GB1590592A

GB1590592A - Coated thermoplastic release foils

Info

Publication number: GB1590592A
Application number: GB17126/77A
Authority: GB
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1977-04-25
Filing date: 1977-04-25
Publication date: 1981-06-03
Also published as: JPS53133260A; DE2816089A1; IT1095315B; NZ186874A; FR2388657A1; ES469124A1; PT67932B; AU3484378A; FR2388657B1; IT7822678A0; DE2816089C2; NL7804390A; SE7804529L; BE866349A; LU79513A1; AU517493B2; PT67932A

Description

(54) COATED THERMOPLASTIC RELEASE FOILS (71) We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British company do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to mould release foils suitable for use in the production of moulded articles from a curable resin, to processes for their production, and to moulding processes employing such foils.

Release agents are employed in a variety of industrial processes to reduce or prevent adhesion between two surfaces. In the production of moulded articles from curable unsaturated polyester resins, self-supporting plastics foils are sometimes employed to reduce or prevent adhesion between the mould surface and the moulded article and also to impart a satisfactory surface finish upon the surface of the article. Such articles are often reinforced with particulate fillers or fibrous additives such as glass rovings, articles reinforced with glass rovings often being termed glass-reinforced polyester (grp). Articles moulded from glass-reinforced polyester are commonly made by applying a mixture of glass rovings and an unsaturated polyester resin, together with a curing catalyst for the resin, to a mould and allowing or causing the resin to cure. In such a process, a release foil such as an uncoated polyethylene terephthalate film may be positioned between the glass-reinforced polyester and the mould surface. However, it has been found that the release of polyethylene terephthalate films from some unsaturated polyester resins is not entirely satisfactory, especially when the moulded article has a shape which includes small radii.

The present invention relates to a mould release foil having improved release properties in comparison with an uncoated polyethylene terephthalate film, to a process for the production of the foil and to a moulding process employing such foil.

According to the present invention, a process for the production of a mould release foil comprises applying a release layer to at least one surface of a self-supporting thermoplastics film, said release layer comprising a film-forming interpolymer of styrene or a derivative thereof with an amount of one or more comonomers effective in rendering the interpolymer film-forming and drying and coalescing the release layer into a continuous layer having a thickness not exceeding 0.5 micron.

According to another aspect of the present invention, a mould release foil comprises a self-supporting thermoplastics film coated on one or both surfaces with a continuous release layer comprising a film-forming interpolymer of styrene or a derivative thereof with an amount of one or more comonomers effective in rendering the interpolymer film-forming, the or each release layer having a thickness not exceeding 0.5 micron.

Such mould release foils exhibit excellent release properties from moulded articles produced from curable resins by a moulding process in which the curable resin is cured in situ against the release layer of the mould release foil. The mould release foils according to this invention may be used for the production of moulded articles from any suitable curable resin, such as curable unsaturated polyester resins, phenolic resins and epoxy resins. The mould release foils are particularly effective in the production of moulded articles from curable unsaturated polyester resins.

The invention also relates to a process for the production of a moulded article which comprises locating a mould release foil as defined in the preceding paragraph against a mould surface such that a release layer of the release foil is positioned on the side of the release foil remote from the mould surface, applying a curable unsaturated polyester resin to said release layer, allowing or causing the resin to cure, and stripping the release foil from the cured resin, said thermoplastics film being dimensionally stable under the conditions for curing the resin.

The nature and chemistry of the curable unsaturated polyester resins forms no part of this invention: the resins are already well known in the art. The agents for initiating and/or catalysing the curing operation and the conditions for curing are also well known. A description of suitable resins and agents and conditions for curing them is given in 'Polyesters' - Volumne 2. Unsaturated Polyesters and Polyester Plasticisers, Parkyn, Lamb and Clifton, Illiffe 1967.

