Degradation of polydimethylsiloxane fluids in the environment--a review

Due to their insolubility in water and high adsorption coefficient, liquid polydimethylsiloxanes (PDMS) discharged as effluent will adsorb to particulate matter and, therefore, will become a component of sewage sludge during waste water treatment. The subsequent environmental fate of PDMS will depend on the fate of the sludge. Due to increasing practices of soil amendment with sewage sludge the principal environmental compartment receiving PDMS fluids is the soil. Degradation of PDMS is a common process taking place in many different types of soils. It occurs through a unique combination of environmental degradation processes. Initial hydrolysis of PDMS is catalysed by clay minerals, the principal component of soil. The primary hydrolysis product, dimethylsilanediol (DMSD), is then either biodegraded, or evaporated into the atmosphere, where it is subsequently oxidised in the presence of sunlight. The end products in both cases are expected to be CO2, SiO2 and H2O.     

Speciation and quantitation of degradation products of silicones (Silane/Siloxane Diols) by gas chromatography—mass spectrometry and stability of dimethylsilanediol

Gas chromatography-mass spectrometry (GC-MS) methodology was developed to speciate and quantitate several degradation products of polydimethylsiloxane (PDMS) in soil. We have demonstrated that the major degradation product,viz., dimethylsilanediol, can be readily analyzed by GC-MS without derivatization as commonly practiced in analyzing such materials. A mixture of linear siloxane diols (n = 1–5, wheren is the number of Me₂SiO units), and cyclic dimethylsiloxanes (n = 4–6) was resolved by GC-MS. We also found that peak identity of various diols required that GC-MS is done in the chemical ionization (CI) mode, since the electron impact (EI) ionization mode produced similar mass fragmentation patterns for diols and cyclics containing the same number of silicon atoms. For siloxane diols, detection limits ranged from 100 pg (forn = 1) to 1 ng (for n = 5). For cyclics, the detection limit was about 1 pg. Dimethylsilanediol, known to be unstable even in the solid state, was shown by NMR techniques to be stable in aqueous solution at <0.1% concentration. A 100-ppm solution was stable for over a year. Purity check for dimethylsilanediol is best carried out by Si-29 solid-state NMR technique.