Using FTIR-ATR Spectroscopy to Investigate Additives in Plastics

Plastic products that are commercially available contain polymers such as polypropylene and polyethylene as their main components along with several trace components that are added for improving performance and maintaining quality.

Here, an evaluation of additives used in plastic bags is introduced taking advantage of the high S/N ratio of the Shimadzu FTIR IRTracer-100 spectrophotometer.

Shimadzu IRTracer-100 FTIR Spectrophotometer and ATR Attachment MIRacle A

Single reflection ATR is used commonly as an infrared spectroscopy method for non-destructive easy evaluation of additives in plastic products. Since measurement can be conducted without any need for pretreatment of the sample, the single reflection ATR method is widely used in various applications, including contaminant identification.

Since the penetration depth of the infrared light into the sample surface using the ATR method is on the order of a few microns, this method can be used effectively, especially for additives that are localized on the sample surface.

The reduction of the content ratio of the target component causes its peak intensity in the measured infrared spectrum to decrease accordingly, so an instrument with a high S/N ratio is required to obtain good evaluation results.

The high 60000:1 S/N ratio of the Shimadzu FTIR IRTracer-100 ensures that stable and clear peak information is obtained from additives present even at trace levels. Figure 1 shows an image of the Shimadzu FTIR IRTracer-100 and Figure 2 shows an image of the MIRacle A Single Reflection ATR Accessory.

Figure 1. Shimadzu IRTracer-100 FTIR Spectrophotometer

Figure 2. MIRacle A Single Reflection ATR Accessory

Measurement

Using the single reflection ATR method, the surface of commercially available plastic bags is measured. An image of the plastic bags is shown in Figure 3. Table 1 shows the measurement conditions used and the measurement results and spectrum search results are shown in Figure 4.

Figure 3. Plastic Bags

Figure 4. Infrared Spectrum and Search Result for Plastic Bag

Table 1. FTIR Measurement Conditions

The results obtained from analysis of the plastic bag are consistent with the library spectrum of polyethylene indicating that the principal component is polyethylene.

Figure 5 shows an expanded view in the vicinity of the baseline of Figure 4. It is believed that the arrow- indicated peaks in the Figure are derived from additives present in the plastic bags, and these are consistent with the spectra of aliphatic amides such as oleamide. Aliphatic amides are one type of substance added to resins to serve as a lubricant.

After the measurement was completed in Figure 4, the plastic bag sample was removed from the ATR prism, and without washing the prism, another measurement was taken. The results are shown in Figure. 6. The results resemble the spectrum of the aliphatic amide shown in Figure 5, indicating the possible transfer of the additive in the plastic bag to the ATR prism.

The peaks associated with the aliphatic amide that appeared in the measurement results of Figures 5 and 6, respectively, show very weak intensity with absorbance values less than 0.010 A. Also, there is a peak in the vicinity of 1631 cm^-1, a region that easily reflects the presence of water vapor in the air.

Figure 5. Expanded Infrared Spectrum of Figure 4 and Spectrum of Oleamide

Figure 6. Infrared Spectrum of Substance Transferred to ATR Prism

Investigation of Repeatability of Small Peaks

It is possible to gain an understanding of content level using the height and area value of the peak originating from the target component, but for small peaks derived from additives, it is important to grasp the measurement repeatability. A plastic bag sample in close contact with the ATR prism was used to take 10 continuous repeat measurements.

The peak area values of the peak in the vicinity of 1631 cm^-1 and the calculated CV values are shown in Table 2. The integration was repeated twenty times and one time, and these values were compared, respectively, with those obtained with the Shimadzu I RPrestige-21. The better stability of measurement values obtained with the IRTracer-100 is attributed to its higher S/N ratio.

Table 2. Peak Area and CV Values

Conclusion

An evaluation of trace additives in plastic resin was done. The high S/N ratio of the Shimadzu FTIR IRTracer-100 enabled stable and clear acquisition of minute peaks originating from additives present at minute levels.

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

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