+
+
Langmuir 1996, 12, 1245-1250
1245
Evolution of Surface Composition, Porosity, and Surface Area of Glass Fibers in a Moist Atmosphere P. Trens, R. Denoyel,* and E. Guilloteau† Centre de Thermodynamique et de Microcalorime´ trie du CNRS, 26, rue du 141e` me RIA, 13331 Marseille Cedex 03, France, and Laboratoire CNRS/Saint Gobain “Surface du verre et interfaces” BP 135, 39, quai Lucien Lefranc 93303 Aubervilliers, France Received July 3, 1995. In Final Form: November 9, 1995X The aim of this work is to study the influence of moisture on the surface chemistry and texture of C and E glass fibers. Thermal analysis (in the conditions of controlled rate transformation analysis) coupled with mass spectroscopy, surface area determination (by krypton adsorption at 77 K), adsorption of water by gravimetry, and AFM were used to characterize both novel and aged fibers (a few days at 50 °C in 95% humidity). Whereas aging has little effect on E glass fibers which can be interpreted by the appearance of both a small roughness and hydrophilic superficial sites, the changes observed on the C glass fibers are drastic. The surface area is increased by at least 10 times, micropores appear (as shown by gas adsorption), and clusters (evidenced by AFM) are formed on the surface. The migration of sodium and calcium from the bulk toward the surface (where they are stabilized under the form of carbonates in agreement with thermal analysis results) could explain the formation of both micropores and clusters.
Introduction In most of their applications, glass fibers belong to a composite system where the interfaces play a predominant role.1-3 For glass fiber reinforced polymers, e.g., “coated” glass fibers for insulation, the mechanical properties or the resistance to aging is very dependent on the chemistry and the structure of the fiber surface. Indeed, like silica, glass fibers show a partial solubility under moisture. In the case of silica, Iler proposed the dissolution mechanism given in Scheme 1.4 More recently, in medium pH conditions (≈7) where dissolution is not easily observed, Vigil et al.5 have interpreted unusual surface forces between two silica surfaces by the growing of a silica gel layer (10-20 Å) in the presence of water. This silica gel porous layer has been shown to thicken with an increase of pH.6 In the case of glass, such phenomena (dissolution, formation of a porous structure) have also been evidenced in the past,7-12 but generally, measurable effects were obtained in more drastic conditions (for example immersion in a basic or acid medium) than a simple contact with pure water (liquid or vapor). The Vycor glass, which is in fact a high silica glass (96% SiO2), is obtained by
dissolution of a glass in acid conditions where all components are soluble except silica.13 Nevertheless, it is observed that the properties of systems containing glass fibers are sensitive to humidity with, for example, a loss of mechanical properties.14 In the present paper, our objective is to analyze structural and chemical changes experienced by glass fibers when they are exposed to humid conditions. With that aim, two glass fibers with typical C and E compositions have been prepared and characterized before and after exposure to humidity (1 week at 50 °C and 90% relative moisture). E glass is known to be more stable than C glass which contains much more sodium. Freshly prepared and aged fibers are characterized by the following methods: (i) surface area determination by krypton or nitrogen adsorption at 77 K, (ii) evaluation of their affinity for water by adsorption gravimetry, (iii) thermal analysis (in the conditions of controlled rate thermal analysis) coupled with a mass spectrometer, and (iv) atomic force microscopy. These methods should allow any change in surface area, porosity, or surface chemistry of our samples after contact with a wet atmosphere to be detected. Experimental Section
* Author to whom correspondence should be addressed. † Aubervilliers. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Plueddemann, E. D. Silane Coupling Agents; Plenum Press: New York , 1991; p 189. (2) Sing, K. S. W. Characterization of powder surfaces. In Silicas; Parfitt, G. D., Sing, K. S. W., Eds.; Academic: New York, 1976. (3) Unger, K. K. Journal of Chromatography Library; Elsevier: Amsterdam, 1979; Vol. 16. (4) Iler, R. K. The chemistry of silica; Wiley Interscience: New York, 1979. (5) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367-385. (6) Axelos, M. A. V.; Tchoubar, D.; Bottero, J. Y. Langmuir 1989, 5, 1186. (7) Akink, M. PhD. Thesis, University of Iowa, Ames, 1977. (8) Donnet, J. B.; Battistella, R.; Chatenet, B. Glass Technol. 1975, 16, 407. (9) Fowkes, F. F.; Dwight, D. W.; Cole, D. A. J. Non-Cryst. Solids 1990, 120, 47. (10) Carman, L. A.; Pantano, C. G. J. Non-Cryst. Solids 1990, 120, 40. (11) Ooi, K.; Miyatake, M. J. Colloid Interface Sci. 1992, 148, 326. (12) Kondo, S.; Igarashi, M.; Nakai, K. Colloids Surfaces 1992, 63, 33.
