SILANOL GROUPS ON SILICA AND THEIR REACTIONS WITH TRIMETHYL CHLOROSILANE AND TRIMETHYLSILANOL WILLIAM J. EAKINS'
Research and Technology Laboratories, Acco Corp., Space Systems Division, Louell, Mass.
The alkyl chlorosilane and alkyl silanol reactions w i t h glass surfaces are basic to an understanding of glass fiber-resin bonding in composites. The surface of Aerosil, as an idealized glass, is characterized as containing silica linkages, hydrogen-bonded silanol, and free silanol groups. The free silanol groups are distinctly more susceptible to attracting all types and degrees of physically absorbable and chemically reactable material. The effect of the presence of the hydrogenbonded silanol on Aerosil surfaces is to promote the reactivity of the free silanols, possibly through the presence of small quantities of hydrogen-bonded water. The activation energy for the reaction probably is one third that required when free silanols are alone. When larger quantities of hydrogen-bonded water are present (one per free silanol site), about 30% of free silanol sites react. Ammonia may be used as a catalyst for reaction between trimethylsilanol and free silanol groups on Aerosil surfaces. About 60% of the free silanol sites react. The limitation in this reaction is the size of the trimethyl silyl group. Full area coverage is likely at the 60% of free silanol sites level. The reaction will occur as low as room temperature, should sufficient time be given.
STUDYING the activity
of glass surfaces experimentally is difficult because there are so many kinds of activity (Y ates and Trebilcock, 1961). There are hydroxyls attached to aluminum oxide groups and boron oxide groups in E-glass as well as silica groups (Eakins, 1962b). Then the migratory nature of some ions like K a - and Al" must be considered (Eakins, 1962a). They tend to go to the surface to reduce the free energy of that surface. Even in silicas with purity of 99.9+% when the surface area per unit volume is low (about 0.2 t o 1.0 meter per gram) as in a drawn quartz filament, some impurities can become concentrated and greatly influence the activity of the surface. I n silica gels and Aerosils, however, where surface areas of 100 to 600 sq. meters per gram are normal, the concentration effect is about one thousandth of these values. The value in using the Aerosil or fume silica as a model surface for a glass coupling agent-resin interface study is then obvious. A model compound, the trimethyl silyl group as a chloride and hydroxylated, is discussed in a study of the coupling agent-glass bond. Silica Surface
The information gathered for examination here is all on fume silica. This type of silica has been rather thoroughly examined (Iler, 1955; Patrick, 1951). Fume silica is made either by vaporizing Si02 a t high temperature or by producing the SiOr vapor by burning tetraethyl silicate or silicon tetrachloride; the product is collected as a white smoke or fume, the extremely small particles
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01095
forming a porous structure as they collide in the atmosphere. Iler (1955) has calculated that if a silicon atom on the surface of amorphous silica must complete its tetrahedral coordination with a silanol group, there are 7.85 OH groups per square millimicron. This value of about eight silanols per square millimicron is representative of fully hydroxylated silica surface. Fume silica (Aerosil) is a porous solid. Milligan and Hachford (1947) have shown that up to 500"C., temperature has no effect on either the specific surface or pore diameter. Davydov et al. (19641, using D2O exchange, showed that the ratio between the quantities of surface to bulk silanol groups depends upon the pretreatment of the sample and the size of the silica particles. The difference in exchange rate of OH groups for OD groups made it possible to separate hydroxyls roughly into bulk and surface classes. Their data indicate that in the fume silicas the bulk silanols are less than 10Lsc the number of surface silanols. Therefore, whether they are included as surface groups is for most calculations relatively unimportant. The concentration of silanol groups on the surface is relatively independent of the history of the silica or the manner in which these groups become dispersed. Evans and White (1967) furnish a curve (Figure 1) for fume silicas in terms of silanols per square millimicron us. temperature. About 5.3 silanols per square millimicron are present a t about 110"C. Davydov et al. (1964) indicate that the surface silanols may be divided into two classes: hydrogen-bonded and free. They distinguish the hydrogen-bonded from the freeVOL. 7 N O . 1 M A R C H
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7.85
I
I
I
7t
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\ M A X I M U M THEORETICAL SILANOL GROUPS
6t
0
FREE SILANOL GROUPS
i
HYDROGEN - BONDED S I L A N O L GROUPS
200
400
600
800
1000
1200
TEMPERATURE, ' C
Figure 1. Hydrogen-bonded and free silanol vs. temperature
