WO2000022232A1

WO2000022232A1 - Colorant application on the wet end of a paper machine

Info

Publication number: WO2000022232A1
Application number: PCT/US1999/023190
Authority: WO
Inventors: Larry O. Hill; John D. Benson; Eric R. Wagner; Mark A. Johnson; Mahendra Mehta
Original assignee: The Mead Corporation
Priority date: 1998-10-14
Filing date: 1999-10-05
Publication date: 2000-04-20
Also published as: EP1194647A1; AU1442600A

Abstract

A method for preparing a colored paper which comprises: flocculating a dispersion of pigment in the absence of fiber to obtain a slurry of the flocculated pigment, combining the slurry of flocculated pigment with paper fibers, and forming a sheet from the flocculated pigment and fiber on the wet end of a paper machine.

Description

COLORANT APPLICATION ON THE WET END OF A PAPER MACHINE

Field of the Invention

The invention relates to an improved process for manufacturing colored papers which provides greater process control over color values. The process is particularly useful in manufacturing saturating grade papers such as colored decor sheets useful in making decorative laminates.

Background

Conventionally decor laminating papers are made by adding dispersed colored pigments to a paper furnish and flocculating the pigments to the fiber using alum or a similar flocculant at or prior to the headbox at the wet end of the paper machine. It is very difficult to control the color of the paper using this process because a host of different variables influence the color of the sheet. Among the variables affecting the color of the paper are the nature and quality of the pulp, the sequence of pigment addition, run to run differences in wet end chemical levels on the paper machine, the sensitivity of pigment mixtures to alum, the difference from one pigment to another in alum demand and others. These variables affect the pigment and fiber interaction which is largely ionic. Certain additives or pigments may reduce the ionic character of the fiber or occupy binding sites that would otherwise be occupied by pigment thus the selection and order of addition of such additives influences the manner in which the pigment will bind to the fiber. In practice, several trial runs are required before a paper which matches the desired color is obtained. For each trial, pigment compositions must be prepared, the compositions must be flocculated to the fiber under conditions that must be carefully controlled, and paper samples must be made and analyzed. With this procedure it is very difficult to make small runs economically. In running the required trials, the paper machine is not used productively and large quantities of pigment and fiber end up as waste or broke because the pigment must be flocculated to the fiber to obtain a reliable indication of what the final color of the paper will be.

When making colored papers other than saturating grade papers, color coatings are sometimes applied to the paper off the paper machine, i.e., after the paper has been formed and dried such as during printing. Alternatively, the paper is dyed (for a discussion of dying papers see Koop, 1, Dyeing of Paper, Bayer Faben, Rev. No. 4/2 (1985)). The methods typically used in preparing and mixing pigments for off machine application to dry paper are not considered effective for applying pigment at the wet end of a paper machine or for making saturating grade papers. On the wet end of a paper machine the pore structure of the fiber mat is very open as the fibers have not been fully consolidated at this point in the papermaking process and the dewatering process is still occurring. Conventional off machine pigment application techniques such as roll or curtain coating, when used at the wet end, result in poor and selective retention and non-uniform distribution or stratification of pigments. In some cases large pigment particles like iron oxides will tend to be retained at the surface of the sheet, while the smaller organic pigment particles will tend to be deposited deeper into the sheet. This is troublesome because many paper colors are created using a mixture of pigment colorants of different types having a gamut of sizes. Color consistency and the ability to color match are very poor when colorant pigments segregate within the fiber network of the paper. The hue of the felt side of the paper is dominated by the larger colorants as they are retained nearest to the felt surface, while the hue of the wire side is controlled by the small sized colorants as they are carried deeper into the sheet. This separation of colorants by particle size and charge within a sheet prevents one from obtaining a uniformly colored paper.

Another reason off machine coating techniques are not generally used in making decor sheets for laminating is because they tend to seal the pores of the sheet and thereby reduces the amount of resin that the sheet can absorb. Accordingly, there is a need for a process for manufacturing colored papers which provides an open sheet suitable for saturation, better and more predictable control over the color of the paper, improved pigment efficiency by improving retention and placing pigment on the surface of the sheet, reduced stratification, less color variation due to sensitivity to wet end chemicals, less sensitivity to process mechanics such as dilution rate, temperature and pH changes, less run to run and within run variation.

Summary of the Invention

The present invention relates to methods for manufacturing colored papers and particularly to colored papers which are useful as decor sheets in forming decorative laminates. The invention also relates to papers prepared by these methods. In accordance with the present invention, instead of flocculating the pigment to fiber as is the conventional wet end practice, the pigment is flocculated with itself or with other pigments or with a spacer in the absence of the fiber, combined with the fiber, and formed into a sheet at the wet end of the paper machine. This improves the retention, efficiency, and uniformity of distribution of pigments applied at the wet end of the paper machine while maintaining the open porous properties of the paper. By preparing structured pigments in this manner and combining the pre-flocculated pigment to the fiber instead of flocculating the pigment to the fiber, the color of the paper is easier to control, fewer trial runs are required, there is less paper machine downtime and because the pigment is not committed to the fiber there is less waste. Furthermore, when conventional color matching algorithms are used to predict the paper color, they more closely predict the paper color. A principal use of colored paper made in accordance with the invention is in decorative laminates of the type used in counter tops, table tops and furniture.

