DE69229794T2 - Process for the production of coated molded articles - Google Patents

Description

Background of the invention a. Field of the invention

This invention relates to molded bodies with high surface hardness and a method for producing such molded bodies. The molded bodies of the invention can be used in various fields of application, including, for example, the field of architecture, the automotive industry and optics.

b. Description of the state of the art

Synthetic resin moldings made of polymethyl methacrylate resin, polycarbonate resin, diethylene glycol bisallyl carbonate resin and the like are lighter in weight than glass products and cheaper than them. Because of these advantages, they are used for a wide range of applications.

However, since such synthetic resin molded articles have insufficient surface hardness, their surfaces are easily damaged by contact with other objects, impact, scratches and the like, resulting in reduced product yield and unsightly appearance. Particularly, where these molded articles are used as optical lenses, fashion glasses, sunglasses, spectacle lenses (such as corrective lenses), window panes and the like, any damage caused to the surfaces reduces their commercial value and/or renders them unusable within a short period of time. Accordingly, there is a great demand for improving the surface hardness of such synthetic resin molded articles. To date, various attempts have been made to meet these needs.

In Japanese Patent Publications Nos. 39691/'77, 5554/'87, 157865/'83, 35675/'88, 36349/'88 and 45094/'91, the abrasion resistance of molded articles is improved by coating them with a coating composition containing a mixture of colloidal silicon dioxide and a hydrolyzable silicon compound.

In Japanese Patent Publication No. 53701/'85, the abrasion resistance and weather resistance of polycarbonate substrates are improved by forming a thermosetting acrylic polymer layer (primer layer) containing an ultraviolet light absorbing agent on the polycarbonate substrates and then forming a coating layer comprising a mixture of colloidal silicon dioxide and a hydrolyzable silicon compound.

However, the coating layers disclosed in Japanese Patent Publication Nos. 39691/'77, 5554/'87, 157865/'83, 35675/'88, 36349/'88 and 45094/'91 have the following disadvantages. (1) When subjected to a Taber abrasion test according to ASTM D-1044 in which a CS-17 truck wheel is used under a load of 250 g and made 5000 revolutions, all of them show haze as high as 10 to 50%. Thus, their abrasion resistance is much lower than that of glass plates which show haze of about 3%. (2) Since the adhesion of the coating layer to the substrate is achieved by dissolving the substrate using a specific solvent, the substrates that can be used are limited by the type of solvent used. (3) The adhesion of the coating layer to the substrate is insufficient. More specifically, the adhesion of the coating layer to the substrate is reduced when the coating layer is subjected to a durability test for a long period of time. In addition, the structure disclosed in Japanese Patent Publication No. 53701/'85 has a disadvantage that the adhesion of the primer layer to the substrate is reduced. if the primer layer contains a large amount of agents for absorbing ultraviolet rays.

Summary of the invention

A first object of the invention is to provide a method for producing a coated molded article having a cross-linked siloxane network structure represented by (-Si-O-Si-) in its surface, exhibiting excellent surface hardness and abrasion resistance, and exhibiting improvement in the adhesion of the coating layer to various substrates.

The above-described object of the invention is achieved by a method for producing an abrasion-resistant molded article having a coating layer formed on the surface region, wherein the surface region is formed from a polymer having structural units derived from a multifunctional monomer containing two or more (meth)acryloyloxy groups in the molecule as defined below, and by curing a coating composition consisting essentially of a silicon dioxide polycondensate obtained by mixing (I) colloidal silicon dioxide and (II) at least one hydrolyzable silicon compound having one of the following general formulas (A) to (F) in such a molar ratio that the average number of hydrolyzable groups calculated from the following equation (I) is in a range of 2.30 to 3.85, and subjecting a cohydrolysis and a polycondensation.

SiR¹aR²b(OR³)c (A)

SiR4dR5e(OR6)f (B)

SiR7gR8h(OR9)i (C)

SiR¹⁰jR¹¹k(OR¹²)l (D)

SiR¹³mR¹⁴n(OR¹⁵)o (E)

SiR¹⁶pR¹⁷q(OR¹⁸)r (F)

wherein R¹, R², R³, R⁵, R⁶, R⁻, R⁴, R⁹⁹, R¹², R¹⁴, R¹⁵, R¹⁵, R¹⁷ and R¹⁴ in formulas (A) to (F) are hydrocarbon radicals of 1 to 15 carbon atoms which may have an ether linkage or an ester linkage; R⁴ in formula (B) is a hydrocarbon radical of 2 to 15 carbon atoms having an epoxy group, R⁵ in formula (C) is a hydrocarbon radical of 1 to 15 carbon atoms having an amino group, R¹⁴ in formula (D) is a hydrocarbon residue of 1 to 15 carbon atoms having a mercapto group, R¹³ in formula (E) is a hydrocarbon residue of 2 to 15 carbon atoms having a vinyl group, R¹⁶ in formula (F) is a hydrocarbon residue of 3 to 15 carbon atoms having a (meth)acryloyloxy group, a, b, d, e, g, h, j, k, m, n, p and q are integers from 0 to 3, c is (4-a-b), f is (4-d-e), i is (4-g-h), 1 is (4-j-k), o is (4-m-n) and r is (4-p-q).

Average number of hydrolysable groups

wherein [A] to [F] are each the number of moles of the hydrolyzable silicon compounds having the general formulae (A) to (F) occurring in the reaction mixture, [I] is the number of moles of the colloidal silicon dioxide occurring in the reaction mixture, and c, f, i, l, o and r are the same integers as defined above for the general formulae (A) to (F).

