US3415671A - Process and apparatus for producing high gloss coated paper - Google Patents

Description

Dec. 10, 1968 J. c. RICE 3,415,671

PROCESS AND APPARATUS FOR PRODUCING HIGH GLOSS COATED PAPER Filed Sept. 29, 1965 INVENTOR Jw/v C P/cE ATTORNEYS United States Patent Oflice 3,415,671 Patented Dec. 10, 1968 3,415,671 PROCESS AND APPARATUS FOR PRODUCING HIGH GLOSS COATED PAPER John C. Rice, Tcnafly, N.J., assignor to Lowe Paper Company, Ridgefield, N.J., a corporation of New Jersey Continuation-impart of application Ser. No. 403,242, Oct. 12, 1964. This application Sept. 29, 1965, Ser. No. 491,138

11 Claims. (Cl. 117-64) ABSTRACT OF THE DISCLOSURE Paper products are produced having a high gloss and smooth finish by the use of highly pigmented coating compositions and by subjecting a paper web in coated and dried stage first to a polishing operation, thereafter to a pre-heating and, finally, while the coated web is in a pre-heated state, to a molding.

This application is a continuation-in-part of my copending application Ser. No. 403,242, filed Oct. 12, 1964 and entitled A Process, now abandoned.

The present invention relates to improvements in a method of producing paper products having a high gloss and smooth finish.

In previous work of this inventor it was found that paper products with a coating composition containing a comparatively large amount of pigment can be produced, which have extremely high gloss, smooth finish, high flexibility and printability, by incorporating in the coating composition, among other components, a thermoplastic resinous component and a nn-thermoplastic adhesive, applying the coating composition to the paper, and subjecting the same to a number of processing steps under specified conditions of temperature and pressure.

In preparing the coating composition, a slurry is first formed with water, the mass being then agitated to form a homogeneous mixture, whereupon it is applied to the paper surface by spraying or other known means of application. The coated paper is then dried, subjected to a brushing operation, and finally molded against a polished metal surface with a resilient backing to maintain contact.

In further development it has been found that there are several factors which need careful control in order to obtain optimum results. These factors are moisture content and temperature of the coating surface at the nip of the molding device, as well as temperature control of the molding surface itself. More particularly it should be noted, that the molding temperature being critical for a given coating formulation, very narrow temperature ranges on the drum surface and on the incoming coated surface have to be observed. It has, further been found that it is advantageous to have the molding take place before all of the moisture from the coating has been allowed to migrate into the base stock.

Another factor influencing the gloss of the paper product is the hardness of the resilient backing roll.

It was further found that yet another factor, by which the gloss of the paper product can be increased is the formation of a loop in the web when it is withdrawn from the molding drum. If, instead of withdrawing the web in a comparatively straight path, it is made to drop under its own weight after it leaves the molding station, a considerable heightening of the gloss is accomplished.

Also, it was discovered that for obtaining optimum conditions, all the operations i.e. coating, brushing, and molding, should be carried out in one continuous operation.

It is therefore, the object of the present invention to provide a process of producing paper products with a high gloss and smooth finish with the use of highly pigmented coating compositions by maintaining strictly controlled operating conditions in a continuous operational procedure. Other objects and advantages will become apparent from the following detailed description with reference to the accompanying drawing, which shows by way of example, two embodiments of an apparatus for carrying out the method according to the invention.

In the drawing, FIG. 1 is a schematic view of the apparatus illustrating the overall arrangement of means for treating the web of paper and the several rollers for mov ing it through the apparatus; and

FIG. 2 is a partial view of another embodiment of the apparatus.

Referring to FIG. 1 of the drawing, the machine according to the invention comprises a first coater hav ing two cooperating rollers 12, and a second coater having two cooperating rollers 14, through which a web of paper 10 is passed continuously from a source not shown. A transfer roll 16 is next provided over which the coated web is conveyed to a drier 18. From the drier the web goes on over a roll 20 past a series of brushes 22. Two guide rollers 24 and 26 convey the web to a number of burners 28, provided for preheating the web before it is passed on to the molding operation. The web reaches the molding drum 30 over a roll 32; the drum is temperature controlled and arranged for cooperation with a pressure roller 34 provided with a resilient facing.

After the molding operation, roller 36 takes the web up and conveys it over two direction changing rollers 38 to a pair of rollers 40 which serve for withdrawing the web which is then passed on to further conventional manufacturing stations.

