Adhesive Composition

United States Patent 119] Yamada et al. [11] 4,196,271 [45] Apr. 1, 1980 [54] ADHESIVE COMPOSITION [75] Inventors: Akira Yamada; Kaoru Kimura, both of Nagoya, Japan [73] Assignee: Toagosei Chemical Industry Co., Ltd., Tokyo, Japan [21] Appl. No.: 753,429 [22] Filed: Dec. 22, 1976 [30] Foreign Application Priority Data Dec. 24, 1975 [JP] Japan .............................. .. 50.153425 Dec. 26, 1975 [JP] Japan .............................. .. 50.155113 [51] Int. Cl.3 ..................... .. C08L 35/06; COBL 35/08 [52] U.S. Cl. .................................... .. 525/242; 106/35; 252/182; 252/188.3 R; 525/256; 525/257; 525/258; 525/259; 525/261; 525/262; 526/213; 526/216; 526/261; 526/291; 526/298 [58] Field of Search ............. .. 106/35, 36, 287 R, 311; 260/4654, 29.6 H, 29.6 M, 45.85 T, 875, 898; 32/1, 11, 14 R, 15; 526/298, 261; 525/242, 262; 252/182, 188.3 R [56] References Cited U.S. PATENT DOCUMENTS T889,004 8/1971 Hyche et al. 168/4 2,045,080 6/1936 Hagedorn .................... .. 260/296 M 3,155,685 11/1964 Prill et al. ................... .. 260/465.4 X 3,527,841 9/1970 Wicker, Jr. et al. 260/465.4 X 3,728,375 4/1973 Coover, Jr. et al. .... .. 260/465.4 3,741,926 6/1973 Jurecic . . . . . . . . . . . . . . . . .. 32/15 X 3,825,580 7/1974 Kato et al. . . . . . , . . . . . . , . , .. 260/465.4 3,961,966 6/1976 Brinkmann et al. . . 106/311 X 3,962,267 6/1976 Suzuki et al. . . . . . . . . . . . .. 32/15 X 4,089,830 5/1978 Tezuka et al. ................... .. 106/35 X FOREIGN PATENT DOCUMENTS 1529105 10/1978 United Kingdom . Primary Examz'ner——Theodore E. Pertilla Attorney, Agent, or Firm—Karl W. Flocks [57] ABSTRACT An adhesive composition comprising a 2-cyanoacrylate and at least one member selected from the group con- sisting of substituted or unsubstituted aliphatic carbox— ylic acids having three or more carboxyl groups, anhy- drides thereof, partial esters thereof, aromatic polycar- boxylic acids having three or more carboxyl groups and anhydrides thereof. This composition has a high impact strength, is suitable for bonding metallic substrates to one another and is also excellent in resistance to heat, weather, and water. 20 Claims, No Drawings 4,196,271 1 ADHESIVE COMPOSITION This invention relates to 2-cyanoacrylate-containing adhesive compositions. 2-Cyanoacrylate-containing adhesives are instanta- neously cured at room temperature with a minute quan- tity of moisture adsorbed on the surface of an adherend and are utilized in various industrial fields in bonding plastics, rubbers, glass, metals, etc. In metal-to-metal bonding, however, the 2-cyanoa- crylate-containing adhesive is used with limit where a high mechanical strength is required, because the adhe- sive is low in impact strength. That is, the adhesive is used only in temporary bonding. The 2-cyanoacrylate-containing adhesive contains an ester of 2-cyanoacrylic acid as the main component and very small amounts of a stabilizer, a thickener, a plasti- cizer, a crosslinking agent, etc. In general, the proper- ties of the adhesive as an instantaneous adhesive are sufficiently improved with an increase in purity of the 2—cyanoacrylate. The modificaton of a 2-cyanoacrylate- containing adhesive by the addition of other compo- nents has heretofore been hardly successful because the l0 15 20 addition of other components results in deterioration of 25 the instantaneous adhesion or deterioration of the stor- age stability. However, the present inventors have con- ducted extensive research on improvement in impact resistance, water resistance, weather resistance, heat resistance and the like in spite of the discrepancy that although the addition of a certain compound is neces- sary for modifying the 2-cyanoacrylate-containing ad- hesive, the reduction in purity of the 2-cyanoacrylate- containing adhesive results in loss of the property as instantaneous adhesive. An object of this invention is to provide a