CA1162562A

CA1162562A - Msa/so.sub.2 stabilization system

Info

Publication number: CA1162562A
Application number: CA000386395A
Authority: CA
Inventors: Luis R. Lizardi; Bernard Malofsky; Chris Mariotti; Ju-Chao Liu
Original assignee: Henkel Loctite Corp
Current assignee: Henkel Loctite Corp
Priority date: 1981-09-30
Filing date: 1981-09-22
Publication date: 1984-02-21
Also published as: GB2107328A; JPS5863771A; GB2107328B; DE3140246A1; BE890553A; FR2515196A1; AU7565681A; FR2515196B1

Abstract

ABSTRACT OF THE DISCLOSURE

A cyanoacrylate adhesive composition containing an organic or inorganic acid with a pKa range of about -12 to about 7, in combination with an acidic gas, as stabilizers. The synergistic effect yields improved stability and shelf-life without affecting adhesive performance.

Description

1 1~2562 1 ll Il ~ B~AC~GROUND OF TH~ INVENTION
. . 1:
1. Field of the Invention This invention relates to cyanoacrylate adhesive composi-tions which have improved stability by the incorporation ofj!
acidic gases in combination with acids with a pKa range between ~i about -12 to abollt 7 therein.
. Descripti~n of the prior art 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 for,mula i .
CH2=C--C--OR
CN o wherein R is a Cl 16 alkyl, cyclalkyl, alkenyl, cycloalkenyl, phenyl, or heterocyclic radical, wnen formed into a thin film, is anionically polymerized in a short period of time due to moisture 1; present in the environment.
The alpha-cyanoacrylate is 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 Ithe monomer is stored at normal temperatures, even in a closed ,vessel relatively free from moisture in the air, it will self-polymerize in a comparitively short time. This may be primarily ~due to residual water impuraties left during the manufacturing process of the monomer. This time period is shortened even more ~when the storing temperature is increased. Therefore, it is necessary to incorporate an anionic polymerization stability agent into the adhesive composition.
, .i I

1 fi~5~2 1 i It is known in the prior art that adhe ive compositions ¦containing monomeric esters of alpha-cyanoacrylic acid can be ¦stabilized by incorporating in the compositions small quantities of an acidic gas, most commonly sulfur dioxide, and a free ~ t radical inhîbitor, most commonly hydroquinone. This technique is described, for example, in U~S. Patent Nos. 2,765,332 to Coover et al, issued October 2, 1956 and 2,794,788 to Coover et al, issued June 4, 1959. It is important to note that acid gas, such l as sulfur dioxide has traditionally been introduced into the monomeric ester during the depolymerization stage of their prepara-,tion to insure stability of hot monomer vapors. This method is well known and described in U.S. Patent Nos. 2,765,33~, 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 No.
1~ 1,159,548, among others. In these known methods, sulfur dioxide is added in concentrations from about 0.001% to about 0.01~ by w~ight of the adhesive compositions. A greater concentration has an adverse effect in that even a slight excess of sulfur dioxide will cause a serious retardation of speed of cure. At a lesser 2~ concentration serious stability problems are encountered.
Excess acidic gas is removed under high vacuum, leaving a desired level in the adhesive. Similar remarks apply to nitric oxide or ;boron trifluoride.

I Prior art, disclosing the use of a variety of other anionic 2~ ~nhibitors of the Lewis acid type, used 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, amon~ others.
~hese stabilizers include phosphoric acid, picric acid, hydrogen ;fluoride, and various carboxylic acids.
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~1 116~,5~2 ~ ~
¦ It i~ ~lso known i~ the art to add the above mentioned ¦¦stabilizers into the distilled finished monomer. ~UlIUr ~ri-oxide, nitric oxide, and aromatic sulfo~ic 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 these conventional stabilizers as used abo~e in cyanoacrylate compositions is that many of these are very volitile at room ¦
¦temperature and a part thereof esca~es without being dissolved 1~ into the cyanoacrylate monomer. Thus, when an inhibitor-incorpora-ted 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.
SU~ARY O~ THE INVENTION

