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A study of the photobleaching and photoyellowing of paper containing lignin using fluorescence spectroscopy
J. Photochem. Photobiol. A: Chem., 58 (1991)
349-359
349
A study of the photobleaching and photoyellowing of paper containing lignin using fluorescence spectroscopy R. Stephen Davidson+
and
Linda A. Dunn
of ChemistT, City Universi& Northampton Square, London ECIV OHB (UK)
Department
Aiain Castellan
and Aziz Nourmamode
UniversitP de Bordeaux, Laboratoire de Photochimie Organique, CNRS URA 348, F33405 Talence (France)
(Received
July 30, 1990; in revised form October
29, 1990)
Abstract The fluorescence of paper produced using high yield pulp increases on bIeaching with hydrogen peroxide. The new blue fluorescent species help to offset the naturaI yelIow colour of the paper. Irradiation of the paper in the presence of reducing agents leads to further bleaching and a further increase in fluorescence, probably as a result of the photoreduction of groups such as quinones.
1. Introduction Paper containing lignin has a light brown appearance. For many end uses white paper is required and therefore paper containing lignin is chemically bleached, hydrogen peroxide being the most widely used reagent [l]. The visual effect so attained is easily lost by exposure of the bleached paper to light (A<400 nm) which produces a yellow discoloration [2]. The origin of the coloured materials in the paper containing lignin is the lignin, and it is this material that responds to the peroxide bleach giving products which can readily undergo photodegradation. The photoinstability has been attributed to a number of sources: photochemical CY-and P-cleavage reactions of carbonyl compounds [3] which are an integral part of the lignin structure; the presence of quinones [4] which are coloured and undergo a plethora of ill-defined photochemical reactions; sensitizers which produce singlet oxygen on excitation [5]; polyphenolic compounds which can be readily photo-oxidized and photodehydrodimerized [63. Another area of chemistry which is not unrelated to that of paper containing lignin is the photoaging of proteins which leads to photoyellowing [7]. Topics which have been studied in depth include the aging of lens proteins [8] and wool [9], the latter having a complex morphological structure which contains approximately 80% protein [lo]. Whilst much of the degradation of such proteinaceous materials can be attributed to the photoreactions of tryptophan [ll] and its degradation products, it is quite clear that other aromatic amino acids such as phenylalanine and tyrosine play a very important role [123. It has been shown that, in the badly weathered regions of wool fibres, there is an increase in concentration of dityrosine and related compounds ‘Author
to whom correspondence
lOlO-6030/91/$3.50
should be addressed.
0 Elsevier
Sequoia/Printed
in The Netherlands
3.50 [13]. There are also indications that the weathering of wool is due to cross-linking reactions such as the dehydrodimerization reactions of aromatic residues [14]. We have shown, using fluorescence microscopy and spectrofluorometry, that wool fibres become more Atiorescent on peroxide bleaching [15] and on exposure to sunlight. Exposure to light of high intensity (A = 488 nm) and to blue light (A > 400 nm) destroys the long-wavelength absorbing and emitting species [9, 161. In this paper, we report on investigation of the fluorescence of papers containing lignin which were bleached in various ways or were treated with materials which minimized the photodegradation processes.
2. Experimental
details
Corrected fluorescence and excitation spectra of the thin sheets of paper were obtained with a Spex Fluororog fluorometer; fluorescence was observed at 90” to the exciting light. Spectra of thick sheets of paper were obtained with a Perkin-Elmer MPF4 spectrofluorometer. The spectra were not corrected for photomultiplier response or for the variation in intensity of the light output as a function of wavelength. The excitation source was a xenon lamp. Details of the microspectrofluorometer have been described previously [9]. Thin sheets of paper (18 g m*) were kindly provided by Professor Lachenal (CTP, Grenoble). They were composed of a peroxide bleached softwood spruce-fir mixture (60:40) chemi-thetmomechanical pulp (CTMP) (disc refined pulp (10 min at 125 “C; sodium sulphite, 1%)). Bleaching of the pulp was performed according to the usual procedure (consistency, 15%; diethylenetriaminepentaacetic acid (DTPA), 0.25%; sodium silicate, 3%; temperature 60 “C for 4 h; H202, 4%; NaOH, 2.5%). Care was taken to avoid contamination by fluorescent whitening agents usually used in paper making facilities. Thin sheets of cellulose paper were kindly provided by Professor Silvy (Ecole FranGaise de Papeterie, Grenoble). They were composed of a BX-50 regenerated cellulose (extremely pure cellulose fibres). The bleached CTMP thick sheets of paper were prepared at the University of Bordeaux as described in ref. 17. The 1,4-diazabicyclo[2,2,2]octane (Aldrich), thioglycerol (Aldrich) and ascorbic acid (Aldrich) were applied to the paper as 3% solutions in aqueous tetrahydrofuran (1:4, vhr) and then dried. The weight of additive on the paper was found to be approximately 5%. The other additives were applied to the paper so treated and irradiated under the conditions previously described [18]. Details of the photobleaching of paper containing lignin have been previously described [17, 191. Tinuvin 1130 was kindly supplied by Ciba Geigy Ltd. (Manchester).
