Synthesis of curing agent for epoxy resin based on halogenophosphazene

Epoxy resins are, due to their excellent properties (such as chemical resistance, dimensional stability, and heat resistence), widely used in practice. The basic principle of curing epoxy resins with a hardener containing multiple amino groups is the crosslinking reaction between active hydrogen atoms in the hardener and the oxirane groups in the epoxy resin. This study deals with the synthesis and characterization of hexachloro‐cyclo‐triphosphazene derivative and its subsequent use for curing epoxy resins. The new hardener was prepared from hexachloro‐cyclo‐triphosphazene by nucleophilic substitution with isophorone diamine and its curing capability was compared with original isophorone diamine. The prepared derivative hexaisophorone diamino‐cyclo‐triphosphazene (HICTP) provided advantages over conventional curing system, as it improved mechanical properties as well as the flame resistance. Testing of the cured epoxy resin during burning was carried out using dual cone calorimeter, which enables more extensive monitoring of parameters in comparison with testing using oxygen index that has been used in many publications. The epoxy resin cured with the prepared phosphorus containing HICTP exhibits lower values for total heat release, amount of smoke released and oxygen consumed, which may cause a slower flame spread. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42917.     

Curing behavior,mechanical, and flame-retardant properties of epoxy-based composites filled by expandable graphite and ammonium polyphosphate

In this study, 4,4′-Diaminodiphenyl methane (DDM) was exploited as hardener for diglycidyl ether of daidzein (DGED) and epoxy resin mixture (E-D). The composites were produced by blending with different contents of ammonium polyphosphate (APP) and expandable graphite (EG) into E-D resin, named E-D/A-E composites. Firstly, E-D/A-E blends were analyzed by Fourier transform infrared spectroscopy and DSC to evaluate their curing behavior. The TGA was performed on the E-D/A-E composites to analyze their thermal stability, and the thermal properties of the composites were enhanced by adding APP-EG (A-E) flame retardants. The mechanical properties and the fractured surfaces of the E-D/A-E composites were studied by tensile, impact tests and scanning electron microscopy. Finally, through torch burning test, limiting oxygen index (LOI), vertical burning (UL-94) and micro-calorimetry tests, the influence of A-E flame retardants contents on flammability of composites were evaluated. The results showed that the UL-94 V-0 rating was obtained for the E-D/A-E composites and the composites exhibited a self-extinguishing phenomenon. When A-E flame retardants content reached 9.1 wt%, the LOI value reached 34.5%, and total heat release (Total HR) was reduced by 20.2% compared with E-D/A-E 1.     

Superior thermal stability and smoke suppression of epoxy resin composites with graphene/LDH phosphorus-rich hybrids

In this article, graphene/LDH phosphorus-rich triple hybrid was prepared by a mild method and used to effectively improve the thermal stability and smoke suppression of epoxy resin (EP). The graphene was firstly reacted with hexamethylenediamine (HA) and followed by the treatment with the layered double hydroxide (LDH) and NaH₂PO₄ solutions. Compared to the unmodified graphene, the initial decomposition temperature of the triple hybrids increases significantly from 168.6 to 292.5°C. The residual carbon content is greatly improved and the residual mass is up to 84.1%. Elemental analysis reveals the content of phosphorus in EP composites is as high as 10 wt%. In flame retardancy tests, the peak heat release rate of the EP composite with 5 wt% graphene/LDH phosphorus-rich hybrids decreases to 786.15 KW/m², 41.19% drastic reduction compared to that of EP. These results indicate that the triple functionalization process effectively expands the interval distribution of heat release and makes the heat release process more gradual and spread flames smaller. The smoke production rate and total smoke production rate of EP composite with 5 wt% graphene/LDH phosphorus-rich hybrids are 0.32 m²/s and 40.91 m², which are significantly reduced by 65.22 and 57.83%, respectively. This gentle and efficient process provides a new approach to multi-functional design to improve the thermal stability and smoke suppression of resin-based composites.     

