US 3784596 A
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United States Patent 3,784,596 NON-AQUEOUS PAPER COATING COMPOSITIUNS Robert T. Nagler, Prairie du Sac, Wis., assignor to Bergstrom Paper Company No Drawing. Filed Nov. 24, 1971, Ser. No. 202,000 Int. Cl. (108g 51/04 US. Cl. 260-23 R 16 Claims ABSTRACT OF THE DISCLOSURE Paper coating pigment is combined with polymeric binder, organic solvent and rheological modifier to form a uniform, high-solids, water free coating composition of controllable rheology which may be applied to paper, film or web material to produce a smooth coated product with superior properties.
RELATED APPLICATION The invention herein disclosed is related to that described and claimed in copending application Ser. No. 789,616, filed Ian. 7, 1969, by Karl E. Guenther and Donald G. Havekost, now US. Pat. 3,655,608.
BACKGROUND Pigmented coatings are applied to paper stock, e.g., for the purpose of providing an improved surface finish suitable for printing. The coating formulations generally used are aqueous systems containing reatively large proportions of water. Coating with such an aqueous system involves mechanical complexity and increased cost because of the necessity for removing the water to dry the product to an acceptable moisture content.
High quality coated paper for printing must meet a number of requirements dictated by the nature of the printing process. Thus, the requirements are somewhat different depending on whether the printing is to be done by offset, gravure or letterpress methods. For example, paper for offset printing generally must have higher moisture resistance than paper for letterpress or gravure printing because the paper is moistened incident to the offset printing process. In general, however, coated printing paper must be smooth and level, dimensionally stable, strong, moisture resistant, resistant to picking or pulling up of coating or fibres by contact with a tacky inked surface, resistant to blistering when heat is used to speed up drying of the ink, and above all, it must accept ink uniformly without absorbing it excessively. In addition, such properties as opacity, gloss and color are imparted by the coating; requirements for these vary Widely depending on the desired appearance of the finished printed matter but they must be uniform throughout a particular stock.
An uncoated paper surface is not completely smooth but contains higher and lower areas since the thickness of the felted cellulose fibers varies from point to point. The magnitude of these variations in thickness is reduced by the smoothing effect of calendering. However, if the paper is again moistened with water, the cellulose fibers tend to swell and spring back, increasing the magnitude of the variations. To create a smooth and level printing surface, the coating must fill in all of the low areas of the paper; while, to provide a uniform surface for ink reception, the coating must also cover the fibers in the high areas. When a paper is moistened by application of an aqueous coating, the magnitude of the surface irregularities is increased and a larger amount of coating must be applied to create a uniform surface.
Non-aqueous materials suitable for the binder or adhesive component of a coating formulation are generally thick viscous liquids in the form in which they are applied. When combined with the higher proportions of filler or pigment, the viscosity is further increased to a point where it is not mechanically possible to apply the required smooth, uniform coating at economically practical speeds by conventional coating techniques.
Coating webs with polymerizable organic materials to prepare the surface thereof for printing has been accomplished according to Brown et al. (US. Pat. 3,279,- 424). Brown et al. apply a polymerizable material (with necessary catalyst) directly to a web. After smoothing the surface of the polymerizable material, said material is subjected to elevated temperature to effect polymerization. The pot life" of the polymerizable material is a critical factor in such a procedure and precautions must be continually taken to avoid premature polymerization. Moreover, Brown et al. are not concerned with the problem of heavy pigment loading in the applied coating.
Although Winchester (US. Pat. 3,240,619) applies plural coatings to a substrate, he is not concerned with the preparation of printing papers, but with printing over a dried first coating prior to the application of a second coating. His substrate is wallboard.
Polymeric materials are generally inherently non-receptive to most inks. Coatings containing large proportions of polymeric binder thus fail to meet requirements for uniform and controlled ink receptivity.
In the coating of paper, the use of non-aqueous coatings with high solids concentrations has many apparent advantages. Unfortunately, it has not heretofore been possible to employ such coatings because otherwise suitable high-solids mixtures were thick and pasty, difficult to mix uniformly, difficult to pump or transport to the coating applicator, and difficult or impossible to spread over the paper surface in the required thin, level, and uniform layer. These problems have now been overcome.
Whereas pigments coated from most aqueous systems form moisture sensitive films prone by their hydrophilic nature to be easily redissolved or broken, the films or bonds from many polymers of organic nature are not easily affected as proved by wet rub tests. The quick release of any solvents of organic nature, chosen for their high evaporation rates, produces smoother films, smaller pores within the film structure and resin bonding between surface fibers and pigment which is only slightly affected by moisture.
Since employed solvent must be subsequently removed from the coating, the amount of solvent should !be kept to a minimum. The cost of binder is so much greater than the cost of filler or pigment that minimizing the proportion of binder is a matter of economics. Moreover, the preparation of a lighter weight high quality paper, particularly for books and magazines, has a further economic impact in view of the ever-rising mailing costs, which are of material significance.
Base paper used for conventional aqueous coatings is barriered by a sizing treatment to reduce coating-color penetration. When non-aqueous or solvent-type coatings are applied to a base paper, e.g., in manufacturing Electrofax papers, the base paper is barriered against penetration by the solvent. It has been believed to be impossible to obtain a satisfactory coating of either type on a base which has not previously been suitably barriered against the employed coating material. The cost of providing a solvent barrier has heretofore ruled out consideration of solvent-type coating for large-scale production of competitive papers.
lighter weight coated paper with improved printability and methods of preparing same. It is particularly concerned with high quality publication grades for offset, gravure and letterpress printing. However, readily available variations or modifications result in products suitable for copying papers, technical papers, barrier papers and newsprint.
Other substrates, such as film and web material, are also advantageously coated with water-free pigmented coating compositions of this invention. [Non-paper substrates should be resistant to short term effects of solvent in the coating composition] These coating compositions have a liquid phase and are successfully applied to base materials which are not inherently, or by virtue of pretreatments, impervious to penetration by the liquid phase. Further, the compositions are successfully coated at higher levels of solid ingredient content than is possible in practice with conventional water-based pigmented coating compositions.
Illustrative substrates include:
(a) paper-offset and roto grades;
( py P p (c) newsprint;
(d) polymeric films of, e.g., polyvinylchloride-acetate copolymer;
(e) foraminous material of natural or synthetic fiber, e.g.
cellulose fiber or polyolefin fiber;
(f) non-Wovens and woven textile;
(g) synthetic paper (foraminously constituted or filmlike), e.g. polystyrene film paper;
(i) enamel paper.
The compositions contain binder, pigment and dispersing medium. The binder develops its properties as a result of its being a fully-polymerized and preferably solvent-soluble thermoplastic polymer. The pigment is in finely-divided form and includes inorganic solids (including fillers), whether or not the solids have tinctorial value, and/or organic pigment. The dispersing medium comprises solvent for the binder and any employed rheological modifier for the composition. The amount and (perhaps) type of dispersing medium, the proportion and molecular weight of hinder, the proportion of pigment, the type of pigment and the particle size distribution of pigment affect the rheology of the resulting coating composition. Some of these factors are interrelated and even interact.
Processing with coating compositions of this invention provides special and unique advantages in production. Preparation of the compositions is simple, requires minimal apparatus or equipment and can be completely automated or performed by the part-time attention of a single operator. Compositions so prepared are extremely stable and may be stored for considerable periods without de terioration and without requiring continuous agitation, as is necessary for conventional compositions. Significant loss of solvent by evaporation must naturally be precluded during storage.
The polymeric binder inherently contributes to the moisture resistance, strength and dimensional stability. While the binder securely bonds the pigment particles together and to the paper surface, it leaves a uniformly ink-receptive pigment (high quality printing) surface in view of the high proportion of pigment compared to that of polymeric binder. This is accomplished in part by employing a non-aqueous solvent in the coating mixture which dissolves the polymeric binder material and thus distributes it uniformly and in intimate contact with the pigment particles. Use of a non-aqueous system permits production of a smooth and level printing surface without applying large amounts of coating. With a non-aqueous system an increase in magnitude of surface irregularities is avoided and a smaller amount of coating is sufficient to give a uniform and level surface. Coatings of from one to two pounds per side per ream (3300 square feet) produce a coated paper equal or superior in smoothness and printability to those produced by conventional aqueous coatings running from 6 to 10 pounds per side per ream.
The process overcomes difficulties which have heretofore prevented the use of polymeric thermostable binders in the production of high quality printing papers.
Loss of dissolved binder into the paper web is minimized through the use of a coating mixture or coating color with high solids content, i.e., with a small proportion of solvent. This not only reduces the strike-in to acceptable limits, but is economically desirable because of the reduction in the amount of solvent which must be removed from the coated paper after application of the pigment. This economy is further accentuated by the use of organic solvents instead of water, since these solvents have much lower latent heat of vaporization and are removed with an expenditure of less heat energy than the same amount of water.
The coating formulations of this invention are dependent upon the binder, which is a thermoplastic or thermosetting (fully polymerized) polymer dissolved in a suitable organic solvent. The formulations (applied to paper sheet) provide a good printable surface at low coating weights by incorporating a high proportion of pigment (compared to binder) in a non-aqueous system in which r the binder dissolves in the solvent used. These formulations are applied at high solids concentrations by limiting the solvent to that required for the essential fiow properties.
The coating formulations are readily applied, e.g., to a paper sheet substrate by an applicator device, such as that described by Nagler (US. Pat. 3,518,964), which permits close control of the rate of application and uniformity of distribution of the high solids coating while preventing loss of volatile organic solvent and permitting continuous recirculationn of the coating material. Different, and much simplified, drying facilities are used in comparison with those required for conventional lower-solids aqueous coating. For both economic and hygienic reasons facilities for recovery and reuse of the organic solvents are desirable. Glossing rolls, buffers, burnishers, calender stacks, etc., may be used as desired for producing a variety of surface finishes on the coated product.
In general, the process equipment for producing high quality coated paper by this process is less complex and appreciably less expensive to operate than conventional off-machine coaters. The process can be used on webs of a width or operating at a speed at least equal to the capabilities of conventional equipment for producing coated paper of comparable quality. A still further object is to apply a satisfactory coating by a solvent process to a base paper which is not barriered against the solvent, the solvent process being suitable for large-scale production of competitive papers.
An object of this invention is to prepare coated paper with a non-aqueous coating composition having a high solids content. A further object is to provide such a coating composition which is homogenous and which has good flow characteristics under conditions suitable for application to papers. Another object is to provide a light-weight coated paper having the physical properties and ink receptivity of the highest grade heavy-weight printing paper. It is also an object that the produced paper be capable of printing and handling on conventional printing machinery without reducing the speed thereof.
One object of the present invention is to coat paper at high speeds with a non-swelling solvent (to prevent swelling of the base cellulose fibers) on a non-supered base sheet and to obtain a coated paper having high smoothness, high ink holdout and high bulk or resilience (requirements in the trade in general for offset papers). The geometry of paper surfaces and their non-uniformity in x, y and z directions achieve a very important role; solvent coating is used to minimize such non-uniformity. Intertwining fiber of an uncoated paper sheet reveal a fairly rough surface. [Fibers (at normal beating conditions) are from 0.8 to 1.7 millimeter (mm) in length and from 0.07 to 0.08 mm. in thickness] Such a surface is barely capable of reproducing a screen of 30 because the screen dots are smaller than the dimensions of the fibers. Resulting prints cannot be sharp as the dots appear to be segmented and broken up. Printing on such papers with higher screens does not result in quality printing.
Another object is to formulate a pigment coating composition which is not so insoluble as to resist being broken up by a mild chemical treatment; the binder cannot be so thermoset and strong and the pigment cannot be so reactive as to preclude repulping of resulting pigment-coated paper and recycling in a commercially feasible fiber-recovery process; and any polymeric binder must be capable of redispersion and separation from fiber by a slight caustic or acidic treatment. Paper coated with the pigment coating composition is thus deinkable at low acid or base concentrations. The binder (either thermoplastic or thermosetting) is hydrolyzable, but may require small amounts of solvent for emulsification (in addition to acid or base) for final fiber recovery.
A still further object is to prepare a planar substrate having on at least one surface a thin uniform coating of pigment, rheological modifier and binder, the pigment constituting from 70 to 95 percent by weight (based on the total dry coating weight), the binder constituting from 5 to 30 percent by weight (based on the pigment weight) and the rheological modifier constituting from 0.1 to 8 percent by weight (based on the total coating weight). Other objects will be apparent from the description and examples which follow.
Throughout the disclosure percentages vary somewhat depending upon whether they are based on fluid compositions to be applied to substrates or to an essentially dry coating on a coated substrate.
DETAILS Extremely light coating weights of non-aqueous, high solids, volatile solvent, pigmented coating dispersions are applied to substrates varying from foil to paper with an applicator that gives smooth coating lays with a minimum of solvent loss. Coating speeds can be in the same range, e.g. from 500 to 5,000, preferably from 1,500 to 3,000, feet per minute (f.p.m.), as with conventional water-based conversion coaters. The coating machine requires less drying capacity than for water-based coatings in View of the low solvent content, e.g. 5 to 30 percent by weight, of the coating dispersions. This result in a reduction in capital cost for drying equipment and reduced heat energy requirements for drying.
By applying only 4 or less pounds (normally in the range of 1 to 2 pounds) of the non-aqueous coating dispersion per ream side, printing paper surface characteristic are obtained which are at least equal to those of papers that are conventionally coated with from 6 to 10 pounds per ream side of Water-based coatings. The elimination of the adverse effect of water (excluded from said non-aqueous dispersions) on the surface smoothness of paper is a major factor. Water causes base paper surface to be roughened by fiber swelling, requiring a larger weight of coating to fill the surface valleys. The lower coating weight also means a reduction in the amount of required coater drying capacity.
Many conventional water-based coated sheets require a supercalender treatment to achieve desired surface smoothness or surface gloss. The need for such calendering is eliminated or essentially reduced with this process ince the smoothness of the base paper is not impaired by solvent coatings as in the case of water dispersions. The elimination of the supercalendering operation leads to a further reduction in the cost of finished paper. Brush polishing or gloss calendering apparatus may be installed directly 0n the solvent coater to attain any required degree of gloss.
A key to the subject coating development is the preparation of flowable, non-aqueous essentially homogeneous coating dispersions having from about 5 to about 30 percent by weight of binder (based on the weight of the pigment) and a total solids content of from about 50 to about 95, preferably from about '60 to 75, percent by weight (based on the total weight of the coating formulation). Smooth, opaque coatings for printing paper application are achieved at lower cost with coating dispersions having such high levels of pigmentation. The higher pigment loading in such non-aqueous dispersion permits lower tcgtzlil coating weights without sacrificing strength or printa iity.
Paper coated with these formulations has improved dimensional stability over wide range of relative humidity in view of the water-insolubility of the coating. The coating formulations of this invention are useful in preparing (a) water, oil and chemical resistant paper or films, (b) smooth surface without supercalendering, (c) matte or glossy surfaces, (d) printing papers of superior ink holdout, (e) strong, light-weight printing papers, (f) highgloss label or decorative papers, (g) special copy papers, (h) varnish label papers, (i) adhesive-coated papers, (j) adhesive-coated films, (k) higher bulk printing papers due to the elimination of supercalendering and (l) carbonizing papers.
Pigment Pigments, e.g., for the preparation of printing papers constitute a recognized class including, but not limited to, such mineral pigments as zinc oxide, kaolin clay, bentonite, fullers earth, calcium carbonate, titanium dioxide, silicas, barium sulfate and other finely divided mineral substances used as opacifiers and/ or fillers in various types of surface coatings. Many of these pigments are difficult to wet with non-aqueous liquids. There is a tendency for the pigment particles to form undesirably large aggre gates of individual particles and to separate out of the dis person by settling rapidly. However, virtually any pig ment or colorant, organic, e.g. Quinacridone Red and. Phthalocyanine Blue, or inorganic, which can be prepared. in finely divided form (having an average diameter o. from about 0.2 to 5 microns) can be employed in the preparation of the coating dispersion.
The pigment can be a single pigment or a combination of two or more pigments. It is finely divided, i.e. its average particle diameter is from about 0.5 to 3.5 microns (a); the particle size distribution is such that from 25 to 75 percent by weight of the particles are smaller than Lu, from 10 to 35 percent by weight are larger than 5 1., not more than 15 percent by weight are larger than 10 none exceed 30; in size and the balance of particles is in the 1 to 5, range.
The preferred average pigment particle size for typical printing paper coatings is in the range of from 0.5 to 1.0a, with a size distribution such that from 50 to 75 percent by weight of the individual particles are less than 1.0 in size; not more than 10% are larger than 5.0,u and no particles are larger than 10.0;t.
