What is the difference between adsorption and absorption desiccants?

Adsorption desiccants (silica gel, activated clay, molecular sieve) physically trap water on their internal surface and stay solid, with capacity capped by surface area at roughly 25 to 40 percent of weight. Absorption desiccants (calcium chloride) chemically react with and dissolve into the water, reaching 300 to 600 percent of weight and forming a gel. The mechanism determines capacity and behaviour.

Which desiccant has the highest capacity?

Calcium chloride has by far the highest capacity, 300 to 600 percent of its own weight, because it absorbs water chemically rather than adsorbing it on a surface. Silica gel and molecular sieve hold about 25 to 40 percent, and activated clay 25 to 100 percent depending on humidity.

Silica gel or molecular sieve, which is better?

It depends on the target dryness. Molecular sieve has a steep isotherm and adsorbs strongly even below 10 percent RH, so it drives an enclosure to a very low dew point, ideal for sealed electronics and insulating glass. Silica gel works best in the 30 to 70 percent RH mid-range and is cheaper for general low-humidity packs. For shipping containers neither is ideal; calcium chloride is.

How do I calculate how much desiccant I need?

Use the DIN 55473 method: n = (V x b + sum of (m x a)) / c, where V is the enclosure volume, b a moisture/duration factor, m and a the mass and water-release factor of each hygroscopic packing material, and c the working capacity of one desiccant unit (about 6 g of water per DU). It sums the water the air and packing will release and divides by per-unit capacity. BENZ runs this calculation free for your cargo and route.

What is a Desiccant Unit (DU)?

A Desiccant Unit is a standardised quantity of moisture-adsorbing capacity defined by DIN 55473 and MIL-D-3464E, roughly the amount that adsorbs 6 g of water at 23 C and 40 percent RH. It lets desiccants of different types and bag sizes be compared and specified on equal terms.

Technical Reference

Types of Desiccants, Compared

Silica gel, activated clay, molecular sieve and calcium chloride are not interchangeable. They capture water by different physics, hold radically different amounts, and behave differently as humidity changes. This is the engineering reference: how each works, how much it holds, the adsorption isotherm that decides its real-world performance, and how to calculate exactly how much you need with the DIN 55473 method.

Reviewed by the BENZ Packaging technical team · Updated June 2026

Desiccant Range →

First Principles

Adsorption vs Absorption: The Distinction That Decides Everything

Two mechanisms capture water, and confusing them is the root of most desiccant mistakes. Adsorption is a physical process: water molecules adhere to the vast internal surface of a porous solid through Van der Waals forces. Silica gel, activated clay and molecular sieve are adsorbents; they stay solid, do not change chemically, and their capacity is capped by available surface area. Absorption is a chemical process: the desiccant reacts with and dissolves into the water it captures. Calcium chloride is an absorbent; it deliquesces, turning into a brine, and its capacity is governed by how much active material it contains, which is why it reaches 300–600% of its weight where adsorbents plateau at 25–40%.

The second decisive property is the adsorption isotherm, the curve of how much water a desiccant holds at a given relative humidity. Molecular sieve has a steep early isotherm: it grabs water hard at very low RH and drives an enclosure bone-dry. Silica gel rises gradually and performs best in the mid-range. Clay sits below silica gel but is cheap and stable. Calcium chloride does the opposite of all of them, its uptake accelerates as RH climbs toward 100%, which is exactly why it dominates the high-humidity, long-voyage container problem. Choosing a desiccant is choosing the isotherm that matches your enclosure's humidity profile.

The Four Desiccants in Depth

Mechanism, capacity, isotherm and the application each one wins.

Silica Gel (SiO₂)

Amorphous porous silica, ~700–800 m²/g surface area. Adsorbs ~25–40% of its weight, best at 30–70% RH. Non-toxic, widely recognised, indicator grades available. Saturates and plateaus, so it under-performs on long, hot, humid container routes.

