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--= -AD-A257 820 0"" 111111 1111111111 M I 11 1i1U IIII IIIII
EVf.LUi.TION OF EMPP/Et.I IIEQUIREMENTS VERSUS(;ORROSIOI PREVENTION METHODS
I GTRI PROJECT NO. A-8245
U.S. AIR FORCE CONTRACT NO. I-09603-88-R-2808
Wd For DTICRobins Air Force Base C LETE
Wa-kor Hobins Air Logistics Center OCT 2 7 1992WR-ALC/CNC
Georgia 31098-5320
Technical Monitor: Mr. David EllicksTelephone: (912) 926-3284
DSN: 468-3284rpx: (912) 926-6619
Prepered by
Dr. Jan W. •'1och,Mr. Paul M. Hawley
Aat.rials Science & Technology LaboratoryTelephone: (404) 894-8485
Fax: (404) 894-6199
Mr. John K. LVaherMr. Daniel M. LaGcc
M tro n" ",•L. L-ivhorhmental Effects Lab:ratoryTelephone: (404) 894-8228
Georgia Tech Research InstituteGerrgia Irbtitute of Technology
Atanta, Georgia j0332
Jaihu'y 23, 14(7
7 Oct 92 Final Report
Evaluation of EMP/EMI Requirements Versus USAF Contract No.Corrosion Prevention Methods F09603-88-R-2808
Dr Jan W. Gooch Mr John K. DaherMr Paul M. Hawley Mr Daniel M. LaGesse
Georgia Tech Research InstituteGeorgia Institue of Technology GTRI Project No.Atlanta Georgia 30332 A-8245
Air Force Corrosion Program OfficeWR-ALC/CNC N/A215 Page Rd, Suite 232Robins AFB GA 31098-1662
Continuation of GTRI Final Technical ReportContract No. F09603-85-R-0959, 31 Aug 87
Distribution Statement A. Approved for public release;distribution is unlimited.
Final report covers the application of conductive sealants onan E-3 aircraft for nine months and evaluating the ElectromagneticPulse (EMP)/Electromagnetic Interference (EMI) Requirements andcorrosion damage. Also, additional testing was performed on threeconductive sealants for corrosion protection via the salt fogchambers. Using conductive sealants will meet both EMP/EMI andcorrosion requirements.
S92-28056Sill i I~lil!l li I ll lI I H ~I
EMP/EMI Electromagnetic Pulse Corrosion Protection 297Electromagnetic Interference Conductive Sealants
Unclassified U U UL
I d
I EVALUATION OF EMP/EMI REQUIREMENTS VERSUSCORROSION PREVENTION METHODS
GTRI PROJECT NO. A-8245
I U.S. AIR FORCE CONTRACT NO. F09603-88-R-2808
I Prepared For
Robins Air Force BaseWarner Robins Air Logistics Center
WR-ALC/CNCGeorgia 31098-5320
Technical Monitor: Mr. David EllicksTelephone: (912) 926-3284
DSN: 468-3284Fax: (912) 926-6619
Prepared by
I Dr. Jan W. Gooch,Mr. Paul M. Hawley
Materials Science & Technology LaboratoryTelephone: (404) 894-8485
Fax: (404) 894-6199
Mr. John K. DaherMr. Daniel M. LaGesse
Electromagnetic Environmental Effects LaboratoryTelephone: (404) 894-8228
Georgia Tech Research InstituteGeorgia Institute of Technology
Atlanta, Georgia 30332 _. .
January 23, 1992
...
Table of Contents
Page
1.0 INTRODUCTION 1
2.0 OBJECTIVES 1
2.1 Corrosion Prevention Materials and 1EMP/EMI Requirements
2.2 Corrosion Prevention Materials and 2Nuclear Hardening
2.3 Field Testing 2
2.4 Specification for Use of Conductive 2Sealants
2.5 Qualified Products List for Conductive 2Materials
3.0 RESEARCH PLAN 2
4.0 RESULTS AND DISCUSSION 12
4.1 Evaluation of Laboratory Sealant Test Samples 12
4.1.1 DC Resistance Measurements 13
S4.1.2 Shielding Effectiveness Measurements 13
4.1.3 Transfer Impedance Measurements 15
S4.1.4 Relationship of Transfer Impedance to 16Shielding Effectiveness
4.1.5 Laboratory Test Results 18
4.2 Field Test Measurements 20
4.2.1 Location of Test Sites 20
4.2.2 E-3A Field Test Results 21
5.0 CONCLUSIONS 22
6.0 RECOMMENDATIONS 23
7.0 REFERENCES 24
I i
List of Appendicies
Appendix A - Shielding Effectiveness Data
Appendix B - Transfer Impedance Data
Appendix C - Drawings for Transfer Impedance Fixture
jAppendix D - E-3A Tail Panel Transfer Impedance Data
Appendix E - Specification for Conductive Sealants
Appendix F - Qualified Products List for Conductive Sealants
Appendix G - Sealant Manufacturer's Product Information
Ii
List of Tables
Table 1. Test Joints and Identification.
Table 2. D. C. Resistance (Milliohm) Measurements of Test
Samples.
Table 3. Average Shielding Effectiveness (dB) of Test Samples.
Table 4. Transfer Impedance (dB-Ohm) of Test Samples.
Table 5. Location of Test Areas on E-3 Aircraft and ConductiveSealants.
Table 6. Corrosion Areas on E-3 Aircraft and Results ofInspections.
Table 7. Results of D. C. Resistance and Transfer ImpedanceMeasurements on E-3 Aircraft.
iii
List of Figures
Figure 1- Profile of E-3 Aircraft.
Figure 2- Top View of E-3A Aircraft.
Figure 3- Tail section and patch (panel), Vertical fin.
Figure 4- Patch (panel) dimensions.
Figure 5- E-3A aft (and forward) entry door.
Figure 6- Block diagram of the shielding effectiveness testsetup
Figure 7- Block diagram of the transfer impedance testsetup
Figure 8- Front left view of E-3A aircraft showingforward and aft entry doors.
Figure 9- View of the bottom section of the forward entrydoor and corrosion areas.
Figure 10- View of forward door, bottom view of sealinggasket and corrosion areas.
Figure 11- View of open aft entry door.
Figure 12- View of bottom section of the aft entry doorand corrosion areas.
Figure 13- Installation of probes for measurement of dcresistance on bottom section of forward entrydoor.
Figure 14- Forward - right view of the E-3A aircraftshowing vertical tail fin panel.
Figure 15- View of the tail section of the E-3A aircraft,right side, showing vertical tail fin panel andcorrosion area.
Figure 16- View of the tail section, left side, showingaccess panels for measuring transfer impedance.
Figure 17- Preparation of vertical tail fin panel, removalof paint to bare metal, for attachment oftransfer impedance fixture
iv
List of Figures (Continued)
Figure 18- Installation of transfer impedance fixture.
Figure 19- View of fixture and instruments for measuringtransfer impedance.
Figure 20- Ground view of bottom side of rotodome and entrydoor.
Figure 21- Front view of air inlet duct EMP shield.
Figure 22- Comparison of measured Zt using laboratory andfield test fixtures (Rdc = 1 milliohm).
Figure 23- Comparison of measured Zt using laboratoryand field test fixtures (Rdc = 5 milliohms).
v
1.0 INTRODUCTION
At the present, chemical conversion coating (iridite) isused on the aluminum faying surfaces as the only protectivefinish that meets the EMP/EMI requirements. This procedureallows moisture to be trapped between the surfaces, resulting insevere corrosion. The nonconductive corrosion products thatdevelop between these surfaces have proven to increase theimpedance high enough to destroy the nuclear hardness of anyweapon system.
