Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H19/00—Coated paper; Coating material
  • D21H19/80—Paper comprising more than one coating
  • D21H19/82—Paper comprising more than one coating superposed

Definitions

• HSWOS heatset web offset
• Pantone high-light or special color
• Higher quality sheet offset litho printing normally has at least six or more printing units in series.
• the offset printing process especially in perfecting presses, has the capability of simultaneously printing both sides of a paper web at speeds as high as 3000 fpm (ft./min.).
• the "offset printing process” gets its name from the fact that "images” are formed on lithography plates then transferred (offset) to a rubberized printing blanket stretched around a cylinder.
• the inked part of the lithography plate that is also stretched around a cylinder forms the image that gets transferred to the offset blanket then in turn to the paper.
• the non-image areas of the lithographic plate are hydrophilic and are protected against ink adherence by fountain solution (water, gum arabic, surfactant, and acid).
• fountain solution water, gum arabic, surfactant, and acid
• the image is offset from the top lithography plate to the paper by the offset blanket.
• an identical unit "perfects" or contacts the moving paper web from the bottom thus printing both sides simultaneously.
• Printers desire to reduce or eliminate back trap mottle so uniform ink densities result whether they are "solids”, “quarter tones”, “half-tone” or “ 3 /4-tones” or any ink density range in between or transition points within an image.
• Figure 2 is a graph of 2 nd Cyan Tobias mottle versus basecoat solids for various preferred paper or paperboard materials of this invention.
• Figure 4 is a graph of 2 nd Cyan Tobias mottle versus 2 nd Cyan scanner mottle.
• the material exhibits superior 2nd Cyan scanner mottle.
• Scanner mottle is determined using the following procedure: Representative samples are selected from pigment coated paper or paperboard printed under controlled conditions typical of commercial offset litho production with the cyan process ink at a reflection density of 1.35 ⁇ 0.05. A 100 percent solid cyan print reflective image is digitally scanned and transformed through a neural network model to produce a print mottle index number between zero (perfectly uniform ink lay with no mottle) to ten (visually noticeable, objectionable and likely rejectable because of print mottle, a random non-uniformity in the visual reflective density or color of the printed area). Data from this 2 nd Cyan scanner mottle system can be correlated to subjective visual perception (using the zero-to-ten guideline) or can be transformed into equivalent mottle values as measured with a Tobias mottle tester from Tobias Associates using the following equation:
• the 2 nd cyan scanner mottle is less than about 6, preferably less than about 5, more preferably less than about 4 and most preferably from about 2 to about 3.
• the 2 n cyan scanner mottle is preferably 20% lower and the 2 nd Cyan Tobias mottle is preferably 5% lower
• the 2 nd cyan scanner mottle is more preferably 40% lower and the 2 nd Cyan Tobias mottle is more preferably 10% lower
• the 2 nd cyan scanner mottle is most preferably 60% lower and the 2 nd Cyan Tobias mottle is most preferably 15% lower than that of a similar material having the same characteristics except for the presence of the low density thermoplastic particles in the basecoat.
• Coated material 10 preferably has a smoothness of less than 2 as measured using TAPPI test method for Parker Print Surface: T 555 om-99.
• the coated paper has Parker Print Surface preferably less than about 1.5.
• the Parker Print Surface is more preferably less than about 1.3 and most preferably less than about 1.2.
• the Parker Print Surface is more preferably less than about 1.
• the Parker Print Surface is preferably 10% lower, more preferably 20% lower and most preferably 30% lower than the Parker Print Surface of a similar material having the same characteristics except for the presence of the low density thermoplastic particles in the basecoat.
• Coated material 10 preferably has a Sheffield smoothness of less than about 25 as measured by the procedure of TAPPI test method T5380m-l.
• the coated paper has Sheffield smoothness preferably less than about 20.
• the Sheffield smoothness is more preferably less than about 15 and most preferably less than about 12.
• the Sheffield smoothness is preferably 10% lower, more preferably 20% lower and most preferably 30% lower than the Sheffield smoothness of a similar material having the same characteristics except for the presence of the low density thermoplastic particles in the basecoat.
• As one essential component material 10 comprises a paper or paperboard substrate 12.
• any conventional paper or paperboard web having a wide variety of porosities, basis weights, densities, calipers and the like can be used to make substrate 12.
• Such webs and methods and apparatus for their manufacture are well known in the art. See for example "Handbook For Pulp & Paper Technologies", 2 nd Edition, G. A. Smook, Angus Wilde Publications (1992) and references cited therein.
• the paper and paperboard substrate can be made from pulp fibers derived from hardwood trees, softwood trees, or a combination of hardwood and softwood trees prepared for use in a papermaking furnish by any known suitable digestion, refining, and bleaching operations as for example known mechanical, thermo mechanical, chemical and semi chemical, etc., pulping and other well known pulping processes.
• the substrate may also include other conventional additives such as, for example, starch, mineral and polymeric fillers, sizing agents, retention aids, and strengthening polymers.
• fillers that may be used are organic and inorganic pigments such as, by way of example, minerals such as calcium carbonate, kaolin, and talc and expanded and expandable microspheres.
• Other conventional additives include, but are not restricted to, wet strength resins, internal sizes, dry strength resins, alum, fillers, pigments and dyes.
• material 10 comprises a basecoat 14 on a surface of substrate 12.
• Basecoat 14 comprises low density thermoplastic particles dispersed in a polymeric binder.
• low density thermoplastic particles are particles formed from thermoplastic or elastic polymers having a density of less than 1.2 Kg/Liter in a dry state including the void air volume. The density is preferably less than 0.8 Kg/Liter, more preferably less than 0.6 Kg/Liter and most preferably from about 0.3 Kg/Liter to about 0.6 Kg/Liter.
• the low density thermoplastic particles preferably are not expandable and more preferably have a diameter less than about 3 microns, more preferably less than about 2 micron and most preferably from about 0.6 to about 1.5 microns.
• thermoplastic particles make the basecoat more compressible and enhances the beneficial properties of the material 10. Improved properties include reduced 2 nd cyan scanner mottle, enhanced sheet and print gloss and/or enhanced Sheffield and Parker Print smoothness as compared a similar material having the same characteristics except for the presences of the low density thermoplastic particles in the basecoat.
• Low density thermoplastic particles that can be used may vary widely and include, but are not limited to, hollow polymer plastic pigments and binders having a particle size that is at least about 175nm. Examples of these are ROPAQUE® HP1055 and AF1353 from Rohm and Haas and the HS 2000NA and HS 3000NA plastic pigments from Dow Chemical Company.
• the amount of low density thermoplastic particles present in the basecoat may vary widely but is preferably in an amount less than about 30% by weight of the basecoat composition. More preferably, they are present in an amount from about 1 to about 15 % by weight of the basecoat composition most preferably in amount from about 2 to about 10% by weight of the basecoat composition and in amount from about 3 to about 7% by weight of the basecoat composition in the embodiments of choice.
• basecoat 14 includes one or more polymeric binders.
• polymeric binders include those which are conventionally used in coated papers as for example styrene butadiene rubber latex, styrene acrylate, polyvinyl alcohol and copolymers, polyvinyl acetates and copolymers, vinyl acetate copolymers, carboxylated SBR latex, styrene acrylate copolymers, styrene/butadiene/acrylonitrile, styrene/butadiene/acrylate/acrylonitrile polyvinyl pyrrolidone and copolymers, polyethylene oxide, poly (2-ethyl-2-oxazoline, polyester resins, gelatins, casein, alginate, cellulose derivatives, acrylic vinyl polymers, soy protein polymer, hydroxymethyl cellulose, hydroxypropyl cellulose, starches, ethoxylated, oxidized and enzyme converted starches, cationic starches,
• Preferred polymeric binders are carboxylated SBR latexes, polyvinyl alcohol, polyvinyl acetate, styrene/acrylonitrile copolymer, styrene/butadiene copolymer, styrene/acrylate copolymer, and vinyl acetate polymers and copolymers.
• Binder latex particles having a sufficient particle size also provide an initial bulking when included with inorganic or organic bulking pigments.
• Latex particles in general have a particle size from about 100 to about 300 nm for paper coating applications.
• Latex particles having sufficient size to provide compressibility generally have a particle size that is at least 175 nm.
• the size of the latex that provides compressibility is directly proportional to the average size of the inorganic and organic pigments used in basecoats.
• a source of ground calcium carbonate (GCC) used in paperboard basecoats is HYDROCARB® 60 (from OMYA). This ground calcium carbonate is a wet ball milled product having 60% of its particles less than 2 microns.
• additional pigment or fillers are employed to improve the properties of the coated paper and paperboard.
• additional pigments may vary widely and include those inorganic pigments typically used in the coated paper and paperboard such as silica, clay, calcium sulfate, calcium silicate, activated clay, diatomaceous earth, magnesium silicate, magnesium oxide, magnesium carbonate and aluminum hydroxide.
• inorganic particles such as precipitated calcium carbonate having bulky structures such as a rosette crystal can also be included.
• inorganic pigments having a rosette or other bulky structure can be included in the basecoat to make the basecoat have greater initial bulk or thickness.
• the rosette structure provides greater coating thickness, thus improved coating coverage for a given coat weight. This allows for the dried coating to more easily move in the Z- direction when compressed by the hot soft gloss calenders on coated SBS paperboard machines, and thus to form a level coated surface with a reduced number of low spots.
• Preferred inorganic pigments include, but are not limited to, precipitated calcium carbonate, mechanically or chemically engineered clays, calcined clays, and other pigment types that function to lower the average density of the coating when dry. These pigments do not provide compressibility to dried basecoats.
• Coat weight of the basecoat can vary widely and any conventional coat can be used.
• Basecoats are generally applied to paper substrates in an amount from about 4 to about 20gms.
• the coat weight of the basecoat is preferably from about 6 to about 18gms and more preferably from about 7 to about 15gms.
• the thickness of the basecoat can vary widely and any thickness can be used. Generally, the thickness of the basecoat is from about 1.8 to about 9.0 ⁇ m at a minimum, which is figured on the average density and weight ratio of each component in a coating.
• the thickness of the basecoat is preferably from about 2.7 to about 8.1 ⁇ m and more preferably from about 3.2 to about 6.8 ⁇ m.
• the average thickness when applied to an impervious surface would be significantly greater than the theoretical values given here.
• the coating thickness at the rough high spots in the paper may be as low as 2-3 microns while valleys between large surface fiber may have coating thickness as great as 10+ microns. Stiff blade metering of the basecoat attempts to provide a level surface to which a very uniform topcoat is applied.
• topcoat 16 comprises one or more inorganic pigments dispersed in one or more polymeric binders.
• Polymeric binders and inorganic pigments are those typically used in coatings of coated paper and paperboard.
• Illustrative of useful pigments and binders are those used in basecoat 14.
• Coat weight of topcoat 16 can vary widely and any conventional coat can be used.
• Topcoat 16 is generally applied to paper substrates in amount from about 4 to about 20gms.
• the coat weight of the basecoat is preferably from about 6 to about 18gms and more preferably from about 7 to about 15gms.
• the thickness of topcoat 16 can vary widely and any thickness can be used. Generally, the thickness of the basecoat is from about 1.8 to about 9.0 ⁇ m at a minimum, which is figured on the average density and weight ratio of each component in a coating.
• the thickness of the basecoat is preferably from about 2.7 to about 8.1 ⁇ m and more preferably from about 3.2 to about 6.8 ⁇ m at a minimum, which is figured on the average density and weight ratio of each component in a coating.
• the point at which the void volume is filled by binder and additives among all pigments is referred to as the "critical void volume". In the paint industry this point is referred to as the transition from matte to gloss paints.
• the paper or paperboard of this invention can be prepared using known conventional techniques. Methods and apparatuses for forming and applying a coating formulation to a paper substrate are well known in the paper and paperboard art. See for example, G.A. Smook referenced above and references cited therein all of which is hereby incorporated by reference. AU such known methods can be used in the practice of this invention and will not be described in detail.
• the mixture of essential pigments, polymeric or copolymeric binders and optional components can be dissolved or dispersed in an appropriate liquid medium, preferably water.
