Fourier Transform Infrared Spectroscopic Method for Evolved-Gas

20

Downloaded by UNIV OF MASSACHUSETTS AMHERST on March 23, 2016 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch020

Fourier Transform Infrared Spectroscopic Method for Evolved-Gas Analysis Cheng-Yih Kuo and Theodore Provder The Glidden Company, 16651 Sprague Road, Strongsville, OH 44136

Qualitative and quantitative Fourier transform infrared (FTIR) spectroscopic methodologies were developed for monitoring the gases evolved during chemical cure. The evolved-gas profile can be obtained as a function of temperature (nonisothermal dynamic scan) or time (isothermal). The major gas evolved can be identified readily by comparison of the experimental spectra with known spectra in a vapor-phase library. The gases coevolved also can be identified after spectral subtraction. Computer programs written for thermal analysis were adapted to obtain nth-order reaction kinetics parameters from the evolved-gas profile. Application of evolved-gas analysis (EGA)-FTIR to the thermal deblocking and chemical cure of model coatings systems is shown. Results are compared with those obtained from other techniques, such as thermal gravimetric analysis and thin-film FTIR studies.

CJUREREACTIONShave been studied by various techniques, for example, thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). By combining information generated from D S C and D M A methods with the information generated from thin-film Fourier transform infrared (FTIR) spectroscopic studies, progress has been made in the basic understanding of cure reactions in terms of the buildup of physical properties and the changes of chemical functionalities. This approach was successfully applied to the study of the deblocking and subsequent cure of blocked isocyanate-containing coatings (1-3) by using

0065-2393/90/0227-0343$06.00/0 © 1990 American Chemical Society

Craver and Provder; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1990.

344

POLYMER

CHARACTERIZATION


F T I R spectroscopy to follow the changes in the reactive functionalities in a thin film during cure and correlating this information with that generated from thermal analysis ( D S C or D M A ) . However, important facts yet to be determined are the types and quantities of volatile gases given off during the cure reaction. T G A has been used to answer the question of quantity by following the weight changes as a function of temperature or time, but identification of the evolved gases is not clear, especially when two or more volatile gases evolve simultaneously. Coupling F T I R spectroscopy to T G A will enable the in-line identification of the volatile gases evolved from the T G A and provide information on which and what quantity of the volatile gases are given off during cure. This type of information will be extremely useful for the further elucidation of cure mechanisms. T G A - F T I R spectroscopy also can be used to provide valuable information on the identities of thermal degradation byproducts. Although E G A - F T I R spectroscopic results obtained from experimental coupling of T G A and F T I R spectroscopy were reported in the literature (4-12) as early as 1980, dedicated instruments were not commercially available until 1988 (13, 14). This chapter describes straightforward instrumentation and methodology for F T I R spectroscopic monitoring of the gases evolved during chemical cure and illustrates the methodology with some cure reaction kinetics applications.

Experimental Details Instrumentation. Figure 1 shows a schematic diagram of the experimental instrumentation. The sample, in the form of either a viscous liquid or a coated aluminum rectangular strip, is placed in A- X 2-in. stainless steel tubing that is then connected to an empty column in a Varian 3700 gas chromatography (GC) oven with temperature programming. For a dynamic temperature scan, the heating rate was 10 °C/min. The gases evolved from the sample passed through a gold-coated light pipe and were monitored with a mercury-cadmium telluride (MCT) detector cooled with liquid nitrogen. A Digilab FTS-15E FTIR spectrometer was used. The infrared data were collected with standard G C - F T I R software. The total amount of gas evolved is displayed as a Gram-Schmidt response in real time as a function of time or temperature. The instantaneous absorbance spectrum as a function of time or temperature also is obtained. l

Materials. Blocked - N C O cross-linkers were prepared from trimerized isophorone diisocyanate (IPDI) (Veba-Chemie T-1890), which was used as received to synthesize blocked derivatives. In a typical blocking reaction, 100 g of T-1890 (0.15 mol) was dissolved in 100 g of dry ethyl acetate, and 0.1 g of dibutyltin dilaurate (DBTDL) was added. Methyl ethyl ketoxime (MEKO), 48.9 g (0.187 mol), was added over 2 h, during which time the temperature rose to 40 °C. Aliquots were removed, and the amount of unreacted - N C O functionality was determined by infrared absorbance. When the reaction of the - N C O was complete, the mixture was cooled and precipitated into diethyl ether. The product was isolated, dissolved in toluene, and reprecipitated three times. The final product was isolated and dried.



PURGE GAS* a

-

MCT DETECTOR

TO EXHAUST

LIGHT PIPE

IR BEAM

spectroscopic analysis.

TO CRT

Figure 1. Instrumentation for EGA-FTIR

TIME

WAVENUMBER

1/4* x 2"

= C = 3 =

SAMPLE TUBE

VARIAN 3 7 0 0 GC OVEN

DIGILAB FTS-15E FTIR



346

POLYMER

CHARACTERIZATION

Methodology. Qualitative: Evolved-Gas Profile. The evolved-gas profile is represented by the Gram-Sehmidt response (GSR). The GSR is reconstructed from the interferograms by the Gram-Schmidt orthogonalization method (15), which compares the interferograms of the background and the sample. When a sample is not present in the light pipe, the interferogram shows no difference from that of the background, and a flat base line is the result. When a sample is present in the light pipe, the orthogonalization yields a nonzero difference and deviates from the base line. The magnitude of the deviation is a function of the concentration of the sample in the light pipe. Figure 2 shows the GSR as a function of temperature of a hydroxyl (-OH) functional acrylic resin cured with a melamine cross-linker. If only one gas is evolving during cure, the GSR is the evolved-gas profile of that gas. If multiple gases are evolving simultaneously, then the GSR is the resultant profile of all gases. The evolved-gas profile of the individual gas can be reconstructed by looking for the characteristic band of interest, either directly or through spectral subtraction.

S X

o

= 9M1 t^tot

)

(2

and ranges from 0 to 1.0, as is shown in Figure 4. The Arrhenius equation gives the temperature dependence of the rate constant k = A exp (-=!)

American Chemical Society Library

115515th s t , N.W. Craver and Provder; Polymer Characterization Advances in Chemistry; American Chemical Wwhington, D.C. Society: 20036Washington, DC, 1990.

(3)

348

POLYMER

CHARACTERIZATION

'Z

o

CO DC


UJ

> z o o < z o O

Fourier Transform Infrared Spectroscopic Method for Evolved-Gas

Recommend Documents