JPH083288A - Thermosetting composition and method for curing the same - Google Patents

Abstract

PURPOSE:To obtain a composition which has excellent storage stability and cures at a low temp. to give a cured composition free from discoloration by using a specific complex catalyst as a reactive curing catalyst for an epoxidized compound. CONSTITUTION:An epoxidized compound (A) is mixed with a compound (B) having reactive silicon groups and a tin chelate catalyst (C). The epoxy groups in the ingredient (A) especially preferably are alicyclic ones. Examples of the ingredient (A) include a polymer obtained by polymerizing an epoxidized ethylenic monomer optionally with another kind of ethylenic monomer. The reactive silicon groups of the compound (B) each comprises a hydrolyzable group, preferably an alkoxyl, directly bonded to a silicon atom. Examples of the ingredient (B) include a (co)polymer of an ethylenic monomer containing a reactive silicon group. The ingredient (C) is desirably an oxygen compound, preferably capable of forming a keto-enol tautomer. Examples thereof include acetoacetic esters and beta-diketones.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a novel curable composition and a curing method using the curable composition.

[0002]

Conventionally, Lewis acids (eg, aluminum chloride, tin chloride, boron fluoride, boron fluoride ether complex, etc.), metal alcoholates (eg, aluminum trialkoxide, titanium tetraalkoxide, etc.) have been used as a curing catalyst for epoxy resins. , Aluminum chelate compounds (trisacetylacetonatoaluminum, etc.), composites of aluminum chelate compounds and active hydrogen compounds (reactants, etc., of trisacetylacetonatoaluminum and silanol compounds), etc. are known.

However, among these catalysts, the curable composition containing the above-mentioned Lewis acid or metal alcoholate has remarkably poor storage stability at room temperature, and the curability composition containing the aluminum chelate compound or a composite thereof. Each of the compositions has drawbacks such as poor storage stability at room temperature and yellowing of the cured product.

[0004]

The present invention has been made for the purpose of providing a curable composition which is excellent in storage stability and can be cured at a low temperature and can form a cured product without discoloration. is there.

[0005]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that a reactive silicon group-containing compound is used as a reactive curing catalyst for an epoxy group-containing compound. It was found that a curable composition using a composite catalyst in combination with a tin chelate catalyst is excellent in storage stability, low-temperature curability, etc., and is excellent in discoloration resistance of the cured product. It came to completion.

That is, the present invention is a thermosetting composition comprising (A) an epoxy group-containing compound, (B) a reactive silicon group-containing compound, and (C) a tin chelate catalyst, Also, the present invention relates to a curing method characterized by applying an organic solution or dispersion prepared by dissolving or dispersing the thermosetting composition in an organic solvent to a substrate, and then heating and curing the composition.

In the present invention, the reactive silicon group means a silanol group which is a group in which a hydroxyl group is directly bonded to a silicon atom, or a group in which a hydrolyzable group is directly bonded to a silicon atom (a group which produces a silanol group by hydrolysis). ) Means. In the reactive silicon group-containing compound, one kind or two or more kinds of these groups can be present.

In the present invention, examples of the hydrolyzable group directly bonded to a silicon atom include an alkoxyl group represented by the general formula —OR ¹ , an acyloxy group represented by the general formula —OCOR ² , and a general formula — A ketoxime group represented by ON = C (R ³ ) ₂ and the like are included. In each of the above general formulas, R ¹ represents an alkyl group having 1 to 5 carbon atoms, preferably 1 to 3 carbon atoms, such as methyl, ethyl, propyl and butyl,
R ² represents an alkyl group having 1 to ³ carbon atoms, and R ³ represents an alkyl group having 1 to 5 carbon atoms.

Specific examples of preferable hydrolyzable groups include alkoxyl groups such as methoxy group, ethoxy group and propoxy group; ketoxime groups such as acetoxy group such as acetoxy group and propionyloxy group, acetoxime group and propionyloxime group. Groups and the like. Among these hydrolyzable groups, an alkoxyl group having 1 to 3 carbon atoms, which has excellent effects on storage stability and low temperature curability, is particularly preferable.

