A Comparison of Hardboards Manufactured By Semidry-, Dry-, and Wet-Formed Processes

The semidry-formed process was investigated as a possible alternative to the wet- and dry-forming processes of manufacturing hardboards. The objective was to improve performance and/or reduce the amount of resin required. A single wood species, aspen, was fiberized in a small pressurized refiner and used for all hardboard manufacturing trials. In addition to producing water-felted fiber mats, other mats were air felted at 65, 35, and 5 percent moisture content (MC). Fiber mats entering the hydraulic hot-press were either water saturated or had 65, 35, 25, or 5 percent MC. Mats felted and pressed from fibers with high MCs did have some liquid effluent from the hot-press, but nowhere near the quantity expelled from a wet-formed and wet-pressed fiber mat. No problems were encountered in air felting fibers with higher MCs, which also eliminated the dust hazard common to all dry-forming operations. At certain fiber MCs, there was a problem with the hardboard sticking to cauls and wires during hot-pressing. Sticking was not a problem with the wet- and dry-formed fiber mats. Strength properties of the semidry-formed hardboards fell somewhere between the wet- and dry-formed hardboards. Semidry-formed hardboards had resin requirements higher than needed for wet-formed hardboards, but lower than needed for dry-formed hardboards. A powdered phenolic resin was also tried with the semidry-formed hardboards, but the properties were poorer than hardboards manufactured with liquid phenolic resin.

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