Anionic Polymerization Inhibitor for Cyanoacrylate Adhesives

(11) (A) No. 1098533 (45) ISSUED 810331 (52) CLASS 260~453.5 C.R. CL. 400~94 2 (51) INT. CL. C07C 121/413 (19, (CA) cmmm PATENT (12, (54) ANIONIC POLYMERIZATION INHIBITOR FOR CYANOACRYLATE ADHESIVES (72) Schoenberg, Jules E., U.S.A. (73) Granted to National Starch and Chemical Corporation U.S.A. ((21) APPLICATION No. ‘ 330,736 , (22) FILED A ‘ 790627 (30) PRIORITY DATE U.S.A. (934,856) 730319 NO. OF CLAIMS 14 ~ NC DRAWING DISTRIBUTED BY THE PATENT OFFICE, OTTAWA. CCA-274 (3-80) 1038533} An adhesive composition based on 2-cyanoacryiate esters contains M /.\R 31 1981 A55TR as an anionic polymerization inhibitor an acid cheiate formed of boric acid or a derivative thereof and a seiected poiyhydroxy compound. The cheiate is convenientiy formed in situ in the 2—cyanoacry1ate ester, but may also be prepared separately and added to the ester directiy. An exempiary acid cheiate used as stabilizer is prepared from boric acid and pyrogaiioi. The resuiting stabiiized adhesive compositions have a rapid cure rate and may be used on a wide variety of substrates. messes #1042 JULES E. SCHOENBERG 10 20 ANIONIC POLYMERIZATION INHIBITOR FOR CYANOACRYLATE ADHESIVES This invention relates to stabilized 2-cyanoacrylate adhesive com- positions. More particularly, this invention is directed to a novel class of anionic polymerization inhibitors for 2-cyanoacrylate adhesive compositions. Adhesive compositions based on 2-cyanoacrylate esters belong to a class of adhesives known as reactive liquid adhesives. 2-Cyanoacrylate adhesives are single-part, low-viscosity adhesives which are character- ized by features such as (1) their ability to polymerize at room temperature without the use of an added catalyst when pressed between two substrates, (2) their rapid rate of cure, and (3) the strength of the bonds produced with a wide variety of substrates. Conventional A.adhesives, on the other hand, cure, for example, upon application of heat and pressure, addition of catalyst, or evaporation of a solvent. A general review of 2-cyanoacrylate adhesives can be found in I. _ Skeist's "Handbook of Adhesives“, New York: Reinhold Publishing Corporation, 1962, Chapter 31, p.. 409 — 414. Application of the 2-cyanoacrylate adhesive merely involves spreading a small sample thereof in a thin film between two substrates, pressing the substrates together, and allowing the resultant bond to cure. The adhesive develops sufficient strength after a short period of time to hold the substrates together until the adhesive completely polymerizes and builds up to its maximum bonding strength, 9/» / if 10 20 1@9853B Initiation of polymerization (cure) is generally believed to proceed through an anionic mechanism; the 2—cyanoacrylate adhesives have such a great tendency to polymerize that water itself is a sufficiently active initiator. Hence, when the adhesive is applied to a substrate and thereby exposed to atmospheric and surface moisture, cure normally begins within a relatively short period of time, generally less than one minute, and on many surfaces within a matter of a few seconds. The rapid cure rate of the 2-cyanoacrylate adhesives is particularly advantageous in production line applications. Due to their tendency to polymerize, 2-cyanoacrylate adhesive compositions normally contain one or more stabilizers. ‘To prevent anionic polymerization an inhibitor such as an acidic gas or a protonic or Lewis acid is normally added to the composition. Examples of acidic gases used for this purpose include sulfur dioxide, nitric oxide, carbon dioxide, hydrogen fluoride, etc. Known protonic acids include mineral acids such as hydrochloric or sulfuric acid, sulfonic acids, and carboxylic acids such as acetic, trichloroacetic, acrylic, methacrylic, and itaconic acid. Examples of anhydrides which are known anionic polymerization inhibitors are carboxylic acid anhydrides such as itaconic and maleic anhydride; phosphoric anhydrides such as phosphorus pentoxide; antimony pentoxide; sultones; acid chlorides; and the like. Anionic polymerization inhibitors which are Lewis acids include stannic chloride, ferric chloride, and boron trifluoride and its etherate complexes“ Typical patents disclosing these and other stabilizers are U. S. Pat. Nos. 2,756,251; 2,912,454; 2,926,188; 3,728,375; and 3,993,678; Jap. Pat. Publication No. 49~31619; and Ger. Often. 