Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention
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- This invention relates to a-cyanoacrylate adhesive
compositions, more specifically, to such compositions for-
mulated so as to produce bonds which are relatively weak and
can be debonded relatively easily.
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Description of the Prior Art
Adhesive compositions based upon esters of a-cyano- ~ ;;
acrylic acid are well known in the art and have found exten-
10 sive use, because of their rapid cure speed and excelle,nt ~ -
long-term bond strength, to a wide variety of surfaces. For
instance, tensile shear strength of a steel-to-steel bond
typically is greater than about 2,200 psi for ethyl cyano-
acrylate, and significantly higher in pure -tensile strength.
;, Cyanoacrylate adhesive bonds also tend to be relatively "~
brittle; therefore, the adhesive compositions are often
;! plasticized. Typical plasticizers include the various alkyl
esters and diesters, alkyl and aromatic phosphates an~ phos-
phonates of Kodak U.S. Patent 2,784,127 issued March 5, 1957;
the diallyl phthalates of Kodak U.S. patent 3,354,128, issued
November 21, 1967; and the aryl and diaryl ethers of Loctite
U.S. Patent 3,699,127, issued October 17, 1972. These ref-
erences indicate that, in general, plasticizer concentrations
of more than 15-20 percent by weight of the adhesive composi-
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tion should be avoided since at higher concentrations, the
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bond strength would be significantly reduced.
Cyanoacrylate monomer is believed to polymerize
primarily by an anionic mechanism. To control premature
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polymerization, theref~re, ionic inhibitors are customarily
used. These inhibitors are most commonly strong acids, e.g.,
` acidic gases such as nitrogen oxide and sulfur dioxide, at a
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level of about 0.001 to about 0.06 percent by weight (see,
e.g~, Kodak U.S. Patent 2,794,788, issued June ~, 1957).
Other known strong acidic inhibitors are soluble organic
acids, e.g., aryl sulfonic acids, which are typically used
at a level of about 0.001 to about 0.06 percent by weight.
While the use of strong acids such as thosle just mentioned,
is important in obtaining good shelf life for the cyano-
acrylate monomer~ the disadvantage inherent in their use
is that they tend to retard the rate of polymerization, there-
by interfering with the effectiveness of the adhesive.
Obviously, the prior art has been directed tomaximizing bond strength and shelf life. While xapid
reaction rate and good storage stability are always important
in a commercial product, it has not heretofore been recognized
that a high level of bond strength may not always be desirable.
Thus, it would be quite beneficial to have an adhesive formu-
lation which combines the rapid bonding associated with cyano-
acrylates with reduced and controlled bond strength, along with
the ability to be readily debonded, when desired.
Summary of the Invention
According to the present invention, such an
advantageous adhesive composition is provided. This composition,
having controlled bond strength and adapted for debonding,
comprises in admixture:
; (a) at least one polymerizabla ester of an
a-cyanoacrylic acid,
(b) from about 20 percent to about 60 percent -
by weight of the composition of a plasticizer
which is miscible with the ester of part (a);
(c) from about 0.015 percent to about 0.15
percent by weight of the composition of a
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carboxylic acid which is soluble in the
ester of part (a).
The invention also relates to a method for
preparing the above adhesive composition by mixing the
component ingredients or dissolving the plasticizer and
carboxylic acid in the cyanoacrylate. The invention further
relates to the use of such adhesive composition for bonding
objects and subsequently debonding them by applying the
composition to a surface of one of the objects, placing that
surface in abutting relationship with a mating surface until
the adhesive cures, then subjecting the bond to heat or
solvent action to effect debonding.
Description of the Preferred Embodiments
The monomeric a-cyanoacrylate esters of this invention
are those well known to the art and conform to the general
formula~
CN
CH2=C-C-O-R
o
wherein R is a C1 to about C16 alkyl, alkenyl, cyclohexyl,
phenyl, haloalkyl, arylalkyl, acylalkyl group, and the like,
and mixtures thereof. Preferably, a single ester will be
used and R will be a Cl to about C5 alkyl, alkenyl or cyclohexyl
group, most preferably, methyl or ethyl.
The above monomers are generally prepared by the
condensation of an ester of cyanoacetic acid with formaldehyde,
followed by thermal depolymerization and fractional distilla-
tion under vacuum to separate the monomer. Appropriate ~-
procedures are described in, for example, Xodak U.S. Patent
3a No. 2,721,858, issued October 25, 1955; and other references.
The second required ingredient in the adhesive
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composition is a plasticizer. The key property of the
plasticizer is that it be fully miscible, and compatible,
within the concentration range noted below, with the
cyanoacrylate. By the term "compatible" :is meant that the
material does not cause gellation or sett:Ling of the mixture.
