CA1174798A - Coating composition and a method for producing a synthetic resin molded product having an abrasion resistant surface - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This invention relates to a coating composition which comprises a polyfunctional monomer having at least three acryloyloxy groups and/or methacryloyloxy groups in one molecule, a monomer havlng not more than two acryloyloxy groups and/or methacryloyloxy groups in one molecule and optionally a photo-sensitizer and which can form a crosslink-hardened film excellent in abrasion resistance upon curing by irradiation with actinic radiation in air and a method for producing a synthetic resin molded product having an abrasion resistant surface.

Description

l 17~798 BACKGROUND OF TlIE IN ~NTION
This application is a division of Canadian Serial No.

297,617, filed February 23, 1978.
Field of th:e Invention This invention relates to a coating composition capable of orming hard film having excellent abrasion resistance, sur-face smoothness, flexibility, water resistance, heat resistance, solvent resistance, durability and adhesiveness to substrates.
Description of the Prior ~rt Synthetic resin molded products produced from polymethyl methacrylate resin, polycarbonate resin, polyallyldiglycol carbonate resin, polystyrene resin, styrene-acrylonitrile copoly-mer resin (AS resin), polyvinyl chloride resin, acetate resin, acrylonitrile-butadiene-styrene copolymer resin (ABS resin), polyester resin, etc. have various advantages such as light in weight, excellent shock resistance, low cost, easy moldability, etc~ as compared with glass articles and have been developed in various fields of use such as optical uses, e.g., organic plate glasses, covers for lighting fitting, optical lens, spectacle lens, light reflector, mirror and the like, decorative uses, e.g., signs, displays and the like and automotive parts, e.g., name plates, dust cover cases and the like. However, the surfaces of these synthetic resin molded products have insufficient abrasion resistance and thus the surface may be damaged by con-tack and collision with other articles or scratched during transportation of the molded products, installation of parts or use of the products to cause decrease in yield of the products or to damage a beautiful appearance. Especially, when the molded products are used as optical lenses such as those of cameras, magnifying glasses, etc. spectacle lenses such as fashion glasses, sun glasses, lenses for correction of eyesight, etc., window glass, decoration cases, covers, clock lenses, light reflectors, mirrors and the like. The damages to the surface cause an extreme reduction in the conunercial value alld l'endOL'S
them unusable in a short period of time. Thus, a need for 1 ~747~8 improved abrasion resistance exists.
PreYiously, many proposals have been suggested to improve the abrasion resistance of the synthetic resin molded products.
For example, there is a method which comprises coating silicone material or melamine material onto the surface of synthetic resin molded products and hardening it with heat treatment, namely, forming the so-called thermosetting type crosslink-hardened film on the sur~ace of the synthetic resin molded pro-ducts. However, since these methods are.thermosetting type, not only is storage stability of the coating material unsatisfactory, r but formation of crosslink-hardened film requires heating at high temperatures for a long period of time. Therefore, pro-ductivity is low and, furthermore, since the hardenlng reaction proceeds gradually even after the crosslinking and hardening treatment, cracks occur in the crosslink-hardened film on the products or cracks occur at the interface between the film and substrate which reduce adhesiveness to the substrate and cause inferior water resistance and weather resistànce.
Another method comprises coating the surface of synthetic resin molded products with a polyfunctional acrylate or meth-acrylate monomer having at least 2 polymerizable ethylenically uns~turated groups in one molecule as the coating material and irradiating the coated monomer with actinic radiation to produce a crosslink-hardened film on the surface of the synthetic resin molded products by radical polymerization.
Since said pol,vfunctional (meth)acrylate (which means acrylate or methacrylate in this specification) monomers have excellent polymerization activity when irradiated with actinic radiation, there have been proposed as materials for quick-drying ink in U.S. Patents 3,661,614, 3,551,311 and 3,551,246 and British Patent l,198,259. Moreover, U.S. Patents 3,552,986,

2,413,973 and 3,770,490 propose application o.E these poly-~ ~74798 functional.(meth)acrylate monQmers as s.urface modifiers of synthetic resin molded products.
The present applicants have also found that polyfunctional (meth)acrylate monomers have excellent crosslink-hardening poly-merizability and are ef:Eective as materials Eor forminy cross-link-hardening Eilm capable of improving abrasion resistance of the surface oE synthetic resin-molded products and they have made a number of proposals (Japanese Patent Publication No.
42211/63, No. 12886/64, No. 22951/64, No. 14859/64 and No.
22952/64).
As compared with the former method which comprises form-ing a crosslink-hardened film by heat treatment of heat-harden-able coating material, the method which comprises coating these polyfunctional (meth)acrylate monomers as a crosslink-hardenable coating material on the surface of synthetic resin molded products and ixradiating with actinic radiation to form a cross-link-hardened film on the surface of the synthetic resin molded products has various merits. The storage stability of the coat-ing material is good, the crosslink-hardened film can be produced in a short time, in the order of minute or second at room temperature because polymerization crosslink-hardening is efeeted by irradiation with actinic radiation and/ thus, pro-: duetivity is excellent, efficiency and abrasion resistance are excellent, no change of the hardened film occurs with lapse of time, water resistance, weather resistance, initial adhesiveness to the substrate are excellent, etc.
On the other hand, it is known.that this method has thefollowing problems. First, that if formation of the crosslink-hardened film by irradiation with actinic radiation after coating of the eoating material on the surfaee of the syntlletic resin molded products is not carried out in an inert gas atmosphere such as nitrogen gas, carbon dioxide gas, a crosslink-hardened film having a sufficient abrasion resistance cannot be produced 1 ~7~79~

