Characterization of moisture states and transport in MUF resin-impregnated poplar wood using low field nuclear magnetic resonance

Abstract

In this study, the spin–spin relaxation time (T₂) distributions of free water and bound water, as well as the moisture content (MC) profiles of MUF resin-impregnated poplar wood (Populous tomentosa) sample (RI) were investigated by low-field nuclear magnetic resonance, with the aim of providing insights into how MUF resin impregnation affects the moisture states and moisture transport in modified poplar wood during drying. The T₂ curves demonstrated that the resin treatment did not increase the number of peaks in the T₂ distributions, but affected the T₂ value as compared to the control. Above the fiber saturation point (FSP), the T₂₂ (corresponds to the free water in wood rays and wood fibers) of the RI sample exhibited an increase compared to the control, while the T₂₃ (corresponds to the free water in the vessels) was almost unchanged. Below the FSP, a shorter T₂₁ (corresponding to the bound water) of the RI sample was observed compared to the control. The drying curves and MC profiles indicated a significant difference in the moisture transport in the RI sample as compared to the control. The gradually cured resin system in the wood surface layer during drying provided a barrier for the transfer of water in the center layer toward to the surface, causing the resin curing reactions in the surface and core layers to be out of sync. Therefore, a more significant MC gradient was observed for the resin-impregnated sample.

Keywords:

• Resin-modified wood
• low-field nuclear magnetic resonance (LF-NMR)
• spin–spin relaxation time
• drying rate
• moisture content profiles
• resin curing

Acknowledgments

The authors would like to thank the engineers Bing Lu and Hao Ding at Niumag Corp., Ltd. in Shanghai for their technical support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors are grateful for the support of the National Natural Science Foundation of China (31800479), Scientific Research Fund of Hunan Provincial Education Department (19B592) and the Forestry Industry Research Special Funds for Public Welfare Projects (201404501).