Derived Properties of Powders
Dry powder materials are handled in one out of every two factories throughout the world. Every powder has its own set of properties that influence its final quality. In addition to its inherent features, a powder's behaviour is influenced by its interactions with its environment, notably with the atmosphere. The density and flowability of the powder will also be affected by how it is handled (packaging properties, electrostatic charge, surface morphology, etc.).Particle size
Particle size refers to both particle shape and size distribution of a powder. Particle size distribution affects many other properties of the powder (density, ease of flow, solubility, and more).It's critical to distinguish between two types of mass/density -
- The mass of many particles in a granule or powder divided by the entire volume they fill is known as bulk density.
- The ratio of the mass of a volume of material to the volume of that substance is known as density, or mass density.
Density of Powders
The bulk density of a powder refers to the number of particles as well as the space between them. Kilograms per cubic meter or pounds per cubic foot are the units of measurement.When it comes to industrial processes, density is crucial since it dictates the size of equipment and containers (hoppers, silos, bulk bags, sacks). If it is high, it will lower the transportation cost estimated based on volume. It also affects the powder's incredibly beneficial properties, such as hydration.
Flow of Powders
Before any conditioning, unloading, transport, storage, dosing, or even combining procedures, the ability of a powder to flow effectively is a crucial aspect to consider. If the powder's flow characteristics were inadequate, lumps might develop, causing damage to particular equipment.Hygroscopicity of Powders
Hygroscopic powders tend to absorb or adsorb moisture from the air. If the proportion of hygroscopicity in a powder is less than 10%, it is not termed hygroscopic. Lactose powder, calcium chloride, sorbitol, soda, and magnesium oxide are all hygroscopic powders.The production of agglomerates, which can possibly block transfer equipment, might make a powder's flow troublesome. A dehumidification operation will have to be installed upstream in order to reduce humidity exposure in this case.
Solubility of Powders
The solubility of a powder, which is determined by dissolving it in another material to create a homogenous mixture, will have an impact on some industrial processes in which the powder is to be introduced into an aqueous phase.Many factors influence a powder's solubility, including viscosity, air temperature, particle size, and so on.
Wettability of Powders
The time it takes for all of the powder to pass across the air/water contact without being agitated is the wettability of a powder. Another way to say it is that a powder's wettability relates to its ability to absorb water. The liquid-solid contact angle is typically used to describe the wetting of a solid, such as a powder, versus a liquid. For a particular liquid-solid combination, numerous parameters such as pressure, temperature, and humidity will influence wetting.The size of a powder's particles, its density, the presence of air, and other factors can all influence its wettability.
When the powder is utilized as a process ingredient, this property is critical. This is especially essential in the instant beverage sector.
Dispersibility of Powders
The ease with which a powder dissolves in water when stirred is referred to as dispersibility. Powder dispersibility is affected by a number of factors, including particle size, outlet air temperature, protein content, and so on. Industries such as fast food and newborn nutrition are looking for high dispersibility powders to use in their products.Porosity
A substance has pores if there are microscopic holes or voids inside. It is the openings between grains or the pockets of air trapped between grains in a microstructure that are defined as porous. Porosity contributes to corrosion when it absorbs liquids or moisture. For solid granite, porosity is less than 0.01 while peat and clay have a higher porosity. As an alternative to expressing the fraction in percentage terms, multiply it by 100 instead.The void fraction is another name for porosity.
Porosity can be divided into two categories -
- Surface porosity is a type of porosity that occurs on the metal's surface and can be seen with the naked eye.
- Subsurface porosity is a type of porosity that occurs within the metal and may only be found via specialist testing.
Similarly, pores generate poor results in ornamental coatings applied to structural castings, and are the origin of failures in coatings, including surface pitting, spotting, or corrosion. Any of these faults can eventually render a cast part unfit for its intended function. Impregnation is a cost-effective, long-term solution to the difficulties that might arise as a result of casting porosity.
Packing Arrangement
The term porosity, which has already been established, is a way to describe how many air spaces there are in a powder. The porosity, generally represented as a percentage, is the percentage of total volume that is void. The equation connects it to the bulk density, ρb.
Where ρ is the powder's actual density,
When spheres of identical size are packed in a regular pattern, porosity can range from 46 percent for a cubical arrangement to 26 percent for a rhombohedral array. Figure depicts these extremes. The porosity of perfect systems of this sort is unaffected by particle size. Packing isn't always consistent in practice. The most open structure is produced by packing in cubic shape, which happens when the following layer is placed directly on top of the four spheres above.
