Absorption of Water in Thermally Modified Pine Wood As Studied by Nuclear Magnetic Resonance

Thermal modification is an environmentally friendly method to increase the lifetime and improve the properties of timber. In this work, we investigate absorption of moisture in thermally modified pine wood ( Pinus sylvestris ) immersed in water using various nuclear magnetic resonance (NMR) methods. Magnetic resonance images (MRI) visualize the spatial distribution of absorbed free water. Spin–echo spectra measured both below and above 0 °C reveal that thermal modification partially blocks the access of water to cell walls; even modification at 180 °C slightly reduces the amount of bound water, and the amount decreases about 80% in the case of the sample modified at 240 °C. The spectra and MRI show that, above the modification temperature of 200 °C, the amount of free water decreases, indicating that high modification temperature tends to close the pits connecting the wood cells. T 2 relaxation time distributions measured using the Carr–Purcell—Meiboom–Gill sequence show four components, two associated with bound water and two with free water. NMR cryoporometry measurements indicate that the bound water sites are mostly below 2.5 nm in size. A unique combined NMR cryoporometry and relaxometry analysis showed that the size of cell wall micropores is between 1.5 and 4.5 nm, and thermal modification significantly hinders the access of water to the pores.