At school we learn some absolute foundations on which to build our embryonic scientific knowledge: gases are compressible but liquids aren’t. The world is an ordered place and we begin to understand its rules.
Unfortunately for some - interestingly for others - the world of powders fails to conform to such hard and fast rules. Powders shift easily between gas-, liquid- and solid-like behaviours. This can be industrially important, fascinating and frustrating – not always in equal measure!
What Can Powders Do?
Returning to the issue of compressibility, it’s interesting to observe where powders lie along the gas-liquid-solid spectrum. In practice, different powders are variously compressible – some sponge-like and compressed relatively easily by a significant amount, others exerting enormous resistance to compressive force.
This is because certain powders, usually cohesive ones, easily entrain and hold air within their bulk. Compressing them simply squeezes the air pockets, so the powder essentially behaves like a gas. Some powders though are very tightly packed beds of particles containing much less air. Their behaviour when compressed is almost indistinguishable from that of stiff solids.
It’s not only about compressibility, of course. Certain powders are, for example:
- Sensitive to moisture
- Responsive to air
- Easily fluidised
- Permeable
- Sensitive to the flow rate at which they are transported
- Prone to segregation
But other aren’t, or are less so.
Characterise Yourself with One Adjective?
I often use the analogy of athletes when discussing powders because it so instantly highlights the difficulties of choosing one number or parameter to describe them. Like athletes, every powder has a unique, inherent set of characteristics, which manifest themselves in different ways depending on the prevailing environment. Thinking in this way also introduces the idea that we need to match powders with applications that play to their strengths - as you would choose different athletes for different events.
Powder Flow and Powder Coating
In spray coating applications, a fine powder, typically a polymer, is drawn from a storage device before being fluidised and ejected through a charged nozzle onto a substrate. It is essential that the powder can be effectively and consistently fluidised, without the formation of agglomerates that may block the nozzle and affect the charging of the individual particles, leading to poor adhesion or the formation of agglomerates on the substrate. It is also essential to establish a smooth flow from the storage device, as erratic flow into the fluidisation chamber will lead to a poorly fluidised bulk.
Identifying and quantifying which powder properties correlate with the most efficient performance in a process allows new formulations to be optimised without the significant cost of running samples through the process to assess suitability, making considerable savings in terms of time and raw materials, and minimising wastage due to rejected products.
I hope this has whetted your appetite to learn more, and would like to suggest “Batch Variability in Spray Coating Applications” as a good starting point. Alternatively, if you have more specific issues to share then please don’t hesitate to leave a comment or contact me at info@freemantech.co.uk.
