GB2042930A

GB2042930A - Process for Coating Thermosetting Plastics Materials

Info

Publication number: GB2042930A
Application number: GB8003647A
Authority: GB
Original assignee: VERNIS VALENTINE COMP D
Current assignee: VERNIS VALENTINE COMP D
Priority date: 1979-02-05
Filing date: 1980-02-04
Publication date: 1980-10-01
Also published as: GB2042930B; FR2447794B1; FR2447794A1

Abstract

Thermosetting plastics materials, replacing metals in the manufacture of automobile headlights and other articles, can be provided with a smooth surface layer by applying a layer of a thermosetting resin in powder form by spraying or by means of a fluidized bed and then raising the temperature of the powder to cause fusion and polymerisation of the resin.

Description

SPECIFICATION Process for Coating Thermosetting Plastics Materials The present invention relates to a process for the coating of thermosetting plastics materials and is intended, in particular but not exclusively, for the manufacture of headlights for the automobile industry.

It is known that it is suitable, in certain cases, to replace metallic parts by parts made of thermosetting plastics materials, especially in order to achieve a weight saving. Up to now certain disadvantages, amongst which may be mentioned degassing in cases where there is a subsequent treatment under vacuum, the occasionally insufficient surface quality of certain parts obtained by moulding and relative mechanical weakness, have militated against the generalised use of thermosetting plastics materials such as, for example, polyester reinforced with glass fibres, in particular in those fields where the surface state of the article must be perfect, either for physical reasons (which is the case with headlights) or for aesthetic reasons (for example in the case of automobile coachwork).

Thermosetting materials, in addition to their lower cost price, have substantial advantages compared with thermoplastic materials, especially as regards their ability to withstand thermal shocks. in the case of automobile headlights, it is known that when an iodine lamp is mounted in the headlight the temperature of the headlight may rise almost instantaneously in certain areas from 0 to 2000C when the light is switched on.

The present invention seeks to overcome the above-mentioned disadvantages and to provide, inter alia, an improved process for the manufacture of automobile headlights.

According to the invention there is provided a process for coating a thermosetting plastics material, which comprises applying to the thermosetting plastics material by spraying or by means of a fluidised bed, a layer of a thermosetting resin in powder form and then raising the temperature of the powder to cause fusion and polymerisation of the thermosetting resin.

When a headlight is being manufactured, it is sufficient to apply the powder solely on the inner face of the headlight which is to be the reflective surface, thus enabling economies to be realised.

Examples of thermosetting resins are polyurethane resins, polyester resins and epoxy resins, and mixtures of two or more of these. The thermosetting resin applied in powder form may consist for example of a hydroxylated polyester and a polyisocyanate masked with caprolactam, forming a polyurethane on curing. It may alternatively consist of a mixture of polyester (especially carboxylated polyester) and epoxy resins.

Preferably, the powder is applied to the surface to be coated by spraying onto a surface which has been preheated, e.g. to 1300 to 1700C, or by electrostatic spraying onto a cold (i.e. unheated) surface. In the latter case, the surface may be rendered electrically conducting by the application of an electrically conducting solution or by printing, but it may already be electrically conducting, for example as a result of incorporation into the mass to be moulded of an electrically conducting agent such as conductive carbon black.

Suitably, sufficient resin is applied to provide a coating. after polymerisation, having a thickness of 50 to 1 50 microns.

Curing, i.e. fusion and polymerisation, of the thermosetting resin applied in powder form is achieved by raising its temperature. This is suitably achieved by heating to from 1400 to 2200C and a convenient procedure is to pass the article through an oven in this temperature range for 1 5 to 25 minutes.

After application of the powder and curing, the surface has a satisfactory state and the part thus coated may be used directly. In this case, the deposited powder may if desired be pigmented, which avoids the need for pigmenting of the mass. The layer thus obtained may also serve as an under layer to receive a traditional finish layer, for example by vacuum metallization, optionally preceded by application of a varnish and crosslinking and optionally followed by coating with a protective varnish.

The present invention may be applied to all moulded articles of thermosetting plastics material. The application of a paint in pigmented powder enables a finished part to be obtained directly or may serve, whether the powder is pigmented or uncoloured, as under layer for traditional finishes.

Other characteristics and advantages of the invention will appear in the course of the description which follows of particular embodiments given by way of non-limitative examples. The two examples below are concerned with the obtaining of reflecting surfaces of thermosetting plastics material before being mounted in the interior of automobile headlights.