novel, 2-cyanoacrylate-containing adhesive composition. Another object of this invention is to provide an adhesive composition useful for metal-to-metal bond- mg. A further object of this invention is to provide a 2-cyanoacrylate-containing adhesive composition with a high impact strength. Other objects and advantages of this invention will become apparent from the following description. According to this invention, there is provided an adhesive composition comprising a 2—cyanoacrylate and at least one member (as an additive) selected from the group consisting of substituted or unsubstituted ali- phatic carboxylic acids having three or more carboxyl groups, anhydrides thereof, partial esters thereof, aro- matic polycarboxylic acids having three or more car- boxyl groups and anhydrides thereof. Thus, the resistance to impact, water, weather and 30 35 40 45 50 heat of a 2-cyanoacrylate is improved by the addition of 55 the above-said carboxylic acids or anhydrides or partial esters thereof to the 2-cyanoacrylate. The 2-cyanoacrylates for use in this invention include all of the common esters of 2-cyanoacrylic acid, such as alkyl 2-cyanoacrylates such as, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, amyl, n-hexyl, heptyl, n-octyl, 2-ethylhexyl, and dodecyl 2- cyanoacrylates; cycloalkyl 2-cyanoacrylates such as cyclohexyl 2-cyanoacrylate; alkenyl 2-cyanoacrylates such as allyl 2-cyanoacrylate; alkinyl 2-cyanoacrylates such as propargyl 2-cyanoacrylate; aralkyl 2-cyanoa- crylates such as benzyl 2-cyanoacrylate; aryl 2-cyanoa- crylates such as phenyl 2-cyanoacrylate; alkyl Cello- 60 65 2 solve 2-cyanoacrylates such as methyl Cellosolve 2- cyanoacrylate, and ethyl Cellosolve 2-cyanoacrylate; haloalkyl 2-cyanoacrylates such as 2-chloroethyl, hexa- fluoroisopropyl, and trifluoroethyl 2-cyanoacrylates; cyanoalkyl 2-cyanoacrylates such as 2-cyanoethyl 2- cyanoacrylate. The alcohol moiety of the 2-cyanoacry- late has preferably 1 to 12 carbon atoms, more prefera- bly l to 6 carbon atoms. The substituted or unsubstituted aliphatic polycar- boxylic acids having three or more carboxyl groups, anhydrides thereof and partial esters thereof, used as additives in this invention, include compounds in which three or more carboxyl groups are attached to alkanes, alkenes, cycloalkanes, and cycloalkenes which may have one or more substituents such as halogens, cyano, ether, ester, amido, amino, carbonyl, nitro, nitroso, phosphate, phosphite, thioether, sulfonate, urethane, isocyanate, phenyl, naphthyl, furan ring, and hydroxyl. Of the substituents, preferably are hydroxyl, ether, thio- ether, cyano, and ketone groups. Examples of the above compounds are aconitic acid, propane-l,2,3-tricarboxylic acid, l-butene—2,3,4-tricar- boxylic acid, l-hexene-2,4,6-tricarboxylic acid, citric acid, butane-1,2,3-tricarboxylic acid, butane-1,2,3,4—tet- racarboxylic acid, l-pentene-2,4,5-tricarboxylic acid, cyclohexane-1,2,4-tricarboxylic acid, cyclohexane- l,2,4,5-tetracarboxylic acid, tetrahydrofuran-tetracar- boxylic acid, cyclopentane-tetracarboxylic acid, 3- methylcyclohexene-4,5,6-tricarboxylic acid, ethane- l, l ,2,2-tetracarboxylic acid, carboxymethyloxysuccinic acid, 2-oxopropane-l,1,3-tricarboxylic acid, carbox- ymethylmercaptosuccinic acid, thiodisuccinic acid, ni- trilotriacetic acid, ethylenediamine-tetraacetic acid, 1-cyanopropane-1,2,3-tricarboxylic acid, l-phenylpro— pane-1,2,3-tricarboxylic acid, 1,2-dibromo-3-methylcy- clohexane-4,5,6-tricarboxylic acid, polyacrylic acid, polymaleic acid, maleic acid-styrene copolymers, and maleic acid—methyl vinyl ether copolymers; anhydrides of said acids; and partial esters of said acids. Especially preferable for use are aconitic acid, l-butene-2,3,4- tricarboxylic acid, butane-1,2,3,4—tetracarboxylic acid, ethane-1,l,2,2-tetracarboxylic acid, l-hexene-2,4,6- tricarboxylic acid, I-pentene-2,4,5-tricarboxylic acid, citric acid, and anhydrides of these acids. The aromatic polycarboxylic