Modification Mechanisms and Properties of Poplar Wood via Grafting with 2-Hydroxyethyl Methacrylate/N,N'-methylenebis(acrylamide) onto Cell Walls

Fixation of Tripotassium Citrate Flame Retardant Using a Sorbitol and Citric Acid Wood-Modification Treatment

Wood modification has been explored in various ways to enhance dimensional stability and reduce flammability, with a focus on environmentally friendly treatments to meet market demands. This study aimed to investigate the efficacy of new, potential fire-retardant materials. Specifically, the study examined the combination of tripotassium citrate (TPC), a water-soluble and bio-based fire retardant, with sorbitol and citric acid (SorCA), an eco-friendly thermosetting resin previously studied. While TPC is known to control combustion, its application in wood modification has not been thoroughly researched. To assess the fixation and flammability of these fire retardants, tests were conducted on Scots Pine (Pinus sylvestris L.), including chemical analysis, dimensional stability, mechanical properties, flame retardancy, and leaching tests. The combination of SorCA and TPC showed high weight percent gain (WPG) values; however, leaching and anti-swelling efficiency (ASE) tests revealed challenges in fixation stability. The dynamic mechanical properties were reduced, whereas the static strength values were in the same range compared with untreated wood. While TPC exhibited high flame retardancy prior to leaching, its efficacy diminished post-leaching, underscoring challenges in fixation and the need for improved retention strategies. Bunsen burner tests conducted on leached specimens indicated enhanced performance even under severe leaching conditions as per the EN 84:2020 procedure. However, cone calorimetry measurements showed less favorable outcomes, emphasizing the necessity for further investigation into optimizing TPC retention and enhancing treatment efficacy.

Cell Wall Modification Based on Combination Reagents to Improve Dimensional Stability of Wood with High Efficiency

Although γ-methacryloxypropyltrimethoxysilane (MPS) was proved to be an effective reagent for improving the dimensional stability of wood, a bottleneck in ASE value (around 50%) existed. The reason was that MPS with low polarity opened few hydrogen bonds in the amorphous region of cellulose, while these hydrogen bonds could be reopened by water. Therefore, citric acid (CA) is chosen to cooperate with MPS to further enhance the dimensional stability of wood. In this paper, MPS and CA were used to modify wood individually (MW and CW) or with different combinations, that is, one-step modification (M/CW) and two-step modification with MPS first (M-CW) or CA first (C-MW). CA and MPS concentrations were optimized at 5 wt%. The ASE value for M/CW was only 25.74% at a weight percent gain (WPG) of 6.43%, which was only 0.6 times to MW or 0.7 times to CW. For M-CW, the ASE value gradually decreased with the soaking cycles, from 65.64% at a WPG of 9.05% to 51.20%. The C-MW had the best dimensional stability, with the ASE value 75.35% at a WPG of 11.50%. Although it decreased during the first soaking cycle, it stabilized at 62.20% at last. SEM and EDS images showed that the polymer mainly distributed in cell walls and few in cell lumen in C-MW. Thus, the enhanced dimensional stability of C-MW could be explained by CA opening the hydrogen bonds in the amorphous region of cellulose first, which provided more binding sites for MPS.

Strong and Durable Wood Designed by Cell Wall Bulking Combined with Cell Lumen Filling

Traditional wood-polymer composite (WPC) based on the in situ polymerization of ethylene unsaturated monomers in the cellular cavity of wood is significant for the high-value-added utilization of low-quality wood. However, this type of WPC has the problems of volatile monomers, low conversion rates, odor residue, and poor compatibility between the polymer and wood interface, which hinder its promotion and application. In this study, a two-step process of cell wall bulking in combination with cell lumen filling was prepared to modify wood using Maleic anhydride (MAN) as the bulking agent and GMA-EGDMA (molar ratio 2:1) as the active monomer system. The results indicate that the modulus of rupture (MOR) (125.19 ± 8.41 MPa), compressive strength (116.38 ± 7.69 MPa), impact toughness (55.4 ± 2.95 KJ m-2), and hardness (6187 ± 273 N) of the bulking-filling wood composite materials were improved by 54%, 56%, 36%, and 66%, respectively, compared with those of poplar wood. These properties were superior to those of the traditional styrene (PSt)-WPC and even exceeded the performance of Xylosma congesta (Lour.) Merr, a high-quality wood from northeast China. Meanwhile, the mass loss of wood composite materials with bulking-filling treatment was only 2.35 ± 0.05%, and the internal structure remained intact, presenting excellent decay resistance. Additionally, the treatment also significantly improved the thermal and dimensional stability of the wood composites. This study provides a theoretical basis and guidance for realizing the high-value-added application of low-quality wood and the preparation of highly durable wood-based composites.