*Review* **Preparation and Applications of Green Thermoplastic and Thermosetting Nanocomposites Based on Nanolignin**

**Debora Puglia 1,\* , Francesca Luzi <sup>2</sup> and Luigi Torre <sup>1</sup>**


**Abstract:** The development of bio-based materials is of great importance in the present environmental circumstances; hence, research has greatly advanced in the valorization of lignin from lignocellulosic wastes. Lignin is a natural polymer with a crosslinked structure, valuable antiradical activity, unique thermal- and UV-absorption properties, and biodegradability, which justify its use in several prospective and useful application sectors. The active functionalities of lignin promote its use as a valuable material to be adopted in the composite and nanocomposites arenas, being useful and suitable for consideration both for the synthesis of matrices and as a nanofiller. The aim of this review is to summarize, after a brief introduction on the need for alternative green solutions to petroleum-based plastics, the synthesis methods for bio-based and/or biodegradable thermoplastic and thermosetting nanocomposites, along with the application of lignin nanoparticles in all green polymeric matrices, thus generating responsiveness towards the sustainable use of this valuable product in the environment.

**Keywords:** bio-based; thermosetting; thermoplastic; nanocomposites; lignin nanoparticles

#### **1. Introduction**

The rising global demand for fossil resources for nonenergy purposes, as in the case of plastics production, has intensely motivated research to find alternative solutions to petrochemical plastics; however, progress has still not reached a commercially viable scale. The demand for cost-effective, ecofriendly materials has also increased to reduce waste management and pollution issues, thus academic/industry interest in sustainable biobased materials has accordingly exploded in recent years. Even if numerous synthetic biopolymers have been used to this purpose, the need to suit different applications has allowed for progress even in the field of natural biopolymers. According to this, significant progress in lignin valorization and the use of sustainable and natural resources has been accomplished in the last years, in particular with regard to bio-based and biodegradable natural polymers based on this source, which are facing increasing consideration as they are environmentally green and economically reasonable. Replacing petroleum-based-derived materials with sustainable and environmentally friendly materials has been also considered as a crucial activity in the current period, so consideration has been given to the progress of lignin bio-based and/or biodegradable materials with thermomechanical performance that can compete or even surpass the petroleum-based products presently used. Lignin is considered an excellent substitute feedstock for the preparation of chemical products and polymers, even if one of the main difficulties still remaining is the lack of a well-defined structure and the partial flexibility linked to its origin, including extraction fragmentation procedures [1]. Although lignin is presently often considered as a filler or additive, it is hardly appreciated as a natural resource for chemical production. Nevertheless, it may be an outstanding candidate for chemical reactions due to its extraordinary reactivity (i.e.,

**Citation:** Puglia, D.; Luzi, F.; Torre, L. Preparation and Applications of Green Thermoplastic and Thermosetting Nanocomposites Based on Nanolignin. *Polymers* **2022**, *14*, 5470. https://doi.org/10.3390/ polym14245470

Academic Editors: Jesús-María García-Martínez, Emilia P. Collar and Nathanael Guigo

Received: 30 October 2022 Accepted: 10 December 2022 Published: 14 December 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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/).

the presence of abundant aliphatic and phenolic hydroxyl groups) for the preparation of bio-based materials.

Following its fast development, efforts have incessantly been made to advance its compatibility with other additives in multicomponent systems, as in the case of its mixing with thermoplastics and thermosetting green matrices [2]. On the other hand, in parallel to its application at the macroscale as an additive in bio-based polymeric materials, research has advanced to solve these limits and has opened a different perspective towards the use of lignin-based nanomaterials as functional fillers in bio-based matrices [3]. To realize the potential of this material, one of the possible routes to follow includes lignin use in nanocomposite assemblies, where synergic interactions are extremely advantageous [4]. To this end, we review both existing possibilities, i.e., renewable thermoset and thermoplastic polymers based on lignin and the use of nanolignin as the active ingredient in these specific matrices, with particular attention to their application in niche sectors.

#### **2. Synthesis of Bio-Based Polymeric Matrices from Lignin**

Lignin is characterized by a complicated three-dimensional structure, obtained by polymerizing three phenylpropane units that arise from three aromatic alcohols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 1a) [5].

Its complexity can be controlled by accurate deconstruction, so the synthesis of polymers starting from aromatic compounds ideally obtained from lignin has attracted a growing curiosity. A large quantity of polymers obtained from lignin derivatives originate from different products (Figure 1b), gained by applying various depolymerization techniques [6,7]: as a function of the phenol substrates, various chemical changes and polymerization routes can be settled, leading to (semi)aromatic polymeric systems covering a widespread range of diverse thermal and mechanical characteristics.

**Figure 1.** (**a**) Typical fragments of lignin structure with its main monolignols and (**b**) the potential phenolic products from lignin degradation. Reprinted with permission from Refs. [8,9]. Copyright 2019, Springer Nature Switzerland AG & Wiley.

Different approaches can be considered to realize high-performance polymers from lignin. The different adopted depolymerization methods have great prospective to get several monomer precursors, e.g., as alcohols, aldehydes, and acids, from lignin (Figure 2a). Vanillin is one of the most explored precursors. Other derived lignin compounds are ferulic acid, coumaric acid, sinapic acid, vanillic acid, syringic acid, caffeic acid, cinnamic acid, syringaldehyde, guaiacol, syringol, phenol, cresol, and catechol. The different molecules can be arranged as blocks for various polymers (Figure 2b), e.g., polyesters, polycarbonates, phenolic resins, polyurethanes, and epoxy resins. Many reviews extensively refer to the synthesis of lignin-derived compounds and their use in the realization of bio-based

polymers [11–16]: here, we will exclude the case of lignin polymer blends, while the possibility of graft polymerization or lignin conversion of monomers to polymers will be considered.

**Figure 2.** (**a**) Lignin depolymerization or degradation procedures and its derived monomers and polymers; (**b**) Different methods for lignin-based polymer synthesis. Reprinted with permission from Ref. [10]. Copyright 2021, Royal Society of Chemistry.

#### *2.1. Synthesis Routes of Bio-Based Thermoplastics from Lignin-Based Molecules*

The use of valorized lignin increases the potential to realize biodegradable biopolymers, such as polyhydroxyalkanoates, polyhydroxybutyrates, polylactic acid, or nonbiodegradable matrices, as in the case of polyurethane, polyolefins, polyamides, etc. [17] (Figure 3).

**Figure 3.** Schematic representation of lignin valorization on biopolymer production. Reprinted with permission from [17]. Copyright 2019, Elsevier Ltd.

The first case of polymerized vanillic acid to make polyesters was defined in 1955 [18]. Carboxylate was obtained by the conversion of vanillic acid and by the esterification process of a phenolic segment with ethylene dihalides. After that, the carboxylate was chemically esterified with ethylene glycol and reduced to linear polyester, showing a glass transition temperature of 80 ◦C and a melting temperature of 210 ◦C [18]. In 1981, a similar strategy was established by Lange and coauthors to produce vanillic and syringic acidbased polymers. In the second path, the phenolic moiety of vanillic acid interacted with ethylene oxide [19]. In the refining technique, lignin has a complex structure that makes it hard to be directly adapted into high-value materials: nevertheless, with the advanced clarification of the lignin structure and its microbial metabolism, it has been possible to alter the lignin structure to give, for example, high-value-added products by means of biological procedures, as in the case of PHAs. In nature, numerous bacteria have settled various metabolic paths to convert lignin to PHAs with short-, medium-, or long-chain structures; specifically, lignin derivatives can be processed to acetyl-CoA for PHA. Currently, PHAs from lignin or lignin-related aromatic compounds have been obtained by selecting many bacteria, such as *Oceanimonas doudoroffii* (PHA from sinapinic acid and syringic acid), *Cupriavidus basilensis* B-8, *Pseudomonas putida* A514, and *Pseudomonas putida* KT2440. The produced PHA can be adapted to convert into varied chemical precursors, such as alkenoic acids and hydrocarbons, which indicate that lignin can be adapted to become fuel-range hydrocarbons, chemical precursors, and biomaterials [20].

Lignin has been also explored as a natural resource for the synthesis of polyurethanes because it retains hydroxyl groups on its surface [21]. It fits well even into PU chemistries, since it acts both as crosslinking agent, due to the accessibility of numerous hydroxyl groups on each molecule and as a hard segment due to the aromatic nature. The last years have also viewed a reliable tendency that seeks to take advantage of the vast availability of renewable feedstocks, such as lignin derivatives, terpenes, vegetable oils, and polyols as precursors for the synthesis of nonisocyanates polyurethanes (NIPUs). Many approaches have been explored to use the aforesaid renewable resources to synthetize NIPUs combined with the prerequisites of green chemistry [22].

Meng et al. studied a different method that utilizes lignin extracted from the cosolvent lignocellulosic fractionation (CELF) of poplar wood to realize bio-NIPUs [23]. In this method, hardwood poplar is initially reduced in different fractions via a CELF treatment with the aim of recovering a lignin stream rich in phenolic content. The CELF lignin was then aminated by a Mannich reaction [23] and finally reacted with bicyclic carbonates to produce an innovative NIPU. The authors suggested the utilization of the CELF reaction to obtain a lignin rich in phenolic OH groups to raise its reactivity in amination. In addition, the authors studied, via the Mannich reaction, the amination of CELF lignin by using diethylenetriamine and formaldehyde in acidic environments. Successively, as an alternative to the reaction of hydroxyl groups of lignin with isocyanate groups to gain the traditional PU linkage (pathway I, Figure 4), the amine group in the CELF lignin reacted with cyclic carbonate originated from carbonation of epoxides to give lignin-based NIPU (pathway II, Figure 4).

