Modified alkaline lignin for ductile polylactide composites
Graphical abstract
Introduction
PLA has been the most promising substitution for the petroleum-based plastics ascribed to the outstanding biodegradability, high mechanical strength, excellent transparency, and easy processing [[1], [2], [3], [4]]. Nevertheless, the wider extensive applications of PLA are restricted by two important reasons, which are the high cost and the low toughness [[5], [6], [7], [8]].
Recently, diverse toughening modifiers have been applied to toughen PLA by a convenient blending method. Blending PLA with flexible polymers, including poly(butylene adipate-co-terephthalate) [6], poly(butylene succinate-co-adipate) [9], poly(ε-caprolactone) [10] and natural rubber [11], has been considered as the most direct way. In addition, modified starch [12] and silica [13] nanoparticles have been employed as an effective toughening agent for PLA. Nevertheless, a cheaper toughening agent was needed to balance the high price of PLA.
Lignin (L) is produced as a by-product in the agricultural, paper and pulp industries. Nearly 50 million tons of industrial lignin is generated yearly and less than 2.5 million tons are found high value use as additives. Unfortunately, the rest is burned to recover energy or discharged into water [14]. Hence, compared with the toughening methods above, industrial waste lignin would be meaningful as a promising toughening agent for PLA due to the low-cost, abundant, green, and biodegradable characteristic [15,16]. However, the simple blending between PLA and pristine lignin usually lead to poor mechanical properties caused by the poor integration of lignin with PLA [17]. Recently Liu et al. integrated dodecylated lignin in PLA to disperse energy and the elongation at break of lignin/PLA blends were more than 40 times higher than PLA [18]. Also, Song et al. reported a lauryl methacrylate and tetrahydrofurfuryl methacrylate grafted lignin and the addition of 20 wt% modified lignin increased the toughness by ~11 folds from 4.4 to 54.6 MJ·m−3 relative to the PLA matrix [19]. Besides, He et al. fabricated the poly(lactide) (PLA)-lignin composites which exhibited a six-fold increase of elongation at break [20]. Nevertheless, organic solvents such as dichloromethane, dimethyl sulfoxide, and toluene were used in these lignin modifications more or less. And chemical modification in the aqueous solutions would be environmentally friendly and attracting. The alkaline lignin could be modified by melamine and phytic acid according to the Mannich reaction and supramolecular self-assembly technology in aqueous solutions [[21], [22], [23]]. After the modification, it would reduce the ratios of hydrophilic functional groups and improve the compatibility in the composites.
In this work, a bio-based LMP was prepared by a simple and highly-efficient method, which was a toughener for PLA in the presence of PEG400. Moreover, the PLA/LMP/PEG400 ternary blend exhibited superior toughness, e.g the notched impact strength of 10.9 kJ·m−2, the toughness of 65.4 MJ·m−3, and the break strain of 328.8% (3.3, 93.4, and 131.5 times higher than PLA/L binary blend, respectively). Also, the toughening mechanisms of PLA composites were discussed in detail. This work offered a green, cost-effective, and facile strategy for fabricating highly toughening PLA materials.
Section snippets
Materials
PLA (4032D) was purchased from NatureWorks (America). Phytic acid (PA, 70 wt% aqueous solution), Melamine and formaldehyde (HCHO, 37 wt% aqueous solution) were purchased from Aladdin (China). PEG400 was purchased from Sinopharm Group Co., Ltd (China). Alkaline lignin was purchased from Tokyo Chemical Industry Co., Ltd (Japan). Deionized water was made in this lab.
Synthesis of the modified lignin (LMP)
The modified lignin was prepared according to the Mannich reaction and supramolecular self-assembly technology [[21], [22], [23]].
Characterization of L and LMP
The main groups of L and LMP were characterized by FTIR and their elemental components were investigated by XPS. In Fig. 2a, alkaline lignin showed a broad adsorption band at around 3420 cm−1 belonging to the stretching vibration of hydroxyl groups. Besides, the characteristic peaks of lignin's aromatic rings were found at 1581, 1506 and 1451 cm−1 [21,22]. Moreover, the absorption peaks at 1620, 1545 and 1334 cm−1 were attributed to the –CN groups of the substituted melamine and the peaks at
Conclusions
In summary, we have demonstrated a facile, economic and highly efficient approach to fabricate a ductile polylactide composites. The bio-based alkaline lignin was successfully modified by melamine and phytic acid according to the Mannich reaction and supramolecular self-assembly technology in aqueous solutions, confirmed by FTIR and XPS spectra. The PLA/LMP/PEG400 ternary blend was prepared by melt compounding, exhibited superior toughness. With 5 and 10 phr LMP loading, the break strains of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by National Natural Science Foundation of China (21975108), MOE & SAFEA, 111 Project (B13025), and National First-Class Discipline Program of Light Industry Technology and Engineering (LITE2018-19).
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