REINFORCED PLASTICS SYMPOSIUM
SILANE COUPLING AGENTS SAMUEL STERMAN JAMES G. MARSDEN
Dual reactivity of silane coupling agents afords chemical bonding to both the reinforcing agent and the resin base and, hence, provides f o r improvement of physical properties o f resin-based composites ilane coupling agents are used in a wide range of
S applications because of their unique ability to bond
polymers with dissimilar materials such as inorganic oxides-Le., silica and alumina. The bond thus formed has good initial strength as demonstrated by failure of the composite by polymer rupture, and the bond exhibits excellent retention of strength even after severe environmental aging. The siliceous matter or metal may be in the form of fibers, particulate fillers, or massive structures. Almost every type of organic polymer is compatible with silane coupling agents, ranging from thermoset resins through elastomers to thermoplastic resins. The silane may be applied to the substrate as a pretreatment or, in many systems, the silane may be added to the resin (72, 74) where it migrates to the substrate during normal mixing and application procedures. The application of silane coupling agents to promote bonding has led to improved physical properties of composite materials such as filled and reinforced resins, filled elastomers, caulks for adhesion of metal and glass, and resin-coated and painted metal. Use of silane coupling agents in glass reinforced plastics has resulted in a particularly notable improvement of materials performance, and this application is the subject of this paper. Development of Coupling Agents
The earliest attempts to apply organic sizes to glass fiber were carried out in the mid-1930’s by Slayter and Thomas (4)and were directed primarily toward main-
PHOTO COURTESY OF UNION CARBIDE SILICONES D I V I S I O N
Electron micrograph of Union Carbide A 7100 silane cou;bling agent on glass cloth shows an eflectiue loading of less than 0.1% silane remaining after 4 hr. of water extraction treatment
taining the physical integrity of glass immediately subsequent to the drawing operation. A continuation of this approach led to the development of the starch-oil or No. 630 size. This size not only gave protection to the fiber after drawing but was also adequate for the weaving operation. Woven glass fabrics treated with the starch-oil size were used as reinforcement for phenol-formaldehyde resins as late as the early 1940’s. During this same period, Hyde (4) and, shortly thereafter, Biefeld (4) filed patent applications covering the use of silicon-based VOL. 5 8
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materials as components in sizes. These materials were potentially capable of reacting with the glass surface, but not with the matrix resin, and they contributed water repellency and lubricity to the treated glass. I n the mid-l940's, three patent applications were filed that laid the foundation for coupling agents as we know them today. Steinman (4) filed on the use of methallyl silicate, T e Grotenhuis ( 4 ) on vinylsiloxanes, and Goebel and Iler ( 4 ) on methacrylato chromic chloride. These patents were all concerned with improving the adhesion of matrix resins to glass fiber by the use of a coupling agent. Mechanism of Adhesion
The study of the theory of mechanism of adhesion has been pursued from many viewpoints using both empirical and classical methods. R. M . Vasenin (15),in a recent article, attempted to classify most of the current theories of adhesion. His classification is as follows : mechanical, adsorption, diffusion, chemical, and physical. The first three of these involve intermolecular forces, the fourth involves chemical bonds, and the fifth, electrostatic forces. A recent view (79) is that the relative surface tension of substrate and adherent is most important, so that an adhesive of lower surface tension will spread and wet a substrate of higher surface tension with good adhesion, but the reverse is not true. A modification of this latter view has been presented by W. D. Bascom ( 1 ) . He assumes the need of a silane coupling agent and proposes the use of a surfactant to promote better wetting of the glass fiber by the resin with subsequent reduction of voids and an increase in strength. One of the difficulties with much of the classical work on adhesion is that it has been primarily concerned with the strength of adhesion under room temperature conditions. While this is an area of importance in glass reinforced plastics, of equal importance is the retention of properties under more severe conditions. I t is true that no clear unequivocal proof for the mechanism of resin adhesion to glass fiber has been demonstrated; and while several of the suggested mechanisms may contribute to adhesion, the majority of available evidence indicates that the controlling mechanism in most cases is chemical bonding of the resin to the glass through a coupling agent. This is a plausible explanation of the good retention of properties exhibited by reinforced composites containing silane coupling agents. The Chemical Bridge
The dual reactivity of a coupling agent permits chemical bonding to both the glass fiber and resin, thereby forming a chemical bridge across the glass-resin interface. This type of bonding, at this critical region of the composite, produces high mechanical strength and good retention of properties even under severe conditions. Materials capable of performing this function were introduced in the mid-1 940's as methacrylato-chromium chloride complexes and vinylsilanes. Since that time, a number of new silane coupling agents have been developed which 34
INDUSTRIAL AND ENGINEERING CHEMISTRY
afford higher strengths in the resulting composites and reactivity with a wider range of resins. Specific functional requirements of a silane coupling agent are discussed below.
