Stretch Void

Stretch void index (SVI). SVI provides an indication of the number voids in the part. It indicates how well the sintering and coalescence have eliminated small voids, which can be present because of the processing technique orthe properties ofthe resin. Voids directly affect the performance of a tube in the end-use. For example, a void free or low void content part will have a longer flex life and greater flex fatigue resistance than a part containing more voids. 

Orientation sometimes leads to lower permeabiHty values (better barrier properties). Orientation can iacrease packing density, which lowers the diffusion coefficient D it can also iacrease the difficulty of hopping or diffusiag ia a direction perpendicular to the film. In the latter case, movement ia general may be fast, but movement through the film is limited. However, mere stretching does not always lead to orientation of the molecular chains. In fact, stretching can lead to void formation, which iacreases permeabiHty. 

Bladder stretching exercises (done by delaying voiding despite the urge to do so)... 

I don t know. Mark, sitting across the car from me, his long legs stretched out and his hands at rest, seems impossibly, baf-flingly substantial. How dare he stand between me and Adam How dare he block my access to that easy love which I refuse to know can no longer be reciprocal, the love I m still sending into a void because not sending it is worse ... 

Voids within a sample are a major cause of internal haze. We see the effect of voiding when we stretch polymers, such as high density polyethylene and isotactic polypropylene, that have distinct yield points and clearly defined necks (as discussed earlier in this chapter). The... 

Voids often look similar to air bubbles. The appearance of voids in filaments or films, however, results for different reasons. Voids can be produced during stretching in the area of necking by a kind of folding mechanism. The formation of voids may also depend on the generation of a radial gradient structure during solidification of the threads. 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

The effect of the HRH system on adhesion is further illustrated by the micrographs (Figures 7-11) of the same rayon-natural rubber composite with and without HRH. Figures 7-9 show a thin section of the composite without HRH stretched to various elongations with the force applied parallel to the direction of orientation. Many voids form as the strain is increased owing to fiber-matrix bond failures. Both the number and size of voids increase with increasing strain. 

Figures 10 and 11 show the identical composite with HRH stretched to 25 and 100% elongation. In this case only two relatively small voids were formed and not until a strain of 100% elongation was reached. This is a clear demonstration of the effectiveness of the HRH system, especially when it is noted that at any given strain level, the HRH composite was under considerably more stress than the non-HRH composite owing to its considerably higher modulus. Unfortunately, it was impossible to determine the actual stress levels incurred for each micrograph since the rate of deformation could not be controlled and the samples were considerably thinner than those used in Figure 6.

SC = Single crystals SF = Single fibrils (cold stretched) SM = Shearing region V = Voids... 

It seems that the major effect at small X is to orient entire crystalline fibers along the stretching direction the void volume is then decreased and the film is more dense. Something else must occur for X > 2. Apparently, fibers begin to slide past one another without much internal change beyond X 6, cracks appear [88]. Orientation would then be limited by... 

If the craze layer extends with complete lateral constraint, the strain in the craze is related to the change in its density. From a relationship between density and refractive index, an equation between strain in the craze and its refractive index can be derived. Although it is usual to start with the Lorenz-Lorentz equation, this may not be the correct relationship for a material having the structure of the craze (9). For the present purposes a linear relationship is assumed. The error introduced is at most 10% and only a few percent for the stretched craze with a high void content. 

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