WO2008065454A1

WO2008065454A1 - Film

Info

Publication number: WO2008065454A1
Application number: PCT/GB2007/050733
Authority: WO
Inventors: Christopher Stewart Konkel
Original assignee: Innovia Films Ltd
Current assignee: Innovia Films Ltd
Priority date: 2006-12-01
Filing date: 2007-11-30
Publication date: 2008-06-05
Anticipated expiration: 2009-06-01
Also published as: EP2097335A1

Abstract

The present invention concerns a cavitated polymeric film comprising hollow glass microspheres therein in an amount of not more than about 40% by weight of the film.

Description

FILM

The present invention relates to a film, in particular to a low density cavitated film, and relates more particularly to cavitated films and packages produced therefrom comprising glass microspheres, and to a manufacturing method for such films.

Polymer films are extensively used in many industries for countless different uses. With such a variety of uses, it is desirable to introduce features in a film to meet the requirements of a particular use, such as for example functional or aesthetic display features of the film.

One such desirable feature is the ability to produce a cavitated or voided material having a low density whilst maintaining good aesthetic, tactile and thermal properties. A cavitated polymeric film is a polymeric film which contains cavities or "voids" within the material.

One known method of creating voids in a polypropylene film is to incorporate particles into the film. US2004213981 describes a simultaneously orientated polyolefinic film comprising particles in at least one layer thereof, said particles incompatible with said layer to cause the initiation of voids therein when the cast polyolefin is stretched simultaneously in both the MD and TD. The particles have a mean aspect ratio of about 1 (e.g. spherical or boulder-like), with a narrow size distribution. The mean particle size is said to be from about 3 to about 10 microns, (preferably about 6 microns). The disclosed films are substantially free of particles above about 12 microns in size, and optionally also substantially free of particles below about 3 microns in size.

GB1416626 discloses plastic films having low adherence comprising 1 to 50 weight per cent of glass spheres of diameter 4 to 105 microns.

GB1428747 discloses a self-supporting film of a synthetic polymeric material containing, by weight of the polymer, from 0.01 to 5% by weight of glass micro beads having a weight average particle diameter not exceeding 35 microns. This film is said to have the advantage of improving the slip and anti-blocking characteristics of the film to aid prevention of tearing when rewinding and unwinding the film to or from a reel.

It is an object of the invention to provide a cavitated polymer film having a lower density than standard cavitated films whilst providing superior aesthetic, tactile and thermal properties. It is also an object of the present invention to provide useful articles, particularly in the field of packaging, made from or incorporating such films.

According to the present invention there is provided a cavitated polymeric film comprising hollow glass microspheres therein in an amount of not more than about 40% by weight of the film.

It has surprisingly been found that cavitated films of extremely low density, and of excellent appearance and texture, can be produced by using hollow glass microspheres as a cavitating agent, and that by selecting such microspheres with satisfactory crush strength and/or glass content, anticipated problems with crushing of the microspheres during processing are not realised, or at least are minimised to an acceptable degree. It has also surprisingly been found that films made in accordance with the invention can have excellent thermal insulation properties, and the use of such films in thermally insulating packaging is particularly contemplated.

According to the present invention there is provided a packaging article comprising a cavitated polymeric film comprising hollow glass microspheres therein in an amount of not more than about 40% by weight of the film, the packaging article having a thermal insulation function.

There is also provided the use of a cavitated polymeric film comprising hollow glass microspheres therein in an amount of not more than about 40% by weight of the film in a packaging article to provide a thermal insulation function.

There is also provided a method of thermal insulating a packaged product comprising packaging the product at least partially in a cavitated polymeric film comprising hollow glass microspheres therein in an amount of not more than about 40% by weight of the film.

The thermal insulation can be effective in both hot and cold applications. Thus, the film of the invention can be used to package a cold product (ice cream, for example) and provide a thermal insulating function delaying warming of the cold product, or it can be used to package a hot product (ready-to-drink coffee for example) and provide a thermal insulating function delaying cooling of the hot product.

The thermal resistance R of the films in accordance with the invention (measured in Kmm²/W) is preferably at least about 2500, preferably at least about 2750, more preferably at least about 3000 and most preferably at least about 3500.

Preferably the thermal conductivity of the films in accordance with the invention is less than about 0.0001 W/mm.K, more preferably less than about 0.000075 W/mm.K, and most preferably less than about 0.000065 W/mm.K. In some cases the thermal conductivity of films according to the invention may be even lower, less than 0.00005 W/mm.K.

According to the invention there is provided a packaged cold product wherein the film of the invention is used in the packaging.

The cold product may be ice cream, iced drinks or cold drinks, for example.

There is also provided a packaged hot product wherein the film of the invention is used in the packaging.

The hot product may be a hot drink for example. The use of the film in a packaging context may, for example, include its use as a container, sleeve, lid, label or wrapper. The film may be used on its own or may be combined with other materials, for example as part of a laminated structure.

The films of the invention may also be used in connection with an innerseal laminate. Innerseal films are commonly used on packaging containers such as bottles, tubs, cartons, jars, pots, tubes and other types of container to provide a substantially airtight or perhaps even hermetic seal to protect the product inside the container from exposure to the atmosphere and provide tamper evidence in its pre-sale condition. Commonly such a seal is located beneath or within an openable closure of the container such as a lid or cap. The innerseal desirably has properties such as sealability, peelability, printability, moisture transfer prevention, vapour transfer prevention, mechanical strength and cuttability, although the precise requirements of any particular innerseal are determined at least to some extent by its end application.

