Recycle Silicone

Recycle Silicone

1. Introduction

Most polydimethylsiloxane fluids are non-volatile polymeric organosilicon materials consisting of

[- (CH3)2 – SiO- ]n
structural units.

Various PDMS fluids ranging from low to high viscosity are used in a wide range of industrial applications, such as manufacturing textiles, paper and leather goods. Due to this wide range of applications, they can enter the environment in a variety of ways.

2. Applications

PDMS fluids are highly-efficient process aids, able to provide desirable properties at very low concentrations. They often serve as antifoams, softeners, or water repellents. In consumer applications, PDMS fluids can be found in personal-, household- and automotive care products.[1] They are used as conditioners in hair care, softeners in skin care products, additives in polish formulations, water proofers as well as components of other surface treatments. Some PDMS materials are also used as end-products (usually in the industrial market), such as transformer dielectric fluids and heat transfer liquids.

3. Environment and recycling

Since they are non-volatile, PDMS fluids do not evaporate into the atmosphere. Very small amounts of PDMS fluids, which are contained in household products, may be washed from the surfaces to which they have been applied and eventually enter into the soil or a wastewater treatment plant .By instance, personal care products such as shampoos and conditioners are rinsed away after use and consequently the PDMS they contain is carried with wastewater to the treatment site. This treatment site could be a private municipal plant or a septic system . When PDMS fluids are used in industrial applications such as process aids or surface treatments, small quantities can also be found in process wastewater that is carried to the treatment plant. End-use industrial products such as transformer fluids are used in contained applications. These transformer oils are suitable for recycling and are therefore unlikely to enter the environment, except in cases of accidental release.

The fate of PDMS is partly a function of where it enters the environment. A number of studies have shown that PDMS will degrade into lower molecular weight compounds, primarily Me2Si(OH)2 , when in contact with soils [2]. Testing under a variety of representative conditions has confirmed this observation in a wide range of different soils, indicating that the phenomenon is widespread in nature. After only few weeks of soil contact, it has been noted a significant degradation to lower molecular weight compounds. The extent of degradation and the actual rate vary as a function of soil moisture content and clay type. These lower molecular weight degradation products have been shown to further oxidize in the environment, both biologically [3] and abiotically[4] to form naturally-occurring substances : silica, carbon dioxide and water. No effects from PDMS (or its degradation products) have been observed on seed germination, plant growth or plant survival as well as on the plant biomass. Additionally, research has shown no adverse effects from PDMS on terrestrial life forms such as insects or birds, even under highly exaggerated conditions of exposure. Research includes studies on survivability and growth.

Consequently, PDMS fluids pose no known hazard to the environment and they are not classified as hazardous wastes. If PDMS fluids should enter the aquatic environment, they attach to particulate matter and are removed from the water column by the natural cleansing process of sedimentation. PDMS fluids do not partition back into the water column and have no detectable Biological Oxygen Demand (BOD).

Bio-concentration does not represent a significant concern with PDMS. Their molecular size renders them too large to pass through biological membranes in fish or other organisms. Specific testing has shown that PDMS is not toxic and does not bio-accumulate in sediment dwelling organisms or various terrestrial species, including earthworms. In wastewater: Household (on-site) septic systems and municipal treatment plants are both designed to facilitate the natural degradation of waste by microscopic organisms. Biomass (or “sludge”) is generated by this natural degradation, and must eventually be discarded. In a municipal system, treated sludge is typically incinerated, landfilled or used as fertilizer. In the United States, where on-site septic systems are common, the tank is usually pumped out periodically and the biomass is taken to a waste water treatment plant. PDMS fluids from personal care and household products enter these treatment systems as tiny dispersed droplets in wastewater. Because the water solubility of these silicone fluids is essentially nil, they attach to suspended materials in wastewater systems and become a minor part of the sludge. Wastewater treatment monitoring and simulation studies have confirmed that PDMS fluids which enter treatment facilities will be almost completely absent from the treated effluent. PDMS does not inhibit the microbial activity by which wastewater is treated. Test levels, far exceeding those expected in the environment, have shown no effect on the activated sludge process, other than the expected benefits of foam control. PDMS loadings had no effect on the operating parameters (pH, solids, sludge volume index and specific oxygen uptake) or physiological activity of the micro-flora in the model activated sludge units. Sludge digestion operating parameters (suspended solids, gas generation, pH) were also unaffected by loadings of up to 100 mg/kg of PDMS. The ultimate fate of sludge-bound PDMS depends on the sludge disposal technique. If the sludge is incinerated, the silicone content converts to amorphous silica, which presents no further environmental consequence when the ash is landfilled. When treated sludge is used as fertilizer, very small levels of PDMS may be introduced to the soil environment, where it is further subject to soil-catalyzed degradation. Similar soil-catalyzed degradation may also occur if sludge-bound PDMS is landfilled. Overall, PDMS has shown no significant environmental effects. For example, Dow Corning maintains an extensive facility in the U.S. dedicated to health and environmental sciences and was a significant contributor to a handbook on the environmental aspects of organosilicon materials.

