The present invention is directed to paper or paperboard products, comprising a substrate and at least one top ply comprising a composite of microfibrillated cellulose and at least one inorganic particulate material in an amount that is suitable for imparting improved optical, surface and/or mechanical properties to such paper or paperboard products to render them suitable for printing and other end-use demands, to methods of making paper or paperboard products by a process of applying a composite of microfibrillated cellulose and at least one inorganic particulate material on to the wet substrate on the wire at the wet end of a papermaking machine, and to associated uses of such paper or paperboard products.

BACKGROUND OF THE INVENTION

Paper and paperboard products are many and various. There is an ongoing need to make quality improvements in paper and paperboard products having optical, surface and/or mechanical properties, which render them suitable for printing and other end-use demands, and to improve the methods for making such paper and paperboard products having improved printability and surface properties, e.g., by reducing cost, making the process more energy efficient and environmentally friendly, and/or improving recyclability of the paper product.

White top linerboard is conventionally made on a multiformer paper machine. The top layer of a white top linerboard frequently comprises a lightly refined bleached hardwood Kraft (short) fibre, which may contain filler in an amount up to about 20 wt. %. The top layer is conventionally applied to cover the base with a layer to improve the optical appearance of the linerboard and to achieve a surface of high brightness suitable for printing or as a base for coating. A pulp-based layer is conventionally used because the base layer normally comprises either unbleached Kraft pulp or recycled paperboard ("OCC," old corrugated containers), and is thus very rough and unsuitable for coating with conventional equipment. White top linerboards are most often printed flexographically, although some offset printing is used, and inkjet techniques are growing in significance.

With the decline in traditional printing and writing grades, many mills have been looking to convert their graphic paper machines to make linerboard or other packaging products. Conversion of a single layer machine to a multiformer requires a major rebuild and investment, and without this the machine would be limited to making simple linerboard grades. Application of a suitable coating composite to produce a white top linerboard product through a suitable coating apparatus operating at the wet end of the paper machine would provide simple and low cost possibility for the machine to produce economically white top linerboard products. Applying low solids content slurry of microfibrillated

cellulose and organic particulate material to the surface of a linerboard substrate at this point in the linerboard production process would allow the white top linerboard to be drained using existing drainage elements and the resulting white top linerboard to be pressed and dried as a conventional sheet.

Coating onto a wet, freshly-formed substrate presents challenges. Among these challenges, is the fact that the surface of a wet substrate will be much rougher than a pressed and dried sheet. For this reason, the top ply slurry of the composite of microfibrillated cellulose and organic particulate material must create a uniform flow or curtain of the composite material at a suitable flowrate. Moreover, the top ply slurry must be introduced onto the wet web evenly to obtain a contour coat. Once pressed and dried, the top ply must present a surface which is suitable either for printing directly or for single coating. Low porosity and good surface strength are therefore very important properties for the finished white top linerboard.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a paper or paperboard product comprising:

(i) a cellulose-containing substrate; and

(ii) a top ply comprising an inorganic particulate material and at least about 5 wt. % microfibrillated cellulose, based on the total weight of the top ply;

wherein the weight ratio of inorganic particulate material to microfibrillated cellulose in the top ply is from about 20: 1 to about 3: 1 and further wherein the top ply has a brightness of at least about 65% according to ISO Standard 1 1475.

In certain embodiments the paperboard products are a white top paperboard or a white top linerboard.

According to a second aspect of the present invention, there is provided a paper or paperboard product comprising:

(i) a cellulose-containing substrate; and

(ii) a top ply comprising inorganic particulate material in the range of about 67 wt. % to about 90 wt. % and at least about 10 wt. % microfibrillated cellulose, based on the total weight of the top ply, wherein the top ply is present in the paper or paperboard product in an amount ranging from about 15 g/m to about 40 g/m .

In certain embodiments of the second aspect, the top ply is present in the product in an amount ranging from about 20 g/m2 to about 30 g/m2, particularly at least about 30 g/m2.

In certain embodiments of the first and second aspect, the brightness measured (according to ISO Standard 1 1475 (F8; D65 - 400 nm)) on the top ply is increased compared to the brightness measured on the substrate on a surface opposite the top ply.

Advantageously, in certain embodiments the top ply provides good optical and physical coverage over a dark substrate, for example, a substrate of a brightness of 15-25, with the potential to yield an improved brightness of at least about 65%, at least about 70%, or at least about 80% at a coating weight of about 30 g/m2.

In certain embodiments the product comprises or is a paperboard product, and in some embodiments the product is a white top paperboard, containerboard or linerboard product. In addition, improvements in brightness can be made utilizing the first and second aspects at coverages of about 30 g/m to reach brightness levels of 80% or more compared to conventional white top coatings typically requiring 50-60 g/m2 at lower filler loadings of typically 5-15 wt.%.

According to a third aspect, there is provided a paper or paperboard product comprising:

(i) a cellulose-containing substrate; and

(ii) a top ply comprising inorganic particulate material in the range of about 67 wt.% to about 92 wt.% and microfibrillated cellulose in a range of 5 wt.% to about 30 wt.% based on the total weight of the top ply.

