- What is silica gel and how does it work?Silica gel is a solid desiccant (drying agent). It is not a wet gel as the name might suggest, and should not be confused with silicone gel. Silica gel is the most commonly used desiccant for providing efficient protection against excessive humidity in sealed spaces. This granular, vitreous, highly porous form of silica is made synthetically from sodium silicate. Silica gel beads contain millions of tiny pores that adsorb and hold water vapour and remain dry to the touch, even when fully saturated. This highly porous substance can adsorb up to 40 percent of its own weight at 50% RH and 25°C, and can take the relative humidity (RH) in a closed container down to about 40%, which makes this the highest water holding capacity, of any commercially used desiccant available. The adsorption up to 50% RH is a near-linear relationship between the percentage of water uptake and the equilibrium relative humidity. This refined form of silica is produced in granular or beaded forms. Beaded silica gel has a higher mechanical strength than the granular form and is therefore less likely to generate dust. Also, the uniform bead size reduces gas flow channelling and minimises pressure drop. Silica gel is nearly harmless, which is why you find it in food products. Silica, or silicon dioxide (SiO2), is the same material found in quartz. Silica gel is essentially very porous sand. Some types of silica gel will "pop" when exposed to enough water. It can be reactivated and reused by heating - see FAQ reactivating/ drying
Silica gel is produced synthetically by the reaction of a mineral acid, usually sulphuric, and a sodium silicate solution to form a gelatinous precipitate that is washed, and then dehydrated to produce colourless silica gel. Ammonium tetrachlorocobaltate(II) (NH4)2CoCl4 or cobalt chloride CoCl2 may be added to silica gel as a moisture indicator. These indicators change colour depending on hydration. Cobalt Chloride is blue when dehydrated and pink when saturated. Orange silica gel desiccant is the latest alternative to blue indicating silica gel (cobalt chloride). Orange silica gel is impregnated with a safe organic indicator that displays an orange/yellow colour when active and changes to green when the desiccant becomes saturated. Silica gel traps water molecules inside its pores yet remains dry and appears physically unchanged. This synthetic form of sodium silicate and a mineral acid produces an amorphous, micro-porous structure with a distribution of pore opening sizes of roughly 0.3-6 nanometres. Comprised of tiny interconnecting pores, it absorbs water vapour by physical means, not a chemical reaction. Physical adsorption involves relatively weak intermolecular forces (van der Waals forces and electrostatic interactions) between the moisture and surface of the desiccant. Chemi Sorbents, such as calcium oxide, involve an actual chemical bond. Physical adsorption of moisture is typically exothermic. The strength of the adsorptive bonds can thus be measured by the heat of adsorption. The higher the heat of adsorption for moisture on the desiccant, the stronger the bonding and the less easily that moisture can be subsequently removed. These interconnected pores form a vast surface area (around 800 m²/g) that will attract and hold moisture inside the silica gel's crystalline structure by adsorption and capillary condensation, allowing silica gel to absorb up to 30-40% of its weight of water. The adsorption capacity depends on a linear relationship between the percentage water uptake and the equilibrium relative humidity. Much of silica gel's popularity is due to its non-corrosive, non-toxic nature. Hence, non-coloured silica gel has received US government approval for use in food and drug packaging. Beaded silica gel has a higher mechanical strength than the granular form and is therefore less likely to generate dust. However, silica gel is not considered biodegrade in water or soil.
Silica Gel will continue to absorb moisture below freezing, but performs best at room temperatures.
- Visual indication of beads' saturation
- Environmentally friendly organic colour impregnation
- Frequently used in food and pharmaceutical applications
- High crush strength and high porosity
- Remains dry and free flowing, even when saturated
- Chemically inert, non-corrosive and non-toxic
- Does not form any by-products
- Extremely large internal surface area
- Meets all Military Specifications for desiccants (Mil-D-3464, Type I & II)
- Available in any desiccant raw material (i.e. silica gel, clay, molecular sieve and others)
- Prevents mould, mildew, rust and corrosion
- Adsorbs moisture and odour inside a packaged environment
- Non-Tearing, Non-Dusting & Non-Linting packets
- Compliant with FDA and USDA guidelines for packaged desiccants
- Available in Tyvek®, Non-Woven, Kraft, Clear Poly & Sewn Cloth bags
- Silica gel applicationsSilica Gel has numerous uses, many of these in the rubric: the prevention of moisture damage. Desiccant Products, such as silica gel, activated clay or molecular sieves are drying agent used to combat humidity or moisture degradation. Products vulnerable to moisture damage need special protection during transportation, storage, etc. to ensure that it remains effective. Silica Gel packets and clay desiccant's provide a simple, dependable, and economical solution for preventing moisture damage in sealed packaging. Humidity in the space turns into condensation which can often cause irreparable damage. Our silica gel products will help to eliminate rust, corrosion, oxidation and tarnish on tools, jewellery, silverware, coin collections, musical instruments and much more. In addition, silica gel can protect seasonal storage, collectibles, leather, clothing and more from mildew, mould, fungus and odors. Furthermore, photographs, documents, and stamp collections can be protected from staining, spotting and cracking.
