Epoxy-silica Nanocomposites Research Articles

Bicontinuous silica-epoxy nanocomposites by aerogel infusion

Interpenetrated, bicontinuous nanocomposites are formed by fully infusing monolithic mesoporous silica (silica aerogel) with epoxy resin. The long-range connectivity of the silica network facilitates direct load transfer and enforces sample homogeneity. The silica networks are prepared using sol–gel chemistry, informed by new phase diagrams, adapted to maximise reinforcement content in the subsequent bicontinuous composite. The infusibility of the aerogels is correlated to pore characteristics determined by gas sorption, as a function of silica aerogel density. Silica reinforcement loadings of up to 22silica vol.% are fully consolidated, with only a modest reduction in glass transition temperature and no change in cure conditions. The reinforcement improves both hardness (+23 %) and reduced modulus (+17 %) of the baseline resin. These properties increase with aerogel content viaa power law relationship which demonstrates the direct role of the connected silica phase as a reinforcing network and motivates future studies to extend the applicable range.

Structural Properties of Epoxy–Silica Barrier Coatings for Corrosion Protection of Reinforcing Steel

Reinforcement steel extensively applied in civil construction is susceptible to corrosion due to the carbonation process in reinforced concrete and chloride ions diffusion. Epoxy-silica-based coatings are a promising option to guarantee the long-term stability of reinforced concrete structures. In this study, the influence of the proportion between the poly (bisphenol-A-co-epichlorhydrin) resin (DGEBA) and the curing agent diethylenetriamine (DETA) on the structural, morphological, and barrier properties of epoxy–silica nanocomposites were evaluated. To simulate different stages of concrete aging, electrochemical impedance spectroscopy (EIS) assays were performed for coated samples in a 3.5 wt.% NaCl solution (pH 7) and in simulated concrete pore solutions (SCPS), which represent the hydration environment in fresh concrete (SCPS2, pH 14) and after carbonation (SCPS1, pH 8). The results showed that coatings with an intermediate DETA to DGEBA ratio of 0.4, presented the best long-term corrosion protection with a low-frequency impedance modulus of up to 3.8 GΩ cm2 in NaCl and SCPS1 solutions. Small-angle X-ray scattering and atomic force microscopy analysis revealed that the best performance observed for the intermediate DETA proportion is associated with the presence of larger silica nanodomains, which act as a filler in the cross-linked epoxy matrix, thus favoring the formation of an efficient diffusion barrier.

Impact of shear mixing time of epoxy-silica nanocomposites on its electrical and mechanical properties

The shear mixing time was varied to study its influence on electrical and mechanical behaviour of epoxy-silica nanocomposites. Epoxy resin along with 1 wt.% of silica nanofillers were subjected to shear mixing at different time spans. Water droplet-initiated corona inception voltage (CIV) was observed to increase with the shear mixing time, under AC and DC voltages. A marginal increase in contact angle a function of shear mixing time was noticed. The CIV due to water droplet and the contact angle has shown a direct correlation. The epoxy-silica nanocomposite sample fabricated with 7 h of shear mixing time has led to 46.68% increment in CIV under AC voltage and 24.81% increment in contact angle compared to base epoxy resin. The ultimate tensile strength (UTS) and the Young’s modulus of nanocomposites have increased with shear mixing time and at 7 h of shear mixing time has shown increase in ultimate tensile strength and Young’s modulus of 45.78% and 43.77%, respectively, as compared to base epoxy resin. The strain versus time plot obtained from the digital image correlation (DIC) studies has shown that maximum strain occurs at a time period before breakdown has increased with shear mixing time up to 7 h. The time at which the maximum strain occurs has increased to 91.49% before the breakdown of epoxy-silica nanocomposite sample which was prepared with 7 h of shear mixing time as compared to base epoxy resin.

