Enhancement of Flame Retardancy and Mechanical Properties of Polylactic Acid with a Biodegradable Fire-Retardant Filler System Based on Bamboo Charcoal

Abstract

A cooperative flame-retardant system based on natural intumescent-grafted bamboo charcoal (BC) and chitosan (CS) was developed for polylactic acid (PLA) with improved flame retardancy and minimal decline in strength properties. Chitosan (CS) as an adhesion promoter improved the interfacial compatibility between graft-modified bamboo charcoal (BC-m) and PLA leading to enhanced tensile properties by 11.11% and 8.42%, respectively for tensile strength and modulus. At 3 wt.% CS and 30 wt.% BC-m, the crystallinity of the composite increased to 38.92%, or 43 times that of pure PLA (0.9%). CS promotes the reorganization of the internal crystal structure. Thermogravimetric analysis showed significantly improved material retention of PLA composites in nitrogen and air atmosphere. Residue rate for 5 wt.% CS and 30 wt.% BC-m was 29.42% which is 55.1% higher than the theoretical value of 18.97%. Flammability tests (limiting oxygen index-LOI and UL-94) indicated significantly improved flame retardancy and evidence of cooperation between CS and BC-m, with calculated cooperative effectiveness index(Ce) >1. From CONE tests, the peak heat release rate (pHRR) and total heat release (THR) were reduced by 26.9% and 30.5%, respectively, for 3% CS + 20% BC-m in PLA compared with adding 20% BC-m alone. Analysis of carbon residue morphology, chemical elements and structure suggest CS and BC-m form a more stable char containing pyrophosphate. This char provides heat insulation to inhibit complete polymer pyrolysis, resulting in improved flame retardancy of PLA composites. Optimal mix may be recommended at 20% BC-m + 3% CS to balance compatibility, composite strength properties and flame retardance.

Keywords: bamboo charcoal; chitosan; cooperative system; flame retardant; polylactic acid.

Figure 1

Schematic diagram of composite preparation and testing.

Figure 2

SEM images of surface morphology: (a) BC and (b) BC-m.

Figure 3

SEM images of fracture surface morphology: (a₁) FPLA-B1, (a₂) FPLA-B2, (a₃) FPLA-B3, (b₁) FPLA-C31, (b₂) FPLA-C32, (b₃) FPLA-C33, (c₁) FPLA-C51, (c₂) FPLA-C52, and (c₃) FPLA-C53.

Figure 4

Typical particle size distribution of BC-m particles on fracture surface.

Figure 5

Heat flow curves of PLA and its blends.

Figure 6

Diffraction patterns of PLA and FPLA composites.

Figure 7

Average mechanical properties of FPLA-B and FPLA-C composites: (a) tensile strength, (b) tensile modulus, (c) flexural strength, and (d) flexural modulus; as well as typical tensile stress-strain curves of a selected sample: (e) FPLA-B composites and (f) FPLA-C composites.

Figure 8

Typical TGA (a,c) and DTG (b,d) curves of CS, BC; BC-m, PLA, and FPLA composites in N₂.

Figure 9

TGA (a) and DTG (b) curves of FPLA composites in air.

Figure 10

Digital images of PLA and FPLA composites after LOI tests: (a) pure PLA; (b) 3CS/10BC-m/87PLA; (c) 3CS/20BC-m/77PLA; (d) 3CS/30BC-m/67PLA; (e) 5CS/10BC-m/85PLA; (f) 5CS/20BC-m/75PLA; (g) 5CS/30BC-m/65PLA.

Figure 11

Images of carbon residues of PLA and FPLA composites after CONE tests: (a₁,a₂) PLA, (b₁,b₂) FPLA-B2, (c₁,c₂) FPLA-C32, and (d₁,d₂) FPLA-C52.

Figure 12

Typical CONE test results of PLA and FPLA composites: (a) HRR, (b) THR, (c) TSR, and (d) mass.

Figure 13

SEM images of residues on outer surface ((a₁): FPLA-B3; (b₁): FPLA-C33; (c₁): FPLA-C53) and internal surface ((a₂): FPLA-B3; (b₂): FPLA-C33; (c₂): FPLA-C53) from FPLA composites after cone tests.

Figure 14

(a) FTIR curves and (b) XPS spectra of the residue char for PLA, FPLA-B3, FPLA-C33, and FPLA-53.

Figure 15

Spectra for char residues of FPLA-C53: (a) C 1s, (b) O 1s, (c) P 2p, and (d) N 1s.