M. Emília Rosa

The Cellular Structure of Cork from Quercus Suber L.

The main characteristics of the cellular structure of cork from Quercus suber L. are reviewed and complt;mented with new observations of virgin and reproduction cork by scanning electron microscopy. Particular emphasis is given to cell geometry and topology and to the corrugations that are observed in the cell walls. The effect of the growth season in these features is described. Large variations in cell size, wall thickness and corrugations are reported.

Rate effects on the compression and recovery of dimensions of cork

A study of the effect of strain rate on the compression behaviour of cork was carried out, which takes into account the anisotropy of the material. Compression curves at three different rates were obtained for each of the three directions in cork (radial, axial and tangential). Strain-rate sensitivity coefficients,m, were also measured in experiments where the strain rate was suddenly changed during the tests. The values ofm are fairly isotropic, around 0.06. For a given strain rate, the radial direction is stronger (i.e. larger stresses) than the other two, but these are not equivalent, the axial direction being slightly stronger for most of the strain interval between 0 and 80%. The recovery of dimensions following compression in each direction was also studied. The change in the three dimensions with time was monitored, following compression to 30% and to 80% strain in a given direction. In the first case, recovery is almost total after ∼ 20 days, but for 80% compression the deformation is not completely recovered after unloading. The recovery rate decreases appreciably with time and increases with the degree of deformation previously imposed. An equation is proposed that describes the recovery behaviour with a reasonable accuracy.

α-Rhamnosidase and β-glucosidase expressed by naringinase immobilized on new ionic liquid sol–gel matrices: Activity and stability studies

Novel ionic liquid (IL) sol-gel materials development, for enzyme immobilization, was the goal of this work. The deglycosylation of natural glycosides were performed with α-l-rhamnosidase and β-d-glucosidase activities expressed by naringinase. To attain that goal ILs with different structures were incorporated in TMOS/Glycerol sol-gel matrices and used on naringinase immobilization. The most striking feature of ILs incorporation on TMOS/Glycerol matrices was the positive impact on the enzyme activity and stability, which were evaluated in fifty consecutive runs. The efficiency of α-rhamnosidase expressed by naringinase TMOS/Glycerol@ILs matrices increased with cation hydrophobicity as follows: [OMIM]>[BMIM]>[EMIM]>[C(2)OHMIM]>[BIM] and [OMIM]≈[E(2)-MPy]≫[E(3)-MPy]. Regarding the imidazolium family, the hydrophobic nature of the cation resulted in higher α-rhamnosidase efficiencies: [BMIM]BF(4)≫[C(2)OHMIM]BF(4)≫[BIM]BF(4). Small differences in the IL cation structure resulted in important differences in the enzyme activity and stability, namely [E(3)-MPy] and [E(2)-MPy] allowed an impressive difference in the α-rhamnosidase activity and stability of almost 150%. The hydrophobic nature of the anion influenced positively α-rhamnosidase activity and stability. In the BMIM series the more hydrophobic anions (PF(6)(-), BF(4)(-) and Tf(2)N(-)) led to higher activities than TFA. SEM analysis showed that the matrices are shaped lens with a film structure which varies within the lens, depending on the presence and the nature of the IL. The kinetics parameters, using naringin and prunin as substrates, were evaluated with free and naringinase encapsulated, respectively on TMOS/Glycerol@[OMIM][Tf(2)N] and TMOS/Glycerol@[C(2)OHMIM][PF(6)] and on TMOS/Glycerol. An improved stability and efficiency of α-l-rhamnosidase and β-glucosidase expressed by encapsulated naringinase on TMOS/Glycerol@[OMIM][Tf(2)N] and TMOS/Glycerol@[C(2)OHMIM][PF(6)] were achieved. In addition to these advantageous, with ILs as sol-gel templates, environmental friendly processes can be implemented.

Temperature-induced alterations of the structure and mechanical properties of cork

Abstract When cork is heated in air at temperatures in the interval 100–300°C, it swells and its mass decreases. In this paper a detailed study of the effects of heat treatment on the structure, overall dimensions, mass and compression properties of cork (reproduction and virgin cork) is reported. It is observed that the lenticular channels swell while the originally corrugated cell walls become straight. These structural changes, together with the chemical alteration of the cell wall material, explain the observed effects on the mechanical properties.

