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Q: Can you tell me why my linear low-density polyethylene (LLDPE) is currently showing a yield point? In years past, we have not seen this happen.
A: The yield point of HDPE is the first observation of zero slope measured on a stress-strain response curve. In most cases, this occurs between 12% and 19% strain for most HDPE. As you know, the major variation between HDPE and LLDPE is density. In the past, most fabricators converted either HDPE or LLDPE in their respective formulations. Now they are blending these two resins and just staying under the threshold for density. As a result, some “blended LLDPE” is exhibiting a slight yield point whereas in the past there was none.