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Scanning electron microscopy observations of insulation cork agglomerates
Materials Science and Engineering, A 111 ( 1989) 2 1 7 - 2 2 5
217
Scanning Electron Microscopy Observations of Insulation Cork Agglomerates H. P E R E I R A
Departamento de Engenharia Florestal, Instituto Superior de Agronomia, 1399Lisboa (Portugal) E. F E R R E I R A
INFAL - lndfstria de Fabricacdo de Aglomerados Sarl, 2870 Monti]o (Portugal) (Received January 4, 1988; in revised form October 19, 1988)
Abstract
Observations by scanning electron microscopy were made of insulation cork agglomerates and of cork raw material The insulation cork agglomerates are agglomerates of granules of cork from the cork-oak tree (Quercus suber), self-bonded and expanded by autoclave steaming at approximately 300 °C and 40 kPa. In this process, the cork cells expand by unfolding the cell wall corrugations and by stretching the cell walls (concurrently decreasing the cell wall thickness). In the junctions between granules, the cells are compressed against each other and collapsed," the region of cell collapse is restricted to a limited number of cell layers at the boundary between the granules. The cell wall material is thermochemically degraded and a weight loss of approximately 30% of the cork material is observed in the production of these insulation agglomerates. I. Introduction
Insulation cork agglomerates are manufactured by the high temperature compression of granules of cork from Quercus suber. Particle diameters are usually in the range 0.5-1.5 cm. The process involves the direct steaming of the cork particles at approximately 300°C and 40 kPa without the use of any adhesive. Under these conditions, the cork granules expand and self-bond to each other. Autoclaves with hydraulic moving bottoms (or tops) are used and some compression is applied to the cork granules before steaming. The final product density may be varied by the extent of this precompression. Insulation cork agglomerates are known as pure expanded agglomerates since no foreign 0921-5093/89/$3.50
material is added to the cork. They are also known as black agglomerates because of their dark colour. They are produced with the lowest quality corks: mostly with virgin cork, the firstgeneration cork, which is rough, with deep cracks and unsuitable for other uses, and also with low quality reproduction cork as well as with cork wastes and remnants from industrial processing. The cork agglomerates obtained in this way have some remarkable properties: they are light, have a low thermal conductivity and a low permeability to water, and are good sound insulators. Because they are chemically stable, they are resistant to microbial degradation and to attack by rodents, and are very durable. Table 1 summarizes some of the properties of insulation cork agglomerates [1, 2]. Insulation cork agglomerates are used in building and industrial plants in the form of boards, pipe coverings and blocks, for thermal insulators and as sound and vibrational barriers. Utilization in interior decoration for wall or ceiling surfacing is also common. However, it is in cold storage that insulation corkboard presents outstanding advantages in relation to other insulating
TABLE 1 [1, 2]
Some propertiesofinsulation cork agglomerates
Property
Range of values
Density Working temperatures Thermal conductivity
lO0-130kgm 3 - 180to + l l O ° C 0 . 0 3 5 k c a l m - i h -I °C t
Specific heat (20 °C) Thermal diffusivity (20 °C) Permeability to steam
0.4-0.5 kcal kg- i °C i 0.00050-0.00073 m 2h i
(20 °c)
0.002-0.006gm
~h i(mmHg)
i
© Elsevier Sequoia/Printed in T h e Netherlands
218
materials, since it maintains a low conductivity while retaining its physical and chemical properties at very low temperatures. The properties of insulation cork agglomerates relate to the structure and chemical composition of the cork raw material. The structure of cork as well as some of its fundamental mechanical properties have been described previously [3-6]. Cork has a regular cellular structure, with closed cells of approximately prismatic form, arranged in parallel columns and connected base to base. The cell dimensions are small (average prism height 30-40/,m, average base area (4-6)× 10 -6 c m 2) and the lateral faces of the prisms show cell wall corrugations. Suberin is the main chemical component of cork, accounting for approximately 40% of the cell wall material [7]. Together with waxes, which are also present in substantial amounts, it should contribute decisively to the low water permeability of cork cell walls. The transformations brought about in the structure and chemical composition of cork by steam heating in the production of insulation corkboard have not been extensively studied [8]. The purpose of this paper is to discuss some aspects of the structure of insulation corkboard on the basis of scanning electron microscopy (SEM) observations. 2. Experimental details
Samples of insulation cork agglomerates were compared with reference samples of virgin cork and reproduction cork. These were not subjected to any treatment before the observations. The reference samples, obtained from pruned branches (virgin cork) and from low quality reproduction cork of the cork-oak tree (Quercus suber), were prepared as cubes (approximate edge length 2 cm), cut with the edges parallel to the three main directions (i.e. radial, tangential and axial, in relation to the tree), and all sections were observed. The nomenclature used in wood science is followed in this paper whenever we refer to these sections: tangential, radial and transverse sections respectively for the sections perpendicular to the radial, tangential and axial directions. The samples from insulation cork agglomerates were obtained from (a) one insulation corkboard selected from the current production in one industrial unit and (b) two cork agglomerates