because the crosslink-hardening reaction is inhibited by the oxygen in the air. This is an extremely great problem in prac-tical use. Not only does the operation become complicated, but variability occurs in performance which reduces the yield and increases the cost because it is difficult to maintain a con-stantly low oxygen concentration in an atmosphere. The second is that the polyfunctional (meth)acrylate monomers generally have a high viscosity at room temperature, and those which are most effective in lmproving the abrasion resistance have a high viscosity which lowers their coating performance and limits the coating method of the coating material. The surface smoothness of the crosslink-hardened film is not adequate, the film thick-ness is not uniform, control of the film thickness is difficult and it is very difficult to form a thin crosslink-hardened film excellent in adhesiveness to the substrate, having good abrasion resistance, surface smoothness and uniEormity of film thickness.
Thus, synthetic resin molded products having a crosslink-hardened film on the surface obtained by coating polyfunctional (meth)acrylate monomers as a coating material on the surface ofsynthetic resin molded products still have many problems and need to be improved and have not yet been put to practical use.
A need continues to exist therefore, for a coating composition which improves the abrasion resistance of products prepared from synthetic resins.
BRIEF SU_RY OF THE INVENTION
An object of the present invention is to provide a coating composition for synthetic resins which will undergo cross-linking upon irradiation with actinic radiation in an oxygen containing atmosphere.
Another object of the present invention is to provide a method for providing products molded from synthetic resins 1 174~g8 with abrasion resistant surfaces.
These and other objects of the present invention which will become apparent have been attained through a coating com-position which comprises 100 parts by weight of a monomer mixture (A) comprising 30 - 98~ by weight of a polyfunctional monomer having at least three groups selected from acryloyloxy groups and methacryloyloxy group in one molecule and 70 - 2% by weight of at least one monomer selected from the group consisting of methoxyethyl(meth)acrylate, ethoxyethyl(meth)acrylate, butoxy-ethyl(meth)acrylate, methoxydiethylene glycol(meth)acrylate,dipropylene glycol mono(meth)acrylate, and ethyl carbitol(meth)-acrylate, and 0.01 - 10 parts by weight of a photosensitizer per 100 parts by weight of monomer mixture (A) and which can form a crosslink-hardened film excellent in abrasion resistance by irradiation with actinic radiation in an air atmosphere.
An abrasion resistant surface can be formed on synthetic resin molded products by coating said composition on the surface of the synthetic resin molded products and irradiating with actinic radiation to form a crosslink-hardened film having a thickness of 1 - 30~(.
The invention also contemplates a method for producing an abrasion resistant synthetic resin molded product which comprises coating on the surface of a synthetic resin molded product a coating composition which comprises 100 parts by weight of a monomer mixture (A) comprising 30 - 98~ by weight of a polyfunctional monomer having at least three groups selected from acryloyloxy group and methacryloyloxy group in one molecule and 70 - 2~ by weight of at least one monomer selected from the group consisting of methoxyethyl(me-th)acrylate, ethoxyethyl-(meth)acrylate, butoxyethyl(meth)acrylate, methoxydiethyleneglycol(meth)acrylate and dipropylene glycol mono(meth)acrylate, and ethyl carbitol(meth)acrylate, and 0.01 - 10 parts by weight of a photosensitizer per 100 parts by weight ~ ~7479~
of monomer mixture (A), and which can form a crosslink-hardened film excellent in abrasion resistance by irradiation with actinic radiation in an air atmosphere, and then irradiating the coating film with actinic radiation to form a crosslink-hardened film of 1 - 30~ thick on the surface of the synthetic resin molded product.

In both the composition and method above, the poly-functional monomer is at least one polyfunctional (,meth)acrylate selected from the group consisting of poly(meth)acrylates of mono or polypentaerythritol having at least three methacryloy-loxy groups and/or acryloyloxy groups in one molecule wherein the mono or polypentaerythritol has the formula:

X112/ 122 \ / IXn Xll - CH2 ~ f- ~2' o - CH2 - IC ~ CH2 O~ _ o - CH2 ~ F- CH2 X14 IH2112 ~H2 X13~ X23/ \ Xn3 (,wherein at least three of Xll, X12, X13, X22, X23, Xn2' Xn3 and X14 are CH2 = CR - COO - group and the remainder are -OH group, n is an integer of 1 - 5 and R is hydrogen atom or methyl group),.

X

~ 1~4~98 DESCRIPTION OF T~IE PRE~ERRED EMBODIME~TS
The coating composition of this invention primarily com-posed of the (meth)acrylate monomer mixture which is the most important component, and comprises 100 parts by weight of a mono-mer mixture (A) comprising 30 - 98% by weight of at least one polyfunctional monomer selected from the group of compounds having at least three acryloyloxy groups and/or methacryloyloxy groups in one molecule and 70 - ~% by weight of a monomer having not more than two acryloyloxy and/or methacryloyloxy groups in one molecule and 0 - 10 parts by weight of a photosensitizer.
This coating composition can produce a transparent crosslink-hardened film excellent in abrasion resistance surface smooth-ness, flexibility, water resistance, heat resistance, solvent resistance and adhesiveness to synthetic resin substrate even when it is irradiated with actinic radiation in an oxygen-con-taining atmosphere such as air.
The polyfunctional monomer having at least three acry-loyloxy groups and/or methacryloyloxy groups in one molecule is essential for imparting a high abrasion resistance to the cross-link-hardened fllm formed by irradiation with actinic radiation and it is necessary that the proportion of said polyfunctional monomer in the monomer mixture (~) is within the range of 30 - 98% by weight, preferably 40 - 96% by weight. When the proportion is less than 30% by weight, a crosslink-hardened film having sufficient abrasion resistance cannot be obtained and when more than 98% by weight, the abrasion resistance is satisfactory, but flexibility and adhesiveness to the substrate decreases and causes undesired phenomena such as the formation of cracks and peeling of the film. These tri-or more functional monomers rnay be used alone or in an admixture of two or more other polyfunctional monomers having at least three functional groups with the range as mentioned above.

~ 17aS798 The monomers having not more than two acryloyloxy and/or methacryloyloxy ~roups in one molecule are necessary to increase the adhesiveness of the crosslink-hardened film to the substrate, to impart flexibility to the film and to increase its durability.
Said monomers are present in an amount of 70 - 2% by weight, preferably 60 - 4% by weight in the monomer mixtùre (A). When the content is more than 70% by weight, the abrasion resistance of the hardened film is inferior and when less than 2% by weight, the film has inferior flexibility and cracks occur in the harden-ed film when de~orming strain is applied to the substrate to decrease adhesiveness to the substrate. This monomer may also be used alone or in admixture of two or more other monomers within the range mentioned above.
The polyfunctional monomers used in this invention are those which can be hardened in air and preferably are at least oneipolyfunctional (meth)acrylates which are represented by the following general formula:

X112 / ~22 \ ~ Xn2 CH2 / fH2 / CIH2 Xll-CH2~ C ~ CH2 CH2 - C - CH~ C~l2 ~ I ~ C ¦ X14 (wherein at least three of Xll, Xl2, X13, X22' 23' 2 Xn3 and X14 are CH2 = CR - COO - group and the remainder are -OH group, n is an integer of 1 - 5 and R is hydrogen atom or methyl group) and which are selected from the group consisting of poly(meth)acrylates of polypentaerythritol which have at least three methacryloyloxy and/or acryloyloxy groups in one molecule.
Examples of polyfunctional monomers of tri or l~ her functional represented by the above general formula are pen-taerythritol tri(meth)acryla-te, pentaerythritol tetra(meth)acry-late, dipentaerythritol tri(meth)acrylate, dipentaerythritol ~--` 117~7g~

