Carboxyl groups in pre-treated regenerated cellulose ...

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Mar 25, 2008 - total amount of acidic groups in the fibres, but their accessibility to cationic ... modal (CMD), and lyocell (CLY) were produced by. Lenzing Ag ...
Cellulose DOI 10.1007/s10570-008-9216-6

Carboxyl groups in pre-treated regenerated cellulose fibres Lidija Fras Zemljicˇ Æ Zdenka Persˇin Æ Per Stenius Æ Karin Stana Kleinschek

Received: 7 December 2007 / Accepted: 25 March 2008 Ó Springer Science+Business Media B.V. 2008

Abstract The influence of peroxide bleaching and slack-mercerization on the amount of acidic groups in regenerated fibres (viscose, modal and lyocell) were studied. Conductometric titration was used to determine the total content of acidic carboxylic groups. Polyelectrolyte titration was used for surface and total charge determination, and to obtain information about the charge distribution and accessibilities of charged groups. Changes in fibre crystallinity to pre-treatment processes were characterized using iodine sorption (Schwertassek method) and correlated to treatments and the amount of carboxylic groups. For all three types of fibres the amount of accessible carboxyl groups was lowered by an increase in the degree of crystallinity. Bleaching with hydrogen peroxide causes some oxidative cellulose damage and, therefore, a larger amount of carboxyl groups

Lidija Fras Zemljicˇ, Zdenka Persˇin, and Karin Stana Kleinschek are the members of the European Polysaccharide Network of Excellence (EPNOE). L. F. Zemljicˇ (&)  Z. Persˇin  K. S. Kleinschek Laboratory for Characterization and Processing of Polymers, Institute for Textile Materials and Design, Faculty of Mechanical Engineering, University of Maribor, Smetanova ulica 17, 2000 Maribor, Slovenia e-mail: [email protected] P. Stenius Laboratory of Forest Products Chemistry, Helsinki University of Technology, P.O. Box 6300, 02015 Espoo, Finland

(presumably formed at the end of cellulose chains). Slack-mercerization did not significantly change the total amount of acidic groups in the fibres, but their accessibility to cationic polyelectrolytes, in particular to polymers with high molecular weight was substantially lowered. Keywords Regenerated fibres  Cellulose fibres  Crystallinity  Carboxylic groups  Pre-treatment  Conductometric titration  Polyelectrolyte titration

Introduction The differences between natural and different types of regenerated cellulose fibres lie primarily in the size and orientation of crystalline and amorphous phase, size and shapes of voids, the number of interfibrillar bonds and the nature of impurities (Kra¨ssig 1984, 1992). Impurities in regenerated fibres, such as oils, waxes, antistatic agents, lubricants and stiffening additives originate from the manufacturing process. They can be amphiphilic or hydrophobic, thus lowering the hydrophilicity and adsorptivity of the fibres (Bredereck et al. 1996), which are properties of importance in further processing of the fibres. The chemical and mechanical properties of regenerated fibres can be improved using standard chemical pretreatment processes, such as alkaline washing, bleaching and slack-mercerization. These processes

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modify the fibres both structurally and chemically. Previous research has mainly focused on analysis of the fine structure of regenerated fibres. Thus, it has been shown that pre-treatment and cleaning changes the cellulose crystal structure and the degree of crystallinity (Lewin and Roldan 1971; Blackwell et al. 1977; Schurz and Ja´nosi 1982; Kra¨ssig 1992; Schurz and Lenz 1994; Kreze et al. 2001, 2002; Persin 2001; Sfiligoj Smole et al. 2003; Persin 2004). Much less attention has been given to the reactivity of regenerated fibre surfaces and its correlation with the quality and quantity of dissociable groups in the fibres. These are of importance to fibre accessibility and interactions, which are essential in processing and applications of fibres. Earlier studies of the dissociable groups in regenerated cellulose have mainly focused on the use of zeta potential (Hunter 1981; Jacobasch et al. 1985). Ribitsch et al. (2001) show that fibre cleaning processes such as alkaline or acidic cleaning and petrol ether extraction result in increased adsorption and surface charge density, as a consequence of the higher amount of accessible negative fibre groups. Effects of different treatment processes on regenerated fibre reactivity have been studied using tensiometry by Stana-Kleinschek et al. (2002) and Persin et al. (2004). Pre-treatment of these fibres increases the relative amount of amorphous domains, resulting in an increase of the surface energy and dispersion forces as well as in a reduction of the interaction of the fibres with aromatic cationic surfactant. Dissociating groups in fibres, which in addition to the structural parameters are determinative for fibre adsorption behaviour, were studied by titration (Barzyk et al. 1997; Fras-Zemljicˇ et al. 2006; Fras et al. 2004) for determining the dissociative properties of natural and non-treated regenerated fibres used in textiles manufacturing using conductometric, potentiometric and polyelectrolyte titration. Combination of the three methods yields relevant information about the amount and strength of acidic groups. This work describes the use of conductometric and polyelectrolyte titration to analyse the influence of bleaching and slack-mercerisation on the dissociation properties (the amount of carboxylic groups) of regenerated cellulose fibres (viscose, modal and lyocell). The correlations between these properties and fibre structure were also investigated. Fibre crystallinity was characterized by iodine sorption

