higher temperatures (27% at 250 ~ 49% at 300 62% at 350 until ashing at 450 The polysaccharides are the most heat sensitive components: at 200 ...
Wood Sci. Technol. 26:259-269 (1992)
Wood Science and Technology 9 Springer-Verlag t 992
The thermochemical degradation of cork H. Pereira*, Lisboa, Portugal
Summary. The thermochemical degradation of cork from Quercus suber L. was studied in the temperature range 150 ~ ~ in relation to mass loss, chemical composition and the influence on the cellular structure. The degradation of cork is strongly dependent on temperature and mass losses become significant at 200~ (15% of initial dry weight) and increase rapidly for higher temperatures (27% at 250 ~ 49% at 300 ~ 62% at 350 ~ until ashing at 450 ~ The polysaccharides are the most heat sensitive components: at 200 ~ hemicelluloses disappear and cellulose is degraded to a considerable extent. Suberin is more resistant and degradation starts at approx. 250~ 300~ samples only contain 7% suberin. The cellular structure of cork is also significantly influenced by temperature. Upon heating, cells expand and the cell walls stretch, attaining at 250~ a maximum cell volume increase corresponding to a factor of approximately 2. Above 300 ~ the structure of cell walls is considerably changed and show profound physical damage; in the later stages of pyrolysis, a cellular structure is no longer observed.
Introduction The thermochemical degradation of lignocellulosic materials, a n d especially of wood, has been the subject of extensive research and it is fairly well k n o w n which components are reactively involved at the different temperatures, even if the reaction mechanisms might prove still too complex. The thermal behaviour of lignocellulosics depends on their chemical composition and reflects the responses to temperature of the main components, cellulose, hemicelluloses and lignin, as it has been shown for wood and bark of different species (Nguyen et al. 1981; Shafizadeh, Chin 1977). The thermal decomposition by way of pyrolysis techniques has been also suggested as an analytical tool in research of wood components (Faix, Meier 1989). The studies have not included cork or other suberinised materials although significant differences in relation to the thermal behaviour of wood are to be expected considering their chemical composition and cellular structure. In the cork from the cork-oak tree (Quereus suber L.), the main chemical c o m p o n e n t is suberin a m o u n t i n g to approximately 40% of the cell wall and cellulose represents less than 10%; extracWe are grateful to Mrs. Joaquina Ferreira for her help with the chemical analysis. The research was financially supported by the Junta Nacional de Investiga~o Cientifica e Tecnol6gica (JNICT) and by the Instituto de Ci~ncia e Tecnologia dos Materiais (ICTM)
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fives, mainly waxes and tannins, are present in significant amounts (Pereira 1988). The structure shows closed and thin-walled cells, arranged regularly without intercellular voids and with cell walls that are corrugated and flexible (Pereira et al. 1987). Research on the effect of temperature on cork has started recently and includes studies on cellular structure and mechanical properties (Rosa, Fortes 1987; Rosa et al. 1991), mass loss (Rosa, Fortes 1988; Pereira, Ferreira 1989) and on mass spectrometry of outgasing products on heating up to 250~ (Bento et al. 1991). A better knowledge on the thermal behaviour of cork might prove important since it is used in building, both as a surfacing material for floors and walls and as a thermal and acoustic insulator. Some phases of cork industrial processing also make use of temperature and of a certain degree of thermochemical degradation. This is the case namely in the production of expanded agglomerates, the most important insulation cork product, where granules of cork are treated in closed autoclaves with superheated steam at approximately 300~ to obtain an expansion of the cork granules and their temperature-induced self-bonding (Pereira, Ferreira 1989). This paper reports results on the thermal decomposition of cork in air, in the temperature range 150 450 ~ in relation to the effect on its chemical composition and cellular structure.
