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Infrared Spectroscopic Analysis of Textile Materials Degradation Using Photoacoustic Detection Charles Q. Yang Department of Textiles, Merchandising and Interiors, The University of Georgia, Athens, Georgia 30602
Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) was used for the studies of oxidation and degradation in various textile fabrics. All the infrared spectroscopic data demonstrate that FT-IR/PAS is able to differentiate the near surface of a textile sample from its bulk. Therefore, it can be used to determine the distribution of degradation products between the near surfaces of fabrics and their bulk. FT-IR/PAS appears to be a reliable qualitative analytical technique for textile samples and does not suffer band distortion found in diffuse reflectance infrared spectra of some textile samples.
Introduction Infrared spectroscopy has a wide range of applications to the analysis of textile samples (O'Connor, 1968; Berni and Morris, 1983). The sampling techniques commonly used include potassium bromide or potassium chloride pellets (OConnor et al., 1957;Higgins, 1957; McCd et al., 1967),mineral oil mulls of finely ground samples (Hurtubise, 1959), and attenuated total reflectance (ATR) (McCall et al., 1966). Photoacoustic detection was extended to the mid-infrared region in the early 1980s. In a photoacoustic spectroscopy (PAS)experiment, acoustic signals generated by the absorbed infrared radiation are detected and Fourier transformed to yield a single beam spectrum. On the basis of the photoacoustic signal generation sequence, FT-IR photoacoustic spectroscopy (FT-IR/PAS) inherently demonstrates a number of advantages over the conventional transmission and reflectance techniques. Samples are not altered during the analysis. Opaque samples can be examined without experiencing difficulties with the detection of transmitted IR radiation. Rough, brittle, or other intractable solid samples can be analyzed without sampling difficulties, whereas ATR, the common alternative sampling technique, requires a flat or deformable surface. FT-IR/PAS can also be used to investigate the surface layers of solid samples (Yang et al., 1987,1990; Yang, 1991). In this research, FT-IR/PAS was evaluated as a nearsurface analytical technique for the studies of oxidation and degradation in a variety of textile fabrics. Representative data are presented and discussed here. Experimental Section Instrumentation. A Nicolet 2ODXB FT-IR spectrometer with an MTEC Model 100 photoacoustic cell and a Spectra-Tech Collector diffuse reflectance accessory was used for most of the lT-IR measurements. Resolution for all the spectra presented was 8 cm-'. The average number of scans was 250. A 6-mm sample cup was used for PAS data collection. Carbon black was used as a reference material, and helium was used to purge the photoacoustic cell prior to collecting data. The mirror velocity used was 0.139 cm/s for PAS, 0.278 cm/s for diffuse reflectance, and 0.556 cm/s for transmission experiments. No smoothing function or base-line correction was used. The data of the silk were collected with an IBM IR-98 spectrometer. The optical velocity used was 0.235 cm/s. Materials. (1)A melt blown polypropylene nonwoven fabric was placed beneath an ozone-free 254-nm-wavelength ultraviolet light at room temperature. The lamp used was a Sylvania GTE germicidal lamp. The radiation intensity at a distance of 2.5 cm from the bare tube is approximately 18 pW/cm2.
(2) The cotton fabric used for thermal degradation and the outdoor weathering test was a desized, bleached 100% cotton print cloth (Testfabrics Style 400). The fabric was mounted at 45O to horizontal facing south for 100 h in July for the outdoor weathering test. (3) The fire retardant cotton fabrics were provided by the US. Department of Agriculture Southern Regional Research Center. The cotton fabric was first treated with an aqueous solution consisting of 26.4 9% tetrakis(hydroxymethy1)phosphoniumchloride (THPC), 8.3% urea, 3.0% Na2HP04,and 3.7% NaOH. The treated fabric was then dried at 85 "C, cured at 160 OC for 2 min, and washed to remove the unreacted reagents. A portion of the washed fabric was also passed through a bath containing 14.3% H202to oxidize the polymeric finish and finally washed in hot tap water. The fabric exposed to outdoor weather conditions was mounted at 45" to the horizontal facing south for 42 days (June-July, New Orleans, LA). The fabric was also put in an Atlas weatherometer for 100 h. The weatherometer was operated at 68 "C with light from a carbon arc. Experimental details were reported elsewhere (Soignet et al., 1978; Soignet and Benerito, 1979). (4) The silk fabric was an undyed, woven (160 X 160), degummed fabric containing no-twist silk filament yarns (Testfabrics Style 604). The silk fabric was immersed in a 0.25 M sulfuric acid solution at 20 "C for 8 h. (5) Fabrics were ground in a Wiley Mill to pass a 40 mesh screen to form powders.
