Estudo da influência das condições de síntese nas propriedades de híbridos orgânico-inorgânicos puros e modificados por Ce(IV)
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Data
2018-06-11
Autores
Santos, Fábio Cesar dos [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
Revestimentos híbridos orgânicos-inorgânicos para proteção contra corrosão de ligas de alumínio são alternativas promissoras para os atuais métodos baseados na passivação com cromatos. A partir da otimização da eficiência de polimerização (Ep) da fase orgânica e utilizando o processo sol-gel, foram preparados híbridos com diferentes razões polímero/sílica e concentrações de cério, sendo estudados em termos da nanoestrutura híbrida formada e seu efeito na estabilidade térmica e desempenho anticorrosivo dos revestimentos. Os híbridos foram preparados pela polimerização radicalar em tetrahidrofurano (THF) do metacrilato de metila (MMA) e 3-metacriloxipropiltrimetoxisilano (MPTS), utilizando o peróxido de benzoíla (BPO) como iniciador térmico, seguida da hidrólise e policondensação ácida do tetra etoxi silano (TEOS) e MPTS. O nitrato de cério e amônio (CAN) foi adicionado a etapa de hidrólise/policondensação para obtenção das amostras dopadas. Após a mistura das fases orgânica e inorgânica, as soluções híbridas foram utilizadas para depositar via dip-coating os revestimentos siloxano-PMMA sobre a liga de alumínio Al2024-T3. Na primeira etapa, utilizando um planejamento fatorial de experimentos, foram estudadas 4 variáveis na reação de polimerização da fase orgânica (temperatura, razão BPO/MMA, tempo e razão MMA/MPTS) e os parâmetros otimizados foram fixados para uma investigação detalhada da razão molar polímero/sílica (MMA/MPTS e TEOS/MPTS) na faixa de 2 a 10. No segundo momento, uma das condições otimizadas MMA/TEOS foi dopada com diferentes concentrações de cério. As propriedades nanoestruturais obtidas com o espalhamento de raios-X em baixos ângulos (SAXS) dos filmes não suportados, foram correlacionadas com a estabilidade térmica avaliada pela termogravimetria (TG), e com a proteção contra a corrosão dos revestimentos híbridos (2 - 6 µm espessura), avaliada por espectroscopia de impedância eletroquímica (EIS) em solução salina/ácida (NaCl 3,5 % + HCl pH 3). Os resultados de TG, espectroscopia Raman e de fotoelétrons excitados por raios-X mostraram que maior razão BPO/MMA induz maior polimerização do que o tempo de reação prolongado e que ambos são muito mais eficazes que a temperatura e razão MMA/MPTS. A termogravimetria mostrou alta estabilidade térmica (temperatura de perda de 5 % de massa em 287 °C) para as composições híbridas com menores frações de sílica, que apresentaram menor distância entre os domínios siloxano presentes na matriz polimérica. A adição de cério aos híbridos também promoveu aumento da distância dentre os domínios siloxano e consequentemente promoveu a diminuição da estabilidade térmica. As medidas eletroquímicas mostraram que os revestimentos com Ep próximo a 100 % e uma relação MMA/TEOS igual a 7 proporcionaram a melhor proteção contra corrosão com módulo de impedâcia (|Z|) de 25 GΩ cm2, sendo cerca de 7 ordens de grandeza maior que o |Z| do substrato Al2024-T3 não recoberto. No caso dos revestimentos dopados, observou-se que a maior resistência contra a corrosão (> |Z|) foi obtida para as amostras em que a espectroscopia de absorção de raios-X (XAS) identificou maior abundância do cério no estado de oxidação Ce4+.
