Curing studies of epoxy resins with phosphorus‐containing amines

The curing system of diglycidyl ether of bisphenol A (DGEBA) with two phosphorus‐containing amine compounds—bis(3‐aminophenyl)methyl phosphine oxide and bis(4‐aminophenyl)‐bis(9,10‐dihydro‐9‐oxa‐10‐oxide‐10‐phosphaphenanthrene‐10‐yl)methane—was studied with differential scanning calorimetry under isothermal and nonisothermal conditions and compared with the DGEBA/diamino diphenyl methane system. The isoconversional method was used to evaluate the dependence of the effective activation energy on the extent of conversion. Modulated differential scanning calorimetry and dynamic mechanical thermal analysis were used to study the phenomena of vitrification and gelation. The thermal and flame‐retardant properties were evaluated, and the limiting oxygen index values of the phosphorylated resins, above 30, confirmed that phosphorus‐containing epoxy resins are effective flame retardants. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1676–1685, 2006     

Synthesis of novel boron‐containing epoxy–novolac resins and properties of cured products

Epoxy–novolac resins were synthesized by modifying a commercial novolac resin with epichlorohydrin. These epoxy–novolac resins were characterized and further modified with different contents of bis(benzo‐1,3,2‐dioxa‐borolanyl)oxide or bis(4,4,5,5‐tetramethyl‐1,3,2‐dioxa‐borolanyl)oxide. The boron‐containing epoxy–novolac resins were autocatalytically crosslinked or crosslinked with BF₃MEA and their thermal stability and flame retardancy were determined by thermogravimetric analysis and limiting oxygen index (LOI) values. These LOI values for the bis(benzo‐1,3,2‐dioxa‐borolanyl)oxide derivatives were higher than the boron‐free novolac resins, which shows the benefit of the presence of boron. To test the role of boron in the enhancement of flammability, scanning electronic microscopy and energy‐dispersive X‐ray spectroscopy images were made. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6332–6344, 2006     

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     

Epoxy resins from novel monomers with a bis‐(9,10‐dihydro‐9‐oxa‐10‐oxide‐10‐phosphaphenanthrene‐10‐yl‐) substituent

A new diepoxide and a new diamine, both bearing bis‐(9,10‐dihydro‐9‐oxa‐10‐oxide‐10‐phosphaphenanthrene‐10‐yl‐)‐substituted methylene linkages, were prepared through the reaction of 9,10‐dihydro‐oxa‐10‐phosphaphenanthrene‐10‐oxide with benzophenone derivatives via a simple addition reaction followed by a dehydration reaction. These two compounds were used as monomers for preparing cured epoxy resins with high phosphorus contents. The resultant epoxy resins showed high glass‐transition temperatures (between 131 and 196 °C). All of the cured epoxy resins exhibited high thermal stability, with 5% weight loss temperatures over 316 °C, and excellent flame retardancy, with limited oxygen index values of 37–50. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 359–368, 2002     

Flame retardant epoxy resins based on diglycidyloxymethylphenylsilane

Silicon‐containing epoxy resins were prepared from diglycidyloxymethylphenyl silane (DGMPS) and diglycidylether of bisphenol A (DGEBA) by crosslinking with 4,4′‐diaminodiphenylmethane (DDM). Several DGMPS/DGEBA molar ratios were used to obtain materials with different silicon contents. Their thermal, dynamomechanical, and flame‐retardant properties were evaluated and related to the silicon content. The weight loss rate of the silicon‐containing resins is lower than that of the silicon free resin. Char yields under nitrogen and air atmospheres increase with the silicon content. The LOI (limited oxygen index) values increased from 24 for a standard commercial resin to 36 for silicon‐containing resins, demonstrating improved flame retardancy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5580–5587, 2006     

Preparation,thermal properties,and flame retardance of epoxy–silica hybrid resins

