Rheology and Rheological Properties – Synthetic Rubber

Today, we would like to share valuable information about rheology.  

Rheology refers to the study that analyzes the flow of emulsion and the deformation of solids when stress and strain are applied. (In the simplest terms..!) 

To identify rheological properties, equipment such as a rheometer is used. There are many types of rheometers, including versatile rheometers and capillary rheometers which can control stress or strain.

Applications  

1. Polymers – Melt Measurements

Typically, determining the rheological properties of polymers is done in diluted solutions or melted polymer states, which play a crucial role in determining molecular structure (molecular weight, molecular weight distribution, and branching), processing behavior, and the performance of the final product.

To measure the rheological properties of molten polymers, a melt index is commonly used. It is a single viscosity measurement experiment and practically represents the amount of material passing through a capillary under standard conditions (temperature, pressure, and time). The melt index is the simplest and fastest method to distinguish relative differences between polymers during quality control and is frequently used. However, it has limitations in accurately distinguishing differences in molecular structure and providing information on how the material behaves during processing.

Polymers show viscoelastic properties under changes in stress, strain, and temperature. A representative example is synthetic rubber (PDMS), which behaves like a liquid over time due to gravity and transforms into a stationary state. When shaped into a ball and bounced, it acts like an elastomer, but if pulled sharply, it breaks like a plastic solid. Understanding these rheological properties of polymers is vital to comprehending how they aggregate, deform, and respond over time during and after processing.

To measure properties such as stress or deformation, rotational rheometers that can control these factors are generally used.

• Measurement of viscoelasticity (G’, G”, tan delta) as a function of frequency (time) and temperature
• Measurement of molecular structure (molecular weight, molecular weight distribution, branching) using frequency range and creep/recovery tests (zero-shear viscosity)
• Measurement of the influence of long-chain branching on linear viscoelastic properties (zero-shear viscosity, steady-state recoverable compliance)

Capillary rheometers are also used for the following measurements:

• Direct reproduction of conditions obtained during processing to measure shear viscosity as shear rate increases from a low state
• Measurement of melt fracture, die swell, and surface defects (shark skin) caused by elastic properties when high shear rates occur during processing
• Measurement of tensile viscosity and melt strength, which are vital parameters in polymer processing