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What is silica gel ?

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 microporous 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.


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.

Self-indicating silica gels have a moisture sensitive additive added to the basic silica gel. This indicator
changes colour as moisture is adsorbed thus giving a visual indication as to the activity level of the silica gel.

There are a number of different self-indicating silica gels:

Orange To Dark Green.
Using an organic indicator this silica gel offers a very strong orange (dry) to dark green (near exhausted)
colour change. Mostly used in a low dust beaded form with granular versions also available.

Orange to Colourless.
Using iron salts as the additive, this high capacity desiccant changes from a deep orange (dry) to
a very pale yellow colour in an exhausted state.
The indicator is very robust, making this silica gel well suited to repeated regeneration and reuse.
Available in granular and beaded physical forms.

Both orange self-indicating silica gels are non-toxic materials developed following
the reclassification of blue to pink self-indicating silica gel as a toxic material in the year 2000.

Blue to Pink.
It is the cobalt in the cobalt chloride indicator that is responsible for the toxic labelling
and disposal restrictions on this product.
It is still though considered as non-hazardous for transportation purposes.
Changing from blue (dry) to pink in a near-exhausetd state, it is supplied in 2 - 5 mm beaded
or 3 - 6 mm granular forms. Please ask for the current Safety Data Sheet.

All self-indicating silica gels are sold in
bulk packs or can be packed into sachets.

Advantages of silica gel as a desiccant.

Silica gel has many other properties that recommend it as a desiccant.

- It will adsorb up to 40% 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-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.

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