Partial exchange of ozone by electron beam irradiation shows better viscosity control and less oxidation in cellulose upgrade scenarios

Electron beam irradiation (EBI) is an alternative treatment for intrinsic viscosity (IV) control in cellulose pulps, but has never been integrated in full bleaching sequences for comparison to conventional methods.

Available online 7 April 2021

0144-8617/© 2021 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Partial exchange of ozone by electron beam irradiation shows better

viscosity control and less oxidation in cellulose upgrade scenarios

Oliver P Sarosia,1, Daniela Bammera,1, Elisabeth Fitza,1, Antje Potthastb,*

aKompetenzzentrum Holz GmbH, Altenbergerstraße 69, A-4040, Linz, Austria

bInstitute of Chemistry of Renewable Resources, Department of Chemistry, University of Natural Resources and Life Sciences, Konrad-Lorenz-Straße 24, A-3430, Tulln,

Austria

A R T I C L E I N F O

Keywords:

Kraft pulp

Xylan

Hexenuronic acid

Chemical-free

Bleaching

A B S T R A C T Electron beam irradiation (EBI) is an alternative treatment for intrinsic viscosity (IV) control in cellulose pulps, but has never been integrated in full bleaching sequences for comparison to conventional methods Both euca-lyptus kraft (EK) paper pulp and beech sulfite (BS) dissolving pulp were subjected to totally chlorine free (TCF) bleaching sequences comprising either EBI, ozone, or both for IV control Additionally, effects of EBI on hex-enuronic acid (HexA) and xylan were investigated IV was adjusted to 450–500 mL g− 1 and properties including carbonyl content, kappa, brightness, alkali-resistance and sugar composition were compared Pulps produced with EBI had a higher alkali-resistance, uniformity and less cellulose oxidation However, the degree of bleaching (DoB) was low without the use of ozone HexA content in a birch pulp was halved by EBI Isolated xylans were more resistant to irradiation than cellulose with little decrease of molar masses and moderate oxidation

1 Introduction

With an increasing demand for environmentally friendly textiles, the

market for regenerated cellulose fibers (RCF) is projected to show a 4.2

% compounded annual growth rate, despite the recent events

sur-rounding COVID-19 (GlobeNewswire, 2020) To enter the RCF market,

the upgrade of existing low-value paper pulp mills to high-value

dis-solving pulp mills or swing mill operation has been considered in the

past (Lundberg, Axelsson, Mahmoudkhani, & Berntsson, 2012; Sappi

Limited, 2013)

Converting paper pulp to dissolving pulp requires the removal of

hemicellulose through either acid prehydrolysis before pulping or

alkaline extraction or enzymatic degradation during bleaching (

Hut-terer, Schild, & Potthast, 2016; Hutterer, Kliba, Punz, Fackler, &

Pot-thast, 2017; Sixta, 2006) Dissolving pulp generally requires a high

content of α-cellulose, a brightness above 90%ISO, a narrow molecular

weight distribution (MWD), and low IV Thus, an upgraded paper mill or

swing mill needs a method for IV reduction, which can be achieved by adjusting the process parameters, such as cooking intensity, ozone charge during TCF bleaching or dwell times during steeping and accel-erated aging However, intensified cooking conditions lead to a sub-stantial cellulose yield loss and a conversion of α-cellulose to undesirable alkali-soluble fragments (Agarwal & Gustafson, 1997; Kubes, Fleming, Macleod, Bolker, & Werthemann, 1983) Furthermore, alkaline steeping and pulp pre-aging oxidize the pulp, introducing both carbonyl and carboxyl groups, which are responsible for pulp brightness reversion (Ahn et al., 2019; Kvarnl¨of, S¨oderlund, & Germgård, 2006; Mozdynie-wicz, Nieminen, & Sixta, 2013)

Ozone rapidly decomposes under aqueous conditions during bleaching, yielding various radical and peroxo-species (Staehelin, Buehler, & Hoigne, 1984) Ozone itself and all subsequent radical spe-cies can cleave the glycosidic bond of cellulose, especially if the tem-perature is raised and transition metals such as iron and cobalt are present, favoring the Fenton-type decomposition of ozone (Kishimoto &

Abbreviations: BS, beech sulfite pulp; CCOA, [2-(2-aminooxyethoxy)-ethoxy]-amide; DoB, degree of bleaching; DP, degree of polymerization; DS, degree of

substitution; EBI, electron beam irradiation; ECF, elemental chlorine free; EK, eucalyptus kraft pulp; FDAM, 9H-fluoren-2-yl-diazomethane; IV, intrinsic viscosity; HexA, hexenuronic acid; KX, kraft xylan; Mn, number average molecular weight; Mw, weight average molecular weight; MWD, molecular weight distribution; odtp, oven-dried ton of pulp; SX, sulfite xylan; REG, reducing end group; RCF, regenerated cellulose fiber; TCF, totally chlorine-free

* Corresponding author at: Muthgasse 18, Department Chemie, A-1190, Vienna, Austria

E-mail addresses: o.sarosi@wood-kplus.at (O.P Sarosi), d.bammer@wood-kplus.at (D Bammer), e.fitz@wood-kplus.at (E Fitz), antje.potthast@boku.ac.at

(A Potthast)

1 Werkstraße 2, A-4860, Lenzing

Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

https://doi.org/10.1016/j.carbpol.2021.118037

Received 8 February 2021; Received in revised form 15 March 2021; Accepted 1 April 2021

