Azo initiator selection to control the curing order in dimethacrylate/epoxy interpenetrating polymer networks

Differential Scanning calorimetry (DSC) and Fourier‐transform infrared (FT‐IR) spectroscopic studies have been undertaken of the cure of interpenetrating polymer networks (IPNs) formed with imidazole‐cured diglycidyl ether bisphenol‐A (DGEBA) and with either diethoxylated bisphenol‐A dimethacrylate (DEBPADM) or bisphenol‐A diglycidyl dimethacrylate (bisGMA), polymerized by a range of azo initiators (AIBN64, VAZ088, VR110 and AZO168). Due to the differing decomposition rates of the azo initiators, the neat dimethacrylate resin either cured faster than (with AIBN64 and VAZO88), or similar to (VR110), or slower than (AZO168), the neat epoxy resin. In the neat DGEBA/1‐methyl imidazole (1‐MeI), DEBPADM/AIBN64, DEBPADM/VAZO88 and DEBPADM/VR110 resins, close to full cure was achieved. For the neat, high‐temperature DEBPADM/AZO168 resin, full cure was not attained, possibly due to the compromise between using a high enough temperature for azo decomposition while avoiding depolymerization or decomposition of the methacrylate polymer. IPN cure studies showed that, by appropriate initiator selection, it was possible to interchange the order of cure of the components within the IPN so that either the dimethacrylate or epoxy cured first. In the isothermal cure of the 50:50 DEBPADM/AIBN64:DGEBA/1‐Mel IPN system, the cure rate of both species was less than in the parent resins, due to a dilution effect. For this system, the dimethacrylate cured first and to high conversion, due to plasticization by the unreacted epoxy, but the subsequent cure of the more slowly polymerizing epoxy component was restricted by the high crosslink density developed in the IPN. After post‐curing, however, high conversion of both reactive groups was observed and the fully cured IPN exhibited a single high‐temperature T_g, close to the T_g values of the parent resins. In the higher‐temperature, isothermal cure of the 50:50 DEBPADM/VR110:DGEBA/1‐Mel IPN system, the reactive groups cured at a similar rate and so the final conversions of both groups were restricted, while in the 50:50 DEBPADM/AZO168:DGEBA/1‐Mel system it was the epoxy which cured first. Both of these higher‐temperature azo‐initiated IPN systems exhibited single T_gs, indicating a single‐phase structure; however, the T_gs are significantly lower than expected, due to plasticization by residual methacrylate monomer and/or degradation products resulting from the high cure temperature. Copyright © 2004 Society of Chemical Industry     

Synthesis of epoxy resin–silica nanocomposites provided from perhydropolysilazane as a curing reagent and the precursor of silica domain

Epoxy resin–silica nanocomposites with spherical silica domains with 29.0 nm in diameter in an epoxy resin matrix were synthesized from Bisphenol‐A type epoxide monomer, 2,2‐bis(4‐glycidyloxyphenyl)propane (DGEBA), and perhydropolysilazane (PHPS, ? [Si₂? NH]_n? ). The volume fraction of silica domain in the composite varied from 5.4 to 37.8 vol % by varying the feed ratio of PHPS to the epoxide monomer. The reaction mechanism of epoxy group and PHPS was investigated by using glycidyl methacrylate as a model compound of the epoxy monomer by ¹H‐nucular magnetic resonance and Fourier transform infrared spectrometry. Ammonia gas provided by the decomposition of PHPS with moisture converted PHPS to silica and cured the epoxy monomer. The curing of epoxy monomer preferably proceeded than the conversion of silica. The addition of 1,4‐diaminobutane drastically accelerated the rate of curing; white and hard epoxy resin–silica nanocomposites were obtained. The good thermal stability of the composite prepared with DGEBA/PHPS/1,4‐diaminobutane was observed by thermogravimetric analysis. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Crosslinking of mixtures of DGEBA with 1,6-dioxaspiro[4,4]nonan-2,7-dione initiated by tertiary amines. Part IV. Effect of hydroxyl groups on initiation and curing kinetics

