Adhesive compositions having improved stability and surface insensitivity are made from a blend of (a) a 2-cyanoacrylate ester adhesive, (b) a polyoxyalkylene glycol, glycol ester, or glycol ether, and (c) fumed silica having no surface treatment.

BACKGROUND OF THE INVENTION 
The field of art to which this invention pertains is cyanoacrylate 
adhesives. 
Alkyl 2-cyanoacrylates, or alpha-cyanoacrylic acid esters, polymerize at 
room temperature in the absence of added catalysts when pressed into a 
thin film between two adherends. The highly polar character of the formed 
polymers permits the formation of very strong bonds between a wide variety 
of materials. 
In adhesive applications, alkyl 2-cyanoacrylates are polymerized by anionic 
mechanism. This anionic polymerization of alkyl 2-cyanoacrylates can be 
initiated even at low temperatures by very weak bases, such as water and 
alcohol. The bonding action observed when a cyanacrylate adhesive is 
placed between two adherends is the result of this anionic polymerization. 
The catalytic effect of minute amounts of water vapor or other weak bases 
is maximized by spreading the adhesive into a very thin film. The presence 
of basic substances accelerates the bonding action of the 
2-cyanoacrylates, whereas the presence of acidic materials inhibits the 
polymerization and can even render the adhesive inactive. The set time 
(elapsed time after which two surfaces that have been pressed together can 
no longer be moved) for cyanacrylate adhesives varies from several seconds 
for glass to three minutes or longer for wood. The residual moisture in 
glass produces bonds having extremely rapid set times whereas the slight 
acidity in wood, as well as wood porosity, produces bonds having somewhat 
slower set times. 
Various attempts have been made to shorten the setting time for 
2-cyanoacrylate adhesives when used on acidic and/or porous surfaces. In 
U.S. Pat. No. 4,170,585, the setting time for 2-cyanoacrylate adhesives is 
accelerated by adding to the 2-cyanoacrylate (1) polyethylene glycols 
having a degree of polymerization of at least 3 and (2) non-ionic surface 
active agents having a poly(ethyleneoxy) moiety also having a degree of 
polymerization of at least three. 
In U.S. Pat. No. 4,307,216, adhesive compositions made from 2-cyanoacrylate 
esters and monoacrylates of glycol monoethers are described. 
2-Cyanoacrylate ester adhesives can be modified with diacrylic acid ethers 
of oxyethylene glycols as described in U.S. Pat. No. 3,692,752. 
Aliphatic polyols, aliphatic polyether polyols, and derivatives thereof are 
used to modify 2-cyanoacrylate adhesives as described in U.S. Pat. No. 
4,377,490. 
Dicyanoacrylate esters of glycols and polyoxyethylene glycols are disclosed 
in British Pat. No. 1,048,906. 
The 2-cyanoacrylate ester monomer is a low viscosity liquid which possess 
high flowability and can penetrate into very small gaps. However, this 
high flowability is a disadvantage when the adhesive is applied to 
vertical, inclined or porous surfaces. For such uses, the adhesive 
compositions have been thickened by various means. As described in 
Japanese Kokai No. 54-107,993, a number of compounds, such as polyols, 
e.g., sorbitol, polyethers, e.g., polyethylene glycol dilauryl ether, 
polypropylene glycol dioctyl ether, polyethylene glycol dimethacrylate, 
aluminum salts of aliphatic acids and silicon dioxides, e.g., silica gel, 
are disclosed which can be used to thicken cyanoacrylate adhesive 
compositions. 
In U.S. Pat. No. 3,607,542, adhesive compositions which can be applied to 
surfaces submerged in water are made from 2-cyanoacrylate esters, fillers, 
e.g., calcium carbonate, barium carbonate and carbon black, and, as an 
optional component, fumed silica. Such pastes can be applied up to about 
four hours after the components are mixed together. 
Thixotropic cyanoacrylate adhesives which are storage stable are made from 
the 2-cyanoacrylate monomer and fumed silicas which are surface treated 
with siloxanes or silanes as described in U.S. Pat. Nos. 4,477,607 and 
4,713,405. 
There is a continuing search for thixotropic cyanoacrylate adhesive 
compositions which are storage stable and which are rapidly curable on 
porous, inactive, or acidic surfaces. 
SUMMARY OF THE INVENTION 
This invention is directed to thixotropic adhesive compositions. In one 
aspect, this invention pertains to stable thixotropic 2-cyanoacrylate 
esters adhesives. In another aspect, this invention relates to stable, 
thixotropic 2-cyanoacrylate ester adhesives which are relatively surface 
insensitive and cure rapidly when applied to a variety of surfaces. 
The adhesive compositions of this invention are made from (a) a 
2-cyanoacrylate ester; (b) an alkylene or a polyoxyalkylene glycol, glycol 
ether, or glycol ester wherein the alkylene group contains 2 or 3 carbon 
atoms; and (c) fumed silica having no surface treatment. 
