Hard elastic polypropylene

The superstructure of hard elastic polypropylene (HEPP) fibre was investigated, By means of SAXS studies, stress in various media, and birefringence determinations, a more precise model is suggested. 

Chou, G.J., Hiltner, A. and Baer, E., The role of surfaces stresses in the deformation of hard elastic polypropylene. Polymer, 1986, 27 369 ... 

Stress depressions (Ao) of hard elastic polypropylene, hard elastic high impact polystyrene, and Gore-Tex , a nonelastic, porous. Teflon material were measured when these polymers, under load, were subjected to changes in their environment from air to various nonswelling liquids. The stress depressions were studied as a function of liquid surface tension and viscosity and the strain imposed on the materials. 

Many investigators have invested a sizeable amount of effort in the study of specially processed polymers, known collectively as hard elastic, since their patenting in the mid 1960 s [1]. These materials have an unusual combination of physical and mechanical properties. Figure 1 shows the loading cycle of hard elastic polypropylene. 

Typical hard elastic behavior demonstrated by loading cycle of hard elastic polypropylene. Note hysteresis loop. 

The first hard elastic polymers were made from crystalline lamellar materials, and were processed via melt spinning and stress crystallization, followed by annealing under tension. The common morphology of these polymers is shown for hard elastic polypropylene [3] in Figure 2a. The structure consists of rows of lamellae oriented perpendicularly to the direction of draw, o Between these lamellae are microfibrils (ca. 100-500A... 

In this study, we closely examine the stress depression phenomenon as a function of environmental surface tension, strain, and viscosity. These results are correlated with changes in the void content as the specimens are strained. Three systems are investigated hard elastic HIPS, hard elastic polypropylene and Gore-Tex, a non-elastic (see Figure 5) but fibrillated Teflon material (see micrograph in Figure 2c). 

Ordinary HIPS, supplied by the Dow Chemical Company, hard elastic polypropylene from Celanese Corporation, and Gore-Tex, supplied by Gore and Associates, were the... 

The stress depressions observed for hard elastic polypropylene and Gore-Tex were completely reversible... 

Because of its low evaporation rate, silicone oil remains entrapped in the pores of these materials. This imbibing quality was utilized to obtain a rough estimate of the void volume fraction of hard elastic HIPS and polypropylene as a function of strain. These values are only estimates since the oil may not penetrate the smaller pores in the materials. The method, however, does offer a simple alternative to other techniques such as mercury porosimetry. As shown in Fig. 11, the results for hard elastic polypropylene are, in fact, reasonably close to void volume fraction measurements determined by mercury penetration [4]. Note that a linear relationship exists between the void volume fraction and strain for hard elastic polypropylene the slope of the line is about one. On the other hand, void volume estimates for hard elastic HIPS are much lower overall (slope is about one-half). Furthermore, the crazed HIPS has a small initial void content at zero strain, whereas no measurable voids were detected for the unstrained hard elastic polypropylene film. 

Despite an increase in void volume with strain, the stress depressions for hard elastic polypropylene are essentially strain independent. On the other hand, crazed HIPS showed significant elongation dependencies in its stress change. Apparently, subtile differences in the deformations of these materials are occurring. 

Fig. 6b. The effect of environmental surface tension on the stress depression in hard elastic polypropylene (HEPP) using water ethanol mixtures.

Unfortunately, calculated and experimental stress depression values for hard elastic polypropylene are substantially different. Unless the fibril volume is much larger or the fibril diameter much smaller than previously reported [4-9], there is no suitable explanation for the discrepancies between the calculated and experimental values using this model. 

Equation 1 further predicts that the stress depression is independent of strain. As seen in Figure 8, hard elastic polypropylene obeys this relationship (the raw stress depression values were also approximately constant). For hard elastic HIPS under high tension the stress depressions are independent of strain. Below about 25% elongation, however, the stress depression decreases with strain. The effect may be due to a larger fibril diameter at low strains, or an increase in the volume fraction of the fibril phase with elongation. Again, the differences in process history contribute greatly to the differences in strain response. 

This surface phenomenon is independent of strain for hard elastic polypropylene for all elongations and for hard elastic HIPS above 25% strain. 

The values of stress depression predicted by Equation 1 for hard elastic polypropylene, are, however, far lower than experimentally determined. 

From the literature [4,12], fibril concentrations of 0.2 and 0.5 are assumed for HIPS and hard elastic polypropylene. Our own microscopic investigations of Gore-Tex revealed a fibril concentration of 0.5 using these values, estimates of df(y) were 1.03 x lO"" ... 

Hard-elastic polypropylene has an interesting combination of mechanical and physical properties [2] including ... 

Relaxation and cyclic deformation behavior of the hard-elastic polypropylene is of particular interest [2]. If the fiber is allowed to relax, the stress needed to continue the deformation is different from the initial stress. The stress increment can be divided into a permanent and transient part. As the recovery properties of the fibers begin to deteriorate, the permanent increment changes from a negative to a positive value and the transient component decreases to zero. Stress relaxation of... 

The results of recent rheo-optical FTIR investigations of the exceptional mechanical properties of hard-elastic polypropylene films are discussed separately in Chapter 7.2.3. 

Fig. 55a and b. FTIR polarization spectra of a hard-elastic polypropylene film recorded during extension to 130% strain with light polarized alternately parallel and perpendicular to the direction of elongation... 

Stribeck N, Ndchel U and Funari S S (2009) Melting and crystallization of differently oriented sets of crystallites in hard-elastic polypropylene, Macromolectttes 42 2093-2101. 

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