GB2308318A - Polymeric films - Google Patents

Abstract

Polymeric films consist of at least one layer 1 of a thermoplastic polymer having thereon a layer of a dispersion of a particulate filler 2 in a dispersant 3, the dispersant consisting of or containing a tackifier. The dispersion can form an outer layer or an internal layer of the films. Such films have good barrier properties to oxygen and other gases and are of particular use in the ensiling of vegetable matter such as hay and straw. If the dispersant is migratory and/or volatile it can enable the dispersion to concentrate, thereby enabling dispersions with low concentrations of filler to concentrate.

Description

Polymeric Films This invention concerns polymeric films, and in particular polymeric films having gas barrier properties.

Thermoplastic polymers have found wide acceptance as raw materials for the production of polymeric films, especially for packaging purposes. Thus they are easy to form into films using simple forming techniques such as extrusion, and they are in general relatively inexpensive. In addition, different polymers often have different properties from those of other polymers, thereby enabling films to be produced having different properties or which have properties which result from a combination of the properties of the combination of polymers from which they are made.

Polyalkenes, in particular polymers derived from major amounts of ethylene or propylene, have the advantage for film formation that they can readily be formed into films by melt extrusion, and in general the resulting films present a good barrier to water vapor, especially after the films have been oriented by stretching. Such films can usually be stretched using relatively low forces even at room temperature, unless they have previously been subjected to high degrees of orientation.

However, films made solely from polyalkenes usually present a relatively low barrier to oxygen even when highly oriented.

Other polymers have been found to present a good barrier to oxygen, but they present a relatively poor barrier to water vapor. Examples of such polymers include nylons and ethylene/vinyl alcohol copolymers. However, such polymers do have disadvantages, very thin layers of ethylene/vinyl alcohol copolymers, for example less than 2um thick, presenting a very high barrier to oxygen, but this oxygen barrier falls dramatically if the ethylene/vinyl alcohol copolymer is exposed to moisture.

This problem can be overcome by sandwiching a layer of an ethylene/vinyl alcohol copolymer between two layers of polyalkenes, thereby preventing water vapor from contacting the ethylene/vinyl alcohol copolymer layer. However, due to the incompatibility between the relatively polar ethylene/vinyl alcohol copolymers and the relatively non-polar polyalkenes, adhesive or so-called tie layers have to be used between them in order to prevent delamination. Examples of such adhesive layers are layers of copolymers of ethylene or propylene with unsaturated carboxylic acids or esters or anhydrides thereof.

Despite the high barrier both to water vapor and to oxygen which such multi-layer films can exhibit, such properties generally require the polyalkylene layers to have been oriented by stretching. However, although layers of polyalkenes can be readily oriented by stretching, layers of ethylene/vinyl alcohol copolymers tend to break up due to fibrillation of the copolymer layer when they are stretched, thereby destroying the oxygen barrier created by the ethylene/vinyl alcohol copolymer layer.

This can be avoided by first producing the necessary oriented layers of polyalkenes and then extrusion laminating the two layers together using an ethylene/vinyl alcohol copolymer, tie layers being used to bring about adhesion between the polyalkene layers and the ethylene/vinyl alcohol copolymer. However, such a process is complicated compared with processes in which the respective layers of the final film are coextruded and then stretched. In addition, the film is then relatively inextensible and therefore unsuitable for packaging processes which require an easily extensible stretch film.

Proposals have been made hitherto for avoiding the rupture of layers of ethylene/vinyl alcohol copolymers when they are stretched. In particular it has been proposed to blend the ethylene/vinyl alcohol copolymers with polyalkenes using a compatibilizer to reduce the inherent incompatibility between the blended polymers. However, the use of compatibilizers complicates film manufacture, and it also increases costs.

Nylon films in general provide a very high barrier to oxygen, and although they can be oriented, the force necessary to stretch such films at room temperature is very high.