Samples. We used C glass fibers and E glass fibers provided by Saint Gobain Recherche. These glass fibers were selected because they are often used in a lot of applications (E glass fibers for composites and C glass fibers for insulation). The surface composition of these glass fibers are shown in Table 1. The main differences between the two kinds of glass are the compositions in aluminum, calcium, and sodium. E glass is rich in aluminum and calcium whereas C glass is rich in sodium. Aging is performed in thermoregulated box at 50 °C in the presence of 90% humidity and dried under room conditions. After 1 week, fibers are dried for 24 h in ambiant conditions. They are stored in closed bags before analysis. Thermal Analysis. The controlled rate transformation analysis procedure was used with equipment already described15 which allows the sample to be heated in such a way that the rate of outgassing remains constant. To this equipment, described (13) Nordberg, M. E. J. Am. Ceram. Soc. 1944, 27, 299. (14) Huang, R. J.; Demirel, T.; Mc Gee, T. D. J. Am. Ceram. Soc. 1973, 56, 87. (15) Trens, P. Ph.D. Thesis, Universite´ de Provence, Marseille, France, 1994.
+
+
1246
Langmuir, Vol. 12, No. 5, 1996
Trens et al. Scheme 1
Table 1. Composition of E and C Glass Fibers (ESCA Data Given by Saint Gobain Recherche) atomic composition in % E glass fibers C glass fibers
Si
Al
Ca Mg Na K
19.4 5.4 5.8 0.3 1.3 19.9 1.4 1.4 4.4
B
O
3.3 63.9 2.2 70.8
as a controlled rate evolved gas detection system,16 or CR-EGD, can be attached a mass spectrometer (Ribovac SB) which allows an analysis of the gas composition during the heat treatment. In fact, this is the overall rate of gas evacuation which is set at a given value (in the following experiments the gas is a mixture of water vapour and carbon dioxide), whereas the gas composition can change along the experiment. This method is used owing to its efficiency in separating decomposition steps.17 The thermal analysis curve is simply the recording of temperature as a function of time. If only one gas is eliminated during the experiment, the time scale can be transformed to a weight loss scale. In the present work, we kept such a representation, even if we are in the presence of a gas mixture, since the weight loss is mainly due to water departure. Gas Adsorption. The gravimetric experiments were realized with an equipment constructed in-house from a magnetic compensation apparatus (SETARAM, model 10-8). It allows adsorption-desorption cycles after a thermal treatment of the samples by in situ CRTA to be realized. This apparatus has been already described.18 It is used here for water vapor adsorption. Its sensitivity is around 1 µg. All experiments were made at 25 °C. The volumetric experiments were carried out at 77 K with a constructed in-house apparatus which is described in the literature by Grillet et al.19 The adsorptive introduction was performed by using a quasi-equilibrium procedure which is made possible thanks to an extremely slow constant introduction of gas (about 2 cm3‚h-1). Due to the expected low surface area of fibers, adsorption of krypton was mainly used here because of its low vapor pressure at 77 K (around 1.6 mbar) which reduces dead volumes corrections as compared, for example, with nitrogen. Surface areas were derived from adsorption isotherms by applying the BET law assuming an average area of 0.192 nm2 per krypton atom. For all adsorption isotherms, the treatment temperature was 140 °C during 2 h under a pressure of 0.1 Pa. Surface Morphology. AFM imaging was realized with a Park instrument in the laboratory “Surface du Verre et Interfaces”, Unite´ mixte CNRS-Saint Gobain, Aubervilliers, France. Diameters of glass fibers were determined by scanning electron microscopy (ISI, Super III-A).
Results (a) CRTA Experiments. Figure 1 shows the CRTA (16) Borde`re, S.; Rouquerol, F.; Rouquerol, J.; Estienne, J.; Floreancig, A. J. Therm. Anal. 1990, 36, 1651. (17) Borde`re, S. Ph.D. Thesis, Universite´ de Provence, Marseille, France, 1989. (18) Rouquerol, J.; Davy, L. Thermochim. Acta 1978, 24, 84. (19) Grillet, Y.; Rouquerol, F.; Rouquerol, J. J. Chim. Phys. 1977, 74, 191.