Nakatsubo, 1998; Ni, Kang, & van Heiningen, 1996; Tripathi, Bhardwaj,

& Ghatak, 2018) Initially, the ozone treatment shows a higher reaction

rate towards residual lignin and after its depletion, cellulose degradation

is promoted, offering dose-dependent IV control (Kang, Zhang, Ni, & van

Heiningen, 1995; Lachenal, Mishra, & Chirat, 2013) However, an

increased rate of cellulose degradation is accompanied by a similar rate

of undesirable carbonyl group formation due to the low selectivity of

ozone and its decomposition products (Pouyet, Chirat, Potthast, &

Lachenal, 2014; Zhang, Ni, & van Heiningen, 2000) Since ozone has

such a rapid reaction rate, its diffusion into the pulp and thus cellulose

degradation is limited to amorphous and crystalline surface regions,

leading to broadening of the MWD and, in extreme cases, to a bimodal

distribution of cellulose due to the unequal treatment of different

cel-lulose fractions (Berggren, Berthold, Sj¨oholm, & Lindstr¨om, 2001)

In contrast, γ-irradiation and EBI homogeneously penetrate the pulp

on a macroscopic and microscopic level, leading to ionization in both

amorphous and crystalline regions (Hammer, Christensen, Conroy, King,

& Pogue, 2011; Yang, Zhang, Wei, Shao, & Hu, 2010) EBI causes radical

formation throughout the cellulose molecule, which leads to cleavage of

the glycosidic bond at statistically distributed positions along the

cel-lulose chain (Burkart, 1999) Not only can the IV be adjusted to a

desirable level through a dose-effect relationship, but also the dispersity

(Ð) is reduced (Sarosi, Bischof, & Potthast, 2020; Sixta, Andrea, & Kraft,

2007) Additionally, EBI treatments of cellulosic pulps have a high yield

while avoiding degradation to the monomer level and a statistical

preference for longer cellulose chains, retaining high contents of α

-cel-lulose at irradiation intensities of 20 kGy or below (Sarosi et al., 2020;

Yang et al., 2010) EBI has been reported to have oxidative effects,

leading to the formation of carbonyl groups on the cellulose backbone at

elevated irradiation levels of 50 kGy and above (Henniges, Hasani,

Potthast, Westman, & Rosenau, 2013) However, by differentiation

be-tween newly formed reducing end groups (REGs) and keto groups from

oxidation by the CCOA method (R¨ohrling, Potthast, Rosenau, Lange,

Ebner et al., 2002), it has been shown that at low irradiation levels of 20

kGy or below, the oxidation by EBI is lower than the detection limit,

refuting its long believed highly-oxidative nature (Henniges et al.,

2013) In a previous study, the protective effect of residual pulp

com-ponents, such as hemicellulose and lignin, towards chain scission and

oxidation were investigated (Sarosi et al., 2020) Hemicellulose was

found to protect cellulose from chain scission and backbone oxidation,

which was attributed to its sacrificial function as a protective barrier,

shielding cellulose from radicals from the surrounding water While for

lignosulfonates the antioxidant properties of residual lignin had a

beneficial effect, lignin in kraft pulps caused backbone oxidation

Another important species to observe is HexA It is formed from

4-O-methylglucuronic acid during alkaline kraft pulping and causes

additional consumption of bleaching chemicals and brightness

insta-bility when not removed (Jiang, Lierop, & Robson, 2000; Rosenau et al.,

2017) HexAs are hydrolyzed during hot acid and ozone stages, and

while their role in pulping and bleaching has been sufficiently studied

(Antes & Joutsimo, 2015; Brogdon, 2009; Gomes, Longue, Colodette, &

rarely been investigated (Tsuji-Katsukawa, Miyawaki, & Koyanagi,

2012)

The goal of this study was to highlight benefits and drawbacks of IV

control by EBI when using it as an integral part of TCF bleaching

se-quences as a full or partial replacement of ozone Experimental focus

was put on the controlled reduction of IV to provide a chemical-free and

more homogeneous and more flexible alternative to ozone for cellulose

depolymerization in paper pulp upgrade applications The final pulp

properties of a eucalyptus (Eucalyptus globulus) kraft paper and a beech

(Fagus sylvatica) sulfite dissolving pulp were compared after using EBI

and/or ozone for IV adjustment Further experiments were performed to

elucidate the impact of EBI on isolated xylan and HexAs Using EBI as a

partial or full replacement of ozone may facilitate IV control especially

in high-Mw kraft pulps, which allows for process debottlenecking and

chemical savings during bleaching

2 Experimental

2.1 Pulps

Two different pulps were used for this study Both oxygen-bleached

eucalyptus (Eucalyptus globulus) kraft paper pulp (EK) and oxygen- bleached beech (Fagus sylvatica) sulfite dissolving pulp (BS) were

generously contributed from an industry partner as research samples Both pulps were never-dried and lab-washed In between use, all pulps were stored at − 20 ◦C After each bleaching stage, the IV was measured

to assure correct progression HexA-rich birch kraft pulp was generously contributed by the University of Natural Resources and Life Sciences (Vienna, Austria)

2.2 Xylan

Beech sulfite xylan was isolated from steeping lye by acidic precip-itation Eucalyptus kraft xylan was prepared by cold caustic extraction and acidic precipitation of a fully bleached paper pulp 300 g of air-dried pulp were subjected to CCE by swirling at 2.5 % consistency for 30 min

at 25 ◦C, using 10 % (w/v) NaOH lye The dissolved xylan was separated

by filtration and regenerated by acidification to pH 2.5 using 20 % H2SO4 The sediment xylan was purified by 4 repeated cycles of centrifugation and stirring in fresh softened water Finally, the xylan was dried at 60 ◦C over-night, which resulted in a yield of 22 g xylan

