Differences between sapwood and heartwood of thermally modified Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) under water and decay exposure: Dissertation

Thermal modification methods have been developed to increase the biological durability and dimensional stability of wood. The aim of this research was to study the differences between sapwood and heartwood of thermally modified Scots pine (Pinus sylvestis) and Norway spruce (Picea abies) under water and decay exposure. The effects of the modification temperature and wood coating were also examined. Several tests were carried out in the laboratory and field with three different complementary research materials. The main research material consisted of sapwood and heartwood of Scots pine and Norway spruce thermally modified at temperatures of 170ºC, 190ºC, 210ºC and 230ºC. The reference materials were untreated sapwood and heartwood of pine and spruce, larch, bangkirai, Western red cedar, merbau and pressure-treated wood materials, depending on the test. Thermal modification decreased the water absorption of sapwood and heartwood of spruce in relation to the modification temperature in a floating test. The water absorption of sapwood and heartwood of pine either decreased or increased, however, depending on the modification temperature. Pine sapwood absorbed more water, and very quickly, than the other wood materials, whilst pine heartwood was the most water-repellent material in the test. In general, the wettability of the thermally modified wood materials measured as contact angles only decreased with samples that had been modified at a very high modification temperature (230ºC) compared with the untreated reference wood materials. The decay resistance of thermally modified wood materials was studied in a laboratory brown-rot test with two fungi (Coniophora puteana and Poria placenta) and two incubation times (6 and 10 weeks), and in a soft-rot test with unsterile soil for 32 weeks. The fungal durability was also evaluated after 1, 2 and 9 years of exposure in the lap-joint field test. In general, the thermal modification increased the fungal durability in all the cases: the higher the modification temperature, the higher the resistance to fungal attack. Significant differences were detected between the different tests and wood materials. A very high thermal modification temperature (230ºC) was needed to achieve resistance against decay comparable to that of the durability classes 'durable' or 'very durable' in the soft-rot test. The brown-rot test resulted in slightly better durability classes than the soft-rot test, which means that, already at lower temperatures (190-210ºC), thermal modification clearly increases resistance to brown-rot attack, especially with pine materials. The results after nine years of exposure in the lap-joint field test had a good correlation with the results in the laboratory test with brown-rot fungi. The effects of the level of thermal modification and decay exposure on the bending strength of wood materials were investigated using small samples. On average, the thermal modification and fungal exposure both reduced the strength. The effect of decay exposure on strength was more significant however. It can be concluded that untreated wood material is stronger than thermally modified wood material until the wood is exposed to decay fungi. The water absorption decreased and the biological durability increased with samples that had been coated with wood oil before the tests. In this study, significant differences between the properties of thermally modified sapwood and heartwood of pine were detected in water and decay exposure. The differences between the sapwood and heartwood of spruce were notably smaller. The modification temperature had a remarkable effect on the properties of wood; this effect was not linear in every case however. As concluded, the wood species, sapwood and heartwood portions, and thermal modification temperature obviously have an influence on the biological and physical properties of thermally modified wood. These factors should be taken into account in production processes and applications as well as in testing.