ABSTRACT An hierarchical structure, composed of a ternary cocontinuous polymer blend, where carbon nanotubes are mostly localized in one of the phases through π–π interactions, is fabricated by direct melt mixing of polyamide 12 and... more
ABSTRACT An hierarchical structure, composed of a ternary cocontinuous polymer blend, where carbon nanotubes are mostly localized in one of the phases through π–π interactions, is fabricated by direct melt mixing of polyamide 12 and polypropylene, as the two major components of the ternary blend, together with pyridine-modified poly(ethylene-co-methacrylic acid) as the minor component that can form strong interactions with the CNTs via π–π interactions and confined the percolated network at the polyamide/polypropylene interface. The hierarchical structure was designed by means of surface energies, and the obtained morphology was verified using electron microscopy. This ternary structure has lower electrical resistivity as compared to cocontinuous binary composites. Different polymer viscosities were used in this study in order to emphasize the importance of kinetics during cocontinuous morphology formation.
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With recent developments in nanotechnology, nanocomposite adhesives offer numerous advantages compared to conventional adhesive materials. Moreover, with the emergence of commercial nanomaterials such as: Nanoclays (NCs), Carbon nanotubes... more
With recent developments in nanotechnology, nanocomposite adhesives offer numerous advantages compared to conventional adhesive materials. Moreover, with the emergence of commercial nanomaterials such as: Nanoclays (NCs), Carbon nanotubes (CNTs), Nanosilica (NS), Polyhedral oligomeric silsesquioxanes (POSS), Tungsten disulfide (WS2) Fullerenes and Nanotubes, and Graphene nanoparticles (GNPs), new potential routes have been opened to tailor adhesives in the nanoscale range. This update will review and analyze the unique fracture mechanisms and the resultant properties of various nanocomposite adhesives based on epoxies, polyurethanes, silicones, polyimides and acrylics containing: NCs, CNTs, NS, POSS, WS2 and GNPs and their current and potential applications.
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Abstract Hybrid superhydrophobic sol-gel coatings were synthesized and applied on glass substrates. Low surface energy was obtained by using hydrophobic organo-functionalized silanes and roughness was formed by alkyl modified silica... more
Abstract Hybrid superhydrophobic sol-gel coatings were synthesized and applied on glass substrates. Low surface energy was obtained by using hydrophobic organo-functionalized silanes and roughness was formed by alkyl modified silica nanoparticles (NPs). However, these superhydrophobic coating (SHCs) are known to exhibit lack of interfacial adhesion of the NPs and limited interfacial adhesion with the underlying substrates, as demonstrated by their ease of removal in harsh environments. In this study, hybrid hydrophobic sol-gel and hydrophobic silica NPs based SHCs were prepared with the objective to obtain enhanced interfacial adhesion both between the silica NPs and the sol-gel matrix and between the sol gel matrix and glass substrates. Consequently, three curing processes were studied: Thermal, radiation, and dual curing. Dual-cured SHC presented the highest mechanical and environmental durability. It retained its superhydrophobicity (SH) for 17 tape peels and for 1000 h under accelerated weathering conditions. Failure analysis indicated that strong interfacial bonding was formed at the interface between the alkyl modified silica NPs and the hybrid hydrophobic sol gel matrix due to the interfacial free radical chemistry as well as between the sol gel coating and the glass substrate due to condensation chemistry. Moreover, the dual-cured hybrid sol-gel demonstrated transparent SHCs with over 90 % optical transmittance, water contact angle (CA) of 160˚ and a correspondingly sliding angle (SA) of ∼0˚. Morphological and roughness analyses revealed a uniform, hierarchic structure consisting of micro- and nano-sized asperities with Gaussian peak density distribution, like the Lotus leaf surface. According to ATR-IR analysis, dual curing process combines the mechanisms of chain polymerization of functional vinyl silane by radiation and thermal condensation polymerization of fluoro-silane to form a siloxane network. Hence, only chain or only condensation mechanisms lead to partial crosslinked network and low interfacial adhesion of the NPs to the sol-gel system. The dual-cured coating's failure appeared to be more ductile, compared to the brittle fracture and the delamination that were observed for thermal or radiation-cured coatings, respectively.
