Characterizing Plasticizers by TG-IR

Plasticizers are additives that are added to polymeric material to increase flexibility. Phthalates, for instance, are normally added to hard polyvinyl chloride (PVC) plastics to make them soft and flexible. When a plasticizer is added to the polymer, many of the properties are changed, for instance, the glass transition temperature (Tg) changes significantly, the hardness and strength are reduced, and the processability is enhanced to a large extent.

Plasticizers typically contain tiny molecules. They move to the surface and easily evaporate from the polymer matrix upon heating, or the plasticizers evaporate over time. The smell of a new car is one standard example of plasticizer evaporation. Here, the plasticizer evaporates from the interior polymer components of the car. However, this additive can prove harmful to human health. As a result, restrictions often apply to some types of phthalates such as in children’s toys in the United States and European Union. It is important to know the plasticizers added to the polymer product and its content.

Thermogravimetric Analysis

A standard method used for studying weight loss during heating is thermogravimetric analysis or TGA. This technique gives the percentage of weight loss precisely and quantitatively. However, simply using the TGA technique will not give a complete picture of the evolved gas’ chemical components. The TG-IR hyphenation is capable of detecting the evolved off gas from TGA, and provides a more detailed picture of material characterization. In this analysis, a sample was obtained from a paint and varnish producer, who needed information regarding the plasticizer and its content. This sample is a mixture of plasticizer, solvent, and polymers.

Instrumentation

For this analysis, a PerkinElmer® Pyris™ 1 TGA and Spectrum™ 100 system coupled with an advanced TL 8000 transfer line was used (Figure 1).

The TL 8000 transfer line couples a Spectrum 100 FT-IR to a Pyris 1 TGA.

Figure 1. The TL 8000 transfer line couples a Spectrum 100 FT-IR to a Pyris 1 TGA.

This system offers the following benefits:

  • Control unit is integrated with a mass flow controller, flow smoothing system, particle filters, vacuum pump with exhaust line, and separate transfer line and gas cell temperature controllers.
  • Insulated heated transfer line is integrated with SilcoSteel® liner, which can be easily replaced.
  • Heated zero-gravity-effect 'ZGCell' gas cell includes several functions like efficient sample area purging, automatic accessory identification, and low volume.
  • Spectrum Timebase software is specifically designed for time-resolved experiments.
  • The Pyris software allows automatic activation of infrared (IR) data collection.

Results and Discussion

The TG-IR technique was applied to study both samples. In Figure 2, the red TGA curve represents the sample without plasticizer, and the blue TGA curve represents the same sample containing about 8% of plasticizer. Also, the derivative of weight loss is displayed so that the weight loss event can be easily identified.

TGA and its derivative curve. Red curve: sample without plasticizer; Blue curve: sample with about 8% plasticizer.

Figure 2. TGA and its derivative curve. Red curve: sample without plasticizer; Blue curve: sample with about 8% plasticizer.

In the case of the red TGA curve, the first weight loss is because of the solvents; the second weight loss is due to nitrocellulose polymer; and the third and final weight loss is attributed to polyacrylate and polyurethane polymers. In the case of the blue TGA curve, the first loss of weight is due to the solvents; the second weight loss is from nitrocellulose polymer; the third weight loss appears to be from the added plasticizer, and the fourth and final weight loss is from polyacrylate and polyurethane polymers.

All the components were detected from gaseous fragments in TG-IR. So by comparing the red curve with the blue curve, it can be seen that the only difference is the weight loss due to plasticizer from the blue curve. The content of plasticizer is determined to be 8.48%.

The IR spectrum at 13 minutes.

Figure 3. The IR spectrum at 13 minutes.

The IR spectrum at 13 minutes is used because from TGA curve it can be seen that the evolved gas at this point is from the plasticizer. By looking at this IR spectrum
and comparing it to reference spectrum (Figure 4), it can be shown that the plasticizer used here is citric acid, tributylester, acetate.

Unknown plasticizer spectrum vs. reference spectrum.

Figure 4. Unknown plasticizer spectrum vs. reference spectrum.

Conclusion

The TG-IR hyphenation technique has often been used for the polymer industry, in this case identifying plasticizers in paint. TG-IR provides a more complete picture of material characterization as it integrates the strength of both FT-IR and TGA analysis. The combination of the PerkinElmer TG- IR instrument and the TL 8000 transfer line is easy to use and has many design features. It provides high-quality and consistent results, and offers a proven option for this type of analysis.

This information has been sourced, reviewed and adapted from materials provided by PerkinElmer.

For more information on this source, please visit PerkinElmer.

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