[show abstract][hide abstract] ABSTRACT: The microstructure and fracture performance of an anhydride-cured epoxy polymer modified with two poly(styrene)-b-1,4-poly(butadiene)-b-poly(methyl methacrylate) (SBM) block copolymers were investigated in bulk form, and when used as the matrix material in carbon fibre reinforced composites. The ‘E21’ SBM block copolymer has a higher butadiene content and molecular weight than the ‘E41’. A network of aggregated spherical micelles was observed for the E21 SBM modified epoxy, which became increasingly interconnected as the SBM content was increased. A steady increase in the fracture energy was measured with increasing E21 content, from 96 to 511 J/m2 for 15 wt% of E21. Well-dispersed ‘raspberry’-like SBM particles, with a sphere-on-sphere morphology of a poly(styrene) core covered with poly(butadiene) particles, in an epoxy matrix were obtained for loadings up to 7.5 wt% of E41 SBM. This changed to a partially phase-inverted structure at higher E41 contents, accompanied by a significant jump in the measured fracture energy to 1032 J/m2 at 15 wt% of E41. The glass transition temperatures remained unchanged with the addition of SBM, indicating a complete phase separation. Electron microscopy and cross polarised transmission optical microscopy revealed localised shear band yielding, debonding and void growth as the main toughening mechanisms. Significant improvements in fracture energy were not observed in the fibre composites, indicating poor toughness transfer from the bulk to the composite. The fibre bridging observed for the unmodified epoxy matrix was reduced due to better fibre–matrix adhesion. The size of the crack tip deformation zone in the composites was restricted by the fibres, hence reducing the measured fracture energy compared to the bulk for the toughest matrix materials.
Journal of Materials Science 08/2013; 48(19). · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: Multi-walled carbon nanotubes, with a typical length of 140μm and a diameter of 120nm, have been used to modify an anhydride-cured
epoxy polymer. The modulus, fracture energy and the fatigue performance of the modified polymers have been investigated. Microscopy
showed that these long nanotubes were agglomerated, and that increasing the nanotube content increased the severity of the
agglomeration. The addition of nanotubes increased the modulus of the epoxy, but the glass transition temperature was unaffected.
The measured fracture energy was also increased, from 133 to 223J/m2 with the addition of 0.5wt% of nanotubes. The addition of the carbon nanotubes also resulted in an increase in the fatigue
performance. The threshold strain-energy release-rate, G
th, increased from 24J/m2 for the unmodified material to 73J/m2 for the epoxy with 0.5wt% of nanotubes. Electron microscopy of the fracture surfaces showed clear evidence of nanotube debonding
and pull-out, plus void growth around the nanotubes, in both the fracture and fatigue tests. The modelling study showed that
the modified Halpin–Tsai equation can fit very well with the measured values of the Young’s modulus, when the orientation
and agglomeration of the nanotubes are considered. The fracture energy of the nanotube-modified epoxies was predicted, by
considering the contributions of the toughening mechanisms of nanotube debonding, nanotube pull-out and plastic void growth
of the epoxy. This indicated that debonding and pull-out contribute to the toughening effect, but the contribution of void
growth is not significant. There was excellent agreement between the predictions and the experimental results.
Journal of Materials Science 05/2012; 46(23):7525-7535. · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: Rubber toughened epoxies are used in a wide range of applications including adhesives when toughness is a crucial property.
It is well known that the cavitation of the rubber particles is an important process to optimise the toughness of such materials.
This article describes the development of a predictive model to describe the dependence of rubber particle cavitation on particle
size. The model is developed using a combination of experimental observations and finite element simulations. Predictions
have been obtained for both uniaxial loading conditions and the triaxial loading conditions expected ahead of a crack. The
model has been extended to consider the cavitation of nano-sized ‘rubber’ particles.
