Synthesis of DOPO‐based pyrazine derivative and its effect on flame retardancy and thermal stability of epoxy resin

Eco-Friendly One-Pot Supramolecular-Assembly of P-N Flame Retardant for Fire-Safe Epoxy Resin

Flame retardant treatment of epoxy resins (EP) to reduce their flammability for extending their range of applications attracts considerable attention. However, the synthesis process of conventional flame retardants is complicated and involves organic hazardous solvents. Meanwhile, how to ensure both the flame-retardant and mechanical properties is a long-standing and actual difficult problem. In this work, a supramolecular flame retardant (named ATPFR) is facilely created by one-pot reaction, using cheap and accessible raw materials in an ecologically benign aqueous solvent. ATPFR is applied to improve the fire safety of EP. With only 5 wt% ATPFR addition, EP can reach the limiting oxygen index of 28.5% and the UL-94 V-0 rating with a significant "blow-out effect." The cone calorimetry test reveals that the EP thermoset with 5 wt% ATPFR has a 75.8% reduction in the peak heat release rate (p-HRR) and a 67.3% reduction in the peak smoke production rate (p-SPR), respectively, compared with the pure EP. Additionally, EP composites with the small amount of ATPFR exhibit a slight decrease and maintain good mechanical properties. Therefore, the facile synthesis and application of this supramolecular flame retardant provide a reliable way for the construction of polymer materials with environment-friendly and effective flame-retardant system.

Amino Phenyl Copper Phosphate-Bridged Reactive Phosphaphenanthrene to Intensify Fire Safety of Epoxy Resins

To improve the compatibility between flame retardant and epoxy resin (EP) matrix, amino phenyl copper phosphate-9, 10-dihydro-9-oxygen-10-phospha-phenanthrene-10-oxide (CuPPA-DOPO) is synthesized through surface grafting, which is blended with EP matrix to prepare EP/CuPPA-DOPO composites. The amorphous structure of CuPPA-DOPO is characterized by X-ray diffraction and Fourier-transform infrared spectroscopy. Scanning electron microscope (SEM) images indicate that the agglomeration of hybrids is improved, resisting the intense intermolecular attractions on account of the acting force between CuPPA and DOPO. The results of thermal analysis show that CuPPA-DOPO can promote the premature decomposition of EP and increase the residual amount of EP composites. It is worth mentioning that EP/6 wt% CuPPA-DOPO composites reach UL-94 V-1 level and limiting oxygen index (LOI) of 32.6%. Meanwhile, their peak heat release rate (PHRR), peak smoke production release (PSPR) and CO₂ production (CO₂P) are decreased by 52.5%, 26.1% and 41.4%, respectively, compared with those of EP. The inhibition effect of CuPPA-DOPO on the combustion of EP may be due to the release of phosphorus and ammonia free radicals, as well as the catalytic charring ability of metal oxides and phosphorus phases.

Effect of 3-methoxysalicylaldehyde on transparency and mechanical properties of EP modified with aminopyridine-based DOPO derivative

Two effective flame retardant additives hsalaminopyridine phosphaphenanthrene (HAD) and methoaminopyridine phosphaphenanthrene (MAD) were respectively prepared with DOPO, aminopyridine, salicylaldehyde and 3-Methoxysalicylaldehyde. Subsequently, HAD and MAD was added into epoxy resin (EP) respectively, and flame retardancy of EP/HAD and EP/MAD thermosets were dramatically enhanced. The result of cone calorimeter (CC) test revealed that both HAD and MAD showed perfect smoke suppression performance. In respect of transmittance and mechanical properties, there was a huge difference between HAD and MAD. The introduction of HAD dramatically harmed transmittance and mechanical properties of EP, while, EP/10%MAD was transparent and nearly possessed the same mechanical properties with pure EP.

A Study on the Synthesis, Curing Behavior and Flame Retardance of a Novel Flame Retardant Curing Agent for Epoxy Resin

In this work, a flame retardant curing agent (DOPO-MAC) composed of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide DOPO and methyl acrylamide (MAC) was synthesized successfully, and the structure of the compound was characterized by FT-IR and 1H-NMR. The non-isothermal kinetics of the epoxy resin/DOPO-MAC system with 1% phosphorus was studied by non-isothermal DSC method. The activation energy of the reaction (Ea), about 46 kJ/mol, was calculated by Kissinger and Ozawa method, indicating that the curing reaction was easy to carry out. The flame retardancy of the epoxy resin system was analyzed by vertical combustion test (UL94) and limiting oxygen index (LOI) test. The results showed that epoxy resin (EP) with 1% phosphorus successfully passed a UL-94 V-0 rating, and the LOI value increased along with the increasing of phosphorus content. It confirmed that DOPO-MAC possessed excellent flame retardance and higher curing reactivity. Moreover, the thermal stability of EP materials was also investigated by TGA. With the DOPO-MAC added, the residual mass of EP materials increased remarkably although the initial decomposition temperature decreased slightly.

Transparent low-flammability epoxy resins using a benzoguanamine-based DOPO derivative

We successfully prepared a highly effective flame-retardant additive called hsalbenzoguanamine phosphaphenanthrene (HDPD) through salicylaldehyde and nitrogen-rich benzoguanamine. The introduction of HDPD into epoxy resin (EP) sharply enhanced the flame retardancy of EP/HDPD thermosets. The introduction of 6 wt% HDPD into EP succeeded in reaching the V-0 rating. Limited oxygen index results revealed the high flame-retarding performance of HDPD. Cone calorimeter test data revealed that heat and smoke released from EP/6 wt% HDPD thermoset were significantly restrained. In addition, EP/6 wt% HDPD thermoset demonstrated excellent transmittance and mechanical strength. The transmittance of EP/6 wt% HDPD was assessed from 520 to 800 nm. The results showed that transmittance of EP/6 wt% HDPD were nearly 90% of the control group.

A bio-based phosphaphenanthrene-containing derivative modified epoxy thermosets with good flame retardancy, high mechanical properties and transparency

Phosphorus-containing flame retardants have received huge interest for improving the flame retardant behavior of epoxy resins (EP) over the past few decades. However, a satisfactory flame retardant effect requires high loading of most phosphorus-containing flame retardants, resulting in the deterioration of the thermo-mechanical properties of the flame retardant epoxy materials. To obtain the flame retardant EP with excellent comprehensive properties, a furfurylamine-derived bis-DOPO derivative (FA-bis-DOPO) was synthesized from bio-mass as a co-curing agent for the flame retardant EP. The incorporation of FA-bis-DOPO improved the mechanical strength, the storage modulus and the glass transition temperature of the flame retardant epoxy materials, owing to its stiffness and reactivity with the epoxy matrix to enhance the crosslinking density. The EP material containing 5.0 wt% of FA-bis-DOPO had an LOI of 31.0% and UL-94 V-0 rating, while the pristine EP had an LOI of 23.5% and failed in the UL-94 test, manifesting the high flame retardant efficiency of FA-bis-DOPO. Besides, the cone calorimeter results demonstrated that the PHRR, THR, and TSP values of the EP/FA-bis-DOPO-5.0 were 28.0%, 27.3%, and 9.9% lower than those of the pristine EP, respectively. The flame retardant mechanism of FA-bis-DOPO could be attributed to the combined vapor and condensed phase mechanisms which involved the interruption of the combustion chain reaction by quenching radicals and the inhibition of the transfer of pyrolytic volatile products by catalytic formation of an intact and compact char layer.

Phosphorus-containing flame retardants have received huge interest for improving the flame retardant behavior of epoxy resins (EP) over the past few decades.

A phosphorus-containing hyperbranched phthalocyanine flame retardant for epoxy resins

A hyperbranched phosphorus-containing copper phthalocyanine compound (DOPO-CuPc) was successfully synthesized and used as the flame-retardant additive to prepare flame-retarded epoxy thermosets. The addition of DOPO-CuPc led to a significant enhancement of the flame retardant properties of the epoxy resin. The 15DOPO-CuPc/EP composite obtained a LOI value of 35.8%, and the UL-94 rose from NR to V-0 rating. And the addition of DOPO-CuPc resulted in early decomposition of the epoxy thermoset, but the residual char at 700 °C reached 27.7%. The flame retardant mechanism was further investigated. It was found that DOPO-CuPc could release phosphorus-containing radicals and non-combustible gases in the gas phase to exert gas-phase flame retardant activity. In the condensed phase, the epoxy thermoset formed the expanded honeycomb-like char layer during combustion and the presence of copper phthalocyanine contributed to the stability of the char layer.

