Efficiency of curcumin, a natural antioxidant, in the processing stabilization of PE: Concentration effects

Abstract

The stabilising efficiency of curcumin was studied in polyethylene during processing and under oxygen at high temperature. The effect of the natural antioxidant was investigated at concentrations of 0 to 1000 ppm in combination with a phosphonite secondary antioxidant (Sandostab P-EPQ) of 1000 and 2000 ppm, respectively. The polymer was homogenized with the additives then processed by six consecutive extrusions taking samples after each processing step. The samples were characterized by FT-IR spectroscopy, melt flow index, colour, and OIT measurements. Compared to the effect of pure phosphorous antioxidant, the melt stability of PE is increased already at 5 ppm curcumin content. The melt as well as the high temperature oxidative stability (OIT) of the polymer are controlled by both types of antioxidants. Curcumin hinders the oxidation of polyethylene and the formation of long chain branches during processing, which can be attributed to the fact that curcumin is not only a hydrogen donor but its unsaturated linear moiety can also scavenge alkyl and oxygen centred macroradicals. Curcumin discolours polyethylene already at small concentrations but the colour fades with increasing number of extrusions.Keywords: curcumin, natural antioxidant, polyethylene, stabilisation, phosphonite 3 IntroductionStabilisation of polyolefins with natural antioxidants got into the focus of attention recently because of the unknown effects of the reaction products of synthetic phenolic antioxidants on human health [1]. Health safety has vital importance in many application areas, like food contacting objects (e.g., packaging materials, water pipes), medical applications, toys, etc. The small molecular mass additives used in polyolefins for stabilisation, colouring, or antiblocking are generally polar compounds, therefore their solubility is small and migrate onto the surface of the polymer during application [2]. Their dissolution into contacting substances cannot be avoided but any harmful effect must be prevented.Among the natural antioxidants, first α-tocopherol was studied extensively for the stabilisation of polyolefins [e.g., 3-9]. Intensive research resulted in the application of α-tocopherol for the stabilisation of ultra high molecular mass polyethylene (UHMWPE) used as total joint implant [e.g., 10-15]. Even an ASTM standard specification was implemented for medical grade UHMWPE blended with vitamin E [16]. The study of different natural antioxidants in polymers has been widely extended lately. The goals of the investigations are mainly the improvement of the safety of packaging materials and the development of functional packaging [e.g., 17-25].Curcumin, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, is the principal curcuminoid of Curcuma longa rhizomes (turmeric). The powdered root is used as a spice, food colorant, and food preservative. The effects and reactions of curcumin have been the subject of investigation in the fields of biology, medicine, pharmacology, and in the food industry yielding a la...

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