|[Home] [Journal papers]|
Thomas Rosenau, Antje Potthast, Paul Kosma; Hans U. Suess, Norbert Nimmerfroh
CRI procedure. The CRI method has been recently published [3,4,5] and was used without modification . It was applied to 3 kg of each pulp to obtain sufficiently "large" amounts of chromophores.
Pulp bleaching. All trials were made with industrial pulp samples taken after oxygen delignification. D and P stages were run in plastic bags in water baths. Ozone was added at low concentration (~1%) in air to well fluffed pulp in a fluidized bed reactor . Eop stages were conducted in a pressurized high-shear mixer with O2 pressure at 0.3 MPa. All trials (except ozone) were made at 10% consistency. Chemicals were added as diluted, preheated solutions to preheated pulp for improved mixing. Brightness was measured with ISO 2470. Reversion testing was made with hand sheets prepared at pH 6 on a Buchner funnel with a weight of 280 g/m², Tappi method UM 200 (105°C, 4h) and Paptac E.4P for humid aging (100°C, 100% humidity, 1h) were used.
All chromophores were identified by NMR and mass spectroscopy, for details see .
In reversion, sample 1 shows a better stability compared to sample 2. Jääskeläinen  detected carbonyls in D bleached pulp, however no traces of such compounds were found in finally peroxide bleached pulp. The final chlorine dioxide stage obviously leaves quinoid structures in sample 2, while those are cleaved by the alkaline peroxide treatment of sample 1. The good stability of sample 3 is explained with the combination of high intensity oxidation in D and the destruction of remaining traces of quinones (or their precursors) in the final P stage.
Figure 1: Chromophores isolated from pulp sample 1, bleached finally with P 1P2.
Using the CRI chromophore isolation procedure, the compounds shown in Figures 1 to 3 were isolated from the extracts of the pulp samples. Each of the eight different compounds in these figures was unambiguously identified based on the MS and NMR (1H) spectra, and additionally confirmed by comparison with authentic samples (comparison of MS and NMR spectra and chromatographic behavior). Authentic samples were either commercially available or chemically synthesized according to standard techniques.
Pulp 1 contained compounds 1 – 5, and thus the highest number of different chromophores in the three samples. It should be noted that the number of chromophores does not necessarily correlate with the overall chromophore concentration, nor does it automatically imply a lower brightness. Reliably stating concentration differences at such low levels appears meaningless. It just can be concluded that more chromophoric structures survived the bleaching sequence used – or were regenerated by aging after-wards – than in the case of the other two pulps. Interestingly, almost all structures contain the 2 -hydroxy-1,4-benzoquinone structure, a typical "primary chromophore", which is rather insensitive towards the applied bleaching process. All of the structures are also distinguished by strong intramolecular hydrogen bonds extending from an α or β -hydroxyl group to the carbonyl acceptor.
Figure 2: Chromophores isolated from pulp sample 2, finally bleached D 1D2.
Pulp 2 contained only one primary chromophore, 2,5-dihydroxy[1,4]benzoquinone, which was also found in pulp 1. In addition, two compounds containing chlorine (6 - 7) were detected, which evidently had formed from 1 by mono chlorination (compound 6) and bis chlorination (compound 7). As they are formed by follow-up chemistry, these two compounds can well be regarded as secondary chromophores. Compared to pulp 1, the variety of chromophores has been largely reduced and secondary chromophores due to the chlorine-containing bleaching stage were generated.
Figure 3: Chromophores isolated from pulp sample 3, finally bleached D(hot)P.
In pulp 3, only two secondary chromophores were found, no primary chromophores remained. Thus the very high temperature in the second D stage effectively degraded other chromophores or their precursors. Compound 7, the bis chlorination product of 1, was detectable in very low amounts only. Compound 8 is likely to be a ring opening and chlorination product of the primary chromophores isolated, or of similar structures. NMR data are indicative of a (2E,4Z)-configuration, but X-ray structure analysis would be needed for unambiguous confirmation of the configuration. The chromophore number and overall concentration in pulp 3 were the lowest of the three pulp samples tested.
Compounds 1 – 5 are typical condensation products which can generally be obtained mainly from mono-saccharidic carbohydrates upon thermal, acidic or basic treatment in the presence of air; so-called "Theander-products" [7-13]. They have been formed by cellulose degradation during pulping and were further modified during subsequent bleaching stages. As mentioned, compounds 1 – 5 are typical primary chromophores, as their formation is caused just by acidic/basic/thermal treatment of polysaccharides and is thus largely independent of the actual reaction medium present.
It has long been known that alkaline, acidic, or thermal treatment of monosaccharides and also cellulose in the presence of oxygen or other oxidants gives rise to EPR-active species already after a few minutes of reaction time. The most prominent of the single-electron species produced is 2,5-dihy-droxysemiquinone , which upon further one-electron oxidation readily forms chromophore 1. This compound, which was found in almost all cellulosic materials investigated so far, can be seen as an "elementary chromophore precursor structure". On one hand it can be readily degraded to smaller, reactive fragments; on the other hand it is readily able to condense to larger structures, so that in any case other chromophores and highly condensed structures are formed from 1 by a complex interplay of fragmentation / condensation sequences.
Similar hydroxyl-[1,4]benzoquinone and hydroxyl-[1,4]naphthoquinone moieties have been found as chromophores in a wide range of other cellulosic products. This is reasonable since these primary chromophores are generated from low-molecular weight carbohydrates (cellulosic and hemicellulosic degradation products) by degradation / condensation in general, and are thus independent of the specific pulp source. The 2-hydroxy-[1,4]benzoquinone moieties show a peculiar reactivity, which can be char¬acterized by three features:
• resonance stabilization causing the absence of localized double bonds and increased inertness towards agents that attack double bonds,
• susceptibility toward electrophilic attack (attack e.g. by cationic species), which is increased in basic media, leading to substitution in position 3,
• sensitivity toward nucleophilic attack (attack e.g. by H-X or X- followed by H+, respectively), which is increased in acidic media, affording cyclohexenediones that are readily re-oxidized to the benzoquinone substituted in position 3.
• In the case of similar 3-substitutents, it cannot be decided just from the structure of the product whether 3-substitution occurred according to an electrophilic attack or according to a nucleophilic substitution / reoxidation mechanism.
The chromophores isolated from the three pulp samples allow one to draw conclusions on the efficiency of the used bleaching stages with regard to both chromophore removal and chromophore re-formation, so-called "brightness reversion". Compounds 1 – 5 present in PP-bleached pulp 1 were evidently rather inert towards this type of bleaching sequence. The DD stage as used for pulp 2 reduced the number of primary chromophores (only 1) and produced secondary chromophores (6, 7) at the same time. These chromophores are process-specific for the D stage as indicated by the presence of chlorine in the chromophores. At present, it cannot be definitely decided whether the applied D stage generally consumed (destroyed) the primary chromophores or whether these primary chromophores were just converted into secondary ones. However, the fact that less secondary chromophores – both with regard to number and concentration – than primary ones in pulp 1 were found, indicated that a large part of the primary chromophores was indeed removed, and only a smaller part remained still present as secondary chromophores. From pulp 3, the smallest number and concentration of chromophores was isolated, the two chlorine-containing compounds 7 and 8. Evidently, the applied D(hot)P bleaching combination was most effective in removing chromophores and preventing their regeneration. A final peroxide stage removes remaining chromophores (quinones) more effectively than chlorine dioxide. The destruction of chromophores with chlorine dioxide is improved by the higher reaction temperature (85°C vs. 75°C) in the D stage .
The observation of low chromophore amounts in pulp 3 requires some mechanistic consideration. If we assume that the D stage leaves a chromophore composition in principle similar to that of pulp 2 (after DD stages), maybe just on a lower level, it was the final P stage that was responsible for the chromophore reduction. This seems to be a contradiction to the outcome in the case of pulp 1, and to the above statement that the efficiency of peroxide is limited in the case of 2-hydroxy-[1,4 ]benzoquinone moieties. How¬ever, it must be kept in mind that the reactivity of 3-chloro-substituted 2-hydroxy-[1,4]benzoquinones, such as 6 and 7 (and of 3-substituted 2-hydroxy-[1,4]benzoquinones in general), is significantly changed in comparison to the non-substituted parent compounds. The 3-substituted derivatives generally behave more like "proper" para-benzoquinones rather than showing the special stabilization of 2-hydroxy-[1,4]benzoquinones. They are thus much more sensitive towards reagents destroying unsaturated, conjugated structures.
Among the tested bleaching variations, the conclusion of the sequence with the stages DP seems most suited to destroy the primary chromophores and to prevent formation of new ones during aging (brightness reversion). A hot D stage, by chlorination of the 2-hydroxy-[1,4]benzoquinone structures in position 3, withdraws the special stabilization of these peroxide-resistant structures, rendering the subsequent P stage effective again.
6. Model substance oxidation
The identification of 2.5-dihydroxy-quinone derivatives as remaining trace substances in fully bleached pulp allows a simple analysis of the potential to improve their removal in bleaching. For a lab study the readily available parent compound was used. A solution of 2.5-dihydroxy-quinone in acetone was sprayed on fully bleached pulp. This resulted in a drop of the brightness and a shift to the reddish color of the quinone. The a* value increases. Reversion tests demonstrate the impact of the quinone (Table 2). Under humid conditions (Paptac E.4P) changes were pronounced. Under dry (UM 200) reversion conditions the impact was smaller. This demonstrates the importance of humidity (water) in reversion. Color generation requires water.
Pulp, sprayed with the quinone solution, was oxidized with chlorine dioxide, peracetic acid and ozone. This was followed by a P stage. Figure 4 illustrates the recovery of the initial brightness and the changes in brightness stability. All three oxidants destroy the quinone and brightness increases to the initial level. Notable differences result for the brightness stability. While chlorine dioxide and peracetic acid simultane¬ously lower the post color number and lift brightness, the treatment with ozone results in a very high, however, very unstable brightness. Obviously ozonation destroys the quinone compound but generates reaction products very prone to color formation. However, reversion improves significantly once an additional peroxide stage is applied. Following the D or Paa stage peroxide bleaching dominantly improves brightness and further lowers the post color number. After an ozone application a peroxide treatment not only pushes brightness up to the highest level, it also yields the by far best stability. Thus the most favorable process for additional brightness with very good stabilization is the combination of Z and P.
7. Conditions for effective brightness stabilization
A final treatment of pulp with ozone was already recommended . As seen above, treating fully bleached pulp with ozone indeed results in a nice brightening effect. Unfortunately, stability of this brightness is very poor. In humid aging most of the brightness gained is lost rapidly. An additional treat-ment is required to stabilize brightness. Figure 5 has the solution. These trials were conducted with a pulp bleached to 88.7 %ISO with a D1 stage. A small amount of ozone (1 kg/t) lifts this brightness to more than 91 %ISO. Doubling the ozone charge gives 92 %ISO. Again, stability is very poor. Bleaching lifts brightness by 3 points, humid reversion decreases it by 5 points. On the other hand, these losses are more than adjusted by an extraction and bleaching step using alkaline peroxide. A treatment with 2 kg/t of ozone yields the same brightness as the treatment with 1 kg ozone plus 2.5 kg/t H2O2, however , there is a significant difference in reversion losses. The combination ZP cuts the losses by more than half. Thus the treatment with peroxide perfects the stabilization process.
Figure 5: Impact of ozone addition on brightness and brightness stability in comparison to an ozone/peroxide treatment. Pulp pre bleached with the stages D0EpD1, final peroxide charge constant at 2 kg/t
Figure 6: Impact of post treating a hotD0EopD1D2 bleached pulp with Z/P, ozone and hydrogen peroxide, (ozonation at room temperature, constant charges of 1 kg/t H2O2 and 4 kg/t NaOH, 75 °C, 0.5 h, 10% cons.)
The additional treatment with ozone and peroxide could be added to an existing sequence. It improves brightness and especially its stability. Trials conducted with hotD0EopD1D2 bleached pulp already started at the very high brightness of 92.5 %ISO. As usual after a final D stage this pulp had a moderate stability, following humid reversion post color number was only 0.287. The treatment with small amounts of ozone, 0.5 kg/t to 1.5 kg/t, and directly added alkaline peroxide (constant 1 kg/t) lifted the brightness above the 94 %ISO level. Figure 6 has these data. The post color numbers become extremely low. Standard pulp nowadays has a brightness of >90 %ISO and loses about 2 points of brightness in reversion. There are some grades available, like photo paper pulp, with a brightness of 92 %ISO. A pulp with a brightness of 94 %ISO and losses of about one point in humid reversion – or a post color number below 0.2 – in fact describes a new class of pulp.
