GB2273064A

GB2273064A - Method of coating polyolefin molded part

Info

Publication number: GB2273064A
Application number: GB9325033A
Authority: GB
Inventors: Yasuhiro Fujii; Tadahiro Kato; Hiroyuki Onoyama; Kenji Iwahashi
Original assignee: Kansai Paint Co Ltd
Current assignee: Kansai Paint Co Ltd
Priority date: 1992-12-07
Filing date: 1993-12-07
Publication date: 1994-06-08
Anticipated expiration: 2013-12-07
Also published as: JPH06172565A; GB2273064B; DE4341694A1; GB9325033D0

Description

2273064 Method of Coating Polyolefin Molded Par This invention relates to

a method of coating a polyolefin molded part, in which the polyolefin molded part is not pretreated or simply is washed with an aqueous medium prior to coating without needing to be washed with a vapor of an organic'solvent such as a harmful trichloroethane.

The polyolefin molded parts have been widely used in shell parts of the automobile, for example, a bumper, etc., because of their reasonable price and high recyclability, and are coated with various coating compositions from the standpoints of appearance and protection.

However, the polyolefin constituting the polyolefin molded part shows unsatisfactory adhesion properties to a coating composition because of its low polarity and inactiveness. For the purpose of solving the above problem, the polyolefin molded part has been washed with a 1,1,1trichloroethane vapor for etching its surface, resulting in increasing a bond strength with a 1 coating. However, the trichloroethane itself is harmful to the human body and shown as one of the causes of breaking of the ozonosphere in the atmosphere, with the result that it has been decided in accordance with the revision of the Montreal protocol that use of trichloroethane should be completely inhibited from the end of 1995 on.

A preferred embodiment of the present invention may provide a method of coating a polyolefin molded part, which makes it possible to form a coating having film performanoes equal to or higher than those of the conventional method needing the pretreatment by use of the harmful trichloroethane without needing the pretreatment by washing with the harmful trichloroethane, that is, without any pretreatment or with a simple and safety pretreatment by washing with an aqueous medium.

The present invention provides a method of coating a polyolefin molded part, which method comprises coating onto the polyolefin molded part. (preferably without any pretreatment or with a pretreatment by washing with an aqueous medium) an undercoating containing, as a major component, a composition consisting of 100 par;s by weight of (A) an unsaturated dicarboxylic acid-modified chlorinated polypropylene having an acid value of 3 to 80, a degree of chlorination in the range of 18 to 28 % by weight as a chlorine content and a weight-average 2 0 A molecular weight of 30,000 to 170,000, 1 to 50 parts by weight of (B) an epoxy resin having a weightaverage molecular weight of 250 to 2,000 and an epoxy equivalent of 90 to 1,200, and preferably 2 parts by weight or less of (C) a catalyst, followed by coating a topcoating.

The polyolefin molded part used in the present invention may be prepared by molding in an arbitrary shape a resin obtained by polymerizing at least one olefin having a'bout 2 to 10 carbons, for example, ethylene, propylene, butylene, hexene, etc., and may include, for example, shell parts of the automobile such as a bumper, a spoiler, a grille and the like, not being limited thereto.

According to the present invention, on coating a coating composition onto the above polyolefin molded part, there is no need of washing the polyolefin molded part with a vapor of the toxic trichloroethane or the like prior to coating. That is, the polyolefin molded part may be coated as it is molded without any pretreatment, but, if needed, may be washed with water at room temperature, warm water, an aqueous solution or with an aqueous dispersion of an acidic compound such as phosphoric acid, sodium phosphate or the like, or an alkaline compound such as sodium hydroxide, sodium silicate, sodium carbonate or the like by dipping there into or spraying therewith.

3 The undercoating used in the present invention is a coating composition to be coated directly onto the polyolefin molded part prior to coating a topcoating, and is an undercoating containing, as a major component, a composition consisting of 100 parts by weight of (A) an unsaturated dicarboxylic acid-modified chlorinated polypropylene having an acid value of 3 to 80, a degree of chlorination in the range of 18 to 28 % by weight as a chlorine content and a weight-average molecular weight of 30,000 to 170,000, and 1 to 50 parts by weight of (B) an epoxy resin having a weight-average molecular weight of 250 to 2,000 and an epoxy equivalent of 90 to 1,200.

The above component (A) is a component constituting the undercoating and is an unsaturated dioarboxylio aoid-modified chlorinated polypropylene having an acid value of 3 to 80, a degree of chlorination in the range of 18 to 28 % by weight as a chlorine content, and a weight-average molecular weight of 30,000 to 170,000.