Stress relaxation and creep of cork

The stress relaxation and creep behaviour of cork under compression were characterized in tests done with the compression axis parallel to each of the three principal directions in the tree (radial, tangential and axial). All stress relaxations lead to a linear variation of stress with the logarithm of time, the slopes being nearly independent of stress and direction of compression. Creep stresses in the range 0.36 to 1.72 MPa were used. The strain rate continuously decreases during creep, from initial values around 10−4sec−1 to ∼ 10-7 sec−1 after 8 h, but its dependence on the creep stress and direction of compression is not simple, mainly because different deformation regimes may operate during a single creep test. Compression loading-relaxation-unloading cycles were imposed on specimens, with compression either in the radial or in the tangential direction, with the purpose of simulating the performance of a cork stopper. A “work softening” is observed, i.e. the resistance decreases in successive compressions, particularly between the first two. This is explained in terms of an increased undulation of the cell walls produced in the first compression.

An introduction to solid foams

This article focuses on the main features of solid cellular materials (solid foams) that are relevant for applications, namely their structure, their weight, and their mechanical and thermal properties. Emphasis will be given to the work of Professor Manuel Amaral Fortes in these areas.

Nucleation and glide of dislocations in a monodisperse two-dimensional foam under uniaxial deformation

Abstract Experimental observations of the deformation of a monodisperse two-dimensional (2D) monolayer foam in tension/compression are reported. The foam samples are bound by two parallel walls at a variable separation, w. At critical values of w, dislocations (5/7 pairs) nucleate at the periphery of the foam which then glide along directions at 60° to the walls. The dislocations may suffer reflection at the walls, with a change in Burgers vector. As a result of the glide process, the number of close-packed rows of cells parallel to the walls changes by one and a neck develops. Rearrangements of the bubbles to more stable configurations are also observed, following dislocation glide. A detailed analysis of the topology of nucleation, glide and reflection of dislocation is undertaken. The stress—strain relation is derived and used to calculate the yield stress of the honeycomb foam from the experimentally measured strains at which topological transitions occur. The yield stress is considerably lowered by t...

Growth stresses and strains in cork

SummaryA detailed study of the growth stresses and strains in the cork shell of the cork-oak was undertaken based on experimentally determined constitutive relations for cork in tension and compression. The stresses depend on the thicknesses of the cork shell and of the back layer around the cork shell, on the radius of the trunk and on its increase due to growth. The circumferential stresses in the cork shell and back layer are tensile and increase with increasing distance to the tree axis. The radial stresses are compressive and decrease with increasing distance to the tree axis. The strains due to growth are not recovered when the cork boards are removed, unless the boards are heated, for example, by immersion in boiling water. Other consequences of the growth stresses are analysed, such as the occurrence of corrugations in the lateral cell walls of cork, the variation of width of the successive growth rings and the occurrence of cracks in the back layer and outer cork layers.

Operational stability of naringinase PVA lens-shaped microparticles in batch stirred reactors and mini packed bed reactors-one step closer to industry.

The immobilization of naringinase in PVA lens-shaped particles, a cheap and biocompatible hydrogel was shown to provide an effective biocatalyst for naringin hydrolysis, an appealing reaction in the food and pharmaceutical industries. The present work addresses the operational stability and scale-up of the bioconversion system, in various types of reactors, namely shaken microtiter plates (volume ⩽ 2 mL), batch stirred tank reactors (volume <400 mL) and a packed-bed reactor (PBR, 6.8 mL). Consecutive batch runs were performed with the shaken/stirred vessels, with reproducible and encouraging results, related to operational stability. The PBR was used to establish the feasibility for continuous operation, running continuously for 54 days at 45°C. The biocatalyst activity remained constant for 40 days of continuous operation. The averaged specific productivity was 9.07 mmol h(-1) g enzyme(-1) and the half-life of 48 days.

The form of a constitutive equation of plastic deformation compatible with stress relaxation data

Abstract Harts approach to constitutive equations of plasticity and the experimental results relevant to his formalism are reanalyzed, with special emphasis on the consequences of the scaling relation observed in the family of relaxation curves of a large number of materials. Complete constitutive equations containing a single structure variable are proposed, which describe very well the experimentally determined relaxation and tensile test curves. An interpretation of the structure variable is given in terms of the density of obstacles to dislocations. Finally, the equations are generalized to include recovery and then applied to describe dislocation creep.