Rheological behavior of the poly(lactic acid) (PLA) composites filled with ground chestnut shell (CN) was investigated. Application of various measuring techniques: small amplitude oscillatory shearing rheometry and capillary rheometry, allowed to determine in detail changes of rheological behavior and potential processing limitations of fully biodegradable composites modified by an organic waste filler. Different influence of the ground chestnut shell filler on PLA-based composites flow behavior was observed during rotational and capillary rheometry. Incorporation of particleshaped natural filler resulted in strong increase of composites’ complex viscosity. However, due to occurrence of significant wall slip during capillary flow, materials containing ground chestnut were characterized by improved processability which result in increased melt flow index (MFI).

Keywords: poly(lactic acid), chestnut, composite, rheology

Nowadays, polymers are one of the most widely used materials. They play a significant role in production of packaging, consumer products or automotive parts. Ever growing utilization of plastics brings about increasing production of nonbiodegradable waste. Although extensive development of novel highly efficient recycling methods, tons of plastic waste are dumped on landfills every year. In the era of rapidly decreasing amount of non-renewable resources a new class of environmentally-friendly materials needs to be employed in mass production, even though the petrol-based polymers comprise of a small percent of overall crude oil consumption (4-6%). Utilization of bio-based, biodegradable polymers, including poly(lactic acid) (PLA), polyhydroxyalkanoate (PHA), poly(butylene adipate-co-terephthalate) (PBAT) and thermoplastic starch, has recently gained a lot of attention of researchers and producers alike, yet the market share of those biopolymers still remains little.^1-8 The reason of this situation is relatively high price and low thermomechanical stability and in some cases brittleness of aforementioned materials.^6-8
Polymeric composites have always been developed to improve materials’ properties, give them a defined characteristics or lower the price. Apart from using conventional, inorganic fillers, incorporation of organic fillers into polymeric matrix is another way to lower the impact of plastics on the environment. Lignocellulosic fillers, i.e. particles and fibers originating from different parts of plants are reported to increase mechanical and physicochemical properties as well as decrease density of thermoplastic and thermosetting polymeric composites.^7,9-13 Natural-based fillers also lower the price of composite final products, in comparison to pure polymeric materials,¹⁴ they are fully renewable and their environmental impact is considerably low.¹⁵ Furthermore, the possibilities of implementation are very wide, as there are various types of natural fillers, which can be used in polymeric composites. Different examples of chemically modified¹⁶ and raw,¹⁷ fibrous¹⁸ and particle-like,¹⁹ specially prepared and originating from agricultural waste fillers are described in the literature. The latter is especially interesting from economical and ecological point of view. What’s more, composites comprising of waste filler and polymeric resin may reveal advantageous properties.^17,19,20 An example of a agricultural waste filler is horse chestnut shell powder. Horse chestnut (Aesculus hippocastanum) is a large tree, cultivated in parks and gardens of northern hemisphere. Shells of its fruits contain a saponin called aescin, used in production of medicines. Several papers describe application of horse chestnut powder in polyester²¹ and polypropylene²² composites. In a previous study, we described thermal and mechanical properties and morphology of ground chestnut shell powder-filled PLA.²³ Even though the results of our investigation were promising, further insight into the characteristics of the researched material is needed. Apart from mechanical and thermal stability of composites, rheological and processing properties are crucial in assessment of the possibilities of industrial utilization of composites. Therefore, the aim of this study is a complex evaluation of rheological properties of biodegradable, bio-based composites comprising of horse chestnut powder embedded in PLA matrix.