In the present invention, the epoxy group in the component (A) is an aliphatic type epoxy group in which an ether group is bonded between the carbon atoms of a linear hydrocarbon group, or between the carbon atoms of an alicyclic hydrocarbon group. The epoxy group may be any type of alicyclic type epoxy group to which an ether group is bonded, but when it is an alicyclic type epoxy group, it may be subjected to cationic polymerization reaction or cationic addition depending on the other components (B) and (C). Since the reaction occurs easily and quickly, it is characterized by excellent low temperature curability.

The curable composition of the present invention comprises (A) an epoxy group-containing compound, (B) a reactive silicon group-containing compound, and (C) a tin chelate catalyst.

The respective components to be blended in the composition of the present invention will be described in more detail below.

Compound (A) As the compound (A), any conventionally known epoxy group-containing compound can be used as long as it has an epoxy group, and in particular, the number of epoxy groups is average in one molecule. Suitable are those having an average molecular weight of about 1 or more, preferably about 2 to 100, and an average molecular weight of about 130 to 100,000.
If the number of the epoxy groups is less than about 1 on average in one molecule, low-temperature curability is deteriorated, which is not preferable.

The compound (A) is, for example, a polymer of an epoxy group-containing ethylenically unsaturated monomer and, if necessary, other ethylenically unsaturated monomer; bis (3,4)
-Epoxycyclohexylmethyl) adipate, bis (3,4-epoxy-6-methylcyclohexylmethyl) adipate, 3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate, 3, 4-epoxycyclohexenecarboxylic acid ethylene glycol diester, 3,
Alicyclic epoxy compounds such as 4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate; (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, trimethylolpropane triglycidyl ether,
Aliphatic epoxy compounds such as epoxidized polybutadiene and triglycidyl isocyanurate; resins such as bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol B type epoxy resin, hydrogenated bisphenol type epoxy resin and phenol novolac type epoxy resin Can be mentioned.

The polymers of the above-mentioned epoxy group-containing ethylenically unsaturated monomer and, if necessary, other ethylenically unsaturated monomers, have an average molecular weight of about 8,000 to 10 in particular.
Those in the range of 10,000 are preferred.

In the above-mentioned polymer, examples of the epoxy group-containing ethylenically unsaturated monomer include glycidyl (meth) acrylate, methylglycidyl (meth) acrylate, allyl glycidyl ether, and 3,4-epoxycyclohexylmethyl (meth) acrylate. Etc. are suitable.

The other ethylenically unsaturated monomers copolymerized with the epoxy group-containing ethylenically unsaturated monomer as required include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, Butyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, stearyl (meth) acrylate,
C _1-24 alkyl (meth) acrylates such as cyclohexyl (meth) acrylate or cycloalkyl (meth) acrylates; perfluorobutylethyl (meth) acrylate, hydroxyethyl (meth) acrylate, perfluorooctylethyl (meth) acrylate , Perfluoroisononylethyl (meth) acrylate, perfluorodecylethyl (meth) acrylate and other perfluoroalkyl group- or perfluoroalkenyl group-containing ethylenically unsaturated monomers; styrene, vinyltoluene and other aromatic vinyl monomers; Nitrile monomers such as (meth) acrylonitrile; amide monomers such as (meth) acrylamide and N-butoxymethyl (meth) acrylamide.

Compound (B) As the compound (B), any conventionally known reactive silicon group-containing compound can be used as long as it contains a reactive silicon group. Is preferably about 1 or more in one molecule, preferably about 2 to 300 on average, and an average molecular weight of about 76 to 100,000. When the number of the reactive silicon groups is less than about 1 on average in one molecule, low temperature curability is deteriorated, which is not preferable.

