Method of producing paper and cardboard

The present invention relates to a method according to the preamble of claim 1 for producing paper and cardboard.

According to such a method in manufacturing of paper or cardboard is used a bleached cellulosic pulp, which comprises hemicelluloses on a paper or cardboard machine.

Low strength of the paper web causes runnability problems in papermaking. Bonding degree of the fiber network controls the mechanical properties of paper. The network would have no cohesion if there were too few bonds between the fibers. Thus, bonding degree of the fiber network should be increased to improve strength properties and runnability of paper.

Xylan in pulp plays an important role both in fiber morphology and paper physics [I].

The bonding between papermaking fibers is conventionally considered primarily to be due to hydrogen bonds. Carboxylic groups are often components of hydrogen bonding. High contents of carboxylic acid groups on fiber surfaces increases inter-fiber bond strength. The concentration of acidic groups on the surface of the fiber has a relatively greater effect on bonding and tensile strength of the dried sheet than the total concentration acidic groups.

Uronic acid groups in hemicelluloses, e.g. in xylan of hardwoods, are the main carboxylic group constituents in wood. The higher xylan content of birch wood compared, for example, to eucalyptus, makes a controlled manipulation of the xylan content in the fiber wall or on the fiber surface of pulp favorable with respect to tailor-make birch pulp properties for different paper and board grades.

During cooking some resorption of dissolved hemicelluloses on pulp fibers takes place. Adsorbed xylan is claimed to contribute to the mechanical strength of paper by enhancing interfiber bonding [2]. Xylan also plays an important role in determining the chemical interactions between fibers, water, and a variety of chemicals used in the papermaking process.

The effects of xylan on the strength properties of pulps are claimed to depend on the degree of polymerization of xylan, the amount of xylan, its chemical structure as well as its distribution in the fiber wall [3]. Xylan located especially on the outer surface of fibers is generally considered to have an important effect on fiber-fiber bonding and thus on the strength properties of pulp.

Phenomena and effects of xylan resorption on pulp fibers have been reviewed and discussed in some studies. They include e.g. behavior of xylan during kraft pulping, mechanism and effects of conditions on resorption of xylan, distribution of the desorbed xylan in pulp and influence of the adsorbed xylan on the properties of pulp.

Based on the above, it is an aim of the present invention to provide a new method of producing cellulosic and lignocellulosic pulp.

The present invention is based on the concept of reducing the hemicellulose content of bleached cellulosic pulp, such as bleached pulp of deciduous wood raw-material, by selectively removing an essential portion of the hemicelluloses, in particular xylans, contained in the bleached pulp to produce a modified pulp.

The hemicellulose can be extracted from the bleached pulp at alkaline conditions. For extracting hemicellose of birch pulp, primarily an alkaline agent, such as an alkali metal hydroxide or carbonate, or a hydrolytic enzyme, such as xylanase, can be used.

We have found that the properties of the extracted pulp containing less hemicellulose is changed in such a way that bulk and optical properties and tear resistance are improved, which improves the applicability of the pulp for certain uses.

The extracted xylane can be adsorbed onto the fibre surface of other papermaking pulps, such as pulps of deciduous and coniferous chemical pulps, chemimechanical pulps and mechanical pulps. Thereby an increased bonding between the fibres is reached and the strength properties of the fibrous mixtures or of the papers or cardboard manufactured therefrom is obtained. The extracted hemicellulose can also be employed for producing various chemicals, such as furfural and xylitol from xylane.

Even if a great number of studies have been carried on the adsorption of hemicellulose onto fibers and the influence thereof on the properties of the pulp, provision of hemicellulose by extraction of bleached pulp and its use for readsorption or for other uses has not been described in the art.

More specifically, the method according to the present invention is mainly characterized by what is stated in the characterizing part of claim 1.

Considerable advantages are obtained by the present invention. Thus, the profitability of paper or board making is improved by using pulps with tailored properties for specific grades. After having right fibers raw material for the paper grade in question and homogenous cooking in pulping, properties of kraft pulp fibers are mainly determined by their chemical composition. As a rule, changes to the composition of bleached pulp (contents of cellulose and hemicelluloses) are resulted mainly by modifications in the cooking process and somewhat in the bleaching stages.

