Aldehydes are known to be present in PET preforms and PET bottles. However, the analysis of these aldehydes is difficult because of the high crystallinity of PET and because of its insolubility in common solvents. Titration of acetaldehyde was investigated comparatively by four different methods: by the solid/gas headspace technique (s/g HS), accepted industrially as the standard method, by colorimetry, based on complexation with pararosaniline dye, by colorimetry, based on derivatization as 2,4-dinitrophenyhydrazones, and by headspace liquid/gas chromatography ( l / g HS) . Similar results were achieved by the last three methods. In contrast, the standard solid/gas headspace chromatography yielded data that was one order of magnitude smaller. The presence of formaldehyde in PET preforms was detected by liquidlgas headspace chromatography only. 0 1994 John Wiley & Sons, Inc.
I NTRO DUCT10 NIn 1954, the US. Food and Drug Administration approved poly (ethylene terephthalate) , also known as PET, for applications in the food industries. In 1973, this approval was extended to food packaging, and PET is now used worldwide for the bottling of many drinks. However, various volatile compounds are formed during the molding of PET bottles by the injection-blowing technique; acetaldehyde amounts for 80%. The presence of residual acetaldehyde appears as a critical shortcoming and restrains the use of PET bottles to flavored carbonated drinks. The unpleasant taste produced by the released acetaldehyde affects dramatically the organoleptic quality of unflavored mineral water and precludes the use of PET bottles, despite their attractive appearance. Regulations limit at 5 ppm the acceptable AA content in PET-injected preforms that are used to blow bottles. An assay of AA, present in PET preforms, is standardized. The approved method is based on the use of solid/gas headspace chromatography (s/g HSGC), applied to fragments ~~ * To whom correspondence should be addressed. of preforms obtained by grinding at low temperature, in order to reduce the evaporation of volatile compounds and to avoid secondary reactions, such as oxidation. Typically, the resulting powdered PET is sieved and heated in a closed vial, equipped with a rubber septum. The entrapped gas above the solid phase is finally injected in the gas chromatograph via a headspace acce~sory.~*~ Calibration is done with AA aqueous solutions of known concentrations. Suppression of preliminary thermal treatment has been proposed to improve the procedure' and to prevent extra degradation during the preliminary thermal treatment. Despite this improvement, data produced by the standard procedure are still viewed suspiciously, because of the absence of control of the evaporation of AA at the grinding and sieving stages and because of the possibility of uncomplete desorption of AA.From a general viewpoint, aqueous solutions of acetaldehyde can be titrated by using different methods, including spectrophotometric ones. Procedures have been proposed, which were based on the use of thi...