i; Briefly summarized, the present invention involves the in-corporation, by means of dissolution of an acid selected from the group of sulfonic acids, phosphorus acids, phosphonic acids, and carboxylic acids, with a pKa range between about -12 (negative twelve) to about 7 (seven), and an acidic gas, into at least one 2~ monomeric ester of 2-cyanoacrylic acid. The concentration of acidic gas (e.g., SO2) is normally within the range of about 0.0005% to about 0.01% by weight of the composition while-the s2me concentration ranges apply to the acids. The results of adding these stabilizers, in combination with each other, to the cyanoacrylate composition, is a synergistic stability effect, ~in~reasing cyanoacrylate adhesive composition shelf-life and I stability far beyond what either known stabilizer has heretofore I accomplished by itself. The stability reached does not negatively interfere, at the proper range of concentrations and pKa of the sta~ility components, with the cure speed, strength or general ~ erformance of the adhesive composition ~ ¦

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DE~ S~T~T~N ~ T~ INVENTION

The esters of 2-cyanoacrylic acid which are used in the lnvention described herein can be prepared by the method des-cr~bed in U.S. Patent No. 2,467,927 to Ar~is, issued April 19, 1949. This method involves the preparation of an intermediate polymer throuyh the condensation reaction between formaldehyde , ¦and an alkyl, cyclohexyl or phenyl ester of cyanoacetic aci~. , ¦The condensation reaction is promoted ~y the presence of a basic j !condensation catalyst, such as ammonium hydroxide, quinoline, lipiperidine and diethyl amine. Moderate warming, such as to about 50C to initiate the reaction, also may 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.
1- The monomeric ester is separated by fractional distillation at a reduced pressure of 1 to 10 millimeters of mercury absolute pxessure, or less, generally in the presence o~ an acidic polymeri-zation inhibitor such as phosphorous pentoxide. The monomer Vapors are condensed into a vessel which contains the desired 2~ 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. Patent No. 2,721,85~
Ito Joyner et al, issued October 25, 1955, and 3,254,111 to Hawkins ,let al, issued May 31, 1966.
In the adhesive composition, the esters of 2-cyanoacrylic acid can be used singly or in combination. Preferably, a single lester is used, selected from the group consisting of Cl to C5 lalkyl and cyclohexyl esters of 2-cyanoacrylic acid~ The single ; ~most desirable ester is ethyl-2-cyanoacrylate.
1 As me~tioned above, the dual stabilizer system produces a synergistic af ect which up to now has not been known. Such an ~ffect was surprising in view of the fact that the reasonably 1~q2s62 ~ I
~k~lled chemist would antici~ate an additive stabilization effect rather than a synergistic one~ based on the respective roles of the individual stabilizers.
There are probahly a number of factors which contribute to the effectiveness of this stabilizer system in cyanoacrylate I ~
adhesives. While not~ wishing to be bound to any theory, it is ¦ J¦
believed that the most preferred acidic gas, S02, forms a hydrate with the water present in the cyanoacrylate monomer, attaching to up to seven (7) water molecules for each S02 molecule. The water 13 ,lis generally present in the monomer as an impurity from the manufacturing process in concentrations gen~rally from about 400 to l,OOOppm. This water residue may alone; or in combination with moisture from the environment, initiate the anionic poly-merization of the monomer. 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 anoiniç initiation is decreased.
~ It is further hypothesized that the acid component of the stabilizer forms hydronium ions when in the ~resence of the water 2~ residue. The result of this reaction is to decrease the free water content and produce the acidic hydroniun ion which, along with ~the S02, serves as a stabilizer. When any acid is present in cyanoacrylate adhesive compositions in excessively high con-,: ~
,~centrations, it has a deleterious effect on the adhesive sta-.;
2~ bility. The reason for this is thought to be due to the role the acid plays in catalyzing an hydrolysis of the cyanoacrylate 'Imonomer, to form cyanoacrylic acid and an alcohol. ~hen the i,alcohol concentration reaches a high enough level, it will se}ve ,ns a catalyst to the anionic polymerization. It is here that the j ioint role of the S02 and acid combination can be clearly shown.

