Predicting Thermal Degradation of Polymers

We can identify how specific process parameters such as applied shear or various temperatures can affect the quality of your polymer melt during processing. Our lab is equipped with both a high temperature oven (500°C) and nitrogen purge system for investigating temperature driven changes in rheology of your polymer including depolymerisation, polymerisation and oxidation. Contact us to discuss how we can help investigate thermal degradation of your polymers.

3 images - On the left a brightly coloured sample has been on the rheometer at 200°C for 5 minutes. The middle picture shows discolouration after the sample has been on for 25 minutes. On the far right the sample is hardly recognisable having been on the rheometer for 120 minutes.
A polymer underdoing various stages of thermal degradation in air. On the left the sample was freshly placed onto the rheometer and is brightly coloured. The middle picture shows discolouration due to exposure to oxgen. The sample on the right has undergone shear as well as exposure to oxygen in high temperature and has completely degradated.

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High temperature oxidation is a common problem when working with certain polymer melts at high temperature. Oxidation can cause discoloration or changes in quality and performance of a polymer material, making it unsuitable for its initial use.

For some elastomers, a combination of high temperature and water content can cause chain scission or further polymerisation. This becomes a difficult problem to solve if the melt temperature of the material occurs within the boundary that these chemical processes occur.

Oscillatory Time Sweeps and Temperature Cycling for Thermal Degradation

In an oscillatory time sweep, the response to deformation of small amplitude oscillations are tracked over time to quantify the rate of change in the material under a specific condition. E.g. at prolonged high temperatures in an air environment vs. an inert gas environment. This can be very useful for tracking the stability of a sample while it is in a specific environment and getting a rate of change.

A plot showing thermal degradation during an oscillatory time sweep. On the left hand of the plot storage and loss modulus remain unchanged as a function of time, however when air is introduced into the chamber on the right hand side we can see a crossover storage over loss modulus.
Schematic showing idealised time sweep of a polymer in a high temperature environment. In an inert gas e.g. N2 (left hand side) we can see that the material appears stable and unchanging. When air is introduced into the chamber, we can see the product appears to become more stiff and elastic.

A temperature cycling test as the name suggests, involves ramping the temperature between a high and low value for a set number of cycles over a set period of time (generally limited by the rate it takes for a sample to reach thermal equilibrium with the heating chamber). Comparing the gradient and peaks of each cycle can give you an idea of the robustness of the material as temperature is repeatedly cycled. You can also compare the material in an inert gas environment against its less than ideal operating conditions and quantify the extent the conditions degrade the rheological behaviour of the polymer.

An oscillatory temperature swing test for testing robustness of a polymer or displaying thermal degradation as a result of repeated temperature cycling. In an inert gas, we can see the polymer melts and solidifies with little changes between cycles. In a reactive gas like air, we can see the polymer initially melts, but then stiffens irreversibly, with subsequent cycles showing the material displays elastic dominant behaviour.
A schematic representing an idealised demonstration of a polymer degradation in air versus an inert gas environment using a temperature cycling method. Phase angle is a measure of elastic dominant behaviour in a material, with lower phase angles indicating a more elastic (or more solid-like) behaviour. In air (line shown in blue), we can see the material irreversibly changes becoming more solid and less sensitive to changes in temperature.

High temperature oscillatory frequency sweeps for a before and after snapshot

A rheometer with the environmental chamber opened.

High shear rheology for some polymer melts can sometimes be practically challenging. Due to the high strain involved, the measurement itself can damage the sample or cause it to fail at the edges, providing erroneous results. Oscillatory frequency sweeps are the preferred method for characterising polymers in the melt condition due to the low strains involved, and the ability to relate data back to microstructural properties including average molecular weight and average molecular weight distribution.

The high sensitivity of oscillatory rheometers makes them suitable for early detection of physicochemical changes occurring within the polymer, such as change in molecular weight average or molecular weight distribution. The maximum of storage modulus is a good indicator of molecular weight distribution, with higher values indicating a narrow distribution and lower values indicating a broad distribution.

If the storage modulus-loss modulus crossover point occurs at a lower frequency than the benchmark, it indicates the sample possesses a higher molecular weight average. A crossover point that occurs towards the right of angular frequency indicates a material with a lower molecular weight.

Cox-Merz ‘Rule’ for Viscosity

It is still possible to obtain flow data or viscosity data from oscillatory frequency sweeps using the Cox-Merz ‘rule’ on qualifying products. The idea being that a steady state shear viscosity at a given shear rate will be the same as the complex viscosity at the same frequency.

A plot showing the interchangeability between complex viscosity and a viscosity shear rate profile. This might not always be possible.

The zero-shear viscosity, or plateau viscosity is a highly sensitive way of measuring molecular weight of high molecular weight polymers, the viscosity being proportional to molecular weight, MW3.4 . In addition to observing the zero-shear plateau, the onset of non-Newtonian behaviour (generally shear thinning) is an indicator of the distribution, with early onset generally being related to a broad distribution.

If you would like to discuss how our capabilities can be used to help investigate the stability of your polymer at high temperature, then please feel welcome to contact us.

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