WO2009091406A1

WO2009091406A1 - Coated paperboard with enhanced compressibility

Info

Publication number: WO2009091406A1
Application number: PCT/US2008/051436
Authority: WO
Inventors: Frederick J. Renk; Vladislav A. Babinsky; Jr. Stanley H. Mcgrew
Original assignee: Meadwestvaco Corporation
Priority date: 2008-01-18
Filing date: 2008-01-18
Publication date: 2009-07-23

Abstract

A coated paper or paperboard with improved compressibility such that its surface smoothness property may be enhanced with a reduced surface pressure level is disclosed. The coated paper or paperboard includes a paper stock and a compressible coating layer. The improvement in Parker Print smoothness of the disclosed coated board is at least 30% higher than an improvement of typical coated board at a same basis weight and a same compressing pressure level. When the surface pressure increases from 5 kgf/cm2 to 20 kgf/cm2, Parker Print smoothness of the disclosed coated paperboard increases at least 1.2 units. At the same increased level of surface pressure, commercial board of a same weight shows an enhanced Parker Print smoothness of less than 1.2 units.

Description

IN THE UNITED STATES PATENT AND TRADEMARK OFFICE International Patent Application For

COATED PAPERBOARD WITH ENHANCED COMPRESSIBILITY

BACKGROUND OF THE INVENTION

[0001] Paper is manufactured by an essentially continuous production process wherein a dilute aqueous slurry of cellulosic fiber flows into the wet end of a paper machine and a consolidated dried web of indefinite length emerges continuously from the paper machine dry end. The wet end of the paper machine comprises one or more headboxes, a drainage section and a press section. The dry end of a modern paper machine comprises a multiplicity of steam heated, rotating shell cylinders distributed along a serpentine web traveling route under a heat confining hood structure. Although there are numerous design variations for each of these paper machine sections, the commercially most important of the variants is the fourdrinier machine wherein the headbox discharges a wide jet of the slurry onto a moving screen of extremely fine mesh.

[0002] The screen is constructed and driven as an endless belt carried over a plurality of support rolls or foils. A pressure differential across the screen from the side in contact with the slurry to the opposite side draws water from the slurry through the screen while that section of the screen travels along a table portion of the screen route circuit. As slurry dilution water is extracted, the fibrous constituency of the slurry accumulates on the screen surface as a wet but substantially consolidated mat. Upon arrival at the end of the screen circuit table length, the mat has accumulated sufficient mass and tensile strength to carry a short physical gap between the screen and the first press roll. This first press roll carries the mat into a first press nip wherein the major volume of water remaining in the mat is removed by roll nip squeezing. One or more additional press nips may follow.

[0003] From the press section, the mat continuum, now generally characterized as a web, enters the dryer section of the paper machine to have the remaining water removed thermodynamically. [0004] Generally speaking, the most important fibers for the manufacture of paper are obtained from softwood and hardwood tree species. However, fibers obtained from straw or bagasse have been utilized in certain cases. Both chemical and mechanical defiberizing processes, well known to the prior art, are used to separate papermaking fiber from the composition of natural growth. Papermaking fiber obtained by chemical defiberizing processes and methods is generally called chemical pulp whereas papermaking fiber derived from mechanical defiberizing methods may be called groundwood pulp or mechanical pulp. There also are combined defiberizing processes such as semichemical, thermochemical or thermomechanical. Either of the tree species may be defiberized by either chemical or mechanical methods. However, some species and defiberizing processes are better economic or functional matches than others.

[0005] An important difference between chemical and mechanical pulp is that mechanical pulp may be passed directly from the defiberizing stage to the paper machine. Chemical pulp on the other hand must be mechanically defiberized, washed and screened, at a minimum, after chemical digestion. Usually, chemical pulp is also mechanically refined after screening and prior to the paper machine. Additionally, the average fiber length of mechanical pulp is, as a rule, shorter than that of chemical pulp. However, fiber length is also highly dependent upon the wood species from which the fiber originates. Softwood fiber is generally about three times longer than hardwood fiber.

[0006] Applying a paper coating is a very common way to enhance the surface properties of paper. However, paper-coating equipment can be very complex and expensive. Typically, coating weights from about 2-6 lbs/ 1000 ft² are required to substantially enhance surface properties of the paper. Such a relatively high coat weight can result in substantial expenses with respect to coating materials and result in an increase in the basis weight of the finished paper. A high coat weight is usually required because lower coating weights are typically not uniform enough to provide the desired improvement in surface properties. Non- uniformity associated with low coating weights can be particularly problematic when coating unbleached board. Because of the brown fibers in the unbleached board, it is particularly important that the coating, which is typically white, cover the brown board completely. Preferably, the final coated surface should be uniform to provide acceptable appearance and printing properties.

