Chemical stability of inviscid melt-spun (IMS) fibers of calcia-alumina in aqueous media

Materials Chemistry and Physics, 34 (1993)

219

219-227

Chemical stability of inviscid melt-spun calcia-alumina in aqueous media

(IMS) fibers of

Kyung-Yol Yon”, Brian S, Mitcheilb, Stanley A. Dunnb and James A. Koutskya’b’* aMute~a~ Science Program, ~nive~i~ %epartment of Chemical Engineering,

(Received

June 22, 1992; accepted

of ~co~~~ad~o~ Madison, University of W~consi~-Mad~on,

October

ulf 53706 (USA) Madison, WI 53706

(USA)

8, 1992)

Abstract Vitreous fibers of CaO/Al,03 (46.5/53.5 wt.%) prepared by inviscid melt spinning (IMS) are shown to be susceptible to attack and eventual dissolution by water. The attack proceeds with the formation of a hydrate coating, which thickens with time at the expense of the underlying oxide fiber. With exposure to saturated lime water a limited initial attack takes place, which then comes to a halt. A coating is formed that is thin, continuous and fine grained. The coating, composed chiefly of CafOH), and a much smaller proportion of aluminum, appears to offer protection in the more corrosive aqueous environments. The strong basic nature and favorable mechanical properties of these fibers make them especially of interest in the concrete industry, where the similarly strong basic nature of the Portland cement matrix suggests a degree of compatibility.

Introduction It is well known that silica-based glass fibers are generally not stable in highly alkaline environments such as Portland cements. Majumdar and Ryder [1] studied the aIkali resistance of various glass fibers using two alkaline solutions: 1N NaOH at 100 “C for 1.5 h, and saturated Ca(OH), at 100 “C for 4 h. In the case of E-glass fiber, they reported a 59% reduction in diameter in NaOH solution and a 9% reduction in Ca(OH), solution. Recently the kinetics of noncongruent dissolution of E-glass fiber in saturated Ca(OH), solution were studied [2] and it was found that the fiber dissolution rate was a linear function of time. According to their results, there was about 22% relative weight loss of E-glass fiber at 1000 h immersion at 25 “C. As a possible alternative to silica-based glass fiber, alumina-based systems attracted attention because of their high moduli, relatively high temperature stability and possibility of inclusion into highly alkaline Portland cements. it was observed that calcium aluminate with a few % SiO, can be made as a glass 13, 41. Brown and Onoda have also made fibers of calcium aluminate with a small amount of silica by hand-drawing the melt under supercooled conditions, whereby both silica and the lower temperature stabilize the molten stream by increasing its viscosity. The water durability of calcium *Author

to whom correspondence

0254-0584/93/$6.00

should be addressed.

aluminate glass (30 A1,03, 60 CaO, 4 SiO,) was studied by measuring the weight loss in water at 100 “C. A 0.5% weight loss after 1 h boiling and a 0.8% weight loss after 2 h were reported. In water at room temperature, no weight loss was observed after 24 h [3]. The fiberization, via melt spinning, of various ceramics containing little or no network formers such as silica is difficult, or not practically possible, because of the instability of low-viscosity stream. In practice, the glass fibers must be drawn from a melt having a viscosity of around 1000 poise for low-cost, large-scale production [5]. For low-viscosity melts (
0 1993 - Elsevier Sequoia. All rights reserved

