Correlating Rheological Measurements with Dynamic Oscillation

The Thermo Scientific™ HAAKE™ MiniLab Series micro-compounder is well-known for being the ideal equipment, when it comes to compounding and processing tiny sample quantities of around 7 ml. The HAAKE MiniLab 3 is a small conical twin-screw extruder fitted with a bypass valve and a back flow channel, which allows users to regulate the residence time of the sample in the compounder.

Figure 1. The HAAKE MiniLab 3 micro-compounder.

The back flow channel’s patented design has two pressure transducers and a slit capillary flow channel, which are used to measure the pressure drop in the capillary. A shear stress can be calculated from the pressure drop and the geometry of the slit capillary. A shear rate is correlated from the designated screw speed and the measured back pressure. The shear stress and the shear rate values are used to calculate the relative sample viscosity at varied screw speeds. This article shows the correlation between the relative viscosity measurements completed on the new HAAKE MiniLab 3 micro-compounder, with results of absolute rheological measurements taken with a high end rheometer the Thermo Scientific™ HAAKE™ MARS™ 60 rotational rheometer.

Materials and Sample Preparation

For this study, two different LDPE grades (Lupolen 1800H and Lupolen 1800S from LyondellBasell) have been blended in different ratios. To have consistent sample preparation and a homogenous product, all samples have been pre-compounded on a Thermo Scientific^TM Process 11 Twin-screw Extruder.

Table 1. Compounding of the samples.

Testing Equipment

a) HAAKE MiniLab 3 micro-compounder with pneumatic ram feeder

Set of co-rotating screws
Software: Thermo Scientific™ HAAKE™ PolySoft OS
Software for MiniLab
N₂ purge

b) HAAKE MARS 60 rotational rheometer

20 mm parallel plates measuring geometry
Controlled test chamber (CTC) for temperature control
N₂ purge

Test Conditions and Test Procedure

Extruder

Sample weight: 6.5 g
Feeding speed: 50 rpm
Testing temperature: 190 °C
Inert gas: N₂ purge
Testing speed: Speed program from 50 rpm to 350 rpm (controlled via the HAAKE PolySoft OS Software)

Rheometer

Testing temperature: 190 °C
Testing mode: Frequency sweep in controlled deformation (CD) mode
Deformation: 1%
Frequency range: 0.1 - 628 rad/s

After a preheating time of about 10 minutes, the pressure transducers of the MiniLab micro-compounder have to be calibrated at the chosen measuring temperature, to avoid any temperature effect on the pressure measurement. The extruder barrel and the pneumatic feeding piston are continually purged by a constant nitrogen flow to avoid the occurrence of oxygen during the tests, to stop degradation of the LDPE samples. The sample is then fed into the running extruder using the pneumatic feeding piston. After 1-2 minutes, the extrusion pressure is equilibrated, which signifies that the sample is correctly molten and ready for the rheological test. The rheological test itself is performed and regulated by the HAAKE PolySoft OS Software. The software launches the pre-programmed measuring steps shown in Figure 2, checks when steady state conditions are attained and then measures the pressure drop between the pressure transducers in the slit capillary channel to calculate the shear stress.

Figure 2. Pressure drops in slit capillary of HAAKE MiniLab 3 micro-compounder at different rotational speeds.

Results of Rheological and Extruder Tests

Figure 3 shows the results of two rheological tests with sample 1 (“LDPE1800 H”). Each measuring point in the flow curve corresponds to one screw speed on the HAAKE MiniLab micro-compounder. With growing shear rate the viscosity drops, because of the typical shear-thinning behavior of polymer melts. Furthermore, the viscosity curves of the two independent tests are almost identical, which shows the excellent repeatability of the test technique.

Figure 3. Apparent viscosity η_αππ and apparent shear stress τ_αππ as a function of the apparent shear rate for low density polyethylene. The results of two independent runs with the same polymer are presented.

The results of the rheological measurements conducted with all five compounded samples in one diagram can be seen in Figure 4. It is easy to see how the sample viscosity diminishes with the increase of LDPE1800S and the decreasing quantity of LDPE1800H in the compound.

Figure 4. Flow and viscosity curves for all compounds.

Relative Measurement vs. Absolute Measurement

It is remarkable how the relative test results produced on the HAAKE MiniLab 3 correlate to the results performed on an absolute Rheometer. To verify this, one of the compounds (Sample 3 “LDPE1800 H1S1”) was analyzed in an oscillatory frequency sweep experiment on a HAAKE MARS 60 rheometer. For the comparison of the tests with the HAAKE MiniLab micro-compounder and HAAKE MARS rheometer the rule of Cox-Merz relation is applied. Empirically the two Scientists who gave the Cox-Merz relation their names discovered that the steady-shear viscosity measured as a function of shear rate could be directly compared to the dynamic complex viscosity measured as a function of angular velocity:

This relationship was found to be valid for many polymer melts and polymer solutions, but it rarely gives reasonable results for suspensions. The benefit of this Cox-Merz relation is that it is technically simpler to work with frequencies than with shear rates. Most of the time polymer melts cannot be calculated at shear rates lower than 50 1/s in a rotational rheometer in open sensor systems such as cone and plate, or parallel plate because of the elastic effects enountered. Therefore instead of measuring a flow curve in steady-state shear, one can more easily use the complex viscosity of dynamic testing¹.

Conclusion

Figure 5 clearly illustrates how the relative viscosity measurement done on a HAAKE MiniLab micro-compounder compares with the absolute data acquired from a high end rotational rheometer. The experiment proves that measuring the change in flow behavior using pressure sensors in the slit capillary channel removes possible influences from screw forces and thereby supplies reliable rheological information. Measuring the rheological data directly during compounding offers a two-fold benefit to the Researcher. Firstly, it saves time when the measuring happens directly in the extruder and sample preparation can be neglected. But also structural variations within the sample that happen during compounding can be noticed directly and valuable process information can be provided on the spot. The HAAKE MiniLab 3 micro-compounder gathers the rheological data under process conditions. When a wider measuring range is needed, the HAAKE MARS rheometer is an ideal complementary extension of the experimental setup.

Figure 5. Comparison of viscosity data obtained from a measurement with a HAAKE MiniLab 3 extruder and a HAAKE MARS rheometer.

Figure 6. The HAAKE MiniLab 3 micro-compounder.

Reference and Further Reading

1. Schramm, Gebhard / A Practical Approach to Rheology and Rheometry, 2nd Edition, Karlsruhe 2004, p. 124.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.