[0007] Coated paper used for printing is generally required to have high level of gloss, excellent smoothness, and excellent printability. Under the pressure for high-speed printing spurred by improved printing techniques in recent years, there is a strong demand for the coated paper to have high stiffness, in addition to the above-mentioned quality characteristics relating to the runnability and printability.

[0008] If the coated paper has a high stiffness, it can smoothly pass through highspeed printing machines with fewer feeding jams. As the paper stiffness increases further, the bulky feel is a perceived quality improvement by customers. Higher stiffness paper can be advantageously used in books, magazines, and catalogues, because it provides a feel of hardness or heaviness similar to a hardcover book.

[0009] Stiffness has close relation with the basis weight of paper and density. There is a general trend that stiffness increases as the basis weight increases, but decreases as the paper density increases. High stiffness achieved by increasing basis weight may improve the feel of a book, however would result in a book becoming heavier than necessary. Therefore, coated paper with high stiffness but moderate basis weight is desirable. Paper with moderate basis weight is also more economical because lower amount of raw material (fiber) is utilized. In addition, shipping costs based on weight are less for low basis weight paper.

[0010] In addition to high stiffness, coated paper for printing is often required to have high gloss and smoothness. For coated paper to have such quality characteristics, density must be increased to some extent due to the calendaring required. However, any increase in paper density by calendaring will inevitably reduce the stiffness. The calendaring process always deteriorates the stiffness of paper by significantly reducing caliper. Thus, the relationship between gloss and stiffness and between smoothness and stiffness are inversely proportional to each other. [0011] Improvements in the calendaring process including moisture gradient calendaring, hot calendaring, soft calendaring, and belt calendaring slightly improve stiffness for a given caliper but does not change the fundamental ratio between caliper, stiffness, smoothness, and printing properties.

[0012] Various proposals have been made to improve the stiffness of coated paper for printing without calendaring. For example, several proposals include high rate mixing of softwood pulp in the raw stock, addition of specially engineered fibers in the raw stock, addition of highly branched polymers within the raw stock, and high rate compounding of starch or copolymer latex with a high glass transition temperature ("Tg") within the coating formulation.

[0013] However, potential drawbacks to these methods of stiffness improvement are that although useful in improving paper stiffness, they could potentially degrade the smoothness, gloss, and/or printability of the coated paper obtained.

[0014] Accordingly, it is desirable to have a paper or paperboard having enhanced surface properties such as smoothness, gloss, and/or printability properties, while maintaining stiffness property at a reduced basis weight

SUMMARY OF THE INVENTION

[0015] The present disclosure relates to coated paper or paperboard having improved compressibility such that its surface smoothness property may be enhanced with a reduced surface pressure. The coated paper or paperboard includes a paper stock and a compressible coating layer. The improvement in Parker Print smoothness of the disclosed coated board is at least 30% higher than an improvement of typical coated board having same basis weight and under same level of compressing pressure. When the surface pressure increases from 5 kgf/cm² to 20 kgf/cm², Parker Print smoothness of the disclosed coated paperboard increases at least 1.2 units. At the same increased level of surface pressure, commercial boards of the same weight have an enhanced Parker Print smoothness of less than 1.2 units. DESCRIPTION OF THE DRAWING

[0016] FIG.l is a diagrammatic view of spray devices located in the drying section of a paper machine in accordance with one aspect of the present disclosure; [0017] FIG.2 is a graph showing a comparison in the Parker Print smoothness improvements of the disclosed coated boards and of the commercial coated boards, when the pressure applied onto the paperboard surface increased from 5 kgf/cm² to 20 kgf/cm²: Samples 1-5 are coated boards of the present disclosure, and Samples 8-9 are commercial coated boards.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0018] The present disclosures now will be described more fully hereinafter, but not all embodiments of the disclosure are shown. Indeed, these disclosures may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. The detailed description is not intended to limit the scope of the appended claims in any manner.

[0019] The coated paper-based product with enhanced compressibility of the present disclosure includes:

(i) a paper stock; and (ii) a compressible coating layer, characterized by an improvement in Parker Print smoothness of at least 30% higher than an improvement of a coated paperboard at a same basis weight and a same compressing pressure level.

[0020] Any paper stock may be used in the present disclosure. These include, but are not limited to, bleached board, unbleached board, No. 1-3 printing grade, release liner grades, and printable overlay grade. The paper stock may have a basis weight in a range of about 10 g/m²to about 700 g/m². [0021] The coated paper or paperboard of the present disclosure may comprise a single coating layer, or a multiple layer of a single coating composition, or a multi-layer coating of different coating compositions.

[0022] In one embodiment of the present disclosure, the compressible coating layer includes nano fibers. The compressible layer of nano fibers may be coated to the paper-based substrate at various coat weights. Additionally, the coat weight of the nanofibers may be about 10 times less than conventional coating materials. In one embodiment, the nanofibers layer may be applied at a coating weight of from about 0.05 to about 20 g/m² (0.01 Ib/ 1000 ft² to about 4 lb/1000 ft²). In another embodiment, the coat weight of the nanofibers layer is from about 1 g/m² to about 10 g/m² (0.2 lb/1000 ft² to about 2 lb/1000 ft²) by on dry weight. In yet another embodiment, the coat weight of the nanofibers layer is from about 2 g/m² to about 6 g/m² (0.4 lb/1000 ft² to about 1.2 lb/1000 ft²) based on dry weight. The thickness of the coating layer comprising nanofibers may be in range of about 1-200 micrometers.

[0023] Any known methods of producing nanofibers may be used in the present disclosure. The coated layer may be applied onto the paper stock using various coating applications. These include, but are not limited to, electrospinning, and spray coating.

[0024] Electrospinning is a process based on the use of high voltages (e.g., 10-

100k V) to generate ultra fine fibers with diameters in the range of about 5 nm to 5000 nm, although fibers with diameters as small as 3 nm have been produced by this method. This technology utilizes an electric field to generate sufficient surface charge to overcome the surface tension in a viscous drop of polymer melt, solution, and/or gel. This creates a jet of solution that comes from the orifice that is drawn down by acceleration to a grounded collection device located on the other side of the web. In the electrospinning device, the jets or slot are located on the side of the moving paper web opposite the grounded collection device. Typically, there is contact and some friction between the paper web and the electrode thereby inducing a charge on the web as the web advances past the stationary electrode. The discharging component of the electrospinning device can include a plurality of nozzles or the discharging component can be in the shape of a slot. Typical cross sectional areas for electrospinning nozzles can range from about 0.1mm² to about 10mm² and can be of any usable shape although the nozzles are typically round. Discharging components having a slot shape typically have a width of between about 10-1000 microns. The diameter of the fibers produced by electrospinning are typically at least one order of a magnitude smaller and have larger surface area to volume ratios than fibers made by conventional extrusion techniques. Electrospinning is described in more detail in U.S. Pat. Nos. 2,158,416; 2,323,025; 6,641,773 and U.S. Pat. App. Pub. No. 2004/0223040, the disclosures of which are hereby incorporated by reference

[0025] The nanofϊbers may be produced using an electrospinning device installed online to a paper making process. The electrospinning device may be located in a certain zone of the paper machine where the dryness of the paper web will be favorable for application of nanofibers generated by electrospinning.

[0026] In one embodiment of the present disclosure, nanofϊbers are produced from the electrospinning device installed on the paper machine in the area of dryness from about 40% to 100% by weight.

[0027] In another embodiment of the present disclosure, nanofibers are produced from the electrospinning device installed on the paper machine in the area of dryness from about 50% to about 80% by weight.

[0028] In yet another embodiment of the present disclosure, nanofϊbers are produced from the electrospinning device installed on the paper machine in the area of dryness from about 55 % to about 75 % by weight.

[0029] The coating layer comprising nanofϊbers should be adequately bonded to paper or paperboard substrate in order to prevent separation during use. The contraction of fiber structure of the substrate during drying will help to secure the nanofϊbers in the fiber structure of the paper forming inner bonding between the paper web and the nanoweb. For example, the electrospinning device may be positioned at various points of the forming paper web. It may be positioned between or in place of one of the sets of press rolls or it may be located between the last press rolls and the dryer section.

[0030] Additionally, the electrospinning device may be located in the drying section of the paper machine. The nano fibers are typically already dry when they contact the paper web; therefore, no additional energy is required to dry the nanofϊber web after application. The electrospinning device may be located in the dryer section such that there is a plurality of drying cylinders on each side of the device. For example, there may be from about 5 to 15 drying cylinders installed before the electrospinning device and from about 10 to 100 drying cylinders after the electrospinning device.

[0031] The electrospinning device may also be positioned relative to the paper machine so as to apply the nano fibers to either surface of the forming paper web. More than one electrospinning device may be employed to apply nano fibers to both sides of the forming paper web, to apply multiple layers of nano fibers or to apply nano fibers in different locations on the web.

[0032] Bonding of nano fibers and paper or paperboard may be enhanced by selection of an appropriate material used for the nano fibers having a certain melting temperature and certain thermo-mechanical and hardening characteristics. In one embodiment, the polymers used in forming the fibers of the nanoweb may have a melt index range of about 0.5 g/10 min to about 250 g/10 min. In another embodiment, the polymers have a melt index range of about 50 g/10 min to about 150 g/10 min. In another embodiment, the polymers have a melt index range of about 50 g/10 min to about 100 g/10 min. Melt Index can be determined in accordance with ASTM D 1238.

[0033] Suitable nano fibers may be cellulosic-based nano fibers or synthetic nano fibers. A variety of materials may be used in forming nano fibers of the present disclosure. Materials such as biopolymers (collagen, fibrinogen), natural polymers (alginate, cellulose derivatives, etc.), chitosan, bicompatible polymers, polycaprolactone, polyethylene oxide and the like may be used. Furthermore, the following materials may also be used: aluminum oxide, ferrofluid composite, silica, aluminosilicate, organosilicas, TiO₂ (anatase and rutile), TiN, Nb₂Os, Ta₂Os, TiN oxide - fluorinated and non-fluorinated, indium, tin oxide, V₂Os, mixed oxides (Mn, Ga, Mo, W, Zn), PEO/synthetic hectorite.

[0034] Polymer materials use in the present disclosure may be addition polymer and condensation polymer materials such as polyolefm, polyacetal, polyamide, polyester, cellulose ether and ester, polyalkylene sulfide, polyarylene oxide, polysulfone, modified polysulfone polymers and mixtures thereof. In one embodiment of the present disclosure, the polymers used for the formation of nano fibers include, but are not limited to, polyethylene, polypropylene, poly (vinylchloride), polyacrylic resin, poly (methyl methacrylate), polystyrene, and copolymers thereof (including ABA type block copolymers), poly (vinylidene fluoride), poly (vinylidene chloride), polyvinylalcohol in various degrees of hydrolysis (87% to 99.5%) in crosslinked and non-crosslinked forms.

[0035] Block copolymers may also be used in the present disclosure. With such copolymers the choice of solvent swelling agent is important. The selected solvent is such that both blocks were soluble in the solvent. One example is an ABA (styrene-EP-styrene) or AB (styrene-EP) polymer in methylene chloride solvent. If one component is not soluble in the solvent, it will form a gel. Examples of such block copolymers are Kraton^® type of AB and ABA block copolymers including styrene/butadiene and styrene/hydrogenated butadiene (ethylene propylene), epsilon-caprolactam/ethylene oxide, polyester/ethylene oxide and polyurethanes of ethylene oxide and isocyanates.

[0036] Addition polymers like polyvinylidene fluoride, syndiotactic polystyrene, copolymer of vinylidene fluoride and hexafluoropropylene, polyvinyl alcohol, polyvinyl acetate, amorphous addition polymers, such as poly (acrylonitrile) and its copolymers with acrylic acid and methacrylates, polystyrene, poly (vinyl chloride) and its various copolymers, poly (methyl methacrylate) and its various copolymers, may be solution spun with relative ease because they are soluble at low pressures and temperatures.

[0037] The fine nano fiber formation described herein may be improved by the presence of oleophobic and hydrophobic additives, as these additives form a protective layer coating or penetrate the surface to some depth to improve the nature of the polymeric material. The important characteristics of these materials are the presence of strong hydrophobic groups that may have oleophobic character. Strongly hydrophobic groups include, but are not limited to, fluorocarbon groups, hydrophobic hydrocarbon surfactants or blocks and substantially hydrocarbon oligomeric compositions. These materials can be manufactured in compositions that have a portion of the molecule that tends to be compatible with the polymer material. These sections of the polymer can form a physical bond or an association with the polymer, while the strongly hydrophobic or oleophobic groups form a protective surface layer that can reside on the surface or become alloyed with or mixed with the polymer surface layers.

[0038] Suitable nano fibers for use in the present disclosure may have an average diameter of less than about 1000 nanometers. In one embodiment, the nano fibers have an average diameter range of about 5 nanometers to about 700 nanometers. In another embodiment, the nano fibers have an average diameter range of about 5 nanometers to about 500 nanometers. In yet another embodiment, the nanofibers have an average diameter range of about 200 nanometers to about 300 nanometers. One skilled in the art will appreciate that the fiber average diameter accounts for variations along the length of the fiber and the fact that the cross sectional shape of the fibers may have various forms such as circular, elliptical, flat or stellato.

[0039] Bonding between nanofibers and paper or paperboard may be improved by providing a binder between the layer of nanofibers and the paper-based substrate. Such binders are not particularly limited so long as they are compatible with the materials used in forming the nanofibers. Examples of suitable binders include, but are not limited to, polyvinyl alcohol and polyvinyl pyrrolidone. The binder compositions may comprise one or more adjunct materials or optional ingredients to improve the application process or binding of the nanofibers and the paper-based substrate.

[0040] The binder compositions, having the appropriate concentration and viscosity, can be determined by one of ordinary skill in the art. In one embodiment of the present disclosure, the glass transition temperature (Tg) of the binders is about 1O⁰C to about 100 ⁰C. In another embodiment of the present disclosure, the glass transition temperature (Tg) of the binders is about 2O⁰C to about 50 ⁰C.

[0041] The binder may be applied to the paper-based substrate by any conventional technique known to those skilled in the art. For example, the binder may be sprayed onto the surface of the paper-based substrate. The binder may be applied to the paper-based substrate before application of the coating layer comprising nano fibers.

[0042] The amount of binder applied is not particularly limited. In one embodiment of the present disclosure, the amount of binder is from about 0.05 g/m² to about 15 g/m² (0.01 Ib/ 1000 ft² and about 3 Ib/ 1000 ft²) based on dry weight. In another embodiment of the present disclosure, the amount of binder is from about 0.05 g/m² to about 2 g/m² (0.01 lb/1000 ft² to about 0.4 lb/1000ft²).

[0043] The improvement in smoothness of the paper or paperboard having a layer of coating comprising nanofibers may be achieved even without calendaring. Application of a coating comprising nanofibers in combination with calendaring may also provide additional flexibility in producing the coated paper.

[0044] In one embodiment of the present disclosure, a paper having a Parker Print smoothness of from about 6.0 to about 9.0 is coated with a layer comprising nanofibers to provide a coated paper with a Parker Print smoothness of less than about 5.0.

[0045] In another embodiment of the present disclosure, a paper having a Parker Print smoothness of from about 6.0 to about 9.0 is coated with a layer comprising nanofibers to provide a coated paper with a Parker Print smoothness of less than about 3.0.

[0046] In yet another embodiment of the present disclosure, a paper having a Parker

Print smoothness of from about 6.0 to about 9.0 is coated with a layer comprising nanofibers to provide a coated paper with a Parker Print smoothness of less than about 2.0.

[0047] Furthermore, a paper stock may be calendered to a Parker Print smoothness of between about 2 and 6 microns prior to application of the nano fibers coating to provide further improvements in smoothness. Parker Print smoothness is determined in accordance with TAPPI standard T 555 om-99.

[0048] In addition to a coating layer comprising nanofibers, the coated paper or paperboard of the present disclosure may include a layer of spray-applied coatings. A variety of spray coatings devices may be used in the present disclosure. Examples of spray coating devices include, but are not limited to, air atomized, mechanically atomized, electrostatic, or ultrasonic spray. Typically, the drop size will fall within the range from about 10 - 200 microns. Numerous spray devices may be installed in the machine direction (MD) and/or the cross direction (CD) on the web. The resulting spray patterns typically overlap to provide uniformity of coverage.

[0049] The spray devices may be installed in any orientation to apply spray to the web. For example, the spray device may be located above or below the moving paper web. In one embodiment of the present disclosure, the spray device(s) may be positioned so as to apply spray coating to one or both sides of a web moving vertically. Additionally, a combination of the aforementioned spraying devices may be used.

[0050] The spray devices may be located on certain number of the drying cylinders of a paper machine. In one embodiment, the spray devices are located in the area of paper dryness of about 45% to about 90%. In another embodiment, the spray devices are located in the area of paper dryness of about 70% to about 90%.

[0051] The cylinders of the lower row of cylinders may be used. Each cylinder may be equipped with a spray device and also with a drying device that heats the coated surface of paper after paper passes the area of spray application. The multi-step application of the spray coating provides a coating or film that appears to be a single layer even under a microscope. One advantage of the coating is that it typically is more uniform and provides a uniform surface for printing. Furthermore, less energy is required to dry several thin films or layers as compared to one thick layer.

