Improving the properties of wood by surface densification

Kristiina Laine

Improving the properties of wood by surface densification

Julkaisun otsikon käännös: Puun ominaisuuksien parantaminen pinnan tiheyttä lisäämällä

Kristiina Laine

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Puun mekaaniset ominaisuudet korreloivat positiivisesti puun tiheyden kanssa. Alhaisen tiheyden puulajien laatua voitaisiinkin parantaa tiheyttä lisäämällä. Väitöskirjatyön tavoitteena oli tutkia massiivipuun tiheyden lisäämistä puun pinnalla, jossa ominaisuudet, kuten pinnan kovuus, ovat erityisen tärkeät. Pinnalta tiheä puu voisi sopia kohteisiin, jossa etenkin pinta on kulutuksen kohteena, kuten lattiat ja työtasot. Väitöskirjatyössä puun tiheyttä lisättiin puristamalla puun huokoista rakennetta kasaan kuumapuristimella. Puristuminen kohdennettiin yhdelle pinnalle lämmittämällä vain toinen puoli puristimessa ja käyttämällä kohtuullisen kuivaa puuta, jolloin lämmön vaikutuksesta puu muovautui vain toiselta pinnalta. Tulokset osoittivat, että tiheys kasvoi läheltä lämmitettyä pintaa ja tiheysprofiilin muodostumista pystytään kontrolloimaan muuttamalla puristusprosessin parametreja, kuten lämpötilaa tai puristimen sulkeutumisaikaa. Tiheysprofiilin muoto sekä prosessiparametrit korreloivat hyvin puun ominaisuuksien kanssa, kuten pinnan kovuuden, joka lähes kaksinkertaistui puristetulla pinnalla. Mikroskooppitutkimus osoitti, että pintapuristus ei aiheuta merkittäviä halkeamia soluseiniin. Puristettu tila ei ole sellaisenaan pysyvä, vaan palautuu vedelle altistettaessa. Pieni osa puristuksesta ei kuitenkaan palautunut vedessä uitettaessa, vaan oli pysyvää - todennäköisesti molekyylisidosten uudelleen järjestäytymisen sekä ligniinin muovautumisen takia. Puristuksen palautuminen saatiin vähennettyä merkittävästi lämpökäsittelemällä puuta puristuksen jälkeen. Sekä puristaminen että lämpökäsittely vähensivät kosteuden sitoutumista puuhun. Lisäksi hystereesi pieneni kosteussyklejä toistettaessa, mikä johtui todennäköisesti purkautuvista sisäisistä jännityksistä, jotka olivat syntyneet sekä puristuksen että lämpökäsittelyn aikana. Väitöskirjatyössä saadut tutkimustulokset edistävät merkittävästi puun puristusprosessin suunnittelun optimointia haluttujen ominaisuuksien saavuttamiseksi. Tutkimustulokset lisäsivät ymmärrystä puun käyttäytymisestä puristuksessa ja korkeassa lämpötilassa sekä modifioidun puun käyttäytymisestä vaihtelevassa ilmankosteudessa.

Julkaisun otsikon käännös	Puun ominaisuuksien parantaminen pinnan tiheyttä lisäämällä
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Hughes, Mark, Vastuuprofessori Rautkari, Lauri, Ohjaaja
Kustantaja	Aalto University
Painoksen ISBN	978-952-60-5842-9
Sähköinen ISBN	978-952-60-5843-6
Tila	Julkaistu - 2014
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

lämpökäsittely
pinnan modifiointi
puun modifiointi
tiheyden kasvattaminen

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-60-5843-6Lisenssi: Määrittelemätön

Biotalousinfrastruktuuri
Seppälä, J. (Manager)
Kemian tekniikan korkeakoulu
Laitteistot/tilat: Facility

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@phdthesis{84a6f7019d6149b7a33f420ae1eba4eb,

title = "Improving the properties of wood by surface densification",

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.",

keywords = "densification, surface densification, thermal modification, wood modification, l{\"a}mp{\"o}k{\"a}sittely, pinnan modifiointi, puun modifiointi, tiheyden kasvattaminen, densification, surface densification, thermal modification, wood modification",

author = "Kristiina Laine",

year = "2014",

language = "English",

isbn = "978-952-60-5842-9",

series = "Aalto University publication series DOCTORAL DISSERTATIONS",

publisher = "Aalto University",

number = "133",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Improving the properties of wood by surface densification

AU - Laine, Kristiina

PY - 2014

Y1 - 2014

N2 - 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.

AB - 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.

KW - densification

KW - surface densification

KW - thermal modification

KW - wood modification

KW - lämpökäsittely

KW - pinnan modifiointi

KW - puun modifiointi

KW - tiheyden kasvattaminen

KW - densification

KW - surface densification

KW - thermal modification

KW - wood modification

M3 - Doctoral Thesis

SN - 978-952-60-5842-9

T3 - Aalto University publication series DOCTORAL DISSERTATIONS

PB - Aalto University

ER -

Improving the properties of wood by surface densification

Abstrakti

Tutkimusalat

Pääsy asiakirjaan

Sormenjälki

Laitteet

Biotalousinfrastruktuuri

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