COATING COMPOSITIONS

Abstract: According to the present invention there is provided a coating composition comprising from 30% to 70% by weight of a novolac-type phenolic resin having a melting point in the range of 70-l30°C or resol-type phenolic resin having a gel time of at least 45 to l80 sec. at l80°C; and 70-30% by weight of a multi-epoxide group containing compound having at least 2 epoxide groups and having an epoxide equivalent weight of l00 to l200.

Description: This invention relates to coating compositions, to a process for their preparation and to their use particularly for the preparation of metallized articles. Coating compositions for use in the manufacture of metallized coatings are known. For example, a polyurethane resin obtained by crosslinking a hydroxylated saturated polyester by means of an adduct based on a masked polyisocyanate and a polyol, together with a polyester-epoxy resin obtained by reacting a carboxylated polyester with an epoxy resin or an epoxy resin hardened by an optionally accelerated or substituted polyamide is disclosed in GB-A-2058l4l. We have now discovered a novel coating composition which can be used in the manufacture of metallized articles. This coating composition is described in Claim 1. The metallized surface obtained has a particularly good optical quality and moreover the metallized layer so produced is particularly stable to heat. The compositions of this invention can be used for the production of metallized surfaces for decorative purposes, for example metallized parts for cars (in particular, bumpers and window frames), containers and container caps and trophies. The compositions can also be used for preparing optical quality reflectors, for example, mirrors and lamp reflectors (particularly headlamp reflectors for motor vehicles). Examples of Novolak-type phenolic resins having a melting point in the range of 70-130°C are SF 118 (manufactured by Schenectady), Alnoval PN 320, Alnoval PN 430 and Alnoval PN 822. The Alnoval series are manufactured by Hoechst. Preferably the resin is one which has a melting point of less than 120°C. Preferably it has a melting point in the range of 83-88°C. In particular, it is Alnoval PN 320. The multi-epoxide group-containing compound can be a low molecular weight molecule for example triglycidyl isocyanurate which has three epoxide groups, a molecular weight of 322 and an epoxide equivalent weight of 107. The multi-epoxide group-containing compound can be an epoxy resin based on bisphenol A and epichlorhydrin or bisphenol F and epichlorhydrin. Preferably it is based on bisphenol A and epichlorhydrin. Preferably the epoxide equivalent weight is at least 500 and is less than l000. Preferably the resin has a softening point between 50 and l00°C. More particularly, the epoxy resin is one having an epoxide equivalent weight of 605 to 645 and a softening point between 59 and 65°C. Examples of epoxy resins of this type are Araldite GT 7072 (equivalent weight 570-595); softening point 55-65°C), Araldite GT 7203 (equivalent weight 605-645; softening point 59-65°C), and DER 663U (equivalent weight 730-840; softening point 88-98°C) and DER 664U (equivalent weight 875-975; softening point 95-l05°C). The Araldite GT resins referred to are manufactured by CIBA-Geigy. The DER resins referred to are manufactured by the Dow Chemical Company. Preferably the resin is Araldite GT7203. The compositions can also contain other additives which facilitate the coating process, for example, an air release agent, in particular benzoine, a flow agent, in particular Acronal 4F, Modaflow and BYK Powderflow, a catalyst for the epoxide/phenol reaction, for example, Rutapox VE260. Benzoine is available from Rhone-Poulenc. Acronal 4F is available from BASF. Modaflow is available from Monsanto and BYK Powderflow is available from BYK-Goulden-Lomberg. The compositions can be prepared by conventional processes. For example, the ingredients can be mixed dry and melted as they are extruded by a single or twin feed extruder. The extrudate is then pressed and allowed to harden. The hard material is then milled into a powder using a hammer mill or a ball mill. The compositions can be used to form a metallized coating on an article by depositing on a surface of the article, a film of the composition, allowing the film to cure, subjecting the cured coating to a corona discharge and thereafter flash metallizing the cured coating. The coating composition, which is in the form of a powder, can be applied by any standard powder coating technique. For example, it can be sprayed with a gun, especially an electrostatic gun, or it can be applied from an electrostatic fluidised bed. In practice the coating is applied in a thickness of at least 40 µm. In practice the thickness of the coating will not exceed l50 µm. Preferably it is from 80-l20 µm. Preferably the curing process is facilitated by baking the coated article. The temperature and the time for which the article is baked depends upon the material from which it is manufactured. The article will be heated to a temperature at which curing is promoted but less than that at which distortion occurs. In general, the temperature is at least 200°C. Where the article is made from sheet moulding compound, it is baked at 220°C for about 30 minutes. Where the article is metal it is heated to about 230°C for about 20 minutes. Some plastic materials are conductive. These can be coated electrostatically. For non-conductive electrostatic materials, the article to be coated is pre-heated to a temperature above the melting point of the powder to facilitate the coating process. Where appropriate, the coating can be subjected to UV radiation to promote curing. When the coating has cured, the article is subjected to a corona discharge and low pressure. The discharge is carried out from l-3 minutes at a pressure of less than 1.33 Pa (l0⁻² mm Hg), and preferably at a pressure of below 0.013 Pa (l0⁻⁴ mm Hg). The metallization process is carried out by a standard flash metallization. The following Examples illustrate the invention: EXAMPLES Example l Non-reactive novolac phenolic resin SFP ll8 (40 parts) by weight was blended with the epoxy resin Araldite GT 7203 (60 parts by weight), Rutapox VE 2600 catalyst (0.24 parts by weight) and benzoine (0.40 parts by weight). The mixture was extruded hot through a twinfeed extruder at a temperature at which the blend is melted. The extrudate was allowed to cool and solidify. The solid mass was pulverised in a hammer mill. The granulometry of the powder obtained was determined on an "Alpine" apparatus. The powder contains 3 to 7% of particles of diameter greater than 80 microns, l0-l8% of particles of diameter greater than 63 microns and 60-75% of particles of diameter greater than 32 microns. Example 2-l3 The compositions of Examples 2-4 were prepared by the process described in Example l along the amounts of phenolic resin and epoxy resin given in Table l below and using Rutapox VE 2600 catalyst (0.24 parts by weight) and benzoine (0.40 parts by weight). Example l4 The compositions of Examples l to l3 can be used to prepare a metallized headlamp reflector with a metal housing. A metal headlamp parabola was degreased with alkali and rinsed with demineralised water. The cleaned parabola was then passed through a zinc phosphate solution, rinsed and dried in an oven at about l20°C. The dried parabola was then coated with the composition using an electrostatic gun. The coated parabola was heated to 220-230°C for 20 mins to cure the coating. The metal layer is deposited by vacuum metallization. The parabola with the cured coating was placed into a vacuum chamber and the chamber evacuated. A block of aluminium in the chamber was caused to sublime by the application of heat using an electric current. The sublimed aluminium deposited on the pararbola. Example l5 The compositions of Examples l to l3 can be used to prepare a metallized headlamp reflector with a housing prepared from conducting sheet moulding compound (SMC). The parabola is cleaned with a hydrocarbon solvent, toluene or xylene or by U.V. irradiation. The cleaned parabola is preheated to 220°C for 30 minutes and cooled to 40°C. The powder is applied by electrostatic spray and cured as described in Example l4. The aluminium layer is deposited also as described in Example l4.

