Machinery, optical and electronic apparatus, military goods and weapons systems, pharmaceutical products and many others need protection against corrosion and deterioration caused by high humidity and condensation. This protection is needed during both storage and transport, all over the world, but especially when climate zones with high temperatures and relative humidity (RH) levels are encountered.
The use of desiccants, preferably in combination with a barrier material, provides this protection, simply and cheaply, over long periods of time.
In Europe, a system of norms for the methods and articles was introduced a number of years ago, called the DIN classification system. The use of desiccants is included in this system, under norms DIN 55473 and 55474. Altico supplies a range of desiccants complying with these two DIN norms. They are activated, calcium rich clay and silica gel which are not poisonous, chemically indifferent and environmentally friendly.
Apart from complying with DIN 55473 and 55474, Altico’s desiccants also comply with MIL-D- 464 (US Army) and TL 6850-008 (German Army).
Characteristics
Desiccants are granules of between 0,3 and 6,3 mm in which there are countless numbers of pores. One gram of desiccant can have a surface area of 1 m². The less water they contain themselves, the more active they are ie, the more water vapour they can adsorb from the surrounding air. Air dried by desiccants will not release condensation on sensitive surfaces when the temperature drops.
Sufficient protection is achieved when the RH in a closed package does not exceed 40% during storage or transport. This is easily achievable with the correct use of desiccants. Rust forming commences when the RH exceeds 40%.
Desiccant unit
According to DIN 55473, a desiccant unit is the measure for the adsorption capacity of the desiccant. At a temperature of 22°C, at least the following quantities of water vapour must be absorbed:
* 3 grams at 20% Relative Humidity
* 6 grams at 40% Relative Humidity
This unit weighs approximately 33 grams.
Desiccants are packed in sachets containing 1/6 of a unit (5 grams), 1/3 (10 grams), (15 grams) and larger bags containing 1 (30), 2 (60), 4 (125), 8 (250), 16 (500), 32 (1000) and 80 (2500) units (grams). It should be borne in mind that, depending on the size of the area that is to be protected, 10 bags of 8 units are generally more effective than 1 bag of 80 units.
Humidity indicators
These are usually small pieces of cardboard with a number of coloured circles impregnated with cobalt chloride, each indicating a certain percentage RH ranging from 10% to 80%. As the humidity increases, the chemically impregnated circles change from blue (dry) through lavender to pink (humid). When it decreases, the process is reversed.
They are also supplied in aluminium or plastic ‘windows’, which can be screwed into the packaging material and through which the RH inside the package can be read.
Application of desiccant bags
* The area to be protected must be hermetically sealed or as close thereto as possible. Outside openings must be closed off with Alu-foil or rubber strips (eg, sliding windows).
* The desiccant bags must be placed as high as possible inside the area that is to be protected, spread evenly from front to back.
* Internal compartments or lockers must be opened.
* Try to avoid articles of wood, paper, cardboard etc, leaving inside the area; these can contain up to 20% of their weight in water which will be released into the air when it warms up and will condense when it cools off. If there are wooden struts or supports inside the object/package, additional bags of desiccants have to be used.
Practical calculation method
To calculate the number of desiccant units required for a particular application, one must determine the following:
1. The Moisture Vapour Transmission Rate (MVTR) of the packaging material
For 200 micron polyethylene, this is 0,4 grams per square metre per 24 hours. For 400 micron polyethylene, it is 0,2 grams per square metre per 24 hours. For Alufoil A30, it is 0,1 grams per square metre per 24 hours
2. The surface area of the packaging material in square metres.
3. The total number of days for which protection is required.
The formula that is applied is:
Number of desiccant units = (MVTR X surface area X days)/6
where the divisor 6 is the amount of water in grams which 1 unit has to absorb to keep the RH at 40%.
In case there is cardboard/wooden dunnage inside the package, add 20 units of desiccant per kilogram of dunnage. In case it concerns a hermetically sealed object (eg, metal cases etc,), 5 units of 30 grams must be used per m³ of space.
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