A Study of the Effect of Nitrogen and Air Plasma Immersion Ion Implantation Treatments on the Properties of Carbon Fiber

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A review on the development and properties of continuous fiber/epoxy/aluminum hybrid composites for aircraft structures

Weight reduction and improved damage tolerance characteristics were the prime drivers to develop new family of materials for the aerospace/ aeronautical industry. Aiming this objective, a new lightweight Fiber/ Metal Laminate (FML) has been developed. The combination of metal and polymer composite laminates can create a synergistic effect on many properties. The mechanical properties of FML shows improvements over the properties of both aluminum alloys and composite materials individually. Due to their excellent properties, FML are being used as fuselage skin structures of the next generation commercial aircrafts. One of the advantages of FML when compared with conventional carbon fiber/epoxy composites is the low moisture absorption. The moisture absorption in FML composites is slower when compared with polymer composites, even under the relatively harsh conditions, due to the barrier of the aluminum outer layers. Due to this favorable atmosphere, recently big companies such as EMBRAER, Aerospatiale, Boing, Airbus, and so one, starting to work with this kind of materials as an alternative to save money and to guarantee the security of their aircrafts.

Atmospheric plasma treatment of carbon fibers for enhancement of their adhesion properties

Plasma processing of carbon fibers (CFs) is aimed to provide better contact and adhesion between individual plies without decrease in the CF mechanical resistance. This paper deals with surface modification of CFs by an atmospheric pressure dielectric barrier discharge (DBD) for enhancing the adhesion between the CF and the polymeric matrix. The scanning electron microscopy of the treated samples revealed many small particles distributed over entire surface of the fiber. These particles are product of the fiber surface etching during the DBD treatment that removes the epoxy layer covering as-received samples. The alteration of the CF surface morphology was also confirmed by the Atomic force microscopy (AFM), which indicated that the CF roughness increased as a result of the plasma treatment. The analysis of the surface chemical composition provided by X-ray photoelectron spectroscopy showed that oxygen and nitrogen atoms are incorporated onto the surface. The polar oxygen groups formed on the surface lead to the increasing of the CF surface energy. The results of interlaminar shear strength test (short beam) of CFs/polypropylene composites demonstrated a greater shear resistance of the composites made with CFs treated by DBD than the one with untreated fibers. Both the increase in surface roughness and the surface oxidation contribute for the enhancement of CF adhesion properties. © 2012 IEEE.

Evaluation of the chemical interaction between carbon nanotubes functionalized with TGDDM tetrafunctional resin and hardener DDS

In this work, the chemical interaction between carbon nanotubes (MWCNT) functionalized with acyl chloride (SOCl2) and polymer chain tetrafuncional N,N,N′,N′-tetraglycidyl-4,4′- diaminodiphenylmethane (TGDDM) and hardener 4,4′diaminodiphenyl sulfone (DDS) has been monitored by Fourier transform infrared spectroscopy (FTIR) with a attenuated total reflectance (ATR) coupled. MWCNT were obtained from the pyrolysis of a mixture of camphor and ferrocene into a oven. The functionalization process was done by oxidative treatment in order to incorporate carboxylic group over the walls of MWCNT, before to be used SOCl2. The functionalized carbon nanotubes were evaluated by X-ray photoelectron spectroscopy (XPS), Raman and transmission electron microscopy (TEM). Nanostructured composites were processed by using epoxy resin with MWCNT in varying percentages. In this work it was observed that different percentages of functionalized nanotubes modify the interaction between the composite matrix and curing agent, where can be observed that in specimens with content less than 1 wt% MWCNT the chemical bond occurs preferentially from the opening of the SO double bond of the hardener and when is used MWCNT content higher than 1 wt% there is little chemical interaction with the SO bond of the hardener and most MWCNT binds to amine. © 2013 Elsevier Ltd.

