1 Biodegradable polymer-based natural fiber composites

Acknowledgments: The authors thank the Federal University of ABC (UFABC) and the São Paulo Research Foundation (FAPESP) (2020/13703-3 and 2021/08296-2), and National Council for Scientific and Technological Development (305819/2017-8). The authors also thank the technical support of the Multiuser Experimental Center of UFABC (CEM-UFABC), CECS (UFABC), and REVALORES for assistance.

Abstract

The increase in global environmental impacts has driven the development of eco-friendly and biodegradable materials for the next product generation. The use of synthetic fibers presents ecological concerns, while natural fiber application as a reinforcing no-material in composite development has stood out as a viable alternative. Natural fibers present attractive properties such as biodegradability, renewability, high specific strength, and specific modulus that make this material suitable for composites development in different areas such as packaging, automotive, sports, aerospace, medical devices, and so on. However, some challenges still limit its broad application, including poor interfacial adhesion between matrix and natural fibers, poor compatibility between non-polar matrix and natural polar fiber, poor moisture absorption, fire resistance, low impact resistance, low thermal stability, and low durability, which need to be addressed before processing. Furthermore, during the manufacture of this type of composite, the processing conditions, fiber loading, and inherent fiber properties must be evaluated, as all these factors affect the properties of the composites and may promote defects in the products. The wide variation in the characteristics of composites based on natural fibers presents a significant challenge to understanding the properties of these systems and ways to optimize them. This review seeks to infer, analyze, and optimize the characteristics of composite materials reinforced with natural fibers in relation to different types and sources of natural fibers, processing, modification techniques, physical, and mechanical behavior toward sustainable products. Therefore, this review aims to better understand the behavior of green composites and promote the increased use of renewable resources in advanced materials.

1.1 Introduction

Global plastic production has grown considerably, with a market responsible for the annual production of approximately 450 million metric tons [1]. Plastic materials are composed of polymeric structures and combine good mechanical properties, inert characteristics, low density, and low cost, making them highly attractive in different applications, with commonly observed single-use applications. However, despite the convenience that these materials present, their inadvertent use has promoted unexpected consequences to the environment, raising global attention about these materials [2]. Due to their inert properties, the polymeric materials usually applied are nonbiodegradable, which imposes ineffective end-of-life (EoL) options associated with recent price rises of raw material that have considerably impacted the plastic industry [3].

Conventional synthetic polymers have become a disadvantage in the EoL phase, due to their accumulation in the environment for a long time [4]. Furthermore, due to the scarcity of petroleum resources and emerging environmental concerns, a synergistic motivation has been encouraged in the development of new products that conciliate sustainable properties and reduce petroleum dependence [5]. This process is also stimulated by the growing consumer awareness and concerns, disseminated by green marketing that promotes new guidelines on recycling, social influence, and changing cognitive values, leading consumers to prefer ecologically correct products [6].

Therefore, there is an urgency to develop ecologically correct technologies applied in various sectors to minimize environmental impacts among the polymeric materials [7]. Biopolymers are renewable and eco-friendly products that usually present biodegradable properties and can be used to replace petroleum-based polymers [8]. However, these materials usually present lower impact strength, tensile strength, permeability, and thermal stability when compared to conventional materials [9]. Thus, the best approach to improve the properties and commercial importance of biopolymers is the incorporation of micro or nano reinforcements [10], resulting in eco-friendly polymeric composites or biopolymer composites [11].

These biopolymer composites offer attractive opportunities for automotive applications [12], aeronautical [13], biomedical [14], and food packaging [1]. Compared to pristine polymers, these composites are lighter and have improved rheological, thermal, mechanical, and barrier biodegradation properties [15].

Abdillah e Charles [16] developed arrowroot starch/iota-carrageenan films at different concentrations. The results showed that the developed composites presented superior mechanical properties than pristine cassava starch film and improved barrier properties, tensile strength, and swelling properties [16]. Khoo, Chow, and Ismail [17] compared the properties of sugarcane bagasse fiber cellulose nanocrystals (SBFCNC) and microcrystalline cellulose-derived cellulose nanocrystals (MCC-CNC), evaluating the effects of both materials after incorporation into poly(lactic acid) (PLA). They observed that CNC types influence the tensile, thermal, and UV protection properties of PLA nanocomposites. In this study, the performance of SBFCNC was superior to that of MCC-CNC in terms of tensile properties and UV protection for PLA bio nanocomposites, demonstrating its potential as bio-based nanofiller for PLA [17]. Research in this field has indicated that biodegradable polymer composites are receiving improved applications due to their excellent mechanical properties, compatibility, and biodegradability [18, 19].

Thus, the main objective of this chapter is to highlight the main properties of biopolymers, along with the incorporation of ecologically correct micro or nano reinforcements, the modifications that reinforcements can undergo to enhance their properties accompanied by conventional and less common processing techniques for the manufacture of biocomposites. This chapter also discusses the main advanced applications of such biocomposites to meet the sustainable demands of advanced materials.

1.1.1 Polymers and their properties

A polymer is a macromolecule with high molecular weight that is composed of repeating structural units (called monomers) that, usually, are connected by covalent chemical bonds. Polymers can be classified as synthetic, semisynthetic and natural. Synthetic polymers are synthesized in the laboratory by a process called polymerization, in which the monomers react under a controlled environment originating the polymer chain. Some examples of synthetic polymers are nylon, polyethylene, polystyrene, synthetic rubber, polyvinyl chloride, and Teflon, among others. On the other hand, natural polymers are found in nature, generally from plants and from animal sources. The modification of natural polymers by chemical treatment to change their properties leads to semisynthetic polymers [20].

Nowadays, every industry aims to reduce fossil fuel-based materials, leading to a crescent need for environment-friendly sustainable materials and their development [21]. Most polymers...