Treatment of metals to enhance adhesive bonding

A process for enhancing adhesive bonding of a metal substrate, particularly aluminum, which comprises treating the aluminum substrate, after optional cleaning thereof in an alkaline solution, with an ammoniacal solution of a copper salt, e.g. copper sulfate, to form a cuprammonium, i.e. [Cu(NH.sub.3).sub.4 ]SO.sub.4, solution containing the complex Cu(NH.sub.3).sub.4.sup.++ ion. Such treatment provides a controlled etch of the aluminum substrate and increases the surface area thereof, resulting in enhanced reactivity of each surface with an adhesive during adhesive bonding, and providing a strong adhesive bond between the adhesive coating and the metal substrate. When such cuprammonium treatment of an aluminum surface is followed by electrodeposition of an organic coating, as described in U.S. Pat. No. 4,180,442, prior to application of an adhesive coating, a strong adhesive bond having consistently high lap shear values is obtained.

BACKGROUND OF THE INVENTION 
This invention relates to surface treatment of a metal substrate, 
particularly aluminum, prior to application of an adhesive to the 
substrate, and is particularly directed to a process for the treatment of 
a metal substrate such as aluminum, to enhance subsequent adhesive 
bonding, by means of a chemical treatment which permits a controlled 
etching of the substrate surface and a uniform increase in surface area. 
The adhesion of polymers (as adhesives) to metal substrates is, generally, 
a complex process. Usually, the metal surface has to be pretreated, and 
the adhesive has to have sufficient activity for the mating of the two to 
result in an optimum bond. Substrates, e.g. metals, have to be prepared 
for this union of such two dissimilar materials as a polymer and a metal 
by first cleaning and then etching the metal, and, as in the case of 
aluminum, by anodizing (oxidizing) or other surface treatment such as the 
so-called FPL etch, employing a solution of sodium dichromate in sulfuric 
acid. These treatments have the effect of increasing surface area in order 
to enhance mechanical interaction between the polymer and the metal 
substrate. Where mechanical interactions alone between the metal and 
polymer coating are involved, due primarily to surface roughening, the 
resulting bond strength is relatively weak. 
Thus, surface treatment of metal substrates for subsequent application to 
adhesive bonding or painting has become a very important problem. With the 
advent of controlled oxidation techniques, i.e. anodization or other 
oxidative etching techniques, such as the well-known FPL etch noted above, 
or phosphoric acid anodization, among others, surface preparation has 
become a relatively simple matter. However, when bonding or coating on 
such a prepared surface, the majority of the methods take advantage of 
physical or mechanical bonding to the substrate, resulting in a relatively 
weak bond between the metal and polymer adhesive, as opposed to having a 
true chemical bond between the substrate and the adhesive. 
U.S. Pat. No. 4,180,442 discloses electrodeposition of a coating of an 
organic compound or polymer on a metal such as aluminum. The polymer 
coating has a strong chemical bond to the metal substrate, and also has 
functional groups for bonding to an adhesive, enabling a total chemical 
bond to be formed via such coating between the metal substrate and the 
adhesive. However, lap shear values measuring the strength of the adhesive 
bond to such coated substrates have varied from as high as over 5300 psi 
to as low as about 3000 psi. Thus, in order to obtain more consistently 
high lap shear values, it became evident that surface roughness or high 
surface area is highly important. 
In adhesive bonding to a metal, such as aluminum, it is usually necessary 
to pretreat the metal surface by cleaning it with a degreaser, followed by 
an alkaline clean and etch, and then by a deoxidizing process, followed by 
either anodizing it or subjecting it to a surface treatment such as FPL 
etch. Subsequently, these surfaces are normally treated with a primer to 
effect a mechanical bond between the substrate and the primer, and then an 
adhesive is applied to the primer. In the process of the above patent one 
proceeds from the deoxidized surface directly to the electrodeposition of 
the organic coating without passing through the surface preparation 
technique of oxidation. It is because of this that in the practice of the 
process of the above patent, electrodeposition takes place principally on 
a surface of variable roughness, thereby resulting in variable values in 
the lap shear bond tests. 
Since the electrodeposition of a coating procedure according to the above 
patent does not utilize an oxidized, e.g. anodized surface as a precursor 
to the electrodeposition step, the surface of the substrate is only as 
rough as it is when received from the mills. The only pretreatment of the 
substrate normally is to subject the substrate surface to a cleaning and a 
deoxidizing process, as noted above. 
