Thermosetting power compositions for the preparation of low-gloss coatings

Abstract

The invention concerns powder thermosetting coating compositions comprising (a) an amorphous polyester containing carboxyl groups, rich in isophthalic acid and neopentylglycol, optionally branched with a polycarboxylic acid or a polyol containing at least three functional groups; (b) a semicrystalline polyester containing carboxyl groups prepared from one or several saturated aliphatic dicarboxylic acids with linear chain, and from a saturated aliphatic diol with linear or cyclic chain optionally branched with a polycarboxylic acid or with a polyol containing at least three functional groups, and having a melting point (Tm) of at least 40° C. and an acid value of 5 to 50 mg of KOH/g, and (c) a cross-linking agent. Said compositions are useful for preparing powder paints and varnishes providing excellent quality coatings whereof the brilliance measured at an angle of 60°, according to the ASTM D 523 standard is always less than 50%, that is half-gloss or matt coatings.

1 . Thermosetting powder coating compositions comprising, as binder, a blend of an amorphous polyester, of a semicrystalline polyester and of a crosslinking agent, characterized in that the binder comprises: 
 (a) an amorphous polyester containing carboxyl groups, rich in isophthalic acid, prepared from an acid constituent comprising from 55 to 100 mol % of isophthalic acid, from 0 to 45 mol % of at least one dicarboxylic acid other than isophthalic acid and from 0 to 10 mol % of a polycarboxylic acid containing at least 3 carboxyl groups and from an alcohol constituent comprising from 60 to 100 mol % of neopentylglycol, from 0 to 40 mol % of at least one dihydroxylated compound other than neopentylglycol and from 0 to 10 mol % of a polyhydroxylated compound containing at least 3 hydroxyl groups, the said amorphous polyester having a glass transition temperature (T g ) of at least 50° C. and an acid number of 15 to 100 mg of KOH/g;    (b) a semicrystalline polyester containing carboxyl groups prepared either 
 (b1) from a saturated aliphatic dicarboxylic acid with a linear chain having from 4 to 16 carbon atoms and from a saturated aliphatic diol with a linear chain having from 2 to 16 carbon atoms and optionally from a polycarboxylic acid containing at least 3 carboxyl groups or from a polyol containing at least 3 hydroxyl groups, or  
 (b2) from 40 to 100 mol % of a saturated aliphatic dicarboxylic acid with a linear chain having from 10 to 16 carbon atoms and from 0 to 60 mol % of an aliphatic dicarboxylic acid with a linear chain having from 4 to 9 carbon atoms, calculated with respect to the total of the dicarboxylic acids, from a cycloaliphatic diol having from 3 to 16 carbon atoms and optionally from a polycarboxylic acid having at least 3 carboxyl groups or from a polyol having at least 3 hydroxyl groups,  
 the said semicrystalline polyesters having a melting point (T m ) of at least 40° C. and an acid number of 5 to 50 mg of KOH/g; and  
   (c) a crosslinking agent.    
     
     
         2 . Compositions according to  claim 1 , characterized in that the amorphous polyester (a) has a glass transition temperature of 50 to 80° C. and an acid number of 30 to 70 mg of KOH per gramme.  
     
     
         3 . Compositions according to either of claims  1  and  2 , characterized in that the amorphous polyester (a) has at least one of the following characteristics: a number-average molecular weight of between 1100 and 11,500 and a melt viscosity of 100 to 15,000 mPa.s at 200° C.  
     
     
         4 . Compositions according to any one of  claims 1  to  3 , characterized in that the acid constituent of the amorphous polyester (a) contains, apart from isophthalic acid, up to 45 mol % of a dicarboxylic acid chosen from terephthalic acid, phthalic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid and mixtures of these compounds.  
     
     
         5 . Compositions according to any one of  claims 1  to  4 , characterized in that the alcohol constituent of the amorphous polyester (a) contains, apart from neopentylglycol, up to 40 mol % of a dihydroxylated compound chosen from ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 2-methyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,4-cyclohexanedimethanol, hydrogenated bisphenol A, neopentylglycol hydroxypivalate and mixtures of these compounds.  
     