The method for producing abrasion-resistant molded articles comprises the steps of (a) providing a molded article having a surface formed from a polymer, the polymer having structural units derived from a multifunctional monomer containing two or more (meth)acryloyloxy groups in its molecule, (b) irradiating the surface of the polymer with ultraviolet light having a wavelength of 300 nm or less, (c) subjecting the irradiated surface to an alkali treatment (d) coating the irradiated and alkali-treated surface with a coating composition consisting essentially of a silica polycondensate obtained by mixing (I) colloidal silica and (II) at least one hydrolyzable silicon compound having any of the above general formulas (A) to (F) in such a molar ratio that the average number of hydrolyzable groups calculated from the above equation (1) is within a range of 2.30 to 3.85, and then subjecting the coating composition to cohydrolysis and polycondensation, and (e) curing the coating composition.

Short description of the drawings:

Fig. 1 is a sectional view showing a substrate 1 having a polymer layer 2 formed thereon for use in the method for producing abrasion-resistant molded articles according to the present invention;

Fig. 2 is a sectional view showing the structure of an abrasion-resistant molded article according to the present invention, which was prepared by forming a surface region 3 having a certain amount of acidic groups in the surface of the polymer layer 2 and then forming a coating layer 4 thereon;

Fig. 3 is a diagram showing the state of the polymer structure during the steps of ultraviolet light irradiation and alkali treatment in the process of the invention;

Fig. 4 is a sectional view showing an example of the ultraviolet light exposure step in which the polymer layers formed on both sides of a substrate are irradiated with ultraviolet light through a glass plate.

Description of the preferred embodiments

In the practice of the invention, a polymer layer 2 (Fig. 1) is first formed by polymerizing a monomer composition containing a multifunctional monomer having two or more (meth)acryloyloxy groups in the molecule. Examples of the multifunctional monomer include the following compounds: 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, 3-methylpentanediol di(meth)acrylate, diethylene glycol bis(β-(meth)acryloyloxypropionate), trimethylolethane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, Tri(2-hydroxyethyl)isocyanatdi(meth)acrylat, Pentaerythritoltetra(meth)acrylat, 2,3-Bis(meth)acryloyloxyethyloxymethylbicyclo[2.2.1]heptan, Poly-l, 2-butadiendi(meth)acrylat, 1,2-Bis(meth)acryloyloxymethylhexan, Nonaethylenglykoldi(meth)acrylat, Tetradecaethylenglykoldi(meth)acrylat, 10-Decandioldi(meth)acrylat, 3,8-Bis(meth)acryloyloxymethyltricyclo[5.2.10]decan, hydrogeniertes Bisphenol-A-di(meth)acrylat, 2,2-Bis(4-(meth)acryloyloxydiethoxyphenyl)propan, 1,4-Bis((meth)acryloyloxymethyl)cyclohexan, Bisphenol-A-diglycidyletherdi(meth)acrylat und epoxidized bisphenol A di- (meth)acrylate. These multifunctional monomers can be used alone or as a mixture of two or more.

If necessary, they may be copolymerized with monofunctional monomers. It is desirable that the monomer composition contains such a multifunctional monomer in an amount of not less than 30% by weight. The term "(meth)acryloyloxy group" as used herein means an acryloyloxy group or a methacryloyloxy group. When a monofunctional monomer is used alone to form a polymer layer, subsequent irradiation with ultraviolet light and alkali treatment fail to obtain a sufficient amount (i.e., within a range from 0.02 to 0.2 umol/cm²) of acidic groups. Thus, the use of a multifunctional monomer is an essential feature of the invention.

Then, the polymer layer 2 (Fig. 1) is irradiated with ultraviolet light having a wavelength of 300 nm or less to dissolve or break apart a part of the crosslinked molecular chains. Since the bonding energy of the crosslinked molecular chains of the polymer layer varies with the composition of the polymer, it is impossible to generally specify the wavelength of ultraviolet light required to break apart the crosslinked molecular chains. However, it is desirable to use ultraviolet light having a photon energy of about 4 eV or greater. Thus, it is necessary to use ultraviolet light having a wavelength of 300 nm or less, which corresponds to the photon energy specified above. It is understood that a sufficient amount of acidic groups is not generated by the irradiation with ultraviolet light alone.

Subsequently, the polymer layer irradiated with ultraviolet light is subjected to an alkali treatment. It is generally said that the hydrolysis of the crosslinked polymers proceeds slowly. However, the inventors found that the polymer layer, after being degraded into low molecular weight fragments by irradiation with a wavelength of 300 nm or less, is easy to hydrolyze. The reaction mechanisms believed to take place are schematically illustrated in Fig. 3. The aqueous alkaline solutions that can be used for this alkali treatment include, for example, aqueous solutions of sodium hydroxide, potassium hydroxide and the like, and such solutions additionally contain suitable solvents such as alcohols. The optimum conditions for the alkali treatment cannot be generally specified since they vary according to the amount of exposure to ultraviolet light, the composition and geometry of the region of the molded article irradiated with ultraviolet light. may vary. However, sodium hydroxide, for example, is preferably used in the form of an aqueous solution having a concentration of 0.1 to 50% by weight, and more preferably 1 to 30% by weight. The temperature for the alkali treatment is generally within a range of 0 to 100°C, and preferably 20 to 80°C. The time period for the alkali treatment is generally within a range of 0.01 to 100 hours, and preferably 0.1 to 10 hours.

In the invention, the part of the polymer layer 2 in which acidic groups have been generated by the above-described ultraviolet light exposure and alkali treatment is referred to as surface region 3 (Fig. 2). This surface region means either (1) the surface portion 3 of the polymer layer 2 formed on a substrate 1 made of a multifunctional monomer (as shown in Fig. 2) or (2) the surface portion of a molded article consisting entirely of a polymer formed of a multifunctional monomer (not shown).

