Craze phenomena

Some other craze phenomena now being recognized as intrinsic crazes (crazes II) have already been discussed in Chapter 2. They will be put into context in a later section of this chapter. 

The second half of this volume is reserved to a discussion of specific craze problems encountered in practical application of polymer materials. J. A. Sauer and C. C. Chen analyze the fatigue behavior (mostly of rubber modified polymers). They show quantitatively the important effects of test variables and sample morphology on fatigue response. K. Friedrich gives an overview on the shear and craze phenomena in semicrystalline polymers. 

Recently, Dettenmaier and Kausch have observed an intrinsic craze phenomenon in bisphenol-A polycarbonate (PC), drawn to high stresses and strains in a temperature region close to the glass transition temperature, T. This type of crazing is not only initiated under extremely well defined conditions which reflect specific intrinsic properties of the polymer but also produces numerous crazes of a very regular fibrillar structure. These crazes were called crazes II in order to distinguish them from the extrinsic type of craze, called craze I. As shown by the schematic representation in Figure 1, a detailed quantitative analysis of intrinsic crazes in terms of craze initiation and microstructure was possible. The basis of this analysis and the results obtained are reviewed in this article. 

Valuable information on the intrinsic craze phenomenon has been obtained from studies on pre-oriented material The effect of pre-orientation on craze initiation is central to the discussion of the molecular craze mechanism in Section 4.2. To facilitate the presentation of the experimental results, the following nomenclature is introduced. Quantities which refer to the pre-orientation and to the crazing experiment are labelled with indices one and two, respectively. If no index is used, the quantity refers to the result of both experiments. 

The author is well aware of the fact that many aspects which have been treated in the extensive literature on extrinsic crazing have not been considered in this article and that more information is needed for a comprehensive account of the observed craze phenomenon. For instance the recent work on the intrinsic crazing of PC and on related phenomena which has been re wed here has primarily been based on structural considerations. It is believed that future work on the kinetics of craze formation and on the underlying molecular dynamics of the system may contribute considerably to a more detailed account of this phenomenon. Nevertheless, it is hoped that this work has opened up some new paths which may lead to a better understanding of the phenomenon of cavitational plasticity in polymers. 

Harris and Ward have observed conventional crazes nearly normal to the tensile stress axis (tensiile crazes), as well as what appear to be crazes along the shear direction (shear crazes), in uniaxially drawn, crystalline PETP sheets. Tensile crazes were formed parallel to the initial draw direction. The shear crazes which were the first of their kind to be reported, were seen in specimens under all directions of orientation. They formed always at the same angle as that of the shear bands which appeared subsequently upon yielding. An explanation for this shear-craze -phenomenon was offered by Brady and Yeh based on their own studies of crazes and shear bands in amorphous and crystalline, isotactic PS films. First they produced a set of crazes and shear bands by stretching a film in one direction. When this film was then redrawn in a second direction, the authors observed that the stress component, which was... 

Argon and coworkers have recently shown that low molecular weight polybutadiene may cause profile crazing in polystyrene by local plasticization. Since Argon and Cohen review this craze phenomenon in their contribution to this volume (Chapter VII) it will not be considered in this paper. 

The toughening of glassy PS by plasticizing PB diluents presented in Sections 13.6.1 and 13.6.2 differs fundamentally from the usual well-known solvent-induced crazing phenomenon described, e.g., by Kramer and Bubeck (1978),... 

Figure 1 Schematic representation of the crazing phenomenon, (a) Crazed sjjed-men subjected to a tensile force F. The crazes are represented by lines roughly perpendicular to the direction of the applied force, (b) Section of a craze with fibrils, strained by the tensile force F. A crack opening is starting to occm, as evidenced by the breaking of the first fibril on the left-hand side, (c) Multicraze mechanism induced by the presence of rubber particles in a rigid matrix.

All polyethylenes are soft, flexible and resistant to acids and alkalis up to 60°C. They retain this flexibility down to —40°C. Hence they have good resistance to impact even at low temperatures. However, unless correctly formulated they can suffer from environmental stress cracking (ESC), poor adhesion and UV degradation. ESC is the phenomenon which occurs when a thermoplastic is put under stress, e.g. bent, in a particular environment and prematurely cracks or crazes. Alcohol and detergent are examples of agents that can cause ESC in polyethylenes. 

Dettenmaier, M. Intrinsic Crazes in Polycarbonate Phenomenology and Molecular Interpretation of a New Phenomenon. Vol. 52/53, pp. 57—104. 

ESC, described in Section 4.9.4, is a physical phenomenon and is the acceleration of stress cracking by contact with a fluid, i.e., stress cracking will occur without the fluid at sufficiently long times. Ultimately, the slow crack growth that follows crazing reaches a critical point when fast crack growth and failure occurs. This failure, with or without the accelerating effect of a fluid, is a creep rupture effect (see Section 8.12). 

This study (34) implies that a right dispersion of rubber particles may permit optimum stress field overlap that affords lower craze-initiation stresses and therefore can rapidly dissipate the strain energy in the HIPS. A more homogeneous spatial distribution of rubber particles allow for a uniform development of crazes. Prevention of the strain localization phenomenon to avoid the detrimental situation, where crazes prefer to develop in certain areas and quickly lead to a catastrophic crack, could result in a larger total volume of crazed material. Further, Donald and Kramer (22) discovered no crazes nucleating from an isolated rubber particle with diameter smaller than 1 urn because of an insufficient size of stress-enhanced zone. Since Sample-A has a small average particle size it should contain a large number of small rubber particles. Two small rubber... 

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