FAQ Archives – Levosil

⏱ Reading time: 3 minutes

Molecular sieves: selective moisture adsorption and high-temperature resistance

Molecular sieves are synthetic zeolites with calibrated porosity, used to produce desiccant bags for special applications where silica gel or activated clay are not suitable. Types 4A and 3A are the ones normally used for moisture adsorption.

Their use makes sense in two cases: when the environment contains other gases that must not be removed together with the moisture, and when moisture must be adsorbed and retained even at high temperatures. Outside these cases, silica gel or activated clay offer equivalent performance and are easier to handle.

Structure: why adsorption is selective

A molecular sieve is an aluminosilicate with a three-dimensional structure of channels and cavities of regular size. The pore opening determines which molecules can enter and be retained: 3A and 4A sieves adsorb only sufficiently small, polar molecules, such as water.

This regularity is the key difference from silica gel and activated clay, which are non-selective adsorbents: they retain moisture, but also other gases present in the air.

High-voltage switchgear

The application where selectivity is essential is high-voltage switchgear insulated with gas (arc-quenching gas). Here the goal is to remove residual moisture without removing the insulating gas: a non-selective desiccant would adsorb both, altering the composition of the gas and its function. Thanks to its calibrated pores, the molecular sieve solves this specific problem.

High-temperature performance

Unlike the other desiccants used in industrial packaging, molecular sieves adsorb and retain moisture even at high temperatures. Silica gel and activated clay begin to lose effectiveness at around 50 °C; the molecular sieve maintains good desiccant capacity even above 120 °C.

Its regeneration temperature exceeds 300 °C: even at very high temperatures, the material does not release the adsorbed moisture. For the same reason, under normal conditions a molecular sieve is practically impossible to regenerate.

Why they are not used for container protection

In shipping containers, the problem is absorbing large quantities of moisture at ambient temperature: neither selectivity nor heat resistance is needed. The right solution is calcium chloride (Levodry Gel).

Pharmaceutical applications

Molecular sieves are often used in the pharmaceutical sector, but in most cases they offer no real advantage over silica gel. The performance of molecular sieve and silica gel is practically identical (both materials have a desiccant capacity of 20% of their weight under standard test conditions of 40% relative humidity and 23 °C). A molecular sieve may be useful when moisture, but not other gases, must be removed inside the drug package, but these cases are very rare.

It is often pointed out, correctly, that molecular sieves adsorb moisture faster than other desiccants. Under normal conditions and in small packages, however, the difference is barely measurable.

In short

Molecular sieve = synthetic zeolite (typically 3A or 4A) with calibrated porosity and selective moisture adsorption.
Use it when moisture must be removed without capturing other gases, and when moisture must be retained at high temperatures (active at 120 °C, regeneration above 300 °C).
Not needed in containers: the right solution is calcium chloride (Levodry Gel).
In pharmaceutical applications, adsorption performance is comparable to silica gel; the speed advantage is marginal.
Our technical team is available to recommend the most suitable desiccant for the result you need to achieve.

You can contact us now for more information: 0039 011 96 43 430 + info@levosil.com

⏱ Reading time: 4 minutes

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.

If you would like to give us a call, we will handle the calculation for you: 0039 011 96 43 430 — info