Precompounding of lignocellulosic fibers and biopolymer matrix
To improve the fiber—matrix interaction and ensure uniform distribution of lignocel- lulosic fibers within the biocomposites, the use of various compounding techniques prior to the processing of LFBC have been reported in literature. These precompounding techniques are usually employed in the case of short fiber-reinforced biocomposites. Commonly used compounding techniques used prior to compression or injection molding process are extrusion, melt blending, pultrusion, two-roll mill, etc.
The use of extrusion for precompounding of the fiber—matrix mixture prior to injection or compression molding has been reported by several researchers. During the extrusion process, as depicted in Fig. 9.6A, the biopolymer and lignocellulosic fibers are mixed together and introduced into an extruder. The extrusion machine consists of a heated barrel with a rotating screw in it. Extruders can be classified as single or twin-screw extruders based on the number of screws. Twin-screw extruders can be further classified as corotating and counterrotating extruders based on the rotational direction of individual screws. The fiber—matrix mixture fed through the hopper is sucked into the heated barrel by the rotating screw(s) and is carried towards the die end. During this travel, the matrix melts and the fiber— matrix compounding takes place. The temperature of various zones of the barrel can be adjusted as per the optimum temperature range for a particular fiber—matrix combination to avoid thermal degradation of the melt. The biocomposite extruded strands are then pelletized and injection molded into desired products. The major processing parameters which affect the quality of the extruded biocomposite strands are barrel temperature, screw speed, and fiber reinforcement concentration. The extru- date quality also depends upon the screw profile and strand die design.
Bledzki et al. (2009) employed twin-screw and single screw extruders for coating and compounding of PLA-based biocomposites, respectively, prior to injection molding. During the compounding process, the melt temperature of PLA was kept at 200°C at a screw speed of 100 rpm, while compounding was carried out at a temperature of 180°C and screw speed of 20 rpm. Bledzki and Jaszkiewicz (2010) in a different study used single and twin-screw extruders for compounding PHBV- and PLA-based biocomposites, respectively. Kim et al. (2010) compounded bamboo flour and wood flour-reinforced PLA and PBS biocomposites in the presence of compatibilizer, using a laboratory size, corotating twin-screw extruder. Fiber weight fraction of 30% was compounded at a screw speed of 250 rpm and temperature of 190 and 145°C for PLA- and PBS-based biocomposites, respectively. The extrudate was subsequently pelletized and dried in an oven at 80°C for 24 h. Moigne et al. (2014) employed a corotating twin-screw extruder for compounding of flax fibers (20 wt%) and PLA pellets prior to the development of biocomposites. The fiber—matrix mixture was compounded at a barrel temperature profile and screw speed of 60—175°C and 300 rpm, respectively. Anstey et al. (2014) compounded PBS-based biocomposites using a 15 cc micro twin-screw extruder. The PBS—filler mixture in the ratio of 3:1 was compounded at 160°C at a screw speed of 100 rpm. Huda et al. (2006a,b) also employed a mini twin-screw extruder having screw L/D and length of 18 and 150 mm, respectively, for compounding wood flour-reinforced PLA biocomposites. Tokoro et al. (2008) compounded PLA—bamboo fiber biocomposites using a twin-screw extruder (screw L/D = 36 and screw diameter = 30 mm) at 180°C. Borchani et al. (2015) compounded alfa fiber-reinforced poly(butylene terephthalate-co-butylene adipate) and starch-based Mater-Bi® biocomposites using an intermeshing corotating twin-screw extruder (screw L/D ratio of 36 and screw diameter = 25 mm), at a melt temperature of 150°C. The extrudate was extruded at a rate of 10 mm/s in the form of 1 mm strands. These strands were cooled and subsequently pelletized using a pelletizer. Asaithambi et al. (2014) compounded untreated and treated hybrid banana—sisal fiber-reinforced PLA biocomposites using a twin-screw extruder having a screw L/D ratio and screw diameter of 40 and 28 mm, respectively. The barrel temperature profile, screw speed, and residence time was fixed at 140—150—155 —160—165—170—175—170—165—160°C,
100 rpm, and 15 min, respectively. Sykacek et al. (2009) compounded wood flour (up to 65 wt%) and five different biopolymers using a conical counterrotating twin- screw extruder having a screw diameter of 45—90 mm and two shear generating elements. The temperature profile and screw speed during extrusion was selected as 150—170—180—180 (feed to die) and 75 rpm, respectively.