Cyanoacrylate adhesives with improved cured thermal properties utilizing substituted aromatic additive

Abstract: A cyanoacrylate monomer adhesive formulation which has improved thermal properties resulting from the inclusion in the formulation of an effective amount for enhancing the thermal resistance of the cured polymer of a mono, poly or hetero aromatic compound characterized by at least three substitutions on an aromatic ring thereof, two or more of said substitutions being electron withdrawing groups selected from the group consisting of NO₂, CN, CF₃, NR¹₃⁺, SR¹₂⁺, C(=O)R¹, C(=O)OR¹, NO, CCl₃, SO₂, S(=O), SO₃, SO₂R¹, SO₂OR¹ and F, one or more of said substitutions being leaving groups selected from the group consisting of F, Br, Cl, I, NO₂, CN, SOR¹, SO₂R¹ and SO₂OR¹, and R¹ is an optionally substituted hydrocarbon group. Example such compounds have the formula: where L is the leaving group and the W groups are the electron withdrawing groups.

Description: Background Of the Invention Cyanoacrylate monomer adhesives are well known and widely used as "instant adhesives", so-called because of their very rapid polymerization on contact with surfaces containing even very weak anion sources. Basic activators are used to improve sensitivity of the cure of cyanoacrylates (compare Irving Skeist: "Handbook of Adhesives", third edition, page 475, Nostrand Reinhold, New York (1990)). In DE-A-12 29 698 specific heterocyclic base compounds are used in order to accelerate cure within few second. However, a notorious problem of cyanoacrylate polymers is their susceptibility to thermal degradation at only moderately high temperatures. As a consequence, the instant adhesive benefits of cyanoacrylate monomers have not been available in many applications where the bonded substrates may be subjected to intermittent temperatures in excess of 120°C or extended exposure to temperatures of about 80°C. This problem of thermal stability of cyanoacrylate polymers is distinct from the problem of stabilizing monomer formulations against premature polymerization. However, to preserve the instant adhesive benefits of cyanoacrylates, it must be appreciated that improvements in polymer thermal stability should not significantly degrade the storage stability or cure speed of the monomer formulation from which the polymer is derived. Numerous attempts have been made to improve the thermal stability of cyanoacrylate adhesive bonds. In US 3,832,334, the addition of maleic anhydride is said to produce adhesives which have increased thermal resistance while preserving fast cure speed of the cyanoacrylate adhesive. In US 4,196,271, tri-, tetra- and higher carboxylic acids or their anhydrides are said to be useful form improving heat resistance of cured cyanoacrylate adhesives. Phthalic anhydride is reported to improve heat resistance of cyanoacrylate adhesive bonds in US 4,450,265 and benzephenonetetracarboxylic acid or its anhydride are reported to provide a superior heat resistance for cyanoacrylate adhesives in US 4,532,293. According to Chem. Abst., 85 :64138p a cyanoacrylate adhesive which includes a graft copolymer of methyl methacrylate and a fluorine containing rubber as a plasticizer is reported to give improved stability to thermal shocks. Cyanoacrylate adhesives containing elastomeric acrylic rubbers are reported to give improved properties, particularly after exposure to elevated temperatures, in US 4,440,910. In US 4,490,515, cyanoacrylate compounds containing certain maleimide or nadimide compounds are reported to improve the hot strength properties of cyanoacrylate adhesives. Mixtures of certain sulfone compounds and a dicarboxylic acid or dicarboxylic anhydride are said to greatly improve heat resistance of cyanoacrylate adhesives in JP 55/066980. In JP 48/8732 cyanoacrylates containing 3-25% divinyl sulfone are reported to have improved heat resistance. US 4,560,723 describes certain cyanoacrylate adhesives containing a certain treated copolymer toughener, and a "sustainer" compound having certain compatibility properties said to provide improved retention of toughness on heat aging of the cured adhesive. Various substituted aryl compounds are identified as suitable "sustainers," including nitrobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene and bromochlorobenzene. Despite this extensive work in the art, there continues to be a need to identify materials which will improve the heat performance of cyanoacrylate adhesives so as to increase the options available to the formulator and/or provide further improvements over the existing additives known to enhance cyanoacrylate thermal resistance. Summary Of The Invention The inventors herein have discovered