Relation between the network structure and dynamic mechanical properties of a typical amine-cured epoxy polymer

The dynamic mechanical properties of a series of epoxy polymers of known network structure have been investigated. It was shown that the distance between crosslinks could be predicted from either the shift in the glass transition temperature T_g or by use of the dynamic modulus above T_g. The front factor in the equation of state for rubber elasticity was near unity for stoichiometric equivalence of epoxy and amine and increased slowly with excess of either component.     

Positronium annihilation in amine-cured epoxy polymers

Positronium annihilation spectroscopy (PAS) has been used to study the microstructural properties of amine-cured epoxy polymers. We have determined the free-volume “hole” sizes in these polymers by comparing the observed ortho-positronium lifetimes with the known lifetime–free volume correlation for low-molecular-weight systems. The free volumes for four epoxies with different crosslink densities are found to vary significantly over the temperature range between ?78° and 250°C. The free-volume holes for these polymers are found to range from 0.025 to 0.220 nm³. Two important transition temperatures were found: one corresponds to the glass transition temperature T_g determined by differential scanning calorimetry (DSC), and the other occurs about 80–130°C below T_g. The sub-T_g transition temperature is interpreted tentatively as being where hole size reaches dimensions adequate for positronium trapping or else the onset temperature for local mode or side-chain motions. These two transition temperatures plus two additional onset temperatures are found to be correlated with crosslink densities calculated from stoichiometry.     

Positron annihilation in amine-cured epoxy polymers—pressure dependence

Positron annihilation spectroscopy has been used to study free volume in an arnine-cured epoxy as a function of external pressure at temperatures above and below the glass transition temperature. The observed ortho-positronium lifetime τ₃ and formation probability I₃ decreased with increasing pressure. The decrease in τ₃ is interpreted in terms of a corresponding decrease in average free-volume hole size over the range from 0.135 to 0.045 nm³. The fractional free-volume and the free-volume compressibility in the epoxy are calculated as functions of pressure at 100°C.     

Enthalpy relaxation in a partially cured epoxy resin

The enthalpy relaxation of a partially cured (70%) epoxy resin, derived from diglycidyl ether of bisphenol-A cured by methyl-tetrahydrophthalic anhydride with accelerator, has been investigated. The key parameters of the structural relaxation (the apparent activation energy Δh*, the nonlinearity parameter x, and the nonexponentiality parameter β) are compared with those of the fully cured epoxy resin. The aging rates, characterized by the dependences of the enthalpy loss and peak temperature on log(annealing time), are greater in the partially cured epoxy than they are in the fully cured resin at an equivalent aging temperature (T_a = T_g − 20°C). There is a significant reduction in Δh*, from 1100 kJ mol⁻¹ for the fully cured system to 615 kJ mol⁻¹, as the degree of cure is reduced. The parameter x determined by the peak-shift method appears essentially independent of the degree of cure (x = 0.41 ± 0.03 for the partially cured resin compared with 0.42 ± 0.03 obtained previously for the fully cured resin), and does not follow the usually observed correlation of increasing x as Δh* decreases. This invariability of the parameter x seems to indicate that it is determined essentially by the local chemical structure of the backbone chain, and rather little by the supramolecular structure. On the other hand, the estimated nonexponentiality parameter β lies between 0.3 and 0.456, which is significantly lower than in the fully cured epoxy (β ≅ 0.5), indicative of a broadening of the distribution of relaxation times as the degree of cross-linking is reduced. Like the parameter x, this also does not follow the usual correlation with Δh*. These results are discussed in the framework of strong and fragile behavior of glass-forming systems, but it is difficult to reconcile these results in any simple way with the concept of strength and fragility. © 1996 John Wiley & Sons, Inc.     