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(ISV), as originally proposed by Schwertassek (1931, 1960, 1961).

Experimental Starting materials The regenerated cellulose fibres, i.e. viscose (CV), modal (CMD), and lyocell (CLY) were produced by Lenzing Ag, Lenzing, Austria. The linear densities of all fibres were 1.3 dtex and the fibre lengths were 39 mm (viscose and modal fibres), and 38 mm (lyocell fibres). Average molecular weights, Mw, and crystallinities are given in Table 1. Mw was determined by viscosimetry (Kra¨ssig 1984; Klemm et al. 1998) and crystallinity index was determined using wide-angle X-ray scattering. For details, see Sfiligoj Smole et al. (2003). Pre-treatments The fibres were pre-treated by conventional pretreatment processes commonly used in textile praxis: first alkaline washing, and then bleaching or slackmercerisation (Table 2). The fibres were washed with distilled water until the conductivity of the water was \3 lS/cm. Table 1 Structural parameters of untreated and pre-treated regenerated cellulose fibres (Persin 2001, Sfiligoj Smole et al. 2003) Structural parameters

Average molecular weight

Crystallinity index xeqa%

CV

(45.9 ± 0.7) 9 103

47.1

CMD

(84.2 ± 1.8) 9 103

48.3

CLY

(10.9 ± 0.3) 9 104

73.4

CVb CMDb

(43 ± 2) 9 103 (53.6 ± 1.5) 9 103

47.5 57.9

CLYb

(7.4 ± 0.6) 9 104

77.4

CVa

(45.1 ± 1.6) 9 103

71.6

CMDa

(69 ± 25) 9 103

74.0

Non-treated fibres

Bleached fibres

Slack-mercerised fibres

CLYa a

(11 ± 0.9) 9 10

4

83.9

Crystallinity index xeq calculated from the equatorial curve according to X-ray wide angle diffraction (2h = 5° - 45°)

Cellulose Table 2 Conditions used in the pre-treatment of regenerated cellulose fibres

Alkaline washing

Bleaching

Slack-mercerisation

1 g/L Na2CO3

6 mL/L of 30% H2O2

40 g/L NaOH

1 g/L Sandoclean PC (wetting agent, anionic)

2 mL/L Tanatex Geo (mineral stabiliser for H2O2 stabilisation)

7 mL/L Tanawet BC (wetting agent, anionic)

pH = 10.9 t = 30 min

pH = 10.7 t = 30 min

pH = 12.8 t = 1 min

T = 60 °C

T = 98 °C

T = 10 °C

Preparation of fibres for analysis Before titrations all untreated and pre-treated regenerated cellulose fibres were ground into small particles. Then they were ion-exchanged into acid form by suspending them in 0.1 M HCl for 15 min. Excess acid was removed by washing with deionised water until the conductivity of the filtrate was \3 lS/ cm. Most of the water was removed by filtering (Bu¨chner funnel). The ion-exchanged fibres were stored (4–6) °C in wet form and were not dried before titration. Chemicals Polyelectrolytes Total charge was determined using Poly(1,5-dimethyl1,5-diazaundecamethylene) bromide, ‘‘Polybrene’’, (molecular weight Mw = 8,000), from Sigma and Poly(dimethyldiallyl-ammonium) chloride, ‘‘PDMAAC’’, (Mw [ 100,000), from Ciba was used for surface charge determination. Excess cationic polymer titration was titrated with sodium poly(ethylenesulfonate) (PesNa) from MT Instruments Oy. Other chemicals Ion exchanged, distilled and de-gassed water and analytical grade chemicals were used in all analyses.

