Tear strength of paper

What is the tear strength of paper?
The tear strength of paper means the resistance of a paper sheet to tearing force that it is subjected to. It is another important basic physical property of paper and paperboard. It is measured in both machine direction (MD) & cross direction (CD) and expressed as mN (mili Newton). Machine direction means the direction of the paper web which is running on the machine during paper making. On the other hand cross direction means the direction, which is perpendicular to the paper sheet that is running on the machine during paper making.

Factor of the tear strength of paper
Tearing resistance depends on the degree of fiber refining, related to inter fiber bonding, the fiber strength, the fiber length, the quality and quantity of fillers used. Among of them fiber length and fiber bonding are most important factor. Longer fibers increase the tear strength because it is able to distribute the stress over more fibers and more bonds, whereas short fibers concentrated the stress in a smaller region. The direction of the fiber is another important parameter for tear strength. It is larger in lateral direction of the fiber than in longitudinal direction. Appropriate refining is increased the tearing strength, whereas insufficient and extra refining decreased. It is also decreased by the uses of more filler.

Applications
The tearing strength property is significant factor for many applications involving cover papers, wrapping, toughness of packaging papers, bond papers, envelope papers, printer and converter.

Tear factor and its calculation
Tear factor is calculated as tear strength per unit basis weight and expressed as mN/g/m2 or dm2 . Tearing factor = Tearing strength/grammage
If the tearing strength = x mN
basis weight = y g/m2
Then the tear factor = x/y mN* m2/g or 100*x/y dm2

Types of instruments
There are two types of instruments are used for tear strength measurement, for example Elmendorf & Trouser tear device. Elmendorf tear test are used most commonly.