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Moisture sorption in artificially aged wood-plastic composites
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. (Byggnadsmaterial)ORCID iD: 0000-0001-7014-6230
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials. (Byggnadsmaterial)ORCID iD: 0000-0002-9156-3161
2012 (English)In: BioResources, ISSN 1930-2126, Vol. 7, no 1, 1283-1293 p.Article in journal (Refereed) Published
Abstract [en]

Moisture sorption in wood-plastic composites (WPCs) affects their durability and dimensional stability. In certain outdoor exposures, the moisture properties of WPCs are altered due to e. g. cracks induced by swelling and shrinkage of the components, as well as UV degradation or biological attack. The aim of this work was to study the effect of different artificial ageing routes on the moisture sorption properties of WPCs. Extruded WPCs were prepared with either unmodified or acetylated wood and recycled high-density polyethylene (HDPE). The WPC samples were artificially aged involving water soaking, artificial weathering, and white-or brown-rot decay in different combinations. After the ageing, the samples were conditioned in either 65% or 90% relative humidity (RH) until equilibrium moisture content was reached. A dynamic moisture sorption analyzer was used to monitor the sorption rate of samples subjected to a climate change from 65% to 90% RH. Scanning electron microscopy was used to study the surface morphology of the aged composites. Results showed that the artificial weathering caused cracking of the HDPE matrix at the composite surface, as well as a wood-matrix debonding, resulting in an increased moisture sorption rate. The WPC samples subjected to white-rot decay showed the highest moisture sorption rate.

Place, publisher, year, edition, pages
2012. Vol. 7, no 1, 1283-1293 p.
Keyword [en]
Wood-plastic composites, Acetylation, Artificial ageing, Moisture sorption, Diffusion, High-density polyethylene, Scanning electron microscopy
National Category
Composite Science and Engineering
URN: urn:nbn:se:kth:diva-66649ISI: 000304041200101ScopusID: 2-s2.0-84859024330OAI: diva2:484433

QC 20150630

Available from: 2012-01-27 Created: 2012-01-27 Last updated: 2015-06-30Bibliographically approved
In thesis
1. Characteristics of wood plastic composites based on modified wood: Moisture properties, biological performance and micromorphology
Open this publication in new window or tab >>Characteristics of wood plastic composites based on modified wood: Moisture properties, biological performance and micromorphology
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biobased materials made from renewable resources, such as wood, play an important role in the sustainable development of society. One main challenge of biobased building materials is their inherent moisture sensitivity, a major cause for fungal decay, mold growth and dimensional instability, resulting in decreased service life as well as costly maintenance. A new building material known as wood-plastic composites (WPCs) has emerged. WPCs are a combination of a thermoplastic matrix and a wood component, the former is usually recycled polyethylene or polypropylene, and the latter a wood processing residual, e.g. sawdust and wood shavings.

The objective of this thesis was to gain more insight about characteristics of WPCs containing a modified wood component. The hypothesis was that a modified wood component in WPCs would increase the moisture resistance and durability in outdoor applications. The study comprises both injection molded and extruded WPC samples made with an unmodified, acetylated, thermally modified or furfurylated wood component in a polypropylene (PP), high density polyethylene (HDPE), cellulose ester (CAP, a cellulose ester containing both acetate and propionate substituents) or polylactate (PLA) matrix. The WPCs were prepared with 50-70 weight-% wood. The emphasis was on studying the moisture sorption, fungal resistance and micromorphological features of these new types of composites. Water sorption in both liquid and vapor phases was studied, and the biological performance was studied both in laboratory and in long term outdoor field tests. Micromorphological features were assessed by analyzing of the wood component prior to and after processing, and by studying the composite microstructure by means of a new sample preparation technique based on UV excimer laser ablation combined with scanning electron microscopy (SEM).

Results showed that the WPCs with a modified wood component had a distinctly lower hygroscopicity than the WPCs with unmodified wood, which resulted in less wood-plastic interfacial cracks when subjected to a moisture soaking-drying cycle. Durability assessments in field and marine tests showed that WPCs with PP or CAP as a matrix and 70 weight-% unmodified wood degraded severely within a few years, whereas the corresponding WPCs with a modified wood component were sound after 7 years in field tests and 6 years in marine tests. Accelerated durability tests of WPCs with PLA as a matrix showed only low mass losses due to decay. However, strength losses due to moisture sorption suggest that the compatibility between the PLA and the different wood components must be improved. The micromorphological studies showed that WPC processing distinctly reduces the size and changes the shape of the wood component. The change was most pronounced in the thermally modified wood component which became significantly reduced in size. The disintegration of the modified wood components during processing also creates a more homogeneous micromorphology of the WPCs, which may be beneficial from a mechanical performance perspective. Future studies are suggested to include analyses of the surface composition, the surface energy and the surface energy heterogeneity of both wood and polymer components in order to tailor new compatible wood-polymer combinations in WPCs and biocomposites.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. xvii, 46 p.
Trita-BYMA, ISSN 0349-5752 ; 2012:2
Wood plastic composites, WPC, acetylation, thermal modification, furfurylation, moisture sorption, biological durability, UV excimer laser, micromorphology
National Category
Composite Science and Engineering
urn:nbn:se:kth:diva-105217 (URN)978-91-7501-554-5 (ISBN)
Public defence
2012-12-07, B2, Brinellvägen 23, KTH, Stockholm, 13:00 (English)

QC 20121119

Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2012-11-19Bibliographically approved

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