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Coated Norway Spruce: Influence of Wood Characteristics on Water Sorption and Coating Durability
Linnaeus University, Faculty of Technology, Department of Forestry and Wood Technology.ORCID iD: 0000-0002-4760-3787
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wood used outdoors is often degraded and discoloured by microorganisms as a natural part of its life cycle, particularly when exposed to high levels of moisture for prolonged times. In this case, the application of a coating (i.e. paint) is an option for increasing the service life of the wood.

Norway spruce (Picea abies (L.) Karst.) is commonly used for outdoor applications in Sweden. Earlier studies have shown that uncoated spruce heartwood is less prone to moisture sorption in outdoor exposure, resulting in lower moisture content (MC) levels, as compared to sapwood. However, studies related to the above mentioned characteristics are rather limited for coated spruce, especially including the influence of outdoor exposure (i.e. weathering).

The aim of this thesis is, therefore, to increase the knowledge of how heartwood and sapwood of different densities influence on the durability of coated Norway spruce for outdoor use. Different types of coatings with alkyd-, acrylic-, flour- (calcimine paint), or linseed-oil-based resin were included. The objectives were to study the water sorption (including MC variation) behaviour and crack formation of uncoated and coated heartwood and sapwood of different densities.

Furthermore was an objective to study the microbial growth on the surface of similar samples of coated spruce in outdoor exposure. The used methods included wetting and liquid permeability experiments, accelerated water absorption (with samples floating freely in water), and outdoor field test. The field method lasted between three to five years and involved monitoring of the MC variation, the crack formation and the microbial growth on the samples.

The results based on wetting measurements using octane as the adsorbed liquid showed no difference in liquid permeability between the spruce heartwood and sapwood samples of comparable densities, and indicated a similar level of pit aspiration (closure). The common flow path between two cells of conifers occurs through the pits. Still, the sapwood samples had in general a clearly higher water sorption rate than heartwood samples. It could be concluded that the increased sorption was presumably caused by a lowered water surface tension, most likely by a contamination effect of the water by surface-active sapwood extractives rather than differences in morphology of heartwood and sapwood.

However, no clear difference in water absorption and MC levels was seen between coated heartwood and sapwood in the field study or in the accelerated water absorption study. Thus, it is suggested that a coating hinders the surfaceactive extractives to lower the water surface tension, resulting in a similar water absorption behaviour of coated heartwood and sapwood. The influence of density on water sorption of coated spruce was similarly to uncoated spruce, meaning the low-density samples had a higher MC than the high-density samples in the field tests. Furthermore, a one-year weathering of the coated and uncoated samples caused a larger increase in water sorption of high-density heartwood in the accelerated water absorption study.

The field study on uncoated and calcimine-coated spruce showed a higher number of cracks on the high-density samples than on the low-density samples. Additionally, within each density group, a larger number of cracks were seen on sapwood samples as compared to heartwood samples. High-density samples with an alkyd- or an acrylic coating also showed a higher number of cracks. As expected, the formation of cracks on the samples increased their water sorption significantly. The microbial growth was higher on sapwood than on heartwood samples with a white coloured alkyd coating.

