Silica gel is a granular, porous form of silica made synthetically from sodium silicate. Despite the name, silica gel is a solid. Silica gel is packed in semi‐permeable plastic. In this form, it is used as a desiccant to control local humidity in order to avoid spoilage of some goods. Because of poisonous dopants and their very high adsorption of moisture, silica gel should not be eaten. If consumed, the pure silica gel is unlikely to cause acute or chronic illness, but would be problematic. Total Pack’s line of foodgrade desiccant does not include any poisons that would cause long term harm to humans if consumed in the quantities normally included with the items of food.
Silica gel's high surface area (around 800 m²/g) allows it to absorb water readily, making it useful as a desiccant (drying agent). Once saturated with water, the gel can be regenerated by heating to 120 °C (250 °F) for two hours. Some types of silica gel will "pop" when exposed to enough water.
In many items from leather to pepperoni, moisture encourages the growth of mold and spoilage. Condensation may also damage other items like electronics and may speed the decomposition of chemicals, such as those in vitamin pills. By adding packets of silica gel, these items can be preserved longer.
Silica gel may also be used to keep the relative humidity inside a high frequency radio or satellite transmission system waveguide as low as possible. Excessive moisture buildup within a waveguide can cause arcing inside the waveguide itself, damaging the power amplifier feeding it. Also, the beads of water that form and condense inside the waveguide change the characteristic impedance and frequency, impeding the signal. It is common for a small compressed air system (similar to a small home aquarium pump) to be employed to circulate the air inside the waveguide over a jar of silica gel.
Silica gel is also used to dry the air in industrial compressed air systems. Air from the compressor discharge flows through a bed of silica gel beads. The silica gel adsorbs moisture from the air, preventing damage to the compressed air users due to condensation or moisture. The same system is used to dry the compressed air on railway locomotives, where condensation and ice in the brake air pipes can lead to brake failure.
Silica gel is sometimes used as a preservation tool to control relative humidity in museum and library exhibitions and storage.
Silica gel is non toxic, non flammable, and non reactive and stable with ordinary usage. It will react "with hydrogen fluoride, fluorine, oxygen difluoride, chlorine trifluoride, strong acids, strong bases, and oxidizers". Silica gel "is irritating to the respiratory tract", "May cause irritation of the digestive tract", and dust from the beads may cause irritation to the skin and eyes, so precautions should be taken .. Some of the beads may be doped with a moisture indicator, such as cobalt(II) chloride, which is toxic and may be carcinogenic. Cobalt (II) chloride is deep blue when dry (anhydrous) and pink when moist (hydrated).
Typically used in food and pharmaceutical applications
High crush strength and high porosity
Remains dry and free ﬂowing even when saturated
Has relatively high capacity for moisture at temperatures below 77°F (25°C) with relative humidity above 40%
Chemically inert, non corrosive and non toxic
DuPont TYVEK as the packaging material is used for their Silica Gel Desiccants.