Colored Paste

1. Field of the Invention

The present invention relates to a colored paste.

2. Description of the Prior Art

There is a known colored paste which is formed by mixing a heat-resistant polyimide resin precursor with a pigment, as disclosed in Japanese Patent Laid-open No. 46325/1983.

The conventional colored paste derived from a polyimide precursor has a disadvantage of being poor in storage stability and also in application properties for uniform spin coating because of its excessive thixotropy ascribed to the coiled molecular structure.

The present invention was completed to address the above-mentioned problem. Accordingly, it is an object of the present invention to provide a colored paste which is superior in storage stability and application properties for uniform coating.

The present invention is embodied in a colored paste which comprises a silicone ladder polymer, a solvent, and an organic pigment insoluble in said solvent, said silicone ladder polymer being a polymer having an average molecular weight of 1.0.times.10.sup.3 to 3.5.times.10.sup.5 represented by the formula (I) below: ##STR1## (where R.sup.1 to R.sup.4 each denotes a hydrogen atom or lower alkyl group, R.sup.5 and R.sup.6 each denotes an aryl group, alkenyl group, or alkyl group, as many R.sup.5 and R.sup.6 as n may be the same or different from one another, and n is an integer of 5 to 1600.) and said organic pigment being mixed in a ratio of 5 to 400 parts by weight to 100 parts by weight of said polymer.

The colored paste of the present invention contains a silicone ladder polymer having an average molecular weight of 1.0.times.10.sup.3 to 3.5.times.10.sup.5 represented by the formula (I) below: ##STR2## (where R.sup.1 to R.sup.4 each denotes a hydrogen atom or lower alkyl group, R.sup.5 and R.sup.6 each denotes an aryl group, alkenyl group, or alkyl group, as many R.sup.5 and R.sup.6 as n may be the same or different from one another, and n is an integer of 5 to 1600.)

Having the siloxane skeleton of ladder structure, this polymer is rigid and has good heat resistance and low thixotropy. In addition, owing to the siloxane skeleton in the molecular structure, this polymer has a transmission higher than 90% for visible light in the region of wavelength longer than 400 nm. Moreover, this silicone ladder polymer is chemically stable and takes on the ladder structure when it is in the form of solution. Therefore, the colored pasted made with this polymer is superior in storage stability.

The colored paste of the present invention contains as the base polymer a silicone ladder polymer represented by the formula (I) below. ##STR3##

In Formula (I), R.sup.1 to R.sup.4 each denotes a hydrogen atom or lower alkyl group. Examples of the lower alkyl group include methyl group, ethyl group, and propyl group. R.sup.5 and R.sup.6 each denotes an aryl group (such as phenyl group), alkenyl group (such as vinyl group and allyl group), or alkyl group (such as methyl group and ethyl group). As many R.sup.5 and R.sup.6 as n may be the same or different from one another. The subscript "n" is an integer of 5 to 1600, preferably 50-1000. With the value of n smaller than 5, the colored paste is poor in film-forming properties. With the value of n greater than 1600, the colored paste is poor in solubility.

According to the present invention, the silicone ladder polymer should have an average molecular weight of 1.0.times.10.sup.3 to 3.5.times.10.sup.5, preferably 1.0.times.10.sup.4 to 2.0.times.10.sup.5.

Examples of the silicone ladder polymer include polyphenylsilsesquioxane, polymethylphenylsilsesquioxane, polyethylphenylsilsesquioxane, polyphenylvinylsilsesquioxane, polymethylvinylsilsesquioxane, and polyethylvinylsilsesquioxane.

In this connection, the silicone ladder polymer shown by Formula (I) may be obtained by following the procedure disclosed in Japanese Patent Application No. 2868/1990; for example, water drops are dropped into an organic solvent solution of phenyltrichlorosilane and an unsaturated functional group containing trichlorosilane for hydrolysis and the hydrolysis products are washed by water and subjected to heating under nucleophilic reagents and then to dehydration-condensation reaction to obtain said silicone ladder polymer.

The colored paste of the present invention contains a solvent in which the above-mentioned polymer is dissolved and the organic pigment mentioned later is dispersed. Examples of the solvent include aromatic hydrocarbons (such as anisole and xylene), ketones (such as methyl isobutyl ketone and acetone), ethers (such as tetrahydrofuran and isopropyl ether), N-methyl-2-pyrrolidone, and N,N-dimethylacetamide. (These solvents are desirable from the standpoint of dissolution of the silicone ladder polymer.) Other examples of the solvent include aromatic hydrocarbons (such as anisole, xylene, and benzene), ketones (such as methyl isobutyl ketone and acetone), ethers (such as tetrahydrofuran and diethyl ether), ethyl cellosolve, N-methyl-2-pyrrolidone, and N,N-dimethylacetamide. (These solvents are desirable from the standpoint of dispersion of the pigment.) These solvents may be used alone or in combination with one another.

