The present invention relates to a photocurable putty for repairing sheet metal. More particularly, the present invention relates to a thin applying type putty for repairing a dent of about 3 mm or less.

A putty for repairing sheet metal of an automobile generally is a two-pack type, and the process of mixing a principal agent with a curing agent is required before use. A curing reaction begins by mixing in a two-pack type putty, and therefore, predetermined amounts of the principal agent and the curing agent must be taken and homogeneously mixed and, furthermore, the resulting mixed putty must be applied using a spatula rapidly within a gelation time.

Since a rate of the curing reaction is remarkably influenced by an ambient temperature, a gelation time, a pot life, and an abradable time of the putty are also influenced and changed depending on the ambient temperature. Therefore, the disadvantage occurs that the operation time required for repairing sheet metal remarkably varies depending on a season.

A general medium type putty, for example, requires a pot life of 6 to 8 minutes and an abradable time of 20 to 30 minutes at 200°C, and a polyputty requires further time for curing. Such a time restriction is a large burden in a procedure for repairing sheet metal.

The present invention solves the above conventional problems, and an object of the present invention is to provide a photocurable one-pack type putty for repairing sheet metal, which cures rapidly by harmless visible light. The present invention provides a photocurable putty for repairing sheet metal comprising:

(a) 100 parts by weight (33.3% to 73.5% by weight based on the total amount of the components (a) to (c)) of a bisphenol A type epoxy di(meth)acrylate;

(b) 1 to 100 parts by weight (0.7% to 33.3% by

weight based on the total amount of the components (a) to (c)) of a photopolymerizable (meth)acrylate;

(c) 35 to 100 parts (25.7% to 33.3% by weight based on the total amount of the components (a) to (c) by weight of a photopolymerizable urethane oligomer having two or more (meth)acryloyloxy groups per molecule; and

(d) 0.01 to 10 parts by weight of α-diketone as a photoinitiator, 0.01 to 10 parts by weight of tertiary amine as a photosensitizer and 0.01 to 5 parts by weight of an onium salt as a photoreaction accelerator, each amount being based on 100 parts by weight of the total amount of the components (a) to (c), and achieves the objects.

The present invention provides photocurable one-pack type putty for repairing sheet metal, which cures rapidly by harmless visible light. When a photocurable resin of the present invention is used as a putty for repairing the panel of an automobile, the photocurable resin is one-pack type, and can cure and adhere, extremely rapidly, by irradiation of visible light. Therefore, workability is good in comparison with a conventional putty, and the time for repairing is decreased. The photocurable putties of the present invention contain one or more bisphenol A type epoxy di(meth)acrylate monomers.

Examples thereof include one selected from the group consisting of 2,2- bis[4-(methacryloxyethoxy)phenyl) propane, 2,2-bis[4-

(methacryloxydiethoxy)phenyl] propane, 2,2-bis[4(methacryloxypolyethoxy)phenyl) propane, 2,2-bis[4(acryloxydiethoxy)phenyl] propane, bisphenol A diglycidyl dimethacrylate, 2,2-bis[4-(methacryloxyhydroxypropyl polypropyloxy)phenyl] propane, and the like, or a mixture thereof. Preferred is bisphenol A diglycidyl dimethacrylate of which a homopolymer of said monomer has a glass transition temperature of about 170°C, because the resulting cured putty has excellent abrasion resistance, flexibility, and heat resistance. The term "glass transition temperature" means a temperature at which tan δ becomes maximum when a sample is measured by using a dynamic viscoelasticity measuring apparatus ("RSA-II" manufactured by Leometrix Co.) in a three point bent mode and a frequency of 1 Hz. The term "putty" means a

substance that is in a thixotropic and viscous (for example, 1,000 to 2,000,000 cps), but flowable state before curing. On the other hand, the term "cured putty" applies to the same substance which is not flowable after curing.

