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Recovery of used cork stoppers
Colloids and Surfaces A: Physicochem. Eng. Aspects 344 (2009) 97–100
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Recovery of used cork stoppers M. Emília Rosa ∗ , M.A. Fortes Departamento de Engenharia de Materiais, Instituto Superior Técnico, ICEMS – Instituto de Ciência e Engenharia de Materiais e Superfícies, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
a r t i c l e
i n f o
Article history: Received 2 October 2008 Received in revised form 11 February 2009 Accepted 12 February 2009 Available online 6 March 2009 Keywords: Cork Stoppers Foams Recovery Buckling
a b s t r a c t Cork is a natural cellular material. As fabricated cork stoppers for wine bottles are cylindrical with a uniform cross-sectional diameter. Upon removal from a bottle, the stopper shows a smaller and slightly non-uniform diameter. If a used cork stopper is immersed in water or water vapour at a sufficiently high temperature, the original dimensions can be recovered in a time interval of a few minutes. The kinetics of dimensional recovery in air and water (liquid and vapour) at different temperatures were experimentally determined. Water absorption is determinant of a fast recovery rate. A simple viscoelastic model is used to explain the experimental results. Since the mechanical properties are also restored, the stopper can be re-used, provided the damage due to the cork-screw used to remove the stopper is confined to its bulk. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Natural cork stoppers are by far the main product of the cork industry. Other applications are the agglomerates made from cork granulates. The annual world production of natural cork stoppers is above 1010 . At present, the very small percentage of collected stoppers is granulated and mixed with cork granules from other origins (e.g. low quality cork) to produce cork agglomerates, including those for the manufacturing of agglomerated cork stoppers. Corkoak tree (Quercus suber L.) grows only in very particular regions (Mediterranean region, mostly Portugal and Spain). The corkboards are removed from the tree in 9-year intervals and there are big oscillations in annual cork world productions. In the last years, the world area of cork-oak has decreased mostly due to fire and tree diseases. The recovery of used cork stoppers would contribute to a waste reduction and to a more sustainable development [1]. The cells in cork are generated by a meristematic tissue – the phelogen – which surrounds the trunk of the cork tree. Each cell of the phelogen generates in succession prismatic cork cells which fill space. These cells have a short life and soon become hollow. The structure of cork is thus cellular, with rows of closed prismatic cells in the radial direction, each cell having, on average, 14 neighbours [1–3]. The new cork cells are pushed (compressed) against the layer of older dried cells, and since they still have soft walls, they buckle. This leads to corrugations or undulations, specially in the lateral cell walls. Further compression of cork, e.g. when a cork stopper is
∗ Corresponding author. Tel.: +351 21 8418105; fax: +351 21 8418132. E-mail address: [email protected] (M.E. Rosa). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.02.035
inserted into a bottle neck, leads to an increase in the amplitude of the corrugations, and to a reduction in linear dimensions (and volume). Heating cork (as extracted from the tree) in water, at 100 ◦ C, as in the industrial “boiling” operation, leads to an expansion of ≈15% in the radial direction (along the rows of cells and parallel to the radius of tree trunk) and ≈6% in the transverse directions (perpendicular to the radius of the tree trunk), caused by a decrease in the amplitude of the cell wall undulations [4]. Cork stoppers are made from boiled cork. When boiled cork is heated in air or water vapour at temperatures above 200 ◦ C, a further expansion occurs (about 15–20% in all directions) and the cell walls become fully straight [5,6]. A cylindrical cork stopper strains by ≈25–30% in its radial direction (along its radius) when inserted into a bottle neck (initial stopper diameter of 24 mm in a neck of 18 mm diameter, are typical values for wine bottles). The deformation of a cork stopper is localized at its periphery (in a layer of thickness ≈0.25n , where n is the neck diameter), where the cells are heavily corrugated, the bulk cells being little affected by compression (strain localization) [1]. Fig. 1 shows a SEM image of the top transverse section (section perpendicular to stopper axis) of a stopper inside the bottle neck. The transition between heavily corrugated cells close to the neck and “normal” cells in the bulk is clearly seen. Upon removal from the bottle, the stopper rapidly recovers part of the strain, but its diameter is still only about 80% of the original diameter. Recovery is slightly larger at the wetter bottom than at the drier top, but the difference is small. If the removed stopper is heated in boiling water it fully recovers its original dimensions and may even exceed them. A stopper with the dimensions of an unused stopper can thus be obtained.