**Figure 4.** Synthesis procedures for polyurethanes obtained from lignin via (**I**) isocyanates pathway and (**II**) nonisocyanates way. Reprinted with permission from [23]. Copyright 2022, Elsevier Ltd.

According to this, the mechanical behavior of NIPUs based on lignin can be definitely varied from rigid to elastic by basically changing the lignin constituents of the polymeric material. The thermal characteristics of NIPUs were enhanced thanks to the addition of aminated lignin, and NIPU containing 55 and 23 wt% of lignin exhibited a high elongation at break (~140%) and tensile strength (~1.2 MPa), respectively. The obtained experimental results reveal that the reaction of cyclic carbonate with aminated lignin can be considered as a significant strategy for the synthesis of lignin-based NIPU with a relatively high lignin amount [23]. Lignin can also work as a polyol in the polyester synthesis, and hydroxylbased chemistries can be adapted to give terminal hydroxyl groups, carboxylic acid, acyl groups, and epoxy groups. Polyester copolymers based on lignin can be obtained by the reaction of hydroxyl groups with additional reactive functional groups (diacyl chloride, dicarboxylic acid, adipic acid, and/or phthalic anhydride): in the case of branched ligninbased poly (ester-amine), reactions of triethanolamine, lignin, and adipic acid have to be considered [24]. Copolymerization of vinyl monomers and lignin is usually applied to realize lignin-based vinyl polymeric systems by different radical initiation routes [25].

Graft copolymerization, or in general, derivatization reactions, can generate new lignin containing thermoplastics by incorporating technical lignin and synthetic material. By appropriately choosing low-glass-transition temperature chemistries, lignin derivatives with variable thermomechanical properties can be obtained. On the other hand, the new progress on controlled/living polymerization and specific and effective synthetic techniques (e.g., click chemistry) propose new pathways for the design and realization of high-performance thermoplastic materials based on lignin derivatives having functional properties [26]. Even with a fruitful commercialization of many lignin-containing thermoplastics, constant efforts are still required to advance and create a new generation of lignin-based thermoplastics with precise structures, strong melt workability, good mechanical, and thermal performances.

#### *2.2. Synthesis Routes of Bio-Based Thermosets from Lignin-Based Molecules*

Thermosetting polymers can be produced from many bio-based resources, as in the case of vegetable oils, lactic acid, and citric acid, giving polymeric materials with more than 90% of a renewable amount [27]. Various bio-based thermosetting resins have been considered through the manipulation of virgin renewable feedstock; therefore, research on how to properly convert residual biomass to stimulate the production of new materials with remarkable properties is of great impact. Many functionalization approaches, including chemical or physical modifications, have been taken into account to broaden the application fields of these new resins with a singular glance to their processability and recyclability [28,29]. Referring to lignin, its aromatic structure gives molecular rigidity, and providing highglass-transition temperatures and stiffness, as well the presence of aliphatic and aromatic hydroxyl functionalities, is a crucial characteristic for application where high thermal stability and high network levels are required [30]. Nevertheless, employing lignin as a raw material is still a noteworthy duty, being characteristically heterogeneous in its native form. Moreover, the processes considered to obtain lignin from biomass permanently alters the assembly of the lignin backbone, which is cleaved, fractionated, and assembled, making it practically nonidentifiable from its source in nature. Additionally, new functional groups are inserted through, for example, oxidation reactions, forming carboxylic acids, aldehydes, and ketones [31]. Oxypropylation, allylation, epoxidation, acetylation, and silylation are a few of the pathways for the modification of technical lignin found in the literature, which makes it compliant to be incorporated, by compatibilization, in polymeric matrices.

Two approaches are currently considered to obtain lignin-based thermosets. The first method utilizes lignin itself or incorporates lignin with other components to synthesize copolymers, with the key restriction that the reactivity of bulk lignin is lower than that of monomers. To overcome these restrictions, another method was developed that uses aromatic molecules obtained directly from lignin depolymerization. This approach enables molecular design and structural modification and can increase the performance of the resulting polymeric materials, as in the case, for example, of vanillin and other derivatives [32]. The main task in both cases is still to obtain reproducible and well-characterized fractions. These prospective polymer feedstocks, on the other hand, have their own limited challenges in terms of yields, prepolymerization reactions, and workability. The review from Feghali et al. [14] presents an overview on polymers obtained from lignin-based model compounds and depolymerized lignin (vanillin, vanillic acid, aromatic acid, quinones, and aromatic aldehydes by oxidation reaction): an extensive multiplicity of high-performance polymeric systems, such as polyurethane (PU), epoxy resin, phenol formaldehyde, and polyester, exhibiting good thermal and mechanical characteristics, can be synthesized with lignin as the macromonomer [33]. For example, Fersosian [34] obtained high-yield phenolic monomers through the selective cleaving of the β-O-4 bond of native lignin for the synthesis of lignin-based epoxy resin; however, the severe depolymerization condition, the low monomer yields, and the high separation costs limit the industrial use of this strategy (Figure 5a).

Compatibility and reactivity of lignin with compounds can be enhanced by means of chemical changes (e.g., propoxylation, phenolation, demethylation, and esterification reactions). Nevertheless, steric-hindrance influence and the limited compatibility of ligninbased epoxy resin weakened the crosslinking density, deteriorating the thermomechanical behavior of the thermosetting polymeric materials. Consequently, limitations such as the complex process, limited effect, low-lignin loading, and waste liquid recovery are unavoidable and still need to be solved. In lignin-modified phenolic resins, lignin is considered as the phenol able to react with formaldehyde in basic conditions or as an aldehyde to react with phenol in acidic conditions. Nevertheless, the replaced amount of phenol is partial due to scarce reactive sites and steric hindrance in lignin. To overcome these restrictions, lignin-derived phenols have been exploited to produce phenolic resins [35] (Figure 5b). The production of renewable, green, and sustainable phenolic resins based

on lignin-derived monomers, having the potential to substitute traditional polymers, is currently under intensive study.

#### (**b**)

**Figure 5.** (**a**) Schematic representation and chemical structure of lignin-based epoxy resin and (**b**) schematic representation of the renewable phenolic resin synthesis based on lignin-derived monomers. Reprinted with permission from [34,35]. Copyright 2021, Elsevier Ltd.

#### **3. Nanolignin as Filler in Polymeric Nanocomposites**

Lignin nanoparticles have received much attention in the last years concerning the effort to utilize and apply lignin into more valued sectors [36]. In order to progress in the suitable use of lignin into different fields, it is required to ensue with chemical changes, fractionations to produce homogeneous materials, as previously described, or realize precipitated material with submicron particles for easier dispersion and enhanced features. The academic interest moved to the preparation of lignin nanoparticles (LNPs) by discovering their potential uses [37,38]. To date, the studied routes for the synthesis of LNPs are essentially chemical-based procedures which include, but are not limited to, acid-catalyzed, flash and nanoprecipitation, dialysis, solvent exchange, antisolvent process, W/O microemulsion processes, homogenization, and sonochemical synthesis. These methods have their profits and restrictions when they are utilized for LNP extractions. Therefore, the synthesis should be selected proficiently in order to yield LNPs of chosen sizes and dimensions [37,39]. Different methods, such as freeze-drying and thermal stabilization, interfacial crosslinking, polymerization and emulsion, and microbial- and enzyme-mediated, have been also considered as appropriate for the production of lignin nanoparticles. A comprehensive list of procedures that can be implemented has been reviewed in a few recent papers [40–43]. The procedures may give rise to appropriate advantages, but even distinctive faults regarding industrial use, since in some cases huge contents of solvents are necessary for the purification before precipitation, precipitation itself, and downstream processing, and in other cases a limited scalability of nanolignin production steps is manifest. Regardless the production yield, the research of suitable combination of LNPs with green matrices has progressed and prospective applications have been found

and developed due to the specific characteristic of this material that can be considered for numerous potential applications (high thermal stability, manifest antioxidant properties, biodegradability, and UV-absorption features).

#### *3.1. Nanolignin as Functional Filler in Thermoplastic Green Nanocomposites: Properties and Applications*

The use of nanolignin as a reinforcing phase in macromolecules (both natural and synthetic) is a key methodology to advance in the realization of sustainable polymeric composite systems. Recently, lignin was utilized as a nanoscaled reinforcement to improve the structural characteristics of polysaccharides, proteins, natural rubber, and synthetic polymeric matrices [44,45]. In this context, the advance in the modification of lignin-based materials to give nanocomposites is, in the last decades, evident, due to the growing interest of the academic and industrial area. To provide few examples, Yang and coauthors proposed the use of lignin nanoparticles (LNP) in poly (lactic acid). They selected two different amounts (1 and 3% wt.) of nanofillers to be utilized in the polymer. Data obtained from antimicrobial analysis demonstrate the ability to hinder the growth of *Xanthomonas axonopodis pv*. *vesicatoria* and *Xanthomonas arboricola pv. pruni* Gram-negative bacteria over time, to positively influence the innovation, and to induce a positive effect against hazardous bacterial plant pathogens. The disintegration test under composting conditions revealed that the tested formulations reach a value up to 90% after 15 days; however, the presence of LNPs did not affect the disintegrability of different films, as shown in Figure 6a,b. The presence of LNPs did not affect the migration value, and accordingly the polymeric systems can be regarded as appropriate for the food packaging sector [46]. Data obtained from antimicrobial analysis demonstrate the ability to hinder the growth of *Xanthomonas axonopodis pv. vesicatoria* and *Xanthomonas arboricola pv. pruni* Gram-negative bacteria over time, to positively influence the innovation, and to induce a positive effect against dangerous bacterial plant pathogens. The disintegration test under composting conditions revealed that the tested formulations reach a value up to 90% after 15 days; however, the presence of LNPs did not affect the disintegrability of different films, as shown in Figure 6a,b. The presence of LNPs did not affect the migration value, and accordingly the films can be regarded as appropriate for application in the food packaging sector [46].