Journal of Materials Science 04/2012; 45(14):3882-3894. · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: The distribution of particles within modern materials must be defined to understand the change in properties attained by their
addition. Two methods of analysis, which use different size scales, are presented here. These methods are applied to characterise
the dispersion of multi-walled carbon nanotubes in a thermoplastic-toughened epoxy polymer. First, the greyscale method uses
transmission optical micrographs, and calculates the ratio of the variance/mean of the greyscale values. Higher values indicate
a greater degree of clustering; lower values may be described as showing a ‘better’ distribution of nanotubes, hence allowing
the results to be ranked. This method is relatively easier to carry out, but care must be taken to use a consistent small
thickness of sample. Secondly, the quadrat analysis uses transmission electron micrographs of the same materials, after identifying
the centre of each nanotube observed. This defines the distribution on the scale of the nanotubes. Peaks in the relationship
between the ratio of the variance/mean and cell size are related to microstructural features such as agglomeration. This scale
is expected to be related to the scale of microstructural deformation mechanisms which determine global material properties.
Journal of Materials Science 04/2012; 46(9):3108-3118. · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: The present paper investigates the effect of adding silica nanoparticles to an anhydride-cured epoxy polymer in bulk and when
used as the matrix of carbon- and glass-fibre reinforced composites. The formation of ‘hybrid’ epoxy polymers, containing
both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber microparticles, is also discussed.
The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. The
fracture energy of the bulk epoxy polymer was increased from 77 to 212J/m2 by the presence of 20wt% of silica nanoparticles. The observed toughening mechanisms that were operative were (a) plastic
shear-yield bands, and (b) debonding of the matrix from the silica nanoparticles, followed by plastic void-growth of the epoxy.
The largest increases in toughness observed were for the ‘hybrid’ materials. Here a maximum fracture energy of 965J/m2 was measured for a ‘hybrid’ epoxy polymer containing 9wt% and 15wt% of the rubber microparticles and silica nanoparticles,
respectively. Most noteworthy was the observation that these increases in the toughness of the bulk polymers were found to
be transferred to the fibre composites. Indeed, the interlaminar fracture energies for the fibre-composite materials were
increased even further by a fibre-bridging toughening mechanism. The present work also extends an existing model to predict
the toughening effect of the nanoparticles in a thermoset polymer. There was excellent agreement between the predictions and
the experimental data for the epoxy containing the silica nanoparticles, and for epoxy polymers containing micrometre-sized
glass particles. The latter, relatively large, glass particles were investigated to establish whether a ‘nano-effect’, with
respect to increasing the toughness of the epoxy bulk polymers, did indeed exist.
Journal of Materials Science 04/2012; 45(5):1193-1210. · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: Measuring quantitatively the nanoparticle dispersion of a composite material requires more than choosing a particular parameter
and determining its correspondence to good and bad dispersion. It additionally requires anticipation of the measure’s behaviour
towards imperfect experimental data, such as that which can be obtained from a limited number of samples. It should be recognised
that different samples from a common parent population can give statistically different responses due to sample variation
alone and a measure of the likelihood of this occurring allows a decision on the dispersion to be made. It is also important
to factor into the analysis the quality of the data in the micrograph with it: (a) being incomplete because some of the particles
present in the micrograph are indistinguishable or go unseen; (b) including additional responses which are false. With the
use of our preferred method, this article investigates the effects on the measured dispersion quality of nanoparticles of
the micrograph’s magnification settings, the role of the fraction of nanoparticles visible and the number of micrographs used.
It is demonstrated that the best choice of magnification, which gives the clearest indication of dispersion type, is dependent
on the type of nanoparticle structure present. Furthermore, it is found that the measured dispersion can be modified by particle
loss, through the limitations of micrograph construction, and material/microscope imperfections such as cut marks and optical
aberrations which could lead to the wrong conclusions being drawn. The article finishes by showing the versatility of the
dispersion measure by characterising various different spatial features.
