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Abstract(s)
Este trabalho foi desenvolvido no âmbito dos materiais aplicados aos substratos têxteis,
especificamente à produção de fibras de origem sintética à nanoescala, recorrendo à tecnologia
de eletrofiação. A técnica de eletrofiação ou fiação eletrostática é um método direto, que
permite produzir fibras contínuas com diâmetros bastante reduzidos, até à gama dos
nanómetros. A versatilidade do processo de eletrofiação, a facilidade de implementação à
escala do laboratório e, essencialmente, as propriedades dos materiais obtidos conduziram, nos
últimos anos, à eletrofiação de uma grande variedade de polímeros, simples ou em combinação,
e à sua aplicação como nanomateriais. Os mantos fibrosos obtidos por eletrofiação, devido à
sua elevada área de superfície em relação ao volume, com uma estrutura à nanoescala, elevada
porosidade, e pequeno tamanho dos poros, o que possibilita diferentes modos de interação com
outros materiais, encontram aplicações em diversas áreas, incluindo roupas de proteção,
filtração e separação, fibras de reforço em materiais compósitos, aplicações óticas e
eletrónicas, dispositivos biomédicos, revestimento de feridas, sistemas de distribuição de
fármacos e a engenharia de tecidos. A tecnologia da eletrofiação tem inúmeras aplicações em
várias indústrias, incluindo a indústria têxtil. Há um potencial considerável na aplicação da
eletrofiação na produção de nanofibras, acabamentos funcionais e de têxteis com aplicações
biomédicas.
O processo de eletrofiação é, no entanto, caracterizado pela oscilação descontrolada e caótica
do jato eletricamente carregado de solução de polímero, o que leva à formação de contas,
fibras com contas e fibras irregulares nos mantos de nanofibras. Esta trabalho de investigação
teve como objetivo geral o desenvolvimento e otimização de um sistema de eletrofiação para
controlar a deposição de nanofibras eletrofiadas de poliamida 6, 6.6 e 11, através da utilização
de uma série de aros de metal, colocados ao longo da trajetória do jato carregado de solução
de polímero, ligados a uma segunda fonte de alimentação de alta tensão. A alteração/
modelação do campo eletrostático, como resultado da introdução dos aros metálicos, permitiu
um maior controlo sobre as instabilidades a que o jato de solução de polímero é sujeito durante
o processo de eletrofiação, possibilitando a formação de nanofibras mais uniformes e a redução,
simultaneamente, da área de deposição do manto fibroso. Ao longo dos trabalhos
desenvolvidos, foi investigado o efeito da adição de aros metálicos com diferentes tensões
aplicadas e de alguns dos parâmetros do processo de eletrofiação na aparência morfológica e
no diâmetro médio das nanofibras.
A tecnologia de eletrofiação utiliza uma fonte de alta tensão (5 – 50 kV) para produzir um
campo elétrico entre a extremidade de uma agulha carregada eletricamente e uma placa
coletora onde são depositadas as fibras. A polaridade do elétrodo ligado à agulha pode ser
positiva ou negativa (tendo o painel coletor a polaridade contrária). À medida que se forma
uma gota de solução polimérica na ponta da agulha (por onde se faz passar um caudal constante) as cargas elétricas acumulam-se na superfície da gota, ocorrendo uma
destabilização da sua forma hemisférica, que afunila em forma de um cone (cone de Taylor).
Quando a tensão aplicada excede o valor do potencial elétrico crítico, a partir do qual as forças
eletrostáticas superam a tensão superficial da solução polimérica, é ejetado um jato
eletricamente carregado de solução de polímero a partir da ponta do cone de Taylor. O jato
segue, inicialmente, uma trajetória quase retilínea até que é sujeito a instabilidades de flexão,
seguindo, então, uma trajetória complexa e aleatória até que é depositado no coletor.
No processo de eletrofiação de uma solução polimérica, as variáveis que podem influenciar a
morfologia das fibras obtidas são diversas, entre elas: parâmetros relacionados com o polímero
(peso molecular); parâmetros relacionados com a solução (concentração, viscosidade, tensão
superficial, condutividade); parâmetros relacionados com o processo em si (potencial
eletrostático aplicado, distância de deposição, caudal de alimentação); e parâmetros
ambientais (temperatura, humidade relativa, convecção do ar circundante).
