Modification of Surface Roughness by Various Varnishes and Effect on Light Reflection

The modification of microscopically rough surfaces by commonly used varnishes and resultant changes in light reflection from these surfaces were studied using laser scanning confocal microscopy, stylus profilometry and specular gloss and distinctness-of-image gloss measurements. Natural resins as well as synthetic low molecular weight and polymeric resins were studied. The molecular weights of the resins were determined using size-exclusion chromatography. All the low molecular weight resins, with weight-average molecular weights in the range 800-6500, including the natural resins, essentially produced smooth surfaces and eliminated high-frequency as well as low-frequency roughness, while polymeric resins, with weight-average molecular weights between about 25000 and 210000, produced rougher surfaces, eliminating high-frequency roughness but not all low-frequency roughness. The spatial frequency range of the roughness of the polymeric coatings decreased with increasing molecular weight. As a result, low molecular weight varnishes reduced small-angle scattering about the specular reflection to a greater extent than the polymeric coatings. Polymeric varnishes, therefore, demonstrated lower specular gloss and, in particular, lower distinctness-of-image gloss. Distinctness-of-image gloss values showed good correlation with the weight-average molecular weights of the resins. The power spectral density of the surfaces coated with low molecular weight resins showed a sharper falloff at low spatial frequencies than the polymeric resins.