2.3 Bleaching stages 2.3.1 Irradiation

EBI was conducted at room temperature as described in a previous study (Sarosi et al., 2020) Irradiation was performed at Mediscan GmbH (Kremsmünster, Austria) using a Rhodotron TT100-IBA-X electron accelerator according to EN ISO 13485 and ISO 11137 Pulps were prepared for irradiation by forming pulp sheets with a thickness of 2− 3

cm, a diameter of 20 cm and a moisture content of 72–75 % The pulp sheets were attached to cardboard panels and positioned in vertical irradiation trays To guarantee homogeneous irradiation, total pulp thickness was kept below 4 cm The irradiation dose was set to (and verified by a dosimeter as) either 1.25 kGy (1.30 kGy) or 2.5 kGy (2.52 kGy) for the beech sulfite pulp and 5.0 kGy (5.2 kGy) or 10.0 kGy (10.3 kGy) for the EK pulp Irradiated pulps were swirled in hot water at 5 % consistency for 5 min and then filtered For irradiation of HexA-rich birch kraft pulp and xylan, ~1.5 g of dry sample was filled into micro-reaction tubes (PP) and attached to cardboard sheets before passing through the electron beam Additional irradiation levels were set to (and verified as) 50.0 kGy (50.2 kGy), 100 kGy (101.3 kGy) or 200 kGy (202.0 kGy) Doses above 50 kGy were applied by multiple passes at reduced doses on alternating sides

2.3.2 A-stage

EK pulps received a sulfuric acid stage for metal removal before ozone or peroxide treatment For hot acid treatments, preheated pulp and softened water were mixed in plastic bottles (PP) at 3.0 % consis-tency and the pH was slowly adjusted to 2.5 using sulfuric acid (100 g

L− 1) The bottles were sealed, mixed thoroughly by vigorous shaking and incubated for 30 min at 60 ◦C (A) or 90 ◦C (A-hot), depending on whether or not an IV reduction, respectively, was desired, with repeated shaking after 15 min The reaction was terminated by vacuum filtration

of the bottle contents over a quartz frit and washing of the pulp 4–6 times with double the pulp volume of hot, softened water and vacuum suction between washes

2.3.3 Z-Stage

Ozone bleaching was conducted in a medium-consistency (MC)

mixer with closed lid and thorough pulp fluidization The treatment and

subsequent pulp workup were performed by trained lab workers The

treatment conditions were the following: 4.0–16.9 kg odtp− 1 (kilogram

per oven-dried ton of pulp) of ozone charge, depending on the type of

pulp and treatment route, pH 2.5, 45 ◦C, 10 % consistency The total

ozone charge for each pulp was split into smaller partial charges and IV

was measured in-between charges while the pulp remained in the

reactor Once the IV was adjusted to a satisfactory level, the pulp was

filtered and washed 4–6 times with double the pulp volume of hot,

softened water Actual ozone consumption was determined by streaming

the gaseous phase before and after bleaching through a potassium iodide

solution and titrating against sodium thiosulfate using starch as

indicator

2.3.4 P-stage

Every sequence was terminated by a strong hydrogen peroxide stage

with constant charges to both increase the DoB and stabilize the IV by

β-elimination of carbonyl “hot-spots” on the cellulose backbone (Yang,

2016) Hydrogen peroxide bleaching was conducted in plastic bottles

(PP) The treatment conditions were the following: 4 kg odtp− 1

hydrogen peroxide, 6 kg odtp− 1 sodium hydroxide, 10 % consistency, 90

◦C, 180 min with thorough shaking every 30 min Both pulp and

soft-ened water were pre-heated and the calibrated amounts of hydrogen

peroxide (~35 %, exact content was measured before use) and sodium

hydroxide stock solution (50 g L− 1) were added to the water just before

mixing The bottle was sealed, shaken vigorously and incubated at 90 ◦C

for 180 min with additionally shaking every 30 min The reaction was

terminated by vacuum filtration of the bottle contents over a quartz frit

and washing of the pulp between 4–6 times with double the pulp volume

of hot, softened water Contact with atmospheric oxygen at high

alka-linity was minimized by working quickly and keeping the pulp covered

during workup Residual hydrogen peroxide and alkalinity were

deter-mined by titration as described above or against 0.1 M HCl, respectively

In cases where the peroxide was not completely consumed due to low

pH, the bleaching stage was repeated, employing the calculated residual

hydrogen peroxide charge After hydrogen peroxide treatment, the

bleaching sequence was terminated by swirling the pulp at 5 %

consis-tency for 10 min in hot water and adjusting the pH to below 3.0 using

SO2-infused water with subsequent washing

2.4 Pulp analysis

Kappa number is measured according to TAPPI T236cm-85 where

pulp chromophores are reacted with a fixed amount of KMnO4 solution

and its consumption is evaluated by neutralization with KI followed by

titration against Na2S2O3 Pulp, brightness, is analyzed according to ISO

2470-1:2009 where the reflectiveness of uniform pulp sheets is

measured optically and IV was were measured according to TAPPI

T236cm-85, ISO 2470-1:2009 and ISO 5351 5351, respectively, which

uses rheological data from pulp solutions in cupri-ethylenediamine and

a flow-through viscometer Alkali-resistant fractions R10 and R18 were

determined according to DIN 54355, which analyzes the pulp solubility

in 10 % or 18 % NaOH solution, respectively Cellulose crystallinity was

measured by FT-Raman spectroscopy with reference data from X-ray

wide angle scattering (R¨oder et al., 2006)