Many manufacturers include silica gel packets in their electronics packaging which helps protect the electronics from moisture damage. Silica gel decreases moisture in the air which can limit the growth of mould and reduce spoilage. Silica gel may be used to dry electronic devices which had become wet. Placing the device in an airtight container with some silica gel, would remove most moisture, more effectively than air drying and more safely than heat. Storing electronic instruments in a sealed environment (plastic bag) with some silica gel prevents condensation and corrosion, which might cause serious damage.
Vitamins and pills are often packaged with silica gel packets to prevent damage or decomposition due to moisture in the bottle. If a bottle of vitamins contained any moisture vapour and was cooled rapidly, the condensing moisture would ruin the pills. Silica gel packets are also used to help keep dried food, like beef jerky and pepperoni, fresh. In museum and library exhibitions and storage, silica gel is used as a preservation tool to control relative humidity (RH) invaluable air tight frames. Leather goods are often transported with silica gels to help protect them from moisture damage. Storing leather goods with silica gel packets can also help protect them from moisture and make them last much longer.
If you have ever seen dried flowers and wondered how they managed to dry them and maintain the colours and shapes, they probably used silica gel. Covering the flowers with loose silica beads while they are still vibrant and firm dries them quickly, which helps maintain the colour and shape of the flowers. Silica gel can also be placed in a gun safe to help prevent moisture damage to both guns and ammunition.
Dry bags or dry boxes used in water sports, should contain silica gel to dry bags or dry boxes and absorb any moisture. Adding a silica gel packet to an underwater camera case absorbs moisture from within the case and limit or eliminates condensation from the camera's window.
Silica gel is most commonly encountered desiccant in everyday life, as clear beads packed in a semi-permeable plastic. In this form, it is used as a desiccant to control local humidity to avoid spoilage or degradation of some goods. Because of poisonous doping agents (see below) and their very high adsorption of moisture, silica gel packets usually bear warnings for the user not to eat the contents. If consumed, pure silica gel is unlikely to cause acute or chronic illness, but if more than just a few beads are ingested the drying properties could cause some complications. Food-grade desiccant should not include any poisons which would cause long-term harm to humans if consumed in the quantities normally included with the items of food.
Silica in this form can be easily purchased for applications such as keeping tools rust free in damp environments, long term storage, and preservation of dried food. Silica gel can be used anywhere, but it is really only effective in an enclosed environment. In a situation where new air (and moisture) is frequently added, an impractical large amount of silica gel and frequent changes to dried silica gel would be required to keep relative humidity at low levels.
Commonly protected items
See the list below for other areas where the silica gel dehumidifiers can be helpful in eliminating moisture. Depending on application, different volume or mixed silica gel may be packaged inside the following substrates; Tyvek, Non-Woven, Plastic Film, Paper, Sewn Cloth, or Reinforced Paper.
- Tool boxes - help prevent rust in damp environments
- Storage containers
- Safes and vaults
- Silver drawer
- Stored vehicles
- Cargo holds on RVs, boats, etc
- Collectibles (stamps, coins, etc.)
- Metal and machine parts
- Industrial equipment
- Pharmaceuticals and vitamins
- Organic Liquid Drying
- Insulated Windows
- Flower Drying
- Telephone Cable Splicing
- Indication of Relative Humidity
- Insulated Glass
- Natural Gas Dehydration
- Military instruments
- Circuit Boards
- Photographic equipment and film
- Pet foods
- Optical devices
- Medical equipment
- Leather products
- Packaged Gas & Liquid Dehydrators
- Hydrocarbon Recovery Units
- High Pressure Gas Drying Cartridges
- Cryogenic Purification
- Liquid Petroleum Gas Dryers
- Dynamic Drying of Refrigerants
- Does silica gel come ready to use?Yes. The silica gel does not need to be 'generated' before use. We sell it in a dry, airtight container, ready for use.