In one embodiment of the invention, a coating mixture of colored pigments is flocked together, mixed with a thickener and a surfactant, and applied to a fibrous web on the wet end of a paper machine using a curtain or slot coater. In a second embodiment, the pigment is pre-flocculated, it is added to fiber (but it is not flocculated to the fiber), for example, by feeding a slurry of the pre-flocculated pigment continuously to a pulp stream that feeds the primary head box, and the mixture of pre-flocculated pigment and fiber is formed into a sheet on the wet end of a paper machine.

The term "pre-flocculated" refers to flocculating the pigment prior to adding the pigment to the fiber. In the third embodiment, pre-flocculated colored pigment slurries are mixed with fiber in batch process and the mixture of pigment and fiber is formed into a sheet on a paper machine. In still another embodiment, a slurry of a pre-flocculated colored pigment is mixed with fiber and applied to the paper machine from a secondary head box.

The term "wet end" as used herein refers to any location on the paper machine in the paper manufacturing process prior to the dryer can and particularly includes the addition of the flocculated pigment to the pulp stream feeding the head box, to the pulp in the fan box, in the beaters, or in a storage chest, addition of the pre-flocked pigment to the white water and applying a mixture of the flocked pigment and fiber as a surface coating by means of a secondary headbox or a slot orifice coater such as a curtain coater prior to the dryer can, e.g., between the dandy roll and the wet press, as well as application of the pigment at a combination of these points prior to the dryer can, for example addition at the beaters and by application from a curtain coater.

The term "saturating paper" is used herein as it is used in the art, namely to refer to a paper having a porosity, resin saturation time, and resin capacity which make it desirable for use in a variety of applications including, but not limited to, making decorative laminates. Maxim. N. F., Decorative Laminating Paper, TAPPI Plastic Laminates, p. 15-18, 1985, describes saturating papers. In general these papers can be characterized as papers having a basis weight or about 40 to 90 lbs./3000 sq. ft., a Gurley porosity of about 1 seconds to 300 seconds, extract pH of about 3 to 10, a resin saturation rate of 1 second to 5 minutes, and resin pickup of about 20 to 80% of the weight of the saturated paper. The properties will vary with the application for the paper and the saturating resin that is used. In the most typical embodiments of the invention, the paper exhibits a Gurley porosity of about 1 to 100 seconds, a resin saturation time of about 1 to 100 seconds and a resin capacity of about 30 to 60% by weight of the saturated paper. Included in these papers are print base and low pressure grades having porosity of 40 to 300 seconds, and higher pressure grades, having a porosity of 3 to 30 seconds.

The term "slot coater or slot orifice coater" as used herein includes coaters in which the coating passes through an orifice and forms a curtain which falls on the web and coaters in which the coating is extruded through a slot where it forms a bead which contacts the web.

One process in accordance with the invention comprises: flocculating a mixture of pigments or a mixture of one or more pigments and a spacer into a structured macroparticle or cluster, optionally milling them to smaller pigment clusters to control color and runnabihty, mixing the structured pigments with the paper fiber and forming the mixture of fiber and flocked pigments into a sheet on the wet end of a paper machine. In another process the pre-flocculated pigment is mixed with a thickener and a surfactant and applied to the web using a slot orifice coater.

Detailed Description of the Invention

Flocculation Step

The flocculation step is conducted in the absence of paper fiber. A stabilized dispersion of blended pigments (and preferably spacers) forms large flocks or clusters of mixed pigment particles. Light scattering and absorption of colored pigments are particularly sensitive to this flocculation step. It is important to maintain spacing between the individual particles of the same color or index of light refraction in the cluster to insure good color strength and opacity. For this reason, the pigments are preferably flocked to a spacer to form a structured pigment having a core/shell structure as described in more detail later in accordance with a preferred embodiment of the invention. Prior to the flocculation step, the colored pigments are blended together to form a well mixed slurry. The pigments typically have a particle size in the range of about 0.01 to 1.0 microns. Since most of the particles at this stage of the process are well dispersed, low to moderate shear is sufficient for this step. Mixing equipment can range from a low speed mixer to an in-line shear unit. Increasing the shear is necessary if an undispersed pigment is added to the slurry of dispersed pigments. To flocculate the pigment, the pigment is dispersed in water in an amount of about 10 to 70% by and preferably about 15 to 35% by using a mixer such as a Cowles blade mixer or a Silverson rotor-stator mixer. Normally the pigments are supplied commercially as dispersions and have been treated with a dispersing agent.

