Interaction of organosilanes with glass fibers

102

Journal of Non-Crystalline Solids 120 (1990) 102-107 North-Holland

INTERACTION

OF ORGANOSILANES

WITH GLASS FIBERS

G.M. NISHIOKA H&N Instruments, Inc., PO Box 955, Newark, OH 43055, USA

Systematic reaction of a glass surface with monofunctional organosilanes can be achieved by multiple coat-cure treatments. A gradual decrease in the polarity of the glass surface, measured by its wetting properties, monitors this reaction. A saturated surface results after ten coat-cure treatments, yielding a surface whose hydrogen bond component to the work of adhesion against water (Wnn20) = 25 ergs/cm2. This value is in contrast with conventional trifunctional organosilane treated surfaces having wH20 = 50 ergs/cm 2. The importance of controlling the energetics of the glass surface is discussed for a variety of processes.

1. I n t r o d u c t i o n

c h r o m a t o g r a p h i c s u p p o r t s [4] a n d the d i s p e r s a b i l ity of silica fillers [4]. The difficulty in obtaining a quantitative meas u r e of t h e o r g a n o s i l a n e - g l a s s r e a c t i o n is d e m o n s t r a t e d i n fig. 1. T h e glass s u r f a c e is r e p r e s e n t e d b y the s i l i c o n - o x y g e n - m e t a l ( M ) b o t t o m layer. S h o w n f r o m left to right are v a r i o u s r e a c t i o n s t h a t c a n o c c u r at the o r g a n o s i l a n e - g l a s s interface.

T h e i n t e r a c t i o n of o r g a n o s i l a n e s w i t h glass surfaces affects i m p o r t a n t p r o p e r t i e s of a v a r i e t y of m a t e r i a l s , s u c h as the electrical r e s i s t a n c e of glass-organosilane-resin composite circuit board c o u p o n s [1,2], the wet s t r e n g t h of fiberglass r e i n f o r c e d plastics [3], the s e p a r a t i o n efficiency of

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G.M. Nishioka / Interaction of organosilanes with glass fibers

First, at position 1, no reaction occurs; the surface hydroxyl group is hydrogen bonded to water. Second, at position 2, an organosilane molecule reacts at two points on the glass surface, replacing two polar hydroxyl groups with two less polar functions. Third, at position 3, an organosilane reacts at one point on the glass surface, replacing one surface hydroxyl group. Fourth, organosilanes polymerize and form only one covalent bond with the glass surface. A nonpolar umbrella forms over other surface hydroxyl groups (positions 5 and 6), preventing their reacting with organosilane. The reactions occurring at positions 2 and 3 are considered desirable; the coupler molecule has effectively reacted and is strongly bonded to the glass surface. The reaction at position 4 is not as desirable since it hinders the reaction of other surface hydroxyl groups. A quantitative measurement of these organosilane-glass reactions, although important, is obviously difficult and has not been accomplished. A study of the organosilane-glass interaction is greatly simplified if a monofunctional organosilane is studied. These contain only one functional group capable of reacting with the glass surface. Monofunctional organosilanes offer several advantages. (1) There is a constant stoichiometry in the organosilane-glass reaction; an organosilane molecule can react with only one surface hydroxyl group. (2) Polymerization is not possible. Non-reacting organosilanes dimerize and are easily washed off the glass surface. Umbrella configurations do not form; hence shielded hydroxyls cannot arise. Wetting experiments are meaningful on

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such surfaces, since a direct relation exists between the surface energy of the substrate and the number of reacted hydroxyls. These advantages are illustrated in fig. 2. Contact angles against fibers can be very precisely measured using a wetting balance [5]. If a filament is immersed in a liquid, and the additional force on the filament due to surface tension is measured, the contact angle of the liquid on the solid can be determined. Experimentally, two contact angles are measured: an advancing angle as the liquid advances onto the fiber, and a smaller receding angle, as the liquid recedes from the fiber. This is accomplished by placing the liquid on an elevator, and measuring the angle as the liquid moves up (onto the fiber), and down (away from the fiber). If the contact angle of methylene iodide is measured against a filament, then a measure of the dispersion forces (nonpolar forces) occurring at the fiber surface can be obtained. The following analysis by Fowkes applies [6]: Wa = 2 oMfo-~v d,

