Thixotropic Cyanoacrylate Compositions

United States Patent 119] Litke [541 [75] [73] [21] [22] [51] [52] [58] [561 THIXOTROPIC CY ANOACRYLATE COMPOSITIONS Alan E. Litke, Naugatuck, Conn. Loctite Corporation, Newington, Conn. App]. No.: 523,275 Filed: Aug. 31, 1983 Int. C13 ......................... .. C08K 9/04; C08K 5/54 U.S. Cl. .................................. .. 523/212; 524/533; 524/850; 525/295 Field of Search .............. .. 523/212; 524/533, 850; 525/295 Inventor: Assignee: References Cited U.S. PATENT DOCUMENTS 3,607,542 9/1971 Leonard et al. .................. .. 524/555 3,663,501 5/1972 Adams et al. 523/212 3,839,065 10/1974 Overhults et al. ................ .. 523/212 [11] Patent Number: 4,477,607 [45] Date of Patent: Oct. 16, 1984 3,395,077 7/1975 Leonard eta]. .......... .. 524/425 3,940,362 2/1975 Overhults .. .. 523/212 4,075,535 2/1973 Kogler . . . . . . . . . . .. 523/139 4,102,945 7/1978 Gleave .......................... .. 525/315 4,130,911 1/1930 Bullock ............................... .. 106/35 4,130,913 1/1930 Takeuchietal.. .... .. 523/211 4,320,047 3/1932 Murphy ............................. .. 523/457 Primary Examiner—Herbert Lilling Attorney, Agent, or F1'rm—Walter J. Steinkraus; Eugene F. Miller [57] ABSTRACT Cyanoacrylate compositions which employ fumed sili- cas treated with polydimethylsiloxane or trialkoxyalk- ylsilane are stable and exhibit an unexpectedly high thixotropic ratio. Such compositions are useful in adhe- sive applications or, when stabilized so as to prevent polymerization in contact with moisture, in latent fin- gerprint developing applications. l0 Claims, No Drawings 4,477,607 ‘I I THIKOTROPIC CY AN OACRYLATE COMPOSITIONS BACKGROUND OF THE INVENTION Cyanoacrylate adhesives based on esters of a-cyanoa- crylic acid have gained wide acceptance in recent years for a broad range of industrial and consumer uses. The cyanoacrylate esters themselves, however, are very low viscosity liquids which makes the use of unfilled com- positions difficult. Such compositions often migrate from the bondline or are absorbed into porous surfaces. Accordingly, there have been developed a variety of thickened cyanoacrylate adhesive compositions which incorporate organic polymers and/or inorganic fillers to reduce adhesive migration. Examples of such compo- sitions are described in U.S. Pat. Nos. 3,607,542; 3,896,077, 4,105,715, 4,180,913 and in Chemical Ab- stracts 89:117907c; 89:216475u; 91:40425c; and 92:95114b. In the art of thickened cyanoacrylates it is sometimes desirable that the composition display thixotropic prop- erties. Thus, under high shear conditions the composi- tion can be processed and easily applied to the substrate but once applied, will display significantly reduced migratory tendencies. It is also well known that the inclusion of fumed silicas in many organic liquid com- positions produces thixotropic effects. The use of such silicas has been reported in cyanoacrylate compositions. At least certain of the prior art cyanoacrylate compo- sitions employing fumed silicas have displayed stability problems, however. Thus U.S. Pat. No. 3,607,542 de- scribes organically filled cyanoacrylate compositions in which fumed silica is an optional ingredient. These compositions are reportedly stable for only up to 4 hours. Furthermore, the same patent states that silica by itself does not readily mix with cyanoacrylate monomer to form a paste. Cyanoacrylate formulations also occasionally find non-adhesive applications. Thus, for instance, the va- pors of methyl and ethyl cyanoacrylates have found use for developing latent fingerprints in law enforcement applications. For such applications it would be desirable to develop a nonflowable form of cyanoacrylate mono- mer so that small open containers of the monomer can be placed throughout a room or automobile to release vapors without the danger of accidental monomer spill- age. Desirably the thixotropic additives will also be kept to a minimum so as not to substantially" reduce the monomer vapor pressure. ' In certain applications it has been discovered that hydrophobic silicas produced by treatment of fumed silica with dimethyldichlorosilane can be used to give thixotropic cyanoacrylate compositions with improved stability. However, these fillers add cure retarding strong acid to the cyanoacrylate composition. Also, the thixotropic ratio of these silicas in cyanoacrylate esters (the ratio of apparent viscosity is measured under speci- fied high