Acrylic adhesive composition comprising an alpha amino phosphonic acid or salt having improved stability

The present invention comprises an acrylic adhesive composition having improved stability comprising polymerizable acrylic monomers in combination with polymerization initiators therefor; the improvement further comprises incorporating in the homogenous mixture of acrylic monomers and an initiator, including other substances to augment its performance as an adhesive,an effective amount of an inhibitor, being hydroquinone, and an effective amount of accelerator, being alpha amino phosphonic acid, or an alkali metal salt thereof.

BACKGROUND OF THE INVENTION 
The present invention relates to improved acrylic adhesive compositions. 
The present invention further relates to acrylic adhesive compositions 
containing free radical initiators, having improved stability. 
The present invention also relates to two component acrylic adhesive 
systems, i.e., a system in which the initiator and accelerator are kept 
separately prior to use. 
All polymerizable adhesives have a limited shelf life. When stored at room 
temperature, the polymerizable adhesives ultimately undergo 
polymerization. However, premature gelation, a highly undesirable feature, 
can occur if the adhesive is not suitably stabilized. In some cases, the 
effect on components of an adhesive upon aging will result in a reduction 
of the rate at which the adhesive can cure when the accelerator is added. 
In general, the tendency of an adhesive, or polymerizable monomer system to 
gel or polymerize, increases as the temperature increases. Conversely, 
lower temperature storage of the adhesive material results in an extension 
of the shelf life. In fact, in many segments of the adhesive market, 
refrigeration of adhesive products is often required. However, this 
technique to increase adhesive shelf life is both cumbersome and 
expensive. Moreover the adhesives must be rewarmed to working 
temperatures, usually room temperature, after refrigeration in order to 
attain a reasonable rate of cure or fixture time. 
In the prior art, approaches are disclosed which utilize the addition of 
various substances in order to stabilize polymerizable adhesives, but many 
of these additives adversely affect the speed of cure of the adhesive, or 
they result in a deleterious lowering of the adhesive strength. 
An ideal stabilization system is one which incorporates additives which 
have no adverse effect on cure speed or the adhesive physical properties, 
while imparting to the adhesive the desired shelf life. Much of the 
technology related to stabilization of acrylic adhesives has been 
developed for the "anaerobic" adhesives, where contamination of the 
composition by various metal ions has a negative effect on storage 
stability. In particular, transition metals having two or more valences 
are considered deleterious, and it is believed that such transition metals 
function as reductants by reacting with a free radical initiator, in 
particular, hydroperoxides, in a redox system. 
Inhibitors previously utilized for free radical polymerizations include: 
quinones, hydroquinones, catechols, sterically-hindered phenols, 
phenothiazines, aminophenols, inorganic salts or oxides, quinone-oximes, 
nitroso-amines, etc. Of these, the most commonly used to stabilize 
commercial monomers are the quinones, hydroquinones and hydroquinone alkyl 
ethers. These inhibitors are effective barriers to polymerization of 
stored monomers. However, for the monomers containing peroxidic materials, 
such as typical free radical initiators, not only are the phenolic or 
amine inhibitors required, but also materials to remove or inactivate 
substances that may cause premature polymerization. In particular, 
transition metals or their ions are known to decompose hydroperoxides to 
produce radical products, which, in turn, initiate polymerization. 
In the case of acrylic adhesives, or acrylic monomer compositions that cure 
and hold substrates together, the curing or hardening reaction is a 
polymerization of the monomers or oligomers. Premature formation of free 
radicals, by a redox reaction involving the peroxide or hydroperoxide, 
must be avoided if the adhesive is to have an adequate shelf storage life. 
Thus, in addition to the typical free radical inhibitor, as described 
above, the incorporation of a second substance to inactivate contaminating 
transitional metal ions is often required for adequate storage stability 
of compositions containing acrylic monomers and/or oligomers, and a 
peroxidic polymerization initiator, such as a hydroperoxide. 
U.S. Pat. No. 3,971,765 discloses that alpha amino-carboxylic monomers of 
the ethylenediamine tetraacetic acid (EDTA) type can be used to improve 
the stability of anaerobic adhesive formulations. Anaerobic adhesives are 
adhesives that are designed to cure or polymerize in the absence of 
oxygen. 