Example 1 Into the interior of a parabolic support, made of reinforced thermosetting polyester, there is projected a powder constituted by an uncoloured thermosetting polyurethane resin crosslinking from 1600C and comprising essentially a hydroxylated polyester and a polyisocyanate masked with caprolactam. The support had been previously preheated to a temperature of from 1600 to 1800C. This temperature falls towards 1300--1700C when the support is transferred to the powder-applying station which projects the powder pneumatically. The preheating ensures cohesion of the layer of applied powder.The polymerisation of the powder coating is effected by passage of the support through an oven at an ambient temperature of between 1 500 and 2200C over a period of 1 5 to 20 minutes.

Then, after dipping in a hydrosoluble varnish of the type "Aquaval 08793" manufactured by Compagnie de Vernis Valentine, the viscosity of which is 30 seconds cut (Standard AFNOR No. 4), the piece is left to drain for 5 to 20 minutes and it is then cured in an oven in which the ambient temperature is between 1 500 and 2200C over a period of 1 5 to 20 minutes. This layer of varnish improved the photometric properties and the maintenance of the metallisation at elevated temperature. There then follows a metallisation under vacuum with aluminium which forms the reflecting surface proper. This surface may itself be protected by a layer of a hydrosoluble or conventional transparent varnish by, for example, dipping, sprinkling or spraying.

Example 2 Supports or parts such as those described in Example 1 are first rendered conductive on their internal surface by immersion in or sprinkling with a conducting solution, for example a solution of quaternary ammonia, optionally followed by drying.

A layer of uncoloured thermosetting resin comprising a mixture of carboxylated polyester resin and epoxy resin in powder form is then projected by the electrostatic method under a voltage of from 40 to 90 kV. Curing of the powder is then carried out. Fusion of the powder levels the surface and then polymerisation occurs. This is achieved by keeping the part in an oven at a temperature from 1500 to 2200C for 1 5 to 20 minutes.

Thereafter either metallisation under vacuum is carried out directly or the resulting layer is coated with a hydrosoluble (e.g. conventional) varnish of aikyd melamine type which is polymerised before the metallisation proper.

In the two Examples which have just been described, the layer of resin deposited has a thickness of the order of 1 00 microns, which improves the mechanical strength properties of the headlight, especially with respect to vibrations.

Of course modilFications may be made to the embodiments that have been described, in particular by the substitution of equivalent technical means, without thereby departing from the scope of the invention.

Claims

1. A process for coating a thermosetting plastics material, which comprises applying to the thermosetting plastics material by spraying or by means of a fluidised bed, a layer of a thermosetting resin in powder form and then raising the temperature of the powder to cause fusion and polymerisation of the thermosetting resin.

2. A process according to claim 1, wherein the thermosetting resin is a polyurethane resin, a polyester resin, an epoxy resin or a mixture of two or more of these.

3. A process according to claim 1, wherein the thermosetting resin comprises a hydroxylated polyester resin and a polyisocyanate masked with caprolactam.

4. A process according to claim 1, wherein the thermosetting resin comprises a carboxylated polyester resin and an epoxy resin.

5. A process according to any of the preceding claims, wherein the fusion and polymerisation of the thermosetting resin are achieved by passage through an oven having a temperature of from 1400 to 2200C for 15 to 25 minutes.

6. A process according to any of the preceding claims, wherein the thermosetting resin powder is sprayed onto the plastics material which has been preheated to a temperature of 1300 to 1 700C.

7. A process according to any of claims 1 to 4, wherein the application of the thermosetting resin is effected by electrostatic spraying in the coid, the surface of the plastics material to be covered being electrically conducting or having been rendered electrically conducting by the application of a conducting solution or by printing.

8. A process according to any of the preceding claims, wherein sufficient thermosetting resin is applied to provide a coating having, after polymerisation, a thickness of from 50 to 150 microns.

9. A process for the manufacture of an automobile headlight having its reflecting surface constituted by a metal layer supported on a thermosetting plastics materials, wherein a thermosetting plastics material having the shape of an automobile headlight is coated on the surface which is to form the reflecting surface by a process according to any of the preceding claims and the thus coated surface is metallized under vacuum.

10. A process according to claim 9, wherein a coating process according to any of claims 1 to 6 is followed by application to the thus coated surface of a varnish, which, after crosslinking, is coated with a reflecting coating by metallization under vacuum.

7 A process according to claim 9 or 10, wherein the metallic layer deposited under vacuum is coated with a protective varnish.

12. A process for coating a thermosetting plastics material carried out substantially as described with reference to either of the foregoing Examples.

13. An automobile headlight obtained by a process claimed in any of the preceding claims.