To flock the dispersion, a flocculating agent is added under moderate shear. This destroys the pigment dispersions by ionically interacting with the dispersing agent, thus allowing the pigments to combine in large flocks. The ideal structure is a pigment cluster in which individual particles of the same materials are always separated by particles of a different color, but this ideal condition does not always occur. Particles of like material may be adjacent to each other in the cluster, reducing color strength and color efficiency. Thus, while in theory, a pigment can be flocked with itself, this is not an efficient use of the pigment which is one of the more expensive additives used in papermaking. Accordingly, more typically a mixture of pigments containing a spacer such as TiO₂ or MgAl silicate is flocculated. This maintains spacing among the individual color particles and improves color strength and opacity. The use of these spacers improves the efficiency of the pigment by providing a substrate upon which the pigment particles can be spaced such that more of the pigment particles are present on a surface from which light can be reflected. By separating the pigment particles with a transparent spacer in this manner, the pigment contributes more color through absorbance. By "transparent spacer" is meant a spacer which appears transparent in the laminate, e.g., it has an index of retraction which approximates that of the saturating resin. Still more preferably the flocculated pigment exhibits a core/shell construction in which a shell of colored pigment particles is flocculated around one or more spacer particles. The amount of pigment and spacers in any mixture will depend on the color that is described but typically the spacer will make up about 50 to 99% of the pigment mixture based on the total weight [pigment and spacer] in the blend. In accordance with the invention, the pigment is flocked into a structured particle having a size of about 1 to 40 microns and more typically 1 to 10 microns. Pigment particle size is important as described above. Distribution of particle size affects color brilliance. The narrower the distribution around the minimum size needed for retention the greater color efficiency that can be obtained. The narrow distribution improves the efficiency and uniformity of distribution of the pigments.

To achieve the core/shell construction, a sequential addition of spacers followed by pigments in the flocculation process can be used. However, because the flocculent demand of spacers tends to be much lower than the flocculant demand of pigment particles, upon mixing spacers and pigment particles and adding a flocculating agent, the major portion of the pigment particles will flocculate to the surface of the inorganic spacer particles providing the core/shell construction. Consequently, sequential addition of pigments can be used but is not essential in all cases to provide the desired core/shell structure.

The flocculent demand for some typical pigments and TiO₂ spacer is shown in the following Table.

Table 1

Flocculent Demand

Measured Demand Pigment (% Flocculant on dry pigment)

Pigment Blue 15 8.2

Pigment Violet 19 10.3

Pigment Yellow 155 9.3

TiO₂ 0.2 In flocculating the colored organic pigment with the inorganic spacer, the amount of colored organic pigment and inorganic spacer is preferably adjusted to promote formation of the core/shell structure. If too much inorganic spacer is added, the paper may have a chalky appearance. If too much organic pigment is present, the organic pigment may flock to itself resulting in areas of high color concentration, i.e., spotting and poor utilization of the pigment. Preferably about 50 to 99 parts spacer particles and about 1 to 50 parts colored pigment particles are used in the flocked pigment. More preferably, about 5 to 100 parts colored pigment particles are used per 100 parts spacer particles. The spacer particles preferably have a particle size of about 0.1 to 5 microns. The colorant pigment particles will generally be organic and obtained in particle sizes of 0.01 to 0.1 microns.

Substantially any pigment or spacer that has previously been used in making structured pigments or colored papers should be useful in practicing the invention. Examples of pigment particles that can be used in the invention include the organic colorants described in Herbst, Willy and Hunger, Klaus, Industrial Organic Pigments. VCH (a Wiley Company), 2d Ed., 1997, and include organic pigments such as azo pigments, naphthol pigments, metal complex pigments, benzimidazolone pigments, disazo condensation pigments, azo pigment lakes, isoindolinone pigments, phathocyanine pigments, quinacridone pigments, perylene and perenne pigments, diketopyrrol pigments, thioindigo pigments, and anthraquinone pigments such as flavanthrone pigments and anthanthrone pigments. Inorganic colored pigments are also useful in the invention. Examples of inorganic pigments are disclosed in Endriss, Hartmuth, Inorganic Colored Pigments Today, Vincentz Verlag, 1998 and include red, yellow , and black iron oxides, lead chromates, chromium oxides, ultramarine blue, violet, and red, rutile nickel, manganese or chromium oxide, nickel, cobalt, chromium and manganese spinel, iron blue, cadmium sulfides, bismuth vanadates and cerium sulfides.

Representative examples of spacers are titanium dioxide, clay (e.g., Ansilex), talc, ethyl cellulose, urethane pigments, insoluble starch, calcium carbonate, zirconium carbonate, aluminum oxide, iron oxide, bentonite, kaolin, aluminum hydroxide, magnesium oxide, silica (e.g., Sansil), diatomaceous earth, amorphous sodium magnesium aluminosilicate (e.g., Hydrex) organic microcapsules (e.g., of the type used in carbonless paper applications), polymeric hollow microspheres (e.g., of the type manufactured by Rohm and Haas under the trademark ROPAQUE), inorganic microcapsules, potassium titanates, zinc sulfate, barium sulfate, and urea- formaldehyde pigment.