**Figure 6.** Disintegrability values (**a**) and visual images (**b**) of PLA and PLA binary and ternary nanocomposites at different incubation times in composting conditions. Reprinted with permission from Ref. [46]. Copyright 2016, Elsevier Ltd.

Chollet and coauthors considered the nanolignin as a new additive for flame-retardancy of poly (lactic acid) [47]. Lignin nanoparticles (LNPs) have been obtained from Kraft lignin microparticles by considering a dissolution–precipitation process. Micro- and nanolignins chemistries were altered by functionalizing the external surface with diethyl chlorophosphate (LMP-diEtP and LNP-diEtP, respectively) and diethyl (2-(triethoxysilyl)ethyl) phosphonate (LMP-SiP and LNP-SiP, respectively) to improve their flame-retardant effect in PLA. The results of inductively coupled plasma (ICP) spectrometry demonstrated that a great content of phosphorus was grafted onto the nanoparticles. Nevertheless, phosphorylated lignin nanoparticles limited PLA degradation during melt processing and the nanocomposite systems were shown to be relatively stable from the thermal point of view.

The use of lignin nanoparticles was largely applied and investigated as a method to develop new multifunctional, innovative materials in the food packaging sector. LNPs have been confirmed to provide enhanced mechanical, thermal, and antioxidant characteristics to the polymers in which they are incorporated depending on their particle size [48]). Lizundia et al. proposed the development of poly(l-lactide) (PLLA)-based nanosystems realized by the solvent casting method and combining LNPs with various metal oxide nanoparticles, such as WO3, Ag2O, Fe2O3, TiO2, and ZnFe2O4 [49] (Figure 7a). It was found that the formulations based on nanolignin and ZnFe2O4 particles exhibited the best antioxidant behavior. Radical scavenging activity was also observed in ternary-based nanocomposites, where lignin and metal oxide nanofillers operated together synergically to boost the functional properties (Figure 7b). The antimicrobial activity of binary nanocomposites containing metal oxide NPs was correspondingly strong against PLLA, but it was only persistent for a few ternary nanocomposite films in a time result that was more obvious for *S. aureus* than for *E. coli* (Figure 7c). Lignin nanoparticles can protect towards UV light while allowing visible light to get through, and they can exceed the UV protection effect of numerous inorganic nanoparticles (Figure 7d).

**Figure 7.** (**a**) TEM images of metal oxide (Fe2O3) and lignin nanoparticles (LNPs); (**b**) antioxidant activities of PLLA-based systems, antimicrobial activities; (**c**) antimicrobial activities of PLLA binary systems; (**d**) UV–vis spectra of PLLA binary and ternary films. Reprinted from [49]. Copyright 2020 American Chemical Society.

The central role of lignin nanoparticles as UV barrier filler was also investigated by Yang and coauthors [50]. They combined LNPs in PLA and polycaprolactone (PCL)-based formula-

tions to increase packaging ductility and UV barrier properties. LNPs and caprolactone were first diluted in toluene to obtain a homogeneous solution and then purged with N2 gas. The process was maintained at 120 ◦C for two days after the l-lactide addition. The addition of PCL determined an increase of the elongation at break up to 185%, an initial decrease of tensile strength that gradually increased to 280% after the addition of the LNP-P(LA-CL) copolymer. The toughness also rose 1.5 times above the PLA/PCL. Similar results were observed for the crystallization values and UV protection. Nanolignin–PLA/PCLbased systems can be utilized in the food packaging industry as an impact-resistant and UV-protectant material.

Cavallo et al. suggested the use of polylactic acid (PLA) films containing 1 wt% and 3 wt% of lignin nanoparticles (pristine (LNP) chemically modified with citric acid (caLNP) and acetylated (aLNP)). The different polymeric films were produced by extrusion and filming, and after that, the formulations were analyzed by determining the overall performance needed for the food packaging sector [51]. The obtained data indicated that all lignin nanoparticles induced UV-blocking, and antioxidant and antibacterial (against Grampositive *Micrococcus luteus* and Gram-negative *Escherichia coli* bacteria) behavior to the PLA films, and a higher consequence was indeed found when increasing the filler content. Acetylation (aLNP) of the fillers moderately limited the antioxidant characteristics and the UV protection of the obtained composite systems, but it affected positively the nanoparticles distribution and aggregation, improving ductility and aesthetic quality of the films by decreasing at the same time the characteristic dark color of the lignin. Migration tests and disintegration test realized in simulated composting conditions of the nanocomposites showed that, irrespectively of their system, the realized active nanocomposites behaved likewise to neat PLA.

The use of LNPs was proposed as a valid possibility to develop promising wound dressing. Pahlevanneshan and coauthors [52] proposed the design and characterization of porous nanocomposite based on polyurethane (PU) foam synthesis. Moreover, the developed materials containing nanolignin coated with natural antimicrobial propolis for wound dressing. The antimicrobial effect was observed adding the extract to the polymeric foams, and all foams showed high biocompatibility toward L929 fibroblast cells, with the highest cell viability and cell attachment in the case of PU-LNP/propolis extract. In vivo wound-healing results, obtained by using Wistar rats' full-thickness skin wound model, showed that PU-LN/EEP has advanced wound-healing efficiency when compared to foams (Figure 8a–c) [52].

**Figure 8.** (**a**) Visual images of the wounds after 1 and 10 days of postoperation for the PU, PU-LNP, and PU-LNP/EEP groups. (**b**) Histograms of the wound closure for PU, PU-LNP, and PU-LNP/EEP groups. The data are expressed as mean ± standard deviation, (n = 8, \*: *p* < 0.05, ns: not significant). (**c**) H&E-stained sections of skin specimens from the wound site of PU, PU-LNP, and PU-LNP/EEP groups. Arrows (a and b letters) designate the epidermis and dermis layers, respectively. Green and blue arrows indicate the keratin layer and sebaceous gland, respectively. Reprinted from Ref. [52]. Copyright 2021 MDPI AG.

#### *3.2. Nanolignin as Functional Filler in Thermosetting Green Nanocomposites: Properties and Applications*

Lignin can be blended, considered as a filler in a composite/nanocomposite formulation, both in its native form or chemically modified, combined in the presence of particular additives: in all cases, it has been proved that lignin can beneficially improve the overall performance of the resulting polymers. While the literature reports numerous examples of LNPs in thermosets [53–55], limited cases of nanolignin incorporation in green-based thermosetting matrices can be found. In their paper, Wang et al. [56] considered a simple and fast synthesis method to synthetize bio-based epoxy resin obtained from vanillyl alcohol; after that, vanillin-based epoxy resin (VE) was additionally reinforced by lignin-containing cellulose nanofibrils (LCNFs) with different weight contents. The authors experimentally observed that a significant improvement in the thermomechanical performance of the nanocomposites was attained with a low amount of nanofibril addition, confirming the possibility of assembling environmentally friendly and sustainable bio-based epoxy lignin nanocomposites with superior properties (Figure 9a).

A new approach was adopted to realize lignin phenol formaldehyde (LPF) resin: in the paper of Chen et al. [57], the preparation of nanolignin with a high specific surface area and porous structure was arranged, and this nanofiller was then utilized as a valid phenol substitute combined with formaldehyde to produce a wood adhesive. Data showed that replacement of phenol by nanolignin could enhance the thermal characteristic of the resin, and in parallel, the modification of the curing schedule of the prepared lignin-based resin was considered.

In a quite recent paper [58], a simple foaming process to realize lignin-based polyurethane foams (LPUFs) was also considered: in that specific case, bio-based polyether polyols partially replaced petroleum-based raw components. Traces of phenolic hydroxyl groups (about 4 mmol) in lignin functioned as a direct reducing component and capping agent to silver ions by forming in situ silver nanoparticles (Ag NPs) within the LPUF skeleton. The lignin polyurethane/Ag composite foam (named as Ag NP-LPUF) was characterized by modulated thermomechanical and antibacterial properties, confirming the possibility of using these antimicrobial composite foams to encourage wound healing of full-thickness skin defects (Figure 9b).

Another example of lignin nanoparticle exploitation in a novel manner is represented by the study reported by [59], where the authors considered the realization of water-based, solvent-free, and multiresistant surface coatings: due to the presence of hydroxyl groups, the nanolignin acted as a hardener and no binder was required to realize adhesion to the substrate. In the case of the wood substrate, the particle morphology permitted proficient water repellency with a low coating weight, since the coating maintained the surface roughness of the wooden substrate while providing additional hydrophobicity.

