Why Is It so Easy to Tear a Newspaper Vertically but Not Horizontally?
The principal raw material required for the manufacture of paper is cellulose pulp. It is obtained from any one of the following plant or plant materials; wood, bamboo, cereal, straw, reeds, tow, bagasse and cotton rags.
The cellulose molecules in the pulp are linear unbranched homopolysaccharides and are called micro fibrils. Three processes are employed for the preparation of cellulose pulp namely mechanical, chemical and semi-chemical.
The most common process is semi-chemical, in which the cell wall portion of the employed plant materials is cooked with water, lime and sulphur dioxide.
During this process lignin fibers made of cellulose fibrils are removed from the digesting cell wall of the plant materials used for pulp making, leaving behind the cellulose fibers only. The fibers are placed on printers in pulp form, consisting of 80 to 90 percent water – the newsprint dries while in the machine.
The printing machines are designed to line up the fibers in a horizontal position to add tear strength to the sheet vertically. The basic purpose of lining up the fibers in one direction is simply to add stability to sheet when the press is running.
The finished newspaper has a grain, just as a piece of meat or linen has a grain. When you rip the newspaper vertically, you are tearing with the grain, or more accurately, between grains.
The paper qualities like tear and folding resistance are dependent on the physical characteristics of the elements constituting the cell walls of the plant materials used for making the paper pulp.
These physical characters are the lengths of the element, ratio of the element length to its diameter and ratio of the cavity to the element diameter, which determines the flexibility.
Pulps made from cell wall materials with shorter elements have better sheet forming properties and the higher the flexibility of the element the denser and stronger the sheet. The pulp obtained from plant cell wall materials, which have thick walled elements in them, can give papers with better tear resistance and folding resistance.
On the other hand the papers of thin walled and longer elements can easily be torn vertically. This is because, when the pulp slurry which is spread into mat and rolled into sheets, the cellulose fibrils in it are mostly laid down in linear fashion.
After folding these papers horizontally and pressing them in the fold, they can easily be torn in horizontal plane also, since they have less folding resistance.