Folding endurance

The flexibility of buccal films is an important physical characteristic needed for easy application on the site of administration. This flexibility is measured quantitatively in terms of folding endurance and is determined by repeatedly folding the film at 180° angle of the plane at the same plane until it breaks or is folded to 300 times without breaking. The number of times the film is folded without breaking is computed as the folding endurance value. 

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

Faiz-festigkeit, /. folding endurance, -ziegel, m. interlocking tile. 

Figure 1. Flex crack test for film using the MIT Folding Endurance Tester...

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. 

The mechanical properties have not been reported conclusively. Most of the available data provided was based on fold endurance testing. Although it is an important property that should be determined, studies of tensile, tear and burst strengths are important as well. 

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.

Figure 1 Effect of relative humidity on fold endurance of wood pulp waterleaf sheets aged at 90°C.

An essentially physical change in cellulose, which can effectively reduce its moisture content, can indeed account for the observed increase in fold endurance loss under cyclic RH conditions,... 

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