CA1200638A - Toughened cyanoacrylates containing elastomers rubbers - Google Patents

Abstract

ABSTRACT

Disclosed are cyanoacrylate adhesive compositions containing elastomeric polymers as fillers. Preferred fillers are the acrylic rubbers. Filler concentration range from about 0.5 - 40% by weight of the composition. Beneficial results include improved toughness of the cured compositions, as measured by impact, peel and tensile strength properties, particularly after exposure elevated temperatures which would ordinarily severely degrade the adhesive strength.

Description

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TOUGHENED CYANOACRYLATES CONTAINING ELASTOMERS RUBBERS
BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to cyanoacrylate adhesive compositions which are improved in toughness by incorporation of certain organic fillers or thickeners.

2. Description of the Prior Art Liquid cyanoacrylate compositions have long been known in the art as excellent adhesives. One of their primary short-comings, however, has been their brittleness after cure.
A variety of fillers has been incorporated into cyano-acrylate adhesive compositions to bring about certain changes in properties. U.S. Patent No. 2,794,788 teaches thickening of cyanoacrylate adhesives by dissolving therein quantities of polymeric alkyl cyanoacrylates, as well as other compounds including polyacrylates, methacrylates and cellulose esters such as acetate, propionate and bukyrate.
U.S. Patent No. 3,836,377 notes among the additional known thickeners polyvinyl ethers such as polyvinyl-methyl ether. U.S.
Paten~ No. 3,692,752 discloses ~hickened cyanoacrylate solutions containing certain polyether acrylates/methacrylates, acrylic/
methacrylic esters of bislhydroxyalkyl) phosphonic acid deriva-tives, and acrylic/methacrylic esters of tris(hydroxyalkyl) cyanuric acid derivatives.
The preceding references relate to thickened homogeneous solutions containing organic compounds as ~hickeners. Various inorganic materials have also been proposed as fillers, which also have the effect of thickeniny the composition. Thus, U.S.
Patent No. 3,663,501 teaches preparation of a dental cement containing inert, finely-divided solids such as fused silica, quartz and alumina. Similarly, U.S. Patent No. 3,607,542 teaches the preparation of a water-resistant cyanoacrylate paste containing insoluble, inert fillers such as salts or calcium, ,~4 063~

titanium, zinc, tin, aluminum, iron and copper, among others.
Non-soluble orqanic fillers are disclosed by U.S. Patent No.
4,105,715.
U.S. Patent No. 4,102,945 discloses a cy~noacrylate adhesive composition thickened by a copolymer or terpolymer resin capable of being dissolved or solvated by the cyanoacrylate monomer, resulting in significantly improved peel strength. Preferred thic~eners are acrylonitrileb~tadiene-styrene terpolymers, meth-acrylate-butadiene-styrene terpolymers, and vinylidene chloride-acrylonitrile copolymers.

Canadian Patent No. 1,156,791, issued November 8, 1983, discloses acrylic rubber fillers, such as those of this invention, as toughening agents for adhesive compositions based upon acrylic monomers with peroxy ini~iators, such ~s, e.g., anaerobic compositions.

It has now been found that use of elastomeric polymers as fillers in cyanoacrylate systems results in suprisingly beneficial properties.

SUMMARY OF THE INVENTION
According to the invention there is provided a curable adhesive composition having improved toughness when cured, comprising: ~a) a monomeric ester of 2-cyanoacrylic acid, and (b) about 0.5% to about 20% by weight of the composition of an elastomeric polymer selected from the group consisting of elastomeric copolymers of a lower alkene monomer and (i) acrylic acid esters (ii) methacrylic acid esters or (iii) vinyl acetate, said composition having improved toughness over the corresponding unfilled adhesive composition. It will be appreciated that the upper concentration limit is related inversely to the molecular weight of the rubber and, therefore, could exceed 20% by weight if a low molecular weight rubber having suitable performance were used.

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i The compositon also preferably contains one or moreknown acidic and free radical inhibitors, and optionally other func~ional additives for such purposes as further improving thermal resistance, providing color, accelerating the cure reaction, providing cross-linking, etc.
The compositions of this invention exhibit substantially increased toughness in comparison with control cyanoacrylate adhesives. It has also been unexpectedly found that the compositions have excellent hot streng~h, i.e., resistance to thermal degradation of strength properties. This finding was particularly surprising since the addition of low glass transition rubbex would not be expected to improve hot strength. It has also been ound that these compositions retard or eliminate what may be termed post-cure embrittlement or loss of toughness properties which occurs with cyanoacrylate adhesive bonds upon exposure to heat and then cooling to room temperature.

DETAILED DESCRIPTION OF THE INVENTION
The benefits of this invention are achievable wlth essentially all adhesive compositions based upon cyanoacrylate estersO Most commonly, the este~s have the formual:

CN
CH2=C-COOR

wherein R represents a C1 l~alkyl, cycloalkyl, alkenyl, cyclo-alkenyl, phenyl or heterocyclic tsuch as furfuryl) radical.
Naturally, the above R group can contain any linkages or sub-stituents which do not adversely affect the monomer in the per-formance of its intended function in the cyanoacrylate adhesive 063~ ;

compositions, such as strongly basic substituents which mayadversely affect the stability of the adhesive compositions. For purposes of this invention, the preferred monomers are those wherein R is a cyclohexyl or a Cl to C6 alkyl or alkenyl radical.
Most preferred are methyl and ethyl cyanoacrylates. The esters may be used singly or in admixture.
The above monmeric esters of 2-cyanoacrylic acid can be prepared by methods known in the art, such as those described in U.S. Patent Numbers 2,467,927 and 3l254,111.
Cyanoacrylate ester adhesive compositions generally contain an anionic inhibitor, e.g., an acidic substance, soluble in the ester of 2-cyanoacrylic acid, which inhibits anionic polymeri-zation. A number of suitable inhibitors of anionic polymerization are well known in the ar~.
The best known axe the soluble acidic gases such as sulfur dioxide, sulfur trioxide, nitirc oxide, and hydrogen 1uoride.
More recently, inhibitors involving organic sultones have been developed, the sultone being generally represented by the formula O S=O
~ X J

