Stabilized Cyanoacrylate Adhesives

 United States Patent [19] [11] 4,182,823 Schoenberg [45] Jan. 8, 1980 [54] ANIUNIC POLYMERIZATION INHIBITOR FOR CYANOACRYLATE ADHESIVES OTHER PUBLICATIONS {7s] Inventor: mes E. Schoenberg, Scotch Plains, §i"§3,8?1’9(g99)73,_1§‘3£§22_ . ' _ Primary Examiner-—Stanford M. Levin [731 Awflfleei N310“! St31'¢h_3nd Chem!“-I Attorney, Agent, or Firm—-Edwin Szala; Janet E. Hasak Corporation, Bndgewater, NJ. ' [57] ABSTRACI‘ pl] App!‘ No‘: 934356 An adhesive composition based on 2-cyanoacrylate [22] Filed; Am 13, 1973 esters contains as an anionic polymerization inhibitor an acid chelate formed of boric acid or a derivative thereof 1“. m2 ...................... .. 40; and a selected compound_ The chelate is U.S. G. ............................... .. 298; cgnvenjenfly formed in situ the 2.cyanoac!-ylate es- 423/500 ter, but may also be prepared separately and added to field Of s€al'Ch .................... .. the estcr dil-ect]y_ An exgmplary acid chelate used as . stabilizer is prepared from boric acid and pyrogallol. [563 References Cited The resulting stabilized adhesive compositions have a US PATENT DOCUMENTS rapid cure rate and may be used on a wide variety of 2,912,454 ‘ll/1959 McKeever ...................... .. 260/465.4 Substrates- 2,926,l88 2/1960 McKeever et al. ..... .. 260/465.4 3,728,375 4/ 1973 Coover et al. .................. .. 260/465.4 15 Claims, N0 Drawings 4,182,823 1 ANIONIC POLYMERIZATION INHIBITOR FOR CYANOACRYLATE ADHESIVES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to stabilized 2-cyanoacrylate adhesive compositions. More particularly, this inven- tion is directed to a novel class of anionic polymeriza- tion inhibitors for 2-cyanoacrylate adhesive composi- tions. 2. Description of the Prior Art 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 characterized 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 adhe- sives, on the other hand, cure, for example, upon appli- cation of heat and pressure, addition of catalyst, or evaporation of a solvent. A general review of 2-cyanoa- crylate adhesives can be found in I. Skeist’s “Handbook of Adhesives”, New York: Reinhold Publishing Corpo- ration, 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 to- gether, and allowing the resultant bond to cure. The adhesive develops sufficient strength after a short per- iod of time to hold the substrates together until the adhesive completely polymerizes and builds up to its maximum bonding strength. Initiation of polymerization (cure) is generally be- lieved to proceed through an anionic mechanism; the 2-cyanoacrylate adhesives have such a great tendency to polymerize the water itself is a sufficiently active initiator. Hence, when the adhesive is applied to a sub- strate 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 par- ticularly 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 inhibi- tor 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 hydrochlo- ric 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; phos- phoric anhydrides such as phosphorus pentoxide; anti- mony 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 5 10 15 20 25 30 35 45 50 55 60 65 2 3,993,678; Jap. Pat. Publication No. 49-31619; and Ger. Offen. No. 2,307,834. Free radical polymerization is generally inhibited in the 2-cyanoacrylate adhesives, if necessary, by adding phenolic-type compounds such as hydroquinone, pyro- gallol, or t-butyl catechol thereto. The acids used to stabilize the adhesive against ani- onic 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, it is an object of the present invention to provide a class of effective anionic polymerization inhibitors for 2-cyanoacrylate adhesive" compositions which do not significantly retard the cure rate of the adhesive. SUMMARY OF THE INVENTION The above and related objects are achieved in the preparation of 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 ll Hy_C=(|3--C-OR CN 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 the che- late and containing no reactive groups which can initi- ate anionic polymerization. Preferably, the boric acid derivative is an anhydride or borate ester derivative, and, most preferably, boric oxide 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 or- tho-dihydroxybenzenes, ortho-hydroxybenzoic acids, aliphatic ct-hydroxy acids, and aliphatic dicarboxylic acids. In one embodiment of this invention, the preformed chelate is added to the 2-cyanoacrylate 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 adhe- 4,182,823 3 sive. The preferred amounts of boric acid compound and polyhydroxy compound added are 5-1000 ppm. and 5-500 ppm., respectively, based on monomeric ester. The boric acid chelates of this invention are them- selves well known in the art; and those formed in aque- ous solution are described, for example, in “Advances in Carbohydrate Chemistry” (edited by W. Pigman and M. Wolfrom), Vol. 4, New York: Academic Press Inc., 1949, pp. 189-210. 