A more targeted chemical composition of pulp fibers, including surface charge of fibers, can be achieved, when the adjustment of hemicelluloses in pulp fibers is carried out in one well-controlled stage after fluctuating cooking and bleaching stages. Thus, hemicelluloses can be taken from one bleached pulp and transferred to another resulting in two pulps with varying properties, one pulp with properties related to low hemicelluloses content in the fiber wall and the other with properties related to high hemicelluloses content on the fiber surface (Fig. 1). Location of hemicelluloses in fiber wall may be favorably influenced by this hemi-transfer.

Further, the hemicellulose fraction removed from the bleached cellulose pulp can be utilized, for example for producing fine chemicals, such as pentose monomers or dimers, or the hemicelluloses obtainable from extraction of bleached cellulose pulp can be used at the wet end of the papermachine as a chemical in a fashion similar to starch.

Next the invention will be discussed in more detail with reference to the attached drawing which shows a scheme for extraction and transfer of hemicelluloses in birch pulps according to the invention.

As discussed above, the present invention concerns a method of producing paper or cardboard from bleached cellulosic pulp, which contains hemicelluloses. Typically, the hemicellulose composition of bleached cellulosic pulps depends on the wood species and on the pulping conditions. The term "hemicelluloses" generally covers comparatively short-chained polysaccharides consisting mainly of xylosyl-, glucosyl-, galactosyl-, arabinosyl- or mannosyl-residues. The hemicelluloses are partially soluble, in particular in alkaline media. The predominant hemicellulose component in deciduous wood species is xylan or xyloglucan, which typically contains glucuronic acid groups in position 4 of the anhydroxylose unit. As known in the art, depending on the pulping conditions, the acidic side chains of pulp xylan are not exclusively composed of the 4-O-methyl- [alpha] -D- glucuronic acid or [alpha] -D-glucuronic acid present in native wood, but an essential part of the 4-methyl glucuronic acids have during pulping been converted to an unsaturated derivate thereof, viz. 4-deoxy- [alpha] -L-threo-4hexenuronic acid, or hexenuronic acid, (HexA), which can also be found in the bleached pulp, as explained in EP Patent No. 0 764226, the contents of which are herewith incorporated by reference.

Other hemicelluloses present in bleached cellulosic pulps include glucomannan and arabinogalactan, the latter of which is typically present in coniferous wood materials.

The present invention particularly concerns the treatment of bleached cellulosic pulp comprising a pulp prepared by alkaline pulping of a raw-material predominantly composed of deciduous tree, such as a bleached cellulosic pulp prepared by kraft pulping or a similar pulping process of wood of the Betula species. As known in the art, in the kraft pulping the wood raw-material is cooked in an aqueous solution of sodium hydroxide and sodium sulfide, known as white liquor, to selectively dissolve the lignin. The yield of kraft pulping is typically about 45 %. Bleaching can be carried out with oxygen containing bleaching chemicals, such as ozone and peroxides, with chlorine containing bleaching chemicals, such as chlorine dioxide or chlorine gas, or combinations of these bleaching chemicals. However, the present invention is not limited to the use of kraft pulping in combination with conventional bleaching methods, but is extends to any combination of cooking with bleaching which produces a bleached pulp that contains a significant portion of hemicelluloses.

In the bleached pulp the hemicellose content is up to about 25 % by weight of oven dried material, a typical concentration in bleached pulp of deciduous wood species being about 15 to 22 % by weight. According to the present invention a significant part of the hemicelMoses are removed by extraction before the pulp is used for making paper or cardboard. The term "significant part" designates in this context a removal of, preferably at least 5 wt-% and less than 50 wt-%, in particular about 7 to 30 wt-%, of the hemicelMoses present in the pulp. Typically, some 8 to 20 wt-% are removed for the purpose of the present invention.