The anionic homopolymerization of DGEBA epoxy resin and its anionic copolymerization with a bislactone was studied using two alternative tertiary amines, 1-methylimidazole (1MI) and dimethylaminopyridine (DMAP) as initiators. 1MI caused slower cure than DMAP in neat DGEBA and DGEBA-bislactone formulations. Studies of the influence of the hydroxyl concentration in the DGEBA oligomer on its homopolymerization explain descrepancies in the literature regarding the ability of these initiators to produce full cure of the epoxy groups. In contrast, in the copolymerization of DGEBA-bislactone formulations, full cure could be readily achieved with either 1MI or DMAP as initiators, irrespective of the hydroxyl content. FTIR and DSC experiments show that this behaviour is associated with the formation of the carboxylate anion which plays an important part on the curing kinetics and the completion of cure.     

Effect of microencapsulated curing agents on the curing behavior for diglycidyl ether of bisphenol A epoxy resin systems

Microcapsules containing a curing agent, 2‐phenyl imidazole (2PZ), for a diglycidyl ether of bisphenol A (DGEBA) epoxy resin were prepared by a solid‐in‐oil‐in‐water emulsion solvent evaporation technique with poly(methyl methacrylate) (PMMA) as a polymeric wall. The mean particle size of the microcapsules and the concentration of 2PZ were about 10 μm and nearly 10 wt %, respectively. The onset cure temperature and peak temperature of the DGEBA/2PZ–PMMA microcapsule system appeared to increase by nearly 30 and 10°C, respectively, versus those of the DGEBA/2PZ system because of the increased reaction energy of curing. The former could take more than 3 months at room temperature, whereas the latter was cured after only a week. The values of the reaction order (a curing kinetic parameter) for DGEBA/2PZ and DGEBA/2PZ–PMMA microcapsules were quite close, and this showed that the curing reactions of the two samples proceeded conformably. The curing mechanism was investigated, and a two‐step initiation mechanism was considered: the first was assigned to adduct formation, whereas the second was due to alkoxide‐initiated polymerization. The glass‐transition temperature of DGEBA/2PZ was 165.2°C, nearly 20°C higher than the glass‐transition temperatures of DGEBA/2PZ–PMMA microcapsules and DGEBA/2PZ/PMMA microspheres, as determined by differential scanning calorimetry measurements. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Rheological,mechanical, and morphological studies of epoxy/poly(methyl methacrylate) semi‐interpenetrating polymer networks

Semi‐interpenetrating polymer networks (semi‐IPNs) of epoxy resin and poly(methyl methacrylate) (PMMA) were synthesized. Methyl methacrylate (MMA) was polymerized by free radical mechanism with azo‐bis‐isobutyronitrile in the presence of oligomeric epoxy resin (DGEBA), and hexahydrophthalic anhydride as crosslinking agent. The gelation and vitrification transitions during cure/polymerization processes have been examined using parallel‐plates rheological technique. From differential scanning calorimetry and rheological techniques, it was suggested that both curing and polymerization processes occur simultaneously. However, the gelation time was longer for the semi‐IPN than those observed for the cure of pure DGEBA or polymerization of MMA. The gelation time increased significantly when 5% of MMA was employed, suggesting a diluent effect of the monomer. Higher amount of MMA resulted in a decrease of gel time, probably because of the simultaneous polymerization of MMA during the curing process. Structural examination of the semi‐IPNs, using scanning electron microscopy, revealed phase separation in nanoscale size for semi‐IPNs containing PMMA at concentrations up to 15%. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Preparation and properties of urethane acrylate-epoxy interpenetrating polymer networks containing silica nanoparticles

Summary In this study, interpenetrating polymer networks (IPNs) and IPN composite materials were prepared by in situ polymerization of urethane dimethacrylate (UDMA) and bisphenol-A diglycidyl ether epoxy resin (DGEBA) with or without silica nanoparticles. Dynamic mechanical analysis (DMA), three-point bending test, thermogravimetric analysis (TGA) and visible spectrometry were performed to evaluate the physical properties of the resulting IPNs and IPN composite materials. The IPNs showed high transparency and higher elastic modulus and strength than that of each homopolymer at ratio of UDMA/ DGEBA is 70/30. The IPN composites maintained high transparency in spite of the addition of silica nanoparticles. Moreover, elastic modulus and surface hardness of the IPN composites increased with increasing silica content.     

