Ivana K. Partridge

Phase separation in epoxy resins containing polyethersulphone

Abstract Scanning electron microscopy and dynamic mechanical spectroscopy have been used to study phase separation of dissolved polyethersulphone (PES) from trifunctional and tetrafunctional epoxy resins during curing. No phase separation was observed at high concentrations of the tetrafunctional resin. Observations of nodules on fracture surfaces, and of multiple peaks in the dynamic mechanical spectra provided evidence for a separate, crosslinked, PES-rich phase in the remaining materials. Despite the variety of morphologies obtained in mixtures of PES with different hardeners and resins, modulus and fracture toughness showed little dependence upon composition.

Mixed-mode fracture in an interleaved carbon-fibre/epoxy composite

Commercial IMS carbon-fibre/modified-thermoset prepregs (Fibredux 927) were interleaved with the same matrix resin in order to study the mechanisms and the extent of toughening achievable by this technique. Unidirectional laminates were tested in mode I, mode II and in mixed mode by means of the modified NASA mixed-mode bending rig. Significant increases in delamination resistance were found in all modes of loading when 50 and 200 μm thick layers of the resin were placed in the central crack path of the laminates. The fracture data were fitted to a simple failure criterion and related to the predicted plastic zone sizes. Fractographic examination documents the gradual change in the micromechanisms of failure as the imposed loading mode changes. The crack path is observed to oscillate between the two ply boundaries, via the resin-rich layer, raising questions as to the micromechanical interpretation of the fracture data.

Cure modeling and monitoring of epoxy/amine resin systems. I. Cure kinetics modeling

The cure kinetics of four epoxy/amine systems including commercial RTM6 and F934 resins have been investigated under both isothermal and dynamic curing conditions. Differential Scanning Calorimetry (DSC) was the thermoanalytical technique used to determine the cure kinetics of these resin systems. The complexity of the cure reactions, illustrated by the results, was attributed to the variety of chemical reactions between the epoxy and the amine groups. Various cure kinetics models were implemented in order to achieve an accurate description and simulation of the cure profiles obtained from the DSC measurements in the chemically controlled region. These varied from simple nth order kinetic models to complicated combinations of nth order and autocatalytic kinetic schemes. The mathematical techniques used to evaluate the parameters of the kinetic models varied from simple linear regression to non-linear regression and peak analysis. The resulting fits were in a good agreement with the experimental results for all the resin systems and for all the experimental conditions used.

Modelling the cure of a commercial epoxy resin for applications in resin transfer moulding

An analytical procedure has been developed for modelling the kinetics of the cure process of a commercial epoxy resin for resin transfer moulding (RTM) applications, using differential scanning calorimetry (DSC) in the isothermal and dynamic modes to obtain the experimental database. The overall reaction rate of the epoxide groups with amines was determined and fitted by an autocatalytic kinetic model. An improvement of the model to allow for diffusion limitation effects results in a good agreement between experimentally determined and predicted reaction rates. A non-linear least squares regression analysis method based on Marquardts algorithm was used to fit the DSC reaction rate data with an appropriate model and to evaluate the activation energies and the reaction orders for this particular resin system. The Di Benedetto equation was utilised to establish the relationship between conversion and glass transition temperature (T g ), required to develop the diffusion-dominated part of the model.

Round-robin interlaminar fracture testing of carbon-fibre-reinforced epoxy and PEEK composites

Abstract This paper summarizes results from a series of tests performed in eighteen laboratories with three specific aims: first, to establish the reproducibility of values from mode I and mode II tests carried out on two materials in different laboratories; secondly to investigate the differences between two data analyses; and finally to examine the influence of specimen thickness on mode I and mode II values. The materials tested, both unidirectional, were a relatively brittle carbon-fibre/epoxy laminate and a thermoplastic composite, carbon fibre/PEEK.

Rubber toughening of plastics: Part 9Effects of rubber particle volume faction on deformation and fracture in HIPS

Electron microscopy of a high-impact polystyrene (HIPS) polymer, containing 8.5 wt% polybutadiene, shows that the volume fraction, ϕ, of composite rubber particles is 35%. The rubber particle size distribution has 8 median diameter of 1.6 µm. By making a series of blends between this HIPS and polystyrene, it is shown that Youngs modulus decreases linearly with ϕ. Dilution with polystyrene results in a sharp drop in notched Charpy impact strength. The relevance of these data to the interpretation of structure-property relationships for a wide range of HIPS morphologies is discussed.

Phase separation in styrenated polyester resin containing a poly(vinyl acetate) low-profile additive

Abstract Scanning electron microscopy has been used to observe morphology in styrenated polyester resins containing poly(vinyl acetate) (PVA). Resins containing 8% PVA form composite spherical particles which occupy 35 vol% of the total material. It is concluded that these particles consist of resin sub-inclusions embedded in the continuous matrix of polyester resin. Increasing the PVA content to 16% results in a phase inversion: PVA forms the matrix, and the resin is present as spherical particles. These observations are interpreted with the aid of a ternary diagram.

Delamination behaviour of Z-pinned laminates

Abstract The paper presents the results of Mode I and Mode II delamination fracture testing carried out on ‘Z-fibre’ reinforced unidirectional beams of IMS/924 laminates. The presence of this new type of through-the-thickness reinforcement is shown to result in dramatic increases in the resistance to crack propagation under both forms of loading, in comparison with the control specimens. In particular, the catastrophic failure associated with the ENF shear loading configuration is suppressed in the pinned samples. The effects of Z-pin diameters and areal densities are investigated. The results cast doubt on the applicability of the standard LEFM treatment of data from this kind of a 3-D composite.

Cure modeling and monitoring of epoxy/amine resin systems. II. Network formation and chemoviscosity modeling

The glass transition temperature (Tg) advancement and the chemoviscosity development under isothermal conditions have been investigated for four epoxy/amine systems, including commercial RTM6 and F934 resins. Differential scanning calorimetry (DSC) was the thermoanalytical technique used to determine the Tg advancement and rheometry the technique for the determination of the chemoviscosity profiles of these resin systems. The complex cure kinetics were correlated to the Tg advancement via an one-to-one relationship using Di Benedettos formula. It was revealed that the three-dimensional network formation follows a single activated mechanism independent of whether the cure kinetics follow a single or several activation mechanisms. The viscosity profiles showed the typical characteristics of epoxy/amine cure. A modified version of the Williams-Landel-Ferry equation (WLF) was adequate to model the viscosity profiles of all the resin systems, in the temperature range 130 to 170°C, with a very good degree of accuracy. The parameters of the WLF equation were found to vary in a systematic manner with cure temperature. Further correlation between Tg and viscosity showed that gelation, defined as the point where viscosity reaches 104 Pas, occurs at a unique Tg value for each resin system, which is independent of the cure conditions.

Monitoring autoclave cure in commercial carbon fibre/epoxy composites

Embedded dielectric sensors and thermocouples have been used to monitor the state of cure, in real time, for a range of current commercial epoxy resin carbon fibre reinforced composites. This paper summarises the methodology and data analyses involved in the monitoring procedure and gives specific examples of the results obtained. The generic rules governing the relationships between dielectric properties and material transformations for epoxy-based resins are stated. The problem of reaction exotherms in thick sections and the resulting uneven temperature and property distributions are quantified for two specific resin types. The reduction in interlaminar shear strength, resulting from the presence of the embedded dielectric sensors, is shown to be 15% for a brittle resin system and 5% for a tough resin system. The results and information obtainable from the dielectric cure monitoring are set against the background of the development of a comprehensive overall model for composite cure.