*2.3. Board Testing*

The boards were conditioned at 20 ◦C and 65% relative humidity prior to testing mechanical properties—internal bond strength (IBS), modulus of rupture (MOR) and modulus of elasticity (MOE), resistance to axial withdrawal of screws and physical properties—

thickness swelling after 24 h immersion in water [40–43]. In addition, thickness swelling was also determined after 48 h immersion in water.

#### 2.3.1. Internal Bond Strength

The wide faces of the 50 by 50 mm samples were glued to slotted aluminum blocks that were then pulled apart on a universal Zwick-Roell Z020 universal testing machine (Zwick-Roell, Kennesaw, GA, USA) and the load required to achieve separation was recorded.

#### 2.3.2. Flexural Tests

The 50 by 350 mm long beams were tested in third-point loading at a span of 320 mm at a loading rate of 3 mm per minute. The load and deflection were continuously recorded, and the resulting data were used to calculate modulus of rupture (MOR) and modulus of elasticity (MOE).

#### 2.3.3. Screw Withdrawal Test

The screw withdrawal tests were performed on the faces and edges of 75 mm square sections using 4.25 mm diameter MDF screws at a withdrawal speed of 2.5 mm/min. The tests were conducted with a 10 kN capacity INSTRON-4486 test machine. A 2 mm pilot hole was drilled prior to inserting the screws to a depth of 17 mm in the panels, leaving 1 mm of the screw above the panel surface for testing. Six replicate specimens were tested for each panel type.

#### 2.3.4. Thickness Swelling

Samples of 50 by 50 mm were weighed and their dimensions were measured with digital calipers (to the nearest 0.01 mm) before being immersed in distilled water. Differences in dimensions were measured after 24 and 48 h of immersion and changes were used to calculate % thickness swell.

#### *2.4. Statistical Analysis*

Statistical analysis was conducted using SPSS software program, version 24.0 (IBM, Armonk, NY, USA, 2018). One-way ANOVA was performed to identify significant differences at the 95% level of confidence, with Duncan's multiple range test grouping.

#### **3. Results and Discussion**

The mechanical properties of the single layer particleboards made from various wood/PO leaves combinations are shown in Table 1. It can be seen that using higher levels of PO leaves resulted in inferior board properties. An incorporation of up to 10% PO leaves did not significantly affect the mechanical properties of the board. In this regard, cluster analysis based on mechanical properties demonstrated close clustering of boards with only 10% PO with the control boards (that is, boards with no PO content) (Figure 3A). Internal bond strength (IBS) was more severely affected in comparison to the bending properties and the screw withdrawal resistance. Similar observations were also made by Grigoriou [44] with straw-based panels, by Papadopoulos and Hague [37] with flax-based panels, and by Hague et al. [45]. The significant reduction in IBS, especially in boards that contained 50% PO leaves, can be attributed to the fact that PO leaf chips are mainly comprised of relatively thin, short walled and weak cells [5]. Consequently, PO leaves are relatively weak and vulnerable to critical defects inside the panel structure, and therefore a rapid decrease in the IBS of the panel is observed as the PO leaves content increases. In addition, it must be pointed out that in boards made with 50% PO leaves, visible checks and cracks (internal blows) occurred in the core section of the mat, as clearly highlighted in Figure 4.


**Table 1.** The mechanical properties of various board types. Standard deviations are given in parentheses. The different letters show which values are statistically different at the 5% level.

\*, ⊥ vertical to the surface, // parallel to the surface.

**Figure 3.** Cluster analyses of the four board types based on the mechanical properties (**A**), and based on all physical and mechanical properties (**B**).

Cluster analysis based on mechanical properties also illustrated distinct difference clustering of boards with higher PO contents (25% and 50%) with those containing lower PO contents (0% and 10%) (Figure 3A). The contour and surface plots demonstrated a close

relationship between the mechanical properties, showing the nearly uniform effect of the increase in PO content on all properties studied here (Figure 5A,B).

**Figure 5.** Contour (**A**) and surface (**B**) plots, based on mechanical properties of the four board types.

The thickness swell values after 24 h immersion in water are summarised in Table 2. The results showed that the incorporation of PO leaves up to 25% did not significantly affect the dimensional stability of the boards. It is to be mentioned that no water-repelling agent was used in this study. What can be deducted from the data presented in Table 2 is that boards made from 50% wood particles and 50% PO leaves showed the best thickness swelling values. In fact, this value is significantly different from the corresponding value of boards made from pure wood chips. This tendency remained the same after 48 h immersion in water. Cluster analysis clearly demonstrated distinct different clustering of boards containing 50% PO with the other three board types, indicating the significant effect of the increase in PO content on the overall properties of the boards (Figure 3B). Two possible explanations can be offered for this behaviour; firstly, the great resistance to water that PO leaves have as an aquatic plant contributed in a decrease in thickness swelling, and secondly, their flat shape served as a coating layer, which in turn protected the internal part of the board.


**Table 2.** The physical properties of various board types. Standard deviations are given in parentheses. The different letters show which values are statistically different at the 5% level.

#### **4. Conclusions**

This study made an approach to investigate the technical feasibility of manufacturing particleboards from seaweed leaves (*Possidonia oceanica*—PO). The use of such a material may benefit both socioeconomic and environmental development since these leaves settle on seashores and decay, and therefore they are generally considered to be a waste material of no industrial value. An incorporation of up to 10% PO leaves did not significantly affect the mechanical properties of the board. Internal bond strength was more severely affected than the other mechanical properties. The results showed that the incorporation of PO leaves up to 25% did not significantly improve the dimensional stability of the boards. Markedly, boards made from 50% wood particles and 50% PO leaves showed the best thickness swelling values. It is suggested that a higher resin dosage and an alternative resin system, such as isocyanates, may improve the panel's properties and could allow a higher content of PO leaves to be incorporated in the panel. Such strategies have been successfully employed in the commercial manufacture of panels from cereal straws.

**Author Contributions:** Methodology, E.R. and G.N.; Validation, E.R. and G.N.; Investigation, E.R., A.M., and D.K.; Writing—Original Draft Preparation, E.R., A.M., and D.K.; Writing—Review and Editing, G.N., H.R.T., and A.N.P.; Visualization, E.R., A.M.; D.K., and G.N.; Supervision, G.N. and A.N.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

