GB2050200A - Process for improving the adhesion of paint to polyolefin surfaces - Google Patents

Abstract

Surfaces of articles made of a polyethylene polypropylene, crystalline dipolymers of ethylene with propylene, blends of two or more of the above, and blends of any of the above with elastomeric ethylene/ propylene copolymers or with EPDM copolymers are primed for painting by coating with chlorinated polypropylene and exposing to ultraviolet radiation. This treatment improves both dry and wet adhesion of paint to surfaces of articles made of single polyolefins as well as of their blends with elastomeric copolymers in which the latter constitute at most 50% of the total polymer.

Description

SPECIFICATION Process for improving the adhesion of paint to polyolefin surfaces This invention relates to a process for modifying the surface of articles made of certain polyolefins and polyolefin blends to improve adhesion of paints and similar protective or decorative coatings thereto.

Polyolefins such as polyethylene, polypropylene, and ethylene/propylene copolymers have found many applications, for example, in household articles, toys, appliances, furniture, and automobile parts. It is often desirable to apply to the exterior surface of a polyolefin article a protective or decorative coating, especially paint, but it is necessary to first prepare the surface either by mechanical treatment or by use cf a primer. It is known to use as primers various ultraviolet sensitizers, for example, benzophenone or trichloroethylene, then to irradiate the surface with ultraviolet light. It is also known to use primers which do not require ultraviolet irradiation, for example, a chlorinated polyolefin sold by Eastman Kodak company and believed to be a chlorinated polypropylene.

While most prior art methods of chemical surface modification are adequate for certain polyolefin materials, they are not entirely satisfactory for others. Thus, it is easier to prime the surface of a vulcanized EPDM copolymer than of polypropylene or polyethylene. EPDM copolymers are either copolymers of ethylene, propylene, and a nonconjugated diene having only one polymerizable double bond or copolymers of ethylene, propylene, a diene having only one polymerizable double bond, and a nonconjugated diene in which both double bonds are polymerizable.

Since it is frequently preferred to use a polymer blend, rather than a single polymer, in fabricating a polyolefin article, the selection of a suitable primer will thus depend on the composition of the blend. For example, blends of polypropylene with EPDM copolymers have been found to be particularly useful for automobile bumpers, bumper inserts, and automobile trim. Because those automobile parts are exposed to varying atmospheric conditions, to rain water, and to hot water in automatic car wash installations, good adhesion of paint is particularly difficult to achieve. If the paint finish blisters or peels, the part is unsatisfactory both to the car manufacturer and to the car owner.Accordingly, there is great need in the industry for a primer that would provide good dry and wet paint adhesion to surfaces or articles made from a polyolefin or polyolefin blend in which polyethylene or polypropylene is the only or the predominating polymer.

SUMMARY OF THE INVENTION According to the present invention, there is now provided a process for improving the adhesion of paint to a surface of an article made of polyolefin selected from: (1) polyethylene, polypropylene, and crystalline dipolymers of ethylene and propylene; (2) blends of at least two members of the group recited in (1) above; and (3) blends of at least one member of the group recited in (1) above with at least one elastomer selected from the class of ethylene/propylene dipolymers and EPDM copolymers in which the total amount of elastomeric polymers is at most about 50 weight percent of the final polymer blend; said process comprising the steps of: (a) cleaning said surface; (b) applying to the cleaned surface a thin layer of a chlorinated polypropylene, and (c) exposing the chlorinated polypropylene coated surface to ultraviolet radiation.

DETAILED DESCRIPTION OF THE INVENTION The polyolefins to which the surface modification process of the present invention is applicable are well known to the art and are readily available commercially from one or more sources. There is abundant patent literature which discloses processes for making both high and low density polyethylene; isotactic and syndiotactic polypropylene; both crystalline and elastomeric ethylene/propylene dipolymers; and EPDM copolymers, both tri- and tetrapolymers. These patents are well known to a polymer chemist. The preferred crystalline ethylene/propylene dipolymers are the hard, impact-resistant grades, which contain up to 22 weight percent ethylene. The preferred elastomeric ethylene/propylene copolymers contain about 30-75% ethylene, 60-90% propylene, and 0-10% nonconjugated diene or dienes. These copolymers are rubbery and substantially amorphous.Nonconjugated dienes having only one polymerizable double bond, which can be copolymerized with ethylene and propylene to EPDM copolymers have about 6-22 carbon atoms and include, for example, acyclic dienes such as: 1,4hexadiene, 1,9-octadiene, 11-ethyl-i, ii -tridecadiene, 9-ethyl- ,9-undecadiene, 8-ethyl-1,8-de- cadiene, 10-ethyl-i ,9-dodecadiene, 1 2-ethyl-i, 1 2-tetradecadiene, 1 3-butyl-1, 1 2-heptadecadiene, and 15-ethyl-l,l 5-heptadecadiene; and cyclic dienes such as: dicyclopentadiene, 5 butenyl-2-norbornene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene, 2-ethyl-2,5-norbornadiene, and 1 5-cyclo-octadienes. The preferred diene in this category is 1,4-hexadiene.

