Characterisation and effects of new anionic groups formed during chemithermomechanical pulping of spruce

Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 1–7

Review

Characterisation and effects of new anionic groups formed during chemithermomechanical pulping of spruce Jonas Konn a,∗ , Andrey Pranovich a , Pedro Fardim b , Bjarne Holmbom a a

˚ Akademi University, Porthansgatan 3, FI-20500 Turku/Abo, ˚ Laboratory of Wood and Paper Chemistry, Process Chemistry Centre, Abo Finland b Laboratory of Fibre and Cellulose Technology, Abo ˚ Akademi University, Porthansgatan 3, FI-20500 Turku/Abo, ˚ Finland Received 30 May 2006; received in revised form 6 September 2006; accepted 29 September 2006 Available online 5 October 2006

Abstract The amount of total anionic groups (TAGs) in laboratory-scale chemithermomechanical pulps (CTMP) were assessed by Methylene Blue (MB) and quinoline sorption. The amounts of surface anionic groups (SAGs) were determined by MB sorption followed by X-ray photoelectron spectroscopy (XPS) and by polyelectrolyte (PE) titration. The content of uronic acids in the pulps was determined by acid methanolysis and gas chromatography (GC) and the methyl-esterification degree of pectins by alkaline hydrolysis. Pulps were produced by chemical pretreatments of sapwood chips of Norway spruce (Picea abies) prior to refining in a small-scale batch refiner. The chips were pretreated with alkaline, sulphite, alkaline sulphite and alkaline peroxide liquors. The TAGs in the pulps increased linearly with alkali and sulphite dosages. Sulphonic groups in lignin and new carboxyl groups formed in pectins accounted for a major part of the new anionic groups. New anionic groups were also created in the fibre material due to lignin oxidation in alkaline peroxide treatment. Pretreatment with sulphite resulted in a lower surface lignin content compared to pretreatment with alkali or alkaline sulphite. This may indicate more preferential fibre separation in the primary wall layers than in the middle lamella or be a result of redeposition of alkali-dissolved lignin onto fibre surfaces. The amount of SAGs, determined by XPS, was doubled for the chemically pretreated pulps compared to a reference TMP. This is probably a result of primary wall exposure and substantial pectin demethylation. Sulphonation did not result in more SAGs. © 2006 Elsevier B.V. All rights reserved. Keywords: Chemithermomechanical pulping; Anionic groups; Sulphonation; Pectin demethylation; Surface chemistry

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Wood raw material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Chemithermomechanical pulping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Bulk characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Surface characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Pulp and paper properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Formation of anionic groups in chemithermomechanical pulping processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Effects of process conditions on the fibre surface chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. The role of AGs in chemimechanical pulps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗

Corresponding author. Tel.: +358 2 215 4888; fax: +358 2 215 4868. E-mail addresses: [email protected] (J. Konn), [email protected] (A. Pranovich), [email protected] (P. Fardim), [email protected] (B. Holmbom). 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.09.047

