The improvement of flame retardancy and compatibility of PBAT/PLLA via a hybrid polyurethane

Poly (butylene adipate-co-terephthalate)/poly (L-lactic acid) (PBAT/PLLA) is one of the most important biodegradable polymer combinations; however, they are flammable with heavy melt dripping and incompatible. To achieve the objective of flame retardation and compatibility, a hybrid polyurethane (PU) with multiple flame retardation elements is synthesized via a new ring-opening polymerization (ROP) method and integrated into PBAT/PLLA film. The PU not only dissolves in different organic solvents at mild temperature but also improves the compatibility of PBAT/PLLA. As PU with respect to PBAT/PLLA is 20 wt%, the limiting oxygen index (LOI) and UL-94 reach 25.5 % and V-0 rating, respectively. In cone calorimeter test, the peak heat release rate (pHRR) of PU/PBAT/PLLA is ahead of PBAT/PLLA, and the total heat release (THR) decreases to 25.85 MJ/m2. The fire safety is achieved successfully. The initial pyrolysis of PU promotes the formation of a seed carbon layer; it continuously breaks down into a series of phosphorus‑oxygen radicals and generates different inert gases, while the pyrolytic solid products accelerate the carbonization to form the carbon/silicon composite layer. Then the polymeric combustion is braked completely. Besides, the PU can also tune the mechanical properties of PBAT/PLLA film and enhance its hydrophobicity. This work opens a new window for developing multifunctional flame retardant and paves the way for the richening engineering application of PBAT/PLLA.

Synergistic effect of stereo-complexation and interfacial compatibility in ammonium polyphosphate grafted polylactic acid fibers for simultaneously improved toughness and flame retardancy

Flammability and poor toughness of unmodified PLA limit its applications in various fields. Though ammonium polyphosphate (APP) is a green and effective flame retardant, it has poor compatibility with the matrix, leading to a decrease in mechanical properties. Stereo-complexation greatly improves the strength and heat resistance of traditional PLA. However, the effect of flame retardants on the formation of stereo-complexed crystals and the impact of stereo-complexation on flame retardancy have not been studied previously. In this research, PDLA chains were first in-situ reacted with APP particles for improved interfacial compatibility. By utilizing the characteristic of PLA enantiomers that can form stereo-complexed crystals, near-complete stereo-complexed PLA fibers with flame retardancy were produced via clean and continuous melt spinning. The compatibility between PDLA-g-APP and PLLA matrix was studied by SEM, rheological analyses and DSC. Strength and flexibility of the fibers were simultaneously enhanced compared to traditional PLA due to the synergistic effect of interfacial compatibility and stereo-complexation. Compared to traditional PLA, the peak heat release rate and total heat release in microcalorimetry test were reduced by 33 % and 22 %, respectively. The flame-retardant fibers achieved a V-0 rating in the UL-94 test, and an increase in LOI value from 19.4 % to 28.2 %.

Toward flame-retardant and toughened poly(lactic acid)/cross-linked polyurethane blends via the interfacial reaction with the modified bio-based flame retardants

Incorporating bio-based flame retardants into polylactic acid (PLLA) to improve flame retardancy has always been the focus of research, but the improvement of flame retardancy is usually at the expense of mechanical properties. How to successfully balanced the material's mechanical and combustion properties has puzzled many scholars. Herein, ammonium polyphosphate (APP) and chitosan (CS) were used as acid source and carbon source respectively. Biological flame retardant APP@CS was designed and synthesized by electrostatic self-assembly method. In addition, toughened PLLA composites were prepared by reactive blending with the in-situ formed polyurethane (PU) as toughening phase. The results show that the CS shell not only reduces the hydrophilicity of the flame retardant, but also has good flame retardant property because of its excellent char forming property. The addition of 10 phr APP@CS can endow PLLA/crosslinked PU (CPU) with UL-94 V-2 rating and a LOI value of 24.9 %. Interestingly, CS shell participates in the in-situ reaction, which improves the mechanical properties of the composite with elongation at break of 74 %, which is higher than that of sample doped with the same amount of APP. This work provides guidance for the high performance modification of PLLA and is expected to expand the practical application range.

Recent advances in flame retardant and mechanical properties of polylactic acid: A review

The large-scale application of ecofriendly polymeric materials has become a key focus of scientific research with the trend toward sustainable development. Mechanical properties and fire safety are two critical considerations of biopolymers for large-scale applications. Polylactic acid (PLA) is a flammable, melt-drop carrying, and strong but brittle polymer. Hence, it is essential to achieve both flame retardancy and mechanical enhancement to improve safety and broaden its application. This study reviews the recent research on the flame retardant functionalization and mechanical reinforcement of PLA. It classifies PLA according to the type of the flame retardant strategy employed, such as surface-modified fibers, modified nano/micro fillers, small-molecule and macromolecular flame retardants, flame retardants with fibers or polymers, and chain extension or crosslinking with other flame retardants. The functionalization strategies and main parameters of the modified PLA systems are summarized and analyzed. This study summarizes the latest advances in the fields of flame retardancy and mechanical reinforcement of PLA.