tetra(meth)acrylate, dipentaeLythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, e-tc.
Polyfunctional ~onomers such as trimethylolpropane tri-(meth)acrylate, trimethylolethane tri(meth)acrylate, pentaglycerol tri(meth)acrylate, although these are similar polyfunctional acrylate monomers, are inferior in air-hardenability with actinic radiation and so it is difficult to produce crosslink-hardened film excellent in abrasion resistance, in an air atmosphere.
The monomers, having not more than two (meth)acryloyloxy groups in one molecule that is, contain 1 or 2 (meth)acryloyloxy groups which are used in combination with said polyfunc-tional monomers, are necessary to impart flexibility to the crosslink-hardened film, increase durability and further increase adhesive-ness to the substrate without reducing abrasion resistance of the film and examples thereof are as follows:
; That is, examples of monomers having two (meth)acryloy-loxy groups in one molecule are ethylene glycol di(meth)acrylate, 1,3-propylene glycol di(meth)acrylate, 1,4-butanediol. di(meth)-acrylate, 1,6-hexanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate~ tetraethy-lene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 2,2 bis-(4-acryloxyethoxyphenyl) propane, 2,2 bis-(4-methacrloxy-phenyl) propane, 2,2 bis-(4-acryloxyethoxyphenyl) propane, 2,2 bis-(4-methacryloxyethoxyphenyl) propane, 2,2 bis-(4-acryloxydiethoxy-phenyl) propane, 2,2 bis-(4-methacryloxydiethoxyphenyl) propane, 2,2 bis-(4-acryloxypropoxyphenyl) propane, 2,2 bis-(4-methacry-.
loxypropoxyphenyl) propane, 2,2 bis-(4-acryloxy-(2-hydroxypropoxy) phenyl) propane, 2,2 bis-(4-methacryloxy-(2-hydroxypropoxy) phenyl) propane, 2,2 bis-(4-acryloxy-(2-hydroxypropoxycthoxy)phcnyl) propane, 2,2 bis-(4-methacryloxy-(2-hydroxypropoxyethoxy)phenyl) propane, etc.

Examples of monofunctional monomers having one (meth)-acryloyloxy groups in one molecule are me-thyl (meth)acrylate, _g_ ethyl (meth)acrylate, propyl ("meth.)acrylate, butyl ~meth)acrylate, isobutyl (,meth)ac:rylate, t-butyl ('meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl'(meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, methoxyethyl (meth)acrylate, ethoxy-ethyl (meth)acrylate, butoxyethyl (,meth)acrylate, 2-hydroxyethyl (meth)acryla-te, 2-hydroxypropyl (meth)acrylate, 2-hydro~y-3-chloropropyl (meth)acrylate, glycidyl (meth)acrylate, 2-chloro-ethyl (meth) acrylate, ethylcarbitol (meth) acrylate, tetra-hydro~urfuryl (meth)acrylate, benzyl (meth)acrylate, 1,4-buty-lene glycol mono(meth)acrylate, etc.
Among the monomers having not more than two (meth)acry-loyloxy groups in one molecule as enumerated above, the following monomers are especially preferred because even when these monomers are irradiated with actinic radiation in air, they are superior to other monomers in polymerization activity. That is, one group : of these monomers represen-ted by the general formula:

1 ~ CH3 2C C C - (Xl - OX2..... 0Xn) - O~ C

Rl CH3 .Y2o YlO)- C - C = CH2 O' (wherein Rl is hydrogen atom or methyl, Xl, X2..... , Xn and Yl, Y2..... , Ym are same or different alkylene groups of not more than six carbon atoms or those in which one hydrogen atom is substituted with hydroxyl group, n and m are integers o~ 0 - 5), another group of these monomers have a boiling point of 150C or higher at normal pressure and a viscosity of not more than 20 centipoises at 20C and in which the group bonded to the (meth)-acryloyloxy group has ether bond or hydroxyl group, e.g., 1 17~798 methoxydiethylene glycol (meth)acrylate, ethoxyethyl (meth)-acrylate, butoxyethyl (meth)acrylate, dipropylene glycol mono-(meth)acrylate, glycidyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 2-hydroxyethyl (meth)acrylate ! 2-hydroxypropyl (meth)acrylate, 1,4-butylene glycol mono(meth)acrylate, ethyl-carbitol (meth)acrylate, diethylene ~lycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraetllylelle ~lycol di(meth)acrylate and dipropylene glycol di(meth)acrylate.
Therefore, the coating compositions, which use the monomer mixtures of said preferred monomers and said poly-functional monomers which have at least three (meth)acryloyloxy groups in one molecule and which are capable of polymerizing upon irradiation with actinic radiation in an air atmosphere, have such good characteristics that not only are the compositions excellent in coating workability, uniform film forming ability and storage stability, but also they can form transparent crosslink-hardened films excellent in abrasion resistance, surface smoothness, flexibility, water resistance, heat resis-tance, solvent resistance, durability and adhesiveness to the substrate by irradiation with actinic radiation even in an air atmosphere.
The above are essential components of the monomer mixture (A) which is one component of the coating composition of the present invention. However, if necessary, at least one other vinyl monomer which is copolymerizable with said monomer mixture and has a polymerization activity upon irradiation by actinic radiation may be added in an amount of up to 50% by weight to impart antistatic properties, fog resistance and other functions to the crosslink-hardened film. Examples of these monomers are quaternary aml~ni~m salts of (meth)acrylic esters, mono(meth)acrylic ester of polyethylene glycol, (meth)-acrylic esters of phosphoric acid, dimethylaminoethyl methacry-late, etc.

~ 17479~
The coating composition of this invention may be used in admixture with organic solvents having specific characteris-tics, if necessary. The organic solvents used in combination not only have good eEfects on coating operability at coating of the coating composition on the surface of the synthetic resin molded products, uniform fllm forming ability and storage sta-bility, but also have an action of increasing adhesiveness of the crosslink-hardened film to the substrate. For example, they have a great effect on the adhesiveness of the hardened film to the substrate and durability of the film when synthetic resin molded products improved in their abrasion resistance of the surface by forming crosslink-hardened film are subjected to severe repeated tests comprising dipping in warm water -~ dipping in cold water -~ drying at high temperature. This is a surprising fact and reason for such effect is not clear. However, it is presumed that one of the reasons is a subtle interaction of the organic solvents on the substrate and the polyfunctional monomers, formation of uniform crosslink-hardened film very excellent in surface smoothness or synergistic action oE them.
Conventionally, in the method for produclng a crosslink-hardened film by coating polyfunctional (meth)acrylate monomers on the surface of the synthetic resin molded products and the like and irradiating the coating~Ji~ actinic radiation causes a very rapid crosslink-hardening polymerization reaction, thus, use of organic solvents with the polyfunctional (meth)acrylate monomers may result in some of the organic solvent remaining in the crosslink-hardened film to damage surface smoothness of the hardened film and so such method has been studied without the use of organic solvents. As the result of detailed research in an attempt to make good use of the merits in coating wor]cability imparted by the use of organic solvents, it has been found that the organic solvents can be used only when they meet the foll~ing 1 17479~

re~uirements and furthermore they have unexpected effects on the adhesiveness and durability of the crosslink-hardened film as mentioned above. That is, the organic solvents which can be used in admixture with the coating composition of this invention must meet the following two conditions.
1. They can form a homogeneous solution in admixture with the monomer mixture (A).
2. They have a boiling point of 50 - 200C under normal pressure.
The first condition that they form a homogeneous solution in admixture with the monomer mixture (A) is -the natural and the most important condition and, for example, the organic solvents of saturated hydro-carbons such as n-hexane, n-heptane, cyclo-hexane cannot be used because they do not produce homogeneous solution. The second condition that they have a boiling point of 50 - 200C at normal pressure is the requirement necessary for forming crosslink-hardened film excellent in uniform film forming ability and surface smoothness when coated on the surface of synthetic resin molded products. When the boiling point under normal pressure is lower than 50C, the surface of the substrate is cooled by the latent heat of the organic solvents volatilized from the film after coating the coating composition which con-denses water in the air which damages surface smoothness of the film. When the boiling point is greater than 200 C, since volatilization of the organic solvents from the film is very slow there is the problem in workability and since volatilization of the remaining organic solvents at the time irradiation with actinic radiation to form the crosslink-hardened film by poly-merization is not balanced, uniformity and surface smoothness of the crosslink-hardened film is lost or the organic solvents re-main in the crosslink-hardened film which whitens the film.

