Mirele Horsth de Paiva Teixeira
DEVELOPMENT OF AN ECO-FRIENDLY
MULTIFUNCTIONAL NANO-TiO2-COATED COTTON FABRIC
Thesis presented to the Graduate Program in Materials Science and Engineering of the Federal University of Santa Catarina as a requirement for obtaining the Master's Degree in Materials Science and Engineering. Advisor:
Prof. Dachamir Hotza, Dr.-Ing. Co-advisor:
Prof. Sergio Y. Gómez González, Dr. Eng.
Florianópolis 2019
Mirele Horsth de Paiva Teixeira
DEVELOPMENT OF AN ECO-FRIENDLY
MULTIFUNCTIONAL NANO-TiO2-COATED COTTON FABRIC
This Thesis was presented to obtain the Master's Degree Title in Materials Science and Engineering and approved in its final version by
the Graduate Program in Materials Science and Engineering of the Federal University of Santa Catarina.
Florianópolis, March 4th, 2019.
______________________________________ Prof. Dr. Guilherme Mariz de Oliveira Barra
Program Dean Examiners:
___________________________ Prof. Dr.-Ing Dachamir Hotza
Advisor/PGMAT-UFSC (Video conference)
_________________________ Prof. Dr. Eng. Sergio Y. Gómez
González
Co-Advisor/POSENQ-UFSC
_____________________________ Prof. Dr. Luís Antonio Lourenço
Uniasselvi
__________________________ Prof. Dr. Luiz Fernando
Belchior Ribeiro UFSC - Araranguá
This work is dedicated to my parents for providing me with education, values, and all the support to be here.
ACKNOWLEDGMENTS My sincere thanks to:
First and foremost, my parents Rachel Horsth de Paiva Teixeira and Anselmo Gonçalves Teixeira, who have always worked hard to ensure that my brother and I had access to the best schools, and have supported and encouraged us to walk our path with honesty. Especially to my mother who kept our family foundations firm after my father's death and to my brother Anselmo Horsth de Paiva Teixeira for all the support and examples of life in which I mirror myself.
My boyfriend, Julio Cesar Fiorio Vettorazzi, who has been supporting me since I was accepted in the Master's Program despite the distance in the last two years.
My advisers, Dachamir Hotza and Sergio Y. Gómez González, for the mentoring, confidence and opportunity conferred on me. Their knowledge, enthusiasm, and feedback have been vital for this work. Also, they gave me the freedom to work independently and always knew when conducting me in the right direction.
Mr. Jemerson Jung de Freitas and the company Teka S.A. for the donation of the fabric used in this work.
The Coordination for the Improvement of Higher Education Personnel (CAPES) for funded this project.
The Federal University of Santa Catarina, mainly to the Graduation Program in Materials Science and Engineering, for the opportunity of professional growth.
The Labmassa coordinators, for allowing the development of this work in the Labsin-Labmassa infrastructure, and to the Labmassa's companions, especially to Luis Antonio Lourenço, Carlos Rafael da Silva Oliveira, Fernando Giacomini, Éllen Francine Rodrigues, Jéssica Mulinari, Wagner Artifon, Celso Jr. de Castro Viera and Alexandre Barbosa da Silva for collaborating in my work, for the patience and friendship during the thesis progress.
My longtime friends Andrey Escala Alves, Maria Eduarda Lacerda, Lucas César Oliveira, Raphael and Gabriel Vasconcelos de Sousa for their support at this stage and for the visits for old time's sake.
"Se fosse fácil achar o caminho das pedras, tantas pedras no caminho não seria ruim." (Humberto Gessinger) "Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid."
ABSTRACT
In this work, a multifunctional cotton fabric was developed by immobilizing titanium dioxide nanoparticles (TiO2-NPs) using an
eco-friendly bath (citric acid as spacer, and sodium hypophosphite as catalyst). This was achieved by employing textile industry techniques and assessing systematically the effects of nanoparticles, spacer and catalyst concentrations on the finishing bath along different curing temperatures. Analysis of variance (ANOVA) and surface response methodology (SRM) were used to investigate multiple cycles of methylene blue photocatalytic degradation and flame retardancy. The results disclosed an excellent performance of the treated fabric for various applications: flame-retardant (char content enhanced by 1000%), photocatalyst for dye removal (>90% of contaminant abatement and feasibility for multiple reuses), self-cleaning agent for intense stains (up to 80% of stain vanishing), and bacterial inhibition in dark condition (~25% of bacteria growth reduction). Moreover, those properties were maintained after at least 5 washing cycles.
Keywords: TiO2 functionalized fabric, photocatalysis, flame retardancy,
RESUMO
Neste trabalho, um tecido multifuncional de algodão foi desenvolvido através da imobilização de nanopartículas de dióxido de titânio (TiO2
-NPs) utilizando um banho de acabamento ecologicamente amigável (ácido cítrico como reticulante e hipofosfito de sódio como catalisador). Foram empregadas técnicas da indústria têxtil e avaliados sistematicamente os efeitos da concentração de nanopartículas, reticulante e catalisador no banho de acabamento em diferentes temperaturas de cura. A análise de variância (ANOVA) e a metodologia de superfície de resposta (SRM) foram utilizadas para investigar múltiplos ciclos de degradação fotocatalítica de azul de metileno e retardância a chama. Os resultados revelam um excelente desempenho do tecido tratado para várias aplicações: retardante de chama (teor de resíduo carbonoso aumentado em 1000%), remoção fotocatalítica de solução corante (>90% remoção e viabilidade para múltiplos reusos), autolimpeza de manchas intensas (até 80% de remoção), e inibição bacteriana no escuro (~25% de redução no crescimento bacteriano). Além disso, essas propriedades foram mantidas por pelo menos 5 ciclos de lavagem.
Palavras-chave: Tecido funcionalizado com TiO2, fotocatálise,
RESUMO EXPANDIDO Introdução
Os desafios da globalização e a crescente demanda por produtos cada vez mais sofisticados impulsionam a pesquisa e a indústria para o desenvolvimento de soluções inovadoras, funcionais e ecologicamente corretas. Neste contexto, a nanociência e nanotecnologia são apontadas como rotas-chave para o desenvolvimento de materiais nanoestruturados e funcionalizados. A incorporação de nanopartículas em produtos comerciais têm se tornado cada vez mais comum em diversos setores, tais como eletrônicos, embalagens, cosméticos, materiais médicos e óticos. Mais recentemente, essa abordagem foi proposta para materiais têxteis, se tornando o foco de vários grupos de pesquisa. O desafio da produçõ e comercialização de têxteis nanofuncionalizados está relacionado à durabilidade e eficiência do revestimento, impulsionando a busca por revestimentos mais duráveis, multifuncionais e ecologicamente corretos. Nesse sentido, os ácidos policarboxílicos (PCA), que são compostos ecologicamente amigáveis, vem sendo estudados como agentes capazes de potencializar a fixação de partículas de óxidos metálicos na superfície de têxteis celulósicos, estendendo a durabilidade do nano-revestimento. As nanopartículas de óxido metálico, particularmente as nanopartículas de TiO2 (TiO2-NPs),
oferecem uma ampla variedade de aplicações tais como na obtenção de superfícies autolimpantes, retardante de chamas, dispositivos de purificação de água e ar, na produção de revestimentos antibacterianos, entre outras aplicações. O TiO2 é um óxido semi-condutor, não tóxico,
altamente disponível, biocompatível e de custo relativamente baixo. Neste contexto, o objetivo deste trabalho foi desenvolver um tecido de algodão funcionalizado com TiO2-NPs pelo método
exhaust-pad-dry-cure, e, investigar os efeitos das concentrações de TiO2-NPs, ácido
cítrico e hipofosfito de sódio, e da temperatura de cura no desenvolvimento de um tecido de algodão com multi-propriedades. O retardamento de chamas e a propriedade fotocatalítica foram avaliados por um delineamento experimental fatorial completo, variando as condições de funcionalização. A autolimpeza de manchas intensas, o efeito antibacteriano na ausência de luz (sem ativação do TiO2), assim
como durabilidade do revestimento a vários ciclos de lavagem também foram avaliados e discutidos.