and low shear conditions) is quite low. There- fore, when very high thixotropic effects are desired, such as when a pasty composition is desired, the amount of acid introduced by the silica can substantially reduce the cure time of the composition. Also, for latent finger- print developing type applications, higher silica levels may result in lower monomer vapor pressures. Commercially available fumed silicas are also known which have been treated with hexamethyldisilazane. It has discovered that thixotropy ratios are also very low 5 10 15 20 25 30 35 45 50 55 65 2 for these materials and that at least some of these materi- als tend to destabilize cyanoacrylate compositions. The destabilization effect is thought to result from residual ammonia or amine in the filler. Accordingly there exists a need for a cyanoacrylate thixotrope, stable to the cyanoacrylate monomer, which has a significantly improved thixotropy ratio over dimethyldichlorosilane or hexamethyldisilazane treated silicas, and which does not adversely affect the fixture time of adhesive formulations. SUMMARY OF THE INVENTION The present application relates to cyanoacrylate com- positions which employ fumed silicas treated with a polydimethylsiloxane or a trialkoxyalkylsilane as a thixotropic additive. It has been unexpectedly found that such silicas when incorporated into cyanoacrylate compositions do not adversely effect the stability of the composition, display a significantly higher thixotropy ratio than the previously mentioned treated silicas and, in adhesive compositions, do not adversely effect fix- ture time even at levels of about 10-12% where the compositions become pasty and very difficult to stir or apply uniformly. . The inventive compositions may consist primarily of an appropriately stabilized cyanoacrylate ester mono- mer and the specified silica. However, it is preferred that a small amount of an organic polymer such as poly- methylmethacrylate be dissolved in the monomer. The inventive compositions are also useful in nonad- hesive applications such as nonflowable latent finger- print developing formulations. DETAILED DESCRIPTION OF THE INVENTION Fumed silicas which have been found to impart de- sired thixotropic properties to the inventive composi- tions fall into two categories. The first, and most pre- -ferred, are polydimethylsiloxane treated silicas such as Cab-O-Sil N70-TS TM, sold by the Cabot Corporation. The-silica has a carbon content of 5 weight percent and a surface area of 70M2/gm according to the manufac- turer. The second category of silicas usable in the inventive compositions are trialkoxyalkylsilanes. An example is Aerosil R805 TM , an experimental product available from Degussa Corporation. Aerosil R805 TM is a trime- thoxyoctylsilane treated silica having a surface area of 150 m2/ gm. The superior thixotropic performance of the inven- tive cyanoacrylate compositions is best obtained at sil- ica levels of about 4-8%. These properties were demon- strated by comparative testing of an ethyl cyanoacry- late formulation containing 6% of a 0.4-0.5 million mw polymethylmethacrylate and 6% hydrophobic silica. The composition was stabilized with 5 ppm methanesul- fonic acid (MSA) approximately 2500 ppm hydroqui- none and 5 ppm S02. The formulations were prepared with the aforementioned Cab-O-Sil N70-TS TM and Aerosil R805 TM as well as two hexamethyldisilazane treated silicas (Tellenox 500 TM solb by Tulco, Inc. and Wacker HDK 2000 TM sold by Wacker-Chemie) and three-dimethyldichlorosilane treated silicas (Aerosils R972 TM , R974 TM and R976 TM , all sold by Degussa). Table I gives the results of comparative Brookfield viscosity and acid level (calculated as ppm MSA) deter- minations. 4,477,607 3 4- TABLE I Brookfield Viscosity 2.5 RPM 20 RPM helipath helipath Ratio Total Comp. Silica (spindle) (spindle) 2.5/20 Acid A Cab-O-Sil N70-TS m 2.9 x 105 (TB) 4.2 x 104 (TB) 6.9 27 13 Aerosil R805 m 1.4 x 105 (TB) 2.8 x 1 (TB) 5.0 32 c Tellenox 500 m 4.4 x 104 (TB) 2.1 x 104 (TB) 2.1 26 D Wacker HDKZOOO m 3.3 x 102 (TA) 3.0 x 102 (TA) 1.1 29 E Aerosil R972 m 1.2 x 104 (Tc) 3.8 x 103 (TC) 3.2 33 F Aerosil R974 m 2.1 x 104 (TC) 5.2 x 103 (TC) 4.0 40 G Aerosil R976 m 2.6 x 104 (TC) 3.1 x 103 (TC) 3.2 39 As can be seen from the table, compositions A and B, which are within the invention, show much higher low shear viscosity and substantially higher thixotropic ra- tios than compositions C—G which are not within the invention. It was also observed that, when