A flocculating agent is typically added to the mixture of dispersed pigments at room temperature. The flocculant is preferably added at one time. The speed of the mixer is adjusted to avoid foaming. Generally, the flocculants are used at about 1 to 8% by weight based on oven dry pigment weight. However, the amount of flocculant needed to create a cluster or agglomerate depends on the surface area and charge of the particles, the amount and charge density of the dispersant and flocculant used. For example, because of the small size and large surface area of organic pigments, they are typically dispersed with a strong dispersant and thus take a larger amount of flocculant than titanium dioxide. Each pigment may be supplied with different types and amounts of dispersants.

Alum and cationic polymers are often used as flocculants but other flocculants can be used including polyethyleneimine (PEI) special polymers such as Polymin 971 from BASF; polyethyleneimine linear polymer such as Polymin 282 L form BASF; quaternary polyvinlypyrolidone, poylvinylimidazole, polyvinylcaprolactam, polyvinylacetate co- polymres such as Luviquat MS 370 from BASF; quaternary trialkyl amine polymers such as Luviquat PZ-11 from BASF; alum and aluminum chlorohydrate polymer; water soluble salts of di- and tri-valent metals such as calcium, magnesium; chromium, aluminum, manganese, iron, cadmium, zinc, etc.; poly-sulfonium salts and poly-phosphonium salts such as poly(2- methyl-2-butylsullfonium) chloride and poly(diallyldiphenylphosphonium) bromide; poly- DMDNAC (dimethyldiallylammonium chloride) polymers such as Cat-Floe from ECC; polyacrylamides (PAMs) such as Hydraid from ECC; polyethylene oxides (PEOs); use of these flocculants can be enhanced by soluble arylsulfonate or lignosulfonate anionic salts such as Stylus from ECC (modified kraft lignin); polyacrylic acids (PAAs) such as K- 130 from Rhom and Haas; cationic pyridine polymers such as poly-l,2-dimethyl-5-vinyl- pyridine-methylsulfate (PPS); cationic starches such as Catostarch from National Starch; cationic modified lignins such as those made by reacting aminoalkyl epoxides or organic amines and formaldehyde with lignin; wet strength amino polymers with epichlorohydrin such as Kymene from Hercules; oxonium polymeric salts such as poly(pyridinium tetrafluoroborate); ionene polymers (methylated polyethylene polyamines) such as poly(3- ionene) chloride; oxonium salts can be grafted to ionene polymers to create conductive flocculants that serve two functions - retention of color and render the laminate useful for anti-static surfaces such as are required in rooms operating sensitive medical instruments. Other flocculants are described in Xuehui, S: Novel Polyimide Ionene: Synthesis and Characterization of Polymides Containing Aromatic Bipyridinium Salt. Polymer 38(18):4737-4741 1997 and Katritzky, AR, Polymers bv Reaction of BisCPyrylium Salts) with Diamines: A Novel Approach to Ionene Polymers. JPolym Sci Part A 26(12):3323- 3326. 1988.

Milling of Pigment Flocks:

After the flocculation step, the pigments are in a flocked state in which they may have a very broad particle size distribution. In order to develop the most consistent color and pigment retention on the paper machine's wet end, the pigment flocks are preferably milled to a uniform size. Milling breaks up oversized pigments and forces undersized pigments to come into contact with one another and thereby cluster and form a flocked pigment having a more uniform average particle size. This reduction in the pre-flocked pigment size is accomplished by controlling the shear and solids. After shearing, the particles remain grouped in small clusters of the various materials. The clusters are much larger than the particle size of the original dispersed pigments. The milling of particle clusters is used to optimize the final pigment cluster for retention and uniformity, while the flocculation step is used to optimize the pigments for efficiency in opacity and color. In some cases the pigment size distribution after flocculation may be acceptable and it may not be necessary to mill the pigment clusters. Milling can be accomplished by passing a slurry of the previously flocked pigment through a refiner. Typical milling equipment is a rotor-stator mixer that forces flocked pigment particles through a low-clearance shear zone. Typically, particles resulting from the shear process range from about 2 to 8 microns in diameter. Particle size distribution is important for runnabihty through the slot gap in the curtain coater head and for uniformity of retention on the wet web. As a result of these requirements a desirable particle size range is typically greater than 1 micron and less than 10 microns.

Cluster Size Stabilization and Viscosity Control

In order to prevent the pigment clusters from reforming into large flocks or settling and insure a homogeneous mixture, a thickener may be added. A thickener is especially recommended to control the viscosity of the coating when the pigment is applied at a secondary head box or slot coater. Coating viscosity is an important property in that it assists in achieving a uniform coating by controlling the penetration of the coating into the loosely formed fibrous mat. In addition, coating viscosity is an important parameter in optimizing the operation of coater application system. Colorants can tend to streak and lump when applied at the wet end of the paper machine when applied by curtain coating. After coating, water still needs to be removed from the fiber mat on the paper machine wire with vacuum. Thus coating viscosity is used to control color and pigment retention.

