The objective of this work was to evaluate selected physical and mechanical properties of experimental particleboards manufactured from pine and spruce with charcoal particles in their core layer. For all the manufactured boards the average density was 750 kg m-3, while the mass share of charcoal in the core layer was changed (0%, 10% and 50%). The manufactured panels were tested with respect to their mechanical and physical properties, including formaldehyde emission. The results indicated that the share of charcoal significantly influenced mechanical properties, swelling, and water relations of the boards. In addition, a test on formaldehyde emission from panels were carried out, which revealed that the charcoal share has a considerable impact on the amount of formaldehyde released by the manufactured boards. The 50% content of charcoal caused about 80% reduction of formaldehyde emission.
Wood composites are expected to attain higher and higher quality standards as their market demand increases. Emphasis is placed not only on their mechanical properties, but also on the characteristics affecting human health, like the emission of the carcinogenic formaldehyde, which sometimes caused the withdraw of such wood products from the market. Regulations on formaldehyde emissions from wood products is constantly becoming more strict, preventing the use of UF-(urea-formaldehyde) bonded panels in interiors. Indeed, UF resin is widely used as adhesive in wood composites industry, because of its low cost and high reactivity. It is mainly used for production of plywoods, particleboards, and medium density fiberboards, and represents approximately 60% of wood adhesive market (
A common method used for lowering formaldehyde emissions is the reduction of the molar ratio between formaldehyde and urea (
Recently,
According to
A further method aimed at decreasing formaldehyde emission from UF resin is the application of wood-derived bio-oil in the production of three-layered plywood (
The above mentioned results suggest the existence of a correlation between the presence of thermally processed wood, including charcoal, in selected wood-based composites structure, and formaldehyde emission from these composites. The goal of this study was to evaluate selected physical and mechanical properties of experimental particleboards manufactured from pine and spruce with different content of charcoal particles in the core layer.
Three layer particleboards with dimensions 320 × 320 × 12 mm3 and average assumed density of 750 kg m-3 were produced from industrial particles for particleboards production. The moisture content of the particles was about 5%. The raw material for particles was 95% chips of softwood species such as Scots pine (
At least 10 samples of each panel variant were used in testing their mechanical and physical properties. As for density profile measurements, 3 samples of each variant were used, and 2 samples of each variant for formaldehyde emission. Mechanical tests were carried out by using a computer-controlled INSTRON universal testing machine. The following parameters were assessed:
Modulus of Rupture (MOR) and Modulus of Elasticity (MOE) in static bending, according to
Internal bond (IB), according to
Screw withdrawal resistance (SWR), according to
Thickness swelling (TS) after 2 and 24 h of soaking in water was measured according to
where
Formaldehyde emission (FE) was measured according to
The analysis of variance (ANOVA) was carried out to test for differences among different panel types (panels 0, panels 10, panels 50) in mean values of all the measured variables, except formaldehyde emission and density profile. Linear regression (α = 0.05) was applied to investigate the relationship between formaldehyde emission and the charcoal particle content of the tested panels.
The density gradient between face and core layers increased with increasing the charcoal content in the core layer (
The modulus of rupture (MOR) of tested panels significantly decreases with increasing the share of charcoal particles in the core layer (
The decrease of MOR observed for panels 10 and panels 50 is likely caused by the brittleness of charcoal particles embedded in their core layer. In fact, the bending strength of flat materials, such as particleboards, is known to depend on the tension/compression strength of face layers, as well as on the shear strength of the core zone. It was observed that in the panels 50 (highest content of charcoal) the damage during bending occurs due to shear forces in the core layer, while in the panels 0 the damage occurred in the face layers. During the pressing of layers, charcoal particles may provoke cracks at different scale in the panels, due to their lower thermoplasticity compared to wood particles. This could still determine a high compression resistance of the panels, but a lowered resistance to shear.
