High bonding strengths are obtained for metals and fiber-reinforced organic resin composites with no significant loss in thermo-oxidative stability of the adhesive resin

Bell, V. L.

Progar, D. J.

St. Clair, T. L.

English

NASA Technical Reports Server

October 1975	 B75-10257 
NASA TECH BRIEF 
Langley Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Process for Preparing Polylmide Adhesives 
S 
.
Bonded structures are advantageous to aerospace 
vehicles due to their lightness of weight and excellent 
resistance to fatigue and corrosion. Advanced aircraft 
experience skin temperatures approaching 300° C, 
and future spacecraft could require adhesives for use 
at that temperature and above. 
A process has been developed for preparing 
aromatic polyimides for use as adhesives in bonding 
metals and fiber-reinforced organic resin composites. 
High bonding strengths are obtained with no resultant 
significant loss in thermo-oxidative stability of the 
adhesive resin, and these polyimide adhesives retain a 
high proportion of their adhesive strength when 
formulated with various additives and when used at 
elevated temperatures. 
Producing polymers for use as adhesives involves 
the addition of an equimolar quantity of a suitable 
aromatic dianhydride to a stirred solution of an 
appropriate aromatic diamine (or equivalent mixture 
of two or more diamines) in certain water-miscible or 
alcohol-miscible ether solvents. In certain instances, 
when a highly-viscous polyamic-acid intermediate 
polymer precipitates from the ether solvent, the 
addition of very small amounts of water or alcohol 
generally leads to redissolution of the polyamic-acid to 
give a viscous polymer solution. At other times, 
depending upon the choice of monomers, the 
polyamic-acid intermediate polymer does not become 
insoluble, and a smooth viscous polymer solution is 
formed directly. The choice of diamines and 
dianhydrides inappropriate for this process is signaled 
by failures, such as the following: 
a. One or both monomers fail to dissolve in the ether 
solvents. 
b. A mismatch between the rate of polymerization of 
the reactants and the rate of precipitation of the 
polymer causes the incompletely-reactea insoluble
monomer to become coated by precipitating poly-
mer and, thus, inactivated before the completion 
of polymerization. 
c. The polyamic-acid intermediate polymer is pre-
cipitated from the solution, usually as a powder, 
before a suitable degree of polymerization has 
been reached. 
The solvents useful for this process are of the 
aliphatic acyclic and cyclic ether types, such as 
tetrahydrofuran, m-dioxane and p-dioxane, mono-
glyme, diglyme, triglyme, and tetraglyme. These 
solvents have a high affinity for the polar chemical 
groups of the aromatic polyamic-acid intermediates. 
The diamines which are most suitable for use with 
ether solvents are those of the dianilino variety, 
usually with both of the amino groups situated meta 
to the divalent X groups (e.g., methylene, carbonyl, or 
oxy) joining the two aromatic rings as follows: 
^^ X -^^ 
NH2	 NH2 
Two specific examples of such diamines are 
3, 3'-diaminobenzophenone and 3,3'-diaminodiphen-
y1methane. The most useful dianhydrides for use in 
the synthesis of the resins by the ether process are 
those of the bisdiphthalic anhydride variety, where the 
two phthalic anhydride functions are also separated 
by a divalent group, such as carbonyl or oxy. Two 
specific examples of appropriate dianhydrides are 
3, 3'-benzophenone tetracarboxylic dianhydride and 
4,4'-oxydiphthalic anhydride. 
The polyamic-acid solutions are applied to suitably-
prepared substrate surfaces, either as pure adhesive 
resins or in various formulations with fillers, 
thickeners, or the like, with or without supports such 
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19750000257 2020-03-19T20:15:04+00:00Z
as glass fabric. The substrates are allowed to stand to 
permit some or all of the solvent to evaporate. To be 
bonded, the treated surfaces are assembled together 
by means of clamps or in a press, and heat is applied 
to complete the evaporation of the solvent. This leads 
to conversion of the intermediate polyamic-acid to the 
more thermally-resistant cyclic polyimide, during the 
course of which the bonding of the substrate occurs. 
Although the bonding cycle can be varied over a wide 
range of time, temperature, and pressure, a typical 
bonding operation would initially involve the drying of 
the primed surfaces in air at room temperature. This 
would be followed by bonding of the joint at pressures 
from 30 to 200 psi (0.2X10 6
 to 1.38x106
 N/rn2), with 
thermal treatment, under pressure, up to tempera-
tures of 200° to 300° C. Times of 1 hour or more, at 
the required temperature and pressure, are sufficient 
to post-cure the bonded joint. 
This process is different from the usual procedure 
for polyirnide preparation in the necessity of using 
ether solvents. This is in contrast to the usually used 
solvents of the N,N-dialkylcarboxylamide class. The 
use of the proper ether solvents leads to molecular 
weights of the polyamic-acid prepolymers which result 
in superior bonding properties. Also, the ether 
solvents are more readily volatilized from the polymer 
at the optimum states of cyclization of the 
polyamic-acids to the polyirnides.
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Langley Research Center 
Mail Stop 139-A 
Hampton, Virginia 23665 
Reference: B75-10257 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
Patent Counsel 
Langley Research Center 
Mail Stop 313 
Hampton, Virginia 23665 
Source: Donald J. Progar and Vernon L. Bell

Langley Research Center and

Terry L. St. Clair of

Virginia Polytechnic Institute & State University

(LAR-1 1397)
fl 
Categories: 08 (Fabrication Technology) 
B75-10257	 04 (Materials)


Process for preparing polyimide adhesives

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