Material and methods
The thermochemical degradation of cork was studied in samples of virgin cork obtained from prunings of cork-oak (Quercus suber L.). Decomposition was determined gravimetrically by mass loss after isothermal treatment in a temperature controlled furnace, using 2 g samples (mesh size 14-20), at temperatures from 150 ~ to 450~ and with treatment times from 5 to 300 minutes. The determination of the chemical composition of the original cork and of the treated samples followed methods previously described (Pereira 1988). Extractives were determined by successive soxhlet extraction with dichloromethane, ethanol and water. Suberin was determined in extractive-free material after depolymerization by methanolysis: 3% NaOCH 3 in CH3OH was used, the filtrates acidified to pH 6 and evaporated to dryness, the residue suspended in water and extracted with C13CH; the extracts were evaporated and determined gravimetrically as suberin. The determination of lignin was made using a standard acid hydrolysis on the desuberinised residue: the acid insoluble lignin was determined gravimetrically as the hydrolysis residue, and the acid soluble lignin by UV absorption at 280 gm. The monosaccharides in the hydrolysis liquor were separated and determined as alditol acetates by gas-liquid chromatography. IR spectra were recorded using KBr pellets. Determination of carbon, hydrogen and oxygen by elemental analysis in some of the heat treated cork samples was done at Laboratdrio Nacional de Engenharia e Tecnologia Industrial, Lisboa, Portugal, The effect of the thermochemical degradation on the cellular structure of cork was followed by observation using scanning electron microscopy. The samples were prepared by coating with a gold film of approximately 200 ~ thickness. Observations were made of the tangential, radial and transverse sections. Measurements of cell dimensions, i.e. of cell wall thickness, were made directly on the SEM photographs.
261
Thermochemical degradation of cork Results and discussion
The extent of the thermal decomposition of cork in air, as measured by mass loss, is determined by the temperature of the treatment. At temperatures lower than 200 ~ the mass loss of cork is small: treatment at 150 ~ for 30, 120 and 300 minutes reduced the dry weight of the samples by 0.6%, 1.0% and 1.3% respectively and at 175 ~ by, respectively, 1.6%, 3.0% and 3.7%. Results for the temperature range 200-350~ are shown in Fig. 1: for instance, at 200 ~ approximately 15% and at 300 ~ about 55% of the initial cork dry weight is lost as combustion gases after 2 hours. The mass loss increases with treatment time, with a higher rate for the first 60 minutes. At temperatures above 350~ the mass loss is rapid, and a sample treated for 30 minutes at 400 ~ loses 72.9% of the initial dry weight. At 450 ~ the combustion process is complete, leaving an ash residue of 2.7% of the initial cork. The results agree with values reported for the thermal degradation of cork in air using a thermobalance, where significant mass loss began at temperatures of approximately 200-250~ and proceeded rapidly until 500~ where the material was reduced to ashes (Rosa, Fortes 1988). The chemical analyses of some of the thermally degraded samples are summarised in Tables I and 2. The results obtained show that the thermal treatment affected the chemical components of cork profoundly, to an extent that depended on the mass loss, and therefore on temperature. However, and before analysing the results obtained, some comments should be made on the accuracy of their assignment to individual chemical components in the case of heat-treated samples. For instance, the method used for suberin determination includes depolymerization by methanolysis, and the recovery of the monomers in an organic phase. The suberin is determined gravimetrically, and therefore all material
70 350 ~
60
.-A
5O
300 ~ o
40
,..I
250 ~
30 2o
.-A 200 ~
10
10
20
30
40
TREATMENT
50 TIME,
60
70
I
I
80
90
100
MiN
Fig. 1. Thermal degradation of cork (14-20 mesh) measured by mass loss in % of initial dry weight, in function of treatment time and temperature
H. Pereira
262
Table 1. Mass loss and chemical composition of cork after 60 minute treatments at different temperatures, in % of sample dry weight Untreated % Mass loss, % of initial cork Extractives Total Dichloromethane Ethanol Water Suberin a Polysaccharides Insoluble lignin a Soluble lignin"
200 ~ %
250 ~ %
300 ~ %
350 c'C %
-
14.7
27.4
49.2
62.1
14.4 3.7 6.0 4.7 35.8 24.6 20.0 2.0
5.1 2.2 2.3 0.7 62.8 4.6 22.2 2.2
3.4 1.4 1.5 0.4 41.6 0.9 42.1 2.7
1.8 0.2 0.8 0.8 6.7 0.3 80.7 2.1
2.0 0.4 0.6 1.1 1.1 84.7 3.4
See text for details and discussion. Suberin includes all material obtained after methanolysis by extraction with an organic phase. Insoluble lignin is the residue of acid hydrolysis of samples previously submitted to methanolysis and soluble lignin is determined by UV absorption at 280 gm in the acid hydrolysate Table 2. Mass loss and chemical composition of cork after treatment at 200 ~ times, in % of sample dry weight
Mass loss, % of initial cork Extractives Total Dichloromethane Ethanol Water Suberin Polysaccharides insoluble lignin Soluble lignin
Untreated
Treatment, min
%
10 %
14.4 3.7 6.0 4.7 35.8 24.6 20.0 2.0
30 %
90 %
3.2
14.7
16.1
15.1 5.4 6.1 3.7 51.3 10.5 16.4 1.6
14.1 5.6 5.5 2.9 53.2 7.0 17.2 1.3
