Thermal Degradation of Cotton Cellulose Studied by Fourier

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29 Thermal Degradation of Cotton Cellulose Studied by Fourier Transform Infrared-Photoacoustic Spectroscopy Charles Q . Yang and James M . Freeman 1

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Department of Textiles, Merchandising and Interiors, The University of Georgia, Athens, GA 30602 Department of Chemistry, Marshall University, Huntington, WV 25755 1

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Cotton textiles are exposed to many types of heat damage during manufacture and consumer use. Therefore, it is important to understand the nature of thermal oxidation and degradation in cotton textiles. In this research, cotton fabrics heated at three different temperatures (180, 210, and 240 °C) were studied using Fourier transform infrared-photoacoustic spectroscopy. Ketone, aldehyde, carboxylic acid, ester, anhydride, and unsaturated hydrocarbon structures were identified at different stages of thermal oxidation. Anhydride was first formed in the near surface of the cotton fabric during this process. At 180 °C, the oxidation products (ketone, aldehyde, carboxylic acid, and ester) were homogeneously distributed between the near surface of the fabric and the bulk. At 240 °C, however, more cotton cellulose was oxidized in the near surface than in the bulk. The acceleration of thermal oxidation of cotton cellulose by increasing temperature was also observed.

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ANALYSIS O F T H E R M A L OXIDATION O F C O T T O N C E L L U L O S E u n d e r dif ferent conditions has l o n g b e e n o f interest to chemists ( 1 - 4 ) . C o t t o n textiles are exposed to many types o f heat damage d u r i n g manufacture a n d consumer use, such as singeing, excess heat setting i n finishing, overdrying, a n d i r o n i n g (5). Therefore, it is important to understand the nature o f t h e r m a l oxidation

0065-2393/93/0236-0693$06.00/0 © 1993 American Chemical Society

In Structure-Property Relations in Polymers; Urban, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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and degradation i n cotton fabrics. K n o w l e d g e about t h e r m a l degradation o f cotton cellulose is also useful f o r the development o f fire-retardant cotton fabrics ( 6 ) a n d the preservation o f historical artifacts ( 7 ) . In the past, i n f r a r e d spectroscopy has b e e n used to study the degradation o f cotton cellulose at elevated temperature (7-12). I n this research, F o u r i e r transform i n f r a r e d - p h o t o a c o u s t i c spectroscopy ( F T I R - P A S ) was used to study the t h e r m a l oxidation a n d degradation o f cotton fabrics. Since the early 1980s w h e n photoacoustic detection was extended to the m i d - i n f r a r e d region, (13). I n the past, F T I R - P A S has b e e n successfully used i n o u r research as a near-surface analytical technique f o r a variety o f chemically m o d i f i e d textile fabrics, yarns, a n d fibers (14-9). A textile sample is first g r o u n d into a p o w d e r using a W i l e y m i l l . B o t h the textile sample a n d the p o w d e r sample are analyzed b y F T I R - P A S . C o m p a r i son o f the photoacoustic infrared spectrum o f the textile sample a n d the spectrum o f the corresponding p o w d e r sample enables evaluation o f the distribution o f c h e m i c a l species between the near surface a n d the b u l k o f the textile sample. I n this research, the distribution o f oxidation products b e tween the near surface o f a cotton fabric a n d its b u l k was d e t e r m i n e d b y this method.

Experimental Details Instrumentation. A F o u r i e r transform infrared spectrometer ( N i c o let 20 D X B ) w i t h a photoacoustic c e l l ( M T E C M o d e l 100) was used to collect all the spectra. Resolution f o r a l l the spectra was 8 c m , a n d the average n u m b e r o f scans was 500. C a r b o n black was used as a reference material a n d h e l i u m was used as the p u r g i n g a n d c o n d u c t i n g gas i n the photoacoustic cell. T h e m i r r o r velocity was 0.139 c m / s ; n o base-line correction or smoothing function was used. - 1

Materials. T h e cotton fabric was a desized, bleached cotton p r i n t cloth (Testfabrics style 400). C o t t o n fabrics were heated i n a radiant gravity oven. T h e temperature variation was w i t h i n ± 3 °C. P o w d e r e d samples were obtained b y grinding the fabric samples i n a W i l e y m i l l to pass a 40 m e s h screen. Results and Discussion T h e photoacoustic infrared spectra o f the cotton fabric heated at 180 °C f o r different times are shown i n Figures 1 and 2. T h e development o f a carbonyl b a n d at 1730 c m is observed i n the initial stages o f thermal oxidation ( F i g u r e 1). T h e intensity o f the carbonyl b a n d at 1730 c m increased as the -

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Figure 1. Photoacoustic infrared spectra of the cotton fabric heated at 180 °C for different times (hours): A, 0; B, 6; C, 15; D, 48; E, 72.