As pointed out earlier, it is believed that trace metal ion contamination 
adversely affects the shelf life of the adhesive or at least renders the 
performance of the adhesive unreliable. The EDTA presumably chelates the 
metal ions, thereby rendering them ineffective in terms of reacting with 
the peroxidic initiator. The concentration levels taught in the '765 
patent makes this technology undesirable. The '765 patent also points out 
the possibility of future contamination, i.e., if the chelator/sequestrant 
is insoluble in the adhesive mixture and separates from the mixture it 
cannot tie up newly introduced metal ions. 
U.S. Pat. No. 3,991,261, assigned to Henkel and Cie, discloses the use of 
N-alkyl-c-aryl nitrones as stabilizers for anaerobic adhesives. These 
compounds are claimed to enhance adhesive stability while not interfering 
with normal anaerobic cure chemistry. 
U.S. Pat. No. 3,962,372, assigned to DuPont, states that Hypalon.TM.(Dupont 
Chemical Company)-based two-part adhesives, e.g., see U.S. Pat. No. 
3,890,407, are relatively unstable and set up or cure prematurely. Many 
adhesive stabilizers known to the art were found to adversely affect the 
rate of polymerization or cure time. An exception to this class is 
butylated hydroxy toluene (BHT). Quinone compounds were found to be 
ineffective stabilizers. In each case the preferred peroxide was cumene 
hydroperoxide (CHP). 
U.S. Pat. No. 4,034,145, assigned to Henkel and Cie, discloses the use of a 
percarboxylic acid in order to improve the storage stability of the 
anaerobic adhesive without adversely affecting the curing properties. 
U.S. Pat. No. 4,262,106, assigned to Loctite, discloses a process for 
treating anaerobic adhesive mixtures containing a sulfimide, such as 
saccharin, with an insoluble chelating agent, in order to improve the 
stability of anaerobic compositions. The technology in the '106 patent 
taught is both expensive, as large amounts of chelating agent are 
required, and cumbersome, as a separate step to remove the excess agent is 
required. 
U.S. Pat. No. 4,038,475, assigned to Loctite, deals with the use of soluble 
chelators in conjunction with a quinone type inhibitor, such as 
naphthoquinone, in a formulated anaerobic composition. The soluble 
chelators claimed are derivatives of alpha and beta-amino caroboxylates, 
but chelators containing a --C.dbd.N-- linkage are excluded since they 
interfere with the speed of cure. The '475 patent also teaches that some 
quinone-type inhibitors adversely affect the speed of cure of adhesive 
compositions. 
Another U.S. patent, i.e., U.S. Pat. No. 4,103,081, also assigned to 
Loctite, discloses the use of nitrobenzene and chlorinated nitrobenzenes 
as stabilizers for anaerobic compositions. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved acrylic 
adhesive composition. 
An object of the present invention is to provide an acrylic adhesive 
composition containing free radical initiators, having improved stability. 
A further object of the present invention is to provide an improved 
two-component acrylic system, i.e., an adhesive system in which the 
initiator is kept separate from the monomers and other additives until 
just prior to its use. 
An object of the present invention is to provide an improved stabilization 
system for polymerizable acrylic adhesive mixtures containing free radical 
initiators. 
Another object of the present invention is to provide an improved 
stabilizer system for acrylic adhesives. 
The above and related objectives are achieved in the present invention 
comprising an acrylic adhesive composition having improved stability. The 
improvement comprises incorporating in the homogenous mixture of monomers 
and initiator, including other substances to augment its performance as an 
adhesive, an effective amount of hydroquinone, and an effective amount of 
an alpha amino phosphonic acid or alkali metal salt thereof. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The acrylate monomers constitute an essential ingredient of the adhesive, 
in that their polymerization converts the liquid adhesive to a solid. The 
major components of the acrylate monomer portion of the adhesive are 
esters of acrylic and methacrylic acids, with preference for the esters of 
methacrylic acid. These esters may be derived from monofunctional or 
polyfunctional alcohols, the latter giving rise to and providing for 
crosslinking. A large number of monomers, both monofunctional and 
polyfunctional, are available commercially and well known to those skilled 
in the art. A choice from this broad category of monomers is dependent 
upon end use application requirements, substrates to be bonded, viscosity 
required, and similar requirements. 
