Abstract:
Cyanate-functional oxazolinylpolysiloxanes are useful for toughening high performance resin systems such as cyanates, epoxies, and bismaleimides without substantial loss of heat resistance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates to high performance thermosetting resin systems. More particularly, the subject invention relates to heat-curable resin systems containing cyanate ester-functional oxazolinylpolysiloxanes. 
     2. Description of the Related Art 
     High performance thermosetting resins based upon cyanate esters find application in many areas where high strength and heat resistance are important. Such resins, which contain monomers having two or more cyanate ester (cyanate) groups, polymerize to form highly cross-linked triazine structures. Unfortunately, the high strength and heat resistance which results from the polymer structure also causes the polymer to be brittle, and subject to impact induced damage. 
     Modification of these resin systems to improve their flexibility and reduce susceptibility to impact induced damage has been but partially successful. Copolymerization with epoxy resins, bismaleimide resins, and modification with cyanate-reactive acrylonitrile-butadiene elastomers have imparted greater toughness to cyanate resin systems, but generally with considerable loss of heat resistance. 
     SUMMARY OF THE INVENTION 
     It has now been discovered that cyanate resins and other resin systems may be successfully modified through incorporation of a cyanate-functional oxazolinylpolysiloxane into the resin system. The resulting toughened resin systems show a considerable increase in flexibility without a concomitant decrease in heat resistance. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The cyanate-functional oxazolinylpolysiloxanes of the subject invention are readily prepared by the catalyzed or uncatalyzed reaction of at least a molar equivalent of a di- or polycyanate functional monomer with an epoxy-functional polysiloxane. 
     In the claims, the term &#34;polycyanate&#34; shall include dicyanates as well as cyanate-functional monomers containing more than two cyanate groups. 
     The cyanate-functional monomers are well known to those skilled in the art. These monomers are generally prepared by reacting a cyanogen halide with a di- or polyhydric phenol or similar compound. Examples of phenols which are commonly used to prepare the cyanate resins include mononuclear phenols such as hydroquinone and resorcinol; the various bisphenols, i.e. bisphenol A, bisphenol F and bisphenol S; and the various phenol and cresol based novolac resins. Examples of the method of preparation and of specific cyanate functional monomers may be found in U.S. Pat. Nos. 3,448,079, 3,553,244 and 4,663,398. Particularly preferred are the cyanates of hydroquinone, bisphenol A, bisphenol F, 2,2&#39;,6,6&#39;-tetramethylbisphenol F, bisphenol S, and the phenolic novolac resins, and the di- and polyphenols which are derived from the reaction products of phenol and dicyclopentadiene in the presence of Friedel-Crafts catalysts as disclosed in U.S. Pat. No. 3,536,734, hereinafter referred to as phenolated dicyclopentadiene. The cyanate functional resins are generally used in an amount at least equivalent to the number of moles of epoxy groups in the epoxy-functional siloxane and preferably in excess. For example, to one mole of a linear siloxane or polysiloxane terminated at both ends with epoxy functionality will be added two moles or more of a dicyanate. The amount of excess cyanate may be adjusted depending upon the particular application or the degree of toughness required. 
     The epoxy-terminated siloxanes may be prepared by methods well known to those skilled in the art. See, for example, J. Riffle, et. al., Epoxy Resin Chemistry II, ACS Symposium Series No. 221, American Chemical Society, pp. 24-25. Generally speaking, the epoxy functional polysiloxanes are prepared by the equilibrium polymerization of the readily available bis (3-glycidoxypropyl)tetramethyldisiloxane with a cyclic siloxane oligomer, preferably octamethylcyclotetrasiloxane or octaphenylcyclotetrasiloxane. 
     The equilibrium polymerization generally proceeds in the presence of a catalyst such a tetramethylammonium or tetrabutylammonium hydroxide or the corresponding siloxanolates. Particularly preferred is tetramethylammonium siloxanolate. The reaction proceeds readily at temperatures from about 50° C. to about 200° C., preferably from about 80° C. to about 150° C. 
     Reaction of the cyanate-functional monomer with the epoxy-functional siloxane occurs at elevated temperatures, e.g. from about 80° C. to about 250° C., preferably from about 130° C. to about 210° C. to yield a cyanate-functional oxazolinylpolysiloxane. The reaction sequence may be illustrated as follows: ##STR1## wherein each R may be, for example, a C 1  -C 6  lower alkyl, C 1  -C 6  lower alkoxy, C 1  -C 6  haloalkyl, vinyl, allyl, allyloxy, propenyl, propenyloxy, acetoxy, C 5  -C 10  cycloalkyl, or aryl radical. 
     Catalysts are not necessary for the reaction between the cyanate monomer and the epoxy-functional polysiloxane. However, if desirable, metal catalysts such as tin octoate, dibutyltindilaurate, dibutyltindiacetate, or compounds of lead or zinc which catalyze triazine formation from cyanates may be used. Other catalysts which may be useful include metal acetylacetonates, metal alkyls such as butyl titanate and propyl aluminum, metal chlorides such as tin(IV) chloride; imidazoles, particularly 2-substituted and 2,4-disubstituted imidazoles, and tertiary amines such as N,N-dimethylbenzylamine, triethylenediamine, N-methylmorpholine and the like. The catalysts, when utilized, are generally present to the extent of from about 1.0×10 -6  to about 2.0 wt. percent, preferably from about 1.0×10 -3  to about 0.5 weight percent, and most preferably from about 1.0×10 -2  to about 0.5 weight percent. 
     The examples which follow illustrate the practice of the subject invention. These examples are by way of illustration only, and should not be interpreted as limiting the scope of the invention in any way. 
     EXAMPLE 1 
    
