words if melamine and polyester polyol reacts first the isocyanates will react only with epoxy resin the blocked isocyanate components used in this invention have a molecular weight of 150 to 500 per blocked nco group and further the blocked nco group has at least three active sites per chain so that cross linking and thermosetting properties are established in the resulting polyurethane binder the epoxy resin used in this binder has an average molecular weight of 1000 to 5000 and contains only secondary hydroxyl groups preferably the epoxy resin is a product of condensation of epichlorohydrin and bisphenol a the use of a secondary hydroxyl group of epoxy resin is to prevent chemical interaction with melamine resin the ratio of blocked isocyanate to epoxy is controlled in such a way that one weight part of the nco group of blocked isocyanate is reacted with one weight part of secondary oh group from the epoxy resin technically speaking when the two crosslinked polymer networks are blended together a characteristic cell structure should be exhibited where the first crosslinked network makes up the cell walls and the second crosslinked network provides the contents of the cells the actual size of the cellular structures depends on the crosslinked density of the two network polymers varying from about 1000 to 500 angstroms this variation by a factor of three depends upon the weight ratio of the components it is preferred that the total weight of melamine polyester is 25 of the total weight and the total weight.