The bisphenol A type epoxy di(meth)acrylate is used in an amount of 33% to 73.5% by weight, based on the weight of the total polymerizable monomers and oligomers, that is, the above components (a) to (c). When the amount of the bisphenol A type epoxy di(meth)acrylate is less than 33 parts by weight, the modulus of the cured putty becomes small and the cutting ability thereof becomes poor. On the other hand, when the bisphenol A type epoxy di(meth)acrylate exceeds 73.5 parts by weight, the amount of the urethane oligomer becomes relatively small, and adhesiveness to the coated sheet metal part to be repaired becomes poor.

Any photocurable or photopolymerizable (meth)acrylate monomer or combinations thereof, may be useful in the present invention. Examples of useful (meth)acrylate monomers include glycidyl methacrylate, benzyl methacrylate, tetrahydrofurfuryl methacrylate, cyclohexyl methacrylate, t-butyl acrylate, and the like.

(Meth)acrylate monomers having a polar group, such as a hydroxyl group, a phosphoric acid group, or a carboxyl group are preferred for improved adhesion of the resulting cured putty to sheet metal.

Examples of the (meth)acrylate monomer having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyI (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxyoctyl (meth)acrylate, and the like. A preferred (meth)acrylate monomer is hydroxyethyl methacrylate of which a homopolymer of said monomer has a glass transition temperature of about 55°C, and is preferably copolymerized with bisphenol A glycidyl dimethacrylate to provide a copolymer having a glass transition temperature of 70 to 150°C (for example, bisphenol A diglycidyl dimethacrylate-hydroxyethyl methacrylate (70:30) having a glass transition temperature of 115°C ("3000M" manufactured by Kyoei Co., Ltd.,

Japan)).

Examples of (meth)acrylate monomers having a phosphoric acid group include those having the tradenames "KAYAMER PM-1 ", "KAYAMER PM-2", "KAYAMER PM-21", and the like, manufactured by Nippon Kayaku Co., Ltd., Japan. Use of a (meth)acrylate monomer having a phosphoric acid group also results in a cured putty having improved adhesion to sheet metal.

A preferred (meth)acrylate having a phosphoric acid group is "KAYAMER PM-21 ", manufactured by Nippon Kayaku Co., Ltd. Since "KAYAMER PM-21 " (meth)acrylate monomer is a long-chain compound, the resulting cured putty remains flexible and the putty's adhesion to sheet metal and the old coating remaining at the part to be repaired is further improved.

When a (meth)acrylate monomer having a phosphoric acid group is used, the other polymerizable monomer and oligomers, (a) and (c), preferably do not have a hydroxyl group or a glycidyl group. If both a phosphoric acid group and a hydroxyl and/or glycidyl group are present in a putty, the groups react and cause the putty to cure, which deteriorates the putty's storage stability.

Examples of bisphenol A type epoxy di(meth)acrylates which have neither a hydroxyl group nor a glycidyl group include 2,2-bis[4-(methacryloxyethoxy)phenyl] propane, 2,2-bis(4-(methacryloxydiethoxy)phenyl] propane, 2,2-bis[4- (methacryloxypolyethoxy)phenyl] propane, and the like. Preferred is 2,2-bis[4- (methacryloxyethoxy)phenyl) propane of which a homopolymer of said monomer has a glass transition temperature of about 160°C.

Examples of (meth)acrylate monomers having a carboxyl group include acrylic acid, phthalic acid monohydroxyethyl (meth)acrylate, β- methacryloyloxyethyl hydroxysuccinate, and the like. Putties of the present invention further contain a photocurable urethane oligomer having two or more (meth)acryloyloxy groups per molecule. In a coating of an automobile, a two-pack type acrylic urethane coating composition is generally used for repairing application, and an acrylic melamine or an alkyd melamine coating composition is used for a new car. Accordingly, by incorporating a segment of a urethane oligomer into the putty, adhesion between the putty and the remaining coating on the part of sheet metal to be repaired can be obtained.