Methods

solution was filtered and the concentration of iodine in the filtrate was determined by titration with 0.02 N sodium thiosulphate. ISV (mg absorbed iodine/g cellulose), and the degree of crystallinity were calculated from the difference in iodine concentration between the filtrate and a blank solution, as described by Hessler and Power (1954) and Schwertassek (1961).

Determination of acidic groups and charge distribution Conductometric and polyelectrolyte titration was used for determination of acidic groups and their accessibility. For a detailed description of the methods, see Fras et al. (2004). Conductometric titration One gram of wet fibres were suspended in 500 mL 1 mM NaCl, acidified with 0.5 mL 0.1 M HCl and titrated at 25 ± 0.5 C with 0.1 M NaOH added from a precision burette in steps of 0.04 mL over 1-min intervals under argon atmosphere, using a Metrohm 712 conductometer. Typically, titrations finished at pH & 10.5. This ensured reliable extrapolation of the linear parts of the titration curves (Sjo¨stro¨m and Enstro¨m 1966; Katz et al. 1984). Elimination of impurity effects was ensured by blank titration without fibres (Fras et al. 2004). The total amount of acidic groups was obtained from the intersection of the second and the third linear parts of the titration curve. The total acidic group content (X) was calculated from:

Determination of iodine sorption value and degree of crystallinity

X ¼

Iodine sorption (ISV) was determined by the Schwertassek method. The fibres were first treated with KI3 solution. After an equilibration period (2 h), the

where COH is the concentration and Vt is the volume of NaOH consumed at the second intersection point and mdry is the oven-dry weight of fibres (Fras et al. 2004).

COH Vt ; mdry

ð1Þ

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The reported amounts of acid groups are the mean values of five separate titrations. Variation between analyses was \2% in all cases.

Table 3 Iodine sorption values (ISV) and crystallinity indexes calculated from ISV (xISV) of untreated, bleached, and slackmercerised viscose, modal and lyocell fibres Fibre type

Polyelectrolyte titration

ðVtit  V0 Þ  Cpolymer Vtot mdry Vsample

ð2Þ

where V0 is the volume of PesNa consumed by the blank, Vtit is the volume of PesNa solution consumed in titration of the fibres, Cpolymer is the concentration of charged groups on the cationic polymer, Vtot is the total volume of filtrate, Vsample is the volume of the titrated filtrate suspension and mdry is the dry mass of the sample. The acidic groups (charge) on the fibres were determined by extrapolation of saturation coverage for least four different polymer concentrations to zero polymer concentration (Laine et al. 1994, 1996; Laine and Stenius 1997). Polybrene adsorption was used for total charge evaluation. PDMAAC adsorption was used to assess

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xISV %

Non-treated fibres

All acidic groups were first converted into their sodium form by suspending the fibres in 1 9 10-3 M NaHCO3 at pH 9 (adjusted with 0.1 M NaOH). Excess electrolyte was removed by washing the fibres with deionised water until the conductivity of the supernatant was \2 lS/cm. About 0.3 g of dry fibres was weighted into a 100 mL beaker. Forty millilitres of 0.01 M NaCl was added and pH was adjusted to 7. An excess of cationic polymer (1.36 g/L PDMAAC or 2.35 g/L Polybrene) was added to the suspension. After equilibration (stirring) for 30 min the suspensions were filtered through weighted filter paper on a Bu¨chner funnel and washed with water so that the total amount of filtrate was 55 mL. Ten millilitres of filtrate was titrated with PesNa (1 g/L) using a Mu¨tek particle charge detector to detect the end point (zero potential). Blank samples (cationic polymers only), were titrated in the same way as the fibres, so that any effects on polymer adsorption by the glassware and glass fibre filter could be eliminated. The fibres on the filter paper were oven-dried at 105 °C for at least 4 h and allowed to cool down in a desiccator. The amount of charged groups (Q) on the fibres corresponding to the adsorbed amount of polymer was calculated from: Q ¼

ISV, mg/g

CV

137 ± 2

66.7 ± 0.5

CMD

130 ± 2

68.4 ± 0.6

CLY

123 ± 2

70.1 ± 0.4

Bleached fibres CVb

126 ± 2

69.4 ± 0.4

CMDb

126 ± 0.5

69.5 ± 0.1

CLYb

101 ± 4

75.5 ± 1.0

Slack-mercerised fibres Cva

97 ± 2

76.5 ± 0.6

CMDa CLYa

92 ± 5 88 ± 2

77.7 ± 1.1 78.6 ± 0.5

the charge on the external surface (Laine and Stenius 1997; Fras et al. 2004). All presented values are the mean values of three parallel determinations.

