The effects of CMC attachment onto industrial and laboratory-cooked pulps

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1 The effects of CMC attachment onto industrial and laboratory-cooked pulps Elisabeth Duker, Elisabet Brännvall and Tom Lindström KEYWORDS: Surface carboxymethylation, CMC,, Kraft pulp, Paper strength properties, Shape factor, Zero-span tensile strength SUMMARY: The effect of surface carboxymethylation of industrial and laboratory-cooked pulp was studied regarding attached amount, fibre properties and paper sheet strength. The strength development was compared with the effects of. Attachment of CMC was shown to be equally effective with industrial pulp as with laboratory-produced pulp. The attachment level was 100% for both pulp types and no difference in paper strength enhancement could be detected. CMC attachment had a small impact on sheet density, especially compared to. Moreover, surface carboxymethylation was shown to increase the shape factor, reduce the number of kinks per fibre and to increase the rewetted zero-span index. This straightening effect of CMC was interpreted in terms of an increase in carboxyl group repulsion on the fibre surface and is probably a factor contributing to the increase in paper strength. Differences in fibre curl between industrial and laboratory-cooked pulp decreased when CMC was attached to the fibre surface. This may explain why no differences in CMC efficiency could be detected. ADDRESS OF THE AUTHORS: Elisabeth Duker (elisabeth.duker@stfi.se): STFI-Packforsk AB, Box 5604, SE Stockholm, Sweden. Elisabet Brännvall (bettan@polymer.kth.se): Royal Institute of Technology, Fibre- and Polymer Technology, SE , Stockholm, Sweden. Tom Lindström (tom.lindstrom@stfi.se): STFI-Packforsk AB, Box 5604, Stockholm, Sweden. Corresponding author: Elisabeth Duker Industrial and laboratory-cooked pulps differ in many ways, not least with regard to their strength properties. A well-known concept used to relate the strength of an industrially produced pulp to the strength of its corresponding laboratory pulp is the term strength delivery. It has been shown that, at a given tensile strength, the tear strength of an industrially produced pulp is 65-90% of the tear strength of its corresponding laboratory pulp (MacLeod 1987; MacLeod et al. 1987; MacLeod and Pelletier 1987; MacLeod 1990a; MacLeod 1990b; Hakanen and Hartler 1995). Laboratory pulps also have a higher tensile strength than industrially produced pulps at a given level of beating (MacLeod 1987; MacLeod and Pelletier 1987; MacLeod 1990a; MacLeod et al. 1995; Brännvall and Lindström 2006). These results imply that the process differences between pulp production in a mill system and laboratory pulp production are important for the pulp properties. In industrial cooking, pulps are more exposed to mechanical stress. The passage through pumps and screw presses generates a high amount of curl (Page et al. 1985; Page and Seth 1988; Hakanen and Hartler 1995). The term fibre curl is a broad categorisation of different deformations, all of which change the shape of the fibre. Paper sheets made of fibres with a high curl index have a lower sheet density and a lower tensile index, than sheets made of straighter fibres. This is explained by a decreased in the number of contact points between curled fibres (Page et al. 1985; Page and Seth 1988; Ellis et al. 1995; Mohlin et al. 1996; Trepanier 1998). The pulping conditions also differ a lot between the pulp mill and laboratory pulping. The ionic strength in the cooking liquor (Na +, Ca 2+ and K + ) is, for instance, ca. 1.5 times higher in an industrial system, due to incomplete processes in the recovery cycle, and this may lead to somewhat different pulp characteristics. For example, a high content of calcium ions may precipitate lignin (Sundin and Hartler 2000). Hence, an industrial pulp is likely to have a higher lignin content on the fibre surface than a laboratory-cooked pulp. Another difference in the cooking conditions is the addition of black liquor in industrial cooking, which has been shown to increase the amount of xylose and to decrease the amount of glucose on the fibre surface. This difference is maintained also after ECF bleaching (Sjödahl et al 2004). Moreover, black liquor addition may lead to a higher