The rheological behavior of the coating color is affected by the proportion of pigment in the total formula and by the proportions of pigment particles of various sizes which are present in the pigment. The resistance (which is presented by the particles of the solid phase) to free motion of the molecules of the liquid phase is the cause of the change in flow properties. The more particles there are present, the more resistance will be encountered. Since small particles have more surface area per unit weight than large particles, the smaller particles also will create more resistance to flow.
High proportions of very fine particles give a very thick and thixotropic coating color. High proportions of particles larger than 5.0a decrease the uniformity of ink receptivity. If appreciable proportions of very large particles, i.e. 10 to 30 are present, the surface smoothness may be adversely affected and the color may also show unsatisfactory rheological properties, such as dilatancy, which prevents uniform spreading.
Different grades or types of pigments contain different size ranges and sometimes different shapes of individual particles. Kaolin clays, which are the principal pigment ingredient in coating colors, are available in a wide range of size distributions. These ranges can also be somewhat modified by blending several different types together. However, factors other than rheology must be considered in selecting pigment particle size ranges, since the uniformity of ink receptivity of the coating, the brightness, and other important end use properties are related to pigment particle size.
Binder The binder is any organic-solvent-soluble synthetic polymer which has adhesive properties and is capable of wetting pigment. Solubility in the solvent selected must be sufficient for the proportion of polymer required for good binding action. The solvent should be one which is economically feasible and which presents a minimum of hazard when employed on a commercial scale. There is also a solvent-diluent" aspect. Reference in this regard is made, e.g., to Example 3, in which a mixture of toluene (which is a solvent for polyvinylacetate) is shown. The xylene does not dissolve the binder, but is miscible with and acts to thin or dilute the solution of binder in toluene. This can affect rheology and also helps to control solvent strike-in and binder migration.
In addition to the essential adhesive characteristics, the polymer is preferably pale in color and has good color stability and pot life. The polymer is preferably thermostable over the temperature range to which any coated substrate will ordinarily be subjected. The binder is a fully pre-polymerized resin; it is ordinarily a thermoplastic, but can also be an organic-solvent-soluble thermosetting resin. [As used herein, polymer includes all synthetic resins, such as homopolymers, copolymers and polycondensates.]
Many synthetic thermoplastics are suitable for use as a binder. A significant characteristic of the binder is whether coating compositions prepared therewith are de-inkable by standard processes. Although, e.g. styrene polymers, such as polystyrene, and vinyl polymers, such as polyvinylacetate (PVAC), constitute two classes of binder which are particularly well-suited for the subject coating compositions, the polyvinylacetates are and the polystyrenes are not de-inkable by standard processes. Those binders which are so de-inkable are preferred. [Whether any particular binder is de-inkable is readily ascertained by a standard routine determination with a minimum of effort] One of the original prime objects of one facet of the project which led to this invention was the formulation of coating compositions (for paper) which would permit recycling of the paper from recovery of broke and eventual reuse in paper making. For this purpose the binder resin must be hydrolyzable to reconstitute intermediate polymers or prepolymers. Binder resins suitable for paper-coating compositions of the instantly contemplated type are not limited to those suitable for recycling and include:
(a) styrene polymers, e.g. polystyrene;
(b) vinyl polymers, e.g. polyvinylacetate;
(c) vinyl copolymers, e.g. plyvinylchloride-acetate;
(d) non-crosslinked polyester resins, e.g. polyglycolmaleate;
(e) polyether resins, e.g. polyphenylene oxide;
(f) acrylic polymers, e.g. polymethylmethacrylate;
(g) cellulosics, e.g. cellulose acetate.
Exemplary solvents for the enumerated types of resins are indicated by the same identifying letter:
(a) benzene; (b) toluene;
(c) methyl-ethyl-ketone; (d) acetone;
(e) perchlorethylene; (f) butylacetate;
Generally, the enumerated polymer types are soluble in one or more of the following classes of solvents: ketones, esters, aromatic hydrocarbons and chlorinated hydrocarbons.
The following classes of polymeric substances are illustrative of those which are deinkable and suitable for recycling as noted above:
(1) cellulosics, e.g. cellulose acetate;
(2) vinyl polymers, e.g. polyvinylacetate;
(3) acrylics, e.g. polymethylmethacrylate; and
(4) soluble linear polyesters, e.g. glycol esters of maleic and fumaric acids and of saturated fatty acids.
Whether any particular polymer is hydrolyzable is either known or is readily ascertainable by simple available test procedures. The determination of hydrolyzability of polymers is not the subject matter of this invention. A feature of one aspect of the invention is the use (as binder) of resin which is hydrolyzable so that resulting pigmentcoated paper can be recycled for reuse in paper-making.
Each of the noted binder resins is suitably employed as such or in admixture with a resin derivative, plasticizer, internal plasticizer, stabilizer (against heat and/or light) and/or hardener. Suitable members of these categories vary from binder to binder and, per se, do not constitute an essential part of the subject invention. Such members are known to the artisan.
All of the noted resin binders are available in a wide variety of compositions and molecular weights; solubility and other properties are dependent upon such factors. Generally, each of the contemplated resins is soluble in ketones, esters and aromatic hydrocarbon, i.e. in the form thereof of interest.