Figure 1. CRTA curve ot the E glass fiber. Weight loss (in %) versus temperature.
curve of E glass fibers. Between 20 and 400 °C, the weight loss is monotonous whereas a clear transition is observed between 450 and 580 °C. The first part (20-400 °C) looks like curves obtained for silicas with low OH content like Aerosil.20 The step around 500 °C could be attributed to a loss of water (in fact, surface hydroxyls) provided by other oxides. For instance, it has been shown in the literature that boehmite AlO(OH) can be decomposed at about 450 °C.21 The thermal analysis of aged E glass fibers leads to the same curve, meaning that this fiber is not chemically altered by moisture. The C glass fiber behavior is quite different. In the case of the freshly prepared fiber (Figure 2), a well defined short step can be observed at about 120 °C. With the aim to obtain more information about this step, we linked a mass spectrometer to the CRTA apparatus in order to determine the composition of evolved gas. The results are reported in Figure 3, where partial pressures of both water and carbon dioxide are plotted versus temperature. It is clear that the step of Figure 2 is accompanied by a peak on both CO2 and H2O partial pressure curves. There is also a CO2 enrichment of the evolved gas between 200 and 400 °C, but without any concomitant step in Figure 2. The appearance of CO2 is probably a result of the decomposition of carbonates like sodium carbonate which appear because of both the basic character of C glass fibers and the migration of internal sodium toward the surface. The equation of decomposition could be the following: (20) Legrand, A. P.; Hommel, H.; Tuel, A.; Vidal, A.; Balard, H.; Papirer, E.; Levitz, P.; Czernichowski, M.; Erre, R.; Van Damme, H.; Gallas, J. P.; Hemidy, J. F.; Lavalley, J. C.; Barres, O.; Burneau, A.; Grillet, Y. Adv. Colloid Interface Sci. 1990, 33, 91-330. (21) Rouquerol, J.; Rouquerol, F.; Ganteaume, M. J. Catal. 1975, 36, 99.
+
+
Glass Fibers in a Moist Atmosphere
Langmuir, Vol. 12, No. 5, 1996 1247
Figure 2. CRTA curve of the C glass fiber. Weight loss (in %) versus temperature.
Figure 3. Composition of the evolved gas during the CRTA experiment obtained by mass spectroscopy.
2NaHCO3 f Na2CO3 + H2O + CO2
(A)
Na2CO3 f Na2O + CO2
(B)
It has been effectively shown that reaction A occurs between 100 and 150 °C.22 Phenomena are more pronounced in the case of the aged C glass fiber (Figure 4). The total weight loss is larger (2% vs 0.02%), showing a higher water content. The transition observed at 120 °C in the case of the pristine C glass fiber is now located around 100 °C and corresponds to a much more important weight loss. The water adsorption isotherms will bring us more information about this transition. Step 1 (Figure 5) is mainly due to a water loss as indicated by the disappearance of the CO2 peak at the corresponding temperature. Steps 2 and 3 are a complex combination of H2O and CO2 departure: during step 2, CO2 partial pressure almost reaches the level of H2O partial pressure, whereas at the end of step 3, CO2 has nearly disappeared from the evolved gas (Figure 5). These results show that aging of C glass fibers leads to a high content in both water and carbonates. We present in Figure 6 the AFM images of C glass fiber before and after aging. These images seem to show the appearance, on the aged fiber, of both a roughness and some clusters (sizing 1 µm). These clusters could corroborate the superficial formation of hydrated carbonates. Surface Area Determination:Adsorption of Krypton and SEM. The aim of these experiments is the determination of the specific surface areas. Adsorption isotherms of krypton on the new or aged glass fibers are presented in Figures 7 and 8. For all samples, a good linearity of the BET plot is obtained in the range 0.1 < p/p° < 0.35. These curves allow us to calculate the specific (22) Atlas of Thermoanalytical Curves; Liptay, G., Ed.; Akademiai Kiado´: Budapest, 1974; tome III, p 127.
Figure 4. CRTA curve of the aged C glass fiber. Weight loss (in %) versus temperature.
Figure 5. Composition of the evolved gas during the CRTA experiment obtained by mass spectroscopy.
Figure 6. AFM pictures of C glass fibers before and after aging (supplied by Saint Gobain-CNRS laboratory).
surface areas and the CBET parameter, which is an indication of the affinity between the adsorptive gas and adsorbent (Table 2).23
+
+
1248
Langmuir, Vol. 12, No. 5, 1996
Trens et al.
Figure 7. Adsorption isotherm of krypton on pristine or aged C glass fiber at 77 K.
Figure 9. Adsorption desorption isotherm of water on C glass fiber. The inset shows the adsorption desorption isotherm at the low-pressure range.