In the first case (1), there is no particular limitation on the type of material constituting the substrate 1, but organic polymer materials are preferred. However, composites consisting of inorganic and organic materials can also be used for the substrate 1. Useful organic materials include acrylic resins, vinyl chloride resins, polycarbonate resins, polyester resins, and the like. Where the polymer layer 2 (Fig. 1) is formed by applying a multifunctional monomer as described above to the surface of the substrate 1, the thickness of the polymer layer 2 is preferably not less than 1 µm, and more preferably not less than 3 µm.

The term "acidic groups" as used herein means carboxyl and hydroxyl groups. In the invention, the amount of acidic groups is expressed as the number of micromoles of a basic dye present on a unit area of surface area (ie in umol/cm2). This can be achieved according to the following procedure.

(1) A 0.1 N sodium acetate buffer solution (pH 4.5) is prepared.

(2) Using this buffer solution, prepare a 1.0 g/L methyl violet solution.

(3) A molded body with dimensions of 50 mm x 50 mm² is immersed in the above solution (at 25ºC) for 72 hours.

(4) The molded body is removed from the solution and washed with water.

(5) After washing, the molded body is rubbed dry.

(6) The dye is extracted by soaking the molded article in N,N-dimethylformamide for 24 hours.

(7) The absorbance of the dye extract is measured at a wavelength of 587 nm.

(8) Separately, a calibration curve is constructed using a series of dye solutions in N,N-dimethylformamide. Using this calibration curve, the concentration of the basic dye per unit area of the surface area of the molded article is determined.

In the invention, the acidic groups serve to increase the bonding strength between the surface region 3 and the coating layer 4 (Fig. 2). In order to achieve sufficient bonding strength, 0.02 to 0.2 µmol/cm² of acidic groups are required. In the invention, the acidic groups appear in the surface region 3 (Fig. 2) and the inner region of the polymer layer 2 contains few acidic groups.

When the polymer layer 2 (Fig. 1) is subjected to other conventional methods for introducing acidic groups (such as plasma treatment, photoinitiated graft polymerization and chromium treatment) instead of the method of the invention comprising a combination of ultraviolet light irradiation and alkali treatment, significant differences can be achieved with respect to the number of acidic groups and the ease of the operation, good results cannot be obtained. After the alkali treatment, the molded article is usually washed with water. If necessary, the molded article can be neutralized by washing with an inorganic or organic acid. The acid groups thus produced neither show any change in number even when heated at 80ºC for 100 hours, nor do they bury themselves.

Subsequently, the surface region 3 (Fig. 2) is coated with a coating composition consisting essentially of a silicon dioxide polycondensate obtained by mixing colloidal silicon dioxide and at least one hydrolyzable silicon compound having one of the following general formulas (A) to (F) in such a molar ratio that the average number of hydrolyzable groups calculated from equation (1) below is in a range of 2.30 to 3.85, and by subsequently subjecting them to cohydrolysis and polycondensation in the presence of water and an organic solvent (and a hydrolysis catalyst and a condensation catalyst if this should be necessary). Subsequently, the coating composition is heat treated to further increase the degree of polymerization of the silicon dioxide polycondensate and thereby obtain a coating layer 4 (Fig. 2).

SiR¹aR²b(OR³)c (A)

SiR4dR5e(OR6)f (B)

SiR7gR8h(OR9)i (C)

SiR¹⁰jR¹¹k(OR¹²)l (D)

SiR¹³mR¹⁴n(OR¹⁵)o (E)

SiR¹⁶pR¹⁷g(OR¹⁸)r (F)

wherein R¹ to R¹⁸ and a to r are as defined above.

The average number of hydrolysable groups

wherein [I], [A] to [F] and c to r are as defined above. During this process, the hydroxyl, epoxy, amino and/or mercapto groups present in the constituents of the coating layer 4 (Fig. 2) chemically react with the acidic groups present in the surface region 3 (Fig. 2) to form chemical bonds and the properties such as adhesion are further improved.

On the other hand, the crosslinking density of the polymer layer 2 (Fig. 2) is maintained at the same level as that achieved during polymerization. The properties of the polymer layer 2 and the coating layer 3 cooperate to produce a molded article which as a whole has very good abrasion resistance and durability. It is desirable that the thickness of the surface region 3 extending from the outer surface to the inner surface is within a range of 0.01 to 1 µm. If the thickness of the surface region 3 having acidic groups is greater than 1 µm, the desired hardness cannot be achieved, and if it is less than 0.01 µm, the coating layer shows poor adhesion due to the decrease of the acidic groups.

Where silicon compounds within the scope of the above general formulas (A) to (D) are used, a heating device may be used to cure the coating composition applied to the surface region and thereby form a coating layer, as will be described in more detail later.

The silicon compounds represented by the general formula (A) include, for example, the following compounds: tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, methyltriethoxysilane, ethyl trimethoxysilane, ethyltriethoxysilane and phenyltrimethoxysilane.

The silicon compounds represented by the general formula (B) include, for example, glycidoxymethyltrimethoxysilane, glycidoxymethyltriethoxysilane, β-glycidoxyethyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane and γ-glycidoxypropyltriethoxysilane.

