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Characteristics of wood plastic composites based on modified wood: Moisture properties, biological performance and micromorphology
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.ORCID iD: 0000-0001-7014-6230
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.
Series
Trita-BYMA, ISSN 0349-5752 ; 2012:2
Keyword [en]
Wood plastic composites, WPC, acetylation, thermal modification, furfurylation, moisture sorption, biological durability, UV excimer laser, micromorphology
National Category
Composite Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-105217ISBN: 978-91-7501-554-5 (print)OAI: oai:DiVA.org:kth-105217DiVA: diva2:570385
Public defence
2012-12-07, B2, Brinellvägen 23, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20121119

Available from: 2012-11-19 Created: 2012-11-19 Last updated: 2012-11-19Bibliographically approved
List of papers
1. Micromorphological studies of modified wood using a surface preparation technique based on ultraviolet laser ablation
Open this publication in new window or tab >>Micromorphological studies of modified wood using a surface preparation technique based on ultraviolet laser ablation
2009 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 4, no 1-2, 46-51 p.Article in journal (Refereed) Published
Abstract [en]

The objective of this paper is to demonstrate an ultraviolet (UV) laser ablation technique as a tool for sample preparation in microscopy studies of modified wood. Improved techniques for studying the microstructure of modified wood are crucial for a deeper understanding of many of their physical, mechanical and durability properties. The surface preparation technique is described in this paper. An illustration of micrographs of the micromorphology and polymer distribution in some examples of modified wood is also presented. It is clearly demonstrated that in contrast to conventional surface preparation techniques used for light microscopy and scanning electron microscopy, i.e. razor blade and microtome cutting techniques, UV laser ablation does not introduce any mechanically induced microcracks and redistribution of polymers or other mobile substances in the prepared surface. Results also show that, in particular, this technique seems to be suitable for studying polymer distribution in resin-impregnated wood, as well as detection of microcracks in modified wood cell walls.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2009
Keyword
Cell wall damage, Micromorphology, Modified wood, SEM, UV laser
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-76712 (URN)10.1080/17480270903151233 (DOI)2-s2.0-70449372641 (Scopus ID)
Note
QC 20120207Available from: 2012-02-06 Created: 2012-02-06 Last updated: 2017-12-08Bibliographically approved
2. Process-related mechanical degradation of the wood component in high-wood-content wood-plastic composites
Open this publication in new window or tab >>Process-related mechanical degradation of the wood component in high-wood-content wood-plastic composites
2012 (English)In: Wood and Fiber Science, ISSN 0735-6161, Vol. 44, no 2, 145-154 p.Article in journal (Refereed) Published
Abstract [en]

Micromorphological studies of wood plastic composites (WPC) are crucial for deeper understanding of their physical, mechanical, and durability properties. The objective of this study was to examine process-related mechanical degradation of the wood component in an extruded high-wood-content WPC. WPC with approximate to 70% wood content and three distinctly different ground wood components were manufactured by a conical extrusion technology, ie WPC were prepared with an unmodified, acetylated, or thermally modified wood component. Size and shape of wood components were determined before and after the extrusion process. Micromorphology of WPC samples was studied using a scanning electron microscope (SEM) and a surface preparation technique based on UV laser ablation. This micromachining technique was also applied to prepare that specimens for micromechanical analysis using a tensile stage mounted in a SEM. Results show that extrusion processes cause a significant mechanical degradation of the wood component. Degradation was most pronounced for the thermally modified wood component, and interestingly, this resulted in a more homogenous WPC micromorphology compared with WPC with unmodified and acetylated wood components. WPC with thermally modified wood also exhibited the highest micromechanical strength.

Keyword
WPC, particle size, micromorphology, acetylated wood, thermally modified wood, mechanical degradation
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-95266 (URN)000303231800004 ()2-s2.0-84859513136 (Scopus ID)
Note

QC 20150630

Available from: 2012-05-21 Created: 2012-05-21 Last updated: 2017-12-07Bibliographically approved
3. Micromorphology, moisture sorption and mechanical properties of a biocomposite based on acetylated wood particles and cellulose ester
Open this publication in new window or tab >>Micromorphology, moisture sorption and mechanical properties of a biocomposite based on acetylated wood particles and cellulose ester
Show others...
2007 (English)In: Wood Material Science and Engineering, ISSN 1748-0272, Vol. 3-4, no 2, 106-117 p.Article in journal (Refereed) Published
Abstract [en]