Typical monofunctional monomers include, methyl methacrylate, hydroxyethyl 
methacrylate, tetrahydrofurfuryl methacrylate, trimethyl cyclohexyl 
methacrylate, cyclohexyl methacrylate, methacrylic acid, isobornyl 
methacrylate, ethoxyethyl methacrylate, dicyclopentenyloxyethyl 
methacrylate, and non-acrylates such as N-vinyl pyrrolidone and vinyl 
acetate. Typical polyfunctional monomers include, methacrylate esters of 
polyethylene glycols, such as the esters of triethylene glycol, 1,6-hexane 
diol, propylene glycol, 1,3- and 1,4-butylene glycol, and 
1,12-dodecanediol. 
Other preferred monomers include the acrylate and methacrylate esters of 
oligomers prepared by reacting epoxy- or isocyanate-terminated with 
hydroxyl-containing monomers. Particularly preferred are methacrylates 
obtained by reacting hydroxyethyl- or hydroxypropyl methacrylate with 
urethane prepolymers obtained by reaction of an excess of diisocyanate 
with a polyester or polyether glycol. Particularly preferred are 
methacrylate oligomers having a molecular weight from about 400 to about 
4000. 
Urethane-acrylates/methacrylates suitable as monomers in compositions for 
adhesives according to the present invention are described in U.S. Pat. 
No. 3,425,988 and U.S. Pat. No. 4,018,851, the disclosures of these 
patents being incorporated herein by reference. 
Similarly, a number of elastomers, can be employed depending upon end use 
applications, process viscosity, and other end use considerations. The 
level of such an additive, relative to the amount of monomer, also effects 
performance and properties of the adhesive, and will vary from about 5 
percent to about 60 percent by weight of the total reactive mixture, 
although a range of from about 15 percent to about 30 percent by weight of 
the total reactive mixture is preferred for most commercial adhesives. 
Typical elastomeric agents include: natural and synthetic rubbers, such as 
chlorosulfonated polyethylenes, butadiene-acrylonitrile copolymers, 
triblock copolymers, such as Shell's Kraton.TM.(Shell Oil Company) series, 
linear thermoplastic polyurethanes, and ethylene-acrylate copolymers 
series, i.e., VAMAC.TM. series, as well as oligomers, which are 
vinyl-terminated compounds derived from epoxy resins or diisocyanates 
reacted with hydroxyl-terminated low molecular weight entities. The 
oligomers are preferably terminated with methacryloyl or acryloyl groups. 
In a typical synthesis, the precursor molecule, containing epoxy or 
isocyanate terminal groups, is reacted with an hydroxyalkyl methacrylate. 
Again, the particular additive selected, and the concentration level that 
it is used in the formulation, depends on many factors such as the 
viscosity required during dispensing and the properties desired after 
curing. 
To promote polymerization of the acrylate-methacrylate materials, a free 
radical initiator is incorporated in an amount sufficient to initiate 
polymerization or cure of the adhesive composition. Preferred free-radical 
initiators are those with high temperature ten hour half lives; these 
include the hydroperoxides of the formula ROOH, wherein R is a hydrocarbon 
radical containing up to 18 carbon atoms. Typical examples of such 
initiators are cumene hydroperoxide, t-butyl hydroperoxide, 2,5-dimethyl 
bexane peroxide and the like. Cumene hydroperoxide is an especially 
preferred free-radical initiator. Additionally, blends of hydroperoxides 
with peresters, such as t-butyl perbenzoate, or t-butyl peroxymaleate can 
be advantageously employed. The level of the initiator used in the present 
invention may vary from about 1 to about 10 percent by weight, with levels 
of from about 2 to about 4 percent by weight being preferred. 
