IE51599B1 - Stabilised cyanoacrylate adhesive composition - Google Patents

Description

This invention relates to cyanoacrylate adhesive compositions which have improved stability.

Liquid cyanoacrylate compositions have long been known in the art as excellent adhesives. It is well known that an alpha-cyanoacrylate represented by the general formula CH,=C-C-OR - I n CN 0 wherein R is a C^g alkyl, cycloalkyl, alkenyl, cycloalkenyl, phenyl, or heterocyclic radical, when formed into a thin film, is anionically polymerised in a short period of time due to moisture present in the environment.

Alpha-cyanoacrylates are widely used in many fields for bonding and sealing all kinds of organic and inorganic materials. However, one of the traditional shortcomings has been that when the monomer is stored at normal temperatures, even in a closed vessel relatively free from moisture in the air, it will self-polymerize in a comparatively short time. This may be primarily due to residual water impurities left during the 2o manufacturing process of the monomer. This time period is shortened even more when the storing temperature is increased. Therefore, it is necessary to incorporate a stabiliser against anionic polymerization into the adhesive composition.

It is known in the prior art that adhesive 51509 -3compositions containing monomeric esters of alphacyanoacryllc acid can be stabilized by incorporating in the compositions small quantities of an acidic gas, most comonly sulphur dioxide, and a free radical inhibitor, most commonly hydroquinone. This technique is described, for example, in U.S. Patent Nos. 2,765,332 and 2,794,788. It is important to note that the acid gas, such as sulphur dioxide has traditionally been introduced into the monomeric esters during the de10 polymerization stage of their preparation to ensure stability of hot monomer vapours. This method is well known and described in U.S. Patent Nos. 2,765,332, 3,335,482, 2,467,927, 3,221,745, 3,178,399, 2,926,188, German Patent No. 1,807,895, French Patent No. 1,504,240, and British Patent Specification No. 1,159,548, among others. In these known methods, sulphur dioxide is added in concentrations from about 0.001* to about 0.011 by weight of the adhesive compositions. A greater concentration has an adverse effect in that even a slight excess of sulphur dioxide will cause a serious retardation of speed of cure. At a lesser concentration serious stability problems are encountered. Excess acidic gas is removed under high vacuum, leaving the desired level in the adhesive. Similar remarks apply to nitric oxide or boron trifluoride.

Prior art, disclosing the use of a variety of other anionic inhibitors of the Lewis acid type, used -4 · in the intermediate, depolymerization steps of cyanoacrylic monomer manufacture, is found in U.S. Patent Nos. 3,751,445, 2,467,926, 2,763,677, 2,756,251, 2,514,387, 2,467,927, and 3,221, 745, among others. These stabilizers include phosphoric acid, picric acid, hydrogen - f luoride, and various carboxylic acids.

It is also known in tho art to add tne above mentioned stabilizers into the distilled finished monomer. Sulphur trioxide, nitric oxide, and aromatic sulphonic acids are a few other types of acidic substances that have been employed to stabilize monomer compositions.

One of the difficulties encountered in the incorporation of the conventional stabilizers referred to above in cyanoacrylate compositions is that many of them are very volatile at room temperature and a part thereof escapes without being dissolved into the cyanoacrylate monomer. Thus, when an inhibitor-incorporated alpha-cyanoacrylate is stored for a long period of time, the inhibitor may spontaneously escape from the monomer, rendering it difficult to impart complete stability to alpha-cyanoacrylates.

The present invention involves the incorporation, by means of dissolution, of an acid together with an acidic gas, into.at least one monomeric ester of 225 cyanoaerylie acid.

The present invention provides a stabilized cyanoacrylate adhesive composition -3comprising at least one monomeric ester of 2-cyanoacrylic acid, 0.0005% to 0.01% by weight of an acid with a pKa of -12 to 7 which is an aliphatic sulphonic acid and 0.0005% to 0,01% by weight of a dissolved acidic gas.

The concentration of acidic gas (e.g., S02) is within the range of 0.0005% to 0.01% by weight of the composition while the same concentration range applies to the sulphonic acid. The results of adding these stabilizers, in combination with each other, to the cyanoacrylate composition, is a synergistic stability effect, increasing cyanoacrylate adhesive composition shelf-life and stability far beyond what either known stabilizer has heretofore accomplished by itself. The stability reached does not negatively interfere, at the proper range of concentrat15 ions and pKa of the stability components, with the cure speed, strength or general performance of the adhesive composition.