696



1735

Figure 2. Photoacoustic infrared spectra of the cotton fabric heated at 180 °C for different times (hours): A , 123; B, 225; C, 295; Ό, 415; E, 515.


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exposure t i m e was increased. W h e n the cotton fabric was heated f r o m 123 to 295 h , the 1 7 3 0 - c m b a n d intensity c o n t i n u e d to increase w h i l e the intensi ties o f the bands a r o u n d 2900 c m " (aliphatic C — H stretching) a n d the bands i n the 1 4 3 0 - 1 3 1 0 - c m region (aliphatic C — II bending) were re d u c e d (Figures 2 A - C ) . T h e bands i n the 1 4 3 0 - 1 1 0 0 - c m " region (aliphatic C — H b e n d i n g a n d C —Ο stretching) b r o a d e n e d a n d started to overlap i n F i g u r e 2 C . T h e infrared spectroscopic data indicated that b o t h the p r i m a r y a n d secondary alcohols w e r e oxidized to f o r m carbonyls at this stage. A b a n d at 1615 c m also developed together w i t h the increase i n intensity o f the 1730-cm b a n d d u r i n g this p e r i o d ( F i g u r e 2). T h e b a n d at 1635 c m associated w i t h b e n d i n g o f cellulosic hydroxyls was overlapped b y the newly developed b a n d at 1615 c m (Figures 2 A a n d B ) . W h e n the exposure t i m e reached 415 h , the bands a r o u n d 2900 c m were no longer discernible, ( F i g u r e 2 D ) . A l s o i n F i g u r e 2 D the bands i n the 1 4 3 0 - 1 1 0 0 - c m region are observed to overlap to f o r m one b r o a d b a n d . A l s o , a decrease i n the intensity of the b r o a d b a n d at 3360 c m ( O - H stretching) i n F i g u r e 2 D indicates that dehydration o c c u r r e d w i t h i n the cellulose molecules. T h r e e n e w bands at 1846, 1774, a n d 905 c m were also observed i n Figures 2 D a n d E. - 1

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T o determine the nature of the c h e m i c a l changes caused b y c o n t i n u e d heating, the cotton fabric heated at 180 °C for 48 h was treated w i t h a 0 . 1 - M aqueous solution o f N a O H for 5 m i n . T h e photoacoustic i n f r a r e d spectrum o f the cotton fabric thus treated showed a decrease i n the 1 7 3 0 - c m band intensity a n d the formation o f the 1 6 1 5 - c m b a n d , w h i c h is due to carboxyl ate carbonyl ( F i g u r e 3 B ) . Treatment o f the fabric w i t h N a O H converted earboxylie acid to carboxylate. A decrease i n the 1 7 3 0 - c m b a n d intensity i n F i g u r e 3 B i n d i c a t e d that the 1 7 3 0 - c m b a n d i n F i g u r e 3 A was partially contributed b y earboxylie a c i d carbonyl. T h e same p h e n o m e n o n was also observed for the cotton fabric heated at 180 °C for 72 h (Figures 3 D and E ) . T h e fabric heated at 180 °C for 48 h was also treated w i t h s o d i u m borohydride i n ethanol at reflux for 1 H. T h e i n f r a r e d spectrum o f the fabric thus treated ( F i g u r e 3 C ) shows a further slight decrease i n the 1 7 3 5 - c m band intensity due to the reduction of carbonyls o f ketone a n d aldehyde. A weak b a n d at 1730 c m i n F i g u r e 3 C , w h i c h r e m a i n e d after the treatment o f the fabric b y s o d i u m b o r o h y d r i d e , was likely due to ester carbonyl. - 1