that were produced in experimental conditions using a bench-scale autoclave. This autoclave had internal dimensions of 20 cm x 10 cm x 14.5 cm and functioned as a replica of the industrial autoclaves. All the agglomerates were prepared from raw material consisting of 75% virgin cork from branches and 25% reproduction cork (refuse and shavings), which is a current industrial furnish composition. The granulometric distribution of the cork granules was as follows: above 15.9 mm, 2.0%; 15.9-12.7 mm, 14.5%; 12.7-9.5 mm, 32.0%; 9.5-6.4 mm, 30.3%; 6.4-2.8 mm, 20.5%; less than 2.8 ram, 0.7% (per cent dry weight). The steam temperature and treatment time used for each case were as follows (a) industrial insulation agglomerate (thermal insulation type): 300°C, 40 kPa, 20 min; (b) experimental cork agglomerate: 300°C, 20 min; (c) experimental cork agglomerate: 250 °C, 40 min. For the samples treated in the experimental autoclave, the mass loss after steam heating was calculated as the percentage of dry weight in relation to the initial cork. The density was determined for all samples. Samples used for observations in the scanning electron microscope were cut with razor blades and the surface was coated with a gold film of approximately 200 A thickness. Measurements of cell dimensions, i.e. of cell wall thickness, were made directly on the SEM photographs. Cell wall thickness corresponds to half of the value measured from cell lumen to cell lumen. 3. Results and discussion
The samples of cork raw material that were observed showed a structure of regularly arranged prismatic cells, stacked in rows by connecting bases, as described previously [5]. Figure 1 exemplifies this cellular structure of untreated cork and is shown here to allow comparison with the steam-treated samples. In the tangential section (Fig. l(a)), the cork cells are seen as polygons, mostly with five, six or seven faces, in a honeycomb-type arrangement. In the radial and transverse sections, the cells are rectangularly shaped and arranged in rows parallel to the tree radial direction (Fig. l(b)). The lateral faces of the prisms show corrugations of the cell walls, as clearly seen in the transverse and radial sections. The year growth rings are marked by late-cork cells, which have thicker walls, reduced prism
219
Fig. 1. Anatomical characteristics of untreated cork from the cork-oak tree (Quercus suber). (a) Tangential section of reproduction cork showing "honey-comb"-type arrangement of cells. (b) Radial section of reproduction cork showing "brick-layered"-type arrangement of cells. Corrugation of the lateral cell walls is clearly visible. (c) Region of transition between two annual growth increments, shown in a radial section of reproduction cork. Late-cork cells have thicker cell walls, reduced prism height and no wall corrugation; early-cork cells have thinner walls, larger prism height and corrugations. (d) Transverse section of virgin cork showing one complete growth increment.
height and show little or no wall corrugation (Fig. l(c)). In virgin cork from branches, growth rings are in.general smaller than in reproduction cork; the cell arrangement is more irregular and the corrugation of cell walls more intense (Fig l(d)). The observations of the insulation cork agglomerates are summarized in Figs. 2-9. No
differences were found between the industrially produced cork agglomerate and the cork agglomerate prepared in the experimental autoclave under the same conditions (300 °C). The photographs shown to exemplify the structural features of insulation cork agglomerates were therefore chosen indifferently from one or the other sample.
220
Macroscopically it may be observed that the insulation cork agglomerates are made up of cork granules bonded to each other but with frequent intergranular spaces; in relation to the cork raw material, the granules significantly increased in volume and turned a dark brown, almost black, colour. The voids that occur between granules result from the irregular shape of the cork particles. The fraction of these intergranular spaces is of course related to the amount of precompression applied to the cork granules before steaming and contributes in large measure to the density of the final product. In the case studied, which corresponds to the normal cork agglomerates used for thermal insulation, the area of the intergranular spaces measured in sections of the agglomerates corresponds to approximately 16% of the total. When observed at the cellular level with the scanning electron microscope two regions corresponding to different structure and cell forms could be distinguished within each granule, as shown in Figs. 2 and 3: one region comprises the exterior part of the granule, where junctions with the adjoining granules occur, and the other comprises the interior part of the granule. In the region of the junctions between granules, the cork cells are compressed against each other and the regular structure of the cork is disturbed. In this region, as a result of compression, the cells are
Fig. 2. Two adjoining granules in insulation cork agglomerate. Arrow shows region of junction between granules.
distorted, heavily wrinkled and completely collapsed, as seen in Figs. 4 and 5. The area of collapse extends to a variable number of cell layers but generally does not exceed 10-20 cell layers per granule. This provides a good example of the capacity of cork to absorb compression energy
Fig. 3. Higher magnification of two adjoining granules in insulation cork agglomerate showing the two different regions of cellular structure and arrangements.
Fig. 4. Distortion of the cellular structure of a cork granule caused by compression against another granule.
221
Fig. 5. Bucklingand collapse of cells in region of compression betweengranules.
