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Coating

( Originally Published 1920 )



A COMPREHENSIVE definition of a coated paper might be —"a base paper to which a coating of any kind has been applied." This would include a wide variety of papers, used for a multitude of purposes, and a description of all of these would produce a lengthy volume. For the purpose of the present story attention will be confined to such coated papers as are generally used for printing books, magazines and advertising matter, with brief mention of associated products where desirable.

Coated paper is said by several authors to have originated with the Chinese, but no dates are given, and it must be assumed that their papers were of a crude type and used for wall coverings. European wall papers—or hangings, as they were called, because they were not pasted to the wall but merely hung like tapestries—were at first printed on uncoated paper, and an English journal states that it was about 1650 when they came into use. These early hangings were quite crude and a marked improvement was made when a flat ground-coat was applied to the paper which was then printed with a decorative design. Such papers were made in the United States in 1824, and this may be assumed as the approximate date at which paper was first coated in America. It seems highly probable that the great improvement caused by printing wall paper over a ground coat was responsible for the coating of other types of paper to obtain a similar improvement in printing.

Early coated papers were made sheet by sheet, brushing on the coating mixture by hand. Many of the papers were used for highly glazed labels or for colored box coverings, and the mixture applied became known as "color." In many plants this name persists to this day, even when the coating contains only enough coloring matter to give some tone of white. About 1852 a machine was built for coating paper in a continuous web and this soon replaced the messy operation of hand coating for both wall and glazed papers.

Printing on coated paper was at first by the lithographic process and its adaptation for letterpress printing was said to be due to the trial of a glazed coated paper to pull a proof of a woodcut which was not giving satisfactory results on a super-calendered plain paper. The improvement in the finer lines of the woodcut was so great that a coated paper for this type of work was developed about 1881 and was called "fine cut" by the producer, because of its especial advantage for finely cut, wood-block illustrations. This paper was therefore already available for use when halftone plates were invented about 1882-1885, and was not developed because of its need for halftone printing. It is probable, however, that with-out the demand caused by the use of halftones the coated paper industry would have had a relatively slight development.

It appears that the first paper coated on both sides was made in 1874 or 1875 and this is also apparently the first application of coated paper for book printing. This paper was coated on each side separately and it was not until considerably later that coating machines were developed to apply the coating on both sides of the paper in a single operation.

The essential feature of a coated paper which makes it superior for printing is the smoothness of its nearly plane surface. No matter how heavily a plain paper is calendered, or how smooth its surface appears to be, there are always roughnesses which prevent perfect contact of the halftone dots of the printing plate and which injure the appearance of the illustration. When a coating is applied it fills in the irregularities of the base paper surface and after calendering it presents a much more uniform surface to the printing plate, and thus permits a far more accurate reproduction of the dots of the halftones.

The materials universally used for coating a printing paper are one or more mineral pigments combined with one or more adhesives. The latter cause the pigments to stick to the surface of the paper firmly enough to withstand the pull of the printing ink. The selection of pigment and adhesive combinations is responsible to a very great extent for the resulting characteristics of the finished paper and its success or failure in the printing field. It can influence such properties as color, brightness, gloss, opacity, surface smoothness, ink receptivity, rate of ink drying and folding characteristics of the paper.

Practically all of these materials are purchased by the paper coater and as they are all subject to variations, either in their raw material or their process of manufacture, very careful supervision is necessary during their use.

Some of the properties inherent in these materials, or imparted by them to the paper, may be of interest.

Clay, and its origin and preparation, have been mentioned in the chapter on Stock Preparation. Clays differ very considerably in chemical composition, but in general are hydrous aluminum silicates. The chemical differences in clays have practically no bearing on its use as a coating pigment. Clays also differ greatly in their natural particle sizes, and these differences are considerably widened by modern methods of preparation, where a given clay may be separated into fractions of coarser and finer particles. This is a very important point since the particle size has a great influence on the finish which a coating will take—the smaller the particles, the higher the gloss. Clay is probably the most widely used of all the coating pigments, and the property next in importance to particle size is color, which should be as white and bright as possible. Clay has a moderately good hiding power and it imparts good printing properties to the paper, but for the highest grade papers it is usually blended with other pigments.

Precipitated chalk is a manufactured calcium carbonate, which may be prepared in a number of ways. The particle size can be made to vary from extremely fine to relatively coarse and the finish which it imparts to the coating varies correspondingly from very high to semi-dull. As compared with clay it tends to increase the brightness, opacity and ink receptivity of the coating. This pigment, whose use started with dull-finish papers, is becoming increasingly important in high-finish products.

Satin white is an artificial pigment prepared by the action of aluminum sulfate on hydrated lime. It is a very fine-grained pigment of high brightness, and it imparts high gloss and whiteness to the coating. It is not particularly opaque and it is strongly alkaline; also it is not easily prepared, is relatively expensive, and is difficult to use. It was formerly considered essential for coatings requiring high gloss, but its place is now largely taken by the very fine particle sized clays and precipitated chalks.

Titanium pigments include the dioxide alone or precipitated on calcium or barium sulfate. The dioxide is the most commonly used, largely because of its very high brightness and covering power (opacity). When used in mixture with other pigments, as is nearly always the case, it does not affect the gloss obtained to any extent, but it is one of the harder pigments and when used in large proportions would tend to reduce finish.

Blanc fixe is a precipitated barium sulfate. It is a heavy pigment of good brightness and covering power but it will not take as high a gloss as other equally fine-grained pigments. It was formerly used as an ingredient in many printing papers, but its present use is largely confined to high grade photographic papers, and to chart papers where its abrasiveness is helpful when the records are made with a metal stylus.

Zinc pigments have high brightness and good covering power and in these respects are similar to titanium dioxide. They are sometimes used in coatings, but are not among the most commonly used ingredients.

For colored papers numerous colored mineral pigments are used where permanence is required, while for greater brilliance, but less permanence, color lakes or soluble dyes are used.

The adhesive used to hold the pigment on the surface of the very early wall papers was probably starch or some vegetable gum, but in making white paper for general printing purposes animal glue was the first to be extensively used. Glue is prepared by extracting various animal tissues with boiling water, and its properties vary considerably according to the material used and the care taken in its manufacture. It is a nitrogen-containing material and is subject to putrefactive changes unless kept in a dry condition; this is also true of paper in which it is used. Glue is seldom used in coating paper today.

About 1893 casein began to replace glue as a coating adhesive.

Casein is prepared from skim milk by natural souring, or by the addition of acid, followed by washing, pressing and drying. This also is a protein, and subject to spoilage, as with glue, but with proper attention to cleanliness and the use of simple preservatives no serious difficulty is encountered. Casein is not soluble in water alone, and the presence of alkali is required to dissolve it for use; the alkalies generally used are ammonia, soda ash, caustic soda, borax and sodium phosphate, but a number of others can be used if desired. Casein solutions are usually prepared by mixing the casein with water, adding the alkali and warming to about 130°F. A considerable excess of alkali should be avoided, as it darkens the color and tends to cause decomposition of the protein molecule. Casein replaced glue, partly because of its lower price and partly because it can be rendered quite insoluble in the finished coating either by formaldehyde, or by compounds which develop formaldehyde during drying of the coating.

Starch was the next adhesive to break into the coating game and samples consisting chiefly of starches were being offered as early as 1904. Natural starches when cooked in water produce pastes which are too thick to make coating mixtures with the proper flow characteristics, and it is necessary to modify them by some form of chemical treatment before they can be used successfully. One such modified starch was prepared as early as 1906 by treating with calcium hypochlorite and was used for many years by a large paper coater. Coating starches are now prepared by oxidizing treatments, by dextrinization or by treatment with enzymes, and in one or another of these forms are used to a much greater extent than casein. This is largely due to their wide application in machine coated papers, for which they have been found especially suitable. In the early days of starch coatings much unsatisfactory paper was made, but the prejudice which this caused has disappeared and starch coatings are now generally used and accepted without question.

Soybean protein is a possible substitute for casein in coatings. It has about the same adhesive strength as casein, but imparts a slightly poorer color to the paper. It requires a relatively strong alkali to develop its full adhesive strength and unless carefully manufactured is somewhat tricky to use, as it is somewhat sensitive to over-heating when in solution.

Soybean flour is an unpurified product made by grinding soy-beans from which the oil has been extracted. It contains only about 50 to 55 per cent usable protein, the rest being very finely ground cellular tissues, etc. It is sometimes used as an extender for casein, but is almost never used alone in printing papers.

Several synthetic resins are sometimes used as adhesives in coating. One of the most interesting is polyvinyl alcohol which is soluble in water, not subject to putrefaction or fermentation, and has a much greater adhesive strength than casein. But for its high cost it would probably be much more widely used.

Synthetic resin latices have also come into frequent use in the last few years to improve the gloss, flexibility and wet rub resistance of the coating. They are seldom used alone, but are generally blended with other adhesives in sufficient amount to give the desired improvement.

With this wide choice of materials available and in actual use it is hopeless to try to give any detailed coating formulas, and it must suffice to say that they vary from a simple mixture of clay and casein to more complicated formulas which may contain three or four pigments as well as two or more adhesives. In addition to these essential materials, others are used in the coating mixture for special effects. Dyes or pigments are added to produce the desired hue in the paper; various materials are used in small amounts to reduce foaming of the mixture and thus eliminate froth pits in the paper; wetting agents are sometimes used to aid the even application of the mixture to the paper surface; and in some cases substances are added to produce a desired physical characteristic in the mixture, or to render the final coating waterproof.

Before preparing a coating mixture it is customary to disperse the pigment in water, and dissolve the adhesive separately. With pigments such as clay it is necessary to use some dispersing agent to obtain a free flowing mixture of high solids content. Such agents may be sodium silicate, or some of the phosphates, and only very small amounts need be used-0.2 to 0.5 per cent on the dry clay. When precipitated chalk is used, casein acts as a reasonably good dispersing agent.

The chief requirements of a coating mixture are freedom from coarse particles or lumps of pigment, and a smooth-flowing mixture of the proper viscosity and solids content. It is, of course, necessary to have the pigment-adhesive ratio such that the paper will take the proper finish on calendering and will print without the coating picking off by the tacky ink. A very thorough mixing or grinding of the pigment suspension and of the coating mixture itself is beneficial in producing the desired flow and viscosity. Mixing the clay at very high solids in a heavy duty mixer is also of much help. At best some of the properties desired cannot be obtained because of the incompatibility of some of the necessary ingredients, and the final result is often a compromise.

Coated paper is today made in two different ways. The first to be developed was that carried out by transferring the base paper, or "raw stock," as it is sometimes called, to a separate plant and completing the work there. Coated papers of the highest quality are still produced in this way, which is generally called "conventional coating." The second method is called "machine coating" because it is carried out as a part of paper machine operation and at the regular speeds of paper production. Although the idea of machine coating was mentioned as early as 1893 no really practical application of it was made until about 1938. Machine coated paper was at first quite inferior to that made by conventional methods, but with the skill and experience developed during the last fifteen years its quality is becoming much nearer to that of the good grade conventional coateds.

Coating was first applied in continuous operation to only one side of the sheet and later machines were developed to coat both sides simultaneously. In conventional coaters there are many different ways to apply the color and spread it uniformly on the paper. Most of these meter the proper amount of color onto the sheet by passing it between rolls whose distance apart can be accurately controlled, and which thus measure the amount passing to the paper. These are followed by brushes working back and forth across the sheet to spread the color, or by rolls revolving rapidly in a direction counter to the flow of the sheet, which accomplish the same result. One coater applies an excess of the color and then removes that not desired by a knife-like blast of air; this is known as the "air brush" coater.

When paper is coated on one side only it can pass from the coater directly to a traveling conveyor which carries it in loops suspended on sticks through a heated drying line. From this it emerges fully dry to be wound in rolls. Such loop lines may be straight away, in which case the roll is wound at the opposite end to the coater, or there maybe a turn around in the line so that the dry paper returns to a winder at the same end of the system as the coater. When the paper is coated on both sides the wet coating would be injured by contact with the sticks, so it is necessary to support the wet web over a series of hot air blasts until it is sufficiently dry to permit contact without injury. It is drawn over this section of the drying system by a traveling, perforated apron to which it is held by suction, and from this point goes to the loop lines as usual.

In all these coaters the weight of coating applied is determined by weighing a sheet of definite size before and after coating. If the weight of coating per ream thus found is not correct the amount is changed by adjusting the metering rolls or changing the air pressure of the air brush coater. At the same time, especially when starting an order, a piece of the coated paper is calendered and tested for coating strength by the sealing wax test or by a printing operation. This test shows whether the coating contains too much or not enough adhesive and makes it possible to adjust the amount to the most satisfactory point for economy and good printing characteristics. Too great a proportion of adhesive is not only wasteful, but it reduces the gloss obtainable on calendering and lowers the printing quality of the sheet.

Conventional coating methods are more versatile than those of machine coating. They are capable of applying widely varying amounts of coating from a few pounds per ream to as much as 30 pounds or more. They employ mixtures containing from 30 to 45 per cent of solid material, and because of these low solids, substances can be used which are not applicable in machine coating because they do not produce mixtures of sufficient fluidity at the high solid content demanded. It is also possible to change quickly from one type of body stock, or color, to another, and thus produce various kinds of paper on one machine. The speed at which conventional coaters operate is much lower than paper machine speeds--300 to 350 feet per minute is a fairly high speed. This limit is imposed more by the ability to dry the coating than by any trouble in applying and spreading the color. If the paper is coated on only one side it can be dried at high temperature in a tunnel drier at speeds as high as 900—1100 feet per minute.

The character of the body stock used has a great influence on the quality of the coated paper produced. Among the important properties are strength, formation, finish, sizing, porosity, opacity, color and brightness. The optimum for all of these properties cannot be obtained in a single sheet, so body stock is always a compromise, planned to emphasize the features most important in the finished paper. For example, the more closely formed the body stock, the smoother the coating and the more uniform the finish, but to obtain these results some reduction in strength and folding qualities must be permitted. Color and brightness are much more important than one would suppose, since the coating would be expected to mask them completely. Such complete masking can be effected only when several times the customary amount of coating is applied, and the paper is then so heavy, stiff and brittle that it is no longer serviceable for most printing purposes. Most body stocks are made from a mixture of long and short chemical wood fibers, beaten relatively lightly—just enough to prevent fuzz on the paper surface, but not enough to make the paper hard to handle on the coaters.

Machine coated papers were first planned with the idea of filling the pores of the body stock but still leave most of the paper surface predominantly fibrous. Such coatings were of very light weight—probably about 3 pounds per ream—and the paper was only slightly better than uncoated, and far inferior to conventional coated papers. From this starting point the weight of coating has been increased to 8-10 pounds per ream per side and the quality is fully as good as some of the conventional coateds.

The coating mixture for machine coating usually consists of clay and starch, either an oxidized or dextrinized product. It must have the right type of flow so that it spreads evenly on the paper, but becomes immobile as soon as the paper is released from the pressure of the coating rolls; otherwise an undesired pattern appears in the coating. Machine coat mixtures usually contain 60 to 65 per cent solids. The pressure of the coating rolls tends to drive the moisture into the paper, and this, together with the small amount present, permits the paper to leave the coater in such a condition that it may make contact with the drying cylinders with-out serious sticking. To accomplish all this, very careful selection of both clay and starch is necessary and the control of every step in preparing the color must be far more exact than similar operations in conventional coating.

The coating equipment in machine coating is located between banks of driers in the paper machine. Some coaters treat both sides of the paper simultaneously, while others coat one side, pass the paper over a few driers and then coat the second side. In some the color is applied by a device very similar to the ink distribution rollers on a printing press; in others it is applied by two heavy, rubber-covered rolls between which the paper passes as the coating goes on. Coaters of these types operate at speeds as high as 1000-1200 feet per minute, which aids very greatly in producing a low cost coated paper.

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