acids used as additives in this invention are those which are generally known and in which three or more carboxyl groups, a part or the whole of which may form anhydrides, are attached to an aromatic nucleus which may have one or more other substituents such as, for example, nitro, ester, ether, hydroxyl, halogen, alkyl, aryl, aralkyl, alkene and mercapto groups. Examples of the above compounds are hemimellitic acid, hemimellitic anhydride, trimellitic acid, trimellitic anhydride, trimesic acid, prehnitic acid, prehnitic anhy- dride, mellophanic acid, pyromellitic acid, pyromellitic anhydride, benzene—pentacarboxylic acid, mellitic acid, naphthalene-1,4-,5-tricarboxylic acid, naphthalene- l,4,5,8-tetracarboxylic acid, pyridine-2,3,4-tricarboxy- lic acid, pyridine-2,3,4,5-tetracarboxylic acid, and ben- zophenone-tetracarboxylic acid. Of these compounds, preferable are trimellitic acid, pyromellitic acid, ben- zophenone~tetracarboxylic acid, and anhydrides of these acids. Although the exact function of the above-mentioned additives in the present adhesive composition is not clear, these additives have three or more carboxyl groups in the free or anhydride form, and hence they 4,196,271 3 are very reactive. They are considered to have some effect on the anionic polymerization which takes place when the adhesive composition is cured. It is also con- sidered that these additives are incorporated into the cured polymer in some way to improve markedly the adhesiveness of the latter to the substrate surface, thereby improving the impact resistance and other properties of the cured adhesive in the case of bonding metals through the adhesive. The amount of the present additives added is in the range of from 0.1 to 50,0()0, preferably from I to 5,000 ppm. based on 2-cyanoacrylate. If the amount exceeds the upper limit of this range, both the setting time and the bonding strength are undesirably decreased. The present adhesive composition may contain other additives usually used in conventional 2-cyanoacrylate- containing adhesives, such as stabilizers, thickeners, plasticizers, and crosslinking agents. Stabilizers such as sulfur dioxide, sulfonic acids, sultones, lactones, boron fluoride, hydroquinone, hydroquinone monomethyl ether, catechol, and pyrogallol may be added in a pro- portion of 1 to 1,000 ppm. When a high—viscosity adhesive is required, 2 to 9 percent of polymers such as methyl methacrylate poly- mer, 2-cyanoacrylate polymer, and acrylic rubber may be added as a thickener. Plasticizers such as dioctyl phthalate, sebacates, and phosphates may be added to improve the flexibility of the cured polymer. In some cases, small amounts of polyfunctional vinyl monomers such as alkylene diacrylates, alkylene dime- thacrylates, trimethylolpropane triacrylate, triallyl iso- cyanurate and the like are added to make a three-dimen- sional structure in the cured polymer. The thermal resis- tance can be improved by the addition of such a mono- mer. The adhesive composition of this invention is used in the field where a high impact strength is required, as in the bonding of metals such as steel, stainless steel, cop- per, brass, aluminum, zinc, tin, ferrite, and the like. It can also be used as an instantaneous adhesive similarly to conventional 2-cyanoacrylate-containing adhesives in bonding other substrates such as plastics, wood, rub- ber, glass, porcelain, leather, paper, cloth, and yarn. The present adhesive is also improved, as compared with conventional 2-cyanoacrylate-containing adhe- sives, in resistance against water, heat, weather, vibra- tion, and chemicals. It manifests thus long-awaited per- formance characteristics of both instantaneous adhe- sives and structural adhesives. The uses for the present adhesive include, in addition to those for the conventional 2-cyanoacrylate-contain- ing adhesives, locking of a threaded fastener or bolt- and—nut; fixing of the portions of a rotary member such as