IE53977B1 - Improved hot strength cyanoacrylate adhesive composition - Google Patents

Abstract

An adhesive composition comprises (a) at least one polymerizable cyanoacrylate monomer, (b) 0.1 to 10% by weight of a polymerizable acrylate ester e.g. allyl methacrylate and (c) 0.1 to 20% by weight of a maleimide derivative of formula wherein R<7> and R<8> are from alkyl, cycloalkyl, aralkyl or arkaryl. The compositions may polymerize with ring-opening of the maleimide. They have excellent hot strength properties for long periods at 121 DEG C (250 DEG F) or more.

Description

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.

Adhesive and sealant compositions based on cyanoacrylate monomers are known in the art. Typical examples of patents relating to cyanoacrylate adhesives are U.S. Patents 2,784,215 to Joyner, 2,794,788 to Coover et al., and Patent Specification No. 31537 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 shelf15 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 time, 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 temperatures to retain their usefulness. - 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 di-functional cyanoacrylate monomers were used. Further, 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 U.S. Patent 3,832,334, the problem of thermal resistance was solved by the addition of maleic anhydrides and their substitution products. Additionally, it has been known in the prior art to include cross-linking agents such as allyl2-cyanoacrylate or polymerizable acrylate esters to improve thermal properties. However, none of the prior art has shown the improved thermal properties at 121°C (25O°F) that the present invention is able to demonstrate. Thus, the need for such a useful adhesive is evident and would prove useful in many applications.

The present invention provides adhesive or sealant compositions, which are normally liquid in the uncured state, and upon cure exhibit a significantly improved hot strength at elevated temperatures and improved resistance to thermal degradation. The compositions comprise: (a) at least one polymerizable cyanoacrylate monomer of the general formula CN I I CH2=C-COOR wherein R is Cllg alkyl, C3_lg cycloalkyl, C2_lg alkenyl, C3-16 cycloalkenyl, phenyl or a heterocyclic radical having up to 16 carbon atems and containing at least, one ring with a hetero atom selected from nitrogen, oxygen and sulphur; (b) 0.1% to 10% by weight of the composition of at least one polymerizable acrylate ester monomer of the general formula - 4 533 77 II CH_=C-C00R2 2 I R' wherein R1 is H or c1_4 alkyl, R2 is H, Cj_lg alkyl, hydroxyC1_16alkyl, C3_lg cycloalkyl, C216 alkenyl, aralkyl, aryl or alkaryl each having up to 16 carbon atoms or tetrahydrofurfuryl; ΙΟ or wherein R. is H, Cj_4 alkyl or hydraxyC^alkyl or R OCH. R6 is H, halogen or C1-4 alkyl; R4 is H, OH or R^O; R^ is CH2=CR§C=O; m is an integer of 1 to 8; k is an integer of 1 to 20; and p is 0 or 1; (c) 0.1% to 20% by weight of the total composition of an additive of any one of the formulae 539 77 wherein R is alkyl, cycloalkyl, aralkyl, alkaryl or Q aryl and R is alkylene, cycloalkylene, arylalkylene, alkarylene or arylene each of R and R having up to 200 carbon atoms; and (d) an anionic polymerization inhibitor. The composition may optionally also contain a free-radical polymerization inhibitor.

The present invention solves the problems of the prior art adhesives, particularly their inability to retain their structural integrity at elevated temperatures (hot strength), as well as their low resistance to thermal degradation through heat aging. The compositions disclosed herein are useful for use with a variety of surfaces, particularly steel, aluminum, phenolics, epoxies, and thermoplastic materials. The shelflife stability and cure speed are excellent as well.

Subject to the above limitations:The preferred monomer which conforms to general formula I is ethyl cyanoacrylate, but a mixture of monomers of this formula can be used.

The polymerizable acrylate ester monemers useful in this invention may be mono- or polyfunctional, or a mixture of both. At least one acrylate ester monomer must be present in the composition of the invention, in an amount of 0.1% to 10% by weight of the total composition. The preferred amount is 1 to 5% most preferably about 1% by weight.

