friendly materials include waste paper sludge (WPS) and ink‐eliminated sludge (IES). These materials are used in high‐density polyethylene (HDPE)/wood flour composites [47]. Research shows that, the flexural properties of the HDPE/wood flour composites increased when WPS was added as an additive. This increase in flexural strength could be further increased by using a compatibilizer such as malleated anhydride grafted polyethylene (MAPE) in between the reinforcement and the matrix. Also, there occurs a decrease in the swelling and water absorption properties when this WPS and IES are added as fillers/additives. It has been found out that the overall mechanical properties of IES‐added composites show better results than that of the WPS‐added composites. The optimum loading condition of IES‐added composites is 60 wt%. These results show a promising outcome that these composites could be used for the preparation of biobased composites.
As these HDPE/wood flour composites find its applications in the exteriors of construction industries, their amount of durability could be calculated by investigating the damage caused due to sunlight and other weathering conditions [48]. Artificial weathering experiments could be carried out on the surface of these composites and by carefully investigating the chemical changes happening on the surface, help in finding out the durability. Further, it has been proved that, when carrying out tests such as X‐ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR), the amount of weathering damage to the wood flour‐reinforced HDPE is increased to 16 times faster than unreinforced plastic. The outcomes confirm that the degradation mechanism is triggered in the polymer chain due to the presence of carbonyl groups and leads to polymer chain scission once the surface oxidation is started. There is an overall reduction in the crystallinity of the polymer as the time increases and when the chain scission mechanism has been initiated.
2.5.2 Non‐biobased Fillers/Reinforcements with Biobased Polymers
The development of biobased composites, in which non‐biobased additives or fillers are reinforced with the biobased polymers has been restricted to the addition of reinforcements to PLAs [48, 49]. When PLA is reinforced with fillers such as hydroxyapatite fibers (HAF) (up to 60 wt%), there is a significant improvement in their Young's modulus under compression molding conditions at 180 °C. The maximum value of Young's modulus attained was about 10 GPa for PLA/HAF composites [49]. For heavily loaded composites, a brittle behavior was observed and a covalent fiber–matrix interaction was revealed [49]. When PLA has been reinforced with carbon nanotubes (CNTs), there occurs a significant rise in the Young's modulus and hardness of the composites [50]. Purified CNTs are able to better disperse in a PLA matrix than nonpurified CNTs, thus resulting in better thermomechanical properties.
2.5.3 Biobased Filler/Reinforcement and Biobased Polymer
Biobased composites with natural fiber reinforcements have several advantages over those with synthetic reinforcements and has become one of the most popular research topics of the decade. Further, natural polymers derived from renewable resources are ecofriendlier and less expensive compared to synthetic polymers. Hemp, keratin, pure cellulose fibers, and flax are used to produce composites with very good dampening properties and high flexibility [51, 52]. In the current research scenario, focus is on enhancing the fiber–resin interactions, neglecting fiber orientation. The poor uniformity in the fiber length and orientation in biobased composites results in huge variations in the mechanical properties of similar samples and hence represents one of the major limitations for its use as a commercial product. Researchers are carrying out both physical and chemical surface treatments for presenting them successfully in the market as a competitive alternative to synthetic fibers. Each component in the natural fiber composite has been separated and studied individually for optimizing the fiber–matrix interactions and also to completely understand their physical properties. Thermotreatment [53], calendaring [54], fiber stretching [55], and electric discharge [56] are some of the common optimization methods available. The methods indicated herein are used to improve the surface of the fibers physically on a macroscale and they do not alter or induce any chemical changes in the fibers. In some cases, they could also be used to improve the adhesion with the polymer matrix and also fiber orientation. Shorter fibers from high cellulose content fibers and plant particulates could be filtered by using Solvent extraction technique.
In some cases, in order to improve the fiber–matrix interactions, the chemical treatment of the fibers is also carried out. Alkaline treatments of fibers can be used as a pretreatment method in addition to the chemical modifications that are carried out. Alkalis like sodium hydroxide could be used for treating the fibers in which the cellulose lattice is converted into a more stable form [57]. It has been found that the mechanical properties of flax/epoxy polymers are increasing at a significant rate than the nontreated samples after alkaline treatment [58]. On the other hand, if there is no stretching of the fibers during the NaOH/water alkaline treatment, then it results in the shrinkage of the fibers [59]. Water can be substituted with ethanol for overcoming this problem [59]. Another method used for enhancing the interfacial adhesion between the reinforcements and the polymer matrix is the use of coupling agents, such as organosilanes which is a common chemical modification method [60]. Even though there occurs a decrease in the crystallinity of the reinforcements, there is still an increase in the tensile properties of the PLA/wood flour composites. Morphological analysis also proves that there is a stronger interfacial bonding that occur in the developed composites.
2.6 Conclusion
It is a well‐known fact that the research activities related to the development of novel biobased polymers have been increasing drastically over the past few decades. Nowadays, the trend has changed from using simpler systems having thermoplastic materials reinforced with natural fibers to more advanced composites with carefully engineered biobased matrices or which are completely of biobased origin (i.e. both the matrix and reinforcements are biobased). Advancements in the polymer and composite processing techniques have led to significant progress in the development of novel biobased composites, which have both industrial and commercial applications. PLA and PLA‐based composites are the most predominating materials used as biobased biomedical equipment, as there is only little focus or work done in other materials such as hydrogels, which are used as scaffolds in cell growth and tissue regeneration. Expectations are in such a way that the development of novel biobased polymers will continue its ladder climbing as it finds more interesting applications in the biomedical field, especially for cell scaffolding and drug delivery, as reported in this chapter.
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