Modified Epoxy Systems Research Articles

The Curing Behavior and Adhesion Strength of the Epoxidized Natural Rubber Modified Epoxy/Dicyandiamide System

The curing properties and adhesive strengths of the epoxidized natural rubber (ENR, 25 mole percent epoxidation) modified epoxy systems are studied with differential thermal calorimetry (DSC), scanning electron microscopy (SEM), and lap shear strength (LSS) measurement. The results of DSC analyses indicate that the curing exotherm, the curing rate, the reaction order, and the glass transition temperature of the epoxy system are affected by the presence of reactive ENR. From SEM micrographs, it is obtained that a second spherical rubber phase is formed during cure and the particle size of the rubber phase is increased by increasing the curing temperature and the ENR content. The changes of the volume fraction of the rubber phase and the Tg of the cured systems indicate that the mutual dissolution between epoxy resin and ENR happens and which changes with the curing temperature and the ENR content. The LSS of adhesive joints prepared with the ENR modified adhesives are all lower than those of the unmodified epoxy system, and decrease with increasing the amount of ENR added because of the limited compatibility of the ENR with the epoxy matrix.

BISMALEIMIDES (N, N′-BISMALEIMIDE-4, 4′-DIPHENYLMETHANE AND N, N′-BISMALEIMIDEO-4, 4′-DIPHENYLSULPHONE) MODIFIED BISPHENOLDICYANATE–EPOXY MATRICES FOR ENGINEERING APPLICATIONS

ABSTRACT An interpenetrating network of cyanate ester (CE)-bismaleimide (BMI)-epoxy matrix systems was developed. Epoxy systems modified with 4%, 8%, and 12% (by wt) of cyanate ester were made by using epoxy resin and cyanate ester with diaminodiphenylmethane as curing agent. The cyanate-ester-toughened epoxy systems were further modified with 4%, 8%, and 12% (by wt) of bismaleimides, namely N, N′-bismaleimido-4, 4′-diphenylmethane and N, N′-bismaleimido-4, 4′-diphenylsulphone. BMI-CE-Epoxy matrices were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and heat deflection temperature (HDT) analysis. The DSC thermograms of cyanate-ester-modified epoxy and BMI-modified epoxy during cure show unimodal reaction exotherms. The studies indicate that thermal stability and resistance to water absorption of epoxy resin has been enhanced by the introduction of cyanate ester. The incorporation of bismaleimide into unmodified epoxy and CE epoxy enhanced the thermal and mechanical properties according to its percentage content. The morphologies of fractured surfaces of modified epoxy systems were compared with the unmodified epoxy system.

Development of Cocontinuous Morphologies in Initially Heterogeneous Thermosets Blended with Poly(methyl methacrylate)

Morphology and phase separation process in blends of a network-forming reactive polymer, poly(aminopropylmethylsiloxane) (PAMS), in a poly(methyl methacrylate) (PMMA)-modified epoxy system were studied using optical, epifluorescence and scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical thermal analysis (DMTA). The thermoset system was bisphenol A diglycidyl ether (DGEBA) with different PMMA percentages, 2−10% w/w. Phase separation and reaction advancement were monitored in situ. At the concentration studied, PMMA does not influence the kinetics of the curing process, but it strongly affects the reactive compatibilization between DGEBA and PAMS. The morphology obtained consists of a continuous thermoplastic-rich phase surrounding thermosetting connected polyhedral particles of 5−15 μm. This cocontinuous morphology is observed independently of the percentage of PMMA. Results show that the morphology is strongly influenced by the diffusion and viscosity conditions during reactive compatibilization and phase separation. An increase in PMMA content leads to a decrease in the thermosetting polyhedral particle size. In contrast, an increase in curing temperature leads to bigger sizes. The addition of thermoplastic polymers to initially nonhomogeneous reactive blends is a potential route for generating cocontinuous morphologies irrespective of the thermoplastic content.