a bearing, a motor shaft, a pulley, a gear, or an axle to be press-fitted to another member; sealing of a gasket, a flange, or a pipe joint; and potting. The invention is concretely explained below refer- ring to Examples which are merely illustrative but not limitative. In the Examples, all parts and percentages are by weight unless otherwise specified. EXAMPLE 1 Ethyl 2-cyanoacrylate was admixed with 1,000 ppm. of l-hexene-2,4,6-tricarboxylic acid, 500 ppm. of hydro- quinone, and 50 ppm. of sulfur dioxide. The resulting mixture was stirred for 2 hours at room temperature and then allowed to stand to obtain a composition. The IO 15 20 25 35 45 SS 60 65 4 composition thus obtained was subjected to adhesion test in the following way. Ten pairs of steel test pieces (ASTM D950-54; bond- ing area, I inl), which had been ground with a flat file, were bonded at 25° C. and 60% relative humidity and then allowed to stand under the same conditions for 24 hours. The setting time of the adhesive was less than one minute. The test thereof according to ASTM D950-54 showed an impact strength of more than 150 kg-cm/in? (all of the test specimens were not peeled.) The stability of the composition was satisfactory. Examples 2 to 16 and Comparative Example I Ethyl 2-cyanoacrylate was admixed with 1,000 ppm. of each of the aliphatic polycarboxylic acids shown in Table 1,300 ppm. of hydroquinone, and 10 ppm. of sulfur dioxide. The resulting mixture was stirred for one hour at 40° C. Steel test pieces (ASTM D950-54) were bonded through the thus obtained composition under the same conditions as in Example I and tested for impact strength according to the same method as in Example I. The results obtained are shown in Table 1. Table I Impact Example Aliphatic strength, No. polycarboxylic acid kg-cm/inz Stability 2 I-Butene-2,3.4- I45 Good tricarboxylic acid 3 Aconitic acid > 150 " 4 Aconitic anhydride >150 " 5 l-Pentene-2,4,5- > 150 " tricarboxylic acid 6 Ethane-l.l,2,2- > I50 " tetracarboxylic acid 7 Carboxymethyloxysuccinic > 150 acid 8 Tetrahydrofuran-tetrm > 150 " carboxylic dianhydride 9 Thiodisuccinc acid > I50 " I0 Nitrilotriacetic acid 85 " 1 I Propane-l,2,3- I43 " tricarboxylic acid 12 Partial ester of >150 " butane-I,2,3,4- tetracarboxylic acid (n-butyl esterilicution degree, 25%) I3 3-Methylcyclohexene- > 150 4,5,6»tricarboxylic acid I4 Polyacrylic acid 85 " I5 2-0xo-propane-l,l.3- > 150 " tricarboxylic acid I 6 Carboxymethyl- 140 " mercaptorsuccinic acid Compara- tive — 45 Example I EXAMPLE 17 To methyl 2-cyanoacrylate were added 200 ppm. of citric acid, 200 ppm. of hydroquinone monomethyl ether, 10 ppm. of methanesulfonic acid, and 2% of methyl methacrylate polymer. The resulting mixture was stirred for 3 hours at room temperature to obtain a composition having good stability. In the same manner as in Example I, 10 pairs of test pieces were bonded through the above composition and tested for impact strength. All of the test specimens 4,196,271 5 showed an impact strength of more than 150 kg-cm/in2. The setting time of the adhesive was 50 seconds. 6 formed in the same manner as in Example 19 are shown in Table 2. Table 2 CN Impact | Thickener or Aromatic polycarboxylic strength CHz=C—C02R plasticizer acid or anhydride thereof (steel- Amount Amount Amount steel Example added. added. added. bond), No. R parts Compound parts Compound parts kg-cm/inz 20 Ethyl 99.9 — — Benzene-penta- 0. I > 150 carboxylic acid 21 " 99.95 — — Pyromellitic 0.05 > 150 anhydride 22 " B9.95 Me metha- l0 Pyromellitie 0.05 > l50 crylate anhydride polymer “ Comp. " 90 Me metha- l0 — 0.01 28 Example 2 crylate polymer 23 " 99.99 — — Mellitic acid 0.01 > [50 24 Iso- 99.9 — — Pyromellitic D. l > I50 butyl anhydride Comp. lso~ l00 — — — — 32 Example butyl 25 Ethyl 99.99 — — Pyromellitic 0.01 142 anhydride EXAMPLE 18 n Butyl 2 cyanoacrylate was admixed with 1,000 EXAMPLE 26 ppm. of prpane-1,2,3-tricarboxylic acid, 300 ppm. of hydroquinone, and 2% of