wherein X is an organic radical joining the ---S(02)0 group in a 4, 5, or 6 membex heterocyclic ring, preferably a 5 member heterocyclic ring. Preferably, X is a hydrocarbon group, although it can contain any substituents or linkages which do not adversely afect the sultone for its intended use as a stabilizer of the adhesive composition. Another excellent class of stabilizers are the organic sulfonic acids, preferably having a molecular weight less than about 400. To be optimally useful ~~ _ 63b~ -as a stabilizer in the adhesive composi.tions, the sulfonic acid should have a pKA value (dissociation constant in water) or less than about 2.8, and preferably less than abo~t 1.~.
Recently the beneficial effec~s of certain stabilizer formulations based on sulfonic acids in combination with sulfur dioxide have been disclosed. Particularly preferred for the 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 about 5 to about 100, more preferably about 10 to about 50, parts per million (based on monomer weight). The preferred concentrations of So2 range from about 15 to about 50 ppm for either acid.
While not essential, the cyanoacrylate adhesive compositions of this invention generally also contain an inhibitor of free radical polymerization. The most desirable of these inhibitors are of the phenolic type, such as quinone, hydroquinone, t-butyl catechol, p-methoxy-phenol, etc.
The above inhibitors. may be used within wide ranges, but the following general guidelines are representative of common practice, all figures being weight percent of the adhesive composition: acidic gases -from about 0.001~ to about 0.06~ by weight; sultones -from about 0.1% to about 10~ by weight;
sulfonic acids from about 0.0005% to about 0.1% by weight; free radical inhibitors ~from about 0.001% to about 1%.
Other com~on additives for cyanoacrylate adhesive compo-sitions are placticizers, Plasticizers serve to make the cured bonds less brittle and, therefore, more durable. The most common of these plasticizers are Cl to ClOalXyl esters of dibasic ,:, acids such as sebasic acid and malonic acid~ Other plasticizers, such as diaryl ethers and polyurethanes, also may be u5ed, and a variety of other plasticizers is also known.
In addition to the monomer, or mixture of mono~ers, the sPcond required ingredient i~ an elastomeric polymer filler of at least one of the types described below. In order to perform suitably, the polymer, or mixture of polymers must be "compatible" with the cyanoacrylate monomer. By thP term "compatible" is meant that the polymers do not significantly interfere with the storage stability of the adhesive composition.
Moreover, as will be discussed further below, it appears that the pol~mers should be at least partially solvated by the cyanoacrylate monomer so that a homogeneous solution or suspension is formed. Molecular weight is considered to be a significant parameter in selecti~g suitable polymeric fillers of the types specified; however, selection of molecular weight is deemed to be a matter of choice. Obviously, such factors as solubility and thickening ability are affected, but selection 4f molecular weight in these respects is well within the skill of the art based upon routine experimentation.
Each of the fillers of this invention is unique in that it imparts a higher toughness to standard adhesive bonds formed using a cyanoacrylate adhesive composition containing it than the toughness of similar bonds formed using ~he same composition without any fillers, or using the same composition filled by means other than members of the said ~roup of alternative fillers.
As already indicated, the fillers of this invention are organic polymers which are elastomeric, i.e., rubbery, in nature. Numerous chemical species fall into this category but, for illustration purposes, the following species have been found to be particularly useful: acrylic rubbers (which are the preferred species); polyester urethanes; ethylene-vinyl ~t~01~38 acetates; fluorinated rubbers; isoprene-acrylonitrile polymers; chlorosulfinated polyethyleneq; and homopolymers of polyvinyl acetate.
The acrylic xubb~rs of the instant invention may be selected ~rom a wide range of suitable materials. Most frequently these rubbers are either: (i) homopolymers of alkyl esters of acrylic acid; (ii) copolymers of~ another polymerizable monomer, such as lower alkenes, with an alkyl ester of acrylic acid or with an al~oxy ester of acrylic acid; (iv~ copolymers of alkyl esters of acrylic acid; (v) copolymers of alkoxy esters of acrylic acid; ~vi) mixtures of any of the above (i)-~v). Other unsaturated monomers which may be copolymerized with the alkyl and alkoxy esters of acrylic include dienes, reactive halogen containing unsaturated compounds and other acrylic monomers such as acrylamides. It will be understood that es~ers of methacrylic acid tend to be relatively brittle; however, to the extent they provide beneficial results in toughness and thermal resistance, they are intended to be included within the scope of this invention.
The choice of the elastomer will, to a large degree, dictate various properties and characteristics of the adhesive composition and such choices are easily determined through general experimentation and known methods within the art.
It is most effective to use elastomers whose molecular weight averages more than about 100,000, but any molecular weight greater than 5,000 would be expected to effect an improvement. As a principle of general guidance, the molecular weight should be high enough to produce toughening but not so high that the adhesive is very stringy and difficult to apply. It is also best to choose an elastomer whose Mooney viscosity is between 20 and about 60, and whose glass transition temperature (Tg) is 15 C or less. The Mooney viscosity is defined as the amount of torque or resistance required to revolve a rotor at a constant speed ~2~638 in a polymer at a constant temperature. The Mooney viscosities of the pre~erred rubbers of the instnat invention are described by the manufacturing as ~L(1~4), The (1~4) symbol is to indi-cate the time involved in measuring the polymer viscosity. The "1" indicates a one minute pre-heatinq time, which is to bring the rotor to the designated temperature, The "4" indicates that the torque reading is to be taken after four minutes of rotating the rotor. The readings are measured on a scale usually of 0-100. There are no specific units involved. These specific limitations are not absolute and various acrylic rubbers which do not fall within them may be within the scope of this invention.
One preferred group of acrylic rubbers consists of the copolymers of ethyl acrylate with 2-chloroethyl vinyl ether in the approximate molecular ratio of 95:5, respectively. One such acrylic rubber is manufactured by the B. F. Goodrich Company, and is marketed under the name Hycar~ such as Hycar 4021. Other preferred acrylic rubbers are the copolymers of methyl acrylate and ethylene, manufactured by Du Pont, under the name of Vamac such as Vamac N123 and Vamac B124. A third group of preferred rubbers is manufactured by American Cyanamid under the name Cyanacxy1~and includes rubbers known as Cyanacryl R, Cyanacryl L
and Cyanacryl C. It has been found that Cyanacryl rubbers, when used "as is," tend to destabilize the cyanoacrylate monomer.

This problem can usually be corrected by washing the rubber with dilute HCL, rinsing and oven drying it prior to adding it to cyanoacrylate.