Fig. 6. Regionof bonding betweentwo granules in insulation cork agglomerate.
and to accommodate large distortion and collapse without long-range deformation [4]. The bonding between the cork granules results from their pressing against each other and from some sort of cell wall fusion between adjoining cells from each granule. In well-bonded granules, as shown in Fig. 6, it is not possible to distinguish
the dividing line between granules or to discriminate the cells pertaining to each granule. The role played by the chemical components of cork in the bonding between granules is not yet clarified. One hypothesis that has been put forward is that the cork waxes are involved in the process by melting during the steaming and subsequently hardening. However, it is possible to produce agglomerates using pre-extracted wax-free cork granules which differ very little from the normal ones [91. It therefore seems more probable to consider the involvement of the phenolic components of cork (especially of tannins) by condensation reactions with aldehyde groups, in a way similar to the synthesis of phenol-formaldehyde resins. In fact, tannins extracted from cork have already been used as the phenolic source for synthesis of phenol-formaldehyde resins [10]. Except for the region of the junctions between granules, the cork cells in the insulation agglomerate show a large increase in volume when compared to cells in the untreated cork samples. The quantification of this cell expansion is not easily assessed owing to the large variability in cell dimensions and to the difficulty in cutting sections for the SEM observations that are perpendicular to the radial, tangential and axial directions, since these are not clearly distinguished in the expanded granules. However, the following ranges of values were measured in the expanded cork cells in the interior of granules: "prism' height 35-50 jim, base area ( 6 - 1 0 ) × 1 0 ('cm 2. When compared to the untreated samples, which have cells with 30-4(I jim prism height and (4-7)× 10 -~' cm e base area, these values represent an increase in cell volume that should not be far from 100%. Earlier studies of insulation cork agglomerates [8] in thin cuttings using microtomy and transmission microscopy also report the expansion of cork cells, with linear dimensional variations of approximately + 30%. Recently, in thermal treatments of cork cubes at 300 °C in air, Rosa and Fortes [6] reported volume increases of approximately 80%. These authors found smaller increases in the radial direction, i.e. in the direction of prism heights, of approximately 10% only. However, for their heat treatments they used reproduction cork previously submitted to the industrial operation of 'boiling', which consists of immersing the cork planks in boiling water. In this process, cork expands mainly in the radial direc-
222
tion, with dimensional variations along this direction that may attain + 15% [11], and this means that in the referred heat treatments a preexpanded cork material was being used, thereby explaining the smaller variations recorded. A remarkable feature of the expanded cork cells in the insulation agglomerates is the flattening of corrugations and the straightening of the cell walls. The corrugations shown by the lateral faces of the prisms in untreated cork disappear during the steaming procedure, as clearly seen if Figs. 7 and 8 are compared with Fig. l(b). This fact was also recorded for the heat treatments of cork in air [6]. As a consequence of expansion and cell wall flattening, the form of the cells changes to a rounder shape, and a certain ballooning of cells may be observed, as shown in Fig. 8. Expansion occurs to a much lesser extent in late-cork cells, which originally show smaller dimensions, little corrugation and thicker cell walls. The year growth rings can still be seen in the expanded cork granules of insulation agglomerates, as shown by Fig. 8. The wall thickness of the cork cells decreases during the steaming at 300 °C. While in untreated cork, the wall thickness is on average 1 ~m (in early cork), in the expanded cells of insulation agglomerates the wall thickness decreased to
approximately 0.5/~m. In some regions, especially in the heavily distorted cells in the region of cell collapse between granules, the wall thickness might be reduced to values as low as 0.2/~m (Fig. 5) where the walls are strongly stretched. The stretching of walls is not the only cause of the observed decrease in cell wall thickness. In fact, material is removed from the cell walls and a total mass loss of cork of 20%-30% is recorded in the production of insulation agglomerates. This mass loss corresponds to the thermal degradation of the chemical components of the cork into smaller molecules that are volatile in these conditions. The thermochemical degradation of cork components during the production of insulation agglomerates has been reported to be selective [8]. Polysaccharides (hemicelluloses and cellulose) seem to be less resistant to thermal degradation and are removed to a greater extent than the other components. Suberin and lignin, on the other hand, remain the most stable cell wall structural components at this temperature. As a result of this thermochemical degradation, some cells show cracks and holes in their cell walls, clearly seen on the interior surface of the cells. This may be observed in Fig. 7 and, at a higher magnification, in Fig. 9. The influence of steam temperature on the process of expansion and bonding of cork
Fig. 7. Cells in the interior of a cork granule in insulation cork agglomerate. Cells have increased in volume and wall corrugations have disappeared.
Fig. 8. Part of one growth ring in the interior of a cork granule in insulation cork agglomerate. Arrow shows latecork cells.
223
Fig. 9. Expanded cells in cork insulation agglomerate showing cracks and holes in the cell walls.
granules was studied by observation of an agglomerate produced at a lower temperature, but otherwise the same conditions. A steam temperature of 250 °C was used. The cork agglomerate obtained in this way had a cork-brown colour, much lighter than the very dark brown colour of the usual insulation corkboard. The SEM observations exemplified by Figs. 10 and 11, show that the cells suffered expansion but the dimensional increase and the unfolding of corrugations proceeded to a lesser extent than in the insulation agglomerates previously described. The cell wall thickness remained similar to the thickness of cells in the untreated cork raw material. In the region of the junctions of the granules, compression of cells was observed but extended to a smaller number of cell layers. The differentiation of the different granules was possible and in many cases the bonding between granules did not occur, as seen in Fig. 11. When steaming was carried out at the lower temperature, the thermochemical degradation of cell wall components did not proceed to a significant extent. In fact only 5% of the initial weight was lost in the steam, as compared with the 30% loss in the case of insulation agglomerates produced at 300 °C (Table 2). The difference in mass loss during steaming at the two temperatures is also apparent if the densities of the materials are considered. As also
Fig. 111. Junction of three cork granules in cork insulation agglomerate produced by steaming at 25(I °C.