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M.E. Rosa, M.A. Fortes / Colloids and Surfaces A: Physicochem. Eng. Aspects 344 (2009) 97–100
Fig. 1. SEM image of part of the top surface of a natural cork stopper inserted into a bottle neck, showing the transition region between heavily corrugated cells (on the left) and normal, undeformed cells (on the right).
This paper investigates the recovery of dimensions in cork stoppers, heated at temperatures up to 100 ◦ C in (liquid) water, water vapour and air. Particular emphasis is put on the evaluation of the kinetics of recovery and on the long term amount of recovery. The changes in mass and in temperature inside the stopper have also been measured as functions of time. 2. Materials and methods Most (natural) cork stoppers used in the experiments were removed from commercial wine bottles of a particular label. Their moisture content, measured by drying at 100 ◦ C in air to constant mass, was around 6%. The average diameter and height of the unused stoppers were 24 mm and 38 mm, respectively. For the used stoppers, an average diameter was calculated from measured diameters at the top, at the bottom and at half-height (after removal from the bottle neck, the stabilised stopper diameter is not uniform: the recovery of dimensions depends on moisture content [1]). The height and mass of each stopper were also measured. The used stoppers were immersed in different media at atmospheric pressure: air and water vapour, at 100 ◦ C, and water at temperatures between 60 ◦ C and 100 ◦ C. The stoppers were removed every minute, up to 10 min, and their dimensions (average diameter and height) and mass were measured during immersion at 1-min intervals. In the case of water immersion, the wetting water layer at the surface was eliminated by contact with filter paper. Similar experiments were undertaken with unused natural cork stoppers. In other experiments, aimed at assessing the depth of penetration of water, dried cubes of cork (edge length ≈20 mm) were immersed in water at 90 ◦ C for various time intervals t. After removal from the hot water bath, the cubes were weighed to determine their water content M/M0 , and subsequently slices, approximately 5 mm thick, were cut from each cube face. The remaining small cube (≈10 mm edge) was weighed and then dried at 100 ◦ C in air to constant mass to determine its water content. The temperature Tc at the centre of a stopper immersed in a bath at temperature Ts = 90 ◦ C was measured with a thermocouple inserted in an axial hole drilled to half-height of the stopper. Heating Tc (t) curves were obtained with stoppers immersed in water, sand and oil baths in different experiments using different stoppers.
Fig. 2. Kinetics of water absorption by used cork stoppers. Water at temperature (◦ C): () 100; () 90; () 80; (䊉) 60; () 100 (water vapour).
3. Results The mass, diameter and height changes versus time of immersion in different media are plotted in Figs. 2, 3a and 3b, respectively. All changes are indicated as a percentage of the initial value (stabilised after removal from the bottle, in the case of used stoppers). Fig. 2 shows that the treatments in water (liquid and vapour) cause a large mass increase which is due to the water absorption. Heating in air at 100 ◦ C causes a small mass decrease due to loss of absorbed
Fig. 3. Effect of heat treatment on the dimensions of unused and used cork stoppers. (a) Diameter and (b) height. Treatment: (♦) water 100 ◦ C, unused; () water 100 ◦ C, used; () water 90 ◦ C, used; () water 80 ◦ C, used; (䊉) water 60 ◦ C, used; () water vapour 100 ◦ C, used; () air 100 ◦ C, used.