Among the monofunctional polymerizable acrylate ester monomers (formula II) which are useful, are hydroxyethyl methacrylate, hydroxypropyl methacrylate, isobomyl methacrylate, methyl methacrylate, tetrahydrofurfuryl methacrylate, - 6 39/7 and butyl methacrylate. Hydroxyethyl methacrylate, hydroxypropyl methacrylate, methyl methacrylate and allyl methacrylate are preferred.

The polymerizable polyacrylate esters utilized in 5 accordance with the invention 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, di-(penta10 methylene glycol) dimethacrylate, tetraethylene diglycerol diacrylate, diglycerol tetramethacrylate, tetramethylene dimethacrylate, ethylene dimethacrylate, neopentyl glycol diacrylate and trimethylol propane triacrylate. Of these, the preferred monomers are triethylene glycol dimethacrylate and polyethylene glycol dimethacrylate. 8 The nature of R and R in the additive (c) is not critical for purposes of this invention. The groups R7 and Q R may be exceptionally large radicals i.e.» containing up 7 8 to 200 carbon ' atoms. Preferably Rz and R will contain from 6 to about 100 carbon atoms, most preferably, 6 to about 50 carbon atoms.

It has been found that resistance to thermal oxidative 7 8 degradation is improved if R or R is aromatic; however, this is not required for the general improvement of this invention to be realized. It will, of course, be understood that both 7 8 R and R 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. The useful concentration range for this additive is 0.1 to 20%, preferably 1 to 5% by weight of the total composition and most preferably about 2%.

Without wishing to be bound by any one theory, it is believed that the improved hot strength properties obtained from the instant composition are a result of the unique combina35 tion of the above maleimides with the polymerizable acrylate esters, both of which are vital constituents in the cyanoS 3 9 7 7 - 7 acrylate adhesive or sealant composition. Theoretically, the maleimide additive reacts with the cyanoacrylate monomer during polymerization. The maleimide ring opens to participate in the reaction and grafts to the cyanoacrylate chain during its polymerization.

As referred to above it is not the intention that the invention should be bound by any particular chemical theory, but 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 sites, and the acrylic ester monomer. Cross-linkages are thus formed. The superior ability to maintain structural properties, such as tensile strength, at temperatures of 121°C (25O°F) or more for long periods of time, as well as to resist the general effects of thermal aging, is atrributed to this unique interaction.

Generally, the amount of the maleimide additive to be used is 0.1 to 20% by weight of the composition, but the preferred amount is 1 to 5%, 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: where R is a phenylene group. Ε. I, DuPont de Nemours & Co., This compound is manufactured by under the trade name HVA. 3 S 7 7 - 8 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 trioxide and nitric oxide, can be incorporated as conventional inhibitors of anionic polymerization. However, it is preferred that a combination of sulfur dioxide and an acid selected from sulfonic acids, phosphorus acids, phosphonic acids, and carboxylic acids, with a PKa range of about -12 (negative twelve) to about 7 (seven) be used.

The most preferred components of the combination are sulfur dioxide with a sulfonic acid preferably methane sulphonic acid or hydroxypropane sulphonic acid, both present in the range of 0.005 to 10% by weight of the composition, preferably 0.005 to 0.1%. The preferred proportion of SOj to methane sulfonic acid is 20:50.

It is optional, but recommended, that an inhibitor of free-radical polymerization, preferably selected from hydro20 quinones, benzoquinones, naphthoquinones, phenanthraquinones, anthraquinones and substituted derivatives of any of the foregoing be incorporated into the adhesive as well. Hydroquinone is the most preferred.

Generally, the amount of such inhibitors is 0.17 to 10% by weight of the composition, 0.17 to 5% being preferred, and 0.95% being most preferred.

The composition may contain a sulphimide or a tertiary amine, present in an amount of 0.1 to 10% by weight of the composition as an accelerator of free-radical polymerization.