Neste trabalho de investigação foram estudados os efeitos da variação da concentração da
solução polimérica, da distância agulha-coletor, da tensão aplicada entre a agulha e o coletor,
do caudal de alimentação, bem como o efeito da introdução de uma segunda fonte de
alimentação ligada a aros circulares metálicos numa configuração em série ou em paralelo,
com distribuição de tensão crescente ou decrescente, localizados ao longo da trajetória
agulha-coletor, sobre a morfologia e diâmetro médio de fibras produzidas por eletrofiação de
soluções poliméricas de poliamida 6 (PA6), poliamida 6.6 (PA6.6) e poliamida 11 (PA11).
Para este trabalho experimental foi projetado um sistema protótipo de eletrofiação,
desenvolvido no laboratório do Departamento de Ciência e Tecnologia Têxteis da Universidade
da Beira Interior. O sistema era constituído por duas fontes de alimentação de alta voltagem,
um dosímetro PHD 2000 Infusion da Harvard Apparatus, um painel coletor (que consistiu em
uma rede de cobre de dimensões 10 × 12 cm, revestida com folha de alumínio) em suporte
móvel de cortiça, aros circulares de aço inoxidável com 10 cm de diâmetro em suporte ajustável
de cortiça, seringas de vidro de 5 mL e agulhas de ponta romba de aço inoxidável e conector
de polipropileno, calibre 25 (diâmetro interno de 0,25 mm e diâmetro externo de 0,52 mm).
A viscosidade das soluções poliméricas foi medida usando um reómetro RheoStress® RS 150 em
ensaios de varrimento de tensão de corte com uma geometria cone-prato apropriada e uma
temperatura ambiente controlada de 23 ºC. A condutividade foi medida com um condutivímetro
de bancada inoLab® Level2.
A análise dos mantos de nanofibras foi efetuada por microscópio eletrónico de varrimento
(SEM), sendo que o diâmetro médio das nanofibras foi calculado usando uma amostra
de 50 medições aleatórias de fibras em cada imagem, obtida com ampliação de 20000× para a
PA6 e PA6.6 e de 10000× para a PA11. A análise da porosidade dos mantos de nanofibras foi realizada com o programa NIM (Nanofiber Images Measurer), desenvolvido especificamente para
este trabalho.
A utilização do sistema de eletrofiação desenvolvido neste trabalho, na variante com os aros
metálicos ligados à segunda fonte de alimentação com aplicação de valores de tensão
crescentes, provou ser bem-sucedido na obtenção de nanofibras mais uniformes, sem formação
de contas, com diâmetros reduzidos e menor desvio padrão. Com este sistema, foram
produzidas nanofibras de poliamida 6, 6.6 e 11 com diâmetros médios de 71.7±13.3 nm,
91.1±15.8 nm e 145.7±23.7 nm, respetivamente.
This work was developed in the scope of the materials applied to the textile substrates, specifically to the production of nanoscale synthetic fibers, using electrospinning technology. The electrospinning or electrostatic spinning technique is a direct method, which allows to produce continuous fibers with very small diameters, up to the nanometer range. The versatility of the electrospinning process, the ease of implementation on a laboratory scale and, essentially, the properties of the materials obtained have, in recent years, led to the electrospinning of a wide variety of polymers, simple or in combination, and their application as nanomaterials. Fibrous mats obtained by electrospinning, due to their high surface to volume ratio, with a nanoscale structure, high porosity, and small pore size, which enables different modes of interaction with other materials, find applications in several areas, including protective clothing, filtration and separation, reinforcement fibers in composite materials, optical and electronic applications, biomedical devices, wound dressing, drug delivery systems and tissue engineering. Electrospinning technology has numerous applications in various industries, including the textile industry. There is considerable potential in the application of electrospinning in the production of nanofibers, functional finishes and textiles with biomedical applications. The electrospinning process is, however, characterized by the uncontrolled and chaotic oscillation of the electrically charged jet of polymer solution, which leads to the formation of beads, fibers with beads and irregular fibers in the nanofiber mats. This research work had as its general objective the development and optimization of an electrospinning system to control the deposition of electrospun nanofibers of polyamide 6, 6.6 and 11, through the use of various metal rings, placed along the trajectory of the charged jet of polymer solution, connected to a second high-voltage power supply. The