Flame retardant epoxy resin based on bisphenol A epoxy resin modified by phosphoric acid

Flame retardant epoxy resin (FREP) were synthesized from phosphoric acid and bisphenol A epoxy resin (BAEP). The structure of the FREP was characterized using FTIR and ³¹P NMR. Then, several FREP/BAEP mass ratios were used to obtain materials with different phosphorus contents. The properties of the thermosetting materials were evaluated by limiting oxygen index, UL 94, cone calorimeter test, and thermogravimetric analysis. The results showed that the heat release rates and smoke production rates decreased greatly, and char residue increased with the increasing of FREP. It indicates that good flame retardant properties are related to the formation of a protective phosphorus-rich char layer.     

Synthesis and characterization of liquid crystalline epoxy and co-polymerisation with a non-mesomorphic epoxy resin

A novel liquid crystalline epoxy resin based on the imine group was synthesised and structurally characterised by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. The mesogenic behaviour of the monomer was measured by differential scanning calorimetry (DSC) and polarised optical microscopy (POM), and presented various textures in the extensive temperature range. Methyl nadic anhydride (MNA) was employed to cure the liquid crystalline epoxy resin and the curing process was investigated using POM and wide-angle X-ray diffraction (WAXD). Information about distribution of liquid crystalline epoxy resin in the blending system resulted from the FT-IR Imaging System, indicating that molecules of liquid crystalline epoxy resin can agglomerate to form anisotropic domains. The improvement in mechanical properties of diglycidyl ether of biphenol A (DGEBA) modified with liquid crystalline epoxy was achieved. Scanning electronic microscopy (SEM) showed that an extremely rough and highly deformed fracture surface can be obtained. DGEBA modified with liquid crystalline epoxy resin was characterised by dynamic mechanical analysis (DMA) for its thermal properties. The results indicate that the presence of the liquid crystal phase influences glass transition temperature (Tg).     

Synergy in flame-retarded epoxy resin

The potential synergists aluminium diethylphosphinate (AlPi), boehmite (AlO(OH)) and melamine polyphosphate (MPP) were compared in flame-retardant epoxy resin (EP)/melamine poly(magnesium phosphate) (S600). The pyrolysis, the fire behaviour as well as the chemical interactions in the gas and condensed phases were investigated by various methods. Flammability was investigated by cone calorimeter and oxygen index (OI). The thermal and thermo-oxidative decomposition were studied by thermogravimetric analysis coupled with FTIR spectrometer. The special focus was on the investigation of structural changes in the condensed phase via solid-state NMR of ²⁷Al and ³¹P nuclei. By the comparison of epoxy resin with only one additive or with S600 in combination with AlPi, AlO(OH) or MPP, it was possible to calculate the synergy index. The best performance in terms of fire behaviour was observed for EP/S600/MPP with a PHRR (peak heat release rate) of 208 kW m^?2 due to slight synergy. In the case of THE (total heat evolved), clear synergy occurred for EP/S600/AlPi and EP/S600/AlO(OH). By solid-state NMR, different phosphates and aluminates were identified, indicating the chemical interactions between S600 and AlPi, AlO(OH) or MPP. The systematic multi-methodical approach yielded insight into the synergistic effects in the flame-retarded epoxy resin.     

Fracto-emission from neat epoxy resin

Fracto-emission is the emission of particles (e.g. electrons, photons, ions, neutral species) due to crack growth in materials. These emissions can be related to a number of fracture related phenomena including microcracking, crack speed of dynamic crack growth, locus of fracture (in filled materials), and potentially the extent of crack branching. Here, we examine the emission of electrons, positive ions, and photons during and following the fracture of a neat epoxy resin subjected to tensile and tensile impact loading in vacuum. Experiments which detect correlations of crack tip position and emission intensity show that the emissions occur during and following fracture. We also illustrate that observed variations of the fracture surface morphology under different loading conditions correlated with characteristics of the photon and charged particle emission. For example, regions of the surface exhibiting the highest degree of surface roughness resulted in more intense emission.     