Fig. 11. Two contacting cork granules in cork insulation agglomerate produced at 25(I °C, showing that bonding has not occurred.
seen in Table 2, the agglomerates produced with 300°C steam (both the industrial and experimental samples) have densities of approximately 120 kg m-3; this value correlates well with the densities of the untreated cork raw materials ( 199-215 kg m- 3) if a mass loss during the treatment of 25%-30% is accounted for, in addition
224 TABLE 2 Densities of samples and mass loss during the steaming for production of the cork agglomerates
Sample
Density (kgm ~)
Mass loss (% dryinitial weight)
Industrial insulation corkboard (300 °C) Experimental cork agglomerate (300 °C) Experimental cork agglomerate (250 °C) Virgin cork Reproduction cork
120
25-30 a
tion, which is decisive in establishing the structure and properties of cork [12]. However, more experimental research on the properties of cork, including insulation cork agglomerates, is under way [13].
115
30.7
4. Conclusions
195
5.3
215 199
"Approximate value, obtained from industrial production data.
to a void percentage of approximately 16%. In the case of the cork treated at 250 °C, the density was 195 kg m -3, much closer to the density of the untreated cork, in agreement with the low mass loss at this temperature. The thermochemical degradation of the cork components during steaming for the production of insulation agglomerates seems necessary to allow a full expansion of cells. On one hand, the presence of volatile degradation products with low molecular weights is required to build up the internal pressure of the gaseous atmosphere inside the cells and cause the unfolding of the cell walls, as calculated by Rosa and Fortes [6]. On the other hand, since this thermochemical degradation is selective [8], it may be argued that the partial loss of the cellulose matrix is necessary to allow the stretching of the cell walls and therefore the surplus expansion of the cells in addition to the unfolding of corrugations. Only with such an increased expansion can the granules increase sufficiently in volume to be compressed against each other and bonded. This means that the process conditions for the production of insulation corkboard will have to be set in order to achieve this result, e.g. a minimum steam temperature will have to be used. The changes in the microstructure of cork and in the chemical composition of the cell walls which occur during the production of insulation cork agglomerates will result in different properties for the cork. It is known that the flattening of cell wall corrugations by expansion in boiling water reduces the strength of cork and the Young's moduli in the three directions [11]. The effect of high temperature has a very drastic effect in reducing the strength [6], since it affects not only the structure but also the chemical composi-
When subjected to direct steam heating at 300 °C and 40 kPa, as used to produce insulation cork agglomerates, cork cells expand by unfolding of cell wall corrugations and stretching of the cell walls. The volume of cells increases and the thickness of the cell walls decreases. At this temperature a significant material loss of approximately 30% occurs by thermochemical degradation of the cell walls. In these conditions, the cork granules are compressed against each other and bonded. In the region of contact between granules, the cork cells are heavily distorted and collapsed. However, collapse is restricted to a limited number of cell layers in the interior of the granules. At lower temperatures, the thermochemical degradation is not sufficient to allow the expansion necessary to achieve the bonding between granules. Steam temperature is therefore an important control parameter in the production of insulation cork agglomerates.
Acknowledgments We are grateful to Luis Barros and M. Emflia Rosa for their help in sample preparation for scanning microscopy and to M. A. Fortes for his critical revision of the manuscript. We also thank Isabel Leitfio who typed the manuscript. The research was financially supported by the Junta Nacional de Investigaqfio Cientffica e Tecnol6gica (JNICT) and by the Instituto de Ci~ncia e Tecnologia dos Materials (ICTM), Lisbon, Portugal.
References 1 A. Andrade, Thermic and acoustic insulation, 1962 (Junta Nacional da Cortiqa, Lisbon). 2 H. Medeiros, A B C Insulation corkboard (Fundo Fomento de Exportaqfio, Lisbon). 3 J. V. Natividade, Subericultura, 1950 (Direcqfio Geral dos Serviqos Florestais e Aquicolas, Lisbon). 4 L. J. Gibson, K. E. Easterling and M. E Ashby, The structure and mechanics of cork, Proc. R. Sot'. London, Ser. A, 377(1981 ) 99-117. 5 H. Pereira, M. E. Rosa and M. A. Fortes, The cellular structure of cork from Quercus suber L., IA WA Bull., 8 (1987)213-218.
225 6 M. E. Rosa and M. A. Fortes, Temperature induced alterations of the structure and mechanical properties of cork, Mater. Sci. Eng., 100 (1987) 69-78. 7 H. Pereira, Chemical composition and variability of cork from Quercus suber L., Wood Sci. Technol., 22 (1988) 211-218. 8 E. P. Ferreira, and H. Pereira, Algumas alteraq6es anatdmicas e quimicas da cortiqa no fabrico de aglomerados negros, Cortica, 576 (1986) 274-279. 9 J. M. Lanuza and L. V. Fernandes, Notas sobre las ceras de corcho y la influencia de su extraccion en los aglomerados, Anales del Instituto Florestal de lnvestigaciones y Experiencias (Madrid) (1966) 25(t-269.