An alternative method of imparting oxygen barrier properties to polyalkene films is to incorporate a layer of a metal into the film structure. Metal foils, for example aluminum foils, provide polymeric films with an extremely high barrier to gases, including both oxygen and water vapor. However, films including layers of metal foils have the disadvantage that apart from the complicated processing required for their manufacture, the metal foil layer can readily become damaged by handling, thereby leading to failure of the gas barrier. In addition, such films clearly cannot be stretched, stretch wrap films typically being required in use to undergo elongations of 20 to 200%.

As an alternative to metal foils, polymeric films can be provided with vacuum deposited layers of metals to improve the gas and water vapor barrier properties of the films. However, such layers usually have micro perforations through the metal layers where metal has not been deposited, leading to reduced gas barrier properties. Furthermore, stretching such films even by only a few percent after manufacture brings about an even greater breakdown in the barrier properties of such films.

A still further alternative proposal for increasing the barrier properties of polymeric films is to include a layer which incorporates a particulate filler, the filler particles acting as a physical barrier to the passage of gases through the films.

However, in order to achieve good gas barrier properties using filled layers, high filler loadings are required, particularly if the films are to be stretched. As will be appreciated, stretching a film increases its area, with a simultaneous and proportionate reduction in its thickness. This results not only in a reduction in the thickness of the filled layer, but a separation of the filler particles, thereby leading to a reduction in the barrier which can be achieved by the filled layer. Although in theory higher filler loadings could be used, there are practical problems in doing so since processing problems can occur as the amount of polymer used to disperse the filler is reduced. Furthermore, stretching filled polyolefins can under certain conditions lead to the formation of microvoids which in turn can result in a reduction in the barrier properties of the films, particularly if the resulting voids do not have closed cells.

Many prior proposals for increasing the gas barrier properties of polymeric films therefore result in films which are relatively inflexible and have at best only relatively low stretchability.

According to the present invention there is provided a polymeric film comprising at least one layer of a thermoplastic polymer having thereon a layer of a dispersion of a particulate filler in a dispersant, the dispersant comprising a tackifier.

The layer of the dispersion of the particulate filler in general provides films in accordance with the present invention with improved gas barrier properties compared with analogous films made from the same thermoplastic polymer or polymers alone and having substantially the same thickness as films of the present invention including the layer of the dispersion. Depending upon the intended end use, even relatively small improvements in barrier properties can be an advantage, for example where the cost of using a dispersion of a particulate filler in a dispersant is less than the polymer saved by down-gauging the film to achieve substantially the same barrier properties for the resultant film. The number of layers of film required to provide a required degree of barrier can also be reduced.

The tackifier serves to adhere the dispersion to the thermoplastic polymer. Furthermore, it generally serves as a binder for the filler particles so that they are held on the surface of the thermoplastic polymer, thereby to impart barrier properties to the films of which they form a part. The use of a tackifier also enables the films to flex, stretch, shrink or otherwise distort without substantially affecting the internal cohesion of the dispersion itself or adversely affecting the overall barrier properties of the films.

The dispersant can be substantially non-migratory into the thermoplastic polymer layer, for example it can be a low molecular weight hot melt adhesive that is barely tacky at room temperature, but it is preferred to use a dispersant which is at least partially migratory into the thermoplastic polymer layer, for example a liquid tackifier. Blends of non-migratory and migratory tackifier can also be used. As will be appreciated by those skilled in the art, the ability of a tacky liquid dispersant to migrate into one type of thermoplastic polymer need not necessarily be the same as that with another thermoplastic polymer, and a dispersant which is highly migratory into one type of thermoplastic polymer may be substantially non-migratory into another type of thermoplastic polymer. The dispersant can also be volatile so that it evaporates from the dispersion, thereby leading to a concentration of the filler particles in the remaining dispersant.