Enthalpic, entropic, and square gradient contributions to the surface energetics of amine-cured epoxy systems

The evolution of surface tension during polymerization of three amine-cured epoxy systems was investigated. Due to the chemical reaction of the epoxy groups with primary and secondary amines, the energetic status of an epoxy-amine system increased during polymerization. At the same time, the polymerization process induced entropic variations, also contributing to the evolution of surface energetics. A simple relation expressing the surface tension as a function of the bulk energy, the entropy of the system, and the square gradient of the polymer density was derived. The bulk and surface energetics were expressed in terms of solubility parameter and surface tension, respectively. The former was predicted using the Van Krevelen group contribution method, while the latter was directly measured using the Wilhelmy wetting method. Results indicated that, in all the three epoxy-amine systems under investigation, a unique relationship combining the surface tension, the bulk energy, the entropy, and the density square gradient of the system could be used. On the basis of the present study, and taking into account all contributory factors, it was concluded that the enthalpy component to the surface energetics is the dominant contribution.     

Curing kinetics of bio-based epoxy resin-toughened DGEBA epoxy resin blend

In the present study, TEIA bioresin was blended with the diglycidyl ether bisphenol A (DGEBA) epoxy resin in different ratios (i.e. 10, 20, 30, 40 mass%), cured with methylhexahydrophthalic anhydride curing agent in the presence of 2-methylimidazole catalyst. The optimized composition of DGEBA and TEIA bioresin blends system was employed as an adhesive strength. The adhesive strength of the TEIA-modified DGEBA epoxy resin blend system was increased from 4.14 to 6.31 MPa on an aluminium substrate compared to the DGEBA epoxy resin. The curing kinetics of non-isothermal, DGEBA epoxy resin and its bio-based blend systems were investigated employing differential scanning calorimetry. An increase in the peak temperature and reduction in a heat of curing as well as activation energy in DGEBA epoxy resin were observed with the addition of TEIA bioresin content. The activation energy (E_a) of the DGEBA resin and their bio-based blend system were obtained from Kissinger and Flynn–Wall–Ozawa methods.

     

Aspects of epoxy resin curing reactions

The progress that has been made in establishing the mechanism of the curing reaction between epoxides and amines, particularly through the use of calorimetric methods (DSC), is discussed. A novel application of tritium-labelled compounds to the study of the reaction of epoxides with imidazoles is also presented. The radiochemical method has produced an enhanced understanding of the reaction mechanism and has given kinetic data in good agreement with DSC measurements.     

Thermal stability and toughening of epoxy resin with polysulfone resin

The cure behavior, thermal stability, and mechanical properties of diglycidylether of bisphenol A (DGEBA)/polysulfone (PSF) blends initiated by 1 wt % N‐benzylpyrazinium hexafluoroantimonate as a cationic latent catalyst were investigated. The DGEBA/PSF content was varied within 100/0–100/40 wt %. Latent properties were studied through the measurement of the conversion as a function of the curing temperature, and the cure activation energy (E_a) was studied by the Kissinger method with a dynamic differential scanning calorimetry analysis. The thermal stabilities, largely based on the integral procedural decomposition temperature (IPDT) and decomposed activation energy (E_t), were investigated by the measurement of thermogravimetric analysis. For the mechanical properties of the casting specimens, the critical stress intensity factor (K_IC) test was performed, and their fractured surfaces were examined with scanning electron microscopy. E_a, IPDT, E_t, and K_IC increased with PSF increasing in the neat epoxy resin up to 30 wt %. However, there was a marginal decrease in the blend system in both the thermal and mechanical properties due to the phase separation between DGEBA and PSF. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 121–128, 2001     