Thermal decomposition kinetics of an epoxy resin with rubber‐modified curing agent

Thermal decomposition kinetics of diglycidyl ether of bisphenol A (DGEBA)/4,4′‐methylene dianiline (MDA) system with rubber‐modified MDA was studied by the methods of Ozawa, Kissinger, and Friedman, and the kinetic parameters were compared. The thermal decomposition data of the cured epoxy resin were analyzed by thermogravimetric analysis (TGA) at different heating rates. TG curves showed that the thermal decomposition of the epoxy system occurred in one stage regardless of rubber‐modified MDA content. The apparent activation energies for the DGEBA/MDA system with 10 phr of rubber‐modified MDA, as determined by the Ozawa, Kissinger, and Friedman methods, are 184, 182, and 222 kJ/mol, respectively. The thermal stability of the epoxy system increased with the increasing content of rubber‐modified MDA, which has four benzene rings with high thermal resistance due to the resonance structure. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 479–485, 2001     

Cure kinetics and modeling of an epoxy resin cross‐linked in the presence of two different diamine hardeners

An epoxy resin diglycidyl ether of bisphenol A (DGEBA) is cross‐linked with the help of two aromatic diamine 4,4′‐diaminodiphenylsulfone (DDS) + 4,4′‐methylenebis 3‐chloro 2,6‐diethylaniline (MCDEA) of nearly equal flexibility but different reactivities. The ratio of the two amines is varied while keeping the stoichiometry of the epoxy/amino hydrogen groups constant. The experimental cure kinetics are studied at four different isothermal temperatures. Their modeling is carried out by a phenomenological Kamal‐Sourour kinetic model. The procedure is two‐fold: 1) linear combinations of the values of rate constants from the two neat thermosets (based on only one amine) and 2) values calculated directly from isothermal cures of reactive amine mixtures. A good correlation was observed between the experimental data and the model predictions (both procedures). These amine formulations provide “mixed” epoxy thermosets and will be used later to control thermoset/thermoplastic blend morphologies for which reaction kinetics need to be predicted. POLYM. ENG. SCI. 45:1581–1589, 2005. © 2005 Society of Plastics Engineers     

Cationic polymerization of diglycidly ether of bisphenol a resins initiated by benzylsulfonium salts

Benzylsulfonium salts are latent thermal cationic initiators that dissociate on heating to form benzyl cations that can initiate polymerizations. This paper describes the cure behavior of commercial epoxy resins using 1 -(p-methoxybenzyl)tetrahydrothiophenium hexafluoroantimonate [ 2 ]. The thermal cationic cure of bisphenol A diglycidylether (DGEBA) resins was investigated using differential scanning calorimetry (DSC). DSC revealed complex cure behavior as indicated by multiple exotherms. The resins cured rapidly at low initiator concentrations with gelation occuring in 3.5 and 1.5 min at 75 and 85 °C, respectively, at conversions of epoxy groups α = 0.2–0.3. Cure kinetics were evaluated from both dynamic and isothermal DSC measurements. The effects of initiator concentration, isothermal cure temperature and heating rate on the cure behavior and mechanisms, especially involving potential termination pathways, are discussed.     

Acetone—Formaldehyde—Phenol based IPNs for Glass Fibre Reinforced Composite Applications

Abstract

Three component IPNs Glass fibre reinforced composites (GRC) have been prepared from acetone-formaldehyde-phenol (AF-P) resin, Diglycedyl ether of bisphenol-A (DGEBA) (a commercial epoxy resin) and methyl methacrylate (MMA) (a vinyl monomer). The curing catalyst hexamethylene tetramine (HMTA) for AF-P, radical initiators 2,2′-azobisisobutyronitrile (AIBN) for MMA and curing catalyst 4,4′-diamino diphenyl methane (DDM) for epoxy resin were employed. All the IPN GRCs were characterized in terms of their resistance to chemical reagents, thermal behaviour (DSC, TGA) and mechanical properties.     

