Silica gel is an amorphous form of silicon
dioxide, which is synthetically produced in the form
of hard irregular granules (having the appearance
of crystals) or hard irregular beads. A micro porous
structure of interlocking cavities gives a very high
surface area (800 square meters per gram). It is this
structure that makes silica gel a high capacity desiccant.
Water molecules adhere to the gels surface because
it exhibits a lower vapour pressure than the surrounding
air. When an equilibrium of equal pressure is reached,
no more adsorption occurs. Thus the higher the humidity
of the surrounding
air, the greater the amount of water that is adsorbed
before equilibrium is reached. It is in these
higher humidity conditions (above 50% Relative Humidity)
that stored or in-transit items are susceptible to
damage.
The beauty of silica gel is the physical adsorption of water vapour into its internal pores. There is no chemical reaction, no by products or side effects. Even when saturated with water vapour, silica gel still has the appearance of a dry product, its shape unchanged.
Advantages of silica gel as a desiccant
Silica gel has many other properties that recommend it as a desiccant.
- It will adsorb
up to one third of its own weight in water vapour.
This adsorption efficiency is approximately 35%
greater that typical desiccant clays, making silica
gel the preferred choice where weight or efficiency
are important factors.
- It has an almost indefinite shelf life if stored
in airtight conditions.
- It can be regenerated and reused if required.
Gently heating silica gel will drive off the adsorbed
moisture and leave it ready for reuse.
- It is a very inert material, it will not normally
attack or corrode other materials and with the exception
of strong alkalis and hydrofluoric acid is itself
resistant to attack.
- It is non-toxic and non-flammable.
- It is most frequently and conveniently used packed
in a breathable sachet or bag. These are available
in a wide range of sizes suitable for use with a
wide range of applications.
Standard white silica gel is referred to as being non-indicating. As it adsorbs moisture it remains physically unchanged. Non-indicating silica gel is both cheap and effective, available loose in bulk packs or packed in sachets.
indicating silica gels are colored
gels whose colour changes as they adsorb moisture.
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