Polymerization-Induced Viscoelastic Phase Separation in Polyethersulfone-Modified Epoxy Systems

The polymerization-induced phase-separation process of polyethersulfone (PES)-modified epoxy systems was monitored in situ continuously on a single sample throughout the entire curing process by using optical microscopes, time-resolved light scattering (TRLS), scanning electronic microscopes (SEM), and a rheometry instrument. At specific PES content a viscoelastic transformation process of phase inversion morphology to bicontinuous was found with an optical microscope. The rheological behavior during phase separation corresponds well with the morphology development. Light-scattering results monitoring the phase-separation process of systems with final phase inversion morphology show a typical exponential decay procedure of scattering vector qm. The characteristic relaxation time of phase separation can be described well by the WLF equation.

Dynamic Process of Water Sorption in a Thermoplastic Modified Epoxy Resin System

In the present work, dynamic water sorption processes in a series of poly(ether imide) (PEI) modified diglycidyl ether bisphenol A/(4,4‘-diaminodiphenyl)sulfone (DGEBA/DDS) systems were investigated. The clear picture of the existing state and the diffusion process of water molecules in the blends was first elucidated by two-dimensional (2D) correlation analysis (based on attenuated total reflection Fourier transfer infrared (ATR-FTIR) measurement), and the novel technique used here could be the most effective as well as convenient way to perform the dynamic analysis on such investigations. Furthermore, by means of scanning electron microscopy (SEM), the delicate relationship between dynamic water sorption behavior and the morphology resulting from phase separation was also discussed. Two states of water as free/bound water were found in the systems, the diffusion sequence of which suggested not only composite-dependence but also morphology-dependence.

Morphology effect on water sorption behavior in a thermoplastic modified epoxy resin system

Water sorption behavior in polyetherimide (PEI) modified diglycidyl ether bisphenol-A/4,4′-diaminodiphenyl sulfone (DGEBA/DDS) systems was investigated by gravimetric analysis, positron annihilation lifetime spectroscopy and scanning electron microscopy. The equilibrium water uptake showed strong composition-dependent, which suggested that hydrophilic groups rather than free volume were more significant in determining ultimate water sorption. While besides the number of hydrophilic groups and fractional free volume, morphology induced by phase separation was another key factor that decided the value of diffusion coefficient, which was chiefly responsible for the anomalous diffusion behavior observed at the beginning of co-continuous phase. In addition, morphology not only had the function of decreasing fractional free volume, but also changed the number of hydrophilic groups in epoxy rich regions, which obviously distinguished water sorption behavior in the blends from that in single component systems.

Preparation and characterization of bismaleimide‐modified bisphenol dicyanate epoxy matrices

AbstractAn intercrosslinked network of epoxy matrix systems modified by cyanate ester (CE) and bismaleimide (BMI) was developed. Epoxy systems modified with 4%, 8%, and 12% (by weight) cyanate ester were made by using epoxy resin and cyanate ester, with diaminodiphenylmethane as the curing agent. The reaction between the cyanate ester and the epoxy resin during the cure process of cyanate ester–modified epoxy systems was studied using Fourier transform infrared spectroscopy. The cyanate ester–toughened epoxy systems were further modified with 4%, 8%, and 12% (by weight) bismaleimide (N,N′‐bismaleimido‐4,4′‐diphenylmethane). BMI–CE–Epoxy matrices were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis, and heat deflection temperature analysis. The matrices, in the form of castings, were characterized for their mechanical properties such as tensile strength, flexural strength, and unnotched Izod impact test as per ASTM methods. Mechanical studies indicated that the introduction of cyanate ester into epoxy resin improved the toughness and flexural strength, with a reduction in tensile strength and glass‐transition temperature, whereas the incorporation of bismaleimide into epoxy resin influenced the mechanical and thermal properties according to its percentage content. However, the introduction of both cyanate ester and bismaleimide influenced the mechanical properties according to their percentage content. DSC thermograms of cyanate ester–modified epoxy and BMI‐modified epoxy showed unimodal reaction exotherms. The thermal degradation temperature and heat distortion temperature of the cured BMI‐modified epoxy and CE‐epoxy systems increased with increasing bismaleimide content. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1596–1603, 2003

