Improving Mechanical Properties of PLA/PBAT Bionanocomposites: Compatibilizer, Nanoparticle Type, Synergy & Pre-treatment Full Factorial Study

To circumvent the drawbacks of polylactic acid (PLA) polymer; blending it with rubbery polymers has been an effective way to achieve superior thermal stability; heat distortion; solvent resistance; mechanical properties; and even a higher biodegradation rate [1] [2] [3]. In this study; PLA (PLE 005-1; NaturePlast; France) is enhanced by: a) blending it with polybutylene adipate terephthalate (PBAT); a biodegradable rubbery biopolyester (PBE-006 NaturePlast; France); at selected ratios; b) adding a selected compatibilizer (Lotader AX-8900; Terpolymer: Ethylene-Methyl Acrylate-Glycidyl Methacrylate); and c) dosing a combination of cellulose nanofibrils (Freeze-dried experimental grade powder; University of Maine; CNF) and graphitic nano-carbon nitride (in-house synthesized g-C3N4) [4] at chosen concentrations. Such concentrations and ratios were determined by a detailed design of experiment (DOE). The compounding of the blends was conducted by high-speed extrusion using a Process 11 Thermofisher Scientific Twin-screw Extruder (11-mm in diameter and L/D: 40) at optimized processing conditions and screw configuration (i.e.; optimal specific energy of mixing). The neat PLA (our control; tested dried as molded; DAM) exhibited a Young’s modulus (YM); an ultimate tensile strength (UTS); an elongation-at-break (EB); and a toughness value of 3.955±0.398 GPa; 75.97±5.38 MPa; 2.93±0.26%; and 1.380±0.19×106 J/m3; respectively. Interestingly; at an 80/20 PLA/PBAT ratio; the blends displayed an EB and toughness value of 125.80±16.08%; and 65.05±9.13×106 J/m3; respectively. These values correspond to about a 4166% and 4613% increase with respect to the control’s properties; respectively. In addition; this blend ratio retained virtually all the stiffness and strength of the pure PLA. Significant phase separation occurred between the PLA and PBAT domains as observed by SEM; which may be responsible for the effective toughening effect attained [5]. To engineer the interphase of those two polymers; a compatibilizer (Lotader AX-8900) was incorporated in situ during the extrusion of the PLA/PBAT (80/20) blend. The experimental design was constructed in Design-Expert software (version 13); using an optimal custom response surface methodology (RSM) with a point exchange algorithm and I-optimality criterion (I-OC) [6] [7] [8]. The concentration of Lotader was considered as a factor with a continuous level ranging from 0.5 wt%-low to 5 wt%-high. The study was focused on four mechanical responses: YM; UTS; EB; and toughness. The analysis of variance (ANOVA) of the predicted models was significant; with R2 values of 0.8787; 0.9246; 0.9855; and 0.9831; respectively. The normality of the residual plots and the insignificant lack of fit validated the accuracy of the predicted model equations. Numerical optimization trials (Criteria: YM; UTS; and Toughness-to be in range; and EB-to be maximized) showed the PLA/PBAT (80/20) with 2-wt% Lotader content as the optimum one (desirability 1.00 among 20 solutions) which exhibited 223.36±6% EB and 104.41±12.25 J/m3 toughness. Our findings revealed a remarkable increase of 7523% in EB and of 7465% in toughness with respect to the PLA control. More importantly; the compatibilized blend; compared to the PLA/PBAT (80/20) blend; displayed improvements of 77% and 60%; respectively for EB and toughness. Nevertheless; the YM and UTS of the compatibilized blends decreased by about 12% and 9% with respect to the PLA control; and 4% and 8% as compared to the PLA/PBAT blend; respectively. A novel aspect of this study is the development of a model equation that predicts mechanical responses based on the compatibilizer content. Based on these results; the study was further extended by reinforcing the blends with g-C3N4 and CNF nanomodifiers. A full-factorial (24 run) design was constructed to investigate the individual and the synergistic impact of the additives. Additionally; the effect of pretreatment prior to extrusion blending was assessed to determine the interaction of the compatibilizer with the nanomodifiers. A comparative study of mechanical properties based on four pretreatment methodologies validates pre-dispersing the nanomodifiers into the minor polymeric phase (i.e.; PBAT). Three numeric factors with two levels (compatibilizer (L-0%; H-2%); g-C3N4 (L-0; H-1%); CNF (L-0%; H-1%)) and one categoric factor (pretreatment (L-No; H-Yes)) were used. Thus; the influence of compatibilizer and pretreatment of the minor polymeric phase with nanomodifiers appeared to be dominant factors influencing the interphase and both polymeric domains and; as a result; the properties of both the elastic and plastic regions of the resulting bionanocomposites. Moreover; graphitic nano-carbon nitride (at only 1 wt%) enhanced the YM and UTS by 76% (6.977±1.277 GPa) and 61% (121.52±18.53 MPa a value higher than most engineering plastics) compared to the PLA control; and 93% and 63% compared to the PLA/PBAT (80/20) blend. To the best of our knowledge; this is the first study reporting 7523% ductility enhancement of a brittle PLA grade with concurrently improved of both elastic and ultimate properties; and that emphasizes the outmost importance of a pre-dispersing step of the nano-reinforcements into the minor polymeric phase; followed by the compounding of both polymeric phases (nano-PBAT and PLA) to achieve superior qualities in PLA. References [1] S. 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