Results and Discussion A melt brown nonwoven polypropylene fabric was exposed to ultraviolet (UV) radiation at 254 nm for 120 h. Both the front surface facing the UV radiation source and the back surface of the polypropylene fabric were analyzed by FT-IR/PAS (Figure lA,B). The fabric was also ground into a powder using a Wiley Mill. The photoacoustic infrared spectrum of the powder sample is presented in Figure 1C. For comparison, the spectrum of the nondegraded polypropylene fabric is shown in Figure 1D. A broad carbonyl band is shown in the spectrum of the front surface of the degraded fabric (Figure 1A). Further infrared spectroscopic studies of the polypropylene fabric indicate that the band at 1720 cm-l is contributed by carbonyls of a carboxyl, ketone, and aldehyde, while the shoulder at 1775 cm-' is due to an anhydride carbonyl (Yang and Martin, 1991). The broad band around 3430 cm-l in Figure 1A is probably associated with the stretching of hydroxyl groups of alcohol and peroxide formed in the fabric (Yang and Martin, 1991). It c8n be seen that the intensities of all the bands of the oxidation products are reduced in the spectrum of the powder sample (Figure IC) and reduced even further in the spectrum of the back surface of the fabric (Figure 1B).
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Figure 2. Photoacoustic infrared spectra of the nonwoven polypropylene fabric heated at 120 O C for 36 h: (A) fabric; (B)powder.
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Figure 1. Photoacoustic infrared spectra of the nonwoven polypropylene fabric exposed to ultraviolet radiation for 120 h (A) front Photoacoustic infrared spectrum side; (B)back side; (C) powder. (D) of the unexposed nonwoven polypropylene fabric.
In a PAS experiment, the infrared radiation absorbed by a sample is first converted to heat. Only the heat generated within one thermal diffusion length from a sample’s surface is able to transfer to the sample’ssurface, cause pressure variation, and generate photoacoustic signals (bencwaig, 1980). Therefore the thermal diffusion length is the effective sampling depth of FT-IR/PAS when the thickness of a sample is greater than the thermal diffusion length. Because the thermal diffusion length for polymeric materials is in the range of a few micrometers, the photoacoustic infrared spectrum of a fabric sample represents a few micrometers of the fabric’s near surface. When the fabric is ground to a powder, the near surface and the bulk are thoroughly mixed. Since the thickness of the fabric is over 200 pm, the amount of substances within a few micrometer’s near surface of the fabric is very small compared with the substances in the bulk. Consequently, the photoacoustic infrared spectrum of the powder represents mainly the bulk of the fabric. The infrared spectroscopic data demonstrate that the highest degree of oxidation occurred in the front near surface of the fabric facing the UV radiation source, whereas the lowest degree of oxidation occurred in the back
near surface of the fabric. The infrared spectra (Figure 1) indicate that there is a gradient in the degree of oxidation across the bulk of the fabric from the front surface to the back surface. The change in the degree of oxidation in the fabric is due to change in the intensity of the UV radiation in the fabric. While the UV radiation was penetrating through the fabric, ita intensity decreased due to absorption and scattering. The nonwoven polypropylene fabric was extracted by methylene chloride for 2 h to remove the antioxidant from the fabric. The extracted fabric was heated in an oven at 120 “C for 36 h. The photoacoustic spectra of the thermally oxidized polypropylene fabric and ita powder sample are demonstrated in Figure 2. Bands due to hydroxyls of an alcohol and peroxide (3424 cm-l), carbonyls of a carboxyl, ketone, and aldehyde (1715 cm-l), and carbonyls of an anhydride (1770 cm-’) are shown in the spectrum of the fabric sample (Figure 2A). The intensities of the carbonyl and hydroxyl bands in the spectrum of the powder sample (Figure 2B) appear to be similar to those in the spectrum of the fabric sample (Figure 2A), indicating that the bulk of the polypropylene fabric has the same degree of oxidation as the near surface of the fabric. Evidently, thermal oxidation of polypropylene fabric is homogenous between the