Honey, hand cream, toothpaste, sweet jelly, grease bitumen and candle wax: Depending on their physical behavior, all materials can be placed on a scale from highly viscous to semi-solid and solid. Consistency measurement is used to determine the degree to which a material resists deformation by an applied force. This article presents the basic principles of consistency determination with a penetrometer.
Consistency describes the degree to which a semi-fluid or semi-solid material resists deformation by an applied force. The deformation behavior of a material depends on the material’s inner structure. The inner structure is characteristic for each material and can be influenced by outside forces as well as the ambient conditions. What is an inner structure? Anything that is dissolved or dispersed in a material contributes to its inner structure. This can be solvent molecules, more complex polymeric structures, small particles, gas bubbles, etc.
The ambient condition that influences the consistency most is the temperature. For some substances a decrease of 1 degree Celsius (°C) or Kelvin (K) can cause a consistency increase of 10 %.
A typical device which is used for the determination of consistency is a penetrometer.
Fig. 1: Samples in a row from fluid (left) to solid (right)
Substances we encounter, within the fluid or solid state of matter, can be described according to their consistency, see Figure 1. A semi-fluid or solid material needs a certain mechanical stress at which 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 mechanical stress is removed. Once the yield point is passed, a fraction of the deformation will be permanent and nonreversible. Typical semi-fluid and semi-solid samples are butter, grease, ointment, and petrolatum. A fluid (liquid) has no such yield point.
We come across consistency all the time in everyday life without even thinking about it. Examples which illustrate the importance of consistency measurement are given in section 4.
A simple method to determine the consistency of different products is by means of a 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 sample for five seconds at a standard temperature of 25 °C (77 °F).
The result is the depth, in tenths of a millimeter (Penetration Unit, PU), at which the cone sinks into the grease and this expresses the consistency. High penetration values indicate softer materials.
Penetration unit = PU = 0.1 mm
The deeper the cone sinks into the material, the softer the sample and the lower the consistency.
Fig. 2: Grease penetration measurement[1]
Fig. 3: Penetrometer with hollow cone
Fig. 4: Test body selection
A penetrometer is very versatile and a lot of different penetration bodies can be used with this instrument, depending on the consistency of the sample.
The general rule for test kit selection is: The harder (more solid) the material, the smaller the cone angle should be. Therefore, a needle is used for bitumen and a perforated disk penetrator is used for liquid-like samples.
The consistency of semi-solid and solid bituminous materials is tested up to 500 Penetration Units (PU). The standard methods used are ASTM D5 and EN 1426. Find out how to use the penetrometer PNR 12 for consistency determination of semi solid materials like bitumen here.
Penetration is the basic value for the bitumen classification systems worldwide. Using this value, conclusions can be drawn about the performance of bitumen as a binder for road construction. This helps when selecting the bitumen binder best suited to the climatic conditions of the respective region. Typical negative consequences of neglecting the consistency are:
The behavior of the bitumen in the medium temperature range can be assessed by means of the penetration test. The typical test temperature is 25 °C.
Another value related to the consistency of bitumen is the “Penetration Index”. The value is obtained by calculation from needle penetration and the ring-and-ball softening point using a softening point tester.
Fig. 5: Bitumen testing in a water bath
The consistency of grease from almost fluid to semi-fluid to hard can be measured in a range up to 475 Penetration Units. The standard methods for grease are ASTM D217, ISO 2137, IP 50, IP 179, IP 310, FTM 791-311, FTM 791-313; for petrolatum the methods ASTM D937 and ISO 2137 are used.
The penetration value ensures the adequate feeding of the friction area; the long-term performance of mechanical elements depends heavily on the selection of the right grease according to the penetration class. Hard grease will not work properly for a bearing that needs to be lubricated as it will not adequately grease the whole contact area. If the grease’s consistency is too low, it may leak away and parts will run dry, causing friction and wear to increase on the contact area. In case of central lubrication systems, the penetration is used for the observation of the pumpability of fluid grease.
