The present invention relates to a process in accordance with the preamble of claim 1 for preparing chemical pulp. The present invention also relates to an enzyme composition for said use.

Softwood pulp generally has the drawback of an unfavourable fibre length distribution. This negatively influences the formation and thus the printability and other properties of the paper and board, manufactured using said pulp. Traditionally a portion of hardwood pulp is added to the softwood pulp to balance the fibre length distribution. The use of thinning material or felling of young forests has also been contemplated, but this has sometimes unwanted effects on the total forest economy. It is also possible to subject the softwood pulp to extensive mechanical treatment but this unevitably increases the amount of fines in the pulp, thereby negatively affecting the dewatering properties of the pulp. There is presently an apparent need for new processes to obtain a softwood pulp for making paper and board with a shorter average fiber length and a more narrow fibre length distribution. The use of enzymes is well known in the pulp and paper industry. Xylanases are presently used as a pretreatment to facilitate bleaching. Cellulases are also know to be used in the processing of recycled fibre to digest fines and improve dewatering on the paper machine.

Traditional use of enzymes in pulp- and paper applications has thus been for purposes of reduction of fines or remowal of other unwanted components. Early patent documents, e.g. US-2 280 307, describes the use of cytase for solubilizing hemicellulose. An advantage is said to be that the cellulose fibres are not affected. The process according to US 3 041 246 uses cellulases to obtain fibrillation of cotton linters without appreciable shortening of the fibres. US 2 280 307 describes enzymatic treatment of wood pulp for Ihe purpose of dissolving non-cellulosic materials while exerting only a minimum of destruc¬ tive action on the cellulosic material.

More recent documents also describe enzymatic treatment of paper pulp, e.g. CA 758 488, that describes simultaneous beating and enzymatic treatment. The enzyme is continuosly added during the beating operation and the description underlines the positive synergistic effect achieved in simultaneous beating and enzymatic treatment. This document does not contain any specific mentioning of controlled fibre shortening. Since beating is normally used with the goal to obtain strength with minimum input of energy, fibre shortening should rather be an undesired effect.

WO 94/20667 describes an enzymatic process for pretreatment of wood raw-

material which makes it possible to reduce the specific energy consumption of mechanical pulping and to improve the technical properties of the fibres. The inventors describe a special enzyme preparation exhibiting simultaneously both celiobiohydrolase activity and mannanase activity. Thereby the hydrolysis of insoluble cellulose is avoided and the strength properties of the fibres not impaired. Again, fibre shortening would rather be a negative effect. Further, according to the examples presented in the description of WO 94/20667, this process requires a cooler and more dilute pulp than what normally is the case in industrial pulp- and paper production. Additionally the long treatment times given in the examples suggest that very large storage volumes are necessary. WO 92/18688 relates to a cellulase preparation with a high content of endogluca¬ nase and a little or no celiobiohydrolase for use for treatment of paper pulp, inter alia to improve the drainage properties of the pulp. It is obvious that such an enzyme treatment ^• mainly solubilizes the fines, thus improving the dewatering. It is noted, also in this description, that the damage to the cellulose fibres in the pulp is less, because of the low celiobiohydrolase activity.

Other documents defining the state of the art are e.g. Evitkov et al., in Ferment. Spirit. Prom.. 4(1979) 31-34 and Ghose et al., Biotechnol. Bioeng., 1 (1979) 131-146. Both these documents concern the production of fermentable sugars of different types of fibrous waste. Ghose et al. discuss methods for hydrolyzing bagasse and conclude, that the synergistic action of exoglucanase, endoglucanase and xylananse is effective for hydrolysis of bagasse but not for pure cellulose. The above method is further directed to the production of glucose, that is a full hydrolysis of the fibres. Evitkov et al. are also driving the hydrolysis as far as possible, with the intention to produce fermentable sugars of fibrous wastes. Said documents concern an non-specific and very far reaching degradation of the fibres, and are thus not applicable to the production of paper and board, where the undisputable goal is to preserve the fibre structure.

