GB2238967A - Process for coating an article - Google Patents

Abstract

A method of decorating an article (other than an intended unrevealed face of a plastics sheet before moulding thereof), for example a cellular uPVC plank, comprises applying to the article a preferably coloured or pigmented layer as a liquid film or curtain of a high molecular weight resin in a volatile solvent mixture having relatively volatile and relatively persistent components.

Description

PROCESS AND APPARATUS FOR COATING AN ARTICLE FIELD OF THE INVENTION This invention relates to a process and apparatus for coating an article, e.g. a board or other article of extruded cellular plastics material such as cellular MPVC.

BACKGROUND TO THE INVENTION Our application PCT/GB89/00623 (the disclosure of which is incorporated herein by reference) discloses a method and apparatus for coating film which is inexpensive and enables a moulder to produce a wide range of differently coloured articles without having to stock a large range of coloured sheets. The invention disclosed in that application provides a method of decorating an intended unrevealed face of a plastics sheet before moulding thereof, which comprises applying to the sheet a coloured or pigmented layer that is highly stretchable under moulding conditions for the sheet. In a preferred aspect of the above process the coating material is formed into a liquid film or sheet which is deposited on the substrate.In a further aspect the invention described in that application provides apparatus for the curtain coating of articles comprising a coating head having chamber and a slit leading from the chamber for discharge of a curtain of liquid coating material, wherein at least the inner surface of the chamber is of a plastics material that readily releases the coating liquid, means within the chamber being movable to expel the paint and mechanically clean the chamber when use of the coating liquid is to be discontinued.

SUMMARY OF THE INVENTION It has now been found that the coating formulations of PCT/GB 88/00623 are of value for the coating of articles in general, including shaped articles1 and in particular plastics faced boards and architectural boards of synthetic plastics materials for indoor or outdoor use for use where wooden planking or boards would previously have been used.

The coating formulations enable colours to be produced locally in relatively short runs to meet customer needs, avoid the need to have to stock a range of differently coloured planking, and improve resistance to weathering in outdoor use.

The present invention provides a method of decorating a plastics article (other than an intended unrevealed face of a plastics sheet) or an article partly of plastics such as a plastics faced or coated fibre or chipboard, which comprises: applying to the article as a liquid film or curtain pigmented or unpigmented solution of a high molecular weight resin whose hardness is similar to that of the article in one or more volatile organic solvents; and allowing the solution to dry so that the article is coated with a layer of the pigmented resin.

DESCRIPTION OF PREFERRED FEATURES The article is preferably wholly of thermoplastics material and it may have a cellular plastics core and at least one smooth face on which the liquid film is coated.

The invention has particular value for the coating of articles of cellular pPVC that are for outdoor use where the improved weather resistance and protection from UV degradation imparted by the coating is of value. The article may be shaped1 for example by extrusion, and may be a plank, a fascia board, a soffit board, a barge-board, a skirting board, an architrave, a window board or a trim.

Such products are sold, for example, by DURAFLEX TUFCON of Cheltenham, England under the name TUFCEL PLANKING. Such articles are sometimes sold with a protective film, whose removal gives rise to electrostatic charges when removed.

The method of the invention may therefore further comprise the preliminary steps of removing a protective plastics film from the article to be coated and treating the article, before application of coating liquid, so as to remove electrostatic charge. Preferably also the liquid film or curtain is treated to remove electrostatic charge before it is applied to the article. Charge removal may be by alpha-emitting ionisation or de-static bars positioned adjacent the article or the curtain of coating liquid as the case may be.

Preferably the coating is applied to the article as a liquid film or curtain that comprises a high molecular weight resin in a volatile solvent mixture. The coating may be pigmented with TiO2 or a mixture of TiO2 and china clay.

To avoid loss of volatile organic solvents, the liquid film is fed from the coating head onto the article through a chamber that surrounds the liquid film, the interior of the chamber becoming saturated or nearly saturated with solvent mixture. The volatile solvents used preferably have boiling 0 points below about 80 C and the solvent mixture used is selected to have relatively volatile and relatively persistent components such that it can dry finger hard in a few minutes at ambient or slightly elevated temperatures but retains a degree of fluidity until it is completely dry so that there is not premature skinning-over of an underlayer containing entrapped solvent which could be detrimental to subsequent moulding.