Cross-linked Salen-based polyphosphazenes (Salen-PZNs) enhancing the fire resistance of epoxy resin composites

In this work, two cross-linked Salen-based polyphosphazenes (Salen-PZNs: Salen-PZN-1 microspheres and layered Salen-PZN-2), which contains Salen-Schiff base and phosphazene components, were synthesized. The results showed that the peak heat release rate and total smoke production of 3% Salen-PZN-1/epoxy resin (EP) (3 wt% addition) were reduced by 23.8% and 87.3%, respectively. Meanwhile, after introducing the layered Salen-PZN-2 into EP, efficient flame resistance was obtained. The results of thermogravimetric analysis/infrared spectrometry proved that the harmful gasses of 5% Salen-PZN-1/EP composites were reduced during the combustion. The possible flame retardancy mechanism was considered to be the synergy of phosphate group catalysis, release of nitrogen-containing noncombustible gasses and gas phase quenching. Therefore, this work provides a method for preparing polymers with highly efficient flame-retardant properties.     

Layer-by-layer assembled bagasse to enhance the fire safety of epoxy resin: A renewable environmental friendly flame retardant

In an effort to make rational use of resources, wasted bagasse were collected, and then fabricated by layer-by-layer (LbL) assembly to prepare an environmentally benign flame retardant for epoxy resin (EP). Six chitosan/ammonum polyphosphate bilayers were successfully deposited on the surface of bagasse, which endows EP/6BL@BF composites with a limiting oxygen index value of 24.1% and V-1 rating in UL-94 test. Compared with untreated bagasse, cone calorimetry test reveals that the incorporation of 6BL@BF reduces the peak heat release rate and total heat release by 64.6% and 13.2%, respectively. Besides, expandable graphite (EG) was combined with 6BL@BF to further enhance the fire safety and thermal stability of EP composites. Furthermore, the gas and condensed phase analysis of EP/6BL@BF/EG-3 composite was investigated by TG-IR, scanning electronic microscopy, Laser Raman spectroscopy, and X-ray photoelectron spectroscopy, and the flame retardant mechanism of 6BL@BF/EG was proposed. Finally, the mechanical properties of EP composites were investigated systematically.     

Structure,thermal, and mechanical properties of DDM‐hardened epoxy/benzoxazine hybrids: Effects of epoxy resin functionality and ETBN toughening

Bifunctional, trifunctional, and tetrafunctional epoxy (EP) resins were hardened with stoichiometric amount of 4,4′‐diaminodiphenyl methane in presence and absence of benzoxazine (BOX). The EP/BOX ratio of the hybrid systems was constant, viz. 50/50 wt %. For the bifunctional EP, the EP/BOX range covered the ratios 75/25 and 25/75 wt %, as well. Epoxy‐terminated liquid nitrile rubber (ETBN) was incorporated in 10 wt % in the systems with trifunctional and tetrafunctional EP, and in 10, 15, and 20 wt % in the EP/BOX with bifunctional EP to improve their toughness. Information on the structure and morphology of the hybrid systems was received from differential scanning calorimetric, dynamic‐mechanical thermal analysis, atomic force microscopic, and scanning electron microscopic studies. The flexural, fracture mechanical properties, thermal degradation, and fire resistance of the EP/BOX and EP/BOX/ETBN hybrids were determined. It was found that some homopolymerized BOX was built in the EP/BOX conetwork in form of nanoscale inclusions, whereas ETBN formed micron scaled droplets of sea‐island structure. Incorporation of BOX improved the charring and fire resistance, enhanced the flexural modulus and strength, reduced the glass transition (T_g), the fracture toughness, and energy. Additional modification with ETBN decreased the charring, fire resistance, flexural modulus and strength, as well as T_g, however, improved the fracture toughness and especially the fracture energy. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Novel multi-element DOPO derivative toward low-flammability epoxy resin

