Crazes formation process

It has been proven by other authors that the P-relaxation is the mechanism governing the craze formation process for stationary as well as for propagating crazes in the slow crack speed region Furthermore, also from crack speed measurements (K, vs a) in the slow propagation regime performed on PMMA in the temperature range of —60 °C to -1-80 an activation energy appropriate for the P-process has been derived... 

Lately the large attention is given to macromolecular entanglements fluctuation network influence on crazes formation process in polymers [20], Much less is known about entanglements network influence on shear deformation zones (ZD) formation processes, which by their essence are crazes, without microvoids. The authors of Ref [21] fulfilled quantitative estimation of such influence on deformation processes in ZD on the example of polyarylatesulfone (PASF) samples in the form of films with a sharp notch. In this analysis two models of macromolecular entanglements network were used of binary hooking s [20] and cluster ones [22], The PASF films were prepared with the aid of nine various solvents that allows their structure wide enough variation [21]. 

The first quantitative study of deformation mechanisms in ABS polymers was made by Bucknall and Drinkwater, who used accurate exten-someters to make simultaneous measurements of longitudinal and lateral strains during tensile creep tests (4). Volume strains calculated from these data were used to determine the extent of craze formation, and lateral strains were used to follow shear processes. Thus the tensile deformation was analyzed in terms of the two mechanisms, and the kinetics of each mechanism were studied separately. Bucknall and Drinkwater showed that both crazing and shear processes contribute significantly to the creep of Cycolac T—an ABS emulsion polymer—at room temperature and at relatively low stresses and strain rates. 

The profound effect of water on tree growth that is so widely reported may be expected with the electrokinetic model on the basis of three principal effects. Water filling the crazes as they develop will help prevent their collapse. Water, as a good solvent for ionic species, will be an excellent medium to facilitate entry of surface-active agents, which, by a process similar to that of environmental stress cracking, will advance the void and craze formation caused by the electric field. Water, with its high relative permittivity, will distort and locally enhance electric fields in the neighbourhood of the voids and crazes where it accumulates. Whether one or other of these effects dominate in a particular situation depends on the exact nature of the specimen and its environment. 

Attempts have been made to correlate crack speed a with time t. An analysis of the fracture behavior of thermoplastics shows that it is essentially determined by craze formation and stretching the fibrils up to fracture. Therefore, the time involved in this process is considered to be the relevant time t which may be calculated by ... 

Fig. 22a—c. Microtome section of an intersperujitic crack path in bulk coarse spherulitic PP 1120 a. Figure b indicates as an SEM-micrograph taken from the surface of the specimen the interspherulitic craze formation prior to the cracking process, c shows a site of shear along a spherulite boundary oriented under an angle of about 60° to the horizontal crack direction... 

Fracture processes are associated with deformations. In glassy thermoplastics craze formation is the most frequent pre-failure deformation process. Just ahead of the crack tip, where the stresses are particularly concentrated, molecular chains of the polymer are drawn out of their amorphous arrangement in the bulk material into fibrils (see Fig. 1.1 and Chapter 1) under the action of the principal tensile stress... 

According to more recent theories, the toughness of high impact polystyrene is caused by flow and energy dissipation processes in the continuous polystyrene phase. The rubber particles act as initiating elements. Considerable differences in the thermal expansion coefficients and in the moduli of the polystyrene phase on the one hand and of the rubber particles on the other lead to an inhomogeneous stress distribution in impact polystyrene. Stress maxima create zones of lower density, called crazes (3), in which the polystyrene molecules are extended parallel to the direction of stress. Macroscopi-cally craze formation appears as whitening the flow processes result in irreversible deformation (cold flow). 

The OsO technique is also effective to enhance observations of the craze formation in ABS, thus providing a better understanding of the mbber toughening [Kato, 1967]. Furthermore, it was also applied to understand the metal plating process of ABS [Kato, 1968]. The ABS sample in Figure 8.2... 

In the separation of an adhering system at or near an interface, in terms of a craze mechanism, there will be four differences from separation within a bulk polymer. One is, that interfacial voids (or proto-voids) may exist. These can act as cavitation nuclei and interfacial craze formation, starting from such nuclei, would be orders of magnitude more rapid than crazing by homogeneous nuclea-tion. Such cavitation could also be more rapid than the processes that occur in the Taylor instability mechanism, particularly if it should happen that the voids at the interface formed a two-dimensional continuum. Patches of low-energy matter in the solid surface can also be loci of void initiation, even if no voids are present before loading. 

The structure of ABS is similar to that of HIPS but with a SAN matrix instead of the PSt matrix in HIPS. PB grafted with SAN acts as a compatibilizer between the rubber particles and the SAN matrix. The rubber particle morphology in ABS can be similar to that in HIPS, with salami-type particles, but ABS particles can also be of the core-shell type, with a core of solid PB and a shell of graft copolymer, especially if the ABS is produced by the emulsion process [34]. In addition to craze formation, an important fracture mechanism in ABS polymers is shear yielding, which leads to tougher materials [46]. 

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