[0052] In one embodiment of the present disclosure, the dryness of the coating film/layer after each cylinder is at least 50% dry. In another embodiment of the present disclosure, the dryness of the coating film/layer after each cylinder is at least 60% dry. In yet another embodiment of the present disclosure, the dryness of the coating film/layer after each cylinder is from about 70% to about 80% dry. Drying may continue on the subsequent drying cylinders, which can reduce energy consumption on the air dryer. The lower row of drying cylinder would not have felts in this configuration.

[0053] The drying devices are not particularly limited and suitable systems are known to those of ordinary skill in the art. Examples of drying devices include non-contact devices such as those utilizing infrared, heated air, dielectric energy, etc.

[0054] The spray-applied coating layer may be of any coat weight. In one embodiment of the present disclosure, the spray coated layer may be applied at a coating weight of from about 0.05 to about 20 g/m² (0.01 lb/1000 ft² to about 4 lb/1000 ft²). In another embodiment of the present disclosure, the spray coated layer may be applied at a coating weight of from about 0.2 to about 10 g/m² (0.04 lb/1000 ft² to about 2 lb/1000 ft²). In yet embodiment of the present disclosure, the spray coated layer may be applied at a coating weight of from about 2 to about 6 g/m² (0.4 lb/1000 ft² to about 1.2 lb/1000 ft²) based on dry weight.

[0055] FIG. 1 illustrates a dryer section 10 of a paper machine with a plurality of drying cylinders 12 arranged in an upper row and a lower row. In the embodiment shown, each drying cylinder 12 in the lower row of cylinders is provided with a spray device 14 and a drying device 16. In operation, the paper web 18 advances through the dryer section 10 by alternately moving between the cylinders 12 in the upper row to the cylinders 12 in the lower row throughout the dryer section. Press rolls 20 are positioned at various locations on the drying cylinders 12 to improve smoothness of the finished paper by pressing the wet paper against the heated surface of the cylinders 12. The repeated cycles of pressing, drying and coating improves uniformity of the finished coating and smoothness of the coated paper or paperboard.

[0056] The coating formulation may be designed to be almost dry when electrostatic application is used and a thin layer of coating is applied. The requirements for drying in this case would be minimal. The location of sprayers on the lower drying cylinders facilitates gradual application of the required amount of coating but at the same time utilization of the heating from the drying cylinders for drying the web. Although the spray coating is applied from multiple cylinders, the finished coating layer typically will form a coating layer that is uniform and appears as a single layer. The uniform coating layer provides an improved surface for printing. Furthermore, the location of the web on cylinders during application will help to improve the hold out of the coating on the surface of paper because the evaporating steam is moving to the exterior surface of the web, resisting the penetration of the coating into the sheet of paper. Improvement of the hold out will reduce the coat weight required for printing properties, will provide more possibilities for coating formulating, and improve paper quality.

[0057] To improve smoothness of paper prior to coating, press rolls may be installed on one or more of the drying cylinders and may typically be located on the upper row of the drying cylinders. The paper may experience a press drying process that leads to improved smoothness because wet paper is pressed against the heated surface of the cylinder. Linear pressure of 10-100 kN/m, more particularly 20-30 kN/m, may be used in this nip. The paper according to this method will experience cycles of pressing, drying, and coating that will result in uniform application and good paper smoothness. The treatment of the web by press roll on the drying cylinder will improve the smoothness and uniformity of absorption that will lead to better uniformity of appearance and smoothness of the subsequently applied coating.

[0058] The coatings may be applied to the disclosed paper and paperboard using a combination of spray coating and electrospinning, resulting in a reduced coated weight of electrospinning coverage while still maintaining the unique uniform absorption ability of an electrospun fiberweb that translates into superior printing properties.

[0059] The spray application is providing superior hold out of the formulation on the surface because of quick and controlled drying of the liquid coating during the spray process. Multi-layer coating will create superior uniformity of the coated layer.

[0060] The coated paper or paperboard of the present disclosure exhibits excellent paper gloss, smoothness and print gloss. As a result of the coating process described herein, these improved properties can be obtained in a paper or paperboard using substantially lower intensity of calendaring which results in corresponding higher caliper and higher stiffness of the finished paper. FIG 2. shows an improvement of Parker Print smoothness of the board when pressure applied onto the paperboard surface increased from 5 kgf/cm² to 20 kgf/cm². Samples 1-5 are coated boards of the present disclosure, and Samples 8-9 are commercial coated boards. Each board has the same basis weight. The improvements of Parker Print smoothness of the disclosed coated boards are at least 1.4 point; whereas, the improvement for the commercial coated boards are less than 1.2 points. The improvement of Parker Print smoothness of the disclosed boards is at least 30% higher than that of the commercial board at the same basis weight.