S~nce th~ S02 functions to tie up water molecules by forming a hydrate, there are less water molecules to participate in the - 6 - .

~1 ! hydrolysis of process of the acld and ther~fore less alcohol, if ¦ any, i~ prcduce~. ~ro~ t~.~se theoretical mechanisms, a reasonably I ~killed person in the art would be expect~ ~o anticipate a ¦ simple additive effect, since presumably, the two inhibiting ; agents serve to stabilize the cyanacrylate adhesive composition, each by tying up or reacting with any water present in the compo~
sition. It would seem then that the net result would be to l ¦
simply decrease the total amount of residue water that could potentially prematurely initiate polymerization and decrease ' I 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 of the adhesive ~omposition would be the sum of the individual stabilizer's effects. Thus, for examDle, if a certain concentration o~ SO2 alone yielded stable composition -5 for y number of days and a certain concentration stable for x number of days, the additive effect would be to o~tain a composi-tion stable for x plus y number of days. Yet, this expected result is not at all what is present in the instant invention, but rather a much more pronounced stability duration.
The synergestic effect will obviously not be apparent if there is very little moisture present in the stabilized cyano-acrylic composition, or if the container in which the comPosi-tion is stored is one which is impermeable to moisture or air, such as glass or aluminum tubes or bottles. This is due to the fact that in cases where there is very little moisture present in ~the composition, less than 40nppm for example, the So2 compon~nt ilof the stabilization system will function adequately enouqh to attach to most of, if not all, of the water molecules. Thus, the results in these cases will not demonstrate the profound synergism `I 11 . . _ because there is so little water present to call into play both the acid and SO2 roles. Since SO2 can react with up to seven (7~ molecules of water, there would be little water for the acid to react with. However, in practice plastic containers which are gen¢rally permeable to air and moisture, such as polythylene, are used. In these types of containers the syn-ergistic effect is quite apparent, as will be seen from the examples.
The preferred acidic gas in the invention is sulfur dio-xide in the range from about 0.0005% to about 0.01% by weight of the c~osition. The preferred range is about betwcen 0.0005~ to about 0.004% by weight of the composition; the most preferred range being from about 0.0005~ to about 0.002~o by weight.
The acid component of the stabilization system preferably is present in a pKa range from about -12 to about 7. The most preferred range is about -12 to about 2. The acids are gene-rally selected from the group of sulfonic, phosphorus, phos-phonic and carboxylic acids. Of these, the preferred acids are methane sulfonic acid, benzene disulfonic acid, p-toluene-sulfonic acid, and hydroxy propane sulfinc acid, benzene phos-phorus acid, benzene phosphonic acid, oxalic acid and penta-fluorobenzoic acid. The most preferred acids are those of the sulfonic group, in particular methane sulfonic acids. Con-centrations of the above acids can range from about 0.0005%
to about 0.01~ by weight of the composition. The preferred concentrations range is from about 0.0005% to about 0.004 by weight, while the most preferred range is from about 0.0005 to about 0.002% by weight. A mixture of any of the suitable '''~;

acids may also be incorporated with the acidic gas. Composi-tions of this invention arc prepared from the cyanoacrylate monomers by a conventional mixing operation. The cyanoacry-late monomer is distilled into a vessel containing a predeter-mined amount of acid and S02 is then added.

