GB2028228A

GB2028228A - Method of coating

Info

Publication number: GB2028228A
Application number: GB7926161A
Authority: GB
Original assignee: Roehm GmbH Darmstadt
Current assignee: Roehm GmbH Darmstadt
Priority date: 1978-07-26
Filing date: 1979-07-26
Publication date: 1980-03-05
Also published as: IT7968462A0; NL7905450A; DE2832676A1; IT1121001B; GB2028228B; FR2434023A1; FR2434023B1; JPS5559929A

Abstract

A method of coating a polycarbonate sheet with an adherent synthetic resin protective layer which comprises extruding a polycarbonate sheet and a synthetic resin protective layer therefor, and bringing the said extruded synthetic resin layer into contact with a surface of the said extruded polycarbonate sheet while both are at an elevated temperature resulting from the extrusion, the elevated temperature, being sufficient, if necessary in combination with the application of pressure, to effect adhesion, upon contact, between the polycarbonate sheet and the said synthetic resin layer. The above synthetic resin layer is advantageously based on a (meth)acrylate homopolymer or copolymer. The synthetic resin layer is preferably provided with a scratch- resistant layer, e.g. based on a silicon resin or a cross-linked hydroxylated, fluorine-containing polymer. The polycarbonate sheet and synthetic resin layer are conveniently extruded from the same multiple die.

Description

SPECIFICATION Method of coating The present invention relates to a method of coating a polycarbonate sheet with an adherent synthetic resin protective layer.

Polycarbonate mouldins are distinguished by excellent Iighttmnsmittance, high mechanical strength and dimensional stability under heat, outstanding dimensional accuracy and a relatively low level of water absorption. Consequently, polycarbonate panels are used for example in the construction of impact-resistant light-weight building elements, light domes, glazing elements etc.

However, with polycarbonate glazing element, as with other mouldings, the substantially lower scratch-#resistance of polycarbonate materials, as compared with silicate glass, is disadvantageous.

Also, intense ultra-violet radiation leads to damage to the surface region of polycarbonate materials, which manifests itself in the dulling of glossy surfaces, brought about by the occurence of very fine hairline cracks.

Since the processing of polycarbonates is mainly effected by injection moulding and, to a lesser extent, by thermoforming, i.e. at relatively high temperatures, the problem cannot generally be solved by admixing ultra-violet absorbers with the resins before processing. One proposed solution to this problem was to provide the surface of the polycarbonate moulding with a protective covering containing an ultra-violet absorber (see W. S. Christopher and P. W. Fox in "Polycarbonates", Reinhold Publishing Company, New York (1962)).

In US-PS 3,582,398 it is suggested that the optical properties (light transmittance) of polycarbonate mouldings can be improved by the application of transparent coatings of polymethyl methacrylate (PUMA), preferably containing ultra-violet absorbers. Coating may be effected on an industrial scale by dipping, spraying or pouring, using a solution of the resin in an inert volatile solvent.

Ultra-violet-inhibiting coatings may also be obtained by the hot pressing together of polycarbon ate panels and films of acrylate copolymers. For example, German OLS 1,953,276 proposes the use of a prefabricated polyacrylate film, 0.025 to 0.25 mm thick, preferably consisting of a methyl methacrylate-ethyl acrylate copolymer containing 0.25 to 5% by weight of an ultra-violet absorber. To achieve a firm adhesion, the temperature should be at least 1 630C and the pressure at least 1.4 kg/cm2. The higher the temperature employed, the smailerthe pressure and the holding time under pressure that are required. Such coating may be applied for example by pressing on heated rollers.Alternatively, the manufacture of the composite product may be carried out by the common vacuum forming of the acrylate polymer film and polycarbonate film or by injection moulding, the polycarbonate being injected to bring it into contact a film of acrylate polymer, the two materials being joined together and pressed simultaneously in one operation.

The provision of the above-mentioned poly(meth): acrylate coatings, particularly in combination with ultra-violet absorbers, may substantially improve the weather resistance of polycarbonate mouldings, but does not necessarily solve the problem of providing a scratch-resistant coating for the polycar- bonate materials.

A scratch-resistant coating for polycarbonate mouldings may be obtained according to German OLS 1,963,278 by applying a coating based on hydroxylated, flourine-containing polymers crosslinked with methyl melamine.

However, it has been found that with conventional scratch-resistant coatings problems arise with regard to adhesive strength and weather-resistance.

