Analysis of the delignification process influence on the dimensional stability of densified wood: a systematic review

Background: One of the challenges in the development of densified wood products is maintaining their dimensional stability. Several treatment methods are employed to achieve this stability, including the delignification process. This paper analyzes the efficiency of the delignification process on the improving of the densified wood dimensional stability through a systematic literature review. This research was based on the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and conducted using Scopus, Web of Science and Google Scholar databases. After the eligibility criteria application and bias analysis, the systematic search sample resulted in 54 articles. The meta-analysis was conducted through a narrative synthesis, including the densification process, wood species, apparent density, delignification process, other wood treatments, and dimensional stability. Results: The delignification process was used in approximately 26% of the 54 articles analyzed and these numbers occurred after 2018. Of the studies that related dimensional stability to delignification, sodium hydroxide (NaOH) was used as a reagent in 50% of the cases. The combination of NaOH and Na2SO3 in the alkaline treatment of wood was the most used (28.5%). The treatments used proved effective in the partial removal of lignin and hemicellulose, improving the dimensional stability of the species analyzed. Conclusion: The increasing number of publications in China indicates that the Chinese market has a significant demand for densified wood. Although partial removal of lignin and hemicellulose improves the dimensional stability of densified wood, some studies have shown that excessive removal of lignin can negatively impact the mechanical characteristics of the material.