2 extractive content on the fibre surfaces (Dai et al. 2006). When industrial and laboratory-pulped fibres are compared, it has also been found that the fibre surface in industrial pulps has a higher xylan content. At the same time, the surface charge density is lower than that of laboratory-produced pulp. This has been shown to be due to a decrease in the amount of hexenuronic acid groups (Brännvall and Lindström 2006). Considering the different surface characteristics of industrial and laboratory-cooked pulps, it can be assumed that the addition of chemical strength additives will, depending on pulp type, lead to differences in adsorption behaviour. Most strength additives act in the joint between fibres and it is therefore probable that a pulp with a higher curl will have a lower ability to fully utilize the strength-enhancing potential of a strength additive. The aim of this study was to investigate the effect of CMC attachment to a bleached, industrially produced kraft pulp and to its corresponding laboratory cooked pulp. CMC is a well-known strength additive that, given the right conditions, can be irreversibly attached to the fibre surface (Laine et. al 2000). Both the influence of pulp type on the adsorbed amount and the effect of CMC on the paper strength properties were studied. The results were compared with the effect of. Experimental Materials Industrial softwood chips (spruce and pine) from the Södra Mönsterås Pulp Mill, Mönsterås, Sweden, were employed in the study. The chips used for the laboratory cooking were air dried before being screened by hand to remove over-thick chips, chips with knots, and bark. An industrially produced, unbleached pulp, cooked from the same batch of chips in a continuous digester, was also obtained from the mill. The industrial pulp had a kappa number of 27 and a viscosity of 1250 ml/g. In the surface carboxymethylation process a commercial CMC (Hercules, Sweden), with a degree of substitution of 0.4 and a molecular weight of Da, was used. Methods Laboratory pulping The aim of the laboratory pulping was to simulate the industrial process with regard to time, temperature and chemical charges. As pulping liquors in mills normally have a higher inorganic content, the laboratory cook was performed at a higher ionic strength. The ionic strength, measured as the sodium ion concentration, was achieved by adding NaCl. The conditions during pulping, and the kappa number and viscosity of the resulting pulp are given in Table 1.

3 Table 1. Conditions in the laboratory cook Process Parameter [OH - ] 1.2 mol/l [HS - ] 0.26 mol/l [Na + ] 2.06 mol/l [Cl - ] 0.6 mol/l Liquor-to-wood ratio 4:1 Temperature 156 o C Cooking time 5 h H-factor 1550 Pulping was performed in a circulation laboratory digester. After steaming for 5 min, the circulation of the cooking liquor was started. The increase in temperature was approximately 1 ºC/min until the cooking temperature was reached and the cooking was then continued until an H-factor corresponding to the desired kappa number was attained. The cook was terminated by emptying the digester of the black liquor and cooling it with deionised water. The delignified chips were washed in the digester for approximately 12 h, using deionised water, and thereafter defibrated in a water-jet NAF defibrator. The pulp was centrifuged to a dry solids content of 25 30%. The kappa number of the laboratory-cooked pulp was 24. Oxygen delignification and bleaching of pulp Subsequent to pulping, both the industrial and the laboratory-produced pulp were oxygen delignified. In both cases, the pulps were disintegrated in a laboratory disintegrator for revolutions (ISO 5263:1995), dewatered on a Büchner funnel and thereafter placed in plastic bags. Sodium hydroxide, magnesium sulphate and deionised water were mixed and added to each pulp. The charge of NaOH and MgSO 4 was 2.7 weight-% and 0.5 weight-% respectively and the final pulp consistency was 12%. The chemicals were mixed into the pulp by kneading and 80 g (O.D.) of pulp was thereafter placed in a two-litre Teflon-coated steel autoclave. At room temperature, the oxygen charge was 6 bars. The temperature used during delignification was 100 ºC and the total treatment time was minutes (warming + reaction time). After completed oxygen delignification, the ph of the residual liquor was approximately 12 and the kappa number was ca. 12 for the laboratory-cooked pulp and ca. 15 for the industrial pulp. The oxygen-delignified pulps were bleached according to a D(EP)DD sequence using the conditions listed in Table 2. The bleaching was performed in plastic bags. Both pulps were washed between bleaching stages by dilution and dewatering on a Büchner funnel and thereafter by displacement. The washing procedure between each stage was repeated twice.