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T h r e e carbonyl bands at 1846, 1775, and 1735 c m " were shown i n the infrared spectrum o f the cotton fabric heated at 240 °C for 20 h ( F i g u r e 4 A ) . W h e n the fabric was treated w i t h distilled water at 25 °C for 15 m i n , the two carbonyl bands at 1846 and 1775 c m disappeared completely ( F i g u r e 4 B ) . A n increase i n the 1 7 3 5 - c m b a n d intensity a n d broadening i n the hydroxyl stretching b a n d at 3 3 6 0 - c m w e r e also observed i n F i g u r e 4 B . It can be c o n c l u d e d that the two bands at 1846 a n d 1775 c m are due to an anhydride. Treatment o f the fabric w i t h water converted the anhydride to the corresponding earboxylie acid. A s a result, the two anhydride carbonyls at 1

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Figure 3. Photoacoustic infrared spectra of the cotton fabric heated at 180 °C for different times: A , 48 h, untreated; B, 48 h, treated with 0.1-M NaOH; C, 48 h, treated with NaBH ; D, 72 h, untreated; E , 72 h, treated with 0.1-M NaOH. 4


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Figure 4. Photoacoustic infrared spectra of the cotton fahnc heated at 240 °C for 20 h: A , untreated; B, treated with water; C, treated with 0.1-M NaOH; D,


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1846 a n d 1775 c m disappeared, w h i l e the carboxyl carbonyl b a n d at 1735 cm increased its intensity. T h e broadening of the 3 3 6 0 - c m " b a n d is also due to the formation o f carboxyl groups. T h e b a n d at 905 c m in Figure 4A is probably associated w i t h the C — O — C b e n d i n g o f the anhydride. This b a n d disappeared w h e n the fabric was treated w i t h water ( F i g u r e 4 B ) . - 1

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T h e frequencies o f the two carbonyl bands o f some unstrained anhy drides a n d cyclic anhydrides reported i n the literature are listed as follows


(20):

A c e t i c anhydride C a p r i o c anhydride

1824, 1825,

1748 1760

Succinic anhydride M a l e i c anhydride Phthalic anhydride

1865, 1848, 1845,

1780 1790 1775

Naphthalene-1, 2dicarboxylic anhydride

1848-1845, 1783-1779

T h e influence o f r i n g strain induces a shift o f the carbonyl bands to higher frequencies, whereas α , β conjugation results i n a l o w e r i n g o f the two carbonyl frequencies (20). T h e anhydride f o r m e d i n the cotton, w h i c h has carbonyl stretching frequencies at 1846 a n d 1775 c m " , appears to be an unsaturated (possibly aromatic) five-member cyclic anhydride. T h e b a n d at 905 c m due to the C — O — C b e n d i n g i n F i g u r e 4 A also indicates that the anhydride f o r m e d i n the cotton was probably a cyclic anhydride (21). W h e n the cotton heated at 240 °C for 20 h was treated w i t h a 0 . 1 - M solution o f N a O H i n ethanol for 1 m i n at r o o m temperature, the intensity o f the 1 7 3 5 - c m " carbonyl b a n d was r e d u c e d whereas the 1 6 1 5 - c m " carbonyl b a n d intensity was increased i n the infrared spectrum ( F i g u r e 4 C ) . T h e anhydride carbonyl bands at 1846 a n d 1775 c m " also disappeared i n F i g u r e 4 C . Treatment o f the fabric w i t h N a O H at r o o m temperature converted b o t h the anhydride a n d the carboxyl to carboxylates. 1

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T h e fabric heated at 240 °C for 20 h was treated i n a 0 . 1 - M solution o f N a O H i n methanol at reflux for 30 m i n . T h e carbonyl bands 1846, 1775, a n d 1735 c m disappeared i n the spectrum o f the fabric thus treated ( F i g u r e 4 D ) . T w o strong bands at 1586 a n d 1385 c m " associated w i t h the asymmet ric a n d symmetric stretching o f carboxylate carbonyl were shown i n the same spectrum ( F i g u r e 4 D ) . It can be c o n c l u d e d that the b a n d at 1735 c m " i n F i g u r e 4 C , w h i c h was not changed b y the treatment of the fabric w i t h N a O H at r o o m temperature, was due to ester carbonyl. W h e n the fabric was treated i n a N a O H solution at reflux, all three types o f carbonyls (e.g., anhydride, stage of t h e r m a l oxidation, the aldehyde a n d ketone identified at an earlier stage o f degradation w e r e further oxidized to earboxylie acid, ester, a n d anhydride. - 1

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T h e cotton fabric heated at 240 °C for 20 h was also treated i n a 0 . 1 - M solution o f b r o m i n e i n carbon tetrachloride for 30 m i n at r o o m temperature.


