Cellulose is the most abundant, renewable and sustainable biopolymer on the planet, having many desirable characteristics that make it a suitable material for many applications. Cellulose fibrils from renewable resources have been gaining increasing interest due to their sustainable and biodegradable nature, combined with other mechanical, optical, thermal, and fluidic properties. Cellulose fibrils are therefore attractive for the production of a variety of materials, from composites to porous membranes and gels, filaments and films. The investigation developed throughout this work aimed to explore both the rheological behavior of nanofibrillated cellulose (NFC) suspensions using different liquid media, and mechanical, optical and barrier properties of structures produced from this material.
Regarding the investigation of NFC suspensions’ rheological behavior, the undertaken study focused on the morphological effects of the fibrils present in aqueous suspensions, on the addition of ethanol or acetone to the medium and on the increase of the medium’s ionic strength through the addition of high NaCl concentrations. For this purpose, two different NFC suspensions were used: a carboxymethylated NFC aqueous suspension obtained from Innventia (Stockholm, Sweden) (NFC-carb); and a NFC aqueous suspension produced in the laboratory using a commercial eucalyptus bleached sulfite pulp, subjected to a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediated oxidation pretreatment in a TEMPO/NaBr/NaClO system, which was subsequently subjected to high pressure homogenization at the RAIZ (Institute for Research on Forest and Paper) facilities (NFC-TEMPO). The morphological analysis of the suspensions through SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) allowed to infer a higher percentage of nanoelements in the NFC-TEMPO suspension compared to the NFC-carb. The physicochemical analysis of the fibrils on both suspensions was performed: the intrinsic viscosity was 397 ml/g for NFC-carb and 85 ml/g for NFC-TEMPO, corresponding to cellulose polymerization degrees of 696 and 149, respectively; the quantification of carboxylic groups was 698 μmol/g for NFC-carb and 1900 μmol/g for NFC-TEMPO. These results confirmed that the used suspensions were morphologically and physicochemically very different.
In order to study the effect of the liquid medium’s characteristics in which the nanofibrils are suspended, on the rheology of the suspension, the addition of ethanol or acetone to the medium was explored, and concentrations between 2.5 and 40% of ethanol or acetone were investigated, using the NFC-carb. On the other hand, to study the effect of increasing ionic strength on the suspensions’ rheological behavior, NaCl was added in order to obtain concentrations between 50 mM and 1000 mM, for both studied nanofibrils (NFC-carb and NFC-TEMPO). The solids content of both studied suspensions was adjusted to 1.3%. The suspensions were subjected to rheological measurements in flow and oscillatory modes, using a tension-controlled rheometer, equipped with a cone and plate geometry, with either smooth or rough surfaces. In the later case, sandpaper with known roughness was attached to the surface of both tools.
The study of the geometry roughness effect on the NFC-carb suspensions’ rheological behavior showed higher shear stresses and viscosity for the rough geometry at lower shear rates (0.05 to 5 s-1), but at higher rates (up to 100 s-1) the measurements were less influenced by either flow instabilities or the geometry’s roughness, closely representing the bulk properties of the suspension. The subsequent rheological measurements were carried out with sandpaper with a roughness of 58.5 μm.
Regarding the study of ethanol or acetone addition effect on the NFC-carb suspension, results showed that an addition of 2.5 wt.% of either solvent decreased the viscosity and dynamic modulus, while a 40 wt.% addition increased the shear stress for values higher than those of the corresponding aqueous suspensions, particularly at low or moderate shear rates, indicating higher interfibrillar interaction. The suspension containing 40wt.% ethanol enabled to further double the storage modulus and allowed the extension of the gel-like behavior to higher shear stress values.
Concerning the effect of ionic strength, increasing the NaCl concentration from 50 mM to 100 mM drastically increased the viscosity of both NFC-carb and NFC-TEMPO suspensions, while the network’s energy storage modulus the in the elastic region linearly increased with the increase of NaCl concentration from 100 mM to 1000 mM, suggesting improved interfibrillar interaction with increasing salt concentration. This result is in agreement with the expected decrease in electrostatic repulsion between fibrils due to salt addition. In practical terms, the suspensions elastic domain was extended from 10 Pa to 50 Pa with the addition of 40 wt.% of acetone, and to above 500 Pa with NaCl addition at a concentration of 1000 mM.
Despite the much lower cellulose degree of polymerization of the NFC-TEMPO sample, compared to that of the NFC-carb (149 vs 696), the NFC-TEMPO aqueous suspension without added salt exhibited a markedly higher shear stress over the entire studied shear rate range. This result evidences the importance of particle size, as well as specific surface area, on the suspension’s rheological behavior, as the NFC-TEMPO sample has a higher nanoelements (nanofibrils) content than the NFC-carb (based on SEM and TEM analysis).
In addition to film production, NFC gel can be used to produce filaments for potential applications in the textile industry. In the present work, the effect of suspension’s solids content and spinning rate on the mechanical properties of filaments produced via wet spinning were studied. For this purpose, two commercial eucalyptus bleached pulps, one obtained by a kraft process and the other one obtained by a sulfite process, were used as sources of cellulose fibers to produce NFC suspensions. Firstly, both pulps were submitted to a refining process in a PFI mill. Then, the refined sulfite pulp was subjected to TEMPO-mediated oxidation in a TEMPO/NaBr/NaClO system, while the refined kraft pulp was subjected to TEMPO-mediated oxidation in a TEMPO/NaClO/NaClO2 system. Both oxidized pulps were subsequently subjected to high pressure homogenization in the RAIZ facilities. The initial characteristics of the pulps and the different applied oxidative treatments led to products with very different degrees of cellulose polymerization. For the sulfite pulp, the cellulose degree of polymerization in the NFC-s was 202, while the NFC resulting from kraft pulp (NFC-k) had a cellulose degree of polymerization of 848. The highest cellulose degree of polymerization of the NFC-k is a consequence of the higher initial value of the pulp and also the less aggressive oxidation treatment with the TEMPO/NaClO/NaClO2 system. The suspensions’ solids content was adjusted to a range from 2.5 to 3.22 % and were used to produce cellulosic filaments via wet spinning, with spinning rates of 0.45 to 1.7 m min-1 into a 1M NaCl coagulation bath, followed by an ethanol fixation bath. Some of the produced filaments were also subjected to an additional water bath, in order to wash out the salt still present in the filaments. The filaments ends were pinned, maintaining the filaments’ length and were air dried under standard conditions of temperature and relative humidity. Subsequently, filaments’ mechanical performance was tested.
The filaments were void-free and had an approximately circular cross section, which was not substantially altered by the spinning rate variation in the studied range, nor by the reimmersion in the washing bath and second drying.
The results showed that an increase in wet spinning rate improved the mechanical performance of the filaments, indicating some level of fibril alignment. On the other hand, the increase in the suspensions’ solids content did not translate into an improvement in the filaments’ strength, due to the higher viscosity and reduced alignment capacity of the fibrils. The filaments produced from the NFC-s suspension exhibited superior mechanical performance, despite the much lower cellulose degree of polymerization. On the other hand, the microscopic analysis of the suspension shows a higher fibrillation extent and a higher nanoelements content, which again highlights the role of fibril morphology in the structure/performance of the filaments.
After additional washing of the filaments, and as a consequence of removing some amount of salt, the linear mass and cross-sectional diameter of the filaments significantly decreased, thus a drastic increase in tensile strength, tenacity and strength-to-failure was observed due to interfibrillar interaction reestablishment. A broader and deeper exploration of filaments production from NFC gels requires a more advanced experimental setup, namely one that allows to increase the spinning rate, which was not available.
With regard to cellulose films, in addition to mechanical properties, barrier properties are very important in several applications. Thus, at a given stage of the undertaken work, the effect of hot calendering on the physical and water vapor barrier properties of bacterial and vegetable cellulosic films was studied. The production of bacterial cellulose was carried out using Gluconacetobacter xylinus at the University of Minho facilities and the films were produced and tested at the University of Beira Interior. To produce the films, bacterial cellulose was fragmented in a blender and the solid content of the resulting suspension was adjusted to 0.354 %. Part of this suspension was subjected to ozonization in order to improve the optical properties of the resulting films.
Films were produced by vacuum filtration from non-ozonized (BC) and ozonized (BCO) bacterial cellulose suspensions. After draining the water, the filter cakes were adhered to metal discs and pressed between blotting papers using a procedure analogous to paper production. Then, the films were dried overnight, between perforated metal rings under pressure applied to the ends of the sheets to prevent the films from shrinking, under standard conditions of temperature and relative humidity. For properties comparison, vegetable cellulose films were also produced from highly refined bleached kraft eucalyptus pulp with approximately the same grammage as the bacterial cellulose films. To study the calendering effect, some films were subjected to a hot calendering process.
Results showed that BC and BCO suspensions formed much denser films than those from vegetable cellulose, forming much more compact and closed structures, in agreement with the smaller dimension of the constituent elements. As a consequence of the high density, the Young's modulus and tensile index of BC and BCO films were much higher than those obtained for vegetable cellulose.