4 Table 2. Conditions in bleaching D EP D D Pulp consistency 10.0% 10.0% 10.0% 10.0% ClO 2 charge, % a Cl 2.8% 2.0% 1.0% H 2 O 2 charge 0.3% EDTA MgSO % Buffer, % of total weight 10.0 % 10.0 % Sodium acetate/acetic acid NaOH 1.5 Temperature 70 o C 70 o C 70 o C 70 o C Time 30 min 60 min 120 min 120 min Final ph Pulp pre-treatment Prior to surface carboxymethylation, both the industrial and the laboratory-produced pulp were washed. In the first washing stage, 0.01 M HCl was added to each suspension in order to remove metal ions adsorbed onto the pulps. The ph was adjusted to 2 using HCl and the pulp was kept at this ph for 30 minutes. The pulps were then washed until the conductivity of the filtrate was less than 5μS/cm. In a second washing stage, the pulps were washed to their Na + -form with 1 mm NaHCO 3. The ph was adjusted to 9 using NaOH and the pulp was kept at ph 9 for 30 minutes. Subsequently, the pulps were washed with deionized water to a conductivity of less than 5 µs/cm. This was done to remove water-soluble substances, e.g. lignin and hemicelluloses, from the pulps. In the final washing stage the pulps were transferred to their Ca 2+ -form and the CaCl 2 -concentration in the aqueous phase was adjusted to 0.05 M. The pulps were left to stand for 15 minutes and then washed with deionized water until the conductivity of the filtrates was less than 5 µs/cm. CMC attachment In order to improve mixing between pulp and CMC during surface carboxymethylation, the CMC was dissolved in water g of the CMC used was dissolved in 400 ml deionized water and continuously stirred overnight. Surface carboxymethylation was carried out in accordance with the method developed by Laine et al. (2000). The pre-washed pulps and the CMC-solution were placed in contact in a two-litre autoclave. CaCl 2 was added and the ph was adjusted. The conditions used during surface carboxymethylation are shown in Table 3. The autoclave was placed in a pre-heated glycol-bath to stabilise the temperature and ensure a certain amount of stirring. After being in contact with the CMC solution, each pulp was washed with cold deionized water to a conductivity of less than 5 µs/cm. Table 3. Conditions during surface carboxymethylation Process parameter CMC addition 20 mg CMC/g fibres Pulp consistency 2.1% Temperature 120 o C Exposure time 2 h ph 8 Electrolyte concentration (CaCl 2 ) 0.05 M

5 A reference sample of both the industrial and the laboratory-cooked pulp was treated under the same conditions, but without the addition of CMC. In order to compare the effects of CMC with those achieved by beating, the untreated industrial reference pulp was beaten in a PFI refiner. The pulp was washed to its calcium form and then beaten in accordance with SCAN- C 24:96 using 0, 1000 and 3000 revolutions. Analyses The water retention value (WRV) of the industrial and the laboratory-produced pulp was determined according to SCAN-C 62:00. The WRV is a measure of pulp swelling (Scallan 1983) and was determined both for the reference pulps and for the surface carboxymethylated pulps. The measurements were made in deionized water and the pulp was in its Ca 2+ -form. Conductometric titration, in accordance with the method devised by Katz and co-workers (Katz et al. 1984), was used to determine the total amount of charged groups, e.g. carboxyl acid groups. The surface charge density of both CMC-grafted and reference fibres was determined by the polyelectrolyte titration procedure, optimised by Horvath et al. (2006) from a method originally described by Wågberg et al. (1985). In addition, the fibre curl was measured as the shape factor using the STFI FiberMaster (Karlsson and Fransson 1994). Finally, handsheets were made in accordance with SCAN-C26:76, although with some modifications. All the sheets were made in deionized water, with the pulp in its Ca 2+ -form. The grammage of the handsheets was always 80 g/m 2, with one exception. The handsheets to be used in Z-strength testing had a grammage of 120 g/m 2 and, in order to vary the sheet density, they were also wet pressed differently. Half of them were wet pressed only once, using 400 kpa and a pressing time of 5 minutes, and the other half were wet pressed three times, using 400 kpa and a pressing time of 5 minutes. Subsequently to wet pressing, all the handsheets were dried under restraint in a conditioned room (50% RH and 23 C). The physical properties of the sheets were tested according to the following standards: grammage ISO 536:1995, structure thickness and sheet density SCAN-P 88:01, tensile strength SCAN-P 67:93, Z-strength SCAN-P 80:98 and zero span tensile strength ISO 15361:2000. The Primary data were treated in accordance with SCAN-G 2:63, using a confidence interval with 95% significance. Results Fibre modification using CMC Attachment of CMC to cellulosic fibres leads to an increase in the surface charge density. In Table 4, the results of all the charge measurements are summarized and the effect of CMC on the WRV is also shown. The term heated reference refers to the pulp treated the same way as the CMC carboxymethylated pulp, but with no CMC addition. As can be seen, surface carboxymethylation resulted in an increase in both total and surface charge. The magnitude of the surface selectivity, defined as the number of charges introduced to the surface of the cellulose fibres during surface carboxymethylation compared to the total number of charges introduced, implies that the fibre modification was surface selective.