Regarding the modulus of elasticity (MOE), significant differences were found between panels 0 and panels 50, as well as between panels 10 and panels 50. In this case, the lower bending strength of charcoal particles is counterbalanced by their rigidity (strongly connected to brittleness), as well as by the higher densification of face layers. In particular, no MOE reduction was detected for panels 10, probably because of the close-to-optimal content ratio of stiff charcoal particles densified closer to the surface layers, which results in a higher load and comparable low deflection. However, according to
The differences in density profile described above might potentially affect the internal bond (IB) of the tested panels. However, we found no significant decrease of IB strength by increasing from 0 to 10% the charcoal particles content in the core layer (
Breaks in the surface layer (1-2 mm in depth) were observed in few samples during this test, especially in panels 0. This may indicate that the press closing speed was too slow, so the surface layers dried out before full pressure was applied. In fact, the surface density of reference panels is lower compared to the core layer. Contrastingly, in panels 50 (where the densification of face layers was higher) the cracks occurred in the core layer, which had the lowest density and a higher charcoal content. It is worth noting that the IB standard deviation decreased with increasing the charcoal content of the panels (
The screw withdrawal resistance (SRW) significantly decreased with increasing charcoal particles content in the core layer of particleboards. As compared with reference panels (0), SWR decreased by about 30% for panels 10 and over 54% for panels 50 (
After soaking in water for 2 hours, thickness swelling (TS) was observed to increase with increasing the charcoal particle content in the core layer, with significant differences between panels 0 and panels 50 (
After 24h of soaking, the only significant difference in TS was observed between panels 0 and panels 10. Longer soaking time did not cause significant thickness change of panels 50. After 24h of soaking, the highest thickness swelling (34%) was reached by panels with 10% of charcoal. The reason of the high swelling in thickness of panels 10 can be their higher density, compare to the other tested panels. Since the thickness of panels 50 was over 13.5 mm (thus having a lower density), these panels could not reach high thickness swelling. It is worth to mention that according to
Water absorption increased as the charcoal content of the core layer of panels increases (
The results of formaldehyde emission from the tested panels are shown in
A similar effect of formaldehyde content reduction was observed by
In this study, the increasing content of charcoal particles in the core layer of particleboards resulted in: (i) a higher densification of face layers; (ii) a significant reduction of the modulus of rupture and a non-significant increase of the modulus of elasticity when adding 10% of charcoal; (iii) a non-significant increase of the internal bond of panels with 10% of charcoal content, as compared with panels without charcoal, as well as a significant internal bond reduction for 50% charcoal content panels compared to reference panels; (iv) a significant, almost linear reduction of screw withdrawal resistance; (v) an increase of thickness swelling and water absorption; (vi) a significant, linear reduction of formaldehyde emission. Since charcoal particleboard panels did not meet most of the standard requirements for furniture, their use for interior equipment is recommended for their remarkably lowered formaldehyde emission.
The following abbreviations were used throughout the manuscript:
APTES: aminpropyltriethoxysilane;
BUF: bio-oil urea-formaldehyde;
EMC: equilibrium moisture content;
FE: formaldehyde emission;
IB: internal bond;
LPL: low pressure laminate;
MDF: medium density fiberboard;
MOE: modulus of elasticity;
MOR: modulus of rupture;
MPS: methacryloxypropyltrimethoxysilane;
MUF: melamine-urea-formaldehyde;
NCC: nano crystalline cellulose;
PFO: phenol-formaldehyde oligomers;
PVC: polyvinyl chloride;
RH: relative humidity;
SWR: screw withdrawal resistance;
TS: thickness swelling;
UF: urea-formaldehyde;
UV: ultraviolet;
WA: water absorption.
Part of this work has been done under statutory research of Department of Technology and Entrepreneurship in Wood Industry, Faculty of Wood Technology, Warsaw University of Life Sciences - SGGW, Poland.
Density profiles of reference panels (0), panels 10 (10% of charcoal particles in the core layer) and panels 50 (50 % of charcoal particles). Average density values (kg m-3) are in parenthesis.
Modulus of rupture (MOR) and modulus of elasticity (MOE) of panels with different content of charcoal particles.
Internal bond (IB) of panels with different content of charcoal particles.
Screw withdrawal resistance (SWR) of panels with different content of charcoal particles.
Thickness swelling (TS) of panels with different content of charcoal particles.
Water absorption (WA) of of panels with different content of charcoal particles.
Results of the linear regression between the charcoal content of the tested panels and their formaldehyde emission.