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1. Introduction The formation of new anionic groups (AGs) in refining and bleaching of chemimechanical pulps is highly influenced by the process conditions. Alkali and sulphite dosage as well as pretreatment temperature affect the release of wood components and the degree of de-esterification and sulphonation of the pulps. Chemithermomechanical pulps (CTMP) have higher total anionic group (TAGs) contents than any other papermaking pulps [1]. This is because of two reasons. First, there is no major destruction or removal of uronic acids from the pulps during processing. Second, new AGs are introduced by sulphonation (sulphonic groups) or converted into their free form by deesterification reactions of pectins (uronic acids). Even if the chemimechanical pulps contain about five times more AGs than kraft pulps, the CTMP fibres are significantly bulkier and stiffer and produce paper of notably lower strength. Most of the native lignin remains in the CTMP fibres. The lignin most probably restricts fibre swelling and decreases the fibre flexibility, fibre wall collapse and fibre-to-fibre bonding. The papermaking properties of chemimechanical pulps are dependent on the location and amount of AGs in the fibre material. The bulk AGs, inside the cell walls, affect the swelling properties and the softening behaviour of the wood material and as a consequence the cell wall cohesion, which is important for the response in refining and for the paper strength properties [2,3]. The AGs located on fibre surfaces (SAGs) influence the electrochemical interactions between fibres and chemical additives, and the fibre-to-fibre bonding in papermaking [4,5]. Sulphonation is a major chemical modification of the fibre material in softwood CTMP. The topochemistry of softwood sulphonation has shown a higher sulphur concentration in the middle lamella than in the cell wall, in line with the higher lignin concentration [6,7]. However, at low sulphite additions, preferential sulphonation may occur in the P and S1 layers, owing to the penetration and diffusion pathways of chemicals [8,9]. In native wood, most of the carboxyl groups stem from uronic acid residues. The main uronic acids in softwoods are 4-Omethylglucuronic acid units bound to xylans and galacturonic acid units in pectins [10–12]. The galacturonic acid is to a large extent esterified or as lactones (30–70%) in native wood, but will be hydrolysed under alkaline conditions (in pretreatment and/or bleaching), increasing the number of TAGs in the fibres [12]. Thus, pectins can be considered to be a key wood component in chemimechanical pulping, because of their chemical structure and location in the primary wall, middle lamella and around the pit pores of fibres [13,14]. The generation of new AGs by demethylation will give rise to a swelling of the wood material [15]. A weakening of the wood structure in the primary wall region may be essential for the selective fibre separation in chemimechanical pulping [16]. In this work, we have assessed the origin, formation and role of total (TAGs) and surface (SAGs) anionic groups in chemithermomechanical pulping. The TAGs and sulphonic acid groups were determined by Methylene Blue (MB) and quinoline sorption, respectively. TAGs were also determined by polyelectrolyte (PE) titration using a low molar mass polymer (polybrene) on

selected pulps. SAGs were determined by MB and quinoline sorption in combination with X-ray photoelectron spectroscopy (XPS). SAGs were also quantified by polyelectrolyte titration using a high molar mass polymer (pDADMAC) on selected pulps. 2. Experimental 2.1. Wood raw material Spruce sapwood boards (2.5 cm × 10 cm) were chipped in a re-chipper (Kone BC 5853). The chips were screened (SCANCM 40:01) and bark residues and knots were removed by hand. The accept chips (length 0.7–4.5 cm and thickness <8 mm) were stored frozen at −25 ◦ C until use. 2.2. Chemithermomechanical pulping Chips were steamed with water-saturated steam at 105– 110 ◦ C for 10 min in a steaming cylinder, before compression to approximately one-third of the initial chip bed volume in a hydraulic press. The impregnation liquor was added and the pressure released. Alkaline (A), sulphite (S), alkaline sulphite (AS) and alkaline peroxide (AP) liquors were used. A reference pulp (TMP) was obtained by pretreatment with water. The impregnation liquor volume was 3 L, including the chip water content. DTPA (0.2% on o.d. wood), MgSO4 (0.05%) and sodium silicate (3%) was, in addition to alkali and peroxide, added to the alkaline peroxide pretreatments. Heat treatments at 40, 60 and 80 ◦ C for 30 min (excluding heat-up time) were carried out in a rotating batch digester. Refining was done in a Wing Defibrator [17]. The chips were preheated in the refiner (120–130 ◦ C for 5 min) to allow softening of the chips prior to refining at 125–135 ◦ C (745 rpm). Each pretreatment batch was refined to four freeness levels, ranging from 750 to 400 mL. The discharged pulps were dewatered with recirculation of the filtrate. The obtained pulps were used for chemical analysis. Pulp and paper properties were determined on screened pulps. The pulps were hot-disintegrated (ISO 5263) and screened in a vertical centrifugal screen with 0.15 mm slots prior to determination of freeness (CSF) according to ISO 5267-2:1980 and hand-sheet (10 cm × 10 cm, 60 g m−2 ) forming according to a modified ISO 5269-1979 standard. 2.3. Bulk characterisation The overall content of uronic and sulphonic acid groups in the fibre material was determined in order to quantify their contributions to the ionic charge. Uronic acid units in pulps and process waters were determined by acid methanolysis and gas chromatography (GC) [18]. The methylation degree of the galacturonic acid (GalA) was determined as the amount of released methanol after complete de-esterification of the pulps [19]. Methanol was quantified by headspace solid-phase microextraction (SPME) and GC [20]. The sulphonic acids groups were determined by quinoline sorption [21] and the TAGs by Methylene Blue sorption [1,22]. Polyelectrolyte back-