The NLGI (National Lubricating Grease Institute) in the USA has established consistency numbers or grade numbers, ranging from 000 to 6, corresponding to specified ranges of penetration numbers. A penetration of 180 would represent solid grease, while a penetration of 450 would mean that the sample is fluid.
| NLGI class | Penetration at 25 °C (mm/10) | General consistency | Use scope |
|---|---|---|---|
| 000 | 445 to 475 | fluid | GE, Z |
| 00 | 400 to 430 | semi-fluid | GE, Z |
| 0 | 355 to 385 | very soft | GE, Z |
| 1 | 310 to 340 | soft | GE, WL, GL, Z |
| 2 | 265 to 295 | medium-soft | WL, GL, Z |
| 3 | 220 to 250 | semi-solid | WL, GL |
| 4 | 175 to 205 | solid | WL, WP |
| 5 | 130 to 160 | hard | WP |
| 6 | 85 to 115 | very hard | . |
GL = sliding friction bearing lubrication
WL = rolling-contact bearing and wheel bearing lubrication
WP = water-pump lubricant
GE = gear lubrication
Z = forced circulation lubrication
Fig. 6: Grease working machine
Additionally, a grease worker is necessary for the grease sample preparation. The treatment in a grease worker exposes the sample to high mechanical stress in a comparatively short time. The penetration is measured before and after applying many double strokes (up to 1,000,000) of a perforated plate in the greased-filled cup placed in the working machine. A high difference of the values stands for low shear stability, which indicates a poor performance of the grease in practice. If the consistency of grease becomes too low (because of the shear stress in the mechanical aggregate), it may leak away from the area to be lubricated and parts will run dry; friction and wear will increase on the contact area.
Fig. 7: Wax test kit
Waxes differ in consistency. The consistency can have a significant effect on other physical properties. The standard methods for consistency measurement are ASTM D1321, DIN 51579, and IP 376. A brief presentation of the globally used ASTM test is given below:
Fig. 8: Micro-cone set; cup assembled
Consistency determination of pharmaceutical products and cosmetics, such as ointment and creams, but also semi-liquid and semi-solid materials, is performed using methods of to the European Pharmacopoeia 2.9.9, the US Pharmacopoeia, and the WHO (UN-World Health Organization).
According to the European Pharmacopoeia Method 2.9.9 two different test kits can be used:
The advantages of the micro-cone kit compared to the hollow-cone kit are:
The penetrometer is further used with different test kits for other medical products and cosmetics such as balm, gel, gelatin products, dental plastic paste, bone cement, soap, toothpaste, lotion, make-up, powder blush, compact powder, eye shadow, lipstick, mascara, and hair styling wax/gel.
The term “texture” is frequently used when it comes to food. It describes the appearance, feel, or consistency of a surface or the food itself (mouthfeel). The penetration test provides information on the texture. Consistency gives food the typical mouthfeel that is experienced when it is swallowed. The sensory expectations of the customer have to be fulfilled. A good spreadability of butter, margarine, cheese, honey, and other food must be ensured. However, if a substance is too liquid it may run off the bread slice. If the consistency of ketchup is not optimal, it will not flow out of the bottle easily.
Due to its easy handling, the penetration method is employed in a wide scope of applications for food. Conclusions concerning the following can be drawn from the penetration value:
The NLGI system (see point 4.2) was also adopted outside of the grease industry; find examples for food applications below.
| NLGI number | Penetration at 25 °C mm/10 | Appearance | Consistency food (analog) |
|---|---|---|---|
| 000 | 445 to 475 | fluid | cooking oil, ketchup |
| 00 | 400 to 430 | semi-fluid | apple sauce |
| 0 | 355 to 385 | very soft | mustard |
| 1 | 310 to 340 | soft | tomato paste |
| 2 | 265 to 295 | medium-soft | peanut butter |
| 3 | 220 to 250 | firm | vegetable shortening |
| 4 | 175 to 205 | very firm | frozen yogurt |
| 5 | 130 to 160 | hard | smooth pâté |
| 6 | 85 to 115 | very hard | cheddar cheese |
A lot of different penetration test kits are used for chemical products such as adhesives, sealants, and glue pastes. Especially for the evaluation of processes such as kneading, pumping, mixing, filling, and brushing, it is essential to optimize the energy balance by adjusting the consistency.
The penetration test is used for adjusting the right consistency:
Consistency measurement with a penetrometer provides information about the texture of semi-solid materials with little effort. The simple rheology concept of the device, short measurement time, and very good repeatability of the results open up a wide application field.