Bio-Plastics – Impact-Modified

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Bio-plastics – Impact-modified

Diversity of bio-plastics

As part of the HiBiKuS project funded by the Federal Ministry of Research, Technology and Space (BMFTR), Polymer Service GmbH Merseburg, Exipnos GmbH Merseburg and the Fraunhofer Institute for Microstructure of Materials and Systems (IMWS) are investigating how semi-crystalline bio-based plastics can be specifically modified to expand their range of applications [1].

Bio-plastics are a diverse class of polymers obtained from renewable raw materials. Unlike conventional polymers based on fossil raw materials, they offer ecological advantages, such as lower greenhouse gas emissions and the possibility of composting. Examples of bio-based polymers include polylactide acid (PLA), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyamide 11 (PA11), starch derivatives, chitin and chitosan, and cellulose. They are used in a wide range of areas, increasingly also for technical components [2].

However, bio-based and biodegradable plastics are often associated with limited mechanical properties.

Necessity of crystallisation control and impact modification of PBS

One aim was therefore to produce fully bio-based impact-modified polymers with good overall mechanical performance and advantageous morphology‒property relationships. The bio-based semi-crystalline polymer polybutylene succinate (PBS) was used as the matrix material [3‒4], a linear aliphatic polyester that is more biodegradable than polylactide acid (PLA), even in fresh and sea-water. However, methods for characterising the morphology of isothermally crystallised PBS have shown (see Figure 1) that the spherulitic morphology of PBS varies greatly within a relatively small temperature range, from rather fine spherulitic to very coarse spherulitic.

Fig. 1:

Microstructure of PBS depending on the cooling rate and nucleation

The grain boundaries that form in coarse spherulitic morphology are defects in the regular structure that have a negative influence on properties. In addition, this behaviour during crystallisation can lead to unacceptable warping in plastic components during the injection moulding process. The pronounced influence of processing conditions (e.g. cooling conditions, see Figure 1) on the morphology and properties of PBS therefore requires control of the crystallisation process (e.g. through nucleation, see Figure 1) and/or efficient impact modification, as described below.

Fig. 2:

Natural rubber latex investigated

Impact modification of PBS using modified natural rubber latexes

One basic way to modify impact strength is to add natural rubber latex (Figure 2) to the PBS during compounding. However, using unmodified natural rubber latex didn't get the results we wanted (Table 1 and Figure 3): this method didn't increase the notched impact strength compared to pure PBS.

Table 1: Procedures for improving the toughness of PBS

Procedure	Feasibility	Impact on toughness
Physical cross-linking of latex particles by electron irradiation|	Not possible
Injection moulding using unmodified latexes	Possible, but pronounced agglomeration of latex particles	Barely increased notch impact strength compared to pure PBS
Emulsion of latexes with variation of emulsifier and procedure; injection moulding using modified latexes	Easy to implement; significantly reduced agglomeration of latex particles	Significantly increased notch impact strength compared to pure PBS

Modifying PBS with natural rubber latex in a solution process with additional direct injection moulding proved to be a viable way of improving the mechanical properties, including toughness, even at low temperatures on a pilot plant scale (batch size: 100 kg) (see Table 1). Compared to pure PBS, this fully bio-based impact-resistant PBS shows greatly improved toughness at ‒20 °C and room temperature (see Figure 3). This behaviour is associated with a relatively low tendency of the primary latex particles to agglomerate due to their modification and is related to a transition from unstable (as is typical for pure PBS) to stable crack propagation. The newly developed impact-resistant PBS exceeds the threshold value for notched impact strength of 15 kJ/m² required in the automotive industry and can therefore be used in similar applications to impact-modified polypropylene (PP), with which it is comparable in terms of processability and property profile. Its advantages over PP are its good recyclability and biodegradability.

Fig. 3:

Toughness of PBS materials

Outlook

The approach described here enables the production of mechanically robust, fully bio-based and biodegradable injection moulding materials. These impact-resistant bio-plastics thus make an important contribution to replacing oil-based plastics and expand the potential of sustainable material alternatives.

See also

• Bio-Plastics
• Smart materials
• Microscopic structure
• Toughness
• Notched impact test

References

[1]	Lach, R., Henning, S. Putsch, E., Putsch, P., Kotter, I.: Gezielte Schlagzähmodifizierung von Polybutylensuccinat mit Naturkautschuk ‒ Kerbschlagzähigkeit auf Niveau von schlagzähmodifiziertem Polypropylen. Plastverarbeiter 06 (2025) 52‒54; https://p7f.vogel.de/wcms/68/3d/683d75ce2cb64/plastverarbeiter--ausgabe-6-2025-v2.pdf#msdynmkt_trackingcontext=e2340da7-6702-4985-b8a0-24c216040000
[2]	Niaounakis, M: Biopolymers: Applications and Trends. William Andrew Publishing, (2015); ISBN 978-0-323-35399-1; https://doi.org/10.1016/C2014-0-00936-7
[3]	Aliotta, L., Seggiani, M., Lazzeri, A., Gigante, V., Cinelli, P.: A brief review of poly (butylene succinate) (PBS) and its main copolymers: Synthesis, blends, composites, biodegradability, and applications. Polymers 14/4 (2022) 844; https://doi.org/10.3390/polym14040844
[4]	Xu, J., Guo, B.-H.: Poly(butylene succinate) and its copolymers: Research, development and industrialization. Biotechnology Journal 5/11 (2010) 1149‒1163; https://doi.org/10.1002/biot.201000136

Weblinks

• European Bioplastics e. V.: https://www.european-bioplastics.org/
• Wikipedia – The Free Encyclopedia: Biodegradable plastic https://de.wikipedia.org/wiki/Biologisch_abbaubarer_Kunststoff
• Umweltbundesamt: https://www.umweltbundesamt.de/biobasierte-biologisch-abbaubare-kunststoffe#22-sind-biobasierte-kunststoffe-nachhaltiger-als-konventionelle-kunststoffe
• Bio-Plastics Europe: https://bioplasticseurope.eu/about
• BiopolymerWiki: https://biopolymerwiki.hof-university.de/