6 Table 4. Charge and WRV measurements Total charge [μekv/g] Surface charge [μekv/g] Surface selectivity [%] WRV [g/g] Untreated, laboratory-cooked reference Heated, laboratory-cooked reference CMC-treated, laboratory-cooked pulp Untreated, industrial reference Heated, industrial reference CMC-treated, industrial pulp The attached amount of CMC was calculated from the total charge measurements, i.e. conductometric titration. During surface carboxymethylation of the industrial and the laboratory-produced pulp, all the CMC added, 20 mg CMC per gram fibre material, was attached. Hence, the attachment level was 100% for both pulp types. Differences in surface charge density are reflected in the water retention values. As can be seen in Table 4, the industrial reference pulps (untreated and heated) had a lower water retention value than the laboratory reference pulps. Surface carboxymethylation led to an increase in WRV and the difference between the two pulp types was levelled out. Fibre dimensions and the fibre shape were evaluated for all the pulps, using the STFI FiberMaster. The results of this evaluation are presented in Table 5. As expected, the industrially pulped fibres had a lower shape factor and contained more kinks than the laboratory-pulped fibres. The shape factor of both the industrial and the laboratory-cooked fibres increased when CMC was attached to the pulps. CMC attachment also resulted in fewer kinks. These results were somewhat unexpected. Neither fibre length nor fibre width was affected by the treatment. The increase in fibre length, seen in Table. 5., is due to the fact that the CMC pulps contain fewer kinks. Table 5. Effect of CMC on fibre dimensions and fibre shape Fibre length [mm] Fibre width [mm] Shape factor [%] Number of kinks per fibre Untreated, laboratory-cooked reference Heated, laboratory-cooked reference CMC-treated, laboratory-cooked pulp Untreated, industrial reference Heated, industrial reference CMC-treated, industrial pulp Paper sheet strength properties Fibre modification with CMC had a strong impact on paper strength. The influence of CMC on the tensile index for sheets made of industrial and laboratory-produced pulp, is shown in Fig. 1. The effect of is shown for comparison.

7 Tensile index [Nm/g] Sheet density [kg/m 3 ] Figure 1. The effect of CMC attachment (20 mg/g) on paper sheet tensile strength, when added to a bleached, never-dried and industrially produced pulp and to a bleached, never-dried and laboratory- produced pulp. The effect of (0, 1000 and 3000 revs.) on the untreated industrial reference pulp is shown for comparison. The confidence intervals are of 95% significance. As can be seen, sheets produced from the laboratory-cooked reference had a higher tensile strength than sheets made from the industrial pulp. Attachment of CMC increased the tensile index by more than 100% for both pulp types. Compared to, the effect of CMC on sheet density was small, as reported earlier (Laine et al. 2002a). Fig. 2 shows the effect of surface carboxymethylation and on strain to failure. Strain to failure [%] Sheet density [kg/m 3 ] Figure 2. The effect of CMC attachment (20 mg/g) on strain to failure, when added to a bleached, never-dried and industrially produced pulp and to a bleached, never-dried and laboratory-produced pulp. The effect of (0, 1000 and 3000 revs.) on the untreated industrial reference pulp is shown for comparison. The confidence intervals are of 95% significance. Paper sheets made from the laboratory-cooked reference pulp had a slightly lower strain to failure value, than sheets made from the industrially produced reference pulp. This relation prevailed also after CMC attachment, which increased the strain to failure for both the industrial and the laboratory pulp by approximately 85%. PFI beating had a small effect on strain to failure and, at a given sheet density, the strain to failure value for the CMC-modified pulps was considerably higher. Fig. 3 shows the effect on tensile stiffness index of CMC attachment, together with the effect of PFI beating. Paper sheets produced from the laboratory-cooked reference pulp had higher tensile stiffness index than sheets made from the corresponding industrial pulp. Surface carboxymethylation of industrial and laboratorycooked pulp resulted in an increase in tensile stiffness index. The relative increase was approximately 20% for

