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- Improving Colloidal Stability of Sepiolite Suspensions: Effect of the Mechanical Disperser and Chemical DispersantPublication . Alves, Luís; Ferraz, Eduardo; Santarén, Julio; Rasteiro, Maria G.; Gamelas, José A. F.To allow the use of fibrous-like clays, as sepiolite, in different applications, their disaggregation and the formation of stable suspensions are crucial steps to enhance their performance significantly, e.g., in cellulose nanofibrils/clay composite formulations, enabling an adequate mixture of the matrix and filler individual components. Three distinct physical treatments of dispersion (magnetic stirring, high-speed shearing, and ultrasonication) and four different chemical dispersants (polyacrylate, polyphosphate, carboxymethylcellulose, and alginate, all in the form of sodium salts) were tested to improve the dispersibility and the formation of stable suspensions of sepiolite. Two sepiolite samples from the same origin but with different pre-treatments were evaluated. The particle size and suspension stability were evaluated by dynamic light scattering, zeta potential measurements and optical microscopy. Additionally, the sepiolite samples were initially characterized for their mineralogical, chemical, and morphologic properties. Of the three physical dispersion treatments tested, the ultrasonicator typically produced more stable suspensions; on the other hand, the biopolymer carboxymethylcellulose showed a higher ability to produce stable suspensions, being, however, a smaller particle size obtained when polyphosphate was used. Remarkably, 47 out of 90 prepared suspensions of sepiolite stayed homogeneous for at least three months after their preparation. In sum, the combination of a high energy dispersing equipment with an appropriate dispersing agent led to stable suspensions with optimal properties to be used in different applications, like in the composite production.
- Design of cellulose nanofibre-based composites with high barrier propertiesPublication . Alves, Luís; Ramos, Ana; Ferraz, Eduardo; J. T. Ferreira, Paulo; G. Rasteiro, Maria; A. F. Gamelas, JoséGas barrier properties are very relevant in composite materials for applications so diverse such as food packaging, electronics, or old document restoration. In the present work, four different types of cellulose nanofibres (CNFs), two types of clay minerals used individually (sepiolite) or combined (sepiolite + kaolinite), and the influence of pH, were explored in the production of composite films. Neat CNFs, only mechanically treated or prepared by enzymatic pre-treatment, gave films with good mechanical and barrier properties, but the addition of minerals led to a dramatic loss of these properties. Contrarily, the use of thin and functionalized fibrils (TEMPO-oxidised or cationized CNFs) gave composite films with good mechanical, thermal and barrier properties. Superior oxygen barrier properties (oxygen transmission rate (OTR) < 0.4 cm3 m−2 day−1) were obtained using TEMPO-oxidised CNF and 20% sepiolite, and, in general, for all the composite films containing the TEMPO CNF (OTR ≤ 1.8 cm3 m−2 day−1). The cationic CNF-based composites also showed a very good oxygen barrier (OTR ≤ 8.2 cm3 m−2 day−1). The high oxygen barrier could be explained by the compactness of the films and better entanglement of the more fibrillated nanocelluloses with the mineral particles. A decrease in the pH of the suspensions led to a decrease in the film preparation time, without a major negative impact on the composite film’s properties.
- Effect of the dispersion state of minerals on the properties of cellulose nanofiber-based composite filmsPublication . Alves, Luís; Ramos, Ana; Ferraz, Eduardo; Sanguino, Pedro; Santaren, Julio; Rasteiro, Maria Graça; Gamelas, José AFThe dispersion state and the efficiency of the mixture of the different components in a composite film have an important impact on its mechanical and optical properties. In the present work, the impacts of different dispersion treatments on the disaggregation state of fibrous clay particles in water, and on the properties of related cellulose nanofiber (CNF)-based composite films, were evaluated. X-ray diffraction studies, performed on samples of sepiolite and palygorskite, revealed only minor changes in the diffraction pattern when the minerals were subjected to ultrasonic treatment, with or without the addition of different chemical dispersing agents. Conversely, microscopic studies revealed important differences in the dispersion state of the samples, induced by the addition of the different dispersants, showing an improvement in the disaggregation of the mineral crystals. The composite films prepared with sepiolite (and carboxymethylcellulose, as chemical dispersant) dispersed using ultrasonic treatment, and different types of CNF, showed improved optical and mechanical properties when compared with composites of the same counterparts prepared with sepiolite dispersed using a high-speed shear disperser.
- Stabilization of Palygorskite Aqueous Suspensions Using Bio-Based and Synthetic PolyelectrolytesPublication . Ferraz, Eduardo; Alves, Luís; Sanguino, Pedro; Santarén, Julio; Rasteiro, Maria G.; Gamelas, José A. F.Palygorskite is a natural fibrous clay mineral that can be used in several applications, for which colloidal stability in aqueous suspensions is a key point to improve its performance. In this study, methods of magnetic stirring, high-speed shearing, and ultrasonication, as well as different chemical dispersants, combined with these methods, namely carboxymethylcellulose, alginate, polyphosphate, and polyacrylate, were used to improve the dispersibility and the formation of stable suspensions of palygorskite in different conditions of pH. The stability and particle size of suspensions with a low concentration of palygorskite were evaluated by visual inspection, optical and electron microscopy, dynamic light scattering, and zeta potential measurements. Moreover, the palygorskite used in this work was initially characterized for its mineralogical, chemical, physical, and morphological properties. It was found that more stable suspensions were produced with ultrasonication compared to the other two physical treatments, with magnetic stirring being inefficient in all tested cases, and for higher pH values (pH of 12 and pH of 8, the natural pH of the clay) when compared to lower pH values (pH of 3). Remarkably, combined with ultrasonication, carboxymethylcellulose or in a lesser extent polyphosphate at near neutral pH allowed for the disaggregation of crystal bundles of palygorskite into individualized crystals. These results may be helpful to optimize the performance of palygorskite in several domains where it is applied.
- Filmes de celulose nanofibrilada com incorporação de minerais: uma nova geração de materiais para embalagens alimentares e eletrónica impressaPublication . Alves, Luís; Aragão, António; Ferraz, Eduardo; Santarén, Júlio; Ferreira, Paulo J. T.; Ramos, Ana; Rasteiro, Maria G.; Gamelas, José A. F.
- Filmes de Celulose Nanofibrilada com Incorporação de Minerais como Substitutos de Plásticos em Embalagens Alimentares: Potencialidades e DificuldadesPublication . Alves, Luís; Ferraz, Eduardo; Santarén, Julio; J. T. Ferreira, Paulo; Ramos, Ana; Rasteiro, Maria G.; A. F. Gamelas, JoséCeluloses nanofibriladas (CNF) são nanomateriais com propriedades promissoras para produção de embalagens alimentares, sendo substitutos lógicos de polímeros de base petroquímica, especificamente plásticos. Contudo, estes materiais apresentam algumas limitações, como o seu elevado custo. A combinação de CNF com minerais argilosos representa uma excelente abordagem para a produção de materiais de elevado valor acrescentado com custos aceitáveis, possibilitando ainda a melhoria de algumas propriedades dos materiais, como por exemplo o efeito barreira a gases. Para a produção de filmes compósitos podem ser seguidas duas estratégias: evaporação de solvente ou filtração seguida de prensagem a quente. Os filmes obtidos por filtração apresentam propriedades mecânicas superiores às dos obtidos por evaporação de solvente e são preparados mais rapidamente. Assim, esta técnica apresentase como a mais apropriada e eficiente para a produção de filmes compósitos com boas propriedades mecânicas