This invention relates to thermoplastic coating compositions and in particular to such compositions in which the principal film-forming ingredients are three-component b ^' leήds of acrylic polymers , cellulose acetate butyrate and monomeric plasticiser.

According to the present invention we now provide a liquid coating composition comprising a mixture of (a) a solution in volatile organic liquid of a copolymer of from 75-90% methyl methacrylate, 1.5 - 4.0% dimethyl aminoethyl methacrylate and 8 - 23.5% of monomer selected from butyl meth¬ acrylate, butyl acrylate and lauryl methacrylate, which copolymer shall have a glass transition temperature of 80° - 95°C; (b) a solution in volatile organic liquid of cellulose acetate butyrate which has a viscosity of 0.5 - 5.0 sec when measured according to Test Method D1343 of the American Society for Testing and Materials , and a hydroxyl value of 2 maximum; and (c) a monomeric ester plasticiser compatible with the selected mixture of the said components (a) plus (b) such that the non-volatile constituents of (a) , (b) and

(c) comprise respectively 43.5 - 64.0%, 15.0 - 25.5% and 21.0 - 31.5% of the total weight of non-volatile material in (a) plus (b) plus (c) .

We further provide a process of preparing a liquid coating composition by blending

(d) a solution in volatile liquid solvent of a copolymer of from 75-90% methyl methacrylate,

1.5 - 4.0% dimethyl aminoethyl methacrylate and 8 - 23.57, of monomer selected from butyl meth¬ acrylate, butyl acrylate and lauryl methacrylate, which copolymer shall have a glass transition temperature of 80° - 95°C;

(e) a solution in volatile liquid solvent of cellulose acetate butyrate which has a viscosity of 0.5 - 5.0 sec when measured according to Test Method D1343 of the American Society for Testing and Materials, and a hydroxyl value of 2 maximum; and

(f) a monomeric ester plasticiser compatible with the selected mixture of the said components (d) plus (e) such that the non-volatile constituents of (d) , (e) and (f) comprise respectively 43.5 - 64.0%, 15.0 - 25.5% and 21.0 - 31.5% of the total weight of non-volatile material in (d) plus (e) plus (f) .

The compositions of this invention provide the principal film-forming components of liquid coating compositions, which are particularly suitable for use as automotive lacquers. Paint films deposited there¬ from exhibit particularly useful combinations of hardness, flexibility, gloss and in particular, adhesion to other paint substrates.

The composition range of components which will confer these desirable properties is unusually narrow and to confer the optimum combination of hardness and adhesion for some uses we have found that it must be limited even further to the following preferred limits. The preferred liquid coating composition comprises a mixture of

• (a) a solution in volatile organic liquid of a copolymer of from 75-80% methyl meth¬ acrylate, 2.5 - 3.5% dimethyl aminoethyl methacrylate and 1.5 - 2.0% butyl meth¬ acrylate, which shall have a glass transition temperature of 80° - 95°C.

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(b) a solution in volatile organic liquid of cellulose acetate butyrate which has a viscosity of 0.5 - 5.0 sec when measured according to Test Method D1343 of the American Society for Testing and Ma erials , and whi-ch has a hydroxyl value of 2 maximum; and

(c) a compatible monomeric ester las-ticiser-i selected from butyl benzyl phthalata and dipropyl ne glycol dibenzoate. such that the non-volatile constituents of (a) , (b) and (c) comprise respectively 43.5 - 64.0%, 15.0 - 25.5% and 21.0 - 31.5% of the total weight of non-vσlarile material in (a) plus (b) plus (c) The preparation of component (a) is a conventional copolymerisation process which may be performed using standard polymerisation techniques. The gla^s transition temperature may be measured by using, for example a Differential Thermal Mechanical Analyser, but for the purpose of determining compliance with our invention, it is satisfactory to calculate the value from the known composition using published Tg values for homopoly ers of the individual components.

As is usual with liquid coating composition, these will normally contain from 60-80% by weight of volatile solvent.

Pigments may also be incorporated in the coatings in conventional manner.

The invention is illustrated by the following examples, in which all components are expressed by weight:

EX&HPI 1

Preparation of a coating composition according to the invention and comparative tasting against a commercial lacquer not according to this invention. An acrylic -copolymer solution of the following composition was prepared by conventional addition polymerisation techniques: monomer composition methyl methacrylate S3 parts n-butyl methacrylate 15 It dimethyl aminoethyl methacrylate 2" 1 solids 407, by ^• weight solvent composition methyl ethyl etone 30 parts toluene 70 t_

132.5 parts of this solution was blended with 83 parts of a 2S7ό (by weight) solution of cellulose acetate butyrate (0.5 second grade) in methyl ethyl ketone and toluene (30 .- 70 by weight) and 25.0 parts of n-butyl benzyl phthalate, to give a solution of film-forming polymer wherein the non-volatile constituents of the copolymer solution, the cellulose acetate butyrate solution and the plasticiss-r ccαsrised 537 _a , 22% and 25 of the weight of the total non-volatile content. The solution so-prepared was pigment d to a pigment/binder ratio of 60 with titanium dioxide pigment to produce a white lacquer.