silanol class as follows: The degree of hydroxylation of the silica surface directly affects approaching molecules having atoms with lone electron pairs and *-bonding potential. White (1965) illustrates with infrared spectra on silica the perturbation of the silanol group with benzene and the hydrogen-bonding effected between the silanol group and acetone. These compounds are readily displaced by vacuum treatment. Between 200" and 4OO0C., the silicas absorb with relatively unchanging capacity. For benzene, ethylene, diethyl ether, and nitrogen, however, the absorption capacity of silica dehydroxylated a t temperatures over 400°C. drops sharply as compared to the absorption of silica preheated t o the 150" to 200" C. range. At 400" C., only about 505 of the hydroxyls present a t 100" to 150°C. remain on the surface (Figure 1). Infrared spectra indicate that these hydroxyls are free silanol groups (Davydov et al., 1964; White, 1963). Thus, the free silanol groups tend to be the sites a t which the absorption takes place. The number of these groups on the silica surface a t temperatures under 400" C. is constant, estimated from infrared to be 2.6 per sq. mp. All these sites are not equally available for absorption or reaction. Some are in the pores on a surface but are not available, particularly for the larger molecules. Another aspect of the silica surface emerges when we consider the areas having no silanol groups (with Si-0Si- bonds only), those having hydrogen-bonded (paired) silanol groups, and those having free silanol groups. Figure 2 shows the number of silanols in each category. Davydov (1964) estimates that each bound silanol occupies about 7 sq. A.; each free silanol, about 33 sq. A. Thus, from geometric considerations, the free silanol is favored as a reaction site. How many of these free silanol surface groups are available for reaction? Consideration of the data of Johannson et al. (1965) sheds some light on this. Using 0.1Mtetrabutylammonium hydroxide in benzene-methanol solution to perform nonaqueous titrations of fume silica surfaces, he found that 0.6 i 0.2 (six trials) silanol sites per sq. mp neutralize on silica dried for 12 hours a t 500°C. in air. Now, since an estimated coverage of ( B u ) 3 O H on silica is 78 sq. A., and each theoretical silanol site occupies about 13 sq. A. (7.85 OH per sq. mp), the ( B U ) ~ N O H covers five theoretical silanol sites in addition to the one reacting. If the (Bu)?NOH reacts with 0.6 OH per sq. 40
I & E C PRODUCT RESEARCH A N D D E V E L O P M E N T
mp, the total number of sites covered is 3.6 OH per sq. m p . The (Bu14NOH groups thus cover 45% of the total area-as a result of their size and of the silica surface geometry, which makes sites unavailable to such large groups unable to penetrate the pores. When a new sample heated in the same manner was washed with 0.1% acetic acid before titration, it was found to have only half of the number of silanol sites, 0.3 i 0.1 OH per sq. mp (based on four trials). Assuming that the sites titrated are proportional t o the total number of sites present that would react in a similar manner, we can conclude that half the total (hydrogen-bonded plus free) silanol sites reacted with the acid. The total actual number of silanol sites, as determined by infrared, is 5.3 OH per sq. mp; half of this number, about 2.6 OH per sq. mp, is the same as the number of free silanol sites determined by infrared. Thus, since the acid would react first with the free silanol sites, we can infer that under the above conditions the acid reacts only with the free silanol sites. The above also suggests that, if the group carrying the reactivity is small enough to penetrate t o the site, all the free silanol sites are available for reaction. Model Coupling Agent-Silica Surface Reaction
The choice of a model coupling agent is difficult. The trimethyl silyl group present in trimethyl chlorosilane and trimethylsilanol is chosen for several reasons. First, since its activity is singular, it cannot chain-extend. Also, methyl is the smallest hydrocarbon group. However, the size and reactivity of the trimethyl silyl group define its coverage of a silica surface and reduce below the maximum value the number of silica sites that it will cover and react with. The expected reactions follow:
(CH3)$iC1 + HO1Si-(CHJ3Si-O-Si I
I
+ HC1
Evans and White (1967) did their work on Aerosil preheated in a vacuum to 500'C. Thus (Figure l), they had degassed the silica surface until only the free silanols remained. They found that a t temperatures over 30OOC. the reaction proceeded, requiring an activation energy of 37 i 3 kcal. per mole. Under this temperature only physical adsorption took place. In contrast, Davydov et al. (1964) degassed the Aerosil a t 200" C., made it react a t 57" C. in boiling trimethyl chlorosilane, and claimed an activation energy requirement
n I I +
7 . 8 5 . THEORLTICA! YUWBER OF SltAkCiS 151-CHI
m F 2 IF SURFACE I S COMPLETELY H Y D R O X l l A T E D
\
2.6 - IOTAi VUVECR O f FREE S l t A h O L S W 2 CONSWIUT 10410'C
Figure 2. Relative numbers of surface silanols (Si-OH) at 110" to 150" C.
