About Molecular Sieve Desiccant Powder
Molecular sieve desiccant is an activated synthetic zeolite supplied as a fine powder. Its crystal structure contains pores of a uniform size, typically 3 or 4 ångström, that admit small water molecules and hold them firmly, while larger molecules such as solvents, monomers and polymer chains are excluded. This selectivity is what separates molecular sieve from general-purpose desiccants.
The grade is used in moisture-sensitive systems where a reactive desiccant such as calcium oxide would be unsuitable. In two-component polyurethane coatings, elastomers, adhesives and casting resins, water reacts with isocyanate to form carbon dioxide, which causes bubbles, pinholes and loss of gloss. A small addition of molecular sieve powder to the polyol component captures that water before mixing. It is also used in high-performance plastics compounding and in coatings and sealants where storage stability depends on very low residual moisture.
Molecular sieve keeps adsorbing at low relative humidity, where other desiccants lose most of their capacity. It does not swell or dissolve, and the adsorbed water is retained at normal processing temperatures. Obsidian Minerals supplies the powder in an activated condition, tests water adsorption, particle size and moisture on every batch in an NABL-accredited laboratory, and packs it in moisture-protective packaging. Pore type and particle size can be discussed on request.
Typical specifications
| Parameter | Typical Value | Test Method |
|---|---|---|
| Zeolite type | 3A or 4A (on request) | — |
| Static water adsorption | ≥ 20 % w/w | Internal method, 25 °C |
| Particle size, D50 | 2–6 µm | Laser diffraction |
| Loss on ignition (as supplied) | ≤ 3.0 % | Internal method, 575 °C |
| Bulk density | 0.40–0.60 g/cm³ | Internal method |
| pH (5 % slurry) | 9.5–11.0 | Internal method |
Typical values for guidance. Guaranteed limits are stated on the Technical Data Sheet and batch CoA. Values apply to freshly opened, activated material.
Key features & benefits
Selective water adsorption
Molecular sieve desiccant pores admit water but exclude larger molecules, so the powder dries a resin or polyol without taking up solvents, plasticisers or reactive components of the formulation.
Works at low humidity
Zeolites retain high adsorption capacity at low relative humidity, allowing them to bring residual moisture in resins and compounds down to levels that silica gel or clays cannot reach.
Non-reactive and non-swelling
The powder does not react with isocyanates, epoxies or polymers and does not swell, so it can be added to sensitive systems without side reactions or viscosity drift in storage.
Fine, dispersible powder
A D50 of a few microns allows the powder to disperse in liquid resins and polymer melts with standard mixing, without visible grit in thin films or moulded surfaces.
Applications
Two-component polyurethanes
Added to the polyol side of PU coatings, floorings, elastomers and casting resins to remove water that would react with isocyanate, preventing bubbles, pinholes and gloss loss.
Adhesives and sealants
Used to improve storage stability of moisture-sensitive adhesives and sealants by scavenging water introduced with fillers, pigments and plasticisers during production, which helps limit premature curing and skinning in the pack.
High-performance plastics
Dosed into compounds of moisture-sensitive engineering polymers to reduce hydrolysis-related defects and surface splay, complementing standard resin drying. Effect should be confirmed by moulding trials.
Coatings and inks
Incorporated into solvent-borne coatings and inks that contain moisture-sensitive resins or curing agents, helping maintain viscosity, pot life and film performance during storage and after the container is opened.
Usage & handling
- Typical dosage: 1–5 % on the polyol or resin component, set by its water content and target.
- Disperse with the fillers and pigments under good mixing, then allow time for adsorption before packing.
- Expect slight warming on contact with water; this is normal adsorption heat.
- Reseal opened packs immediately; exposure to humid air uses up adsorption capacity.
Packaging & supply
- 25 kg HDPE or paper bags with moisture-barrier liners on request
- 1 MT or 1.2 MT jumbo (FIBC) bags
- Custom pack sizes on request
Frequently asked questions
What is the difference between 3A and 4A molecular sieve?
The number refers to the approximate pore opening in ångström. Type 3A admits water but excludes most other molecules, which makes it the safer choice in systems containing small reactive molecules such as methanol. Type 4A has slightly larger pores and a high water capacity, and is widely used in polyurethanes and general drying. We can advise on the type for your formulation.
How much molecular sieve powder should I add to a polyurethane system?
Typical additions are about 1–5 % of the polyol component, depending on its water content and the moisture brought in by fillers and pigments. Measuring water content, for example by Karl Fischer titration, before and after addition is the most reliable way to fix the dose.
Can molecular sieve be used instead of calcium oxide in rubber?
It can be used, but calcium oxide is usually more economical for rubber compounds cured at high temperature. Molecular sieve is preferred where a non-reactive desiccant is required, where the water must be removed at very low humidity, or where alkaline reaction products would be a problem.
Is the powder supplied activated?
Yes. The powder is supplied in an activated, low-moisture condition and packed to protect it from humid air. Its adsorption capacity is used up if the pack is left open, so it should be used promptly and resealed. Guidance on reactivation is available on request for users who need it.