Antioxidants their analysis in plastics

Polymer degradation can result in discoloration and other associated visual defects (such as gels and black specks), inconsistent flow characteristics and a general deterioration in physical properties. Molecular weight can increase as a result of crosslinking reactions or decrease as a result of chain scission reactions. 

Antioxidants function by preferentially reacting with the radical intermediates, thereby protecting the polymer and extending its usage life. There are two types of antioxidants that are typically used in commercial polymer stabilization primary and secondary. The majority of primary antioxidants are either hindered phenolics or secondary aryl-amines. Both hindered phenolics and aryl-amines have one or more reactive OH and NH groups. The hydrogen atoms which are liberated from the alcohol or amine groups readily react with free radicals to form stable species. Secondary antioxidants are usually phosphites or thioesters. 

They tend to react preferentially, preventing the regeneration of new free radicals from the decomposition of the hydroperoxides. Primary and secondary antioxidants are often used together because they exhibit a synergism which provides an effective mechanism for polymer stabilization. 

Characterization of chemical additives such as antioxidants in plastics is often described as a daunting and challenging task. The need for complete compositional analysis of additive packages in industrial plastics for both research and quality control applications has led to the development of numerous analyte-specific test procedures in recent years. The methodology employed in these analyses must overcome many obstacles the relative instability and high reactivity of many types of additives, the residence of the additives in what is essentially an insoluble polymer material, and the relatively low concentration of these additives in the polymer matrix [1]. In addition, the analysis technique used must be specific and should not be susceptible to interference from other additives which may also be present in the polymer. 

Analytical techniques for the quantitative determination of antioxidants in plastics generally fall into two classes indirect (or destructive) and direct (or non-destructive). Indirect or destructive methods, as the name implies, require a significant alteration to the sample so that the antioxidant can be removed from the plastic material for subsequent detection. Direct or non-destructive methods, on the other hand, involve the direct examination of the plastic sample with minimal sample preparation. 

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Antioxidants for poly(ethylene terephthalate) Antioxidants their analysis in plastics ... 

Chromatographic Analysis of Antioxidants in Polymeric Materials and Their Migration from Plastics into Solution... 

Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

A simpler, lower cost technique which can readily separate the antioxidants from plastic extracts and give a qualitative analysis is thin layer chromatography (BS6630, 1985). In thin layer chromatography (TLC), the stationary phase is comprised of a thin layer of adsorbent such as cellulose, alumina, or silica gel on a plastic sheet, thick aluminium foil, or a glass plate. A small spot of solution containing the sample is applied to a plate, about 1 cm from the base. The plate is then placed in a sealed container which holds a suitable solvent, such as ethanol, so that it does not come into contact with the spots. The solvent moves up the plate by capillary action and meets the sample mixture, which is dissolved and is carried up the plate by the solvent. Components in the sample mixture travel at different rates due to differences in solubility in the solvent, and due to differences in their attraction to the plate. 

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