The function of the plasticizer is to wea]cen the ultimate
bond strength of the cured adhesive; therefore, the plastici-
zer is to be used in relatively high concentration, namely,
from about 20 percent to about 60 percent by weight of the
total adhesive composition~ Preferably, the plastici~er
content will be about 30 percent to about 40 percent by
weight. A considerable variety of plasticizers are suitable
for use in this invention and their selection accordin~ to
the criteria above stated will be a matter of straightforward
experimentation in any given case. The preferred plasticizers
; fall into various broad classes, as follows:
(a) aryl and diaryl ethers, such as diphenyl ether
~` ;t and its halogen-substituted derivatives; each
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aryl ring with its substituents, if any, may
contain up to about 20 carbon atoms;
(b) alkyl and alkenyl esters of acids of the
fumaric acid series, such as dialkyl fumarate
and diallyl maleate; each alcoholic portion ;~
may contain up to about 12 carbon atoms, but
preferably about 4 to 8 carbon atoms
- (c) alkyl and alkenyl esters of acids of the
; oxalic acid series, such as diallyl oxalate,
dioctyl azelate, and diallyl sebacate; the
number of carbon atoms in the alcoholic portion
will preferably be as in (b), above;
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~ (d) alkyl and alkenyl esters of acids of the
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phthalic acid series, whether ortho, meta or
para, such as diallyl phthalate, dibutyl ;
phthalate, isobutyl phthalate, and ethyl hexyl
phthalate; again the number of carbon atoms in
the alcoholic portion will preferably be as in :
(b), above;
(e) alkyl and alkenyl esters of various other
unsaturated dicarboxylic acids, such as
diallyl itaconate. ;
Of the above plasticizers, those of groups (a~, ~c)
and (d) are especially preferred. Mixtures of the above ~`
plasticizers can also be used. It will be understood that ,~
the above list of plasticizers is not intended to be llmiting.
Other plasticizers can be used, provided that they meet the
miscibility and compatibility criteria described above.
The third required ingredient in the adhesive
composition is a carboxylic acid or polycarboxylic acid. The
key property of the acid is that it be soluble, within the
concentration range noted below, in the cyanoacrylate. The
acid should be relatively weak, that is, its pRA should not be~
below about 1.0 and preferably should be in the range of
about 1.3 to about 6Ø The precise function of the carboxylio ~;
acid is not well understood; however, without intending to be
bound to any particular theory, it is believed that the acid
tends to limit polymer chain length. However, the acid may
; ~ also provide surface effects which tend to enhance adhesion
and thereby regulate the weakening effect of the plasticizer.
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, The most desirable acids are those which, in addition to being ~
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soluble, do not substantially contribute to degradation of
bond strength during storage of bonded par~ts over pexiods
- of days or weeks, or substantially increase fixture or cure
'~ time. The acid also appears to aid in disassembly of the
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bond, as by application o~ solvent, where desired.
The concentration of -the carboxylic acid may vary
from about 0.015 percent to about 0.15 percent by weight of
the adhesive composition. The acid concentration chosen for
any given situation will depend on the particular properties
of the acid in relation to the above criteria. The preferred
concentration, however, will usually be in the range of about
0.025 percent to about 0.075 percent by weight. It will also
be appreciated that since the acid and the plasticizer ha~e a
certain degree of individual ability to control bond strength,
a considerable range of adhesive properties can be obtained
by varying the proportions of these ingredients, as will be
found by straightforward experimentation.
The preferred soluble organic acids fall into the
following categories:
(a) alkyl and alkenyl monocarboxylic acids and
their halo and cyano derivatives, such as
mono- and dichloroacetic acid, cyanoacetic
acid, and oleic acid; the alkyl or alkenyl
moiety preferably will have from 1 to about
25 carbon atoms, more preferably from 1 to
about 17 carbon atoms;
(b) aromatic monocarboxylic acids, such as benzoic
acid and phenyl acetic acid, and the halo and
lower alkyl and alkenyl derivatives (e.g.,
from 1 to about 4 carbon atoms) of such acids,
for example, toluic acid and chlorobenzoic acid;
(c) aliphatic dicarboxylic acids of the oxalic acid
series, such as oxalic acid and malonic acid,
3Q wherein the aliphatic portion may be substituted
with halo and lower alkyl and alkenyl groups
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(e.g., from 1 to about 4 carbon atoms).
A variety of additional materials may also be present
in the adhesive composition. As has been mentioned, the cyano-
acrylate monomer will usually contain an inhibitor of anionic
polymerization , such as an acidic gas. More recently, sultone
stabilizers have been developed (see Kodak U.S. Patent 3,742,018,
issued June 26, 1973). It is also usually desirable to include
an inhibitor of free radical polymerization, such as compounds
of the phenolic type, for example, hydroquinone, its monomethyl
ether, p-tert.-butyl catechol, and the like. In some applica-
tions it may be advantageous to include dyes and viscosity
modifiers as well. These materials, as well as the concentra-
tions of their use, are known in the art and form no part `
of the present invention. A worthwhile summary of known ~ ;
formulations may be seen in U.S. Patent 3,699,127 issued
October 17, 1972.
