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Users of the microbond test assume that a microbond resin droplet's properties are equivalent to a macroscale specimen. However, there is currently no standardised methodology for determining the cure state of droplet specimens used in the microbond test. In this paper, we present a technique for microbond test users to better understand the properties of thermoset droplet specimens. Utilising a conventional benchtop spectrometer, a novel sample preparation technique involving curing epoxy droplets on thin-steel filaments allowed for high-throughput determination of the microbond droplet cure state. The parity between steel filament and glass fibre microbond samples was confirmed by infrared microspectroscopy. It is shown that cure schedules used in manufacturing composite parts produced microbond droplets with degrees of cure lower than that of bulk matrix specimens subjected to an identical thermal history. Testable microbond droplets could only be prepared for commercial resin systems when introducing a room temperature pre-curing time of at least 2 h. It is concluded that microbond testing should be supported by some droplet cure state characterisation methods to ensure that interfacial effects are not artefacts of droplet sample preparation. [ABSTRACT FROM AUTHOR]

The results of a study of environmental ageing on the apparent interfacial shear strength of glass fibre–vinyl ester microbond samples are presented. The scale related issues of estimating water absorption levels in microbond samples are reviewed and discussed and a solution based on Fickian diffusion is given. Interphase strength of microdroplets aged in water at 23°C was relatively stable up to 6 days exposure after which strength degradation was fairly rapid. Immersion in water at 50°C resulted in an immediate 20–30% irreversible loss of strength. Further exposure beyond 2 days led to total loss of integrity of the microbond samples. Electron microscope images of aged microdroplets appeared to show the development of a bi-phasic microstructure. The presence of two phases in small scale vinyl ester polymer samples was confirmed by DMA on thin films. [ABSTRACT FROM AUTHOR]

The microbond test was used to investigate the interface strength (IFSS) in various glass fibre-vinylester (VE) composite systems and a number of issues related to the sample preparation were identified. Cure schedules that produce well-reacted VE polymers on the macroscale did not result in cured microdroplets. Hence, the microbond test could not be carried out on samples with the same cure history as macroscale composites. Testable microdroplet samples could only be obtained when resin cure was carried out under an inert atmosphere. Higher IFSS values were obtained by raising the final temperature of the cure schedule. Glass fibres with a full sizing gave significantly higher apparent IFSS values comparedf to bare fibres or fibre coated with only silane coupling agent. It was discovered that the measured IFSS of VE compatible glass fibres was approximately doubled when fibres were mounted using epoxy glue instead of cyanoacrylate glue. This phenomenon appears to be related to the deposition of cyanoacrylate vapours onto the surface of the fibres during sample preparation. It is concluded that great care must be taken in ensuring that effects observed using the microbond test are evidence of real material characteristics and not artefacts of sample preparation. [ABSTRACT FROM AUTHOR]

It is often suggested that optimization of the fibre-matrix interfacial adhesion is essential to improve the mechanical performance of natural fibre reinforced thermoplastic compounds. The use of such composites in many applications often requires characterisation of performance over a range of temperatures. This paper presents a study on the measurements of the interfacial adhesion of coir fibre with various polyolefin matrices at room temperature. We then present a novel adaptation of a dynamical mechanical analyser to enable the measurement of interfacial shear strength over the temperature range of −40°C to 120°C. Results from these measurements are presented and discussed in terms of the relationship of apparent IFSS to residual thermal stresses in such thermoplastic composites. [ABSTRACT FROM AUTHOR]

The work presented focuses on the need to better understand the effect environmental conditions, namely humidity, have on the interfacial shear strength (IFSS) of composites. The IFSS was first characterised using the microbond test method in a laboratory atmosphere. A specialised rig was then designed and manufactured in order to carry out the microbond test in a dynamic mechanical analyser with a humidity chamber accessory. Due to the well-known moisture absorbing nature of polyamide matrices, the effect of a range of conditions from 0 to 90% relative humidity was investigated. Results presented for both methods are based on the use of a Polyamide 6 matrix. Steel fibres are used extensively for method development and comparison throughout this work. Finally, successful testing on the primary system of fully sized HiPer-Tex™ glass fibre polyamide 6 effectively shows that humidity influences the measured apparent IFSS. [ABSTRACT FROM AUTHOR]

Organosilanes are often used in commercial sizings for glass fibres to provide wettability with the resin and promote strong interfacial adhesion to the matrix in a fibre reinforced polymer composite. The silane treatment is introduced as part of a complex deposition from an aqueous emulsion immediately at the spinaret and determines the optimum properties of the cured composite. To understand the interaction of organosilanes contained in sizings for glass surfaces, XPS was used to investigate the adsorption of γ-aminopropyltriethoxysilane (APS) from a simple sizing system containing a polyurethane (PU) film former. It has been found that both APS and the sizing (containing APS and PU) deposits on E-glass fibre surfaces contained components of differing hydrolytic stability. The differences observed in the AFM images of APS coated E-glass fibres before and after water extraction also confirmed that the APS deposit contained components with different water solubility. [ABSTRACT FROM AUTHOR]

A number of analytical techniques were applied to investigate changes to the surface of unsized boron-free E-glass fibres after thermal conditioning at temperatures up to 700 °C. Novel systematic studies were carried out to investigate the fundamental strength loss from thermal conditioning. Surface chemical changes studied using X-ray photoelectron spectroscopy (XPS) showed a consistent increase in the surface concentration of calcium with increasing conditioning temperature, although this did not correlate well with a loss of fibre strength. Scanning electron microscopy fractography confirmed the difficulty of analysing failure-inducing flaws on individual fibre fracture surfaces. Analysis by atomic force microscopy (AFM) did not reveal any likely surface cracks or flaws of significant dimensions to cause failure: the observation of cracks before fibre fracture may not be possible when using this technique. Fibre surface roughness increased over the whole range of the conditioning temperatures investigated. Although surface roughness did not correlate precisely with fibre strength, there was a clear inverse relationship at temperatures exceeding 400 °C. The interpretation of the surface topography that formed between 400–700 °C produced evidence that the initial stage of phase separation by spinodal decomposition may have occurred at the fibre surface. [ABSTRACT FROM AUTHOR]