The results of the particle size distributions of OH are shown in Fig. 1. The median particle length of OH was 8.51 mm (Fig. 1a). A large proportion of the particle lengths were concentrated around the median value (50% percentile), which indicated a homogeneous particle length. The particle width distribution, on the other hand, was slightly broader (Fig. 1b). The husks had a median width of 1.31 mm in a range from 0.22 to 1.96 mm.
Table 1 shows the mechanical and physical properties of particleboards P1 to P5. The values for the IB and MOR increased at press temperatures from 120 to 140°C. From temperatures of 160°C and higher, these values were reduced again. TS and WA showed a general decline from P1 to P4. MOE, otherwise, increased with increasing temperature, but only marginally after 160°C. The minimum requirements for general application in dry areas (P1), according to EN 312:2003, could not be achieved at any of the press temperatures examined.
The falling IB after 140°C press temperature indicated a potential over-curing of the adhesive and a deterioration of the inner cohesion and bonding capability. Likewise, a brown discolouration of the panels (Fig. 1c) increased from a pressing temperature of 160°C, which was not noticeable when the adhesive was integrated into fibreboards in a previous study (Neitzel et al., 2023). The DAS-based adhesive, in combination with a fibrous material, resulted in fibreboards with comparable properties to those bonded to a melamine urea-formaldehyde resin (Neitzel et al., 2023). In the present study, the OH panels bonded with the DAS-based adhesive showed a reduction of IB at temperatures above 140°C. OH have high extractive content, for example, proteins (Schmitz et al., 2020). A high protein content in combination with short starch molecules can trigger the so-called Maillard reaction (Moreno et al., 2017), which usually imparts a distinctive flavour and brown colour, and it was noticed above pressing temperatures of 140°C (Fig. 1c). Above 180°C, processes such as caramelization accelerate degradation and reduce internal bonding capacity in the panels (Patil and Netravali, 2019).
Table 1
Mechanical and physical properties of oat husk particleboards (P1-P5). IB = internal bond, MOR = modulus of rupture, MOE = modulus of elasticity, TS = thickness swelling, WA = water absorption
| Panel code | Press temperature | IB | MOR | MOE | TS | WA |
| 2 h | 24 h | 2 h | 24 h |
| | (°C) | (kPa) | (MPa) | (MPa) | (%) | (%) | (%) | (%) |
| P1 | 120 | 23.5 | 3.1 | 640 | 71.9 | 96.5 | 41.0 | 77.5 |
| P2 | 140 | 24.2 | 4.0 | 1382 | 91.8 | 113.9 | 72.1 | 118.2 |
| P3 | 160 | 15.7 | 3.8 | 1819 | 103.8 | 135.7 | 82.9 | 142.8 |
| P4 | 180 | 16.5 | 3.4 | 1878 | 128.0 | 126.6 | 94.5 | 153.4 |
| P5 | 200 | 13.1 | 1.2 | 1916 | 113.1 | 106.3 | 86.1 | 127.0 |
The particle size distribution confirmed a homogeneous and coarse particle geometry and amplified the poor results of the adhesive. Coarse sunflower seed husks showed reduced TS, WA, MOR and IB in contrast to the fine sunflower seed husk particles in particleboards (Cosereanu et al., 2015). Due to their concave geometry, the coarse OH particles produced local agglomerations of adhesive, which affected compactness, and reduced contact surfaces, thus resulting in poor internal bonding capacity. The highly porous panel structure also facilitated WA. The water resistance is also related to the bonding quality of the respective adhesive (Neitzel et al., 2023). Therefore, the low IB values led to reduced water resistance. The decreased TS for P4 and P5 after 24 h compared to 2 h and the WA results of P5 might be due to the problematic measurement on fragile samples. The OH panels had low MOR, especially at 200°C, which can be attributed to the lower cellulose content of the OH (Schmitz et al., 2020). However, the MOE increased with increasing temperature. Similar results were observed in the coarse sunflower seed husk particleboards (Cosereanu et al., 2015). The overall results suggested that the best compromise for using a DAS-based adhesive in particleboards from OH could be using a pressing temperature of 140°C.