Title: Coated Norway spruce – influence of wood characteristics on water sorption and coating durability
Subject: Forestry industry production systems
Faculty: Faculty of technology
Date: Thursday 14 March 2019 at 1.00 pm
Place: Room M1083 (Södrasalen), building M, Växjö
External reviewer: Professor Carsten Mai, University of Göttingen, Germany
Examining committee: Dr Karin Sandberg, RISE, Sweden; research professor Gry Alfredsen, NIBIO, Norway; dr Emil Engelund Thybring, University of Copenhagen, Denmark
Chairperson: Professor Johan Bergh, Department of forestry and wood technology, Linnaeus University
Supervisor: Associate professor Åsa Blom, Department of forestry and wood technology, Linnaeus University
Examiner: Professor Stergios Adamopoulos, Department of forestry and wood technology, Linnaeus University
Spikning: Thursday 14 february 2019 at 9.30 am
A protective coating (i.e. paint) can prolong the service life of outdoor wood. Studies on Norway spruce (Picea abies (L.) Karst.) have shown that uncoated heartwood is less prone to water sorption, crack formation and microbial growth as compared to sapwood. However, studies related to the above mentioned characteristics are rather limited for coated spruce, especially including the effect of 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 spruce for outdoor use. Four types of coatings (alkyd, acrylic, calcimine and linseed oil) were included. Methods used were wetting and liquid permeability measurements, accelerated water absorption test and outdoor field test.
The results from the wetting measurements showed that wood of similar permeability (similar density and pit closure) still had a higher water sorption rate of sapwood than heartwood samples. It could be concluded that the increased sorption was presumably caused by the lowered water surface tension, most likely by a contamination effect of the water by surface-active sapwood extractives. However, no such difference in water absorption was seen between the coated wood. Thus, it is suggested that a coating hinders the extractives to lower the water surface tension, resulting in a similar water absorption behaviour of the coated heartwood and sapwood.
The influence of density on water sorption in the field test of coated spruce was similarly to uncoated, meaning the low-density samples had a higher moisture content than the high-density samples. Furthermore, weathering caused a larger increase in water sorption of the high-density heartwood samples in the accelerated water absorption study.
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, the largest number of cracks was seen on sapwood. The microbial growth was higher on sapwood than on heartwood samples with a white coloured alkyd coating. 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.