Coating composition, paper product having flexible coating and method for manufacturing a paper product

A paper product is described that includes a paper substrate having a first surface and a second surface; a coating layer provided on the first surface of the paper substrate, the coating layer comprising a binder component and a pigment component, wherein the weight ratio the binder component to the pigment component is between about 0.25:1 to about 1:1; and wherein the paper product exhibits a gloss of at least about 20 to 75 according to TAPPI T480 om-92, a Print-surf roughness of at least about 0.5 to 2.5 according to TAPPI T555 pm-94, and a density of at least about 12 to 18 according to TAPPI T410 om-93 and TAPPI T411 om-89. A method for manufacturing a paper product is provided.

FIELD OF THE INVENTION

The invention relates to a coating composition, a paper product having a flexible coating, and to a method for manufacturing a paper product having a flexible coating. In particular, the coating composition provides a flexible coating when applied to a paper substrate and can receive printing while reducing the occurrence of cracking at the fold.

BACKGROUND OF THE INVENTION

When coated paper products are folded, there exists a tendency for cracks to develop at the fold. This cracking can be referred to as flex cracking or cracking at the fold. Paper products that are often subjected to folding are often found in magazines, books, bags, and boxes. Flex cracking or cracking at the fold can be either cosmetic or may lead to a complete paper failure at the location of the fold. A cosmetic cracking can be characterized by a small disruption of the coating layer at the location of the fold, making the fold unappealing, especially if the fold is in a heavily inked area.

Factors that effect cracking at the fold include changes in humidity as a result of seasonal changes and the chemistry of the paper substrate, the coating, the ink, and the techniques for processing and manufacture. Cracking at the fold may become more noticeable if heavy colors are used at fold areas.

Several techniques are available for reducing cracking at the fold. One technique involves selecting a latex for the coating layer. Another technique is to increase the moisture in the coated paper. Increasing the moisture level can lead to other coating defects, such as, blister. These techniques are described by Attal, “Ask Dow,” It's All About Paper, Vol. 2, Issue 1, April 2001, Page 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A paper product can be provided according to the invention having a flexible coating that reduces the occurrence flex cracking. The term “flex cracking” refers to the existence of cracking at the fold of a paper product when the paper product is folded. By controlling the coating composition that is applied to a paper substrate and by controlling the finish of the coated paper product, flex cracking can be reduced.

The coating composition can be applied to a paper substrate to provide a coated paper product. The coating composition is available as a dispersion containing pigment, binder, and water as the largest components of the composition. Additional components that can be incorporated into the coating composition include additives such as rheology modifiers, lubricants, defoamers, dyes, dispersants, insolubilizers, and preservatives.

The coating composition is provided as a flowable dispersion that can be applied to a surface of a paper substrate to form a coating layer thereon. The coating composition should be sufficiently flowable to allow it to be applied using conventional paper coating equipment, such as, a metering film size press.

The pigment component of the coating composition provides a surface on the coating layer that accepts printing. There should be a sufficient amount of pigment in the coating composition so that the resulting coating layer can receive printing in commercial printing facilities. A concentration of pigment in the coating composition that is too high can result in rheology problems during application of the coating composition and can result in the possibility of dusting if there is pigment that is not bound to the paper substrate. The pigment component is generally provided as a particulate dispersed in the binder component and the water. Examples of the pigment component include delaminated clay, kaolin clay, titanium dioxide, calcium carbonate (precipitated and natural ground), calcined clay, alumina trihydraie, precipitated amorphous silica, talcum, gypsum, plastic pigments, exters, and mixtures thereof. The coating composition can include the pigment component in an amount of between about 20 wt. % and about 48 wt. %. In addition, the pigment component can be provided in an amount of between about 32 wt. % and about 43 wt. %, and between about 36 wt. % and about 40 wt. %, and at about 38 wt. %. The weight percent is based upon a 100% solids component. The pigment component can include a combination of delaminated clay and another pigment. The pigment component can include at least about 80 wt. %, based on the weight of the pigment component, of delaminated clay, and can include up to about 20 wt. %, based on the weight of the pigment component, of another pigment such as titanium dioxide. When titanium dioxide is included in the pigment component, it is preferably included in an amount of at least about 2 wt. % based on the weight of the pigment component.

