Biopolymer Blends

Compared to the standard fossil-based materials the field of biopolymers is relatively new. Particularly, the field of biodegradable polyesters is, yet, in its childhood shoes for material property variations of a specific polymer. While in the conventional material world, the variation of grades is backward integrated into the polymerization process producing copolymers or MFI variations in homopolymers, different bio-polyester grades are mainly produced by additivation or by blending of different homopolymers yielding the desired properties. To be able to predict the resulting material properties when blending different polymers, the properties of the respective resins need to be known. In an ideal system when the resins are perfectly miscible and soluble in each other the resulting properties are largely the average of the sum of the respective contributions (not looking at special interaction such as sc-PLA, see blog post “Crystallization Kinetics of semicrystalline Polyesters”). If one resin is rigid, and the second resin is flexible, the blend will be semi-rigid/semi-flexible. How good the compatibility of two given polymers is, depends on two factors. 1: the free surface energy of the polymers, 2: the melt viscosity difference. It is a necessary condition for the polymers to be miscible that the surface energies are similar. Otherwise, a phase separation up to macro level will be observed. It is a sufficient condition if the melt viscosities of the two blended resins are similar. If both requirements are fulfilled, a monophasic blend, also called polymer solution will be obtained. If the latter requirement is not fulfilled a biphasic blend known as “composite” will be obtained (see. Figure 7). Within a biphasic material system, two extreme morphologies can be found. On the one hand co-continuous phases, which form three-dimensional networks within the carrier resin, and, on the other hand, dispersed phases within a continuous phase can be found.??

It is important to note that we are discussing the compatibility during the mixing process. That is, the material properties require to fulfil the above-mentioned principle under given extruder conditions. Within a biphasic material system, two extreme morphologies can be observed. First, co-continuous phases can be found, which form three-dimensional networks within the carrier resin. If one polymer would be selectively removed a sponge-like structure remains. Second, dispersed phases within a continuous phase are observed. To give an example from our product portfolio: We imagine a blend from unpolar, low surface energy PE and a highly polar, high surface energy thermoplastic starch. Both polymers behave very differently in the melt state. Thermoplastic starch does not melt. Yet, the two polymers can be made compatible (we will go deeper into this topic in a later blog). The common ground of both polymers is that they show a shear-thinning behavior. By knowing the materials and the process very well it is possible to adjust the introduced shear such that the sufficient condition (see above) is met. As discussed earlier the introduced shear stress needs to be balanced concerning the shear sensitivity of the materials in question. There are tricks to meet the necessary conditions as well, which will not be elaborated at this point. Ultimately, the thermoplastic starch could be found as a dispersed phase within a continuous PE phase. An example of a co-continuous morphology are blend of PLA and PHB.?

Many currently known bio-based polyesters are also biodegradable (e.g. PLA, PHAs, PBS, etc.). While talking about this polymer class it is important to note that while degradable polymers are made to degrade, they will always do so under real conditions. That means the shelf life is always limited even if no particular biodegradation stimulus is introduced. There are means to delay the degradation process by keeping them dry, cool, and under a vacuum or inert gas atmosphere. As this is not always done in industry, biopolymer suppliers usually give shelf-lives of the resin of no longer than 6 months. Coming back to biopolymer blends, the shelf-life, and the biodegradation rate might be strongly influenced by the partner resin. If the partner resin rapidly degrades it often creates favorable conditions for the carrier resin to degrade, as well, by increased accessible surface area (sponge example) and by changing the pH inside the structure. This also applies to biopolymer composites.?

As soon as the polymer blend is becoming biphasic one could already consider it a biopolymer composite. However, we do not want to omit the transition from a monophasic blend. One possibility for increasing the adhesion of the components in a biphasic system is reactive extrusion. Reactive extrusion is already well-known in the field of thermosets but is becoming increasingly important in compounding, as well. Here, chain extenders or transesterification agents can be used to make the two phases “overlap” on a molecular level. The newest developments are aiming to backward integrate the compounding towards polymerizing the co-polyester within the extrusion process. Overall, the impact of the partner resin on the final mechanical properties can be ranked according to biphasic < biphasic chemically compatibilized < monophasic. We will cover biopolymer composites in the next issue of our blog.?

Dr. Rudi Eswein

Director of Sustainability