silica gel, not really a gel then ??
No, silica gel isn't a gel
like a hair gel. It is a solid material that comes in granular
or beaded forms. These granules or beads are sorted according to their
But does silica gel get wet as it adsorbs water ?
No it doesn't, it traps water
molecules inside its pores yet remains dry and appears physically unchanged.
Silica gel is a desiccant
Yes, it adsorbs water vapour
by physical means, not a chemical reaction.
Water vapour is attracted inside the silica gels crystalline structure.
and it changes colour as
it adsorbs moisture ?
The basic white non-indicating
silica gel doesn't.
But self-indicating silica gels do. They are impregnated with moisture sensitive
change colour as they adsorb moisture. For example, an orange silica gel which
changes to dark green
as it adsorbs moisture. The purpose of this being to indicate when the silica gel is
unable to take up any more moisture (orange = dry, dark green = near exhausted in this
Does silica gel comes
ready to use ?
silica gel does not need to be 'generated' before use.
It is always sold (loose or in sachets) in a dry condition, ready for use.
How long can silica gel
be usefully stored ?
The shelf life of silica gel
is between 4 and 12 months (although practical experience has shown
that it can often far exceed this). The crucial factor is to keep it in a sealed
environment until it is
needed for use - it will, of course, adsorb moisture from any environment.
Although the moisture uptake rate is not fast, small sachets can be particularly
vulnerable to poor storage
conditions - it is recommended that they are not left open to the atmosphere for longer
than 15 minutes.
How much water vapour can
silica gel hold ?
Silica gel will adsorb up to
40% of its' own weight in moisture.
Can silica gel be used
Theoretically yes, but it is
really only effective in an enclosed environment. In a situation where
(and moisture) is constantly being introduced, an unpractical amount of silica gel will
likely be needed to
keep the Relative Humidity at low levels.
What is Relative Humidity
Air will always contain water
vapour. At any given temperature there will be a saturation point at which
no more moisture can be retained (any excess will condense). The Relative Humidity of air
is the actual
moisture content expressed as a percentage of this saturation content. The aim of using
silica gel is to keep
the Relative Humidity at levels below 50% of the saturation capacity. At these levels,
corrosion and mould
growth will not be promoted. Problems are often caused if the temperature drops as the
of air at the new temperature becomes much lower.
Is silica gel dangerous ?
(white) silica gel is non-toxic and non-flammable, it is very inert
with a very high melting point.
It is very much like sand and thus can safely be sent by any means of transport.
Self-indicating (orange) silica gels (orange to green and orange to colourless) are
similarly non-toxic and non-flammable.
Self-indicating (blue to pink) silica gel was reclassified by the
European Union and as of 01/07/2000
has been classified as a toxic material due to the heavy metal cobalt which forms part of
the moisture sensitive indicator.
How can silica gel work
when it's inside a sachet ?
The materials used to make
silica gel sachets are specifically chosen to be breathable.
Water vapour passes through them to be adsorbed by the silica gel.
Why are there so many
different size sachets ?
The larger the item that is
being protected, the more silica gel is needed.
A range of sachets allows for a single sachet to be used in most cases.
And which size sachet
should I use ?
It will depend on the
application. For a well sealed item, a rule of thumb is that 5 grams of silica gel
needed for every cubic foot of volume of the package. This converts to 170 grams per cubic
Can you get
non-indicating and self-indicating silica gel sachets ?
Yes. The orange
self-indicating silica gels can be seen through the sachet material.
How is silica gel
Self-indicating silica gels
when they have become saturated can be regenerated by heating at 100 - 120°C
until they return to their original colours. The heating literally drives off the adsorbed
can be carried out repeatedly, although eventually the crystals will lose their colour.
self-indicating silica gel sachets, only the minimum necessary heat should be used. This
will prevent the
sachet material from deteriorating. Although non-indicating silica gel can be regenerated
in exactly the same
way, it is not apparent when the silica gel is regenerated other than by checking its
weight - it will return
to its original dry weight when completely regenerated.
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