10 H. Pereira and V. Prata, Utilizaqfio de extractos taninosos da cortiqa como fonte fen61ica no fabrico de resinas, Cortica, 511 (19811119-125. 11 M. E. Rosa, H. Pereira and M. A. Fortes, Effects of hot water treatments on the structure and properties of cork, submitted to Wood Fiber Sci. 12 H. Pereira and V. Marques, 1988. The effect of chemical treatments in the structure of cork, IA WA Bull., 9 (1988) 337-345. 13 M. A. Fortes, M. Ashby and H. Pereira, Structure, properties and new applications of cork, Research Contract, Programme Wood as a Raw-Material, 1988 (Commission of the European Communities, Brussels).
217
Scanning Electron Microscopy Observations of Insulation Cork Agglomerates H. P E R E I R A
Departamento de Engenharia Florestal, Instituto Superior de Agronomia, 1399Lisboa (Portugal) E. F E R R E I R A
INFAL - lndfstria de Fabricacdo de Aglomerados Sarl, 2870 Monti]o (Portugal) (Received January 4, 1988; in revised form October 19, 1988)
Abstract
Observations by scanning electron microscopy were made of insulation cork agglomerates and of cork raw material The insulation cork agglomerates are agglomerates of granules of cork from the cork-oak tree (Quercus suber), self-bonded and expanded by autoclave steaming at approximately 300 °C and 40 kPa. In this process, the cork cells expand by unfolding the cell wall corrugations and by stretching the cell walls (concurrently decreasing the cell wall thickness). In the junctions between granules, the cells are compressed against each other and collapsed," the region of cell collapse is restricted to a limited number of cell layers at the boundary between the granules. The cell wall material is thermochemically degraded and a weight loss of approximately 30% of the cork material is observed in the production of these insulation agglomerates. I. Introduction
Insulation cork agglomerates are manufactured by the high temperature compression of granules of cork from Quercus suber. Particle diameters are usually in the range 0.5-1.5 cm. The process involves the direct steaming of the cork particles at approximately 300°C and 40 kPa without the use of any adhesive. Under these conditions, the cork granules expand and self-bond to each other. Autoclaves with hydraulic moving bottoms (or tops) are used and some compression is applied to the cork granules before steaming. The final product density may be varied by the extent of this precompression. Insulation cork agglomerates are known as pure expanded agglomerates since no foreign 0921-5093/89/$3.50
material is added to the cork. They are also known as black agglomerates because of their dark colour. They are produced with the lowest quality corks: mostly with virgin cork, the firstgeneration cork, which is rough, with deep cracks and unsuitable for other uses, and also with low quality reproduction cork as well as with cork wastes and remnants from industrial processing. The cork agglomerates obtained in this way have some remarkable properties: they are light, have a low thermal conductivity and a low permeability to water, and are good sound insulators. Because they are chemically stable, they are resistant to microbial degradation and to attack by rodents, and are very durable. Table 1 summarizes some of the properties of insulation cork agglomerates [1, 2]. Insulation cork agglomerates are used in building and industrial plants in the form of boards, pipe coverings and blocks, for thermal insulators and as sound and vibrational barriers. Utilization in interior decoration for wall or ceiling surfacing is also common. However, it is in cold storage that insulation corkboard presents outstanding advantages in relation to other insulating
TABLE 1 [1, 2]
Some propertiesofinsulation cork agglomerates
Property
Range of values
Density Working temperatures Thermal conductivity
lO0-130kgm 3 - 180to + l l O ° C 0 . 0 3 5 k c a l m - i h -I °C t
Specific heat (20 °C) Thermal diffusivity (20 °C) Permeability to steam
0.4-0.5 kcal kg- i °C i 0.00050-0.00073 m 2h i
(20 °c)
0.002-0.006gm
~h i(mmHg)
i
© Elsevier Sequoia/Printed in T h e Netherlands
218
materials, since it maintains a low conductivity while retaining its physical and chemical properties at very low temperatures. The properties of insulation cork agglomerates relate to the structure and chemical composition of the cork raw material. The structure of cork as well as some of its fundamental mechanical properties have been described previously [3-6]. Cork has a regular cellular structure, with closed cells of approximately prismatic form, arranged in parallel columns and connected base to base. The cell dimensions are small (average prism height 30-40/,m, average base area (4-6)× 10 -6 c m 2) and the lateral faces of the prisms show cell wall corrugations. Suberin is the main chemical component of cork, accounting for approximately 40% of the cell wall material [7]. Together with waxes, which are also present in substantial amounts, it should contribute decisively to the low water permeability of cork cell walls. The transformations brought about in the structure and chemical composition of cork by steam heating in the production of insulation corkboard have not been extensively studied [8]. The purpose of this paper is to discuss some aspects of the structure of insulation corkboard on the basis of scanning electron microscopy (SEM) observations. 2. Experimental details
Samples of insulation cork agglomerates were compared with reference samples of virgin cork and reproduction cork. These were not subjected to any treatment before the observations. The reference samples, obtained from pruned branches (virgin cork) and from low quality reproduction cork of the cork-oak tree (Quercus suber), were prepared as cubes (approximate edge length 2 cm), cut with the edges parallel to the three main directions (i.e. radial, tangential and axial, in relation to the tree), and all sections were observed. The nomenclature used in wood science is followed in this paper whenever we refer to these sections: tangential, radial and transverse sections respectively for the sections perpendicular to the radial, tangential and axial directions. The samples from insulation cork agglomerates were obtained from (a) one insulation corkboard selected from the current production in one industrial unit and (b) two cork agglomerates
