Low-temperature curable composition
FIELD OF THE INVENTION 
     The present invention relates to a curable composition. More particularly, it relates to a low-temperature curable composition which is suitable for a binder of a paint for plastics. 
     BACKGROUND OF THE INVENTION 
     A large quantity of plastics has recently been used, and a demand for a paint for use in plastics have been increasing. The paint for plastics is generally required to cure at low-temperature and to provide a coated film superior in hardness, flexibility and acid resistance. 
     A two-package urethane paint has hitherto widely been used for plastics, because it cures at low-temperature and forms an acid resistant coated film. However, the two-package urethane paint has a problem that a process for coating is complicated and an isocyanate component is toxic. On the other hand, a melamine paint is superior in workability because it is one-package, but it provides a coated film having poor acid resistance. 
     Japanese Laid-Open Patent Publication No. 63-123471 discloses a paint which forms a coated film having excellent flexibility, but hardness of the coated film is insufficient. Japanese Laid-Open Patent Publication No. 62-140684 discloses a paint which forms a coated film having excellent hardness, but flexibility of the coated film is insufficient. 
     As described above, there has never been reported a paint which cures at low-temperature and provides a coated film which is superior in hardness, flexibility and acid resistance. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a low-temperature curable composition which is useful for a binder of a paint for plastics. 
     The present invention provides a low-temperature curable composition comprising: 
     (a) an acyl carbamate group-containing acrylic polymer having repeating units represented by the formula: ##STR1## wherein, R 1  is a hydrogen atom, or an alkyl group having 1 to 4 carbon atoms such as a methyl group and an ethyl group; R 2  is an alkyl group having 1 to 12 carbon atoms such as a methyl group, all ethyl group, a propyl group, a butyl group, an 2-ethylhexyl group and a dodecyl group; an alkoxycarbonylalkyl group having 1 to 12 carbon atoms such as a methoxycarbonylmethyl group and an ethoxycarbonylmethyl group; an alkoxyalkyl group having 1 to 12 carbon atoms such as a methoxyethyl group, an ethoxyethyl group, a butoxyethyl group, a hexyloxyethyl group, a methoxypropyl group, a methoxybutyl group and a 2-ethylhexyloxyethyl group; an acyl alkyl group having 1 to 12 carbon atoms such as an 2-acetyl-1,1-dimethylethyl group (MeCOCH 2  C(Me) 2  -); an alkyleneimino group (RR&#39;C═N--) having 1 to 12 carbon atoms such as a methylethylamino group, a methylisobutylimino group, a cyclohexylimino group and a methylphenylimino group; an aryl group having 6 to 25 carbon atoms such as a phenyl group, a 2,6-di-t-butyl-4-methyl phenyl group and naphthyl group; and an aralkyl group having 7 to 12 carbon atoms such as a benzyl group and a phenetyl group; and the formula: ##STR2## wherein, P, Q, Y and Z is independently a group selected from the group consisting of a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 1 to 20 carbon atoms, an aryl group having not more than 10 carbon atoms, an aralkyl group having not more than 10 carbon atoms, a halogen atom, a cyano group and --COOR a  (R a  is an alkyl group having 1 to 20 carbon atoms, an aryl group having not more than 10 carbon atoms, or an aralkyl group having not more than 10 carbon atoms; 
     wherein, the acyl carbamate group-containing acrylic polymer is composed of m (m is a positive integer) repeating unit of the formula (I) and n (n is 0 or a positive integer) repeating unit of the formula (II), and has a content of an acyl carbamate group of 0.5 to 6.5 mmol/g and a molecular weight of 2,000 to 10,000; and 
     (b) a lactone-modified polyol having a molecular weight of 200 to 20,000, represented by the formula: ##STR3## wherein A is a residue of a polyol; k is an integer of 1 to 5; 1 is an integer of 1 to 3; and x is an integer of not less than 2; 
     wherein the low-temperature curable composition comprises the acyl carbamate group-containing acrylic polymer (a) and the lactone-modified polyol (b) in an amount that a mol ratio of acyl carbamate group to hydroxyl group becomes 0.5 to 2.0. 
     A paint prepared from the low-temperature curable composition of the present invention is superior in low-temperature curability, because transesterification of the acyl carbamate bond proceeds at low temperature. Furthermore, the paint has high safety, because it contains no isocyanate. The cured coated film has satisfactory combination of hardness and flexibility, and has acid resistance because it is non-melamine system. Accordingly, there was provided a low-temperature curable composition which is useful for binders of paint for use in plastics, composites of plastic and steel, etc. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The acyl carbamate group-containing acrylic polymer, which is used as a main component in the low-temperature curable composition of the present invention can be obtained, for example, by polymerizing an ethylenically unsaturated monomer having an acyl carbamate group (hereinafter it is referred to as &#34;an acyl carbamate group-containing monomer&#34;), represented by the formula: ##STR4## wherein R 1  and R 2  are the same as defined above; alone, or polymerizing the acyl carbamate group-containing monomer with the ethylenically unsaturated monomer different from the acyl carbamate group-containing monomer. 
     The acyl carbamate group-containing monomer may be obtained by reacting an ethylenically unsaturated monomer having an acyl isocyanate group, which is represented by the formula: ##STR5## wherein R 1  is the same as defined above; with a hydroxyl group-containing compound represented by the formula: 
     
         R.sub.2 --OH 
    
     wherein R 2  is the same as defined above. 
     The ethylenically unsaturated monomer having an acyl isocyanate group may be obtained by the method described in Japanese Laid-Open Patent Publication No. 60-115557, and it may also be produced via an intermediate described in Die Nakromoleklare Chemie, 131(1970), 247-257 (No. 3199). Generally, the ethylenically unsaturated monomer having an acyl isocyanate group can be produced by reacting α-alkylacrylamide with oxalyl halide. The reaction is generally conducted at a temperature of 0° to 80° C. in the presence of an inert solvent such as hydrocarbon halide. 