that certain aromatic compounds, distinct from those utilized in the formulations of US 4,560,723, provide cyanoacrylate adhesive compositions with improved thermal resistance properties. The invention is further distinguished from that of US 4,560,723 in that the improvement provided by the aromatic compounds is not limited to polymer toughened formulations. Furthermore, unlike previously reported cyanoacrylate formulations utilizing substituted aromatic carboxylic acids or anhydrides or aromatic sulfonic acids (e.g., US 3,652,635 (nitrobenzene sulfonic acid); US 4,377,490 (nitrobenzoic, nitrovanilic and nitrophthalic acids); and US 4,196,271 (polycarboxylic acids or anhydrides, optionally substituted with nitro or halo groups)), the aromatic additive compounds used in the invention are non-acidic and do not substantially retard fixture speeds of the adhesive. The invention comprises a cyanoacrylate monomer adhesive formulation which has improved thermal properties resulting from the inclusion in the formulation of an effective amount for enhancing the thermal resistance of the cured polymer of a mono, poly or hetero aromatic compound, other than an aromatic base compound, characterised by at least three substitutions on an aromatic ring thereof, two or more of said substitutions being electron withdrawing groups selected from the group consisting of NO 2 , CN, CF 3 , NR 1 3 + , SR 1 2 + , C(=O)R 1 , C(=O)OR 1 , NO, CCl 3 , SO 2 , S(=O), SO 3 , SO 2 R 1 , SO 2 OR 1 and F, one or more of said substitutions being leaving groups selected from the group consisting of F, Br, Cl, I, NO 2 , CN, SOR 1 , SO 2 R 1 and SO 2 OR 1 , and R 1 is an optionally substituted hydrocarbon group. Example such compounds have the formula: where L is the leaving group and the W groups are the electron withdrawing groups. The aromatic compounds are suitably employed at levels in the range of 0.5% - 15% by weight of the formulation, preferably at least 1.0% and more typically 3% - 10% by weight of the formulation. Detailed Description of the Invention: The α-cyanoacrylate adhesive compositions of this invention contain as their principal ingredient one or more α-cyanoacrylate monomers of the formula: where R represents a C 1-16 alkyl, cycloalkyl, alkenyl, alkynyl, cycloalkenyl, alkaryl, aralkyl or aryl group, any of which may be optionally substituted or, interrupted with non-basic groups, such as oxo, halo, silicon and ether oxygen, which do not interfere with the stability and functioning of the monomer as an adhesive. Specific examples of the groups for R are a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a pentyl group, a hexyl group, an allyl group, a methallyl group, a crotyl group, a propargyl group, a cyclohexyl group, a benzyl group, a phenyl group, a cresyl group, a 2-chlorobutyl group, a trifluoroethyl group, a 2-methoxyethyl group, a 3-methoxybutyl group and a 2-ethoxyethyl group. Ethyl cyanoacrylate is the preferred monomer for use in the inventive compositions. A single α-cyanoacrylate monomer or a mixture of two or more of these α-cyanoacrylate monomers can be used. For a number of applications, the above α-cyanoacrylate monomer alone is not sufficient as an adhesive, and at least some of the components set forth below are typically added. (1) An anionic polymerization inhibitor; (2) A radical polymerization inhibitor; (3) A thickener; (4) Special additives such as cure accelerators, plasticizers tougheners and heat stabilizers; (5) Perfumes, dyes, pigments, etc. A suitable amount of the α-cyanoacrylate monomer present in the adhesive composition is about 75 to 99 by weight, based on the total weight of the adhesive composition. An anionic polymerization inhibitor is added to the α-cyanoacrylate adhesive composition, e.g. , in an amount of 1 to 1000 ppm based on the total weight of the adhesive composition, to increase the stability of the adhesive composition during storage, and examples of known inhibitors are sulfur dioxide, sulfur trioxide, nitric oxide, hydrogen fluoride, and certain sultones. Particularly preferred for purposes of this invention are combinations of methane sulfonic acid (MSA) or hydroxypropane sulfonic acid (HPSA) with sulfur dioxide. Preferred concentrations of sulfonic acids range from 5 to about 100, more preferably 10 to 50, parts per million (based on monomer weight). The preferred concentrations of SO 2 range from 15 to 50 ppm for either acid. While not essential, the cyanoacrylate adhesive compositions of this invention generally also contain an inhibitor of the free radical polymerization. The most desirable of these inhibitors are of the phenolic type, such as quinone, hydroquinone, t-butyl catechol, p-methoxyl-phenol, etc. The