Using Melt Viscosity of Polyethylene Terephthalate to Solution Intrinsic Viscosity

PET (polyethylene terephthalate), a polyester, is a hygroscopic resin that takes in moisture from the surrounding atmosphere when exposed to air at ambient conditions. Water absorption has a large impact on the rheological characteristics, including flow behavior, of PET plastic melts.

IV Measurement of Polymers

In most cases the additional moisture increases the plasticity of the PET melt, resulting in a higher flowability. As a result of hygroscopic behavior PET’s melt flow rate (MFR) parameters are highly dependent on the moisture content of PET and the conditions used to dry it, though these are rarely stated by PET suppliers. However, the intrinsic viscosity (IV) of the PET is often provided by suppliers and can be used to classify PET into different resin grades.

Measurement of the IV of a polymer is usually achieved by dissolving a small volume of sample into an appropriate solvent, such as phenol, at ambient conditions. The use of a phenol solvent and a dilute solution viscometer means the IV measurement taken is independent of the resin’s moisture content, meaning the sample does not need to be dried before testing. Whilst this method is highly useful it also requires the use of volatile and hazardous solvents, which is not desirable.

LCR Capillary Rheometer Series

The LCR capillary rheometer series in the Dynisco^® range can correlate shear viscosity data to the IV of PET polymers. This correlation is based on the joint relationship between zero-shear viscosity and polymer molecular weight, and the relationship between IV and molecular weight. This is because the viscoelastic behavior of polymers is, in part, the result of chain entanglement and molecular motion. Polymers that have a higher molecular weight tend to exhibit more chain entanglement, and this can result in a greater IV or shear viscosity.

For polymers with a high level of chain entanglement, the Fox-Flory equation can be used to describe the relationship between molecular weight and zero-shear viscosity.¹

Where η₀ is zero-shear viscosity, and K is a polymer type-dependent constant. The relationship between the molecular weight of a polymer and its IV is described by the Mark-Houwink equation.²

Where η is the intrinsic viscosity, and K՛ and α are Mark-Houwink parameters and depend on a particular polymer-solvent system. For most polymers the value of α is within 0.5-0.8. It is clearly seen that, the changes in molecular weight of a polymer with high entanglement, affect melt viscosity more than intrinsic viscosity.

As equations (2) and (4) both refer to molecular weight it is possible to use these equations to determine the relationship between the melt viscosity and the intrinsic viscosity. This is achieved by combining the equations to give:

As it can be seen, for a high entangled, linear and monodisperse polymer with narrow molecular weight distribution, the plot of logarithm of intrinsic viscosity versus logarithm of zero-shear viscosity gives a straight line with the slope of α/3.4 .

Figure 2 (Credit: Reilly et al.³) illustrates this relationship for a sample of PET at different levels of moisture content. IV measurements were taken using phenol/TCE (60/40) solvent and a dilute solution viscometer at ambient conditions. Zero-shear viscosity measurements were taken using a capillary rheometer at 285 °C.

Figure 1. Plot of shear viscosity versus shear rate for different PET samples and their resulted values of intrinsic viscosity.

Using the discussed relationships, the LCR capillary rheometer series from Dynisco^® can measure the IV of PET polymers using a shear sweep test at 285 °C. The samples must be prepared by drying to the required moisture content level.

As demonstrated in Figure 2 the flow curve produced (of apparent shear viscosity vs. apparent shear rate) must be fitted to a cross model to provide a zero-shear viscosity value.

Figure 2. Relationship between zero-shear viscosity and intrinsic viscosity of PET samples with various moisture contents.

ViscoIndicator Online Rheometers

The ViscoIndicator online rheometers from Dynisco^® use a similar method to monitor the rheological properties of polymers in extrusion lines in real time, including information on the IV of extrudates. Figure 3 shows the IV and shear viscosity measurements of a variety of PET samples taken using a ViscoIndicator online rheometer. Extrusion took place using a Dynisco^® REX sampling extruder at a constant head pressure and screw speed.

Information such as this is of particular use to the PET recycling industry, where the quality of the regrind must be continually monitored throughout processing.

Figure 3. Shear viscosity and intrinsic viscosity measurements of PET extrudates using a Dynisco^® ViscoIndicator online rheometer.

The use of melt rheometers to determine the IV of samples is popular in the plastics recycling and PET processing industries because it removes environmental concerns and the complicated disposal associated with using noxious solvents. In addition, melt flow methods are more easily integrated into real-time measurements for process control.⁴

References

1. Dealy, J. M., & Wissbrun, K. F. (2012). Melt rheology and its role in plastics processing: theory and applications. Springer Science & Business Media.
2. Sperling, L. H. (2005). Introduction to physical polymer science. John Wiley & Sons.
3. Reilly, J. F., & Limbach, A. P. (1994). Correlating melt rheology of PET to solution intrinsic viscosity. KAUTSCHUK UND GUMMI KUNSTSTOFFE, 47, 271-271.
4. Whelan, T., & Brydson, J. (2202). “Practical Rheology Handbook.” Edited by. De Laney D. E., 3rd Edition, Scribd, www.scribd.com/doc/81343769/Practical-Rheology-Handbook.

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.