CELLULOSE PROTECTORS FOR IMPROVING OZONE BLEACHING - REVIEW

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1 WOOD RESEARCH 49 (4): 2004 CELLULOSE PROTECTORS FOR IMPROVING OZONE BLEACHING - REVIEW MLCHAL JABLONSKY, MILAN VRSKA, SVETOZAR KATUSCAK FACULTY OF CHEMICAL AND FOOD TECHNOLOGY, DEPARTMENT OF CHEMICAL TECHNOLOGY OF WOOD, PULP AND PAPER, SLOVAK REPUBLIC ABSTRACT Cellulose protectors (CPs) are able to eliminate an influence of degradating processes in ozone bleaching. Published literature reviews on using ozone in bleaching of pulps, issued till the year 1992, have not contained any systematic classification of cellulose protectors by chemical groups. It has not dealt with evaluation of the effect of applied additives on the change of viscosity, kappa number and brightness either. In this work, we present a survey of the additives applied in ozone bleaching from the year 1963 to Used CPs were systematically classified to 7 main chemical groups. The influence of used additives is evaluated on the basis of the change of viscosity, kappa number and the brightness compared to the ozone bleaching without additives as for their positive and negative effect. The group of carboxylic acids can be ranked among the most effective additives, as regards the aspect of viscosity, elimination of lignin and brightness. Inorganic additives that were assessed usually have no positive influence on the protection of cellulose against degradation. KEY WORDS: ozone bleaching, cellulose protector, degradation, additive, hydroxyl radical INTRODUCTION The use of ozone as a bleaching agent for chemical pulps has been studied extensively in the 70th. Ozone has been used either or with other bleaching agents in a multistage process, primarily to replace delignification stages that use elementary chlorine. Ozone is a strong oxidating agent that reacts with almost any organic material, including lignocellulosic material. Ozone reactions are thought to be selective toward lignin. Ozone plays a key role in the development of closed bleaching processes. The negative influence of ozone in pulp bleaching lies in its degradative effect on cellulose. A decrease of polymerizing degree can be avoided by the use of additives that work as cellulose protectors (Liebergott and Lierop 1978, Medwick et al. 1992). The mentioned authors published a thorough review in 1978, which discussed the effects of pulp consistency, ph, time and temperature on the ozonization of hardwood and softwood kraft and kraft-oxygen pulps in great detail. Additional publications by Liebergott et al. (1992a, 1992b) and Medwick et al. (1992) from 1992 were literature reviews which covered: studies on ozone bleaching according to pulp type, the effect of main reaction variables, studies on carbohydrate/preserving additives and pretreatments, studies of bleaching sequences containing ozone, and pilot plant studies. They may 71 Drevo 4_2004 O41123.indd 71 («) 04/11/23 1:16:11 PM

