Field of the Invention

The present invention relates to a method for making a printing paper product by multilayer technique starting from aqueous furnish containing fibres and strength additive. The invention also relates to a printing paper product made from the furnish and containing fibres and strength additive.

Background of the Invention

Printing papers made by multilayer technique need to have high surface strength and internal strength because of the mechanical stresses caused by the printing process, in addition to good imprint quality. This is especially true for papers intended for heatset web offset (HSWO). Internal bonding strength is especially needed for resisisting blistering of paper when water inside of paper is evaporating during drying of printed paper. In the multilayer technique, the furnish is fed by a feeding device (headbox) in separate layers which are combined before the formation zone or right after the start of the formation zone. When the furnish is kept as separate flows to the start of the formation, different compositions can be used in the flows forming the middle layer and surface layers. The layered flows are combined in a sufficiently early stage that different substances can migrate between the layers, especially from the middle layer to surface layers, and strengthen the internal bond of the paper product in z direction.

It is also desirable to increase the proportion of filler in the printing paper and thereby to reduce the use of fibres. However, the filler increase of paper is limited by internal bonding strength, which is weakened if too large proportion of filler is used.

Summary of the Invention

Thus, there is a need for a novel manufacturing method where the internal bonding strength of a multilayer paper can be raised, especially in cases where the aim is to use filler is in large proportion to replace papermaking fibres.

Said need is satisfied according to a method where in the aqueous furnish comprising fibres and strength additive, nanofibrillar cellulose and strength additive is fed in a middle layer to form a multilayer printing paper. The printing paper made from the furnish by multilayer technique through dewatering has a target basis weight and comprises the strength additive and the fibril cellulose migrated from the middle in z-direction to surface layers of the printing paper. By using nanofibrillar cellulose in the furnish together with the strength additive, an increase of the internal bonding strength of a paper made by multilayer technique is observed, and this is particularly advantagegous, if the furnish contains filler. In the furnish, the flow forming the middle layer can comprise water, nanofibrillar cellulose and strength additive, and the flows forming the surface layers can comprise fibres and filler. The same basic furnish can be used for the surface layers, and the furnish for the middle layer is prepared separately.

According to another embodiment, the same basic furnish is used for all three layers, and additives are added to the flows differently so that nanofibrillar cellulose and strength additive is added to the flow forming the middle layer. This basic furnish common to all layers can contain fibres and filler.

The principle is that the flow forming the middle layer contains strength additive and nanofibrillar cellulose in larger proportion than the surface layers on both sides of it, and during the dewatering the nanofibrillar cellulose and strength additive migrate in z-direction to the surface layers along with the water, increasing the internal bond in Z-direction. When the nanofibrillar cellulose, whose retention is poor because of its size, is introduced in the middle of the web, its retention is improved. Preferably the nanofibrillar cellulose is only in the flow forming the middle layer. Strength additive can be originally present in the surface layers if that is used in the basic furnish. The furnish contains advantageously filler. The compositions of the different flows can be arranged so that the proportion of filler is larger in the flows forming the surface layers. This can be made by using the same furnish for the surface layers (furnish comprising filler) and a different furnish for the middle layer (furnish with no filler). Alternatively, the amount of filler can be the same in all flows, and other materials, especially strength additive and fibril cellulose, are added differently in the flow forming the middle layer and in the flows forming the surface layers. As stated above, the nanofibrillar cellulose is used preferably only in the middle layer. Thus, if the same basic furnish is divided to two surface layer flows and one middle layer flow, the fibril cellulose is added only to the middle layer flow.

According to one embodiment, the strength additive is cationic strength additive which can be a cationic polymer (polyelectrolyte). Cationic starch is widely used dry strength additive in papermaking and can be used according to one embodiment of the method.