This lacquer was tested according to the methods outlined below. A commercial acrylic refinish lacquer having the same overall general composition but in which the acrylic polymer of this example was replaced with an equal weight, based on solids content, of a methyl methacrylate homopolymer was also tested according to these methods.

Adhesion:.test"

The lacquer was thinned with the abovementioned solvent to a viscosity of 15-17 sec. in a Ford 4 viscosity cup and sprayed on to steel test panels coated with a baked,, aged and lightly sanded film of commercial thermoset acrylic enamel. The film was dried in air for 24 hours and then subjected to the adhesion test of the American Society for Testing and Materials, test method D3359. Heat distortion test

The lacquer was applied to a primed steel panel and the panel was spotted along its length with a I solution of egg albumen. The spots were allowed to dry overnight and the panel was then placed for 6 minutes on a thermal copper bar with a temperature gradient of from 35°C to _. 65°C between its ends. The panel was washed and inspected for distortion resistance, the temperature to -which eachalbumen spot was heated being easily determined by its position on the bar. A coating composition is_ considered to have acceptable distortion resistance if only those spots corresponding to temperatures above about 40°C show signs of distortion. Recoat craze test The lacquer was applied to a primed steel panel, allowed to air dry and aged in an oven at 65°C for 16 hours. The panel was then cooled to 25°C and a 175 μ film of the lacquer (thinned to spraying viscosity) was drawn down. The film was allowed to dry and examined for crazing (a network of fine inter¬ connecting cracks on the surface of the dry film) . The panel was returned to the oven for seve -days after which it was again withdrawn, cooled to 25°C and the abovementioned procedure repeated. The panel and

the thinned lacquer used for the draw-downs ware then cooled to 7°C and a third film drawn down, allowed to dry and examined for crazing.

Both lacquers performed satisfactorily in the heat distortion and recόat craze test but whereas the lacquer prepared according to the invention exhibited almost- no loss of adhesion in the adhesion test, the commercial lacquer exhibited very poor adhesion.

^• EXAMPLE 2

Use of a number of different acrylic polymers. A number of coating compositions was prepared according to Example 1 but in each case the copolymer in that -example was replaced by one of the following copolymers;

(a) methyl methacrylate (87%) , n-butylmeth- acrylate (10%) , dimethylr*πτπnoethyl- methacrylate (DMAEMA) (3%) .

(b) methyl methacrylate (92%) , laurylmeth- acrylate (6%) , DMAEMA (2%) ;

(c) methyl methacrylate (92%), butylacrylate (6%), DMAEMA. (2%)

(d) methyl methacrylate (90%) , ethyl- acrylate (8%) , DMAEMA (2%) ; (e) methyl methacrylate (95%) , n-butyl methacrylate (3%) , DMAEMA (2%) where (a) (b) (c) are according to the invention, (d) includes a monomer which is not according to the invention and (e) has proportions of comonomers which lie outside the Oermitted limits.

The coating compositions thus prepared were subjected to the testing procedures of Example 1. The compositions (a) , (b) and (c) gave good results in all tests whereas the composition (d) performed poorly in the adhesion and _. heat distortion tests and the composition (e) in the adhesion and recoat craze test.

EXAMPLE 3 Effect of changing the proportions of the components of the coating composition.

Example l::was repeated but the proportions of the components were altered such that the ratio of copolymer/cellulose acetate butyrate/plactici≤er was 69/6/25 instead of the 53/22/25 ratio of that example. When subjected to the testing procedures outlined in Example 1 the resultant coating composition exhibited unacceptable recoat crazing.

EXAMPLE 4 Use of various types of cellulose acetate butyrate (CAB) .

Example 1 was repeated twice, in each case replacing the CAB of that example by an identical quantity of

(a) a CAB whose hydroxyl value was 3; and (b) a CAB, whose viscosity was 0.2 sec.

^' When tested by the procedures outlined in Example 1, the composition containing (a) exhibited poor adhesion and that containing (b) poor recoat crazing resistance. Prior to testing, it was also noticeable that each composition had poor gloss when compared to the composition of Examnle 1.

EXAMPLE 5 Use of various plasticisers. Example 1 was repeated, replacing the butyl benzyl phthalate plasticiser of that example by an identical quantity of

(a) dipropylene. glycol dibenzoate

(b) "Santiciser" 141 (trade mark) ex Monsanto Co. - a commercially available monomeric plasticiser (c) 34% coconut glycerol alkyd resin

(d) saturated neo-pentylglycol-adipic acid polyester. The compositions containing (a) and (b) performed satisfactorily when tested according to the procedures of Example 1 but those containing (c) and (d) exhibited poor adhesion.