Novel flame‐retardant thermosets: Diglycidyl ether of bisphenol A as a curing agent of boron‐containing phenolic resins

Boron‐containing novolac resins were synthesized by the modification of a commercial novolac resin with different contents of bis(benzo‐1,3,2‐dioxaborolanyl)oxide. These novolac resins were crosslinked with diglycidyl ether of bisphenol A (DGEBA), and their thermal, thermodynamomechanical, and flame‐retardant properties were evaluated. The boron‐containing novolac resins were less thermally stable than the unmodified novolac resin. Their modification degree and DGEBA content were related to the crosslinking density of the materials. The boron‐containing novolac resins generated boric acid at high temperatures and gave an intumescent char that slowed down the degradation and prevented it from being total. They also showed good flame‐retardant properties. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1701–1710, 2006     

Flame‐retardant epoxy resins with high glass‐transition temperatures from a novel trifunctional curing agent: Dopotriol

A novel phosphorus‐containing trifunctional novolac (dopotriol) was synthesized through the addition reaction of 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene 10‐oxide and rosolic acid. The structure of dopotriol was confirmed with NMR spectroscopy and elemental analyses. The dopotriol was blended with phenol novolac in the ratios of 10/0, 8/2, 6/4, 4/6, 2/8, and 0/10 to serve as a curing agent for diglycidyl ether of bisphenol A. Thermal properties, such as the glass‐transition temperature, thermal decomposition temperature, and flame retardancy, moisture absorption, and dielectric properties of the cured epoxy resins were evaluated. The activity and activation energy of curing were studied with the methods of Kissinger and Ozawa by dynamic differential scanning calorimetry scans. The glass‐transition temperatures of the cured epoxy resins were 138–159 °C, increasing with the phosphorus content. This is rarely seen in the literature after the addition of a flame‐retardant element. The flame retardancy increased with the phosphorus content, and a UL‐94 V‐0 grade was achieved with a phosphorus content of 1.87%. Similar dielectric properties and moisture absorption were observed for these phosphorus‐containing epoxy resins, and this implied that the addition of phosphorus to epoxy did not affect the dielectric properties and moisture absorption. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2862–2873, 2005     

Synthesis of a novel phosphorus‐containing curing agent and its effects on the flame retardancy,thermal degradation and moisture resistance of epoxy resins

A novel phosphorus‐containing compound diphenyl‐(1, 2‐dicarboxylethyl)‐phosphine oxide defined as DPDCEPO was synthesized and used as a flame retardant curing agent for epoxy resins (EP). The chemical structure of the prepared DPDCEPO was well characterized by Fourier transform infrared spectroscopy, and ¹H, ¹³C and ³¹P nuclear magnetic resonance. The DPDCEPO was mixed with curing agent of phthalic anhydride (PA) with various weight ratios into epoxy resins to prepare flame retardant EP thermosets. The flame retardant properties, combustion behavior and thermal analysis of the EP thermosets were respectively investigated by limiting oxygen index (LOI), vertical burning tests (UL‐94), cone calorimeter measurement, dynamic mechanical thermal analysis and thermogravimetric analysis (TGA) tests. The surface morphologies and chemical compositions of the char residues for EP thermosets were respectively investigated by scanning electron microscopy and X‐ray photoelectron spectroscopy (XPS). The water resistant properties of the cured EP were evaluated by putting the samples into distilled water at 70°C for 168 hr. The results revealed that the EP/20 wt% DPDCEPO/80 wt% PA thermosets successfully passed UL‐94 V‐0 flammability rating and the LOI value was as high as 33.2%. The cone test results revealed that the incorporation of DPDCEPO effectively reduced the combustion parameters of the epoxy resin thermosets, such as heat release rate and total heat release. The dynamic mechanical thermal analysis test demonstrated that the glass transition temperature (Tg) decreased with the increase of DPDCEPO content. The TGA results indicated that the incorporation of DPDCEPO promoted the decomposition of epoxy resin matrix ahead of time and led to a higher char yield and thermal stability at high temperatures. The surface morphological structures and analysis of the XPS of the char residues of EP thermosets revealed that the introduction of DPDCEPO benefited the formation of a sufficient, compact and homogeneous char layer with rich flame retardant elements on the epoxy resin material surface during combustion. The mechanical properties and water resistance of the cured epoxy resins were also measured. After water resistance tests, the EP/20 wt% DPDCEPO/80 wt% PA thermosets retained excellent flame retardancy, and the moisture adsorption of the EP thermosets decreased with the increase of DPDCEPO content in EP thermosets because of the existence of the P–C bonds and the rigid aromatic hydrophobic structure in DPDCEPO. Copyright © 2015 John Wiley & Sons, Ltd.     