Improving flame retardancy of in-situ silica-epoxy nanocomposites cured with aliphatic hardener: Combined effect of DOPO-based flame-retardant and melamine

Silica-epoxy nanocomposites were prepared via an “in-situ” sol-gel synthesis process and a phosphorus (P) flame-retardant i.e. 6H-dibenz[c,e][1,2]oxaphosphorin,6-[(1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-yl)methoxy]-, 6-oxide (DP) and melamine (Mel) were further added to the matrix to improve its fire performance. The main components of epoxy resin were bisphenol A diglycidyl ether (DGEBA) and isophorone diamine (IPDA) hardener. The addition of DP as well as silica alone into the epoxy system stopped the melt dripping phenomena in the vertical fire test (UL 94), however, the addition of melamine was crucial for achieving the highest fire classification (UL 94-V0 rating). The presence of DP and Mel in the silica-epoxy nanocomposite promoted a large reduction (ranging from 53% up to 80%) in the heat release rate (HRR) and a delay (up to 31%) in the ignition time in the cone calorimetry experiments. Improved fire performance of the epoxy system was attributed to i) a condensed phase activity of silica, DP and melamine to form a protective thermal barrier during combustion and ii) a minor gas phase flame inhibition activity of DOPO component of DP. The mechanical characterization of the epoxy nanocomposites through tensile tests showed that the addition of DP increases the stiffness of the epoxy resin, resulting in a strong increase of Young modulus (up to 32%) and in a slight decrease of fracture strength, elongation at break and toughness. An increased glass transition temperature (up to 8%) of the epoxy system possibly due to hydrogen bonds and polar interactions of DP with the matrix was also observed.

Effect of the Silicon-Containing Phase on the Hierarchy of the Structure of Epoxy–Silica Nanocomposites

Structural features of epoxy–silica nanocomposites obtained by cationic polymerization were studied by small-angle X-ray scattering. An increase in the amount of the inorganic phase in this system leads to greater aggregation of the silica particles. The structure of such systems is described satisfactorily be a model with multilevel fractal aggregation. Fractal analysis was used to identify the type of fractal aggregates of each structural level and to determine their dimensions. Aggregates with mean radius 10-13 nm are formed at the first structural level, while silica particles form aggregates with mean radius 55-67 nm at the second level. The mean radius of aggregates at the third level exceeds 250 nm.

Ladder-Structured Polysilsesquioxane/Al2O3 Nanocomposites for Transparent Wear-Resistant Windows

As a protective layer for deformable displays, we synthesized ladder-type polysilsesquioxanes (LPSQs) containing cyclic epoxy as a curable unit. The mechanical properties after photo- and thermal-curing of LPSQs with a small amount of added Al2O3 nanoparticles were compared with those of the pure LPSQs. The prepared LPSQ-Al2O3 nanocomposites and the pure LPSQs exhibited comparable optical transparencies and thermal stabilities. In addition, the degree of conversion of the applied epoxy units in LPSQs and the resulting mechanical properties, as monitored by Fourier transform infrared spectroscopy and nanoindentation tests, indicated that the addition of nanoparticles to LPSQs moderately enhanced the epoxy conversion rate and remarkably improved the wear resistance, including hardness, after photo-/thermal-curing processes. The LPSQ-Al2O3 nanocomposites achieved higher wear resistance than epoxy-silica nanocomposites containing similar curable functional groups and reinforcing fillers (silica). The excellent mechanical properties of the LPSQ-Al2O3 nanocomposites could be attributed to three-dimensionally interconnected networks of organic-inorganic hybrid-type chemical structures in the LPSQ as well as additional reinforcement from amine-functionalized Al2O3 nanoparticles covalently interconnected with the LPSQ. We believe that the devised LPSQ-Al2O3 nanocomposites could serve effectively as a wear-resistant platform for deformable display windows.

A micromechanical study on the existence of the interphase layer in particle-reinforced nanocomposites

In this paper, the effective elastic properties of nanoparticles reinforced nanocomposites including effects of an interphase layer around the nanoparticles are determined using analytical micromechanical approach. The present micromechanical model is based on the Green’s function and the interface operator. By using the interface operator, the interphase layer and the nanoparticle are combined as a single spherical inclusion and then the effective properties of the composite are calculated by using the Green’s function. To assess the presented method, epoxy-silica nanocomposites are thoroughly studied in this paper. Since there is no direct method to determine physical and mechanical properties of the interphase layer, we have presented a method to determine these properties, namely volume fraction and elastic modulus of the interphase layer, for epoxy-silica nanocomposites by correlating the experimental data for effective elastic properties of the epoxy-silica nanocomposites and those obtained by the presented model. Applying the presented method to various experimental data available in the literature, epoxy-silica nanocomposites are characterized into nanocomposites with, and without an interphase layer. The presence or absence of the interphase layer is concluded by the value of its volume fraction determined by the present method. It is observed that nanocomposites with surface-modified silica nanoparticles yield perfect bonding with the matrix and do not form an interphase layer. On the other hand, nanocomposites with raw silica nanoparticles (without surface modification) yield an interphase layer between the nanoparticles and the matrix which is softer than the matrix.