The Hycar rubbers are high molecular weigh~ rubbers, typ-ically having a Mooney viscosity at 100C of between about 25 and 60 ML(1+4), and a glass transition temperature range of about -15C to About -40C. ~ycar 4021 has a Mooney vlscosity of about 40 minimum and a glass transition temperature (Tg) of ~z~a~3~

about -15C.
Vamac Nl23 has a Mooney viscosity of about 30 and a glass transition temperature (Tg) of abou~ -24C; Vamac B124 has a ~ooney viscosity of about 20. Cyanacryl R is reported to have a Mooney viscosity of about 42 to about 51 and a glass transition temperature (Tg) of about -18C, while Cyanaaryl L and C have Mooney viscosities between about 30-48 and glass transition temperatures of -24C and -32C respectively. These data have been procurred from the manufacturers' technical literature.
The concentration range of elastomeric polymer should be about 0.5 to about 20 percent by weight, preferably 1.5 to about 15 percent, based on the weight of the composition.
While beneficial effects will he realized with rubbers which are merely "compatible," as defined herein, it is preferred that the rubbers be dissolved in the monomer. All of the above preferred acrylic rubbers are solid materials which are preferably masticated on a mill prior to dissolution in the acrylic ester monomers. Mastication aides the dissolution by breaking down the molecular weight and reducing the physical and chemical cross-links. Properties of the cured adhesive composition will vary somewhat with the degree of mastication, the effects of which may be determined with routine experimen-tation for any given acrylic rubber. These solid rubbers should show little or no sign of phase separation once ~ully dissolved in the monomer.
The improved toughness of the cured compositions o~ this invention is manifested through various physical properties, e.g., 180 peel strength, impact strength and tensile shear strength.
These strengths are useful propexties of an adhesive bond, ~~ _ being parameters of what is loosely referred to as ~he bond strength. Ref~rring for simplicity to the procedures of American Standard Test Methods, peel Qtrength is determined in accordance with ASTM No. D 903-49; impact strength is determined in accordance with ASTM No. D-950; and tensile shear strength is determîned in accordance with ASTM No. D-1002. The reader is referred to these standards for a full description of the tests.

Examples The following examples are intended to illustrate, not limit, the invention.
Example 1 The typical procedure for preparing an adhesive composition of this invention is as follows:
1. A high-purity (i.e., over 99% pure) alkyl cyanoacrylat-monomer is stabilized to 30 ppm MSA.
2. The elastomer is diced into small pieces and the mon-omer is heated to 45-50C.

3. The- elastomer is added in several equal portions over a period of an hour; the batch tempexature is raised to 50-55C and maintained there.

4. The batch is stirred or milled, as necessary, for 3-4 hours at temperature until the elastomer is dissolved.

5. The batch is cooled to room temperature and addition-ally stabilized with 40 ppm SO2.
In order to ensure that the composition so prepared has adeqùate stability for commercial use, a sample of the compo-sition is placed in a test tube and kept in a water bath at 82C until incipient gellation can be detected in the composition.

Table 1: Physical Properties of Filled AdbesiYes Adhesive A B C D E X-l X-2 X-3 F G H
CA Type ethyl ethyl ~ethyl ~ethyl ethyl ethyl ethyl ethyl .5tabilizer HPSA ~PSA HPSA HPSA 2~5A MSA/5O2 2~5A/S02 MSA/5O2 Level, ppm 30 18 22 22 10 30/40 30/40 30/40 Filler P2~YA - PKM~ PNMA - B-124 B-124 B-124 Level, parts 5 - 5 3 - 10 10 10 Impact Strength P~x~n 5emp. cure 24 hrs. 9.0 10.5 10.3 9.0 10.4 8.7 9.7 8.0 9.1 9.9 g.5 5 days 8.5 lQ.5 11.0 10.1 8.7 7.8 7.8 10.2

6 days 10.0 10.2 10.6 Po~t-cured ~ 250F
2 hrs. 9.~ 1.5 9.0 1.1 8.5 13.7 12.6 13.3 1.4 4.3 5.7 24 hrs. 2.0 2.1 1.2 1.0 1.8 1.0 2.0 2.2 180 Peel Strength Room Iemp. cure 24 hrs. 23.0 14.0 32.3 31.6 4.0 5.5 20.0 ~ 6 days 34.3 32.0 10.0 3.0 18.0 Post-cured ~ 250F ~,~
2 hrs. 39.5 41.0 0 1.0 1.0 - O
24 hrs. 12.0 I~nsile Sh~ar Strength Room Temp. CuL~e S days 3020 3050 3690 3690 3010 30qO 3380 2930 3370 3080 Post-cured ~ 250F
, 2 hr~. 2750 2350 3190 590 2190 3770 3870 1700 2050 1730 24 hrs. 1410~ 1430 550 160 1470 3920 360 1070 1330 Tested a 250 F
Post-cured ~ 250F
1 hr. 620 1890 2430 2280 2170 2 hrs. 105 1600 2430 2270 2160 ~Z~)0~38 ~

