A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness in its vicinity. Desiccants are most effective in closed environments. They are an effective way to control the moisture content inside product packaging’s and transport containers.
Sorbent materials used as desiccants have a particular affinity for water molecules, able to bond free water molecules from their direct surrounding. We can divide desiccants into two categories:
Absorbing water, either physically or chemically bonding water molecules into the body of the desiccant material
Adsorbing water, physically bonding water molecules to the surface of the desiccant material.
Absorbent desiccant
Absorption of moisture from the surrounding air occurs through chemical reaction with the bulk of the absorbent material. The water molecules are embedded into the absorbent’s material structure.
As the moisture is absorbed within the whole of the material, absorbent desiccants have a high-water adsorption capacity up to a multiple of their net weight. They are slow acting desiccant types which can actively absorb moisture for up to 3 months and are effective in all temperature settings which can regularly occur during transport and storage conditions.
Given these properties of absorbent desiccants, they are well suited for high moisture environments, like goods shipped during ocean transport, where the cargo is exposed to moisture for extended periods of up to 3 months under extreme temperature conditions between 0°C and 60°C inside the ocean container.
Calcium chloride (CaCl2) is a deliquescence salt which in its pure form can absorb up to 10 times the amount of water of its net weight, forming a hydrate (salt brine).
Today most Calcium chloride desiccants are a mix of anhydrous CaCl2 with starch where the absorbed moisture is transformed into a gel.
Calcium chloride desiccants are effective in a wide temperature range between 0°C and 90°C and can absorb moisture for up to 90 days and absorb up to 300% of their dry weight.
These properties make Calcium chloride desiccant the perfect solution to protect cargo from moisture damage during long haul ocean shipments.
Magnesium chloride (MgCl2) is a deliquescence salt which in its pure form can absorb up to 6 times the amount of water of its net weight, forming a hydrate (salt brine).
Available Magnesium chloride desiccants are mainly of a dry-dry type, where the desiccant always stays dry. The chemically changed anhydrous MgCl2 absorbs moisture and turns from a powder into dry lumps without a liquid stage.
Magnesium chloride dry-dry desiccant have similar properties as Calcium chloride desiccant but with a lower absorption capacity of max. 200% of its net weight and a shorter effective application time of up to 60 days.
Magnesium chloride desiccants are a suitable alternative to Calcium chloride desiccant for application in ocean transport conditions. Due to the higher material costs of MgCl2, this desiccant type is a good choice for users who don’t accept the gel stage of the more common Calcium chloride desiccants.
Adsorbent desiccants
Adsorption of moisture from the surrounding air occurs on the large capillary surface structure. Through the mechanism of surface energy, the water molecules stick to the surface area of the adsorbent, forming a thin film on the adsorbent material surface.
Adsorbent desiccants have a comparable low water adsorption capacity between 20% and 40% of their net weight. They are fast acting desiccant types which reach saturation within 3 days and are only effective in temperature setting between 15°C and 30°C.
Given the properties of adsorbent desiccants, they are not suited for goods shipped in ocean containers, where the merchandise is exposed to moisture for extended periods of up to 3 months.
Silica gel is a chemically manufactured amorphous and porous form of silicon dioxide. Type A Silica gel with an average pore size of 2.4 nanometers has a strong affinity for water molecules and is widely used as a desiccant.
The adsorption capacity of Silica gel is limited to a maximum of 40% of its dry weight. It has a fast moisture absorption rate but short moisture absorption cycle of only 1 to 3 days to reach saturation. Adsorption is most efficient in a narrow temperature range between 15°C and 30°C. Under higher temperatures, adsorbed moisture will re-evaporate back into the environment.
Some Silica gel desiccants contain an indicator substance that change color when water has been adsorbed. Special attention needs to be taken with blue indicator Silica gel, as it may contain cobalt(II) chloride, in EU listed as a substance of very high concern and banned as an indicator substance with Silica gel.
Dry clay desiccant also known as Bentonite or Montmorillonite desiccants, are manufactured through the controlled refining of magnesium aluminum silicate, an abundant mineral clay composition retrieved through open mining.
The adsorption capacity of dry clay is limited to a maximum of 30% of its dry weight. Comparable to Silica gel, dry clay has a fast moisture absorption rate with short moisture absorption cycle of only 1 to 3 days to reach saturation. Adsorption is most efficient in a temperature range below 50°C. Under higher temperatures, adsorbed moisture will re-evaporate back into the environment.
Molecular sieves are synthetic zeolite materials made of aluminosilicates and engineered with pores of precise and uniform structure and size. This allows molecular sieves to be specifically made to adsorb a targeted molecule, like water. Molecular sieves work in ultra-low moisture and high temperature environments. The adsorption capacity of molecular sieve is limited to 20% of its net weight. Molecular sieves are mainly used in technical applications to dry liquids and gases or within packaging where an ultra-low environment of 10% relative humidity or less is required, like for certain medical instruments.