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. Unex- pectedly, 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 adhe- sives, which compounds are Lewis acids, the chelates employed herein are protonic acids. DESCRIPTION OF THE PREFERRED EMBODIMENTS As used herein, the term “adhesive composition” refers to an adhesive comprising at least 65% by weight, based on the total composition, of a monomeric ester or mixture of esters of 2-cyanoacrylic acid of the general formula given hereinabove. For purposes of this inven- tion, 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 iso- or n-butyl group. Especially preferred esters for preparing the adhesives of this invention are methyl and ethyl 2-cyanoacrylates due to their commercial availability. The above-mentioned monomeric esters of 2-cyanoa- crylic acid may be prepared by methods well known in the art such as those described in U.S. Pat. Nos. 2,467,926; 2,467,927; and 3,254,111; the disclosures of which are incorporated herein by reference. As the novel feature of this invention, an acid chelate formed from reaction of a boric acid compound (i.e., boric acid or its derivatives) and a polyhydroxy com- pound defined hereinbelow is used to inhibit anionic polymerization of 2-cyanoacrylate esters. These che- lates may be prepared separately and added to the 2- cyanoacrylate ester directly, or they may be formed in situ in the ester. When prepared separately, the chelates are typically produced, in a known procedure, by heat- ing the boric acid or derivative thereof with a stoichio- metric 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 reac- tion 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 par- ticular, water, when boric acid or boric oxide are em- ployed) 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 of this invention, the chelate is conveniently prepared in situ in the 2-cyanoacrylate ester by dissolving appropriate amounts of the polyhy- droxy and boric acid compounds in the ester at room temperature. The formation of the chelate, which is at 10 15 20 25 30 35 45 50 55 60 65 4 all times in equilibrium 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 poly- hydroxy compound. It is, however, possible that a che- late of the A2B type could be formed when a polyhy- droxy compound having several reactive hydroxyl groups such as tartaric acid is employed. While the present invention is not to be limited by any one theory, it is believed that the chelates described herein have one or more of the following general structures reported in the literature: l .w) ll wherein the carbon atoms may be directly bonded to each other or separated by one or more atoms. It can be seen that Structures (I) and (III) represent strong pro- tonic acids, the former being an AB-type chelate and the latter an AB2-type chelate having a spirane struc- ture with boron as the central atom. The practitioner will recognize that more than one type of polyhydroxy compound may be used to form the AB-type chelate, if the equilibrium is favorable, in which case an unsym- metrical spirane will be obtained. It is also within the scope of this invention to employ a protonic acid, AB- type chelate connecting all four oxygen atoms together, which chelate could be formed if four hydroxyl groups of one molecule of polyhydroxy compound reacted with one molecule of boric acid compound to form the spirane. The compounds which are used to provide the boron atom in the chelate are ortho-boric acid (commonly known, and referred to herein, as boric acid), or any derivatives thereof which form boric acid and/or tetra- valent monoborate ions in the 2-cyanoacrylate ester, or are able to react directly with the polyhydroxy com- pound to form the chelate. The preferred boric acid derivatives for this invention 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 4,182,823 5 such as carboxylic acids. Examples of borate ester de- rivatives include alkyl borates, which can be mono-, di-, or trisubstituted, such as, e.g., triethyl borate, and the like. The derivatives herein which are particularly pre- ferred are boric oxide and trialkyl borates. 