Objetivos
multifuncional e nanotecnológico através da sua modificação superficial empregando o ácido cítrico como agente de reticulação, na imobilização de nanopartículas de TiO2. Os objetivos específicos envolvem: a
desengomagem do tecido de algodão, a funcionalização e estudo da influência dos parâmetros de funcionalização nas propriedades do tecido, a estimação do efeito da concentração de ácido cítrico e da temperatura de cura na esterificação do tecido de algodão, a avaliação da influência dos parâmetros de funcionalização na remoção fotocatalítica de contaminantes em fase líquida e na viabilidade de múltiplos ciclos de reutilização, a investigação do efeito do tratamento de funcionalização na propriedade de retardância de chama, a avaliação da capacidade de autolimpeza na remoção de manchas orgânicas intensas, a verificação do efeito antimicrobiano do tecido funcionalizado na ausência de luz, e a apreciação da eficácia da ancoragem das TiO2-NPs na superfície têxtil
após ciclos de lavagem. Metodologia
O tecido felpudo de algodão cru, utilizado com suporte para as nanopartículas, foi submetido a um processo de desengomagem oxidativa para remover a goma, cera e sugidades naturais das fibras de celulose. O tecido desengomado foi funcionalizado em duas etapas: esterificação do ácido cítrico seguida de imobilização das TiO2-NPs. As
soluções do agente ligante foram preparadas dissolvendo diferentes concentrações (30, 60, 90 g/L) de ácido cítrico (CA) em água destilada, utilizando hipofosfito de sódio (40% em peso relacionado a CA) como catalisador da esterificação, sob agitação magnética, à temperatura ambiente. Amostras (15 × 15 cm) do tecido foram adicionadas nos canecos de teste juntamente com as soluções de ácido cítrico numa relação de banho de 20:1. Os canecos foram fixados no equipamento de tingimento em canecos e aquecidos a 60 ºC durante 30 min. Em seguida, o excesso de líquido das amostras têxteis foi removido (pick-up de 100%) através do processo de foulardagem, e as amostras foram secas a 100ºC por 4 min, e, curadas em diferentes temperaturas (125, 150, 175 ºC) em forno rama. O procedimento de imobilização das TiO2-NPs foi
realizado similarmente ao tratamento com ácido cítrico, salvo pela temperatura e tempo de tratamento que foram 75 ºC e 45 min, respectivamente. Após a segunda etapa de funcionalização, as amostras de tecido foram enxaguadas três vezes em banho ultrassônico (20 min, 30 ºC) para eliminação de TiO2-NPs fracamente ancoradas,e, por fim,
retardância de chama e fotocatalítico na remoção do corante azul de metileno como um modelo de poluente aquático em quatro ciclos de uso. O teste de retardância de chama foi realizado no calorímetro de pirólise e combustão segundo a ASTM D7309:2013, e, o teste fotocatalítico foi desenvolvido em um reator de ultravioleta C, de acordo com a ISO 10678:10 (E) adaptada. Os efeitos principais e de interação entre os parâmetros de funcionalização foram avaliados para as duas propriedades supracitadas através da ANOVA, e, as melhores condições de funcionalização para cada um das respostas foram determinadas pela metodologia de superfície de resposta. A amostra que apresentou o melhor desempenho fotocatalítico foi ainda avaliada quanto ao seu caráter autolimpante na remoção de manchas intensas, antibacteriano contra S. aureus, e de solidez a cinco ciclos de lavagem. Para os testes de autolimpezas, as amostras foram deixadas de molho por 12 h em soluções de azul de metileno, café e molho de soja. Em seguida foram secas em estufa à 35 ºC e expostas a radição ultravioleta A, com medidas de K/S ao longo do tempo de exposição. No teste antibacteriano, amostras têxteis foram adicionadas a suspensões contendo bactérias do tipo S. aureus e acomodadas em um banho termostático por 24 h na ausência de luz. Posteriormente, as suspensões remanescentes foram plaqueadas, incubadas e o número de colônias por volume de solução foi determinado por contagem. O teste de durabilidade do recobrimento foi realizado de acordo com a ISO 105-C06:2010 adaptada, simulando cinco ciclos de lavagem doméstica. Amostras selecionadas foram caracterizadas por ATR-IR com a finalidade de verificar a esterificação do ácido cítrico na superfície têxtil, MEV e EDS para análise morfológica e micro-análise química superficial, ICP-MS com o objetivo de determinar o conteúdo de TiO2
efetivamente ancorado ao têxtil antes e após ciclos de lavagem, e, TGA para avaliar o comportamento termo-oxidativo das amostras.
Resultados e Discussão
Os resultados mostraram que após a desengomagem, o tecido tornou-se mais branco e hidrofílico. Além disso, a gramatura aumentou de 564,46 para 717,90 g/m2 em detrimento do inchamento e encolhimento dos fios de algodão. Análises por ATR-IR confirmou a presença de ligação do tipo éster na superfície dos têxteis funcionalizados, e mostrou uma tendência ao aumento da esterficação com o incremento da temperatura de cura e em níveis mais baixos de ácido cítrico. Sendo a reação de esterificação endotérmica, aumentos na temperatura de cura foverecem sua ocorrência. Ademais, como a celulose contém uma quantidade finita
e determinada de hidroxilas, a adição de 30 g/L de ácido foi suficiente para promover a esterificação com os grupos hidroxila disponíveis. O teste retardância de chama revelou o perfil da taxa de liberação de calor ao longo da temperatura de combustão. Em geral, observou-se uma tendência à redução do calor total liberado e incremento na formação de resíduo carbonoso após a combustão nas amostras tratadas nos níveis mais altos de temperatura de cura e concentração de ácido cítrico (e hipofosfito de sódio). Acredita-se que as nanopartículas de TiO2
funcionariam como material inerte enquanto que o hipofosfito de sódio (vinculado a concentração de ácido cítrico) atuaria diretamente como agente retardante de chama, ao interagir com a celulose fixando o contéudo fosforoso na estrutura celulósica do têxtil. Esta tendência foi confirmada estatísticamente pela ANOVA, onde o efeito de interação da temperatura de cura e concentração de ácido cítrico (e hipofosfito de sódio) foi o mais significativo à 95% de confiança. Verificou-se através da metodologia de superfície de resposta que a melhor condição de funcionalização para a retardância de chamas, fixando a concentração de TiO2 em 5 g/L, é obtida para níveis mais altos de temperatura de cura e
concentração de ácido cítrico e hipofosfito de sódio. Os resultados do TGA corroboraram com os resultados acima, evidenciando uma redução da temperatura de onset e incremento no teor de resíduo carbonoso. De fato, observou-se experimentalmente uma redução de 43% no pico de liberação de calor e 24% no calor total liberado, bem como um aumento de 1000% na teor de resíduo carbonoso da amostra tratada com 90 g/L de ácido cítrico, 54 g/L de hipofosfito de sódio, 5 g/L de TiO2 e curada à
175 ºC em relação a amostra não tratada. Os testes fotocatalíticos mostram que o maior percentual de remoção do azul de metileno nos quatro ciclos de uso (maior que 90%) foi obtido para a amostra tratada com 30 g/L de ácido cítrico, 18 g/L de hipofosfito de sódio, 15 g/L de TiO2 e curada à 175 ºC. A ANOVA revelou que todos os fatores de
funcionalização e suas interações são significativos à 95% de confiança, sendo o efeito principal da temperatura de cura e da concentração TiO2
os mais significativos. A metodologia de superfície de resposta apontou que a melhor condição de funcionalização para a resposta fotocatalítica ocorre nos níveis mais altos de temperatura de cura e de concentração de TiO2, fixando a concentração de ácido cítrico em 30 g/L. Esse dado
corrobora com o resultado de ATR-IR, onde observou-se que o incremento da temperatura de cura favorece a esterificação, que por sua vez, promove uma ancoragem efetiva das TiO2-NPs. As curvas
superior a 0.97. Observou-se que, com exceção da amostra produzida com 90 g/L de ácido cítrico, 54 g/L de hipofosfito de sódio, 5 g/L de TiO2 e curada à 125 ºC, todas as amostras mantiveram suas constantes
de taxa de velocidade (kapp) razoavelmente inalteradas ao longo dos ciclos de uso. A drástica redução no kapp da amostratratada com 90 g/L de ácido cítrico, 54 g/L de hipofosfito de sódio, 5 g/L de TiO2 e curada à
125 ºC foi acompanhada de uma elevada redução da atividade fotocatálica específica (PMB), provavelmente devido a lixivição de TiO2
da superfície têxtil, ao longo dos ciclos, em detrimento da baixa temperatura de cura. Já a amostra tratada na melhor condição de funcionalização, apresentou tanto o kapp como o PMB elevados ao longo
dos ciclos de uso, atinguindo um kapp médio de 0,977 h
-1
. A amostra que apresentou melhor resultado fotocatalítico foi selecionada para os testes de autolimpeza e antibacteriano por apresentarem mecanismos de ação similares. Os resultados do teste de autolimpeza mostraram um redução de 80, 35 e 16% na intensidade das manchas de azul de metileno, café e molho de soja, após 9 h de exposição ao ultravioleta para a amostra manchada com azul de metileno, e, 48 h, para as manchas de café e molho de soja. Manchas orgânicas não gordurosas são excitadas por radiação acima de 290 nm, e, doam elétrons para o TiO2, o que favorece
a sua remoção fotocatalítica em relação a manchas gordurosas como as de café e molho de soja. Os resultados antibacterianos revelaram um aumento de 5 x na atividade antibacteriana do têxtil tratado em relação ao têxtil sem tratamento, mesmo sem a ativação previa do TiO2 com
ultravioleta. Contudo, um teste de médias mostrou que não houve diferença significativa entre a redução bacteriana causada pela amostra tratada com ácido cítrico e TiO2, e, apenas tratada com ácido cítrico.