J. Konn et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 1–7

titration using 1,5-dimethyl-1,5-diazundecamethylene polymethobromide (polybrene) with a molar mass of about 8 × 103 were performed on a selection of Na2 -EDTA washed pulps. Different amounts of polymer were added to pulps suspended in 10−5 M NaHCO3 at pH 7.5. The suspension was stirred for 2 h followed by filtration on a GF 50 glassfibre filter (Schleicher and Schuell). The filtrates were titrated with potassium polyvinyl sulphate, PVSK (Wako Pure Chemical Industries Ltd., Japan), using a M¨utek particle charge detector (PCD). A blank sample was titrated for each polymer dose in order to quantify the PE sorption to the filter and glassware. The saturation level (SL) of the PE was estimated based on sorption isotherms [23,24]. 2.4. Surface characterisation

The carbohydrate coverage was calculated as the total area subtracted by the lignin coverage (Eq. (3)). φcarbohydrates = 100% − φlignin

(3)

SAGs were determined by XPS on MB and quinoline labelled pulps. The concentrations of C, O, S and N on the fibre surfaces were measured using a pass energy of 93.6 eV, 20 min measurement time, and sensitivity factors provided by the instrument manufacturer. The Shirley background was used in all cases [28]. PE back-titration using poly-diallyldimethyl ammonium chloride (pDADMAC) with a molar mass of 1 × 105 to 3 × 105 was performed on Na2 -EDTA washed pulps [23,24]. 2.5. Pulp and paper properties

The chemical composition of the fibre surface and the SAGs were determined in order to identify the different anionic groups that were accessible at the fibre surfaces. The surface coverage by extractives, lignin and carbohydrates was estimated using Xray photoelectron spectrometry. Spectra were obtained in low and high resolution mode using a Physical Electronics Quantum 2000 ESCA instrument [25]. An extracted filter paper was used to monitor the carbon contamination in the instrument chamber [25]. Three different spots of 400 ␮m × 400 ␮m were analysed on both untreated samples and Soxhlet-extracted samples (acetone extraction for 60 h). The pass energy was 187 and 23.5 eV for low and high resolution, respectively. Charge compensation was applied using a combination of an ion gun bombarding and a low energy electron flood gun. Curve fitting of C 1s peak was performed using a Shirley background, Gauss–Lorentzian character and a full width at half-maximum (FWHM) of 0.9–1.5 eV. Binding energy (BE) of all spectra was related to C1 (C C, C H) at 285 eV. The following BE, relative to the C1 position, were employed for the respective groups, 1.7 ± 0.2 eV for C2 (C O), 3.1 ± 0.3 eV for C3 (C O, O C O) and 4.6 ± 0.3 for C4 (O C O). Surface coverage of extractives was determined using C1 relative areas (Eq. (1)) of extracted and unextracted samples [26]. φext = C1before extraction − C1after extraction

(1)

The surface coverage by lignin was also calculated using the C1 relative areas of the extracted sample (Eq. (2)), φlignin = (C1after extraction − X) ×

3

100 49

(2)

where X is the instrument-dependent correction factor for surface contamination, determined from the C1 relative area measured for an extracted filter paper [25], i.e. 4% for this instrument. The value of 49% is the estimated amount of C1 present in spruce milled wood lignin [27]. Good correlations have been found for the C1 method and the common O/C method for the surface coverage of lignin on spruce TMP [25]. Since no major altering of the chemical structure of the spruce lignin is expected under the pretreatment conditions used in this work, and since the major part of the native lignin remains in the pretreated pulps, we assume that the C1 method is applicable on these samples.

