Thermoplastic processing of Polyhydroxyalkanoates with CITROFOL®

Biopolymers have a variety of possible applications due to their renewable characteristics and their potential biodegradability. Biopolymers are polymers that are either bio-based, biodegradable, or both. Unmodified biopolymers suffer from thermo-mechanical sensitivity during processing and often show poor physical and chemical resistance. Plasticisers can improve the handling and processing of biopolymers. Ideally, they are bio-based and biodegradable to tailor the properties of biopolymers into the desired range, like the CITROFOL® esters. Citrate esters are already applied in bio-based polymers with a positive impact on the processing and final product properties. Besides their compatibility with various polymers, their quick compostability without harm to air, soil or water is of benefit as well.

PHAs are naturally occurring linear polyesters, which are biodegradable and readily compostable. More than 150 types of PHAs can be produced by microbial fermentation of carbon-based raw materials like corn or sugar cane. They can be used in a variety of applications like packaging and agriculture and are therefore promising alternatives for petrochemical polymers. One of the most common PHA type is Poly(3-hydroxybutyrate-co-3-hydroxy- valerate) (PHBV), shown in figure 1. Genuine PHB is a brittle material with poor mechanical properties. Plasticised PHB has physical and mechanical properties comparable to polypropylene (PE).

To investigate the influence of different plasticisers on the processing of PHBV, a two-step manufacturing process was conducted. As plasticisers CITROFOL® AI (triethyl citrate), CITROFOL® BI (tributyl citrate) and CITROFOL® BII (tributyl O-acetylcitrate) were used in different concentrations (10, 15, 20 wt.%) and compared to the benchmark polyethylene glycol (PEG) 1000. PHBV pellets (ENMAT Injection Molding Grade Y1000P) were provided by Tianan Biologic Material (China). In the first step, PHBV was melted in a co-rotating twin-screw extruder and the respective plasticiser was added using a liquid dosage unit. The mixture was extruded, granulated and analysed for thermal properties and viscosity. Afterwards, selected plasticised granulates were melted again, films were prepared via cast film extrusion and subsequently their mechanical properties were characterised.

PHBV was successfully compounded with all tested plasticisers and concentrations. Moreover, the CITROFOL® esters exhibit a significant advantage. As opposed to the benchmark PEG 1000 (solid at room temperature), they are clear, practically colourless, oily liquids, and do not require an additional melting step. The measured motor load of the compounder during the processing of plasticised PHBV is shown in figure 2.
Generally, the motor load of the extruder device is significantly reduced by the addition of plasticisers. All citrate esters exhibit a similar tendency. The lowest motor load was achieved with 20% plasticiser content (reduction by more than 50%). This may result in an easier processing as well as lower energy consumption, contributing to a more sustainable process. The decrease in the motor load is due to the strong drop in viscosity of the plasticised polymer melt, measured separately to verify the results.

The viscosity of the granulates was determined as it is also a critical factor for evaluating the processing of polymers. Figure 3 shows the changes in viscosity for compounds with 20% plasticiser and the unplasticised material. In all cases, the viscosity decreases with the addition of plasticiser. The citrate esters exhibit a higher plastification efficiency than PEG 1000. The results correlate very well with the measured motor load during the processing. Low viscosities may enable an easier processing and an improved incorporation of higher amounts of fillers that are essential for the production of tailor-made polymer compounds. In addition, low viscosities are especially advantageous for the production of thin-walled parts.

Thermal properties like glass transition temperature and melting temperature are also critical parameters as they give an indication if the processing window can be expanded with the addition of plasticisers. This allows an enhanced processing of the polymer material. With increasing plasticiser concentration, the melting temperature is reduced (figure 4). The level of reduction depends on the type of plasticiser. Citrate esters demonstrate a higher efficiency compared to PEG 1000, i.e. stronger reduction, which leads to a larger gap between degradation temperature and melting temperature. This results in an easier processing of the polymer melt.

The plastification of polymer matrices have an influence on the mechanical properties of the produced parts. To show the proof of concept and in order to analyse the change on the properties of PHBV when plasticised with CITROFOL® AI, cast foils were prepared and the elongation at break and tensile strength were determined. Figure 5 shows, that the tensile strength decreases with increasing plasticiser concentration, whereas the elongation at break increases.

In summary, this study demonstrates that the citrate esters CITROFOL® AI, BI and BII are suitable plasticisers for the processing of PHBV. The addition leads to a broader processing window as well as to improved thermal properties. For CITROFOL® AI an improvement of the mechanical properties was shown. Nevertheless, the produced films are very inelastic expressed by the rather low elongation at break values. Therefore, product and process optimisations are necessary in order to realise a broader applicability of these materials.

Jungbunzlauer is one of the world's leading producers of biodegradable ingredients of natural origin, which enable its customers to manufacture healthier, safer and more sustainable products. Jungbunzlauer belongs to the largest global producers of citric acid and citrate esters, which are well-known under the brand CITROFOL®. Product innovation and continuous process improvements in our state-of-the-art plants result in unique high quality products. Citrate esters have an excellent toxicological and eco-toxicological profile, but also provide good versatility and compatibility with numerous polymers. They are particularly characterised by highly efficient solvation, low migration and non-VOC attributes. CITROFOL® grades offer a sustainable alternative to petrochemical- based plasticisers. Therefore, they are the preferred choice for sensitive products like toys, medical devices, food packaging, pharmaceutical applications and personal care. Moreover, all CITROFOL® esters are non-GMO, vegan, kosher and halal.