Toughened Cyanoacrylate Compositions Containing Elastomeric Rubbers

United States Patent [191 0’Connor [54] [75] [73] [21] [22] [5 1] I [52] [53] [56] TOUGHENED CY ANOACRYLATES CONTAINING ELASTOMERIC RUBBERS Inventor: Assignee: Appl. No.: Filed: John T. O’Connor, Wallingford, Conn. Loctite Corporation, Newington, Conn. 339,952 Jan. 18, 1982 Int. Cl.3 .................... .. C08L 23/08; C08L 51/06; C08L 35/04 U.S. Cl. .................................. .. 525/295; 525/244; 525/256; 525/259; 525/261; 525/266; 525/276; 524/533; 524/850 Field of Search ....................................... .. 525/295 References Cited U.S. PATENT DOCUMENTS 2,794,788 3,507,822 4,038,345 4,042,442 6/1957 4/1970 7/1977 8/1977 Coover et al. ...................... .. 260/17 Miyami .............. .. .. 260/31.8 O’Sul1ivan et al. .. 525/295 Dombroski et al. .............. .. 525/295 [11] 4,440,910 [45] Apr. 3, 1984 4,102,945 7/1978 Gleave .............................. .. 525/315 4,196,271 4/ 1980 Yamada et al. . 525/242 4,313,004 1/ 1982 Kluger et al. ..................... .. 564/491 OTHER PUBLICATIONS “Cyanoacrylate Adhesives” Coover et al., B. Adhesives Materials pp. 569-579. Primary Examiner-—Carman J. Seccuro Attorney, Agent, or Firm—Walter J. Steinkraus; Eugene F. Miller [57] ABSTRACT Disclosed are cyanoacrylate adhesive compositions containing elastomeric polymers as fillers. Preferred fillers are the acrylic rubbers. Filler concentration range from about 0.540% by weight of the composition. Beneficial results include improved toughness of the cured compositions, as measured by impact, peel and tensile strength properties, particularly after exposure elevated temperatures which would ordinarily severely degrade the adhesive strength. 8 Claims, No Drawings 4,440,910 1 TOUGHENED CY ANOACRYLATES CONTAINING ELASTOMERIC’ RUBBERS ‘ ‘ BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to cyanoacrylate adhesive compositions which are improved: in toughness by in- corporation of certain organic fillers orthickeners. 2. Description of the Prior Art 3 Liquid cyanoacrylate compositions have long been known in the art as excellent adhesives. One of their primary shortcomings, however, has been their brittle- ness after cure. M A variety of fillers has been incorporated into cyano- _acrylate adhesive compositions to bring about certain changes in properties. U.S. Pat. No. 2,794,788 teaches thickening of cyanoacrylate adhesives by dissolving therein quantities of polymeric alkyl cyanoacrylates, as well as other compounds including polyacrylates, meth- acrylates and cellulose esters such as acetate, propionate and butyrate. U.S. Pat. No. 3,836,377 notes among the additional known thickeners polyvinyl ethers such as-polyvinyl- methyl ether. U.S. Pat. No. 3,692,752 discloses thick- ened cyanoacrylate solutions containing certain poly- ether acrylates/methacrylates, acrylic/methacrylic es- ters of bis(hydroxyalkyl) phosphonic acid derivatives, -and acrylic/methacrylic esters of tris(hydroxyalkyl) ' cyanuric acid derivatives. The preceding references relate to thickened homo- ‘geneous solutions containing organic compounds as thickeners. Various inorganic materials have also been proposed as fillers, which also have the effect of thick- ening the composition. Thus, U.S. Pat. No. 3,663,501 teaches preparation of a dental cement containing inert, finely,-divided solids such as fused silica, quartz and ‘aluminaL‘Similarly, U.S. Pat. No. 3,607,542 teaches the preparation of a water-resistant cyanoacrylate paste containing insoluble, inert fillers such as salts of cal- cium, titanium, zinc, tin, aluminum, iron and copper, among others. Non-soluble organic fillers are disclosed by U.S. Pat. No. 4,105,715. U.S. Pat. No. 4,102,945 discloses a cyanoacrylate adhesive composition thickened by a copolymer or . terpolymer resin capable of being dissolved or solvated by the cyanoacrylate monomer, resulting in signifi- cantly improved peel strength. Preferred thickeners are acrylonitrilebutadiene-styrene