220

exposed fibers gave no noteworthy signs of degradation due to hydration from water in the vapor state [ll, 121. This seeming inertness in the face of an appreciable chemical potential for spontaneous hydration could be due to (1) formation of a hydrate layer which is impervious to water vapor and/or (2) the presence of the initial carbon IMS sheath, which might be similarly impervious. The potential use of these fibers as a composite reinforcement and the use of liquid water exposure in associated materials and practices suggested the desirability of clarifying the behavior of the fibers with respect to aqueous media. Furthermore, there has been growing concern about negative health effects caused by exposure to fine manmade fibers. Recently, Adachi et al. [13] evaluated the biological effects of inhaling manmade fibers. They studied six manmade fibers, including two types of amorphous silicate (Rock wool and fiberglass), three types of monocrystalline whisker (potassium titanate fiber, calcium sulfate fiber and basic magnesium sulfate fiber) and a metaphosphate polymeric fiber. The results showed that four of the six fibers (fiberglass, calcium sulfate fiber, basic magnesium sulfate fiber and metaphosphate fiber) have the potential to induce not only intrapleural tumors but also intraperitoneal tumors after inhalation. Health concerns over water-insoluble fiber reinforcements have stimulated work to find fibers that have complete water solubility. Furthermore, fibers that are water soluble have the potential to lead to enhanced recycling of matrices from fiber-reinforced composites and production of textiles that can readily be disintegrated and disposed of. In this work, the time-dependent morphological changes in various simple aqueous systems and the solution behaviors were examined, employing newly prepared samples of calcia/alumina (46.5/53.5 wt.%) IMS and redrawn IMS (RIMS) fiber; the mechanisms are discussed in terms of chemistry and morphology.

Experimental Melt preparation

A mixture of reagent grade calcia and alumina powders was calcined at 1200 “C for 6 h. The dry powders were mixed in a composition of 46.5 wt.% calcia and 53.5 wt.% alumina, then homogenized by ball milling in a sealed polyethylene jar. Approximately 364 g of powder were melted in a graphite crucible at 1505 “C, then quenched by being poured onto a cold steel plate. ZMS jiberization

Fibers were produced by inductively remelting this mixture in a carbon crucible and forcing the melt under

pressure through a fine orifice into a special atmosphere, propane. The propane pyrolized at the hot surface of the nascent stream and deposited a carbonaceous sheath. The sheath stabilized the cylindrical geometry of the stream, preventing droplet formation prior to solidification. The sheath was required because the viscosity of the melt was too low to stabilize against droplet formation prior to solidification. Solidification produced continuous fibers with diameters of 235 to 315 pm, with a noticeable dark skin of carbon on the surface. The moduli of the as-spun CA IMS fibers were around 10 Mpsi. This continuous fiber is referred to as inviscid melt spun (IMS) fiber. Redrawing of ZMS fibers (RIMS)

Samples of the calcia/alumina IMS fiber were subsequently redrawn in air through a small platinum wire coil furnace. The temperature of the redrawing unit was maintained at 1200 “C and feed rates for this unit were on the order of 0.25 cm see-l. These resulting fibers are referred to as redrawn IMS (RIMS) fibers. The diameters of the RIMS fibers were 35-70 pm, and the moduli of the CA RIMS fibers were around two to three times higher than those of the as-spun IMS fibers. It was noted that the carbon skin associated with the IMS fibers appeared visually to be absent from the RIMS fibers. Exposure tests

A series of chemical stability experiments on the calcia/alumina IMS and RIMS fibers was performed using timed immersions in distilled water, saturated calcium hydroxide and unsaturated solutions of calcium hydroxide. Fiber samples measuring 4 to 5 cm in length were immersed in about 50 ml of the respective test solution. Samples and solutions were held at room temperature (ca. 25 “C), without stirring. The combination of test conditions employed are summarized in Table 1. At the end of the exposure period, the fibers were removed from the test solution and dried in the open air at room temperature. In order to avoid disturbing the surfaces of the fibers, the adhering test solution was neither wiped nor rinsed off. The amount of solute depletion was estimated from the thickness of the hydrate layer and the concentration of the solution, TABLE

1. Summary of chemical

Test solution (2.5 “C, no stirring)

-log (pH)

water, distilled Ca(OH)*, dilute Ca(OH)*, saturated

7 12.1 12.7

[H+]

stability test conditions Exposure

time

(h) 0.5, 1.5, 26, 96 144, 331 1, 1.5, 144, 331, 1000, 1257

221

and it turned out to be less than 1 ppm. After drying, fiber samples were examined using a JEOL JSM-35C SEM equipped with a Tracer Noshes energy dispersive X-ray spectrometer. The water solubility of CA IMS fiber was studied by monitoring the weight loss in distilled water at six different water/fiber weight ratios (1163:1,1207:1,110:1, 126:1, 49:l and 53:l).