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Abstract Owing to their outstanding variety of properties, nanoparticles attracted considerable interest for developing a new age of novel polymer nanocomposites and adhesives. Among them, inorganic nanotubes (INTs) of tungsten disulfide... more
Abstract Owing to their outstanding variety of properties, nanoparticles attracted considerable interest for developing a new age of novel polymer nanocomposites and adhesives. Among them, inorganic nanotubes (INTs) of tungsten disulfide (WS 2 ) and carbon nanotubes (CNTs) have been identified as unique candidates for many industrial applications by virtue of their superior mechanical, thermal and tribological properties. In this work, a comparative study between INT-WS 2 and multi-walled carbon nanotubes (MWCNTs) regarding the properties of structural polyurethane (PU) adhesives is presented. Specifically, we evaluate the thermomechanical properties, chemical and micro-phase structure, bonded joints performance in lap shear and peel modes, and fracture mechanisms, with aiming to highlight some of the differences between these nanotubes. The added content spanned over the range of 0.3, 0.6, 0.9 and 1.2 wt% of INT-WS 2 , while CNTs are incorporated at the same volumetric fractions (e.g., 0.1, 0.2, 0.3 and 0.4 wt%). According to the results of the tests, in all measured aspects, INT-WS 2 outperformed the carbon counterparts. With regards to the thermomechanical attributes, the hard segment Tg of the PU increased by 14 °C at 0.6 wt% INTs, whereas CNTs recorded only modest improvement of 8 °C at the nanocomposite loaded with 0.4 wt%. The structural bonding performance indicates an improvement of 117% and 38% in the shear and peel strengths at 0.9 and 1.2 wt% INTs, respectively, while CNTs show moderate changes. Interestingly, the changes are also reflected by the interplay between the hard and soft segments of the PU, and different fracture mechanisms operating within the nanocomposites. At any rate, the combined attributes of >20 MPa in the shear strength and approximately 2 N/mm in (average) peel strength, targets the PU/INT nanoadhesives as promising alternative for conventional epoxy systems which has similar shear strength with inferior peel strength.
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The effects of nano- and micro-roughness as well as the chemical composition of surface treatments on self-cleaning properties and their suitability for ice repellency were investigated. In the first part of the study, the thermodynamics... more
The effects of nano- and micro-roughness as well as the chemical composition of surface treatments on self-cleaning properties and their suitability for ice repellency were investigated. In the first part of the study, the thermodynamics of water wettability of coated surfaces was evaluated both theoretically and experimentally. Accordingly, relationships were derived between the interfacial adhesion strength of a liquid drop to a polymer surface of a given composition, the mass of the drop, the measured contact angles, and the sliding angle. To verify the proposed model various hydrophobic coatings having different surface nanoand micro-roughnesses were prepared using polycarbonate as a substrate.The surface treatment was based on fluoroalkylsilane. Roughness was introduced by means of silica (microparticle) and POSS — Polyhedral Oligomeric Sil Sesquioxane (hybrid inorganic-organic nanoparticle) using a two tier topography. Using optimal compositions and processing conditions trans...
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ABSTRACT Closed-cage (fullerene-like) nanoparticles (NPs) of WS2 are currently produced in large amounts and were investigated as additives to thermoplastics and thermosetting polymers. The nanoinduced morphology and the resulting... more
ABSTRACT Closed-cage (fullerene-like) nanoparticles (NPs) of WS2 are currently produced in large amounts and were investigated as additives to thermoplastics and thermosetting polymers. The nanoinduced morphology and the resulting enhanced fracture toughness of epoxy/WS2 nanocomposites were studied. The morphology of the epoxy nanocomposites was induced by controlled WS2 surface chemistry. The WS2 NPs used were either untreated or chemically treated with acryloxy, which is compatible, and alkyl silane, which is incompatible, respectively, with the epoxy matrix. In the case where the acryloxy silane was used to treat the WS2 particles, good dispersion and compatibility were obtained in the epoxy resin. Moreover, a distinct nodular morphology was induced on fracture as a result of nucleation by the compatible NPs. In the case where the alkyl silane treatment was used cavitation morphology was induced, following mechanical loading, which is the result of incompatibility with the epoxy resin. The fracture toughness results showed an increase of 70% for nanocomposites contains alkyl-treated WS2 compared with the neat epoxy. Modeling of the nodular morphology enabled the determination of optimal concentration of the WS2 in epoxy (0.3% by weight). Two main fracture mechanisms were observed, crack bowing around the nodular boundaries in the case of compatibility between the nanoparticle and the epoxy and particle-induced cavitation in the case of incompatibility, respectively. These results are of significant importance both for epoxy-based adhesives and fiber composites. POLYM. ENG. SCI., 53:2624–2632, 2013. © 2013 Society of Plastics Engineers
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ABSTRACT Improvement of superhydrophobic surfaces durability is a critical key for potential commercial applications such as self-cleaning and ice repellency. In this study, functionalization of silica nanoparticles by photoreactive... more
ABSTRACT Improvement of superhydrophobic surfaces durability is a critical key for potential commercial applications such as self-cleaning and ice repellency. In this study, functionalization of silica nanoparticles by photoreactive benzophenone groups was made in order to covalently bond nanoparticles to polymer substrates to obtain durable coatings. Upon ultraviolet irradiation reactive excited triplet benzophenone species are formed enabling them to react with the polymer matrix through hydrogen abstraction. Two matrices were studied: radiation-curable urethane acrylate and epoxy. The bonding of the particles to the surfaces was evaluated using atomic force microscope nanomanipulation and cross-section analysis. The results have shown a greater stability of the photoreactive silica nanoparticles. Copyright © 2014 John Wiley & Sons, Ltd.