Obviously the presently known processes for enzymatic treatment of paper pulp are not related to controlled fibre shortening in the production of paper and/or board. Known processes do not adress the same problem that the present invention intends to solve. Contrary to all conventional practices, the present inventors worked to find an enzyme preparation and to develop a treatment process that actually would cause localized damage to the cellulose fibres. The purpose was to find a way of controlling the fibre length in softwood pulp, making it usable for the production of fine paper or board with improved essential qualities, e.g. better formation and better print quality. The present in-

vention mainly concerns a process for treatment of chemical softwood pulp, preferably bleached chemical softwood pulp. The process and enzyme preparation according to the present invention can, of course, also be applied to unbleached chemical pulp but then the enzyme dosage and/or process conditions as treatment time, temperature and pH have to be adjusted accordingly. For reasons of simplification, the present description will deal only with the treatment of bleached softwood pulp. Naturally the process and enzyme preparation according to the present invention, when necessary, can be applied also to pulps including recycled fibres as well as virgin fibres and mixtures, in varying propor¬ tions, of the two. It has now surprisingly been shown that the simultaneous use of endo- and exocellulases in a treatment step before the beating operation results in a controlled fibre shortening in softwood pulp.

The fibres present in the chemical pulp are always to some extent mechanically damaged, i.e. the fibres show more or less sharp kinks and bends in their three dimensio- nal structure. It has now been shown that a combination of endo- and exocellulase activity, e.g. endo- and exoglucanase activity, attacks these damaged points and causes changes in the cellulose crystal structure. Thus, local weakening of the fibre is achieved. In other words the enzymatic treatment introduces local damage, in the following called "breakage domains" on the fibres. Later, when the pulp is subjected to mechanical action in the form of beating, using conventional equipment for this purpose, the fibres break in a controlled fashion at these breakage domains. This surprising effect results in a shorter average fiber length and a more narrow fibre length distribution of the pulp, which better corresponds to the fibre length distribution of hardwood pulp. This in turn is favourable in the production of paper and board, e.g. the production of printing papers. Normally, con- ventional beating of softwood pulp, without previous enzymatic treatment, results in a fibre length distribution with a large content of very short fibre fragments, so called fines, and a substantial amount of unnecessarily long fibres. This unfavourable fibre length dis¬ tribution causes irregularities in the paper and reduces for example the print quality. Con¬ trolled fibre shortening according to the present invention for the first time makes it possible to use softwood pulp alone for the production of high quality printing papers.

The invention is further illustrated in the following preferred embodiments, examples and figures, wherein

Figure 1 is a graphic representation showing zero-span tensile index as a function of enzymatic treatment at different enzyme dosages and temperatures,

Figure 2 is a graphic representation showing the fibre length distribution of untreated pulp compared to the fibre length distribution of pulp subjected to enzymatic treatment according to the present invention.

The extent of fibre shortening can be controlled through several means. Firstly, according to one embodiment of the invention, the ratio of endocellulase activity to exo- cellulase activity can be varied. Endoglucanase can be used together with a trace amount of exocellulase and vice versa, preferably in an interval of 1 : 100 to 100: 1. More preferably, the amount of exocellulase activity is significantly larger than the amount of endocellulase activity. Most preferably the ratio of exocellulase to endocellulase is in the interval of about 4: 1 to 5: 1 , which gives a synergistic effect. An optimal ratio will depend on the actual enzymes used and their sources and this optimal ratio can easily be determined by a person skilled in the art.

