Shear viscosity and Capillary Rheometer

In summary, shear viscosity is a measure of a molten plastic's resistance to flow when subjected to shear forces, and is affected by both temperature and shear forces. Shear viscosity is typically measured using a capillary rheometer, which calculates the viscosity based on the output flow rate of the molten plastic and the velocity gradient. The capillary rheometer indirectly calculates shear stresses by knowing the pressure applied at the top of the molten plastic. The melt index (MFI) is a parameter used to compare the viscosity of polymers within the same family, but it may be misleading when comparing different polymers. Some polymers are rated using intrinsic viscosity (IV), which is a measure of the molecular weight and is not affected by shear thin
  • #1
fog37
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Hello Forum,

I have some conceptual doubts about shear viscosity and would like some help if possible. In general, viscosity depends both on temperature and on shear forces (more strongly on shear forces).

"Shear viscosity" is the viscosity that a molten plastic assumes when the molten fluid is subjected to shear forces that put it in motion, make the fluid layers slide one over the other. This happens when the molten plastic is pushed through a channel/pipe or mixed it. For most fluids, the shear viscosity decreases with shear (thinning).

That said, a capillary rheometer is the device used to measure shear viscosity versus shear strain rate (1/s). This device pushes the molten plastic through a small orifice once a certain pressure is applied at the top. Based on the molten plastic output flow rate, the shear viscosity is calculated. Inside a cylindrical pipe the molten plastic assumes a parabolic profile (more or less). This velocity profile represent the velocity gradient. The v gradient is related to the shear rate of deformation of the plastic. The smaller v gradient the less the fluid layers interact with each other. By knowing the shear strain rate of deformation and the shear forces we can determine the viscosity (slope of the graph).

How does the capillary rheometer calculate the shear stresses? Indirectly, by knowing the pressure applied at the top of the molten plastic?

How does the output flow rate from the rheometer orifice relate to the v gradient and to the shear strain rate of deformation? The larger the output flow rate the larger the strain rate?

thanks!
fog37
 
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  • #2
Hi fo37,

What is the context of your questions? Is this for schoolwork, or for measurements in your company's lab?
 
  • #3
fog37 said:
How does the capillary rheometer calculate the shear stresses? Indirectly, by knowing the pressure applied at the top of the molten plastic?

In the capillary rheometer, you are trying to determine the shear stress at the wall as a function of the shear rate at the wall. The shear stress at the wall is related to the pressure drop by (neglecting the exit effect):

$$\tau = \frac{DΔP}{4L}$$
How does the output flow rate from the rheometer orifice relate to the v gradient and to the shear strain rate of deformation? The larger the output flow rate the larger the strain rate?

Determining the shear rate at the wall from measurements of the flow rate and wall shear stress is done by well-established techniques. As a first approximation, the shear rate at the wall is just the Newtonian result, 8V/D. However, this is usually not accurate enough. The techniques for determining the shear rate at the wall are discussed in any book on rheology. Google Rabinowitch equation.

If you have a commercial viscometer, it should come with a users manual. You can also probably Google Capillary Viscometer or Capillary Rheometer and get the gory details.

Chet
 
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  • #4
Thank you Chet.

So, to calculate the shear viscosity we need data for the shear stress and shear strain which are indirectly obtained. It makes sense that the normal compressive pressure at the top will induce shear, tangential forces at the wall of the channel. But exactly at the wall the molten polymer should adhere to the wall (non slip condition) no matter the type of viscosity, correct?

The melt index (or MFI or MFR) is a parameter calculated at a single shear stress condition and serves to compare polymers of the same family (the lower MFI the more viscous). Comparing the MFI of different polymers can be misleading, correct?

Some polymers are not rated using MFI but using IV (intrinsic viscosity of solution viscosity). Do you know why?

best,
fog37
 
  • #5
fog37 said:
Thank you Chet.

So, to calculate the shear viscosity we need data for the shear stress and shear strain
No. Shear rate, not shear strain.
which are indirectly obtained. It makes sense that the normal compressive pressure at the top will induce shear, tangential forces at the wall of the channel. But exactly at the wall the molten polymer should adhere to the wall (non slip condition) no matter the type of viscosity, correct?
Yes. So?
The melt index (or MFI or MFR) is a parameter calculated at a single shear stress condition and serves to compare polymers of the same family (the lower MFI the more viscous). Comparing the MFI of different polymers can be misleading, correct?
Yes, sometimes. It's a quick and dirty method.
Some polymers are not rated using MFI but using IV (intrinsic viscosity of solution viscosity). Do you know why?
You need to get yourself a textbook on Polymer Science. Otherwise, you are wasting your time speculating about these things.

IV is a measure of the molecular weight of the polymer. It is related to the zero shear viscosity of the polymer, but doesn't relate to either shear thinning behavior of the polymer or to polymer viscoelasticity.

Chet
 

1. What is shear viscosity?

Shear viscosity is a measure of a fluid's resistance to flow when a shear force is applied. It is also known as dynamic viscosity and is represented by the symbol η (eta).

2. How is shear viscosity measured?

Shear viscosity is typically measured using a capillary rheometer, which applies a constant shear stress to a fluid and measures the resulting shear rate. The ratio of shear stress to shear rate is the shear viscosity.

3. What factors affect shear viscosity?

The shear viscosity of a fluid is affected by temperature, pressure, and the presence of additives or impurities. In general, higher temperatures and pressures decrease viscosity, while impurities can increase it.

4. What is a capillary rheometer?

A capillary rheometer is a type of instrument used to measure the shear viscosity of fluids. It consists of a cylindrical chamber with a small, narrow capillary tube, and a mechanism for applying a controlled shear stress to the fluid.

5. Why is shear viscosity important in materials science?

Shear viscosity is an important property in materials science because it affects a fluid's flow behavior, which is crucial in processing and manufacturing. Understanding the shear viscosity of a material can help scientists and engineers design and optimize processes for producing materials with desired properties.

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