Lebo, Stuart E., Jr.

Lonsky, Werner F. W.

McDonough, Thomas Joseph

Medvecz, Patrick J.

Scholarly Materials And Research @ Georgia Tech

THE INSTITUTE OF PAPER CHEMISTRY, APPLETON, WISCONSINIPC TECHNICAL PAPER SERIESNUMBER 281THE OCCURRENCE AND LIGHT INDUCED FORMATION OF ORTHO QUINONOIDLIGNIN STRUCTURES IN WHITE SPRUCE REFINER MECHANICAL PULPS. E. LEBO, JR., W. F. W. LONSKY, T. J. McDONOUGH, AND P. J. MEDVECZMARCH, 1988The Occurrence and Light Induced Formation of Ortho-QuinonoidLignin Structures in White Spruce Refiner Mechanical PulpS. E. Lebo, Jr., W. F. W. Lonsky, T. J. McDonough, and P. J. MedveczPortions of this work were used by SEL as partial fulfillment of therequirements for the Ph.D. degree at The Institute of Paper Chemistry.This manuscript will be presented at the International PulpBleaching Conference in Orlando, Florida on June 5-9, 1988Copyright, 1988, by The Institute of Paper ChemistryFor Members OnlyNOTICE & DISCLAIMERThe Institute of Paper Chemistry (IPC) has provided a high standard of professional service and has exerted its best effortswithin the time and funds available for this project. The information and conclusions are advisory and are intended only forthe internal use by any company who may receive this report. Each company must decide for itself the best approach to solv-ing any problems it may have and how, or whether, this reported information should be considered in its approach.IPC does not recommend particular products, procedures, materials, or services. These are included only in the interest ofcompleteness within a laboratory context and budgetary constraint. Actual products, procedures, materials, and services usedmay differ and are peculiar to the operations of each company.In no event shall IPC or its employees and agents have any obligation or liability for damages, including, but not limited to,consequential damages, arising out of or in connection with any company's use of, or inability to use, the reported informa-tion. IPC provides no warranty or guaranty of results.THE OCCURRENCE AND LIGHT INDUCED FORMATION OFortho-QUINONOID LIGNIN STRUCTURES IN WHITE SPRUCEREFINER MECHANICAL PULPS. E. Lebo, Jr.,* W. F. W. Lonsky,**T. J. McDonough, and P. J. MedveczThe Institute of Paper ChemistryAppleton, WI 54912ABSTRACTA sensitive method, based on reaction with tri-methyl phosphite, was developed for the determina-tion of ortho-quinonoid structures in pulp. It wasused to show that structures of this type areresponsible for most of the color of white sprucemechanical pulp and for most of the increase incolor associated with light induced yellowing. Theuntreated pulp contained 5-6 quinone units per 100lignin C9 units, which were responsible for about75Z of its color. Upon irradiation with simulatedsunlight for 20 hr, the number increased to about10 and the increase accounted for about 75% of theincrease in color. The quantum efficiency ofquinone formation was 0.08 moles per mole of pho-tons absorbed.INTRODUCTIONIncreasing wood costs and predicted shortages infuture wood supplies have continued to stimulateinterest in the production and use of high yieldpulps. However, a major problem associated withthe use of such pulps, both bleached and unbleach-ed, is that they "yellow" upon exposure to sun-light. This light induced yellowing (or reversion)increases the absorbance of these pulps in thevisible region of the spectrum, and hence, de-creases pulp brightness. This, in turn, limitstheir use as substitutes for fully bleached chemi-cal pulps.The photochemical and chemical reactionsinvolved, while of economic significance, are notfully understood. Results obtained with model com-pounds (1-6), isolated lignins (7,8), and ground-wood pulps (9-10), have led to theories suggestingthat ortho-quinonoid lignin structures are formedduring the photoyellowing process (Fig. 