Note: Descriptions are shown in the official language in which they were submitted.
~L~'iJV~
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,
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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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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
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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
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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
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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
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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
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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,
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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.
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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
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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.
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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.
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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, ~'