Thus, the adhesion of a coating based on hydroxylated, fluorine-containing polymers cross-linked with methyl melamine on the polycarbonate surface has been found to be clearly inferior to that of (meth)acrylate polymers, especially under weathering. Even after relatively short weathering periods, a tendency towards loosening of the coating and increased susceptibility of the surface to damage are observed.

A problem has therefore hitherto existed in the provision of scratch-resistant, firmly adhering surface coatings for polycarbonate mouldings.

The hitherto proposed processes for the provision of such coatings possess certain disadvantages. The application of the coating from an organic solvent is unsatisfactoryfrom a technical and ecologial point of view. Although the application of a film under pressure and heating approaches more closely the desired objective, it has not been possible for conventional processes of the state of the art to be fully satisfactory for use an industrial scale.

Production of the film and the polycarbonate moulding and the joining together have hitherto been carried out in separate operations. In conventional processes, a minimum temperature of 1630C is generally required for pressing; however, the practice is to increase the temperature in order to be able to reduce the duration of the operation.

The above-mentioned possibilities of the common vacuum forming of polyacrylate film and polycarbonate film and of injection moulding against a polyacrylate film are naturally restricted to vacuumformed or injection-moulded mouldings. Moreover, under the conditions required for vacuum forming a uniform, stable and firmly adhering coating may not necessarily be produced in all cases.

In addition to the above considerations, it is desirably that the coating process can be carried out continuously and without excessive expense.

It is object of the present invention to provide a new and improved method of coating extruded polycarbonate sheets with an adherent synthetic resin protective layer and optionally with a further scratch-resistant layer on the said synthetic resin layer.

According to the present invention, we provide a method of coating a polycarbonate sheet with an adherent sythetic resin protective layer which comprises extruding a polycarbonate sheet and a synthetic resin protective tayertherefor, and bringing the said extruded synthetic resin layer into contact with a surface of the said extruded polycarbonate sheet while both are at an elevated temperature resulting from the extrusion, the elevated temperature, being sufficient, if necessary in combination with the application of pressure, to effect adhesion, upon contact, between the polycarbonate sheet and the said synthetic resin layer.

The coated polycarbonate sheets produced in accordance with the invention have been found to meet substantially all of the requirements noted above.

Examples of particular forms of polycarbonate sheets which may be processed in accordance with the present invention include films and panels of polycarbonate resins.

The polycarbonate materials employed in the above-described method according to the invention are poly-condensation products of carbonic acid with polyhydric alcohols, especially dihydric phenols, such as dioxy-diphenyl alkanes, with a molecularweight above approximately 12,000. Particularly preferred polycarbonates include those based on (4,4'-dihydroxy-diphenyl)-2, 2-propane and its derivatives substituted in the phenyl nuclei by halogen or alkyl.

The synthetic resin protective layer is preferably composed of extrudable homopolymer or copolymers based on (meth)acrylates, particularly methyl methacrylate and/or methyl acrylate, as well as acrylates of homologous alcohols with 2 to 4 carbon atoms, especially ethyl and butyl acrylate, benzyl acrylate, dimethacrylates of diols such as ethandioland 1,4-butandiol-methacrylate. Furthermore, copolymers with acrylonitrile, styrene, maleic acid anhydride, butadiene-styrene and butadiene-amethyl-styrene may also be employed in the method according to the present invention.

The synthetic resin layers is conveniently applied in a thickness of 10 to 100F, preferably 50 . The thickness of the applied layer may be of critical importance for industrial use, since beyond a certain coating thickness, the transmission of cracks into the layer situated thereunder may occur.

The polycarbonate sheet and synthetic resin protective are conveniently co-extruded from an extruder containing a multiple die. If desired, a plurality of synthetic layers may be co-extruded in this manner, and adhesion between adjacent layers effected in a similar manner to that described above for adhesion between the polycarbonate sheet and the said synthetic resin layer adjacent thereto. In this case, poly(meth)acrylates which have good adhesion to the polycarbonate surface are advantageously used as the layer adjacent to the polycarbonate. At least one of the synthetic resin layers, preferably the poly(meth)acrylate layer, advantageously contains one or more ultra-violet absorbers, for example those mentioned in US-PS 3,594,262.

According to a preferred embodiment of the method according to the present invention, the synthetic resin protective layer is itself provided with a scratch-resistant coating for example one based on silicon resins or based on hydroxylated, fluorinecontaining polymers cross-linked with methyl melamine, as described in German OLS 1,963,278.

The application of the scratch-resistant coating on to the synthetic resin layer may be effected in conventional manner, for example by the application of a coating solution.

