Folding endurance measures the durability of paper (maps, bank notes, archival documents etc.) when repeatedly folded under constant load. The direction of the grain in relation to the folding line, the type of fibers used, the fiber contents, the caliper of the test piece etc., as well as which type of folding tester that is used affect how many double folds a test piece can take. Standards on which folding endurance test complies:
- ISO 5626: Paper – Determination of folding endurance
- ASTM D2176-97: Standard Test Method for Folding Endurance of Paper by the M.I.T. Tester
- ASTM D643-97: Standard Test Method for Folding Endurance of Paper by the Schopper Tester
- TAPPI Test Method T 551: Folding endurance of paper (MIT tester)
- TAPPI Test Method T 423: Folding endurance of paper (Schopper type tester)
A Schopper tester is used to determine the number of times a paper can be folded until it breaks. The folding strength is quoted as the number of double folds and the folding endurance is the log10 of the number of double folds. The ageing characteristics of a paper can be assessed by measuring fold endurance before and after accelerated environmental ageing.