The sugar composition of pulps and aqueous samples was analyzed

by total hydrolysis and HPLC as previously described (Sarosi et al.,

2020)

Measuring the MWD was coupled to fluorescence labeling of

carbonyl or carboxyl groups, as described in the CCOA or FDAM method,

respectively (Bohrn et al., 2006; R¨ohrling, Potthast, Rosenau, Lange,

Borgards et al., 2002) Instrumentation, settings and data evaluation was

performed as previously described (Sarosi et al., 2020) For fluorescence

detection of FDAM-labelled samples, a different fluorescence detector,

RF 535 (Shimadzu, Japan), was used at λex 280 nm and λem 312 nm

As a rough estimation, the number of REGs can be calculated from

GPC data according to:

M n

where Mn is the number average molecular weight

HexA analysis relied on the available TAPPI standard using spec-troscopic quantification after hydrolysis (Chai, Zhu, & Li, 2001; TAPPI,

2007)

3 Results & discussion

3.1 Bleaching sequence conduct and observations

The goal of the adjustment was to achieve IV between 450–500 mL

g− 1 (degree of polymerization (DP) 1045–1205), which is suitable for RCF processes To investigate the differences between EBI and ozone during a full bleaching sequence, three approaches were chosen for each pulp Table 2 summarizes the different sequences for each pulp Oxygen- delignified pulps were used as the starting point for all experiments The first variant used EBI as a full replacement of ozone Electron doses were calculated based on dose-effect fit functions from previous experiments with similar pulps (Sarosi et al., 2020) The second approach used both EBI and ozone in lower doses to represent the integration of EBI in an existing bleaching sequence and to highlight potential chemical savings and quality improvements Finally, the third sequence applied a con-ventional sequence based on ozone as reference Xylan removal from the paper pulps can be performed by cold caustic extraction and enzymatic treatments after TCF bleaching, but was omitted in this study to avoid the interference of effects such as additional IV reduction, changes in Ð,

or alteration of the chemical composition (Duan, Verma, Li, Ma, & Ni,

2016; Hutterer et al., 2016)

Fig 1 shows the IV decrease caused by each stage EBI of the kraft pulp delivered values within the calculated target range A dose of 10 kGy halved the IV, while the 5 kGy treatment achieved 87 % of that IV reduction This is in accordance with previous studies, since EBI is known to randomly cleave the cellulose chain, which is expressed by a linear increase of the number of chain scissions and an exponential decrease of IV and weight average molecular weight (Mw) (Chen, Ma, Li, Miao, & Huang, 2017; Henniges, Okubayashi, Rosenau, & Potthast,

2012) Since BS had a lower initial IV, a smaller irradiation dose was applied However, the sulfite pulp showed a similar IV decrease by both 1.25 and 2.5 kGy, despite precise dose control This may be ascribed to the high sensitivity of Mw at the initial, low dose range

In some cases, the IV was slightly increased after hot acid (A) or the first ozone (Z) stage One explanation may lie in the hydrolysis and removal of short chains, thus increasing the median chain length However, the sugar outflow (Fig 2, Table 3) did not indicate significant carbohydrate release in those stages and it remains unclear where this IV increase originates from The ozone stages were conducted by applying stepwise charges with IV measurements in between, which allowed for precise control When comparing the IV reduction capacity of ozone treatments expressed as the quotient of %IV reduction and ozone dose (data not shown), the presented results fall in line with numbers that other researchers have found during the ozone treatment of comparable pulps (He, Liua, & Tian, 2018; Pouyet et al., 2014; Tripathi et al., 2018)

By incremental application of ozone charges it is shown that dose-normalized %IV reduction of BS pulps is larger after the first charge, while EK pulps receive an equal reduction intensity with each charge This may indicate that BS pulp has approximated a kinetic bottleneck after the first ozone stage In contrast, the high residual lignin and xylan content in EK partially protect cellulose from degradation, which inhibits %IV decrease, resulting in a lower rate across all ozone stages Additionally, data from the literature suggests that fiber acces-sibility and pulp reactivity of a paper pulp is typically much lower than that of a dissolving pulp, limiting ozone diffusion in the former (He

et al., 2018; K¨opcke, Ibarra, & Ek, 2008; Miao et al., 2015) Finally, the

peroxide stage (P) required an increased proportion of alkali to

compensate for β-elimination reactions due to the higher number of keto

groups from EBI and ozone stages The final IVs of EK-mixed, EK-ozone

and BS-EBI were within the 450–500 mL g− 1 target, while EK-EBI,

BS-mixed, and BS-ozone delivered lower values, which allows the EBI

or ozone doses to be lowered in those cases (Fig 1)

Carbohydrate release was monitored by measuring the sugar

composition of crude bleaching filtrates, both before and after total

hydrolysis, to differentiate monomeric and polymeric fragments,

respectively (Fig 2) EBI is known to release predominately oligomeric

and polymeric hemicelluloses from the pulp (Sarosi et al., 2020) The

extent of hemicellulose release is roughly correlated to the pulps’

hemicellulose content and to the irradiation dose Acid and ozone stages

released minimal amounts of polymeric cellulose and hemicellulose

with the exception of EK-ozone (Fig 2, E), which shows cellulose

degradation to the monomer level, likely due to its highest ozone dose

and longest dwell time in the reactor at acidic conditions During

hydrogen peroxide bleaching, the alkaline conditions facilitate removal

of low-molecular fragments, which mostly consist of hemicellulose

Overall yield loss during bleaching, calculated as total sugar outflow

from the pulp, was lowest in BS pulps, with values slightly above 0.1 %,

and highest in EK pulps, with values of 0.8 % for EK-EBI and EKI-mixed and 1.8 % for EK-ozone

