METHOD OF IMPROVING PROPERTIES OF PAPER

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method of improving the internal bond strength of paper and paper products.

[0002] The internal bond strength refers to the ability of paper to resist stresses in the thickness direction of the paper. When the Z direction strength of paper/cardboard is exceeded, the structure of the paper breaks down even if the surface remained intact. The internal bond strength is a particularly important property in the printing of paper products and cardboards. The internal bond strength may be measured by various physical tests, such as a Scott bond test, according to the T 569 pm-00 standard.

[0003] For instance in offset printing and cardboard processing, in corrugation, folding and coating stages, paper/cardboard is exposed to great forces in its cross machine direction. Owing to the needs associated with either the manufacture or use particularly of printing papers and cardboards, their internal bond strength should thus be at a sufficient level.

[0004] The internal bond strength of paper products may be improved in various ways, such as by paying attention to web formation, wet pressing and fibre orientation or by improving the strength of fibre bonding e.g. by adding various chemical fillers or additives or by refining the pulp.

[0005] It is also known per se to improve the strength properties, such as the internal bond strength, of wood pulp or e.g. recycled pulp by admixing fractionated non-wood pulp or wood pulp with the pulp.

[0006] "Control of bagasse pulp quality by fractionation and refining" by El-Sharkawy, K., Rousu, P., Haavisto, S. & Paulapuro, H., Appita Journal 60(5), 2007, 404 to 409, 415 describes addition of fractionated bagasse pulp to softwood pulp, whereafter the bagasse pulp was refined together with the softwood pulp. Fractionation of the bagasse pulp was carried out by pressure sieve fractionation, wherefrom accept and reject fractions were recovered. It is mentioned that the reject fraction contained mainly long and coarse fibres, while the accept fraction contained mainly short fibres, fines, and some of the thin fibres. Different amounts of the accept fraction were added to the softwood pulp prior to refining the pulp. It was found that the addition of bagasse pulp increases the density of paper. It is also stated that this results in an improved internal bond strength. [0007] "Upgrading of recycled paper with oil palm fibre soda pulp" by Wanrosli, W. D., Zainuddin, Z. & Roslan, S., Industrial Crops and Products 21 (3), 2005, 325 to 329 describes addition of oil palm pulp to recycled pulp. It was found that upon adding 20% unrefined oil palm pulp or 10% refined oil palm pulp, both the tensile strength and the tearing strength of the recycled pulp improved.

[0008] "Fiber fractionation as a tool for making better quality paper from agricultural residues pulp" by Sood, Y. V., Tyagi, S., Payra, I., Neethiku- mar, R., Tyagi, R., Kumar, A. & Kulkarni, A. G., Cellulose Chem. Technol., 41 (1 ), 77 to 83 (2007) discloses fractionation of bleached pulp which contained 90% a wheat straw pulp and bagasse pulp mixture and 10% hardwood pulp. The pulp was fractionated into two fractions, namely accept and reject, by a Bauer McNett device (aperture size 0.297 mm), the reject was refined, and from both fractions, laboratory sheets were prepared using laboratory sheet moulds. The sheets thus obtained were used for making three-layer paper by sandwiching a sheet made from the accept between two sheets made from the reject. It was found that the three-layer paper thus obtained had an improved tensile and tearing strength, bulk, and stiffness, for instance.

[0009] It has also been found that it is possible to improve e.g. the strength of the middle layer of a three-layer cardboard by using therein, instead of a mixture of chemithermomechanical pulp and long-fibrous cellulose, a fine fraction of birch cellulose fractionated by a hydrocyclone (Panula-Ontto, S. & Fuhrmann, A., "Effect of fractionation and refining on board and tensile stiffness of board". SP-2 Sustain-pack seminar. Latest innovation in cellulose packaging, Verona, Italy, 18 April 2007). The birch cellulose pulp was used refined.

[0010] It is also known that the components of non-wood pulps may have both positive and negative overall effects on the properties of the total pulp. By removing only a small portion of fines of a certain type, for instance, the drainability and brightness of the total pulp may be improved (Fl Patent Application No. 20075735, publ. 18 April 2009).

[0011] A problem with the above-described methods is that the non- wood pulp compositions have not been sufficiently characterized in order to enable the internal bond strength of paper and paper products to be improved in a sufficient and repeatable manner by adding non-wood pulp. Additional refining stages, for instance, have also been necessary. Brief description of the invention

[0012] An object of the invention is thus to provide a method of improving the internal bond strength of paper and other paper products in a re- peatable manner. The object of the invention is achieved by a method which is characterized by what is stated in the independent claim. Preferred embodiments of the invention are disclosed in the dependent claims.

