CA2076500A1 - Thermosetting coating composition and method of forming coating therefrom - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The thermosetting paint composition containing (a) a hydroxyl group containing resin; (b) an acid catalyst; (c) a highly alkyl-etherified melamine resin capable of forming carbonium ions in the presence of the acid catalyst; and (d) a blocked polyisocyanate having active hydrogen at a baking temperature of a wet coated layer, the active hydrogen being substitutable with the carbonium ions formed from the highly alkyl-etherified melamine resin, the amount of the acid catalyst being, on a solid basis, 0.3-5 weight % based on the total amount of the resin components.

Description

BACKGROIIND OF THE INVENTION
The present invention relates to a thermosetting paint composition and a method of forming a coated layer (film) 5 from such a thermosetting paint composition, and more particularly to a thermosetting paint composition capable of providing a coated layer having excellent acid resistance and scuff (mar) resistance as well as good appearance, which is suitable for automotive topcoats, coats of building exteriors, etc., 10 and a method of forming a coated layer (film) from such a thermosetting paint composition.
Recently, the deterioration of surfaces of automobiles, buildings, etc. by acid rain has becorne a serious problem. For instance, automotive topcoats conventionally 15 formed from acrylic resin-type paints, polyester resin-type paints, etc. are stained with spots, discoloration, etc. when brought into contact with the acid rain, and in extreme cases the topcoats themselves are destroyed.
Coatings having high resistance to acid rain can be

2 0 obtained from silicone resin paints, fluorine resin paints, polyurethane resin paints, etc. However, these resins are generally very expensive and poor in paintability. Thus, various rneasures have been taken for conventional coatings formed from acrylic resin-type paints or polyester resin-type paints to 2 5 prevent their cleterioration by acid rain.
F;or instance, Japanese Patent Laid-Open No. 3-172368 discloses a coatirlg composition having a high resistance to ~cid rain, which comprises an acrylic resin and a particular amount of a polyfunctional blocked polyisocyanate of non-yellowing type.
However, automotive topcoat is required to have a good scuff (mar) resistance in addition to a good acid rain 5 resistance. For instance, if there is dust on the coat surfaces of automobiles at the time of cleaning by car washing machines, small scut`fs may be produced on the coat surfaces, thereby deteriorating their appearance. Thus, attempts have been made to improve the scuff (mar) resis~ance of the topcoats. For instance, Japanese Patent Laid-Open No. 2-142867 discloses a method of improving the scuff (mar) resistance of a film formed from a melamine resin and an acid catalyst for increasing a cross-linlcing density in the coated film to make the surface h ard er .
However, to meet the requirements of an acid rain resistance and a scuff (mar) resistance simultaneously, it would not be satisfactory to simply conduct the above two methods at the same time, because the resulting coated film would show extreme yellowing and wrinkle, and the paints are poor in 2 0 storage stability. In view of these problems, such paints eannot practically be used for automotive topcoats.
Development has been conducted to provide paint compositions capable of satisfying both acid rain resistanee and seuff (mar) resistance. For instance, Japanese Patent Laid-Open 2 5 No. 1-158079 cliscloses a composition comprising a particular acrylic copolymer, an alkyl ether-substitutecl melamine resin, and if necessary a blockecl polyisocyanate. This reference has a deseription to the effeet that the bloeked polyisoeyanate may be h ~3i ~ ~ 3 a polyisocya~ e ~locked with oxirne or alcohol In Examples of this reference, polyisocyanntes blocked with oxime are used. In the composition of Japanese Patent Laid-Open No. 1-158079, an acid catalyst such as paratoluene sulfonic acid, etc. is used.
Japanese Patent Laid-Open No. 2-86671 discloses a composition cornprising a hydroxyl group containing acrylic copolymer, a polyisocyanate and an alkyl ether-substituted melamine resin. The hydroxyl group containing acrylic copolymer contains a vinyl monomer having a sulfonic acid group which acts to accelerate the hardening of the rnelamine resin .
Coatings formed from the above two compositions show relatively good acid resistance and scuff (mar) resistance, but fail to maintain good appearance for a long period of time as 15 required on luxurious automobiles.
Further, Japanese Patent Laid-Open No. 2-242867 discloses a thermosetting paint composition consisting essentially of (A) a hydroxyl group containing resin, (B) an amino resin, and (C) a blocked polyisocyanate, a temperature at which the 20 reaction of (A) and (C) starts to take place being within the range of between -20C and -~50C relative to a temperature at which the reaction of (A) and (B) starts to take place. The specification of this reference describes: The paint composition is characterized by adding a blocked polyisocyanate (component 2 5 (C)) as a supplemental cross-linking agent to an organic solvent-type paint containing the components (A) and (~) as vehicle cornponents. As a reslllt, hydroxyl groups remaining in the cornponent (A) after reaction with the component (B) are reacted

- 3 -2 ~ 7 ~ ~3 ~ ~
wi~h the compol-ent (C) isol~ltecl from a blocking agen~, thereby reducing the amOUrlt of the hydroxyl groups, which leads to improvement of water resistance ancl acid resistance. Since urethane bonds formed by the reaction between the hydroxyl groups and the blocked polyisocyanate are chemically stable7 the resulting coated layer shows excellent chemical resis~ance, water resistance, as well as excellent mechanical strength and surface conditions (sagging, flowing, etc.). Incidentally, Japanese Patent Laid-Open No. 2-242867 has Example in which an isocyanate blocked with ethyl acetoacetate is used. However, the isocyanate blocked with ethyl acetoacetate is subjected to dissociation under a baking condition of about 120 160C for 30 minutes"
and ~he dissociated isocyanate is reacted with the componen~ (A) (hydroxyl group containing resin) ~o form urethane bonds.
Acco-rding to the inventors' research, however, it has been found that many of the blocked polyisocyanates which are dissociated at a relatively low temperature such as a baking temperature (specifically about 120-160C) cause the deterioration of significant coating properties such as extreme - 2 0 yellowing, wrinkling, etc. of the topcoat film, and make paints containing them less stable in storage. Therefore, the thermosetting paint composition of Japanese Patent Laid-Open No. 2-242867 fails to provide a coated layer (film) c,apable of maintaining good appearance as required for the automotive topcoats for a long periocl of time.

O~JEC'l' AND SIJM~lAR~ OlF THI~ INVENTION

s~pJ~
Accorclingly, all object of the present irlvention is to provide a thermosetting paint composition capable of satisfying the requirements of excellent acid rain resistance and scuff (mar) resistance and good film appearance simultaneously.
Another object of the present invention is to provide a method of forming a coated layer (film) satisfying the above requirements from the above thermosetting paint composition.
As a result of intense research in view of the above objects, the inventors have found that by adding, to a thermosetting paint composition comprising a hydroxyl group containing resin and a highly alkyl-etherified melarnine resin~ a blocked polyisocyanate having active hydrogen remaining even at a baking temperature of the coated layer (film) and an acid catalyst, the resulting coated layer (film) can show good coating appearance without yellowing. They also have found that a substitution reaction of active hydrogen proceeds between the blocked polyisocyanate and the remaining functional groups of the highly alkyl-etherified melamine resin which have not participated in a reaction with the hydroxyl group containing 2 0 resin in the process of baking, whereby a self-condensation reaction of the remaining melamine resin in the coated layer (film) is prevented to provide the hardened coated layer with excellent acid rain resistance and scuff (mar) resistance. The present invention has been completed based on these findings.
2 5 Thus, the thermosetting paint composition according to thc present invention comprises:
(a) a hyclroxyl grollp containing resin;
(b) an acicl catalyst;

2 ~ rd~
(c) a highly allcyl-etherified melamine resin capable of forming carbonium ions in the presence of the acid catalyst;
and (d) a blocked polyisocyanate having active hydrogen at a baking temperature of a wet film, the active hydrogen being substitutable with the carbonium ions formed from the highly alkyl-etherified melamine resin, the amount of the acid catalyst being, on a solid basis, 0.3-5 weight % based on the total amount of the resin components.
The method of forming a coated film comprising the steps of;
( 1~ coating, onto a substrate, a thermosetting paint composition comprlslng;
(a) a hydroxyl group containing resin;
( b) an acid catalyst;
(c) a highly alkyl-etherified melamine resin capable of forming carbonium ions in the presence of the acid catalys~;
and (d) a blocked polyisocyanate having active hydrogen at a 2 0 baking temperature of a wet coated layer, the active hydrogen being substitutable with the carbonium ions formed from the highly alkyl-etherified melamine resin, the amount of the acid catalyst being, on a solid basis, 0.3-5 weight % based on the total amount of the resin components;
2 5 and (2) baking the wet coated layer to cause (i) a cross-tinking reaction between the hydroxyl group containing resin ancl the highly nlkyl etherified melnmine resin, (ii) the formation of ~ ~ 7 ~
carbonillrrl ions from tlle remaining functional groups of the highly alkyl-e~herified melamine resin in the presence of the acid catalyst, and (iii) the rcaction between the carbonium ions and the blocked polyisocyanate.

DETAII,ED DESCRIPTION OF THE INVENTION
The present invention will be explained in detail below .
[A] Reaction of Thermosetting Paint Composition As described above, the thermosetting paint composition of the present invention comprises a hydroxyl group containing resin, an acid catalyst, a highly alkyl-etherified melamine resin, and a blocked polyisocyanate having active hydrogen remaining at a baking temperature of the wet coated 15 layer. In the process of hardening of the wet coated layer formed from the above thermosetting paint composition, the hydroxyl group containing resin is subjected to a cross-linking reaction with the highly alkyl-etherified melamine resin.
Along with the reaction, the following reaction proceeds.
2 0 First, the remaining functional groups of the highly alkyl-etherified melamine resin which have not participated in the above cross-linking reaction with the hydroxyl group containing resin are subjected to an active hydrogen substitution reaction with the blocked polyisocyanate. By this reaction, the self-condensation of 2 5 the remaining highly alkyl-etllerified melamine resin can be prevented. The above active hyckogen substitution reaction is represented by the following formulae (1) ancl (2):

~s~
,COR, C -t N~C~ N,C~N
Il l + 1~ C C ~ RlOH (1) ~C~N~C~ ~ ~

C-~ ,C-N-C-OR2 N`'~ I ~ O

,C~ "C~ + --N-C-OR~ _ IN ~N + H+ . . . ~2) whereill Rl-represents an alkyl group having 1-4 carbon atorlls, S and R2 represents a hydrocarbon group derived from the blocking agent.
In the reaction (1) the melamine resin generates carbonium ions in the presence of the acid catalyst, and in the reaction (2) the carbonium ions are reacted with the blocked 10 polyisocyanate, whereby the active hydrogen atoms of the blocked polyisocyanate are substituted with the carbonium ions.
The degree of the above reaction can be assessed by measuring the infrared spectroscopic absorption of imino groups in the reaction product of the melamine resin and the polyisocyanate.
The thermosetting paint composition of the present invention is characterized by containing the acid catalyst, the highly alkyl-etherified melamine resin generating the carbonium ions in the presence of the acid catalyst, the blocked polyisocyanate having active hydrogen substitutable with the carbonium ions at a baking 2 0 temperature of the wet coated layer, such that the active hydrogen substitlltion reaction proceeds along with the reaction of the paint composition. Accordingly, the reactiorl mechanisrn of the I'~'J ~ L7 i~ ~
therrnosettil)g paint composition of the present invention is apparently clifferent t`rom the reaction mechanism of the conventional thermosetting paint cornposition containing a melamine resin and a blocked polyisocyanate, in which the blocked 5 polyisocyanate acts as a supplemental cross-linking agent, reacting with the hydroxyl groups of the hydroxyl group containing resin to form urethane bonds.
[B] Components of Thermosetting Paint Composition (a) Hydroxyl group containing resin The hydroxyl group containing resin usable in the present invention is a resin having at least 2 hydroxyl groups acting as reaction sites with the highly alkyl-etherified melamine resin in one molecule. Specific examples of such - hydroxyl group containing resins include hydroxyl group 15 containing acrylic resins, hydroxyl group containing polyester resins, hydroxyl group containing fluorine resins, hydroxyl group containing amido resins, etc. Among them, the hydroxyl group containing acrylic resins and the hydroxyl group containing polyester resins are particularly preferable.
2 0 ( I ) Hydroxy I group containing acrylic resin The hydroxyl group containing acrylic resins usable in the present invention are those having a number-average molecular weight of 1,200-12,000, a hydroxyl value of 50-200 and an acid value of 0-50. Such hyc'roxyl group containing acrylic 2 5 resins can be procluced by a usual method from at least one of the rnonomcrs (i) ancl at least one selected fronn the group consisting of the monomers (ii) and (iii):