In order to obtain cured epoxy resin (EP) with satisfactory thermal stability and flame retardancy, a multi-element P/N/Si-containing flame retardant (DPAK) was synthesized by a facile way and was used as a reactive flame retardant to prepare flame-retardant EP. The flame-retardant efficiency of DPAK was subsequently evaluated by limiting oxygen index (LOI), UL-94, and cone calorimeter (CC) test. With a low incorporation amount of DPAK (4 wt%), the resultant EP achieve to UL-94 V-0 rating, and the corresponding LOI value reached to 30%, which was higher than that of EP containing DOPO (2.9 wt%). More importantly, the thermogravimetric analysis (TGA) revealed their higher thermal stability than those of EP containing DOPO. Furthermore, dynamic mechanical analysis (DMA) shown the maintained glass transition temperature of DPAK-EP. The increase of CO/CO₂ ratio in the CC test for the DPAK-EP samples proved the gas-phase activity of DPAK. Additionally, DPAK showed evidence of condensed phase activity by increasing char residue in TGA and CC test. The scanning electronic microscope together with the energy dispersive X-ray spectrometer (SEM–EDX) and X-ray photoelectron spectroscopy (XPS) exhibited that DPAK promoted the formation of compacted phosphorus-silicon char layer. Subsequently, TG-FTIR results indicated that DPAK-EP produced lesser combustible gases than neat sample did, improving flame-retardant properties of epoxy resin.     

Flame‐retardant,glass‐fabric‐reinforced epoxy resin laminates fabricated through a gradient distribution mode

In general, epoxy resin (EP) glue mixed with a high content of flame retardants is used to coat glass fabrics layer by layer to prepare fire‐retardant printed circuit boards (PCBs). However, the addition of the flame retardants not only increases the cost but also greatly deteriorates the processability and mechanical properties of the PCBs. In this study, a gradient distribution mode of composite flame retardants was designed and applied in EP‐based PCB composites. Unlike the traditional uniform distribution mode, in which flame retardants are evenly distributed in every resin layer, the gradient mode concentrates a higher content of the flame retardants on the surface layer, and the concentrations are gradually reduced along the thickness. In this way, the surface resin can quickly form a condensed charring barrier to hold back fire; this effectively protects the underlying resin, which has lower contents of flame retardant. The results of this study show that PCB prepared by the gradient mode obtained satisfactory flame retardance (a UL94 V‐0 rating) with only a 3.5 wt % total amount of flame retardant; this value was much lower than that (6.3 wt %) of composites featuring a uniform distribution. Additionally, the gradient mode also maintained the mechanical properties of PCB better. The tensile, impact, and flexural strengths of the gradient distribution system were obviously higher than those of the uniform distribution one with the same content of flame retardant. On the basis of the mode, a more economic and efficient technology was developed to manufacture flame‐retardant layered PCB. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44369.     

The degradation and charring of flame retarded epoxy resin during the combustion

To study flame retardant mechanism of epoxy resin (EP) by octaphenyl silsesquioxane and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide, an experimental method is set up to investigate the residue of the EPs, which is extinguished during the cone calorimeter testing at different times. The chemical structures of the residues are investigated by the FTIR and XPS. The breakdown of EPs network and formation of new crosslinking structure are supported by the FTIR analysis. The changes of C and O concentrations in the condensed phase during the combustion are investigated by XPS in detail. Moreover, formation of organic carbon is uncovered by the plasmon loss curves based on XPS that could track the carbon crosslinking. These results exhibited a whole degradation and charring process of EP during the combustion: degradation of EP chain, more crosslinking charring, and thermal oxidation of the char. Furthermore, a program of combustion and degradation process of EPs is described in this research. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4119–4128, 2013     

Novel strategy for molybdenum disulfide nanosheets grown on titanate nanotubes for enhancing the flame retardancy and smoke suppression of epoxy resin

