## 2 Materials and Methods
### Visualization and Numerical Analysis of Different Phases in WPC Sample  
Phases in tomograms from the scans of the WPC sample before and after impregnation with sodium iodide (NaI) were visualized using the volume rendering software Drishti, which assigns colors and transparency to voxels (3D pixels) based on their intensity value [25]. Drishti was used to create 2D images and 3D animations of the WPC sample. Numerical analyses of volumes of different phases in the WPC sample used the software Avizo (Thermo Fisher Scientific, Waltham, MA, USA). For the analysis of phase volumes, the central region (2.56 mm long) of the untreated cylindrical core, which had the best contrast, was selected. The matching region within the NaI-impregnated core was also selected. Both data sets were thresholded, and the percentages of the different phases were calculated. The wood volume from the second data set was adjusted to account for the swelling of wood caused by NaI. This adjustment involved the following. First, calculation of the plastic content of the WPC before impregnation, which is assumed to remain constant as a result of labeling, is required (1):  
\[P_{0}(\%)=\frac{V_{PW0}-\frac{V_{W1}}{1+S^{W}}}{V_{0}} \tag{1}\]  
where \(P_{0}\) is the plastic content before impregnation, \(V_{0}\) is the total volume before impregnation, \(V_{PW0}\) is the volume of plastic and wood before impregnation, \(V_{W1}\) is the wood volume after impregnation, and \(S^{W}\) is the swelling ratio of wood. According to [PERSON] [26], \(S^{W}\) can be calculated using Equation (2):  
\[S^{W}=(M_{f}-M_{0})G_{0}^{w} \tag{2}\]  
where \(M_{f}\) is the moisture content of wood at the saturation point, \(M_{0}\) is the moisture content of the wood before impregnation, and \(G_{0}^{w}\) is the specific gravity of the wood cell wall substance. Equation (1) can then be modified as follows:  
\[P_{0}(\%)=\frac{V_{PW0}}{V_{0}}-\frac{V_{W1}}{V_{0}+(M_{f}-M_{0})G_{0}^{w}V _{0}} \tag{3}\]  
\(V_{PW0}\), \(V_{0}\), and \(V_{W1}\) were obtained from the X-ray CT scan. Given that the initial moisture of wood in WPC is \(\sim\)1-1.5% [27], we assume \(M_{f}\) = 30%, \(M_{0}\) = 1.5%, and \(G_{0}^{w}\) = 1.53. The plastic content was then calculated as 35.10%. The adjusted proportions of the components before impregnation are shown in Table 1.  
To calculate the extent of the interconnected void network, we used Avizo's label analysis module, which measured the percentage volume of the largest 3D object representing voids. The same module was used to calculate the percentage of wood particles that were connected. In addition, Avizo was used to calculate the number and sizes of the zinc borate particles before and after impregnation of the WPC sample. A series of 760 continuous 2D tomograms were selected from the central region of the WPC core before impregnation, and a matching series of tomograms were selected from the core after treatment. Analysis of variance was used to analyze the effect of NaI impregnation on the number and sizes of zinc borate particles in the WPC sample. Statistical computation used Genstat (v. 20) (VSN International, Hemel Hempstead, UK).