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Characterisation of thermally modified wood for use as component in biobased building materials
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The building sector shows growing interest in biobased building materials. Wood components, here defined as ground or milled wood, i.e. by-products (residuals/residues) from wood processing, such as sawdust or shavings, are valuable raw materials for new types of durable biocomposites suitable for outdoor building applications. An important research question related to such composites is how to characterise and enhance molecular interactions, i.e. adhesion properties, between wood and binder components. Another challenge is the hygroscopicity of the wood component, which can lead to dimensional changes and interfacial cracks during exposure to varying moisture conditions. Thermal modification of wood reduces its hygroscopicity, thereby, increasing its durability, e.g. its dimensional stability and resistance to biodeterioration. The hypothesis is that the use of thermally modified wood (TMW) components in biocomposites can enhance their durability properties and, at the same time, increase the value of residues from TMW processing. The main objective of this thesis is to study and analyse the surface and sorption properties of TMW components using inverse gas chromatography (IGC), dynamic vapour sorption (DVS), X-ray photoelectron spectroscopy (XPS), and the multicycle Wilhelmy plate method. The aim is to gain a better understanding of the surface and sorption characteristics of TMW components to enable the design of optimal adhesion properties and material combinations (compatibility) for use in biocomposites, especially suitable for outdoor and moist building material applications. Samples of TMW and unmodified wood (UW) components of Norway spruce (Picea abies Karst.) and Scots pine (Pinus sylvestris L.) heartwood were prepared and analysed with respect to surface energetics, hygroscopicity, liquid sorption and resulting swelling. The work also included analysis of surface chemical composition, as well as influences of extractives and moisture sorption history. The effect of using TMW components in a wood plastic composite (WPC) exposed to a series of soaking-drying cycles in water was studied with a focus on water sorption, swelling and micromorphological changes. The IGC analyses indicate that TMW components of spruce have a more heterogeneous surface energy character, i.e. a distinctly higher dispersive part of surface energy for low surface coverages, than do UW components. This is suggested to be due to the higher percentage of hydrophobic extractives present in TMW samples. Lewis acid-base analysis indicates that both UW and TMW components from spruce have a predominantly basic character and an enhanced basicity for the latter ones. Results show that both the hygroscopicity and water liquid uptake are lower for TMW than for UW samples. Unexpectedly, a significantly lower rate of water uptake was found for the extracted UW of pine heartwood than for non-extracted samples. In the former case, this is presumably due to contamination effects from water-soluble extractives, which increase capillary flow into wood voids, as proven by a decrease in water surface tension. Water uptake as well as swelling was significantly reduced for the WPCs with TMW and hot-water extracted UW components compared with the WPCs with UW components. This reduction also resulted in fewer wood component-polymer interfacial cracks in the WPCs with the modified wood components.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. iii, 50
Series
TRITA-ABE-DLT ; 1818
Keywords [en]
Thermally modified wood (TMW) components, surface properties, inverse gas chromatography (IGC), water sorption, dynamic vapour sorption (DVS)
National Category
Materials Chemistry
Research subject
Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-233569ISBN: 978-91-7729-860-1 (print)OAI: oai:DiVA.org:kth-233569DiVA, id: diva2:1241790
Public defence
2018-09-19, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
EnWoBio - Engineered Wood and Biobased Building Materials Laboratory
Funder
Swedish Research Council Formas, EnWoBio 2014-172
Note

QC 20180824

Available from: 2018-08-24 Created: 2018-08-24 Last updated: 2018-08-24Bibliographically approved
List of papers
1. Surface energy characterization of thermally modified spruce using inverse gas chromatography under cyclic humidity conditions
Open this publication in new window or tab >>Surface energy characterization of thermally modified spruce using inverse gas chromatography under cyclic humidity conditions
2015 (English)In: Wood and Fiber Science, ISSN 0735-6161, Vol. 47, no 4, p. 410-420Article in journal (Refereed) Published
Abstract [en]

The surface energy of unmodified and thermally modified spruce wood components was researched at dry and moist conditions using inverse gas chromatography. The results indicate a more pronounced heterogeneous nature of the thermally modified wood surfaces in terms of the dispersive (nonpolar) component of the surface energy, compared with that of the unmodified wood surfaces. The dispersive component of the surface energy of the thermally modified wood ranged between 44 and 38 mJ/m(2) corresponding to an increase in surface coverage from a low level and up to about 10%. Suggested explanations for the more distinct heterogeneity of the thermally modified wood sample are related to chemical changes of the wood substance which seem to result in certain micromorphological features observed by scanning electron microscopy as alternated fracture surfaces created in the grinding process; and also possible changes or redistribution of the wood extractives. An increase of the MC, representing a change from a dry condition of approximately 0% RH to ca 75% RH, of both the unmodified and thermally modified samples seemed to have a marginal influence on the dispersive component of the surface energy. Possible implications of the results in this study can be found in the tailoring of new compatible and durable material combinations, for example, when using thermally modified wood residuals as a component in new types of biocomposites.

