Coating Composition for Mold Coat of Molded Polyurethane Resin

1. Field of the Invention

This invention relates to a coating composition for mold coat of a molded polyurethane resin, and more particularly to such a coating composition which has long pot life and high drying ability while having high weatherability.

2. Description of Prior Art

Hitherto so-called mold coat coating process has been proposed and put into practical use in molding polyurethane resin, in which a coating composition is coated on the inner or molding surface of a mold for polyurethane resin, prior to molding of polyurethane resin; and thereafter the coated coating composition is transferred onto the outer surface of the molded polyurethane resin during molding of the polyurethane resin. There are two types of coating compositions for the mold coat process, i.e., a single liquid type one and a two liquid type one. These types of the mold coat coating compositions will be listed hereinafter.

The single liquid type coating composition (polyurethane lacquer):

a resin constituent: polyurethane compound as addition polymer of glycols (adipate, hexane adipate and the like) and diisocyanates (no yellowing type isophorone diisocyanate, xylene diisocyanate and the like);

a solvent: a mixture of cyclohexanone, methylethyl ketone, xylene, toluene, isopropanol, ethyl acetate, butyl acetate and the like;

a pigment: carbon black (or organic or inorganic colouring pigment to be used for automotive vehicle body in case of other than black):

a dispersant: high molecular weight polycarboxylic acid salt, polyaminoamide phosphate, or alkylamine salt of polycarboxylic acid; and

a light resisting agent: a light stabilizer of benzophenone, benzotriazole or hindered amine.

The two liquid type coating composition:

a principal component:

a resin component (a): polyester polyol (having a molecular weight of 800 to 3,000, hydroxyl value of 12 to 150 and an acid value of not more than 4) which is a polymer of polybasic acid (adipic acid, dicarboxylic acid such as phthalic acid) and polyol of polyhydric alcohol (polyol having OH group at its chain end, such as ethylene glycol, trimethylol propane and the like);

a resin constituent (b): a mixture of polyester polyol (or the resin constituent (a)) and acryl polyol whose principal ingredient is alkyl acylate containing stylene or alkylmethacrylate;

a resin constituent (c): a mixture of the above-mentioned resin component (b) and a copolymer of vinyl chloride and vinyl acetate, or of the above-mentioned resin component (b) and cellulose resin such as cellulose acetate butyrate; and/or

a resin constituent (d): acryl polyol of hydroxyacryl copolymer (having a molecular weight of 3,000 to 10,000, an acid value of 2 to 9 and a hydroxy value of 5 to 30) of acrylate containing hydroxy such as hydroxyalkyl methacrylate and hydroxyalkyl acrylate, and acrylic esters (MMA, butyl methacrylate and the like);

solvent: a mixture of ester solvent such as ethyl acetate, butyl acetate or the like, ketone solvent such as methylisobutyl ketone, methylethyl ketone or the like, and aromatic hydrocarbon such as toluene, xylene or the like, and glycol ether such as cellosolve acetate, butyl cellosolve acetate or the like;

additives: a surface improving agent of polysiloxane copolymer such as silicone oil or the like; a pigment dispersant of alkylamine salt of polycarboxylic acid or polycarboxylic acid salt; and optionally a light stabilizer of benzophenone, benzotriazole or hindered amine;

pigment: inorganic pigment of carbon black, titanium dioxide or the like, organic pigment of Phthalocyanine Blue, quinacridone red or the like, extender pigment of calcium carbonate, barium sulfate or the like, and optionally inorganic substance such as silica or the like and organic substance such as polyethylene wax or the like (these substances are usually used as delustering agent); and

a hardener component:

a resin constituent: aliphatic diisocyanate prepolymer (for example, no yellowing diisocyanate such as hexamethylene diisocyanate, isophorone diisocyanate, xylene diisocyanate or the like), aromatic diisocyanate such as T.D.I, hydrogenenated T.D.I, or so-called soft isocyanate of addition polymer of aliphatic diisocyanate and higher alcohol or polyester to introduce a soft segment into a structure for softening purpose; and

a solvent constituent: ester solvent such as ethyl acetate, butyl acetate or the like, aromatic hydrocarbon such as toluene, xylene or the like, glycol ester such as cellosolve acetate or the like, or a mixture of them.

