Investigation of the wood/phenol-formaldehyde adhesive interphase morphology

This work addresses the morphology of the wood/Phenol-Formaldehyde (PF) adhesive interphase or scale of PF penetration in wood. The objective is to develop methods for revealing ever smaller levels of wood/resin morphology. Dynamic techniques that are commonly utilized in polymer blend studies are implemented on wood/PF composites. These are Dynamic Mechanical Analysis (DMA) and solid state NMR using CP/MAS. PF resin molecular weight is manipulated to promote or inhibit resin penetration in wood, using a very low (PF-Low) or a very high (PF-High) molecular weight PF resin.; With DMA, the influence of PF resin on wood softening is investigated. It is first demonstrated that the cooperativity analysis according to the Ngai coupling model applies for wood softening. PF-Low does not influence wood softening temperature. However, intermolecular coupling is enhanced in the PF-Low composite, suggesting that PF-Low constrains wood relaxation. Surprisingly, PF-High significantly lowers the sample softening temperature. With CP/MAS NMR, none of the resins significantly affects the cross-polarization time (TCH), suggesting that angstrom scale penetration does not significantly occur. However, PF-Low alters wood polymer spin lattice relaxation time in the rotating frame, HT1rho, indicating nanometer scale interactions at the interphase. PF-High has no significant effect on wood HT1rho. Interestingly, PF-Low disrupts the spin coupling typical of wood components, while PF-High has no effect on spin coupling of wood components. Finally, the PF-Low resin T1rho lies within the range of wood relaxations suggesting spin coupling, while the PF-High resin T1rho appears outside the range of wood relaxations and is therefore not spin coupled with wood.; The CP/MAS NMR and DMA studies converge to identify nanometer scale penetration and in-situ cure of PF-Low in wood. On the other hand, PF-High forms overall separate domains from wood, although the low molecular weight tail of PF-High may penetrate wood on a nanoscale. The lower softening temperature in the PF-High composite may stem from this low molecular weight fraction penetrating wood intimately but remaining uncured or unconnected to the bulk PF-high resin phases.