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T h e intensity o f the 1615-em b a n d was r e d u c e d a n d the 3 1 0 0 - c m " b a n d disappeared i n t h e spectrum o f the fabric thus treated ( F i g u r e 5). T h i s observation demonstrated that the 1 6 1 5 - c m " b a n d has contributions f r o m the aliphatic C = C structures. A d d i t i o n o f b r o m i n e to t h e C = C structures resulted i n the reduction i n the 1 6 1 5 - e m " b a n d intensity a n d disappearance o f the 3 1 0 0 - c m " b a n d (aliphatic unsaturated C — H stretching). T h e b a n d at 1615 c m " a n d the shoulder at 1578 c m " i n the spectrum o f the b r o m i n e treated cotton fabric ( F i g u r e 5) were possibly due to aromatic ring structures ( 2 2 ) . T h e unsaturated aliphatic structures were probably f o r m e d b y the dehydration i n t h e cellulosic molecules d u r i n g the t h e r m a l degradation process, because a reduction i n the Ο — H stretching b a n d at 3360 c m " was observed i n the spectra o f the cotton fabric heated at 180 °C (Figures 2 D a n d E ) . T h e oxidation products o f cotton cellulose a n d the corresponding stretch i n g b a n d frequencies are s u m m a r i z e d as follows: 1

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1730-1735

A l d e h y d e a n d ketone carbonyl

1730-1735

Carboxylic acid carbonyl E s t e r carbonyl

1735 1846, 1775

C y c l i c anhydride carbonyl A l i p h a t i c a n d aromatic C = C

1615 3100

Unsaturated C — H (aliphatic)

W 1615 CO

4000

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Figure 5. Photoacoustic infrared spectra of cotton fabric heated at 240 °C for 20 h and treated with bromine in CCl . 4


702


T h e acceleration o f the t h e r m a l degradation o f cotton cellulose b y increasing temperature was demonstrated i n F i g u r e 6. T h r e e cotton fabrics were heated for 6 h at different temperatures. W h e n the fabric was heated at 180 °C, little oxidation resulted as shown b y a very weak carbonyl b a n d at 1730 c m " i n F i g u r e 6 A . T h e carbonyl b a n d at 1730 c m " was still weak i n 1


