The corrosive alkaline surface layer of an epoxy resin product formed by the curing of the epoxy with an aliphatic amine is eliminated by first applying a non-solvent to remove most or all of the free unreacted amine and then applying a layer of a chemical reagent to neutralize the unused amine or amine functional groups by forming a substituted urea. The surface then may be rinsed with acetone and then with alcohol. The non-solvent may be an alcohol. The neutralizing chemical reagent is a mono-isocyanate or a mono-isothiocyanate. Preferred is an aromatic mono-isocyanate such as phenyl isocyanate, nitrophenyl isocyanate and naplthyl isocyanate

Lee, S. Y.

English

NASA Technical Reports Server

United States Patent [191 
Lee 
4,362,769 
[451 Dec, 7, 1982 
[54] METHOD OF NEUTRALIZING THE 
CORROSIVE SURFACE OF AMINE-CURED 
EPOXY RESINS 
[75] Inventor: Sheng Y. Lee, Silver Spring, Md. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 293,412 
[22] Filed: Aug. 14, 1981 
[51] Int. Cl .3  ............................................... B05D 3/00 
[52] US. Cl. .................................... 427/322; 427/340, 
427/352; 427/400; 427/407.1 
[58] Field of Search ................. 528/73, 109, 121, 123, 
528/119; 427/336, 337, 340, 352, 399,400, 
393.5, 407.1, 385.5, 322 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,361,715 1/1968 Vogt et al. .......................... 528/121 
3,377,308 4/1968 Oertel et al. ....................... 260/32.6 
3,407,175 10/1968 Presley et al. ........................ 260/47 
3,525,779 8/1970 Hawkins ............................. 260/830 
3,558,422 1/1971 Hamilton et al. ................... 161/185 
3,715,326 2/1973 Traubel et al. ............... 260/2.5 AY 
4,009,113 2/1977 Green et al. .......................... 252/95 
4,085,161 4/1978 Sekmakas et.al. .............. 260/837 R 
4,260,727 4/1981 Noyd ................................ 528/73 X 
Primary Examiner-Thurman K. Page 
Attorney, Agent, or Firm-Ronald F. Sandler; John R. 
Manning 
ABSTRACT 1571 
The corrosive alkaline surface layer of an epoxy resin 
product formed by the curing of the epoxy with an 
aliphatic amine is eliminated by first applying a non-sol- 
vent to remove most or all of the free unreacted amine 
and then applying a layer of a chemical reagent to neu- 
tralize the unused amine or amine functional groups by 
forming a substituted urea. The surface then may be 
rinsed with acetone and then with alcohol. The non-sol- 
vent may be an alcohol. The neutralizing chemical rea- 
gent is a mono-isocyanate or a mono-isothiocyanate. 
Preferred is an aromatic mono-isocyanate such as 
phenyl isocyanate, nitrophenyl isocyanate and naplthyl 
isocyanate. 
8 Claims, No Drawings 
https://ntrs.nasa.gov/search.jsp?R=19830025768 2020-03-22T01:04:43+00:00Z
4.362.769 
31 
METHOD OF NEUTRALIZING THE CORROSIVE 
SURFACE OF AMINE-CURED EPOXY RESINS 
ORIGIN OF THE INVENTION 
The invention described herein was made by an em- 
ployee of the United States Government and may be 
manufactured and used by or for the Government for 
governmental purposes without the payment of any 
royalties thereon or therefor. 
TECHNICAL FIELD 
The invention relates generally to a process for elimi- 
nating the corrosive surface of amine-cured epoxy res- 
ins, and more particularly to the elimination of the cor- 
rosive alkaline surface which is frequently formed when 
amines are used to cure epoxy resins. 
BACKGROUNDART 
There are many applications where it is desirable to 
employ an amine-cured epoxy resin for bonding, coat- 
ing, embedding or casting. These applications include 
the bonding or coating of electrical conductors, the 
embedding of electrical conductors in the epoxy resin 
or the casting of epoxy resin with an electrical conduc- 
tor within. These electrical conductors are typically 
copper or aluminum. Under many process conditions, 
however, a surface is formed on the epoxy resin product 
which is highly corrosive to both copper and aluminum. 
The problem of corrosive surface formation is particu- 
larly acute when aliphatic amines are used as a curing 
agent. This problem has been previously recognized 
and determined to be due to the formation of an alkaline 
surface which includes unreacted amine and unused 
amine functional groups, either free or in the form of 
carbonates. 
Previous efforts have been made to solve the corro- 
sive surface problem. These efforts have included cur- 
ing the amine-cured epoxy resin in a dry container at an 
elevated temperature but the results have not been com- 
pletely satisfactory. High temperature curing does tend 
to leave less surface alkalinity but usually is not satisfac- 
tory. Moreover, the method may not be practicable in 
many situations and high temperature curing may have 