5.1 2.2 2.3 0.7 62.8 4.6 22.2 2.2
4.5 2.3 2.0 0.2 58.7 1.5 26.1 0.9
with different times, in
Treatment, min
%
%
30 %
60 %
90 %
12.6 0.7 0.8 8.5 1.6 0.4
5.7 0.5 0.4 3.1 0.8 0.2
3.7 0.4 0.3 1.7 0.7 0.1
3.0 0.3 1_2 0.1 -
1.5 -
I0
Glucose Galactose Mannose Xylose Arabinose Rhamnose
60 %
2.0
Table 3. Composition of cork polysaccharides after treatment at 200 ~ % of sample dry weight Untreated
with different
--
Thermochemical degradation of cork
263
extracted with this organic solvent after methanolysis will be weighed and designated "suberin". Similarly, the determination of insoluble lignin relies on the acid hydrolysis of the desuberinised cork, by which polysaccharides are hydrolysed to monosaccharides and lignin remains as an insoluble residue, also determined gravimetrically. In this case the method will designate as "lignin" all material that remains as insoluble residue. It follows that the values shown in Tables 1 and 2 for the thermally degraded cork do not represent contents of native suberin and lignin, with chemical composition as in the original cork, but refer also to molecules which have undergone chemical transformation and may include, for instance, condensation products with other components, as discussed below. The results presented in Table 1 show that, upon heating, the content of extractives in cork decreases rapidly, and that after treatment at 200 ~ only 30% of the initial extractives remain in the sample. The rapid loss of polysaccharides in the heat treated samples is also clearly seen in Table 1. At 200~ only 8% of the initial amounts are still present, and they practically disappear at higher temperatures. A similar conclusion was also drawn from the comparison of the chemical composition of cork and of expanded cork agglomerate, which had been produced using 300~ steam (Ferreira, Pereira 1986). It is known that hemicelluloses have lower thermal stability than cellulose and lignin, and that their degradation starts at approximately 180 ~ (Nguyen et al. 1981). This was confirmed for cork, where hemicellulose-derived monosaccharides, i.e. xylose, could be found only in the very early stages of heating, as shown in Table 3. The degradation of cellulose takes place within a broader temperature interval, ranging from about 160~ to 360~ with significant mass loss at temperatures considerably higher than for hemicelluloses. In the lower temperature range, it is considered that cellulose reactions involve the thermoxidative depolymerization in the amorphous region (Nguyen et al. 1981). In cork, cellulose degradation, as monitored by the decrease in the amount of glucose in the samples, was important already in the lower temperature range of 200-250~ Nothing is known on the extent of the amorphous region in cork cellulose, but this result may be an indication of a low crystallinity and a high reactivity. Suberin is more heat resistant, and in fact the amount of the material determined as suberin increases for mass losses of the samples up to approximately 25%. The increase in the amount of the methanolysis products may be due to heat-induced condensation reactions of suberin with extractives, hemicelluloses or with their degradation products. The content of "suberin" decreases afterwards, and at 300 ~ the samples yielded only 6.7% of methanolysis products. As regards "lignin", its content 9increased steadily with temperature and with the extent of degradation. Again here, the results refer also to the formation of condensation products that remain insoluble under the conditions used, and should be considered more as a charred product. The results of the elemental analysis allowed the calculation of the following formulas for the samples treated at 200 ~ 250 ~ 300 ~ and 350 ~ respectively: CH1.4800.27 , CH1.0700.28 , CH0.7300.29 and CH0.35Oo.2s. With increasing temperature the atomic ratio C : H increases considerably, and this is a further indication of temperature-induced condensation reactions. The initial stages of the thermochemical degradation may be followed using the results summarised in Table 2, for samples treated at 200~ for different times,
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corresponding to mass losses in the range 2 to 16% of dry weight. Polysaccharides are very rapidly lost even when the mass loss of the material is small. This supports the idea of their condensation with suberin, and subsequent determination as methanolysis products; in fact, the carbohydrate loss corresponds approximately to the increase in the suberin content. Cellulose is more temperature resistant, as has been discussed above, and the monosaccharide composition of the samples (Table 3) shows that glucose increases significantly its relative weight and is the only sugar present for a global mass loss of 17% after 90 minutes at 200~ On the other hand, lignin content decreases in the early stages, and the condensation reactions mentioned above do not seem to be important at 200 ~ This agrees with reports of lignin thermal decomposition showing that, in the low temperature range, it is mostly reactions with the lignin side-chains which are involved, the condensation increasing with higher temperatures (Faix et al. 1988; Funaoka et al. 1990). The IR-spectra of original and 250 ~ cork show significant differences in the absorption patterns. The absorption band at 1,030 cm-1, assigned