5 The polyhydroxy compounds applicable to this in- vention 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 com- pounds must contain at least two hydroxyl groups and be capable of forming a stable chelate with boric 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 acids. The preferred types of hydroxyl groups are those which are slightly acidic, because any unreacted (non-chelated) hydroxyl groups which are non-acidic may act to initiate poly- merization, thus destabilizing the adhesive, or may un- dergo 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: 10 15 20 25 30 -$=(|l-OH and 0:-(‘I-01-I When the hydroxyl groups are strongly acidic, how- ever, 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. Not all of the compounds falling within these categories, however, will form the desired chelates, as is explained further in detail hereinbelow. A deciding factor in determining the suitability of the polyhydroxy compound as a chelating agent for the boric acid is the stereochemistry of the hydroxyl groups, i.e., their orientation with respect to each other. In general, the polyhydroxy compounds 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 exam- ple, to be suitable as polyhydroxy compounds herein, aliphatic polyols (i.e., alcohols) must have at least two hydroxyl groups which are favorably situated for 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 fa- vorably 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. 35 45 50 55 65 6 As to the applicable aliphatic hydroxy acids, the a- hydroxy acids such as, e.g., tartaric acid, a-hydroxya- cetic acid, and whydroxypropionic acid are suitable polyhydroxy compounds, herein; whereas the B- hydroxy acids such as B-hydroxyacetic acid, lactic acid, and /3—hydroxybutyric acid do not normally form che- lates; With the alicyclic polyhydroxy compounds, forma- tion of chelates depends on the size of the cyclic ring of the compound and on the relative orientation of the hydroxyl_ groups. While five-membered-ring polyhy- droxy compounds (e.g., cyclopentane-1,2-diols or 2- hydroxycyclopentane carboxylic acids) having the hy- droxyl groups in the cis positions form chelates, the corresponding trans isomers do not because of unfavor- able geometry. Thus, while the cis isomers of cyclopen- tane-l,2-diol, l-methylcyclopentane-1,2-diol, indane- 1,2-diol, tetramethylenesulfone-2,3-diol, 2-methyltet- ramethylenesulfone-2,3-diol, l,4-dimethyltetramethy- lenesulfone-2,3-diol, 2-hydroxycyclopentane carboxylic acid, and 5-methyl-2-hydroxycyclopentane carboxylic acid form chelates with boric acid, their trans counter- parts are inactive. ' . In the case of the six-membered-ring, alicyclic poly- hydroxy compounds, however, neither the cis nor the trans isomers are effective as chelating agents. Thus, for example, cyclohexane-1,2-diols and 2-hydroxycy- clohexane carboxylic acids do not form chelates with boric acid compounds, regardless of the stereochemis- try of the hydroxyl groups. Because the substituents in aromatic compounds are rigidly held in the plane of the benzene ring, the aro- matic polyhydroxy compounds suitable herein must contain at least one hydroxyl group which is ortho to another hydroxyl group or carboxyl group on the ring. Thus, ortho-dihydroxybenzenes and ortho-hydrox- ybenzoic acids (and their condensed aromatic analogs) form chelates, whereas the meta- and para-isomers of these compounds do not.‘ As specific examples, 1,2- hydroxybenzene (catechol), 1,2,3-trihydroxybenzene (pyrogallol), 1,2,4-trihydroxybenzene (hydroxyhy- droquinone), l,2- and 2,3-dihydroxynaphthalene, o- hydroxybenzoic acid (salicylic acid), 2,4-hydroxyben- zoic acid, 3,4-dihydroxybenzoic acid (protocatechuic acid), 2,4,5-trihydroxybenzoic acid, and 3,4,5-trihy- droxybenzoic acid (gallic acid) and esters thereof are effective chelating agents; while 1,3-dihydroxybenzene (resorcinol), 1,4-dihydroxybenzene (hydroquinone), 1,3,5-trihydroxybenzene ‘ (phloroglucinol), 1,3-dihy- droxynaphthalene, and m- and p-hydroxybenzoic acid are unable to form chelates. Certain diacids with adjacent carboxyl groups or carboxyl groups separated by one carbon atom are also suitable polyhydroxy compounds herein, if the other criteria mentioned above are met, but only when the carboxyl groups are properly oriented to form the che- late. Preferred diacids herein are aliphatic dicarboxylic acids. Specific compounds within the categories given above which are particularly preferred are pyrogallol, catechol, salicyclic acid, tartaric acid, and oxalic acid. The amount of chelate to be added to the adhesive when the chelate is prepared separately depends on the molecular weight of the chelate, the 2-cyanoacrylate ester employed, and the degree of stabilization required. Generally, however, the concentration range of inhibi- tor used to stabilize the ester effectively is 10-600 ppm. of the total composition. 