In certain embodiments the weight ratio of inorganic particulate to microfibrillated cellulose in the top ply is from about, 8: 1 to about 1 : 1, or from about 6: 1 to about 3: 1, or from about 5 : 1 to about 2 : 1 , or from about 5 : 1 to about 3 : 1 , or about 4: 1 to about 3: 1 , According to a fourth aspect of the present invention, there is provided a method of making a paper or paperboard product, the method comprising: (a) providing a wet web of pulp; (b) providing a top ply slurry onto the wet web of pulp, wherein: (i) the top slurry is provided in an amount ranging from 15 g/m2 to 40 g/m2 and (ii) the top ply slurry comprises a sufficient amount of microfibrillated cellulose to obtain a product having a top ply comprising at least about 5 wt. % microfibrillated cellulose based on the total weight of top ply; (iii) and the top slurry comprises inorganic particulate material and microfibrillated cellulose. In additional embodiments, the top ply comprises at least about 10 wt. %, at least about 20 wt. %, or up to about 30 wt. %, based on the total weight of the top ply.

According to a fifth aspect, the present invention is directed to the use of a top ply comprising at least about 20 wt. % microfibrillated cellulose, based on the total weight of the top ply, as a white top layer on a paperboard substrate. In additional embodiments, the present invention is directed to the use of a top ply comprising up to about 30 wt. % microfibrillated cellulose, based on the total weight of the top ply, as a white top layer on a paperboard substrate. In certain embodiments the present invention is directed to the use of a top ply comprising inorganic particulate material in the range of about 67 wt. % to about 92 wt. % and microfibrillated cellulose in a range of about 5 wt. % to about 30 wt. % based on the total weight of the top ply.

According to a sixth aspect, the present invention is directed to forming a curtain or film through a non-pressurized or pressurized slot opening on top of a wet substrate on the wire of the wet end of a paper machine to apply a top ply to a substrate to manufacture a paper or paperboard product of the first to third aspects.

In certain additional embodiments, the composite of microfibrillated cellulose and inorganic particulate materials may be applied as a white top layer or other top layer. Advantageously, the process may be performed utilizing low cost equipment for application such as a curtain coater, a pressurized extrusion coater, secondary headbox or pressurize or unpressurized slot coater compared to applying a conventional secondary fibre layer or coating to a dry or semi-dry paper or paperboard product. Moreover, the existing drainage elements and press section of a paper machine such as the drainage table of a Fourdrinier machine may be utilized for water removal. The top ply of microfibrillated cellulose and inorganic particulate material has the ability to stay on top of the substrate and to provide good optical and physical coverage at a low basis weight of the paper or paperboard product.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the formation of sheets produced at varying grammage according to Example 1.

Fig. 2 is a graph summarizing the brightness of sheets produced at varying grammage according to Example 1.

Fig. 3 is a graph summarizing PPS Roughness of sheets produced at varying grammage according to Example 1.

Fig. 4 is a plot of brightness versus coating weight levels for Trials 1 -4 of Example 2.

Fig. 5 is a scanning electron microscope image of a substrate coated with a 35 g/m2 top ply comprising 20 wt. % microfibrillated cellulose and 80 wt. % ground calcium carbonate applied to a 85 g/m2 substrate at trial point T2.

Fig. 6 is a scanning electron microscopic image of a substrate coated with a 48 g/m2 of a top ply comprising 20% wt. % microfibrillated cellulose, 20 wt. % ground calcium carbonate and 60 wt. % talc applied to a 85 g/m2 substrate at trial point T4.

Fig. 7 presents a cross-section of a Flexography printed sample.

DETAILED DESCRIPTION OF THE INVENTION

It has surprisingly been found that a ply comprising a composite of inorganic particulate material and microfibrillated cellulose can be added onto a paper web in the wet-end of a paper machine (such as a Fourdrinier machine), immediately after the wet line forms and, where the web is still less than 10-15 wt. % solids. The top ply paper or paper board made by the disclosed process provides advantageous optical properties (e.g., brightness) as well as light-weighting and/or surface improvement (e.g., smoothness and low porosity, while maintaining suitable mechanical properties (e.g., strength for end-use applications.

By "top" ply is meant that a top ply is applied on or to the substrate, which substrate may have intermediary plies or layers below the top ply. In certain embodiments, the top ply is an outer ply, i.e., does not have another ply atop. In certain embodiments, the top ply has a grammage of at least about 15 g/m2 to about 40 g/m2.

By "microfibrillated cellulose" is meant a cellulose composition in which microfibrils of cellulose are liberated or partially liberated as individual species or as smaller aggregates as compared to the fibres of a pre-microf!brillated cellulose. The microfibrillated cellulose may be obtained by microfibrillating cellulose, including but not limited to the processes described herein. Typical cellulose fibres (i.e., pre-microfibrillated pulp or pulp not yet fibrillated) suitable for use in papermaking include larger aggregates of hundreds or thousands of individual cellulose microfibrils. By microfibrillating the cellulose, particular characteristics and properties, including but not limited to the characteristics and properties described herein, are imparted to the microfibrillated cellulose and the compositions including the microfibrillated cellulose.

There are numerous types of paper or paperboard possible to be made with the disclosed compositions of microfibrillated cellulose and inorganic particulate materials and by the manufacturing processes described herein. There is no clear demarcation between paper and paperboard products. The latter tend to be thicker paper-based materials with increased grammages. Paperboard may be a single ply, to which the top ply of a composite of microfibrillated cellulose and inorganic particulate material can be applied, or the

paperboard may be a multi-ply substrate. The present invention is directed to numerous forms of paperboard, including, by way of example and not limitation, boxboard or cartonboard, including folding cartons and rigid set-up boxes and folding boxboard; e.g. a liquid packaging board. The paperboard may be chipboard or white lined chipboard. The paperboard may be a Kraft board, laminated board. The paperboard may be a solid bleached board or a solid unbleached board. Various forms of containerboard are subsumed within the paperboard products of the present invention such as corrugated fibreboard (which is a building material and not a paper or paperboard product per se), linerboard or a binder's board. The paperboard described herein may be suitable for wrapping and packaging a variety of end-products, including for example foods.

