TECHNICAL FIELD

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 11475.

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/m2to about 40 g/m2.

In certain embodiments of the second aspect, the top ply is present in the product in an amount ranging from about 20 g/m2to 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 11475 (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/m2to reach brightness levels of 80% or more compared to conventional white top coatings typically requiring 50-60 g/m2at 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/m2to 40 g/m2and (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.

