J. H. Hodgkin

Thermoplastic toughening of epoxy resins: a critical review

Thermoplastic toughening of epoxy resins has been actively studied since the early 1980s with considerable progress in property improvement and understanding having been made since then. The main advantage in using thermoplastics to toughen epoxy resins is that their incorporation need not result in significant decreases in desirable properties such as modulus and yield strengths as is generally the case when rubbers are used as toughening agents. However, the predominant criteria for achieving optimum toughness enhancement in the thermoplastic toughening of epoxy resins are still not all that clear from the literature. This review has focused upon the importance of the thermoplastic endgroups, the materials morphology, the ductility of the matrix and the chemical structure of the thermoplastic, it summarizes what the authors believe are the important requirements for good thermoplastic toughening.

Toughening of a trifunctional epoxy system: Part VI. Structure property relationships of the thermoplastic toughened system

This paper examines the effect of the addition of PSF upon the final properties and network structure of the TGAP/DDS system after cure and post-cure. It also compares the differences in the network structure and properties of the modified system between samples in which the epoxy resin and thermoplastic had been prereacted and those which had been simply mixed together. The thermal properties of the network structure were investigated using dynamic mechanical thermal analysis while the chemical structures were characterised using near infra-red spectroscopy. Physical properties such as water uptake, density and mechanical properties such as toughness, modulus, compressive strength and yield stress were measured.

Quantitative analysis of the cure reaction of DGEBA/DDS epoxy resins without and with thermoplastic polysulfone modifier using near infra-red spectroscopy

Abstract Near infra-red spectroscopy techniques were used to study the cure reactions of various epoxy resin formulations based on diglycidyl ether of bisphenol A (DGEBA) resins cured with 4,4′-diaminodiphenyl sulfone (DDS) hardener. Stoichiometric and non-stoichiometric DGEBA/DDS resin formulations, using neat as well as thermoplastic toughened systems containing two phenolic hydroxyl terminated polysulfones with different molecular weights were involved in this study. The infra-red absorption spectra of the prepared formulations were obtained on an FTi.r. spectrometer operating in the region of 4000–11000cm−1 and were analysed according to methods described in this paper. The chemical group peaks of interest in a DGEBA/DDS spectrum were identified by a comparative study with individual spectra of DGEBA and DDS monomers. Where necessary, special model compounds were used to identify unknown bands, such as the primary amine band at 4535 cm−1. The absorption bands of interest were integrated to quantify the areas and then converted to molar concentrations. This series of quantitative analysis of the major chemical groups in several resin systems, led us to understand not only the reaction mechanism in each system but also the cure kinetics, which showed strong dependence on the formulation of the system. In this paper, the details of the quantitative analysis of the infra-red spectrum for various systems and the reaction mechanisms observed in stoichiometric DGEBA/DDS resin formulations are described.

Toughening of a trifunctional epoxy system Part III. Kinetic and morphological study of the thermoplastic modified cure process

The effect of thermoplastic addition on the cure kinetics and morphology of an epoxy/amine resin were investigated using differential scanning calorimetry (d.s.c.), dynamic mechanical spectroscopy and transmission electron spectroscopy. The results obtained from d.s.c. were applied to the Arrhenius, autocatalytic and diffusion controlled different kinetic models and showed that the effect of thermoplastic addition on the rate of cure was rather modest. The cure mechanism remained broadly autocatalytic in nature regardless of PSF concentration although at higher concentrations and lower cure temperatures, the mechanism became far more diffusion controlled. The residual miscibility of the epoxy/amine and thermoplastic phases within each other, however, caused the ultimate cure conversions and the conversions at gelation and vitrification to decrease with increasing PSF content while the Arrhenius activation energy increased with increasing cure conversion with increasing PSF content.

Cure kinetics of elementary reactions of a diglycidyl ether of bisphenol A/diaminodiphenylsulfone epoxy resin: 2. Conversion versus time

Abstract The kinetics of the elementary reactions (primary amine-epoxy and secondary amine-epoxy) in addition to the overall reaction have been studied in a diglycidyl ether of bisphenol A (DGEBA) epoxy resin system, cured with diaminodiphenylsulfone (DDS) hardener. The conversion versus time data for the elementary reactions and for the overall reaction were obtained by monitoring the changes in concentration of epoxy, primary and secondary amine groups during cure using near-infra-red spectroscopy. The data were evaluated using nth-order and autocatalytic kinetic models. The primary amine-epoxy reaction (linear polymerization) was described well by the nth-order kinetic model over the whole range of conversion of the primary amine, whereas the secondary amine-epoxy reaction was described by both the nth-order kinetic and first-order autocatalytic models in the ranges of initial 30% conversion at 130°C cure, and 80% conversion at 205°C cure. The kinetic parameters for each reaction were also obtained. The reactivity ratio of the secondary amine to the primary amine for a non-catalysed reaction increased with increasing cure temperature, whereas the opposite trend was observed for the autocatalysed reaction.

