# UV-Induced Aging in Thermochromic Pigment-Integrated Food-Grade Polymers: A Performance Assessment
## 3 Results
### Color Stability  
To document color changes, the injection-molded bars' colors and surface topography were examined after exposure to UV radiation for 500 and 1000 h. The total color difference (\(\Delta E^{*}_{\text{ab}}\)) for thermochromic polymer specimens subjected to UVA was measured in ten replicates for each sample condition. Table 3 presents the comparative CIE \(L^{*}a^{*}b^{*}\) parameters and total color difference (\(\Delta E^{*}_{\text{ab}}\)) values for thermochromic polypropylene blends (PP98/TP2 and PP92/TP8) following UVA exposure under indoor (RS 559-934) and outdoor-simulated (QUV/Solar Eye) conditions at 500 and 1000 h. These values are the means of ten replicates per condition and reflect the time-dependent progression of surface discoloration and pigment instability. Figure 5: Visual appearance of the PP98/TP2 and PP92/TP8 impact test specimens before and after UV exposure: (**A**) PP98/TP2 prior to UV exposure; (**B**) PP98/TP2 after 500 h of simulated outdoor UV exposure; (**C**) PP98/TP2 after 1000 h of simulated outdoor UV exposure; (**D**) PP92/TP8 prior to UV exposure; (**E**) PP92/TP8 after 500 h of simulated outdoor UV exposure; (**F**) PP92/TP8 after 1000 h of simulated outdoor UV exposure. Figure 6: Samples mounted in metal frames prior to accelerated aging in the QUV weathering chamber. The measured color changes--expressed as \(\Delta L^{\star}\), \(\Delta a^{\star}\), \(\Delta b^{\star}\), and \(\Delta E^{\star}\)ab--represent deviations from the initial (0 h) color state and demonstrate the extent of discoloration and pigment shift after 500 and 1000 h of UVA exposure. Across both formulations and exposure environments, \(\Delta E^{\star}\)ab values increased with exposure duration, confirming the cumulative effects of UVA-induced photodegradation. Under the RS 559-934 chamber (360 nm), which simulates milder indoor lighting, PP98/TP2 exhibited a moderate color change, with \(\Delta E^{\star}\)ab rising from 19.29 at 500 h to 23.50 at 1000 h. The corresponding changes in \(\Delta L^{\star}\) (14.22 \(\rightarrow\) 19.58) and \(\Delta a^{\star}\) (8.16 \(\rightarrow\) 6.03) indicate progressive surface lightening and a reduction in red tones, while \(\Delta\)b* increased from 11.38 to 13.53, suggesting enhanced yellowing. The degradation was more pronounced under the QUV/Solar Eye exposure (340 nm). PP98/TP2 exhibited higher \(\Delta E^{\star}\)ab values of 26.36 and 30.29 after 500 and 1000 h, respectively, corresponding to intensified lightening (\(\Delta L\) = 24.62 at 1000 h) and continued chromatic shifts. PP92/TP8 showed a similar but more severe trend. Under the RS 559-934 exposure, \(\Delta E^{\star}\)ab increased from 17.82 to 27.57 between 500 and 1000 h, with a substantial increase in \(\Delta L\) from 13.14 to 24.78 and a sharp decrease in \(a^{\star}\) from 7.51 to 3.27, indicating greater fading of red hues. Under the QUV exposure, PP92/TP8 reached the highest color difference of all the samples: \(\Delta E^{\star}\)ab = 34.07 at 1000 h, with \(\Delta L^{\star}\) rising to 30.34 and \(a^{\star}\) decreasing to 4.09. These changes confirm the greater photo-instability of this formulation, likely due to its higher thermochromic pigment loading and associated matrix interactions [78]. The literature cites a \(\Delta E^{\star}\)ab value of 3.3 as the threshold for perceptible and acceptable color change in industrial contexts [79]. Values above 5.0 are considered clearly noticeable and are often deemed unacceptable in industrial applications [80]. In this study, several samples far exceeded this threshold following UV exposure, indicating substantial visual discoloration likely to impact the functional appearance of consumer packaging. To support the interpretation of these color differences, RGB values were calculated from the average \(L^{\star}a^{\star}b^{\star}\) coordinates and are included in Table 3. These triplets provide an approximate visual reference for the degree of discoloration observed under each UV exposure condition.