These results look very promising. Unfortunately, they are based on conditions not suitable for mill scale. Laboratory ozonation was conducted at ambient temperature. Figure 7 compares post bleaching with ozone addition at ambient temperature and at 75°C. Pre bleaching used the sequence DD0EopD1P. P brightness was only 89.2 %ISO. P stage brightness stability was already high (post color # improved from 0.529 after D1 to 0.194 after P). The graph shows a positive impact of the high temperature Z stage on final brightness. However, it has a lower stability in humid aging. An analysis of another pulp's viscosity (Figure 8) shows a decrease with higher temperature. Obviously, the higher temperature favors the formation of poorly stable oxidation products, which under aging conditions readily convert into chromophores. Reaction selectivity decreases with higher temperature.
Figure 7: Impact of ozonation temperature in Z/P post bleaching. Ozonation either at ambient tempera-ture or at 75°C, P stage without intermediate washing constant with 1 kg/t H 2O2, 4.5 kg/t NaOH, 75°C, 0.5 h
Figure 8: Impact of temperature in Z on final brightness and viscosity after Z/P (ozone charge at 1 kg/t, H2O2 at 1 kg/t, 3 kg/t NaOH, 75°C, 10% cons., 0.5 h)
Figure 9: Impact of time and temperature in the final P stage on brightness; pulp pre bleached with the sequence D0EpD1 to 88.6 %ISO brightness; ozone charge at 1 kg/t, 2 kg/t H2O2 added without inter-mediate washing, constant 4 kg/t NaOH
The remaining question is the "best" temperature in peroxide bleaching. As temperature and time are interrelated, in principle low temperature can be compensated by an extended reaction time. Figures 9 and 10 describe the impact of P stage conditions on brightness and reversion. The data show a minor effect of temperature. There is a very small positive contribution of high temperature to brightness development, however differences are below half a brightness point. The impact of higher temperature on the extraction and removal of potential chromophores is more pronounced. The recommendation therefore is to operate the peroxide stage at ≥75°C to allow about one hour retention time. In case time is available (~2 h) 60°C to 70°C should be sufficient.
Figure 10: Impact of time and temperature in the final P stage on brightness stability (humid reversion, E.4P)
Figure 11: Pulp and water flow in a Z/P post treatment sequence with low water and steam demand
Consequently an implementation of ozone and hydrogen peroxide as brightness stability boosters requires more than just a mixer and a pipe for ozone addition ahead of the P stage. It is important to keep ozonation temperature low, ideally ≤ 50°C. This would require a cooling down – heating up approach for both stages. The cooling water and steam demand would result in a rather inefficient, unattractive process. A simple solution describes Figure 11. Modern bleach plants use wash presses. These require much less washing water for an effective displacement, and can lower the pulp's temperature effectively with relatively moderate water input. Their additional advantage is the discharge of the pulp at high consistency. Following a wash press ozone addition can take place without problems at lower temperature and under high consistency conditions. A medium consistency P stage can be added directly without washing. Steam is required to increase the temperature. Nevertheless, steam demand is affected by the consistency in the P tower, which can be at the high end of medium consistency. A partial recycle of hot displaced water from the wash press after the P stage can decrease the steam demand. The partial closure of this water loop is possible, as very little additional organic material dissolves.
The compounds "surviving" a normal ECF bleaching sequence are destroyed with the application of high temperature in the D1 stage and a subsequent cleavage of still existing quinones with alkaline hydrogen peroxide. The implementation of a very moderate amount of ozone into a "normal" ECF bleaching sequence between the D1 stage and the final P stage is an ideal option for the production of extremely bright pulp with a similarly extreme stability. The "surviving" chromophores most likely originate from oxidized carbohydrates.
• The CRI method for chromophore isolation from cellulosic material was applied successfully for the identification of chromophores present in bleached and subsequently aged pulps.
