Intrinsic Flame-Retardant and Thermally Stable Epoxy Endowed by a Highly Efficient, Multifunctional Curing Agent

Synthesis of a furfural-based DOPO-containing co-curing agent for fire-safe epoxy resins

A furfural-based DOPO-containing flame retardant, 6,6′-(((methylenebis(4,1-phenylene))bis(azanediyl))bis(furan-2-ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (MBF-DOPO), was synthesized and utilized as a co-curing agent of 4,4′-diaminodiphenyl methane (DDM) for fire-safe epoxy thermosets. For the cured epoxy resin containing 4.0% MBF-DOPO, the limiting oxygen index (LOI) reached 32.9% (with the V-0 rating in UL-94 test), and the peak heat release rate and total smoke production values were respectively decreased by 29.3% and 33.6%, compared to pure epoxy resin. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) results confirmed that the furfural-based flame retardant MBF-DOPO promoted the charring formation of the epoxy matrix, which effectively isolated the gas and heat transfer during combustion and thus enhanced the fire-safety performance of the epoxy thermosets. This work provides an effective route for synthesizing a furfural-based flame retardant, which possesses great potential for application in fire-safe epoxy thermosets.

A novel furfural-based DOPO-containing flame retardant MBF-DOPO was successfully synthesized and acted as a co-curing agent for fire-safe epoxy resins.

Flame Retardant Epoxy Composites on the Road of Innovation: An Analysis with Flame Retardancy Index for Future Development

Nowadays, epoxy composites are elements of engineering materials and systems. Although they are known as versatile materials, epoxy resins suffer from high flammability. In this sense, flame retardancy analysis has been recognized as an undeniable requirement for developing future generations of epoxy-based systems. A considerable proportion of the literature on epoxy composites has been devoted to the use of phosphorus-based additives. Nevertheless, innovative flame retardants have coincidentally been under investigation to meet market requirements. This review paper attempts to give an overview of the research on flame retardant epoxy composites by classification of literature in terms of phosphorus (P), non-phosphorus (NP), and combinations of P/NP additives. A comprehensive set of data on cone calorimetry measurements applied on P-, NP-, and P/NP-incorporated epoxy systems was collected and treated. The performance of epoxy composites was qualitatively discussed as Poor, Good, and Excellent cases identified and distinguished by the use of the universal Flame Retardancy Index (FRI). Moreover, evaluations were rechecked by considering the UL-94 test data in four groups as V0, V1, V2, and nonrated (NR). The dimensionless FRI allowed for comparison between flame retardancy performances of epoxy composites. The results of this survey can pave the way for future innovations in developing flame-retardant additives for epoxy.

A biobased Schiff base from protocatechualdehyde and its application in flame-retardant, low-smoke epoxy resin systems

Herein, a new renewable Schiff base flame retardant 4,4'-((1E,1'E)-((oxybis(4,1-phenylene))bis(azanylylidene))bis(methanylylidene))bis(benzene-1,2-diol) (PH-ODA) was prepared by the reaction of protocatechualdehyde with 4,4'-diaminodiphenyl ether (ODA). PH-ODA (acting as a carbonization agent) combined with ammonium polyphosphate (APP) were used as intumescent flame retardants for commercial bisphenol A epoxy resin (DGEBA). For the cured epoxy resin containing 7.5% APP and 2.5% PH-ODA, the limiting oxygen index (LOI) reached 29.9% (with the V-0 rating in UL-94 test), and the peak heat release rate and total smoke production were respectively decreased by 88.1% and 68.3%, compared with pure epoxy resin. The enhancement of fire-safety performance was due to PH-ODA/APP promoting the formation of a compact intumescent char structure. It was also found that the synergism between PH-ODA and APP was helpful to enhance the fire resistance of the epoxy matrix. This work provides a facile and sustainable route for synthesizing Schiff base compounds from biomass-derived resources, possessing great potential for application in highly-effective intumescent flame retardants.