Results Several minutes after IMS fibers were immersed in distilled water or aqueous solutions, the carbon sheath was observed to flake off. Examination of the fiber via SEM after 1.5 h immersion in water revealed the formation of superficial hydrate layers having significant surface cracking (Fig. l(a)). A fiber exposed for an equal time to saturated calcium hydroxide solution, however, exhibits a uniform textured hydrate layer, without cracks (Fig. l(b)). RIMS fibers were seen to behave similarly to their precursor IMS fibers when exposed to distilled water and to saturated lime water (with the exception, of course, that they shed no carbon sheath, since it apparently was oxidized during the redrawing process). They both form hydrate films, and have surface cracks and uniform textures. However, at a necessarily higher magnification, other differences may be noted. The cracked hydrate layer associated with distilled-water exposure is relatively thick and the substrate fiber is of correspondingly reduced diameter (Fig. 2(a)). By contrast, the diameter of the fiber exposed to saturated lime water, even for long exposures, is nearly the same as its initial diameter and the hydrate layer is thin as well as uniformly textured (Fig. 2(b)). Calcia-alumina IMS fibers were exposed to distilled water for still longer periods of time to further elucidate the surface attack mechanism. The scanning electron micrographs show progressive uniform attack of the fiber and a thickened, layered sheath structure subjected to frequent cracking and rupture (Figs. 3(a) and 3(b)). Eventually the fiber loses its integrity, the RIMS fiber more rapidly than the IMS fiber because of its greater surface-to-volume ratio. Subsequent longer-term exposures to saturated lime were carried out to determine whether the hydrate layers were indeed formed in sihr while the fibers were immersed in the solution and not artifacts of the drying procedure. These studies showed that growth continues beyond 1 h but ceases before 100 h. The major proportion of the coating growth appears to occur when the fiber resides in the saturated solution and very little from deposition of solute after withdrawal and drying of the sample (< 1 ppm).

(4

@I Fig. 1. Scanning electron micrographs (SEM’s) Ca0-A1203 (46.553.5 wt.%) after immersion distilled water; (b) saturated lime water.

of IMS fibet‘S of for 1.5 h in (a)

Fig. 2. SEM micrographs of redrawn IMS fibers (RIMS) of calcia-alumina after immersion in: (a) distilled water for 0.5 h; (b) saturated.lime water for 1 b.

SEM micrographs of IMS fibers given bng-term exposure (1000 h) to saturated lime water revealed that the uniformly textured film, once formed, stabilized and protected the fiber substrate from further attack (Figs. 4(b), 4(d) and 4(e), Fibers protected with such a hydrate layer, which formed during 1257 h storage in saturated lime, withdrawn and reimmersed in distilled water for 1 h, showed no visually detectable dissolution, nor any sign of the cracking otherwise characteristic of exposure of virgin IMS fiber to water (Fig. 4(f)). EMS fibers exposed to pH 12.1 lime water (approximately l/4 saturated) resembled those exposed to distilled water more than those immersed in saturated lime. The hydrate layer was cracked and did not have a uniform texture. It differed from those generated in distilled water, however, in exhibiting sporadic clusters of relatively large crystalline plates. These were of hexagonal symmetry. They tended to stand up sharply but not always perpendicularly to the surface of the fibres. Their orientation with respect to the fiber axes was random (Fig. 4(a)). Qualitative chemical analyses by energy dispersive X-ray spectrometer (EDS) were performed at several points on exposed and unexposed IMS fibers (Table 2) to see if there are appreciable variations in the

Fig. 3. SEM micrographs of IMS fibers of calciaimmersion in distilled water for: (a) 26 h; (b) 96

lina after

223

(4 (conrinued)

(0

(e)

Fig. 4. SEM micrographs of IMS fibers of calcia-alumina after immersion for 144 h in lime water [(a) dilute (pH 12.1) and (b) saturated (pH 12.7)], showing EDS analysis points corresponding to the analyses in Table 2: after immersion for 331 h in lime water [(c) dilute (pH 12.1) and (d) saturated (pH 12.7)J; after immersion in saturated lime water for (e) 1000 h and (f) 1257 h, followed by immersion in distilled water for 1 h.

