J. López

Effect of water sorption on the structure and mechanical properties of an epoxy resin system

The characteristics of sorption and diffusion of water in an amine-cured epoxy system based on tetraglycidyl diaminodiphenylmethane and a novolac glycidyl ether resin were studied as a function both of the polymer microstructure, known from previous works, and the temperature. Water-sorption experiments and dynamic mechanical analysis (DMA) were performed. Tensile stress–strain and Rockwell hardness tests were conducted to investigate the effects of absorbed water on the mechanical properties of the material. Competing effects of the sorption of water in the free volume and of strong interactions between water molecules and polar groups of the network were used to explain the diffusional behavior observed, which followed Ficks second law. DMA analysis seemed to be sensitive to the water effects and the viscoelastic behavior was related both to the water-sorption processes and to the microstructure of the system. An important impact of water uptake on the tensile properties at break was also appreciated.

Effect of poly(styrene-co-acrylonitrile) on the curing of an epoxy/amine resin

Abstract The cure kinetics and the miscibility have been studied in blends of poly(styrene-co-acrylonitrile) (SAN) with tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and 4-4′-diaminodiphenylsulphone (DDS) as hardener. Differential scanning calorimetry (DSC) was the technique used for dynamic and isothermal experiments. Binary mixtures of SAN with the epoxy resin show complete miscibility over the whole composition range, displaying a single glass transition temperature. Kinetic analysis were performed using three kinetic models: Kissinger, Flynn–Wall–Ozawa and the phenomenological model developed by Kamal. Activation energies and kinetic parameters were determined by fitting experimental data. Diffusion control is incorporated to describe the cure in the latter stages, predicting the cure kinetics over the whole range of conversion. The autocatalytic mechanism was observed for the neat system and the epoxy-SAN blends. The reaction rates for the epoxy blends were found to be lower than of the neat epoxy. The reaction rates and the maximum conversions decreased when SAN contents increase, due to reduction of mobility of the reacting species.

Kinetic studies of the effect of ABS on the curing of an epoxy/cycloaliphatic amine resin

Using differential scanning calorimetry (DSC), we have studied, under isothermal and dynamic conditions, the kinetics of the cure reaction for an epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) modified with different contents of acrylonitrile-butadiene-styrene (ABS) and cured with 1,3-bisaminomethylcyclohexane (1,3-BAC). Kinetic analysis were performed using three kinetic models: Kissinger, Flynn-Wall-Ozawa, and the phenomenological model of Kamal as a result of its autocatalytic behavior. Diffusion control is incorporated to describe the cure in the latter stages, predicting the cure kinetics over the whole range of conversion. The total heats of reaction were not influenced by the presence of ABS. The autocatalytic mechanism was observed both in the neat system as well as in its blends. The reaction rates of the blends and the maximum conversions reached did not change too much with the ABS content. Blending ABS within the epoxy resin does not change the reaction mechanism of the epoxy resin formation.

Cure kinetics of amine-cured diglycidyl ether of bisphenol: A epoxy blended with poly(ether imide)

An epoxy based on the diglycidyl ether of bisphenol A (DGEBA) has been modified with poly(ether imide) (PEI) and cured with 1,3-bisaminomethylcyclohexane (1,3-BAC). The curing kinetics of the neat resin and of a PEI/epoxy blend were analysed and compared by differential scanning calorimetry. The results showed that the PEI has effect on curing reaction, reaching less conversion in this process than for the neat system. The experimental data for both systems showed an autocatalytic behaviour and the phenomenological model of Kamal was proposed to obtain the activation energies for the rate constants. The values of the activation energies for the PEI/epoxy blend system are higher than the values for the neat system. Diffusion control is incorporated to describe the cure in the latter stages predicting the cure kinetics over the whole range of conversion.

Physical Aging of a Tetrafunctional/phenol Novolac Epoxy Mixture Cured with Diamine. DSC and DMA measurements

The physical aging of a system containing tetraglycidyl-4-4′-diaminodiphenylmethane (TGDDM), with a multifunctional novolac glycidyl ether resin hardened by 4,4′-diaminodiphenylsulphone (DDS) has been investigated by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Samples fully cured were aged at temperatures between 200 and 250°C, during periods of time from 1 to a maximum of 336 h. Furthermore, the dynamic mechanical relaxation behaviour annealed at temperature of 220°C, was studied, aging during 24 and 168 h. The effect of the enthalpy relaxation during DSC heating scan is shown by the presence of an endothermic peak whose position and intensity depends on the aging conditions, both temperature and time. DSC studies suggest that enthalpy relaxation increases gradually with aging time to a limiting value for each temperature where structural equilibrium is reached. DMA results show that the effect of aging is to cause chain stiffening and a decrease in the height of the peak value of the loss factor.