terpolymers, methacry- late-butadiene-styrene terpolymers, and vinylidene chlorideacrylonitrile copolymers. It has now been found that use of elastomeric poly- mers as fillers in cyanoacrylate systems results in supris- ingly beneficial properties. SUMMARY OF THE INVENTION According to the invention there is provided a cur- T able adhesive composition having improved toughness ‘when cured, comprising: (a) a monomeric ester of 2- cyanoacrylic acid, and (b) about 0.5 to about 20 parts by weight of the composition of an elastomer, preferably an acrylic rubber, said composition having improved toughness over the corresponding unfilled adhesive composition. It will be appreciated that the upper con- centration limit is related inversely to the molecular weight of the rubber and, therefore, could exceed 20 5 l0 15 20 25 30 35 45 50 55 65 2 parts by weight if a low molecular weight rubber hav- ing suitable performance were used. The composition also preferably contains one or more known acidic and free radical inhibitors, and op- tionally other functional additives for such purposes as further improving thermal resistance, providing color, accelerating the cure reaction, providing cross-linking, etc. The compositions of this invention exhibit substan- tially increased toughness in comparison with control cyanoacrylate adhesives. It has also been unexpectedly found that the compositions have excellent hot strength, i.e., resistance to thermal degradation of strength prop- erties. This finding was particularly surprising since the addition of low glass transition rubber would not be expected to improve hot strength. It has also been found that these compositions retard or eliminate what may be termed post-cure embrittlement or loss of toughness properties which occurs with cyanoacrylate adhesive bonds upon exposure to heat and then cooling to room temperature. DETAILED DESCRIPTION OF THE 3 INVENTION The benefits of this invention are achievable with essentially all adhesive compositions based upon cyano- acrylate esters. Most commonly, the esters have the formual: CN I CH2=C—COOR wherein R represents a C1.15alkyl, cycloalkyl, alkenyl, cycloalkenyl, phenyl or heterocyclic (such as furfuryl) radical. Naturally, the above R group can contain any linkages or substituents which do not adversely affect the monomer in the performance of its intended func- tion in the cyanoacrylate adhesive compositions, such as strongly basic substituents which may adversely af- fect the stability of the adhesive compositions. For pur- poses of this invention, the preferred monomers are those wherein R is a cyclohexyl or a C1 to C5 alkyl or alkenyl radical. Most preferred are methyl and ethyl cyanoacrylates. The esters may be used singly or in admixture. The above monmeric esters of 2-cyanoacrylic acid can be prepared by methods known in the art, such as those "described in U.S. Pat. Nos. 2,467,927 and 3,254,1 1 1. Cyanoacrylate ester adhesive compositions generally contain an anionic inhibitor, e.g., an acidic substance, soluble in the ester of 2-cyanoacrylic acid, which inhib- its anionic polymerization. A number of suitable inhibi- tors of anionic polymerization are well known in the art. The best known are the soluble acidic gases such as sulfur dioxide, sulfur trioxide, nitric oxide, and hydro- gen fluoride. More recently, inhibitors involving or- ganic sultones have been developed, the sultone being generally represented by the formula 4,440,910 3 wherein X is an organic radical joining the —S(O2)O——— group in a 4, 5, or 6 member heterocyclic ring, prefera- bly a 5 member heterocyclic ring. Preferably, X is a hydrocarbon group, althoughiit can contain any substit- uents or linkages which do not adversely affect the sultone for its intended use as a stabilizer of the adhesive composition. Another excellent class of stabilizers are the organic sulfonic acids, preferably having a molecu- lar weight less than about 400. To be optimally useful as a stabilizer in the adhesive compositions, the sulfonic acid should have a pKA