Place, publisher, year, edition, pages
SOC WOOD SCI TECHNOL, 2015
Keywords
Thermally modified wood, inverse gas chromatography, humidity cycling, Norway spruce, dispersive surface energy, surface energy heterogeneity, BET-specific surface area
National Category
Materials Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-172956 (URN)000363992000011 ()2-s2.0-84991704745 (Scopus ID)
Funder
Swedish Research Council Formas, EnWoBio 2014-172
Note

QC 20151202

Forskningsfinansiärer: Stiftelsen Nils och Dorthi Troëdssons forskningsfond (Projektnr 793/12 Hydro-termo-mekanisk modifiering av trä). EcoBuild Institute Excellence Centre

Available from: 2015-09-04 Created: 2015-09-03 Last updated: 2018-08-24Bibliographically approved
2. Water vapour sorption characteristics and surface chemical composition of thermally modified spruce (Picea abies karst)
Open this publication in new window or tab >>Water vapour sorption characteristics and surface chemical composition of thermally modified spruce (Picea abies karst)
Show others...
2016 (English)In: International Wood Products Journal, ISSN 2042-6445, E-ISSN 2042-6453, Vol. 7, no 3, p. 116-123Article in journal (Refereed) Published
Abstract [en]

The objective of this work was to study the hygroscopicity and surface chemical composition of thermally modified (TM) spruce. An effort was also made to study if those features were influenced by a previous exposure to a significant increase in relative humidity (RH). TM and unmodified Norway spruce (Picea abies Karst) samples, both in solid and ground form, were prepared. Water vapour sorption characteristics of the ground samples were obtained by measuring sorption isotherms using a dynamic vapour sorption (DVS). The surface chemical composition of the solid samples, both acetone extracted and non-extracted, were analysed using X-ray photoelectron spectroscopy (XPS). The DVS analysis indicated that the TM wood exposed to the 75% RH revealed a decrease in isotherm hysteresis. The XPS analysis indicated a decrease of acetone extractable or volatile organic components and a relative increase of non-extractable components for the samples exposed to the increased RH condition.

Place, publisher, year, edition, pages
Taylor & Francis, 2016
Keywords
Dynamic vapour sorption (DVS), Extractives, Norway spruce, Surface chemical composition, Thermally modified wood, X-ray photoelectron spectroscopy (XPS)
National Category
Materials Engineering Chemical Sciences Wood Science
Identifiers
urn:nbn:se:kth:diva-188633 (URN)10.1080/20426445.2016.1160590 (DOI)000382319700002 ()2-s2.0-84981554847 (Scopus ID)
Note

QC 20160711

Available from: 2016-06-15 Created: 2016-06-15 Last updated: 2018-08-29Bibliographically approved
3. Liquid sorption, swelling and surface energy properties of unmodified and thermally modified Scots pine heartwood after extraction
Open this publication in new window or tab >>Liquid sorption, swelling and surface energy properties of unmodified and thermally modified Scots pine heartwood after extraction
2018 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 72, no 3, p. 251-258Article in journal (Refereed) Published
Abstract [en]

The effect of extractives removal on liquid sorption, swelling and surface energy properties of unmodified wood (UW) and thermally modified Scots pine heartwood (hW) (TMW) was studied. The extraction was performed by a Soxtec procedure with a series of solvents and the results were observed by the multicycle Wilhelmy plate method, inverse gas chromatography (IGC) and Fourier transform infrared (FTIR) spectroscopy. A significantly lower rate of water uptake was found for the extracted UW, compared with the unextracted one. This is due to a contamination effect in the latter case from water-soluble extractives increasing the capillary flow into the wood voids, proven by the decreased water surface tension. The swelling in water increased after extraction 1.7 and 3 times in the cases of UW and TMW, respectively. The dispersive part of the surface energy was lower for the extracted TMW compared to the other sample groups, indicating an almost complete removal of the extractives. The FTIR spectra of the extracts showed the presence of phenolic compounds but also resin acids and aliphatic compounds.

Place, publisher, year, edition, pages
Walter de Gruyter, 2018
Keywords
extractives, FTIR, inverse gas chromatography (IGC), liquid sorption, multicycle Wilhelmy plate method, Scots pine, surface energy, swelling, thermally modified wood
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-224686 (URN)10.1515/hf-2017-0064 (DOI)000426254900009 ()2-s2.0-85037846034 (Scopus ID)
Funder
Swedish Research Council Formas, EnWoBio 2014-172
Note

QC 20180326

Available from: 2018-03-26 Created: 2018-03-26 Last updated: 2018-08-24Bibliographically approved
4. Sorption and surface energy properties of thermally modified spruce wood components
Open this publication in new window or tab >>Sorption and surface energy properties of thermally modified spruce wood components
2018 (English)In: Wood and Fiber Science, ISSN 0735-6161, Vol. 50, no 3, p. 346-357Article in journal (Refereed) Published
Abstract [en]