Examples of the compound (B) include methoxytrimethylsilane, ethoxytriethylsilane, propoxytripropylsilane, butoxytributylsilane,
Monofunctional silane compounds such as methoxytrioctylsilane, methoxytriphenylsilane, methoxytribenzylsilane, triphenylhydroxysilane; dimethoxydimethylsilane, dimethoxydiethylsilane, diethoxydibutylsilane, dipropoxydipropylsilane, dimethoxydilaurylsilane, Dimethoxydiphenylsilane,
Bifunctional silane compounds such as dimethoxydibenzylsilane, methoxybenzyloxydipropylsilane, methoxy2-ethylhexyloxydipropylsilane, diphenylsilanediol; trimethoxymethylsilane, triethoxyethylsilane, tripropoxypropylsilane, trimethoxystearylsilane , Trimethoxyphenylsilane, trimethoxybenzylsilane, methoxydibenzyloxypropylsilane, methyltrihydroxysilane, phenyltrihydroxysilane and other trifunctional silane compounds; tetramethoxysilane, tetraethoxysilane,
Tetrapropoxysilane, tetrabutoxysilane, trimethoxybenzyloxysilane, triethoxybenzyloxysilane, dimethoxydi2-ethylhexyloxysilane, tetrahydroxysilane and other tetrafunctional silane compounds; the above-mentioned trifunctional silane compounds and / or tetrafunctional silane compounds. Low condensate (about 2 to 20 mer); vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris (2-methoxyethoxy) silane, γ- (meth) acryloyloxypropyltrimethoxysilane, γ- (meth) acryloyl Oxypropyltriethoxysilane,
Reactive silicon group-containing ethylenically unsaturated monomers such as γ- (meth) acryloyloxypropylmethyldimethoxysilane and β- (meth) acryloyloxyethylpropyltrimethoxysilane, and optionally other ethylenically unsaturated monomers And the like.

Tin Chelate Catalyst (C) The tin chelate catalyst (C) is a compound in which an organic chelating agent is chelated to part or all of the tetravalent valence of tin element.

As the organic chelating agent, a conventionally known organic chelating agent can be used, and the organic chelating agent includes a compound containing at least one atom selected from oxygen atom, sulfur atom and nitrogen atom. To be done. Among the above, a compound containing an oxygen atom, and further a compound capable of forming a keto / enol tautomer are preferable.

Examples of the compound capable of forming the keto-enol tautomer include β-diketones (acetylacetone etc.), acetoacetic acid esters (methyl acetoacetate etc.),
Malonic acid esters (ethyl malonate, etc.), ketones having a hydroxyl group at the β-position (diacetone alcohol, etc.), aldehydes having a hydroxyl group at the β-position (salicylaldehyde, etc.), esters having a hydroxyl group at the β-position ( Methyl salicylate, etc.) and the like can be used. Particularly, preferable results are obtained by using acetoacetic acid esters and β-diketones.

The tin chelate catalyst (C) is usually an organic chelating agent in an amount of 1 to 1 with respect to 1 mol of a tetravalent tin compound.
It can be prepared by mixing at a molar ratio of about 0 mol and heating if necessary.

Examples of the tin compound include organic acid salts, alkoxides, hydroxides, halides, sulfides, and combinations of these.

The organic acid salt and alkoxide of the above tin compound are represented by the following general formula (I) (R ⁴ ) _n Sn (OR ⁵ ) _4-n (I) (wherein R ⁴ is the same or different). Represents a monovalent hydrocarbon group having 1 to 20 carbon atoms, R ^{5 s} are the same or different,
A monovalent hydrocarbon group having 1 to 20 carbon atoms or

[0026]

Embedded image

Is shown. Moreover, n shows the integer of 0-2. ) Are included.

The monovalent hydrocarbon group having 1 to 20 carbon atoms of the tin compound represented by the above general formula (I) is an aliphatic hydrocarbon group, an alicyclic hydrocarbon group or an aromatic hydrocarbon group. And any combination of these groups. The aliphatic hydrocarbon group may be linear or branched, and may contain an unsaturated bond, for example,
Methyl, ethyl, n-propyl, iso-propyl, n
-Butyl, iso-butyl, tert-butyl, n-pentyl, iso-pentyl, tert-pentyl, ne
Examples thereof include o-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, dodecyl, tridecyl and tetradecyl groups.

Examples of the alicyclic hydrocarbon group include a cyclohexyl group and the like. Examples of the aromatic hydrocarbon group include phenyl, methylphenyl, dimethylphenyl, toluyl and xylyl groups. In addition, examples of the group in which the aliphatic hydrocarbon group and the aromatic hydrocarbon group are combined include a benzyl group and a phenethyl group.
Among these hydrocarbon groups, an alkyl group having 1 to 20 carbon atoms and a phenyl group are preferable.

