Film stacking method
The film stacking method is the simplest processing route for the fabrication of LFBC laminates (Fig. 9.8). In this process, biopolymer matrix films and lignocellulosic fiber mats (woven or plain or random) are alternatively stacked and compressed between the heated mold plates. Plackett et al. (2003) developed PLA—jute fiber biocomposites using the film stacking technique. PLA pellets were first converted into 0.2 mm thick PLA sheets using an extruder. Jute fiber mats (40 wt%) were then stacked between PLA films in a frame/mold and precompressed at 3.3 MPa for 15 s. The precompressed stack of PLA films and jute fiber mats was then placed between heated platens (180—220°C) under a pressure of 400 Pa for 3—10 min. After heating, the mold/frame was again compressed (3.3 MPa) at 60°C for 1 min to obtain 2 mm thick biocomposites. Sawpan et al. (2011, 2012) also used an extruder to convert PLA pellets into 5 mm thick sheets. A hand carding machine was used to align industrial hemp fibers. PLA films and industrial hemp fiber-based biocomposites were then prepared by the film stacking process at varying fiber weight fractions (30, 35, and 40%). The PLA films and industrial hemp fiber mats were then alternately stacked in a mold (220 X 150 X 3.5 mm3) and precompressed (2 MPa) at 185°C for 5 min using a compression molder. The mold pressure was then increased to 5 MPa and thereafter cooled at room temperature. Bajpai et al. (2012, 2013) developed 4 mm thick, PLA-based biocomposites incorporating sisal, Grewia optiva, and nettle fibers (fiber weight fraction of 20%), using the film stacking method. PLA pellets were compression molded into 1 mm thick PLA films. PLA films and fiber mats were then stacked alternatively within the mold. The PLA-fiber stack was then compressed at a pressure of 4 MPa for 8 min at 180°C. The compression pressure was further increased to 6 MPa and the mold was allowed to cool at room temperature. Huda et al. (2008) converted PLA pellets into 1 mm thick PLA sheets using a compression molder at 190°C. Three layers of kenaf fiber mats were stacked alternatively between four sheets of PLA films and compressed (4.8 MPa) at 190°C for 12 min. The mold was further compacted at
11.7 MPa for 5 min and cooled at room temperature. Three millimeter thick biocomposites were then removed from the mold at 90°C. Hu et al. (2010) developed randomly oriented short jute fiber (10—15 mm)-reinforced PLA biocomposites using the film stacking method. Jute fibers with varying fiber volume fractions (30, 40, and 50%) were stacked alternatively between PLA films and placed between a compression molder (1.3 MPa) at 170°C for 10 min to obtain 4—5 mm thick biocomposites. Islam et al. (2010) developed long hemp fiber (30 wt%)-reinforced PLA biocomposites, by stacking fiber mats in between PLA films and compressing the stack (1 MPa) at 170°C for 10 mins.
Fiber orientation and distribution
Compression molded specimens incorporating fiber mats can be tailor-made, using the film stacking method as it offers the best control to the designer over fiber orientation within the biocomposites. Continuous lignocellulosic fibers can be woven at different angles and types according to the load-bearing requirement. Research studies have reported the use of carding process to align nonwoven fibers before the film stacking method. No fiber—matrix—tooling interaction in the case of biocomposites developed using film stacking method is observed. The biocomposites developed using the film stacking method, having fibers aligned in the direction of applied load, are likely to bear more load as compared to the randomly aligned short fiber-reinforced (precompounded) biocomposites.