The results align with the earlier studies on dust from modified wood performed by different authors. There is a general opinion that the thermal modification of wood does not influence the sizes of dust particles generated during sanding-modified wood. Other factors, such as species, wood density, and processing parameters, are more crucial in woo dust generation. However, some studies report an increase in the generation of fine wood dust during the mechanical processing of thermally modified wood. The most important examples of earlier results are described below.
Kučerka and Očkajová (2018) studied the dust of Sessile oak (Quercus petraea) and Norway spruce (Picea abies). The wood, after the thermal modification in four variants of temperatures, 160, 180, 200, and 220°C, by 3h, was sanded on a vertical narrow-belt sander with P80 paper. The set of sieves with aperture sizes 2000, 1000, 500, 250, 125, 80, 63, and 32 µm was used in the sieve analysis. The fine particle size fraction (≤ 80 µm) of oak dust showed the highest mass share for all treatment temperatures; simultaneously, the lowest values of the dust fraction with a size of ≤ 80 µm were obtained at the processing temperature of 220°C for both wood species studied. The authors pointed out that the increase in treatment temperature does not significantly affect the amount of dust generated. The authors explained the high amount of the finest particles created during sanding by the decreased wood density resulting from increased temperature during the modification process.
Očkajová et al. (2019) studied the relationship between the density of untreated wood, wood after thermal modifications at four temperatures (160, 180, 200, and 220°C), and the properties of dust created by sanding with a belt sander using P80 sanding paper. Three species of wood were tested: Sessile oak (Quercus petraea), Norway spruce (Picea abies), and Meranti (Shorea acuminata). A set of sieves, 2000, 1000, 500, 250, 125, 80, 63, and 32 µm, was used to assess the dust particle size distribution. The fraction of fine particles (≤ 80 µm) in thermally modified oak dust was similar to that of untreated wood's fine dust. The mass of this fraction was 92–95%. A significant decrease in the mass share of this fraction was observed in the dust from wood modified at 220° C. The highest mass share of dust with a particle size of ≤ 80 µm for other wood species was measured in dust from modified wood at a temperature of 160° C (87% - meranti, 93% - spruce). This share decreased with increasing temperature of wood modification. A similar influence of temperature was observed in the smallest sieve fraction (32 µm) and on the bottom of the sieve (a fraction with the finest particle sizes < 32 µm). The authors explained the sieve analysis results with the reduced density of thermally modified wood.
Processing technology influences the particle size distribution of wood dust. Očkajová et al. (2020b) studied the granulometric composition of chips and dust from the longitudinal milling and sanding of thermally modified oak and spruce wood at four modification temperatures: 160, 180, 200, and 220°C. Sieve analysis Included 2000, 1000, 500, 250, 125, 80, 63, and 32 µm sieves. The results showed that the residual curves show the difference in particle size distribution in the two technologies. The sanding dust residue curves shift to the right due to the increased temperature of the wood treatment (higher mass share of large particles), while the milling dust residue curves shift to the left due to the increased temperature of the wood treatment (higher mass of small particles). This observation is also confirmed by earlier literature reports on oak sanding dust (Marková et al. 2016; Očkajová et al. 2018b).
Očkajová et al. (2020a) studied the particle size distribution in dust generated during longitudinal milling of thermally modified spruce and oak. The variables in the experiment were the modification temperature (160, 180, 200, and 220°C), and the feed rate (6, 10, and 15 m·min− 1). The particle size distribution measurements were made with a set of sieves (2000, 1000, 500, 250, 125, 80, 63, and 32 µm). The authors observed that the mass share of the coarse, fine, and very fine dust fractions changes with increasing wood modification temperature. The results of the study showed that the amount of sieve fraction with particles ≤ 80 µm increased in oak and spruce only after applying the highest temperatures, i.e. 200 and 220°C.
Kminiak and Dzurenda (2019) investigated the changes in the particle size distribution of wood chips due to thermal wood treatment. They used maple (Acer pseudoplatanus) wood particles formed during milling in a 5-axis CNC machining center for analysis. The analysis showed that more than 2/3 of the dust particles produced were coarse-grained fractions > 100 µm. The mass share of particles with a size smaller than 125 µm, did not exceed 2.5%. The thermally modified maple wood did not form the finest dust particles. The results did not confirm the thesis suggested by earlier authors that changes causing an increased share of fine dust fraction occur as a result of the heat treatment on the wood's chemical structure.
The above-cited research results suggest that the thermal modification reduces the dust fraction with a particle size ≤ 80 µm. An example is the cited study of dust waste from sawing oak and pinewood (Dzurenda et al. 2010), milling and sanding oak, and spruce wood (Očkajová et al. 2020b, a). Furthermore, in scientific publications on the dust generated during thermally modified wood (oak, spruce, meranti), based on the sieve analysis performed, a lower particle content of particles with the smallest size was found in the modified dust from the wood at the highest temperature (220 ° C) (Kučerka and Očkajová 2018; Očkajová et al. 2019). The absence of the finest particles, i.e. < 32 µm, was also found. Kminiak and Dzurenda (2019) proposed a similar conclusion based on their research on dust from milling maple wood. The authors also stated that there were no significant differences in particle sizes for modified and untreated wood dust.
Additionally Mikušová et al. (2019) investigated the influence of various thermal treatment temperatures on the size distribution of wood dust created by a hand-held belt sander. Test samples were made of meranti (Shorea accuminata) wood. The untreated and thermally modified samples at temperatures of 160, 180, 200, and 220°C samples were compared using the optical and gravimetric methods. The mass share of the finest particles in tested wood dust was highest at the treatment temperature of 160°C. However, the authors stated that mass proportion mass was not significantly influenced by thermal treatment.
The occurrence of the finest particles (≤ 10 µm) in the dust created by untreated and thermally modified wood of five species (aspen, fir, maple, ash, and poplar), was also investigated. Thermal modification of wood did not affect the amount of these particles in the air (Aro et al. 2019). (Majka et al. 2022) compared the dust from untreated beechwood to the dust from thermally modified beechwood (200°C, 3 h). The dust were separated into four sieve fractions with grain sizes < 25 µm, 25–80 µm, 80–250 µm, and > 250 µm. The authors studied whether the thermal modification changes the particle size distributions and whether all four dust sieve fractions contain the finest particles. The wood materials were sanded with P120 paper. Both types of tested dust had similar particle size distributions. Based on measurements using a laser particle sizer, the presence of particles < 10 µm in each of the four fractions was found
Such statements contradict the research results described by other authors. An example of such research is the work of (Hlásková et al. 2018). On the basis of the sieve analysis results, these authors found that the increased modification temperature of beech wood resulted in a reduced mass share of the smallest particles in the dust created in sanding. However, in the dust of wood modified at higher temperatures, the microscopic image analysis showed a higher content of the finest particles. The use of the laser diffraction analysis method to assess the mass share of the finest particles in the undersieve fraction (containing the smallest particles) also allowed the conclusion that when milling modified pine wood, the modification temperature influences the higher mass share of the finest particles in the resulting dust (Piernik et al. 2019).
Therefore, there is still uncertainty as to whether and to what extent, depending on the type of wood and the method of processing, thermal modification affects the increase in the mass share of fine dust. This is a reason for further research in this area so that the knowledge base is large enough to clearly assess this impact.