In the organotin compound of the general formula (I), R ⁵ is

[0031]

Embedded image

Those having an ester bond are excellent in storage stability and low temperature curability. Further, in the general formula (I), it is preferable that R ⁴ is an alkyl group having 1 to 20 carbon atoms or a phenyl group.

Further, in the above general formula (I), in particular,
The one in which n is 0 or 1 is excellent in storage stability and low temperature curability.

Examples of the tin compound represented by the general formula (I) containing an ester bond include, for example, tin tetraacetate, tin tetraoctate, tin tetralaurate, butyltin triacetate, butyltin tributyrate and butyltin trihexylate. , Butyltin trioctate, butyltin trilaurate, octyltin triacetate, octyltin tributyrate, octyltin trihexylate, octyltin trioctate,
Octyltin trilaurate, phenyltin tributyrate, phenyltin trilaurate, dibutyltin diacetate, dibutyltin dibutyrate, dibutyltin dihexylate, dibutyltin dioctate, dibutyltin dilaurate, dioctyltin diacetate, dioctyltin dibutyrate, dioctyl Examples thereof include tin dihexylate, dioctyl tin dioctate, and dioctyl tin dilaurate. In addition, examples containing an ether bond include butyltin trimethoxy, butyltin tributoxy, octyltin trimethoxy, phenyltin trimethoxy, dibutyltin dimethoxy, dioctyltin dimethoxy, dioctyltin dibutoxy, tributyltin butoxy, and the like.

Among the above, tin tetraoctate, tin tetraacetate, butyltin triacetate, butyltin tributyrate, butyltin trioctate, butyltin trilaurate, octyltin triacetate, octyltin tributyrate and the like are particularly preferable.

Examples of the halide in the above tin compound include a stannic chloride compound and the like. As the sulfide, for example, the general formula _{^{(II) (R 4) n}} Sn (SR 6) 4-n (II) ( In the formulas, R ⁶ are the same or different, having from 1 to 20 carbon atoms
A monovalent hydrocarbon group or -C _m H _2m COOR ⁴ groups. m represents an integer of 1 to 4. R ⁴ and n have the same meanings as described above. ) Are included. The monovalent hydrocarbon group having 1 to 20 carbon atoms for R ⁶ includes the same ones as described above.

Specific examples of the sulfide include C _{4
H 9 Sn (SCH 2 COOC 8} H 17) 3, (C 4 H 9)
₂ Sn (SCH ₂ COOC ₈ H ₁₇ ) ₂ , C ₈ H ₁₇ Sn
(SCH ₂ COOC ₈ H ₁₇ ) ₃ , (C ₈ H ₁₇ ) ₂ Sn
(SCH ₂ COOC ₈ H ₁₇ ) ₂ , (C ₄ H ₉ ) ₂ Sn
_{(SC 4 H 9) 2,} C 4 H 9 Sn (SC 4 H 9) 3 and the like.

Examples of the tin chelate catalyst (C) include tetravalent tin and an organic chelating agent in a 2 mol chelate bond, for example, a compound represented by the formula:

[0039]

Embedded image

(In the above formula, A is the same or different and represents R ⁴ , OR ⁵ or SR ⁶ ,

[0041]

[Chemical 4]

Shows a chelate ring structure formed by an organic chelating agent. In addition, R ⁴ , R ⁵ and R ⁶ have the same meanings as described above. ) Can be used.

Preferred specific examples of the tin chelate catalyst (C) include, for example, monobutylbis (ethylacetoacetate) tin acetate, monobutylbis (acetylacetonato) tin octate, monobutylbis (ethylacetoacetate) tin laurate, and bis (propylacetoacetate) tin. Diacetate, bis (propylacetoacetate) tin dilaurate, bis (acetylacetonato) tin diacetate, bis (acetylacetonato)
Examples include tin dioctate, bis (acetylacetonato) tin dilaurate, bis (ethylacetoacetate) tin dioctate, bis (ethylacetoacetate) tin diacetate, bis (ethylacetoacetate) tin dilaurate, and bis (propylacetoacetate) tin dioctate. it can.