trimethylolpropane triacry1- ate to obtain a composition having good stability. In the same manner as in Example 1, test pieces were bonded through the above composition and tested for impact strength. The impact strength was found to be more than 150 kg-cm/inz. The same procedure as above was repeated except that the propane-l,2,3-tricarboxy- lic acid was not added. The impact strength obtained was only 40 to 50 kg-cm/inz. EXAMPLE 19 To 99.99 parts of ethyl 2-cyanoacrylate containing 10 ppm. of sulfur dioxide and 300 ppm. of hydroquinone as stabilizer was added 0.01 part of pyromellitic anhy- dride. The resulting mixture was brought into solution by shaking at room temperature to obtain an adhesive. Ten pairs of steel test pieces, the bonding areas of which had been ground with a smooth-cut file, were coated with the above adhesive and each pair of pieces were adhered while mildly pressing with fingers. The adhe- sive was cured in 40 sec. The test specimens thus ad- hered were allowed to stand at 20° C. and 50% relative humidity for 24 hours, and tested for impact strength in the same manner as in Example 1. Eight of the ten test specimens showed an strength higher than 150 kg-cm/inz, which is the maximum value of the tester and showed no peeling. The remaining two specimens also showed impact strengths as high as 140 and 145 kg-cm/in2, respectively. For comparison, a conventional ethyl 2-cyanoacrylate-containing adhe- sive was tested under the same conditions as mentioned above and found to have an impact strength of only 30 kg-cm/inz. Examples 20 to 30 and Comparative Examples and 3 Adhesive compositions were prepared in the same manner as in Example 19, except that the 2-cyanoacry- lates and carboxylic acids or anhydrides shown in Table 2 were used. The results of impact strength test per- 30 35 40 45 55 60 65 An adhesive was prepared by adding 0.] part of naph- tha1ene-l,4,5,8-tetracarboxylic acid to 99.9 parts of ethyl 2-cyanoacrylate containing as stabilizers 20 ppm. of p-toluenesulfonic acid and 300 ppm. of hydroquinone methyl ether. Ten pairs of steel test pieces, the bonding areas of which had been ground with a smoothcut file, were coated with the above adhesive and each pair were bonded to obtain test specimens. The thus adhered specimens were allowed to stand under an atmosphere at 20" C. and 50% relative humidity for 24 hours, and then tested for impact strength in the same manner as in Example 1. All of the test specimens showed no peeling and had an impact strength of more than 150 kg-cm/in2. The above adhesive showed good stability, no gelation having been observed after storage for several months at room temperature. EXAMPLE 27 An adhesive composition was prepared by adding 0.05 part of prehnitic anhydride to 99.95 parts of ethyl 2-cyanoacrylate containing 100 ppm. of hydroquinone alone and subjected to the impact test in the same man- ner as in Example 19. The impact strength was higher than l50 kg-cm/in2. The stability of the above adhesive composition was so good as to be comparable to that of the composition containing sulfur dioxide. EXAMPLE 28 To ethyl 2-cyanoacrylate containing 500 ppm. of hydroquinone monomethyl ether and 100 ppm. of S0; was added 300 ppm. of benzophenone-tetracarboxylic anhydride to obtain an adhesive. Each of ten pairs of steel test pieces were bonded through the resulting adhesive, and allowed to stand in an atmosphere at 20° C. and 50% relative humidity for 24 hours, after which the bonded test pieces were subjected to measurement of impact strength in the same manner as in Example 1. The average impact strength of the bonded pieces was 135 kg-cm/inz. 4,196,271 7 EXAMPLES 29 to 32 To ethyl 2-cyanoacrylate containing 500 ppm. of hydroquinone and 100 ppm. of S0; was added a poly- carboxylic acid (or anhydride) as shown in Table 3 to obtain an adhesive. Using the thus obtained adhesives, the same test as in Example 1 was effected to obtain the results