A novel epoxy system was developed through the in situ curing of bisphenol A type epoxy and 4,4′‐diaminodiphenylmethane with the sol–gel reaction of a phosphorus‐containing trimethoxysilane (DOPO–GPTMS), which was prepared from the reaction of 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) with 3‐glycidoxypropyltrimethoxysilane (GPTMS). The preparation of DOPO–GPTMS was confirmed with Fourier transform infrared, ¹H and ³¹P NMR, and elemental analysis. The resulting organic–inorganic hybrid epoxy resins exhibited a high glass‐transition temperature (167 °C), good thermal stability over 320 °C, and a high limited oxygen index of 28.5. The synergism of phosphorus and silicon on flame retardance was observed. Moreover, the kinetics of the thermal oxidative degradation of the hybrid epoxy resins were studied. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2354–2367, 2003     

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     

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     

Synthesis and properties of thermosetting polymers from a phosphorous‐containing fatty acid derivative

A new phosphorous‐containing fatty acid diepoxide was obtained from 10‐undecenoyl chloride and 10‐(2′,5′‐dihydroxyphenyl)‐9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide and crosslinked with 4,4′‐diaminodiphenylmethane and bis(m‐aminophenyl)methylphosphine oxide. The properties of the thermosetting materials were evaluated by differential scanning calorimetry, dynamic mechanical thermal analysis, thermogravimetric analysis, and limiting oxygen index (LOI). Thermal and thermooxidative degradation was studied by gas chromatography/mass spectrometry, FTIR, ³¹P magic angle spinning NMR spectroscopy, and scanning electron microscopy. LOI values indicate good flame‐retardant properties that are related to the formation of a protective phosphorous‐rich layer that slowed down the degradation and prevented it from being total. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5630–5644, 2006     

Flame‐retardant epoxy resins: An approach from organic–inorganic hybrid nanocomposites

Phosphorus‐containing epoxy‐based epoxy–silica hybrid materials with a nanostructure were obtained from bis(3‐glycidyloxy)phenylphosphine oxide, diaminodiphenylmethane, and tetraethoxysilane in the presence of the catalyst p‐toluenesulfonic acid via an in situ sol–gel process. The silica formed on a nanometer scale in the epoxy resin was characterized with Fourier transform infrared, NMR, and scanning electron microscopy. The glass‐transition temperatures of the hybrid epoxy resins increased with the silica content. The nanometer‐scale silica showed an enhancement effect of improving the flame‐retardant properties of the epoxy resins. The phosphorus–silica synergistic effect on the limited oxygen index (LOI) enhancement was also observed with a high LOI value of 44.5. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 986–996, 2001     

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     

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     

Flame‐retardant epoxy resins with high glass‐transition temperatures. II. Using a novel hexafunctional curing agent: 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene 10‐yl‐tris(4‐aminophenyl) methane

We synthesized a novel phosphorus‐containing triamine [9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene 10‐yl‐tris(4‐aminophenyl) methane (dopo‐ta)] from the nucleophilic addition of 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene 10‐oxide and pararosaniline chloride, using triethylamine as an acid receiver. We confirmed the structure of dopo‐ta by IR, mass, and NMR spectra and elemental analysis. dopo‐ta served as a curing agent for diglycidyl ether of bisphenol A (DGEBA) and dicyclopentadiene epoxy (hp7200). Properties such as the glass‐transition temperature (T_g), thermal decomposition temperature, flame retardancy, moisture absorption, and dielectric properties of the cured epoxy resins were evaluated. The T_g's of cured DGEBA/dopo‐ta and hp7200/dopo‐ta were 171 and 190 °C, respectively. This high T_g phenomenon is rarely seen in the literature after the introduction 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 1.80 wt % for DGEBA/dopo‐ta/diamino diphenylmethane (DDM) systems and 1.46 wt % for hp7200/dopo‐ta/DDM systems. The dielectric constants for DGEBA/dopo‐ta and hp7200/dopo‐ta were 2.91 and 2.82, respectively, implying that the dopo‐ta curing systems exhibited low dielectric properties. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5971–5986, 2005     