Benzoxazine Resin/Carbon Nanotube Nanostructured Composite's Degradation Kinetic

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Fatigue crack growth rate in mode I of a carbon fiber 5HS weave composite laminate processed via RTM

Delamination or crack propagation between plies is a critical issue for structural composites. In viewing this issue and the large application of woven fabrics in structural applications, especially the ones that requires high drapeability to be preformed in a RTM mold cavity such as the asymmetric ones, e.g HS series, this research aimed in dynamically testing the carbon fiber 5HS/RTM6 epoxy composites under opening mode using DCB set up in order to investigate the crack growth rate behavior in an irregular surface produced by the fabric waviness. The evaluation of the energy involved in each crack increment was based on the Irwin-Kies equation using compliance beam theory. The tests were conducted at constant stress ratio of R=0.1 with displacement control, frequency of 10 Hz, in accordance to ASTM E647-00 for measurement of crack growth rate. The results showed large scatter when compared to unidirectional carbon fiber composites due to damage accumulation at the fill tows.

Suitability of carbon fiber-reinforced polymers as power cable cores: Galvanic corrosion and thermal stability evaluation

The increasing demand for electrical energy and the difficulties involved in installing new transmission lines presents a global challenge. Transmission line cables need to conduct more current, which creates the problem of excessive cable sag and limits the distance between towers. Therefore, it is necessary to develop new cables that have low thermal expansion coefficients, low densities, and high resistance to mechanical stress and corrosion. Continuous fiber-reinforced polymers are now widely used in many industries, including electrical utilities, and provide properties that are superior to those of traditional ACSR (aluminum conductor steel reinforced) cables. Although composite core cables show good performance in terms of corrosion, the contact of carbon fibers with aluminum promotes galvanic corrosion, which compromises mechanical performance. In this work, three different fiber coatings were tested (phenol formaldehyde resin, epoxy-based resin, and epoxy resin with polyester braiding), with measurements of the galvanic current. The use of epoxy resin combined with polyester braiding provided the best inhibition of galvanic corrosion. Investigation of thermal stability revealed that use of phenol formaldehyde resin resulted in a higher glass transition temperature. On the other hand, a post-cure process applied to epoxy-based resin enabled it to achieve glass transition temperatures of up to 200 degrees C. (C) 2014 Elsevier Ltd. All rights reserved.

Dielectric behaviour of CaCu3Ti4O12-epoxy composites

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Changes in the surface of four calcium silicate-containing endodontic materials and an epoxy resin-based sealer after a solubility test

Aim To compare the changes in the surface structure and elemental distribution, as well as the percentage of ion release, of four calcium silicate-containing endodontic materials with a well-established epoxy resin-based sealer, submitted to a solubility test. Methodology Solubility of AH Plus, iRoot SP, MTA Fillapex, Sealapex and MTA-Angelus (MTA-A) was tested according to ANSI/ADA Specification 57. The deionized water used in the solubility test was submitted to atomic absorption spectrophotometry to determine and quantify Ca2+, Na+, K+, Zn2+, Ni2+ and Pb2+ ions release. In addition, the outer and inner surfaces of nonsubmitted and submitted samples of each material to the solubility test were analysed by means of scanning electron microscopy and energy-dispersive spectroscopy (SEM/EDX). Statistical analysis was performed by using one-way anova and Tukeys post hoc tests (a = 0.05). Results Solubility results, in percentage, sorted in an increasing order were -1.24 +/- 0.19 (MTA-A), 0.28 +/- 0.08 (AH Plus), 5.65 +/- 0.80 (Sealapex), 14.89 +/- 0.73 (MTA Fillapex) and 20.64 +/- 1.42 (iRoot SP). AH Plus and MTA-A were statistically similar (P > 0.05), but different from the other materials (P < 0.05). High levels of Ca2+ ion release were observed in all groups except AH Plus sealer. MTA-A also had the highest release of Na2+ and K+ ions. Zn+2 ion release was observed only with AH Plus and Sealapex sealers. After the solubility test, all surfaces had morphological changes. The loss of matrix was evident and the filler particles were more distinguishable. EDX analysis displayed high levels of calcium and carbon at the surface of Sealapex, MTA Fillapex and iRoot SP. Conclusions AH Plus and MTA-A were in accordance with ANSI/ADAs requirements regarding solubility whilst iRoot SP, MTA Fillapex and Sealapex did not fulfil ANSI/ADAs protocols. High levels of Ca2+ ion release were observed in all materials except AH Plus. SEM/EDX analysis revealed that all samples had morphological changes in both outer and inner surfaces after the solubility test. High levels of calcium and carbon were also observed at the surface of all materials except AH Plus and MTA-A.

