Polyester polyols of low molar mass, their preparation and use in coating compositions

A mixture comprising

• A) low molar mass polyester polyols having a weight-average molar mass Mw of not more than 2000 g/mol, with a hydroxyl number of from 80 to 300 mg/g and an acid number of from 5 to 35 mg/g, comprising mole fractions in % [=mol/100 mol] of structural units derived from:
• a) from 1 to 45% of an aliphatic polycyclic polyhydroxy compound having at least two hydroxyl groups per molecule;
• b) from 5 to 50% of a branched aliphatic acyclic dihydroxy compound;
• c) from 30 to 50% of an aliphatic cyclic polycarboxylic acid;
• d) from 0 to 30% of an aliphatic acyclic or monocyclic polyhydroxy compound having three or more hydroxyl groups per molecule;
• e) from 0 to 10% of a polyfunctional compound selected from the group consisting of aliphatic linear and monocyclic dihydroxy compounds, aliphatic linear and branched dicarboxylic acids, aromatic dicarboxylic acids and polycarboxylic acids having three or more carboxyl groups per molecule;
• and
• f) from 0 to 10% of a monofunctional unit selected from the group consisting of monocarboxylic acids;
• and monoalcohols, the mole fractions indicated in each case under a), b), c), d), e) and f) adding up to 100%, and
• B) a copolymer prepared separately or in the presence of said polyester polyol by free-radical addition polymerization, the monomer mixture on which the copolymer is based comprising
• at least one olefinically unsaturated monomer g), which is an alkyl ester of an aliphatic linear, branched or cyclic α,β-unsaturated monocarboxylic acid or an alkyl diester of an olefinically unsaturated aliphatic linear, branched or cyclic dicarboxylic acid of 1 to 20 carbon atoms in the alkyl radical and 3 to 10 carbon atoms in the acid residue of the ester,
• at least one hydroxyalkyl ester h) of said mono- or dicarboxylic acids specified under g), in which the hydroxyalkyl radical is derived from an at least dihydric aliphatic linear, branched or cyclic alcohol of 2 to 15 carbon atoms,
• at least one carboxylic acid i) selected from the group consisting of said monocarboxylic and dicarboxylic acids specified under g), in nonesterified form or, in the case of the dicarboxylic acids, in a form in which said dicarboxylic acid is monoesterified with one of the alkyl radicals specified under g) or one of the hydroxyalkyl radicals specified under h), and optionally,
• at least one further olefinically unsaturated monomer j) selected from the group consisting of vinylaromatic compounds vinyl halides, unsaturated nitriles, amides and diamides of the acids specified under i), vinyl ethers of aliphatic linear, branched or cyclic alcohols of 1 to 18 carbon atoms and olefinically unsaturated ketones of 4 to 21 carbon atoms,
• wherein the said polymerisation is conducted in the presence of cyclic compounds k) that can react with at least one of the monomers h) and i) under formation of acid-functional or hydroxy-functional copolymerisable monomers.