Results Crystallinities The crystallinities calculated from the ISV of fibres are given in Table 3. The results are in good agreement with the X-ray crystallinities given in Table 1. Acidic groups and fibre charge The amounts of acidic groups in the fibres, determined by conductometric and polyelectrolyte titration, are shown in Table 4. The trend is similar for all fibres: viscose fibres contained the highest amount of carboxyl groups, followed by the modal fibres and then the lyocell fibres. The initial amount (untreated fibres) of acidic groups in the modal fibres was about half of that in the viscose fibres and 30-35% higher than in the lyocell fibres. Bleaching increased the charge of all fibres, obviously because oxidation by H2O2 introduces new carboxyl groups. The influence of slackmercerization on the amount of acidic groups was insignificant.

Cellulose Table 4 Amounts of acidic groups in regenerated cellulose fibres obtained by conductometric and polyelectrolyte titrations

Method

Conductometric titration Acidic groups, mmol/kg

Polyelectrolyte titration Total charge, mmol/kg

Surface charge, mmol/kg

Total charge/surface charge ratio

4.7 ± 0.4

5:1

Untreated fibres CV

48.6 ± 1.0

24 ± 1.4

CMD

27.2 ± 0.5

16 ± 0.8

3.5 ± 0.1

5:1

CLY

20.6 ± 0.5

15 ± 0.9

3.5 ± 0.1

4:1

Bleached fibres CVb

57.1 ± 1.5

29 ± 1.2

7.5 ± 1.0

4:1

CMDb

49.1 ± 0.9

23.5 ± 1.5

6 ± 0.5

4:1

CLYb

32.2 ± 0.2

21 ± 0.5

5 ± 0.5

4:1

Slack-mercerised fibres Total charge: from titration with Polybrene. Surface charge: from titration with PDMAAC

Cva CMDa

44.5 ± 0.5 26 ± 0.3

17 ± 1.0 11.5 ± 0.8

2.5 ± 0.4 2.5 ± 0.5

7:1 5:1

CLYa

19.2 ± 0.5

9 ± 0.9

2 ± 0.3

5:1

Trends in the total charge determined by polyelectrolyte titration were similar to those determined by conductometric titration.

Discussion

to experimental uncertainties: crystallinity as determined by ISV is indirect method based on wet fibre properties, while X-ray diffraction is a direct method based on dry fibre properties. However, the two methods correlate reasonably well; the crystallinity index calculated from ISV results tends to rise when the X-ray index rises.

Structural parameters Correlation between crystallinities from X-ray diffraction and ISV

Crystallinities of different pre-treated regenerated cellulose fibres

Figure 1 shows that the ISV crystallinity indexes (Table 3) were larger than X-ray crystallinity indexes (Table 1). The difference can probably be attributed

Untreated fibres The ISV crystallinities of untreated viscose and modal fibres are (within experimental error) almost equal while it is about 4% higher for

85

75

ISV

,%

80

x

Fig. 1 Correlation of crystallinity index gained by wide-angled X-ray scattering to crystallinity index calculated from ISV results. The R2-value of the trendline shown is 0.75. The mean uncertainty of xISV is assumed to be 0.6%

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60 40

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90

xeq, %

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lyocell fibres. The differences between untreated regenerated fibres are due to the different spinning conditions (Kra¨ssig 1992; Klemm et al. 1998).

Titrations

Bleached fibres The ISV crystallinity of viscose was increased about 4% by bleaching. The effect on the crystallinity of modal fibres was even smaller: 1.5%. Bleaching of lyocell fibres resulted in a more substantial increase: 8%. Bleaching may cause some oxidative cellulose damage in viscose and modal fibres and, therefore, any increase in crystallinity is hindered (Persin 2004).

Conductometric and polyelectrolyte titrations are compared in Fig. 2. The reproducibility of conductometric titration, which is a direct method, (coefficients of variation 0.5–3%) was much better than the reproducibility of polyelectrolyte titration, which is an indirect method (variation coefficients 4–16%). In Fig. 2 the uncertainties are set to ±0.7 mmol/kg for conductometric titrations and ±1 mmol/kg for polyelectrolyte titrations. The general trend is that an increase in the amount of acidic group determined by conductometric titration is accompanied by an increase in total charge determined by adsorption of the low-molecular weight polymer (Polybrene), but the correlation is not particularly good. More importantly, adsorption of the low Mw polymer detected, on average, is only around half of the amount of acidic groups detected by conductometric titration. This indicates that charges in the smallest pores of regenerated fibres are inaccessible even for a polyelectrolyte with a molecular weight as low as Mw = 8,000. This may be due to either the size of the polymer molecules or a slow adsorption process. In addition, even very small amounts of impurities remaining on the fibre surface may cover some