Representative nonconjugated dienes in which both double bonds are polymerizable are 1,5hexadiene, 1 , 4-pentadiene, and 2, 5-norbornadiene.

Curable homopolymers and copolymers are normally cured before the surface modification process of the present invention is carried out. Curing methods are well known in the art. Thus, amorphous polyethylene, polypropylene, and ethylene/propylene dipolymers are curable with conventional peroxide systems. The EPDM copolymers, whether tripolymers or tetrapolymers, can be cured either with a peroxide or with sulfur by methods well known in the art. Crystalline polyethylene and polypropylene ordinarily are not cured.

The chlorinated polypropylene preferred in the process of the present invention has a number average molecular weight of about 5,000-50,000 and contains about 5-25% chlorine by weight. Commercially available chlorinated polypropylenes, sold by Eastman Chemical Products, Inc., Rochester, New York, under the designation CP-343-1 and CP-515-2, are suitable in the instant process. The first of these two commercial products is a chlorinated polypropylene which is believed to contain cyclic anhydride groups in the polymer backbone and has a number average molecular weight of about 1 5,000. Its chlorine content is about 1 5 weight percent. The second commercial material has about the same molecular weight and chlorine content but contains no cyclic anhydride groups in its backbone.Chlorinated polypropylene can be readily prepared by solution, melt, or solid polymer chlorination at moderate to high temperatures according to well-known techniques as reviewed for example in Raff and Doak, "Crystalline Olefin Polymers II," p. 239. Interscience Publishers, New York, N.Y., 1964.

For practical application of the chlorinated polypropylene it is preferred to use a solution of the chlorinated polypropylene in a suitable solvent, for example, an aromatic hydrocarbon, a chlorinated hydrocarbon, or a solvent blend. Alicyclic hydrocarbons can be used in solvent blends and are preferred because of their low toxicity and ready availability at a low cost.

Tetrahydrofuran may also be used. Suitable alicyclic hydrocarbon diluents include cyclohexane and methylcyclohexane. While chlorinated hydrocarbons, both aliphatic and aromatic, can be used, they are less desirable on account of their greater toxicity. The concentration of the chlorinated polypropylene in the solution can be about 0.1 to 5.0% by weight but 1.0-2.5% is peferred.

The polyolefin surface to be treated according to the process of this invention must be thoroughly clean. Cleaning can be accomplished by scrubbing with water and soap or detergent or by solvent-cleaning with, for example, a hydrocarbon, a chlorinated hydrocarbon, a ketone, or another suitable solvent. The cleaned surface is then allowed to dry, optionally at an elevated temperature. A solution of chlorinated polypropylene is then applied to the clean surface, and the solvent is allowed to evaporate, again optionally at an elevated temperature. Finally, the primed surface is exposed to ultraviolet radiation.

Most commercial sources of ultraviolet radiation can be used in the process of the present invention, although the most efficient radiation is obtained within the wavelength range of about 200-400 nm. It is preferred to adjust the radiation exposure time so that the radiation energy received by the surface is about 0.10-1.0 J/cm2. The optimum radiation energy range is 0.2-0.5 J/cm2. The surface that has been primed with chlorinated polypropylene and irradiated with UV is now ready for painting. The quality of paint adhesion can be determined in both dry and wet adhesion tests, as explained below in the Example.

The polyolefins and polyolefin blends which can be advantageously treated according to the process of this invention can be either uncompounded or compounded in the usual manner, for example, with carbon black and other fillers, process oils, plasticizers, and similar ingredients.

However, the proportions of individual polyolefins in the blends recited in the Summary of the Invention and in the claims, are based on the polyolefins themselves, not on the weight of a compounded polyolefin composition.

This invention is now illustrated by the following examples of certain preferred embodiments thereof, where all parts, proportions and percentages are by weight unless otherwise indicated.

Polymer melt flow rate was determined according to ASTM D-1238-73 (Condition L).

I. EXAMPLES Example 1 A filled polyolefin composition was prepared by mixing the following ingredients in an internal mixer for two minutes at 150"C.

Parts EPDM Rubber(') 31.7 Polypropylene2) 38.7 HAF Black 29.6 (1) Medium viscosity, non-crystalline copolymer of 35.5 mol % propylene/63 mol % ethy lene/i.5 mol % 1,4-hexadiene having a Mooney viscosity (my1+4) = 39 at 121'C.