For two part acrylic adhesives, the cure or polymerization is effected by 
contacting the monomer solution containing a free radical initiator with 
an activator. The activator, which can be mixed with the adhesive 
composition just prior to use, or can be applied to one surface to be 
mated with a second surface, which is coated with adhesive to affect an 
adhesive joint. 
Suitable activators include aldehyde-amine condensates, such as VANA X 
808.TM., sold by the R. T. Vanderbilt Company, or tertiary amines, such as 
dimethylaniline, or transition metal salts or acetylacetonates, or 
thioureas such as tetramethyl thioureas or acetyl thiourea. 
To achieve stability of the adhesive compositon, that is, to avoid 
premature gelation during storage, various inhibitors and stabilizers or 
chelators may be added, as discussed earlier. In the practice of this 
invention the preferred stabilizing system consists of a mixture of 
hydroquinone and an alpha amino phosphonic acid, or sodium salt thereof. 
The amount of hydroquinone found to be effective, is from about 300 to 
about 1000 parts per million (0.03 to 0.10 percent by weight) of the 
acrylic composition. Additionally, from about 300 to about 1000 parts per 
million (0.03 to 0.10 percent by weight) of alpha amino phosphonic acid, 
or an alkali metal salt thereof, is incorporated in the acrylic 
composition. The preferred alpha amino phosphonate is nitrilo 
(trismethylene phosphonic acid), also known as aminotri(methylene 
phosphonic acid), or the penta sodium salt thereof. The free acid is sold 
by Monsanto Industrial Chemicals Co. under the tradename, DEQUEST 
2000.TM., and the penta sodium salt thereof is called DEQUEST 2006.TM.. 
These DEQUEST compositions are excellent chelators in that they form very 
stable complexes with transition metal ions that might otherwise react to 
decompose the hydroperoxide initiators. 
It was observed that acrylic adhesives, that is, the compositions 
containing polymerizable monomers and a peroxide, e.g., cumene 
hydroperoxide, frequently gel in less than one week at room temperature in 
the absence of stabilizing additives or polymerization inhibitors. 
Screening of selected known free radical inhibitors involved using an 
accelerated stability test. This test consists of placing a small test 
tube containing approximately two grams of an oxidant-containing adhesive 
composition, along with a selected inhibitor, in an oven maintained at 
82.degree. C. and observing the time for the fluid mixture to gel or 
polymerize. Compositions having a stability of one hour or less are 
regarded as unstable. In general, the greater the stability at 82.degree. 
C., the longer the shelf life of the adhesive. Stability times of about 
four hours at 82.degree. C. is considered equivalent to stability at room 
temperature for one year. Comparisons of time to gel for various 
compositions containing additives affords a sound basis for evaluating the 
effectiveness of the additive or stabilizer.

The present invention is further exemplified below by several examples 
thereof in accordance with the preferred embodiments of the invention. 
However, it is understood that the invention is not limited to the 
examples included herein, but that equivalents will be apparent to those 
skilled in the art. In the following examples, and throughout this 
application, all parts and percentages are by weight unless otherwise 
indicated, and all temperatures are reported in degrees Celsius, unless 
otherwise specified. 
EXAMPLES 
EXAMPLE 1 
Effect of Hydroquinone and its Alkyl Derivatives on Acrylic Monomer 
Stability 
A solution of cumene hydroperoxide in dicyclopentenyloxyethyl methacrylate, 
a monomer often used in adhesive formulations, was made up to contain 3.3 
percent cumeme hydroperoxide by weight. This solution was then divided 
into portions, and to each portion was added an inhibitor at a molar 
concentration equivalent to 1000 ppm of hydroquinone. These solutions 
containing inhibitor were placed in small glass tubes which were placed in 
an oven maintained at 82.degree..+-.1.degree. C. The time for the 
solutions to gel was noted and the results are recorded in Table 1 below. 
TABLE 1 
______________________________________ 
Inhibitor Hours to Gel 
______________________________________ 
None 2 
Hydroquinone 57 
t-Butyl hydroquinone 
44 
Methyl hydroquinone 
44 
Di-t-butyl hydroquinone 
19 
Di-t-amyl hydroquinone 
19 
Trimethyl hydroquinone 
12 
______________________________________ 
It can be seen from the results shown in Table 1 above that hydroquinone 
and monoalkyl substituted hydroquinones are effective polymerization 
inhibitors. However, hydroquinone is a particularly preferred inhibitor. 