(i) Ethylenic monomers corltailling hydroxyl group~s ~s~cQ~as hydroxymethyl acrylatc, hyclroxymethyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, N-methylol acrylarnine, etc.;
(ii) Ethylenic monomers containing carboxyl groups such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, etc.; and (iii) Ethylenic monomers copolymerizable with the above monomers (i) and (ii) such as alkyl acrylates and alkyl methacrylates including methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, n-dodecyl acrylate, etc., acrylonitrile, methacrylonitrile, styrene, etc.
When the number-average molecular weight of the hydroxyl group containing acrylic resin is less than 1,200, sufficient cross-linking does not take place in the resulting coated layer, so that the coated layer shows poor properties. On the other hand, 2 0 when it exceeds 12,000, the paint composition shows an undesirably increased viscosity, and a coated layer having enough surface smoothness and evenness cannot be obtained. The number-average molecular weight of the hydroxyl group containing acrylic resin is preferably 1,500-10,000, and more preferably 2 5 3,000-6,000.
With respect to the hyclroxyl value, when it is less th~n 50, the resulting coate(l layer suffers from a poor adhesion and a decreased hardness. On the other hand, when it exceeds 200, the - ~0 -~7~
resulting coated layer suffers frorn a decrease in a water resistance and an acid resist.lnce due to the hydroxyl groups remaining in the coated layer. The hydroxyl value of the hydroxyl group containing acrylic resin is preferably 70-16û, and more preferably 80-120.
With respect to the acid value, when it exceeds 50, the thermosetting paint composition has low storage stability and shows too high a reaction speed due to too many functional groups remaining in the coated layer, so that the resulting coated layer shows a poor acid resistance, lacking in surface smoothness. The preferred acid value of the hydroxyl group containing acrylic resin is ()-30.
(2) Hydroxyl group containing polyester resin The hydroxyl group containing polyester resins usable in the present invention are those having a number-average molecular weight of 200-10,000, a hydroxyl value of 50-350, and an acid value of 3-50. Since the hydroxyl group containing polyester resins are produced by an esterification reaction of polyvalent alcohols and polybasic acids or their anhydrides, they contain hydroxyl groups in their molecules.
2 0 The polyvalent alcohols which may be used in the production of the hydroxyl group containing polyester resins according to the present invention include ethylene glycol, propylene glycol, butylene glycol, 1,6-hexanediol, diethylene glycol, dipropylene glycol, neopentyl glycol, triethylene glycol, glycerin, Z 5 trimethylol ethane, trimethylol propnne, pentaerithritol, dipentaerithritol, etc.
The polybasic acids or their anhydrides which may be used in the present invention inclucle phthalic acicl, phthalic ~7~
anhydride, isophthalic .Icid, terephthalic acid, succinic acid, succinic anhydricle, adipic acill, azelaic acid, sebacic acid, tetrahydrophthalic anhydride~ hexahydrophthalic anhyclride, maleic anhydlide, fumaric acid, itaconic acid, trimellitic anhydride, etc~
The hydroxyl group containing polyester resin usable in the present invention is produced by a reaction in which a molar ratio of ~he hydroxyl groups of the above polyvalent alcohol to the carboxyl groups of the above polybasic acid or its anhydride is 1.2-1.8, so that the polyester has residùal hydroxyl groups in the 1 0 rnolecule.
When the number-average molecular weight of the hydroxyl group containing polyester resin is less than 200, sufficient cross-linking does not take place in the coated layer, and the resulting coated layer does not have suf-ficient properties. On the other hand, when it exceeds 1,000, the viscosity of the paint composition is undesirably increased, and the resulting coated layer does not have a good appearance such as surface evenness, etc. The number-average molecular weight of the hydroxyl group containing polyester is preferably 300-6,000, and more preferably 300-3,000.
2 0 With respect to the hydroxyl value, when it is less than 50, the resulting coated layer has poor adhesion and decreased hardness. When it exceeds 350, the resulting coated layer suffers from a decrease in a water resistance and an acid resistance due to the remaining hydroxyl groups. The preferred hydroxyl value of 2 5 the hydroxyl group containing polyester resin is 70-280.
With respect to the acicl value, when it is less than 3, the resulting coated film has poor hardness. On the other hand, when it exeeeds 50, the thermosetting paint composition has low ~ J~
storage stability ancl shows too high a reactiorl speed due to too many functional groups remaining in the coated layer, so that the resulting coated layer (film) shows poor acid resistance and surface smoothness (appearance). The preferred acid value of the hydroxyl group containing polyester resin is 5-20.
(b) Acid catalyst The acid catalysts used to accelerate the reaction in the present invention mean those acids functioning to activate the highly alkyl-etherified melamine resin so that the highly alkyl-etherified mélamine resin can generate carbonium ions. Typical examples of the acid catalysts include carboxylic acids such as acetic acid, lactic acid, succinic acid, oxalic acid, maleic acid, decanedicarboxylic acid, (meth)acrylic acid, e~c.; sulfonic acids such as paratoluene sulfonic acid, dodecylbenzene sulfonic acid, dinonylnaphthalene (di)sulfonic acid, etc.; esters of organic ~lkyl phosphates such as dimethyl phosphate, dibutyl phosphate, dimethyl pyrophosphate, dibutyl pyrophosphate, etc. Among these organic acids, sulfonic acids, particularly dodecylbenzene sulfonic acid, paratoluene sulfonic acid and dinonylnaphthalene (di)sulfonic 2 0 acid are preferable to achieve a high cross-linking density.
The acid ca~alyst is preferably blocked with a blocking agent which can be dissociated at a baking temperature of a usual coated layer. If an unblocked acid catalys~ is used, the ~hermosetting paint cornposition shows poor storage stability.
2 5 As the blocking agent, an arnine is preferable. The amines usable in the present invention include prima~y, secondary or tertiary ,alkyl.lrnines, alkanolamines, alicyclic amines, N-heterocyclic amines~ etc., each having 40 or less carbon atoms.