-8a-~ 1 6 2 5 6~
It i3 freauently beneficial to inc~r~a~e cort~ ~ o~.c~
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 ~henolic type such as quinone, hydroquinones, p-tert-butyl catechol, p-methoxy phenol, 2:6-di~
tert-butyl-para cresol and 2,2-methylene-bis-(4-methyl-6-tert-butyl) phenol. These inhibitors may be used at concentrations from about 0.001~ to about 1~ by weight of the adhesive compo-l 5ition. Most preferably, they are used-in the range of about 0.005% to about 0.1% by weight of the adhesive composition.
Thickeners and plasticizers may be added in pro~ortions easily determined by one skilled in the art. ~,enerally, they are used in the range of about 1% to about 10% by weight of the adhesive composition, the volatile solvent from 0% to about 25 or more by weight of the adhesive composition.
~mong the most common thickeners are acrylate resins such as polymethylmethacrylate and polyethylamethacrylate. Other suitable thickeners include polymeric alkylcyanoacrylate cellulose esters such as cellulose acetate aI~d cellulose butyrate, and polyvinyl ethers such as polyvinylmethyl ether. The most common , plasticizers are Cl to Cl0 alkyl esters of dibasic acids such as l~ sebacic acid and malonic acid. Other plasticizers such as poly-, alkyether and polyurethanes also may be used.
A previ-ously mentioned, the stability of a cyanoacrylate adhesive composition is affected by atmospheric moisture as well a~ residual manufactured moisture. Thus, the type and size ,container in which you choose to store the adhesive will affect ~the stability to some degree.
l ; .
I ', 9 ~ ~ 62~62 The following examples are given to demonstrate the synergistic stability effect found when an acid gas and an acid are dissolved in the cyanoacrylate composition. The cyanoacrylate monomer used in all examples was ethyl-2-cyanoacrylate prepared by methods described above. Theexperiments of all examples were run at 70C to accelerate aging affects.
EXAMPLE I
Concentrations from O to 0.0080% by weight of methane sulfonic acid and S02 respectively, were added to a cyanoacrylate composition and packaged in 3 gram aluminum tubes and were kept at 70C. The adhesive composition was found to have remained stable and unreacted and maintained satisfactory performance up to 61 days. The following table shows the duration of stability expressed in days with each of the chosen combinations of stabilizers' concentrations.
As previously explained, when either MSA (methane sulfonic acid) or S02 is used alone in a container impermeable to air, such as an aluminum or glass tube, then the synergistic effect will not be apparent. This i5 evidenced by points in both Table I and II texample II).
Table I shows the composition stable after 55 days with Sppm of S02 and Oppm 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 S02 and 5 and 10 ppm of MSA
have remained stable longer than 16 days to date, which is further evidence of the above-mentioned rationale.

r ~ 1 Il i 11!;25~2 ~able 1 ¦ A. Stability of MSA/S02 stabilized CA in 3 gram Al tube at 70C
(days).
I . ' , 1 ___ _ _ l MSA/S02 o ppm 2.5 5 10 20 ~ _ . .; _ _ .
Oppm . 14 55 S~ ~
I ___ . _ ~
. 2.5 6~ 46 50 5656 .j ........... .-- _ _ _ j;
~. 5 6 50 56 6156 ,. . _ _ . .~
. 10 8 50 S6 6156 ~ _ . _, 12 56 56 56 ~8 , _ _ . ..

1. Concentrations are expressed in ~arts per million 2~ Stability is expressed in days.

EXAMPLE II

~ .
Table II
A. Stability of MSA/S02 stabili~ed in a test tube at 70C
'. (days).
i ! _ _ I!
MSA/S02 O 2.5 5 10 20 . , ___ . __ _ ll O 14 47 56 5~
,. ~,. __ .. ._ _ 2.5 13 46 56 58 56 1~ - ----- , --~
1~ 5 4 50 56 61 56 1 !
., __ .... = _I
i' 10 6 50 56 56 56 ij ~ _ __ . --_ i 20 9 50 ~6 56 50 l 1. . .......... _ .