a deletereous effect on the epoxy resin itself because too 
high a peak exotherm can burn the resin itself or it may 
destroy any embedded electronic components. 
STATEMENT OF THE INVENTION 
Accordingly, an object of the present invention is to 
provide a new and improved process for making amine- 
cured epoxy resin products. 
Another object of the invention is to provide a new 
and improved process for the elimination of unreacted 
amine and unused amine functional groups from the 
surface of amine-cured epoxy resin products. 
A further object of the invention is to provide a new 
and improved process for the elimination of corrosive 
surface ingredients from amine-cured epoxy resin prod- 
ucts. 
In accordance with the invention, a process is pro- 
vided for making amine-cured epoxy resins which are 
free from the presence of a corrosive alkaline surface 
2 
amines are employed for the curing process and allows 
curing at room temperature. 
DETAILED DESCRIPTION O F  THE 
INVENTION 5 
Epoxy resins are frequently amine-cured. Amine- 
cured epoxy resins find a broad application in bonding, 
coating, embedding and casting. The bisphenal A type 
epoxies have proved very popular for use in the amine- 
10 cured process because the reaction is predictable. Rep- 
resentative epoxies in this group include EPON 815 and 
EPON 828 which are Shell Oil Company trademarked 
products. These epoxies are represented by the general 
formula: 
15 
.o n cy3 n .o. 
CH2--CAZ--O-( / \  0 )-;-( O)-0--CHz-CHz / \  
u cH3 I20 
Unfortunately, when epoxy resins are amine-cured, an 
alkaline corrosive layer is often formed on the surface 
of the cured product, especially where aliphatic amines 
25 are employed. The corrosive layer includes unreacted 
amine and unused amine functional groups. This is par- 
ticularly likely to occur when the product is cured in air 
whereupon the moisture and carbon dioxide in the air 
retard the curing reaction of amines on the surface, 
30 either by dilution effect or by the formation of amine 
carbonate salt. Additionally, the amine concentration in 
the surface layer may be inadvertently raised if the 
amine is relatively volatile and vacuum deaeration has 
been applied for an unnecessarily long period. Surface 
35 corrosiveness is particularly serious when aliphatic 
amines are chosen as the curing agent. The aliphatic 
amines may be shown by a typical formula H2N-R-NH- 
Typical aliphatic amines chosen for this classical 
40 reaction may be the amidoamines, marketed under the 
mark VERSAMID, a trademark of General Mills, Inc., 
diethylene triamine, marketed under the mark DETA, a 
trademark of the Allied Chemical Corporation, triethyl- 
ene tetraamine, marketed under the mark TETA, a 
45 trademark of the Allied Chemical Corporation, m-xyly- 
lene diamine (MXDA) and 1,3-bis (aminomethyl) cyclo- 
hexane (I,3-BAC). 
Aromatic amines usually present no significant prob- 
lem with respect to the formation of a corrosive layer 
50 because they typically require a high temperature cur- 
ing process where the humidity is low. Generally, the 
aromatic amines are much less hydrophilic and do not 
react significantly with carbon dioxide. In contradis- 
tinction, the alphatic amines are usually hydroscopic 
55 and very reactive toward carbon dioxide. This makes 
them very vunerable when exposed to air. It is the mois- 
ture and carbon dioxide that hinder the desired curing 
reaction of the aliphatic amine at the surface of the 
product. Nevertheless, the aliphatic amines are often 
60 the curing reagent of choice because they allow the 
epoxy resin to cure at room temperature and at a rapid 
rate. The resulting product will have a relatively high 
modulus of elasticity and bond strength. Room temper- 
ature curing may save cost and avoid the necessity of 
R-NH2. 
- .  
layer by treating the epoxy resin surface with mono- 65 elevated temperature curing which may be detrimental 
isocyanates or mono-isothiocyanates to eliminate both to the embedded electronic components. Corrosiveness 
the unreacted amine and the unused amine functional appears to be the only significant disadvantage of using 
groups. The process is especially useful when aliphatic the aliphatic amines. 
4,362,769 
3 
The alkaline corrosive layer on the surface can not be 
washed away with a solvent because the amines used 
for curing to effect polymerization and crosslinking are 
polyfunctional. The amine functional groups that are 
exposed to air may be left unreacted while other func- 
tional groups of the same molecule may have reacted 
and are tied to the polymer network. The alkaline sur- 
face cannot be eliminated by using less than the stoichi- 
metric amount of amines for the curing reaction. The 