Comparative study of boron compounds and aluminum trihydroxide as flame retardant additives in epoxy resin

The aim of this study was to investigate and compare the flame retardant properties of boron compounds with respect to aluminum trihydroxide (ATH) in an epoxy system based on bisphenol A epichlorohydrin‐based epoxy resin and cycloaliphatic polyamine‐based hardener. Six different boron compounds including colemanite (C), ulexite (U), boric acid (BA), boric oxide (BO), melamine borate (MB) and guanidinium nonaborate (GB) were used as flame retardant additive. The flame retardant properties of epoxy‐based composites were investigated using limiting oxygen index (LOI), UL 94 standards both in vertical and horizontal position, thermogravimetric analysis, cone calorimeter and scanning electron microscopy. According to flammability test results, boron compounds except for C and U showed better performance than ATH. According to the LOI results, 40% BA containing sample had the highest LOI value of 28.5, while 30% MB, 35% GB and 40% BA containing samples had the highest UL 94V rating (V0). According to the cone calorimeter test results, all boron containing samples had better fire performances than ATH containing sample; 40 wt% BO containing sample showed the lowest peak heat release rate, average heat release rate and total heat release values. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermal and flame retardant properties of epoxy resin cured by a novel phosphorus-containing 4,4′-bisphenol novolac curing agent

A novel flame retardant curing agent for epoxy resin (EP), i.e., a DOPO (9,10-dihydro-9-oxa-10-phosphaphenan-threne-10-oxide)-containing 4,4'-bisphenol novolac (BIP-DOPO) was synthesized and characterized by Fourier transform infrared (FTIR), ¹H NMR, ³¹P NMR spectroscopy, and gel permeation chromatography. The epoxy resin cured by BIP-DOPO itself or its mixture with a commonly used bisphenol A-formaldehyde novolac resin (NPEH720) was prepared. The flame retardancy of the cured EP thermosets were studied by limiting oxygen index (LOI), UL 94 and cone calorimeter test (CCT), and the thermal properties by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results show that the cured epoxy resin EPNP/BI/3/1, which contains 2.2% phosphorus, possesses a value of 26.2% and achieves the UL 94 V-0 rating. The data from cone calorimeter test demonstrated that the peak release rate, average heat release rate, total heat release decline sharply for the flame retarded epoxy resins, compared with those of pure ones. DSC results show that the glass-transition temperatures of cured epoxy resins decrease with increasing phosphorus content. TGA indicates that the incorporation of BIP-DOPO promotes the decomposition of epoxy resin matrix ahead of time and leads to higher char yield. The surface morphological structures of the char residues reveal that the introduction of BIP-DOPO benefits to the formation of a continuous and solid char layer on the epoxy resin material surface during combustion.     

Vanillin‐derived phosphorus‐containing compounds and ammonium polyphosphate as green fire‐resistant systems for epoxy resins with balanced properties