Figure 8. Adsorption isotherm of krypton on pristine or aged E glass fiber at 77 K. Table 2. Specific Surface Area Obtained by Adsorption of Krypton at 77 K and SEM samples
specific area/ m2‚g-1 a
geometric area/ m2‚g-1 b
pristine C glass fiber aged C glass fiber pristine E glass fiber aged E glass fiber
0.34 4.11 0.20 0.30
0.33 121 0.30 12
a
Figure 10. Adsorption desorption isotherm of water on aged C glass fiber. The inset shows the adsorption desorption isotherm at the low-pressure range.
CBET 15 10
Obtained by adsorption of krypton at 77 K. b Obtained by SEM.
For the pristine fibers, the specific surface areas are very close to those derived from the radius measured by SEM (assuming a density of 2.3 g‚cm-3), showing that there is apparently no porosity accessible to krypton atoms. The influence of moisture on the specific surface area is very different between the two kinds of fibers. In the case of E glass, the surface is only increased by a few percent, whereas in the case of C glass fibers, the specific surface area is multiplied by 12. These results will be corroborated by the adsorption of water (see next paragraph). The vertical slope of the adsorption isotherm at the origin and the very large CBET value in the case of the aged C glass could indicate the creation of either a microporosity or highly energetic sites. Adsorption of Water. The water adsorption-desorption isotherms are presented in Figures 9-12. For all systems, the affinity of the water is relatively high as indicated by the slope at the origin, and a hysteresis occurs during the desorption which shows an important part of irreversibility. Nevertheless, the comparison between C and E samples shows a quite different behavior evidenced on one hand by the shape of curves at p/p° above 0.5 and on the other hand by the influence of aging on adsorption capacities. (23) Gregg, S. J.; Sing, K. S. W. Adsorption Surface Area and Porosity; Academic Press: London, 1982.
Figure 11. Adsorption desorption isotherm of water on E glass fiber. The second adsorption is carried out after a heat treatment at 140 °C (the same as the one used before the first adsorption).
In order to compare quantitatively these results, we have reported in Table 3 the specific surface areas from the krypton adsorption experiments, the specific adsorbed amount of water (in mg‚g-1 at p/p° ) 0.1), and the surface concentration of water (in mg/m2 of krypton surface area). We have also derived a surface area by applying the BET law to these water adsorption isotherms (last column of Table 2). Freshly prepared samples (C and E) show a simple behavior: BET surface areas are relatively close to each other for krypton and water vapor and are in agreement with apparent surface areas derived from SEM pictures. Aging has a slight influence on E adsorption capacities. The krypton surface area slightly increases whereas the surface concentration of water at p/p° ) 0.1 is multiplied by 3. This is in fact mainly due to an increase in the affinity of the aged fiber for water since the water surface
+
+
Glass Fibers in a Moist Atmosphere
Langmuir, Vol. 12, No. 5, 1996 1249
The partial solubility of the fiber in water leads to (i) the formation of a microporosity (made evident by the difference of surface area between water and krypton and by the shape of the krypton adsorption isotherm), (ii) an increase of specific surface area, and (iii) the formation of carbonates. These carbonates seem to appear as large clusters on the fiber surface (see AFM pictures). In fact, during aging, CO2 from the atmosphere can react with the basic aqueous phase issued from the reaction of water with the fiber. For example, we may have the following reactions:
Figure 12. Adsorption desorption isotherm of water on aged E glass fiber.
H2O + Na2O f 2NaOH
(C)
CO2 + NaOH f NaHCO3
(D)
2NaHCO3 f Na2CO3 + H2O + CO2
(E)
Table 3. Water Adsorption Results sample
adsorbed amount/ mg‚g-1 a
As/m2‚g-1 b
As/m2‚g-1 c
C glass fiber aged C glass fiber E glass fiber second adsorption aged E glass fiber
0,08 5,00 0,07 0,15 0,16
0,34 4,11 0,20 / 0.30
0,20 15,1 0,25 / 0,55
a Water adsorbed amount at p/p° ) 0.1. b BET surface area from krypton adsorption. c BET surface area from water adsorption.