The urethane oligomer is used in an amount of 14.9 to 49.8 parts by weight, preferably 20 to 40 parts by weight, based on 100 parts by weight of the total polymerizable monomer and oligomer. When an amount of the urethane oligomer is less than 14.9 parts by weight, adhesiveness between the old coating remaining at the part to be repaired and the cured putty is poor. On the other hand, when the amount of urethane oligomer exceeds 49.8 parts by weight, the modulus of the cured putty is lower, which results in a deterioration of the abrasion resistance of the resulting cured putty.

It is preferred that a combination of high-molecular weight and low- molecular weight urethane oligomers are used in the present invention. Since the high-molecular weight urethane oligomer is a long-chain compound, the resulting cured putty is flexible, and the putty's adhesion to sheet metal and a coating is further improved. On the other hand, low-molecular weight urethane oligomers increase the crosslinking density of the cured putty, improving high temperature performance.

The high-molecular weight urethane oligomer has a number-average molecular weight of not less than 5000, and a number-average molecular weight of preferably 8000 to 10000. When a number-average molecular weight of the high- molecular weight urethane oligomer is less than 5000, the flexibility of the resulting cured putty is poor.

Examples of useful high-molecular weight urethane oligomers include the UV curing type urethane acrylates having the tradenames "UX-2201", "UX-330P, and "UX-7101" (manufactured by Nippon Kayaku Co., Ltd., Japan) and a urethane oligomer having the tradename "UA-4000" (manufactured by Shinnakamura Kagaku Co., Ltd., Japan). A preferred urethane oligomer is "UA-4000", having a molecular weight of about 8000.

The high-molecular weight urethane oligomer is used in the present invention in an amount of 4 to 100 parts by weight, preferably 50 to 70 parts by weight, based on 100 parts by weight of the total amount of the polymerizable oligomer. When the amount of the high-molecular weight urethane oligomer in the

invention is less than 40 parts by weight, the adhesion between the old coating remaining at the part of sheet metal to be repaired and the cured putty decreases.

The low-molecular weight urethane oligomer has a number-average molecular weight of not more than 1500, and preferably has a number-average molecular weight of 1000 to 1500.

Examples of useful low-molecular weight urethane oligomers include the NK oligourethane having the tradenames "U-6HA", "U-324A", "UA-122P", and "UA-5201", manufactured by Shin-nakamura Kagaku Co., Ltd., Japan. A preferred low molecular weight urethane oligomer is a bifunctional polycarbonatediol- modified urethane prepolymer having the tradename "UA-122P", having a molecular weight of about 1100, and a tetrafunctional urethane oligomer, having the tradename "U-4HA", having a molecular weight of about 590, both manufactured by Shin-nakamura Kagaku Co., Ltd., Japan.

The low-molecular weight urethane oligomer is used in an amount of 0 to 60 parts by weight, preferably 30 to 50 parts by weight, based on 100 parts by weight of the total amount of the polymerizable oligomer. When an amount of the low-molecular weight urethane oligomer exceeds 60 parts by weight, the amount of the high-molecular weight urethane oligomer becomes relatively small and the flexibility of the cured putty becomes poor, resulting in poor adhesion between the cured putty and the coating remaining on the area of sheet metal to be repaired.

A putty of the present invention can also contain various photopolymerizable catalysts which are known to those skilled in the art. It is preferred that at least a photoinitiator, a photosensitizer, and a photoreaction accelerator are present in the putties of the present invention in order to obtain good photocurability.

A preferred photoinitiator is α-diketone, and those having a maximum absorption wavelength within a visible range are particularly preferred.

Examples thereof include those selected from the group consisting of diacetyl, acetyl benzoyl, benzyl, camphorquinone, α-naphthyl, p,p'- dimethoxybenzyl, 2,3pentanedione, 1,2-phenanthrenequinone, 1,4-

phenanthrenequinone, 3,4-phenanthrenequinone, 9,10phenanthrenequinone and naphthoquinone, or a mixture thereof. Preferred is camphorquinone.

The α-diketone is used in an amount of 0.01 to 10 parts by weight, preferably 0.1 to 5 parts by weight, based on 100 parts by weight of the total polymerizable monomer and oligomer. When an amount of the α-diketone is less than 0.01 parts by weight, the efficiency of photoinitiation is low and it becomes hard to use practically. On the other hand, when the amount of α-diketone exceeds 10 parts by weight, storage stability is poor.

