Cooling Crazing

The rapid cooling of certain plastic products can result in frozen in stresses and strains (particularly with injection molding). The stresses may decay with time in a viscoelastic manner. However, they will act like any other sustained stress to aggravate cracking or crazing in the presence of aggressive media and hostile environments like UV radiation. 

A single craze is produced at a running crack-tip in PMMA at 20 °C (or perhaps at any temperature from —25 to 70 °C), at a l.Opm/s velocity, under static loading. The craze is kept stretched and cooled to —25 °C to freeze-in any subsequent relaxation. The load-thickness curve of that craze is measured at 11 Hz. 

Another craze produced under the same conditions, instead of being kept stretched during cooling, is left unloaded over a given period of time. Thus the craze fibrils could relax and change their entanglement density. Subsequently the craze is cooled down to —25 °C and analysed at 11 Hz as described above. 

Fig. 12. Craze growth rate as a function of applied stress for SB5/S4 blend containing 18 vol.% PB (O) quenched sample, (O) slow-cooled sample, ( ) sample isothermally aged 7 days at 87 °C and ( ) sample isothermally aged 5 days at 87 °C solid line and dashed line show predictions of the cavitation model and the meniscus convolution model, respectively...

The objective of this test method is to measure the cohesive stress and the time to failure of a crystalline polymer craze layer under rapid, uniform extension. The method is an impact variant of the Full Notch Creep test used by Fleissner [12], Duan and Williams [13], Pandya and Williams [14] and others. The specimen (Fig. 2), a square-section tensile bar, is injection moulded. At the mid-plane of the gauge length a sharp, deep circumferential notch reduces the cross-section to about one fifth of its original area. This notch plane is formed by a moulded-in, hardened steel washer. Specimens were injection moulded at 210°C into a warm (100°C) mould and air cooled to 40 C using a hold pressure of 45-50 bar. 

The influence of injection moulding on a preliminary orientation of the ciystallites and their deformation we described recently (Schneider, 2010). It was shown, that injection moulded specimens in injection direction as well as the compression moulded and quickly cooled specimens are highly stretchable. By contrast the specimens transversal to the injection direction fail very soon. Here in some cases even crazing could be observed before failure. 

Cup and mold temperatures were raised by increments of 20°C for each blend increment of 20 weight % PPO from a lower limit of 250°C for PS, the copolymers, PpClS, and PoClS to a high of 340 C for unblended PPO. After injection, molds were removed, placed on a large metal plate, and allowed to slowly cool to ambient temperature (23°C). In this manner, blends were cooled at rates comparable to the controlled rates used in previous DSC studies of these blends (6). Studies of n -hexane induced crazing of PS/PPO molded specimens using the above techniques indicated no preferential orientation of crazes whereas rapidly quenched samples... 

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