The propylene used in the above component (A) may include homopolymer of propylene, copolymers of propylene with at least one of other olefins selected from, for example, ethylene, pentene, hexene, octene, decene, etc. A propylene content in the above copolymer is 40 % by weight or more, preferably 60 % by weight or more.

The unsaturated dicarboxylic acid-modified chlorinated polypropylene may be obtained by graft- 4 modifying the above polypropylene with an unsaturated dicarboxylic acid or an anhydride thereof, followed by chlorinating, but an order between the graft-modification and chlorination may be reverse or simultaneous.

An unsaturated dicarboxylic acid to be graftmodified may include a compound having two carboxyl group and at least one of a carbon-carbon double bond in one molecule and a cyclic anhydride thereof. Specific examples of the unsaturated dicarboxylic acid may include maleic acid, fumaric acid, itaconic acid, citraconic acid, allyl succinic acid, mesaoonic acid, glutaconic acid, tetrahydrophthalic acid, and anhydrides thereof.

The above graft-modification may be carried out by a known process per se, for example, by reacting polypropylene in the form of an organic solution with an unsaturated dicarboxylic acid in the presence of a freeradical generator. It is necessary for modification with the unsaturated dicarboxylic acid to be controlled so that an acid value (mg/KOH) of the resulting component (A) may be in the range of 3 to 80, preferably 10 to 50. When the acid value is less than 3, the adhesion properties to the polyolefin molded part may undesirably be reduced. When the acid value is more than 80, a resulting coated film may undesirably show an unsatisfactory water resistance.

The chlorination of the graft-modified polypropylene may be carried out by introducing chlorine gas into an organic solution of the graft-modified polypropylene, followed by reacting preferably at 50 to 1200C - It is necessary for the chlorination to be carried out so that a chlorine content in the component (A) may be in the range of 18 to 28 % by weight, preferably 20 to 25 % by weight. When the chlorine content is less than 18 % by weight, solubility in an organic solvent is undesirably reduced, and the storage stability and coating workability of the undercoating become poor. When the chlorine content is more than 28 % by weight, the adhesion properties to the polyolefin molded part and organic solvent resistance are undesirably reduced.

The weight-average molecular weight of the component (A) is in the range of 30,000 to 170,000, particularly 50,000 to 130,000. When less than 30, 000, the adhesion properties to the polyolefin molded part, water resistance and solvent resistance are reduced, and when more than 170,000, atomization on spray coating becomes undesirably poor.

The component (B) is an epoxy resin having two or more of epoxy group in one molecule, and having a weightaverage molecular weight of 250 to 2,000 and an epoxy equivalent of 90 to 1,200. Specifically, the component (B) may include, for example, aliphatic epoxy resin such as an etherification product between bivalent, or trivalent or more aliphatic alcohol and epichlorohydrin, 6 bisphenol based epoxy resin, hydrogenated bisphenol based epoxy resin, cycloaliphatic epoxy resin, and the like.

The component (B) has a weight-average molecular weight of 250 to 2,000, preferably 300 to 1500, and an epoxcy equivalent of 90 to 1,200, preferably 120 to 1,000. When the weight-average molecular weight is less than 250, the water absorption properties of the coated film is undesirably increased, resulting in reducing water resistance, and when more than 2,000, compatibility with the chlorinated polypropylene resin is undesirably reduced. When the epoxy equivalent is less than 90, water resistance is undesirably reduced, and on the other hand, when more than 200 compatibility with the chlorinated polypropylene resin is undesirably reduced.

The undercoating used in the present invention contains, as the essential components, the component (A) and the component (B), and a ratio of an amount of the component (A) to that of the component (B) is such that the component (B) is in the range of I to 50 parts by weight, preferably 2 to 10 parts by weight per 100 parts by weight of the component (A). When the component (B) is less than one part by weight, solvent resistance is undesirably reduced, and when more than 50 parts by weight, the adhesion properties to the polyolefin molded part and water resistance undesirably become poor.

In addition to the component (A) and the component (B), the undercoating in the present invention, if 7 needed, may contain a catalyst (C).

The catalyst as the component (C) is effective for promoting the cros. slinking reaction between the component (A) and component (B). The reaction between carboxyl group in the component (A) and epoxy group in the component (B) results in crosslinking and curing, and addition of the component (C) is advantageous in that the crosslinking reaction takes place easily and at low temperatures. Examples of the catalyst may include phosphonium chloride, tertiary amine, aluminium chelate and the like. The amount of the component (C) is 2 parts by weight or less, particularly 0.05 to 1.0 part by weight.