near surfaces of the fabric and the bulk. When a cotton fabric was treated with an aqueous solution containing tetrakis(hydroxymethy1)phosphonium chloride (THPC), urea, and NaOH, THPC was first converted to tri(hydroxymethy1)phosphine (THP) with the release of formaldehyde. Cotton cellulose was then cross-linked by THP with the formation of amide groups (Soignet et al., 1978; Soignet and Benerito, 1979). When the finished cotton is oxidized with hydrogen peroxide, the phosphine groups in the cross-linkages were converted to phosphine oxide (Soignet and Benerito, 1979). The finish (THPC) imparted flame retardant properties to cotton fabrics. The photoacoustic infrared spectrum of the cotton fabric treated with THPC/urea is shown in Figure 3A. The two bands at 1658and 1560 cm-I in Figure 3A can be assigned to amide I and amide I1 absorption due to the
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Figure 3. Photoacoustic infrared spectra: (A) the cotton fabric treated with THPC/urea; (B) the cotton fabric treated with THPC/urea and hydrogen peroxide.
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Figure 5. Photaacoustic infrared spectra of the cotton fabric treated with THPC/urea and hydrogen peroxide and then weathered outdoors: (A) front side; (B) back side; (C) powder.
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Figure 4. Photoacoustic infrared spectra of the cotton fabric treated with THPC/urea and hydrogen peroxide, and then weathered in a weatherometer: (A) front side; (B)back side; (C) powder.
secondary amide groups in the cross-linkages between cotton cellulose molecules. In the photoacoustic infrared
spectrum of the cotton fabric finished and then oxidized with hydrogen peroxide (Figure 3B), the two amide bands shift to 1664 and 1558 cm-', respectively. The cotton fabric treated with THPC/urea and hydrogen peroxide was exposed in a weatherometer for 100 h. Both the front surface facing the radiation source in the weatherometer and the back surface of the fabric were analyzed using Fl'-IR/PAS (Figure 4A,B). The fabric was ground in a Wiley Mill and the powder was also examined by FT-IR/PAS (Figure 4C). It is seen in Figure 4A that the weathering causes an increase in the intensity of the amide I band (1663 cm-') in the spectrum of the front surface of the fabric. In the spectra of the back surface of the fabric and the powder (parts B and C of Figure 5, respectively), however, the amide I band intensity is not increased. Obviously, the weathering effects in the weatherometer were concentrated on the front near surface of the fabric which faced the radiation source in the weatherometer. The degree of degradation in the bulk and the back side of the fabric was relatively low. The photoacoustic infrared spectra of the fabric treated with THPC/urea and hydrogen peroxide and exposed to outdoor weathering are presented in Figure 5. An increase in the intensity of the amide I band (1664 cm-l) is obsewed in both the spectrum of the front surface of the fabric (Figure 5A) and the spectrum of the back surface of the fabric (Figure 5B). Contrary to the weathering in the weatherometer, outdoor weathering affected both sides of the fabric. There is less degradation in the bulk of the fabric as shown by a weaker amide I band in the spectrum of the powder (Figure 5C) than in the spectra of the fabric
620 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992
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CM-' 1 Figure 6. Photoacoustic infrared spectra of (A) the cotton fabric weathered outdoors for 100 h and (B) the unexposed cotton fabric; (C) the difference spectrum, A-B. WAVENUMBERS
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(Figure 5A,B). This is another example demonstrating the capability of FT-IR/PAS to differentiate near surfaces of textile fabrics from their bulk. When the degree of degradation in the near surface of a textile fabric is so low that no obvious difference can be observed between the spectrum of the exposed fabric and the spectrum of the unexposed fabric, spectral subtraction
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Figure 8. Infrared spectra of the nonwoven polyethylene fabric: (A) photoacoustic (in intensity, arbitrary unit); (B) diffuse reflectance (in Kubelka-Munk unit); (C) transmission (in absorbance unit, potassium bromide pellet).
