Micro-crazing

Often the most obvious effect of exposure to sunlight is colour fading this is essentially related to the pigments or dyestuffs, and not to the polymer. The other effects are the results of surface oxidation of the polymer, in a similar way to heat ageing. The first visible effect is discoloration, in the direction of yellow and then brown. Micro-crazing develops concurrently, but may not become evident until a general loss of gloss is apparent. How... 

In practical use, the oven and fireplace sight panels are subjected to stresses mainly by the corrosive effects of the oven atmosphere due to the combustion of, for example, fuel (so-called micro-crazing). The intensity of the said stresses is a function of the influential factors compiled in Table 3.6. 

When the stress that can be bom at the interface between two glassy polymers increases to the point that a craze can form then the toughness increases considerably as energy is now dissipated in forming and extending the craze structure. The most used model that describes the micro-mechanics of crazing failure was proposed by Brown [8] in a fairly simple and approximate form. This model has since been improved and extended by a number of authors. As the original form of the model is simple and physically intuitive it will be described first and then the improvements will be discussed. 

Dispersed elastomeric micro-particles preferentially break (bright spots) with the development of crazes around the broken rubber domain as evident from Fig. 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

Lu H, Schmidt MA, Jensen KF (2001) Photochemical Reactions and On-Line UV Detection in Microfabricated Reactors. Lab Chip 1 22-28 Manz A, Harrison DJ, Verpoorte EMJ, Fettinger JC, Ludi H, Widmer HM (1991) Miniaturization of Chemical-Analysis Systems - A Look into next Century Technology or just a Fashionable Craze. Chimia 45 103-105 McCreedy T (1999) Reducing the Risks of Synthesis. Chem Ind 15 588-590 McCreedy T (2000) Fabrication Techniques and Materials Commonly Used for the Production of Microreactors and Micro Total Analytical Systems. Trac Trends Anal Chem 19 396-401... 

For these reasons, PMMA and its maleimide and glutarimide copolymers represent very suitable materials for investigating the effect of the chemical structure and of the solid state molecular motions on the plastic deformation, the occurrence of the various micro-mechanisms of deformation (chain scission crazes, shear deformation zones, chain disentanglement crazes), as well as the fracture behaviour. 

Before discussing specific aspects of micro deformation and fracture in bulk polyolefins, some basic notions of microdeformation and the micromechanics of fracture mediated by generation and breakdown of cavitated or fibrillar deformation zones or crazes are introduced. SCG in PE and rate-depen-dent fracture in iPP are then considered in more detail. 

Fig. 12 a and b. Material data of the micro region at the crack tip as derived by the application of the Dugdale model to measured craze sizes (Fig. 11) a craze stress 0 b creep modulus E... 

In this Section the kinetics of craze growth at crack tips in air will be considered in some detail. We shall not be concerned with the initiation phase and any micro mechanism (e.g. leading to craze initiation. 

III. The third step of direct longitudinal transmission of strain onto connected crystalline blocks leads to a perfect stretching of these fibrils. Because of the alignment of the molecules the fibers in this condition should possess a strength about 1 to 2 orders of magnitude higher than the yield stress of randomly distributed folded polycrystals. As the fibrils are able to stabilize the enhanced micro-void volume between them, a lateral coalescence of these voids finally provides a local deformation zone in the shape of a craze as known from amorphous polymers. 

In Eq. (10) that gives the toughness in dilatational plasticity the factors C and A are dependent on craze microstructure and will not vary significantly. The stress and temperature dependence of the craze velocity while quite determinate in the interface convolution process of craze matter production will also be quite sensitive to micro-structural detail of phase distribution in block copolymers. The appUed stress = Y ... 

In KRO-3, a very different situation exists. The crazes are macroscopically indistinct and wavy. Whitening of the sample is more homogeneously distributed. The micro-... 

Figure 20 shows a typical craze microstructure in these strained di-block copolymers, that for PS 600/PB 256. Examination revealed that the scale of the craze micro-... 

The crazing process does not induce large-scale energy dissipation in pure Polypropylene so the cavitation / dewetting process is the main toughening mechanism in CaC03 filled Polypropylene. The basic micro-mechanisms depend on the material properties and on the loading conditions. 

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