In certain embodiments, the product is or comprises containerboard, and the substrate and top ply are suitable for use in or as containerboard. In certain embodiments, the product is or comprises one of brown Kraft liner, white top Kraft liner, test liner, white top test liner, brown light weight recycled liner, mottled test liner, and white top recycled liner.

In certain embodiments, the product is or comprises cartonboard.

In certain embodiments, the product is or comprises Kraft paper.

In certain embodiments, the substrate comprises a paperboard product or is suitable for use in or as a paperboard product. In certain embodiments, the substrate is suitable for use in a white top paperboard product, for example, as linerboard. In certain embodiments, the product comprises or is a paperboard product, for example, linerboard. In certain embodiments, the product comprises or is a white top paperboard product, for example, linerboard. In such embodiments, the paperboard product may be corrugated board, for example, having the product comprising substrate and top ply as linerboard. In certain embodiments, the paperboard product is single face, single wall, double wall or triple wall corrugated.

Unless otherwise stated, amounts are based on the total dry weight of the top ply and/or substrate.

Unless otherwise stated, particle size properties referred to herein for the inorganic particulate materials are as measured in a well-known manner by sedimentation of the particulate material in a fully dispersed condition in an aqueous medium using a Sedigraph 5100 machine as supplied by Micromeritics Instruments Corporation, Norcross, Georgia, USA (telephone: +1 770 662 3620; web-site: www.micromeritics.com), referred to herein as a "Micromeritics Sedigraph 5100 unit". Such a machine provides measurements and a plot of the cumulative percentage by weight of particles having a size, referred to in the art as the 'equivalent spherical diameter' (e.s.d), less than given e.s.d values. The mean particle size d50 is the value determined in this way of the particle e.s.d at which there are 50% by weight of the particles which have an equivalent spherical diameter less than that d50 value.

Alternatively, where stated, the particle size properties referred to herein for the inorganic particulate materials are as measured by the well-known conventional method employed in the art of laser light scattering, using a Malvern Mastersizer S machine as supplied by Malvern Instruments Ltd (or by other methods which give essentially the same result). In the laser light scattering technique, the size of particles in powders, suspensions and emulsions may be measured using the diffraction of a laser beam, based on an application of Mie theory. Such a machine provides measurements and a plot of the cumulative percentage by volume of particles having a size, referred to in the art as the 'equivalent spherical diameter' (e.s.d), less than given e.s.d values. The mean particle size d50 is the value determined in this way of the particle e.s.d at which there are 50% by volume of the particles which have an equivalent spherical diameter less than that d50 value.

Unless otherwise stated, particle size properties of the microfibrillated cellulose materials are as measured by the well-known conventional method employed in the art of laser light scattering, using a Malvern Mastersizer S machine as supplied by Malvern Instruments Ltd (or by other methods which give essentially the same result).

Details of the procedure used to characterise the particle size distributions of mixtures of inorganic particle material and microfibrillated cellulose using a Malvern Mastersizer S machine are provided below.

Top ply

In certain embodiments, the top ply comprises at least about 5 wt. % microfibrillated cellulose, based on the total weight of the top ply. In certain embodiments, the top ply comprises from about 5 wt. % to about 30 wt. % microfibrillated cellulose, for example, 5 wt. % to about 25 wt. %, or from about 10 wt. % to about 25 wt. %, or from about 15 wt. % to about 25 wt. %, or from about 17.5 wt. % to about 22.5 wt. % microfibrillated cellulose, based on the total weight of the top ply.

In certain embodiments, the top ply comprises at least about 67 wt. % inorganic particulate material, or at least about 70 wt. % inorganic particulate material, or at least about 75 wt. % inorganic particulate material, or at least about 80 wt. % inorganic particulate material, or at least about 85 wt. % inorganic particulate material, or at least about 90 wt. % inorganic particulate material, based on the total weight of the top ply, and, optionally, from 0 to 3 wt. % of other additives.

In certain embodiments, the microfibrillated cellulose and inorganic particulate material provide a top ply grammage of from about 15 g/m to about 40 g/m . In this and other embodiments, the weight ratio of inorganic particulate to microfibrillated cellulose in the top ply is from about 20: 1 , or about 10: 1 , or about 5: 1 , or about 4: 1 , or about 3: 1 or about 2: 1.

In certain embodiments, the top ply comprises from about 70 wt. % to about 90 wt. % inorganic particulate material and from about 10 wt. % to about 30 wt. % microfibrillated cellulose, based on the total weight of the top ply, and optionally up to 3 wt. % of other additives.

In certain embodiments, the top ply is optionally may contain additional organic compound, i.e., organic compound other than microfibrillated cellulose.

In certain embodiments, the top ply is optionally may contain cationic polymer, anionic polymer, and/or polysaccharide hydrocolloid.

In certain embodiments, the top ply is optionally may contain wax, polyolefins, and/or silicone.

In certain embodiments, the top ply is devoid of an optical brightening agent.

In certain embodiments, the top ply consists essentially of inorganic particulate material and microfibrillated cellulose, and as such comprises no more than about 3 wt. %, for example, no more than about 2 wt. %, or no more than about 1 wt. %, or no more than about 0.5 wt. % of additives other than inorganic particulate material and microfibrillated cellulose. In such embodiments, the top ply may comprise up to about 3 wt. % of additives selected from flocculant, formation/drainage aid (e.g., poly(acrylamide-co-diallyldimethylammonium chloride, Polydadmac®), water soluble thickener, starch (e.g., cationic starch), sizing agent, e.g., rosin, alkylketene dimer ("AKD"), alkenylsuccinic anhydride ("ASA") or similar materials and combinations thereof, for example, up to about 2 wt. % of such additives, or up to about 1 wt. % of such additives, or up to about 0.5 wt. % of such additives.

In certain embodiments, we have found that adding small amounts of retention/drainage aids, such as poly(acrylamide-co-diallyldimethylammonium chloride) solution (Polydadmac®), as opposed to much greater amounts used in normal papermaking, the lowered amount of retention aid provides microscale flocculation with no visible negative impact on formation of the substrate, but results in positive impacts on dewatering. This results in significant improvements in dewatering speed.

In certain embodiments, the top ply consists of inorganic particulate material and microfibrillated cellulose, and as such comprises less than about 0.25 wt. %, for example, less than about 0.1 wt. %, or is free of additives other than inorganic particulate material and microfibrillated cellulose, i.e., additives selected from flocculant, formation/drainage aid (e.g.,poly(acrylamide-co-diallyldimethylammoniumchloride) solution (Polydadmac®)), water soluble thickener, starch (e.g., cationic starch) and combinations thereof.

The microfibrillated cellulose may be derived from any suitable source.

In certain embodiments, the microfibrillated cellulose has a d5o ranging from about 5 μπι to about 500 μπι, as measured by laser light scattering. In certain embodiments, the microfibrillated cellulose has a d50 of equal to or less than about 400 μιτι, for example equal to or less than about 300 μιη, or equal to or less than about 200 μηι, or equal to or less than about 150 μηι, or equal to or less than about 125 μιη, or equal to or less than about 100 μπι, or equal to or less than about 90 μιη, or equal to or less than about 80 μιη, or equal to or less than about 70 μηι, or equal to or less than about 60 μπι, or equal to or less than about 50 μπι, or equal to or less than about 40 μιη, or equal to or less than about 30 μπι, or equal to or less than about 20 μπι, or equal to or less than about 10 μπι.

In certain embodiments, the microfibrillated cellulose has a modal fibre particle size ranging from about 0.1-500 μηι. In certain embodiments, the microfibrillated cellulose has a modal fibre particle size of at least about 0.5 μιη, for example at least about 10 μπι, or at least about 50 μπι, or at least about 100 μηι, or at least about 150 μηι, or at least about 200 μιη, or at least about 300 μπι, or at least about 400 μηι.

Additionally or alternatively, the microfibrillated cellulose may have a fibre steepness equal to or greater than about 10, as measured by Malvern. Fibre steepness (i.e., the steepness of the particle size distribution of the fibres) is determined by the following formula:

Steepness = 100 x (d30/d7o)

The microfibrillated cellulose may have a fibre steepness equal to or less than about 100. The microfibrillated cellulose may have a fibre steepness equal to or less than about 75, or equal to or less than about 50, or equal to or less than about 40, or equal to or less than about 30. The microfibrillated cellulose may have a fibre steepness from about 20 to about 50, or from about 25 to about 40, or from about 25 to about 35, or from about 30 to about 40.

The inorganic particulate material may, for example, be an alkaline earth metal carbonate or sulphate, such as calcium carbonate, magnesium carbonate, dolomite, gypsum, a hydrous kandite clay such as kaolin, halloysite or ball clay, an anhydrous (calcined) kandite clay such as metakaolin or fully calcined kaolin, talc, mica, huntite, hydromagnesite, ground glass, perlite or diatomaceous earth, or wollastonite, or titanium dioxide, or magnesium hydroxide, or aluminium trihydrate, lime, graphite, or combinations thereof.

In certain embodiments, the inorganic particulate material comprises or is calcium carbonate, magnesium carbonate, dolomite, gypsum, an anhydrous kandite clay, perlite, diatomaceous earth, wollastonite, magnesium hydroxide, or aluminium trihydrate, titanium dioxide or combinations thereof.

An exemplary inorganic particulate material for use in the present invention is calcium carbonate. Hereafter, the invention may tend to be discussed in terms of calcium carbonate, and in relation to aspects where the calcium carbonate is processed and/or treated. The invention should not be construed as being limited to such embodiments.

The particulate calcium carbonate used in the present invention may be obtained from a natural source by grinding. Ground calcium carbonate (GCC) is typically obtained by crushing and then grinding a mineral source such as chalk, marble or limestone, which may be followed by a particle size classification step, in order to obtain a product having the desired degree of fineness. Other techniques such as bleaching, flotation and magnetic separation may also be used to obtain a product having the desired degree of fineness and/or colour. The particulate solid material may be ground autogenously, i.e. by attrition between the particles of the solid material themselves, or, alternatively, in the presence of a particulate grinding medium comprising particles of a different material from the calcium carbonate to be ground. These processes may be carried out with or without the presence of a dispersant and biocides, which may be added at any stage of the process.

Precipitated calcium carbonate (PCC) may be used as the source of particulate calcium carbonate in the present invention, and may be produced by any of the known methods available in the art. TAPPI Monograph Series No 30, "Paper Coating Pigments", pages 34-35 describes the three main commercial processes for preparing precipitated calcium carbonate which is suitable for use in preparing products for use in the paper industry, but may also be used in the practice of the present invention. In all three processes, a calcium carbonate feed material, such as limestone, is first calcined to produce quicklime, and the quicklime is then slaked in water to yield calcium hydroxide or milk of lime. In the first process, the milk of lime is directly carbonated with carbon dioxide gas. This process has the advantage that no by-product is formed, and it is relatively easy to control the properties and purity of the calcium carbonate product. In the second process the milk of lime is contacted with soda ash to produce, by double decomposition, a precipitate of calcium carbonate and a solution of sodium hydroxide. The sodium hydroxide may be substantially completely separated from the calcium carbonate if this process is used commercially. In the third main commercial process the milk of lime is first contacted with ammonium chloride to give a calcium chloride solution and ammonia gas. The calcium chloride solution is then contacted with soda ash to produce by double decomposition precipitated calcium carbonate and a solution of sodium chloride. The crystals can be produced in a variety of different shapes and sizes, depending on the specific reaction process that is used. The three main forms of PCC crystals are aragonite, rhombohedral and scalenohedral (e.g., calcite), all of which are suitable for use in the present invention, including mixtures thereof.

In certain embodiments, the PCC may be formed during the process of producing microfibrillated cellulose.

Wet grinding of calcium carbonate involves the formation of an aqueous suspension of the calcium carbonate which may then be ground, optionally in the presence of a suitable dispersing agent. Reference may be made to, for example, EP-A-614948 (the contents of

which are incorporated by reference in their entirety) for more information regarding the wet grinding of calcium carbonate.

When the inorganic particulate material of the present invention is obtained from naturally occurring sources, it may be that some mineral impurities will contaminate the ground material. For example, naturally occurring calcium carbonate can be present in association with other minerals. Thus, in some embodiments, the inorganic particulate material includes an amount of impurities. In general, however, the inorganic particulate material used in the invention will contain less than about 5% by weight, or less than about 1% by weight, of other mineral impurities.

The inorganic particulate material may have a particle size distribution in which at least about 10% by weight of the particles have an e.s.d of less than 2 μη , for example, at least about 20% by weight, or at least about 30% by weight, or at least about 40% by weight, or at least about 50% by weight, or at least about 60% by weight, or at least about 70% by weight, or at least about 80% by weight, or at least about 90% by weight, or at least about 95% by weight, or about 100% of the particles have an e.s.d of less than 2 μπι.

In another embodiment, the inorganic particulate material has a particle size distribution, as measured using a Malvern Mastersizer S machine, in which at least about 10% by volume of the particles have an e.s.d of less than 2 μηι, for example, at least about 20% by volume, or at least about 30% by volume, or at least about 40% by volume, or at least about 50% by volume, or at least about 60% by volume, or at least about 70% by volume, or at least about 80% by volume, or at least about 90% by volume, or at least about 95% by volume, or about 100% of the particles by volume have an e.s.d of less than 2 μηι.

Details of the procedure used to characterise the particle size distributions of mixtures of inorganic particle material and microfibrillated cellulose using a Malvern Mastersizer S machine are provided below.

In certain embodiments, the inorganic particulate material is kaolin clay. Hereafter, this section of the specification may tend to be discussed in terms of kaolin, and in relation to aspects where the kaolin is processed and/or treated. The invention should not be construed as being limited to such embodiments. Thus, in some embodiments, kaolin is used in an unprocessed form.

Kaolin clay used in this invention may be a processed material derived from a natural source, namely raw natural kaolin clay mineral. The processed kaolin clay may typically contain at least about 50% by weight kaolinite. For example, most commercially processed kaolin clays contain greater than about 75% by weight kaolinite and may contain greater than about 90%, in some cases greater than about 95% by weight of kaolinite.

Kaolin clay used in the present invention may be prepared from the raw natural kaolin clay mineral by one or more other processes which are well known to those skilled in the art, for example by known refining or beneficiation steps.

For example, the clay mineral may be bleached with a reductive bleaching agent, such as sodium hydrosulfite. If sodium hydrosulfite is used, the bleached clay mineral may optionally be dewatered, and optionally washed and again optionally dewatered, after the sodium hydrosulfite bleaching step.

The clay mineral may be treated to remove impurities, e. g. by flocculation, flotation, or magnetic separation techniques well known in the art. Alternatively the clay mineral used in the first aspect of the invention may be untreated in the form of a solid or as an aqueous suspension.

The process for preparing the particulate kaolin clay used in the present invention may also include one or more comminution steps, e.g., grinding or milling. Light comminution of a coarse kaolin is used to give suitable delamination thereof. The comminution may be carried out by use of beads or granules of a plastic (e. g. nylon), sand or ceramic grinding or milling aid. The coarse kaolin may be refined to remove impurities and improve physical properties using well known procedures. The kaolin clay may be treated by a known particle size classification procedure, e.g., screening and centrifuging (or both), to obtain particles having a desired d50 value or particle size distribution.

The substrate

The substrate (and the microfibrillated cellulose) may be derived from a cellulose-containing pulp, which may have been prepared by any suitable chemical or mechanical treatment, or combination thereof, which is well known in the art. The pulp may be derived from any suitable source such as wood, grasses (e.g., sugarcane, bamboo) or rags (e.g., textile waste, cotton, hemp or flax). The pulp may be bleached in accordance with processes which are well known to those skilled in the art and those processes suitable for use in the present invention will be readily evident. In certain embodiments, the pulp is unbleached. The bleached or unbleached cellulose pulp may be beaten, refined, or both, to a predetermined freeness (reported in the art as Canadian standard freeness (CSF) in cm3). A suitable stock is then prepared from the bleached or unbleached and beaten pulp.

In certain embodiments, the substrate comprises or is derived from a Kraft pulp, which is naturally (i.e., unbleached) coloured. In certain embodiments, the substrate comprises or is derived from dark Kraft pulp, recycled pulp, or combinations thereof. In certain embodiments, the substrate comprises or is derived from recycled pulp.

