GB2058141A

GB2058141A - Production of smooth aluminium coatings on metal substrates

Info

Publication number: GB2058141A
Application number: GB8021132A
Authority: GB
Original assignee: Equipements Automobiles Marchal SA
Current assignee: Equipements Automobiles Marchal SA
Priority date: 1979-06-29
Filing date: 1980-06-27
Publication date: 1981-04-08
Also published as: ES8305226A2; ES492869A0; FR2304690A1; IT1128830B; DE3024371A1; IT8068016A0; FR2304690B1; DE3024371C2; FR2460339A2; FR2460339B2; JPS6330071B2; JPS5645785A; GB2058141B

Description

SPECIFICATION Process for the production of smooth aluminium coatings on metal substrates The present invention relates to a process for producing smooth aluminium coatings on metal substrates.

French published Patent Application No.

2,304,690 relates to a process for treating a metallic surface, intended to be aluminised, in order to obtain, in particular, vehicle headlight reflectors, characterised by the fact that, firstly, a layer of powdered resin is applied to the said surface; secondly, the layer of resin is melted and hardened on its support; and, thirdly a coating of brilliant aluminium is deposited on the pretreated surface in a known manner. According to this process, a polyurethane, polyester or epoxy resin is used in particular to form the layer of resin.

The Applicants have now discovered that aluminium coatings having excellent smoothness characteristics can be obtained by using certain resins.

Accordingly, the present invention provides a process for producing a smooth aluminium coating on a metal substrate which process comprises the steps:- a) applying to the metal substrate to be coated a layer of powdered resin material; b) melting the powdered resin material and allowing the thus produced melted resin material to harden on the metal substrate; and c) depositing an aluminium coating onto the treated substrate, wherein the resin material is a polyurethane resin obtained by cross-linking a hydroxylated saturated polyester by means of an adduct based on a masked polyisocyanate and a polyol, 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.

In order to form the hydroxylated polyester, adipic acid may, for example, be used as the aliphatic polyacid, and terephthalic, isophthalic or orthophthalic acid as well as the esters and hydrngenated and halogenated derivatives of such acids, and, in particular tetrahydrophthalic acid and hexahydrophthalic acid, as aromatic polyacid.

As polyol, there may be used, inter alia, ethylene glycol, propylene glycol, neopentyl glycol, trimethylolpropane or pentaerythritol.

The hydroxylated polyester formed from these constituents will advantageously have a hydroxyl index of between about 50 and 80 and an acid index of about 4 to 6. Terephthalic acid and neopentyl glycol are preferably used to form this polyester.

The hydroxylated polyester is next cross-linked by an adduct based on a polyol and a masked polyisocyanate, such as toluene diisocyanate, hexamethylene diisocyanate or isophorone diisocyanate, masked for example by caprolactam.

The product obtained by this cross-linking will preferably have a melting point range of 65 to 1 000C and an unblocking temperature of 1 60 to 1800C.

In order to obtain a layer of resin having a particularly favourable degree of smoothness, it is desirable to add to this polyurethane resin a small amount (0.5 to 10%) of an epoxy resin such as a resin formed from epichlorohydrin and bisphenol A whose melting point range is of the order of 80 to 1 100C.

In order to prepare the second type of resin used according to the invention, a carboxylated polyester is first of all formed by reacting a polyol with an aliphatic or aromatic polyacid preferably chosen from the polyacids mentioned above, in proportions such that a polyester having an acid index of about 30 to 50 is obtained. This carboxylated polester will advantageously have an average molecular weight of about 1000 to 6000 and a melting point of the order of 80 to 1000C.

Terephthalic acid, possibly mixed with isophthalic hexahydrophthalic and/or tetrahydrophthalic acid, and neopentyl glycol, possibly mixed with trimethylolpropane, will preferably be used to form the carboxylated polyester.

In order to obtain the epoxy polyester, the carboxylated polyester is mixed in the appropriate proportions with respect to the ratio of acid equivalent index to epoxy index, with an epoxy resin preferably having a molecular weight of about 1100 to 2000 and a melting point range of the order of 90 to 1000C.

Finally, the third type of resin used according to the invention is obtained by cross-linking an epoxy resin by means of a polyamide, such as dicyandiamide, possibly accelerated or substituted. The starting epoxy resin, for example a resin based on epichlorohydrin and bisphenol A, will advantageously have a molecular weight between 1100 and 2000, and preferably between 1300 and 1500.

In order to prepare the powdered resin for use, conventional additives such as spreading agents, in particular of the polyacrylate type, are of course added to the various resins mentioned above, and if necessary inert and anticorrosion pigments are also added if it is desired to obtain a layer of pigmented and non-colourless resin. As spreading agent, there may be mentioned in particular the product "Modaflow" produced by Monsanto Limited, which is used in an amount of about 0.5 to 2%.

The constituents, namely resin and additives, which are in a solid state, are mixed and dispersed in an extruder, for example of the Buss PR 100 type. The mixture is rolled and cooled, and is then ground, for example in an ACM type mill marketed by Mikropull, to a grain size of about 2 to 100 y.

The powder thus obtained is preferably applied electrostatically, as mentioned in French Patent Application No. 2,304,690, and when stoved between about 180 and 2200 provides a perfectly smooth film having an average thickness of about 30 to 80 y. The Applicants have found that in order to obtain a high degree of smoothness, the resin must be formulated so that the end of the polymerisation occurs a sufficiently long time after the end of the melting, and accordingly the powdered resin should not be too reactive. Of course, the powdered resin must nevertheless possess a certain degree of reactivity in order for the necessary polymerisation temperature not to be too high.