2,307,834- 10 20 1lG>98.E:38»e3 Free radical polymerization is generally inhibited in the 2-cyano- acrylate adhesives, if necessary, by adding phenolic—type compounds such as hydroquinone, pyrogallol, or tfbutyl catechol thereto. The acids used to stabilize the adhesive against anionic polymerization must be used with great discretion. Very strong acids, if added in large amounts, can lead to overstabilization of the adhesive; however, weak acids are generally not so effective as the stronger acids in achieving stabilization. In addition, all of the acids which act as anionic polymerization inhibitors exert a retarding effect on the cure rate of the adhesive, to a greater or lesser degree, depending on the specific acid used. Carboxylic acids, for example, generally retard the cure rate to a large extent. Accordingly, a class of effective anionic polymerization inhibitors for 2-cyanoacrylate adhesive compositions is provided herein which do not significantly retard the cure rate of the adhesive. The invention resides in the preparation of an adhesive composi- tion comprising a mixture of: A. at least 65% by weight of a monomeric ester of 2~cyanoacrylic acid of the general formula: 0 H2C =‘C - C — OR all wherein R is an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, a cyclohexyl group, or a phenyl group; and i A B. a stabilizing amount of an acid chelate formed from reaction of boric acid, or a derivative thereof, with a polyhydroxy compound capable 10 20: 1398533 -4- of forming the chelate and containing no reactive groups which can initiate anionic polymerization. Preferably, the boric acid derivative is an anhydride or borate ester derivative, and, most preferably, boric okide or a trialkyl borate. The applicable polyhydroxy compound generally has at least two hydroxyl groups attached to adjacent carbon atoms or to carbon atoms separated by one atom, depending on the orientation of the hydroxyl‘ groups, to form a five- or six-membered—ring chelate with the boric acid. It is preferred that at least one of the hydroxyl groups of the polyhydroxy compound which react to form the chelate is slightly acidic (e.g., attached to a carbon atom containing a multiple bond), to avoid destabilization of the adhesive. The preferred polyhydroxy compounds herein are ortho-dihydroxybenzenes, ortho-hydroxybenzoic acids, ali- phatic¢(-hydroxy acids, and aliphatic dicarboxylic acids. In one embodiment, the preformed chelate is added to the 2-cyano- acrylate adhesive in a separate step, in an amount of 10-600 ppm., based on the amount of 2-cyanoacrylate ester. In another embodiment, the chelate is formed in situ by adding the boric acid compound and polyhydroxy compound separately to the ester in quantities sufficient to provide a stabilizing amount of chelate in the adhesive.» The preferred amounts of boric acid compound and polyhydroxy icompound added are 5-1000 ppm. and S-500 ppm., respectively, based on monomeric ester. The boric acid chelates herein are themselves well known in they art; and those formed in aqueous solution are described, for example, in "Advances in Carbohydrate Chemistry" (edited by N. Pigman and M. lwolfrom), Vol. 4, New York: Academic Press Inc., 1949, pp. 189-210. 10 20 fi@‘E.“853‘£$ These acid chelates, which are known to be much stronger acids than boric acid, act as very effective stabilizers against anionic polymerization of 2—cyanoacrylate adhesives, being superior to the boric acid or the polyhydroxy compound alone. Unexpectedly, the chelates used herein are found to cause very little retardation in the cure rate of the adhesive. Unlike the boron trifluorides or etherates thereof used in the prior art as stabilizers for 2—cyanoacrylate adhesives, which compounds are Lewis acids, the chelates employed herein are protonic acids. As used herein, the term “adhesive composition“ refers to an adhesive comprising at least 65% by weight, based on the total composi- tion, of a monomeric ester or mixture of esters of 2—cyanoacrylic acid of the general formula given hereinabove. For purposes herein, the preferred ‘esters are those wherein the R group is an alkyl or alkenyl group having 1 to 4 carbon atoms, and, more particularly, a methyl, ethyl, allyl, or jsg; or Q-butyl