Content of the material
What is silica gel?
Silica gel is a desiccant – also known as a drying agent. The small beads inside each sachet are able to absorb moisture or condensation.
You might be wondering what the purpose of silica gel is for manufacturers. Silica gel sachets can be found in anything from packets of pepperoni to medications, as these products can be in warehouses and stores for quite some time. Moisture can easily build up and spoil the goods. Instead, the packets absorb any moisture and prevent mould growth.
Silica gel can significantly reduce moisture damage done to books and hard-copy records during storage or transport. It is important to note that, for any storage duration, cardboard boxes are not adequate protection against moisture. The Library of Congress (LOC) states that relative humidity during storage for books should be about 35 percent. Although some consideration must be made for the humidity levels and temperature ranges of the storage environment, the LOC’s recommended humidity target can be readily achieved by placing bulk silica gel into the proper storage containers along with the books.
Silica gel’s high specific surface area (around 750–800 m2/g (230,000–240,000 sq ft/oz)) allows it to adsorb water readily, making it useful as a desiccant (drying agent). Silica gel is often described as “absorbing” moisture, which may be appropriate when the gel’s microscopic structure is ignored, as in silica gel packs or other products. However, material silica gel removes moisture by adsorption onto the surface of its numerous pores rather than by absorption into the bulk of the gel.
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 sachets 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, 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 system caused by condensation or moisture. A similar setup is used to dry the compressed air for brake systems on railway locomotives, where condensation and ice in the brake air pipes could lead to brake failure.
Silica gel is sometimes used as a preservation tool to control relative humidity in museum and library exhibitions and storage.
In chemistry, silica gel is used in chromatography as a stationary phase. In column chromatography, the stationary phase is most often composed of silica gel particles of 40-63 micrometers (μm) diameter. In this application, due to silica gel’s polarity, non-polar components tend to elute before more polar ones, and this process is called normal phase chromatography. However, when hydrophobic groups (such as groups with long, C18 chains) are attached to the silica gel, then polar components elute first and the method is referred to as reverse phase chromatography. Silica gel is also applied to aluminum or plastic sheets for thin layer chromatography.
In some cases, chelating groups have been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous media. Chelating groups can be covalently bound to polyamines, which in turn have been grafted onto a silica gel surface, producing a material of greater mechanical integrity. Silica gel may also be combined with alkali metals to form a reducing agent.
Silica gel is also used as cat litter, by itself or in combination with more traditional materials, such as clays including bentonite. It is trackless and virtually odorless, but expensive.
Why You Should Keep Silica Gel Packets
If you normally throw these packets away, it’s time to break that habit. There are so many uses for silica gel around the house. Silica gel is incredibly absorbent with the ability to absorb over 40 percent of its own weight in water. The materials surrounding the bag are well protected against the penetration of moisture.
Tip: Collect silica gel packets in an airtight container and leave them in a dark and dry space so that you always have a small supply when needed.
How much silica gel should I use? [ edit
Several factors will influence the amount of silica gel required for a specific exhibit application. Refer to the publication below to estimate the amount of moisture sorbent needed for your cases. Manufacturers of the gels could also be consulted regarding their respective products.
The amount of silica gel can be proportionately increased for the following mitigating factors:
- There is a very wide humidity differential between the case and the exhibit area.
- The RH fluctuation parameter to be maintained is extremely tight.
- The ambient temperature will fluctuate more than 20° F.
- The air volume of the case is very large relative to the size of the object(s).
- It is necessary to extend the time between silica gel maintenance (or eliminate the need altogether).
- ↑ Feldmann, Maryann, and Pierre Desrochers. “Research Universities and Local Economic Development: Lessons from the History of the Johns Hopkins University” Industry and Innovation 10(1): 5-24, March 2003. Retrieved September 27, 2007.
How does silica gel work? [ edit
Silica gel can absorb up to 40% of its weight at high humidity levels. Its moisture absorbing capacity is based on large surface area created by surface pores and by interstitial micro pores that contain capillary channels. Polar water molecules drawn into the channels by capillary action are stored there until released. Silica gel can be used to maintain a specific relative humidity level (e.g., a low level as a desiccant) or to maintain an RH range (e.g., levels between 40 to 60% RH). Before use, the gel must be conditioned to the desired RH. The effectiveness of the silica gel also depends on:
- a tightly-sealed exhibit case, with little or no air leakage to the ambient exhibit space;
- a maintenance compartment to store an adequate amount of gel, accessible for maintenance purposes; the gel can be hidden behind a panel in the bottom, ceiling, sides, or back of the case;
- air inside the case circulating over the silica gel;
- a method to contain the gel; silica gel should never touch collection objects, nor should you touch the gel with your bare hands;
- exposure of the gel over as wide a surface area as possible;
- temperature; silica gels absorb best at temperatures below 77° F (25° C).
Is silica gel poisonous?
Although the packets have a huge ‘DO NOT EAT’ sign, silica gel is actually non-toxic, so on its own it’s not actually poisonous. However, you should obviously keep them inaccessible to animals and small children who could eat them, as they can potentially have traces of other bad chemicals, and are a choking hazard.
Alberto Tolentino Pineda Getty Images
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