     The ethylenically unsaturated monomer having an acyyl isocyanate group has a possibility of conducting various reactions. Therefore, when this monomer is reacted with a blocking agent, there has been assumed the proceeding of various side reactions, such as dimerization, trimerization, polymerization of the ethylenically unsaturated monomer having an acyl isocyanate group itself; polymerization of the formed acyl carbamate group-containing ethylenically unsaturated monomer; reaction of an amide-form NH group of the formed acyl carbamate group-containing ethylenically unsaturated monomer with the ethylenically unsaturated monomer having an acyl isocyanate group; etc., in addition to and/or in place of the desired addition reaction between the acyl carbamate group-containing monomer and blocking agent. However, it was confirmed that the above desired reaction proceeds preferentially within a temperature range which does not exceeds 100° C., actually. Particularly, the desired reaction only proceeds quantitatively at relatively low temperature which does not exceed room temperature (0° to 30° C.) and various side reactions to be assumed can be substantially avoided. 
     The hydroxyl group-containing compound used in the present invention may be a suitable aliphatic, alicyclic or aromatic monoalcohol, phenol, a substituted phenol having a substituent except for halogen, and an oxime. Examples thereof include aliphatic alcohols having 1 to 10 carbon atoms, e.g. various alcohols such as methyl, ethyl, chloroethyl, propyl, butyl (n-butyl and t-butyl), amyl, hexyl (n-hexyl and cyclohexyl), heptyl, octyl, nonyl, 3,3,5-trimethylhexyl, 2-ethylhexyl, decyl, etc.; aromatic alkyl alcohols such as phenyl carbinol, methyl phenyl carbinol, benzyl alcohol, phenethyl alcohol, etc.; ether bond-containing alcohols such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethylhexyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, butylene glycol monomethyl ether, diethylene glycol monobutyl ether, etc.; phenol itself; substituted-phenols having a substituent except for halogen, in which the substituent does not adversely affect the product, such as cresol, 2,6-di-t-butyl-p-cresol, naphthol, xylenol, nitrophenol, ethyl phenol, t-butyl phenol, etc.; acyl group-containing alcohols such as diacetone alcohol, etc.; ester group-containing alcohols such as methyl glycolate, etc.; oximes such as methyl ethyl ketoxime, methyl isobutyl ketoxime, acetophenone oxime, cyclohexanone oxime, etc.; pyrrolidones; lactams such as ε-caprolactam, etc. Small amount of comparatively nonvolatile higher monoalcohols or phenols may optionally be used as a blocking agent of the present invention. Examples thereof include nonyl phenol, monofunctional polyethylene and polypropylene oxide, e.g. Carbowax 550, etc. 
     The hydroxyl group-containing compound is generally liquid at room temperature and it can acts as a reaction medium. However, an inert solvent is generally used as a reaction medium whether the hydroxyl group-containing compound is liquid or solid. The inert solvent may be any one that do not adversely affect the reaction, and various solvents can be used. For example, it may be selected appropriately from aliphatic hydrocarbons such as pentane, hexane, heptane, etc.; aromatic hydrocarbons such as benzene, toluene, xylene, etc.; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, decalin, etc.; hydrocarbon solvents such as petroleum ether, petroleum benzine, etc.; hydrocarbon halide solvents such as carbon tetrachloride, chloroform, 1,2-dichloroethane, etc.; ether solvents such as ethyl ether, isopropyl ether, anisole, dioxane, tetrahydrofuran, etc.; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, acetophenone, isophorone, etc.; esters such as ethyl acetate, butyl acetate, etc.; acetonitrile; dimethylformamide; dimethyl sulfoxide; etc. These may be used alone or in combination thereof. 
     The reaction is generally carried out preferably at -20° to 100° C., advantageously at about room temperature (0° to 30° C.). A side reaction may occur at a high temperature such as 100° C. or more. On the other hand, the reaction ratio disadvantageously becomes small when the temperature is too low. Tin and amine catalysts may be used in the reaction, but it is not necessary to use the catalyst, usually. 
     The acyl carbamate group-containing monomer may be polymerized alone to obtain a homopolymer, and it may be polymerized with the ethylenically unsaturated monomer having no active hydrogen which is different from the acyl carbamate group-containing monomer to obtain a copolymer. It is not preferred to use a monomer having an active hydrogen because it can reacts with the acyl carbamate group. 
     The ethylenically unsaturated monomer having no active hydrogen which is different from the acyl carbamate group-containing monomer is represented by the formula: ##STR6## wherein P, Q, Y and Z are the same as defined above. 