The esters oE 2-cyanoacrylic acid which are used in the invention described herein can be prepared by the method described in U.S. Patent No. 2,467,927. This -6method involves the preparation of an intermediate polymer through the condensation reaction between formaldehyde and an alkyl, cyclohexyl or phenyl ester of cyanoacetic acid. The condensation reaction is promoted by the presence of a basic condensation catalyst, such as ammonium hydroxide, quinoline, piperidine and diethyl amine. Moderate warming, such as to about 50°C to initiate the reaction, may also be desirable. Water and organic solvents are removed as best as possible, generally by distillation, following which the intermediate polymer is thermally decomposed to produce the monomeric ester of 2-cyanoacrylic acid. The monomeric ester is separated by fractional distillation at a reduced pressure of 1 to 10 millimeters of mercury absolute pressure, or less, generally in the presence of an acidic polymerization inhibitor such as phosphorous pentoxide. The monomer vapors are condensed into a vessel which contains the desired inhibitors for use in the final adhesive product, as disclosed above and discussed more fully below.

Other similar manufacturing processes are disclosed, for example, in U.S. Patents Nos. 2,721,858 and 3,254,111.

In the adhesive composition, the esters of 2cyanoacrylic acid can be used singly or in combination. Preferably, a single ester is used, selected from the group consisting of to Cg alkyl and cyclohexyl esters of 2-cyanoacrylic acid. The single most desirable ester is ethyl-2-cyanoacrylate. -7As mentioned above, the dual stabilizer system produces a synergistic effect which up to now has not been known. Such an effect is surprising in view of the fact that the reasonably skilled chemist would anticipate an additive stabilization effect rather than a synergistic one, based on the respective roles of the individual stabilizers.

There are probably a number of factors which contribute to the effectiveness of this stabilizer system in cyanoacrylate adhesives. While not wishing to be bound to any theory, it is believed that the most preferred acidic gas, SO?, forms a hydrate with the water present in the cyanoacrylate monomer,, attaching to up to seven water molecules for each SO? molecule.

The water is generally present in the monomer as an impurity from the manufacturing process in concentrations generally from 400 to 1,000 ppm. This water residue ay alone, or in combination with moisture from the environment, initiate the anionic polymerization of the eononer. Such an effect is undesirable and greatly decreases the shelf-life of the adhesive composition.

By forming the hydrate with the water molecules, the potential anoinic initiation is decreased.

It can be further hypothesized that the acid component of the stabilizer forms hydronium ions when in the presence of the water residue. The result of this reaction is to decrease the free water content and S1599 -8produce the acidic hydronium ion which, along with the 5O2, serves as a stabilizer. When any acid is present in cyanoacrylate adhesive compositions in excessively high concentrations, it has a deleterious effect on the adhesive stability. The reason for this is thought to be due to the role the acid plays in catalyzing an hydrolysis of the cyanoacrylate monomer, to form cyanoacrylic acid and an alcohol. When the alcohol concentration reaches a high enough level, it will serve as a catalyst to the anionic polymerization.

If is here that the joint role of the SO2 and acid combination can be clearly shown. Since the SO2 functions to tie up water molecules by forming a hydrate, there are less water molecules to participate in the hydrolysis process of the acid and therefore less alcohol, if any, is produced. From these theoretical mechanisms, a reasonably skilled person in the art would be expected to anticipate a simple additive effect, since presumably, the two inhibiting agents serve to stabilize the cyanoacrylate adhesive composition, each by tying up or reacting with any water present in the composition. It would seem then that the net result would be to simply decrease the total amount of residual water that could potentially prematurely initiate polymerization and decrease stability. Thus, the person skilled in the art would anticipate that by balancing the amount of the respective stabilizers in a proper way, the stability -9of the adhesive composition would be the sum of the inidividual stabilizer's effects. Thus, for example, if a certain concentration of SO? alone yielded stable composition for y number of days and a certain concentration stable for x number of days, the additive effect would be to obtain a composition stable for x plus y number of days. Yet, this expected result is not at all what is provided by the present invention, but rather a much more pronounced stability duration.

The synergistic effect will obviously not be apparent if there is very little moisture present in the stabilized cyanoacrylic composition, or if the container in which the composition is stored is one which is impermeable to moisture or air, such as glass or aluminium tubes or bottles. This is due to the fact that in cases where there is very little moisture present in the composition, less than 400 ppm for example, the SO?.component of the stabilization system will function adequately enough to attach to most of, if not all, of the water molecules. Thus, the results in these cases will not demonstrate the profound synergism because there is so little water present to call into play both tbe acid and SO? roles. Since SO? can react with up to seven molecules of water, there would be little water for the acid to react with. However, in practice plastic containers which are generally permeable to air and moisture, such as polyethylene, are used. In -10these types of-containers the synergistic effect is quite apparent, as will be seen from the examples.

The preferred acidic gas in the invention is sulphur dioxide in the range from 0.0005% to 0.01% by weight of the composition. The preferred range is trom 0.0005* to 0.004» by weight of the composition; the most preferred range is from 0.0005% to 0.002% by weight.

The aliphatic sulphonic acid component of the 10 stabilization system is present in a pKa range from -12 to 7.

The most preferred range is -12 to 2. Of these, the preferred acids are methane sulphonic acid and hydroxy propane sulphonic acid. The most preferred acid is methane sulphonic acids. Concentrations of the above acids can range from 0.0005% to 0.01% by weight of the composition. The preferred concentrations range is from 0.0005% to 0.004% by weight, while the most preferred range is from 0.0005% to 0.002% by weight. A mixture of any of the suitable acids may also be incorporated with the acidic gas. 2o Compositions of this invention are prepared from the cyanoacrylate monomers by a conventional mixing -11operation. The cyanoacrylate monomer may be distilled into a vessel containing a predetermined amount of acid and SO? is then added.

It is frequently beneficial to incorporate certain other ingredients in the adhesive composition to impart specific characteristics to the adhesive or the cured adhesive product. Free radical polymerization inhibitors are generally included, the most desirable being of the phenolic type such as quinone, hydroquinones, p-tert-butyl catechol, p-methoxy phenol, 2:6-di-tertbutyl-para cresol and 2,2-methylene-bis-(4-methyl-6tert-butyl) phenol. These inhibitors may be used at concentrations from 0.0014 to 14 by weight of the adhesive composition. Most preferably, they are used in the range of 0.0054 to 0.14 by weight of the adhesive composition.

Thickeners and plasticizers may be added in proportions easily determined by one skilled in the art. Generally, they are used in the range of 14 to 104 by weight of the adhesive composition, λ volatile solvent may be present in an amount from 04 to about 254 or more by weight of the adhesive composition.

Among the most common thickeners are acrylate resins such as polymethylmethacrylate and polyethylamethacrylate. Other suitable thickeners include polymeric alkylcyanoacrylate cellulose esters such as cellulose acetate and.cellulose butyrate, and polyvinyl ethers -12such as polyvinylmethyl ether. The most common plasticizers are to C^o alkyl esters of dibasic acids such as sebacic acid and malonic acid. Other' plasticizers such as polyalkylether and polyurethanes also may be used.

As previously mentioned, the stability of a cyanoacrylate adhesive composition is affected by atmospheric moisture as well as residual moisture from the manufacturing process. Thus, the type and size of container in which the adhesive is stored will affect the stability to some degree.

The following examples are given to demonstrate the synergistic stability effect found when an acid gas and an acid are dissolved in cyanoacrylate compositions The cyanoacrylate monomer used in all examples was ethyl 2-cyanoacrylate prepared by methods described above.

The experiments of all examples were run at 70°C to accelerate aging effects.

EXAHPLE I Concentrations from 0 to 0.0080% by weight of methane sulphonic acid and.SOj respectively, were added to a cyanoacrylate composition and packaged in 3 gram aluminium tubes and were kept at 70°C. The adhesive composition was found to have remained stable and un25 reacted and maintained satisfactory performance up to 61 days. The following table shows the duration of stability expressed in days with each of chosen -1351599 combinations of stabilizer concentrations.