    
     Preparation of Tetramethylammonium Siloxanolate 
     Into a 250 ml three neck round bottom flask equipped with a mechanical stirrer and reflux condenser are placed 118.6 g (0.4 mol) octamethylcyclotetrasiloxane and 18.6 g (0.1 mol) tetramethylammonium hydroxide pentahydrate. The mixture is stripped of water over a period of 48 hours by means of a flow of nitrogen while stirring at 70° C. The resulting viscous syrup is used as a polymerization catalyst without further purification. 
     EXAMPLE 2 
     Preparation of Epoxy-Functional Polysiloxane Copolymer 
     To a 2 liter three neck round bottom flask equipped with a mechanical stirrer and reflux condenser are charged 534.4 g octamethylcyclotetrasiloxane, 534.4 g octaphenylcyclotetrasiloxane, 90.7 g bis[3-glycidoxypropyl]tetramethyldisiloxane, and 12.0 g tetramethylammonium siloxanolate from Example I. The resulting mixture is stirred at 80° C. for 48 hours under nitrogen. During this period, the viscosity is observed to increase and then reach a stable value. The catalyst is then destroyed by heating to 150° C. for 4 hours. After cooling to room temperature, the filtered reaction mixture is extracted twice with methanol (300 ml×2) to remove unreacted cyclic oligomers. The product is then dried in vacuo at 1 torr and 150° C. The product is a viscous oil (1100 g) having an epoxy equivalent weight (EEW) of 1210. 
     Example 3 
     Preparation of Epoxy-Functional Polydimethylsiloxane 
     Using the procedure of Example 2, a reactor is charged with 18.3 g bis[3-glycidoxypropyl]tetramethyldisiloxane, 182.0 g octamethylcycloetrasiloxane, and 1.4 g tetramethylammonium siloxanolate. The product is a colorless, viscous oil (180 g, EEW =2200). 
     Example 4 
     Heat-Curable Resin Adhesive 
     A heat-curable resin adhesive composition is prepared by mixing 20.0 g of the epoxy-functional polysiloxane from Example 2 with 180.0 g 2,2&#39;,6,6&#39;-tetramethylbisphenol F dicyanate in a 500 ml glass reactor. The mixture is heated, with vigorous stirring, to 190° C. and maintained at that temperature for 5 hours under nitrogen. After cooling to 70° C., 5.25 g of fumed silica (CAB-O-SIL® M-5, a product of the Cabot Corporation), 0.6 g copper acetylacetonate and 8.0 g of a novolac epoxy resin (DEN® 31, a product of the Dow Chemical Company) are added. The homogenous mixture is coated on a 112 fiberglass carrier. 
     Comparison Example A 
     Heat-Curable Resin Adhesive 
     A resin composition similar to that of example 4, but without the cyanate-functional oxazolinylpolysiloxane modifier, is prepared by admixing 180 g 2,2&#39;,6,6&#39;-tetramethylbisphenol F dicyanate, 5.25 g CAB-O-SIL® M-5, 0.6 g copper acetylacetonate, and 8.0 g DEN® 431. 
     The adhesives prepared in Example 4 and Comparison Example A are used to bond aluminum sheets. The resins are cured by heating for 4 hours at 177° C., 2 hours at 220° C. and 1 hour at 250° C. Single lap shear strengths are measured by ASTM method D-1002. Results are presented in Table I below. 
     
