Characterizing the Processing Behavior of PVC Dry Blends, Part 2 (Lab Mixer Reproducibility)

When it comes to testing of PVC formulations for fusion behavior, compound stability, and processing behavior, the laboratory mixer is still the ideal measuring tool. However, laboratory mixers are very sensitive to any changes to the compound formulation, such as the resin itself, the additives used, and the added fillers. Mishandling of the material, or any changes in testing conditions, will also have a significant influence on the measuring results. Reliable test results strongly depend on the reproducibility of the test procedures.

The application note, PVC Mixer Tests: Reproducibility and Influence of Test Conditions presents an investigation using a rheometer system. A dry blend of rigid PVC, normally used for the production of window profiles, was selected  as a sample material.

Test procedure and measuring results

Typically, a PVC  dry blend is a mixture of the basic PVC resin, fillers, processing aids, plasticizers, stabilizers, inner and outer lubricants and other additives. Each component has an effect on the processing behavior of the compound.

One important factor is the fusion behavior of such a compound. Changes in the fusion behavior have a direct effect on the degree of gelation of the final product. For a PVC window profile this would result in a deterioration of the mechanical properties, or problems in the welding behavior.

Unlike a normal thermoplastic polymer, PVC does not melt when exposed to higher temperatures. PVC needs additional shearing and compression forces to bring the PVC particles to agglomerate and finally to form a homogenous melt.

In a laboratory mixer, this fusion behavior can be studied by measuring the drive torque, and recording this torque over the mixing time. The rheometer system software used in the experiment guides the user through this test, records the measurement data and evaluates all points of interest of the resulting measurement curve. Graphs are presented in the application note showing an evaluation of such a torque curve.

At the beginning of the test, the loading of the PVC powder into the mixer causes an instantaneous increase of the torque. After that, the powder distributes in the mixer chamber and some parts of the compound (e.g. waxes) melt due to the high mixer temperature. Both effects lead to a drop of the torque and a first minimum. Due to the increase of mass temperature and the introduced shear energy, the PVC starts to combine to bigger agglomerates. This causes an increase of viscosity, which leads to an increase of torque. This process results in a second torque maximum. The PVC dry blend forms a homogeneous melt. Due to additional increase of the sample temperature, caused by frictional heating, the torque drops again until it comes to a constant torque. A balance between the increase of temperature caused by dissipation and decrease of temperature caused by heat conduction through the chamber wall is reached. The torque which is adjusting here is a relative value for the melt viscosity of the sample.

To check the reproducibility of mixer tests, five tests were done under the same conditions, using the same PVC dry blend. Test conditions included the influence of mixer temperature, mixer speed, and mixer load. These tests show that test with the laboratory mixer are well reproducible, very sensible to changes in compound formulations, but also very sensitive to changes in handling and measuring conditions.  Download the application note to view comparison charts and sample software reports.

Editor’s Note:  You can read Part 1 here: