Paper folding endurance

Folding endurance refers to the number of folds a paper can withstand before failure when tested (TAPPI T423). 

Book ASTM Stand., Folding Endurance of Paper by the MIT Tester, ... 

One of the most industrially important characteristics of papers is their chemical stability, which enables them to withstand degradation with its consequential loss of tensile and tear strength and fold endurance under normal conditions of use. However, this stability is not absolute. Cellulose is susceptible to oxidation and the glycosidic linkage is susceptible to hydrolysis. In order to protect book papers from acid degradation, they must not be exposed to acid. Acids are generated from the alum-rosin size as well as from such... 

Accordingly, a complete book preservation program should consider (a) complete deacidification (b) improving fold endurance, tear and tensile strength of paper and (c) inhibiting oxidation. 

Barly findings from this work were published in the Paris Conference proceedings of the XXC on Adhesives and Consolidants (13). This paper gave examples of the results obtained with different acrylic and methacrylic monomers. The monomers were selected because they had suitable physical characteristics and were already used in conservation work. The problems which occur with aged and woody based papers were highlighted. This paper also indicated that the use of certain solvents such as methanol to improve monomer penetration had a detrimental effect on the strength of the paper, in particular its fold endurance. 

Initial work used Whatmans filter paper (cotton based) as a control and Figure 2 shows that as the X polymer deposited within the paper structure increases the fold endurance also Increases. Figure 3 shows a similar relationship for a lignin containing (wood based) paper, although the strength increases are usually less dramatic. 

Figure 2. Dependence of fold endurance on percentage weight gain for Whatmans filter paper using EA MMA 5 1.

Figure 4. Loss of degree of polymerization and folding endurance and rise in hot-alkali-soluble matter in filter paper during exposure to "daylight" fluorescent lamps and subsequent thermal aging.

The condition of the cellulose fiber in a pulping operation is often followed by measuring the viscosity—degree of polymerization (DP) of the cellulose. In the present work, an increase in brittleness of the paper is the property of interest, and this will be followed by the change in folding endurance. Oxidation of the cellulose will show up by the increase in the carboxyl group. This will be estimated by the pH and titration values of the slurried fiber. 

The results of the work are shown in the tables and curves. As shown in Figures 3 and 3A and Table III (paper at pH 4.8), there was an increase in folding endurance from 1470 to 2010 on spraying the sheet with 0.0314% Cu [(CH3 COO)2-H20]. Such an increase is usual when a calendered sheet is wetted and relaxed. The expected rapid loss of folding endurance for a sheet of this pH showed up in both dry and humid ovens. 

Fold Loss Rate. A direct way of evaluating the effect of the morpholine process on the stabilization of paper is to measure the rate at which its folding endurance deteriorates before and after treatment. Figure 4 typifies the effect. The two lines are for the same paper aged in a dry oven at 100°C the line with the lesser slope reflects morpholine-treated... 

Life Expectancy Estimates. If the end of the useful life of a paper is arbitrarily defined as a folding endurance of one double fold, rate-of-fold-loss lines such as those in Figure 4 can be extrapolated until they reach one double fold, and the corresponding accelerated-aging intercepts on the time axis can be compared directly. This gives a measure of the extent of improvement resulting from treatment. 

Studies over a number of years have now shown that papers of adequate permanence can be made from wood pulps if the wood is deligni-fied without undue degradation and is carefully bleached and purified. Wood pulp papers sometimes do not have the initial high physical test values, particularly folding endurance, that can be achieved with sheets of cotton fibers, but the relative rates of loss on aging often approach those exhibited by cotton papers. 

The folding endurance is primarily a measure of the response of fiber-fiber bonds and, to a lesser extent, that of the fibers to alternate compression and tension during the folding. The strength, flexibility, and other physical characteristics of these bonds are profoundly affected by any of the factors associated with aging. As noted above, this may be attributed to the more poorly organized structure and lower crystallinity and, consequently, greater accessibility of the bonds in the paper structure. 

The moisture content of the paper has a strong effect on the folding endurance test. The swelling and shrinkage which accompany adsorption and desorption of water vapor cause shifting and rearrangement of... 

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