Effect of polyisocyanate level on strength properties of wood fiber composite materials

The development of strong, durable, and hydrophobic wood composite materials that can be manufactured in a technically simple and efficient operation have been sought by wood scientists and industry for decades. Polyisocyanate resins possess unique characteristics that may enable the manufacture of performance-driven and hybrid wood composites for partially fulfilling the desired attributes. The objectives of this study were to examine the effectiveness of using increasing levels of diphenylmethane diisocyanate (MDI) resin (3%, 10%, 20%, and 30%) for improving strength properties of wood fiber composites. The strength properties of MDI-consolidated fiber composites are compared with those consolidated with phenol-formaldehyde (PF) resin at comparable resin levels. Strength properties evaluated were modulus of elasticity (MOE), modulus of rupture (MOR), and internal bond (IB) of specimens subjected to dry, water-immersion (wet), and ASTM D-1037 accelerated-aging treatments (AAT). Results indicate that an MDI level near 20 percent may be the most efficient in maximizing strength properties of MDI- consolidated wood fiber composites. Dry-MOE and dry-MOR were maximized at MDI resin levels near 10 and 20 percent, respectively, while dry-IB was maximized at the 20 percent level. Resin level had no effect on AAT-MOE. AAT-MOR and AAT-IB were maximized at the 10 percent and 20 percent levels, respectively. Wet-MOE and wet-MOR were most efficiently maximized at the 30 and 20 percent levels, respectively. There were practically no differences between MDI- and PF-consolidated wood fiber composites in MOEs under the dry, water-immersed, and AAT conditions at each of the four resin levels. However, there were significant differences between MDI and PF in MOR and IB under all test conditions; the MDI-consolidated composites were far superior to the PF- consolidated composites.

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