The use of a dispersant which is at least partially migratory into the thermoplastic polymer layer, or has a component which is at least partially lost be evaporation, enables the dispersions to be applied at a lower concentration of filler in the dispersant than is required for the finished film. This can avoid problems which can occur in trying to apply dispersions of filler in the dispersant which have higher concentrations of filler, for example as a result of their higher viscosity. Thus the dispersion used to produce films in accordance with the present invention can be applied by spraying it on to the thermoplastic layer into which the dispersant is to migrate and allowing the dispersant to migrate into the thermoplastic layer, or to evaporate, thereby concentrating the filler in the remaining dispersant.

Films in accordance with the present invention are preferably produced by applying to a thermoplastic polymer layer a dispersion of a filler in a dispersant, and if the dispersant is at least partially migratory into the polymer layer, or is volatile, allowing the dispersant to migrate into the thermoplastic polymer layer or to evaporate.

Films in accordance with the present invention are preferably at least partially oriented during the film forming process or in a subsequent operation, and this can in general be achieved without destroying the integrity of the layer of the dispersion.

This is in contrast with hitherto proposed films using polyvinyl alcohol as a barrier layer. Indeed, orientation can be positively beneficial, for example when it causes alignment of the filler particles in the dispersion. There can also be end uses of films of the present invention in which they are shrunk to tighten them around an article which has been wrapped, and the use of a tackifier as the dispersant in the barrier layer in general serves to facilitate such shrinkage without a significant reduction in barrier properties.

If desired, films in accordance with the present invention can include a heat seal layer to enable them to be heat sealed to themselves or to other surfaces.

Particularly preferred films in accordance with the present invention have a relatively high degree of flexibility when compared with highly oriented polypropylene films. An especially preferred polymeric film for use as the thermoplastic polymer layer is therefore of blown or cast polyethylene. Such films can in general be used as stretch wrap films, and the tackifier in the dispersion can provide the films with cling properties. If desired, additional tackifier can also be added to at least one of the thermoplastic polymer layers used to form films of the present invention.

Any of a wide variety of thermoplastic polymers can be used for the thermoplastic polymeric layer. Thus the layer of the thermoplastic polymer can be in the form of a mono-web, or it can be a layer of a multi-layer film. However, when it is the outer layer of a multi-layer film, it is often preferred that the dispersant should be capable of migrating into one or more of the layers of the said thermoplastic polymer. More particularly, it is often preferred that the dispersant should be capable of migrating into all of the layers of such a multilayer film whether such films consist of two, three, four, five or more polymeric layers, particularly when it is desired that the films of the present invention have an outer tacky surface remote from the layer of the dispersion. When multi-layered films are used, all of the layers will in general be made of thermoplastic polymers, but different polymers can be used for the respective layers. If desired, multi-layered films can be used which include a layer which inhibits or even prevents migration of the dispersant through the films, for example using high density polyethylene.

The film having the dispersion thereon can be selected to have desired physical properties, for example toughness, flexibility or extensibility. However, the thermoplastic polymers which can be used in accordance with the present invention can be any of a variety of polymers known in the art for producing polymeric films, for example polyalkenes, polyvinyl chloride or polyurethanes. Examples of polyalkenes which can be used include polymers derived from at least one of ethylene, propylene, butene-l and higher alpha-olefins containing from 5 to 10 carbon atoms, e.g. low density polyethylenes, linear low density polyethylenes and polypropylenes, and co- and terpolymers of propylene and/or ethylene optionally with butene-l.

The dispersant is preferably capable of at least partially migrating into the thermoplastic polymer or polymers of the polymeric layer, and/or being at least in part volatile, this leading to an increase in the concentration of the particulate in the remaining dispersant. However, it is generally preferred that some at least of the dispersant should remain as a dispersing phase for the particulate filler after substantial migration has occurred as otherwise the layer of dispersed filler can lack sufficient cohesion to remain a coherent layer.

In other words if the filler particles do not remain adhered to the film, the barrier properties of the films would be impaired.

