Molecular weight environmental stress cracking resistance

Polyethylene (PE) is globally produced and consumed as commodity polymer. It has high-impact strength, low brittleness temperature, flexibility, and outstanding electrical properties. It is partially crystalline, with increase in molecular weight, and it provides better tensile and environmental stress-cracking resistance [3]. 

PE is described by its density, strength, molecular weight, crystallinity, and melts flow or rheological characteristics. Environmental stress-cracking resistance of PE is specific application oriented. Rheological characterization involves measurement of viscosity and viscoelasticity of the molten polymer (Table 4.1). 

An alternative approach was adopted by Kurelec et al. [84] who determined true stress-true strain curves at 80°C for a range of polyethylenes. It was shown that the slope of the tensile curve above the natural draw ratio (called the strain-hardening modulus) correlated well with the measured stress crack resistance (Figure 13.31). These results are entirely consistent with those obtained by Capaccio and co-workers and Ward and co-workers described above. Kurelec et al. found similar effects on the environmental stress cracking resistance (ESCR) performance with regard to short chain branches, and elaborated these in terms of the exact nature of the branches, particularly with regard to bimodal molecular weight distribution polymers. 

Figure 13.31 Environmental stress cracking resistance (ESCR) plotted against strain-hardening modulus for a range of EIDPEs, both catalyst based with broad molecular weight distribution, and bimodal. Points labelled reproducibility are repeated tests on the same sample. (Reproduced from Kurelec, L, Teeuwen, M., Schoffeleers, El. et al. (2005) Strain hardening modulus as a measure of environmental stress crack resistance of high density polyethylene. Polymer, 46, 6369. Copyright (2005) Elsevier Ltd.)...

The nature of the failures indicated that they were the result of solvent stress cracking. This is a well-recognized phenomenon, although the exact mechanism for such failure is not fnlly understood. This type of failure has not been encountered in pipe made from virgin, specification grade ABS. The failures are a direct result of using reprocessed materials, contrary to the requirements of the ASTM specification. The factors that contributed to the low environmental stress crack resistance were the distribution of the acrylonitrile, the rubber particle size, and the lower molecular weight. 

Compared to other types of polyethylenes, high molecular weight polyethylenes show improved impact strength, abrasion resistance and environmental stress cracking resistance. HMW-HDPE finds application in film, piping and blow-moulded containers. UHMWPE is used for components of material handling equipment and for container linings. 

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