The silicon compounds represented by the general formula (C) include, for example, aminomethyltrimethoxysilane, aminomethyltriethoxysilane, aminomethyltripropoxysilane, aminomethyltributoxysilane, aminoethyltrimethoxysilane, aminoethyltriethoxysilane, aminoethyltripropoxysilane, aminoethyltributoxysilane, aminopropyltrimethoxysilane, aminopropyltriethoxysilane, aminopropyltripropoxysilane, aminopropyltributoxysilane, N-aminomethylaminomethyltrimethoxysilane, N-aminomethylaminomethyltriethoxysilane, N-aminomethylaminomethyltripropoxysilane, N-aminomethylaminomethyltributoxysilane, N-β-aminoethyl-β-aminoethyltrimethoxysilane, N-β-aminoethyl-β-aminoethyltriethoxysilane, N-β-aminoethyl-β-aminoethyltripropoxysilane and N-β-aminoethyl-β-aminoethyltributoxysilane.

The silicon compounds represented by the general formula (D) include, for example, γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, γ-mercaptopropyltripropoxysilane, γ-mercaptopropyltributoxysilane, mercaptomethyltrimethoxysilane, mercaptomethyltriethoxysilane, mercaptomethyltripropoxysilane, mercaptomethyltributoxysilane, mercaptoethyltrimethoxysilane and mercaptoethyltriethoxysilane.

Where silicon compounds are used within the scope of the above general formulas (E) and (F), not only a heating means but also a means for exposing to actinic radiation may be used to cure the coating composition and thereby form a coating layer.

The silicon compounds represented by the general formula (E) include, for example, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltripropoxysilane and vinyltributoxysilane.

The silicon compounds represented by the general formula (F) include, for example, methacryloyloxymethyltrimethoxysilane, methacryloyloxymethyltriethoxysilane, methacryloyloxymethyltripropoxysilane, methacryloyloxymethyltributoxysilane, methacryloyloxyethyltrimethoxysilane, methacryloyloxyethyltriethoxysilane, methacryloyloxyethyltripropoxysilane, methacryloyloxyethyltributoxysilane, methacryloyloxypropyltrimeth oxysilane, Methacryloyloxypropyltriethoxysilane, Methacryloyloxypropyltripropoxysilane, Methacryloyloxypropyltributoxysilane, Acryloyloxymethyltrimethoxysilane, Acryloyloxymethyltriethoxysilane, Acryloyloxymethyltripropoxysilane, Acryloyloxymethyltributoxysilane, Acryloyloxyethyltrimethoxysilane, Acryloyloxyethyltriethoxysilane, Acryloyloxyethyltripropoxysilane, Acryloyloxyethyltributoxysilane, acryloyloxypropyltrimethoxysilane, Acryloyloxypropyltriethoxysilane, acryloyloxypropyltripropoxysilane and Acryloyloxypropyltributoxysilane.

Also useful are silicon compounds obtained by replacing the trialkoxysilane groups in the above compounds with dimethoxymethylsilane, dimethoxyethylsilane, dimethoxypropylsilane, diethoxymethylsilane, diethoxyethylsilane, diethoxypropylsilane, dimethoxybutylsilane, diethoxybutylsilane, dibutoxymethylsilane, dipropoxyethylsilane, dipropoxypropylsilane, dipropoxybutylsilane, dibutoxymethylsilane, dibutoxyethylsilane, dibutoxypropylsilane, dibutoxybutylsilane, and like groups.

Colloidal silica (i.e. silica particles) is commercially available as a "colloidal silica solution" which comprises silica particles dispersed in water or an alcohol/water mixture. To improve the dispersion stability of the silica particles, the solution is an acidic or alkaline pH. In the invention, the optimum particle diameter of the colloidal silica (or silica particles), the content of the silica particles in the colloidal silica solution (ie, its solid content) and the pH of the colloidal silica solution can be selected as desired. In particular, the optimum colloidal silica solution can vary according to the type of hydrolyzable silicon compound used. Usable commercially available products include Cataloid S (solid content: 20 to 30 wt%; pH: 8 to 10; manufactured by Catalysts and Chemicals Industries Co., Ltd.), OSCAL-1432 (solid content: 30 wt%; pH: 2 to 3; manufactured by Shokubai Kasei Kogyo KK), Snowtex C (solid content: 30 wt%; pH: 8 to 10; manufactured by Nissan Chemical Industries, Ltd.), IPA-ST (solid content: 30 wt%; pH: 2 to 3; manufactured by Nissan Chemical Industries, Ltd.), and the like. All of these commercially available colloidal silicas have an average particle diameter of 10 to 20 µm.

Colloidal silica (i.e., silica particles) is used in such an amount that the average number of hydrolyzable groups (as will be described below) is in a range of 2.30 to 3.85. Moreover, in the composition for forming the silica polycondensate containing colloidal silica (I) and at least one hydrolyzable silicon compound (II), the colloidal silica (I) is desirably present in an amount of 10 to 40% by weight. If the amount of the colloidal silica (I) is less than 10% by weight, the surface hardness of the coating layer tends to be insufficient. When the amount of the colloidal silica (I) is greater than 40 wt%, cracks tend to be generated in the coating layer under severe environmental conditions.

The hydrolyzable silicon compounds that can be used as the component (II) are classified into four classes according to the number of hydrolyzable groups in the molecule. They range from monofunctional silicon compounds having one hydrolyzable group to tetrafunctional silicon compounds having four hydrolyzable groups. In the invention, the weight ratio of colloidal silicon dioxide (I) to the hydrolyzable silicon compound (II) is limited based on experimental results, assuming that (I) the hydrolyzable silicon compounds are completely hydrolyzed in solution and their hydrolyzable groups are all converted to -OH groups, and (2) the colloidal silicon dioxide is regarded as a tetrafunctional silicon compound. The average number of hydrolyzable groups is a value calculated from the above equation (I) and corresponds to the average cross-linking density of the silica polycondensate (the coating layer) cured by heat treatment. In the invention, as described above, the colloidal silica (I) and the hydrolyzable silicon compound (II) are mixed in such a molar ratio that the average number of hydrolyzable groups is in a range of 2.30 to 3.85. If the average number of hydrolyzable groups is greater than 3.85, cracking occurs in the heat-treated coating layer, and if the average number of hydrolyzable groups is less than 2.30, the heat-treated coating layer does not have sufficient abrasion resistance. Preferably, an average number of hydrolyzable groups within a range of 3.30 to 3.60 is used to obtain a coating layer having excellent abrasion resistance and durability.

A silicon dioxide polycondensate is then obtained by subjecting the mixture described above to cohydrolysis and polycondensation. This cohydrolysis and polycondensation can be carried out by stirring the The hydrolysis reaction may be carried out in the presence of water and an organic solvent (and a hydrolysis catalyst and a condensation catalyst, if necessary) at a temperature ranging from room temperature to reflux temperature for a period of about 1 to 10 hours. Where a compound having the general formula (A), (B), (D), (E) or (F) is used as the hydrolyzable silicon compound (II), it is desirable to adjust the pH of the mixture to 4 to 5 in order that the mixture is rapidly hydrolyzed and can be stored for a long period of time. This pH adjustment may be accomplished by adding an acid as a hydrolysis catalyst. Specific examples of useful acids include citric acid, benzoic acid, acetic acid, hydrochloric acid and nitric acid. Where a compound having the general formula (C) is used, such an acid need not be used.

The condensation catalyst used as required is a latent catalyst which is inactive, for example in the form of a solution, and exhibits its effect upon heating, and may be selected from amine salts of carboxylic acids and quaternary ammonium salts of carboxylic acids. Specific examples thereof include dimethylamine acetate, ethanolamine acetate, dimethylaniline formate, tetraethylammonium benzoate, sodium acetate and sodium propionate. The condensation catalyst is preferably used in an amount of about 0.05 to 1% by weight based on the total weight of the mixture.

The amount of water added for hydrolysis may include any suitable amount sufficient for hydrolysis.

The organic solvents that can be used for the purpose of cohydrolysis and polycondensation include alcohols such as methyl alcohol, ethyl alcohol and isopropyl alcohol; ketones such as acetone and methyl ethyl ketone; and ethers such as tetrahydrofuran.

The coating composition used in the invention consists essentially of the silica polycondensate obtained by the cohydrolysis and polycondensation described above. If desired, a catalyst that promotes the reaction between the acidic groups appearing in the surface region and the functional groups (i.e., epoxy, amino and/or mercapto groups) appearing in the hydrolyzable silicon compounds may be added to the coating composition. To promote the reaction with the epoxy groups, for example, a perchloric acid compound known as a catalyst for ring opening of the epoxy groups may be used. Specific examples of such perchloric acid compounds include ammonium perchlorate, perchloric acid, magnesium perchlorate, potassium perchlorate, sodium perchlorate, zinc perchlorate and aluminum perchlorate.

Furthermore, as described in Japanese Patent Publication No. 28094/'88, an alkyl (meth)acrylate polymer may also be added to the coating composition to further improve the adhesion of the coating layer without impairing its appearance (for example, transparency and smoothness). The alkyl (meth)acrylate polymer preferably has a molecular weight characterized by an intrinsic viscosity [η] within a range of 0.01 to 0.30 g/L, and may be a homopolymer of a monomer selected from alkyl (meth)acrylates having an alkyl group of 1 to 8 carbon atoms or a copolymer of such monomers. Specific examples of the alkyl (meth)acrylate polymer include homopolymers and copolymers of the following compounds: methyl (meth)acrylate; Ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, amyl methacrylate, hexyl methacrylate, octyl methacrylate, 2-ethyl-1-hexyl (meth)acrylate, 3-pentyl acrylate, 3-methyl-1-hexyl (meth)acrylate and 3-methyl-1-butyl (meth)acrylate. These polymers can be used alone or as a mixture of two or more.

To apply the above-described coating composition to the surface portion, any of various techniques such as spray coating, spin coating and dip coating may be used. However, for molded articles having a simple structure, dip coating is preferred, and for molded articles having a complex structure, spray coating is preferred.

Subsequently, the coating composition applied to the surface region is cured in accordance with a conventional method to form a coating layer. During this process, the reactive functional groups present in the silica polycondensate (for example, the epoxy, amino and/or mercapto groups occurring in the compounds having the general formulas (A) to (F)) chemically react with the acidic groups (carboxyl and hydroxyl groups) present in the surface region, resulting in enhanced adhesion. Useful conventional methods include the application of heat and exposure to actinic radiation such as ultraviolet light and γ rays.

Where one of the hydrolyzable silicon compounds represented by the general formulae (A) to (F) is used, the coating layer can be formed by curing by means of the application of heat. This can be done, for example, by heating the coated molded article in an oven at 100 to 130°C for a period of time ranging from about 10 minutes to about 10 hours.

Where vinyl- or (meth)acryloyloxy-containing silicon compounds with the general formula (E) or (F) are used, the coating layer can advantageously be prepared by curing by means of exposure to actinic radiation. This exposure to actinic radiation can be carried out under suitable conditions which are well known in the art.

After the coating layer is cured by the application of heat or exposure to actinic radiation, it may have a thickness of 1 to 30 µm, and preferably 1 to 10 µm. If the thickness of the coating layer is less than 1 µm, the resulting molded article does not have sufficient surface hardness and abrasion resistance. If the thickness of the coating layer is more than 30 µm, the coating layer shows a decrease in adhesion and is prone to cracking.

The invention will be explained in more detail with reference to the following examples. In these examples, the coated molded articles were evaluated according to the following method.