of only about 10 kcal. per mole (activation energy determined by his associate, I . Yu. Babkin). Hydrogen-bonded silanol groups in equilibrium with free silanol groups also assume an equilbrium with water hydrogen bonded to the free silanol groups. The equation showing this relationship follows:
[
hydrogen-bonded silanol groups )
{
[
surface silica groups
free silanol groups +
1
----)
hydrogen-bonded water silanol groups
1
\
\
4
+
- SI 0
\
' \
\
I n Group I , considerable additional energy is required to dehydrate the double silanol site and, when completed, silanol is not available. I n Group 11, however, the dehydration of the site takes relatively low energy and gives one active silanol site for each water molecule desorbed. Davydov et al. (19641, using deuterium exchange before treatment with trimethyl chlorosilane and spectrally analyzing before and after modification, showed that the band a t 3750 cm.-', representing the free silanols, disappeared after modification. By means of the deuterium determination, they further showed that almost all the free OH react with (CH J ,SiCl, although few (but some) hydrogen-bonded silanols showed this activity. They gave, however, no specific values. Kolb and Koelling (1966) have published results that are also in contrast to those of Evans and White. Their fume silica was preheated to 500°C. for 24 hours but not evacuated. They used infrared analysis, but also examined the treated samples after immersion in boiling distilled water, boiling toluene, boiling 0 . 5 5 aqueous acetic acid, and heated air. The oxy bond between the Aerosil and trimethyl silyl was indicated as stable under these conditions. (The test period is not given, but was probably 2 hours.) Golubenkova et al. used a microporous filament of silica (20 to 25 sq. meters per gram), probably equivalent to acid-leached and ovened E-glass with silica content of about 9 9 5 . Dipping the presumably nondegassed filament in boiling trimethyl chlorosilane for 2 hours, they took infrared spectra of the chlorosilane reagent and of the solvent-washed, then 0.5' boiling ammonia-washed, fiber. They found that the band a t 2965 cm. ' , characteristic of the trimethyl silyl groups, was maintained in strong intensity even through the boiling, 0.5'1. ammonia (2-hour) treatment. Heating to 200°C. in vacuum also had no effect. Evans and White (1967) show, on the other hand, that Aerosil preconditioned by heating to 450" C. will reversibly adsorb but will not react with the trimethyl chlorosilane under 300°C. This information is summarized in Tables I and 11. Johannson et al. (1966) have data from which can be calculated the probable effect of water on this reaction (their Tables XVIII and XIX). Table 11, item 7, shows that Aerosil preconditioned by immersion in boiling water (changing its area-volume by N L from 410 to 363 sq. meters per gram) and subsequent drying a t 80" C. in room atmosphere can be treated with a trimethyl chlorosilane
Mo
>z
Ef-
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I
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e
VOL. 7 N O . 1 M A R C H
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Table II. Reactivity with (CH3)3SiCI and (CH3)JSiOH and Silanols on Fume Silica Surfaces
Item No. 1 2 3 4 5 6 7 8
Reacting Molecule (CH.J)?SiCl (CH?)?SiCl (CH1)?SiC1 (CH?),SiOH (CH,I)SiOH (CH,)$iCl (CHq),SiC1 (CHI),2Xl
Preconditioning Ref. of Silica, C. (Mm. Hg) 450 (lo-') 450 (lo-') 450 (lo-') * 250 (lo-') 250 (lo-') b 500 (760) ' Boiling H?O, 2HR, 80 (760) ' T-water vapor, RT 75.C. (lo-'), 4 hr. O
Reacting Medium Vapor Vapor Vapor Vapor Vapor-NH3 Dry hexane Dry hexane
Temp., C. 450 325 30 25 25 25 25
Time Hours Days Days 24 hr. 24 hr. 24 hr. 24 hr.
Dry hexane
25
24 hr.
Estimated Cocerage at 59 Sq. A . Molecule. ( L 92 92 100 55
Silanol Sites on Which Gmup I s Absorbed; Reacted; sq. mp sq. mw
... ...
1.6 1.6
34 46
1.7 0.9 0.2 0.2 0.03
0.1 1.6 0.6 0.8
46
...
0.8
100
...