An engineering study was performed by GTRI entitledElectromagnetic Pulse (EMP) and Electromagnetic Interference(EMI) Versus Corrosion Control Methods, 31 August 1987. Themajor conclusions from this investigation were:
A. Specially formulated, two-part liquid-applied sealantscontaining metal fillers with adjusted galvanicpotentials can protect "bare" aluminum surfaces after1000 hours of salt spray exposure per ASTM B117.
B. 2.5 milliohms of dc resistance can be maintained usingselected sealants.
C. EMI/EMP shielding effectiveness is related to dcresistance and to the geometry of the structural bond.
This study was successful in that it formed relationshipsbetween shielding effectiveness, d.c. transfer impedanceresistance and bond geometry. Also, it demonstrated thatselected liquid-applied sealants were capable of protectingaluminum surfaces from corrosion while maintaining 2.5 milliohmsof resistance across the bond. It was recommended that furtherresearch be conducted to identify multiple sealant materials andproducts that would protect a range of aircraft and weaponsystems. Also, "real world testing" was encouraged to field testthe sealants on actual aircraft and weapon systems. It wasthought that the completion of these tasks would satisfypotential users of conductive sealants for structures requiringEMP/EMI hardening and corrosion protection. References a. - n.were consulted during the course of this project.
2.0 OBJECTIVES
2.1 Corrosion Prevention Materials and EMP/EMI Requirements
An objective was to evaluate the best corrosion preventionmaterials and processes to be compatible with the EMP/EMIrequirements on aircraft.
2.2 Corrosion Prevention Materials and Nuclear Hardening
An objective was to determine the best corrosion preventionmaterials which will effectively protect against corrosion andprovide nuclear hardness.
2.3 Field Testing
An objective was to determine the effects of a field leveltest performance of these materials on an actual weapon system inservice.
2.4 Specification for Use of Conductive Sealants
An objective was to provide a draft military-typespecification for the use of these materials on aircraft andweapons systems.
2.5 Qualified Products List for Conductive Sealants
An objective was to provide a draft military-typespecification for the qualification of these materials foracquisition by the Air Force.
3.0 RESEARCH PLAN
The following tasks represent the research plan used forcompletion of the above objectives and the research project.
Task 1 - Preparation of Research Plans
Before the research was initiated, detailed plans,schedules, cost estimates, and functional assignments foranalysis and test phases were prepared and submitted to theSponsor. The research plan was submitted to the Sponsor thirtydays after initiation of the contract. The Sponsor submitted toGTRI all comments on the research plan thirty days afterreceiving the plan. After an acceptable plan was agreed upon byboth parties, the research was initiated.
The plan included input from the Sponsor and interested AirForce sources including, but not limited to, the following:
Mr. Sid Childers-MSAMr. Gary Stevenson-MSAMr. John C. Zentner-ASD/ENACEMr. George A. Slenski-MLSA
2
(continued from above)Materials Integrity BranchSystems Support DivisionAir Force Wright Aeronautical LaboratoriesWright Patterson Air Force Base, Ohio
Mr. Calvin Moore-MMETMTelephone (405) 736-5008Headquarters Oklahoma City - Air Logistics CenterMaterials Analysis BranchEngineering DivisionTinker Air Force BaseOklahoma 73145
Task 2 - Preparation of Test Joints and EMI/EMP MeasurementEauipment
The test joints for the dc resistance and shieldingeffectiveness test setups were identical to those used inReference m. In addition, a test fixture was designed andconstructed to evaluate the transfer impedance of the test jointsas in Reference m.
Chemical conversion coatings3 over No. 7075 aluminum were beutilized before sealing since aluminum used for aircraft andweapon systems is usually treated with a chemical conversioncoating.
Disbonds were intentionally designed into some of the jointsto allow corrosive media access to the bond line and sealedconductive joint. Disbonds occur in the actual weapon systems.This procedure did better evaluate and measure the corrosiveeffects on electrical resistance and the extent of corrosionadvancement between the conductive sealant-metal interface.
In the GTRI report, liquid-applied conductive sealants gavesatisfactory corrosion resistance. Therefore, we proposed thecontinued use of these materials over the use of other methods aswire screen in primer coatings, inhibited conductive coatings, orgaskets. However, we did not eliminate these materials from thestudy. The application using liquid applied sealants is mosteconomical and labor-saving.
Task 3 - Evaluation of Polymeric Conductive Sealants
This task consisted of the evaluation of polymeric sealantswhich had the potential of providing high conductivity andcorrosion protection for aluminum surfaces. Conductive sealantsconsisted of a polymeric matrix filled with a conductive media,
3
I
I e.g., silvered aluminum powder. The powder was dispersed withinthe liquid, uncured polymer to provide electrical conductivity inorder to achieve the desired level of shielding effectiveness.Liquid, two-component polymeric matrix was required to maintainshelf life and avoid a moisture cure system which is notapplicable for large joints. Fillers are required which possessgalvanic potentials similar to that of aluminum so as not to
-- produce a corrosive galvanic cell. Surface treatment of aluminumand blending fillers have been successful for this purpose. Amatrix of polymeric materials and fillers with correspondingrmeasured properties were prepared under this task. This taskconsisted of identifying a range of conductive sealant materialsand testing them according to Task 4.
Task 4 - Accelerated Testing of Conductive Sealants
Accelerated testing of the sealed aluminum joints wasperformed per ASTM B117 and ASTM G-85-85 Salt Spray Chamber for aminimum of 2000 hours or until failure. Electrical dc resistanceby the 4-point probe method, transfer impedance and shieldingeffectiveness was measured throughout the exposure time in theSalt Spray Chamber. Resistance, transfer impedance and shieldingeffectiveness versus exposure time was reported.
Task 5 - Plan for Conducting Field Tests
GTRI developed a test plan for conducting field tests onaircraft. The purpose of these tests was to evaluate theeffectiveness in the field of selected conductive sealantmaterials with respect to corrosion and EMP/EMI requirements.First, several bare aluminum faying surfaces on each system wereselected for application of the conductive sealant materials.Next, the resistance of each joint was measured with a doubleKelvin bridge milliohmmeter before application of the sealantmaterial. The conductive sealant material was then liquid-applied on the bare aluminum faying surfaces, cured, and theresistance of the joint was remeasured. The resistance of thejoints was measured again after a one-year period.
Corrosion effects was determined by visual inspection and byfield optical microscope with photographic capability. EMP/EMIeffectiveness evaluation was nearly as straightforward ascorrosion evaluation. DC resistance measurements are the mostpractical measurements which can be made for an actualinstallation (which is the primary reason why resistance is theparameter specified in MIL-B-5087B for evaluation of bondeffectiveness by the contractor). As described above, dcresistance measurements were performed before and afterapplication of the sealant materials and after a two-year period.Complicating the dc resistance measurement was the problem that
4
I
3 the actual resistance of the joint containing the conductivesealant materials was shorted out by either the conductivefasteners used to construct the joint or by a variety of otherparallel paths set up by metallic structures which are in contactwith both faying surfaces.
It should be noted (as described in the GTRI final technicalreportm on Contract No. F09603-85-R-0959) that the shieldingeffectiveness of a joint, and hence the effectiveness againstEMP/EMI sources, cannot be determined simply from a knowledge ofthe dc resistance. A low dc resistance is not sufficient forgood shielding effectiveness. Electrical continuity must beobtained across the entire joint area for effective shieldingover a broad frequency range. This requirement is notnecessarily met by a low dc resistance value nor was it to beverified solely by a dc resistance measurement. Consequently,measurements in addition to the required dc resistancemeasurements were beneficial in terms of evaluating theeffectiveness of the conductive sealants with respect to EMP/EMI
* requirements.
