Natural antioxidants as stabilizers for polymers

The additive packages routinely used today usually consist of a phenolic antioxidant and a phosphorus or sulfur containing secondary stabilizer. Several years ago some questions were raised about the health and environmental hazard of these additives and industry has not offered an alternative solution yet. Nature produces a large number of antioxidants, which play a key role in radical reactions taking place in the human body. The substances containing these antioxidants are used in natural medicine for ages and they are applied in increasing quantities also in the food industry. The application of natural antioxidants for the protection of polymers is in its infancy, the information available is limited and often contradictory. This review paper summarizes published results, analyzes them and points out the advantages and drawbacks of the approach. Although a wide variety of compounds have been added to polymers to improve their stability, the most promising candidates are the carotenoids, the flavonoids, other natural phenols and phenolic polymers including lignin. Available results indicate that flavonoids are much more efficient stabilizers than the hindered phenols used in industrial practice. On the other hand, most of the natural antioxidants discolor the polymer and their solubility is limited. Nevertheless, natural antioxidants can be efficiently used in specific applications, but further research is needed to explore all their advantages and include them into additive packages used in practice.