Method for producing paper pulp, paper pulp so produced and paper and cardboard manufactured from said pulp

Abstract: The invention relates to a noil consisting of fibrils for use in paper manufacture, to be mixed in the paper pulp. The noil is produced from cellulose fibre by refining and consists of noil fibrils that, in respect of size distribution, mainly correspond to wire screen fraction P50, i.e. 50 mesh, and the noil constitutes 0.1 -15w-% of the paper pulp after mixing.

Description: The present invention relates to a method for producing paper pulp as defined in the preamble of claim 1. Moreover, the invention relates to paper pulp and paper and graphic cardboard containing it. Today, the trend of development of paper products is increasingly determined by customers and legislative measures. The buyers of information paper want to economise on postage and reduce the amount of waste produced. Further, waste processing charges depending on weight have been imposed on packaging materials. Generally, it seems that energy taxes and environmental protection taxes are being added as an extra imposition to the price of paper products. For these reasons, paper buyers want paper products of a lower grammage which still meet high quality requirements. In this application, information paper refers to different kinds of printable paper and cardboard, coated or uncoated, manufactured on paper and cardboard machines, e.g. printing paper and graphic cardboard. Because of the general trend of development described above, there is a need to produce high-quality information paper using a reduced amount of raw material. When the grammage of paper is reduced, its opacity becomes a critical property. Opacity can be increased by increasing the filler content of the paper, but this generally reduces its strength properties. Therefore, there is a need to alter the paper structure in a way that allows important product properties to be maintained at the same time. For paper-based communication to remain competitive with respect to electric communication, further improvements in the impression quality of paper products are required. - These general trends of development impose very high requirements on the raw materials of paper and on paper production processes. In order to meet the requirements, intensive efforts have been made in recent times to further develop paper raw materials and their production processes. Specification FI 931584 (corresponding to GB-A-2 265 916) presents a composite product for use as filler in paper and a method for its manufacture, based on precipitation of calcium carbonate on the surface of cellulose fibres. The fibres are mainly whole chemical pulp fibres, individual microfibrils are only present on fibre surfaces. From specification FI 953238 (corresponding to WO-A-97/01670), a paper filler is known that consists of porous aggregates formed by calcium carbonate particles precipitated on noil fibrils produced by refining from cellulose fibre and/or mechanical pulp fibre; as to their size distribution, the noil fibrils correspond to wire screen fraction P100. The filler gives the paper better optical and strength properties and a lower grammage than earlier calcium carbonate based fillers. For the manufacture of this product, calcium carbonate must be precipitated on the surface of noil fibrils. The textbook J.P. Casey "Pulp and Paper; Chemistry and Chemical Technology, 3 rd ed. vol. II, 1980, pages 826-846, 854-865, 915-920, 935-940 discloses generally beating and refining of pulp and the main sheet properties (strength and other mechanical properties, sheet formation and density related properties) that are controlled by this processing. Document US-A- 5228954 discloses cellulose pulp compositions of selected morphology for improved paper strength potential. Document US-A- 4692211 discloses high strength, cellulosic-gel-containing kraft paper which is made by mechanically refining kraft papermaking pulp containing 1-10 % hydrated cellulosic gel binder. Document US-A- 5387319 discloses flat, fibrous, supple substrate which is difficult to tear produced from cellulose pulp having a degree Schopper-Riegler > 80 and which contains at least 30 w-% artificial fibers. The present invention discloses a noil for use in paper manufacture which is mixed with paper pulp and which meets the requirements specified above and is easier to manufacture than corresponding kinds of noil and/or fillers known before. The invention discloses a new use of noil that gives the paper better strength properties, especially a better tensile strength and a better interlaminar strength as well as a lower grammage than commonly known prior-art use of noil and/or filler and that is still easier and cheaper to manufacture than corresponding types of noil and/or filler known in prior art. The noil which, when used in paper manufacture, makes it possible to add to the paper an increased amount of pigments to improve the optic properties