SOFT AND TOUGH

Another factor that dictates the mechanical performance of polymers is the appearance of chain orientation. In the case of fibers, it is customary to pre-stretch in order to achieve a high degree of orientation in the axial direction. In films or other profiles made by extrusion or injection molding, chain orientation is usually obtained in the flow (machine) direction. This is manifested by an increase in tensile strength and a decrease in elongation in the machine direction, while the opposite is trae in the transverse direction. This calls for measurements in both directions. In two-dimensional objects this phenomenon is undesirable, as it leads to asymmetric properties (anisotropy). Similar behavior may be found in reinforced materials (with long fibers), in 

FIGURE 4-30 Stress-strain graph of a soft and tough material 

The sensitivity to temperature is paramount in the case of thermoplastics, but less important with thermosets. In order to retain strength and toughness (over a broad temperature range) with thermoplastics, one has to shift to more rigid chemical stmctures, to higher crystallinity or partial cross-linking. This is the domain of those unique materials called engineering polymers. 

The practical conclusion is to increase the mass of polymers, such as using thicker walls in pipes or containers. The ideal is to derive new engineering polymers that exhibit high strength even if not reinforced. The reinforced polymers (namely, the composite materials) excel in strength and stiffness. 

In practice, it is essential to match the type of measurement with the actual performance of the material (tension, flexure or compression), in spite of the popularity of the tensile test. Sometimes, it is desirable to follow the response to shear stresses. The relationship between the modulus of elasticity E (Young s) and the shear modulus G, is given by  

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Fig. 4. Types of stress—strain curves (a) soft and weak (b) hard and brittle (c) soft and tough (d) hard and strong and (e) hard and tough.

The chemical picture of this modified collagen is not yet complete, but its mixed structural regions already hint at similar new synthetic materials. For the NIH, which has supported this biotechnological research, the hope is that this unique material, with its variable elasticity, will lead to more comfortable and pliable artificial skin. Ultimately, it could also improve the quality of such goods as footwear and radial tires, where softness and toughness are equally desirable. 

Dow Chemical has launched a range of foams which are said to exceed industry standards for softness and toughness. This article supplies brief details of the foams which are based on Dow s Insite catalyst technology. Synergy Soft Touch Foams are produced using Dow s Index Interpolymers, a new thermoplastic polymer family based on the copolymerisation of ethylene and styrene. The foams are offered in three grades of softness, and other properties include shock absorption, vibration damping and insulation. 

AP shell intended to penetrate face-hardened armor is usually provided with a slightly blunted nose which is fitted with a cone-shaped cap made of soft and tough forged alloy steel, face hardened. Such shell is called APC (armor-piercing capped). The cap of this shell is fitted with a conical cup, called windshield or ballistic cap, (also called false ogive) the purpose of which is... 

Wide range of mechanical properties from soft and tough material to rigid... 

During the application of the system to the skin, the membrane should intimately touch the skin and should not break. Therefore, soft and tough mechanical properties are requested for the polymers. 

We may consider two extremes in regard to heat effects in the failure of a bulk polymer (1) adiabatic systems and (2) isothermal systems. And we may consider two qualitative extremes with regard to polymer hardness and strength the polymer may be (a) hard and brittle, or (b) soft and tough. Some examples of the four combinations of these extremes are ... 

Soft and tough (rubbers and plasticized PVC) Low Low Moderate High... 

Soft and Tough Low modulus but exhibits high elongation and energy to break. Typical polymers are polyethylene or flexible (plasticized) PVC. (See Figure 4-30.)... 

In contrast to the pearlite structure, which is lamellar (Figure 2.15), tempered martensite contains the carbide particles as a spheroidal dispersed phase. While the tempered martensite is soft and tough, the parent martensite is hard and abrasion resistant. 

Fig. 6.18 Illustration of five conventional stress-strain curves of polymer materials under constant strain rates. 1 hard and brittle, 2 hard and tough, 3 hard and strong, 4 soft and tough, and 5 soft and weak...

Blending polymer is similar to alloying metal, to obtain the complementary advantages of two materials. For example, polystyrene (PS) appears hard and brittle, while polybutadiene (PB) appears soft and tough. By adding amphiphilic... 

There are many cases where the initial particle size of the sample requires preliminary reduction. Classical machines for this purpose are jaw crushers, normally used with samples of medium to extreme hardness as well as brittle to hard-tenacious samples, and cutting mills, used with samples of soft and tough make-up. 

At high temperatures, the kinetic studies of polymer crystallization is mostly focused on the nucleation. In this region, heterogeneous nucleation takes place and the resulted mechanical properties of the semicrystalline polymers are usually hard and brittle. While in the region of low temperatures, as a result of high density of small crystallites, the semicrystalline polymers become soft and tough. 

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