Analyzing Thermoplastics with a Kinexus Rotational Rheometer

One of the principle reasons that polymers are used so widely is because they are relatively cheap to form into complex shapes in the molten state which is a huge change in rheology from a solid starting material through to a solid end product. Learn how to determine flow curves, carry out creep tests and measure viscous and elastic properties.

One of the principle reasons that polymers are used so widely is because they are relatively cheap to form into complex shapes in the molten state which is a huge change in rheology from a solid starting material through to a solid end product. However, we need to understand how they flow when being processed in this manner. In our previous article, several rheological properties of polymers have been discussed. Now, we will have a look at three properties that can be determined by the Kinexus rotational rheometer.

Rotational rheometers instruments normally require a small specimen of the material to be tested in the form of a disk – typical dimensions being 25mm diameter and 1mm thick. The sample is placed between a pair of parallel plates or upper cone and lower plate the temperature of which can be maintained accurately to mimic the conditions experienced by the sample whilst being processed [1].

The Kinexus by NETZSCH is capable of a number of test types to allow for full characterization of a material over a range of temperatures and flow rates. Examples of the types of tests available are:

Determining flow curves

Flow curves measure the shear viscosity versus shear rate or shear stress. At sufficiently low shear rates, a constant value for the viscosity will be attained. This so called zero shear viscosity has been shown to depend on the average molecular weight of the polymer and the length of the plateau (how high a rate before the viscosity decreases) is known to reflect the width of the molecular weight distribution [1].

Figure 1: Flow curve for LDPE at 190°C showing low shear rate plateau for viscosity. The magnitude of the zero shear viscosity is related to the average molecular weight of the polymer

Determine zero shear viscosity with creep tests

Creep tests (application of constant stress for a defined period of time feature) an alternative means for determination of the zero shear viscosity. When combined with recovery testing (removal of the stress), these tests enable the amount of elasticity in the sample to be measured because a material will, due to its “elasticity”, recoil and attempt to recover its original shape [1].

Figure 2: Creep (blue) & recovery (red) curve of polypropylene at 190ºC allow for zero shear viscosity to be determined and equilibrium recoverable compliance

Measure viscous and elastic properties

Small amplitude sinusoidal oscillatory testing as a function of test frequency is a rapid and frequently used method to measure the viscous and elastic properties of a polymer. Two parameters are most often reported – elastic (storage) modulus and viscous (loss) modulus (G’’) which represent the relative degrees of the material to recover (elastic response) or flow (viscous response) as the rate of deformation (test frequency) changes. A typical response for a polymer melt is to exhibit elastic-dominated behavior at high frequencies and viscous-dominated behavior at low frequencies. This means a critical frequency exists at which the two responses are equal.

This is obviously a well-defined point and conveniently, this “cross-over” frequency and modulus has been shown to depend on the molecular weight and molecular weight distribution of some linear polymers. A potential advantage of utilizing this point as a quality control tool is that the cross-over of elastic and viscous moduli occurs at significantly higher frequencies than the point at which a constant value of shear viscosity occurs. Testing times for oscillation testing are typically reduced when compared to performing flow curve measurements or creep tests [1].

Figure 3: Frequency sweep for polypropylene at 190ºC. The cross-over point is determined by the average molecular weight and molecular weight distribution

Kinexus rotational rheometers are the preferred choice when the requirement is to obtain information concerning the molecular structure and how this affects processing characteristics. In particular, the ability to easily extract information about the average molecular weight and molecular weight distribution via measurement of the viscoelastic properties makes the rotational rheometer a powerful tool.

After covering the basics of analyzing thermoplastics with a Kinexus rotational rheometer, the next blog article will show two examples to illustrate how visco-elastic characterization of polymers has solved real processing problems.

Source

[1] Rheology Testing of Polymers and the Determination of Properties Using Rotational Rheometers and Capillary Extrusion Rheometers (azom.com)

Thanks to Dr Bob Marsh (former employee of Malvern Panalytical) as the original author of this article!

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