Thus, to provide chemical bonding from the metal through to the adhesive, 
as opposed to the physical bond formed with oxides, a chemical treatment 
is needed that will etch the surface, as well as deposit a chemical 
compound that will function as an "active" surface for the 
electrodeposition process. This, then, would create a high surface area 
substrate that has a chemical compound attached chemically, and that will, 
in turn, chemically bond to the adhesive. 
Numerous methods are known for increasing the surface area of a substrate, 
and roughening a surface, such as by sanding, for example, but it is 
difficult to perform in a production line system. Furthermore, it does not 
create a uniform surface. Aside from the oxidation methods described 
above, there are very few other techniques for increasing surface area in 
a uniform manner. 
Generally, in a galvanic cell, one element is present as an ionic species 
and the other is metal. Then, if the electromotive force of the cell is 
such that one part of the cell has a lower reduction potential than the 
other, the reducing element will reduce the ion in solution to the 
metallic state and the reducing element will become oxidized and go into 
solution. Thus, it is well known that metallic zinc will reduce copper 
ions to copper metal, and the zinc metal will go into solution. Similarly, 
metallic aluminum will reduce copper ions to copper metal, and the 
aluminum will go into solution. In the case of a sheet of aluminum (or its 
alloy), in a copper salt solution, there will result a severely pitted 
aluminum sheet with large masses of copper attached to its surface, and 
this action will continue as long as there are copper ions in the 
solution. 
Accordingly an object of the present invention is the treatment of a metal 
surface such as aluminum by a chemical procedure to enhance adhesive 
bonding to the metal. Another object is the provision of procedure for 
chemically etching the surface of a metal such as aluminum so as to 
uniformly attack the surface of the metal and to increase the surface area 
of the metal in a uniform manner. A further object is to provide a 
chemical treatment for metals such as aluminum, which will etch the 
surface uniformly, and also deposit a chemical compound which will also 
function as an "active" surface for the electrodeposition process of the 
type described in the above patent. Yet another object is a chemical 
treatment procedure for a substrate such as aluminum which creates a high 
surface area substrate having a chemical compound attached chemically 
thereto which will in turn chemically bond to an adhesive. A still further 
object is the provision of a chemical treatment procedure of the type 
noted above which uniformly attacks the surface of a substrate and 
provides a high surface area, in conjunction with electrodeposition of a 
coating to provide a chemical bond for an adhesive coating resulting in 
more consistently high lap shear values in adhesive bonding tests. 
SUMMARY OF THE INVENTION 
The invention concept resides in a method of obtaining a controlled surface 
reaction such that no pitting of the aluminum surface occurs, and in the 
absence of large masses of copper depositing on and growing out of the 
substrate surface. More specifically, the above objects and advantages are 
achieved by a process involving an electrochemical oxidation reaction 
which permits a controlled "etching" of the substrate, e.g. aluminum, 
surface to occur with the consequent deposition of a moiety or chemical 
compound on the resulting "etched" surface which prevents the etching from 
proceeding too far and from becoming destructive. 
The invention concept preferably involves the use of an ammoniacal solution 
of a copper salt, e.g. copper sulfate, dissolved in ammonium hydroxide to 
form a cuprammonium solution, i.e. [Cu(NH.sub.3).sub.4.sup.++ ]SO.sub.4 
solution. In this form, the cupric ion is tied up as a complex, and it is 
not readily available for the redox reaction that would normally occur 
with Cu.sup.++ and aluminum metal. In fact, the concentration of the 
simple cupric ion at 25.degree. C. is 10.sup.-14 molar, when complexed 
with ammonia (i.e. of the cupric ions present in solution, only 1 in 
10.sup.11 atoms is not combined with ammonia) [C. Immerwahr, Z. anorg. 
Chem 24, 269 (1900)]. Thus, whether the small amount of free cupric ions 
present are capable of a slow, controlled oxidation of the aluminum 
surface, or if the cuprammonium ion is capable of a unique "etching" of 
the aluminum is not absolutely known. What is known, however, is that the 
surface texture of the aluminum changes to a somewhat highly convoluted 
form, indicative of some morphological change, and a consequent enhanced 
reactivity of the surface in adhesive bonding. This ties in with the fact 
that cuprammonium ion is known to be a strong oxidizing agent. 