2 be seen as a survey of cellulose protectors for improving ozone bleaching. They also introduce changes in viscosity, degree of delignification (kappa number) and brightness. Ozone as an individual oxidative reagent when compared with polysaccharides reacts 105 times faster with substances of the lignin type. The reason for degradation of a number of reactions is formation of hydroxyl (free) radicals (Ragnar et al. 1997, Zhang et al. 2000, Ragnar 2000). These radicals are the results of reactions with lignin. The cause of the radical formation in ozone bleaching has been attributed to the self-decomposition of ozone (Zhang 1994). This reaction is slow in acidic media. However, the selectivity of the reaction of hydroxyl radical with carbohydrate of lignin type, expressed as a ratio of rate constants kl / kc of radicals, is less than 5-6 (Hoigne and Bader 1983a and 1983b, Ek et al. 1989, Solinas et al. 1994, Zhang 1994, Bouchard et al. 1995, Ragnar 2000). The radicals formed in reactions between ozone and lignin or in the decomposition of ozone in water promote the unwanted attack of ozone on carbohydrates, which may cause an unacceptable decline in strength properties. The purpose of many studies, listed above, is to identify conditions that minimize the ozone-carbohydrate reactions. The aim of this work is the systematic classification of used additives by the chemical groups and evaluation of the influence of CPs in ozone bleaching. In this work, we evaluate present the additives applied in ozone bleaching from the year 1963 to Mechanisms of cellulose protection Contribution concerns of the use of cellulose protectors in ozone bleaching, usually mention various mechanisms for the protection of cellulose against degrading reactions. Many authors explain the protective influence of used additives just as a hypothesis of possible mechanisms for the protection of cellulose, or an effect in the process of pulp ozonization. Mechanisms of cellulose protectors effects are as follows: 1. The protective effect on degradation of the cellulose may be interpreted as the scavenging of hydroxyl radicals (Walling and El-Taliwai 1973, Pan 1984, Lachenal and Bokstrom 1986, Kang et al. 1995, Gierer and Zhang 1993, Magara et al. 1994, Cogo et al. 1999, Bouchard et al. 2000). 2. Physical factors such as changed solubility of oxidation agents in the presence of CPs and changes in the pk a of functional groups, primarily in lignin (Lindholm 1987). 3. Selective adsorption - CPs may be adsorbed on cellulose surfaces, thereby acting as a protective barrier for the cellulose. In this way they presumably coat and reduce the surface area of the available cellulose (Cogo et al. 1999, Allan et al. 2000, Van Heiningen and Violette 2003). 4. The formation of compounds between CPs and cellulose which prevent the degradation of cellulose (Katuscak et al. 1971a and 1971b and 1972a, Kamishima et al. 1977a). 5. Decreased accessibility of cellulose to oxidation agents (decreased swelling in the presented CPs in cellulose) (Mbachu and Manley 1981, Kamishima et al. 1982a and 1982b, Bouchard et al. 2000, Roncero et al. 2003a). 6. CPs may lead to a diminution in the extent of the oxidant to the preferred reaction site (Allan et al. 2000). 7. The reduction of carbonyl groups to hydroxyl groups or the removal of carbonyl groups also stabilizes the cellulose chain (Osawa and Schuerch 1973). 8. The removal of metal cations (Soteland 1977, Kamishima et al. 1977b, Lachenal and Bokstrom 1986, Chirat 1993). 72 Drevo 4_2004 O41123.indd 72 -fs64-04/11/23 1:16:15 PM

3 O/one bleaching with additives A lot of articles has been devoted to finding an additive or a pretreatment that would protect the cellulose and make the ozone react more preferentially with the lignin in the fiber. In this investigation, the application of various inorganic and organic additives before and during ozone bleaching in order to improve pulp qualities was studied. These substances are expected to decrease or completely eliminate the degradative reactions that affect the decrease of selectivity and efficiency of ozone bleaching. The main task of additives is to prevent the degradation of polysaccharides. Besides the cellulose protection, they have also studied from the viewpoint of increasing brightness and the amount of eliminated lignin. Some of the additives are able to prevent the degradation of polysaccharides, but on the other side, their application to the system of ozone bleaching lacks the required elimination of lignin or brightness rise, or both. Tables 1 and 2 summarize the results obtained. They present additives which were experimentally applied in ozone bleaching in the period from 1963 to The applied additives are ranked according to chemical groups. None could be identified as preventing degradation of cellulose while promoting the removal of a large portion of the lignin. The influence of used additives is evaluated on the basis of the positive(+) or negative(-) effect on the following main characteristics of pulps: the change of viscosity, the change of lignin content expressed as the change of kappa number and the change of brightness compared to the ozone bleaching without additive use. Used CPs were systematically classified to 7 main chemical groups, namely to alcohols, carboxylic acids, carbonyl compounds, organic compounds with nitrogen, other organic compounds and inorganic compounds. Ozone bleaching with alcohols Alcohol impregnation of pulp before ozone bleaching has been shown to be very effective for improving selectivity (Bouchard et al. 2000). Most of examinated alcohols have a positive effect on removal lignin and at the same time they prevent carbohydrate degradation during ozone bleaching. The most applied alcohols are methanol and ethanol (Fujii et al. 1986, Berg et al. 1995, Griffin and Van Heiningen 1998, Bouchard et al. 2000, Hagglund 2001, Meredith 1980, Kamishima 1982a and 1985a, Solinas et al and 1997). The addition of methanol (80-100% on pulp) showed some chance of commercial application (Kamishima et al. 1977b). There is a continuous research to find a way of improving application of additives to decrease their amount in ozone bleaching stage. One possibility lies in the addition of additives to the ozone gas stream (Bouchard et al. 2000). Other frequently evaluated additives are CPs such as ethylene glycol, isopropanol and tert-butyl alcohol (Meredith 1980, Solinas et al and 1997, Murphy and Norris 1996, Hagglund 2001, Kassebi and Gratzl et al. 1982). Ethylene glycol increases the selectivity during ozone delignification much more than methanol, and its effect is optimal at ph 3 and 35wt% ethylene glycol in the reaction system (Johansson et al. 2000). In the presence of tert-butyl alcohol an improvement, was found in efficiency and selectivity of ozonization (Zhang et al. 1997b, Cogo et al. 1999). According Hoigne and Bader (1983a and 1983b) tert-butyl alcohols inhibits the radical decomposition of ozone by scavenging hydroxyl radicals. Cogo et al. (1999) found that tert-butyl alcohol was not consumed during the ozonization process. 73 Drevo 4_2004 O41123.indd 73 («) 04/11/23 1:16:18 PM

