Preparing and Analyzing PET with Additives

PET samples with several additives and plain resin were mixed for a specific amount of time. While the mixing took place, the composition/decomposition was measured in the integrated slit capillary. The mixture was then transferred to a micro injection molding machine so as to prepare disc-shaped test specimens. With these discs, rheological tests of the polymer melt were conducted later on a rotational rheometer. The aim was to demonstrate that a test in a micro compounder with just 7 g of sample can be used for a rapid screening of PET and additives and to provide an indication for the chemical recycling of the polymer.

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Methods

Sample Preparation

The Thermo Scientific™ HAAKE™ MiniLab Micro Compounder with co-rotating screws (Figure 1) was used to prepare the mixtures of PET with additives at 270 °C with a screw speed of 50 rpm. For 15 minutes, the sample was mixed by re-circulating. During the mixing process the pressure drop was monitored in the slit capillary of the backflow channel (Figure 2).

Figure 1. HAAKE MiniLab Micro Compounder.

Figure 2. HAAKE MiniLab Micro Compounder backflow channel built as slit capillary with two pressure sensors.

Injection Molding of the Test Specimens

After the mixing process was completed, the polymer was directly extruded into the heated cylinder of the Thermo Scientific™ HAAKE™ MiniJet Pro System (Figure 3) for injection molding of test specimens (20 mm Ø and 1.5 mm thickness) for additional rheologial tests. The heated cylinder's temperature was 270 °C and the mold was heated to 80 °C. The samples had been injected with 500 bar for five seconds and post pressure of 300 bar for five seconds.

Figure 3. HAAKE MiniJet System and molds.

Rheological Test

The rheological tests were performed using 20 mm parallel plates and a gap of 1.4 mm on a Thermo Scientific™ HAAKE™ MARS™ Rheometer with an electrical heated oven at 270 °C under nitrogen atmosphere. All samples had been primarily tested in an amplitude sweep to determine the linear viscoelastic range. New test specimens were used for frequency sweeps from 0.1 to 46 Hz. The deformation for all tests was 0.5%, thus in a safe regime of the linear viscoelastic range for all samples.

Figure 4. Thermo Scientific™ HAAKE™ MARS™ Rheometer

Results

In the recirculation mode, the pressure profile can be monitored over a period of time by the pressure difference of the two pressure sensors, built in backflow channel (see Figure 2). At the start of the test, material is filled into the micro compounder. This causes a pressure peak. After all the material is filled in and the temperature equilibrated, the pressure profile over time can refer to a reaction of the polymer. A decrease of the pressure over time shows an alteration of the material. For plain PET, for instance, this can be a reaction of the polymer with water (moisture) where the polymer degrades. A reduction of the pressure is in line with a lower viscosity of the PET. When the pressure increases over time it is a sign of a condensation reaction of the PET increase in the chain length or branching which results in a higher viscosity. The samples for the rheological test were prepared with material that had been recirculated for 15 minutes in the HAAKE MiniLab Micro Compounder. The final pressure value can be correlated with complex viscosity |η*| of a dynamic oscillatory test done with a rheometer. For the plain PET illustrated in Figure 5 after the loading peak, the pressure drop shows a decomposition of the PET. After 15 minutes, pressure is nearly constant with a value of approximately 18 bar. In Figure 6, the frequency sweep for the same sample indicates that the loss modulus G" is significantly higher than the storage modulus G'. The slight bumpy curve of G" is because of the fact that the phase shift δ is nearly 90° and the smallest changes have big influences on G". The complex zero shear viscosity |η*| is 200 Pas.

Figure 5. Pressure dependence of PET with no additives.

Figure 6. Frequency sweep of PET with no additives.

Figure 7 shows the PET with 1% 1,2,4-Benzenetricarboxylic anhydride after the loading peak, a pressure increase which correlates with the condensation reaction of the PET. After 15 minutes, the pressure continues to increase with a value of approximately 15 bar. Compared to the plain PET it is marginally less of an indication of a lower viscosity.

Figure 7. Pressure dependence of PET with 1% 1,2,4-Benzenetricarboxylic anhydride.

A look at the frequency sweep in Figure 8 for the same sample illustrates that G' and G" are getting closer. This goes along with a lower δ of approximately 85° at low frequencies. The PET gains more elasticity. The |η*| is 150 Pas at low frequencies. Compared to the plain PET, the additive is accountable for the lower pressure and the lower |η*| on the one hand, but on the other hand the additive brought about a reaction of the PET.

Figure 8. Frequency sweep of PET with 1% 1,2,4-Benzenetricarboxylic anhydride.

The pressure dependence of PET with 1% 1,2,4-Benzenetricarboxylic anhydride and 1% meta-Dioxazolinebenzene in Figure 9 illustrates the pressure decrease, and afterward an increase after the loading peak. The end pressure with 55 bar is considerably higher compared to the plain PET and the compound with 1% 1,2,4-Benzenetricarboxylic anhydride as an additive. The pressure fluctuation at the end of the test is because of a rubbery morphology. The frequency sweep in Figure 10 illustrates the common trend of G' and G" for a viscoelastic material. The |η*| with nearly 2800 Pas is over 10 times higher compared to the plain PET and the compound with PET and 1% 1,2,4-Benzenetricarboxylic anhydride as an additive. An observation of the change of δ from 88° at low to 52° at high frequencies shows a higher elastic behavior coming near the crossover. The mixture of both additives reveals first a decomposition of the PET followed by a reaction to build up a new structure. It is very likely that the molecular weight is considerably higher. The rise in pressure and |η*| correlates well in comparison to the tests of plain PET and the compound with 1% 1,2,4-Benzenetricarboxylic anhydride.

Figure 9. Pressure dependence of PET with 1% 1,2,4-Benzenetricarboxylic anhydride and 1% meta-Dioxazolinebenzene.

Figure 10. Frequency sweep of PET with1% 1,2,4-Benzenetricarboxylic anhydride and 1% meta-Dioxazolinebenzene.

Conclusion

The HAAKE MiniLab Micro Compounder is a suitable instrument to screen the effects of various additives. Only a small quantity of sample (7 g) is needed. Just viewing the pressure dependence provides a first revelation of the functionality of the additives. The time required for one test is reasonable. If additional rheological or other physical tests have to be done, the transfer of the polymer melt into the HAAKE MiniJet Pro System is possible. Different test specimens can be prepared rapidly and are reproducible. Additional studies on the molecular weight and distribution, either by comprehensive rheological tests for example Time Temperature Superposition compared with GPC data, could really establish the assumptions.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

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