S.G. Prolongo

Nanoreinforced Epoxy Adhesives for Aerospace Industry

Adhesive joints of carbon fiber/epoxy laminates were studied using an epoxy resin as the adhesive. In order to enhance the mechanical and electrical properties of epoxy adhesives, they were modified by the introduction of carbon nanofibers (CNFs). Also, different surface treatments, such as grit blasting, peel ply, and plasma, were applied to the laminates. The CNFs addition slows down the curing reaction of the epoxy adhesive although the final conversion is still high. The contact angle of nanoreinforced adhesives on the surface of treated laminates is lower than that of the neat epoxy resin. However, this increase of wettability scarcely modified the lap shear strength. Plasma treatment causes an important increase of the surface energy of laminates, markedly increasing the joint strength. The fracture mechanisms of the adhesive joints tested in the present study are strongly dependent on the surface treatment applied to the laminates.

Study of the effect of substrate roughness on adhesive joints by SEM image analysis

The effect of adherend surface roughness on epoxy bonded aluminium joints has been studied. Several epoxy adhesives were tested to evaluate the influence of adhesive nature on the roughness effect. The aluminium pre-treatments applied were abrasion and impression processes, generating different texture levels. The abrasion with grinding papers of different grain sizes provides surfaces with high density of low summits. In contrast, the surfaces subjected to impression process present low density of very deep valleys. The roughness measurements were made by image analysis of micrographs obtained by Environmental Scanning Electron Microscopy (ESEM). In addition to the average roughness, other surface descriptors were determined to characterise completely the surfaces, allowing to evaluate the influence on the adhesive strength of different texture variables, such as the height, shape and density of the peaks and valleys which constitute the roughness profile. An optimum value of surface roughness was found for the joint strength measured by the lap shear tensile test. An increase of adherend roughness causes an increase of effective area but, at the same time, decreases the ability for adhesive penetration. It has also been shown that the joint strength depends on the main characteristics of the adherend surface, such as the density and depth of the protuberances. Finally, it has been found that the roughness effect seems be influenced by the adhesive nature. Different epoxy adhesives with similar mechanical properties present different joint strengths due to their different penetration abilities.

Rheological Behaviour of Nanoreinforced Epoxy Adhesives of Low Electrical Resistivity for Joining Carbon Fiber/Epoxy Laminates

Epoxy resins reinforced with carbon nanofibers (CNF) and nanotubes (CNT) were prepared and evaluated as adhesives of carbon fiber/epoxy laminates. Different percentages of nanofiller (0.1–3 wt%) have been tested. The viscosity of the non-cured nanoreinforced epoxy mixtures increased with the nanofiller content. On the other hand, the thermal treatment at high temperatures of the mixtures of amino-functionalized CNTs and epoxy monomer also caused an increase of their viscosity — this is likely due to the chemical reaction between the oxirane groups of the epoxy and the amine groups of the nanofiller. The joint strength of the carbon fiber/epoxy laminates bonded with nanoreinforced epoxy adhesives was analyzed by means of the single lap shear test. The shear strength of these joints was similar to that of the one made with unfilled epoxy resin. However, observation by Scanning Electron Microscopy of the fracture surfaces of the adhesive joints confirmed that the incorporation of carbon nanofillers caused the cohesive fractures inside the laminates (light-fiber tear failure). The electrical conductivity was drastically increased by the addition of nanofillers, especially CNTs.

Effect of the epoxy/amine stoichiometry on the properties of carbon nanotube/epoxy composites

The effect of different amine/epoxy ratios on different properties of an epoxy resin was evaluated. Also, different amounts of carbon nanotubes (CNT) were added in order to analyse which of these two factors is more relevant. Dynamic thermomechanical analysis and flexural tests were carried out. The results obtained for the epoxy resin are in agreement with that reported by several researchers. The glass transition temperature of the resin and composites is maximal for the amine/epoxy stoichiometric ratio, while the highest glassy storage and flexural modulus correspond to the epoxy-rich systems, showing that the most crosslinked network is not the stiffest one. The effect of changing the stoichiometry is more relevant than adding CNT to the epoxy resin used in this work. However, the addition of CNT causes more remarkable changes in the epoxy-rich resin, promoting an increase of the glass transition temperature and the elastic modulus.

Surface Pretreatments for Composite Joints: Study of Surface Profile by SEM Image Analysis

The main objective of this research was to obtain an improved understanding of the factors which contribute to bond strength in adhesive joints of carbon fiber/epoxy composites. For this, the most common surface treatments for composites were applied including grit blasting, peel ply and plasma. Also, the untreated adherends were analysed. The topography of the studied surfaces was analysed with a profilometer and through image analysis of scanning electron micrographs. Besides measuring the average roughness, other surface parameters were determined in order to characterise the shape, distribution, height and density of summits on the surfaces. In view of the results obtained, it was confirmed that average roughness alone was not sufficient to interpret the results; the adhesion between the adhesive and the adherends does not depend only on the mechanical interlocking effect. It was confirmed that plasma treatment provides the highest joint strength due to the higher surface energy of the composite when is treated by this method. In contrast, peel ply gives poor results. The joints whose adherends were treated by grit blasting also showed high strength values due to a high density of summits on the surface.