DESCRIPTION OF THE INVENTION 
The 2-cyanoacrylate esters useful in this invention are those having the 
formula 
##STR1## 
wherein R is a straight chain or branched chain alkyl group having 1 to 12 
carbon atoms which can be substituted with a substituent such as a halogen 
atom or an alkoxy group, a straight or branched chain alkenyl group having 
2 to 12 carbon atoms, a cycloalkyl group, an aralkyl group or an aryl 
group. Specific examples of such groups include methyl, ethyl, isopropyl, 
butyl, hexyl, lauryl, allyl, cyclohexyl, benzyl, phenyl, 2-chloroethyl, 
2-methoxyethyl, and the like. 
Particularly preferred cyanoacrylate esters are alkyl 2-cyanoacrylates 
wherein the alkyl group contains one to four carbon atoms. 
The alkylene glycols useful in this invention are ethylene glycol and 
propylene glycol-1,2. The polyoxyalkylene glycols useful in this invention 
are polyoxyethylene gylcols and poly(oxypropylene-1,2) gylcols wherein the 
oxyalkylene group has a degree of polymerization of at least two. There 
polyoxyalkylene gylcols are made by polymerizing ethylene oxide and 
propylene oxide-1,2. Examples of such polyoxyalkylene glycols are 
diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene 
glycol, tetraethylene glycol, tetrapropylene glycol, polyethylene glycols 
having molecular weights up to about 1000 and polyethylene glycols having 
molecular weights up to about 4000. Additional polyoxyalkylene glycols 
useful in this invention are block copolymers of ethylene oxide and 
propylene oxide-1,2 wherein the molar ratio of ethylene oxide to propylene 
oxide varies between 1:99 to 99:1. The molecular weights of the 
polyoxyalkylene glycols useful in this invention are limited by the 
solubility of the glycols in the cyanoacrylate monomer. Polyethylene 
glycols having molecular weighs up to about 600 and polypropylene glycols 
having molecular weights up to about 2000 are preferred. 
The polyoxyalkylene glycol ethers useful in this invention are mono and di 
alkyl or aryl ethers of alkylene and polyoxyalkylene glycols having the 
same definition as defined hereinbefore. The alkyl groups are straight or 
branched chains having 1 to 18 carbon atoms, and the aryl groups are 
alkaryl or aralkyl groups having six to about 20 carbon atoms. Examples of 
suitable ethers are polyethylene glycol monomethyl ether, polypropylene 
glycol dimethyl ether, polyethylene glycol monobutyl ether, polypropylene 
glycol monolauryl ether, polypropylene glycol dimethyl ether, polyethylene 
glycol dipropyl ether, and the like. 
Alkylene and polyoxyalkylene glycols esters useful in this invention are 
the mono and diacrylic or mono and dimethacrylic acid esters of alkylene 
and polyoxyalkylene glycols having the same definition as described 
hereinbefore. 
The fumed silicas useful in this invention are those which have no surface 
treatment and which have a particle size of about 0.007 to 0.05 
micrometers, a surface area of about 50-400 sq. meters/gram and a silica 
content of at least 99 percent. Fumed silicas are prepared by the 
following methods: (1) in high-temperature arc processes wherein silica is 
used as part of the electrodes and wherein the silica is vaporized and 
recondesnsed as a fine silica dust; (2) in plasma jet processes, wherein 
crystalline silica such as sand is fed directly to the high temperature 
plasma jet; (3) in flame-hydrolyic processes, wherein silicon 
tetrachloride is reacted with hydrogen and oxygen in a flame to form a 
very fine silica plus hydrochloric acid. 
In preparing the compositions of this invention, the alkylene or 
polyoxyalkylene glycol compound is blended with the 2-cyanoacrylate ester 
monomer in the amount of about 0.05 to about 3 parts by weight based on 
100 parts by weight of the 2-cyanoacrylate ester and preferably about 1 to 
about 2 parts by weight. The fumed silica is used in the amount of about 2 
to about 10 parts by weight on the same basis and, preferably, about 3 to 
about 6 parts by weight. 
Anionic polymerization inhibitors can be added to the adhesive compositions 
of this invention so as to increase the storage stability of the adhesive. 
Examples of such inhibitors are sulfur dioxide, aromatic sulfonic acids, 
aliphatic sulfonic acids, sultones, carbon dioxide, and the like. Such 
anionic polymerization inhibitors are used in the amount of about 5 to 
about 500 ppm based on the total weight of the adhesive. 
Free radical polymerization inhibitors can also be added to the adhesive 
compositions of this invention in order to capture radicals which are 
formed in storage by light, heat or other means. Such inhibitors are the 
well known free radical polymerization inhibitors which are exemplified by 
hydroquinone and hydroquinone monomethyl ether. These inhibitors are added 
in the amount of about 500 to about 5000 ppm based on the total weight of 
the adhesive composition. 