Therefore, it is necessary that the organic solvents used have a boiling point of 50 - 200C, preferably 60 - 150 C
under normal (i.e., atmospheric) pressure.
The amount of organic solvent mixed with the coating composition varies depending on the purpose of use, but is preferably within the range of 95 - 10 parts by weight per 5 - 90 parts by weight of the monomer mixture (A) (totally 100 parts by weight). When less than 10 parts by weight, the effect is small and when more than 95 parts by weight, controlling of thickness of the crosslink-hardened film becomes difficult, or abrasion resistance becomes inferior.
The organic solvents which can be used here must meet said conditions and examples thereof are alcohols such as ethanol, isopropanol, normal propanol, isobutyl alcohol, normal butyl alcohol, etc., aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene, etc., ketones such as acetone, methyl-ethylketone, etc., ethers such as dioxane, etc., acid esters such as ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl ~ propionate, etc. These organic solvents may be used alone or as a mixed solvents of 2 or more if the boiling point and amount of the mixed solvent are within the ranges satisfying said re-quirements.
Furthermore, polymerizable monomers such as methyl acrylate, ethyl acrylate, methyl methacrylate, styrene, etc.
may also be used as the organic solvent if a specific purpose is to be attained and they meet the same conditions as for the organic solvents and have the same effects.
Some of these organic solvents may fog the substrate which should be transparent, dissolve out dyes or pigments of colored substrate to cause discoloration or easily produce cracks in the substrate itself. Therefore, it is necessary to select the organic solvents depending upon the kind of substrate on 1 ~74798 - which a crosslink-hardened film is formed and its u6e.
In order to form a crosslink-hardened film by coating the coa-ting composition or the coating composition blended with organic solvents on the surface of synthetic resin molded pro-ducts it is necessary to irradiate the coated composition with actinic radiation such as ultraviolet rays, electron ray, radiant ray, ~tc., among which irradiation with ultraviolet rays is the most preferred, practical, crosslink-hardening method.
When ultraviolet rays are used as the actinic radiation for crosslink-hardening of the coating film, it is necessary to add a photosensitizer capable of initiating the polymerization reac-tion of said coating co~position by irradiation with ultraviolet radiation. Specific examples of the photosensitizer are carbonyl compounds such as benzoin, benzoinmethyl ether, benzoinethyl ether, benzoinisobutyl ether, benzoinisopropyl ether, acetoin, butyrion, toluoin, benzil, benzophenone, p-chlorobenzophenone, p-methoxybenzophenone, etc., sulfur compounds such as tetra-methylthiuram monosulfide, tetramethylthiruam disulfide, etc., azo compounds such as azobisisobutyronitrile, azobis-2, 4-dimethylvaleronitrile, etc., peroxide compounds such as benzoyl peroxide, di-tertiary b~tyl peroxide, etc. These photosensitizers may be used alone or in admixture of 2 or more. The amount of the photosensitizers to the coating com-position is 0 - 10 parts by weight, preferably 0.01 - 10 parts by weight per 100 parts by weight of the monomer mixture (A) or sum of the monomer mixture (A) and organic solvent (B). Addition of too much photosensitizer causes coloration of the crosslink-hardened film or reduction of weather resistance.
Furthermore, if necessa~y, additives such as antistatic agents, surfactants, storage stabilizers, etc. may be suitably added to the coating composition used in this invention.

~ 1~4798 The synthetic resin molded products used in this in-vention include various synthetic resin molded products regardless of thermoplastic or thermosetting resins and specific examples are sheet, film, rod and various injection molded products produced from polymethyl methacrylate resin, polycarbonate resin, poly-allyldiglycol carbonate resin, polystyrene resin, acrylonitrile-styrene copolymer resin, polyvinyl chloride resin, acetate resin, ABS resin, polyester resin and the like.
Among these molded products, those produced from poly-methyl methacrylate resin, polycarbonate resin, polyallyldiglycol carbonate resin and the like are often used because of their characteristics such as optical properties, heat resistance, shock resistance, etc., and furthermore, demand for improving abrasion resistance is great, and so these molded products are especially preferred as those used in this invention.
The molded products used in this invention may be used as they are, but if necessary, they may be subjected to pretreat-ments such as washing, etching, corona discharging, irradiation with actinic radiation, dyeing, printing, etc.
As methods for coating said coating composition on these synthetic resin molded products, brush coating, curtain coating, spray coating, rotating coating, dip coating, etc.
are employed. Each of these methods has its merits and limita-tions and it is necessary to properly choose the coating method depending on the desired performance of the synthetic resin molded products and uses thereof. For example, when it is desired to impart abrasion resistance to only a part of tlle objective synthetic resin molded products, brush coating and curtain coating are suitable. When shape of the surface of the molded products is complicated spray coating is suitable, when the molded products are relatively flat and symmetrical rotating coating is suitable, and when the molded products are ln the form of rod or sheet dip coatiny is suitable.
The amount of the coating material coated on the sur-face of the synthetic resin molded products varies depending on use of the molded products produced or content of the monomers in the coating composition, but preEerably it is coated so that the thickness of the crosslink-hardened film formed on the surface of the synthetic resin molded products is within the range of 1 - 30~. When the thickness is less than 1~, abrasion resis-tance is inferior and when more than 30~, flexibility of the filmis damaged and cracks are liable to occur in the film whereby strength of the molded products themselves may be reduced.
As mentioned above, there are various methods ~or coating the coating composition, among which especially the dip coating method is simple and excellent in productivity and furthermore it can relatively freely change the thickness of the crosslink-hardened film although it might be restricted somewhat depending on the shape of the synthetic resin molded products. However, in order that the dip coating method can be applied and the merit of this method can be utilized, the coating material is required to meet the following various con-ditions.