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the spectrum o f the fabric heated at 210 °C ( F i g u r e 6 B ) . W h e n the fabric was heated at 240 °C, however, carbonyl bands o f anhydride, ester, earboxylie acid, a n d bands d u e to C = C stretching a n d unsaturated C — Η stretching w e r e presented i n the i n f r a r e d spectrum ( F i g u r e 6 C ) . T h e distribution o f the oxidation products between the near surface o f the cotton fabric a n d its interior was also investigated using F T I R - P A S . T h e cotton fabric heated at 180 °C f o r different times was g r o u n d into powders. T h e carbonyl b a n d intensities i n the i n f r a r e d photoacoustic spectra o f the powders ( F i g u r e 7) appeared to be the same as those o f the fabric samples ( F i g u r e 1). L i t t l e difference was seen between the spectra o f the fabric samples a n d the spectra o f the corresponding p o w d e r samples f o r the cotton fabric heated at 180 °C u p to 225 h ( F i g u r e 8). I n a P A S experiment, m o d u l a t e d I R radiation absorbed b y a sample is first converted to heat. W h e n the heat propagates to the sample surface, a n d subsequently into the gas w i t h i n a photoacoustic cell, it causes pressure variation a n d generates an acoustic signal. W h e n a sample's thickness is larger than the t h e r m a l diffusion length, only the heat generated w i t h i n the first t h e r m a l diffusion length f r o m the sample's surface c a n propagate to the surface a n d generate photoacoustic signals ( 2 3 ) . Because the t h e r m a l d i f f u sion length o f cotton i n the m i d - i n f r a r e d region is i n the range o f a f e w micrometers f o r the optical velocity u s e d i n this research (0.278 c m / s ) ( 1 4 ) , micrometers o f the near surface o f the fabric. W h e n the fabric was g r o u n d into a p o w d e r , the near surface a n d b u l k w e r e m i x e d . Because the diameter o f the cotton y a r n i n the fabric is approximately 300 μ ι η , the photoacoustic i n f r a r e d spectrum o f the p o w d e r sample represents m a i n l y the b u l k . T h e similarity between the spectra o f the fabric samples a n d the spectra o f the p o w d e r samples indicated that the oxidation o f cotton cellulose was homoge neous between the near surface a n d the b u l k . Differences are seen b e t w e e n the spectrum o f the cotton fabric heated at 180 °C f o r 415 h a n d the spectrum o f the corresponding p o w d e r (Figures 9 A a n d B ) . T h e two anhydride carbonyl bands at 1846 a n d 1775 c m shown i n the spectrum o f the fabric sample ( F i g u r e 9 A ) were too weak to b e recogniz able i n the spectrum o f the corresponding p o w d e r sample ( F i g u r e 9 B ) . T h e b a n d at 905 c m associated w i t h the C — O — C b e n d i n g o f the anhydrides appeared to b e m u c h weaker i n the spectrum o f the p o w d e r ( F i g u r e 9 B ) than i n the spectrum o f the fabric ( F i g u r e 9 A ) . It is obvious that the anhydride was first f o r m e d i n the near surface. T h e same p h e n o m e n o n was also seen i n the fabrics heated at 210 °C a n d 240 °C, respectively. -

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T h e photoacoustic i n f r a r e d spectra o f the cotton fabric heated at 240 °C for 2.5 h a n d the corresponding p o w d e r are presented i n F i g u r e 10. T h e carbonyl b a n d at 1735 c m " i n the spectrum o f the fabric ( F i g u r e 10A) appears to b e more intense than the same b a n d i n the spectrum o f the p o w d e r ( F i g u r e 10B). E v i d e n t l y , the cotton cellulose i n the near surface o f the fabric h a d a higher degree o f oxidation than the cotton cellulose i n the 1




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Figure 7. Photoacoustic infrared spectra of the powders of the cotton fabric heated at 180 °C for different times (hours): A, 0; B, 6; C, 15; D, 48; E, 72.



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WAVENUMBERS (cm") Figure 8. Photoacoustic infrared spectra of the cotton fabric heated at 180 °C for 225 h: A, fabric; B, powder.

b u l k o f the fabric. It was n o t e d that w h e n the cotton fabric was heated at 180 °C f o r 225 h , the spectrum o f the fabric sample ( F i g u r e 8 A ) appeared to be similar to the spectrum o f the p o w d e r sample ( F i g u r e 8 B ) . T h e i n f r a r e d spectroscopic data o f the cotton fabric heated at 240 °C ( F i g u r e 10) i n d i c a t e d that more oxidation products (ketone, aldehyde, earboxylie acid, a n d ester) w e r e f o r m e d i n the near surface o f the fabric than i n the b u l k . T h e inhomogeneous oxidation o f cotton cellulose between the near surface a n d the b u l k at a higher temperature is p r o b a b l y d u e to t h e acceleration o f


706



1735

4000

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WAVENUMBERS

1000

400

fcm" ) 1

Figure 9. Photoacoustic infrared spectra of the cotton fabric heated at 180 °C for 415 h: A, fabric; B, powder.

oxidation at the higher temperature. W h e n the fabric was oxidized at 180 °C, the surface o f the fabric into its bulk. A s a result, the degree o f oxidation i n the near surface a n d i n the b u l k was similar, as demonstrated i n Figures 7 a n d 8. W h e n the fabric was o x i d i z e d at 2 4 0 °C, however, the oxidation o f cotton cellulose was drastically accelerated. T h e amount o f oxygen diffusing f r o m the surface o f the fabric into the b u l k was not enough to supply the r a p i d oxidation o f the cotton cellulose i n the b u l k . O n l y the cotton cellulose i n t h e near surface was oxidized w i t h abundant oxygen. Consequently, t h e



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100 0 400 1

WAVENUMBERS (cm" ) Figure 10. Photoacoustic infrared spectra of the cotton fabric heated at 240 °C for 2.5 h: A, fabric; B, powder. degree o f oxidation o f cotton cellulose i n t h e b u l k o f the fabric was lower than that i n the near surface.