Flame retardants from vanillin when utilized together with ammonium polyphosphate (APP) yield excellent synergistic flame retardancy toward epoxy resins. Bisphenol A epoxy resins have been widely used due to their excellent mechanical properties, chemical resistance, electrical properties, adhesion, etc., while they are flammable. Environment‐friendly and bio‐based flame retardants have captured increasing attention due to their ecological necessity. In this paper, 3 bio‐based flame retardants were synthesized from abundant and more importantly renewable vanillin, and their chemical structures were determined by ¹H NMR and ¹³C NMR. They were used together with APP (an environment‐friendly commercial flame retardant) to improve the fire resistance of bisphenol A epoxy resin. With the addition APP content of 15 phr, the modified bisphenol A epoxy resin could reach UL‐94V0 rating during vertical burning test and limit oxygen index values of above 35%, but reducing APP content to 10 phr, the flame retardancy became very poor. With the total addition content of 10 phr, the epoxy resins modified by 7 to 9 phr APP and 1 to 3 phr bio‐based flame retardants with epoxy groups or more benzene rings showed excellent flame retardancy with UL‐94V0 rating and limit oxygen index values of around 29%. The T_gs of the epoxy resins could be remained or even increased after introducing bio‐based flame retardants, as the control; those of APP alone‐modified epoxy resins compromised a lot. The green synergistic flame‐retardant systems have a great potential to be used in high‐performance materials.     

Novel flame‐retardant thermosets: Phosphine oxide‐containing diglycidylether as curing agent of phenolic novolac resins

Phosphorus‐containing novolac–epoxy systems were prepared from novolac resins and isobutyl bis(glycidylpropylether) phosphine oxide (IHPOGly) as crosslinking agent. Their curing behavior was studied and the thermal, thermomechanical, and flame‐retardant properties of the cured materials were measured. The T_g and decomposition temperatures of the resulting thermosets are moderate and decrease when the phosphorous content increases. Whereas the phosphorous species decrease the thermal stability, at higher temperatures the degradation rates are lower than the degradation rate of the phosphorous‐free resin. V‐O materials were obtained when the resins were tested for ignition resistance with the UL‐94 test. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3516–3526, 2004     

Phosphorylated cellulose/Fe3+ complex: A novel flame retardant for epoxy resins

A novel flame retardant containing cellulose, phosphorus and ferrum complex (Cell‐P‐Fe) was successfully synthesized and then it was used as flame retardants in epoxy resins (EP). Due to the present of acid sources and carbon sources, the Cell‐P‐Fe exhibits improved thermal stability and flame retardant properties. The EP/Cell‐P‐Fe composites with 10 wt% of Cell‐P‐Fe show remarkably improved LOI and UL‐94 values compared with the flame retardants without ferrum. At the loading of 10.0 wt% flame retardants, the char yield for EP/Cell‐P‐Fe composites increased to 29.1 wt%, indicating the improved thermal stability at high temperature. Moreover, thermogravimetric analysis, morphology of char residues and FTIR results demonstrate that stable char layers are formed on the surface of the composites during the combustion, attributing to the catalytic carbonization effect of Fe and phosphorus and the present of cellulose as carbon source. The stable char layers, which can protect the underlying materials from heat and oxygen, play an important role in the flame retardancy enhancement.     

Development of novel flame‐retardant thermosets based on boron‐modified phenol–formaldehyde resins

Boron‐containing novolac resins were prepared through the modification of a commercial novolac resin with different contents of bis(benzo‐1,3,2‐dioxaborolanyl) oxide. Their thermal and flame‐retardant properties were measured. Then, they were crosslinked with hexamethylenetetramine, and their thermal, thermodynamomechanical, and flame‐retardant properties were evaluated. Their modification degree was related to the segmental motion of the materials. The crosslinking of the boron‐modified novolac resins with hexamethylenetetramine was slower and not as extensive as that of commercial novolac resins because the nitrogen from intermediate species coordinated with boron. The thermal degradation of the boron‐containing novolac resins generated boric acid at high temperatures and gave an intumescent char that slowed the degradation and prevented it from being complete. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3503–3512, 2006     

Towards the development of thermally latent novolac-based char formers for ABS resins

Novel novolac-based char formers for ABS resins, [PN-PI], [PN-BPI], [CN-PI] and [CN-BPI], were prepared from phenol formaldehyde novolac (PN), cresol formaldehyde novolac (CN), phenyl isocyanate (PI), and 4-biphenyl isocyanate (BPI) via a simple urethane-forming reaction. The four compounds were used as thermally latent char formers for this study. Thus, a two component system employing novolac-isocyanate adduct as a char former, and tetra-2,6-dimethyl phenyl-resorcinol diphosphate (DMP-RDP) as a phosphorous-based flame retardant was blended with ABS, and the thermal degradation behaviour and flame retardancy were determined by thermogravimetric analysis (TGA) and LOI test. The mixtures show a synergistic effect between DMP-RDP and novolac-isocyanate adduct on the flame retardance enhancement of ABS. Those containing higher molar mass CN-BPI adduct are found to be most flame retardant, and a LOI value as high as 35 is obtained.     

