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Introduction to pVT-measurement

By Tim Haake posted 02-28-2017 10:29

  
For all our members here in the Carolinas, please note that PCE "Polymer Center of Excellence" has a fantastic application lab in Charlotte available for your use!
One of their instruments, a GOETTFERT RG20 capillary rheometer, comes with the option to run PVT (Pressure Volume Temperature) measurements, according to ISO 17744

Introduction to PVT-measurement
Beside rheological parameters, for example thermodynamic properties, the specific volume of the polymer as a function of temperature and pressure, can be determined using the PVT option. This data are particularly important for the simulation of the injection molding process, but can also be used to describe any process of shrinkage during cooling as for example in extrusion.
The measurement can be performed in two ways:
Isothermal – at constant temperature and changing pressure or isobaric – at constant pressure with different cooling rates.
This procedure is standardized in ISO 17744 for piston operated instruments. Especially for the injection molding process the isobaric cooling simulates the process best, as the mold is filled and cooled, while under constant pressure. Especially in the transition region between melt and solid, isobaric and isothermal determined PVT diagrams show big differences. The option to be able to run a PVT measurement, especially under isobaric test conditions according ISO 17744, using a capillary rheometer, is unique.
In case of an isothermal test, the measurement is started at room temperature, as shown in figure below: IsothermThe sample has been filled, heated up to melt temperature and was slowly cooled back down to room temperature. The pressure is then increased stepwise before the next temperature is set. This measurement cycle is repeated for every temperature step.

In contrast, isobaric measurement (see figure below) is performed at constant pressure whereby volume changes within the cycle as material is being cooled. 
Isobar
The device is heated again after reaching the low temperature level and the next pressure level is performed. Measurement of an isobaric diagram needs only 2/3 of the time as is used for the evaluation of isothermal diagrams.

Comparison between isothermal and isobaric measurements
Plastics can be divided into partially crystalline (PE, PP) and amorphous (PS, PC) plastics. PA and PET come in two types, amorphous and partially crystalline.
Especially for partially crystalline plastics, PVT diagrams differ whether they have been determined isobaric or isothermal. For isobaric measurement recrystallization time is reduced because of fast cooling rates, where less crystalline areas are generated. Hereby the specific volume in the transition between melt and solid is influenced.
PVT diagramm HDPE, isotherm/isobar

Above figure shows the comparison between isothermal and isobaric test procedure for a HDPE sample run under isothermal-isobaric conditions. First both diagrams are nearly identical in the melt area. In the transition between melt and solid significantly lower shrinkage is observed looking at the isobaric diagram with cooling at a rate of 15 K/min. This reduced shrinkage goes on with the solid area. Generally, reduction of shrinkage is higher with faster cooling rates and the slope of the isobar diagram becomes smoother in the transition area.

For more information, please contact PCE, or GOETTFERT.
Tim Haake, General Manager GOETTFERT Inc
tim@goettfert.com
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