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

Highly flame retardancy, barrier, mechanical and persistent antibacterial polylactic acid film with high-parallel interconnected thousand layered cake architecture

In this work, a bio-based material (CGP) is obtained by combing chitosan, gelatin and polyvinyl alcohol through a simple solution mixing to simultaneously address polylactic acid film (PLA)' flammability and poor barrier, toughness and antibacterial properties by soaking. The results of open fire testing show that modified PLA films can effectively prolong the combustion time, improve the thermal stability and reduce the release of heat in the cone calorimeter test. For the PLA sample after soaking for 5 times (PLA-5) in particular, it can reduce the peak heat release rate (pHRR) and total heat release (THR) values to 85.8 kW/m2 and 1.3 MJ/m2 from the values of 129.5 kW/m2 and 1.8 MJ/m2 for PLA, respectively. Structural analysis suggests that CGP primarily operates in the condensed phase by forming physical barriers. Meanwhile, the modified PLA films can exhibit superior barrier effects, which indicate the oxygen transmission rate value of PLA-5 decreases to 0.9 cm3/(m2·day) from the 392.5 cm3/(m2·day) of raw PLA film. Moreover, the PLA-5 also have excellent toughness (the value increased to 200.5 % from 31.0 %) and persistent antibacterial effects (it still has 100 % sterilization after 500 days).

A nimble strategy for enabling bioderived flame retardants with strikingly enhanced interfacial compatibility in poly (lactic acid) composites

The utilization of bioderived flame retardants in biodegradable poly (lactic acid) (PLA) has profound practical implications for extending the widespread application of PLA composites and protecting the environment. Nevertheless, there are still certain challenges that require prompt attention, especially the ineffectiveness of bio-based flame retardants and their deterioration of the mechanical properties of PLA. This work introduced triglycidyl isocyanurate (TGIC), which has multiple epoxy functions, into the self-assembly process of phytic acid (PA) and chitosan (CS). The epoxy-modified bioderived flame retardant PA@CS-TGIC (PCT) was well dispersed in the PLA matrix and had a strong interfacial adhesion, while also TGIC had a synergistic char-forming effect. By compounding epoxy-modified ammonium polyphosphate (MAPP), 3%PCT/MAPP-PLA composites may reach a LOI value of 28.8 % and UL-94 V-0 rating. Simultaneously, the melting droplets had been considerably reduced. Tensile strength of the 3%PCT/MAPP-PLA composites was 67.0 MPa, 10.8 % higher than that of pure PLA. This work paves a new avenue for the development of PLA composites with robust mechanical and flame retardant properties.

Enhancing the flame retardancy of polylactic acid nonwoven fabric through solvent-free transparent coating

Polylactic acid (PLA) nonwovens, recognized as eco-friendly substitutes for petroleum-based synthetic fibers, face a significant challenge due to their inherent flammability. This work addresses this concern by synthesizing a hyperbranched polyphosphoramide flame retardant (TPDT) through a one-step polycondensation process without using solvent and catalyst. TPDT is subsequently applied to PLA nonwovens using a dip-pad finishing technique. Notably, with a mere 7 wt% weight gain of TPDT, the PLA nonwovens exhibit a substantial increase in the limited oxygen index (LOI) value, reaching 32.3 %. Furthermore, the damaged area in the vertical burning test is reduced by approximately 69.2 %. In the cone calorimeter test, 17 wt% weight gain of TPDT results in a 51.4 % decrease in peak heat release rate and a 56.0 % reduction in total heat release compared to the control PLA. Additionally, char residue increases from 1.5 wt% to 31.1 wt% after combustion. The strong affinity between TPDT and PLA molecules persists even after repeated abrasion, ensuring sustained flame retardancy. Importantly, the introduction of TPDT also imparts increased softness to the PLA nonwovens. This work addresses this concern by synthesizing a hyperbranched polyphosphoramide flame retardant (TPDT) through a solvent-free, catalyst-free, and one-step polycondensation process.

Preparation of phosphorus-doped chitosan derivative and its applications in polylactic acid: Crystallization, flame retardancy, anti-dripping and mechanical properties

The topic of biobased flame-retardant PLA has always been of great interest. In our study, we successfully synthesized a phosphorus-containing chitosan derivative (PCS) and combined it with aluminum hypophosphate (AP) to create an effective flame-retardant PLA system. PCS acted as an enhancer, enhancing the thermal performance, crystallinity, and toughness of PLA/AP. Compared to PLA modified with 12 wt% AP achieving UL-94 V-2 level and 24.3 % of limited oxygen index, PLA containing 3 wt% PCS and 9 wt% AP achieved UL-94 V-0 level and limited oxygen index of 28 %. The system testing studies such as CCT, Raman, XPS, and TG-IR results indicated that PLA/AP/PCS exhibited a dual flame-retardant mechanism of condensed and gas phases.