Other agents such as thickeners, plasticizers and diluents are also known in the art and may advantageously be incorporated where functionally desirable, provided only that they do not interfere with the functioning of the vital additives for their intended purposes. The present compositions exhibit good shelf-life stability, e.g., they normally remain 3 9 7 7 - 9 liquid at room temperature in the uncured state. This, of course, can be determined by simple experimentation.

EXAMPLES The following examples are given to demonstrate the compositions within the scope of the invention disclosed herein. These examples are not intended to be limitations on the scope of the invention.

Below is a table of the ingredients in each of the compositions used in the examples. All percentages are by weight based on the amount of ethyl cyanoacrylate, which comprises the rest of the adhesive composition.

Table 1 Compositions Additives (control) (control) A *B C *D (controls) E F G HVA — — 2% 2% — 4% 2% allyl meth- — — 1% 1% 2% — 5% acrylate inhibitors 0.2% 0.2% 0.2% 0.2% 0.2% 0.2% 0.2% ‘Compositions B & D had approximately 5% thickening agents to increase their Brookfield viscosity to 200 mPaS (200 cps) at 25°C, using a No. 2 spindle.

Example 1 Hot strengths of control composition A and B from Table I were measured at 121°C (25O°F). Grit-blasted and solvent washed steel laps were used to prepare lap shear samples with these compositions and the samples were then aged and tested at 121°C (25O°F). The lap shear samples were allowed to cure for 24 hours at room temperature before heat aging.

After 1 hour, the lap shear tensile strength of composition A was 1300 psi. After 25 hours, the strength dropped to 700 psi, and after 48 hours, the strength was 650 psi.

Composition B was similarly tested and the results are tabulated in the table below: 3 9 Ύ 7 - 10 Table II HOT STRENGTH AFTER AGING AT 121°C (25O°F) Tested at 121°C (250°F) Kg/cm2 (psi)/% strength retention Composi- tion room tenp. control 1 HR 24 HRS 48 HRS 72 HRS 96 HRS 1 WK 2 WKS 3 WKS 4 WKS B 228.5 (3250)/ 78.7 (1120)/ 48.5 (690)/ 45.7 (650)/ 47.1 (670)/ 28.1 (400)/ 52.0 (740)/ 49.2 (700)/ 35.2 (500)/ 36.6 (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 121°C (25O°F) beginning with the first hours of aging and continuously declining until there is little structural integrity at the bondline.

These compositions are 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 present 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 121°C (250°F). The results are tabulated in the table below: 39 7 7 - 11 co ου ΟΥ id Μ· ΟΥ • Ο ΙΟ CM * Ο ιη ου drl'f a ο •d •d W Ο Ο ϋ \ X 00 CA • o • o <*> g r- cn <#> H O m to rd in id rd id in id id m X \ r* CA oo O • o CM g • CM t*> oy in <* cm cn m Ο m O OY rd d* rd rd in xx X X in *-x 2 • o rd kJ CM Ο Λ» cn p* dp id in in O d· r* id rd in dd X X CM o in*-» • o • o in ca id d1 <#> ου O dP Ol K o^o id i** in W id id in id id in *-* ·—* X σ» <χ m*-» • o • o io ca in CM dP rdf** dP r· K O in cn cn co o m id id If) rd id IO X X rd *>x OY xx. • o • o CO CA id 00 OP cn cm dp d* 05 rd in in id in CM w id rd in id id in \ X d· co σ\ O oo CM K • in & • 00 dP K rr cn r> I- cn d1 OY rd d* σι H 'T ·*** X»» X X OY XX rd 05 co o • o » • in dP id in dP OHO OMh co id d* rd id d· ft 6 H Φ 0 X d m 10 xx 44 • O dp • O dP ε c cm ω O 00 id O 0 0 Ο» o rd id O Q U CM CM H cm cn H 05 & Λ C ο •Η ti Φ > +» ti φ W S § Η S ft 8· Φ Λ XJ 44 Φ til ti Φ ti 4J 44 (ti (0 XJ 4J « Φ (ti U •d Ό ti Φ 44 O Φ > O Φ XJ 44. ti (ti ti •d ti a XJ ti» ti Φ ti ω O XJ Φ XJ ti O (0 •d d (ti ti) e o ο φ d 539 (7 - 12 Example 3 Lap shear specimens were prepared as in the previous examples using compositions E and F and G.