Computational Thermomechanical Properties of Silica⁻Epoxy Nanocomposites by Molecular Dynamic Simulation.

Silica–epoxy nanocomposite models were established to investigate the influence of silane coupling agent on the structure and thermomechanical properties of the nanocomposites through molecular dynamics simulation. Results revealed that incorporating silica nanoparticles into a polymer matrix could improve thermomechanical properties of the composites and increase their glass transition temperature and thermal conductivity. Their thermomechanical properties were further enhanced through silane coupling agent modification on the surface of fillers. Compared with that of pure epoxy, the glass transition temperatures of the silica–epoxy composites with grafting ratios of 5% and 10% increased by 17 and 28 K, respectively. The thermal conductivities of the two models at room temperature respectively increased by 60.0% and 67.1%. At higher temperature 450 K, thermal conductivity of the nanocomposite model with a high grafting ratio of 10% demonstrated a considerable increase of approximately 50% over the pure epoxy resin (EP) model. The elastic and shear modulus of the nanocomposite models decreased at temperatures below their glass transition temperatures. These observations were further addressed in the interpretation from three aspects: segmental mobility capability, radial distribution function, and free volume fraction. Our computational results are largely consistent with existing experimental data, and our simulation model got fully validated.

Determination of the Mechanical Properties of Epoxy Silica Nanocomposites through FEA-Supported Evaluation of Ball Indentation Test Results

In the present paper a continuous Finite Element Analysis (FEA) simulation method of the ball indentation hardness test is introduced in order to describe the deformation behavior of nanosilica composites and with this to extract precisely the material's stress-strain behavior. The developed procedure demonstrate in particular the adequacy of this method to determine the nanocomposites' elastic modulus which is compared with Halpin-Tsai and Lewis-Nielsen models as well as with experimental measurements taken from uniaxial tensile tests. The fracture area of all the tensile specimens was examined using a scanning electron microscope (SEM). It is shown that the correlation between the experimental results, the semi-empirical models and the FEA computational models concerning the elastic modulus values was satisfactory with very small deviations.

Investigation of dielectric breakdown in silica-epoxy nanocomposites using designed interfaces

Adding nano-sized fillers to epoxy has proven to be an effective method for improving dielectric breakdown strength (DBS). Evidence suggests that dispersion state, as well as chemistry at the filler-matrix interface can play a crucial role in property enhancement. Herein we investigate the contribution of both filler dispersion and surface chemistry on the AC dielectric breakdown strength of silica-epoxy nanocomposites. Ligand engineering was used to synthesize bimodal ligands onto 15nm silica nanoparticles consisting of long epoxy compatible, poly(glycidyl methacrylate) (PGMA) chains, and short, π-conjugated, electroactive surface ligands. Surface initiated RAFT polymerization was used to synthesize multiple graft densities of PGMA chains, ultimately controlling the dispersion of the filler. Thiophene, anthracene, and terthiophene were employed as π-conjugated surface ligands that act as electron traps to mitigate avalanche breakdown. Investigation of the synthesized multifunctional nanoparticles was effective in defining the maximum particle spacing or free space length (Lf) that still leads to property enhancement, as well as giving insight into the effects of varying the electronic nature of the molecules at the interface on breakdown strength. Optimization of the investigated variables was shown to increase the AC dielectric breakdown strength of epoxy composites as much as 34% with only 2wt% silica loading.

Dielectric properties of nanosilica filled epoxy nanocomposites

This paper presents the development of epoxy-silica nanocomposites and characterized for dielectric properties. The effect of nanosilica loading (0–20 wt%), frequency, temperature and sea water aging on these properties was studied. Transmission electron microscopy (TEM) analysis of the samples showed an excellent dispersion. However, at higher silica loading TEM showed inter-contactity of the particles. The dielectric constant (ε′) increased with silica loading and reached an optimum at about 10 wt%. The ε′ of the nanocomposites showed linear decrease with frequency whereas AC conductivity (σ ac) increases. The σ ac and ε′ increased marginally with temperature and sea water aging.

