TOWARD SOLVENTLESS LIQUID COATINGS

Results of an exploratory study of solventless (100% reactive) liquid polyester/melamine resin and polyester/polyisocyanate coatings are des cribed. An investigation of how the chemical structure and the molecul ar shape of oligomeric polyester resins affects their viscosities foun d that slender molecules have very low viscosities. ''Linear'' oligoes ter diols composed entirely of diacids and diols of the structures HOO C (CH2)(n)COOH and HO(CH2)(m)OH with M(n) of 700 or below have viscosi ties below 800 mPa . s at 25 degrees C, low enough for potential use i n solventless coatings. Physical properties of oligoester diols were s tudied, and quantitative relationships among viscosity (eta), temperat ure (T), molar mass (M(n)), glass transition temperature (T-g), and me lting point (T-m) were demonstrated. The next step was to formulate so lventless coatings with linear oligoester diols. An arbitrary goal was to keep viscosity below 800 mPa . s at 25 degrees C without adding or ganic solvents. A two-package solventless coating made from a linear o ligoester diol and a low viscosity polyisocyanate crosslinker had a vi scosity of 560 mPa . s at 25 degrees C and excellent film properties; its VOC was 8 g/L. Polyester/melamine resin coatings with similar visc osities can be formulated, but their coating films tend to be soft and to have poor adhesion to metal. Film properties can be improved by ad ding hardeners to the formulations, but viscosity increases. Viscosity of these solventless formulations can be further reduced by adding mo dest amounts of water (3 to 20 wt%). This expedient makes it possible to formulate one-package ''water-thinnable solventless'' coatings with reasonably good film properties from combinations of linear oligoeste r diols, hardeners, and melamine resins. Two-package coatings containi ng polyisocyanate crosslinkers can also be thinned with water with lit tle effect on pot life.