Coreacting PMUF/isocyanate resins for wood panel adhesives

PMUF/pMDI resins coreacting during curing give good results as thermosetting wood adhesives. TMA experiments indicate their best performance to be around the relative weight proportions of 15/85 pMDI/PMUF. However, the results of laboratory particleboards bonded with this adhesive system indicate a much lower sensitivity to the relative proportions of the two components, except in the case of the IB after 2 h boiling which improves markedly passing from 0% pMDI to 20%-25% pMDI. This indicates that addition of small proportions of pMDI markedly improves the exterior grade performance of PMUF resins. The performance of any formaldehyde resin coupled with pMDI appears to be dependent from its molar ratio, as does the optimal relative proportion of isocyanate to formaldehyde resin. This clearly indicates the reaction of the hydroxymethyl group of the formaldehyde resin with the isocyanate group. This effect appears to be valid regardless of the used type of formaldehyde resin.