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 (E_r) 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 E_r 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.

KEYWORDS:

• Adhesive for wood
• interphases
• mechanical properties
• nanoindentation
• temperature
• wood

Acknowledgments

The authors gratefully acknowledge the financial support of the Doctorate Fellowship Foundation of Nanjing Forestry University, a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions; UTIA 2015 Innovation Grant; and Zhejiang Key Laboratory of Wood Science and Technology. The authors would also like to thank Dr. T. G. Nieh and D. Wu for assisting with the nanoindentation instrument.