Why take rheology measurements?

Anyone starting to learn to think rheologically should first ask themselves the following question: Why do viscosity measurements?

The answer to this question lies in the experience of thousands of people who have made such measurements, showing how useful it can be prior knowledge of the behavior of materials, or of the effects produced by changes in manufacturing processes, changes in formulation. , aging phenomena, etc. The area that most frequently justifies measurements of rheological properties is the area of quality control, where raw materials must be identical lot after lot. Typically, flow behavior is an indirect measure of the quality and conformance of a product. Another reason justifying the flow behavior studies is that they make it possible to justify the feasibility of the production processes. For example, a highly viscous liquid will require more energy to be pumped than a less viscous product. Knowledge of rheological properties is thus useful for defining pumping and piping systems. Rheology has been shown to be the most sensitive method for characterizing materials because flow behavior is very directly related to physical properties such as molecular weight and molecular weight distribution. These relationships are useful for example for molecular syntheses because they allow visible relative differences without having to carry out molecular mass measurements. Rheological measurements are also useful for monitoring the course of a chemical reaction. Such measurements can be carried out as a quality test during production or to trace, mark out and control the progress of the process. Rheological measurements make it possible to study chemical, mechanical and thermal treatments, the influence of additives, or to follow the evolution of a cooking reaction. They are also the means of predicting or controlling a quantity of material properties, its behavior and performance in end-use situation.

Think rheo-logically

To begin with, consider the question: Is it possible to correlate a rheological data with a parameter of the production process or an aspect of product behavior?

To determine this, you need to develop a certain “instinct” to understand the physical and chemical phenomena that affect the rheological response. For the moment, consider that this information is known and that several possibilities have been identified. The next step is to collect preliminary rheological data in order to determine what type of flow behavior is characteristic of the phenomenon studied. At the most basic level, this involves making measurements with any Brookfield viscometer available and drawing some conclusions based on the description of the different types of rheological behavior described in Chapter 4. Once the type of behavior at the flow has been identified, it is possible to better understand how the components of the system interact (more information on what affects the rheological properties can be found in section 4.7). The data thus obtained can then be compared to one of the mathematical models that have been used successfully with the Brookfield devices. Most of these models are presented in Chapter 5. These mathematical models are classified from the simplest to the most complex. Some simply involve drawing curves on graph paper, others involve calculating a ratio between two numbers. Some are quite sophisticated and require the use of a programmable calculator or a computer. These types of analysis are the best way to get the most information from your measurements and often results from these analyzes one or two "constants" which summarize the measurements and can be used to quantify product or process performance. . As soon as a correlation is found between the rheological data and the behavior of the product, the procedure can be reversed and the rheological data can be used to predict the performance and behavior of the product.

Three schools of thought regarding viscosity measurements

In our experience, there are three schools of thought regarding the use of viscometers in rheology applications. We present them here and invite you to choose the one that suits you, keeping in mind that none of the three is the best and that each has its own advantages.

The pragmatic school

The first school of thought is the most practical. People who attend this school are only interested in the fact that the Brookfield viscometer generates numbers which gives useful information about the product or process. These people have little regard for rheological theories and measurement parameters expressed in absolute terms. Typical applications in this category are quality control or production control applications.

The theoretical school

The second school of thought involves a more theoretical approach. Those who adhere to this school know that some models of Brookfield viscometers will not directly produce absolute viscosity measurements at shear rates set for non-Newtonian products. However, they consider that it is possible to find correlations between the apparent viscosities (figure indicated by the dial of the viscometer) and important parameters concerning their production or their process. Most users follow this school of thought. Application rheology documentations are teeming with remarks such as "I know that the data is not absolutely and formally defined, but keeping this data in mind, I process the rheology information as it is measured". In most cases this produces highly satisfactory results and eliminates the need to purchase highly sophisticated and excessively expensive rheology equipment.

Academic school

The third school of thought is rather academic in nature. Proponents of this school demand that all measurement parameters, particularly shear rate and stress, be known and well defined. They need equipment with well-defined geometries such as cone / plane geometry or coaxial geometry. The equipment in the Brookfield range meeting these criteria are Wells-Brookfield cone / plane viscometers, CAP viscometers, ULA, SSA, DIN, SAA hairspring adapters, thermosel system, R / S, R / S-CPS and rheometers. PVS. With this equipment, the shear rate is defined and precise measurements of absolute viscosity are thus directly obtained. So here is our take on the three schools of thought regarding viscosity measurements. You will need to think in terms that correspond to one or the other of these schools according to your knowledge, approaches, goals and according to the material in your possession. Brookfield viscometer users all fall into one of these 3 categories.