Additional means of characterizing the bonding effectivenessof conductive sealants included shielding effectivenessmeasurements and transfer impedance measurements. Shieldingeffectiveness testing is the most direct method of evaluatingconductive sealants performance with respect to EMP/EMIrequirements. However, in certain instances, shieldingeffectiveness testing may not be feasible. For example, adequateroom may not exist inside of a missile to place the receivingantenna which is required to perform standard (e.g. MIL-STD-285)shielding effectiveness measurements. Another difficultysurrounds the fact that illumination of the system duringshielding effectiveness measurements will expose other shieldflaws which will make isolation of the joint-under-test extremelydifficult or impossible. Consequently, the use of shieldingeffectiveness measurements were carefully assessed in terms ofits feasibility and applicability to bonding effectivenessevaluation of the conductive sealant materials.
An approach which was applicable to the present problem isto measure the transfer impedance of the conductive sealant bondsover a wide frequency range. Transfer impedance measurementsmade over a broad frequency range and at several different pointsalong the length of the joint would provide a better indicationof bond effectiveness than a dc resistance measurement. Relativecomparisons were made between bonds with conductive sealantsapplied versus those without conductive sealants applied.
GTRI proposed to perform the required dc resistancemeasurements and to supplement these measurements with additionalmeasurements which more effectively evaluated the effectivenessof the sealant materials in protecting against EMP/EMI sources.
*5
Selection of the particular joints to be tested includedconsideration as to the testability of the joint from both a dcresistance and an transfer impedance/shielding effectivenessmeasurement perspective. The existence of parallel conductingpaths were identified and documented, and their impact on the dcresistance measurements was assessed.
Task 6 - Visits to Air Force Installations
GTRI personnel visited the Oklahoma Air Logistics Center inorder to conduct field testing on identified aircraft. The firstvisit to this installation was made early in the program. Duringthis first visit, the panels/joints were selected and allnecessary information regarding these panels (such as location,total faying surface area, fastening technique, number offasteners, existence of parallel conducting paths, etc.) wereobtained for test planning purposes. After the test plans werecomplete, a second visit was made to apply the sealant materialsand to make the electrical performance measurements both beforeand after application of the sealants. The third visit wasplanned to be made one year after the second visit to re-evaluatethe performance of the conductive sealant materials. However,the final inspection was not possible for 24 months since theplane was on mission.
Personnel who directly assisted GTRI at OC-ALC were:
Lt. James KihleU.S. Air Force/OC-ALC/MMKRA(405) 736-2748DSN: 336-2748
Frank J. Parker(OC-ALC/MMKRA)(405) 736-2748DSN: 336-2748
Pam Kennedy(OC-ALC/MMKRA)(405) 736-3660DSN: 336-3660The areas that were selected on the E-3A aircraft for
testing of the conductive sealants are listed below and are shownin relationship to the aircraft in Figures 1 and 2.
a. Vertical Tail Fin (see Figures 3 and 4)
Corrosion in the form -•f pitting around the fastening
surfaces of the fin have been regularly observed.
b. Forward Entry Door (see Figure 5)
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Severe pitting adjacent to the elastomeric gasket has beenregularly observed, especially on the bottom sections wherecondensation collects.
c. Aft Entry Door (see Figure 5)
(Same as in b.)
d. Rotodome Air Inlet Duct EMP Shield (see Figure 2)
The vent has been observed to corrode badly and producelarger passages due to failure of the metal.
All of these areas have exhibited documented notoriouscorrosion problems. The bonds in these area all requireelectrical conductivity. GTRI theorized that, if the proposedconductive sealants could solve the corrosion problem, then theelectrical conductivity could be maintained as well asmaintaining the electromagnetic shielding.
Task 7 - Preparation of Sealant Material Specification andQualified Products List.
After Tasks 1-6 were completed and data was accumulated andanalyzed, then a draft, military type performance specificationwas prepared for Air Force use. This was followed by a draft,qualified products lists for these materials. Also, an AMSspecification based on the results of this work was prepared.
4.0 RESULTS AND DISCUSSION
4.1 Evaluation of Laboratory Sealant Test Samples
The dc resistance, shielding effectiveness, and transferimpedance of each test sample were measured. The shieldingeffectiveness and transfer impedance plots are contained inAppendix A and Appendix B, respectively. Three differentmaterials were evaluated: (1) Chomerics 4375-27-4, (2) PRC 1764A-2, and (3) PRC 1764 Class B. Four samples of each materialwere evaluated for a total of twelve different test samples.Iridited aluminum substrates were used in three of the foursamples whereas one of the four samples was assembled with barealuminum substrates. The test samples were coded foridentification purposes as listed in Table 1.
The electrical measurements were made shortly after the testsamples were assembled and the sealants were cured. Next, thesamples were placed in a salt-fog chamber for acceleratedcorrosion testing. The samples were periodically removed from
12
the salt-fog chamber (ASTM B 117), dried, and the surfaces whichcontact the electrical test fixtures were cleaned. The dcresistance measurements were performed at approximately one weekintervals. The transfer impedance and shielding effectivenessmeasurements were repeated at approximately two week intervalswith a total salt fog exposure time of 2000 hours. A briefdescription of the dc resistance, shielding effectiveness, andtransfer impedance measurement techniques are briefly describedbelow.
4.1.1 DC Resistance Measurements
The dc resistance of the test samples was measured with adouble Kelvin bridge milliohmmeter. The double Kelvin bridge(General Electric Mfg.) is a four terminal instrument which iscapable of accurately measuring very small resistance values (inthis case, as low as 0.1 mQ). The terminals of the milliohmmeterwere clipped or contacted directly to the metal plates of thetest joint. All resistance measurements were made after thesealant material had cured so that the resistance value had timeto stabilize.
4.1.2 Shielding Effectiveness Measurements
The shielding effectiveness of each test sample wasdetermined as the difference (in dB) between the electromagneticfield coupled through an aperture with and without the samplepresent over the aperture. (This measurement technique conformsto the basic procedure set forth in MIL-STD-285n.) A blockdiagram of the shielding effectiveness test setup is illustratedin Figure 6. This test setup enabled accurate and repeatableswept frequency measurements to be made on the test joint samplesover the 500 - 1000 MHz frequency range. A transmitting antenna(capacitively-loaded dipole) was placed in the center of ashielding effectiveness test enclosure and a receiving antenna(log conical spiral) was positioned outside of the test enclosureat a distance (5 1/2 feet) which meets the far-field criterionover the entire test frequency range. The log conical spiralreceiving antenna was directed towards a 6 inch square aperturelocated in one wall of the enclosure. The purpose of the testchamber was to isolate the transmitting and receiving antennassuch that the only coupling between them was through theaperture. To minimize unwanted coupling, all permanent seams ofthe test enclosure are welded and finger stock is installedaround the lid of the enclosure and around the periphery of thetest panel port (aperture). The inside of the test enclosure waslined with ferrite absorbing tiles which serve to dampenenclosure resonances and to simulate free-space conditions forthe transmitting antenna.
