Laminates of polymeric films to thermoplastics resins are well know in the polymer art. However, certain polymeric films present problems in forming such laminates due to the nature of the polymers used to make the films and the nature of the thermoplastics polymer to which they are to be laminated.

Fluorine-containing polymeric films, and in particular those derived from polyvinylidene fluoride, often impart desirable properties to laminates derived from them, for example weather resistance. However, bonding polyvinylidene fluoride containing films directly to thermoplastics resins is generally unsatisfactory due to the poor bond produced between the polyvinylidene fluoride containing material and thermoplastics and thermosetting resins in general.

It has been proposed hitherto to improve the bonding of polyvinylidene fluoride containing films to thermoplastics resins by the use of films having one or more further polymeric layers which have good adhesion to polymeric layers including polyvinylidene fluoride, the latter forming the outer layer of the resulting laminate. The further layer or layers adhere to the polyvinylidene fluoride containing layer, and the further layer or layers adhere to the thermoplastics resin at the temperature of extrusion of the thermoplastic resin and at which bonding to the film is effected.

Whilst satisfactory bonding of polyvinylidene fluoride containing films to thermoplastic polymers can be achieved by such techniques, bonding between these polyvinylidene fluoride containing films and thermosetting resins is generally unsatisfactory due to the lower temperatures which are generally encountered during the curing of thermosetting resins.

According to the present invention there is provided a polymeric film comprising a first outer layer which comprises a fluorine-containing polymer, and a second outer layer bonded to the first outer layer, the second outer layer comprising a polymeric material which adheres to thermosetting resins during the curing of the thermosetting resins at temperatures of not more than 130°C.

The present invention further provides laminates of a polymeric film in accordance with the present invention adhered to a thermosetting resin, adhesion of the film to the resin having been effected during curing of the thermosetting resin at temperatures of not more than 130°Co Films in accordance with the present invention are preferably vacuum formable to enable them to be formed to the shape of a mold before applying a thermosetting resin system to the formed film.

Preferred fluorine-containing polymers which can be used for the first outer layer include homo-and copolymers containing units derived from vinylidene fluoride, the copolymers preferably containing at least 50% by weight of units derived from vinylidene fluoride. The first outer layer can also be made from blends of polyvinylidene fluoride and/or copolymers containing units derived from vinylidene fluoride with other thermoplastic polymers, the amount of polyvinylidene fluoride present in the blends preferably being at least 50% by weight of the blend. Preferred thermoplastic polymers for use in such blends include acrylic resins.

Particularly preferred vinylidene fluoride-containing copolymers include units derived from both vinylidene fluoride and hexafluoropolymer (such as Kynar Flex@ from Arkema or SOLEFO PVDF Copolymers from Solvay).

The first outer layer may also contain additives used in the polymeric film art, for example antioxidants, UV stabilisers, pigments and fillers, e. g. talc, zinc sulfide (ZnS), or titanium dioxide, plasticizers, impact modifiers, etc.

The second outer layer of films of the present invention should be capable of adhering to, or being compatible with, thermosetting resin systems which cure at temperatures below 130°C. In use, the second outer layer will in general be in contact with the thermoset resins.

The second outer layer of films of the present invention is preferably heterogeneous, consisting of at least two discrete phases with one exhibiting compatibility with the first outer layer containing fluorine-containing polymers and the second showing adhesion or compatibility with the thermosetting resin.

The second phase preferably represents at least 10% by weight of the second outer layer.

The overall structure preferably has a glass transition temperature (Tg) which allows high elongation of the films at temperatures between room temperature and 100°C with normal atmospheric pressure on one side and a vacuum on the other.

One of the phases forming the second outer layer will in general be selected from materials which adhere to the first outer layer of films of the present invention when there is no intermediate layer, and to the adjacent intermediate layer when one or more intermediate layers is present.

The other phase will in general be selected from materials having good adhesion to the thermoset resin. The polymers forming this phase should in general contain reactive groups which can form covalent links with the thermosetting resins.

Polymer compatibility with thermoplastic resins generally requires that they contain polar but non-reactive groups whereas compatibility with thermosetting resins generally requires them to contain reactive polar groups which facilitate the production of covalent links with the thermosetting resins. Polymers with reactive groups therefore tend to act as incompatibilizers for thermoplastics resins.