The polycarbonate material and synthetic resin e.g. the (meth)acrylate polymer or copolymer, may be employed in the conventional form and quality suitable for extrusion, for example as granulates.

The resulting extrudate may also be formed conventionally e.g. by the use of commercial extruders and multiple dies of suitable dimensions.

Adhesion between the polycarbonate moulding and the synthetic layer(s) is brought about in the course of the extrusion process, the heat content of the extruded material, which is generated in the extrusion operation, being generally large enough to guarantee the adhesive connection between the adjacent layers.

The co-extrusion according to the invention is advantageously followed by calendering.

The ultra-violet absorber(s) employed in the synthetic resin layer may be admixed with the resin, in as homogeneous distribution as possible, before extrusion or supplied continuously during extrusion.

The coatings obtained by the process according to the invention are superior to the products manufactured by conventional processes especially with regard to adhesion and, primarily, with regard to their optical properties.

The method according to the present invention will now be further described with reference to the accompanying drawing which is a cross-section of an apparatus which is suitable for carrying outa preferred embodiment of the method according to the invention.

The apparatus shown in the drawing is used for the preparation of polycarbonate panels with synthetic resin coatings. The essential parts for the carrying out of the invention include a multi-die nozzle in which the sheet thickness is adjusted by baffle elements 1,2,3 and 4. The total thickness of the panel is adjusted by two moveable lips 5 and 6.

The adjustment of the baffle elements as well as the lips can be effected by screws 7,8,9, 10, 11 and 12.

The material flows through channels 13, 14, 15 and 16 arranged in the interior of the nozzle. The heating of the multi-die nozzle is effected by resistance heating elements whereby the temerature of the individual sheets can be adjusted independently of each other.

Using the above-described apparatus, polycarbon ate panels of different thicknesses can be coextruded with synthetic resin layers of between 10 to 100Ft.

Films and panels of a polycarbonate-based materials can be co-extruded with (meth) acrylate polymers with particular success. This also applies when one or more UV-absorbers have been added to the co-extruded polymers.

it is also possible to carry out the co-extrusion when the final layer on both of the outer sides of the panels or films contain UV absorbers as well as when the intermediate layer also contains UV absorbers.

The working temperature in the extrusion process depends upon the type of materials employed. With (meth) acrylates the temperature lies, as a rule, in the range of 240-280 C. For polycarbonate materials, the temperature generally lies between 260 and 310 C.

Within the given range the temperature should be adjusted such that optimal bonding and optical quality for the co-extruded material is achieved.

Optimal temperature and thickness is achieved if a bubble-free homogenous length of material is obtained from the multi-die nozzle and the calendar rollers.

Claims

1. A method of coating a polycarbonate sheet with an adherent synthetic resin protective layer which comprises extruding a polycarbonate sheet and a synthetic resin protective layer therefor, and bringing the said extruded synthetic resin layer into contact with a surface of the said extruded polycarbonate sheet while both are at an elevated temperature resulting from the extrusion, the elevated temperature being sufficient, if necessary in combination with the application of pressure, to effect adhesion, upon contact, between the polycarbonate sheet and the said synthetic resin layer.

2. A method as claimed in claim 1 wherein the said synthetic resin layer is composed of a hompolymer or copolymer of one or more (meth)acrylate monomers.

3. A method as claimed in claim 2 wherein the said synthetic resin layer is composed of a hompolymer or copolymer of methyl methacrylate and/or methyl acrylate.

4. A method as claimed in any of the preceding claims wherein the said synthetic resin layer contains one or more utitra-violet absorbers.

5. A method as claimed in any of the preceding claims wherein the said synthetic resin layer has a thickness of 10 to 100p.

6. A method as claimed in any of the preceding claims wherein the said polycarbonate sheet is extruded in the form of a panel or film.

7. A method as claimed in any of the preceding claims in which the polycarbonate sheet and synthetic resin layer are co-extruded from a multiple die.

8. A method as claimed in any of the preceding claims wherein the polycarbonate sheet and adherent synthetic resin layer are subjected to a calendering operation.

9. A method as claimed in any of the preceding claims wherein a scratch-resistant layer is applied to the said extruded synthetic resin layer.

10. A method as claimed in claim 9. wherein the said scratch-resistant layer is based on a silicon resin.

11. A method as claimed in claim 9 wherein the said scratch-resistant layer is based on a hydroxylated fluorine-containing polymer cross-linked with methylamine.