1. Abrasion resistance

Using a Taber abrasion tester (manufactured by Toyo Seiki Seisakusho K.K.), a CS-17 truck wheel was pressed against a sample with a load of 250 g and rotated at 1000, 3000 and 5000 revolutions. Then, the sample was washed with a neutral detergent and its total light transmittance (Tt) and haze (H) were measured with a transmissometer Model HR-100 (manufactured by Murakami Color Technology Laboratory).

2. Liability

The adhesion of the coating layer of a sample was evaluated by means of an adhesive tape peeling test. Specifically, 11 parallel cuts were made in the surface of the sample with a cutting knife at intervals of 1.5 mm, in two mutually orthogonal directions each, to form a total of 100 squares in the coating layer. A strip of cellophane adhesive tape (manufactured by Nichiban Co. Ltd.) was applied to the squares under pressure and quickly peeled upward. The number of unremoved squares among the 100 squares was counted and was used as an index of the adhesion of the coating layer.

3. Durability (a) Heat shock test

Using a heat shock tester, a sample was subjected to thermal cycles in which the sample was exposed to a temperature of -30ºC for 2 hours in the atmosphere and then to a temperature of 80ºC for 3 hours. After the sample had been subjected to five thermal cycles, its appearance was examined and its total light transmittance (Tt) and haze (H) were measured.

(b) Accelerated weathering test

Using a sunshine weatherometer (manufactured by Suga Testing Machines Co., Ltd.) with a black panel temperature of 63ºC, a sample was subjected to an accelerated weathering test for 500 hours in which the sample was cyclically exposed to 12 minutes of water spraying and 48 minutes of drying. Thereafter, its adhesion was evaluated and its appearance was visually observed for changes.

(c) Water immersion test

A sample was immersed in warm water at a temperature of 60ºC for 100 hours. The adhesion and abrasion resistance of the sample were then assessed.

Production of substrates with acidic groups in their surface areas

1) Monomer solutions were prepared by adding 35 parts by weight of isopropyl alcohol and 20 parts by weight of toluene to 45 parts by weight of each of monomer composition Nos. 1 to 5 shown in Table 1. Substrates were immersed in each of the monomer solutions and then taken out at a speed of 1 m/min to form a monomer solution film on their surfaces. The substrates comprised polymethyl methacrylate (PMMA) sheets (sold by Mitsubishi Rayon Co., Ltd. under the trade name name Acrylite L) and polycarbonate (PC) sheets (sold commercially by Mitsubishi Rayon Co., Ltd. under the trade name Dialite).

Then, as shown in Fig. 4, each substrate having the monomer solution film thereon was sandwiched between two glass plates having a thickness of 1.5 mm (commercially available under the trade name BK 7), and irradiated with ultraviolet light from a high pressure mercury vapor lamp from outside the glass plates to cause photopolymerization of the monomers. The reason why the substrate was covered with the glass plates is that the monomer solution film can be cured only in the absence of air. Under these conditions, ultraviolet light having wavelengths of less than 300 nm was absorbed by the glass plates. The integrated energy of the ultraviolet light at 365 nm was 2.508 mJ/cm2 and the thickness of the photocured polymer layer 2 (Fig. 1) was 20 µm. For the purpose of irradiation, an ultraviolet light irradiator Model UV5003 (manufactured by Mitsubishi Rayon Engineering Co., Ltd.) was used. This light source emits ultraviolet light with a main wavelength of 365 nm and also wavelengths of 300 nm or less. The integrated energy of the ultraviolet light was measured using an ultraviolet actinometer Model UV-350 (manufactured by Oak Seisakusho K. K.). This actinometer (Model UV-350) has a peak sensitivity wavelength of 360 nm and a measurable wavelength range of 320 to 390 nm.

(2) Using a high-pressure mercury lamp, the polymer layer formed as described in section (1) was irradiated with ultraviolet light containing short-wavelength light of 300 nm or less to partially break apart the cross-linked molecular chains. For this purpose, the surface of the polymer layer was directly irradiated with ultraviolet light from the high-pressure mercury lamp. The integrated energies of the light applied to the polymer layer irradiated ultraviolet light are shown in Table 1. For the purpose of irradiation, the same ultraviolet light irradiation apparatus (Model UV5003) as described above was used. The integrated energies of ultraviolet light were measured by means of an ultraviolet actinometer Model UV-350 and a Model UV-350-25 (manufactured by Oak Seisakusho KK). The latter actinometer (Model UV-350-25) had a maximum sensitivity wavelength of 254 nm and a measurable wavelength range of 241 to 271 nm.

Then, the polymer layer irradiated with ultraviolet light was hydrolyzed by soaking it in a 10 to 20 wt% aqueous solution of sodium hydroxide (at a temperature of 25 to 50°C). The conditions of the hydrolysis and the measured amounts of the acidic groups thus produced are shown in Table 1.

As can be seen from the data marked (A), (E), (I), (M) and (R), the amount of acidic groups was insufficient when no alkali treatment of the polymer layer was carried out.

Preparation of coating composition solutions: < Coating composition solution No. I to III>

A colloidal silica solution containing 30 wt% of colloidal silica dispersed in isopropyl alcohol (commercially available from Nissan Chemical Industries, Ltd. under the trade name IPA-ST), γ-glycidoxypropyltrimethoxysilane (commercially available from Toshiba Silicone Co., Ltd. under the trade name TSL-8350), first-grade tetraethoxysilane (commercially available from Katayama Chemical Industries Co., Ltd.), γ-glycidoxypropylmethyldiethoxysilane (commercially available from Shin-Etsu Chemical Co., Ltd. under the trade name KBE-402), isopropyl alcohol and 1/1000 N hydrochloric acid were mixed according to each of the formulations shown in Table 2. This mixture was heated to 70°C and stirred at this temperature for 7 hours. After the After the resulting solution was cooled to room temperature, magnesium perchlorate as a catalyst for ring opening of epoxy groups was added thereto and dissolved therein to prepare a coating composition solution. The average number of hydrolyzable groups of the coating composition solutions Nos. I to III are shown in Table 2 as calculated from the above equation (1). These coating composition solutions Nos. I to III were all adjusted to a pH of 4.5.