3. Results
and discussion
In the initial studies, very thin sheets of paper were prepared and the fluorescence spectra were recorded at room temperature. Suprisingly, pure cellulose paper fluoresces more strongly than bleached CTMP (Table 1, Figs. 1 and 2) even though fibre arrangement and carbohydrate composition are different for the two materials. The CTMP contains hemicellulose. The presence of lignin appears to shift the fluorescence spectrum (keeping the excitation wavelength at 330 nm) slightly to the red. This is
351 TABLE
1
Relative
quantum yields of fluorescence
Bleached Cellulosic Bleached Bleached Bleached
CTMP paper CTMP CTMP C’IYMP
1.0 17.6 1.6 23.5 50.4
paper paper + DABCO paper + ascorbic acid paper + thioglycerol
350
250
for thin sheets of paper
450 Wavelength
650
550 (nm)
Fig. I. Corrected fluorescence and excitation spectra of pure cellulose recorded at room temperature.
Excitation
250
Emission
350
450 Wavelength
Fig. 2. Corrected
fluorescence
550
650
(nm)
and excitation
spectra of CAMP recorded
at room temperature.
due to the fact that the fluorescence from the paper containing lignin originates from chromophores which are different from those responsible for fluorescence in pure cellulose. Unfortunately the identity of these chromophores is unknown. The lower fluorescence yield of bleached CTMP paper and its different emission spectrum indicate that the lignin quenches the fluorescence of the cellulose and, in all probability, the quantum yield of fluorescence of the lignin is low. Addition of 1,4-diazabicyclo[2,2,2]octane (DABCO), which quenches excited states by electron transfer [20],
3.52
changes the shape of the emission spectrum, but quenching is not observed (Fig. 3). The changes in the spectrum may be due to a pH effect or suppression of fluorescence from particular fluorescent species. When additives such as ascorbic acid (Fig. 4) and thioglycerol (Fig. 5) are applied to the paper, similar emission spectra are observed, but in these cases the fluorescence intensity is increased and considerably so in the case of thioglycerol (Table 1, Fig. 4). Both of these compounds photostabilize the paper [17, 191 and bleach it. The latter may be the reason for the increase in intensity, since inner filter effects are reduced and also quenching groups, eg. carbonyl, may either be reduced or at least complexed with the additives, thereby decreasing their quenching ability. Further studies were carried out using thicker papers as employed in the bleaching experiments [17, 191. Using diffuse reflectance UV-visibie spectroscopy, the spectra of the paper containing lignin, before and after bleaching with hydrogen peroxide, were recorded (Fig. 6). The fluorescence spectrum (Fig. 7) of the unbleached paper is dependent on the excitation wavelength, showing that more than one chemical species is responsible for the emission. Since fluorescence can be observed on the fluorometer with an excitation wavelength of 450 nm and with light of wavelength 488 nm on the microspectrofluorometer, it is clear that long-wavelength absorbing and
= i 250 Fig.
350
450 Wavelength
550
650
(nm)
3. Corrected fluorescence and excitation spectra of CTMP paper treated with
DAEICO.
_I 250
350
450 Wavelength
Fig. 4. Corrected
fluorescence
and
550
650
(nm)
excitation
spectra
of CAMP
paper treated with ascorbic acid.
353
250
350
450 Wavelength
550
650
(nm)
Fig. 5. Corrected fluorescence and excitation
spectra of CXMP paper treated with thioglycerol.
l.O-
0% unb,..sk.d
0.E E z =
0.4-
a 9 0.2 ‘. 200
300
400 WAVELENGTH
Fig. 6. UV-visible peroxide.
500
600
(mm)
diffuse reflectance
of CT’MP paper before and after bleaching with hydrogen
fluorescent species are contained in the paper. When the paper is bleached with hydrogen peroxide, fluorescence in the 400-550 nm range is increased (Table 2). This result is reminiscent of that obtained in a study of peroxide-bleached wool [15]. However, the peroxide treatment of the paper does not markedly increase the fluorescence generated by excitation at 450 nm, but neither does it decrease it. The absorption spectra (Fig. 6) indicate that some of the whitening is due to peroxide degradation of some of the coloured species; from Table 2 it can be seen that there is a greater contribution from fluorescence in the blue region which helps to offset the yellow colour. This effect can also be -seen with wool and may be thought of as being due to the production of natural fluorescent whitening agents. The increase in fluorescence by treatment with peroxide is puzzling; however, if the peroxide, via advantageous
354
E cn
“In
?!f E -
.‘,.__,
I
1
400
450
Fig.7. Fluorescence TABLE
I
550
spectra of unbleached
CTMP
600
650
paper as a function of excitation
wavelength.
2
Quantum Paper
I
500 EMISSION (ml
yields of fluorescence
for various papers relative
Excitation
and treatment
Unbleached
to bleached
CTMP
CTMP
wavelengths
(nm)
370
400
450
0.49
0.56
0.89
1.0
1.0
1.0
0.82
1.12
2.57
Bleached CTMP irradiated 24 h in 2% solution of potassium metabisulphite
4.84
4.02
2.04
Bleached CTMP irradiated 24 h in 2% solution of Rongalit C
3.37
2.98
1.38
Bleached CTMP irradiated 2% solution of Blankit D
2.48
1.62
1.55
8% on weight of paper Rongalit C applied to bleached CTMP, dried and irradiated 24 h
0.71
1.04
2.47
4.2% on weight of paper Tinuvin 1130 applied to bleached CTMP, dried and irradiated 24 h
0.39
1.28
2.45
“The quantum yield at each of the excitation material.