Accordingly, there is also provided in accordance with the invention an innerseal laminate comprising a voided biaxially oriented polypropylene film having a thickness of from 20 μm to 500 μm (for example from 50 μm to 250 μm), a density in the range of from 0.2 to 0.9 g/cm³, and at least one other property selected from: 1. A yield of at least about 40 cm²/g 2. Light transmission of less than about 20%

3. Tensile strength (at break) of at least about 50 MPa in one or both of the machine and transverse directions

4. Elongation (at break) of at least about 50% in one or both of the machine and transverse directions

5. 1% Secant modulus of at least about 1000 in one or both of the machine and transverse directions, the polypropylene film being voided (or cavitated) with hollow glass microspheres.

The invention further provides an innerseal laminate comprising a cavitated polymeric film comprising hollow glass microspheres therein. The innerseal laminate may comprise a metallic layer, such as a metallic foil (eg aluminium) layer, and may further comprise a sealing layer, such as an induction sealing layer. The laminate may also comprise one or more polyester (eg PET) layers. The invention also concerns a container sealed with such an innerseal laminate.

The cavitated polymeric film of the invention preferably comprises hollow glass microspheres therein in an amount of not more than 35% by weight of the film. For example, the film may comprise from about 0.5% by weight to about 35% by weight of hollow glass microspheres, from about 5% by weight to about 35% by weight of hollow glass microspheres, from about 5% to about

30% by weight of hollow glass microspheres, or from 7.5% to about 25% by weight hollow glass microspheres. The glass microspheres preferably have an average isotactic crush strength of greater than about 500psi, more preferably greater than 1 ,000psi, still more preferably greater than 2,500psi and most preferably greater than 5,000psi. The average glass content of the microspheres is preferably at least about

5%, more preferably at least about 10% v/v, yet more preferably at least about

15% v/v, more preferably at least about 20% v/v, for example from about 20% v/v to about 50% v/v. The glass microspheres may have an average diameter of from about 5 to about 75 micron, for example from about 15 to about 40 micron, preferably from about 15 to about 30 μm.

The films of the invention are found to be very low density, and yet with a range of excellent properties in appearance and texture. The film density is preferably less than about 0.6 kg/dm³, more preferably less than about 0.55 kg/dm³, and most preferably less than about 0.50 kg/dm³.

We have also found that the cavitated films according to the invention are more resistant to thermal transfer than a corresponding film of otherwise similar characteristics and formulation produced in the absence of hollow glass microspheres.

The films used in the present invention, prior to deposition of any coating and/or skin or lamination layer may comprise any suitable polymeric filmic substrate, such as films made from biopoiymers [e.g. polylactic and/or cellulosic films (e.g. microbial and/or regenerated cellulose film)]; thermoplastic films; polymeric films (for example films comprising: polyolefins [e.g. polypropylene and/or polyethylene] polyurethanes, polyvinylhalides [e.g. PVC], polyesters [e.g. polyethylene terephthalate-PET], polyamides [e.g. nylons] and/or non-hydrocarbon polymers) ; and/or multilayer and/or composite sheets formed by any suitable combinations and/or mixtures of thereof. Suitable filmic substrates therefore include polyolefinic films, but also polyester films, polyurethane films, cellulosic and PLA films.

The film may therefore comprise a cellulosic material, polymeric material and/or thermoplastic polymer, and may conveniently comprise polymers of low surface energy. More preferably the sheet comprises a homopolymer, a crystalline polymer and/or a polymer of randomly oriented amorphous noncrystalline polymer chains. Most preferably the sheet comprises: polyolefins [e.g. polypropylene and/or polyethylene] polyurethanes, polyvinylhalides [e.g. polyvinyl chloride (PVC)], polyesters [e.g. polyethylene terephthalate-PET], polyamides [e.g. nylons] and/or non-hydrocarbon polymers).

Conveniently the polyolefin films to be used with the present invention may comprise one or more polyolefins [e.g. polypropylene homopolymer, polyethylene homopolymer (e.g. linear low-density polyethylene-LLDPE) and/or polypropylene/polyethylene copolymer(s); optionally in one or more layers]. The constituent polymers and/or layers in a film of the present invention may be oriented, blown, shrunk, stretched, cast, extruded, co- extruded and/or comprise any suitable mixtures and/or combinations thereof. Preferred films comprise a major proportion of polypropylene and/or an olefin block copolymer containing up to about 15% w/w of the copolymer of at least one copolymerisable olefin (such as ethylene). More preferred films comprise polypropylene homopolymer, most preferably isotactic polypropylene homopolymer.

Films may optionally be cross-linked by any suitable means such as electron beam (EB) or UV cross-linking, if necessary by use of suitable additives in the film.

The definition of polyolefin, as intended herein, is a polymer assembled from a significant percentage, preferably ≥50% by weight of one or more olefinic monomers.

