UV Cured Waterborne Dispersions for Coatings
While UV-cured PU coatings are not a dominant coating technology, its use is growing for wood, metal, and plastic coatings in particular. UV-cure technology will grow within its application space as the properties of the coatings improve. The concept of these coatings is that they allow for lower coating viscosity than two-part solventless formulations, and emit less VOC than solvent-borne formulations or those having unreacted monomers within the formulation. While radical-induced polymerizations resulting from UV irradiation can be fast, the need to evaporate water from the coating can slow down manufacturing line speeds [51, 52]. However, as formulations improve and solids levels increase, the waterborne coatings become an increasingly attractive option in some venues. In addition, the dual cure nature of these coatings allows for tuned cross-link densities, and the possibility of higher final coating cross-link densities at lower initial viscosity than obtained with other coating technologies .
The UV-cure potential of PU s is achieved by incorporating hydroxyl functional acrylate monomers into the prepolymer synthesis. There are many acrylates to choose from, but an available monomer that maximizes the available cross-link potential is the triacrylate of penterythritol. The reactive components are controlled to optimize
FIGURE 10.21 Illustrative composition of building blocks for making a UV curable polyurethane coating. The addition of fatty alcohol triglyceride might enhance solvent resistance.
structure and performance. An example of the PU prepolymer components including the polyacrylate monomer are pictured in Figure 10.21. The composition is conventional with the possible exception of castor oil (the triglyceride of ricinoleic acid— see Chapter 2) that could potentially provide additional solvent resistance [54, 55]. The addition of the triacrylate needs to be controlled to prevent it from causing a maldistribution of molecular weights . The isocyanate is added in amounts to control the final dispersion hard segment volume.
Upon completion of prepolymer formation, the prepolymer is added to cold water under high shear as described in detail previously, and the diamine chain extender (i.e., 1,2 propylene diamine or ethylene diamine) added. An appropriate photoinitiator is added last and incorporated with agitation. Depending on the photoinitiator, this may be more than 1% of the coating weight. A coated panel is then dried and subsequently passed under UV light exposure; time and energy are variables . Properties of coated panels as a function of UV treatment versus no treatment are presented in Table 10.12. Coating performance data is a function of many parameters including hard segment volume, and other characteristics of the PU backbone, along with coating processing and substrate preparation.