Hydrostatic Failure In PVC

Introduction
Polyvinyl Chloride (PVC) is a flexible polymer commonly modified with additives to enhance impact resistance and reduce notch sensitivity. Processing aids are often used in transparent PVC applications to improve transparency. Stress whitening, believed to be caused by cavitation around particles, can occur in transparent PVC formulations with high molecular weight processing aids. Stress whitening may also occur in PVC formulations without processing aids, likely due to microvoid formation around insoluble additives.

High molecular weight processing aids

Cavitation of processing aid particles and intense shearing enhances toughness in PVC blends, similar to other ductile polymers like polycarbonate/PMMA blend. The hydrostatic stress component triggers cavitation, making the triaxial stress field at a notch ideal for studying localized failure processes before fracture. The semicircular notch shape is well-suited for investigating pre-fracture failure as it promotes cavitation while minimizing crack propagation. By slowly applying tensile load to transparent specimens, researchers can observe the initiation and interaction of two yielding modes, shearing and dilation, within the visible damage zone near the notch root. A core-yielding zone with visible slip lines is clearly present in PVC, similar to other polymers. Stress whitening occurs near the tip of the slip line in PVC, away from the notch root.

Chlorinated polyethylene (CPE)
In contrast, stress whitening in PVC/CPE blends starts at the notch surface without prior shearing, resulting in a crescent-shaped damage zone at the notch root. The initiation and growth pattern of these zones in the blends remain consistent across various subambient temperatures. The study specifically investigates the influence of temperature and the high molecular weight processing aid or CPE content on the formation of damage zones in PVC and PVC/CPE blends.

Stress whitening


Stress whitening in rigid PVC is typically caused by interfacial failure in second-phase constituents like stabilizers, lubricants, and other processing additives. The extent of stress whitening in uniaxial tension can vary significantly based on the resin formulation and resin itself due to the presence of residue of surfactant and catalyst. In a triaxial stress state, the likelihood of stress whitening increases, and in thick notched specimens, internal crazing may occur after stress whitening. Stress whitening did not start at the notch but instead occurred at a distance from the notch root, specifically at the tip of a shear yielding zone that extended from the notch.

The order of failure events at the notch root, including core yielding and stress whitening, was determined by the interplay between shear yield stress and cavitation stress at the notch root. The onset of stress whitening was identified by a critical mean stress. Depending on the failure events sequence at the notch, either an elastic or a plastic analysis was applied. The critical mean stress for blends decreased slightly with high molecular weight processing aid or CPE and was significantly lower in the blends compared to PVC. The critical volume strain for PVC was greater than that in blends.

Conclusion
An in-depth analysis of the damage zone formation ahead of a semicircular notch in Polyvinyl Chloride (PVC) and its transparent blends with high molecular weight processing aid/CPE revealed two distinct failure mechanisms: shear yielding and stress whitening. Stress whitening in the blends was attributed to particle cavitation, while in unmodified PVC, it was linked to the expansion of preexisting microvoids. The initiation of stress whitening and the sequence of failure events at the notch were influenced by the particle in the blend and temperature. At lower temperatures, stress whitening began at the notch root, resulting in a crescent-shaped damage zone. Conversely, at ambient temperatures and above, shear yielding occurred first, with stress whitening starting at a distance from the notch root at the tip of the yielded zone.

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Author
Dr. Muralisrinivasan Natamai Subramanian
Trainer, Polymerupdate Academy