The definition of copolymer herein, is a polymer assembled from two or more monomers. Such polymers may include, but are not limited to, polyethylene homopolymers, ethylene-α-olefin copolymers, polypropylene-α-olefin copolymers, polypropylene homopolymers, ethylene-vinyl acetate copolymers, ethyiene-methacrylic acid copolymers and their salts, ethylene-styrene polymers and/or blends of such polymers. The polymers may be produced by any suitable means, for example one or more of free radical polymerisation

(e.g. peroxy compounds), metallocene catalysis and/or coordination catalysis

(e.g. Ziegler and/or Natta catalysts and/or any variations thereof).

Polymeric resins used to produce the films of the present invention are generally commercially available in pellet form and may be melt blended or mechanically mixed by well-know methods known in the art, using commercially available equipment including tumblers, mixers and/or blenders. The resins may have other additional resins blended therewith along with well- know additives such as processing aids and/or colorants. Methods for producing polyolefin films are well-know and include the techniques of casting films as thin sheets through narrow slit dies, and blown-film techniques wherein an extruded tube of molten polymer is inflated to the desired bubble diameter and/or film thickness.

For example to produce a polymeric film the resins and additives may be introduced into an extruder where the resins are melt plastified by heating and then transferred to an extrusion die for formation into a film tube. Extrusion and die temperatures will generally depend upon the particular resin being processed and suitable temperature ranges are generally known in the art or provided in technical bulletins made available by resin manufacturers. Processing temperatures may vary depending upon process parameters chosen.

Thus, the polymeric film can be made by any process known in the art, including, but not limited to, cast sheet, cast film, or blown film. This invention may be particularly applicable to films comprising cavitated or non-cavitated polypropylene films, with a block copolymer polypropylene/polyethylene core and skin layers with a thickness substantially below that of the core layer and formed for example from random co-poiymers of ethylene and propylene or random terpolymers of propylene, ethylene and butylene. The film may comprise a biaxially orientated polypropylene (BOPP) film, which may be prepared as balanced films using substantially equal machine direction and transverse direction stretch ratios, or can be unbalanced, where the film is significantly more orientated in one direction (MD or TD). Sequential stretching can be used, in which heated rollers effect stretching of the film in the machine direction and a stenter oven is thereafter used to effect stretching in the transverse direction. Alternatively, simultaneous stretching, for example, using the so-called bubble process, or simultaneous draw stenter stretching may be used.

A film of the present invention may be oriented by stretching at a temperature above the glass transition temperature (Tg) of its constituent polymer(s). The resultant oriented film may exhibit greatly improved tensile and stiffness properties.

Conveniently a film comprising a propylene homopolymer is oriented at a temperature within a range of from about 145°C to165°C. Orientation may be along one axis if the film is stretched in only one direction, or may be biaxial if the film is stretched in each of two mutually perpendicular directions in the plane of the film. A biaxial oriented film may be balanced or unbalanced, where an unbalanced film has a higher degree of orientation in a preferred direction, usually the transverse direction. Conventionally the longitudinal direction (LD) is the direction in which the film passes through the machine (also known as the machine direction or MD) and the transverse direction (TD) is perpendicular to MD. Preferred films are oriented in both MD and TD. Orientation of the film may be achieved by any suitable technique. For example in the bubble process the polypropylene film is extruded in the form of a composite tube which is subsequently quenched, reheated, and then expanded by internal gas pressure to orient in the TD, and withdrawn, at a rate greater than that at which it is extruded, to stretch and orient it in the MD. Alternatively a flat film may be oriented by simultaneous or sequential stretching in each of two mutually perpendicular directions by means of a stenter, or by a combination of draw rolls and a stenter. A preferred oriented film comprises biaxially oriented polypropylene (known herein as BOPP), more preferably the BOPP film described in EP 0202812.

The degree to which the film substrate is stretched depends to some extent on the ultimate use for which the film is intended, but for a polypropylene film satisfactory tensile and other properties are generally developed when the film is stretched to between three and ten, preferably, seven or eight, times its original dimensions in each of TD and MD.

After stretching, the polymeric film substrate is normally heat-set, while restrained against shrinkage or even maintained at constant dimensions, at a temperature above the Tg of the polymer and below its melting point. The optimum heat-setting temperature can readily be established by simple experimentation. Conveniently a polypropylene film is heat-set at temperatures in the range from about 100⁰C to about 160⁰C. Heat-setting may be effected by conventional techniques for example by means one or more of the following: a stenter system; one or more heated rollers (e.g. as described in GB 1124886) and/or a constrained heat treatment (e.g. as described in EP 023776).

The film may comprise a major proportion of polypropylene such as isotactic polypropylene homopolymer, but also may comprise coextruded multilayer films where the polymer of at least one layer is isotactic polypropylene homopolymer, and the polymer of one or both outer layers is a surface layer polymer having different properties to the isotactic polypropylene homopolymer.

The sheet of the present invention may consist of only one layer, or the sheet may be multi-layered i.e. comprise a plurality of layers. The layers can be combined by lamination or co-extrusion. Preferably the sheet comprises at least three layers where at least one layer(s) are sandwiched between other layers such that none of such sandwiched layer(s) form either surface of the sheet.

A film of the invention may also be made by lamination of two coextruded films.

One or more layers of the film may be opaque or transparent depending on the end use of the film. Such layers may also comprise voids introduced by stretch orienting such a layer containing spherical particles of a material higher melting than and immiscible with the layer material (e.g. if the layer comprises isotactic polypropylene homopolymer, then such particles may be, polybutylene terephthalate, as shown, for example, in US 4632869 and US 4720716).

