US2425231A

US2425231A - Method of coating paper and composition therefor

Info

Publication number: US2425231A
Application number: US519542A
Authority: US
Inventors: Gilbert K Dickerman; Robert W Riley
Original assignee: Consolidated Water Power and Paper Co
Current assignee: Consolidated Water Power and Paper Co
Priority date: 1944-01-24
Filing date: 1944-01-24
Publication date: 1947-08-05
Anticipated expiration: 1964-08-05

Description

Patented Aug. 5, 1947 METHOD OF COATING PAPER. AND COIVIPOSITION THEREFOR Gilbert K. Dickerman and Robert W. Riley, Wisconsin Rapids, Wis., assignors to Consolidated Water Power & Paper Company, Wisconsin Rapids, Wis., a corporation of Wisconsin No Drawing. Application January 24, 1944, Serial No. 519,542

13 Claims. 1

This is a continuation in part of our co-pending application for patent, Serial N 0. 439,766, filed April 20, 1942, which is a continuation in part of our application, Serial No. 280,732, filed June 23, 1939. The present invention relates particularly to an improved paper coating composition of the mineral type for forming a mineral coated surface upon paper which is receptive to fine printing, and a method of using such coating material in a machine or roll coating operation.

It is well known that the printing quality of a sheet of paper is dependent upon the uniform flatness and character of the printing surface. In some machine coating operations of the roll coating type, uniformity is different of attainment. However, because of the speed of such processes and the economies effected thereby, imperfections in the coated surface have been heretofore tolerated.

Typical machine coating operations of the roll coating type, to which this invention is adapted, are exemplified by the process set forth in the patents to Peter J. Massey, Nos. 1,921,368 and 1,921,369, and the apparatus of the Massey et al., Patent No. 2,105,981.

The Massey coating process is potentially operable at relatively high speeds, up to 1,000 linear feet per minute web speed or higher, as opposed to conventional coating methods heretofore practiced where the maximum web speed seldom exceeds 200 to 400 feet per minute. As a consequence the Massey process is capable of being performed in conjunction with, and as a step in,

the process of making paper, or more strictly speaking, making coated paper, as opposed to coating paper which has been previously separately made.

It is an object of the present invention to provide a coating composition particularly adapted to take the fullest advantage of the highest speed potentialities of the Massey or machine coating operations, and to enable the coating operation to effectively take place without sacrifice in character or quality of the resulting coating formed on the paper, at high paper making speeds of from about 800 to about 1,000 linear feet per minute.

For use in machine coating operations of the class herein contemplated, the coating material must be flowable in the sense that it can be or metering rolls and transferable in controlled quantities from roll surface to roll surface and from theapplicator or coating roll surface to the paper web surface in controlled amount and in such form as to require no further working or smoothing after application to the web surface.

In addition to having the foregoing characteristics, the coating composition of the present inventionenables the formation therefrom of preformed films, on the rolls of the coating apparatus, of relatively high density which facilitates transfer to the paper web at such high speeds. The composition is also adapted to be formed in relatively thin films of high solids content and to be coated on paper in a relatively heavy weight at such high speeds, without the formation of ridges, which ridges would otherwise be formed with prior thicker films of conversely greater water content. Our low water content coating composition additionally contributes to the ability to effectively operate at high speeds by reason of the fact that the excess of water that must be rapidly absorbed by the sheet to prevent sticking on the dryers, is at a minimum, and of course the low moisture content minimizes the amount of water necessary to be evaporated from the sheet, with attendant added advantage in the operation cost. In all of those prior coating processes wherein the coating material, after having been initially applied to the surface of the web, is brushed,

- scraped or subjected to an air stream traveling at relatively high velocity to smooth or mold the adaptable for the reception of fine printing, the coating material employed must be substantially liquid, the term being employed in its strict sense as connoting a substance which is substantially a true Newtonian liquid. In a true Newtonian liquid the rate of shear is directly proportional linearly to the shearing stress and the coating liquid has a zero yield point. It is this characteristic of the coating materials heretofore employed which renders them suitable for after working" coating operations, that is, the type of coating operations hereinbefore described. If the coating materials had an appreciable yield point or were of relatively low mobility, they could not be smoothed on the web by brushing, scraping, etc.

The coating material of the present invention is a plastic or pseudo-plastic, having an appreciable yield point or apparent yield point and being in general of lesser mobility than the liquid coating materials heretofore employed. In our process of employing our coating material theexistence of an appreciable yield point or apparent yield point is used to advantage in maintaining the coating material relatively immobile or static, so to speak, after it has been initially applied to the web, since it is contemplated that inbefore described, must possess those transfer properties which exist in a coating material which has a yield point or apparent yield point between predetermined limits and has a mobility within predetermined limits. Such a coating material can be transferred from roll surface to roll surface, or from the applicator roll surface to the web to be coated without streaking and without the aqueous content of the coating material, normally relatively low, from leaving the coatin too rapidly or remaining too long in the coating material.

We have found that our coating material functions satisfactorily if, in addition to its critical limitations of total solids and adhesive content, it has a yield point or apparent yield point between about 500 and 5,200 dynes per square centimeter and has a range of mobility between about 0.0575 to 0.750 rhe, (mobility being the rate of change of shearing rate with respect to shearing stress as obtained throughout the linear regime of the relationship. Expressed mathematically,

where the rate of shear, s, is expressed in centimeters per second per centimeter and the shearing stress I, is expressed in dynes per square centimeter.) These values of yield point and mobility have been derived from measurements obtained on the Stormer viscosimeter modified as follows:

A variable weight was used as the load on the driving pulley; a solid brass cylindrical rotor was substituted for the standard rotor, the cylinder having a radius of 1.03 centimeter, a length of 3.20 centimeters and a shaft diameter of 0.6 centimeter; and the standard baffles cup was replaced by a jar having an inside diameter of about 8.5 centimeters and filled to a depth of about 6 centimeters, the rotor being immersed in the coating material in the center of the jar to a depth of 1 centimeter on the rotor shaft. The force exerted at the rotor surface wascomputed to be 5.23 dynes per square centimeter per gram applied weight. The viscous shear, which is the linear rate of shear per unit distance between shearing planes or centimeters per second per centimeter, was determined by calibration of the instrument agagnst fluids of known viscosity, and was found to e where t represents the period of one revolution of the rotor.

That characteristic of our coating material which indicates that it falls within the maximum and minimum limits of yield point and within the maximum and minimum limits of mobility hereinbefore expressed will hereinafter be referred to as body." Thus, coating materials heretofore employed having not less than 0.50 the mobility and substantially zero yield point, have no body. Body may be expressed in the dimensions of viscosity but differs from viscosity in that it takes into account both mobility and a definite yield point or apparent yield point within a predetermined range, whereas viscosity contemplates the relationship of the rate or shear and shearing stress for a material having zero yield point.

We have found that a coating material made in accordance with our invention has a body of about 5 to 150 seconds as measured on a Stormer viscometer modified as above, using a 500 gram load on the driving pulley and expressed as the period of revolutions of the rotor, the operations being performed at 40 C., and the expres-' sion body as used in the specification and claims hereof shall be considered as defined herein.

The coating materials having a body of 5 to seconds contemplated herein are of the type comprising a mineral pigment and an adhesive in an aqueous carrier. A suitable deflocculating or dispersing agent may be employed, particularly where the type of mineral pigment is such that a suitable slurry cannot be made without it. Auxiliaries, or materials for special purposes may also be added to meet certain conditions. The pigments which may be used comprise clay, calcium carbonate, calcium sulphate, barium sulphate, titanium oxide, zinc sulphide, satin white, zinc oxide, aluminum hydrate or mixtures thereof. The effect of the individual pigments upon the body of coating material may difier, for instance, some pigments, other conditions remaining the same, may tend to raise the body of the material more than others. The quantity of pigment, in general, appears to have a greater effect upon the apparent yield of the coating material than upon the mobility thereof. However, it does to a proportional extent exert an influence upon both factors which constitute the body of the material.

Some types of pigments, such as refined coating clay, generally require the aid of a dispersing agent to form a defiocculated suspension in water. Others, such as a water dispersing grade of titanium dioxide may be suspended in water in a deflocculating condition without such aid. If, 7

however, a deflocculating agent is necessary, the deflocculating or dispersing agents contemplated comprise sodium silicate, tetra-sodium pyrophosphate, di-sodium phosphate and tri-sodium phosphate, sometimes supplemented by an agent such as caustic soda, soda ash, or a soap, such as ammonium stearate, sodium oleate or sodium stearate. In those types of mineral pigments which contain residual coagulants, 0 to 6 parts of dispersing agent per 970 quarts of mineral pigment (bone dry weight) has been found to be satisfactory.

The body of the coating material may be controlled to a predetermined extent by the solids content thereof, the solids content having a pronounced efiect upon the apparent yield point of the pseudo-plastic coating material, e. g., as the percentage of solids increases the apparent yield point increases.

The adhesive employed may comprise starch,

casein, soy bean protein, glue or mixtures thereof, and the amount of adhesive employed relative to the remaining ingredients of the coating material greatly influences the mobility of the material. For example, a high viscosity starch will produce a coating of relatively low mobility and vice versa, other factors being equal. Prepared starches can be obtained with a wide range of viscosities, but for control purposes it is advantageous to modify an untreated starch so as to obtain a paste with the desired characteristics which may be varied at the discretion of the operator.

The control of mobility may also be exercised by the use of auxiliaries added to the coating material. For instance, the viscosity of a. starchsized coating can be increased, thereby decreasing th mobility of the coating, through the use of various soaps and fatty acid or fatty alcohol derivatives, such as ammonium or sodium stearates, sulfated fatty alcohols, sulfonated castor oils, or the like. A relatively small amount of these materials is sufficient to produce a. large effect on the mobility of the coating. The auxiliary material 'may be added to the coating mixture either before or following the addition of the starch, although it is preferred to add it after the adhesive, since the largest effect is produced in this way, and the mixing operation is simplified.

The amount of soap employed is logically based upon the adhesive, since its chief effects are the result of a definite interaction between the soap and the adhesive. Preferably 2.0 to parts of soap based on 100 parts of adhesive is employed. However, depending upon the result desired no soap may be used, or as high as 30 parts soap per 100 parts adhesive may be employed. When using starch as the adhesive, ammonium stearate is the preferred soap, although the other auxiliaries hereinbefore set forth also produce desired resuits,

The kind of starch employed is a factor if the mobility of the coating composition is to be controlled vby the effect of the auxiliaries upon the starch, since not all starches are thickened to the same extent by additions of soap. Raw tapioca or pearl starch, modified by the action of enzymes is preferred, although chlorinated and other prepared starches exhibit the same quality of sensitiveness to soap.

The adhesive content of the coatin material is also dependent to a large degree upon the use requirements of the paper. A relatively large ad hesive content, for instance, gives a high pick test, and low absorbency for printing inks and a low adhesive content gives a relatively low pick test, and a high degree of absorbency for ink.

The adhesive content of our coating material for paper ranges between about 60 to 300 parts per 970 parts of pigment (bone dry) and the preferred range is between 120 and 200 parts adhesive to 970 parts pigment. Below about 60 parts adhesive solids to 970 .parts pigment the coating material doesnot have the desired body hereinbefore specified, and above about 300 parts of adhesive solids to 970 parts pigment the coating becomes more grease-proof and resistant to ink and therefore makes the surface a poor one for printins.

Other agents when added to starch-sized coating mixtures may be employed to control the mobility of the coating composition. For instance, dispersions of casein, locust bean gum, soluble alginates, methyl cellulose, ethyl cellulose and other water-soluble resins and solutions of borax may be employed as mobility control agents.

For the purpose of the aforementioned highest speed roll or machine coating operations as contemplated by the present invention and the simultaneous production of fiat coatings of highly printable quality it is essential that our coatin material have a relatively high solids content, such as for example from at least about 50 to about 60-62 per cent solids.

One of the advantages of such high solids content is that a heavier weight of coating can be applied per pass. For example with a 45% solids content, 6 pounds per side per ream can be applied, whereas-with a 60% solids content 13 pounds per side per ream may be applied.

A further advantage. as previously indicated, is that with such greater densities better films can be formed, or preformed, on the coating or applicator roll which facilitates transfer therefrom and application to the passing paper web. With higher solids content, and conversely lower water content, relatively thin films can be formed, the advantage of such thin films being that they are smoother as distinguished from films of relatively higher water content which must be thicker to transfer properly and deposit a desired amount of solids and which consequently are conducive to undesirable formation of ridges.

Another and most important advantage is the ability to operate at the aforementioned high speeds. Thus, for example, with a solids content of about 53% the coating operation has been successfully conducted at a linear web speed of 930 feet per minute, as distinguished from the necessity to slow down to, for example, a linear speed of about 750 feet per minute when utilizing a solids content of 45%, and to even appreciably slower speeds with still lower solids content. It will be evident by the increase of solids content of 8% (between the 45% solids composition and the 53% solids composition) that a 24% increase in speed of production is obtained, without sacrifice in quality. Such increase in speed of coating enables the production of the paper web at like high speeds, and a consistently high, and if desired continuous, speed of coating, or over all high speed production of coated paper, The tremendous commercial and economical factors involved will be readily apparent as a result of our improved coating composition, and viewed in the light of the foregoing comparison the provision of a successful coating composition of at least about 50% solids content renders operations with compositions even of 45% solids content commercially disadvantageous under competitive conditions.

Added to the foregoing is the advantage of lower water content which entails lower costs of evaporation, in addition to the operational benefits of ability of the web to readily absorb the requisite amount of water from the coating composition so as to prevent sticking on the dryers, which is a factor in the ability to conduct the coating operation at such indicated high speeds.

As a general rule it is desirable to have as high a body and solids content as possible and the upper limits herein expressed are considered ing away from the coating rolls; the penetration of the coating material into the sheet is too slow causing a build-up of coating material on the distributing and coating rolls and sticking of the coated sheet to the driers; and at high speeds the problem of penetration is particularly acute.

As examples of our coating composition the following are set forth for the purpose of illustration, but not of limitation:

This coating material has a body of approximately 150 seconds; a total solids content of 56.6 per cent; an adhesive content of 140 parts based on 970 parts of bone dry mineral pigment; a mobility of 0.09 rhe, and an apparent yield of 5130dynes per square centimeter. This coating material is ideally adapted for the application of a relatively heavy coating to an absorbent sheet at relatively high speed.

Pounds of Pounds of Material Bone Dry Water Coating Clay 1,000 070 so 25% Starch Paste 560 140 420 Tetra Sodium Pyro Phosphate (Anhydrous) 2.0 2.0 Water 400 400 Total 1,962 1,112 850 This coating material has a body of approximately 6.2 seconds; a solids content of 56.7 per cent; a mobility of 0.28 rhe; and an apparent yield of 600 dynes per square centimeter. This coating material is ideally adapted for the application of a relatively heavy coating to a comparatively non-absorbentor resistant sheet at relatively high speed.

Pounds of Pounds of Material Bone Dry water Coating Clay 1,000 970 30 25% Starch Paste 320 80 240 Water 700 700 Total 2, 020 1, 050 970 This coating material has a body of approximately seconds; a total solids content of 52 per cent; and an adhesive content of 80 parts based on 970 parts of bone dry mineral pigment. This coating material is ideally adapted for the application of a relatively lighter coating to an absorbent sheet at relatively high speed.

We claim as our invention:

1. A mineral coating composition adaptable for the coating of paper by the machine coating method in a high speed paper making process which comprises, an aqueous dispersion of a mineral pigment, a defiocculating agent for said pigment, from about 60 to about 300 parts of enzyme modified starch adhesive based on 970 parts of bone dry mineral pigment, and about 2 to 10 parts of ammonium stearate based on 100 parts Ti 8 of said starch, said composition having asolids content of from at least about 50 to about 60-82 percent, an apparent yield point of between 500 and 5,200 dynes per square centimeter and a range of mobility between about 0.0575 and 0.750 rhe.

2. A mineral coating composition adaptable for the coating of paper by the machine coating method in a high speed paper making process which comprises, an aqueous dispersion of a mineral pigment, a small amount. of tetra-sodium pyrophosphate as a deiiocculating agent for said pigment, from about 120 to about 200 parts enzyme modified starch adhesive based on 970 parts of bone dry mineral pigment, and about 2 to 10 parts of ammonium stearate based on parts of said starch, said composition having a solids content of from at least about 50 to about 60-62 per cent, and being of a thixotropic nature.

3. The process of coating paper webs in a continuous paper making process which comprises, continuously spreading onto a rotating surface a controlled amount of a mobile, dense, thin, smooth coating composition film, said coating composition comprising an aqueous dispersion of a mineral pigment, a deflocculatlng agent for said pigment, from about 60 to about 300 parts of enzyme modified starch adhesive based on 970 parts of bone dry mineral pigment, and about 2 to 10 parts of ammonium stearate based on 100 parts of said starch, said composition having a solids content of from at least about 50 to about 60-62 per cent, an apparent yield point of between 500 to 5,200 dynes per square centimeter and a range of mobility between about 0.0575 and 0.750 rhe, and transferring said composition in the form of a preformed film from said rotating surface to the surface of a paper web passing forwardly at a linear speed of from about 800 to about 1,000 feet per minute to provide the web with a highly printable smooth surface.

4. The process of coating paper webs in a continuous paper making process which comprises, continuously spreading onto a rotating surface a controlled amount of a mobile, dense, thin, smooth coating composition film, said coating composition comprising an aqueous dispersion of a mineral pigment, a small amount of tetrasodium pyrophosphate as a. deflocculating agent for said pigment, from about to about 200 parts of enzyme modified starch adhesive based on 970 parts of bone dry mineral pigment, and about 2 to 10 parts of ammonium stearate based on 100 parts of said starch, said composition having a solids content of from at least about 50 to about 60-62 per cent, and being of a thixotropic nature, and transferring said composition in the form of a preformed film from said rotating surface to the surface of a paper web passing forwardly at a linear speed of from about 800 to about 1,000 feet per minute to provide the web with a highly printable smooth surface.

5. A mineral coating composition adaptable for the coating of paper by the machine coating method in a. high speed paper making process which comprises, an aqueous dispersion of mineral pigment, a small amount of deflocculating agent for said pigment, from about 120 to about 200 parts enzyme modified starch adhesive based on 970 parts of bone dry mineral pigment, and about 2 to 10 parts of ammonium stearate based on 100 parts of said starch, said composition having a solids content of from at least about 50 to about 60-62 per cent.

6. A mineral coating composition adaptable for 9 the coating of paper by the machine-coating method at a relatively high linear speed which comprises, a, mineral pigment and a modified starch in aqueous dispersion, the composition being of a thixotropic nature and having a solids content of from at least about 50 to about 60-62% and a starch content of about 60 to 300 parts based on 970 parts of bone dry mineral pigment.

7. A mineral coating composition adaptable for the coating of paper by the machine-coating method at a, relatively high linear speed which comprises, a mineral pigment and a chlorinated starch in aqueous dispersion, the composition being of a thixotropic nature and having a solids content of from at least about 50 to about (50-62% and a starch content of about 60 to 300 parts based on 970 parts of bone dry mineral pigment.

8. A mineral coating composition adaptable for coating paper with a thin, dense, printable surface film by the machine-coating method at a paper web speed of from about 800 to about 1000 linear feet per minute which comprises, a mineral pigment and a modified starch in aqueous dispersion, said composition being of a thixotropic nature and having a solids content of from at least about 50 to about 60-62% and a starch content of about 120 to 200 parts based on 9'70 parts of bone dry pigment.

9. A mineral coating composition adaptable for the coating of paper by the machine-coating method at a relatively high linear-speed which comprises, a mineral pigment and a modified starch in aqueous dispersion, the composition being of a. thixotropic nature and having a solids content of from at least about 50 to about (50-62%,

a starch content of about 60'to 300 parts based on 9'70 parts of bone dry mineral pigment and from about 1 to about 6 parts of pigment dispersing agent based on 970 parts of bone dry mineral pigment.

10. A mineral coating composition adaptable for the coating of paper by the machine-coating method at a relatively high linear speed which comprises, a mineral pigment and a modified starch in aqueous dispersion, the composition being of a thixotropic nature and having a solids content of from at least about 50 to about 60-62%, a starch content of about 60 to 300 parts based on 970 parts of bone dry mineral pigment and about 2 to 30 parts of ammonium stearate based on 100 parts of starch.

11. The process of coating paper webs in a. continuous paper-making process which comprises, continuously spreading onto a rotating surface a controlled amount of a mobile, dense, thin, smooth film of a, coating composition of thixotropic nature, said coating composition comprising mineral pigment and modified starch in aqueous dispersion having a solids content of from at least about to about -62% and a starch content of about 60 to 300 parts based on 9'70 parts of bone dry mineral pigment, and transferring said composition in the form of a preformed film from said rotating surface to the surface of a paper web passing forwardly at a linear speed of from about 800 to about 1000 feet per minute.

12. Paper coated with the dry residue of a film of coating composition in accordance with claim 5.

13. Paper receptive to fine printing characterized by smooth mineral pigment coating of substantially uniform texture and flatness, compacted and bonded to the paper web surface, said coating being the dried residue of a film of coating composition in accordance with claim 6.

GILBERT K. DICKERMAN. ROBERT W. RILEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,194,216 Coppock Mar. 19, 1940 2,185,859 Massey Jan. 2, 1940 2,124,372 Kesler July 19, 1938 2,293,690 Harrigan Aug. 18, 1942 2,024,123 Baker Dec. 10, 1935 FOREIGN PATENTS Number Country Date 496,775 Great Britain Dec. 6, 1938 OTHER REFERENCES De Guehery, Technical Association Sec. of Paper Trade Journal June 30, 1938, pages 76-77.