Recent Progress in Hybrid Biocomposites: Mechanical Properties, Water Absorption, and Flame Retardancy

materials Review Recent Progress in Hybrid Biocomposites: Mechanical Properties, Water Absorption, and Flame Retardancy Mohsen Bahrami * , Juana Abenojar and Miguel Ángel Martínez Materials Science and Engineering and Chemical Engineering Department, University Carlos III de Madrid, 28911 Leganes, Spain; [email protected] (J.A.); [email protected] (M.Á.M.) * Correspondence: [email protected]; Tel.: +34-91-6249401 Received: 4 October 2020; Accepted: 12 November 2020; Published: 15 November 2020 Abstract: Bio-based composites are reinforced polymeric materials in which one of the matrix and reinforcement components or both are from bio-based origins. The biocomposite industry has recently drawn great attention for diverse applications, from household articles to automobiles. This is owing to their low cost, biodegradability, being lightweight, availability, and environmental concerns over synthetic and nonrenewable materials derived from limited resources like fossil fuel. The focus has slowly shifted from traditional biocomposite systems, including thermoplastic polymers reinforced with natural fibers, to more advanced systems called hybrid biocomposites. Hybridization of bio-based fibers/matrices and synthetic ones offers a new strategy to overcome the shortcomings of purely natural fibers or matrices. By incorporating two or more reinforcement types into a single composite, it is possible to not only maintain the advantages of both types but also alleviate some disadvantages of one type of reinforcement by another one. This approach leads to improvement of the mechanical and physical properties of biocomposites for extensive applications. The present review article intends to provide a general overview of selecting the materials to manufacture hybrid biocomposite systems with improved strength properties, water, and burning resistance in recent years. Keywords: biocomposites; green polymers; hybrid composites; natural fibers; mechanical properties 1. Introduction Nowadays, global environmental safety, ecological concerns, recyclability, eco-efficiency, and economic factors are fundamental driving forces to increase the employment of bio-based materials [1,2]. The EU Bioeconomy Strategy [3] is one of the developed plans or policies to accelerate the UN Sustainable Development Goals (SDGs). This strategy focuses not only on shifting from fossil resources to renewable raw materials but also on recycling renewable bio-based raw materials and innovating in the production and consumption of materials [3]. In this regard, biocomposites are among the potential sectors for the bioeconomy. Renewable and sustainable biomaterials as an alternative for petroleum-based materials prevent generating carbon dioxide causing global warming. They would be a solution to alleviating the petroleum supplies’ uncertainty, which are about to decline shortly [4,5]. Within recent years, biopolymers or green polymers have received tremendous interest among biomaterials. The term biopolymer is a polymer of monomeric molecules derived from natural resources, including biological systems or living organisms. Moreover, biopolymers can be found in nature and daily life, such as natural rubber, starch, cotton, leather, wool, etc. [6–8]. Biopolymers are renewable, environmentally friendly, or/and biodegradable materials, which can be divided into two broad groups of natural and synthetic (Figure1) with the following origins [9–13]: Natural biopolymers extracted from biomass (e.g., polysaccharides, proteins) • Materials 2020, 13, 5145; doi:10.3390/ma13225145 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 48 materials, which can be divided into two broad groups of natural and synthetic (Figure 1) with the followingMaterials 2020 origins, 13, 5145 [9–13]: 2 of 46 Natural biopolymers extracted from biomass (e.g., polysaccharides, proteins) Synthetic biopolymers produced by a micro-organismmicro‐organism or bacteria (e.g., bacterial cellulose, • polyhydroxyvalerate, polyhydroxybutyrate) Synthetic biopolymers synthesized from renewable bio bio-based‐based monomers or mixed sources of • biomass andand petroleum petroleum (e.g., (e.g., polylactic polylactic acid, aliphatic acid, polyester,aliphatic aliphatic-aromaticpolyester, aliphatic copolyesters)‐aromatic Biodegradablecopolyesters) polymers that are derived from nonrenewable resources (petroleum sources) • (e.g.,Biodegradable polycaprolactone) polymers that are derived from nonrenewable resources (petroleum sources) (e.g., polycaprolactone) Figure 1. Origins of biopolymer materials [[©MB,©MB, JA, MAM_UC3M2020] MAM_UC3M2020].. Despite the advantages of biopolymer materials, they have somesome limitations compared to petroleum-basedpetroleum‐based materials,materials, such