Multiple-layer films of the invention may be prepared in a range of thicknesses governed primarily by the ultimate application for which a particular film is to be employed. For general use films, having a mean thickness from about 10 μm to about 500 μm, preferably from about 15 μm to about 400 μm are suitable. For certain applications, such as packaging, preferred films have a mean thickness of from about 25 μm to 360 μm, most preferably from about 50 μm to about 350 μm.

If desired, before coating a sheet of the present invention (e.g. with a gas barrier coating of the present invention and/or any other coating and/or layer) may be subjected to a chemical or physical surface-modifying treatment to ensure that the coating and/or layer will better adhere to the sheet thereby reducing the possibility of the coating peeling or being stripped from the sheet. Known prior art techniques for surface pre-treatment prior to coating comprise, for example: film chlorination, i.e., exposure of the film to gaseous chlorine ; treatment with oxidising agents such as chromic acid, hot air or steam treatment; flame treatment and the like. A preferred treatment, because of its simplicity and effectiveness, is the so-called electronic treatment in which the sheet is passed between a pair of spaced electrodes to expose the sheet surface to a high voltage electrical stress accompanied by corona discharge.

Optionally if even adhesion of the coating is desired an intermediate continuous coating of a primer medium and/or anchor coating can be applied to a sheet surface treated by any of the methods described herein. Primer materials may comprise titanates and poly (ethylene imine) and may be applied as conventional solution coatings [such as poly (ethylene imine) applied as either an aqueous or organic solvent solution, e.g. in ethanol comprising about 0.5 wt. % of the imine]. Another primer medium comprises the interpolymerised condensation acrylic resins prepared in the presence of a Ci-βalkanol as described in either; GB 1134876 (condensing aminoaldehyde with an interpolymer of acrylamide or methacrylamide with at least one other unsaturated monomer); or in GB 1174328 (condensing aminoaldehyde with acrylamide or methacrylamide, and subsequently interpolymerising the condensation product with at least one other unsaturated monomer).

The film may comprise one or more additive materials. Additives may comprise: dyes; pigments, colorants ; metallised and/or pseudo metallised coatings (e.g. aluminium); lubricants, anti-oxidants, surface-active agents, stiffening aids, gloss-improvers, prodegradants, UV attenuating materials (e.g. UV light stabilisers); sealability additives; tackifiers, anti-blocking agents, additives to improve ink adhesion and/or printability, cross-linking agents (such as melamine formaldehyde resin); adhesive layer (e.g. a pressure sensitive adhesive); and/or an adhesive release layer (e.g. for use as the backing material in the peel plate method for making labels). Further additives comprise those to reduce coefficient of friction (COF) such as a terpolymer described in US 3753769 which comprises from about 2% to about 15% w/w of acrylic or methacrylic acid, from about 10% to about 80% w/w of methyl or ethyl acrylate, and from about 10% to about 80% w/w of methyl methacrylate, together with colloidal silica and camauba wax. Still further additives comprise slip aids such as hot slip aids or cold slip aids which improve the ability of a film to satisfactorily slide across surfaces at about room temperature for example micro-crystalline wax. Preferably the wax is present in the coating in an amount from about 0.5% to about 5.0% w/w, more preferably from about 1.5% to about 2.5% w/w. The wax particles may have an average size conveniently from about 0.1 μm to 0.6 μm, more conveniently from about 0.12 μm to abut 0.30 μm.

Yet further additives comprise conventional inert particulate additives, preferably having an average particle size of from about 0.2 μm to about 4.5 μm, more preferably from about 0.7 μm to about 3.0 μm. Decreasing the particle size improves the gloss of the film. The amount of additive, preferably spherical, incorporated into the or each layer is desirably in excess of about

0.05%, preferably from about 0.1% to about 0.5%, for example, about 0.15%, by weight. Suitable inert particulate additives may comprise an inorganic or an organic additive, or a mixture of two or more such additives.

Suitable particulate inorganic additives include inorganic fillers such as talc, and particularly metal or metalloid oxides, such as alumina and silica. Solid glass or ceramic micro-beads or micro-spheres may also be employed. A suitable organic additive comprises particles, preferably spherical, of an acrylic and/or methacrylic resin comprising a polymer or copolymer of acrylic acid and/or methacrylic acid. Such resins may be cross-linked, for example by the inclusion therein of a cross-linking agent, such as a methylated melamine formaldehyde resin. Promotion of cross-linking may be assisted by the provision of appropriate functional groupings, such as hydroxy, carboxy and amido groupings, in the acrylic and/or methacrylic polymer.

Yet still further additives comprise fumed silica for the purpose of further reducing the tack of a coating at room temperature. The fumed silica is composed of particles which are agglomerations of smaller particles and which have an average particle size of, for example, from about 2 μm to about

9μm, preferably from about 3 μm to about 5 μm, and is present in a coating in an amount, for example, from about 0.1% to about 2.0% by weight, preferably about 0.2% to about 0.4% by weight.