value (dissociation constant in water) of less than about 2.8, and preferably less than about 1.5. Recently, the beneficial effects of certain stabilizer formulations based on sulfonic acids in combination with sulfur dioxide have been disclosed in copending U.S. patent application Ser. No. 06/157,149, filed June 6, 1980. Particularly preferred for purposes of this in- vention are combinations of methane sulfonic acid (MSA) or hydroxypropane sulfonic acid (HPSA) with sulfur dioxide. Preferred concentrations of sulfonic acids range from about 5 to about 100, more preferably about 10 to about 50, parts per million (based on mono- mer weight). The preferred concentrations of S02 range from about 15 to about 50 ppm for either acid. While not essential, the cyanoacrylate adhesive com- positions of this -invention generally also contain an inhibitor of free radical polymerization. The most desir- able of these inhibitors are of the phenolic type, such as quinone, hydroquinone, t-butyl catechol, p-methoxy- phenol, etc. The above inhibitors may be used within wide ranges, but the following general guidelines are representative of common practice, all figures being weight percent of the adhesive composition: acidic gases—from about 0.001% to about 0.06% by weight; sultones—from about 0.1% to about 10% by weight; sulfonic acids—- from about 0.0005% to about 0.1% by weight; free radical inhibitors—~from about 0.001% to about 1%. Other common additives for cyanoacrylate adhesive compositions are placticizers. Plasticizers serve to make the cured bonds less brittle and, therefore, more dura- ble. The most common of these plasticizers are C1 to C1oalkyl esters of dibasic acids such as sebasic acid and malonic acid. Other plasticizers, such as diaryl ethers and polyurethanes, also may be used, and a variety of other plasticizers is also known. In addition to the monomer, or mixture of monomers, the second required ingredient is an elastomeric poly- mer filler of at least one of the types described below. In order to perform suitably, the polymer, or mixture of polymers must be “compatible” with the cyanoacrylate monomer. By the term “compatible” is meant that the polymers do not significantly interfere with the storage stability of the adhesive composition. Moreover, as will be discussed further below, it appears that the polymers should be at least partially solvated by the cyanoacry- late monomer so that a homogeneous solution or sus- pension is formed. Molecular weight is considered to be a significant parameter in selecting suitable polymeric fillers of the types specified; however, selected of mo- lecular weight is deemed to be a matter of choice. Obvi- ously, such factors as solubility and thickening ability are affected, but selection of molecular weight in these respects is well within the skill of the art based upon routine experimentation. Each of the fillers of this invention is unique in that it imparts a higher toughness to standard adhesive bonds 10 15 20 25 30 35 45 50 55 65 4 formed using a cyanoacrylate adhesive composition ‘containing it than the toughness of similar bonds formed using the same composition without any fillers, or using the same composition filled by means other than mem- bers of the said group of alternative fillers. As already indicated, the fillers of this invention are organic polymers which are elastomeric, i.e., rubbery, in nature. Numerous chemical species fall into this cate- gory but, for illustration purposes, the following species have been found to be particularly useful: acrylic rub- bers (which are the preferred species); polyester ure- thanes; ethylene-vinyl acetates; fluorinated rubbers; isoprene-acrylonitrile polymers; chlorosulfinated poly- ethylenes; and homopolymers of polyvinyl acetate. The acrylic rubbers of the instant invention may be selected from a wide range of suitable materials. Most frequently these rubbers are either: (i) homopolymers of alkyl esters of acrylic acid; (ii) copolymers of another polymerizable monomer, such as