Preparation and characterizations of interpenetrating polymer network hydrogels of poly(ethylene oxide) and poly(methyl methacrylate)

Interpenetrating polymer network (IPN) hydrogels based on poly(ethylene oxide) and poly(methyl methacrylate) were prepared by radical polymerization using 2,2‐dimethyl‐2‐phenylacetophenone and ethylene glycol dimethacrylate as initiators and crosslinkers, respectively. The IPN hydrogels were analyzed for sorption behavior at 25°C and at a relative humidity of 95% using dynamic vapor sorption. The IPN hydrogels exhibited a relatively high equilibrium water content in the range of 13–68%. The state of water in the swollen IPN hydrogels was investigated using differential scanning calorimetry. The free water in the hydrogels increased as the hydrophilic content increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 258–262, 2003     

Study on curing kinetics and curing mechanism of epoxy resin based on diglycidyl ether of bisphenol a and melamine phosphate

The curing kinetics of the diglycidyl ether of bisphenol A/melamine phosphate (DGEBA/MP) was analyzed by the DSC technique. The Kissinger and Flynn–Wall–Ozawa methods were applied to determine the dynamic kinetics of the DGEBA/MP system. The activation energies obtained by these two methods were 83.9 and 85.6 kJ/mol, respectively. An autocatalytic equation was applied to determine the isothermal curing kinetics of the DGEBA/MP system. The DGEBA/MP system exhibits autocatalytic behavior in the isothermal curing procedure, whose kinetics fits well with the autocatalytic mechanism. The obtained isothermal curing activation energy of the DGEBA/MP system was 110.0 kJ/mol. The curing mechanism of DGEBA with melamine phosphate was investigated using FTIR, ¹³C solid‐state NMR, and ³¹P solid‐state NMR. It involved an epoxide–amine reaction, etherification of phosphoric acid and epoxy, dehydration, and thermal oxidation of the hydroxyl group of the DGEBA/MP system. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 892–900, 2004     

Dielectric relaxation during isothermal curing of epoxy resin with an aromatic amine

The isothermal curing behavior of an epoxy resin system has been monitored by a dielectric measurement whose sensor consisted of a vertical parallel-plate cell based on a three-terminal method. The materials used in the isothermal cure were the diglycidyl ether of bisphenol-A (DGEBA), which was purified from Epon 825 by recrystallization, and 4,4′-diamino diphenyl methane (DDM). A dielectric relaxation was observed during each isothermal cure at temperatures of 70,80,90, and 100°C, which were below the glass transition temperature (T_g) of the reactive DGEBA–DDM system at the gel point. The relaxation is considered to be caused by the transformation from a liquid state to an ungelled glassy state as a result of an increase in molecular weight because gelation followed by a rubbery state did not exist in the temperature range studied. The dielectric relaxation for the DGEBA–DDM system fits the empirical model of the Havriliak–Negami equation. The T_g of the DGEBA–DDM system, which was estimated from the dielectric relaxation time, agreed with the one experimentally determined by differential scanning calorimetry (DSC).     

In-process control of epoxy composite by microdielectrometric analysis

The curing of an epoxy prepolymer based on the diglycidyl ether of bisphenol A (DGEBA) with dicyandiamide (DDA) as the hardener and imidazole as the catalyst agent was analyzed using microdielectrometry, differential scanning calorimetry, viscosity measurements, and insolubles in THF for gel-point detection. Interpreting dielectric data with respect to chemorheology continues to be the subject of scientific discussion. The focus of this issue is to give an industrial point of view on the collected on-line dielectric measurements during an epoxy/fiber glass composite cure. Hence, isothermal polymerizations of DGEBA/DDA/imidazole resin were examined and dielectric properties such as ionic conductivity were related to the cure kinetics by conversion through an experimentally established equation. This mathematical model was used to predict reaction advancement of epoxy processing under nonisothermal cure conditions. This model is shown to be able to forecast both isothermal and nonisothermal cure data of unaged resin. According to these results, cure monitoring was carried out on prepregs. Whereas some deviations of the law were observed at the time of the last stage of the cure, good correlation was obtained for the reaction rate during the in-mold process curing time.     