A novel hybrid consisting of a molybdenum disulfide (MoS₂) coating on a titanium dioxide nanotube (TNT) surface (MoS₂–TNT) was synthesized by a hydrothermal method. The MoS₂, TNTs, and MoS₂–TNT hybrid were incorporated into epoxy resin (EP) to study their effects on its thermal performance and flame retardancy. Thermogravimetric analysis results show that the char yield at 700 °C of EP–MoS₂–TNTs was obviously increased compared with that of the EP–MoS₂ or EP–TNTs; this indicated that MoS₂–TNTs had a good carbonization effect. The limiting oxygen index, cone calorimetry, and smoke density tests showed that MoS₂–TNTs effectively improved the flame retardancy and smoke suppression of EP. This was attributed to the physical barrier effect of MoS₂ and the adsorption of TNTs. Moreover, the flame retardancy and smoke suppression of the EP–MoS₂–TNTs were better than those of the EP–MoS₂ or EP–TNTs alone with the same amount of addition; this indicated that there was a synergistic effect between MoS₂ and TNTs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46064.     

Synthesis and properties of a novel phosphorous‐containing flame‐retardant hardener for epoxy resin

An aryl phosphinate dianhydride 1,4‐bis(phthalic anhydride‐4‐carbonyl)‐2‐(6‐oxido‐6H‐dibenz[c,e][1,2]‐oxaphosphorin‐6‐yl)‐phenylene ester (BPAODOPE) was synthesized and its structure was identified by FTIR and ¹H‐NMR. BPAODOPE was used as hardener and flame retardant for preparing halogen‐free flame‐retarded epoxy resins when coupled with another curing agent. Thermal stability, morphologies of char layer, flame resistance and mechanical properties of flame‐retarded epoxy resins were investigated by thermogravimetric analysis, SEM, limiting oxygen index (LOI), UL‐94 test, tensile, and charpy impact test. The results showed that the novel BPAODOPE had a better flame resistance, the flame resistance and char yield of flame‐retarded epoxy resins increased with an increase of phosphorus content, tensile strength and impact strength of samples gradually decreased with the addition of BPAODOPE. The flame‐retarded sample with phosphorus contents of 1.75% showed best combination properties, LOI value was 29.3, and the vertical burning test reached UL‐94 V‐0 level, tensile strength and impact strength were 30.78 MPa and 3.53 kJ/m², respectively. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Flame retardance and origin of bismaleimide resin composites with green and efficient aluminum phosphates

Developing green and high efficiency inorganic flame retardants is the trend of preparing flame retarding polymer composites. Aluminum phosphates (t‐hAP) with uniform, small dimension, and hexagonal structure were facilely synthesized, which have similar size (1–2 μm) but different structures from commercial spherical‐like aluminum phosphate (cAP). The flame retardance of bismaleimide (BD)/t‐hAP and BD/cAP composites were intensively investigated. t‐hAP is proved to have much better flame retarding effect than cAP, but also exhibits advantages over Mg(OH)₂ and Al(OH)₃. With only 5 wt % addition of t‐hAP into BD resin, the peak and total heat releases as well as total smoke production significantly reduce 42.3, 47.8, and 67.3%, respectively; besides, better data are obtained as the loading of t‐hAP increases to 10 wt %. These attractive data result from three effects induced by t‐hAP. Besides the better protection role of sheet structure, the strong hydrogen bonding between t‐hAP and BD resin endows the composite with good dispersion of t‐hAP and high crosslinking density; moreover, t‐hAP releases H₂O and NH₃, diluting flammable gases during combustion. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41089.     

