Thermogravimetry: the evaluation and classification of plastics

Materials World magazine
,
1 Feb 2017

Dr G Kaiser, Dr E Moukhina and Dr A Schindler discuss the importance of thermogravimetric analysis for polymer blends and mixtures.

In order to increase efficiency in laboratory work, simplification and standardisation of test procedures is ongoing. Particularly high demands are made in instrument software. Usability and automatic evaluation routines are of key importance. 

When it comes to thermogravimetric analysis, it is now possible to create instrument-independent measurement methods to automatically detect, evaluate and classify thermal effects. Tests on a sample material can be carried out with various thermogravimetric instruments (of the same type) under identical conditions, leading to identical results. Typing errors during transfer of measurement programmes from one lab to another or varying evaluation results caused by different users, within the same lab, can be eliminated. 

Thermogravimetry (TG) or thermogravimetric analysis (TGA) is an established method for compositional analysis of polymer blends and mixtures. With TGA, it is possible to determine the amount of volatiles – such as plasticiser – the polymer proportion, and the filler content within a polymer. Additionally, it delivers information about the thermal stability of a plastic material or about its decomposition kinetics. A series of national and international standards, such as ISO 11358 or ASTM E1131, give recommendations for sample preparation and for the selection of suitable measurement conditions.       

TGA measures the amount and rate (velocity) of change in the mass of a sample as a function of temperature or time in a controlled atmosphere. The technique can analyse materials that exhibit either mass loss or gain because of decomposition, oxidation or loss of volatiles. It is especially useful for the study of polymeric materials, including thermoplastics, thermosets, elastomers, composites, films, fibres, coatings and paints.

Generally, mass changes can be detected after a period of time (at an isothermal temperature) or within a certain temperature interval (upon heating). Dynamic measurements performed during heating prevail. The occurring evaporation/decomposition pattern allows for conclusions to be drawn on the polymer type and the nature of the volatile. However, the decomposition temperatures of many polymers are quite close to each other and the resulting TGA curve is therefore not always clear enough for a distinct analysis.

In such cases, coupling to gas analysis systems,such as a Fourier-Transform infrared spectrometer provides accurate results. TGA can also determine the ash content of a sample by changing the gas atmosphere from nitrogen to air or oxygen. If the sample, after finishing the polymer decomposition, is cooled back to 300–400°C under nitrogen and reheated in air atmosphere, TGA is even capable of distinguishing between pyrolytic carbon and added carbon black.

Quality control

To find out whether the present TGA curve conforms to reference measurements, users take, in most cases, the classical route via evaluation and comparison of the experimental data with documented ones. However, if there are regions within the TGA curve where the evaluation limits cannot be clearly defined, the calculation can yield different mass-loss values, although the base material remains the same. 

Figure 1Figure 1 displays a TGA measurement on a part made of wood polymer compound (WPC), with polyethylene as the polymer matrix material. 

Between the second and third mass loss step (around 400–420°C), the TGA curve slightly declines, resulting in a curve that does not precede parallel to the x-axis. This allows variable positioning of the evaluation limits (with a fixed evaluation limit at 204°C and a second one varying between 401–418°C), creating a mass difference of 1.3%. This difference could be decisive in pass or fail tests.  

Things can be altered using AutoEvaluation, developed by NETZSCH, Germany, the first autonomous acting evaluation software for thermal analysis. The operator obtains user-independent and objective results with just one click. AutoEvaluation can also be integrated directly into a measurement method. In this case, the evaluation starts automatically as soon as the measurement has finished. The outcome is an evaluated curve, which is, for example, available as an analysis results sheet.

The curve (Figure 2) is based on a method carried out with the NETZSCH TG 209 F3 Tarsus. As each measurement method is instrument-independent, it can also be used on any other thermobalance systems from the polymer line, such as the TG 209 F1 Libra. This puts both material manufacturer and material processors in the position to always refer to exactly the same measurement programme.  

Figure 2Classification of polymers

In a nitrogen atmosphere, polyvinyl chloride (PVC) exhibits a multi-step decomposition in temperatures of 200–550°C. The decomposition profile depends on whether the base polymer contains plasticiser (PVC-P) or not (PVC-U). For example, Figure 3 illustrates a TGA measurement on a PVC-P granulate (light blue curve), which was recently delivered. To evaluate if this material complies in its composition to the previous sample, the Identify software, based on a database of libraries can help. Some of these libraries are included in the instrument package, but further ones can be created, edited and expanded by the user themselves to be adapted to each task. 

All test runs, which should serve as a comparison, were combined in advance in a statistical class named PVC-P. Both the evaluated curve and the individual curve with the highest similarity or the average curve of the most similar statistical class are displayed. In Figure 3, the average curve of the statistical class PVC-P (violet) is shown. There is a similarity of 99.46% (Figure 4), which concludes that the new material matches the previous lot.

The statistical classes PVC and PVC-U, which are also present in the similarity table are rated ‘less similar.’ The PVC class is an inherent part of the software, containing data of various PVC-types from a range of sources. The two statistical classes PVC-P and PVC-U are user-defined.  

Figure 3The AutoEvaluation and Identify systems can automatically evaluate and classify thermogravimetric curves to obtain objective, reproducible results and leave more time for other tasks. The possibility to create user-defined libraries and statistical classes allows this database system to be applied to material identification as well as quality control or failure analysis testing. 

Dr Gabriele Kaiser studied Chemistry in Erlangen, Germany. Currently, she is working as Head of Scientific and Technical Communication Department at NETZSCH-Gerätebau. 

Dr Alexander Schindler studied Physics at the University of Bayreuth, Germany. He started at NETZSCH in 2003, where he is now working in the research and development department. 

Dr Elena Moukhina studied Physics with specialisation on Theoretical Physics at the Yaroslavl State University, Russia. Since 2002, Moukhina is Software Project Manager at NETZSCH. 

Figure 41 TGA measurement on a WPC part (consisting of polyethylene and wood). Sample mass is 13.5mg, crucible material is Al2O3, at a heating rate of 10K/min, change of gas atmosphere (from nitrogen to air) at 600°C. The DTG curve (dashed-dotted line) represents the first derivative of the TGA curve with respect to time.

2 Results of the TGA curve from figure one using AutoEvaluation – declaration of the individual mass-loss steps and the residual mass at 798°C. The small step in the beginning (DTG peak at 140°C) can be related to water loss (moisture), followed by the decomposition of wood (DTG peak at 366°C) and polymer (DTG peak at 482°C). After switching over to air – combustion of the pyrolytic carbon (DTG peaks at 641°C). The polymer content is around 28%.

3 TGA measurement on PVC-P, sample mass at 10.45mg, crucible material – Al2O3, heating rate at 10K/min, change of gas atmosphere
(from nitrogen to oxidising conditions) at 800°C. Database comparison between the current measuring curve (light blue) and the average curve of the statistical class PVC-P (violet) by means of the Identify system.

4 Identify hit list – the class PVC-P has the highest similarity to the measurement curve.