Polymer Reinforced Wood-Based Composite Prepared By In-Situ Polymerization Of Active Monomers

Traditional wood-polymer composite (WPC) is one of wood-based composites reinforced by polymer from in-situ free radical polymerization of vinyl or acrylic monomers. It was originated in the1960s, and also recognized by American as one of top ten scientific discoveries in1960s. This method can both endow wood with mechanical properties and durability, which is quite significant for value-added applications of low-quality wood. However, there're also several disadvantages for the method, such as easier volatilization of the traditional monomers, lower conversion rate, remaining odor of the resulted WPC, which all limit the extensive application of the wood-polymer composite.To address these problems, and aiming at developing polymer reinforced wood-based composite with cost effectiveness, this research first tried to originally explore the relationship between structure of polymer from traditional monomers and wood performance, and then generally choose several monomers to prepare polymer reinforced wood-based composites. With the characterization by SEM and analysis by FTIR, several compatible monomers and their combination were preliminary ascertained. Afterward, in consideration of targeting monomers (system) capable of both treating solid wood and veneer for surface decoration of wood-based panel, a series of the panels decorated with polymer modified veneer on the basis of the above selected monomers were prepared. Depending on the bonding strength and SEM characterization results, the optimized ratio of active monomers and the corresponding reaction conditions were finally determined. Furthermore, under the optimized craft, two fast-growing wood, Poplar Wood (Populus ussuriensis Kom) from north of China and Muchelia macclurei wood from south of China, were prepared into polymer reinforced wood-based composites by the above active monomer system. Their micro structures characterized by SEM, FTIR and XRD, and the mechanical properties as well as durability were further combined to assess the rationality and applicability of the method. Finally, in order to meet the higher requirement of durability on wood when it was applied in some special conditions, a novel method-"bulk-grafting polymerization" was designed to modify wood based on the optimized monomers system. And its effectiveness was also evaluated by the micro structure characterized by SEM, FTIR and XRD, and the properties (mechanical properties and durability) of the resulted polymer reinforced wood-based composite. The main results and conclusions are as follows:(1) Four traditional monomers (system):methyl methacrylate (MMA), styrene (St), vinyl acetate (VAc) and combined system of MMA and St, were respectively used to prepare traditional wood-polymer composites. From the test results of properties, it can be known that these monomers couldn't effectively improve the properties of wood; especially, the resulted lower impact toughness even less than that of untreated wood. The characterization and analysis from SEM, FTIR and XRD indicated that the interfacial interaction between polymer and wood matrix was quite weak due to these monomers lack of active groups capable of chemically reacting with wood components or strongly physically absorbing wood matrix, and thus affected the improvement of wood properties. Additionally, the polymers from these monomers also have disadvantages due to limitations of the monomers'structures, which further negatively affect the properties of wood-polymer composites.(2) Depending on the characterization of interfacial interaction by SEM and impact fracture mode and toughness, the active monomer, glycidyl methacrylate (GMA) and ethyl glycol dimethyl methacrylate (EGDMA) was chosen as an interfacial modifier and an impact toughness modifier of traditional wood-polymer composite, respectively. Combining GMA and EGDMA, polymer reinforced wood-based composite was finally prepared through their copolymerization. SEM and FTIR characterization for the composite, and the impact toughness result indicated that the created difunctional monomers system—(GMA+EGDMA) could effectively improve the interface and impact toughness of traditional wood-polymer composite.(3) According to the bonding strength of three kind of wood-based panels covered with modified veneers, and in consideration of impact toughness as well as cost, the optimized ratio of GMA to EGDMA for the difunctional monomer system was determined as2:1(mol:mol). And the optimized hot-pressing craft was temperature:130℃; pressure:0.65MPa; time:15min and content of Man as catalyst:6%(mass percent of total monomers). Under the optimized craft,, all the bonding strengths of the wood-based panel covered by polymer modified veneers were higher than the corresponding national standard values; and its hardness, abrasion resistance, modulus of rupture were totally improved by about15%-51%over those of untreated veneer covered panel. The SEM observation also showed strong interfacial interaction between veneer and wood-based panel.(4) Polymer reinforced Poplar wood-based and Muchelia macclurei wood-based composites were both prepared by the polymerization of difunctional monomers system (2GMA+EGDMA) under the optimized conditions, and their structures were also characterized and analyzed by SEM, FTIR and XRD. The results indicated that the difunctional monomers copolymerized and filled in wood cell lumen and the resulted polymer grafted onto wood cell walls as an amorphous form, representing stronger interfacial interaction between wood and polymer phases. The testing results of properties for the two composites showed that the mechanical properties (modulus of rupture, modulus of elasticity, compression strength parallel to the grain, impact toughness, abrasion resistance and hardness) were whole improved by0.5～1.5times compared to untreated wood, achieving or even higher than those of normally high-quality wood in northeast of China. And their dimensional stability under long continuing immersion time of228h in distilled water and decay resistance against fungi were also evidently improved compared to those of untreated wood. The TG curves showed higher thermal stability of the two composite than untreated wood.(5) In the wood modification experiment with the combined two-step method—"bulk-grafting polymerization", Man was first fixed to bulk wood cell wall by dissolved in acetone, and then the optimized reaction condition for Man and wood was further determined in terms of ASE as temperature:110℃and reaction time:8h. Under the optimized craft, weight gain of Man modified wood was14.72%, and its volume gain was8.96%. Characterization and analysis by SEM-EDX, FTIR and XRD further indicated that GMA and EGDMA copolymerized in wood cell lumen, and the resulted polymer grafted onto wood matrix as an amorphous form, resulting in the improvement of interfacial interaction between the two phases. The properties testing results of the bulk-grafting polymer reinforced wood-based composite showed that the modulus of rupture, modulus of elasticity, compression strength parallel to grain, hardness, abrasion resistance and impact toughness was respectively improved1.18,1.14,1.25,1.94,2.22and0.57times compared to untreated wood, whole of which reached or even far higher than those of normally high-quality wood in northeast of China. The decay resistance and dimensional stability of the composite were both remarkably improved than those of untreated wood, and also higher than those of wood treated by traditional preservatives (inorganic boron preservative and organic IPBC preservative) and dimensional stabilizer (polyethylene glycol), respectively. TG analysis also indicated the thermal stability of the composite was improved in a certain degree.In conclusion, this research successfully prepared polymer reinforced wood-based composite with good performance by creating active monomer system, whole performance of which achieved those of normally high-quality wood in northeast of China. And the monomers with high conversion rate (high boiling point) are nontoxic. The method can treat not only solid wood with environmental operation, but also veneer (for modifier veneer covered wood-based panels without additional requirement of adhesives). The prepared polymer reinforced wood-based composites can be applied as value-added products in areas of construction, transport and top grade interior decorations. Additionally, a novel combined two-step method:'bulk-grafting polymerization', was proposed on the basis of the above researches; and the corresponded polymer reinforced wood-based composites showed good mechanical properties, excellent dimensional stability, decay resistance as well as slightly improved thermal stability, which can be applied in the fields of outdoor public place and interior bathroom where special requirements are needed for wood durability.