Claims: A coating composition comprising from 30-70% by weight of a novolak-type phenolic resin of melting point in the range of 70-130°C and from 70-30% by weight of a multi-epoxide group containing compound having an epoxide equivalent weight of 100 to 1200. A composition as claimed in claim 1 where the epoxy resin has an epoxy equivalent weight of 605 to 645 and a softening point of 59 to 65°C. A composition as claimed in claim 1 or claim 2 where the epoxide resin is based on bisphenol A and epichlorhydrin. A composition as claimed in any one of claims 1 to 3 comprising a novolak-type resin and the resin has a melting point of 80-100°C. A composition as claimed in any one of claims 1 to 4 and containing a catalyst for the reaction between the phenolic resin and the epoxide. A process for forming a metallized coating on an article which comprises depositing on a surface of the article, a film of a composition as claimed in any one of claims 1-5, allowing the coating to cure, subjecting the cured coating to corona discharge and thereafter flash metallizing the cured coating. A process for preparing a composition as claimed in any one of claims 1-5 which comprises dry blending the epoxide resin and the phenolic resin. A light reflecting article comprising a surface coated with a layer of a coating as claimed in any one of claims 1-5 and a layer of metal.

Inventors: CHAVASSET, GEORGE, VO-THAN, MAURICE
Applicants: HOLDEN EUROPE SA
Application year: 1987
Main classification: F21V7
IPC codes: C23C14, C23C4, C09D163, B05D5, C09D161, F21V7