Preparation and characterization of siliconized epoxy‐1,2‐bis(maleimido)ethane intercrosslinked matrix materials

AbstractNovel intercrosslinked networks of siliconized epoxy‐1,2‐bis(maleimido)ethane matrix systems are developed. The siliconization of epoxy resin is carried out by using 5–15% hydroxyl‐terminated poly(dimethylsiloxane) with γ‐aminopropyltriethoxysilane as a crosslinking agent and dibutyltin dilaurate as a catalyst. The siliconized epoxy systems are further modified with 5–15% 1,2‐bis(maleimido)ethane and cured by using diaminodiphenylmethane. The prepared neat resin castings are characterized for their mechanical properties. Mechanical studies indicate that the introduction of siloxane into these epoxy resins improves the toughness with a reduction in the stress–strain values, whereas incorporation of bismaleimide (BMI) into the epoxy resin improves the stress–strain properties with a lowering of the toughness. The introduction of both siloxane and BMI into the epoxy resin influences the mechanical properties according to their content percentages. Differential scanning calorimetry (DSC), thermogravimetry, and heat distortion temperature analyses are also carried out to assess the thermal behavior of the matrix materials that are developed. DSC thermograms of the BMI modified epoxy systems show unimodal reaction exotherms. The glass‐transition temperature, thermal degradation temperature, and heat distortion temperature of the cured BMI modified epoxy and siliconized epoxy systems increase with increasing BMI content. The water absorption behavior of the matrix materials is also studied. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3808–3817, 2003

Influence of Dipole Interactions on Mechanical Behavior of Modified Epoxy Resins

In this article, Influence of dipole interactions on tensile properties, fracture toughness and glass transition of epoxy resins was investigated by comparing the poly ethylene glycol (PEG) and carboxyl-terminated butadiene acrylonitrile (CTBN) modified epoxy systems. The infrared spectroscopy (IR) spectrums proved that there existed polar interactions between epoxy network and PEG chain. The results showed that dipole interactions played a significant role in influencing on toughness and strength of epoxy resins but had little effect on their glass transition temperature.

Mechanical, thermal and morphological behavior of bismaleimide modified polyurethane‐epoxy IPN matrices

AbstractAn intercrosslinked network of bismaleimide modified polyurethane‐epoxy systems were prepared from the bismaleimide having ester linkages, polyurethane modified epoxy and cured in the presence of 4,4′‐diaminodiphenylmethane. Infrared spectral analysis was used to confirm the grafting of polyurethane into the epoxy skeleton. The prepared matrices were characterized by mechanical, thermal and morphological studies. The results obtained from the mechanical and thermal studies reveal that the incorporation of polyurethane into the epoxy skeleton increases the mechanical strength and decreases the glass transition temperature, thermal stability and heat distortion temperature. Whereas, the incorporation of bismaleimide having ester linkages into polyurethane modified epoxy systems increases the thermal stability, tensile and flexural properties and decreases the impact strength, glass transition temperature and heat distortion temperature. Surface morphology of polyurethane modified epoxy and bismaleimide modified polyurethane‐epoxy systems were studied using scanning electron microscopy. Copyright © 2003 John Wiley & Sons, Ltd.