1 ~7~798 That is, it must satisfy such conditions that viscosity of the coating material can be relatively freely controlled, the coating material is excellent in film-forming property by dip coating, viscosity of the coating material does not change with lapse of time and the coating material is excellent in storage stability.
The coating compositions of this invention, especially those in which an organic solvent is incorporated meet said requirements and have a good adaptability to the dip coating method which can form a transparent crosslink-hardened film excellent in abrasion resistance, surface smoothness, flexi-bility, durability, water resistance, heat resistance, solvent resistance and adhesiveness to the substrate.
Furthermore, for some uses of the synthetic resin molded products on which the crosslink-hardened film is formed, very excellent surface smoothness is required and furthermore it is required to stand severe conditions, e.g., bending working under suitable heating, machinlng such as cutting or perforating, appllcation of a great deformation strain at fitting of parts or -1 17~798 during use. In such case, it is naturally necessary that the crosslink-hardened film per se is excellent in charac-teristics such as flexibility and adhesiveness to the substrate resin, and as the second factor the thickness of the hardened film is an im-portant factor. That is, the thinner film has the higllcr clura-bility against these external actions, but too thin film has lower abrasion resistance. Therefore, thickness of the crosslink-hardened film is desirably 1 - 9~ when it is to be subjected to severe conditions.
As the conventional technical level according to which the conventional polyfunctional (meth)acrylate monomers or mix-tures thereof are used as crosslink-hardened film forming ma-terial, it has been impossible to form the thin crosslink-hardened film excellent in abrasion resistance, surface smoothness, uniformity of thickness, transparency and appearance on the surface of the synthetic resin molded products.
It has now become possible to form a thin transparent crosslink-hardened film of 1 - 9~ in thickness which is e~cellent in abrasion resistance, surface smoothness, uniformity of thick-ness, appearance and adhesiveness to the substrate by coating acoating composition of the present invention to which an organic solvent has been added, and which is prepared so as to have a viscosity of 10 centipoise or less at 25C, on the surface of synthetic resin molded products by the dip coating method and crosslink-hardening it. This is one of the important aspects of this invention.
Next, the coating composition is coated by a suitable method depending upon the shape of the synthetic resin molded products or desired properties and is crosslink-hardened by irradiation with actinic radiation. When the coating composition to which an organic solvent is added is used as a coating 7 g 8 material, preferably, the synthetic resin molded products coated with said coating composition is placed under specific conditions to volatilize and release more than 50% by weight of the organic solvent contained in the film coated on the surface of the syn-thetic resin molded products and thereafter this is irradiated with actinic radiation. When the coated film containing more than 50% by weight o the organic solvent is irradiated with actinic radiation, there may occur undesired phenomena such as loss of surface smoothness of the formed crosslink-hardened film, generation of bubbles in the ilm, retention of the organic sol-vent in the crosslink-coated film which causes whi-tening of the film, etc.
For formation of the crosslink-hardened ilm actinic radiation such as ultraviolet rays emitted rom light sources, e.g., xenon lamp, low pressure mercury lamp, medium pressure mer-cury lamp, high pressure mercury lamp or ultra-high pressure mercury lamp, and radiant rays, e.g., ~ rays, ~ rays, y rays and electron rays ordinarily taken out from electron accelerator of 20 - 2000 kv must he ;rradiated on the coatcd film to crosslin~-harden it.
From practical and working viewpoints, ultraviolet ra-diation is most preferred as the source for irradiation.
~ s the atmosphere in which actinic radiation is ir-radiated, inert atmospheres such as nitrogen gas, carbon dioxide gas, etc. or atmospheres having reduced oxygen concentration may, of course, be employed, but the coating composition of this in-vention can form a crosslink-hardened film excellent in character-istics such as abrasion resistance, etc. even in the usual air atmosphere. The temperature of the atmosphere may be warmed to such a degree to cause no deformation which is harmful for the substrate synthetic resin molded products.

1 17479~

The coating composition.of this invention, and the syn-thetic resin molded products having a crosslinked-hardened film produced using the coating composition, are excellent in surface smoothness and appearance and excellent in surface hardness, abrasion resistance and mar resistance. Furthermore, the crosslink-hardened film formed on the surface is a transparent and uniform filrn having flexibility, is excellent in adllesiveness to the substrate, and is not peeled off and does not form cracks even under severe conditions. Thus, this is very useful for organic window glasses, covers for lighting fixtures, light ; reflectors, mirrors, lenses for eyeglasses, lenses for sunglasses, optical lenses, lenses for watches, etc.
This invention will be explained.in more detail in the following Examples. Measurements and evaluations in the Examples were conducted as follows:
1. Abrasion reslstance (a) Surface hardness . . . Pencil hardness in ac-~ cordance : JISK 5651 - 1966 (b) Mar test . . . . . . . Mar test by steel wool of #000.
0 . . . . . . . . . . The surface lS hardly ~: marred by light rubbing . . . . . . . . . . The surface is slightly ~ marred by light rubbing X . . . . . . . . . . The surface is greatly marred by light rubbing (in the same degree as the substrate resin is marred.) ~ 17~798 2. Adhesiveness Peeling test of crosscu-t adhesive cellophane tape on the crosslink-hardened film. That is, eleven cutting lines which reach the substrate are cut at an interval of 1 mm in ver-tical and horizontal directions of the Eilm to produce 100 squares of 1 mm and an adhesive "CELLOPH~NE"~ tape is put thereon. Then, this tape is rapidly peeled off. This is repeated 3 times on the same portions.
0 . . . . . . . . . No squares are peeled off after repetition of 3 times.
. . . . . . . . . 1 - 50 squares are peeled off after repetition of 3 times.
X . . . . . . . . . 51 - 100 squares are peeled off after repetition of 3 times.

3. Flexibility (Maximum bending angle) A~crosslink-hardened film is formed on the surface of a sheetlike molded product of 2 - 3 mm thick, from which a test piece strip of 6 mm in width and 5 cm in length is cut.
Force is applied from both ends of this strip to give bending deforming strain thereto and the angle formed by the test piece ; to the horizontal plane when cracks are generated is measured.
This is "maximum bending angle" and with increase of this angle, the flexibility of the film is greater.

4. Thermal cycle test A molded product having a crosslink-hardened film on the surface is dipped in a warm watex of 65C for 1 hour, immediately thereafter dipped in an ice water of 0C for 10 min-utes and then hot-air dried at 80C for 1 hour. This is repeated several times and thereafter various tests are conducted.

`:~
117479~ ;

5. Smoothness of surface 0 . . . . . . . . . Smoothness of the surface of the film is very good and is considered to be a specular sur~ace . . . . . . . . . Uniformity of the surface of the film is good, but -there are slight irregularities and this surface cannot be consld-ered to be a specular surface.
X . . . . . . . . . There are clear irregularities on the surface and the surface lacks smoothness.
Having now fully described the invention, a more com-plete understanding can be obtained by reference to certain specific~examples, which are included for purposes of illustra-tion only and are not ïntended to be limiting unless otherwise specified.
Example 1 -The coating compositions as shown in Table 1 were prepared. They were coated on one surface of a methacrylic resin sheet of 2 mm thick (trade name: ACRYLITE manufactured by Mitsubishi Rayon Co., Ltd.) by a bar coater and irradiated with ultraviolet rays from a high pressure mercury arc lamp (H02-L21, 2KW manufactured by Iwasaki Electric Co., Ltd.) at a distance of 15 cm from the surface for 15 seconds in the atmos-pheres as shown in Table 1. The results are shown in Table 1.
As is clear from Table 1, the coating compositions of this in-vention exhibited good hardenability even in an air atmosphere.
On the other hand, the coating compositions other than those of this invention, e.g., experiment Nos. 5, 7 and 8 did not harden in an air atmosphere.