     Examples thereof include the followings: 
     Monoolefinic and diolefinic hydrocarbons, i.e. monomers having only a hydrogen atom and a carbon atom, such as styrene, α-methyl styrene, α-ethyl styrene, isobutylene (2-methyl-propane-1), 2-methyl-butene-1,2-methyl-pentene-1,2,3-dimethyl-butene-1,2,3-dimethyl-pentene-1,2,4-dimethyl-pentene-1,2,3,3-trimethyl-heptene-1,2,3-dimethyl-hexene-1,2,4-dimethyl-hexene-1,2,5-dimethyl-hexene-1,2-methyl-3-ethyl-pentene-1,2,3,3-trimethyl-pentene-1,2,3,4-trimethyl-pentene-1,2-methyl-octene-1,2,6-dimethyl-heptene-1,2,6-dimethyl-octene-1,2,3-dimethyl-decene-1,2-methyl-nonadecene-1, ethylene, propylene, butylene, amylene, hexylene, butadiene-1,3, isoprene, etc.; 
     Halogenated monoolefinic and diolefinic hydrocarbons, i.e. monomers having a carbon atom, a hydrogen atom and at least one halogen atom, such as α-chlorostyrene, α-bromostyrene, 2,5-dichlorostyrene, 2,5-dibromostyrene, 3,4-dichlorostyrene, ortho-, meta- and para-fluorostyrene, 2,6-dichlorostyrene, 2,6-difluorostyrene, 3-fluoro-4-chlorostyrene, 3-chloro-4-fluorostyrene, 2,4,5-trichlorostyrene, dichloromonofluorostyrene, 2-chloropropene, 2-chlorobutene, 2-chloropentene, 2-chlorohexene, 2-chloroheptene, 2-bromobutene, 2-bromoheptene, 2-fluorohexene, 2-fluorobutene, 2-iodopropene, 2-iodopentene, 4-bromoheptene, 4-chloroheptene, 4-fluoroheptene, cis- and trans-1,2-dichloroethylene, 1,2-dibromoethylene, 1,2-difluoroethylene, 1,2-diiodoethylene, chloroethylene (vinyl chloride), 1,1-dichloroethylene (vinylidene chloride), bromoethylene, fluoroethylene, iodoethylene, 1,1-dibromoethylene, 1,1-difluoroethylene, 1,1-diiodoethylene, 1,1,2trifluoroethylene, chlorobutadiene and other halogenated diolefinic compounds; 
     Esters of organic acids and inorganic acids, such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl valerate, vinyl caproate, vinyl enanthate, vinyl benzoate, vinyl toluate, vinyl p-chlorobenzoate, vinyl o-chlorobenzoate and similar vinyl halobenzoates, vinyl p-methoxybenzoate, vinyl p-ethoxybenzoate, methyl crotonate and ethyl tiglate; 
     Methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, amyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 3,5,5-trimethylexyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate and phenyl (meth)acrylate; 
     Isopropenyl acetate, isopropenyl propionate, isopropenyl butyrate, isopropenyl isobutyrate, isopropenyl valerate, isopropenyl caploate, isopropenyl enanthate, isopropenyl benzoate, isopropenyl p-chlorobenzoate, isopropenyl o-chlorobenzoate, isopropenyl o-bromobenzoate, isopropenyl m-chlorobenzoate, isopropenyl toluate, isopropenyl α-chloroacetate and isopropenyl α-bromopropionate; 
     Vinyl α-chloroacetate, vinyl α-bromoacetate, vinyl α-chloropropionate, vinyl α-bromopropionate, vinyl α-iodopropionate, vinyl α-chlorobutyrate, vinyl α-chlorovalerate and vinyl α-bromovalerate; 
     Allyl chloride, allyl cyanide, allyl bromide, allyl fluoride, allyl iodide, allyl chloride carbonate, allyl nitrate, allyl thiocyanate, allyl formate, allyl acetate, acetate propionate, allyl butyrate, allyl valerate, allyl caploate, allyl 3,5,5-trimethyl hexoate, allyl benzoate, allyl (meth)acrylate, allyl crotonate, allyl oleate, allyl chloroacetate, allyl trichloroacetate, allyl chloropropionate, allyl chlorovalerate, allyl lactate, allyl pyruvate, allyl aminoacetate, allyl acetoacetate, allyl thioacetate, methallyl esters corresponding to these allyl esters, and esters derived from alkenyl alcohols such as β-ethyl allyl alcohol, β-propyl allyl alcohol, 1-buten-4-ol, 2-methyl-buten-4-ol, 2-(2,2-dimethylpropyl)-1-buten-4-ol and 1-penten-4-ol; 
     Methyl α-chloro(meth)acrylate, methyl α-bromo(meth)acrylate, methyl α-fluoro(meth)acrylate, methyl α-iodo(meth)acrylate, ethyl α-chloro(meth)acrylate, propyl α-chloro(meth)acrylate, isopropyl α-bromo(meth)acrylate, amyl α-chloro(meth)acrylate, decyl α-chloro(meth)acrylate, methyl α-cyano(meth)acrylate, ethyl α-cyano(meth)acrylate, amyl α-cyano(meth)acrylate and decyl α-cyano(meth)acrylate; 
     Dimethyl maleate, diethyl maleate, diallyl maleate, dimethyl fumarate, diethyl fumarate and diethyl glutaconate; 
     Organic nitriles such as acrylonitrile, methacrylonitrile, ethacrylonitrile, crotonitrile and oleonitrile; and 
     Polymerizable amides such as (meth)acrylamide and N,N-dimethylaminopropyl(meth)acrylamide. 
     The polymerization is carried out by the radical reaction in the presence of the above inert solvent. A conventional radical catalyst is suitably used as a polymerization catalyst. Examples thereof include azobisisobutyronitrile, benzoyl peroxide, cumene hydroperoxide, tetramethylthiuram disulfide, 2,2&#39;-azobis-(4-methoxy-2,4-dimethylvalelonitrile), acetylcyclohexylsulfonyl peroxide, 2,2&#39;-azobis-(2,4-dimethylvaleronitrile), etc. The catalyst is generally formulated in an amount of from 0.1 to 10 parts by weight, based on an amount of the monomer. 
     The polymerization temperature is generally 20° to 200° C., preferably 80° to 150° C. In case of the polymerization, other additives such as polymerization modifier, etc. may optionally be formulated. 
     It is preferred that the molecular weight is controlled within a range of 2,000 to 10,000, particularly 2500 to 6000. When the molecular weight exceeds 10,000, viscosity of the resulting polymer becomes too high and workability of the resulting paint becomes poor. On the other hand, when the molecular weight is smaller than 2,000, hardness of the coated film becomes poor. The term &#34;molecular weight&#34; as employed in this specification represents the number molecular weight. 
     In addition, it is preferred that the resulting acyl carbamate group-containing acrylic polymer has an acyl carbamate group content of 0.5 to 6.5 mmol/g, preferably 0.5 to 3.5 mmol/g. When the amount of the acyl carbamate group is smaller than 0.5 mmol/g, hardness of the resulting coated film becomes poor. On the other hand, when the amount exceeds 6.5 mmol/g, flexibility of the resulting coated film becomes poor. 