Silane to Glass Bonding. Silanes bond chemically to glass by the condensation of an active group on silicone, such as OH, C1, OR, or OAc, with a silanol on the glass surface. Supporting evidence for this type of reaction is difficult to obtain in practical commercial systems. This conclusion is based on a large number of empirical results and a limited amount of more basic work. Islinger (6) has studied the vapor phase chemisorption of chlorosilanes on E-glass fibers and has concluded that siloxane bonds are formed at the interface. Infrared techniques have been used by White (17) to study the reaction of chlorosilanes with high surface silicas. He concludes that a reversible physical reaction takes place at room temperature but that irreversible chemisorption occurs at higher temperatures. Sterman and Bradley (10) studied the tenacity of the silane glass bond by aqueous and solvent extraction of silane-finished E-glass cloth. Their work indicated that some of the finish can be removed by this technique, but that a tightly bonded quantity of silicone remains equivalent to two to four theoretical monolayers. Theoretically, one hydrolyzable group per silicon atom would be sufficient if the coupling agent could be applied under anhydrous conditions. Obviously, this anhydrous condition is not present in commercial systems where the preferred application solvent is water. The use of silanes in water results in the hydrolysis of the silane to a silanol, which is the active species in bonding to glass. Competitive with the condensation of coupling agent silanol with a silanol on the glass surface is a condensation of silanols on two coupling agent molecules. Therefore, the use of a silane coupling agent having one or two hydrolyzable groups per silicon atom can result in the formation of di- or polysiloxanes having little or no ability to bond to glass under the normal use conditions. For these reasons and because of the solubility requirement for aqueous application, all commercially used coupling agents have three hydrolyzable groups per silicon atom. The group on silicon that is hydrolyzed to form the silanol has little effect on the performance of the coupling agent in the resulting composite. I t is, therefore, chosen on the basis of handling ease, nature of hydrolysis byproducts (HC1, HOAc, or ROH), and, occasionally, by ease or feasibility of synthesis. Table I shows the essentially equivalent performance of a series of trifunctional vinylsilanes in a standard 12-ply 181-style glass cloth reinforced polyester laminate. This equivalency of performance of different hydrolyzable groups is typical for many types of silanes in a number of resin systems. Resin Reactivity, The variety of organofunctional groups presently available on silanes provides at least one (and often several) coupling agent that is highly
reactive with all of the current thermoset resin systems. Unlike the hydrolyzable groups discussed in the preceding section, these groups must be bonded to silicon in a hydrolytically and thermally stable manner. The chemistry and performance of coupling-agent types can best be discussed in terms of specific resin systems, but the following list is representative of silanes that have proved commercial utility in glass reinforced plastics and the resin systems in which they are commonly used.
Commercially Available Silane Coupling Agents
Thermoset Resins of Common Use
CHZ=CHSi= CHZO
I /I
CH z=C-C-O
CH 2CHzCH zSi= Polyes ter, diallyl phthalate CHZ-CHCH~OCHZCH~CH~S~=
\/ 0
TABLE I . EFFECT O F X O N FLEXURAL STRENGTH O F ViSiX3 FINISHED GLASS CLOTH REINFORCED POLYESTER RESIN COMPOSITESa
Flexural Strength, P.S.I. x 10-8 Dry I 8-Hr. Boil 68.5
58.1
65.0
59.0
66.0
57.0
69.0
61 .O
64.0
57.0
72.2
57.8
a Laminates constructed from 12 plies of glass and a general-purpose polyester resin.
TABLE I I . FLEXURAL STRENGTH O F V A R I O U S GLASS RE I N FORCED POLY ESTER RES I N COMPOS lTESa
Flexural Type of Glass Reinforcement
P.S.I.
x
35-45
32
Woven roving
55
40
Satin weave cloth
62
55
Unidirectional roving
70
150
Chopped strand mat
Flexural Strength, P.S.I. x 10-3 8-Hr. Boil
Silane Finish 112 (heat-cleaned cloth)
CH3 0
I
I1
CH2=C--COCH&H&H2Si=
10-8
1
1
61 Dry .O 43.5 69.0
87.0
I 1
1
23.0 29.7 61.0
79.0
a Laminate constructed from 12 plies of glass and a general-purpose polyester resin.
