Effect of Lignin Plasticization on Physico-Mechanical Properties of Lignin/Poly(Lactic Acid) Composites

Abstract

Kraft lignin (KL) or plasticized KL (PKL)/poly(lactic acid) (PLA) composites, containing different lignin contents and with and without the coupling agent, were prepared in this study using twin-screw extrusion at 180 • C. Furthermore, ε-caprolactone and polymeric diphenylmethane diisocyanate (pMDI) were used as a plasticizer of KL and a coupling agent to improve interfacial adhesion, respectively. It was found that lignin plasticization improved lignin dispersibility in the PLA matrix and increased the melt flow index due to decrease in melt viscosity. The tensile strength of KL or PKL/PLA composites was found to decrease as the content of KL and PKL increased in the absence of pMDI, and increased due to pMDI addition. The existence of KL and PKL in the composites decreased the thermal degradation rate against the temperature and increased char residue. Furthermore, the diffusion coefficient of water in the composites was also found to decrease due to KL or PKL addition.PLA is considered one of the most promising biopolymers due to its excellent properties, which include biodegradability, biocompatibility, and renewability, as well as good mechanical properties [10][11][12]. PLA is derived from agricultural products (e.g., corn and potato) and is usually produced by ring-opening polymerization of lactide and condensation of lactic acid without polluting the environment during the production process [6,10,13]. However, despite its properties, the application of PLA has been limited because of its higher price and lower resistance to heat and water [11,13]. It has been proposed that compounding PLA with filler can improve its properties and remove some of its drawbacks [5,[11][12][13][14][15]. Recently, some studies have introduced lignin to improve the performance of PLA and reduce the cost [10,13,14].Lignin is one of the most abundant biopolymers, accounting for nearly 25% of lignocellulosic biomass [16][17][18][19]. It is usually easily obtained as a byproduct of the pulping industry and has some positive properties, including being biodegradable, non-toxic, and low-cost and having low density and excellent thermal and moisture resistance [20][21][22][23]. Due to these properties, research on lignin application for bioplastics has increased. Lignin has been known to have positive effects on composite properties. Some studies have observed that lignin addition enhances resistance for heat and moisture [14,15,24]. Furthermore, lignin has also been utilized as a stabilizer to prevent oxidation on plastic composites [25]. However, some studies have reported that the presence of lignin can deteriorate the mechanical properties of lignin-based composites. Lignin has been found to be incompatible with most aliphatic polyesters, including PLA, PBS, and PCL, thus deteriorating the mechanical properties of the composites [26][27][28]. However, it has been found that this strength deterioration, which is caused by lignin addition, can be overcome by adding coupling agents [26,[29][30][31]. Isocyanate coupling agents, such...

Thermal Stability and Decomposition Mechanism of PLA Nanocomposites with Kraft Lignin and Tannin

“…However, in both cases, a deterioration in the tensile and impact strength of the composites was demonstrated, especially for fillers loading above 5 wt%, whereas in the second case a decrease in the degree of crystallinity was also observed. Park et al [ 37 ] synthesized PLA-lignin and lignin plasticized composites. They found that the filler decreases the thermal degradation rate against temperature.…”

Esterification of Lignin Isolated by Deep Eutectic Solvent Using Fatty Acid Chloride, and Its Composite Film with Poly(lactic acid)

“…The elastic modulus of PLA composite film with DL or EDL was also decreased with increasing content of DL or EDL. Park et al [ 6 ] prepared kraft lignin/PLA composites via twin-screw extrusion with different ratios of lignin and PLA. As kraft lignin loading was increased to 30%, the tensile strength and elastic modulus deteriorated because of weak interfacial bonding between kraft lignin and PLA.…”

“…The diffusion coefficient (D) of water into the composite films can be determined using Fick’s second law [ 6 ]. For the case of one-dimensional diffusions from the sheets, Equation (2) was used.…”

“…It has some advantages including biodegradability, biocompatibility, nontoxicity, low density, high carbon content, thermal resistance and oxidation resistance [ 1 , 2 , 3 ]. Due to these properties, it has great potential utilization in bioplastics, films, carbon fibers, various chemicals, and so on [ 3 , 4 , 5 , 6 , 7 ]. Lignin is mainly produced from black liquor, which is a by-product of the pulping process through a regeneration process by precipitation [ 8 ].…”

Insights into the Potential of Hardwood Kraft Lignin to Be a Green Platform Material for Emergence of the Biorefinery

“…The aromatic structure of lignin provides rigidity and stiffness to materials [ 60 ]. Moreover, lignin has been found to be incompatible with most aliphatic polyesters, thus deteriorating the mechanical properties of any resultant composites besides contributing to rougher and fractured surfaces [ 61 ]. Mechanical testing of 100% lignin fibers has revealed tensile strengths of 23 MPa and a Young’s modulus of 3.9 GPa [ 62 ] These values are comparable to those reported for modified softwood kraft lignin thermoplastic (25 MPa and 1.5 GPa, respectively) [ 63 ].…”