Thermoplastic Processing of PLA/Cellulose Nanomaterials Composites

Abstract

Over the past decades, research has escalated on the use of polylactic acid (PLA) as a replacement for petroleum-based polymers. This is due to its valuable properties, such as renewability, biodegradability, biocompatibility and good thermomechanical properties. Despite possessing good mechanical properties comparable to conventional petroleum-based polymers, PLA suffers from some shortcomings such as low thermal resistance, heat distortion temperature and rate of crystallization, thus different fillers have been used to overcome these limitations. In the framework of environmentally friendly processes and products, there has been growing interest on the use of cellulose nanomaterials viz. cellulose nanocrystals (CNC) and nanofibers (CNF) as natural fillers for PLA towards advanced applications other than short-term packaging and biomedical. Cellulosic nanomaterials are renewable in nature, biodegradable, eco-friendly and they possess high strength and stiffness. In the case of eco-friendly processes, various conventional processing techniques, such as melt extrusion, melt-spinning, and compression molding, have been used to produce PLA composites. This review addresses the critical factors in the manufacturing of PLA-cellulosic nanomaterials by using conventional techniques and recent advances needed to promote and improve the dispersion of the cellulosic nanomaterials. Different aspects, including morphology, mechanical behavior and thermal properties, as well as comparisons of CNC- and CNF-reinforced PLA, are also discussed.

Keywords: cellulose nanomaterials; composites; functionalization; polylactic acid (PLA); properties.

Figure 1

Synthesis and chemical structures of lactide stereoisomers and copolymers.

Figure 2

The commonly used chemical modification of CNM in PLA nanocomposites.

Figure 3

The first row presents optical microscopic images; the second row shows visual appearance; and the third row presents SEM images of film composites—P1, P2, P3 and P4 (Reprinted from [75]; Copyright ©2018, Springer Nature).

Figure 4

Schematic representation of the melt spinning of PLA and PLA/CNC fibers. Reprinted with permission from [70].

Figure 5

(a) Scanning electron microscopy (SEM) image of PLA fibers obtained at 400 m min⁻¹; (b) Transmission electron microscopy (TEM) of cellulose nanocrystals; (c) schematic of the coating procedure employed on the PLA fiber surface; and SEM images of: (d) noncoated; (e) PLA/PVAc; (f) PLA-CNCs-65; (g) PLA-CNCs-75; (h) PLA-CNCs-85; and (i) PLA-CNC-95 fibers. Reprinted from [84] Copyright ©2018 American Chemical Society.

Figure 6

Photos of the 3D printed objects with the MNC/PLA composites: the MNC/PLA composite 3D printing wire rods (a); the 3D printed material subjected to planing and sawing (b); samples of 3D printed objects, including double-balls standing on the shelf, buckets, half-baskets, and sticks in elongated and dumbbell shape that were used for the testing of mechanical properties (c); the 3D printed solid ball floated on the water (d); and the 3D printed solid ball with 2 cm diameter (e). Reprinted with permission from [86].

Figure 7

(a) Schematic representation of undrawn tape and (b,c) SEM and AFM images of undrawn tape respectively after etching; and (d) schematic representation of tape indicating drawing direction and (e,f) SEM and AFM of drawn oriented tape respectively after etching; and (g) Penning’s model representing “shish-kebab” structure. Reprinted with permission from [88].

Figure 8

Typical stress-strain curves of the nanocomposite tapes: (a) different drawing ratio (DR) drawn at 40 °C with drawing speed (DS) of 50 mm/min; (b) DR 2.0 drawn at different temperatures with DS of 50 mm/min; and (c) DR 2.5 drawn at 40 °C with different speeds. Reprinted with permission from [88].

Figure 9

Storage modulus for PLA/LA-CNC nanocomposites at 23 and 70 °C. Reprinted with permission from [52].

Figure 10

Tan Delta of modified and unmodified CNCs. Reprinted with permission from [52].

Figure 11

(a) Weight (%) versus temperature graph obtained from the TG data; and (b) DTG plots of PLA with different acid hydrolyzed CNC nanocomposite at β = 10 °C/min. Reprinted by permission from [100].