Polypropylene Copolymer Containing Cross-Linkable Antioxidant Moieties with Long-Term Stability under Elevated Temperature Conditions

Despite its commercial success, isotactic polypropylene (PP) is not suitable for the applications that require long-term exposure to high-energy conditions, such as elevated temperatures, UV radiation, or high electric fields, due to the combination of polymer chain oxidative degradation, incompatibility with polar additives (antioxidants, stabilizers, etc.), and low material softening temperature. This paper presents a new solution that can simultaneously address both chemical and physical limitations. The idea is to develop a new PP-HP copolymer that contains some specific hindered phenol (HP) groups, homogeneously distributed along the polymer chain. These PP-bound HP pendant groups can not only effectively protect PP chains from the oxidative degradation but also engage in a facile cross-linking reaction to form a 3-D network structure during the oxidation reaction. One accelerated oxidation test in air at 190-210 °C shows this distinctive advantage. While a commercial PP polymer (containing common antioxidants and stabilizers) degrades within 1 h, a PP-HP copolymer with about 1 mol % (9 wt %) HP groups shows almost no detectable weight loss after 1000 min. In an ASTM endurance test under a targeted application temperature (140 °C in air), the commercial PP shows 1% weight loss within about 10 days. On the other hand, this new PP-HP lasts for 10⁵ days (4 order increase) under the same condition. In the strain-stress curve measurement, the PP-HP film also shows no detectable change in tensile strength and modulus after constant heating the polymer film at 140 °C in air for 1 week. Overall, the experiment results present the potential of expanding PP applications into a much higher temperature range (>140 °C) under oxygen oxidative environments.