Curing characteristics of carboxyl functionalized glucose resin and epoxy resin

The curing characteristics of carboxylic functionalized glucose resin (glucose maleic acid ester vinyl resin: GMAEV) and epoxy resin have been studied using DSC and FTIR methods. Exothermic reactions attributed to esterification and etherification reactions of the hydroxyl and carboxyl functionalities of GMAEV with the epoxy groups were identified. Exothermic reactions showed very different patterns according to the degree of carboxyl group substituent of GMAEV. The results showed that esterification reaction occurs in the early stage of cure and then etherification followed after completion of the esterification. A cured matrix containing epoxy resin and 50 wt.% of GMAEV was prepared and characterized. The cured matrix showed thermal stability up to 300 °C. The average glass transition temperature and storage modulus of the matrix were as high as 95 °C and 2700 MPa, respectively. The cured matrix of epoxy resin and GMAEV with higher degree of carboxyl group was found to have a lower density due to the formation of bulky groups in the crosslinks.     

Block copolymer modified novolac epoxy resin

The phase behavior of uncured and cured mixtures containing stoichiometric amounts of Epon164 novolac epoxy resin and 4,4′‐methylenedianiline combined with a nearly symmetric poly(methyl acrylate‐co‐glycidyl methacrylate‐b‐polyisoprene) diblock copolymer was investigated with small‐angle X‐ray scattering and transmission electron microscopy. A series of morphologies were documented as a function of the copolymer concentration, which ranged from pure diblock to 2.5 wt % in the thermoset resin. Ordered lamellae bordered a wide multiphase region followed by disordered wormlike micelles that transformed continuously into vesicles at the lowest compositions. The thermal curing of this pentafunctional epoxy system to complete conversion had little impact on the phase behavior of the mixtures, and this was consistent with previous experiments with difunctional epoxy and the same hardening agent. However, changing the epoxy component led to gross changes in the phase behavior that were interpreted with the concept of a wet‐to‐dry brush transition. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1994–2003, 2003     

Study on curing of novolac epoxy resin

The optimization of proportions of novolac epoxy resin, Dobeckot E4 and polyamide hardener, EH411 has been established by DSC and the data indicates that resin-polyamide, 100∶40 and 100∶50, appear to be optimum where ‘extent of cure’ is maximum. The kinetic parameters for these formulations have been evaluated using isothermal and dynamic modes by employing DSC. The rate constants have been evaluated for curing process of these formulations using isothermal DSC mode in the temperature range of 70°–90°C. These have also been predicted at 20°±1°C (room temperature) by extrapolating the data obtained at elevated temperatures. A comparison of the predicted values with the experimental values shows that there is a good agreement between them.     

Dynamic mechanical properties of glass-filled epoxy resin

Composite specimens were prepared using soda glass beads and a purified epoxy resin cured with 1,3-propylene diamine. Some beads were treated with a silane coupling agent. The dynamic mechanical properties of these specimens were measured in the temperature range ?190 to +180°C using a free-oscillation torsion pendulum. The dynamic mechanical relaxation spectrum showed no feature that could be attributed to the formation of a new interfacial phase and the torsional moduli were unaffected by the use of the coupling agent. Increasing the glass content of the specimens decreased the damping and increased the modulus. An attempt was made to predict the composite modulus using the Kerner equation. When the specimens were immersed in boiling water, two effects were noted. First, water was absorbed in the epoxy resin matrix and changes in the dynamic spectrum were observed. Second, in samples filled with untreated glass debonding occurred and the presence of free water at the interface was indicated by the appearance of a new peak near 0°C.     

Low-temperature thermal decomposition of an epoxy resin

The thermal decomposition of the epoxy resin system, MY720 (70 wt % tetraglycidyl diaminodiphenyl methane), cured with Eporal (diamino diphenyl sulfone), was investigated at 125–215°C. The primary decomposition product was propenal. Several other aldehydes were observed; however, they are formed during cure and are not thermal decomposition products. The apparent activation energy for the production of propenal was 67 kJ/mol (16 kcal/mol). Possible mechanisms for the source of this compound are discussed.     

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