lower alkenes, with an alkyl ester of acrylic acid or with an alkoxy ester of acrylic acid; (iv) copolymers of alkyl esters of acrylic acid; (V) copolymers of alkoxy esters of acrylic acid; (vi) mixtures of any of the above (i)-(v). Other unsatu- rated monomers which may be copolymerized with the alkyl and alkoxy esters of acrylic include dienes, reac- tive halogen-containing unsaturated compounds and other acrylic monomers such as acrylamides. It will be understood that esters or methacrylic acid tend to be relatively brittle; however, to the extent they provide beneficial results in toughness and thermal resistance, they are intended to be included within the scope of this invention. ~ . The choice of the elastomer will, to a large degree, dictate various properties and characteristics of the adhesive composition and such choices are easily deter- mined through general experimentation and known methods within the art. It is most effective to use elasto- mers whose molecular weight averages more than about 100,000, but any molecular weight greater than 5,000 would be expected to effect an improvement. As a principle of general guidance, the molecular weight should be high enough to produce toughening but not so high that the adhesive is very stringy and difficult to apply. It is also best to choose an elastomer whose Moo- ney viscosity is between 20 and about 60, and whose glass transition temperature (Tg) is 15° C. or less. The Mooney viscosity is defined as the amount of torque or resistance required to revolve a rotor at a constant speed in a polymer at a constant temperature. The Moo- ney viscosities of the preferred rubbers of the instant invention are described by the manufacturing as ML(1+4). The (l+4) symbol is to indicate the time involved in measuring the polymer viscosity. The “1” indicates a one minute pre-heating time, which is to bring the rotor to the designated temperature. The “4” indicates that the torque reading is to be taken after four minutes of rotating the rotor. The readings are mea- sured on a scale usually of 0-100. There are no specific units involved. These specific limitations are not abso- lute and various acrylic rubbers which do not fall within them may be within the scope of this invention. One preferred group of acrylic rubbers consists of the copolymers of ethyl acrylate with 2-chloroethyl vinyl ether in the approximate molecular ratio of 95:5, respec- tively. One such acrylic rubber is manufactured by the B. F. Goodrich Company, and is marketed under the name Hycar, such as Hycar 4021. Other preferred acrylic rubbers are the copolymers of methyl acrylate 4,440,9 10 5 and ethylene, manufactured by Du Pont, under the name of Vamac, such 'as‘Vamac N123 and Vamac B124. A third group of preferred rubbers is manufactured by American Cyanamid under the name-Cyanacryl and 6 ‘tine experimentation for any given acrylic rubber. These solid rubbers should show little or no sign of phase separation once fully dissolved in the monomer. The improved toughness of the cured compositions includes rubbers known as Cyanacryl R, Cyanacryl L 5 of this invention is manifested through various physical and Cyanacryl C. It has been found that Cyanacryl properties, e.g., 180° peel strength, impact strength and rubbers, when used “as is,” tend to destabilize the cya~ tensile shear strength. noacrylate monomer. This problem can usually be cor- These strengths are useful properties of an adhesive rected by washing the rubber with dilute HCL, rinsing bond, being parameters of what is loosely referred to as and oven drying it prior to adding it to cyanoacrylate. 