I~ no gellation occurs for at leas~ two days at ~2C, the stability is considered satisfactory.
It will be understood that the procedure given above may not be optimum for all elastomers within the scope o~ this invention. For instance, the minimum adequate temperature may be found to be higher or lower, or the period of dissolution may be longer or shorter than one hour. Also, it may not be necessary in all cases to obtain complete solution of the elastomer in order to achieve the beneficial effects of this inventionO Determination of such factors is considered to be a matter of routine experimentation within the skill of the art and cannot be conveniently described here for all useful formulations.
Example 2 Using the procedure descri~ed in Example 1, several experi-mental adhesive compositions were prepared. Table I sets out a varie~y of comparitive data showing the beneficial results of using Vamac B-124, an acxylic rubber of this invention. Compo-sitions containing this filler are designated X-l, X-2 and X-3.
Of particular interest are the superior thermal properties of these compositions. Samples A-C are commercially available cyanoacrylat~ adhesives of Loctite Corporation, Newington, Connecticut. Samples A, C and D contain polymethyl methacrylate (PMMA), a commonly used filler of the prior art. Samples F-H
are cyanoacrylate adhesive compositions of other manufacturers available through normal commercial channels. Sample E is cyanoacrylate monomer in "neat'l form, i.e., not ~ormulated into a commercially saleable adhesive composition.-Impact strength data are expressed in ft.-lbs. per sq. inch;
peel strength data are in lbs. per inch of width; tensile shear strength data are in lbs. per sq. inch. Concentrations of materials are based on the composition as a whole, except for the stabilizer concentrations in ppm, which are based on the monomer weight.

lZ~631!~ ' Example 3 Using the procedure of Example 1, a vaxiety of elastomeric polymers are made into adhesive compositions. Tests similar to those shown in Example 2 indicate that general improvement in toughness is obtained over control compositions not containing a filler of this invention. Elastomers included in this Example, and tvpical of the useful ela~tomers, are shown in Table II.

Table II Some Other Useful Elast~omers Trade Mark Chemical Species Suggested Conc~ntration ~ange,~ Sc,uroe Q-thane PA-lO Polyester-ureth~ne 0.5 - 2.5 K.J. ~uinn, Inc., Malden, M~
Polyester-urethane 0.5 - 2.5 Polyester-urethane 3.0 - 3.5 Vamac N-123 Ethylene-methyl acrylate 3.0 - 3.5 E.I. duPont de Nemours, Wilmin~ton, DE

~ynathene EV-907 Ethylene-~inyl aoe tate 4.0-ll.0 U.S. Industrial Cnemicai Co , New York, NY
;

- - Polyvinyl a oe tate (MW=195,000) 16.0 - 17.0 Scientific Polymer Products, Inc.
Ontario, NY -~

Viton C-lO Fluorinated rub~er 15.0 - 30.0 E.I. duPont de Ne,Dours, Wilmington, DE ~
E-60 Fluorinated rubber 15.0 - 30.Q QB

~rynac 833 Iscprene-acryl~nitrile Less than l.0 Polypar, Inc., AXron, ~'

Claims (8)

Claims:

1. A curable adhesive comprising a substantially solvent-free mixture of:
(a) a monomeric ester of 2-cyanoacrylic acid, and (b) about 0.5% to about 20% by weight of an elasto-meric polymer selected from the group consisting of elastomeric copolymers of a lower alkene monomer and (i) acrylic acid esters (ii) methacrylic acid esters or (iii) vinyl acetate.

2. A composition of claim 1 wherein the elastomeric polymer is a copolymer of a lower alkene monomer with an alkyl ester of acrylic or methacrylic acid.

3. A composition of claim 1 wherein the elastomeric polymer is a copolymer of a lower alkene monomer with an alkoxy ester of acrylic or methacrylic acid.

4. A composition of claim 1 containing in addition a stabilizing amount of an acidic stabilizer and a free radical inhibitor.

5. A composition of claim 1 wherein the monomeric ester is methyl cyanoacrylate or ethyl cyanoacrylate.

6. A composition of claim 1 wherein the concentration of the elastomeric polymer is about 1.5% to about 15% by weight.

7. A composition of claim 1 wherein the elastomeric polymer is an ethylene-methyl acrylate copolymer.

8. A composition of claim 1 wherein the elastomeric polymer is an ethylene-vinyl acetate copolymer.