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Flickr varying compositions of the coatings used. White paint, which
contains a filler, likely contributes to the increased bulk density.

Relative Hardness and Density Profile

The surface with clear varnish exhibited a higher relative
hardness of 0.15, whereas the surface coated with white
varnish had a relative hardness of 0.11. This information is
crucial when considering the fractional composition of the
particles after milling.
More complex coatings tend to be more brittle, which
can increase the proportion of smaller fractions, specifically
For industrial particles, most fractions fall within the 1–4 those less than two mm, or even dust. The values obtained
mm range. In contrast, particles obtained from reclaimed are consistent with other studies.

wood window frames are reduced during shredding, with Another study investigated the effect of temperature on
most fractions ranging from 0.25–4 mm, particularly for those various coatings (varnishes were applied to the heated wood),
with white paint. including cellulose lacquer, synthetic varnish, polyurethane
A similar trend is observed with particles from clear varnish, and water-based varnish.
varnish-painted window frames; however, these contain a The initial heat treatment of the wood did not contribute
higher proportion of fine fractions than white-painted particles. to the relative hardness of the varnishes used.
The higher density and brittleness of varnishes compared Varnished wooden surfaces are exposed to various external

to wood likely contribute to the shortening of the particles. A variables depending on their application. To avoid economic
similar trend of particle shortening was observed in studies loss, it is critical to select varnishes that provide optimal
where particleboards were subjected to repeated milling. performance matched to the specific area of application.
These particleboards contained laminate, which has a high Studies report that the enrichment of the varnish with TiO2
density similar to varnish. Additionally, the adhesive content and Al2O3 nanofillers promotes a reduction in the thickness
increased with each successive batch of particleboard made of the surface layer and thus contributes to the hardness of
from particleboard residues. the coating and the ability to absorb UV radiation.
Once cured, this adhesive also has a higher density than The relative hardness for the clear varnish and the white
wood. Consequently, there was a significant shortening of wood paint used in the tests was shown. The density profile of the

fibres, as confirmed by the studies of Wronka and Kowaluk. white paint, which has a lower density and does not penetrate
The industrial particles, free from contamination, recorded the wood as deeply as the clear varnish (orange line), which
the lowest bulk density (145 kg m−3). may be due to the presence of fillers in the paint.
The blue bar represents the bulk density values of window The lack of fillers in the paint makes the paint penetrate
frame mill particles painted with clear paint (177 kg m−3). This the wood better, which can affect some test results, such as
value is higher than industrial particles but lower than the bulk water absorption or bulk density.
density of particles from milled window frames painted with Paint layer thickness can vary greatly depending on solvent

white paint (192 kg m−3). evaporation, surface tension gradients, and paint application
The differences in these values may be attributed to the processes, and also, the temperature of the liquid top coating