TABLE 2. Elemental aqueous media”

EDS analyses

of IMS fibers exposed

to

Analysis location

Calcium (at.%)

Aluminum (at.%)

CalAl fraction

Normalized G/Al ratio

unexposed point 1 point 2 point 3

46 56 57 71

54 44 43 29

617 514 413 512

1 1.5 1.6 2.9

fiber

The water solubility of CA IMS fiber was studied by monitoring the weight loss in distilled water at six different water/fiber ratios. The pH of each solution was measured after 6 h immersion and the amount of fiber dissolved in distilled water was plotted with respect to the pH. The results are given in Fig. 5, with the equilibrium concentration of CaO calculated from the solubility product of Ca(OH),.

“See Figs. 4(a)-(b).

Discussion

atomic ratio of calcium and aluminum associated with the several features observed. The ratio was determined on the hydrate layer of exposed fibers at three points, indicated by numbered arrows on the side views of Fig. 4(a) (unsaturated lime water) and Fig. 4(b) (saturated lime water). The ratio on an unexposed fiber was determined for reference. The Ca/Al ratios at the three numbered sites are all seen to be higher than that of the reference (unexposed) fiber. The normalized Ca/Al ratio of point 3 on the hydrate layer produced in saturated lime water is nearly three times higher than that of the reference fiber.

Chem&ry

The strong affinity of both metal oxides for water virtually guarantees rapid hydration of the fiber surface upon exposure to water in either the liquid or the vapor form. The carbon sheath on the IMS fiber was seen to offer little hindrance to the interaction of the fiber with liquid water. Other observations further indicate that the carbon is not bonded tightly, nor should it be, judging by its chemical dissimilarity to the metal oxides [8] and its relative thermochemical stability [9]. The hydrates will undoubtedly form in humid air regardless of the presence of the carbon sheath. The

225

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Immersion time (hour) 0.1120

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9 $ c

0.1100~

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11.6

11.8

12

12.2

12.4

112.6

PH

Fig. 5. Weight 126:l; (c) 49:l

change of CA IMS fibers in distilled water, and 53:l. (d) Relation between the amount

with water/fiber weight ratios of (a) 1163:l and 1207:l; of fiber dissolved in distilled water and the pH.

low solubility and lack of hygroscopicity of the two hydrates precludes their acquiring any additional water from humid air. Upon immersion in aqueous media, however, the hydrates are free to go into solution via liquid transport in accordance with their solubilities and their solution dependencies upon the other solutes present. As such solution takes place, more metal oxide is exposed to hydration and further solution. In neutral water, calcium hydroxide is many orders of magnitude more soluble than the aluminum hydrate. At equilibrium, it forms strongly basic solutions, pH cu. 12.7: CaO + H,O -

Ca(OH), Ca2+ + 20H-

(pK,, = 5.1)

(1)

Aluminum, which is amphoteric, exhibits its acidic form as the appreciably soluble aluminate ion under basis conditions of this order [lo]: Al,O, + H,O -

2A10,H 2AlO,- +2H+

(p&, = 15)

(2)

In the present oxide mixture, there is sufficient excess of the calcia to ensure neutralization of the alumina to form the aluminate anion: AlO,H+OH-

-

AlO,- +H,O

(3)

(b)

11O:l

and

At pH 12.7, the equilibrium concentration of the aluminate ion is calculated to be 0.005 M, assuming no third-ion interference. If insoluble salts form between the aluminate and calcium ions, the solubility might of course be lower. When the IMS fiber is first introduced into neutral water, the hydrate of the calcia, probably already formed, will proceed into solution, raising the pH locally and neutralizing and mobilizing the adjacent alumina hydrate as the aluminate ion. A freshly exposed surface of the metal oxides will then be opened to water attack to repeat the process of hydration, dissolution, neutralization and mobilization. Although the fiber will ultimately dissolve, the observed buildup of the hydrate layer on its surface suggests that initially, at least, the rate of diffusion of water through the hydrate layer to form more hydrate at the fiber oxide surface is faster than the rate of solution of the hydrate into the quiescent water phase. During immersion in saturated lime water, the calcia-alumina IMS fibers certainly first form the respective hydrates, just as in plain water. The aluminate ion will also form and proceed to establish equilibrium between solution and alumina hydrate. However, the solid-phase calcium hydroxide will be unable to dissolve, because the surrounding solution is already saturated with respect to this solute. On the contrary, calcium