- What to do if silica gel has been ingested?We recommend contacting your local Poison Control Centre or seeking medical advice/attention. You can contact the Queensland Poisons Information Centre at 13 11 26 and they will provide you with advice based on who or what ingested the silica gel and how much of it. Based on this information, they will provide advice based on your situation. Silica gel is a non-toxic material. See also http://www.health.qld.gov.au/PoisonsInformationCentre/
- Is silica gel hazardous?Silica gel is inert, non-toxic, non-flammable, non-reactive, and stable with ordinary usage. The substance was in existence as early as the 1640s as a scientific curiosity and is safe to use for protecting foods, medicines, sensitive materials and much more. It will react with hydrogen fluoride, fluorine, oxygen difluoride, chlorine trifluoride, strong acids, strong bases, and oxidisers. 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 small amount of a moisture indicator, such as cobalt(II) chloride, which is toxic, and may be carcinogenic, and irritating to skin and the respiratory tract. Cobalt (II) chloride is deep blue when dry (anhydrous) and pink when moist (hydrated). We, along with several other retailers, offer a safer alternative which is an indicating silica gel that changes from orange to green and does not contain cobalt chloride.
Note: Crystalline silica dust can cause silicosis but synthetic amorphous silica gel is non-friable, and so does not cause silicosis.
- What is indicating silica gel?Indicating silica gel, was clear silica gel which had at least some of the beads impregnated with a chemical that changes colour with moisture content. When the colour has changed, the silica gel is at least partially saturated and needs to be replaced or reactivated by drying. This can be useful when long term humidity control is needed, like in a display case.
- What is "Saturation" and "Equilibrium Capacity"?Technically, for most practical purposes, these two terms cover the point at which a desiccant no longer absorbs moisture. Saturation is when the desiccant is full of water vapour and cannot adsorb more, even if there is excess moisture present. Very dry silica gel can reduce relative humidity further than silica gel which already carries some water. Equilibrium capacity applies to desiccant which has acquired so much moisture that the air retains an equally strong hold on water molecules as the desiccant. Adding more desiccants, of equilibrium capacity, cannot reduce relative humidity any lower. Exchanging the desiccant with very dry material is the obvious solution.
- What is Relative Humidity?Air always contains water molecules. At any given temperature the saturation point is where no more moisture is retained and excess moisture is condensed. The relative humidity of air is the moisture content expressed as a percentage of this saturation content. Silica gel is used to restrain relative humidity to below 50% of saturation capacity. At these low levels, mould growth and corrosion will not be promoted. Since the saturation point is temperature dependent, a drop in temperature may cause an increase in relative humidity.
- Disposal of silica gel products?The Silica gel can simply be thrown into the normal garbage. If the silica gel has come into contact with a toxic or hazardous material, it could potentially adsorb the material and become toxic. In this event, the silica gel should be disposed of in the same manner as the hazardous material that contaminated it.
- How much silica gel should be used in various applications?These are estimates and relate to situations when used in an airtight container. The amount of silica gel or desiccant required will depend on several factors including: volume contents of container, the chemical characteristics of the product, physical properties of the container, and conditions in which container will be stored and used. The amount will depend on the application however for a well sealed item, it is suggested that 200 grams of silica gel is needed for every cubic metre of volume of the package - assuming that specimen and space were moderately dry when the silica gel was added.
- How does moisture cause damage to a product?Moisture trapped within a product package or leaking into it during storage and shipping can result in harm to the item. The maintenance of their physical product integrity, as well as the stability of many pharmaceutical formulations and diagnostic reagents, is often closely tied to the moisture conditions of the package environment.
In poorly sterilised situations, moisture may promote the growth of moulds, mildews and fungi. Products that use polymers are prone to swelling in high humidity conditions, due to water causing intermolecular bonds between polymer chains to weaken. If a very water soluble solid, such as sugar coating, is exhibited to the right conditions, dissolution can trigger irreversible water uptake and subsequent deliquescence.