can be used to extract information from the infrared spectroscopic data. By comparison of the spectra of the exposed and unexposed fabrics, the common spectral features due to the bulk of the fabric can be canceled out. Therefore,the remaining bands mbe interpreted in terms of the near-surface species due to the degradation of the fabric. A cotton fabric was weathered outdoors for 100 h. The photoacoustic infrared spectrum of the weathered fabric and that of the unexposed fabric appeared to be identical. However, when the spectrum of the unexposed cotton fabric (Figure 6B) is subtracted from the spectrum of the weathered cotton fabric (Figure 6A), carbonyl bands due to various oxidation products of cotton cellulose are observed in the difference spectrum (Figure 6C). The bands at 1722 and 1704 cm-l in the difference spectrum (Figure 6C) are probably due to the carbonyls of aldehyde/ketone and carboxylic acid, respectively,formed as the oxidation products of the primary and secondary alcohols in the cellulose molecules. Shown in Figure 7A,B are the photoacoustic infrared spectra of the silk fabric treated with a 0.25 M sulfuric acid for 8 h at 20 OC and the untreated silk fabric, respectively. Even though the two spectra appear to be similar, a band at 1735 cm-' shown in the difference spectrum (Figure 7C) indicates that carbonyls were formed as the oxidation products of the silk by sulfuric acid. Presented in Figure 8 are the photoacoustic, diffuse reflectance, and transmission infrared spectra of a melt blown nonwoven polyethylene fabric. Evidently, spectral distortion occurs in the diffuse reflectance spectrum
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(Figure 8B) in the hydrocarbon stretch frequency region (2800-3000 cm-') due to specular reflection. The photoacoustic infrared spectrum (Figure 8A), however, does not have artifacts and band distortion. Photoacoustic signals are generated by the infrared radiation absorbed by a sample. Therefore, a photoacoustic infrared spectrum resembles the transmission spectrum of the same sample and does not suffer band frequency shift and spectral distortion caused by band frequency shift. FT-IR/PAS proves to be a reliable qualitative analytical technique for solid samples.
Conclusions The photoacoustic infrared spectroscopic data indicate the following. (1)FT-IR/PAS is able to differentiate the near surfaces of textile samples from the bulk. Therefore, it can be used to determine the distribution of degradation products between the near surfaces of fabric samples and their bulk. (2) Combination of FT-IR/PAS and a spectral subtraction technique enhances the capability of FT-IR/PAS as a near surface analytical technique. (3) Photoacoustic infrared spectra resemble transmission spectra and do not suffer band distortion found in diffuse reflectance infrared spectra of some textile samples. Acknowledgment
I am grateful to the Spectroscopy Society of Pittsburgh for providing a grant for this research project. Special thanks are extended to Linette K. Martin, Department of Chemistry, Marshall University, for collecting some of the data. Registry No. THPC, 124-64-1;H2NCONH2,57-13-6;polyethylene (homopolymer), 9002-88-4;polypropylene (homopolymer), 9003-07-0.
