GB2092167A - Plasticized resin compositions for coating metal surfaces - Google Patents

Abstract

A synthetic resin composition suitable for coating metal surfaces comprises a dispersion of synthetic resin, such as polyvinyl chloride or vinyl chloride copolymer, in a plasticizer, such as a phthalate ester of a C6-C12 alcohol, and a polymerizable diluent, such as an acrylic ester. Under the curing action of heat or other curing means the diluent polymerises rather than volatilises and is incorporated in the hardened coating. The composition may be coated on such metals as steel, iron, copper, zinc, nickel or brass in the form of articles such as coils, planar surfaces, wires, meshes, and cylindrical or rod-like articles.

Description

SPECIFICATION Coating a metal surface with a synthetic resin This invention relates to the coating of a metal surface with a synthetic resin. In particular, it relates to the coating of the surface of a metal article such as a steel coil with a protective layer of synthetic resin.

It is known to apply a synthetic resin coating (e.g. of plasticised polyvinyl chloride) to a metal surface by preparing a plastisol consisting of a dispersion of synthetic resin in a liquid medium which is a plasticiser for the resin, possibly together with other ingredients such as stabilizers, fire retardant additives and pigments, using the plastisol with diluent in the ratio typically of 80 parts plastisol to 20 parts diluent for a 100 micron coating, reverse roller-coating the mixture onto the metal surface, and subsequently removing the diluent by the application of heat.

During the removal of the diluent the liquid part of the plastisol is absorbed into the resin and curing takes place.

The diluent is provided to reduce the viscosity of the plastisol to an acceptable level. This is important in the case of, for example, a 100 micron coating because the pigment concentration must be high enough to give the necessary opacity for a thin coating, and this may increase the viscosity of the plastisol to a level which makes its application to a surface difficult and may cause problems in the integrity of the final coating. A similar problem will arise if there is in the plastisol a high level of some other filler such as an inorganic fire retardant. The final removal of the diluent is an inconvenient part of the process in that its removal uses energy and takes time, and the diluent represents a financially noticeable proportion of the coating system which has to be paid for but which is lost during the application stage.In some relatively expensive coating machines the diluent e.g. white spirit, may be burnt off and its heat of combustion recycled or otherwise used. This nevertheless needs a relatively high capital outlay.

It is an object of the present invention to overcome these disadvantages by providing a plastisol that has a desirable viscosity during application but results in a substantially reduced waste of diluent.

According to a first aspect of the present invention there is provided a synthetic resin composition for coating metal surfaces, the composition including a dispersion of synthetic resin in a liquid plasticiser and a polymerizable diluent.

Preferably the synthetic resin is polyvinyl chloride and/or a polyvinyl chloride copolymer, and the polymerizable diluent is an acrylic ester. The polymerizable diluent may be an acrylic ester of a monohydric alcohol. The polymerizable diluent is preferably 125% by weight of the synthetic resin.

The composition may contain up to about 5% by weight of a volatile solvent. The volatile solvent is preferable a hydrocarbon or mixtures thereof, having a boiling point between 50--2000C.

According to a second aspect of the present invention there is provided a method of coating a metal surface with a synthetic resin composition comprising the steps of preparing a dispersion of a synthetic resin in a liquid plasticiser, mixing the dispersion with a polymerizable diluent, applying the mixture to the metal surface, and subsequently polymerising the polymerizable diluent content of the mixture so as to provide an adherent hard coating of the resin on the metal surface.

The polymerization may be carried out by the application of heat, preferably for between 20-100 seconds at a temperature in the range 180--3000C.

In a first example a piastisol according to the following composition was prepared: pts by weight High molecular weight paste polymer pvc 100 Plasticizer: a phthalate ester of a C6C,2 alcohol 40 Stabilizer LC68 by Lankro Chem. Co. 2-4 Diluent: a butyl dimethacrylate ester of mol. wt 226 4 White spirit 6 The plastisol was prepared in a high-speed mixer by adding the liquid plasticizer and other ingredients to a paste grade pvc resin having a K value in the region of 70 and a mean particle size in the region of 50 microns. The plastisol was found to have a commercially acceptable shelf life.

The plastisol was found to have a viscosity in the region of 30 poise measured by a Brookfield Viscometer having a number 4 spindle rotating at 20 rpm.

A coil of clean steel sheet was coated with the plastisol to a thickness of 100 microns by a reverse roller-coating technique and then heated for 50 seconds in an oven at 2400 C. The cured coating on the coil was found to be adherent and of good quality, with a Shore A hardness of about 90.