< Coating composition solution No. IV>

15.7 g of N-(β-aminoethyl)aminopropyltrimethoxysilane (commercially available from Shin-Etsu Chemical Co., Ltd. under the trade name KBM-603), 50.0 g of γ-glycidoxypropyltrimethoxysilane (TSL-8350), 56.0 g of an isopropyl alcohol-dispersed colloidal silica solution (IPA-ST), 547.4 g of isopropyl alcohol and 4.3 g of purified water were mixed and this mixture was stirred at room temperature (25°C) for 7 hours. In order to promote hydrolysis and polycondensation, the resulting solution was allowed to stand at room temperature for 5 days and was used as a coating composition solution. The average number of hydrolyzable groups of this solution was 3.5. The silane compound TSL-8350 was used in an amount equal to that of its epoxy equivalent. The solid content of the solution at the time of mixing was 13.0 wt% based on its total weight. The reason why the formulation described above was used is that the workability period becomes extremely short when the solid content at the time of mixing is greater than 20 wt% and the amount of water added is greater than 30 wt% of the stoichiometric amount required for hydrolysis. When the solution is kept at room temperature after it is prepared, it remains colorless and clear for 10 days, but becomes increasingly cloudy thereafter. <

Coating Composition Solution No. V>

270.0 g of γ-mercaptopropyltrimethoxysilane (KBM-803), 162.0 g of γ-glycidoxypropyltrimethoxysilane (TSL-8350), 412.0 g of an isopropyl alcohol-dispersed colloidal silica solution (IPA-ST) and 222.5 g of 1/1000 N hydrochloric acid were mixed, and this mixture was stirred at room temperature (25°C) for 1 hour. Then, the resulting solution was stirred at 70°C for 3 hours to promote hydrolysis. After the solution was cooled to room temperature again, 3.0 g of magnesium perchlorate was dissolved therein to prepare a coating composition solution. The average number of hydrolyzable groups of this solution was 3.50 and its solid content at the time of mixing was 30% by weight based on its total weight.

< Coating composition solution No. VI>

70.0 g of γ-methacryloyloxypropyltrimethoxysilane (commercially available from Nippon Unicar Co., Ltd. under the trade name A-174), 100.0 g of an isopropyl alcohol-dispersed colloidal silica solution (IPA-ST) and 30.2 g of 1/1000 N hydrochloric acid were mixed. This mixture was stirred at 70°C for three hours to promote hydrolysis. After the resulting solution was cooled to room temperature again, 0.5 g of benzoin isopropyl ether and 0.5 g of methyl phenyl glyoxylate were added as photoinitiators to prepare a coating composition solution. The average number of hydrolyzable groups of this solution was 3.641.

Examples 1 to 18

In Examples 1 to 5, 7 to 15, 17 and 18, a resin plate having acidic groups (one of (A) to (U) in Table 1) was immersed in one of the coating composition solutions Nos. I to V and taken out at a speed of 50 cm/min. Then, the coated resin plate was cured in an oven at 105°C for 3 hours to obtain a coated molded article. In Examples 6 and 16, a resin plate having acidic groups ((D) or (Q) in Table 1) was immersed in the coating composition solution No. VI and taken out at a speed of 1 m/min. To evaporate the solvent from the coating film, the coated resin plate was allowed to stand in an oven at 80°C for 5 minutes. Then, the coated resin plate was irradiated with ultraviolet light having a wavelength of 365 nm with an energy of 3000 mJ/cm2 using an ultraviolet light irradiation device (UV-5003) to obtain a coated molded article. The results of evaluation of the coated molded articles thus obtained are shown in Table 3.

Comparison example 1

According to the formulation for the coating composition solution VII shown in Table 4, various components (i.e., IPA-ST, GPTMSi, TEOSi, IPA, and 1/1000 N HCl) were mixed. This mixture was heated to 70°C and stirred at this temperature for 7 hours. After the resulting solution was cooled to room temperature, magnesium perchlorate (in the amount shown in Table 4) as a catalyst for ring opening of epoxy groups was added thereto and dissolved therein to prepare a coating composition solution. A resin plate ((C) in Table 1) was immersed in this coating composition solution and taken out at a speed of 1 m/min. The coated resin plate was thermally cured in an oven at a temperature of 105°C for 3 hours. However, after the heat treatment, it was found that the coating layer had cracks.

Comparison example 2

According to the formulation for the coating composition solution VIII shown in Table 4, various ingredients (i.e., IPA-ST, GPDMSi, IPA and 1/1000 N HCl) were mixed. This mixture was heated to 70°C and stirred at this temperature for 7 hours. After the resulting solution was cooled to room temperature, magnesium perchlorate (in the form given in Table 4) was added thereto. A resin plate ((C) in Table 1) was immersed in this coating composition solution and taken out at a speed of 1 m/min. The coated resin plate was thermally cured in an oven at a temperature of 105°C for 3 hours. The resulting coating layer was not even due to poor leveling properties and was found to lack abrasion resistance in that the coating layer was easily damaged by pressing with a #0000 steel wool.