3.2 Comparison of final pulp properties 3.2.1 IV, MWD and carbonyl group distribution

After completing each bleaching sequence, the pulps were analyzed

by the CCOA method (Table 4, Fig 3) The underlying GPC data shows, that all pulps have an Mw of 227–295 kg mol− 1, which translates to calculated IVs of 561–685 mL g− 1 This deviates significantly from the measurements by the cupri-ethylenediamine (CUEN) method, which showed similar ratios between the pulps, albeit 150–200 mL g− 1 lower numbers due to β-elimination in the CUEN solution In both EK and BS pulps, dispersity is significantly higher when ozone was used for IV control This effect is aggravated in the EK pulps, where a stronger IV reduction was required This is a result of the diffusion behavior and high reactivity of ozone, which limits polysaccharide degradation to short-chain amorphous and outer crystalline regions, leaving long-chain core regions mostly intact Thus, EK-ozone shows the strongest peak broadening as it required the highest dose of ozone Additionally, it is the only pulp that showed slight degradation to the monomer level during the Z-stage, caused by the prolonged dwell times in the acidic reactor as a consequence of the step-wise process conduct (Fig 2) EBI treatments led to a statistically distributed increase of carbonyl groups, raising the carbonyl profiles by both backbone oxidation and new REG This is most visible in, but not limited to, the low-Mw region While carboxy group formation by EBI has been reported for amorphous cotton materials, lignocellulosic pulps barely undergo carboxyl group formation at low irradiation doses, especially in hardwood pulps in which a high hemicellulose content is present (Bouchard, M´ethot, & Jordan, 2006) This is caused by the limited diffusion of molecular ox-ygen inside the material Hence, EBI in the present study delivers carbonyl groups as the major oxidation product at the given irradiation levels

In analogy to the IV reduction, oxidation is heterogeneous for chemical treatments (Potthast, Rosenau, & Kosma, 2006) The carbonyl profiles (Fig 3, A, B) of both EK and BS reference ‘ozone’ pulps display the highest carbonyl group content in the low-Mw region This is due to

Table 1

General properties of the investigated hardwood pulps before treatment

(kg mol − 1 ) IV (mL g

− 1 ) Kappa number Brightness (%) R10; R18 (%) Hemicellulose content (%)

Oxygen-bleached eucalyptus kraft pulp (EK) 466 943 8.6 63.5 89.3; 91.6 19.0

Oxygen-bleached beech sulfite pulp (BS) 336 643 1.9 77.5 89.1; 93.8 3.4

Table 2

Pulp treatment sequences and electron and ozone dose *The first number is the

measured actual dose that was required and the second number in parentheses is

the initially calculated dose requirement

Pulp Bleaching

sequence Electron dose (kGy)* Ozone charge (kg odtp − 1 ) Name

EK OO-EBI-A-P 10.3 (10.0) – EK-EBI

EK OO-EBI-A-Z-P 5.2 (5.0) 6.4 EK-mixed

BS EO-EBI-Z-P 1.3 (1.25) 4.4 BS-mixed

O = oxygen delignification; EBI = electron beam irradiation stage; A = hot

sulfuric acid stage; A(hot) = A-stage with increased temperature; E = alkaline

extraction stage; Z = ozone stage; P = hydrogen peroxide stage

Fig 1 IV of eucalyptus kraft (A) and beech sulfite (B) pulps after EBI, hot acid, incremental ozone, or hydrogen peroxide stages, respectively Error bars correspond

to 1.0 % inherent standard error of the method

the overwhelming generation of REG in low-Mw fragments, which is also

reflected in a larger fraction of DP < 200 species Oxidation of REG in

the low-Mw region to carboxylic acid and lactone species, which are not

covered by CCOA labelling, was secondary, as the carbonyl profiles of

“ozone” variants indicated more carbonyl groups than “EBI” and

“mixed” in that area

The IV reduction caused by the P-stage is greater in EBI variants due

to the facilitated backbone carbonyl formation in the high-Mw region Chain cleavage through β-elimination was shown to have a greater effect

on IV if the keto group is located on a long chain than on a short chain

In ΔDSCO (degree of carbonyl substitution including REG) plots (Fig 3, C, D) the respective CCOA signals of pulps from ‘EBI’ and ‘mixed’ treatment routes were subtracted from the signal of the conventional

“ozone” variants to compare the oxidation profiles caused by each

Fig 2 Progressive sugar release from all bleaching stages of eucalyptus kraft and beech sulfite pulps Bleaching filtrates were analyzed before and after total

hydrolysis, to differentiate between monomeric and polymeric carbohydrates, respectively A = EK-EBI; B = BS-EBI; C = EK-mixed; D = BS-mixed; E = EK-ozone; F = BS-ozone

approach As indicated by the absolute carbonyl group contents

(Table 4), progressive replacement of ozone by EBI for IV control lead to

a lower carbonyl group profile throughout the MWD The low-Mw region

showed the strongest differences with ozone-treated pulps featuring

more REG due to aforementioned diffusion limitations Only BS-EBI

shows a moderately higher carbonyl group content than BS-ozone

around a log(Mw) of ~3.8 In bleaching sequences with combined

treatments of ozone and hydrogen peroxide, a greater number of

carbonyl groups disappears through oxidation of REGs than in sequences

with lower or without ozone stages Yet, the higher number of total

carbonyl groups of reference “ozone” pulps permits the conclusion that

“mixed” and “EBI” variants exhibit a lower amount of true cellulose

backbone oxidation and thus better brightness stability (Ahn et al.,

2019)