[0013] The invention relates to a method of improving the internal bond strength of paper manufactured from wood pulp by adding a certain type of non-wood pulp composition to the wood pulp.

[0014] According to the invention, it has been found that a certain type of non-wood pulp, when admixed with wood pulp, significantly improves the internal bond strength of paper without substantially affecting other paper technological properties. A certain type of straw pulp composition in particular has been found to be a good source for internal bond strength. An advantage of the method according to the invention is that additional refining stages can be avoided and no extra chemical additives or additional components are necessary.

Definitions relating to the invention

[0015] "Parenchymatous cells" refer to parenchymatous cells and the like of non-wood pulp.

[0016] "Epidermis cells" refer to epidermis cells and the like of non- wood pulp.

[0017] "Accept" and "accept fraction" refer to a fraction that has passed through a sieve or a screen or that has been separated by a hydrocyc- lone as a hydrocyclone top product.

[0018] "Reject" and "reject fraction" refer to a fraction that has been retained on a sieve or screen or in a hydrocyclone as a hydrocyclone bottom product.

[0019] Unless otherwise stated, the amounts, proportions and ratios of the components have been calculated by weight.

Detailed description of the invention

[0020] The invention relates to a method of improving the internal bond strength of paper manufactured from wood pulp by adding to the wood pulp a non-wood pulp composition having the following properties: it contains fibres whose length/width ratio lies within a range of 40 to 150, parenchymatous cells whose length/width ratio lies within a range of 1 to 20, and epidermis cells whose length/width ratio lies within a range of 1 to 20, and

the amount of the fibres is at least 25%, preferably at least 33%, the amount of the parenchymatous cells is 20 to 35%, and the amount of the epidermis cells is 10 to 25% of the total non-wood pulp composition, as calculated by weight.

[0021] In a preferred embodiment of the invention, the amount of the parenchymatous cells in the composition is larger than that of the epidermis cells.

[0022] The non-wood pulp composition also contains small amounts of other common cell types of non-wood pulp, such as various vessel elements, tracheids, cork cells, and hair cells.

[0023] The proportion of non-wood pulp composition to be added is 5 to 35%, preferably 5 to 25%, most preferably 25 to 35%, as calculated from the wood pulp.

[0024] The non-wood pulp composition may be added to the wood pulp at any suitable stage prior to paper manufacture. It may be added to bleached or unbleached pulp.

[0025] Typically, the non-wood pulp composition to be added is produced by fractionating chemical non-wood pulp. The fractionation may be carried out using fractionation based e.g. on screening, classification or pressure difference. Suitable industrial devices for these include screens, sieves, such as pressure sieves, and hydrocyclones (vortex cleaners). The pressure sieve may be e.g. an aperture sieve.

[0026] In an embodiment of the invention, fractionation is carried out using hydrocyclone fractionation. Preferabiy, the hydrocyclone fractionation is carried out by a hydrocyclone whose diameter is 30 to 70 mm, e.g. 30 to 50 mm or 50 to 70 mm.

[0027] In another embodiment of the invention, fractionation is carried out using pressure sieve fractionation. The aperture size of the pressure sieve is preferably within a range of 0. 5 to 0.6 mm.

[0028] In one preferred embodiment of the invention, a combination of pressure sieve fractionation and hydrocyclone fractionation is used. First, pressure sieve fractionation into accept and reject is carried out, whereafter the accept is further fractionated into accept and reject by using a hydrocyclone. The accept separated from the pressure sieve fractionation accept by the hydrocyclone is recovered and added to the wood pulp. In still another embodiment of the invention, the accept separated from the pressure sieve fractionation reject by the hydrocyclone is recovered and added to the wood pulp.

[0029] When the combination of pressure sieve fractionation and hydrocyclone fractionation is used, the pressure sieve fractionation is preferably carried out using a pressure sieve whose aperture size is 0.15 to 0.6 mm. The fractionation of the accept obtained from the pressure sieve fractionation is preferably carried out using a hydrocyclone whose diameter is 30 to 50 mm. The fractionation of the reject obtained from the pressure sieve fractionation is preferably carried out using a hydrocyclone whose diameter is 50 to 70 mm.

[0030] It was found that fine fibres (the fibre length/width ratio being within a range of 40 to 150) and a positive ratio of parenchymatous cells to epidermis cells (the amount of parenchymatous cells being larger than that of epidermis cells) have a positive effect on the internal bond strength. The accept fraction of the hydrocyclone fractionation is a typical representative of such a composition. A fraction that prior to the hydrocyclone stage has also passed through a pressure sieve has an even stronger positive effect on the internal bond strength.

