The Basics of Capillary Rheometry

What data can be obtained by means of a capillary rheometer?

• Melt viscosity

Viscosity can be thought of as the fluidity of a liquid, or how much it resists flow. Viscosity, η, is expressed as the ratio of shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress (force per unit area) to the shear rate (rate change of shear StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain). [2]

• High shear rate behavior

The shear rate is the rate at which a fluid is sheared or deformed during flow. In more technical terms, it is the rate at which fluid layers move past each other. If someone quickly rubs a very thin layer of ointment, cream, or lotion on the skin, for example, then the shear rate is much higher than if that material is slowly squeezed out of its tube. [3]

• Extensional properties

Extensional flow occurs when the material is not in contact with solid boundaries, as is the case during drawing of filaments, fibers, films, sheets or inflating bubbles. Converging flows at the inlets of dies are also extensional in nature. [4] Extensional properties include extensional-StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain rate and extensional viscosity.

• Melt fracture (Flow Instability)

Melt fracture is defined as the phenomenon caused by excessive shear StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress exerted on the molten resin that leads to roughness in the extrudate. [5] It is an undesired surface effect that can also affect the part properties. Since it occurs at high shear stresses for the material under investigation, it can be reduced or eliminated by reducing the throughput.

• StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.Stress RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. Relaxation (Relative)

StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.Stress RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation is a time-dependent decrease in StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress under a constant StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain. This characteristic behavior of the polymer is studied by applying a fixed amount of deformation to a specimen and measuring the load required to maintain it as a function of time. [6]

• Melt strength

Melt strength can be described as the resistance of the polymer melt to stretching. The melt strength of a material is related to the molecular chain entanglements of the polymer and its resistance to untangling under StrainStrain describes a deformation of a material, which is loaded mechanically by an external force or stress. Rubber compounds show creep properties, if a static load is applied.strain. The polymer properties affecting the resistance to untangling are molecular weight, molecular-weight distribution (MWD) and molecular branching. As each property increases, the melt strength is improved at low shear rates. [7] It is an important property for successful extrusion of plastic materials.

• Die swell

Die swell occurs when a material flows out of the capillary die. One way to explain die swell is to consider the ability of the polymer melt to remember its flow history. The idea is to imagine a fluid element moving from the reservoir into a capillary die as a short, fat cylinder being squeezed into a long, slender cylinder. If the residence time of the fluid element in the die is shorter than the time of its fading memory (RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation time), it will try to return to its original shape and produce the die swell effect. [8]

• pvT behavior and compressibility

pvT investigates the relationship between pressure and volume in a material. It further gives an indication of how compressible a polymer melt is. As polymers are processed at high temperatures and pressures, the relationship between pressure, volume and temperature is of great importance.