3.2.2 Alkali-resistance R10 and R18

Pulp alkali-resistance in 10 % (R10) or 18 % (R18) NaOH showed

distinct differences between each treatment variant (see supplementary

information) Cellulose chain cleavage inevitably leads to a decrease in

the R10 and R18 fractions However, since EBI statistically favors longer

chains and ozone is diffusion-limited to outer areas for the fiber, the

alkali-resistance of the “EBI” variant is less compromised than in the

“ozone” pulp This is indicated by the MWD (Fig 3, A, B), where pulps

with EBI displayed lower Ð and less shifts towards dissolution limits in

10 % and 18 % NaOH, respectively However, EK-EBI and EK-ozone

display equal R10 and R18 fractions with EK-mixed having high R10

and R18 fractions with on average 2.2 % and 1.3 % higher values,

respectively, than the other two BS-mixed had similar R10 and R18

values to BS-ozone due to their equal ozone stages and the low

irradi-ation dose of the former Hence, the behavior of those pulps during

alkalization and viscose making may be the same BS-EBI is more alkali-

resistant with R10 and R18 fractions on average 1.7 % and 0.4 % higher, respectively, than the other two BS variants Due to the sparse avail-ability of data in the literature on alkali-resistance of pulps after EBI, one must consider other means of determining alkali-resistance, such as measuring the α-cellulose content (Ritter, 1929) Other researchers have found an exponential decrease in the α-cellulose content of sugarcane bagasse of up to 80 % by EBI with a dose of up to 40 kGy (Ribeiro, Oikawa, Mori, Napolitano, & Duarte, 2013) Such behavior was not indicated in the present results Another study suggested that γ-irradi-ation in doses of up to 10 kGy had no effect on the α-cellulose content of bamboo paper kraft pulp, with minor reductions at 30 kGy and strong reductions beyond that irradiation level (Yang et al., 2010) Generally,

an increasing level of irradiation causes polymeric chain length to fall below the dissolution limit in 10 % or 18 % NaOH, respectively How-ever, at the given irradiation levels in both EK and BS pulps, the bulk of holocellulosic chain length remained above the dissolution limit in both

EK and BS pulps at the given irradiation levels

3.2.3 Pulp sugar composition

The sugar composition of the final pulps (Fig 4, Table 5) was very similar within each group of EK or BS variants This is reasonable, since the yield loss of all treatment sequences was below 1.8 % Marginal differences were observed in the EK series, where xylan content decreased from 17.1 % in EK-ozone over 17.0 % in EK-mixed to 16.7 %

in EK-EBI, but glucose recovery decreased in the same order from 80.3 % over 79.9 % to 78.9 %, respectively These small differences can be ascribed to side reactions such as formation of furfural and HMF during sample preparation Compared to the respective starting material, EK pulps lost between 1.8–2.2 % of hemicellulose content, while BS did not

a show significant reduction (Table 1) The pulp crystallinity index of all pulps was 54.7–55.9 % and 54.0–54.5 % for EK and BS pulps,

Table 3

Sugar release data of each individual bleaching stage of eucalyptus kraft and beech sulfite pulps *Measured values were lower due to formation of hydrox-ymethylfurfural during total hydrolysis

Pulps Stage Cellulose monomeric (mg odgp − 1 ) Cellulose polymeric (mg odgp − 1 ) Hemicellulose monomeric (mg odgp − 1 ) Hemicellulose polymeric (mg odgp − 1 )

EK-mixed

EK-ozone

A-

Table 4

CCOA data of all final pulps

Pulps M n

(kg mol − 1 ) M− 1w ) (kg mol Mmolz (kg − 1 ) DP<100

(%)

DP<

200 (%)

DP<

2000 (%)

DP>

2000 (%)

Ð IV (mL g − 1 ) REG (μmol g − 1 ) C = O (μmol g − 1 )

REG = reducing end groups; IV = intrinsic viscosity; DP = degree of polymerization; Mn, Mw or Mz =number, weight or z-average molecular mass

respectively, with differences between the treatment-variants well below the relative standard deviation If pulp crystallinity can be assumed as one indicator for pulp reactivity, differences of the latter between the pulps may be minimal (Ferreira, Evtuguin, & Prates, 2020) While reactivity measurements according to Fock or Treiber were not possible in the current study, other researchers have provided detailed insight A study by Gondhalekar et al found that crystallinity of a hardwood dissolving pulp was decreased by up to 9% and reactivity was increased by 11 % at a dose of 5 kGy (Gondhalekar, Pawar, & Dhumal,

2019) Based on this data, reactivity changes by EBI can be assumed to

be equal to those by ozone treatments

3.2.4 Degree of bleaching (DoB)

The pulps’ DoB was quantified by kappa number and ISO brightness measurements (Fig 5) Out of the EK pulps, only EK-ozone with the conventional sequence reached brightness values that meet dissolving pulp specifications BS-mixed and BS-ozone showed similar and high DoB due to their equal Z-stage EK-EBI, EK-mixed and BS-EBI showed DoB below dissolving pulp specifications due to a lack of sufficient chromophore removal or destruction EBI at the employed doses does not introduce enough disruptions in the extended π-electron system of lignin and other chromophores to have an effect on pulp brightness (Sarosi et al., 2020) In contrast with cellulose, lignin shows good radical stabilization and is resistant against lower irradiation doses and thus remains mostly intact in the pulp after irradiation (Dizhbite, Telysheva, Jurkjane, & Viesturs, 2004; Faustino, Gil, Cecília, & Duarte, 2010) While EBI is known to form reactive oxygen species in atmospheric oxygen and water, such as ozone, peroxyl and hydroxyl radicals, the amount of generated bleaching agents in moist pulps is too low to have a

