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Wood-plastic composites made from thermally modified spruce wood components and effects of exposure to water-soaking-drying cycles
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
(Aalto University, Department of Chemical and Metallurgical Engineering)
(Aalto University, Department of Chemical and Metallurgical Engineering)
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(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 [en]
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: urn:nbn:se:kth:diva-233547OAI: oai:DiVA.org:kth-233547DiVA, id: diva2:1241494
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
In thesis
1. Characterisation of thermally modified wood for use as component in biobased building materials
Open this publication in new window or tab >>Characterisation of thermally modified wood for use as component in biobased 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
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:nbn:se:kth:diva-233569 (URN)978-91-7729-860-1 (ISBN)
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

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