The test joint was fastened to the test enclosure andelectromagnetically sealed by three rows of finger stock located
13
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I on the enclosure flange area. Conductive copper tape was alsoapplied around the periphery of the test sample as an additionalprecautionary measure to prevent the electromagnetic field fromleaking around, rather than through, the test sample. A 6-dBattenuator was used at the transmitting antenna input port toensure a stable 50-ohm impedance at the power amplifier output.An HP 8510B network analyzer was used to provide the sweptfrequency output source and receiver. The power amplifierprovided approximately 46 dB of RF gain which corresponded to 4watts output power with -10 dBm (-10 decibels relative to 1.0milliwatt) input power. A low noise, high gain amplifier (noisefigure = 2 dB; gain = 35 dB) was used to improve the sensitivityof the spectrum analyzer and thereby increase the overall dynamicrange.
The test enclosure and receiving antenna were located in ashielded anechoic chamber in order to isolate the test antennasfrom the external electromagnetic environment and to minimizeerrors due to ground and/or wall reflections. The isolationcharacteristics of this chamber were 100 dB or greater over thefrequency range of 10 MHz to 1000 MHz and absorption waseffective for RF frequencies greater than 300 MHz. The testenclosure was connected to the outside of the anechoic chambervia a 1/2 inch copper pipe/flange assembly and a coaxial cablewas routed through the copper pipe to connect the power amplifieroutside the enclosure to the attenuator and transmitting antennalocated inside the test enclosure. This cable routingconfiguration was used to prevent RF energy from coupling thecoaxial cable to the receiving antenna and swamping the signalwhich is coupled through the test sample. In this way, a dynamicrange of approximately 100 dB was achieved. (The dynamic rangeof the test setup is the ratio of the signal level coupled to thereceiving antenna with the aperture open to the signal level
I received with a thick metal plate fastened over the aperture.)
4.1.3 Transfer Impedance Measurements
One measure of the RF performance of shielding materials(sealants, EMI gaskets, etc.) is surface transfer impedance.Electromagnetic fields incident on the exterior of a shieldinduce surface currents. When these surface currents encounteran imperfectly sealed aperture or seam, a voltage is inducedacross the seam on the interior of the shield. These interiorvoltages then act as secondary sources and reradiate internal tothe shield thereby degrading the performance of the shield. Thetransfer impedance of the conductive sealant joint may be definedas follows:
15
where:
Zt = transfer impedance (in Q-m);Vo = induced output voltage (in V); ands = surface current density (in A/m).
The transfer impedance of each test sample was measuredusing a transfer impedance test fixture. The drawings for thetest fixture, which has an upper frequency limit of 200 MHz, areprovided in Appendix C. A block diagram of the overall transferimpedance test setup is shown in Figure 7. The transferimpedance was measured over the 100 kHz to 200 MHz frequencyrange with an HP 3577A network analyzer. The RF output of thenetwork analyzer was terminated in a matched 50 ohm loadresistor. The Reference Channel of the network analyzer was usedto measure the input voltage which was then used to calculate thecurrent flowing through the 50 ohm resistor and, thus, the testsample. Channel A of the network analyzer was used to measurethe voltage induced across the test sample. The transferimpedance was then found by dividing the voltage induced acrossthe test sample by the current flowing through the test sample.All mathematical operations were performed by the networkanalyzer. The transfer impedance as a function of frequency wasplotted on a digital plotter. An IBM compatible PC computer wasused to control all test instrumentation.
The transfer impedance data is presented in units of dBO.In order to compare the data with other data measured using testfixtures with different dimensions, one must convert this data tounits of 0-m. Since the perimeter of the test joint wasapproximately 53 cm, it was necessary to subtract 5.5 dB (20 log0.53) from the data presented in the plots to yield thenormalized transfer impedance.
4.1.4 Relationship of Transfer Impedance to ShieldingEffectiveness
Shielding effectiveness is dependent on many variablesincluding the properties of the shield (shield geometry, seamdimensions, seam orientation) as well as the properties of theelectromagnetic wave (impedance, polarization, and incidenceangle). Therefore, the relationship between Z, and SE can beextremely complex for practical shield geometries and fieldconditions. However, as a simple rule of thumb, the followingrelationship was used:
16
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SE(dB) = 22.5 - Z t (dBC'm) (2)
IThe constant (22.5 dBQ-m) in Equation 2 was empirically
derived from the measured data and thus applies to the particulargeometry and field conditions used during our tests. Since theupper frequency limit of the transfer impedance measurements was200 MHz and the lower frequency limit of the shieldingeffectiveness measurements was 500 MHz, the 200 MHz transferimpedance data was extrapolated to 500 MHz using the 200 MHzamplitude and slope. The constant in Equation 2 was thencalculated for each test sample by adding the 500 MHz shieldingeffectiveness value to the extrapolated 500 MHz transferimpedance value. The resultant values were then statisticallyanalyzed to have an average value of 22.5 dBQ m and a standarddeviation of 3.0 dB. As an example, Sample A2 (t = 377 hrs) hada transfer impedance of -52.5 dbQ at 200 MHz and an extrapolatedtransfer impedance of -50 dbQ at 500 MHz. Normalizing thisextrapolated impedance by the perimeter length yielded a transferimpedance of -55.5 dBO-m which, when inserted in Equation 2,yielded a predicted shielding effectiveness at 500 MHz of 78 dB.This predicted value was chose to the measured shieldingeffectiveness value for A2 at 500 MHz of 75 dB.
1 4.1.5 Laboratory Test Results
D. C. Resistance Test Results
A summary of the dc resistance test results is provided inTable 2. The Chomerics 4375-27-4 samples provided reasonablygood performance. The dc resistance values for the iriditedsamples (Al, A2, and A3) were initially in the range of 0.42 -0.67 mg and gradually increased with salt-fog exposure time to1.17 - 2.50 mQ at 2000 hours. The non-iridited sample (A-4)initially had an immeasurably low resistance (< 0.1 mQ), but theresistance increased to 28 mQ after 2000 hours. The PRC 1764 A-2sealant samples provided excellent performance. The dcresistance values for both the iridited samples (Cl, C2, and C3)and the non-iridited sample (C4) were consistently less than 1.5mO through the duration of the testing and were typically lessthan 0.1 mO. The PRC 1764-B sealant samples also providedexcellent performance. The dc resistance values for both theiridited samples (D1, D2, and D3) and the non-iridited sample(D4) were consistently less than or equal to 1.3 mQ through theduration of the testing and were often times less than 0.1 mO.
* 18
I
U
3 Shielding Effectiveness Test Results
A summary of the shielding effectiveness test results isprovided in Table 3. The shielding data was averaged over the500 - 1000 MHz frequency range and this average shielding valuewas used in the summary table. The Chomerics 4375-27-4 samplesprovided reasonably good shielding performance over the 2000 hoursalt-fog test period. The average shielding effectiveness valuesfor the iridited samples (Al, A2, and A3) were initially in therange of 76 to 88 dB and gradually decreased with salt-fogexposure time to 61 - 73 dB at 2000 hours. The non-iriditedsample (A-4) initially had an immeasurably high shieldingeffectiveness value (> 90 dB) but the shielding decreased rapidlywith salt-fog exposure to a value of 55 dB after 2000 hours. ThePRC 1764 A-2 sealant samples provided excellent shieldingperformance. The shielding effectiveness values for both theiridited samples (Cl, C2, and C3) and the non-iridited sample(C4) exceeded the measurement dynamic range (> 90 dB) through theduration of the testing. The PRC 1764-B sealant samples alsoprovided excellent shielding performance. The shieldingeffectiveness values for both the iridited samples (Dl, D2, andD3) and the non-iridited sample (D4) were consistently greaterthan 90 dB for the duration of the test period.
