Physical properties of green composites based on poly-lactic acid or Mater-Bi® filled with Posidonia Oceanica leaves
Introduction
Over the past few years, bioplastic-based composites gained a rising interest, due to the possibility to reduce the overall environmental impact. In particular, green composites, which can be defined as biodegradable polymers reinforced with natural fibers [1], [2], attracted increasing attention since they allow the valorization of agricultural or marine waste, providing products that present lower price, full biodegradability and/or compostability, and practically negligible hazards for the workers during the overall manufacturing process. Beyond the several advantages in terms of eco-sustainability, adding natural fibers to a biodegradable polymer matrix may improve some mechanical features, such as stiffness, thus allowing these so-called green composites to replace conventional composite materials [2], [3], [4], [5], [6]. Several studies involved natural fibers and particles as fillers for biopolymeric matrices. In the scientific literature it was reported that organic fillers such as wood flour, flax, hemp, jute, kenaf, ramie and sisal, and lignocellulosic fillers obtained from agricultural waste or by-products such as okra, coir, pineapple, Posidonia Oceanica and artichoke can be used to prepare green composites that exhibit higher mechanical performance if compared with those of neat matrices [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Nevertheless, as generally observed in polymer-based composites (or nanocomposites), the extent of the improvements achieved was found to depend on some key-factors: (i) chemical-physical properties of starting components; (ii) filler dispersion (and eventually fibers orientation); (iii) matrix/fiber adhesion [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [26]. Moreover, differently from composites containing synthetic/inorganic fillers, green composites may display high data scattering, due to the intrinsic heterogeneities affecting raw lignocellulosic fillers. In fact, each particle may display a variable content of lignin, cellulose and hemicellulose, as well as differences in terms of shape, morphology and surface roughness that usually result in different capability to transfer stresses, i.e. to interact/connect to the polymer matrix. Among the biopolymers investigated as matrices for green composites, polylactic acid (PLA) is considered one of the most attractive one, due to its physical properties, renewability, biodegradability. Therefore, it finds applications in many areas of science and technology, ranging from biomedicine to packaging [27], [28], [29], [30], [31], [32], [33], [34], [35]. Another promising synthetic biopolymer is MaterBi® (MB), a commercial biodegradable polymer based on complexed starch and synthetic polymers. MB finds important applications (mainly for packaging), owing to its interesting mechanical properties, especially in terms of stretchability and toughness, dramatically higher than other plasticized starch-based biopolymers [36], as well as a good thermal stability and processability, and full biodegradability, as reported in many studies [37], [38], [39], [40]. In the frame of this work we evaluated the feasibility of using Posidonia Oceanica leaves (POL) waste as a filler for renewable, biodegradable composites. In fact, Posidonia Oceanica is a widely distributed, fast-growing sea grass species, peculiar of Mediterranean Sea [9]. Due to its availability and tendency to accumulate under the form of decomposing leaves on Mediterranean beaches, it could be achieved very economically on a large scale and, furthermore, its reuse/valorization allows reducing the disposal issues [9]. In particular, two samples of ground POL, having different granulometry, aspect ratio and therefore surface area, were added to two different biodegradable matrices: a brittle, rigid polymer, PLA, and a ductile, extremely stretchable resin, MB. The systems investigated are extremely complex owing to the singular morphology of POL fillers – displaying a porous platelet architecture – and their aforementioned intrinsic heterogeneity, which poses several issues in terms of reliability [19]. Furthermore, a detailed study was carried out by examining the processing-structure-property relationship of these systems, by taking into account several phenomena, such as filler aspect ratio after processing, polymer degradation, and capability of the polymer to enter the void channels of the filler porous platelets. Finally, two properties of interest, such as elastic modulus and toughness, were analyzed by a full factorial design, aiming at measuring the effect of filler size, filler content and type of polymer, as well as their mutual interactions, on the properties of biocomposites. In fact, the modelling of complex systems is crucial for the fabrication of materials engineered with specific characteristics [41].
Section snippets
Materials and preparation
POLs used in this study were collected from the seacoast of Mondello in Palermo (Sicily, coordinates: 38°06′56″N13°21′41″E) and successively washed, ground, meshed and dried under fume hood overnight at room temperature. After this stage, POL particles were sieved using two different mesh size, i.e., from 150 μm to 300 μm (labeled as A) and from 75 μm to 150 μm (labeled as B). They are characterized by an apparent density equal to 0.42 g/cm3 and a real density, measured by helium pycnometer,
Results and discussion
SEM micrographs of microparticles are reported in Fig. 1A–D together with a digital micrograph of raw leaves and ground fibers. More in detail, an overview of POL-A and POL-B granulometry is provided in panels A and B, respectively. The particles can be regarded as porous parallelepipeds having a practically constant thickness equal to 25 μm and variable width and length. POLs display a characteristic morphology, consisting of a series of parallel vascular bundles, as clearly visible in
Conclusions
Within the frame of this work, the feasibility of using POLs as fillers for PLA and MB was investigated. More in particular, the effect of type of polymer, filler aspect ratio and concentration on elastic modulus and toughness of corresponding composites was statistically analyzed, together with a close inspection of the relationship between viscosity, degradative phenomena, intraphase and final aspect ratio of the fillers. Generally speaking, adding POLs determined a stiffening effect, more
Data availability statement
Some rough/processed data that support our study are available from the corresponding author on reasonable request.
Acknowledgments
This work was financially supported by the project RL-INSTM GREENPACK (PUBRL16SCA – PUB16SCASG).
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