The self-supporting thermoplastics film may comprise any suitable material, e.g.

polyamides, polycarbonates, polysulphones and highly polymeric film-forming linear polyester of one or more dibasic aromatic carboxylic acids and one or more dihydric alcohols. Suitable linear polyesters may be produced by condensing one or more dicarboxylic acids or their lower alkyl (less than 6 carbon atoms in the alkyl group) diesters, e.g. terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- and 2,7-naphthalene dicarboxylic acid, succinic acid, sebacic acid, adipic acid, azelaic acid, diphenyl dicarboxylic acid, and hexahydroterephthalic acid, or bis-p-carboxyl phenoxy ethane, optionally with a monocarboxylic acid, such as pivalic acid, with one or more alkylene glycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and 1,4-cyclohexane-dimethanol.

The preferred linear polyester is polyethylene terephthalate. Films of polyethylene terephthalate employed in the foils of this invention may be stretched in mutuallv perpendicular directions to impart molecularly orientation and dimensionally stabilised by heat setting, e.g. as disclosed in British Specification 838708 and may have a thickness of up to about 175 microns. Films having thicknesses up to about 50 microns are generally suitable for mould release use and preferably have a thickness of about 20 microns.

Styrene or styrene derivatives may be interpolymerised with one or more suitable comonomers to produce the release interpolymers employed according to this invention, such as acrylic acid, methacrylic acid, alkyl esters of acrylic acid or methacrylic acid in which the alkyl group comprises up to 10 carbon atoms, glycidyl acrylate, glycidyl methacrylate, hydroxy ethyl methacrylate, acrylamide, methacrylamide, acrylonitrile and methacrylonitrile.

Interpolymers comprising 30 to 85 mole % of styrene have film-forming properties such that a continuous release layer is formed on the foil and also provide satisfactory release properties whilst avoiding the initiation of tears in the release layer during release from the moulded article.

Interpolymers which have been found to exhibit satisfactory release properties include a copolymer of 40 mole % butyl acrylate/60 mole % styrene; a copolymer of 60 mole % butyl acrylate/40 mole % styrene; a copolymer of 67 mole % butyl acrylate/33 mole % styrene; and a copolymer of 18 mole % butyl acrylate/82 mole % styrene.

The release layer may be applied to the plastics film, preferably as an aqueous latex, by any suitable coating technique and apparatus and heated to dry the coating and fuse the release polymer into a continuous layer.

When the plastics film comprises a linear polyester film such as a molecularly oriented and heat-set polyethylene terephthalate film, the release layer may be applied to the film during its process of production and preferably prior to the commencement of the stretching operation by which molecular orientation is achieved or after stretching has been effected in one direction and before stretching in a direction substantially perpendicular thereto. Such a coating operation is particularly useful in providing the thin release layers, i.e. layers not exceeding 0.5 micron thick, which are employed according to this invention since the stretching which is imposed upon the film after the application of the release layer also has the effect of reducing the thickess of the release layer thereby providing a convenient means of achieving the desired layer thickness. Transverse stretching and heat setting of polyethylene terephthalate films are generally effected in a heated stenter oven and the temperatures employed therein are generally sufficient to dry and coalesce the release layer according to the invention.

Release layers having thicknesses up to 0.1 micron provide especially beneficial release properties. The prefered release layer thickness is 0.01 to 0.05 micron.

The release layer thicknesses according to the present invention will be determined by the concentration of the coating latex and coating conditions and will be readily achieved by those skilled in the art.

In the production of moulded articles by means of the release foils according to the invention, the curable resin, e.g. an unsaturated polyester resin, may contain finely divided particulate fillers and/or fibrous reinforcement such as glass fibres. Pigments may also be included where appropriate. The moulded article may be provided with a gel coat which may be pigmented for decorative purposes and is also produced from a curable resin bonded to a layer or core which may contain finely divided particulate fillers and/or fibrous reinforcement. Such an article is produced by moulding the gel coat adjacent the release layer of the mould release foil and applying the other layer or core to the surface of the gel coat remote from the release foil. The moulded article may alternatively have a reinforced resin-rich surface layer which may be obtained by locating a reinforcing fibrous material, e.g. a surfacing tissue of a saturated polyester, adjacent the release layer of the release foil.

