the discharge station at a linear speed of preferably 1.0 to 500 m per minute. The discharge electrodes can be positioned 0.1 - 10.0 mm from the surface of the moving film.

The layers of a composite film according to the invention may conveniently contain any of the additives that are conventionally used in the manufacture of polymeric films. Thus, agents such as colorants, pigments, voiding agents, lubricants, antioxidants, antiblocking agents, surface active agents, slip agents can be incorporated as appropriate into the substrate and / or heat sealable layer (s). , gloss enhancing agents, prodegradants, ultraviolet stabilizers, viscosity modifiers and dispersion stabilizers.

In this specification the following test methods have been used to determine certain properties of film compounds:

The characteristics of the protrusions peaks were measured using a Topo-3D Profiler (manufactured by Wyko Corporation). The statistical analysis of the cuispids (that is, of the peaks of the projections) was carried out establishing a cut-off height of 5 nm and a threshold value> 5 nm, in order to obtain the number of cuispids or peaks of the outgoing. The mean height (the height measured from the mean level of the surface of the heat sealable layer) of an overhang peak was calculated from the resulting probability plot as the value in nanometers for which the height of 50% of all peaks is below. The shape, that is, the height / width ratio, of an overhang peak was determined by measuring the height of a overhang peak (as defined above) and dividing it by the width of the base of the peak measured at medium surface level of heat sealable layer. A mean height / width value was calculated for 10 typical overhang peaks.

The heat seal strength was measured by sealing the heat sealable layer to itself at 140^°C for 1.0 second under pressure of 103 kPa (15 psi) (psi = pounds per inch²), cool to room temperature and measure the force required under linear tension per unit width of the seal to separate the sealed films at a constant rate of

4.23 mm / second.

The coefficient of static friction of the heat-sealable layer was measured against itself by an inclined plane method based on the ASTM D 4516-87 test, using an IPST Model (Specialist Engineering, Welwyn, UK).

The wide angle turbidity was determined as the percentage of transmitted light that deviates from normal to the surface of the film by an average amount of more than 2.5^° arc during passage through the film, essentially following the ASTM D 1003-61 test, using a Hazegard XL211 Turbidimeter (BYK Gardner, USA).

The invention is illustrated below with reference to the accompanying drawings, in which:

Figure 1 is a schematic sectional elevation view, not to scale, of a polymer film having a heat sealable layer adhered directly to a first face of the substrate.

Figure 2 is a similar schematic elevation view of a polymer film with an additional heat sealable layer adhered to the second face of the substrate.

Referring to Figure 1 of the drawings, the film comprises a polymer substrate layer (1) having a heat sealable layer (2) attached to one face (3) thereof, with particulate material (4) directly under the heat sealable layer surface.

The film of Figure 2 also comprises an additional heat-sealable layer (6) attached to the second face (5) of the substrate (1). The additional heat-sealable layer (6) also comprises particulate material (7) directly under the surface of the layer.

The invention is further illustrated below with reference to the following Examples.

Example 1

Film composites were prepared comprising polyethylene terephthalate as a substrate and a heat sealable layer comprising a copolyether of 82 mole% ethylene terephthalate / 18 mole% ethylene isophthalate.

The aforementioned polyesters were prepared using a conventional process that comprised the direct esterification of ethylene glycol with an acid (that is, with terephthalic acid in the case of polyethylene terephthalate,

ES 2 139 589 T3 or with a mixture of 82 mol% terephthalic acid and 18 mol% isophthalic acid in the case of the copolyester), followed by polycondensation. After completion of the polycondensation, the polymer was chopped into small granules suitable for extrusion.

Film composites were produced from the aforementioned polyesters by a duct coextrusion process in which the polyethylene terephthalate and copolyether streams supplied by separate extruders were joined in a tube leading to the manifold of an extrusion die, and were simultaneously extruded through the die under laminar flow conditions and without intermixing. The film compound emerging from the extrusion die was immediately rapidly quenched on a rotating, water-cooled metal drum having a polished surface, and was stretched to 3.6 times its original dimension in the extrusion direction at a temperature about 90 ° C. The stretched film was then coated with 8% by weight aqueous silica dispersion of an average particle size of 40 nm (Aerosil K330, Degussa) applied on the surface of the heat sealable layer. The longitudinally stretched film was then cross-stretched in a stenter oven to 4.2 times its original dimension at a temperature of about 100^°C. The film composite was finally thermoset under dimensional fixation in a stenter oven at a temperature of approximately 225^°C.

The resulting film composite consisted of a thermoset, biaxially oriented polyethylene terephthalate substrate and an amorphous copolyether layer. The final film thickness was 15 µm, the copolyester layer being 3 µm thick. The film was subjected to the test methods described above, and exhibited the following characteristics:

1) Peaks of the protrusions on the surface of the heat-sealable layer

Number = 700 per mm²Average height = 25 nm Average height / width ratio = 2.2 x ₁₀- 3

2) Turbidity = 2.7%

3) Coefficient of static friction of the heat-sealable layer = 0.63

4) Heat seal strength of heat sealable layer = 275 Nm^-1.

Example 2

The procedure of Example 1 was repeated, except that the polyethylene terephthalate substrate layer contained 1550 ppm of china clay of an average particle size of 0.8 μm, and the heat-sealable copolyether layer additionally contained 1250 ppm of china clay. an average particle size of 0.8 μm. The resulting film was subjected to the test methods described above, and exhibited the following characteristics:

1) Peaks of the protrusions on the surface of the heat-sealable layer

Number = 739 per mm²Average height = 63 nm

2) Turbidity = 6.7%

3) Coefficient of static friction of the heat-sealable layer = 0.56

4) Heat seal resistance of heat sealable layer = 250 Nm^-1.

Example 3

The procedure of Example 1 was repeated, except that the polyethylene terephthalate substrate layer contained 18% by weight, based on the weight of the polymer, of a finely divided particulate barium sulfate filler having an average particle size. 0.4 μm. The final film thickness was 250 µm, with the copolyester layer being 40 µm thick. The resulting film was subjected to the above-described test methods, and exhibited the following characteristics:

1) Peaks of the protrusions on the surface of the heat-sealable layer

Number = 710 per mm²Average height = 26 nm

2) Turbidity - not applicable, opaque film

3) Coefficient of static friction of the heat-sealable layer = 0.58

4) Heat seal strength of heat sealable layer = 1400 Nm^-1.

Example 4

AND^í This is a comparative example not in accordance with the example. The procedure of Example 2 was repeated, except that the silica coating step was omitted, and the polyethylene terephthalate substrate layer contained no china clay filler. The thickness of the final film was 75 µm, with the copolyester layer being 12 µm thick. The resulting film was subjected to the test methods described above, and exhibited the following characteristics:

1) Peaks of the protrusions on the surface of the heat-sealable layer

Number = 26 per mm²Average height = 26 nm

2) Turbidity = 0.3%

3) Coefficient of static friction of the heat-sealable layer => 1.2

4) Heat seal strength of heat sealable layer = 590 Nm^-1.

The above examples illustrate the improved properties of the film composites according to the present invention.