     
         6 . Compositions according to any one of  claims 1  to  5 , characterized in that the semicrystalline polyester (b) has a melting point of between 40 and 90° C., an acid number of 5 to 30 mg of KOH per gramme and a hydroxyl number which does not exceed 5 mg of KOH per gramme.  
     
     
         7 . Compositions according to any one of  claims 1  to  6 , characterized in that the semicrystalline polyester (b) has at least one of the following characteristics: a number-average molecular weight of between 2200 and 25,000 and a melt viscosity of from 50 mPa.s at 100° C. to 10,000 mPa.s at 150° C.  
     
     
         8 . Compositions according to any one of  claims 1  to  7 , characterized in that the semicrystalline polyester (b1) is the polyesterification product of 1,12-dodecanedioic acid with 1,6-hexanediol.  
     
     
         9 . Compositions according to any one of  claims 1  to  7 , characterized in that the semicrystalline polyester (b2) is the polyesterification product of 1,12-dodecanedioic acid with 1,4-cyclohexanediol or 1,4-cyclohexanedimethanol.  
     
     
         10 . Compositions according to any one of  claims 1  to  9 , characterized in that the amount of amorphous polyester (a) represents approximately 60 to 87% by weight and the amount of semicrystalline polyester (b) represents approximately 13 to 40% by weight calculated with respect to the total weight of the amorphous polyester (a) and of the semicrystalline polyester (b).  
     
     
         11 . Compositions according to any one of  claims 1  to  9 , characterized in that the amount of amorphous polyester represents 60 to 82% by weight and the amount of semicrystalline polyester (b) represents 18 to 40% by weight calculated with respect to the total weight of the amorphous polyester (a) and of the semicrystalline polyester (b).  
     
     
         12 . Compositions according to any one of  claims 1  to  11 , characterized in that the crosslinking agent (c) is a polyepoxidized compound.  
     
     
         13 . Compositions according to any one of  claims 1  to  11 , characterized in that the crosslinking agent (c) is a beta-hydroxyalkylamide.  
     
     
         14 . Compositions according to any one of  claims 1  to  13 , characterized in that the ratio of the amount of amorphous polyester (a) and of semicrystalline polyester (b) containing carboxyl groups, on the one hand, to the amount of crosslinking agent (c), on the other hand, is such that there are from 0.5 to 1.5 equivalents of carboxyl groups per equivalent of functional groups in the crosslinking agent.  
     
     
         15 . Compositions according to any one of  claims 1  to  14 , characterized in that the crosslinking agent (c) is present in an amount of approximately 4 to 25% by weight calculated with respect to the total weight of the binder.  
     
     
         16 . Compositions according to any one of  claims 1  to  15 , characterized in that the binder comprises from 45 to 83% by weight of amorphous polyester (a), from 13 to 30% by weight of semicrystalline polyester (b) and from 4 to 25% by weight of crosslinking agent (c).  
     
     
         17 . Compositions according to any one of  claims 1  to  15 , characterized in that the binder comprises from 45 to 78% by weight of amorphous polyester (a), from 18 to 30% by weight of semicrystalline polyester (b) and 4 to 25% by weight of crosslinking agent (c).  
     
     
         18 . Varnishes and paints obtained from the thermosetting powder compositions according to any one of  claims 1  to  17 .  
     
     
         19 . Process for obtaining a low-gloss coating on an article, characterized in that a thermosetting powder coating composition according to any one of  claims 1  to  17  is applied to the said article and in that the article thus coated is cured at a temperature of 140 to 200° C. for a time which may be as long as 30 minutes.  
     
     
         20 . Satin and matt coatings obtained from thermosetting powder compositions according to any one of  claims 1  to  17 .  
     
     
         21 . Articles entirely or partially coated using the process according to  claim 19.