Flame retardancy and heat resistance of phenol-biphenylene-type epoxy resin compound modified with benzoguanamine

The flame retardancy and heat resistance of a phenol-biphenylene-type epoxy resin compound, which forms a self-extinguishing network structure, were increased by the inclusion of a benzoguanamine-modified phenol biphenylene resin. The benzoguanamine-modified phenol biphenylene resin contains a benzoguanamine unit to release non-flammable nitrogen substances during ignition and to increase the resin's reactivity toward epoxy resins, and biphenylene units to keep the resin's thermal degradation and water resistance. The addition of the benzoguanamine-modified phenol biphenylene resin in the epoxy resin compound improved the epoxy resin compound's flame retardancy and heat resistance, and also increased its glass transition temperature while maintaining its water resistance and mechanical properties. Copyright © 2003 John Wiley & Sons, Ltd.     

Fire Retardancy of Aluminum Hydroxide Reinforced Flame Retardant Modified Epoxy Resin Composite

A novel phosphorous containing flame retardant epoxy resin is synthesized by modifying the epoxy resin initially with phosphoric acid and further with aluminum hydroxide (ATH) to enhance the fire retardancy of the modified epoxy resin. The several phosphorous modified epoxy resin to ATH mass ratios were used to study the effect of ATH addition on epoxy. Thermal and mechanical properties. The structure of the modified flame retardant epoxy resin was characterized using Fourier-transform infrared spectroscopy (FTIR) while thermal degradation behavior and flame retardant properties were examined using thermo-gravimetric analysis (TGA) and UL-94 testing. Furthermore, ultimate tensile strength and young modulus were analyzed to study the effect of ATH addition on mechanical properties. The findings indicated that fire retardancy of ATH reinforced modified ep oxy resin is higher than virgin and phosphorous modified epoxy resin and depicted eminent flame retardant properties with suitable mechanical properties.

     

Flame‐retardant epoxy resins from o‐cresol novolac epoxy cured with a phosphorus‐containing aralkyl novolac

A novel phosphorus‐containing aralkyl novolac (Ar‐DOPO‐N) was prepared from the reaction of 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) first with terephthaldicarboxaldehyde and subsequently with phenol. The chemical structures of the synthesized compounds were characterized with Fourier transform infrared, ¹H and ³¹P NMR, and elemental analysis. Ar‐DOPO‐N blended with phenol formaldehyde novolac was used as a curing agent for o‐cresol formaldehyde novolac epoxy, resulting in cured epoxy resins with various phosphorus contents. The epoxy resins exhibited high glass‐transition temperatures (159–177 °C), good thermal stability (>320 °C), and retardation on thermal degradation rates. High char yields and high limited oxygen indices (26–32.5) were observed, indicating the resins' good flame retardance. Using a melamine‐modified phenol formaldehyde novolac to replace phenol formaldehyde novolac in the curing composition further enhanced the cured epoxy resins' glass‐transition temperatures (160–186 °C) and limited oxygen index values (28–33.5). © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2329–2339, 2002     

Preparation of phosphorus‐containing phenolic resin and its application in epoxy resin as a curing agent and flame retardant

For the sake of improving the flame retardancy of epoxy resin (EP), a novel phosphorus‐containing phenolic resin (PPR) synthesized in our group instead of conventional phenolic resin (PR) was used to cure EP in the present research. The curing processes and the corresponding crosslinking structure and mechanical performance were investigated by differential scanning calorimeter and dynamic mechanical thermal analysis. Because of the introduction of flame‐retarding elements including P and Si, PPR exhibited higher charring capacity in the condensed phase, which is helpful to construct a char layer of higher quality. Correspondingly, PPR‐cured EP displayed remarkably improved flame retardance as compared to conventional PR‐cured EP through the related evaluations including limiting oxygen index, vertical burning test, and microscale combustion colorimeter. As a multifunction agent, it is believable that PPR possesses potential commercial value to prepare flame‐retardant EP with high performance.     

Effect of a triazine ring‐containing charring agent on fire retardancy and thermal degradation of intumescent flame retardant epoxy resins

A triazine ring‐containing charring agent (PEPATA) was synthesized via the reaction between 2,6,7‐trioxa‐l‐phosphabicyclo‐[2.2.2]octane‐4‐methanol (PEPA) and cyanuric chloride. It was applied into intumescent flame retardant epoxy resins (IFR‐EP) as a charring agent. The effect of PEPATA on fire retardancy and thermal degradation behavior of IFR‐EP system was investigated by limited oxygen index (LOI), UL‐94 test, microscale combustion calorimetry (MCC), thermogravimetric analysis (TGA) and thermogravimetric analysis/infrared spectrometry (TG‐IR). The glass transition temperatures (T_g) of IFR‐EP systems were studied by dynamic mechanical analysis (DMA). The LOI values increased from 21.5 for neat epoxy resins (EPs) to 34.0 for IFR‐EP, demonstrating improved flame retardancy. The TGA curves showed that the amount of residue of IFR‐EP system was largely increased compared to that of neat EP at 700 °C. The new IFR‐EP system could apparently reduce the amount of decomposing products at higher temperatures and promotes the formation of carbonaceous charred layers that slowed down the degradation. Copyright © 2010 John Wiley & Sons, Ltd.     

Synergistic flame‐retardant effect and mechanisms of boron/phosphorus compounds on epoxy resins

To explore the component synergistic effect of boron/phosphorus compounds in epoxy resin (EP), 3 typical boron compounds, zinc borate (ZB), boron phosphate (BPO₄), and boron oxide (B₂O₃), blended with phosphaphenanthrene compound TAD were incorporated into EP, respectively. All 3 boron/phosphorus compound systems inhibited heat release and increased residue yields and exerted smoke suppression effect. Among 3 boron/phosphorus compound systems, B₂O₃/TAD system brought best flame‐retardant effect to epoxy thermosets in improving the UL94 classification of EP composites and also reducing heat release most efficiently during combustion. B₂O₃ can interact with epoxy matrix and enhance the charring quantity and quality, resulting in obvious condensed‐phase flame‐retardant effect. The combination of condensed‐phase flame‐retardant effect from B₂O₃ and the gaseous‐phase flame‐retardant effect from TAD effectively optimized the action distribution between gaseous and condensed phases. Therefore, B₂O₃/TAD system generated component synergistic flame‐retardant effect in epoxy thermosets.     

Flame‐retardant performance and mechanism of epoxy thermosets modified with a novel reactive flame retardant containing phosphorus,nitrogen, and sulfur

A novel phosphorus‐containing, nitrogen‐containing, and sulfur‐containing reactive flame retardant (BPD) was successfully synthesized by 1‐pot reaction. The intrinsic flame‐retardant epoxy resins were prepared by blending different content of BPD with diglycidyl ether of bisphenol‐A (DGEBA). Thermal stability, flame‐retardant properties, and combustion behaviors of EP/BPD thermosets were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), limited oxygen index (LOI) measurement, UL94 vertical burning test, and cone calorimeter test. The flame‐retardant mechanism of BPD was studied by TGA/infrared spectrometry (TGA‐FTIR), pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS), morphology, and chemical component analysis of the char residues. The results demonstrated that EP/BPD thermosets not only exhibited outstanding flame retardancy but also kept high glass transition temperature. EP/BPD‐1.0 thermoset achieved LOI value of 39.1% and UL94 V‐0 rating. In comparison to pure epoxy thermoset, the average of heat release rate (av‐HRR), total heat release (THR), and total smoke release (TSR) of EP/BPD‐1.0 thermoset were decreased by 35.8%, 36.5% and 16.5%, respectively. Although the phosphorus content of EP/BPD‐0.75 thermoset was lower than that of EP/DOPO thermoset, EP/BPD‐0.75 thermoset exhibited better flame retardancy than EP/DOPO thermoset. The significant improvement of flame retardancy of EP/BPD thermosets was ascribed to the blocking effect of phosphorus‐rich intumescent char in condensed phase, and the quenching and diluting effects of abundant phosphorus‐containing free radicals and nitrogen/sulfur‐containing inert gases in gaseous phase. There was flame‐retardant synergism between phosphorus, nitrogen, and sulfur of BPD.     