I Transfer Impedance Test Results
A summary of the transfer impedance test results is providedin Table 4. Separate entries are provided for 100 kHz (lowesttest frequency) and 200 MHz (highest test frequency). Note thatthe low frequency (i.e., 100 kHz) transfer impedance correspondsto the dc resistance of the test joint. The Chomerics 4375-27-4samples provided reasonably good performance. The transferimpedance values for the iridited samples (Al, A2, and A3) wereinitially in the range of -65 to -54 dBQ (0.6 - 2.0 mO) andI gradually increased with salt-fog exposure time to -59 to -36 dBQ(1.1 - 15.8 mQ) at 2000 hours. The non-iridited sample (A-4)initially had transfer impedance values of -87 (or less) to -79dBQ (0.04 - 1.1 mQ) but the transfer impedance increased rapidlyto a range from -32 to -22 dBQ (25.1 - 79.4 mQ) after 2000 hours.The PRC 1764 A-2 sealant samples provided excellent performance.The transfer impedance values for both the iridited samples (Cl,C2, and C3) and the non-iridited sample (C4) ranged from -99 to -70 dBQ (0.01 - 0.3 mO) and showed very little degradation inperformance after 2000 hours. The PRC 1764-B sealant samplesalso provided excellent transfer impedance performance. Thetransfer impedance values for both the iridited samples (Dl, D2,and D3) and the non-iridited sample (D4) ranged from -91 to -68dBQ (0.03 - 0.4 mQ) and showed very little degradation inperformance after 2000 hours.
I* 19
I
I
1 4.2 Field Test Measurements
4.2.1 Location of Test Sites
The test sites on the E-3A aircraft are shown in Figure 1-6,and actual photographs of the test sites are attached.I
Figure 8- Front left view of E-3A aircraft showing3 forward and aft entry doors.
Figure 9- View of the bottom section of the forward entrydoor and corrosion areas.
Figure 10- View of forward door, bottom view of sealinggasket and corrosion areas.
I Figure 11- View of open aft entry door.
Figure 12- View of bottom section of the aft entry doorand corrosion areas.
Figure 13- Installation of probes for measurement of dcresistance on bottom section of forward entrydoor.
Figure 14- Forward - right view of the E-3A aircraftshowing vertical tail fin panel.
Figure 15- View of the tail section of the E-3A aircraft,right side, showing vertical tail fin panel andcorrosion area.
Figure 16- View of the tail section, left side, showingaccess panels for measuring transfer impedance.
Figure 17- Preparation of vertical tail fin panel, removalof paint to bare metal, for attachment oftransfer impedance fixture.
3 Figure 18- Installation of transfer impedance fixture.
Figure 19- View of fixture and instruments for measuringtransfer impedance.
Figure 20- Ground view of bottom side of rotodome and entrydoor.
Figure 21- Front view of air inlet duct EMP shield.
3 20
I
3 DC resistance and transfer impedance measurements wereperformed on the E-3A aircraft (Tail No. 77-351). In order toperform the transfer impedance measurements in the field, aspecialized field transfer impedance test fixture was designedand fabricated. A photograph of the field test fixture is shownin Figure 18. In order to validate the functionality of thefield test fixture, transfer impedance measurements on laboratoryU samples were performed with both the laboratory and field testfixtures. Comparative plots of the lab and field test fixturesfor high impedance (RdC = 1 0) and low impedance (R = 5 mQ) testsamples are shown in Figures 22 and 23, respectively Note thatthe close agreement occurs at low frequencies where the"effective" perimeter is essentially the same for both fixtures.However, at high frequencies, the field test fixture essentiallyconcentrates surface current over a smaller region resulting inan increase in impedance of approximately 10 dB (implying areduction in effective perimeter from the lab fixture value of0.53 m to a value of 0.17 m). Overall, the field test fixtureprovides reasonable and accurate transfer impedance data,particularly when the reduction in effective perimeter is takeninto account.
4.2.2 E-3A Field Test Results
The test areas on the E-3A aircraft are listed in Table 5with respect to the type of conductive sealant used. The resultsof a visual inspection using a field microscope are contained inTable 6. Significant corrosion and pitting was observed duringthe initial inspection. All surfaces were lightly abrasivelycleaned with sand paper to remove products of corrosion beforeapplying the conductive sealants. After 24 months, the samesurfaces were reinspected and no indications of corrosion werepresent in all cases.
The dc resistance measurements generally indicate that theconductive sealants maintained conductance and preventedcorrosion. However, there are some measurements which requireexplanation. The aft entry door did not show corrosion after 24months of service in contrast to the initial inspection, but thedc resistance rose to 4.6 milliohms in one measurement. First,the dc resistance measurements are actually a measurement acrossthe metal fasteners where no sealant was used; and measurementacross sealant and fasteners after 24 months. Severe corrosionwas observed initially and none after 24 months. Consideringthese circumstances, GTRI believes that the 1.4-4.6 milliohmmeasurement is an indication of good conductivity and corrosionprevention although 2.5 milliohms and below would be preferred.
Referring to the air vent EMP shield, GTRI cleaned thesurfaces from products of corrosion, applied conductive sealantto these surfaces and remounted the shield which had some severedamage in the center section. Measurements were made and
21
reported as shown in Table 7. After the'measurements were made,the shield was removed, and the corroded section replaced usingnonconductive sealant by maintenance workers at OC-ALC. The 11.3milliohm measurement is explained by the presence ofnonconductive sealant on the surfaces and around the fasteners.No additional corrosion was observed.
The transfer impedance data for the tail panel measurementsare provided in Appendix D. Note that when the tail panel wasassembled with the Chomerics 4375-27-4 sealant, the anodized(nonconductive) finish was inadvertently left on prior toapplication of the sealant. Thus, the only conductivity from thepanel to the aircraft was through the rivet fasteners leavingnonconductive slots which are indicated by the inductiveimpedance (Z increases monotonically with frequency) noted iný11 of the piots. However, very little change in transferimpedance was noted over the 2 year period between measurementsindicating that the conductive sealant provided excellentcorrosion prevention characteristics.
S.0 CONCLUSIONS
The conclusions which may be drawn from this study are basedon the previous laboratory study, Electromagnetic Pulse andElectromagnetic Interference Versus Corrosion Protection Methods(31 August 1987), and the above field report. The above fieldstudy utilized the successful conductive sealant materials forapplication on an E-3 aircraft. Visual inspection, andelectrical measurements including d.c. resistance and transferimpedance, were performed on four corrosion prone areas tomeasure the effectiveness of the sealant materials.
The conclusions follow:
A. Visual corrosion was arrested on all areas tested usingboth conductive sealant materials.
B. Transfer impedance and d.c. resistance measurementsindicated that the corrosion was arrested in all testareas.
C. Corrosion was successfully monitored using d.c.resistance and transfer impedance; a correlationwas made.
D. The 2.5 milliohm d.c. resistance requirement isachievable using conductive sealants and for-up toat least 24 months of field service.
22
I
E. Salt spray (ASTM Bll7) testing on test panels correlateto field results; the accelerated method is a anacceptable method of evaluating materials for this
* application.
F. The use of the proven conductive sealants have thepotential of saving the Air Force significantmaintenance time in replacing parts and costs ofparts/materials.
S 6.0 RECOMMENDATIONS
Based on the successful results of the above report andI those reported in reference m., we recommend the following:
A. Use of the tested sealant materials on corrosion -prone joints and bonds which require the dc 2.5milliohm dc resistance. GTRI feels comfortable inrecommending these materials for extensive use onequipment which will result in significant saving inparts replacement maintenance.