The moulded articles produced according to this invention may comprise flat sheets or boards. Sheets may be moulded with a shaped or profiled form. The mould release foils are particularly suitable for the moulding of profiled sheets having tightly radiused surfaces, e.g. "box-shaped" profiles.

Moulded articles such as sheets and boards may be moulded between two mould release foils appliedito opposing surface of the sheet or board.

The moulded articles according to this invention may be produced by a batch or continuous process.

Production by a batch process may, for example, be achieved by placing a mould release foil according to the invention in a suitable mould so that the release layer is remote from the mould surface. A gel coat of a curable resin, such as an unsaturated polyester resin, or a layer of a surfacing tissue such as a saturated polyester and/or a layer of glass-reinforced curable resin, such as an unsaturated polyester resin, containing if required a suitable curing agent, is then applied to the release layer and cured.

In a continuous process for the production of sheet laminates according to this invention, a gel coat of a curable resin, such as an unsaturated polyester resin, or a layer of a surfacing tissue such as a saturated polyester and/or a layer of a curable resin, such as an unsaturated polyester resin, containing glass-fibre reinforcement may be cast continuously onto the mould release layer of a moving web comprising a mould release foil according to the invention. The cast resin is caused or permitted to cure and the mould release foil stripped from the sheet by virtue of the inherent release properties of the release layer.

In one embodiment of the invention, a moulded article is produced in the form of a profiled sheet having a form suitable for cladding buildings. A particularly suitable profiled sheet has a sine-wave cross-section. Such a profiled sheet may be produced by moulding the curable resin, such as an unsaturated polyester resin, between two release foils according to the invention, partially curing the resin, and then shaping or profiling the sheet by passing it over a suitably shaped former before completing the curing of the resin.

The invention is further illustrated by the following examples.

Examples I to 4 Aqueous coating latices comprising the release polymers specified in Table 1 were prepared containing 3.0% by weight of the release polymer and 0.5% by volume of a non-ionic surfactant which is commercially available as 'Lissapol' (registered Trade Mark) N.

Polyethylene terephthalate films were melt extruded and quenched to the amorphous state on a cooled rotating drum. The resulting films were stretched in the direction of extrusion to about 3.5 times their original length. Each film was coated on one side with an aqueous latex prepared as described above by a roller coating technique and passed into a stenter oven where the coating was dried. The dried coated film was then stretched sideways about 3.5 times its original width and finally heat set at a temperature of about 210 C. The resulting coating had coalesced into continuous layers of approximate thickness 0.025 micron in the finished foils which had overall thicknesses of about 20 microns.

Each release foils was employed in the production of a moulded sheet from glass-reinforced unsaturated polyester resin. The edges of samples of each release foil, produced as described above, were folded over the edges of two glass plates about 30 cm square and securely fastened to the plates by means of a thermally resistant pressuresensitive adhesive tape. The foils were shrunk onto the glass plates by heating in an oven at 1200C for 15 minutes. A piece of glass fibre mat approximately 22 cm square (available commercially as 'Supremat' (registered Trade Mark) from Fibreglass Limited) was placed centrally over the release layer of one of the foil/glass plate assemblies and a curable unsaturated polyester resin formulation as specified below was applied to the mat and spread over it with a roller: Resin (commercially available as 'Impol' F920 from Imperial Chemical Industries Limited) 100 g Accelerator (commercially available as NL49/ST from Novadel Limited) 0.25 g Methyl ethyl ketone peroxide catalyst 1.0 g The second foil/glass plate assembly was placed over the layer of resin with the release foil adjacent the resin/mat layer. The foil/glass plate assemblies were pressed gently together and held together with thermally resistant pressure sensitive adhesive tape. The assemblies were placed in an oven at 1200C for 40 minutes to cure the resin. After cooling, the glass plates were separated from the release foils by slitting the foils around the edges of the plates. The foils were then separated from the moulded sheets by peeling them apart. It was found that the foils separated readily at the release layer/resin interface without tearing of the release layer and without failure at the release layer/film interface. The surfaces of the moulded sheets were glossy and free from blemishes and release layer residues.

