Method for curing resin

Abstract: In a method for curing a resin by utilizing an intermolecular or intramolecular crosslinking reaction, the improvement wherein the crosslinking reaction is a ring opening polymerization reaction between lactone structures and/or a ring opening addition reaction between a lactone structure and an active hydrogen-containing functional group.

Description: This invention relates to a novel method for curing a resin. There has been hitherto well known a method for three-dimensionally crosslinking and curing a syntheric resin by using a curing agent (crosslinking agent) such as a melamine resin or a polyisocyanate compound. However, when using the melamine resin, by-products such as alcohols, aldehydes, water occur during a curing reaction, deteriorating physical properties of a cured product. Meanwhile, when using the polyisocyanate compound, a system is a two-package system which is intricate to treat (see, for example, U.S. Patent 4,756,975). Encyclopedia of Polymer Sciene and Engineering, Vol. 12, pp. 36-41 (John Wiley & Sons, New York 1988) describes the ring opening polymerization of cyclic esters such as lactones. However, this document does not teach or suggest anything regarding ring opening crosslinking reactions between lactone structures. The present inventors have made extensive studies to eliminate the above conventional defects in curing the synthetic resin and develop a novel curing method in which a system is a one-package system and occurrence of by-products is little observed in the curing reaction. As a result, they have this time found that the above object can be achieved by a curing method chiefly utilizing a ring opening reaction of a lactone structure, and completed this invention. Thus, according to this invention, there is provided a method for curing a resin by utilizing an intermolecular or intramolecular crosslinking reaction of a resin, characterized in that the crosslinking reaction is a ring opening polymerization reaction between lactone structures wherein the resin contains at least 2 lactone structures in the same molecule. The resin is generally crosslinked and cured by, for example, chemically bonding molecules of a resin having a straight-chain structure (including also a chain structure with a branched chain) in three-dimensional network such that the molecules of said resin are bridged intermolecularly and/or intramolecularly. This invention is characterized in that the resin is crosslinked and cured chiefly utilizing a ring opening reaction of a lactone structure contained in a straight-chain resin. The lactone structure is preferably a 4- to 9-membered cyclic functional group having an ester linkage in a ring. The resin is presumably crosslinked and cured such that the lactone ring is opened at the ester linkage site either by heating or with a catalyst, and then attacks the other lactone structure to form a polyester structure (ring opening polymerization), as shown in the following reaction scheme A. wherein R denotes a saturated aliphatic hydrocarbon group having preferably 2 to 7 carbon atoms, denotes a resin body and R' denotes an alkyl group such as an ethyl group. As the crosslinking and curing reaction usually little proceeds at room temperature, there are advantages that a resin which can be cured according to this invention is excellent in storage stability and usable as a one-package type. Moreover, compared with an ordinary method for curing a resin with a curing agent such as an amino resin, the method of this invention little or never allows formation of by-products in the crosslinking and curing reaction. In consequence, when the method of this invention is applied to paints, a coating surface having excellent smoothness can be formed and an internal stress of a coating film is low so that a coating surface excellent in adhesion to a surface being coated can be provided. The lactone structure can be formed by the following methods which are known per se. However, they are not critical in this invention. (1) Formation by an oxidation reaction and a reduction reaction (a) Formation by peroxidation of a cyclic ketone A cyclic ketone enlarges the ring and forms a lactone by a peracid. (b) Formation by oxidation of a cyclic ether A cyclic ether is oxidized with chromic acid, t-butyl chromate or ruthenium oxide to form a lactone. (c) Formation by reduction of an acid anhydride When an acid anhydride is reduced with metallic sodium-alcohol, lithium aluminum hydride or lithium aluminum tri-t-butoxy hydride, a lactone structure is formed. It is also formed using chlorotris-(triphenylphosphine) rhodium. (2) Formation by ring closure of a carboxylic acid or its derivatives (a) Formation from a hydroxycarboxylic acid or its esters When a hydroxycarboxylic acid is treated with an acid, dehydration takes place to form a lactone. For example, a gamma-hydroxycarboxylic acid and a delta-hydroxycarboxylic acid provide a gamma-lactone and a delta-lactone respectively. Examples of the acid are mineral acids such as sulfuric acid and hydrochloric acid, and organic acids such as p-toluenesulfonic acid. The lactone can be formed using an acid anhydride (e.g. acetic anhydride and trifluoroacetic anhydride) or phosgene. (b) Formation from an unsaturated carboxylic acid or its esters A beta, gamma-unsaturated carboxylic acid is