Calendering substantially increased the transparency of the films and ozonation was effective in increasing the brightness and transparency of the BCO films regarding the BC films. However, both processes slightly decreased the mechanical performance of the films. The water vapor transfer rate was lower for bacterial cellulose films than for vegetable cellulose films and decreased by 70% with calendering. Concluding, hot calendering can be used to obtain bacterial cellulose (or nanofibrillated vegetable cellulose) films suitable for various applications, where the water vapor barrier represents an important functionality. Ozonation can be used as a way to improve the optical properties of the films, although it is necessary to establish a compromise between the benefits of ozonization and calendering, and the decrease in the mechanical performance of the films induced by these processes.
Subsequently, research was carried out in order to provide hydrophobicity to films made from a NFC suspension, through coating with stearic acid (SA) and particles of modified precipitated calcium carbonate (PCC). The NFC used in this work, oxidized with a TEMPO/NaBr/NaClO system, contained a cellulose with a degree of polymerization of 367 and a carboxylic group content of 997 μmol/g.
An SA aqueous suspension was prepared following a procedure that involved raising the temperature of a SA/water mixture and sonicating it, followed by cooling in an ice bath. The modification of the PCC was carried out by adding PCC to the priorly made SA suspension and subjecting it to ultrasounds. After drying in an oven, the modified PCC was collected and manually ground, resulting in a fine white powder. Both the SA aqueous suspension and the modified PCC were used as a coatings in the production of hydrophobic NFC films. The films were produced by vacuum filtration and later surface-modified, resulting in three different sets: a first set of neat, uncoated NFC films; a second set coated with a layer of stearic acid (NFC-SA) and a third set coated with a layer of stearic acid and an additional layer of modified PCC (NFC-LBL). The films were dried overnight, between perforated metal rings under pressure applied to the ends of the sheets, under standard conditions of temperature and relative humidity. Subsequently, some of the coated films were subjected to heat treatment in an oven at 68ºC or 105ºC. Next, the films were analyzed in terms of their mechanical performance, water vapor and oxygen barrier properties, as well as static/dynamic contact angle measurements. The results showed that the coatings and heat treatments did not affect the mechanical performance of the films. Heat treatments decreased the water vapor transmission rate and oxygen permeability of both NFC-SA and NFC-LBL films. Coating with SA rendered hydrophobic films as the measured static contact angle reached 122°. Non-heat-treated NFC-LBL films achieved a contact angle of up to 150º and a contact angle hysteresis of 3.1º. The NFC-LBL films treated at 68°C exhibited near superhydrophobicity, with a contact angle of up to 140° and a contact angle hysteresis of 5°. The coating of NFC-LBL films without heat treatment was easily removed by friction, but the heat treatment turned the films’ coating the hydrophobicity persistent, resisting to friction and handling.
Taking into account that the production of nanofibrillated cellulose films through filtration is a slow process, based on previous rheology studies it was decided to study the effect of ethanol addition to the suspension on the drainage time during the production of NFC films by vacuum filtration and the mechanical, optical and barrier properties of the resulting films. For this, aqueous / alcoholic suspensions of NFC with an ethanol content up to 75 wt.% were used in the production of films, monitoring the drainage progression by measuring the collected filtrate. The films were air-dried under standard conditions of temperature and humidity or in an oven at 70ºC for 4 hours. Mechanical and optical performance as well as water vapor barrier properties were tested.
The results showed that the filtration time drastically decreased with increasing ethanol content, from about 2 hours to 2 minutes for ethanol suspensions with 0 (zero) and 75 wt.%, respectively. The increase in ethanol content did not significantly affect the mechanical performance of air-dried films, despite the increase in the global porosity of the structures. On the other hand, oven drying provided the films with tensile properties superior to those exhibited by air-dried films, both produced from purely aqueous suspensions, despite the increase in elongation at break and the higher value of specific light scattering coefficient. The water vapor transmission rate increased for films produced from 75 wt.% ethanol suspensions and was even higher for oven dried films. The combination of drying the films in an oven and producing the film from an alcoholic/aqueous suspension provided the opportunity to manipulate the transparency of the film and the water vapor transmission rate, preserving the mechanical performance of the films. Briefly, with this study, it was possible to develop a relatively fast and simple method to produce NFC films with mechanical and barrier properties with potential for several applications, such as membranes, where mechanical strength, toughness, low density, controlled permeability and porosity and high specific area are important, fuel cells, liquid purification and filtration, tissue engineering, protein immobilization and separation, and protective clothing.
In short, the investigation carried out throughout this work allowed to prove the great potential of cellulosic fibrils in numerous applications, mainly in the manufacture of films. Regarding calendering, the present work demonstrated that this unitary operation is highly effective in improving the water vapor barrier properties of cellulosic fibril films. With regard to the production of films by vacuum filtration from highly fibrillated cellulose suspensions, the most reported difficulty by several authors referenced in the literature is the fact that this is an extremely slow process, making the production of NFC films impractical at the industrial level. The present work presents important developments regarding this aspect, suggesting a simple modification from the purely aqueous medium of suspensions to a medium with a mixture of water/ethanol. This technique, in addition to drastically reducing film formation time and using a solvent that is easily and quickly recovered by evaporation, preserves the mechanical properties of the produced films. Another aspect that complicates the application of NFC in several areas and, consequently, of films made from NFC, is its hydrophilic nature. The present work developed methods for the production of NFC films with strong hydrophobic properties, resistant to handling and friction, applying surface coatings of stearic acid and precipitated calcium carbonate, combined with a heat treatment, in an environmentally friendly technique and possibly industrially scalable.A celulose é o biopolímero mais abundante, renovável e sustentável do planeta, possuindo muitas características desejáveis que a tornam um material adequado para diversas aplicações. As fibrilas de celulose obtidas a partir de recursos renováveis têm vindo a ganhar um crescente interesse devido à sua natureza sustentável e biodegradável, combinada com outras propriedades mecânicas, óticas, térmicas e fluídicas. As fibrilas de celulose são, portanto, atraentes para o fabrico de diversos materiais, desde compósitos a membranas porosas e géis, filamentos e filmes. A investigação desenvolvida ao longo deste trabalho visou explorar quer o comportamento reológico de suspensões de celulose nanofibrilada (NFC) usando diferentes meios líquidos, quer as propriedades mecânicas, óticas e de barreira das estruturas produzidas a partir deste material.
Relativamente à investigação do comportamento reológico de suspensões de NFC, o estudo realizado focou-se nos efeitos da morfologia das fibrilas presentes em suspensões aquosas, na adição de etanol ou acetona ao meio e no aumento da força iónica do meio através da adição de elevadas concentrações de NaCl. Para tal, foram utilizadas duas suspensões diferentes de NFC: uma suspensão aquosa de NFC carboximetilada obtida através da Innventia (Estocolmo, Suécia) (NFC-carb); e uma suspensão aquosa de NFC produzida no laboratório usando uma pasta comercial de eucalipto ao sulfito branqueada, submetida a um pré-tratamento de oxidação mediada por 2,2,6,6-tetrametilpiperidina-1-oxil (TEMPO) num sistema TEMPO/NaBr/NaClO, que posteriormente foi submetida a homogeneização de alta pressão nas instalações do RAIZ (Instituto de Investigação da Floresta e do Papel) (NFC-TEMPO). A análise morfológica das suspensões através de SEM (Scanning Electron Microscopy) e TEM (Transmission Electron Microscopy) permitiu inferir uma maior percentagem de nanoelementos na suspensão NFC-TEMPO relativamente à NFC-carb. Foi efetuada a análise físico-química das fibrilas presentes em ambas as suspensões: a viscosidade intrínseca foi de 397 ml/g para a NFC-carb e 85 ml/g para a NFC-TEMPO, correspondendo a graus de polimerização da celulose de 696 e 149, respetivamente; a quantificação de grupos carboxílicos foi de 698 μmol/g para a NFC-carb e 1900 μmol/g para a NFC-TEMPO. Estes resultados confirmaram que as suspensões usadas tinham propriedades morfológicas e físico-químicas substancialmente diferentes. meio, tendo-se investigado concentrações entre 2,5 e 40% de etanol ou acetona, utilizando a NFC-carb. Por outro lado, para o estudo do efeito do aumento da força iónica no comportamento reológico das suspensões, foi adicionado NaCl com vista a obter concentrações entre 50 mM e 1000 mM, tendo-se estudado ambas as nanofibrilas (NFC-carb e NFC-TEMPO). O teor de sólidos das duas suspensões estudadas foi ajustado para 1,3 %. As suspensões foram submetidas a medições reológicas nos modos de fluxo contínuo e oscilatório, usando um reómetro controlado por tensão, equipado com uma geometria de cone e prato, com superfície lisa ou rugosa, sendo que, no caso da geometria rugosa, papel de lixa com rugosidade conhecida foi colado na superfície de ambas as peças.
O estudo do efeito da rugosidade da geometria no comportamento reológico das suspensões NFC-carb evidenciou tensões de cisalhamento e viscosidade mais elevados para a geometria rugosa a taxas de cisalhamento mais baixas (0.05 a 5 s-1), mas a taxas mais elevadas (até 100 s-1) as medições foram menos influenciadas quer por instabilidades de fluxo, quer pela rugosidade da geometria, aproximando-se das