[0061] The coated paper or paperboard of the present disclosure may further include another coating layer similar to a conventional top coating layer containing filler, binder, and/or special additives. Roll, blade, curtain, or check coating equipment known to those or ordinary skill in the art may utilized to apply the conventional coating layer.

[0062] The top coating, when present, may be applied at a coat weight of from about 2 to about 50 g/m² _. In another embodiment, the coat weight of the top coating is from about 5 to about 30 g/m². In yet another embodiment, the coat weight of the top coating is from about 10 to about 20 g/m².

[0063] In one embodiment of the present disclosure, the coated paper or paperboard includes a coating layer comprising nano fibers at a coat weight of 1-10 g/m², a layer of spray coating at a coat weight of 0.2-10 g/m²; and a layer of top coating at a coat weight of 5-30 g/m². By contrast, conventional coated paper with comparable surface properties as described herein typically requires from about 25 to about 60 g/m² of coating.

[0064] The coated paper or paperboard of the present disclosure typically exhibit improved stiffness and printing properties yet require only minimal calendaring, if any. The disclosed coated paper or paperboard may exhibit an improvement of stiffness of more than about 10-40%, while maintaining smoothness as compared to a paper or paperboard containing a conventional coating layer only.

EXAMPLES

[0065] The present disclosure is described in more detailed by reference to the following non-limiting examples.

[0066] The paper stock was formed from unbleached kraft pulp consisting of hardwood or softwoods or a combination thereof. Brightness of the paper stock was typically about 14% to 22% for unbleached fibers and about 70% to 85% for bleached fibers before application of the coating comprising nano fibers. The basis weight of the paper stock may range from about 10 to about 700 g/m2 . The paper stock may also contain organic and inorganic fillers, sizing agents, retention agents, and other auxiliary agents as is known in the art.

[0067] The coating was spray-applied to the paper stock using electrostatic spraying at an air pressure of 50 psi and a voltage of 40 kV. Other spray coating techniques may be used, such as air-atomized or ultrasonic. The coat weight for the spray coating layers was 4

Ib/ 1000 ft². A typical spray coating formulation was as shown in Formulation 1. Formulation 1

Formulation 2

Formulation 3

Formulation 4

Formulation 5

[0068] The treated paper or paperboard was further coated with nanofibers using an electrospinning device at a voltage of 20-100 KV and an electrical current of 50-400 microampere. The flow rate is 0.5-3 mil/min per emitter; a target distance of 8-15 inches; a temperature of 70-200⁰F, and a relative humidity of 45-65%. The nanofibers produced had an average diameter of about 25 - 200 nm. Several formulations for coatings comprising nanofibers were used: Formulations 2, 3, 4, and 5.

[0069] The resulting coated paper was passed through calendaring to further enhance surface properties. The calendaring required for the disclosed coated paper to develop the same printing properties as are characteristic for conventionally coated paper was much less intensive. An example would be calendaring at a pressure of 20 - 50 kN/m for coated board with two-three coating layers. Before calendaring, minimal calendaring may be applied to provide uniform thickness of the paper web in cross direction. Pressure of between about 20 - 60 kN/m was recommended for this step. Hot calendaring may be used, typically at a temperature range of about 80 - 150⁰C.

[0070] After coating, a gloss calendaring is usually used. For the proposed structure, a lower pressure gloss calendaring may be used in comparison with conventional coating. The lower pressure is possible because the spray and electrospun layers of coating are more bulky, plastic, and resilient than conventional coatings. Therefore, the same smoothness will be achieved when using lower pressure, thus, preserving bulk and improving stiffness.

[0071] It is to be understood that the foregoing description relates to embodiments are exemplary and explanatory only and are not restrictive of the disclosure. Any changes and modifications may be made therein as will be apparent to those skilled in the art. Such variations are to be considered within the scope of the disclosure as defined in the following claims.

Claims

We Claim:

1. A coated paper-based product with enhanced compressibility, including:

(a) a paper stock; and (b) a compressible coating layer, characterized by an improvement in Parker Print smoothness of at least 30% higher than an improvement in Parker Print smoothness of a paper-based product at a same basis weight and a same compressing pressure level, wherein the Parker Print smoothness is determined in accordance with TAPPI standard T 555 om-99.

2. The product of Claim 1, wherein the paper stock comprises a substrate selected from the group consisting of bleached board, unbleached board, No 1-3 printing grade, release liner grade, printable overlay grade, and combinations thereof.