In preparing the synthetic polymer, such as polyvinylacetate, polymerization is controlled in known ways to obtain products which are polymerized to different degrees; different fully-polymerized products from the same monomeric building blocks vary from each other in molecular weight. If the polymer consists of relatively few molecules of monomer linked together, the molecular weight is low; as the number of molecules of monomer in a single molecule of polymer increases, the molecular weight of the polymer increases. As the molecular weight, or chain length, of the polymer increases, the viscosity of solutions containing equal weights of polymer also increases. Other properties of the polymer also change with changes in molecular weight. As it increases, the softening temperature increases and the resin becomes harder and more brittle. These changes in properties also change the effects they product in coating colors. Polyvinylacetate is marketed in a wide range of molecular weights, and values or characteristics intermediate between them can be obtained to some extent by mixing different grades. [The grade of any particular resin is measured in terms of the viscosity of a molar solution of the resin in a defined organic solvent. The only true grade comparisons are those made between polymers having the same monomeric building blocks and dissolved in the same solvent.]
For polyvinylacetate (dissolved in benzene) the optimum molar viscosity is between approximately 6 centipoises (cp.) and 28 cp. [A molar solution is one containing 1 gram (g.) mole of solute per liter of solution. Molar viscosity is the viscosity of a molar solution] Organicsolvent solutions (of resins) having molar viscosities between 5 and 50 cp. are useful for the subject coating compositions.
The minimum amount of binder in a formulation is limited by the adhesive quality developed; there must be sufficient binder present to prevent pigment in the coating layer from picking, piling or dusting during the printing process. The maximum quantity is limited by economics, since the resin binder, e.g. polyvinylacetate, is the most expensive ingredient. Since the viscosity of the binder solution in the solvent depends on both the amount and the molecular weight of binder present, these factors affect the viscosity and rheology of the color formulation. A binder concentration of approximately percent by weight (based on the weight of the pigment) is ordinarily satisfactory.
Dispersions (coating compositions) of the type described here do not have a viscosty in the sense that Newtonian fluids, such as water or glycerine, do. These dispersions are non-Newtonian, and the viscosity varies with the shear rate which exists under a given set of conditions. The behavior of a coating color should be plastic flow, in which the viscosity decreases with increasing shear rate. This means that the dispersion is thick and viscous when standing in a tank under no shear, slightly more fluid when flowing through a pipe under low shear, and very fluid when exposed to extremely high shear forces in the nip of a doctor blade. With this characteristic it will flow and level readily under the blade but set up without further running or sagging as soon as the blade shear region is passed. It is also desirable that the flow characteristic be thixotropic; i.e., that there be a slight time factor in the change of viscosity with shear rate.
Dispersing medium The dispersing medium is an organic solvent for the binder. A solvent for one binder may or may not be suitable for a different binder. Appropriate solvents for all contemplated resins are known from, e.g., published texts [Schmidt, A. X., and Marlies, C. A., Principles of High- Polymer Theory and Practice, Chapter 6, page 218 (particularly page 252), McGraw-Hill Book Company, Inc., 1948; Modern Plastics Encyclopedia 1969-1970, vol. 46, No. 10A pages 1006 and 1007, October 1969]. The organic solvent is a volatile liquid, i.e. one with a boiling point at about 80 C. or less, or a volatile liquid combination of two or more individual ingredients and is preferably, but not necessarily, non-polar.
In addition, effective and important changes in the rheology of a coating color formulation are effected by a proper selection and proportioning of minor quantities of rheological modifiers, which are incorporated as part of the dispersing medium and fall into one or more of the following categories:
(a) Cationic surfactant, e.g. alkyl amines or alkyl-aryl quaternary ammonium compounds, such as n-dodecylamine acetate;
(b) Anionic surfactant, e.g. alkali-metal salts of alkyl or alkylarylsulfonic acid, such as sodium lauryl sulfonate;
(c) Nonionic surfactant, e.g. polyesters and polyethers of fatty acids, such as trimethyl nonanol ethoxylate;
(d) Lubricant, e.g. soluble or insoluble metal stearates,
such as lead stearate;
(e) Antifoaming agent, e.g. polyalkylsiloxanes, such as polymethylsiloxane.
Contemplated compositions contain from 0.1 to 8.0 percent by weight (based on total composition weight) of at least one (and preferably a combination) of these rheological modifiers. In some cases a single modifier is effective in more than one category. For example, cationic surfactants which are amino salts or ammonium soaps of fatty acids, e.g. octadecylamine and ammonium stearate, amines, soaps or esters of naturally-occurring mixtures of saturated and unsaturated fatty acids, such as cottonseed or coconut oils and lard oil, having carbon chains with from about 8 to about carbon atoms combine lubricant and surfactant effects.
Cationic surfactants tend to reduce viscosity at low shear rates and to reduce the degree of thixotropy of the dispersion. Anionic surfactants tend to increase viscosity 10 at low shear rates. Nonionic surfactants tend to increase viscosty throughout a wide range of shear rates. The various surfactants are used in varying proportions with any pigment/ binder combination. The effects of rheological modifiers are interacting (with each other and with other formulation variables) both as to a specific modifier and the percentage thereof present in a particular formulation.
Pigment treatment The first requirement for a coating color is that the solid phase of the dispersion, which consists of fine particles of the pigment, must be effectively dispersed in the liquid phase. Where the liquid phase is an organic substance, the techniques commonly used for aqueous systems which involve ionization phenomena in a highly polar and ionic liquid are not applicable.
To achieve dispersion, the pigment particles must first be effectively wetted by the liquid phase. Further, some means of preventing flocculation (formation of aggregations of particles with eventual separation or settling) must be employed. The actual mechanism of flocculation is not certain. Electrostatic charge phenomena, surface energy forces and molecular or Van der Waals forces are probably all involved. From a practical standpoint, it is immaterial whether the particles are mutually attracted by some force effective over appreciable distances or whether they only tend to adhere when they come into contact.
A surfactant or wetting agent (rheological modifier) is used to accomplish the wetting of the pigment. The surfactant can be any surfactant which enables the employed pigment to be wetted by the organic solvent. If the surfactant does not also serve as a dispersing agent for the pigment in the organic solvent, any dispersing agent (for the pigment in the solvent) which is compatible with the system can be used for this purpose.