Average fibre length (length-weighted) was measured using a Kajaani FibreLab 3.5 analyser (Metso Automation Inc.). Water retention value (WRV) was determined according to SCAN-C 62:00. Tear and tensile indexes and the specific volume to weight ratio (bulk) of hand-sheets were determined according to the ISO 5270-1979 standard. The determined pulp and paper characteristics were compared at the same freeness level (500 mL) [29]. 3. Results and discussion 3.1. Formation of anionic groups in chemithermomechanical pulping processes The amount of TAGs in the reference pulp (TMP) was 96 ␮mol g−1 (Table 1). Considering a methylation degree of galacturonic acid (GalA) as approximately 60% [30], the uronic acids comprised 97% of the TAGs in this sample. Pectins are demethylated under alkaline conditions and free carboxyl groups are formed. It is also conceivable that the increase in TAGs by the formation of new carboxyl groups in pectins is counteracted by pectin dissolution. The alkaline treatments removed up to 35% of the pectins (as GalA) from the fibres. However, only about one-third of this amount was quantified as GalA in the process waters [30]. This divergence will affect the calculation of the methylation degree and the total amount of residual pectins in the analysed pulps. In Table 1 the contribution of GalA to TAGs was calculated based on two different assumptions: (I) all undetermined pectin was dissolved in the process water after the alkaline treatment and (II) all undetermined pectins remained within the fibre material. The “true” TAG contribution could then be expected be somewhere between these two boundary values. Alkaline (A) pretreatment with 2% NaOH (on dry wood) increased the TAGs to 151 ␮mol g−1 , primarily due to the formation of new AGs by pectin demethylation. Between 69% (I) and 86% (II) of the TAGs in this pulp could be assigned to the uronic acids (Table 1) depending on the above mentioned assumptions. No changes were found in the amounts of 4-O-methylglucuronic acid (4-O-MeGlcA) and glucuronic acid (GlcA). The difference in the amount of uronic acids and the TAGs determined by Methylene Blue sorption, assigned “other

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Table 1 Origin of TAGs in chemithermomechanical pulps determined by Methylene Blue sorption Sample

TMP-1 A-2 A-3 S-4 S-5 S-6 AS-7 AS-8 AS-9 AS-10 AS-11 AS-12 AP-13 AP-14 AP-15 AP-16 AP-17 AP-18

TAGs (␮mol g−1 )

Chemical dosage (%) NaOH

Sulphite

H2 O2

0 1 2 – – – 1 1 1 2 2 2 1 1 1 2 2 2

– – – 2 4 6 2 4 6 2 4 6 – – – – – –

– – – – – – – – – – – – 2 4 6 2 4 6

96 131 151 125 143 151 164 179 195 175 200 224 139 144 144 158 167 169

Sulphonic acids (␮mol g−1 )

Uronic acids (␮mol g−1 )

Other AGs (␮mol g−1 )

4-O-MeGlcA

GlcA

GalA I

GalA II

(I)

(II)

– – – 13 21 29 12 21 30 8 17 24 – – – – – –

53 52 51 56 53 57 60 60 59 51 56 61 54 56 52 54 56 59

7 7 7 7 6 6 7 7 6 6 5 6 6 7 6 7 7 6

34 56 47 37 43 39 50 60 46 50 53 56 46 46 48 53 50 60

34 67 72 44 61 55 62 69 72 75 75 76 57 52 56 67 61 66

2 17 46 13 19 20 34 31 54 60 68 76 33 35 38 45 55 45

2 5 21 6 2 4 22 22 28 35 46 56 22 29 29 31 44 39

Sulphonic acids were determined by quinoline sorption and uronic acids by carbohydrate analysis. The methyl-esterification degree of GalA was determined by total alkaline hydrolysis; (I), all undetermined GalA in water phase; (II), all undetermined GalA in the fibre material.

