The DOPO derivative-conjugated flame retardant 4, 4'-{1'', 4'' - phenylene - bis [amino - (10‴ - oxy -10‴-hydro-9‴-hydrogen-10‴ λ5 -phosphaphenanthrene-10''-yl)-methyl]}-diphenol (P-PPD-Ph) with two hydroxyl groups was synthesized. Polylactic acid conjugated flame-retardant composites with P-PPD-Ph were papered by using a twin-screw extruder. The flame-retardant properties of polylactic acid-conjugated flame-retardant composites were investigated. The flame-retardant properties of PLA-conjugated flame-retardant composites were characterized by the limiting oxygen index (LOI) and the vertical burning test (UL94). The results showed that the PLA-conjugated flame-retardant composites achieved a V-0 rating (UL-94, 3.2 mm) when the conjugated flame retardant was added at 5 wt%, and increase in LOI value from 22.5% to 31.4% relative to composites without added conjugated flame retardant. The flame-retardant mechanism of PLA-conjugated flame-retardant composites were further studied by TG-FTIR, the results showed that the P-PPD-Ph promoted the PLA-conjugated flame-retardant composites to decompose and also released fragments with quenching and dilution, which suggests that P-PPD-Ph for PLA-conjugated flame-retardant composites mainly play a role of the gas-phase flame retardant.

Polylactic acid (PLA) is a new type of biodegradable material with good mechanical properties, and is widely used in many fields. However, as PLA is highly flammable, it is necessary to conduct flame‐retardant modification research on PLA. Phosphorus heterophilic flame retardants are low smoke, non‐toxic and have high flame‐retardant efficiency, and also have broad application prospects. In this study, a phosphazene flame retardant, 9,10‐dihydro‐9‐oxa‐10‐phosphazene‐10‐yl‐hydroxy‐phenol (abbreviated as DOPO‐PHBA), was synthesized. The PLA/ECE/DOPO‐PHBA flame‐retardant composites were prepared by adding DOPO‐PHBA and epoxy chain extender (ECE) into PLA. Thermal, mechanical, and flame‐retardant properties, as well as microscopic morphology of the PLA flame‐retardant composites were characterized and analyzed, and the flame‐retardant mechanism was discussed. Results show that when the flame‐retardant DOPO‐PHBA is added at 5 wt%, the PLA composites can reach the V‐0 level of combustion, and the corresponding LOI value is 30.0% at this time, and the LOI value increases from 22.5% to 33.6% with the increase of the flame‐retardant content. In addition, PLA composites still have good mechanical properties, and the cone heat, carbon residue and the thermal decomposition process shows that the flame‐retardant causes a two‐phase flame‐retardant mechanism on PLA with the gas and condensed phases acting in synergy. High flame retardancy is mainly attributed to free radical quenching, gas dilution, and the thermal barrier caused by the carbon layer. This work provides a simple and scalable method for the preparation of high‐performance flame‐retardant PLA materials.

The conjugated flame retardants have rarely been studied. A conjugate flame-retardant 4, 4'-{1″, 4″-phenylene-bis [amino- (10‴-oxy10‴-hydro-9‴-hydrogen- 10‴λ5-phosphaphenanthrene-10″-yl)-methyl]}-diphenol (P-PPD-Ph) was synthesized and added into the polylactic acid (PLA) matrix. The P-PPD-Ph-conjugated flame-retardant structure was tested by FTIR, 1H, and 31P NMR analysis. The thermal and rheological properties of PLA/P-PPD-PH-conjugated flame-retardant composites were investigated. The results showed that P-PPD-Ph-conjugated flame retardant affects PLA/P-PPD-PH-conjugated flame-retardant composites for promoting the formation of a carbon layer when the P-PPD-Ph-conjugated flame-retardant content was 15% and the residual carbon ratio for PLA/P-PPD-PH-conjugated flame-retardant composites increased by 4.2%.