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Types of Desiccants

1. SILICA GEL DESICCANT
The most commonly known and used desiccant is silica gel which is a form of silica dioxide (SiO2), a naturally occurring mineral. It will work from below freezing to past the boiling point of water, but performs best at room temperatures (70-90?F) and high humidity (60-90%). Its performance begins to drop off over 100?F, but will continue to work until approximately 220?F. It will lower the relative humidity in a container to around 40% at any temperature in its range until it is saturated. Silica gel will absorb up to 40% of its weight in moisture. Some forms are approved by the FDA for direct food use (check with your supplier to be sure). It recharges easily (see below in the indicating silica gel text) and does not swell in size as it adsorbs moisture.

2. INDICATING SILICA GEL DESICCANT

In the retail trade, the most commonly found form of silica gel is indicating silica gel which is small white crystals looking much like granulated sugar with small pink or blue colored crystals scattered throughout. This is ordinary silica gel with the colored specks being coated with cobalt chloride, a heavy metal salt. When the gel has absorbed approximately eight percent of its weight in water the colored crystals will turn from blue to pink making an easy visual indicator of whether the gel has become saturated with moisture. Because cobalt is a heavy metal, indicating silica gel is not food safe and should be kept from spilling into anything edible.

The indicating silica gel will still adsorb up to 40% of its weight in water vapor just like the non-indicating type will but once it has gone past the 8% level and the crystals have turned pink there is no way to tell how close it is to saturation. This isn't necessarily a problem, you'll just have to treat like the other non-indicating desiccants and either weigh it to determine adsorption or use a humidity indicator card. These cards are made to show various humidity ranges and can be had from many desiccant and packaging suppliers.

When saturated, both varieties of silica gel can be dried out and used again. This is done by heating the crystals in an oven at a temperature of no more than 300?F (149?C) for approximately three hours or until the crystals turn blue. Dehydrating the desiccant may also be accomplished by heating in a microwave oven. Using a 900 watt oven heat the crystals for three minute intervals until the color change occurs. The exact amount of time necessary will depend upon the oven wattage. Spreading the desiccant in a broad pan in a shallow layer will speed the process. Heating to 325?F (149?C) or more, or using a microwave oven over 900 watts can damage the gel and render it unable to adsorb moisture.

If your desiccant is packaged, particularly if packaged in Tyvek, do not heat it above 250?F (121?C) or you could damage the material. This leaves a fairly narrow temperature window since silica gel will not begin to desorb moisture below 220?F (104?C). It's a good idea to use a reliable oven thermometer to check your oven temperature as the thermostats in home ovens are often off by more than twenty five degrees. Start with the packets in a cold oven and raise the temperature to 245?F (118?C), keeping it there for twenty four hours. Spread the packets so they are not touching and keep them at least 16 inches from any heating elements or flames so that radiant heat does not damage the packaging. Tyvek should not be microwaved.

3. CLAY DESICCANT

Although not typically found for sale on the retail market, clay desiccant is fairly common in commercial and industrial use. The primary reason for this seems to be that it is inexpensive compared to any other form of desiccant. Some mail order suppliers offer it for retail sale.

The material is Montmorillonite clay, composed primarily of magnesium aluminum silicate, a naturally occurring mineral. After mining it is purified, reduced to granules and subjected to a controlled dehydration process to increase its sorbent porosity. It recharges easily and does not swell as it adsorbs water vapor. It works well at low and room temperatures, but has a rather low ceiling temperature. At 120?F it will begin to desorb or shed the moisture it has adsorbed. This is an important consideration for storage in hot areas.

Subject to a degree of variability for being a natural material, clay desiccant will adsorb approximately 25% of its weight in water vapor at 77?F and 40% relative humidity.

