# NaOH-Only Pretreated Wood Densification: A Simplified Sulfite-Free Route Across Wood Species

## TL;DR

This study shows that using only NaOH to pretreat wood before densification improves its mechanical properties without complex chemicals.

## Contribution

A simplified NaOH-only pretreatment method for wood densification is introduced, avoiding complex chemical systems.

## Key findings

- NaOH-only pretreatment modifies hemicellulose and interpolymer bonds without dissolving lignin or hemicellulose.
- Densified wood achieved higher density and mechanical strength due to lumen collapse and compact microstructure.
- The method reduces chemical complexity and material loss compared to traditional pretreatment approaches.

## Abstract

The development of high-performance wood-based materials has attracted increasing interest as a means of enhancing the mechanical properties of wood for structural applications. Mechanical densification combined with chemical pretreatment is an effective approach; however, many reported methods rely on complex multi-component chemical systems or severe chemical conditions designed to dissolve lignin or hemicelluloses. In this study, a simplified NaOH-only pretreatment followed by hot-press densification was investigated, targeting selective cell-wall plasticization rather than extensive polymer dissolution. Juniper (Juniperus communis), hawthorn (Crataegus monogyna), and birch (Betula pendula) were used as samples of softwood and hardwood species. Wood specimens were pretreated in 1 M NaOH at 145 °C for 10–30 min and subsequently densified by radial compression. Changes in chemical composition were evaluated by HPLC after acid hydrolysis and FTIR spectroscopy, while microstructural changes were examined using SEM. Physical and mechanical properties were assessed through density measurements and three-point bending tests. The results show that NaOH-only pretreatment induces hemicellulose deacetylation and modification of interpolymer linkages without substantial changes in the main wood polymer contents. Densification resulted in effective lumen collapse and a compact microstructure, leading to a significant increase in density and mechanical properties. Overall, the results demonstrate that efficient wood densification and mechanical enhancement can be achieved by promoting polymer mobility through selective cleavage of interpolymer bonds, using a simplified, single-alkali pretreatment that reduces chemical complexity and material loss while avoiding extensive lignin or hemicellulose dissolution.