Acid functional polyester resins and lower temperature curable powder coating compositions comprising them

A process for the preparation of linear carboxylic acid functional polyester resins, comprising:

• (a) at least one aromatic and/or cycloaliphatic dicarboxylic acid compound A, comprising two aromatic and/or secondary aliphatic carboxyl groups in the 1,2-position, or anhydride thereof;
• (b) at least one hydroxyl monocarboxylic acid B, comprising two aliphatic hydroxyl groups, which groups each independently may be primary or secondary hydroxyl groups and wherein the carboxylic acid group is a tertiary carboxyl group, and a cycloaliphatic diol B2;
• (c) an aliphatic dicarboxylic acid C, comprising an alkylene group having from 3 to 8 carbon atoms and having two terminal aliphatic carboxyl groups;
• and
• (d) an aromatic monocarboxylic acid D selected from the group of benzoic acid, orthotoluic acid, and paratoluic acid;
• wherein the molar ratio of the compounds A:B1+B2:C:D is X:Y+X+Z1:Y:Z2, wherein X ranges from 1 to 8, Y ranges from 1 to 8, Z1 ranges from −3 to 3 and Z2 ranges from 1 to 2,
• by:
• (i) reacting A with B1, whereby the molar amounts of A and B1 are equal at a temperature from 100 to 200° C. under a slight vacuum during a period of from 2 to 8 hours, thus allowing the reaction to initiate and proceed under continuous removal of water,
• (ii) adding B2 and C, increasing the temperature from about 170 to about 190° C. and proceeding under continuous removal of water, and
• (iii) decreasing the temperature to 140° C., adding D, increasing the temperature from about 170 to about 190° C., and proceeding under continuous removal of water until at least 90% of all the hydroxyl groups supplied into the reaction mixture have been reacted.