4. CALCIUM OXIDE DESICCANT

Also known as "quicklime" or "unslaked lime", calcium oxide is a slow, but strong adsorbent. It is efficient at low humidities and can drop moisture vapor to below 10% relative humidity. Qucklime is caustic and must be carefully handled, particularly with regards to dust inhalation and exposure to skin and eyes. It expands as it soaks up water vapor and this must be taken into account when packaging it. It will adsorb up to about 28% of its weight in moisture, but does it slowly over a period of several days rather than a matter of hours like other desiccants. It is most effective when used in high humidity environment where a very low humidity level is desired. It will release a fair amount of heat if exposed to direct (liquid) moisture or extreme humidities.

Calcium oxide can be recharged, but I do not have any details on how to go about this other than roasting at fire temperatures.

For expedient use, quicklime can be manufactured from clean, pure lime stone (calcium carbonate) or pickling lime (calcium hydroxide) available in the canning sections of many grocery and hardware stores.

5. CALCIUM SULFATE DESICCANT

Also known as the mineral gypsum and commercially as Drierite, calcium sulfate is another naturally occurring mineral. It is produced by the controlled dehydration of gypsum (CaSO4). It is chemically stable and does not readily release its adsorbed moisture. It has a low adsorbency capacity, only approximately 10% of it weight. It can be regenerated, but apparently not easily so.

For expedient use, gypsum is commonly used in household drywall and Kearny mentions using this source in his Nuclear War Survival Skills. This makes only a so-so desiccant and you'd be much better off to use a more suitable choice but in an emergency it can get the job done.

6. Montmorillonite Clay
Montmorillonite clay is a naturally occurring adsorbent created by the controlled drying of magnesium aluminum silicate of the sub-bentonite type. This clay will successfully regenerate for repeated use at very low temperatures without substantial deterioration or swelling. However, this property causes clay to give up moisture readily back into the container as temperatures rise.

Clay is a good basic desiccant that works satisfactorily below 120°F (approximately 50°C). Above 120° F, there is a possibility that the clay will give up moisture rather than pulling it in, so anticipated storage and transportation conditions should be considered. The upside to clay is that it is normally the least expensive desiccant per pound.

Clay is highly effective within normal temperature and relative humidity ranges. Its appearance is that of small gray pellets. Care should be taken to be sure that any low level impurities in the clay are not incompatible with the packaged product.

7.Molecular Sieve Desiccant
Molecular sieves are porous crystalline aluminosilicates, a synthetic desiccant that has a very strong affinity for moisture molecules. The distinctive feature of the molecular sieve structure, as compared to the other desiccants, is the uniformity of the pore size openings in the crystal lattice structure.

There is no pore size distribution with molecular sieves. As part of the manufacturing process, the pore size on the molecular sieve particles can be controlled. The most commonly used pore size is 4 angstroms (4A) although 3 angstroms (3A), 5 angstroms (5A) and 10 angstroms (13X) are available. This feature allows the selection of a molecular sieve product which can adsorb water, yet exclude most other molecules, such as volatile organics, which might be present in the package.

For example, Type 3A molecular sieve's structure, with a 3 angstrom pore opening, allows moisture adsorption, but excludes most hydrocarbons. Type 4A molecular sieve has a slightly higher moisture capacity, but adsorbs molecules as large as butane. Type 13X molecular sieve has a different crystal structure from the types 3A and 4A, and has a pore opening of about 10 angstroms. This allows for the adsorption of a wide range of organic molecules as well as moisture.

The selective adsorption characteristics of molecular sieves can be useful when it is necessary to dry a package without removing other desirable compounds from the system. Molecular sieve can hold moisture to temperatures well past 450°F (230°C), and because of its high affinity for moisture, molecular sieve is able to bring the relative humidity in packages down as low as 10% RH.

The United States FDA has not approved molecular sieve for direct contact with consumable items, although in Europe molecular sieve is used with pharmaceuticals. Being man-made rather than naturally occurring, molecular sieve is slightly higher in cost per unit, but due to its extremely large range of adsorptive capabilities, it might often be the best value, especially in areas of low relative humidity.

Lack of government approval for the use of molecular sieves in food and drug packaging has limited its more widespread use. Independent testing suggest that molecular sieves meet government requirements. Presumably, however, the industry has been unwilling to fund the expensive testing required for government approval.

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