¦ As observed from Table II, results similar to Table I were obtained when the container used was a test tube. The cyano-9 1~6~5fi~ I I
j ~crylate adhe~ive composition was found to have excell~nt cure ¦~ ~peed and bond stren~th af*er each o- ~he time periods listed.

EX~MPLE III.
~his example demonstrates the stability effect when poly-ethylene bottles, permeable to air, are used as containers. The ¦ tempera~ures remain the same as the above examples. As shown ¦ from Table IIIA the optimum values of concentrations are 20ppm of both SO2 and MSA. At the same concentration (20ppm) the expected !/ additive effect would yield a value of only 13 days while 29 is 1~ realized by synergism. Table IIIB shows a stability of 56 days at 20ppm of both MSA and SO2. This is twice what is expected if an additive effect were present. Similar stability values are obtained wlth other combinations of MSA/SO2 concentrations.

~5 Table III
A. Stability of MSA/SO2 stabilized CA in 1 Oz. polyethylene bottle at 10C (day) 2.5 1 5 .- 3 4 9 . _ . . ~ .
2.5 2 611 18 2~
_~_ ~ __ __ .
_ 4 4* 4* 18 4 5* 5* 7* 29 ~ _ _ __ 4 4* 5* 7* 29 . I
~1 __--. . ._. I _ __ - ~Package failure due to severe stress cracks.
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i l - 12 ~

. __ I 1 1 6~5~2 . .
B. Stability of MSA/S02 stabilized CA in 1/3 oz polyethylene bottle at 70C ~day) il r - ~ 2.5 1 ~ I ~ ~
li O . 10 8 ~ _ ~ 1~
.1 ..... . __ _ ; ll 2.5 5 26 35 27 S6 I~ 5 lO 33 41 32 l9 "....... . .
.~ lO ll 56 29 11~ 56 . ,. . . .
: 20 14 50 ll* 53 56 .. . _ *PacXage failure due to severe stress cracks The following example shows concentrations of 10 and 40~pm of various acids in combination with Oppm and lOppm of S02.

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!

i 13 - `
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1 1 6~56~
EXAMPL~ IY
Table IV Stability of Ethyl CA stabilized with S02 and acids l i at 70C (day) ¦ t Acid/S02 ppm O lO
_ _ _ _ 2 2I
Glass Tubel oz Btl C71ass Tube 1 oz Btl ;
i~ ~SA lOppm 6 4 - 56 7 40ppm - 4 _ _ pentafluorobenzoic q.
acid lOppm 1 1 57 4 ~ 40ppm l - 1 4 Oxalîc acid lOppm 1 1* 59 17 40ppm l 1* S q 10 EPSA lOppm 36 9 S49 12 Benzene phosphonic acia lOppm 2 2 56 6 40ppm 13 16 56 17 Benzene ~hosphorous acid lOppm l l 52 16 40ppm l 1 48 lo Benzene disulfonic acid lOppm 22 14 27 17 " p-toluenesulfonic .
; acid lOppm 6 7 56. 15 , 40ppm 22 12 48 15 i *Package failure

3 lHydroxy propane sulfonic acid polyethylene bottle il . i Il .
Il ' . , ~1 14 - .

. . .

,5~2 ~ he above table demonstrates the dramatic eff~ct obtained for various acids in combination with SO2 at levels of 10-40 ppm . ¦
As seen from the table, lOppm of Methane Sulfonic acid and lOppm f S2 added to the ethyl cyanoacrylate composition gave a stable product with good performance (bond strength/ cure speed~ after ¦¦ 56 days at 70C in a glass container. Similar results were ¦ ~¦
i obtained for other sulfonic phosphorous and phosphoric acids.