However, difficulties have been encountered in the above-listed single and two liquid type mold coat coating compositions, as follows: In the above single liquid type mold coat coating composition, the bond of the principal chain of the structure is an ester bond and therefore tends to be broken by hydrolysis. In the above two liquid type mold coat coating composition, the embodiment whose principal component is polyester polyol has ester bonds as the principal chain bonds and has benzene ring in the structure. The embodiment whose principal component is acryl polyol has a rigid molecular chain. Accordingly, both compositions are lower in reaction rate and therefore inferior in drying ability which is required for a coating composition for mold coat. This requires simultaneous use of reaction accelerator in order to improve drying ability, particularly to accelerate initial drying. As a result of use of the reaction accelerator, the single liquid type coating composition is inferior in moisture resistance, hot water resistance and weatherability. Similarly, the above two liquid type coating compositions respectively including, as the principal component, polyester polyol and acryl polyol have defective characteristics as shown in Table 1.

It is an object of the present invention to provide an improved coating composition for use in a mold coat process, overcoming difficulties encountered in conventional similar coating compositions.

Another object ofthe present invention is to provide animproved coating composition for use in a mold coat process, which is suitable for amolded polyurethane resin, particularly for an automotive vehicle exterior RIM (Reaction Injection Molded) polyurethane resin.

A further object of the present invention is to provide an improved coating composition in use for a mold coat process, which is excellent in physical properties such as weatherability, hot water resistance, moisture resistance and chemicals resistance, and excellent in coating characteristics such as pot life and open time (time between spraying the coating composition and closing a mold in a molding cycle), while providing excellent coated appearance of a resultant polyurethane resin product.

According to the present invention, the coating composition is used in a mold coat process wherein the coating composition is coated initially on the inner or molding surface of a mold prior to molding of polyurethane resin and then during molding the coating composition is transferred onto the surface of the molded polyurethane resin. The coating composition is comprised of a principal component including a resin constituent whose principal ingredient is an addition polymer of at least one compound selected from the group consisting of isophorone diisocyanate, hexamethylene diisocyanate and hydrogenerated methane diisocyanate, and polycarbonate polyol; and a hardener component including a resin constituent whose principal ingredient is a polymer of diisocyanate.

Hence, by virtue of the principal component of the present invention, (a) Ester or ester bond is removed or reduced, and therefore the coating composition is excellent in resistance to hydrolysis decomposition; (b) Bonding energy in the principal bond is higher; (c) Molecular weight is higher (more than 10,000), and (d) Coating film is higher in strength or excellent in balance between extension (not less than 100%) and tensile strength (200 kg/cm.sup.2), thereby providing a high weatherability coating film.

By virtue of the hardener component of the present invention, a reaction system in which reaction occurs under the action of cross linking agent becomes possible after formation of the coating film upon evaporation of the solvent. This enhances two contradictary functions of prolongation of pot life and quick drying, thereby making it possible to shorten the open time. Accordingly, operational efficiency of coating or painting is improved. Additionally, by using the thinner having a good diluting ability for the principal and hardener components and having a high evaporation rate, the operational efficiency of coating or painting is further improved.

Furthermore, the coating composition according to the present invention solidifies within a short period of time and offers a predetermined strength, and therefore it does not flow under the pressure of polyurethane resin or molding material when the polyurethane resin is poured to the cavity of the mold. Besides, by suitably increasing molecular weight of the principal composition, the solvent is easily separable from the resin constituent while concordance of the coating film with the mold releasing agent is good, thus providing good appearance of the coating film.

Consequently, the coating film of the coating composition of the present invention has a high weatherability which is required for the mold coat or coating film of a molded RIM-polyurethane resin product. Therefore, the coating composition of the present invention is suitable for automotive exterior plastic parts and for a variety of paints.

Moreover, since the coating composition of the present invention has a quick drying ability though having a long pot life, the open time can be shortened. As a result, the time of a molding cycle is greatly shortened. In addition, coloring of the coating composition of the present invention can be facilitated thereby making it possible to exhibit the feeling of an integrated or one-piece member even with other parts, thereby offering a good design effect while providing lustrous molded plastic products.

A coating composition according to the present invention is used in a mold coat coating process wherein the coating composition is coated initially on the inner or molding surface of a mold for polyurethane resin, prior to molding of the polyurethane resin, in which the coating composition is transferred onto the surface of the molded polyurethane resin. The coating composition is comprised of a principal component including a resin constituent whose principal ingredient is an addition polymer (referred to as "urethane polyol" hereinafter) of at least one selected from the group consisting of isophorone diisocyanate, hexamethylene diisocyanate and hydrogenated methane diisocyanate, and polycarbonate polyol; and a hardener component including a resin constituent whose principal ingredient contains polymer of aliphatic diisocyanate.