Isso provavelmente se deve a ionização de espécies derivadas da esterifcação do ácido cítrico, que atuariam na destruição da parede celular bacteriana causando sua morte. O resultado da resistência à lavagem foi avaliada pela quantificação de Ti por ICP-MS, que demonstrou que houve uma variação na concentração de Ti no têxtil de 865±19 mg/g antes de lavagem, para 886±107 mg/g após a lavagem. Essa baixa variação indica que acabamento nano-funcional do têxtil produzido com 30 g/L de ácido cítrico, 18 g/L de hipofosfito de sódio, 15 g/L de TiO2 e curado à 175 ºC apresenta durabilidae por pelo menos
cinco ciclos de lavagem, e, recorrobora com a elevada atividade fotocatalítica apresentada pela amostra inclusive após quatro ciclos uso.
Considerações Finais
Foi possível obter um tecido felpudo 100% algodão atoalhado para múltiplas aplicações através da rota de processamento exhaust-pad-dry-cure empregando um banho de acabamento ecológico. A análise estatística foi fundamental na avaliação dos parâmetros de funcionalização e na determinação das melhores condições para o desenvolvimento de cada uma das propriedades avaliadas. Destaca-se que o têxtil nanotecnológico produzido através de uma rota comumente empregada no processo de tingimento têxtil, apresentou propriedades diferenciadas como retardância de chama, autolimpeza e atividade fotocatalítica e antibacteriana, além de excelente solidez à lavagem, o que corrobora com sua possível aplicação em produtos comerciais em um futuro próximo.
Palavras-chave: Tecido funcionalizado com TiO2, fotocatálise,
LIST OF FIGURES
Figure 1- Fabric classification. (Source: Own authorship) ... 24 Figure 2 - Schematic illustration of cellulosic chain structure. (Source: ZANROSSO, 2016) ... 25 Figure 3 - Structure of textiles: (a) woven, (b) welf knitted, (c) warp knitted, (d) nonwoven. (Source: GONG; OZGEN, 2018) ... 26 Figure 4 - Classification of terry towel fabrics. (Source: YILMAZ; POWELL; DURUR, 2005) ... 27 Figure 5 - Scheme of cellulose cross-linking by a spacer and TiO2
anchoring. (Source: MEILERT; LAUB; KIWI, 2005)... 30 Figure 6 - Illustration of photocatalysis mechanism. (Source: Own authorship)... 37 Figure 7 - Representative scheme of the oxidative desizing process. (Source: Own authorship) ... 45 Figure 8 - Sampling procedure for fabric weight measurement. (Source: AMERICAN SOCIETY FOR TESTING AND MATERIALS, 2017) . 46 Figure 9 - Iodine test on the terry-towel fabric (a) before and (b) after desizing procedure. (Source: Own authorship) ... 52 Figure 10 - ATR-IR spectra of the pristine cotton fabric and the samples treated with different concentrations of citric acid and curing temperatures. (Source: Own authorship) ... 54 Figure 11 - Heat release rate (HRR, W/g) curves for untreated (control) and treated terry-towel cotton fabric samples according to the 23 factorial design. (Source: Own authorship) ... 56 Figure 12 - Contour plot of THR (kJ/g) response of the treated cotton fabric with different concentrations of SHP and curing temperature, fixing TiO2 concentration at 5 g/L. (Source: Own authorship) ... 59
Figure 13 - TGA and DTA plots for pristine and treated (90:54:175:5) fabrics. (Source: Own authorship)... 60 Figure 14 - SEM microscopy of (a) pristine cotton fabric, (b) treated fabric (90:54:175:5) before and (c) after flaming test. (Source: Own authorship)... 62
Figure 15 - EDX spectroscopy of (a) the treated cotton fabric (90:54:175:5) before and (b) after flame retardancy test. (Source: Own authorship) ... 62 Figure 16 - Scheme of the intermediate reactions during cotton crosslinking process with CA: (a) itaconic acid formation, (b) cyclic anhydride creation in presence of SHP, (c) sodium hypophosphite ionization and (d) crosslink reaction of cellulosic chains with CA in presence of SHP. (Source: Own authorship) ... 63 Figure 17 - Methylene blue photocatalysis curves for 4 using cycles of untreated and treated terry-towel cotton fabric samples according to the 23 factorial design. (Source: Own authorship) ... 64 Figure 18 - Contour plot of DR1-4 (%) response of the treated cotton
fabric with different concentrations of TiO2 and CA, fixing curing
temperature at the center point. (Source: Own authorship) ... 67 Figure 19 - Langmuir-Hinshelwood pseudo-first-order kinetic plots for methylene blue photocatalytic removal. (Source: Own authorship) ... 68 Figure 20 - Pseudo-first order constant (kapp) and specific photoactivity value (PMB) for samples 7, 3 and 2 along the four reuses. (Source: Own authorship) ... 69 Figure 21 - Self-cleaning curves of the photocatalytic removal of (a) methylene blue, (b) coffee and (c) soy sauce stains. (Source: Own authorship) ... 71 Figure 22 - Photocatalytic self-cleaning mechanism of organic stains. (Source: Own authorship) ... 72 Figure 23 - Elemental analysis of Ti in sample 7 before and after washing fastness test by ICP-MS. Ti concentration is presented in mass of element per mass of fabric. (Source: Own authorship) ... 74
LIST OF TABLES
Table 1- Comparative data concerning the main polycarboxylic acids used as cellulose crosslinking agents. ... 31 Table 2 - Levels and factors of the 23 experimental design... 47 Table 3 - Weight per area of terry towel fabric before and after desizing procedure. ... 52 Table 4 - Experimental matrix design and responses. ... 55 Table 5 - Analysis of variance (ANOVA) for total heat release (THR, W/g). SS = sum of squares; df = degree of freedom; MS = mean square. ... 57 Table 6 - Combustion data obtained from TGA and PCFC analyses. TpHRR = temperature on pHRR. ... 61 Table 7 - Analysis of variance (ANOVA) for methylene blue photodegradation percentage between the first and fourth reusing cycle (DR1-4). ... 66 Table 8 - Antibacterial efficiency of textile specimens. ... 73
LIST OF ABBREVIATIONS AND NOTATIONS
A Area
Ag-NPs Silver nanoparticles ANOVA Analysis of variance
APTMS Aminoprophyltrimethoxysilane ATR -IR Attenuated total reflection infrared BHI Brain heart infusion
BLB Black light blue
BTCA 1,2,3,4 butenetetracarboxylic acid C0 Initial concentration
CA Citric acid
Ct Concentration in time t
D Cuvette length
DK/S Relative color strength
DR1-4
Photocatalytic dye removal between the first and fourth reuse cycle
DTA Differencial thermal analysis
e- Electron
EDX Energy-dispersive X-ray diffractometry
h+ Hole
HRR Heat release rate
ICP-MS Inductively coupled plasma mass spectroscopy K/S Color strenght (Kubelka-Munk equation) Kapp Constant
L:G Liquors-to-goods ratio (dye bath volume in ml in relation to material dyed weight g)
MA Maleic acid
MB Methylene blue
MIC Minimum inhibitory concentration PAA Polyacrylic acid
PCAs Polycarboxylic acids
PCFC Pyrolysis combustion flow calorimeter PEG Polyethylene glycol
PMB Specific photoactivity (on methylene blue)
SA Succinic acid
SEM Scanning electron microscopy SHP Sodium hypophosphite SRM Surface response methodology
t Time
Tcuring Curing temperature
TEA Triehanol amine
TGA Thermalgravimetric analysis THR Total heat release
TiO2-NPs TiO2 nanoparticles
UPF Ultraviolet protection factor UV Ultraviolet
UV-Vis Ultraviolet visible
v Volume
WRA Wrinkle recovery angle Wt% Weight percentage ZnO-NPs Zinc oxide nanoparticles
[CA]S Citric acid concentration in solution
[SHP]S Sodium hypophosphite concentration in solution
[TiO2]SP TiO2 concentration in suspension
∆Absdar Absorbance variation under dark condition
∆Absirr Absorbance variation under UV-light
∆t Time variation
ε Molar extinction coefficient
SUMMARY 1 INTRODUCTION ...21 2 OBJECTIVES ...23 2.1 GENERAL OBJECTIVE...23 2.2 SPECIFIC OBJECTIVES ...23 3 LITERATURE REVIEW ...24 3.1 TEXTILE MATERIALS ...24
3.2 FABRIC FUNCTIONALIZATION STRATEGIES ...27
3.2.1 Polycarboxylic acids as cellulose crosslinking agents...29
3.2.2 Economic and ecological aspects in the selection of a polycarboxylic acid ...30 3.3 FUNCTIONAL PROPERTIES ...32 3.3.1 Durable-press ...32 3.3.2 Flame retardancy ...32 3.3.3 Antibacterial effect...34 3.3.4 Photocatalytic performance ...36 3.3.5 Self-cleaning ...38
3.4 NANOTECHNOLOGY APPLIED TO FABRICS ...38
4 MATERIALS AND METHODS ...44
4.1 MATERIALS ...44
4.2 METHODS ...44
4.2.1 Fabric oxidative desizing ...44
4.2.2 Fabric weight per area determination ...45
4.2.3 Fabric surface functionalization ...46
4.2.3.1 Attachment of the eco-friendly spacer………...46
4.2.3.2 Immobilization of TiO2 nanoparticles………...…...46
4.2.4 Experimental design ...47
4.2.5 Chemical, thermal and morphological analyses ...48
4.2.7 Photocatalytic activity on dye removal ... 49 4.2.8 Self-cleaning ... 50 4.2.9 Antibacterial effect ... 50 4.2.10 Washing fastness ... 51
5 RESULTS AND DISCUSSION ... 52
5.1 FABRIC DESIZING AND WEIGHT PER AREA ... 52 5.2 ATTACHMENT OF SPACER ON THE COTTON FABRIC ... 53 5.3 EXPERIMENTAL DESIGN ANALYSIS ... 54 5.4 FLAME RETARDANCY PROPERTY ... 55
5.4.1 Effect of curing temperature and CA, SHP and TiO2
concentration on flammability ... 55 5.4.2 Thermogravimetric profile ... 59
5.5 MICROSTRUCTURAL ANALYSIS ... 61 5.6 PHOTOCATALYTIC ACTIVITY ON WATER
DECONTAMINATION ... 64 5.6.1 Effect of curing temperature and CA, SHP and TiO2
concentration on terry-towel cotton fabric photocatalytic activity ... 64 5.6.2 Photocatalysis kinetics ... 67
5.7 SELF-CLEANING PERFORMANCE ... 69 5.8 ANTIBACTERIAL EFFECT ... 72 5.9 WASHING FASTNESS OF TiO2-NPs-TREATED TERRY COTTON
FABRIC ... 73
6 CONCLUSIONS ... 75
REFERENCES ... 76
21
1 INTRODUCTION
The globalization challenges and worldwide competition currently move research and industry toward creating innovative environmentally friendly solutions and high-performance products. For this scope, nanoscience and nanotechnology might be applied to the development of nanostructured and functionalized materials (HARIFI; MONTAZER, 2012a; HASHEMIKIA; MONTAZER, 2012). Thousands of labeled commercial goods, such as electronics, packaging, cosmetics, medical and optical materials contain different nanoparticles in their composition and this number goes up day-by-day (ASMATULU; TWOMEY; OVERCASH, 2012; RADETIĆ, 2013; VANCE et al., 2015). This approach was recently embraced for textile materials, and has become the focus of several research groups (HASHEMIKIA; MONTAZER, 2012; PASQUI; BARBUCCI, 2014; RADETIĆ, 2013; VANCE et al., 2015; WANG et al., 2015).
Metal oxide nanoparticles, particularly TiO2 nanoparticles (TiO2
-NPs), offer a variety of applications and can be efficiently used to obtain self-cleaning surfaces, to enhance flame retardancy, to produce water and air purification devices, and to generate antibacterial coatings, among other applications. The interest in the immobilization of TiO2-NPs on
textile materials is growing continuously mainly due to their photocatalytic activity and the combination of the other afore mentioned properties. In addition, TiO2 is a non-toxic, highly available,
biocompatible, and relatively low-cost material. Despite its potential and added value benefits, the commercialization of such functionalized textiles lies on the coating durability and efficiency, which requires more research efforts (CARP; HUISMAN; RELLER, 2004; KARIMI et al., [s.d.]; LI et al., 2018; NAZARI et al., 2009; RADETIĆ, 2013; VANCE et al., 2015).
Formaldehyde-free linking agents such as polycarboxylic acids (PCA) have been pointed out as eco-friendly spacers, capable of enhancing the attachment of TiO2 particles on the textile surface
extending the durability of the nano-coating. Polycarboxylic acids, especially citric acid (CA), combined with phosphorus-based catalysts, such as sodium hypophosphite (SHP), have been shown to be effective substitutes for formaldehyde-based linkers. During the curing process, the carboxyl groups of PCA form ester bonds with the hydroxyls of cellulose chains via intermediate anhydrides. These ester links are also resistant to various home launderings, thereby conferring durability (GALOPPINI, 2004; LESSAN; MONTAZER; MOGHADAM, 2011; SCHRAMM; VUKUŠIĆ; KATOVIĆ, [s.d.]; YANG; WANG; KANG, 1997).
The aim of this study was to investigate the effect of TiO2-NPs,
CA and SHP concentrations, as well as curing temperature on the development of a multi-property terry towel cotton fabric, through exhaustion-pad-dry-cure method using textile industry equipment. Flame retardancy and photocatalytic properties were evaluated by a full-factorial experimental design varying the functionalization conditions. Some features of the treated fabrics including flame retardancy, water decontamination capacity, self-cleaning and antibacterial properties, as well as washing durability were assessed and discussed.
23
2 OBJECTIVES
2.1 GENERAL OBJECTIVE
This work proposes the development of nanotechnological multifunctional cotton fabric by its surface modification using citric acid as an eco-friendly crosslinking agent on TiO2 nanoparticles
immobilization.
2.2 SPECIFIC OBJECTIVES
The following objectives, specific to the development of functionalized cotton fabrics using nanotechnology principles, are pursued:
To desize the cotton fabric before functionalization treatment;
To functionalize and study the influence of functionalization parameters (crosslinking agent, catalyst and TiO2-NPs
concentrations, and curing temperature) on fabric properties;
To estimate the effect of citric acid concentration and curing temperature on cotton fabric surface esterification;
To evaluate the functionalization parameters on photocatalytic removal of contaminants in the liquid phase and the feasibility for multiple reuse cycles;
To investigate the functionalization treatment effect on flame retardancy property;
To assess the self-cleaning ability in the removal of intense organic stains;
To verify the antimicrobial effect of the functionalized fabric in dark condition; and
To appraise the effectiveness of the anchoring of TiO2-NPs on
3 LITERATURE REVIEW 3.1 TEXTILE MATERIALS
Textile materials have been an integral part of everyday life since prehistoric times. They are among the most versatile materials in society, being used for clothing, household articles, and for a wide range of industrial applications such as tires reinforcement, composite materials, filtration and insulation media (HORROCKS; ANAND, [s.d.]; VANCE et al., 2015).
Textile fabrics are flexible materials that consist in a network of interlaced yarns, originated by a set of fibers. They can be classified through different criteria. In Figure 1, three classic categorizations are shown, regarding fibers origin, manufacturing method and applications (FURTADO, 2015; HORROCKS; ANAND, [s.d.]; SALEM, 2010).
Figure 1- Fabric classification. (Source: Own authorship) Manufacturing Method
Woven (e.g. Twill) Knitted (e.g. Tricot) Nonwoven (e.g. Felt)
Fibers Origin
Natural
Vegetable (e.g. Cotton) Animal (e.g. Silk) Mineral (e.g. Asbestos)
Man-made
Regenerated (e.g. Viscose) Synthetic (e.g. Polyester)
Inorganic (e.g. Metallic)
Applications
Apparel (e.g. Clothes) Household (e.g. Bedding)
Technical
Technological (e.g. self-cleaning)
25 Textile fibers are derivated from natural sources (animal, vegetable or mineral), or man-made, developed by regeneration of natural compounds, synthetic polymers or inorganic sources. Among natural fibers, cotton represents half of the global consumption. Brazil has been the 5th largest producer and the 4th largest exporter of cotton in the world in the last five years (―Cotton in the Era of Globalization and Technological Progress‖, 2017). Cotton's appeal lies not only in economic benefits but also in its excellent properties, to mention a few: comfort, softness, natural ventilation, high hydrophilicity, static resistance, easy dyeing, renewable origin (―Cotton in the Era of Globalization and Technological Progress‖, 2017; MISNON et al., 2014). Cotton consists mainly of cellulose (80–90% wt), which is a polysaccharide formed by a linear chain of thousands of glucose units (C6H10O5), and some non-cellulosic components such as pectins (0.4–
1.2%), waxes (0.4–1.2%), proteins (1.0–1.9%), ashes (0.7–1.6%), and other miscellaneous compounds (FURTADO, 2015; WANG et al., 2006; YLHSIEH; GORDON, 2006).
As aforementioned, intermolecular bonds of several cellulosic chains form the fibers structure as illustrated in Figure 2. It can be seen that each glucose unit has three hydroxyl groups on its surface (positions 2, 3 and 6) that are potentially available for reactions. However, the occurrence of intermolecular hydrogen bonds in the crystalline regions of cellulose molecule hinders reactions in mild conditions. Nevertheless, the reactivity could be enhanced by applying heat and/or catalysts (LAM; KAN; YUEN, 2012; YLHSIEH; GORDON, 2006).
Figure 2 - Schematic illustration of cellulosic chain structure. (Source: ZANROSSO, 2016)
Regarding the manufacturing method, the main textile structures are knitted, nonwoven and woven as shown in Figure 3.
Figure 3 - Structure of textiles: (a) woven, (b) welf knitted, (c) warp knitted, (d) nonwoven. (Source: GONG; OZGEN, 2018)
Knitted fabrics are produced by the interpenetration of loops formed by one or more threads generating patterns such as tricot. Nonwoven fabrics originate from the intertwining of layers of yarns which are attached by physical or chemical processes, for example, felt. Woven fabrics are produced by weaving at least two sets of yarns that intersect in an angle of 90º. The horizontally arranged yarns are called wefts, and the vertically ones are called warps.Woven fabrics can be also sub-classified as simple structured, when formed by only one warp and weft set; or, complex structured, when they have more than one set of warp and weft. Denim is an example of simple structured woven fabric and terry towel fabrics is sub-classified as a complex structured (FURTADO, 2015; GONG; OZGEN, 2018).
Terry towel is a woven fabric product, topped with loop pile covering one or both entire sides forming strips, checks or other patterns. Moreover, it can contain end hems or fringes and side hems or selvedges (AMERICAN SOCIETY FOR TESTING AND MATERIALS, 2013b). It can be categorized according to its weight, manufacturing method, finishing and pile presence on the surface as presented in Figure 4. Futhermore, terry towel cotton fabric features include great absorbancy due to a high surface area; crease resistance and dullness (YILMAZ; POWELL; DURUR, 2005).
27
Figure 4 - Classification of terry towel fabrics. (Source: YILMAZ; POWELL; DURUR, 2005)
3.2 FABRIC FUNCTIONALIZATION STRATEGIES
The growing demand and sophisticated customers boost the rising concern for hygienic living, leading also to the development of novel products able to meet the market expectations (LAM; KAN; YUEN, 2012; RADETIĆ, 2013).
In the textile sector, the challenge lies in improving fabric functionalities creating anti-odour and protective finishes, wellness finishes that release fragrances or cosmetics, and medical finishes that deliver drugs (GUGLIUZZA; DRIOLI, 2013; LAM; KAN; YUEN, 2012).
Fabric functionalization strategies pursue good adhesion, uniformity and stability of the deposited finish, without significant losses in the fabric intrinsic properties. In this sense, the most usual techniques cited in the literature are: plasma treatment (BOZZI et al., 2005); sol-gel
Weight Very heavy (>550 g/m2) Heavy (450 - 550 g/m2) Medium (350 - 450 g/m2) Light (250 - 350 g/m2) Manufacture Method Woven Weft knitted Warp knitted Finishing Velour Printed Embroider Appliqués
Pile on the surface One side
(JOSHI; BHATTACHARYYA, 2011) electrospinning (DAS et al., 2014); exhaust (CLARK, 2011); and pad-dry-cure (LAM; KAN; YUEN, 2012; RADETIĆ, 2013).
Plasma treatment consists in exposing a fabric to an ionized gas discharge containing active and/or neutral species. The active species can modify fabric surface by creating new functional groups or increasing them in volumetric density. Plasma treatment is considered as a pre-treatment, since the functional finishing should be added afterwards (BHAT et al., 2011; BOZZI et al., 2005; MIHAILOVIĆ et al., 2011; QI et al., 2007).
In sol-gel method, a colloidal finishing solution (sol) acts as the precursor for an integrated network (gel) through hydrolysis and polycondensation reactions. Applied to textile, the challenge is to develop the crystallization of the finishing at low temperatures (BRZEZIŃSKI et al., 2012; GRANCARIC et al., 2017; JOSHI; BHATTACHARYYA, 2011; MESSAOUD, 2011; WANG et al., 2015).
Electrospinning is a non-woven manufacture technique that allows incorporating various substances into polymeric nanofibers during their manufacturing from polymeric solutions. It is an important and versatile tecnique, however, applied for man-made fabrics solely (DAS et al., 2014).
Exhaust method consists in placing a fabric in contact with a finishing bath for a predetermined time, generally with agitation and temperature. The objective is to induce the migration of the finishing material from the bath to the fibers. This is the most used approach for textile dyeing (CLARK, 2011; LI et al., 2018).
In the pad-dry-cure technique, a fabric is initially dipped in a finishing bath (e.g. exhaust method), and then the excess water is mechanically removed (an automatic padder is usually used). Following, the fabric is dried and cured in order to fix the finishing material. This method is the most used in literature for fabric functionalization, especially those which are cellulose-based and use croslinking agents (AFZAL; DAOUD; LANGFORD, 2013; ALAY; GENC, 2015;
CHAUDHARI; MANDOT; PATEL, 2012; HASHEMIKIA;
MONTAZER, 2012; KARIMI et al., [s.d.]; KARTHIK; RATHINAMOORTHY; MURUGAN, 2011; LESSAN; MONTAZER; MOGHADAM, 2011; MEILERT; LAUB; KIWI, 2005; MENGAL et al., 2016; NAZARI, 2014; NAZARI; MONTAZER; MOGHADAM, 2012; RADETIĆ, 2013; TANG; JI; SUN, 2016; ZELJKO B et al., 2013).
29 3.2.1 Polycarboxylic acids as cellulose crosslinking agents
Polycarboxylic acids (PCAs) are organic compounds having two or more carboxyl groups (-COOH) in their molecule. In the textile sector, PCAs have been used to crosslinking cellulose through esterification of cellulose OH-groups. Cellulose esterification by PCAs takes place in two main steps: dehydration of two carboxyl groups forming cyclic anhydride intermediates, and reaction of the cyclic intermediates with cellulose hydroxyl groups. PCAs containing 3 or more COOH-groups are pointed out as the most effective spacers for cellulose. In addition, carboxyl groups that were not used in the covalent esterification bond can be meant to anchor other substances such as TiO2, which presents a well-known
strong electrostatic interaction with carboxylic groups (DHANANJEYAN et al., 2001; FURTADO, 2015; HARIFI; MONTAZER, 2012a; MEILERT; LAUB; KIWI, 2005; RADETIĆ, 2013; ZANROSSO, 2016). The schematic interation of PCAs, TiO2 and cellulose is shown in Figure
5.
The main factors that affect PCAs esterification are the size of the acid molecule and the catalyst used. In relation to the size, polycarboxylic compounds with low molecular weight, such as citric acid (CA), maleic acid (MA), succinic acid (SA) and 1,2,3,4-butanotetracarboxylic acid (BTCA) cause less steric hindrance, therefore better esterification (HASHEMIKIA; MONTAZER, 2012; KARIMI et al., [s.d.]; YANG; WANG; KANG, 1997; SCHRAMM; VUKUŠIĆ; KATOVIĆ, [s.d.]; DEHABADI; BUSCHMANN; GUTMANN, 2013; NAZARI; MONTAZER; MOGHADAM, 2012; HARIFI; MONTAZER, 2012a). Furthermore, the appropriate catalyst accelerates the formation of the intermediate anhydrides, which are reactive species related to the esterification process by the weakening of the hydrogen bonds. For this application, alkali salts of phosphoric, polyphosphoric, phosphorous and hypophosphoric acids showed high catalytic efficiency. Moreover, many researchers indicated sodium hypophosphite as the most effective catalyst for crosslinking cotton by polycarboxylic acids (HARIFI; MONTAZER, 2012a; MORRIS; MORRIS; TRASK-MORRELL, 1996; YANG; WANG; KANG, 1997; YUN LU; YANG, 1999)
Figure 5 - Scheme of cellulose cross-linking by a spacer and TiO2 anchoring.
(Source: MEILERT; LAUB; KIWI, 2005)
3.2.2 Economic and ecological aspects in the selection of a polycarboxylic acid
The recent desire for green processing routes and eco-friendly materials has led to the substitution of traditional formaldehyde-based crosslinking agents with polycarboxylic acids. However, PCAs, although formaldehyde-free, do not always have a green production route. Moreover, some of these compounds are cost intensive, thus reducing the feasibility of their application into marketable products (HARIFI; MONTAZER, 2012b). In this sense, Table 1 presents a comparison of the main PCAs used as spacers in terms of their manufacturing route and cost.
According to Table 1, CA is the most eco-friendly spacer due to its biosynthetic origin. MA, SA, and BTCA are commercially produced by non-renewable sources using petroleum derivatives (e.g. benzene) and/or very aggressive chemicals (e.g. nitric acid), although green synthesis routes have been sought (CHI et al., 2016; LEONEL; CERADA, 1995; ZHU et al., 2016). Despite CA does not present the lowest molecular weight, many authors report good durability of the functional properties developed by using it. Furthermore, in economic terms, citric acid is at least 4-fold more accessible than the other PCAs. Those factors point out its potential for use in functionalized textile products (HARIFI; MONTAZER, 2012b; HSIEH et al., [s.d.]; KARTHIK; RATHINAMOORTHY; MURUGAN, 2011; LEONEL; CERADA, 1995; NAZARI; MONTAZER; MOGHADAM, 2012; YANG; WANG; KANG, 1997).
31 Table 1- Comparative data concerning the main polycarboxylic acids used as cellulose crosslinking agents.
Chemical Citric Acid (CA) Maleic Acid (MA) Succinic Acid (SA) 1,2,3,4 butanetetr a-carboxylic acid (BTCA) Nº of carboxyls 3 2 2 4 Synthesis Method Bio-synthesis using Aspergillus niger (LEONEL; CERADA, 1995) Hydrolysis of maleic anhydride, obtained from oxidation of benzene (LOHBECK et al., 2000) Acidic hydrogenation of maleic acid (CORNILS; LAPPE, 2000) Nitric acid oxidation of cyclo-alkene dicarboxylic acid (NATION AL TOXICOL OGY PROGRA M, 1989) Molecular Weight 192.12 116.07 118.09 234.16 CAS Number 77-92-9 110-16-7 110-15-6 1703-58-8 Vendor Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Cost (USD / 100 g)* 6.30 29 41.4 70.20 * https://www.sigmaaldrich.com/united-states.html, accessed in 01/20/2019.
3.3 FUNCTIONAL PROPERTIES
The surface modification of ordinary materials through nanoparticles immobilization is capable of generating numerous new functionalities in the original material. In this section, some of the principal functionalities developed essencially in textile materials are highlighted.
3.3.1 Durable-press
Durable-press (also called easy-care) refers to both the wrinkle-resistance and the finishing process that originates crease wrinkle-resistance. This functionality is particularly indicated to cotton fabrics due to its low creasing resistance, especially in presence of moisture.
Wrinkle effect on cotton fabrics is caused by slippage between their linear cellulosic chains in detriment of breaking and distorted re- formed hydrogen bonds in the amorphous (i.e., less packed) region under stress or/and moisture. In this sense, durable-press processing is based on chemical reactions of a crosslinking agent with OH-groups of cellulose in the presence of heat and catalyst, forming covalent bonds between chains, thus preventing them from sliding (DEHABADI; BUSCHMANN; GUTMANN, 2013; HARIFI; MONTAZER, 2012a; KARTHIK; RATHINAMOORTHY; MURUGAN, 2011; LAM; KAN; YUEN, 2012; YUEN et al., 2009).
Traditional cross-linking agents, based on formaldehyde, have been considered carcinogenic, mutagenic, toxic, and cause mechanical properties deterioration. Thus, PCAs are widely studied as a feasible alternative due to their cost-effectiveness, availability, resistance to acids and chlorine, and they are more eco-friendly. Furthermore, TiO2-NPs
have been used as a co-catalyst with phosphorous-based catalysts to improve wrinkle resistance properties (HARIFI; MONTAZER, 2012a; KARTHIK; RATHINAMOORTHY; MURUGAN, 2011).
3.3.2 Flame retardancy
Fabric flammability is dependent of the fibers origin, manufacturing method, linear, and thread density and environmental conditions. Related to the fibers, cellulose is a non-volatile material, which means that cellulosic fibers are not directly related to fire initiation. However, they have low flame resistance, and when ignited, an intense thermal degradation undergoes and the flame is propagated due to volatile
33 and combustible compounds formation. The large cotton utilization and the continuing implementation of flammability standards have been driving the need for flame-retardant cotton (MOHAMED; EL-SHEIKH; WALY, 2014).
Flame-retardants are chemical compounds added to combustible substrates to provide more resistance to ignition. They are also indicated to prevent ignition and to slow fire spread. However, they are not designed to completely stop fire progress, although they may lower the heat release rate (BEARD; ANGELER, 2010).
Flame retardancy can be achieved by different physical and chemical pathways. The physical mechanisms include (BEARD; ANGELER, 2010; LAM; KAN; YUEN, 2012):
Cooling, when the retardant agent acts dissipating heat by endothermic processes such as fusion or sublimation;
Formation of a protective layer (coating), when the substrate is protected from the heat and oxygen with a solid or gaseous protective layer formed either during the finish bath or after been heated; and
Dilution, when the flame-retardant agent evolves noncombustible gases such as water vapors, CO2, ammonia, resulting in the
dilution of the fuel.