The stock from which the substrate is prepared may contain other additives known in the art. For example, the stock contains a non-ionic, cationic or an anionic retention aid or microparticle retention system. It may also contain a sizing agent which may be, for example, a long chain alkylketene dimer ("AKD"), a wax emulsion or a succinic acid

derivative, e.g., alkenylsuccinic anhydride ("ASA")., rosin plus alum or cationic rosin emulsions. The stock for the substrate composition may also contain dye and/or an optical brightening agent. The stock may also comprise dry and wet strength aids such as, for example, starch or epichlorhydrin copolymers.

The product

In certain embodiments, the substrate has a grammage which is suitable for use in or as a containerboard product, for example, a grammage ranging from about 50 g/m2 to about 500 g/m . In this and other embodiments, the top ply may have a grammage ranging from

2 2 2 2

about 10 g/m to about 50 g/m , particularly about 15 g/m to 40 g/m ' and more particularly about 20 g/m2 to 30 g/m2.

In certain embodiments, the substrate has a grammage of from about 75 g/m2 to about 400 g/m , for example, from about 100 g/m to about 375 g/m , or from about 100 g/m to about 350 g/m2, or from about 100 g/m2 to about 300 g/m2, or from about 100 g/m2 to about 275 g/m2, or from about 100 g/m2 to about 250 g/m2, or from about 100 g/m2 to about 225 g/m2,

2 2

or from about 100 g/m to about 200 g/m . In this and other embodiments, the top ply may

2 2 2 have a grammage ranging from about 15 g/m to 40 g/m , or from about 25 g/m to 35 g/m2.

In certain embodiments, the top ply has a grammage which is equal to or less than 40 g/m2, or equal to or less than about 35 g/m2, or equal to or less than about 30 g/m2, or equal to or less than 25 g/m2, or equal to or less than 22.5 g/m2, or equal to or less than 20 g/m2, or equal to or less than 18 g/m2, or equal to or less than 15 g/m2.

In certain embodiments, the top ply has a grammage which is equal to or less than 40 g/m2, or equal to or less than about 35 g/m2, or equal to or less than about 30 g/m2, or equal to or less than 25 g/m2, or equal to or less than 22.5 g/m2, or equal to or less than 20 g/m2, or equal to or less than 18 g/m2, or equal to or less than 15 g/m2.

Advantageously, the application of a top ply comprising inorganic particulate material and microfibrillated cellulose enables manufacture of a product, for example, paperboard or containerboard, having a combination of desirable optical, surface and mechanical properties, which are obtainable while utilising relatively low amounts of a top ply having a high filler content, thereby offering light-weighting of the product compared to conventional top ply/substrate configurations. Further, any reduction in mechanical properties which may occur following application of the top ply may be offset by increasing the grammage of the substrate, which is a relatively cheaper material.

Therefore, in certain embodiments, the product has one or more of the following:

(i) a brightness measured (according to ISO Standard 1 1475 (F8; D65 - 400 nm)) on the top ply which is increased compared to the substrate absent of the top ply or measured on the substrate on a surface opposite the top ply and/or a brightness measured on the top ply of a least about 60.0 % according to ISO Standard 1 1475 (F8; D65 - 400 nm);

(ii) a PPS roughness (@1000 kPa) measured on the top ply of no more than about

6.0 μηι and/or a PPS roughness (@1000 kPa) measured on the top ply which is at least 2.0 μηι less than the PPS roughness of the substrate absent the top ply.

In certain embodiments, a brightness measured on the top ply is at least about 70.0 %, for example, at least about 75.0 %, or at least about 80.0 %, or at least about 81.0 %, or at least about 82.0 %, or at least about 83.0 %, or at least about 84.0 %, or at least about 85.0 %. Brightness may be measured using an Elrepho spectrophotometer.

In certain embodiments, the product has a PPS roughness (@1000 kPa) measured on the top ply of less than about 5.9 μηι, for example, less than about 5.8 μηι, or less than about 5.7 μπι, or less than about 5.6 μπι, or less than about 5.5 μιη. In certain embodiments, the PPS roughness is from about 5.0 μηι to about 6.0 μιη, for example, from about 5.2 μηι to about 6.0 μιη, or from about 5.2 μηι to about 5.8 μηι, or from about 5.2 μηι to about 5.6 μπι.

In certain embodiments, the top ply has a grammage of from about 30 to 50 g/m2, a brightness of at least about 65.0 %, and, optionally, a PPS roughness of less than about 5.6 μηι.

In certain embodiments, the product comprises a further layer or ply, or further layers or plies, on the ply comprising at least about 50 wt. % microfibrillated cellulose. For example, one or more layers or plies, or at least two further layers or plies, or up to about five further layers or plies, or up to about four further layers or plies, or up to about three further layers or plies.

In certain embodiments, one of, or at least one of the further layers or plies is a barrier layer or ply, or wax layer or ply, or silicon layer or ply, or a combination of two or three of such layers.

Another advantageous feature of the disclosed top ply coated substrates comprising microfibrillated cellulose and inorganic particulate material is improved printing on the top ply. A conventional white top liner typically has a white surface consisting of a white paper with relatively low filler content, typically in the 5-15% filler range. As a result, such white top liners tend to be quite rough and open with a coarse pore structure. This is not ideal for receiving printing ink.

Fig. 6 below illustrates the printing improvements realized by application of the top ply of the present invention comprising microfibrillated cellulose and organic particulate material. Overall, the use of such a ply may provide a 'greener' packaging product because the low porosity of the ply may allow for improved properties in barrier applications that enable non-recyclable wax, PE and silicon, etc., coatings to be replaced by recyclable formulations, to obtain an overall equal or improved performance from the non-recyclable counterparts.

