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:

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

An Analect Diamond 20 FTIR with ATR attachment equipped with a KRS-5 45

o

crystal 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

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Coloring Technology for Plastics

Figure 1. FTIR microscope spectra of kaolin vs. polyethylene vinyl acetate.

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.

Fourier Transform Infrared Spectroscopy

193

Figure 2. Standard curve for % kaolin vs. absorbance ratio

(slope=0.0514, intercept=0.0118, R

2

=0.995.

Table 1. Summary of results of

ashing prepared samples

Calculated %

kaolin in hot melt

Average % ash of

samples at 500

o

C

0

0.03

1

0.80

5

4.19

10

8.47

20

17.68

30

26.61

40

35.59

50

44.74

Figure 3. ATR spectra for 1, 10, 30, and 50% 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 128

o

C 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 500

o

C 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 CH

2

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.

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Coloring Technology for Plastics

Figure 4. FTIR microscope spectrum of ashed (500

o

C) kaolin in PVAc.

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 500

o

C 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.

A Primer on Colorful Additives

195