We face consistency all day long without even thinking about it, for example in the morning when we use toothpaste or face creams. Creams, in particular, have different consistencies. Liquid-like lotions soak into the skin easily – a good example here is sunscreen lotion. More solid-like ointments stick on the skin. This is useful for treating open wounds and cuts, for example. Butter, margarine, cheese, honey and other foods need to be spreadable but not so runny that they drip off the bread or knife.
Another good example in daily life is the bitumen which is used for road construction and roofing. In warm climates, the consistency of soft bitumen can lead to rutting on the road or to bitumen dropping off roof shingles because of insufficient stiffness of the bitumen binder. Very hard bitumen consistency can cause fatigue cracks on the road surface under cold climatic conditions.
All of these examples show how important it is to have the right consistency of the specific material.
But the first question should be: what is consistency exactly?
In the literature you find the definition that consistency describes the degree to which a semi-fluid or semi-solid material resists deformation by an applied force. This measurement of consistency is called penetration.
The relationship of consistency to viscosity is obvious. The property of a fluid or semi-solid substance which causes it to resist flow is called viscosity. Therefore, the application fields of viscometers, dynamic shear rheometers and penetrometers overlap.
The deformation behavior of a material depends on the material’s inner structure. This inner structure is characteristic for each material and can be influenced by outside forces as well as the ambient conditions.
The inner structure is everything that is dissolved or dispersed in a material which contributes to its inner structure. This can be solvent molecules, more complex polymeric structures, small particles or even gas bubbles.
The ambient condition that influences the consistency most is the temperature. For some substances a decrease of 1 °C or °K can cause a consistency increase of 10 %.
The second question should be: what is the difference between fluid, semi-solid and solid material?
Every single substance on earth can be placed on a scale from fluid to solid.
A semi-fluid or solid material needs a special stress before it begins to deform elastically or plastically. The point of plastic deformation is called the yield point. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Typical semi-fluid and semi-solid samples are butter, grease, ointment and petrolatum. A fluid (liquid) has no such yield point.
The classification of different greases, for example, is defined by the NLGI (National Lubricating Grease Institute) in the USA. They have established consistency numbers or grade numbers, ranging from 000 to 6, corresponding to specified ranges of penetration numbers.
|NLGI Class||Penetration Number (mm/10)||General Consistency||Use Scope|
|000||445 – 475||fluid||GE, Z|
|00||400 – 430||semi-fluid||GE, Z|
|0||355 – 385||very soft||GE, Z|
|1||310 – 340||soft||GE, WL, GL, Z|
|2||265 – 295||medium-soft||WL, GL, Z|
|3||220 – 250||semi-solid||WL, GL|
|4||175 – 205||solid||WL, WP|
|5||130 – 160||hard||WP|
|6||85 – 115||very hard|
|GL = sliding friction bearing lubrication
WP = water-pump lubricant
WL = rolling-contact bearing and wheel bearing lubrication
GE = gear lubrication
Z = forced circulation lubrication
How can you measure consistency?
There are different instruments which measure consistency. For this blog article we picked out the penetrometer.
A typical example of consistency determination is the penetration measurement of grease. A cone assembly of given weight (150 g) is allowed to sink into a grease for 5 seconds at a standard temperature of 25 °C (77 °F). The depth, in tenths of a millimeter (Penetration Unit, PU) to which the cone sinks into the grease is the penetration.
Consistency measurement with the penetrometer can be carried out on a huge variety of samples, including pasty, creamy, semi-solid or highly viscous samples. For each sample you need different test kits.
There is one general rule for choosing the right test kit: The harder (more solid) the material is, the smaller the cone ankle should be. Therefore, a needle is used for bitumen and a perforated disk penetrator is used for liquid-like samples.