Characterization Methodologies for Obtaining a Reliable Indicator for the Environmental Stress Cracking Resistance of Polyethylene

Environmental stress cracking (ESC) is one of the most common failure mechanisms in polymers where a sudden catastrophic premature brittle fracture occurs. This type of fracture develops through a slow crack growth mechanism caused by internal molecular heterogeneities in the polymer structure, promoted during the manufacturing or production of a specific plastic part. The resistance to such failure is of critical importance especially for applications where structural integrity is essential. A complete understanding of the molecular structure-property relationship of the polymer is required in order to evaluate the performance of a particular polymer that is subjected to conditions where ESC is prone to occur. Furthermore, reliable characterization methodologies are needed to predict the environmental stress cracking resistance (ESCR) of polymers. Currently, ESCR is reported based on unreliable and extremely time-consuming testing methods such as the notch constant load test (NCLT) or the bent strip test (BST). In both tests, notched polymer specimens are subjected to a certain load in the presence of an aggressive fluid and elevated temperatures. The time of failure is recorded and is reported as the ESCR. In this study, we aimed to investigate the structural properties which control the ESCR of polyethylene (PE) resins. The first part of the investigation involved development and modification of an alternative testing method to predict the ESCR of various types of PE resins in a more practical, reproducible, and reliable fashion (related to the strain hardening behaviour of PE under uniaxial tension). The second part of the investigation was a comprehensive rheological study, both in shear and extension, in order to identify potential correlations between rheological properties and findings from the initial stage of the study (related to the extent of strain hardening in both melt and solid phases). The third part of the study involved identifying a reliable crystalline structural indicator, namely, the lamella lateral surface area (LLSA), which could yield a better prediction for the ESCR. X-ray scattering analyses were conducted in order to identify the effect of processing and post-processing temperature on the extent of LLSA. A correlation between the processing factors and the extent of interlamellar entanglements and, subsequently, ESCR was finally developed.

Hardening Stiffness (HS) Test as an Indicator of ESCR

A range of commercially available rotomolding and pipe grades of LLDPE and HDPE were selected. The interlamellar entanglements (a combination of long tie-molecules and the physical chain entanglements) are believed to control the slow crack growth involved in ESC of PE resins. The extent of such entanglements was previously investigated by monitoring the strain hardening behaviour of PE resins in the solid state through a uniaxial tensile test ^[1]. A correlation was developed between the slope of the line (referred to as Hardening Stiffness (HS)) obtained from the load-displacement curve of a tensile test and ESCR. In order to modify the test for a better prediction of HS, two experimental designs were adopted: firstly, a D-optimal factorial design to investigate the significance of specimen dimensions, strain rate, and the molecular weight (MW) of PE resins, and secondly, a completely randomized central composite design to investigate the sensitivity of HS to short chain branching content (SCB). Based on the results, weight average molecular weight (Mw), strain rate, specimen width and thickness, along with some of their interactions were found to be the significant factors. The design of the test was based on two main criteria: (1) to reduce the effect of sample dimensions on HS, and (2) to better reflect the effect of molecular structure (Mw and SCB) on HS. The developed test ^[2] was found to provide a more reliable and consistent ESCR picture without the drawbacks of the subjective notching process and presence of aggressive fluids. Further, in order to extend the developed methodology to different types of PE (LLDPE with different comonomer content), a correction factor was developed (corrected HS or cHS) ^[2]. This cHS is believed to be a better indicator for ESCR of bulk PE resins as it takes both Mw and branching effects into account (see Figure 1).

Rheological Behaviour of the Resins

Rheological studies were conducted to identify a possible relationship between findings from the melt and solid studies (focus on extent of interlamellar entanglements). From the shear studies, a correlation between a normalized characteristic relaxation time (λ_N) as a measure of network mobility and ESCR (represented by cHS)) was established (see Figure 2) ^[3]. This outcome was critical for predicting the ESCR of PE samples with similar molecular weight properties, but different comonomer content (linear low density PE (LLDPE)). Extensional viscosities were evaluated from entry flow measurements and Sentmanat extensional rheometry. Cogswell, Binding, and Gibson methodologies were used to identify the steady state extensional viscosities from entrance pressure drops using a capillary rheometer. It was found that the extensional viscosity in the melt is a better tool to detect differences in molecular structure of PE resins with similar molecular weight properties. Steady state extensional viscosity obtained from entry flow measurements was dominated by the effect of molecular weight. A correlation was further acknowledged between plateau values of extensional stress and the HS of the LLDPE resins from the extensional measurements. It was found that the PE that showed higher extent of strain hardening in the melt had a higher ESCR. This is mainly due to the fact that the increase in molecular properties such as molecular weight, molecular weight distribution, and branching content will increase the ESCR of PE. Similarly, strain hardening is directly a function of these properties and any increase in such factors will enhance the extent of strain hardening (both in melt and solid states). Therefore, a direct relation between the degree of strain hardening (both in solid and melt) and ESCR is useful, given that PE resins exhibit strain hardening behaviour.

Temperature Effect on Lamella Lateral Surface Area

Inter-lamellar links, which are critical to ESCR of PE, must “anchor” lamellae as the term suggests. Theorization on the ESCR behaviour from the perspective of interconnectivity between crystalline and amorphous phases, led our research into the study of the relationship between the lamella lateral surface area (LLSA) and ESCR. Unlike crystallinity and lamella thickness that predominantly show SCB effects, LLSA calculations take into account both SCB and MW influences. Hence this investigation can be applied over resins of a wide range of MW and MWD. Our work showed that an increasing ESCR is associated with an increasing LLSA of PE. A larger lamella lateral surface area increases the probability of inter-lamellar linkage formations, which leads to improved phase interconnectivity and hence higher ESCR for polyethylene. We carried out an investigation with four high density polyethylene (HDPE) resins. The effects of processing (controlled cooling) and post-processing temperature (annealing) on LLSA at three different cooling rates and two different annealing temperatures were evaluated. Wide and small angle X-ray scattering (WAXS & SAXS) and differential scanning calorimetry (DSC) were utilized in order to characterize the crystal structure of the modified resins. WAXS and SAXS were used to determine the crystallinity and the lamella thickness of the resins, respectively. DSC analysis was utilized to verify the results obtained from the X-Ray diffraction analyses.

References

[1] Cheng, J.; Polak, M.A.; Penlidis, A. J Macromol Sci, Part A, Pure & Appl Chem 2008, 45, 599

[2] Sardashti, P.; Tzoganakis, C.; Polak, M.A.; Penlidis, A. J Macromol Sci, Part A, Pure & Appl Chem 2012, 49, 689.

[3] Sardashti, P.; Tzoganakis, C.; Zatloukal, M.; Polak, M.A.; Penlidis, Adv in Polym Tech (2013, under review)

Figure 1: ESCR vs. cHS

Figure 2: λ_N vs. SCB/ cHS