Deliverable D2.3 - Model for quality control and optimization of solid wood and fibre impregnation with bioPCM

The objective of the work in Deliverable 2.3 was to optimize the impregnation parameters, weight percentage gain of the selected bioPCM (i.e., ethyl palmitate, EP) and thermal characteristics; the above parameters were elaborated in models for prediction and control of the impregnation quality in solid wood and fibres. BioPCM retention was optimized following the outcomes of the Tasks 2.1, 2.2 and 2.3 as well as the feedback from WP4. The effect of thermal modification (TM) and microwave treatment (MW) on solid wood lamellae were studied comprehensively by scanning electron microscopy (SEM) and the compound middle lamellae (CML) found the most susceptible part of the wood cell wall to be modified in form of checks, buckling and delamination. The above micro changes ensure additional retention of EP when impregnated, which was proven initially in Deliverable 2.1. The impregnated wood fibres were studied by light microscopy and two dyes to visualise the penetration and location of EP in the wood cells. Due to the properties of the impregnated bioPCM, it was hypothesized in Deliverable 2.2 that EP cannot penetrate the nanopores in the wood cell wall and the main transport path during impregnation found to be through the cell wall openings (pits); a fact proven by microscopy observations. Quantification of EP in fibres was difficult because it is highly variable and depends on the wood species, anatomical elements and physical status of fibre (torn, damaged or relatively untouched after refining). Two approaches were applied to model the impregnation of solid wood lamellae. Impregnation process of phase change material in wood lamellae was theoretically modelled by using a Washburn equation and adapted further to macroscopic model where the wood pore size distribution data obtained from X-ray micro-computer tomography (CT) were employed. The model was calibrated and verified using experimental results of ethyl palmitate impregnation of Scots pine sapwood. The model allows selection of impregnation parameters (pressure, duration) and calculates the retention of EP in solid wood. The second statistical modelling analysed data of 9 impregnations to show the relations between density, weight percentage gain and leaching of EP. Both models revealed that porosity and wood density are the property that determine the amount of impregnated EP while leaching is related to the impregnation parameters. Deliverable 2.3 contributes with the two models that can be used for practical impregnations of EP but also can be adjusted to other wood species and liquids. A computational fluid mechanics (CFM) model was developed to demonstrate the impregnation of EP in a single spruce fiber. The simulations revealed clear differences between early- and latewood fibre impregnation behaviour arising from pit geometry and flow resistance. Importantly, the surface tension and capillary effects increased as lumen filling progressed and significantly influenced the impregnation efficiency. Impregnated lamellae with various retention of EP were combined to imitate the middle layer of parquet and tested regarding thermal performance of the impregnated bioPCM by measuring material’s T-history. The measured melting enthalpy corresponded well to the calculated values. Deliverable 2.3 completes the work in WP2 and contributes to the understanding of the chain from the initial biomaterial (solid wood and fibres), the pre-treatments to improve permeability and impregnability, and selection of appropriate impregnation schedule to ensure an optimal amount of EP as a source of thermal energy storage in building materials.