The dispersant should be tacky so that it adheres the filler particles to the thermoplastic polymer layer. The use of a liquid dispersant in general serves not to impede the stretchability or shrinkability of the films of the present invention, and this can be a particular advantage where the films of the present invention are intended to be stretchable or shrinkable. Tacky materials which can be used for the purpose include tackifiers known in the adhesives and polymeric film arts, for example for providing cling films and stretch wrap films with self adhesion. Examples of tackifiers which can be used include low molecular weight polyalkenes and also polyamides and polyesters formulated to function as tackifiers.

These tackifiers may also be selected for their intrinsic barrier properties, thereby contributing to the barrier properties of the films.

Examples of tackifiers which can be used for polyolefin films include low molecular weight polyolefins, for example low molecular weight poly-isobutylene, terpene resins, coumarone resins, coumarone-indene resins, rosin, petroleum resins, atactic polypropylene and glyceryl esters of long chain fatty acids, for example glyceryl mono-oleate or glyceryl monostearate. Examples of tackifiers which can be used for polyvinyl chloride films include di-esters of dicarboxylic acids, for example di-esters of phthalic or adipic acids.

The dispersant can consist of a tackifier alone. However, it is often preferred to include a diluent, for example to modify the viscosity of the dispersion, e.g. to facilitate application of the dispersion to the thermoplastic polymer layer, or to provide a substantially non-migratory content to the dispersion when the tackifier migrates into the thermoplastic polymer layer.

Examples of substantially non-migratory diluents which can be used with thermoplastic polyolefin layers include, for example, high molecular weight poly-isobutylenes compared with the low molecular weight poly-isobutylenes which might be used as a migratory dispersant.

As will be appreciated by those skilled in the art, the concentration of the particulate filler in the dispersion can be varied according to various factors such as the shape and size of the particles, the degree of barrier required for the films, the nature of the dispersant and its rate and final extent of migration into the polymer layer, and the overall properties required of the films.

In general, lamella filler particles are preferred, that is particles with two dimensions substantially greater than their third dimension, since they have a high aspect ratio which results in an increased path length for gas molecules trying to diffuse through the layer of the dispersion. Preferred lamella filler particles include particles of mica, graphite or huntite, the latter possibly associated with hydromagnesite. Mixtures of filler particles can also be used.

The size of the filler particles will usually depend upon their shape since lamella particles can have a larger maximum dimension than the thickness of the layer in which they are present. The preferred maximum dimension of lamella filler particles is generally about 20Wm whereas the preferred particle size of substantially spherical particles is less than about 1im. The aspect ratio of lamella filler particles is preferably from 1.5:1 to 20:1, and more preferably less than about 10:1.

When the dispersion is capable of migration into the thermoplastic polymer layer and/or it is volatile and can evaporate from the dispersion, the dispersion preferably contains from 10 to 90 percent by weight of filler particles before migration occurs, and more preferably from 25 to 60 percent. However, if the dispersant is substantially nonmigratory or non-volatile, or if it is migratory and migration has not occurred, or volatile and evaporation has not occurred, it is preferred that the concentration of filler particles in the non-migrated and non-evaporated dispersant is from 25 to 70, preferably 35 to 60 percent by weight.

If desired, migration of the dispersant into the thermoplastic polymer layer can be enhanced by the inclusion within the thermoplastic polymer layer of an absorbent which itself can absorb migrating dispersant. This can be of particular advantage if the dispersant does not migrate strongly into the thermoplastic polymer and migration is desired. Examples of absorbents which can be used for the purpose include carbon black.

The thickness of the layer of the dispersion of the particulate filler in the migratory dispersant should be such that the filler particles after migration of the dispersant provide the film with improved barrier to gases, and this in turn will depend upon the amount of particles in the dispersion, the intrinsic barrier properties of the dispersion itself and the migratory properties of the dispersant. As will be appreciated by those skilled in the art, excessive amounts of migrated dispersant can be disadvantageous, for example as a result of its plasticizing effect on the film, thereby reducing the physical strength of the film. In order to reduce this effect whilst providing sufficient dispersant to enable the dispersion to be applied to the thermoplastic polymer layer it is generally preferred to include in the dispersant a diluent having little or no migratory properties into the particular film. The use of such a diluent also has the advantage of preventing substantially complete migration of the dispersant into the film which could result in a substantial loss of cohesion between the film and the particulate filler. As will be appreciated by those skilled in the art, migration of the migratory dispersant into the thermoplastic polymer layer or layers in general has the effect of causing the layers to expand.