has been arbitrarily
Bleached
CTMP
(hydrogen
Bleached
CTMP
24 h irradiation
peroxide)a
24 h in
wavelengths
set at 1 for this
traces of iron salts, leads to hydroxyl radical production, further phenolic groups will be produced, and dehydrodimerization of phenolic residues, via phenoxyl radical formation, may occur 1211. When the bleached paper is irradiated, yellowing occurs which is accompanied by an increase in the fluorescence produced on excitation at 400 and 450 nm (Fig. 8, Tabie 2). This result again parallels that obtained when wool is weathered in natural or artificial daylight. Tyrosine-rich proteins produce long-wavelength fluorescing species on irradiation via the formation of dityrosine and trityrosine etc. [22]. The very large increase (2.5 times) in the fluorescence produced on excitation at 450 nm is clearly due to the formation of long-wavelength absorbing species. Quinones are normally
355
._ \ I
400
450
sbo
550 I
-_ ..._ ..-_,
600
650
EMISSIONLl
Fig. 8. Fluorescence spectra of bkached CTMP function of excitation wavelength.
paper, following UV irradiation for 24 h, as a
non-fluorescent due to the n,T* nature of their lowest excited singlet state. However, if the quinones are part of a large aromatic structure, the lowest excited state may be of T,T* in nature and consequently fluorescence can be observed. In addition, in the presence of suitable substituents, as in the case of l-hydroxy-9,10-anthraquinone [23], fluorescence can be observed due to hydrogen bonding raising the energy of the
n,rr* singlet state. Examination of the long-wavelength fluorescence by microspectrofluorometry [16] has shown that irradiation with high intensity light destroys the fluorescent species. Once again there is a parallel with wool, since following irradiation, there is a regeneration of the fluorescent species (not to the 100% level) (Figs. 9(a)
and 9(b)). This regeneration can be attributed to two processes: (i) back oxidation in the dark of the photoreduced species and (ii) diffusion into the irradiated area of fluorescent species originally present in the non-irradiated area. In earlier work, we have shown that irradiation of peroxide-bleached wool in solutions of certain reducing agents (Rongalit C, Blankit D and potassium metabisulphite) leads, according to yellowness index (YI) measurements, to photobleaching; however, this is not due to an increase in fluorescence in the 400-450 nm region [24]. Examination of the fluorescence spectra of paper irradiated in the presence of reducing agents was most revealing (Fig. 10). With both Rongalit C and potassium metabisulphite, there was a substantial increase (Table 2) in the fluorescence in the 380-500 nm region using excitation wavelengths of 370 and 400 nm and, furthermore, the emission spectra generated using these excitation wavelengths were identical. Although fluorescence can be produced using an excitation wavelength of 450 nm, the presence of these longer-wavelength absorbing species is offset by the blue effect of the emitters which absorb in the 350-400 nm region. Blankit D also acts as a photobleach by causing an increase in the fluorescence in the 400-450 nm wavelength range (Fig. 11); however, in this case, the spectral distribution of the emission (A,,= 370 nm) is different from that produced by the Rongalit C and potassium metabisulphite. Given that all three reagents operate in similar ways to sulphur dioxide, it is strange that Blankit D does not generate the same species from the lignin as the other two reagents. A possible explanation for reagents reduce
the increase in fluorescence quinones to give fluorescent
in the 400-450 phenols.
nm
region
is that
the
LOG~O
IRRADIATION
TIME
(S)
Fig. 9.(a) Bleaching of the fluorescence of yellowed paper by 488 nm light. fluorescence in the dark following irradiation with 488 nm light.
(b) Recovery
of the
When the Rongalit C is impregnated into the paper and the paper irradiated, yellowing occurs; however, from brightness index measurements it can be seen that the Rongalit C exhibits a protective effect [25]. Examination of the fluorescence spectra (Fig. 12) produced on excitation at 370,400 and 450 nm shows that “natural fluorescent are not generated and that longer-wavelength absorbing species brightness agents” are produced. It may be that the natural fluorescent whitening agents are produced Somewhat similar results have been obtained with but undergo photodegradation. Tinuvin 1130, which acts as a UV screen [25]. Thus the longer-wavelength emissions increase in intensity with relatively little emission in the 400450 nm region.
4. Conclusions
the
The fluorescence of bleached CTMP is not markedly affected quencher DABCO, but is substantially enhanced by adsorbed
by the presence of ascorbic acid and
357
400
450
500
550
EMISSIONlnml
Fig. 10. Fluorescence spectra of bleached CIMP paper irradiated in the presence of 2% solutions of Rongalit C and potassium metabisulphite. Excitation wavelength, 370 nm.
L
400
1
450
500 EMISSIONInml
I
550
I
600
Fig. 11. Fluorescence spectra of bleached CIMP paper, following UV irradiation for 24 h in a 2% solution of Blankit D, as a function of excitation wavelength. thioglycerol; these compounds reduce some of the coloured species which may contribute to the inner filter effect and act as quenchers. Bleaching of paper containing lignin using hydrogen peroxide increases the fluorescence in the 420-450 nm region. As a consequence, the yellowing effect is offset in the same way as when fluorescent whitening agents are used to decrease the perceptiveness of yellowing. The presence of the reducing agents Rongalit C, potassium metabisulphite and Blankit D in aqueous solution leads to a dramatic increase in the amount of blue emitting species on irradiation. Impregnation of Rongalit C into the paper does not have this effect, but the rate of photoyeliowing is reduced (this is also observed with Tinuvin 1130). Reducing agents such as sodium borohydride bleach paper; this is probably due to the reduction of quinoid species by borohydride giving fluorescent phenols. The photobleaching effect described in this paper is most probably a result of the reduction of carbonyl compounds and the prevention of the formation of peroxidic species which
I
!
