The Effect of Sequential Ammonia and Methyl Formate Injection on the Strength Development of Phenol-Formaldehyde Bonds to Wood

It has been reported in a companion paper that gaseous anhydrous ammonia may readily penetrate compressed wood fiber mats and that subsequent cell-wall softening is very rapid. Literature also shows that alkaline catalyzed phenol-formaldehyde (PF) resins of the resole type are stable at pH levels above nine. Methyl formate has, however, been shown to accelerate the rate of PF cross-linking, and its volatility (sea level boiling temperature 33°C) makes it well suited to vapor phase injection into resinated mats at near ambient temperatures. It was hypothesized at the outset of the reported investigation that mats may be treated with gaseous ammonia to affect softening without stimulation of resin cure, and that the PF could be subsequently catalyzed with methyl formate in the vapor phase. Before adopting this strategy in a sealed pressing system, the strength development characteristics of miniature PF-to-wood bonds exposed first to ammonia and then to methyl formate were explored. Bond formation and testing were conducted with an Automated Bonding Evaluation System (ABES) equipped with a computer-controlled fluid injection accessory. Results, in the form of room-temperature strength development plots, show that ammonia does not greatly stimulate PF cure (1.65 MPa after 600 s), but application of methyl formate significantly hastens strength development (3.5 MPa after 150 s). These results are aiding in the development of techniques for the rapid room-temperature formation of strong and dimensionally stable fiber composites using sealed pressing with sequential chemical injection