In the thermosetting composition of the present invention, the compounding ratio of the compound (A), the compound (B) and the tin chelate catalyst (C) is about 0 for the compound (B) per 100 parts by weight of the compound (A). 1 to 200 parts by weight, preferably about 1 to 1
100 parts by weight, and the tin chelate catalyst (C) is about 0.
A range of 001 to 20 parts by weight, preferably about 0.01 to 15 parts by weight is suitable.

When the compound (B) and the tin chelate catalyst (C) are less than the above-mentioned ranges, the low temperature curability deteriorates,
On the other hand, if it exceeds the above range, further low temperature curability is not obtained, and the finished appearance and durability of the cured product are also deteriorated, which is not preferable.

The thermosetting composition of the present invention contains a colorant, a filler, an organic solvent, if necessary, in addition to the components of the compound (A), the compound (B) and the tin chelate catalyst (C) described above.
An ultraviolet stabilizer, an ultraviolet absorber, a fluidity modifier and other additives can be added.

The thermosetting composition of the present invention can be cured at a temperature of 60 ° C. or higher, and its application is not particularly limited and can be selected from a wide range of fields and applied. Particularly preferably, it can be applied to the field of paints, for example.

Next, the curing method of the present invention will be described.

The curing method of the present invention is carried out by applying an organic solution or dispersion prepared by dissolving or dispersing the above-mentioned thermosetting composition in an organic solvent onto a substrate and then heating to cure it. be able to. At this time, the solid content concentration is
It is preferably about 20 to 99% by weight.

The organic solvent for dissolving or dispersing the thermosetting composition may be appropriately selected depending on the kind of the thermosetting composition, but is usually an ester type, a ketone type, an ether type, an alcohol type or a hydrocarbon type. A solvent such as can be used.

The base material is not particularly limited as long as it is soluble in an organic solvent and does not melt or deteriorate by heating at about 60 ° C., and a conventionally used material is selected and used. be able to. Specific examples thereof include treated or untreated metals, plastics, papers, fibers, and coated products of these.

As a method for applying to the substrate, for example, a usual coating means such as spray coating, brush coating, roller coating, dip coating and the like can be used. Thereby, the coating is applied so that the dry film thickness is in the range of about 1 to 100 μm.

The heat curing is usually carried out at a temperature of about 60 ° C. or higher, preferably about 100 to 300 ° C. Also, the heating time varies greatly depending on the temperature,
For example, it is usually about 10 seconds at about 250 ° C, or about 6 seconds.
At 0 ° C., it takes about 3 hours.

[0054]

The thermosetting composition of the present invention does not react at room temperature (about 20 ° C.), but the reaction rapidly occurs when a temperature of about 60 ° C. is applied, so that the composition is stable during storage. It has excellent properties, excellent low temperature curability, and excellent discoloration resistance of the cured product. The reason for this is not clear, but the heat causes the tin compound and the reactive silicon group-containing compound to combine to form a complex of tin and silicon, which is then subjected to a cationic polymerization reaction or cationic addition of an epoxy group. It is presumed that it acts as a curing catalyst that accelerates the reaction.

[0055]

EXAMPLES Hereinafter, production examples, examples and comparative examples will be described.
The present invention will be described more specifically.

Production Example 1 Production of Epoxy Group-Containing Compound (i) Using a reaction vessel equipped with a reflux device and a stirrer, 3,4-
Epoxy cyclohexyl methyl methacrylate 400
g, 600 g of butyl methacrylate and 10 g of azoisobutyronitrile are added to xylene 1000 at 110 ° C.
g, and allowed to react for 5 hours to give a number average molecular weight of 10,
000 acrylic resins (containing about 20 epoxy groups in one molecule) were produced.

Production Example 2 Production of Epoxy Group-Containing Compound (ii) Using a reaction vessel equipped with a reflux device and a stirring device, 284 g of glycidyl methacrylate and 71 of butyl methacrylate were used.
11 g of a mixture of 6 g and 10 g of azoisobutyronitrile
The mixture was added dropwise to 1000 g of xylene at 0 ° C. and reacted for 5 hours to produce an acrylic resin having a number average molecular weight of 10,000 (containing about 20 epoxy groups in one molecule).