Fluorescence probes for investigation of epoxy systems and monitoring of crosslinking processes

The fluorescence behavior of 1,1′‐dimethyl‐2,2′‐carbocyanine and p‐N,N‐dimethylamino‐styryl‐2‐ethylpyridinium was investigated in several epoxy systems. Time‐correlated single photon counting was used for all fluorescence measurements to obtain the rate constant for viscosity or mobility‐dependent nonradiative processes of the probe. Microviscosity effects were discussed on the basis of a model describing the microfriction between matrix and probe molecules. The probes investigated are able to detect the glass‐transition temperature of the materials investigated. These compounds also show a dependence on the mobility in the glassy state. The probes applied in this work also can be used to monitor the crosslinking process of several epoxy systems containing 4,4′‐diaminodiphenylmethane (DDM) as curing agent. The epoxides used for the crosslinking process were 2,2′‐[(1‐methylethylidene)bis(4,1‐phenyleneoxymethylene)bis‐oxiranemethaneamine] [common name, diglycidyl ether of bisphenol A (DGEBA)], N‐oxiranylmethyl‐N‐phenyl‐oxiranylmethane [common name, diglycidyl aniline (DGA)], and epoxy novolacs of different functionality. The networks obtained have a different morphology, which was studied by the fluorescence probe technology. The structure of the epoxy compound has an important influence on the probe behavior because both network density and size of the free volume influence the photochemical behavior of the probe. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1367–1386, 1999     

Preparation and properties of epoxy/amine hybrid resins from in situ polymerization

Homogeneous and transparent epoxy/amine hybrid resins were successfully obtained through the in situ curing of bisphenol A epoxy and hexakis(methoxymethyl)melamine with 2 wt % (3‐glycidoxypropyl)trimethoxysilane as a facial coupling agent. The hybrid resins showed good miscibility, high glass‐transition temperatures, good thermooxidative stability, and good flame retardance. The outstanding properties of the hybrid resins may lead to their use in high‐performance green electronic products. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1868–1875, 2004     

Enhanced thermal properties and flame retardancy from a thermosetting blend of a phosphorus‐containing bismaleimide and epoxy resins

Epoxy resins modified by an organosoluble phosphorus‐containing bismaleimide (3,3′‐bis(maleimidophenyl) ­phenylphosphine oxide; BMPPPO) were prepared by simultaneously curing epoxy/diaminodiphenylmethane (DDM), and BMPPPO. The resulted epoxy resins were found to exhibit glass transition temperatures as high as 212 °C, thermal stability at temperatures over 350 °C, and excellent flame retardancy with Limited oxygen index (LOI) values around 40. Incorporation of BMPPPO into epoxy resins via the thermosetting blend was demonstrated to be an effective way to enhance the thermal properties and flame retardancy simultaneously. Copyright © 2003 John Wiley & Sons, Ltd.     

Preparation and properties of high performance epoxy–silsesquioxane hybrid resins prepared using a maleimide–alkoxysilane compound as a modifier

An alkoxysilane compound possessing maleimide moiety (MSM) was prepared from N‐(4‐hydroxyphenyl)maleimide and 3‐glycidoxypropyltrimethoxysilane and was used as a modifier of epoxy resins. In situ curing epoxy resins with MSM resulted in epoxy resins with good homogeneity. Just 5–10 wt % of MSM is sufficient to yield high glass transition temperature (165 °C), good thermal stability above 360 °C, and high flame retardancy (LOI = 30) to bisphenol‐A‐based epoxy resins. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5787–5798, 2005     

Flame retardant epoxy resins based on diglycidyl ether of (2,5-dihydroxyphenyl)diphenyl phosphine oxide