(1)

where Wa is the work of adhesion of the fiber to the liquid, o m is the surface tension of methylene iodide, and o d is the dispersion force component to the surface energy of the fiber. The term OF d is a measure of the amount of dispersion forces (or nonpolar forces) that are present at the fiber surface. Equation 1 is used as follows. Wa is measured for the fiber against methylene iodide. Since the

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G.M. Nishioka / Interaction of organosilanes with glass fibers

104

surface tension of methylene iodide is known, eq. 1 can be used to solve for ova. The work of adhesion can be split into nonpolar and polar components: W a = W 2 -'l- WaH

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filament in methylene iodide and water, a quantitative measure of the dispersion and polar forces acting at the filament surface is obtained. The effect of systematic treatment of glass fibers with monofunctional organosilanes on these forces is examined in this paper.

2. Experimental methods

2.1. Apparatus The wetting balance at H & N Instruments is depicted in fig. 3. The wetting balance consists of three groups of instruments: (1) the microbalance and elevator; (2) the data collection and analysis system, consisting of a Keithley 193 DMM and personal computer, plotter and printer; and (3) associated sensors (platinum thermometers, hygrometer).

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G.M. Nishioka / Interaction of organosilanes with glass fibers

The wetting experiment consists of moving a liquid surface up and down along the length of a single filament. A section of filament about 2 cm in length is glued to a small metal hook and hung vertically from the measuring arm of a Cahn 2000 recording microbalance. The combined mass of hook and fiber are tared off electronically. The wetting liquid is placed in a 10 cm 3 beaker on a R a m e - H a r t Precision Drive Elevator centered directly beneath the fiber. The elevator is capable of raising and lowering the liquid container at speeds ranging from 0.5 to 10 ~ m / s and is equipped with a Hewlett-Packard 7 DCDT-1000 Displacement Transducer which generates a voltage proportional to the elevator's vertical position. Output from the microbalance and transducer is sent to the computer and stored. The elevator platform is moved into position with a fast-drive motor so that the liquid surface lies 0.1 mm beneath the end of the filament. A precision drive motor, preset to move the platform at a velocity of 2 ~tm/s, is then engaged. The computer, activated simultaneously with the precision drive, collects one transducer reading and one microbalance reading every second as the elevator advances the liquid front along 5 mm of fiber length. The precision drive is then reversed and the meniscus recedes until the fiber is free of the liquid. The microbalance output is displayed on the computer terminal during the 90 min immersion period for a visual record of the experiment. A small Lucite box on the elevator platform which encases the filament, the liquid container and the bottom of the microbalance hangdown tube maintains environmental control. A hinged panel on the front of the box permits access to the filament and wetting liquid. The microbalance and elevator are encased in a larger container made from Lexan and aluminum frame. The outer box contains shelving for constant humidity solutions and for sample conditioning prior to immersion. Humidity levels, maintained between 30% and 50% RH, are monitored with a Panametrics Model 2100 Hygrometer. Fiber diameters are measured using a B & L Galen oil immersion microscope. Surface tension measurements are obtained with a Wilhelmy Plate.

105

2.2. Materials

Three monofunctional organosilanes were used: trimethylethoxysilane ( ( C H 3 ) 3 S I O C 2 H 5)), butyldimethylethoxysilane ( C 4 H 9( C H 3)2 SiOC2Hs), and p h e n y l d i m e t h y l e t h o x y s i l a n e (Ph(CH3)2SiOC2Hs). These were purchased from Petrarch Scientific, had a stated purity of 97% and were used as received. E-glass cloth was obtained from J.P. Stevens. The cloth was heat cleaned at 500 ° C for 48 h to remove the binder used in its manufacture. 2.3. Procedure

Cloths were coated as follows. A 0.1M solution of organosilane in a 95% ethanol in water solution was prepared and adjusted to pH = 5 with glacial acetic acid. A swatch of heat cleaned cloth weighing several grams was immersed for 10 s in the solution. The cloth was then suspended in an oven at 1 6 0 ° C for 10 min. The cloth was then either redipped in the organosilane solution and again cured, or placed in a Soxhlet extractor. Samples of cloth were subjected to varying numbers of c o a t / cure treatments in order to vary the extent of reaction of the glass surface with the organosilane. Following the c o a t / c u r e step, the cloth was washed with petroleum ether C in a Soxhlet extractor for 48 h. All unreacted organosilanes and dimers have been shown to be removed by this procedure [7]. After Soxhlet extraction, the cloth was dried in a desiccator for several days. Wetting properties were determined by measuring the wetting of glass fibers in methylene iodide and water. Following the standard analysis by Fowkes [6], wetting against methylene iodide yields the dispersion force component to the surface energy of the fiber, and wetting against water yields the work of adhesion of the fiber against water, and the hydrogen bond component to the work of adhesion against water.

