Russell J. Varley

Morphology, thermal relaxations and mechanical properties of layered silicate nanocomposites based upon high-functionality epoxy resins

This paper investigates the possibility of improving the mechanical properties of high-functionality epoxy resins through dispersion of octadecyl ammonium ion-modified layered silicates within the polymer matrix. The different resins used are bifunctional diglycidyl ether of bisphenol-A (DGEBA), trifunctional triglycidyl p-amino phenol (TGAP) and tetrafunctional tetraglycidyldiamino diphenylmethane (TGDDM). All resins are cured with diethyltoluene diamine (DETDA). The morphology of the final, cured material was probed by wide-angle X-ray scattering, as well as optical and atomic force microscopy. The α- and β-relaxation temperatures of the cured systems were determined using dynamic mechanical thermal analysis. It was found that the presence of organoclay steadily decreased both transition temperatures with increasing filler concentration. Further, the effect of different concentrations of the alkyl ammonium-modified layered silicate on the toughness and stiffness of the different epoxy resins was analyzed. All resin systems have shown improvement in both toughness and stiffness of the materials through the incorporation of layered silicates, despite the fact that it is often found that these two properties cannot be simultaneously achieved.

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.

Studies on blends of epoxy-functionalized hyperbranched polymer and epoxy resin

An epoxy-functionalized hyperbranched polymer (HBP) was used to toughen a conventional epoxy resin, diglycidyl ether of bisphenol A (DGEBA) cured with diethyltoluene-2,6-diamine (DETDA). There was little change in gel time as a result of addition of HBP, even though the HBP reacts at a slower rate with amine hardeners compared to DGEBA alone. Phase separation was investigated for various HBP contents and as a function of cure conditions as well. The thermal and dynamic viscoelastic behavior of the modified matrices have been examined and compared to the DGEBA epoxy matrix. It appears that the HBP which phase separates does not react as fully as when it is reacted with the amine alone. Nonetheless, good improvement in impact strength as a result of incorporation of HBP were observed and explained in terms of morphological behavior for a DGEBA matrix modified with various amounts of HBP.

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.

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.

Toughening of a trifunctional epoxy system: 1. Near infra-red spectroscopy study of homopolymer cure

Abstract The cure mechanism of a trifunctional epoxy resin with an amine catalyst has been studied using near infra-red spectroscopy. The concentrations of primary and secondary amine and epoxide groups have been monitored directly as a function of cure by the use of this technique. This was compared to thermal analysis of the cure process at a range of cure temperatures by d.s.c. The conversion of the various functional groups and the increase in the glass transition temperature was found to be uniquely determined by the degree of cure and unaffected by cure temperature. The primary amines were found to be largely reacted by gelation, followed by the commencement of the secondary amine reaction. By monitoring hydroxyl production it was possible to determine the degree to which side reactions (reactions other than the main epoxy/amine reaction) occurred. These side reactions, such as etherification, occurred at conversion values above gelation and become prominent at high levels of crosslinking when the reactive functionalities become strongly diffusion controlled and their movement sterically hindered. At these high conversions unreacted secondary amines become trapped in the glassy network and these groups may result in disadvantageous properties of the final material.

Effect of ionic content on ballistic self-healing in EMAA copolymers and ionomers

A specific class of poly(ethylene-co-methacrylic acid) copolymers and ionomers (EMAA) possesses a unique ability to instantly self-heal following ballistic projectile puncture. The present study has comprehensively explored the relationship between ionic content, morphology and the self-healing property response to high energy impact, using a range of EMAA copolymers which vary from non-ionic to highly neutralized. DSC, DMA, FTIR and rheological methods were used to understand the thermal, rheological, chemical and microstructural responses which can occur during temperature variations experienced during high energy impact. Ballistic puncture testing probed the self-healing response over a wide range of temperatures, from −50 °C to 140 °C, allowing the production of a self-healing “phase-style diagram” identifying regions of self-healing success or failure as a function of ionic content and impact temperature. Moderate ionic content proved the most beneficial to healing for tests below the order–disorder transition (Ti = 40 °C); while above and into the melt (Tm = 90 °C) healing improved with increasing ionic content. Finally, a previously established, instrumented non-ballistic puncture method was used to record the tensile forces and displacements during a simulated puncture experiment allowing delineation of elastic vs. elastomeric (flow) properties as a function of ionic content. Overall, the relative mobility and strength of the associative regions whether arising from hydrogen bonding in the case of the non-ionic species, or ionic association in the case of the neutralized ionomers, was demonstrated to control the self-healing performance when subjected to high energy impact.

Self-healing of delamination fatigue cracks in carbon fibre–epoxy laminate using mendable thermoplastic

This article examines the self-healing repair of delamination damage in mendable carbon fibre–epoxy laminates under static or fatigue interlaminar loading. The healing of delamination cracks in laminates containing particles or fibres of the mendable thermoplastic poly[ethylene-co-(methacrylic acid)] (EMAA) was investigated. The results showed that the formation of large-scale bridging zone of EMAA ligaments along the crack upon healing yielded a large increase (~300%) in the static mode I interlaminar fracture toughness, exceeding the requirement of full restoration. The mendable laminates retained high healing efficiency with multiple repair cycles because of the capability of EMAA to reform the bridging zone under static delamination crack growth conditions. Under fatigue loading, healing by the EMAA was found to restore the mode I fatigue crack growth resistance, with the rates of growth being slightly less than that pertinent to the unmodified laminate. The EMAA bridging zone, which generated high toughness under static loading conditions, does not develop under fatigue loading because of rapid fatigue failure of the crack bridging ligaments. Similar to the multiple healing capability of EMAA under static loading, multiple healing of delamination fatigue cracks is confirmed, with the fatigue crack growth rates remaining approximately unchanged. This study shows that EMAA was capable of full recovery of fatigue crack growth resistance and superior healing efficiency for static loading.

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.