Mechanical Properties and Degrading Behaviors of Aluminum Hypophosphite-Poly(Lactic Acid) (PLA) Nanocomposites

Simultaneously enhance the fire safety and mechanical properties of PLA by incorporating a cyclophosphazene-based flame retardant

The application of poly(lactic acid) (PLA) has been limited in flame-retardant field, and flame-retardant modification usually deteriorates its mechanical properties. In this study, a reactive flame-retardant hexa(ethylene oxide)-cyclotriphosphazene (HCCP-EP) was synthesized and used to improve the fire retardancy of PLA. As a result, the limiting oxygen index of PLA increased from 19.5% to 27.3% with an addition of 3 wt% HCCP-EP, and the PLA/HCCP-EP blend reached to underwriters laboratories (UL)-94 V-0 rating. The cone calorimeter test results showed that the peak heat release rate and total heat release of PLA decreased by 12.6% and 18.5%, respectively. Interestingly, the tensile strength of PLA increased slightly after the incorporation of HCCP-EP. The improved mechanical properties are ascribed to the fine dispersion of HCCP-EP and the coupling reaction between the epoxy groups of the HCCP-EP and the terminal groups of PLA during the melt processing.

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

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.

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

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.

Development of Biodegradable Flame-Retardant Bamboo Charcoal Composites, Part II: Thermal Degradation, Gas Phase, and Elemental Analyses

Bamboo charcoal (BC) and aluminum hypophosphite (AHP) singly and in combination were investigated as flame-retardant fillers for polylactic acid (PLA). A set of BC/PLA/AHP composites were prepared by melt-blending and tested for thermal and flame-retardancy properties in Part I. Here, in Part II, the results for differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), thermogravimetry-Fourier transform infrared spectrometry (TG-FTIR), X-ray diffraction (XRD), and X-ray photoelectron analysis (XPS) are presented. The fillers either singly or together promoted earlier initial thermal degradation of the surface of BC/PLA/AHP composites, with a carbon residue rate up to 40.3%, providing a protective layer of char. Additionally, BC promotes heterogeneous nucleation of PLA, while AHP improves the mechanical properties and machinability. Gaseous combustion products CO, aromatic compounds, and carbonyl groups were significantly suppressed in only the BC-PLA composite, but not pure PLA or the BC/PLA/AHP system. The flame-retardant effects of AHP and BC-AHP co-addition combine effective gas-phase and condensed-phase surface phenomena that provide a heat and oxygen barrier, protecting the inner matrix. While it generated much CO₂ and smoke during combustion, it is not yet clear whether BC addition on its own contributes any significant gas phase protection for PLA.

Development of Biodegradable Flame-Retardant Bamboo Charcoal Composites, Part I: Thermal and Elemental Analyses

In this study, bamboo charcoal (BC) was used as a substitute filler for bamboo powder (BP) in a lignocellulose-plastic composite made from polylactic acid (PLA), with aluminum hypophosphite (AHP) added as a fire retardant. A set of BC/PLA/AHP composites were successfully prepared and tested for flame-retardancy properties. Objectives were to (a) assess compatibility and dispersibility of BC and AHP fillers in PLA matrix, and (b) improve flame-retardant properties of PLA composite. BC reduced flexural properties while co-addition of AHP enhanced bonding between PLA and BC, improving strength and ductility properties. Adding AHP drastically reduced the heat release rate and total heat release of the composites by 72.2% compared with pure PLA. The formation of carbonized surface layers in the BC/PLA/AHP composites effectively improved the fire performance index (FPI) and reduced the fire growth index (FGI). Flame-retardant performance was significantly improved with limiting oxygen index (LOI) of BC/PLA/AHP composite increased to 31 vol%, providing a V-0 rating in UL-94 vertical flame test. Adding AHP promoted earlier initial thermal degradation of the surface of BC/PLA/AHP composites with a carbon residue rate up to 40.3%, providing a protective layer of char. Further raw material and char residue analysis are presented in Part II of this series.

Curing Kinetics and Thermal Stability of Epoxy Composites Containing Newly Obtained Nano-Scale Aluminum Hypophosphite (AlPO2)

For the first time, nano-scale aluminum hypophosphite (AlPO₂) was simply obtained in a two-step milling process and applied in preparation of epoxy nanocomposites varying concentration (0.1, 0.3, and 0.5 wt.% based on resin weight). Studying the cure kinetics and thermal stability of these nanocomposites would pave the way toward the design of high-performance nanocomposites for special applications. Scanning electron microscopy (SEM) and transmittance electron microscopy (TEM) revealed AlPO₂ particles having domains less than 60 nm with high potential for agglomeration. Excellent (at heating rate of 5 °C/min) and Good (at heating rates of 10, 15 and 20 °C/min) cure states were detected for nanocomposites under nonisothermal differential scanning calorimetry (DSC). While the dimensionless curing temperature interval (ΔT*) was almost equal for epoxy/AlPO₂ nanocomposites, dimensionless heat release (ΔH*) changed by densification of polymeric network. Quantitative cure analysis based on isoconversional Friedman and Kissinger methods gave rise to the kinetic parameters such as activation energy and the order of reaction as well as frequency factor. Variation of glass transition temperature (T_g) was monitored to explain the molecular interaction in the system, where T_g increased from 73.2 °C for neat epoxy to just 79.5 °C for the system containing 0.1 wt.% AlPO₂. Moreover, thermogravimetric analysis (TGA) showed that nanocomposites were thermally stable.