So, 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 size.
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 then ?
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 indicators which change colour as they adsorb moisture. For example, orange silica gel changes to dark green as it adsorbs moisture. The purpose of this being to indicate when the silica gel is saturated and unable to take up any more moisture (orange = dry, dark green = near exhausted in this case).
Does silica gel comes ready to use ?
Yes. The 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 anywhere ? Theoretically yes, but it is really only effective in an enclosed environment. In a situation where new air (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 saturation content of air at the new temperature becomes much lower.
Is silica gel dangerous ?
Non-indicating (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 (blue) silica gel has been reclassified by the European Union and as of 01/07/2000 has been classified as a toxic material. More here. New safe self-indicating silica gels are available.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 are needed for every cubic foot of volume of the package. This converts to 170 grams per cubic meter. Can you get non-indicating and self-indicating silica gel sachets ? Yes. The blue and orange self-indicating silica gels can be seen through the sachet material.
How is silica gel regenerated ?
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 moisture. Regeneration can be carried out repeatedly, although eventually the crystals will lose their colour. When regenerating 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
