The Physical And Chemical Aspects Of Paper
( Originally Published 1920 )
THE size and weight of a sheet of paper of any given quality and finish are its most obvious features, and when we speak of the weight of a sheet of paper we refer not to the one sheet, but to the weight of one ream of similar sheets. Most papers are ordered on a basis of ream weight for a specified size, as, for example, 25 by 38, 50-pound. Blanks, cardboards and cover-papers, especially the first two, are more frequently ordered on a basis of bulk, as two-ply, three-ply, etc., and thick or double thick in the case of covers. The thinner covers are usually designated by their ream weight, though frequently quoted, as are the heavy-weight covers, the blanks and cardboards, in price by the hundred sheets.
The reason for this difference is probably that such stocks are sold in comparatively small lots, so that it is simpler to bill them in accordance with the number of sheets than to figure the weight of a small number of sheets and multiply by the pound price.
Another thing which facilitates the system is that these kinds of paper are carried in standard stock sizes, as the majority of orders are too small to be made in special sizes.
The relation between thickness and weight of a given paper is approximately a direct ratio. For example, given a sheet of machine finish 25 by 38, 50-pound, four sheets of which bulk .011 of an inch, the bulk of the same finish and quality in 25 by 38, 60-pound, can be approximately ascertained by the equation 50 : .011 :: 60 : x, the answer of which is .0132.
The difference in bulk between two papers of the same weight depends on:
1. The finish.
2. The percentage of mineral filler.
3. The nature and treatment of the fiber.
For example, on a bulk of .015 of an inch to four sheets a super-calendered paper would weigh about 65 pounds, a high machine finish about 60 pounds, a text or medium finish about 50 pounds, an antique about 40 pounds. In other words, the density of any given piece of -paper is proportionate to the amount of calendering it receives. Naturally, the antique paper, lightly pressed and uncalendered, is loose for texture and full of minute air pockets; so that it is light for bulk, while the supercalenderedr paper is squeezed to a hard, dense sheet containing little air space.
If the proportion of mineral filler is great, the weight will be still greater in proportion to the bulk, as the specific gravity of the mineral is greater than that of the fiber, and the fine particles tend to fill completely the small interstices between the fibers, so that the air space is reduced to a mini-mum. If, in addition, a surface coating is added, we get a paper with the highest possible percentage of filler, and consequently a glazed coated paper has less bulk in proportion to its weight than any other kind. Such paper contains from 30 to 40 percent of mineral.
The nature of the fiber brings about a difference, in that some fibers have thicker walls and smaller canals than others. The treatment causes a variation, in that a quick beating with sharp knives leaves the fibers more nearly in their original shape than a prolonged beating with dull knives, which breaks down the structure of the fibers and draws them out into minute fabrillae.
The strength of a paper of given quality will also to a certain extent be proportionate to the duration of beating; as well as the amount of pressing and calendering received. The amount of sizing and the drying also, affect its strength.
An antique paper, having large air spaces and loosely knit as it is, has not the tensile strength it would possess if pressed and calendered to a greater density.
The addition of loading adds to the weight without increasing the strength, as it has no binding properties. More-over, the bulk, in proportion to the weight, is lessened by the introduction of filler.
Consequently it is axiomatic, that of two given papers of equal weight, finish and quality of fiber, the one containing the less filler will be the stronger, as well as bulkier. The addition of filler, however, increases the opacity, gives mellowness, and improves the printing quality by equalizing the texture of the surface.
The addition of sizing tends to increase the strength of paper, owing to its adhesive properties, but if liberally used it detracts from the mellowness and gives the sheet a tinny "character."
The length of the fiber also affects the strength, as long fibers give greater strength and better folding quality than short. It is not possible to get as close formation with long as with short fibers.
Hence occasions frequently arise wherein customers ask for characteristics which are somewhat contradictory.
A desires a light, bulky paper with a high finish, but a bulky paper with high finish must, in the nature of things, be heavy.
B desires a very strong, thin, but opaque paper. It is obvious that the strength of a thin, opaque paper can be but a relative factor, while thinness and opacity are irreconcilable features.
C inquires .for a closely formed sheet, with good folding qualities, but the first characteristic is only to be gained at the expense of the latter.
D wishes to print half-tones on an antique paper. In this case modern printing inventions have bridged over some of the obstacles of the past, and the offset press and extra-deep engravings have brought this last requirement within the realms of possibility, but unless resort is had to these new methods, the requirements again are irreconcilable to each other.
It is evident, however, that only through technical paper information can one solve such problems as necessitate a compromise capable of giving the maximum possible satisfaction.
The structure of paper, machine made, results in the greater proportion of the fibers in the formed sheet lying in the direction of the flow of the stuff. This determines what is called the "grain" of the paper. When paper is in the roll the grain of course is lengthwise of the web, but in the sheet the cutting and slitting may be arranged so as to leave the grain either lengthwise or crosswise of the sheet. This is an important consideration for a number of reasons.
In the first place, it is easier to tear the paper with the grain than across, as the fibers are parted rather than fractured in this way. This is a point which might be utilized by printers when printing detachable coupons.
Perhaps the most important consideration is the great difference in folding qualities. Many a paper will fold very nicely with the grain and crack badly if folded the other way.
Again, a great difference is noticeable in the flexibility of books, dependent largely on whether the grain runs parallel or at right angles to the binding. If flexibility is desired, the grain should run parallel to the back of the binding. Occasionally a wide-paged pamphlet, especially of light-weight paper, is improved by the rigidity to be gained from having the fibers run at right angles to the binding. It is also true that this increases the strength of the binding, as the sewing or wire stitching passes around more fibers than if the grain ran up and down the page.
Not infrequently does the middle signature of a pamphlet pull loose from the binding. Usually in such cases the paper is not strong anyway, but it could have had more resistance had the grain run at right angles to the binding.
The tensile strength of a strip of paper is greater with the grain, but its elasticity is greater across the grain.
A convenient way to ascertain the direction of the grain in papers that do not show it clearly by folding is to cut two narrow strips a few inches long, hold them by one end so that they coincide. When held horizontally, if the loose ends do not part, it indicates that the lower paper has its grain in the long dimension. If the lower paper has its grain crosswise, the loose end will sag away from the top strip, because, as above remarked, a paper is more flexible across the grain. This test may be applied either to sized or unsized papers.
Another test is to cut a small square and moisten one side; the paper will curl into a little cylinder and the grain runs parallel to the length of the cylinder. This test can-not be applied to an unsized paper.
This leads us to a consideration of the effects of moisture and humidity on paper.
It will be recalled from the chapter on Paper-Making (No. VI) how plastic paper is in its moist stage, and how tenacious of water are the cellulose fibers. It will also be recalled that there is considerable shrinkage across the web of the paper from the time it leaves the wire to the moment it is reeled. In fact, the very thing which makes paper-making a possibility is the shrinking of each individual fiber, occasioned by the expulsion and evaporation of the water, which has served as a carrier from the machine chest to the wet end of the machine.
This propensity of each individual fiber does not cease when the paper is made, but persists forever. A cellulose fiber will absorb moisture from the air in proportion to the relation humidity, just as the hair in a barometer is continually shrinking or expanding as the weather changes.
A definite percentage of moisture is normal to a cellulose fiber in proportion to the moisture in the air. The fiber swells as it absorbs, and shrinks as it gives off water.
In a sheet of paper, where thousands of fibers lie side by side, the combined expansion is distinctly noticeable in the changing dimensions of the sheet. This gives rise to difficulties in securing accurate register in color-printing, owing to atmospheric changes. The manufacturer may minimize this difficulty by a careful formation of the paper and the regulation of the drying, so as to turn out the paper as nearly as possible containing an average normal percentage of moisture.
The same conditions are responsible for wavy edges, which occur principally along the cross-grain dimension of the sheets. The ends of the fibers, being exposed, easily absorb moisture as paper lies in a pile, but the moisture seldom permeates more than a few inches into the pile. Therfore, the larger part of each sheet is unaffected, but the fibers exposed to the air expand when absorbing moisture increasing the area of the exposed end and, consequently, causing it to assume a wavy formation which is suggestive of a ruffle.
When feeding such sheets to a cylinder press, much trouble may arise if the waves occur along the "gripper edge," which is usually on the longer dimension of the sheet. In some instances the difficulty may be avoided by ordering paper with the grain running the long way of the sheet, which also offers another advantage in relation to securing close register, namely this: the area of the sheet in square inches will increase least through atmospheric expansion which occurs across the grain if the cross-grain dimension is the lesser.