above inhibitors may be used within wide ranges, but the following general guidelines are representative of the adhesive composition: acid gases, from 0.001% to 0.06% by weight; sultones, from 0.1% to 10% by weight; sulfonic acids, from 0.0005% to 0.1% by weight; and free radical inhibitors, from 0.001% to 1%. A thickener frequently is added to increase the viscosity of the α-cyanoacrylate adhesive composition. The α-cyanoacrylate monomer generally has a low viscosity of about several centipoises, and therefore, the adhesive penetrates into porous materials such as wood and leather or adherends having a rough surface. Thus, good adhesion strengths are difficult to obtain. Various polymers can be used as thickeners and examples include poly(methyl) methacrylate, methacrylate-type copolymers, acrylic rubbers, cellulose derivatives, polyvinyl acetate and poly(α-cyanoacrylate). A suitable amount of thickener is generally about 20% by weight or less based on the total weight of the adhesive composition, preferably 3%-10% based on the total weight of the adhesive composition. A number of conventional polymer additives may also be added for toughening purposes. Examples include acrylic elastomers, acrylonitrile copolymer elastomers and fluoro elastomers. In appropriate amounts such materials may serve as both thickener and toughener. Certain fumed silica fillers may also be usefully employed as cyanoacrylate thickeners. Various hydrophobic silicas are reported in the art to give satisfactory storage stable gels with cyanoacrylates. See, for instance US 4,477,607, US 4,533,422, US 4,686,247, US 4,837,260 and GB 2,204,872, all incorporated herein by reference. Preferred silicas are polydimethylsiloxane, octyltrimethoxysilane or hexamethyldisilazane treated silicas such as Cab-O-Sil® TS-720 (Cabot Corp.), Aerosil® R202 (Degussa Co.), Aerosil® R805 (Degussa Co.) or Cab-O-Sil® TS-530 (Cabot Corp.). Mixtures of such silicas may also be employed. Suitable levels of such silicas are in the range of 3%-12% based on total composition weight, preferably 4%-8%. As examples of cure accelerators there are known, for instance, calixarene compounds as described in US 4,556,700 and US 4,695,615 and silacrown compounds as described in US 4,906,317. Other accelerators are well known to those skilled in the art. The thermal property enhancing aromatic additives utilized in the invention are mono, poly or hetero aromatic compounds, other than an aromatic base compound, characterised by at least three substitutions on an aromatic ring thereof, two or more of said substitutions being electron withdrawing groups (W) selected from the group consisting of NO 2 , CN, CF 3 , NR 1 3 + , SR 1 2 + , C(=O)R 1 , C(=O)OR 1 , NO, CCl 3 , SO 2 , S(=O), SO 3 , SO 2 R 1 , SO 2 OR 1 and F, one or more of said substitutions being leaving groups (L) selected from the group consisting of F, Br, Cl, I, NO 2 , CN, SOR 1 , SO 2 R 1 and SO 2 OR 1 , and R 1 is an optionally substituted hydrocarbon group. Suitably R 1 is an optionally substituted alkyl or aryl group, optional substitutions including alkoxy, halo, cyano, and nitro groups. Where R 1 is an aromatic group it may be the base aromatic group of the molecule to form, for instance, a sulfone compound. Acidic functional groups, such as carboxylic acid, phenolic hydroxyl and anhydride groups are generally detrimental to the fixture speed of the cyanoacrylate adhesives and therefore should usually be excluded from the aromatic additive compounds used in the invention. Specific examples of aromatic compounds useful in the invention include: 2,4-dinitrofluorobenzene; 2,4-dinitrochlorobenzene; 2,4-difluoronitrobenzene; 3,5-dinitrobenzonitrile; 2-chloro-3,5-dinitrobenzonitrile; 4,4'-difluoro-3,3'-dinitrophenyl sulfone; pentafluoronitrobenzene; pentafluorobenzonitrile; α,α,α,2-tetrafluoro- p -tolunitrile; and tetrachloroterephthalonitrile. These compounds are usefully employed at levels in the range of 0.5% - 15% by weight of the formulation, preferably at least 1.0% and more typically 3% - 10% by weight of the formulation. The cyanoacrylate polymers of the invention which include an aromatic additive as defined above have higher thermal decomposition temperatures than compositions employing no additive. By way of contrast, polymers of the invention can be readily formulated to provide an onset of decomposition temperature when heated at 10°C per minute of at least 200°C whereas additive free polymers provide a decomposition onset temperature of only 155°C. Additionally, typical polymers of the invention are characterized by a weight loss of 25% or less, frequently 20% or less when heated at 150°C for 900 minutes, as opposed to a 