Blowing‐out effect and temperature profile in condensed phase in flame retarding epoxy resins by phosphorus‐containing oligomeric silsesquioxane

A series of flame retardant epoxy resins (EPs) containing phosphorus‐containing oligomeric silsesquioxane are prepared, and an interesting blowing‐out effect is detected in flame retardant EPs. The temperature profiles show that blowing‐out effect slows the heat transfer from the fire to the unburned matrix; furthermore, this blowing‐out effect can even take away some heat from the surface zone by the spurting gases. The thermo gravimetric analyzer and Fourier transform infrared spectrometer result shows that the spurting gases during the blowing‐out effect have a high content of CO₂, which could reduce the combustion capability of the jetting gases. The flame retardancy of these EPs is tested by limit oxygen index and UL‐94. The incorporation of 2.5 wt% phosphorus‐containing oligomeric silsesquioxane into EP gives a remarkable blowing‐out effect, which results in a significant enhancement of limit oxygen index value and UL‐94 rating. The flame retardancy mechanism of blowing‐out effect is quite different from the traditional mechanisms. The char strength and morphology of EP composites are also investigated to explain the mechanism of the blowing‐out effect. Copyright © 2013 John Wiley & Sons, Ltd.     

Mechanism Studies on Water Sorption and Permeation in Epoxy Resin by Impedance Spectroscopy. II. Cure Kinetics of O-Cresol Novolac Resin with Esterfied Phenol Novolac Resin

Abstract

To study the effect of water affinity of the cured epoxy resin on water sorption and permeation in the cured epoxy resin, a novel hardener (esterfied phenol novolac was synthesized and used for obtaining the cured product without free hydroxyl group. Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FT-IR) were used to study the cure kinetics of o-cresol novolac epoxy resin using esterfied phenol novolac resin as curing agent in the presence of 2-methylimidazole as accelerator. Some kinetic parameters of the curing reaction such as the reaction order, activation energy, and frequency factor were obtained in the temperature range studied. The results show that this curing process is a first-order kinetic mechanism, which was different with that cured with phenol novolac resin.     

磷酸酯双三聚氰胺盐阻燃环氧树脂的燃烧性能和阻燃机理

以季戊四醇、三氯氧磷、三聚氰胺为原料合成了[1-氧-4-亚甲基-2,6,7-三氧-1-磷杂双环(2,2,2)辛烷]磷酸酯双三聚氰胺盐阻燃剂,将该阻燃剂加入到环氧树脂中制成阻燃环氧树脂。用TG、SEM、EDS和FT-IR进行表征,并采用极限氧指数法和垂直燃烧法测试材料的燃烧性能,结果表明,极限氧指数和垂直燃烧性能随阻燃剂含量的增加而提高,当阻燃剂含量达到30%时,氧指数达到36,垂直燃烧性能达到V-0级;阻燃剂对材料的成炭量影响不大,但改变了炭层的组成和物理性质,燃烧过程中形成的含有P、O、N的粘性高聚物将炭层连接在一起,起到了隔热、隔氧作用,发挥了凝聚相阻燃作用。此外,阻燃环氧树脂在燃烧过程中有NH3等不燃气体逸出,有效地稀释了气相中的氧气浓度,发挥了气相阻燃作用,对材料的阻燃有协同作用。