Literature Cited Berni, R. J.; Morris, N. M. Infrared Spectroscopy. In Analytical Methods for a Textile Laboratory; Weaver, J. W., Ed.; American
Association of Textile Chemists and Colorists: Research Triangle Park, NC, 1983;Chapter 11. Higgins, H. G. The Use of Potassium Chloride Disk in the Infrared Examination of Fibrous Cellulose and Other Solid Materials. Aust. J. Chem. 1957,10,496-501. Hurtubise, F. Application of Infrared Spectroscopy to Cellulose Chemistry. Can. Tert. J. 1959,76,53-60. McCall, E. R.; Miles, S. H.; O'Connor, R. T. Frustrated Multiple Reflectance Spectroscopy of Chemically Modified Cotton. Am. Dyest. Rep. 1966,55,400-404. McCall, E. R.; Miles, S. H.; O'Connor, R. T. An Analytical Method for the Identification of Nitrogenous Crosslinking Reagents on Cotton Cellulose. Am. Dyest. Rep. 1967,56,35-39. OConnor, R. T. Absorption Spectroscopy. In Amlytical Methods for a Textile Laboratory; Weaver, J. W., Ed.; American Association of Textile Chemists and Colorists: Research Triangle Park, NC, 1968; Chapter 10. O'Connor, R. T.; Dupre, E. F.; McCall, E. R. Infrared Spectrophotometric Procedure for Analysis of Cellulose and Modified Cellulose. Anal. Chem. 1957,29,998-1005. Rosencwaig,A. General Theory of the Photoacoustic Effect in Condensed Media: the Gas-Microphone Signal. Photoacoustics and Photoacoustic Spectroscopy; Wiley: New York, 1980; pp 94-98. Soignet, D. M.; Benerito, R. R. Surface and Bulk Properties of Flame Retardant Cotton Fabric. Part 11. Effect of Exposure to Atmospheric Conditions. J. Fire Retard. Chem. 1979,6,225-238. Soignet, D. M.; Benerito, R. R.; Berni, R. J. Surface and Bulk Properties of Flame Retardant Cotton Fabrics. Part I. Effect of the Flame Retardant Treatment. J. Fire Retard. Chem. 1978,4, 161-173. Yang, C. Q. Comparison of Photoacoustic and Diffuse Reflectance Infrared Spectroscopy as Near-Surface Analysis Technique. Appl. Spectrosc. 1991,45,102-108. Yang, C. Q.;Martin, K. L. Thermal and Photo Oxidation of Polypropylene Fibers Studied by FT-IR Photoacoustic Spectroscopy. In Book of Papers, 1991 AATCC International Conference;American Association of Textile Chemists and Colorists: Research Triangle Park, NC, 1991;p 261-268. Yang, C. Q.; Bresee, R. R.; Fateley, W. G. Near-Surface Analysis and Depth Profiling by FT-IR Photoacoustic Spectroscopy. Appl. Spectrosc. 1987,41,889-896. Yang, C. 8.; Bresee, R. R.; Fateley, W. G. Studies of Chemically Modified Poly(ethy1eneterephthalate) Fibers by FT-IRPhotoacoustic Spectroscopy and X-ray Photoelectron Spectroscopy. Appl. Spectrosc. 1990,44,1035-1039. Received for review May 29, 1991 Accepted November 4,1991
Solubility of Synthesis and Product Gases in a Fischer-Tropsch SASOL
Wax Jeffrey S. Chou and Kwang-Chu Chao* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283
Solubility of hydrogen, carbon monoxide, methane, carbon dioxide, ethane, and ethylene is reported at five pressures from 10 to 50 atm and at temperatures from 200 to 300 "C in a Fischer-Tropsch SASOL wax which is primarily a mixture of n-paraffins. The experimental solubility is compared with Huang et al.'s correlation.
Introduction Solubility of the synthesis gases (hydrogen and carbon monoxide) and of the product gases (carbon dioxide and light hydrocarbons) in Fischer-Tropsch reactor wax is required for understanding the reaction kinetics of Fischel-Tropsch synthesis reactions and for reactor design. Previous studies on gas solubility in high paraffins and heavy waxes are of limited scope. Peter and Weinert (1955) and Deimling et al. (1984) measured gas solubility in paraffin wax at high tempera-
tures. Albal et al. (1984) reported solubility of synthesis gases in a Gulf wax at 348-523 K. Masumoto and Satterfield (1985) studied the solubility of synthesis gases in n-odacosane,phenanthrene, and fomblin YR. Gasem and Robinson (1985) measured solubility of carbon dioxide in high paraffins. Tsai et al. (1987,1988) and Huang et al. (1988a,b) reported solubility of methane, ethane, and carbon dioxide in high paraffii and in a Mobil wax. Chou and Chao (1989) measured solubility of ethylene in high paraffins. Huang et al. (1988~)determined solubility of
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