One of the major issues in a long-term perspective for the use of wood-plastic composites (WPCs) in outdoor applications is the moisture sensitivity of the wood component and the consequent dimensional instability and susceptibility to biological degradation of the composite. In this work, the effects of using an acetylated wood component and a cellulose ester as matrix on the micromorphology, mechanical performance and moisture uptake of injection-moulded WPCs have been studied. Composites based on unmodified and acetylated wood particles, specially designed with a length-to-width ratio of about 5-7, combined with both cellulose acetate propionate (CAP) and polypropylene (PP) matrices were studied. The size and shape of the wood particles were studied before and after the processing using light microscopy, and the micromorphology of the composites was studied using a newly developed surface preparation technique based on ultraviolet laser irradiation combined with low-vacuum scanning electron microscopy (LV-SEM). The water vapour sorption in the composites and the effect of accelerated weathering were measured using thin samples which were allowed to reach equilibrium moisture content (EMC). The length-to-diameter ratio was only slightly decreased for the acetylated particles after compounding and injection moulding, although both the unmodified and the acetylated particles were smaller in size after the processing steps. The tensile strength was about 40% higher for the composite based on acetylated wood than for the composite with unmodified wood using either CAP or PP as matrix, whereas the notched impact strength of the composite based on acetylated wood was about 20% lower than those of the corresponding unmodified composites. The sorption experiments showed that the EMC was 50% lower in the composites with an acetylated wood component than in the composites with an unmodified wood component. The choice of matrix material strongly affected the moisture absorptivity of the WPC. The composites with CAP as matrix gained moisture more rapidly than the composites with PP as matrix. It was also found that accelerated ageing in a Weather-Ometer® significantly increased the moisture sensitivity of the PP-based composites.

Keyword
Acetylation, Cellulose acetate propionate, Mechanical performance, Micromorphology, Moisture sorption, UV excimer laser, Wood-plastic composite
National Category
Dentistry
Identifiers
urn:nbn:se:kth:diva-7838 (URN)10.1080/17480270801906175 (DOI)2-s2.0-49549093826 (Scopus ID)
Note

Uppdaterad från submitted till published(20101116) QC 20150716

Available from: 2007-12-14 Created: 2007-12-14 Last updated: 2015-10-13Bibliographically approved
4. Moisture sorption in artificially aged wood-plastic composites
Open this publication in new window or tab >>Moisture sorption in artificially aged wood-plastic composites
2012 (English)In: BioResources, ISSN 1930-2126, E-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.

Keyword
Wood-plastic composites, Acetylation, Artificial ageing, Moisture sorption, Diffusion, High-density polyethylene, Scanning electron microscopy
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-66649 (URN)000304041200101 ()2-s2.0-84859024330 (Scopus ID)
Note

QC 20150630

Available from: 2012-01-27 Created: 2012-01-27 Last updated: 2017-12-08Bibliographically approved
5. Improved Durability and Moisture Sorption Characteristics of Extruded WPCs made from Chemically Modified Wood
Open this publication in new window or tab >>Improved Durability and Moisture Sorption Characteristics of Extruded WPCs made from Chemically Modified Wood
Show others...
2007 (English)In: Proceedings of the 9th International Conference on Wood & Biofiber Plastic Composites., 2007, 251-256 p.Conference paper, Published paper (Refereed)
National Category
Dentistry
Identifiers
urn:nbn:se:kth:diva-7839 (URN)
Conference
the 9th International Conference on Wood & Biofiber Plastic Composites, Madison, WI, USA, 2007, May 21-23
Note

QC 20101116

Available from: 2007-12-14 Created: 2007-12-14 Last updated: 2014-09-24Bibliographically approved
6. Moisture sorption, biological durability, and mechanical performance of WPC containing modified wood and polylactates
Open this publication in new window or tab >>Moisture sorption, biological durability, and mechanical performance of WPC containing modified wood and polylactates
2012 (English)In: BioResources, ISSN 1930-2126, E-ISSN 1930-2126, Vol. 7, no 4, 4575-4585 p.Article in journal (Refereed) Published
Abstract [en]

Biological durability is an important feature for wood-plastic composites (WPC) intended for outdoor applications. One route to achieving WPC products with increased biological durability is to use wood preservative agents in the formulation of the WPC. Another option could be to use a chemically modified wood component that already exhibits increased resistance to biological degradation. There is also a need to use biobased thermoplastics made from renewable resources, which would decrease the dependency on petrochemically-produced thermoplastics in the future. The objective of this study was to examine moisture sorption properties, biological durability, and mechanical performance of injection-molded WPC samples based on acetylated or thermally modified wood components and a polylactate matrix. The biological durability was evaluated in a terrestrial microcosm (TMC) test according to ENV 807, followed by mechanical evaluation in a center point bending test. The moisture sorption properties were investigated via both water soaking and exposure in a high-humidity climate. Low or negligible mass losses were observed in the TMC test for all WPC samples. However, the mechanical evaluation after exposure in the TMC test showed 35-40% losses in both strength and stiffness for the WPC containing an unmodified wood component.

Keyword
Acetylation, Thermal modification, Polylactate, Moisture, Biological durability, Wood plastic composite
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-100362 (URN)000311336800010 ()2-s2.0-84872821717 (Scopus ID)
Note

QC 20150630

Available from: 2012-08-07 Created: 2012-08-07 Last updated: 2017-12-07Bibliographically approved
7. Biological outdoor durability of WPC with chemically modified wood
Open this publication in new window or tab >>Biological outdoor durability of WPC with chemically modified wood
Show others...
2012 (English)In: Proceedings of the 6th European conference on wood modification / [ed] Jones, D., Militz, H., Petrič, M., Pohleven, F., Humar, M. and Pavlič, M., 2012, 47-54 p.Conference paper, Published paper (Refereed)
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-103705 (URN)
Conference
6th European conference on wood modification
Note

QC 20121119

Available from: 2012-10-18 Created: 2012-10-18 Last updated: 2012-11-19Bibliographically approved

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