A siloxane is any chemical compound composed of units of the form R₂SiO, where R is a hydrogen atom or a hydrocarbon group. They belong to the wider class of organosilicon compounds.

Siloxanes can have branched or unbranched backbones consisting of alternating silicon and oxygen atoms -Si-O-Si-O-, with side chains R attached to the silicon atoms.^[1] More complicated structures are also known, for example eight silicon atoms at the corners of a cube, connected by 12 oxygen atoms as the cube edges.^[2]

The word siloxane is derived from the words silicon, oxygen, and alkane.

Polymerized siloxanes with organic side chains (R ≠ H) are commonly known as silicones or as polysiloxanes. Representative examples are [SiO(CH₃)₂]_n (polydimethylsiloxane) and [SiO(C₆H₅)₂]_n (polydiphenylsiloxane). 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.

Contents 1 Naming 2 Applications 3 Siloxanes in biogas 4 References 5 External links

[edit] Naming

M-units: (CH₃)₃SiO_½, D-units: (CH₃)₂SiO, T-units: (CH₃)SiO₂

Cyclic siloxanes	Linear siloxanes
D3: hexamethylcyclotrisiloxane	MM: hexamethyldisiloxane
D4: octamethylcyclotetrasiloxane	MDM: octamethyltrisiloxane
D5: decamethylcyclopentasiloxane	MD2M: decamethyltetrasiloxane
D6: dodecamethylcyclohexasiloxane	MDnM: polydimethylsiloxane

[edit] Applications

Siloxanes can be found in products such as cosmetics, deodorant, defoamers, 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.

[edit] Siloxanes in biogas

In internal combustion engines, siloxanes are oxidised to silicon dioxide which then forms deposits on pistons and cylinder heads which 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]

[edit] References

[edit] External links

• EPA report: Siloxane D5 in Dry-cleaning

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