The extraction is typically carried out using an aqueous solution or dispersion of an alkaline substance or an enzyme or a mixture thereof. It is preferred to carry out extraction at a consistency of about 0.1 to 25 %, preferably about 0.5 to 20 %, in particular about 1 to 15 %.

For the extraction, an aqueous solution containing an alkaline substance can be employed, such a substance being selected from the group of alkali metal and earth alkaline metal hydroxides and carbonates or mixtures thereof. In addition to solutions or dispersions of pure chemicals, also various pulping liquors or solutions (e.g. bleaching or washing solutions) from alkaline bleaching steps can be employed. Of the pulping liquors, white liquor or green liquor can be particularly mentioned.

Typically, the aqueous solution contains 0.01 to 5 M of the alkaline substance, a preferred range of concentration being about 0.5 to 3 M for alkali metal hydroxides, such as sodium hydroxide.

The extraction with an aqueous solution or dispersion of an alkaline substance or a mixture of alkaline substances is, according to one embodiment, carried out at ambient temperature and pressure for 1 min to 24 hours, preferably about 5 min to 12 hours, in particular about 10 min to 6 hours. According to another embodiment, the extraction is carried out at an elevated temperature of 20 to 150 ⁰ C for 1 min to 24 hours, preferably about 5 min to 4 hours.

In addition, or instead of an alkaline substance, the aqueous extraction solution can contain at least one hydrolytic enzyme capable of releasing hemicelMoses from the pulp.

Examples of such enzymes are hydrolases selected from the group of hemicellulases and pectinases. A particularly interesting group of enzymes comprises xylanases.

According to the present invention, the hemicelluloses are selectively removed from the pulp essentially without degrading them. In practice, it has been found that the extraction liquor contains but a small proportion of mono- or disaccharides derived from the hemicelluloses.

Although the extracted pulps exhibit interesting and valuable properties, which can be utilized as such for making specific paper and cardboard products, as will be discussed below, the invention also provides for the use of the removed hemicelluloses. The extraction products are therefore recovered and subjected to further processing. Typically, the hemicelloses are recovered in the form of an aqueous dispersion or solution.

According to a first embodiment, the hemicelluloses are contacted with cellulosic or lignocellulosic fibres under conditions conducive to adsorption of them onto the fibres. The fibres which the hemicelluloses are adsorbed on include fibres of chemical, chemimechanical or mechanical pulps used for the production of paper and paperboard products. In this embodiment, hemicelluloses taken from a first bleached pulp can thereby transferred to a second, optionally also bleached pulp, resulting in two pulps with varying properties, one pulp with properties related to low hemicelluloses content in the fiber wall and the other with properties related to high hemicelluloses content on the fiber surface. According to a particularly advantageous alternative, the same pulp can thereby be used for preparing two different kinds of paper or board qualities.

The hemi-transfer embodiment can be carried out by recovering the hemicelluloses in the form of an aqueous dispersion which then is fed as such, or optionally after concentrating or dilution to desired consistency, to the headbox of a paper or cardboard machine where they are contacted with the fibres in the furnish. The hemicelluloses can be adsorbed onto the fibres at a consistency of about 0.05 to 20 %, preferably about 0.1 to 20 %, in particular about 0.5 to 10 %.

The adsorption of the hemicelluloses onto the fibers is carried out at alkaline conditions, preferably mildly alkaline conditions, the pH being about 7 to 13, in particular about 7.5 to

10. It is possible to operate at essentially neutral conditions, also. However, it has been found that the adsorb amount increases somewhat at increased pH.

The concentration of hemicelluloses in the aqueous dispersion or solution used for adsorption of hemicelluloses onto the fibres is preferably about 0.1 to 50 g/1, such as about 0.5 to 30 g/1, in particular about 1 to 20 g/1, more particularly about 1.5 to 15 g/1.

When birch pulp (or pulp of other wood species of the Betula species or other species rich in xyloglucans) is being extracted, the recovered hemicelluloses comprise mainly xylan. The hemicelluloses generally, in particular xylan, have interesting properties when adsorbed onto pulp fibres. In particular, it has been found that they can be adsorbed onto cellulosic or lignocellulosic fibres used for paper or cardboard in order to improve mechanical strength properties, including tensile strength, bonding and elasticity. These properties will be examined more closely in connection with the examples.