shown in Table 3. Table 3 Amount lmpact Example added strength Sta- No. Additive (ppm) (kg-cm/inz) bility Aconitic acid 100 29 Benzophenone-tetra > I50 Good carboxylic anhydride 500 Butane-l,2.3-,4- tetracarboaylic 300 30 anhydride > 150 Good Trimellitic acid 500 l-Butene-2,3,4- Jl tricarboxylic acid 200 Pyromellitic > l50 Good anhydride 203 Pyromellilic acid I00 32 Benzophenone-tetnr 140 Good carboxylic anhydride l(X) EXAMPLE 33 An adhesive was prepared by mixing 30% of methyl 2-cyanoacrylate and 70% of ethyl 2-cyanoacrylate with l,000 ppm. of trimellitic anhydride and 1,000 ppm. of hydroquinone. Using the adhesive, the same impact strength test as in Example l was effected to find that the impact strength was more than 150 kg-cm/inz. EXAMPLE 34 An adhesive was prepared by mixing 60% of methyl 2-cyanoacrylate and 40% of ethyl 2-cyanoacrylate with 300 ppm. of thiodisuccininc acid, 50 ppm. of p-toluene sulfonic acid, 500 ppm. of catechol and 10 parts, per 100 parts of the two 2-cyanoacrylates, of polymethyl meth- acrylate having an average molecular weight of about 700,000 at 50° C. Using the adhesive, the same impact test as in Example 1 was effected to find that the impact strength of each test specimen was more than 150 kg- cm/inl. What is claimed is: 1. An adhesive composition comprising a 2-cyanoa- crylate and, as an impact strength improving additive, at least one member selected from the group consisting of substituted and unsubstituted aliphatic carboxylic acids having three or more free carboxyl groups, anhy- drides thereof, partial esters thereof having at least three free carboxyls, aromatic polycarboxylic acids having three or more free carboxyl groups and anhy- drides thereof, said additive being present in an amount sufficient to improve impact strength. 2. The adhesive composition according to claim 1, wherein the 2-cyanoacrylate is an alkyl 2-cyanoacry- late, an alkenyl 2-cyanoacrylate, a cycloalkyl 2-cyanoa- crylate, a cycloalkenyl 2-cyanoacrylate, an alkenyl 2- cyanoacrylate, an aralkyl 2-cyanoacrylate, an aryl 2- cyanoacrylate, an alkyl Cellosolve 2-cyanoacrylate, a haloalkyl 2-cyanoacrylate, or a cyanoalkyl 2-cyanoa- crylate. 3. The adhesive composition according to claim 2, wherein the 2-cyanoacrylate has an alcohol moiety containing 1 to l2 carbon atoms. 5 l0 15 20 25 30 35 45 50 55 65 8 4. The adhesive composition according to claim 3, wherein the alcohol moiety of the 2-cyanoacrylate has I to 6 carbon atoms. 5. The adhesive composition according to claim 1, wherein the 2—cyanoacrylate is methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, amyl, n-hexyl, heptyl, n-octyl, 2-ethylhexyl, dodecyl, cyclo- hexyl, allyl, propargyl, benzyl, phenyl, methyl Cello- solve, ethyl Cellosolve, 2-chloroethyl, hexafluoroiso- propyl, trifluoroethyl, or 2-cyanoethyl 2-cyanoacrylate. 6. The adhesive composition according to claim 1, wherein the additive is at least one member selected from the group consisting of substituted and unsubsti- tuted aliphatic carboxylic acids having three or more carboxyl groups, anhydrides thereof and partial esters thereof. 7. The/adhesive composition according to claim 6, wherein the additive is at least one compound selected from the group consisting of alkanes, alkenes, cycloal- kanes, and cycloalkenes each having three or more carboxyl groups; alkanes, alkenes, cycloalkanes, and cycloalkenes each having three of more carboxyl groups and at least one substituent selected from the group consisting of halogens, cyano, ether, ester, amido, amino, carbonyl, nitro, nitroso, sulfide, phosphate, phosphite, mercaptan, sulfonate, urethane, isocyanate, phenyl, naphthyl, furanyl, and hydroxyl; and anhy- drides and partial esters of these compounds. 