     In contrast with the above process, the acyl carbamate group-containing acrylic polymer of the present invention can also be prepared by polymerizing the ethylenically unsaturated monomer having an acyl isocyanate group to form an acrylic polymer having an acyl isocyanate group by first, followed by blocking the acyl isocyanate group. Conditions, reagents and preferred characteristics to be used for the reaction are the same as described above. 
     Meanwhile, the acyl carbamate group introduced into an acrylic backbone according to the method of the present invention easily reacts with a compound containing an active hydrogen by heating. Particularly, the acyl carbamate group can be cureed with an compound containing plural active hydrogens at low temperature, in comparison with a conventional blocked isocyanate group. Accordingly, the acyl carbamate group is a stable equivalent of a highly reactive acyl isocyanate group, and it is easily handled. 
     The lactone-modified polyol used in the low-temperature curable composition of the present invention is represented by the following formula; ##STR7## 
     In the above formula, A is a residue of polyol of which hydroxyl group was reacted with lactone. Accordingly, x is an integer of not less than 2. The k and l are the same as defined above, and k represents the number of lactone ring members, and l represents the addition mole number of polyols per hydroxyl group. 
     The lactone-modified polyol of the present invention is obtained by ring-opening adding a lactone having 3 to 7 carbon atoms, preferably 4 to 6 carbon atoms, to a polyol in the presence of an acid catalyst. 
     One embodiment of the lactone-modified polyol of the present invention is a lactone-modified polyol of lower polyhydric alcohol, represented by the above formula wherein A is a residue of a lower polyhydric alcohol having 2 to 15 carbon atoms, preferably 2 to 6 carbon atoms. 
     Examples of the lower polyhydric alcohol which can be used include difunctional alcohols such as ethylene glycol, diethylene glycol, propanediol, butanediol, hexanediol, etc.; trifunctional alcohols such as trimethylolpropane, glycerin, 1,2,3-heptanetriol, etc.; tetrafunctional alcohols such as pentaerythritol, mesoerythritol, etc.; hexafunctional alcohols such as dipentaerythritol, mannitol, etc. Examples of the lactone which can be used include propiolactone, butyrolactone, valerolactone, caprolactone, hexylolactone, etc. Among them, caprolactone is particularly preferred due to its easiness of industrial production. The acid catalyst may be any one which is generally used for the ring-opening addition reaction, and examples thereof include Lewis acids such as tin catalyst, e.g. dibutyltin dilaurate, etc. 
     The reaction can be carried out, for example, by conventional modification methods such as a process which comprises simultaneously charging the lower polyhydric alcohol, lactone and acid catalyst and heating the mixture to the reaction temperature with stirring, or a process which comprises adding dropwise the polyhydric alcohol in the lactone and acid catalyst, which are previously heated to the reaction temperature. 
     It is preferred that the resulting lactone-modified polyol of lower polyhydric alcohol has a number-average molecular weight of 200 to 1500, particularly 300 to 600, and a hydroxyl group amount of 3 to 8 mmol/g, particularly 6 to 8 mmol/g. When the number-average molecular weight is smaller than 200, hardness of the resulting coated film becomes poor. On the other hand, when the number-average molecular weight exceeds 1500, viscosity of the resulting polymer becomes too high and workability of the resulting paint becomes poor. The molecular weight and the hydroxyl group amount can be generally controlled by changing a kind and a mixing ratio of the polyhydric alcohol and lactone. 
     The lactone-modified polyol of lower polyhydric alcohol which can be used for the present invention is commercially available from Daicel Chemical Industries Co., Ltd under the trade name of &#34;Placcel&#34; series. Examples thereof include difunctional polyols such as &#34;Placcel 205&#34; having a number-average molecular weight of 500, &#34;Placcel 208&#34; having a number-average molecular weight of 800, etc.; trifunctional polyols such as &#34;Placcel 303&#34; having a number-average molecular weight of 300, &#34;Placcel 305&#34; having a number-average molecular weight of 500, &#34;Placcel 306&#34; having a number-average molecular weight of 600, &#34;Placcel 308&#34; having a number-average molecular weight of 800, etc.; tetrafunctional polyols such as &#34;Placcel 404&#34; having a number-average molecular weight of 400, &#34;Placcel 406&#34; having a number-average molecular weight of 600, &#34;Placcel 407&#34; having a number-average molecular weight of 700, etc.; hexafunctional polyols such as &#34;Placcel 604&#34; having a number-average molecular weight of 400, &#34;Placcel 609&#34; having a number-average molecular weight of 900, etc. 
     Another embodiment of the lactone-modified polyol of the present invention is an acrylic lactone-modified polyol represented by the above formula, wherein. A is a residue of an acrylic polyol. 
     For example, such an acrylic lactone-modified polyol can be prepared by polymerizing a lactone-modified ethylenically unsaturatedmonomer having a terminal hydroxyl group (hereinafter it is referred to as &#34;a hydroxyl group-containing lactone-modified monomer&#34;), which is represented by the formula: ##STR8## wherein R 3  is a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, such as methyl group and an ethyl group; y is an integer of 1 to 10, and k and l are as defined above; alone, or polymerizing the hydroxyl group-containing lactone-modified monomer with the ethylenically unsaturated monomer different from the hydroxyl group-containing lactone-modified monomer. 
     The hydroxyl group-containing lactone-modified monomer is obtained, for example, by ring-opening adding 1 to 5, preferably 1 to 2 above described lactones to an ethylenically unsaturated monomer having a hydroxyl group, which is represented by the formula: ##STR9## wherein R 3  and y are the same as defined above. Examples of the ethylenically unsaturated monomer having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxypropyl (meth)acrylate, etc. 