Efficient Adsorption of Dyes Using Polyethyleneimine-Modified NH2-MIL-101(Al) and its Sustainable Application as a Flame Retardant for an Epoxy Resin

Metal-organic frameworks (MOFs) exhibit highly designable properties and have been used in wide applications. To further improve their performance, the modification of MOFs is an effective method. However, the modification process is usually complicated. Besides, the sustainable use of MOFs is difficult to achieve due to the complicated recycling treatment. Herein, we designed a polyethyleneimine (PEI)-modified NH₂-MIL-101(Al) composite (PEI@NH₂-MIL-101(Al)). This composite showed excellent dye removal performance of methyl orange (MO, 89.4%) and Direct Red 80 (DR80, 99.8%). Remarkably, the dye removal application of PEI@NH₂-MIL-101(Al) also acted as a modification process toward flame retardant application. Thus, the dye-adsorbed PEI@NH₂-MIL-101(Al) composite (MO-PEI@NH₂-MIL-101(Al) and DR80-PEI@NH₂-MIL-101(Al)) was sustainably used as an effective flame retardant for an epoxy resin (EP) at low additions (4.0 wt %). The limiting oxygen values of EP/MO-PEI@NH₂-MIL-101(Al) and EP/DR80-PEI@NH₂-MIL-101(Al) increased to 26.5 and 26.7%, respectively. The heat release and the smoke production of dyes-PEI@NH₂-MIL-101(Al)/EP were greatly reduced compared with those of EP. This strategy provides a simple and effective modification method for MOFs. Meanwhile, the modified MOF composite can achieve sustainable application, giving full play to the advantages of MOFs.

Synthesis of chitosan-based flame retardant and its fire resistance in epoxy resin

A novel flame retardant CCD was synthesized by the condensation between cinnamalde and chitosan, followed by the addition reaction with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO), and CCD was named from the initials of the three raw materials. The intermediate product CC and the target product CCD was then characterized by infrared spectrometry and elemental analysis. The flame retardancy of EP thermosets modified by CCD was dramatically improved. Epoxy resin (EP) with 10wt% CCD passed vertical burning (UL-94) V-0 rating and possessed limited oxygen index (LOI) value of 31.6 %. Cone calorimeter test exhibited the introduction of 3.5g CCD into 35g EP decreased the total heat release by 38.8 % and decreased the total smoke product by 72.0 %. XPS, FTIR, SEM and Raman tests were proceeded to determine the char residue for EP/10 % CCD thermoset, and the results showed that char residue for EP/10 % CCD thermoset possessed dense and compact structure which played a positive effect in blocking the exchange of heat and gas.

Recent Developments in the Flame-Retardant System of Epoxy Resin

With the increasing emphasis on environmental protection, the development of flame retardants for epoxy resin (EP) has tended to be non-toxic, efficient, multifunctional and systematic. Currently reported flame retardants have been capable of providing flame retardancy, heat resistance and thermal stability to EP. However, many aspects still need to be further improved. This paper reviews the development of EPs in halogen-free flame retardants, focusing on phosphorus flame retardants, carbon-based materials, silicon flame retardants, inorganic nanofillers, and metal-containing compounds. These flame retardants can be used on their own or in combination to achieve the desired results. The effects of these flame retardants on the thermal stability and flame retardancy of EPs were discussed. Despite the great progress on flame retardants for EP in recent years, further improvement of EP is needed to obtain numerous eco-friendly high-performance materials.

Novel Phosphorus-Nitrogen-Containing Ionic Liquid Modified Metal-Organic Framework as an Effective Flame Retardant for Epoxy Resin

Metal-organic frameworks (MOFs) have shown great potential in flame retardant applications; however, strategies for fully exploiting the advantages of MOFs in order to further enhance the flame retardant performance are still in high demand. Herein, a novel MOF composite was designed through the generated cooperative role of MOF (NH₂-MIL-101(Al)) and a phosphorus-nitrogen-containing ionic liquid ([DPP-NC₃bim][PMO]). The ionic liquid (IL) was composed of imidazole cation modified with diphenylphosphinic group (DPP) and phosphomolybdic acid (PMoA) anions, which can trap the degrading polymer radicals and reduce the smoke emission. The MOF acts as a porous host and can avoid the agglomeration of ionic liquid. Meanwhile, the -NH₂ groups of NH₂-MIL-101(Al) can increase the compatibility with epoxy resin (EP). The framework is expected to act as an efficient insulating barrier to suppress the flame spread. It was demonstrated that the MOF composite (IL@NH₂-MIL-101(Al)) is able to effectively improve the fire safety of EP at low additions (3 wt. %). The LOI value of EP/IL@NH₂-MIL-101(Al) increased to 29.8%. The cone calorimeter results showed a decreased heat release rate (51.2%), smoke production rate (37.8%), and CO release rate (44.8%) of EP/IL@NH₂-MIL-101(Al) with respect to those of neat EP. This strategy can be extended to design other advanced materials for flame retardant.