The binder component is provided to adhere the pigment component to the paper substrate. It is believed that prior art coating compositions minimize the amount of the binder component in the coating composition because the binder component is considered to be the most expensive component of the coating composition. The applicants discovered that by increasing the concentration of the binder component in the coating composition and enhancing the finishing technique, it is possible to decrease the occurrence of flex cracking. In general, the applicants have found that the binder component can be provided in an amount of at least about 8 wt. % in the coating composition to provide a coating layer on a paper substrate that, when processed according to the invention, exhibits reduced flex cracking compared with prior art coating compositions containing a lower level of the binder component and not being processed according to the invention. The coating composition can contain between about 8 wt. % and about 30 wt. % of the binder component. In addition, the binder component can be provided in an amount of between about 13 wt. % and about 20 wt. %, and between about 15 wt. % and about 18 wt. %, and at about 17 wt. %. The weight percent is based upon a 100% solids component.

Exemplary binder components that can be incorporated into the coating composition include starches, proteins, water-soluble polymers, synthetic binders and mixtures thereof. Exemplary starches include corn starch, potato starch, wheat starch, tapioca starch, unmodified pearl starch, oxidized starch, ethylated starch, and cationic starch. Exemplary proteins include soy based proteins, and casein. Exemplary water soluble polymers include polyvinyl alcohol and alkali soluble emulsions. An exemplary alkali soluble emulsion includes solubilized polyacrylate polymer. Exemplary synthetic binders include styrene butadiene, polyvinyl acetate, styrene acrylate, and acrylates.

The coating composition includes water in an amount sufficient to provide the coating composition with desired flowability properties. That is, the coating composition should be sufficiently flowable to allow it to be applied to a paper substrate and form a continuous coating on the paper substrate. It is believed that an insufficient amount of water will create rheology problems that can lead to streaking. In addition, too much water can reduce coverage of the binder component and the pigment component on the paper substrate. It is expected that the water will be present in the coating composition in an amount of between about 35 wt. % and 60 wt. %, and can be present in an amount of between about 40 wt. % and about 55 wt. %, between about 43 wt. % and about 47 wt. %, and at about 45 wt. %. Alternatively expressed, it is believed that the coating composition can have a solids content of between about 40 wt. % and about 65 wt. %, where the “solids content” refers to the non-water components in the coating composition. The solids content can be between about 45 wt. % and about 60 wt. %, between about 53 wt. % and about 57 wt. %, and at about 55 wt. %.

Another way of characterizing the amounts of the binder component and the pigment component can be as parts by weight or as a weight ratio where the binder component and the pigment component are expressed on a dry, weight basis. In general, it is believed that the coating composition can include between about 25 parts to about 100 parts of the binder component per 100 parts of the pigment component. In addition, the weight ratio of the binder component to the pigment component can be between about 0.25:1 and about 1:1, and can be between about 0.35:1 and about 0.7:1.

Various components that can be provided in the coating composition including the pigment component, the binder component, and the various additives are identified in Lehtinen, Esa,Pigment Coating and Surface Sizing of Paper, Helsinki: Fapet Oy, 2000. The entire disclosure of this publication is incorporated herein by reference.

The coating composition can be applied to the paper substrate either online during the manufacture of the paper substrate or off-line after the manufacture of the paper substrate. It is advantageous that the coating composition can be applied to the paper substrate soon after the paper substrate is manufactured and without having to ship the paper substrate to a separate facility for application of the coating composition.

Now referring toFIG. 1, a process for the manufacturer of a paper product according to the invention is shown at reference numeral10. It should be understood that schematic diagram10is an exemplary schematic diagram and includes many of the operations carried out in commercial paper making facilities. The equipment used in a particular operation may very from facility to facility, but it is expected that the general operations will be present.

The starting material12generally includes wood pulp14. The wood pulp can include a blend of hard wood and soft wood fibers. The wood pulp can be provided as cellulose fiber from chemical pulped wood, and can include a blend from coniferous and deciduous trees. The wood pulp14can be processed through a refining operation16and through a cleaning operation18. The cleansed pulp20is then applied through a head box22onto a fourdrinier machine24to provide a paper base sheet26. Certain additives can be added prior to the head box22and can be referred to as “wet end chemistry.” Wet end additives can be provided for sizing, strength, water resistance, and/or oil resistance. Exemplary water resistance additives include rosin and alkylketene dimer (AKD). Exemplary oil resistance additives include fluorochemicals. Exemplary strength additives include urea formaldehyde and polyamide.