of the paper being produced, and which is easier to manufacture than prior-art types of noil and/or filler that meet corresponding criteria. Further the type of noil whose production does not require any large additional investments and which can be produced using equipment generally already existing in a paper factory. The object of the invention is to disclose a method for producing a paper pulp containing said noil. A further object of the invention is to disclose a paper pulp containing said noil, to be used in the manufacture of information paper. Yet another object of the invention is to disclose a paper type manufactured using said noil. The invention. as described in the claims is based on the unexpected observation made during investigations that paper having the desired optical, strength and grammage properties can be produced by mixing the paper pulp with noil produced by refining from pulp fibre, preferably chemical pulp fibre, consisting of noil fibrils that, in respect of size distribution, mainly correspond to wire screen fraction P50, the amount of noil forming 0.1 - 15. w-%, more preferably 0.5 - 10 w-%, most preferably 2 - 4.5 w-% of the paper pulp. A new feature in the invention is particularly the fact that noil, added to paper pulp, gives the paper the desired, improved strength and/or grammage properties. The desired optic properties is achieved by adding required pigments into the paper pulp without impairing the strength properties of the paper. Before, a corresponding type of noil has been used in the manufacture of transparent paper sorts, such as butter paper and baking paper. However, the amounts of noil used in these cases have been substantially larger than in the present application. Moreover, in the present application, the noil makes it possible to achieve expressly improved optic properties because an improved paper strength allows the use of an increased filler content; in transparent paper sorts, such optic properties have not been aimed at. The noil, used in the method and paper pulp of the present invention differs from the filler presented in specification FI 953238 (corresponding to WO-A- 97/01670) in that, according to the present application, the desired strength and grammage properties are achieved by using noil without precipitation of pigment on noil, in other words, the invention makes it possible to add the pigment into the pulp in the conventional manner, whereas in specification FI 953238 the pigment, i.e. calcium carbonate, is precipitated onto noil fibrils. According to the present application, the noil can be mixed as such in the paper pulp; the pigment can be added into the paper pulp before the noil is mixed or after it has been mixed, or it may be added in conjunction with the mixing of the noil. The mixing can be performed e.g. in a machine tank or in some other suitable, e.g. separate mixer or container. According to a preferred embodiment of the invention, the noil, used in the method or in the paper pulp consists of noil fibrils mainly corresponding to wire screen fraction P100. The amount of noil in the paper pulp is 0.1-15 w-%, preferably 0.5 - 10 w-%, most preferably 2-4.5 w-% of the pulp, calculated in terms of dry matter. The mass ratio of pigment to noil is e.g. 0.1 - 20, preferably 0.2 - 10, most preferably 2 - 6. In this description, all mass ratio values have been calculated in terms of dry matter. The noil is preferably produced by refining cellulose fibre to a Schopper number > 80, preferably to a Schopper number between 80 - 90, or even to a higher Schopper number value. In addition to the noil, conventional pigments, e.g. kaolin, talcum, titan dioxide, ground calcium carbonate, precipitated calcium carbonate (PCC), chalk, synthetic silicate (such as aluminium silicate, aluminium magnesium silicate), barium sulphate, aluminium hydroxide or in general any pigment, can be used in the paper pulp. Mixed and/or composite pigments can also be used. Different calcium carbonates and kaolin are especially well suited for this purpose. Particularly good results have been achieved using precipitated and ground calcium carbonates, e.g. calcium carbonates having a scalenohedral crystal structure. The consistency of the noil is preferably of the order of 0.4 - 10 w-%. The noil can be advantageously produced from cellulose fibres by refining so that the edge load is of the order of 0.1 - 8 Ws/m. The refining can be effected using any known type of refiner, e.g. a conical refiner or a disk refiner. The specific energy consumption in the refining process is of the order of 10 - 1000 kWh/t, preferably 100 - 700 kWh/t, most preferably 10 - 300 kWh/t. The noil can be used in the manufacture of any kind of paper or cardboard produced on a paper or cardboard machine. The noil is especially well suited for use in the manufacture of information paper or cardboard, e.g. in the manufacture of printing paper and copying paper sorts having a grammage of the order of 