Researchers from Washington State University, part of the NSF-supported Industry– University Cooperative Research Center for Bioplastics and Biocomposites (CB2), considered the use of a deep eutectic solvent to extract oligomeric lignin (nanoDESL) from plant biomass at a high yield and also nanosized [60]. NanoDESL shows narrower molecular size dimensions, distribution, and structural characteristics of traditional lignin. Oxypropylation of lignin was also optimized: it has been revealed that the use of polar aprotic solvents for the oxypropylation coupled with nanoDESL significantly promotes the oxypropylation reaction toward the synthesis of semiflexible PU. It was observed that the lignin-based PU containing ~20 wt% nanoDESL realized using polyol had density and compressive force comparable to the "standard" PU foam. The researchers are investigating how to enhance the reaction yield, with the goal of including 40 wt% lignin-based polyol into semiflexible foams: the potential of nanoDESL-based PU for adhesive, sealant, and coating applications is also explored by also considering their environmental toxicity and biodegradability issues. Using lignin as a source for PU synthesis not only encourages a circular economy but may also lead to the design of more ecofriendly end-of-life routes for PU plastics.

(**b**)

**Figure 9.** (**a**) Vanillin-based epoxy resin (VE) reinforced with lignin-containing cellulose nanofibrils (LCNFs) and the results of mechanical performance of the nanocomposites produced by considering different LCNF contents; (**b**) Schematic drawing of Ag NP-LPUF composite foam preparation by lignin liquefaction and one-step foaming. Reproduced with permission from [56,58]. Copyright 2020 and Copyright 2022, American Chemical Society.

All these studies provide awareness on potentialities of lignin nanosized fractions and allows for the design of reproducible and foreseeable material characteristics. It is essential to know these characteristics if we would exploit lignin as a raw material for a sustainable and innovative design. These preliminary and updated works confirm that nanolignin, if combined with green thermosetting matrices, can give fully green nanocomposites and, if effective, multifunctionality is often achieved in the presence of this nanoscaled filler.

#### **4. Conclusions and Future Perspectives**

This review, divided into two main sections, firstly provided an overview of preparation and applications of green thermoplastic and thermosetting nanocomposites based on lignin, and thereby a glance to the use of nanolignin in biopolymeric nanocomposites was also considered. Even if various structures and different properties of lignin at the nanoscale effectively can be challenging and interesting from the research point of view, the preparation and application of nanolignin-based green composites in high-value sectors are still in their infancy. Limiting factors include the achievement of uniform dispersion, and, additionally, the morphology, size, and chemistry of lignin nanoparticles, which need to be the prerequisites for the high-value-added and multifield applications of lignin. The structural and functional properties of lignin are the key points for its conversion into aromatics, polymers, and high-performance materials. Regarding this issue, we should emphasize that the production of thermoplastics or thermosetting polymers from the depolymerized lignin still involves the use of several chemicals, often causing a high environmental impact for the synthesis of bio-based and/or biodegradable polymers. Consequently, methods to chemically functionalize lignin to useful products without the use of expensive reagents or complicated synthetic routes must still be identified, with the main aim of competing with commercial commodity polymeric materials. The success of synthesizing thermoplastics and thermosetting from lignin opens up, on the other hand, new avenues to incorporate lignin as a component of value-added polymers while utilizing renewable resources. To balance the negative impact of chemically treated lignin and lignin derivatives to produce bio-based matrices, the use of lignin nanoparticles (obtained applying green process) in lignin-derived polymeric nanocomposites can be considered as a valid strategy to guarantee multifunctionality in sustainable biopolymeric matrices.

**Author Contributions:** Conceptualization, D.P.; methodology, D.P. and F.L; writing—original draft preparation, D.P. and F.L.; writing—review and editing, D.P. and F.L; supervision, L.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


### *Review* **Sustainable Materials Containing Biochar Particles: A Review**

**Giulia Infurna \* , Gabriele Caruso and Nadka Tz. Dintcheva \***

Dipartimento di Ingegneria, Università degli Studi di Palermo, Edificio 6, 90128 Palermo, Italy **\*** Correspondence: giulia.infurna@unipa.it (G.I.); nadka.dintcheva@unipa.it (N.T.D.)

**Abstract:** The conversion of polymer waste, food waste, and biomasses through thermochemical decomposition to fuels, syngas, and solid phase, named char/biochar particles, gives a second life to these waste materials, and this process has been widely investigated in the last two decades. The main thermochemical decomposition processes that have been explored are slow, fast, and flash pyrolysis, torrefaction, gasification, and hydrothermal liquefaction, which produce char/biochar particles that differ in their chemical and physical properties, i.e., their carbon-content, CHNOS compositions, porosity, and adsorption ability. Currently, the main proposed applications of the char/biochar particles are in the agricultural sector as fertilizers for soil retirement and water treatment, as well as use as high adsorption particles. Therefore, according to recently published papers, char/biochar particles could be successfully considered for the formulation of sustainable polymer and biopolymerbased composites. Additionally, in the last decade, these particles have also been proposed as suitable fillers for asphalts. Based on these findings, the current review gives a critical overview that highlights the advantages in using these novel particles as suitable additives and fillers, and at the same time, it shows some drawbacks in their use. Adding char/biochar particles in polymers and biopolymers significantly increases their elastic modulus, tensile strength, and flame and oxygen resistance, although composite ductility is significantly penalized. Unfortunately, due to the dark color of the char/biochar particles, all composites show brown-black coloration, and this issue limits the applications.

**Keywords:** biochar particles; sustainable materials; polymers; biopolymers; asphalts

#### **1. Introduction**

Nowadays, the conversion of polymer waste, food waste, and biomass aimed at reducing their impact on the environment gives them a second life, and changing from a linear economy to a circular economy is being widely investigated [1,2]. Different thermochemical decomposition processes leading to the recovery of fuels and residual solid phase have been taken into consideration, including those methods that could be profitable for some applications, such as for soil remediation, as additives, for use as synthetic carbonaceous particles, for the formulation of polymer and biopolymer-based composites, as additives for asphalts, etc.

Therefore, this review reports on the use of biochar particles, coming from different sources, for the formulation of composites and asphalts. See Figure 1 for more detail. The first part of the review deals with the considered methods to produce biochar particles and their main properties; the second and thirst parts are related to the formulation of polymer-based and biopolymer-based composites, respectively; and the fourth part is focused on the use of biochar particles as new additives for asphalts systems.

**Citation:** Infurna, G.; Caruso, G.; Dintcheva, N.T. Sustainable Materials Containing Biochar Particles: A Review. *Polymers* **2023**, *15*, 343. https://doi.org/10.3390/ polym15020343

Academic Editors: Jesús-María García-Martínez and Emilia P. Collar

Received: 30 November 2022 Revised: 30 December 2022 Accepted: 3 January 2023 Published: 9 January 2023

**Copyright:** © 2023 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/).

**Figure 1.** Different feedstocks used for the production of biochar particles and their adoption as suitable additives and fillers in polymers, biopolymers, and asphalts.

#### **2. Biochar Particles: Production, Characteristics, and Properties**

An opportunity to convert solid/food waste and biomass includes the thermochemical decomposition processes that are being used with increasing frequency. The main thermochemical decomposition processes explored are slow, fast, and flash pyrolysis, torrefaction, gasification, and hydrothermal liquefaction. All of these processes essentially generate: *i.* a solid phase, named char or biochar (in the case of biomass feedstock), *ii.* fuel, a mixed liquid phase of the heaviest hydrocarbon, *iii.* syngas, and a mixed gas phase of the lightest hydrocarbons are produced [3–6]. Of course, depending on the chemical composition of the treated materials (i.e., biomass, mixed waste, synthetic polymers), and depending on the operative condition (i.e., temperature process, heating rate, presence or absence of oxygen, residence time), the relative ratio between these three main products could change. Slow pyrolysis, conducted in the absence of oxygen, is characterized by slow heating rates and long residence times, as well as atmospheric pressure with an operating temperature that can vary from 350 to 800 ◦C; the necessary energy to pyrolyze the feedstock is usually provided internally by combusting a portion of the feedstock. The main product is a high-carbon solid char, and the coproducts are watery, low molecular weight liquid and a low energy combustible gas [7–10]. Fast pyrolysis, like slow pyrolysis, is conducted in the absence of oxygen with a temperature range between 400 and 600 ◦C. In contrast to slow pyrolysis, it uses a very high heating rate under a vacuum atmosphere, a short residence time, and the rapid quenching of vapor, since the main goal of this process is to produce bio-oil [7,11,12]. Flash pyrolysis is a batch process with an operative temperature range between 300 and 800 ◦C, and is similar to slow pyrolysis but with a high heating rate that uses moderate pressure (between 2 and 25 atm) to condense volatile elements and to promote secondary formation, since the aim of this process is to produce a biocarbon liquid fraction or biochar solid phase [13]. Torrefaction is a slow pyrolysis method with a

lower temperature range, between 200 and 300 ◦C, that mainly removes water and some volatiles from the biomass to produce a "brown" char that is easy to ground and is a stabilized and friable biomass. Gasification is characterized by a high process temperature (between 750 and 1800 ◦C) with a limited and controlled oxygen concentration (normally calculated as the amount relative to stoichiometric combustion) [14] and/or steam [15]; as the name suggests, the primary products are a non-condensable gas mixture, called syngas, which is essentially composed by the presence of CO, H2, with a smaller amount of carbon dioxide, methane, and other low molecular weight hydrocarbons [16]. Lastly, hydrothermal liquefaction is a process conducted in the presence of water, with a 250–450 ◦C temperature range under 100–300 bar; the main product of this process is called bio-crude, which is an energy-dense intermediate renewable source equivalent to oil that can be fractionated to a variety of liquid fuels [17]. Under this thermochemical process, the biomass is involved in depolymerization reactions (hydrolysis, dehydration, or decarboxylation), which produce insoluble products, such as bio-crude oil or bio-carbon, as well as volatile components (CO2, CO, H2 or CH4) or soluble organic substances (mainly acids or phenols). All these processes and their differences are summarized in Table 1.

