5.4 The properties of the particleboards based on a hybrid between the chips
5.4.1.2 The bending properties
The modulus of rupture (MOR) of the manufactures particleboardss of the various production variants (bean shells substitutions), is presented in Figure 50. The particleboards of cacao tree prunings in each of the pan-els’ variants achieved higher MOR compared to the boards of umbrella tree wood and the boards of the industrial wood chips. Amongst the vari-ants with 40 percent substitution of their SL chips with the chips of bean shells, the MOR of the PMDI bonded boards (5% SL, 3% CL) was signif-icantly higher (p<0.005) compared to the boards of the variant bonded with UF-resin.
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Despite the low IB values of the panel variant with the 20:20 percent sub-stitution of bean shell in the core layer and surface layers of the panels of the various wood materials, they achieved unexpectedly higher MOR values, with the boards of cacao tree prunings exceeding the 11 N/mm2 minimum value for DIN EN 312-2 (2010). The MOR values of the PB variant manufactured by substituting the surface layer chips with the chips of bean shell (40% BS in SL), bonded with PMDI (5% SL, 3% CL), also exceeded the 11 N/mm2 value for particleboards of 13 mm – 20 mm as required by DIN EN 312-2 (2010). None of the boards of the variant bonded with UF-resin could achieve the bending strength value of 11 N/mm2.
Figure 51 presents the bending modulus (MOE) as a function of sample stiffness of the manufactured particleboards of the various variants. The values of the MOE of the particleboards of the variant bonded with PMDI (5% SL, 3% CL), and the surface layers substituted by bean shell chips (40% BS in SL), did not differ significantly (p>0.1801) from one another.
A similar trend was also observed for the UF-resin bonded boards (40%
BS in SL). The results also indicate that amongst the panel variant with the surface layers substituted by bean shell chips (40% BS in SL), the stiffness properties of those bonded with PMDI (5% SL, 3% CL) did not differ significantly (p>0.05) from their UF-resin bonded counterparts. For the PMDI bonded variant (4% SL, 4% CL), with the 20:20 percent substi-tution of the surface layers and the core layer by beans shells, shows that
151 the boards of industrial wood chips achieved higher MOE (2072 N/mm2) followed by the boards of cacao tree prunings (1905 N/mm2).
PMDI UF-resin PMDI
0 2 4 6 8 10 12 14
Modulus of rupture [N/mm2 ]
Cacao tree prunings Umbrella tree wood Industrial wood chips
40% Bs in SL 40% Bs in SL 20% Bs on SL and
20% Bs on CL (5% SL, 3% CL) (10% SL, 8.5% CL) (4% SL, 4% CL)
650 kg/m3 650 kg/m3 650 kg/m3
The umbrella tree wood boards of this variant had the least value of MOE (1765 N/mm2), making them the only boards that failed to achieve the
Figure 50: The modulus of rupture (MOR) of the UF-resin and PMDI bonded particleboards manufactured from umbrella tree wood, cacao tree prunings and industrial wood chips with the various substitutions of bean shells (Bs) on the surface layer (SL) and the core layer (CL). Horrizontal line indicates the minimum value stipulated by DIN EN 312-2(2010).
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1800 N/mm2 minimum value for MOE as stipulated by the DIN EN 312-2 (2010) for particleboards of 13 mm - 20 mm thickness.
PMDI UF-resin PMDI
Cacao tree prunings Umbrella tree wood Industrial wood chips
40% Bs in SL 20% Bs on SL and
20% Bs on CL 40% Bs in SL
(5% SL , 3% CL) (10% SL , 8.5% CL) (4% SL , 4% CL)
650 kg/m3 650 kg/m3 650 kg/m3
5.4.1.3 Thickness swelling and water absorption
Thickness swelling (TS) and water absorption (WA) are some of the key parameters in determining the dimensional stability of wood-based com-posites. The TS and WA (after 24 hours) of the manufactured hybrid par-Figure 51: The modulus of elasticity (MOE) of the UF-resin and PMDI bonded particleboards manufactured from umbrella tree wood, cacao tree prunings and industrial wood chips with the various substitutions of bean shells (Bs) on the surface layer (SL) and the core layer (CL). Horrizontal line indicates the minimum value stipulated by DIN EN 312-2(2010).
153 ticleboards of the different wood materials with the various substitutions of the surface layers and core layers with the chips of bean shell bonded with the various binder systems are presented in Figure 52 and Figure 53 respectively.
Cacao tree prunings Umbrella tree wood Industrial wood chips
40% Bs in SL 40% Bs in SL 20% Bs on SL and
20% Bs on CL (5% SL, 3% CL) (10% SL, 8.5% CL) (4% SL, 4% CL)
650 kg/m3 650 kg/m3 650 kg/m3
As can be seen from Figure 52, the values of the 24 hours TS of the parti-cleboard variants bonded with PMDI were significantly lower than the
Figure 52: The thickness swelling (TS) after 24 hours of the UF-resin and PMDI bonded particleboards manufactured from umbrella tree wood, cacao tree prunings and industrial wood chips with the various substitutions of bean shells (Bs) on the surface layer (SL) and the core layer (CL).
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variant bonded with UF-resin (p<0.001). This is because the methylene bonds formed between the UF-resin and the furnish are easily hydrolyzed compared to the urethane bridges formed between PMDI and the furnish.
For the particleboards variant bonded with PMDI (5% SL, 3% CL) with their surface layers substituted by bean shells (40% BS in SL), the 24 hours thickness swelling of the boards of the various materials did not differ significantly from one another. A similar trend was also observed for the boards of the variant bonded with PMDI (4% SL, 4% CL) with 20:20 percent substitution of the core layer and the surface layers by the chips of bean shell.
Conversely, for the PB variant bonded with UF-resin (with 40% BS in SL), the 24 hours thickness swelling values of the panels of industrial wood chips (42.3%) were significantly higher (p<0.001) compared to the TS of the boards of umbrella tree wood (34.6%) and and the TS of the boards of cacao tree prunings (37.2%). The TS of the PB variant with the surface layers substituted by bean shells (40% BS in SL), bonded with PMDI (5% SL, 3% CL), showed the lowest TS rate compared to the other two variants. However, there is no required limit for TS and WA accord-ing to the DIN EN 312-2(2010).
Similar to the thickness swelling, the 24 hours WA of the PB of the vari-ant bonded with PMDI were significvari-antly lower than the WA of the boards of the variant bonded with UF-resin (p<0.001). Also, amongst the variants bonded with PMDI, the boards of the variant with the surface layers substituted by bean shell chips (40% BS in SL) had significantly
155 lower WA values (p<0.001) compared to the boards of the variant manu-factured by substituting both the surface layers (20% w/w) and the core layers (20% w/w) with bean shells chips.
PMDI UF-resin PMDI
Cacao tree prunings Umbrella tree wood Industrial wood chips
40% Bs in SL 20% Bs on SL and prunings were observed to be slightly higher compared to the boards con-taining the chips of umbrella tree wood. The 24 hours WA values of the boards of umbrella tree wood did not differ significantly from the boards Figure 53: The water absorption (WA) after 24 hours of the UF-resin and PMDI bonded particleboards manufactured from umbrella tree wood, cacao tree prunings and industrial wood chips with the various substitutions of bean shells (Bs) on the surface layer (SL) and the core layer (CL).
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of industrial wood chips in each of the variants bonded with PMDI (p>0.5000). However, for the variant bonded with UF-resin, the WA of the boards containing industrial wood chips was much higher than the WA of the boards containing umbrella tree wood
5.4.2 Formaldehyde release of the bean shells-wood chips hybrid PB