Improving fire behavior and smoke suppression of flame-retardant PBS composites using lignin chelate as carbonization agent and catalyst

A copolymer of alkali lignin and polyacrylamide was fabricated by graft copolymerization, and further chelated with Fe³⁺. The obtained lignin chelate with star structure was used as synergistic agent and catalyst with the incorporation of intumescent flame retardants (IFRs) to prepare flame-retarded poly (butylene succinate) (PBS) composites. The replacement of IFR by lignin chelate favors the enhancement of mechanical performance, resulting in the synchronous improvement of tensile and flexural properties. Compared with the specimen used 25 wt% IFRs (75P/25I), the tensile strength, flexural strength, and modulus of specimen prepared by 23 wt% IFRs and 2 wt% lignin chelate (75P/23I/2LC) exhibited significant increased. A synergistic effect between IFR and lignin chelate occurred when they were combined with an appropriate ratio. When IFR and lignin chelate loadings were 24 and 1 wt%, respectively (75P/24I/1 LC), the limited oxygen index (LOI) value of 36.2% and UL-94 V0 rate of composite could be achieved. Compared with 75P/25I, the char residue mass of 75P/24I/1 LC increased by 35.7%. Moreover, SEM images indicated that a more compact, smooth, and continuous char layer of 75P/24I/1 LC could be formed during combustion. The peak heat release rate (pHRR) and total heat release (THR) of 75P/24I/1 LC decreased by 58.9% and 9.9% compared with PBS, also decreased by 10.3% and 4.8% compared with 75P/25I, respectively. TGIR and FTIR analysis also indicated that lignin chelate exhibited an excellent synergistic effect with IFRs, and gave PBS a good flame retardancy by making contribution to char-formation and gas-phase flame retardancy. This study provides an alternative way for the application of natural polymers such as lignin in flame retardant materials.     

Effect of functionalized graphene oxide with phosphaphenanthrene and isocyanurate on flammability,mechanical properties,and thermal stability of epoxy composites

Maleic anhydride, 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and 1, 3, 5-triglycidyl isocyanurate functionalized graphene oxide (GO) was prepared in this paper. The resultant phosphorus-nitrogen functionalized GO called GOMT was homogeneously dispersed and incorporated into diglycidyl ether of bisphenol A to prepare composites. The char residue of GOMT/EP composites increased and its LOI value increased to 28.1% with UL-94 V-1 rating. T _g of composite containing 1 phr GOMT increases to 165.6 °C, and the storage modulus of the sample with 3 phr GOMT was increased by 19% compared with pure EP. Furthermore, the elastic modulus and flexural strength of epoxy composite with 5 phr GOMT were increased by 17.9 and 26.7% at room temperature, respectively. Besides, the incorporation of GOMT into EP significantly reduces the PHRR and THR of the matrix. Therefore, the as-designed GOMT not just obviously enhances the flame retardancy with low loading but raises the mechanical behavior and thermal stability of epoxy resins. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48761.     

Excellent role of Cu2O on fire safety of epoxy resin with ammonium polyphosphate based on the construction of self-intumescent flame retardant system

This work reports an effective self-intumescent flame retardant system for epoxy resin (EP) based on the remarkable synergistic effect between Cu₂O and ammonium polyphosphate (APP). The effect of Cu₂O/APP on improving EP's fire performance was evaluated by limited oxygen index (LOI), UL-94, and cone calorimeter test. The optimal mass ratio of Cu₂O: APP was shown to be 2:8. With 15 wt% total flame retardant loading, the EP with optimum Cu₂O/APP formulation reached V-0 classification and high LOI (33.5%), while the EP with APP only got NR and low LOI (26.5%). Additionally, the pHRR, total heat release, total smoke production, CO production of the EP with optimum Cu₂O/APP formulation were primarily decreased. All the improvements were ascribed to the formation of the self-intumescent char layer of EP resulted from the catalyzing effect of Cu₂O for char formation and CO to CO₂ conversion. These findings will consolidate approaches for conferring flame retardancy to flammable polymers or their blends.     

