Effect of wood chemical characteristics on bond quality. Part VIII. Impact of wood chemical characteristics on curing behaviour of PF, MDI, and UF resins
A total of 16 wood species were characterized for pH, acid buffer capacity, and base buffer capacity. These species are used in the manufacture of particleboard, medium density fiberboard (MDF), and oriented strand board (OSB) in Canada. The effect of wood characteristics on the curing behaviour of phenol-formaldehyde resin (PF), methylene diphenyl diisocyanate (MDI), and urea-formaldehyde (UF) resin were investigated by means of differential scanning calorimetry (DSC) analyses and gel tests.
First of all, it was observed that wood chemical characteristics influenced PF resin curing behaviour in terms of onset curing temperature (Te), exothermic peak temperature (Tp), and exothermic reaction heat measured by DSC. The extent of this influence on resin curing also depended on resin properties. High viscosity phenol-formaldehyde resin (PF H) cured faster than low viscosity phenol-formaldehyde resin (PF L) in the absence of wood, in terms of lower Te, Tp, and reaction heat. This is because PF H is a more advanced resin than PF L. In the presence of wood, including aspen, white birch, yellow birch, red maple or a mix of these species, PF H cured faster than PF L. However, the resin curing behaviour was influenced by wood species. For PF H, adding white birch and the mixed species seemed to retard resin curing as indicated by higher Te and Tp. This was likely due to the low pH and/or high base buffer capacity of these species. In the case of PF L, adding wood did not seem to interfere with resin curing, but rather promoted resin curing, regardless of wood species. This result can probably be attributed to the higher pH value of PF L, which made PF-L more tolerant of the influence of the wood species or wood chemical characteristics than PF-H. Statistical analysis also indicated that the difference in Te between PF-L and PF-H decreased with increased wood base buffer capacity or decreased wood acid buffer capacity.
Next, the effect of the above-mentioned four wood species and their mix on MDI curing behaviour was evaluated by DSC analysis. The study showed that wood chemical characteristics significantly influenced MDI curing. Aspen, having the highest acid buffer capacity and the lowest base buffer capacity, yielded the highest Tp and the lowest reaction heat; white birch, having the lowest pH and highest base buffer capacity resulted in the lowest Te. In addition, yellow birch, with the lowest acid buffer capacity, yielded the highest reaction heat. Statistical analysis showed that a strong correlation exists between pH and Te, and an adverse correlation exists between acid buffer capacity and reaction heat. Moreover, an apparent correlation was observed between pH and Tp, while an adverse correlation was observed between base buffer capacity and both Tp and Te. These results suggest that an increase in wood acidity promotes MDI curing.
Of the 16 wood species characterized, pH ranged from 3.74 to 6.32, acid buffer capacity ranged from 1.62 to 6.93 mEq, and base buffer capacity ranged from 4.44 to 16.22 mEq. Both gel test and DSC analysis indicated that an increase in wood pH and acid buffer capacity or a decrease in base buffer capacity appeared to retard UF resin curing. The effect of wood chemical characteristics on UF resin curing using two different catalysts was also evaluated by DSC. Ammonium chloride (NH4Cl, 10% aqueous solution) is a salt of strong acid and weak base, while acetic acid (CH3COOH) is a weak organic acid. Ammonium chloride seemed to be more effective than acetic acid in promoting resin curing in the presence of wood. At least 1.50% acetic acid was required in order to achieve a level of resin curing similar to that observed for 0.25% ammonium chloride. Increasing the catalyst content from 1.50 to 5.0% did not seem to influence the resin curing rate with ammonium chloride catalyzed resin but continue accelerating the resin curing rate with acetic acid catalyzed resin.