Pre-treatment with Ionic Liquids or Organic Superbases to Reduce the Spring-Back and Set-Recovery of Surface-Densified Scots Pine

• Compressing the surface of a wooden board results in a layer of densified wood cells a few millimetres thick and a substantial increase in hardness. Surface densification of soft woods such as Scots pine opens up new market opportunities for high-value wood products, but wide-spread commercialisation is unfortunately hindered by the lack of an industrially viable densification process, the major obstacle being the recovery of the densified wood cells upon exposure to moisture. This so-called set-recovery is due to insufficient plastic deformation of the crystalline cellulose, which is kept in a meta-stable state by the surrounding hemicellulose and lignin matrix. In recent years, the dissolution of cellulose in ionic liquids became increasingly popular. These ionic liquids are primarily based on imidazolium rings or on mixtures of an organic superbase and a carboxylic acid. Our hypothesis is that partial dissolution of the crystalline cellulose will largely prevent the set-recovery of densified wood, and the objective of this study was to evaluate the effect of ionic liquid and superbase pre-treatment on the elastic spring-back and set recovery of surface-densified wood. Specimens of Scots pine (Pinus sylvestris L.) we retreated with ionic liquids or superbases diluted in ethanol, and then densified in a hot press at temperatures between 200°C and 270°C. The set-recovery was measured after one and two wet-dry cycles. The spring-back when the press was opened was completely eliminated. Depending on the pressing temperature, the set-recovery could be reduced from about 90% for the control group to only about 10% for the chemically treated specimens. A positive correlation between the reduction in set-recovery and the concentration of the chemical solution was also observed. The treatment and densification process presented in this study can be seen as a precursor to the development of a continuous densification process. A closed system is not necessary and both the chemical treatment and the densification stage have the potential to be performed in a continuous manner. Further studies are however needed to optimize the treatment and understand its underlying mechanisms.

Ämnesord

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Trävetenskap (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Wood Science (hsv//eng)

Nyckelord

wood compression

densification

wood modification

shape-memory effect

Träteknik

Wood Science and Engineering

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