CA1212199A - Hot strength cyanoacrylate adhesive composition - Google Patents

Abstract

A B S T R A C T
An adhesive composition having improved thermal properties, especially hot strength, comprising (a) at least one polymeri-zable cyanoacrylate monomer, (b) about 0.1 to about 10% by weight of at least one polymerizable acrylate ester and (c) about 0.1 to about 20% by weight of an additive selected from the group consisting of the following compounds:

Description

- 2 Background Of The Injection 1. Fiend of the Invention This invention relates to certain liquid adhesive and sealant compositions which, upon cure, have improved structural strength properties at elevated temperatures and improved resistance to thermal degradation.
2. Prior Art Adhesive and sealant compositions based on cyanoacrylate monomers are known in the art. Typical examples of patents relating to cyanoacrylate adhesives are US. Patents 2,784,215 to Junior 2~794/788 to Cover et aloe and British Patent 1,196,069 to O'Sullivan. Cyanoacrylate adhesive compositions are extremely sensitive, and great care must be exercised in their formulation. Cure (polymerization) is generally considered to be initiated by an anionic mechanism, with water being a sufficiently strong base to initiate the cure under most circumstances. The adhesives remain shelf-stable items of commerce as long as they are suitably packaged, but when placed on a substrate to be bonded and exposed to atmospheric and surface moisture, cure generally is instituted in a relatively short period of tome, generally less than one (1) minute and on many surfaces, within a matter of seconds. This exceptional cure speed offers numerous advantages, particularly to those who use adhesive bonding in production line applications. However, a major shortcoming which has heretofore limited the areas of applicability of cyanoacrylate adhesives has been the relatively low thermal resistance of the cured bonds. Bonded assemblies frequently are exposed to continuous operating temperatures substantially above normal room temperature, and adhesive assemblies must retain reasonable strength for substantial periods of time at these elevated to azures to retain their usefulness.

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- 3 In addition to strength retention by the adhesive at elevated temperatures (i.e., hot strength?, the adhesive bonds must not be unduly affected by continuous or repeated exposure to elevated temperatures (resistance to heat degradation). In the prior art it has not been possible to prepare a cyanoacrylate adhesive composition which produced substantial cross-linking on cure, even when ostensively difunctional cyanoacrylate monomers were used. Further t because of the extreme reactivity of the cyanoacrylate monomer, there have been substantial limitations upon addition of other ingredients, such as cross-linking agents or co-monomers, in order to improve the above-described thermal properties.
In US. Patent 3,832,3~4, the problem of thermal resistance was solved by the addition of malefic androids and their substitution products. Additionally, it has been known in the prior art to include cross-linking agents such as ally-2-cyanoacrylate or polymerizable acrylate esters to improve thermal properties. However, none of the prior art has shown the improved thermal properties at 250F that the instant invention is able to demonstrate. Thus, the need for such a useful adhesive is evident and would prove useful in many applications.
SUMMARY OF THE INVENTION
mere is provided an adhesive and sealant composition, which is normally liquid in toe uncured state, and upon cure exhibits a significantly improved hot strength at elevated erasures and improved resistance to they'll degradation. This position comprises (a) at least one polymerizable \

cyanoacrylate, (b) about 0.1~ to about 10~ by weight of the composition of at least one difunctional or monofunctional polymerizable acrylate ester, I about 0.1% to about 20% by weight of the composition of an additive selected from the group consisting-of the following compounds:

O O
if if if ¢ N-R7 _R8 N 3 ,. .. ..
O O O
o O O
I ..

N-R -O O
wherein R7 and R8 are selected from the group consisting of alkyd, cycloalkyl, aralkyl, alkaryl, (d) an anionic polymerization inhibitor; and (e) optionally a free-radical polymerization inhibitor.
The instant invention solves the problems of the prior art adhesives, particularly their Liability to retain their structural integrity at elevated twitters (hot strength, as well as their low resistance to they'll degradation through heat aging. me compositions disallowed herein are useful in a variety of surfaces, particle steel, aluminum, phenolics, epoxies, and th~plastic materials. The shelf e stability of cure speed are excellent as well.