Polymers having good adhesion to fluorine-containing polymers include acrylic resins, (homopolymers and copolymers), modified fluorine-containing polymers (for example fluorine-containing polymers bearing functional groups such as OH, carboxylic acid, carboxylic anhydride and NH2), polyamide elastomers, polyester elastomers, polycaprolactones, polyurethane elastomers, ethylene/vinyl acetate copolymers, for example Elvaloy copolymers, acrylonitrile/butadiene/styrene resins, styrene/butadiene/methyl methacrylate (SBM) block copolymers, carboxy terminated butadiene/acrylonitrile (CTBN) rubbers, core-shell polymers, for example Paraloid polymers, e. g.

Paraloid EXL2300, etc.

Polymers exhibiting good adhesion to thermosetting resins, and can be used to form the second phase of the second outer layer of films in accordance with the present invention, include polymeric materials which include reactive groups, for example hydroxy, primary amino, carboxylic acid, carboxylic anhydride or epoxy groups.

Preferred polymers of this type include modified fluorine- containing polymers, (for example fluorine-containing polymers bearing functional groups such as OH, epoxy, carboxylic acid, carboxylic anhydride, NH2), polyvinyl butyral (PVB), phenoxy resins, polymer modifiers, for example the Fusabond series from Dupont, ionomeric resins such as Surlyn from Dupont, polyvinyl alcohol (PVOH), core-shell polymers, for example Paraloid polymers, e. g. Paraloid EXL2314, modified ethylene/vinyl acetate resins, etc.

The blend forming the second outer layer can contain additives used in the polymeric film art, for example impact modifiers, compatibilizers and plasticizers.

Optionally, one or more layers may be present between the first and second layers of films of the present invention, for example to promote adhesion between the first and second layers, thereby forming a continuously cohesive film structure.

When inner layers are present, they are preferably formed from blends of the polymers used to form the first and the second outer layers of the films.

When inner layers are present they can in general contain additives used in the polymeric film art, for example antioxidants, UV absorbers, pigments and fillers, for example ZnS or titanium dioxide, impact modifiers, etc.

Films in accordance with the present invention typically exhibit percentage elongations at break of from 50 to 300% at temperatures between room temperature and 100°C, and they can be from 100 to 500%, this enabling them to be vacuum formed during the manufacture of large components such as wind turbine blades and boats.

The first outer layers of films of the present invention are preferably from 10 to 300um thick, and the second outer layers of films of the present invention are preferably from 10 to 500um thick. When present, the intermediate layer or layers are each preferably from 10 to 100um thick.

Films of the present invention can be produced by known methods, for example by coextrusion of melts of layers of the respective polymers of the respective layers of the films, followed by cooling to form the films. The films can be produced by casting or blowing of the coextrudate.

The second outer layer of films of the present invention preferably have a surface pattern and/or texture for facilitating the evacuation of air at the interface between the films and the thermosetting resin during curing of the latter.

Films of the present invention can be used to provide outer layers to articles made from thermoformable resins. A particularly preferred use is as a vacuum formable outer layer for such articles. However, they can also be used as repair coatings which are applied to thermoset adhesive layers on articles which are to be repaired.

Prior to vacuum forming of films of the present invention the mold which is to be used can be coated with a surface control material to modify the texture and/or appearance of the film on the finished moulded article which is produced such as its gloss.

After vacuum forming of a film in accordance with the present invention, formation of molded articles from thermosetting resin systems can be effected using known methods. Examples of such methods include spray lay-up, wet lay-up, vacuum bagging, resin transfer molding, infusion processes (SCRIMP, RIFT, VARTM, etc), prepreg molding, autoclave, or resin infusion. The surface against which the thermosetting resin system is formed is then the second outer surface of a film of the present invention.

In some cases it may be necessary to apply a protective layer to the second outer surface of the films before applying the thermosetting resin composition, for example when using unsaturated polyester resins or vinyl ester resins containing up to 30% by weight of styrene monomer. Styrene monomer is a low boiling material with high mobility and so it can migrate to the interface between the film of the present invention and the thermoset resin, thereby reducing adhesion therebetween.

Styrene monomer can also lead to delamination of the first and second outer layers of films of the present invention.

Protective layers which can be applied to the second outer layer to reduce these undesirable effects caused by free styrene monomer include thermosetting adhesives based on epoxy, acrylic, polyurethane, phenolic, and silicone resins, but others can be used. The coating materials can be applied with or without a solvent, and they can be one or two part adhesives. They can also be applied as wet systems or in the form of films.

The thermosetting adhesives can include additives known in the adhesives art, for example fillers, air-release agents, colorants, and rheology and impact modifiers.

