Sample preparation of PET Preforms for analysis
Formaldehyde is a chemical compound widely used in the industry, and it is employed for the production of plastics, to improve the properties of textiles or for the conservation of biological material. But formaldehyde is of concern to the human health. For example, it may trigger allergies, and since 2004 it is rated as carcinogenic for humans. The maximum allowed content for baby garments is 20 ppm, according to the Öko-Tex-Standard 100. Also, the plastic bottles from the beverage industry are tested for their formaldehyde content. For this analysis the preforms are taken from one of the preliminary steps during the fabrication of the bottles. The determination is only possible after the formaldehyde has been extracted from the sample. In order to perform this step reproducibly, it is necessary to shred the sample, and take special precautions that formaldehyde, being a highly volatile substance, does not evaporate. Grinding requires mechanical energy, which may conduct to a rise in temperature, and therefore to a possible evaporation of formaldehyde. In order to avoid this possible interference, cryo-grinding is the method of choice. In the first step, the preforms are cautiously chopped in the RETSCH cutting mill SM 100 with a bottom sieve of 6 mm. The granulate obtained by this method is then fed into a RETSCH CryoMill at -196 °C, being cooled with liquid nitrogen, and ground very finely within a few minutes. This procedure guarantees, that during the grinding process the sample will not be heated, avoiding any possible escape of formaldehyde. Also, due to the cooling, the material is embrittled, and the fracturing characteristics are considerably improved. After a very short period of cryo-grinding the sample is reduced to a powder with a grain size of less than 100 µm, from which the formaldehyde may be extracted. The determination is then performed spectroscopically by a color reaction e.g. with MBTH or by the Schiff Test.