Desiccants – Function, Types and Applications

During the storage and transport of metallic goods, climatic conditions can cause corrosion damage. It is therefore essential to protect products safely against moisture. A proven solution for this is desiccants, which can be used flexibly and easily and are ideally suited, among other things, to protecting metal parts and electronic components from moisture.
As simple and flexible as the use of desiccants may be in everyday packaging operations, a great deal of expertise lies behind them. In this interview, Marco Nickel, Managing Director of TROPACK Packmittel GmbH, provides a comprehensive overview of the following questions:

• How do desiccants work?
• What types of desiccants are there, and what are they used for?
• How do I know how much desiccant needs to be used?
• Are desiccants environmentally friendly, and how should they be disposed of?
• What new challenges exist in desiccant packaging, and what are the trends — for example ESD and automation?

1.How do desiccants work?

There are two categories of desiccants that function in fundamentally different ways:

1) Physically acting desiccants
Physically acting desiccants adsorb* moisture by taking up water vapour in their pores. These physically acting desiccants do not change their form during the adsorption process; in other words, they remain solid, retain their size with no change in volume, and are chemically inert.
From the outside, it is not possible to tell whether physically acting desiccants are saturated or still active. The exception is silica gels with indicators, such as orange gel or ruby gel, which change colour as adsorption progresses.
Through the adsorption of atmospheric moisture, these desiccants change only in weight: the more water vapour is adsorbed, the heavier the desiccants become.

2) Chemically acting desiccants
Chemically acting desiccants absorb* moisture and condensation water as part of a chemical reaction. These desiccants usually use calcium chloride as the key active substance. Calcium chloride is predominantly effective in environments with very high relative humidity and also absorbs condensation water.
Chemically acting desiccants change their form during absorption. Granulate, for example, often turns from an unsaturated state into a soft or gel-like mass after absorption, more than doubling its volume.
 

2. What types of desiccants are there, and what are they used for?

1) Physically acting desiccants
Physically acting desiccants can, almost without exception, come into direct contact with sensitive products such as food and medicines, as well as components such as electronics and optics. Physically acting desiccants are used in primary packaging or as a component in ongoing operation within machines and systems.

Within the group of physically acting desiccants, the following types are distinguished:

1. Clay: Clay is the “standard desiccant” for desiccant bags and the most commonly filled desiccant in desiccant bags that comply with DIN 55473 — from the smallest 1/6-unit bags to the largest 32-unit desiccant bags.
   Clay is a natural product mined from bentonite deposits around the world.
2. Silica gel: Due to its significantly higher adsorption capacity compared with clay, silica gel is the “standard desiccant” for very small desiccant bags — from 0.25 g to 5 g filling weight — or for applications with very limited installation space. Higher adsorption capacity means that less space is required.
   In the pharmaceutical, diagnostics and food industries, silica gel is often the preferred desiccant. It is also filled into DIN 55473 desiccant bags, but in that case has the same adsorption capacity as DIN 55473 desiccant bags filled with clay.
   Silica gel is an amorphous silicon dioxide that is produced chemically by acidifying water glass.
3. Orange gel / ruby gel (Silica gel with indicator): Silica gels can be combined with a colour indicator for visual monitoring of the desiccant’s saturation level. Based on the colour change of the desiccant, the user can see when the silica gel is saturated and needs to be replaced.
   Orange gel and ruby gel are suitable for contact with food. These indicator gels are used, among other things, in applications where important parameters for calculating the desiccant requirement are unknown and where it is not clear exactly how much moisture needs to be adsorbed.
   Thanks to the possibility of visual monitoring, orange gel and ruby gel are also ideally suited for long-term packaging.
4. Molecular sieve: Molecular sieve is characterised by special kinetic properties and is suitable for rapid adsorption — “sharp drying” — as well as applications with low relative humidity below 40% RH, for example cold chains.
   In molecular sieve, the diameter of the pore, i.e. the “channel” in which moisture is bound, is precisely defined. The pore diameter is described in ångströms and, in addition to moisture uptake, also enables the adsorption of other molecules with a defined molecular diameter, such as gas decomposition products.
   Molecular sieve is a crystalline aluminosilicate that is produced chemically by combining aluminium oxide with silicon dioxide.

2) Chemically acting desiccants
Chemically acting desiccants, especially calcium chloride salts, are not suitable as primary packaging materials because they can damage products in the event of direct contact.

The main application of chemically acting desiccants is container desiccant bags, which are placed outside primary packaging — such as wooden crates, big bags or closed cardboard boxes — during sea transport.

The purpose of these bags is, in particular, to absorb condensation water dripping from the inside of the container roof. For use in sea containers outside primary packaging, chemically acting desiccants are particularly well suited because they are especially efficient at high relative humidity levels and also absorb condensation water, often referred to as “container rain.”

The absorption capacity can reach up to 300% of the desiccant’s own filling weight at very high relative humidity levels above 80% RH.
 

3. How do I know how much desiccant needs to be used?

There is a German industrial standard — DIN 55474 — in which a formula is defined for calculating the required number of desiccant units. This formula can be used to calculate the amount of desiccant required for transport and/or long-term storage in order to adequately protect products in packaging against moisture.

The following parameters are taken into account in this formula: