This research project originated from interest in assessing the potential of enzyme technology (particularly laccase-based systems) for the biomodification of sisal specialty fibres by using environmentally friendly processes. This doctoral work focused on two different research lines, namely: biobleaching and enzymatic functionalization of sisal pulp fibres. The study was started by assessing the use of natural, potentially cost-effective phenolic compounds as substitutes for expensive, potentially toxic laccase mediators. The tendency of natural phenols to either promote delignification or couple onto pulp was examined with a view to assessing their potential for either bleaching or functionalizing sisal fibres. In the biobleaching study, totally chlorine free (TCF) sequences were implemented in order to compare the efficiency of a selected natural mediator and a well-known synthetic mediator, both in the presence and absence of a xylanase pre-treatment. The effluents resulting from each stage in the sequence were analysed with a view to assessing the environmental impact of the laccase treatments ¿a scarcely explored aspect of biobleaching sequences. The xylanase stage proved highly efficient in reducing the HexA content of sisal fibres and in boosting the bleaching effect of the laccase treatments. The proposed TCF sequences provided high-cellulose sisal pulp with brightness above 80% ISO and a reduced HexA content; also, they exhibited improved performance and a reduced impact on effluent properties relative to the use of the synthetic mediator. Two different approaches to fibre functionalization were explored, namely: lignin modification (biografting) and cellulose modification (laccase¿TEMPO oxidation). Biografting of phenolic compounds was for the first time studied in sisal pulp. Covalent binding of the originally assayed phenolic compounds to sisal fibres during the laccase treatment was exposed by a novel analytical approach based on pyrolysis-GC/MS. The phenolic compound showing the highest tendency to couple to fibres was selected to investigate biografting under different reaction conditions and to evaluate the extent of phenol coupling via various pulp properties. Biografting efficiency was enhanced by refining the fibres prior to the enzyme treatment, which provided improved strength-related properties in the resulting paper. The use of the laccase-TEMPO system to oxidatively modify cellulose and improve strength-related properties in sisal pulp was for the first time evaluated as an environmentally friendly alternative to existing halide-based systems. The first part of this study revealed that the laccase¿TEMPO system considerably improved wet strength in sisal pulp by effect of the formation of a substantial amount of aldehyde groups in cellulose chains that facilitated inter-fibre bonding through hemiacetal linkages. The influence of process variables on various properties of the oxidized fibres and resulting paper was assessed by using a three-variable statistical plan. The conditions maximizing functionalization and the improvement in paper strength properties were used to design treatments of increased efficiency that exposed the potential of laccase¿TEMPO oxidation for biorefining pulp fibres. Analytical methods including pyrolysis-GC/MS, polyelectrolyte titration, conductimetric titration, carbohydrate determination by HPLC, fibre morphology analysis by SEM and thermogravimetry were used to both characterize the raw material and gain a better understanding of the reaction mechanisms behind the different laccase-based treatments. Some of the analyses were performed by collaborating research groups at IRNAS (Seville, Spain) and the Department of Chemical Engineering of the University of Huelva (Spain). Also, part of this doctoral work was conducted at the Institute of Paper Science and Technology of the Georgia Institute of Technology (Atlanta, USA).

Este trabalho avalia as vantagens do uso das fibras curtas de polpa de Eucalipto tanto como alternativa às fibras longas de polpa de Pinus, como também para fibras sintéticas, tradicionalmente usadas no reforço de materiais cimentícios. Os efeitos da morfologia (comprimento, largura, fibrilação, conteúdo de finos, número de fibras por grama, etc.) das fibras celulósicas no processamento, no desempenho mecânico e físico e na microestrutura dos compósitos de fibrocimento foram avaliados. Os compósitos foram avaliados antes e após ciclos de envelhecimento acelerado. Fibras de Eucalipto apresentaram melhor dispersão na matriz cimentícia e forneceram maior densidade de fibras em massa ou em volume, em relação às fibras de Pinus. As fibras curtas permitem um reforço efetivo da matriz frágil, diminuindo a propagação das fissuras, o que contribuiu para o melhor desempenho mecânico dos compósitos após envelhecimento. Estes resultados promissores mostram o potencial apresentado pelas fibras curtas de Eucalipto para reduzir custos, em vista da substituição parcial das fibras sintéticas em processos de cura ao ar, e durante o refino da polpa celulósica. O efeito do branqueamento das fibras também foi avaliado, e mostrou que as fibras branqueadas de Eucalipto são mais reativas para se ligarem por pontes de hidrogênio com a matriz cimentícia. Fibras branqueadas melhoraram a interface entre fibra e matriz, embora apresentassem mais sinais de mineralização (re-precipitação de produtos de hidratação dentro das fibras) do que as fibras não-branqueadas. O refino da polpa celulósica foi utilizado para modificar as propriedades morfológicas das fibras de Eucalipto e Pinus. Os resultados mostraram que são necessárias maiores intensidade de refino na polpa de Pinus para obter valores de retenção de sólidos do cimento similares àqueles obtidos com fibras não-refinadas de Eucalipto. O refino aumentou a capacidade das fibras de capturar as partículas minerais, melhorando a aderência das fibras com a matriz. Esta melhor interface entre fibra e matriz melhorou as propriedades mecânicas dos compósitos aos 28 dias de cura, mas os tornou mais frágeis após os ciclos de envelhecimento acelerado. A modificação química da superfície das fibras foi realizada com o objetivo de melhorar as ligações entre fibra e matriz e diminuir a mineralização da fibra dentro dos compósitos. Esta modificação química foi realizada com Metacriloxipropiltri-metoxisilano (MPTS) e Aminopropiltri-etoxisilano (APTS) e mostrou influenciar significativamente a microestrutura dos compósitos. Ciclos de envelhecimento acelerado diminuíram o módulo de ruptura (MOR) e a tenacidade (TE) dos compósitos com fibras não-modificadas e modificadas; entretanto, compósitos reforçados com fibras modificadas com MPTS apresentaram fibras sem produtos de hidratação do cimento em seu interior, enquanto que fibras modificadas com APTS apresentaram acelerada mineralização. Fibras mineralizadas tornam os compósitos mais frágeis após os ciclos de envelhecimento acelerado. Estas observações são, portanto, muito úteis para o entendimento da contribuição de diferentes condições das fibras (composição química, resistência mecânica, morfologia e propriedades de superfície) para os mecanismos de aderência entre fibras e matriz cimentícia, de mineralização das fibras e de degradação dos compósitos de fibrocimento. / This work evaluates the advantages of using hardwood short fibre pulp (Eucalyptus) as alternative to softwood long fibre pulp (Pinus) and synthetic fibres, traditionally used in reinforcement of cement based materials. The effects of cellulose fibre morphology (e.g., length, width, fibrillation, content of fines and number of fibres per gram) on the processing, on the mechanical and physical performance and on the microstructure of fibre-cement composites were evaluated. Composites were evaluated before and after accelerated ageing cycles. Eucalyptus pulp fibres were better dispersed in the cement matrix and provided higher number of fibres per unitary weight or volume, in relation to Pinus long fibre pulp. The short reinforcing elements lead to an effective crack bridging of the fragile matrix, which contributes to the improvement of the mechanical performance of the composite after ageing. These promising results show the potential of eucalyptus short fibres for reducing costs by both the partial replacement of expensive synthetic fibres in air curing process and the energy savings during pulp refining. The effects of pulp bleaching were also evaluated, and showed that Eucalyptus bleached fibres are more reactive to bond with the cement matrix by hydrogen bonds. Bleached fibres improved the fibre-matrix interface, although they presented more signals of fibre mineralization. Mechanical refining was used to change the morphological properties of Eucalyptus and Pinus pulps. Results show that high levels of refining were necessary for Pinus pulp to obtain cement retention values similar to those obtained by unrefined Eucalyptus pulp. The mechanical refining increased the capacity of the fibres to capture the mineral particles improving the adherence of the fibres with the matrix. This improved fibre-matrix interface led to better mechanical properties at 28 days of cure, but turned brittle the composites after 200 ageing cycles. The chemical surface modification of cellulose pulp fibres was done in order to improve fibre-matrix bonding and to decrease fibre mineralization into the composite. Surface modification of the cellulose pulps was performed with Methacryloxypropyltri-methoxysilane (MPTS) and Aminopropyltri-ethoxysilane (APTS) and showed significant influence on the microstructure of the composites. Accelerated ageing cycles decreased modulus of rupture (MOR) and toughness (TE) of the composites with unmodified and modified fibres, however composites reinforced with MPTS-modified fibres presented fibres free from cement hydration products, while APTS-modified fibres presented accelerated mineralization. Higher mineralization of the fibres led to higher embrittlement of the composite after accelerated ageing cycles. These observations are therefore very useful for understanding the contribution of the different fibre conditions (chemical composition, mechanical strength, morphology and surface properties) to the mechanisms of fibre-matrix adherence, fibre mineralization and degradation of fibre-cement composites.

Este trabalho avalia as vantagens do uso das fibras curtas de polpa de Eucalipto tanto como alternativa às fibras longas de polpa de Pinus, como também para fibras sintéticas, tradicionalmente usadas no reforço de materiais cimentícios. Os efeitos da morfologia (comprimento, largura, fibrilação, conteúdo de finos, número de fibras por grama, etc.) das fibras celulósicas no processamento, no desempenho mecânico e físico e na microestrutura dos compósitos de fibrocimento foram avaliados. Os compósitos foram avaliados antes e após ciclos de envelhecimento acelerado. Fibras de Eucalipto apresentaram melhor dispersão na matriz cimentícia e forneceram maior densidade de fibras em massa ou em volume, em relação às fibras de Pinus. As fibras curtas permitem um reforço efetivo da matriz frágil, diminuindo a propagação das fissuras, o que contribuiu para o melhor desempenho mecânico dos compósitos após envelhecimento. Estes resultados promissores mostram o potencial apresentado pelas fibras curtas de Eucalipto para reduzir custos, em vista da substituição parcial das fibras sintéticas em processos de cura ao ar, e durante o refino da polpa celulósica. O efeito do branqueamento das fibras também foi avaliado, e mostrou que as fibras branqueadas de Eucalipto são mais reativas para se ligarem por pontes de hidrogênio com a matriz cimentícia. Fibras branqueadas melhoraram a interface entre fibra e matriz, embora apresentassem mais sinais de mineralização (re-precipitação de produtos de hidratação dentro das fibras) do que as fibras não-branqueadas. O refino da polpa celulósica foi utilizado para modificar as propriedades morfológicas das fibras de Eucalipto e Pinus. Os resultados mostraram que são necessárias maiores intensidade de refino na polpa de Pinus para obter valores de retenção de sólidos do cimento similares àqueles obtidos com fibras não-refinadas de Eucalipto. O refino aumentou a capacidade das fibras de capturar as partículas minerais, melhorando a aderência das fibras com a matriz. Esta melhor interface entre fibra e matriz melhorou as propriedades mecânicas dos compósitos aos 28 dias de cura, mas os tornou mais frágeis após os ciclos de envelhecimento acelerado. A modificação química da superfície das fibras foi realizada com o objetivo de melhorar as ligações entre fibra e matriz e diminuir a mineralização da fibra dentro dos compósitos. Esta modificação química foi realizada com Metacriloxipropiltri-metoxisilano (MPTS) e Aminopropiltri-etoxisilano (APTS) e mostrou influenciar significativamente a microestrutura dos compósitos. Ciclos de envelhecimento acelerado diminuíram o módulo de ruptura (MOR) e a tenacidade (TE) dos compósitos com fibras não-modificadas e modificadas; entretanto, compósitos reforçados com fibras modificadas com MPTS apresentaram fibras sem produtos de hidratação do cimento em seu interior, enquanto que fibras modificadas com APTS apresentaram acelerada mineralização. Fibras mineralizadas tornam os compósitos mais frágeis após os ciclos de envelhecimento acelerado. Estas observações são, portanto, muito úteis para o entendimento da contribuição de diferentes condições das fibras (composição química, resistência mecânica, morfologia e propriedades de superfície) para os mecanismos de aderência entre fibras e matriz cimentícia, de mineralização das fibras e de degradação dos compósitos de fibrocimento. / This work evaluates the advantages of using hardwood short fibre pulp (Eucalyptus) as alternative to softwood long fibre pulp (Pinus) and synthetic fibres, traditionally used in reinforcement of cement based materials. The effects of cellulose fibre morphology (e.g., length, width, fibrillation, content of fines and number of fibres per gram) on the processing, on the mechanical and physical performance and on the microstructure of fibre-cement composites were evaluated. Composites were evaluated before and after accelerated ageing cycles. Eucalyptus pulp fibres were better dispersed in the cement matrix and provided higher number of fibres per unitary weight or volume, in relation to Pinus long fibre pulp. The short reinforcing elements lead to an effective crack bridging of the fragile matrix, which contributes to the improvement of the mechanical performance of the composite after ageing. These promising results show the potential of eucalyptus short fibres for reducing costs by both the partial replacement of expensive synthetic fibres in air curing process and the energy savings during pulp refining. The effects of pulp bleaching were also evaluated, and showed that Eucalyptus bleached fibres are more reactive to bond with the cement matrix by hydrogen bonds. Bleached fibres improved the fibre-matrix interface, although they presented more signals of fibre mineralization. Mechanical refining was used to change the morphological properties of Eucalyptus and Pinus pulps. Results show that high levels of refining were necessary for Pinus pulp to obtain cement retention values similar to those obtained by unrefined Eucalyptus pulp. The mechanical refining increased the capacity of the fibres to capture the mineral particles improving the adherence of the fibres with the matrix. This improved fibre-matrix interface led to better mechanical properties at 28 days of cure, but turned brittle the composites after 200 ageing cycles. The chemical surface modification of cellulose pulp fibres was done in order to improve fibre-matrix bonding and to decrease fibre mineralization into the composite. Surface modification of the cellulose pulps was performed with Methacryloxypropyltri-methoxysilane (MPTS) and Aminopropyltri-ethoxysilane (APTS) and showed significant influence on the microstructure of the composites. Accelerated ageing cycles decreased modulus of rupture (MOR) and toughness (TE) of the composites with unmodified and modified fibres, however composites reinforced with MPTS-modified fibres presented fibres free from cement hydration products, while APTS-modified fibres presented accelerated mineralization. Higher mineralization of the fibres led to higher embrittlement of the composite after accelerated ageing cycles. These observations are therefore very useful for understanding the contribution of the different fibre conditions (chemical composition, mechanical strength, morphology and surface properties) to the mechanisms of fibre-matrix adherence, fibre mineralization and degradation of fibre-cement composites.