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 contribution||Puun ominaisuuksien parantaminen pinnan tiheyttä lisäämällä|
|Publication status||Published - 2014|
|MoE publication type||G5 Doctoral dissertation (article)|
- surface densification
- thermal modification
- wood modification