3. The product of Claim 1 , wherein the paper stock has a basis weight in a range of about 10 g/m² to about 700 g/m².

4. The product of Claim 1, wherein the compressible coating layer includes nanofϊbers.

5. The product of Claim 4, wherein the nanofϊbers comprise a material having a melt index range of about 0.5 g/10 min to about 250 g/10 min.

6. The product of Claim 4, wherein the nanofϊbers comprise a material selected from the group consisting of biopolymer, natural polymer, chitosan, bicompatible polymer, polycaprolactone, polyethylene oxide, and combinations thereof.

7. The product of Claim 4, wherein the nanofϊbers comprise a material selected from the group consisting of aluminum oxide; ferrofluid composite; silica; aluminosilicate; organosilicas; TiO₂; TiN; Nb₂Os; Ta₂Os_; TiN oxide; indium; tin oxide; V₂Os; mixed oxides of Mn, Ga, Mo, W, Zn, and combinations thereof; PEO/synthetic hectorite; and combinations thereof.

8. The product of Claim 4, wherein the nano fibers comprise a material selected from the group consisting of polyolefin, polyacetal, polyamide, polyester, cellulose ether, cellulose ester, polyalkylene sulfide, polyarylene oxide, polysulfone, modified polysulfone polymer, polyethylene, polypropylene, poly (vinylchloride), polyacrylic resin, poly (methyl methacrylate), polystyrene, poly (vinylidene fluoride), poly (vinylidene chloride), polyvinylalcohol, styrene- ethylene propylene -styrene copolymer, styrene- ethylene propylene copolymer, styrene/butadiene copolymer, styrene/hydrogenated butadiene copolymer, epsilon-caprolactam/ethylene oxide copolymer, polyester/ethylene oxide copolymer, polyurethanes of ethylene oxide and isocyanates, polyvinylidene fluoride, syndiotactic polystyrene, copolymer of vinylidene fluoride and hexafluoropropylene, polyvinyl alcohol, polyvinyl acetate, poly (acrylonitrile), acrylonitrile- acrylic acid copolymer, acrylonitrile- methacrylate copolymer, polystyrene, poly( vinyl chloride), poly(methyl methacrylate), and combinations thereof.

9. The product of Claim 4, wherein the nanofibers further comprise an oleophobic additive.

10. The product of Claim 4, wherein the nanofibers have an average diameter of less than about 1000 nanometers.

11. The product of Claim 1 , wherein the compressible coating layer has a coating weight range of about 0.05 g/m² to about 20 g/m² based on dry weight.

12. The product of Claim 1, wherein the compressible coating layer has a thickness in a range of about 1 micrometers to about 200 micrometers.

13. The product of Claim 1 , further including a binder between the stock paper and the compressible coating layer.

14. The product of Claim 13, wherein the binder comprises a polymer selected from the group consisting of polyvinyl alcohol, polyvinyl pyrrolidone, and combinations thereof.

15. The product of Claim 13, wherein the binder has a glass transition temperature in a range of about 10⁰C to about 100 ⁰C.

16. The product of Claim 13, wherein an amount of the binder is in a range of about 0.05 g/m² to about 15 g/m² based on dry weight.

17. The product of Claim 1, further including a layer of top coating.

18. The product of Claim 17, wherein the top coating layer has a coating weight range of about 2 g/m² to about 50 g/m² based on dry weight.

19. A method of producing a coated paper-based product with enhanced compressibility having an improvement in Parker Print smoothness of at least 30% higher than an improvement in Parker Print smoothness of a paper-based product at a same basis weight and a same compressing pressure level comprises an application of a compressible coating layer onto a paper substrate.

20. The method of Claim 19, wherein the application of the compressible coating layer onto the paper substrate comprises a step of spray coating, electrospinning, or combinations thereof.

21. The method of Claim 19, wherein the compressible coating layer is applied onto the paper substrate using a spray device located on a papermaking machine in an area having a dryness of the paper is in a range of about 45% to about 90%.

22. The method of Claim 21, wherein the spray device comprises a member selected from the group consisting of air atomized spray device, mechanically atomized spray device, electrostatic spray device, ultrasonic spray device, and combinations thereof.

23. The method of Claim 21, wherein the compressible coating layer is sprayed in a drop size range of about 10 microns to about 200 microns.

24. The method of Claim 19, further comprising a step of passing the paper coated with the compressible coating layer through a drying device, wherein the coated paper is at least 50% dry after passing through the drying device.