Applicable equipment could include:
1. aircraft
2. missiles
3. electronic/communications equipment
4. ground equipment
B. Pretesting and evaluation of sealant materials using thethe GTRI test joints and methods.
C. GTRI strongly recommends an advanced field study usingconductive sealants extensively on test aircraft toconfirm the cost saving advantages of corrosioninhibiting and conductive polymeric sealant.
i D. A specification (see Appendix E) that is refined anddeveloped for acquisition use; and an American MaterialsSpecification (AMS) developed from this specification.
23
7.0 REFERENCES
a. TO 1-1-1, Cleaning of Aerospace Equipment. (Basic Issue.29 Jun 79, Change 18, 3 Sep 87).
b. TO 1-1-2, Corrosion Prevention and Control by AerospaceEquipment. (Basic Issue, 3 March 83, Change 7, 16 Apr 87).
C. AFSC Design Handbook 1-4, Electromagnetic Compatibility.(January 1987).
d. MIL-STD-454J, Standard General Requirements for ElectronicEquipment, 26 Feb 87.
e. MIL-STD-461C, Electromagnetic Emission and SusceptibilityRequirements for the Control of ElectromagneticInterference, 1 Apr 87.
f. MIL-STD-462, Electromagnetic Interference CharacteristicsMeasurement of: 9 Feb 71.
g. MIL-STD-469, Radar Engineering Design Requirements,Electromagnetic Compatibility: 30 Mar 67.
h. MIL-HDBK-253, Guidance for the Design and Test of SystemsProtected Against the Effects of Electromagnetic Energy,28 Jul 78.
i. MIL-S-5002C, Surface Treatments and Inorganic Coatings forMetal Surfaces of Weapons Systems, 28 Aug 78.
j. MIL-C-5541, Chemical Conversion Coatings on Aluminum Alloys,
14 Apr 81.
k. MIL-STD-889B, Dissimilar Metals, 21 Nov 79.
1. MIL-STD-1568A, Materials and Processes for CorrosionPrevention and Control in Aerospace Weapons Systems, 24Oct 79.
m. "Electromagnetic Pulse and Electromagnetic InterferenceVersus Corrosion Protection Methods," by Georgia TechResearch Institute, Atlanta, GA, 31 Aug 87.
n. MIL-STD-285, "Attenuation Measurements for Enclosures,Electromagnetic Shielding, for Electronic Test Purposes,Method of," 25 Jun 56.
24
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Table 5. Location of Test Areas on E-3 Aircraft andConductive Sealants
Test Area Type of Sealant
Entry Door, Forward Products Research 1761 A/B
Entry Door, Aft go "
Air Vent EMP Shield Chomerics 4375-27-4 A/Bin Rotodome
Vertical Tail Fin
30Ii
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IFigure 8. Front-left view of E-3 aircraft showing forward
* and aft entry doors.
IIIII 33
I
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I IL - :Figure 9. View of forward door, bottom view of sealing
i gasket and corrosion areas.
IIII
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n Figure 10. View of the bottom section of the forward entryI door and corrosion areas.
-- 35
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Figure 11. View of open aft entry door.
IIIII* 36
I
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I _ ,_
I
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i Figure 12. View of bottom section of the aft entry door and
corrosion.i
i 37
I
IIII
III -!,I1Ii -
I Figure 13. Installation of probes for measurement of dc
resistance on bottom section of forward entry* door.
1
* 38
I
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* 39
I
70351
Figure 15. View of the tail section of the E-3A aircraftshowing vertical tail fin panel and corrosionarea.
40
III-U
I
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I ;
Figure 16. View of the tail section, left side, showing the---
access panel for measuring transfer impedance.i
3 41
IIIIIIIIIIIIIII Figure 17. Preparation of vertical tail fin panel, removal
of paint to bare metal, for attachment of* transfer impedance fixture.
I* 42
I
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1
IFigure 18. Installation of transfer impedance fixture.
* 43
IIIIII,Iit • -I' )"mI /
I2
m Figure 19. View of fixture and instruments for measuring
transfer impedance.
I 44
i
i
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I Figure 20. Ground view of bottom side of rotodome and entrydoor.
I 45
I I• i l n im i H L R l l ain lg H I
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i Figure 21. Front view of air inlet duct EM? shield.
Iwi
i
I
I(a) Lab Test Fixture ResultsI
REF LEVEL /OIV MARKER 100 600. 80BHz02.610. Ia . G66dB MAG (UOF) - 1. 02Sdm
260. 66UDB 13. 66dB MARKER 100 300.300HzS10 Ohm MAG (03) D. 433dB
"rITflrMtl3i ill ihm
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10 mOhm
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looK IM PB loo•mSTART 160 O66. WON1= STOP 23691 000 00. 13.0-HzI
3 (b) Field Test Fixture Results
REF LEVEL /1IV MARKER 190 332. 920s-
20. 000dm 10. 000dB -IAG (UOF) 0. 222d420. 000dB 10. 080dB MARKER 106 000.806Hz
10 Ohm MA (03) 0. 433id
I Ohm -
100 mOhm lit
10 Ohm -
1 mOshm -
0. 1 m~hm __ -- MSTART 130 900. 09013 STOP 236 000 00•. 003H=
IFigure 22. Comparison of Measured Z, Using Lab and Field Test
Fixtures (R,= 1 Ohm)
l47
(a) Lab Test Fixture Results
RE LEVEL /DIV MARKER 188 888. 888Hz23. 28ed .AG U ts -43. 015J82...8.dB 18 88ldBe MMARKER 1iSO 3 3800 8Hs
10 Ohm MAG (03) 8. 4334dB
*1 Ohm
100 mOhm-
1.0 mOhm- I80Kl IM I BMii i i i il
START 188 388. 838HU STOP 200 W93 MO8.388Hz
I(b) Field Test Fixture Results
REF LEVEL /DIVI%.3Beds 13. il is
10 Ohm 2.3d 0 3d
U~ lOhm
3 ~100 mOhm
I OK i lii l oomii
3 START 102 613. 8•0H= STOP 280 8•8 988.838Hz
IFigure 23. Comparison of Measured Z, Using Lab and Field Test
Fixtures (Rd= 5 milliohm)
I48I
IIIIIIII
I APPENDIX A
i SHIELDING EFFECTIVENESS DATA
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III
I APPENDIX C
I DRAWINGS FOR TRANSFER IMPEDANCE TEST FIXTURE
IIII
I cl
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camw aqx -woco wr YN akkvwm braMO bwo 6W wo
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T am"caftTransfer Impedance Test FixtureV~_ _ _ _ __ _ _ _ _
Liectrnics and Computer Systems LabElectromagnetic Compatibility Division
KRS 1/31/89 1 Rev. 2
* C2
Coaxial Connector Mounting Bracket (2 Rea'd)
'I i,, I +
-m
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WE I k krw I f I I %) M
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Aluminum Threaded Stop Collar
Bottom Viamewm 4 a I ) Os
SNote~~~~As: emblyce Drawngtsr ingbloos
IRK Transfer Impedance Test Fixture
V.,lme u=-.m m~
Iecacs a Computer Systems Lab
to.
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Notes: Referenced drawing $'s a's in balloons. ]ElcU'omagnedc Compatibility DivisionRefer to Dwg. #5 for detailed assemblgy •>rm,•.,omlfl'tr~UCtoflS. XRPS /2$ 3.-Rcv.