1). It iswell known that such structures are highly colored,and their formation would account for at least apart of the color developed when mechanical pulpsare exposed to light. In addition, it has beensuggested that such structures may account for thenatural color of wood and wood pulps (9-10).The extent of the contribution of these struc-tures to the color of wood pulps, however, has notbeen quantitatively evaluated. In this work, amethod was developed to study both the occurrenceand the light induced formation of ortho-quinonoidlignin structures in situ. The method was thenused to estimate their contribution to the color of*Now with Reed Lignin, Inc., 100 Highway 51 South,Rothschild, WI 54474.**Now with Kimberly-Clark, B400-4, Roswell, GA30076.unirradiated and sunlight irradiated white sprucemechanical pulp. It was also used to estimate thequantum efficiency of 312 nm UV light inducedortho-quinone formation.Method of ortho-quinone analysisSince ortho-quinonoid structures in lignin arelikely to be strongly colored, the occurrence ofonly a few of them could account for the color ofyellowed high yield pulps. To detect such smallamounts in the chemically complex pulp fibers, wesought a method that was both sensitive and speci-fic for these structures.The method we developed to fill this need isbased on the known reaction between ortho-quinonesand trivalent phosphorus compounds (11-13). Forexample, we reacted trimethyl phosphite with3-methoxy-l, 2-ortho-benzoquinone to form, in quan-titative yield, the colorless oxyphosphoraneadduct, I, as shown in Fig. 2 (13). Upon additionof water, I was quantitatively converted to thecorresponding cyclic phosphate triester, II (alsocolorless). These reactions are significantbecause the ortho-quinone becomes "tagged" withphosphorus, an element foreign to the fiber wall,and one for which sensitive microanalytical tech-niques are available.A further advantage of the phosphorus tag isthat it allows one to check the assumption that thereagent is specific for ortho-quinonoid structures.This is possible because the phosphorus-31 nucleusis paramagnetic, and its structural environmentcan, therefore, be characterized by nuclear magneticresonance (NMR). Thus, the 3 1 p NMR spectrum of theoxyphosphorane adduct, I, displayed a single signalat a chemical shift (relative to 85% H3 P0 4 ) of-45.8 ppm, and the corresponding value for thecyclic phosphate triester, II, was 11.1 ppm. Thesechemical shifts are characteristic (14); a secondreaction with 3,5-di-tert-butyl-l,2-ortho-benzo-quinone, for example, gave products with corres-ponding values of -46.5 and 12.1 ppm.To further test this method, we oxidized asample of white spruce mechanical pulp with Fremy'ssalt (potassium nitrosodisulfonate), which is knownto introduce ortho-quinonoid structures into lignin(15,16). When the oxidized pulp was reacted withtrimethyl phosphite, it was rapidly bleached, andthe resulting pulp produced a single, strong 31pNMR signal at 13.2 ppm, characteristic of a cyclicphosphate triester structure.Occurrence of ortho-Quinonoid Lignin StructuresWe investigated the occurrence of ortho-quinonoidlignin structures by reacting a white spruce mecha-nical pulp (WSMP) with trimethyl phosphite. Asshown in Table 1, the Kubelka-Munk light absorptioncoefficients of the pulp decreased significantlyupon exposure to this reagent. Its 31P NMRspectrum had a strong signal at 12.4 ppm (71.5% ofthe total signal). A second, weaker signal wasobserved at 4.2 ppm (28.5% of the total signal).On the basis of the results described above, thesignal at 12.4 ppm was attributed to cyclicphosphate triester formation. The origin of thesecond signal is not known. It appears to be1 1Lig0 0Fig. 1 Proposed mechanism for the light induced formation of or tho-quinonoid lignin structures (10).lignin related, however, since a similarly