Examples of thickeners are caboxymethylcellulose, microcrystalline cellulose, sodium alginate, hydroxypropyl cellulose, polyvinyl alcohol and starch. Other examples include: polycarboxylates (e.g., copolymers of maleic anhydride and ethylene or methylene vinyl ether,poly(acrylic) acids, hydrolyzed poly(acrylonitrile)s, hydrolyzed poly(acrylamide)s, alkali swellable latexes); cellulosic thiceners (e.g., methyl-, ethyl-, hydroxyethyl- and hydroxypropyl-cellulose, sodium carboxymethyl cellulose, natural gums such as guar, arabic, tragacanth, algin, agar, pectin hemicellulose, xanthan, gellan, welan, rhamsan, dextran, pullulan, curdlan, scleroglucan, chitin, carrageenan, karaya, ghatti, tahla, scleroglucan, hydroxyethylstarch, hydroxypropylstarch, cationic starch, starch acetate, starch phosphate, oxidized starch, aloe gum, chia gum, flaxseed gum, okra gum, psyllium seed gum, quince seed gum, tamarind seed gum, stractan, cactus, bacterial cellulose such as Cellulon from Monsanto); proteins (e.g., casein, soy protein, gelatin, egg white albumin), associative thickeners (e.g., hydrophobically modified ethoxylated polyurethanes (HEURs), polystyrene sulfonate) or poly(vinyl sulfonate) combined with xanthan gum, poly(acrylic) acid with alkyl amine polymers).

The thickener is used on an as needed basis. Critical coating parameters that limit thickener use are final viscosity, aluminum oxide grit loading, curtain stability, compatibility with other coating components especially flocculants, pH and temperature. The thickener is preferably used in an amount effective in producing a viscosity of 30 to 250 centipoise. When the slurry of the pre-flocculated pigment is added directly to the pulp instead of being coated on the paper machine, the coating can be easily agitated until the time of addition and a thickener is not required in these applications.

Modification of Surface Tension

Particularly for applications of slurries of pre-flocculated pigment from a secondary headbox or curtain coater, it is desirable to add surfactants that reduce the surface tension of the coating and thus promote dewatering of coating and deeper penetration of the pigments into the sheet, which improves the runnabihty of fiber mat through the wet press. Improving the dewatering of the coating also reduces mottle of the pigments on the surface of the paper. It is important that the surface of the sheet is not sealed by the coating. If excessively wet coating remains on the surface of the paper as it enters the wet press, more web breaks tend to occur from the coating sticking to the top roll of the press section, thus delaminating the web. Also, the wet press induces a film split pattern, causing excessive mottle, if the paper surface is too wet as it enters the press section. Thus, addition of a surfactant to control the surface tension of the coating is recommended to increase the dewatering rate for improved operation at the press section and to insure colorant pigments are uniformly distributed. Surface active wetting agents also enhance curtain uniformity and stability. Wetting of the land area of the coating head is a critical factor. With poor wetting, holes or blips often appear in the curtain. Surface tension of the coating at make down is in the range of 65 to 80 dynes/cm. In order to obtain good curtain uniformity surface tension should be lowered to 35 to 60 dynes/cm.

Typically, .05 to 1% surfactant (as received/wet weight) is needed to achieve curtain stability. Surfactant is usually added in line just prior to curtain coating. Use of Aerosol OT-75 or Triton X-100 (nonionic surfactant) or anionic surfactants such as triethanolamine dodecylbenzene sulfonate are particularly useful in curtain coating.

Surfactants that are suitable for curtain coating flocked pigment on the wet end of a paper machine should be compatible with the thickener and the flocculant. Suitable surfactants include amphoteric, zwitterionic, non-ionic and anionic surfactants with anionic being more preferred. The surfactant should produce a surface tension of less than 60 dynes/cm at 0.1% or at the critical micelle concentration in water for this application. Suitable surfactants generally fall in the range of 4 to 22 HLB (hydrophihc lipophilic balance) units but more commonly 11-20 units HLB and pH 6-8 at 1% aqueous dispersion and include salts of dodecylbenzene sulfonate such as monoisopropanolamine, mono- and diethanolamine, calcium, magnesium, ammonium, sodium, potassium, etc.; C8 to C12 alkyl polysaccharide ethers such as Henkel's APG Glyoside series; DuPont's modified ethylene oxide polymers such as Merpol and Zonyl;polyethylene glycol ethers of cetearyl alcohol, Ceteareth-5 (CAS No. 68439-49-6); disodium stearyl sulfosuccinamate (CAS 14481-60-8); sodium alkyl sulfonate; fatty alcohol ethoxylate; sodium alkyl diaryl sulfonate; oxazoline- type compd (CAS 28984-69-2; 75499-49-9 amphoteric); disodium capryloamphodiacetate (amphoteric); sodium cocoamphopropionate (amphoteric); sodium C12-15 alkoxypropyl iminodipropionate (amphoteric); PEG-3 C13 fatty alcohol phosphate ester (anionic/nonionic); alkyl polyglycoside (CAS 68515-73-1); polysorbate 20 (CAS 9005-64-5 nonionic); PEG-20 sorbitan isostearate (CAS 66794-58-9 nonionic); sodium alkane sulfonates based on n- paraffm (anionic); PEG-4 stearate (CAS 9004-99-3 nonionic); PEG-10 glycol tallate; ethoxylated hydrog, tallow amide (nonionic); PEG- 13 hydrogenated tallow amide (CAS 68155-24-8); potassium perfluoroalkyl sulfonate (anionic); PEG-15 rosin acid (nonionic); nonoxynol-6; CAS 9016-45-9 (nonionic); sodium lauryl sulfoacetate (anionic); alkyl phenol ether sulfate, sodium salt (anionic); sodium dinonyl sulfosuccinate (anionic); dioctyl sodium sulfosuccinate (anionic); sodium heptadecyl sulfate (anionic); PEG-POP ether (nonionic); sodium isostearoyl-2-lactylate (anionic); trifunctional polyoxyalkylene glycol (nonionic); sodium dodecyl diphenyl ether disulfonate (anionic); sodium decyl diphenyl ether disulfonate (anionic); sodium 2-ethylhexylsulfate (anionic); polyalkylene oxide-modified polymethylsiloxane (nonionic); PEG-6 caprylic/capric glycerdies; CAS 52504-24-2 (nonionic); phosphated aromatic ethoxylate (anionic); and Cl l-15 pareth-7 (CAS 68131-40-8 nonanionic).

