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��Practical Analysis Techniques of Polymer Fillers by Fourier Transform Infrared Spectroscopy (FTIR) Barbara J. Coles, Caryn J. Hall Hauser, Inc. ABSTRACT The identification of polymers by FTIR is often complicated by the presence of fillers. How- ever for kaolin clay, an FTIR analysis should be able to identify the filler and predict its con- centration using a standard curve. The resulting percentage is more reliable than a simple ash, which may change the chemical composition of the filler. INTRODUCTION In the growing plastics industry, there is often need to identify polymer formulations. Whether the analysis is done to reproduce the material, identify another supplier, or provide insight into the cause of failure, the filler is an important aspect. Fillers are used for several reasons; to extend the amount of polymer for overall cost reduction, to add structural stability or impart specific physical characteristics to the polymer such as chemical, temperature, or flame resistance, or to add color to a polymer. Several commonly occurring fillers include: silicates, aluminum trihydrate, calcium carbonate, fiberglass, and talc. These fillers have characteristic FTIR bands which can be easily identified within a spectrum of the polymer. The amount of filler present in the formulation can be of great importance to the performance of the polymer. THEORY FTIR is a powerful analytical tool. Not only does it provide qualitative identification, but also quantitative information. The use of FTIR to quantify the amount of filler present in a poly- mer formulation should follow Beer's Law: 192 Coloring Technology for Plastics A = abc where: A = absorbance, a = absorptivity (a constant specific to the material), b = thickness of sample, c = concentration. The challenge in FTIR quantitative analysis of polymers is the thickness of the sample. The use of peak ratios standardizes the absorbance signal and eliminates the thickness vari- able. Attenuated total reflectance (ATR) and microscope FTIR were the two methods chosen to acquire the FTIR spectra. The filler content in the polymer was confirmed by ashing. DESCRIPTION OF EQUIPMENT AND PROCESS Figure 1. FTIR microscope spectra of kaolin vs. polyethylene vinyl acetate. An Analect Diamond 20 FTIR with ATR attachment equipped with a KRS-5 45ocrystal as well as a XAD-Plus Microscope attachment was used to acquire the FTIR spectra. Kaolin powder was chosen for its peaks by microscope FTIR at 3695, 3668, 3652, 3618, 1115, 1032, 1008, 937, and 913 cm-1 as well as its distinctive shape above 3600 cm-1 and in the Fourier Transform Infrared Spectroscopy 193 Table 1. Summary of results of ashing prepared samples Calculated % Average % ash of kaolin in hot melt samples at 500oC 0 0.03 1 0.80 5 4.19 10 8.47 20 17.68 Figure 2. Standard curve for % kaolin vs. absorbance ratio (slope=0.0514, intercept=0.0118, R2=0.995. 30 26.61 40 35.59 50 44.74 Figure 3. ATR spectra for 1, 10, 30, and 50% kaolin in PVAc. fingerprint region. Kaolin also has a distinctive peak around 540 cm-1 in ATR spectra. A hot melt (polyethylene/vinyl acetate) was chosen because of the relative lack of interferences with kaolin (see Figure 1) and the ability to easily mix various amounts of filler. 194 Coloring Technology for Plastics Figure 4. FTIR microscope spectrum of ashed (500oC) kaolin in PVAc. The standards were prepared by weighing appropriate amounts of kaolin and hot melt into aluminum dishes to achieve filler percentages of 0, 1, 5, 10, 20, 30, 40, and 50%. The alu- minum dishes were then heated on a hot plate to 128oC to melt the hot melt. The kaolin and hot melt were then mixed together and formed into thick films using an 8 mil draw down bar on silicone release paper. Portions of the films were cut and analyzed by both ATR and Micro- scope FTIR and portions were ashed at 500oC in a muffle furnace overnight and allowed to cool in a desiccator. PRESENTATION OF DATA AND RESULTS ATR was the most consistent tool to obtain a good correlation of peak ratios to percent filler. We obtained a standard curve with an R squared value of 0.995 using the kaolin peak at 540 cm-1 and the CH2 stretch of polyethylene vinyl acetate at 2847 cm-1. The standard curve is shown in Figure 2 and the overlay of the ATR spectra for 1 to 50% filler is shown in Figure 3. Summary of results of the ashing of the samples is shown in Table 1. A Primer on Colorful Additives 195 INTERPRETATION OF DATA The microscope FTIR, while providing better resolution of kaolin, did not have a large enough sampling area and so was subject to small shifts in concentration of filler within the sample. ATR was not as sensitive to kaolin as microscope FTIR, but provided a larger sam- pling area and more consistent results. The ashing of the samples at 500oC produced an unanticipated event. Kaolin clay holds water even when considered bone dry. This water was liberated from the clay when it was ashed. Aluminum silicate underwent a partial transformation to aluminum oxide and silicon oxide. Figure 4 is the FTIR spectrum of the ashed material. This raises a problem with simply doing percent filler by ashing when the filler is kaolin clay; the results can be 12-20% low based on percent water and degree of conversion. Also, FTIR of the ashed material could be misinterpreted as silicates rather than aluminum silicate. CONCLUSIONS It is possible to predict percent kaolin by ATR examination. This method may also apply to other fillers in polymers. It is important to identify the type of filler in polymers to get an ac- curate picture of the polymer. However, care must be taken when a polymer is ashed then the ash analyzed by FTIR as the composition of the filler could change during the ashing process. An FTIR spectrum should be taken both before and after an ashing process. REFERENCES 1 N. B. Colthup, L. H. Daly, S. E. Wiberley, Introduction to Infrared and Raman Spectroscopy. 3rd Ed. Boston: Academic Press, Inc., 1990. 2 R. Gaechter, and H. Mueller, Plastics Additives Handbook. 2nd Ed. New York: Hanser, 1987.

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