reactedholocellulose sample produced no clear signals.Fig. 2 Reaction of trimethyl phosphite with 3-methoxy-l,2-ortho-benzoquinone (13).Phosphorus microanalysis of three separatesamples yielded a value of 0.39 ± 0.056 gram ofphosphorus per 100 grams of dry pulp. From the NMRdata, the amount of phosphorus associated withnaturally occurring ortho-quinonoid structures wascalculated to be 0.27%, or 71.5% of the totalphosphorus present in the pulp. From this and theKlason lignin content (26.9%) the number of molesof ortho-quinonoid structures per gram of ligninwas calculated to be 3.3 x 10 - 4 . Assuming a unitweight for spruce lignin of 185 g lignin/mole of C9units (17), we then estimated that the pulp con-tained 5-6 ortho-quinonoid lignin structures per100 C9 lignin units.Table 1. Kubelka-Munk absorption coefficients ofWSMP before and after reaction with tri-methyl phosphite.Kubelka-Munk AbsorptionCoefficient (cm2 /g)Sample 415 nm 457 nm 520 nmUnreacted WSMP 141 48.9 20.2Tr imethyl phosphitereacted WSMP 74.8 13.2 5.01The light absorption measurements of Table 1indicate that these ortho-quinonoid structurescontribute significantly to the "natural" color ofthe pulp. From these measurements, we estimatethat their contribution to the absorbance was about50% at 415 nm, and about 75% at 457 and 520 nm.The formula used in these estimations wask - kbcontribution to absorbance = kwhere k is the absorption coefficient of the pulpbefore reaction and kb is the absorption coef-ficient of the pulp after reaction (i.e., bleachingwith trimethyl phosphite. Detailed UV-visiblespectroscopic analysis (18) indicated that themajority of the reduction in pulp color resultedfrom the reaction of trimethyl phosphite with ortho-quinonoid structures and not from the reaction oftrimethyl phosphite with the structure giving riseto the signal at 4.7 ppm. This was particularlytrue for the two longer wavelengths examined.Sunlight Induced Formation of ortho-quinonoidLignin StructuresUpon exposure to simulated sunlight for 20 hoursthe WSMP underwent severe yellowing (reversion).The results presented in Table 2 show that theassociated increases in visible light absorptionwere partially reversed when the exposed pulp wasreacted with trimethyl phosphite. The 3 1 p NMRspectrum of the exposed and reacted pulp had astrong signal at 12.6 ppm (95% of the total signal)and a weak signal at 2.1 ppm (5% of the totalsignal). As in the two earlier pulps, we attri-buted the signal at 12.6 ppm to the formation ofcyclic phosphate triesters. The origin of thesecond signal is not known; it may have originatedfrom the same structure found in the unirradiatedand reacted pulp.Table 2. Kubelka-Munk absorption coefficients ofsunlight irradiated WSMP before and afterreaction with trimethyl phosphite.Kubelka-Munk AbsorptionCoefficient (cm2 /g)Sample 415 nm 457 nm 520 nmIrradiated WSMP 442 186 68.4Irradiated andreacted WSMP 170 42.3 15.0these additional structures to the increase inabsorbance of this pulp to be 70-80% when measuredat wavelengths of 415, 457, and 520 nm. The for-mula used in these estimations wascontribution to reversion =whereIk,k - absorption coefficient of unrevertedunreacted pulpfr = absorption coefficient of irradiated(reverted) pulpLb = absorption coefficient of reacted(bleached) pulprb = absorption coefficient of irradiatedreacted pulp.andandThus, ortho-quinonoid structures are responsiblefor most of the color formed when white sprucemechanical pulp is exposed to simulated sunlight.The increased formation of cyclic phosphatetriester structures after irradiation shows thatthe formation of ortho-quinonoid structures is animportant component of the photoyellowing mechanism.To confirm that the increased phosphorus incorpora-tion was a direct result of irradiation, we used x-ray microanalysis (scanning electron microscopywith energy dispersive