DETAILED DESCRIPTION OF THE INVENTION
_ AND PREFERRED EMBODIMENTS

The cyanoacrylate monomers useful in this invention are represented by the general formula:

I ON
CH2=C-COOR
wherein R is Clue alkyd, cycloalkyl~ alkenyl, cycloal-Kenya, phenol or heterocyclic radical. The preferred monomer which conforms to the general formula is ethyl cyanoacrylate, but a mixture of the above can be useful The polymerizable acrylate ester monomers useful in this invention may be moo- or polyfunctional, or a mix-lure of both, and conform to the general formulas:

CH2=C-COOR2 II I

wherein Al is H, SHEA or lower alkyd, R is H, alkyd, alkoxy, cycloalkyl, alkenyl, aryalkyl, alkaryl or airlocks group;
or R13~ R3 R o ( 2~mt R4J~ C OUR

wherein R3 is H, Of 4 alkyd or hydroxyalkyl or R50CH2-;
R6 is H, halogen or Of 4 alkyd; R4 is H, OH or R50-; R5 is CH2=CR6C=O; m is an integer, preferably 1 to 8; k is an integer, preferably 1 to 20; and p is O or 1.

There must be at least one acrylate ester monomer present in the instant composition, generally in the amount of about 0.1~ to about 10% by weight of the total composition. The preferred amount is about 1 to about 5%, most preferably about 1% by weight.
Among the monofunctional polymerizable acrylate ester monomers (formula II) useful, are hydroxyethyl methacrylate, hydroxypropyl methacrylate, isobornyl methacrylate, methyl methacrylate, tetrahydrofurfuryl methacrylate, and bottle methacrylate; hydroxyethyl, hydroxypropyl and methyl methacrylate and alkyd methacrylate being prefarrPd.
The polymerizable polyacrylate ester utilized in accordance with the inventioll and corresponding to the above general formula III are exemplified by, but not restricted to, the following materials diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, dipropylene glycol dimethacrylate, dip (pentamethylene glycol) dimethacrylate, tatraethylene diglyceroldiacrylate, diglycerol tetramethacrylate, tetramethylene dimethacrylate, ethylene dimethacrylate, neopentyl glycol diacrylate and trimethyol propane triacrylate. Of these, the preferred monomers are triethylene glycol dimethacrylate and polyethylene glycol dimethacrylate.
Another ingredient essential to the instant invention is an additive selected from the group consisting of any of the following structures:

I

- pa -O O

Jo No ~N-R7 O O
O O

I NUB

O O o the nature of R7 and R8 jot critical for purposes of this invention end may ye any organic radical which does not Contain any group which will adversely affect the composition for purposes disclosed herein Most commonly, R7 aloud I are S selected from the group consisting of alkyd, çycloalkyl, aralkyl, alkaryl, aureole, airlocks, alkoxy, any of which may be , exceptionally large radicals; e.g., containing up to about 200 carbon atom or more; preferably they will contain from 6 to Jo bout 100 carbon atoms, most preferably, 6 to about 50 carbon atoms.
It has been found that resistance to thermal oxidative degradation is improved if I or I is aromatic; however, this ; is not required for the general improvement of this invention I to be realized. It will of course, be understood that both R7 and R8 can consist of relatively complicated moieties, provided only that they do not contain functionality which interferes ¦ with the performance of the additive for its intended purposes.
j The useful concentrations range for this additive is about 0.1 : 1!1 to about 20%, preferably about 1 to about 5% by weight of the total composition and more preferably about 2%.
ii Without wishing to be bound by any one theory, it is be lived that the improved hot strength properties obtained from the instant composition are a result of the unique combination I; of the above maleimides with the polymerizable acrylate esters, !
j both of which are vital constituents in the cyanoacrylate adhesive composition, Theoretically, the maleimide additive reacts with the cyanoacrylate monomer during polymerization.
The maleimide ring opens to participate in thyroxine and grafts to the cyanoacrylate chain during its polymerization.

i ! _ 7 _ i!
It ' ' '' .

