US3399074A

US3399074A - Cast coating process using raw starch as adhesive

Info

Publication number: US3399074A
Application number: US649741A
Authority: US
Inventors: Bruce C Gottwald; John M Haigh
Original assignee: Albemarle Paper Manufacturing Co
Current assignee: Albemarle Paper Manufacturing Co
Priority date: 1967-06-28
Filing date: 1967-06-28
Publication date: 1968-08-27
Anticipated expiration: 1985-08-27

Description

United States Patent O ABSTRACT OF THE DISCLOSURE Process for cast coating a smooth surface on a web material comprising coating web surface with low viscosity coating slurry containing raw starch as essentially l the adhesive portion and then pressing the slurry coated web surface against a smooth heated surface until a substantially tack-free smooth hard surface is formed on the web. Smooth surface may be heated to temperature of from 200 F. to about 325 F. and slurry coated surface pressed against heated smooth surface at pressure of from about 10 to about 200 p.s.i.

Cross references to related applications This application is a continuation of application Ser. No. 296,683, filed July 23, 1963, and now abandoned.

Background of the invention The manufacturer today, as is Well known, utilizes a colorful and attractive packaging or wrapping for his product, which is appealing to the prospective purchaser, for the reason that such eye catching packages are a significant factor in the sale of any individual product. Competitive products are often side by side on the retail shelf, each vying for the attention of the consumer. Thus, more attractive wrappings or packages have been sought by manufacturers to sell their product without adding appreciably to the cost of the product.

One of the most desirable type of packages from the standpoint of attractiveness is a cast coated paper such as was disclosed in the basic Bradner patent, U.S. 1,719,- 166. In accordance with that process, a suitable coating for the paper web is applied thereto in the form of a slurry and the resultant wet coated paper pressed against a =highly polished casting surface, such as a chromium plated drum. Among the important facets of this prior process was the need in the coating slurry of a suitable binder having, ab initio, suicient adhesive properties so that when applied the coating would adhere or stick to the casting drum. Thus, conventional binders, such as precooked or gelatinized starch or the like, were used to form rather viscous syrupy coating slurries which were caused to initially adhere to the drum by the application of pressure at a nip roll. Thereupon the wet coated paper was pressed against the heated casting surface so that the coating was dried into a very smooth, glossy surface iinish. In the course of this drying process the gelatinized binder, such as precooked starch, is caused to shrink so that con comitantly the cohesive forces within the coating itself ultimately become suiciently great as to overcome distortion due to sticking of the coating against the casting surface. Thereafter the coated paper could be separated from the casting surface without marring the smooth glossy finish on the paper.

Another important facet of this prior process was the 3,399,074 Patented Aug. 27, 1968 ice necessity of keeping the temperature of the casting surface below 212 F. This need arose because generation of steam and other vapors within the coating would readily force the same away from the drum or casting surface before the coating was sufficiently dry. This in turn would lead to distortions and spotty appearances in the surface of the coating itself. In aggravated cases the coating would actually be blown away from the surface causing a splitting of the coating as well as undesirable distortions and imperfections in surface quality. Furthermore, part of the coating would adhere to the casting surface and part would remain on the paper so that use of the roughened casting surface would mar the incoming coated stock.

One of the reasons for the splitting of the coating is that the usual viscous aqueous coating, such as precooked starch or a mixture of precooked starch and clay, has higher adhesive properties than cohesive properties while wet. However, the reverse is true after the coating has dried. Thus, only after the coating has dried can the coated stock be safely withdrawn from the heated casting surface.

As can be readily appreciated, this prior practice possessed a number of inherent shortcomings. In the rst place, the rate of drying was much slower than would be desirable on a commercial basis. In short, the use of temperatures below 212 F. for effecting drying of the coating necessarily resulted in a relatively low production rate, eg., in the order of 50-80 feet per minute. Subsequent etforts such as disclosed in Montgomery, U.S. 2,568,288, and Hart, U.S. 2,919,205, did effect some improvements in the rate of producing cast coated paper, but nevertheless, even these improvements still leave much to be desired. Generally speaking, the most advanced process as reilected by the above Hart patent enables the achievement of production rates of about 20() feet per minute. Needless to say, production rates substantially higher than this are a significant desideratum in the art.

The excessively high viscosities of the coating slurries constituted another limitation of this prior practice. -For example, aqueous dispersions of precooked or gelatinized starch have such high viscosities even at low solids content, that the starch had to be chemically modified in some manner in order to prepare acceptable coating binders. However, conventionally used chemical treatments of starch tend to markedly reduce its binding strength, the use for which it is intended. Even when utilizing chemically treated starch, the precooking gives rise to a high viscosity slurry which is difficult to handle and to apply evenly and uniformly to the paper web. Therefore, here again there is a distinct need in the art for a method of cast coating of paper and the like which would eliminate the necessity for the use of precooked starches and other similar highly gelatinous binders.

Still another shortcoming inherent in this prior practice was the difficulties often encountered when utilizing a resin in the coating in order to give it Wet strength. In conventional coating systems the precooked starch mixture was kept hot to reduce its viscosity as much as possible and if such coating also contained a resin for wet strength it was essential to keep the pH of the mixture on the neutral or alkaline side. If the pH of the coating slurry was lowered the resin cured prematurely and therefore the desired wet strength was not aorded to the ultimate coated paper. Also, this premature curing of the resin further adversely a'ected the stability of the coating mixture and compounded the problems associated with its application to the paper web. Therefore, another important contribution to the art would be the provision of a cast coating process wherein wet strength resins can be employed irrespective of the pH of the coating slurry.

Consequently, it is an object of this invention to provide an improved process for cast coating paper and like material while achieving advantages heretofore unobtainable.

Another object of this invention is to provide a method of cast coating paper and the like which is essentially free of the limitations and shortcomings referred to above.

A further object is to provide a novel method of cast coating paper and like material which enables the use of slurries in which raw starch is the incipient binder for the coating.

A still further object of this invention is to provide a method for the production of cast coated paper of excellent quality at high production rates involving, inter alia, the use of non-viscous unheated coating slurries containing as an inchoate binder uncooked starch which need not but may be a chemically treated starch.

An additional object of this invention is to provide a method for the rapid production of cast coated paper of excellent quality enabling and involving, inter alia, the use of non-viscous unheated resin-containing coating slurries containing uncooked starch as an inchoate binder, which slurries are characterized by being insensitive to changes in pH and by having suitable stability prior to and during use.

Other important objects of this invention will be apparent from the ensuing description and the appended claims.

In accordance with this invention, we accomplish the above and other objects through. the use of a new and improved method of cast coating paper and the like. This method comprises (l) Coating a paper web surface with a hardenable coating slurry containing raw starch as an inchoate binder for the coating;

(2) Pressing said coated paper surface against a smooth casting surface heated to a temperature within the range of from about 200 F. to about 325 F. with a pressure sufficient to maintain said coating against said smooth casting surface while permitting the vaporized liquid from the coating to pass through said web and away from said coating; and

(3) Maintaining said pressure for a suliicient time until the raw starch is converted in situ into an active binder for the coating and sufficient volatile liquid has been driven away from the surface of the coating whereby the cohesive strength of the coating will resist any distortion due to further drying.

To achieve a strong coating at a high production rate and with a relatively low input of heat, it is preferable to maintain the smooth casting surface at a temperature within the range of about 220 to about 280 F., temperatures of from about 230 to about 250 F. lbeing particularly preferred.

Pursuant to this invention, the application of the pressure forcing the wet coated paper surface against the very hot casting surface serves a multiplicity of functions. In the first place, this pressure serves to hold or aix the applied initially non-viscous dispersion coating substantially immobile with respect to the web so that the coating remains uniformly distributed thereon. This in turn insures that the coating will not acquire a roughened, irregular or spotty configuration prior to the time that it is hardened into the finished coating. In the second place, the application of this pressure keeps the coating tightly pressed against the very hot casting surface so that the initially ungelatinized starch is rapidly gelatinized in situ and thereupon caused to set in situ through shrinkage into a highly smooth coating having the requisite cohesive strength. This in turn enables the dried coating to be rapidly withdrawn from the hot casting surface without incurring sticking or other undesirable physical phenomena' resulting in disruption of the desired finish. In short, the necessary release properties are thereby afforded. Still another function served by the application of the pressure is that it prevents the coating from being forced away prematurely from the hot casting surface by the steam and other vapors generated by the boiling of the water and other liquids contained in the coating. This beneficial feature manifests itself in the elimination of blowing or splitting of the coating.

Another important feature of this new process is that the vaporized water is allowed to pass through the paper web and away from the coating as it is being dried. This contributes in very large measure to insuring that the coating will acquire the desired smooth glossy surface finish without encountering such undesirable manifestations as spots, blisters, cracks, or the like. To accomplish this use may be made of methods and apparatus such as are described in our prior copending applications Ser. Nos. 77,083, now U.S. Patent 3,110,612, and 77,174, now abandoned, both filed Dec. 20, 1960, all disclosures of which are incorporated herein by the foregoing reference. In brief, the paper surface to which the liquid coating has been applied is pressed against the hot casting surface (e.g., a heated drier drum) with an appropriate pressing surface, such as a pressure belt or a soft flexible roller, while maintaining a vapor receiving web between the coated paper surface and the pressing surface. In this way the vaporized liquids from the coating (mainly steam) are caused to pass into the spaces provided within the vapor receiving web.

The process of the present invention not only retains all of the advantages attendant the processes described in the above referred to copending applications, but concomitantly achieves a number of other important advantages. For example, relatively large amounts of uncooked starch binder can be applied with no difiiculty to the surface of the paper web because the viscosity of even 50 weight percent suspensions of uncooked starch remains extremely low. To illustrate, a 50 percent aqueous suspension of uncooked starch had a Brookfield viscosity at l0 r.p.m. (80 F.) of 3,000 centipoises whereas a 25 percent aqueous colloidal dispersion of cooked starch had a Brookfield viscosity of 6,700 centipoises under the same test conditions. Furthermore, there is no need for a preliminary cooking of the starch or for heating the coating slurry prior to its application so as to reduce its viscosity, and there is no difiiculty with the stability of the coating mixture or slurry. Less water is required so that drying is easier and more rapid. Another advantage is that raw starches are cheaper than chemically modified starches. Moreover, resins for wet strength are readily incorporated into the present coating slurries without need for pH control to obviate premature curing of the resins.

So far as is known, this is the first cast coating process employing uncooked starch as a binder for the coating.

'Ihe coating composition applied to the paper and like webs in the practice of this invention is a non-viscous predominantly aqueous coating slurry in which uncooked starch is essentially the only binder ingredient. As noted above, the uncooked starch is therefore an inchoate binder since it becomes active during the drying of the coating while in contact with the relatively hot smooth casting surface. Hence, the binder or adhesive ingredient of the coating composition is raw starch, such as corn starch, potato starch, rice starch, or modified forms thereof, such as acetylated, oxidized, chlorinated, enzyme converted, or like treated ungelatinized starches such as are commonly available as articles of commerce. The coating slurries may contain from about 5 to about 50 percent by weight of the starch suspended or dispersed in the predominantly aqueous medium. In preferred embodiments of this invention the coating slurry will also contain a minor amount (e.g., from about 10 to about 30 percent based on the weight of the starch used) of one or more wet strength resins. Particularly suitable resins for this use include urea-formaldehyde, melamine-formaldehyde, and like resins. Whether or not the slurry contains the resin, the liquid coating slurry may contain a suitable pigment, such as finely-divided clay, titanium dioxide, calcium carbonate, satin white, Iblanc fixe, and the like, or suitable mixtures of these. Such pigments are preferably utilized in amounts rangingfrom about 20 to about 90 weight percent, based on the weight of the solids making up thecoating composition. Other ingredients which may be used include release agents, including dispersing agents (c g., gum arabic, sodium hexametaphosphate, tetrasodium pyrophosphate, alkyl ketene dimer, and the like). Use may also be made of such materials as stearate base defoamers, vwhich are rknown to exhibit release properties, as well as small amounts of dyes or optical bleaches. The nature of these various ingredients and the proportions in which they are used will be readily apparent to those skilled in the art. Exemplary of preferred coating compositions for use in this invention are:

Composition A: Parts Clay 90 Titanium dioxide 10 Uncooked starch 20 Soy protein l Sodium hexametaphosphate 0.03 Urea-formaldehyde resin 5 Ammonium stearate 3 Composition B:

Clay 60 Calcium carbonate 30 Uncooked starch 10 SBR latex 10 Casein 1 Sodium hexametaphosphate 0.03 Urea-formaldehyde resin 5 Ammonium stearate 3 To demonstrate the advantages of utilizing uncooked starch formulations pursuant to this invention a series of Brookfield viscosity tests was conducted. In these tests measurements were made of the viscosities of formulations of Composition A when utilized in various proportions with water. For comparative purposes the same measurements were made on a series of aqueous systems in which Composition A was modified only to the extent that cooked starch was utilized in place of the uncooked starch. The results of these experiments are shown graphically in the figure of the drawing in which the ordinate represents the Brookfield viscosity (l r.p.m.', 80 F.) and the abscissa represents the percentage of the coating solids employed in the various aqueous formulations. It will be seen from this figure that the viscosity of the cooked starch formulations sharply increases withincreasing coating solids concentration whereas the uncooked starch formulation had a much lower viscosity even at substantially higher solids concentration. Thus, with coating equipment which is capable of utilizing coating slurries having a Brookfield viscosity of, for example, no more than 4,000 centipoises, the manufacturer is limited with the cooked starch formulation to an aqueous system containing no more than about 38 percent of solids. But, on the other hand, when utilizing an uncooked starch formulation pursuant to this invention, the foregoing equipment limitations do not constitute a problem as the uncooked starch formulation can be employed to a concentration as high as 57 percent solids.

As noted above, the coating slurries utilized in the practice of this invention possess excellent stability characteristics. By way of example, Composition A exhibits only a l0 percent increase in Brookfield viscosity on standing at room temperature for 24 hours whereas the corresponding composition containing cooked starch increases in viscosity by at least about 30 percent under the same conditions.

In applying the foregoing coating slurries used in this invention, recourse may be had to conventional apparatus for this purpose, such as use of a roll coater, an air knife, a blade coater, or combinations of these, or the like. Such apparatus will be adjusted so as to uniformly apply the coating slurry to the incoming paper or like web, preferably just before the coated web is brought into intimate contact with the casting surface. The amount of coating slurry so applied will depend to some extent upon the characteristics of the web being coated and the desired characteristics and thickness of the cast coating to be affixed thereto, but generally speaking, will range from about 5 to about 25 pounds per 3,000 square feet when utilizing rough kraft type paper as the material to ibe cast coated.

As noted above, the coating slurries are not precooked prior to the time they are initially applied to the incoming paper or like web. Preferably, the coating slurries are formed by mixing the appropriate amounts of the particular ingredients at or near room temperature (e.g., 50L90 F.) so as to provide a stable slurry which is suitable for application to the incoming paper or like web under similar temperature conditions. v

The characteristics and properties ofthe web to which the cast coating is applied may vary to suit the needs of the occasion. Thus, the paper so used will range from 40 pound magazine or label paper to 28 point box board.

Suitable methods of pressing the initially coated web against the hot cast coating surface and for enabling the liquid vapors thereby generated to escape from the surface of the drying coating have been referred to both above and in our prior copending application Ser. No. 77,083. Hence, these aspects of the present process need not be amplified here except to indicate that it is preferable to employ a continuous pressing surface, especially a tiexible belt having suicient structural strength to effect the necessary amount of pressing of the coated web against the hot casting surface. The casting surface itself is preferably a chromium plated drum of l0 to l2 feet in diameter, although the diameter of such drum may be larger or smaller, depending upon such factors as the desired production rate, belt strength, drum temperature, and the like. Machine finished drums will also permit the production of a paper surfaced with a smooth cast coat. The coated paper and like webs are pressed against these casting surfaces utilizing pressures ranging from about 10 to about 200 pounds per square inch, pressures of from about 40 to about 80 pounds per square inch being particularly suitable and preferred.

Preferably interposed between the coated paper and the continuous pressing surface is suitable backing means, such as any relatively thick (approximately 1A" thick layer) continuous felt belt or other smooth surfaced continuous belt-like layer composed, for example, of wool, cotton, fiber glass, or like closely knit woven or pressed material. When such backing means are employed in conjunction with the preferred drum type casting surfaces, it is desirable to utilize the mechanical arrangement described in FIG. l of our prior copending application Ser. No. 77,083 so as to urge such backing means against the drum surface through a substantial circumferential arc (e.g., to 300) of the drum. It will be understood that `other suitable means may be used for effecting vthe necessary pressure and for enabling the vapor to escape from the coating as it is being dried.

Heating of the casting surface is preferably accomplished by utilizing super-heated steam, although other suitable methods of heating, such as electrical or gas heaters, may be used. The source of the heat is generally maintained on that side of the casting surface which does not directly contact the coated web.

Prior to the start of a casting operation, it is necessary to insure that the casting surface is free from foreign objects, such as dirt, grit, and other particulate matter.

Accordingly, the casting surface is generally thoroughly washed, preferably in conjunction with the use of a buffing compound or other surface cleanser containing a suitable release agent so as to achieve the application of a very thin film of release agent upon the casting surface. However, a feature of this invention is that the use of such release agents is unnecessary, particularly when a suicient quantity of a release agent is employed in the coating slurry itself.

During the casting operation itself, the pressure is continuously maintained upon the coated web and against the casting surface until the coating has dried or hardened through the in situ conversion of the binder, and the release of suicient vapors into the vapor receiving web. Thereupon, the dried cast coated web is released from the casting surface.

The production rates achievable by the practice of this invention are appreciably higher than could be accomplished by previously known cast coating processes even though those processes necessarily involved the use of pregelatinized binders. It is interesting to note, for example, that the most advanced previously known process described in Hart, U.S. 2,919,205, required a special gellable coating which had to be gelled before the coating was contacted by the drier drum. Even so, the production rate was limited to about 200 feet per minute. The present process however requires the gelation of the binder to occur in situ and, nevertheless, has been found capable of achieving production rates as high as 400 feet per minute and even higher production rates are entirely feasible. The quality of the cast coated finishes produced by the practice of this invention is excellent and hardly distinguishable in appearance from the best coatings made by the slower processes known heretofore.

Comparative experiments were conducted in order to demonstrate some of the additional tangible advantages made available to the art for the first time by virtue of this invention. In these experiments an -attempt was made to utilize uncooked starch as an inchoate binder using previously known cast coating procedures such as that described in Bradner, U.S. 1,719,166. The same coating solution using the same amount .of the same uncooked starch was also utilized in accordance with the process of this invention. More particularly, the following uncooked raw starch coating composition was utilized in all experiments:

Parts Clay 100 Uncooked raw starch 25 Protein 2 Commercially available release agent 3 Water 90 Thereupon the dried coatings were tested for pigment binding strength by use of Dennison waxes, a series of numbered waxes of graded strength or pull. This test involves melting the wax, placing it on the coating, allowing it to cool for 15 minutes and then removing it with an upward pull. The binder strength is expressed as the highest numbered wax which shows no coating pick. Therefore, the higher the value of the Dennison wax pick the stronger the coating.

It was found that the oven dried and the infrared dried coatings (enumerated as 1-3, inclusive, above) had no binder strength as they picked at the lowest wax, No. 2. In other words, under the specified conditions the coatings were dewatered long before the starch gelatinizing temperature was reached.

The cast dried sample utilizing the Bradner process (Item 4 above) exhibited a wax pick value of only 4 and the surface of the coating was found to be of poor quality. In fact, the coating did not display the mirror image of the chrome plated drum nor did it possess the conventional cast appearance. Consequently, when attempting to utilize this prior process in association with uncooked raw starch binder, it was found that the adhesive power of the starch was not developed to an extent sufficient to cause the coating to adhere to the casting drum during the short nip pressure contact time.

The samples from Procedure 5 above (i.e., the steamed and then oven dried samples) showed no adhesive development during drying as the coatings picked with the No. 2 wax. Therefore, the procedure recommended in the prior art of exposing starch to direct steam at atmospheric pressure for a matter of seconds is totally insufficient to develop the necessary adhesive properties when utilized in conjunction with a raw uncooked starch coating.

In sharp contrast to the foregoing results, the pressure cast samples resulting from the practice of this invention afforded the following results in the Dennison wax pick test:

Temperature of casting drum F: Dennison wax pick 212 7 240 8 Not only did the process of this invention enable the development of excellent adhesive strength during drying but in all cases the coatings possessed the high quality mirror finish of the chrome casting surface.

As is well known in the art, cast coat'ngs produced by previously known procedures are generally characterized as having relatively high porosity so that they tend to absorb printing inks very rapidly. Such absorption has been a shortcoming of the cast coated paper stocks and considerable efforts have heretofore been expended in ameliorating this problem either through modification of the coating procedure or through the development of special inks for use with the relative porous cast coatings. On the other hand, the coatings produced by the process of the invention possess a relatively non-porous surface and are -therefore suitable for printing operations even when utilyizing conventional inks. These and relatedv advantages of this invention will now be readily apparent to those skilled in the art.

What is claimed is:

1. A process for cast coating a smooth surface on a web material comprising, in combination,

(a) coating a web surface with a hardenable low viscosity coating slurry containing an adhesive consisting essentially of raw starch,

(b) pressing the slurry coated web surface against a smooth casting surface heated to a temperature of from about 200 F. to about 325 F. with a pressure sufficient to maintain said coating against said smooth casting surface while permitting the vaporized liquid from the coating slurry to pass through said web and away from said coating; and

(c) maintaining said pressure for a sutiicient time to convert in situ said slurry coated web surface into a smooth hard substantially permanently tack-free surface bonded to said web and to drive sufficient volatile liquid away from the surface of the coating whereby the cohesive strength of the coating will resist distortion with a further drying.

2. The process of claim 1 further characterized by 9 said low viscosity beingv from about 10 to about 4,000 centipoises.

3. The process of claim 1 further characterized by said raw starch being present in said slurry in a concentration up to about 60 percent by weight of said slurry.

4. The process of claim 1 further characterized by said temperature being from about 220 to about 280 F.

5. The process of 4claim 1 further characterized by said pressure being from about 10 to about 200 p.s.i.

1 0 References Cited UNITED STATES PATENTS 3,110,612 11/1963 Gottwald et al. 117-64 3,205,091 9/1965 Williams et al. 117-156 X WILLIAM D. MARTIN, Primary Examiner.

M. LUSIGNAN, Assistant Examiner.