Role of Crystallinity on Failure Variability in ASTM D1693 Bell Test

Abstract

Academically and industrially observed variability in failure times obtained from the ASTM D1693 ‘Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics’ (aka Bell Test) for high-density polyethylene (HDPE) was hypothesized in this project to be primarily caused by variation in crystallinity within molded test plaques. The broad molecular weight distribution of polyethylene inherently limits the ability to completely eliminate such variability, but specimen preparation irregularities related to uneven cooling during compression molding were felt to be a major contributing factor and one that was controllable if the hypothesis proved true. This thesis work attempted to validate the hypothesis about crystallinity’s role in test variability by proposing alternative cooling strategies aimed at improving the uniformity of test results.

Three cooling protocols – cold quenching, slow cooling (annealing), and a standard method – were evaluated to understand the hypothesis by observing cooling patterns. Bell Test results showed that varied crystallinity based on these different protocols did vary the failure time of the molded specimens by as much as 54% and indicated that rapid cooling via quenching could significantly reduce this variability (23%) relative to the standard method, giving direction on how to improve the molding procedures. A subsequent double-pressing technique is also conceived and evaluated at the time, involving the application of pressure near the crystallization temperature, which was found to further improve failure time (33%) relative to the standard method.

With no convenient test for assessing the distribution of crystallinity over a relatively large area, a bulk-state test method was devised based on dynamic mechanical analysis (DMA). Results revealed a more consistent spatial distribution of crystallinity when specimen data clustered closer together in Cole-Cole plots. The close correlation between variability in DMA data versus Bell Test data was taken as validation of the methodology. Differential scanning calorimetry (DSC) was conducted but generally found to be uninformative in this study for explaining variability in the Bell Test results.

To test the extent to which more uniform cooling, at a high rate of cooling, could decrease failure variability, a new cooling plate design was considered to produce HDPE specimens with more uniform crystallinity. The original cooling plate design for the compression molding unit was used to prepare a finite element simulation using Ansys Fluent, which was validated with experimental thermal imaging data. Testing of new potential designs was subsequently followed before a final version was created. The new cooling plate design, which intentionally did not differ too dramatically from the standard but corrected for the unevenness of cooling, was coupled with a revised cooling protocol, to create new Bell Test samples. The Bell Test variability by the new plate and best cooling protocol decreased by 95% relative to the standard cooling protocol of the original plate.

The outcomes of this study support the implementation of a revised cooling protocol and improved cooling plate, both of which aim to reduce failure time variability and enhance the reproducibility of Bell Test results in industrial and academic settings.