spectrometry, SEM-EDS) tosee how phosphorus incorporation varied withdistance from the irradiated surface of the sheet.The results (Fig. 3) showed clearly that the sur-face layers, where most of the radiation isabsorbed, contained most of the cyclic phosphatetriester structures. This shows, in turn, thatortho-quinonoid structures are formed in an amountthat increases with the amount of UV radiationabsorbed.Phosphorus microanalysis of the above pulpyielded a value of 0.47 ± 0.025 gram of phosphorusper 100 grams of dry pulp. From the NMR data, theamount of phosphorus associated with the ortho-quinonoid lignin structures present in the pulp wascalculated to be 0.45%, or 95% of the total phos-phorus present in the pulp. Calculations similarto those described above yielded a value for thispulp of 10-11 ortho-quinonoid lignin structures per100 C9 lignin units. Thus, we estimate that uponexposure to 20 hours of simulated sunlight, 4-6additional ortho-quinonoid structures were formedper 100 Cg lignin units. From the absorption datain Tables I and 2, we estimated the contribution ofFig. 3 Z-direction phosphorus distribution in a160 g/m 2 sheet irradiated for 20 hours withsimulated sunlight and reacted with tri-methyl phosphite. 1 layer = 40 pm.Quantum EfficiencyThe quantum efficiency of ortho-quinone formation,defined here as the number of moles of ortho-quinonoid lignin structures formed per mole of pho-tons absorbed, is a quantity of both theoreticaland practical interest. If exposure conditions areknown, quantum efficiency values at various wave-lengths allow one to predict the extent ofyellowing. Such values would also be useful in thedesign of optical brighteners for high yield pulps.In an attempt to determine this value, weirradiated sheet samples with known amounts ofnearly monochromatic UV light. We then determinedthe number of moles of ortho-quinonoid ligninstructures generated in each sample by reactionwith trimethyl phosphite. It has been shown thatthe UV component of sunlight (i.e., light in the290-400 nm wavelength range) induces yellowing, andthe wavelength range 310-315 nm is most effective3(18). Accordingly, we used light within this rangein these experiments. The optical system used alsoallowed incident light flux to be measured andcontrolled (18).From the results obtained (Table 3), the quan-tum efficiency of ortho-quinone formation at thiswavelength was calculated to be 0.08 mole of ortho-quinonoid lignin structures per mole of photonsabsorbed.Table 3. Effect of irradiation with 312 nm lighton the amount of ortho-quinonoid struc-tures in a sample of white sprucemechanical pulp.Number of Photons Moles of QuinoneAbsorbed x 10-18 Present x 1060 1.57.27 2.110.7 3.1CONCLUSIONSMost of the color of white spruce mechanical pulpcan be attributed to ortho-quinonoid lignin struc-tures. When the pulp is exposed to sunlight, addi-tional ortho-quinonoid lignin structures areformed. At an incident wavelength of 312 nm, 0.08mole of ortho-quinonoid lignin structures areformed per mole of photons absorbed. They areresponsible for most of the increase in color asso-ciated with the light-induced yellowing of thepulp.EXPERIMENTAL PROCEDURESThe white spruce mechanical pulp used was preparedin a 12-inch Sprout-Waldron laboratory refiner fromfresh chips. It was EDTA treated for 24 hours,ethanol/benzene extracted, and washed with dis-tilled water. Handsheets were then formed accord-ing to the procedures described in TAPPI testmethod T 218 om-83 (basis weight = 160 g/m 2 ) or T205 Om-81 (basis weight = 30 g/m 2 ).Kubelka-Munk absorption coefficients weredetermined using the method of "black backing"described by Polcin and Rapson (19). Measurementsof Ra, and Ro were made on a Perkin-Elmer Model 320spectrophotometer equipped with a Hitachi integrat-ing sphere accessory. Pressed BaSO4 pellets wereused as