In this case, the photosensitizer which is suitable to be used in combination with the α-diketone is tertiary amine.

Examples of useful tertiary amines include one selected from the group consisting of N,N-dimethylaniline, N,N-diethylaniline, N,N-dimethyl-p-toluidine, N,N-dimethylamino-p-ethyl benzoate, N,N-dihydroxyethylaniline, triethylamine and N,N-dimethylhexylamine, or a mixture thereof. A preferred tertiary amine is a compound in which an aromatic group is substituted directly with N, for example,

N,N-dimethylamino-p-ethyl benzoate.

The tertiary amine is used in an amount of 0.01 to 10 parts by weight, and preferably 0.1 to 5 parts by weight, based on 100 parts by weight of the total polymerizable monomer and oligomer. When an amount of the tertiary amine is less than 0.01 parts by weight, the efficiency of photoinitiation is low and it becomes hard to use practically. On the other hand, when the tertiary amine exceeds 10 parts by weight, storage stability is poor.

A preferred photoreaction accelerator is onium salt.

Examples of useful photoreaction accelerators include one selected from the group consisting of triphenylsulfonium hexafluoroantimonate, triphenylsulfonium hexafluorophosphate and diphenyliodonium hexafluorophosphate, or a mixture thereof.

The onium salt is used in an amount of 0.01 to 5 parts by weight, and preferably 0.1 to 1 parts by weight, based on 100 parts by weight of the total polymerizable monomer and oligomer. When an amount of the onium salt is less than 0.01 parts by weight, the efficiency of photoinitiation is low and it becomes

hard to use practically. On the other hand, when the amount of onium salt exceeds 5 parts by weight, storage stability is poor.

A putty of the present invention can contain a filler which does not inhibit light transmissibility. The filler having a high light transmissibility is preferred. The filler is used in order to improve impact resistance, to impart thixotropy, to prevent cure shrinkage, and to improve abrasive ability.

Examples of useful fillers include an inorganic small hollow spherical filler, a zinc oxide ultrafine particle, a titanium oxide ultrafine particle, a barium sulfate ultrafine particle, and the like. A preferred small hollow spherical filler is a glass bubble consisting of soda-lime borosilicate glass, for example, "Glass Bubbles", manufactured by Minnesota Mining and Manufacturing Company. Such a glass bubble has a particle size of 10 to 100/μm, and preferably a size of 40 to 80/μm on average.

The glass bubble is used in an amount of 5 to 20 parts by weight, and preferably 10 to 15 parts by weight, based on 100 parts by weight of the total polymerizable monomer and oligomer. When an amount of the glass bubble exceeds 20 parts by weight, elongation of the resulting putty becomes poor, and irregularities may be formed on a surface of the cured putty. On the other hand, when the amount of the glass bubble is less than 5 parts by weight, the effect of inhibiting cure shrinkage is not obtained.

In addition, a putty of the present invention may contain an ultrafine particle of zinc oxide having an average particle size of about 0.02 μm and high transmissibility, thereby providing good spreadability resulting in a repaired surface having a fine texture. Zinc oxide can generally be added in an amount of 0.1 to 20 parts by weight, and is preferably 5 to 10 parts by weight.

A putty of the present invention can contain a thixotropic agent. The thixotropic agent is used to control fluidity and spreadability, and to prevent sedimentation of the filler.

Examples of useful thixotropic agents include an ultrafine particle of high- purity anhydrous silica which can impart thixotropy by an interaction of silanol groups present on a surface of the particle. Preferred is a silica ultrafine particle of

which the surface is hydrophobically treated, having a particle size of about 7 to 30 nm. When the ultrafine silica particle of which its surface is hydrophobically treated is used, sedimentation of the glass bubbles is prevented. Such a silica is commercially available from Nippon Aerogyl Co., Ltd., Japan, under the tradename of"Aerogy".

