## 3 Results
### Polytetrafluoroethylene  
Full frequency spectra for both refractive index and loss coefficient for PTFE can be seen in Figure 1. It is evident at low frequencies that the PTFE pellets are in good agreement \begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Manufacturer True Density (g/cm\({}^{3}\))** & \(\phi_{\mathrm{true}}\) (g/cm\({}^{3}\))** \\ \hline PTFE & 2.15 & 2.07 \(\pm\) 0.01 \\ HDPE1 & 0.94 & 0.96 \(\pm\) 0.02 \\ HDPE2 & 0.94 & 0.95 \(\pm\) 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Solid true density of PTFE and the two HDPE grades. Measured values are the mean \(\pm\) one standard deviation (\(n=3\)). with the loss coefficient of solid material whereas the refractive index is slightly higher. Oscillations in the pellet spectra are artifacts due to internal reflections within the sample producing a Fabry-Perot effect. The oscillation period or free spectral range (\(FSR\)) is determined by the sample thickness and refractive index, given by [26,27]:  
\[FSR=\frac{c}{2n_{\text{extracted}}L} \tag{7}\]  
For the purposes of subsequent analysis, mean values over a range (2.45-2.55 THz) were used due to the lack of spectral features and relatively high signal-to-noise ratio (SNR). Figure 2 presents the refractive index (a) and loss coefficient (b) of PTFE pellets compacted at various pressures (159, 235, 319, 352, 392, 436 and 478 MPa). The refractive indices of pellets made using the compaction simulator (Figure 2a) are either in good agreement or slightly above the values in the literature with the exception of the pellets that were compacted at a pressure of 159 MPa. Consistently higher refractive index values suggest a difference in material density compared with the values in the literature. Raman spectra of the PTFE pellets and solid reference samples were acquired to determine whether a change in amorphous or crystalline character had occurred (Figure A1); no such change was observed. Therefore, the increased refractive index is attributed to the different production method used for the powder compared with that for the solid polymer. The loss coefficient values for compaction pressures > 350 MPa are within the reference ranges. It can also be observed that higher compaction pressures lead to more consistent loss coefficient values. The refractive index value at a compaction pressure of 159 MPa is significantly lower than expected and exhibits high variability across ten pellets. This lower refractive index suggests a large sample porosity created by inter-particle pores. These inter-particle pores reduce in size with increasing compaction pressure. The measured RI approaches the refractive index of the solid material as the porosity decreases with increasing compaction pressure. Evidence of sample cracks can be observed in microscope images (Figure A2b). The high calculated standard deviation is due to cracks in the samples produced at this lower pressure, particularly noticeable in those made using the compaction simulator. These cracks cause inaccuracies in the determination of the calculated refractive index due to changes in sample thickness. The cracks in these samples also affect the loss coefficient, which is higher in samples compacted at lower pressures. The samples fabricated using the hydraulic press with a vacuum had refractive indices either slightly above or in agreement with the values in the literature. Loss coefficient values were also in good agreement with the values in the literature. However, it should be noted that creating samples in such a die requires care when releasing the pressure to prevent sample cracking. The dwell time difference between the compaction methods  
Figure 1: Refractive index (**a**) and loss coefficient (**b**) spectra of PTFE pellets. Oscillations in spectra are artifacts due to internal reflections. must also be considered where the hydraulic press had a larger dwell time compared with the compaction simulator, thus explaining the better performance at lower pressures.