The paper base sheet26can be characterized as a non-woven web and can be considered continuous in the machine direction. The paper base sheet26can be processed through a wet press section28to remove water, and then through a drier section30to further reduce the water content and provide a paper substrate32. The paper substrate32can be dried to a moisture level of between about 0.5 wt. % to about 5 wt. %.

The paper substrate32is processed through a size press34for the application of a coating composition onto the paper substrate32to provide a coated paper product36. The size press34is provided as a metering film size press35. The coated paper product36is then dried in a second drier section38and calendered in a machine calender40to provide a calendered paper product42. AlthoughFIG. 1shows a supercalender46off line from the paper making and coating line, the supercalender46can be provided on line. The calendered paper product42is then taken up on a reel44. The calendered paper product42can then be processed by a supercalender46where the rolls are provided at a temperature of at least about 240° F. to provide a supercalendered paper product50. The supercalendered paper product50can be taken up on a reel52. In general, it is expected that the supercalender46will have rolls provided at a temperature of between about 240° F. and about 500° F. The rolls of the supercalender46can be provided at a temperature of between about 300° F. and about 500° F. It is expected that the rolls of the supercalender can be provided at a temperature of about 400° F. The supercalendered paper product50can then be sent to a winder for cutting and subsequent distribution.

The coating composition can be applied to the paper substrate having a weight of between about 20 lbs./3,000 ft.2and about 100 lbs./3,000 ft.2using the metering film size press to provide coat weights of between about 2 lbs./3,000 ft.2to about 4 lbs./3,000 ft.2the coat weight can be between about 2.5 lbs./3,000 ft.2and about 3.5 lbs./3,000 ft.2, and can be provided at about 3 lbs./3,000 ft.2. In contrast, many prior art coated paper products that exhibit undesirable levels of cracking at the fold are manufactured with coat weights of 5 to 10 lbs./3,000 ft.2.

It is believed that the combination of the coating composition, the low coat weight, and the finishing step of calendering at a roll temperature of between about 240° F. and about 500° F. improves the flex cracking resistance of the resulting coated paper product. By calendering at a higher temperature than conventional machine calendering, and by providing a coating composition according to the invention, it is possible to provide a lower coat weight or a thinner coating layer on the paper substrate. It is believed that the coating composition, when calendered at a roll temperature of at least about 240° F., will provide a more extensible coating that stretches instead of cracking. In addition, it is believed that the surface will be more of a hybrid surface, and that the fibers in the paper substrate will act to reinforce the surface.

It is expected that supercalendering the coating composition will improve flex crack resistance by allowing the manufacturer to produce a coated paper product having a coat weight that is lower than traditional coated paper products. By reducing the coat weight while retaining a desired print surface, it is expected that flex crack resistance will improve. In addition, it is believed that supercalendering the coated paper product provides a better distribution of the coating layer across a paper substrate and provides better binding of the pigment to the paper fibers. As a result of better binding, it is believed that there will be a lesser tendency of the pigments to break away from the fibers when subjected to folding.

It is expected that supercalendering can cause the paper substrate to become thinner, and that a thinner paper substrate will be more desirable because flex cracking will be less likely to occur while the paper substrate retains its physical properties. In addition, it is expected that a thinner paper product having desired physical properties will be a more desirable product than another paper product having the same physical strength properties but being thicker.

A supercalender operation is generally considered to be different from a machine calendering operation. During machine calendering, the pressure between the nips is generally less than about 1,000 pli (pound per lineal inch) and is commonly between about 200 pli and 400 pli. Supercalendering involves nip pressures between about 1,000 pli and about 4,000 pli. It is expected that the paper product according to the invention can be achieved at nip pressures of up to 2,500 pli. In addition, a supercalendering operation generally includes alternating soft rolls and steel rolls wherein the steel rolls are often provided at a temperature in excess of 240° F. As shown inFIG. 1, the exemplary supercalender46includes top and bottom steel rolls60and62, and alternating soft rolls64and steel rolls66. The coated paper product70can be threaded through the rolls in any desired configuration by use of the carrier rolls72.

The resulting coated paper product can be characterized as exhibiting a gloss of at least about 20 according to TAPPI T480 om-92, a Print-surf roughness of less than about 2.5 according to TAPPI T-555 pm-94, and a density of at least 12 according to TAPPI T410 om-93 and TAPPI T411 om-89. The coated paper product can be characterized as exhibiting a gloss of between about 20 and about 75, a Print-surf roughness of between about 0.5 and about 2.5, and a density of between about 12 and about 18.