3 a ~
'I`heir specific c~amp1es are ethylamine, diethylamine, triethylamine, ethanol.lmine, diethanolamine, triethanolamine, n-, iso-, sec-, or tert- butylamine, N,N-dimethyl stearylamine, tri-isopropanolamine, etc.
Commercially available acid catalysts are PTSA (Nacure 2500X manufactured by King), DDBSA (Nacure 522S7;'lmanufactured ~,t by King), DNNSA (Nacure 1015 manufactured by King), DNNDSA
(Macure 155 manufactured by King), phosphate-type (Nacure 4054 manufactured by King), etc.
(G) Highly alkyl-etllerified melamine resin The highly alkyl-etherified melamine resins usable in the present invention are melamine resins whose amino groups are completely or at least 50% bonded wi~h alkyl ether groups. The amount of the alkyl ether groups is expressed as an average percentage of the alkyl ether groups actually bonded to the triazine ring, assuming that it is 100% when all of the hydrogen atoms of the amino groups in the melamine resin are substituted by alkoxy groups. Since there are three amino groups per each triazine ring, "50% or more of the alkyl ether 2 0 groups" means that 3 or more alkyl ether groups are bonded to one triazine ring on average. When the amount of the alkyl ether is less than S0% in the highly alkyl-etherified melamine resin, the resulting coated layer (film) shows poor acid resistance .
Specific examples of such highly alkyl-etherified melamine resins include those having amino groups bonded with methylol groups whose H atoms are substitwtecl with alkyl 5 ~ ~
groups such as a me~hyi glOLlp, an ethyl group, arl n-butyl group, an isobutyl group, etc.
Such highly alkyl-etherified melamine resin can be produced by an addition reaction or an addition-condensa~ion reaction between the melamine resin and an aldehyde such as formaldehyde, paraformaldehyde, etc. and then ~y etherification with a monovalent alcohol having 1-4 carbon atoms.
The highly alkyl-etherified melam;ne resin preferably has a number-aver.lge molecwlar weight of 2,000 or less. When the number-average molecular weight exceeds 2,000, ~he viscosity of the paint composition is unclesirably increased, resulting in a coated layer (film) having a poor appearance such as poor surface smoothness, etc. The preferred number-average molecular weigh~
of the highly alkyi-etherified melamine resin is 450-1,200.
As described above, the highly alkyl-etherified melamine resin generates carbonium ions in the presence of the acid catalyst, and the carbonium ions are subjected to an active hydrogen substitution reaction with the blocked polyisocyanate.
(d) Blocked polyisocyanate 2 0 If reaction is caused by the above hydroxyl group containing resin and the highly alkyl-etherified melamine resin having 50% or more of alkyl ether groups per one triazine ring, the resulting coated klyer (film) would not have desired acid resistance. Thus, the blockecl polyisocyanate having active hydrogen cap.lble of reacting with the carbonium ions generated from the highly alkyl-etherified melamine resin is added in the present inven~ion.

The blocked polyisocyanate used in ~he present invention has active hydrogen remaining at a baking temperature of the coated layer (film). Accordingly, the blocked polyisocyanate is substantially reacted not with the hydroxyl groups of the hydroxyl group containing resin, but with the highly alkyl-etherified melamine resin. Thus, the active hydrogen (hydrogen atoms in amino groups) of the blocked polyisocyanate is substituted witll carbonium ions generated from the highly alkyl-etherified melamine resin (refer to the formulae ( 1 ) and (2)).
The blocked polyisocyanates usable in the present invention include aliphatic polyfunctional isocyanates such as non-yellowing type hexamethylene diisocyanate (HMDI)?
alicyclic polyfunctional isocyanates such as isophorone diisocyanate (IPDI), other isocyanates such as diphenylmethane-

4,4'-diisocyanate (MDI), hydrogenated MDI, etc. The functional groups (NCO) of these polyisocyanate are partially or totally blocked with blocking agents which are not thermally dissociated.
2 0 The blocking agent should be chemically stable in the process of forming a wet film, and retain active hydrogen even at a baking temperature. The preferred blocking agents are alcohols, -caprolactam, phenols, etc. and alcohols are particularly preferable. Alcohols usable as blocking agents are 2 5 methanol, ethanol, isopropyl alcohol, butanol, 2-ethylhexanol, cyclohexanol, etc.
The polyisocyanate blocked with alcohols are generally dissociated at about 1 80-200C, though this dissoci~ltion tenlper.lture may change slightly depending on the types of alcollols. Since usllal baking conditions applicable to the thermosetting paint composition of this type are 120-160C and 15-30 minutes, the active hydrogen can remain in the

5 polyisocyanate under such baking conditions.
Particularly preferable as blocked polyisocyanate are HMDI and IPDI both blocked with Z ethylhexanol or methanol.
[C] Proportions of Components in Therrnosetting Paint Composition 1 0 ( 1 ) Weight ratio of (a) to (c) The weight ratio of component (a)/component (c) on a solid basis is preferably within the range of 40/60-90l10.
When it is less than 40/60 (a melamine resin exists too much), the resulting cs~ateci layer (film) shows drastically decreased acid 15 resistance and poor strength. On the other hand, when the weight ra~io of (a)/(c) exceeds 90/10, the coated layer (film) is not sufficiently hardened. The weight ratio of (a)/(c~ is more preferably within the range of 50/50-85/15, further preferably 55/45-80/20, and most preferably 60/40-75/25.
20 (2) Weight ratio of (d) to [(a)+(c)]
The weight ratio of component (d) to [component (a)+component (c)] on a solid basis is preferably within the range of 1.5/100-50/100. When it is less than 1.5/100, the substitution reaction of the active hydrogen of the blocked 2 5 polyisocyanate cloes not proceed sufficiently, resulting in a coated klyer (film) with poor acid resistance and scuff (mar) resist.lnce. On the other hand, when the weight ratio of (d)/~(a)+(c)l exceeds 50/100, the coatecl layer (film~ is not sut`ficiently harclellecl, also resulting in a coated layer (film) with poor acid resistance and scut`f (mat) resistance. The weight ratio of (d)/[(a)+(c)l is more preferably within the range of 5/100-30/100, and further preferably 10/100-20/100.
(3) Weight ratio of (b) to [(a)-~(c)+(d)l The amount of the acid catalyst (component (b)) expressed by a weight ratio of (b) to [(a)~(c)+(d)] on a solid basis is preferably within the range of 0.3/100-5/100. In other words, the amount of the acid catalyst is 0.3-5 weight % based on the total amount of the resin components. When it is less than 0 3 weight %, the above-described active hydrogen substitution reaction does not proceed sufficiently, resulting in a poor hardness oiF a coated layer (film). On the other hand, when it exceeds 5 weight %, the resulting coated layer (film) is too hard and brittle. The amount of component (b) is more preferably within the range of 0.5-3 weight %, and further preferably 0.5-2 weight %.
(4) Hydroxyl group containing resin When both of the hydroxyl group containing acrylic 2 0 resin ancl the hyclroxyl group containing polyester resin are used, a weight ratio of the hydroxyl group containing acrylic resin to the hydroxyl group containing polyester resin is preferably 100/0-60/40.
~D] Other Components 2 5 In aclclition to the above components, the thermosetting paint cornposition of the present invention may contain metallic pigments, inorgallic pigments, organic pigments, etc.