The nanofibrillar cellulose can be nanofibrillar cellulose made from chemically unmodified fibres. If the strength additive is cationic strength additive, the nanofibrillar cellulose is preferably anionically charged nanofibrillar cellulose. Anionically charged nanofibrillar cellulose is nanofibrillar cellulose where the cellulose molecules are modified so that they contain anionic groups. The hydroxyl groups of the cellulose can be for example oxidized to carboxylate groups. One example of anionically charged nanofibrillar cellulose which can be used is nanofibrillar cellulose where the carboxylate groups are the result of catalytic N-oxyl mediated oxidation, such as 2,2,6,6-tetramethyl-1 - piperidine N-oxide -mediated ("ΤΕΜΡΟ''-mediated) oxidation of cellulose. The carboxylate groups obtained through oxidation make the cellulose also labile to such extent that the fibres can be disintegrated to fibrils with less energy. Another alternative for the anionically charged nanofibrillar cellulose is fibril cellulose where the cellulose is carboxy methylated. This nanofibrillar cellulose can also be made by chemical modification (carboxymethylation) of the fibres and subsequent disintegration of the fibres to fibrils. According to one embodiment, the nanofibrillar cellulose is used in the furnish in an amount of 0.1 ...5 wt-%, preferably 0.5...2.0 wt-% of the dry weight of the base paper (uncoated weight). The method is particularly advantageous if the paper product made contains filler which replaces part of the papermaking fibres to reach the target basic weight. Because the internal strength is increased, more filler (about 10 percentage units more) can be incorporated in the base paper of the paper product without deteriorating the strength below values normally required. According to one embodiment, the paper product contains filler in an increased amount of 30...35 wt-% on the weight of the base paper.

The furnish and consequently the paper product made form the furnish can also contain other additives, in addition to the materials mentioned above. These include especially retention aids which can be used especially in the flows forming the surface layers.

In the method, the nanofibrillar cellulose acts as a kind of strength additive increasing the internal bonding strength of a printing paper made by separate furnish flows. It also acts as a kind of fibrillar retention aid binding the filler, if present, to the fibrous network of the papermaking fibers, which are normal-size fibres and can be made of variety of pulps which in turn can be based on many cellulosic raw materials. The fibrillar cellulose is characterized by considerably smaller size compared with the papermaking fibers. Especially if the furnish comprises cationic strength additive, the nanofibrillar cellulose is characterized also by anionic charge on the surface of the fibrils due to anionic groups, especially carboxylates.

Detailed Description of the Invention

The paper product is a printing paper produced by multilayer technique and having increased internal bond due to the introduction of nanofibrillar cellulose and strength additive in the middle of the paper. In the multilayer technique the furnish flows which have differing compositions are kept as separate "layers" which are combined at the start of the dewatering when the fibril cellulose and strength additive migrate towards the surfaces of the paper web being formed. The dewatering for the initial formation of the paper web takes place in two opposite directions through a pair of moving foraminous supports, such as formig fabrics. The mechanical properties of the printing paper make it suitable for printing in many processes, such as in HSWO.

The multilayering technique can be accomplished using a so-called multilayer headbox. Layers are formed by means of layering vanes, which keep the furnish flows separate. Two adjajent layers are formed at the end of the resceptive vane keeping them separate. In making paper from three parallel furnish flows by a multilayer headbox, two layering vanes are thus used.

The purpose of the middle layer in the multilayer technique is to introduce the additives contributing to the internal bonding strength (fibril cellulose, polymeric strength additive) inside the paper web and thus improve their retention. The middle layer is not intended to be a "bulk" layer, and the weight of the middle layer flow, calculated as dry matter, is smaller than the combined weight of the surface layer flows calculated as dry matter. The weight of the middle layer flow can be smaller than the weight of either surface layer. In the extreme case, only circulation water, strength additive and nanofibrillar cellulose is used for the furnish of the middle layer, and papermaking fibres forming the structural fibrous network of the paper, are brought in the surface layer flows, possibly together with filler. The basis weights of the printing papers made by the multilayer technique range from 70 to 350 g/m ² , preferably 90 to 150 g/m ² . They are normally coated papers, and the above basis weights refer to the combined weight of the base paper and coating. The papermaking fibres of the furnish for making the base papers by the method can be mechanical pulp, chemical pulp, or a suitable mixture of these.

Thus, to increase the internal bonding strength and the retention of the filler nanofibrillar cellulose is used together with the strength additive in the flow forming the middle layer. The order of addition of the materials depends on the method of preparation of the furnish. The furnish can be prepared separately for the middle layer, or the strength additive and the nanofibrillar cellulose can be added to the middle layer flow which is the same basic furnish as the surface layer flows.