4 Structure according to chemical groups 3 1 Caiboxylic acids Monocarboxylic acids Dicalboxylic acids Dialcohols! 2-(2-ethoxyethoxy)ethanol Tab. 1: Organic compound added to pulp before and in combination with an ozone stage. Cellulose protector viscosity Kappa number brightness Methanol References 6,9,21,27,28,32-35,36, 37,39-42,48,59,62,64, 83,84,89,95,62,83,84,93,102 Ethanol ,83,84 Propanol ,62,83,84 Isopropanol ,62 1-butanol butanol ,28,62,103 tert-butyl alcohol methyl-l -propanol pentanol hexanol heptanol + Ethylene glycol Propanediol ,4-butanediol Glycerol Diethylene glycol Pentaerythritol butoxyethanol (2-butoxyethoxy)ethanol Formic acid + ;- +;- + ; -,60, /9,95 Acetic acid + ; - +;- + 28,32,62,64,83,84 Propionic acid - ; ,4,38,40 n-butyric Acid ,40 /-Butyric acid n-valeric acid /-Valeric acid Oxalic acid + + ;- + 74,75,95 Malonic acid + ; Succinic acid + -; + + ; - Glutaric acid Adipic acid + ; - + -; + Unsaturated Maleic acid + ; - + -; + acids Itaconic acid frt. Ill ag 3,6,9,15,33,,44,49, 50,60,65,68,79,89,95,10 3 1,7,10,11,15,20-22, 33,35,36,38-42,55,56, Glycolic acid + ; Lactic acid + ;- + + Malic acid , Tartaric acid + ; , Glyoxylic acid Pyruvic acid + ; it* Levulinic acid Sfr'S viscosity - positive effect on viscosity, increases viscosity against to ozone bleaching without additive. Kappa number - positive effect on Kappa number, decreases Kappa number against to ozone bleaching without additive. brightness - positive effect on brightness, increases brightness against to ozone bleaching without additive. (+ positive effect, - negative effect) 74 Drevo 4_2004 O41123.indd 74 04/11/23 1:16:21 PM

5 Structure according to chemical groups ", Aromatic acids il af '3 3 '$ Tab. 1: cont. - Organic compound added to pulp before and in combination 'with an ozone stage. Cellulose protector viscosity Kappa number brightness Tetrahydrofuran Ethylene oxide 1,4-Dioxane Trioxane Diethylether 15 Methylethyl ketone - 95 Acetone References 15,87,92 Formaldehyde ,89,95 95 Salicylic acid U.95 Benzoic acid +; - - +;- 35.4U Terephthalic acid ; U p-hydroxybenzoic acid + ;- - - ; U Vanillic acid + ;- - ; + - ; Protocatechuic acid + ;- - - ; + Formamide - p-phenylenediamine - Pyridine - N-cyclohexyl pyrrolidinone Thriethanolamine N-methyltaurine Urea-Methanol Tetramethylurea + + 1,84,85,95 DTPA - ; ,11,69,95 EDTA Mg-EDTA - 15, DMF Nitromethane - - ; + viscosity - positive effect on viscosity, increases viscosity against to ozone bleaching without additive. Kappa number - positive effect on Kappa number, decreases Kappa number against to ozone bleaching without additive. brightness - positive effect on brightness, increases brightness against to ozone bleaching without additive. (+ positive effect, - negative effect) 75 Drevo 4_2004 O41123.indd 75 04/11/23 1:16:23 PM