EXAMPLE 2 
Effect of Combination of Pentasodium Salt of Nitrilo (Trismethylene 
Phosphonic Acid) and Hydroquinone on Acrylic Monomer Stability 
To solutions of dicyclopentenyloxyethyl methacrylate, containing 3.4 
percent by weight of cumene hydroperoxide, were added (a) various 
inhibitors at a concentration of 0.05% by weight and (b) a chelator, the 
pentasodium salt of nitrilo (trismethylene phosphonic acid) at the level 
of 465 ppm of solution (0.0465 percent by weight). This latter material 
(NTMPA) is sold by Monsanto Chemical Co. under the trade name of DEQUEST 
2006.TM.. The solutions were subjected to stability testing as described 
in Example 1. The results are shown in Table 2 below. 
TABLE 2 
______________________________________ 
Inhibitor Time to Gel (hrs.) 
______________________________________ 
None 0-1 
Hydroquinone (HQ) 6 
Catechol 3-4 
Butylated hydroxytoluene 
1-2 
Benzoquinone 1-2 
Monomethyl ether of HQ 
1-2 
1,4-Naphthoquinone 
0-2 
______________________________________ 
The advantages of using hydroquinone in combination with NTMPA are clearly 
demonstrated in Table 2 above. 
EXAMPLE 3 
Effect of Combination of NTMPA and Hydroquinone on Acrylic Monomer 
Stability 
The monomer solution used for stability testing in this example comprised 
94% tetrahydrofurfuryl methacrylate, 2.6% triethyleneglycol dimethacrylate 
and 3.4% cumene hydroperoxide. To portions of this solution were added 
0.1% by weight of various inhibitors, plus 400 ppm NTMPA. Stability 
testing was conducted as described in Example 1. The results are shown in 
Table 3 below. 
TABLE 3 
______________________________________ 
Inhibitor Time (hrs) 
% Gel Observed 
______________________________________ 
None 1 100 
Hydroquinone (HQ) 
23 None 
Benzoquinone 6 50 
t-Butyl catechol 
3 100 
Methyl ether of HQ 
3 25 
Catechol 2 25 
1,4-Naphthoquinone 
2 100 
______________________________________ 
It is evident that the combination of hydroquinone and NTMPA is superior in 
imparting stability to acrylic monomers. It should be noted that addition 
of 475 ppm of ethylene diaminetetraacetic acid, with no inhibitor, gave a 
solution that completed gelled in one hour. Further, the solution 
containing benzoquinone was found to cure much more slowly when treated 
with a reductant than the solution containing hydroquinone. 
EXAMPLE 4 
Effect of Combination of ATMP and Hydroquinone on Acrylic Monomer Stability 
To portions of a solution of triethyleneglycol dimethracrylate containing 
3.4% cumene hydroperoxide was added hydroquinone (HQ) and the nitrilo 
(trismethylene phosphonic acid) (ATMP) to achieve the additive levels 
shown in Table 4. Each of these portions was then tested for stability 
according to the procedure of Example 1. Results are shown below Table 4. 
______________________________________ 
HQ conc. (ppm) 
ATMP conc. (ppm) 
Time to Gel (hrs) 
______________________________________ 
None None 1 
75 100 1 
75 300 1-2 
225 75 2-3 
225 300 5 
______________________________________ 
It is evident that at levels of HQ and ATMP both exceeding 220 ppm 
satisfactory stabilization of certain acrylic monomers containing peroxide 
can be achieved. ATMP or aminotri (methylene phosphoric acid) is sold by 
Monsanto Industrial Chemicals Co. as DEQUEST 2000. 