In order to further evaluate the release characteristics of the release layers, second samples of the moulded sheets were prepared in the manner described above with the modification that polyester tissues (available from Fothergill and Harvey Limited) were located on both sides of the glass fibre mat before the application of the resin. The evaluation of release properties is more critical with such a polyester tissue moulded into the sheet. The release behaviour of each release foil was graded as shown in Table 1 according to the following scheme: Release grade Behaviour A Foil releases cleanly from moulding with little resistance to peeling.

B Some resistance to peeling and slight tearing of foil.

C Approximately 50% of the foil area releases from the moulding.

D Slight peeling at the edges of the foil but mostly bonded to foil.

E No release.

TABLE 1 Release Example Release interpolymer Grade 'Primal' (registered Trade Mark) P376 (commercially available from Rohm & Haas Ltd, 1 analysis copolymer of butyl A acrylate/styrene) 'Texicryl' 13.926 (commercially available from Scott Bader Co 2 Ltd, analysis of butyl acrylate/ A styrene copolymer) A copolymer of 67 mole % butyl 3 acrylate/33 mole % styrene A A copolymer of 18 mole % butyl 4 acrylate/82 mole % styrene A Examples 5, 6 and 7 The procedure of Examples 1 to 4 was repeated employing the aqueous coating latex of Example 1, i.e. a latex containing 'Primal' (registered Trade Mark) P376, but applying different release layer thicknesses, as shown in Table 2. The release grades were determined according to the procedure described in relation to Examples 1 to 4 and are also shown in Table 2.

TABLE 2 Example Release layer thickness Release micron Grade 5 0.01 A 5 0.08 B 7 0.4 D Comparative Examples 1, 2 and 3 Moulded sheets of glass-reinforced unsaturated polyester resin were prepared as described in relation to Examples 1 to 4 using the foils indicated in Table 3 in place of the release foils according to the invention. The release grade was evaluated as prescribed for Examples 1 to 4.

TABLE 3 Polymer Comparative Coating coating Release Example Polymer thickness coating Grade micron No polymer coating uncoated biaxially 1 oriented polyethylene - D terephthalate film Released at film/ 2 Styrene homopolymer 0.025 homopolymer interface A copolymer of 47 mole % 3 methyl methacrylate/ 0.025 E 53 mole % styrene In the case of Comparative Example 1, a conventional uncoated biaxially oriented polyethylene terephthalate film gave poor release properties of Release Grade D.

The polymeric coatings employed in Comparative Examples 2 and 3 were non-filmforming and discontinuous in nature. The styrene homopolymer of Comparative Example 2 provided ready release but the adhesion between the homopolymer and the film surface failed with the result that patches of the homopolymer adhered to the surface of the moulded sheet.

No release could be achieved from the methyl methacrylate/styrene copolymer coating employed in Comparative Example 3.

Example 8 Example 1 was repeated employing a conventional curable phenolic resin in place of the curable unsaturated polyester resin described therein. After curing the resin, it was found that the moulded sheet released readily from the release layer of the release foil without tearing of the release layer of failure of the adhesion between the release layer and the polyethylene terephthalate film in the release foil. The surfaces of the moulded sheet were glossy and unblemished.