Thermal and Morphological Properties of Bisphenol Dicyanate–Epoxy–Bismaleimide Intercrosslinked Matrix Materials

Intercrosslinked network of cyanate ester–bismaleimide modified epoxy matrix systems was developed. Epoxy systems modified with 4%, 8%, and 12% (by wt.) of cyanate ester were made by using epoxy resin and cyanate ester with diaminodiphenylmethane as curing agent. The reaction between cyanate ester and epoxy resin during the cure process of cyanate ester modified epoxy systems was studied using FTIR. The cyanate ester toughened epoxy systems were further modified with 4%, 8%, and 12% (by wt.) of bismaleimide (N,N′‐bismaleimido‐4,4′‐diphenylmethane). BMI–CE–epoxy matrices were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and heat deflection temperature (HDT) analysis. The studies indicate that the thermal stability and resistant to water absorption behavior of epoxy resin has been enhanced by the introduction of cyanate ester. However, glass transition temperature and heat deflection temperature are found to decrease with increasing cyanate ester concentration, whereas, the incorporation of bismaleimide into epoxy resin enhanced the thermal properties according to its percentage content. However, the introduction of both cyanate ester and bismaleimide influences the thermal properties according to their percentage content. Differential scanning calorimetry thermogram of cyanate ester modified epoxy and BMI modified epoxy during cure show an unimodel reaction exotherms. The fractured surface of cyanate ester modified epoxy systems reveals the presence of homogeneous morphology, and a smooth fractured surface is observed with increasing bismaleimide content.

Studies on the Phase Separation of a Polyetherimide Modified Epoxy Resin. VI. Effect of Surface Energy on Reaction‐Induced Phase Separation of Epoxy Resin Modified with Polyetherimide

The effect of surface energy on the phase separation of polyetherimide modified epoxy systems is studied by time‐resolved light scattering (TRLS), differential scanning calorimeter (DSC), scanning electron microscopy (SEM), and rheometer. Scanning electron microscopy shows that phase separation of F‐blend system goes further and finally, smaller epoxy‐rich particles and thicker polyetherimide (PEI)‐rich layers are generated. Differential scanning calorimeter and TRLS experiments indicate that the F‐PIP/Epoxy blend has a lower curing rate and phase separation rate than the P‐PIP/Epoxy blend. Rheological behavior displays that the viscosity of F‐PIP/Epoxy blend is lower than that of P‐PIP/Epoxy blend. By studying the properties of different PIP surface the F‐PIP and P‐PIP give different surface energy and it could be concluded that kinetics is a control factor in the phase separation of the two blends due to their difference of surface energy of PEI chain.

The development of a low temperature cure modified epoxy resin system for aerospace composites

Abstract It has been recognised within the composites industry that there are economic advantages in moving away from a conventional autoclave cure at high temperature. A low temperature cure (LTC) resin system has been developed that can be cured at 85 °C initially then subsequently further cured at 175 °C, for a short period. For such resins there is a requirement to achieve the same toughness properties as conventional thermoplastic toughened high temperature cured (HTC) material. The development of an LTC resin system has required a special catalyst systems and appropriate choice of a poly(ethersulphone) co-polymer, in terms of molecular structure, molecular weight and chain ends. Toughness development in thermoset–thermoplastic blends requires an appropriate development of phase morphology during cure. Morphology control has been achieved in LTC materials. Application of linear elastic fracture mechanics measurements of toughness in association with transmission electron microscopy and scanning electron microscopy of the fracture surface has demonstrated links between the toughness and the phase morphology and the chemistry. Small angle neutron scattering has enabled the development of the phase morphology during the low temperature curing process to be monitored. The presence of phase morphology in the resin means that the influence of phase or interface boundaries on the environmental stress cracking (ESC) properties might be a critical issue. This issue has been addressed and investigated for three environments, namely air, demineralised water and dichloromethane at 23 °C. The LTC material is shown to perform in a similar manner to a comparable conventional HTC resin system.

