Hse reported a correlation between penetration and contact angle for PF and southern pine wood [53]. The author employed 36 formulations to determine the contact angle, and its influence on cure time, heat of reaction, plywood shear strength, percent wood failure, bondline thickness, and cure shrinkage. Penetration was not measured, but assumed to be inversely proportional to bondline thickness (thickness of cured adhesive between the veneers). Penetration increased with increasing caustic content. There were no clear trends observed for penetration in relation to adhesive solids content or formaldehyde–phenol mole ratio.
In the case of powdered adhesives, such as powdered PF used in OSB manufacture, it has to melt before penetration. Johnson and Kamke [54] noted that powdered PF resin remained on the surface of wood strands during the blending process and was able to flow and penetrate only during steam injection hot-pressing.
Frazier et al. [55] noted that low MW of pMDI resin would promote penetration into wood cell walls. They further hypothesized that the MDI forms an interpenetrating network of polyurea and biuret linkages within the cell wall. Swelling of the cell wall by pMDI was also observed [48].
The use of resin with low MW components has the potential for deeper penetration than that with high MW. Stephen and Kutscha separated a commercial PF resin into two MW fractions (approximately ±1000 MW) [56]. They observed the penetration of the resin by placing a drop of resin on the surface of aspen. The authors reported that no penetration through the wood was observed for the high MW fraction of the resin, while a penetration of one to two cells deep occurred for the low MW PF fraction. The addition of NaOH improved the penetration of the high MW fraction, which the authors assumed was due to swelling of the cell wall by the NaOH. The improved penetration may also have been due to a lower viscosity as a result of NaOH addition [57]. Gollob et al. reported that PF bond performance was associated with penetration in Douglas-fir plywood [58]. They noted that higher MW formulations tended to dry out and had little penetration.
Zheng studied the penetration of the blends of MDI and PF into yellow-poplar and southern pine [59]. The penetration of the adhesive blends was characterized by a phase separation, with pMDI penetrating deeper. PF tended to bulk the lumens and remain at the interface of the bondline. In general, the blends resulted in less penetration than either of the neat resins. The author attributed the reduction in penetration to increased MW, and subsequent increased viscosity, due to the formation of urethane bonds between the PF and the PMDI.
MW distribution of resin systems will impact their ability for cell wall penetration. Laborie [32] reported evidence of cell wall penetration for two PF formulations, one that had a number average Mn of 270 and a weight average Mw of 330. The other PF had Mn and Mw values of 2840 and 14,200, respectively. The more highly condensed PF resin had a broad MW distribution, including a low Mw component that was similar to the low MW PF resin. Using dynamic mechanical analysis, the author concluded that both resin systems penetrated the cell wall.
1.9 Effect of Processing Parameters on Penetration
Processing factors that can influence the adhesive penetration are open assembly time, pressing time, temperature, and consolidation pressure involved in wood-based composite manufacture. There can be a complex interaction between processing parameters, adhesive formulation, and wood characteristics. Sernek et al. reported increasing penetration of UF resin into beech as open assembly was increased [31]. Temperature influences penetration by affecting resin viscosity and cell wall permeability. At the same time, in the case of thermosetting adhesives, polymerization increases viscosity, countering the temperature effect on liquid viscosity.
White studied the influence of consolidation pressure (from 3 to 1000 kPa) on penetration and consequently on the fracture toughness of southern pine blocks bonded with resorcinol-formaldehyde [52]. Increasing consolidation pressure increased penetration into earlywood, but had an erratic effect on latewood. Low permeability of the latewood may result in the adhesive squeezing of the adhesive out of the bondline during consolidation. Increasing consolidation pressure reduced fracture toughness of the latewood specimens but had no significant influence on the earlywood specimens.
Johnson and Kamke [54] reported increased penetration of PF into yellow-poplar strands as a result of steam injection pressing. The authors suggested that condensate from the steam dilutes the resin and steam pressure forces the resin deeper into the wood.
The use of radio-frequency heating of a veneer composite caused a reduction in penetration of UF resin in comparison to matched samples produced using conduction heat in a platen [31]. The authors noted that the rate of polymerization was much faster using radio-frequency heating and thus reduced the time for penetration.
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