The thixotropic agent is used in an amount of 0.1 to 10 parts by weight, and preferably 5 to 8 parts by weight, based on 100 parts by weight of the total polymerizable monomer and oligomer. When an amount of the thixotropic agent is less than 0.1 parts by weight, sedimentation of the filler is not prevented. On the other hand, when the thixotropic agent exceeds 10 parts by weight, fluidity of the putty becomes poor, and therefore, it becomes difficult to spread the putty using a spatula.

A silane coupling agent can be added to a putty of the present invention in order to improve adhesiveness to metal. For example, γ-methacryloxypropyl trimethoxysilane is added in an amount of about 0.1 to 5 parts by weight, and preferably 0.5 to 1.0 parts by weight, based on 100 parts by weight of the total polymerizable monomer and oligomer.

The above components such as bisphenol A type epoxy di-(meth)acrylate, (meth)acrylate, a urethane oligomer, a photopolymerization catalyst, a filler, a thixotropic agent, and any other components, are mixed by using a suitable method known to those skilled in the art to obtain a putty of the present invention. For example, all components described above may be kneaded while defoaming by using a vacuum stirrer.

When sheet metal is repaired, the resulting putty is spread at the part to be repaired. The thickness of the spread putty at one time is preferably not more than

3 mm. When the thickness exceeds 3 mm, light transmissibility is poor, thereby much time may be required to cure, and the putty may not be cured completely.

Then, the putty spread is cured by irradiation. It is preferred that visible light is used in view of the safety of work. The wavelength of visible light to be used varies depending on a kind of the polymerizable monomer, oligomer and

photopolymerization catalyst. Visible light having a wavelength of 300 to 1200 nm, particularly 400 to 500 nm is generally preferred.

Examples of an irradiation source which emits such light include a halogen lamp, a xenon lamp, a tungsten filament electric bulb, a carbon arc lamp, a fluorescent lighting, a mercury lamp, and the like. The distance for irradiation is generally 10 to 30 cm and the time for irradiation is generally 1 to 10 minutes. Sunlight may be used as a light source.

The cured putty is abraded by the method known to those skilled in the art and a repaired surface is formed.

Examples

The following examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. In the following examples, "parts" are by weight unless otherwise stated.

Example 1

Forty-two parts of bisphenol A diglycidyl (meth)acrylate, 18 parts of hydroxyethyl methacrylate, 20 parts of a urethane oligomer "UA4000", manufactured by Shin-nakamura Kagaku Co., Ltd. Japan, 20 parts of a urethane oligomer "UA122P" manufactured by Shin-nakamura Kagaku Co., Ltd., 0.25 parts of camphorquinone, 0.25 parts of N,N'-dimethylamino-p-ethyl benzoate, 0.25 parts of triphenylsulfonium hexafluorophosphate, 15 parts of a glass bubble having a particle size of 40 μm on average and 7 parts of Aerogyl™ R972 silica were charged in a vessel, and the mixture was uniformly kneaded while defoaming by using a vacuum stirrer to prepare a putty.

A steel plate having a thickness of 0.7 mm ("SPCC-SB" manufactured by Nippon Test Panel Kogyo Co., Ltd., Japan) was slightly abraded, with an abrasive (about #80), degreased with the detergent for a painted surface using "Silicon Off* manufactured by Nippon Paint Co., Ltd., Japan. The putty was spread in 1 mm thick thereon. Then, the spread putty was irradiated using two halogen spot lights

(360 W) from a distance of 20 cm for 5 minutes to cure.

Curing time of the putty, abrasive ability of the cured putty and adhesiveness to sheet metal and to a coating were examined by the following method. The results are shown in Table 1.

Curing time

According to the same manner as that described above, the spread putty (1 mm in thickness) was irradiated using two halogen spot lights (360 W) from a distance of 20 cm at a temperature of 5, 25 and 35°C, and measured a time required to curing the putty completely.

Abrasive ability test

The cured putty was cut once under a load of 500 g, using a crosscut tester and a tungsten needle. Depth of a trace of cutting was measured at three positions using a surface roughness tester and the resulting value on average was taken as evaluation.

