Water uptake of thermally modified Norway spruce/Upijanje vode toplinski modificirane norveske smreke

Thermal modification of wood has been commercially available for almost twenty years but the complete mechanism of improved durability is still not completely understood. It is known that the temperature and duration of the modification influences the properties of the final products. There are several potential reasons for the increased durability of the modified wood. In recent research in particular, water exclusion efficiency has been identified as one of the key mechanisms. In order to elucidate this presumption, specimens made of Norway spruce heartwood were thermally modified at 6 different temperatures (160°C, 180°C, 190°C, 200°C, 210°C and 230°C) for three hours according to the Silvapro® procedure. Control specimens were left unmodified. Three sets of tests were performed: (a) samples were soaked in water for 4 days and then positioned on load cells and allowed to dry until a constant mass was achieved; (b) short term water uptake was determined with a tensiometer and (c) wood-water interactions were verified using constant gravimetric moisture measurement during outdoor exposure. As expected, the degree of modification was reflected in the moisture content of the wood during testing. Short and medium term water uptakes correlated quite well with the performance of wood in outdoor applications. On the other hand, long term tensiometer measurements were not in line with either short term water uptake or outdoor measurements. Key words: load cell, Picea abies, tensiometer, thermal modification, water uptake, field testing Toplinski modifi cirani drvo dostupno je na trzistu vec gotovo dvadeset godina medutim, potpuni mehanizam poboljsane trajnosti drva jos uvijek nije sasvim razjasnjen. Poznato je da temperatura i trajanje modifi kacije utjecu na svojstva konacnog proizvoda. Nekoliko je razloga za povecanje trajnosti toplinski modifi ciranog drva. U recentnim istrazivanjima ucinkovito je odstranjenje vode identifi cirano kao jedan od kljucnih mehanizama za poboljsanje trajnosti drva. Kako bi se razjasnila ta pretpostavka, uzorci drva srzi norveske smreke toplinski su modifi cirani pri sest razlicitih temperatura (160, 180, 190, 200, 210 i 230°C) tijekom tri sata prema postupku Silvapro®. Kontrolni su uzorci ostali nemodifi cirani. Provedena su tri ciklusa ispitivanja: a) uzorci su namakani u vodi tijekom cetiri dana, zatim su postavljeni na senzor tezine da se osuse do konstantne mase; b) kratkorocno upijanje vode odredeno je tenziometrom i c) inter