Rongxian Ou

Thermoplastic deformation of poplar wood plasticized by ionic liquids measured by a nonisothermal compression technique

Abstract The in situ thermoplasticization of poplar wood with ionic liquids (ILs) has been investigated. The thermoplastic deformation of wood samples treated with four types of ILs at various concentrations was determined through nonisothermal compression tests by means of a rotational rheometer. Results show that increasing the concentration of ILs reduced softening temperature and increased deformation compared to the untreated control. Scanning electron microscopy revealed that plastic deformation of wood cells from the applied compression stress varied, depending on cell type, and occurred without cell wall fracture. X-ray diffraction analysis of compressed wood showed that wood treated with ILs exhibits a greater crystallinity index than the untreated control. The recovered strain in compressed samples decreased with increasing temperature and concentration of ILs to 18% weight percent gain (WPG) and then decreased slightly to 36% WPG. In treated samples, the combined wood/IL blends demonstrated less thermal stability than wood and ILs alone. Results also show that plastic deformation of IL-treated wood resulted in viscous buckling of unfractured cell walls. This deformation mode likely resulted from the disintegration of intermolecular and intramolecular hydrogen bonding between cell wall polymers through the combined effect of ILs, pressure, and high temperature.

Reinforcing effects of modified Kevlar® fiber on the mechanical properties of wood-flour/polypropylene composites

Kevlar® fiber (KF) is a synthesized product with strong mechanical properties. We used KF as a reinforcement to improve the mechanical properties of wood-flour/polypropylene (WF/PP) composites. KF was pretreated with NaOH to improve its compatibility with the thermoplastic matrix. Maleated polypropylene (MAPP) was used as a coupling agent to improve the interfacial adhesion between KF, WF, and PP. Incorporation of KF improved the mechanical properties of WF/PP composites. Treatment of KF with NaOH resulted in further improvement in mechanical strength. Addition of 3% MAPP and 2% hydrolyzed KF (HKF) led to an increment of 93.8% in unnotched impact strength, 17.7% in notched impact strength, 86.8% in flexure strength, 50.8% in flexure modulus, and 94.1% in tensile strength compared to traditional WF/PP composites. Scanning electron microscopy of the cryo-fractured section of WF/PP showed that the HKF surface was rougher than the virgin KF, and the KF was randomly distributed in the composites, which might cause a mechanical interlocking between KF and polypropylene molecules in the composites.

Material pocket dynamic mechanical analysis: a novel tool to study thermal transition in wood fibers plasticized by an ionic liquid (IL)

Abstract The investigation of phase transition in powdered materials by dynamic mechanical analysis (DMA) is not straightforward because powders are difficult to prepare in a solid compact form without altering their structure and properties. In this study, a material pocket (MP) method has been applied to provide physical support to powdered samples for DMA testing (MP-DMA). Poplar wood strips and four types of wood particles [native wood flour (WF), α-cellulose (αC), holocellulose (HC), and particles without hemicelluloses (HR)] were treated with an ionic liquid (IL), 1-ethyl-3-methylimidazolium chloride ([Emim]Cl), to a weight percent gain (WPG) of 36%. Results show that all four [Emim]Cl-treated wood particles exhibited three apparent transition peaks over the measured temperature range. Paracrystalline cellulose, hemicelluloses, and lignin all exhibited a glass transition temperature (Tg) at approximately 85°C due to the plasticizing effect of [Emim]Cl. The transition peak at a higher temperature may be due to the melting of crystalline cellulose in [Emim]Cl. MP-DMA is an effective tool for direct monitoring the phase transition of powdered lignocellulosics. This provides new insight into the interactions of ILs and cell wall polymers, and the method established can be easily extended to other systems based on powdered samples.