AGs”, was found to increase with alkali and sulphite dosage in the pretreatments. Possible hydrolysis of lactones and splitting of lignin–carbohydrate ester bonds [31,32] is included in this group, as well as the charge contribution of fatty and resin acids, however, found to be less than 1 ␮mol g−1 for all pulps by extraction and GC quantification [33]. Pretreatments with sulphite (S) or alkaline sulphite (AS) resulted in TAGs between 125 and 224 ␮mol g−1 depending on the alkali and sulphite dosage. The amount of introduced sulphonic acid groups for S and AS was in the range 8–33 ␮mol g−1 . The amount of new AGs increased linearly with the amount of introduced sulphonic acids (Fig. 1). However, the sulphonic groups accounted for only 20–50% of the new AGs created.

Apparently there is a significant amount of other AGs formed in pretreatment with sulphite. Extensive demethylation of pectins already under the mild conditions in sulphite pretreatments was recently reported [30]. The increase in AGs in the sulphite pretreatments could be almost entirely accounted for by the introduction of sulphonic groups and by the formation of new AGs in the GalA. However, the sulphonic and uronic acids comprised only 65–85% of the TAGs after AS pretreatments (Table 1). Prereatment with alkaline peroxide (AP) resulted in a significant increase in TAGs compared to treatments with only alkali (Fig. 2). The TAGs was approximately 30 ␮mol g−1 higher for AP than for A at the same total alkali consumption independently of the peroxide dosage. The contribution of uronic acids on the

Fig. 1. New AGs introduced by lignin sulphonation. (Symbols) light coloured symbols indicate pretreatment at 60 ◦ C and dark coloured symbols at 80 ◦ C. The TAGs for TMP was 96 ␮mol g−1 . Introduced sulphonic acids accounted for 20–50% of the formed anionic groups in chemithermomechanical pulping.

Fig. 2. TAGs for pulps pretreated with alkaline and alkaline peroxide liquors. The pretreatment temperatures were 40 ◦ C (only for A), 60 and 80 ◦ C. Oxidative reactions by peroxide increased the TAGs at the same alkali consumption.

J. Konn et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 1–7

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Table 2 O/C ratios and relative areas of C 1s peak components (C1, C2, C3 and C4) of untreated and extracted CTMP samples, average and standard deviation (S.D.) Sample

O/C

Relative areas of C 1s peak components (%)

Fibre surface coverage (%)

C1

C2

C3

C4

Extractives

TMP-1 Unextracted Extracted

0.54 (0.02) 0.56 (0.01)

32.8 (2.3) 30.3 (1.8)

52.9 (4.3) 53.9 (1.6)

12.0 (1.6) 12.5 (2.5)

2.2 (0.4) 3.2 (0.8)

2.5

A-3 Unextracted Extracted

0.50 (0.03) 0.55 (0.03)

36.3 (1.4) 26.1 (1.9)

50.6 (1.1) 52.9 (3.2)

9.8 (0.1) 16.6 (1.8)

3.4 (0.3) 4.5 (0.4)

10.2

S-6 Unextracted Extracted

0.54 (0.02) 0.56 (0.02)

29.3 (0.9) 16.6 (2.5)

61.7 (1.9) 66.9 (1.5)

6.8 (1.4) 11.7 (3.9)

2.2 (0.4) 4.9 (0.1)

12.7

AS-11 Unextracted Extracted

0.53 (0.03) 0.55 (0.01)

35.7 (2.2) 23.2 (1.7)

52.7 (2.3) 59.1 (3.1)

9.3 (0.4) 13.2 (3.4)

2.3 (0.6) 4.5 (1.5)

12.5

Carbohydrates

Lignin

46.3

53.7

54.9

45.1

75.3

24.7

60.8

39.2

The calculated surface coverages by extractives, lignin and carbohydrates are also presented.