The method according to the present invention generally leads to a lowering of the total yield of the bleached cellulosic pulp by a maximum of 15 %, preferably by a maximum of 10 %, preferably about 1 to 8 % by weight. After the extraction of hemicelluloses the pulp can be used as a raw-material for the production of various paper and cardboard products, in particular uncoated and coated paper or cardboard products having a grammage of 25 to 500 g/m ² , (for papers typically about 40 to 180 g/m ² , and for cardboards typically about 180 to 450 g/m ) exhibiting one or several improved properties from the group of brightness, resistance against yellowing, drainability and water retention. These properties will also be discussed in more detail below. The papers can be used e.g. as printing papers and the board materials e.g. as liners in FBB products. One particularly interesting application of the invention is for the production of fine papers from the extracted pulp.

The extracted hemicelluloses represent an extremely pure raw-material - pure with respect to the polysaccharide composition and lacking extractives etc. - and they can be used as a raw-material for the manufacture of chemicals. Examples of such fine chemicals include, in the case of xylan, xylitol and furfural. Xylitol is a sweetening agent with interesting properties and very diverse applications. Xylitol can be prepared from xylose in a batchwise, three-phase hydrogenation process. It is also possible to produce xylitol from xylose by fermentation. Furfural can be prepared commercially by dehydration of pentose

sugars, such as xylose. It is the aldehyde of pyromucic acid and has properties similar to those of benzaldehyde. The major application of furfural is being use as a feedstock for furfuryl alcohol which is, again employed in the production of thermosetting furan resin and furan cement.

Summarizing the above:

The results show that the higher xylan content of birch wood compared e.g. to eucalyptus makes it favorable for a controlled manipulation of xylan content in the fiber wall or on the fiber surface to improve exploitation of birch pulp in paper and board making. By changing the content and location of hemicelluloses in fibers by removal of a hemicellulose fraction (xylan) from one bleached birch kraft pulp and by transferring it onto another significant changes in chemical and physical properties of the fibers are achieved. As a result, the technical properties of pulp for paper and board can be changed and usability of a bleached birch kraft pulp improved with respect to a plurality of paper and board grades.

The usefulness of the invention is manifested in many ways:

Alkali extraction of a birch kraft pulp mainly removes xylan from the pulp and increases cellulose content and viscosity of the pulp. The amounts of extractives are also significantly reduced which prevents yellowing almost totally. Fibers become lighter and their fiber width and wall thickness is reduced, indicating improved formation capability and reduced flocculation tendency of fibers in papermaking.

Sorption of xylan onto kraft pulp fibers have an opposite, but not as significant effect on fiber properties as the high xylan amount in polysulfide pulp fibers.

A reduced xylan content improves initial pulp properties like drainage, bulk and light scattering. Although some properties, like tensile strength, bonding, elasticity and dimension stability, are also to extent detrimentally affected, by subjecting the pulp to more beating aiming, e.g., at reaching a predetermined tensile strength level, many of these changes in pulp properties can be compensated.

Sorption of xylan onto birch kraft fibers changes many pulp properties according to the same lines as the increased xylan amount in polysulfide fibers, but to a lesser extent. Some initial pulp properties like tensile strength, bonding and elasticity are improved. By xylan sorption, a significant improvement can be obtained in Scott bond, which can be increased with up to 50 % at a certain tensile strength level compared to the original kraft pulp or to the polysulfide pulp. The substantially increased surface charge by the xylan sorption probably contributes to this high bonding, but this is only one possibility.

According to the results obtained in connection with the present invention, properties of bleached birch kraft pulp can be manipulated in a controlled way by transferring xylan from one pulp onto another. Exploitation of these changes in a pure bleached birch pulp, e.g. better bonding, will be further studied in pulp mixtures expressing certain paper and board grades.

The present results are in line with earlier findings that after a uniform cooking and gentle post-treatment, the properties of birch kraft pulp produced from a constant raw material source are mainly affected by the pulp yield, i.e. by the content of hemicellulose (xylan).