4,182,823 7 The chelate which is formed in situ by adding the reactant compounds to the adhesive must be present in an amount sufficient to stabilize the adhesive. This amount cannot be precisely defined, as it will depend on the equilibrium constants of the reaction between poly- 5 hydroxy and boric acid compound, which constants in turn depend on the 2-cyanoacrylate ester and the partic- ular reactants used. The absolute and relative amounts of polyhydroxy and boric acid compounds to be added depend on many factors, including the specific com- pounds and ester used, and the degree of stabilization required. The amount of polyhydroxy compound added depends on, for example, its strength as a chelating agent, molecular weight, solubility, the number, type and acidity of its hydroxyl groups, and the amount of 15 boric acid added. The polyhydroxy compound is typi- cally used in amounts of from about 5 to 500 ppm., based on the weight of 2-cyanoacrylate ester, but more or less may be added as necessary to obtain the desired degree of stabilization without incurring loss of cure rate. The amount of boric acid compound added to the adhesive is also determined by many factors, but is generally from about 5 to 1000 ppm., with more or less being added if needed or desired, depending on the molecular weight and equilibrium constant of the par- ticular compound employed. For example, boric acid is typically added in amounts of about 5 to 100 ppm., while an octyl borate would be required in considerably greater amounts to provide the same or comparable molar equivalents of boron. The molar ratio of polyhy- droxy to boric acid compound is likewise dependent on the equilibrium of the chelating reaction involved and thus on the types of compounds and 2-cyanoacrylate esters used. If the polyhydroxy compound, for example, is a moderately strong acid, i.e., with pKa less than 3, the molar ratio should be low to prevent, or at least minimize, retardation of cure by uncomplexed acid. Similarly, if the polyhydroxy compound is neutral, its concentration should be kept low relative to the boric acid compound to prevent initiation of polymerization or transesterification with the 2-cyanoacrylate ester. However, if the polyhydroxy compound is also em- ployed as a stabilizer against free radical polymeriza- tion, the molar ratio of polyhydroxy to boric acid com- pound will normally be high so that an excess of free radical polymerization inhibitor will be present. In gen- eral, the relative amount of polyhydroxy compound will range from about 0.1 to 10 moles per mole of boric acid compound. The chelates used as stabilizers in this invention may 50 be used alone or in conjunction with other inhibitors of anionic polymerization. It is generally desirable with the relatively volatile methyl and ethyl 2-cyanoacry- lates to include a gaseous inhibitor such as sulfur dioxide in the formulation to prevent the polymer from forming on the walls of the container above the liquid level. Generally, sulfur dioxide is used as a process stabilizer in the synthesis of the 2-cyanoacrylate ester, as typically described in U.S. Pat. No. 2,756,251. The sulfur dioxide passed through the system during several stages of the processing builds up to a high concentration, which is then removed by pulling vacuum on the ester upon completion of the synthesis procedure. It may not be necessary to add inhibitors of free radi- cal polymerization to the adhesive when the polyhy- droxy compound used to form the chelate is a phenolic compound such as catechol or pyrogallol, which are effective free radical scavengers themselves. Excess 20 25 30 35 45 55 10 60 65 8 polyhydroxy compound may be present from the reac- tion in situ, or the chelate may partially hydrolyze in the adhesive to regenerate the polyhydroxy compound, which will act to inhibit free radical polymerization if it is a scavenger. However, under certain demanding storage conditions, or when the polyhydroxy com- pound used is ineffective as a free radical scavenger (e.g., salicyclic acid), it may be desirable to add a free radical inhibitor. As a suitable inhibitor for this purpose, any one of a wide variety of compounds known in the art to stabilize 2-cyanoacrylate adhesive compositions against free radical polymerization is applicable. Such inhibitors include phenolic compounds such as hydro- quinone, t-butyl catechol, catechol, p-methoxyphenol, and the like. The conventional free radical polymeriza- tion inhibitor, like the anionic polymerization inhibitor, is normally added during the processing of the 2- cyanoacrylate ester. Hence, a free radical polymeriza- tion inhibitor is generally introduced into the distillation vessel and the receiver to stabilize the ester in the syn- thesis thereof. As a result, commercially available 2- cyanoacrylate esters may already contain a certain amount of a conventional free radical polymerization inhibitor such as those mentioned hereinabove. More such inhibitor, however, may be added thereto if greater stability is desired. The total amount of such inhibitor which will be effective for stabilization pur- poses will range from 10 to 500 ppm. of the total com- position. There may also be present in the adhesive composi- tions of this invention various other optional ingredients including, for example, plasticizers and thickeners. Plas- ticizers improve the aging characteristics of the cured bonds by lessening the brittleness thereof. For best per- formance the amount of plasticizer to be used should not exceed 20% by weight of the total composition. Suitable plasticizers include monofunctional and difunc- tional aliphatic esters of acids having 1 to 10 carbon atoms such as, for example, dimethyl- and dioctylseba- cate, and esters of malonic acid, difunctional aromatic esters, and alkyl and aromatic phosphates and phospho- nates. Thickeners, which may be used in amounts of up to 25% by weight, depending in part on their thickening power at room temperature, serve to increase the vis- cosity of the adhesive so that it may be more easily applied. Among the suitable thickeners for this purpose are included, for example, polymeric alkyl 2-cyanoacry- lates, cellulose esters including cellulose acetate buty- rate, acrylate resins such as poly (methyl methacrylate) and poly (ethyl methacrylate), and poly (vinyl alkyl ethers) such as poly-(vinyl methyl ether). The adhesive compositions of the present invention generally may be prepared by adding a given amount of the preformed chelate to the 2-cyanoacrylate ester and mixing at room temperature until the chelate is thor- oughly dissolved in the ester. Other anionic polymeriza- tion inhibitors desired may already be present in or added to the ester before the chelate is dissolved therein. Any further optional ingredients desired, in- cluding the free radical polymerization inhibitor, may be added either prior to or following the addition of the acid chelate. In the in situ variation the boric acid com- pound and polyhydroxy compound are mixed in with the 2-cyanoacrylate ester at room temperature in the desired proportions. If boric acid itself is used, it may need to be ground before being added to the ester to aid in its solubility. As with the addition of the preformed chelate, any optional ingredients may be added before 4,182,823 9 or after addition of the boric acid and polyhydroxy compound. The resultant adhesive composition may be used in a variety of applications, including the bonding of house- hold articles, precision instruments, optical lenses, and 5 the like. I The following examples will demonstrate the efficacy of the 2-cyanoacrylate adhesive compositions of this invention. In these examples all percentages and parts are given by weight, unless otherwise specified. The present adhesive compositions are evaluated on the basis of the following two test procedures: 1. SET TIME TEST One drop of test adhesive is placed near one edge of a 2.54 cm. by 2.54 cm. by 0.48 cm. phenolic chip. The 15 mating surface of a second chip of the same dimensions is quickly placed thereover and positioned such that half of each chip overlaps the other chip. The lamina- tion is immediately clamped together by means of a #50 medium spring clip. At 15-second intervals an attempt is made to pull the two chips apart using a light peel force. The “set time” is related to the cure rate, and is defined as the time interval between the initial application of the adhesive and the final time at which the chips can no longer be pulled apart manually. II. AGED VISCOSITY TEST The test adhesive is sealed into an aluminum tube and aged for 10 days at 70° C. If the initial viscosity of the 2-cyanoacrylate ester is already somewhat high, the aging time is reduced to 7 days. Kinematic viscosity of 30 the adhesive, expressed in centipoise (cps.), is then mea- sured using a calibrated #300 Cannon-Fenske viscome- ter. A low viscosity indicates good stability, while a viscous sample indicates poor stabilization of the adhe- sive. 10 20 25 35 EXAMPLE 1 This example illustrates the effectiveness of the stabi- lizer of this invention using the in situ variation. Five samples of the 2-cyanoacrylate adhesive compo- sitions were prepared by adding the indicated amount of the given additive in Table I to a quantity of methyl 2-cyanoacrylate containing about 100 ppm. hydroqui- none as free radical polymerization inhibitor