Improving the properties of wood by surface densification

Kristiina Laine

    Research output: ThesisDoctoral ThesisCollection of Articles

    Abstract

    Mechanical properties generally correlate positively with wood density and, therefore, it might be beneficial to increase the density of otherwise low-density wood species to improve certain characteristics. The aim of the dissertation was to examine the possibility to improve wood properties, such as hardness, by increasing the density of wood, a modification method referred to as 'densification'. The purpose was to target the densification at the very surface of wood, where the property improvements are most needed. Surface densified wood would be suitable in applications where the surface is generally exposed to use, such as flooring or worktops. An increase in density was obtained by reducing the porosity of wood by compressing the structure in a laboratory scale hot press. Densification was targeted at the surface by heating only one plate in the press and utilising wood of relatively low moisture content to inhibit the softening of the sample throughout the thickness. It was discovered that during surface densification the deformation occurs mainly in the earlywood cells close to the heated plate. The density profile development can be controlled by the parameters used in the densification process. Furthermore, the process parameters and density profile formation correlate directly with property changes, such as hardness, which was almost doubled by surface densification. Microscopic analysis with SEM suggested that no significant damage occurs in the cell wall during densification. The densified state tends to recover when exposed to high humidity. However, a minor part of the densification was considered permanent and irrecoverable after water soaking - most likely due to rearrangement of the molecular bonds and viscoelastic flow of the matrix during densification. Recovery of the densified state was significantly reduced by thermally modifying the samples after densification. Both, densification and thermal modification reduce the hygroscopicity of wood. In addition, hysteresis was reduced after repeated humidity cycles, possibly resulting from relaxation of the inner stresses which were developed during densification as well as during thermal modification. The knowledge obtained through this dissertation enhances optimization of the wood densification process for desired property improvements. The results provided knowledge of solid wood behaviour under compression and at high temperature, as well as, behaviour of modified wood in changing humidity conditions.
    Translated title of the contributionPuun ominaisuuksien parantaminen pinnan tiheyttä lisäämällä
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    Supervisors/Advisors
    • Hughes, Mark, Supervisor
    • Rautkari, Lauri, Advisor
    Publisher
    Print ISBNs978-952-60-5842-9
    Electronic ISBNs978-952-60-5843-6
    Publication statusPublished - 2014
    MoE publication typeG5 Doctoral dissertation (article)

    Keywords

    • densification
    • surface densification
    • thermal modification
    • wood modification

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