A U T H0 RS Sam uel Sterm an is Sufiervisor, Product Development, Resins, Monomers, and Functional Fluids; and James G. Marsden is Group Leader of the Product Development Grou@for Resins and Monomers at Union Carbide’s Silicones Division Research and Development Laboratories in Tonawanda, N . Y.
0
0
S
-CH2CH2Sis=
Polyester, epoxy, melamine
NH2CH&H&H&i=
H
I
NHzCHzCHzNCH2CHzCHzSk
Epoxy, melamine, phenolic
The relative performance of a number of silanes in several resin systems is presented in the following sections, based on results obtained on woven glass cloth reinforced laminates. This choice of composite allows the use of readily available heat-cleaned glass fabric which offers good reproducibility and allows the effect of the silane to be studied free of possible interference by other materials. Woven glass reinforced laminates limit the number of mechanical properties that can be meaningfully tested and the use of a coupling agent as a finish does not evaluate its compatibility with and utility in a complete binder system. The absolute strength values obtained for a given resin system depend on the type as well as the amount of glass reinforcement used. Table I1 shows the strength values obtained with various forms of reinforcements in a polyester resin ( 3 ) . These values seem somewhat conservative when compared to the values obtained with the newer coupling agents, but they illustrate the trend of type of reinforcement us. mechanical strength. I t has been shown empirically that the relative performance of a series of coupling agents is essentially the same regardless of the form of glass used. Therefore, it appears that the evaluation of silanes in a woven glass laminate system can provide meaningful design data if it is realized that additional work is required to convert such systems to a complete binder system. Matrix Resins
The choice of the matrix resin in all-glass reinforced composites depends on a great many factors including strength requirements, method of fabrication, electrical requirements, use conditions (temperature, humidity), and cost. Silane coupling agents have demonstrated significant improvements in and retention of general mechanical and electrical properties in glass reinforced composites of all thermosetting resins. I n some of these systems the improvement of a specific property is of equal or greater importance than over-all performance improvement. For example, the use of y-aminopropyltriethoxysilane as a finish in glass cloth reinforced phenolic resin laminates produces dry and wet strength improvements VOL. 5 % NO. 3
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112 (heat-cleaned cloth)
23.0
CH2-CH-CH20CH2CH2CH2Sk
58.0
\ /
1
0
I
1
i a Laminates constructed from 12 plies polyester resin.
of
71.0
58.0
glass and a general-purpose
TABLE V. EFFECT O F SILANES O N T H E MECHANICAL PROPERTIES O F 181-TYPE GLASS CLOTH REINFORCED EPOXY RESIN LAMINATES"
Flexural Strength, P.S.I. x 1 0 - 2 Dry 1 72-Hr. Boil
Silane Finish
1
112 (heat-cleaned cloth) NH2CH2CH2CHZSir
78.0
29.0
90.0
54.0
i 101.0
1
66.0
I 87.0
CH2-CHCH2-OCH2CH2CH2SiE
56.0
\ / 0
65.0 a Laminates constructed ,from 72 plies of glass and an amine-hardened glycidyl ether-type epoxy resin.
TABLE V I . EFFECT O F SILANE FINISHES O N M E CHANICAL PROPERTIES O F GLASS CLOTH REI N F ORCED T H E R M O PLAST I C RES I NS"
I
I
Flexural ,ength wement lcer Control Wet0 Temp., ' F . ~
Resin
Silane
100.0
95.0
70.0 (200)
Polyvinyl chloride
(HOCH2CH2)l83.0 NCH~CH~CHZSi=
100.0
...
Nylon
(HOCHzCH2)nNCH2CHzCHr Sir
110.0
160.0
150.0 (400)
Polycarbonate
30.0
60.0
20.0 (250)
Polymeth) methacrylate
45.0
90.0
25. 0 (200)
145.0
228.0
145.0(150)
Polystyren
Acrylonitrilebutadienestyrene
OCHzCH2CHz- ' Si=
Ij
CHz-CHCH2-
\/ 0
OCHzCHzCHa Sir
a Laminates contain 7 7 plies of 181-type glass cloth, 40-45y0resin. b Control is heat-cleaned glass cloth reinforcement. C Immersion f o r 76 hr. in water at 720" F.