In a second example a plastisol according to the following composition was prepared: pts by weight High molecular weight paste polymer pvc 100 Plasticizer: a phthalate ester of C6-C12 alcohol 40 Stabilizer LC68 by Lankro Chem. Co. 2-4 Diluent: a butyl dimethacrylate ester of mol. wt. 226 4 Pigment 10 White spirit 8 The plastisol was prepared in the same manner as described in the first example.

The plastisol was sprayed onto a rotating section of a steel road pole (that had been previously coated with an adhesive primer) using an airless spray gun. When a uniform coating of 200 microns had been achieved a curing operation was carried out by means of radiant panels at about 2400C for about 50-60 seconds, giving a section of road pole having a well adhered pvc coating of excellent quality with a Shore A hardness of about 90.

It was found that the polymerisable diluent helped to reduce the viscosity of the plastisol to a level at which it could be sprayed. The diluent polymerised during the curing stage instead of volatilising, as is the case with conventional diluents, and the effect of this is that thicker coatings can be applied without the risk of bubbling or foaming during the curing stage.

It will be seen that if a polymerizable diluent were not used in the examples it would be necessary to use extra volatile solvent (white spirit) which would be burnt off during the curing. The invention thus reduces the wastage of volatile solvent.

In alternative embodiments the plastisol may contain alternative pvc or pvc copolymer formulations, alternative plasticizers (e.g. a phthalate ester derived from a mixture of C6-C12 alcohols), uitraviolet absorbers, fillers, pigments, fire retardants (e.g. antimony compounds, borates), or other methacrylate ester derivatives of monohydric alcohols, although other polymerisable diluents are not excluded.

Polymerisation of the diluent may be effected in some cases by means other than heat, such as catalysis.

A plastisol on a steel substrate would, as indicated above, typically be subjected to a curing regime of 50 seconds at 240--2500C. However, for special purposes the curing regime could be in the range 20-100 seconds at 180--3000C.

The amount of volatile solvent, if any, present may be varied to suit specific requirements. Other hydrocarbons having boiling points in the range 50-2000C may be used.

Compositions of the type according to the invention may be applied to substrates other than steel, such as iron, copper, zinc, nickel, or brass. The compositions may be applied by methods other than roller coating or spraying, such as dipping.

The method of coating according to the invention may be applied to a variety of articles and shapes such as coils, planar surfaces, wires, meshes, cylindrical and rod-like articles, and fabricated articles,

Claims (13)

1. A synthetic resin composition suitable for coating metal surfaces, the composition including a dispersion of synthetic resin in a liquid plasticiser and a polymerizable diluent.

2. A composition as claimed in claim 1 wherein the polymerizable diluent is an acrylic ester.

3. A composition as claimed in claim 2 wherein the acrylic ester is an ester of a monohydric alcohol.

4. A composition as claimed in claim 3 wherein the monohydric alcohol is butyl alcohol.

5. A composition as claimed in claim 4 wherein the diluent is butyl dimethacrylate ester.

6. A composition as claimed in any preceding claim wherein the resin is selected from the group comprising polyvinyl chloride and polyvinyl chloride copolymer.

7. A composition as claimed in any preceding claim wherein the plasticiser is a phthalate ester of a C6-C12 alcohol or mixture thereof.

8. A composition as claimed in any preceding claim wherein the polymerizable diluent is 125% by weight of the synthetic resin.

9. A composition as claimed in any preceding claim further containing up to 5% by weight of a hydrocarbon solvent having a boiling point in the range 50-2000C.

10. A method of coating a metal surface with a synthetic resin composition comprising the steps of (i) preparing a dispersion of a synthetic resin in a liquid plasticiser, (ii) mixing the dispersion with a polymerizable diluent, (iii) applying the mixture to the metal surface, and (iv) polymerizing the polymerizable diluent content of the mixture so as to provide an adherent hard coating of the resin on the metal surface.

11. A method as claimed in claim 10 wherein the synthetic resin composition is as claimed in any one of claims 2-9.

1 2. A method as claimed in claim 11 wherein the polymerization is carried out by the application of heat for 20-100 seconds at a temperature of 180--3000C.

13. A method as claimed in claim 12 wherein the metal is iron or steel, the synthetic resin composition is as claimed in any one of claims 3-9, and the polymerisation is carried out by the application of heat for 50 seconds at a temperature of 24O-2500C.

1 4. A synthetic resin composition substantially as described herein with reference to the examples.

1 5. A method of coating a metal surface with a synthetic resin composition substantially as described herein with reference to the examples.