6 Structure according to chemical groups w O O 1 1 o Carboxylic derivates Derivates of saccharides Tab. 1: cont. - Organic compound added to pulp before and in combination 'with an ozone stage. Cellulose protector viscosity Kappa number brightness References Ethyl acetate Methyl acetate - + Acetic anhydride - Sodium acetate - Mg-Acetate Peracetic acid Sodium formate 26 Galacturonic acid - 33 Glucose + Methyl cellosolve + Dextrin - CMC - Dialdehyde starch Starch - Lignin p-benzoquinone + Hydroquinone - o-s 3 g Antraquinone o Pyrogallol + Diethylenetriaminepentaphoshonic acid 1 DMSO Hexadecyl trimethyl ammonium bromide Nonyl trimethyl Ammonium bromide Dodecyl trimethyl ammonium bromide Benzoyl peroxide 51 + Di(tertbutyl)peroxide 51 + Sulfamic acid Thioglycolic acid Citric acid L-cystine 53 Lecithin Acrylonitrile 53 Butadiene 53 DABCO 26 Polyethylene glycol 26 Cyclohexan 16 - Benzene 16 - NH 2 NH 2 - NH 2 OH.HC1 + viscosity - positive effect on viscosity, increases viscosity against to ozone bleaching without additive. Kappa number - positive effect on Kappa number, decreases Kappa number against to ozone bleaching without additive. brightness - positive effect on brightness, increases brightness against to ozone bleaching without additive. (+ positive effect, - negative effect) Other organic compounds Drevo 4_2004 O41123.indd 76 04/11/23 1:16:25 PM

7 Inorganic compounds : Inorganic compounds compound added to pulp before and in combination with an ozone stage. Structure Cellulose protector viscosity Kappa number brightness Na 2 CO ,95 MgC0 3 - MgS0 4 - MnSO 4 - FeSO 4 - CuSO 4 - Na 2 SO 4 - (NH 4 ) 2 S0 4 - A1 2 (S0 4 ) 3 - NaBH,96 4 -; + KBH, + 96 MgO - NaCl - CoCl 2 - ZnCl 2 - Mn(NO 3 ) 2 - NH^Oj + KI - I 2 - P KH 2 PO 4 - Na 2 B 4 O 7 - Na 2 SiO 3 - (NH 4 ) 6 Mo 7 O 24.4H 2 O NaCIO C1O Na 2 O H H 2 SO H 2 SO H 3 P0 3 1 HjBOj References S other HNO 3 + 6, ,99 C1 2 SO C NO + SW + NO viscosity - positive effect on viscosity, increases viscosity against to ozone bleaching without additive. Kappa number - positive effect on Kappa number, decreases Kappa number against to ozone bleaching without additive. brightness - positive effect on brightness, increases brightness against to ozone bleaching without additive. (+ positive effect, - negative effect) studies 77 Drevo 4_2004 O41123.indd 77 04/11/23 1:16:27 PM