117~79~ ;
Example 2 The coating compositions as shown in Table 2 were pre-pared. In these compositions were dipped methacrylic resin sheets (manufactured by Mitsubishi Rayon K.IC.) and the slleets wcrc ta~cn up thererom at a speed of 0.5 cm/sec to form a film thereon.
These were left as they were for 10 minutes. Then, both surfaces of these sheets irradiated with ultraviolet rays from the same high pressure mercury arc lamp as used in Example 1 at a distance of 15 cm from each surface for 15 seconds in the atmospheres as shown in Table 2. The results obtained are shown in Table 2.
As is clear from Table 2, the coating compositions of this invention had good hardenability in an air atmosphere while the coating compositions other than those of this invention did not harden in an air atmosphere.
Example 3 The coating compositions as shown in Table 3 were pre-pared and methacrylic resin cast sheets 3 mm thick were dipped therein. Then, these plates were slowly taken up therefrom at a speed of 0.5 cm/sec to form a coating film of said composition on the surface of the sheets. Both coated surfaces of these sheets were irrzdiated with ultraviolet rays from a 2 KW high pressure mercury arc lamp at the distance of 15 cm from each coated sur-face for 15 seconds in an air atmosphere to form a transparent crosslink-hardened film. The results obtained are shown in Table 3.
As is clear from the results, the sheets obtained in accordance with the method of this invention (experiment Nos. 1-3) had well balanced properties. On the other hand, when the ranges of the components in the monomer mixture were outside those of this invention, the obtained films were inferior in hardness, flexibility and adhesiveness to the subs-trate.

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Example 4 ._ A coating composition comprising 25 parts by weight of dipentaerythritol pentaacrylate, 25 parts by weight of pentaery-thritol triacrylate, 50 parts by weight 2,2 bis-(4-acryloxyethoxy-phenyl) propane and 2 parts by weight of benzoinisobutyl ether was coated on the surface of a plastic artificial tooth produced by molding a dental methacrylic resin by a brush so that thickness of the film was 14-16 ~O
This was fixed to the rotating axis in the quartz tube fitted with a rotation driving body, then said rotating axis was rotated at a rate of 5 rpm and while passing air through the tube a ar infrared rays of 300 W was irradiated for 2 minutes and subse-quently a high pressure mercury lamp of 100 W was irradiated for 10 minutes obliquely and from above outside the quartz tube to form a crosslink-hardened film on the surface of the plastic artificial tooth. Thus obtalned molded product was excellent in the result of steel wool mar test and adhesiveness of the hardened film.
Example 5 360 parts by weight of dipentaerythritol pentaacrylate, 40 parts by weight of 2,2 bis-(4-acryloxydiethoxyphenyl) propane and 20 parts by weight o~ benzoinethyl ether were mixed with agitation at 60C to obtain a monomer mixture. This monomer mixture was mixed with an organic sol~ent comprising 340 parts by weight of isopropyl alcohol and 60 parts by weight of xylene at ratios as shown in Table 4 to obtain homogeneous coating com-positions.

A methacrylic resin cast molded plate 3 mm thick was dipped in each o these coating compositions and was slowly taken - 31 ~

~ 17~798 out at a rate of 0.5 cm/sec to for~ a coated film of said coating composition on the surface of the molded plates.
This was left to stand at toom temperature of 25C for 30 minutes and thereafter was fitted to a driven body which can be transferred into a high output power opposing ultraviolet rays irradiating box in which two high pressure mercury lamps of 2 KW
were opposed and air was passed. Then, the driven body was set so that the time of irradiation of ultraviolet rays was 15 sec-onds and the driven body was passed throùgh the irradiating box to form a crosslink-hardened film on the surface of the molded plate. Properties of thus obtained molded products are shown in Table 4.
As is clear from the above results, when the dip coating method was employed, the thickness of crosslink-hardened film could relatively easily be controlled by adjusting viscosity of the coating composition and surface smoothness and uniformity of the film were excellent.

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z ~' '' - 3 4 1 1~4798 Example 6 40 parts by weight of dipentaerythritol pentaacrylate, 40 parts by weight of pentaerythritol tetraacrylate, 12 parts by weight of pentaerythritol triacrylate, a parts by weight of 2,2 bis-(4-acryloxypropoxyphenyl) propane, 4 parts by weight of benzoinethyl ether and 300 parts by weight of the organic sol-vents shown in Table 5 were mixed to obtain homogeneous coating compositions. ~ methacrylic resin cas-t molded pla-te of 2 mm thick was dipped in each of these compositions and then was slowly withdrawn therefrom to form coated film on the surface of the molded plate. This was left to stand for 10 minutes in a box in which a warm air of 40C was passed and then was irradiated with ultraviolet ray using the same high output power opposing ultraviolet rays irradiating apparatus as used in Example 5 for 15 seconds to form a crosslink-hardened ilm on the surface of the;molded product.
Various properties of -the obtained molded products were measured and the results are shown in Table 5.
As is clear from the results of Table 5, when organic solvents other than those of this invention are used, formation of coated film, adhesiveness of hardened film and appearance of the hardened film are inferior.
Example 7 10 parts by weight of dipentaerythritol hexacrylate, 29 parts by weight of dipentaerylthritol Fentaacrylate, 10 parts by weight of pentaerythritol tetramethacrylate, 10 parts by weight of 2,2 bis-(4-methacryloxyethoxyphenyl) propane, 40 parts by weight of isopropyl alcohol and 10 parts 1 174~9~1 n ~? ~' n ~ e O ~ rt _~
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~ 1~479~

by weight of toluene were mixed to produce a homogeneous solution.
A sensitizer mixture of 0.4 part by weight of benzoinisobutyl ether, 0.4 part by weight of benzoinethyl ether and 1.2 parts by weight of benzophenone as a photosensitizer was dissolved in said solution to obtain a coating composition. This was s~ray coated on the ou-ter surface of a commercial methacrylic resin injection molded product of 3 mm in thickness, 6 cm in diameter and 5 cm in height. This was left to stand at room temperature of 25C for 30 minutes. Then, this was exposed to a light source from a high pressure mercury lamp of 2 KW at a distance of about 30 cm from the coated surface for 20 seconds in an air atmosphere to form a crosslink-hardened film of 11 ~ in an average thickness on the outer surface of said molded product.
Abrasion resistance of the outer surface of thus obtain-ed molded product was 8H in pencil hardness and this was also excellent in steel wool mar test. Furthermore, adhesiveness of the hardened film was such that no peeling occurred at crosscut adhesive cellophane tape test and no change was seen even after five thermal cycle tests.
Example 8 In a coating composition comprising 20 parts by weight of dipentaerythritol pentaacrylate, 10 parts by weight of 2,2 bis-(4-acryloxyethoxyphenyl) propane, 10 parts by weight of 2,2 bis-4-(2 hydroxy-3- acryloxypropoxy phenyl) propane, 55 parts by weight of n-butyl alcohol, 15 parts by weight of toluene and 1.5 parts by weight of benzoinisobutyl ether was dipped a polycarbon-ate plate-like molded product of 2 mm thick to form a coated film.
This was irradiated with light from a high pressure mercury lamp in the same manner as in Example 5 to form a crosslink-hardened film on the surface of the molded product.
The surface of thus obtained molded product was very excellent I 17~79~