It has been found, as shown in greater detail hereinafter, that 
cuprammonium treatment of aluminum to obtain a controlled etched and 
uniformly increased surface area, followed directly by bonding with an 
adhesive, such as an epoxy adhesive, provides a strong chemical bond 
between the adhesive and the substrate surface, having high lap shear 
values. However, it has been found that cuprammonium treatment, followed 
by electrodeposition, particularly provides more consistently good lap 
shear test results when adhesive bonding was effected after the 
electrodeposition of organics on the aluminum substrate. 
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The metal substrate, particularly aluminum, to be subjected to treatment 
according to the invention, is first subjected to a cleaning and 
deoxidizing procedure. The initial cleaning step for removal of grease and 
surface dirt can be carried out by treatment with Chlorosolve, a mixture 
of dichloromethane and isopropyl alcohol, or with methyl ethyl ketone 
(MEK). Thereafter, the substrate is treated in an alkaline cleaner. For 
this purpose, caustic alkali, e.g. sodium hydroxide, can be employed, 
noting that this material is very active particularly on aluminum 
substrates. 
Another illustrative cleaning procedure can include treatment first with 
methyl ethyl ketone followed by treatment with "Alconox", a slightly basic 
soapy material containing a phosphate other than trisodium phosphate, 
followed by treatment with NaOH. 
Another cleaning procedure is the use of "Comet", a commercial alkaline 
cleaner containing a gritty material as an abrasive plus a hypochlorite, 
optionally followed by treatment with NaOH solution. 
In carrying out the cuprammonium ion oxidation of the substrate, preferably 
aluminum, according to the invention, the solution employed can range in 
copper ion concentration, from 0.001 to 1.5 molar. A preferred copper ion 
concentration ranges from about 0.1 to 1.0 molar. The ratio of ammonia to 
copper in such solution can be the stoichiometric ratio of 4 to 1 for the 
complex Cu(NH.sub.3).sub.4.sup.++ ion. However, such ratio can range from 
2 to 1 to 8 to 1, preferably 4 to 1 to 8 to 1. The above noted 
stoichiometric ratio of 4 to 1 of ammonia to copper particularly is 
employed for obtaining greater lap shear values for the adhesive bond. 
Thus, Table 1 below illustrates lap shear values obtained following bonding 
with an epoxy resin, of an aluminum substrate initially treated according 
to the invention by oxidation in a cuprammonium solution at various molar 
copper ion concentrations ranging from 0.001 to 1.0, employing a ratio of 
ammonia to copper ranging from 1 to 1 to 8 to 1. 
TABLE I 
______________________________________ 
Molar Copper Ion 
Ratio of Lap Shear 
Concentration NH.sub.3 to Cu 
Values, psi 
______________________________________ 
1.0 4:1 5,796 
0.6 8:1 4,895 
0.6 4:1 5,024 
0.3 4:1 4,763 
0.3 8:1 4,500 
0.2 8:1 5,225 
0.2 1:1* 4,700 
0.2 2:1 4,335 
0.2 4:1 5,120 
0.1 8:1 4,690 
0.1 4:1 4,980 
0.001 4:1 4,000 
______________________________________ 
*Some pitting on surface 
From the above table, it is seen that consistently high lap shear values 
were obtained for the copper ion concentrations ranging from 0.1 to 1.0 
molar, and generally best results were obtained between the stoichiometric 
ratio of 4:1 to 8:1 of ammonia to copper in the solution. 
As previously noted, the cuprammonium oxidation procedure of the present 
invention can also be employed in conjunction with electrodeposition of an 
organic coating or polymer, as described in above U.S. Pat. No. 4,180,442 
or in Application, Ser. No. 317,162, filed Nov. 2, 1981, by Norman R. 
Byrd. The disclosure of the above mentioned patent and application are 
incorporated herein by reference. Thus, as disclosed in the above patent, 
electrodeposition is carried out employing a metal substrate, e.g. 
aluminum, as the anode, the cathode being, for example, platinum or 
carbon. Either direct current or alternating current can be employed in 
the electrodeposition, preferably direct current. Electrolytic voltages 
employed can range from about 1 to as high as 300 volts. The 
electrodeposition reaction is carried out according to this patent in 
non-aqueous media. 