10 the bond strength. Referring for simplicity to the proce- The Hycar rubbers are high molecular weight rub- dures of American Standard Test Methods, peel bers, typically having a Mooney viscosity at 100° C. of strength is determined in accordance with ASTM No. between about 25 and 60 ML(l +4), and a glass transi- D 903-49; impact strength is determined in accordance tion temperature range of about — 15° C. to about —‘40° with ASTM No. D-950; and tensile shear strength is C. Hycar 4021 has a Mooney viscosity of about 40 mini- 15 determined in accordance with ASTM No. D-1002. mum and a glass ‘transition temperature (Tg) of about The reader is referred to these standards for a full de- - -15" C. scription of the tests. Vamac N123 has a Mooney viscosity of about 30 and a glass transition temperature (Tg) of about —20° C.; V EXAMPLES - Vamac B124 has a Mooney viscosity of about 20.,Cya- 20 The following examples are intended to illustrate, not nacryl R is reported to have a Mooney viscosity of limit, the invention. _ . about 42 to about 51 and a glass transition temperature ‘ (Tg) of about —— 18° C., while Cyanacryl L and C have EXAMPLE 1 Mooney viscosities between about 30-48 and glass tran- The typical procedure for preparing an adhesive sition temperatures of -24” C. and —32° C. respec- 25 composition of this invention‘ is as follows: ' tively. These data have’ been procurred from the manu- 1. A high-purity (i.e., over 99% pure) alkyl cyanoac- facturers’ technical literature. rylate monomer is stabilized to 30 ppm MSA. The concentration range of elastomeric polymer 2. The elastomer is diced into small pieces and the should be about 0.5 to about 20 percent by weight, monomer is heated to 45°—50° C. preferably 1.5 to about 15 percent, based on the weight 30 3. The elastomer is added in several equal portions of the composition. 3 over a period of an hour; the batch temperature is While beneficial effects will be realized with rubbers raised to’ 50°—55° C. and maintained there. which are merely “compatible,” as defined herein, it is 4. The batch is stirred or milled, as necessary, for 3-4 preferred that the rubbers be dissolved in the monomer. hours at temperature until the elastomer is dis- All of the’ above preferred acrylic rubbers _are solid 35 solved. ' L materials which are preferably masticated on a mill 5. The batch is cooled to room temperature and addi- prior to dissolution in the acrylic ester monomers. Mas- tionally stabilized with 40 ppm S02. tication aides the dissolution by breaking‘ down the In order to ensure that the composition so prepared molecular weight and reducing the physical and chemi- has adequate stability for commercial use, a sample of cal cross-links. Properties of the cured adhesive compo- 40 the composition is placed in a test tube and kept in a sition will vary somewhat 'with the degree of mastica- water bath at 82° C. until incipient gellation can be tion, the effects of which may be determined with rou- detected in the composition. .TABLE 1 Physical Progrties of Filled Adhesives ~ Adhesive A B C D E x-1 x-2 x-3 F G H CA Type ethyl ethyl methyl methyl ethyl ethyl ethyl ethyl Stabilizer HPSA HPSA HPSA HPSA MSA MSA/S02 MSA/S02 MSA/SO; Level, ppm 30 18 22 22 10 30/40 30/40 30/40 Filler PMMA —— PMMA PMMA — B-124 13-124 13-124 0 Level, parts 5 -— 5 3 — 10 10 10 Impact Strength Room Temp. cure 24 hrs. 9.0 10.5 ’ 10.3 9.0 10.4 8.7 9.7 8.0 9.1 9.9 9.5 5 days 8.5 10.5 11.0 10.1 8.7 7.8 7.3 10.2 6 days 10.0 10.2 10.6 Post-cured 250' F. 2 hrs. 9.6 1.5 9.0 1.1 8.5 13.7 12.6 13.3 1.4 4.3 5.7 ‘24 hrs. ~ 2.0 2.1 1.2 1.0 1.8 1.0 2.0 2.2 180° Peel Strength Room Temp. cure» 7 V 24 hrs. . 23.0 14.0 32.3 31.6 4.0 5.5 20.0 6 days A . 34.3 32.0 10.0 3.0 18.0 Post-cured @ 250' F. 2 hrs. 39.5 41.0 0 1.0 1.0 24 hrs. 12.0 Tensile Shear Strength Room Temp. Cure 5 days 3020 3050 3690 3690 3010 3090 3380 2930 3370 3080 Post-cured @ 250° F. 2 hrs. 2750 2350 3190 590 2190 3770 3870 1700 2050 1730 24 hrs. 1410 1430 550 160 1470 3920 360 1070 1330 4,440,910 7 ,8 TABLE I-continued Physical Properties of Filled Adhesives Adhesive A B C _ D E X-l X-2 X-3 F G H Tested a 250" F. Post~cured 250° F. 1 hr. 620 1890 2430 2280 2170 2 hrs. 105 1600 2430 2270 2160 If no ellation occurs for at least two da s at 82° C. the ' g y » A ’ 1° EXAMPLE 3 stability is considered satisfactory. It will be understood that the procedure given above may not be optimum for all elastomers within the scope of