alteration/ modeling of the electrostatic field, as a result of the introduction of the metal rings, allowed for greater control over the instabilities to which the polymer charged jet is subjected during the electrospinning process, allowing the formation of more uniform nanofibers and simultaneously reducing the deposition area of the fibrous mat. Throughout the work developed, the effect of adding metallic rings with different applied voltages and some of the parameters of the electrospinning process on the morphological appearance and the average diameter of the nanofibers was investigated. The electrospinning technology uses a high-voltage source (5 - 50 kV) to produce an electric field between the end of an electrically charged needle and a collecting plate where the fibers are deposited. The polarity of the electrode connected to the needle can be positive or negative (with the collector panel having the opposite polarity). As a drop of polymeric solution is formed at the tip of the needle (through which a constant flow is passed) the electrical charges accumulate on the surface of the drop, destabilizing its hemispherical shape, which tapers in the shape of a cone (Taylor cone). When the applied voltage exceeds the value of the critical electrical potential, from which the electrostatic forces exceed the surface tension of the polymeric solution, an electrically charged jet of polymer solution is ejected from the tip of the Taylor cone. The jet initially follows an almost straight path until it is subject to bending instabilities, then follows a complex and random path until it is deposited in the collector. In the electrospinning process of a polymeric solution, the variables that can influence the morphology of the obtained fibers are diverse, among them: parameters related to the polymer (molecular weight); parameters related to the solution (concentration, viscosity, surface tension, conductivity); parameters related to the process itself (applied electrostatic potential, deposition distance, flow rate); and environmental parameters (temperature, relative humidity, surrounding air convection). In this research work, the effects of the variation of the concentration of the polymeric solution, the needle-collector distance, the tension applied between the needle and the collector, the flow rate, the effect of the introduction of a second hi-voltage power supply connected to circular metal rings in a series or parallel configuration, with increasing or decreasing voltage distribution, located along the needle-collector path, on the morphology and average diameter of fibers produced by electrospinning of polymer solutions of polyamide 6 (PA6), polyamide 6.6 (PA6.6) and polyamide 11 (PA11), were studied. For this experimental work, an electrospinning prototype system was developed in the laboratory of the Department of Textile Science and Technology at the University of Beira Interior. The system consisted of two high voltage power supplies, a PHD 2000 Infusion dosimeter from Harvard Apparatus, a collector panel (consisting of a 10 × 12 cm copper network, covered with an aluminum foil) on a mobile cork support, 10 cm diameter stainless steel circular rings on adjustable cork support, 5 mL glass syringes and blunt stainless steel needles and polypropylene connector, 25 gauge (0.25 mm inner diameter and outer diameter 0.52 mm). The viscosity of the polymer solutions was measured using a RheoStress® RS 150 rheometer in shear stress sweeping tests with an appropriate cone-plate geometry and a controlled ambient temperature of 23 ºC. Conductivity was measured with an inoLab® Level2 bench conductivity meter. The analysis of the nanofiber mats was performed by scanning electron microscope (SEM), where the average diameter of the nanofibers was calculated using a sample of 50 random measurements of fibers in each image, obtained with a 20000x magnification for PA6 and PA6.6 and 10000x for PA11. The porosity analysis of the nanofiber sheets was carried out with the NIM program (Nanofiber Images Measurer), developed specifically for this work. The use of the electrospinning system developed in this work, in the variant with the metal rings connected to a second power supply with application of increasing voltages, proved to be successful in obtaining more uniform nanofibers, without forming beads, with reduced diameters and less standard deviation. With this system, polyamide 6, 6.6 and 11 nanofibers were produced with average diameters of 71.7 ± 13.3 nm, 91.1 ± 15.8 nm and 145.7 ± 23.7 nm, respectively.