The following non-limiting examples illustrate the invention:

Preparation of pulps Cooks

Chips from normal pulpwood of birch from southeastern Finland (code Koivu-12) were used in cooking experiments. Over-sized and fine fractions were removed on a wire screen with openings of 16x32 mm and 6x6 mm. From fresh chips of the accept fraction samples of 4.5 kg (as. o.d.) were weighted for the cooks and preserved frozen in a refrigerator. Before cooking the samples were melt at room temperature for a couple of days. Cooking experiments were carried out in KCL's 15 L rotating autoclaves under following conditions:

Table 1

A polysulfide liquor was prepared at KCL oxidizing a laboratory made white liquor with the Moxy-method. Analysis of liquors before and after the oxidation were as follows:

Table 2

White liquor Polvsulfide liαuor

Effective alkali g/i 74.7 104.5

Na ₂ S g/i 115.7 47.4

Sulfidity % 88.5 37.7

Na ₂ S ₂ O ₃ g/i n.d. 30.8

Polysulfide-S ε/i n.d. 18.6

After cooking the pulp was washed with deionized water over night, disintegrated and screened on a plane screen with 1.0 and 0.3 mm slots. Pulp yield, screenings, kappa number, brightness and viscosity of pulp and residual alkali in black liquor were determined. The results of birch cooks are given in Table 3.

Bleaching experiments

The birch pulps were bleached with the sequence OD(EO)DND to the target brightness 90+ %.

Oxygen delignification (O-stage) was carried out in KCL' s 4OL DELFI reactor equipped with indirect oil bath heating and with inverter controlled mixer. The process conditions were as follows:

Table 4

Magnesium sulfate used as an inhibitor was added to pulp in plastic bag before heating the pulp to the reaction temperature by microwave oven. The heated pulp was placed into the reactor, alkali with additional water was charged and the pulp slurry was mixed 30 seconds at a rate of 300 rpm. The reactor was pressurized with oxygen and the pulp slurry was mixed 30 seconds at a rate of 600 rpm. During the whole reaction time the pulp was mixed after every 15 minutes for 18 seconds at a rate of 300 rpm at the balanced reaction temperature.

After the reaction, the pressure was released and dilution water was added to the pulp slurry directly after sampling and pH-measurement. The pulp slurry of 5% consistency was poured onto suction filter (Buchner type) immediately. After dewatering the pulp was washed twice with ten fold volumes of water. The washing was finished when water did not come out anymore from the pulp pad. Finally the pulp was centrifuged up to 22-25% dry matter content.

The handsheet was prepared for determination of kappa number, brightness and viscosity (Table 3). Pulp yield and residual alkali in O-stage liquor were determined, too.

ECF bleaching

After oxygen delignification the birch pulps were bleached by the D(EO)D _N D -sequence using conventional laboratory technique. (EO)-stage was carried out in DELFI reactor in the same manner as O-stage. DO-, Dl- and D2-stages were performed in KCL' s 18L air bath reactors. Rotation (40 rpm) of reaction vessel took care of the mixing of pulp and chemicals during the bleaching stage. After bleaching stages final pH and residual chemical were measured. The pulp was diluted 5% consistency and washed in standard way. The pulp sample was taken and acidified with SO2-water at 2% consistency and a handsheet was prepared at pH 4,5. Deionized water was used thoroughly in bleaching sequence. In Dlu-stage the neutralization followed Dl -stage without intermediate washing.

Table 5. Bleachin conditions

Bleaching results are indicated in Table 3 above.

Extraction and precipitation experiments

The alkali extraction experiments of bleached birch pulps to remove hemicelluloses (xylan) were made by treating the pulp in a plastic beaker with mixing under a nitrogen atmosphere using following conditions: Table 6

The extracted pulp was washed with 2 x 1 L H ₂ O + IL 1% acetic acid + 2 x 1 L H ₂ O before the yield determination, analysis and testing.