this invention. For instance, the minimum adequate temperature may be found to be higher or lower, or the period of dissolution may be longer or shorter than one hour. Also, it may not be necessary in all cases to obtain complete solution of the elastomer in order to achieve 15 Using the procedure of Example 1, a variety of elas- tomeric polymers are made into adhesive compositions. Tests similar to those shown in Example 2 indicate that general improvement in toughness is obtained ‘over control compositions not containing a filler of this in- vention. Elastomers included in this Example, and typi- cal of the useful elastomers, are shown in Table II. TABLE II Trade blame Chemical Species Some Other Useful Elastomers Suggested Concentration Range % Source Q-thane PA-10 Polyester-urethane O.5—2.5 K.J. Quinn, Inc., Malden, MA PA-30 Polyester-urethane » O.5—2.5 PS-80 Polyester-urethane 3.0-3.5 Vamac N-123 Ethylene-methyl acrylate 3.0—3.5 E.I. duPont de Nemours, Wilmington, DE Vynathene EY-907 ‘. ’ Ethylene-vinyl acetate 4.0-1l.0 U.S. Industrial Chemical Co.. . New York, NY ——- —- ‘ Polyvinyl acetate (MW: l95,000) 16.0-17.0 Scientific Polymer Products, Inc. Ontario, NY Viton C-l0 Fluorinated rubber l5.0—30.0 E.I. duPont de Nemours E-60 Fluorinated rubber 15.0-30.0 Wilmington, DE Krynac 833 Isoprene-acrylonitrile less than 1.0 Polypar, Inc., Akron, OH the beneficial effects of this invention. Determination of I claim: such factors is considered to be a matter of routine experimentation within the skill of the art and cannot be conveniently described here for all useful formulations. EXAMPLE 2 Using the procedure described in Example 1,‘ several experimental adhesive compositions were prepared. Table I setsout a variety of comparative data showing the beneficial results of using Vamac B-124, an acrylic rubber of this invention. Compositions containing this filler are designated X-1, X-2 and X-3. Of particular interest are the superior thermal properties of these compositions. Samples A-C are commercially available cyanoacrylate adhesives of Loctite Corporation, New- ington, Conn. Samples A, C and D contain polymethyl methacrylate (PMMA), a commonly used filler of the prior art. Samples F-H are cyanoacrylate adhesive com- positions of other manufacturers available through nor- mal commercial channels. Sample E is cyanoacrylate monomer in “neat” form, i.e., not formulated into a commercially saleable adhesive composition. Impact strength data are expressed in ft.-lbs. per sq. inch; peel strength data are in lbs. per inch of width; tensile shear strength data are in lbs. per sq. inch. Con- centrations of materials are based on the composition as a whole, except for the stabilizer concentrations in ppm, which are based on the monomer weight. 35 40 45. 50 55 60 65 1. A curable adhesive comprising a substantially sol- vent-free mixture ofi (a) a monomeric ester of 2-cyanoacrylic acid, and (b) about 0.5% to about 20% by weight of an elasto- meric polymer selected from the group consisting " of elastomeric copolymers of a lower alkene mono- mer and (i) acrylic acid esters (ii) methacrylic acid esters or (iii) vinyl acetate. . 2. A composition of claim 1 wherein the elastomeric polymer is a copolymer of a lower alkene monomer with an alkyl ester of acrylic or methacrylic acid. 3. A composition of claim 1 wherein the elastomeric polymer is’ a copolymer of a lower alkene monomer with an alkoxy ester of acrylic or methacrylic acid. 4. A composition of claim 1 containing in addition a stabilizing amount of an acidic stabilizer and a free radi- cal inhibitor. . 5. A composition of claim 1 wherein the monomeric ester is methyl cyanoacrylate or ethyl cyanoacrylate. 6. A composition of claim 1 wherein the concentra- tion of the elastomeric polymer is about 1.5% to about 15% by weight. ' 7. A composition of claim 1 wherein the elastomeric polymer is an ethylene-methyl acrylate copolymer. 8. A composition of claim 1 wherein the elastomeric polymer is an ethylene-vinyl acetate copolymer. * * =|‘- * *