*Article* **Rheological Characterization and Modeling of Thermally Unstable Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)**

**Silvia Lajewski \*, Annika Mauch, Kalman Geiger and Christian Bonten**

Institut für Kunststofftechnik, University of Stuttgart, Pfaffenwaldring 32, 70569 Stuttgart, Germany; a\_mauch@t-online.de (A.M.); kalman.geiger@ikt.uni-stuttgart.de (K.G.); christian.bonten@ikt.uni-stuttgart.de (C.B.)

**\*** Correspondence: silvia.lajewski@ikt.uni-stuttgart.de; Tel.: +49-711-685-62831

**Abstract:** Presently, almost every industry uses conventional plastics. Its production from petroleum and extensive plastic pollution cause environmental problems. More sustainable alternatives to plastics include bioplastics such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), which is produced by bacteria and is biodegradable even in seawater. High temperature sensitivity as well as massive thermal degradation cause difficulties during the processing of PHBV. The aim of this work is to create a detailed rheological characterization and master curves to gain deeper knowledge about the material and its processing parameters. The rheological characterization was performed with frequency sweeps in the range of 0.1 rad/s to 628 rad/s and time sweeps over 300 s. Creating master curves at the reference temperature of 180 ◦C with the software IRIS delivers Carreau and Arrhenius parameters. These parameters allow for a calculation of the master curves for all other temperatures by means of the temperature shift factor. Moreover, the rheological measurements reveal a minimum rheological measurement temperature of 178 ◦C and a surprisingly high activation energy of 241.8 kJ/mol.

**Keywords:** bioplastics; PHBV; rheological characterization; plastics processing