M.E. Rosa, M.A. Fortes / Colloids and Surfaces A: Physicochem. Eng. Aspects 344 (2009) 97–100
Fig. 4. Water content of cubes immersed in water at 90 ◦ C. () Original cubes and () internal cubes.
water [1,5]. Fig. 2 also shows that for the treatments in water, the mass change increases as the temperature increases. This is due to a faster rate of water absorption [4]. For the same temperature (100 ◦ C) the mass increase is larger in liquid water than in water vapour. The mass changes of the unused stoppers (not shown) are similar to those obtained with deformed stoppers, for the same medium and treatment duration. Fig. 3a shows that the heat treatments of used stoppers with liquid water cause a diameter expansion that increases as the temperature increases, but the diameter expansion is smaller in water vapour compared to liquid water, for the same temperature and time interval. The diameter expansion of the unused stoppers is smaller than that of the used stoppers (see Fig. 3a). Comparison of the results of Fig. 3a (diameter change) and b (height change) shows that the height expansion is much smaller than the diameter expansion for the used stoppers, due to the anisotropy of strain in the bottle neck. The unused, undeformed stoppers show similar changes in the diameter and height, the latter being larger than for the used stoppers (see Fig. 3b). This is because the Poisson effect (when a compression stress is imposed on a specimen, usually a constriction along the direction of the applied stress (axial) and elongations in the directions perpendicular to it (lateral) will appear; Poisson effect relates the lateral and the axial strains) in the compression of the used stopper increases the height of the used stoppers compared to that of the unused stopper. Results of the experiments with cork cubes are shown in Fig. 4 in which the moisture content is plotted as a function of time of immersion in water at 90 ◦ C of the original large cubes and of the small internal cubes. It is apparent that water hardly penetrates to the centre of the original cube sample and concentrates in the surface layers. Note that the thickness of the slices is comparable to that of the heavily deformed surface layer of a used stopper. The increase with time of the temperature Tc at the centre of a stopper immersed in a hot water bath is shown in Fig. 5. The initial temperature of the stopper was 23 ◦ C and that of the water bath was 90 ◦ C. The temperature near the surface was not measured; it is believed to be close to the temperature of the surrounding medium. Similar Tc (t) curves were obtained for immersion in sand and in oil at the same temperature. This shows that water diffusion into the stopper does not have an effect on the temperature change. 4. Discussion Corrugated cell walls in cork are a consequence of buckling under the compressive growth stresses in the tree when the cells are still soft. When the cells die and stiffen, residual stresses remain in the bent walls. These bending stresses can be estimated from the curvature 1/ of the corrugations using results from the elementary elasticity theory of bending of beams. Their maximum absolute val-
99
Fig. 5. Temperature increase with time at the centre of a natural cork stopper immersed in water at 90 ◦ C.