I
4oll
450
sbo
550
sbo
EMISSICINL..)
Fig. 12. Fluorescence spectra of bleached CTMP paper, treated irradiated for 24 h, as a function of excitation wavelength.
with 8 wt.%
Rongalit
C and
undergo further photodegradation to give long-wavelength absorbing species. Rongalit C, potassium metabisulphite and Blankit D do not increase the fluorescence of CTMP in the dark, which indicates that changes in the surface pH are not responsible for the effects. When the paper is impregnated with the reagents, the rate of diffusion of the reagents to the site of peroxide or hydroperoxide production is reduced; thus quinoid and other species can initiate photodegradation prior to being reduced by the reagents.
Acknowledgments This
work
was funded
by the
EEC
(Contract
No.
MAlB
0127-C
[EDB]).
References 1 D. Lachenal, C. de Choudens and L. Bourson, Tappi, 70 (3) (1987) 119-122. J. A. van den Akker and W. A. Wink, Pap. Trade J., 121 (1945) 101-105. A. Castellan, N. CoIombo, Ph. Fornier de Violet, A. Nourmamode and H. Bouas-Laurent, 5th ht. Sytnp. on Wood and Paper Chemistry, Raleigh, NC, May 21-25, 1989, Tappi Proc., 1 (1989) 421-430, and references cited therein. 3 A. Castellan, P. Girard and C. Vanucci, .l. Wood Chem. TechnoL, 8 (1988) 73. C. Vanucci, P. Fomier de Violet, H. Bouas-Laurent and A. Castellan, J. Photochem. PhotobioI., 41 (1988) 251. A. Castellan, N. Colombo, C. Cucuphat and P. Former de Violet, Hokforschung, 43 (1989) 179. 4 G. S. Furman and W. F. W. Lonsky, J. Wood Chem. Technol., 8 (1988) 165-189, 191-208. 5 I. Forsskahl, C. Olkkonen and H. Tylli, J. Photochem. Photobiol. A: Chem., 43 (1988) 337-344. G. Brunow and M. Sivonen, Pap. Puu, 57 (1975) 215. G. Gellerstedt and E. Pettersson, Acta Chem. Stand., 29 (1975) 1005-1010. D. N. S. Hon and G. T. Chang, Wood Sci. Technol., 16 (1982) 193-201. 2 P. Nolan,
359 6 A. Castellan,
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21
22 23 24 25
N. Colombo, A. Nourmamode, J. H. Zhu, D. Lachenal, R. S. Davidson and L. A. Dunn, J. Wood Chem. Techno!., 20 (1990) 461-493. J. A. Maclaren and B. Milligan, Wool Science, Science Press, Marrickville, Australia, 1981, Chapter 13. C. M. Rao, B. Balasubramanian and B. Chakrabarti, Phorochem. PhorobioL, 46 (1987) 511-515. S. Collins, R. S. Davidson, M. H. Hilchenbach and D. M. Lewis, submitted for publication. J. A. Maclaren and B. Milligan, Wool Science, Science Press, Marrlclrville, Australia, 1981, Chapter 1. F. G. Lennox and R. J. Rowlands, Phorochem. Phorobioi., 9 (1969) 359. A. S. Inglis and F. G. Lennox, T&r. Res. J., 33 (1963) 431. J. A_ Maclaren, Text. Rex J., 33 (1963) 773. K. S. Stewart, Ph.D. Thesis, Queens University, Belfast, 1987. M. S. Otterbum and P. E. Gargan, J. Chromarogr., 303 (1984) 429. L. A. Bettison, M.Sc. T&es& University of Otago, Dunedin, New Zealand, 1985. S. Collins, R. S. Davidson, P. Greaves, M. A. Healey and D. M. Lewis, .I. Sot. Dyers Coiour., IO4 (1988) 348-352. S. Collins, R. S. Davidson, M. H. Hilchenbach and D. M. Lewis, submitted for publication. P. Fomier de Violet, A. Nourinamode, N. Colombo and A. Castellan, Cell&. Chem. Technol., 23 (1989) 535. P. Fomier de Violet, A. Nounnamode, N. Colombo, J. H. Zhu and A. Castellan, Celhd. Chem. Technol., 24 (1990) 225-235. A. Castellan, R. S. Davidson and L. A. Dunn, J. Phorochem. Phorobiol. A: Chem., 58 (1991) 263-273. R. S. Davidson, in R. Foster (ed.), Molecular Association, Vol. 1, Academic Press, London, 1975, pp. 216-318. R. S. Davidson, in V. Gold and D. Bethel1 (eds.), Advances in Physical Organic Chemise, Vol. 19, Academic Press, London, 1983, pp. l-130. C. W. Bailey and C. W. Dence, Tuppi, 52 (1969) 491. A. Kunai, S. Hata, S. Ito and K. Sasaki, J. Am. Chem Sot., 208 (1986) 6012-6016. R. W. Pero and C. W. Dence, I. Wood Chem. Technol., 3 (1983) 195-222. A. V. Pantar, M. V. R. Rao, M. Atreyi and S. Kumar, Phorochem. PhorobioL, 43 (1986) 591-594. H. H. Dearman and A. Chan, J. Chem. Phys-, 44 (1966) 416417. S. Collins and R. S. Davidson, unpublished results, 1989. R. S. Davidson, L. A. Dunn, A. Castellan, N. Colombo, A. Nourmamode and J. H. Zhu, submitted for publication.