[0031] If pulp drainability is at a critical level, the accept separated from the pressure sieve reject by a hydrocyclone may be used for improving the internal bond strength, since said fraction has no effect on the drainability of the pulp.

[0032] In the method according to the invention, the non-wood pulp composition is added to the wood pulp unrefined. The non-wood pulp composition is not refined together with the wood pulp, either.

[0033] According to one further aspect of the invention, the invention relates to a method of improving the internal bond strength of paper made from wood pulp by adding to the wood pulp a non-wood pulp composition obtained by fractionating the non-wood pulp by a hydrocyclone whose diameter is 30 to 70 mm, by recovering the hydrocyclone accept, and by adding the hydrocyclone accept to the wood pulp in an amount of 5 to 35%, preferably 15 to 25%, most preferably 25 to 35%, as calculated from the wood pulp.

[0034] According to yet another further aspect of the invention, the invention relates to a method of improving the internal bond strength of paper made from wood pulp by adding to the wood pulp a non-wood pulp composition obtained by a method comprising the following steps of:

fractionating the non-wood pulp by a pressure sieve whose aperture size is 0.15 to 0.6 mm, thus obtaining a pressure sieve accept and a pressure sieve reject,

fractionating the thus obtained pressure sieve accept by a hydrocyc- lone whose diameter is 30 to 50 mm, thus obtaining a hydrocyclone accept and a hydrocyclone reject,

recovering the thus obtained hydrocyclone accept and adding it to the wood pulp in an amount of 5 to 35%, preferably 15 to 25%, most preferably 25 to 35%, as calculated from the wood pulp.

[0035] Typically, the flow rate in the hydrocyclone fractionation of the pressure sieve accept is within a range of 1.2 to 1.5 l/s.

[0036] According to yet another further aspect of the invention, the invention relates to a method of improving the internal bond strength of paper made from wood pulp by adding to the pulp a non-wood pulp composition obtained by a method comprising the following steps of:

fractionating the non-wood pulp by a pressure sieve whose aperture size is 0. 5 to 0.6 mm, thus obtaining a pressure sieve accept and a pressure sieve reject,

fractionating the thus obtained pressure sieve reject by a hydrocyclone whose diameter is 50 to 70 mm, thus obtaining a hydrocyclone accept and a hydrocyclone reject,

recovering the thus obtained hydrocyclone accept and adding it to the wood pulp in an amount of 5 to 35%, preferably 15 to 25%, most preferably 25 to 35%, as calculated from the wood pulp.

[0037] Typically, the flow rate in the hydrocyclone fractionation of the pressure sieve reject is within a range of 1.8 to 2.2 l/s.

[0038] In the present invention, non-wood pulp refers to pulp made from herbaceous plant fibres, bast fibres, leaf fibres or fruit seed fibres. Examples of useful herbaceous-plant-based fibres include straw, e.g. cereal straw (wheat, rye, oat, barley, rice, com), reeds, e.g. reed canary-grass, common reed, paper reed, sugar cane or bagasse, and bamboo, as well as grasses, e.g. esparto, sabai and lemon grass. Examples of bast fibres include flax, such as stems of common flax and stems of oil flax, hemp, east india hemp, kenaf, jute, ramie, paper mulberry tree, gambi fibre, and mitsumata fibre. Examples of leaf fibres include manilla hemp and sisal. Examples of fruit seed fibres include cotton seed hair and cotton linter fibres, kapok, and coconut fibre.

[0039] Herbaceous plants growing in Finland and useful in the present invention include common reed, reed canary-grass, timothy, cock's-foot, yellow melilot, awnless brome, red fescue, white melilot, red clover, goat's rue, and medick.

[0040] Particularly preferably, pulp manufactured from fibres of herbaceous plants, such as straw cellulose pulp, is used. In an embodiment, pulp manufactured from annual herbaceous plants is used. In another embodiment, pulp manufactured from perennial non-wood plants is used. According to the invention, agricultural waste material, including e.g. the aforementioned cereal straw, may also be used.

[0041] The non-wood pulp may be pulp manufactured by any chemical methods, such as a sulphate, sulphite or soda method. The pulp may also be pulp manufactured by solvent-based methods, such as formic-acid- based and/or acetic-acid-based, alcohol-based or ionic methods. The pulp may be bleached or unbleached pulp.

