Léa Lopes

Role of conformation and acetylation of xanthan on xanthan-guar interaction

The synergistic effect obtained by mixing xanthan and guar solutions were examined by low shear viscosity measurements in relation to the temperature. Native and deacetylated xanthan samples were used in mixtures in which the total polymer concentrations were 1 g/litre and 0·5 g/litre. Gelation was observed for temperatures lower than 15°C for the native xanthan-guar system (weight ratio 11) in 10−2 m NaCl and at 22–24°C for the same system in water; in this last case, it is known that the xanthan is in the disordered conformation. For a mixture of deacetylated xanthan-guar, gelation was observed at a temperature below 26°C in water. The results confirm that there is a stronger interaction between deacetylated xanthan and guar than native xanthan and guar because of enhanced xanthan-guar gum backbone association in the former case.

Optimization of culture conditions for exopolysaccharides production in Rhizobium sp. using the response surface method

The combined effects of the processing parameters for exopolysaccharides production by Rhizobium sp. was studied using the experimental design and response surface methodology. The experiments were carried out using a fermenter with 20 L capacity, as the reactor. All processing parameters were online monitored. The temperature [(30 ± 1)oC] and pH value (7.0 ± 0.1) were kept constant throughout the experimental time. As statistical tools, a complete 2 3 factorial planning with central point and response surface were used to study the interactions among three relevant variables of the fermentation process: calcium carbonate concentration, aeration and agitation. The processing parameters setup for reaching a maximum response for exopolysaccharides production was obtained when applying the highest values for calcium carbonate concentration (1.1 g/L), aeration (1.3 vvm) and agitation (800 rpm). In addition, the combination of these optimum processing parameters yielded Y P/S (g/g) = 0.35.

Effect of Process Parameters on Production of a Biopolymer by Rhizobium sp

The production of biopolymers by a Rhizobium strain was studied under batch and bioreactor conditions. The best viscosity levels were obtained under low mannitol concentrations as well as low agitation and aeration conditions. Infrared spectra indicated the presence of chemical groups characteristic of microbially produced biopolymers, including C = O and O-acetyl groups. Thermogravimetric analysis showed the characteristic degradation profiles of the exopolysaccharide produced (T(onset) = 290degreesC). The experimental design showed that a low substrate concentration (10.0 g/L), and low aeration (0.2 vvm) and agitation (200 rpm) levels should be used. The maximum yield of the process was a Yp/s (g/g) of 0.19 +/- 0.1, obtained under optimized conditions.

Influence of the method of purification on some solution properties of welan gum

The origin and some consequences of the presence of aggregates and impurities (deriving from the fermentation procedure) on the solution properties of welan gum have been studied. For this purpose, a method of purification was developed for a fermentation welan broth. It was verified that this method is ineffective for the dissociation of aggregates in the commercial product. This demonstrates the irreversible nature of the intermolecular interactions of welan samples isolated by precipitation in the presence of impurities. The purified products were characterized (Mw, [eta], kH) and the results were compared to those obtained from commercial samples of welan. The influence of the method of purification on the rheological properties was analysed.

Development and characterization of micellar systems for application as insect repellents.

N,N-diethyl-meta-toluamide (DEET) is a widely used insect repellent due to its high efficacy. In this work, micellar systems based on poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer were developed and studied for the purpose of controlling the release and cutaneous permeation of DEET, using concentrated solutions of the copolymer Pluronic F127 to form thermoreversible gels. The formulations presented thermoreversible gelation above 5°C and altered rheological behavior at 15 and 25°C. The presence of the drug drastically changed the sol-gel transition temperatures. The micrographs suggest that DEET induced the formation of anisotropic structures, and Maltese Crosses were observed. The formulation containing 10wt% DEET and 15wt% Pluronic F127 presented sustained drug release for up to 7h. DEET release profile followed the Higuchi kinetics model. There was a reduction of approximately 35% in the amount of DEET absorbed through the skin after 6h. About 62% of DEET from the formulation consisting of Pluronic F127 and DEET remain retained on the skin. The anisotropic structure may constitute a barrier to diffusion and thereby controlling the drug release effectively. These tests suggest that the tested samples exhibit safety profile greater than some commercially available products.

