Stabilization of γ-sterilized low-density polyethylene by synergistic mixtures of food-contact approval stabilizers

Int J Ind Chem DOI 10.1007/s40090-017-0117-0 RESEARCH Stabilization of c-sterilized low-density polyethylene by synergistic mixtures of food-contact approval stabilizers Sameh A. S. Alariqi1 • Niyazi A. S. Al-Areqi1 • Elyas Sadeq Alaghbari1 • R. P. Singh2 Received: 7 October 2015 / Accepted: 17 March 2017 Ó The Author(s) 2017. This article is an open access publication Abstract In our previous studies, we have found the synergistic combinations of stabilizers which follow different mechanisms of stabilization and are approved for food contact applications. The present attempt is to test the potentials of those systems in stabilizing c-sterilized lowdensity polyethylene (LDPE). The results were discussed by comparing the stabilizing efficiency of mixtures with and without phenol systems as well as with their counterparts of isotactic polypropylene (iPP) and ethylene-propylene copolymers (EP) matrices. LDPE has been melt-mixed with tertiary hindered amine stabilizer (tert-HAS), oligomeric HAS stabilizer, phenolic and organo-phosphite antioxidants and subjected to c-sterilization. Stabilization in terms of changes in oxidation products, tensile properties, yellowing and surface morphology was evaluated by FT-IR spectroscopy, Instron, colorimetry, and scanning electron microscopy (SEM), respectively. The results of the present study confirm the validity of those systems for protecting various polyolefins against c-sterilization. The results showed that the synergism, antagonism and the trend in stabilization efficiency of the binary, ternary and quaternary stabilizer systems were almost similar in LDPE, iPP and EP matrices. The binary system of oligomeric HAS and tert-HAS has shown the antagonistic effect of & Sameh A. S. Alariqi R. P. Singh 1 Department of Chemistry, Faculty of Applied Science, University of Taiz, P. O. Box: 4007, Taiz, Yemen 2 Division of Polymer Science and Engineering, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India stabilization, whereas their combination with organophosphite has exhibited synergistic effect even at higher doses of c-sterilization. The combination of oligomeric HAS, tert-HAS, organo-phosphite and hindered phenol exhibited improved stabilization efficiency than single or binary additive systems. The phenol systems have shown long term of stability than that of phenol-free systems. It was found that the consumption of oligomeric stabilizer significantly depends on the components of stabilization mixture. It was concluded that the stability of polyolefins (LDPE, iPP and EP) against c-sterilization can be achieved by blends of different stabilizers which are approved for food contact applications. Keywords LDPE  c-Sterilization  Stabilization  Oligomeric HAS stabilizer  Synergistic mixtures Introduction LDPE is one of the most popular polymers in the manufacturing of food packaging and medical disposables, because it exhibits high transparency, good mechanical properties, low cost, good sealability and chemical resistance, and can be employed over a wide temperature range. Treatment with gamma radiation is becoming a common process for the sterilization of food packaging and medical plastics. The most commonly validated dose used for sterilization is 25 kGy [1]. However, using c-radiation for sterilization of packaging and medical plastics is known to result in physical changes, including embrittlement, stiffening, softening, discoloration, odor generation and a decrease in molecular weight [2–5]. The degradation of sterilized plastics continues for a long time during their shelf life and service, which is called post-degradation or 123 Int J Ind Chem post-sterilization. Radiation-induced changes in the physical properties of a packaging material and medical plastics should not impair its function and the degradation products as well as the utilized additives should be non-toxic [6]. The radiation stability of polyolefins can be done at different stages of degradation process by adding very small amounts of additives (0.05–0.5% w/w) called as ‘stabilizers’. They are radical scavengers, antioxidants and hydroperoxide decomposers which follow different action mechanism. Antioxidants are incorporated in the polymer formulation to inhibit the attack of oxygen during the processing and c-sterilization of the polymer [7]. Phenols, phosphites, or amine compounds are used as antioxidants depending on the free radicals expected to form. Phenolic antioxidants (Primary antioxidants) are generally radical scavengers or H-donors such as Irganox-1010. They are extremely effective at preserving physical properties of polymer during and after c-sterilization, but at the expense of yellow color formation [8]. Consequently, antioxidants such as hindered phenols are unacceptable medically and for the food packaging because of the intense yellow discoloration which results from the formation of compounds such as stibenequinones upon c-sterilization. Secondary antioxidants (organo-phosphites) are typically hydroperoxide decomposers (i.e. Irgafos-168) inhibiting oxidation by decomposing the hydroperoxides to form stable products. Unlike primary antioxidants, secondary antioxidants are inadequate if they used alone, so they are usually used in combination with primary antioxidants to get synergistic effects [9]. An organo-phosphite may be used as a shortterm antioxidant to protect the polymer during processing, while phenolic antioxidants are used for long-term protection. Hindered amine stabilizers (HAS) are widely used radical scavengers having multifunctional capabilities for scavenging radicals. A substituted piperidine was found to give good protection (little yellowing or embrittlement) against c-irradiation as well as post-irradiation storage under accelerated test conditions (60 °C in air) [10]. The efficiency of stabilizers is very much dependent upon the type and the grade of polymer in which they are compounded; thus the judicious selection of stabilizers is very important in the formulation of plastic [11]. These additives are not chemically bound to the polymer matrix and migrate or leach out under the influence of physicochemical factors such as temperature, sterilization and type of solvents and pH of the packaged product [12]. Unfortunately, the toxicological data on most of the stabilizers are either not available or incomplete and for many antioxidants are available from feeding studies only [7]. Polymer stabilization is a dynamic process resulting in many transformed and degradation products which are potentially leachable and extractable [12]. Many antioxidants and stabilizers act sacrificially and are converted to 123 oxidation products during the process of stabilization [13]. In fact, there is a little knowledge regarding the toxicity of antioxidant transformation products; thus there is a doubt that they may be more toxic than the antioxidants from which they are derived [13]. It is urged tha (...truncated)