Posted in Silicone Recycling

Silicone Oil Recycling

Silicone oil has many different names such as : silicone fluid, polydimethylsiloxane, dimethicone, dimethylpolysiloxane, dimethyl siloxane, PDMS. From a chemical point of view, probably that polydimethylsiloxane name best describes the silicone oil structure.: Poly (it is a polymer), dimethyl (two methyl (CH3) groups) , siloxane (Silicone and Oxygen).

The chemical structure of polydimethylsiloxane is:

where n= number dimethylsiloxane units.

The customers of polydimethylsiloxane will be aware that the silicone oil is available in a number of different viscosities. The most common silicone oils are are that of viscosities 100cSt (centistoke), 350cSt and 1000cSt. The number of dimethylsiloxane units depicts the size of the polymer chain which determines the viscosity. The smaller the polymer chain the lower the viscosity (e.g. 100 cSt) , the bigger the polymer chain the higher the viscosity (e.g.1000cSt).

An Allcock product in the Allcosil 200 range has the lowest viscosity equal to 0.65cSt. Allcosil 200/0.65 has only two units, this means that it is a dimer, and not a polymer. The dimer is hexamethyldisiloxane.

The chemical structure of hexamethyldisiloxane is:

It is clear that that the dimer has a structure very similar to the structure of polydimethysiloxane .The polymer chain length also has an effect on the other properties of the oil as shown in Table no.1 below :

Table no.1.

(1000000 cSt, longest polymer chain.)

It may be observed from the table no.1 that the polymer chain length becomes less important in effecting the properties at viscosities higher than 50cSt. According to the FDA Regulation 21 CFR, certain viscosities are food grade. Silicone oils have many uses due to their lubrication, dielectric and water repellent properties.

2. Silicone Emulsion

Silicone Emulsion contains 3 ingredients; polydimethylsiloxane, emulsifier & water. The key ingredient is the emulsifier, which encapsulates the polydimethylsiloxane oil and holds it in suspension.

A typical emulsifier used in making silicone emulsions is ethoxylated glycol ethers. These emulsifiers have a hydrophobic (dislikes water) and hydrophilic (likes water) part in their structure. The hydrophobic faces the polydimethylsiloxane and the hydrophillic faces the water. This creates a barrier between the water and the polydimethylsiloxane. Here below is represented a picture that shows how an emulsifier works. A surfactant (i.e. surface active agent) is just a type of emulsifier.

Unfortunately emulsifiers, due to their chemical make-up, are vulnerable to bacteria. Bacteria can digest certain emulsifiers causing the polydimethylsiloxane to float to the top or disrupt the pH enough for non-digestible emulsifiers to split from the oil. Splitting in an emulsion can be seen and smelt. Bacterial growth causes smell and usually the emulsion smells like sour milk.

Emulsions are a key mould release/lubricant and antifoams for many industries. For instance, Allcock biggest selling emulsion is Allcosil 435 FG, which is sold to the food packaging industry. It is food grade and kosher certified.

3. Recycling

Research and review of peer-reviewed scientific studies indicate one should begin to be cautious about silicone for the following reasons [2] .

• Silicones are not completely inert or chemically unreactive and can release toxic chemicals. They can leach certain synthetic chemicals at low levels, and the leaching is increased with fatty substances, such as oils. Evidence of contamination from silicone was found in wine and edible oil foods. Materials such as aluminium, platinum, magnesium and calcium were found to have leached into food when testing was carried out on silicone bakeware.
• Fluid silicone studies indicated release of siloxanes, one of which – cylcopentasiloxane – is considered toxic and persistent. This siloxane, also known as D5, is used as a softener in cosmetics, and according to the U.S. Environmental Protection Agency may also be carcinogenic. [3]
• Silicone tubing commonly used for medical applications has been shown to leach several chemicals, including dioctyl phthalate . DOP Silicone intravenous devices have been shown to leach silicone and cause local inflammation [4].
• Silicones are likely not completely inert and may cause local inflammation. as suggested in this study on the breakdown of silicone joint implants.[5]
• Silicone oils have a low recycling rate.
  Silicone does not biodegrade or decompose (certainly not in our lifetimes). It is recyclable, but not likely through a local municipal recycling program. Preferably it should be taken to a specialized private recycling facility.

An alternative process is mentioned for removing silicones present on fibers, yarns or textile sheet materials (substrates), wherein an aqueous preparation is allowed to act on the substrates. Thus, the treated substrates are rinsed to remove the residual silicone and the said preparation contains from 0.5 to 10% by weight of surface-active quaternary ammonium compounds and from 0.5 to 5% by weight of alkali hydroxide, respectively based on the aqueous preparation. The process is suitable, in particular, for the recovery and recycling of fibrous material and silicones from airbags or coated fabric materials from the ready-making of airbags.[6]

Another process is mentioned for recycling a silicone compound, wherein the silicone compound is decomposed by employing an alkyl carbonate and a compound containing active hydrogen group in the presence of a catalyst. Following this procedure, the silicone monomers and/or silicone oligomers are recovered from the decomposed silicone compound. Silicone monomers and/or silicone oligomers are produced without producing any byproduct, and no step for removal by separation is needed in an after process. Therefore the implementation of the said process is claimed to be easy.[7]

TMC Industries uses both centrifugal molecular stills and packed column stills in order to recover silicone oils. Using centrifugal molecular stills, the thin-film, short-path, high-vacuum technology, TMC Industries can restore contaminated vacuum fluids to their original state with exceptionally high yields. Using packed column technology, TMC can purify many fluorocarbon solvents by removing contamination and restoring them to their original high purity. Inert and semi-inert fluids can be reclaimed many times without affecting their performance capabilities.[8]

References:

[1] www.allcocks.co.uk
[2] www.lifewitoutplastic.com
[3] 2005 Report commissioned by the United Kingdom Food Standards Agency : “Chemical migration from silicones used in connection with food-contact materials and articles”
[4] Archives of Disease in Childhood: “Plastic migration from implanted central venous access devices” , a 1999 study
[5] Oral Surgery, Oral Medicine, Oral Pathology : “ Silicone-induced foreign body reaction and lymphadenopathy after temporomandibular joint arthroplasty”, a 2006 study.
[6] US Patent no. 20090126122 A1
[7] US Patent no.6,172,252
[8] www.tmcindustries.com