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ABSTRACT The distribution of residual stresses in quenched modified poly(phenylene oxide) (PPO) specimens was investigated. Quenching was carried out from temperature level above Tg to various temperatures below Tg. As expected,... more
ABSTRACT The distribution of residual stresses in quenched modified poly(phenylene oxide) (PPO) specimens was investigated. Quenching was carried out from temperature level above Tg to various temperatures below Tg. As expected, compressive stresses were measured at the surface layers while tensile stresses were in the inner layers. The ratio between the tensile and compressive stresses varied, depending on the thermal history. The level of residual surface stresses was found to depend on both the total temperature difference during cooling and the initial specimen temperature. At constant initial temperature the surface stresses are proportional to the total temperature difference, whereas, at constant final temperature the surface stresses are inversely proportional to the total temperature difference. An empirical correlation describing the surface stresses as a function of thermal history has been suggested. The differences between present theories and experimental data are discussed. emphasizing the apparent discrepancy regarding the influence of initial temperature above Tg on the level of residual stresses.
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ABSTRACT The distribution of density and tensile properties in quenched modified poly(phenylene oxide) specimens was investigated. Quenching was carried out from temperature level above Tg to below Tg temperatures. Simultaneous to buildup... more
ABSTRACT The distribution of density and tensile properties in quenched modified poly(phenylene oxide) specimens was investigated. Quenching was carried out from temperature level above Tg to below Tg temperatures. Simultaneous to buildup of residual stresses, profiles of density and tensile properties were observed. The profiles were obtained using the layer removal technique, which was found not to affect the measured properties. Quenching of the material results in a steep density gradient in the surface layers. Correspondingly, the tensile modulus increases significantly from the surface to the inner layers and so are also the ultimate tensile properties. This behavior could be accounted for neither by the conventional packing volume approach nor by superposition of internal and external stresses. However, observations of the fracture surfaces are very supportive and indicate that the fracture initiation sites are influenced by the residual stresses. Hence, the mechanical behavior is strongly affected by both density and residual stresses profile. Density is the controlling factor in determining the elastic properties whereas residual stresses determine the ultimate strength and fracture mechanism.
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The orientation development characteristics in shear flow of wholly aromatic thermotropic liquid crystalline polymers were studied. Based on the analysis of shear flow in a capillary followed by elongational flow, an experimental setup... more
The orientation development characteristics in shear flow of wholly aromatic thermotropic liquid crystalline polymers were studied. Based on the analysis of shear flow in a capillary followed by elongational flow, an experimental setup has been proposed to determine the shear-induced orientation. Experimental results have indicated that shear-induced orientation depends on total shear strain. However, orientation enhancement due to shear flow is much less effective compared with orientation buildup in the case of elongational flow. Thermotropic liquid crystalline polymers that exhibited higher orientability in elongational flow exhibited also higher orientability in shear flow. Finally, to obtain high, shear orientation in processing of thermotropics, special care should be exercised in the design of dies and molds and in optimizing processing conditions, so that high shear rates and/or long shearing times should prevail.
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3 ECCM-8 421 The effect of transcrystallinc interface on the mechanical properties of composite materials N. Klein, H. Nuriel and C ... As a result many-research groups are currently engaged in investigating the interfacial layer or... more
3 ECCM-8 421 The effect of transcrystallinc interface on the mechanical properties of composite materials N. Klein, H. Nuriel and C ... As a result many-research groups are currently engaged in investigating the interfacial layer or interphase, and layer thicknesses of the order of ...