Wet End Operations

In accordance with the invention, the flocculated pigment is added to the fiber at the wet end of the paper machine. This addition can be accomplished in several different operations. In accordance with one embodiment of the invention, a slurry of pre-flocculated pigment is applied to the surface of the wet paper mat on the wire using a secondary headbox or using a curtain slot coater which is preferably situated on the paper machine before the wet press. In other embodiments of the invention, the flocculated pigment is mixed with the fiber before the fiber is deposited onto the wire. This process can be accomplished in the beaters, in a storage chest or fan pump, or pigment can be continuously added to a pulp stream which feeds the headbox. Each of these embodiments of the invention is discussed below in more detail.

In applying the pigment to the surface of the paper using a curtain slot coater, the flocculated pigment slurry contains about 10 to 35% solids and is applied in an amount of about 2 to 15 lbs. pigment per 3000 sq. Ft.. As discussed above, a surfactant must be added to and uniformly mixed with the slurry in order to achieve a uniform and stable curtain which coats the surface of the paper with minimal coating defects such as fish eyes, streaks, etc. The surfactant also facilitates dewatering such that the coating does not seal the sheet and prevent or unacceptably delay resin impregnation. The curtain coating and wet press parameters and operating conditions are adjusted in a conventional manner to maximize retention. The preferred location for the slot curtain coater die is on the wet end of the paper machine just past the dandy roll. Other locations may be before the dandy, between the last vacuum box and wet press or anywhere between the dandy and the last vacuum box on the paper machine.

Wet end vacuums and coating viscosity factors control color and pigment retention. Coating conditions that increase the time that the coating resides on the surface or within the fiber matrix before it is extracted by backside vacuum enhances retention and efficiency of color. A combination of higher viscosity which retards flow through the web and placement and strength of wet end vacuum away from the site of curtain contact with the web promote retention of color. Retention factors such as viscosity and wet end vacuum need to be balanced with the operating stability limits of factors such as the curtain coating head and the need to efficiently dewater the web before it reaches the wet press, otherwise coating pick and web breaks will occur.

In another embodiment of the invention, the slurry of the flocculated pigment is applied from the secondary head box. For application from a secondary head box the slurry preferably contains 1 to 20% solids and more typically about 1 to 5% solids. The secondary headbox is located between the primary headbox and the dandy roll. When applying the slurry as a coating from a curtain coater or a secondary head box, it is important to avoid sealing the sheet so as to unacceptably impede resin impregnation.

The flocculated pigment can also be mixed with the fiber to provide a furnish composition which is applied to the paper machine wire from the primary head box. The flocked pigment may be added to the furnish at any point in the formation or dilution of the fiber. For example, a slurry of pre-flocculated pigment can be added to the white water and used to dilute the pulp in any of the stock chests (e.g., the thick stock chest, the thin stock chest, or the machine chest), or the slurry can be added to the pulp in the beater or fan pump. When mixing the slurry with the pulp for application to the paper machine wire, the flocked pigment slurry does not need to contain thickener or surfactant and typically contains about 15 to 30%) solids. The slurry fed to the head box as part of the furnish contains about 0.5 to 0.7% solids.

The invention is illustrated in more detail by the following non-limiting examples:

Example 1

A typical wet end coating is prepared by mixing the following components in a Cowles mixer at a final solids of 10 to 20%.

