Physical properties of green composites based on poly-lactic acid or Mater-Bi® filled with Posidonia Oceanica leaves

Abstract

This work focuses on the evaluation of Posidonia Oceanica leaves as effective reinforcing agent for ecofriendly, fully biodegradable polymer composites. Posidonia leaves were washed, ground and sieved in order to achieve two different size distributions and aspect ratios. They were then added to either a stiff or a ductile biodegradable polymer matrix, respectively poly-lactic acid (PLA) and MaterBi® (MB), at two different filler contents (10 wt% and 20 wt%). The materials were fully characterized from a spectroscopic, morphological, rheological, and mechanical point of view. In particular, the outcomes of tensile tests were statistically analyzed by using a Full Factorial Design in order to examine the main effect of type of polymer, filler size and filler content (as well as their mutual interactions) on two properties of great concern, such as elastic modulus and toughness.

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].