Cure kinetics of wood phenol-formaldehyde systems

Jinwu Wang

doi:10.7273/000005698

This project aims to develop kinetic models for chemical and mechanical cure development and correlate chemical and mechanical degrees of cure in order to create a comprehensive cure model that encompasses both of these tasks. With these objectives, the cure processes of two commercial phenol-formaldehyde (PF) resol resins with differing molecular weights were evaluated using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) under isothermal and linear heating regimes. For both resins, cure was characterized in the neat state with DSC, in mixtures of PF/wood flour with DSC, and as a bondline between two wood substrates with DMA. The synergy of DSC and DMA techniques picked up the phase transitions of PF curing processes and characterization were comparable between the two techniques. During a DSC temperature scan, PF resols typically exhibited two exotherms, while in wood/PF mixtures another small exotherm appeared in a lower temperature range, indicating the impact of wood/PF interactions. In contrast, DMA offered a quantitative view of the adhesion mechanics from which the glass transition of uncured resin, gelation, and vitrification points were inferred. An analytical solution was developed to estimate the in situ shear modulus of the adhesive layer during the curing process, a change estimated from 0.01 to 16MPa. The maximum storage modulus and the ratio of maximum to minimum storage modulus were recommended for direct evaluation of wood-PF system.; Model-fitting kinetics of nth order and autocatalytic models can be reasonably applied to the DSC data, while autocatalytic, Prout-Tompkins, and Avrami-Erofeev models have been successfully applied to describe cure development in the DMA. The activation energy of PF curing processes in the neat state was around 85-100 kJ/mol and decreased to 50-70 kJ/mol in the presence of wood. However, it was the model-free kinetics of the Kissinger-Akhira-Sunnose, Friedman and Vyazovkin methods that offered insight into the cure mechanisms of commercial PF resoles and predicted the cure development under isothermal and linear heating regimes for both mechanical and chemical degrees of cure. Mechanical cure development has been correlated with chemical advancement in an empirical equation analog to the Weibull cumulative function. After either mechanical or chemical cure development is characterized, the other can be estimated through connection of the correlation equation between the mechanical and chemical degrees of cure.

Cure kinetics of wood phenol-formaldehyde systems

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