Moisture transport in coated wood

Moisture accumulation inside wood causes favorable conditions for decay. Application of a coating alters the moisture sorption of wood and prevents accumulation of moisture. This paper presents the results of a nuclear magnetic resonance (NMR) study on the influence of a coating on the moisture absorption of wood. NMR allows to determine both local wood moisture content and rate of water absorption during water absorption and desorption of coated and uncoated wood. In contrast to weighing, both quantities are measured dynamically and non destructively with high spatial and temporal resolution in relatively short experiments. In addition, NMR relaxometry distinguishes between moisture in lumina and moisture in cell walls, which allows to accurately characterize sorption processes in wood. In the present study, samples with a diameter of 20 mm and a height of 10 mm, are studied in a 4.7 T NMR scanner with a spatial resolution of 33 ± 3 μm. Several commonly used wood-coating combinations are studied. Water is placed on the tangential side of samples equilibrated at 22% relative humidity while the wood moisture content (MC) is monitored for 24 h. This research shows that the sorption behavior of coated wood depends on the specific combination of wood and coating. Additionally, the amount of water that is absorbed in a coating may have a strong influence on the moisture content of the coated wood. We found that the water absorption of a hardwood dark red meranti sample is diffusion dominated. Application of a waterborne acrylic coating has no influence on this absorption process, which is attributed to the large water uptake of the coating. A solvent borne alkyd coating that absorbs very little water is found to strongly reduce the water uptake of the meranti studied. The waterborne coating reduces the water uptake of pine by preventing capillary water uptake of rays present in this softwood. The solvent borne alkyd coating further reduces uptake. Uncoated spruce also absorbs water by capillary suction. On this wood, the acrylic coating strongly reduces the water uptake; comparable to alkyd coated spruce. This is a result of the pits of spruce which became aspirated during drying. Application of a coating might fixate the aspirated pits, resulting in a structure with low permeability.