and a small amount of sulfur dioxide for protection from vapor 45 phase polymerization. Each adhesive formulation was tested for set time and aged viscosity using the proce- dures described above. The results are indicated in Table I. 40 TABLE I 50 Adhesive A B C D E I. Additive (ppm.): Phosphorus pemoxide 0 10 0 0 O ortho-Boric acid 0 0 43 o 43 55 Pyrogallol O O 0 200 200 II. Set Time (sec.) 15 45 15 15 15 III. Aged Viscosity (cps.) 6.4 3.8 5.1 5.0 3.6 As the results clearly show, boric acid and pyrogal- 60 lol, which are weak acids (pK,, of about 9) somewhat improve the stability of the adhesive. When combined, however, these two compounds form an acidic chelate which is a far more effective stabilizer than either com- pound alone. In its stabilizing effect, this chelate is com- parable to phosphorous pentoxide, a well-known inhibi- tor. With regard to effect on cure rate, however, the chelate is superior to phosphorus pentoxide because the 65 10 latter causes an undesirable retardation in cure, while the chelate does not adversely affect the cure rate. EXAMPLE 2 This example illustrates the stabilizer of this invention using a different monomer. Four samples of 2-cyanoacrylate adhesive composi- tions were prepared by adding either 100 ppm. hydro- quinone or 180 ppm. pyrogallol as free radical polymer- ization inhibitor to a butyl 2-cyanoacrylate monomer which was freshly distilled under a nitrogen atmo- sphere. A given amount of boric acid was added to two of these compositions. Each adhesive was then tested for set time and aged viscosity as in Example 1, and the results are indicated in Table II. TABLE II Amount of Free Radical Boric Acid Set Time Aged Viscosity Inhibitor Added (ppm.) (sec.) (cps.) 0 15 6.3 Hydroquinone { 26 15 6.3 0 15 5.7 Pyrogallol { 26 15 4.0 Due to the presence of a free radical inhibitor in all of the adhesives (which is required for stability at 70° C.), the aged viscosity obtained for each adhesive mostly reflects stabilization against anionic polymerization. Because the hydroxyl groups of hydroquinone are para to each other and thus cannot form acidic chelates with boric acid, hydroquinone and boric acid together are ineffective as a stabilizer, whereas pyrogallol and boric acid form a chelate which acts as a suitable anionic polymerization inhibitor. EXAMPLE 3 This example illustrates the use of various aromatic polyhydroxy compounds which can be used in accor- dance with this invention. 6 Six samples of 2-cyanoacrylate adhesive composi- tions were prepared by adding boric acid and the indi- cated polyhydroxy compound in the indicated amounts to an ethyl 2-cyanoacrylate monomer containing 75 ppm. hydroquinone and a small amount of sulfur diox- ide to prevent vapor phase polymerization. The molar concentrations of each polyhydroxy compound added were approximately the same, and they were four times greater than the molar concentration of boric acid. Each adhesive was evaluated for set time and aged viscosity as in Example 1, with the results summarized in Table III. TABLE III Amount of Amount of Set Aged Polyhydroxy Polyhydroxy Boric Acid Time Viscosity Compound Compound (ppm.) (ppm.) (sec.) (cps.) none 0 0 15 17.8 none 0 20 15 17.8 Catechol 136 20 15 9.2 Pyrogallol 177 20 15 7.4 Salicylic acid 190 O 30 13.0 Salicylic acid 183 20 30 6.5 It can be seen from the results that all of the adhesives containing the chelates are effectively stabilized over the control adhesive containing boric acid alone. Sali- cylic acid, which is a moderately strong acid, suffi- 4,182,823 11 ciently stabilizes the adhesive, but retards the cure rate when used alone at these concentrations. However, when boric acid is added together with the salicylic acid, the chelate formed thereby acts as an even more effective stabilizer which does not further reduce the 5 cure rate. EXAMPLE 4 This example illustrates the effect of increasing the 12 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 deriva- tives in forming the chelate stabilizers of this invention. Four samples of 2-cyanoacrylate adhesive composi- tions designated as Samples A-D in Table VI were prepared by adding the indicated amount of either boric relative amount of boric acid on the stability of ethyl 10 oxide (Samples A and B) or triethyl borate (Samples C 2-cyanoacrylate. and D to the same 2-c anoacr late monomer used in Y Y The 2-cyanoacrylate ester of Example 3 was used to Example 5. To Samples B and D were added 180 ppm. prepare nine adhesive compositions by adding the given pyrogallol. The set time and aged viscosity of each amount of pyrogallol and boric acid thereto. The result- adhesive was determined as in Example 5, and the re- ing