that were produced in experimental conditions using a bench-scale autoclave. This autoclave had internal dimensions of 20 cm x 10 cm x 14.5 cm and functioned as a replica of the industrial autoclaves. All the agglomerates were prepared from raw material consisting of 75% virgin cork from branches and 25% reproduction cork (refuse and shavings), which is a current industrial furnish composition. The granulometric distribution of the cork granules was as follows: above 15.9 mm, 2.0%; 15.9-12.7 mm, 14.5%; 12.7-9.5 mm, 32.0%; 9.5-6.4 mm, 30.3%; 6.4-2.8 mm, 20.5%; less than 2.8 ram, 0.7% (per cent dry weight). The steam temperature and treatment time used for each case were as follows (a) industrial insulation agglomerate (thermal insulation type): 300°C, 40 kPa, 20 min; (b) experimental cork agglomerate: 300°C, 20 min; (c) experimental cork agglomerate: 250 °C, 40 min. For the samples treated in the experimental autoclave, the mass loss after steam heating was calculated as the percentage of dry weight in relation to the initial cork. The density was determined for all samples. Samples used for observations in the scanning electron microscope were cut with razor blades and the surface was coated with a gold film of approximately 200 A thickness. Measurements of cell dimensions, i.e. of cell wall thickness, were made directly on the SEM photographs. Cell wall thickness corresponds to half of the value measured from cell lumen to cell lumen. 3. Results and discussion
The samples of cork raw material that were observed showed a structure of regularly arranged prismatic cells, stacked in rows by connecting bases, as described previously [5]. Figure 1 exemplifies this cellular structure of untreated cork and is shown here to allow comparison with the steam-treated samples. In the tangential section (Fig. l(a)), the cork cells are seen as polygons, mostly with five, six or seven faces, in a honeycomb-type arrangement. In the radial and transverse sections, the cells are rectangularly shaped and arranged in rows parallel to the tree radial direction (Fig. l(b)). The lateral faces of the prisms show corrugations of the cell walls, as clearly seen in the transverse and radial sections. The year growth rings are marked by late-cork cells, which have thicker walls, reduced prism
219
Fig. 1. Anatomical characteristics of untreated cork from the cork-oak tree (Quercus suber). (a) Tangential section of reproduction cork showing "honey-comb"-type arrangement of cells. (b) Radial section of reproduction cork showing "brick-layered"-type arrangement of cells. Corrugation of the lateral cell walls is clearly visible. (c) Region of transition between two annual growth increments, shown in a radial section of reproduction cork. Late-cork cells have thicker cell walls, reduced prism height and no wall corrugation; early-cork cells have thinner walls, larger prism height and corrugations. (d) Transverse section of virgin cork showing one complete growth increment.
height and show little or no wall corrugation (Fig. l(c)). In virgin cork from branches, growth rings are in.general smaller than in reproduction cork; the cell arrangement is more irregular and the corrugation of cell walls more intense (Fig l(d)). The observations of the insulation cork agglomerates are summarized in Figs. 2-9. No
differences were found between the industrially produced cork agglomerate and the cork agglomerate prepared in the experimental autoclave under the same conditions (300 °C). The photographs shown to exemplify the structural features of insulation cork agglomerates were therefore chosen indifferently from one or the other sample.
220
Macroscopically it may be observed that the insulation cork agglomerates are made up of cork granules bonded to each other but with frequent intergranular spaces; in relation to the cork raw material, the granules significantly increased in volume and turned a dark brown, almost black, colour. The voids that occur between granules result from the irregular shape of the cork particles. The fraction of these intergranular spaces is of course related to the amount of precompression applied to the cork granules before steaming and contributes in large measure to the density of the final product. In the case studied, which corresponds to the normal cork agglomerates used for thermal insulation, the area of the intergranular spaces measured in sections of the agglomerates corresponds to approximately 16% of the total. When observed at the cellular level with the scanning electron microscope two regions corresponding to different structure and cell forms could be distinguished within each granule, as shown in Figs. 2 and 3: one region comprises the exterior part of the granule, where junctions with the adjoining granules occur, and the other comprises the interior part of the granule. In the region of the junctions between granules, the cork cells are compressed against each other and the regular structure of the cork is disturbed. In this region, as a result of compression, the cells are
Fig. 2. Two adjoining granules in insulation cork agglomerate. Arrow shows region of junction between granules.
distorted, heavily wrinkled and completely collapsed, as seen in Figs. 4 and 5. The area of collapse extends to a variable number of cell layers but generally does not exceed 10-20 cell layers per granule. This provides a good example of the capacity of cork to absorb compression energy
Fig. 3. Higher magnification of two adjoining granules in insulation cork agglomerate showing the two different regions of cellular structure and arrangements.
Fig. 4. Distortion of the cellular structure of a cork granule caused by compression against another granule.
221
Fig. 5. Bucklingand collapse of cells in region of compression betweengranules.