such as as relatively relatively low low stiff ness,stiffness, tensile tensile strength, strength, permeability, permeability, and thermal and thermalstability [stability8,9]. Therefore, [8,9]. Therefore, to modify to biopolymers’ modify biopolymers properties’ andproperties commercial and importance,commercial “biopolymerimportance, “biopolymercomposites” composites” are tremendously are tremen exploreddously by explored adding reinforcementby adding reinforcement materials withinmaterials the within micro the or micronanoscale or nanoscale [14–18]. These[14–18 composites]. These composites have a large have assortment a large asso ofrtment applications of applications in construction, in construction, medicine, medicine,electronics, electronics, packaging, packaging, and automotive and automotive sectors [19 –sectors24]. [19–24]. Composite materials systems are made from one or more discontinuousdiscontinuous reinforcing phases embedded in in a a continuous continuous matrix matrix phase. phase. Biopolymer Biopolymer composites composites or orbiocomposites biocomposites are are referred referred to as to compositesas composites in inwhich which at atleast least one one constituent constituent is is bio bio-based‐based or or biodegradable biodegradable [25]. [25]. The The focus focus of this review will be onon thethe fibrousfibrous compositescomposites ratherrather thanthan laminatelaminate andand particulateparticulate composites.composites. Fibrous composites cancan be be made made of naturalof natural/biofiber/biofiber and syntheticand synthetic fiber. Biocompositefiber. Biocomposite systems systems can be classified can be classifiedinto partial into biodegradable partial biodegradable and complete and complete biodegradable biodegradable biocomposites biocomposites on the basison the of basis the matrixof the matrixand fibers and (Figure fibers 2(Figure)[ 2,5,26 2)]. In[2,5,26]. the complete In the complete biodegradable biodegradable biocomposites, biocomposites, bio-fibers arebio‐ employedfibers are withemployed the matrices with the are matrices made ofare biodegradablemade of biodegradable polymers polymers such as renewable such as renewable biopolymer biopolymer matrices (e.g.,matrices cellulosic (e.g., cellulosic plastic, soy plasti plastic,c, soy starch plastic, plastic) starch or plast petro-basedic) or petro biodegradable‐based biodegradable polymer matricespolymer (e.g.,matrices aliphatic (e.g., co-polyester,aliphatic co polyesteramides).‐polyester, polyesteramides). However, in However, the partial in biodegradable the partial biocomposites,biodegradable biocomposites,bio-based matrices bio are‐based reinforced matrices with are synthetic reinforced fibers, with or non-biodegradable synthetic fibers, polymersor non‐biodegradabl matrices suche polymersas traditional matrices thermoplastic such as traditional polymers (e.g.,thermoplastic polypropylene, polymers polyethylene) (e.g., polypropylene, and thermoset polyethylene) polymers (e.g.,and thermoset epoxy, polyester) polymers are (e.g., reinforced epoxy, polyester) with bio-fibers are reinforced [5,26]. Itwith is worth bio‐fibers noting [5,26]. this It classification is worth noting of thisbiocomposites classification does of notbiocomposites estimate time does or amountnot estim ofate degradation, time or amount which of depends degradation, on many which parameters, depends onsuch many as environment, parameters, temperature,such as environment, and micro-organisms temperature, and and can micro be defined‐organisms by standard and can degradation be defined bytests. standard This classification degradation emphasizes tests. This classification that if both fibers emphasizes and matrix that beif both 100% fibers biodegradable and matrix materials, be 100% biodegradablea composite will materials, decompose a composite at the end will of its decompose life and completely at the end go of back its intolife and the naturalcompletely environment. go back intoOtherwise, the natural if one environment. of them be non-biodegradable,Otherwise, if one of them the composite be non‐biodegradable, doesn’t show the full composite biodegradability doesn’t properties.show full biodegradability Recently, the blending properties. of two Recently, or more the polymers blending reinforced of two or with more one polymers or more reinforced fibers that producewith one hybridor more composites fibers that materialsproduce hybrid has become composites an eff materialsective way has to become manipulate an effective biocomposites way to propertiesmanipulate and biocomposites maintain the properties balance among and maintain ecology-economy-technology. the balance among ecology The‐ presenteconomy study‐technology reviews. the recent progress of hybrid biocomposites in improving

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