US2725307A - Process of finishing mineral-adhesive coated paper - Google Patents

Description

Nov. 29, 1955 B. E. SOOY ETAL 2,725,307

PROCESS OF FINISHING MINERAL-ADHESIVE COATED PAPER Filed Sept. 26 1951 INVENTORS- Am/mwa 1 Soar 47 6. IVs/veg,

data M ATTORNEY$ United States Patent ice PROCESS OF FINISHING MINERAL-ADHESIVE COATED PAPER Brainard E. Sooy, Middletown, and Harry C. Fisher, Cincinnati, Ohio, assignors to The Gardner Board and Carton Co., Middletown, Ohio, a corporation of Ohio Application September 26, 1951, Serial No. 248,372

' 13 Claims. (Cl. 117-65) This is a continuation-in-part of our copending application entitled Process of Finishing Mineral-Adhesive Coated Paper, Serial No. 72,172, filed January 22, 1949, now U. 5. Patent 2, 611,717 dated September 23, 1952.

Our invention relates to the problem of producing wettype calendered finishes on paper or boxboard coated with adhesive-mineral coatings, and has for its objects the obtaining of superior finishes, improved physical properties and new effects, all as will be pointed out hereinafter.

In the processes of coating paper or board with finely divided mineral matter to enhance its surface characteristics and appearance, the coating mixture itself comprises a dispersion of the mineral matter in a water solution of adhesive. The mineral matter usually is clay with or without admixture of other finely divided mineral matters, pigments and the like. The adhesive can be casein, animal glue or other proteinaceous material, starch, dextrine and similar substances. The adhesive is prepared by dissolving it in water, using chemicals to assist as may be necessary; and the mineral matter, also in water, is dispersed in the adhesive solution by mechanical means. The dispersion may be assisted by the use of dispersing agents. Other materials may be included in the coating mixture to smooth it and give it body.

The physical properties of the coatings formed when these mixtures are applied to and dried on the surface of paper or board are dependent in part on the nature of the surface to which they are applied, upon the ratio of adhesive to mineral matter and also upon the intrinsic strength of the adhesive. If the ratio of adhesive to mineral matter is small, the porous character of the minerals prevails in the applied coating, whereas with increasing adhesive content the absorptive power of the minerals decreases. The adhesive not only binds the minerals to the paper surface in the dried coating but also reduces their porosity.

Depending upon the method used to apply the wet coating mixture to the paper or board surface and upon the coated results desired, the concentration of solids in the coating mixture may vary from a few per cent, say, around up to around 60% or 70%, more or less; this is well understood in the art.

In United States Patent No. 2,419,207 to Harry C. Fisher there are disclosed steps in a process of making coated board or paper wherein the freshly made web is treated with a plasticizable, film forming, water-borne substance, for example, starch, or starch and polyvinyl alcohol, while on equipment such as a breaker stack of calenders, so that the surface is smoothed by plasticization and working. Then, immediately, a mineral coating is applied to it in an aqueous vehicle. The Web is then dried and calendered in one or more stacks of finishing calender rolls. Where the coating is applied by printing, as with a roll, the solids concentration in the coating mixture is preferably high, say, around 50% and upwards; and a very thin layer of mineral coating may be produced.

Where a coating mixture is imprinted upon the surface .of paper or board as by a roll, a characteristic pat- 2,725,307 Patented Nov. 29,1955

terned surface of the coating results due to the manner in which the wet film of coating breaks away from the imprinting roll. The brush distribution of applied coating and smoothing rolls, used in some processes, has in part the purpose of breaking up patterns. If the coating mixture is low in viscosity, the pattern is minimized by a partial flowing together of the wet mixture. If the coating mixture is stiff, the applied film may be strongly patterned. In general, the heavier the thickness of the applied film, the more prominent is its surface pattern. Absofptivity of the paper surface plays its part. Thus there are optimum conditions to meet to produce a coating of usable thickness on the surface of paper or board and with a minimum of pattern. The patterning dificulty is one reason for the desirability of a further after-treatment for the coated board.