area is not even doubled. The consequence of the wet treatment seems to be an increase of roughness and the appearance of hydrophilic sites (perhaps OH groups). Noticeably, the second adsorption isotherm of Figure 11 is very close to the adsorption isotherm on the aged fiber of Figure 12, leading to the conclusion that a few hours (in fact the time to describe the adsorption desorption isotherm part above p/p° ) 0.8) in humid conditions at 25 °C are as efficient as 1 week at 50 °C and 95% humidity to alter the E glass fibers. Oppositely, aging has a large effect on water adsorption capacities of C glass fibers. The krypton surface area has been multiplied by 12, but water capacity (in mg‚g-1 at p/p° ) 0.1) by 60. In the same manner, the BET surface area is 15 m2‚g-1 in the case of water vs 4.1 m2‚g-1 for krypton. This could be interpreted by the presence of micropores accessible to water and not to krypton. The behavior of C glass fibers above p/p° ) 0.5 is very particular since a clear step appears at a pressure of 0.67 (0.55 on the desorption branch) (Figures 9 and 10). Such a defined step suggests capillary condensation in mesopores, but no hysteresis was observed on the nitrogen adsorption desorption isotherm (not presented here). Moreover, the position of this step on the p/p° axis is the same on new and aged sample, indicating that the same phenomenon occurs on both samples (the amplitude is only changed). Discussion These results stress the difference of behavior between C and E fibers toward water. E glass fibers are not deeply modified by exposure by water. We can observe a slight increase of surface area which can correspond to an increase of roughness and the appearance of hydrophilic sites, probably OH groups. These fibers being prepared at high temperature, the formation of these OH groups is analogous to the rehydroxylation of a silica which has been heated at high temperature.20 C glass fibers are much more sensitive to humid conditions. This is a well-known result for Na-rich fibers, but with the help of the various techniques used here, it is possible to have informations on the evolution at the microscopic level.
During thermal analysis, the decomposition reactions are observed, i.e., reactions E and F:
Na2CO3 f Na2O + CO2
(F)
Noticeably carbonates are already present on the new fibers. It shows that aging already occurs in ambient conditions during the time (1 h) of preparation of experiments. The height of the first step of the thermal analysis graphs (Figures 2 and 4) increases with aging, and the temperature at which it appears decreases. At the same time, the corresponding CO2 peak on the partial pressure plot disappears. This first step probably corresponds mainly to water departure even for the new fiber (the pressure scale of Figure 3 is logarithmic). The weight loss value (5 mg‚g-1) in the case of the aged fiber is close to the adsorbed amount at p/p° ) 0.1, suggesting that the origin of this first step could be the emptying of the microporosity evidenced by adsorption results. During aging, there is both an extension of micropores (shown by adsorption results and increase of the first step height) and an enlargement of their size since the first step appears at a lower temperature. Indeed, the activation energy (i.e., the temperature) necessary to empty the porosity decreases when the pore diameter increases whereas the porous volume increases when the porous diameter increases. NaHCO3 is decomposed under vacuum at 100 °C.22 Thus, its decomposition could be at the origin of the peak obtained at 100 °C in the case of the new fiber. Nevertheless, this peak disappeared on the aged fiber, raising concerns about the stability of NaHCO3 during the moistening treatment. Other carbonates compounds are more stable since they decomposed above 200 and 400 °C for the new and aged fiber, respectively. They could be at the origin of clusters observed on AFM pictures which could be small crystallites of Na2CO3 or CaCO3. Finally, the mechanism of aging could be the following: Na2O is attacked by water, creating pores filled with a basic aqueous solution. Due to high-humidity conditions, a film of water is certainly present on the fiber surface. This film is initially poorer in ions than the pores where the attack of oxides occurs. This may lead to a migration of ions from inside the pores to the external surface of the fiber by osmotic flow. This basic liquid film can dissolve CO2 from the atmosphere leading to the formation of carbonates which precipitate during the drying of fibers, then the formation of clusters. The migration of sodium
+
1250
+
Langmuir, Vol. 12, No. 5, 1996
and calcium during aging has been recently shown by SIMS on C glass fibers of the same origin.24 Conclusion This work shows a very important difference between E and C glass fibers toward wet conditions. When the silicas are new, textures are similar. Indeed, the specific surface areas determined by krypton or water adsorption are very close to that derived from SEM images. When the silicas are aged by wet atmosphere, we observe important differences. The E glass fiber has a good (24) Rouyer, E.; Lehue´de´, P.; Chopinet, M.-H. Proceedings of XVII International Congress on Glass, Beijing, 1995; Vol. 3.
Trens et al.
resistance toward moisture, as shown by the small evolution of CRTA curves and specific surface areas. On the other hand, the C glass fiber is completely altered by moisture which leads to the formation of both micropores and carbonates clusters. Acknowledgment. This work has received a financial support from Saint Gobain Recherche. The authors wish to thank Dr. H. Arribart from Laboratoire mixte Saint Gobain-CNRS and Drs. P Chartier and E. Dallies from Saint Gobain Recherche for fruitful discussions. LA950531E