Films of the present invention can be of a variety of thicknesses, for example they can be as thin as lOum or as thick as 500um or more, such thick films sometimes being referred to in the art as sheet rather than film.

It is particularly preferred that films in accordance with the present invention have the layer of the dispersion of the particulate filler in the dispersant as an intermediate layer between the specified layer of thermoplastic polymer and another polymer layer, thereby to contain the dispersion within an overall film structure. Such other polymer layers are preferably selected to allow the dispersant to migrate into them. If such migration occurs, the concentrating effect of the migration on the dispersion can be particularly advantageous in increasing the concentration of the particulate material in the layer of the dispersion. However, the further thermoplastic polymer layer can be such that the dispersant which is migratory into the other polymer layer is substantially non-migratory into it.

Sandwiching the layer of the dispersion of the particulate filler between two thermoplastic layers has the additional advantage that the dispersion is physically protected by the thermoplastic layers, thereby preventing abrasion of the particulate filler when the films are in use. This also avoids the possibility of the layer of the dispersion on one surface of the thermoplastic polymer layer from adhering to the reverse surface of the film, for example after it has been wound into a roll.

Especially preferred films in accordance with the present invention consist of two layers of thermoplastic polymers, preferably blown polyethylene, each having a tackifier on their outer surfaces, and with a layer of a particulate filler dispersed in a tackifier between the said two layers. The amount of tackifier within the respective outer layers is preferably from 0 to 20% of the weight of the respective layers, and more preferably from 4 to 15%.

Films in accordance with the present invention can be produced by known methods. They can, for example, be produced by coating a dispersion of a particulate filler in a selected dispersant on to a surface of a pre-formed polymeric film or a film as it is being formed, if desired followed by the application of a further pre-formed film on to the free surface of the coating of the dispersion. Alternatively, and depending upon the nature of the dispersant, the various layers, including the dispersion itself, can be coextruded from the appropriate compositions to form a film having the desired layers. In some instances it may, however, be preferred to spray a dispersion of the particulate filler in the dispersant directly on to the film, for example as the film is being formed. However, the dispersion can be applied subsequent to film formation, and even as the film is being applied to articles which are being packaged in the film. The thermoplastic polymer layer or layers can be, or can form a part of, a cast film, or a mono- or biaxially stretched film, stretched films being produced for example by blowing or sequential stretching.

Before use, films in accordance with the present invention will often be left for a period of time so that the dispersant can migrate into the thermoplastic polymer layer or layers, thereby concentrating the filler particles in the remaining dispersant and improving the barrier properties of the films. However, there are end uses where the dispersion can be applied to the thermoplastic layer or layers as late as during application of the film to articles as they are being wrapped in the films.

Films in accordance with the present invention can be used for a variety of end uses where enhanced barrier properties are desired. However, particularly preferred end uses are those which do not require the very high levels of barrier such as can be achieved using layers of ethylene/vinyl alcohol copolymers, but higher levels than are inherent in the polymer of the films themselves. For example, the present invention is particularly preferred for enabling a down-gauging of polyethylene films to be achieved, especially polyethylene stretch films, whilst maintaining or even improving the same barrier properties as are achieved with the thicker films, cost savings thereby being made.

An especially preferred end use of films in accordance with the present invention is as an overwrap for ensiling of vegetable matter, for example hay and in particular straw, straw usually having inherently insufficient moisture or nutrients to enable the ensiling process to take place if it is merely enclosed in a layer of a polymeric film. More particularly, films in accordance with the present invention can be used to contain straw treated with additional moisture and nutrients in the necessary moist, anaerobic environment to achieve the required properties in the ensiled straw.

