Meaning
Dimensionless numbers in rheology compare the extensional viscosity of a fluid to its shear viscosity to identify deviations from Newtonian behavior in the melt. Flow characterization in multiple dimensions provides a more complete understanding of how a polymer melt will behave in complex geometries. The trouton ratio is defined as the extensional viscosity divided by the shear viscosity at the same strain rate.
For a simple Newtonian fluid, this value is exactly three. In most polymer melts, the ratio is much higher and increases with the rate of deformation. The boundary of the concept is reached when the flow is no longer purely extensional or purely shear.
This value governs the stability of processes that involve stretching, such as blow moulding or film extrusion. It helps engineers identify materials that show strain-hardening behavior.
Comparative Metric
Behavior of the melt under different types of stress is essential for predicting the final part quality. The trouton ratio highlights the difference between the resistance to being pushed through a die and the resistance to being pulled out of it. Most polymers are non-Newtonian, meaning their molecules are long and can become entangled.
These entanglements have a much larger effect on the extensional flow than on the shear flow. A high ratio indicates that the material will be very stable when stretched, which is desirable for forming large parts. Processors use this number to compare the suitability of different resin grades for specific applications.
It provides a more nuanced view than a single viscosity measurement.
Flow Morphology
Molecular structure of the polymer is the primary driver of the observed ratio. Materials with a high degree of long-chain branching, such as low-density polyethylene, exhibit very high trouton ratio values. This is because the branches become caught on each other when the material is elongated, creating a high resistance to further stretching.
Linear polymers like high-density polyethylene or polypropylene have much lower ratios and are more prone to thinning and breaking. This relationship explains why some materials are easier to process into thin films or large bottles. Engineers use the ratio to optimize the die design and the cooling rates.
Regrind can lower the ratio by breaking the long branches that provide the melt strength. Understanding the morphology of the melt allows for the prediction of how different additives will affect the final behavior.
Processing Consequence
Economics of high-speed manufacturing are tied to the stability of the polymer melt during stretching. The trouton ratio acts as a guide for selecting the best resin for a given process window. If the ratio is too low, the material may experience draw resonance or necking during the stretching stage.
This leads to uneven wall thickness and a higher rate of rejects. By choosing a material with a higher ratio, a producer can run the line faster while maintaining the product quality. This balance is necessary for maximizing the output and the profitability of the operation.
The ratio remains a fundamental parameter for anyone involved in the science of polymer flow.