Some or all of the desired additives listed above may be added together as a composition to coat the sheet of the present invention and/or form a new layer which may itself be coated (i.e. form one of the inner layers of a final multi- layered sheet) and/or may form the outer or surface layer of the sheet. Alternatively some or all of the preceding additives may be added separately and/or incorporated directly into the bulk of the sheet optionally during and/or prior to the sheet formation (e.g. incorporated as part of the original polymer composition by any suitable means for example compounding, blending and/or injection) and thus may or may not form layers or coatings as such, These conventional other coatings and/or layers may thus be provided on top of or underneath the gas barrier coatings of the present invention and may be in direct contact thereto or be separated by one or more other intermediate layers and/or coats. Such additives may be added to the polymer resin before the film is made, or may be applied to the made film as a coating or other layer. If the additive is added to the resin, the mixing of the additives into the resin is done by mixing it into molten polymer by commonly used techniques such as roll-milling, mixing in a Banbury type mixer, or mixing in an extruder barrel and the like. The mixing time can be shortened by mixing the additives with unheated polymer particles so as to achieve substantially even distribution of the agent in the mass of polymer, thereby reducing the amount of time needed for intensive mixing at molten temperature. The most preferred method is to compound the additives with resin in a twin-screw extruder to form concentrates which are then blended with the resins of the film structure immediately prior to extrusion.

Formation of a film of the invention (optionally oriented and optionally heat-set as described herein) which comprises one or more additional layers and/or coatings is conveniently effected by any of the laminating or coating techniques well known to those skilled in the art.

For example a layer or coating can be applied to another base layer by a coextrusion technique in which the polymeric components of each of the layers are coextruded into intimate contact while each is still molten. Preferably, the coextrusion is effected from a multi-channel annular die such that the molten polymeric components constituting the respective individual layers of the multi-layer film merge at their boundaries within the die to form a single composite structure which is then extruded from a common die orifice in the form of a tubular extrudate.

A film of the invention may also be coated with one or more of the additives described herein using conventional coating techniques from a solution or dispersion of the additive in a suitable solvent or dispersant. An aqueous latex, (for example prepared by polymerising polymer precursors of a polymeric additive) in an aqueous emulsion in the presence of an appropriate emulsifying agent is a preferred medium from which a polymeric additive or coating may be applied.

Coatings and/or layers may be applied to either or both surfaces of the sheet. The or each coating and/or layer may be applied sequentially, simultaneously and/or subsequently to any or all other coatings and/or layers. If a gas-barrier coating of the present invention is applied to only one side of the sheet (which is preferred) other coatings and/or layers may be applied either to the same side of the sheet and/or on the reverse (other) side of the sheet.

A coating composition may be applied to the treated surface of sheet (such as the polymer film) in any suitable manner such as by gravure printing, roll coating, rod coating, dipping, spraying and/or using a coating bar. Solvents, diluents and adjuvants may also be used in these processes as desired. The excess liquid (e.g. aqueous solution) can be removed by any suitable means such as squeeze rolls, doctor knives and/or air knives. The coating composition will ordinarily be applied in such an amount that there will be deposited following drying, a smooth, evenly distributed layer having a thickness of from about 0.02 to about 10 μm, preferably from about 1 to about 5 μm. In general, the thickness of the applied coating is such that it is sufficient to impart the desired characteristics to the substrate sheet. Once applied to the sheet a coating may be subsequently dried by hot air, radiant heat or by any other suitable means to provide a sheet of the present invention with the properties desired (such as an optionally clear; optionally substantially water insoluble; highly oxygen impermeable coated film useful, for example in the fields of authentication, packaging, labelling and/or graphic art).

It would also be possible to use combinations of more than one of the above methods of applying additives and/or components thereof to a film. For example one or more additives may be incorporated into the resin prior to making the film and the one or more other additives may be coated onto the film surface.

In a multi-layer film in accordance with the invention having at least a substrate layer and a skin layer, the skin layer may be preferably ink printable. The skin layer has a thickness of from about 0.05 μm to about 2 μm, preferably from about 0.1 μm to about 1.5 μm, more preferably from about 0.2 μm to about 1.25 μm, most preferably from about 0.3 μm to about 0.9 μm.

The hollow glass microspheres in the present invention may comprise any suitable low density hollow microspheres. Exemplary hollow glass microspheres are available from 3M™ under the designation Scotchlite™ Hollow glass microspheres S60, which are made from water-resistant and chemically-stable soda-lime-borosilicate glass. They have an average diameter of 30 microns, an isostatic crush strength of 10,000 psi, a true density (i.e. the density of a single hollow microsphere, if measurable) of 0.60 g/cc, and a bulk density, taking into consideration the packing of a multitude of hollow microspheres, of 0.38 g/cc.

Embodiments of a film and according to the present invention will now be described, by way of example only, with reference to the accompanying drawings and examples, in which:

Figure 1 is a graph showing the relationship between the shear rate and the viscosity for both a known polypropylene homopolymer and a masterbatch sample of the new material comprising hollow glass microspheres;

Figure 2 is a graph showing the phase angle results for both a known polypropylene homopolymer and a masterbatch sample of the new material comprising hollow glass microspheres;

Figure 3 is a graph showing the results from the thermogravimetric analysis for both a known polypropylene homopolymer and a masterbatch sample of the new material comprising hollow glass microspheres; Figure 4 is a graph showing the relationship between the concentration of hollow glass microspheres in the core of a material and its opacity;

Figure 5a is an illustration of the cavity shape formed in a film incorporating hollow glass microspheres from above a web made according to the present invention at 1% loading of hollow microspheres;

