# UV-Induced Aging in Thermochromic Pigment-Integrated Food-Grade Polymers: A Performance Assessment
## 3 Results
### Tensile Properties After UV Exposure  
PP98/TP2 (Table 4) exhibits moderate variations in tensile properties across different UV exposures. The corresponding trends are visualized in Figure 7 for clearer comparison. The tensile property data presented in Table 4 highlight the degradative effects of indoor UVA exposure on thermochromic polypropylene blends (PP98/TP2 and PP92/TP8) using the RS 559-934 (360 nm) device. Both blends exhibited a progressive reduction in the tensile strain at break and the maximum tensile stress with increasing exposure duration, indicating a decline in ductility and tensile strength due to photo-oxidative degradation [83]. For PP98/TP2, the strain at break decreased from 95.69% (control) to 81.49% after 500 h and 60.11% after 1000 h of UVA exposure. The corresponding maximum tensile stress values declined from 29.45 MPa to 25.64 MPa (\(-12.94\%\)) over the same period. A similar degradation pattern was observed for PP92/TP8, with the strain at break decreasing from 87.25% to 44.79%, tensile stress--from 28.89 MPa to 22.31 MPa (\(-22.77\%\)) following 1000 h of exposure. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Specimen** & **UV Device** & **Exposure** & **Tensile Strain at** & **Maximum** & **Young’s** \\
**ID** & **UV Device** & **Hours (h)** & **Break (\%)** & **Tensile Stress (MPa)** & **Modulus (MPa)** \\ \hline  & None (control) & 0 & 95.69 (\(\pm\)3.81) a  & 29.45 (\(\pm\)0.85) a  & 653.82 (\(\pm\)9.57) a  \\ PP98/TP2 & RS 559-934 (360 nm) & 500 & 81.49 (\(\pm\)3.52) b  & 27.57 (\(\pm\)0.76) b  & 660.12 (\(\pm\)10.34) b  \\  & RS 559-934 (360 nm) & 1000 & 60.11 (\(\pm\)3.34) c  & 25.64 (\(\pm\)0.69) c  & 672.97 (\(\pm\)11.21) c  \\ \hline  & None (control) & 0 & 87.25 (\(\pm\)3.63) a  & 28.89 (\(\pm\)0.81) a  & 653.10 (\(\pm\)8.72) a  \\ PP92/TP8 & RS 559-934 (360 nm) & 500 & 72.13 (\(\pm\)3.38) b  & 26.94 (\(\pm\)0.78) b  & 660.82 (\(\pm\)9.94) b  \\  & RS 559-934 (360 nm) & 1000 & 44.79 (\(\pm\)2.92) c  & 22.31 (\(\pm\)0.73) c  & 672.33 (\(\pm\)11.06) c  \\ \hline  & & & & & & \\ \end{tabular}
\end{table} 
Table 4: The effect of UV exposure on the tensile strength of PP98/TP2 and PP92/TP8 post 0, 500, and 1000 h.  
These changes are consistent with chain scission [84] and surface embrittlement [85] induced by prolonged UV exposure. The observed increase in Young's modulus across both formulations suggests progressive stiffening, likely due to oxidative crosslinking and crystalline rearrangement [86]. A shift toward more brittle behavior is indicated by the mechanical reaction, especially in the pigment-rich PP92/TP8 formulation, which demonstrated larger strain and stress reductions [87]. The necessity for efficient stabilizing techniques to maintain mechanical integrity in thermochromic systems meant for light-exposed applications is highlighted by these findings. These trends are consistent with the molecular mechanisms of photo-oxidative degradation in polypropylene systems. Free radicals form predominantly at tertiary carbon sites upon UVA exposure, initiating chain scission, reducing molecular weight, and disrupting the polymer matrix's load-bearing continuity. This manifests as decreased tensile strength and elongation at break. Additional covalent connections between polymer chains may be formed concurrently by oxidative crosslinking, especially at the material surface. This causes local stiffening, which accounts for the observed rise in Young's modulus following extended exposure. Such a shift from ductile to brittle behavior has been well-documented in UV-aged polyolefins [88; 89; 90], and is accelerated in pigment-rich systems such as PP92/TP8, where pigment-matrix interactions can catalyze radical formation and propagation. These mechanisms support the mechanical performance trends observed in this study and reinforce the importance of UV stabilizer integration in thermochromic packaging applications.