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Reviewed and updated by ICD - Week 24/2026
Desiccant is a material that adsorbs and holds water vapor inside a sealed space, used to prevent mold, rust and product damage during storage and transport. It is the most common and cheapest moisture-control solution in packaging, and especially vital for sea-freight exports - where humidity and temperature swings can ruin a whole shipment even when carefully boxed.
What desiccant is and how it adsorbs moisture
Desiccant works by adsorption: the material’s surface has countless micropores that trap water molecules from the air, pulling the relative humidity (RH) inside sealed packaging down to a safe level (typically below 50-60% RH). When humidity drops below the dew point, water no longer condenses on the goods - that is how desiccant stops mold and rust.
One gram of silica gel has an internal surface area of 500-800 m², so a small amount captures a lot of water. Adsorption capacity is expressed as a percentage of weight: silica gel, for example, holds 20-35% of its own weight under normal conditions.
“Container rain” - why goods get damp even when sealed
Many shippers load dry goods yet find mold or rust on arrival. The cause is container rain: by day the sun warms moisture out of the wooden pallet, cartons and goods; by night the container walls cool, condensing that vapor into droplets that fall on the cargo like rain. A long sea voyage can repeat this cycle dozens of times.
Desiccant adsorbs this vapor before it condenses, so it is the main line of defense against container rain - even more effective combined with floor liners and sealed packaging.
Common types of desiccant
Each type has a different capacity and cost; choose by environment and transit time:
| Type | Capacity (% of weight) | Characteristics and use |
|---|---|---|
| Silica gel | 20-35% | Clean, non-toxic, fast; small sachets for parts, electronics, pharma |
| Clay (bentonite) | 20-28% | Low cost, stable at normal temperatures; general industrial goods |
| Container desiccant (CaCl2) | 150-300% (gels) | Very high uptake, hanging bags; fights container rain on long sea hauls |
| Molecular sieve | ~22% | Adsorbs deeply at low RH; highly moisture-sensitive, fully sealed goods |
Note: silica gel and clay hold a small percentage but retain well at high RH; CaCl2 adsorbs many times its weight by forming a gel, making it the top choice for containers.
How to calculate how much desiccant to use
The amount depends on the volume, seal quality, initial humidity and transit time. Two reference methods:
By sealed package volume: about 1-2 desiccant units per 0.03 m³ (one standard unit ≈ 33 g of silica gel, holding ~6 g of water at 40% RH). A 0.3 m³ box, for example, needs about 10-20 units.
By sea container: use hanging container desiccant. Reference:
| Container | Short voyage (under 2 weeks) | Long voyage (over 4 weeks) |
|---|---|---|
| 20ft | ~4-6 kg | ~8-10 kg |
| 40ft / 40HC | ~8-10 kg | ~14-18 kg |
Damp goods such as timber, produce and textiles need more; the tighter the packaging, the less you need.
How to place desiccant correctly
Add desiccant the moment you seal, and do not leave it exposed because it adsorbs ambient moisture before doing its job. Distribute it evenly around the goods and keep it near the most moisture-sensitive surfaces. In a container, hang desiccant bags along both walls and behind the doors - where condensation is heaviest. Combine with floor liners and sealed packaging (PE film, foil bags) for maximum effect. For metal goods, pair it with VCI anti-corrosion.
Common mistakes when using desiccant
First, using too little for the volume and transit time - the desiccant saturates mid-journey and stops working. Second, sealing goods and wooden pallets while still damp - that internal moisture drives container rain and overwhelms the desiccant. Third, placing desiccant in leaky packaging, so it adsorbs outside air instead of protecting the cargo. Fourth, using plain silica gel for long sea containers when high-capacity CaCl2 is the right choice.
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Frequently asked questions
Which desiccant is best?
It depends: silica gel is the cleanest and most common for parts, electronics and pharma; for long sea containers use CaCl2 container desiccant for its many-times-weight capacity.
How much desiccant for a 20ft container?
About 4-6 kg for short voyages and 8-10 kg of container desiccant for voyages over 4 weeks, more for damp goods like timber and produce.
Why do sealed goods still mold during export?
Because of container rain: day-night temperature swings condense vapor that drips onto the cargo. You need enough desiccant plus sealed packaging and floor liners.
Can silica gel be reused?
Yes. Dry it at 120-150°C for a few hours to reuse it several times; clay and CaCl2 types are usually single-use.
Is desiccant toxic?
Silica gel is inert and non-toxic but not for eating; sachets carry warnings. CaCl2 forms a solution after adsorbing, so keep it from direct contact with goods.
Should desiccant be used with VCI?
For metal goods use both: VCI lays an anti-rust vapor layer on the surface while desiccant lowers overall humidity - the two mechanisms complement each other.
Contact ICD Vietnam
ICD Vietnam Industrial Manufacturing Co., Ltd - pallets, plastic crates, stretch film, forklifts and packaging solutions.
Hotline: 0983 797 186 / 090 345 9186
Email: sales@icdvietnam.com.vn | Zalo: Chat on Zalo