Slack-mercerized fibres The largest change occurred for viscose fibres with a 15% increase in crystallinity index. Slack-mercerisation has the smallest effect on lyocell fibres, a 12% increase. A possible reason for the significant increase in the crystallinity index due to slack-mercerization is that this treatment takes place in a strongly alkaline solution (Table 2). This would result in much more extensive solvation of the hydroxyl groups in the cellulose than the two other treatments, in particular in the amorphous domains. On drying, the swollen chains would stick together, resulting in increased crystallinity (Persin 2004). Note that the effect is more marked for the viscose and modal fibres, which are initially less crystalline than lyocell.

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-COOH, mmol/kg

Fig. 2 Correlation between conductometric and polyelectrolyte titrations. The R2-value of the trendline shown is 0.78. The uncertainties are set to ±0.7 mmol/kg for conductometric titrations and ±1 mmol/kg for polyelectrolyte titrations

Comparison of titration methods

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0 0

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Total charge, mmol/kg

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carboxyl groups and block the ion-exchange reaction with polyelectrolytes (Kra¨ssig 1984, 1992). These results are in agreement with earlier studies of adsorption of two dyes with the same structure but different molecular weights (Mw = 1373. 07 g/mol and Mw = 624.55 g/mol) on non-treated, bleached and slack-mercerised fibres (Persin 2004). These studies showed that the adsorbed amount of higher molecular weight dye was only 50% the amount of low molecular weight dye. Conductometric titration: effect of pre-treatments on carboxylic groups Conductometric titration implies the neutralization of all acidified ionisable groups by reaction with hydroxyl ions. Because both acidification and neutralisation involve small and rapidly diffusing ions it can be assumed that this titration gives a measure of the total amount of acidic groups in the fibres. Moreover, it can be assumed that in purified regenerated fibres these are exclusively carboxyl groups. Untreated fibres The amount of carboxylic groups in fibres subjected to chemical modification during manufacture (viscose, modal) is higher than in the mildly treated lyocell fibres (Table 1). This is not surprising, since viscose and modal fibre manufacture includes processes that may lead to oxidation and/or depolymerization of cellulose; i.e.: alkaline and oxidation treatments as well as xanthogenation and desulphuration. In contrast, the lyocell fibres are spun using organic solvent only. To this solvent antioxidants are added in order to control reaction. Bleached fibres As for non-treated regenerated fibres, the bleached fibres that had been treated chemically during manufacture (CVb, CMDb) contained more carboxyl groups than the solventspun fibre (CLYb). The amount of carboxyl groups in all of the regenerated fibres was increased by bleaching. The hydrogen peroxide used for bleaching introduces new carboxyl groups by nonselective oxidation. Slack-mercerised fibres The amount of acidic groups in the slack-mercerised fibres was almost the same as in the untreated fibres. This is somehow surprising; treatment in strong alkali could, in

principle, lead to undesirable peeling reactions with concomitant formation of carboxylic groups (Sjo¨stro¨m 1992; Ale´n 2000). However, in comparison to known literature data (Lewin et al. 1983) the slack-mercerised treatment conditions used in order not to damage fibres were mild (Table 2): (i) short treatment time; (ii) lower alkali concentration and (iii) lower treatment temperature. As the results of average molecular weights show (Table 1), this treatment did not result in significant peeling and therefore, no additional acidic groups were introduced into fibres. Polyelectrolyte titration: effect of pre-treatments on charge Total (accessible) charge The bleaching process introduced new charges accessible to the low molecular weight polymer; it increased by 20% in the viscose fibres and by 46% and 40% in the modal and lyocell fibres, respectively. These increases were relatively lower than the increases in carboxylic groups detected by conductometric titration. Thus, the bleaching process resulted in oxidation of fibre domains that were not accessible to the polymer. Slack-mercerisation resulted in a substantial reduction of the amount of accessible charge. The total charge was reduced by around 30% in the viscose fibres and by about 35% in the modal and lyocell fibres. A likely reason is the higher crystallinity of slack-mercerized fibres (Table 1), which should reduce the ability of a polyelectrolyte to penetrate into the fibre. Surface charge The surface charges, i.e. the charges accessible to the high molecular weight PDMAAC, of untreated modal and untreated lyocell fibres, were equal and both of them were lower than those of untreated viscose fibres. The approximate ratios between total and surface charges for each fibre sample are shown in Table 4. It can be concluded that most of the carboxyl groups, independent of pre-treatment, are located in the internal parts of the regenerated fibres. Bleached fibres had the highest amounts of surface charge. This may be due to oxidation or to removal of hydrophobic surface impurities. The latter effect seems less likely, since the ratio of total to surface charge is not much affected by bleaching.