[0042] The wood pulp to which the non-wood pulp is added may be any chemical or mechanical softwood or hardwood pulp, such as pine, spruce, birch, eucalyptus, acacia, and aspen pulp or a mixture of any one thereof. The pulp may also be recycled pulp, possibly mixed with any one of the aforementioned pulps.

[0043] Paper or other paper products is/are manufactured from mixed pulp obtained according the invention by employing conventional methods. According to the present invention, the paper and paper products may be any paper products, including cardboard. Particularly preferably, the present invention is suitable for the manufacture of printing papers and coated papers, such as magazine papers of different types.

[0044] The invention further relates to manufacture of paper pulp by adding the above-described non-wood pulp composition to wood pulp. Preferred embodiments of this aspect are the same as those disclosed above. The invention also relates to manufacture of paper and other paper products by using the thus obtained mixed pulp containing wood pulp and the above- described non-wood pulp composition.

[0045] The invention also relates to a use of the above-described non-wood pulp composition for manufacturing paper pulp, paper, and other paper products. Preferred embodiments of this aspect are the same as those disclosed above.

[0046] The following examples are set forth to clarify, not to restrict, the invention.

Example 1. Internal bond strengths of wheat straw pulp and different fractions thereof

[0047] First, wheat straw pulp was fractionated by a fine pressure sieve (aperture size 0.2 mm) into two fractions, i.e. into accept, which was the fine fraction passed through the pressure sieve, and into reject, which did not pass through the pressure sieve. Next, using hydrocyclones, both fractions were further divided into two fractions, obtaining a total of four different fractions: the pressure sieve accept was divided by a hydrocyclone (diameter about 40 mm ja flow rate 1.4 l/s) into accept (PS accept→ HC accept) and reject (PS accept→ HC reject), and the pressure sieve reject was divided by a hydrocyclone (diameter about 60 mm ja flow rate 2.0 l/s) into accept (PS reject → HC accept) and reject (PS reject→ HC reject).

[0048] Table 1 shows weight proportions of said pulp fractions on the total pulp and Table 2 shows cell type compositions. The cell types were determined in accordance with the ISO 9184-1 -7 standard by using Fiber Atlas by Marja-Sisko llvessalo-Pfaffli in the identification. More than 5 000 cells were calculated from each sample.

Table 1. Mass proportions of wheat straw pulp fractions

Pulp Fraction PS Reject→ PS Reject→ PS Accept→ PS Accept→

HC Accept HC Reject HC Accept HC Reject

Mass Proportion, 32.6 27.0 19.7 20.7

% Table 2. Compositions of wheat straw pulp fractions

[0049] The cell type calculation revealed clear differences between the pulp fractions. The pressure sieve accept contained clearly less actual fibres and more of both parenchymatous cells and epidermis cells than the pressure sieve reject. The most vessel elements were found in the pressure sieve reject that had ended up in the hydrocyclone accept. The most parenchymatous and epidermis cells were found in the pressure sieve accept that had ended up in the hydrocyclone reject. Half of the fraction that had ended up in both the pressure sieve and the hydrocyclone accept was comprised of fine fibres, and the fraction also contained considerable amounts of parenchymatous cells and epidermis cells. The hydrocyclone affected the fractionation of the pressure sieve accept such that the hydrocyclone accept contained relatively more fibres than the reject, whereas the largest fraction of the reject consisted of epidermis cells.

[0050] Laboratory sheets were prepared from the original pulp and different pulp fractions in a laboratory sheet mould. Table 3 shows the lengths, widths, amount of fines, length/width ratio, linear density, and internal Scott bond strengths of particles of the original pulp and the different pulp fractions. A Lorenz-Wettre fibre analyzer was used for determining cell dimensions, and widths of the fibres were determined microscopically. The Scott bond strength was determined by the T 569 P 00 standard. In Table 3, the definition L(l) means length-weighted fibre length, and L(n) means arithmetic fibre length. Table 3. Dimensions and internal bond strengths of components of origi nal wheat straw pulp and pulp fractions

[0051] It can be seen in Table 3 that cells that ended up in the pressure sieve reject were physically longer than cells that ended up in the accept. The internal bond strengths show that the internal bond strength of the original pulp was lower than that of any single fibre fraction. The pulp fraction that ended up in the pressure sieve and hydrocyclone accept bonded so well that it was impossible to determine the internal bond strength of paper manufactured therefrom by a Scott bond device.