'Euan.5 and White, 1967. 'Johannson et al., 1965, 'Johannson et ai..1966.
solution a t room temperature in dry hexane to give a bond that is resistant to both exchange with other trimethyl silyl groups and to hydrolysis by boiling water [59 sq. meters per gram was used to convert these data. This is the value experimentally determined by Evans and White (1967).]. Item 8 of Table I1 shows that tritiated water vapor treatment a t room temperature, followed by drying a t the same temperature, gives the identical trimethyl chlorosilane adsorption values. (I assume that the bonding is as strong as with the boiling water treatment.) The tritiated water adsorbed by the Aerosil (410 sq. meters per gram) was 0.388 mmole per 100 sq. meters. This may be calculated as exchanging 4.7 silanols per sq. mp. I t is assumed that the -0T substituted by this treatment for only the free silanols (2.6 OH per sq. mp) on the surface (as Davydov found -OD tended t o do) and that 1.0 molecule of water was left to hydrogen-bond to the free silanols. These water molecules would be removed by the drying effect of the trimethyl chlorosilane in anhydrous hexane. After treatment with the silane-hexane solution, the tritiated water remaining on the surface was determined as equivalent to 1.04 OH per sq. mp. The trimethyl chlorosilane reacted with 0.83 OH per sq. mp, accounting with the remainder for 1.9 OH per sq. mp. The hydrogen-bonded water, therefore, has left the Aerosil surface as a result of the modifying treatment. The trimethyl silyl oxy bond formed with the Aerosil surface is stable in boiling water (Table I ) . In the light of the earlier work (Golubenkova et al., 1966), these data indicate that when one water molecule is available a t free silanol sites, the sites will react with trimethyl chlorosilane either alone or in anhydrous solvent. The reaction of trimethyl chlorosilane on silica degassed a t 450°C. has been studied by Evans and White (1967). They found that a t temperatures of 300°C. and over the reaction forms a heat- (500" C.) and moisture-stable bond that is destroyed by air a t temperatures over 400" C. Johannson et aL(1965) show that trimethylsilanol, a much less reactive compound (that physically is absorbed a t 25" C. to silica degassed a t 250" C. in vacuum), will chemisorb to silica a t 25°C. in the presence of KH, vapor and hydrogen-bonded silanol (Table I , items 4 and 5). When these data are calculated on the same basis as the Evans and White d a t a (59 sq. A. per molecule coverage), the agreement is perfect. The reacted groups 42
I & E C PRODUCT RESEARCH
AND
DEVELOPMENT
(1.6 per sq. mp) are those that are not displaced by soaking in liquid (CH,) $iOH. The action of the NH? is seen as a simple metathesis exchange with a low energy requirement. There are two simultaneous reactions:
-
1. Sit-OH + NH, Sit", + H20 2. (CHd3SiOH + NH3 (CH,)3SiNH2+ HzO --$
The trimethyl silazine reacts with the free silanol,
3. SiI-OH
+ H2NSi(CH3),
+
Sit-OSi(CH3)3 + NHs
to regenerate ammonia. The ammonia reacts with the water to form ammonium hydroxide. Geometric considerations suggest that Reaction 2 can be several deciorders of magnitudes more likely than Reaction 1. The trimethylsilanol can be in excess, its molecules being small and mobile, those of the surface silanols being fixed and available only from one side. Kolb and Koelling (1966) present spectral evidence that non- or slightly hydrolyzed trimethyl ethoxysilane in aqueous solution applied t o Aerosil over 5 hours a t room temperature loses its ethyl group. This loss indicates the retention of only the trimethoxy configuration on the silica surface. An explanation may be that, as in other cases, th6 hydrogen-bonded water on the free silanols attracts and promotes the bonding reaction preferentially to these sites. The siloxane bonds formed are stable to organic materials, water-dilute acids, and moderate heating. Conclusions
Fume silica is about 70% hydroxylated a t temperatures around 100"C. These hydroxyls can be divided into two classes: free and hydrogen-bonded, each present in equal amounts a t around 100" C. The hydrogen-bonded hydroxyls do not exist a t temperatures in excess of 400" C. The free hydroxyls or silanols, in contrast to the hydrogen-bonded silanols, have a physical attraction for atoms having lone electron pairs or r-bonds in compounds. They also preferentially attract compounds having hydro-
gen bonding capability and preferentially react with acetic acid up to possibly 100% of free silanols available. The data on the application of the trimethyl silyl group are summarized in Figures 3 and 4. The formation of the oxy linkage between the trimethyl silyl group and the free silanol groups on the Aerosil surface can be thought of as requiring considerable energy of activation (37 kcal. per mole for trimethyl chlorosilane) if only heat is used. If, however, hydrogen-bonded silanol, in equilibrium with hydrogen-bonded water or water from another source, is present, the reaction will proceed with a lower activation energy requirement (about 10 kcal. per mole). Reactions a t room temperature are possible.