Such a process can be achieved with thick polyethylene films, but the present invention enables the process to be achieved using thinner films or fewer layers, thereby reducing the amount of polymer used and thereby the cost of the films. Polyethylene films are therefore particularly preferred as the thermoplastic polymer layer or layers for films of the present invention.

Embodiments of films in accordance with the present invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which the various insets show outer layers of the films to an enlarged scale and in which:- Figs. la and lb show cross-sections through a first film in accordance with the invention using substantially spherical filler particles, the films being shown after manufacture and after migration, respectively; Figs. 2a and 2b show a film similar to that shown in Figs la and 2a, but with a lamella rather than a substantially spherical filler; Fig. 2c shows the film of Fig. 2b after the film of Fig. 2b had been stretched by twice its length; Figs 3a and 3b show two films similar to those of Fig. 2a with their respective dispersions in contact, Fig. 3a being immediately after contact and Fig. 3b being after migration; Fig. 4 shows a further embodiment of film in accordance the present invention including two multi-layer polymeric webs with a layer of dispersion therebetween; and Fig. 5 shows a yet further embodiment of film in accordance with the present invention.

Referring to the drawings, in each case, a polymeric mono-web 1 of an ethylene homopolymer has on one surface a dispersion of spherical particles 2 or lamella particles 4 in a dispersant 3 consisting of a tackifier which is capable of migration into the mono-web 1. In Figs. la to 2b the mono-web has a thickness of 25cm, and the layer of the dispersion of particles 2, 4 in the dispersant 3 has a thickness of 1.5cm.

After allowing the dispersant 3 to migrate into the mono-webs 1, the thicknesses of the layers of the dispersions had reduced, as shown in Figs ib and 2b, but the particles of the dispersions had become more closely packed.

Stretching of the film of Fig 2b by a factor of two, the thickness of the mono-web 1 had reduced from 25Wm to 12.5cm.

Although the thickness of the dispersion on the mono-web was similarly reduced by a factor of two, the concentration of lamella particles was still sufficient to provide the films with good gas barrier properties, for example to oxygen.

The film structures of Figs 3a and 3b are similar to those of Figs 2a and 2b, but with another mono-web 1 on the layer of the dispersion of particles 4 in the dispersant 3, sandwiching the dispersion between the two mono-webs 1.

The film shown in Fig. 4 consists of two multi-layer polymeric webs 5 with a dispersion of particles 4 in a dispersant 3. Each web 5 consists of a layer 6 into which the dispersant 3 migrates, a barrier layer 7 which inhibits further migration of the dispersant through the multi-layer web, and outer heat seal layers 8.

As will be seen from Figs lb, 2b, 2c and 3b, dispersant 3 from the dispersion of the particles 2, 4 in the dispersant 3 has migrated through the respective layers of the mono-webs 1, not only reducing the thickness of the dispersion on the mono-webs, but resulting in a layer of the dispersant on the surface of the mono-webs 1 remote from the original layers of the dispersion.

The films of Figs lb, 2b, 2c, 3b and 4 have both good gas barrier properties resulting from the presence of the respective layers of filler particles, and good water vapor barrier properties resulting from the filler particles and the polymer used to form the respective mono-webs 1. In addition, the films of Figs Ib, 2b, 2c and 3b have cling properties resulting from the presence of the tackifier 3 on the surfaces of the mono-webs 1 remote from the dispersion of the particles 2, 4 in the tackifier dispersant 3. The films of Fig 4 do not have cling properties because migration of the dispersion through the films is prevented by the barrier layers 7.

