# UV-Induced Aging in Thermochromic Pigment-Integrated Food-Grade Polymers: A Performance Assessment
## 2 Materials and Methods
### Color Stability Analysis (CIE L*a*b*)  
Spectrophotometric color measurements were conducted to evaluate the UV-induced chromatic changes in thermochromic polypropylene blends. A handheld X-Rite SP62 spherical spectrophotometer (X-Rite SP62, Grand Rapids, MI, USA) was used for all measurements. Before analysis, the instrument was calibrated using a zero tile and a standard white calibration tile in accordance with the SP60 user manual. Color values were collected in the Commission Internationale de l'Eclairage (CIE) chromaticity coordinate system (\(L^{*}\), \(a^{*}\), \(b^{*}\)), where \(L^{*}\) describes lightness (0 = black, 100 = white), \(a^{*}\) represents the red-green chromaticity, and \(b^{*}\) corresponds to the blue-yellow chromaticity. The differences show the variations in the \(L^{*}\), \(a^{*}\), and \(b^{*}\) values between the sample and the standard (\(\Delta L^{*}\), \(\Delta a^{*}\), and \(\Delta b^{*}\)). Equation (1) was used to determine the total color difference (\(\Delta E^{*}_{\text{ab}}\)):  
\[\Delta E^{*}_{ab} = \left[\left(\triangle L^{*}\right)^{2}+\left(\triangle a^{*} \right)^{2}+\left(\triangle b^{*}\right)^{2}\right]^{1/2} \tag{1}\]  
where \(\Delta E^{*}_{\text{ab}}\) is the color difference between two colors; \(\Delta L^{*}\) is the difference in lightness between the two colors; \(\Delta a^{*}\) is the difference in the red-green axis; and \(\Delta b^{*}\) is the difference in the yellow-blue axis.  
Using OncolorTM (v.6.3.4.4 QC-Lite) (Hunter Associates Laboratory, Inc., Reston, VA, USA), the color coordinates \(L^{*}\), \(a^{*}\), and \(b^{*}\) were captured in accordance with ISO/CIE 11664-4:2019 [67]. Data were collected for ten replicate specimens per test condition, with the mean values and standard deviations calculated. To aid visual interpretation, RGB values were derived from the average CIE \(L^{*}a^{*}b^{*}\) coordinates using the built-in lab2 rgb function in MATLAB (version R2024a) software (The MathWorks, Inc., Natick, MA, USA). This function transforms the color space from CIE 1976 \(L^{*}a^{*}b^{*}\) to the red, green, and blue (RGB) values using the D65 standard illuminant, in alignment with the CIE conversion protocols. RGB values are reported as integer triplets (0-255) for each sample group. The color difference connected to the test specimens may be identified using these coordinates. Equation (1) was used to compare the color coordinates of the simulated indoor and outdoor UVA-exposed samples with those of the unexposed food-grade material samples, which were used as a reference, to determine the color difference, which is represented by the distance metric \(\Delta E^{*}_{\text{ab}}\). The decision to use ten replicates for colorimetric testing was based on the need for greater statistical resolution in capturing surface discoloration trends, which are often more variable than bulk mechanical properties. Mechanical testing followed ISO standards, with five replicates prescribed for tensile testing (Section 2.8) in accordance with ISO 527-2 [57] and ten replicates prescribed for Charpy impact testing (Section 2.9) as per ISO 179-1 [56].