Flex life

Modified ETEE is less dense, tougher, and stiffer and exhibits a higher tensile strength and creep resistance than PTEE, PEA, or EEP resins. It is ductile, and displays in various compositions the characteristic of a nonlinear stress—strain relationship. Typical physical properties of Tef2el products are shown in Table 1 (24,25). Properties such as elongation and flex life depend on crystallinity, which is affected by the rate of crysta11i2ation values depend on fabrication conditions and melt cooling rates. 

A combination of excellent chemical and mechanical properties at elevated temperatures results in rehable, high performance service to the chemical processing and related industries. Chemical inertness, heat resistance, toughness and flexibiUty, stress-crack resistance, excellent flex life, antistick characteristics, Htfle moisture absorption, nonflammability, and exceptional dielectric properties are among the characteristics of these resins. 

However, conventional systems ia natural mbber do provide better flex life than EV cures, and this is one of the limitations of EV curiag. The short monosulftde bonds are less able to rearrange to reheve localized stresses which build duriag flexing, whereas the longer S bonds can. This abiUty for stress rehef is thought to be the mechanism for the superior flex life of conventional cures. 

If natural mbber compounds are subjected to thermal aging plus fatigue, the conventional systems perform no better than EV systems. The compromise obtained by usiag semi-EV systems iavolves the balance between heat aging and flex life. 

If flammabiHty is an issue, Hquid chloroprene polymers (eg, Du Pont PB or Denki LCR-H-050) can be used. They cocure and, for that reason, are nonvolatile and nonextractable. They are particularly useful in hard compounds where they do not detract from physical properties as much as nonreactive plastici2ers (132,133). Methacrylate esters have been used as reactive plastici2ers (qv). Por example, hexa(oxypropylene)glycolmonomethacrylate can be used as a reactive plastici2er to enhance flex life without increasing hardness (134). 

Two propylene oxide elastomers have been commercialized, PO—AGE and ECH—PO—AGE. These polymers show excellent low temperature flexibihty and low gas permeabihty. After compounding, PO—AGE copolymer is highly resiUent, and shows excellent flex life and flexibiUty at extremely low temperatures (ca —65°C). It is slightly better than natural mbber in these characteristics. Resistance to oil, fuels, and solvents is moderate to poor. Wear resistance is also poor. Unlike natural mbber, PO—AGE is ozone resistant and resistant to aging at high temperatures. The properties of compounded ECH—PO—AGE he somewhere between those of ECH—EO copolymer and PO—AGE copolymer (22). As the ECH content of the terpolymer increases, fuel resistance increases while low temperature flexibihty decreases. Heat resistance is similar to ECH—EO fuel resistance is similar to polychloroprene. The uncured mbber is soluble in aromatic solvents and ketones. 

Dispersion polymer, which leads to products with improved tensile strength and flex life, is not easily fabricated by the above techniques. It has, however, been found possible to produce preforms by mixing with 15-25% of a lubricant, extruding and then removing the lubricant and sintering. Because of the need to remove the lubricant it is possible to produce only thin-section extrudates by this method. 

In addition, the polymers are noted for their outstanding flex life, toughness and stress eraeking resistance. 

While detailed discussion of the merits of PEN fibres is largely outside the scope of this book mention may be made of the success in preliminary trials of (yacht) sailcloths made from PEN fibre. PEN fibres have a modulus roughly 2.5 X that of PET, exhibit excellent flex life and also show very good UV resistance. It is understood that the one yacht fitted with PEN sailcloth in the 1996 Olympics won the gold medal in its event. 

Forming the hinge cross-section by using an extruder die results in a hinge with poor flex life. Because hinges are formed in the direction of the polymer flow, they cannot be sufficiently oriented when flexed. However, if an extruded hinge is formed by the take-off mechanism while the polypropylene... 

The land of the hinge should be at least 1.5 mm (0.06 in.) wide for a proper flow pattern and at least wide enough so that when the product is bent in service it will not develop strains. Too short a land length will cause the hinge to have limited flex life. 

The tensile strength of NR compounds in the presence of cross-link promoters such as dichlorobenzene is increased as compared to the sulfur-accelerator and peroxide-curing systems. The retention of the maximum tensile strength at elevated temperamres is greater for radiation cured than for chemically vulcanized NR [326,327]. Also reported are a higher abrasion resistance [328] and a lower flex life in the case of radiation-cured system. Effect of phenoxy ethyl acrylate (PEA)... 

Excellent Bonding to Metals and Fabrics Excellent Vibration Damping Characteristics Excellent Flex Life in Dynamic Applications Excellent Weatherablllty Fungus Resistant Flame Resistant... 

For example, the original flex life ranges from 10000 up to 100000 cycles but decreases after ageing, to 1000 cycles after 500 h at 150°C. 

Resilient (elastic) with a high flex life (flexible lifetime)... 

Throughout the text we will relate polymer structure to the properties of the polymer. Polymer properties are related not only to the chemical nature of the polymer, but also to such factors as extent and distribution of crystallinity, distribution of polymer chain lengths, and nature and amount of additives, such as fillers, reinforcing agents, and plasticizers, to mention a few. These factors influence essentially all the polymeric properties to some extent including hardness, flammability, weatherability, chemical stability, biological response, comfort, flex life, moisture retention, appearance, dyeability, softening point, and electrical properties. 

The retention of the maximum tensile strength at elevated temperatures is greater for radiation cured than for chemically cured natural rubber. The physical properties after high-temperature aging are not improved, however. Lower flex life and higher abrasion resistance of radiation cross-linked NR were reported. Other properties, such as permanent set, hardness, and resilience, were found to be nearly equal. 

This property can be improved by the addition of antiozonants. Finer fillers give better flexing characteristics. Low sulphur or sulphurless cures, though good for heat ageing, are very bad for increasing flex life. Polysulphide links are preferred to mono or di-sulphide cross links. 

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