Enhancing the dimensional stability and durability of wood polymer composites
Wood has been used as an important engineering material for a long time because of its excellent properties (e.g., high strength-to-weight ratio, renewable and environmentally benign nature, aesthetically pleasing character, and low processing cost). Wood, however, is susceptible to swelling and shrinkage under certain conditions (e.g., changing relative humidity). It is also vulnerable to damage from micro-organisms and insects,1 which causes dimensional instability and reduces long-term durability. Such damage may even render the material entirely ineffective.
Wood polymer composites (WPCs) have been shown to be a successful approach for improving the engineering performance of wood.2, 3 These composites are prepared by impregnating wood materials with low-molecular-weight molecules and unsaturated monomers, and subsequent in situ polymerization. It is commonly accepted that dimensional stability increases greatly when modification takes place in the cell walls of wood rather than in cell lumens. However, previous studies have shown that with traditional polymerization methods, the reaction mainly occurs in wood cell lumens and without grafting on the wood matrix.4 This results in poor compatibility between the wood and the polymers.
The aim of our study, therefore, was to graft polystyrene to poplar wood cell walls through the process of free-radical copolymerization and thus fabricate WPCs with very high dimensional stability.5 Graft copolymerization is by far the most popular method of achieving compatibility between two or more components.6 We used methacryloyl chloride to swell the wood cell walls and for the esterification reaction with hydroxyl groups. In this way chemical bonds were formed with the wood matrix and the quantity of hydroxyl groups in the wood cell walls was stoichiometrically reduced. We then sequentially copolymerized the chemically bonded methacryl groups with polystyrene—in situ in the wood lumens—to enhance the desired properties of the resultant WPCs. We therefore used three different types of wood sample in this study, i.e., wood treated with methacryloyl chloride (MCT-wood), with styrene (ST-wood), and with methacryloyl chloride/styrene copolymerization (MCST-wood).
We used scanning electron microscopy (SEM) to analyze the polymer distribution and morphology of our wood samples (see Figure 1). The SEM images reveal that the poplar wood exhibited porous structures and distinct cell size distributions. We also find that the cell wall thicknesses of the MCT-wood samples were slightly higher than those of the untreated sample. This result indicates that methacryl groups had successfully been grafted on the wood cell walls. Our images also show the majority of the cell lumens were filled with the polymer that formed in situ and thus demonstrates the favorable distribution of polymer in the wood sample cross sections.
We conducted water uptake (WU) measurements on both our treated and untreated wood samples. The results—shown as a function of time in Figure 2—indicate that water absorption increased over time for both the treated and untreated samples. With increased immersion time, more water could penetrate into the wood voids and capillaries (until an equilibrium was reached), which resulted in the increased WU capacity.7 In addition, our results show that the untreated wood samples had a much higher water absorption than the MCST-wood samples. Theoretically, the MCT-wood samples should have absorbed less water than the untreated samples because methacryl groups were chemically grafted onto the wood cell wells (and the quantity of hydroxyl groups was thus reduced). Our results show, however, that the MCT-wood samples absorbed more water than the untreated samples. This is likely a result of hemicellulose degradation caused by hydrochloric acid in the methacrylation process. In contrast, the MCST-wood samples had a much lower WU throughout the experiment (mainly because the void spaces decreased and resulted in lower water absorption).
To evaluate the dimensional stability of our treated wood samples, we determined anti-swelling efficiency (ASE) indices (see Figure 3). Overall, we find that the ASE increased as the methacryloyl chloride concentration increased (i.e., as hydroxyl groups were esterified by methacryloyl groups, the water absorption of the treated wood was reduced). Our results also show that as the methacryloyl chloride concentration increased, there were fewer remaining hydroxyl groups, which made the treated wood more hydrophobic. The ASE of the MCST-wood samples was much higher than the MCT-wood samples, probably because the covalent-attached polymer on the cell walls (from copolymerization) restricted the cell walls from shrinking in response to moisture variation. In addition, the polymer is less hygroscopic than wood, and therefore less water was absorbed in the more humid conditions.8
In this study we have demonstrated a new method for the fabrication of WPCs. We used free-radical copolymerization methods to graft polystyrene to poplar wood cells Our results confirm that graft copolymerization significantly improves the interfacial compatibility between polymer and wood, and thus results in a more effective and stable modification of WPC samples. Furthermore, we have found that the physical and mechanical properties of WPCs (e.g., dimensional stability, hydrophobic characteristics, and surface hardness) improved significantly after the grafting copolymerization process. In our future work we will continue to explore the methods of wood modification and thus produce additional high-performance wood materials.
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- K. Wang, Y. Dong, Y. Yan, S. Zhang and J. Li, Improving dimensional stability and durability of wood polymer composites by grafting polystyrene onto wood cell walls, Polym. Compos., 2016. First published online: 17 January
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