The undercoating in the present invention contains the component (A) and component (B) as the essential components, may contain, if needed, the catalyst (C), and further a color pigment, extender pigment, etc. The undercoating may be prepared Py dissolving or dispersing the above components into an organic solvent. The organic solvent is not particularly limited and may include any organic solvents conventionally used in the coating composition. Examples of the organic solvent may include aromatic hydrocarbon solvents such as toluene, xylene and the like, ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone and the like, ester solvents such as ethyl acetate, butyl acetate, cellosolve acetate, butyl cellosolve and the like, alcohol solvents such as 8 J- methanol, ethanol, isopropanol and the like, and the like.

The method of the present invention comprises coating the undercoating consisting of the above composition onto the polyolefin molded part, followed by coating a topcoating.

The topcoating may include, for example, a coating composition containing, as a base component, polyester resin or acrylic resin having a plurality of hydroxyl groups in one molecule, and further containing, as a crosslinking agent, melamine resin, polyisocyanate compound or blocked polyisocyanate compound. The above coating composition, if needed, may contain color pigments, metallic pigments, etc., and may be dissolved or dispersed into an organic solvent and/or water to be used. The above topcoating is crosslinked and cured at room temperature or by heating.

The hydroxy group-containing polyester resin used in the above topcoating may be obtained, for example, by esterifying a polyvalent oarboxylic acid or its anhydride having two or more carboxyl groups in one molecule and a polyvalent alcohol having two or more hydroxyl groups in one molecule, if needed, along with monocarboxyliG acid and/or monoalcohol, and preferably has a hydroxyl value of 40 to 120 mg/KOH, an acid value of 2 to 18 mg/KOH and a weight-average molecular weight of 5,000 to 30,000.

The hydroxyl group-containing acrylic resin may be obtained, for example, by polymerizing a hydroxyl group- containing monomer having one or more of hydroxyl group and polymerizable double bond in one molecule respectively as an essential component, and, if needed, a functional group-containing monomer having at least one of at least one functional group selected from carboxyl group, glycidyl group and amino group and of polymerizable double bond respectively in one molecule and a non-functional monomer free of the above functional groups. The acrylic resin preferably has a hydroxyl value of 40 to 120 mg/KOH, an acid value of 4 to 10 mg/KOH and a weight-average molecular weight of 5,000 to 30,000.

The amino resin and the polyisocyanate or the blocked polyisocyanate may include ones conventionally used in the coating composition.

The ratio of the amount of the hydroxyl groupcontaining polyester or acrylic resin as the base component of the amount of the crosslinking agent may arbitrarily be varied depending on intended uses, but preferably the base component is in the range of 50 to 90 % by weight and the crosslinking agent is in the range of 50 to 10 % by weight based on a total solid content of both components.

The method of coating the polyolefin molded part according to the present invention may be carried out as follows. The undercoating is coated onto the polyolefin molded part without any pretreatment or with washing with an aqueous medium to a cured film thickness of preferably 1 0 i to 30 /zm, particularly 10 to 20 p m by the airless spray, air-mixed spray, air spray, etc., followed by leaving to stand for several minutes at room temperature or heating at 80 to 120 C for 5 to 60 minutes to be cured or half-oured, coating the topcoating thereonto to a cured film thickness of preferably 20 to 50 ILm, particularly 25 to 35 u m in the same manner as in the undercoating, and by leaving to stand at room temperature or heating at 80 to 140 OC to obtain a cured film. That is, the undercoating may be coated onto the polyolefin molded part without subjecting the surface of the polyolefin molded part to any treatment after the completion of molding as it is, or with dipping the polyolefin molded part into water at normal temperature or hot water at 70'G or lower, or with spraying water at normal temperature or hot water at 701C or lower onto the polyolefin molded part after the completion of molding. It is unnecessary for the above water or hot water to contain any components other than water.

The present invention can provide a method of coating a polyolefin molded part, which makes it possible to form a coating having film performances equal to or higher than those of the conventional method needing the pretreatment by use of the harmful trichloroethane without needing the pretreatment by use of the harmful trichloroethane, that is, without any pretreatment or with a simple and safety pretreatment by washing with an 1 1 aqueous medium.

The present invention will be explained more in detail by the following Examples and Comparative Examples in which "%II and"Part" means "% by weight" and "part by weight" respectively. Preparation Examples of Component (A) and Comparative Component (A) Two Component (A)s, i.e. A-1 and A-2, and three Comparative Component (A)s, i.e. A-3, A-4 and A-5 were prepared by graft-modifying homopolymer of propylene or copolymer of 80 % of propylene with 20 % of ethylene by use of maleic anhydride as the unsaturated dicarboxylic acid or without graft-modifying, followed by chlorination as shown in table 1. The resulting degrees of chlorination as the chlorine content, acid values and weight-average molecular weights are shown in Table 1 respectively.

