## 3 Results  
### Visualization of Phases in WPC before and after Impregnation with NaI  
Impregnation of the WPC sample with sodium iodide made it possible to clearly differentiate wood and polymer as hypothesized. Before impregnation of the WPC sample, it was possible to clearly identify air and zinc borate, which are colored light blue and white in Figure 2a,c,e, respectively, but there was a lack of contrast between wood and plastic. In particular, it was difficult to distinguish smaller wood particles from the plastic matrix. However, larger wood particles could be seen in the unimpregnated sample because of their cellular characteristics. As a result, it was possible to see that the WPC contained both softwood (Figure 2a, arrowed) and hardwood particles (Figure 2c, arrowed).  
The unimpregnated sample contained voids within and around wood particles, particularly larger particles, whereas such voids were absent from the impregnated sample. Interfacial voids around particles in the unimpregnated sample were connected creating a network of cracks within the WPC (Figure 2e). Numerical analysis of the network revealed that 61.8% of the voids formed a single connected network. A similar analysis revealed that almost all (99.75%) of the wood particles in the WPC were connected. As expected, zinc borate (ZB) was easy to see in the unimpregnated sample because it is far denser than plastic or wood. ZB occurred mainly as discrete particles between rather than within or around wood particles (Figure 2). However, occasionally, large wood particles were encased in a layer of zinc borate particles (Figure 2c,d). Zinc borate particles were clearly smaller and less numerous in the WPC sample after it was impregnated with sodium iodide. Accordingly, analysis of variance showed a highly significant (\(p<0.001\)) effect of impregnation with sodium iodide on the number of ZB particles and their average diameter in the selected volume within the WPC. These observations are supported by analysis of ZB particles in the entire data set, which showed that the number of particles decreased from 1,130,639 to 512,731 as a result of labeling. Similarly, the average volume and diameter of the ZB particles dropped from 1204.4 \(\upmu\)m\({}^{3}\) and 10.4 \(\upmu\)m (before labeling) to 770.8 \(\upmu\)m\({}^{3}\) and 8.8 \(\upmu\)m, respectively, after labeling.  
Figure 1: Labeling and restoration of wood particles. (**a**) Original grayscale tomogram; (**b**) thresholded volume showing segments (yellow); (**c**) three different types of labels: blue, a wood particle with more than one segment; green, a particle with only one segment; magenta, an unseparated particle that shared a segment with another particle. (**d**) Results of the manual restoration of labeled particles.  
The improved contrast between wood and polymer in the impregnated sample made it possible to see two features that were not apparent in the unimpregnated sample. Firstly, there were areas where wood particles met that contained little or no polymer (Figure 3a,b). Secondly, wood particles in the WPC were not randomly oriented. Instead, many of the larger particles were oriented in approximately the horizontal direction in length (Figure 3c,d).  
Figure 2: Two-dimensional (2D) tomographic images of the wood plastic composite: before (**a,c,e**) and after (**b,d,f**) impregnation with sodium iodide; brown = wood; navy blue = plastic; light blue = void; white = zinc borate.