John F Hardy

The effect of the morphology of urea-formaldehyde pigments in paper filler applications

Abstract White polymeric pigments, based on urea-formaldehyde condensation products, are described. These pigments are composed of nonporous “particles” which are partially coalesced to form agglomerates of controlled size. While the “particles” are nonporous, mercury porosimetry studies indicate that the pigment agglomerates have a pore volume of 2 cm 3 /g with a modal pore diameter of 0.3–0.5 μm. Automated image analysis shows that the mean agglomerate size of the pigments can be varied from about 5 to 20 μm. The morphology of the urea-formaldehyde pigments makes them highly suitable for use in paper filler applications where they exhibit high first-pass retention and provide opacity and bulk. Porosimetry studies show that in paper the pigment agglomerates maintain their pore structure and increase the apparent interfiber void volume of the sheet. Calendering reduces both the agglomerate pore volume and the apparent interfiber void volume and shifts their modal pore and void diameters to smaller sizes. The effect of such changes on the bulk and opacity of paper is discussed.