349-359
349
A study of the photobleaching and photoyellowing of paper containing lignin using fluorescence spectroscopy R. Stephen Davidson+
and
Linda A. Dunn
of ChemistT, City Universi& Northampton Square, London ECIV OHB (UK)
Department
Aiain Castellan
and Aziz Nourmamode
UniversitP de Bordeaux, Laboratoire de Photochimie Organique, CNRS URA 348, F33405 Talence (France)
(Received
July 30, 1990; in revised form October
29, 1990)
Abstract The fluorescence of paper produced using high yield pulp increases on bIeaching with hydrogen peroxide. The new blue fluorescent species help to offset the naturaI yelIow colour of the paper. Irradiation of the paper in the presence of reducing agents leads to further bleaching and a further increase in fluorescence, probably as a result of the photoreduction of groups such as quinones.
1. Introduction Paper containing lignin has a light brown appearance. For many end uses white paper is required and therefore paper containing lignin is chemically bleached, hydrogen peroxide being the most widely used reagent [l]. The visual effect so attained is easily lost by exposure of the bleached paper to light (A<400 nm) which produces a yellow discoloration [2]. The origin of the coloured materials in the paper containing lignin is the lignin, and it is this material that responds to the peroxide bleach giving products which can readily undergo photodegradation. The photoinstability has been attributed to a number of sources: photochemical CY-and P-cleavage reactions of carbonyl compounds [3] which are an integral part of the lignin structure; the presence of quinones [4] which are coloured and undergo a plethora of ill-defined photochemical reactions; sensitizers which produce singlet oxygen on excitation [5]; polyphenolic compounds which can be readily photo-oxidized and photodehydrodimerized [63. Another area of chemistry which is not unrelated to that of paper containing lignin is the photoaging of proteins which leads to photoyellowing [7]. Topics which have been studied in depth include the aging of lens proteins [8] and wool [9], the latter having a complex morphological structure which contains approximately 80% protein [lo]. Whilst much of the degradation of such proteinaceous materials can be attributed to the photoreactions of tryptophan [ll] and its degradation products, it is quite clear that other aromatic amino acids such as phenylalanine and tyrosine play a very important role [123. It has been shown that, in the badly weathered regions of wool fibres, there is an increase in concentration of dityrosine and related compounds ‘Author
to whom correspondence
lOlO-6030/91/$3.50
should be addressed.
0 Elsevier
Sequoia/Printed
in The Netherlands
3.50 [13]. There are also indications that the weathering of wool is due to cross-linking reactions such as the dehydrodimerization reactions of aromatic residues [14]. We have shown, using fluorescence microscopy and spectrofluorometry, that wool fibres become more Atiorescent on peroxide bleaching [15] and on exposure to sunlight. Exposure to light of high intensity (A = 488 nm) and to blue light (A > 400 nm) destroys the long-wavelength absorbing and emitting species [9, 161. In this paper, we report on investigation of the fluorescence of papers containing lignin which were bleached in various ways or were treated with materials which minimized the photodegradation processes.
2. Experimental
details
Corrected fluorescence and excitation spectra of the thin sheets of paper were obtained with a Spex Fluororog fluorometer; fluorescence was observed at 90” to the exciting light. Spectra of thick sheets of paper were obtained with a Perkin-Elmer MPF4 spectrofluorometer. The spectra were not corrected for photomultiplier response or for the variation in intensity of the light output as a function of wavelength. The excitation source was a xenon lamp. Details of the microspectrofluorometer have been described previously [9]. Thin sheets of paper (18 g m*) were kindly provided by Professor Lachenal (CTP, Grenoble). They were composed of a peroxide bleached softwood spruce-fir mixture (60:40) chemi-thetmomechanical pulp (CTMP) (disc refined pulp (10 min at 125 “C; sodium sulphite, 1%)). Bleaching of the pulp was performed according to the usual procedure (consistency, 15%; diethylenetriaminepentaacetic acid (DTPA), 0.25%; sodium silicate, 3%; temperature 60 “C for 4 h; H202, 4%; NaOH, 2.5%). Care was taken to avoid contamination by fluorescent whitening agents usually used in paper making facilities. Thin sheets of cellulose paper were kindly provided by Professor Silvy (Ecole FranGaise de Papeterie, Grenoble). They were composed of a BX-50 regenerated cellulose (extremely pure cellulose fibres). The bleached CTMP thick sheets of paper were prepared at the University of Bordeaux as described in ref. 17. The 1,4-diazabicyclo[2,2,2]octane (Aldrich), thioglycerol (Aldrich) and ascorbic acid (Aldrich) were applied to the paper as 3% solutions in aqueous tetrahydrofuran (1:4, vhr) and then dried. The weight of additive on the paper was found to be approximately 5%. The other additives were applied to the paper so treated and irradiated under the conditions previously described [18]. Details of the photobleaching of paper containing lignin have been previously described [17, 191. Tinuvin 1130 was kindly supplied by Ciba Geigy Ltd. (Manchester).