ues are of the order of Ew/2, where w is the wall thickness and E is Young’s modulus of the cell wall material at room temperature. The corresponding strain is around 10% (for = 5w). For the cork stoppers used in the experiments, the amplitude a and wavelength of the corrugations in the deformed layer of a stopper inside the bottle neck (Fig. 1) are of the order of h/5 and h/3, respectively (h is the cell height). In the non-deformed region of a stopper (Fig. 1) the amplitude is smaller, around h/10 for a wavelength around h/2. Note that the maximum curvature 1/ of a sinusoidal wall is proportional to a/2 . When cork is heated at 100 ◦ C in water or water vapour (but not in air), the cell walls straighten, as the experiments show. The residual stresses are progressively removed and the curvature 1/ and amplitude a of the corrugations decreases. The kinetics of straightening depends greatly on the presence of water (liquid water being more efficient than vapour) and also on temperature. In the time duration of the experiments (up to 10 min) water penetrates into the layer of deformed cells and alters their mechanical properties. The polymeric constituents of the cell walls in cork are viscoelastic and a wall can be modelled by a Voigt solid, with elastic (modulus E) and viscous (viscosity ) elements associated in parallel. An ˙ applied stress relates to strain and strain rate by = Eε + ε. The internal stresses int in the cell walls at room temperature, when is very large, correspond to the Eε term, with absolute value int = Ew/2, as discussed above. Water absorption and high temperatures are expected to greatly decrease because the main components of cork are polymeric; E/ thus increases. The internal stresses and strains are released. The residual strain ε decreases with time, with Eε + ε˙ = 0
(1)
since there are no external stresses ( = 0). Integration of Eq. (1) gives, since |ε| = w/2, = 0 exp
E
t
(2)
where 0 is the radius of curvature of the corrugations at time t = 0. The characteristic time that governs the kinetics of recovery is thus = /E. It depends on temperature and on the amount of absorbed water. Our experimental results suggest that decreases by many orders of magnitude when the temperature increases from 20 ◦ C to 80–100 ◦ C and the water content increases to 10–20%. As increases, the amplitude a of the corrugations decreases and their wavelength increases. Since the actual length of a wall is fixed, the height of the initially corrugated cells and thus the diameter of the stopper increase. As discussed above, strain in a cork stopper localizes near the surface in contact with the neck. It is this region that has the larger contribution to dimensional recovery when the stopper is heated in water or water vapour. This is apparent in Fig. 3a; the percentual
100
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diameter increase of an unused stopper is considerably smaller than that of a used stopper. 5. Conclusions The results obtained in experiments with used cork stoppers, described in this paper, show that used cork stoppers can recover the dimensions of unused stoppers. Full recovery in boiling water is achieved in a few minutes and is likely to be due to a large decrease in the viscosity of the cell wall material caused by water absorption. Temperature also lowers viscosity in addition to accelerating water absorption. The residual stresses are partly eliminated and the amplitude of the cell wall corrugations decreases. It is known [4,6] that these treatments do not disturb the cell wall composition and do not affect the mechanical properties since the cellular structure is recovered to that of the unused stopper. Recovery of used cork stoppers with an intact lateral surface (not damaged by the cork-screw used to remove the stopper from the bottle neck) should then receive serious consideration, since they can be brought back to their original dimensions and mechanical properties. On the other hand, the bulk region around the cork-screw is compressed and heals when the recovered stopper is re-inserted into a bottle. The more damaged top of the stopper can be removed by cutting, with a small decrease in the stopper height. Experi-
ments with recovered stoppers proved that their sealing ability was not affected by the treatments. Finally, the possible colouring of used cork stoppers at their bottom (e.g. in red wine bottles) may be removed, for example, by an appropriate chemical treatment (probably, the immersion in water at 90–100 ◦ C, will also remove the colouring). Acknowledgements The authors acknowledge L.O. Faria and A.M. Deus for helpful discussions and P.I.C. Teixeira for comments on the manuscript. References [1] M.A. Fortes, M.E. Rosa, H. Pereira, A Cortic¸a, IST Press, Lisboa, Portugal, 2004. [2] H. Pereira, M.E. Rosa, M.A. Fortes, The cellular structure of cork from Quercus suber L., International Association of Wood Anatomists Bulletin 8 (1987) 213–218. [3] L.J. Gibson, M.F. Ashby, Cellular Solids. Structure and Properties, second ed., Cambridge University Press, Cambridge, UK, 1997. [4] M.E. Rosa, H. Pereira, M.A. Fortes, Effects of hot water treatment on the structure and properties of cork, Wood and Fiber Science 22 (1990) 149–164. [5] M.E. Rosa, M.A. Fortes, Temperature induced alterations of the structure and mechanical properties of cork, Materials Science and Engineering 100 (1988) 69–78. [6] M.E. Rosa, M.A. Fortes, Effects of water vapour heating on the structure and properties of cork, Wood Science and Technology 23 (1989) 27–34.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Recovery of used cork stoppers M. Emília Rosa ∗ , M.A. Fortes Departamento de Engenharia de Materiais, Instituto Superior Técnico, ICEMS – Instituto de Ciência e Engenharia de Materiais e Superfícies, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
