Siloxane
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A siloxane is any chemical compound composed of units of the form R2SiO, where R is a hydrogen atom or a hydrocarbon group. A siloxane has a branched or unbranched backbone of alternating silicon and oxygen atoms -Si-O-Si-O-, with side chains R attached to the silicon atoms.[1]
The word siloxane is derived from the words silicon, oxygen, and alkane.
Siloxanes can be found in products such as cosmetics, deodorant, water repelling windshield coating, food additives and some soaps. They occur in landfill gas and are being evaluated as alternatives to perchloroethylene for drycleaning. Perchloroethylene is widely considered environmentally undesirable.
Polymerized siloxanes with organic side chains (R ≠ H) are commonly known as silicones or as polysiloxanes. Representative examples are [SiO(CH3)2]n (dimethylsiloxane) and [SiO(C6H5)2]n (diphenylsiloxane). These compounds can be viewed as a hybrid of both organic and inorganic compounds. The organic side chains confer hydrophobic properties while the -Si-O-Si-O- backbone is purely inorganic. Dimethylpolysiloxane is added to vegetable oil as an antifoaming agent. McDonalds uses this type of vegetable oil to cook certain products[2]. In addition, automobile ignition modules, which heat up during use, dissipate heat by having their backings coated with Dimethylpolysiloxane (also known as a "thermo grease"). Acting as a "thermo grease" in this application, Dimethylpolysiloxane allows high heat transfer between the transistor ignition module and the distributor backing plate that it is mounted on which effectively cools the switching transistor (ignition module) during operation which enhances and extends its useful life. (There are some discussions expressing uncertainty about "di-electric grease" (in general) for this ignition application but they all do not have equal properties in simultaneously conducting heat well yet also remaining electrically non-conductive. Dimethylpolysiloxane has this property and may be considered falling into the category of a "di-electric grease" though properties vary for substances in this group.
[edit] Siloxanes in biogas
In internal combustion engines deposits on pistons and cylinder heads are extremely abrasive and cause damage to the internal components of the engine. Engines can require a complete overhaul at 5,000 h or less of operation. Deposits on the turbine of the turbocharger will eventually reduce the components efficiency.
Stirling engines are more resistant against siloxanes, though deposits on the tubes of the heat exchanger will reduce the efficiency.[3][4]
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