98% weight loss under identical conditions for an additive free composition. Other additives which may be added to the cyanoacrylate adhesive compositions of the invention are plasticizers. Plasticizers serve to make the cured bonds less brittle and, therefore, more durable. The most common of these plasticizers are C 1 to C 10 alkyl esters of dibasic acids such as sebacic acid and malonic acid. Other plasticizers, such as diaryl ethers and polyurethanes, also may be used, and a variety of other plasticizers are also known. The plasticizers, as well as cyanoacrylate compatible perfumes, dyes, and pigments, may be added depending on desired uses in amounts which do not adversely affect the stability of the α-cyanoacrylate monomer. The use of such additives is within the skill of those practicing in the cyanoacrylate adhesive art and need not be detailed herein. EXAMPLES The effects of various aromatic compound additives on the thermal properties of cured cyanoacrylate polymers were investigated in several ways. Thermal analyses of cyanoacrylate polymers to which had been added amounts of additive as shown in Table I were conducted dynamically to determine temperature at which decomposition onset occurred and isothermally at 150°C to determine relative weight loss of polymer on heat aging. Fixture speeds and 82°C stability were performed on monomer formulations containing the additives to ascertain whether the additive affected the cure speed or storage stability of the formulation. Thermal analysis was done using two different instruments. the DuPont 2100 Thermal System with 951 Thermogravimetric Analyzer attached, and Seiko SSC5245HM2 controller attached to TG/DTA220 Thermogravimetric Differential Thermal Analyzer. Isothermal thermal runs were started from 30°C and heated at 50°C/minute up to 150°C and held at that temperature for 900 minutes under 250 cc/min nitrogen gas flow. Temperature dynamic runs were started at 30°C and heated at 10°C/min up to 450°C under 250 cc/min nitrogen gas flow. Samples for dynamic analyses were all prepared by intimate mixing with a mortar and pestle of a prepolymerized cyanoacrylate polymer and the additive followed by heating of the mixture at 100°C for two hours. Samples for isothermal analyses were either prepared in the same way or from a film prepared by placing a droplet of a monomer formulation containing the additive between pressed Santoprene™ blocks for 72 hours at room temperature, post-curing at 100°C for 2 hours to completely polymerize all residual monomer present, and then separating the resulting polymer film. Both methods of sample preparation were found to give equivalent results. Freshly distilled ethyl cyanoacrylate monomer containing methane sulfonic acid (10 ppm), sulfur dioxide (2 ppm) and hydroquinone (3000 ppm) was used in preparing thermogravimetry analysis samples. Fixture speeds were measured on formulations containing a polymethyl methacrylate (6 wt%), hydrophobic silica (6 wt%), calixerene (0.5 wt%) and silacrown (0.5 wt%) in ethyl cyanoacrylate monomer. Results are summarized in Table I. Additive % Onset of decomposition temp (°C) % Wt loss at 150°C in 900 min 82°C Stability (days) Fixture speed (sec) Balsa wood Cow leather Invention Examples 2,4-Dinitrofluorobenzene 5 208 16 20 25 40 2,4-Dinitrochlorobenzene 5 200 15 20 25 40 3,5-Dinitrobenzonitrile 5 200 15 20 23 38 2,4-Difluoronitrobenzene 8 187 53 20 22 34 2-Chloro-3,5-dinitro benzonitrile 10 205 15 20 23 38 Pentafluoronitrobenzene 8 203 15 20 22 35 Pentafluorobenzonitrile 8 201 15 20 22 35 α,α,α,2-Tetrafluoro- p -tolunitrile 6 195 25 20 22 35 Tetrachloroterephthalonitrile 5 190 45 20 22 35 4,4'-Difluoro-3,3'-dinitrophenyl sulfone 8 200 25 20 22 35 Comparative Examples None (control) - 155 98 20 22 35 Phthalic anhydride 5 155 96 20 40 50 3-Nitrophthalic anhydride 3 170 70 20 40 50 Tetrafluorophthalic anhydride 2 190 50 20 43 56 2,4-Dinitrophenol 5 164 76 18 60 100 4-Bromochlorobenzene 10 155 96 20 25 40 1,2,4-Trichlorobenzene 10 155 96 20 25 45 Nitrobenzene 20 155 96 20 25 45 1,2,4,5-Benzenetetracarboxylic dianhydride 2 155 96 20 60 100 From Table 1 it can be seen that the onset of thermal decomposition was substantially improved when an aromatic additive having two or more electron withdrawing groups and one or more leaving groups, as defined above, was employed but there was no improvement in the thermal decomposition onset temperature when prior art aromatic additives having only electron withdrawing or only leaving groups as defined herein was utilized, even when the additive had three or more such substituents. It can also be seen that anhydride and phenolic hydroxyl groups are detrimental to fixture speed of the adhesive.

