PU Coating Formulations
Coatings often have a decorative and a protective function. Since many coatings are exposed to the environment, especially to light, they are often formulated with aliphatic isocyanates to circumvent photolytic degradation mechanisms (see Section 2.2.3). Manufacturers of aliphatic isocyanates have modified aliphatic monomers to provide cross-linking potential for coatings by trimerizing isocyanates into isocyanurate rings. The resulting ring structures are very thermodynamically stable,
FIGURE 10.20 Structure of aliphatic isocyanate tamers (IPDI and HDI) commonly employed in decorative and protective polyurethane coatings.
have the potential to create three-dimensional networks through three-functional cross-links, improve solvent resistance, and promote formation of clear amorphous/ glassy films by inhibiting phase separation of any chain extended phase. Figure 10.20 are the structures of aliphatic isocyanate trimers commonly employed for PU coatings. H12MDI that is used in limited coating applications is not commercially supplied as a trimer. The use of HDI trimer or IPDI trimer is a matter of coating requirements, and it is not unusual for a coating formulator to mix them in a single coating to obtain optimized properties. The linear aliphatic HDI trimer is employed to impart flexibility, and uniform reactivity. The cycloaliphatic IPDI trimer produces enhanced coating rigidity and higher film hardness. The secondary isocyanate of IPDI is slightly less reactive than the primary isocyanates of the HDI trimer, and results in longer pot life and slower reactivity as measured by tack-free times.
Two-part Solvent-Borne Coating
The two-part solvent-borne coating formulation depicted in Table 10.9 is one that might be used for an automotive top coat, refinish, or general industrial coating. It utilizes a linear acrylic polyol with a binder Tg of 48 °C, OH number of 59.4 (OH equivalent weight 944), an Mn of 2941 g/mol dissolved in a xylene-n-butyl acetate mixture with a viscosity of 2200 cps at 40% solids. The total solids content of the formulated coating is 55% adjusted with added butyl acetate. Formulations were prepared with trimers of HDI and IPDI. Coatings were applied at room temperature and cured at 60 °C for 3 h. Final coating thickness was 25±2|im for both coatings.
The formulations of Table 10.9 are simplified to demonstrate coater binder properties separate from properties influenced by other components such as pigments or rheological and stabilization additives. The data illustrates the influence of aliphatic isocyanate on coating properties. The disparity of properties between trimers of HDI and IPDI naturally leads formulators to attempt the best of both worlds by blending [44, 45]. While blending of IPDI with HDI does moderate the coating performance between the extremes, it is often not a linear response and the properties of different coating properties do not change with the same concentration function. Thus, while change in Koenig hardness may change linearly with isocyanate composition, the mandrel test of flexibility may not change appreciably until the isocyanate
TABLE 10.9 Formulation and properties of a two-part solvent-borne polyurethane coating
Coating properties are affected in predictable ways by choice of isocyanate cross-linking and formulators often mix them to balance performance.
composition is very close to a concentration limit. In addition, change in the chemical resistance of the coating may follow an "S" curve suggesting some kind of required network formation to affect properties.