Methods of manufacture

A method of making a paper product is provided. It comprises:

(a) providing a wet web of pulp; and

(b) providing a top ply slurry onto the wet web of pulp.

The top ply slurry (i) is provided in an amount ranging from 15 g/m to 40 g/m ; and (ii) the top ply slurry comprises a sufficient amount of microfibrillated cellulose to obtain a product having a top ply comprising at least about 5 wt. % microfibrillated cellulose and (iii) the top ply slurry comprises at least about 67 wt. % inorganic particulate material.

This method is a 'wet on wet' method which is different than conventional paper coating methods in which an aqueous coating is applied to a substantially dry paper product (i.e., 'wet on dry').

2

In certain embodiments, the top slurry is provided in an amount ranging from 15 g/m to 40 g/m2.

In certain embodiments, the top ply slurry comprises a sufficient amount of microfibrillated cellulose to obtain a product having the strength properties required for meeting end-use demands. Typically this would mean a top ply comprising at least about 5 wt. % microfibrillated cellulose, based on the total weight of top ply (i.e., the total dry weight of the top ply of the paper product).

The top ply slurry may be applied by any suitable application method. In an embodiment, the top ply slurry is applied through a non-pressurized or pressurized slot applicator having an opening positioned on top of a wet substrate on the wire of the wet end of a paper machine. Examples of known applicators which may be employed include, without limitation, air knife coaters, blade coaters, rod coaters, bar coaters, multi-head coaters, roll coaters, roll or blade coaters, cast coaters, laboratory coaters, gravure coaters, kisscoaters, slot die applicators (including, e.g. non-contact metering slot die applicators jet coaters, liquid application systems, reverse roll coaters, headbox, secondary headbox, curtain coaters, spray coaters and extrusion coaters.

In certain embodiments, the top ply slurry is applied using a curtain coater. Further, in certain embodiments in which the top ply slurry is applied as white top liner layer, the use of a curtain coater may eliminate the need for a twin headbox paper machine and the associated cost and energy.

In certain embodiments, the top ply slurry is applied by spraying, e.g., using a spray coater.

Use of high solids compositions is desirable in the method because it leaves less water to drain. However, as is well known in the art, the solids level should not be so high that high viscosity and leveling problems are introduced.

The methods of application may be performed using a suitable applicator such as an air knife coater, blade coater, rod coater, bar coater, multi-head coater, roll coater, roll or blade coater, cast coater, laboratory coater, gravure coater, kisscoater, slot die applicator (including, e.g. a non-contact metering slot die applicator and a non-pressurized or pressurized slot applicator), jet coater, liquid application system, reverse roll coater, headbox, secondary headbox, curtain coater, spray coater or an extrusion coater, to apply the top ply slurry to the substrate.

In an embodiment, the top ply slurry is applied a coating to the substrate by a non-pressurized or pressurized slot opening on top of the wet substrate on the wire of the wet end of a paper machine, for example a Fourdrinier machine.

In certain embodiments, the wet web of pulp comprises greater than about 50 wt. % of water, based on the total weight of the wet web of pulp, for example, at least about 60 wt. %, or at least about 70 wt. %, or at least about 80 wt. %, or at least about 90 wt. % of water, based on the total weight of the wet web of pulp. Typically, the wet web of pulp comprises about 85-95 wt. % water.

In certain embodiments, the top ply slurry comprises inorganic particulate material and a sufficient amount of microfibrillated cellulose to obtain a paper product having a top ply comprising at least about 5 wt. % microfibrillated cellulose, based on the total weight of the top ply and such that the paper product has sufficient microfibrillated cellulose to obtain a paper product with the strength properties needed for its end-use application. In certain embodiments, the top ply slurry comprises a sufficient amount of inorganic particulate material to obtain a paper product having a top ply comprising at least about 67 wt. % of inorganic particulate material, based on the total weight of the top ply of the paper product. In such embodiments the objective is to incorporate as little microfibrillated cellulose with as much inorganic particulate material as possible on the surface of the substrate material as a top layer. Accordingly, ratios of 4: 1 or greater of inorganic particulate material to microfibrillated cellulose in the top ply are preferred.

In certain embodiments, the top ply slurry has a total solids content of up to about 20 wt. %, for example, up to about 15 wt. %, or up to 12 wt. %, or up to about 10 wt. %, or from about 1 wt. % to about 10 wt. %, or from about 2 wt. % to 12 wt. %, or from about 5 wt. % to about 10 wt. %, or from about 1 wt. % to about 20 wt. %, or from about 2 wt. % to about 12 wt. %. The relative amounts of inorganic particulate material and microfibrillated cellulose may be varied depending on the amount of each component required in the final product.

Following application of the top ply slurry and appropriate dwell time, the paper product is pressed and dried using any suitable method.

Methods of manufacturing microfibrillated cellulose and inorganic particulate material

In certain embodiments, the microfibrillated cellulose may be prepared in the presence of or in the absence of the inorganic particulate material.

CLAIMS

A paper or paperboard product comprising:

(i) a cellulose-containing substrate; and

(ii) a top ply comprising an inorganic particulate material and at least about 5 wt. % microfibrillated cellulose based on the total weight of the top ply, wherein the inorganic particulate material content is about 67 wt. % to about 92 wt. % based on the total weight of the top ply, and further wherein the brightness measured (according to ISO Standard 1 1475 (F8; D65 - 400 urn)) on the top ply is at least about 65%.