Figure 5b is an illustration of the cavity shape formed in a film incorporating hollow glass microspheres from the side of a web made according to the present invention at 1% loading of hollow microspheres.;

Figure 6 is a graph showing the results of thermal transfer tests on sample materials made in accordance with the present invention;

Figure 7 is a graph showing the heat seal strength for single web samples of the present invention for a first material comprising 1% bubbles and a second material comprising 2% bubbles wherein the samples have been sealed to themselves at a pressure of 15 PSI applied for 0.5 seconds;

Figure 8 is a graph showing the heat seal strength for single web samples of the present invention for a first material comprising 1% hollow glass microspheres and a second material comprising 2% hollow glass microspheres wherein the samples have been sealed to themselves at a pressure of 5 PSI applied for 0.2 seconds; Figure 9 is a graph showing the thermal insulation properties of films in accordance with the invention;

Figure 10 depicts a laboratory experiment for investigating the thermal insulating properties of films in accordance with the invention;

Figure 11 is a graph showing the insulative properties of films in accordance with the invention;

Figure 12 is a graph showing the insulative properties of films in accordance with the invention;

Figure 13 is a graph showing the thermal resistance properties of films in accordance with the invention;

Figure 14 depicts in schematic form an experimental setup for measuring the frangibility of the films in accordance with the invention; and

Figure 15 depicts the experimental procedure of the Figure 14 setup in operation.

Examples

Masterbatch Preparation and Properties Commercially available hollow glass microspheres, such as Scotchlite™ S60 hollow glass microspheres available from 3M were used in the formation of a cavitated film. These hollow glass microspheres have an average diameter of about 30 μm, but may have individual diameters ranging from about 5 to about 100 μm. These hollow glass microspheres were pre-processed as a masterbatch into a Melt Flow Index (MFI) 3 polypropylene homopolymer.

An MFI 8 masterbatch was also satisfactorily prepared.

Initial screening of the material showed that the material could be produced in polypropylene masterbatches with little or no unsatisfactory breakage of the hollow glass microspheres in a range of glass loading percentages of up to about 40% by weight, for example in the range of from about 20 to 40%. Master batch samples of 30% and 40% glass loading respectively were combined with polypropylene and processed though a multilayer extruder to produce almost identical ribbons at approximately 10% overall glass loading. The 40% master batch was selected for further experimentation. The flow characteristics of the masterbatch material were investigated and compared with the unloaded polypropylene. The results are shown in Figures 1 and 2. The rheological properties (Figure 1) of the masterbatch are similar to polypropylene, but at low shear rates the presence of the hollow glass microspheres acts to thicken the material. A similar explanation may be made for the phase angle results shown in Figure 2. Thermogravimetric analysis of the masterbatch was carried out to verify that negligible degradation of the material occurred at the anticipated processing temperatures. The results are shown in Figure 3.

Preparation of Film

Polypropylene was blended with the master batch at different feed ratios to giving a final hollow glass microsphere loading in the resulting film of 1 , 2, 5, 10 and 15% by weight. The initial run condition was chosen to be substantially the same as that for a standard cavitating film mixture. This core polymer blend and a terpolymer of propylene, ethylene and butylene as the skin polymers were coextruded from a triple channel annular die to form a polypropylene film having a core layer comprising hollow glass microspheres and skin layers on either surface thereof. The resultant polypropylene tube was then cooled by passage over a mandrel within the tube, and externally quenched by passage through a water bath surrounding the mandrel, heated to stretching temperature, expanded by internal gas pressure, and withdrawn from the expansion zone at a rate greater than that at which it was fed thereto, so that the tube was stretched to seven times its original dimensions in both the direction of extrusion and in a direction transverse thereto. The stretched tubular film was then opened out to form flat film which was subsequently heat-set at a temperature of 120⁰C. on a matt-surfaced roller heat-setter of the kind described in GB-A-1124886 to form an opaque voided BOPP film suitable for use as a substrate in the example herein. The film was extracted either as a single web film, or as a laminated film approximately double the thickness of the single web. Examples 1 to 15

Films with the following microsphere loadings were prepared;

*(in which SW refers to "single web")

Two sets of results using slightly different processing conditions are set out below in Table 1 and Table 2, and show that increasing the level of hollow glass microspheres in the feed increases the thickness of the resulting blown film, and reduces its density. Increasing the percentage of hollow glass

10 microspheres up to or greater than 5% changes the appearance of the sample giving it a more uniform appearance. Increasing the percentage of hollow glass microspheres was found also to increase the metallic sheen of the resulting material.

15 Table 1

Table 2

As can be seen from the results below in Tables 3 and 4 increasing the amount of hollow glass microspheres in a sample causes an increase in narrow angle haze and a reduction in average gloss and wide angle haze. Addition of the hollow glass microspheres to the material causes the material to take on a metallic sheen. Table 3

Table 4

* Sample is too opaque for analysis.

Measurement of the samples opacity shows good correlation between concentration of hollow glass microspheres and sample thickness as shown in Figure 4. At higher percentages the opacity appears to be levelling off.

Samples were submitted for optical and scanning electron microscopy.

From the standard microscopic results it was possible to compare cavity size versus that of the glass bubble creating it. The 1% Glass bubble containing films showed that there was a 900 percent increase in size around the bubble (i.e. a bubble of 10 microns gave a cavity of 100 microns) in the plane of the web. Scanning electron microscope (SEM) analysis showed that increasing the amount of hollow glass microspheres in the sample increases the final film thickness whilst reducing the density. Samples containing 1% Glass Bubble's were viewed through the optical microscope and each glass bubble in the sample generates a cavity that is approximately 10 times its own size. SEM analysis of samples through the thickness of the web showed that increasing the amount of hollow glass microspheres in the sample increases the amount of cavitation, hence the final film thickness increases whilst reducing the density. From all of the microscopic evidence obtained it was concluded that the overall shape of cavities formed using hollow glass microspheres is substantially as schematically shown in Figures 5a and 5b, wherein the each glass bubble 10 forms a cavity 12 in the material.

Results in Tables 5 and 6 show both oxygen transmission rate (OTR) and water vapour transmission rate (WVTR) for the various films made. There appears to be a dramatic increase in oxygen transmission rate when the density of the film drops to below 0.35 g/cm3 in conjunction with an increase in thickness to above approximately 100 microns. This jump in transmission rate is not observed for moisture. One possible explanation for this could be that the film structure changes from a single strata of voids through the film thickness to a multi strata structure where each void is significantly thick verses the overall film thickness and each individual void is separated by a thin layer of polymer. Permeation through this kind of structure will favour a permeant that has good solubility in the film substrate material as well as good diffusion through the substrate material as the multiple polymer/air interfaces require the permeant to solublise and desolublise many times. This favours oxygen transmission over water vapour transmission and may therefore explain the measured results. An alternative or additional explanation may concern the entrainment of air in through the cut side of the film thus giving variation in the OTR test baseline for oxygen, and higher OTR values.

Table 5

Table 6

As expected the increase in thickness and cavitation of a material have increased the resistance to thermal transfer. Samples of the film have been glued to the surface of a standard plastic cup; the cup was filled with 1 pint of water and placed in an 850 watt microwave where it was heated for 6

5 minutes. Results were taken every minute for 6 minutes, a comparison of temperature between the cup and the label (cup temperature - label temperature) was made. As can be seen from Figure 6, increasing the percentage of hollow glass microspheres in material causes a corresponding increase in temperature difference between cup and label. This effect seems

10 to reduce with increasing concentration - samples containing 5 and 10% loadings are similar.

Tables 7 and 8 below show that there is a rapid drop off in material tensile properties when only a small concentration of hollow glass microspheres is 15 added. At a 1-2% glass bubble loading the Young's Modulus of the material is roughly half of its original value where as at 10% it is approximately 25% of its starting value.

20 Table 7

Table 8

Puncture resistance of glass bubble filled films were measured using an internal test method. The averaged results for each film type showed a rapid drop in puncture resistance (maximum load) and in the elasticity of the polymer (displacement of punch at maximum load) as the percentage of hollow glass microspheres in the sample increased. In the results Tables 9 and 10 below there is no correction for thickness in these samples and thus the results are for a given film. Table 9

Table 10

Results for the heat sealing of the single web samples to themselves follow the expected trends - samples sealed at higher pressure and for greater times form better seals. As can be seen from Figures 7 and 8, both the 1% hollow glass microspheres sample and the 2% bubble sample seem to reach a maximum seal strength of around 300. In both cases the higher the percentage of hollow glass microspheres in the core the better the heat seal achieved at a given temperature. Assuming a heat seal of 150 is required for a seal to form the 2% sample achieves this by 115°C at 15 PSI 0.5 sec and 130⁰C at 5 PSI 0.2 sec, whereas the 1% sample achieves it at 130⁰C at 15 PSI 0.5 sec and 145°C 5 PSI 0.2 sec.

Thermal Insulation Properties

Three 500ml beer cans were opened and their contents emptied. The cans were then washed with deionised water. 500 ml of deionised water was then added to the cans, together with a magnetic mixer bar. The film sample was cut to a label sleeve shape (222.5 x 125 mm) and wound tightly around the can, and secured with tape. The labelled cans were then placed in a fridge for a minimum of 18 hours at 6°C.

A water bath was heated to 36°C and the water pumped around a rubber tubing system using a peristaltic pump. The tube was immersed in the water bath for 30 minutes prior to each run. After immersion the tube was removed, dried and wrapped tightly around the cold labelled can, 4Yz times. The temperature of the water in the can was taken using a thermocouple suspended midway through the can. The results are shown in Figure 9. The 350 μm sample is a film sample corresponding to Example 14 and the 110 μm sample is a film sample corresponding to Example 4.

An experimental apparatus was set up as shown in Figure 10. The film sample of Example 14 was stuck to the thermocouples and the can as shown in the Figure. Ice (200 g) was then placed into the can and the can filled to level using a saturated water salt mixture. Probes were then added and the recorder started. Results were recorded from all 4 points shown. The results are shown in Figure 11.

Examples 16 to 21 A number of further film samples were prepared in accordance with the previously described method and further thermal conductivity measurements were conducted using the Kes Thermolab Il Method with the following parameters:

• Test area 25 sq cm • Temperature difference 10⁰C

• 2 samples of each material were tested 4 times each and the average values recorded.

The results are shown in Table 11 Table 11

^*Example 16 was prepared with a smaller grade of Scotchlite™ glass bubbles than the grade used in the other Examples

#Example 19 was a metallised film

$Example 20 was a label film with a laminated printable layer. Further tests were conducted using some of these samples on the apparatus shown in Figure 10. The results are shown in Figure 12.