Correction: Tailored high performance shape memory epoxy–silica nanocomposites. Structure design

Correction for ‘Tailored high performance shape memory epoxy–silica nanocomposites. Structure design’ by S. Ponyrko, et al., Polym. Chem., 2016, 7, 560–572.

Tailored high performance shape memory epoxy–silica nanocomposites. Structure design

The shape memory (SM) properties of polymers depend on material toughness and the efficiency of SM performance is governed by morphology and viscoelasticity.

Nanoindentation, compression and fractural characterization of highly dispersed epoxy silica nanocomposites

Uniaxial compression tests were conducted in this study in order to investigate the compressive properties of the unmodified and nanomodified epoxy with silica nanoparticles. An instrumented nanoindentation technique was also used to investigate its applicability for measuring mechanical properties for such nanocomposite systems. Additionally, the fracture toughness of the unmodified and nanomodified epoxy was measured with single-edge-notch bending tests. The Young’s modulus was found to significantly improve with addition of silica nanoparticles and increase with increasing filler content when measured with both compression and nanoindentation tests. The modulus obtained from the nanoindentation testing was 4–8% higher than the one obtained from the compression tests. As expected, the addition of silica nanoparticles had a significant impact on the fracture toughness and fracture energy and increased with increasing filler content, while observation of the fracture surfaces using SEM suggested that the nanoparticles affected the fracture behavior of such epoxy systems. Implementation of an analytical model in the current nanomodified epoxy network as compared with the experimental fracture energy results showed that the 15% matrix void growth, from debonded nanoparticles, as well as the matrix shear banding are the governing mechanisms of energy absorption.

Fluorescence probes the early formation of network at the interface of epoxy–silica nanocomposite during curing

The effect of surface modified silica on the cure behavior at the interface of epoxy–silica nanocomposites has been analyzed monitoring the fluorescence of the dansyl probe located at different distances from the silica surface by means of molecular tethers. FTIR analysis revealed the catalytic role of the surface hydroxyls, which is modulated by the surface modification. Fluorescence results show that network formation and associated phenomena as gelation and vitrification occurs first at the interface. The exchange of branched species throughout the permeable interface spreads its influence to the whole system until vitrification is reached.

Anti-corrosion hybrid coatings based on epoxy–silica nano-composites: Toward relationship between the morphology and EIS data

This work reports on design and manufacture of organic–inorganic hybrid coatings based on diglycidyl ether of bisphenol A (DGEBA) epoxy resin pursuing hydrolyzation of tetraethoxysilane (TEOS) through a sol–gel process. The resulting hybrid materials were cured to be used as potential anticorrosive coatings. The assigned materials were modified molecules made of DGEBA and 3-aminopropyl triethoxylsilane (APTES), in which the molar ratio of epoxide group of DGEBA to NH of APTES varied in the order of 2:1, 4:1, 8:1 and 16:1. In the next stage, the APTES-modified DGEBA precursors were added to different amounts of pre-hydrolyzed TEOS, i.e. 7.5, 12.5 and 17.5 wt%, as inorganic part of the resulting hybrid. The mixtures were subsequently cured at room temperature by a cycloaliphatic amine based curing agent to yield transparent epoxy–silica hybrid coatings. Microstructure assessment of the hybrid materials, before and after curing, was performed using FTIR and 29Si NMR spectroscopies. The morphology of the epoxy–silica hybrid coatings has also been studied by scanning electron microscopy (SEM). The anti-corrosive measurements on the resultant coatings were conducted based on electrochemical impedance spectroscopy (EIS). The mechanical properties evaluation such as micro-hardness measurements and pull-off adhesion tests of the cured samples were also carried out. The thermal properties of the cured hybrid coatings were evaluated using thermogravimetric analysis (TGA). The results showed that the concentration of APTES and pre-hydrolyzed TEOS play an important role in determining the morphology as well as the mechanical and thermal properties of coatings. The EIS results corresponding to these effects reaffirmed that the corrosion resistance of the hybrid coatings improved with increasing the inorganic phase content.

