Small Angle Neutron Scattering Reveals Wood Nanostructural Features in Decay Resistant Chemically Modified Wood

Rebecca E Ibach,Sai Venkatesh Pingali,Nayomi Z Plaza

doi:10.3389/ffgc.2021.814086

Abstract

While it is known that modifying the hydroxyls in wood can improve the decay resistance; what is often missing in the literature is whether these modifications alter wood nanostructure, and how these changes correlate to the improved decay resistance. Here, we used small angle neutron scattering (SANS) to probe the effects of alkylene oxide modifications on wood nanostructure. Southern pine wood samples were chemically modified to various weight percentage gains (WPG) using four different alkylene oxides: propylene oxide (PO), butylene oxide (BO), epichlorohydrin (EpH), and epoxybutene (EpB). After modification, the samples were water leached for 2 weeks to remove any unreacted reagents or homopolymers and then equilibrium moisture content (EMC) was determined at 90% relative humidity (RH) and 27°C. Laboratory soil block decay evaluations against the brown rot fungus Gloeophyllum trabeum were performed to determine weight loss and biological efficacy of the modifications. To assist in understanding the mechanism, SANS was used to study samples that were fully immersed in deuterium oxide (D2O). These measurements revealed that the modifications altered the water distribution inside the cell wall, and the most effective modifications reduced the microfibril swelling and preserved the microfibril structure even after being subject to 12 weeks of brown rot exposure.

Highlights

Outdoor exposure of non-durable wood species often results in wood decay
For a chemical modification of solid wood to be considered biologically effective the weight loss caused by the 12-week fungal exposure should be less than 5%
EpB and EpH were the most effective at reducing the weight loss, and even at low modification levels (WPG < 14%) the weight loss was reduced to less than 5% compared to the 65% WL observed for the unmodified wood samples

Summary

Introduction

Outdoor exposure of non-durable wood species often results in wood decay. Brown rot fungi, in particular, are responsible for a large part of the timber decayed in service and these affect all wood polymers but target primarily the wood carbohydrates (Goodell et al, 2020). It is thought that alkylene oxides attach to the available wood hydroxyl groups and that this modification makes the modified wood less susceptible to decay (Rowell and Ellis, 1984). EpB, and BO have similar chemistries and have been shown to provide increased protection by lowering the moisture content of the chemically modified wood cell walls (Ibach and Plaza, 2019), but EpB has a C = C that could be used for incorporating new chemistries such as UV protection. Whereas EpH attaches to the wood hydroxyls and provides adequate protection by modifying the lignocellulosic substrate without significantly lowering the sample’s MC (Ibach and Lee, 2002). The mechanisms behind decay resistance of alkylene oxide modified woods are still unclear, even though this knowledge could inform the development of new wood protection chemistries

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Discussion

Conclusion