Cross-linkable compounds based on saturated polyesters

Abstract

The invention relates to compounds based on saturated polyester and cross-linking agent. The compositions comprise a saturated polyester with terminal hydroxyl groups and a cross-linking agent consisting of a dianhydride with ester groups, the product of reaction between a trimellitic anhydride and a diol. Use of the compounds as a binder for powder paints applicable to the automobile, electrical household goods, agricultural machinery and lead industries.

We claim: 
     
       1. A binder comprising a saturated polyester with terminal hydroxyl groups and a cross-linking agent, characterized in that the polyester has a softening temperature within the range of 70° to 130° C., a hydroxyl number within the range of 150 to 250 and an acid number below 10, and that the cross-linking agent comprises a dianhydride with ester groups, which is the product of a reaction between trimellitic anhydride and a diol and which has a softening temperature within the range of 60° to 130° C. 
     
     
       2. A binder according to claim 1, characterized in that the polyester is the product of a reaction between at least one dicarboxylic acid selected from the group consisting of aromatic dicarboxylic acid and lower alkyl ester of aromatic dicarboxylic acid with one or more diols. 
     
     
       3. A binder according to claim 2, characterized in that at least one acid selected from the group consisting of saturated aliphatic acid and aromatic or aliphatic acid with more than two carboxylic groups are associated with the aromatic dicarboxylic acid. 
     
     
       4. A binder according to claim 2, characterized in that one or more triols are associated with the diol. 
     
     
       5. A binder according to claim 1, characterized in that the polyester is the product of a reaction between ethylene glycol polyterephthalate of high molecular weight and a triol. 
     
     
       6. A binder according to claim 1, characterized in that the cross-linking agent is obtained by reacting trimellitic anhydride with a diol at a temperature within the range of 180° to 250° C. and in the molar ratio trimellitic anhydride/diol of 2/1 to 1.5. 
     
     
       7. A binder according to claim 6, characterized in that the diol is an aliphatic diol having 2 to 10 carbon atoms. 
     
     
       8. A binder according to claim 1, characterized in that their cross-linking temperature is from 140° to 200° C. for from 5 to 30 minutes. 
     
     
       9. A method of obtaining the binder of claim 1, comprising mixing a polyester having terminal hydroxyl groups with the cross-linking agent formed by reacting trimellitic anhydride with a diol at a temperature within the range of 180° to 250° C. and in the molar ratio of 2 moles of trimellitic anhydride to 1 to 1.5 moles of the diol. .Iadd. 
     
     
       10.  A cross-linking agent comprising a dianhydride with ester functions, said cross-linking agent being the product of the reaction between trimellitic anhydride and a diol, at elevated temperatures, and wherein such reaction the molar ratio of the trimellitic anhydride to the diol is not in excess of 2. .Iaddend. .Iadd.11. The cross-linking agent as defined by claim 10, wherein the diol is an aliphatic diol containing from 2 to 10 carbon atoms. .Iaddend. .Iadd.12. The cross-linking agent as defined by claim 11, wherein the diol is selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol and neopentyl glycol. .Iaddend. .Iadd.13. The cross-linking agent as defined by claim 11, said composition having a softening temperature of approximately 60°  to 130°  C. .Iaddend. .Iadd.14. The cross-linking agent as defined by claim 13, said cross-linking agent having a softening temperature of approximately 100°  to 120°  C. .Iaddend. .Iadd.15. The cross-linking agent as defined by claim 11, said cross-linking agent having an approximately theoretical 
     
     
        acid number. .Iaddend. .Iadd.16.  The cross-linking agent as defined by claim 10, wherein the molar ratio of trimellitic anhydride to diol ranges from 2/1 to 2/1.5. .Iaddend. .Iadd.17. The cross-linking agent as defined by claim 13, wherein the diol is ethylene glycol. .Iaddend. .Iadd.18. The cross-linking agent as defined in claim 17, wherein the molar ratio of trimellitic anhydride to ethylene glycol is 2/1. .Iaddend. .Iadd.19. The cross-linking agent as defined by claim 17, wherein the molar ratio of trimellitic anhydride to ethylene glycol is 3/1.8. .Iaddend. .Iadd.20. A process for preparing a cross-linking agent comprising a dianhydride with ester functions, which process comprises reacting trimellitic anhydride with a diol at elevated temperatures, and wherein the molar ratio of the trimellitic anhydride to the diol is not in excess of 2. .Iaddend. .Iadd.21. The process as defined by claim 20, wherein the reaction is conducted at temperatures ranging from 180°  to 250°  until no further water is liberated. .Iaddend. .Iadd.22. The process as defined by claim 21, wherein the temperatures range from 220°  to 
     
     
        240°  C. .Iaddend. .Iadd.23.  A coating composition comprising that cross-linking agent as defined by claim 10. .Iaddend. .Iadd.24. The coating composition as defined by claim 23 which comprises a paint. .Iaddend. .Iadd.25. A cross-linked polymer comprising the condensation product of (1) the cross-linking agent as defined by claim 10, and (2) a saturated polyester with terminal hydroxyl groups. .Iaddend. .Iadd.26. The cross-linked polymer as defined by claim 25, wherein the saturated polyester (2) has a softening temperature of from 70°  to 130°  C., a hydroxyl number of from 150 to 250 and an acid number below 10. .Iaddend. .Iadd.27. The binder as defined by claim 2, wherein the at least one dicarboxylic acid is selected from the group consisting of terephthalic acid and isophthalic acid. .Iaddend. .Iadd.28. The binder as defined by claim 3, wherein the at least one acid associated with the aromatic dicarboxylic acid is selected from the group consisting of adipic 
     
     
        acid, succinic acid and sebacic acid. .Iaddend. .Iadd.29.  The binder as defined by claim 3, wherein the at least one acid associated with the aromatic dicarboxylic acid is selected from the group consisting of trimellitic acid, trimellitic acid anhydride, pyromellitic acid, pyromellitic acid anhydride and butanetetracarboxylic acid. .Iaddend. .Iadd.30. The binder as defined by claim 4, wherein the one or more triols are selected from the group consisting of glycerol, trimethylolpropane and trihydroxyethylisocyanurate. .Iaddend. .Iadd.31. The binder as defined by claim 2, wherein the one or more diols are selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, butanediol, bishydroxyethyl-bisphenol A, neopentyl glycol and hydrogenated bisphenol A. .Iaddend. .Iadd.32. The binder as defined by claim 1, wherein the saturated polyester and the cross-linking agent are present in proportions corresponding to their respective hydroxyl and acid numbers. .Iaddend. .Iadd.33. The cross-linked polymer as defined by claim 25, wherein the condensation is effected at a temperature of from 140°  to 200°  C., for from 5 to 30 minutes. 
     
     
        .Iadd.  .  A coated substrate, the coating comprising the cross-linked polymer as defined by claim 25. .Iaddend. .Iadd.35. The coated substrate as defined by claim 34, the coating comprising a paint. .Iaddend. .Iadd.36. A coated substrate, the coating comprising the binder as defined by claim 1. .Iaddend. .Iadd.37. The coated substrate as defined by claim 36, the coating comprising a paint. .Iaddend. .Iadd.38. The cross-linking agent as defined by claim 11, said composition having an acid number of about 518 in aqueous medium and about 336 in alcoholic medium. .Iaddend. .Iadd.39. The cross-linking agent as defined by claim 11, said composition having an acid number of about 455 in aqueous medium and about 280 in alcoholic medium..Iaddend.