2.1 VOC solvent borne 2K clear coats based on star oligoethers

The European product directive 2004/42/CE on 2K (2-component) car repair coatings forces the customers to use 3.5 lbs/gal volatile organic compounds (VOC) products for topcoats and clear coats as of 2007. There will be a new discussion on solvent emission levels as of 2008 in view of actual technologies available and the EC commission may come up with more stringent proposals. The main binders used in 3.5 VOC 2-pack refinish topcoats and clear coats are acrylic polyols sometimes combined with polyester based diluents which are cross-linked with 1,6-hexamethylene diisocyanate trimer (HDI-trimer). This technology will not allow formulating substantial lower VOC products (e.g. 2.1 lbs/gal) at an acceptable overall balance of potlife, overall drying, appearance and durability. It is believed today that those emission levels and product performance can only be reached:-In solvent borne systems with oligomers if reactive functional groups, like hydroxyl or amine, are blocked. The blocking of the functional groups result in sufficient long potlife and unblocking takes place upon hydrolysis during film formation.-In water borne systems with standard polyol dispersions and emulsions.Both solutions will have limitations as far as paint application conditions because of the dependence of either the hydrolysis reaction (in solvent borne) or the evaporation of water (in water borne) on air humidity. Other issues related to water borne are the reactivity of the polyisocyanate with water causing foaming and film defects remain unsolved. Technologies depending on a fast hydrolysis reaction with water will probably also be limited to unpigmented formulations. It would therefore be an advantage if a standard hydroxyl–isocyanate binder system could be developed that can be used in low VOC pigmented and unpigmented 2K formulations.In this paper we will discuss hydroxyl functional, branched polyethers that allow formulating regular 2K clear and top coats at 2.1 lbs/gal VOC with an acceptable overall balance of potlife, appearance, durability and productivity impossible or difficult to obtain with acrylic or polyester polyols. The structure of the polyether oligomer has to be branched (at least four functional) to get the overall property balance. The polyether oligomers are prepared by ring-opening polyetherification giving narrow functionality and molecular weight distributions and a favorable solids/viscosity relation. The ratio of mono epoxyester to polyol and type of polyol will determine functionality, molecular weight (Mw) and hydroxyl number (OH#) of the polyether oligomer. The paper will comment on relation solids/viscosity with glass transition temperature (Tg), functionality (F) and Mw, all-important factors to get the property balance. We will show how final clear coat properties correlate with some initial oligomer/cross-linker properties in view of Mw, Tg and F of oligomer and type of polyisocyanate hardener.