TABLE IV HOT STRENGTH AFTER AGING AT 121°C (25O°F) Kg/cm2 (PSI) LAP SHEARS TESTED AT 121°C (25O°F) Composition E 1 HR 4 HRS I DAY 6 DAYS 50.6 (720) 38.0 (540) 62.6 (890) 73.8 (1050) F 66.8 (950) 64.7 (920) 84.4 (1200) 80.1 (1140) G 62.9 (895) 67.5 (960) 94.4 (1343) 113.2 (1610) As indicated in Table I, composition E is similar to the control composition A, except that E has allyl methacrylate as an additional ingredient. Composition F is also similar to the control composition A, except it contains the additional ingredient HVA. Composition G is an embodiment of the present invention.

The results of the above Table IV indicate that the use of HVA or an acrylic ester additive in the adhesive composition by themselves produces very similar hot strength results. The composition of the present invention demonstrates an improved hot strength, particularly after 6 days at elevated temperatures Example 4 This example demonstrates the improved heat resistance properties of the composition of the invention. Steel lap shears were prepared with compositions A, F and G in the same manner as before, and heat aged at 121°C (25O°F) for up to five (5) days. The lap shears were then allowed to cool to room temperature and pulled. The results are shown below: 3 9 7 7 - 13 TABLE V HEAT RESISTANCE kg/cm2 (PSI) Composition Room Temp. Days exposure at 121°C (25O°F) 1 2 3 4 5 A (control 112.5 (1600) 43.1 (613) 38.7 (550) F 142.4 (2025) __ — 71.5 (1017) 64.5 (917) 63.3 (900) G 115.4 (1642) 124.8 (1775) 126.5 (1800) 134.5 (1913) 111.9 (1592) As evidenced by the above table, composition G of the present invention is 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 HVA in its formulation (see Table I). This example clearly illustrates that the compositions of the invention yield improved heat resistance characteristics over the prior art compositions.

Example 5 This last example shows results from a thermal stability test (thermogram). This test involves heating a sample of the cured composition and measuring its weight loss. The weight loss is proportional to degradation of the polymer. The analysis was performed using a Perkin Elmer TGS using the following conditions: Temperature range Heating range Sample size Purge gas Chart speed 40°C - 400°C °C/min. approx. 9mg. air at 40oc/min. flow mm/min.

The results in the chart are shown as Figs. 1 and 2 of the accompanying drawings. These results indicate composition G of the present invention retains nearly 50% of its weight at 280% and 40% of its weight at 300°C. It is apparent from the - 14 table below and figures 1 & 2, that the compositions of this invention have two steps of decomposition, one of which starts at 160°C and represents 48% of the total sample, and the other of which starts at 28O°C and represents 42% of· the sample weight. This can be contrasted with one step decomposi tion of the prior art. The prior art composition (control A) shows total degradation, 98.8% weight loss at 26O°C.

TABLE VX % Weight Lost at T°C 120 140 160 180 200 220 240 260 280 300 320 360 380 400 0 0 0 0.2 0.2 0.3 0.5 0.8 1.7 2.2 7.8 13.0 25. 78 46.8 98.8 53.8 99.3 59.8 72 86 89 90.