Nanofibrillar cellulose refers to a collection of isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. Nanofibrillar cellulose has typically a high aspect ratio: the length might exceed one micrometer while the number-average diameter is typically below 200 nm. The diameter of nanofibril bundles can also be larger but generally less than 5 μιτι. The smallest nanofibrils are similar to so called elementary fibrils, which are typically 2-12 nm in diameter. The dimensions of the fibrils or fibril bundles are dependent on raw material and disintegration method. The nanofibrillar cellulose may also contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of nanofibrillar cellulose from cellulose raw material, cellulose pulp, or refined pulp is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. The nanofibrillar cellulose is preferably made of plant material. One alternative is to obtain the fibrils from non-parenchymal plant material where the fibrils are obtained from secondary cell walls. One abundant source of cellulose fibrils is wood fibres. The nanofibrillated cellulose is manufactured by homogenizing wood-derived fibrous raw material, which may be chemical pulp. The disintegration in some of the above- mentioned equipments produces fibrils which have the diameter of only some nanometers, which is 50 nm at the most and gives a dispersion of fibrils in water. The fibrils can be reduced to size where the diameter of most of the fibrils is in the range of only 2-20 nm only. The fibrils originating in secondary cell walls are essentially crystalline with degree of crystallinity of at least 55 %.

The nanofibrillar cellulose used can be chemically unmodified. The nanofibrillar cellulose used can altenatively be nanofibrillar cellulose containing anionically charged groups (anionically charged nanofibrillar cellulose). Such anionically charged nanofibrillar cellulose can be for example chemically modified nanofibrillar cellulose that contains carboxyl groups as a result of the modification. Cellulose obtained through N-oxyl mediated catalytic oxidation (e.g. through 2,2,6,6-tetramethyl-1 -piperidine N- oxide) or carboxymethylated cellulose are examples of anionically charged nanofibrillar cellulose where the anionic charge is due to a dissociated carboxylic acid moiety. Anionically charged nanofibrillar cellulose is typically produced by modifying pulp chemically, whereafter the fibres of the pulp are disintegrated to fibril cellulose. The filler can be any filler used in paper manufacturing, e.g. precipitated calcium carbonate (PCC), ground calcium carbonate (GCC), kaolin clay, talc or gypsum.

In the method, the filler is used in the furnish in an amount which results in the final filler content of of 30...35 wt-% on the weight of the base paper. The nanofibrillar cellulose is used in the furnish in an amount of 0.1 ...5 wt-%, preferably 0.5...2.0 wt-% on the dry weight of the base paper.

The strength additive is usually a cationic strength addditive, which is a strongly cationic polymer (polyelectrolyte), and it can be any dry strength additive used in paper manufacturing, such as cationic starch or cationic polyvinylamine. Preferably, the cationic polyelectrolyte is cationic starch (CS). The strength additive is added in an amount of 0.1 ... 2.5 wt-%, preferably 0.5...1 .0 wt-% of dry weight of the base paper.

The retention aid, which is used especially in the furnish flows forming the surface layers, is also a cationic polymer (polyelectrolyte) , and it can be any retention aid used in paper manufacturing used to improve the retention of fillers and fines in the paper. It can be cationic polyacrylamide (CPAM), polydimethyldiallyl ammonium chloride (PDADMAC), or polyethylene-imine (PEI). Also, the combinations of these different polyelectrolytes can be used.

The following examples were carried out to illustrate the present invention. The examples are not intended to limit the scope of the invention. Preliminary tests

In first studies based on SC furnish, cationic starch was dosed equally in all paper layers but chemically unmodified nanofibrillar cellulose only in middle layer of paper in one test. Internal bond strength improved. The results are shown in Figs. 1 and 2, where Fig. 1 shows the tensile index and filler amount of papers made in different methods, and Fig. 2 shows the bonding strength (Scott Bond) of the same papers. The furnish had the same composition layer by layer for each paper ecxept for the nanofibrillar cellulose whose percentage was varied between layers. Filler (kaolin clay) was dosed in equal amounts in all layers. Nanofibrillar cellulose was dosed to the furnish so that its total amount was 6 wt-% of the fibres. The leftmost paper is reference test where no nanofibrillar cellulose was used, the second one is a test where all nanofibrillar cellulose was dosed in the middle layer, the third one is a test where the nanofibrillar cellulose was distributed to surface layers only, and the fourth one is a test where all nanofibrillar cellulose was distributed equally between the layers. The second test (nanofibrillar cellulose in the middle layer only) reached the same tensile index level with higher filler content (Fig. 1 ).

Next, WFC furnish was used for pilot paper machine trial at pilot plant. At first step only cationic starch readily present in base furnish was used and a reference level for trial was produced. In next step additional starch was dosed into middle layer of paper together with circulation water. Filler content increased but internal bond strength remained practically unchanged.

In next step anionically charged nanofibrillar cellulose and cationic starch were dosed together alongside circulation water into middle layer of paper. A clear increase in internal bond strength of paper was observed.