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A desiccant sachet is considered “expired” when it has adsorbed all the moisture it can hold: at that point, it stops working.

The speed at which it saturates does not depend on chemical degradation of the material, but on how much moisture it encounters.

In a well-sealed package, with little incoming moisture, the same sachet can protect the contents for months. In a permeable package, or one filled with damp materials, it can be exhausted within a few days.

The desiccant does not expire. It saturates.

You might be tempted to look for an expiration date, as on a yogurt pot. But silica gel and activated clay do not chemically degrade over time: they adsorb water vapour until they reach their maximum capacity, then they stop.

The right question, then, is not “how long does the desiccant last”, but “how much moisture will reach the desiccant, and how quickly?”.

The packaging determines the duration

The desiccant sachet always works together with the packaging. Its job is to remove the moisture contained inside the packaging and capture the moisture that penetrates from outside over time.

If the barrier material enclosing the desiccant sachet and the goods to be protected lets large amounts of moisture through, the desiccant will saturate quickly.

The key parameter is the water vapour permeability of the packaging material (indicated on technical datasheets as WDD or WVTR, in g/m² per day): the more permeable the barrier, the more moisture enters each day, the sooner the desiccant saturates.

For this reason it is essential to always use quality barrier materials, suitable for the intended purpose and for the protection duration required.

The hidden moisture inside the packaging

There is also the moisture that the packaging itself already contains from the start. Pallet wood, cardboard, paper fillers: they are all hygroscopic materials that hold water and release it slowly into the air inside the package.

Never place damp materials inside the packaging. Every gram of water contained in wood or cardboard is a gram the desiccant will have to adsorb: capacity wasted from the start.

How long it lasts, in numbers: calculation according to DIN 55474

Estimating the required amount of desiccant — and therefore the duration of protection — is not done by eye: the DIN 55474 standard provides the calculation formula. But the formula only works if you have all the data. Here they are:

• V — internal volume of the packaging (m³)
• b — moisture contained in the internal air at closure (g/m³)
• m — mass of hygroscopic materials inside the packaging: wood, cardboard, fillers (kg)
• c — moisture content of these materials (g/kg)
• A — surface area of the barrier packaging (m²)
• WDD — water vapour permeability of the barrier material (g/m² · day)
• t — required protection duration (days)
• a, e — factors defined by the standard: adsorption capacity per desiccant unit and climatic correction factor

The result is the number of DIN desiccant units to insert in the packaging to guarantee protection for the entire time t.

We must accept that industrial packaging always has some water vapour permeability, whether higher or lower.

If the quantity resulting from the DIN calculation is correctly applied, you have the guarantee that the packaging will be preserved for the desired duration.

DIN 55474 applies to desiccants used inside packaging. For sizing desiccants used inside shipping containers, the reference is the CTU Code: we have covered this in a dedicated article.

In practice

• The desiccant does not expire: it saturates as a function of the moisture it encounters.
• Duration is determined by the packaging: a barrier is needed to isolate both the goods and the sachet from the outside environment.
• Check the water vapour permeability (WDD) of the barrier material: this is the parameter that governs how much moisture enters each day. A low moisture barrier will require a larger number of desiccant sachets; a very high moisture barrier (such as aluminium Triplex) will require a smaller quantity.
• No damp materials inside the package: wet wood and cardboard consume the desiccant before its time.
• To calculate quantity and duration, use DIN 55474, with all the required data: without them, any estimate is a gamble.

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