8 Ozone bleaching with carboxylic acids Most of used carboxylic acids shows positive effect on viscosity, the removal of lignin and brightness. According to Kamishima et al. (1982a and 1985a) among twenty-seven organic compounds, the oxalic acid belongs to the most effective and its optimum ph is 2. It provides high yield of bleached pulps and also increases contents of a-cellulose and the pentosans (Kamishima et al. 1983b). Contrary Lidholm (1989) declared that oxalic acid improved the high consistency of ozone bleaching process only slightly. Mbachu and Manley (1981) found that acetic and formic acid pretreatments use less ozone to reach a given kappa number than does a sulfuric acid-treated pulp. The positive effect on the change of viscosity can be seen in these substances from the group of monocarboxylic acids (n-butyric acid, i-butyric acid, n-valeric acid, i-valeric acid) and dicarboxylic acids as oxalic, succinic and glutaric acid (Kamishima 1982a and 1985a). Improvement of selectivity has been attributed to the decreased accessibility of the cellulose to ozone due to a very poor swelling effect of organic acids (Mbachu and Manley 1981, Lachenal and Bokstrom 1986). Mentioned acids also provides possibility of radical scavenging (Tibbling 1993, Zhang et al. 1997b). Ozone bleaching with carbonyl compounds Cogo et al. (1999) found that acetone and formaldehyde (Tibbling 1993) increased selectivity ozone bleaching. The presence of dioxane does not significantly change the ozone reaction efficiency but leads to a dramatic reduction of cellulose degradation (Van Heiningen et al. 1994). Ozone bleaching with aromatic acids Aromatic acids with more amount of ozone added achieve a better brightness but donit avoid degradation of cellulose (Kamishima et al. 1977b and 1982a). Aromatic acids like salicylic acid (Vivero and Blanco 2001), benzoic acid, p-hydroxybenzoic acid, vanillic and protocatechuic acid seem to prevent viscosity loss somewhat (Kamishima et al. 1977b and 1982a). Ozone bleaching with organic compounds with nitrogen Presence of N-methyltaurine derivate at a proper amount of added ozone has favourable effect on pulp yield, its brightness, kappa number, viscosity and strength properties (Rutkowski and Szopinski 1983 and 1984). Tan and Solinas (1996) examined the effect of N-alkylated compounds such as N-cyclohexyl pyrrolidinone and tetramethylurea and proved a significant improvement in viscosity, kappa number reduction and the brightness development. Other compounds like ureamethanol, DMF (Kamishima et al. 1977b and 1982a), EDTA (Chirat 1993, Parthasarathy and Glenn 1995, Vivero and Blanco 2001) and DTPA (Solinas et al and 1997, Vivero and Blanco 2001) were used as effective CPs in ozone bleaching. According to Kamishima et al. (1977b) the formation of clathrate compounds between urea and cellulose prevented the degradation of carbohydrates. The application of a chelant such as DTPA or EDTA before the ozone stage, however, was shown to be best at ph 3 rather then ph 8, suggesting that acid sequestering is best for metal ion removal (Allison 1982). 78 Drevo 4_2004 O41123.indd 78 («) 04/11/23 1:16:28 PM

9 Ozone bleaching with other organic compounds The pulp bleached with ozone and peracetic acid improved the ozone bleaching and strength properties (Rothenberg et al. 1975, Fuhrman et al. 1997, Parthasarathy and Glenn 1995, Rautonen 1997). Peracetic acid reacts with lignin and opens up the pulp fiber structure (Parthasarathy and Glenn 1995). Citric acid (Andersson et al. 1992, Vivero and Blanco 2001) and sulfamic acid (Kamishima et al. 1977a, Vivero and Blanco 2001) has been shown to be effective for improving selectivity and efficiency of ozone bleaching. Organic compounds such as glucose, methyl cellosolve, p-benzoquinone, pyrogallol, hydroxylamine hydrochloride (Kamishima et al. 1977a and 1985a) benzoyl peroxide, di(tert-butyl)peroxide (Liebergott 1973), DMSO (Kamishima et al. 1977a and 1985a, Allison 1985, Lindholm 1987 and 1989, Cogo et al. 1999, Vivero and Blanco 2001) and antraquinone (Rutkowski and Szopinski 1983 and 1984) may be preventing to carbohydrates degradation at ozone bleaching. Eckert et al. (1978) found that in the presence of cationic surfactant process of lignin removal and brightness enhancement may be improved. Ozone bleaching with other inorganic compounds Inorganic compounds that are effective cellulose protectors in oxygen delignification offered no protection during ozone bleaching. Addition of small amounts of metal such as iron, copper and nickel salts increased cellulose degradation (Kamishima et al. 1977a, 1985b). Some chemicals (e.g. magnesium compounds) used as an effective cellulose protector in oxygen delignification are defeated in ozone bleaching. Heavy metals may decompose the ozone, leaving apportion of it unavailable to react with lignin (Soteland 1974). The addition molybdates (Ragnar 2000, Agnemo 2002) and P4 (Wang et al. 1997) to an ozone bleaching stage can markedly reduce the viscosity loss and increase the brightness of the pulp. Inorganic acids like sulfuric acid (among studies), boric acid (Vivero and Blanco 2001), Caro's acid (Zhang et al. 1995), nitric acid (Pan 1984) seemed to have positive effect in ozone bleaching. Combination of ozone and chlorine dioxide may allow reduction of chlorine dioxide amount and improvement of ozone bleaching (Tsai 1990, Chirat 1995, Chirat and Lachenal 1997, Chirat et al. 1997, Millar et al. 2002, Toven 2003). Acidic peroxide treatment at ph 2 E 3 just prior to ozone application also improved the efficiency of an ozone delignification stage (Liebergott et al. 1973, 1992b, Rothenberg et al. 1975). 79 Drevo 4_2004 O41123.indd 79 («) 04/11/23 1:16:32 PM