     The hydroxyl group-containing lactone-modified monomer, which can be used in the present invention, is commercially available from Daicel Chemical Industries Co., Ltd. under the trade name of &#34;FM&#34; series obtained by adding lactones to 2-hydroxyethyl methacrylate and &#34;FA&#34; series obtained by adding lactones to 2-hydroxyethyl acrylate. Examples thereof include &#34;FM-1&#34; having 1 added caprolactone in an average, &#34;FM-2&#34; having 2 added caprolactones in an average, &#34;FM-3&#34; having 3 added caprolactones in an average, &#34;FM-4&#34; having 4 added caprolactones in an average, &#34;FM-5&#34; having 5 added caprolactones in an average, FA-1&#34; having 1 added caprolactone in an average, &#34;FA-2&#34; having 2 added caprolactones in an average, &#34;FA-3&#34; having 3 added caprolactones in an average, &#34;FA-4&#34; having 4 added caprolactones in an average, &#34;FA-5&#34; having 5 added caprolactones in an average, etc. 
     Examples of a comonomer of the hydroxyl group-containing lactone-modifled monomer include the ethylenlcally unsaturated monomer having no active hydrogen which is different from the acyl carbamate group-containing monomer, described for preparing an acyl carbamate group-containing acrylic polymer, the above described ethylenically unsaturated monomer having hydroxyl group, a carboxyl group-containing monomer such as acrylic acid, methacrylic acid, and an amide group-containing monomer such as acrylamide, methacrylamide. 
     The polymerization can also be carried out in the same manner as that described above, but it is preferred that a number-average molecular weight of the resulting polymer is controlled within a range of 2,000 to 20,000. When the number-average molecular weight is smaller than 2,000, hardness of the resulting coated film becomes poor. On the other hand, when the number-average molecular weight exceeds 20,000, viscosity of the resulting polymer becomes too high and workability of the resulting paint becomes poor. It is preferred that a hydroxyl group amount of the resulting polymer is controlled within a range of 0.5 to 3.5 mmol/g. When the hydroxyl group amount is not within this range, curing properties and physical properties of the resulting paint becomes poor. 
     The acrylic lactone-modified polyol of the present invention can also be obtained by polymerizing the above described ethylenically unsaturated monomer having a hydroxyl group to prepare an acrylic polymer having a hydroxyl group by first, and then ring-opening adding the above described lactone to the acrylic polymer. 
     The acrylic polymer having a hydroxyl group can be prepared in the same manner as that described above, except for using an ethylenically unsaturated monomer having a hydroxyl group in place of the hydroxyl group-containing lactone-modified ethylenically unsaturated monomer. 
     The low-temperature curable composition of the present invention can be obtained by mixing the acyl carbamate group-containing acrylic polymer thus obtained with the lactone-modified polyol. A mixing ratio is preferably selected so that a molar ratio of the acyl carbamate group contained in the acyl carbamate group-containing acrylic polymer to the hydroxyl group contained in the lactone-modified polyol becomes 0.5 to 2.0, particularly 0.5 to 1.5. When the mixing ratio deviates from the above range, an amount of the unreacted functional group increases, therefore, physical property of the coated film, such as hardness, etc. may become poor. 
     Organic solvents can optionally be contained in the low-temperature curable composition of the present invention so as to control its viscosity to a level which is suitable for coating. Furthermore, additives known to those skilled in the art for coating, such as anti-corrosive pigments (e.g. borate, phosphate, chromate, molybdate, etc.), coloring pigments (e.g. titanium oxide, carbon black, quinacridone, cyanine blue, etc.), surface modifiers, ultraviolet absorbers, antioxidants, pigment sedimentation inhibitors, etc., may be included in the low-temperature curable composition of the present invention to provide a paint of the present invention. 
     The paint of the present invention can be applied and cured by a process known to those skilled in the art. For example, the coated film of the present invention can be obtained by applying the paint on a substrate using a spray or a bar coater, followed by baking at 100° to 140° C. for 10 to 60 minutes. 
    
    
     EXAMPLES 
     The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. 
     Production Example 1 
     Preparation of &#34;Polymer A&#34; Having Acyl Carbamate Group 
     A 300 ml four-necked flask was equipped with a thermometer, a stirrer, a nitrogen introducing tube and a 200 ml dropping funnel and methyl isobutyl ketone (MIBK, 100 g) was charged in the flask, which was heated to 100° C. N-methacryloyl hexyloxyethyl carbamate (MAHC, 70 g), methyl methacrylate (MMA, 3 g), 2-ethylhexyl acrylate (EHA, 27 g) and N,N&#39;-azobisisobutyronitrile (AIBN, 5 g) were mixed, and the mixture was charged in the 200 ml dropping funnel and added dropwise at constant speed over 90 minutes. After completion of dropping, the mixture was stirred at 100° C. for 30 minutes and 0.5 g of AIBN dissolved in MIBK was added dropwise over about 10 minutes, followed by stirring at 100° C. for 90 minutes to obtain the &#34;polymer A&#34; having acyl carbamate group, having a nonvolatile content of 48%, a polymer conversion ratio of 93%, a number-average molecular weight of 2500, Tg of -15° C. and an acyl carbamate amount of 2.72 mmol/g. 
     Production Examples 2 to 8 
     Preparation of &#34;Polymers B to H&#34; Having an Acyl Carbamate Group 
     According to the same manner as that described in Production Example 1 except for using the monomer composition shown in the following Table 1, &#34;polymers B to H&#34; having an acyl carbamate group were obtained. The characteristic values of the resulting polymers are also shown in Table 1. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                  Production Example No.                                  