a reference.Irradiations were conducted with an ORIEL 1000watt solar simulator (distance from source = 50 cm,total irradiance 85 mw/cm2 , roughly equivalent tonatural sunlight) or with light from optical equip-ment designed to provide nearly monochromatic light(18). Both units were located in humidity (50%)and temperature (21°C) controlled labs. "Thick"handsheet samples (basis weight = 160 g/m 2) wereused in these experiments.The reaction of trimethyl phosphite with irra-diated or unirradiated pulp was performed inanhydrous dichloromethane. Sheet samples wereWiley milled to 20 mesh and dried under vacuum overP205 for 48 hours. While under dry nitrogen, 0.25-0.30 g of each sample was added to 10 mL of a 10Xsolution of trimethyl phosphite in anhydrous di-chloromethane. The resulting samples were sealedand allowed to react for one week at 20°C. Whilestill under nitrogen, the samples were washedseparately with two 500 mL portions of anhydrousdichloromethane and one 1.0 L portion of anhydrousether. The washed samples were then dried undervacuum and stored under dry nitrogen.Solid state 3 1p NMR analyses were performed bySpectral Data Services, Inc. (818 Pioneer,Champaign, IL 61820). An inert environment wasmaintained during sample preparation and analysis.Microanalyses were conducted by Micro-Tech Labs,Inc. (4117 Oakton St., Skokie, IL 60076). SEM-EDSanalyses were conducted by the EM lab at TheInstitute of Paper Chemistry (P.O. Box 1039,Appleton, WI 54912).ACKNOWLEDGMENTSThe assistance of Dr. G. Turner in performing 31PNMR analyses and E. Foxgrover in performing SEM-EDSanalysis is gratefully acknowledged. We also thankDr. E. W. Malcolm and Dr. D. R. Dimmel for helpfuldiscussions.Portions of this work were used by S. E. Lebo,Jr. as partial fulfillment of the requirements forthe Ph.D. degree at The Institute of Paper Chemistry.Other portions were used by P. J. Medvecz, Jr. aspartial fulfillment of the requirements for theM.S. degree at The Institute of Paper Chemistry.LITERATURE CITED1. Hosoya, S., Hatakeyama, H., and Nakano, J. J.Japan Wood Res. Soc. 16: 140(1970).2. Hosoya, S., Seike, K., and Nakano, J. J. JapanWood Res. Soc. 22: 314(1970).3. Pew, J. C. and Conners, W. J., Tappi 54(2):245(1971).4. Gierer, J. and Lin, S., Svensk Papperstid. 75(7): 233(1972).5. Imsgard, F., Falkehag, S. I., and Kringstad,K. P., Tappi 54(10): 1680(1971).6. Gellerstedt, G. and Pettersson, E., SvenskPapperstid. 80: 15(1977).7. Falkehag, S. I., Marton, J., and Adler, E.,Adv. Chem. Ser. 59: 75(1966).8. Hemra, L. and Lundquist, K., Acta Chem. Scand.27: 365(1973).9. Leary, G. L., Nature 217: 672(1963).10. Lonsky, W. F. W., The Institute of PaperChemistry, unpublished work.11. Ramirez, F., Acc. Chem. Res. 1: 168(1968).12. Ramirez, F., Smith, C. P., Pilot, J. F., and44Gulati, A. S., J. Org. Chem. 33(10): 3787(1968).13. Medvecz, P. J. Independent Study, TheInstitute of Paper Chemistry, Appleton, WI,1987.14. Crutchfield, M. M., Dungan, C. H., Letcher, J.H., Mark, V., and Van Wazer, J. R. P31Nuclear Magnetic Resonance. New York, NY,Interscience Publishers, 1968:227-448.15. Adler, E. and Lundquist, K., Acta Chem. Scand.15: 223(1961).16. Lebo, S. E., Jr., to be published.17. Bjorkman, A. and Person, B., Svensk Papperstid.60(5): 158(1957).18. Lebo, S. E., Jr. The occurrence and lightinduced formation of ortho-quinonoidstructures in high yield pulp lignins.Doctoral Dissertation, Appleton, WI, TheInstitute of Paper Chemistry, 1987.19. Polcin, J. and Rapson, H., Tappi 52(10): 1965(1969).5 5

The occurrence and light induced formation of ortho-quinonoid lignin structures in white spruce refiner mechanical pulp

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The occurrence and light induced formation of ortho-quinonoid lignin structures in white spruce refiner mechanical pulp

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