TAGs was 70–80% which is approximately the same ratio as for A and AS pretreatments. The AGs formed by oxidation reactions in the AP pulps were included in the group “other AGs” since their specific amount was not known. 3.2. Effects of process conditions on the fibre surface chemistry The data for surface coverage by extractives, lignin and carbohydrates are presented in Table 2. The chemically pretreated pulps had clearly higher surface coverage by extractives than the reference TMP. However, it should be mentioned that the surface coverage by extractives is generally lower for CTMP than for TMP samples [34] since more extractives and lignin is expected to dissolve from the fibre surfaces under alkaline conditions. Thus, our results could be explained by the dissolution of fatty and resin acids due to the high pH in the pretreatment and refining conditions and further redeposition onto fibre surfaces during dewatering or in hand-sheet forming after acidification [35]. The surface coverage by lignin was lowest after pretreatment with sulphite (S-6). This may indicate more preferential fibre separation in the primary wall or S1 layer after sulphonation. Mildly sulphonated fibres have previously been found to be completely free of middle lamella fragments indicating fibre separation preferentially in the primary wall [8]. Dissolution of proteins from the primary wall has also been reported to increase the fracture sensitivity in this particular region [16]. The higher surface coverage by lignin after alkaline sulphite (AS-11) and alkaline (A-3) pretreatments indicated more fibre separation occurring in the middle lamella region. Dissolution of lignin at high pH followed by lignin redeposition onto the fibre surfaces [35] may also explain the higher lignin surface coverage after alkaline pretreatments. The reference TMP had, however, the highest lignin coverage. This may be due to the refining temperature (125–135 ◦ C), being in the range of lignin Tg , resulting in fibre separation in the middle lamella region. The detection of SAGs is much dependent on the analytical method used. The proximate analysis depth of the XPS technique

for fibre surfaces is 3–10 nm [28], corresponding to about 0.1% of the spruce cell wall width. The N/S ratio obtained in XPS spectra of MB-labelled samples was close to the theoretical value for the MB molecule. The SAGs on the reference TMP was found to be 1.2 ␮mol g−1 (Table 3). The amount of SAGs was doubled for the chemically pretreated pulps, probably as a result of substantial pectin demethylation and primary wall exposure. These results were consistent with results obtained on industrial TMP and CTMP samples [28]. Sulphonation did not, however, result in more SAGs as indicated by nitrogen concentration of the fibre surfaces of Q-labelled samples (results not shown). On the contrary, sulphite pretreatment (S-6) resulted in lower amounts of sulphonic groups on the fibre surface compared the alkaline sulphite pretreated pulp (AS-11). The amount of SAGs may be too low for precise quantification with quinoline on the surfaces. This might be due to losses of SAGs by dissolution of lignin from the fibre surfaces after sulphonation. Polyelectrolyte titration has successfully been used for quantitative analysis of TAGs and SAGs in cellulosic materials [36]. PE titration with polybrene resulted in similar TAG results as the MB sorption (Table 4). However, PE sorption resulted in slightly lower TAGs for TMP and for the sulphite-pretreated pulp (S-6). These lower values may be due to limited accessibility of the polymer into the fibre wall. This finding was accentuated for the SAGs determined by pDADMAC titration. The SAGs share of the TAGs was much higher for the PE titrations compared to the MB–XPS method. The SAGs determination by pDADMAC titration usually results in much higher values than the MB–XPS Table 3 Atomic concentrations (at%) of N and S, O/C and N/S ratios for Methylene Blue (MB) labelled pulp samples, average and standard deviation (S.D.) Sample

O/C

N (at%)

S (at%)

N/S

SAGs (␮mol g−1 )

TMP-1 A-3 S-6 AS-11

0.49 (0.01) 0.64 (0.04) 0.59 (0.04) 0.56 (0.03)