easily ring-closed in the presence of an acid to form a gamma-lactone. Examples of the acid. are sulfuric acid and trifluoroacetic acid. 4-Pentenoic acid forms a gamma-lactone and 5-methyl-4-hexenoic acid forms a gamma-lactone. When an olefinic carboxylic acid is reacted with a peracid, a hydroxylactone is formed. Moreover, an ester having a cyclopropyl group can be lactonized with an acid. (c) Formation from an olefinic diazo ester A carbene formed by decomposition of a diazo ester having a double bond is added to a double bond in a molecule to form cyclopropyl lactone. A diazo malonic acid ester also forms a lactone. A compound having a cyclopropylcarbonyl group can be converted into a gamma-lactone with a cyclopropane ring cleaved. (3) Formation by carbonylation and introduction of a carboxylic acid residue (a) Formation by carbonylation of acetylene carbinol When an alcohol having terminal acetylene is carbonylated, a lactone is formed. For example, when beta-hydroxyacetylene is carbonylated with carbon monoxide, alpha-methylene lactone results, and butenolyd is formed from ethenyl carbinol. Further, when vinyl alcohol is carbonylated, a lactone is obtained. (b) Formation by a reaction between a ketone and a ketene An aldehyde or a ketone is condensed with a ketene in the presence of a Lewis acid as a catalyst to form a beta-lactone. Dimethyl ketene is condensed with cyclopropane to form spiro-beta-lactone. Dichloroketene is concensed with an aldehyde to form alpha,alpha-dichloro-beta-lactone. An unsaturated delta-lactone is formed by a reaction of an alpha,beta-unsaturated aldehyde and/or a ketone with a ketene. (c) Formation by introduction of a carboxylic acid residue into an epoxide Active methylene compounds such as a malonic acid ester, ethyl cyanoacetate and ethyl acetoacetate form corresponding gamma-lactone derivatives respectively. Oxirane is condensed with a ketene to form a gamma-lactone. As a method to bone the thus formed lactone structure to a resin molecule, it is possible to introduce a substituent in the lactone structure and chemically bond the lactone structure to a resin substrate utilizing the substituent. The substituent is introduced into the lactone structure by e.g. carboxylation or hydroxymethylation of an alpha-position of the lactone ring. A hydroxyl group can be added by subjecting a beta-position of the lactone ring to a Michael condensation. An isocyanate group can also be introduced by adding a polyisocyanate compound to the hydroxyl group. The substituent (e.g. a hydroxyl group, a carboxyl group and an isocyanate group) introduced into the lactone structure as above is reacted with a functional group introduced in the resin substrate, thereby making it possible to introduce the lactone into the resin substrate. The functional group that can be introduced into the resin substrate may be any group that is mutually reacted with the substituent of the lactone structure. Examples of such group are a hydroxyl group, a carboxyl group, an epoxy group and an isocyanate group. The resin substrate into which these functional groups are introduced is not limited in particular, and can optionally be selected depending on the use purpose of the resin. Examples thereof are an acrylic resin, a vinyl resin, a polyester (alkyd) resin, a polyamide resin, a polyurethane resin, an epoxy resin and a fluorine resin. It is advisable that the resin substrate has a number average molecular weight of usually 200 to 100,000, preferably 200 to 50,000 and more preferably 200 to 20,000. The above functional group can be introduced into such resin substrate in a manner known per se. A preferable combination of the functional group of the resin substrate for introducing the lactone structure into the resin substrate and the substituent of the lactone structure is listed below. Functional group of a resin substrate Substituent of a lactone structure Hydroxyl group Carboxyl group, epoxy group, isocyanate group Carboxyl group Hydroxyl group, amino group, epoxy group Epoxy group Carboxyl group, hydroxyl group, thiol group, amino group, hydroxyl group Isocyanate group Hydroxyl group, amino group Another method to introduce the lactone structure into the resin substrate is that a polymerizable unsaturated monomer containing a lactone structure is singly polymerized or copolymerized with another polymerizable monomer. Examples of the polymerizable unsaturated monomer containing the lactone structure are monomers represented by formulas wherein R 1 denotes H or CH 3 and R 2 denotes a C 1-8 hydrocarbon group. The other monomer copolymerizable with these monomers is preferably the polymerizable unsaturated monomer (the vinyl monomer or the acrylic monomer) used in forming the vinyl resin or the acrylic resin. Further, when maleic anhydride is copolymerized with allyl alcohol, ring closure is conducted in polymerization to obtain a lactone structure-containing vinyl polymer as shown in the following reaction scheme C. The number of the lactone structure being introduced can be at least 2, preferably 3 to 30, more preferably 5 to 15 per molecule. The method of this invention crosslinks and cures the resin based chiefly on the ring opening reaction of the lactone structure. To be concrete, the ring opening polymerization reaction between the lactone structures (see Reaction Scheme A) is taken. Accordingly, it