(a~ Metallic pigments The metallic pigments usable in the present invention include mica, finely divided metal flakes such as aluminum flakes, bronze flakes, tin flakes, gold flakes, silver 5 flakes, copper flakes, titanium flakes, stainless steel flakes, nickel flakes, chromium flakes, flakes of alloys of these metals, metal compound powders such as cobalt sulfide, manganese sulfide, titanium sulfide, etc., finely divided metal flakes coated with plastics, etc. These flakes preferably have an average 10 diameter of 1 0-45 ,um.
(b) Inorganic pigments The inorganic pigments usable in the presen~
inYention include transparent iron oxide such as Sicotrans red L2915D, Sicotrans yellow L1915 and 1916 available from BASF, 15 carbon black, titanium dioxide, yellow iron oxide, red iron oxide~
etc.
(c) Organic pigments The organic pigments usable in the present invention include Chromophthal red A213 available from Ciba GeigyT~
2 0 transparent pigments having dark color of red, blue, purple, brown, black, etc. such as Chromophthal violet ~and Irgazin yellow 3R available from Ciba Geigy, phthalocyanine blue, phthalocyanine green, quinacridoné, indanthrone, isoindolenone, perylene, anthrapyrimicline, benzimidazolone, etc.
2 5 (d) Organic solvents The organic solvents aclded to the thermosetting paint composition of the present invention to adjust its viscosity rnay be organic liquids or their mixtures commonly available as ., , ~7~ ,? ~
paint reducers. Org.lnic solvents having high boiling points are, for instance, alcohols such as ethylene glycol monobutyl ether, ethylene glycol rnonobutyl ether acetate, etc.; aromatic hydrocarbons such as solvesso 100, solvesso 150, etc.; ethers S such as carbitol acetate, butyl carbitol, etc. Organic solvents having low boiling points are, for instance, esters such as ethyl acetate, butyl acetate, etc.; aromatic hydrocarbons such as toluene, xylene, etc.; ketones such as methyl ethyl ketone, methyl isobutyl ketone, acetone, etc.; alcohols such as methanol, butanol, isopropanol, ethylene glycol monomethyl ether, etc.
[E] l~lethod of F'orming Coated Layer (Film) from Thermosetting Paint Composition The thermosetting paint composition of the present invention may be coated in the same manner as in usual automotive painting. First, a substrate is subjected to a chemical treatment tG form a chemical layer. After coating an electrodeposition layer and an intermediate layer successively on the chemical layer, the thermosetting paint composition of the present invention is coated as a topcoat paint. Alternatively, the 2 0 thermosetting paint composition may be applied as a clear topcoat onto a usual non-metallic or metallic basecoat to maintain a high durability. In this case, either or both of the electrodeposition layer and an intermediate layer may be omitted.
2 5 With respect to application conditions, they may be the same as conventional ones used for the automotive coating.
In a case where the paint composition of the present invention is applied as a non-metallic topcoat or a clear coat onto the ~76~
basecoat, an air spray method or an electrostatic coating method is preferable. In this case, the viscosity of ~ne paint is preferably 20-30 seconds/#4 Ford Cup (hereinafter referred to as #4FC) at 20C~
Baking conditions of the wet coated layer may be substantially the same as in a case where usual melamine resin or polyisocyanate is used alone as a cross-linking agent; 120-1 60C for about 15-30 minutes.
As described above, since the thermosetting paint composition of the present invention contains the component (d) in addition to the components (a) and (c), the following special reaction takes place in the process of forming a film. First, when the wet film formed from the thermosetting paint composition of the present invention is baked, a cross-linking reaction between the hydroxyl groups of the hydroxyl group containing resin and the melamine resin proceeds, but substantially no reaction takes place between the blocked polyisocyanate and the hydroxyl group containing resin because the blocked polyisocyanate retains active hydrogen in the baking process of the coated layer 2 0 (film) (namely, the blocking agent is kept bonded to the polyisocyanate). However, since the blocked polyisocyanate is reactive to the melamine resin, the remaining functional groups of the melamine resin which have not participated in a reaction with the hydroxyl group containing resin (component (a)) are 2 5 reacted with the blocked polyisocyanate. In other words, the above-described active hydrogen substitution reaction proceeds.
By this reaction, the self-condensation of the component (c) can be prevented, thereby providing a coated layer (film) with e;ccellent acicl resistance arld scuff (mar) resistance. Incidentally, if the self-conclens.ltion of the cornponent (c) takes place, tne resulting coated layer (film) would show a drastically decreased acid resistance.
The presen~ invention will be explained in further detain by the following Examples.