Examples 22 to 37 A number of further film samples were prepared in accordance with the previously described method and the thermal resistance of these films was investigated using the Kes Thermolab Il apparatus. The results are shown in Table 12 and in Figure 13. Table 12

Key M Metallised W Sample contains TiO2 in the core of the material

Examples 38 to 47 Ultrasound Seal Strength

Tensile strengths of Ultrasound sealed samples were determined using 1 cm wide ultrasonically seal sample strips. Samples were tested on an lnstron with a 100 newton load cell. The results below are an average of 5 tests samples and give an example of the materials seal strength. The results are shown in Table 13 Table 13

Examples 48 to 52

Tear Strength and Puncture Resistance

The tear strength and puncture resistance of a number of film samples was measured, using the procedure already described in relation to puncture resistance, and using the ASTM D 1938 trouser tear test method for tear strength. The results are shown in Table 14. Table 14

Example 53

Laser Printability

Normal polypropylene films cannot be printed using laser printers due to melting of the polypropylene. Surprisingly, samples of the glass bubble filled materials can successfully be printed on a laser printer. Samples of 350 micron cavitated material were successfully printed on a HP LaserJet 5 printer. The uncoated material showed very limited curl after printing whilst the calcium carbonate coated variant of the film passed through and showed no visible after-effects. Both samples received the standard HP LaserJet 5 test print which was clearly legible.

Examples 54 to 57 Frangibility A film sample was stuck in place as shown in Figure 14 and the film peeled off the tape by hand pulling the tape from one end, as shown in Figure 15. The distance to the first continuous sample of residual film sample on the tape was measured with a calibrated ruler and recorded. Each Test was repeated 5 times as can be seen from the results Table 15. Table 15

Thus, certain films in accordance with the invention are found to have sufficient frangibility to have potential application in a security context, for example as tamper evident means.

Claims

1. A cavitated polymeric film comprising hollow glass microspheres therein in an amount of not more than about 40% by weight of the film.

2. A cavitated polymeric film according to claim 1 comprising hollow glass microspheres therein in an amount of not more than 35% by weight of the film.

3. A cavitated polymeric film according to claim 2 comprising from about 0.5% by weight to about 35% by weight of hollow glass microspheres.

4. A cavitated polymeric film according to any one of claims 1 to 3 wherein the glass microspheres have an average isotactic crush strength of greater than about 500psi.

5. A cavitated film according to any one of claims 1 to 4 wherein the average glass content of the microspheres is at least about 5% v/v.

6. A cavitated film according to claim 5 wherein the average glass content of the microspheres is at least about 10% v/v.

7. A cavitated film according to claim 6 wherein the average glass content of the microspheres is at least about 20% v/v.

8. A cavitated film according to claim 7 wherein the average glass content of the microspheres is from about 20% v/v to about 50% v/v.

9. A cavitated film according to any one of claims 1 to 8 having a thickness of from about 10 μm to about 500 μm.

10. A cavitated film according to any one of claims 1 to 9 having a density of less than about 0.6 kg/dm³.

11. A cavitated film according to claim 10 having a density of less than about 0.55 kg/dm³.

12. A cavitated film according to claim 10 having a density of less than about 0.50 kg/dm³.

13. A cavitated film according to any one of claims 1 to 12 being more resistant to thermal transfer than a corresponding film in the absence of hollow glass microspheres.

14. A cavitated film according to any one of claims 1 to 13 wherein the glass microspheres have an average diameter of from about 5 to about 75 micron.

15. A film according to claim 14 wherein the glass microspheres have an average diameter of from about 15 to about 40 micron.

16. A film according to claim 15 wherein the glass microspheres have an average diameter of from about 15 to about 30 μm.

17. A film according to any one of claims 1 to 16 wherein the polymeric film comprises a polypropylene homopolymer.

18. A film according to any one of claims 1 to 17 being a multilayer film having a core layer, the hollow glass microspheres being present in the core layer.

19. A film according to claim 18, the hollow glass microspheres being present only in the core layer.

20. A film according to any one of claims 1 to 19 wherein the thermal resistance R measured in K.mm²/W is at least 2500.

21. A packaging article comprising a cavitated polymeric film according to any one of claims 1 to 20.

22.A packaging article according to claim 21 , the packaging article having a thermal insulation function.

23.A packaging article according to claim 22 or claim 23, being or comprising a container, sleeve, lid, label, wrapper or innerseal laminate.

24. Use of a cavitated polymeric film according to any one of claims 1 to 20 in a packaging article to provide a thermal insulation function.

25.A method of thermal insulating a packaged product comprising packaging the product at least partially in a cavitated polymeric film according to any one of claims 1 to 20.

26.A packaged cold product wherein a cavitated polymeric film according to any one of claims 1 to 20 is used in the packaging.

27.A packaged cold product according to claim 26 wherein the cold product is selected from ice cream, iced drinks or cold drinks.

28. A packaged hot product wherein a cavitated polymeric film according to any one of claims 1 to 20 is used in the packaging.

29.A packaged hot product according to claim 28 wherein the hot product is a hot drink.

30. Use of a frangible film according to any one of claims 1 to 20 as a tamper evident means.