Stiffness, significance

There is a definite trend in the suppHer industry to develop multifimctional materials with value-added properties. One suppHer has introduced a modified hydrocarbon resin that softens, improves filler incorporation, improves mill banding, and increases tack in the green compound while imparting increased low strain (<100%) stiffness, significantly improving tear resistance in the vulcanized compound. 

Fatigue failure may occur when a specimen fractured into two parts was softened and/or its stiffness significantly reduced by thermal heating or cracking. Sometimes, for different reasons, a large number of cycles elapses from the first formation of microscopic cracks to complete fracture. In this case, the fatigue failure is... 

In the case of the fibrous laminate not much work has been done, but it has been observed that a significant loss of stiffness in boron—aluminum laminate occurs when cycled in tension—tension (43,44). Also, in a manner similar to that in the laminated PMCs, the ply stacking sequence affects the fatigue behavior. For example, 90° surface pHes in a 90°/0° sequence develop damage more rapidly than 0° pHes. In the case of laminates made out of metallic sheets, eg, stainless steel and aluminum, further enhanced resistance against fatigue crack propagation than either one of the components in isolation has been observed (45). 

Content of Ot-Olefin. An increase in the a-olefin content of a copolymer results in a decrease of both crystallinity and density, accompanied by a significant reduction of the polymer mechanical modulus (stiffness). Eor example, the modulus values of ethylene—1-butene copolymers with a nonuniform compositional distribution decrease as shown in Table 2 (6). A similar dependence exists for ethylene—1-octene copolymers with uniform branching distribution (7), even though all such materials are, in general, much more elastic (see Table 2). An increase in the a-olefin content in the copolymers also results in a decrease of their tensile strength but a small increase in the elongation at break (8). These two dependencies, however, are not as pronounced as that for the resin modulus. 

Of course, the above appHes when staying within a given tread design and at equivalent tread stiffness. Tire designs that allow water to be channeled out of the tread mbber/road contact area are significantly superior to those that do not, even when more contact area is available. Tread element stiffness plays an important role in traction capabiUties as softer treads have less column stiffness and reduce tread void areas, ie, there is more mbber on the road but this is offset if water channeling is significantly reduced. 

Blends with styrenic block copolymers improve the flexibiUty of bitumens and asphalts. The block copolymer content of these blends is usually less than 20% even as Httie as 3% can make significant differences to the properties of asphalt (qv). The block copolymers make the products more flexible, especially at low temperatures, and increase their softening point. They generally decrease the penetration and reduce the tendency to flow at high service temperatures and they also increase the stiffness, tensile strength, ductility, and elastic recovery of the final products. Melt viscosities at processing temperatures remain relatively low so the materials are still easy to apply. As the polymer concentration is increased to about 5%, an interconnected polymer network is formed. At this point the nature of the mixture changes from an asphalt modified by a polymer to a polymer extended with an asphalt. 

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