ALUMINUM TRIHYDROXIDE (ATH) - A UV TRANSPARENT FILLER FOR UV-CURABLE COATINGS

Unlike other extenders, aluminum trihydroxide (ATH) absorbs very little ultraviolet radiation over the wavelength range which is commonly used to activate most UV photoinitiators (220 to 400 nm). This is demonstrated by comparing the total transmittance (diffuse and specular) of UV-cured polyurethane films with and without the use of ATH as an extender. Difference spectra between filled and unfilled systems show that ATH does not interfere with the transmittance of UV light. On the other hand, a comparable study with a silica-filled system indicates that a higher proportion of UV light is absorbed. Solid state C-13 NMR and FTIR spectra of ATH-filled, silica-filled, and neat systems show that the final chemical components of the crosslinked polymer matrices are similar. However, FTIR analyses of residual acrylate absorbances near 830 cm-1 show that the ATH-filled systems exhibit higher degrees of cure than both the neat and the silica-filled systems. This agrees with differential photocalorimetry (DPC) studies which show that the enthalpy of cure increases in the presence of ATH. Diffuse reflectance studies of neat powders indicate that ATH exhibits higher reflectivity than silica in the UV range. Thus, the difference in UV absorption behavior between the filled and neat polymer films, and the difference in the degree of cure can be attributed to differences in filler absorption and reflectivity. Taken together, these trends indicate that, unlike other fillers, ATH can function as a UV transparent extender tor UV-curable polymer applications.