Synthesis of a novel phosphazene-based flame retardant with active amine groups and its application in reducing the fire hazard of Epoxy Resin

A novel compound containing active amine groups of polyphosphazene (PBFA) was successfully synthesized and applied as a reactive flame-retardant additive in epoxy (EP) resin. It was synthesized from N-aminoethylpiperazine and hexachlorocyclotriphosphazene using a simple method, and its structure was well-characterized. The results indicated that introducing PBFA into EP composites significantly improves the resistance to fire and suppresses smoke generation. An EP composite with 9.0 wt% PBFA can pass the vertical burning tests V-0 rating, the peak heat release rate and total heat release of the sample decreased by 46.7% and 29.3%, respectively. Moreover, it decreased the total smoke release by 48.0%. Thermogravimetric analysis showed that the presence of PBFA can accelerate EP decomposition at comparatively low temperatures and lead to the formation of a stable char layer, which protects the matrix from fire, therefore improving the amount of char residue at 800 °C. The degree of small molecule degradation characterized by gas chromatograph/mass spectrometer, which was lower than that of pure EP, demonstrating that PBFA reduces the risk of fire. The glass transition temperature of EP composites increased with the amount of PBFA increasing owing to the presence of active amine groups. Notably, its mechanical properties were not degraded.

Self-Extinguishing Resin Transfer Molding Composites Using Non-Fire-Retardant Epoxy Resin

Introducing fire-retardant additives or building blocks into resins is a widely adopted method used for improving the fire retardancy of epoxy composites. However, the increase in viscosity and the presence of insoluble additives accompanied by resin modification remain challenges for resin transfer molding (RTM) processing. We developed a robust approach for fabricating self-extinguishing RTM composites using unmodified and flammable resins. To avoid the effects on resin fluidity and processing, we loaded the flame retardant into tackifiers instead of resins. We found that the halogen-free flame retardant, a microencapsulated red phosphorus (MRP) additive, was enriched on fabric surfaces, which endowed the composites with excellent fire retardancy. The composites showed a 79.2% increase in the limiting oxygen index, a 29.2% reduction in heat release during combustion, and could self-extinguish within two seconds after ignition. Almost no effect on the mechanical properties was observed. This approach is simple, inexpensive, and basically applicable to all resins for fabricating RTM composites. This approach adapts insoluble flame retardants to RTM processing. We envision that this approach could be extended to load other functions (radar absorbing, conductivity, etc.) into RTM composites, broadening the application of RTM processing in the field of advanced functional materials.

Spontaneous Shape Alteration and Size Separation of Surfactant-Free Silver Particles Synthesized by Laser Ablation in Acetone during Long-Period Storage

The technique of laser ablation in liquids (LAL) has already demonstrated its flexibility and capability for the synthesis of a large variety of surfactant-free nanomaterials with a high purity. However, high purity can cause trouble for nanomaterial synthesis, because active high-purity particles can spontaneously grow into different nanocrystals, which makes it difficult to accurately tailor the size and shape of the synthesized nanomaterials. Therefore, a series of questions arise with regards to whether particle growth occurs during colloid storage, how large the particle size increases to, and into which shape the particles evolve. To obtain answers to these questions, here, Ag particles that are synthesized by femtosecond (fs) laser ablation of Ag in acetone are used as precursors to witness the spontaneous growth behavior of the LAL-generated surfactant-free Ag dots (2–10 nm) into different polygonal particles (5–50 nm), and the spontaneous size separation phenomenon by the carbon-encapsulation induced precipitation of large particles, after six months of colloid storage. The colloids obtained by LAL at a higher power (600 mW) possess a greater ability and higher efficiency to yield colloids with sizes of <40 nm than the colloids obtained at lower power (300 mW), because of the generation of a larger amount of carbon ‘captors’ by the decomposition of acetone and the stronger particle fragmentation. Both the size increase and the shape alteration lead to a redshift of the surface plasmon resonance (SPR) band of the Ag colloid from 404 nm to 414 nm, after storage. The Fourier transform infrared spectroscopy (FTIR) analysis shows that the Ag particles are conjugated with COO– and OH– groups, both of which may lead to the growth of polygonal particles. The CO and CO₂ molecules are adsorbed on the particle surfaces to form Ag(CO)_x and Ag(CO₂)_x complexes. Complementary nanosecond LAL experiments confirmed that the particle growth was inherent to LAL in acetone, and independent of pulse duration, although some differences in the final particle sizes were observed. The nanosecond-LAL yields monomodal colloids, whereas the size-separated, initially bimodal colloids from the fs-LAL provide a higher fraction of very small particles that are <5 nm. The spontaneous growth of the LAL-generated metallic particles presented in this work should arouse the special attention of academia, especially regarding the detailed discussion on how long the colloids can be preserved for particle characterization and applications, without causing a mismatch between the colloid properties and their performance. The spontaneous size separation phenomenon may help researchers to realize a more reproducible synthesis for small metallic colloids, without concern for the generation of large particles.