# Alien Wood Species as a Resource for Wood-Plastic Composites
## 3 Results and Discussion
### Surface and Internal Structure of WPC Boards  
According to the methodology presented above, we investigated the surface and internal structure of WPC boards made of wood species mixture (F series). First Scanning Electron Microscopy (SEM) observations were performed on the unflattened surface of WPC boards. In this case, we could only observe flat homogeneous surface and we could not recognize any wood structure, especially at lower magnification (100\(\times\)) (Figure 11a). Rarely, in some areas, we could observe some voids on the surface. A detailed analysis (1000\(\times\) magnification) revealed some wood anatomical structure which were, however, difficult to recognize and could not enable the identification of wood species (Figure 11b). The main objective of the microscopic analysis of the WPC structure was to observe the interaction and adhesion between the polymer matrix and the small wood particles embedded in it. To enhance the observation of the anatomical structures of the wood, WPC cross-section were pre-treated. A first overview of a WPC pre-treated cross-section was made on the region of interest (ROI) by a Confocal Laser Microscope (Figure 12). On Figure 12 we can clearly observe wood particles. We could recognize the polymer matrix between the wood particles and within the wood pores. In the WPC cross-section we could not detect any voids. To observe in detail and with high resolution the interaction between wood particles and polymer we used SEM. Various regions of interest (ROI), which we observed at SEM, are marked A1 to A4 in Figure 12.  
In Figure 13 the internal WPC board structure at 100\(\times\) and 250\(\times\) magnification is shown (ROI marked on Figure 11). In this case, the structure of wood is clearly identifiable, so we can recognize the arrangement of the vessels as well as the wood fibres and tracheids. According to previous wood structure analysis of IAPS [16; 17] we can identify boxelder map (_Acer negundo_) cross section. This proved that with our SEM methodology it is possible to identify the wood species used for WPC production although the wood particles are small. A larger magnification (500\(\times\) and 2500\(\times\)) in Figure 14 revealed interaction between wood particles and polymer matrix. The embedding of wood components in a polymer matrix, as well as lumens filled with polymer, is clearly visible. In Figure 14b is shown a detailed cross-section structure, in which we can see that all fibres, as well as ray parenchyma  
Figure 11: Scanning Electron Microscopy observation on the unflattened surface of WPC boards; (**a**) homogeneous surface with no recognizable wood structure, (**b**) higher magnification image revealing some wood structures. cells, are filled with polymer. The absence of considerable voids between wood fibre and matrix indicates good compatibility and good interfacial adhesion. The detailed structural observation of WPC showed that the WPC production process used (flat pressing) produced a fine and filled structure without large any major voids or fibre pull out, and with good interfacial adhesion, so that the moisture resistance, mechanical properties and thermal stability were relatively high. The experience gained in the production of flat pressed WPC was used to carry out a pilot production of 3D shaped composites. Following the described procedure, we produced several different 3D-shaped WPC products made from invasive alien woody plants (Figure 15). Figure 12: Confocal Scanning Laser image of a cross section WPC board. A1 to A4 are ROI analysed by Scanning Electron Microscopy (scale bar is 500 \(\upmu\)m). Figure 13: Scanning Electron Microscopy image of a cross section WPC board; (**a**) A1 analysed ROI, (**b**) A2 analysed ROI. Based on the results of this research and the newly gained experience, we will further develop the production of wood-based composites from invasive species, analysing the impact of particle morphology differences in order to optimise (increase) the proportion of wood in WPC.