Fig. 6. Regionof bonding betweentwo granules in insulation cork agglomerate.
and to accommodate large distortion and collapse without long-range deformation [4]. The bonding between the cork granules results from their pressing against each other and from some sort of cell wall fusion between adjoining cells from each granule. In well-bonded granules, as shown in Fig. 6, it is not possible to distinguish
the dividing line between granules or to discriminate the cells pertaining to each granule. The role played by the chemical components of cork in the bonding between granules is not yet clarified. One hypothesis that has been put forward is that the cork waxes are involved in the process by melting during the steaming and subsequently hardening. However, it is possible to produce agglomerates using pre-extracted wax-free cork granules which differ very little from the normal ones [91. It therefore seems more probable to consider the involvement of the phenolic components of cork (especially of tannins) by condensation reactions with aldehyde groups, in a way similar to the synthesis of phenol-formaldehyde resins. In fact, tannins extracted from cork have already been used as the phenolic source for synthesis of phenol-formaldehyde resins [10]. Except for the region of the junctions between granules, the cork cells in the insulation agglomerate show a large increase in volume when compared to cells in the untreated cork samples. The quantification of this cell expansion is not easily assessed owing to the large variability in cell dimensions and to the difficulty in cutting sections for the SEM observations that are perpendicular to the radial, tangential and axial directions, since these are not clearly distinguished in the expanded granules. However, the following ranges of values were measured in the expanded cork cells in the interior of granules: "prism' height 35-50 jim, base area ( 6 - 1 0 ) × 1 0 ('cm 2. When compared to the untreated samples, which have cells with 30-4(I jim prism height and (4-7)× 10 -~' cm e base area, these values represent an increase in cell volume that should not be far from 100%. Earlier studies of insulation cork agglomerates [8] in thin cuttings using microtomy and transmission microscopy also report the expansion of cork cells, with linear dimensional variations of approximately + 30%. Recently, in thermal treatments of cork cubes at 300 °C in air, Rosa and Fortes [6] reported volume increases of approximately 80%. These authors found smaller increases in the radial direction, i.e. in the direction of prism heights, of approximately 10% only. However, for their heat treatments they used reproduction cork previously submitted to the industrial operation of 'boiling', which consists of immersing the cork planks in boiling water. In this process, cork expands mainly in the radial direc-
222
tion, with dimensional variations along this direction that may attain + 15% [11], and this means that in the referred heat treatments a preexpanded cork material was being used, thereby explaining the smaller variations recorded. A remarkable feature of the expanded cork cells in the insulation agglomerates is the flattening of corrugations and the straightening of the cell walls. The corrugations shown by the lateral faces of the prisms in untreated cork disappear during the steaming procedure, as clearly seen if Figs. 7 and 8 are compared with Fig. l(b). This fact was also recorded for the heat treatments of cork in air [6]. As a consequence of expansion and cell wall flattening, the form of the cells changes to a rounder shape, and a certain ballooning of cells may be observed, as shown in Fig. 8. Expansion occurs to a much lesser extent in late-cork cells, which originally show smaller dimensions, little corrugation and thicker cell walls. The year growth rings can still be seen in the expanded cork granules of insulation agglomerates, as shown by Fig. 8. The wall thickness of the cork cells decreases during the steaming at 300 °C. While in untreated cork, the wall thickness is on average 1 ~m (in early cork), in the expanded cells of insulation agglomerates the wall thickness decreased to
approximately 0.5/~m. In some regions, especially in the heavily distorted cells in the region of cell collapse between granules, the wall thickness might be reduced to values as low as 0.2/~m (Fig. 5) where the walls are strongly stretched. The stretching of walls is not the only cause of the observed decrease in cell wall thickness. In fact, material is removed from the cell walls and a total mass loss of cork of 20%-30% is recorded in the production of insulation agglomerates. This mass loss corresponds to the thermal degradation of the chemical components of the cork into smaller molecules that are volatile in these conditions. The thermochemical degradation of cork components during the production of insulation agglomerates has been reported to be selective [8]. Polysaccharides (hemicelluloses and cellulose) seem to be less resistant to thermal degradation and are removed to a greater extent than the other components. Suberin and lignin, on the other hand, remain the most stable cell wall structural components at this temperature. As a result of this thermochemical degradation, some cells show cracks and holes in their cell walls, clearly seen on the interior surface of the cells. This may be observed in Fig. 7 and, at a higher magnification, in Fig. 9. The influence of steam temperature on the process of expansion and bonding of cork
Fig. 7. Cells in the interior of a cork granule in insulation cork agglomerate. Cells have increased in volume and wall corrugations have disappeared.
Fig. 8. Part of one growth ring in the interior of a cork granule in insulation cork agglomerate. Arrow shows latecork cells.
223
Fig. 9. Expanded cells in cork insulation agglomerate showing cracks and holes in the cell walls.
granules was studied by observation of an agglomerate produced at a lower temperature, but otherwise the same conditions. A steam temperature of 250 °C was used. The cork agglomerate obtained in this way had a cork-brown colour, much lighter than the very dark brown colour of the usual insulation corkboard. The SEM observations exemplified by Figs. 10 and 11, show that the cells suffered expansion but the dimensional increase and the unfolding of corrugations proceeded to a lesser extent than in the insulation agglomerates previously described. The cell wall thickness remained similar to the thickness of cells in the untreated cork raw material. In the region of the junctions of the granules, compression of cells was observed but extended to a smaller number of cell layers. The differentiation of the different granules was possible and in many cases the bonding between granules did not occur, as seen in Fig. 11. When steaming was carried out at the lower temperature, the thermochemical degradation of cell wall components did not proceed to a significant extent. In fact only 5% of the initial weight was lost in the steam, as compared with the 30% loss in the case of insulation agglomerates produced at 300 °C (Table 2). The difference in mass loss during steaming at the two temperatures is also apparent if the densities of the materials are considered. As also
Fig. 111. Junction of three cork granules in cork insulation agglomerate produced by steaming at 25(I °C.
