Environmental durability of PET and PET/PEN blends for outdoor applications

The photodegradation of poly(ethylene terephthalate)(PET), poly(ethylene 2,6-naphthalene dicarboxylate)(PEN) and their blends have been investigated in terms of different weathering factors such as UV irradiation time and condensation. PET/PEN blends were made with a twin-screw extruder. Samples for study were then made with an injection molding machine or compression molded. Transesterificaiton reactions occur during the blending of PET and PEN. A critical value of degree of randomness has to be achieved to make sure that the properties of materials are not a function of the level of transesterification, but a function of the composition of blends. Degree of randomness was examined with nuclear magnetic resonance. Mechanical properties of PET and PEN/PET blends have been evaluated before and after UV irradiation. The results show that after exposure for equivalent UV irradiation times, blends containing PEN exhibit better mechanical properties than those of pure PET. Fourier transform infrared spectroscopy and UV absorption spectroscopy were used to study the mechanisms of photodegradation of these materials. Changes in carboxyl endgroup –COOH generation of PET, PEN and the blends, as a result of progressive UV irradiation time, were examined since carboxylic acid endgroups accompany molecular weight drop and chain-scission. After equivalent exposure conditions, PET shows more carboxyl acid endgroup generation than PEN. Blending PEN with PET produces less carboxylic acid endgroup generation than that of PET. An end-group analysis technique using FTIR has been used for end-group determination in PET, PEN and blends in the solid states. With this technique both –COOH, –OH endgroups were determined. The number-average molecular weight, Mn, was then calculated from the concentration of these two endgroups. The number of chain-scission was also determined. The results of calculations demonstrate that the extent of the photodegradation of PEN is much less than that of PET and blending with PEN improves the UV barrier properties of PET significantly. Photolysis in vacuum was conducted to understand the role of oxygen in the photodegradation reaction. The results suggest that for PET, the presence of the oxygen has inhibited the crosslinking reactions to form a bi-phenyl structure which accounts for the yellowing of PET. PEN tends to form bi-naphthalene structures causing yellowness to occur as a result of exposure to UV in air environment. The mechanisms of yellowing and carboxyl endgroups generation are discussed. Studies of the depth of photodegradation of these materials have shown that the “screen effect” of PEN is much better than that of PET and that blending PEN with PET at concentration as low as 5% will decrease the depth of photodegradation of PET considerably.