Experimental characterization of carbon-fiber-reinforced polymer laminates

The goal of this thesis is to make static tensile test on four Carbon Fiber Reinforced Polymer laminates, in such a way as to obtain the ultimate tensile strength of these laminates; in particular, the laminates analyzed were produced by Hand Lay-up technology. Testing these laminates we have a reference point on which to compare other laminates and in particular CFRP laminate produced by RTM technology.

Mode II fracture energy in the adhesive bonding of dissimilar substrates: carbon fibre composite to aluminium joints

The end-notched flexure (ENF) test calculates the value of mode II fracture energy in adhesive bonding between the substrates of same nature. Traditional methods of calculating fracture energy in the ENF test are not suitable in cases where the thickness of the adhesive is non-negligible compared with adherent thicknesses. To address this issue, a specific methodology for calculating mode II fracture energy has been proposed in this paper. To illustrate the applicability of the proposed method, the fracture energy was calculated by the ENF test for adhesive bonds between aluminium and a composite material, which considered two different types of adhesive (epoxy and polyurethane) and various surface treatments. The proposed calculation model provides higher values of fracture energy than those obtained from the simplified models that consider the adhesive thickness to be zero, supporting the conclusion that the calculation of mode II fracture energy for adhesives with non-negligible thickness relative to their adherents should be based on mathematical models, such as the method proposed in this paper, that incorporate the influence of this thickness.

Automatic quantification of matrix cracking and fiber rotation by X-ray computed tomography in shear-deformed carbon fiber-reinforced laminates

The deformation and damage mechanisms of carbon fiber-reinforced epoxy laminates deformed in shear were studied by means of X-ray computed tomography. In particular, the evolution of matrix cracking, interply delamination and fiber rotation was ascertained as a function of the applied strain. In order to provide quantitative information, an algorithm was developed to automatically determine the crack density and the fiber orientation from the tomograms. The investigation provided new insights about the complex interaction between the different damage mechanisms (i.e. matrix cracking and interply delamination) as a function of the applied strain, ply thickness and ply location within the laminate as well as quantitative data about the evolution of matrix cracking and fiber rotation during deformation

New insights into NO-carbon and N2O-carbon reactions from quantum mechanical calculations

Density functional theory calculations were used to investigate the mechanisms of NO-carbon and N2O-carbon reactions. It was the first time that the importance of surface nitrogen groups was addressed in the kinetic behaviors of the NO-carbon reaction. It was found that the off-plane nitrogen groups that are adjacent to the zigzag edge sites and in-plane nitrogen groups that are located on the armchair sites make the bond energy of oxygen desorption even ca. 20% lower than that of the off-plane epoxy group adjacent to zigzag edge sites and in-plane o-quinone oxygen atoms on armchair sites; this may explain the reason why the experimentally obtained activation energy of the NO-carbon reaction is ca. 20% lower than that of the O-2-carbon reaction over 923 K. A higher ratio of oxygen atoms can be formed in the N2O-carbon reaction, because of the lower dissociation energy of N2O, which results in a higher ratio of off-plane epoxy oxygen atoms. The desorption energy of semiquinone with double adjacent off-plane oxygen groups is ca. 20% less than that of semiquinone with only one adjacent off-plane oxygen group. This may be the reason why the activation energy of N2O is also ca. 20% less than that of the O-2-carbon reaction. The new mechanism can also provide a good qualitative comparison for the relative reaction rates of NO-, N2O-, and O-2-carbon reactions. The anisotropic characters of these gas-carbon reactions can also be well explained.

