The text contained herein was written by Helen U. Kiely, and delivered by Joseph H. Burgen before the Joint Session, Connecticut Valley Mill Superintendents and Printing House Craftsmen, March 5th, 1927.
American Writing Paper Company
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PROBABLY the most serious single source of trouble to the lithographers, printers and converters is found in the curling or buckling of paper. I am glad to say that any troubles printers may have been experiencing well probably be lessened appreciably as the warm weather comes and artificial room heating removed. The importance of moisture and humidity control in relation to the elimination of these difficulties is increasingly recognized throughout the trade and will be more universally applied within the next few years.
Relative Humidity could involve a very technical study. It is hard to visualise and understand just what it means without becoming somewhat technical and hence uninteresting to many. I will try to explain very simply what is meant by Relative Humidity and how it effects paper and causes trouble which the paper maker can in no way remedy.
The terms "dryness" and "wetness," applied to air are purely relative and indicate the proportion of water vapor actually present in comparison with what the air could contain at the same temperature. At the Dew Point the air is saturated and the water is deposited as dew, mist, or even rain. It holds all the water it can. As the temperature rises the dew disappears and is held in suspension until the temperature is lowered when it is again deposited. This condition is observed in the early morning in summer. In the sweating of the tumbler of cold water we have another example of the existence of water vapor in the atmosphere. The ice water chills the temperature around the glass and the moisture is deposited. In the winter time you can see your breath when you breathe as the breath is warmer than the air and it condenses. From this we see the words "dry" or "moist" as applied to atmosphere have a purely relative significance depending upon the temperature. They involve a comparison between the amount of water vapor actually present and that which the air could hold if saturated at the same temperature. The ratio of these two quantities is called the "Relative Humidity." When we say a Relative Humidity of 65% we mean that the amount of water vapor actually present is 65% of what the air might have contained at the given temperature of say 70 F if it had been saturated.
The Psychrometer is an instrument used for measuring Relative Humidity. It consists of two thermometers, one covered with muslin and kept moist with water. The rate of evaporation from the moist muslin depends upon the quantity of moisture in the air. The more rapid the evaporation of water the greater the cooling and hence the greater difference in the readings of the two thermometers.
It would be very easy for any printer or paper man to take a series of readings around his print shop and record the variation in temperature and relative humidity. It would be very interesting and enlightening to many to see the variation in the amount of water in the different rooms and realize how the paper is being dampened and dried through the converting processes. The following tables are the measured humidities in various parts of the paper mill:
|Outdoors (dry day)||84 F.||37%|
|Outdoors (rainy day)||73 F.||73%|
This tabulation affords an excellent illustration of the increase in humidity on lowering the temperature of the air, also of the variation with the weather.
At various relative humidities paper holds different percentages of water and will absorb or give up water until it comes to equilibrium with the existing temperature of the room.
|% Relative Humidity||% Moisture in Paper|
Paper on coming off the driers of a paper machine contains from 3-5% moisture. Paper from the loft contains from 2-3% moisture, depending on loft conditions. These moisture ranges are the ones that are suitable for the ordinary ranges of humidities ordinarily encountered. When such paper is packed for shipping the moisture present in the sheet is evenly distributed and there is no curling as the paper in the finishing process has been seasoned under the normal atmospheric conditions for this period of the year and consequently there is no tendency for curling or waving as the paper is in equilibrium with the moisture conditions present.
In general the tendency of a paper storehouse is to be several degrees lower in temperature than a print shop, where the warming stoves for stones, bunsens for copper plates and heating systems assist in raising the temperature during the working period. The heating developed during the day is highest at closing time. It must be remembered that when a room is warmed the quantity of moisture is not diminished but the humidity of the air is lessened because the saturated capacity of the air is so enormously increased. With the falling temperature at closing time the moisture which had previously not been noticed is apparent in the form of a damp.
Apart from the system of heating and ventilation the amount of moisture varies during the course of the day and year being at its maximum of absolute moisture in the summer at the hours of 8 A. M. and 8 P. M. while the minimum occurs at 3 A. M. and 3 P. M. This is due to ascending currents of air which carry the moisture upwards. The relative moisture is on the average greater in higher than in lower latitudes; it is at its minimum in the hottest and maximum in the coolest parts of the day. It also varies with different regions, being less in the center of the continent than near the coast.
During the winter humidity conditions in the press rooms and print shops are particularly bad for while the average outdoor humidity may be fairly high (50-60%) the small amount of moisture contained in the air at the low temperatures encountered during the winter months causes a serious reduction in relative humidity on being heated inside the building. As an example to illustrate this point, the air at 40 F. and 50% relative humidity, which are normal winter conditions contains but 18 grains of moisture in a pound of dry air. On heating such air to 75 F without changing the actual amount of moisture present the relative humidity ha been reduced from 50% to 15% because the capacity of the air for holding moisture has been so enormously increased by raising the temperature.
For this reason, during the winter months the floor should be sprinkled with water at frequent intervals or buckets of water should be allowed to evaporate on radiators, etc., unless other more adequate arrangements for humidification are provided.
In the summer conditions frequently are just the converse. At the temperatures prevailing in the summer even with normal humidity, there is a considerable quantity of moisture available in the air. When this air is brought inside and cooled to any extent, especially if it enters a cool cellar, the temperature is rapidly reduced, the quantity of moisture remaining constant, the humidity must increase. Providing the temperature drop is great enough, especially near walls or any cold objects the air will contain more water than it can hold at that temperature and the vapor will be deposited in the form of a fog or damp, which will see tle out on the paper disturbing the equalized moisture conditions and resulting in curls and buckles.
An example based on figures perhaps will help make this idea clearer. A comfortable, normally hot day in the summer would have a temperature of 900 F. and a humidity of 60%. It is only necessary to drop the temperature of air at this condition to 74 F. to cause over 100% humidity and consequent condensation of water vapor.
From this example the slight change in temperature required to completely transform the humidity conditions is evident. If, therefore, in the absence of adequate methods of controlling humidity, differently behaving paper is run on a day suited for its condition, better results will be obtained. Even though the temperature drop was not great enough to cause super-saturation of the air, it is enough to increase greatly the relative humidity which will affect the paper. It must be borne in mind that the moisture content of a sheet of paper is dependent not only on the relative humidity of its environment but also on the temperature. It is, therefore, not good practice to use cellars for storerooms or to subject the paper to great temperature and humidity change without inviting curling and buckling difficulties.
What has this talk on humidity to do with paper curling and buckling? Just this--Paper possesses the property of absorbing and giving off moisture until the moisture in the paper and the moisture in the atmosphere are in equilibrium. The behavior of paper under given atmospheric conditions is, therefore, dependent on the relative amounts of water present in the atmosphere and in the paper. That makes it impossible for the paper maker to manufacture paper that will satisfy all condition in the trade. No two rooms have the same operating conditions and no two localities the same outdoor conditions. Therefore, paper cannot be made that will not curl under certain conditions.
The so-called curling of papers is generally found to be in one direction-toward the wire side as a, result of certain conditions encountered in forming the sheet of paper on the paper machine wire.
In forming a sheet on the traveling wire the stock suspension surges out on the wire at a considerable velocity, dependent on the head of water behind the slices and the speed of the machine. The natural flow of the stock would tend to induce the fibers to arrange themselves mostly lengthwise. A lateral shake, therefore, is given to the traveling wire to felt the fibers together. The stock nearest the bottom attaches itself to the wire before the effect of the lateral shake can distribute the proper proportion of the fibers in a horizontal direction. The middle and upper layers contain a larger proportion of horizontally lying fibers which tends to make the sheet of more uniform structure in each succeeding layer from the bottom. This more equal distribution of lengthwise and crosswise fibers near the top of the sheet of itself resists any curl in the direction toward the top of the sheet.
In addition the stock next to the wire settles against the smooth surface of the wire, thereby making the wire side of the sheet comparatively level. On the top of the sheet, however, the stock settles in clots, forming microscopic hills and valleys on the top surface or felt side of the sheet. In spite of the smoothing effects of the press rolls and calender rolls in smoothing the microscopic lumps into the sheet of paper the felt side of the sheet has more surface and consequently is longer than the wire side. This fact of itself is responsible for a tendency of the sheet to curl toward the wire or shorter side the instant the surface becomes moistened. Other things being equal a sheet made from shorter stock will curl more easily under variable humidity conditions than one made from longer stock.
It has been pointed out above that the greater proportion of the fibers tends to arrange themselves lengthwise in the web resulting in a distinct grain in the paper. It is a well-known fact that the fibers expand far more diametrically than they do lengthwise on being moistened thereby giving rise to a wave in the direction of the machine or grain.
Furthermore when the water is removed from the web during drying, there is a tendency for the traveling and gradually drying web to contract in width, this contraction lengthwise being restrained by reason of the tension created by the traveling and drying apparatus, but being unrestrained in the cross direction.
As a result of these two factors a sheet will always expand more in the cross grain direction than in the length on being exposed to changing moisture conditions thereby causing any curling to take place in the direction of the grain or machine direction.
The effect of the sizing on the natural curl of the paper is of considerable importance. Sizing tests invariably indicate that the wire side is slacker sized than the felt side due to the action of the suction boxes in removing the rosin size precipitate from the fibers. This is particularly true in case the stock is free. As a consequence, the sheet will absorb more glue or gum on the wire side which on being subjected to moisture or drying is more flexible than the stock and will tend to cause a curl in the direction of the bottom or wire side of the sheet.
The slowness or condition of beating that the stock has been subjected to also has an important influence on the curling. A paper stock that has been hydrated or slowed shrinks considerably on being dried which makes the sheet more susceptible to stretching on being moistened. Another property of a hard, slow stock is its lack of absorbency which does not permit of water passing through the sheet but rather contains it on the outside surfaces thus rendering it also more susceptible to curling.
If the paper is allowed to stand about the print shops and press rooms in high piles in either a drier or more humid atmosphere, the exposed surfaces of the paper gradually assume the moisture condition corresponding to the prevailing humidity conditions. Inasmuch as the piles are tightly packed it is only the top surface and edges of the sheets that have an opportunity to adjust their moisture content to these conditions and consequently with a condition of variation in moisture content between the edges and inside of the sheet, the waving and buckling is inevitable.
Another consequence of piling is a result of the height of piles. The weight being concentrated at the center of gravity of the pile, which is the center, causes an uneven pressure on the center and edges of the pile which also results in further wavy edges.
An example of the effect of uneven distribution of moisture through a sheet of paper is available in a comparison between piles of stored ledger paper and a cockled bond. The ledger paper having a high finish will be packed very tightly and closely and consequently will be wavy and buckled. The cockled bond, however, because of the cockle will not pack closely in a pile of the same height, more air is allowed into the pile and as a result very little buckling is observed.
In order to eliminate these effects outlined above it is suggested that the piles of paper be made lower and the paper crossed with alternate layers in different directions. In fact any procedure to thoroughly aerate the paper cannot fail to help eliminate the waving and distortion of the edges.