ANSWER 1:
A sheet of material made up of many discrete cellulose fibres felted (bonded together) is known as paper.
Paper is either `hand-made' or `machine-made'. The machine used for making paper could be either a cylinder machine or fourdrinier. Newsprint is invariably made on a Fourdrinier.
Tearing strength (`Internal tearing resistance') is the average force, in grams, required to tear a single sheet of paper under standardized conditions. The fibre orientation in a sheet of paper determines the tearing strength of the paper. If the orientation is at random, the tearing strength will be almost the same in all the directions. This is the case in `hand-made' and `cylinder-made' papers.
On the other hand, if the orientation is in one direction, the tear strength will be a minimum along that direction and a maximum along the cross (perpendicular) direction. This is the case in `fourdrinier-made' paper.
In the case of a paper made on the fourdrinier — essentially, a long continuous wire screen — the fibre orientation is mainly along the direction of travel on the machine, that is, in the machine direction. Hence, in the machine direction, the tear is obtained just by separating the felted fibres, without any significant cutting of the fibres.
But, in the cross direction, the `tear' is obtained mainly by cutting the fibres. Therefore, a much greater force, vis-à-vis the first case, would be needed.
Thus, a paper is easily torn `vertically' (in the machine direction) but not `horizontally' (in the cross direction). Contrastingly, paper is generally stronger (greater tensile strength) in the machine direction than in the cross direction.
ANSWER 2:
Paper is either `hand-made' or `machine-made'. The machine used for making paper could be either a cylinder machine or fourdrinier. Newsprint is invariably made on a Fourdrinier.
Tearing strength (`Internal tearing resistance') is the average force, in grams, required to tear a single sheet of paper under standardized conditions. The fibre orientation in a sheet of paper determines the tearing strength of the paper. If the orientation is at random, the tearing strength will be almost the same in all the directions. This is the case in `hand-made' and `cylinder-made' papers.
On the other hand, if the orientation is in one direction, the tear strength will be a minimum along that direction and a maximum along the cross (perpendicular) direction. This is the case in `fourdrinier-made' paper.
In the case of a paper made on the fourdrinier — essentially, a long continuous wire screen — the fibre orientation is mainly along the direction of travel on the machine, that is, in the machine direction. Hence, in the machine direction, the tear is obtained just by separating the felted fibres, without any significant cutting of the fibres.
But, in the cross direction, the `tear' is obtained mainly by cutting the fibres. Therefore, a much greater force, vis-à-vis the first case, would be needed.
Thus, a paper is easily torn `vertically' (in the machine direction) but not `horizontally' (in the cross direction). Contrastingly, paper is generally stronger (greater tensile strength) in the machine direction than in the cross direction.
ANSWER 2:
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 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 determine 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. But, after folding these papers horizontally and pressing them in the fold, the can easily be torn in horizontal plane also, since they have less folding resistance.
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 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 determine 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. But, after folding these papers horizontally and pressing them in the fold, the can easily be torn in horizontal plane also, since they have less folding resistance.
Published in The Hindu on April 18, 2002.