Transparent epoxy resin material with excellent fire retardancy enabled by a P/N/S containing flame retardant

An epoxy resin (EP) with excellent fire retardancy, good transparency, and satisfactory thermal stability has been obtained by introducing a new N/P/S containing flame retardant (HBD) into EP composites. When the phosphorus content was 0.48 wt%, EP/HBD reached V-0 rating with the limiting oxygen index of 33.5%. The cone calorimeter test (CC) indicated that the incorporation of HBD resulted in 1.5 times increase in ignition time, a 50% decrease in the maximum of heat release rates, 40% reduction of total heat release, and 50.7% decrease in total smoke production compared with EP. Besides, the fire-resistant behavior of EP/8% HBD is much better than the EP materials modified by similar P/N/S flame retardants reported in literature. The fire-retardant mechanism of HBD on EP was also analyzed by Raman, scanning electron microscope, Py-GC/MS, and Fourier transform infrared spectroscopy. The results show that HBD plays an important role in the formation of a dense intumescent carbon layer and gas phase quenching.     

环氧复合材料低温固化剂研究进展

低温固化(固化温度低于100℃)可获得高尺寸稳定性、低成本、力学性能优良的环氧复合材料,室温储存期长的低温固化剂体系是低温固化环氧复合材料的关键。重点探讨了微胶囊型、潜伏性以及其它改性环氧低温固化剂,展望了其未来前景及发展方向。     

高耐热透明阻燃环氧树脂固化体系研究

在以双酚A/四溴双酚A混合环氧树脂(EP)与甲基六氢邻苯二甲酸酐(MeHHPA)构成的胶料基础上,通过加入氯桥酸酐,制备出了具有良好耐热性和阻燃性的透明EP固化物。结果表明:与未加入氯桥酸酐体系相比,当w(氯桥酸酐)=20%(相对于固化剂质量而言)时,EP固化物的总燃烧时间由21 s降至2 s,而氧指数由31%增至37%,玻璃化转变温度(Tg)从123℃提高到148℃,负荷热变形温度(HDT)从108℃增加到129℃。     

DOPO-based curing flame retardant of epoxy composite material for char formation and intumescent flame retardance

Phosphorus-containing flame retardant (HBAEA-DOPO) for epoxy resin was synthesized by addition reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) with bis[2-(4-hydroxybenzylideneamino)ethyl]amine (HBAEA) that was synthesized via 4-hydroxybenzaldehyde with diethylenetriamine. HBAEA-DOPO was mixed with 4,4′-diaminodiphenyl sulfone to co-cure the epoxy resin of diglycidyl ether bisphenol A. The silane modified nano-silica (nano-SiO₂) was used to reinforce the epoxy resin. Thermal stability and dynamic mechanical properties of the cured epoxy materials were studied with the use of thermogravimetric analysis and dynamic mechanical thermal analysis. Flame retardance and burning behavior were evaluated by the limiting oxygen index (LOI), vertical burning test, and the cone calorimetry. The cured epoxy materials have excellent thermal stability, and the temperatures at the maximum weight loss rate are over 384.0°C. The characteristic temperature corresponding to 5.00 wt% of thermal decomposition reaches 341.5°C as 1.00 wt% of phosphorus content is loaded. Flame retardant grade meets the V-0 level. The fire residue mass gradually increases with HBAEA-DOPO and nano-SiO₂. The characteristics of high flame retardance and smoke suppression of HBAEA-DOPO and nano-SiO₂ on the cured epoxy composites have been demonstrated to be related to char formation and intumescent flame retardance in the condensed phase.     

Synthesis and characterization of epoxy film cured with phosphorous‐containing phenolic resin

Through the electrophilic addition reaction of ? P(O)? H and C?C, a series of novel phosphorus‐containing phenolic resins bearing maleimide (P‐PMFs) were synthesized and used as curing agent for preparing high performance and flame retardancy epoxy resins. The structure of the resin was confirmed with FTIR and elemental analysis. Thermal properties and thermal degradation behaviors of the thermosetted resin was investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The epoxy resins exhibited high glass transition temperature (143–156°C), goof thermal stability (>330°C) and retardation on thermal degradation rates. High char yields (700°C, 52.9%) and high limited oxygen indices (30.6–34.8) were observed, indicating the resins' good flame retardance for the P‐PMFs/CNE cured resins. The developed resin may be used potentially as environmentally preferable products in electronic fields. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3813–3817, 2007