RESUMO A disponibilidade de novos materiais, processos e equipamentos sempre traz perspectivas favoráveis ao mercado. A busca por processos alternativos e pela redução do custo da obtenção de precursores de materiais de alto desempenho tem se tornado primordial para o desenvolvimento do setor industrial de produtos com valores mais acessíveis, visando suprir a demanda de fibras de carbono de uso geral em um mercado que vem apresentando contínua expansão. As fibras de poliacrilonitrila (PAN) obtidas pelo processo de extrusão mostra potencial na redução de custo como precursor de fibras de carbono, em relação ao processo via fiação a úmido utilizado atualmente. Essa tecnologia inovadora permite a fiação por fusão da PAN empregando a glicerina (proveniente da produção do biodiesel) como seu principal plastificante, e dispensando o emprego de solventes altamente tóxicos. Assim sendo, o presente trabalho visou obter copolímeros de PAN que fossem adequados ao processo de fiação por extrusão convencional. Para tanto, foi utilizado para a obtenção dos copolímeros de PAN o processo de polimerização em suspensão aquosa e a caracterização foi realizada por meio da técnica de Cromatografia de Permeação em Gel (GPC). Os resultados mostraram que a técnica de GPC foi apropriada para determinar as propriedades moleculares dos copolímeros de PAN e que o comonômero estireno (ES) foi adequado para a polimerização, gerando copolímeros de PAN-co-ES com Mn > 40.000 g/mol e Mw > 130.000 g/mol.

ABSTRACT The availability of new materials, processes and equipment always brings favorable perspectives to the market. The search for alternative processes and the precursor production cost reduction of high performance materials has become essential for the development of the industrial activity to produce the general purpose carbon fiber to supply the continuously expanding domestic market. The extrusion process of PAN fibers shows potential of cost reduction as it is a precursor of carbon fibers, in relation to the wet spinning method used up to now. This innovative technology enables spinning by melting PAN using glycerin (from biodiesel production) as the primary plasticizer, dispensing the use of highly toxic solvents. Thus, the present work aimed to obtain PAN copolymers that are suitable for the conventional extrusion spinning process. Therefore, PAN copolymer was copolymerized by aqueous suspension and the characterization was performed by Gel Permeation Chromatography (GPC) analysis. The results showed that GPC technique was suitable for determining of the PAN copolymers molecular properties and that styrene monomer was suitable for polymerization, generating PAN-co-ES copolymers with Mn > 40,000 g / mol and Mw > 130,000 g / mol.

ABSTRACT This paper presents the study and development of a firing test used to evaluate the behavior of a solid rocket motor. The motivation for the development of a subscale solid rocket motor with end burning propellant grain geometry arose from the need to evaluate the nozzle inserts of graphite for the possible replacement with the carbon fiber-reinforced carbon composite. These subscale solid rocket motors, simulating full scale motor operating time, but with mass flow far below, aim to determine the ablative characteristics of composite materials as a function of operating time. The objective was to correlate the mass flow between subscale solid rocket motors and full scale using insert data materials such as graphite and carbon fiber-reinforced carbon composites, which have ablative characteristics determined in subscale solid rocket motors used at the Instituto de Aeronáutica e Espaço. The critical section to evaluate the test device is rocket nozzle throat region. Analysis of the materials of the subsonic and supersonic nozzle insert parts was performed after the burning tests. It was found the formation of a thin layer of material deposited after the test. The deposited coating layer was analyzed by electron dispersive x-ray analysis and scanning electron microscopy. The results analyzed by these methods showed that there were aluminum and carbon in the coating. Finally, the material was analyzed by x-ray diffraction, and the results showed the presence of aluminum oxide. It was also noticed that, because of the unexpected coating deposition forming material in exit conical and throat of the insert that the effect of ablation was not observed.

ABSTRACT Many dynamic engineering systems have interferences and interactions that causes wear under friction effect. Carbonaceous materials, such as polycrystalline graphites, are of great interest in engineering systems due to their inherent lubricant properties and their versatility as a material for applications in the areas of mechanics, transportation, energy systems and sensitive technologies. In this work the tribological behavior of three identical pairs, pin-on-disc, of commercial polycrystalline graphites were evaluated, in a controlled environment at 20 °C and 50 % RH. Results of tribological behavior were correlated considering the prevalence of basal plane, identified by polarized light optical microscopy, as a function of the tribological behavior, measured by tests with pin on disc tribometer. The results showed that the coefficient of friction behavior is directly related to the basal plane mosaic domain and the extension of the damage generated on this plane due to the contact between the tribological pair. The materials studied presented an increase in the friction coefficient values, 0,10; 0;13 and 0,23 with increasing area of the basal plane mosaic domain, respectively 55; 64 and 81% respectively.