FIG. 2 illustrates the right hand portion of an apparatus in which the left-hand portion, i.e. the portion up to roll 32, is identical with that of the embodiment shown in FIG. 1. 30 designates again the molding drum which is reached by the web over the roll 32; the pressure roller 34 is arranged for cooperation with the molding drum 30, as described above.

As the web leaves the molding drum, it is taken up by a direction changing roll 36a and passed to another idler roller 42, from where it is made to drop under its own weight to form a loop generally designated by 44. This loop formation is caused when the apparatus is first set up for operation. The web is made to descend to a controlled level 46, before its ascending leg is passed over a further roller 48 from where it is passed on to a rewinder, cutter or other processing device (not shown).

Since the length of the loop is subject to variations when the weight of the loop changes, e.g. due to a differing water content or coating thickness, means are provided for the control of the level of the loop. In the example illustrated herein, a sensing device is provided, e.g. an electric eye, which may comprise a source of light 54 and a plurality of sensing elements. In FIG. 2, two such sensing elements 50 and 52 are shown, between which the length of the loop may vary. However, it should be understood that any desired number of elements could be provided in case a greater variation of the length of the loop is desirable.

When the loop passes the upper sensing element 50 as by reaching the level 46, and cuts off the light from source 54, the operator may receive a warning signal at an indicating device (not shown) connected to respond to said sensing element 50. He may then adjust the rewinding (or other) device for changing the speed of web withdrawal. Alternatively, the change of speed may be postponed until the level of the loop passes the sensing element 52.

Instead of manual adjustment, automatic speed-changing means may be provided to respond to the different sensing means.

It should be understood that this interposition of a loop of web has been found to increase the degree of gloss of the treated paper quite considerably. The improvement is considered to be due to the control of the slippage which takes place between the coated surface of the paper and the highly polished surface of the molding unit at the nip. The greater the slippage the less mirror finish on the coated surface. Even though the actual numerical gloss might not be affected by this slippage the cast or mirror image imparted by the polished surface will be greatly affected by excess slippage.

Therefore, by introducing the free loop, as described above, any excessive pulling out of the nip by other units (e.g., wind-up rolls, pinch rolls, tension devices) has been eliminated. The molding unit will then tend to push or extrude out the glazed paper from the nip rather than have it pulled out by foreign exertion. The length of the loop may vary to a certain extent as long as the loop is capable of performing its main function, i.e. of reducing slippage.

Apart from the control of the length of the loop, there are a number of other factors which have to be closely controlled in carrying out the process according to the invention. One of these is the temperature and the moisture content of the coated surface at the molding nip between rollers and 34. Since, as mentioned before, it is important that the molding should be effected before all the moisture has passed from the coating into the base stock, it has been found best to dry the coating by applying heat to the back of the incoming web in order to retain moisture in the coating rather than allow it to pass into the base stock. This is accomplished by the t arrangement of the burners 28 ahead of roller 32. It should, however, be understood that the heat can likewise be applied by other means, such as a steam-heated roller, or by infra-red radiation.

It is estimated that the over-all moisture content of the stock and the coating is between 9% and 11% by weight at the point when the coated paper reaches the brushes 22. More of this moisture is contained in the coating than in the base stock, and it is desirable to maintain that condition. Intense heat is therefore being applied to the back of the moving web where it passes the gas burners, whereby the temperature of the surface is raised just before it reaches the molding nip, while, at the same time, moisture is being driven toward the coated surface. Molding can then be effected at higher speeds than usual with a minimum danger of the stock sticking to the molding surface.

Under the old process of separate operations, the moisture content of the paper and coating came to equilibrium between the coating and finishing operations. It was, therefore, necessary to run at slower speed in order to prevent sticking of the coating when a high moisture content (such as 12%) was used during molding. The new machine is designed to retain as much moisture in the coating layer as practicable while simultaneously keeping the base stock moisture low. This is accomplished by molding immediately after coating before the moisture from the coating has penetrated completely into the base stock by drying from the back and by pre-he'aters on the back of the web immediately preceding the molding drum. Under these conditions an overall moisture content of base stock plus coating might be 8%, but the moisture of the coating layer at this point is estimated at 12%. In this way, the small amount of water contained in the coating is evaporated into the dry base stock quickly, permitting separation from the drum without sticking at a faster speed.

The optimum temperature of the coated surface ranges from 155 to 165 F., just before it reaches the molding nip. This temperature can be adjusted by arranging the proper number of burners or other heating means, and providing thermostatic temperature control for the desired degree of temperature once it is reached.

The calender is designed for a pressure range from 760 to 1600 lbs./lin. inch. The normal operating pressure is 1000 to 1200 lbs./lin. inch.

As regards the optimum temperature of the drum surface, this ranges from 215 to 225 F. For this, likewise accurate temperature control means have to be provided. The drum may either be heated internally by superheated steam of the required temperature, or controlled gas heating means or electrical means may be applied for this purpose. With the temperature of about 220 F., a pressure of about l0001200 lbs./lin. inch and a speed of 200 r.p.m., the gloss of the paper ranges from 50-55 on a Gardner 20 Gloss Meter.

Another factor which has to be considered is the material of the drum, the wall thickness, and the diameter of the drum. We prefer to use a steel drum having a wall thickness of 2 in. and a diameter of 45 in. It has been found that with the above parameters for the drum, a suitable optimum heat transfer rate can be obtained which may be computed by the formula:

450 (+10%) B.t.u./hr./ F./Sq. ft./in.

The drum diameter is, of course, not limited to 45". A molding operation can be carried out at a higher speed if a larger diameter drum is used. The speed can be calculated in terms of dwell time of the paper in contact with the drum surface. In other words, a drum having twice the diameter, assuming the same amount of wrap, is capable of producing the same gloss at twice the speed.

As mentioned above, the gloss of the paper product is, furthermore, influenced by the hardness of the backing roll which forms the nip with the molding roll. The backing roll is a steel roll with a covering of rubber or other resilient material. The thickness of the covering is from to 1, the preferred value being /8". The hardness of the layer measured by P & I plastometer ranges from 4-12, the preferred range being 4-8, which we found will produce highest gloss and smoothest surface.

It has been stated above that the length of the loop may vary. However, the leg of the loop must be long enough to act as a counterbalance preventing the coated paper or board from adhering to the polished drum and doubling back into the nip.

This force-weight-counterbalance relationship will vary with the type of base stock being used (e.g., paper vs. board), speed, drum diameter and possibly winder tension before the molding unit. (See the specific example below.)

As stated before, the paper products to be produced contain a large amount of pigments consisting of fine clay and/or other pigments. The clay and other pigments constitute at least of the composition. Clays of the kaolinite type, having a particle size of at least below 2 microns, are preferred. Other pigments such as titanium dioxide, zinc sulfide, calcium carbonate, etc. may be added to improve brightness, opacity, or ink receptivity of the coating. The coating formulation preferably contains a non-thermoplastic adhesive such as protein, casein, polyvinyl alcohol, etc. to improve adhesion to the substrate. The preferred range for the adhesion is 1 to 5% of the weight of clay plus pigment.

Thermoplastic polymers such as acrylics, vinyl copolymers and synthetic rubber polymers are an essential part of the coating composition. The preferred range for the thermoplastic polymer is between 10 and 15% base on the weight of the clay plus pigment. The total of thermoplastic polymer and adhesive comprises from 12 to 20% of the weight of the clay plus pigment.

The following examples show by way of illustration formulations for the composition of the coating and the way of operating with different coating weights and related operating speeds.

Formula 1: Parts Kaolin clap 80.0 Rutile TiO 20.0 Acrylic latex 12.6 Casein 2.3 Microcrystalline wax 3.2 Calcium stear-ate 1.0 Water 110.0 Weight coat 4-5 lbs. pigment/1000 sq. ft.

Formula 2:

Kaolin clay 90 Rutile Ti 5 Calcium carbonate 5 Vinyl acrylic copolymer latex 12 Protein 1.5 Parafiin wax 2.5 Calcium stearate 1.5 Water 120 Weight coat 7-8 lbs. pigment/1000 sq. ft.

Formula 3:

Kaolin clay 75 Talc 5 Anatase Ti0 20 Styrene butadiene 10 Polyvinyl acetate 5 Casein 2 Microcrystalline wax 1.5 Calcium stearate 1.3 Water 135 Weight coat 3 /z-4 /2 lbs/1000 sq. ft.

Basic weight (per M sq. ft.):

I (lbs)..- 90 II (lbs) 50 III (lbs) 70 Speed:

I (f. p. m.) 200 II (f. p. m.) 400 III (f. p. m.) 350 Drum material: ..Steel Drum diameter, inches 45 Wall thickness, inches 2 Drum surface temp., F. 215-225 Pressure, lbs./lin. inch 1000-1200 Total moisture at point of brushing, percent 10 Total moisture content at point of molding, percent 8 Temperature of coated surface just 1 This temperature is achieved by using gas burners on the back of web having an output of 2500 B.t.u. hr./in. of wldtlu An example of optimum loop length using Formula 1, above, under operation conditions I, would be approximately 20 feet total as measured from rolls number 42 to 48 in FIGURE 2. When running a web 48" wide the counterbalancing weight force of this 20 foot total loop would be approximately 7.2 pounds.

As mentioned before, the paper products made in accordance with the invention have an improved gloss which on 21 Gardner 20 Gloss Meter gives a reading that ranges from 45-60. a

The foregoing disclosure relates only to preferred embodiments of the invention and is intended to include all changes and modifications of the examples described within the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A method of producing paper products having a high gloss and smooth finish by the use of a highly pigmented coating composition containing at least 70% by Weight, of pigment and further containing a weight quantity of a thermoplastic polymer equal to 10 to 15% of the weight of the pigment and a weight quantity of a nonthermoplastic adhesive equal to 1 to 5% of the weight of the pigment, the total weight quantity of the thermoplastic polymer and the adhesive comprising 12 to 20% of the weight of the pigment, and water, which comprises subjecting a paper web coated with said composition and dried to a water content of 9 to 11% by weight, based on the weight of the web, to a polishing by brushing operation, thereafterv passing the web through a pre-heating zone in which the coating is heated to a temperature of to F. and, while at said temperature, on to a molding station wherein molding is effected by molding means having a molding surface heated to a temperature of 215 to 225 F., the pre-heating facilitating evaporation of water in the coating during the molding step.

2. The method according to claim 1, wherein the moisture content in the paper product is brought down to about 8% by weight of the paper product, including the coating, at the point where molding sets in.

3. The method according to claim 1, wherein the pressure maintained during the molding operation is from 1000 to 1200 lbs./lin. inch.

4. The method according to claim 1, in which the web is withdrawn from the molding station under tension sufficiently low to prevent slippage of the web in the molding station.

5. The method according to claim 1, wherein said loW tension withdrawing is effected bymaking the web leaving the molding station form a loop located immediately after the molding station and of predetermined length by dropping under its own weight.

6. The method according to claim 5, wherein a variation in the length of the loop is controlled by speed adjustment of the web withdrawal.

7. An apparatus for producing paper products having a high gloss and smooth finish by the use of highly pigmented coating compositions, which comprises in combination means for applying and drying said coating composition on a web of paper, means for polishing the coated surface, means for pro-heating said web from the uncoated side to heat the coated surface to a temperature to facilitate evaporation of water in the coating during subsequent molding and to prevent a major amount of water from migrating from the coating composition into the paper base before molding takes place, a polished metal drum and a backing roll arranged for cooperation for molding said coating, means for feeding the web product through said apparatus in continuous flow so that the coated web is fed to said drum and said backing roll while in the pre-heated state and means for withdrawing the web from the molding station under tension sutficiently low to prevent slippage of the web in the molding station, said withdrawing means comprising means for permitting the web to drop under its own weight immediately after the molding station to form a loop and for regulating the length of said loop.

8. An apparatus according to claim 7, wherein said backing roll is provided with a resilient covering having a hardness of 4-12 as determined by a P & J plastometer.

9. An apparatus according to claim 8, wherein said backing roll is provided with a rubber covering having a thickness of A3 to 1" and a hardness of 4-8 as determined by a P & J plastometer.

10. The apparatus according to claim 7, in which the means for controlling the length of said loop is located intermediate between the molding station and the Withdrawal means.

7 8 11. The apparatus as claimed in claim 10, wherein the 2,829,980 4/1958 Redd 117-64 loop controlling means are electrically operated. 3,015,581 1/ 1962 Gallino 1l746 References Cited MURRAY KATZ, Primary Examiner. UNITED STATES PATENTS 5 U S C1 XR Re. 11,461 1/1895 Sparks 117-64 1,583,788 5/1926 Gilman 11764 117-46, 65.2, 155, 156; 11847, 60, 112

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