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ABSTRACT An excimer laser may be used for preadhesion treatment of aluminum alloys. This method presents an alternative to the use of ecologically unfriendly chemicals involved in conventional anodizing pretreatments.Experimental results... more
ABSTRACT An excimer laser may be used for preadhesion treatment of aluminum alloys. This method presents an alternative to the use of ecologically unfriendly chemicals involved in conventional anodizing pretreatments.Experimental results indicate that preadhesion laser surface treatment significantly improved the shear strength of modified-epoxy bonded aluminum specimens compared with untreated and anodized substrates. The best results were obtained with laser energy of about 0.2 J/Pulse/cm where single lap shear strength was improved by 600-700% compared with that of untreated Al alloy, and by 40% compared with chromic acid anodizing pretreatment.The mode of failure changed from adhesive to cohesive as the number of laser pulses increased during treatment. The latter phenomenon has been correlated with morphology changes as revealed by electron microscopy, and chemical modification as indicated by Auger and infrared spectroscopy.It can be concluded that the excimer laser has potential as a precise, clean and simple preadhesion treatment of Al alloys.
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The viscoelastic related properties of four structural adhesives were studied in their bulk form. All four adhesives were based on rubber-toughened epoxy resin with a thermoplastic carrier. Two of the adhesives were commercial film... more
The viscoelastic related properties of four structural adhesives were studied in their bulk form. All four adhesives were based on rubber-toughened epoxy resin with a thermoplastic carrier. Two of the adhesives were commercial film adhesives (120°C curing systems) and the other two were formulated by us from commercially available constituents. The first formulation is a high-temperature-curing system based on cyclo-aliphatic resin and anhydride hardener, toughened with carboxy-terminated butadiene elastomer. The second self-prepared formulation is a special room-temperature-curing adhesive for elevated temperature service, based on a blend of trifunctional and tetrafunctional expoxies cured with triethylene tetramine toughened with amine-terminated butadiene elastomer. The latter formulation was also prepared, in addition to the carrier-containing composition, without the thermoplastic carrier.As expected for viscoelastic materials, it was found that the yield stress and modulus decreased with temperature. The rate of loading had a pronounced effect on the yield stress which increased with increasing loading rates, and a negligible influence on the modulus. The rate-temperature effects on the yield stress were shown to obey the superposition as described by Eyring's theorem of viscosity. Consequently, the activation energy and activation volume were determined. The high-temperature-curing adhesives comprising a carrier exhibited higher activation energies compared with the room-temperature-curing formulation and other epoxy adhesives cured with aliphatic amines or polyamides reported in the literature.
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ABSTRACT Humidity absorbed by epoxy film adhesives during low temperature storage or exposure to atmosphere may result in reversible changes and irreversible modifications. Vacuum treatment may partially remedy the reversible changes. The... more
ABSTRACT Humidity absorbed by epoxy film adhesives during low temperature storage or exposure to atmosphere may result in reversible changes and irreversible modifications. Vacuum treatment may partially remedy the reversible changes. The consequences of vacuum drying are manifested in enhancement of both the peel and shear properties of bonded joints (Part I and Part II of this series of papers) and the thermal, physical and mechanical properties of the bulk adhesive, characterized in the present study.Experimental results have shown that the bulk properties of structural epoxy based adhesives are highly correlated with the aging processes caused by water absorption in the prepolymerized adhesive. Applying the vacuum process is harmful to fresh unaged adhesive due to devolatization of low molecular species of the film adhesive.The characterization of bulk properties for the purpose of following the aging and recovery processes is advantageous, since the bulk is independent of geometrical and interfacial effects which dominate in the case of property evaluation of the adhesive in a bonded joint.
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Ice adhesion to surfaces is detrimental to electrical and communication cables and to air and ground transportation vehicles. Hence, anti-icing materials and mechanical and heating means have been used for de-adhesion of ice to various... more
Ice adhesion to surfaces is detrimental to electrical and communication cables and to air and ground transportation vehicles. Hence, anti-icing materials and mechanical and heating means have been used for de-adhesion of ice to various substrates. Alternatively, icephobic surface treatments have been developed to reduce the adhesion of ice to solid surfaces. However, the understanding of ice anti-adhesion phenomenon is lacking with respect to the effects of surface roughness and chemical composition. Consequently, the effects of roughness on the nanoscale and microscale as well as of the chemical composition of surface treatments on ice repellency were investigated. In the first part of the study, the thermodynamics of water wettability of coated surfaces was theoretically evaluated. Accordingly, relationships were derived between the interfacial adhesion strength of a liquid drop to a polymer surface of a given composition, the mass of the drop, the measured contact angles, and the...
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Experimental studies have been conducted with a 2 1/3 oz reciprocating-screw injection molding machine to check the validity of a proposed theoretical model for injection molding. A semi-circular cavity has been employed in the study to... more
Experimental studies have been conducted with a 2 1/3 oz reciprocating-screw injection molding machine to check the validity of a proposed theoretical model for injection molding. A semi-circular cavity has been employed in the study to achieve spreading ...
