¹³C CP/MAS NMR analysis of cure characteristics of phenol formaldehyde resin in the presence of wood composite preservatives and wood: effect of ammonium pentaborate and copper compounds

Abstract

The effect of typical wood composite preservatives, ammonium pentaborate (APB), nanosize copper oxide and basic copper carbonate, on the cure characteristics of phenol formaldehyde (PF) resin in the presence of wood was evaluated by solid-state ¹³C nuclear magnetic resonance with cross-polarization and magic angle spinning (CP/MAS). With the introduction of APB the absorption intensity and peak area of PF resin at 129.5 ppm was reduced, and the carbons in methylene bridges shifted from 65.8 to 73.5 ppm, which were the result of hydrogen bond formation between ammonium in APB and oxygen of phenolic hydroxyl, as well as coordination bond between the boron in APB and oxygen in phenolic hydroxyl and/or unreacted hydroxymethyl. In addition, the peak area at 152.7 ppm increased with the addition of poplar powder for the overlap of cellulose, hemicellulose, and lignin chemical shifts with the active groups in PF resin. However, the connection status of critically active chemical groups of condensed polymer structure in cured PF resin such as the existence of phenolic ring, phenolic hydroxyl, methylene bridges, and hydroxymethyl linkage are unchanged. Combined with relative increase in the amount of carbon in methylene bridges from 2.42 to 2.56, drop in number of carbons of unreacted hydroxyls from 1.19 to 1.07, and the reported increase in physical and mechanical properties, the nanosize copper oxide improved the curing degree of PF. Furthermore, the similar analysis indicated that basic copper carbonate delayed the curing degree of PF.