I C4
114 In. FIrture Flange
0 0 -0
31 N. bob
2 1 1 iI
208Wgh. 6L mm0 oi
2R YSmle im•~ e•'4-3I*I
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am to Ow M it for p =A be &A "W W1~M6 ~ __________________________ Ift I" .1ift * ~ a Of akVVM I ift 0d
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Section "p-p-
Transfer Impedance Test Fixture
1/ gIn. Tog P1"
, • •• .. .. . .. .... ... ect 'omagneu~c Compatibility Division
I " R] 2/7/S89 4-Rev. I
2 C5
o
General Notes and Assembly Steps
IGeneral Notes:
1. The inside and outside diameter dimensions shown for the brans pipe are suggested only. If standard siz-ws are available, the on. that most closely matches the dimensions shown. I a size with dimensions oth-
er than those shown is selected, it is necessary to modify the other parts which mate to the brass pipesuch that they mate correctly with it. Specifically, the hole cut kilo the 114" thick top aluminum plate mustbe modified to reflect the now O.D. dimension of the pipe. (The hole should not be cut to the 0.0. dimen-sion. Rather, the hole should be cut to facilitate the tapping to mate with the threads of the pipe). Also.the aluminum threaded stop collar hole I.D. dimension must be modified (in the same fashion as the 1/4'plate hole). Finally, the large I.D. of the wood spacer must be modified to match the O.D. of the brass pipe(this part has no thread). Note that the minimum I.D. of the brass pipe must be 7/87.
2. Both the top and flange 1/4" aluminum plates can be cut from the same sheet (because the u.rtout of theflange is precisely the same size as the top plate). If it is convenient to cut the flange piece in two, andthen weld it back together to accomplish the cut out, then this is acceptable.
3. The threading of the stop collar and the hole in the 1/4' top plate should not be done until the stop collar iswelded to the top plate. This is necessary because these threads must match up so that the brass pipecan be screwed into the resulting threaded hole.
4. Do not glue brass pipe, wood spacer, and brass contact plates together. The wiring shown on drawing #1must be installed first. This will be done by EMCD personnel
Assembly Steps:
1. Fabricate the aluminum 1/4" top plates (2 req'd) and 1/4" flanges (2 req'd). Cut flange pieces into two piec-es if necessary, and if done, weld back together (insuring that mating surfaces are flush). Drill flangeholes, and drill brass pipe hole in top plates. Do not tap pipe holes yet.
2. Fabricate the aluminum stop collars, as shown on the drawings, but do not tap larger inside diameter yet.
3. Weld stop collars to top plates, centering brass pipe holes.
4. Thread top plate and stop collars simultaneously. See general notes above for important notes on thisstep.
5. Fabricate side walls from 1/8 thick aluminum plate. Wold sides to jr as shown on drawing. Note thatonce welding is complete, will have two side pieces (one for the top shell and one for the bottom shell).
6. Fabricate coaxial connector brackets from 1/8" thick aluminum plate.
7. Weld flange pieces to sWe wafn pieces, weld 1/4' thick top plates to side walls, and weld coaxial connectorbrackes to top 1/4- plates.
8. Cut to length four brass pipes, thread Io match top Viaplate/stop collar threaded holes. Fabricate four wooden Transfer Impedance Test Fixturespacers and brass contact plates. Do not glue these OMpieces together. (Wiring must be connected first. This Elewonics and Computer Systems Labwill be done by EMCO personnel.) Ekxctomagneuic Compauibi Sy Division
I2/7/8
C6
W ood S12a er T readed B rass Pipe
(4 r~g'dl 4 vad. allI different tengihs)
74 h
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Transfer Impedance Test Fixtureor#""_ _ _ _ _ _ _ _
Elctonics and Comnputer Systcms LabElto1T3gfCLic Cornpatbility Division
Seto G G RS 2/7/89 6
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IIIIIII
I
IAPPENDIX E
I SPECIFICATION FOR CONDUCTIVE SEALANTS
i
I
II
I El
I
I
3 Aerospace Materials Specification
SEALANT, ELECTRICALLY CONDUCTIVEAND CORROSION-INHIBITING
I 1.0 SCOPE
1.1 General
This specification covers the requirements for liquid appliedelectrically conductive two-part sealants for bonding barealuminum or chemical conversion coated aluminum surfaces. Theconductive sealant is effective over a continuous operationtemperature of -55 0 C (-67*F to 248 *F).
E 1.2 Classification
The two-part sealant shall be particle (electricallyconductive) filled elastomers including, but not limited to,3 polysulfide and polyurethane.
I 2.0 APPLICABLE DOCUMENTS
2.1 Government Documents
3 U.S. Government publications are available from: CommandingOffice, Naval Publications and Forms Center, 5801 Table Avenue,Philadelphia, PA 19120.
2.1.1 Military Specifications, Standards and Handbooks: Thefollowing documents listed with the Department of Defense andI other agencies shall apply.
MIL-STD-285 Attenuation Measurements for Enclosures,Electromagnetic Shielding, for ElectronicTest Purposes, Method of
MIL-STD-794 Parts and Equipment, Procedures Packaging3 and Packing of
MIL-C-5541 Chemical Conversion Coating on Aluminum3 Alloy
MIL-S-8802 Sealing Compound, Temperature Resistant,Integral Fuel Cell Cavities, High Adhesion
MIL-C-38736 Compound Solvent, For Use in Integral FuelTanks
I MIL-C-83528 Gasketing Materials, Conductive, ShieldingGasket, Electronic, Elastomer, EMI/RFII
I
1 2.1.2 Federal Documents::
QQ-A-250/12 Aluminum Alloy 7075, Plate and Sheet
QQ-A-250/13 Aluminum Alclad Alloy 7075, Place and Sheet
CCC-C-419 Cloth, Duck, Cotton, Unbleached, PliedYarns, Army and Numbered
TT-M-261 Methyl ethyl Ketone, Technical
TT-1735 Isopropyl Alcohol
L-P-378 Film Properties
ASTM D 2196 Rheological Properties of Non-NewtonianI Fluids
ASTM D 429 Rubber Property - Adhesion to RigidSubstrates, Method A - Rubber Part Assembled
* Between Two Parallel Metal Plates
2.2 American Standard Testing Methods:
I ASTM publications are available from American Society forTesting Materials. 1916 Race street, Philadelphia, PA 19103.
ASTM B117 Salt Spray (Fog) Testing
ASTM D412 Tension Testing of Vulcanized Rubber
ASTM D991 Test Methods for Rubber Property - VolumeResistivity of Electricaly Conductive andAntistatic Products.
ASTM D1084 Viscosity of Adhesives
ASTM D2137 Rubber Property - Brittleness Point ofFlexible Polymers and Coated Fabrics
ASTM D2240 Indention Hardness of Rubber and Plastics byMeans of a Durometer, Test for
ASTM D3182 Rubber - Materials, Equipment and Plasticsby Means of a Durometer, Test for
2.3 SAE Publications
SAE publications are available from: SAE, 400 CommonwealthDrive, Warrendale, PA 15096
2.3.1 Aerospace Material Specifications:
AMS 235L - Standard Test Methods
AMS 2825 - Material Safety Data Sheets
2.4 Other Documents:
Final Reports of Georgia Tech Research Institute, No. a-4324, and Contract No. F02060-85-66007, WR-ALC, Robins AirForce Base, 31 August 1987.
Manufacturer's publications for application of sealantsand material safety data sheets, available from individualsupplier of materials.
3.0 TECHNICAL REQUIREMENTS
3.1 Sealant: The sealant shall consist of two parts, a basecompound and a separate curing agent which, when mixed inproper proportion will cure at room temperature to a rubberysolid.