The chemical processes are based in gas or solid phase reaction described as follows:
Gas phase reaction, where the combustion process is disrupted by decreasing the concentration of volatile products and by changing combustion point temperature, which results in the cooling of the system, reducing and eventually abolishing the supply of flammable gases; and
Solid phase reaction, where the flame-retardant agent can reduce the pyrolysis temperature, or build up a char layer protecting the substrate against oxygen or even increase the formation of non-volatile compounds.
Currently, the most used flame-retardant agents are based on halogenated, mineral, phosphorous or nitrogen compounds. Concerning cotton fabric application, there are two main commercial products based on tetrakis(hydroxymethyl)phosphonium chloride (THPC) (commercially named "Pyrovatex CP"), which act forming insoluble polymers that are physically trapped inside the fibers, and, N-methylol dimethylphosphono propionamide ("Pyrovatex CP New"), which is chemically bonded to the cellulose using a cross-linking agent (EL-HADY; FAROUK; SHARAF, 2013; MOHAMED; EL-SHEIKH; WALY, 2014; WU; YANG, [s.d.]).
Halogen-containing flame-retardants, such as Pyrovatex CP, are durable, low-cost, and cause little damage in the physical and mechanical properties of the textile substrate. Nevertheless, this class of flame-retardants has been pointed out as highly toxic and also harmful due to the formation of potentially carcinogenic by-products during combustion (BEARD; ANGELER, 2010; EL-HADY; FAROUK; SHARAF, 2013; LAM; KAN; YUEN, 2012; MOHAMED; EL-SHEIKH; WALY, 2014; WU; YANG, [s.d.]).
The introduction of eco-friendly finishes in the textile industry, eventually through nanotechnology, is replacing the traditional harmful flame-retardants by functionalized coatings (LESSAN; MONTAZER; MOGHADAM, 2011; MOHAMED; EL-SHEIKH; WALY, 2014).
3.3.3 Antibacterial effect
Microorganisms growth on textiles can generate unpleasant odors, low fabric durability and high risk of contamination and infections; the latter is more critical in clinical environments (ABDEL-HALIM et al., 2010; GAO; CRANSTON, 2008; LAM; KAN; YUEN, 2012; WIENER-WELL et al., 2011; WINDLER; HEIGHT; NOWACK, 2013). In fact, according to Wiener-Well et al., 2011, 60% of hospital uniforms are colonized by potentially pathogenic bacteria and the areas of greater contamination corresponded to the pockets and cuffs, which can lead to hands recontamination, raising the risk of cross infections. In order to avoid or limit these and other negative effects, the development of antimicrobial fabrics has been widely investigated (GAO; CRANSTON, 2008; WINDLER; HEIGHT; NOWACK, 2013).
Although antimicrobial textile materials are not a recent phenomenon, nowadays they have their use expanded to various sectors such as hospital materials (uniforms, coats, bedding, mattresses), air filters, household (towel, curtains, mattress liners), and majorly (85% of the total produced) in clothing and footwear (DASTJERDI;
35 MONTAZER, 2010; GAO; CRANSTON, 2008; WINDLER; HEIGHT; NOWACK, 2013).
Typically, antimicrobial tests are focused on the evaluation of gram-positive and/or gram-negative bacteria growth. The main differences regarding these two groups of bacteria are related to the cell wall and cell organization. Gram-positive bacteria (e.g. Staphylococcus aureus or S. aureus) posses a relatively thick cell wall (20-80 nm) composed of multiple layers of peptidoglycan and a single plasma membrane. Otherwise, gram-negative bacteria (e.g. Escherichia coli or E. coli) have a thin layer of peptidoglycan (2-6 nm) and a complex cell wall structure with two cell membranes: an outer membrane, and a plasma membrane. This cell wall composition makes gram-negative bacteria more resistant to chemical compounds (LAM; KAN; YUEN, 2012).
Antimicrobial agents can perform in different ways against microorganisms: clotting proteins, breaking cell membrane, removing groups necessary for enzymatic functioning and also competing for the same substrate. Thus, different substances, such as synthetic organic compounds (e.g. triclosan), natural compounds (e.g. chitosan), bioactives from plants, noble metals (e.g. Ag) and oxide nanoparticles are employed for this function (GAO; CRANSTON, 2008; LAM; KAN; YUEN, 2012; SIMONCIC; TOMSIC, 2010; WINDLER; HEIGHT; NOWACK, 2013).
Regarding nano-antimicrobial agents, nano-silver is the most used due to its effectiveness against a wide spectrum of microorganisms. Silver antibacterial mechanism is based in suppressing bacterial respiration process, and, in oxygen-charged aqueous media, Ag catalyzes the complete destructive oxidation of microorganisms. ZnO has been also studied as an antibacterial coating on account of its low toxicity to human beings and high efficiency, especially on S. aureus inhibition. ZnO antibacterial activity is related to the generation of hydrogen peroxide (H2O2) from its surface. Thus, ZnO-NPs lead to higher H2O2 generation
and so the antibacterial activity. Another widely studied nano-antibacterial agent is TiO2 -NPs. The photocatalytic reaction using TiO2
has been found to be effective for killing bacteria and inactivating viruses. TiO2 antibacterial effect is based on photo-induced processes,
which generate highly reactive species, mainly hydroxyl radicals and superoxide ions that can decompose peptidoglycan layer destroying bacteria cell membrane resulting in bacteria death by leakage of intracellular substances (LAM; KAN; YUEN, 2012).
Moreover, antimicrobial agents have a biocidal or biostatic effect, with good or bad durability, i.e. releasable or fixed. Biostatic action is based on inhibition of microbial growth and it is accessed when
the concentration of antimicrobial agents in the substrate is at least equal to the minimum inhibitory concentration (MIC). Contrarirwise, biocidal action entails microorganism death as a result of active agent concentration greater than the MIC. Furthermore, the antimicrobial agents can be physically immobilized, called in this case as releasable antimicrobials. This category represents the majority of the antimicrobial products in the textile sector. The antimicrobial activity of such products is attributed to their gradually-controlled release from the textile material into their action field. Antimicrobial agents with good durability are attached to the fabric by chemical bonds. Since these agents are not suppose to be released, the probability of microbes developing resistance to them is small (ABDEL-HALIM et al., 2010; LAM; KAN; YUEN, 2012; SIMONCIC; TOMSIC, 2010).
3.3.4 Photocatalytic performance
Realizing the importance of keeping the planet clean, the progressing in technologies applied to the mitigation of environmental contamination is increasingly demanded. Photocatalysis is a well-known technique for pollutant degradation based on the acceleration of photocatalytic reactions in the presence of a photocatalyst (CARP; HUISMAN; RELLER, 2004; HERRMANN, 1999).
The photocatalytic reaction mechanism, illustrated in Figure 6, starts by activating the photocatalyst with photons having energy equal or higher than the photocatalyst band gap, and ensues in the following main stages:
Adsorption of contaminants on the photocatalyst surface;
Reactions in the adsorbed phase; and
Desorption of reaction products.
Photons absorption leads to electrons (e-) excitation in the photocatalyst surface, and the migration of them from valence to the conduction band, leaving holes (h+) in their original positions. The as-created electron-hole pairs can get trapped and react with compounds adsorbed on the photocatalyst: On one hand, the holes act by capturing electrons from compounds adsorbed on the photocatalyst surface generating hydroxyl radicals (OH•) and cations of hydrogen (H+). On the other hand, electrons form superoxide radicals (O2
-•
) with the oxygen dissolved in the medium. The reaction of superoxide radical with water
37 molecules generates hydroxyl ions (OH-) and peroxide radicals (HOO•), which further react with H+ giving rise to hydroxyl radicals and ions. The OH• species generated by the redox reactions decompose the contaminants (C) present in the medium to simpler compounds such as CO2 and H2O, which are further desorped. Hence, photocatalysis can
decompose common organic matters in air and water media such as odour molecules and coloring substances in the form of solutions and stains, as well as, bacteria and viruses deactivation (LAZAR; VARGHESE; NAIR, 2012; RADETIĆ, 2013; SAAD et al., 2016; WANG et al., 2015).