As previously explained, when either MSA (methane sulphonic acid) or SO? is used alone in a container impermeable to air, such as an aluminium or glass tube, then the synergistic effect will not be apparent. This is evidenced by points in both Table I and II (example II). Table I shows the composition stable after 55 days with 5 ppm of SO? and 0 ppm of MSA. Thus, there is no significant change in stability when there is no moisture absorbed by the composition from the environment. Data points for both Tables I and II at 10 ppm of SO2 and 5 and 10 pfm of MSA have remained stable longer than 61 days to date, which is further evidence of the above-mentioned rationale.

Table I A. Stability of MSA/SO? stabilized CA in 3 gram Al tube at 70*C (days).

SiSA/SO? 0 ppm 2.5 5 10 20 0 ppm 14 55 56 47 2.5 62 46 50 56 56 5 6 50 56 61 56 10 8 50 56 61 56 20 12 56 ' 56 56 58 1.

Concentrations are expressed in parts per million Stability is expressed in days. -14EXAMPLE II Table II A. Stability of MSA/SO^ stabilized in a test tube at 70°C (days).

MSA/S02 0 2.5 5 10 20 0 14 47 56 58 2.5 13 46 56 58 56 5 4 50 56 61 56 10 6 50 56 56 56 20 9 50 56 56 50 As observed from Table II, results similar to Table I were obtained when the container used was a test tube. The cyanoacrylate adhesive composition was found to have excellent cure speed and bond strength after each of the time periods listed.

EXAMPLE III This example demonstrates the stability effect when polyethylene bottles, permeable to air, are used as containers. The temperatures remain the same as the above examples. As shown from Table IIIA the optimum values of concentrations are 20 ppm of both SOj and MSA. At the same concentration (20 ppm) the expected additive effect would yield a value of only -1513 days while 29 is realized by synergism. Table IIIB shows a stability of 56 days at 20 ppm of both MSA and SO?· This is twice what is expected if an additive effect were present. Similar stability values are obtained with other combinations of MSA/SO? concentrations. Table III A. Stability of MSA/SO? stabilised CA in 1 oz. polyethylene bottle at 70eC (day) MSA/SO? 0 2.5 5 10 20 0 2 3 4 9 2.5 2 6 11 18 29 5 4 4* 4* 18 11* 10 4 5* 5* 7* 29 20 4 4* 5* 7* 29 •Package failure due to severe stress cracks.

B. Stability of MSA/SO? stabilised CA in 1/3 oz polyethylene bottle at 70°C (day) C“— MSA/SO? 0 2.5 5 10 20 0 10 8 - 14 2.5 5 26 35 27 56 5 10 33 41 32 19 10 11 56 29 11* 56 20 14 50 11* 53 56 * Package failure due to severe stress cracks

Claims (5)

1. A stabilized cyanoacrylate adhesive composition comprising at least one monomeric ester of 2-cyanoacrylic acid, 0.0005% to 0.01 % by weight of on ocid with o pKo of -12 fo 7 which is on oliphotic sulphonic 5 acid and 0.0005% to 0.01% by weight of a dissolved acidic gas.

2. A composition as claimed in claim 1 wherein the ester of 2-cyanoocrylic ocid has the formula CN I CH 2 =C·-COOR where R represents o alkyl, cycloalkyl, alkenyl, cycloalkenyl, phenyl 10 & heterocyclic radical.

3. A composition as claimed in claim 1 or 2 wherein the sulphonic ocid is methane sulphonic acid.

4. A composition as claimed in claim 4, wherein the methane sulphonic acid is present in a concentration of from 0.0005% to 0.002% by 15 weight. 5. A composition as claimed in claim 1 or 2 wherein the sulphonic ocid is hydroxy propone sulphonic acid. 6. A composition os claimed in claim 5, wherein the hydroxy propane sulphonic ocid is present in a concentration of from 0.0005% to 0.002% by 20 weight. 7. A composition as claimed in any of claims 1 to ό wherein the oddlc gas Is SO^. S. A composition as claimed in any of claims 1 to 7 which additionally contains on inhibitor of free radical polymerization. - 17 9. A stabilized cyanoacrylate adhesive composition substantially as described with reference to any of the Examples. 10. A method of making a stabilized cyanoacrylate adhesive composition as claimed in any of claims 1 to 9 which comprises

5. Dissolving the acid and the acidic gas in the monomeric ester of 2-cyanoacrylic acid.