Due to the desire to minimize the use of biocides, we are increasingly using alternative methods for wood preservation, including thermal modification. The production of thermally modified wood is increasing, but we still do not have a fully developed unified method for determining the degree of modification that would be required to control the quality of thermally modified wood. The degree of thermal modification of wood correlates with the loss of dry wood mass during this process.
During heating of thermally modified samples in the thermoanalyzer, processes that have not yet taken place during thermal modification at elevated temperature occur in the samples. The higher the degree of previous thermal modification, the lower the mass loss during thermogravimetric measurement in a given temperature range and the lower the moisture content, because the number of hydroxyl groups decreases during modification and consequently they are less hydrophilic.
The course of thermal decomposition is influenced by several factors, including particle size. Smaller particles have a larger surface area, so the rate of mass loss in a given temperature range is higher than for samples with a larger particle size.
As part of my diploma work, I wanted to test whether two different particle sizes of thermally modified oak wood had a noticeable effect on the rate of weight loss in a given temperature range. Samples of thermally modified oak were prepared first. These were heat treated in the absence of oxygen at temperatures of 170°C, 180°C, 190°C, 200°C, 205°C and 210°C. We chose oak because this type of wood is quite widespread in Slovenia and can be used for the production of a wide range of products. The prepared samples are standard ones because we can easily calculate the weight loss during thermal modification by weighing the samples before and after thermal modification. The thermally modified samples were then milled into two different sized fractions, 0.5 mm and 1.0 mm, weighed into crucibles and exposed to a humid atmosphere with a certain humidity. After saturation, the samples were thermogravimetrically analyzed in an inert atmosphere and in the temperature range between 20°C and 600°C. From the data obtained, curves or calibration lines were constructed showing the dependence of the weight loss of the samples on the heat treatment. The obtained calibration lines can then be used to determine the mass loss during thermal modification for unknown samples, which is proportional to the degree of thermal modification.