A thickener in addition to the fumed silica can also be added to the 
adhesive composition. Such thickeners are non reactive organic polymers 
which are soluble in the cyanoacrylate monomers. Examples of such 
thickeners are poly(methyl methacrylate), copolymers of methylmethacrylate 
with vinyl or acrylic monomers, acrylic rubbers, cellulose derivatives, 
and the like. The amount of thickener used is about 2 to about 20 parts by 
weight based on 100 parts by weight of the cyanoacrylate monomer. 
Additional components which can be added to the adhesive compositions are 
plasticizers, perfumes, dyes, pigments and the like. 
The adhesive compositions of this invention can be used to bond a variety 
of adherends, such as wood, chromate-treated metal surfaces as well as 
other metallic materials, ceramics, plastics, rubbers, leather, and paper. 
The adhesive compositions of this invention have fast setting times and 
excellent storage stability as exhibited by a slow rate of syneresis.

The invention is described in more detail by the following examples. Parts, 
percentages and ppm (parts per million) are all by weight unless otherwise 
designated. 
EXAMPLE 1 
To a suitable container were added 88.9 parts of ethyl 2-cyanoacrylate and 
15 ppm of sulfonic acid stabilizer. 5.66 parts of powered poly(methyl 
methacrylate), were then added and the mixture was gently heated and 
stirred until the polymer dissolved. 1.18 parts of polypropylene glycol 
dimethacrylate (M.W. 620) were then added. Fumed silica, 4.25 parts, was 
baked at 150.degree. C. to dry it and was added hot to the blend. The 
resulting adhesive composition was a homogeneous, thixotropic gel having a 
viscosity (Brookfield HBT TA Spindle) at 10 rpm of 16,000 cps and at 1 rpm 
of 104,000, a viscosity index of 6.5. When applied to a vertical surface, 
a 1/8 inch bead did not flow on a vertical glass plate. 
When stored at 82.degree. C. in borosilicate glass tubes, the adhesive was 
stable exhibiting no thickening after more than 4 days and slight 
syneresis in 5 days. 
The adhesive composition was applied to the following surfaces and the 
speed of the cure was measured. 
______________________________________ 
Balsa wood 10-20 seconds 
Cherry wood 15-20 seconds 
Ceramic 25-30 seconds 
Paper 40-60 seconds 
______________________________________ 
EXAMPLE 2 
An adhesive composition was formulated exactly as Example 1 except the 
polypropylene glycol dimethacrylate was left out. The properties of this 
composition compared to those of Example 1 are as follows: 
______________________________________ 
Example 1 Example 2 
______________________________________ 
Appearance Homogeneous, Grainy 
thixotropic gel 
paste 
Stability Slight Severe 
in borosilicate 
syneresis Syneresis 
glass tubes in 5 days in less than 1 day 
Flow 1/8" bead does 
1/8" bead 
on vertical not flow flows 
glass plate 
Speed of Cure 
Balsa wood 10-20 seconds &gt;1 minute 
Cherry wood 15-20 seconds 30-60 seconds 
Ceramic 25-30 seconds &gt;&gt;1 minute 
Paper 40-60 seconds &gt;&gt;1 minute 
______________________________________ 
EXAMPLE 3 
Adhesive compositions were formulated using the same procedure and 
components as described in Example 1 except oxyethylene glycol 
dimethyacrylates having varying degrees of polymerization of the 
oxyethylene group were used. The formulations were left undisturbed for 5 
days after which the amount of exuded liquid was a measured. These results 
are shown in the table. The degree of polymerization of the oxyethylene 
group [CH.sub.2 CH.sub.2 O].sub.n is shown in the column under "n". 
EXAMPLE 4 
______________________________________ 
Glycol dimethacrylate Amount of exudate 
Example 
Oxyethylene Unit 
n % of formulation 
______________________________________ 
3A ethylene 1 1.00 
3B diethylene 2 0.23 
3C triethylene 3 0.24 
3D polyethylene 10 0.04 
______________________________________ 
Adhesive compositions were formulated using the procedure described in 
Example 1 except 0.2 part of 1,2-polyoxypropylene glycols of varying 
degrees of polymerization were used. The flow characteristics, as 
described in Example 2 were measured before and after 85 days aging under 
ambient conditions. Examples 4D and 4E were made with 1,3-propanediol 
using 0.2 part in 4D and 1.54 parts in 4E. These results were as follows: 
______________________________________ 
1/8 inch bead flow 
in 20 minutes 
Example Glycol n Initial 
85 Days 
______________________________________ 
A Polypropylene 
34 No flow 
No flow 
4B Polypropylene 
17 No flow 
No flow 
4C Polypropylene 
12 No flow 
No flow 
4D 1,3-Propanediol 
1 Exudate 
Severe 
flow exudate 
flow 
4E 1,3-Propanediol 
1 Slight Severe 
exudate 
flow 
______________________________________ 
The principles preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrative and changes may be made by those skilled in 
the art without departing from the spirit of the invention.