The electrodeposition reaction of such patent accordingly comprises passing 
an electric current through a non-aqueous electrolyte in which said 
aluminum substrate is the anode, the aluminum of said substrate being 
capable of forming an organometallic compound, said electrolyte containing 
an organic compound having a labile hydrogen, and capable of generating an 
anion during electrolysis, and an inert organic solvent for said compound 
and electrodepositing a coating of said compound on said metal substrate, 
wherein said anion is reacted with and is chemically bonded to the metal 
of said substrate. The electrolyte contains an organic compound selected 
from the class consisting of (a) carboxyl substituted aminosilanes, (b) 
butylamine reaction products with phthalic anhydride, decylamine reaction 
products with phthalic anyhdride, and carboxyl terminated 
butadieneacrylonitrile copolymer, and (c) phenyl phosphonic acid, and 
dimethyl phosphite, as their triethylamine salts, and sulfonic acid 
compounds. 
The electrodeposition of the organic coating on the metal substrate, e.g. 
aluminum, takes place very rapidly, e.g. within about 1 minute and forms 
an organometallic compound, either monomeric or polymeric with the metal 
substrate, and such coating facilitates subsequent adhesive bonding to the 
substrate. 
A similar electrodeposition process is disclosed in the above application 
Ser. No. 317,162 employing a non-aqueous or aqueous electrolyte containing 
a phosphorylated amide having a labile hydrogen, and capable of generating 
an anion during electrolysis, said phosphorylated amide being in the form 
of (a) an organic polymer consisting of a poly (phosphinohydrazide), a 
poly (phosphinoguanide) or a poly (phosphinoureide), including 
homopolymers and copolymers thereof, and their thio analogs, or (b) a 2:1 
molar adduct of two moles of a nitrogen-containing compound of the group 
consisting of hydrazine, guanidine and urea, and its thio analog, to one 
mole of an organic phosphite or phosphonate, and their thio analogs, and 
electrodepositing a coating of said amide on said metal substrate wherein 
said anion is reacted with and is chemically bonded to the metal of said 
substrate. The number of recurrent units in the homopolymers and 
copolymers can range from about 3 to about 20. 
A particularly preferred amide of the above class is the reaction product 
from one mole of dimethylphosphite with one mole of hydrazine hydrate, or 
two moles of hydrazine hydrate with one mole of dimethylphosphite. A 
non-aqueous electrolyte containing an inert organic solvent such as 
dimethylformamide or ethylene glycol is preferably employed, 
electrodeposition being carried out at a voltage ranging from about 1.5 to 
about 100 volts, for a period up to not more than about 5 minutes. The 
phosphorylated amide forms an organo-metallic compound with the aluminum 
of the anode during electrodeposition. 
A series of tests, the results of which are shown in Table II below, were 
carried out to provide lap shear values when adhesive bonding was effected 
after cuprammonium ion oxidation of aluminum according to the invention, 
with or without subsequent electrodeposition, as compared to control 
surfaces of aluminum (a) which had no treatment other than cleaning with 
Chlorosolve or methyl ethyl ketone(MEK); (b) which were cleaned by normal 
processing procedure including solvent wipe with MEK, followed by alkaline 
cleaning with a proprietary cleaner (Turco 4215) at 
145.degree.-155.degree. F. for 15 minutes and rinsing in warm water at 
110.degree. F., followed by treatment with a deoxidizer such as Amchem 
616; and (c) which were cleaned by the conventional procedure of (b) 
above, followed by FPL etch or oxidizing treatment, employing a solution 
of sodium dichromate in sulfuric acid. 
To obtain lap shear values, the various control strips and strips treated 
with cuprammonium solution according to the invention, with and without 
electrodeposition, were bonded with FM73 (epoxy tape) adhesive in an 
autoclave at 40 psi and 240.degree. F. for 90 minutes. In each test, a 
pair of strips of the same type were adhesively bonded together, and the 
cured adhesively connected pairs of strips were then subjected to single 
lap shear tests. 