Rhombohedral packing, which is made by creating the following layer around the sphere seen in a broken line in Figure, is the closest.
Nonetheless, the porosity of coarse, isodiametric particles with a small range of sizes is very consistent, ranging between 37 and 40 percent. Lead shot, for example, has the same porosity as finely graded sand. Because some fine particle packing in the interstices between the coarsest particles is feasible with broader size ranges, porosity reduces. Fine powders do not have these effects. Because of their more cohesive nature, the porosity of the particles increases as they become finer, and the size distribution has no effect.
Because open packing and bridging become more prevalent as particle shape departs from sphericity, porosity increases in any uneven array. A flaky substance with a porosity of roughly 90%, such as crushed mica, packs well. If the surface of the particles is rough, the porosity will rise.
Chance packing happens when powders are poured, and porosity is affected by the speed of the process and the degree of agitation. If the powder is poured slowly, each particle can find a stable place in the developing surface. The porosity will be low, the interstitial volumes will be small, and there will be a lot of interactions with the surrounding particles. Because there isn't enough time for stable packing when pouring is done fast, bridges form as particles clash, resulting in a higher porosity bed. Open packing and the creation of bridges are both hampered by vibration. When densely packed powder beds are required, it is frequently used.
Packing at a border is not the same as packing in a powder's bulk. Normally, the border generates a zone of more open packing, which can span many particle layers. When particles are compressed into compact volumes, this is critical. If the particles are relatively large, the zone of expanded packing and low bulk density will be large, and the weight of material that fills the space will decrease as the particle size rises under these conditions. The inverse is true with finer powders: as the particle size decreases, the weight of powder required to fill a volume decrease. As a result, a small volume has a maximum capacity for a certain particle size. The size of the space in which the particles are squeezed determines this.
Bulkiness and Densities
A powder's bulk density is the ratio of its mass to its volume, including the contribution of inter-particulate void volume. As a result, the bulk density is determined by the density of powder particles as well as voids in the spatial arrangement of particles in the powder bed. Because the measurements are taken using cylinders that provide volume in mL, bulk density is generally reported in grammes per milliliter (1 g/mL = 1 g/cm3 = 1000 kg/m3). The bulk qualities of a powder are determined by the sample's production, treatment, and storage, or how it was handled. The particles can be packed in a variety of ways to achieve a variety of bulk densities. As a result, the untapped and tapped bulk densities are distinguished.To determine the untapped bulk density of a powder, the volume of a known mass of powder sample in a graduated cylinder (Method 1), or the mass of a known volume of powder that has been passed through a volumeter into a cup (Method 2) or has been introduced into a measuring vessel is used (Method 3).
Even the slightest perturbation in the powder bed can result in a change in the bulk density, especially for cohesive powders. The untapped bulk density is often difficult to measure with good reproducibility in these cases, and it is critical to specify how the determination was made when reporting the results.
Flow Properties
Powders can be free-flowing or adhere to one other ("sticky"). Powder flow is blamed for a slew of typical production issues, including:- Powder is transferred using heavy machinery such as a hopper.
- Excess entrapped air inside powders, uneven powder flow, capping, or lamination
- Uneven powder flow increases particle friction with the die wall, producing lubrication issues and increasing the possibility of dust contamination during powder transport.
- issues with non-uniformity (segregation) in mixing powder flow
Tests to evaluate flowability of powders
1- The compressibility index of Carr
A known volume of powder is placed in a graduated glass cylinder and tapped repeatedly for a set period of time. After tapping, the volume of powder is measured.
Tapped density - Poured or bulk density x 100/Tapped density Equals Carr's index (percentage)
Bulk density is defined as the weight divided by the bulk volume.
Weight / real volume = tapped density
Tests to evaluate flowability of powders
percent compressibility of the flow description
The flow is excellent 5–15
excellent 16-18
good 19-21
bad 22-35
Very bad 36-40
> 40 is really bad.
Adding glidant between 1.25 and 1.5 usually improves flow. A value of 1.5 or above implies weak flow (= 33% Carr). Powders that are more cohesive and less free-flowing, such as flakes. Coarse spheres, for example, have little interparticle friction. A value of less than 1.25 denotes good flow (= 20% Carr).
Tapped density /Poured or bulk density = Hausner ratio
Interparticle friction was linked to the Hausner ratio:
Tests to determine a powder's flowability include - 2 - Hausner ratio: > 1.5 more glidant has little effect
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