3. Results and discussion

Typical wetting data are shown in fig. 4. The left part of the graph represents the advancing

G.M. Nishioka / Interaction of organosilanes with glass fibers

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angle; after 42 min the elevator reverses and a receding angle is measured. The elevator moves at 2 t,m/s, sampling 5 mm of fiber surface. The results of the systematic coat-cure treatments on wetting properties are summarized in fig. 5. Plotted on the vertical scale is W~2 o, the hydrogen bond (or polar) component to the work of adhesion of the filament against water. This can be considered a measure of the polarity of the fiber surface. This is plotted against the number of coat-cure cycles the cloth was subjected to. Each point is derived from the average of ten wetting measurements each in water and methylene iodide.

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The general trend of all three samples is a decrease in the polarity of the glass surface as the number of coat-cure cycles increases. This is presumably due to the gradual reaction of the polar surface hydroxyl groups with the organosilane. It appears that ten coat-cure treatments are sufficient to form a surface saturated with the organosilane. It is clear that the smaller organosilane (trimethylsilane) does not reduce polarity as much as the larger organosilanes (butyldimethyl- and phenyldimethyl-silane). This is not unexpected, since the relatively polar glass network is not as well covered by the smaller organosilane. In a recent patent [8], glass cloths were treated with varying amounts of monofunctional organosilanes using this coat-cure technique. Electrical circuit boards were constructed from these cloths, and their electrical resistance was measured• It was found that a systematic increase in electrical resistance was obtained, as the number of coat-cure cycle treatments increased. Electrical conduction through composite circuit boards is thought to occur at the glass-resin interface; presumably the polarity of the glass surface controls the formation of this electrically conductive layer• In related-work, Wesson and Jen [9] constructed fiberglass reinforced plastic composites using different trifunctional organosilanes as coupling agents. They found a linear increase in the wet strength retention of the composite as the fiberglass polarity (W~2o) decreased. Apparently the susceptibility of the glass-resin interface to water induced degradation is measured by W~: o. It is interesting to note that the values of W~: o reported for glass fiber surfaces treated with trifunctional organosilanes is - 50 e r g s / c m 2. Figure 5 indicates that W~2o of a saturated surface is lowered to 25-30 e r g s / c m 2. Therefore, considerable improvements in the reaction of trifunctional organosilanes with the glass surface may be possible. The systematic change in W~2o made possible with monofunctional organosilanes could find use in a variety of investigations. For example, the importance and control of W ~ o in reinforcements used in composites could lead to taildring of the fiber-matrix interface to achieve improved

G.M. Nishioka / Interaction of organosilanes with glass fibers mechanical properties. The control of WHo on the s u r f a c e of c h r o m a t o g r a p h i c s u p p o r t s c o u l d c o n s i d e r a b l y i m p r o v e v a r i o u s s e p a r a t i o n processes. O t h e r processes i n v o l v i n g a solid i n t e r f a c e c o u l d b e n e f i t as well f r o m c o n t r o l l e d r e a c t i o n w i t h organosilanes.

References [1] J.T. Huneke and J.K. Dorey, in: Proc. 10th NATAS Conf., Boston, MA (1980) p. 175. [2] K. Kadotani, Composites (1980) p. 199.

107

[3] M.E. Schrader, in: Surface Characteristics of Fibers and Textiles, ed. M.J. Schick (Dekker, New York, 1977). [4] S. Ross and G.M. Nishioka, in: Emulsions, Lattices, and Dispersions, eds. P. Becher and M.N. Yudenfreund (Dekker, New York, 1978). [5] S.P. Wesson and A. Tarentino, J. Non-Cryst. Solids 38&39 (1980) 619. [6] F.M. Fowkes, in: Chemistry and Physics of Interfaces, ed. S. Ross (American Chemical Soc., Washington, DC, 1965) p. 1. [7] W.A. Aue and C.R. Hastings, J. Chromat. 42 (1969) 319. [8] G.M. Nishioka and S.P. Wesson, US patent no. 4,455,440 (1984). [9] S.P. Wesson and J.S. Jen, Proc. 16th Nat. SAMPE Tech. Conf. 16 (1984) 375.