Fig. 11. Two contacting cork granules in cork insulation agglomerate produced at 25(I °C, showing that bonding has not occurred.
seen in Table 2, the agglomerates produced with 300°C steam (both the industrial and experimental samples) have densities of approximately 120 kg m-3; this value correlates well with the densities of the untreated cork raw materials ( 199-215 kg m- 3) if a mass loss during the treatment of 25%-30% is accounted for, in addition
224 TABLE 2 Densities of samples and mass loss during the steaming for production of the cork agglomerates
Sample
Density (kgm ~)
Mass loss (% dryinitial weight)
Industrial insulation corkboard (300 °C) Experimental cork agglomerate (300 °C) Experimental cork agglomerate (250 °C) Virgin cork Reproduction cork
120
25-30 a
tion, which is decisive in establishing the structure and properties of cork [12]. However, more experimental research on the properties of cork, including insulation cork agglomerates, is under way [13].
115
30.7
4. Conclusions
195
5.3
215 199
"Approximate value, obtained from industrial production data.
to a void percentage of approximately 16%. In the case of the cork treated at 250 °C, the density was 195 kg m -3, much closer to the density of the untreated cork, in agreement with the low mass loss at this temperature. The thermochemical degradation of the cork components during steaming for the production of insulation agglomerates seems necessary to allow a full expansion of cells. On one hand, the presence of volatile degradation products with low molecular weights is required to build up the internal pressure of the gaseous atmosphere inside the cells and cause the unfolding of the cell walls, as calculated by Rosa and Fortes [6]. On the other hand, since this thermochemical degradation is selective [8], it may be argued that the partial loss of the cellulose matrix is necessary to allow the stretching of the cell walls and therefore the surplus expansion of the cells in addition to the unfolding of corrugations. Only with such an increased expansion can the granules increase sufficiently in volume to be compressed against each other and bonded. This means that the process conditions for the production of insulation corkboard will have to be set in order to achieve this result, e.g. a minimum steam temperature will have to be used. The changes in the microstructure of cork and in the chemical composition of the cell walls which occur during the production of insulation cork agglomerates will result in different properties for the cork. It is known that the flattening of cell wall corrugations by expansion in boiling water reduces the strength of cork and the Young's moduli in the three directions [11]. The effect of high temperature has a very drastic effect in reducing the strength [6], since it affects not only the structure but also the chemical composi-
When subjected to direct steam heating at 300 °C and 40 kPa, as used to produce insulation cork agglomerates, cork cells expand by unfolding of cell wall corrugations and stretching of the cell walls. The volume of cells increases and the thickness of the cell walls decreases. At this temperature a significant material loss of approximately 30% occurs by thermochemical degradation of the cell walls. In these conditions, the cork granules are compressed against each other and bonded. In the region of contact between granules, the cork cells are heavily distorted and collapsed. However, collapse is restricted to a limited number of cell layers in the interior of the granules. At lower temperatures, the thermochemical degradation is not sufficient to allow the expansion necessary to achieve the bonding between granules. Steam temperature is therefore an important control parameter in the production of insulation cork agglomerates.
Acknowledgments We are grateful to Luis Barros and M. Emflia Rosa for their help in sample preparation for scanning microscopy and to M. A. Fortes for his critical revision of the manuscript. We also thank Isabel Leitfio who typed the manuscript. The research was financially supported by the Junta Nacional de Investigaqfio Cientffica e Tecnol6gica (JNICT) and by the Instituto de Ci~ncia e Tecnologia dos Materials (ICTM), Lisbon, Portugal.
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225 6 M. E. Rosa and M. A. Fortes, Temperature induced alterations of the structure and mechanical properties of cork, Mater. Sci. Eng., 100 (1987) 69-78. 7 H. Pereira, Chemical composition and variability of cork from Quercus suber L., Wood Sci. Technol., 22 (1988) 211-218. 8 E. P. Ferreira, and H. Pereira, Algumas alteraq6es anatdmicas e quimicas da cortiqa no fabrico de aglomerados negros, Cortica, 576 (1986) 274-279. 9 J. M. Lanuza and L. V. Fernandes, Notas sobre las ceras de corcho y la influencia de su extraccion en los aglomerados, Anales del Instituto Florestal de lnvestigaciones y Experiencias (Madrid) (1966) 25(t-269.
10 H. Pereira and V. Prata, Utilizaqfio de extractos taninosos da cortiqa como fonte fen61ica no fabrico de resinas, Cortica, 511 (19811119-125. 11 M. E. Rosa, H. Pereira and M. A. Fortes, Effects of hot water treatments on the structure and properties of cork, submitted to Wood Fiber Sci. 12 H. Pereira and V. Marques, 1988. The effect of chemical treatments in the structure of cork, IA WA Bull., 9 (1988) 337-345. 13 M. A. Fortes, M. Ashby and H. Pereira, Structure, properties and new applications of cork, Research Contract, Programme Wood as a Raw-Material, 1988 (Commission of the European Communities, Brussels).