0.6 (0.1) 0.9 (0.0) 1.0 (0.2) 0.9 (0.1)

0.2 (0.0) 0.3 (0.0) 0.3 (0.1) 0.3 (0.0)

3 (0.1) 3 (0) 3.1 (0.2) 3 (0)

1.2 (0.2) 2.20 (0.1) 2.46 (0.4) 2.22 (0.1)

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J. Konn et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 1–7

Table 4 TAGs were determined by MB sorption and polybrene (PE) titration, average and standard deviation (S.D.) Sample

TAGs by MB sorption (␮mol g−1 )

SAGs by MB–XPS (␮mol g−1 )

TAGs/SAGs (MB) (%)

TAGs by PE sorption (␮mol g−1 )

SAGs by PE sorption (␮mol g−1 )

TAGs/SAGs (PE) (%)

TMP-1 A-3 S-6 AS-11

96 (1) 151 (5) 151 (1) 200 (2)

1.2 2.2 2.5 2.2

1.2 1.4 1.6 1.1

77 (5) 156 (4) 128 (4) 195 (3)

9.5 20.0 11.5 20.0

12 13 9 10

SAGs were determined by MB–XPS and PE (pDADMAC) titrations. The ratio TAGs/SAGs was calculated for MB sorption and PE sorption separately. Table 5 Physical properties of pulps and hand-sheets compared at a freeness of 500 mL, average and standard deviation (S.D.) Sample

SEC (MWh tonnes−1 )

Fibre length (mm)

Tear index (mNm2 g−1 )

Tensile index (Nm g−1 )

Density (tonnes m−3 )

WRV (g g−1 )

TMP-1 A-3 S-6 AS-11

1.77 (0.12) 2.96 (0.14) 3.25 (0.14) 3.28 (0.25)

2.24 (0.07) 2.42 (0.05) 2.51 (0.03) 2.54 (0.03)

5.7 (0.3) 6.4 (0.1) 7.4 (0.3) 7.1 (0.4)

16.1 (0.6) 18.0 (0.4) 17.9 (0.5) 18.0 (0.7)

0.23 (0.02) 0.26 (0.02) 0.24 (0.03) 0.25 (0.02)

1.07 (0.02) 1.15 (0.02) 1.12 (0.01) 1.12 (0.03)

method, probably due to penetration of the polymer into pores and voids of the fibres [37]. 3.3. The role of AGs in chemimechanical pulps The AGs formed in the chemical pretreatments were found to increase the specific refining energy demand (Table 5). This is an indication of increased fibre flexibility and compressibility. Also the fibre lengths were preserved to a higher degree at higher TAG contents. This is probably due to a more selective fibre cleavage and less fragmentation of the fibres. Both the hand-sheet tear and tensile strengths were found to increase with increasing TAGs. The sulphite-pretreated pulp (S-6) had the highest tear index. The tensile index was, however, not higher for the S-6, despite the lower surface lignin coverage compared to AS-11 and A-3. The tensile index is apparently more related to the specific density of the paper sheet. The water retention value was clearly increased for the chemically pretreated pulps compared to reference TMP and in good correlation with the sheet density. The predominant location of TAGs in inner regions of the fibre wall will also have an impact on the wet-interactions in the paper machine. Papermaking is a dynamic process in which different chemicals are added in a very short time sequence. The SAGs will preferentially interact with cationic paper chemicals and inorganic cations due to their accessibility. However, a competition for surface sites and displacement of certain additives is also possible, once that many organic and inorganic cations are used in papermaking. On the other hand, the inner AGs increase the water absorption into the fibre wall. As consequence, the paper drying will require more energy and the speed of the paper machine will be reduced in comparison with kraft pulps. 4. Conclusions Chemical pretreatment of spruce chips resulted in a doubling of the total anionic groups. About 70–80% of the TAGs could be accounted for by uronic and sulphonic acids after chemical pretreatment. Oxidative reactions by peroxide also generated new

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