                       TABLE I______________________________________        Single Lap Sheer Strength (psi)Adhesive Formulation          20° C.                       177° C.______________________________________Example 4      3100         3300Comparison Example A          2000         2510______________________________________ 
    
     Example 5 
     Heat-Curable Modified Bismaleimide Resin Adhesive 
     A heat-curable bismaleimide adhesive composition is prepared by first admixing 10.0 g of the epoxy-functional polysiloxane of Example 2 with 70.0 g 2,2&#39;,6,6&#39;-tetramethylbisphenyl F dicyanate in a 500 ml reactor. After heating at 190° C. for 5 hours with vigorous stirring, the mixture is cooled to 150° C. and 10.0 g of the bismaleimide of 4,4&#39;-diaminodiphenylmethane is added. After stirring for 30 minutes, the mixture is allowed to cool to 70° C. and 3.2 g CAB-O-SIL® M-5, 0.43 g zinc naphthenate, and 2.0 g benzylalcohol are added. After coating onto a 112 glass fabric, the adhesive is used to bond aluminum. The cure cycle is identical to that used previously for Example 4 and Comparison Example A. Single lap shear strength (ASTM 1002) are as follows: 
     
                       TABLE II______________________________________Temp        Shear Strength (psi)______________________________________ 20° C.       2890177° C.       3060205° C.       3270______________________________________ 
    
     Example 6 
     Heat-Curable Resin Formulation 
     A heat-curable cyanate resin formulation is prepared by stirring together 6 g of bisphenol A dicyanate and 16.0 g of the epoxy functional silicone of Example 3 at 150° C. under nitrogen for 5 hours. To the resulting homogenous but opaque mixture is added 0.079 g zinc octoate. The resulting mixture is cured at 177° C. for 4 hours, then 205° C. for an additional 4 hours. The resin showed good adhesion to both aluminum and glass. Thermogravimetric Analysis (TGA) of the cured elastomer and of cured bisphenol A dicyanate are presented in Table III which indicates that despite large quantities of modifier, the finished elastomer has virtually the same heat resistance as the polymerized cyanate resin itself. 
     
                       TABLE III______________________________________            TGA (°C.) in AirResin              5% Wt. Loss                         10% Wt. Loss______________________________________Modified Cyanate of Example 6              430        440Bisphenol A dicyanate              440        445______________________________________ 
    
     The cyanate-functional oxazolinylpolysiloxane modifiers of the subject invention may be used to toughen a number of high performance resin systems. Due to the variety of reactions in which the cyanate radical may take part, these modifiers may be used, for example, in epoxy resin systems, cyanate resin systems, and bismaleimide resin systems, to name a few. Such resin systems are well known to those skilled in the art. 
     The toughened resin systems find use as laminating resins, as matrix resins in high performance, fiber reinforced prepregs, as potting and encapsulating resins, and as structural adhesives. When used in prepregs, traditional fiber reinforcement such as carbon/graphite, fiberglass, boron, and other fibers may be used in woven or non-woven form, as a mat, or as collimated fiber tows. Rovings and yarns may also be used. The use of such fiber reinforcement is commonplace in the aerospace and transportation industries.