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Without intending to be bound to any particular chemical theory, it is believed that as the cyanoacrylate cures, the maleimide is incorporated into the cyanoacrylate chain.
Subsequent elevated temperatures are believed to induce a second stage of polymerization between the grafted maleimides, which have unsaturated cites, and the acrylic ester monomer.
Cross-linkages are thus formed. The superior ability to maintain structural properties, such as tensile strength, at temperatures of 250F or more for long periods of time, as well as to resist the general effects of thermal aging, is attributed to this unique interaction.
Generally, the amount of the maleimide additive to be used is about 0~1 to about 20% by weight of the composition, but the preferred amount is about 1 to about 5% t the most preferred amount being about 2%. Amounts of about 2% or less readily dissolve at room temperature into the cyanoacrylate and acrylic ester monomer. Above this amount, the additive may remain in suspension in the liquid composition, still serving its function and producing the desired properties, however.
Among the maleimide additives preferred is the following structure:

O O J' _R8_ no O O
where R8 is a phenol group. This compound is manufactured by E. I. Dupont de Numerous & Co., under the trade mark HA.

I. .

It is important to maintain proper stability of the composition without losing the advantage of fast cure.
The stability can be controlled by the use of known inhibitors of anionic polymerization.
Standard acidic gases, such as sulfur dioxide, sulfur trioxides and nitric oxide, can be incorporated as convent tonal inhibitors of anionic polymerization. However, it is preferred that a combination of sulfur dioxide and an acid selected from the group of sulfonic acids, phosphorus acids, phosphoric acids, and carboxylic acids, with a Pea range of about negative twelve) to about 7 (seven) be used. This inhibiting system is disclosed in Canadian Patent 1,1~2,5620 The most preferred components of the combination are sulfur dioxide with methane sulfonic acid, both present in the range of about 0.005 to about 10% by weight of the composition, but most preferably in the range of about 0.005 to about 0.1~. The preferred pro-portion ox SO to methane sulfonic acid is 20:50.
It is optional, but recommended, that an inhibitor of free-radical polymerization, selected from the group con-sitting of hydroquinones, benzoquinones, naphthoquinones, phenanthraquinones, anthraquinones, and a substituted come pound of any of the foregoing, be incorporated into the adhesive as well. Hydroquinone is the most preferred.
Generally, the amount of such inhibitors is about 0.17 to about 10~ by weight of the composition, 0.17 to 5% being preferred, and 0.95% being most preferred.
Other agents such as thickeners, plasticizers, delineates, etc. are also known in the art and may pa advantageously be incorporated where functionally desirable, provided only that they . g _ I;

do not interfere with the functioning of the vital additives for their intended purposes. The instant compositions exhibit good shelf-life stability, e.g., they normally remain liquid at room temperature in the uncured state. This, of course, can be determined by simple experimentation.
In the drawings illustrating the invention, Figures 1 and 2 are thermograms of the present invention and the prior art, respectively.

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. EXAMPLES
The flying examples are given to demonstrate the compositions within the scope of the inventicm disclosed herein. These examples are not intended to be limitations on the scope of the invention.
Below it a table ox the ingredients in each of the c~mpo-Chinese used in the examples. ~11 percentages are by weight based on the amount of ethyl cyanoacrylate. which comprises the : rest of the adhesive composition.

table I
Compositor Additives control) (control) (controls) % by weight _ A _ ED C D E F G

HA __ __ 2 2 _ 4 Jo 2 Lowe eighth-acrylate - __ 1% 1% I __ 1 I
.. inhibitors 0.2~ 0.2~ 0.24 0~2~ 0.2% 0.2~l 0.2%