Examplese_l-13~ Compar2tive Exarnples_ 1 6 A dull steel plate having a th;ckness of 0.8 mm treated with a zinc phosphate chernical was subjected to electrodeposition coating using a cationic electrodeposition paint [POWERTOP U-50 manufactured by Nippon Paint Co., Ltd.] at a dry film thickness of about 25 llm.
Next, the resulting electrodeposition-coated layer was coated with an inteTmedia~e coating paint [ORGA S-90 SEALER7~
manufactured by Nippon Paint Co., Ltd.l at a dry film thickness of about 40 !lm by an air-spraying method and baked at 140QC for 30 minutes. Thereafter, an acrylic resin basecoat paint [SIJPERLUCK, manufactured by Nippon Paint Co. Ltd.] was applieel at a dry film thickness of about 20 ,um, and subjected to setting for 3 minutes.
Next, each of the resulting clear paint compositions shown in Table 1 was adjusted so that it had a viscosity of 2S seconds by FORDCUP
~4, and applied at a dry film thickness of about 40 ~m under spraying pressure of 5 kg/cm2 by means of an electrostatic spray 2 5 machine (AUTO REA, manufactured by Ransberg Gemma). hfter setting t`or 7 minutes, it was baked at 1~()C for 30 minutes.
For comparison, a thermosetting paint composition eontaining no polyisocyanate (Connparative Examples 1, 5 and 6), a thermosetting pairlt colllposition containing a polyisocyanate blocked with MEK oxime (Comparative Example 2), a thermosesting paint composition containing no acid catalyst (Comparative Examples 3 and 4) were prepared, and coated and baked in the 5 same manner as in Example 1.
Incidentally, in Example 11, a 2-coat, 2-bake system was used. In Example 8 and Comparative E~xample 5, a topcoat paint shown in Table l was coated at a dry film thiclcness of about 40 ~lm directly on the above intermediate coat, and set for about 7 mirlutes and then baked at 140C for 30 minutes.

3 $~ ~
Table l - I
Example No. 1 2 3 4 5 Component (a) Acrylic Resin-l(l) 60 60 40 81 60 Acrylic Resin-2(2) Acrylic Resin-3(3) - - - - -Polyester Resin- 1(4) Polyester Resin-2(5) Component (b) Acid Catalyst-1(6) 1 1 0 S 3 Acid Catalyst-2(7) Component (c) Melamine Resin-1(8)3 2 - 5 4 1 4 14 Melamine Resin-2(9) - - - -Melamine Resin-3(1) - 2 4 _omponent (d) IC-1(1l) 8 16 6 5 IC-2(12) - - - 16 IC-3(l3) IC-4(14) IC-5(15) Pigment(l6) Wel ~h t R atio (a)/(c) 65/35 71/29 43/57 85/lS 71/29 (d)(17) 8.7/10019/100 6.4/100 5/10019/100 (b)(t8~ I/lO0 1/100 0.5/lO0 3/100 1/100 - 2~ -.

~7~50~
Table 1-2 ~ç~, 6 7 8 9 1 0 Come~nent (a~
Acrylic Resin- 1(1) - 6 0 - 4 6 Acrylic Resin-2(2) 4 7 Acrylic Resin-3(3) - - - - 5 5 Polyester Resin-1(4) - - 7 0 Polyester Resin-2(5) 8 Component ~b) Acid Catalyst- 1(6) 1 1 1 2 2 Acid Catalyst-2(7) Component ~c) Melamine Resin- I (8) 3 6 - 2 4 31 3 6 Melamine Resin-2(9) - 2 4 Melamine Resin-3(10) Component (d) IC-1(1l) 9 16 6 23 9 IC-2(l2) - - - - - :
IC-3(13) Ic-4t l 4) IC-5(15) Pigment(16) - - 3 - - ;
Wei~ht Ratio ~ :
(a)/(c) 60/40 71/29 74/26 60/40 :~60/40 (d)(17) 9.:9/100 19/1006.4/100 30/100 9.91100 b)(l8) 1/100 1/100 1/100 2/100 : 2/100 , ..

:

~7~
~ ble 1 -3 No. Ex. I I Ex. 12 Ex. 13 (Com. Ex I Com. Ex. 2 Component (a~
Acrylic Resin- 1(1) 6 0 6 0 6 0 6 0 6 0 Acrylic Resin-2(2) - - - ~ -Acrylic Resin-3(3) - - - - -Polyester Resin- 1(4) Polyester Resin-2(s) Component (b~
Acid Catalyst- 1(6) - 2 2 Acid Catalyst-~(7) Component (c) Melamine Resin- 1(8) 3 2 2 8 2 8 4 0 3 2 Melamine Resin~2(9) Melamine Resin-3(10) Component (d) IC-1(1l) 8 IC 2(l2) IC-3(13) - 8 IC-4(l 4) 1 4 - .
Ic-S(ls) - - 16 Pigment(16) Weight Ratio (a)/(c) 65/35 68/32 68132 60/4065/35 (d)(17) 8.7/100 16/100 18/100 0/1008.7/100 (b)(l8) 1/lO0 2/100 2/100 l/lO01/100 Ç~
'I'.able 1-4 Comparative Exam,ple No. 3 4 5 6 Component (a~
Acrylic Resin-l(l) 60 60 - -Acrylic Resin-2(0 - - - 4 7 Acrylic Resin-3(3) - - - -Polyester Resin-1(4) - - 7 0 Polyester Resin-2(5) - - - g Component (b) Acid Catalyst- 1(6) _ -Acid Catalyst-2(7) - - - -Component (c) Melamine Resin-1(8) - - - 4 5 Melamine Resin-2(9) - 2 4 3 0 Melamine Resin-3(10)3 2 - - -Component (d) IC-1(1l) 8 16 IC-2(l2) IC-3(13) IC-4(l4) IC-5(1s) Pigment(16) - - 3 ,Wei~ht_Ratio (a)/(c) 65/35 71/29 70/3055/45 (d)(17) 8.7/100 19/100 0/1000/100 (b)(18) 0/100 0/100 l/lO01/100 Note:
(1): Acrylic resin (copolymer of styrene, ethylhexyl methacrylate, n-butyl methacrylate and hydroxyethyl methacrylate, and having a number-average molecular weight of 47000, a hydroxyl value of 95 and an acid value of 22).
(2): Acrylic resin (copolymer of styrene, ethylhexyl methacrylate, ethylhexyl acrylate and hydroxyethyl methacrylate, and having a number-average molecular weight of 2,000, a hydroxyl value of 100 and an acid value of 20).
(3): Acrylic resin (copolymer of styrene, lauryl methacrylate, hydroxybutyl methacrylate and isobutyl methacrylate, and having a number-average molecular weight of 4,000, a hydroxyl value of 95 and an acid value of 0).
(4): Polyester resin (copolymer of phthalic anhydride, isophthalic aGid, trimethylol ethane and neopentyl glycol and having a number-average molecular weight of 3,000, a hydroxyl value of 80, and an acid value of 5).
(5~: Polyester resin (copolymer of hexahydrophthalic anhydride and neopentyl glycol and having a number-average molecular weight of 500, a hydroxyl value of 270, and an acid value of 10).