3. Results
and discussion
In the initial studies, very thin sheets of paper were prepared and the fluorescence spectra were recorded at room temperature. Suprisingly, pure cellulose paper fluoresces more strongly than bleached CTMP (Table 1, Figs. 1 and 2) even though fibre arrangement and carbohydrate composition are different for the two materials. The CTMP contains hemicellulose. The presence of lignin appears to shift the fluorescence spectrum (keeping the excitation wavelength at 330 nm) slightly to the red. This is
351 TABLE
1
Relative
quantum yields of fluorescence
Bleached Cellulosic Bleached Bleached Bleached
CTMP paper CTMP CTMP C’IYMP
1.0 17.6 1.6 23.5 50.4
paper paper + DABCO paper + ascorbic acid paper + thioglycerol
350
250
for thin sheets of paper
450 Wavelength
650
550 (nm)
Fig. I. Corrected fluorescence and excitation spectra of pure cellulose recorded at room temperature.
Excitation
250
Emission
350
450 Wavelength
Fig. 2. Corrected
fluorescence
550
650
(nm)
and excitation
spectra of CAMP recorded
at room temperature.
due to the fact that the fluorescence from the paper containing lignin originates from chromophores which are different from those responsible for fluorescence in pure cellulose. Unfortunately the identity of these chromophores is unknown. The lower fluorescence yield of bleached CTMP paper and its different emission spectrum indicate that the lignin quenches the fluorescence of the cellulose and, in all probability, the quantum yield of fluorescence of the lignin is low. Addition of 1,4-diazabicyclo[2,2,2]octane (DABCO), which quenches excited states by electron transfer [20],
3.52
changes the shape of the emission spectrum, but quenching is not observed (Fig. 3). The changes in the spectrum may be due to a pH effect or suppression of fluorescence from particular fluorescent species. When additives such as ascorbic acid (Fig. 4) and thioglycerol (Fig. 5) are applied to the paper, similar emission spectra are observed, but in these cases the fluorescence intensity is increased and considerably so in the case of thioglycerol (Table 1, Fig. 4). Both of these compounds photostabilize the paper [17, 191 and bleach it. The latter may be the reason for the increase in intensity, since inner filter effects are reduced and also quenching groups, eg. carbonyl, may either be reduced or at least complexed with the additives, thereby decreasing their quenching ability. Further studies were carried out using thicker papers as employed in the bleaching experiments [17, 191. Using diffuse reflectance UV-visibie spectroscopy, the spectra of the paper containing lignin, before and after bleaching with hydrogen peroxide, were recorded (Fig. 6). The fluorescence spectrum (Fig. 7) of the unbleached paper is dependent on the excitation wavelength, showing that more than one chemical species is responsible for the emission. Since fluorescence can be observed on the fluorometer with an excitation wavelength of 450 nm and with light of wavelength 488 nm on the microspectrofluorometer, it is clear that long-wavelength absorbing and
= i 250 Fig.
350
450 Wavelength
550
650
(nm)
3. Corrected fluorescence and excitation spectra of CTMP paper treated with
DAEICO.
_I 250
350
450 Wavelength
Fig. 4. Corrected
fluorescence
and
550
650
(nm)
excitation
spectra
of CAMP
paper treated with ascorbic acid.
353
250
350
450 Wavelength
550
650
(nm)
Fig. 5. Corrected fluorescence and excitation
spectra of CXMP paper treated with thioglycerol.
l.O-
0% unb,..sk.d
0.E E z =
0.4-
a 9 0.2 ‘. 200
300
400 WAVELENGTH
Fig. 6. UV-visible peroxide.
500
600
(mm)
diffuse reflectance
of CT’MP paper before and after bleaching with hydrogen
fluorescent species are contained in the paper. When the paper is bleached with hydrogen peroxide, fluorescence in the 400-550 nm range is increased (Table 2). This result is reminiscent of that obtained in a study of peroxide-bleached wool [15]. However, the peroxide treatment of the paper does not markedly increase the fluorescence generated by excitation at 450 nm, but neither does it decrease it. The absorption spectra (Fig. 6) indicate that some of the whitening is due to peroxide degradation of some of the coloured species; from Table 2 it can be seen that there is a greater contribution from fluorescence in the blue region which helps to offset the yellow colour. This effect can also be -seen with wool and may be thought of as being due to the production of natural fluorescent whitening agents. The increase in fluorescence by treatment with peroxide is puzzling; however, if the peroxide, via advantageous
354
E cn
“In
?!f E -
.‘,.__,
I
1
400
450
Fig.7. Fluorescence TABLE
I
550
spectra of unbleached
CTMP
600
650
paper as a function of excitation
wavelength.
2
Quantum Paper
I
500 EMISSION (ml
yields of fluorescence
for various papers relative
Excitation
and treatment
Unbleached
to bleached
CTMP
CTMP
wavelengths
(nm)
370
400
450
0.49
0.56
0.89
1.0
1.0
1.0
0.82
1.12
2.57
Bleached CTMP irradiated 24 h in 2% solution of potassium metabisulphite
4.84
4.02
2.04
Bleached CTMP irradiated 24 h in 2% solution of Rongalit C
3.37
2.98
1.38
Bleached CTMP irradiated 2% solution of Blankit D
2.48
1.62
1.55
8% on weight of paper Rongalit C applied to bleached CTMP, dried and irradiated 24 h
0.71
1.04
2.47
4.2% on weight of paper Tinuvin 1130 applied to bleached CTMP, dried and irradiated 24 h
0.39
1.28
2.45
“The quantum yield at each of the excitation material.