!! * Compositions I) had approximately I ~hickeninq .
;, agents to increase their Brook field viscosity to 200 ens at 25C, using a No. 2 spindle Example 1 Not strengths of control compositions A and B from Table I were measured it 250 . Grit-blasted and solvent washed steel lops were used to prepare lap shear samples with these 1. compositions end the samples were then aged end tested at ~50~.
he lap shear mules were allowed Jo cure or 24 hours at room emper~ture before heat g.
. After 1 hour, the lop err tensile length ox ~omposi-Zion was 1300 psi. After US hour, the length propped to : . 700 pi, an after 4B o'er thy trying way 650 Paid 121~i~9 Composition B was similarly tested and the results are tabulated in the table below:

Table II

Tested at:250F --psi/% strength retention . . .. _ . room tempt 24 48 72 96 1 2 3 4 position control HO HIS HIS HIS HIS WACO WAS WOKS WOKS
B 3250/1120/ 690/ 650/ 670/ 400 740 700~ 500/ 520/
100%34~ 21~ 20~ 21~ 12% 23% 22~ 17% 16%

As noted from the data above, there is a significant decrease in the hot strength properties as measured by lap shear tensile at 250F beginning with the first hours of aging and continuously declining until there is little structural integrity at the bond line.
These compositions are ones typical of the prior art and clearly demonstrate the degradation of tensile strength at elevated temperature. This example is for the purpose of comparison with the compositions of the instant invention (see Examples 2 and 3).

Example 2 Compositions C and D from Table I were used to prepare grit-blasted steel lap shears, which were subsequently aged and tested at 250F. The results are tabulated in the table below:

a aye a 1 'O ox L _ owe r o o UP 3 2 o a 8 Jo a L

¦ I . x I . /

example 3 . , ,: Lap shear specimen were prepared as in the previous examples using compositions E end P and I.

TABLE IV

.._ I (P s I) LAP SHEARS TESTED AT 250~F

Composition HO HIS_ DAY _ DAYS

F . 950 9201200 1140 ; C 895 ' 96013~3 1610 I , As indicated in Table I, composition E is fiimilax to the I! control composition A, except that E has ally methacrylate as an additional ingredient. Composition is also similar to thy control composition A, except it contains the additional in-gradient HA. Composition C is an embodiment of the instant I, invention.
The results of the above Table IV indicate that the use of EVA or an acrylic ester additive in the adhesive composition by themselves produce very similar hot length results. The j composition of the instant invention demonstrates an improved : hot strength, particularly after 6 days at elevated temperature;
i-Jo I Al this example demonstrates the improved heat resistance properties of the Nanette ~ompositi~n~ steel l a p shears were prepaid with compos~tlons I, P, pa G in the tame manner I

! :
. .. . ... . . . . .. .. . .. ...

~2J
before, end heat aged at 2~00F for up to five (5) days. The lap shears were then allowed to cool to room temperature and pulled. The results are shown below:

TABLE V
S HEAT RESISTANCE
(PSI ) Room Days exposure at 2500F
Composition Temp. 1 2 I 4 5 A (control) 1600613 550 -- Jo Jo i 16421775 18001913 1592 As evidenced by the above table, composition G of the instant invention us superior in its heat resistance capability ' than those compositions of the prior art. Composition A was a typical prior art cyanoacrylate adhesive composition, and composition F had HA in its formulation (see Tale I). This jig example clearly illustrates that the instant compositions yield ' improved heat resistance characteristics over the prior art compositions.
!:

20 i This last example shows results from a thermal stability i test (thermogram). This test involves heating a sample of the if cured composition and measuring its weight loss. The weight ¦, loss is proportional to degradation of the polymer. The analysis was performed using a Perking Elmer US using the following if conditions:
!' I

lo Temperature range 40C - 400C
Heating range 20C/min.
Sample size approx. 9mg.
Purge gas air at 40cc/min. flow Chart speed 10 mm/min.

The results in the chart below indicate composition G
of the instant invention retains nearly 50% of its weight at 280% and 40% of its weight Attica. It is apparent from the table below and figures 1&2, that the compositions of this invention have two steps of decomposition, one which starts at ICKY and represents 48~ of the total sample, and another which starts at 280C and represents 42% of the sample weight.
Contrast this with one step decomposition of the prior art.
The prior art composition (control A) shows total degradation, 98.8% weight loss at 260C.