Comparison of the curing kinetics of a DGEBA/acid anhydride epoxy resin system using differential scanning calorimetry and a microwave‐heated calorimeter

The cure of an epoxy resin system, based upon a diglycidyl ether of bisphenol‐A (DGEBA) with HY917 (an acid anhydride hardener) and DY073 (an amine–phenol complex that acted as an accelerator), was investigated using a conventional differential scanning calorimeter and a microwave‐heated power‐compensated calorimeter. Dynamic cure of the epoxy resin using four different heating rates and isothermal cure using four different temperatures were carried out and the degree of cure and reaction rates were compared. The cure kinetics were analyzed using several kinetics models. The results showed different activation energies for conventional and microwave curing and suggested different reaction mechanisms were responsible for curing using the two heating methods. Resins cured using conventional heating showed higher glass transition temperatures than did those cured using microwave heating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2054–2063, 2007     

Curing of diglycidyl ether of bisphenol‐A epoxy resin using a poly(aryl ether ketone) bearing pendant carboxyl groups as macromolecular curing agent

BACKGROUND: Reactive thermoplastics have received increasing attention in the field of epoxy resin toughening. This paper presents the first report of using a novel polyaryletherketone bearing one pendant carboxyl group per repeat unit to cure the diglycidyl ether of bisphenol‐A epoxy resin (DGEBA). The curing reactions of DGEBA/PEK‐L mixtures of various molar ratios and with different catalysts were investigated by means of dynamic differential scanning calorimetry and Fourier transform infrared (FTIR) spectroscopy methods. RESULTS: FTIR results for the DGEBA/PEK‐L system before curing and after curing at 135 °C for different times demonstrated that the carboxyl groups of PEK‐L were indeed involved in the curing reaction to form a crosslinked network, as evidenced by the marked decreased peak intensities of the carboxyl group at 1705 cm^?1 and the epoxy group at 915 cm^?1 as well as the newly emerged strong absorptions of ester bonds at 1721 cm^?1 and hydroxyl groups at 3447 cm^?1. Curing kinetic analysis showed that the value of the activation energy (E_a) was the highest at the beginning of curing, followed by a decrease with increasing conversion (α), which was attributed to the autocatalytic effect of hydroxyls generated in the curing reaction. CONCLUSION: The pendant carboxyl groups in PEK‐L can react with epoxy groups of DGEBA during thermal curing, and covalently participate in the crosslinking network. PEK‐L is thus expected to significantly improve the fracture toughness of DGEBA epoxy resin. Copyright © 2009 Society of Chemical Industry     

Synthesis and characterization of diphenylsilanediol modified epoxy resin and curing agent

A series of diphenylsilanediol modified epoxy resins and novel curing agents were synthesized. The modified epoxy resins were cured with regular curing agent diethylenetriamine (DETA); the curing agents were applied to cure unmodified diglycidyl ether of bisphenol A epoxy resin (DGEBA). The heat resistance, mechanical property, and toughness of all the curing products were investigated. The results showed that the application of modified resin and newly synthesized curing agents leads to curing products with lower thermal decomposition rate and only slightly decreased glass transition temperature (T_g), as well as improved tensile modulus and tensile strength. In particular, products cured with newly synthesized curing agents showed higher corresponding temperature to the maximum thermal decomposition rate, comparing with products of DGEBA cured by DETA. Scanning electron microscopy micro images proved that a ductile fracture happened on the cross sections of curing products obtained from modified epoxy resins and newly synthesized curing agents, indicating an effective toughening effect of silicon–oxygen bond.     

