The antioxidant role of α‐tocopherol in polymers. I. The nature of transformation products of α‐tocopherol formed during melt processing of LDPE

Abstract

SYNOPSISThe antioxidant role of a-tocopherol (vitamin E) in low-density polyethylene (LDPE), and the nature of its transformation products formed during extrusion of the polymer are investigated. The melt stabilizing effectiveness of a-tocopherol was found to be very high, higher than that of the commercial hindered phenol antioxidants, Irganox 1076 and 1010, after single and multiple extrusion. The high antioxidant activity of a-tocopherol as a melt stabilizer is due, at least in part, to its transformation products. The importance of the processing history and the parent antioxidant concentration on the transformation products is discussed. Transformation products of a-tocopherol were analyzed after each of the four extrusion passes of the polymer. These were fractionated, analyzed, and characterized by HPLC and spectroscopy, respectively. The main products formed are diastereoisomers of dimers and trimers, as well as aldehydes; the relative concentration of each was shown to depend on processing severity (number of extrusion passes) and the initial concentration of a-tocopherol. The dihydroxydimer was found to be formed at a high concentration relative to the other products and proportional with the initial concentration of tocopherol. Based on both the identity and distribution of transformation products, a mechanism is proposed for the melt stabilization effect of a-tocopherol in LDPE.

Antioxidant effectiveness of vitamin E in HDPE and tetradecane at 32°C

“…Additives and their conversion products can be studied directly in situ, without recourse to extraction. Extraction is often non-quantitative (for example, if grafting has occurred) and can cause changes in additives and products during the extraction process [6][7][8][9]. There is one major drawback to identification and quantification of changes in a semi-crystalline polymer from small bands by spectral subtraction.…”

“…Vitamin E has been used as an efficient melt flow stabiliser in [4,7,22]. Also it has shown potential for the reduction of off-flavours related to polymer oxidation during processing [22].…”

“…However, these analyses are often complex. Also extraction may be far from quantitative, insensitive to products that become grafted to the polymer and may lead to changes in the additive or its conversion products as a result of extraction conditions [6][7][8][9]. Direct analyses on the solid polymer by spectroscopic techniques such as Fourier transform infrared (FTIR), coupled with spectral subtraction, and electron paramagnetic resonance (EPR) can overcome these drawbacks [1,9,10].…”

Natural antioxidants as stabilizers for polymers

“…The authors [99] stabilized UHMWPE against γ-radiation also with rosemary extract and pointed out that 5000 ppm of the extract ensures the long term stability of the product. Cerruti [55] claimed that the flavonoid content of red grape seeds resulted in a more efficient thermo- [12]. The natural additive efficiently hindered the decrease of the melt flow rate of polyethylene during multiple extrusions even at low additive concentrations.…”

“…The beneficial effect of these compounds is well known, the risk of the formation of harmful byproducts is much smaller during their use than in the case of synthetic phenolic antioxidants. The application of natural antioxidants for the protection of polymers is in its infancy, the only compound used routinely in practice is α-tocopherol [12]. The substance hinder the degradation reactions of polyethylene efficiently and in spite of its high price, it is used for the stabilization of medical devices made from ultra-high molecular weight polyethylene (UHMWPE) [13].…”