Durability of Aluminium Adhesive Joints Bonded with a Homopolymerised Epoxy Resin

The durability of epoxy–aluminium joints that use a homopolymerised epoxy resin was studied, and the effects of relative humidity, temperature, and salt concentration were analysed. The adhesive properties were measured by lap–shear tests, and the water uptake of the epoxy resin was determined by gravimetric measurements. Ageing and degradation effects on the epoxy resin and on the aluminium substrates were also analysed. The homopolymerised epoxy resin absorbs little water (1.5 wt%) because of its nonpolar network structure. The water uptake is enhanced by increasing relative humidity and temperature; however, the joint strength remains constant because of epoxy plasticization. A saline environment is damaging to the adhesive joints, because of metal corrosion, but was not significantly harmful to the epoxy resin, because of a lower diffusion coefficient of salt water. The Tg decrease of the epoxy adhesive due to water absorption depends only on the amount of absorbed water and is independent of the hydrothermal ageing conditions.

Epoxy Adhesives Modified with Graphene for Thermal Interface Materials

Thermal interface materials (TIMs) are extensively used to improve thermal conduction across two mating parts. In this study, the viability of using epoxy resins reinforced with graphene nanoplatelets (GNPs) as TIMs is evaluated. Different GNPs contents are added using a mini-calander to achieve a homogeneous dispersion. The addition of GNPs induces an increase of the glass transition temperature and the storage modulus referenced to the thermosetting matrix. Furthermore, the introduction of high GNPs contents (10 wt%) causes a dramatic increase of the thermal diffusivity (300%) and electrical conductivity (∼10−2 S/m). GNP/epoxy adhesives present enhanced wettability upon aluminum adherends, compared with the strength of joints bonded with neat epoxy adhesives. The introduction of high GNPs contents induces a change of the failure mechanism of joints, from adhesive for neat epoxy resin to cohesive mode.

Optimum Dispersion Technique of Carbon Nanotubes in Epoxy Resin as a Function of the Desired Behaviour

The use of carbon nanostructures for epoxy matrices modification has been widely studied, nevertheless there are several alternative methods for manufacturing that try to avoid difficulties related to their tendency to keep entangled. The use of the calendering approach and high shear mixing alternatives is common for dispersing these nanoreinforcements. The present article compares these two methods as well as possible synergies from the use of the two alternatives together. It has been found that the dispersion technique used modifies the final dispersion level reached as well as on the final properties of the different nanocomposites. Nevertheless, this effect depends on the type of nanoreinforcement (structure and functionalization) and the property measured. Results suggest that each carbon nanostructure requires an individual design of the dispersion stage to get the optimum properties. Thus, the optimum technique may be different depending on the final desired properties, and the dispersion cycle should be designed carefully depending of the final material aim and the nanostructure used. Nevertheless, typical dispersion cycles are currently applied for different type of nanoreinforcements.

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are proposed to create the electrical network in this work: (a) the incorporation of the nanoplatelets into the epoxy matrix and (b) the coating of the glass fabric with a sizing filled with the same nanoplatelets. Both types of multiscale composite materials, with an in-plane electrical conductivity of ~10-3 S/m, showed an exponential growth of the electrical resistance as the strain increases due to distancing between adjacent functionalized graphene nanoplatelets and contact loss between overlying ones. The sensitivity of the materials analyzed during this research, using the described procedures, has been shown to be higher than commercially available strain gauges. The proposed procedures for self-sensing of the structural composite material would facilitate the structural health monitoring of components in difficult to access emplacements such as offshore wind power farms. Although the sensitivity of the multiscale composite materials was considerably higher than the sensitivity of metallic foils used as strain gauges, the value reached with NH2 functionalized graphene nanoplatelets coated fabrics was nearly an order of magnitude superior. This result elucidated their potential to be used as smart fabrics to monitor human movements such as bending of fingers or knees. By using the proposed method, the smart fabric could immediately detect the bending and recover instantly. This fact permits precise monitoring of the time of bending as well as the degree of bending.

Strength and Durability of Epoxy-Aluminum Joints

The adhesive strength and durability of adhesively-bonded aluminum joints in wet environments was analyzed. A2024-T4 alloy was subjected to two different surface treatments based on etching with chromic-sulfuric acid (FPL) and with sulfuric acid-ferric sulfate (P2). Small differences were observed in the lap shear strength as a function of the applied surface treatment. However, durability in humid environments was higher for the joints whose adherends were treated with P2. Although the amount of water absorbed by the epoxy adhesive is lower in saline environments, the effects on the glass transition temperature of the epoxy adhesive and on the lap shear strength of the joints are more marked than the effects caused by aging with distilled water. Finally, a new epoxy adhesive with a siloxanic hardener was tested, obtaining good mechanical properties, high glass transition temperature, moderate values of lap shear strength, and high durability in wet environments.