Reactions requiring higher activation energy, such as between trimethylsilanol and free surface silanols, are catalyzed in the presence of N H I to form bonds with all available sites. For the trimethyl silyl group, the available sites are 60‘; of the total free silanol sites. Reactions a t room temperature are possible between some trimethyl alkoxy silanes and Aerosil silanol sites. Acknowledgment
The author thanks Charles Hughes and Lawrence McA1lister for editing his paper.
literature Cited
i
L
?CAI ENERGY + OTHER ENERGY. TOTAL ENERGY 1 0 A C T l V A T E
Figure 3. Activation energy differentials based on differences in treatment Level A
Heat alone
Level B. Hydrogen-bonded silanol, less heat Level C. Hydrogen-bonded silanol, hydrogen-bonded water, time a t RT Level D. Hydrogen-bonded silanol, N H 3
r
7m
1
I z 0 4
*HBS-PYDROGEN-BONDED
SILA\OI
i
RfACTIO\ OBIAI\ED BY etVAhi ANDIVHITI
0 DAVYDOV OGNUBERKOVA
6JOHAL\SON +
JOHAU‘ISOh OK3LBAND KdLLI\G
I
I
1
1
\
Y. 8.
S.*
IhCREASIYG ENERGY FRW OTHER SOURCES
*
Figure 4. Energy requirements for reaction to bond trimethylsilyl group to free silanol group on Aerosil (fume silica) through oxy linkage
Davydov, V. Ya., Kiselev, A. V., Zhuravlev, L. T., Trans. Faraday SOC.60, 2254-64 (1964). Eakins, W. J., SPE Trans. 2, 354-62 (October 1962a). Eakins, W. J., “Studying the Reactivity of Various Fiberglass Surfaces as Formed after Heat Soaking and after Mild Acid Leaching,“ Proceedings of 17th Annual Conference, Reinforced Plastics Division, Society of the Plastics Industry, February 196213. Evans, B., White, T. E., “Adsorption of Methylchlorosilanes a t Silica Surfaces,” Proceedings of Conference on Fundamental Aspects of Fiber-Reinforced Plastics, NonMetallic Materials Division, Air Force Materials Laboratory, Dayton, Ohio, postprint Section X I , January 1967. Golubenkova, L. I., Nikonova, S. N., Shadadash, A. N., Akutin, M. S., “Study of Processes of Interaction between Coupling Agents, Resin Binders and Glass,” Fifth International Plastics Conference, London, November 1966. Iler, R . K., “Colloidal Chemistry of Silica and the Silicates,” Cornell IJniversity Press, Ithaca, N. Y., 1955. Johannson, O., Stark, F., Baney, R., et al., “Investigation of the Physical Chemical S a t u r e of the MatrixReinforcement Interface,” Air Force Materials Laboratory, AFML-TR-303, Part I , September 1965. Johannson, P., Stark, F., Vogel, G., “Investigation of the Physical Chemical Yature of the MatrixReinforcement Interface,” Air Force Materials Laboratory, AFJIL-TR-65-303, Part 11, July 1966. Kolb, K. E., Koelling, J. G., “Study of the Glass-SilaneEpoxy System,” Proceedings of 21st Annual Meeting, Reinforced Plastics Division, Society of the Plastics Industry, Section 13-D, February 1966. Milligan, W . O., Hachford, H . H . , Jr., J . Phys. Colloid Chem. 51, 333 (1947). Patrick, W. A,, “Properties of Highly Desiccated Silica Gel,” Symposium on Catalyst and Reaction Mechanisms, Catalyst Club of Philadelphia, University of Pennsylvania, April 28, 1951. White, T. E., “Study of the Reactions of Surface Hydroxyl Groups of Silica by Means of Infrared Spectrography,” Proceedings of 20th Annual Conference, Reinforced Plastics Division, Society of the Plastics Industry, Section 3-B, February 1965. White, T. E., Pilkington Bros., unpublished work, 1963. Yates, P. C., Trebilcock, J., “Chemistry of Chromium Complexes Used as Coupling Agents in Fiberglass Resin Laminates,” Proceedings of 16th Annual Conference, Reinforced Plastics Division, Society of the Plastics Industry, Section 8-B, February 1961. RECEIVED for review June 23, 1967 ACCEPTED October 24, 1967 VOL. 7 NO. 1 M A R C H
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