Testing PET Preforms for Consumer Safety

Formaldehyde is a chemical widely used in the industrial production of materials like plastics, for the processing of fabrics or to preserve biological materials. However, formaldehyde is considered by many standards to be a health hazard. It can cause allergic reactions and was classified as carcinogenic in 2004. According to the German standard Oeko-Tex 100, baby clothes must not contain more than 20 PPM of this substance.

The beverage industry tests plastic bottles for formaldehyde as part of their quality control process. This is usually performed on the preforms for the beverage bottles. Before the actual analysis can be done, the formaldehyde must be extracted from a sample. To do this reproducibly, the sample first has to be comminuted. Because formaldehyde is a volatile substance, the mechanical energy input which is necessary for the grinding process can lead to its evaporation. To avoid this effect, cryogenic grinding is the method of choice.

The first step, however, is the short and gentle preliminary grinding of the preforms in a cutting mill, such as Retsch’s SM 100, with a bottom sieve of 6mm. The obtained granules are then pulverised in the Retsch CryoMill at – 196°C with liquid nitrogen. The cooling ensures that the sample does not warm up during grinding, thus preserving the formaldehyde. Moreover, the sample is embrittled which improves the milling process. The CryoMill quickly grinds the preforms down to a powder of 100 microns and less which can then be used for extraction of the formaldehyde. The actual spectroscopic analysis is carried out by colour reaction (for example, with MBTH or Schiff test). Retsch crushers and mills are ideal tools for representative and reproducible size reduction and homogenisation of solids in the context of quality control.