(2)1 0-1 2 melt flow rate homopolypropylene.

Plaques of the above thermoplastic composition were injection-molded at 232"C. Two plaques were cleaned in a power wash cycle and primed as follows: On both plaques the primer consisted of a light spray coat of a 1 % solution in toluene of chlorinated polypropylene (CP-343-1, Eastman Chemical Products Inc.). The dry weight of the chlorinated polypropylene deposited on each plaque was 0.095 mg/cm2. After the primer had been applied and the plaques allowed to dry, one of the two plaques was additionally exposed to UV radiation from a medium pressure quartz-jacketed mercury vapor lamp, radiating power at the rate of about 80 watts per cm of lamp length. The exposure was adjusted so that the plaque received all the radiation accompanying an irradiation of 0.33 J/cm2 at the wavelength of 365 nm. The other plaque received no UV radiation.Both plaques were then painted with a flexible alkyd enamel (Durethane(E) 100, PPG Industries) to give a dry film thickness of 22 cm. The enamel was baked for 40 minutes at 120"C.

The plaques were tested as follows: (i) Dry adhesion. A lattice pattern of cuts was made through the paint over a small test area of the sample. The lattice consisted of a series of parallel cuts about 1.5 mm apart containing two sets of cuts at right angles to each other and a third set diagonally to the first two. Reinforced adhesive tape was then pressed into the paint over the area of the lattice cuts, then quickly ripped off. Adhesion was rated from 100% to 0% according to the area of paint left adherent to the substrate.

(ii) Wet adhesion. The center of the plaque was scribed with an "X". The plaque was then placed about 5 cm below and at an angle of 45 to the nozzle of a steam generator, so that the nozzle, capable of delivering wet steam at about 0.4 MPa gauge pressure, was aimed at the center of the "X". The test consisted of measuring the time taken for the steam blast to initiate a paint adhesion failure at the center of the "X" and was run for a maximum of 1 80 seconds.

This test is particularly useful for evaluating the ability of painted parts on the outside of cars to withstand exposure to an automatic car wash. The following results were obtained: No UV With UV Dry Adhesion: 90% 100% Wet Adhesion: 10 s > 180 s EXAMPLE 2 Plaques were prepared and tested as in Example 1, except that the following thermoplastic composition was used: Parts EPDM Rubber(') 50 Polypropylene(2) 50 (1) Partially crystalline copolymer of 18 mol % propylene/81 mol % ethylene mol % 1,4 hexadiene having a Mooney viscosity (ML2+,0)= 60 aq 121"C.

(2) Same as in Example 1.

Results: No UV With UV Dry Adhesion: 95% 100% Wet Adhesion: 50 s > 180 s EXAMPLE 3 Plaques were prepared and tested as in Example 1, except that the following thermoplastic composition was used: Parts Propylene Homopolymer") 99 HAF Black 1 (1, 4 melt flow rate.

Test results were as follows: No UV With UV Dry Adhesion: 10% 10% Wet Adhesion: 10 s 20 s EXAMPLE 4 Plaques were prepared and tested as in Example 1, except that the plaques were baked for 3 hours at 105"C. and that the following composition was used: Parts EPDM Rubber of Example 2 20 High Density Polyethylene 80 (melt index 2.8) Test results were as follows: No UV With UV Dry adhesion: 0 99% Wet adhesion: < 5s > 180 s EXAMPLE 5 This example compares the use of two different commercial chlorinated polypropylenes: CP-343-1, and CP-515-2, both supplied by Eastman Chemical Products, Inc. Plaques having the same composition as described in Example 1, were cleaned in a power wash cycle and then lightly spray-coated with one or the other of the two chlorinated polypropylenes as 1 % solutions in toluene.After drying, the plaques were exposed to UV radiation as described in Example 1.

Two coats of flexible alkyd paint were applied: about 18,um of a neutral grey base coat, which was baked for 20 minutes at 15"C., and about 45 m of a white top coat, which was baked for 40 minutes at 115"C.

The following test results were obtained: Chlorinated Polypropylene: CP-343-1 CP-515-2 Weight deposited, mg/cm2: 0.09 0.10 Dry Adhesion (a), 100% 100% Dry Adhesion (b) very good good Wet Adhesion (c), > 60 s > 60 s Wet Adhesion (d) Pass Pass (a) As described in Example 1.

(b) A "V" shaped cut was made through the paint, and the paint peeled back from the apex of the "V" with a sharp knife. The results reflect a qualitative asessment of the force required to peel the paint.

(c) As described in Example 1. The test was stopped at 60 seconds.

