Carlo Naddeo

Chemical and morphologial modifications of irradiated linear low density polyethylene (LLDPE)

Films of linear low density polyethylene (LLDPE) produced with the spherilene technology were exposed to accelerated weathering through UV irradiation at 60°C for increasing times. A different series of films were only annealed at 60°C, to differentiate the thermal effects from those due to the UV irradiation. FT–IR analysis was used to investigate the change of the chemical structure. It was found that, following the initial production of hydroperoxides, the degradation is dominated by the formation of carbonyl and vinyl species. This process proceeds slowly up to 150 h of irradiation and afterwards it accelerates, completely degrading the material. The samples exposed more than 150 h are mechanically very fragile and soon fail. Structural analysis of the irradiated samples showed that there is a perfection of the elementary cell of polyethylene, principally along the a axis, and a consistent increase of crystallinity. The first effect was found also in thermally treated samples, whereas the second is more consistent in the irradiated samples. Also the dimension of the crystals increase much more for the irradiated samples than for the annealed ones. All the effects are mostly evident for samples after 150 h of irradiation, that is when the degradation is accelerated. The chain scission due to the photo-oxidation makes the amorphous chains more mobile and free for further crystallization, and this happens mainly when the degradation occurs simultaneously in many chains.

Development of epoxy mixtures for application in aeronautics and aerospace

This work describes a successful attempt toward the development of composite materials based on nanofilled epoxy resins for the realization of structural aeronautic components providing efficient lightning strike protection. The epoxy matrix is prepared by mixing a tetrafunctional epoxy precursor with a reactive diluent which allows the moisture content to be reduced and facilitates the nanofiller dispersion step. The reactive diluent also proves to be beneficial for improving the curing degree of nanofilled epoxy mixtures. It increases the mobility of reactive groups resulting in a higher cure degree than the epoxy precursor alone. This effect is particularly advantageous for nanofilled resins where higher temperature treatments are needed, compared to the unfilled resin, to reach the same cure degree. As nanofiller, different carbon nanostructured fiber-shaped fillers are embedded in the epoxy matrix with the aim of improving the electrical properties of the resin. The results highlight a strong influence of the nanofiller nature on the electrical properties especially in terms of electrical percolation threshold (EPT) and electrical conductivity beyond the EPT. Among the analyzed nanofillers, the highest electrical conductivity is obtained by using multiwalled carbon nanotubes (MWCNTs) and heat-treated carbon nanofibers (CNFs). The achieved results are analyzed by considering the nanofiller morphological parameters and characteristics with respect to the impact on their dispersion effectiveness.

Mechanical and transport properties of irradiated linear low density polyethylene (LLDPE)

Abstract Films of a new linear low density polyethylene (LLDPE) were exposed to accelerated weathering in a UV chamber at 60°C for increasing times. The changes in the mechanical properties, due to the degradation were followed by determining the elastic modulus ( E ), the stress at the yield point ( σ y ), the post yield stress drop (PYSD), the stress at the breaking point ( σ b ), the strain at the breaking point ( ϵ b ), and the toughness ( t ). They were correlated to the exposure time and to the carbonyl index, previously determined on the same samples. All the mechanical parameters were found to be very sensitive to the chemical degradation and to the morphological changes. Furthermore the curves of the mechanical parameters as a function of the exposure time allowed the determination of the point beyond which the material becomes useless. This point corresponds to 150 h of irradiation and to a carbonyl index of 0.3. The transport properties, diffusion and sorption, of a non polar molecule, n -pentane and a more polar molecule, dichloromethane were also correlated to the changes in the molecular structure.

Healing efficiency and dynamic mechanical properties of self-healing epoxy systems

Several systems to develop self-repairing epoxy resins have recently been formulated. In this paper the effect of matrix nature and curing cycle on the healing efficiency and dynamic mechanical properties of self-healing epoxy resins were investigated. We discuss several aspects by transferring self-healing systems from the laboratory scale to real applications in the aeronautic field, such as the possibility to choose systems with increased glass transition temperature, high storage modulus and high values in the healing functionality under real working conditions.

Influence of carbon nanoparticles/epoxy matrix interaction on mechanical, electrical and transport properties of structural advanced materials

The focus of this study is to design new nano-modified epoxy formulations using carbon nanofillers, such as carbon nanotubes, carbon nanofibers and graphene-based nanoparticles (CpEG), that reduce the moisture content and provide additional functional performance. The chemical structure of epoxy mixture, using a non-stoichiometric amount of hardener, exhibits unique properties in regard to the water sorption for which the equilibrium concentration of water (C eq) is reduced up to a maximum of 30%. This result, which is very relevant for several industrial applications (aeronautical, shipbuilding industries, wind turbine blades, etc), is due to a strong reduction of the polar groups and/or sites responsible to bond water molecules. All nanofillers are responsible of a second phase at lower glass transition temperature (Tg). Compared with other carbon nanofillers, functionalized graphene-based nanoparticles exhibit the best performance in the multifunctionality. The lowest moisture content, the high performance in the mechanical properties, the low electrical percolation threshold (EPT) have been all ascribed to particular arrangements of the functionalized graphene sheets embedded in the polymeric matrix. Exfoliation degree and edge carboxylated groups are responsible of self-assembled architectures which entrap part of the resin fraction hindering the interaction of water molecules with the polar sites of the resin, also favouring the EPT paths and the attractive/covalent interactions with the matrix.