To isolate xylan for the precipitation experiments a bleached birch sulfate pulp was extracted with alkali under the conditions above but having a pulp charge of 300 g and the alkalinity 1.0 mol/1. The yield of pulp after the extraction was 84.7%. Combined washing waters were acidified to pH 4-5 with cone, acetic acid. The precipitate was separated by centrifuging. The amount of recovered precipitate was 295 g containing 15.0% dry matter with 70.7% xylose.

The precipitation experiments of xylan on a birch sulfate pulp were made in KCL' s Quantum Mark reactor under following conditions:

The xylan precipitate was first dissolved in 200 ml IN NaOH (0.01 N NaOH in the two latest experiments 1482 S6 and 1482 S7), then mixed with the pulp slurry containing NaAc 1 mol/1. The alkalinity in the mixture was 0.1 and 0.001 mol NaOH/1, respectively. The mixture was poured into a reactor, where air was removed by nitrogen gas before heating in 30 min to the temperature. In two first experiments (1482S and 1482S2) the temperature was 120°C, in later experiments 80°C. In two first experiments, after the treatment the pulp was first filtrated, then washed with 2 x 1 L H ₂ O + IL 1% acetic acid + 2 x 1 L H ₂ O before the yield determination etc. In later experiments pH in the slurry after the treatment was adjusted, if necessary, to about 7 with acetic acid before the filtration and washings. In the last experiment (1482 S7) no acetic acid was used after the treatment or in washing.

The following analysis were made for pulps before and after the treatments:

Table 8

Carbohydrates Internal

Acetone extracts SCAN-CM 49

Carboxyls Internal

Charge, surface Internal

ISO brightness of pulp ISO 3688, ISO 2470

Viscosity (CED-solution) SCAN-CM 15

Brightness reversion, pc-number TAPPI T 260

Results of the extraction and precipitation experiments are compiled in Tables 9 and 10.

Table 9. Results of alkaline extractions of DEDD bleached birch kraft pulps

Pulp Kraft Extraction

Pulp No 1482 1482U3 1482U2 1482U1

Code DEDD NaOH 1% NaOH 2% NaOH 4%

NaOH, % in liquor - 1 2 4

Yield of extraction, % - 99.1 94.3 83.8

Pulp yield, % on wood 49.6 49.1 46.8 41.6

Brightness, % 90.8 89.9 90.3 91.0

Viscosity, ml/g 1160 1100 1160 1270

Acetone extraxts, % 0.41 0.17

Total carbohydrates, mg/100 mg 94.0 94.8 98.2 97.0

Carbohydrate composition

-glucose, % 76.0 76.8 80.0 88.9

-xylose, % 24.0 23.2 20.0 11.1

-mannose, % - +

Cellulose, % 75.5 76.3 79.5 88.6

Xylan, % 24.5 23.7 20.5 11.4

Glucomannan, % -

Uronic acids, mmol/kg 33

-MeGIcA, mmol/kg 33

-HexA, mmol/kg +

Carboxyls, mmol/kg 79.2 78.9 57.3 50.2

Total charge of fibres, μeq/g fibres -21.8 -24.9 -28.3 -18.6

PC number 0.38 0.023

Table 10. Results of xylan precipitation on DEDD bleached birch kraft pulp.

Pulp Kraft Precipitation

No 1482 1482 S2 1482 S 1482 S3

Code DEDD XyI 0 g/l XyI 2 g/l Xy1 10 g/l

Xylan, g/l - 0 2 10

Yield of precipitation, % - 92.3 100.3 105.5

Pulp yield, % on wood 49.6 45.8 50.1 52.3

Brightness, % 90.8 80.8 86.9 85.2

Viscosity, ml/g 1160 1070 1090 1030

Acetone extraxts, % 0.41

Total carbohydrates, mg/100 mg 94.0 95.6 93.0 92.6

Carbohydrate composition

-glucose, % 76.0 79.3 75.2 72.8

-xylose, % 24.0 20.7 24.8 27.2

-mannose, % -

Cellulose, % 75.5 78.8 74.7 72.2

Xylan, % 24.5 21.2 25.3 27.8

Glucomannan, % -

Uronic acids, mmol/kg 33.0

-MeGIcA, mmol/kg 33

-HexA, mmol/kg +

Carboxyls, mmol/kg 79.2 63.5 87.3 93.9

Total charge of fibres, μeq/g fibres -21.8 -27.6 -33.5 -35

Beating and testing of pulps

The bleached pulps were beaten in PFI. The original birch sulfate and polysulfide pulp were beaten with 0, 250, 500 and 1000 revolutions, all other pulps only with 0 and 500 revolutions. The unbeaten and beaten pulps were analyzed for:

