George L. Fix Raytheon Co. Equipment Development Laboratories Sudbury, Mass. 01776
Snap Crack,
Blister and Peel Surface Finish Elastomeric Cushion Coat Zinc Chromate Epoxy Primer Epoxy Sealer Coat
Aluminum Flame Spray
Epoxy Glass Laminate
Figure 1 . Cross section of the failed paint system of a large radar antenna
Each of us has experienced anger and frustration from an elastomer, adhesive or paint t h a t failed to perform to our expectations. T h e knowledge t h a t such failures are a common experience may provide a topic of conversation to share with others, but it offers little consolation when a laborious personal project is suddenly doomed to failure. Material failures are far more important to a corporation, however, for its continued business depends on the reliability of its products. This is particularly true for manufacturers of military hardware. T h e combination of stringent performance requirements with state-ofthe-art technology, electronics and materials makes military hardware the most challenging commodity a corporation could elect to manufacture. When a material fails, and some do, the demand for high reliability in military hardware requires t h a t the "root cause" of the failure be accurately determined and reproduced. T h e personnel assigned to study the root cause obviously vary with the nature of the failure—mechanical, metallurgical, electronic, or chemical. Failure analysis of organic materials is one of many activities undertaken by materials engineers, many of whom are actu-
ally chemists cleverly disguised in a white shirt, brown shoes and a fully outfitted pocket saver. An extremely important search for a root cause occurred in the early sixties because of serious problems with polyurethane encapsulants. T h e encapsulants were employed to isolate electronic components electrically and support t h e m during shock and vibration. Many materials performed admirably in the laboratory but failed in the stifling heat and humidity of South Vietnam. T h e root cause: Hydrolytic degradation transformed the polyurethane encapsulants into a soft, sticky and relatively conductive ooze. T h i s root cause determination identified some basic types of organic materials as " 4 - F " and led to hydrolytic stability testing of materials for use in military electronics. T h e light weight and high density of today's electronics have significantly increased the application of organic materials. At the same time the performance capabilities of organic materials have increased so dramatically t h a t some plastics are actually stronger t h a n steel. Fortunately, the analytical instrumentation available to the materials engineer has kept pace with the advancement of other technolo-
1312 A • ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
gies. Surprisingly to some within the electronics industry, a well-equipped materials engineering laboratory includes an ESCA/Auger spectrometer, a differential scanning calorimeter, a high pressure liquid chromatograph, and a Fourier transform infrared spectrometer.
Adhesive and Cohesive Failures In analyzing a failed adhesive, encapsulant or elastomer, the type of failure mechanism determines the selection of the analytical techniques applied to the problem. For example, an adhesive failure occurs at the interface between t h e adhesive and t h e substrate. Adhesive failures are generally best investigated by surface-sensitive techniques such as multiple internal reflectance infrared spectrometry, ESCA and Auger spectrometry. A cohesive failure occurs within the adhesive. Cohesive failures require evaluation of bulk properties and most often utilize solvent extraction, liquid chromatography, thermal analysis and infrared spectrometry. Also, a very powerful tool for evaluating both types of failures is the scanning electron microscope (SEM) equipped with an X-ray fluorescence analyzer. Semi0003-2700/80/A351-1312$01.00/0 © 1980 American Chemical Society
The Analytical
Approach
Edited by Jeanette G. Grasselli
An Analytical Approach
quantitative information on the inorganic constituents of adhesive failures is easily obtained and often provides a basis for the rest of the investigation. S E M photographs of the fracture surface of cohesive failures readily reveal voiding and noncharacteristic modes of fracture propagation.
Paint Adhesive Failure An interesting example of a catastrophic adhesive failure occurred last winter with the polyurethane paint system of a large radar antenna. As the photograph shows, the paint stripped from the antenna surface like
shelving paper, all 250 square ft of it. Beneath the peeled paint was a yellow-brown liquid, visible in the photograph on the exposed section of the antenna surface. Figure 1 shows the cross section of the antenna, epoxy sealer coat and polyurethane paint system. Samples of the paint film, the yellow-brown liquid and scrapings of the epoxy sealer coat were collected for analysis. Understandably, we did not section samples of the epoxy glass laminate of the antenna, since it was in excellent condition. T h e free paint film was found to have excellent adhesion between
layers. Solvent extraction, liquid chromatography, infrared spectrometry and differential scanning calorimetry revealed no problem with the paint system. ESCA analysis of the primer surface disclosed only the normal constituents of an epoxy-based zinc chrom a t e primer. T h e yellow-brown liquid was an aqueous solution containing two discrete constituents, one yellow and one brown. Infrared spectrometry, S E M X-ray fluorescence, and wet chemical methods were all used for its analysis. T h e yellow solute was identified as water-soluble chromâtes, a normal constituent of primers for aluminum surfaces. Their presence was not surprising since their function is to bleed, should moisture contact the primer surface. An excess of water soluble chromâtes can cause similar failures, however, and the primer was initially suspect. T h e brown solute was found to be an amine. Since both the primer and the sealer coat are amine cured liquid epoxy based systems, the origin of the water soluble amine was uncertain. T h e scrapings of the epoxy sealer coat were analyzed by solvent extraction, liquid chromatography, and infrared spectrometry. These analyses revealed t h a t the sealer coat had contained too much amine curing agent. To check the cause of the failure, the conditions were reproduced. T h e improperly formulated sealer coat was duplicated, applied to aluminum test panels, cured and overcoated with the paint system. No exposure to humidity, however, could duplicate the catastrophic failure of the antenna. Since the facilities of the vendor who applied the sealer coat and paint system were as cool as 50 ° F during the winter months, the misproportioned sealer coat was applied to cold aluminum test panels. Once again humidity exposure would not cause duplicate catastrophic failure in the test panels. Although a discrete problem with
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980 · 1313 A
Pin Area
Epoxy Staking
Aluminum Frame
Contact Area
Plastic Insulator
Figure 2. Cross section of a wire wrap connector. Epoxy staking is applied to pre vent the subsequent encapsulation of the pin area from reaching the contact sur faces of the connectors
the epoxy sealer coat had been identi fied analytically, reproducing the fail ure mechanism seemed impossible. Amidst all of the theories and specula tion, one technician's observation sur faced. The unused mass of the adhe-
^^^^^^
10 1 2 | 10" —. > § 1010
ο
•f
107
1 S
M
Initial — I
109
(Ohms/c
=
m
F J
I
A - . After Humidity