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Claims

Claim:

1. A stabilized cyanoacrylate adhesive composition which comprises at least one monomeric ester of 2-cyanoacrylic acid and at least about 0.0005% to about 0.01% by weight of a dissolved acidic gas and about 0.0005% to about 0.01% by weight of an acid selected from the group of sulfonic acids, phosphorus acids, phosphonic acids and carboxylic acids.

2. The composition of claim 1 wherein the ester of 2-cyanoacrylic acid has the formula wherein R represents a C1-16 alkyl, cycloalkyl, alkenyl, cyclo-alkenyl, phenyl or heterocyclic radical.

3. The composition of claim 2 wherein the sulfonic acid is Methane Sulfonic acid, present in concentrations between about 0.0005% to about .002% by weight.

4. The composition of claim 2 wherein the sulfonic acid is hydroxy propane sulfonic acid, present in concentrations between about 0.0005% to about 0.002% by weight.

5. The composition of claim 2 wherein the sulfonic acid is benzene disulfonic acid, present in concentrations between about 0.0005% to about 0.002% by weight.

6. The composition of claim 2 wherein the sulfonic acid is p-toluene sulfonic acid, present in concentrations between about 0.0005% to about 0.002% by weight.

7. The composition of claim 2 wherein the phosphorous acid is benzene phosphorous acid, present in concentrations between about 0.0005% to about 0.002% by weight.

8. The composition of claim 2 wherein the phosphonic acid is benzene phosphonic acid, present in concentrations between about 0.0005% to about 0.002% by weight.

9. The composition of claim 2 wherein the carboxylic acid is oxalic acid, present in concentrations between about 0.0005%
to about 0.01% by weight.

10. The composition of claim 2 wherein the carboxylic acid is pentafluorobenzoic acid, present in concentrations between about 0.0005% to about 0.01% by weight.

11. The composition of claim 2 wherein the acidic gas is SO2, present in concentrations between about 0.0005% to about 0.01% by weight.

12. The composition of claim 2 which additionally contains a free radical polymerization inhibitor.

13. A stabilized cyanoacrylate adhesive composition which comprises at least about one monomeric ester of 2-cyanoacrylic acid and at least about 0.0005% to about 0.01% by weight of an acid with a pKa of about 12 to 7 and about 0.0005% to about 0.01%
by weight of a dissolved acidic gas.

14. The composition of claim 13 wherein the ester of 2-cyanoacrylic acid has the formula wherein R represents a C1-16 alkyl, cycloalkyl, alkenyl, cyclo-alkenyl, phenyl or heterocyclic radical.

15. The composition of claim 14 wherein the acid is methane sulfonic present in concentrations between about 0.0005% to about 0.01% by weight.

16. The composition of claim 14 wherein the acid is p-toluensulfonic acid, present in concentrations between about 0.0005% to about 0.01% by weight.

17. The composition of claim 14 wherein the acid is benzene phosphorus acid, present in concentrations between about 0.0005%
to about 0.01% by weight.

18. The composition of claim 14 wherein the acid is penta-fluorobenzoic acid, present in concentrations between about 0.0005% to about 0.01% by weight.

19. The composition of claim 14 wherein the acid is oxalic acid, present in concentrations between about 0.0005% to about 0.01% by weight.

20. The composition of claim 14 wherein the acid is hydroxy propane sulfonic acid, present in concentrations between about 0.0005% to about 0.01% by weight.

21. The composition of claim 14 wherein the acid is benzene phosphonic acid, present in concentrations between about 0.0005%
to about 0.01% by weight.

22. The composition of claim 14 wherein the acid gas is SO2, present in concentrations between about 0.0005% to about 0.01% by weight.

23. The composition of claim 14 which additionally contains a free radical polymerization inhibitor.

24. A method of making a stabilized cyanoacrylate adhesive comprising dissolving into said cyanoacrylate an acid with a pKa between about minus twelve (-12-) to about over seven (7) and an acidic gas in the concentration of about 0.0005% to about 0.01%
by weight of the monomer.