a r t i c l e
i n f o
Article history: Received 2 October 2008 Received in revised form 11 February 2009 Accepted 12 February 2009 Available online 6 March 2009 Keywords: Cork Stoppers Foams Recovery Buckling
a b s t r a c t Cork is a natural cellular material. As fabricated cork stoppers for wine bottles are cylindrical with a uniform cross-sectional diameter. Upon removal from a bottle, the stopper shows a smaller and slightly non-uniform diameter. If a used cork stopper is immersed in water or water vapour at a sufficiently high temperature, the original dimensions can be recovered in a time interval of a few minutes. The kinetics of dimensional recovery in air and water (liquid and vapour) at different temperatures were experimentally determined. Water absorption is determinant of a fast recovery rate. A simple viscoelastic model is used to explain the experimental results. Since the mechanical properties are also restored, the stopper can be re-used, provided the damage due to the cork-screw used to remove the stopper is confined to its bulk. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Natural cork stoppers are by far the main product of the cork industry. Other applications are the agglomerates made from cork granulates. The annual world production of natural cork stoppers is above 1010 . At present, the very small percentage of collected stoppers is granulated and mixed with cork granules from other origins (e.g. low quality cork) to produce cork agglomerates, including those for the manufacturing of agglomerated cork stoppers. Corkoak tree (Quercus suber L.) grows only in very particular regions (Mediterranean region, mostly Portugal and Spain). The corkboards are removed from the tree in 9-year intervals and there are big oscillations in annual cork world productions. In the last years, the world area of cork-oak has decreased mostly due to fire and tree diseases. The recovery of used cork stoppers would contribute to a waste reduction and to a more sustainable development [1]. The cells in cork are generated by a meristematic tissue – the phelogen – which surrounds the trunk of the cork tree. Each cell of the phelogen generates in succession prismatic cork cells which fill space. These cells have a short life and soon become hollow. The structure of cork is thus cellular, with rows of closed prismatic cells in the radial direction, each cell having, on average, 14 neighbours [1–3]. The new cork cells are pushed (compressed) against the layer of older dried cells, and since they still have soft walls, they buckle. This leads to corrugations or undulations, specially in the lateral cell walls. Further compression of cork, e.g. when a cork stopper is
∗ Corresponding author. Tel.: +351 21 8418105; fax: +351 21 8418132. E-mail address: [email protected] (M.E. Rosa). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.02.035
inserted into a bottle neck, leads to an increase in the amplitude of the corrugations, and to a reduction in linear dimensions (and volume). Heating cork (as extracted from the tree) in water, at 100 ◦ C, as in the industrial “boiling” operation, leads to an expansion of ≈15% in the radial direction (along the rows of cells and parallel to the radius of tree trunk) and ≈6% in the transverse directions (perpendicular to the radius of the tree trunk), caused by a decrease in the amplitude of the cell wall undulations [4]. Cork stoppers are made from boiled cork. When boiled cork is heated in air or water vapour at temperatures above 200 ◦ C, a further expansion occurs (about 15–20% in all directions) and the cell walls become fully straight [5,6]. A cylindrical cork stopper strains by ≈25–30% in its radial direction (along its radius) when inserted into a bottle neck (initial stopper diameter of 24 mm in a neck of 18 mm diameter, are typical values for wine bottles). The deformation of a cork stopper is localized at its periphery (in a layer of thickness ≈0.25n , where n is the neck diameter), where the cells are heavily corrugated, the bulk cells being little affected by compression (strain localization) [1]. Fig. 1 shows a SEM image of the top transverse section (section perpendicular to stopper axis) of a stopper inside the bottle neck. The transition between heavily corrugated cells close to the neck and “normal” cells in the bulk is clearly seen. Upon removal from the bottle, the stopper rapidly recovers part of the strain, but its diameter is still only about 80% of the original diameter. Recovery is slightly larger at the wetter bottom than at the drier top, but the difference is small. If the removed stopper is heated in boiling water it fully recovers its original dimensions and may even exceed them. A stopper with the dimensions of an unused stopper can thus be obtained.
98
M.E. Rosa, M.A. Fortes / Colloids and Surfaces A: Physicochem. Eng. Aspects 344 (2009) 97–100
Fig. 1. SEM image of part of the top surface of a natural cork stopper inserted into a bottle neck, showing the transition region between heavily corrugated cells (on the left) and normal, undeformed cells (on the right).
This paper investigates the recovery of dimensions in cork stoppers, heated at temperatures up to 100 ◦ C in (liquid) water, water vapour and air. Particular emphasis is put on the evaluation of the kinetics of recovery and on the long term amount of recovery. The changes in mass and in temperature inside the stopper have also been measured as functions of time. 2. Materials and methods Most (natural) cork stoppers used in the experiments were removed from commercial wine bottles of a particular label. Their moisture content, measured by drying at 100 ◦ C in air to constant mass, was around 6%. The average diameter and height of the unused stoppers were 24 mm and 38 mm, respectively. For the used stoppers, an average diameter was calculated from measured diameters at the top, at the bottom and at half-height (after removal from the bottle neck, the stabilised stopper diameter is not uniform: the recovery of dimensions depends on moisture content [1]). The height and mass of each stopper were also measured. The used stoppers were immersed in different media at atmospheric pressure: air and water vapour, at 100 ◦ C, and water at temperatures between 60 ◦ C and 100 ◦ C. The stoppers were removed every minute, up to 10 min, and their dimensions (average diameter and height) and mass were measured during immersion at 1-min intervals. In the case of water immersion, the wetting water layer at the surface was eliminated by contact with filter paper. Similar experiments were undertaken with unused natural cork stoppers. In other experiments, aimed at assessing the depth of penetration of water, dried cubes of cork (edge length ≈20 mm) were immersed in water at 90 ◦ C for various time intervals t. After removal from the hot water bath, the cubes were weighed to determine their water content M/M0 , and subsequently slices, approximately 5 mm thick, were cut from each cube face. The remaining small cube (≈10 mm edge) was weighed and then dried at 100 ◦ C in air to constant mass to determine its water content. The temperature Tc at the centre of a stopper immersed in a bath at temperature Ts = 90 ◦ C was measured with a thermocouple inserted in an axial hole drilled to half-height of the stopper. Heating Tc (t) curves were obtained with stoppers immersed in water, sand and oil baths in different experiments using different stoppers.
Fig. 2. Kinetics of water absorption by used cork stoppers. Water at temperature (◦ C): () 100; () 90; () 80; (䊉) 60; () 100 (water vapour).
3. Results The mass, diameter and height changes versus time of immersion in different media are plotted in Figs. 2, 3a and 3b, respectively. All changes are indicated as a percentage of the initial value (stabilised after removal from the bottle, in the case of used stoppers). Fig. 2 shows that the treatments in water (liquid and vapour) cause a large mass increase which is due to the water absorption. Heating in air at 100 ◦ C causes a small mass decrease due to loss of absorbed
Fig. 3. Effect of heat treatment on the dimensions of unused and used cork stoppers. (a) Diameter and (b) height. Treatment: (♦) water 100 ◦ C, unused; () water 100 ◦ C, used; () water 90 ◦ C, used; () water 80 ◦ C, used; (䊉) water 60 ◦ C, used; () water vapour 100 ◦ C, used; () air 100 ◦ C, used.
