Painting of a molded urethane part

Abstract

Molded urethane automobile parts containing an internal waxy mold release agent that normally causes "fisheying" in urethane paints used to topcoat such parts are effectively painted by first applying a thin clear coat of the thermosetting vehicle of a urethane-type paint free of pigment, at least partially curing the clear coating to bind the lubricant or mold release agent, followed by application of the pigmented final color coat of a urethane paint and fully curing both coats.

Description

INTRODUCTION

Certain automobile interior and exterior parts are now made from molded urethanes such as the so-called thermoplastic urethanes and the self-skinning urethane foams. Such parts after molding are painted to color match the color of the automobile.

Internal mold release agents are customarily used in the manufacture of such parts, such as waxes added to the molding formulations which migrate, bleed or bloom to the surface during molding and aid in release of the part during demolding. When a paint comprising a vehicle and a pigment is applied to such a molded surface the mold release agent can be absorbed by the paint film and effect the forces that hold the pigment in dispersion in the paint such that "fisheying" or "crawling" will occur in the paint film, i.e., the pigment in the paint tends to separate in areas where the mold release agent or lubricant is present on the surface of the part such that pigment pockets or voids result.

The mold release agents that are preferred are usually esters or amides of C.sub.12 -C.sub.24 fatty acids. Once they have migrated to the surface and served their mold release function, they are quite difficult to remove. Reasonably cleaning with solvents or detergents will often not reliably do so.

THIS INVENTION

It has now been found that when such molded parts are to be painted with a urethane type paint that contains a dispersed pigment, the deleterious effect of the mold release agent on the surface can be blocked by first applying a clear coat of only the urethane vehicle of the paint and at least partially heat curing the clear coat following which the part can be painted in the normal manner. It has been found that the lubricant on the surface of the part does not prevent wetting and uniform film formation by the urethane vehicle and that once the clear film is at least partially cured this seems to "lock" the lubricant in and prevent further migration of any significant amount thereof that would be harmful to the subsequently applied paint film.

In one situation where Chevrolet "B" and Cadillac bumper end fillers were being manufactured, rejects because of unsatisfactory paint film formation were reduced from an average of 10 to 12 per cent to under 3 per cent by the use of the barrier coat technique of this invention.

Full curing of the clear barrier coat prior to the application of the final topcoat does not appear to be required. Heating of the film to a true film temperature in a range of 180.degree. to 260.degree.F appears to adequately bind the lubricant while leaving the barrier coat film sufficiently uncured to bind well with the topcoat when it is applied. The barrier when partially cured in this manner will not normally pass the xylene rubbing test although in some cases it may be sufficiently cured to do so. Too hard a cure of the clear barrier coat may prevent the topcoat from effectively bonding with it. After the topcoat is applied both films are fully cured by heating them to a temperature above 225.degree.F.

The clear barrier coat film while continuous need not have a thickness of greater than 0.5 mil as film thicknesses in the order of 0.2 to 0.4 mil seems to lock adequately the mold release lubricant in against migration. Thicker films may contribute to cold flex cracking. The top pigment-containing paint film can have the customary thickness, usually about one mil or more.

Generally it is desirable to remove as much as possible of the mold release agent by solvent degreasing, detergent washing or the like so as to minimize the chance of strike through.

Because both the barrier coat and the paint contain a solvent, it is preferred to heat the part to a temperature greater than 130.degree.F prior to their application and to allow the part to air flash after the application of the barrier coat and after the application of the paint.

While the paints used in the present inventions are referred to as urethane paints or urethane lacquers, they are more properly identified as urethane-polyester-extended melamine lacquers the vehicles of which are cross-linkable or heatcurable.

DRAWING

Attached to and forming a part hereof is a drawing which schematically illustrates the significant steps of the process of this invention and is self-explanatory.

In the drawing the first block, block 10, is labeled "Mold Urethane Part with Internal Mold Release Agent." Block 11 is labeled "Clean Surface" and block 12 is labeled "Apply Clear Coat of Urethane Vehicle." Block 13 is labeled "Partially Cure Clear Coat." Block 14 is labeled "Apply Color Coat of Vehicle Plus Pigment" and block 15 is labeled "Fully Cure Both Films."

EXAMPLE

Test blocks 8 inches .times. 8 inches .times.1/8 inch are prepared by injection molding of a thermaplastic urethane, Roylar E-2B sold by Uniroyal Chemical, Naugatuck, Connecticut, 60770, to which 0.3 weight percent of ethylene-bis-stearamide is added as a mold release agent. Advawax 280 F sold by Cincinnati Milacron Chemicals, Inc. Reading, Ohio 45215 could as well be used. The surface of the test blocks are cleaned by washing with a xylene soaked rag.

The clear lacquer used is Durathane 100 applied by an air gun as supplied by the manufacture, Pittsburgh Plate and Glass Chemical Company. The color coat used is Durathane 100 dark metallic green (WV-4517) also applied by air gun as supplied.

The test blocks are preheated to 130.degree.F prior to application of the first coat and are allowed to air flash for about 5 minutes after the first coat is applied.

One block is coated with the color coat after the solvent washing and after the air flash it may be noticed that unacceptable fisheying has occurred.

The 8 inch .times. 8 inch surface of another preheated block is first coated with a 0.3 mil film (dry) of the clear Durathane 100, air flashed and baked 30 minutes at 200.degree.F. After allowing some cooling but while the part is still warm the surface is sprayed with a one mil film (dry) of the color coat, air flashed for a few minutes then baked at 240.degree.F for 30 minutes. Paint film formation will be noticed to be good and the sample will pass all of the usual tests such as flex cracking, aging, weathering, and the like.

Claims

We claim:

1. In the painting with a paint of a molded urethane part made from a moldable urethane formulation containing a wax of a type and in an amount sufficient to serve as a mold release agent, said paint comprising a urethane lacquer vehicle with a pigment dispersed therein, the improvement comprising:

A. cleaning a surface of said part to at least in part remove said wax therefrom:

B. applying a clear continuous barrier coat to said surface of said vehicle free of pigment as a film having a thickness under 0.5 mil, dry;

C. at least partially curing said barrier coat by heating to a film temperature in the range of 180.degree.-260.degree.F, and

D. applying said paint as a film and curing said film by baking at a film temperature in excess of 225.degree.F.

2. The process of claim 1 wherein said barrier coat and said paint contain a solvent and are applied to said part after the part has been heated to a temperature greater than 130.degree.F, and the part is allowed to air flash after application of the barrier coat, and after application of said paint prior to baking.

3. The process of claim 1 wherein said wax is selected from the group consisting of esters and amides of c.sub.12 -C.sub.24 fatty acids and said film of said paint has a thickness of at least 1 mil, dry.

4. The process of claim 1 wherein said wax is ethylene-bis-stearamide.