The tendency of a mineral coating, especially one containing moderate amounts of adhesive, is to dry with a matt or dull surface. For some useses a harder, glossier and denser surface is desired. The coated board of the patent referred to above, when finished in a dry calender stack, has had a coating which has a matt or dull surface, comparatively, and which is excellent for many purposes. It can be printed with oil or varnish base inks to a dull or glossy film of dry ink, or to a glossy finish with overprint varnish depending upon the quality of coating mixture. It is, however, desirable in many instances to harden the coating to reduce the possibility of marking when passed over equipment for handling and processing the the coated board, and to produce a smoother and glossier surface which will lend itself better to the production of highly uniform matt printed surfaces or of a glossier and smoother printed film with gloss inks and varnishes.

The mineral coated boards of commerce ordinarily cannot be given a wet finish in a calender stack. The coated board of the patent just described, when coated with a non-waterproof starch-clay mixture has hitherto received only a dry finish by passing it through the nips of one or more finishing stacks of steel calender rolls, with the exception that steam, issuing at gentle pressure from a perforated pipe extending across the calender stack at an appropriate hip and blown onto the coating of the web just before it enters said nip, has been employed with an improvement of the finish and without picking or sticking difliculties in the stack. But the introduction of water ino the calender box of a sack of finishing calenders through which the coated paper or board is passing soon results in a dissolving off of coating substances into the water, a picking oil? of the coating onto the rolls, and sticking together of the coating itself over the surface and back of the board.

The copending applications of Harry C. Fisher and Brainard E. Sooy, Serial No. 558,042, now abandoned, filed October 10, 1944, and Serial No. 738,772, filed April 1, 1947, now Patent 2,515,340, dated July 18, 1950, teach procedures by which the starch of a starchclay coating on paper or board can be insolubilized and rendered waterproof. There is also a known process of coating webs in which the adhesive has to be intrinsically water resistant. In this process the coating mixture is applied and doctored onto the still damp web early in the heated dryer roll section of the paper machine, and the coated web is passed around and contacts the alternate remaining dryer rolls, passing thence through a calender stack. Starch does not perform well in this process because even though it be treated to become insoluble in water eventually, it remains soluble in the coatingfor some little time, and fouls up the heated dryer rolls of the board machine. The process employs casein or other proteinaceous adhesive material of a kind which is intrinsically water-resistant but can 3 be dispersed for coating purposes; the dispersion reverts to the water-resistant condition as soon as the water of the coating mixture is reduced or removed, and complete insolubility is achieved by use of an agent like formaldehyde.

The coated web of the latterprocess, as well as the coated webs of the copending applications referred to above, may be calendered in the presence of water without fouling the calender stack rolls if the coatings have first been dried and sufliciently insolubilized. However, if the coating, or rather its adhesive, is completely insoluble in water, water is ineffective on the calender stack and the wet calendering action does not produce the highly smooth, even, glossy, surface frequently desired. Only to the extent that the surface of the coating is softened and dissolved, more or less, by the liquid used on the calender stack can the surface become plastic and workable into a smooth glossy condition by the action of the calender roll nips. This is especially true with insolubilized starch, and to a lesser extent with casein, which latter is more plastic by nature.

Aside from the difficulties and expense of employing adhesives which are or become substantially completely insoluble in water, since wet finishes could hitherto only be applied to mineral coated boards which had been thoroughly dried and had lost whatever plasticity the coating as such possessed, it has not hitherto been possible to apply a water finish to coated boards in such manner as to take advantage of the smoothing action of calender finishing as applied to a coating having resident plasticity. In the process of this invention we have found a way in which a water finishing operation may be correlated with coating operations'in such manner as to obtain the ultimate in smoothness, removal of surface pattern, hardness and glossiness of coating, whether ultimately insoluble in water or not, and without the fouling of the stack of calender rolls.

Prior attempts to calender a mineral coated sheet in which starch or other binder is not completely water resistant, in the presence of a calender box full of water, have been disastrous either immediately or after a short time has elapsed. The coating mixture may be loosened from the surface of the sheet thus destroying the coated surface. Often stickiness of the adhesive in the coating develops at once, and this causes the coating to pick ofii the web, stick to the calender rolls and adhere to other parts of the coated web on the stack. Methods have been suggested'in which a rubberdoctor is placed against the surface of a steelcalender roll between its nip and the lower calender box containing the water, whereby the doctor could control the amount of Water entering the nip against the coated web surface. It was thought thata strict control of the amount of the applied water would reduce the sticking troubles. The success of this procedure depended upon the skill with which it was employed; it was infrequently successful; and in many instances the amount of water which could be tolerated at the nip of the rolls was not suificient to enhance the smoothing effect and produce commercial improvement in the coating.