**Table 1.** Thermochemical processes and their main differences in terms of operative conditions, time of reactions, and primary products.


The focus of this work is biochar, which is essentially a carbon-made material that can potentially be produced through any thermochemical process, as a primary or auxiliary coproduct, and from any feedstock. Feedstocks could include building materials, agricultural waste, forestry residues, municipal solid waste etc.

Biochar could be described as being divided into a "carbon" fraction, which includes carbon, hydrogen, and oxygen bonded together in different forms, and an ash inorganic fraction. For each thermochemical process employed, the temperature process, heating rate, and residence time affect the quality and the quantity of primary products and auxiliary co-products, and an operative parameter needs to be tailored to the feedstock, since the composition of potential biochar results may be affected by the feedstock characteristics. The primary analysis normally performed to characterize the feedstock is the operative temperature, and the relative char quality is the proximate analysis. This thermogravimetric analysis gives information about feedstock moisture content relative to the mass lost until 110 ◦C; volatile matter relative to the mass lost in an inert atmosphere at 950 ◦C; fixed carbon relative to mass lost in the air at 750 ◦C; and the remaining part relative to ash amount. Elemental analysis is normally employed to characterize the quality of char in terms of carbon content. This is a technique in which a sample is combusted at a very high temperature in a little chamber with an excess oxygen content, and the gasses relative to the combustion are trapped and, depending on the number of sensors available, it is possible to have, in terms of percentage in weight, information about carbon, hydrogen, nitrogen, CHN element amount (relative CO2, H2O, NO), sulfur content, CHNS, oxygen/sulfur contents, and CHNOS.

As discussed above, biochar is a carbon-rich material which can be prepared from various waste feedstock. Municipal solid waste and agricultural waste are only two examples of the many organic wastes that may be utilized as feedstock to create biochar. Sludge is a solid waste that must be treated and disposed of, since it is produced during the wastewater treatment process. However, because it includes abundant carbon and nutrients such as ammonia, it is a viable feedstock for the synthesis of biochar [18]. The high carbon content, high cation exchange capacity, vast surface area, and stable structure of biochar are only a few of its benefits [19].

In general, organic or synthetic material can be used as feedstock with different processes depending on the physiochemical characteristics and the product composition.

The value of a particular type of biomass depends on the chemical and physical properties of the molecules from which it is made. Biomass is the main feedstock used in the literature for BC production because of different advantageous reasons. First, for environmental reasons, biomass is more readily available in a renewable way, either through natural processes or as a product of human activities. Furthermore, when produced by sustainable means, biomass produces approximately the same amount of carbon during conversion as is taken up during plant growth, which reduces the CO2 amount in the atmosphere [20].

Biomass is mainly composed of three different organic compounds: cellulose, hemicellulose, and lignin, which give different mechanical and physiochemical properties to the woods. Cellulose makes up between 40% and 50% of the weight of dried wood and gives the biomass its strength [21]. Hetero polymers coexist with cellulose in plant cell walls to form hemicellulose. They contain several sugar monomers, including glucose, mannose, galactose, and xylose, and have lower molecular weights than glucose. Hemicellulose makes up anywhere from 20% to 35% of the bulk of dried wood [22]. The secondary cell wall of plants is made of lignin, which is a complex chemical compound. It is a kind of cross-linked resin that is amorphous, and it accounts for 15% to 30% of the mass of hardwoods. Depending on how much cellulose, hemicellulose, and lignin they contain, various biomass feedstocks have variable volatile matter concentrations and heating values, as well as different feedstock properties [23,24].

Biochar has received increasing attention due to its specific characteristics, such as high carbon content, cation exchange capacity, large specific surface area, and stable structure.

With different types of feedstocks, biochar has different physiochemical characteristics. The most typical processes for producing biochar are pyrolysis, gasification, and hydrothermal carbonization. Acid, alkali, oxidizing substances, metal ions, carbonaceous compounds, steam, and gas purging can all modify biochar. The environmental application fields determine the modification techniques to use.

The primary method used by biochar to remove organic and heavy metal contaminants is adsorption. The physiochemical characteristics of biochar, such as surface area, pore size distribution, functional groups, and cation exchange capacity, are strongly related to its adsorption ability, whereas physiochemical characteristics alter according to the production circumstances [25].

In general, biochar produced at high temperatures has a higher surface area and carbon content, mainly due to the increase in micro-pore volume caused by the removal of volatile organic compounds [26]. However, biochar yields decrease with temperature increases [27]. Therefore, an optimal strategy is required in terms of biochar yields and adsorption capacity. To sum up, the direct chemical composition of products and bioproducts is strictly connected to operative conditions (i.e., temperature, pressure and heating rate), which depend on the thermochemical process employed.

The physiochemical characteristics of biochar have been adjusted using metal ions, acids, alkalis, and oxidizing agents to make them better for various environmental processes [28].

Biochar has been widely employed in environmental applications, such as soil remediation, carbon sequestration, water treatment, and wastewater treatment because of its unique properties, which include high surface area, recalcitrant, and catalysis.

Common wastes, such as sludge and agricultural wastes, are produced in great quantities in the world. Sludge production alone reached 6.25 million tons in 2013 in China [29].

Converting common household wastes into biochar could be an option for environmental sustainability. Different feedstock has different proportions of element composition, and thus exhibits different properties, so the biochar derived from different feedstocks has various performances. The ways to deal with these wastes are directly linked to the impact they have on the environment.

Distinct feedstocks show varied qualities due to the different proportions of their elemental makeups, and, as a result, the biochar produced from those feedstocks performs differently. For instance, the pH (9.5) and potassium content (961 mg kg<sup>−</sup>1) of straw-derived biochar were greater than those of wood biochar (349 mg kg−1) [30]. Additionally, the biochar made from straw had more volatile material than non-volatile material, which is easier to remove during the pyrolysis process. Therefore, the high volatile component of the feedstock may contribute to poor biochar yields. Additionally, the content of pig and cow manures differed in terms of proportions [31]. Moreover, volatile content can be more easily removed than non-volatile content during pyrolysis. Therefore, the feedstock containing a high content of volatile content may result in low yields of biochar.

The type of feedstock has a significant effect on the physiochemical properties of biochar [32]. Therefore, the content of carbon in biochar is an important parameter, and different feedstocks can be converted into char using thermochemical decomposition processes, as was already described before (see Table 2).




**Table 2.** *Cont.*

Due to different compositions (carbon with the presence of alkali metals, e.g., Li, Na, and K or alkaline metals, e.g., Ca, Mg, and Ba metals) depending on the nature of the feedstock, biochar can have versatile properties leading to many applications, including bioenergy (co-gasification, co-firing, and combustion), chemical use (as a catalyst or catalyst support), agronomy (regarding water retention, plant nutrients, or soil conditioner), pharmacological use (regarding the adsorption of drugs and toxins), environment remediation (regarding carbon sequestration and the sorption of pollutants), and as biomaterials for the production of bio-composites, fuel cells, and photovoltaic plants [46].

#### **3. Polymer-Based Composites Containing BCp**

As was already explained above, the final properties of BC particles depend on several factors, such as the nature of feedstock used to produce BC particles, the type of process employed, and the relative operative condition. The final content of fixed carbon and ash (which involves the milling and sieving process used to control the dimension of the final particles), their surface area, and pore volume consequently determine the final properties of the composites, and this also depends on the polymer matrix employed in terms of interfacial adhesion, dispersion, thermal and mechanical stability, and ageing protection efficiency. In order to assess how the pyrolysis temperature and type of feedstock could determine a difference in the final properties of composites, Das et al. [47] added biochar particles as a co-filler for producing wood plastic polypropylene-based composites. The authors also pyrolyzed different feedstocks (landfill pine sawdust, sewage sludge, and poultry litter) at different temperatures, with the aim of identifying a routing process for waste employing. In fact, when keeping the landfill pine wood weight percentage constant at 30 wt.%, 24 wt.% of biochar particles were obtained through the following procedures: *i.* pine wood was pyrolyzed through a two-step parallel reactor chamber with a retention time of 60 min at 900 ◦C for a high-temperature activation regime (TCP 900); *ii.* using the same reactor, a torrefaction regime reached pyrolyzing at 350 ◦C (TCP 350); *iii.* the same feedstock was pyrolyzed by means of an auger reactor with a retention time of 10 min