                  1  2  3  4  5  6  7  8                                  
                  Label of polymer                                        
                  A  B  C  D  E  F  G  H                                  
__________________________________________________________________________
Monomer Composition (g)                                                   
Methyl methacrylate                                                       
                  3     5  10 10 10 5  3                                  
2-Ethylhexyl acrylate                                                     
                  27                   17                                 
2-Ethylhexyl methacrylate                                                 
                     25          20 15                                    
n-Butyl methacrylate    25 20 20    50                                    
Styrene              5                                                    
Ethyl N-methacryloylcarbamate                                             
                     70                80                                 
Butoxyethyl N-methacryloyl-                                               
                           70                                             
carbamate                                                                 
Hexyloxyethyl N-methacryloyl-                                             
                  70                30                                    
carbamate                                                                 
2-Ethylhexyl N-methacryloyl-                                              
                        70                                                
carbamate                                                                 
Methoxycarbonylmethyl N-      70                                          
methacryloylcarbamate                                                     
Isobutoxyimine N-methacryloyl-   70                                       
carbamate                                                                 
AIBN (g)          5.0                                                     
                     5.5                                                  
                        5.5                                               
                           5.5                                            
                              5.5                                         
                                 5.5                                      
                                    3.5                                   
                                       5.5                                
Nonvolatile content (%, MIBK)                                             
                  48 48 50 49 45 45 48 50                                 
Conversion rate (%)                                                       
                  93 97 96 93 92 90 94 95                                 
Number-average molecular weight                                           
                  2500                                                    
                     2800                                                 
                        3000                                              
                           3500                                           
                              2600                                        
                                 2600                                     
                                    6000                                  
                                       3800                               
Amount of acyl carbamate (mmol/g)                                         
                  2.72                                                    
                     4.46                                                 
                        2.90                                              
                           3.06                                           
                              3.48                                        
                                 3.10                                     
                                    1.17                                  
                                       5.10                               
__________________________________________________________________________
 
    
     Production Example 9 
     Preparation of Lactone-Modified Polyol 1 
     A 300 ml four-necked flask was equipped with a thermometer, a stirrer, a nitrogen introducing tube and a 200 ml dropping funnel and methyl isobutyl ketone (MIBK, 100 g) was charged in the flask, which was heated to 100° C. FM-1 (44 g, manufactured by Daicel Chemical Industries Co., Ltd.), n-butyl acrylate (nBA, 27 g), methyl methacrylate (MMA, 29 g) and N,N&#39;-azobisisobutyronitrile (AIBN, 2.5 g) were mixed, and the mixture was charged in the 200 ml dropping funnel and added dropwise at constant speed over 90 minutes. After completion of dropping, the mixture was stirred at 100° C. for 30 minutes and 0.25 g of AIBN dissolved in MIBK was added dropwise over about 10 minutes, followed by stirring at 100° C. for 90 minutes to obtain a lactone-modified polyol 1 having a nonvolatile content of 48%, a polymer conversion ratio of 100%, a number-average molecular weight of 8800, Tg of 0° C. and a hydroxyl group amount of 1.78 mmol/g. 
     Production Examples 10 to 13 
     Preparation of Lactone-Modified Polyols 2 to 5 
     According to the same manner as that described in Production Example 9 except for using the monomer composition shown in the following Table 2, lactone-modified polyols 2 to 5 were obtained. The characteristic values of the resulting polymers are also shown in Table 2. 
     
                       TABLE 2                                                     
______________________________________                                    
               Production Examples                                        
               9    10     11     12   13                                 
               Label of polymer                                           
               1    2      3      4    5                                  
______________________________________                                    
Monomer composition (g)                                                   
Methyl methacrylate                                                       
                 29     31     27   28   28                               
n-Butyl acrylate 27     5      55   17   17                               
FM-1             44            18   55   55                               
FM-2                    64                                                
AIBN (g)         2.5    2.5    2.75 2.75 1.8                              
Nonvol. content (%, MIBK)                                                 
                 48     47     48   47   47                               
Polymer conversion (%)                                                    
                 100    100    100  100  100                              
Mn               8800   7900   9200 8500 15000                            
Amount of OH group                                                        
                 1.78   1.78   0.72 2.23 2.23                             
(mmol/g)                                                                  
______________________________________                                    
 
    
     Production Example 14 
     Preparation of Acrylic Polyol 6 
     A 300 ml four-necked flask was equipped with a thermometer, a stirrer, a nitrogen introducing tube and a 200 ml dropping funnel and methyl isobutyl ketone (MIBK, 100 g) was charged in the flask, which was heated to 100° C. 2-Hydroxyethyl methacrylate (23 g), n-butyl acrylate (nBA, 48 g), methyl methacrylate (MMA, 29 g) and N,N&#39;-azobisisobutyronitrile (AIBN, 2.5 g) were mixed, and the mixture was charged in the 200 ml dropping funnel and added dropwise at constant speed over 90 minutes. After completion of dropping, the mixture was stirred at 100° C. for 30 minutes and 0.25 g of AIBN dissolved in MIBK was added dropwise over about 10 minutes, followed by stirring at 100° C. for 90 minutes to obtain an acrylic polyol 6 having a nonvolatile content of 48%, a polymer conversion ratio of 100%, a number-average molecular weight of 8200, Tg of 0° C and a hydroxyl group amount of 1.78 mmol/g. 
     Production Example 15 
     Preparation of Acrylic Polyol 7 
     According to the same manner as that described in Production Example 14 except for using the monomer composition shown in the following Table 3, an acrylic polyol 7 was obtained. The characteristic values of the resulting polymer are also shown in Table 3. 
     
                       TABLE 3                                                     
______________________________________                                    
                     Production                                           
                     Examples                                             
                     14     15                                            
                     Label of polymer                                     
                     6      7                                             
______________________________________                                    
Monomer composition (g)                                                   
2-Hydroxyethyl methacrylate                                               
                       23                                                 
4-Hydroxybutyl acrylate         26                                        
n-Butyl acrylate       48       30                                        
Methyl methacrylate    29       44                                        
AIBN (g)               2.5      2.5                                       
Nonvol. content (%, MIBK)                                                 
                       48       48                                        
Polymer conversion (%) 100      100                                       
Mn                     8200     6800                                      
Amount of OH group (mmol/g)                                               
                       1.78     1.78                                      
______________________________________                                    
 
    
     Example 1 
     The &#34;polymer A&#34; having an acyl carbamate group (65 g) obtained in Production Example 1 and lactone-modified polyol 1 (100 g) obtained in Production Example 9 were mixed at room temperature. The resulting mixture was applied on a tin plate using a bar coater (#75), and left at room temperature for 30 minutes. The applied tin plate was then baked at 120° C. for 30 minutes to obtain a cured coated film. A gel fractional ratio of the coated film was 95%. Hardness, flexibility and acid resistance of the resulting coated film were measured by the following methods. A result of the pencil hardness test was HB, the folding test was 0T, the impact resistance test was 30 kgcm, and the acid-resistance test was good. 