A low cost and environmental friendly extrusion process of the Polyacrylonitrile (PAN) polymer was viabilized by using the 1,2,3-propanetriol (glycerol) as a plasticizer. The characterization of the fibers obtained by this process was the object of study in the present work. The PAN fibers were heat treated in the range of 200 °C to 300 °C, which is the temperature range related to the stabilization/oxidation step. This is a limiting phase during the carbon fiber processing. The characterization of the fibers was made using infrared spectroscopy, thermal analysis and microscopy. TGA revealed that the degradation of the extruded PAN co-VA fibers between 250 °C and 350 °C, corresponded to a 9% weight loss to samples analyzed under oxidizing atmosphere and 18% when the samples were analyzed under inert atmosphere. DSC showed that the exothermic reactions on the extruded PAN co-VA fibers under oxidizing synthetic air was broader and the cyclization started at a lower temperature compared under inert atmosphere. Furthermore, FT-IR analysis correlated with thermal anlysis showed that the stabilization/oxidation process of the extruded PAN fiber were coherent with other works that used PAN fibers obtained by other spinning processes.

Carbon fiber reinforced carbon (CFRC) composites, also called carbon/carbon (C/C) composites are materials with superior characteristics such as low density, good thermal shock resistance, high strength and low ablation under severe environments. Due to their properties, CFRC composites are ideal candidate in the high temperature fields. By pyrolysis process, carbon fiber phenolic resin composites are converted in C/C composites. The phenolic resin is a non-conductor (electrical resistivity of 1012Ω.m) and during heat treatment of carbon fiber phenolic resin composite, it is converted to a carbon matrix (a conductor). This conversion was accomplished by electrical resistivity measurements using four-probe method according to ASTM C611-98 with final electrical resistivity of 0.04 mΩ.m. Microstructure of carbon fiber phenolic resin composite was assessed by using optical microscopy and image analysis. Pore volume was evaluated and the results were compared with thermal gravimetric analyses. The values of activation energy (Ea) during pyrolysis were 1.153kJ.mol–1 and 10.860kJ.mol–1.

Polyacrylonitrile fiber encompasses a broad range of products based on acrylonitrile (AN) which is readily copolymerized with a wide range of ethylenic unsaturated monomers giving rise to polymers with different characteristics and applications. Such products can be designed for cost-effective, flame and heat resistant solutions for the textile industry, aircraft and automotive markets. In the present work acrylonitrile was copolymerized with vinylidene chloride (VDC) by conventional suspension polymerization process via redox system, with an initial content of 10%/mass of the VDC monomer. The copolymer average molecular weight was obtained by Gel Permeation Chromatography (GPC) and by intrinsic viscosity analysis. To control the polymerization process continuously, qualitative and quantitative analysis of the chloride content in the PAN AN/VDC copolymer structure was accomplished by using X-ray fluorescence and potentiometric titration techniques. A good correlation was found between these two techniques, leading to a straightforward verification of VDC in the polymer structure. The thermal behavior of PAN AN/VDC copolymer was performed by Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). The results showed that VDC monomers exhibited a nearly stoichiometric reaction with acrylonitrile, copolymerizing about 90% of its initial mass. VDC changed significantly the polyacrylonitrile thermal behavior, decreasing the polymer degradation temperature by about 40-50°C.

Nano-carbon materials, such as carbon nanotubes and carbon nanofibers, are being thought to be used as multifunctional reinforcement in composites. The growing of carbon nanofiber at the carbon fiber/epoxy interface results in composites having better electrical properties than conventional carbon fiber/epoxy composites. In this work, carbon nanofibers were grown in situ over the surface of a carbon fiber fabric by chemical vapor deposition. Specimens of carbon fiber/nanofiber/epoxy (CF/CNF/epoxy) composites were molded and electrical conductivity was measured. Also, the CF/CNF/epoxy composites were tested under flexure and interlaminar shear. The results showed an overall reduction in mechanical properties as a function of added nanofiber, although electrical conductivity increased up to 74% with the addition of nanofibers. Thus CF/CNF/epoxy composites can be used as electrical dissipation discharge materials.

