Analysis Of Failure Mechanisms In Platelet-Reinforced Composites

The short-term mechanical strength of platelet-reinforced polymer composites was modeled using classical two-dimensional stress-transfer analysis. The stress field in the platelet and at the platelet/matrix interface was described in the presence of a matrix crack perpendicular to the interface. Modeling takes into account the tensile strength of the platelet, its adhesion to the matrix, and also considers the internal stress state resulting from processing. Platelet rupture and interface delamination were considered to be the two key failure mechanisms, depending on the ratio of platelet strength to interface strength. The transition between the two failure events was predicted to occur at a critical platelet length, the value of which depends on the elastic properties of the platelet and matrix, on the platelet geometry and strength, on the platelet/matrix adhesion, and on the internal stress state. The approach was applied to the case of low volume fraction silicon oxide platelets/poly(ethylene terephthalate) composites, where the size of the platelets was accurately controlled either below or above the predicted critical length. Compression molded composites, with perfect alignment of the platelets, and injection molded composites, were prepared and tested. The toughness of the compression molded composites was found to be accurately predicted by the strength model, with a 100% increase in the case of platelets smaller than the critical length compared to larger platelets. Injection molded composites with platelets larger than the critical length were found to fail without yielding. By contrast, when the platelets were smaller than the critical length, the injection molded composites exhibited excellent ductility. The general agreement obtained between the predicted and observed toughening transition shows the importance of filler size and stress state on the strength of platelet-reinforced composite

Radical photoinduced cationic frontal polymerization in porous media

Two different interpenetrating phase composites were produced using a radical photoinduced cationic frontal polymerization process. The composites were based on polyurethane (PU) and aluminium open-cell foams impregnated with a formulation of a cycloaliphatic epoxy with different concentrations of a cationic photoinitiator and a thermal initiator. The influence of both types of initiators on the frontal polymerization features was systematically evaluated for the PU foam. It was found to occur only when the concentration of both initiators was greater than 0.5 wt%, leading to full conversion of the epoxy in the whole volume of the 15 mm thick composite samples within less than 100 s. The maximum temperature reached by the propagation front was in the range 275–305 °C depending on the type of formulation, leading to pores in the epoxy phase and extensive degradation of the PU phase. In the case of the opaque aluminium foam, an additional layer of pure resin was required on the UV-exposed surface, which corresponded to a critical mass of a few grams to ensure sufficient heat generation and trigger the front propagation. © 2020 Society of Chemical Industry

Durability of Nanosized Oxygen-Barrier Coatings on Polymers

Research on silicon oxide thin films developed as gas-barrier protection for polymer-based components is reviewed, with attention paid to the relations between (i) coating defects, cohesive strength and internal stress state, and (ii) interfacial interactions and related adhesion to the substrate. The deposition process of the oxide from a vapor or a plasma phase leads in both cases to the formation of covalent bonds between the two materials, with high adhesion levels. The oxide coating contains nanoscopic defects and microscopic flaws, and their respective effect on the barrier performance and mechanical resistance of the coating is analyzed. Potential improvements are discussed, including the control of internal stresses in the coating during deposition. Controlled levels of compressive internal stresses in the coating are beneficial to both the barrier performance and the mechanical reliability of the coated polymer. An optimal coating thickness, with low oxygen permeation and high cohesive strength, is determined from experimental and theoretical analyses of the failure mechanisms of the coating under mechanical load. These investigations are found relevant to tailor the interactions and stress state in the interfacial region, in order to improve the reliability of the coating/substrate assembly

Biaxial fragmentation of thin silicon oxide coatings on poly(ethylene terephthalate)

Crack patterns of 53 nm and 103 nm thick silicon oxide coatings on poly(ethylene terephthalate) films are analyzed under equibiaxial stress loading, by means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The biaxial stress and strain are modeled from classical elastic membrane equations, and an excellent agreement is obtained with a finite element method. In the large pressure range, the derivation of the biaxial strain from force equilibrium considerations are found to reproduce accurately the measured data up to 25% strain. The examination of the fragmentation process of the coating under increasing pressure levels reveals that the crack onset strain of the oxide coating is similar to that measured under uniaxial tension. The fragmentation of the coating under biaxial tension is also characterized by complex dynamic phenomena which image the peculiarities of the stress field, resulting in considerable broadening of the fragment size distribution. The evolution of the average fragment area as a function of biaxial stress in the early stages of the fragmentation process is analyzed using Weibull statistics to describe the coating strengt

