Stability, Safety Aspects, and Environmental Impacts

Siloxane polymers possess a number of properties that seem almost contradictory. One example is the tremendous stability and durability shown by polysiloxanes in a wide variety of applications. One reason for this stability is the fact that the chain is already in a high oxidation state, and reduction takes place only at very high temperatures. Most scientists 

Examples of polysiloxane degradation studies include the role of surfactants in suppressing aging of silica-PDMS gels, the effects of pigments on the stability of montmorillonite-PDMS composites, and the use of NMR and mass spectrometry to characterize degradation processes.  

Degradation can occur in water, in air, and particularly in the soiP when polymers come into contact with one or more reactive species, such as nitrate ion present in natural waterways. Nitrate is a source of atomic oxygen and, from it, hydroxyl radicals, which initiate the degradation process. Another reagent is ozone, split by UV (ultraviolet) light into oxygen atoms, followed again by the production of hydroxyl radicals. 

Even when methyl radicals are replaced by silanol units, the surface of the material does not remain hydrophilic (water-wettable) very long. Either the silanol groups condense with other silanol units to restore the siloxane structure or unmodified chain segments migrate to the surface. In any case, a self-repair mechanisms underlies the recoverability of siloxane surfaces. 

Clay minerals present in many soils have high interfacial areas with strongly acidic groups on their surfaces. These materials can react with siloxane chains and reorganize them into much smaller molecules. In fact, water readily reacts with the Si-0 bond in the presence of catalytic amounts of either acids or bases. Some of these small molecules are volatile enough to evaporate into the atmosphere. Others become capped with silanol (-SiOH) groups that frequently makes them water-soluble, and 

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