accuracy of the products produced is the spray layup technique, which is a modified version of the hand layup technique.
Idicula et al. [11] used short randomly oriented banana and sisal fibers as reinforcing materials for developing a polyester‐based hybrid biocomposite. The composites were developed using a compression molding machine. They found that, the composites made by using compression molding techniques are having very good impact strength when compared to those produced using resin transfer molding. Sreekumar et al. [12] developed biocomposites using a compression molding machine and investigated their mechanical properties. Polyester has been used as the matrix and sisal fiber as reinforcement. They compared the mechanical properties of these developed composites with that of the composites made using resin transfer molding. They found out that the strength of the composites made by compression molding technique is lower than that made from resin transfer molding technique. Kim et al. [13] used an injection molding machine for developing biobased composites using pineapple and cassava flour as reinforcements and PLA as matrix. Here they found out that the tensile and flexural strength has increased and it is attributed to the use of compatibilizer with flour as reinforcement. This helped to increase the interfacial adhesion between the matrix and the fillers. Feldmann et al. [14] developed biobased composites by using biorenewable polyamides as matrix and cellulosic fibers as reinforcements. The difference in this work is that, the authors have used only a single‐step pultrusion process instead of the conventional two‐step pultrusion process. The resultant composites from this single‐step pultrusion process showed very good tensile strength when compared to the composites made from other processes. Sevkat et al. [15] developed a hybrid hollow shaft‐like composite using the filament winding method. The advantage with this method is that changing the fiber loading can be achieved with much ease even when the production is running. Hence, it is concluded that hybridization could be easily achieved by using the filament winding technique. When the shapes of the composites becomes complex with higher fiber percentage, then resin transfer molding (RTM) technique could be used. Dweib et al. [16] developed biocomposites by using soybean oil as the matrix material and cellulose fiber as reinforcement. The authors, in order to manufacture composites of roof top structural applications with large dimensions, utilized a modified vacuum‐assisted resin transfer molding technique. In order to eliminate the need for using a solid mold, the authors used a nylon bagging sheet for covering the entire part.
Table 2.2 Fabrication methods to produce biocomposites.
| S. no. | Method of fabrication | Key point | Features |
|---|---|---|---|
| 1. | Hand lay‐up | Involves the use of an open mold in which the resin and reinforcements are placed manually and a mild pressure is applied using the roller in order to remove the voids. | Tooling cost is very low and there is no size constraint. Examples range from small tubs, medium storage tanks to larger boats, etc. |
| 2. | Spray lay‐up | A typically developed spray gun is used for spraying the mixture of chopped fiber and resin in an open mold. | This is a faster method compared to the hand lay‐up technique. Also, semi‐automatic in nature. Examples range from small tubs, medium storage tanks to larger boats, etc., with more accuracy. |
| 3. | Compression molding | It is a method by which the resin and reinforcement are heated by a closed metallic mold structure in which the reinforcement and resin are initially placed in the lower half of the metallic mold and the pressure is applied by closing both the halves of the mold. The various controlling parameters include time, temperature, and pressure. | Specific features include lower postprocessing requirements and higher part design flexibility. The applications are structural components and automobile components. |
| 4. | Injection molding | The mixture of reinforcement and resin are pushed through a screw extruder that pushes the compounds through a closed mold at the end of a nozzle. On through the way, the resin and reinforcement mixtures are heated up by a heater surrounding the extruder. | Specific features of the process include high volume production and very fast cycle time. Applications include floor pans, bumpers, automobile body panels, etc. |
| 5. | Pultrusion method | Consists of a long continuous chain of fibers wound over a drum. It is being pulled through the path way of the resin and passed through a heated die. The shape of die controls the cross‐section of the product. | Specific features include constant cross‐sectional shape products. Applications are channels, long rods and extremely high fiber loading beams. |
| 6. | Vacuum bag molding | Consists of a vacuum bag that is used to cover the mold. Used for producing laminates from prepregs or wet lay‐ups. This method helps to improve the mechanical properties of laminates. | Specific features include uniform degree of consolidation and includes no void content. |
Boey et al. [17] used an autoclave process to develop thermoset composites. The study was carried out on the developed composites by considering the effect of vacuum on the voids present. Once the composites reach an initial pressurization range, there occurs a significant decrease in the void content of the composite. Some other techniques that are being used for developing these biobased composites include tube rolling, automated fiber placement (AFP), and automated tape laying (ATL).
2.4 Fabrication Techniques of Biobased Composites
Many a time, in order to improve the overall thermomechanical properties of the developed composites, either natural polymers such as cellulose are used as resins in addition to the reinforcing fibers or synthetic polymers are used for polymerization. Some of the unique advantages of these biobased materials are that they offer good biodegradability and biocompatibility. This helps them to find their application in the field of biomedical engineering and also in tissue engineering. When used in the biomedical field, inside the human body, these scaffolds that are engineered must be capable of degrading in a way which is nontoxic to the user. It should also be capable of mimicking the body's own extracellular matrix (ECM) in order to allow the proliferation of human body cells and also to avoid an immune response. The rate of gradation of these scaffolds must take place in a proper way. Hence, it becomes difficult for us to synthesize these biobased polymers alone. Nanoparticles such as hydroxyapatite (HA) could be used for nanoscale hybridization of natural and synthetic materials [18] or carbon nanotubes [19]. The following are the most common fabrication methods used for developing biobased composites.
They are:
1 Solvent Casting and Particulate Leaching (SCPL)
2 Emulsion Freeze Drying
3 Electrospinning
4 Blow Film Extrusion
5 3D printing
2.4.1 Solvent Casting and Particulate Leaching
During Solvent Casting and Particulate Leaching (SCPL) technique, the polymer matrix is found to be dispersed with soluble particles [20]. As soon as the material is consolidated, an appropriate solvent washes away the dissolved particles, which then leads to a porous material [20]. But on the other hand, when the parts become thicker,