Preparation and characterization of chain‐extended bismaleimide modified polyurethane–epoxy matrices

AbstractIntercrosslinked networks of bismaleimide (BMI) modified polyurethane–epoxy systems were prepared from chain‐extended BMI and polyurethane modified epoxy and cured in the presence of 4,4′‐diaminodiphenylmethane. Infrared spectral analysis was used to confirm the grafting of polyurethane onto the epoxy skeleton. The prepared matrices were characterized by mechanical, thermal, and morphological studies. The results, obtained from the mechanical and thermal studies, reveal that the incorporation of polyurethane into epoxy increases the mechanical strength and decreases the glass‐transition temperature and thermal stability. The incorporation of chain‐extended BMI into polyurethane modified epoxy systems increases the thermal stability and both tensile and flexural properties, and decreases the impact strength and glass‐transition temperature. Surface morphologies of polyurethane modified epoxy and chain‐extended BMI modified polyurethane– epoxy systems were studied by scanning electron microscopy. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1562–1568, 2003

Toughening of cycloaliphatic epoxy resins by poly(ethylene phthalate) and related copolyesters

AbstractPoly(ethylene phthalate) (PEP) and poly(ethylene phthalate–co‐ethylene terephthalate) were used to improve the brittleness of the cycloaliphatic epoxy resin 3,4‐epoxycyclohexylmethyl 3,4‐epoxycyclohexane carboxylate (Celoxide 2021™), cured with methyl hexahydrophthalic anhydride. The aromatic polyesters used were soluble in the epoxy resin without solvents and effective as modifiers for toughening the cured epoxy resin. For example, the inclusion of 20 wt % PEP (MW, 7400) led to a 130% increase in the fracture toughness (KIC) of the cured resin with no loss of mechanical and thermal properties. The toughening mechanism is discussed in terms of the morphological and dynamic viscoelastic behaviors of the modified epoxy resin system. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 388–399, 2002; DOI 10.1002/app.10363

Synthesis and characterization of siliconized epoxy-1,3-bis(maleimido)benzene intercrosslinked matrix materials

Intercrosslinked network of siliconized epoxy-1,3-bis(maleimido)benzene matrix systems have been developed. The siliconization of epoxy resin was carried out by using various percentages of (5–15%) hydroxyl-terminated polydimethylsiloxane (HTPDMS) with γ-aminopropyltriethoxysilane (γ-APS) as crosslinking agent and dibutyltindilaurate as catalyst. The siliconized epoxy systems were further modified with various percentages of (5–15%) 1,3-bis(maleimido)benzene (BMI) and cured by using diaminodiphenylmethane (DDM). The neat resin castings prepared were characterized for their mechanical properties. Mechanical studies indicate that the introduction of siloxane into epoxy resin improves the toughness of epoxy resin with reduction in the values of stress–strain properties whereas, incorporation of bismaleimide into epoxy resin improves stress–strain properties with lowering of toughness. However, the introduction of both siloxane and bismaleimide into epoxy resin influences the mechanical properties according to their percentage content. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and measurement of heat distortion temperature were also carried out to assess the thermal behavior of the matrix samples. DSC thermogram of the BMI modified epoxy systems show unimodel reaction exotherms. The glass transition temperature ( T g), thermal degradation temperature and heat distortion temperature of the cured BMI modified epoxy and siliconized epoxy systems increase with increasing BMI content and this may be due to the homopolymerization of BMI rather than Michael addition reaction. The morphology of the BMI modified epoxy and siliconized epoxy systems were also studied by scanning electron microscopy.

Morphology profiles generated by temperature gradient in PMMA modified epoxy system

AbstractA diglycidyl ether of bisphenol‐A (DGEBA) epoxy resin was modified with 15 wt% of poly(methylmethacrylate) (PMMA) and cured with a stoichiometric amount of 4,4′‐diamino diphenyl methane (DDM). The reactive mixture was cured in a heated mold with different gradients of temperature. Temperature profiles in the mold were imposed by generation of a heat flux from the base, supported on a hot plate, and the top, cooled with water; they were measured along the mold. Depending on the thermal history in each position of the mold, the competition between the phase‐separation process and reaction kinetics produces opaque or transparent zones. Phase separation can also occur in the postcure process while the gelation does not take place before. Therefore, a thermoset plate with gradient of morphology and properties was obtained. Mass fractions of PMMA dissolved in the matrix were calculated with the Fox equation from glass transition temperatures measured along the mold. They were related to morphologies developed during curing. The superposition of the phase diagrams with the conversion‐temperature trajectories during cure permitted an explanation of the morphology gradients generated.

