Main results : Quercus suber L.
The health status also affects the number and area of pores, especially in the cork oak groves of the coast, where the coefficient of porosity ranges from 3. The effect of tree vitality on the formation of pores in the cork oak phellogen is lower in the mountain than in the coast. No significant effects were found for any of the two factors neither on the annual growth rate nor on the thickness of the cork. Research highlights : Results lead to the conclusion that the effect of health status on traumatic phellogen formation and activity is clear but not uniform.
Further studies are necessary for a deeper understanding of the effect of stress situations on pore formation and characteristics. Cork Oak Woodlands on the edge. Ecology, adaptive management, and restoration. Island Press. Auclair AND, Extreme climatic fluctuations as a cause of forest dieback in the pacific Rim. Water, Air, and Soil Pollution 66 : Bakry M, Abourouh M, Nouvelles donnees sur le deperissement du chene-liege Quercus suber L. Ann Rech For Maroc Benest M, Bonneau M, Landmann G, La Recherche 19 : Bouhraoua R, Villemant C, Bouhraoua RT, Cambini A, Valutazione dei danni causati dagli insetti defogliatori alla querci da sughero.
Density, buoyancy, thermal insulation, fire behavior, compression, and permeability are discussed.
The density of cellular materials is expressed as their solid mass fraction and the density of the solid. The density of air-dried cork is usually about to kg m -3 , but a broader range of values can be observed in nature, as influenced by several factors. The density of the solid i.
As the cell wall density varies only slightly, the differences in cork density are derived from its structural features such as cell size and cell wall corrugation Fig. The average dimensions of earlycork and latecork cells indicate densities of and kg m -3 , respectively. The higher density of the latecork layer is clearly seen in Fig. The effect on density depends on the corrugation parameter the quotient between the length of the corrugated wall and the length of the wall if it were straightened in a way such that the density is higher when cells are more corrugated.
The straightening of the cell walls, by thermal treatments or boiling in water, will decrease cork density; on the contrary, treatments that increase the cellular corrugation will yield denser corks e. Regarding the porosity resulting from lenticular channels, the general tendency is toward higher density values in corks with more and larger lenticular channels. In fact, lenticular channels contain a filling material, and in most cases they are bordered by thicker cells Fig.pracperquini.tk
Cork : biology, production and uses
Cork has been used since antiquity as a floatation device. Another reason for the floating capacity of cork is the very small diffusion of water into it: the diffusion coefficient of water in cork has been found to be between 1. Its solid fraction is small, and the gas enclosed in the cells of cork has low thermal conductivity. The cells are small and closed, which eliminates convection. Radiation is reduced through repeated absorption and reflection at the numerous cork cell walls. In comparison with other synthetic insulation foams, cork has smaller cells but higher density, which results in comparable heat transfer properties.
The chemical composition of the cell wall of cork imparts appreciable thermal stability as compared to that of synthetic polymers e. This allows cork to be used as an insulation layer in case of fire. Under compression, cork exhibits a behavior typical of cellular materials, with some peculiarities.
Cork: Biology, Production and Uses - [PDF Document]
This process is practically fully reversible. This region corresponds to the buckling of cells. Figure 9 exemplifies what occurs in cork, at the cellular level, during compression along the stress-strain curve. Although each point is located in the plateau region of the stress-strain curve, they correspond to different intensities of cellular buckling. Compression does not cause failure of the cork cells, and even in the densification phase, the cell walls do not fracture. The recovery of the original dimensions after stress removal is rapid and is associated with the unfolding of buckled cell walls.
Although anisotropic, the compressive behavior of cork in different directions is similar. It does exhibit higher strength in the radial direction than in the non-radial i. The variation in the dimensions in the directions perpendicular to the direction of compression i. It is logical that the relative proportion of cell walls, or in other words, the solid fraction as given by cork density, influences compression.
The chemical structure of the cork cell wall explains this behavior.
