D2.1 - Intermediate report on the optimal pre-treatment parameters for solid wood and wood fibres to facilitate the penetration of bioPCM

The objective of the work on Deliverable 2.1 was to optimize and upscale the physical incorporation of bioPCM into solid wood (for parquet) and fibres/particles (for wallboards) by impregnation. The specific objective was the selection between thermal or microwave pre-treatment of solid wood, fibres and particles to optimise the uptake of phase change material (PCM) and reduce its leaching. The pre-treatment of wood fibres and particles was carried out by only thermal modification. Wood fibres, particles and sawn boards of Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), beech (Fagus sylvatica L.) and poplar (Populus alba, Populus tremula L.) were delivered to the laboratories at CNR (Italy) and SLU (Sweden). The thermal modification (TM) was carried out according to the patented thermo-vacuum process in the laboratory at CNR. The thermo-vacuum process was applied to the sawn boards at three modification temperatures of 180, 200 and 210ºC. Woof fibres and particles were treated only at 160 ºC. Microwave (MW) treatments of the boards were carried out in a 30 kW laboratory plant at SLU operating at a frequency 2.45 GHz. The applied power was 10, 15, 20, 25 and 35% of the maximum power. After the TM and MW treatments, the materials were impregnated with ethyl palmitate (EP) by full cell impregnation, involved vacuum and pressure to ensure maximum uptake of the impregnation liquid. Leaching of the impregnated solid wood, fibres and particles was carried out in a cycling temperature mode between 5 and 45ºC and duration of 116 h. The effect of the treatments was characterized by changes in wood porosity, EP uptake, leaching and visualized by scanning electron microscopy (SEM) (only applied to spruce wood). The significant effect of TM and MW treatments that increased the wood porosity was proven by means of X-ray micro-CT (computer tomography) and pycnometry. SEM images showed a general loss of tracheid integrity and fractures in the rays. The compound middle lamellae between adjacent anatomical elements was modified by revealing ruptures and disruptions in the regions between the axial anatomical elements and ray/tracheid interphases. It was concluded that both TM and MW treatments had an effect on solid wood by changing the structure of wood cell wall that allowed higher uptake of EP. The effect of TM was more significant than MW and improved the uptake in spruce, poplar (at 180, 200 and 210ºC) and beech at 210ºC. MW was efficient when the applied power was higher than 25% for spruce and 20% for poplar. The effect of TM and MW was not significant on Scots pine wood. Leaching of EP was not significantly decreased for spruce, Scots pine and poplar; only beech showed significant decrease of leaching after TM. A trend of leaching decrease was observed with decreased uptake, particularly well-illustrated for Scots pine and beech wood. Spruce showed leaching less than 2%, while the leaching from poplar wood did not show a relation to the uptake. The impregnated fibres demonstrated a high percent of leaching, e.g., 24.7% leached EP from chemical-thermo-mechanical pulp fibres. The use of TMP fibres reduced the leaching to 5.3% due to their more intact lignin structure. Wood particles and sawdust showed an uptake of 1.2 and 1.12 g/g EP, which was significantly lower than that of the pulp fibres having more than 2 g/g EP. Consequently, the leaching of EP was significantly lower in comparison to the fibres and even the leaching from solid wood; the recorded values were 2.5 and 3.3% for the particles and sawdust. The study recommended that spruce solid wood treated by TM (200°C) and MW (35%), untreated and TM Scots pine (180°C) and untreated and TM (180°C) beech wood should be tested further as a middle layer in 3-layer parquet in a pilot-scale experiment. Wood particles and sawdust were found more suitable than fibres for wallboard production involving PCM due to their natural porosity, lower leaching than that from fibres and the diversity of material properties.