Question: What can we do to prevent caking?
When our powders are packaged into bags at our plant, they appear to be free of agglomerates. However, our customers report that when the bags are opened at their facilities, they observe an excessive amount of caked material. This results in rework and potential losses in sales. We suspect that moisture is the cause, but our bags include a vapor barrier, and our records show that we are indeed producing dry materials.
Answer: Even if your product seems dry, unwanted agglomeration may still occur over time due to moisture migration. Moisture migration occurs whenever there is a difference in water activity (the equilibrium relative humidity of the air in the void spaces between the powder particles) and is often a consequence of temperature gradients. Transient temperature gradients may occur when the powder is packaged at elevated temperatures and then stored or transported under ambient conditions. If the bags are stored outdoors or in a warehouse that has poor temperature control, temperature cycling may occur.
Consider the case where a powder is packaged at a temperature higher than ambient. As the powder cools, material closest to the fabric (for example, polymer, paper or cloth) will be colder than powder at the center of the bag. As a result, the relative humidity of the interstitial air will be highest nearest the fabric exposed to ambient temperatures. The moisture in that air will adsorb onto the surface of the particles in an effort to establish equilibrium. This lowers the absolute humidity of the air closest to the fabric, thereby providing a concentration driving force for further transfer of moisture from the center of the bag towards the fabric.
Once moisture migration takes place, there are three mechanisms that can cause the particles in a powder to bond, resulting in caking:
1. With many powders, water can act as a plasticizer, which will soften the material, leading to an increase in interparticle contact area, a greater number of contacts between neighboring particles, and a decrease in distance between adjacent particles. The resulting increase in cohesive strength can be dramatic if the bulk material is one where plasticization causes its glass transition temperature to fall below ambient.
2. When some powders are exposed to high relative-humidity air, the water adsorbed by the powder collects between powder particles due to capillary condensation, forming liquid bridges. If the entire void is filled with liquid, a negative capillary pressure will exist, resulting in additional attractive forces between particles.
3. Solid bridges between particles can form when soluble matter in liquid bridges precipitate during cooling or drying. In general, capillary condensation must first occur in order for solid bridging to take place.
To analyze this problem and develop a solution, the root cause must be uncovered. Shear cell tests that measure the powder’s gain in cohesive strength when stored at rest can be conducted under controlled conditions in a laboratory. Such an analysis will reveal powder moisture contents that must be avoided to prevent caking.
Some possible solutions include:
1. Cooling the powder to ambient temperatures before packaging.
2. Conditioning the powder prior to packaging by injecting low-humidity air as the powder flows through a mass-flow silo.
3. Drying the powder to a sufficiently low level such that if moisture migration takes place, any local increase in powder moisture content will not cause agglomeration due to plasticization or liquid or solid bridging.
4. Adding a parting agent, such as magnesium oxide or silicon dioxide to your powder to minimize interparticle contact area.
Of course, there are other causes of caking, such as the presence of impurities or attrition of powder particles. Again, shear cell testing can be used to determine if contaminants or fines are the cause of unwanted agglomeration.
Greg Mehos is a project engineer at Jenike & Johanson, Inc. (Tynsboro, Mass.; www.jenike.com), an engineering firm specializing in the storage and flow of powders and bulk granular solids.