at 470 ◦C (PSD470); *iv.* the same pilot plant was used for the same feedstock with the same retention time at 420 ◦C (PSD420); *v.* sewage sludge biochar was produced with a pyrolysis temperature of 680 ◦C and a retention time of 10 min; *vi.* biochar from chicken litter was produced at 450 ◦C and 20 min of residence time. The fixed biochar concentration of 24 wt.% was determined thanks to a previous work of the same research group [48], in which BC produced from pine wood was added to the PP matrix at different loadings that ranged from 6 to 30 wt.%. In that study, a BC content of 24 wt.% showed a general improvement in tensile and flexural strengths, as well as in the young modulus of the final composite. Taking into account these results, it was found that an increase in tensile strength and the moduli was strongly related to the increase in surface area. Moreover, the presence of residual minerals (i.e., CACO3 found in BC from chicken litter and the relative ash content) increased the impact strength of the composite and exhibited a lower heat release rate under the combustion regime compared to other composites. These results can be explained by considering that inorganic particles generally hinder the diffusion of oxygen through the matter, which creates a physical barrier between the combustible and the oxidizing agent, thus allowing the BC composites a possibility of being used in a flame-retardant field. Additionally, thanks to maleic anhydride grafted polypropylene/maleated anhydride polypropylene (MAPP) being used as a coupling agent, a general good dispersion of BC particles and an infiltration of polypropylene into biochar pores was observed for all composites. Without wood presence for the PP-based composites, a flameretardant ability in pine wood biochar was established through a study by Das et al. [49], in which various BC loads, from 0 to 35 wt.% of the composites, exhibited increasingly stable compact char structures during controlled combustion tests that hid the O2 diffusion in a polypropylene matrix. Moreover, the addition of char significantly reduced peak heat release and smoke production. The increase in flame retardant ability conferred by the presence of biochar particles in wood polypropylene composites was also explored in the presence of conventional inorganic flame retardants, such as magnesium hydroxide and ammonium polyphosphates [50,51]. On the other hand, in the presence of biochar, the two flame retardants particles were trapped into BC pores instead of in polypropylene with a final reduction in PP flow during processing and consequent reduction in interfacial adhesion and relative mechanical stability. Furthermore, the addition of biochar particles in wood polypropylene composites bestowed the composite with resistance toward water. This result remained valid without exceeding a threshold concentration, up to which the composites became more susceptible to water. In addition, it has been found that high pyrolysis temperatures generate more hydrophilic particles, due to absorption through the capillary action of pores [52]. Moreover, the reason for adding biochar particles to a Wood Polypropylene Composite (WPC) is that WPC usually suffers of thermal instability and thickness swelling, due to the high hydrophilic behavior of wood dust. Ayrilmis et al. [53] progressively reduced wood dust concentrations from 60 to 0 wt.% while respectively increasing commercial *Quercus* char flour concentrations from 0 to 60 wt.%, wherein thickness swelling was reduced by the 50% after 30 days when wood dust was completely substituted by BC dust, which increased the global dimensional stability. The same stabilization behavior was established for the water absorption after 30 days of WPC substation with BC dust, which decreased from ca. 21% for the PP/wood composite to ca. 15% for the PP/char composites. The same result of dimensional stabilization and global improvement of resistance to thermal degradation by adding BC particles to WPC was also confirmed through a study conducted by DeVallance et al. [54,55]. A variation in final composite properties related to pyrolysis temperature was also highlighted in a polypropylene/poly (octene-ethylene) copolymer (POE) (70/30 wt.%) blend [56], in which 10 and 20 wt.% of high-temperature pyrolyzed biocarbon (HTBioC) and low temperature pyrolyzed biocarbon (LTBioC) from *Miscanthus* were added. The HTBioC showed a lower presence of functional groups on the char surface, as well as a higher porosity with a relative increase in surface area that promoted better compatibility to the polymer blend, while also having a significantly better stiffness–toughness balance in the composite compared to the LTBioC.

Giorcelli et al. [57] also studied the relationship between the pyrolysis temperature and the electrical conductivity of biochar particles with the intention to use biochar particles as a filler in epoxy resin-based conductive composites. The residues of *Miscanthus* were pyrolyzed at 650, 700, and 750 ◦C and activated by CO2 to increase surface area. The residues were characterized, and, then, 20 wt.% of the particles were added into epoxy resins for electrical characterization. As was already shown by other studies, it was found that an increase in pyrolysis temperature corresponded to and increase in carbon content with a corresponding reduction in other elements (i.e., O, Mg, Si, K, Ca) [46], and the ratio between the disordered and graphitized structure of the carbon structure increased with the increase in pyrolysis temperature. All of these properties led to an increase in the conductivity of biochar particles as a function of pyrolysis temperature, with a consequent increase in electrical performance for composites obtained with the addition of particles produced at higher pyrolysis temperatures. In the same lox viscosity epoxy resin LPL (Cores Ocean), two different biochars obtained by pyrolyzing Maple tree waste at low and high temperatures (600 and 1000 ◦C) were added at different weight percentages in order to improve the mechanical properties of the resin [58]. It was observed that the addition of a small amount of carbon fillers, lower than 2 wt.%, increased the load bearing capacity of the epoxy matrix, while also modifying the mechanical properties of the polymer matrix; on the other hand, a concentration equal to or higher to 2 wt.% transformed the pristine epoxy resin from brittle to a ductile composite, which was different from what was already seen for polypropylene-based composites. The optimum filler level depends on the type of polymer, the pyrolysis temperature, the type of feedstock used for biochar production, and the presence of other additives in composite production. When adding a curing agent (i.e., cycloaliphatic polyamine) and an embedding medium during epoxy-based composite processing, it is possible to increase the filler content above a critical level that normally lowers tensile strength. This configuration was found at a critical level of chars obtained from natural substances equal to 25 wt.%, and, in composites with plastic waste char, the critical level appeared to be reasonably low, equal to 15 wt.% [59]. Nevertheless, plastic waste char, or PWC (made from the pyrolysis of polyethylene terephthalate, PET), due to the terephthalic acid in the char structure, increased the global conductivity of a polymer composite [60].

Another way to activate biochar particles has been explored by Zhang et al. [61] by means of the impregnation of biomass feedstock before the carbonization process in an H3PO4 solution. In that work, biochar from rice husk, obtained pyrolyzing at 600 ◦C, was compared with activated biochar by varying the H3PO4 concentration in the activation solution. Generally, the activation of BC improved the thermal stability of the resulting composites, but a different concentration of activating agent affected the characteristic of the biochar in terms of chemical and morphological structure. In fact, a low concentration of H3PO4 improved the porous structure, which improved the resulting mechanical properties, thanks to better adhesion between particles and the HDPE polymer matrix, including flexural properties, rigidity elasticity, creep resistance, and anti-stress relaxation. On the other hand, a high concentration of H3PO4 in the activation solution generated fouling in the porous structure, which reduced all mechanical properties.

To further improve electrical properties, a carbonization process of charcoal from three different biomasses [62] has been performed with high fill ultra-high molecular weight polyethylene/linear low density poly ethylene UHMWPE/LLDPE [63]. For example, starting with charcoal coming from bamboo pyrolysis, which was further carbonized at 1100 ◦C in a muffle furnace in the absence of air, particles with irregular shapes have been obtained with a global transformation of their amorphous structure into a graphite-like structure with a higher crystallinity grade. This result was simultaneously confirmed by an increase in the three diffraction peaks at around 24.6◦, 43.7◦, and 50.1◦, which were associated with the C (002), C (100), and C (004) diffractions, respectively, of the graphitic structure through XRD analysis and with an increase in the intensity ratio of the D-to-G peak obtained through Raman spectroscopy, which revealed a defective graphitic structure

and turbostratic crystallites in the BC1100 particles. Moreover, the high temperature carbonization generated a high specific surface, due to the creation of a nanoporous structure. These particles were added into a UHMWPE blended with a LLDPE as a flow accelerator for reducing the melt viscosity of the UHMDPE and improving the processability of the composite and final particle dispersions. The highly filled composites (with a carbon load up to 80 wt.%) showed excellent electromagnetic interference shielding performance, and one of the highest values reported for conductive polymer composites was found; in fact, at maximum biochar concentration, a conductivity of 107.6 S/m was found. The same further-carbonized bamboo charcoal particles were used with high fill UHMWPE to produce scaffolds for cells proliferation. In that study [64], the raw bamboo charcoal and bamboo charcoal carbonized at 800 ◦C and 1100 ◦C were over pyrolyzed in a muffle furnace in the absence of air. Additionally, in that case, thanks to high temperature and the relative development of the surface into a nanoporous structure, crystallinity, hardness, and thermal stability were found to be higher for the biochar carbonized at the higher temperature. On the other hand, for better biocompatibility, achieving a high temperature of pyrolysis is not useful, because, at low temperature, biochar enables the composite to exhibit better hydrophilicity and higher specific surface energy, which promotes protein adhesion and cell proliferation. Moreover, globally, the composite obtained by adding to the UHMWPE showed good mechanical properties and friction performance that make it appropriate for use in orthopedic applications.

Arrigo et al. [65] performed an extra carbonization process of a torrefied coffee powder was into a tubular furnace, and the waste was pyrolyzed at 700 ◦C for 1 h under a nitrogen atmosphere, which was then added to high density polyethylene, HDPE, to understand the interaction between HDPE and BC from spent coffee grounds, as well as how the filler content influenced the rheological and thermal behavior of the resulting composites. The authors subjected BC/HDPE composites with different BC loads (up to 7.5 wt.%) to SEM analysis and rheological characterization that employed different flow fields, including linear and non-linear dynamic shear flow, which resulted in clear confinement of the polymer chains onto the surface of particles and into the porous structure of particles, as well as a pseudo solid-like behavior of the BC/HDPE composites due to the formation of a network.

To reduce waste for environmental purposes, Kane et al. [66] recently compared recycled high-density polyethylene, rHDPE, with and without biochar to look at both improving mechanical properties and environmental impact. From a mechanical point of view, the tensile behavior of the rHDPE was significantly altered by the addition of biochar particles coming from wood forestry residues, which increased the strength and stiffness through the global increase of crystallinity of the rHDPE through the reinforcing action of the polymer matrix, and, thanks to a good interface adhesion, that led to a polymer interlocking with the porous structure of the biochar. Moreover, by means of life cycle assessment, it has been noticed that the addition of biochar as a filler reduces the global amount of plastic spent to produce a product, which of course provides a benefit in terms of global warming potential when referring to the CO2 emitted for plastic production. It has been calculated that rHDPE reached a 0 kg CO2 equivalent by adding less than 40 wt.% of biochar particles, which obtained a composite with a similar strength and stiffness obtained by adding 40–50 wt.% of biochar particles to virgin HDPE [67].