TABLE VI

% Weight Lost at TIC

Cup. 120 140 160_180 200 220_ 740 2~0 280 3~0 ~320 360 80 400 . .. . . ... .

G 0 0 0.2 0.5 1.7 7.8 25. 46.8 53.8 59.8 72 86 89 90.2 A 0 0.2 0.3 0.8 2.2 13.0 78 98.8 99.3

Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An adhesive composition having improved thermal properties comprising:
(a) at least one polymerizable cyanoacrylate monomer of the general formula I wherein R is C1-16 alkyl, cycloalkyl, alkenyl, cycloal-kenyl, phenyl or heterocyclic radical;
(b) about 0.1 to about 20% by weight of the total composition of at least one polymerizable acrylate ester monomer of the general formula:

II
wherein R1 is H, CH3 or lower alkyl, R2 is H, alkyl, alkoxy, cycloalkyl, alkenyl, aryalkyl, alkaryl or aryloxy group;

III
wherein R3 is H, C1-4 alkyl or hydroxyalkyl or R5OCH2-;
R6 is H, halogen or C1-4 alkyl; R4 is H, OH or R5O-;

R5 is CH2=CR6C=O; m is an integer, preferably 1 to 8; k is an integer; and p is 0 or 1;
(c) about 0.1 to about 20% by weight of the to-tal composition of an additive selected from the group consisting of wherein R7 and R8 are selected from the group consisting of alkyl, cycloalkyl, aryalkyl, alkaryl, aryl, aryloxy, alkoxy; and (d) an anionic polymerization inhibitor present in the amounts of about 0.1% to about 10% by weight of the total composition.

2. The adhesive composition of claim 1, wherein K
is 1 to 20.

3. The adhesive composition of claim 2 which inclu-des a free-radical polymerization inhibitor in amounts of about 0.1% to about 10% by weight of the total composition.

4. The adhesive composition of claim 2, wherein the polymerizable cyanoacrylate monomer is ethyl cyanoacrylate.

5. The adhesive composition of claim 4, wherein the polymerizable acrylate ester monomer is allyl meth-acrylate, present in the amount of about 1% to about 5%
by weight of the total composition.

6. The adhesive composition of claim 5, wherein the additive (c) is represented by the formula:

wherein R8 is phenyl.

7. The composition of claim 6 wherein the additive (c) is present in the amount of about 2% by weight of the total composition.

8. The adhesive composition of claim 2, wherein the ionic polymerization inhibitor is a combination of an acid and sulfur dioxide, in a ratio of 50:20.

9. The adhesive composition of claim 8, wherein the acid is sulfonic acid, present in concentrations between about 0.005% to about 0.002% by weight of the composition.

10. The adhesive composition of claim 9, wherein the sulfonic acid is methane sulfonic acid.

11. The adhesive composition of claim 10, wherein the sulfonic acid is hydroxy propane sulfonic acid.

12. The adhesive composition of claim 2, wherein the free-radical polymerization inhibitor is selected from the group consisting of hydroquinones, benzoquinones, naphtho-quinones, phenanthraquinones, and anthraquinones.

13. The adhesive composition of claim 2, wherein the polymerizable acrylate ester monomer corresponding to formula I is hydroxyethyl methacrylate, hydroxypropyl methacrylate, isobornyl methacrylate, methyl methacrylate, tetrahydrofurfuryl, methacrylate, or butylmethacrylate.

14. The adhesive composition of claim 2, wherein the polymerizable acrylate ester monomer corresponding to formula II is tetraethylene glycol dimethacrylate, triethylene glycol dimethacrylate, polyethylene or glycol dimethacrylate.

15. The adhesive composition of claim 2, wherein the composition contains a sulfimide or a tertiary amine, present in the amount of about 0.1 to about 10% by weight of the composition, as an accelerator of free-radical polymerization.