8 both pulp types. had a greater impact on tensile stiffness index than CMC, but it was accompanied by significant sheet densification. Tensile Stiffness index [knm/g] Sheet density [kg/m 3 ] Figure 3. The effect of CMC attachment (20 mg/g) on paper sheet tensile stiffness index, when added to a bleached, never-dried and industrially produced pulp and to a bleached, never-dried and laboratory-produced pulp. The effect of (0, 1000 and 3000 revs.) on the untreated industrial reference pulp is shown for comparison. The confidence intervals are of 95% significance. Finally, the effect of CMC attachment on the Z-strength at two different wet pressing levels was investigated and compared to the effect of. Fig. 4 and Fig. 5 show results for the lower and the higher pressing level respectively Low pressing level Z-strength [kpa] CM C att achment Sheet density [kg/m 3 ] Figure 4. Low pressing level. The effect of CMC attachment (20 mg/g) on paper sheet Z-strength, when added to a bleached, never-dried and industrially produced pulp and to a bleached, never-dried and laboratory-produced pulp. The effect of (0, 1000 and 3000 revs.) on the untreated industrial reference pulp is shown for comparison. The confidence intervals are of 95% significance.

9 Z-strength [kpa] High pressing level Sheet density [kg/m 3 ] Figure 5. High pressing level. The effect of CMC attachment (20 mg/g) on paper sheet Z-strength, when added to a bleached, never-dried and industrially produced pulp and to a bleached, never-dried and laboratory-produced pulp. The effect of (0, 1000 and 3000 revs.) on the untreated industrial reference pulp is shown for comparison. The confidence intervals are of 95% significance. At the lower pressing level, a small difference in Z-strength between the industrial and laboratory reference pulp can be seen. Surface carboxymethylation increased the Z-strength of both pulps to the same extent but, compared to beating, the effect of CMC was poor. At the higher pressing level, the difference between the industrial and the laboratory-produced pulp was levelled out. Compared to the lower pressing level, attachment of CMC had a large effect on Z-strength for both pulp types. The Z-strength enhancement achieved by CMC attachment was of the same magnitude as the enhancement achieved by, but with less densification. Effect of CMC on fibre shape Table 5 reveals a large effect of CMC on fibre shape. In order to further investigate the importance of this straightening effect, the rewetted zero-span tensile strength of paper strips made from both the industrial and laboratory-produced pulp was determined. The results are shown in Fig. 6 Rewetted zero-span index [Nm/g] Figure 6. The effect of CMC attachment (20 mg/g) on rewetted zero-span index, when added to a bleached, never-dried and industrially produced pulp and to a bleached, never-dried and laboratory-produced pulp. The confidence intervals are of 95% significance. It is evident that surface carboxymethylation of fibres increased the rewetted zero-span index significantly for both pulp types. However, the largest effect was obtained with CMC attachment to the laboratory-produced