group. Especially preferred esters for preparing the adhesives herein are methyl and ethyl 2-cyanoacry- lates due to their commercial availability. The above-mentioned monomeric esters of 2—cyanoacrylic acid may be ~ prepared by methods well known in the art such as those described in U.S. Patent Nos. 2,467,926; 2,467,927; and 3,254,111. As the novel feature herein, an acid chelate formed from reaction of a boric acid compound (i.e., boric acid or its derivatives) and a polyhydroxy compound defined hereinbelow is used to inhibit anionic polymerization of 2-cyanoacrylate esters. These chelates may be pre- pared separately and added to the 2-cyanoacrylate ester directly, or they may be formed in situ in the ester. when prepared separately, the 10 20 1038533 chelates are typically produced, in a known procedure, by heating the boric acid or derivative thereof with a stoichiometric amount, or a molar excess, of the appropriate polyhydroxy compound, generally in a solvent, until reaction is complete. The lower—boiling by-product thereby produced is driven off, during or after reaction; and the chelate product is then isolated from the reaction mixture and added directly to the 2-cyanoacrylate ester. The main advantage of preparing the chelate in a separate step is that the reaction by—product(s) (in particular, water, when boric acid or boric oxide are employed) is not introduced into the 2-cyanoacrylate ester. According to U.S, Pat. No. 3,728,375, the presence of water in the 2—cyanoacrylate ester has a detrimental effect on the performance of the adhesive. In another embodiment, the chelate is conveniently prepared in_ situ in the 2-cyanoacrylate ester by dissolving appropriate amounts of the polyhydroxy and boric acid compounds in the ester at room tempera- ture. The formation of the chelate, which is at all times in equili~ brium with the polyhydroxy and boric acid compounds, occurs rapidly. The exact structure of the chelates used as stabilizers herein cannot be determined with certainty and varies with the polyhydroxy and boric acid compounds and relative amounts thereof used. In general, the chelate is of the AB or-AB2 types, wherein A is the boric acid compound and B is the chelating ligand from the polyhydroxy compound. i It is, however, possible that a chelate of the A28 type could be formed when a polyhydroxy compound having several reactive hydroxyl groups such as tartaric acid is employed. while no one theory is being espoused, it is believed that the chelates described herein have one or .more of the following general structures reported in the literature: 10 20 so 'with boron as the central atom. H0\,0—C— ,o—c— (a) /13‘ HI’ :_‘- H0 « B \ I + H20 no o—c— O-(|3- (I) (II) I I - I I —c—o\,o—c- _\ -c—o\,o—c— ‘“ -c'-o»B~o-I- ”* ~— —(!:-o’B I. I I I H0" (III) (IV) I‘ I _ E _ 0‘ ,0 - E _ B-0-B (c) ~T-0’ *0-(I3- .(V) wherein the carbon atoms may be directiy bonded to each other or separated by one or more atoms. It can be seen that Structures (1) and (III) represent strong protonic acids, the former being an AB—type cheiate and the Iatter an AB2—type cheiate having a spirane structure The practitioner will recognize that more than one type of polyhydroxy compound may be used to form the AB- ‘ type cheiate, if the equiiibrium is favorabie, in which case an unsym- cmetricai spirane wiil be obtained. Also meant to be inciuded is a protonic acid, ABetype cheiate connecting aI1 four oxygen atoms together, I which cheiate couid be formed if four hydroxyi groups of one moiecuie of poiyhydroxy compound reacted with one moiecuie of boric acid com- pound to form the spirane. The compounds which are used to provide the boron atom in the m‘ in. 1098533 chelate are ortho—boric acid (commonly known, and referred to herein, as boric acid), or any derivatives thereof which form boric acid and/or tetravalent monoborate ions in the 2—cyanoacrylate ester, or are able to react directly with the polyhydroxy compound to form the chelate. The preferred boric acid derivatives herein are anhydrides and borate ester derivatives. As used herein, the term “anhydrides" refers not only to boric acid anhydrides such as boric oxide (B203) and meta- boric acid (HBO2), but also includes mixed anhydrides of boric acid and other acids such as carboxylic acids. Examples of borate ester derivatives