According to the present invention the endocellulase is preferably endoglucanase and the exocellulase is preferably exoglucanase. The endoglucanase and exoglucanase can be used in a clean, isolated form or as a mixture of enzymes from different sources. Also commercial enzyme mixtures exhibiting the above specified activities can be used. One

® example of such a commercially available product is Celluclast 1 ,5L from Novo Nordisk

A/S. Obviously also other commercial or independently developed enzyme products can be available to a person skilled in the art. Several endo- and exoglucanases are known and can be used according to the present invention. Microbial enzymes are preferred for economical reasons. The enzymes should be active and stable at the conditions, especially the pH and temperature, that prevail during the pulp treating processes. Examples of suitable enzymes are those derived from the microorganisms listed in Table 1 and Table 2. Table 1 : Examples of cellulase-producing fungi

Agaricus bisporus

Ascoboulus furfuraceus

Aspergillus aculeatus, A. fumigatus," A. niger, A. phoenicis, A. terreus and A. wentii Botryodiploida theobromae

Chaetomium cellulolyticum, C. globosum and C. thermophile

Chrysosporium lignorum

Cladosporium cladosporioides

Coriolus versicolor Dichomitus squalens

Eupenicillium javanicum

Fo es famentarium

Fusarium moniliforme, F. solani and Fusarium spp.

Table 1. continued

Humicola grisea and H. insolens

Hypocapra merdaria Irpex lacteus

Lenzites trabea

Mycellophtora thermophila

Myriococcum albomyces

Myrothecium verrucarla Neocallimastix frontalis

Neurospora crassa

Paecilomyces fusisporus and P. variotly

Papulaspora thermophilia

Pellicularia filamentosa Penicillium chrysogenum, P. citrioviride, P. funicolosum, P. notatum,

P. pinophilium, P. variabile and P. verruculosum

Pestalotiopsis versicolor

Phanerochaete chrysosporium

Phialophora malorum Phoma hibernica

Physarum polycephalum

Pleurotus ostreatus and P. sajor-caju

Podospora deciplens

Polyporus schweinitzil and P. versicolor Poria placenta

Poronia punctata

Pyricularia orzyzae

Saccobolus trunctatus

Schizophyllum commune Sclerotinia libertiana

Sclerotium rolfsii

Scytalidium lignicola

Sordaria fimicola

Sporotrichum pulverulentum and S. thermophile Stereum sanguinolentum

Talaromyces emersonii

Thermoascus aurantiacus

Thrausiotheca clavata

Torula thermophile Trichoderma koningii, T. pseudokoningii and T. reesei

Trichurus spiralis

Verticillium albo-atrum

Volvariella volvacea

Table 2: Examples of cellulase-producing bacteria ^ Cellulomonas flavigena, C. biazotea, C. cellasea, C. fimi, C. gelida, C. curtae,

C. uda and C. turbata Bacillus brevis, B. firmus, B. lichenformis, B. pumilus, B. subtilis, B. polymyxa and B. cereus Serrata marcescens

Table 2. continued

'Pseudomonas fluorescens var. cellulosa'

'Cellvibrio viridus, C. flavescens, C. ochraceus, C. fulvus, C. vulgaris and C. gilvus'

Cytophaga hutchinsonii, C. aurantiaca, C. rubra, C. tenulssima, C. winogradskii and C. krzemienlewskoe Herpetosiphon geysericolus Sporocytophaga myxococcoides Streptomyces flavogriseus

'Thermoactinomyces spJ Thermomonospora curvata

(1 The bacteria within prime signs are not validly classified.

The fungi and bacteria listed above are only given as examples. Presently microorganisms of the species Trichoderma and Aspergillus are considered specially suitable for the production of the present enzymes but the scope of the present invention is not limited to the use of the named microorganisms. It is very possible that other enzyme producing microorganisms suitable for the present invention already exist or will be developed using mutation and selection or methods of genetical engineering. It is also likely, that the enzyme producing capabilites of an existing microorganism can be further enhanced through genetical engineering.