Protective layers can be applied to the second outer layer of films of the present invention using known methods, for example coating, lamination, spraying or using a roller. They can also be applied before or after vacuum forming of a film of the present invention.

The protective layers can be partially cured before molding the thermosetting resin, as is effected in wet in-mold gel coating, for example using heat, UV radiation or time. Heat can be provided using infra-red lamps, hot air or other methods known in the art. Final curing of the protective layers will then in general be effected during curing of the thermosetting resin used to form the finished molded article.

The protective layers preferably have thermal and mechanical properties which are as close as possible to those of the thermoset resin of the molded articles.

The following Examples are given by way of illustration only.

Example 1 A two layer polymeric film was produced by coextruding through a slot die melts of a first blend of 35% by weight of polyvinylidene fluoride (Kynar 1000HD-Atofina), 35% by weight of polymethyl methacrylate (DIAKON ST45G8-Lucite International), and 30% by weight of titanium dioxide- (R-TC30 - Huntsmann), and a second blend of 22. 5% by weight of polyvinyl butyral (Mowital B60 HH-Kuraray), 22.5% by weight of polycaprolactone (CAPA6500-Solvay), 37% by weight of polymethyl methacrylate (DIAKON ST45G8-Lucite International), and 18% by weight of an elastomeric core shell polymer (PARALOID EXL2314-Rohm & Haas). The coextrudate was cooled and wound up.

An epoxy panel was made from three plies of glass tissue (Glassweefsel 350g. m~2-Vosschemie) using a two part thermosetting resin system (Epoxy-BK-Vosschemie) with a mix ratio of 100: 60.

Glass tissue plies (Glassweefsel, 350g. m-2-Vosschemie) were laid on the surface of the above film which was formed from the second blend, the respective plies being placed at angles to the ply on which they are placed. The plies were then brushed with a two part thermosetting resin system (Epoxy-BK-Vosschemie) with a mix ratio of 100: 60.

After the lay up and impregnation of the three glass tissue plies, a peel ply layer (econostitch 60''-Airtech) was applied to the laminate, a Dahtar release bag (125 P-3-Airtech) was placed on top, covered by an econoweave 44 (Airtech) to absorb excess resin. This was bagged in a film (SL200-Airtech) and then vacuumed before curing for 12 hours at 60°C.

The resin content of the impregnated glass plies was approximately 50% by weight. The two layers of the film could hardly be removed from the cured glass reinforced resin.

Example 2 A first glass plate having an area of approximately 500cm2 was covered on one surface by film produced in Example 1 with the first layer contacting the plate. A second glass plate substantially identical plate was then connected to the first plate with the film between the two plates, using sealing tape along three sides of the plates and a spacer to keep the plates more or less 2-3mm apart.

The thermosetting resin used in Example 1 was then poured between the two plates so that the resin contacted the second surface of the film, and the resin was then cured for 12 hours at a temperature of 60°C. After curing, the two layer film adhered strongly to the epoxy resin casting.

Example 3 (comparison) An epoxy resin casting was produced as described in Example 2, the two layered film being replaced by a commercially available polyvinylidene fluoride based two-layer film for thermoforming on polymer substrates (Kynar 301 PGM-LT-Atofina).

After curing, adhesion of the film to the epoxy resin was very poor, and the film could be easily peeled from the resin.

Example 4 The two layered film produced in Example 1 was placed flat and sealed around the edges of a glass hemisphere with a diameter of 7.5cm and a depth of 4cm, and having a small exhaust pipe in its centre.

A vacuum was then pulled through the exhaust pipe and maintained until the film had been vacuum formed and tightly spread on the inner surface of the hemisphere. The two dimensional elongation of the film was approximately 100%, and the film displayed a perfectly homogeneous appearance, free from heterogeneities or defects.

Example 5 A three layer polymeric film was produced by coextruding through a slot die melts of a first blend of 80% by weight of PVDF (Solef 21508) and 20% by weight of an acrylic resin (312EA01), a second blend of 60% by weight of an acrylic resin (312EA01), 25% by weight of PVDF (Solef 21508) and 15% by weight of ZnS, and a third blend of 60% by weight of an acrylic resin (312EA01), 27% by weight of caprolactone (CAPA 6500), 10% by weight of polyvinylbutyral (Mowital B60HH-Kuraray) and 3% by weight of Fusabond MC423D (Dupont). The coextrudate was cooled and wound up.