Thomas P. Schuman

Dielectric Properties of Polymer-particle Nanocomposites Influenced by Electronic Nature of Filler Surfaces

The interface between the polymer and the particle has a critical role in altering the properties of a composite dielectric. Polymer-ceramic nanocomposites are promising dielectric materials for many electronic and power devices, combining the high dielectric constant of ceramic particles with the high dielectric breakdown strength of a polymer. Self-assembled monolayers of electron rich or electron poor organophosphate coupling groups were applied to affect the filler-polymer interface and investigate the role of this interface on composite behavior. The interface has potential to influence dielectric properties, in particular the leakage and breakdown resistance. The composite films synthesized from the modified filler particles dispersed into an epoxy polymer matrix were analyzed by dielectric spectroscopy, breakdown strength, and leakage current measurements. The data indicate that significant reduction in leakage currents and dielectric losses and improvement in dielectric breakdown strengths resulted when electropositive phenyl, electron-withdrawing functional groups were located at the polymer-particle interface. At a 30 vol % particle concentration, dielectric composite films yielded a maximum energy density of ~8 J·cm(-3) for TiO2-epoxy nanocomposites and ~9.5 J·cm(-3) for BaTiO3-epoxy nanocomposites.

Fabrication of bio-based epoxy–clay nanocomposites

Epoxy–clay nanocomposites derived from renewable soybean oils and organo modified montmorillonite clay were prepared. Improved efficiency and performance were achieved through application of new, low viscosity, glycidyl esters of epoxidized fatty acids (EGS) as the epoxy monomer and 4-methyl-1,2-cyclohexanedicarboxylic anhydride as the comonomer. Tensile testing showed that 1 wt% of clay improved nanocomposite strength and modulus by 22% and 13%, respectively, compared to neat polymer. Tensile modulus could be increased up to 34% by nanocomposite clay without any sacrifice of strength. Three types of dispersion techniques, mechanical stirring, high speed shearing, and ultrasonication, were carried out to disperse the clay directly into the epoxy or anhydride portion of the thermoset system without the need of additional solvent. Dispersion of the clay particles into monomer was assessed by means of solubility parameters and optical and scanning electron microscopies, and quality of dispersion further confirmed by small angle X-ray scattering and transmission electron microscopy. Sonication dispersion of clay into the epoxy portion was needed to optimize the dispersion and exfoliation of clay and the higher mechanical and thermal strength of the nanocomposites. The nanocomposites’ morphologies were a mix of intercalated and exfoliated structures, dependent on the dispersion technique. The optimum tensile strength and glass transition temperatures of the nanocomposites were a function of clay concentration and dispersion morphology.

Improved Dielectric Breakdown Strength of Covalently-Bonded Interface Polymer–Particle Nanocomposites

Interfacial covalent bonding is an effective approach to increase the electrical resistance of a polymer–particle composite to charge flow and dielectric breakdown. A bifunctional tether reagent bonded to an inorganic oxide particle surface assists with particle dispersion within a thermosetting epoxy polymer matrix but then also reacts covalently with the polymer matrix. Bonding the particle surface to the polymer matrix resulted in a composite that maintained the pure polymer glass transition temperature, compared to modified or unmodified particle dispersions that lacked covalent bonding to the polymer matrix, which depressed the polymer glass transition to lower temperatures. The added interfacial control, directly bonding the particle to the polymer matrix, appears to prevent conductive percolation across particle surfaces that results in a reduced Maxwell–Wagner relaxation of the polymer–particle composite and a reduced sensitivity to a dielectric breakdown event. The inclusion of 5 vol% particles of higher permittivity produces a composite of enhanced dielectric constant and, with surface modification to permit surface cross-linking into the polymer, a polymer–particle composite with a Weibull E 0 dielectric breakdown strength of 25% greater than that of the pure polymer resulted. The estimated energy density for the cross-linked interface composite was improved 260% compared to the polymer alone, 560% better than a polymer–particle composite synthesized using bare particles, and 80% better than a polymer–particle composite utilizing bare particles with a dispersant.

Synthesis and Characterization of Soy-based Nanocomposites

Soy-based nanocomposites were synthesized using soy resin and montmorillonite clay. Two types of dispersion techniques, pneumatic and sonication, were carried out to disperse the nanoclay into the soy-based epoxy resin. Tensile testing of the nanocomposites showed that the nanoclay improved the modulus and the strength by 625% and 340%, respectively. Exfoliation of the nanoclay was investigated by X-ray diffraction. The curing mechanism of soy epoxy resin with different amounts of montmorillonite was studied using differential scanning calorimetry and Fourier transform infrared spectroscopy. Rheological measurements were also conducted to find the suitability of using the soy-based epoxy clay nanocomposites toward composite manufacturing applications. The dielectric constant and the loss factor were also investigated. These soy-based nanocomposites hold great promise as environmentally friendly and low cost materials for structural applications.

Protective coatings for aluminum alloys

Abstract Aluminum is an important material due to its lower density, potentially high strength to weight ratio in mechanical strength, and cyclic flexural performance, depending on alloying elements and microstructure. While alloying yields improved mechanical performance, it can also result in sensitivity to corrosion. The chapter reviews the driving forces for corrosion and surface pretreatments, conversions, and surface coatings for prevention of aluminum or aluminum alloy corrosion. The article discusses how the alloy composition influences corrosion sensitivity and whether and how this sensitivity can be remediated. A few industrial examples of corrosion prevention, corrosion problems, and analysis methods are presented.

Characterization of polyurethane composites manufactured using vacuum assisted resin transfer molding

Glass fiber-reinforced polymer composites have promising applications in infrastructure, marine, and automotive industries due to their low cost, high specific stiffness/strength, durability, and corrosion resistance. Polyurethane (PU) resin system is widely used as matrix material in glass fiber-reinforced composites due to their superior mechanical behavior and higher impact strength. Glass fiber-reinforced PU composites are often manufactured using pultrusion process, due to shorter pot life of PU resin system. In this study, E-glass/PU composites are manufactured using a low-cost vacuum-assisted resin transfer molding process. A novel, one-part PU thermoset resin system with a longer pot life is adopted in this study. Tensile, flexure, and impact tests are conducted on both the thermoset PU neat resin system and E-glass/PU composites. A three-dimensional finite element model is developed in a commercial finite element code to simulate the impact behavior of E-glass/PU composite for three different energy levels. Finite element model is validated by comparing it with experimental results.

Manufacturing of Transparent Composites Using Vacuum Infusion Process

A novel optically-transparent glass fibre reinforced polymer matrix composite has been developed by infusing a clear epoxy resin system of matching refractive index into a conventional E-glass fabric preform. Transparent composites are manufactured using a low cost, environmentally friendly vacuum infusion process. Physical and mechanical tests have been conducted. Transparent composites manufactured using the modified vacuum infusion process had a fibre volume fraction of 40%. Tensile strength and tensile modulus of these composites were 374.9 MPa and 31.74 GPa respectively. The results indicate that the transparent composites possess good physical and mechanical properties. These transparent composites form a good base for developing new generation transparent armour systems.

Optical properties of optically transparent glass-ribbon composites

Optically transparent glass-ribbon composite panels were made by reinforcing a clear epoxy resin with soda-lime silicate glass ribbons, as opposed to using cylindrical fibers, of matching refractive index. Cross-ply (0/90°) optically transparent glass-ribbon composite panels were made by stacking either 64 or 128 glass-ribbon layers and with two-ribbon dimensions. The haze and light transmission were measured between 10℃ and 52℃. Additionally, 610 mm × 910 mm × 18.7 mm flat windshields were made by laminating 1.9-mm thick (0°/90°)40 optically transparent glass-ribbon composite panels to clear polycarbonate panels in an autoclave. The haze and light transmission for the optically transparent glass-ribbon composite panels and for a prototype windshield were measured as a function of temperature while optical distortion was measured at room temperature (22℃) only for the windshield. The haze changed with temperature, with a minimum at the temperature where the refractive index of the glass ribbons and the polymer were equal. The lowest haze value was found for the widest ribbon, while the light transmission was almost constant in the temperature range of study.

Processing of yttrium aluminosilicate (YAS) glasses for dental composites

Two series of silicate glasses were processed to micron-size, sub-micron size, and nanoparticles using three different milling systems: ball milling, attrition, and high-energy milling. The effect of milling time and media size on particle size and contamination were investigated in aqueous and isopropanol suspensions. The particle size was determined using a laser-diffraction particle size analyzer and scanning electron microscopy. The smallest glass particles with a median particle size of 0.3 µm were achieved by a two-step comminution process in a high energy mill.

Impact Characterization of Polyurethane Composites Manufactured Using Vacuum Assisted Resin Transfer Molding

Glass fiber reinforced composites are finding various applications due to their high specific stiffness/strength, and corrosion resistance. Vacuum assisted resin transfer molding (VARTM) is one of the commonly used low cost composite manufacturing processes. Polyurethane (PU) resin system has been observed to have better mechanical properties and higher impact strength when compared to conventional resin systems such as polyester and vinyl ester. Until recently, PU could not be used in composite manufacturing processes such as VARTM due to its low pot life. In the present work, a thermoset PU resin systems with longer pot life developed by Bayer MaterialScience is used. Glass fiber reinforced PU composites have been manufactured using one part PU resin system. Performance evaluation was conducted on these composites using tensile, flexure and impact tests. Finite element simulation was conducted to validate the mechanical tests. Results showed that PU composites manufactured using novel thermoset PU resins and VARTM process will have significant applications in infrastructure and automotive industries.© 2012 ASME