The environmental conditions during the enzymatic treatment are not critical for the scope of invention, but can of course be used to controll the enzymatic reaction. The environmental conditions of the enzymatic treatment are to a certain extent governed by the normal process parameters of the pulping and paper making processes. Simultane¬ ously, the requirements of the enzyme or enzyme mixture have to be considered. A pH in the interval of 2 - 13 is possible, while an interval of 4 - 10 is preferable, depending on the enzymes used. The temperature of the reaction mixture is of considerable importance as it affects the reaction rate of the enzymes. The reaction rate directly influences the time needed for the desired reaction to take place and thus the necessary storage volumes. Depending on the enzymes used, their thermotolerance and thermal optimum, the tempera¬ ture can be in an interval from about 5 - 95 °C, theoretically even higher e.g. about 100 °C, but preferably about 30 - 60 °C and most preferably about 45 - 50 °C. One can anticipate a future development of highly thermotolerant enzymes, which would enable the enzymatic treatment to be performed at considerably higher temperatures. It is possible that this would give rise to unexpected synergistic effects.

A skilled worker can, given the requirements of the enzymes in question,

determine more exactly the optimum environmental conditions for application of the enzyme or enzyme mixture. The presence of heavy metals can also influence the en¬ zymatic activities and should therefore be avoided. A skilled worker with knowledge of both the manufacturing of paper and the usage of enzymes is able to adapt the process to accommodate the enzymatic treatment according to the present invention.

According to one embodiment of the present invention the enzymatic treatment is terminated before the pulp is subjected to the beating operation. The enzymatic treatment is preferably terminated by adjusting the pH of the reaction mixture to an interval outside, preferably above, the functional interval of the enzyme mixture. This functional interval depends on the pH-stability of the enzyme, the temperature and other environmental

® factors. Using the commercially available enzyme product Celluclast the pH -was adjusted to about pH 8 to terminate the reaction.

According to another embodiment of the invention the beating operation is modified in relation to the enzymatic treatment. Generally, enzymatically treated pulp requires a lower input of energy at the beating stage. It is of course of interest both to minimize the energy consumption and the production of fines during beating. A skilled worker can easily optimize the milling in relation to the enzymatic treatment.

According to one embodiment of the invention, only part of the flow of chemical pulp is subjected to enzymatic treatment and then returned to the main flow. According to another embodiment of the invention, the enzyme or enzyme mixture used is bound to a carrier in order to facilitate its removal and possible reuse. Suitable carriers and methods for immobilization of enzymes can be found in the littera- ture.

The present invention also relates to an enzyme composition for treatment of softwood pulp for the production of paper and board where essential product properties are obtained by controlling the fibre length distribution of the pulp, whereby said pulp is first subjected to enzymatic treatment and thereafter subjected to mechanical or equivalent actions in order to effectuate controlled fibre shortening at the breakage domains, characterized in that said enzyme preparation introduces local breakage domains on the fibres.

According to an embodiment of the invention said enzyme preparation exhibits both endocellulosic and exocellulosic activity.

Preferably the ratio of endocellulosic activity to exocellulosic activity in said enzyme preparation is in the interval from 1: 100 to 100: 1. More preferably the exo-

cellulosic activity is significantly larger than the endocellulosic activity and most prefe¬ rably the ratio of exocellulosic activity to endocellulosic activity is in the interval of about 4: 1 to 5: 1.

According to a preferable embodiment of the invention the exocellulosic activity is exoglucanase activity and the endocellulosic activity is endoglucanase activity.

The invention is further described in the following examples which are not in any way intended to limit the scope of the invention set forth in the claims.

Example 1

Laboratory scale experiments were performed to determine the influence of enzymatic treatment on zero-span tensile index. Two series of experiments- w h increasing enzyme concentrations were performed, one at a temperature of 40 °C and the other at a temperature of 50 °C. The treatment time at both temperatures was 2 hours. The

® enzyme mixture used was Celluclast from Novo Nordisk AS, Denmark, with an enzyme activity of 1500 NCU/g enzyme. (One Novo Cellulase Unit (NCU) is the amount of enzym which, under standard conditions, degrades CMC to reducing carbohydrates with a reduction power corresponding to 1 μmol glucose per minute.) The results are presented in Table 3 and graphically in Fig. 1.