2. The product according to claim 1 , wherein the product comprises or is a white top containerboard product.

3. The product a claim 1 , wherein the product comprises or is a white top paperboard product.

4. The product according to claim 2, wherein the substrate has a grammage suitable for use in a containerboard product, comprising a grammage ranging from about 50 g/m2 to about 500 g/m2, and/or the top ply has a grammage ranging from about 15 g/m2 to 40 g/m2.

5. The product according to claim 4, wherein the top ply has a grammage of from about 15 to 40 g/m2.

6. The product according to claim 1 , wherein the substrate comprises recycled pulp, dark kraft, or combinations thereof.

7. The product according to claim 1, wherein the microfibri Hated cellulose is present in the top ply in an amount of about 5 wt. % to about 30 wt. %.

8. The product according to claim 1, wherein the inorganic particulate material and the microfibrillated cellulose comprise greater than 95 wt. % of the top ply, based on the total weight of the top ply.

9. The product according to claim 1 , wherein the top ply comprises at least 70 wt. % of an inorganic particulate material, based on the total weight of the top ply.

10. The product according to claim 1 , wherein the top ply comprises at least about 80 wt. % of an inorganic particulate material, based on the total weight of the top ply.

1 1. The product according to claim 1 , wherein the top ply comprises at least about 10 wt. % to about 20 wt. % microfibrillated cellulose, based on the total weight of the top ply.

12. The product according to claim 1 , wherein the top ply comprises at least one inorganic particulate material selected from the group consisting of: calcium carbonate, magnesium carbonate, dolomite, gypsum, an anhydrous kandite clay, kaolin, perlite, diatomaceous earth, wollastonite, talc, magnesium hydroxide, titanium dioxide, or aluminium trihydrate, or combinations thereof.

13. The product according to claim 12, wherein the inorganic particulate material comprises or is calcium carbonate.

14. The product according to claim 1 , wherein the product has a PPS roughness (@1000 kPa) measured on the top ply of no more than about 6.0 μπι and/or a PPS roughness (@1000 kPa) measured on the top ply which is at least 2.0 μηι less than the PPS roughness of the substrate absent the top ply.

15. The product according to claim 1, wherein the top ply comprises up to about 2 wt. %, in total, of additives selected from the group consisting of: flocculant, formation/drainage aid, water soluble thickener, starch, retention aid and combinations thereof.

16. The product of claiml , wherein top ply is devoid of additional organic compound.

17. The product according to claim 16, wherein the top ply is devoid of cationic polymer, anionic polymer, or polysaccharide hydrocolloid.

18. The product according to claim 1, wherein the top ply is an outer ply.

19. The product of claim 1 , wherein the top ply is devoid of wax, polyolefins, and silicone.

20. A paper or paperboard product comprising:

a cellulose-containing substrate; and a top ply comprising at least about 10 wt. % microfibnllated cellulose based on the total weight of the top ply, wherein the top ply is

2 2

present in the product in an amount of about 15 g/m to 40 g/m .

21. The product according to claim 20, wherein the substrate comprises up to about 1 wt. % retention aid, based on the total weight of the substrate.

22. The product according to claim 20, wherein the top ply consists essentially, or consists of, inorganic particulate and microfibnllated cellulose.

23. The product according to claim 20, wherein the top ply comprises up to about 30 wt. % microfibnllated cellulose, based on the total weight of the top ply.

24. The product according to claim 20, further comprising a further layer or ply, or further layers or plies, on the ply comprising at least about 30 wt. % microfibnllated cellulose, based on the total weight of the top ply.

25. The product according to claim 24, wherein at least one of the further layers or plies is a barrier layer or ply, or wax layer or ply, or silicon layer or ply.

26. A method of making a paper or board product, the method comprising:

(a) providing a wet web of pulp;

(b) providing a top ply slurry onto the wet web of pulp, wherein:

(i) the top ply slurry is provided in an amount ranging from 15 g/m2 to 40 g/m2;

(ii) the top ply slurry comprises a sufficient amount of microfibrillated cellulose to obtain a product having a top ply comprising at least about 5 wt.% microfibrillated cellulose, based on the total weight of the top ply; and

(iii) the top ply slurry comprises a sufficient amount of inorganic particulate material to obtain a product having a top ply comprising at least 67 wt.% inorganic particulate material, based on the total weight of the top ply.

27. The method according to claim 26, wherein the top ply slurry is applied using an applicator suitable to form a film through a non-pressurized or pressurized slot opening on top of a wet substrate on the wire of the wet end of a paper machine.

28. The method according to claim 26, wherein the wet web of pulp comprises greater than about 50 wt. % of water, based on the total weight of the wet web of pulp.

29. The method according to claim 26, wherein the wet web of pulp comprises up to about 1 wt. % of retention aid, based on the total weight of the wet web of pulp.

30. The method according to claim 26, wherein the top ply slurry comprises inorganic particulate material and a sufficient amount of microfibrillated cellulose to obtain a paper product having a top ply comprising at least about 15 wt. % microfibrillated cellulose, based on the total weight of the top ply.

31. The method according to claim 26, wherein the top ply slurry is applied using a pressurized slot opening on top of a wet substrate on the wire of the wet end of a paper machine.

32. The method according to claim 26, wherein the top ply slurry is applied using a curtain coater.

33. The method according to claim 26, further comprising applying a further layer or ply, or further layers or plies, onto the top ply comprising microfibrillated cellulose and inorganic particulate material.

34. The method according to claim 33, wherein at least one of the further layers or plies is a barrier layer or ply, or wax layer or ply, or silicon layer or ply.

35. The product according to claim 20, wherein the top ply consists essentially, or

consists of microfibrillated cellulose.