While PUDs have demonstrated strong growth as a unique technology, it has been inevitable that innovation has explored PUDs blended with other dispersible polymers or hybridized with other polymer systems [48, 49]. One of the common blended or hybridized copolymer systems has been the PU-acrylic waterborne dispersion. This category has been successful because (i) each system has properties that make up for shortcomings in the other, (ii) they have compatible chemistry that makes them easily blended or hybridized, (iii) the acrylic blend can be less expensive than the pure PUD, and (iv) they are already incumbent within the coating market so that barriers to acceptance by consumers of their combination is greatly reduced. Blends tend to exhibit better hardness and weatherability than pure PUDs and better flexibility and solvent resistance than pure acrylic dispersions.
Blending dispersions is the easiest way of making a new product, and this technique is certainly utilized industrially. The acrylate monomers making up the acrylic dispersion are usually formulated to optimize hydrogen-bonding opportunities with the PUD to enhance phase stability and coating performance. Hybridized or alloyed dispersions are produced by creating covalent bonding between the acrylate and urethane polymer chains. This has been done by reacting hydroxy functional acrylic monomers such as hydroxyl ethyl acrylate with diisocyanate and then performing the radical polymerization of the isocyanate-acrylate monomer with the other acrylate monomers for the dispersion . The reactive acrylate dispersion is then reacted with the PUD prepolymer and chain extender to make the new urethane-acrylate
TABLE 10.11 Composition and coating properties of a blend of acrylic and polyurethane waterborne dispersions
polymer backbone. The hybrid backbone may obtain superior phase mixing and coating properties and commercial products do exist; however, the additional associated costs associated with the new product may in some cases create a barrier to implementation. An example of an acrylic-PUD system suitable for blending along with coating properties is given in Table 10.11.