Introduction
A main target of bleaching is to increase the brightness of the pulp. Although the brownish color of unbleached chemical pulps originates mainly from lignin, other chemical components may contribute to the stability of the color of the brightened pulps during transportation or storage through the combined effect of humidity and heat over longer periods of time (Cardona-Barrau et al.
2003). High contents of the uncolored xylan-bound hexenuronic acid (HexA), especially, can potentially lead to yellowing of bleached kraft pulps (Costa and Colodette
2007). Therefore, achieving a high brightness and brightness stability, characteristic for market pulps (Jullander and Brune
1957), translates into effective removal of the residual lignin and HexA from the pulp.
Chemical pulp bleaching is a multistep process, which consists of sequential treatments with oxidants like chlorine dioxide (ClO
2), ozone (O
3) and hydrogen peroxide (H
2O
2), in combination with acids and alkali. It is a common practice to begin the bleaching with ClO
2 (D stage) which, oxidizes specifically phenolic lignin structures and forms simultaneously hypochlorous (HOCl) and chlorous acids (HClO
2) (Tarvo et al.
2010). HOCl is electrophilic in an acidic solution because of the equilibrated generation of chlorine molecule and cation that leads to oxidize both phenolic and non-phenolic lignin structures and HexA. Part of the reaction products are chlorinated. HOCl can also oxidize secondary and primary alcohol groups after oxidation of these groups to a carboxylic acid in cellulose and hemicelluloses into keto and aldehyde groups, respectively (Zhou et al.
2011). On the other hand, HClO
2 may convert the aldehydes further into carboxylic acid groups.
As a strong electrophile, O3 oxidizes both lignin and HexA when applied in pulp bleaching (Z stage). The somewhat low selectivity of Z stage, resulting from the in situ formed reactive oxygen species, especially hydroxyl radical, has limited its wider industrial application. In Z stage, the typical side reactions include formation of carbonyl groups (aldehydes and ketones) and further oxidation product, a carboxylic acid, especially in an aqueous solution in hemicelluloses and cellulose and its subsequent depolymerization.
H
2O
2 is a nucleophile that is commonly applied for increasing the brightness of pulps especially in the end of bleaching sequences. H
2O
2 reacts specifically with unconjugated and conjugated aldehydes and ketones, such as hydroxyquinones formed in other bleaching sequences (Kuitunen et al.
2011). In peroxide bleaching (P stage) traces of transition metals may catalyze the decomposition of H
2O
2 and formation of hydroxyl radicals that may lead to oxidation and depolymerization of hemicelluloses and cellulose similar to Z stage. (Zeronian and Inglesby
1995).
The presence of carbonyl groups in cellulose leads ultimately to its partial depolymerization and deterioration of the strength (Beyer et al.
2006). In addition, the carbonyl groups in cellulose and hemicelluloses are often mainly responsible for the brightness reversion of fully bleached chemical pulps (Röhrling et al.
2002; Sevastyanova et al.
2006; Jullander and Brune
1957; Lewis and Epstein
1962; Zhou et al.
2011; Perrin et al.
2014). The structure and origin of the carbohydrate derived chromophores have been studied in details (Sevastyanova et al.
2006; Forsskahl et al.
2009; Theander and Nelson
1988; Potthast et al.
2005). Some chromophores may also form during the bleaching process and contribute to so-called brightness ceiling that makes it difficult to achieve the highest brightness (Knill and Kennedy
2003; Vikkula et al.
2006). However, the pulp chromophores, such as hydroxyquinones, are difficult to characterize due to their presence in trace amounts (Schedl et al.
2017).
Moving from the use elemental chlorine and hypochlorite as the main bleaching chemicals to chlorine dioxide based elemental chlorine free (ECF) bleaching technology has dramatically reduced the emission of toxic organochlorine substances (Juuti et al.
1996). However, ECF bleaching does not totally avoid the formation of organochlorine compounds because HOCl is always formed as an intermediate in the reactions of ClO
2 with lignin (Gunnarsson and Ljunggren
1996; Lehtimaa et al.
2010). The residual organochlorine in pulps (OX) is a concern in certain end uses, such as food packaging, of the pulp. In addition, OX may lower the brightness stability of bleached pulps (Owens et al.
1994). Regular ECF pulps have OX content of 100–200 ppm (g/odt) while so-called ECF-light pulps contain less than 30 ppm OX.
Recently, in searching ways to control the reactions of the in situ formed HOCl we found that certain tertiary amines (R
3N), such as 1,4-diazabicyclo[2.2.2]octane (DABCO), efficiently catalyzed its reactions with HexA and lignin over a wide range of pH (Chenna et al.
2013). The reactive species of the catalytic system is a chloroammonium cation (R
3N
+–Cl) that is a non-nucleophile (unlike HOCl) and a much stronger electrophile than HOCl (Chenna et al.
2016). Combining the initial catalytic treatment (H
cat stage) with subsequent treatments with ozone and alkaline peroxide led to very low residual lignin and HexA contents and high brightness (Afsahi et al.
2015). Surprisingly, the viscosity of the pulp remained at a high level proposing that cellulose remained almost unreacted during the whole bleaching sequence (H
cat–Z–P).
In this study, we wanted to confirm the observation on the selectivity studying the development of carbonyl group content, molar mass distribution and OX during the course of the sequence. Additionally, the fully bleached pulps were analyzed for their brightness stability and characterized with time-gated Raman spectroscopy (Kostamovaara et al.
2013; Rojalin et al.
2016) to possibly get some understanding on the residual chromophore structures.
Conclusions
The Hcat–Z–P bleaching removes the residual lignin and HexA from hardwood kraft pulps very selectively. In the catalytic stage cellulose remains almost unattacked while the subsequent ozone stage lowers moderately DP of cellulose and introduces some carbonyl groups in the pulp. The peroxide treatment reacts partly with the carbonyl groups without decreasing further DP of cellulose. In spite of their low lignin, HexA and carbonyl group contents, the Hcat–Z–P bleached pulps undergo thermal yellowing during the artificial aging test. The discoloration is accompanied by formation of highly conjugated unsaturated organic structures. The relatively high OX level of the Hcat–Z–P bleached pulps could possibly explain their aging tendency. However, further research is required to reveal the real origin of the phenomenon.
The possibility to reach full brightness with low chemical dosage and short bleaching time without degrading cellulose makes the Hcat–Z–P sequence attractive for bleaching of hardwood kraft pulps although means to control the OX level must be studied and developed.