In accordance with the procedure described in French Patent Application No. 2,304,690, before aluminising the metallic surface, a varnish may be applied to the layer of resin formed in order to improve the adherence of the layer of aluminium and to insulate the layer of resin so as to avoid the danger of salting out, which can occur when the metallic article is heated to fairly high temperatures, i.e. above about 1 600C.

When the types of resin described above are used, an alkyd-melamine varnish, more particularly of the hydrosoluble type, may advantageously be selected as varnish. Such a varnish consists of a carboxylated binder of the alkyd type modified by vegetable fatty acids (for example soya bean acid) and neutralised with an amine (such as dimethylethanolamine, triethanolamine, triethylamine, ammonia, etc.), and to which a melamine-formaldehyde resin etherified by an alcohol has been added.

The brilliant aluminium coating may be deposited onto the metal surface, treated with the resin material, by any known method. The present invention is particularly useful in the manufacture of relfecting surfaces, especially vehicle headlight reflectors.

The following examples illustrate the invention without however limiting its scope.

EXAMPLE 1 Use of a Polyurethane Resin In order to form the starting hydroxylated polyester, 55 parts of terephthalic acid are reacted with 45 parts of neopentyl glycol to obtain a product having a hydroxyl index of 50. 75 parts of this product are mixed with 20 parts of an adduct based on isophorone diisocyanate masked with caprolactam, 2 parts of a spreading agents (Modaflow marketed by Monsanto Limited) and 3 parts of an epoxy resin of the epichlorohydrinbisphenol A type (Epikote 1004 resin marketed by Shell Limited).

When this resin is applied to a metallic substrate according to the technique described in French Patent Application No. 2,304,690 and is stoved, it provides a layer having an excellent degree of smoothness.

EXAMPLE 2 Use of a Polyester-epoxy Resin In order to form the starting carboxylated polyester, 40 parts of terephthalic acid and 13 parts of tetrahydrophthalic acid are reacted with 47 parts of neopentyl glycol to give a product having an acid index between about 40 and 50 and an average molecular weight of about 1200.

60 parts of this product are mixed with 40 parts of an epoxy resin of the epichlorohydrinbisphenol A type (Epikote 1004 resin marketed by Shell Limited) and 1 part of a polyacrylate type spreading agent (Modaflow marketed by Monosanto Limited).

Of course, various modifications may be made by those skilled in the art in the process that has just been described, without going beyond the scope of the invention.

Claims

1. A process for producing a smooth aluminium coating on a metal substrate which process comprises the steps: a) applying to the metal substrate to be coated a layer of powdered resin material; b) melting the powdered resin material and allowing the thus produced melted resin material to harden on the metal substrate; and c) depositing an aluminium coating onto the treated substrate, wherein the resin material is a polyurethane resin obtained by cross-linking a hydroxylated saturated polyester by means of an adduct based on a masked polyisocyanate and a polyol, 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.

2. A process according to claim 1, characterised in that the hydroxylated saturated polyester is obtained by reacting a polyacid chosen from adipic, terephthalic, isophthalic, orthophthalic, tetrahydrophthalic and hexahydrophthalic acid, with a polyol chosen from neopentyl glycol, trimethylolpropane, pentaerythritol, ethylene glycol, and propylene glycol.

3. A process according to claim 2, characterised in that the hydroxylated saturated polyester is obtained by reacting terephthalic acid with neopentyl glycol.

4. A process according to one of claims 1. to 3, characterised in that the hydroxylated saturated polyester has a hydroxyl index of between about 50 and 80.

5. A process according to one of claims 1 to 4, characterised in that the starting polyisocyanate is chosen from toluene diisocyanate, isophorone diisocyanate and hexamethylene diisocyanate.

6. A process according to one of claims 1 to 5, characterised in that the polyisocyanate is masked by caprolactam.

7. A process according to one of claims 1 to 6, characterised in that the polyurethane resin has a melting point range of 65 to 1000C and an unblocking temperature of 160 to 1 800C.

8. A process according to one of claims 1 to 7, characterised in that 0.5 to 10% by weight of an epoxy resin based on epichlorohydrin and bisphenol A is added to the polyurethane resin.

9. A process according to claim 1, characterised in that the carboxylated polyester is obtained by reacting a polyacid chosen from adipic, terephthalic, isophthalic, orthophthalic, tetrahydrophthalic and hexahydrophthalic acids, with a polyol chosen from neopentyl glycol, trimethylolpropane, pentaerythritol, ethylene glycol, and propylene glycol.

10. A process according to claim 1 to 9, characterised in that the carboxylated polyester has an acid index of between about 30 to 50.

11. A process according to one of claims 1, 9 and 10, characterised in that the epoxy resin is a resin based on epichlorohydrin and bisphenol A having a molecular weight of about 1100 to 2000.

12. A process according to claim 1, characterised in that the epoxy resin hardened to a polyamide is a resin based on epichlorohydrin and bisphenol A hardened by dicyandiamide.

13. A process according to one of claims 1 to 12, characterised in that a spreading agent of the polyacrylate type is added to the resin.

14. A process according to one of claims 1 to 13, characterised in that an inert or anti-corrosion pigment is added to the resin.

1 5. A process according to one of claims 1 to 14, characterised in that a varnish of the alkydmelamine type is applied to the layer of resin.

1 6. A process according to claim 15, characterised in that the alkyd-melamine varnish is a hydrosoluble varnish.

17. A powdered resin of the polyurethane, polyester-epoxy or hardened epoxy type, which can be used in a process according to any one of claims 1 to 16.

1 8. Headlight reflectors for vehicles, obtained by a process according to any one of claims 1 to 16.