Films/Flexibles/Bags
Films/Flexibles/Bags PHBV blown films Citrates enable PHBV blown film extrusion without affecting degradability B iopolymers are difficult to process in their pure form, but the use of conventional additives often destroys environmental considerations. By using a biobased and biodegradable plasticizer based on citric acid, it was possible to produce blown films with a PHBV content of almost 70 %. The group of polyhydroxyalkanoates (PHA) is particularly interesting because of their good degradability under various environmental conditions. One of the best known representatives is polyhydroxybutyrate-co-hydroxyvalerate (PHBV). PHBV has a low viscosity, low melt strength and is highly crystalline and brittle in the solid state. Since PHBV is completely biodegradable in soil within a few months, it is a suitable starting material for mulch films and many other applications. Due to the limited property profile, however, the production of blown films made of PHBV was not considered until now. New Additives for Bioplastics Many polymers cannot be processed in their pure form, but must first be compounded into usable plastics with suitable additive systems (see Figure 1). However, if one deals with biodegradable plastics, this contradicts the use of numerous commercially available processing aids, which are not biodegradable themselves. For this reason, IKT (Stuttgart, Germany) is looking for new additives that can be used to improve biobased and biodegradable polymers without losing these advantages over conventional plastics. In the past, citrates based on industrially produced citric acid have proven to be good plasticizers for various plastics. The citrates of Jungbunzlauer (Ladenburg, Germany) are biobased and certified as compostable according to EN 13432. Therefore they may offer an alternative for the use in biopolymers. The aim of this project was to investigate whether the use of a citrate can modify the biopolymer PHBV in such a way that it can be processed into thin, biodegradable films by means of blown film extrusion. Due to the very low viscosity and melt strength of PHBV, it was decided to integrate a small amount of PLA, despite its reduced biodegradability towards PHBV, into the compounds to increase the chances of success of the project. Extrusion and characterization The compounds were produced in a ZSK 26 twin screw extruder from Coperion, Stuttgart, Germany. The PHBV ENMAT Y1000P from TianAn Biologic Materials, China, and PLA 4043D from NatureWorks, USA, was used. A compound with a PHBV/PLA ratio of 75:25 was used as a reference. For modification, CITROFOL BII from Jungbunzlauer, was used. This is a biobased acetyltributyl citrate which is fully biodegradable according to EN 13432. The blown film extrusion was carried out on a laboratory extrusion line from Collins, Maithenbeth, Germany. All composition data refer to mass fractions. The rheological properties of the compounds produced were investigated in a Discovery HR2 hybrid plateplate rheometer (TA Instruments, UK) at a temperature of 185 °C. An analysis of the crystallization behavior was carried out using differential scanning calorimetry (DSC 204, Netzsch, Selb, Germany). In order to determine the mechanical properties, tensile tests were performed on the films on a universal testing machine from Zwick, Ulm, Germany. Evaluation of the Films The compounding showed that the two base polymers selected were compatible and that a homogeneous compound could be produced without the use of further additives. The viscosity of the compounds decreased due to the addition of citrate compared to the non-additivated compound of both polymers. By means of differential scanning calorimetry it was to be examined whether the crystallization behavior changes by the addition of the plasticizer. Strongly pronounced peaks at about 175 °C (120 °C) show the melting (crystallization) of the PHBV fraction, while at about 150 °C the heating curve shows the melting of PLA as a very weak peak. However, it is clearly visible that the modified compounds have a melting temperature about 5 K lower. The resulting reduction in processing temperature not only saves heating costs, but also reduces thermal damage to the material during processing. The most important evidence of the successful modification became clear in the case of film blowing. Without plasticizers, the PHBV-PLA compounds could not be expanded, resulting synthesis compounding converting Figure 1: Polymers are compounded to plastics resource raw material material component monomers polymers plastics plastic parts 14 bioplastics MAGAZINE [06/20] Vol. 15
Films/Flexibles/Bags By: Silvia Kliem, Research Associate Christian Bonten, Professor Institut für Kunststofftechnik Univ. Stuttgart, Germany By: Silvia Kliem, Research Associate Christian Bonten, Professor Institut für Kunststofftechnik Univ. Stuttgart, Germany Biodegradable blown film with biodegradable softener in isolated pieces of film with high film thickness. The compounds with 5 % citrate allowed at least temporarily a continuous extrusion and the inflation of a film tube, film gauges of about 50 µm could be achieved. Only the compound with 10 % citrate could be processed in a stable and continuous process, the average film thickness was below 25 µm. Due to the high crystallinity of PHBV, the films are semitransparent. In contrast to films based on pure PLA, which are often criticized for their loud crackling, they resemble commercially available plastic films made of PE in terms of haptics. The tensile test of the films in Figure 2 shows a clearly changed elongation behavior of the modified compounds towards an elastic behavior with a maximum elongation of just over 5 %. Conclusion Especially in the agricultural sector, there is a great demand for biodegradable films. In addition to suitable polymers, attention must also be paid to the use of environmentally friendly additives. In this project, it was possible for the first time to produce blown films with a mass portion of almost 70 % of very well biodegradable PHBV. By using biobased and biodegradable citric acid ester as plasticizer and a small percentage of PLA as blending partner it can be assumed that the films presented here can be completely metabolized in our environment without leaving residues in the form of microplastics in the soil. Besides an optimization of the formulation and the transfer to industrial scale, the proof of this degradability in further investigations is of great interest. Acknowledgements This project was supported by Jungbunzlauer Ladenburg GmbH. We thank the project partners for the good cooperation. A more detailed version of this article was already published in the magazine “Kunststoffe”, issue 09/2020. www.ikt.uni-stuttgart.de 60 Figure 2: Stress-strain diagram of the blown films produced 50 Stress in MPa 40 30 20 PHBV+PLA 5 % Citrate 10 % Citrate 10 0 0 1 2 3 4 5 Strain in % bioplastics MAGAZINE [06/20] Vol. 15 15
