Building bridging structures and crystallization reinforcement in sodium silicate-modified poplar by dimethylol dihydroxyethylene urea

The mechanical and flame-retardant properties of sodium silicate-impregnated fast-growing poplar have been greatly improved by a wide variety of methods, which has solved the problem of insufficient supply of natural wood to a certain extent. However, sodium silicate is easily leached and has high hygroscopicity, and sodium silicate-modified poplar (SSMP) has low anti-shrink efficiency (ASE), which make it difficult to maintain its dimensional stability. Dimethylol dihydroxyethylene urea (DMDHEU) contains polyhydroxymethyl active groups that are highly reactive with wood fibers and sodium silicate. Therefore, in this study, sodium silicate and DMDHEU were used as a composite modifier to carry out vacuum-pressure-impregnation modification on fast-growing poplar to form a bridging structure. This treatment fixed the sodium silicate and improved the dimensional stability of poplar. Mechanical properties of poplar wood were improved by modifications with sodium silicate and DMDHEU. The dimensional stability was greatly improved, and the fixation of sodium silicate was improved. Compared with the SSMP, the leaching rate and ASE of SS/DDMP were reduced by 48.82% and 41.79%, respectively. XRD, FTIR, and XPS results showed that C–O–C and Si–O–C bonds were formed between DMDHEU and the wood cell walls and sodium silicate. These bonds closely bound the cellulose crystals, which reduced the number of –OH groups being accessible for water and, thus, the moisture absorption of SS/DDMP. In addition, due to the increase in crystallinity, the heat resistance was further enhanced. The cone calorimetry results showed that SS/DDMP had the lowest heat release rate and total heat release. Compared with SSMP, the mean smoke release rate (mean SPR) and total smoke release decreased by 40.38% and 40.83%, respectively. Moreover, the release of CO and CO 2 decreased. In conclusion, compared with other modification methods, the use of SS/DD impregnation to modify poplar has the potential to produce good overall performance of poplar with high-dimensional stability, flame retardancy, and smoke suppression.