Cure kinetics of elementary reactions of a DGEBA/DDS epoxy resin: 1. Glass transition temperature versus conversion

Abstract The influence of elementary reactions (linear polymerization and crosslinking) on the development of the glass transition temperature ( T g ) of diglycidyl ether of bisphenol A/diaminodiphenylsulfone (DGEBA/DDS) epoxy resin system has been analysed quantitatively according to a proposed model. The model was derived on the basis of the reaction mechanisms observed in the curing system and the assumption that the T g has individual linear relationships with the degree of conversion of the two reactions. The conversions of elementary reactions during the cure were obtained experimentally by measuring the changes in the concentrations of epoxy, primary and secondary amine groups with time by near-infra-red spectroscopy. The T g values were measured by differential scanning calorimetry (d.s.c.). In addition, and for the purpose of comparison, the overall conversion of the system was also obtained from d.s.c. in a dynamic scanning mode. The conversion-time data from the two different techniques were consistent. The proposed model successfully predicted the separate increases in T g due to the linear polymerization and due to the crosslinking reactions. The Di Benedetto equation and the viscoelastic model of Gan et al. are also discussed.

Toughening of a trifunctional epoxy system. II. Thermal characterization of epoxy/amine cure

The cure of a trifunctional epoxy resin with an amine coreactant was studied using two thermal analysis techniques: differential scanning claorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). These techniques were used to monitor the development of both the thermal and mechanical properties with cure. Detailed kinetic analysis was performed using a variety of kinetic models: nth order, autocatalytic, and diffusion-controlled. The reaction was found to be autocatalytic in nature during the early stages of cure while becoming diffusion-controlled once vitrification had taken place. By combining the results obtained from DSC and DMTA, the degree of conversion, at which key events such as gelation and vitrification take place, were determined. A TTT diagram was constructed for this epoxy/amine system showing the final properties that can be achieved with the appropriate cure history.

Toughening of trifunctional epoxy system. V. Structure–property relationships of neat resin

This work presents an investigation into the structure–property relationships of a cured highly crosslinked epoxy/amine resin system. The mechanical, physical, and thermal properties of the cured and postcured networks were measured and compared to the chemical structures. Crosslink density was shown to be dependent upon secondary amine conversion and it determined the glass transition temperatures, water uptake, density, toughness, and compressive strength. Other properties such as compressive modulus and yield stress were determined by more short-range molecular motions. Curing at a temperature of 150°C was shown to be the minimum temperature required to “completely” cure the network and achieve optimum mechanical, physical, and thermal properties.

Toughening of a trifunctional epoxy system: IV. Dynamic mechanical relaxational study of the thermoplastic‐modified cure process

Dynamic mechanical spectroscopy has been used to investigate the cure of a thermoplastically modified trifunctional epoxy resin. The complex dissolution, curing behavior, and variations in the glass transition of the thermoplastic (PSF) phase were described, as was the Tg behavior of the epoxy phase. Prereaction of the PSF material with the epoxy resin was found to greatly increase the solubility of the PSF in the epoxy phase with little effect on the concentration of the epoxy monomer dissolving in the PSF phase. The curing behavior of the epoxy component in the thermoplastic phase was also investigated, in addition to changes in the mobility of the network at both gelation and vitrification.

Effect of reinforcing fibres on the morphology of a toughened epoxy/amine system

The effect of reinforcing fibres on the morphology of a thermoplastically toughened tri-functional epoxy resin cured with an aromatic hardener has been studied using optical microscopy. Single and multiple strands of different types of fibres were placed within curing resin matrices containing varying thermoplastic concentrations. The fibres appeared to encourage the development of large particles (significantly larger than would be found in the bulk matrix), which remained at the fibre surface. A plain weave glass fibre tape was also found to greatly affect the final morphology of the same system. The results indicate that the morphology of a thermoplastic modified epoxy/amine resin is significantly affected by reinforcing fibres.