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The flexible suberin macromolecule, with its long-chain linear monomers as shown in Fig. Cork is used for sealing purposes because of its low permeability and high flexibility under compression.
The permeability coefficients were low but varied widely across three orders of magnitude. Water-boiled cork the pre-treatment that all raw cork planks undergo before stopper production exhibited lower permeability than non-boiled cork. The mechanism for the permeability of cork to gases was established as transport processes between cells through the small plasmodesmata channels Fig. The transport followed a Knudsen molecular flow mechanism with negligible contributions of viscous transport to the total flux.
The driving force that regulates gas transport through cork is the gradient of the partial pressure of the gas. A model was developed, based on the morphology of the cork cell structure the cell dimensions and the plasmodesmata features that fitted well the determined experimental values. From studies with ethanol and water vapors and liquids, it was found that these species permeate not only through the small channels of the plasmodesmata but also through the walls of the cork by sorption and diffusion, as schematically represented in Fig.
The overall permeation of water was higher than that of ethanol by approximately 4 times in the vapor phase and 14 times in the liquid phase due to the larger size of the ethanol molecule. The permeation of liquids was higher than the permeation of vapors by a factor of 2. The permeability of cork is of major practical interest for its use as a wine stopper.
The properties of cork are based, as previously discussed, on the features of its cellular structure and its chemical composition. The limits of this natural variation are important to define the material as cork. A large part of the variability in cork performance will be related to the cell prism height and the frequency distribution of its values. One aspect of interest is the estimate of the macroscopic dimensional limit required for the material to exhibit cork-like performance. The minimum particle size required to maintain such performance would be interesting to determine.
When the dimensions of cork particles are reduced, the number of closed cells decreases, the external surface of the particle is enlarged, and consequently, the number of open, through-cut cells increases.
An extreme case is illustrated in Fig. This is of interest due to the increasing production and use of composites in which cork particles are bound with adhesives or combined with other materials. A cork particle of volume 0. This particle size should likely be the smallest size to maintain the typical cork behavior, even when used in cork-derived composites. Figure 12 shows an example of a cork granulate fraction obtained by separation between 0. Scanning electron micrographs of cork granules: left particles ground to below 0.
Data regarding the natural variation of the chemical composition of the cork cell wall exists. Large sampling and chemical analyses of reproduction cork 96 samples, Pereira ; 10 samples, Pereira and virgin cork 40 samples, Pereira allow for insight into the natural variation found in cork and into its overall average chemical composition. The content of suberin in the cell wall is the most important chemical attribute of cork since this is its most unique feature and is directly related to most of the typical properties of cork.
The suberin content is, on average, It is true that some samples have suberin contents outside this interval. Although this leads to variation in its properties, such as its compression variables, abnormal suberin contents still allow the material to behave as cork. Unfortunately, experimental data are not available. However, some estimates may be made using existing chemical data and statistics Table 1. If one considers that the minimum suberin content required to impart the required cork properties is the mean value Therefore, a composite material with such a proportion of cork would still exhibit the known cork performance.
This is certainly a matter for which targeted experimental research is needed. Cork is a natural cellular material of biological origin with an interesting and unique combination of properties.
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It has low density, buoyancy, very low permeability, low thermal coefficients, elasticity, and withstands large deformation without fracture under compression. Thanks are due to Duarte Neiva for helping to design the molecular lignin and suberin models, Rita Teixeira for the TEM figures, and Vanda Oliveira for the microtomography figures.
Achyuthan, K. Anjos, O. DOI: Arno, M. Asensio, A. Natural Prod. Bento, M. Brazinha, C. Membrane Sci. Brunetti, A. Methods in Physics Res. All Pages Books Journals. View on ScienceDirect. Editors: Helena Pereira. Hardcover ISBN: Imprint: Elsevier Science. Published Date: 29th March Page Count: For regional delivery times, please check When will I receive my book? Sorry, this product is currently out of stock. Flexible - Read on multiple operating systems and devices. Easily read eBooks on smart phones, computers, or any eBook readers, including Kindle.
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