Desiccant method
Introduction
According to DIN 55 473, the purpose of using desiccants is as follows: "desiccant bags are intended to protect the package contents from humidity during transport and storage in order to prevent corrosion, mold growth and the like."
The desiccant bags contain desiccants which absorb water vapor, are insoluble in water and are chemically inert, such as silica gel, aluminum silicate, alumina, blue gel, bentonite, molecular sieves etc.. Due to the absorbency of the desiccants, humidity in the atmosphere of the package may be reduced, so eliminating the risk of corrosion. Since absorbency is finite, this method is only possible if the package contents are enclosed in a heat sealed barrier layer which is impermeable to water vapor. This is known as a climate-controlled or sealed package. If the barrier layer is not impermeable to water vapor, further water vapor may enter from outside such that the desiccant bags are relatively quickly saturated, without the relative humidity in the package being reduced.
Desiccants are commercially available in desiccant units. According to DIN 55 473:
"A desiccant unit is the quantity of desiccant which, at equilibrium with air at 23 ± 2°C, adsorbs the following quantities of water vapor:
Introduction
According to DIN 55 473, the purpose of using desiccants is as follows: "desiccant bags are intended to protect the package contents from humidity during transport and storage in order to prevent corrosion, mold growth and the like."
The desiccant bags contain desiccants which absorb water vapor, are insoluble in water and are chemically inert, such as silica gel, aluminum silicate, alumina, blue gel, bentonite, molecular sieves etc.. Due to the absorbency of the desiccants, humidity in the atmosphere of the package may be reduced, so eliminating the risk of corrosion. Since absorbency is finite, this method is only possible if the package contents are enclosed in a heat sealed barrier layer which is impermeable to water vapor. This is known as a climate-controlled or sealed package. If the barrier layer is not impermeable to water vapor, further water vapor may enter from outside such that the desiccant bags are relatively quickly saturated, without the relative humidity in the package being reduced.
Desiccants are commercially available in desiccant units. According to DIN 55 473:
"A desiccant unit is the quantity of desiccant which, at equilibrium with air at 23 ± 2°C, adsorbs the following quantities of water vapor:
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min. 3.0 g at 20% relative
humidity
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min. 6.0 g at 40% relative
humidity
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The number of desiccant units is a measure of the adsorption capacity of the desiccant bag."
Desiccants are supplied in bags of 1/6, 1/3, 1/2, 1, 2, 4, 8, 16, 32 or 80 units. They are available in low-dusting and dust-tight forms. The latter are used if the package contents have particular requirements in this respect.
Barrier films
Barrier films are available in various forms, for example as a polyethylene film or as a composite films with two outer polyethylene layers and an aluminum core. The composite film performs far better with regard to water vapor permeability (WVP), achieving WVP values of below 0.1 (g/m2d). In the composite film, the barrier layers are arranged so as to bring about a considerable reduction in permeability in comparison with a single layer.
In accordance with current DIN standards, water vapor permeability is always stated for both 20°C and 40°C. According to information from the manufacturer, it may be concluded that water vapor permeability rises with increasing temperature and falls with increasing thickness. This problem occurs most particularly with polyethylene films, while aluminum composite films are largely insensitive to rises in temperature.
Placement of desiccant bags
The desiccants should be suspended from strings in the upper part of the climate-controlled package to ensure good air circulation around them.
It is essential to avoid direct contact between the desiccant bag and the package contents as the moist desiccant would promote corrosion.
It is advisable to use numerous small bags rather than fewer large ones, as this increases the available surface area of the desiccant and so improves adsorption of the water.
In order to ensure the longest possible duration of protection, the barrier film must be heat sealed immediately once the desiccant bags have been inserted.
Desiccant bags are always supplied in certain basic package sizes which, depending upon the desiccant unit size, may contain a single bag (of 80 units) or up to 100 bags (of 1/6 unit). The basic outer package should only be opened directly before removal of a bag and must immediately be heat sealed again.
Comparison of advantages and disadvantages of the desiccant method
Advantages
Barrier films are available in various forms, for example as a polyethylene film or as a composite films with two outer polyethylene layers and an aluminum core. The composite film performs far better with regard to water vapor permeability (WVP), achieving WVP values of below 0.1 (g/m2d). In the composite film, the barrier layers are arranged so as to bring about a considerable reduction in permeability in comparison with a single layer.
In accordance with current DIN standards, water vapor permeability is always stated for both 20°C and 40°C. According to information from the manufacturer, it may be concluded that water vapor permeability rises with increasing temperature and falls with increasing thickness. This problem occurs most particularly with polyethylene films, while aluminum composite films are largely insensitive to rises in temperature.
Placement of desiccant bags
The desiccants should be suspended from strings in the upper part of the climate-controlled package to ensure good air circulation around them.
It is essential to avoid direct contact between the desiccant bag and the package contents as the moist desiccant would promote corrosion.
It is advisable to use numerous small bags rather than fewer large ones, as this increases the available surface area of the desiccant and so improves adsorption of the water.
In order to ensure the longest possible duration of protection, the barrier film must be heat sealed immediately once the desiccant bags have been inserted.
Desiccant bags are always supplied in certain basic package sizes which, depending upon the desiccant unit size, may contain a single bag (of 80 units) or up to 100 bags (of 1/6 unit). The basic outer package should only be opened directly before removal of a bag and must immediately be heat sealed again.
Comparison of advantages and disadvantages of the desiccant method
Advantages
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Desiccants provide excellent
corrosion protection to both metallic and nonmetallic items
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Removal of the desiccant on
delivery to the receiver is straightforward, unlike the removal of protective
films in the protective coating method. The package contents are immediately available.
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No particular occupational hygiene
requirements apply as the desiccant is nonhazardous.
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Disadvantages
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Placement of the desiccant bags
and heat sealing of the barrier films are relatively labor-intensive.
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The slightest damage to the barrier
layer may negate the effectiveness of corrosion protection.
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Calculating the required number of
desiccant units is not entirely simple and it is easy to overcalculate.
However, too much protection is better than too little.
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Humidity indicators inside the
package are not very reliable as they are only valid for certain temperature
ranges.
http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm
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