include alkyl borates, which can be mono-, di-, or tri- substituted, such as, e.g., triethyl borate, and the like. The derivatives herein which are particularly preferred are boric oxide and trialkyl borates. The polyhydroxy compounds applicable herein may be aliphatic, alicyclic, aromatic, condensed aromatic (e.g., naphthylic), or arene compounds, all of which can additionally contain unsaturated groups and/or ether, ester or amido linkages, or heteroatoms. To be suitable herein, however, the polyhydroxy compounds must contain at least two hydroxyl groups and be capable of forming a stable chelate with boricg acid. In addition, these compounds must be at least somewhat soluble in the 2—cyanoacrylate ester employed and must have no groups such as amino groups which would initiate anionic polymerization and thus adversely affect the stabilizing properties of the chelate. The term "hydroxyl group" as used herein is meant to include the -OH groups such as are contained in alcohols, phenols, and carboxylic Vacids. ‘The preferred types of hydroxyl groups are those which are 10 20 slightly acidic, because any unreacted (non-chelated) hydroxyl groups which are non—acidic may act to initiate polymerization, thus destabilizing the adhesive, or may undergo transesterification with the 2—cyanoacrylate ester. An example of an acidic hydroxyl group is one attached to a carbon atom containing a multiple bond such as depicted below: _ c = c — on and 0 = c — on I when the hydroxyl groups are strongly acidic, however, the adhesive may become overstabilized, so that very acidic polyhydroxy compounds must be added only in small amounts. The types of polyhydroxy compounds which meet the requirements specified above include polyols, i.e., phenols and alcohols; hydroxy acids; and dicarboxylic acids. these categories, however, will form the desired chelates, as is ex- plained further in detail hereinbelow. A deciding factor in determining the suitability of the polyhy— droxy compound as a chelating agent for the boric acid is the stereo- chemistry of the hydroxyl groups, i.e., their orientation with respect to each other. In general, the polyhydroxy compound must have at least two hydroxyl groups attached to adjacent carbon atoms or to carbon atoms separated by one atom to form a stable five— or six—membered—ring chelate. The specific locations of the hydroxyl groups, however, depend on the polyhydroxy compound itself. For example, to be suitable as polyhydroxy compounds herein, aliphatic polyols (i.e., alcohols) must have at least two hydroxyl groups which are favorably situated for Not all of the compounds falling within, 10 20 3fil§‘i:T”@§.‘.s&:i;$ -10- the formation of a chelate. Non~cyclic 1,2-glycols do not form chelates because of the mutual repulsion of the hydroxyl groups. The presence of additional hydroxyl groups suitably located in the compound prevents this repulsion to enable formation of a chelate with boric acid. As the number of adjacent hydroxyl groups is increased, the two hydroxyl groups become more favorably situated to form the chelate. Thus, ethylene glycol and glycerol do not form boric acid chelates to any significant degree, while erythritol, adonitol, and xylitol are increasingly effective in forming the chelates. As to the applicable aliphatic hydroxy acids, thecxshydroxy acids such as, e.g., tartaric acid,c 60 III. Aged Viscosity 57.5 24.9 58.2 33.7 17.6 18.0 17.7 (cps-) 10 20 -22- Sulfur dioxide is a fairly effective stabilizer, but retards the cure rate of the adhesive considerably. It can be seen from the results that increasing the relative amount of boric acid added to the adhesive above a certain level results in little gain in adhesive stability, with substantial loss in cure rate. EXAMPLE 6 This example illustrates the use of boric acid derivatives in forming the chelate stabilizers herein. Four samples of 2—cyanoacrylate adhesive compositions designated as Samples A-D in Table VI were prepared by adding the indicated amount of either boric oxide (Samples A and B) or triethyl borate (Samples C and D) to the same 2-cyanoacrylate mdnomer used in Example 5. To Samples B and D were added 180 ppm. pyrogallol. The set time and aged vicosity of each adhesive was determined as in Example 5, and the results are indicated in Table VI. UWLEVI Amount of Amount of Amount of Aged L Pyrogallol Boric Oxide Triethyl Borate Set Time Viscosity Sample _ [ppm.