Moisture in electronics causes corrosion and may lead to "short" circuits and other faults. Usually this will not produce sparks, the effects are more subtle, and the device may operate unreliably - sometimes it work, sometimes it doesn't. A situation like this is frustrating, time-wasting and may ruin an expensive instrument.
- Difference between silica gel and clay desiccants?Both bentonite clay and silica gel are desiccants, but there are major differences. Bentonite clay emits dust, whereas silica gel produces minute levels of dust particles. This addition renders silica gel the best choice to be packaged with items that could be adversely affected by dust particles like food or electronics. Silica gel is commonly packaged in cotton and perforated plastic.
Silica gel adsorbs water up to 104°C, beyond that point it releases absorbed moisture. Bentonite clay adsorbs water only to 49°C at which point it begins to release any moisture back into the air. Both silica gel and bentonite clay can be reactivated for reuse. Bentonite clay and silica gel are used in industry, but silica gel is also commonly used in commerce and for domestic applications. Silica gel's absorbent capacity is 40% of its weight; bentonite clay's adsorbent capacity is only 25%. Silica gel's high adsorption capacity is the greatest available and considering its other desirable attributes (low dust, low toxicity and ability to absorb at higher temperatures) makes it the most popular desiccant.
The buffering capacity of silica gel is determined by measuring the moisture gained or lost by silica gel as the RH changes in the surrounding air. For museum applications, where the case will cycle within a controlled RH range, the buffering capacity of silica gel is measured by its moisture holding (MH) value, which takes into account changes in buffering capacity based on whether the RH increases or decreases.
Regular density silica gel is the most common type of silica gel and is universally available. RD silica gel is a form of silica manufactured from sodium silicate and sulphuric acid. Like clay, silica gel is non-hazardous and is capable of adsorbing 40% of its weight in water vapour at 100% humidity. Because of its capacity for high moisture uptake in the low RH range, it is a very effective desiccant. For museum applications, the most important difference is buffering capacity, defined by the gel's MH value. Because of its poor hygroscopic properties around 45-50% RH and above, it is not recommended for museum applications requiring control in the mid to upper RH range. Within the range of 40-55% RH, it has an MH value of 2. Institutions located in areas with well defined wet and dry seasons, the buffering capacity of silica gel may become insufficient and the silica gel employed in sensitive cases may require replenishing. Recording a tray's silica gel net weight before and after drying will give a record of moisture levels. Silica gel has a porous molecular structure that closely resembles a sponge, and has the highest capacity of any commercial desiccant for moisture adsorption.
Molecular Sieve is a manufactured crystalline version of Zeolite containing a network of uniform pores and empty cavities. Molecular sieve is derived from sodium, potassium or calcium alumina silicate. It is a non-hazardous material. Molecular sieve is the desiccant of choice for the most demanding applications; so it is used to remove traces of moisture from absolute ethanol or acetone. Molecular sieve is the most aggressive and expensive of the primary desiccants.
- Reactivating/ drying of silica gelSilica gel products can be dried and reused "as new".Orange silica gel: Dry pellets are orange and turn dark green when near saturation.Blue indicating silica gel: Dry it's dark blue and with increased water uptake it becomes more pale and pink. The most efficient method to reactivate silica gel is with heat. When heat is applied to saturated silica gel, the beads will restore to the previous moisture absorbing capability and return to the original orange colour. Unlimited regeneration cycles are possible.
Silica gel has a very high melting temperature, 1600°C. However, it will lose its chemically bound water and hygroscopic properties if heated above 300°C. In addition, there is a new class of indicator gels, incorporating organic dyes that are heat sensitive and their colour indicating dye will be affected above 125-150°C . Therefore, it is not recommended that orange indicating silica gel is heated above 120°C. Also, if heated above 120°C through several reactivation cycles, the material may disintegrate into powder. This in turn, reduces the adsorption capacity of the material and may eventually lead to the loss of indicating colour within the crystals. The minimum heat necessary should be used when removing moisture from silica gel; ProSciTech recommends regeneration at between 105 °C and 120 °C. This will prevent the silica gel from deteriorating and it may be re-generated numerous times.
Blue indicating silica gel has a higher tolerance and may be dried by heating it to 150°C. Lower heat for regeneration requires longer drying times, but the degradation of silica gel is reduced.