Influence of aggregates on the sedimentation properties of welan gum

SummaryThe water-soluble bacterial polysaccharides are used by industry as emulsifying, stabilizing, thickening and suspending agents, and eventually for their gelling properties. It has been observed that several of these properties may be influenced by the presence of disordered intermolecular associations (aggregates or microgels). The influence of aggregates on the sedimentation properties of welan gum were investigated by means of the falling sphere method. The velocity of the particle sedimentation is affected by the type and size of the aggregates.

Uso de DSC na Determinação de Parâmetros de Vulcanização de Látex de Borracha Natural

Resumo: Os processos de obtencao de artefatos de borracha natural (NR) a partir do latex e a composicao dos sistemas de vulcanizacao sao fundamentais na determinacao da aplicabilidade e qualidade destes artefatos. Varias tecnicas podem ser usadas para a verificacao da eficiencia do processamento e dos sistemas de vulcanizacao. O objetivo deste trabalho foi estudar a eficiencia da vulcanizacao de latex de NR por dois sistemas de vulcanizacao SV1 (sistema convencional) e SV2 (sistema eficiente) em diferentes concentracoes atraves de medidas calorimetricas. A Calorimetria Diferencial de Varre- dura (DSC) mostrou ser uma tecnica eficaz na obtencao de parâmetros de vulcanizacao como velocidade, tempo de pre- vulcanizacao, bem como do excesso de ligacoes cruzadas. Os resultados foram corroborados pelos ensaios de resistencia a tracao e analise optica. De forma geral, o sistema de vulcanizacao convencional SV1, em comparacao ao sistema eficiente SV2 mostrou-se mais adequado para uso, nas condicoes experimentais deste trabalho, por fornecer maior velo- cidade de vulcanizacao, maior seguranca de processo e melhor desempenho mecânico. Palavras-chave: Vulcanizacao, latex de borracha natural, DSC. The Use of DSC in the Determination of Natural Rubber Latex Vulcanization Abstract: The manufacture of natural rubber goods from the latex has significant commercial interest. The processes to obtain these artifacts, the type and amounts of the vulcanization systems used are of utmost importance in determining their quality and suitability. A number of techniques can be used to verify the efficiency of both the processing and the vulcanization systems. The objective of this work was the study of the efficiency of NR latex vulcanization carried out by two vulcanization systems, namely SV1 (conventional system) and SV2 (efficient system) in different concentrations, by differential scanning calorimetry (DSC). The DSC has been found to be a very efficient technique for the acquisition of data which would allow the determination of the vulcanization rate, pre-vulcanization time and the excess of crosslinks in elastomers latexes compositions. The results have been corroborated by stress strength tests and optical analysis. In general, the conventional system, SV1, as compared with the efficient SV2, gave the best results, under the experimental conditions, since it provided a higher vulcanization rate, better process safety and better mechanical behavior.

Estudo da Formação de Ligações Cruzadas por Irradiação Gama em Membranas Híbridas de Polissulfona Bisfenol-A e Sílica Precipitada

In this work the bisphenol-A-polysulfone (PSF) was sulfonated using trimethyl silyl chlorosulfonate [(CH3)3SiSO3Cl] as a mild sulfonating agent in a homogeneous solution of dichloroetane. The sulfonation reaction was confirmed by acid-base titration and FTIR-spectroscopy analysis. The hybrid membranes were obtained by casting the sulfonated bisphenol-A-polysulfone (SPSF) and precipitated silica Tixosil® 333 solutions in N-N-dimethylacetamide. Cross-linking in the hybrid membranes was obtained by irradiation, with doses ranging from 5 to 30 kGy using gamma ray from a 60Co source. The water uptake and the swelling of the membranes were estimated by measuring the change in weight between dry and wet conditions. The conductivity of the membranes in acid form was measured with the ac impedance technique using a PGSTAT30 frequency response analyzer. The hybrid cross-linked membranes have conductivity close to 10-1 S.cm-1 at 100% RH and 80 °C. Electrochemical performances, thermo-mechanical stability and low cost make this cross-linked SPSF hybrid membrane an attractive material for fuel cells using a proton exchange membrane.

EFFECT OF CNSL RESIN ON THE CRYSTALLINITY OF POLYPROPYLENE IN PP/NBR MIXTURES

Blends of polypropylene (PP) and acrylonitrile rubber (NBR) were prepared in a Haake 9000 Rheocord System. Two different phenolic resins were introduced into these blends and the effect of this third component on the mechanical properties was evaluated, as well as the relationship between the crystallization of the polypropylene matrix and the properties analysed.