36
INDUSTRIAL A N D ENGINEERING CHEMISTRY
but its more important contribution may be the retention of strength at elevated temperatures. The two thermoset resins that command the greatest attention in the field of glass reinforcement today are polyester and epoxy resins. Polyester resins comprise a major part of the reinforced plastics market today and epoxy resins represent the highest strength systems. Polyester Resins. The free radical initiated polynierization of unsaturated polyester resins lends itself to modification of the resin by copolymerization with other materials of appropriate reactivity. Indeed, the commercial form of most polyester resins contains a reactive monomer, such as styrene, that serves to reduce the viscosity of the resin and to copolymerize during the cure. Silane coupling agents can react with unsaturated components of polyester resins by the same copolymerization mechanism. Current theories of double bond polymerization predict that optimum copolymerization is obtained from monomers that have similar orders of reactivity. The relative reactivity of monomers may be estimated from q - Evalues derived bl- Schwan and Price (9). In this system, q represents the resonance stabilization of a radical by adjacent groups and ranges 0 to - 4 . 4 kcal./mole, while E indicates the polarity of monomer radical forming the end of a growing chain. These E values range from -0.61 X loplo to +0.76 X in electrostatic units. The results obtained using silanes of varying reactivity as finishes for glass cloth reinforced polyester laminates are shown in Table 11. Because ethyl silane has no double bond reactivity, it is ineffective. Vinylsilane is relatively unreactive and produces marginal improvement in dry strength and good wet strength, while the highly reactive methacryloxypropylsilane produces excellent dry and wet values. Plueddmann et al. (8) have evaluated a number of silanes as finishes in glass cloth reinforced polyester resin and compared the results with the q - E values of Schwan and Price ( 9 ) . Good agreement is obtained between the results predicted from q - E values and those actually obtained. I t would appear from these results that good coreactivity with styrene is more important than coreactivity with the maleate portion of the polyester resin. There is another group of silanes that does not contain any double bonds, yet is effective as coupling agents in polyester resin systems. Many of these compounds are believed to interact with the resin by chain transfer (76). Two such materials are y-glycidoxypropyltrimethoxysilane and ~-(3,4-epoxycyclohexyl)ethyltrimethoxysilane. They are about as effective as vinylsilanes as finishes for glass reinforced polyesters. Typical values are shown in Table IV. Epoxy Resins. The silane coupling agents that are effective in epoxy resins can be divided into two types: (1) those that have the same chemical reactivity as the resin and react with the hardner in the same way, such as the epoxysilanes, and (2) those that are chemically similar to the hardner and react with the epoxy groups of the resin, such as the aminoalkylsilanes. Neither type is preferred over the other on a general
basis, although in a specific resin there may be unexpectedly wide variations in results. Table V shows the results obtained when silanes of both types were used as finishes in a n amine-hardened glycidyl ether bisphenolA-type epoxy resin.
New Systems
T h e foregoing discussion has been directed primarily a t glass reinforced thermosetting resins because they represent the bulk of the present market. There are, however, two new areas that are relatively small now but represent tremendous growth potentials. These are glass reinforced thermoplastics and elastomers. Thermoplastics. Thermoplastics offer many attractive properties including relatively low cost, great versatility of chemical type, and ease of fabrication into end-use products. Several resins have been limited in structural and engineering applications because of low mechanical properties, especially modulus. These deficiencies can be overcome by the use of glass fiber treated with silane coupling agent without adversely affecting the processing characteristics of the polymers. T h e mechanism of silane-resin interaction has been reported on by Sterman and Marsden (77), and while it is not completely understood, empirical results indicate that, in a t least some systems, chemical coreaction similar to that discussed for thermoset resin is operative. I n the case of polystyrene, those silanes capable of undergoing addition polymerization or chain transfer reaction perform best. Processing temperatures of polystyrenes are such that chain scission and free radical generation can occur, and it is suggested that the reaction of the silane is with these active polymer fragments. Nylon, on the other hand, is a “reactive” thermoplastic. T h e silanes that are the most useful with nylon are capable of reacting either with the amido hydrogen of the polymer or by rearrangement of the amide linkage. However, many anomalies remain. Table VI shows the results obtained by reinforcing a series of thermoplastic resins with 181-style glass cloth with and without silane coupling agent. These results show that the incorporation of glass typically doubles the flexural strength of a given resin, while the use of glass with the proper coupling agent doubles this value again. Also of considerable interest is the retention of mechanical strength a t higher temperature produced by the use of coupling agents. Composites containing glass and silane typically have flexural strengths a t 200’ F. and higher, equivalent to the flexural strength of composites without silane a t room temperature. This can possibly increase the useful temperature range of many thermoplastics. While these results were obtained with woven glass cloth, and the market appears to be moving in the direction of chopped glass reinforcement, they are useful in indicating the kind of improvement that is possible. Obviously, there is need for much additional work, particularly in the systems closer to the conditions of actual use.