Cohesive strength of nanocrystalline ZnO:Ga thin films deposited at room temperature

In this study, transparent conducting nanocrystalline ZnO:Ga (GZO) films were deposited by dc magnetron sputtering at room temperature on polymers (and glass for comparison). Electrical resistivities of 8.8 × 10-4 and 2.2 × 10-3 Ω cm were obtained for films deposited on glass and polymers, respectively. The crack onset strain (COS) and the cohesive strength of the coatings were investigated by means of tensile testing. The COS is similar for different GZO coatings and occurs for nominal strains approx. 1%. The cohesive strength of coatings, which was evaluated from the initial part of the crack density evolution, was found to be between 1.3 and 1.4 GPa. For these calculations, a Young's modulus of 112 GPa was used, evaluated by nanoindentation

Tailoring the interfacial interactions in ferroelectric fluorinated polymer/ceramic nanocomposites

In this work composites of PVDF-TrFE containing 60 vol% untreated and surface modified BaTiO3 were produced by solvent casting with two procedures. Their morphology and structure were characterized by scanning electron microscope, X-ray diffraction and differential scanning calorimetry. The effect of the processing conditions and of the surface modification of BaTiO3 on the viscoelastic, dielectric and piezoelectric properties was investigated. The surface modification of BaTiO3 allowed obtaining composite films with low porosity and good filler dispersion, and hence higher storage modulus and lower loss tangent, in a wider processing window. Furthermore it reduced the dielectric losses at low frequency and modified the decay kinetic of the d33 piezoelectric coefficient with respect to composites made with untreated particles

Rheological behavior of concentrated hyperbranched polymer/silica nanocomposite suspensions

The rheological behavior of two hyperbranched polymer/silica suspensions with different dispersion states, surface chemistries, and concentrations of the silica nanoparticles was investigated in terms of viscoelastic properties, activation energy for viscous flow, and yield stress. The viscoelastic properties of both types of suspensions were reduced to a master curve that was a function of the limiting viscosity and shear modulus. A liquid-to-solid transition and correlated activation energy change were found to occur for particle volume fraction in the range of 5-10% for well-dispersed systems and 20-25% for systems where silylated particles were agglomerated. The viscosity of the suspensions was found to be considerably higher than that predicted by the classical percolation model for concentrated particle suspensions; this was argued to result from an immobilized layer of polymer on the surface of the silica particles. The percolation model was therefore modified to include such confined layer in order to predict the viscosity as a function of filler fraction. In the case of silylated particles with weak interactions with the polymer, the model based on an immobilized layer of thickness in the range of 2-5 nm reproduced the data. In the case of well-dispersed particles with strong interfacial interactions, the immobilized layer was correlated to the average distance between adjacent particles. In this case the model predicted an exponential increase of the viscosity with particle fraction and that the whole matrix gelled at particle concentrations larger than 5 vol %, corresponding to a 7.5 nm thick immobilized layer. © 2010 American Chemical Society

Low-Stress Hyperbranched Polymer/Silica Nanostructures Produced by UV Curing, Sol/Gel Processing and Nanoimprint Lithography

Nanocomposite materials based on a HBP and silica are produced using either a dual-cure sol/gel and photopolymerization process or by mixing silica nanoparticles with the HBP. In both cases the conversion of the HBP is independent of composition and obeys a time-intensity superposition with power-law dependence on UV intensity. Optimization of the dual-cure process leads to transparent sol/gel composites with ultrafine structures. These materials systematically outperform the particulate composites, including an increase of the glass transition temperature of 63 degrees C and a process-induced internal stress as low as 2.5MPa. Nano-sized gratings are produced from the sol/gel composites by low-pressure UV nanoimprint lithography

Prediction of the adhesive fillet size for skin to honeycomb core bonding in ultra-light sandwich structures

The formation of resin fillet between honeycomb core cell walls and skin in light sandwich structures was studied to gain a better understanding of the bonding process. A method was developed for tailoring the amount of adhesive between 8 and 80 g/m2. The size of the adhesive menisci and the contact angles between the adhesive and the skin and the core materials were measured. A model was developed to predict the size of the menisci, based on the surface energy of skin and honeycomb materials. When adhesive films were used for bonding, up to 50% of the adhesive did not form the menisci whereas 100 % did when the newly-developed adhesive deposition method was used, which allowed better bonding with lower weight