1 2 Table 1

Component (A)s and Comparative A-1 A-2 A-3 A-4 A-5 Components(A)s Homopolymer Propy- Propy- Propy- Propy- Propy or copolymer lene lene lene lene lene homo- ethy- homo- homo- homo poly- lene poly- poly- poly- mer copo- mer mer mer lymer Unsaturated maleic maleic maleic maleic dioarboxylic anhyd- anhyd- anhyd- anhyd- acid ride ride ride ride Degree of chlorination as 22 25 22 30 22 chlorine content M Acid value 30 50 0 30 30 Weight-average molecular weight 120,000 60,000 120,000 120,000 20,000 Preparation Examples of Component (B) and Comparative Component (B) Two Component (B)s, i.e. B-I and B-2, and two Comparative Component (B)s, i.e. B-3 and B-4 consist of an aliphatic epoxy resin (a,) obtained by etherification between glycerin and epichlorohydrin, bisphenol type epoxy resin (bi) obtained by esterification between bisphenol A and epichlorohydrin, an aliphatic epoxy resin (a2) obtained by etherification between ethylene glycol and epichlorohydrin, and bisphenol type epoxy resin (b2) obtained by esterification between bisphenol A and 1 3 1 epichlorohydrin respectively as shown in Table 2, have the epoxy equivalent and weight-average molecular weight respectively as shown in Table 2.

Table 2

Component (B) and Com- B-1 B-2 B-3 B-4 parative Component (B) Epoxy resin a, bi a2 bp Epoxy equivalent 145 950 76 2000 Weight-average molecu- 302 1400 140 2900 lar weight Preparation Examples of Undercoating and Comparative Undercoating Undercoatings 1 to 4 and comparative undercoatings 5 to 9 were formulated as shown in Table 3.

1 4 1 Table 3

Undercoating and comparative 2 3 4 5 6 7 8 9 undercoating Component (A) and A-1 A-1 A-2 A-2 A-1 A-1 A-3 A-4 A-5 comparative component (A) Amount of Component 100 100 100 100 100 100 100 100 (A) and comparative component (A)(part) Component (B) and B-] B-2 B-] B-2 B-3 B-4 B-2 B-2 comparative component (B) Amount of Component 5 5 7 7 5 5 7 7 (B) and comparative component (B)(part) 1 1 1 Catalyst Phosphonium Chloride Amount of 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Catalyst (part) Titanium White 80 80 80 80 80 80 80 80 Pigment (part) 1 1 1 1 1 Solvent toluene/xylene=1/1 (weight ratio) Viscosity Ford cup No.4: 14 seconds/25 'C B-2 7 U-3 -4 f Examples 1-4 and Comparative Examples 1-6 In Examples 1-4 and Comparative Examples 1-5, the undercoatings 1-4 and comparative undercoatings 5-9 were coated onto a polypropylene bumper with no pretreatment respectively under. the conditions shown in Table 4, followed by coating a topcoating shown in Table 4 under the conditions shown in Table 4.

The above undercoatings 1-4 and comparative undercoatings 5-9 were subjected to tests of storage properties and compatibility, and the results are shown in Table 4.

Film performance tests of adhesion properties of the undercoating film alone, interlaminar adhesion properties, water resistance, gasohol resistance and gasoline resistance were carried out, and the results are shown in Table 4.

In Comparative Example 6, the same experiments and tests as above were carried out except that the above polypropylene bumper was washed with trichloroethane vapor prior to coating the undercoating 1. The results are also shown in Table 4.

1 6 Table 4

Examples and Comparative Examples Examples

Comparative Examples 1 2 3 4 1 2 3 4 5 6 1 2 3 4 5 6 7 8 9 Undercoatings and comparative under coatings Topcoating kind f ilm thickness heat-curing kind 0C, - 10 min.

1) Organic solvent type white coating composition f ilm thickness heat-curing 2) Storage properties . 3) Compatibility . 4) Adhesion properies of undercoating or comparative undercoating alone . 4) Interlaminar adhesion properties p 'G. 30 min.

() 0 0 0 X A 0 0 0 . 0 0 0 0 X X 0 0 0 . 0 0 0 A 0 0 X X 0 .. 1........................................................