8. The adhesive composition according to claim 6, wherein the additive is at least one compound selected from the group consisting of aconitic acid, propane- 1,2,3-tricarboxylic acid, l-butene-2,3,4-tricarboxylic acid, 1-hexene-2,4,6-tricarboxylic acid, citric acid, bu- tane-1,2,3-tricarboxylic acid, butane-l,2,3,4-tetracan boxylic acid, lpentene 2,4,5-tricarboxylic acid, cy- clohexane-1,2,4-tricarboxylic acid, cyclohexane-l,2,4,5- tetracarboxylic acid, tetrahydrofuran-tetracarboxylic acid, cyclopentane-tetracarboxylic acid, 3-methylcy- clohexene-4,5,6-tricarboxylic acid, ethane-l,1,2,2-tet- racarboxylic acid, carboxymethyloxysuccinic acid, 2- oxopropane- l , 1,3-tricarboxylic acid, carboxymethyl- mercaptosuccinic acid, thiodisuccinic acid, nitrilotri- acetic acid, ethylenediamine-tetraacetic acid, l-cyano- propane-1,2,3-tricarboxylic acid, 1-phenylpropane- 1,2,3-tricarboxylic acid, l,2-dibromo-3-methylcy- clohexane-4,5,6-tricarboxylic acid, polyacrylic acid, polymaleic acid, maleic acid-styrene copolymers, ma- leic acid-methyl vinyl ether copolymers, and anhy- drides and partial esters of these compounds. 9. The adhesive composition according to claim 6, wherein the additive is at least one member selected from the group consisting of aconitic acid, l-hutene- 2,3,4-tricarboxylic acid, butane—1,2,3,4-tetracarboxylic acid, ethane-1,l,2,2-tetracarboxylic acid, 1-hexene- 2,4,6-tricarboxylic acid, 1-pentene-2,4,5-tricarboxylic acid, citric acid, and anhydrides of these acids. 10. The adhesive composition according to claim 1, wherein the additive is at least one compound selected from the group consisting of aromatic polycarboxylic acids having three or more carboxyl groups and anhy- drides thereof. 11. The adhesive composition according to claim 10, wherein the additive is at least one compound selected from the group consisting of hemimellitic acid, hemi- mellitic anhydride, trimellitic acid, trimellitic anhy- dride, trimesie acid, prehnitic acid, prehnitic anhydride, mellophanic acid, pyromellitic acid, pyromellitic anhy- dride, benzene-pentacarboxylic acid, mellitic acid, .______:_T_ :, .. , 4,196,271 9 naphthalene-1,4,5-tricarboxylic acid, naphthalene- 1,4,5,8-tetracarboxylic acid. pyridine-2,3,4-tricarboxy- lic acid, pyridine-2,3,4,5-tetracarboxylic acid, and ben- zophenone-tetracarboxylic acid. 12. The adhesive composition according to claim 10, wherein the additive is at least one compound selected from the group consisting of trimellitic acid, pyromel- litic acid, and benzophenone-tetracarboxylic acid. 13. The adhesive composition according to claim 1, wherein the amount of the additive is 0.1 to 50,0()0 ppm. based on 2-cyanoacrylate. 14. The adhesive composition according to claim 1, wherein the amount of the additive is l to 5,000 ppm. based on 2-cyanoacrylate. 15. The adhesive composition according to claim 1, wherein the composition further contains a stabilizer, a thickener, a plasticizer, or a crosslinking agent. 16. The adhesive composition according to claim 1, wherein the 2-cyanoacrylate is methyl cyanoacrylate and the additive is aconitic acid. l0 l5 20 25 30 35 45 50 55 65 10 17. The adhesive composition according to claim 1, wherein the 2-cyanoacrylate is ethyl 2-cyanoacrylate and the additive is aconitic acid. 18. The adhesive composition according to claim 1 further comprising 2—9% of a polymeric thickener. 19. The adhesive composition according to claim 1 further comprising a small amount of an alkylene diac- rylate, alkylene dimethacrylate, trimethylolpropane triacrylate or triallyl isocyanurate. 20. In an adhesive composition comprising as the major ingredient a 1-cyanoacrylate, the improvement wherein said composition further including an impact strength improving additive selected from the group con- sisting of substituted and unsubstituted aliphatic carboxylic free acids having three or more car- boxyl groups, anhydrides thereof, partial ester thereof, aromatic polycarboxylic free acids having three or more carboxyl groups and anhydrides thereof, said additive being present in an amount of 0.1 to 50,000 ppm. of the free acid containing addi- tive based on the amount of 2-cyanoacrylate. t t O l t