Photolytic Degradation of Cellulose Triacetate

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Photolytic Degradation of Cellulose Triacetate CATHERINE S. HSIA CHEN, STANLEY JANKOWSKI, and A L L E N BROTHER Celanese Corp. of America, Summit Research Laboratories, Summit, N. J. The degradation of cellulose triacetate induced by ultra violet radiation was investigated in air and in vacuum, using mercury lamps. Volatile products were ascer tained by mass spectrometry as formed during irradiation, and the resulting polymers were characterized by viscosity measurements and functional group analysis. After a small radiation dose, in vacuum, anhydrous cellulose triacetate yielded carbon monoxide and ketene as volatile products. In air, oxygen uptake was the main initial reaction; virtually no volatile material was detected even after a large total dose. Both in vacuum and in air, residual moisture in the polymer influenced degradation. The photolysis of related monomeric compounds (cellobiose octaacetate and glucose pentaacetate) was studied, and the reaction products were characterized. Tjhotolysis of cellulose, cellulose acetate, and cellulose nitrate, and thermal pyrolysis of cellulosic materials, including cellulose triacetate, have been investigated (I, 2, 3, 4, 7, 8, 9, 10, I I ) , in the case of photolysis of cellulose, rather extensively (I, 2, 3, 4, 8,11). In connection with a study of outdoor degradation of cellulose triacetate in the solid state induced by sunlight, we have investigated the photolytic degradation of this material both in vacuum and in the presence of oxygen. Cellulose tri acetate in dilute solutions does not absorb ultraviolet radiation above wavelength 250 τημ (Figure 1, c). However, in a condensed phase, such as in the form of films, absorption above this wavelength becomes appre ciable ( Figure 1, a and b, for films of 5.8- and 1.3-mil thickness, respec tively); therefore, photolytic changes can be induced by the ultraviolet region of the absorbed radiation. In this investigation such effects have been ascertained by using ultraviolet radiation at 253.7 and 313 τημ. 240 Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

241

Cellulose Triacetate

17. C H E N E T A L . 0.9 0.8h 0.7

0.6

l&J £

S g m

0.5

——

FILM (5.8 MIL.)

b ——

FILM ( 1.3 MIL.)

c

DILUTE SOLUTION IN CH CI (0.35gm IN 100MIL.)

α

\

—

2

2

0.4 0.3 0.2

O.i 313

•253.7

200

250

300

350 400 WAVELENGTH, mp

450

500

Figure 1. Absorption spectra of cellulose triacetate

While 253.7 τημ is of higher energy and more strongly absorbed by solid cellulose triacetate, it is below the cutoff of the sun spectrum received on the earth's surface (290 πΐμ). On the other hand, 313 τημ falls in the most damaging region of the sun spectrum; its photolytic effects should have practical importance. The degradation has been followed with emphasis on identifying the volatile products formed during irradiation by qualitative mass spec trometry. The resulting polymers have been characterized by viscosity measurements and functional group analysis. The primary active species have been examined by electron spin resonance spectroscopy. For a more definitive elucidation of the photolytic degradation of cellulose triacetate, comparative studies of photolysis of related monomeric com pounds, cellobiose octaacetate, cellobiose, glucose pentaacetate, and glucose were undertaken. This communication describes the results and offers some discussion and conclusions.

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

242

IRRADIATION OF POLYMERS

Experimental

Materials. C E L L U L O S E T R I A C E T A T E . Celanese cellulose tria purified by dissolution in reagent grade methylene chloride, followed by filtration and reprecipitation into an excess of reagent grade 2-propanol. The polymer was collected on a Biichner funnel, washed with 2-propanol, and dried. Clear films of — 5-mil thickness were cast from methylene chloride solution and used for the photolytic studies. C E L L O B I O S E O C T A A C E T A T E A N D G L U C O S E P E N T A A C E T A T E . Eas White Label materials were purified by recrystallization from a methylene chloride-methanol mixture.