MOLECULAR MODELING OF EPON 862-DETDA / CARBON COMPOSITES

The thermoset epoxy resin EPON 862, coupled with the DETDA hardening agent, are utilized as the polymer matrix component in many graphite (carbon fiber) composites. Because it is difficult to experimentally characterize the interfacial region, computational molecular modeling is a necessary tool for understanding the influence of the interfacial molecular structure on bulk-level material properties. The purpose of this research is to investigate the many possible variables that may influence the interfacial structure and the effect they will have on the mechanical behavior of the bulk level composite. Molecular models are established for EPON 862-DETDA polymer in the presence of a graphite surface. Material characteristics such as polymer mass-density, residual stresses, and molecular potential energy are investigated near the polymer/fiber interface. Because the exact degree of crosslinking in these thermoset systems is not known, many different crosslink densities (degrees of curing) are investigated. It is determined that a region exists near the carbon fiber surface in which the polymer mass density is different than that of the bulk mass density. These surface effects extend ~10 Å into the polymer from the center of the outermost graphite layer. Early simulations predict polymer residual stress levels to be higher near the graphite surface. It is also seen that the molecular potential energy in polymer atoms decreases with increasing crosslink density. New models are then established in order to investigate the interface between EPON 862-DETDA polymer and graphene nanoplatelets (GNPs) of various atomic thicknesses. Mechanical properties are extracted from the models using Molecular Dynamics techniques. These properties are then implemented into micromechanics software that utilizes the generalized method of cells to create representations of macro-scale composites. Micromechanics models are created representing GNP doped epoxy with varying number of graphene layers and interfacial polymer crosslink densities. The initial micromechanics results for the GNP doped epoxy are then taken to represent the matrix component and are re-run through the micromechanics software with the addition of a carbon fiber to simulate a GNP doped epoxy/carbon fiber composite. Micromechanics results agree well with experimental data, and indicate GNPs of 1 to 2 atomic layers to be highly favorable. The effect of oxygen bonded to the surface of the GNPs is lastly investigated. Molecular Models are created for systems with varying graphene atomic thickness, along with different amounts of oxygen species attached to them. Models are created for graphene containing hydroxyl groups only, epoxide groups only, and a combination of epoxide and hydroxyl groups. Results show models of oxidized graphene to decrease in both tensile and shear modulus. Attaching only epoxide groups gives the best results for mechanical properties, though pristine graphene is still favored.

Novel bio-based amine curing agents for epoxy resins: the way towards fully bio-based composite materials

Epoxy resins are widely used in many applications, such as paints, adhesives and matrices for composites materials, since they present the possibility to be easily and conveniently tailored in order to display a unique combination of characteristics. In literature, various examples of bio-based epoxy resins produced from a wide range of renewable sources can be found. Nevertheless, the toxicity and safety of curing agents have not been deeply investigated and it was observed that all of them still present some environmental drawback. Therefore, the development of new environmentally friendly fully bio-based epoxy systems is of great importance for designing green and sustainable materials. In this context, the present project aims at further exploring the possibility of using bio-based compounds as curing agents for epoxy resin precursors. A preliminary evaluation of several amine-based compounds demonstrated the feasibility of using Adenine as epoxy resin hardener. In order to better understand the crosslinking mechanism, the reaction of Adenine with the mono-epoxy compound Glycidyl 2-methylphenyl ether (G2MPE), was study by 1H-NMR analysis. Then Adenine was investigated as hardener of Diglycidil ether of bisphenol A (DGEBA), which is the simplest epoxy resin based on bisphenol A, in order to determine the best hardener/resin stoichiometric ratio, and evaluate the crosslinking kinetics and conversion and the final mechanical properties of the cured resin. Then, Adenine was tested as hardener of commercial epoxy resins, in particular the infusion resin Elan-tron® EC 157 (Elantas), the impregnation resin EPON™ Resin 828 (Hexion) and the bio-based resin SUPER SAP® CLR (Entropyresins). Such systems were used for the production of composites materials reinforced with chopped recycled carbon fibers and natural fibers (flax and jute). The thermo-mechanical properties of these materials have been studied in comparison with those ones of composites obtained with the same thermosetting resin reinforced with chopped virgin carbon fibers.