1.1 INTRODUCTION: BACKGROUND AND MOTIVATION
In the 20th century, developments in the field of composite materials were driven by the need for robust and highly reliable systems that could provide consistent performance. As a result, composites made from petroleum-based resins, reinforced with engineered fibers, such as glass and carbon, have dominated the industry due to their superior specific strength and stiffness compared to metallics (Akampumuza, 2016). Ceramics and metals which have been used in specific medical applications, such as orthopedic tissue replacement, suffer from being non-biodegradable and have strong limitations in terms of processability (Lopes, 2012). Reinforced polymer composites provide a high degree of structural tailoring with good processing control. However, polymer/synthetic fiber composites are also not biodegradable (Peter, 1998; Chen, 2002; Nair, 2007; Choudhury, 2021). Traditional management, i.e., incineration and land filling, of this kind of waste results in environmental pollution. Incineration generates greenhouse gases (GHGs) hazardous to human health, while landfilling contaminates soil and water, threatening the environment. Among synthetic polymers, thermoplastics offer the possibility of recycling, while thermosets are not easily recycled. Though recyclability is an advantageous property, the recycling potential is usually not fully exploited due to regional availability, contamination and deteriorating properties. Packaging materials, designed for single use, significantly contribute to disposal-related concerns (Davis, 2006; Akampumuza, 2016).
In automotive applications, 25â35% of components can neither be safely disposed of, nor recycled (Ghomi, 2021). This bears increasingly critical relevance as the kerb weight of cars has steadily increased over the years to accommodate customers' requests and stricter safety standards (Akampumuza, 2016). An automobile's mass accounts for 75% of its energy consumption, meaning fuel efficiency has also been negatively affected (Mohanty, 2002).
In order to tackle growing environmental concerns, governments around the globe, in cooperation with environmental bodies, have launched policies and set carbon emission targets. This has sparked an enormous research interest to find renewable material alternatives and provide solutions for companies seeking to meet environmental requirements (Mooney, 2009).
A new class of composite materials, so-called âgreen compositesâ, is investigated as an alternative to petroleum-based composite systems, owing to their renewability and sustainability. Green composites are created when two or more renewable source-based materials are combined. This can include the combination of biopolymers with natural fibers (Ben, 2007; Gejo, 2010).
Ideally, a biopolymer originates from a renewable biological source and provides end-of-life conversion into simpler compounds. This conversion characteristic is referred to as bio-degradability (Kargarzadeh, 2018). The resulting elements include nitrogen, sulfur and carbon, which can then be redistributed (Choudhury, 2021). Natural polymers such as cellulose, chitosan and starch share these characteristics and can be extracted from biomass directly. They are sometimes referred to as âagro-polymersâ. âBiopolyestersâ such as polylactic acid (PLA) and polycaprolactone (PCL) are synthetically obtained from biomass but possess biodegradability (Kargarzadeh, 2018; Choudhury, 2021).
Among emerging biopolymers such as polyamide 11 (PA11) and polyhydroxybutyrate (PHB), PLA has shown great commercial success with a world production of 240 kt/a; a figure which is said to double by 2023 (Birat, 2015; Getme, 2020).
1.2 PLA PRODUCTION AND CHARACTERISTICS
Lactic acid (2-hydroxypropionic acid), CH3âCHOHCOOH, is a hydroxycarboxylic acid and the monomer to PLA. Due to its wide occurrence, it is applied in a multitude of pharmaceutical, food, chemical, textile and leather products (Vickroy, 1985; John, 2009b). Due to the chirality of the lactic acid molecule (two molecule sets existing as non-superimposable mirror images), optically active (plane-polarized light is rotated when passing through the chiral molecules) L- (levorotatory, counterclockwise rotation) and D- (dextrorotatory, clockwise rotation)-enantiomers (mirror images) can be found. As a result, PLA appears as four different stereoisomers: isotactic poly-L-lactic acid (PLLA), isotactic poly-D-lactic acid (PDLA), atactic poly-D, L-lactic acid (PDLLA) and syndiotactic PDLLA (Lopes, 2012; Zhou, 2021). Each type of PLA exhibits different mechanical, thermal, degradable and barrier properties. These physical properties can be tailored to various different applications via the adaptation of processing parameters (temperature, shear, etc.), route of synthesis and the lactic acid source (Liu, 2014; Zhou, 2021). While lactic acid can be manufactured via chemical synthesis (hydrolysis of lactonitrile), 90% of total lactic acid production is fermentative (Adsul, 2007; Gupta, 2007). In order to produce lactic acid via microbial fermentation, bacteria are provided with either pure sugar (lactose, sucrose, glucose) or sugar-containing materials (sugarcane bagasse, molasses, whey) as a carbon source. In 2008, Brazil, the world's largest producer, supplied 130 million tons of bagasse generated from 650 million tons of sugarcane (Lopes, 2012). Fermentation also has the added advantage of low energy consumption, low production temperature and low cost of substrates in comparison to chemical synthesis with strong acids. This process also generates optically pure L- or D-lactic acid, which can produce PLLA and PDLA (Adsul, 2007; John, 2007; Lopes, 2012). Both PLA types are crystallizable, meaning they can show improved mechanical and thermal properties. PLLA, in particular, shows great biocompatibility and high crystallinity, resulting in excellent mechanical and thermal properties (Lopes, 2012; Zhou, 2021). Due to its high crystallinity, the degradation time of PLLA is enhanced compared to the optically inactive PDLLA, which is amorphous. In addition, inflammatory reactions can be triggered by high-crystalline fragments produced during the degradation process of PLLA in the body. In order to avoid the formation of potentially harmful crystalline fragments during degr...
