Powder Business

A powder is a dry, bulk solid composed of a large number of very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms powder and granular are sometimes used to distinguish separate classes of material. In particular, powders refer to those granular materials that have the finer grain sizes, and that therefore have a greater tendency to form clumps when flowing. Granulars refers to the coarser granular materials that do not tend to form clumps except when wet. Typically, a powder can be compacted or loosened into a vastly larger range of bulk densities than can a coarser granular material. When deposited by sprinkling, a powder may be very light and fluffy. When vibrated or compressed it may become very dense and even lose its ability to flow. The bulk density of coarse sand, on the other hand, does not vary over an appreciable range.

Powders are transported in the atmosphere differently than a coarse granular material. For one thing, tiny particles have little inertia compared to the drag force of the gas that surrounds them, and so they tend to go with the flow instead of traveling in straight lines. For this reason, powders may be an inhalation hazard. Larger particles cannot weave through the body's defenses in the nose and sinus, but will strike and stick to the mucous membranes. The body then moves the mucous out of the body to expel the particles. The smaller particles on the other hand can travel all the way to the lungs from which they cannot be expelled. Serious and sometimes fatal diseases such as silicosis are a result from working with certain powders without adequate respiratory protection.

A paste or gel might become a powder after it has been thoroughly dried, but is not considered a powder when it is wet because it does not flow freely. Substances like dried clay, although dry bulk solids composed of very fine particles, are not powders unless they are crushed because they have too much cohesion between the grains, and therefore they do not flow freely like a powder. A liquid flows differently than a powder, because a liquid cannot resist any shear stress and therefore it cannot reside at a tilted angle without flowing (that is, it has zero angle of repose.) A powder on the other hand is a solid, not a liquid, because it may support shear stresses and therefore may display an angle of repose.

Many common powders made in industry are of combustible; particularly metals or organic materials such as flour. Since powders have a very high surface area they can combust with explosive force once ignited. Facilities such as flour mills can be vulnerable to such explosions without proper dust mitigation efforts. Some metals become especially dangerous in powdered form, notably titanium.