*2.4. Optical Characterization*

Transparency was determined as the normalized transmittance according to the standard ASTM D1746 by using a ultraviolet (UV) spectrophotometer Varian Cary 6000i (USA) [54]. For this, samples were cut into rectangular pieces and placed directly in the spectrophotometer test cell. An empty test cell was used as a reference. Five measurements were taken from different samples and the results were averaged to obtain a mean value. Normalized transmittance, in percentage, was calculated as indicated below:

$$\text{Normalized transmittance } (\%) = \frac{\log \%T}{b} \times 100$$

where %*T* is the transmittance at 600 nm and *b* is the thickness of the sample (mm).