to alcoholic hydroxyl groups, as well as absorption at 3,350-3,500 cm-1 corresponding to O - H stretching, decreased after heating, indicating the loss of polysaccharides and condensation of lignin side-chains with phenyl nuclei (Funaoka et al. 1990). The band at 1,720 c m - 1, which corresponds to carbonyl absorption of carboxyl and ester groups, increased by heating, indicating the stability of suberin in these conditions; increase of absorption at 1,720 c m - 1 by heating is also characteristic of lignin (Sudo et al. 1985; Funaoka et al. 1990). The intensity of ethylenic absorption bands increased with a displacement of wave number from 1,640 cm-1 in cork to 1,600 cm-1 in the heated cork; this was also reported for wood as an indication of K-conjugation (Bourgois et al. 1989). The effect that thermochemical decomposition has on cork cellular structure, as shown by scanning electron microscopy, is summarised in Figs. 2 to 8. The cellular structure of untreated cork is shown in Fig. 2 by the tangential and radial sections. Usually, and as previously reported (Pereira et al. 1987), the cells are hexagonal prisms, with corrugated vertical walls, that are stacked base to base in the radial direction in a regular arrangement of rows. When cork is heated, the cells increase their dimensions, and the walls straighten and lose all undulations or corrugations (Fig. 3). This effect can already be noticed in the lower temperature range, when the material loss is negligible. The expansion of cork cells with temperature has also been reported for experiments using hot water and superheated steam (Pereira, Ferreira 1989; Rosa et al. 1991). With increasing temperature, the walls stretch and the regular '~ type arrangement of cells in rows, shown by the radial or transverse sections, is lost; in many regions, and especially where cell walls had many corrugations, these sections become similar to the "honeycomb"-type arrangement of cells, shown by tangential sections. This may be seen in Fig. 4, which includes a complete annual growth ring and where the layers of late-cork cells can be clearly observed, due to their reduced dimensions in the radial direction. The thickness of cell walls decreases at temperatures above 250 ~ and the double wall, from cell lumen to cell lumen, may be reduced to about 1 gm, corresponding to approximately half the value of untreated cork.
Thermochemical degradation of cork
265
Fig. 2a and b. Anatomical characteristics of untreated virgin cork from the cork-oak tree (Quercus suber L.): a Tangential section showing "honey-comb"-type arrangement of cells; b Radial section showing "brick-layered"-type arrangement of cells and the corrugation of the lateral cell walls. The transverse section (not shown) is similar to this section
Fig. 3. Expansion of cells and flattening of walls in a
cork sample heated at 300 ~ (radial section)
The accurate quantification of cell expansion during the heating process was difficult due to the variability o f dimensions but measurements m a d e directly on the S E M p h o t o g r a p h s indicate that cell volume was increased by a factor o f a p p r o x i m a t e ly 2. The same value was reported previously for heating o f cork with superheated steam at 300~ (Pereira, Ferreira 1989). It seems that this value corresponds to a m a x i m u m limit for the expansion o f cells and the stretching o f walls b e y o n d which, i.e. with treatments at higher temperatures, rupture and d e g r a d a t i o n occurs. The dimensional increase o f cells and the stretching o f the walls m a y cause tearing o f the
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H. Pereira
Fig. 4. Cell arrangement in a radial section of a cork sample treated at 300 ~ showing a complete annual growth ring and the region of late-cork cells
Fig. 5. Cracks in a cork sample treated at 350~ (radial section)
Fig. 6. Tear in the cell walls of a sample heated at 300~ (radial section)
Thermochemical degradation of cork
267
Fig. 7 a and b. Cell wall of a sample treated at 350 ~ ities: a Surface of cell wall; b Section of cell wall
showing numerous holes and discontinu-
Fig. 8. Sample heated at 350 ~ showing diffeent pyrolysis stages: charred product (top) and region maintaining the cork cellular structure (bottom)
tissue, and the n u m b e r of cracks found in the heat treated samples increases with temperature (Fig. 5). Physical damage o f the cell walls starts at a p p r o x i m a t e l y 300 ~ In the temperature range 3 0 0 - 3 5 0 ~ the cell walls are torn (Fig. 6) and present numerous discontinuities, such as holes and cracks, with a significant alteration of their structures (Fig. 7). This corresponds to a severe loss o f cell wall material, as seen by the values of mass loss at these temperatures (Table 1), and it is accompanied by the chemical transformation o f the remaining components, as discussed previously. In the temperature range 3 5 0 - 4 0 0 ~ the pyrolysis process is in an advanced stage, corresponding to p r o f o u n d chemical alterations and to the f o r m a t i o n of a charred product, with loss
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o f the cellular structure. This m a y be seen in Fig. 8, which shows the progress of pyrolysis: at the outer region o f the sample complete charring occurred, while in the interior the cellular structure is maintained.