226

hydroxide will tend to pass from solution to solid phase, because its activity in the latter phase is depressed owing to the presence of the alumina hydrate in the solid phase. The resulting lowered activity of the lime in the solid phase provides the driving force for its crystallization from the solution. As time progresses, lime deposits while alumina dissolves as aluminate ion. Thus the solid-phase lime exposed to solution will approach unity, and the driving force for further crystallization from the saturated lime solution will steadily diminish, thus limiting the thickness of the stable hydrate layer. This mechanism explains the relatively low proportion of Al in the hydrate layer formed on the surface of the fiber when exposed to saturated lime water (Table 2). The high proportion of nuclei expected in a saturated solution provides an explanation for the relatively finegrain texture of hydrate layer formed under these conditions. In dilute lime water, dissolution of calcium hydroxide from the surface of the hydrate layer is no longer prohibited, as it is in the presence of saturated lime water. The rate of nucleation of calcium hydroxide crystals is comparatively low; hence fewer and larger crystals are formed than result from exposure to saturated lime water (Fig. 4(a)). What is of interest is that the fine-grained hydrate layer formed by exposure to saturated lime offers promise as a means of protection against exposure to aqueous media and water. The 1 h exposure to water of a protected fiber showed none of the signs of degradation noted elsewhere (Fig. 4(f)). This observation encourages further examination of this area. It is apparent that the inherent water solubility of CA fibers offers an important area of further study for two reasons. First, one needs to understand the interactions of these fibers with water-based systems such as cements, as well as water-based polymers (phenolics and urea-formaldehydes). The second reason has to do with health concerns. At the higher water/ fiber weight ratios (Fig. 5(a)), there is still a decreasing tendency of CA fiber weight even after 6 h immersion in distilled water. In other words, the solution process is still going on in these two systems and the amount of CA fiber in distilled water is lower than the level of equilibrium concentration of CaO (Fig. 5(d)). On the other hand, the other four systems have almost reached the equilibrium state after 2 h immersion (Figs. 5(b) and 5(c)) and the amounts of CA fiber dissolved are all located above the level of equilibrium concentration of CaO. This observation could be explained as follows: Aluminum is amphoteric, being present as A13+ or lower-valence hydroxyl forms at low pH ( <5) and the aluminate anion (AI(OH at higher pH values (> S), and the solubility of AI(O increases linearly with

pH at high pH values [14]. The theoretical solubility of AI(O in water at pH 12.5 can be estimated as 1.2 X 10e4 g ml-’ at 25 “C. If this amount is added to the equilibrium concentration of CaO (5.9~ 1O-4 g ml-l), the total amount of fiber dissolved in distilled water can be obtained as 7.1 x lop4 g ml-’ vs. 8.5 x 10e4 g ml-l. However, as can be seen in Fig. 5(d), there is a discrepancy between the experimental values and the calculated ones. This could be due to the experimental error or solubilities of other types of oxide compounds. Further work will be necessary in order to explain this discrepancy. Morphology

Hydrate growth initiating at the fiber-hydrate interface will result in accumulated tension in the external layers of hydrate as the hydrate layer continues to grow. The tension arises from the molar volume increase accompanying hydration. The tangential stress will be manifest primarily as hoop stress, and initial failure accordingly will tend to occur along longitudinal lines. Longitudinal failure will transform hoop stress into shear stress in the radial plane, primarily at the two ends of the failure fault. Failure in the shear mode will occur as the shear stress accumulates with continuing longitudinal failure and would be expected to predominate in a direction at right angles to the longitudinal axis. Failure cracks would be expected to widen with further exposure, owing to the enhanced access of the water. The built-in gradient of peripheral tension prior to rupture would impart a tendency in the hydrate segments to curl away from their original cylindrical shape. The diffusion of water in the hydration process could account for the uniformity of attack noted in the underlying substrate fiber. The tendency toward a rectilinear grid pattern of cracks is readily seen in many of the micrographs. The crack widening and curl of the segments are also evident. However, these latter effects are undoubtedly exaggerated by the shrinkage occasioned by the partial dehydration accompanying drying of the samples.

