Analysis of PET with Additives [Scientific Poster]

This scientific poster outlines the preparation and analysis of PET with additives in a micro compounder and rheometers.  Polyethylene terephthalate – known in the plastics and polymers industry as PET – is a strong, lightweight, transparent plastic.

We used a minilab micro compounder to screen the effect of different additives. The whole exercise was done to do re-active extrusion in the mini compounder and use recycled PET with additives to give “recycled” PET the similar properties as fresh material. The problem with PET is that after it is recycled a number of times, the material properties change dramatically and molding (e.g. for a bottle) is no longer possible. Instead of dumping/burning the old PET, re-active extrusion could help to restore the conditions so that a further usage is possible.

PET samples with several additives and plain resin were mixed for a specific time. During the mixing period the composition / decomposition was measured in the integrated slit capillary. When the mixture was ready it was transferred to a micro injection molding machine in order to prepare disc shaped test specimens. With these discs, rheological tests of the polymer melt were performed afterwards on a rotational rheometer. The aim was to prove that a test in a micro compounder with only 7 g sample can be used for a fast screening of PET and additives and also gives an indication for the chemical recycling of the polymer.

With a rheometer, the mechanical properties such as the viscosity and the dynamic moduli of polymer solutions, polymer melts as well as semi-solids can be measured as a function of stress, strain, time, frequency, temperature etc., not in only in shear but also in extensional flow.  We used a mini-lab micro compounder system based on a conical, twin-screw compounder with an integrated backflow channel. Due to the channel and a bypass valve, the residence time is well defined. Two pressure transducers are integrated in the backflow channel and allow the measurement of (relative) melt viscosity (counter-rotating screws required).

Sample Preparation

The mixtures of PET with additives were at 270 °C with a screw speed of 50 rpm. The sample was mixed by re-circulating for 15 minutes. During the mixing process the pressure drop in the slit capillary of the backflow channel was monitored.

After the mixing step, the polymer was directly extruded into a heated cylinder of the system for injection molding of test specimens (20 mm ∅ and 1.5 mm thickness) for further rheological tests. The temperature of the heated cylinder was 270 °C and the mold was heated to 80 °C. The samples were injected with 500 bar for 5 sec and post pressure of 300 bar for 5 sec.

The rheological tests were conducted with 20 mm parallel plates and a gap of 1,4 mm with an electrical heated oven at 270 °C under nitrogen atmosphere. All samples were first tested in an amplitude sweep to determine linear visco elastic range. For frequency sweeps from 0.1 to 46 Hz new test specimens were used. The deformation for all tests was with 0.5 % in a safe regime of the linear visco elastic range of all samples.

Results

For the plain PET:  after the loading peak, the pressure drop indicated a decomposition of the PET. After 15 min. pressure was almost constant with a value of approximately 18 bar.  The frequency sweep for the same sample showed that the loss modulus G” was significantly higher than the storage modulus G’. The phase shift δ was almost 90° showing the smallest changes have major influences on G”. The complex zero shear viscosity |η*| was 200 Pas.

The PET with 1% 1,2,4-Benzenetricarboxylic anhydride showed after the loading peak a pressure increase which correlated with condensation reaction of the PET. After 15 min. the pressure was still increasing with a value of about 15 bar. Compared to the plain PET it was slightly lower, an indication of a lower viscosity. A look at the frequency sweep for the same sample showed that G’ and G” were getting closer. This corresponds with a lower δ of about 85° at low frequencies. The PET obtains more elasticity. The |η*| is 150 Pas at low frequencies. Compared to the plain PET, the additive is responsible for the lower pressure and the lower |η*| on the one hand; but on the other hand, the additive induced a reaction of the PET.

We also measured the pressure dependence of PET with 1% 1,2,4-Benzenetricarboxylic anhydride and 1% meta-Dioxazolinebenzene.  Take a look at the scientific poster to see the results of this test, as well as the frequency sweep charts of all the analyses and details of instruments used in the process.

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