Table 11

Drainability, SR-number EN ISO 5267-1

Water retention value, WRV, g/g SCAN-C 62

Fiber distribution, FS-200

- Arithmetic av. fiber length, mm

- Length weighted av. fiber length, mm

- Weight weighted av. fiber length, mm

- Length < 0,2 mm, %

- Coarseness, mg/m

Preparation of laboratory sheets (for physical testing) EN ISO 5269-1

Grammage, g/m ² EN ISO 536 modif.

Bulking thickness, μm ISO 534 Apparent bulk-density, kg/ni ³ ISO 534 Air resistance, Gurley, s ISO 5636-5 ISO-brightness, % ISO 2470 Opacity (65g/m ² ), % ISO 2471 Light-scattering coefficient, mVkg ISO 9416 Light-absorption coefficient, m ² /kg ISO 9416 Color (C/2°) CIE ISO 5631 Tensile index, Nm/g EN ISO 1924-2 Stretch, % EN ISO 1924-2

Tensile energy absorption index, J/g EN ISO 1924-2 Tensile stiffness index, kNm/g EN ISO 1924-2 Modulus of elasticity, N/mm ² EN ISO 1924-2 Tear index, mNm ² /g ISO 1974

Internal bonding strength, modif. Scott, TAPPI T 833 Huygen Internal Bond Tester, J/m ² modif.

Zero-span tensile index, wet, Nm/g ISO 15361 Capillary rise, KIemm method, mm ISO 8787 modif. Sheet shrinkage, % Internal

The results obtained are as follows:

Table 12. Fiber and paper technical properties of extracted and hemi-sorption birch kraft ul s

As will appear from the above tables, by increasing the NaOH concentration up to 1 mol/1 some 15% from the bleached birch kraft pulp and more than 20% from the bleached birch polysulfide was removed during the alkali extraction in the room temperature. Removal concerned mostly xylan, practically no cellulose was dissolved. The intactness of cellulose was reflected in the increased viscosity values after the alkali extraction of the pulps, especially of the polysulfide pulp. Acetone extracts were reduced to a half by the alkali extraction of pulps.

Due to xylan dissolution with its uronic acid groups the carboxyl group content of the kraft pulp decreased almost 40%, whereas the surface charge, originated from carboxyl groups, reduced only minimally. Accordingly, the dissolved xylan came from the fiber wall. Yellowing of pulps was almost totally prevented by the alkali extraction of pulps.

Mere alkaline conditions in the precipitation experiments of xylan on the birch kraft pulp fibers induced some effects from alkaline extraction on pulp properties. However, increasing the xylan concentration in the precipitation up to 10 g/1, an increase of 5% in the pulp yield was achieved. Lowering the alkalinity of precipitation increased the yield.

In the precipitation the carboxyl group content in the kraft pulp increased by 15%, but the charge by almost 70%. After the xylan sorption the kraft had about 40% higher charge than the xylan-rich polysulfide pulp, although its carboxyl content was 20% lower than that of the latter pulp reflecting a sorption of xylan on the fiber surface.

In the kraft pulp the alkali extraction resulted in lowering of the measured fiber length value and increased curliness of fibers, but in beating the fibers were straightened as often experienced in PFI beating. The polysulfide pulp fibers were less curly and slightly longer than the kraft pulp fibers. Although alkalinity induced its effects on fiber length and curliness in the xylan precipitation, the sorption of xylan on fibers somewhat increased the former and decreased the latter.