CELLOBIOSEANDGLUCOSE.

These were Eastman White L Photolysis Reactors and Ultraviolet Sources. For 253.7-m/*, irradia tion, a modified irradiation apparatus purchased from Delmar Co. was used. The reactor was a 2-necked, 500-ml., round-bottomed flask. One neck was an O-ring joint, and the other was a 24/40 joint. A 4- X 1-inch coiled low pressure mercury quartz lamp was placed inside the flask through the O-ring neck, and the joint was sealed with removable O-rings. The reactor was connected directly to the mass spectrometer by the 24/40 joint. The samples were placed inside the flask and irradiated internally. The O-ring was shielded from direct radiation so as not to induce degradation. The estimated output of the lamp was 30 watts, and the ambient temperature within the reactor during irradiation was 70°C. For irradiation at 313 π\μ, the reactor consisted of a 6- X 2-inch o.d. cylindrical borosilicate glass tube fitted directly to the mass spectrometer by a 24/40 joint. A Hanovia analytical lamp, having an estimated output of 325 watts, was mounted externally alongside the tube. The radiation received by the samples inside the tube was filtered through borosilicate glass and was essentially 313 τημ. The ambient temperature within the reactor during irradiation was 25 °C. Some comparative experiments were carried out at 70 °C. with additional heating. There was no de tectable temperature effect between 25° and 70°C, and the results re ported here are for 25 °C. Irradiation and Mass Analysis. The reactor was connected to a Bendix Time-of-Flight mass spectrometer using vacuum-tight Swagelok fittings. A variable leak allowed continuous monitoring of all volatile products generated. Mass spectra were recorded with a Honeywell ultraviolet oscillographic recorder, model 906B. Three high impedance electrometers were used to record the mass range from mass 1 to mass 250 in approximately 1 minute. Qualitative identification was based on molecular ions, ion fragmentation patterns, and empirical formula studies. The differentiation of C O (mass 28) and N (mass 28) was initially established by ionization potential measurement and the molecular ion and a high pressure negative ion analysis for CO. Only CO, under the conditions employed, could produce the negative ion observed. Samples were irradiated in the following manner: Cellulose triace tate, 2- X 2-inch film, was placed under the ultraviolet source for irradia tion. The monomeric compounds were dissolved in suitable solvents and slurried along the reactor walls. Residual solvent was removed under 2


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vacuum. The complete removal of solvent, in all cases, was established by mass spectrometric analysis at a vacuum of 10~ torr prior to irradiation. When free of background, the source was activated, and mass recordings were started after the first 30 seconds. All samples were irradiated for a total of 24 hours. During the first 5 minutes, recordings were made at 1-minute intervals. Then the oscilloscopic readout was monitored, with periodic mass recordings being made during the remainder of the 24 hours. It took 20 minutes to detect the volatiles generated by irradiation in air at 760 torr. Hence, it was impossible to determine the volatiles during the initial stages of irradiation. Therefore, pure oxygen at 150 torr was used in place of air. By this system, mass detection of the volatiles could be made within 30 seconds to 2 minutes. An oxygen atmosphere was therefore used in place of air. Because of the rapid changes in sample pressure and other problems such as differential pumping of the sample, quantitative measurements were extremely difficult, and the results were obtained on a qualitative basis. However, the order of formation of the volatiles could be detected with certainty. 5

Infrared Examination of Samples after Irradiation. Surface effects on the irradiated side of the cellulose triacetate films were examined employing a Perkin Elmer 521 using the ATR technique (5, 6), which allowed a comparison of both sides of an irradiated film. Monomeric compounds were examined in a KBr disk employing a Perkin-Elmer 21. In many cases remaining acetate groups were removed by treating with NaOH. The regenerated materials were examined to assess other groups, whose infrared absorptions occurred at the same wavelengths as the strong ester carbonyl absorptions. Solution Viscosity and Acetyl Value. For cellulose triacetate, the values of [hty /C] C — 0.1% in a 90 to 10 volume mixture of methylene chloride-methanol at 25°C. were measured and denoted as inherent viscosity (I. V.) (deciliters per gram). Acetyl values were determined by base hydrolysis of the acetyl groups of a weighed sample, followed by back-titration to determine the number of milliequivalents of acetic acid. r

