Xinzhou Wang

Evaluation of the effects of compression combined with heat treatment by nanoindentation (NI) of poplar cell walls

Abstract A combination of compression and heat treatment is a modification method that has great potential for improving the mechanical properties and dimensional stability of wood materials in industrial application. The objective of this project was to track changes in the microstructure, chemical composition, cellulose crystallinity, and mechanical properties of the treated poplar cell wall to investigate the mechanism of modification. Poplar boards were compressed at 100°C and subsequently treated in the hot press at 200°C. The results indicated that the treatment contributed to a reduction in porosity without obvious mechanical compression and damage to the cell wall. Hemicellulose degraded, however, and the relative lignin content and cellulose crystallinity increased during the process. The observed increase in relative lignin content and crystallinity may contribute to the improvement of mechanical properties. The longitudinal elastic modulus and hardness of poplar cell walls increased significantly from 12.5 and 0.39 GPa for the control to a maximum of 15.7 and 0.51 GPa for compressed wood with HT, respectively.

The effects of thermal treatment on the nanomechanical behavior of bamboo (Phyllostachys pubescens Mazel ex H. de Lehaie) cell walls observed by nanoindentation, XRD, and wet chemistry

Abstract The effects of thermal treatment of bamboo at 130, 150, 170, and 190°C for 2, 4, and 6 h were investigated in terms of changes in chemical composition, cellulose crystallinity, and mechanical behavior of the cell-wall level by means of wet chemical analysis, X-ray diffraction (XRD), and nanoindentation (NI). Particularly, the reduced elastic modulus (Er), hardness (H), and creep behavior were in focus. Both the temperature and treatment time showed significant effects. Expectedly, the hemicelluloses were degraded and the relative lignin content was elevated, while the crystallinity of the cellulose moiety was increased upon thermal treatment. The Er and H data of the cell wall were increased after 6 h treatment at 190°C, from 18.4 to 22.0 GPa and from 0.45 to 0.65 GPa, respectively. The thermal treatment led to a decrease of the creep ratio (CIT) under the same conditions by ca. 28%. The indentation strain state (εi) also decreased significantly after thermal treatment during the load-holding stage.

In situ identification of the molecular-scale interactions of phenol-formaldehyde resin and wood cell walls using infrared nanospectroscopy

Atomic force microscope infrared spectroscopy (AFM-IR), contact resonance AFM (CR-AFM) measurement, and nanoindentation were combined to identify the interactions between wood cell wall and phenol-formaldehyde resin (PF) on the nanoscale. Significant differences in both chemical structure and mechanics were observed among the cell wall, resin, and interphase regions, indicating that PF resin had diffused to the cell wall effectively. In particular, the maximum penetration depth of the resin in the glueline reached approximately 3 μm, showing that PF resin was able to penetrate into the woods secondary cell wall. The penetrating resin molecules not only dispersed within the cell wall but also reacted with cell-wall polymers, resulting in an increase in the elastic modulus and hardness of the wood cell wall. Nanoscale mechanical interlocks also formed between the resin and the wood cell wall in the interphase region, which may improve adhesion performance in wood-based composites.

Effects of accelerated aging treatment on the microstructure and mechanics of wood-resin interphase

Abstract Plywood panels prepared from loblolly pine with cured phenol resin (PF) and urea-formaldehyde resin (UF) were submitted to accelerated aging and the microstructures and mechanics of wood-resin interphase were studied by nanoindentation (NI) and nanoscale dynamic mechanical analysis (Nano-DMA). The mass loss (ML) of wood, PF and UF resins were 3.4, 5.0 and 4.6% after aging treatment, respectively, and a large amount of microcracks were observed on the surface of wood and resins after aging treatment, which also affected the static mechanics of the cell walls far from the interphase region and the resins in the interphase region. The elastic modulus (Er) and hardness (H) values of the cell wall decreased by 7.2 and 9.5%, respectively, against the untreated control. The storage and loss modulus of the resins decreased significantly after aging treatment. The significant inconsistency in the mechanics, shrinkage and swelling properties of wood cell wall and resin in the interphase region after aging treatment resulted in a decrease of about 47 and 51% on the average bonding strength of the plywood made of PF and UF resins, respectively.

Temperature-dependent mechanical properties of wood-adhesive bondline evaluated by nanoindentation

ABSTRACT Phenol formaldehyde (PF) and urea formaldehyde (UF) were used to prepare wood-adhesive bonds, respectively. The reduced elastic modulus (Er) and hardness (H) of the control wood cell wall, the adhesive, and the cell wall penetrated with an adhesive (CW-adhesive) at the wood-adhesive bondline were measured within a certain temperature range from 20 to 160°C using high-temperature nanoindentation (NI). The results indicated that the wood-PF bondline showed a strong dependence on elevated temperatures, while the wood-UF bondline presented better mechanical stability. A reduction of carbohydrates and increment of lignin in wood resulting from heat treatment at a temperature above 140°C were beneficial to increase the micromechanics of wood cell walls at the bondline. Furthermore, the possible post cross-linking reactions between the wood cell walls and PF adhesive molecules during the long heating period at high temperature made a major contribution to a significant increase in Er and H of the bondline. However, the significant difference in the mechanics of the PF adhesive and CW-PF in bondline after heat treatment negatively affects the interfacial adhesion properties of wood panels.

Effects of thermal modification on the physical, chemical and micromechanical properties of Masson pine wood (Pinus massoniana Lamb.)

Abstract In an attempt to evaluate the effects of thermal treatment on wood cell walls (CWs), Masson pine (Pinus massoniana Lamb.) wood was thermally modified (TM) at 150, 170 and 190°C for 2, 4 and 6 h, respectively. The chemical properties, cellulose crystallinity (CrI) and micromechanics of the control and thermally modified wood (TMW) were analyzed by wet chemical analysis, X-ray diffraction and nanoindentation. The relative lignin content and CrI increased after the TM partly degraded the amorphous wood polymers. The relative lignin content was higher in TMW and the equilibrium moisture content decreased. Moreover, the elastic modulus (Er) and hardness (H) of TMW were lowered along with the creep ratio decrement (CIT) of CWs. However, a severe treatment (e.g. 190°C/6 h) may negatively affect the mechanical properties of CWs caused by the partial degradation of hemicelluloses and also cellulose.