Industrial Crops and Products 46 (2013) 304–310 Contents lists available at SciVerse ScienceDirect Industrial Crops and Products journa l h o me pag e: www.elsevier.com/locate/indcrop Mechanical properties of MMA-hardened hybrid poplar wood Wei-Dan Ding a , Ahmed Koubaa a,∗ , Abdelkader Chaala b a Chaire de recherche du Canada sur la valorisation, la caractérisation et la transformation du bois, Université du Québec en Abitibi-Témiscamingue, 445 BD de l’Université, Rouyn-Noranda (QC), Canada J9X5E4 b Service de Recherche et d’expertise en Transformation des produits forestiers (SEREX), 25, rue Armand-Sinclair, Door 5 Amqui (QC), Canada G5J1K3 a r t i c l e i n f o Article history: Received 9 December 2012 Received in revised form 27 January 2013 Accepted 1 February 2013 Keywords: Hybrid poplar Mechanical properties MMA Hardened wood Static bending Compressive strength a b s t r a c t Wood of fast-growing hybrid poplars, consisting of six 6-year-old clones in one plantation in Quebec, was impregnated with methyl methacrylate (MMA) and then polymerized by heat/catalyst method. The mechanical properties (static bending and compressive strength) of poplar clones and its MMA- hardened wood were investigated. Significant differences were observed among clones in bending and compression strength parallel to grain. Hardening treatment has also considerably improved all strength properties except for strain at rupture in static bending. The effects of density on mechanical properties were inconsistent, especially for hardened wood. Density was not the most important factor affecting strength properties. The improved properties of MMA-hardened hybrid poplar were comparable to some commercial hardwood species. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Wood is a natural and renewable material. It has been used for construction, tools, furniture and artistic medium for thousands of years due to its high strength-to-weight ratio, unique porous struc- ture, and aesthetic characteristics. However, faced with dwindling supply of high-quality lumber from natural forests and stricter environmental regulations, poplar (Populus spp.) and its hybrids have been considered as alternative wood source due to their rapid growth and ease of reproduction (Beaudoin et al., 1992; Koubaa et al., 1998). It represents one of the most widespread broad-leaved species in North America (Balatinecz et al., 2001). It is estimated that poplars account for over 50% of all hardwoods and approxi- mately 11% of the entire Canadian timber resource (Avramidis and Mansfield, 2005). According to a recent report (Parent, 2010), har- vested poplar wood in Quebec was 2,164,000 m 3 in 2009, which was almost 45% of the total hardwood production. Nevertheless, it has long been characterised as a low-density (specific gravity: 0.30–0.39), low-strength wood species (Balatinecz et al., 2001; Mátyás and Peszlen, 1997). Currently, poplar wood is primarily ser- vicing as fibre for pulp and paper industry, and as engineered wood products such as oriented strand board, laminated veneer lumber and structural composite lumber (Balatinecz et al., 2001). To improve the strength of poplar wood, modification of its porous structure is regarded as an effective way. Wood hardening, ∗ Corresponding author. Tel.: +1 8197620971. E-mail address: ahmed.koubaa@uqat.ca (A. Koubaa). which is a method of impregnating chemicals into wood pores and subsequently curing the chemicals in place by radiation or catalyst- thermal treatment, has received considerable attention in the past decade. After wood hardening, not only the strength, but also the density, surface and thermal properties, dimensional stability, and durability of the poplar wood are enhanced (Ding et al., 2008, 2012; Koubaa et al., 2012; Li et al., 2011a,b, 2012a,b, 2013; Gao et al., 2009a,b; Yildiz et al., 2005). Different polymeric monomers and resins have been employed, including methyl methacrylate (Ding et al., 2012; Koubaa et al., 2012; Li et al., 2011a, 2012b, 2013; Yildiz et al., 2005), styrene (Li et al., 2012a,b, 2013; Yildiz et al., 2005), polyurethane resin (Gao et al., 2009a,b), and maleic anhydride (Li et al., 2011b, 2012b). Among those chemicals, methyl methacrylate is the most commonly used monomer due to its cheap price, acces- sibility and low viscosity (Zhang et al., 2006a,b). Therefore, MMA was selected in this study. So far, very few studies on mechanical properties of MMA hard- ened poplar wood have been carried out and the properties were not studied systematically. Yildiz et al. (2005) reported that MMA- hardened 9-year-old poplar wood (P. euramericana cv. I-214) in Turkey had higher compression strength (max. 1.8 times more) and static bending strength (max. 1.5 and 1.4 times more for MOE and MOR) than the untreated depending on the polymer reten- tion rate. Further, no study has compared mechanical properties of MMA-hardened poplar wood among different poplar clones. These studies are necessary to enable the genetic selection of polar clones suitable for wood hardening to achieve higher strength. In this study, we used 6 hybrid poplars (P. maximowiczii × balsamifera M × B, P. nigra × maximowiczii N × M, P. deltoides × nigra D × N) in 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.02.004