Abstract: Advanced carbon fiber hybrid carbon-ceramic matrix composites are realizing their potential in many thermostructural components for aerospace vehicles. This work presents ab-initio predictions of elastic constants and thermal properties for 2.5D carbon fiber reinforced carbon-silicon carbide hybrid matrix composites, by using the homogenization technique. The homogenization technique takes properties of individual components of the composites (fiber and matrix) and characteristics of the geometrical architecture of the preform to perform calculations. Ab-initio modeling of mechanical and thermal properties is very attractive, especially during the material development stage, when larger samples may be prohibitively expensive or impossible to fabricate. Modeling is also useful when bigger samples would be prohibitively expensive or impractical. Thermostructural composites made of 2.5D preforms are easy to manufacture in relation to 3D preforms. Besides, 2.5D preforms are also resistant to thermo cycling and have high resistance to crack propagation in relation to ply stacked composites such as unidirectional (1D) and bidirectional (2D) structures. The calculations were performed by setting an overall carbon fiber volume fraction at 40, 45 and 50 for a 2D stacked composite, and volume fraction in Z-direction of 2, 4 and 6.

To address a critical aspect of the fast growing use of composites in aircraft and aerospace industry, the influence of the porosity on the shear strength of composites property was investigated as a mean for determining the critical values of porosity. Acid digestion techniques were applied to determine the void volume ratio of two families of carbon epoxy laminates (8 and 16 plies). Ultrasonic inspections revealed the corresponding attenuation coefficients. The void morphology was investigated by optical microscopy. Results from Interlaminar shear and Iosipescu shear tests were correlated with the attenuation coefficient to determine critical values of porosity using a modified Mar-Lin fracture criteria. It has been shown that the shear strength decrease with the increase of void volume ratio and the effects are more significant in thicker laminates. This work showed that by using the Mar-Lin criteria the singularity order, which is an indicative of the sensibility to voids in composites, is dependent of type of loading and void distribution.

Polycrystalline graphites are widely used in the metallurgical, nuclear and aerospace industries. Graphites are particulated composites manufactured with a mixture of coke with pitch, and changes in relative proportions of these materials cause modifications in their mechanical properties. Uniaxial tension tests must be avoided for mechanical characterization in this kind of brittle material, due to difficulties in making the relatively long specimens and premature damages caused during testing set-up. On other types of tests, e.g. bending tests, the specimens are submitted to combined stress states (normal and transverse shear stresses). The Iosipescu shear test, is performed in a beam with two 90° opposite notches machined at the mid-length of the specimens, by applying two forces couples, so that a pure and uniform shear stress state is generated at the cross section between the two notches. When a material is isotropic and brittle, a failure at 45° in relation to the beam long axis can take place, i.e., the tensile normal stress acts parallel to the lateral surface of the notches, controls the failure and the result of the shear test is numerically equivalent to the tensile strength. This work has evaluated a graphite of the type used in rocket nozzles by the Iosipescu test and the resulted stress, ~11 MPa, was found to be equal to the tensile strength. Thus, the tensile strength can be evaluated just by a single and simple experiment, thus avoiding complicated machining of specimen and testing set-up.

Ceramic matrix composites (CMCs), constituted of a silicon boron oxycarbide (SiBCO) matrix and unidirectional carbon fiber rods as a reinforcement phase, were prepared by pyrolysis of carbon fiber rods wrapped in polysiloxane (PS) or poly(borosiloxane) (PBS) matrices. The preparation of the polymeric precursors involved hydrolysis/condensation reactions of alkoxysilanes in the presence and absence of boric acid, with B/Si atomic ratios of 0.2 and 0.5. Infrared spectra of PBS showed evidence of Si-O-B bonds at 880 cm-1, due to the incorporation of the crosslinker trigonal units of BO3 in the polymeric network. X ray diffraction analyses exhibited an amorphous character of the resulting polymer-derived ceramics obtained by pyrolysis up to 1000 °C under inert atmosphere. The C/SiBCO composites showed better thermal stability than the C/SiOC materials. In addition, good adhesion between the carbon fiber and the ceramic phase was observed by SEM microscopy

Composite structures for aerospace applications are mainly made by the well-known prepreg technology. In order to achieve adequate prepreg processing schedules, and consequently maximum fiber strength utilization, one has to know in deep the cure kinetics of matrix, which held the fibers together. This work describes a procedure to study the cure kinetic and has as example how aromatic amine hardeners influence the cure kinetics of an epoxy resin used in advanced composites. The investigation was carried out by using the DSC technique and it was found that depending on the system used the cure kinetics of the formulation obeys order n or autocatalytic order.

