Silicone rubber and its vapour permeability

 

 

Silicone rubber has many characteristics which have enabled it to achieve wide use.  Its vapour permeability, however is extremely high which leads to both specific benefits and considerable drawbacks in many applications.  It is an excellent barrier against liquids, but many vapours – such as oxygen, water vapour and hydrogen can pass through most types of silicone almost as if it was not there. This makes it highly suitable for use in some applications, and totally unsuitable for others

Silicone is a polymerized silicone (or polysiloxane) and is based on an inorganic silicon-oxygen molecular chain with organic side groups attached to the (tetravalent) silicon.  These organic side groups allow linkages between chains leading to the option of a very wide variety of properties and compositions.

The vapour permeability of silicone rubber can very drastically (e.g. by five or six orders of magnitude!) due to several factors, including:

• Physical structure
• Temperature
• Specific gas
• Additives and fillers

General silicone properties

• Reasonably stable with temperature – (except for vapour transmission which can rise rapidly)
• Hydrophobic
• Resists weathering and oxidation
• Adhesive
• Electricalyl insulating (though this can vary with filler)
• Anti-foaming of foam stabilizing
• Largely inert and thus safe in many applications

Free volume, or holes, not only exists in the matrix of silicone rubber but can be quite large (for example, by comparison to carbon based polymer systems) and these form and vanish though the movement of polymer chains. This process allows vapours to permeate and can be very temperature dependant. The diffusion of a vapour molecules (gas) through the silicone involves the gas molecules migrating from hole to holes and finally appearing on the other side of the membrane.  (CHRIS I realise it is be more complex than this as it involves both dissolving and evaporation – but I felt that this was a step to far?)

Oxygen permeability comparisons for various Polymers

Polymer	Vapour Permeability (cm3cm/(s*cm2cmHg)	Relative permeability
Silicone	109	272500
Dimethylsilicone rubber	60.0	150000
Fluorosilicone	11.0	27500
Nitrile rubber	8.5	21250
Natural rubber	2.4	6000
Polyethylene, low density	0.8	2000
Butyl rubber	0.14	350
Polystyrene	0.12	300
Polyethylene, high density	0.10	250
Nylon 6	0.004	10
Poly(ethylene terephthalate)	0.0019	4.75
Teflon	0.0004	1

Functional fillers for silicones

Typical applications - based on vapour permeability characteristics

High permeability of silicone is suitable for use in membranes for blood oxygenation, gas separation, drug delivery. It can also be used in wound care dressings, for external prosthetic devices and in some types of contact lens.

Typical areas where silicone is NOT ideal - based on vapour permeability characteristics

In electronics components, water vapour is often the cause of corrosion, fogging and in some cases it can cause side reactions that produce unwanted chemicals such as ammonia. Electronic thermoelectric modules (TEMs) may also fail catastrophically if vapour is allowed of permeate in.

Additionally it is unsuitable for applications where water affects the performance of added fillers. In particular in Light Emitting Diodes (LEDs) the phosphorus added to the silicone that encapsulates the LED to produce white light can absorb moisture and change the light output over time.

Silicones are also not practical for applications that need a low gas permeability performance in extreme environments such as aerospace components, inflatables, ducting, diaphragms and electronics used underground or in marine environments.  Silicone is also not suioted for use in sealants which require an effective water vapour or oxygen seal.

However, in all cases, having a better understanding of the relative differences of vapour permeability, particularly their WVTR (Water Vapour Transmission Rate) between silicone formulations will help immensely with the appropriate silicone selection, or if silicone is even appropriate in the first place.

Link to silicone on Wikipedia