FOLDING ENDURANCE (MIT)
Introduction
The folding endurance test measures the amount of folding that paper will endure before its tensile strength falls below a standard value of one kilogram force. When enduse requirements call for repeated folding, as in the case of map, record, and currency papers, in which the requirement is one of durability or resistance to wear over a prolonged period, the folding endurance test is particularly useful. When the paper or board is required to survive only a few comparatively severe folds, a manual test indicating "foldability" rather than a "folding endurance" test would be more meaningful.
Significance
The folding endurance test measures a combination of tensile strength, stretch, and fatigue properties. This test is also useful for measuring the deterioration of paper upon aging as it is sensitive to changes which show up long before there is a change in tensile, burst, or tearing resistance. Slight increases in relative humidity cause a marked increase in folding endurance. This, coupled with the extremely small area tested, results in wide variations for individual samples. Because of this variability, folding endurance is not often used as a specification unless a tolerance of at least 20% is allowed for highgrade papers and 30% for ordinary papers.
The increase in folding endurance produced by a small increase in beating is so great that significant differences are indicated when no significant difference is found in either tensile or bursting strength.
Test Specimen
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Specimens to be tested shall be cut accurately in each principal direction of the paper with a width of 15 mm (0.59 in) and at least 12.5 cm (5.0 in) long. They shall be initially free from folds, wrinkles or blemishes not inherent in the paper, and the area where the flexing takes place shall not contain any portion of a watermark. The edges of the specimen must be clean cut and parallel. The specimens shall be so selected from the sample secured by the standard sampling method as to be representative of the sample. Test 10 specimens cut from each principal direction of the paper.
Method of Test
Place the oscillating folding head in the position of zero fold. Place on top of the springloaded plunger a weight equivalent to the tension desired on the specimen. Clamp the plunger in position when depressed under the load by tightening the plunger lock screw. Then clamp the specimen firmly and squarely in the jaws with the surface of the specimen lying wholly within one plane and not touching the oscillating jawmountingplate. Handle the specimen by the ends and do not touch it with the hands in the region which is to be folded. Then apply the specified tension to the test strip by removing the weight and loosening the plunger lock screw.
Whenever possible a tension of 1 kgf shall be used, but if this does not afford practical test results, a greater or a smaller tension may be used. Fold the strip at a uniform rate of 175 ± 25 double folds per minute until it is severed at the crease. Record the number of double folds required to sever the specimen. Make 10 tests in each of the principal directions.
Report
Tests made on strips having their length in the machine direction are designated "folding endurance in machine direction". Similar remarks apply to the cross direction. For each paper to be tested, obtain 10 measurements in MD and 10 in CD. Convert these raw data to the logarithm (base 10), using common log tables, a computer, or a handheld calculator. Calculate the average of the logarithms for MD, then for CD. Report these averages to two figures for the mantissa. State clearly if a tension other than 1 kgf was used. Also report 95% confidence intervals.
References
ASTM D 2176
TAPPI T 511
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