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The study of flow and deformation of materials is known as rheology. Rheometers are widely used to measure the rheological properties of fluids. Under the influence of an external force, or stress, a body’s deformation and flow are measured as strain or strain rate, respectively. As a result, rheology is also known as the study of stress-strain relationships in materials.
In a rheometer, the material of interest is placed in a geometric configuration, the environment around it is controlled, and stress, strain, and strain rate can be applied and measured.
The Brookfield Rheometer is the world’s most versatile platform for rheological measurements in wide range of applications. AMETEK Brookfield’s rheometers make viscosity measurements and viscoelastic measurements quick and easy.
Different types of Rheometers
Capillary systems and Rotational systems:
In capillary systems, fluid flows through tubes of circular cross section as a result of a pressure difference between their inlet and outlet, which may be generated by gravity or other mechanical means .The rotational system, on the other hand, is based on the cylindrical, conical, or circular body immersed in liquid that experiences a viscous resistance force when a rotational speed is applied. Rotational rheometers have some advantages over capillaries, including their ability to measure the rate of deformation and tension of shear using small samples of products, as well as their wide range of strain rates, which allows an analysis of time-dependent behavior.
The stress-strain relationship of a material is measured with a rheometer to understand its flow/deformation properties. Most rheometers use the same geometry for measurement (cone, plate, parallel plate, concentric cylinder, etc.) and have a similar measurement range. Despite this, the design of each rheometer differs greatly.
Various types of rheometers are available commercially in Brookfield; each rheometer operates on a different principle such as rotation, creeping, relaxation, swinging, and so on.
Whether the product is a liquid, cream, paste, or powder, manufacturing requires multiple steps to produce a finished product. For pumping, mixing, transferring, and storing the product in shipping containers successfully, R&D must perform rheological analyses flowability and stability at every step.
Rheometers are the scientific tool that R&D uses to evaluate new formulations for liquids and semi-solids. The viscometers used on production floors and in quality control labs measure whether the product complies with specifications. What’s the difference between a rheometer and a viscometer? Are they able to perform the job properly, so there are no flow behavior issues or instabilities in the final product during manufacturing? Viscometers can very likely do the job without issue, and perhaps can do so much more if necessary.
The figure 1 illustrates a cone/plate rheometer. Shears the material over a continuous range of shear stresses and/or shear rates. Because it can handle a wide range of materials, ranging from thin liquids (eye solutions) to semi-solids (rubbing ointments), it is more expensive. However, a rheometer can perform a much more comprehensive battery of tests, starting with visco-elastic deformation and yield stress measurement when the material is at rest, to a full viscosity low curve with thixotropy calculation (time sensitivity to being sheared)when the material is in motion, to recovery and creep (low force low behavior), and finally temperature profiling (how the viscosity property changes with temperature).Consequently, the versatility of this instrument makes it a must have for R&D.
These rheometers have become more affordable in recent years, and their ease of usage in standalone mode or under PC control makes them viable options for both QC and manufacturing.
The most important improvement is that they are durable in high-traffic environments and can even be used on the manufacturing floor by many operators. Several major companies have taken this step because they see the value in following in the footsteps of R&D. Is this enough to rule out standard benchtop viscometers? By any stretch of the imagination, it does not.
Figure 2 shows a popular and inexpensive viscometer that only operates in controlled rate mode (you define the rotational speed of the spindle and the instrument measures the material viscosity). A single point viscosity test is the name for this type of measurement. It provides very little information on the sample’s general low behaviour, but R&D has probably done enough characterization work to know that this single data point will detect the majority of problem batches of material.
As a result, a lot of information that this type of instrument may provide is left on the table, such as a viscosity low curve (how viscosity changes with different shear rates), time sensitivity to shearing (thixotropy), or temperature profiling (how viscosity is affected in different climates).
The use of a viscometer to investigate shear thinning behaviour of materials is a well-established method.
Depending on the thickness of the material, choose two speeds that are a decade apart, such as 1 and 10 rpm or 10 and 100 rpm. By determining the ratio of the former to the later, you may compare the viscosity values observed at each speed. This ratio produces a number greater than 1.0 because the viscosity measured at the lower speed is almost always higher than the second viscosity. The “Thix Index” or”TI” is the term referring to this ratio.
The TI will most likely be close to 1 for off-the-shelf pharmaceutical medicines with a modest variation in viscosity behaviour, such as cough syrups. The TI might be at least 2, and possibly as high as 3 or 4 for extremely shear thinning materials like rubbing ointments. As a result, the TI becomes a very important instrument for improving pharmaceutical product validation and certification.
Manufacturing has strong cause to think about the added value that comprehensive viscosity data can bring to operations. There are a number of different shear rates that take place during processing in manufacturing.
Knowing that the viscosity data is within specified parameters for acceptable flow behaviour ensures that the process will run smoothly. What may appear to be an esoteric subject – “Rheology” – has everything to do with the success of Manufacturing. The usage of online process viscometers in production is a final point to consider.
Once viscosity data regarding changes in material behaviour throughout the production process is available, it becomes possible to choose a control point in the process to monitor and manage viscosity.
Despite the fact that process viscometers have been around for a long time, most manufacturing organisations are unaware of their capabilities. The goal is for the product to be consistent. The corollary is to reduce waste or product that needs to be redone. This is where the viscometer manufacturer can assist you in determining the best position for the online instrument to be installed.
In conclusion, manufacturing stands to benefit from a larger involvement in the application of rheology to improve operations. R&D and QC can work together to make this happen. The goal is to provide a consistent product while simultaneously enhancing processability and meeting customer expectations.
For any additional information or support:
Contact: Mr. Indraneel
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Story source: Smart Labtech Pvt Ltd, Hyderabad