Uniform Polymer Distribution in Papers Saturated with Polymer Solutions via Microwave Power^*

Low-cost overlay papers are often made by continuously saturating paper web with a solution of a thermosetting polymer solution, such as phenol formaldehyde or melamine formaldehyde resin solution, followed by drying the saturated paper in a convection-heated oven. The final properties of the overlay paper so produced, such as the internal bond, are often dependent on the polymer distribution across the cross-section of the paper. A method for determining the polymer distribution in paper by soaking the paper in 80 per cent HF for 7 days to dissolve the cellulosic material, followed by embedment in an epoxy resin, microtoming 10–15 μ thick sections of the sample, and photographing the sections under low magnification, was worked out. Using this method, the resin distribution in papers dried at various drying temperatures under convection heating was compared with the resin distribution in microwave- dried samples. It was found that papers dried initially by microwaves gave a uniform polymer distribution. The papers dried using conventional convection-drying temperatures were starved internally for polymer, most of the polymer having migrated to the surface, presumbly during the dring process.