10 pulp. The increase in rewetted zero-span tensile index correlated well with shape factor, as can be seen in Fig. 7. The effect of is shown in the same figure for comparison. 180 Rewetted zero-span index [Nm/g] Heat treatment Shape factor [%] Figure 7. The correlation between rewetted zero-span index and shape factor, when CMC is attached (20 mg/g) to a bleached, never-dried and industrially produced pulp and to a bleached, never-dried and laboratory-produced pulp. The effect of (0, 1000 and 3000 revs.) on the untreated industrial reference pulp is shown for comparison. The confidence intervals are of 95% significance. Heat treatment (e.g. surface carboxymethylation conditions, but with no CMC addition) of the unbeaten industrial pulp resulted in a decrease in the shape factor, whereas both CMC attachment and increased both the shape factor and the rewetted zero-span index. The increase in rewetted zero-span index with increasing shape factor is independent of the straightening method but, at a given shape factor, the CMCmodified pulp had a higher rewetted zero-span index than the PFI-beaten pulp. Discussion This paper has demonstrated that CMC can be successfully attached onto both industrial and laboratoryproduced pulps. An attachment level of 100% for both pulp types implies that the difference in surface properties between industrial and laboratory produced-pulp does not effect the attachment efficiency. The addition of CMC to both industrial and laboratory produced-pulps has a strong influence on the shape factor and on the kink value (Table 5). The shape factor and kink data indicate that surface carboxymethylation of pulp straightens the fibres. Attachment of CMC introduces more carboxyl groups onto the fibre surface and hence increases the surface charge density (Table 4). The straightening effect of CMC on fibre shape is therefore interpreted in terms of an increase in carboxyl group repulsion. This finding is somewhat surprising, but Lennholm (1995) showed that the adsorption of xylan onto the fibre surface leads to an increase in shape factor, e.g. straighter fibres. It has also been reported that bulk carboxymethylation of softwood kraft pulp increases the shape factor (Zhang and Mohlin 1995). As can be seen from Table 5, the straightening effect of CMC was more pronounced for the industrial pulp, which is probably explained by the fact that this pulp had a lower shape factor originally. Mohlin et al. (1996) demonstrated a strong correlation between the number of fibre deformations and the rewetted zero-span index. The addition of CMC to industrial and laboratory-produced pulp had a large effect on rewetted zero-span (Fig. 6) and there was clearly a correlation between shape factor and rewetted zero-span index (Fig. 7). Since little stress transfer occurs between wet fibres, local defects have a large impact on the rewetted zero-span index (Mohlin et al. 2003) and the higher rewetted zero-span value observed for CMC-

11 treated pulps can therefore be understood in terms of a higher shape factor and fewer kink per fibre. The effect of CMC on fibre shape is clearly of the same order as the effect achieved by moderate. Furthermore, CMC attachment greatly reduces friction between fibres during sheet consolidation (Yan 2006). This in turn leads to fewer residual stresses in the fibre network after forming, which may even be reflected in the rewetted sheet and hence in the rewetted zero-span index measurements. The interpretation of the rewetted zero-span index is not however clear-cut. In Fig. 7 it can also be seen that heat treatment leads to an increase in the rewetted zero-span index with decreasing shape factor. Heat treatment is associated with a small dissolution of substances, leading to a lower charge density. Apart from the higher curl/kink frequency, the effect of heat treatment on fibre strength is not yet known. Fibre swelling, measured as WRV, is affected by both pulp type and by surface carboxymethylation. The fact that the industrially produced reference pulp had a lower WRV than the laboratory reference pulp (Table 4), is probably due to the fact that industrial fibres are subjected to more mechanical stress during the pulping process and therefore have a slightly more damaged cell wall with less ability to retain water (Joutsimo et al. 2006). Attachment of CMC leads to an increase in the WRV and when CMC was attached to the fibres, the difference between the two pulp types was levelled out. The effect of CMC on WRV can be explained by the hypothesis that the CMC molecule is attached to the fibre surface in a bulky conformation containing loops and tails which thus retains water. The effect of CMC on WRV has previously been discussed in detail by Laine et al. (2002a). Paper strength is strongly influenced by the attachment of CMC. The influence of CMC on the in-plane tensile properties is shown, for both the industrial and the laboratory pulp, in Fig. 1-Fig. 3. For comparison, the effect of beating is also shown in these figures. All the strength properties are significantly increased by the addition of CMC and the magnitude of the effect is approximately the same for both pulp types. In contrast to beating, surface carboxymethylation has little effect on paper sheet density. The nature of CMC s strengthenhancing ability is not yet clear. It has earlier been suggested that the attachment of CMC leads to an improvement in the specific bond strength between fibres rather than to an increase in the relative bonded area, at least on the structural levels which affects the lightscattering coefficient (Laine et al. 2002a). The small effect on sheet density supports this conclusion. The impact of fibre shape on strength properties can not however be neglected. Fibre deformations are known to have a large impact on paper strength properties (Perez and Kallmes 1965; Page et al. 1985; Ellis et al and Mohlin et al. 1996). The small differences in strength between the industrial and laboratory reference pulps and between the carboxymethylated industrial pulp and carboxymethylated laboratory pulp (Fig. 1-Fig. 3) can also be explained by differences in the shape factor (Table 5). The effective stress distribution in a paper sheet is impaired by fibre curl, whereas the extensibility of the fibre network is improved. Hence, fibres with a low shape factor give paper sheets with poor tensile strength and tensile stiffness, while strain at break is favoured by fibre curl. Fig. 2 reveals, however, that the strain to failure strongly increases when CMC is attached to the pulps, in spite of the fact that the fibres simultaneously become straighter. This is due to the strong effect of joint strength on the postyield region of the stress-strain curve, where the specific bond strength has a dominant influence on strain to failure. This also explains why beating has only a small effect on strain to failure. The fibre straightening effects of beating greatly diminish the joint-strengthening effects.