Claims: A cyanoacrylate monomer adhesive composition characterised in that the composition includes an effective amount, for enhancing the thermal resistance of the polymerized adhesive, of a mono, poly or hetero aromatic compound, other than an aromatic base compound, characterised by at least three substitutions on an aromatic ring thereof, two or more of said substitutions being electron withdrawing groups selected from NO 2 , CN, CF 3 , NR 1 3 + , SR 1 2 + , C(=O)R 1 , C(=O)OR 1 , NO, CCl 3 , SO 2 , S(=O), SO 3 , SO 2 R 1 , SO 2 OR 1 or F, one or more of said substitutions being leaving groups selected from F, Br, Cl, l, NO 2 , CN, SOR 1 , SO 2 R 1 or SO 2 OR 1 and R 1 is an optionally substituted hydrocarbon group. A composition as claimed in claim 1, wherein said aromatic compound is present at a level of 0.5-15% by weight. A cyanoacrylate polymer composition comprising between 0.5 and 15% by weight of a mono, poly or hetero aromatic compound, other than an aromatic base compound, characterised by at least three substitutions on an aromatic ring thereof, two or more of said substitutions being electron withdrawing groups selected from NO 2 , CN, CF 3 , NR 1 3 + , SR 1 2 + , C(=O)R 1 , C(=O)OR 1 , NO, CCl 3 , SO 2 , S(=O), SO 3 , SO 2 R 1 , SO 2 OR 1 or F, one or more of said substitutions being leaving groups selected from F, Br, Cl, l, NO 2 , CN, SOR 1 , SO 2 R 1 or SO 2 OR 1 , and R 1 is an optionally substituted hydrocarbon group. A composition as claimed in any of claims 1 to 3 wherein said aromatic compound has the formula: where L is the leaving group and the W groups are the electron withdrawing groups. A composition as claimed in any of claims 2 to 4 wherein the level of said aromatic compound is at least 1%. A composition as claimed in claim 5 wherein the level of said aromatic compound is 3-10% by weight. A composition as claimed in any of claims 1 to 6 wherein R 1 is an optionally substituted alkyl or aryl group. A composition as claimed in claim 7 wherein said optional substitutions are selected from alkoxy, halo, cyano or nitro groups. A composition as claimed in any of claims 1 to 6 wherein said aromatic compound is selected from 2,4-dinitrofluorobenzene; 2,4-dinitrochlorobenzene; 2,4-difluoronitrobenzene; 3,5-dinitrobenzonitrile; 2-chloro-3,5-dinitrobenzonitrile; 4,4'-difluoro-3,3'-dinitrophenyl sulfone; pentafluoronitrobenzene; pentafluorobenzonitrile; α,α,α,2-tetrafluoro- p -tolunitrile; or tetrachloroterephthalonitrile. A composition as claimed in any of claims 1 to 8 wherein said aromatic compound is free of carboxylic acid, anhydride and phenolic hydroxyl groups. A cyanoacrylate polymer composition as claimed in claim 3 characterised by an onset of decomposition temperature, when heated at 10°C per minute, of at least 200°C. A polymer composition as claimed in claim 3 characterised by a weight loss of 25% or less when heated at 150°C for 900 minutes. A polymer composition as claimed in claim 12 wherein said weight loss is 20% or less. A polymer composition as claimed in any of claims 1 to 13 characterised in that the mono, poly or aromatic ring compound does not comprise acidic functional groups.

Inventors: ATTARWALA SHABBIR, ATTARWALA, SHABBIR
Applicants: LOCTITE CORP, LOCTITE CORPORATION
Application year: 1993
Main classification: C09J4
IPC codes: C09J4, C08K5