Conclusions C o r k is thermochemically decomposed in air at temperatures above 150 ~ Mass loss is significant at 200 ~ and increases rapidly at higher temperatures, until ashing at 450 ~ The polysaccharides are the most heat sensitive components o f cork. A t 200 ~ hemicelluloses disappear and cellulose is degraded to a considerable extent. Suberin is m o r e resistant and decomposes significantly only after 250~ Temperature-induced alterations in the chemical composition o f the components are likely to occur, e.g. condensation o f c a r b o h y d r a t e products with suberin in the lower temperature range and heavy condensation of all remaining components at higher temperatures. Temperature-induced effects were also observed in the cellular structure of cork. Cell expansion and flattening o f wall corrugations occur at the beginning o f the heating treatment. By approximately 250 ~ the cell walls have stretched, their thickness is reduced, and the cells have attained a m a x i m u m volume increase before physical degradation. A t higher temperatures, and corresponding to severe thermochemical degradation, the structure o f the cell walls is largely destroyed. In the later stages of pyrolysis, the cellular structure is lost.
References Bento, M. E; Cunha, M. A.; Moutinho, A. M. C.; Pereira, H.; Fortes, M. A. 1991: A mass spectrometry study of thermal dissociation of cork. Int. J. Mass Spectrometry (accepted for publication) Bourgois, J.; Bartholin, M. C.; Guyonnet, R. 1989: Thermal treatment of wood: analysis of the obtained product. Wood Sci. Technol. 23:303-310 Faix, O.; Jakob, E.; Till, F.; Sz6kely, T. 1988: Study on low mass thermal degradation products of milled wood lignins by thermogravimetry-mass-spectrometry. Wood Sci. Technol. 22: 323-334 Faix, O.; Meier, D. 1989: Pyrolytic and hydrogenolytic degradation studies on lignocellulosics, pulps and lignins. Holz Roh-Werkstoff 47:67-72 Ferreira, E. P.; Pereira, H. 1986: Algumas altera~6es anat6micas e quimicas da cortiga no fabrico de aglomerados negros. Cortiga 576:274 279 Funaoka, M.; Kako, T.; Abe, I. 1990: Condensation oflignin during heating of wood. Wood Sci. Technol. 24:277-288 Nguyen, T.; Zavarin, E.; Barrall II, E. M. 1981: Thermal analysis of lignocellulosic materials. Part I. Unmodified materials. J. Macromol. Sci.-Rev. Macromol. Chem. C 20:1-65 Pereira, H. 1988: Chemical composition and variability of cork from Quercus suber L. Wood Sci. Technol. 22:211-218 Pereira, H.; Ferreira, E. 1989: Scanning electron microscopy observations of insulation cork agglomerates. Mater. Sci. Eng. A 111:217-225 Pereira, H.; Rosa, M. E.; Fortes, M. A. 1987: The cellular structure of cork from Quercus suber L. IAWA Bull.n.s. 8:213-218 Rosa, M. E.; Fortes, M. A. 1987: Temperature-induced alteration of the structure and mechanical properties of cork. Mat. Sci. Eng. 100:69-78
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Rosa, M. E.; Fortes, M. A. 1988: Thermogravimetric analysis of cork. J. Mat. Sci. Letters 7: 1064-1065 Rosa, M. E.; Pereira, H.; Fortes, M. A. 1991: Effects of hot water treatments on the structure and properties of cork. Wood Fiber Sci. 22:149-164 Shafizadeh, E; Chin, P. S. 1977: Thermal deterioration of wood. In: Goldstein, I. S. (Ed.): Wood technology: Chemical aspects. ACS Symposium Series 43, pp. 57-81. Washington: American Chemical Society Sudo, K.; Shimizu, K.; Sakurai, K. 1985: Characterization of steamed wood lignin from beech wood. Holzforschung 39:281-288
(Received October 24, 1990) Prof. Helena Pereira Universidade T6cnica de Lisboa Instituto Superior de Agronomia Departamento de Engenharia Florestal Tapada da Ajuda P-1399 Lisboa Codex
