Influence of rheology on mixing behavior in static mixers
When choosing the most suitable static mixer, it is essential to consider the rheological properties of the initial components and their mixing ratio. This is particularly important for high-viscosity materials with a high proportion of fillers, which often exhibit non-Newtonian behavior. Therefore, it is essential to have a good understanding of the rheological behavior of the components that need to be mixed.
1.1.Basics of rheology
Table 1 provides a comprehensive classification of material properties:
Table 1 - Rheological Classification of materials, Source: Wikipedia Rheology - Wikipedia
For Newtonian fluids, viscosity is independent of load, meaning that it depends on temperature but not on shear stress. However, only a small group of mostly low-viscosity fluids, such as water, milk, and salad oil, exhibit such constant viscosity. Most fluids experience changes in viscosity under shear stress; these are called non-Newtonian fluids.
These fluids can exhibit either shear-thinning (structurally viscous) or shear-thickening (dilatant) flow behavior. Shear-thinning flow behavior is characterized by a decrease in viscosity as the shear rate increases. This behavior is commonly observed in adhesives and filler materials used in bite registration, endodontics or cementation. On the other hand, shear-thickening refers to an increase in viscosity with increasing shear rate. Materials that typically exhibit such behavior include highly filled dispersions, such as ceramic suspensions, starch dispersions, sometimes dental fillings (dental composites), and special composites for protective clothing.
Non-Newtonian fluids are shear-dependent, so it is important to report measured viscosities together with the exact shear conditions and, ideally, as a function of shear rate. The viscosity can often be approximated section-wise by a power law.
A material is classified as viscoelastic if it exhibits a combination of viscous and elastic behavior. Plasticity refers to the behavior of a material that behaves as a solid under low applied stresses but starts to flow above a certain level of stress, known as the yield stress.
Furthermore, it is important to note that viscosity can depend not only on the shear rate but also on time. When such materials are subjected to shear stresses, their viscosity does not change instantly but rather over time. Time-dependent shear-thinning materials are known as thixotropic, while shear-thickening materials are known as rheopectic.
In addition, the viscosity of a material is also strongly dependent on temperature. Generally, the viscosity of liquids decreases with increasing temperature. This can have a significant impact, as demonstrated by the fact that the viscosity of a typical engine oil decreases by a factor of 3 when the temperature is increased from 23°C to 50°C.
1.2.Impact of rheological properties on the performance of static mixers
The optimum mixing quality that can be achieved depends on various factors, such as the type of mixer, the number of mixing elements, the mixing ratio, and the viscosity ratio of the two components. Optimal mixing quality is typically achieved with a pre-defined mixer when mixing materials with a 1:1 ratio and the same viscosity.
In many cases, however, these ideal conditions do not exist, and a high-viscosity, shear-thinning resin must be mixed with a low-viscosity, Newtonian curing agent. Optimum mixing occurs at equal viscosity, so it is advantageous to select a mixer that operates at a shear rate that results in equal viscosities of the two components.
However, the rheological behavior of the material not only affects the mixing quality, but in particular also the pressure loss in the mixer. For instance, if a high-viscous material needs to be discharged at a high flow rate, the resulting high pressure loss in the mixing element causes significant mechanical loads on the cartridge and the mixer housing. These loads can have negative impact on functionality causing pre-flow of one component, blow-up of the cartridge or mixer housing or even damage to the plastic parts.
To prevent these problems, it is advisable to choose a mixer with a larger diameter. This will significantly reduce pressure loss.
1.3.Material guideline to achieve good mixing results
To ensure that new materials and components are easily mixable, it is beneficial for material manufacturers to work with mixer manufactures already at the formulation stage. At this stage, it is even possible to design a customized mixer for such an application. However, even if an exclusive solution is not desired, good mixability of the components can be ensured in advance, resulting later in shorter mixers with fewer mixing elements and lower pressure drop.
From a rheological point of view, good mixability can be achieved by following these guidelines:
- Both components should have a similar rheological behavior. Similar rheological behavior means that in a log/log diagram the viscosity versus shear strain rate curves are parallel and very close together. This will ensure that a similar mixing quality can be achieved over a wide range of operating conditions.
- The volumetric mixing ratio of both components should be close to 1. Application tests with materials with a high volumetric mixing ratios indicate a much higher number of mixing elements required to achieve adequate mix quality.
- Due to the shear thinning behavior of many materials and the high shear rates in the mixer, the actual viscosity of the material in the mixer will be lower than specified in the data sheet. Typical shear rates in a static mixer are in the range of 20 > S > 200 1/s. However, an actual viscosity that is too high should not be exceeded, as this will at best result in the desired flow rate not being achieved. At worst , it will lead to a loss of function or mechanical failure.
About the author:
"Joachim Schöck has been working as a senior technology expert at Sulzer Mixpac and medmix Switzerland AG for 12 years. His main activity is the optimization and further development of high-precision application and mixing systems. This is done to a large extent using modern simulation tools such as CFD and FEM. Another focus is on the further development of test methods for predicting the mixing quality of 2K adhesives and sealants."
