First normal stress difference and in-situ spectral dynamics in a high sensitivity extrusion die for capillary rheometry via the ʽhole effect'

- A high sensitivity system for capillary rheometry is presented.
- It achieves remarkable pressure and time resolutions to detect melt flow instabilities.
- It can determine the first normal stress difference via the 'hole effect'.
- The first normal stress difference was estimated for a low and a high density polyethylene.
- The presence of the hole influences the onset of instabilities.

The development of a high sensitivity polymer melt extrusion flow instability detection die capable of estimating the normal stress differences of polymer melts during capillary rheometry tests via the 'hole effect' is presented here as proof-of-principle. Two polymer melts, a low density polyethylene (branched topology) and a high density polyethylene (linear topology), were tested in a variety of experimental configurations with the purpose of determining optimal conditions for performing normal stress difference measurements. The data was compared with rotational rheometry oscillatory shear measurements analyzed via the Laun rule. It is shown that it is possible to estimate the first normal stress difference in the capillary die, whereas the second normal stress difference cannot be determined within the experimental errors using the current configuration design. Furthermore, the influence of the induced streamline curvature via the 'hole effect' on the onset and development of melt flow instabilities is simultaneously assessed. It is shown that the hole depth has a stabilizing influence, i.e. the onset of instabilities occurs at higher shear rates, in the long chain branched polymer tested, whereas for the linear polymer tested it has a destabilizing effect on the stick-slip instability i.e. the onset of stick-slip occurs at lower shear rates.