Comparative examples 3 to 7

The results of evaluation of various molded articles outside the scope of the invention are shown in Table 5. In Comparative Example 3, a resin plate ((A) in Table 1) having an insufficient number of acidic groups was coated with the coating composition solution No. III. In Comparative Example 4, a polymethyl methacrylate (PMMA) plate (commercially available from Mitsubishi Rayon Co., Ltd. under the trade name Acrylate L) was directly coated with the coating composition solution No. III. In Comparative Examples 5 to 7, the results of abrasion tests on commercially available surface-cured plates and a glass plate are given. Acrylite MR is a surface-cured acrylic resin plate manufactured by Mitsubishi Rayon Co., Ltd. and Dialite SH is a surface-cured polycarbonate plate manufactured by Mitsubishi Rayon Co., Ltd. The glass plate is a commercial product (sold under the trade name BK-7) with a thickness of 2 mm. Table 1 (Substrates (A) to (U) with acidic groups on them) Table 2 (Coating composition solution No. I-III)

GPTMSi: γ-glycidoxypropyltrimethoxysilane

TEOSi: Tetraethoxysilane

GPDMSi: γ-glycidoxypropylmethyldimethoxysilane

IPA: Isopropyl alcohol

Ageing conditions Average number of hydrolysable groups

70ºC, 7 hours 3.643

70ºC, 7 hours 3,610

70ºC, 7 hours 3,602 Table 3 (Assessment of the properties of the coated molded bodies)

Tt: total light transmittance

H: Turbidity

*: These designations correspond to those given in the column "Amount of acidic groups" in Table 1. Table 4 (Coating composition solution No. VII and VIII) Table 5 (Assessment of properties in comparative examples 3 to 7)

Disclosed are molded articles having a coating layer formed on the surface portion by curing a coating composition consisting essentially of a specific silica polycondensate, the surface portion being formed of a polymer derived from a multifunctional acrylic monomer and having 0.02 to 0.2 µm/cm² of acidic groups therein. The silica polycondensate is obtained by mixing (I) colloidal silica and (II) a specific silicon compound in such a molar ratio that the average number of hydrolyzable groups is in a range of 2.30 to 3.85, and then subjecting them to cohydrolysis and polycondensation. In these molded articles, the coating layer has excellent properties such as hardness and abrasion resistance, as well as good adhesion.

Claims (5)

1. Process for producing abrasion-resistant molded articles, which comprises the following steps: a) providing a shaped body with a surface formed from a polymer, the polymer having structural units derived from a multifunctional monomer containing two or more (meth)acryloyloxy groups in its molecule, b) irradiating the surface of the polymer with ultraviolet light having a wavelength of 300 nm or less, c) subjecting the irradiated surface to an alkali treatment, d) coating the irradiated and alkali-treated surface with a coating composition consisting essentially of a silicon dioxide polycondensate obtained by mixing (I) colloidal silicon dioxide and (II) at least one hydrolyzable silicon compound selected from the group consisting of compounds having the following formulas (A) to (F), SiR¹aR²b(OR³)c (A) SiR4dR5e(OR6)f (B) SiR7gR8h(OR9)i (C) SiR¹⁰jR¹¹k(OR¹²)l (D) SiR¹³mR¹⁴n(OR¹⁵)o (E) and SiR¹⁶pR¹⁷q(OR¹⁸)r (F) wherein R¹, R², R³, R⁵, R⁶, R⁻, R⁴, R⁹⁹, R¹², R¹⁴, R¹⁵, R¹⁵, R¹⁷ and R¹⁴ in formulas (A) to (F) are hydrocarbon radicals of 1 to 15 carbon atoms or hydrocarbon radicals of 1 to 15 carbon atoms having an ether linkage or an ester linkage; R⁵ in formula (B) is a carbon hydrogen radical of 2 to 15 carbon atoms having an epoxy group; R⁷ in formula (C) is a hydrocarbon radical of 1 to 15 carbon atoms having an amino group, R¹⁰ in formula (D) is a hydrocarbon radical of 1 to 15 carbon atoms having a mercapto group; R¹³ in formula (E) is a hydrocarbon radical of 2 to 15 carbon atoms having a vinyl group; R¹⁶ in formula (F) is a hydrocarbon radical of 3 to 15 carbon atoms having a (meth)acryloyloxy group; where a, b, d, e, g, h, j, k, m, n, p and q are integers from 0 to 3, c is (4-ab), f is (4-de), i is (4-gh), l is (4-jk), o is (4-mn) and r is (4-pq); in such a molar ratio that the average number of hydrolyzable groups calculated from the following equation (1) is within a range of 2.30 to 3.85, average number of hydrolysable groups wherein [A] to [F] are each the number of moles of the hydrolyzable silicon compounds of the formulas (A) to (F) occurring in the reaction mixture, [I] is the number of moles of the colloidal silicon dioxide occurring in the reaction mixture, and c, f, i, l, o and r are the same integers as defined above for the formulas (A) to (F); and then subjecting it to cohydrolysis and polycondensation, and e) Curing the coating composition 2. A process for producing abrasion-resistant molded articles according to claim 1, in which the polymer having structural units derived from a multifunctional monomer obtained by polymerizing a monomer composition containing 30% by weight or more of the multifunctional monomer. 3. Process for producing abrasion-resistant molded articles according to claim 1, in which the coating composition additionally contains an alkyl (meth)acrylate polymer. 4. A process for producing abrasion-resistant molded articles according to claim 1, in which the coating composition is cured by the application of heat. 5. Process for producing abrasion-resistant molded articles according to claim 1, in which the hydrolyzable silicon compound (II) comprises at least one compound having the general formula (E) or (F) and the coating composition is cured by exposure to actinic radiation.