The film shown in Fig. 5 has a two relatively thin outer polymeric layers (9, 9'), a relatively thick polymeric core layer 10, and a layer 11 of a dispersion of a particulate filler 12 in a tackifier 13 which acts as a dispersant for the filler particles. The core layer 10 can, for example, be of a linear low density polyethylene, with the two outer layers 9, 9' both typically being formed from blends of linear low density polyethylene, an ethylene copolymer and a tackifier. Such a film is typical of a stretch film for the ensiling of hay, and differential cling can be provided by adjusting the composition of the polymer blends and the relative amounts of tackifier in the two outer layers 9 and 9' of the film.

With the exception of Figs lb, 2b and 3b, the illustrated films also represent films of the present invention in which the tackifier of the dispersion does not migrate into the thermoplastic layer or layers. Figs lb, 2b and 3b also represent films of the present invention in which the tackifier of the dispersion does not migrate into the thermoplastic layers but the films have a tackifier on an outer surface, for example by including the tackifier in an outer layer or layers of the films.

Claims (28)

Claims

1. A polymeric film comprising at least one layer of a thermoplastic polymer having thereon a layer of a dispersion of a particulate filler in a dispersant, the dispersant comprising a tackifier.

2. A film according to claim 1, wherein the particulate filler comprises lamella particles.

3. A film according to claim 2, wherein the lamella particles comprise mica, hydromagnesite or graphite.

4. A film according to any of the preceding claims, wherein the tackifier comprises a liquid tackifier.

5. A film according to any of the preceding claims, wherein the tackifier comprises a hot melt adhesive.

6. A film according to any of the preceding claims, wherein the dispersant is substantially non-migratory into the thermoplastic polymer.

7. A film according to any of claims 1 to 5, wherein the dispersant is at least partially migratory into the thermoplastic polymer.

8. A film according to claim 7, wherein the dispersant includes a diluent.

9. A film according to claim 8, wherein the dispersant has not migrated into the thermoplastic layer.

10. A film according to claim 8, wherein the dispersant has at least partially migrated into the thermoplastic layer.

11. A film according to any of the preceding claims, wherein the thermoplastic polymer layer includes a filler which absorbs the dispersant.

12. A film according to any of the preceding claims, wherein the thermoplastic polymer comprises a polyalkene, polyvinyl chloride or a polyurethane.

13. A film according to claim 12, wherein the thermoplastic polymer comprises a polyalkene and the dispersant comprises a polyolefin, a terpene resin, a coumarone resin, rosin, a petroleum resin or a glyceryl ester of long chain fatty acids.

14. A film according to claim 13, wherein the dispersant comprises a low molecular weight polyisobutylene, glyceryl monooleate or glyceryl mono-stearate.

15. A film according to claim 12, wherein the thermoplastic polymer layer comprises a polyalkene and the diluent comprises a high molecular weight poly-isobutylene.

16. A film according to any of the preceding claims, wherein the thermoplastic layer is part of a multi-layer polymeric film.

17. A film according to claim 16, wherein the multi-layer film includes a layer which acts as a barrier to migration of the dispersant.

18. A film according to claim 16 or claim 17, wherein the multi-layer film includes a filler which absorbs the dispersant.

19. A film according to any of the preceding claims, wherein the layer of the dispersion is between the said layer of the thermoplastic polymer and a further thermoplastic polymer layer.

20. A film according to claim 19, wherein the dispersant is capable of migration into the said further thermoplastic polymer layer.

21. A film according to claim 19 or claim 20, wherein the further thermoplastic polymer layer is a layer of a further multi-layer polymeric film.

22. A film according to claim 21, wherein the further multilayer film includes a layer which acts as a barrier to migration of the dispersant.

23. A film according to claim 19, wherein the dispersant is substantially non-migratory into the said further thermoplastic polymer layer.

24. A film according to any of the preceding claims, having the properties of a stretch film.

25. A film according to any of the preceding claims, which has cling properties.

26. A film according to any of the preceding claims, which has been oriented.

27. A film according to claim 1 substantially as herein described.

28. A method of ensiling vegetable matter which comprises wrapping the vegetable matter in a film according to any of the preceding claims.