Adhesiveness test to sheet metal and coating

An anti-corrosive steel plate of 2.5 cm x 10 cm and 0.8 mm in thickness "Excelite" manufactured by Nippon Test Panel Kogyo Co., Ltd. and a coated plate obtained by applying a two-pack type acrylic urethane coating composition on a steel plate of 2.5 cm x 10 cm and 0.8 mm in thickness "SPCC-SB" manufactured by Nippon Test Panel Kogyo Co., Ltd. were used as a test plate, respectively.

According to the same manner as that described above, a cured putty of 2.5 cm x 2.5 cm and 1 mm in thickness was formed on a surface of each test plate. Then, a surface of the cured putty was adhered using an epoxy adhesive to a surface of the test plate having the same shape, which was likewise treated.

Then, a test plate on which the cured putty was formed and a test plate which was adhered onto the surface of the cured putty later were pulled apart at a rate of 5 mm/minute using a "Tensilon" apparatus manufactured by Olientic Co., and shear adhesive strength of the putty was measured.

Example 2

A putty was prepared and evaluated according to the same manner as that described in Example 1 except that 20 parts of a urethane oligomer "U-4HA" manufactured by Shin-nakamura Kagaku Co., Ltd. was used instead of the urethane oligomer "UA122P". The results are shown in Table 1.

Example 3

A putty was prepared and evaluated according to the same manner as that described in Example 1 except that urethane oligomer ' ^,T JA122P" was not used and 40 parts ofan urethane oligomer "UA4000" was used. The results are shown in

Table 1.

Example 4

A putty was prepared and evaluated according to the same manner as that described in Example 1 except that 42 parts of 2,2-bis[4-

(methacryloxyethoxy)phenyl] propane ("BPE-1" manufactured by Shin-nakamura Kagaku Co., Ltd.) instead of bisphenol A diglycidyl (meth)acrylate, and 18 parts of methacrylate having a phosphoric acid group ("PM-21" manufactured by Nippon Kayaku Co., Ltd.) instead of hydroxyethyl methacrylate were used. The results are shown in Table 1.

Comparative Example 1

A commercially available two-pack type polyputty ("SU putty" manufactured by Kansai Paint Co., Ltd., Japan) was used. A principal agent contains a polyester resin and a styrene monomer as a main component. A curing agent is peroxide. 100 Parts of the principal agent and 2 parts of the curing agent were weighed and mixed uniformly using a spatula.

Two kinds of test plates made according to the same manner as that described above were spread with a putty at a thickness of 1 mm, respectively. Then, the coated putty was left to cure. In curing, the test plate was maintained at a

temperature of 5, 25 and 35°C, respectively, and measured a time required for curing the putty completely.

Abrasive ability of the cured putty and adhesiveness to sheet metal and a coating were tested according to the same manner as that described in Example 1. The results are shown in Table 1.

Comparative Example 2

A putty was prepared and evaluated according to the same manner as that described in Comparative Example 1 except that "Plastaner" manufactured by Nippon Paint Co., Ltd. is used instead of "SU putty" manufactured by Kansai Paint Co., Ltd. The results are shown in Table 1.

Table 1

Curing time (minute) Abrasive Adhesiveness ability (μm) (kgf 'cm ² )

5°C 25°C 35°C Steel ¹ Coating ²

Ex. 1 5 5 5 178 35 >43

Ex. 2 5 5 5 165 30 >33

Ex. 3 5 5 5 162 30 >38

Ex. 4 5 5 5 183 36 >40

C.Ex. 1 100 30 10 128 43 >40

C.Ex. 2 75 20 8 163 35 >38

¹ Anticorrosive steel plate

² Coated plate

Regarding the putties of Examples 1 to 4, curing time does not vary depending on an ambient temperature and shows a short curing time such as 5 minutes, respectively. In addition, they are also superior in abrasive ability. To the contrary, regarding commercially available putties of Comparative Examples 1 and

2, curing time is long and varies depending on an ambient temperature, which results in poor workability.