The coating composition is preferably further comprised of a thinner component for diluting the coating composition, including as a principal ingredient a mixture of ketone, aromatic hyrocarbon, ester and glycol.

It is more preferable that the composition or blending ratio of the principal component is defined as shown in Table 2, and that of the hardener component is defined as shown in Table 3.

To evaluate the coating composition of the present invention, Examples of the present invention will be discussed hereinafter in comparison with Comparative Examples which are not within the scope of the present invention, with reference to Tables 4 and 5 in which content is indicated in "part by weight".

First the principal component and the hardener component (as indicated in Table 4) were mixed and stirred in a container to be sufficiently blended. The thinner (as indicate in Table 4) for dilution purpose was added into the container and stirred to prepare the coating composition having a viscosity of 12 to 15 seconds and a non-volatile content of 9.5 to 11%. Subsequently, the prepared coating composition was sprayed onto the inner or molding surface (whose temperature was adjusted at 65.degree. to 72.degree. C.) of a mold by using an air spray gun having a nozzle opening diameter of 0.8 to 1.3 mm under a condition of air pressure of 2.5 to 3 kg/cm.sup.2 and a discharge amount of 60 to 100 cc/min, to obtain a coating composition film having a predetermined thickness. The molding surface of the mold had been already coated with mold releasing agent before spraying the coating composition. After spraying the coating composition, the mold was immediately closed to confine therein a cavity for molding, and then polyurethane resin as the molding material was poured into the molding cavity.

Upon lapse of 30 seconds (or after completion of reaction and hardening of the molding material) after pouring of the molding material, the mold was opened to take out a molded product whose surface was coated with a mold coat (or the hardened coating composition). The mold coat was formed by being transferred from the molding surface of the mold onto the surface of the molded product.

It is to be noted that the above experiment was conducted under conditions of Examples 1 and 1' shown in Table 6.

The principal component and the hardener component used in Example 1 are shown in Table 4 and described in detail below.

The principal component

Ingredient: Addition polymer of isophorone diisocyanate and polycarbonate polyol;

Characteristics:

Molecular weight: 15,000 to 20,000

OH content: Not more than 4%

Acid value: Not less than 1

General formula: Basic frame ##STR1## Extension/tensile strength: 450-500%/350-400 kg/cm.sup.2

Feature: Higher in resistance against hydrolysis decomposition and in bonding energy; and highly balanced in extendability and strength of coating film.

It has been confirmed that similar characteristics and feature was obtained also in case of using addition polymer of polycarbonate polyol and a mixture of at least two of isophorone diisocyanate, hexamethylene diisocyanate and hydrogenated methane diisocyanate, or in case of using addition polymer of polycarbonate polyol and one of hexamethylene diisocyanate and hydrogenated methane diisocyanate. In the characteristics and feature, the prepared principal component was higher in molecular weight and in resistance to hydrolysis decomposition depending on the fact that ester bond was removed, and higher in bond energy.

The hardener component

Ingredient: Addition polymer of hexamethylene diisocyanate and isophorone diisocyanate;

Characteristics: NCO content: 15 to 16%

Feature: No yellowing and higher drying ability

It will be appreciated that addition polymer of other aliphatic diisocyanate (for example, no yellowing diisocyanate such as xylene diisocyanate) will be used as the hardener component.

Experiments of Examples 2 to 4 were conducted similarly to that of Example 1, using the principal and hardener components and the dilution thinner as shown in Tables 4 and 5 and under conditions of Examples 2 to 4 in Table 6 thereby to obtain resultant molded products with the coating films of the coating compositions. Additionally, experiments of Comparative Examples 1 to 5 were conducted similarly to that of Example 1, using the principal and hardener components and dilusion thinner as shown in Table 5 and under conditions of Comparative Examples 1 to 5 in Table 7 thereby to obtain resultant molded products with the coating films of the coating compositions.

Evaluation Test

The thus obtained coating films of the resultant molded products of Examples 1 to 4 and Comparative Examples 1 to 5 were subjected to evaluation tests as shown in Tables 6 and 7 in order to evaluate the coating compositions according to present invention.

As appreciated from the evaluation test results of Tables 6 and 7, it has been confirmed that the coating films of the coating compositions of Examples 1, 1', 2, 3 and 4 (according to the present invention) are excellent in weatherability, long in pot life and short in open time in molding cycle, as compared with those of Comparative Examples 1, 2, 3, 4 and 5 (not within the scope of the present invention).