Thermal decomposition behavior and the mechanical properties of an epoxy/cycloaliphatic amine resin with ABS

Abstract A bifunctional epoxy resin was modified with several amounts of a thermoplastic based on a mixture of acrylonitrile–butadiene–styrene (ABS) in order to improve the toughness of the resin. Previous to study of the mechanical properties, thermal degradation behavior and the kinetic data of the modified systems were calculated using isothermal and dynamic thermogravimetric methods. The results have shown good agreement between the activation energies obtained from all methods if it is assumed that the mechanism of degradation of these epoxy resins is of sigmoidal type. The mechanical behavior was studied by stress–strain tests. The tensile properties of the blends were measured at different test temperatures. In general, the toughness improved when the test temperature increased. The more effective thermoplastic modification was the blend with 10 phr ABS, because it obtained better mechanical data than the other ones at low test temperatures. The fractography study proved that the blend with 10 phr ABS had the satisfactory morphology in order to get a toughness improvement, but the SEM micrographs also displayed adhesion problems between ABS particles and the epoxy matrix which did not allow the mechanical behavior improvement that was expected.

Thermal properties of amine cured diglycidyl ether of bisphenol A epoxy blended with poly(ether imide)

Thermal properties of an epoxy system containing diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcyclohexane (1,3-BAC) as curing agent modified with poly(ether imide) (PEI) were studied using both dynamic mechanical (DMA) and thermogravimetric (TG) analysis. The effect of thermal degradation in air and in vacuum on the dynamic mechanical and thermogravimetric properties for the DGEBA/1,3-BAC/PEI system was investigated. The results showed that this system can be considered miscible for PEI contents up to 10 phr (phr: number of parts of PEI per hundred parts of the DGEBA epoxy resin), and also that even very small contents of PEI in the blends reduced the thermal stability of the material. Thermal degradation during a short period of time of this material both, in air or in vacuum yields a decrease of thermal stability and an increase of the glass transition temperature with respect to the material without degradation.

TTT isothermal cure diagram of a dyglicidyl ether of bisphenol A/1,3‐bisaminomethylcyclohexane (DGEBA/1,3‐BAC) epoxy resin system

The isothermal cure of an epoxy-cycloaliphatic amine system has been studied following the evolution of both glass transition temperature and conversion. A functional relationship between Tg and conversion is established. The cure reaction is satisfactorily described by a phenomenological model with parameters determined from DSC experiments. By applying the kinetic model, gelation and vitrification curves are calculated and compared with experimental times to gelation and times to vitrification determined at temperatures between 50 and 100°C. The isothermal time-temperature-transformation (TTT) curing diagram including iso-Tg contours has been established.

Degradation Kinetics of an Epoxy/Cycloaliphatic Amine Resin Under Isothermal and Non-isothermal Conditions

A study of an epoxy-cycloaliphatic amine system has been realized using a thermogravimetric technique (TG). Isothermal and non-isothermal (dynamic) methods were employed to determine the kinetic data of this system.Five methods were used for determining the activation energies of this system in the dynamic heating experiments. In two of them (Flynn-Wall-Ozawa, and Kissinger) it is not necessary to have a prior knowledge of the reaction mechanism of the degradation behaviour for this system. In the other ones (Coats and Redfern, Horowitz and Metzger, and Van Krevelen et al.) it is necessary to know this reaction mechanism, besides Criado et al. method was used for determining it.The results have shown that good agreement between the activation energies obtained from all methods can be achieved if it is assumed that the degradation behaviour of this system is of sigmoidal-rate type.

Thermogravimetric Study of Tetrafunctional /Phenol Novolac Epoxy Mixtures Cured with a Diamine

The degradation behaviour of an epoxy system containing both tetraglycidyl-4-4′-diami-nodiphenylmethane (TGDDM) and a multifunctional novolac glycidyl ether resins, which are cured with 4,4′-diaminodiphenylsulphone (DDS) has been studied using thermogravimetric technique (TG).Isothermal and non-isothermal (dynamic) methods were used to determine the kinetic parameters of this system. An isothermal method and five dynamic methods reported in the literature were used to determine the activation energies of the system. Kissinger’s method only requires knowledge of the temperature at which the rate of weight loss is at maximum to calculate the activation energy. The Flynn-Wall-Ozawa method provides the activation energy without any assumption about the reaction order while the other three methods (Coats and Redfern, Horowitz and Metzger and Van Krevelen et al.) require a prior knowledge of the mechanism of degradation for this system to calculate the kinetic parameters.The results obtained by applying these different methods agreed well. In fact, the values of the activation energies provided by the six methods have shown excellent agreement when the degradation behaviour of this system was assumed to be of the deceleratory rate type. The kinetic parameters have been used to estimate the half-life of this system in two different ways.