     Test methods of the cured film 
     (1) Hardness 
     A cured coated film was scratched using a set of hardness-measuring pencils. Maximum hardness of the pencils which do not forms scratch on the surface of the coated film was adopted as the rating of hardness. 
     (2) Flexibility 
     Flexibility was tested according to the following two methods. 
     (a) Impact resistance test 
     Du Pont type drop-weight test described in JIS-K-5400-6.13 was conducted. A value of potential energy calculated according to (weight of the load x height) was adopted as the rating. 
     (b) Folding test 
     The coated plate was folded such that the coated surface faces outward with putting some plates for coating therebetween. The smallest number of the plates inserted between the folded coated plate which makes no failure such as fracture and clack on the coated film was adopted as the rating. 
     (3) Acid resistance 
     (a) 1N sulfuric acid (1 ml) was placed on a coated film, and left at 40° C. overnight. The coated film was visually observed whether abnormality such as blister, rust, etc. is present or not. 
     (b) A coated film was dipped in iN sulfuric acid (1 ml), and left at 40° C. for 38 days. The coated film was visually observed whether abnormality such as blister, rust, etc. is present or not. 
     Examples 2 to 29 and Comparative Examples 1 to 8 
     According to the same manner as that described in Example 1 except for using the formulation shown in the following Table 4 and optionally using a polypropylene (PP) plate as a substrate to be coated in place of the tin plate, a cured coated film was produced and tested, respectively. 
     The PP plate was primed with commercially available chlorinated PP, or corona treated before coating. Two kinds of PP plates which are &#34;M4800&#34; containing a rubber component (manufactured by Mitsui Petroleum Chemical Co., Ltd.) and those containing no rubber component (commercially available) were used. The test results are shown in Table 5. 
     As to the pencil hardness, it should be taken into consideration the fact that the rating of a coated film on the PP plate becomes low as much as 1 level, when compared to the rating of the same coated film on the tin plate. 
     
                                           TABLE 4                                 
__________________________________________________________________________
Acyl carbamate                                                            
              Amount      Amount                                          
                               Equiv.                                     
polymer       (g)  Polyol (g)  ratio.sup.a                                
                                   Substrate                              
__________________________________________________________________________
Ex. No.                                                                   
1     A (P Ex. 1)                                                         
              65   1 (P Ex. 9)                                            
                          100  1/1 Tin plate                              
2     A (P Ex. 1)                                                         
              64   2 (P Ex. 10)                                           
                          100  1/1 Tin plate                              
3     A (P Ex. 1)                                                         
              26   3 (P Ex. 11)                                           
                          100  1/1 Tin plate                              
4     A (P Ex. 1)                                                         
              80   5 (P Ex. 13)                                           
                          100  1/1 Tin plate                              
5     A (P Ex. 1)                                                         
              124  Placcel 305                                            
                          25   1/1 Tin plate                              
6     A (P Ex. 1)                                                         
              142  Placcel 305                                            
                          20   1/0.7                                      
                                   Tin plate                              
7     A (P Ex. 1)                                                         
              116  Placcel 305                                            
                          35   1/1.5                                      
                                   Tin plate                              
8     G (P Ex. 7)                                                         
              173  Placcel 305                                            
                          15   1/1 Tin plate                              
9     H (P Ex. 8)                                                         
              113  Placcel 305                                            
                          45   1/1 Tin plate                              
10    G (P Ex. 7)                                                         
              34   3 (P Ex. 11)                                           
                          112  1/0.5                                      
                                   Tin plate                              
11    H (P Ex. 8)                                                         
              21   3 (P Ex. 11)                                           
                          110  1/0.7                                      
                                   Tin plate                              
12    G (P Ex. 7)                                                         
              87   4 (P Ex. 12)                                           
                          70   1/1.5                                      
                                   Tin plate                              
13    H (P Ex. 8)                                                         
              68   4 (P Ex. 12)                                           
                          83   1/2.0                                      
                                   Tin plate                              
14    B (P Ex. 2)                                                         
              120  Placcel 305                                            
                          40   1/1 Tin plate                              
15    C (P Ex. 3)                                                         
              135  Placcel 305                                            
                          30   1/1 Tin plate                              
16    D (P Ex. 4)                                                         
              129  Placcel 305                                            
                          30   1/1 Tin plate                              
17    E (P Ex. 5)                                                         
              123  Placcel 305                                            
                          30   1/1 Tin plate                              
18    F (P Ex. 6)                                                         
              140  Placcel 305                                            
                          30   1/1 Tin plate                              
19    A (P Ex. 1)                                                         
              129  Placcel 404                                            
                          5    1/1 Tin plate                              
20    A (P Ex. 1)                                                         
              130  Placcel 604                                            
                          5    1/1 Tin plate                              
21    A (P Ex. 1)                                                         
              124  Placcel 305                                            
                          25   1/1 Primed PP (M4800)                      
22    A (P Ex. 1)                                                         
              124  Placcel 305                                            
                          25   1/1 Primed PP                              
23    A (P Ex. 1)                                                         
              124  Placcel 305                                            
                          25   1/1 Corona.sup.b PP (M4800)                
24    D (P Ex. 4)                                                         
              129  Placcel 305                                            
                          30   1/1 Primed PP (M4800)                      
25    E (P Ex. 5)                                                         
              123  Placcel 305                                            
                          30   1/1 Primed PP (M4800)                      
26    F (P Ex. 6)                                                         
              140  Placcel 305                                            
                          30   1/1 Primed PP (M4800)                      
27    D (P Ex. 4)                                                         
              63   1 (P Ex. 9)                                            
                          110  1/1 Primed PP (M4800)                      
28    E (P Ex. 5)                                                         
              60   1 (P Ex. 9)                                            
                          110  1/1 Primed PP (M4800)                      
29    F (P Ex. 6)                                                         