This work was developed in the scope of the materials applied to the textile substrates, specifically to the production of nanoscale synthetic fibers, using electrospinning technology. The electrospinning or electrostatic spinning technique is a direct method, which allows to produce continuous fibers with very small diameters, up to the nanometer range. The versatility of the electrospinning process, the ease of implementation on a laboratory scale and, essentially, the properties of the materials obtained have, in recent years, led to the electrospinning of a wide variety of polymers, simple or in combination, and their application as nanomaterials. Fibrous mats obtained by electrospinning, due to their high surface to volume ratio, with a nanoscale structure, high porosity, and small pore size, which enables different modes of interaction with other materials, find applications in several areas, including protective clothing, filtration and separation, reinforcement fibers in composite materials, optical and electronic applications, biomedical devices, wound dressing, drug delivery systems and tissue engineering. Electrospinning technology has numerous applications in various industries, including the textile industry. There is considerable potential in the application of electrospinning in the production of nanofibers, functional finishes and textiles with biomedical applications. The electrospinning process is, however, characterized by the uncontrolled and chaotic oscillation of the electrically charged jet of polymer solution, which leads to the formation of beads, fibers with beads and irregular fibers in the nanofiber mats. This research work had as its general objective the development and optimization of an electrospinning system to control the deposition of electrospun nanofibers of polyamide 6, 6.6 and 11, through the use of various metal rings, placed along the trajectory of the charged jet of polymer solution, connected to a second high-voltage power supply. The alteration/ modeling of the electrostatic field, as a result of the introduction of the metal rings, allowed for greater control over the instabilities to which the polymer charged jet is subjected during the electrospinning process, allowing the formation of more uniform nanofibers and simultaneously reducing the deposition area of the fibrous mat. Throughout the work developed, the effect of adding metallic rings with different applied voltages and some of the parameters of the electrospinning process on the morphological appearance and the average diameter of the nanofibers was investigated. The electrospinning technology uses a high-voltage source (5 - 50 kV) to produce an electric field between the end of an electrically charged needle and a collecting plate where the fibers are deposited. The polarity of the electrode connected to the needle can be positive or negative (with the collector panel having the opposite polarity). As a drop of polymeric solution is formed at the tip of the needle (through which a constant flow is passed) the electrical charges accumulate on the surface of the drop, destabilizing its hemispherical shape, which tapers in the shape of a cone (Taylor cone). When the applied voltage exceeds the value of the critical electrical potential, from which the electrostatic forces exceed the surface tension of the polymeric solution, an electrically charged jet of polymer solution is ejected from the tip of the Taylor cone. The jet initially follows an almost straight path until it is subject to bending instabilities, then follows a complex and random path until it is deposited in the collector. In the electrospinning process of a polymeric solution, the variables that can influence the morphology of the obtained fibers are diverse, among them: parameters related to the polymer (molecular weight); parameters related to the solution (concentration, viscosity, surface tension, conductivity); parameters related to the process itself (applied electrostatic potential, deposition distance, flow rate); and environmental parameters (temperature, relative humidity, surrounding air convection). In this research work, the effects of the variation of the concentration of the polymeric solution, the needle-collector distance, the tension applied between the needle and the collector, the flow rate, the effect of the introduction of a second hi-voltage power supply connected to circular metal rings in a series or parallel configuration, with increasing or decreasing voltage distribution, located along the needle-collector path, on the morphology and average diameter of fibers produced by electrospinning of polymer solutions of polyamide 6 (PA6), polyamide 6.6 (PA6.6) and polyamide 11 (PA11), were studied. For this experimental work, an electrospinning prototype system was developed in the laboratory of the Department of Textile Science and Technology at the University of Beira Interior. The system consisted of two high voltage power supplies, a PHD 2000 Infusion dosimeter from Harvard Apparatus, a collector panel (consisting of a 10 × 12 cm copper network, covered with an aluminum foil) on a mobile cork support, 10 cm diameter stainless steel circular rings on adjustable cork support, 5 mL glass syringes and blunt stainless steel needles and polypropylene connector, 25 gauge (0.25 mm inner diameter and outer diameter 0.52 mm). The viscosity of the polymer solutions was measured using a RheoStress® RS 150 rheometer in shear stress sweeping tests with an appropriate cone-plate geometry and a controlled ambient temperature of 23 ºC. Conductivity was measured with an inoLab® Level2 bench conductivity meter. The analysis of the nanofiber mats was performed by scanning electron microscope (SEM), where the average diameter of the nanofibers was calculated using a sample of 50 random measurements of fibers in each image, obtained with a 20000x magnification for PA6 and PA6.6 and 10000x for PA11. The porosity analysis of the nanofiber sheets was carried out with the NIM program (Nanofiber Images Measurer), developed specifically for this work. The use of the electrospinning system developed in this work, in the variant with the metal rings connected to a second power supply with application of increasing voltages, proved to be successful in obtaining more uniform nanofibers, without forming beads, with reduced diameters and less standard deviation. With this system, polyamide 6, 6.6 and 11 nanofibers were produced with average diameters of 71.7 ± 13.3 nm, 91.1 ± 15.8 nm and 145.7 ± 23.7 nm, respectively.
Description
Keywords
Eletrofiação Nanofibras Poliamidas Parâmetros do processo Campo eletrostático Instabilidade de flexão Lente eletrostática