Another way to reduce the amount of polyolefin waste and reduce the amount of virgin polyolefin employed in the industrial field has been addressed in a study conducted by Idress et al. [68], in which a recycled poly-ethylene-terephthalate rPET-based composite was produced by adding biochar. The biochar employed in that work came from the high temperature pyrolysis (1100 ◦C) of PET waste under an autogenic pressure of ca. 150 bar. In that study, the researchers were able to extrudate recycled PET and PET/BC composites, which highlighted that the incorporation of biochar enhanced the mechanical properties and provided the PET with thermal properties, which suggests that BC could supply the necessity for commercial graphene materials in polymer composites. As was noticed with

other polyolefins, the responsibility for the improvement in mechanical properties must be referred to the high surface porosity of BC particles and the high affinity between BC particles and the polymer matrix. The improvement in thermal properties must be referred to the known barrier effect of BC.

#### **4. Bio-Polymer-Based Composites Containing BCp**

Bioplastics should be intended as polymers that meet any of two criteria: the polymer is bio-based and/or biodegradable [69]. In the context of a sustainable and circular economy, the recovery of bio-waste and the addition of them in biopolymers, intended as bio-based and biodegradable, for sustainable bio-composites formulation is a challenging issue. Among biopolymer-based composites containing biochar particles, the literature reports a significant number of studies. One of the most biochar-added polyester matrixes is polylactic acid (PLA), which suffers from poor thermal stability and high brittleness that reduces its employment in many fields, i.e., textile, biomedicine, and food packaging [70]. Briefly, the use of BC in PLA can lead to growth in the PLA market, thanks to a global improvement in the mechanical stability of this polymer [33,71,72]. Kane et al. [73] investigated BC-added PLA composites and compared them to high density polyethylene HDPE composites. In contrast to HDPE composites, for PLA/BC composites, the work highlighted an impact of BC in thermal degradation behavior, which was shown through a decrease in onset degradation temperature and a global reduction in melt viscosity of the PLA, which was probably due to the presence of an inorganic element of the BC surface being responsible for catalyzing PLA thermal decomposition. The same behavior has been found by Arrigo et al. [74], in which BC particles derived from spent ground coffee were added in the PLA matrix by processing the composites through melt mixing and solvent casting methods. It was found that the PLA rheological behavior underwent significant alteration when the composites were obtained by melt processing. In fact, the authors reported (see Figure 2) a progressive increase in melt viscosity in composites obtained by solvent casting and a progressive decrease in melt viscosity in composites obtained by melt mixing, as the BC content increased, which, in the last case, suggested a severe reduction in polymer molar mass, due to thermal degradation [75], and PLA preservation when the processing was carried out at room temperature by means of solvent casting. In any case, a strong polymer-filler or filler-filler interaction has been found, which was demonstrated by the appearance of a yield stress behavior.

**Figure 2.** Complex viscosity as a function of frequency for neat poly(lactic acid) (PLA) and biochar (BC)-containing composites obtained through melt mixing (MM) (**A**) and solvent casting (SC) (**B**) [74].

A significant reduction in the molecular mass of poly(3-hydroxybutyrate) (PHB) processed at high temperatures with the addition of biochar particles has been demonstrated by Haeldermans et al. [76]. In their study, different PHB/char with varying BC loads (from 20 wt.% to 50 wt.%) were produced, and their formulation was compared with PHB/thermoplastic starch (TPS)/BC composites. Despite having the best biodegradability

compared to other biopolymers, it is well known in the literature that PHB suffers from a significant reduction in molecular weight after processing, and this limits its application, due to a really small operational processing window [77], e.g., an unprocessed PHB-Mw of 611 Kg/mol and a melt-processed PHB-Mw of ca. 463 Kg/mol [76]. Regrettably, increasing the amount of biochar particles from 20 to 50 wt.% further reduced the PHB-Mw, which achieved a reduction to 218 Kg/mol for 50 wt.% of BC and reduced the global thermal stability of the BC bio-composites. Consistent with molecular weight reduction, a reduction in thermal properties was found, such as a decrease in melting point with a decrease in molecular weight [78]. Thanks to the presence of thermoplastic starch in bio-composites, the decrease in Mw is more gradual and controlled, and molecular weight analysis has shown that, at low BC loads, TPS can act as an intermediator between PHB and PHB by controlling the reduction in molecular weight.

In contrast, the addition of BC particles as a filler in an Ecovio commercial polymer blend containing poly(1,4-butylene adiphat-co-1,4-butylene terephthalate), PBAT, 47 mol% of an aromatic segment, and PLA, 25 mol%, significant increased the application field of BC particles in the biopolymers matrix [79]. In fact, a significant reduction in the resistivity of obtained bio-composites was found as the BC load increased by up to 30 wt.%, which suggests employment of the composites in equipment elements in laboratories for precise measurement, or as an antistatic agent in the packaging industry. Moreover, thermal stability has not been affected by the presence of BC compared to the Ecovio polymer matrix. Moreover, thanks to a global improvement of modulus shown in DMA analysis for all temperature ranges (−50 to 120 ◦C), mechanical properties were higher for composites with respect to the neat matrix, which suggested better mechanical stability.

Regarding the PBAT matrix containing biochar particles, several works have been published. Botta et al. [80] investigated the properties and the filmability of PBAT-based materials that were added to commercial biochar powder used in the food industry that was formed from birch and beech wood pyrolysis. The team performed a preliminary investigation of the prepared PBAT/BC composites by melt mixing with BC loads from 5 wt.% to 20 wt.% of commercial BC, which showed a uniform filler dispersion and a good adhesion within the selected biopolymer matrix, which led to an increase in global mechanical properties. Moreover, DSC analysis clarified how the BC did not influence the PBAT chain structure, which remained almost amorphous despite filler addition, even with the increase in Tm as the filler content increased and revealed compatibility between the filler and matrix. Instead, the rheological behavior of PBAT-based composite results were affected by the presence and the increase in carbonaceous filler, which resulted in a relative increase in melt viscosity, in all ranges of frequency, and suggested an influence of embedded filler on the long-range and short-range dynamics of polymer chains, especially when the BC load was equal to 20 wt.%. At that carbon load, the PBAT underwent a dramatic reduction in its intrinsic ductility and a significant decrease in the break–stretching ratio (BSR), which resulted in a composite with no filmability properties. The same rheological and mechanical behavior has been found by Infurna et al. [37], in which agricultural carob waste was pyrolyzed at three different temperatures (BC280, BC340, and BC400 respectively pyrolyzed at 280, 340, and 400 ◦C) and then added to PBAT at two different concentrations, i.e., 10 and 20 wt.%. In their work, an ageing protection assessment was performed on both the pristine particles and on the BC composites. First, the authors characterized the radical scavenging efficiency by means of 1,1-diphenyl-2-pycryl (DPPH) free radical analysis, in which the three different BC particles were added at constant loads to a methanol solution of DPPH, a stable free radical, and they monitored the disappearance of the free radical UV absorption peak at 517 nm. From their analysis, thanks to residual functional groups on the BC surface after 24 h, the particles obtained at a lower pyrolysis temperature achieved about 100% radical scavenging efficiency, despite the values obtained at 400 ◦C, with higher scavenging kinetics of the BC280. The monitoring of the DPPH UV absorption peak was also performed by increasing the amount of BC in the solution. In that case, it was demonstrated that, from a limited concentration onwards after 24 h, the radical

scavenging efficiency results were comparable between the three different particles. This result is consistent with what has been found in the photooxidation assay of biopolymer composites, as partly shown in Figure 3, in which the variation in mechanical properties as a function of irradiation time was monitored while also extrapolating the half time as the time at which the elongation at break was half of the initial one.

**Figure 3.** The trend of dimension elongation at break with (**a**) 10 wt.% and (**b**) 20 wt.% of filler content in the PBAT matrix [37].

A significant reduction in the ductility of pristine PBAT is shown, which achieved a half time of 14 h. In Figure 3a, the same trend of the DPPH assay is shown, in which the lower the pyrolysis temperature was of the obtained BC particles, the higher the concentration of functional groups on the surface were able to scavenge free radicals from the accelerating weathering test, which resulted in a higher shown resistance of the bio-composites. In conclusion, it was enough to increase the BC load from 10 wt.% to 20 wt.% to lead to a comparable ageing resistance for all bio-composites, and the same results have been found by ATR-FTIR analysis as a function of irradiation time.

Polyvinyl alcohol (PVA)/corn starch/BC bio-composites were successfully formulated by means of the solvent casing method in the presence of citric acid and glutaraldehyde, which was added for fixative effect before the casting period [81]. The interaction between the biopolymer blend and BC particles significantly affected the degradation path of the PVA/starch composites. In fact, a significant decrease in the narrowing of the peak relative to the hydroxy band with the increments of BC load was noticeable. The authors attributed this phenomenon to the good compatibility of the PVA, starch, and BC [82]. As had happened for PLA and PHB-based composites, the global thermal stability of the bio-composites was lower when BC was added to the blend.