In a copending application for patent of Harry C. Fisher and Brainard E. Sooy, Serial No. 613,436, now U. S. Patent No. 2,656,286, filed August 29, 1945, we taught the procedure of sealing the surface of an insolubilized mineral coating on board by coating it with a thin layer of starch, protein, sodium silicate, cellulose compound, etc., applied from water solution as from the water box'of the finishing stack of board machine calendars. We did this to reduce the absorption of the vehicle of oil or varnish base'printing inks into into the mineral coated surface, which, itself, remains non-glossy, thus to enhance the gloss of the dried ink or varnish film printed onto the coated surface. The process operates only with coating that is substantially completely insolubilized, and does not substantially alter the original non-gloss characteristics of the coating.

We have found that while the softening of a water soluble adhesive in a mineral coating on paper or board renders it impracticable to water finish the product through the use of water alone in the water boxes of a calender stack, it is readily possible so to aifect the rate of solubility of the adhesive of the coating in the liquid placed in the water boxes as (a) to attain such softening of the coating as will permit the formation of a true water finish and the elimination of the pattern if any in the coating as initially applied, while (b) preventing so much softening of the coating adhesive as will produce a tacky condition during the passage of the stock through the finishing calender stack. In this way water finishes are attained but the picking action and fouling of the calender rolls discussed above is avoided. We accomplish this by adding to the water to be used in the water boxes an organic material freely soluble or miscible with the water but a non-solvent for the adhesive of the mineral coating.

Preliminary to our discovery, we reasoned that an enhanced smooth glossy surface of coating could result only if surface of the coating were softened and dissolved, as by plain water, at the calender nip and then re-smoothed and compacted by it and the following calender nips. Further, it was reasoned that by substituting for the Water on the calender stack, a liquid in which the coating is insoluble, e. g., methyl alcohol, no enhanced smoothing and glossing of the coated surface could result. Both premises were found to be true. We have now discovcred that for any given coating some simple combination of liquids, two or more, containing a solvent and a nonsolvent for adhesive of the coating, are usable on the calender stack to produce a smooth glossy finish of coated surface as desired, and without fouling the calender rolls or the coated surface itself. These liquids should be ultimately volatile to leave the calendered coated surface without residue or discoloration.

By way of example, when a solution made by mixing 60 parts water and 40 parts methyl alcohol by volume was used in a water box on the top nip of a finishing stack containing seven polished metal calender rolls through which a freshly coated, dried web of paper board was passing, the adhesive of the coating being water soluble, and the coating comprising starch and minerals, the velvety matt surface of coating ordinarily obtained was enhanced to become highly smooth and glossy, almost to a mirror-like degree. The-calender rolls remained clean, and the coated surface did not stick to them, pick or become marred. The calendered surface was dried in a normal manner. In the use of a solution of methyl alcohol and water, the water dissolves and softens the adhesive of the coating, while the alcohol in which the adhesive is insoluble acts as a diluent to retard and control the solvent action, we believe.

Results similar to those just described were obtained with water-methyl alcohol liquid on starch-mineral coated board in which the process of insolubilizing the starch with urea formaldehyde resin was in progress, as in the copending applications of Harry C. Fisher and Brainard E. Sooy, Serial Nos. 558,042 and 738,772 referred to above. Here the retarding action of alcohol or equivalent substance remains important but plays a lesser role because the proofed starch adhesive coating already has lower solubility in water and in consequence less alcohol or other non-solvent for the coating adhesive may be needed.

While it is possible and useful to include a soluble substance inour alcohol-water calender solution and from it deposit a very thin film of said substance over the surface of the coating on the web while it is being calendered, it is not our primary desire to=do so. We prefer to improve the smoothness and glossiness of surface of a coating without the imposition of anything to alter the printingcharacteristics of the particular starch-mineral coating mixture itself. Of course, we can color or dye our alcohol-water solution and use it to color the mineral coating itself.

We do not confine ourselves to methyl alcohol in our alcohol-water type of calender solution, but do state that it has properties valuable for our use. 1 It dissolves in water, is not sticky in solution, does not dissolve the starch-adhesive of the coating, is not too volatile but evaporates awayfrom the calendered coated surface without leaving a residue of stain or odor thereon, and is not too expensive. Acetone can be used but is too volatile and is also expensive. Ethyl alcohol is expensive, but is satisfactory otherwise. On the other hand, 'glycerine does not dissolve the coating but does. not evaporate off at all, leaves a stain on the coating, and makes a sticky solution in water.

As an illustration of a procedure in accordance with our invention, reference is now made to the drawing wherein a web of paper or paperboard 1 is shown leaving the last roll 2 of the heated drying rolls of a paper machine and passing to a calender stack'indicated generally at 3. A plasticizing solution such as a solution of starch or starch and polyvinyl alcohol is applied to the surface of the web on the calender stack 3 as by means of one or more water boxes 4; and the result of the action of the calender roll nips of the stack on the treated surface of thetweb is to plasticize it and prepare it for the reception of the mineral coating. The mineral coating is applied promptly while the surface of the web retains its plasticity, by means of an imprinting roll 5, receiving the coating mixture from a supply 6 by means of metering and doctoring rolls 7. The coated board then passes to a series of heated drying rolls 8. Adjacent the first of these there may if desired be a hood 9 where hot air may be blown against the coated surface of the board to accelerate the initial drying before that surface contacts the second one of the drying rolls 8.

The procedure thus far described is characteristic of the procedure of Patent 2,419,207 referred to above; but we are not restricted to it. Other procedures of imposing the coating may be employed; but we prefer to practice our finishing treatment upon freshly coated board and while the coating retains a resident plasticity. The adhesives employed in the coating mixture may be watersoluble, or water-insoluble, or may be treated to render them water-insoluble in accordance with the procedures of the copending applications referred to above or in other manners.

The calendering of the coated paper or board in the finishing calender stack 10 is done at the pressures usually characteristic of finishing stack calendering, say, varying up to some 300 poundsper lineal inch of nip line of polished steel rolls, depending upon the calender stack and the calender nip concerned. We may use a plurality of finishing stacks if desired, and practice our treatment in connection with one or all of them, any stacks upon which our treatment is not applied serving to impart a further dry finish to the web.

In one or more water boxes 11 on the finishing calender stack 10, we employ water containing an organic agent modifying the rate at which the water can dissolve or soften the adhesive of the mineral coating previously imposed upon the board. An alcohol-water solution, by way of example, is relatively volatile, and since the solutions in the calender water boxes tend to become warmed from the heated condition of the paper or board passing through the calender stack, the solution should be handled in such manner that its alcohol-water ratio is preserved. This is best done by balancing the fiow of solution to the calender nip with its consumption in the nip so as to minimize the quantity of the solution subjected at any time to heating eifects productive of differential evaporation. It is, of course, possible to compensate by additions of alcohol for alcohol lost by evaporation, but this increases the expense. We are not limited to the application of the solution by water boxes on the calender since other methods including spraying the solution upon one or more of the calender rolls or upon the coated surface of the web itself may be employed if desired. As is well understood in the art, a calender stack consists of a plurality of heavy, polished steel rolls in a stand resting by weight at least against each other with the interposition of the paper or board therebetween. A system of compounded weighted levers usually is employed to increase the nip pressure between the rolls. The amount of sub stance which can be applied on a calender stack by means of the water boxes or otherwise is thus effectively strictly limited. Ordinarily, by means of a water box, only about 10 gallons or less of water may be applied per ton of board of thickness .016 in. Loss of organic liquids in our process is thus not economically of great consequence; but solvent recovery procedures may be employed if desired, and in any event, if the fumes of the organic liquid employed are unpleasant, adequate venting means will be used.

We have found that the addition of other substances of quality and in quantity such as will not in themselves affect the character of the reduced solvent action of the water solution of organic material therein, or produce tackiness therein does not in any way inhibit that action which permits us to apply a water finish to mineral coated. webs in accordance with this invention. For example, the solution applied in the water boxes 11 may contain wetting or penetrating agents, acidic or alkaline materials, soluble or dispersible dyestuffs or coloring matters, as well as small amounts of film-forming substances that promote gloss imprinting, such as polyvinyl alcohol, soluble celluloses like carboxymethyl cellulose and hydroxyethyl cellulose, alginic substances, proteins, starches and the like, and by the methods herein taught such solutions can be applied over the mineral coating in a calender finishing operation which forms a wet finish and employs the useful effect of the plasticization of the mineral coating itself. The film-forming agents mentioned above enhance the sealing of the pores of the coating in the finished coated board as treated in accordance with our process.

Again, we may include other materials in the calender box solution. Where it is desired to enhance the waterproof characteristics of the coating, as in the case of coatings which must stand rough handling under moist conditions, or in the case of coatings which are colored and must not smudge or offset if handled under conditions of dampness, we may include a small amount of resinous material in the finishing calender solution with or without any of the several substances mentioned above, in order further to waterproof and harden the surface coating.

We have found that a finishing calender surface application of the alcohol-water type of solution, as herein taught, but with the solution slightly acidified, promotes flat printing with oil or varnish base inks without sacrificing the high quality of gloss ink printing or over-varnishing where otherwise obtainable. A similar action appears to take place with paper or board which has not been coated with mineral adhesive coatings; but a critical feature here is that acid treatments against a cellulosic surface may be undesirable, especially as the board or paper ages. An acid treatment applied over the coating as herein taught, however, does not adversely affect the quality of the board, while producing the printing efifects which have just been described.

Other advantages are inherent in our process. Whereas starch-mineral coated surfaces, particularly those which arenot waterproof, when given the heretofore possible dry calender finishes, marred and marked easily when rubbed or passed over mechanical parts of machinery, scored with the fingernail, and the like, the same coating, calender finished with the alcohol-water type of solution in accordance with our invention, with or without a content of other subsances in-such solutions, no longer can be-marked easily and is more valuableforthat-rea son. The coated'surface-has become-harder, smoother and glossier and ih such-condition performs better where two coated surfaces must rub and turn upon each other.

Our procedure results in producing a smooth, glossy, sized, coated surface on the web, in which the pattern of the coating is largely'redueed, the coating-liav'ingbeensoftened by the solution applied on the calender rolls, then smoothed and compacted'by the calenderrollnips. The resulting sheet has greatly enhanced printingquah ities, good appearance, and can be as bright and white as desired, or can be colored to-desired hues. have other properties as dictated by the nature of the mineral coating itself.

Soluble ordispersed dyestuffs or other coloring matters may be included in-the calender solution to color-the surface of the coating or'to modify it-if already colored by dyestuffs or coloring matters therein, orthereunder as taught in U. S. Patent-No. 2,370,344 entilted lm'parting Color to Coated Paper Board, in the nameof Harry G. Fisher, or-by various combinations of these procedures.

Our procedure operates on mineral co'ated boards in which the adhesive is untreated, water-soluble starch, as well as with coatings in which the adhesive is casein or proteinaceous substances. Furthermore, the calender-ingaction'is not dependent upon any specific previous state of dryness, hardness or plasticity in-the coating itself. Any of thehitherto known mineral coatings'may be calendered in accordance with our process. A great advantage of our process is that the finishing action' occurs while the coating at least retains a degree of plasticity which assists the smoothing action. Nevertheless, the coating as the result of-thetreatment has enhanced hardness and gloss as well as enhanced smoothness. are not limited to the treatment of board immediately after coating. Our process is of value in providing water finishes on any board or paper previously coated with a mineral coating in a binder'having water soluble-characteristicsat the time of our treatment, such'that it may be sufficiently softened for water finishing purposes- It is impracticable to attempt to water finish suclr paper or board in the ordinary way for reasons which have been given above. A- salient feature of our procedure is the water finishing ofmineral coated paper or board by means of water applied during the finishing procedure, where the rate of solubility of the adhesive of the mineral coating in the finishing water so applied, is modified orretarded to the-extent of permitting a softening to occur for finishing purposes, without permitting sufiicient tackiness-to develop to cause pickingand sticking. The primary object of this invention is accomplished by adding tothe'water used in finishing an organic material which will retard the rate of solubility of the adhesive of the coati'ngtherein, and which at the same time will be volatiles We are not limited to the particular organic liquids mentioned above since others will serve in our process; but the alcohols adequately fulfill the requirements of our process and are satisfactorily inexpensive.

Modifications may be made in our invention without departing from=the spiritthereof; Having thus described our invention in certain exemplary embodiments, what we claim as new and desire to secure by Letters Patent-is:

1. A process of producing'water finishes on webs of paper or paperboard coated with amineral-adhesive coat- It will ing, the adhesive of which is a material chosen from a class consisting of amylae'eous and pretiiiaeeonsqau hesivesand is in a watr softnableconditi which comprises passing said webs-through a steels-er arseniccalender rolls and applying to the calender rdlls engaged by the'coatedsurfac'e of said'wb'sawater'sohitioii (if an organic, highly volatile liquid c'a pablof diminishing-the rate of solubility in said w'ater' of the adhesive or said mineral-adhesive c'c wltingz 2. The process claimed in claim 1, wherein the" adhesive of said mineral-adhesive coating' i's' sta rch 3. The process" claimed in claim 1 wherein" the 36-" hesive of said mineral-adhesivecoating i'sstarch, and said substance dissolved in the applied water isalcohol.

4. The process claimed in claim 1, wlierein the adhesive of said mineral-adhesive"coating is' protein:

5. The process of claim 1 in which the solution applied on the said calenders is a solution of alcohol in water.

6. The'process of claim Sin which the proportionsof waterto alcohol inthe calender solutioir areadjusted, the water upwards to adjust an'eifective optimum softening action on the surface of' the' coating, and the" alcohol upwardsto exert a'restrainingeflectupon said softem'ng actionof the water to prevent sticking-or picking of the coated surface on the'surface of "the calender rolls;

7. The process of claim 5 in which the 'solutionoi alcohol in water is made by'mi-xin'g together-two volumes of alcohol with three volumes of water 8. The'process of'claim Sin'whichthe=alcohol is methyl alcohol.

9.- The process of claim-5 in which thealcoholisethyl alcohol;

10. The process of claim 5 in which-the" solution in water is colored by a dyestu'if.

11. The process of claim 5 in whichthe" mineral-adhesive coating is a coating of-mineral-matter-in a'starch adhesive, which is non-waterproof;

12. The process of claim 6 in 'which the' adjusted solution of two liquids is slightly acidified;-

13. A process of producing-coated webs of paper-or paperboard which comprises 'p'assing'thefreshly made webs through a breaker stack of calender rolls; applying to the webs a plasticizable, film-forming; waterb'orne substancechemically inert to-the fibers of the 'web; and working and plasticizing the surface thereof, then duringa persistence of the plasticization, imprinting-upo'n the surfaces of said webs a mineral-adhesive coating; said coating being chosen from-a class consisting ofamylaceous and proteinaceous adhesives of water-softenable character, drying the webs, and further during-apersistence of the plasticizing efiect, passing said webs through a stack of metallic calender rolls and applying to the coated surfaces thereof on saidcalender rolls a water solution of an organic, highly volatile liquidcapable of diminishing the rate of solubility in said water of the adhesive of said mineral-adhesive coating.

References Cited in the file'of this patent UNI-TED --s-"rA-"rEs PATENTS" 1,725,699 Boyce Aug.- 205 1929 2,003,065 Boyce May 28, 1935 2,088,893 Cares Aug. 3, 1937 2,293,278 Cates Aug. 18, 1942 2,304,818 Grupe Dec. 15, 1942' 2,331,922 Montgomery Oct. 19, 1943

Claims (1)

1. A PROCESS OF PRODUCING WATER FINISHES ON WEBS OF PAPER OR PAPERBOARD COATED WITH A MINERAL-ADHESIVE COATING, THE ADHESIVE OF WHICH IS A MINERAL CHOSEN FROM A CLASS CONSISTING OF AMYLACEOUS AND PROTENACEOUS ADHESIVES AND IS IN A WATER-SOFTENABLE CONDITION, WHICH COMPRISES PASSING SAID WEBS THROUGH A STACK OF METALLIC CYLINDER ROLLS AND APPLYING TO THE CALENDER ROLLS ENGAGED BY THE COATED SURFACE OF SAID WEBS A WET SOLUTION OF AN ORGANIC, HIGHLY VOLATILE LIQUID CAPABLE OF DIMINSHING THE RATE OF SOLUBILITY IN SAID WATER OF THE ADHESIVE OF SAID MINERAL-ADHESIVE COATING.

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