(6): Dodecylbenzene sulfonic acid (Nacure 5225 available from King) blocked with diethanol amine.

(7): Dodecylbenzene sulfonic acid (Nacure 5225 available from 2 5 King).

(8): Methylbutyl-substituted methylol-type melamine resin (C~MEL 267, m~lnuf'actured by American Cyanamide, alkyl etller group content: about 80~/o).

(9): Methylbutyl-substitlltecl melamine resin (CYMEL 232, manuf,lctured by American Cyanamide, alkyl ether group content: 90% or more).

(10): Methylbutyl-substituted imino-type melamine resin (CYMEL
253, manufactured by American Cyanamide, alkyl ether group content: about 30%).
( 11 ): 2-ethylhexanol-blocked polyisocyanate (nurate-type HMDI).
(12): Methanol-blocked polyisocyanate (nurate-type HMDI).
(13): MEK oxime-blocked polyisocyanate (nurate-type HMDI) (SUMlDUR-BL4165,'rmanufactured by Sumitomo Bayer~.
(14): Polyisocyanate (CORONATE HX7~anufactured by Nippon Polyurethane Co. (nurate-type HMDI), 2/3 of NCO groups of which are blocked with cyclohexanol).
(15): Polyisocyanate (the above partially blocked polyisocyanate (14), the remaining NCO groups of which are completely blocked with -caprolactam).
(16): MA-10~manufactured by Mitsubishi Carbon Co.).
~17): Weight ratio on a solid basis: (d)/[(a)+(c)].
(18): Weight ratio on a solid basis: (b)/[(a)+(c)~(d)].
Each coated layer (film) thus produced was evaluated with respect to film appearance, yellowing, pencil hardness, acid resistance and scuff (mar) resistance according to the following standards: The results are shown in Table 2.
2 5 ( 1 ) Film appearance Each test piece produced by the above method was observed by the bare eye with respect to defects such as roughness, gloss, blistering, dent, uneven color, cracking, wrinkling, etc., and evalu,lte(l according to tlle following standards:
O: No defects.
x: There was at least one of the above defects.
5 (2) Yellowing A tinplate was coated with a non-yellowing type7 high solid type, white topcoat paint based on a polyester resin and a melamine resin in a thickness of 40 llm, and baked at 140C for 25 minutes to obtain a reference plate. The reference plate was coated with each paint composition of Examples 1-7 and 9-13 and Comparative Examples 1-4 and 6 in a thickness of 40 llm with one side masked, and baked at 1 40C for 30 minutes. The difference in color between the reference surface (masked surface) and the coated surface was measured by an SM color computer (SM-37 15 available from Suga Shikenki K. K.).
(3) Pencil hardness Measured according to JIS K5400 8, 4.2.
(4) Acid resistance 0.6 ml of a 10~weight-% sulfalric acid solution was 2 0 dropped onto each test piece kept horizontal at 70C, and the test piece was kept at 70C for 15 minutes without air flow. After drying, defects such as discoloration, etching and blistering were observed with the naked eye. The evaluation of acid resistance was conducted according to the following standards:
2 5 ~): No traces (defects).
O: Slight traces, but they could be erased by polishing the test piece surface with an ultrafine powder compound (Unicon FMC-81J available from Ishihara Yakuhin K. K.).
X: Substantial traces, wlhich could not be erased by polishing with the above compound.
(5) Scuff (mar) resistance About 1 cm3 of a cleanser liquid [mixture of a foamable-~
type cleanser available from Kaneyo K. K. ~a~rasive: 93%, and~;linear~;
alkylbenzene surfactant: 4%) and water at a ratio of 1/3] was dropped onto two plies of flannel, and the knit was fixed to a vibration head of a ~wear resistance tester. Under a load~ of ~500 g, the vibration head was moved reciprocally 1 0 times. Thereafter, the 20 gloss of a tested poriion of the flannel was mensured; to~
determine the gloss ;retent1on. The evaluation of scuff (mar) resistance was conducted according to the following standards.
Gloss Retention ~): 90 or more.
0: 70: or~ more and less than 90.
40 or~ more and less than 70.
X: ~less ~than ~0.~

:~ :

~. ~ : : :

, : :

; . . .

~ .

2~rl~0~
I`able 2 Film Pencil Acid Scuff (Mar) ~L ~L~ Yellowing Hardne~ss B~s~ Resistance Exam O 0.5 F
2 0 1) . 6 F (~ ~) 3 O 0.5 F ~
4 0 0.~ ) O
O 0.6 F
6 O 0.6 ~ O O
7 O 0.3 F ~ ~
8 0 - HB O o 9 0 0.7 F ~) O
0 7 H (~
1 1 O 0.8 H O
12 O 0.6 H O O
13 O 0.8 H O O
Comparative Exam~e 0 0.5 F x O
2Yellowed 6.4 F X O
3 0 0.4 ~IB X X

O - ~' x O
6 0 0.6 F X O
Note *: The paint composition was not hardened well.

As describecl above in cletail, the coatecl layer obtained 5 by using the thermosetting p~lint composition of the present invention h.acl excellent scuff (mar) resistance and acid resistance, as well as goocl film appe.lrance without suffering from yellowing.
The thermosetting paint composition of the present invention was suitable for an automotive topcoat and a coat for buildings, o~ltdoor 5 constructions, etc. It may also be used for clear coats to be formed on the topcoats made of high-solid paints, metallic paints, etc

Claims (32)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A thermosetting paint composition comprising:
(a) a hydroxyl group containing resin;
(b) all acid catalyst;
(c) a highly alkyl-etherified melamine resin capable of forming carbonium ions in the presence of said acid catalyst;
and (d) a blocked polyisocyanate having active hydrogen at a baking temperature of a wet coated layer, said active hydrogen being substitutable with said carbonium ions formed from said lightly alkyl-etherified melamine resin, the amount of said acid catalyst being, on a solid basis, 0.3-5 weight % based on the total amount of the resin components.