Elastomers. T h e use of glass, with its low elongation and high tensile strength, as a reinforcement for rubber has been a long sought goal. However, the lack of adhesion of rubber to glass and the destructive selfabrasion of the glass fibers have prevented realization of this goal. Recent developments appear to have overcome these problems and offer the promise of glass reinforcement in a variety of rubber mechanical goods, including tires. Marzocchi and Lachut (7) reported the development of a rubber-impregnated glass yarn suitable as reinforcement for several rubber stocks. The use of silane coupling agents to improve the adhesion of a variety of rubbers to glass was reported by Vanderbilt and Clayton (13). This work states that the use of the proper coupling agent produces excellent bonding to glass for 11 different rubber stocks. I n five types of rubber the bonding to glass was strong enough to result in cohesive failure of the rubber on rupture. A companion paper by Clayton and Kolek (2) describes the application of these results to binder compositions and yarn constructions necessary to optimize results in a practical continuous process. While the final and optimum glass reinforcement for rubber has not been developed, great progress has been made and it appears that silane coupling agents will be an important component of the final system. I n addition to glass reinforced systems, silanes are applied in some filled elastomers. They are particularly effective in mineral-filled compositions of the newer ethylene-propylene copolymers and terpolymers. Fusco (5) has reported on the tensile strength improvements obtained by the use of vinyl silanes in silica-filled EPR, and Wolfe and Roche (78) describe the retention of electrical properties under wet conditions and the higher modulus with vinylsilanes in clay-filled EPT. REFERENCES (1) Bascom, W. D., “Some Surface Chemical Aspects of Glass-Resin Composites. Part I-Wetting Behavior of Epoxy Resins on Glass Filaments,” 20th Annuai SPI Preprints, February 1965. (2) Clayton, R. E., Kolek, R. L., Rubber World 151 (5), 95 (1965). (3) De Dani, A., “Glass Fibre Reinforced Plastics,” Interscience, New York, 1960 (4) Eakins, William J., “Glass/Resin Interface: Patent Survey Patent List and General Bibliography ” Plastics Technical Evaluation Center, Sicatinny Arsenal, Dover, N. J., September 1964. (5) Fusco, J. V., “New Cures for EPR,” Rub6er World, pp. 48-54 (February 1963). (6) Islinger, J. S., et al., “Mechanism of Reinforcement of Fiber-Reinforced Structural Plastics and Composites,” WADC Technical Report 59-600. Part I. (7) Marzocchi, A., Lachut, F. J., “Glass Fiber for Reinforcement,’’ Rubber World, pp. 62-64 (December 1962). (8) Plueddmann E. P. “Evaluation of New Silane Cou ling Agents for Glass Fiber Reinforckd Plasiics,” 17th Annual SPI Preprints, Fetruary 1962. ( 9 ) Schwan, T. C., Price, C. C., J . PolymerSci. XL,457-68 (1959). (10) Sterman, S . , Bradley, H. B., “Evaluation of Existing and Development ofNew Analytical Procedures for Use in the Analysis of Finishes for Glass Fabric Material,” WADD Technical Report 60-318,(April 1960). (11) Sterman, S., Marsden, J. G., “The Effect of Silane Coupling Agents in Imy v h g the Pro erties of Filled or Reinforced Thermoplastics,” 21st Annual PE Preprints, $arch 1965. (12) Sterman, S., Marsden J G “Silane Coupling A ents as ‘Integral Blends’ in Resin Filler Systems,” 18;h Anzual SPI Preprints, Fefruary 1963. (13) Vanderbilt B. M., Clayton, R. E., IND. ENa. CHEM.PROD.R E S . DEVELOP. 4 (l), 18 (1965’). (14) Vanderbilt, B. M., Clayton, R. E., “Premixes Based on Hydrocarbon Resins,” 16th Annual SPI Preprints, February 1961. (15) Vasenin, R . M., Adhefives Age 8 (5, 61, 18-22 (1965). (16) Wallace, T., Gretter? R., “Free Radical Rearrangement of Epoxides,” Abstract, 142nd ACS Meeting, 6 5 4 No. 118. (17) White T. E “A Stud of the Reactions of Surface H droxyl Groups of Silica by Meaks of IGfra-Red lpectroscopy,” 20th Annual S%’I Preprints, February 1965. (18) Wolfe, J. R., Jr., Roche I. D “Factors Influencing the Electrical Pro erties of Eth lene-Propylene Te;polyn;)er Vulcanizates,” presented a t 145th d e e t i n g ACS, Jeptember 1963, New York. (19) Zisman, W. A “Surface Chemistry of Glass Fiber Reinforced Plastics,” 19th Annual Meetjhg SPI, February 1964.
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