Flexing endurance

This test describes the behavior of the finish coat on repeated flexing of the leather. 

The test is carried out both on dry and on wet leathers. It is described in DIN 53351 or I.U.P. 20. The rectangular leather specimens are clamped into a flex-ometer (BaUy flexometer. Fig. 11-10) with a fold. The rotational movement of the up- 

After visual examination, the test specimen is evaluated according to the degree of damage of the finish coat 

High flexibility is demanded, for example, by the shoe industry for work and sports footwear. But leathers for the automotive sector (e.g., leather seats) also have to meet such high requirements. 

A variation of this test is flexing endurance at temperatures below freezing. Typical requirements are 30 000 flexes at -10 °C or 10 000 flexes at -20 °C. 

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Vinyl-coated fabrics exhibit high density, extremely low water vapor and air permeabiUty, cold touch, poor flex endurance, and plasticizer migration. However, they have good scratch resistance and colorabiUty and are inexpensive. 

The bonding properties of (Ti02) have been used for size-reinforcing of glass fibers so that they adhere to asphalt or to a PTEE—polysulfide mixture to impart enhanced flex endurance (434—436). Poly(vinyl alcohol) (PVA) solutions mixed with sucrose can be cross-linked with the lactic acid chelate and used generally for glass-fiber sizing (437). 

BiomeF Ethicon (Somerville, NJ) Aromatic co(polyether urea) MDI-PTMEG EDA Outstanding flex endurance Research use only... 

Testing of rubber and elastomers - Flexing endurance test, definitirHis, apparatus, preparation of test pieces (withdrawn replaced by ISO 132 2011)... 

Long flexing endurance Complicated manufacturing process... 

Long flexing endurance Printer, hard disk drive, floppy disk drive, CD drive, VCR... 

As flexible circuits have various mechanical stresses, their conductors also have requirements for more flexibility and toughness than traditional ED copper foils of standard rigid circuit boards. An RA copper foil is the solution for high flexing endurance. It is necessary for dynamic flexing use. RA copper foils are more expensive than ED copper, especially at thicknesses of less than 18 micron. Therefore, high-ductility electrodeposited (HD-ED) copper foils have been developed that have greater flexibility than ED copper foils and a lower cost than RA copper foils. The differences between the copper foils are shown in Table 61.11. 

Material selection Thickness (m) Dimensional accuracy (minimum opening) Reliability (flexing endurance)... 

The properties of raw materials do not represent final properties of the circuits. They undergo change during the manufacturing process, and therefore final properties should be measured at the end of the manufacturing process. Generally, most of electrical, mechanical, and chemical properties are different from those of raw materials. Critical properties such as flexing endurance, insulation resistance, and heat resistance should be measured carefully. 

IPC-TM-650 indicates a flexing endurance test method based on a flexing mode, as shown in Fig. 67.3.Unfortunately,the test method takes a very long time (sometimes more than a month) to give a final result by conductor failure. JIS-C5016 indicates an alternative test method called the MIT test, as shown in Fig. 67.4. This method does not require more than 1 hour for one test. It is not specified by a public industrial standard, however, a new flexing endurance test method for the spiral wiring illustrated in Hg. 67.5 has been required by the mobile equipment manufacturers such as notebook computers or clamshell type cellular phones. 

FIGURE 67.4 JIS-C5016 MIT test method for flexing endurance. R is the radius of the bend during the test... 

FIGURE 67.5 Flexing endurance test for spiral wiring. 

Shoe upper leather requires good flexing endurance and good adhesion of the finish. Rub-fastness is of minor importance. 

Apparel leathers preferably utilize sheep and goat leathers, but in some cases also calf leather. Inevitably, fashion aspects are of primary importance with apparel leathers. Other decisive aspects are wear properties such as softness and feel. With regard to processing, the finish does not have to meet special requirements. One important performance characteristic is the light-fastness of the finish. Flexing endurance and rub-fastness are of lesser importance. 

In contrast, the polyurethane dispersions used in the pigment coat are of medium hardness. A portion of the polyurethane dispersions may also be replaced by polyacrylate dispersions. These must not have an adverse effect on the cold flexing endurance and so the polyacrylate dispersions used must have a low glass transition temperature. The high rub-fastnesses are primarily achieved by the crosslinking of the top coat Useful crossHnkers include, for example, modified aliphatic polyisocyanates. The leathers are strongly embossed to conform the surface structure of the leathers to the interior styling of the car. 

The leather specimens are aged (a) at 50 °C for 7 days or (b) at 80 °C for 3 days. They are then assessed to see whether heat aging has resulted in embrittlement, yellowing or a change in the flexing endurance. 

This test is important for automotive leather in particular. It assesses the effect of light, heat and moisture on the flexing endurance, the rub-fastness and the color fastness of the leather finish. Finished leather strips are eTcposed to a defined dose of radiation in a test chamber at 20 % relative humidity. The color fastness is examined after the first cycle, the flexing endurance after the second cycle, the mb-fastness after the third cycle and the color fastness once more after the last cycle. Requirements differ from one car producer to the other. 

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