Effect of UV
A block plot of the modulus ratio targeting UV is shown in Fig. 12(a). An examination of the arrangement of UV (“1") and no UV (“0") within each block shows that UV radiation is always located at the bottom of each bar, with the modulus ratios below unity. The block plot of the mean modulus ratio [Fig. 12(b)] indicates that 27 of 30 different robustness factor settings show that UV is an important factor in the modulus reduction. Based on binomial considerations, the probability of getting at least 27 of the 30 total settings by chance is 4 x 10~4 %, showing that the role of UV in modulus decrease is statistically significant. In addition, the importance of UV in modulus reduction is further attested to by the consistently large block heights across the various combinations of robustness factors [Figs. 9(d), 12(a), and 12(b)] in that most modulus data for UV exposure are below unity and located at the bottom left corner of the plot. Thus, it is concluded that UV is statistically important across the various robustness factor settings.
The effect of UV in decreasing modulus may be understood based on the fact that UV photons have sufficient energy to rupture the polymeric bonds present in the sealant. Because the sealant chemistry is unknown, it is impossible to propose the exact mode of chain scission. However, it is believed that UV photons are absorbed by chromophores, which are introduced into the polymer backbone during manufacturing in aliphatic type and aromatic type polymers. The chemical structures of the latter contain chromophores capable of absorbing the UV photons, which split the covalent bonds to produce free radicals. The excited chromophores then dissipate the photon energy via chain scission, leading to the rupturing of molecular bonds. The low molecular weight fragments produced by scission reactions directly correlate with the decrease in modulus.