Crosslinking of mixtures of diglycidylether of bisphenol‐A with 1,6‐dioxaspiro[4.4] nonan‐2,7‐dione initiated by tertiary amines: III. Effect of hydroxyl groups on network formation

BACKGROUND: Blends of epoxy resin oligomers, diglycidylether of bisphenol‐A (DGEBA), and a bislactone, 1,6‐dioxaspiro[4.4]nonan‐2,7‐dione (s(γBL)), were anionically copolymerized using two tertiary amines as anionic initiators. Their curing rheology and gelation behaviour were studied to provide a more comprehensive knowledge of the curing of these previously studied systems. RESULTS: The activation energy for gelation was found to be similar to that previously measured using differential scanning calorimetry and appeared to increase in the presence of the bislactone. The reaction rate during copolymerization of DGEBA with s(γBL) was slower than DGEBA homopolymerization alone because the alkoxide attack on the epoxide is faster than the reaction of the carboxylate ion and the epoxy group. The effect of the initiator type on the gel conversion was small and was presumably due to differences in the kinetic chain length caused by amine regeneration from the quaternary amine. For the same initiator and at a constant ratio of DGEBA/s(γBL), an increase in the hydroxyl concentration of the DGEBA oligomer raised the gel conversion. For a DGEBA oligomer with low hydroxyl levels, an increase in the concentration of s(γBL) increased the gel conversion; however, for a DGEBA oligomer with high hydroxyl levels, increasing s(γBL) concentration decreased the gel conversion. CONCLUSION: These results are interpreted in terms of the effect of initiation rate and chain transfer rate on the kinetic chain length. The glass transition temperature of the gel was found to be controlled by the fraction of the aliphatic s(γBL) and the amount of plasticizing sol in the matrix. Copyright © 2009 Society of Chemical Industry     

Preparation of a light color cardanol-based curing agent and epoxy resin composite: Cure-induced phase separation and its effect on properties

A light color cardanol-based epoxy curing agent (MBCBE) was synthesized from cardanol butyl ether, formaldehyde and diethylenetriamine. In comparison, a phenalkamine with a similar structure was also prepared. The chemical structures were confirmed by GC–MS and FTIR. The cure behaviors of diglycidyl ether of bisphenol A (DGEBA) with these two curing agents was studied by differential scanning calorimetry (DSC). The morphology, mechanical properties, thermal properties of the cured epoxies were also investigated. The DSC results indicated that MBCBE is less reactive than the phenalkamine. The morphology of the cured MBCBE/DGEBA consisted of cavities dispersed within a continuous epoxy matrix. The cavities markedly improved the lap shear strength and impact strength of the cured resin. Both the two cured resins indicated a two-stage decomposition mechanism. Compared with PKA/DGEBA, the weight loss of MBCBE/DGEBA at the first stage was mainly resulted from the dispersed phase in the epoxy matrix.     

Kinetic analysis of curing behavior of diglycidyl ether of bisphenol A with imidazoles using differential scanning calorimetry techniques

The curing kinetics and mechanisms of diglycidyl ether of bisphenol A (DGEBA) using imidazole (H‐NI) and 1‐methyl imidazole (1‐MI) as curing agents are studied with differential scanning calorimetry (DSC) under isothermal (90–120°C) and dynamic conditions (50–250°C). The isothermal DSC thermograms of curing DGEBA with H‐NI and 1‐MI curing agents show two exothermic peaks. These peaks are assigned to the processes of adduct formation and etherification. These results indicate that there is no difference in the initiation mechanism of 1‐unsubstituted (H‐NI) and 1‐substituted (1‐MI) imidazoles in the curing reaction with epoxy resin. A kinetic analysis is performed using different kinetic models. The activation energies obtained from DSC scanning runs using the Ozawa and Kissinger methods are similar and in the range of 75–79 and 76–82 kJ/mol for DGEBA/H‐NI and DGEBA/1‐MI systems, respectively. These values compare well with the activation energies obtained from isothermal DSC experiments using the autocatalytic method (74–77 kJ/mol). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2634–2641, 2006