is advisable to perform the method of this invention using the following resin (A). (A) A resin containing per molecule at least 2, preferably 3 to 30, more preferably 5 to 15 (or per kg of the resin, 0.1 to 15 mols, preferably 0.3 to 10 mols. In the method of this invention, the conditions for crosslinking and curing the resin (A) vary with the lactone structure. The heating (baking) temperature is usually at least 80°C, especially preferably 140 to 200°C. When heating is conducted at the above temperature usually for 10 to 30 minutes, the resin is crosslinked and cured. In order to decrease the crosslinking and curing temperature or shorten the heating temperature, said resin may be blended with the following catalyst in an amount of 0.01 to 10 parts by weight per 100 parts by weight of the resin. 1. Catalyst for anionic polymerization. For example, Li, Na, K, Na-naphthalene, Li 2 -benzophenone, K 2 -benzophenone, LiR, NaR, LiH, NaH, K 2 CO 3 , KOH, NaOR, LiOR, acetates of Li, Na and K, and tertiary amines such as pyridine, picolin and quinoline. 2. Catalyst for coordination anionic polymerization: For example, AlR 3 , ZnR 2 , MgR 2 , RMgX, R 2 AlX, RAlX 2 , R 2 Al(OR'), AlEt 3 -H 2 O, Zn(C 2 H 5 ) 2 -H 2 O, Cd(C 2 H 5 ) 2 -H 2 O, Al(OR) 3 , Mg(OR) 2 , Ti(OR) 4 and titanium phosphate. 3. Catalyst for cationic polymerization: AlCl 3 , BF 3 .(C 2 H 5 ) 2 O, other metal halides, CF 3 CO 2 H, acetyl perchlorate, toluenesulfonic acid and phosphoric acid. In the above description, R and R' each denote a hydrocarbon group having 1 to 10 carbon atoms and X denotes a halogen atom. In the method of this invention, the resins (A) to (C) can be blended with a coloring pigment, a metallic pigment and an extender pigment. It is advisable that the resin is used by being dissolved or dispersed in an organic solvent and/or water. Moreover, it is also possible, if required, to asdd to said resin a low-molecular polyol (a number average molecular weight less than 500) such as glycol or triol and a lactone compound (a number average molecular weight less than 500) such as valerolactone or caprolactone. The method of this invention can advantageously be utilized in the field of paints, ink, adhesives, tackifiers and molded articles. Preparation Example 1 A vinyl monomer (212g : 1 mol) represented by formula, 400 g of methyl methacrylate, 130 g of 2-hydroxyethyl methacrylate and 256 g of ethyl acrylate were copolymerized to form a vinyl resin (1) having a number average molecular weight of 15,000. This resin contains 1 mol/kg of a caprolactone structure and 1 mol/kg of a primary hydroxyl group. Preparation Example 2 A compound (260 g: 2 mols) of a structure represented by formula was reacted with 730 g of a polyurethane resin with terminal NCO of a hexamethylenediisocyanate type to form a polyurethane resin (2) containing a terminal valerolactone structure and having a number average molecular weight of 5,000. This resin contains 2 mols/kg of the valerolactone structure. Preparation Example 3 One thousand grams of a bisphenol A-type diepoxy resin having a number average molecular weight 1,000 was reacted with 2 mols of ketene in the presence of a catalytic amount of trifluoroboron etherate to introduce a butylolactone structure in the end. This resin (3) contains 1.85 mols/kg of the butylolactone structure and 1.85 mol/kg of a secondary hydroxyl group. Preparation Example 4 A compound (432 g: 3 mols) of a structure represented by formula was condensed with 300 g of a polyester resin with a terminal carboxyl group having a number average molecular weight of 3,000 to form a polyester resin (4) containing a caprolactone structure and having a molecular weight of 3,500. This resin contains 0.57 mol/kg of the caprolactone structure and 1.0 mol/kg of the carboxyl group. Preparation Example 5 Hydrogenated bisphenol A was oxidized into a diketone which was then reacted with peracetic acid to obtain a resin (5) of a structure represented by formula. This resin contains 7.5 mols/kg of caprolactone and has a number average molecular weight of 268. Example (a) The resin (1) alone was baked at 170°C for 60 minutes. (b) AlCl 2 (3%) was added to the resin (3) alone, and the nixture eras baked at 200°C for 30 minutes. (c) The resin (2) was mixed with 30 % of epsilon-caprolactone, and 1 % of tris(acetylacetonato)aluminum was added thereto. The mixture was baked at 140°C for 30 minutes. The characteristics of the crosslinked cured resins obtained in (a) to (c) are tabulated below. (a) (b) (c) Wiped 20 times with xylol No change Gloss slightly loses No change Tukon hardness 18 5.5 8

Claims: A method for curing a resin by utilizing an intermolecular or intramolecular crosslinking reaction, characterized in that the crosslinking reaction is a ring opening polymerization reaction between lactone structures, wherein the resin contains at least 2 lactone structures in the same molecule. The method of claim 1 wherein the resin contains 3 to 10 lactone structures in the same molecule. The method of claims 1 and 2 wherein the lactone structure is a 4- to 9-membered lactone ring. The method of claims 1 to 3 wherein curing is conducted at a temperature of 140 to 200°C.

Inventors: ISOZAKI OSAMU, ISOZAKI, OSAMU
Applicants: KANSAI PAINT CO LTD, KANSAI PAINT CO. LTD.
Application year: 1991
Main classification: C08J3
IPC codes: C08J3