A noticeable improvement in mechanical properties has been obtained by introducing a suitable concentration of biochar particles in eco-friendly bio-composites manufactured by a green epoxy matrix reinforced with short agave fibers for replacing synthetic materials in structural applications [83]. In that case, with the optimum amount of BC particles (in this case found to be equal to 2 wt.%) added by means of the synergic effect of short fiber and biochar particles, a better adhesion has been achieved between the epoxy matrix and fiber, which was demonstrated by the fiber pull-out test. This aspect involved an increase in the global Young's modulus and tensile strength, as well as an increase in fatigue performance with an increase in fatigue strength by about 67% and fatigue lifetime by at least three orders of magnitude. This remarkable enhancement of mechanical performance increases the possibility of employing a green epoxy matrix fiber reinforced for structural and semi-structural applications, especially in automotive and naval applications.

Green composites have been formulated by adding biochar particles in partially biobased polymers or bio-based polymers that are non-biodegradable. Nagarajan et al. [6] performed a study of the varying particle size distributions of biochar particles produced from the low temperature slow pyrolysis process of *Miscanthus* fibers. After pyrolysis, size-fractionation of the BC was performed with sieves having different openings,

e.g., 300, 212, 150, 125, 75, and 20 μm. The different size-fractioned BCs were added to a poly(trimethylene terephthalate) (PTT) 70/poly(lactic acid) (PLA) 30- ethylene-methyl acrylate-glycidyl methacrylate terpolymer (EMAGMA) polymer blend (85–15), using, in some formulation, an epoxy-functionalized chain extender (CE). A good range of particle size distribution, combined with the presence of the chain extender, helped to obtain a morphology with a better dispersion of the blend components, i.e., particle size distributions of 75–20 μm. Biochar particles under 20 μm in size diameter stabilized the blend morphologies, which showed a coalescence of PLA-EMAGMA particles that turned in smaller and finer morphologies in the presence of CE. This evaluation obtained from SEM image observations was consistent with rheological tests and mechanical analysis, which showed that, with an appropriate BC particle size and shape, morphologies and properties can be tailored to achieve desired properties with solid cost reduction. The partially bio-based PTT was combined with 20 wt.% high temperature bio-carbon from peanut hull pyrolysis, which resulted in superior mechanical performance that could be optimized for non-structural automotive components or electrical housing applications [84]. Also in this case, particle size distribution and particle size of the original biomass can play a crucial role in the resulting biochar, impact the concentration of volatiles and bio-oil from the pyrolysis process [85,86] and, as expected in bio-composite properties, call for a milling process before pyrolysis has been conducted [84]. The addition of peanut hull biochar, with its sheet-like surface morphology with high graphitic carbon content and its relatively low electrical conductivity, can contribute to the improved thermal stability of PTT-based green composites by increasing both flexural and tensile moduli, which suggest nonstructural and anti-static applications.

#### **5. Asphalts Composites Containing BCP**

Using biochar particles as an asphalt binder modifying filler is going to become a new and interesting application field. In fact, nowadays in the construction sector, it is already used as a substitute for cement in mortar or concrete, thanks to its help with accelerating cement hydrating [87] and due to global CO2 mitigation [88]. The new approach regards the addition of asphalt binders as an ageing protector, which are an essential component of asphalt concrete in addition to being the heaviest coproduct of the petroleum refining system, after distillation, to obtain fuels and lubricants [89]. The oxidation of asphalt binders is an inevitable phenomenon that plays an enormous role in the deterioration of asphalt binders. In fact, the life expectancy of usual binders is susceptible to ultraviolet (UV) rays, which cause faster oxidation of asphalts, with a sensitive reduction in rheological characteristics and a loss of rutting properties, which can lead to pavement distress [90]. Walters et al. [91] investigated the impact of added biochar particles (coming from a thermochemical process used to convert swine manure in bio-oil) or nano-clay (Cloesite 30B) on the rheological properties and ageing susceptibility of asphalt binder, and compared the results with a control asphalt (PG 64–22) binder. The introduction of BC to the asphalt binder led to a reduction in asphalt temperature susceptibility, and, regarding the shear susceptibility, its sensitivity decreased by adding 10 wt.% of BC to PG 64–22, which achieved a lower value than control asphalt. Contrary to the addition of BC, the addition of nano-clay generated an impact on the layer spacing, which appeared to be responsible for enchaining the high temperature performance and ageing resistance of asphalt binders. In a second study by Walters et al. [90] a composite with both biochar (3 wt.%) particles and nano-clay (3 wt.%) was produced that resulted in a lower viscosity than the ones with only nano-clay, while the ageing susceptibility was improved significantly. This happens because biochar seems to have a role in the flow modifiers alleviating the stiffening effect of nano-clay, which help the nano-clay to disperse better in an asphalt binder.

Zhao et al. [92] evaluated the properties and performance of asphalt binders and mixtures by adding 5 wt.% and 10 wt.% of biochar from the fast pyrolysis of switchgrass for biofuel production. In their work, the authors found that biochar significantly increased the rutting resistances at high service temperatures of both asphalt binders and asphalt

mixtures. Moreover, the addition of 5 wt.% of biochar may be the optimum in modifying binders in terms of cracking resistance, while 10 wt.% shows little effect in comparison with 5 wt.%. Another study was conducted by the same research group by adding different BC particles to a commonly used asphalt binder (PG 64–22), which resulted in a different type of pyrolysis of switchgrass, while taking into account that BC results as a by-product for biofuel production, and comparing these lab-made BC particles with a commercially activated carbon [93]. In particular, the BC particles were produced by the following techniques: *i.* a microwave reactor in which switchgrass was mixed with silicon carbide to absorb enough microwaves, then the mixture was heated up until 500 ◦C in less than 1 min and the temperature was maintained for 15 min, and, then, after cooling down, the silicon carbide particles were sieved out to finally obtain BC particles with size diameters between 75 and 150 μm; *ii.* a tube furnace method in which feedstock was heated up until 400 or 500 ◦C with a heating rate of 15 ◦C/min to result in BC particles with diameters smaller than 75 μm for particles obtained at 400 ◦C and 500 ◦C, and for the ones obtained at 500 ◦C a diameter range between 75 and 150 μm was obtained as well; *iii.* an activated commercially available carbon was selected for comparison. The composites were characterized in terms of viscosity modification, ageing and fatigue resistance, and rutting properties of un-aged and aged composites. The addition of all bio-modifiers increased the viscosity of the asphalt binder at a high service temperature and exploited a positive effect in ageing resistance at a long time of UV exposition. Globally, except for particles with smaller diameters (<75 μm) produced at 400 ◦C that showed a positive effect on the specific properties or performance of the asphalt binders, the pyrolysis method appeared to have a negligible effect on the degree of modification. Another study published by Zhang et al. [94] focused on varying biochar loads and biochar diameter distributions by comparing the biochar contribution on varying properties with the ones obtained by graphite adding. The biochar used in the work was obtained from waste wood resources, at a temperature ranging from 500 and 650 ◦C, through a pyrolysis plant able to heat at 10 ◦C/s. Then, BC particles were sieved to separate the different size ranges, and the size range of previous work was studied, i.e., between 75 and 150 μm and lower than 75 μm; the biochar contents in PG 58–28 control asphalt were 2 wt.%, 4 wt.%, and 8 wt.%, respectively. The flake graphite with a diameter lower than 75 μm and content of 4 wt.% was also added to PG 58–28 for comparison. As was expected, BC addition resulted in higher porosity and micro-structure compared with dense and smooth graphite. This aspect of course led to a larger and better adhesion interaction in the asphalt binders of BC than that of graphite, and, as a result, BC modified binders had better high-temperature rutting resistance and better anti-ageing properties, especially for the BC-modified binder with a lower BC diameter at a higher content.

In conclusion, it seems that the addition of BC to asphalt seems to increase the thermal resistance during asphalt preparation and their oxygen resistance in service. This is another important result of asphalt reducing viscosity during processing, which helps the dispersion of other asphalts constituents in order to obtain a high-performance pavement.

#### **6. Conclusions**

Char/biochar particles could be considered as a new kind of sustainable particle created from their "waste" feedstocks. Specifically, when also considering the circular principles, the conversion of polymer waste, food waste, and biomasses, through thermal treatment at high temperatures, gives an appropriate second life for these waste materials. Produced biochar particles differ in their physical and chemical properties, e.g., CHNOS compositions, porosity, and adsorption ability, because of the implementation of different thermal treatments, such as slow, fast, and flash pyrolysis; torrefaction, gasification; and hydrothermal liquefaction. Currently proposed applications of the new char/biochar particles are mainly as follows: *i.* as fertilizers for soil retirement, *ii-* as high adsorption particles for water remediation, and *iii.* as suitable fillers for the formulation of polymer/biopolymerbased composites and to produce asphalts. Therefore, this review critically reports on the

current status of char/biochar particles in considering the main advantages and drawbacks in the use of these new particles as suitable fillers for polymers, biopolymers, and asphalts.

The char/biochar particles, being particles mainly composed of carbon atoms and having a large surface, are very useful to formulate composites with improved mechanical resistance, i.e., elastic modulus and tensile strength, as well as improved oxidative and photooxidative resistance, while also considering the particles' radicals scavenging abilities in comparison to the properties of neat matrices. Unfortunately, the main drawback is related to the particle color. Particularly, being black particles, their composites appear mainly with brawn-black coloration, and, obviously, this is a limitation for large scale esthetic applications.

**Author Contributions:** Conceptualization, G.I. and N.T.D.; methodology, G.I.; data curation, G.I. and G.C.; writing—original draft preparation, G.I. and G.C.; writing—review and editing, G.I. and N.T.D.; supervision, N.T.D.; funding acquisition, N.T.D. and G.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** G.I. would like to thank MIUR—Italy (Ministry of Education, University and Research of Italy) for having joined with support by CLEAN—PRIN-20174FSRZS\_002.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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