Silicone Polymers in Nature

Silicone Polymers Environmental Information – Health Environment & Regulatory Affairs (HERA)

Many consumer products containing silicone polymers, or polydimethylsiloxane (PDMS), are used in a fashion, which allows them to enter municipal wastewater treatment plants. Because PDMS is so insoluble in water, it partitions onto the sludge, causing no adverse effect on the operations of the treatment plant [1].

The sludge is then either destroyed by incineration, entombed in a landfill, or spread out on golf courses, woodlands, and agricultural fields as a fertilizer. This latter disposal technique allows PDMS to enter the soil environment.

In soil, the PDMS polymer can hydrolyze to small, water-soluble siloxanols [2, 3, 4], with the ultimate product being the monomeric dimethylsilanediol (DMSD) [4, 5]. This hydrolysis is probably abiotic, because it can take months to years in wet soil, but only days as the soil dries [3, 6].

The phenomenon has been documented in a wide range of soils throughout the U.S. [7] and in 12 common soil minerals [8], meaning that the catalyst is widespread in nature. Although these experiments were done with pure PDMS, the incorporation of PDMS into sludge does not prevent the hydrolysis.

It does, however, make the process more gradual [9], possibly because the PDMS must first diffuse out from the sludge before it can contact the soil surfaces and begin hydrolyzing. If the sludge is first composted, PDMS will remain intact with no effect on the composting process [10], and will then degrade after the compost is mixed in with soil.

Silicone Polymers Silicone in Nature

The hydrolysis product, DMSD, can microbially degrade to CO2 [5, 11, 12] and inorganic silicate [13], the latter of which should merge with the silicate already present in the soil [14]. The production of CO2 from DMSD degradation varied from 0.4-1.6% per week [11].

In addition, DMSD volatilizes from soil at about 1-7% per week [15], with the higher losses occurring from sandy soils. These loss mechanisms suggest that DMSD will not persist in the soil environment. For example, only small amounts of DMSD were found in soils following the hydrolysis of sludge-applied PDMS [9], while an extensive program of field monitoring has found less DMSD than expected in sludge-amended soils showing loss of PDMS [16].

Once in the atmosphere, DMSD is expected to degrade by sunlight-induced reactions, much like other volatile silicones [17, 18, 19]. If it is instead washed out of the air in rainfall, DMSD can be oxidized in water by a similar sunlight-induced reaction [20], or it can be microbially oxidized in soil [5].

Downward movement through the soil profile is not expected because it was not observed in agricultural microcosms during PDMS degradation [9]. Moreover, neither PDMS nor its degradation products harmed soil microorganisms or affected the growth of wheat and soybeans [21].

The above concepts were tested in field plots sprayed with PDMS emulsions [22]. Extensive losses of PDMS (half-lives of 1-2 months), coupled with dramatic decreases in molecular weight, were observed during a typical Michigan summer season.

Only small amounts of DMSD (corresponding to <5% of the original silicone) were found, and deeper sampling revealed that the DMSD had not simply moved downward in the soil profile. This result is thus consistent with laboratory studies showing polymer hydrolysis followed by biodegradation and/ or volatilization of the monomer to natural components (CO2 and inorganic.

The overall reaction of Silicone Polymers

Silicone Polymers in Nature
An apparent contradiction to the degradation of silicone polymers in nature is that these polymers are used for many outdoor applications because of their stability to high temperatures and their resistance to UV and O3 exposure. This stability during the polymer’s intended use is a bulk phenomenon.

However, when PDMS is disposed down-the-drain and is eventually applied to the soil as a component of sludge, it becomes dispersed at low concentrations on soil minerals.

This allows the PDMS to contact the catalysts needed to begin its depolymerization, which eventually results in its complete conversion to natural components. The conclusions in this fact sheet are being further examined in an ongoing program of laboratory and field research.

Silicone Nature Silicone Polymers in Nature

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