} _ §gpm.2 gppm.} §sec.} fcps.) A O 56 0 . 15 ‘46.5 B 180 56 Q 15 23.3 C 0 0 236 15 ' 46.8 b 180 0 236 15 36.8 The results show that boric acid derivatives are also effective in forming chelates which increase adhesive stability without incurring loss in cure rate. EXAMPLE 7 This example illustrates the effect of increasing the relative concentration of boric acid using a different 2~cyanoacrylate monomer. Five samples of a Z-cyanoacrylate adhesive composition were prepared by adding the indicated amount of boric acid and 180 ppm. pyrogallol to a quantity of methyl 2-cyanoacrylate monomer containing 100 ppm. hydroquinone as free radical polymerization inhibitor and a small amount of sulfur dioxide as process stabilizer. The boric acid was soluble at all concentrations. Each adhesive was evaluated for set time and aged viscosity as in Example 5. The results are indicated in Table VII._ 1hBLE'vII Amount of g Boric Acid Set Time Aged Viscosity §ppm.} gsec.) fogs.) O 15 4.9 100 A 15 3.3 200 60 ' 3.3 400 T R > so I 3.2 600 ‘ > 60 3.2 The results indicate that, as in the case of ethyl Z-cyanoacry— late, excess boric acid in the methyl 2-cyanoacrylate~monomer results in little gain in adhesive stability, while the cure rate is retarded significantly. A EXAMPLE 8 This example illustrates the effect of increasing the relative amount of polyhydroxy compound on the stability of ethyl 2-cyanoacry- late. 1G98fi33 -24- The 2-cyanoacrylate ester of Example 5 was used to prepare five adhesive compositions by adding 100 ppm. boric acid and increasing amounts of pyrogallol up to 400 ppm. thereto. The resulting adhesives were evaluated for set time and aged viscosity as in Example 5, and the results are indicated in Table VIII. TABLE VIII Amount of Ratio of Moles Aged Pyrogallol Pyrogallol to Set Time Viscosity §ppm.l Moles Boric Acid §sec.} §cps.} 10 O 0 15 64.6 50 0.25 15 I 42.3 100 0.5 15 33.1 200 1.0 ' 15 35.5 400 , 2.0 30 39.7 The adhesive mixture containing 50 ppm. pyrogallol developed a pink color. It can be seen that the adhesive containing 400 ppm. pyrogallol showed decreased stability and cure speed. However, this amount of pyrogallol may be desirable when additional protection from free radical polymerization is desired or required in the adhesive. 20. I EXAMPLE 9 This example illustrates the use of various aliphatic polyhydroxy compounds to form the acid chelate. The 2-cyanoacrylate monomer of Example 5 having a viscosity of AI4.5 cps. was used to prepare several adhesives by adding boric acid and the indicated polyhydroxy compound thereto in the given amounts. Each adhesive was analyzed for set time and aged viscosity as in Example 5. The results are given in Table IX. 10 20 -25- TABLE IX Amount of Amount of Aged. Boric Acid Polyhydroxy Set Time Viscosity (ppm.} Polyhydroxy Compound Compound (ppm.) (sec.) (cps.) 0 none 0 15 64.5 100 none 0 15 46.9 0 Oxalic acid . 2HgO 30 15 51.1 100 »0xalic acid . 2H20 30 15 19.6 0 L-Tartaric acid 192 , 15 5 79.9 100 L-Tartaric acid 192 g 15 21.1 The results show that oxalic acid, a relatively strong diacid, does not overstabilize the adhesive when used in small amounts with iboric acid. In fact, oxalic acid forms a chelate which is very effec- tive as an anionic polymerization inhibitor. Tartaric acid, which is both a diacid and anczlhydroxy acid, also forms a chelate which acts as a good stabilizer. The_tartaric acid presumably forms a five-membered- ring chelate via the carboxyl and «Ghydroxyl groups (see Kustin and Pizer, "JACS", g; (1969) 317). EXAMPLE 10 This example illustrates the variation wherein the acid chelate is formed in a separate step and added to the 2-cyanoacrylate adhesive. A total of 15.5 g. (0.25 mole) boric acid and 63.1 g. (0.5 mole) pyrogallol were charged into a reaction vessel and refluxed in 1,2-di- chloroethane, using a reverse Dean-Stark receiver to collect the water by-product. Heating was discontinued when 13.5 g. (0.75 mole) water was collected and the reaction mixture became homogeneous. A total of 54.5 g. of product (84% of theoretical yield) crystallized on cooling. 