[0052] Furthermore, from the pulp fraction that ended up in the pressure sieve and hydrocyclone accept (PS accept→ HC accept), the widths and lengths of each particle type, i.e. epidermis cells, parenchymatous cells and fine fibres, were separately determined by microscopic measurements (widths and lengths were measured from microscope pictures). The following results were obtained:

• epidermis cells: width 11 to 21 pm and length 120 to 220 pm

• parenchymatous cells: width 20 to 1 10 pm and length 40 to 300 pm

• fine fibres: width 5 to 1 pm and length at most 0.6 mm. Example 2. Effect of admixing wheat straw pulp fractions on internal bond strength and other paper technological properties

[0053] Wheat straw pulp was fractionated according to Example 1. Eucalyptus pulp manufactured by a sulphate method was refined to a refining degree required by typical fine paper manufacture (SR 27.5). 10% of the proportion of eucalyptus cellulose was replaced with wheat straw cellulose or wheat straw cellulose fractions and, in addition, in one of these fractions (PS accept→ HC accept) 30%. Thus, at one test point, the wheat straw cellulose was used as such while at other test points different pulp fractions were used.

Table 4. Paper technological properties of paper made from eucalyptus cellulose (Ref.) and of six other test points wherein either 10% or 30% of eucalyptus pulp was replaced by either non-fractionated wheat straw pulp (Ref. 10%) or a wheat straw pulp fraction

[0054] The internal bond strengths given as Scott bond measurement results are shown in Figure 1 . In Figure 1 , "Euca" refers to eucalyptus pulp, "WS" refers to wheat straw pulp, "PSrHCa" refers to a fraction PS reject → HC accept, PSrHCr refers to a fraction PS reject→ HC reject, "PSaHCr" refers to a fraction PS accept→ HC reject, and "PSaHCa" to a fraction PS accept→ HC accept.

[0055] The test results show that by replacing 10% of the refined eucalyptus pulp with wheat straw pulp, the internal bond strength of paper manufactured from the mixture can be improved 12.9%. By replacing 10% of the same eucalyptus pulp with a pulp fraction passed through a pressure sieve and a hydrocyclone, the internal bond strength can be improved 25.8%. When the replacement degree was raised to 30%, the internal bond strength could be improved 69.2%.

[0056] Irrespective of whether or not the components in the hydro- cyclone accept had passed a first stage pressure sieve, they affected the internal bond strength positively. In contrast, the pulp fraction that passed through the pressure sieve but not through the hydrocyclone had no effect on the internal bond strength. A pulp fraction consisting of small components (a small average particle size) and having thin fibres and a larger ratio of the parenchymatous cells to the epidermis cells is the most effective fraction to improve the internal bond strength.

[0057] The results also show that when 0% of the eucalyptus pulp used as the reference was replaced with wheat straw pulp, the drainability (SR value) of the pulp was not affected negatively but, in addition to the internal bond strength, the tensile strength, breaking energy, tensile stiffness and smoothness of the paper were improved. On the other hand, a slight decrease of less than 1 % was found in the tearing resistance of the paper. In addition, the air permeability of the paper decreased substantially. In practice, a decrease in air permeability of paper together with an increase in its smoothness may result in a decrease in the amount of coating material or printing ink necessary in paper coating and printing, respectively, and thus in lower costs.

Example 3. Effects of different components of wheat straw pulp on internal bond strength of paper

[0058] The amount of wheat straw pulp fractions was varied in a fibre mixture in order to determine the optimal internal bond strength potential. The proportion of other components (other fractions) in the fibre mixture was kept constant (the same as in the original pulp), and different percentages of each fraction under study were added to said composition. Figure 2 shows the effect of the proportion of pulp fractions on the internal bond strength of paper, as expressed as Scott bond measurement results, and it shows the optimal effect of wheat straw fractions in the pulp mixture. The Scott bond strength of the original wheat straw cellulose is shown in Figure 2 in a uniform line. The abbreviations of Figure 2 mean the same as those in Figure 1.

[0059] It can be seen in Figure 2 that the internal bond strength of the original pulp is weaker than that of any other combination, i.e. the internal bond strength may be improved by changing the original pulp composition. The greatest internal bond strength is achieved by using a double accept, i.e. a fraction which has passed through both a pressure sieve and a hydrocyclone (PSaHCa fraction). (No result from a test point of the 100% PSaHCa fraction is provided because the internal bond strength of paper made from said fraction was too great to be determined by the device in question).

[0060] It is obvious to one skilled in the art that as technology advances, the basic idea of the invention may be implemented in many different ways. The invention and its embodiments are thus not restricted to the above- described examples but may vary within the scope of the claims.