Problem: Polynuclear aromatic hydrocarbons, also known as PAH, are used as plasticizers in many caoutchouc products, such as the rubber grips of tools. Some PAH have a proven carcinogenic effect on humans; this is why there are critical limits for the maximum concentration allowed in consumer products. Phthalates are used as plasticizers in plastics as well, in products such as PVC. Recent tests (by the German consumer magazine Test, issue 09/2009, p. 72ff) have shown that these harmful substances occur in such sensitive products as children’s strollers. Not only PAH was found, but also chlorinated paraffin and flame retardants, which are considered harmful to health.
The analysis of the above-mentioned compounds is usually done with chromatographic methods (HPLC-MS, GC-MS). It is first necessary to extract the compounds in question from the test material (i.e., by solvent extraction). To facilitate the extraction of a small but representative sample, the material must be ground to a particle size of approximately 0.5 mm. Because the material is very elastic and can be fibrous and heat-sensitive, this presents an added challenge in the size-reduction process. Moreover, it must be ensured that volatile components of the sample are not expelled by heat generated during the grinding process. Therefore, it is recommended to process the sample in two steps.
Solution: The best way to prepare such materials for analysis can be demonstrated with the rain cover of a stroller. It is made primarily of PVC, which—partly due to the use of plasticizers such as phthalates—is highly elastic and heatsensitive
First, the entire rain cover is processed quickly and gently using the RETSCH Cutting Mill SM 300 and a 6.0 mm bottom sieve. The mill features a powerful 3 kW drive with very high torque and can process the plastic cover to <5 mm within one minute. The variable speed, which can be set between 700 and 3,000 rpm, can greatly reduce the unfavorable temperature increase that could lead to evaporation of volatile components. Thanks to the fold-back housing, pushfit rotor and smooth interior surfaces, the mill can be cleaned quickly and easily.
Liquid nitrogen must be used as a grinding aid to reduce the elastic material to a fineness of <0.5 mm. Liquid nitrogen helps to embrittle the plastic, improving its breaking behavior. In addition, LN2 cools the sample material to ensure that the volatiles don’t escape. The CryoMill is the ideal instrument for this application. This mill features an integrated cooling system that continually cools the grinding jar before and during grinding. The CryoMill is particularly efficient and safe: the liquid nitrogen is replenished from an autofill system in the exact amount required to keep the grinding chamber at -196°C.
After representative sample division, 2 g of precut rain cover are put into a 50 ml stainless steel grinding jar along with a 25 mm grinding ball. After only four minutes of grinding with the CryoMill, 87 percent of the sample has a fineness of <500 microns. The grinding is done in intervals: two grinding cycles of two minutes each, interrupted by a one-minute cooling cycle. The obtained sample can then be subjected to extraction for the subsequent chromatographic separation.
For more information, go to www.retsch.com.