**1. Introduction**

Due to their complex end-of life management, petroleum-based plastics have caused a serious environmental problem, mainly related to their disposal. It was observed that from 1950 to 2015, less than 10% of the total plastic produced amount was recycled [1]. A possible solution to the waste management problem caused by non-degradable plastics can be obtained by replacing these materials with biodegradable polymers obtained from renewable resources compounded with agro-food waste. In this context, biobased and biodegradable polymers are an interesting solution to preserve petroleum resources and to decrease CO2 emissions [2].

Agro-industry generates large biomass amounts that are not sufficiently and adequately exploited. For example, in the European Union alone, about 700 million tons of agriculture waste is annually produced [3]. The use of plant waste materials as raw materials in the production of biocomposites materials represents an exceptional opportunity for sustainable technological development. In fact, fruit shells and other agricultural waste are potentially important sources for the production of sustainable and competitive biocomposites. These plant by-products are produced in high quantities and crop wastes are rich in different nutritional components that can be valorized. Recently, the utilization of by-products has been increased by food and pharmaceutical manufacturers to produce valuable compounds from such inexpensive resources. In particular, nuts are one of the most important agricultural products due to their different uses within the food industry [4].

Walnut and hazelnut shells have great potential due to their large scale production; considering that about 67% of the total product weight consists of the shell, 646,818 tons

**Citation:** Aliotta, L.; Vannozzi, A.; Bonacchi, D.; Coltelli, M.-B.; Lazzeri, A. Analysis, Development, and Scaling-Up of Poly(lactic acid) (PLA) Biocomposites with Hazelnuts Shell Powder (HSP). *Polymers* **2021**, *13*, 4080. https://doi.org/10.3390/ polym13234080

Academic Editor: Pablo Marcelo Stefani

Received: 3 November 2021 Accepted: 21 November 2021 Published: 24 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of walnut shells, and 353,807 tons of hazelnut shells are produced each year [5]. After the separation of the kernel from the external parts of the fruit, large quantities of peel and shell are generated. These materials are the main part (over 60%) of the nut fruit and are discarded or burned as fuel without any useful application. Unfortunately, this waste material is typically burned directly in situ for heating purposes, while it could potentially be used for the production of both high added-value chemicals and biocomposites. Hazelnut shells are cost-effective byproducts [6] and their exploitation represents a stimulating challenge [7]. To better exploit their potentialities, it is necessary to find other better uses for hazelnut shells [8,9]. Hazelnut shells' composition is very similar to that of other wood-based biomass because cellulose, hemicellulose, and lignin are the main components. Shell grinding allows to produce hazelnut shell powder (HSP) of different sizes and morphologies. HSPs consist of lignin (40–50% by weight), cellulose (25–28%), and hemicellulose (22–30%), but they also contain a fraction of polyphenols (flavonoids and tannins), which can be recovered by hydroalcoholic extraction [10–12]. The shell extracts can be used as natural antioxidants in polymeric matrices as they can act as thermal and photo oxidative stabilizers for different types of polymers, including biopolymers like poly(lactic) acid (PLA) [13,14]. Moreover, the HSP addition enables light biocomposites to be obtained that, in some cases, possess improved mechanical and thermal properties and have enhanced biodegradability, when compared to the pure matrices [15–19]. Furthermore, the incorporation of HSPs into a biopolymeric matrix contributes to reducing the overall biocomposite cost [20]. However, some drawbacks must be mentioned in using agricultural waste for the production of lignocellulosic composites: unstable fiber availability over the year, absence of industrialized processing, and the need for big storage facilities and different necessary pre-treatments [21–24]. For this purpose, in order to take a step forward, the extrusion and injection molding processes considering the biocomposites scaling-up ability were investigated in this paper.

The polymeric matrix chosen for this study was poly(lactic acid) (PLA). In fact, among the biopolymeric matrices commercially available in the market, poly(lactic) acid (PLA) is one of the most attractive and its use in the production of green composites is gaining great importance [25]. PLA can be considered the front runner of the bioplastic market with an annual consumption of about 140,000 tons [26]. What has pushed up the increasing PLA demand are its excellent starting mechanical properties (≈3 GPa of Young's modulus, ≈60 MPa of tensile strength, ≈3% of elongation at break and an impact strength close to 2.5 kJ/m2) that are comparable to those of polystyrene (PS) [27].

Song et al. investigated the addition of walnut shell powder into PLA; they noticed during the biocomposites processing that an increase in the melt fluidity was correlated to the fiber powder addition [28]. This melt fluidity increment can lead to problems during the processing, making impossible or very difficult, for example, the extrusion compounding, the injection molding, the casting extrusion, etc. The evaluation of the fiber/matrix adhesion plays an important role and must be considered. From the processing point of view, fiber-matrix adhesion improvement can be done by chemical fiber pretreatments or in-situ reactive blending. The last option is very interesting for the scaling-up point of view and involves the use, during the extrusion compounding, of coupling agents that are able to modify the polarity and surface tension of the fibers, enhancing the fiber-matrix adhesion [29,30]. The main coupling agents added to improve the fibermatrix adhesion are maleic anhydride (MA), silane, isocyanate, and peroxide [29,31,32]. Commercial chain extender represents another way to improve the fiber-matrix adhesion, thanks to their easy processability during the extrusion compounding; however, they are not bio based and not biodegradable and even if they are introduced in very few amounts, they compromise the totally full bio-based origin of the final biocomposites.

The addition of HSPs into a PLA matrix must be deeply investigated and little work has been done regarding the scaling-up of these biocomposites into semi-industrial extrusion compounding process. For this reason, in this work, firstly the effect of the addition of different amounts (from 20 up to 40 wt.%) of two HSPs with different values of granulome-

try was investigated. The effect on melt fluidity, and thermal and mechanical properties was investigated on a lab-scale. Analytical models were also adopted to evaluate the powder size effect and adhesion between HSPs and PLA matrix. Then, the best selected compositions were extruded into a semi-industrial twin screw extruder, evaluating scaleup feasibility, focusing on the change of melt fluidity and mechanical properties of the scaled-up composites.