Fig 3 Molecular weight distribution and carbonyl group profiles of eucalyptus kraft (A) and beech sulfite (B) pulps from variable treatments and their respective

differential carbonyl group profiles Vertical lines R10 and R18 represent the lower limit of alkali resistance in 10 % or 18 % NaOH solution, respectively ΔDS plots are generated by subtracting the CCOA signal of ‘EBI’ or ‘mixed’ from the reference ‘ozone’ pulp signal, which shows the divergence of carbonyl group profiles within

EK (C) and BS (D) pulps, respectively

Fig 4 Total hydrolysis sugar composition of final eucalyptus kraft and beech

sulfite pulps with variable bleaching sequences

Table 5

Data of total hydrolysis sugar composition of final eucalyptus kraft and beech

sulfite pulps

Pulps Glucose (%) Xylose (%) Mannose (%) Galactose (%)

great effect on kappa number or brightness at the given dose (Cleland &

Galloway, 2015; Gehringer, 1997; Sarosi et al., 2020) Since the “mixed”

variant of each pulp gave a significantly higher DoB compared to the

“EBI” route, especially in the case of EK pulp, a hybrid use of EBI and

ozone in paper pulp upgrade is at least plausible, if the hemicellulose

fraction is removed and the bleaching intensity is adjusted accordingly

3.3 The influence of EBI on isolated xylan

EBI of isolated hemicellulose has rarely been investigated in previous

studies (Chen et al., 2016; Ma et al., 2014) Isolating xylan before

irra-diation may reveal effects that are otherwise overshadowed by the

presence of cellulose Hence, both isolated eucalyptus kraft xylan (KX)

and beech sulfite xylan (SX) were irradiated and analyzed by the CCOA

method The Mw reduction of xylan was less pronounced than for

cellulosic components since the fragment size was already low (Table 6)

Non-irradiated KX had a relatively high Mw, which was reduced to

almost level-off Mw after a dose of just 10 kGy, while SX was barely

affected by EBI (Table 6) Other researchers have observed a similar

resistance of the Mw of hemicellulose towards irradiation (Ma et al.,

2014) The MWD of KX showed peak broadening at high irradiation

levels, which may indicate the simultaneous occurrence of chain

cleavage and cross-linking, with the former being more pronounced

(Fig 6) Carbonyl group content increased moderately for SX and

strongly for KX at high irradiation doses In the latter case, a significant

part of the carbonyl groups originate from newly formed REG The

majority of REG groups of SX are oxidized since it originates from an

acidic magnesium bisulfite process Therefore, a significant portion of

REG in SX are not labeled by CCOA, and the calculated number of REG is more than double the total number of measured carbonyl groups, regardless if Mn or Mw is used for calculation Overall, isolated xylan is more resistant to degradation by EBI than cellulose pulp However, as shown by the sugar analysis of wash filtrates after irradiation (Fig 2), xylan is primarily leached from the pulps This may indicate that xylan

Mw decreases barely below the dissolution limit after EBI, or that the accessibility of xylan fibers otherwise recalcitrant to dissolution is improved by EBI

3.4 The influence of EBI on HexA

The kappa number of EK pulps may be partially impacted by HexA, which is attached to the xylan side chains in kraft pulps (Jiang et al.,

2000; Vuorinen, Fagerstr¨om, Buchert, Tenkanen, & Teleman, 1999) As shown before, EK contains considerable amounts of xylan This gave rise

to a small series of experiments in which a HexA-rich birch kraft pulp was irradiated The birch pulp was fully bleached, yet had a distinct yellow hue caused by the high HexA content The HexA content was traced using three methods: regular kappa measurements, uronic acid groups by the FDAM method, and HexA quantification based on the available TAPPI standard (Bohrn et al., 2006; TAPPI, 2007)

Fig 7 shows that EBI causes a linear increase of uronic acid groups at elevated irradiation doses, which is analogous to the carbonyl group formation found in other studies (Bouchard et al., 2006; Henniges et al.,

2012) As has been shown, kappa numbers decrease with increasing irradiation doses (Sarosi et al., 2020) While the previous data showed a marginal kappa increase at elevated irradiation doses due to carbonyl group formation, it was not observed here

Similar to other parameters like cellulose Mw or kappa number, the HexA content was reduced rapidly at low irradiation doses, with a level- off effect at elevated doses In the dose range of 5–10 kGy, relevant for paper pulp upgrade, the HexA content was reduced by 10–26 %, while the highest reduction of 49 % was observed at a dose of 50 kGy HexA removal by EBI gives rise to improved pulp brightness stability, since HexA is known as a precursor for multiple chromophores (Rosenau

et al., 2017) HexA content reduction is caused either by chemical modification or by removal of hemicelluloses Hemicellulose degrada-tion by EBI to soluble fragments is observed, although the degree of hemicellulose removal can only account for a small part of the decrease

in HexA content (Chen et al., 2016; Ribeiro et al., 2013) Interestingly, HexA content saw a slight increase at 200 kGy indicating the formation

of double bonds, which are sensitive for analysis, at elevated irradiation doses

Since HexAs react with potassium permanganate during kappa determination, they are considered “false lignin,” the extent of which can be calculated (Vuorinen et al., 1999) To determine that fraction, both variables, HexA content and Kappa number, were transformed into permanganate molar equivalents The KMnO4 consumption was directly taken from kappa measurements and a 1:1 ratio was used for calculating KMnO4 equivalents from the HexA concentration (Table 7) (Chai et al.,