Carbon fibres and glass fibres are reinforcements for advanced composites and the fiber strength is the most influential factor on the strength of the composites. They are essentially brittle and fail with very little reduction in cross section. Composites made with these fibres are characterized by a high strength/density ratio and their properties are intrisically related to their microstructure, i.e., amount and orientation of the fibres, surface treatment, among other factors. Processing parameters have an important role in the fibre mechanical behaviour (strength and modulus). Cracks, voids and impurities in the case of glass fibres and fibrillar misalignments in the case of carbon fibres are created during processing. Such inhomogeneities give rise to an appreciable scatter in properties. The most used statistical tool that deals with this characteristic variability in properties is the Weibull distribution. The present work investigates the influence of the testing gage length on the strength, Young's modulus and Weibull modulus of carbon fibres and glass fibres. The Young's modulus is calculated by two methods: (i) ASTM D 3379M, and (ii) interaction between testing equipment/specimen The first method resulted in a Young modulus of 183 GPa for carbon fibre, and 76 GPa for glass fibre. The second method gave a Young modulus of 250 GPa for carbon fibre and 50 GPa for glass fibre. These differences revelead differences on how the interaction specimen/testing machine can interfere in the Young modulus calculations. Weibull modulus can be a tool to evaluate the fibre's homogeneity in terms of properties and it is a good quality control parameter during processing. In the range of specimen gage length tested the Weibull modulus for carbon fibre is ~ 3.30 and for glass fibres is ~ 5.65, which indicates that for the batch of fibres tested, the glass fibre is more uniform in properties.

Phenil glycidyl ether (PGE), a monofunctional diluent, has been used in epoxy resins formulations in order to increase the toughness of the epoxy molded composite. In a systematic study concerning its influence in the cure kinetics of the epoxy resin, it was used in concentrations of 2,5; 5,0; 10 and 20% in relation to a diglycidyl ether bisphenol-A (DGEBA)/diamino diphenil-sulfone (DDS) base matrix. Dynamic and isothermal scanning analysis were carried out using a differential scanning calorimety (DSC) equipment. For all the concentrations of PGE, a n order kinetics was observed, with n varing between 0,35 -- 0,91 as a function of the increase in the PGE concentration.

Os compósitos estruturais com reforço uni ou bi-direcional, ao contrário dos materiais homogêneos e isotrópicos, apresentam tanto baixa resistência ao cisalhamento interlaminar (< 60 MPa) quanto baixa tenacidade à fratura interlaminar. Como decorrência disso estes compósitos são susceptíveis de falha sob baixo carregamento mecânico, e por esse motivo a utilização destes materiais está confinada a componentes estruturais de responsabilidade secundária. A utilização de matrizes com maior tenacidade à fratura em compósitos produz resultados insatisfatórios para aplicações estruturais, pois o ganho na resistência à fratura interlaminar é insuficiente. A utilização de reforços multi-direcionais (preformas) têm gerado resultados significativos nesse particular fazendo com que as pesquisas sejam direcionadas para essa tecnologia. O presente trabalho descreve os métodos que estão sendo utilizados para obtenção de preformas multi-direcionais, tanto para aplicação em compósitos poliméricos quanto para compósitos termo-estruturais. A adição de uma fração volumétrica de fibras equivalente a ~1 - 2% na direção normal ao plano do compósito é suficiente para conferir tanto resistência ao cisalhamento quanto tenacidade à fratura interlaminar, sem prejudicar as propriedades originais em outras direções. A utilização de preformas multi-direcionais permite também maior isotropia de propriedades, maior tenacidade à fratura e resistência às tensões térmicas durante o processamento e em serviço.

Stacked laminated composites structures (uni and bi-direcional), unlike homogeneous isotropic materials, exhibit low shear strength (< 60 MPa) and low interlaminar fracture thoughness. As a consequence, premature failure during service can occur and their use are restricted mainly to secondary non-load bearing structures. The use of high toughness matrices in composites is not sufficient to improve the interlaminar fracture toughness. Presently, the use of multi-axis preforms have shown significant improvements in interlaminar fracture toughness of composites and a lot of research has been done using this technology. The present work describes methods to obtain multi-axis preforms for either polymer or thermo-structural composites. Many investigations have shown that addition of a ~1-2% fibre volume fraction normal to main fibre plane is enough to provide an adequate shear and interlaminar fracture resistance for the composite keeping in-plane properties. Multi-axis preforms provide higher isotropy, higher fracture toughness and resistance to thermal stresses during processing and service.