0 0 0 A 0 0 0 0 0 . 0 0 0 X A A 0 0 0 . 0 0 0 A 0 X X X 0 0 0 0 A 0 X X X 0 5) Water resistance . 6) Gasohol resistance 7) Gasoline resistance 1 7 Remarks in Table 4 are as follows.

1) The topcoating is an organic type white coating composition prepared by mixing 70 parts of a polyester resin having a hydroxyl value of 50 mg/KOH, an acid value of 7 mg/KOH and a number-average molecular weight of 3,500 with 30 parts of a butyl etherified melamim resin.

2) Storage properties:

Samples were stored at 40 OG for 10 days in a closed state to examine the conditions of the samples, and evaluation was made as follows: 0 shows no changes, A shows interlaminar separation, and X shows interlaminar separation and development of spitting. 3) Compatibility:

Transparency of a samples as a clear coating composition free of titanium white pigment wAs examined at the passage of 24 hours after preparation. Evaluation was made as follows: 0 shows no changes, and X shows a considerable development of haze. 4) Adhesion properties:

Adhesion properties of the undercoating film alone and interlaminar adhesion properties between the undercoating film and the topcoating film were tested by cutting and forming 100 squares (1X 1 mm) of a film with a knife, followed by adhering thereonto an adhesive cellophane tape, separating the tape, and by examining a number of remaining squares. Evaluation was made as 1 8 i 1 follows: 0 shows 100 of remaining squares, L shows 95 to 90 of remaining squares, and X shows less than 90 of remaining squares.

5) Water resistance:

A sample was dipped into warm water at 40 C, for 10 days, and then was subjected to the same test of adhesion properties as above to evaluate in the same manner as above. 6) Gasohol resistance:

A sample was dipped into a mixed solvent of gasoline and methanol in a weight ratio of gasoline: methanol=9:1 at 20 IC for 60 minutes to examine the surface of a coating of the sample. Evaluation was made as follows: 0 shows no changes, A shows development of blistering, and X shows development of both blistering and peeling. 7) Gasoline resistance:

A sample was dipped into gasoline at 20 IC for 24 hours to examine the surface of a coating of the sample. Evaluation was made as follows: 0 shows no changes, L shows development of blistering, X shows development of both blistering and peeling.

1 9

Claims

A method of coating a polyolefin molded part, which method comprises coating onto the polyolefin molded part an undercoating containing, as a major component, a composition consisting of 100 parts by weight of (A) an unsaturated dicarboxylic acid-modified chlorinated polyproPYlene having an acid value of 3 to 80, a degree of chlorination in the range of 18 to 28% by weight as a chlorine content and a weight-average molecular weight of 30,000 to 170,000, and 1 to 50 parts by weight of (B) an epoxy resin having a weight-average molecular weight of 250 to 2,000 and an epoxy equivalent of 90 to 1,200.
2. A method as claimed in claim 1, wherein said undercoating further contains a catalyst (C) in an amount of two parts by weight or less per 100 parts by weight of the component (A).
3. A method as claimed in claim 2, wherein said undercoating contains the catalyst (C) in an amount of 0. 05 to 1. 0 parts by weight per 100 parts by weight of the component (A).
4. A method as claimed in claim 2 or 3, wherein the catalyst (C) is selected from a group consisting of phosphonium chloride, tertiary amine and aluminium chelate.
5. A method as claimed in any preceding claim wherein the component (A) has an acid value of 10 to 50.

i
6. A method as claimed in any preceding claim wherein the component (A) has a degree of chlorination in the range of 20 to 25% by weight as a chlorine content.
7. A method as claimed in any preceding claim wherein the component (A) has a weight-average molecular weight of 50,000 to 130,000.
8. A method as claimed in any preceding claim wherein the epoxy resin (B) has two or more epoxy groups in one molecule.
9. A method as claimed in any preceding claim wherein the epoxy resin (B) has a weight-average molecular weight of 300 to 1500.
10. A method as claimed in any preceding claim wherein the epoxy resin (B) has an epoxy equivalent of 120 to 1,000.
11. A method as claimed in any preceding claim wherein the undercoating contains 2 to 10 parts by weight of the component (B) per 100 parts by weight of the component (A).
12. A method as claimed in any preceding claim including application of a topcoating which is a coating composition containing, as a base component, polyester resin or acrylic resin having a plurality of hydroxy groups in one molecule, and further containing, as a crosslinking agent, melamine resin, polyisocyanate compound or blocked polyisocyanate compound.

21
13. A method according to any preceding claim wherein the undercoating is applied to the molded part without any pretreatment or with a pretreatment by washing with an aqueous medium.
14. A method of coating a polyolefin part substantially as described herein and as exemplified in Examples 1 to 4.

22 1