in smoothness and thicknes~ of the hardened film was 5.0 ~.
Pencil hardness of the surface was 6H, adhesiveness of the film was such that no peeling occurred at the crosscut adhesive cel-lophane tape test and both the abrhsion resistance and adhesive-ness of the film did not change even after five thermal cycle tests.
Example 9 In a coating composition comprising 10 parts by weight of dipentaerythritol tetraacrylate, 10 parts by weight of dipenta-erylthritol triacrylate, 4 parts by weight of 2,2 bis-(4-acryloxy-ethoxyphenyl) propane, 1 part by weight of 2,2 bis-(4-acryloxy-phenyl) propane, 55 parts by weight of n-butyl alcohol, 15 parts by weight of xylene and 1.5 parts by weight of benzoinisobutyl ether was dipped a polyallyldiglycol carbonate lense (a lense manufactured by CR-39) and then was slowly withdrawn to form a coatiny film on the surface of said lense.
This WhS put in a quartz tube through which a hot air of 40 C was passed and kept therein for 3 minutes. Thereafter, both surfaces of this lense kept at this state were irradiated with light of a high pressure mercury lamp of lOOW at a distance of 20 cm from the surface of the molded product for 10 minutes.
In this case, for the first 2 minutes, far infrared rays were irradiated on both surfaces of the lense obliquely and from - above outside the quartz ~ube together with ultraviolet ray.
The surface of thus obtained molded product was very excellent in smoothness and thickness of the hardened film was 5.2 ~.
Furthermore, pencil hardness of the surface was 3H, the surface was excellent in the steel wool mar test and adhesiveness of the hardened film was such that no peeling occurred in the crosscut cellophane adhesive tape test.

`-` `` 1 ~747~8 Example 10 12 parts by weight of pentaerythritol tetraacrylate, 13 parts by weight of pentaerythritol triacrylate, 13 parts by weight of 2,2 bis-(4-acryloxydiethoxyphenyl) propane, 2 parts by weight of 1,4 butanediol diacrylate, 2 parts by weight of benzoinisobutyl ether, 50 parts by weight of isopropyl alcohol and 15 parts by weight of toluene were mixed to prepare a homo-geneous coating material.
A methacrylic resin molded product was produced having a crosslink~hardened film of 5.2 ~ thick in the same manner as in Example 5.
This molded product had a very excellent surface smooth-ness and had a pencil hardness of 7H. No peeling of the film occurred at crosscut adhesive cellophane tape test and adhesive-ness of the film was good.
Example 11 5 parts by weight of dipentaerythritol pentaacrylate, 10 parts by weight of pentaerythritol tetraacrylate, 10 parts by weight of pentaerythritol triacrylate, 5 parts by weight of 2,2 bis-(4-methacryloxypropoxyphenyl) propane, 2 parts by weight of benzoinisobutyl ether and 70 parts by weight of a mi~ed sol-vent comprising methyl methacrylate/isopropyl alcohol/toluene =
40/40/20% by weight were mixed to obtain a homogeneous coating composition. A methacrylic resin cast molded plate of 4 mm thick was dipped in said coating composition and was slowly withdrawn therefrom to obtain a coating film on the surface of the molded product.
This was subjected to the hardening treatment in the same manner as in Example 5 to obtain a molded product having a crosslink-hardened film of 5.5 ~ thick.

`` 1 ~7~79~

Surface appearance of said, molded product was very good. This product had a pencil hardness of 8H and was excel-lent in both steel wool mar test and adhesiveness of the film.
No changes occurred in abrasion resistance and adhesiveness.
Example 12 32 parts of dipentaerythritol pentaacrylate, 8 parts of 2-hydro~ypropyl acrylate, 60 parts of isopropyl alcohol and 2 parts of benzoinisopropyl ether were mixed to obtain a homo-geneous coating composition.
A polycarbonate sheet of 3 mm thick was dipped in said coating composition and taken up therefrom at a speed of 0.6 cm/
sec to form a film thereon. Both surfaces of this sheet were irradiated with ultraviolet rays from a 2 KW high pressure mercury arc lamp at a distance of 15 cm from each surface for 11 seconds in an air atmosphere~ As the result, the film was completely hardened and this hardened film was transparent film having a good surface smoothness.
The thickness of the film was 4 ~ and the surface hard-ness of 6H in pencil hardnessO Adhesiveness of the film to the substrate was such that no peeling of the film was caused after the crosscut cellophane tape test was repeated three times on the same portion. ~oreover, no change was seen after the thermal cycle test was effected five times.
Example 13 A coating composition comprising 30 parts of dipenta-erythritol pentaacrylate, 10 parts of tetrahydrofurfuryl acrylate, 60 parts of isopropyl alcohol, 10 parts of toluene and 2 parts of benzoinisobutyl ether was prepared.
A polymethyl methacrylate sheet of 2 mm thick was dipped in said composition and was taken therefrom at a speed of 0.45 cm/sec to form a film. Both surfaces of said sheet were irradi-ated with ultraviolet rays from a 2 KW high pressure mercury arc lamp at a distance of 15 cm from each surface for 11 seconds in 747sa an air atmosphere. ~s the result! the coating film was completelyhardened to obtain a transparent film having a good surface smooth-ness.
The thickness of the film was 3 ~, the surface hardness was 8H in pencil hardness and the adhesiveness to the substra-te was such that no peeling of the film was caused by crosscut cel-lophane tape tesk.
Example 14 A coating composition comprising 20 parts of dipenta-erythritol pentaacrylate, 8 parts of dipentaerythritol tetra-acrylate, 6 parts of 2,2 bis-(4-acryloxydiethoxyphenyl) propane,

6 parts of 2-hydroxyethyl acrylate, 60 parts of isopropyl alcohol, 20 parts of toluene and 3 parts of benzoinisopropyl ether was prepared. A polymethyl methacrylate sheet of 3 mm thick was dipped in said composition and taken up therefrom at a speed of 0.45 cm/sec to form a Eilm thereon. Both surfaces of this sheet were irradiated with ultraviolet rays from a 2 liW high pressure mercury arc lamp at a distance of 15 cm from each surface for 10 seconds in an air atmosphere. As the result, the film was completely hardened to obtain a transparent film having a good surface smoothness.
The thickness of the ~ilm was 4 ~, the surface hardness was 7H in pencil hardness and the adhesiveness to the substrate was such that no peeling of the Eilm was caused by crosscut cellophane tape test. Moreover, no change was seen after thermal cycle.
~ aving now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.