Claims (15)

1. CLAIMS:1. An adhesive or sealant composition having improved thermal properties comprising: (a) at least one polymerizable cyanoacrylate monomer of the general formula CN I I CH 2 =C-COOR wherein R is C 1 _ lg alkyl, C 3 _ lg cycloalkyl, C 2 _ lg alkenyl, C 3 phenyl or a heterocyclic radical having up to 16 carbon atoms and containing at least one ring with a hetero atom selected from nitrogen, oxygen and sulphur; (b) 0.1 to 10% by weight of the total composition of at least one polymerizable acrylate ester monomer of the general formula: CH o =C-C00R 2 I, II R 1 2 wherein R is H or C^^alkyl, R is H, Cj_i g alkyl, hydroxyC 1 _ lg alkyl, C 3 _i g cycloalkyl, C 2 _i g alkenyl, aralkyl, aryl or alkaryl each having up to 16 carbon atoms or tetrahydro furfuryl; or III P wherein R is H, Cj_ 4 alkyl or hydroxy C 3 _. alkyl or R 5 OCH9-; R 6 is H, halogen or C. , alkyl; is H, OH or R O-; R° is CH2=CR°C=O; m is an integer of 1 to 8; k is an integer of 1 to 20; and p is 0 or 1; (c) 0.1 to 20% by weight of the total composition of an additive of any one of the formulae 539 77 wherein R is alkyl, cycloalkyl, aralkyl, alkaryl or aryl, © and R is alkylene,cycloalkylene, arylalkylene, alkarylene or 7 8 arylene each of R and R having up to 200 carbon atoms; and 5 (d) an anionic polymerization inhibitor.

2. A composition as claimed in claim 1 which also contains a free-radical polymerization inhibitor.

3. A composition as claimed in Claim 1 or 2, wherein the polymerizable cyanoacrylate monomer is ethyl cyanoacrylate.

4. A composition as claimed in any of claims 1 to 3, wherein the additive (c) is represented by the formula: 5. 3 9 7 7 Ο wherein R is phenylene.

5. A composition as claimed in Claim 4 wherein the additive (c) is present in an amount of about 2% by weight of the total composition.

6. A composition as claimed in any of Claims 1 to 5, wherein the anionic polymerization inhibitor is a combination of sulfur dioxide, and an acid selected from sulphonic acids, phosphorus acids, phosphonic acids and carboxylic acids with a pKa in the range from -12 to 7.

7. A composition as claimed in Claim 6, wherein the acid is a sulfonic acid, which is present in a concentration from 0.005% to 0.1% by weight of the composition.

8. A composition as claimed in Claim 7, wherein the sulfonic acid is methane sulfonic acid.

9. A composition as claimed in Claim 7, wherein the sulfonic acid is hydroxy propane sulfonic acid.

10. A composition as claimed in any of claims 1 to 9 which contains a free-radical polymerization inhibitor which is a hydroquinone, a benzoquinone, a naphthoquinone, a phenanthraquinone, or an anthraquinone, any of the foregoing being optionally substituted.

11. A composition as claimed in any of claims 1 to 10 wherein the polymerizable acrylate ester monomer of formula XI is allyl methacrylate present in an amount of 1% to 5% by weight of the total composition.

12. A composition as claimed in any of claims 1 to 10, wherein the polymerizable acrylate ester monomer of formula IX 5 3 9 7 7 - 18 is hydroxyethyl methacrylate, hydroxypropyl methacrylate, isobomyl methacrylate, methyl methacrylate, tetrahydrofurfuryl methacrylate or butylmethacrylate.

13. A composition as claimed in any of claims 1 to 10, 5 wherein the polymerizable acrylate ester monomer corresponding to formula III is tetraethylene glycol dimethacrylate, triethylene glycol dimethacrylate, or a polyethylene glycol dimethacrylate.

14. A composition as claimed in any of claims 1 to 13, 10 wherein the conpositicn further contains a sulphimids or a tertiary amine, present in the amount of 0.1 to 10% by weight of the composition, as an accelerator of free-radical polymerization.

15. An adhesive or sealant composition as claimed in claim 1 substantially as described with particular reference to 15 compositions C, D and G.