The Effect of Nanoparticles Percentage on Mechanical Behavior of Silica-Epoxy Nanocomposites

Silica-epoxy nanocomposites are very common among nanocomposites, which makes them very important. Several researchers have studied the effect of nanoparticle’s size, shape, and loading on mechanical behavior of silica-epoxy nanocomposites. This paper reviews the most important research done on the effect of nanoparticle loading on mechanical properties of silica-epoxy nanocomposites. While the main focus is the tensile behavior of nanocomposite, the compressive behavior and flexural behavior were also reviewed. Finally, some of the published experimental data were combined in the graphs, using dimensionless parameters. Later, the best fitted curves were used to derive some empirical formulas for mechanical properties of silica-epoxy nanocomposites as functions of weight or volume fraction of nanoparticles.

Nanoindentation measurements of fumed silica epoxy reinforced nanocomposites

Nanoindentation testing is used in this paper in order to investigate the mechanical properties of specimens with varying weight percentage of fumed silica using a low-viscosity liquid epoxy resin as a matrix. Controversial results are reported in the literature regarding the elastic modulus of fumed silica epoxy nanocomposites using conventional tensile tests, thereby there is much scope in investigating comparatively such systems in a nanometer scale. The results show that the hardness and modulus of the nanocomposites steadily decrease with increasing fumed silica content which was attributed to the severe tendency for cluster formation as studied by optical microscope and scanning electron microscope analysis. The variations in elastic modulus as measured using the nanoindentation testing technique revealed small differences from the tensile test data. A material-depending calibration procedure has been used by utilising the epoxy elastic modulus as measured by the tensile tests in order to account for the indentation inherent uncertainties in the tip shape calibration. The results demonstrate that the nanoindentation testing technique is an effective mechanical testing method for epoxy nanocomposites when an appropriate calibration procedure is followed.

Influence of Nanoparticles on the Coupling Between Optical Dipoles in Epoxy-Silica Nanocomposites During Network Formation

High-performance refractometry and infrared spectroscopy are combined in order to elucidate the gelation process and the glass transition during the network formation of epoxies and epoxy-based nanocomposites. Whereas infrared spectroscopy yields the chemical conversion due to the opening of oxirane rings during the covalent network formation, high-performance refractometry is extremely sensitive to the accompanying changes of the arrangement of the molecular network. In accordance with the Lorentz-Lorenz relationship, the evolution of the refractive index seems to reflect that of the mass density during polymerization of the epoxy-based systems within the limits of a few percent. The slight deviations from the Lorentz-Lorenz relationship, which occur during the gelation of the epoxy-based systems, are attributed to long-ranged dipole-dipole interactions, which respond at optical frequencies. This point of view is supported by the fact that chemically inert silica nanoparticles embedded in the pure epoxy matrix as disturbances for these dipole-dipole interactions are able to diminish or even to suppress totally this excess contribution of the refractive index.

Erosion yield of epoxy–silica nanocomposites at the lower earth orbit environment of the International Space Station

The erosive effect of ultraviolet radiation and atomic oxygen at the lower earth orbit space environment of the International Space Station on Epon 862 based epoxy composites with 12 and 100 nm silica particles was investigated. Although exposure to ultraviolet radiation had a small effect on surface erosion, restricted to 2 µm of the top surface, concurrent exposure to ultraviolet radiation and atomic oxygen resulted in significant erosion. Atomic oxygen erosion of nanocomposites with 1–5 wt% silica particles resulted in a carpet-like residual surface layer whose thickness and morphology were dependent on the size and concentration of the embedded silica particles. The eroded surface of the control epoxy had high surface roughness in the form of 10–40 µm long conical protrusions. With the addition of silica particles, the residual surface layer became fibrous and rich in silica particles, and its density increased with the weight fraction and size of the silica particles. The large and uneven erosion depth of samples exposed to atomic oxygen and ultraviolet radiation resulted in a surface damage layer with average thickness between 5 and 100 µm with significantly reduced mechanical properties compared to the surface of the as-fabricated nanocomposites. The erosion yield of the control epoxy due to atomic oxygen was 4.36 × 10−24 cm3/atom and the addition of silica nanoparticles reduced it significantly to 1.78 × 10−25 cm3/atom. In particular, silica nanoparticles of diameter 12 and 100 nm and weight fraction 5% reduced the erosion yield of the control epoxy by 90% and 96%, respectively.