25 - 350 g/m, or in the manufacture of graphic cardboard sorts in which the surface layer has a grammage of the order of 25 - 350 g/m 2 and the backing or inner layer is any kind of base layer known in the art. The paper pulp to be used may contain any pulp components, pigments, additives or other substances in known proportions of weight, as is generally known in paper manufacturing industry or in relevant literature. In the following, the invention will be described in detail by the aid of a few examples of its embodiments by referring to the attached drawings, wherein Figures 1 - 6 represent the light-scattering coefficient of the paper as a function of its tensile index, and the air permeability, interlaminar strength, CaCO 3 retention, light-scattering coefficient and density as functions of the CaCO 3 content of the pigment used, for certain test paper sorts when the noil of the invention is used together with precipitated calcium carbonate, ground calcium carbonate and without them. EXAMPLE 1 In this experiment, printing paper was produced using a fibrous mixture containing 50 % bleached chemical softwood pulp refined to the value CSF 420 and 50 % groundwood. The noil was produced by refining chemical birch pulp in a Valley laboratory hollander in accordance with the SCAN-C 25:76 standard for 2.5 hours. The refined pulp was screened using a Bauer-McNett screen, and the P200 fraction was saved. The amount of noil added to the pulp was 4.5 w-%. The amounts of filler used were 10 w-% and 20 w-%. The noil and the filler were added into the pulp simultaneously before sheet formation. As retention agents, cationic starch in an amount of 0.65 w-% and silica in an amount of 0.15 w-% were used. From the pulps thus obtained, test sheets were produced and for each sheet the light-scattering coefficient was determined as a function of tensile index and the air permeability, interlaminar strength, CaCO 3 retention, light-scattering coefficient and density were determined as functions of CaCO 3 content. The amounts of noil added into the pulps prepared and the pigments used in different pulp mixtures are presented in Table 1. The measurement results are presented in the form of graphs in Fig. 1 - 6. The measurement results indicate that the addition of noil in the pulp significantly increased the tensile index value. Moreover, the addition of noil reduced the air permeability, increased the interlaminar strength, improved the CaCO 3 retention and slightly increased the density. However, the addition of noil had no significant effect on the light-scattering coefficient. Pulp mixture Addition of noil Pigment used 1 0 % PCC 2 4.5 % PCC 3 0 % GCC 4 4.5 % GCC 5 4.5 % no pigment 6 0 % no pigment PCC = precipitated calcium carbonate GCC = ground calcium carbonate The embodiment examples are intended to illustrate the invention without limiting it in any way.

Claims: Method for producing paper pulp for use in printing paper and graphic cardboard paper manufacture, characterised in that paper pulp containing cellulose fibre and/or mechanical pulp fibre and filler is mixed with noil produced by refining from cellulose fibre and consisting of noil fibrils that, in respect of size distribution, correspond to Bauer-McNett wire screen fraction P50 and the amount of noil forming 0.1 - 15 w-% of the paper pulp. Method as defined in claim 1, characterised in that the noil fibrils correspond to Bauer-McNett wire screen fraction P100. Method as defined in claim 1 or 2, characterised in that the noil is produced by refining cellulose fibre to a Schopper number > 80. Method as defined in claim 3, characterised in that cellulose fibre and/or mechanical pulp fibre is/are refined to a Schopper number in the range 85 - 90. Paper pulp for use in printing paper and graphic cardboard paper manufacture, containing cellulose fibre and/or mechanical pulp fibre and filler, characterised in that the paper pulp contains noil that has been produced from cellulose fibre by refining and consists of noil fibrils corresponding in size distribution to Bauer-McNett wire screen fraction P50 and that the noil constitutes 0.1 - 15 w-% of the paper pulp. Paper pulp as defined in claim 5, characterised in that the noil fibrils correspond to Bauer-McNett wire screen fraction P100. Paper pulp as defined in claim 5 or 6, characterised in that the paper pulp contains pigment, the mass ratio of pigment to noil being 0.1-20. Paper pulp as defined in any one of claims 5 - 7, characterised in that the noil has been produced by refining cellulose fibre to a Schopper number > 80. Paper pulp as defined in claim 6, characterised in that cellulose fibre has been refined to a Schopper number in the range 85 - 90. Printing paper and graphic cardboard manufactured using paper pulp as defined in any one of claims 5 - 9.

Inventors: SILENIUS PETRI, SILENIUS, PETRI
Applicants: REAL CORP M, M-REAL CORPORATION
Application year: 1999
Main classification: D21H11
IPC codes: D21H11, D21H17