It is well accepted that polydimethylsiloxane fluids become permanent residents of sediment, but should not exert adverse environmental effects. Polydimethylsiloxanes fluids are very surface active, because the flexible siloxane linkages permit the alignment of the hydrophobic methyl substituents towards the non-polar phase and of the polysiloxane backbone towards the polar phase. The polar medium is generally water. Other polar media to which polydimethylsiloxanes become attached may be textiles, sewage sludge, sediment , hair, algae and so on. In aqueous environments, polydimethylsiloxanes are adsorbed onto sedimenting particles. Also, in the presence of nitrate ions, which exist at various concentrations in the environment, short chain siloxanes are photo-degraded to the level of silicate within days. The stability of the siloxanes, desirable from a technical point of view, makes the siloxanes very persistent, and once released to the environment the said siloxanes remain for many years.

The volatile siloxanes may account for a significant part of the siloxanes used for cosmetics. The main source of releases of siloxanes to the air is volatile siloxanes used in cosmetics, wax, polishes, and to a minor extent in several other applications. Non-volatile silicone fluids used in cosmetics, wax, polishes, cleaning products and for textile applications (softeners) will end up in wastewater and be directed to wastewater treatment plants to a large extent. The cyclic siloxanes and small-chain linear siloxanes are bioconcentrated ( note that bioconcentration factors for long-chained siloxanes have not been assessed). The estimated bioconcentration factors (BCF) of the small siloxanes range from 340 for HMDS to 40,000 for a phenylated trisiloxane (phenyl trimethicone). The small phenylated siloxanes seem to have very high BCF, and the model estimates indicate that these substances are the most toxic for aquatic organisms. The substances were screened using the PBT profiler screening developed by U.S. EPA (U.S. EPA 2003PBT profiler screening ) in order to make a first comparison between the substances as to persistence, bioaccumulation and toxicity. The mentioned profiler uses a procedure to predict persistence, bioaccumulation and toxicity of organic chemicals on the basis of the chemical structure and physical parameters of the substances combined with experimental parameters for substances with a similar structure. The results for six members of the siloxane family predict the highest bioconcentration factors for the two phenyl siloxanes, one order of magnitudes higher than the values for the cyclic siloxanes and two orders of magnitudes higher than the values for the small linear methyl siloxanes. The predicted toxicity is significantly higher (lowest ChV values) for the phenyl siloxanes as well. The predicted half-life is nearly the same for all substances. Using U.S. EPA’s criteria, the screening indicates that all substances are of high concern as to environmental toxicity and that the phenyl siloxanes are considered very bioaccumulative.

The environmental fate and effects of volatile methylsiloxanes (mainly cyclosiloxanes) and polydimethylsiloxane (PDMS) have been reported as follows:
For octamethylcyclosiloxane: 9 of 11 Fish acute LC50 (14 day): rainbow trout 10 ug/l; sheepshead minnow : >6.3 ug/l Daphnia magna acute EC50 (48 h): >15 ug/l; NOEC 15 ug/l Mysid shrimp acute LC50 (96 h): >9.1 ug/l; NOEC 9.1 ug/l.

For PDMS: Daphnia magna NOEC 572 mg/kg.

Physical effects such as surface entrapment have been observed when testing aquatic invertebrates in clean laboratory water, but similar effects are not expected in natural environments where a large variety of other surfaces provide opportunities for deposition.

All waste must be handled in accordance with the local, state and federal regulations. Legislation regarding waste disposal requirements may differ by country, state and/ or territory. Each user must refer to laws operating in their area. Certain wastes must be tracked in some areas. This material may be recycled if unused, or if it has not been contaminated so as to make it unsuitable for its intended use. If it has been contaminated, it may be possible to reclaim the product by filtration, distillation or some other procedures. Shelf life considerations should also be applied in making decisions of this type. It is important to note that the properties of a material may change in use and recycling or reuse may not always be appropriate. It is prohibited to allow wash water from cleaning equipment to enter drains. All wash water for treatment before disposal must be collected. It is advisable to recycle wherever possible or to consult the manufacturer for recycling options. Residues are to be buried or incinerated at an approved site. If possible, it is advisable to recycle containers or dispose them in an authorized landfill.

References:
[1] Health Environment & Regulatory Affairs (HERA) Ref. n° 01-1034A-01 1/4 © Copyright Dow Corning Corp., 1997
[2] RR Buch and D.N. Ingebrigtson, “Rearrangement of Polydimethylsiloxane Fluids on Soil,” Environmental Science and Technology 13, 676 (1979).
[3] R.G. Lehmann, S. Varaprath, C.L. Frye, “Fate of Silicone Degradation Products (Silanols) in Soil,” Environmental Toxicology and Chemistry 13, 1753 (1994).
[4] C.L. Sabourin, J.C. Carpenter, T.K. Leib, J.L. Spivack, “Biodegradation of Dimethylsilanediol in Soils,” Applied and Environmental Microbiology 62, 4352 (1999)

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