Journal of Materials Science 04/2012; 46(19):6437-6452. · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: The present work investigates the effect on the morphology, fracture performances, and tensile properties of an epoxy polymer when modified with various percentages of a thermoplastic-polymeric toughener. The thermoplastic toughener was a poly(ether sulfone) copolymer with reactive end-groups. It was initially soluble in the epoxy-resin/hardener mixture but phase separated during the curing of the epoxy resin. After the epoxy had cured, the thermoplastic toughener, when present at relatively low concentrations, possessed a spherical-particulate morphology in an epoxy-rich continuous phase. However, as the weight percentage of the thermoplastic was increased the morphology changed to a co-continuous microstructure, and then to a phase-inverted microstructure of epoxy spherical particles in a thermoplastic-rich continuous phase. The Young's modulus and 0.2% proof stress of the epoxy polymer were relatively unaffected by the addition of the thermoplastic, whilst the ultimate tensile strength increased with increasing thermoplastic content. The fracture toughness and fracture energy of the formulations were found to increase steadily with increasing thermoplastic content. This increase was not, however, linked to the observed changes in morphology, but simply to the weight-percentage of the thermoplastic toughener added to the formulation.
[show abstract][hide abstract] ABSTRACT: An anhydride-cured thermosetting epoxy polymer was modified by incorporating 10 wt.% of well-dispersed silica nanoparticles. The stress-controlled tensile fatigue behaviour at a stress ratio of R = 0.1 was investigated for bulk specimens of the neat and the nanoparticle-modified epoxy. The addition of the silica nanoparticles increased the fatigue life by about three to four times. The neat and the nanoparticle-modified epoxy resins were used to fabricate glass fibre reinforced plastic (GFRP) composite laminates by resin infusion under flexible tooling (RIFT) technique. Tensile fatigue tests were performed on these composites, during which the matrix cracking and stiffness degradation was monitored. The fatigue life of the GFRP composite was increased by about three to four times due to the silica nanoparticles. Suppressed matrix cracking and reduced crack propagation rate in the nanoparticle-modified matrix were observed to contribute towards the enhanced fatigue life of the GFRP composite employing silica nanoparticle-modified epoxy matrix.
[show abstract][hide abstract] ABSTRACT: A thermosetting epoxy polymer was modified by incorporating 9 wt% of a CTBN rubber microparticles. The stress-controlled CA tensile fatigue behavior at stress ratio, R = 0.1 for both the neat and the modified epoxy was investigated. The addition of rubber particles increased the epoxy fatigue life by a factor of about three to four times. The rubber particle cavitation and plastic deformation of the surrounding material was observed to contribute to the enhanced fatigue life of the epoxy polymer. Then, the neat and the rubber-modified epoxy resins were infused into a quasi-isotropic, lay-up E-glass fiber, non-crimp fabric in a RIFT set -up to fabricate GFRP composite panels. Further, the stress-controlled CA tensile fatigue tests at stress ratio, R = 0.1 were performed on both of these GFRP composites. Matrix cracking and stiffness degradation was continuously monitored during the fatigue tests. Similar to bulk epoxy fatigue behavior, the fatigue life of GFRP composites increased by a factor of about three times due to the presence of rubber particles in the epoxy matrix. The suppressed matrix cracking and the reduced crack propagation rates in the rubber-modified matrix contribute towards the enhanced fatigue life of GFRP composites employing a rubber-modified epoxy matrix.
Journal of Reinforced Plastics and Composites 01/2010; 29(14):2170-2183. · 0.90 Impact Factor
[show abstract][hide abstract] ABSTRACT: The introduction of nano-silica particles into an epoxy polymer has increased both the initial toughness, as measured by the
fracture toughness, KIc, and also significantly improved the cyclic-fatigue behaviour of the epoxy polymer. Thus, the significant increases recorded
in the values of the range of applied stress-intensity factor at threshold, ΔKth, from the cyclic-fatigue tests for the nano-silica modified materials are very noteworthy, since these increases are accompanied
by significant improvements being recorded in the initial toughness.