As has been indicated, a particular advantage of the
- adhesive compositions of this invention is that their bonds ;
can be destroyed relatively easily, thereby permitting disass-
20 embly of bonded parts. This property is highly desirable in
applications where, for instance, rapid fixturing but weak
ultimate strength is desired or where a small object is to be
adhered to a jig or fixture ~or machining but is subsequently
~- to be detached from the jig or fixture. The compositians of
this invention are uniquely adapted to such needs since they
quickly (e.g., within one hour or less) form a secure bond -
having tensile strength of 1,000 psi or more, but yet the
parts can be very rapidly debonded, typically within 10
minutes or less. This combination of properties greatly
improves efficiency in certain manufacturing operations.
Debonding can be readily accomplished thermally, such as by
heating the bond line to 200-300F by oven or induction heat.
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A variety of solvents, such as nitromethane, ac~tone,
acetonitrile or dimethyl~ormamide, will also dissolve the
bond. A particularly desirable feature of the present
compositions is that the adhered parts may also be debonded
by an aqueous surfactant solution containing, e.g., about 1
to about 5 percent by weight of the surfactant. A wide
variety of water-soluble surfactants may be employed, but a
preferred surfactant is a potassium aryl alkyl sulfonate
sold under the trademark "Minate 415" manuEactured by Stepan
Chemicals, Inc., Northfield, Illinois.
Mild heating (e.g.~ about 50C) of the solution is
helpful. The presence of the carboxylic acid has been found
to be beneficial in this context. While the mechanism :is not
; understood, it appears that it reduces the water resistance
of the bond. Whether a solvent or aqueous bath is used, the
; ~ bath should be agitated. An especially effective form of
agitation is provided by an ultrasonic transducer.
The following examples illustrate the invention
but are not intended to be limiting in any way.
EX~PLE 1
An adhesive composition solution was prepared as
follows. 2.5 gxams of a phenolic free radical inhibitor and
0.25 gram of oxalic acid were dried for about 2 hours at 60C
; and ~mm Hg pressure. These dried ingredients were weighed
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into a clean, dry polyethylene container. Then 175.0 grams
of dibutyl phthalate and 322.25 grams of ethyl cyanoacrylate
~; monomer were weighed into the container. Shaking accelerated
the dissolving of the solids. This composition is referred
to as "A".
The same procedure was followed using dried cyano- ;
acetic acid instead of oxalic acid and diphenyl ether instead
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of dibutyl phthalate. This composition is referred to as
"B".
A third composition, referred to as "C"~ was
prepared using the same procedure except that 0.37 gram dried
benzoic acid was substituted for the oxalic acid.
Typical strength properties for these adhesive
formulations are reported in Table I below for the room
temperature cure times shown. The tensile lap shear test was
; performed according to AST~l A109. This test involves
10 adhering together overlapping surfaces of two steel test ;~
strips. The ends of the so-assembled specimens are pul:Led
with a measuring device such as an Instron tester, and the
tensile shear strength of the bond is measured. A compressive
shear test was used to measure the fixture strength of the
adhesive on various mixed substrates. This test involves
adhering together overlapping surfaces of two test strips
tglass, steel, aluminum, vinyl). One end of the so-assembled ~ ;
specimens is pushed against a "Chattillon" hand gauge and
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~ the compression force to break the bond is measured.
; 20 TABLE I
Tensile Shear (psi) Compressive Shear (psi)
Adhesive 24~hr. cure 15-min. cure 30-sec. cure (steel/glass) c
A 1765 916 more than 15 (glass
substrate failure)
B 1265 890 more than 15 (glass,
substrate failure)
; C 1490 850 more than I5 (glass
substrate failure)
D* 2200 1350 more than 15 (glass
substrate failure)
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*Control: ethyl cyanoacrylate without plasticizer and acid
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EXAMPLE 2
Using the above adhesives, glass discs (about S/8"
to l" diameter) were bonded to mating steel surfaces. Bonds
were fully cured (30-60 minutes at room temperature) and the
parts were then debonded by the following ~ethods. Method A
involves immersing the bond in a 2 percent solution of a
surfactant ("CONTR~D 70" ~ a proprietary anionic surfactant
manufactured by Harleco, Inc., Philadelphia, Pennsylvania) in
water at about 150F in an ultrasonic tank of about 400-500
watt output. Method B involves using an oven or an induction
heater and elevating the temperature of the bondline to about
250F. Method C involves immersing the bond in solvents such
as nitromethane, acetone or dimethylformamide in a 400-500
watt output ultrasonic tank. Typical times for disassembly
o~ these adhesive bonds are listed in Table II, below.
TABLE II
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; Time to Debond, minutes
Adhesive Method AMet_od B Method C
A about 2-3less than l about 2-3
2Q B about 1-2less than l about 1-2
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C less than l less than l about l
D* more than 20 more than lO more than 20
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*Control: ethyl cyanoacrylate without plasticizer and acid
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