TABLE II 
__________________________________________________________________________ 
NO CUPRAMMONIUM 
PRETREATMENT 
ALUMINUM PROCESSING 
OF ALUMINUM CUPRAMMONIUM PRETREATMENT 
TECHNIQUES VOLT- LAP VOLT- CONCENTRATION LAP 
ON 2024T3 CLAD ALUMINUM 
AGE.sup.(6) 
SHEAR (psi).sup.(3) 
AGE.sup.(6) 
OF CUPRAMMONIUM 
SHEAR (psi).sup.3 
__________________________________________________________________________ 
8 
No surface treatment other than 
-- 1,500 -- .1M (Molar) 4,380 
MEK cleaning 
No surface treatment other than 
-- -- -- .01M 4,400 
MEK cleaning 
Cleaned by -- 1,000 -- -- -- 
normal processing procedure 
Cleaned by normal processing 
-- 5,000 -- -- -- 
procedure and given standard 
FPL etch treatment 
Electrodeposition of 1:1 DMP/H.sup.(1) (2) 
1.5 2,000 1.5 .3M 4,700 
" 1.5 1,250 -- -- -- 
" 5 1,100 -- -- -- 
" 7 2,000 7 .3M 4,900 
" 10 3,680 10 .05M 4,800 
" 15 2,250 15 .3M 3,800 
" 20 3,100 20 .005M 4,400 
" 30 2,700 -- -- -- 
" 50 3,000 50 .05M 4,700 
" -- -- 100 .05M (Molar) 5,100 
Electrodeposition of 2:1 H/DMP.sup.(4) 
1.5 1,900 -- -- -- 
" 7 2,200 7 .3M 3,700 
" 15 2,300 15 .3M 3,500 
" 30 1,600 30 .3M 4,000 
" 30 2,700 -- -- -- 
Electrodeposition of 1:1 DMP/H.sup.(5) 
15 3,000 15 .3M 3,500 
__________________________________________________________________________ 
.sup.(1) Reaction product from one mole of dimethylphosphite with one mol 
of hydrazine hydrate (1:1 DMP/H)? 
.sup.(2) Non-aqueous electrodeposition process 
.sup.(3) Average value of three lap shear tests 
.sup.(4) Reaction product from two moles of hydrazine hydrate with one 
mole of dimethylphospite (2:1 H/DMP) 
.sup.(5) Aqueous electrodepositions 
.sup.(6) Electrodeposition voltage 
Note from Table II above, the high lap shear values obtained employing the 
cuprammonium procedure of the invention, and particularly when such 
procedure is employed in conjunction with electrodeposition, wherein 
consistently high lap shear values were obtained, using the reaction 
product of one mole of dimethylphosphite with one mole of hydrazine 
hydrate. 
Other cleaning procedures, prior to cuprammonium treatment, which can be 
utilized include (1) solvent wipe with MEK followed by cleaning in Alconox 
at 5% concentration for 10 minutes at 60.degree. C.; (2) wiping the 
surface with MEK and then scrubbing with Comet followed by a 5 minute 
rinse in deionized water, then a 5 minute dip in sodium hydroxide at 
either a 5%, 10% or 20% concentration, and then a 5 minute rinse in 
deionized water, followed by a 20 second dip in 10% NH.sub.4 OH, then a 5 
minute rinse in deionized water; (3) same as (2) above but without the 
Comet treatment; (4) a combination of (1) and (3) above; and (5) treatment 
with MEK or Chlorosolve followed by treatment with NaOH, a water rinse, 
and then an acid rinse, such as nitric, phosphoric, acetic or sulfuric 
acid solution. 
Although aluminum is the preferred substrate employed according to the 
invention process, the process is applicable to any metallic surface that 
is above copper in the electromotive series and where an oxidized surface 
is required for enhancing adhesive bonding to the substrate. The invention 
process is also applicable to such substrates or surfaces as stainless 
steel, utilizing tetravalent tin as the oxidizing agent, in a solution of 
a soluble tin salt such as stannic chloride. Thus, the concept of 
electrochemical oxidation, although specifically directed in the present 
disclosure to cupric ions and aluminum, has broader applicability to 
various oxidizing agents and metallic substrates or surfaces. 