Conclusions

(1) The calcia-alumina IMS fiber is inherently more basic than the conventional E-glass fibers. The basic chemical nature makes it a candidate for fiber reinforcement for materials such as Portland cement. (2) Calcia/alumina (46.5/53.5 wt.%) fibers made by IMS and RIMS are susceptible to attack to the point of dissolution in some aqueous media. The attack is accompanied by growth of a fiber-enveloping, crystalline hydrate layer. The attack appears to be uniform over

227

the surface of the fiber, as evidenced by the smooth interface between the two solid phases and the preservation of the original cylindrical geometry. The hydrate layer thickens and cracks with continued attack. (3) Exposure to saturated lime water appears to halt the attack after a brief incursion. Shortly after immersion in the saturated lime solution, a thin, fine-grained, adherent, crack-free hydrate layer forms. It then ceases to grow further and seems to prevent further hydration and dissolution of the fiber. Evidence suggests that the protective effect of this layer may extend to subsequent exposures in other aqueous media. (4) It is apparent that the inherent water solubility of CA fibers offers an important area of further study to help understand the interactions of these fibers with systems such as cements as well as water-based polymer solutions and dispersions (phenolics and ureaformaldehydes) and polymer emulsions. The knowledge of the formation of complicated interphases between these fibers and the matrices involved is crucial to understanding the properties of composites.

the contributions to this paper.

of Eric Podlogar and Kurt Baumann

References A. J. Majumdar and J. F. Ryder, Glass Technol., 9 (1968) 78. A. Al Cheikh and M. Murat, Gem. Corrcr. Rex, 18 (1988) 943. S. P. Brown and G. Y. Onoda Jr., Tech. Rept. R-6692, Contract NOW-65-0426-d, U.S. Department of the Navy, Bureau of Naval Weapons, Washington, DC, October 1966. 4 S. P. Brown and G. Y. Onoda Jr., .J. Am. Ceram. Sot., 53 (1970)

311.

5 B. A. Proctor

and B. Yale, Philos.

Ser. A, 294 (1980) 427. 6 S. A. Dunn, L. F. Rakestraw Pat. No. 3 658 979 (1972).

7 R. E. Cunningham, Symp.

London,

and R. E. Cunningham,

L. F. Rakestraw

Ser., 74 (1978)

Trans. R. Sot.

U.S.

and S. A. Dunn, AIChE

20.

8 L. F. Rakestraw, N. W. Harakas, M. R. Sargent and W. J. Privott, AIChE Symp. Ser., 74 (1978) 32. 9 S. A. Dunn and E. G. Paquette, U.S. Pat. No. 4 IO4 355 (1978).

10 S. A. Dunn and E. G. Paquette, Adv. Ceram. Mater., 2 (1987) 804.

11 F. T. Wallenberger,

Acknowledgements

The authors would like to thank the members of The Consortium for Redrawn Inviscid Melt-Spinning (RIMS) and Related Fiber Technology for their financial support: St. Gobain Recherche; Thermal Ceramics, and Toyobo New York Inc. The authors wish to recognize

Lett.,

9 (1990)

N. E. Weston

and S. A. Dunn, Mater.

121.

12 F. T. Wallenberger,

N. E. Weston and S. A. Dunn, J. Non-

Cyst. Solids, 124 (1990) 13 S. Adachi, K. Takemoto

116.

and K. Kimura, Environ. Res., 54 (1991) 52. Water 14 F. N. Kemmer and J. McCallion (eds.), The N&o Handbook, McGraw-Hill, New York, 1979, Chap. 6, pp. 12, 13; see also E. Stumm and J. J. Morgan, Water Qunlity, Wiley-Interscience, New York, 1970.