Table 3: Zero-span tensile index (Nm/g)

Dosage (NCU/ 10 g pulp) 2 h / 40 °C 2 h / 50 °C

0 118 1 18

25 87 45

50 75 26 *

70 51 23 *

100 46 19 *

The accuracy of reading was low for these values.

It can be clearly seen that the effect depends on both dosage and treatment temperature. With an increase in temperature the same effect can be achieved with a significantly lower enzyme dosage. An increase from 40°C to 50°C allows for a reduction of the necessary enzyme dosage from 100 to 25 NCU/ 10 g pulp while maintaining the same result.

Example 2

The ability of the enzymatic treatment to accomplish the desired narrowing of the fiber length distribution was tested in pilot scale experiments. Enzymatic treatment was performed in batches of 400 kg pulp over night. The pH of the pulp was adjusted to 5,0 -

5,5 by addition of H SO4 during disintegration, whereafter the enzyme mixture (Cel-

® luclast from Novo Nordisk AS, Denmark, enzyme activity 1500 NCU/g enzyme) was added to the pulp. Two different enzyme concentrations were used. The enzyme dosages were determined on the basis of previous laboratory experiments and were 7,5 NCU/ 10 g pulp and 18,75 NCU/10 g pulp, respectively. After 13 hours, the reaction was stopped by adjusting the pH to 8,0 - 8,2 by addition of NaOH-pills to the mixture. Subsequently the pulp was beaten in an industrial Disc-refiner at a concentration of 3,5 %. For an overview of the experimental conditions, see Table 4.

Table 4: The experimental conditions

Control Batch 1 Batch 2

Initial pH 5,0 - 5,5 5,0 - 5,5 5,0 - 5,5

Enzyme addition - 7,5 U/10 g pulp 18,75 U/10 g pulp

Treatment time 13 h 13 h 13 h

Temperature 38 - 40 °C 38 - 40 °C 38 - 40 °C

Fibre concentration 3,5 % 3,5 % 3,5 %

Stopping pH 8,0 - 8,2 8,0 - 8,2 8,0 - 8,2

Changes in fibre strength after the enzymatic treatment was observed by measu¬ ring zero-span tensile index on dry and rewetted sheets. Additionally pulp viscosity and shape factor (also known as "curl index") was determined. Pulp viscosity can be seen as a measure, relative to the average chain length for the cellulose. The shape factor on the other hand is the ratio of the projected length of the fiber and its real length, thus giving an indication of the curvature or crookedness of the fibre.

It was seen from the results that a stronger enzymatical treatment caused stronger degradation of the cellulose. This was obvious from the viscosity and zero-span tensile index. The strongest effect was observed for rewetted zero-span tensile index. That the

difference between dry and rewetted zero-span results increased along with increasing enzyme dosage indicates that the degradation took place as local weakening of the fibre, in the form of local breakage domains as discussed previously. The shape factor remained unchanged in Batch 1. In Batch 2, which was subjected to a relatively high enzyme concentration, the fibre deformations have also increased. The average fibre length was not influenced by the enzymatic treatment alone. The results are summarized in Table 5.

Table 5. Changes in fibre properties

Control Batch 1 Batch 2

Pulp viscosity 818 739 677

Zero-span tensile 131 116 101 index, dry (Nm/g)

Zero-span tensile 116 77 47 index, rewetted (Nm/g)

Fibre length (mm) 2,24 2,09 2,22

Form factor (%) 86,7 86,4 85,2

The fibre length distribution after enzymatic treatment and subsequent beating is presented graphically in Fig. 2. The graphs clearly demonstrate the beneficial effect the process according to the present invention has on the fibre length distribution.

Although the invention has been described with regard to its preferred em¬ bodiments, which constitute the best mode presently known to the inventors, it should be understood that various changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention which is set forth in the claims appended hereto.