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. 2021 Oct 6;6(41):26990-27006.
doi: 10.1021/acsomega.1c03393. eCollection 2021 Oct 19.

Intumescent-Grafted Bamboo Charcoal: A Natural Nontoxic Fire-Retardant Filler for Polylactic Acid (PLA) Composites

Liang Zhang  1 , Weisheng Chai  1 , Wenzhu Li  1 , Kate Semple  2 , Ningning Yin  1 , Wenbiao Zhang  1 , Chunping Dai  2
Affiliations

Intumescent-Grafted Bamboo Charcoal: A Natural Nontoxic Fire-Retardant Filler for Polylactic Acid (PLA) Composites

Liang Zhang et al. ACS Omega. .

Abstract

In this work, an alternative flame-retardant filler based on phosphate- and urea-grafted bamboo charcoal (BC-m) at 10-30 wt % addition was aimed at improving the flame retardancy of polylactic acid (PLA) composites. The filler caused only a small reduction in strength properties but a slight increase in the modulus of elasticity of PLA composites. BC-m significantly improved the flame-retardant performance compared with pure BC. The limiting oxygen index (LOI) was 28.0 vol % when 10 wt % of BC-m was added, and 32.1 vol % for 30 wt % addition, which was much greater than the value of 22.5 vol % for 30 wt % pure BC. Unlike pure BC, adding BC-m at 20 wt % or more gave a UL-94 vertical flame test rating of V-0 with significantly reduced melt dripping. The peak heat release rate (pHRR) and total heat release (THR) of BC-m/PLA composites decreased by more than 50% compared with pure PLA, and the values for 20% BC-m were significantly less than that for 25% BC addition. The grafted biochar-based system provides an effective flame retardancy effect by a condensed-phase protective barrier through the rapid formation of a dense, honeycomb-like cross-linked carbonized char layer. The results suggest a promising route to enhancing the flame-retardant properties of biodegradable polymer composites using nontoxic, more environmentally friendly grafted biochar.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Appearance of test specimens: (a) LOI/UL-94 vertical flame, (b) CONE, (c) flexural, and (d) tensile.
Figure 2
Figure 2
Schematic diagram of composite preparation and testing.
Figure 3
Figure 3
Raw material assays: (a) FTIR spectra of BC, BC-o, and BC-m. (b) 13C MAS–NMR spectra of BC and BC-m. (c) XRD patterns of BC and BC-m. (d) Particle size distribution of BC and BC-m. (e) EDAX spectrum of BC and BC-m. (f) Surface micrograph of the outer surface of BC. (g) Surface micrograph of the outer surface of BC-m.
Figure 4
Figure 4
Mechanical properties of PLA and composites (error bars represent standard deviation): (a, b) BC/PLA and (c, d) BC-m/PLA. Typical tensile stress–strain curves of selected samples: (e) BC/PLA and (f) BC-m/PLA.
Figure 5
Figure 5
TG (a, c, e) and DTG (b, d, f) curves for BC, BC-m, PLA, and its blends under a N2 atmosphere.
Figure 6
Figure 6
Digital images of materials after LOI tests: (a) PLA; (b) BC/PLA1; (c) BC/PLA2; (d) BC/PLA3; (e) BC-m/PLA1; (f) BC-m/PLA2; and (g) BC-m/PLA3.
Figure 7
Figure 7
Typical CONE testing curves: (a) HRR, (b) THR, (c) TSR, and (d) mass loss curves of PLA, 25BC/75PLA, and BC-m/PLA composites.
Figure 8
Figure 8
Images of carbon residues from PLA and BC-m/PLA composites after CONE testing: (a1–2) PLA; (b1–2) BC-m/PLA1; (c1–2) BC-m/PLA2; and (d1–2) BC-m/PLA3.
Figure 9
Figure 9
FTIR spectra of gaseous products for the thermal decomposition of PLA and BC-m/PLA composites vs time: (a) CO2; (b) CO; (c) carbonyl compound; (d) hydrocarbons; (e) C–O bond; (f) NH3; (g) P–O bond; and (h) P=O bond.
Figure 10
Figure 10
FTIR spectra of the volatiles from BC-m/PLA3 heated at different temperatures.
Figure 11
Figure 11
SEM images of fractured surface of BC-m/PLA composites before combustion: (a1) BC-m/PLA1; (a2) BC-m/PLA2; and (a3) BC-m/PLA3.
Figure 12
Figure 12
SEM images of residues on the outer surface (a1: BC-m/PLA1; b1: BC-m/PLA2; c1: BC-m/PLA3) and the internal surface (a2–a3: BC-m/PLA1; b2–b3: BC-m/PLA2; c2–c3: BC-m/PLA3) from BC-m/PLA composites after cone tests with different magnifications.
Figure 13
Figure 13
XPS spectra of the char residue of BC-m/PLA composites after the cone calorimeter test.
Figure 14
Figure 14
Spectra of the char residue of BC-m/PLA3 for (a) O 1s, (b) C 1s, (c) P 2p, and (d) N 1s.
Figure 15
Figure 15
Schematic representation of the possible flame-retardant mechanism for BC-m/PLA composites.
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