M.E. Rosa, M.A. Fortes / Colloids and Surfaces A: Physicochem. Eng. Aspects 344 (2009) 97–100
Fig. 4. Water content of cubes immersed in water at 90 ◦ C. () Original cubes and () internal cubes.
water [1,5]. Fig. 2 also shows that for the treatments in water, the mass change increases as the temperature increases. This is due to a faster rate of water absorption [4]. For the same temperature (100 ◦ C) the mass increase is larger in liquid water than in water vapour. The mass changes of the unused stoppers (not shown) are similar to those obtained with deformed stoppers, for the same medium and treatment duration. Fig. 3a shows that the heat treatments of used stoppers with liquid water cause a diameter expansion that increases as the temperature increases, but the diameter expansion is smaller in water vapour compared to liquid water, for the same temperature and time interval. The diameter expansion of the unused stoppers is smaller than that of the used stoppers (see Fig. 3a). Comparison of the results of Fig. 3a (diameter change) and b (height change) shows that the height expansion is much smaller than the diameter expansion for the used stoppers, due to the anisotropy of strain in the bottle neck. The unused, undeformed stoppers show similar changes in the diameter and height, the latter being larger than for the used stoppers (see Fig. 3b). This is because the Poisson effect (when a compression stress is imposed on a specimen, usually a constriction along the direction of the applied stress (axial) and elongations in the directions perpendicular to it (lateral) will appear; Poisson effect relates the lateral and the axial strains) in the compression of the used stopper increases the height of the used stoppers compared to that of the unused stopper. Results of the experiments with cork cubes are shown in Fig. 4 in which the moisture content is plotted as a function of time of immersion in water at 90 ◦ C of the original large cubes and of the small internal cubes. It is apparent that water hardly penetrates to the centre of the original cube sample and concentrates in the surface layers. Note that the thickness of the slices is comparable to that of the heavily deformed surface layer of a used stopper. The increase with time of the temperature Tc at the centre of a stopper immersed in a hot water bath is shown in Fig. 5. The initial temperature of the stopper was 23 ◦ C and that of the water bath was 90 ◦ C. The temperature near the surface was not measured; it is believed to be close to the temperature of the surrounding medium. Similar Tc (t) curves were obtained for immersion in sand and in oil at the same temperature. This shows that water diffusion into the stopper does not have an effect on the temperature change. 4. Discussion Corrugated cell walls in cork are a consequence of buckling under the compressive growth stresses in the tree when the cells are still soft. When the cells die and stiffen, residual stresses remain in the bent walls. These bending stresses can be estimated from the curvature 1/ of the corrugations using results from the elementary elasticity theory of bending of beams. Their maximum absolute val-
99
Fig. 5. Temperature increase with time at the centre of a natural cork stopper immersed in water at 90 ◦ C.
ues are of the order of Ew/2, where w is the wall thickness and E is Young’s modulus of the cell wall material at room temperature. The corresponding strain is around 10% (for = 5w). For the cork stoppers used in the experiments, the amplitude a and wavelength of the corrugations in the deformed layer of a stopper inside the bottle neck (Fig. 1) are of the order of h/5 and h/3, respectively (h is the cell height). In the non-deformed region of a stopper (Fig. 1) the amplitude is smaller, around h/10 for a wavelength around h/2. Note that the maximum curvature 1/ of a sinusoidal wall is proportional to a/2 . When cork is heated at 100 ◦ C in water or water vapour (but not in air), the cell walls straighten, as the experiments show. The residual stresses are progressively removed and the curvature 1/ and amplitude a of the corrugations decreases. The kinetics of straightening depends greatly on the presence of water (liquid water being more efficient than vapour) and also on temperature. In the time duration of the experiments (up to 10 min) water penetrates into the layer of deformed cells and alters their mechanical properties. The polymeric constituents of the cell walls in cork are viscoelastic and a wall can be modelled by a Voigt solid, with elastic (modulus E) and viscous (viscosity ) elements associated in parallel. An ˙ applied stress relates to strain and strain rate by = Eε + ε. The internal stresses int in the cell walls at room temperature, when is very large, correspond to the Eε term, with absolute value int = Ew/2, as discussed above. Water absorption and high temperatures are expected to greatly decrease because the main components of cork are polymeric; E/ thus increases. The internal stresses and strains are released. The residual strain ε decreases with time, with Eε + ε˙ = 0
