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.
Internal tearing resistance is the force perpendicular to the plane of the paper sheet required to tear multiple plies through a specified distance after the tear has been started. An Elmendorf-type tearing tester is used and the result is quoted in mN/ply. The Elmendorf-type tester is a pendulum that applies the force and measures a loss in the potential energy of the pendulum equating to the work done in tearing the paper.
Smithers use an L&W single tear tester for papers and a James Heal Elmatear tester for heavier weight boards.
Tearing index = Tearing strength/grammage, quoted in mN.m2/g.
Smithers also measures tear initiation using a Finch tear attachment on a tensile test machine. This edge tear test conforms to Tappi T470.
There are many applicable standards including BS EN 21974, ISO 1974, Tappi T414,DIN 53128, and SCAN P11. Smithers is UKAS, ISO 17025 accredited for the determination of internal tear strength against British standard BS EN 21974 and International Standard ISO 1974.
Internal tear strength testing of paper is included in Smithers' Paper Proficiency Service.