adhesives were evaluated for set time and aged 15 sults are indicated in Table VI. viscosity as in Example 1, and the results are summa- TABLE VI rized in Table IV. Amount of TABLE IV Amount of Amount of Triethyl Set Aged Amount of Amount of Ratio of Moles Aged Sam— Pyrogallol Boric Oxide Borate Time Viscosity Pyrogallol Boric Acid Pyrogallol to Set Time Viscosity 0 . "‘° (PP'“" (l’l""" (P‘’"‘') (S‘‘‘’‘’ “"5" (ppm.) (ppm.) Moles Boric Acid (sec.) (cps.) A 0 56 0 15 46.5 0 0 ° 15 159 E “*8 53 232 ii 323 180 0 —— 15 19.9 ' 180 5 In 15 “.0 D 180 0 236 15 36.8 180 10 3.3 15 9.8 25 £3 :3 E‘: The results show that boric acid derivatives are also 90 20 2:2 H5 3:7 effective in forming chelates which increase adhesive 180 50 1.8 is 7.3 stability without incurring loss in cure rate. 180 100 0.9 15 6.2 30 EXAMPLE 7 The results indicate that, in the presence of the pyro- T1_1i5 eX3mPle llldstfates the effeet Ofdnefeaslng the gallol’ as fitfle as 5 ppm boric acid gives some improve- relative concentration of boric acid using a different ment in the stability of the adhesive. Increasing the 2'eY_an0aeFYlate m0n0me1'- _ . boric acid concentration up to 100 ppm. effects even _ F1Ve Samples Ofa 2'eYaT193e1‘Y13t_e 3dheS1Ve e0mP0S1' greater stability without compromising cure speed. 35 U0“ Were Prepared by adding the lfldleated amount Of EXAMPLE 5 This example illustrates the effect of further relative increases in boric acid concentration on the stability of ethyl 2-cyanoacrylate. The indicated amounts of pyrogallol and boric acid were added to the 2-cyanoacrylate monomer of Exam- ple 3 which had aged to a viscosity of 10.6 cps. At a 40 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 stabi- lizer. 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. level of .400 ppm. the boric acid'did not completely TABLE VI] dissolve in the monomer. The set time and aged viscos- 45 Amount of ity of each adhesive was determined as in the test proce- B0,,-C Add Se, Time Aged viscosity dures described above, with the viscosity test carried ippm.) (sec.) (cps.) out for 7 days instead of 10 days. A control containing .0 5 4.9 an added amount of sulfur dioxide as a stabilizer was l00 l5 3.3 used for comparison purposes. 50 £3 . . . . > _ The results are indicated in Table V. mo >60 32 TABLE V Adhesive A B C D E F G I. Additive (ppm.): Sulfur dioxide 0 S0 0 ‘D D 0 D ortho-Boric acid 0 0 0 1100 lOO ‘.100 (400 Pyrogallol 0 0 180 D 180 M30 l8O ii. Set Time (sec.) 15 >60 l5 l5 .30 45 ‘>60 iii. Aged Viscosity 57.5 24.9 58.2 33.7 l7.6 18.0 17.7 (cps) Sulfur dioxide is a fairly effective stabilizer, but re- 65 tards 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 The results indicate that, as in the case of ethyl 2- wcyanoacrylate, excess boric acid in the methyl 2- -cyanoacrylate monomer results in little gain in adhesive stability, while the cure rate is retarded significantly. 11,182,823 14 EXAMPLE 3 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. 1 The boric-acid chelate thus obtained was added in the indicatedamount of the ethyl 2-cyanoacrylate mono- mer used .in2Example 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. This example illustrates the effect of increasing the relative amount of polyhydroxy compound on the sta- bility of ethyl 2-cyanoacrylate. 5 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 evalu- ated for set time and aged viscosity as in Example 5, and 10 the results are indicated in Table VIII. TABLE VIII TABLE X _ Amount of Chelate Set Time Aged Viscosity Amount of Ratio of Moles Aged Added (ppm) (Sad) (CPS) Pyrogallol Pyrogallol to Set Time Viscosity (ppm.) Moles Boric Acid (sec.) (cps.) 15 2(0) ° 0 15 5“ 40 15 70:9 50 0.25 15 42.3 200 ,5 349 100 0.5 15 33.1 400 15 29.7 200 l.0 15 35.6 800 60 29 4 400 2.0 30 39.7 20 ' 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 stabi- lize 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, the present invention is seen to provide a novel class of anionic polymerization inhibitors for 2-cyanoacrylate adhesives which are effective stabiliz- ers and do not significantly retard the cure rate of the adhesive. Now that the preferred embodiments of the present invention have been described in detail, various modifi- cations and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the The adhesive mixture containing 50 ppm. pyrogallol developed a pinlr color. It can be seen that the adhesive containing 400 ppm. pyrogallol showed decreased sta- bility and cure speed. However, this amount of pyrogal- lol may be desirable when additional protection from free radical polymerization is desired or required in the adhesive. 