(1)
since there are no external stresses ( = 0). Integration of Eq. (1) gives, since |ε| = w/2, = 0 exp
E
t
(2)
where 0 is the radius of curvature of the corrugations at time t = 0. The characteristic time that governs the kinetics of recovery is thus = /E. It depends on temperature and on the amount of absorbed water. Our experimental results suggest that decreases by many orders of magnitude when the temperature increases from 20 ◦ C to 80–100 ◦ C and the water content increases to 10–20%. As increases, the amplitude a of the corrugations decreases and their wavelength increases. Since the actual length of a wall is fixed, the height of the initially corrugated cells and thus the diameter of the stopper increase. As discussed above, strain in a cork stopper localizes near the surface in contact with the neck. It is this region that has the larger contribution to dimensional recovery when the stopper is heated in water or water vapour. This is apparent in Fig. 3a; the percentual
100
M.E. Rosa, M.A. Fortes / Colloids and Surfaces A: Physicochem. Eng. Aspects 344 (2009) 97–100
diameter increase of an unused stopper is considerably smaller than that of a used stopper. 5. Conclusions The results obtained in experiments with used cork stoppers, described in this paper, show that used cork stoppers can recover the dimensions of unused stoppers. Full recovery in boiling water is achieved in a few minutes and is likely to be due to a large decrease in the viscosity of the cell wall material caused by water absorption. Temperature also lowers viscosity in addition to accelerating water absorption. The residual stresses are partly eliminated and the amplitude of the cell wall corrugations decreases. It is known [4,6] that these treatments do not disturb the cell wall composition and do not affect the mechanical properties since the cellular structure is recovered to that of the unused stopper. Recovery of used cork stoppers with an intact lateral surface (not damaged by the cork-screw used to remove the stopper from the bottle neck) should then receive serious consideration, since they can be brought back to their original dimensions and mechanical properties. On the other hand, the bulk region around the cork-screw is compressed and heals when the recovered stopper is re-inserted into a bottle. The more damaged top of the stopper can be removed by cutting, with a small decrease in the stopper height. Experi-
ments with recovered stoppers proved that their sealing ability was not affected by the treatments. Finally, the possible colouring of used cork stoppers at their bottom (e.g. in red wine bottles) may be removed, for example, by an appropriate chemical treatment (probably, the immersion in water at 90–100 ◦ C, will also remove the colouring). Acknowledgements The authors acknowledge L.O. Faria and A.M. Deus for helpful discussions and P.I.C. Teixeira for comments on the manuscript. References [1] M.A. Fortes, M.E. Rosa, H. Pereira, A Cortic¸a, IST Press, Lisboa, Portugal, 2004. [2] H. Pereira, M.E. Rosa, M.A. Fortes, The cellular structure of cork from Quercus suber L., International Association of Wood Anatomists Bulletin 8 (1987) 213–218. [3] L.J. Gibson, M.F. Ashby, Cellular Solids. Structure and Properties, second ed., Cambridge University Press, Cambridge, UK, 1997. [4] M.E. Rosa, H. Pereira, M.A. Fortes, Effects of hot water treatment on the structure and properties of cork, Wood and Fiber Science 22 (1990) 149–164. [5] M.E. Rosa, M.A. Fortes, Temperature induced alterations of the structure and mechanical properties of cork, Materials Science and Engineering 100 (1988) 69–78. [6] M.E. Rosa, M.A. Fortes, Effects of water vapour heating on the structure and properties of cork, Wood Science and Technology 23 (1989) 27–34.