Determination of percent glycerol monostearate in polypropylene by FTIR

This method is for the determination of the glycerol monostearate (GMS) content and chemically identical antistatic additives in polypropylene where the additive package is known.

The method has been validated over the range of 0.05 to 0.8% GMS content and can be used for either powder or pellet samples. Certain ester-based antioxidants, such as Irganox 1010 and Irganox 3114 have been found to interfere with the determination and therefore a correction factor is necessary for resins containing these additives.

The sample must be pressed into a film or coupon prior to the analysis.

The film is placed in the infrared spectrometer to obtain the spectrum at 4 wavenumber resolution or better. Using the Agilent DialPath or TumblIR accessories, the film or coupon can be inserted into the infrared beam path between the top and bottom crystals. Both these accessories provide a revolutionary new way to measure thin polymer films or liquids. The horizontal mounting provides a simple, fast and reproducible mechanism to mount the sample by simply laying it down flat and rotating the crystal into position, eliminating errors and providing accurate and reliable answers.

The ester absorbance of the GMS additive band is measured at 1739 cm-1 and the absorbance is measured for the reference polypropylene band at 1044 cm-1 to provide a path length or film thickness correction. To obtain the additive concentration in the sample, the ratio of the ester GMS band to the reference band is substituted into a linear regression calibration equation, constructed from measurements of prepared standards with known concentrations of additive. This Beer’s Law calibration is linear through the 0.05 to 0.8% GMS range.

The MicroLab PC FTIR software method, Polymer — Glycerol Monostearate in Polypropylene v1, includes the calibration data from Figure 2. This calibrated method is available with the Agilent 5500 and 4500 Series DialPath or TumblIR FTIR spectrometers, as well as the Cary 630 FTIR spectrometers. This method and software performs all the calculations automatically and reports the final value as % Glycerol Monostearate. The values obtained from triplicate determinations should be averaged to give the final reported concentration. Conclusion.

Excerpt from Agilent Technologies Application Note 5991-0467EN