• the percent solids of the top and basecoat coating formulation can vary widely and conventional percent solids are used.
• the percent solids of the basecoat coating formulation is preferably from about 46% to 69 % because within range excellent scanner mottle characteristics are exhibited by the material with increased drying demands.
• the percent solids in the basecoat coating formulation is more preferably from about 57 to 69% and is most preferably from about 60% to about 68%.
• the percent solids in the basecoat coating formulation in the embodiments of choice is from about 63% to 67%.
• the paper or paperboard web may be calendered to improve the smoothness and other properties of the paper as for example by passing the coated paper through a nip formed by a calender.
• Gloss calenders chromed steel against a rubber roll
• hot soft gloss calenders chromed steel against a composite polymeric surface
• the amount of heat and pressure needed in these calenders depends on the speed of the web entering the nip, the roll sizes, roll composition and hardness, specific load, the topcoat and basecoat weights, the roughness of the under lying rough paperboard, the binder strength of the coatings, and the roughness of the pigments present in the coating.
• topcoats contain very fine particle size clays and ground or precipitate calcium carbonate, binder, rheology aids, and other additives.
• hot soft calenders are 1 m and greater in diameter and are heated internally with very hot heat transfer fluids.
• the diameter of the heated steel roll is directly dependent on the width of the paper machine.
• a wider paper machine of 400" as compared to 300" or 250" wide machines requires much larger diameter rolls so that the weight of the roll does not cause sagging of the roll in the center.
• Hydraulically, internally loaded, heated rolls that are crown compensating are used. Surface temperatures typically used range from 100 to 200°C. The preferable range is 130°C to 185 0 C with nip loads between 20 kN/m and 300 kN/m.
• coated paper or paperboard of the present invention can be used for conventional purposes.
• specific uses of the paper and paperboard include, but are not limited to use in the formation of Bristol, folding carton, aseptic paperboard, double coated free sheet, and any other type of product made from coated paper or paper board.
• Preformed aqueous slurries of the pigments and low density thermoplastic particles are added to a high shear mixer. Then dispersant, binder and viscosity modifier are added to the slurry under shear to form a coating formulation having the desired Brookfield viscosity.
• the viscosity of the basecoat coating formulation is about 1000 centipoises (cps) spindle 4 at 100 revolutions per minute.
• the viscosity of the topcoat coating formulation is about 700 to 800 centipoises (cps) spindle 4 at 100 revolutions per minute. After final mixing, the coatings are ready for casting.
• Controls were made using the same basecoat and topcoat formulations, except that there was no hollow pigment in the basecoat.
• the coating formulations are set forth in the following Table 1.
• the operating temperature of the metal roll surface was 185 0 C; nip load was varied, but sufficient to reach 60% sheet gloss (TAPPI Method, T480, 75° angle meter).
• Substrate 1 identified in Table 3 below required 75 fcN/m to obtain a 60% sheet gloss while Substrate 2 identified in Table 3 below only required 65 fcN/m to reach 60% sheet gloss.
• Substrate 6 required a calendering pressure of 50kN/m to obtain a 60% sheet gloss while
• Substrate 7 required a calendering pressure of 41kN/m to reach 60% sheet gloss.
• topcoat and base coat of the coated paperboard substrates are set forth in the following Table 3.
• the coated substrates identified in Table 3 were evaluated to determine their scanner mottle using a unit 2 cyan solid print from a 6-color offset press.
• the scanner mottle results were transformed into equivalent mottle values as measured with a Tobias mottle tester from Tobias Associates using the following equation:
• Example 2 Using the procedure of Example 1, coating formulations were prepared. The coating formulations used are set forth in the following Table 4. Table 4
• the coated substrates identified in Table 5 were evaluated to determine their 2 cyan solid print gloss at 60° from a 6-color offset printing press as measured by the procedure of TAPPI test method T480, sheet gloss at 75° as measured by the procedure of TAPPI test method T480, sheet gloss Parker Print Surface as measured by the procedure of TAPPI test method: T555 and Sheffield smoothness as measured by the procedure of TAPPI test method T538.
• Table 6 Table 6

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