Rapid Analysis of Polymeric Materials Using FT-IR Spectrometry

Polymers are everywhere. As materials, they play a key role in our modern society where different polymers are suitable for various applications. To obtain the optimal properties for the designated application, different and specific polymeric substances have been developed. The development of copolymers and block-copolymers, expanded even further the family of available polymers and, accordingly, the range of potential applications. Hence, the production of polymers is one of the most active fields in the chemical industry.

To analyze a multitude of possible raw materials, additives, flame-retardants and products, new measurement techniques are required. Fourier transform infrared (FT-IR) spectroscopy is one of the fastest and most precise techniques to inspect incoming goods and perform quality control of raw materials and polymers, respectively. FT-IR spectroscopy offers information about the identity of raw materials, possible contaminations, quality of the products and allows identifying unknown polymeric samples.

High quality IR spectra can be recorded in a few seconds. Usually there is no need of specific sample preparation or of expensive consumables. FT-IR spectroscopy for instance, can help identifying an incoming raw material or validating that a given product is within its specifications. The identification of an unknown polymer is as well extremely easy when using the Bruker polymer library.

Instrumentation

Incoming goods inspection and quality control using IR spectroscopy is mainly performed using the attenuated total reflection (ATR) method, as this is much more straightforward than the traditional transmission one. The IR radiation penetrates slightly (about 1 micron) into the sample surface. The IR detector of the FT-IR spectrometer can then measure the absorbance resulting from the sample. All types of samples (e.g. solids, liquids, powders, pastes, pellets, slurries, fibres etc.) are just put on the accessory before performing the data acquisition. The standard analysis takes just a minute, including sampling, measurement and data evaluation. ATR allows also differentiating between the top layers of a polymer laminate, which is instead not possible when measuring the sample in transmission mode.

Figure 1. ALPHA-P with diamond ATR.

The highly compact ALPHA FT-IR spectrometer with the Platinum diamond ATR-module is a robust, affordable and easy to operate system. Its ergonomic one-finger clamp mechanism enables an extremely easy sampling of solid samples. To provide the user an unobstructed access to the sampling area, the pressure applicator can be rotated by 360°. Diamond is highly robust, chemically inert and hence an ideal material for the analysis of a wide range of samples. In order to measure highly absorbing dark samples such as black polymers, the platinum ATR module can be equipped with a germanium (Ge) crystal plate. Both plates are identified electronically and the setting parameter are loaded accordingly.

The ALPHA spectrometer ensures consistent and reproducible data. A permanent online diagnostics of the spectrometer by the PerformanceGuard offers a “real time” display of the instrument status. The instrument validation (OQ/PQ) is done by fully automated test routines to ensure operation within the specifications. Furthermore, the OPUS software is fully compliant to cGMP and 21 CFR part 11 when operated in a validated environment.

The measurement process itself is very simple using a specially designed Wizard. The user is guided through the analysis procedure by a dedicated software Wizard. Even untrained personnel can measure and evaluate a sample in less than a minute. During the measurement and evaluation process, the Wizard changes its appearance dynamically presenting the functionality to perform at each step.

As an example, Figure 2 shows the appearance of the user interface after two measurements. The Wizard, located on the left side of the OPUS window, now offers the possibility to measure a new background or to proceed with the measurement of a new sample.

Figure 2. OPUS user interface with the Wizard bar on the left.

The first sample considered is polyethylene terephthalate (PET), a thermoplastic widely used in many applications. It is used for instance, as a material for beverage, food and other liquid containers. In the scenario of PET being an incoming good to be inspected, its identity has to be confirmed and its possible contamination with other polymers has to be excluded.

The analysis proceeds as per the following steps:

• First the background spectrum of the clean ATR unit is measured by pressing the “Measure Background” button of the QC-Wizard

  

• Then the sample (sheet, foil, powder, liquid etc.) is placed on the ATR crystal. The “one finger” clamp is applied on solid samples to ensure good contact to the ATR crystal.
• The spectrum is acquired simply by clicking on the “Measure Sample” button:

  

After the measurement, the following buttons are shown on the QC-Wizard bar:

To analyse the spectrum, different options including the “Quick compare” or the “Library search” are availbale. Additionally, printing of the spectra or of the result pages can be easily done via the “Print Report” button. The “Next Sample” button initialises the analysis of another sample. The “Quick Compare” function compares the sample with a reference: either a single spectrum, an average of spectra or a number of different single spectra. When a comparison with one single spectrum or with an average of spectra is selected, the comparison result will be an “OK” or a “Not OK” of the sample. In the PET example considered here, the high correlation of the sample spectrum with the reference data, well above the set threshold, confirms the sample to be PET (Figure 3).

Figure 3. Comparison with an average spectrum of the directory containing previously measured spectra of PET.

The second set of samples considered here are: polycarbonate/acrylonitrile butadiene styrene-blend (PC-ABS), pure polycarbonate (PC) or cellulose acetate (CA), for which a comparison with a set of polymer spectra is performed. To set up the method the data path containing the files of the reference spectra has to be selected. The method results in a hit list, showing those results that are within the desired correlation limit. Results below the threshold are greyed as shown in Figure 4. In the shown example, the sample is clearly assigned to be PC-ABS.

Figure 4. Comparison to several different spectra. Hits below threshold (hit #2 and #3) are greyed indicating that only the first hit is correct.

The third set of samples includes completely unknown polymers that can be identified using a library search. The Bruker ATR-polymer library contains a large number of commercially available plastics and blends. For each compound, the spectrum was recorded with both a Ge- and a Diamond-crystal. The results are shown in figure 5.

Figure 5. Library search using the Bruker Optics ATR-Polymer Library. The result shows that the sample is a polycarbonate/acrylonitrile butadiene styrene-blend. A click on the “Next Sample” button will close the result windows and the QC-Wizard will offer the user to measure a new sample or to measure a new background first.

Measurement of Dark Colored Samples

Highly absorbing substances like carbon black filled rubbers cannot be easily measured with a diamond ATR crystal. When using Diamond with its rather low refractive index (2.4) the penetration depth into the sample is too high, resulting in spectral artefacts. To overcome this limitation Germanium (Ge) is used as an ATR-crystal. It offers a higher refractive index (4.01) in combination with a wide spectral range and high chemical resistivity. Therefore it constitutes an ideal material for highly absorbing samples. Figure 6 shows two spectra of a sample measured with a Diamond- (top) and a Ge-crystal (bottom).

Figure 6. Comparison of a highly absorbing sample measured with a Diamond-crystal (top) and a Germanium-crystal (bottom).

The spectrum using the Ge-ATR does not have derivative-like artefacts that are present when using a diamond-crystal and shows much more pronounced single bands. Due to their low penetration depth, Ge-crystals are also very suitable for measurements of thin surface films with high sensitivity.

Summary

The integration of the compact and robust FT-IR spectrometer ALPHA with a dedicated software Wizard, OPUS quick compare functions and comprehensive spectra libraries result in a reliable and user-friendly analysis system for polymers and their raw materials. Rapid and specific compare methods are set up easily even with just one reference spectrum. Hence, this function is an excellent tool for incoming goods inspection and product quality assurance. Applying a spectrum search on a polymer library, the identification of an unknown polymer sample can be performed in only a few seconds. Bruker Optics FT-IR-spectrometer ALPHA with a Diamond ATR module provides a quick and robust measurement method. Due to the easy handling of the ALPHA instrument and wizard-guided measurement, evaluation and reporting can be performed even by untrained users.

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

For more information on this source, please visit Bruker Optics.