US3181500A - Apparatus for the production of coated paper - Google Patents

Description

y 4, 1955 M. s. PAULSON ETAL 3,181,500

APPARATUS FOR THE PRODUCTION OF COATED PAPER Filed Jan. 25, 1960 3 Sheets-Sheet l FIG. I

- INVENTOR. 23 MORRIS e. PAULSON CARROLL mwnumzau 52,4} w w-mi May 4, 1965 M. G. PAULSON ETAL 3,181,500

APPARATUS FOR THE PRODUCTION COATED PAPER Filed Jan. 25, 1960 3 Sheets-Sheet 2 BLADE ANGLE 25 BLADE EXTENSION I'3/4" CALIPER .020 PRESSURE I-D' FIG. 3

BLADE ANGLE 25 BLADE EXTENSION 1-5/4" CAMPER .020 PRESSURE 4'5 BLADE ANGLE 20 BLADE EXTENSION BLADE ANGLE 20 BLADE EXTENSION Ila/4 CALIPER .OZO PRESSURE 35* 1-3/4" CALIPER .020 PRESSURE 40 FIG. 5 F|G.6

BLADE ANGLE 25 BLADE EXTENSION BLADE ANGLE 25 BLADE EXTENSION 2" CALIPER .025 PRESSURE 70* 2" CALIPER .025 PRESSURE so BLADE ANGLE 25' BLADE EXTENSION 2" CALIPER .005 PRESSURE 20* BLADE ANGLE 25 BLADE EXTENSION 2" CALIPER .OIS PRESSURE 15* FIG IO 9 INVENTOR.

MORRIS G. PAULSON CARROLL M. THUREEN M. e. PAULSON ETAL 3,181,500

3 Sheets-Sheet 3 BLADE ANGLE 25 BLADE EXTENSION I-3/4" CALIPER .085 PRESSURE 15* FIG. I2

INVENTOR. MORRIS G. PAULSON By CARROLL M. THUREEN May 4, I965 APPARATUS FOR THE PRODUCTION OF COATED PAPER Filed Jan. 25. 1960 BLADE EXTENSION CALlPER .ols PRESSURE 20* BLADE ANGLE 25 l-s/4 FIG.

United States Patent 3,181,500 APPARATUS FOR THE PRGDUCTTUN UF CQATED PAPER MorrisG. Paulson and Carroll M. Thureen, international Falls, Minn, assignors to'Minnesota and @ntario Paper '(Iornpany, Minneapolis, Minn.

Filed .l'an. 25, 1960, Ser. No. 4,3?4 1 Claim. (Cl. 118-426) This invention relates to an apparatus and a process for the production of mineral-coated paper having high smoothness and a high gloss. By mineral-coated paper we mean paper bearing a coating consisting essentially of a mineral pigment and a hydrophilic adhesive.

An object of the invention is to produce a paper coating having a superior surface in respect of flatness, smoothness, freedom from imperfection, etc. The raw paper stock, in web form, may be of any suitable kind, and the ingredients contained in the coating applied thereto may be chosenirom a number of different raw materials now commercially in use. The coating mixture may include, for example, mineral pigment such as clay, calcium carbonate, titanium dioxide, or satin white, and a binder of starch, protein, glue, or other suitable material, all contained in an aqueous dispersion.

Another object of the invention is to perform the coating and finishing operation on a fast-traveling web of paper. By the term paper as used herein, We mean to include any and all materials in sheet form whereon a coating may be applied and treated in accordance with the present invention, and no distinction is intended between various kinds or classes of paper such, for example, as printing and photographic papers, or paper or paperboard.

By the treatment given to the paper web, in accordance with this invention, there is produced a finished surface on the coating, either thick or thin, which is smooth, integral and superior to those now commercially available.

This may be accomplished at high operating speeds so as to maintain the cost at a minimum. Many of the factors which are responsible for the advantageous results hereinafter noted are both variable and controllable within certain limits so that it is possible to predetermine to a considerable extent the characteristics of the resulting paper.

In making mineral-coated paper, it is customary first to apply to the body stock a coating mixture containing a pigment and a binding agent suspended in an aqueous suspension. The coating mixture may be applied in various ways; such as, dip, brush, etc. To obtain mineralcoated paper of good quality it is necessary to exercise considerable care in applying the coating mixture in order to secure as even a coating as possible. Various devices have been used to smooth wet coating but such devices fail for various reasons to obtain desired smoothness and gloss in the dried coating. if the coating film is split by the applying or by the smoothing devices, the dried coating will have, when observed with a hand lens, hills and valleys.

One of the essential features of the invention is the moving of the hills into the valleys of a coating without substantially any film splitting. This may be accomplished by a blade while the coating is in a plastic condition-on the exposed surface while adjacent to the paper web the coating is substantially in a set up condition. The exposed surface of the coating should be thixotropic when the smoothing takes place.

According to this invention, a substantially uniform .film of coating is applied to one or both sides of the web while it is supported by any suitable supporting means. The paper is coated when a greater proportion of the i' atented i l/lay 4, 1955 moisture has been removed and superior results are obtained when the paper being coated contains about 2% to about 12% of moisture, it being understood that moisture outside this range, under some conditions, may be used. The moisture from the applied coating strike into the paper web with the results that the coating adjacent the web surface being set up quicker than the exposed coating surface.

Almost any blade can be made to produce a smoothing action by proper'selection of operating conditions. Blade calipers have been tried ranging from about 0.010" to about 0.5" thick. Obviously a compromise must be made between a blade which is so easily deflected that it cannot exert any shearing action on the color and a blade which is so rigid that it cannot conform to the slight variations in basestock caliper which may occur across theweb. The blade caliper should be deter nined by the operating conditions imposed by the basestocl; and the coating material (color).

A wide variety of initial blade angles may be employed, the limitation being that the angle must be so great that abnormally high pressure must be used to create the necessary blade deflection, or so low that only a very slight pressure will cause the tip of the blade to recurve away from the back-up roll causing the color to pump through in an uncontrolled manner.

In theory, any blade length can be used providing that the combination of stillness and angle does not impose too high a pressure for thenecessary blade deflection nor too low a pressure for developing adequate shear of the thixotropic, or pseudo-plastic coated surface to be smoothed properly.

The actual mechanism by which a blade works is considerably difierent from other doctor blade coaters. Most other doctor blade coaters are of the gravure type, that is the blade rides on the high spots of the basestock of previously coated surface and doctors off the excess coating. A process of this nature requires an extremely grit free coating material as the blade is scraping the surface at a comparatively high angle and no opportunity is .present for the grit to slip through the nip. In the present invention, the blade merely reworks the surface of the coating, in substantially the exact amount desired, to pro duce a levelness which cannot be obtained by any conventional coaters.

The exact geometrical relationships between the blade and the back-up roll cannot now be fully described. However, numerous experiments both experimentally and mathematically, have been made to determine What these relationships are. There is sufficient agreement between the two methods of analysis to be reasonably certainthat the deflection of the blade can be treated in the same way as an end-loaded cantilever beam. The curvature of the blade is found-to be a parabola with the tip of the hat side of the blade bearing tangentially against the back-up roll. The tangent to this curve at the tip of the blade approaches the tangent to the back-up roll. Although we have not definitely determined these angles, the static measurement of the angles falls within'the range of '0 to 16 as a maximum. The high angles were obtained with relatively flexible blades which could presumably deflect several more degrees due to the frictional drag of tbecoating. With a blade 05" thick which cannot deflect to any measurable amount in this use, the best operating angle is about 5. At 0 the coating was pumped through and at 10 nearly all the coating was scraped off the surface of the paper. In actual operating experience and practice these tangents to the two respective curves may actually come very close to coinciding. The mechanism by which the coated surface is reworked to produce a smooth surface is not completely understood. According to the Arnold equation, the thickness of the film formed between a pair of rollers is determined by the pressure between the rollers, the viscosity of the coating and the speed of coating. A somewhat similar situation is believed to occur in the blade nip, the difference being only that one of the rollers is replaced by a stationary blade.

It is well known that the surface pattern of a roll coated sheet consists of numerous humps of coating with areas in between which have very little coating thereon. This piling of color material results in the so-called orange-peel pattern. If the still moist surface of coating is made to pass through the nip of the smoothing blade, the humps will be subjected to higher pressures than the surrounding areas, if this pressure is high enough, the coatings at the hump should be spread out to fill the low spots. This is provided that the coating is thixotropic and/or pseudo plastic, in other words can be reworked. The pressure required to accomplish this should follow the same type of relationship found in the Arnold equation. That is, for a given viscosity and speed there is a definite pressure which will pass the film of a given thickness. In an ideal operation this pressure should be such that the thickness of the resulting film should be of the median thickness of the Wet film as applied by the roll coater. This will vary to some extent depending on the smoothness of the basestock. If the basestock is too rough, higher coating weights will be required to achieve maximum smoothness.

One method of determining the exact blade setting which will give the maximum gloss and smoothness is: if the pressure on the blade is brought up slowly the surface of the coating goes through a dull scratchy appearance which gradually improves until a point is reached where the surface takes on an extremely high gloss; as the pressure is increased further the surface again becomes dull but without the scratchy appearance, at this point the smoothing blade passes the maximum amount of grit without marking the sheet. A further increase in pressure results in the so-called pumping of the coating material through strings or splotches which mar the coated surface.

It is suggested that with adequate screening much of the scratching problem can be eliminated. For example, a 40 mesh screen in the system can give fairly good results.

The following tables disclose results which occur when using this invention on a free sheet with a fairly good initial finish on the basestock.

In the drawings:

FIGURE 1 is a diagrammatical representation of a coater and the smoothing blade;

FIGURE 2 is a diagrammatical representation of the backing roll in relationship to the blade;

FIGURES 3 through 12 are illustrating the blade angle as relates to backing roll;

FIGURE 13 is a modified form of blade; and

FIGURE 14 is a view of the rubber diaphragm shown in FIGURE 13.

Roll or other type coated paper have defects noticeable upon microscopic or naked eye examination of the coated paper some of which are:

Crevices--Microscopic irregular shaped holes in the coating having the appearance of shrinkage cracks,

HolesMicroscopic round holes having the appearance of being caused by minute gas bubbles in the coating.

TrenchesThis is a condition readily observed with a microscope, in which a coating, instead of being smooth and uniform, presents a surface containing shallow trenches or valleys that follow certain of the surface fibers of the underlying paper.

StippleA condition of the coating readily seen by the naked eye by transmitted light. The coating is in thick and thin areas spaced about a millimeter or two apart.

Mottled and streaks-When looking over the surface of the paper, numerous small dull blotches may be apparent where the light is not uniformly reflected. Upon examination under a microscope it can frequently be determined that this lack of uniformity is due to concentration of trenches, crevices, or holes in certain areas or lines.

These defects are eliminated or substantially eliminated by the use of the blade as herein disclosed.

In order to avoid confusing the terms used in describing coating colors flow properties are hereinafter defined:

Thixotropy is a reversible solgel structure shown by many colloidal suspensions of high solid contents. When a thixotropic solution is agitated, the absolute viscosity becomes lower as the speed of agitation increases. When the agitation ceases the solution will again gradually set to a firm jell.

Viscosity is the internal resistance of a fluid to flow. The absolute unit of viscosity is poise.

Fluidity is the reciprocal of viscosity.

Referring to the drawings in detail, in FIGURE 1, 16 represents a coater of any suitable type. Coaters disclosed in Patent 2,830,555; Patent 2,861,541; and Patent 2,729,- 192 give excellent results. The continuous paper web after receiving the coating while supported by backing roll 18 has the exposed coating material surface reworked by blade 24. The blade is supported in the desired position by a suitable blade holder 22. The time that should elapse between the application of the coating material to the paper and the reworking of the exposed coating surface depends upon several variables such as solids contents of the coating, the speed of the paper, the viscosity of the coating, the time required for the coating material adjacent the paper to set, and the thixotropic properties of th exposed surface of the applied coating. 1

In FIGURE 2 is shown the blade 24 in solid lines with minimum pressure applied and in dotted lines with an operating pressure applied. The blade 24 is supported by holder 28 and the holder is pivotally mounted at 26. The blade 24 and holder 28 extends the width of the coater and there is attached to opposite ends of the blade holder handle-like member 25. A predetermined pressure is applied to the handle so that the desired reworking of the applied exposed coating surface is accomplished.

In FIGURES 3 to 12 are shown operating conditions of the reworking blade 24 under varying conditions.

It is to be understood that the backing roll may be of metal, rubber, or other suitable material. The backing roll may be heated under certain conditions, and if it is to be heated, of course, a metal roll should be employed.

Papercoaters are often used which are in excess of in width. Where wide coating apparatus are em- 'ployed, it necessitates the use of a very long blade to extend across the width of the coated paper web. It is important that substantially the same pressure be applied to the blade across its entire width. This may be accomplished in several ways, but one of the most effective ways is disclosed in FIGURES 13 and 14. The blade 30 has attached thereto a spacer block 32 and a backing member 34. These are secured together by any suitable means such as members 36. A flexible rubber diaphragm member 38 is secured between the blade 30 and the backing member 34, so as to form an air-tight chamber extending the length of the blade. Fluid under pressure is furnished to the chamber through line or lines 40. The fluid under pressure is controllably supplied thereto to the extent desired so as to urge the blade 30 against the coated surface of the paper. It has been found that better results are obtained if a plurality of lines 40 supply the fluid under pressure to the chamber and that the supply lines are spaced closer together along the middle sections of the chamber. Generally better results will be obtained if the fluid chamber is divided at 42 into several noncommunicating chambers with the chambers adjacent the mid-sections smaller than the chambers adjacent the end sections. The release of the fluid pressure to clear any grit between the blade and the backing roll can be carried out with minimum loss of paper.

While one embodiment of the invention has been described in more or less detail to comply with the requirements of the statute, it is nevertheless desired that the detailed description be considered merely as illustrative, not as limiting, and it is to be understood by those skilled in the art that changes can be made without departing from the invention, as defined in the appended claim.

Release of the fluid pressure is accomplished by any suitable means (not shown) such as a controlled opening to atmosphere in the supply pipe or pipes.

Patent No. 2,830,555 hereinafter referred to shows the applicator roll moving in the same direction as the web of paper.

What is claimed is:

In a coating apparatus for producing coated paper having smooth and glossy surface comprising a roller coating apparatus, a backing roll for supporting paper during coating and smoothing, a smoothing blade contacting the coated paper supported by the backing roll, said blade of a thickness of about 0.010 inch to less than about 0.50 inch and inflatable means contacting an adjacent portion of the smoothing blade contacting the coated paper and holding the blade in close contact with the coated paper, said inflatable means divided lengthwise into separate compartments with the separate compartments increasing in size from the mid-portion toward the ends thereof and means of inflating each of said compartments.

References Cited by the Examiner UNITED STATES PATENTS 2,257,373 9/41 Fanselow 117-1l1 2,312,927 3/43 Murray 117111 2,368,176 1/45 Trist 118-407 2,534,320 12/50 Taylor 117111 X 2,861,541 11/58 Hornbostel 118122 X 2,914,788 12/59 Smith et al 15-2565 3,029,780 4/62 Justus et al. 15256.5

FOREIGN PATENTS 1,819 of 1897 Great Britain.

RICHARD D. NEVIUS, Primary Examiner.

WILLIAM D. MARTIN, Examiner.

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