12 Nor could any difference between industrial and laboratory-produced pulp be detected when the effect of CMC addition on Z-strength was studied (Fig. 4 and Fig. 5). The Z-strength of both pulp types was however strongly influenced by wet pressing. A higher pressing level brings the fibres closer together, the development of fibre-fibre bonds is facilitated and hence, the Z-strength is increased. The effect of CMC addition on Z-strength is greater when a higher pressing level is used. CMC is thought to increase the specific bond strength between fibres (Laine et al. 2002a). This difference in CMC effectiveness between the higher and lower pressing levels is therefore best understood in terms of a larger number of fibre-fibre contact points in the former case. The greater effect of beating than CMC attachment at the lower pressing level is probably due to the strong contribution to Z-strength of fibre flexibility at low pressing levels. The results shown in Fig. 1-Fig. 2 indicate that the strength-enhancing effect of CMC is independent of whether an industrial or a laboratory-cooked pulp is used for surface carboxymethylation. One reason for this finding may be the fibre-straightening effect of CMC. Once the CMC molecules are attached to the fibre surface, the difference in the number of fibre-fibre contacts between industrial and laboratory pulps is levelled out. Conclusions The effects of CMC addition to an industrially cooked pulp and to its corresponding laboratory pulp were examined and compared to the effect of. A CMC attachment level of 100% was reached for both pulp types and it was therefore concluded that the differences in surface properties between industrial and laboratory-produced pulps do not affect the adsorption efficiency. Surface carboxymethylation was also shown to have a large effect on the shape factor and on the number of kinks per fibre. Attachment of CMC straightened the fibres, which in turn led to an increase in the rewetted zero-span index. These results were interpreted in terms of an increase in carboxyl group repulsion on the fibre surface. Finally, it was found that the ability of CMC to improve paper sheet strength properties is independent of whether an industrial or a laboratory cooked pulp is used for surface carboxymethylation. One contributory factor may be the straightening effect of CMC. Differences in fibre shape between industrial and laboratory-produced pulp are levelled out by the attachment of CMC. Literature: Brännvall, E. and Lindström, M. E. (2006): A study on the difference in tensile strength between industrially and laboratory-cooked pulp. Nord. Pulp Paper Res. J. 21(2): 222. Dai, Q., Jameel, H. and Chang, H-M. (2006): Precipitation of extractives onto kraft pulps during black liquor recycling in extended delignification process. J. Wood Chem. Technol. 26(1):35. Ellis, M. J., Allison, R. W. and Kibblewhite, R. P. (1995): The effect of laboratory mixing at medium pulp concentration on radiata pine kraft pulp. Appita 48(5): 358. Hakanen, A. and Hartler, N. (1995): Fiber deformations and strength potential of kraft pulps. Paperi Puu 77(5):339. Horvath, A. E., Lindström, T. and Laine J. (2006): On the indirect titration of cellulosic fibres. Conditions for charge stoichiometry and comparison with ESCA. Langmuir 22(2):824. Joutsimo, O., Wathén, R., Robertsén L. and Tamminen, T. (2006): The influence of fibre ultrastructural damage on pulp and paper properties. In 60 th Appita annual conference and exhibition, pages , Melbourne, Australia. CD-Rom, ISBN X.

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