Certain metal carboxylates, e.g., act as both wetting agents and dispersing agents and thus serve to keep the particles from adhering after they are wetted or dispersed by the liquid phase. The preferred compound (because it is readily available, inexpensive and effective) is zinc naphthenate. Other divalent metal soaps of naphthenic acid, e.g. calcium naphthenate, and its analogues, e.g. benzoates and crotonates, are similarly appropriate.
The metal atom in the metal carboxylate is adsorbed onto the surface of the pigment particle, leaving the organic radical projecting into the surrounding medium. This organic radical makes the pigment organophilic or wettable by the organic liquid. It also prevents or deters individual particles from adhering together.
Very small amounts of metal carboxylate have a distinct effect on dispersability. The quantity required depends on the particle size and size distribution of the pigment, on the method of making the dispersion and possibly on the presence of other ingredients. The proportion of carboxylate in the formulation also affects the total rheology of the coating color; changes in carboxylate content thus afford one means of process control. Coating compositions contain, e.g., from 0.1 to 6.0 percent of wetting and dispersing agent for the pigment, such as metal carboxylate; the formulations generally comprise in the range of from 1.0 to 3.0 percent, with from 1.0 to 2.0 percent surfactant being most common (these percentages being by weight, based on the total coating composition Weight).
The method of dispersing the pigment is important. Mechanical forces must be applied to break up aggregations of pigment particles normally existing in dry pigment. A combination of high shearing forces at low speeds is obtained with a sigma-blade mixer on dough of the proper consistency. High levels of mechanical disaggregation are obtained by kneading a doughy mixture of the pigment and other ingredients with a partial quantity of the total solvent with such a mixer. This seems to give the most consistent and satisfactory product.
Storage-stable coating compositions suitable for applying a uniform thin coating on a substrate are thus obtained. Although the mixture can be prepared by other methods, most seem to have serious disadvantages. Ball-milling, for example, has been found to promote the formation of hard aggregates because of impact forces as well as shear effects on the pasty mixture. A high shear high speed mixer, such as the Cowles, requires a more fluid mixture to function and entrains too much air into the thick and viscous liquid. The efficiency and uniformity of dispersal achieved is greatly alfected by the consistency of this dough stage. It must contain sufficient liquid to be fully wetted while still retaining a stiff plastic consistency. If even slightly too much liquid is used, the dispersion is less complete unless the milling time is considerably lengthened. The proportions of solid to liquid for the best consistency obviously vary with the particular formulation. Approximately 85 percent by weight of solids to 15 percent by Weight of liquid is about average, but the percentage of solids may range from 70 to 95 percent, preferably from 80 to 90 percent by weight (based on total composition weight).
With brief experience the dough stage is readily determined by observing the milling process; once determined, it remains uniform for a given formulation. The dough contains sufiicient solvent to plasticize the binder completely so the mixture is not dry and crumbly; the dough lacks suificient solvent to enable the mixture to flow rather than shear.
All ingredients are included in the composition during the dough-stage milling. The amount of organic solvent therein, however, is sufiiciently reduced so that the cor1- sistency is that of a heavy paste or dough. When the milling is completed, the pigment is completely wetted and the doughy product is homogeneous.
Coating compositions Content Overall Typical Ingredient Pigment, percent Binder, percent Solvent, percent Surfactants, percent Dispersing agents, percent Lubricant, percent Anti foam agent, p.p.m
l Surfactant has lubricant properties.
The ranges indicated cover formulations with binder content based on pigment ranging from 10% to 25% by weight, and total solids levels ranging from 60% to 75% by weight.
The solvent is completely removed from the final coated substrate. Also, perhaps 50% by weight of the surfactants and dispersing agents consists of volatile materials which are removed in drying. The coating compositions (as color) contain at least 50% solids composed by pigment plus binder plus nonvolatile additives. As shown by the above table of ranges, the color may contain slightly less than 50% by weight of pigment, but the finished coating contains well over 50% of pigment.
The coating compositions are applied to a non-compressed dry base sheet from a solvent solution/ suspension of clay and binder to inhibit swelling action of the base sheet which ordinarily results from the application thereto of water-suspended clay and binder. For a smooth coating layer of high ink holdout, binders (polymeric reaction products and film-formers) which occlude the pigment particles (within the coating) as favored ink-receptors are employed. The balance of binder to clay and the relationship thereto of clay dimensions are important factors.
The proportion of binder to pigment must be above a certain minimum level to prevent separation of pigment particles from the coated surface (dusting or piling) and/or separation of sections of the coating layer from the substrate (picking) in the printing operation. If the proportion of binder is excessive, the pigment particles may be so coated with binder that they lose their desirable ink-receptive properties; also, an impermeable film of binder may form which prevents release of volatiles during high temperature drying of heat-setting inks and thus causes blistering or delamination of the sheet.
The optimum proportion of binder to pigment, falling between the extremes cited above, depends upon the shape, size, and size distribution of the pigment particles used since these determine the specific surface area of the pigment. When pigment with high specific surface is employed, an increased proportion of binder is necessary to provide adequate bonding between all particles. Obviously in this case a still larger proportion of binder would have to be used before the undesirable condition of complete coating of all pigment particles with binder would occur.
These factors are fully appreciated by the artisan and are readily worked into each material change in overall operation.
In applying the subject coating compositions, fiber swelling is limited, hydrogen bonding of the base is retained in its pristine strength and the resulting coated surface is coated with minimum penetration of the substrate at light coating weights.
Base paper [50 pound basis weight (3300 square feet); Sheffield Smoothness-SO to K & N Diiferences of 30 to 32; Gurley Densometer of 600 to 900 seconds] yields coated paper of Sheflield Smoothness20 to 30, K & N Differences of 19 to 23 with Gurley Porosities of 2,000 to 20,000 seconds. A considerable increase in smoothness is thus obtained on an uncompressed and resilient base to produce an offset printing sheet of high value in spite of low coat weight.
When pigment-[coarser-particle clay, e.g. SWW clay, or finer-particle clay, e.g. No. 2 coating clay]- based coating is applied (to a paper sheet-substrate or base) from non-aqueous solvent therefor, distortion swelling of fibers (in the substrate) is either non-existent or limited to at most about one twentieth that of water-swelling. The non-aqueous coating solvent does not strike far into the sheet, but remains on the surface because of solubility and swelling factors. The paper itself maintins its original structure; swelling is extremely limited and virtually no densification occurs on drying.
By restricting possible changes in paper structure in the physical sense, the tensile, tear and burst strengths and folding endurance are maintained as in the uncoated sheet; the original thickness of the base remains unaltered. [Conversely, a water-wet sheet would be subject to all kinds of pressure and drying variations] The coating remains fundamentally on the surface. Lower coat weights yield more dense film structure; higher coat weights, less dense values.
On printing, SWW clay-coated paper produces matte print whereas No. 2 (finer) clay-coated paper produces gloss print (paper remaining non-glossy). Whether subsequent printing is flat or glossy is thus effectively controlled by the pigment particle size employed in the pigment coating. A low-density paper with a high-density matte flat pigment coating is obtained.
When larger particle size (SWW) clay is used and an effective glossy coat or a final glossy print is desired, the coated paper must be supered or calendered; such treatment results in a non-bulky base and a thin sheet which yields glossier printing.
The non-aqueous solvent in the pigment coating composition does not distort, swell or penetrate the cellulose structure of the base and does not destroy the original hydrogen bonding of the sheet. The factors involved are thermodynamic in nature; the lack of aflinity of the solvent for cellulose and the insolubility of cellulose in the solvent are fundamental. Reactivity to water is retained; it is aided by the pigment and small pores. The pigmentcoated paper displays a high reactivity to ink with commensurate even wetting and surface hold out.
Pigment particles in the pigment coating are thinly coated with polymer (binder) [or even partly uncoated] to impart a correct balance of water wettability and ink receptivity for oifset printing, which involves alternate wetting by water and ink in a near instantaneous sequence.
Only a very evenly distributed web (evenly overcoated) permits printing with high screens of from 60 to 80 or even 100 dots. [A screen number of 80 requires a surface on which 6400 dots per square centimeter (cm?) can be printed. The surface must be very uniform and smooth. The usual water-suspended coatings achieve quality of oifset and rotogravure at from 8 to 12 grams (g.) per square meter (m?) of coat weight (6 to 9 pounds per 3300 square feet per side). Only with 20 g./m. (13 pounds per 3300 feet per side) is it possible to obtain the highest screens of fineness.] Such a web is produced according to this invention without high coat weights in 'view of the closed, smooth surface, high ink repellency rand porosity values obtainable with materially lower coat weights, with which fine screens (equal to rotogravure) have invariably produced proper dots and clean prints on experimental and web offset printing machines.
In the following examples all parts and percentages are by weight unless otherwise specified. The percentage of binder is based on the weight of pigment used and not on the weight of the complete system; the percentage of solids is based on the weight of those ingredients (pigment plus binder) which remain in the finished coating. In each example the solvent is dependent upon the specific resin (binder) employed; with any change in resin, a correspondingly suitable solvent is employed.
EXAMPLE 1 80.8 lb. of kaolin clay having an average particle size of approximately 3.5 microns and containing approximately 65% of particles less than two microns, 20% of particles between 2 and microns, and of particles larger than 5 microns was charged into a sigma-blade mixer of 10 gallons nominal capacity in which the mixer blades revolved at approximately 50 revolutions per minute. The mixed bowl was provided with a water jacket through which cooling water at a temperature of approximately 60 F. was circulated. [For some embodiments hot water is advantageously circulated for heating, rather than cooling; temperature control means should be available] 12.1 lb. of polyvinylacetate resin granules, having a molar viscosity (in benzene) of approximately centipoises, was charged into the mixer. The mixer was then operated approximately 10 minutes with these dry ingredients to mix and blend them together.
0.81 lb. of an amino-linked complex fatty acid ester (Proprietary name-Sotex-IO) cationic surfactant and 0.81 lb. of technical zinc naphthenate containing 8% zinc was added to and thoroughly mixed with 18.7 lb. of toluol. This mixture was then charged into the operating mixer containing the blended dry ingredients and rapidly formed a stiff doughy mass which undergoes a high shearing action by the kneading effect of the sigma blades. The mass undergoes a temperature rise as a result of the milling action which is limited to a maximum of about 130 F. by the cooling water jacket. The mixer is kept tightly covered to prevent loss of solvent by evaporation. Milling 14 of this dough stage was continued for approximately 60 minutes.
At the end of the dough stage milling operation 26.8 lb. of toluol was introduced into the mixer by trickling flow over a period of approximately 30 minutes. The mixer was stopped and inspected several times during this period; occasionally masses of thick material adhered to the blades which were scraped off into the more fluid contents. The mixer was then operated for an additional 60 minutes after complete addition of the second portion of solvent to assure obtaining a smooth and homogeneous product.
At the end of this period the contents of the mixer constitutes a finished coating color, which may be transferred to storage containers or to the coating color circulation system of a coating machine as desired. The coating color so prepared may be stored for an indefinite period without agitation in closed containers to prevent loss of solvent by evaporation. After extended storage there may be some settling of pigment but the color is readily restored to a uniform homogeneous composition by stirring or agitation.
EXAMPLE 2 Following the same procedures described in detail in Example 1, the following dry ingredients were charged into the mixer:
68.7 lb. of a kaolin clay having an average particle size of 0.8 micron and containing approximately of particles less than 2 microns, 16% of particles between 2 microns and 5 microns, and 4% of particles larger than 5 microns.
8.1 lb. of precipitated calcium carbonate having an average particle size of 0.25 micron;
4.0 lbs. of anatase titanium dioxide having an average particle size of 0.3 micron;
14.1 lb. of polyvinylacetate resin granules having a molar viscosity (in toluol) of approximately 25 centipoises;
3.23 lb. of a coco-amine surfactant and 1.62 lb. of 8% zinc naphthenate was mixed with 19.75 lb. of toluol and added to the mixer to form the dough stage material;
23.0 lb. of toluol was slowly added following the dough stage milling; the finished color was obtained following the final mixing operation.
This example produced a high opacity-high brightness coating color containing approximately 70% solids.
EXAMPLE 3 Following the same procedures described in detail in Example 1, the following dry ingredients were charged into the mixer:
80.8 lb. of a kaolin clay of the specifications indicated in Example 2;
12.1 lb. of polyvinylacetate resin granules, having a molar viscosity (in toluol) of approximately 7 centipoises;
1.6 lb. of an amino-linked fatty acid ester cationic surfactant and 1.6 lb. of 8% zinc naphthenate were mixed with 19.0 lb. of toluol and added to the mixer to form the dough stage material.
A mixture consisting of 8.8 lb. of toluol and 18.5 lb. of xylol was slowly added following the dough stage milling.
EXAMPLE 4 The coating color prepared according to Example 3 was used to produce a coated printing paper on a pilot scale trailing blade oif-machine coater capable of handling a web width of 22 inches. A suitable base paper was coated at Web speeds up to and including 2,400 feet per minute, the maximum speed for the coater as set up.
Using a trailing doctor blade of 0.018 inch (18 mils) gauge or thickness set at an angle of approximately 60 to the tangent of the backup roll, a coating with a dry weight of approximately 1.5 to 2.0 lb. per side per 3,300 ft. was obtained with a blade pressure of 10 pounds per linear inch of blade, at a web speed of 2,400 feet per minute.
What is claimed is:
1. A non-aqueous homogeneous dough-like composition useful for providing paper with an improved surface finish suitable for printing: consisting essentially of finelydivided pigment having a particle size of from 0.2 to microns, a solution of synthetic resin adhesive binder in an organic solvent and ingredient means for completely wetting the pigment with the solution; the resin binder in the solution constituting from 5 to 30 percent by weight based on the weight of the pigment, and the amount of the solvent being only that necessary for a doughy consistency of the composition.
2. A composition according to claim 1 wherein the ingredient means comprises both wetting and dispersing means for the pigment with respect to the solution.
3. A composition according to claim 2 wherein the ingredient means is a metal carboxylate.
4. A composition according to claim 1 wherein the ingredient means comprises cationic surfactant with lubricant properties.
5. A composition according to claim 4 wherein the cationic surfactant is a member selected from the group consisting of fatty acid amino salt, fatty acid ammonium soap and fatty acid ester.
6. A homogeneous storage-stable non-aqueous coating composition consisting essentially of a composition according to claim 1 and sufiicient additional organic solvent means to impart thereto a fluidity suitable for applying a uniform thin coating on a substrate, the binder being suitable for bonding the pigment particles securely together and to the substrate.
7. A coating composition according to claim 6 wherein the ingredient means comprises at least one rheological modifier selected from the group consisting of cationic surfactant, anionic surfactant, nonionic surfactant, lubricant and anifoaming agent.
8. A coating composition according to claim 7 wherein the solvent is a non-polar volatile solvent, the pigment solids constitute at least 50 percent by weight based on 16 the total coating composition weight and the binder has a molar viscosity in the solvent of from 5 to centipoises.
9. A coating composition according to claim 8 wherein the binder is polystyrene.
10. A coating composition according to claim 8 wherein the binder is de-inkable by standard processing.
11. A coating composition according to claim 10 wherein the binder is polyvinylacetate.
12. The use of a composition according to claim 1 to reduce the weight required for quality printing paper.
13. A composition according to claim 1 which is useful for imparting a high-quality printable surface to a planar substrate to which it is applied in a more fluent form.
14. A composition according to claim 13 wherein the ingredient means comprises a metal carboxylate, the metal of which is divalent and is adsorbed onto the pigment and the carboxylate of which is in the form of a naphthenate, benzoate or crotonate.
15. A composition according to claim 6 which is useful for imparting a high-quality printable surface to a planar substrate to which it is applied and which contains from 0.1 to 6.0 percent by weight of wetting and dispersing agent for the pigment.
16. A composition according to claim 15 wherein the ingredient means comprises a metal carboxylate, the metal of which is divalent and is adsorbed onto the pigment and the carboxylate of which is in the form of a naphthenate, benzoate or crotonate.
References Cited UNITED STATES PATENTS 3,117,018 1/1964 Strauss 260-23 X 3,655,608 4/ 1972 Guenther et al 26037 3,530,080 9/1970 Inskip 260-874 3,674,736 7/1972 Lerman et al. 260-41 3,640,750 2/1972 Schutzner 2604l X 3,682,684 8/1972 Newman et al 260-41 X DONALD E. CZAJA, Primary Examiner E. C. RZUCIDLO, Assistant Examiner U.S. Cl. X.R.
1l7l55 UA; 26031.2 R, 41 C