The main conclusion is that heartwood and sapwood of different densities influence the water sorption and durability of coated Norway spruce. However, the principles in water sorption of uncoated heartwood and sapwood could not be applied to coated samples. Overall, the results point out that low-density heartwood could be the best material combination to improve the durability of coated spruce in outdoor use. The knowledge acquired in this thesis can enable an increased service life of coated spruce in outdoor use. The increase in service life is achieved by a careful selection of the wood material regarding the proportion of heartwood and to the choice of wood density. As a concluding remark, the role of surface-active spruce extractives needs to be explored, and a follow-up investigation in the context of water sorption is suggested for future research.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2019. , p. 56
Series
Linnaeus University Dissertations ; 344
Keywords [en]
Heartwood, Sapwood, Wood extractives, Moisture content, Paint, Alkyd, Acrylic, Linseed oil, Calcimine paint, Mould, Fungi, Algae, Weathering
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
URN: urn:nbn:se:lnu:diva-80768ISBN: 978-91-88898-39-5 (print)ISBN: 978-91-88898-40-1 (electronic)OAI: oai:DiVA.org:lnu-80768DiVA, id: diva2:1290893
Public defence
2019-03-13, Södrasalen, Hus M, Växjö, 13:00
Opponent
Supervisors
Available from: 2019-02-21 Created: 2019-02-21 Last updated: 2019-03-06Bibliographically approved
List of papers
1. Liquid sorption characterisation of Norway spruce heartwood and sapwood using a muiticycle Wilhelmy plate metho
Open this publication in new window or tab >>Liquid sorption characterisation of Norway spruce heartwood and sapwood using a muiticycle Wilhelmy plate metho
2018 (English)In: International Wood Products Journal, ISSN 2042-6445, E-ISSN 2042-6453, Vol. 9, no 2, p. 58-65Article in journal (Refereed) Published
Abstract [en]

A multicycle Wilhelmy plate method was applied to study the water and octane sorption behaviour of small Norway spruce veneers. Dry heart- and sapwood samples of varying density were investigated. The results showed a correlation between the porosity and the sorption of octane for all samples, i.e. a higher wood porosity resulted in higher octane sorption. However, no difference in octane sorption was found between heart- and sapwood samples of similar density. The water sorption behaviour was difficult to interpret, probably due to the influence of surface-active wood extractives. It is suggested that the presence of such extractives, particularly in the sapwood samples, increases the sorption of water due to a significant decrease in its apparent surface tension. Hence, the results indicate that the liquid water sorption of spruce heart- and sapwood is strongly influenced by variations in the extractives content rather than by the micromorphology.

Place, publisher, year, edition, pages
Taylor & Francis, 2018
Keywords
Picea abies, water, octane, extractives, permeability, pit aspiration, coating, durability
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-73984 (URN)10.1080/20426445.2018.1467602 (DOI)000440888400004 ()2-s2.0-85046402339 (Scopus ID)
Funder
Swedish Research Council Formas, EnWoBio 2014-172
Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2019-08-29Bibliographically approved
2. The influence of coating color, heartwood and sapwood, on moisture content and growth of microorganisms on the surface during outdoor exposure of Norway spruce boards
Open this publication in new window or tab >>The influence of coating color, heartwood and sapwood, on moisture content and growth of microorganisms on the surface during outdoor exposure of Norway spruce boards
2019 (English)In: Journal of Coatings Technology and Research, ISSN 1547-0091, Vol. 16, no 3, p. 819-826Article in journal (Refereed) Published
Abstract [en]

The use of wood contributes to the global transformation into a bio-based community. There are, however, challenges. The growth of mold, rot fungi, and algae on the coated surface is of major concern due to decreased aesthetic service life and shorter maintenance intervals. The colonization of a coated surface requires the presence of spores, nutrients, and a sufficient amount of water. This work studied the influence of using heartwood and sapwood on the moisture content (MC) and growth of microorganisms on the surface of coated Norway spruce boards [Picea abies (L.) Karst.]. The results revealed a relationship of heartwood samples having a lower MC and a lower or equal degree of biological growth on the coated surface than sapwood samples. The relationship was valid through a range of densities (309–548 kg/m3) and two different coating systems based on either an alkyd or an acrylic resin. Furthermore, the choice of coating color (red compared to white) affected the MC as the red-colored samples had a lower MC, combined with no growth of microorganisms.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Mold Fungi Algae Durability Cladding
National Category
Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
urn:nbn:se:lnu:diva-79712 (URN)10.1007/s11998-018-00165-3 (DOI)000466347800017 ()2-s2.0-85060156167 (Scopus ID)
Available from: 2019-01-22 Created: 2019-01-22 Last updated: 2019-08-29Bibliographically approved

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Sjökvist, Tinh

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