Cold-Curing Epoxy Resins: Aging and Environmental Effects. I - Thermal Properties

The effects of physical and chemical aging have been investigated on two cold-curing epoxy adhesives. One of the two has been physically aged at different temperatures below its glass transition (i.e. 10, 23, 30 and 40°C) and then subjected to enthalpy relaxation measurements in a differential scanning calorimeter (DSC). According to recent literature, DSC studies have suggested that enthalpy relaxation gradually increases with aging time to a limiting value, depending on the temperature, until structural equilibrium is reached. Specimens of both adhesives, previously cured at room temperature for 10 days, have been aged in a dry-dark atmosphere or in a freezer (-20°C) for a prolonged time, measuring Tg and enthalpy relaxation at different time spans. When the structural equilibrium is reached, the samples aged in the dark-dry atmosphere exhibit Tg and relaxation peak values similar to those found in the earlier analysis for samples isothermally cured at 23°C. Samples aged at -20°C, the curing time suggested by suppliers, which was not sufficient to cure the adhesives completely, have glass transition temperatures that are always lower. The cured adhesives have also been exposed to natural weather for up to 36 months. Some samples, taken at different periods of time, have been analyzed in DSC. Other samples have been subjected to a de-aging procedure (24 hours at 50°C) before the thermal analysis. The natural exposition, regarded as chemical aging, determines a cycling change of Tg of both adhesives around average values slightly lower than the initial ones. This behavior cannot be erased by the de-aging procedure, which is able to delete only the effects due to physical aging. Enthalpy relaxation peaks, characteristics of physical aging, are observed only for exposed samples not subjected to de-aging procedure. These samples, finally, are subjected during the natural exposition to aging and de-aging processes taking place in non-isothermal conditions and, therefore, not predictable. The presented study must be regarded as a qualitative analysis of the aging phenomena taking place in cold curing epoxy resins.

Behavior of epoxy composite resins in environments at high moisture content

Three different organo-modified clays have been incorporated by sonication into a high performance epoxy resin before the cross-linking reaction. The X-ray analysis indicated that, depending on the organoclay type, partially exfoliated and partially intercalated composites have been obtained. As shown by the DSC analysis, the clay addition seems to interact with the cross-linking reaction. The incorporation of organoclay into epoxy increased free volume and micro-voids in the samples. Sorption of water in the composite samples resulted higher than that of the pristine resin, whereas the diffusion coefficient is significantly lower. The lower value of diffusion makes the permeability at ambient conditions lower than the pristine resin. The elastic modulus of the composite sample results higher than that of the pristine resin, especially in the temperature region around the glass transition. The presence of organoclay in epoxy matrix decreased the glass transition temperature, whether the nanocomposites were in a dry or wet condition.

Toughening of epoxy adhesives by combined interaction of carbon nanotubes and silsesquioxanes

The extensive use of adhesives in many structural applications in the transport industry and particularly in the aeronautic field is due to numerous advantages of bonded joints. However, still many researchers are working to enhance the mechanical properties and rheological performance of adhesives by using nanoadditives. In this study the effect of the addition of Multi-Wall Carbon Nanotubes (MWCNTs) with Polyhedral Oligomeric Silsesquioxane (POSS) compounds, either Glycidyl Oligomeric Silsesquioxanes (GPOSS) or DodecaPhenyl Oligomeric Silsesquioxanes (DPHPOSS) to Tetraglycidyl Methylene Dianiline (TGMDA) epoxy formulation, was investigated. The formulations contain neither a tougher matrix such as elastomers nor other additives typically used to provide a closer match in the coefficient of thermal expansion in order to discriminate only the effect of the addition of the above-mentioned components. Bonded aluminium single lap joints were made using both untreated and Chromic Acid Anodisation (CAA)-treated aluminium alloy T2024 adherends. The effects of the different chemical functionalities of POSS compounds, as well as the synergistic effect between the MWCNT and POSS combination on adhesion strength, were evaluated by viscosity measurement, tensile tests, Dynamic Mechanical Analysis (DMA), single lap joint shear strength tests, and morphological investigation. The best performance in the Lap Shear Strength (LSS) of the manufactured joints has been found for treated adherends bonded with epoxy adhesive containing MWCNTs and GPOSS. Carbon nanotubes have been found to play a very effective bridging function across the fracture surface of the bonded joints.

Nanocomposites Based on PCL and Halloysite Nanotubes Filled with Lysozyme: Effect of Draw Ratio on the Physical Properties and Release Analysis

Halloysite nanotubes (HNTs) were loaded with lsozyme, as antimicrobial molecule, at a HNTs/lysozyme ratio of 1:1. Such a nano-hybrid was incorporated into a poly (ε-caprolactone) (PCL) matrix at 10 wt % and films were obtained. The nano-composites were submitted to a cold drawn process at three different draw ratios, λ = 3, 4, and 5, where λ is l(final length)/l0(initial length). Morphology, physical, and barrier properties of the starting nanocomposite and drawn samples were studied, and correlated to the release of the lysozyme molecule. It was demonstrated that with a simple mechanical treatment it is possible to obtain controlled release systems for specific active packaging requirements.

Dynamic Mechanical Properties of Structural Self-Healing Epoxy Resins

In this paper, we report the study and characterization of a multifunctional autonomically healing composite containing solid particles of Grubbs’ first generation catalyst and poly(urea-formaldehyde) microcapsules filled with liquid DCPD. This system, already reported in literature, in some respects shows great potential for epoxy structural composites: however, other aspects have to be explored in order to put to use in advanced applications. Here, we have determined the curing process to obtain the best mechanical performance without deactivating the self-repair activity of the material. It has been found that, for the same curing cycle, the presence of catalyst powder causes a slight decrease in the elastic modulus value with respect to the epoxy matrix. A large recovery in this performance is gained for the self-healing specimen, proving that the microcapsules contribute to improve the mechanical characteristics of the self-healing sample.