Journal of Materials Science 07/2007; 42(16):7049-7051. · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: An epoxy resin, cured with an anhydride, has been modified by the addition of silica nanoparticles. The particles were introduced via a sol–gel technique which gave a very well-dispersed phase of nanosilica particles which were about 20 nm in diameter. Atomic force and electron microscopies showed that the nanoparticles were well-dispersed throughout the epoxy matrix. The glass transition temperature was unchanged by the addition of the nanoparticles, but both the modulus and toughness were increased. The measured modulus was compared to theoretical models, and good agreement was found. The fracture energy increased from 100 J/m2 for the unmodified epoxy polymer to 460 J/m2 for the epoxy polymer with 13 vol% of nanosilica. The fracture surfaces were inspected using scanning electron and atomic force microscopies, and the results were compared to various toughening mechanisms proposed in the literature. The toughening mechanisms of crack pinning, crack deflection and immobilised polymer were discounted. The microscopy showed evidence of debonding of the nanoparticles and subsequent plastic void growth. A theoretical model of plastic void growth was used to confirm that this mechanism was indeed most likely to be responsible for the increased toughness that was observed due to the presence of the nanoparticles.
[show abstract][hide abstract] ABSTRACT: Hybrid materials have been formed using an epoxy polymeric matrix and a range of inorganic particles, including mica and organically-modified
montmorillonites (‘organoclays’), with various concentrations of the silicate modifier up to about 30wt.% depending upon
the viscosity increase induced by the presence of the silicate. Wide-angle and small-angle X-ray scattering plus transmission
electron microscopy were used to identify the morphologies produced, which included particulate, intercalated and ordered
exfoliated. The modulus of these composites increased with the weight fraction of silicate. The morphology had a small effect
on the measured modulus; the nano-composites with the ordered exfoliated microstructure showing the highest values of the
modulus for a given volume fraction of silicate. The fracture toughness, Kc, and the fracture energy, Gc, initially increased as the weight fraction of the silicate was increased, but then decreased at relatively high concentrations.
The measured moduli and toughnesses were compared to theoretical predictions. The measured moduli values showed very good
agreement with the predicted values, whilst the agreement for values of the measured fracture energy, Gc, with the predicted values, based upon a crack deflection toughening mechanism, were less convincing. Indeed, analysis of
the fracture surfaces using scanning electron microscopy showed that the main toughening effect of the silicate particles
is due to plastic deformation of the epoxy matrix around the particles.
Journal of Materials Science 01/2006; 41(11):3271-3297. · 2.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: Thermoplastic/epoxy blends were formed using an amine-cured epoxy polymer and a semi-crystalline thermoplastic: syndiotactic polystyrene (sPS). Complete phase-separation of the initially soluble sPS from the epoxy occurred via ‘reaction-induced phase-separation’ (RIPS) or via ‘crystallisation-induced phase-separation’ (CIPS), depending upon the thermal processing history employed. Dynamic mechanical thermal analysis showed that no sPS was retained dissolved in the epoxy polymer. For RIPS, at concentrations of sPS of up to 8 wt%, the sPS is present solely as spherical particles. However, macro phase-separation, giving a co-continuous microstructure, accompanied by local phase-inversion, dominates the RIPS blends containing more than 8 wt% sPS. In the CIPS blends, the sPS is present as spherulitic particles, and this microstructure does not change over the range of sPS concentrations employed, i.e. from 1 to 12 wt% sPS. The pure epoxy polymer was very brittle with a value of fracture energy, GIc, of about 175 J/m2. However, the addition of the sPS significantly increases the value of GIc, though the toughness of the RIPS and CIPS blends differs markedly. For the RIPS blends, there is a steady increase in the toughness with increasing content of sPS and an apparent maximum value of GIc of about 810 J/m2 is obtained for 8–10 wt% sPS. On the other hand, the measured toughness of the CIPS blends increases relatively slowly with the concentration of sPS, and a maximum plateau value of only about 350 J/m2 was measured in the range of 8–12 wt% sPS. The relationships between the microstructure of the RIPS and CIPS sPS/epoxy blends and the measured fracture energies are discussed. Further, from scanning electron microscopy studies of the fracture surfaces and optical microscopy of the damage zone around the crack tip, the nature of the micromechanisms responsible for the increases in toughness of the blends are identified. For the RIPS blends, (i) debonding of the sPS particles, followed by (ii) plastic void growth of the epoxy matrix are the major toughening micromechanisms. The increase in toughness due to such micromechanisms is successfully predicted theoretically using an analytical model. In the case of the CIPS blends, the increase in the value of GIc results from (i) crack deflection and (ii) microcracking and crack bifurcation.