The objective of this work is to study the water vapor sorption and surface energy properties of thermally modified wood (TMW) components, ie wood processing residuals in the form of sawdust. The thermal modification was performed on spruce wood components using a steam-pressurized laboratoryscale reactor at two different temperature (T) and relative humidity (RH) conditions, T = 150 degrees C and RH = 100% (TMW150), and T = 180 degrees C and RH = 46% (TMW180). A dynamic vapor sorption (DVS) technique was used to determine water vapor sorption isotherms of the samples for three adsorption-desorption cycles at varying RH between 0% and 95%. Inverse gas chromatography (IGC) was used to study the surface energy properties of the samples, including dispersive and polar characteristics. The DVS results showed that the EMC was reduced by 30-50% for the TMW samples compared with control samples of unmodified wood (UW) components. A lower reduction was, however, observed for the second and third adsorption cycles compared with that of the first cycle. Ratios between EMC of TMW and that of UW samples were lower for the TMW180 compared with the TMW150 samples, and an overall decrease in such EMC ratios was observed at higher RH for both TMW samples. The IGC results showed that the dispersive contribution to the surface energy was higher at lower surface coverages, ie representing the higher energy sites, for the TMW compared with the UW samples. In addition, an analysis of the acid-base properties indicated a higher KB than KA number, ie a higher basic than acidic contribution to the surface energy, for all the samples. A higher KB number was also observed for the TMW compared with the UW samples, suggested to relate to the presence of ether bonds from increased lignin and/or extractives content at the surface. The KB was lower for TMW180 compared with TMW150, as a result of higher modification temperature of the first, leading to cleavage of these ether bonds.

Place, publisher, year, edition, pages
Society of Wood Science and Technology, 2018
Keywords
Thermally modified wood, dynamic vapor sorption (DVS), inverse gas chromatography (IGC), Norway spruce, surface energy, acid-base properties
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-232780 (URN)000439305600010 ()2-s2.0-85050353079 (Scopus ID)
Funder
Swedish Research Council Formas, EnWoBio 2014-172
Note

QC 20180803

Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2018-08-24Bibliographically approved
5. Wood-plastic composites made from thermally modified spruce wood components and effects of exposure to water-soaking-drying cycles
Open this publication in new window or tab >>Wood-plastic composites made from thermally modified spruce wood components and effects of exposure to water-soaking-drying cycles
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The over-all aim of this work is to gain more insight on the potential to use thermally modified wood (TMW) components in wood-thermoplastic composites (WPCs), ie a new type of biobased building material, here defined as TMWPCs, assumed to have significantly increased moisture resistance and durability related to conventional WPCs. The specific objective was to prepare lab-scale TMWPCs and WPC controls with unmodified wood (UW), and to expose these samples to a series of severe water-soaking-drying cycles to study the effects on the water sorption behavior and resulting dimensional and micromorphology changes. TMW was prepared by thermal modification of a spruce board in an atmosphere of superheated steam at atmospheric pressure with a peak temperature of 210°C (also matched with an UW board as control). TMW and UW components were then prepared by a Wiley mill and thereafter sifted into a smaller (0.20-0.40 mm) and a larger (0.40-0.63 mm) size fraction. A portion of the wood components were also hot-water extracted (HE) with liquid hot-water. Composite samples with these different wood components, polypropylene (PP) matrix, and maleated PP (MAPP) as coupling agent (50/48/2 wood/PP/MAPP ratio) were then prepared by using a Brabender mixer followed by hot-pressing. The matching micromorphology of the composites before and after the soaking-drying cycles was analyzed using a surface preparation technique based on UV-laser ablation combined with scanning electron microscopy (SEM). An effort was also made to study the wood-thermoplastic interfacial behavior in the composites by dynamic mechanical analysis (DMA). The results of the water absorption tests showed, as hypothesized, a significantly reduced water absorption and resulting thickness swelling for the TMWPCs compared with the controls. Similarly, the WPCs with HE-UW components showed a significant reduction in water absorption and thickness swelling compared with the controls. In contrast, the samples with HE-TMW components resulted in only minor moisture property changes. These observations were also in agreement with the micromorphology analysis of the composites before and after the moisture cycling which showed a more pronounced wood-plastic interfacial cracking (de-bonding) as well as other microstructure changes in the controls compared with those prepared with TMW and HE-UW components. The DMA indicated better dispersion and increased interfacial interaction for the WPCs with UW components with the smaller size fraction compared with the larger size fraction. The loss modulus and storage modulus were overall reduced for samples with HE and TMW components compared with those with UW components. Based on these observations it is suggested that a potential biobased building material with increased durability for applications in harsh outdoor environments may be tailored as a TMWPC with a well-defined and comparably small size fractions of TMW components.

Keywords
Thermally modified wood (TMW), wood-plastic composite (WPC), water absorption, dimensional stability, dynamic mechanical analysis (DMA), micromorphology, scanning electron microscopy (SEM), UV-laser ablation
National Category
Composite Science and Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-233547 (URN)
Projects
Engineered Wood and Biobased Building Materials Laboratory (EnWoBio)
Funder
Swedish Research Council Formas, EnWoBio 2014-172
Note

QC 20180903

Available from: 2018-08-23 Created: 2018-08-23 Last updated: 2018-09-03Bibliographically approved

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Källbom Doctoral thesis 2018(3054 kB)168 downloads
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