The following are additional examples of practice of the invention process:

EXAMPLE I 
In order to demonstrate the beneficial effects of the electrochemical 
oxidation by the use of the cuprammonium process, a series of tests were 
initially performed using various cleaning and surface treating 
procedures, but without use of the cuprammonium ion solution. Since the 
first step of the process always consisted of a solvent wiping of the 
aluminum surface with methyl ethyl ketone (MEK), strips of 2024-T3 clad 
aluminum were wiped with MEK, dried and bonded with FM73 epoxy adhesive 
tape. The average lap shear value of three specimens was 1500 psi with one 
percent cohesive failure in the adhesive. The percent cohesive failure is a 
measure of the degree of adhesion to the substrate. The higher the cohesive 
failure values, the better the adhesion. In other words, the failure mode 
is in the adhesive, not at the interface between the adhesive and the 
substrate. 
Another set of 2024-T3 clad aluminum strips were wiped with MEK, dipped in 
a 5 percent solution of "Alconox" for 10 minutes at 60.degree. C., rinsed 
in deionized water for 5 minutes, air dried and bonded with FM73 epoxy. 
The lap shear was an average of 4000 psi. The failure mode was about 65 
percent cohesive. 
In another series of experiments, the Al was first MEK wiped then dipped in 
either a 5 percent, 10 percent or 20 percent sodium hydroxide solution for 
either 2 minutes, 3 minutes, or 5 minutes, rinsed in deionized water for 5 
minutes, air dried and bonded with FM73 epoxy. The average lap shear values 
were 3000 psi with about 50 percent average cohesive failure in the 
adhesive. 
An experiment was also performed where an abrasive was used for surface 
treating the aluminum as part of the cleaning process. In this case, the 
2024-T3 clad Al strips were wiped with MEK and then scrubbed with "Comet". 
This was followed by a 5 minute rinse in deionized water, then the strips 
were air dried and bonded with FM73 epoxy. The average lap shear value was 
2000 psi with about 10 percent cohesive failure in the adhesive. 
EXAMPLE II 
The effect of the cuprammonium ion on the activation of aluminum substrates 
was then examined. In Example I results were given for the lap shear 
obtained on 2024-T3 clad aluminum with only a MEK wipe. The same treatment 
was given to another set of 2024-T3 clad aluminum strips followed by a 10 
minute dip in the 0.2 molar cuprammonium sulfate solution (4:1 ammonium 
hydroxide to copper ion), then a 5 minute rinse in deionized water, dried 
and bonded with FM73 epoxy; the average lap shear was 4300 psi with about 
80 percent cohesive failure. 
EXAMPLE III 
In Example I it was demonstrated that when the aluminum was dipped into a 5 
percent "Alconox" solution at 65.degree. C. for 10 minutes lap shears of 
around 4000 psi were obtained. Using the same process as in Example I, 
viz., a MEK wipe, a 10 minute dip into a five percent solution of 
"Alconox" at 65.degree. C. for 10 minutes, then a 5 minute rinse in 
deionized water and then a 20 second rinse in 10% NH.sub.4 OH, followed by 
10 minutes in 0.2 molar (4:1) cuprammonium ion solution, then a 20 second 
rinse in 10% NH.sub.4 OH, a 5 minute rinse in deionized water and air 
dried, the FM73 bonded strips had an average lap shear of 5100 psi with a 
100 percent cohesive failure of the adhesive bond. 
EXAMPLE IV 
Repeating the same procedure of Example III, but using an 8:1 ratio of 
ammonium hydroxide to copper ion, the lap shear values were about 4940 psi 
with 100 percent cohesive failure of the adhesive bond. 
EXAMPLE V 
In Example I, the effect of sodium hydroxide solution pretreatment on 
aluminum was demonstrated. Repeating the treatment with sodium hydroxide 
but adding a 20 second dip in 10 percent NH.sub.4 OH solution followed by 
a 10 minute dip in cuprammonium sulfate solutions of varying molarities 
ranging from 0.001 to 1:5, and 4:1 ammonia to copper ion ratios, then 
another 20 seconds dip in 10 percent NH.sub.4 OH, a 5 minute rinse in 
deionized water, dried and bonded with FM73 epoxy, the lap shears averaged 
5000 psi with about 90 percent cohesive failure. 
EXAMPLE VI 
In Examples III and V, the effects of "Alconox" and sodium hydroxide, 
separately, were determined. In this example, both the "Alconox" and 
sodium hydroxide pretreatment were combined prior to the cuprammonium dip. 
