Samuel Kenig

The Effect of Tungsten Sulfide Fullerene-Like Nanoparticles on the Toughness of Epoxy Adhesives

In this work the effect of inorganic fullerene-like (closed cages) nanoparticles of tungsten disulfide (IF-WS2) on the mechanical properties and especially on the toughness of epoxy resins, was studied. The epoxy resin used was the well-known DGEBA (di-glycidyl ether of bis-phenol A) cured with polyamidoamine. The epoxy/IF-WS2 nanocomposites were prepared by applying a high shear mixing to obtain a uniform dispersion and homogeneous distribution of the IF nanoparticles in the epoxy matrix. Two mixing procedures were used — a low shear of short duration and high shear with a long mixing time. The resulting epoxy nanocomposites were first characterized for their shear and peel strength using appropriate bonded joints. The experimental results demonstrate that enhanced shear strengths and shear moduli were achieved, together with a significant increase in the peel strengths at low concentrations of the IF-WS2 nanoparticles (more than 100% increase at 0.5 wt% IF-WS2). Above the threshold value of 0.5% IF-WS2 the peel strength decreased sharply. The fractured surfaces of the bonded joints were examined by transmission and scanning electron microscopy in order to characterize the fracture mechanisms and analyze the dispersion level of the nanoparticles within the polymer. The electron micrographs indicated that the presence of the nanoparticles in the epoxy matrix induced fracture mechanisms which were different from those observed in the pristine epoxy phase. These mechanisms included: crack deflection; crack bowing; and crack pinning. Evidence for a chemical interaction between the nanoparticles and the epoxy were obtained by infrared measurements in the attenuated total transmittance mode. The data suggests the formation of new carbon–oxygen–sulfur bonds, which are most likely due to the reaction of the outermost sulfur layer of the IF nanoparticles with the reactive epoxy groups. The observed simultaneous increase in both shear and peel strengths at very low IF-WS2 concentrations, found in this work, could lead to the development of high performance adhesives and to new types of structural and ballistic fiber nanocomposites.

The Effect of WS2 Nanotubes on the Properties of Epoxy-Based Nanocomposites

In this paper we evaluated the effect of embedding inorganic nanotubes (INT) of tungsten disulfide (WS2) in an epoxy matrix, on the mechanical, thermal and adhesion properties of the resulting nanocomposites. The nanotube content spanned a range of values (0, 0.1, 0.3, 0.5 and 1.0 wt%), and the nanotube incorporation process consisted of a combination of both distributive (magnetic stirring) and dispersive (ultrasonic mixing) methods. The adhesion of the nanocomposites to an aluminum substrate was characterized by both a single lap shear and a T-peel test. The fracture toughness (K IC) of the nanocomposites was characterized by a standard compact tension (CT) plane-strain fracture test. The thermal properties of the nanocomposites were determined by dynamic mechanical thermal analysis (DMTA). Overall, the addition of INT-WS2 was found to improve the shear strength and peel properties of the nanocomposite, and to significantly improve its fracture toughness and glass transition temperature. The extent and character of the nanotube–epoxy interaction were examined by electron microscopy, as was the energy dissipation mechanisms during fracture.

Super-hydrophilic coatings based on silica nanoparticles

This work focuses on the use of silica nanoparticles for producing durable, transparent, and super-hydrophilic coatings on painted surfaces. Two methods were studied in detail: bottom-up approach using layer-by-layer (LbL) assemblies of hydroxylated SiO2 nanoparticles, and top-down approach based on hybrid polymer/silica nanoparticles coatings. Of the two approaches studied, only the hybrid polymer/SiO2 nanocomposite coatings containing 50–90%wt. SiO2 exhibited durable super-hydrophilic surface properties less than 5° water contact and sliding angles. In the latter case, a unique micrometer-sized cracking pattern was developed. The LbL-assembled SiO2 coatings showed a gradual degradation over time from the initial super-hydrophilic properties, indicated by the increase of the contact angles from less than 5o to greater than 30o after accelerated aging. To investigate the effect of environmental exposure on developing hydrophilicity, a variety of analytical methods were employed such as: atomic force microscopy, scanning electron microscopy, optical microscopy, and Fourier transform infrared. Experimental results and associated modeling indicated that the combination of micro- and nano-surface roughness and the surface chemical composition were the dominant factors affecting the durability of the hydrophilic attributes of the coatings containing silica nanoparticles.

Characterization of Hybrid Epoxy Nanocomposites

This study focused on the effect of Multi Wall Carbon Nanotubes (MWCNT) content and its surface treatment on thermo-mechanical properties of epoxy nanocomposites. MWCNTs were surface treated and incorporated into two epoxy systems. MWCNTs surface treatments were based on: (a) Titania coating obtained by sol-gel process and (b) a nonionic surfactant. Thermo-mechanical properties improvement was obtained following incorporation of treated MWCNT. It was noticed that small amounts of titania coated MWCNT (0.05 wt %) led to an increase in the glass transition temperature and stiffness. The best performance was achieved adding 0.3 wt % titania coated MWCNT where an increase of 10 °C in the glass transition temperature and 30% in storage modulus were obtained.

The effect of composition and thermodynamics on the surface morphology of durable superhydrophobic polymer coatings

Durable superhydrophobic coatings were synthesized using a system of silica nanoparticles (NPs) to provide nanoscale roughness, fluorosilane to give hydrophobic chemistry, and three different polymer binders: urethane acrylate, ethyl 2-cyanoacrylate, and epoxy. Coatings composed of different binders incorporating NPs in various concentrations exhibited different superhydrophobic attributes when applied on polycarbonate (PC) and glass substrates and as a function of coating composition. It was found that the substrate surface characteristics and wettability affected the superhydrophobic characteristics of the coatings. Interfacial tension and spreading coefficient parameters (thermodynamics) of the coating components were used to predict the localization of the NPs for the different binders’ concentrations. The thermodynamic analysis of the NPs localization was in good agreement with the experimental observations. On the basis of the thermodynamic analysis and the experimental scanning electron microscopy, X-ray photoelectron spectroscopy, profilometry, and atomic force microscopy results, it was concluded that localization of the NPs on the surface was critical to provide the necessary roughness and resulting superhydrophobicity. The durability evaluated by tape testing of the epoxy formulations was the best on both glass and PC. Several coating compositions retained their superhydrophobicity after the tape test. In summary, it was concluded that thermodynamic analysis is a powerful tool to predict the roughness of the coating due to the location of NPs on the surface, and hence can be used in the design of superhydrophobic coatings.