10 20 KGQESBB -25- The boric acid chelate thus obtained was added in the indicated amount to the ethyl 2-cyanoacrylate monomer used in Example 5. The set time and aged viscosity of the adhesive (vs. the control containing no added stabilizer) were determined as in Example 5, and the results are indicated in Table X. TABLE X Amount of Chelate Set Time Aged Viscosity Added m. sec. c s. O 15 101.4 20 15 84.1 40 15 70.9 200 15' 34.9. 400 ~ 15 29.7 \ 800 60 29.4 It can be seen from the results that when the chelate is first prepared and then added to the adhesive in a separate step, the chelate acts very effectively to stabilize the adhesive. However, as in the _in situ variation, when the chelate is added in an amount which gives a boron concentration above a certain value, the cure rate of the adhesive is retarded significantly with little, if any, gain in stability. In summary, a novel class of anionic polymerization inhibitors for 2—cyanoacrylate adhesives is provided which are effective stabilizers sand do not significantly retard the cure rate of the adhesive. 10 15 20 25 _ 103’EI53i3 THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS: 1. An adhesive composition comprising a mixture of: A. at least 65% by weight of a monomeric ester of 2-cyanoacrylic acid of the general formula: - 0 H20 = C - C - OR IN wherein R is an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, a cyclohexyl group, or a phenyl group; and B. a stabilizing amount of an acid chelate formed from reaction of boric acid, or a derivative thereof, with a polyhydroxy compound capable of forming said chelate and containing no reactive groups which can initiate anionic polymerization. 2. The composition of Claim 1 wherein said chelate is present in said 2-cyanoacrylic acid ester in an amount of 10-600 ppm., based on the weight of the ester. 3. The composition of Claim 1 wherein said derivative of boric acid is an anhydride or a borate ester derivative. / 4. The composition of Claim 3 wherein said derivative of boric acid is boric oxide or a trialkyl borate. 5. The composition of Claim 1 wherein said polyhydroxy compound has at least two hydroxyl groups attached to adjacent carbon atoms or to carbon atoms separated by one atom, such that the chelate formed therefrom contains a five- or six—membered ring. 27 10 15 20 10998533 6. The composition of Claim 1 wherein at least one of the hydroxyl groups of said polyhydroxy compound which react to form said chelate is attached to a carbon atom containing a multiple bond. 7. The composition of Claim 1 wherein said polyhydroxy compound is selected from the group consisting of ortho~dihydroxybenzenes, ortho—hydroxybenzoic acids, aliphatic «?hydroxy acids, and aliphatic dicarboxylic acids. 8. The composition of Claim 7 wherein said polyhydroxy compound is selected from the group consisting of pyrogallol, catechol, salicylic acid, tartaric acid, and oxalic acid. 9. The composition of Claim 1 wherein there is additionally present a free radical polymerization inhibitor. 10. The composition of Claim 1 wherein there is additionally present a gaseous anionic polymerization inhibitor. 11. The composition of Claim 1 wherein R of said 2-cyanoacrylic acid ‘ester is a C1 — C4 alkyl group or a C2 — C4 alkenyl group. 12. In a process for preparing 2-cyanoacrylate adhesive compositions, the improvement which comprises adding to said composition a stabilizing amount of an acid chelate formed from reaction of boric acid, or a derivative thereof, with a polyhydroxy compound capable of forming said chelate and containing no reactive groups which can initiate anionic polymerization. 28 10 1fl9853—3 13. In a process for preparing 2—cyanoacry1ate adhesive compositions, the improvement which comprises adding to said composition an amount of boric acid, or derivative thereof, and an amount of a poiyhydroxy compound capabie of forming a cheiate with said boric acid or deriva- tive thereof and containing no reactive groups which can initiate anionic poiymerization, the amounts of said boric acid, or derivative thereof, and said poiyhydroxy compound being sufficient to provide a stabilizing amount of said cheiate in said 2-cyanoacryiate adhesive. 14. The process of Ciaim 13 wherein said boric acid or derivative thereof is added in an amount of 5-1000 ppm. and said poiyhydroxy compound is added in an amount of 5-000 ppm., based on the weight of the 2—cyanoacrylate ester. 2.3 2% fig. SECTON is not Present Cetzte Sectian est Absmie