2. The thermosetting paint composition according to claim 1, wherein said hydroxyl group containing resin is a hydroxyl group containing acrylic resin or a hydroxyl group containing polyester resin.

3. The thermosetting paint composition according to claim 1 or 2, wherein said hydroxyl group containing resin is a hydroxyl group containing acrylic resin having a hydroxyl value of 50-200 and an acid value of 0-50.

4. The thermosetting paint composition according to claim 1 or 2, wherein said hydroxyl group containing resin is a hydroxyl group containing polyester resin having a hydroxyl value of 50-350 and an acid value of 3-50.

5. The thermosetting paint composition according to any one of claims 1-4, wherein said acid catalyst is at least one selected from the group consisting of carboxylic acids, sulfonic acids and organic alkyl phosphates.

6. The thermosetting paint composition according to any one of claims 1-5, wherein said acid catalyst is a blocked acid catalyst.

7. The thermosetting paint composition according to any one of claims 1-6, wherein said highly alkyl-etherified melamine resin is a melamine resin in which 50% or more of hydrogen atoms of amino groups are substituted with alkyl ether groups.

8. The thermosetting paint composition according to claim 1, wherein a weight ratio of (a)/(c) is 40/60-90/10.

9. The thermosetting paint composition according to claim 1 or 8, wherein a weight ratio of (a)/(c) is 50/50-85/15.

10. The thermosetting paint composition according to claim 1 or 8, wherein a weight ratio of (a)/(c) is 55/45-80/20.

11. The thermosetting paint composition according to any one of claims 1, 8-10, wherein a weight ratio of (d)/[(a)+(c)] is 1.5/100-50/100.

12. The thermosetting paint composition according to any one of claims 1 and 8-11, wherein a weight ratio of (d)/[(a)+(c)] is 5/100-30/100.

13. The thermosetting paint composition according to any one of claims 1 and 8-12, wherein a weight ratio of (d)/[(a)+(c)] is 10/100-20/100.

14. The thermosetting paint composition according to claim 1, wherein a weight ratio of (b)/[(a)+(c)+(d)] is 0.3/100-5/100.

15. The thermosetting paint composition according to claim 1 or 14, wherein a weight ratio of (b)/[(a)+(c)+(d)] is 0.5/100-3/100.

16. The thermosetting paint composition according to claim 1, 14 or 15, wherein a weight ratio of (b)/[(a)+(c)+(d)] is 0.5/100-2/100.

17. A method of forming a coated layer comprising the steps of;
(1) coating, onto a substrate a thermosetting paint composition comprising;
(a) a hydroxyl group containing resin;
(b) an acid catalyst;
(c) a highly alkyl-etherified melamine resin capable of forming carbonium ions in the presence of said acid catalyst;
and (d) a blocked polyisocyanate having active hydrogen at a baking temperature of a wet coated layer, said active hydrogen being substitutable with said carbonium ions formed from said highly alkyl-etherified melamine resin, the amount of said acid catalyst being, on a solid basis, 0.3-5 weight % based on the total amount of the resin components;
and (2) baking the wet coated layer to cause (i) a cross-linking reaction between said hydroxyl group containing resin and said highly alkyl-etherified melamine resin, (ii) the formation of carbonium ions from the remaining functional groups of said highly alkyl-etherified melamine resin in the presence of said acid catalyst, and (iii) the reaction between said carbonium ions and said blocked polyisocyanate.

18. The thermosetting paint composition according to claim 17, wherein said hydroxyl group containing resin is a hydroxyl group containing acrylic resin or a hydroxyl group containing polyester resin.

19. The thermosetting paint composition according to claim 17 or 18, wherein said hydroxyl group containing resin is a hydroxyl group containing acrylic resin having a hydroxyl value of 50-200 and an acid value of 0-50.

20. The thermosetting paint composition according to claim 17 or 18, wherein said hydroxyl group containing resin is a hydroxyl group containing polyester resin having a hydroxyl value of 50-350 and an acid value of 3-50.

21. The thermosetting paint composition according to any one of claims 17-20, wherein said acid catalyst is at least one selected from the group consisting of carboxylic acids, sulfonic acids and organic alkyl phosphates.

22. The thermosetting paint composition according to any one of claims 17-21, wherein said acid catalyst is a blocked acid catalyst.

23. The thermosetting paint composition according to any one of claims 17-22, wherein said highly alkyl-etherified melamine resin is a melamine resin in which 50% or more of hydrogen atoms of amino groups are substituted with alkyl ether groups.

The thermosetting paint composition according to claim 17, wherein a weight ratio of (a)/(c) is 40/60-90/10.

25. The thermosetting paint composition according to claim 17 or 24, wherein a weight ratio of (a)/(c) is 50/50-85/15.

26. The thermosetting paint composition according to any one of claims 17, 24 and 257 wherein a weight ratio of (a)/(c) is 55/45-80/20.

27. The thermosetting paint composition according to any one of claims 17 and 24-26, wherein a weight ratio of (d)/[(a)+(c)]
is 1.5/100-50/100.

28. The thermosetting paint composition according to any one of claims 17 and 24-27, wherein a weight ratio of (d)/[(a)+(c)]
is 5/100-30/100.

29. The thermosetting paint composition according to any one of claims 17 and 24-28, wherein a weight ratio of (d)/[(a)+(c)]
is 10/100-20/100.

30. The thermosetting paint composition according to claim 17, wherein a weight ratio of (b)/[(a)+(c)+(d)] is 0.3/100-5/100.

31. The thermosetting paint composition according to claim 17 or 30, wherein a weight ratio of (b)/[(a)+(c)+(d)] is 0.5/100-3/100.

32 . The thermosetting paint composition according to claim 17, 30 or 31, wherein a weight ratio of (b)/[(a)+(c)+(d)] is 0.5/100-2/100.