EXAMPLE 5 
Effect of Combination of NTMPA and Hydroquinone on Acrylic Monomer 
Stability 
To portions of a solution of 3.4% cumene hydroperoxide in 
dicyclopentenyloxyethyl methacrylate was added various amounts of 
hydroquinone and NTMPA to achieve the levels, expressed as ppm, shown in 
Table 5. Each of these portions was then tested for stability according to 
the procedure of Example 1. Results are given in Table 5. 
TABLE 5 
______________________________________ 
HQ conc. (ppm) 
NTMPA conc. (ppm) 
Time to Gel (hrs) 
______________________________________ 
None None 1 
118 100 1-2 
118 300 1-2 
268 75 2-3 
268 300 2-3 
500 500 15-18 
______________________________________ 
It is evident that excellent storage stability can be achieved with 
peroxide-containing acrylic monomers with both hydroquinone and NTMPA 
concentrations over 300 ppm. 
EXAMPLE 6 
Effect of Combination of ATMP and Hydroquinone on Acrylic Monomer Stability 
To portions of a solution of tetrahydrofurfuryl methacrylate containing 
3.4% cumene hydroperoxide was added HQ and ATMP to achieve the levels 
shown in Table 6. Each of these portions was tested for stability 
according to the procedure of Example 1. Results are given in Table 6. 
TABLE 6 
______________________________________ 
HQ conc. (ppm) 
ATMP conc. (ppm) 
Time to gel (hrs) 
______________________________________ 
500 None 1 
0 500 1 
140 100 1 
140 300 2-3 
290 75 2-3 
290 300 3-4 
1000 500 23 
______________________________________ 
It is evident that levels of HQ and ATMP both of over about 300 ppm are 
required to adequately stabilize certain acrylic monomers containing 
peroxide material. 
EXAMPLE 7 
Effect of Inhibitor on Fixture Time 
To solutions of cumeme hydroperoxide, 4% by weight in 
dicyclopentenyloxyethyl methacrylate were added 0.1% by weight (1000 ppm) 
of hydroquinone or benzoquinone. 
Fixture time was determined by adding one drop of activator solution to one 
glass slide and one drop of monomer solution to a second glass slide. The 
slides were placed together at right angles to each other and gently 
rotated back and forth through approximately a .+-.45.degree. angle 
several times, for approximately 6 seconds to ensure good mixing of 
activator and initiator. A stop watch was started as soon as the two 
coated surfaces were brought together. The fixture time was recorded as 
the time when resistance to movement of the mated slides became difficult. 
In Table 7 the fixture times recorded were the average of three tests. 
The activator solution consisted of a 2% by weight solution of acetyl 
thiourea in triethylene glycol dimethacrylate. 
TABLE 7 
______________________________________ 
Inhibitor Fixture Time (secs.) 
______________________________________ 
None 145 
Hydroquinone 141 
Benzoquinone 400 
______________________________________ 
The results demonstrate that hydroquinone is superior to a quinone in terms 
of fixture time for adhesives. 
EXAMPLE 8 
Effect of Stabilizers on Adhesive Fixture Time 
A two-part adhesive was prepared with the adhesive having the following 
composition: 
______________________________________ 
Tetrahydrofurfuryl methacrylate 
66.5 pts. by wt. 
Triethylene glycol dimethacrylate 
8.0 pts. by wt. 
Thermoplastic polyurethane 
15.0 pts. by wt. 
Methacrylic acid 7.5 pts. by wt. 
Cumene hydroperoxide 3.0 pts. by wt. 
Additive None or 0.03 pts. 
by wt. 
______________________________________ 
To the above adhesive composition was added either 0.05 percent by weight 
of hydroquinone (HQ), or aminotris(methylene phosphonic acid) or both. 
Stability of the various compositions was determined according to the 
procedure of Example 1. Fixture time was determined according to the 
procedure in Example 7. 
TABLE 8 
______________________________________ 
HQ ATMP Fixture Time (sec) 
Time to Gel (hrs) 
______________________________________ 
None None 58 1 
500 ppm 
None 71 2 
None 500 ppm 74 1 
500 ppm 
500 ppm 84 4 
______________________________________ 
The results shown in Table 8 demonstrate that stabilization of 
polymerizable adhesive compositions can be achieved with only a modest and 
acceptable increase in fixture time.