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ABSTRACT Failure mechanisms of graphite-fabric reinforced epoxy composites loaded in flexure have been studied, following ageing in dry, wet and hot environments. Experimental results based on mechanical loading to failure, dynamic... more
ABSTRACT Failure mechanisms of graphite-fabric reinforced epoxy composites loaded in flexure have been studied, following ageing in dry, wet and hot environments. Experimental results based on mechanical loading to failure, dynamic mechanical analysis, electron microscopy and infrared spectra indicated that in the case of high temperature ageing in dry conditions the failure mechanism is characterized by shear at short loading spans and by tension at long spans. This failure mode changes to shear delamination after long duration ageing. Wet environments up to 50°C hardly affected the mechanical properties and failure mechanisms compared to those at ambient conditions. However, a reduction in the glass transition temperature was noticeable. Ageing in boiling water was harmful to the 120°C curing composite which was based on a dicyandiamide cured epoxy. Leaching of partially reacted components of the dicyandiamide epoxy system resulted in void formation, a considerable drop in mechanical properties and a change in the failure mechanisms. Enhanced hygrothermal stability of the 120°C dicyandiamide cured system was achieved by curing or post curing at 175°C.
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ABSTRACT Failure mechanisms of fabric reinforced graphite epoxy specimens subjected to flexural loading have been studied. Two factors were characterized with regard to fracture modes; the span-to-thickness ratio and the fibre direction... more
ABSTRACT Failure mechanisms of fabric reinforced graphite epoxy specimens subjected to flexural loading have been studied. Two factors were characterized with regard to fracture modes; the span-to-thickness ratio and the fibre direction with respect to the load axis. Results have shown that as in the case of unidirectional composites, fabric composites exhibit a transition in the failure mode from shear delamination to fibre yield as the span-to-thickness ratio in flexural testing is increased. For off-axis fabric reinforced composites, the fracture curves showed a maximum in the tensile strength as the span length increased, followed by a sharp decline. In general, lower strength values both in shear and tension are typical of off-axis specimens compared to axial loaded ones. Electron micrographs of the fracture surfaces of axial and off-axis loaded specimens supported the observed mechanical behaviour. While the axially loaded specimens showed a clear shear delamination failure for short spans and a tensile failure of fibres for long spans, the off-axis specimen exhibited a mixed mode failure of both delamination and fibre breakage in the case of large span-to-thickness ratios.
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ABSTRACT The effects of tungsten disulphide inorganic nanotubes (INT-WS2) on the bulk and adhesion properties of silicone nanocompositeswere investigated. The nanocomposites were prepared using solvent mixing which enhanced distribution... more
ABSTRACT The effects of tungsten disulphide inorganic nanotubes (INT-WS2) on the bulk and adhesion properties of silicone nanocompositeswere investigated. The nanocomposites were prepared using solvent mixing which enhanced distribution and dispersion of the nanotubes in the high viscosity silicone matrix. The quality of dispersion was evaluated using scanning electron microscopy (SEM) indicating good dispersion state. It was found that the optimal concentration of INT-WS2in the nanocomposites was 2 wt%, exhibiting 33% higher storage modulus, 6% improvement in tensile strength,17% reduction in the coefficient of linear thermal expansion and 13% improvement in shear strength, compared to the neat polymer. Above this threshold value the nanocomposites adhesion properties were reduced.
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ABSTRACT Recently large amounts of inorganic nanotubes (INT) and inorganic fullerene-like (IF) nanoparticles of WS2 became available and methods for their dispersion in different media were developed. In the present work the tribological... more
ABSTRACT Recently large amounts of inorganic nanotubes (INT) and inorganic fullerene-like (IF) nanoparticles of WS2 became available and methods for their dispersion in different media were developed. In the present work the tribological properties of epoxy composite compounded with tungsten disulfide particles of different sizes and morphologies, including quasi-spherical IF nanoparticles, one-dimensional INT as well as micron-size platelets (2H) were investigated. The coefficient of friction and wear loss were measured under dry contact conditions using different tribological rigs. Remarkable reduction in wear and also friction (under high load) was demonstrated for the IF/INT epoxy nanocomposite. The reduced wear is attributed in general to the reinforcement of the polymer matrix by nanoparticles and the simultaneous reduction of the epoxy brittleness. Contrarily, the friction of the neat epoxy sample and epoxy mixed with platelets was accompanied with strong wear and transfer of a polymer film onto the rubbed surfaces. These results are consistent with the recently reported improvements in the fracture toughness, peel and shear strength of the epoxy-nanoparticles (IF/INT) composites.