## 4 Discussion  
Our results only partially support our hypothesis that aqueous sodium iodide is a suitable X-ray micro-CT contrast agent to help visualize the microstructure of a wood plastic composite. Sodium iodide labeling made it possible to: more clearly visualize wood and plastic in the WPC, quantify the levels of different phases (wood, plastic, zinc borate, and voids) in the composite, and measure the geometry of the wood reinforcement. These outcomes of labeling are noteworthy, but sodium iodide was not an ideal contrast agent because it altered the microstructure of the WPC. For example, it swelled wood particles closing interfacial voids, filled any remaining voids with liquid, and dissolved zinc borate particles. Furthermore, the labeling process was lengthy and two separate CT scans were needed to quantify the composite's composition and analyze the shape of wood reinforcement. Future research should seek to overcome some of these limitations possibly by selecting a solvent for sodium iodide or an alternative halogenated chemical that does not swell wood or dissolve zinc borate. Wood or plastic could be labeled prior to the manufacture of the WPC as has been done previously [21], which would eliminate the time-consuming step of impregnating wood with the contrast agent, but this approach is not suitable for visualizing the microstructure of commercial WPCs, which was one of the aims of this study  
Previously, we used X-ray micro-CT to visualize the distribution of the biocide zinc borate in oriented strand board (OSB), a structural wood composite that contains high levels of wood reinforcement and low adhesive content [17]. OSB is manufactured using a completely different process to that of WPCs [32; 33], and zinc borate in OSB is located at the interface between wood and adhesive [17]. The distribution of zinc borate in our WPC was different from that in OSB, but in both composites, zinc borate was dissolved by water, which reduced the size of zinc borate particles. The dissolution of zinc borate in WPC and absorption or complexation of zinc or borate ions with wood that is not fully encapsulated in plastic may explain, in part, why it is an effective biocide for WPC's [17]. Some of our findings also help explain other aspects of the durability of WPCs. For example, previous studies, as mentioned above, have shown that WPCs with larger wood particles absorb more moisture and are more susceptible to decay than composites made from smaller particles [4; 34]. These observations can be explained by our finding that interfacial voids were concentrated around large wood particles and many of the voids were connected, possibly assisting moisture ingress into the WPC, as suggested by [PERSON] et al. [35]. In addition to the network of voids, moisture ingress would also be facilitated by the aggregation of wood particles and the numerous wood-to-wood connections in the composite, which made shape analysis of individual particles difficult. The absorption of moisture into WPCs is the rate-limiting factor restricting decay of the composites [36], and therefore, measures that reduce the porosity of the composite improve the durability of WPCs. In practice, this is achieved in the latest generation of 'capped' WPCs by extruding a layer of pure  
Figure 6: Shape descriptions of the selected 50 particles based on their length, breadth, and thickness (L:B:T) ratios. polymer over the surface of the composites [31]. This barrier approach is highly effective at reducing moisture ingress into WPCs, but in areas where the protective polymer cap is absent, or breached, for example by cut ends and fixings, some moisture absorption can still occur [37]. Therefore, additional measures to reduce the porosity and durability of WPCs may be useful. Our results suggest that reducing the number of large wood particles and the interfacial voids around the particles could have the desirable effect of reducing the porosity and possibly increasing the durability of WPCs. Further, research is needed to fully test this hypothesis.