Summary 1. K e t o n e , aldehyde, earboxylie acid, ester, anhydride, a n d unsatu rated c a r b o n - c a r b o n structures were identified as the oxidation products d u r i n g different stages o f the t h e r m a l degradation o f cotton cellulose at various temperatures.


708


2 . T h e formation o f anhydride o c c u r r e d first i n the near surfaces of the cotton fabrics d u r i n g the thermal degradation process at all three temperatures studied ( 1 8 0 , 2 1 0 , a n d 2 4 0 ° C ) .


3 . T h e oxidation products (ketone, aldehyde, carboxylic acid, a n d ester) were homogeneously distributed between the near sur face a n d the b u l k o f the fabric w h e n the fabric was oxidized at 1 8 0 °C. W h e n the fabric was oxidized at 2 4 0 ° C , however, more oxidation o c c u r r e d i n the near surface o f the fabric than i n the b u l k throughout the degradation process.

J.

References 1. Doree, C. The Methods of Cellulose Chemistry, 2nd ed.; Chapman and Hall: London, England, 1950; p 216. 2. Waller, R. C.; Bass, K. C.; Roseveare, W. E. Ind. Eng. Chem. 1948,40, 138. 3. Garten, V. Α.; Weiss, D. E. Rev. Pure Appl. Chem. 1957,7, 69. 4. Hebeish, Α.; El-aref, A. T.; El-alfi, Ε. Α.; El-rafie, M. H. J. Appl. Polm. Sci. 1979,23, 453-462. 5. Merkel, R. S. Analytical Methods for a Textile Laboratory, 3rd ed.; Weaver, W., Ed.; American Association of Textile Chemists and Colorists: Research Triangle Park, NC, 1985; p 41. 6. Shafizadeh, F. Adv. Carbohydr. Chem. 1968,23, 419-444. 7. Cardamore, J. M.; Gould, J. M.; Gordon, S. H. Textile Res. J. 1987,57, 235-239. 8. Higgins, H. G. J. Polym. Sci. 1958,28, 645-648. 9. Hofman, W.; Ostrowski, T.; Urbanski, T.; Witanowski, M. Chem. Ind. (London) 1960,23, 95-97. 10. Holmes, F. H.; Shaw, C. J. G. J. Appl. Chem. 1961,11, 210-216. 11. O'Connor, R. T. In Analytical Methods for a Textile Laboratory, 2nd ed.; Weaver, J. W., E d . ; American Association of Textile Chemists and Colorists: Research Triangle Park, NC, 1968; p 295. 12. Miller, B.; Gorrie, T. M. J. Polym. Sci. 1971,36, 3-19. 13. Coufal, H.; McClelland, J. F. J. Mol. Struct. 1988,173, 129-140. 14. Yang, C. Q.; Bresee, R. R.; Fateley, W. G. Appl. Spectrosc. 1987,41, 889-896. 15. Yang, C. Q.; Bresee, R. R. J. Coated Fabrics 1987,17, 110-128. 16. Yang, C. Q.; Perenich, Τ. Α.; Fateley, W. G. Textile Res. J. 1989, 59, 562-568. 17. Yang, C. Q.; Bresee, R. R.; Fateley, W. G. Appl. Spectrosc. 1990,44, 1035-1039. 18. Yang, C. Q. Appl. Spectrosc. 1991,45, 102-108. 19. Yang, C. Q. Ind. Eng. Chem. Res. 1992,31, 617-621. 20. Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London, England, 1975; p 144. 21. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991; p 121. 22. Painter, R.; Snyder, M.; Starsinic, M.; Coleman, M.; Kuehn, D.; Davis, A. Appl. Spectrosc. 1981,35, 475. 23. Rosencweig, A. Photoacoustics and Photoacoustic Spectboscopy; Wiley: New York, 1980; pp 93-98. RECEIVED for review M a y 1 4 , 1991. ACCEPTED revised manuscript M a y 30, 1992.


Thermal Degradation of Cotton Cellulose Studied by Fourier

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