There are no restrictions on the amount of the solvent to be used. It should preferably be used in an amount of 2 to 20 parts by weight for 1 part by weight of the silicone ladder polymer so that it readily solubilizes the silicone ladder polymer and the resulting colored paste has good storage stability and application properties for uniform coating.

The colored paste of the present invention contains an organic pigment which is insoluble in the above-mentioned solvent. The pigment should take on any one of the three primary colors (red, green, and blue). In addition, it should have good heat resistance so that it does not undergo decomposition, foaming, and noticeable discoloration when the colored paste is applied to a substrate and heated at 300.degree. C. for 10 minutes.

The pigment should have a particle diameter smaller than 1 .mu.m; otherwise, the resulting colored paste is poor in film-forming properties. Examples of the pigment include quinacridone pigments (such as Color Index No. 73905 pigment red 209 and Color Index No. 46500 pigment violet 19) which take on a red color, phthalocyanine green pigments (such as Color Index No. 74160 pigment green 36 and Color Index No. 74260 pigment green 7) which take on a green color, and phthalocyanine blue pigments (such as Color Index No. 74160 pigment blue 15-3 and Color Index No. 74160 pigment blue 15-4) which take on a blue color.

The organic pigment should be added in an amount of 5-400 parts by weight, preferably 10-300 parts by weight, for 100 pars by weight of the silicone ladder polymer. With an amount less than 5 parts by weight, the pigment does not produce the desired effect of the colored paste. With an amount in excess of 400 parts by weight, the pigment does not disperse uniformly in the colored paste, impairing the application properties and film-forming properties.

The colored paste of the present invention may be optionally incorporated with a small amount of surface active agent and other additives within the ranges which do not adversely affect the performance and heat resistance of the colored paste.

The colored paste of the present invention may be prepared by dissolving the silicone ladder polymer in the solvent and mixing the solution with the organic pigment with stirring for thorough dispersion. The order of dissolution and mixing is not specifically limited. It is possible to mix the pigment with the solvent and then dissolve the silicone ladder polymer in the solvent.

The colored paste of the present invention contains the pigment in the completely dispersed state, has good application properties for uniform coating with very little thixotropy, and is superior in storage stability.

The present invention will be described with reference to the following examples, which are not intended to restrict the scope of the invention.

In a 300-ml flask was placed a solvent as shown in Table 1. In the solvent was completely dissolved a silicone ladder polymer as shown in Table 1 with stirring. To the solution was added an organic pigment as shown in Table 1 with stirring for thorough dispersion. Thus there was obtained a desired colored paste.

The colored paste was applied to a glass plate (4 inches) using a spinner so that the amount of coating was about 2 ml. The coating step was followed by heating at 100.degree. C., 200.degree. C., and 300.degree. C. for 30 minutes each.

The coating film was tested for thickness and surface roughness at the center of the glass substrate and several points 0.5 cm, 1.0 cm, 2.0 cm, and 4.0 cm away from the center, using a surface texture measuring apparatus (Dektak 3030, made by Throne Co., Ltd.). The same experiment as mentioned above was repeated for a colored paste which had been aged at 25.degree.-40.degree. C. for 1 month. The results are shown in Table 2. It is noted that the colored pastes in these examples meet the desired requirements.

Colored pastes were prepared in the same manner as in Examples 1 to 6 except that changes were made in the kind and amount of the solvent, silicone ladder polymer, and organic pigment as shown in Table 1. The coating film of the colored pastes were examined for thickness and surface roughness. The results are shown in Table 2.

In Comparative Example 1, the colored paste was poor in film-forming properties because the silicone ladder polymer had too low an average molecular weight (or a degree of polymerization (n) smaller than 5). In addition, the colored paste gave a coating film which cracked upon heating.

In Comparative Example 2, the colored paste was not homogeneous because the silicone ladder polymer did not dissolve completely in the solvent because it had too high an average molecular weight (or a degree of polymerization (n) greater than 1600).

In Comparative Example 3, the colored paste was poor in film-forming properties and gave a coating film with a rough surface because it was incorporated with an excess amount of the organic pigment which was not uniformly dispersed.

The present invention provides a colored paste composed of an organic solvent, a silicone ladder polymer having good heat resistance and transparency, and an organic pigment having good heat resistance. The colored paste takes on a red color, green color, or blue color according to the pigment used. The colored paste has good storage stability and readily forms a uniform film by spin coating.