has been arbitrarily
Bleached
CTMP
(hydrogen
Bleached
CTMP
24 h irradiation
peroxide)a
24 h in
wavelengths
set at 1 for this
traces of iron salts, leads to hydroxyl radical production, further phenolic groups will be produced, and dehydrodimerization of phenolic residues, via phenoxyl radical formation, may occur 1211. When the bleached paper is irradiated, yellowing occurs which is accompanied by an increase in the fluorescence produced on excitation at 400 and 450 nm (Fig. 8, Tabie 2). This result again parallels that obtained when wool is weathered in natural or artificial daylight. Tyrosine-rich proteins produce long-wavelength fluorescing species on irradiation via the formation of dityrosine and trityrosine etc. [22]. The very large increase (2.5 times) in the fluorescence produced on excitation at 450 nm is clearly due to the formation of long-wavelength absorbing species. Quinones are normally
355
._ \ I
400
450
sbo
550 I
-_ ..._ ..-_,
600
650
EMISSIONLl
Fig. 8. Fluorescence spectra of bkached CTMP function of excitation wavelength.
paper, following UV irradiation for 24 h, as a
non-fluorescent due to the n,T* nature of their lowest excited singlet state. However, if the quinones are part of a large aromatic structure, the lowest excited state may be of T,T* in nature and consequently fluorescence can be observed. In addition, in the presence of suitable substituents, as in the case of l-hydroxy-9,10-anthraquinone [23], fluorescence can be observed due to hydrogen bonding raising the energy of the
n,rr* singlet state. Examination of the long-wavelength fluorescence by microspectrofluorometry [16] has shown that irradiation with high intensity light destroys the fluorescent species. Once again there is a parallel with wool, since following irradiation, there is a regeneration of the fluorescent species (not to the 100% level) (Figs. 9(a)
and 9(b)). This regeneration can be attributed to two processes: (i) back oxidation in the dark of the photoreduced species and (ii) diffusion into the irradiated area of fluorescent species originally present in the non-irradiated area. In earlier work, we have shown that irradiation of peroxide-bleached wool in solutions of certain reducing agents (Rongalit C, Blankit D and potassium metabisulphite) leads, according to yellowness index (YI) measurements, to photobleaching; however, this is not due to an increase in fluorescence in the 400-450 nm region [24]. Examination of the fluorescence spectra of paper irradiated in the presence of reducing agents was most revealing (Fig. 10). With both Rongalit C and potassium metabisulphite, there was a substantial increase (Table 2) in the fluorescence in the 380-500 nm region using excitation wavelengths of 370 and 400 nm and, furthermore, the emission spectra generated using these excitation wavelengths were identical. Although fluorescence can be produced using an excitation wavelength of 450 nm, the presence of these longer-wavelength absorbing species is offset by the blue effect of the emitters which absorb in the 350-400 nm region. Blankit D also acts as a photobleach by causing an increase in the fluorescence in the 400-450 nm wavelength range (Fig. 11); however, in this case, the spectral distribution of the emission (A,,= 370 nm) is different from that produced by the Rongalit C and potassium metabisulphite. Given that all three reagents operate in similar ways to sulphur dioxide, it is strange that Blankit D does not generate the same species from the lignin as the other two reagents. A possible explanation for reagents reduce
the increase in fluorescence quinones to give fluorescent
in the 400-450 phenols.
nm
region
is that
the
LOG~O
IRRADIATION
TIME
(S)
Fig. 9.(a) Bleaching of the fluorescence of yellowed paper by 488 nm light. fluorescence in the dark following irradiation with 488 nm light.
(b) Recovery
of the
When the Rongalit C is impregnated into the paper and the paper irradiated, yellowing occurs; however, from brightness index measurements it can be seen that the Rongalit C exhibits a protective effect [25]. Examination of the fluorescence spectra (Fig. 12) produced on excitation at 370,400 and 450 nm shows that “natural fluorescent are not generated and that longer-wavelength absorbing species brightness agents” are produced. It may be that the natural fluorescent whitening agents are produced Somewhat similar results have been obtained with but undergo photodegradation. Tinuvin 1130, which acts as a UV screen [25]. Thus the longer-wavelength emissions increase in intensity with relatively little emission in the 400450 nm region.
4. Conclusions
the
The fluorescence of bleached CTMP is not markedly affected quencher DABCO, but is substantially enhanced by adsorbed
by the presence of ascorbic acid and
357
400
450
500
550
EMISSIONlnml
Fig. 10. Fluorescence spectra of bleached CIMP paper irradiated in the presence of 2% solutions of Rongalit C and potassium metabisulphite. Excitation wavelength, 370 nm.
L
400
1
450
500 EMISSIONInml
I
550
I
600
Fig. 11. Fluorescence spectra of bleached CIMP paper, following UV irradiation for 24 h in a 2% solution of Blankit D, as a function of excitation wavelength. thioglycerol; these compounds reduce some of the coloured species which may contribute to the inner filter effect and act as quenchers. Bleaching of paper containing lignin using hydrogen peroxide increases the fluorescence in the 420-450 nm region. As a consequence, the yellowing effect is offset in the same way as when fluorescent whitening agents are used to decrease the perceptiveness of yellowing. The presence of the reducing agents Rongalit C, potassium metabisulphite and Blankit D in aqueous solution leads to a dramatic increase in the amount of blue emitting species on irradiation. Impregnation of Rongalit C into the paper does not have this effect, but the rate of photoyeliowing is reduced (this is also observed with Tinuvin 1130). Reducing agents such as sodium borohydride bleach paper; this is probably due to the reduction of quinoid species by borohydride giving fluorescent phenols. The photobleaching effect described in this paper is most probably a result of the reduction of carbonyl compounds and the prevention of the formation of peroxidic species which
I
!
