A molecular model for reversible and irreversible hygroscopicity changes by thermal wood modification

Wim Willems, Michael Altgen, Lauri Rautkari

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Abstract

Heat treatment (HT) is a well-known means to reduce the equilibrium wood moisture content (EMC) at a specified relative humidity (RH). EMC is profoundly decreased by the loss of accessible hydroxyl groups (OHacc) in the wood matrix by thermochemical reactions. However, the obtained EMC reduction after HT can be partly reversible, depending on the ability of the wood matrix polymers to fully mechanically relax during HT. We discuss the results of our earlier experimental study on the OHacc content and the associated EMC decrease at 93% RH by a relaxation inhibiting dry-HT vs. a relaxation enabling wet-HT. New experimental results, showing that OHacc does not significantly change during reversible EMC changes, are added to the discussion. This study quantitatively supports a molecular explanation of the reversible EMC, in which wood moisture is principally bound at sorption sites, composed of two functional groups, constituting a hydrogen-donor/acceptor pair, involving at least one OHacc group. The irreversible part of EMC reduction is assigned to the thermochemical removal of OHacc from the wood matrix. The reversible part is attributed to a process of wood polymer conformal rearrangements, bringing an isolated OHacc group in proximity of another free hydrogen-bonding functional group, creating a site for water sorption.
Original languageEnglish
Pages (from-to)420-425
Number of pages6
JournalHolzforschung
Volume74
Issue number4
Early online date14 Aug 2019
DOIs
Publication statusPublished - 1 Apr 2020
MoE publication typeA1 Journal article-refereed

Keywords

  • crosslinks
  • equilibrium moisture content
  • heat treatment
  • mechanical relaxation
  • sorption sites

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