EFFECT OF FIBER MAT MOISTURE-CONTENT ON STRENGTH PROPERTIES OF POLYISOCYANATE-BONDED HARDBOARD

This paper reports on the effects of fiber mat moisture content (MMC) (8%, 15%, and 22%) on strength properties of hardboard bonded at a 3 percent diphenylmethane diisocyanate (MDI) adhesive level, and compares them to hardboard bonded at phenol-formaldehyde (PF) adhesive levels of 3 and 10 percent and an MMC of 7 percent. Test conditions consisted of dry, 24-hour water-immersion, and two, four, and six cycles of ASTM D-1037 accelerated-aging treatment (AAT). MMCs of 8, 15, and 22 percent had no statistically significant effect on either modulus of elasticity (MOE) or modulus of rupture (MOR) of dry, 24-hour water-immersed, or AAT specimens. Changes in MMC from 8 to 22 percent had no significant effect on internal bond (IB) of dry-tested hardboard, but the higher moisture levels improved IB strength of hardboard subjected to AAT. In most cases, strength properties of hardboard bonded at the 3 percent MDI level were superior to hardboard bonded at the 3 percent PF level, and superior or comparable to hardboard bonded at the 10 percent PF level. Furthermore, the MDI-bonded hardboards were found to be very durable, even after exposure to as many as six cycles of AAT.