Abstract
Elongational flow properties of polymer melts are very important for numerous polymer processing technologies such as blown film extrusion or foam extrusion. Rheotens tests were conducted to investigate the influence of plasticizer content on elongational flow properties of cellulose acetate (CA). Triethyl citrate (TEC) was used as plasticizer. Melt strength decreases whereas melt extensibility increases with increasing plasticizer content. Melt strength was further studied as a function of zero shear viscosity. The typical draw resonance of the Rheotens curve shifts to higher drawdown velocity and the amplitude of the draw resonance decreases with increasing TEC content. With respect to foam extrusion, not only are melt strength and melt extensibility important but the elongational behavior at low strain rates and the area under the Rheotens curve are also significant. Therefore, elongational viscosity as well as specific energy input were calculated and investigated with respect to plasticizer content. Preliminary foam extrusion tests of externally plasticized CA using chemical blowing agents confirm the results from rheological characterization.
Highlights
It is well known that for excellent foam extrusion performance the polymer must fulfill specific thermal and rheological properties
It has to be considered that the dissolved blowing agent (BA) can act as plasticizer, which causes a considerable reduction in the viscosity of the polymer melt [2,3,4,5]
If melt pressure is lower than the solubility pressure required for complete dissolution of the BA in the melt, early nucleation and premature foaming in the die can occur, causing poor morphology and surface quality of the foam [1,6,7]
Summary
It is well known that for excellent foam extrusion performance the polymer must fulfill specific thermal and rheological properties. The addition of low molecular weight plasticizers causes an increase in free volume, in lubricity, and in chain mobility of the polymer [23,24] This is basically accompanied with a decrease in glass transition temperature Tg [23,24]. If extensional flow properties are poor due to too strong plasticization, reduced foam extrusion behavior of the polymer can be the consequence. When discussing the suitability of CA for foam extrusion it is necessary to investigate the influence of plasticizer on thermal and rheological properties, especially on extensional flow behavior. No systematic research has been published so far concerning the extensional flow properties of externally plasticized CA melts. Preliminary foam extrusion tests using chemical blowing agents are shown to verify the results from rheological characterization
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