Claims (23)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A coating composition which comprises 100 parts by weight of a monomer mixture (A) comprising 30 - 98% by weight of a polyfunctional monomer having at least three groups selected from the group consisting of acryloyloxy groups and methacryloyloxy groups in one molecule and 70 - 2% by weight of at least one monomer selected from the group consisting of methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylatce, butoxyethyl (meth)acrylatel methoxydiethylene glycol (meth) acrylate, dipropylene glycol mono(meth)acrylate, and ethyl carbitol(meth)acrylate, and 0.01 - 10 parts by weight of a photosensitizer per 100 parts by weight of monomer mixture (A) and which can form a cross-linked hardened film excellent in abrasion resistance upon irradiation with actinic radiation in an air atmosphere.

2. The coating composition of Claim 1, wherein the polyfunctional monomer is at least one polyfunctional (meth)acrylate selected from the group consisting of poly (meth)acrylates of mono or polypentaerythritol having at least three methacryloyloxy groups and/or acryloyloxy groups in one molecule wherein said mono or polypentaerythritol has the formula:

(wherein at least three of X11, X12, X13, X22, X23,. . . .
Xn2, Xn3 and X14 are CH2 = CR - COO group and the remainder are -OH group, n is an integer of 1 - 5 and R is hydrogen atom or methyl group).

3. The coating composition of Claim 2, wherein the polyfunctional monomer is selected from the group consisting of pentaerythritol tri and tetra(meth)acrylate.

4. The coating composition of Claim 2, wherein the polyfunctional monomer is selected from the group consisting of dipentaerythritol tri, tetra, penta and hexa(meth)acrylate.

5. The coating composition of Claim 1, which comprises 5 - 90 parts by weight of said monomer mixture, said mixture comprising 30 - 98% by weight of a polyfunctional monomer having at least three acryloyloxy groups and/or methacryloyloxy groups in one molecule and 70 - 2% by weight of monomer having not more than two acryloyloxy groups and/or methacryloyloxy groups in one molecule, and 95 - 10 parts by weight of at least one organic solvent which is mixed with said monomer mixture to form a homogeneous solution.

6. The coating composition of Claim 5, wherein the polyfunctional monomer is at least one polyfunctional (meth) acrylate selected from the group consisting of poly(meth) acrylates of mono or polypentaerythritol having at least three methacryloyloxy groups and/or acryloyloxy groups in one molecule wherein said mono or polypentaerythritol has the formula:

(wherein at least three of X11, X12, X13, X22, X23, . . . .
Xn2, Xn3 and X14 are CH2 = CR - COO - group and the remainder are -OH group, n is an integer of 1 - 5 and R is hydrogen atom or methyl aroup).

7. The coating composition of Claim 6, wherein the polyfunctional monomer is selected from the group consisting of pentaerythritol tri and tetra(meth)acrylate.

8. The coating composition of Claim 6, wherein the polyfunctional monomer is selected from the group consisting of dipentaerythritol tri, tetra, penta and hexa(meth) acrylate.

9. The coating composition of Claim 6, wherein the organic solvent has a boiling point of 50°C - 200°C.

10. The coating composition of Claim 6, which has a viscosity of not more than 10 centipoises at 25°C.

11. A method for producing an abrasion resistant synthetic resin molded product, which comprises coating on the surface of a synthetic resin molded product a coating composition which comprises 100 parts by weight of a monomer mixture (A) comprising 30 - 98% by weight of a polyfunctional monomer having at least three groups selected from acryloyloxy group and methacryloyloxy group in one molecule and 70 - 2%

by weight of at least one monomer selected from the group consisting of methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, methoxydiethylene glycol (meth)acrylate, dipropylene glycol mono(meth) acrylate, and ethyl carbitol(meth)acrylate, and 0.01 - 10 parts by weight of a photosensitizer per 100 parts by weight of monomer mixture (A) and which can form a crosslinked-hardened film excellent in abrasion resistance by irradiation with actinic radiation in an air atmosphere and then irradiating the coating film with actinic radiation to form a crosslinked-hardened film of 1 - 30µ thick on the surface of the synthetic resin molded product.

12. The method of Claim 11, wherein the polyfunctional monomer is at least one polyfunctional (meth)acrylate selected from the group consisting of poly(meth)acrylates of mono or polypentaerythritol having at least three methacryloyloxy groups and/or acryloyloxy groups in one molecule wherein said mono or polypentaerythritol has the formula:

(wherein at least three of X11, X12, X13, X22, X23, . . . . .
Xn2, Xn3 and X14 are CH2 = CR - COO - group and the remainder are -OH group, n is an integer of 1 - 5 and R is hydrogen atom or methyl group).

13. The method of Claim 12, wherein the poly-functional monomer is selected from the group consisting of pentaerythritol tri and tetra(meth)acrylate.

14. The method of Claim 12, wherein the poly-functional monomer is selected from the group consisting of dipentaerythritol tri, tetra, penta and hexa(meth)acrylate.

15. The method of Claim 11, wherein the synthetic resin molded product is methacrylic resin, polycarbonate resin, or polyallyl diglycol carbonate resin.

16. The method of Claim 11, wherein said coating composition comprises 5 - 90 parts by weight of said monomer mixture and 95 - 10 parts by weight of at least one organic solvent (B) which is mixed with said monomer mixture to form a homogeneous solution.

17. The method of Claim 16, wherein the poly-functional monomer is at least one polyfunctional (meth)acrylate selected from the group consisting of poly(meth)acrylates of mono or polypentaerythritol having at least three methacryloylox groups and/or acryloyloxy groups in one molecule wherein said mono or polypentaerythritol has the formula:

(wherein at least three of X11, X12, X13, X22, X23, . . . .
Xn2, Xn3 and X14 are CH2 = CR - COO - group and the remainder are -OH group, n is an integer of 1 - 5 and R is hydrogen atom or methyl group).

18. The method of Claim 17, wherein the poly-functional monomer is selected from the group consisting of pentaerythritol tri and tetra(meth)acrylate.

19. The method of Claim 17, wherein the poly-functional monomer is selected from the group consisting of dipentaerythritol tri, tetra, penta and hexa(meth)acrylate.

20. The method of Claim 16, wherein the organic solvent has a boiling point of 50° - 200°C.

21. The method of Claim 16, which comprises coating the coating composition on the surface of the resin molded product, volatilizing and releasing at least 50% by weight of the organic solvent contalned in the coated film and then irradiating the film with actinic radiation to form a crosslink-hardened film of 1 - 30 µ thick on the surface of the molded product.

22. The method of Claim 16, which comprises coating on the surface of the synthetic resin molded product the coating composition having a viscosity of not more than 10 centipoises at 25°C by dipping method and forming a crosslink-hardened film of 1 - 9 µ thick.

23. The method of Claim 16, wherein the synthetic resin molded product is methacrylic resin, polycarbonate resin or polyallyl diglycol carbonate resin molded product.