corrosive surface layer may be removed by mechanical 
means, such as sanding or abrading, but these processes 
are not always practical. Curing in dry air or in an inert 
atmosphere, e.g. nitrogen, or adding a post-cure at an 
elevated temperature will reduce the surface alkalinity 
but frequently insufficiently. 
The invention is a rapid, convenient and effective 
method for chemically converting all the unused amines 
and amine functional groups into neutral, noncorrosive 
substituted urea. The reaction will take place at room 
temperature, in a few minutes, without leaving any 
corrosive residues. The method is simply to treat the 
corrosive alkaline surface layer of the amine cured 
epoxy resin by applying a thin but complete, contiguous 
coating or layer of a chemical reagent which may be 
either a mono-isocyanate or a mono-isothiocyanate. 
The reagent may be either aromatic or aliphatic, cyclic 
or acyclic. Aromatic mono-isocyanates are generally 
preferred because aliphatic mono-isocyanates are less 
reactive while mono-isothiocyanates, whether aromatic 
or aliphatic, have an unpleasant odor. Among the sub- 
stituted or unsubstituted aromatic mono-isocyanates, 
phenyl isocyanate, nitrophenyl isocyanate and naphthyl 
isocyanate are liquids, and readily available. I-Naphthyl 
isocyanate is a preferred reagent because it has a low 
vapor pressure and its vapor is much less irritating than 
some of the other reagents. Nevertheless, the entire 
process, including the epoxy preparation, should be 
carried out under a fume hood. In any event, these 
reagents, as a class, will convert the corrosive alkaline 
layer of the epoxy resin product to a neutral, noncorro- 
sive substituted urea in from one to fifteen minutes at 
room temperature. The reaction will work at any tem- 
perature below the boiling point of the mono-isocyanate 
or mono-isothiocyanate employed as the reagent. 
All the cyanates used are those that will react with 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
the unused amine or amine functional group left on the 6o 
epoxy resin surface. With the mono-isocyanates, NCO 
is the functional group. With the mono-isothiocyanates, 
NCS is the functional group. The reaction may be illus- 
trated as follows: 65 
4 
bondedto 
epoxy mass 
NCO 
partially reacted /' 
diethylene '' 
triamine(DETA) ,e' 
I-naphthyl 
isocyanate 
The effectiveness of the cyanate reagent tresitment 
may be improved by pre-washing the surface to be 
treated with a non-solvent such as alcohol to remove, as 
much as possible, the free unreacted amine on the sur- 
face. Thereafter, the surface should be dried because 
moisture or residual alcohol may consume part of the 
cyanate reagent and result in incomplete neutralization. 
Thereafter, it is advisable to use an excess of the cyanate 
reagent, cover the entire surface and to brush-mix the 
reagent on the surface once or twice during the reagent 
treatment. The surface may then be rinsed, initially with 
acetone, and then with alcohol. It is desirable to collect 
the washings in a receptacle where the excess cyanate 
reagent will be destroyed by the alcohol. The effective- 
ness of the neutralization treatment may be tested sim- 
ply with a piece of moistened pH paper. Before the 
neutralization treatment, the surface may show a pH 
value of from about 9 to 10. After the treatment, the pH 
value will be from about 7 to 8. 
I claim: 
1. A process for making a cured epoxy resin polymer, 
forming said polymer with a mixture of an epoxy 
resin prepolymer and an aliphatic amine, said ali- 
phatic amine acting as a curing agent; 
applying a liquid layer of a mono-isocyanate or 
mono-isothiocyanate reagent to said surface area of 
said polymer; and 
with a corrosion free surface area including: 
rinsing said reagent from said surface area. 
2. The process of claim 1 wherein said surface area of 
said polymer is treated with an aromatic mono-isocya- 
nate reagent. 
3. The process of claim 2 wherein said reagent is 
selected from the group consisting essentially of phenyl 
isocyanate, nitrophenyl isocyanate and naphthyl isocya- 
nate. 
4. The process of claim 2 wherein said reagent is 
I-naphthyl isocyanate. 
5. The process of claim 1 wherein said layer is a con- 
tiguous coating of said reagent on said surface. 
6. The process of claim 1 wherein said surface area is 
treated with a non-solvent for said polymer and dried 
prior to applying said reagent. 
7. The process of claim 6 wherein said non-solvent is 
alcohol. 
8. The process of claim 1 wherein said rinsing is ac- 
complished with acetone and then alcohol. * * * * *  


Method of neutralizing the corrosive surface of amine-cured epoxy resins

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