Coating Preparation:

% Solids Pounds Wet

Titanium Dioxide 60 116 Pigment Yellow 155 32 8.8 Pigment Blue 15 28 13.3 Pigment Violet 19 20 27.7 Tabular Alumina 100 209 PR 971 Flocculant 25 5.1 Avicel/CMC (9:1) 2 840

The titanium dioxide is prepared at 60% solids by adding 0.3% Tetrasodium pyrophosphate (on dry TiO2) and mixing in a Cowles mixer for 10 minutes followed by screening at 325 mesh using a Sweco shaking sieve. The mixture of microcrystalline cellulose (Avicel) and carboxymethyl cellulose (9:1) was prepared in a Cowles mixer at 2% solids. The final coating mixture described above was fed at 60% solids to a 68" curtain coater die at 7 gallons per minute with an in-line feed of 20% triethanolamine dodecylbenzene sulfonate to give a final surfactant level of 0.3% (as received/wet weight).

The preferred location for the curtain coater die is on the wet end of the paper machine just past the dandy roll. Other locations may be before the dandy, between the last vacuum box and we press or anywhere between the dandy and last vacuum box.

Example 2

A flocked colorant mixture was prepared as follows:

A titanium dioxide slurry at 60% solids was prepared by adding 8000 grams dry titanium dioxide to 5300 grams deionized water containing 24 grams tetrasodium pyrophosphate (a dispersant) and mixed at 4000 rpm for 15 min using a Cowles bench top mixer. The resulting slurry was filtered through a 325 mesh screen. Using a Silverson rotor- stator bench top mixer the following colorant mixture was prepared by adding the materials to the mixer in the order and in the amounts indicated:

Grams

1. Titanium dioxide slurry (60% solids) 270

2. Pigment Violet 19 (19.5% solids) 42

3. Pigment Yellow 155 (32% solids) 13

4. Pigment Blue 15 (28% solids) 20

5. Deionized Water 656

The mixture was stirred and 154 grams 10% Polymin PR 971 L (BASF) cationic flocculent polymer was added. A dry mixture of 67.5 g Avicel microcrystalline cellulose and 7.5 g sodium carboxymethyl cellulose was added slowly to 6175 g deionized water at 3200 rpm in a Cowles mixer and mixed 15 minutes. The flocked colorant mixture was added to 4000 grams of thickener using the bench top Cowles mixer. Deionized water (606 grams) was added (2%) to wash out the Silverson mixer and added to the Cowles. Surfactant, triethanolamine dodecylbenzene sulfonate at 25%, (46 grams) was added just prior to running on the paper machine.

A base sheet furnish was prepared. The furnish was refined to 500 cfs and adjusted to pH 9.5. Melamine- formaldehyde wet strength was added at 2400 grams (6% solids). Using a pilot paper machine having a 12 inch web width and operating at 11 feet per minute, the colorant mixture prepared as described above was applied on the wet end of the paper machine by feeding to a stock slurry in the flow box prior to the primary head box and/or by a 12 inch slot curtain coater located between the dandy roll and the wet press. The following operational data were obtained:

In general, feeding the same colorant mixture to the wet end prior to the head box gave a 20 to 40% better overall retention of color than the slot curtain coater method. When the two methods were combined, the total retention of color improved over use of the slot coater alone. Retention (%) of dry solids that was calculated as:

(Pounds/ream weight gain observed X 100)/(Pounds/reams theoretical maximum retention).

The theoretical maximum retension of dry solids was calculated as:

(ml/min flow) X (1/specific gravity) X 3000 sq ft/ream X % Solids/100 X 1 lb/454 g /((ft/min machine speed) X (coating width, ft)).

Effect of Addition Point on Color Efficiency

Colored papers from each of the paper machine runs described above were saturated with melamine-formaldehyde resin and laminated against phenolic resin saturated kraft to produce colored laminates typical of the counter top and table top industry. Color of the laminates were measured using a Hunter Lab Ultrascan XE spectrophotometer and compared to the color obtained by slot coating alone. Color was measured using CIE Lab cool white fluorescent, 10 degree observer.

Table 1. Color Data for Wet End and Slot Coater Color Additional Laminates

Effect of Wet End Addition of Color on Production of Highly Colored Paper from Low Opacity Colors:

Low Opacity coatings are typically light colors with high relative amount of titanium dioxide and low amounts of colored pigments. Because decorative paper is laminated against a dark brown core, even a slight deviation from 100% opacity can throw off the color from standard specifications with light colors. Typically, higher opacity is obtained by either increasing the amount of TiO₂ added to the sheet or increasing the relative amount of colored pigment. In an experiment described below, a low opacity coating was prepared at different solids levels and run on a slot curtain coater and wet end feed and made into laminates in the same manner as described in the previous example. Results are shown in Table 2.

Table 2. Use of Wet End Feed to Achieve Target Opacity

'Slot coater feed = 376 ml min.

²Slot coater feed = 376 ml/min and wet end feed = 120 ml/min. Having described the invention in detail and by reference to specific embodiments thereof it will be apparent that numerous variations and modifications thereof are possible without departing from the spirit and scope of the invention as defined by the following claims:

Claims

1. A method for preparing a colored paper which comprises: flocculating a dispersion of pigment in the absence of fiber to obtain a slurry of the flocculated pigment, combining the slurry of flocculated pigment with paper fibers, and forming a sheet from the flocculated pigment and fiber on the wet end of a paper machine.