              68   1 (P Ex. 9)                                            
                          110  1/1 Primed PP (M4800)                      
C Ex. No.                                                                 
1     A (P Ex. 1)                                                         
              65   6 (P Ex. 14)                                           
                          100  1/1 Tin plate                              
2     A (P Ex. 1)                                                         
              65   6 (P Ex. 14)                                           
                          100  1/1 Tin plate                              
3     A (P Ex. 1)                                                         
              153  TMP.sup.c                                              
                          8    1/1 Tin plate                              
4     Commercially available polyester/melamine                           
                               8/2 Tin plate                              
      curable type clear for plastics                                     
5     A (P Ex. 1)                                                         
              65   6 (P Ex. 14)                                           
                          100  1/1 Primed PP (M4800)                      
6     A (P Ex. 1)                                                         
              65   7 (P Ex. 15)                                           
                          100  1/1 Primed PP (M4800)                      
7     A (P Ex. 1)                                                         
              153  TMP    8    1/1 Primed PP (M4800)                      
8     Commercially available polyester/melamine                           
                               8/2 Primed PP (M4800)                      
      curable type clear for plastics                                     
__________________________________________________________________________
 .sup.a Equivalent ratio of acyl carbamate group to hydroxyl group        
 .sup.b Corona treated                                                    
 .sup.c Trimethylolpropane                                                
 
    
     
                       TABLE 5                                                     
______________________________________                                    
                  Gel    Pen- Impact                                      
Ex.  Curing       frac.  cil  resist.                                     
                                    Folding                               
                                           Acid                           
No.  condition    (%)    hard.                                            
                              (kgcm)                                      
                                    test   resist.                        
______________________________________                                    
1    120° C. × 30 min.                                       
                  95     HB   30    0T     Good                           
2    120° C. × 30 min.                                       
                  98     F    25    2T     Good                           
3    120° C. × 30 min.                                       
                  92     HB   25    1T     Good                           
4    120° C. × 30 min.                                       
                  97     F    22    3T     Good                           
5    120° C. × 30 min.                                       
                  95     F    35    3T     Good                           
6    120° C. × 30 min.                                       
                  92     HB   30    1T     Good                           
7    120° C. × 30 min.                                       
                  99     H    22    3T     Good                           
8    120° C. × 30 min.                                       
                  93     HB   30    1T     Good                           
9    120° C. × 30 min.                                       
                  98     F    20    3T     Good                           
10   120° C. × 30 min.                                       
                  90     HB   20    1T     Good                           
11   120° C. × 30 min.                                       
                  95     F    20    3T     Good                           
12   120° C. × 30 min.                                       
                  93     HB   30    1T     Good                           
13   120° C. × 30 min.                                       
                  97     H    20    3T     Good                           
14   150° C. × 30 min.                                       
                  98     H    25    3T     Good                           
15   120° C. × 30 min.                                       
                  95     HB   30    1T     Good                           
16   120° C. × 30 min.                                       
                  95     HB   30    1T     Good                           
17   100° C. × 30 min.                                       
                  93     HB   30    1T     Good                           
18    80° C. × 30 min.                                       
                  90     HB   30    1T     Good                           
19   120° C. × 30 min.                                       
                  98     HB   30    2T     Good                           
20   120° C. × 30 min.                                       
                  97     HB   28    3T     Good                           
21.sup.P                                                                  
     120° C. × 30 min.                                       
                  95     F    35           Good                           
22.sup.P                                                                  
     120° C. × 30 min.                                       
                  95     F    35           Good                           
23.sup.P                                                                  
     120° C. × 30 min.                                       
                  95     F    35           Good                           
24.sup.P                                                                  
     120° C. × 30 min.                                       
                  95     HB   30           Good                           
25.sup.P                                                                  
     100° C. × 30 min.                                       
                  93     HB   28           Good                           
26.sup.P                                                                  
      80° C. × 30 min.                                       
                  90     H    30           Good                           
27.sup.P                                                                  
     120° C. × 30 min.                                       
                  95     HB   28           Good                           
28.sup.P                                                                  
     100° C. × 30 min.                                       
                  93     HB   28           Good                           
29.sup.P                                                                  
      80° C. × 30 min.                                       
                  95     HB   28           Good                           
C.sup.b 1                                                                 
     120° C. × 30 min.                                       
                  100    F    &lt;5    Fract..sup.a                          
                                           Good                           
C2   120° C. × 30 min.                                       
                  93     HB   3     1T     Good                           
C3   120° C. × 30 min.                                       
                  98     2H   &lt;1.5  Fract. Good                           
C4   120° C. × 30 min.                                       
                  96     3H   50    0T     Poor                           
C5.sup.P                                                                  
     120° C. × 30 min.                                       
                  99     F    &lt;5           Good                           
C6.sup.P                                                                  
     120° C. × 30 min.                                       
                  95     HB   3            Good                           
C7.sup.P                                                                  
     120° C. × 30 min.                                       
                  95     2H   &lt;1.5         Good                           
C8.sup.P                                                                  
     120° C. × 30 min.                                       
                  98     3B   50           Poor                           
______________________________________                                    
 .sup.a Fractured when folded at 90 degree.                               
 .sup.b Comparative Example                                               
 .sup.P PP plate was used as the material to be painted.                  
 
    
     As is apparent from the above results, paints of Comparative Examples 1 to 3 provide a coated film which is superior in hardness but is inferior in impact resistance, and paint of Comparative Example 4 provides a coated film which is superior in impact resistance but is inferior in hardness.