Birefringent coating for strain measurements on large surfaces contains constituents that can be premixed and sprayed as a single component with conventional paint spray equipment. Elevated temperatures are not required for spraying or curing of the coating material which has long pot life

Humphrey, F. T.

Mc~gee, W. M.

English

NASA Technical Reports Server

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December 1966	 Brief 66-10578 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Sprayable Birefringent Coating Enables Strain Measurements on Large Surfaces 
The problem: 
Although birefringent coatings have been widely 
used for photoelastic analysis of surface strains on 
structures, their use has been primarily limited to 
small-area coverage. Two-component spray systems 
have been proposed for large-area coverage because 
short pot life precludes the premixing of conventional 
curing agents and resins in sufficient quantities for 
spraying. Such systems, however, are quite expensive 
and require precise metering of the coating constit-
uents. In addition, these coating compositions must 
be heated to reduce their viscosities sufficiently to 
allow spraying. 
The solution: 
A birefringent coating formulation containing con-
stituents that can be premixed and sprayed as a single 
component with conventional paint spray equipment. 
Elevated temperatures are not required for spraying or 
curing of the coating material. This material has long 
pot life, and cures in a short time after spraying to 
produce a coating with relatively high and stable 
strain-optic coefficient. 
How it's done: 
The formulation consists of an epoxy resin, a 
thixotroping agent, two different curing agents or 
hardeners, and a solvent. Total hardener concentra-
tion is at or near the stoichiometric ratio based on the 
epoxide equivalent of the resins. The unique feature 
of the formulation is provided by using a ketimine as 
the primary curing agent.
In the absence of moisture, the ketimine reacts very 
slowly with the epoxy resin and thus ensures a long 
pot life. However, in the presence of moisture, which 
accumulates during spraying, the ketimine converts to 
an amine and a ketone. The resultant amine reacts 
with the epoxy resin, allowing the sprayed coating to 
cure rapidly, and the ketone is expelled. A second 
amine, which also functions as a hardener in the 
formulation, is used in sufficiently small concentration 
to have no appreciable effect on pot life. This amine 
also serves to accelerate coating gel formation during 
conversion of the ketimine. The solvent reduces the 
viscosity of the mixture sufficiently to allow spraying. 
The thixotroping agent is used to allow thick coats 
to be applied to vertical surfaces. 
Note: 
Inquiries concerning this invention may be di-
rected to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B66-10578 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: W. M. McGee and F. T. Humphrey
of Lockheed Aircraft Corporation
under contract to

Marshall Space Flight Center

(M-FS- 1484) 
Category 03 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19660000576 2020-03-11T17:45:36+00:00Z


Sprayable birefringent coating enables strain measurements on large surfaces

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