Wood Modification With N-Hydroxymethyl Resin-Based Agents

This study aims at improving the dimensional stability and flame retardancy of wood by treating poplar(Populus ussuriensis Kom) and oak (Quercus mongolica Fischer ex Ledebour) wood with1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) combining boric acid (BA), guanylurea phosphate (GUP), methylolated guanylurea phosphate (MGUP), and/or FRW (GUP and BA as the main compounds). The effects of modification on the dimensional stability, moisture sorption, mechanical strength, coating and glueability, and combustion performance were determined.The treating aqueous solutions contain DMDHEU as the modifying agent, magnesium chloride (MgCl2) and BA as the reaction catalysts, and/or GUP, MGUP, and FRW as the flame retardants. They are delivered into wood microstructures using a vacuum-pressure impregnation process, followed with a curing process at120℃. The wood treated with DMDHEU and5%FRW, GUP, or15%MGUP in the presence of1.5%MgCl2exhibited a fixation up to90%. This can be explained by crosslinking between DMDHEU and wood cell wall polymers and self-condensation of DMDHEU in the cell wall. The formaldehyde emission for the wood treated with a DMDHEU concentration not higher than15%measured up to the standard (GB18580-2001) set by the state for interior used materials. In order to further reduce the emission of formaldehyde, a methylolated DMDHEU (mDMDHEU) was also tested for wood modification. The poplar wood treated with mDMDHEU emited significantly lower amount of formaldehyde than this treated with DMDHEU; however, mDMDHEU apparently had a lower fixation in the wood due to bearing less reactive N-methylol and the treated wood therefore exhibited less dimensional stability.Treatments with DMDHEU can impart wood an anti-swelling efficient (ASE) and bulking efficient (BE) up to50and6%, respectively, showing that treatments can dimensionally stabilize wood. Addition of MGUP did not substantially influence the ASE and BE of wood. Compared to the untreated wood, wood treated with increasing concentration of DMDHEU exhibited lesser shrinkage in tangential, radial, and longitudinal directions as they were dried from water saturation state to oven dry state; addition of MGUP resulted in an increased shrinkage. Wood treated with DMDHEU and flame retardant compounds showed a decreased moisture adsorption, as evidenced by FTIR that the amount of hydroxyl groups in wood is reduced. Water and aqueous urea-formaldehyde resin can disperse and wet the modified wood surface well; however, glycerol was more difficult to wet the treated surface than to the surface of untreated controls.DMDHEU treatments led to an increase in wood density, modulus of elasticity in bending, hardness, and compression strength; however, both the bending-and impact strengths were decreased. Dynamic mechanical analysis (DMA) showed that wood treated with DMDHEU and flame retardant compounds exhibited a greater storage modulus than the untreated controls preceding the glass transition temperature, after which presented it an inverse result. The glass transition temperature ranged between211and228℃for the untreated oak wood, between203and250℃for wood treated with DMDHEU alone, between195and239℃for wood treated with DMDHEU and MgCl2, and between220and238℃for DMDHEU and MGUP treated wood, respectively.The plywood made from DMDHEU treated veneers glued by melamine-modified urea-formaldehyde (MUF) adhesive showed a lower gluing strength than this from untreated controls; however, direct compression of impregnated and air dried wood veneers under hot press in the presence of MUF adhesive can get a plywood with an improve gluing strength up to80%. Treatments with DMDHEU alone or combined MGUP did not influence the adhesion of coatings on the treated wood surface.The combustion behavior of untreated and treated wood was evaluated using CONE calorimetry. Treatments with DMDHEU created a combustive material. Therefore, several flame retardant compounds, FRW, GUP, MGUP, and BA were individually or jointly incorporated into DMDHEU solution to improve the flame retardancy of the treated wood. Treatments of wood apparently caused an improved flame retardancy and smoke suppression during combustion and the combinative treatments with BA and GUP or MGUP exhibited the best effect. SEM-EDX micrography showed that DMDHEU and part of flame retardant compounds can penetrate and distribute evenly in the cell walls. TG analysis evidenced that DMDHEU treated wood exhibited a higher pyrolysis temperature and lower residual charcoal than the untreated controls; incorporation of flame retardant compounds can considerably increase the amount of residual charcoal and lower the pyrolysis temperature. The flame retardancy effects may be attributed to formation of charcoal during combustion process catalyzed by flame retardant compounds.Summarily, treatments with DMDHEU can apparently dimensionally stabilize the wood; incorporation of flame retardant compounds efficiently retards the combustion of the treated wood. Considering the wood performance and cost of modifying agents, a formulation used for wood modification containing10%DMDHEU,3%BA, and10%MGUP can be recommended.