unagitated, compositions A and B were nonflowable gels whereas compositions C—G were all ungelled and pourable. Compositions similar to formulation E but com- pounded with sufficient Aerosil R972 TM to produce non-flowable gels comparable to those of compositions A and B in Table I have been prepared and have been observed to require longer fixture times than those of compositions A and B. This is believed to result from increased strong acid imparted by the dimethyldichlo- rosilane treated silica. The significantly higher acid numbers of compositions E—G is evidence that the di- chlorosilane treated silicas do contribute strong acid to the cyanoacrylate formulations. Both hexamethyldisilazine treated silicas had imprac- tically low thixotropic ratios as shown by Table I. Fur- thermore, at least one of the hexamethyldisilazane treated silicas appars to destabilize cyanoacrylate mono- mers. Formulation C, which utilizes the Tellenox 500 TM silica polymerized in less than one day in a sealed tube at 82° C. while compositions A, B and D—G all lasted at least l5 days under the same conditions. It is preferred that the cyanoacrylate compositions of the invention include a minor amount of dissolved or- ganic polymer. Suitable polymers include polyacrylates and polymethacrylates, polycyanoacrylates such as poly(ethyl cyanoacrylate), and poly(vinyl acetate) pol- ymers and copolymcrs. The organic polymers are pref- erably included witin the range of approximately 1-15% of the composition by weight. Preferably, the organic polymers are included in the range of 3—l0%. The inclusion of the organic polymer is recommended in order to prevent or significantly diminish the settling out of the silica from the inventive compositions. The compositions containing dissolved polymer are also observed to produce higher viscosities at equivalent silica concentrations and to recover thixotropic behav- ior faster after agitation than without dissolved poly- mers. Other additives, conventional within the cyanoacry- late formulation art, may be included within the compo- sitions of the invention without departing from the teaching hereof. Examples of such additives need not be 15 20 25 30 ‘35 40 45 50 55 60 65 specified since they are within the skill of those working in the art. As mentioned above, it would be desirable for latent fingerprint developing applications to have a nonflow- able form of cyanoacrylate with high vapor pressure. Gel compositions of the invention which have been stabilized to the point where they will not polymerize on contact with moisture are especially useful for such applications. The gel form pevents spillage and the over-stabilization guards against bonding of fingers (“finger-stick”) are other articles. A typical such formu- lation includes 88% methylcyanoacrylate stabilized with 0.2% methane sulfonic acid and 2500 hydroqui- none, 6% polymethylmethacrylate and 6% polydimeth- ylsiloxane treated silica. From the foregoing it can be seen that the invention is not limited by the specific examples set forth above, the invention being limited only as set forth in the fol- lowing claims. I claim: 1. In a composition comprising an a-cyanoacrylate ester monomer and a thixotropic agent, the improve- ment comprising that said composition is a non-flowa- ble gel, said thixotropic agent is present in an amount of 12% or less and is a fumed silica having a surface treated with a trialkoxyalkylsilane and the said compo- sition has a 2.5 RPM Brookfield viscosity of about 1X 105 or greater. 2. A composition as in claim 1 wherein the silica surface is treated with trimethoxyoctylsilane. 3. A composition as in claim 1 comprising an organic polymer dissolved in said monomer. 4. A composition as in claim 3 wherein said polymer is selected from polyvinyl acetate polymers and copoly— mers, polyacrylates, polymethacrylates and polycyanoacrylates. 5. A composition as in claim 1 stabilized so as to be nonpolymerizable on contact with moisture. 6. A composition as in claim 3 wherein said polymer is present at levels of between 1 and 15% by weight. 7. A composition as in claim 6 wherein said polymer is present at levels of between 3 and 10% by weight. 8. A composition as in claim 7 wherein said polymer is polymethylmethacrylate. 9. A composition as in claim 1 wherein said silica is present in an amount between about 4 and 8% by weight. 10. A composition as in claim 9 wherein said silica is present in an amount of about 6% by weight. Sit ill Ill Ill 31¢