Hydrolysis of Poly(dimethylsiloxanes) on Clay Minerals As Influenced by Exchangeable Cations and Moisture

Silicone polymers undergo clay-catalyzed hydrolytic degradation in soil. This study investigated the effects of moisture levels and exchangeable cations on degradation of a poly(dimethylsiloxane) (PDMS) fluid on clay minerals. Kaolinite, talc, and Arizona montmorillonite saturated with Na +, Ca 2+, or Al 3+ were incubated with 14 C-labeled PDMS (∼2000 μg g - 1 ) at 32% and 100% RH. The hydrolytic products were extracted sequentially and analyzed with HPLC/GPC, GC/MS, and LSC. It was found that PDMS hydrolyzed predominantly through random scission of its Si−O−Si backbone, regardless of clay type, exchangeable cation, or humidity. The hydrolytic degradation had two stages; both were zero-order reactions. The degradation rates in the initial stage rose with an increase in the polarizing power of the exchangeable cations (i.e., Al 3+ ≫ Ca 2+ > Na + ) and decreased humidity. Although high humidity may result in the formation of some volatile cyclic methylsiloxanes on an artificial catalyst, Al-saturated montmorillonite, the ultimate degradation product was otherwise water-soluble dimethylsilanediol. The conclusion was that exchangeable cation type, moisture level, and clay type can all influence the degradation rates and products of silicone polymers.