Preparation of novel soluble poly(ester imide)s containing a trimellitimide moiety and their use as modifiers for aromatic diamine‐cured epoxy resin

AbstractPoly(ester imide)s, prepared by the reaction of phthalic anhydride, N‐(4‐carboxyphenyl) trimellitimide and 1,2‐ethanediol, were used to improve the toughness of bisphenol‐A diglycidyl ether epoxy resin cured with 4,4′‐diaminodiphenyl sulfone (DDS). The poly(ester imide)s include poly(ethylene phthalate‐co‐ethylene N‐(1,4‐phenylene) trimellitimide dicarboxylate)s (PESIs) having 10, 20 and 30 mol% trimellitimide (TI) units, respectively. PESIs having 10 and 20 mol% TI units were effective as modifiers for toughening the cured epoxy resin. For example, the inclusion of 20 wt% of PESI (20 mol% TI unit, M W 19300 g mol−1) led to a 55% increase in the fracture toughness (KIC) of the cured resin (with an increase in flexural strength and modulus) and the modified resin had a particulate morphology. PESI having 30 mol% TI units was not effective because of degradation of the modifier by DDS. The toughening mechanism is discussed in terms of morphological and dynamic viscoelastic behaviour of the modified epoxy resin system.© 2001 Society of Chemical Industry

Toughening of epoxy resins with aromatic polyesters

Polyesters, prepared by direct polycondensation from bisphenol A and aliphatic dicarboxylic acids [adipic acid (AD), suberic acid, sebacic acid (SE), and dodecanedioic acid], were used to improve the toughness of the diglycidyl ether of the bisphenol A/diaminodiphenyl methane epoxy system. Polyesters had the number average molecular weight (Mn) ranging from 4300 to 19,200 g/mole. The epoxy systems modified with the AD system (Mn = 6400 g/mole) and the SE system (Mn = 10,200 g/mole) showed phase separated structures with discrete domains of 0.2 μm, but other systems showed smooth fracture surfaces when observed by scanning electron microscopy. The modified epoxy systems except for the AD system and SE system showed two tan δ peaks corresponding to the α and β transitions of the epoxy resin. The modified epoxy systems showed maximum values of K1c at around 10 wt % of polyester and maximum flexural properties at 5 wt % of polyester. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2464–2473, 2000

Investigation of hydrothermal ageing of a filled rubber toughened epoxy resin using dynamic mechanical thermal analysis and dielectric spectroscopy

The effect of hydrothermal cycling on the water absorption behaviour of a dicyandiamide cured silica filled rubber modified epoxy resin system is reported. The water absorption and desorption behaviour of the resin were monitored gravimetrically. Dielectric and dynamic mechanical thermal analyses were carried out on samples, selected from the batches being exposed, at times which reflect various critical points in the ageing profile. It was observed that although the saturation water uptake for the samples was independent of the hydrothermal history; the equilibrium value of the mass on desorption did depend on the hydrothermal history. Deviations from simple Fickian behaviour were observed for samples, which had undergone a previous absorption–desorption cycle. This phenomenon is interpreted as water being absorbed in the polar rubbery phase of the material and not readily released on dehydration. The dielectric data provide a clear indication of the presence of a heterophase structure in the epoxy resin and are consistent with the postulate that, whilst the initial water absorption occurs reversibly in the epoxy matrix, subsequent absorption of water into the polar rubbery phase is irreversible at 50°C, the temperature used in the study.