Figure 6 - Illustration of photocatalysis mechanism. (Source: Own authorship) Since the end of the 20th century, semiconductors have been extensively studied along its photocatalytic potencial, especially titanium dioxide (TiO2), due to its chemical and biological inertness, water
insolubility, high photocatalytic activity and photostability. TiO2 has
three polymorphic forms: anatase, rutile and brookite. The first two polymorphs crystallize in the tetragonal structure, and the last one presents the orthorhombic arrangement. Of the three mentioned polymorphs, anatase and rutile are the most frequent in the literature. Although rutile has a lower band gap energy (3.02 eV) than anatase (3.20 eV), which leads to an absorption of photons closer to the visible spectrum, its lower band gap also implies a higher rate of recombination of electron-hole pairs and the deactivation of the photocatalyst as a consequence. Nonetheless, TiO2 is often used as nanopowder due to its
large surface area available as reaction sites, which favors photocatalytic efficiency (CARP; HUISMAN; RELLER, 2004; ETACHERI et al., 2015; HERRMANN, 1999; KUMAR; BANSAL, 2013).
Photocatalysis processes can take place with the photocatalyst dispersed in a medium (e.g. water) or immobilized on a support. The immobilization appeared as an alternative to avoid the loss of the photocatalyst at the end of the process, which represents a reduction in the cost of the process and enable its reuse (CARP; HUISMAN; RELLER, 2004; CHONG et al., 2010; RADETIĆ, 2013).
3.3.5 Self-cleaning
The increase in the awareness about waste of water has attracted attention to self-cleaning materials. The self-cleaning technology can be applied to building materials, glasses, metals and textiles. A self-cleaning surface can be classified as (super)hydrophobic or (super)hydrophilic (DAS et al., 2014; GANESH et al., 2011; SAAD et al., 2016; WANG et al., 2015).
Hydrophobic surface repels water with the properties of low wettability and contact angles greater than 90º. For contact angle greater than 150°, the surface is named as superhydrophobic. Hydrophobicity can also be regulated by the surface roughness. In rougher surfaces, the contact angle is increased, and form bumps that trap air between water and the surface. Hydrophobic self-cleaning surfaces are bio-inspirated on the Lotus effect, in which ultrahydrophobicity is achieve by the presence of micro and nanoroughness at the surface, enabling the contact area and the adhesion force between surface and droplet to be significantly reduced. Engineered hydrophobic and superhydrophobic surfaces are frequently produced by chemical or geometrical surface modification (DAS et al., 2014; GANESH et al., 2011; SAAD et al., 2016; W H WONG et al., 2005).
Contrarywise, hydrophilic self-cleaning effect takes advantage of the high surface wettability combined with the photocatalytic mechanism to destroye dirt molecules (RADETIĆ, 2013; SAAD et al., 2016; WANG et al., 2006). For contact angles of water close to 90º, the phenomenon is referred to as superhydrophilicity.
3.4 NANOTECHNOLOGY APPLIED TO FABRICS
Nanotechnology has become one of the most important and promising fields, being pointed as a key technology in the 21st century, encompassing several areas, such as physics, chemistry, biology, and
39 engineering. It is defined as a heterogeneous set of technologies applied to nanometric systems, i.e., those that present at least one dimension equal to or smaller than 100 nm. The manipulation of the matter in quasi-atomic scale allows obtaining new structures, devices, and materials that can present different effects and properties of analogous materials in a macro scale, which has been attracting the interest of scientists and trade (FLEISCHER; GRUNWALD, 2008; LAM; KAN; YUEN, 2012; MOROSE, 2010; SIMONCIC; TOMSIC, 2010; W H WONG et al., 2005).
Nanotechnology plays a key role as a functionalization processing route; conferring innovative properties to traditional products by dispersing nanometric materials on their surface. In this context, the development of nano-functionalized products has expanded rapidly through several segments, such as electronics, pharmaceuticals, cosmetics, polymers, energy and textiles.The latter are pointed out as one of the best substrates for nanotechnology application mainly due to their large surface area (ASMATULU; TWOMEY; OVERCASH, 2012; FLEISCHER; GRUNWALD, 2008; MOROSE, 2010; SAAD et al., 2016; SIMONCIC; TOMSIC, 2010; VANCE et al., 2015). According to Hansen et al., 2015, more than 2 thousand products containing nanomaterials were catalogued in Europe between 2012 and 2015, and, most of them were applied in personal care and clothing categories (≥ 300).
Nanotechnological processes, recently introduced in the textile sector, are a route to incorporate new functionalities, high performance and higher added value to textile materials associated with a low consumption of inputs, directing this industry sector to the development of smart, functional and adaptable textile goods (DASTJERDI; MONTAZER, 2010; GANESH et al., 2011; GUGLIUZZA; DRIOLI, 2013; RADETIĆ, 2013; VANCE et al., 2015; WANG et al., 2015).
Functionalized fabrics comprise a wide range of functional properties, such as flame retardancy, durable press, photocatalytic effect, self-cleaning, antibacterial, among others (BOZZI et al., 2005; DEHABADI; BUSCHMANN; GUTMANN, 2013; GANESH et al.,
2011; HASHEMIKIA; MONTAZER, 2012; JOSHI;
BHATTACHARYYA, 2011; LESSAN; MONTAZER; MOGHADAM, 2011; LI et al., 2018; MEILERT; LAUB; KIWI, 2005; SAAD et al., 2016; SCHRAMM; VUKUŠIĆ; KATOVIĆ, [s.d.]; SIMONCIC; TOMSIC, 2010; W H WONG et al., 2005; WU; YANG, [s.d.]).
Narazi et al. (2009) proposed the development of an ease-care plain-weave cotton fabric (118 g/m2) by surface modification with PCAs, SHP and TiO2-NPs using the pad-dry-cure method. The concentration of
PCAs (CA and BTCA) was varied as well as the curing method (UV-A irradiation, temperature, and UV-A + temperature). The effect of functionalization process was evaluated in the developed of wrikle-resistence and in the physical and mechanic properties. The durable-press effect was assessed in terms of the wrinkle recovery angle (WRA), a parameter that evaluates the capacity of a textile material to recover its smooth appearance after been creased. The authors concluded that all the functionalization parameters affect the response, and easy-care property increased with cross-linking agent and SHP contents. Their best response was obtained with 2.6 wt% TiO2-NPs, 99 g/L BTCA and a combined
curing (UV-A + temperature). Despite BTCA crosslinking agent has presented the best easy-care response and less yellowing, the tensile strength retention and hydrophilicityt were higher for CA-treated samples.
Karthik, Rathinamoorthy & Murugan (2011) also evaluated the easy-care effect on plain-weave cotton fabric (112 g/m2) by using PCAs. The wrinkle-resistant treatment was done with CA, SHP and TiO2-NPs as
a co-catalyst. The authors pointed out that due to the nanometric size, TiO2-NPs could fill the cellulose amorphous regions, which might restrict
the movement of the chains. Their results demonstrated an increase in the WRA with CA, SHP, and TiO2-NPs concentrations. Moreover, the
optimum wrinkle-recovery was achieved in the range of 0.05-0.1% TiO2
-NPs, and for concentration below 0.05% and above 0.1%, no significant improvement as observed. However, this treatment decreases the tensile strength in about 20% and a little yellowing.
Lessan, Montazer & Moghadam (2011) presented a novel flame-retardant plain-weave cotton fabric (128 g/m2), based on MA, SHP, triethanol amine (TEA) and TiO2-NPs through pad-dry-cure
functionalization methodology. In this work, TEA was used with the aim to reduce the yellowing by reducing the formation of unsaturated compounds derivated from MA and SHP at hight temperature. The authors observed that increasing MA and SHP concentrations leads to decrease on flammability, and higher phosphorous content increased the char residue formation. The thermal analysis indicated the presence of SHP leads to decrease the initial and maximum decomposition temperatures. In addition, flame retardancy of the treated fabrics withstood after five washing cycles and the TEA addition incresed the whiteness.
El-Hady, Farouk & Sharaf (2013) presented a novel flame retardant and UV protection for mill-bleached cotton (210 g/m2) and