Utilization of spent coffee grounds as charring agent to prepare flame retardant poly(lactic acid) composites with improved toughness

The objective was to utilize spent coffee grounds (SCG) as charring agent to combine with ammonium polyphosphate (APP) to prepare flame retardant poly(lactic acid) (PLA) composites with improved toughness. PLA/APP-SCG and PLA/APP-SCG/KH560 composites were prepared, and silane coupling agent KH560 was applied to improve particle-matrix interfacial compatibility. The particle-matrix interface, char formation, flame retardancy, mechanical properties and fracture morphology of PLA composites were studied. Results showed that PLA/APP-SCG5% and PLA/APP-SCG20% passed UL-94 V-0 rating, and increase in charred residues was favorable for improving flame retardancy. Improved toughness was also obtained compared to PLA, attributed to debonding of APP from matrix under external force as well as plasticization effect of coffee oil contained in SCG. PLA/APP-SCG5%/KH560 and PLA/APP-SCG20%/KH560 showed smaller elongation at break and impact strength compared to PLA/APP-SCG5% and PLA/APP-SCG20%, respectively. The improved interfacial compatibility was unfavorable for debonding of APP from matrix, and both APP and SCG played the role of enhancing strength, thus decreasing toughness. PLA/APP-SCG/KH560 counterparts were actually set as parallel samples to prove that PLA/APP-SCG composites showed improved toughness with weak interfacial compatibility. This study has provided a practical approach to utilize bio-derived wastes as charring agent to prepare flame retardant PLA composites with enhanced toughness.

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 %.

Poly(ethylene succinate-co-lactic acid) as a Multifunctional Additive for Modulating the Miscibility, Crystallization, and Mechanical Properties of Poly(lactic acid)

Polymer blending offers an effective and economical approach to overcome the performance limitations of poly(lactic acid) (PLA). In this study, a series of copolymers poly(ethylene succinate-co-lactic acid) (PESL) were synthesized, featuring lactic acid (LA) contents that ranged from 20 to 86 wt %. This synthesis involved a one-pot industrial melt polycondensation process using succinic acid (SA), ethylene glycol (EG), and LA, catalyzed by titanium tetraisopropoxide (TTP). The goal was to produce a fully biobased copolymer expected to exhibit partial miscibility with pure poly(lactic acid) (PLA). To assess the capability of PESL copolymers in toughening PLA, we conducted tensile testing on PLA/PESL blends containing 15 wt % PESL. As a result, an elongation at break for the blends with 15 wt % loading of the copolymer PESL72 was directly enhanced to 250% with an ultimate strength of 35 MPa, compared to brittle PLA with less 10% tensile length. The morphological features of interfacial adhesion before and after tensile failure were measured by scanning electron microscopy (SEM). A significant enhancement in the chain mobility of the PLA/PESL blends was further evidenced by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). These findings hold promise for the development of functional packaging materials based on PLA. The proposed copolymer design, which boasts strong industrial feasibility, can serve as a valuable guide for enhancing the toughness of PLA.

A Green Treatment Mitigates the Limitations of Coffee Silver Skin as a Filler for PLA/PBSA Compatibilized Biocomposites

The development of fully renewable and biodegradable composites for short-term applications was pursued by combining a compatibilized poly(lactic acid) (PLA)/poly(butylene succinate-co-adipate) (PBSA) (60:40 wt:wt) blend with coffee silver skin (CSS), an industrial byproduct from coffee processing. An epoxy-based reactive agent (Joncryl ADR-4468) was added as a compatibilizer. CSS was incorporated at 5, 10, and 20 wt% in the blend both in the as-received state and after a simple thermal treatment in boiling water, which was performed to mitigate the negative impact of this filler on the rheological and mechanical properties of the blend. The CSS treatment effectively increased the filler degradation temperature of 30-40 °C, enabling stable melt processing of the composites. It also improved filler-matrix adhesion, resulting in enhanced impact properties (up to +172% increase in impact energy compared to the untreated filler). Therefore, treated CSS demonstrated potential as an effective green reinforcement for PLA/PBSA blends for rigid packaging applications. Future works will focus on studying suitable surface modification of CSS to further increase the interfacial interaction and the tensile quasi-static properties, to fully exploit the capabilities of this renewable material toward the development of eco-friendly composites.