(d) The painted plaque, with an "X" cut in the surface of the paint, was immersed in distilled water for 10 days at 38"C. On removal from the water, the plaque was quickly dried and reinforced pressure sensitive tape pressed over the center of the "X" cut. The tape was then rapidly stripped off. A "Pass" indicated that none of the paint was removed.

It is evident that both types of chlorinated polypropylene gave satisfactory levels of adhesion.

II. COMPARISON EXAMPLES USING BENZOPHENONE AND UV RADIATION AS TAUGHT BY CANADIAN PATENT 974,928 TO G. A. BASEDEN Examples 1-3 above were repeated, but instead of using a chlorinated polypropylene primer, the priming was effected by spraying the plaques with a 5% solution of benzophenone in petroleum naphtha having a boiling range 120-140"C. (dry deposition 0.056 mg/cm2); after the solvent had evaporated, the sprayed surfaces were exposed to UV radiation (approximately 0.5 J/cm2 as measured at a wavelength of 365 nm). This was followed by paint application in which the same paint type, dry film thickness, and paint bake cycle was used as in the corresponding examples of Section I.The following results were obtained: Polyolefin Composition 8 Type Dry Wet As Example in Section I Adhesion, % Adhesion, s 1 30 > 180 2 0 > 180 3 0 < 5 By comparing the above results with those of Examples 1-3 in Section I, it can be seen that chlorinated polypropylene primer treatment followed by ultraviolet radiation is more effective than benzophenone treatment followed by ultraviolet radiation. While results are dramatic for polypropylene blends with EPDM rubber, an improvement can also be seen for polypropylene alone.

III. COMPARISON WITH TRICHLOROETHYLENE AND UV RADIATION AS TAUGHT BY U.S.

PATENT 3,619,245, TO R. A. BRAGOLE Additional plaques identical to those used in Section I. Example 2 above, were primed with trichloroethylene and exposed to UV, as described in U.S. Patent 3,619,246. After the surface of a plaque had been cleaned in a power wash and over dried, trichloroethyiene was brushed onto the surface. A short time afterward (e.g., 1-2 minutes), the surface was exposed to UV radiation of 0.5 J/cm2 at a wavelength of 365 nm. Data supplied by the UV lamp manufacturer (Hanovia Lamp Division, Canrad Precision Industries) indicate that 0.5 J/cm2 at 365 nm is equivalent to an exposure of about 1.43 J/xm2 in the range of 240-400 nm. This lies within the radiation given in the examples of U.S. 3,619,246. A coating of paint was applied and baked, as described in Example 3 in Section I.

The following test results were obtained: Dry Adhesion Wet Adhesion 20% > 180 s It is apparent that the level of dry adhesion obtained with this priming procedure was considerably inferior to that obtained by the chlorinated polypropylene/UV priming process of the present invention.

Claims (11)

1. A process for improving the adhesion of paint to a surface of an article made of polyolefin selected from: (1) polyethylene, polypropylene, and crystalline dipolymers of ethylene and propylene; (2) blends of at least two members of the group recited in (1) above; and (3) blends of at least one member of the group recited in (1) above with at least one elastomer selected from the class of ethylene/propylene dipolymers and EPDM copolymers in which the total amount of elastomeric polymers is at most about 50 weight percent of the final polymer blend; said process comprising the steps of: (a) cleaning said surface, (b) applying to the cleaned surface a thin layer of chlorinated polypropylene, and (c) exposing the chlorinated polypropylene-coated surface to ultraviolet radiation.

2. A process as claimed in Claim 1 wherein the chlorinated polypropylene has a molecular weight of about 5,000-50,000 and contains about 5-25% chlorine by weight.

3. A process as claimed in claim 1 or Claim 2 wherein the chlorinated polypropylene contains cyclic anhydride groups in the polymer backbone.

4. A process as claimed in any of claims 1 to 3 wherein the ultra-violet radiation has a wavelength within the range of about 200-400 nm.

5. A process as claimed in any of claims 1 to 4 wherein the ultraviolet energy received by the polyolefin article surface is about 0.1-1.0 J/cm2.

6. A process as claimed in any of claims 1 to 5 wherein the ultraviolet energy received by the polyolefin article surface is about 0.2-0.5 J/cm2.

7. A process as claimed in any of the preceding claims wherein the polyolefin is a crystalline, impact-resistant, hard grade of ethylene/propylene dipolymer.

8. A process as claimed in any of claims 1 to 6 wherein the polyolefin is a blend of polypropylene with an EPDM copolymer.

9. A process as claimed in any of claims 1 to 6 wherein the polyolefin is a blend of a crystalline, impact-resistant hard grade of ethylene/propylene dipolymer with an elastomer.

10. A process as claimed in claim 1 substantially as hereinbefore described.

11. A process as claimed in claim 1 substantially as hereinbefore described with reference to the Examples.