Acetyl value ==

No. of meq. of

C H 3 C O O H X 0.06005 X 100 wt. of sample

Results

Mass Spectrometric Analysis of Volatiles during Photolysis. Samples investigated were cellulose triacetate, cellobiose octaacetate, glucose pentaacetate, cellobiose, and glucose.

I N T H E P R E S E N C E O F O X Y G E N . At 253.7 τημ ( 7 0 ° C ) : In a no volatile products were observed other than a small amount of C 0 . 2


244

IRRADIATION OF POLYMERS

A significant change was the decrease of the mass 16 and mass 32 ions, indicating a decrease of oxygen in the system. At 313 τημ ( 2 5 ° C ) : In all cases, no measurable volatile material was produced and no decrease in the oxygen content of the system could be detected. I N V A C U U M . At 313 τημ ( 2 5 ° C ) : None of the samples produced any measurable volatile material. At 253.7 τημ ( 7 0 ° C ) : Cellulose Triacetate. Irradiation of cellulose triacetate film generated six volatile products which were identified as C H = C = 0 (ketene), CO, H , C 0 , H 0 , and C H C O O H . Ketene was the first component observed while C O was the second. After several minutes, the ketene concentration decreased, and C O became the major product generated during the remaining irradiation period. Total sample pressure appeared to reach a maximum after 10 minutes and dropped off gradually during the remaining 24 hours. Cellobiose Octaacetate and Glucose Pentaacetate. In both cases the major component observed at 2 to 5 minutes was C H = C = 0 , with lesser amounts of CO, C 0 , C H C O O H , and H 0 . H was not observed. Cellobiose and Glucose. The major product generated at 3 minutes was H 0 , with lesser amounts of CO, C 0 , and H . Both C H = C = 0 and CHaCOOH were totally absent. 2

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Infrared Examination of the Residues. C E L L U L O S E T R I A C E T A ATR infrared spectra of the films irradiated at 313 τημ both in vacuum and in the presence of oxygen showed no change from those before irradiation. ATR infrared spectra of the films after irradiation at 253.7 γημ, both in vacuum and in the presence of oxygen, showed additional absorptions at 5.8 and 7.85 microns, which were attributable to carboxylic acids. In some cases absorptions at 6, 10.4, 11.5, and 12.4 microns were observed, indicating the presence of unsaturation. Figure 2 compares ATR spectra of the irradiated and unirradiated sides of a cellulose triacetate film after 24-hour radiation at 253.7 χημ in vacuum. The ATR spectrum of a control cellulose triacetate film, which is identical with that of the unirradiated side in Figure 2, is given in Figure 3. Figure 4 shows the change of infrared absorptions of a cellulose film cast on a NaCl plate upon irradiation at 253.7 πΐμ in vaccum. The spectra were recorded at 90°C. An increase in OH (3 microns) and a decrease in carbonyl ( 5.7 microns ) absorption were noted.

CELLOBIOSEOCTAACETATEANDGLUCOSEPENTAACET

on both compounds were similar. Samples irradiated at 253.7 and 313 m/A, both in the presence of oxygen and in vacuum, were saponified. Infrared analysis of the saponified residues showed that all samples had carbonyl absorption at 5.76 to 5.78 microns, owing to lactones, ketones, UUCRONS)

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IRRADIATION

Figure 3.

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3000 2500

2000 1800

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OF

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ATR infrared spectrum

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Change of infrared absorptions of a cellulose triacetatefilm(cast curve: original spectrum; solid curve:


( M I C R O N S ) 7

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Photolytic Degradation of Cellulose Triacetate

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