Thermal properties of wood-based panels: specific heat determination
Wood Sci Technol
Thermal properties of wood-based panels: specific heat determination
Łukasz Czajkowski 0 1 2
Wiesław Olek 0 1 2
Jerzy Weres 0 1 2
Ryszard Guzenda 0 1 2
0 Faculty of Agriculture and Bioengineering, Poznan ́ University of Life Sciences , ul. Wojska Polskiego 28, 60-637 Poznan ́ , Poland
1 Faculty of Wood Technology, Poznan ́ University of Life Sciences , ul. Wojska Polskiego 28, 60-637 Poznan ́ , Poland
2 & Łukasz Czajkowski
Ensuring reliability of data on thermal properties of wood-based panels is important for manufacturing processes, especially when it is recommended to shorten the cooling phase and stack the panels in hot conditions. Prediction of the heat transfer during cooling phase and normal or hot stacking based on accurate data is essentially important for attaining panels of required properties. The thermal properties are also required when designing houses, especially low-energy or passive ones. Therefore, a water calorimeter was adequately designed and constructed to ensure improvement in the accuracy of the specific heat measurements. The calorimeter was used to determine the specific heat. The attained accuracy estimated by the relative error was significantly increased, and the error values were less than 2 % for all types of the investigated particleboard and OSB. In case of low-density fiberboard (LDF), the maximum value of the relative error did not exceed 4 %. It was also shown that high accuracy required for the specific heat measurements was achieved for experiments in which high-mass samples were used, in contrast to measurements for such samples in traditional DSC systems. The results for the specific heat were within the range from 1420 to 1450 J/kg K for LDF and all types of particleboard. The effect of the investigated material density on the specific heat was not found. The only exception was in case of OSB for which the specific heat was ca. 1550 J/kg K, and it was approximately 100 J/kg K higher when compared to other panels.
Introduction
Credible data on thermal properties of wood-based panels are of special significance
also during manufacturing processes, e.g., for controlling the cooling phase which is
required before stacking materials already produced. Cooling of panels is especially
important in the case of composites bonded with urea–formaldehyde or melamine–
urea–formaldehyde resins as temperature of a stack as high as 65–70 C may result
in hydrolysis of the resins, and therefore, it can lead to the reduction in the bonding
strength of particles within panels
(e.g., Chapman 2006; Irle and Barbu 2010)
.
However, in the case of panels bonded with phenol–formaldehyde resins it is often
recommended to shorten the cooling phase and stack the panels in hot conditions.
The hot stacking is used to complete the curing process of phenol–formaldehyde
resins, especially in the core layers of the panels (e.g., Carvalho et al. 2010).
Therefore, the accurate prediction of heat transfer during cooling phase as well as
during normal or hot stacking has special importance for obtaining the required
properties of the panels.
High accuracy of thermal properties of wood-based panels is also required when
designing houses, especially low-energy or passive ones. The proper application of
materials forming thermal envelope of a building is one of the most important
problems which is faced during the design process
(e.g., Kosny et al. 2014; Upton
et al. 2008)
. The primary goal of the thermal envelope is to reduce heat losses of a
building in winter and heat gains in summer. The heat transfer reduction can be
relatively easily obtained; however, inaccurate data on the thermal properties of
materials may cause unfavorable and uncontrolled local increase in heat transfer,
which is usually called heat bridges (Piot et al. 2011). The phenomenon is usually
associated with water condensation at surfaces with low temperature. It results in
dramatic decrease in insulating properties of the materials due to increase in their
moisture content. Moreover, the process is self-propelling as the increase in
moisture content causes further increase in thermal conductivity of the materials.
The thermal properties of wood-based panels which are required for heat transfer
calculations usually consist of specific heat and thermal conductivity. The two
properties are, however, determined with different experimental methods. The
specific heat is traditionally measured with calorimetric methods
(e.g., Baryeh
1997)
, while the thermal conductivity values are obtained by the application of
various methods based on principles of steady-state or transient heat transfer
(e.g.,
Sonderegger and Niemz 2009; Zhou et al. 2013)
. In the last decades, the
development of the calorimetric methods resulted in constructing automated
systems for thermodynamic analysis of materials based on the technique of
differential scanning calorimetry (DSC). The most important disadvantage of the
DSC systems, as a (...truncated)