Organic-inorganic hybrid coatings for corrosion protection of aluminum alloys are promising alternatives to the current methods based on chromate passivation. From the optimization of polymerization efficiency (Ep) of the organic phase and using the sol-gel process, hybrids with different polymer/silica ratios and cerium concentrations were prepared, being studied in terms of the hybrid nanostructure formed and its effect on the thermal stability and anticorrosive performance of the coatings. Hybrids were prepared by the radical polymerization in tetrahydrofuran (THF) of the methyl methacrylate (MMA) and 3- methacryloxypropyltrimethoxysilane (MPTS) using benzoyl peroxide (BPO) as thermal initiator, followed by hydrolysis and acid polycondensation of tetraethoxy silane (TEOS) and MPTS. The ceric ammonium nitrate (CAN) was added to the hydrolysis/polycondensation step to obtain the doped samples. After mixing the organic and inorganic phases, the hybrid solutions were used to prepare by dip-coating the siloxane-PMMA coatings onto the Al2024- T3 aluminum alloy. In the first step, using a factorial design of experiments, 4 variables were studied in the polymerization reaction of the organic phase (temperature, BPO/MMA ratio, time and MMA/MPTS ratio) and the optimized parameters were fixed for a detailed investigation of the polymer/silica molar ratio (MMA/MPTS and TEOS/MPTS) in the range of 2 to 10. In the second moment, one of the optimized MMA/TEOS conditions was doped with different concentrations of cerium. The nanostructural properties obtained with the small angle X-ray scattering (SAXS) of the unsupported films were correlated with its thermal stability evaluated by thermogravimetry (TG) and with the corrosion protection of the hybrid coatings (2-6 μm thickness), evaluated by electrochemical impedance spectroscopy (EIS) in saline/acid solution (NaCl 3.5 % + HCl pH 3). The results of TG, Raman and X-ray photoelectron spectroscopy showed that higher BPO/MMA ratio induces higher polymerization than prolonged reaction time and both are much more effective than the temperature and MMA/MPTS ratio. Thermogravimetry showed high thermal stability (temperature of 5% mass loss at 287 °C) for the hybrid compositions with lower silica fractions, which presented smaller distance between the siloxane domains present in the polymer matrix. The addition of cerium to the hybrids also increased the distance between the siloxane domains and consequently promoted a decrease in thermal stability. Electrochemical measurements showed that coatings with Ep close to 100 % and MMA/TEOS ratio of 7 provided the best corrosion protection with impedance modulus (|Z|) of 25 GΩ cm2, being about 7 orders of magnitude higher than the |Z| of the uncoated Al2024-T3 substrate. In the case of doped coatings, it was observed that the higher corrosion protection (> |Z|) was obtained for the samples in which the x-ray absorption spectroscopy (XAS) identified a greater abundance of cerium in the oxidation state Ce4+.
Organic-inorganic hybrid coatings for corrosion protection of aluminum alloys are promising alternatives to the current methods based on chromate passivation. From the optimization of polymerization efficiency (Ep) of the organic phase and using the sol-gel process, hybrids with different polymer/silica ratios and cerium concentrations were prepared, being studied in terms of the hybrid nanostructure formed and its effect on the thermal stability and anticorrosive performance of the coatings. Hybrids were prepared by the radical polymerization in tetrahydrofuran (THF) of the methyl methacrylate (MMA) and 3- methacryloxypropyltrimethoxysilane (MPTS) using benzoyl peroxide (BPO) as thermal initiator, followed by hydrolysis and acid polycondensation of tetraethoxy silane (TEOS) and MPTS. The ceric ammonium nitrate (CAN) was added to the hydrolysis/polycondensation step to obtain the doped samples. After mixing the organic and inorganic phases, the hybrid solutions were used to prepare by dip-coating the siloxane-PMMA coatings onto the Al2024- T3 aluminum alloy. In the first step, using a factorial design of experiments, 4 variables were studied in the polymerization reaction of the organic phase (temperature, BPO/MMA ratio, time and MMA/MPTS ratio) and the optimized parameters were fixed for a detailed investigation of the polymer/silica molar ratio (MMA/MPTS and TEOS/MPTS) in the range of 2 to 10. In the second moment, one of the optimized MMA/TEOS conditions was doped with different concentrations of cerium. The nanostructural properties obtained with the small angle X-ray scattering (SAXS) of the unsupported films were correlated with its thermal stability evaluated by thermogravimetry (TG) and with the corrosion protection of the hybrid coatings (2-6 μm thickness), evaluated by electrochemical impedance spectroscopy (EIS) in saline/acid solution (NaCl 3.5 % + HCl pH 3). The results of TG, Raman and X-ray photoelectron spectroscopy showed that higher BPO/MMA ratio induces higher polymerization than prolonged reaction time and both are much more effective than the temperature and MMA/MPTS ratio. Thermogravimetry showed high thermal stability (temperature of 5% mass loss at 287 °C) for the hybrid compositions with lower silica fractions, which presented smaller distance between the siloxane domains present in the polymer matrix. The addition of cerium to the hybrids also increased the distance between the siloxane domains and consequently promoted a decrease in thermal stability. Electrochemical measurements showed that coatings with Ep close to 100 % and MMA/TEOS ratio of 7 provided the best corrosion protection with impedance modulus (|Z|) of 25 GΩ cm2, being about 7 orders of magnitude higher than the |Z| of the uncoated Al2024-T3 substrate. In the case of doped coatings, it was observed that the higher corrosion protection (> |Z|) was obtained for the samples in which the x-ray absorption spectroscopy (XAS) identified a greater abundance of cerium in the oxidation state Ce4+.
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Palavras-chave
Processo sol-gel, Nanocompósitos (Materiais), Nanoestrutura, Revestimentos protetores, Corrosão e anticorrosivos, Híbridos orgânico-inorgânicos, Revestimentos híbridos orgânico-inorgânicos, Caracterização estrutural, Proteção contra a corrosão, Proteção anticorrosiva, Dopagem com cério