Activated Clay (Montmorillonite)

Natural thermally-activated clay, >200 m²/g. Adsorbs ~25–100% of weight depending on RH, stays solid, nitrite-free and chemically stable, the safe choice in barrier bags with metals and electronics. The basis of BENZPACK C DRY.

Molecular Sieve (Zeolite)

Synthetic crystalline alumino-silicate with uniform pores (3A/4A/13X). Adsorbs strongly even below 10% RH and to very low dew points. The choice for sealed electronics, insulating glass and gas drying where an extremely dry endpoint is required.

Calcium Chloride (CaCl²)

Deliquescent salt that absorbs 300–600% of its weight, accelerating as RH rises, binding water into a leak-proof gel. The only technology that truly eliminates container rain on long ocean routes. The basis of BENZPACK BE DRY and BE DRY ULTRA.

Side-by-Side Comparison

Property	Silica Gel	Activated Clay	Molecular Sieve	Calcium Chloride
Mechanism	Adsorption	Adsorption	Adsorption	Absorption (deliquescent)
Capacity (% of weight)	25–40%	25–100%	20–25%	300–600%
Best RH range	30–70%	10–80%	<1–30% (very dry)	60–100% (high)
Isotherm behaviour	Gradual, plateaus	Gradual	Steep early, then flat	Accelerates with RH
Form when saturated	Solid	Solid	Solid	Leak-proof gel
Nitrite-free / metal-safe	Yes	Yes (BENZPACK C DRY)	Yes	Yes
Best use	Small low-RH packs	Barrier bags, electronics, pharma	Sealed dry endpoints	Shipping containers
BENZPACK product	—	C DRY	—	BE DRY / BE DRY ULTRA

How to Calculate How Much Desiccant You Need (DIN 55473)

The international method that converts a moisture load into a precise number of desiccant units. This is what separates a protected container from a guessed one.

The DIN 55473 Desiccant Unit (DU) Formula

n = ( V × b + Σ ( m_i × a_i ) ) / c

n = number of desiccant units required · V = volume of the enclosure / barrier (m³) · b = a factor for the air's moisture and the required protection duration · m_i = mass of each hygroscopic material inside, wood, paper, cardboard (kg) · a_i = that material's water-release factor · c = the working capacity of one desiccant unit (1 DU adsorbs roughly 6 g of water at 23°C / 40% RH per the standard).

In plain terms: add up the water the enclosed air will hold plus the water the packing materials will release, then divide by how much one unit can take. A “Desiccant Unit” is a defined quantity, not a bag size, which is why DIN 55473 lets you compare desiccants on equal terms.

Worked example, 40ft container, 25-day Asia–Europe route: ~67 m³ internal volume, timber-and-carton packed (heavy hygroscopic load). The calculated moisture load works out to roughly 40–55 DU. With activated clay at 1 DU per small unit that is dozens of bags; with calcium chloride, whose working capacity per kilogram is far higher, that is just 6–8 × 1kg BE DRY units (or 5–6 BE DRY ULTRA). Same moisture load, a fraction of the units, because capacity per unit is what the formula rewards.

Which Desiccant for Which Job

Match the isotherm to the enclosure.

Shipping container, long ocean route

Calcium chloride (BE DRY / BE DRY ULTRA), the only technology with the capacity and the rising isotherm to eliminate container rain.

Electronics / PCB in a barrier bag

Nitrite-free activated clay (C DRY) per J-STD-033C, or molecular sieve where an ultra-dry endpoint is specified.

Pharmaceuticals & diagnostics

Clay or silica gel in controlled formats; nitrite-free, DMF-free chemistry for packaging compatibility.

Sealed optics / insulating glass / gas

Molecular sieve, for its steep low-RH isotherm and very low achievable dew point.

Small consumer / retail packs

Silica gel, recognised, low-cost and effective in small low-humidity enclosures.

Mixed or unsure

Send us the enclosure, materials and route and our engineers run the DIN 55473 calculation and specify the type and quantity, free.

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