The resin system should be tough and desirably has a surface hardness similar to that of the hardest likely substrates. It should be a good adhesive and desirably should adhere to aost thersoplastics materials that are available including acrylic, polystyrene, ABS, PVC, polycarbonate, cellulose acetate and cellulose butyrate sheets. It is desirably pigmentable and should be capable of being highly charged with pigment without destroying its physical or chemical properties so that a film of e.g.20-25 dry thickness will be sufficiently opaque. A coating 75 p thick may typically be applied. The temperature response profile of the coating material and if possible other factors such as its hygroscopic character should be as close as possible to those of the substrate.The coating should also be benign to the substrate and in particular should not give rise to significant crazing, embrittlement or other weakening or damage.

The coating liquid to be applied to the article may contain a high molecular weight resin dissolved in a mixture of volatile solvents, the coating forming by drying without significant increase of molecular weight by polymerisation during the drying process. The best commercially available material at present known to the applicants is a polymethyl methacrylate resin dissolved in an organic solvent system. For toughness the resin used should be of relatively high molecular weight and should be a pure homopolymer.The resin found to be most useful is a PMMA homopolymer of molecular weight about 80000, melting point about 2100C and acid number 7. It is a chemically and temperature resistant resin of satisfactory hardness.

Higher molecular weight grades of PMMA are available, but these may be lower melting or have other disadvantages.

Less preferred materials which may have some utility include methacrylate copolymers e.g. with styrene or a vinyl compound, or butyl acrylate polymers or copolymers.

The high molecular weight of the resin and the volatility of the solvent are an obstacle to the use of conventional coating methods such as spray coating because the coating material then tends to form "strings" as it emerges from the nozzle and partly dries in its journey from the spray gun to the work piece resulting in a coating of pebble-dash appearance. Furthermore, in order not to exhibit irregularities, the initially coated film has to be very even, and the required evenness cannot be achieved by spray coating. Roller coating is a theoretical alternative, but it is difficult to achieve an adequate film thickness. A roller coating unit is costly and the roller material is at risk from attack by the solvents used. For these reasons a curtain coating process has been adopted as described in patent application PCT/GB 88/00623.Where the curtain or film of coating material is applied to an article such as a skirting board or architrave that has a profiled surface or returned edges, means may be provided to adapt the curta n or otherwise direct the coating material onto the article to achieve a sufficiently even coating.

There mas be a need for both a clear resin base mixture for translucent colours and for the darker colours and for a white resin base mixture for white or lightcoloured articles. A suitable formulation for the solution is: Resin 30%w/w Methyl ethyl ketone (MEK) 20% I sobutanol 6X Toluene 40X Ethyl glycol acetate (EGA) 4% The MEK and toluene are mixed with a dispersant which for TiO2 can be 50% by weight of a silicone modified polycarboxylic acid derivative and 50% of a carrier such as di-(2-ethylhexyl)-phthalate.The dispersant with its carrier is present in an amount of 0.3% by weight of the total mixture (including pigment in the case of a white base) and in these quantities the di-(2-ethylhexyl)phthalate does not significantly plasticise the resin. The resin beads or granules are dispersed in the MEK, toluene and carrier mixture system and mixed at 35-400C for 8-10 hours (or less if greater temperatures are used). Except where it is intended to make a white resin base, the higher boiling solvents can be added when the resin has gone into a bead free solution on visual inspection (which should provide a reading of about 4 on the Hegman scale).The higher boiling isobutanol and EGA are also volatile materials but have a greater persistence than the MEK and toluene which if used on their own could lead to premature skinning which would resist further volatilisation of solvent.

A clear resin base may be coloured with temperature stable and high light resistant organic dyestuffs. A white resin base may be pigmented with TiO2 alone but it is preferably pigmented with a mixture of TiO2 and china clay.