• In fully bleached pulp resonance stabilized quinones are the main chromophores surviving ECF bleaching with chlorine dioxide and hydrogen peroxide.
• With ozone their destruction is simple and requires very little chemical.
• In a 4½ stage sequence (hotD0EopD1Z/P) the final Z/P treatment allows up to 94% ISO brightness with unprecedented stability.
 Rosenau, T., Potthast, A., Kosma, P., Suess, H. U., Nimmerfroh, N.; First Isolation and Identification of Residual Chromophores from Aged Bleached Pulp Samples; Holzforschung 61 656-661 (2007)
 Adorjan, I., Potthast, A., Rosenau, T., Sixta, H., Kosma, P. (2004) Discoloration of cellulose solutions in N-methylmorpholine-N-oxide (Lyocell). Part 1: Studies on model compounds and pulps. Cellulose 12(1):51-57.
 Rosenau, T., Potthast, A., Milacher, W., Adorjan, I., Hofinger, A., Kosma, P. (2004) Discoloration of cellulose solutions in N-methylmorpholine-N-oxide (Lyocell). Part 2: Isolation and Identification of Chromophores. Cellulose 12(2):197-208.
 Rosenau, T., Adorjan, I., Potthast, A., Kosma, P. (2005) Isolation and identification of residual chromomophores in cellulosic materials. Macromol. Symp. 223(1):239-252.
 Rosenau, T., Potthast, A., Milacher, W., Hofinger, A., Kosma, P. (2004) Isolation and identification of residual chromophores in cellulosic materials. Polymer 45(19):6437-6443.
 Jääskeläinen, A.-S., Saariaho, A.-M., Matousek, P., Parker, A., Towrie, M., Vuorinen, T.; Characteriza-tion of residual lignin structures by UV Raman Spectroscopy and the possibilities of Raman spectros-copy in the visible region with Kerr-gated fluorescence rejection; 2003 ISWPC, Madison, WI, proceed-ings 139 - 142
 Popoff, T., Theander, O. (1972) Formation of aromatic compounds from carbohydrates. Part 1. Carbo¬hydr. Res. 22:135-149.
 Popoff, T., Theander, O. (1976) Formation of aromatic compounds from carbohydrates. Part 3. Acta Chem. Scand. B 30:397-402.
 Popoff, T., Theander, O. (1976) Formation of aromatic compounds from carbohydrates. Part 4. Acta Chem. Scand. B 30:705-710.
 Popoff, T., Theander, O., Westerlund, E. (1978) Formation of aromatic compounds from carbohydrates. Part 6. Acta Chem. Scand. B 32:1-7.
 Olsson, K., Pernemalm, P.A., Theander, O. (1978) Formation of aromatic compounds from carbohy-drates. Part 7. Acta Chem. Scand. B 32:249-256.
 Theander, O., Nelson, D.A. (1978) Aqueous, high-temperature transformation of carbohydrates rela-tive to utilization of biomass. Adv. Carbohydr. Chem. Biochem. 46:273-326.
 Theander, O., Westerlund, E. (1980) Formation of aromatic compounds from carbohydrates. Part 8.Acta Chem. Scand. B 34:701-705.
 Lagercrantz, C. (1964) Formation of stable radicals in alkaline solution of some monosaccharides. Acta Chem. Scand. 18(5):1321-1324.
 Suess, H.U., Leporini, C.; How to improve brightness stability of Eucalyptus Kraft pulp. Proceedings ABTCP 2003, 36° Congresso Internacional de Celulose e Papel, São Paulo, Brazil
 Chirat, Ch., Lachenal, D.; Other ways to use ozone in a bleaching sequence; Tappi J. 80 (9) 209-214 (1997)
 Nimmerfroh, N.; Suess, H. U., Hafner, V., Überlegungen zum grosstechnischen Einsatz von Ozon zur Zellstoffbleiche; Wochenbl. f. Papierfab. 120 (21) 860 - 868 (1992)
Thomas Rosenau*, Antje Potthast*, Paul Kosma*; Hans U. Suess**, Norbert Nimmerfroh**
*University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria,
**EVONIK Degussa, Hanau, Germany
|[Home] [Journal papers]|