The aluminum was first wiped with MEK then dipped into a 5 percent 
"Alconox" solution at 60.degree. C. for 10 minutes. This was followed by a 
5 minute rinse in deionized water, then a 5 minute dip in 20 percent sodium 
hydroxide solution at room temperature. After a 5 minute rinse in deionized 
water, then a 20 second dip in 10 percent NH.sub.4 OH followed by a 10 
minute dip in a 0.2 molar (4:1) cuprammonium sulfate solution, a 20 second 
dip in 10 percent NH.sub.4 OH, a 5 minute rinse in deionized water and air 
dried, the average lap shears were about 5200 psi with a 98 percent 
cohesive failure in the adhesive bond. 
EXAMPLE VII 
In Example I it was demonstrated that a pretreatment cleaning of the 
aluminum with MEK and "Comet" cleanser gave lap shear values of about 2000 
psi. Repeating the process of Example I with the "Comet" cleanser, but 
adding a 5 minute rinse in deionized water then a 10 minute dip in a 0.2 
molar (4:1) cuprammonium sulfate solution, followed by a 20 second dip in 
a 10 percent ammonium hydroxide solution, then a 5 minute rinse in 
deionized water, air drying and bonding with FM73 epoxy adhesive, the 
average lap shear was 4700 psi with about 80 percent cohesive failure. 
EXAMPLE VIII 
In this Example, a number of tests were evaluated with regard to the effect 
of the length of time the aluminum was in the cuprammonium solution. The 
first test was the effect of immersion in the 0.2 molar (4:1) cuprammonium 
sulfate solution after a pretreatment cleaning with the five percent 
"Alconox" solution. The 2024-T3 clad aluminum was wiped with MEK then 
dipped into a five percent "Alconox" solution for 10 minutes, at 
40.degree. C. The aluminum was then rinsed for 5 minutes in deionized 
water, followed by either a one minute, 15 minute or 30 minute dip in the 
0.2 molar (4:1) cupramonium sulfate solution, then a 5 minute rinse in 
deionized water, air dried and bonded with FM73 epoxy. The lap shears 
averaged about 5000 psi with about 95 percent cohesive failure in the 
adhesive bond. 
In the next test, the aluminum was wiped with MEK, then dipped in 10 
percent sodium hydroxide for three minutes followed by a five minute rinse 
in deionized water, a 20 second dip in 10 percent NH.sub.4 OH solution, 
then a one minute, 15 minute or 30 minute dip in the 0.2 molar (4:1) 
cuprammonium sulfate solution. This was followed by a 20 second dip in 10 
percent NH.sub.4 OH, a five minute rinse in deionized water, air dried and 
bonded with FM73 epoxy. The average lap shear was about 4500 psi with about 
60 percent cohesive failure. 
The next test was concerned with the effect of exposure time to the 
cuprammonium solution after a pretreatment cleaning with both "Comet" 
cleanser and sodium hydroxide. The aluminum was wiped with MEK, scrubbed 
with the "Comet" cleanser, rinsed in deionized water for five minutes, 
then dipped in five percent sodium hydroxide for five minutes followed by 
a five minute rinse in deionized water, a 20 second dip in 10 percent 
NH.sub.4 OH solution, then a one minute, 15 minute or 30 minute dip in the 
0.2 molar (4:1) cuprammonium sulfate solution. This was followed by a 20 
second dip to 10% NH.sub.4 OH, a five minute rinse in deionized water, air 
dried and bonded with FM73 epoxy. The average lap shear was 4780 psi with 
about 85 percent cohesive failure. 
EXAMPLE IX 
Subsequent to the treatment of the aluminum with a cuprammonium solution, 
the bonding of a cuprammonium treated aluminum that also has had an 
organophosphorous polymer electrodeposited onto its surface was evaluated. 