It has been found that the more glossy the surface of the film the less the tendency for granulation, Accordingly the pigments may be sized to a nominal size of 2p and preferably not above 5P. Pigment may be mixed with the above clear base in at 24% w/w pigment and 76% clear base (pigment:binder ratio 1:1). The high P:B ratio and the finely ground state of the pigments gives films of useful opacity in the particular resins selected. The pigments are added before the high boil solvents, and as the latter are mixed in the mixture is tested for viscosity and if necessary adjusted to a Brookfield viscometer viscosity of 200 poise. If the viscosity is above this value, low boiling solvent has been lost and MEK is added to bring it to the required value.If a non-white pigment is required, organic colourants stable up to at least 2400C and of high light-fastness are added.

The viscous base material described above has to be thinned (e.g. for gravity flow to a viscosity of about 100 0 90 seconds at 18 C in a Ford No 4 cup and for pressure flow to a somewhat greater viscosity) by addition of a rapidly evaporable thinner. A suitable rapidly evaporable thinner that may be needed to be added to the base material so that it can form a fluid film or curtain by gravity flow is: Methyl ethyl ketone (MEK) 44% v/v Acetone 38% Ethyl glycol monoethyl ether 18% ( cellosolve )t J If desired 100% MEK can be used as the thinner but if static charges are present on the surface of the article these can give rise to blemishes in the coating.If there is a high static charge on the article e.g. when the protective film has been stripped off and the article has been placed on a belt conveyer, the thinner may be: Methyl ethyl ketone 60% Ethyl glycol acetate 40% However, the latter thinner can lead to increased process times and a small increase in retained solvents.

A coating station for use in the above process is disclosed in PCT/GB 88/00623. It is desirable to minimise solvent loss -that the curtain of coating liquid should be confined for a major portion of its length in a closed chamber in which the atmosphere can become saturated or near saturated with solvent vapours. The above curtain coater or liquid contact coating machine has the advantage that it can provide very fast turnaround on a coating liquid colour change since it is quick to clean, flush and recharge.

Various modifications may be made to the formulations described above without departing from the invention . For example, the resin could simply be dissolved in a mixture qf methyl ethyl ketone and ethyl glycol acetate, preferably in a ratio of three parts of MEK to two parts of EGA.

Furthermore, if the substrate has a cut or unfinished edge, that edge may be sealed and coated as described in our copending application entitled FORMING A COATING ON A SUBSTRATE and filed on 29th November 1989 Beresford case 5065501.

Claims (15)

1. A method of decorating an article wholly or partly of plastics (other than an intended unrevealed face of a plastics sheet) which comprises: applying to the article as a liquid film or curtain a pigmented solution of a high molecular weight thermoformable resin whose hardness is similar to that of the article in a mixture of volatile solvents containing relatively volatile and relatively persistent components so that the solution dries without forming a solvententrapping skin, the ratio of the pigment to resin in the solution being relatively high; and allowing the solution to dry so that the article is coated with a layer of the pigmented resin.

2. A method according to claim 1, wherein the solution is pigmented with TiO2 or a mixture of TiO2 and china clay.

3. A method according to claim 2, wherein the average particle size of the pigment is less than 12 micrometers (7 on the Hegman scale).

4. A method according to any preceding claim, wherein the film is gravity fed onto the article.

5. A method according to any of claims 1-3, wherein the film is pressure fed from a coating head onto the article.

6. A method according to any preceding claim, wherein the liquid film is fed from the coating head onto the article through a chamber that surrounds the liquid film, the interior of the chamber becoming saturated, or nearly saturated, with the solvent mixture.

7. A method according to any preceding claim, wherein the article has a cellular plastics core and at least one smooth face on which the liquid film is coated.

8. A method according to claim 7, wherein the article is of cellular uPVC

9. A method according to claim 7 or 8, wherein the article is shaped.

10. A method according to claim 9, wherein the article is shaped by extrusion through a die.

11. A method according to claim 10, wherein the article is a plank,, a fascia board, a soffit board, a barge-board, a skirting board, an architrave, a window board or a trim.

12. A method according to any preceding claim, wherein the article is coated with a solution containing a high molecular weight polymethacrylate homopolymer.

13. A method according to any preceding claim, comprising the preliminary steps of removing a protective plastics film from the article to be coated and treating the article, before application of coating liquid, so as to remove electrostatic charge.

14. A method according to claim 13, wherein the liquid film or curtain is treated to remove electrostatic charge before it is applied to the article.

15. A method according to any preceding claim, wherein the coating is applied in a thickness of about 75u.