2. The method of claim 1 wherein the dispersion of pigment is a dispersion of at least one colored organic pigment and a spacer.

3. The method of claim 2 wherein the flocked pigment includes a core of one or more spacer particles and a shell of an organic pigment.

4. The method of claim 3 wherein the method includes the additional step of adding a thickener to the flocked pigment before mixing the flocked pigment with the paper fibers.

5. The method of claim 3 wherein the step of combining the flocked pigment and fiber is performed by applying a slurry of the flocculated pigment from a slot coater to a mat of paper fibers on the paper machine.

6. The method of claim 5 wherein said slot coater is located on the paper machine after the dandy roll and before the dryer can.

7. The method of claim 3 wherein the flocked pigment has a particle size of about 1 to 10 microns.

8. The method of claim 1 wherein the method includes the additional step of milling the flocked pigment before combining the flocked pigment with the paper fibers.

9. The method of claim 3 wherein the step of combining the flocked pigment with the paper fiber is carried out by adding flocked pigment to fiber as the fiber is fed to the primary head box on the paper machine.

10. The method of claim 1 wherein the paper sheet is a decor paper for use in decorative laminates.

11. The method of claim 3 wherein the step of combining the flocked pigment with the paper fiber is carried out by adding a slurry of the flocculated pigment to a paper pulp in a beater chest.

12. The method of claim 3 wherein the step of combining the flocked pigment with the fiber is carried out by adding a slurry of the flocculated pigment to a paper pulp in a storage chest.

13. The method of claim 5 wherein the slot coater is a curtain coater.

14. The method of claim 3 wherein the spacer particle is selected from the group consisting of titanium dioxide, clay, talc, ethyl cellulose, urethane pigment, insoluble starch, calcium carbonate, zirconium carbonate, aluminum oxide, iron oxide, bentonite, kaolin, aluminum hydroxide, magnesium oxide, silica, diatomaceous earth, organic microcapsules, polymeric hollow microspheres, inorganic microcapsules, potassium titanates, zinc sulfate, barium sulfate, and urea- formaldehyde pigment.

15. The method of claim 1 wherein the paper is a saturating grade paper.

16. The method of claim 15 wherein said paper exhibits a Gurley porosity of about 1 to 300, a resin saturation time of about 1 to 300 seconds and a resin capacity of 20 to 80% by weight of the saturated paper.

17. A method for preparing a colored saturating paper useful as a decor sheet in a decorative laminate which comprises: flocculating a dispersion of an organic pigment and a spacer so as to form a structured pigment in which the spacer forms a core and the organic pigment is deposited on the spacer as a shell, combining the structure pigment with paper fibers, and forming the structed pigment and paper fibers into a sheet on the wet end of a paper machine.

18. The method of claim 17 wherein the step of combining includes applying the flocked pigment from a slot coater to a mat of paper fibers on the paper machine.

19. The method of claim 18 wherein said slot coater is located on the paper machine after the dandy roll and before the dryer can.

20. The method of claim 17 wherein the method includes the additional step of milling flocked pigment before mixing the flocked pigment with the paper fibers.

21. The method of claim 17 wherein the step of combining the flocked pigment with the paper fiber is carried out by adding a slurry of the flocked pigment to fiber as the fiber is fed to a head box.

22. The method of claim 17 wherein the step of combining the flocked pigment with the paper fiber is carried out by adding the flocked pigment to a paper pulp in a beater chest.

23. The method of claim 17 wherein the step of combining the flocked pigment with the fiber is carried out by adding a slurry of the flocculated pigment to a paper pulp in a storage chest.

24. The method of claim 18 wherein the method includes the additional step of adding a thickener to the flocked pigment before mixing the flocked pigment with the paper fibers.

25. The method of claim 24 wherein the slot coater is a curtain coater.

26. The method of claim 1 wherein the method includes the additional step of adding a surfactant to the slurry of flocculated pigment.

27. The method of claim 26 wherein the surfactant is an anionic surfactant.

28. The method of claim 27 wherein the surfactant is a triethanolamine dodecylbenzene sulfonate.

29. The method of claim 17 wherein the method includes the additional step of adding a surfactant to the slurry of flocculated pigment.

30. The method of claim 29 wherein the surfactant is an anionic surfactant.

31. The method of claim 30 wherein the surfactant is a triethanolamine dodecylbenzene sulfonate.

32. A paper prepared by the method of claim 1.

33. A paper prepared by the method of claim 5.

34. A paper prepared by the method of claim 9.

35. The paper of claim 32 wherein the paper is a colored saturating grade paper useful in forming decorative laminates.

36. A colored paper comprising a mat of cellulosic fibers having dispersed therein a pre-flocculated pigment.

37. The paper of claim 36 wherein the paper is a colored saturating grade paper useful in forming decorative laminates.