Folding Endurance

A paper property that refers to the ability of a paper to be folded repeatedly without tearing. The number of folds it can withstand before it breaks is its folding endurance. Folding endurance varies according to grain direction, and is greater against the grain. In the Schopper method, a metal blade repeatedly folds a strip of paper back and forth between several rollers until it breaks. In the MIT method, an oscillating folding head repeatedly folds a paper sample back and forth until it breaks. The MIT method allows greater variability in the paper samples, and the tension can be adjusted based on the thickness of the sample.

Folding endurance is enhanced by increased paper fiber refining, being a function of the interlacing of the bonds between the paper fibers. Non-fibrous additives such as fillers, sizing, and coatings to the papermaking furnish or finished paper surface reduce folding endurance. Moisture loss also considerably decreases folding endurance. The degree of folding endurance desired depends on the end-use requirements of the paper. The procedures that increase folding endurance also work to the detriment of other, perhaps more desirable paper properties. (See Paper and Papermaking: Paper Properties.)

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