2001) Results indicate that up to one third of the kappa number is caused by HexAs The “false lignin” fraction decreases with irradiation, which indicates that HexAs are more susceptible to degradation by EBI than lignin, which is reasonable considering the structure and radical scavenging activity of the latter (Dizhbite et al., 2004)

4 Conclusion

In the present study, a eucalyptus kraft paper pulp and a beech sulfite dissolving pulp were subjected to different bleaching sequences, con-sisting of either an EBI, an ozone stage, or a combination of both, with the aim of lowering the IV to levels applicable for RCF processes Overall yield loss of the sequences measured by carbohydrate outflow was around 0.1 % for all beech dissolving pulps, 0.8 % for EK-EBI and EK- mixed pulps and 1.8 % for EK-ozone pulp EBI posed a tool for

Fig 5 Kappa number and brightness of eucalyptus kraft and beech sulfite

pulps after their respective treatments Vertical lines represent the upper kappa

limit of 1.0 and lower brightness limit of 90 % that are recognized as typical for

dissolving pulps Error bars indicate the inherent standard error of the kappa

(5%) and brightness (0.2 %) measurement method, respectively

Table 6

Weight average molecular mass, carbonyl group content and calculated REG of

eucalyptus kraft and beech sulfite xylan after irradiation at varying doses

Sample 0 kGy 10

kGy 100 kGy 200 kGy

M w (kg mol − 1 ) KX SX 47.6 5.5 14.4 4.9 11.9 4.4 13.0 4.6

Carbonyl group content

(μmol g − 1 ) KX SX 71.6 67.8 66.1 69.5 89.3 82.7 216.1 84.7

REG calculated from M n

(μmol g − 1 ) KX SX 25.8 183.2 87.2 202.4 83.8 228.8 160.5 218.8

REG calculated from M w

(μmol g − 1 ) KX SX 21.0 156.0 69.3 154.6 83.8 186.9 77.2 180.5

REG = reducing end groups

efficient IV reduction, giving rise to a potential use in upgrading high-IV

paper pulps to dissolving pulps The CCOA measurements supported this

hypothesis, revealing a lower carbonyl group content and improved

carbonyl profiles when progressively replacing the ozone stage with EBI

Additionally, EBI-treated variants had a lower dispersity of molar

masses and a smaller shift towards the low-Mw region due to the

sta-tistically high chance of cleaving long chain cellulose, which is

prefer-able for dissolving pulp This was also expressed by higher alkali-

resistant fractions R10 and R18 in EBI-treated pulps “EK-mixed” pulp

displayed the highest alkali-resistance out of all tested pulps, high uni-formity, the lowest carbonyl group content, a suitable IV, and a bleaching degree slightly below dissolving pulp specifications On the other hand, acid sulfite pulping generates a pulp that already has a low

IV, making the use of EBI for IV control less important, despite similar dispersity and oxidation advantages EBI is a suitable treatment method for IV reduction of high-Mw pulps, which can be used to substitute otherwise intensive chemical treatments and release potential process bottlenecks However, since EBI had no significant effect on lignin at the employed irradiation levels, pulps with EBI-only sequences suffered from low bleaching degrees Herein, the combined use of EBI and ozone posed a good compromise, unifying advantages of both treatments Additional irradiation experiments on HexA-rich birch pulp and isolated xylan samples revealed a reduction of the former of up to 49 % EBI by a dose of 50 kGy While xylan sample’s Mw were barely above or within lower level-off regions, the degree of oxidation increased moderately for beech sulfite xylan and strongly for eucalyptus kraft xylan Overall, changes imparted in xylan by EBI are weaker than in cellulose pulps at equal irradiation doses, especially if the xylan Mw is already low

CRediT authorship contribution statement Oliver P Sarosi: Conceptualization, Investigation, Data analysis,

Writing - original draft Daniela Bammer: Investigation Elisabeth

Fitz: Writing - review & editing, Supervision Antje Potthast:

Concep-tualization, Data analysis, Manuscript review & editing, Supervision

Funding

This research was funded by the Austrian Research Promotion Agency (FFG), Grant Number 844608 Open access was supported by BOKU Vienna Open Access Publishing Fund

Declaration of Competing Interest

The authors report no conflict of interests

Acknowledgements

Financial support was provided by the Austrian government and by the provinces of Lower Austria, Upper Austria and Carinthia, as well as

by Lenzing AG We also express our gratitude to the University of Nat-ural Resources and Life Sciences (BOKU), Vienna and Lenzing AG for their in-kind contributions Furthermore, we thank the responsible project manager at Lenzing AG Robert Bischof Last but not least, special thanks to all lab technicians for their patience and laboratory support, especially Sonja Schiehser and Markus Huemer

Fig 6 Molecular weight distribution and carbonyl group profiles of eucalyptus kraft (A) and beech sulfite (B) xylan after irradiation at varying dose

Fig 7 Birch pulp HexA content, uronic acid content, and kappa values after

EBI with varying dose

Table 7

Permanganate equivalent concentrations of irradiated birch pulp calculated

from Kappa number or HexA measurements and the fraction of “false lignin”

caused by HexAs *The 1.25 kGy sample was determined as an outlier in both

kappa and HexA measurements due to irregularities during measurements

Irradiated birch

pulp sample KMnO“Kappa” (4 equivalent μmol

g − 1 )

KMnO 4 equivalent

“HexA” (μmol g − 1 ) Fraction of “false lignin” caused by

HexA (%)

Appendix A Supplementary data

Supplementary material related to this article can be found, in the

online version, at doi:https://doi.org/10.1016/j.carbpol.2021.118037

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