LDPE, additives Volatiles

Certain preservative systems can be lost from certain plastic packs by sorption and solubility in the plastic and, if they are also volatile, by evaporation from the external surface which is in contact with the atmosphere. Reports, particularly as references in textbooks, often suggested that phenol could not be used as a preservative in lowdensity packs. Subsequent work, many years later, established that the solubility of phenol in polyethylene was relatively low and that sufficient phenol for adequate preservative efficacy could be retained by a number of methods, i.e. coating LDPE internally or externally, or enclosing the pack in an additional barrier overwrap (e.g. paper/foil/PE or similar foil-bearing barriers). 

In packaging, three resins account for the majority of the market for antioxidants PP, PE, and HIPS. For PP, a combination of hindered phenol and phosphite antioxidants is commonly used, with the total concentration normally from 0.08 to 1%, depending on formulation and end use. Clba Specialty Chemicals has developed the phenolic antioxidant family of Irganox for use in PP and PE, and also the Irgafos family, which are phosphite stabilizers used in combination with phenolic antioxidants. For LDPE, BHT, a phenolic antioxidant, is normally incorporated at levels of 50 to 500 ppm however there is a tendency to employ less volatile additives to prevent their migration from the resin. For HDPE and LLDPE, antioxidants less volatile than BHT, such as polyphenols, at higher concentrations, are normally used In combination with phosphites. For HIPS, hindered phenols are used in combination with UV absorbers. Alpha-tocopherol (Vitamin E) is sometimes used as an antioxidant for polyolefins. 

Machalkova [643] has described analysis of polymer composites and rubber blends with emphasis on separation of low-MW additives by instrumental methods. Examples refer to analysis of inorganic filler- or synthetic fibre-reinforced plastics and laminated plastic Aims using PyGC and IR. The versatility of PyGC has further been exemplified by Jones [633] as a thermovolatilisation technique for direct determination of occluded volatiles and low-MW additives in lube oil, novolac resins and HDPE, of plasticisers and vinylchloride in PVC, and of solvent residues in paints and bitumens, etc. Dicumylperoxide (DCP) in LDPE was identified through detection of three main by-products of reaction, acetophenone, a-methylstyrene and 2-phenylpropan-2-ol [633]. 

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