10 CONCLUSION Base on the presented list of additives, we can conclude that the positive effect on the change of viscosity can be seen in these substances: alcohols in general, from the group of carboxylic acids mainly monocarboxylic acids (n-butyric acid, j'-butyric acid, n-valeric acid, j'-valeric acid) and dicarboxylic acids (oxalic, succinic and glutaric acid). The next group with a positive effect is made up of carbonyl compounds (1,4-dioxan, acetone, formaldehyde) and aromatic acids (salicylic, benzoic, p-hydroxybenzoic, vanillic and protocatechuic acids). Inorganic substances show in general a negative effect on the change of viscosity. As for the aspect of kappa number, alcohols, carboxylic acids, organic compounds with nitrogen (N-cyclohexyl pyrrolidinone, N-methyltaurine, urea-methanol, tetramethylurea, DMF) have the positive effect on its decrease. The whole group of carboxylic acids considerably affects the change of brightness. From the other groups of substances we can mention complex making reagents (EDTA, DTPA) and, for example, sulfuric acid that prevents the catalytic effect of transition metals in the formation of free radicals and decomposition of ozone. With regards for viscosity, elimination of lignin and brightness agents the group of carboxylic acids proved to be the most efficient substances. The problem of finding acheap effective commercial inhibitor of carbohydrate depolymerization during ozone delignification is still actual. Further verification of the additives effect has been continuing within the VEGA project. ACKNOWLEDGEMENTS The authors express their thanks to the VEGA agency for financial support of the project No. 1 / 0061 / Drevo 4_2004 O41123.indd 80 («) 04/11/23 1:16:35 PM

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16 ING. JABLONSKY MLCHAL FACULTY OF CHEMICAL AND FOOD TECHNOLOGY, DEPARTMENT OF CHEMICAL TECHNOLOGY OF WOOD, PULP AND PAPER, PvADLINSKEHO 9, BRATISLAVA, SLOVAK REPUBLIC ING. VRSKA MILAN, CSc. FACULTY OF CHEMICAL AND FOOD TECHNOLOGY, DEPARTMENT OF CHEMICAL TECHNOLOGY OF WOOD, PULP AND PAPER, PvADLINSKEHO 9, BRATISLAVA, SLOVAK PVEPUBLIC Doc. ING. KATUSCAK SVETOZAR, CSc. FACULTY OF CHEMICAL AND FOOD TECHNOLOGY, DEPARTMENT OF CHEMICAL TECHNOLOGY OF WOOD, PULP AND PAPER, PvADLINSKEHO 9, BRATISLAVA, SLOVAK PVEPUBLIC 86 Drevo 4_2004 O41123.indd 86 («) 04/11/23 1:16:45 PM