Synthesis of functional polypropylene containing hindered phenol stabilizers and applications in metallized polymer film capacitors

This paper discusses a new family of polymeric stabilizers (antioxidants) containing a polypropylene backbone and several pendant hindered phenol groups (PP-HP), with both random and tapered copolymer microstructures. They are effectively prepared by Steglich esterification of the corresponding hydroxylated polypropylene (PP-OH) copolymers with the hindered phenol molecule containing a carboxylic acid moiety. Some resulting PP-HP copolymers show significantly higher thermal-oxidative stability than the pristine PP polymer and the commercial PP products that contain a small amount of organic hindered phenol stabilizers. The better oxidative stability was also observed in PP/PP-HP blends under elevated temperatures. Overall, the PP chain degradation temperature increases with the increase of HP concentration in the matrix. The effectiveness of the PP-HP stabilizer is attributed to its comparability and cocrystallization with the PP homopolymer, which result in a uniform distribution of hindered phenol moieties. It also allows the relatively higher concentration of hindered phenol (polar) moieties in the PP (nonpolar) matrix without phase separation. In addition to the known advantages of polymer-bonded stabilizers, with low mobility and volatility to prevent loss through diffusion and/or extraction (particularly acute in films and coatings), the PP-HP thin dielectric films with uniform morphology show a higher dielectric constant and maintain low dielectric loss, particularly for the tapered PP-HP copolymers with high crystallinity.