Phosphine oxide-containing epoxy resins were prepared from diglycidyl ether of (2,5-dihydroxyphenyl)diphenyl phosphine oxide and diglycidyl ether of bisphenol A by crosslinking with 4,4′-diaminodiphenylmethane. Several (2,5-dihydroxyphenyl)diphenyl phosphine oxide/diglycidyl ether of bisphenol A molar ratios were used to obtain materials with different phosphorus content. The properties of the thermosetting materials were evaluated by differential scanning calorimetry, dynamic mechanical analysis, thermogravimetric analysis, and limiting oxygen index and related to the phosphorus content. Thermal and thermooxidative degradation was studied by GC/MS, ³¹P MAS NMR spectroscopy, and scanning electron microscopy. Limiting oxygen index values indicate good flame retardant properties that are related to the formation of a protective phosphorus-rich layer that slowed down the degradation and prevented it from being total. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2142–2151, 2007     

A novel halogen‐free co‐curing agent with linear multi‐aromatic rigid structure as flame‐retardant modifier in epoxy resin

A novel halogen‐free 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO)‐containing co‐curing agent, 6,6′‐(1,4‐phenylenebis(((4‐(phenylamino)phenyl)amino)methylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6‐oxide) (DPN) was synthesized via a simple 1‐pot or 2‐step procedure with yield of 86.2% and 70.8%, respectively. The molecular structures of 4,4′‐((1,4‐phenylenebis(methanylylidene))bis(azanylylidene))bis(N‐phenylaniline) (DPN intermediate) and DPN are characterized by FTIR, NMR, and MS. TGA tests show that the char yield of DPN/EP composites raises to 30.9% when the molar ratio of DPN to 4,4‐diaminodiphenyl methane(DDM) is 20:80. T_g values of DPN/EP composites tested by DSC and DMA are similar to neat epoxy resin (EP), which is due to the secondary amine in DPN that participates in the cross‐linking reaction of epoxy resin. The storage modulus in the rubber stage (E′‐190 °C) of flame‐retardant epoxy resin is close to that of neat EP, while their tanδ's are lower, which indicates the similarity of samples' cross‐linking density due to the participation of DPN in the cross‐linking reaction. The results show that when the molar ratio of DPN and DDM is 5:95, the epoxy has a higher T_g value and better mechanical properties than other samples. The introduction of DPN efficiently improves the flame‐retardant properties of epoxy resin with V‐0 rating of UL‐94 vertical burning test, non‐dripping, 41% of limit oxygen index (LOI) value, low peak heat release rate (PHRR), and total heat release (THR).     

Reaction of epoxy resin and hyperbranched polyacids

The condensation reaction between two different epoxy resins and a hyperbranched polyester (MAHP) [poly(allyloxy maleic acid‐co‐maleic anhydride)] was studied. We compared two kinds of diglycidyl ether bisphenol A type of epoxy resins with different molecular weights, that is, epoxy resin GY240 (M = 365 g/mol) and GT6064 (M = 1540 g/mol) in this reaction. The results showed a marked difference in their reaction pattern in terms of ability to form crosslinked polymer networks with MAHP. For the former low‐molecular‐weight epoxy resin, no crosslinking could be observed in good solvents such as THF or dioxane within the set of reaction conditions used in this study. Instead, polymers with epoxide functional degrees between 0.34 and 0.5 were formed. By contrast, the latter high‐molecular‐weight epoxy resin, GT6064, rapidly produced highly crosslinked materials with MAHP under the same reaction conditions. The spherical‐shape model of hyperbranched polymer was applied to explain this difference in reaction behavior. Hence, we have postulated that low‐molecular‐weight epoxy resins such as GY240 are unable to crosslink the comparatively much bigger spherically shaped MAHP molecules. However, using high‐molecular‐weight epoxy resins greatly enhances the probability of crosslinking in this system. Computer simulations verified the spherical shape and condensed bond density of MAHP in good solvents, and submicron particle analysis showed that the average MAHP particle size was 9 nm in THF. Furthermore, the epoxy‐functionalized polyesters were characterized by ¹H NMR and FTIR, and the molecular weights and molecular‐weight distributions were determined by size‐exclusion chromatography. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4457–4465, 2000