25 EXAMPLE 9 30 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 14.5 cps. was used to prepare several adhe- sives by adding boric acid and the indicated polyhy- droxy 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. 35 TABLE IX spirit and scope of the invention are to be limited only d d l ' h f ' ‘- Amount Amount of 40 lgy the appen e c aims, and not by t e oregoing speci of Boric Polyhydroxy Set Aged. ‘canon’ . _ _ Acid Polyhydroxy Compound Time Viscosity What is clalmed 1s: (ppm-) Compound (ppm-) (sec-) (CD5) 1. An adhesive composition comprising a mixture of: 0 none 0 15 64.5 A. at least 65% by weight of a monomeric ester of 100 , "0319 0 15 46-9 45 2-cyanoacrylic acid of the general formula: 0 Oxalic acid . 21-120 30 15 51.1 101) Omaha acid . 21120 30 15 19.6 0 L-Tartaric acid 192 15 79.9 0 100 L-Tartaric acid 192 15 21.1 ll I-I2C=C'3—C-OR The results show that oxalic acid, a relatively strong 50 CN diacid, does not overstabilize the adhesive when used in small amounts with boric acid. In fact, oxalic acid forms a chelate which is very effective as an anionic polymeri- zation inhibitor. Tartaric acid, which is both a diacid and an ct-hydroxy 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 a- hydroxyl groups (see Kustin and Pizer, “JACS”, 91 (1969) 317). . 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 a protonic acid chelate containing a boron atom bonded to at least three oxygen atoms formed from reaction of boric acid, or a compound which forms boric acid or tetrava- lent monoborate ions in the monomeric ester, with a polyhydroxy compound capable of forming said chelate and containing no reactive groups which 55 EXAMPLE 10 This example illustrates the variation of this invention 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 ves- sel and refluxed in 1,2-dichloroethane, using a reverse Dean~Starl< receiver to collect the water by-product. 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 65 10-600 ppm., based on the weight of the ester. 3. The composition of claim 1 wherein said com- pound which forms boric acid or tetravalent monobo- rate ions is an anhydride or a borate ester. 4,182,823 15 4. The composition of claim 3 wherein said com- pound is boric oxide or a trialkyl borate. 5. The composition of claim 1 wherein said polyhy- droxy compound has at least two hydroxyl groups at- tached to adjacent carbon atoms or to carbon atoms separated by one atom, such that the chelate formed therefrom contains a t'1ve- or six-membered ring. 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 polyhy- droxy compound is selected from the group consisting of ortho-dihydroxybenzenes, ortho-hydroxybenzoic acids, aliphatic a-hydroxy acids, and aliphatic dicarbox- ylic acids. 8. The composition of claim 7 wherein said polyhy- droxy 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 addi- tionally present a free radical polymerization inhibitor. 10. The composition of claim 1 wherein there is addi- tionally present a gaseous anionic polymerization inhibi- tor. 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 adhe- sive compositions, the improvement which comprises adding to said composition a stabilizing amount of a protonic acid chelate containing a boron atom bonded to at least three oxygen atoms formed from reaction of boric acid, or a compound which forms boric acid or tetravalent monoborate ions in the monomeric ester, with a polyhydroxy compound capable of forming said 10 15 20 25 30 35 M5 16 chelate and containing no reactive groups which can initiate anionic polymerization. 13. In a process for preparing 2-cyanoacrylate adhe- sive compositions, the improvement which comprises adding to said composition an amount of boric acid, or a compound which forms boric acid or tetravalent monoborate ions in the monomeric ester, and an amount of a polyhydroxy compound capable of forming a pro- tonic acid chelate containing a boron atom bonded to at least three oxygen atoms with said boric acid or boric- acid-forming compound and containing no reactive groups which can initiate anionic polymerization, the amounts of said boric acid, or boric—acid-forming com- pound, and said polyhydroxy compound being suffi- cient to provide a stabilizing amount of said chelate in said 2-cyanoacrylate adhesive. 14. The process of claim 13 wherein said boric acid or boric-acid-forming compound is added in an amount of 5-1000 ppm. and said polyhydroxy compound is added in an amount of 5-500 ppm., based on the weight of the 2-cyanoacrylate ester. 15. The composition of claim 1 wherein said chelate contains a group of the formula -0 o—c— ‘D1’ / \ -0 o—c— wherein the carbon atoms are directly bonded to each other or separated by one or more atoms. # I I II It