Production Example 3 Reactive silicon group-containing compound (a)
Production of "Colcoat ES40" (manufactured by Colcoat, having an average of about pentamer of tetraethyl silicate and having an average of about 12 ethoxy groups in one molecule), 775 g, 225 g of benzyl alcohol and 0.05 g of tris (acetylacetonato) aluminum. The mixture was subjected to 96 g of deethanolization reaction at 160 ° C. for 3 hours to obtain the desired product. This one is 1
It contains on average about 10 ethoxy groups and 2 benzyl ether groups in the molecule.

Production Example 4 Reactive silicon group-containing compound (b)
Using a reaction vessel equipped with a reflux device and a stirrer, 243 g of methacryloxypropyltrimethoxysilane, 757 g of butyl methacrylate and azoisobutyronitrile 10
The mixture of g was added dropwise to 1000 g of xylene at 110 ° C. and reacted for 5 hours to produce an acrylic resin having a number average molecular weight of 10,000 (containing about 30 methoxy groups in one molecule).

Example 1 "ERL-422" which is an epoxy group-containing compound (iii)
1 "(manufactured by Union Carbide Co., trade name, alicyclic diepoxy compound) 200 g (resin solid content 100 g), 30 g of reactive silicon group-containing compound (a) and 1.0 g of butyl bis (ethyl acetoacetate) tin acetate. hand,
The curable composition of the present invention was produced.

Examples 2 to 10 and Comparative Examples 1 to 3 The compositions of the present invention of Examples 2 to 10 and the comparative compositions of Comparative Examples 1 to 3 were prepared according to the formulations shown in Table 1.

The storage stability of each curable composition of Examples and Comparative Examples was examined by the following test method.

Storage stability: evaluated according to the following criteria.

⊚: No thickening at 40 ° C. for 14 days or more, ◯: 40 ° C., slightly thickening in 14 days, Δ: 40 ° C., thickening or gelation in 1 to 13 days, ×: 20 ° C. It gels in 1 hour.

Next, each curable composition of Examples and Comparative Examples was applied to a degreased mild steel plate so that the dry film thickness was about 50 μm, and then heat-cured at 120 ° C. for 30 minutes. The performance of the obtained cured product was examined by the following test method.

Appearance: The presence or absence of abnormalities such as cracks and blurring of the cured film was observed. Those with no abnormality were considered good.

Curing property: The peeled cured film was extracted with an acetone solvent at a reflux temperature for 2 hours, and the residual ratio (%) of the cured film was examined.

Yellowing resistance: The film before heating and the cured film after heating were compared, and yellowing was visually observed, and evaluated according to the following criteria. ◯: Good with no change, ×: Yellowing and bad.

The test results are also shown in Table 1.

[0070]

[Table 1]

[0071]

[Table 2]

In Table 1 above, the reactive silicon group-containing compound (c) represents phenyltrimethoxysilane, and (d) represents diphenylsilanediol.

Claims (6)

[Claims] 1. A thermosetting composition comprising (A) an epoxy group-containing compound, (B) a reactive silicon group-containing compound, and (C) a tin chelate catalyst. 2. The thermosetting composition according to claim 1, wherein the epoxy group is an alicyclic epoxy group. 3. The thermosetting composition according to claim 1, wherein the reactive silicon group is a group in which a hydrolyzable group is directly bonded to a silicon atom, and the hydrolyzable group is an alkoxyl group. 4. The compound (A) is a (co) polymer obtained by (co) polymerizing an epoxy group-containing ethylenically unsaturated monomer and, if necessary, another ethylenically unsaturated monomer. Or the thermosetting composition according to 2. 5. The compound (B) is a (co) polymer obtained by (co) polymerizing a reactive silicon group-containing ethylenically unsaturated monomer and, if necessary, another ethylenically unsaturated monomer. Item 1. A thermosetting composition according to item 1 or 3. 6. An organic solution or dispersion prepared by dissolving or dispersing the thermosetting composition according to any one of claims 1 to 5 in an organic solvent is applied to a substrate and then cured by heating. A curing method characterized by: