Q.1001: What is Paper?
Ans. In simplest term paper is a homogeneous sheet formed by irregularly intervening cellulose fibers.
What is Cellulose Fiber?
An elongated, tapering, thick walled cellular unit, which is the main structural component of woody plants. Fibers in the plants are cemented together by lignin. In British English Fiber is spelled as Fibre.
What makes Paper different from similar materials?
The answer is the Chemical and Physical nature of paper's ingradients, plus the hydrogen bonds that hold the cellulose fibers together. Paper consists essentially of:
Cellulose fibers, which are the main component of the raw material. The individual fibers are present in a network, as can be seen by looking at the torn edge of a piece of paper. These fibers occur in nature, and also containhemicellulose, which is similar to cellulose.
The adhesive power of hemicellulose and cellulose is achieved by hydrogen bonding. Each cellulose fiber is bonded to its adjacent fibers by thousands of hydrogen bonds. They are chemical bonds that are weaker than the chemical ionic and covalent bonds that hold most materials in the world together. Mechanical entanglement of the fibers makes only a minor contribution to holding fibers together.
Lignin, which accompanies the cellulose fibers, contributes towards yield from the fibrous raw material (usually wood). It also improves general strength properties (eg: unbleached softwood kraft pulp) including stiffness (eg: corrugating medium), although by acting as a polymer rather than by inter-fiber bonding. On the negative side, it turns brown during chemical digestion and is also unstable to light. Lignin is therefore removed or bleached, but only to the extent that it cannot be tolerated in a specific grade of paper, because these steps make the paper more expensive.
Paper additives (ie: chemicals) are added to most paper grades to improve specific paper properties.
Paper thus differs from 2 familiar and similar materials:
Textiles, where the fibers are held together mainly by mechanical entanglement. Eg: weaving.
Nonwovens, where the fibers are bonded together either by strong chemical bonding agents, or by melting adjacent fibers, or by mechanical entanglement.
The fibrous raw materials, used for both these 2 materials, are mainly synthetic polymers - with their associated different properties. Even when cellulose fiber is used (eg: cotton, linen or wood pulp), little or no lignin is present.