For this purpose, 2024-T3 clad aluminum was wiped with MEK, dipped in a 
0.05 molar (4:1) cuprammonium sulfate solution for 10 minutes, dipped in a 
10 percent ammonium hydroxide solution for 20 seconds, rinsed in deionized 
water for five minutes and air dried. The aluminum was placed in a 0.25 
molar solution of poly (phosphinohydrazide) (1:1 DMP/H) (prepared by 
reacting one mole of dimethylphosphite with one mole of hydrazine hydrate) 
in 400 mls ethylene glycol and having 0.25 moles of triethylamine. The 
aluminum strips, as the anode, with platinum (or carbon) as the cathode, 
were subjected to a potential of 10 volts for five minutes. The result was 
an electrodeposited coating of the poly (phosphinohydrazide) (1:1 DMP/H) 
onto the cuprammonium treated aluminum. The electrodeposited coating was 
washed in ethylene glycol for five minutes, methyl alcohol for five 
minutes and finally another methyl alcohol wash for five minutes. The 
dried aluminum was then bonded with FM73 epoxy. The average of three lap 
shear specimens was 4800 psi. Repeating the same electrodeposition 
process, but omitting the cuprammonium treatment, the lap shears were 3680 
psi. 
EXAMPLE X 
In another test wherein the aluminum was first treated with a cuprammonium 
solution and then electrodeposited with a phosphinohydrazide compound, 
analogous to Example IX, the aluminum was precleaned with MEK and treated 
with a 0.3 molar solution of cuprammonium sulfate solution, as described 
in Example IX. The aluminum was then placed in a 0.25 molar solution of a 
2:1 H/DMP (prepared by reacting one mole of dimethylphosphite with two 
moles of hydrazine hydrate) in 400 mls ethylene glycol and having 0.25 
moles of triethylamine. The aluminum as the anode, with platinum (or 
carbon) as the cathode, was subjected to a potential of 30 volts for five 
minutes. The result was an electrodeposited coating of the 2:1 
(phosphinodihydrazide) onto the cuprammonium treated aluminum. The 
electrodeposited coating was washed in ethylene glycol for five minutes, 
methyl alcohol for five minutes and finally another methyl alcohol wash 
for five minutes. The dried aluminum was then bonded with FM73 epoxy. The 
average of three lap shear specimens was 4000 psi. Repeating the same 
electrodeposition process, but omitting the cuprammonium treatment, the 
lap shears were 1600 psi. 
X-ray microprobe analysis shows the presence of copper on the surface of 
the aluminum substrate after cuprammonium oxidation according to the 
invention, but it is not metallic copper. Indirect evidence indicates that 
the copper ion has deposited on the aluminum surface in a complex form, 
probably as a copper aluminate, analagous to sodium aluminate, but more 
insoluble and quite stable. This reactive cuprammonium ion is also capable 
of being formed from curpic chloride, cupric hydroxide, cupric acetate, 
cupric nitrate and other cupric salts. Furthermore, other bases, such as 
ethylene diamine, triethylamine and pyridine, for example, will also form 
stable complexes with cupric ions. Although these all form, and can be 
used, the preferred species is the cuprammonium ion; and, in particular, 
the cuprammonium ion formed from cupric sulfate. 
Thus, the invention in its broad aspects is directed to a process for 
enhancing adhesive bonding of a metal substrate by the steps of treating 
said substrate to increase the surface area and to roughen said surface, 
and then applying an adhesive coating to said surface, wherein the 
essential feature comprises the step prior to adhesive bonding, of 
treating said substrate with a solution containing an ion of a metal 
having a lower electromotive force than said substrate metal, to cause 
said substrate metal to become oxidized and go into solution as an ion, 
the metal ion of said solution to deposit on said substrate in a complex 
form. 
From the foregoing, it is seen that the process of the present invention 
particularly employing an aluminum substrate, and an ammoniacal solution 
of a copper salt, preferably copper sulfate, results in a uniform 
"etching" of the aluminum substrate and increases the surface area thereof 
to a uniform convoluted form, wherein the copper of the ammoniacal solution 
forms a compound with the aluminum surface, and providing improved adhesive 
bonding of an adhesive to the aluminum substrate with or without 
electrodeposition of an organic coating on the substrate surface prior to 
adhesive bonding. Thus, the novelty in the invention process resides in 
the ability to oxidize an aluminum surface by formation of a copper salt 
therewith, in preparation for adhesive bonding employing less stringent 
conditions than are normally encountered in making aluminum bondable. 
Furthermore, the difficulties encountered in the use of the process 
described in above application Ser. No. 317,162 of electrodeposition of 
polymers prior to adhesive bonding and in the absence of prior etching of 
the aluminum surface according to the present invention, are reduced, and 
more consistent and higher lap shear values are obtained.