applied sciences  
Sodium Iodide as a Contrast Agent for X-ray Micro-CT of a Wood Plastic Composite  
[PERSON] 1,*\({}^{\copyright}\), [PERSON] 2 and [PERSON] 1,2,*  
1 Department of Wood Science, Faculty of Forestry, University of British Columbia, Vancouver, BC V6T 1Z4, Canada  
2 ARC Training Centre for M3D Innovation, Department of Applied Mathematics, Research School of Physics, The Australian National University, Canberra, ACT 0200, Australia; [PERSON]] Correspondence: [EMAIL_ADDRESS] (D.F. ); [EMAIL_ADDRESS] (P.D.E. ); Tel. : +1-604-822-0517 (D.F)  
Footnote 1: email: {[[https://www.mdpi.com/journal/applsci/code/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdfpdf/pdfpdf/pdfpdf/pdfpdf/pdf](https://www.mdpi.com/journal/applsci/code/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdfpdf/pdfpdf/pdfpdf/pdfpdf/pdf)]([https://www.mdpi.com/journal/applsci/code/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdfpdf/pdfpdf/pdfpdf/pdfpdf/pdf](https://www.mdpi.com/journal/applsci/code/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdf/pdfpdf/pdfpdf/pdfpdf/pdfpdf/pdf))  
by [PERSON] et al. [9], prediction of the properties of polymer composites requires accurate characterization of their microstructural features. Various techniques have been used to characterize the microstructure of composites, including light and electron microscopy, ultrasound imaging, NMR and IR imaging, and various X-ray imaging techniques [10; 11; 12; 13]. X-ray microcomputed tomography (CT) is increasingly used to characterize the microstructure of polymer and aerospace composites and examine either experimentally or computationally the relationship between microstructure and bulk composite properties [13; 14]. X-ray micro-CT has also been used to characterize the microstructure of wood composites, including wood plastic composites [3; 15; 16; 17; 18; 19; 20]. Characterization of wood plastic composites using X-ray micro-CT is challenging because, as pointed out by [PERSON] et al. [3], 'plastics and wood are both weak in X-ray attenuation, and this always causes insufficient contrast in X-ray images and inability to separate wood particles from plastic'. [PERSON] et al. [21] incorporated gold nanoparticles into a WPC to improve the contrast between wood and plastic in a laboratory-made composite. Alternative methods used to make it easier to distinguish wood and plastic are to reduce the wood particle fraction in the WPC or subsample volumes and select larger wood particles that are easier to threshold [8]. Nevertheless, it is challenging to distinguish plastic from small wood particles and discern interfacial regions within WPCs, as pointed out by [PERSON] et al. [8]. Hence, better methods of increasing the contrast between wood and plastic in WPCs are needed. One obvious approach is to label either the plastic or the wood component to increase their X-ray opacity. In previous research, we successfully labeled the thermosetting polymer adhesive, melamine-urea formaldehyde with copper sulfate to distinguish it from wood in particleboard [16]. Similarly, barium sulfate or iodine have also been used as X-ray contrast agents for urea formaldehyde resin in medium-density fiberboard [22]. However, these approaches are only suitable for laboratory-made boards. For commercial particleboard, we took the opposite approach and labeled the wood rather than the polymer by injecting particleboard with the halogenated, low-viscosity, X-ray-opaque solvent bromodform (CHBr\({}_{3}\)) [23]. This approach was only partially successful because the bromodform did not completely penetrate the composite [23]. Nevertheless, in areas where wood particles absorbed bromodform, it was possible to distinguish wood from polymer [23].