Increasing heat vapourises adsorbed moisture and above 105°C is removes all water molecule from silica gel. A porous desiccant like silica gel, removes water from the surrounding air by two mechanisms: multi-layer adsorption and capillary condensation. Multi-layer adsorption is the attraction of thin layers of water molecules to the surface of the desiccant. Since the desiccant is very porous, the surface area is high and significant amounts of water can be attracted and absorbed. Capillary condensation is when the smaller pores become filled with water. Capillary condensation occurs when saturated water vapour pressure in a small pore is reduced by the effect of surface tension.
In a conventional oven, the time of regeneration varies from minutes to hours, depending on temperature and the thickness of the gel within a dish. Although silica gel can be dried in a microwave oven, it is difficult to determine the temperature inside the gel. Particularly the orange gel should not be dried using microwaves since excessive heating denatures the indicator dye. As metal cannot be used in a microwave oven, only glass, ceramic or microwave safe plastic with a high melting temperature should be used to hold the gel.
- Place 'used' silica gel beads into a large tray
- Use an oven at 105 - 120°C for two hours, or continue heating for 30 minutes after the gel turned orange.
- Check the silica gels beads periodically. Caution:the gel beads are very hot to touch.
- Defining "Adsorption" and "Absorption"Absorption is when a substance is chemically integrated into another. For example, when acid is added to water, the acid is diluted and the water cannot be driven off by just heating. The acid cannot be concentrated by driving the water off. We can absorb knowledge as this becomes part of us.
Adsorption applies when one substance is being held inside another by physical bonds only. The term is used in surface chemistry and physics. So, a film of moisture is adsorbed onto cold surfaces. The surface is not chemically changed and when the surface is heated, the adsorbed water will mostly volatilise. Most desiccants do not absorb water or other substances. Instead, they capture molecules by adsorption and capillary action, they sequester them.
- Use and applications of silica gelHow long for silica gel will protect an enclosed area before it needs to be recharged depends on many factors. Deciding factors are the volume of the area, the amount of new, additional air penetrating the area, and if the contents was reasonably desiccated. If the area is not sealed tightly, outside air will infiltrate and shorten the time before the unit needs to be reactivated. Also, if you frequently break the seal and expose the area to new air, it will shorten the time until reactivation is needed.
When silica gel is first placed into an enclosed area, it may be saturated quickly as it adsorbs residual moisture adsorbed on the specimens and chamber surfaces. After the residual dampness is removed, a dry condition can be maintained with less frequent need to reactivate the silica gel.
The more air tight the environment is, the less reactivation times needed. Making a chamber airtight will limit the amount of moisture leaking into the area. Also, avoid frequent opening of the desiccated space. For example, silica gel will work well within a storage trunk, but it will not work as well if the lid is opened and closed several times a day. The adsorption capacity of silica gel can be destroyed by contamination with dust, grease or petroleum products, and also by heating the material above 125°C.
- How does silica gel control relative humidity?All silica gels are hygroscopic, responding to the relative humidity (RH) of the surrounding in the same way as most organic materials, such as paper and wood. The amount of moisture in silica gel will increase as the RH rises, and will decrease when the RH falls. Unlike organic materials that expand and contract with changes in moisture content, silica gel volume remains unchanged. In a near-airtight space like a display case, a quantity of silica gel acts as a buffer hydrating and dehydrating the case's RH. Additionally, silica gel adsorbs and desorbs much larger amounts of moisture when the RH changes under normal conditions.
- Storing silica gelMost damage to stored supplies is caused by humidity trapped within the enclosed storage unit itself. The crucial factor is to keep it in a sealed, air-tight environment until it is needed for use - it will, of course, adsorb moisture from any environment. Although the moisture uptake rate is not fast, silica gel 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.
- Useful life of silica gelSilica gel can indefinitely adsorb and desorb moisture and never has to be replaced. It is the moisture within the silica gel that is used up when it is buffering against changes in RH within an enclosed area. The useful life of silica gel is shortened by excessive heating and fine dust or oil and other substances blocking its fine structure. There are a number of methods for reconditioning silica gel. Some methods require the removal of some or all of the silica gel, and other methods allow for reconditioning silica gel within the case through the addition of a controlled amount of water or additional dry silica gel. Please refer to FAQ - Reconditioning silica gel.