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index was the highest for lyocell fibres followed by modal and viscose fibres, independent of the pretreatment process (Table 1). The amount of –COOH groups determined using conductometric titration is compared with ISV values for untreated and pre-treated regenerated fibres in Fig. 3. Figure 3 shows that the amount of carboxyl groups decreased when the crystallinity increased, whether the fibres were pre-treated or not. This is a very reasonable result: when the fraction of amorphous domains is reduced the amount of carboxyl groups accessible to titration is reduced. Assuming that there are no carboxyl groups in the crystalline regions, it can be concluded that crystallinity is of great importance for the amount of carboxyl groups formed during different stages of fibres processing. Even though accessibility was lowered by increasing crystallinity, bleached regenerated fibres contained the highest amounts of acidic groups. The reason is clearly that oxidation by H2O2 breaks the 1,4 b-glucosidic bond in cellulose molecules resulting in the formation of new end carboxyl groups (Ale´n 2000), more extensively on the fibre surface. This is shown by the reduction of Mw when fibres are bleached (Table 1).

The surface charges of all slack-mercerised fibres were almost equal (i.e. CVa and CMDa 2.5 mmol/kg and CLYa 2 mmol/kg) and on average, around 1 mmol/kg lower than the charges of untreated fibres. This indicates that the PDMAAC was able to partially penetrate into untreated fibres, an effect that should be reduced by the increased crystallinity induced by slack—mercerization. The weight loss after slack-mercerisation treatment could somehow also contribute to the reduction of surface charge. The weight loss by CVa was 1.73%, while by CMDa and by CLYa the weight loss was only 0.1%. Summarising, the surface charge was much lower than the total charge in all regenerated fibres (Table 4). It is likely that, because of the very low specific surface area of regenerated fibres, PDMAAC, which has a very high Mw, can only access certain of the acidic groups on the surface. However it is also evident that most of the carboxyl groups are located in an inner region of the regenerated cellulose fibres. Influence of structural parameters on the amount of carboxyl groups The differences in acid content of the analyzed samples may be explained by different properties, such as average molecular mass, degree of polymerization, crystallinity index (see Table 1), etc. The most important structural parameter in this context is probably the degree of crystallinity. The crystallinity

The use of titration (conductometric and polyelectrolyte) in combination with the determination of

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60 CV

CMD

CLY

CVb

CMDb

Fibre type

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CLYb

Cva

CMDa

CLYa

XISV , %

carboxyl groups, mmol/kg

Fig. 3 Comparison of –COOH groups with XISV for untreated and pre-treated regenerated cellulose fibres. Bars: carboxyl groups; line: crystallinities. Line is drawn for guidance of the eye only

Conclusions

Cellulose

crystallinity by the Schwertassek method is a powerful tool and a new approach for following, in detail, the influence of pre-treatment processes, as for example bleaching and slack-mercerisation, on the accessibility and amount of acidic groups in regenerated cellulose fibres. The content of accessible carboxyl groups in viscose, modal and lyocell fibres is lowered when the degree of crystallinity increases. Bleaching with hydrogen peroxide results in oxidative cellulose damage and, therefore, bleached regenerated fibres contain larger amounts of carboxyl groups (presumably formed at the end of cellulose chains). Slackmercerization does not significantly change the total amount of acidic groups in the fibres, but the accessibility to cationic polyelectrolytes, in particular to polymers with high molecular weight, is substantially lowered. Titrations could be useful for optimisation and control of important physical and mechanical properties of regenerated fibres during different treatment processes. From a practical point of view the fibre functionalisation (higher hydrophilicity, better adsorption capacity, antimicrobial properties, etc.) can be estimated by functional groups titration. Acknowledgments We thank the Finnish Centre for International Mobility (CIMO), which provided financial support for this work and Prof. Janne Laine from Laboratory of Forest Products Chemistry for scientific discussion. We also thank Katja Routanen and Ritva Kivela¨, Laboratory of Forest Products Chemistry, as well as Tanja Kos and Vida Zˇizˇek, Laboratory for Characterization and Processing of Polymers, for their skilful technical assistance.

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