The Applicants' copending application 17127/77 (Serial No. 1590593) describes and claims a mould release foil which comprises a self-supporting thermoplastics film coated on one or both surfaces with a continuous release layer comprising a film-forming homopolymer of an alkyl acrylate in which the alkyl group comprises up to 10 carbon atoms or a film-forming interpolymer of such an alkyl acrylate with one or more ethylenically unsaturated comonomers, said homopolymer or intopolymer having a glass-transition temperature not exceeding 30"C, the or each release layer having a thickness not exceeding 0.5 micron and a moulding process using the mould release foil.

WHAT WE CLAIM IS: 1. A mould release foil which comprises a self-supporting thermoplastics film coated on one or both surfaces with a continuous release layer comprising a film-forming interpolymer of styrene or a derivative thereof with an amount of one or more comonomers effective in rendering the interpolymer film-forming, the or each release layer having a thickness not exceeding 0.5 micron.

2. A mould release foil according to claim 1, in which the self-supporting thermoplastics film comprises a molecularly oriented and heat-set film of polyethylene terephthalate.

3. A mould release foil according to claim 1 or 2, in which the interpolymer of the release layer is formed from styrene or a derivative thereof with one or more comonomers selected from acrylic acid, methacrylic acid, alkyl esters of acrylic acid or methacrylic acid in which the alkyl group comprises up to 10 carbon atoms, glycidyl acrylate, glycidyl methacrylate, hydroxy ethyl methacrylate, acrylamide, methacrylamide, acrylonitrile or methacrylonitrile.

4. A mould release foil according to claim 1, 2 or 3, in which the interpolymer of the release layer comprises 30 to 85 mole % of styrene.

5. A mould release foil according to claim 4, in which the release layer comprises a copolymer selected from a copolymer of 40 mole % butyl acrylate/60 mole % styrene; a copolymer of 60 mole % butyl acrylate/40 mole % styrene; a copolymer of 67 mole % butyl acrylate/33 mole % styrene; or a copolymer of 18 mole % butyl acrylate/82 mole % styrene.

6. A mould release foil according to any preceding claim, in which the release layer has a thickness of 0.01 to 0.05 micron.

7. A mould release foil according to any preceding claim, in which the self-supporting thermoplastics film has a thickness of up to about 50 microns.

8. A process for the production of a mould release foil according to any preceding claim, in which the release layer is applied to one or both surfaces of a self-supporting thermoplastics film, is dried and coalesced into a continuous layer having a thickness not exceeding 0.5 micron.

9. A process for the production of a moulded article, which comprises locating a mould release foil according to any of claims 1 to 7 against a mould surface such that the release layer of the release foil is positioned on the side of the release foil remote from the mould surface, applying a curable unsaturated polyester resin to said release layer, allowing or causing the resin to cure, and stripping the release foil from the cured resin, said thermoplastics film of the release foil being dimensionally stable under the conditions for curing the resin.

10. A moulded article produced by the process according to claim 9.

11. A mould release foil according to claim 1 substantially as described in any one of Examples 1 to 7.

12. A process for the production of a mould release foil according to claim 8 substantially as described in any one of Examples 1 to 7.

13. A process for the production of a moulded article according to claim 9 substantially as described in any one of Examples 1 to 7.

14. A process for the production of a moulded article according to claim 9 substantially as described in Example 8.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. homopolymer of an alkyl acrylate in which the alkyl group comprises up to 10 carbon atoms or a film-forming interpolymer of such an alkyl acrylate with one or more ethylenically unsaturated comonomers, said homopolymer or intopolymer having a glass-transition temperature not exceeding 30"C, the or each release layer having a thickness not exceeding 0.5 micron and a moulding process using the mould release foil. WHAT WE CLAIM IS:

1. A mould release foil which comprises a self-supporting thermoplastics film coated on one or both surfaces with a continuous release layer comprising a film-forming interpolymer of styrene or a derivative thereof with an amount of one or more comonomers effective in rendering the interpolymer film-forming, the or each release layer having a thickness not exceeding 0.5 micron.

10. A moulded article produced by the process according to claim 9.