I
4oll
450
sbo
550
sbo
EMISSICINL..)
Fig. 12. Fluorescence spectra of bleached CTMP paper, treated irradiated for 24 h, as a function of excitation wavelength.
with 8 wt.%
Rongalit
C and
undergo further photodegradation to give long-wavelength absorbing species. Rongalit C, potassium metabisulphite and Blankit D do not increase the fluorescence of CTMP in the dark, which indicates that changes in the surface pH are not responsible for the effects. When the paper is impregnated with the reagents, the rate of diffusion of the reagents to the site of peroxide or hydroperoxide production is reduced; thus quinoid and other species can initiate photodegradation prior to being reduced by the reagents.
Acknowledgments This
work
was funded
by the
EEC
(Contract
No.
MAlB
0127-C
[EDB]).
References 1 D. Lachenal, C. de Choudens and L. Bourson, Tappi, 70 (3) (1987) 119-122. J. A. van den Akker and W. A. Wink, Pap. Trade J., 121 (1945) 101-105. A. Castellan, N. CoIombo, Ph. Fornier de Violet, A. Nourmamode and H. Bouas-Laurent, 5th ht. Sytnp. on Wood and Paper Chemistry, Raleigh, NC, May 21-25, 1989, Tappi Proc., 1 (1989) 421-430, and references cited therein. 3 A. Castellan, P. Girard and C. Vanucci, .l. Wood Chem. TechnoL, 8 (1988) 73. C. Vanucci, P. Fomier de Violet, H. Bouas-Laurent and A. Castellan, J. Photochem. PhotobioI., 41 (1988) 251. A. Castellan, N. Colombo, C. Cucuphat and P. Former de Violet, Hokforschung, 43 (1989) 179. 4 G. S. Furman and W. F. W. Lonsky, J. Wood Chem. Technol., 8 (1988) 165-189, 191-208. 5 I. Forsskahl, C. Olkkonen and H. Tylli, J. Photochem. Photobiol. A: Chem., 43 (1988) 337-344. G. Brunow and M. Sivonen, Pap. Puu, 57 (1975) 215. G. Gellerstedt and E. Pettersson, Acta Chem. Stand., 29 (1975) 1005-1010. D. N. S. Hon and G. T. Chang, Wood Sci. Technol., 16 (1982) 193-201. 2 P. Nolan,
359 6 A. Castellan,
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21
22 23 24 25
N. Colombo, A. Nourmamode, J. H. Zhu, D. Lachenal, R. S. Davidson and L. A. Dunn, J. Wood Chem. Techno!., 20 (1990) 461-493. J. A. Maclaren and B. Milligan, Wool Science, Science Press, Marrickville, Australia, 1981, Chapter 13. C. M. Rao, B. Balasubramanian and B. Chakrabarti, Phorochem. PhorobioL, 46 (1987) 511-515. S. Collins, R. S. Davidson, M. H. Hilchenbach and D. M. Lewis, submitted for publication. J. A. Maclaren and B. Milligan, Wool Science, Science Press, Marrlclrville, Australia, 1981, Chapter 1. F. G. Lennox and R. J. Rowlands, Phorochem. Phorobioi., 9 (1969) 359. A. S. Inglis and F. G. Lennox, T&r. Res. J., 33 (1963) 431. J. A_ Maclaren, Text. Rex J., 33 (1963) 773. K. S. Stewart, Ph.D. Thesis, Queens University, Belfast, 1987. M. S. Otterbum and P. E. Gargan, J. Chromarogr., 303 (1984) 429. L. A. Bettison, M.Sc. T&es& University of Otago, Dunedin, New Zealand, 1985. S. Collins, R. S. Davidson, P. Greaves, M. A. Healey and D. M. Lewis, .I. Sot. Dyers Coiour., IO4 (1988) 348-352. S. Collins, R. S. Davidson, M. H. Hilchenbach and D. M. Lewis, submitted for publication. P. Fomier de Violet, A. Nourinamode, N. Colombo and A. Castellan, Cell&. Chem. Technol., 23 (1989) 535. P. Fomier de Violet, A. Nounnamode, N. Colombo, J. H. Zhu and A. Castellan, Celhd. Chem. Technol., 24 (1990) 225-235. A. Castellan, R. S. Davidson and L. A. Dunn, J. Phorochem. Phorobiol. A: Chem., 58 (1991) 263-273. R. S. Davidson, in R. Foster (ed.), Molecular Association, Vol. 1, Academic Press, London, 1975, pp. 216-318. R. S. Davidson, in V. Gold and D. Bethel1 (eds.), Advances in Physical Organic Chemise, Vol. 19, Academic Press, London, 1983, pp. l-130. C. W. Bailey and C. W. Dence, Tuppi, 52 (1969) 491. A. Kunai, S. Hata, S. Ito and K. Sasaki, J. Am. Chem Sot., 208 (1986) 6012-6016. R. W. Pero and C. W. Dence, I. Wood Chem. Technol., 3 (1983) 195-222. A. V. Pantar, M. V. R. Rao, M. Atreyi and S. Kumar, Phorochem. PhorobioL, 43 (1986) 591-594. H. H. Dearman and A. Chan, J. Chem. Phys-, 44 (1966) 416417. S. Collins and R. S. Davidson, unpublished results, 1989. R. S. Davidson, L. A. Dunn, A. Castellan, N. Colombo, A. Nourmamode and J. H. Zhu, submitted for publication.