Increasing alkalinity in the extraction resulted first a slight increase of cell wall thickness as well as fiber width reflecting alkaline swelling of fibers. Later the dissolution of materials caused diminishing of these fiber dimensions. The polysulfide pulp fibers had

higher values for the fiber width and cell wall thickness than the kraft pulp fibers; but the effect of xylan sorption was negligible.

Due to lightening of fibers in the alkali extraction their formation behavior in papermaking could be estimated to become better. Respectively, heavier fibers in the polysulfide and xylan sorption pulps might have negative effects on formation. Similar estimations were made for the flocculation tendency of fibers in the headbox of paper machine: removal of xylan reduced and increased xylan content enhanced this tendency.

Single fiber strength (as zero-span tensile strength) was diminished when xylan was extracted from the birch pulp. Increasing the xylan content in cooking improved fiber strength in unbeaten pulp, but beating leveled off the differences. The effect of xylan sorption remained negligible.

Tensile index of pulp after a constant beating, describing beatability, was reduced with removal of xylan, as expected. The hemi-rich polysulfide pulp gave clearly higher tensile index than the kraft pulp. Although tensile index was lowered by the alkalinity in the xylan precipitation, the increasing xylan concentration improved it.

Xylan removal by the alkali extraction improved drainability of pulp and notably more with the polysulfide pulp than with the kraft pulp. In the xylan precipitation on birch pulp the effect of alkalinity was evident: drainability was improved in spite of xylan sorption. Only in the case of lowered alkalinity drainability as decreased. Water retention values behaved in a similar way to drainability.

A high NaOH concentration in the alkali extraction gave a noteworthy increase in bulk of unbeaten kraft pulp. The alkaline extraction increased beating demand and respectively increased density, i.e. decreased bulk of pulp at a certain tensile index. On the other hand, the xylan-rich polysulfide pulp, but not the xylan sorption kraft pulp had higher bulk than the kraft pulp at the same tensile index.

The alkaline extraction of pulps reduced bonding of fibers, but at a constant tensile index level, due to increased beating demand, it had slightly improving effect on Scott bond. The most beneficial was the alkali extraction after beating. The 'easily beatable' polysulfide

pulp had even somewhat lower Scott bond than the original kraft pulp at a constant tensile index level. On the other hand, the xylan precipitation improved Scott bond of the birch kraft up to 50%. There is a clear difference in bonding of fibers when xylan is situated mostly in the fiber wall like in polysulfide pulp fibers or on the surface of fibers like after the xylan precipitation.

Modulus of elasticity, after a constant beating of pulp, was reduced by the alkaline extraction of pulp but increased 10 % by the xylan sorption or even more, 30%, by the polysulfide cook.

Compared at certain air permeability the alkaline extraction induced reduced values for the birch kraft pulp but not for the polysulfide pulp. The effect of the xylan sorption was practically disappeared at constant air permeability.

The xylan sorption slightly improved dimensional stability, based on shrinkage of pulp, whereas the effect of the alkali extraction was negative. At a constant tensile index the polysulfide pulp had 20-25% higher dimensional stability than the kraft pulp. By the xylan precipitation dimensional stability of the birch kraft pulp could not be improved to the level of the polysulfide pulp

Light scattering coefficient of the sheet with a constant beating was increased by the alkaline extraction of and reduced by the xylan sorption or by polysulfide cook Different beating behaviors of pulps caused, however, that at a constant tensile index level the light scattering coefficients of the kraft, polysulfide and xylan sorption pulps were equal. The alkaline extraction of pulp resulted in lowered light scattering in this comparison.

REFERENCES

1. Genco, J.M., Busayasakul, N., Medhora, H.K., Robbinds, W. Hemicellulose retention during kraft pulping. Tappi J. 73(1990): 4, 223-233.

2. Simonson, R. The hemicellulose in the sulfate pulping process. Svensk Papperstidn. 74(1971): 21, 691-700.

3. Lai, Y.-Z. Chemical degradation. In: Hon, D.N.-S., Shiraishi, N. (Eds.) Wood and Cellulose Chemistry, Marcel Decker, Inc., New York, 1991, pp. 455523.