Determining Copolymer Levels for Polyethylene and Polyvinylacetate Samples Using Near-Infrared Spectroscopy

Thermal polymers are typically supplied in the form of pellets or chips. It is extremely difficult to perform analysis of these polymers in this form due to the properties of these materials, including strength and resistance to acids and solvents. To perform the analysis, the pellets must be dissolved, a process which can take several hours and in some cases more than a day, depending upon the type of material.

An analysis technique that does not involve any sample preparation would be useful in terms of speed and simplicity of analysis, and reduction of errors from irreproducible sample preparation.

Near-infrared (NIR) spectroscopy is ideal for quantitative polymer analysis as it does not require any sample preparation and is non-destructive. This article discusses the application of NIR spectroscopy for the measurement of copolymer levels in polymer pellets with no sample modification.

Experimental Procedure

In this analysis, a NIRS XDS RapidContent Analyzer is used in reflectance mode from 1100-2500nm to measure the NIR spectra of the samples. The samples are simply fed into a hopper, with the analysis of the contents performed without an operator.

The acquired spectrum is the average of all scans and this sampling method reduces differences due to non-homogeneous sample packing, thereby allowing quantitative analysis without sample grinding.

Experimental Results

The analysis involved the collection of the NIR spectra of polyethylene (PE) and polyvinyl acetate (PVA) to identify the spectroscopic contributions of each of the copolymers, as depicted in Figure 1. The spectra of these materials appear very similar because of the characteristic broad, overlapping absorptions in this spectral region.

Figure 1. NIR spectra of PE and PVA.

Conversion to the second derivative makes spectral comparison easier by improving the features in each spectrum (Figure 2). This math treatment has also inverted absorption peak maxima to second derivative peak minima. Isolate unique absorbance features for PE and PVA are illustrated in Figures 3 and 4.

Figure 2. Conversion to the second derivative makes spectral comparison easier.

Figure 3. Isolate unique absorbance features for PE and PVA.

Figure 4. Isolate unique absorbance features for PE and PVA.

The absorbance (log 1/reflectance) spectra for ethylene vinyl acetate (EVA) copolymer pellets are delineated in Figure 5. The baseline varies drastically because of differences in the intensity of radiation scattered from the pelletized samples.

It is possible to compensate for baseline variations by converting the spectra to the second derivative, as demonstrated in Figure 6. The compensation of the baseline variations enables noticing the chemical variations due to the amount of copolymer.

Figure 5. The absorbance spectra for EVA copolymer pellets.

Figure 6. The baseline variations can be compensated by conversion of the spectra to the second derivative.

Figure 7. The increase in the vinyl acetate absorption near 1680nm.

Figure 8. The decrease in an ethylene absorption at 2050nm.

The increase in the vinyl acetate absorption near 1680nm is delineated in Figure 7, while Figure 8 shows the corresponding decrease in an ethylene absorption at 2050nm. The known vinyl acetate levels and the second derivative spectra are used to perform a linear least-squares regression. The following table shows the results:

In this table, K(0) and K(1) represent the intercept and slope of the regression line, respectively. 'R' is the correlation coefficient and SEC is the standard error of calibration. Figure 9 shows the comparison of the NIR results and the known values graphically.

Figure 9. Comparison of the known values and NIR results.

Conclusion

NIRS can analyse polymer pellets without any sample preparation or reduction in accuracy of the measurement. This was shown for samples of a copolymer blend, whose baseline differences appear at first to be too large to perform the quantitative analysis on the pellets. The analysis also demonstrated the ability to perform quantitative analyses using the averaging of large areas of the samples and the use of derivative math treatments.

NIR spectroscopy can also be used to determine other copolymer pellets, including butyl-ene/terephthalate, styrene/butadiene, and ethylene/propylene. The technique can also monitor pelletized samples of polymer additives, including slip agents, stabilizers and antioxidants.

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

For more information on this source, please visit Metrohm AG.