3.1.1 Base Compound: The base compound shall be an uncatalyzedpolysulfide (polythioether) or polyurethane.
3.1.2 Curing Agent: The curing agent shall be of sufficientlydifferent color from the base compound to easily identify anincompletely mixed system.
3.1.3 Surface Conditioner: Optionally, a surface conditioner
may be included as part of the material package.
3.2 Physical Properties
3.2.1 Unmixed Sealant:
3.2.1.1 Storage Life: The base compound and the curing agent,stored in closed containers at no higher than 25 0 C, when mixedin proper proportions at any time up to three months from thedate of manufacture, shall meet all requirements of thisspecification.
3.2.2 Mixed, Uncured Sealant:
3.2.2.1 Application Time: One-half hour after mixing,application time shall be 15 grams/minute minimum in accordancewith paragraph 4.2.1. The sealant can be either extruded froma tube or troweled from a container.
3.2.2.2 Viscosity: The vendor shall supply a plot of viscosityversus time at 23 °C 5.0 minutes after mixing the two componentsto form the sealant mixture. This viscosity measurement shallbe performed with a Brookfield Viscometer or equivalent usinga No. 1 spindle below 50 RPM. Any of the various models ofBrookfield Viscometers will suffice for this measurement. This
curve will represent the usable period of time before thesealant is too viscous to be workable. The period ofworkability after mixing shall be 1.0 hour minimum at 23*C.(73.4*F). The viscosity 1.0 hour after mixing shall not exceed60,000 centipoise.
3.2.3 Mixed Cured Sealant: The product shall conform to thefollowing requirements:: Tests shall be performed on specimenscut from air-free slabs prepared as in paragraph 4.4.6 andtested in accordance with specified test methods at 25 *C +1-1°C and 45-55% relative humidity.
Requirement Method
3.2.3.1 Tensile strength, dry 50 psi ASTM D 412Method A,Die C
3.2.3.2 Elongation, Min. 50 ASTM D 412Method A,Die C
3.2.3.3 Adhesion 40 psi ASTM D 429Method A
3.2.3.3 Tack Free Time, hrs. 10 para.4.5.5
3.2.3.5 Hardness, 40 ASTM DDurometer A Minimum 2240
3.2.3.6 Thermal Properties:Melting Temperature None para.
4.5.6.1
Decomposition 250 0 C (482.0) para.Temperature, (50% 4.5.6.2
* weight loss)
Brittleness Temperature -55 0 C (-67'F) para.4.5.6.3
3.2.3.7 D.C. Resistance, max. 2.5 milliohm para.after an hour cure and 4.5.1after 2000 hrs. in salt
3.2.3.8 Volume Resistivity, 0.07 ASTM D 991ohm-cm spray per ASTM B Para.117 4.5.7
3.2.3.9 Salt Spray Chamber, no blisters ASTM B 1172000 hrs.
3.2.3.10 Percent Nonvolatile, 95 para.min. (potential 4.5.4shrinkage
4.0 QUALITY ASSURANCE PROVISIONS
4.1 Responsibility for Inspection
The vendor shall have the responsibility for initially testingthe lots of base compound and curing agent before shipping, andthe purchasers shall have the option to re-test whenappropriate.
4.2 Classification for Tests
4.2.1 Acceptance Tests. Tests to determine conformance to thefollowing requirements are classified as acceptance tests andshall be performed on each lot:
Requirement Reference
Hardness 3.2.3.5Application Time 3.2.2.1Tack Free Time 3.2.3.4viscosity 4.5.3D.C. Resistance, 24 Hours 3.2.7
4.2.2 Preproduction Tests.: Tests to determine conformance toall technical requirements of this specification are classifiedas preproduction tests and shall be performed prior to or on theinitial shipment of material to a purchaser; when a change inmaterial, processing, or both require reapproval; and whenpurchaser deems confirmatory testing to be required.
4.2.2.1 For direct U.S. Military procurement, substantiatingtest data and, when requested, preproduction test materialshall be submitted to the cognizant agency as directed by thehprocuring activity, the contracting officer, or the request forprocurement.
4.3 Sampling
4. 3.1 For Acceptance Tests: Sufficient material shall be takenat ramdom from each lot to perform all required tests. Thenumber of determinations for each requirement shall be asspecified in the applicable test procedure.
4.3.1.1 A lot shall be all material from the same batch of rawmaterials processed in one continuous run and presented forvendor's smaller quantities under the basic lot approvalprovided lot identification is maintained.
4.3.2 For Preproduction Tests: As agreed upon by purchaser andvendor.
4.4 Approval:
I
4.4.1 Sample Sealant: Sample sealant material shall beapproved by purchaser after material for production use hasbeen manufactured, so that the material being tested is thesame material to be supplied to the purchaser.
4.4.2 Changes: Vendor shall use ingredients, manufacturingprocedures, processes, and methods of inspection on productionmaterials which are identical to those used in the approvedsample material. If any changes are proposed in theingredients, type of equipment for processing, manufacturikngprocedures, or inspection methods, the vendor shall submit forreapproval a statement of the proposed changes. If requestedby the purchaser, samples of the materials shall be submittedfor approval. No material shall be shipped without writtenapproval by the purchaser.
-- 4.5 Test Methods
4.5.1 D.C. Resistance Measurements and Test Joint Design. Theobjective of this test is to measure the resistance underconditions similar to actual field installations. A chemicalconvection coated, aluminum (70075-T6, MIL-C-5541 Class 3) testjoint shall be constructed for the purpose of assuring that theperformance of the sealant compiy with the requirements of this
_ specification. The total area of the faying surfaces of thetest joint shall not exceed 18 square inches. A suitable test
* joint is shown in Figure 1.
The test joint shall allow the sealant to be applied to one orboth faying surfaces. The d.c. resistance of the sealantmaterial shall be measured after fastening the aluminum platestogether with threaded fasteners torques at standard force (13inch-pounds). The fasteners shall be electrically insulatedsuch that conductivity is achieved solely through the sealantmaterial. All seams shall be directly exposed and visible tothe naked eye. Resistance shall be measured with a milliohmmeter (Kelvin Bridge type) capable of measuring resistancevalues less than 2.5 milliohms with a minimum accuracy andresolution of +/- 0.1 milliohm. The instrument probes shall becontacted directly to the base aluminum plates.
4.5.2 Adhesion of Aluminum Substrates:
Adhesion shall be measured in accordance with ASTM D 429,Rubber Property - Adhesion to Rigid Substrates, Method A -Rubber Part Assembled Between Two Parallel Metal Plates. Thismethod is used to determine adhesion of rubber between twometal plates. The metal plate shall be chemical conversioncoated (MIL-C-5541, Class 3) aluminum alloy (7075-T6).
4.5.3 Viscosity. Viscosity measurements shall be performedwith a Brookfield RVF Viscometer (ASTM D 1084, Method B), at23 0 C +/- 0.5 0 C. The spindle number and RPM shall be chosen shothat very high viscosities can be measured (refer to ASTM-D-
I0
I2196, Rheological Properties ot Non-Newtonion Fluids). The
viscosity measurements shall be taken at 5.0 minute intervals,and a plot of Viscosity versus Time shall be generated to showthe workable time of the sealant mixture at 25 0 C beforegelation or unworkability.
4.5.4 Percent Nonvolatile Content. A 3-5 gram sample ofcompound shall be transferred to a cup approximately 70 mm indiameter and 3 mm deep. The sample shall be distributed evenlyover the bottom surface of the cup. A fitted cup shall beplaced immediately over the cup and weighed to the nearestmilligram. The weight of the cover and cup shall have beendetermined after drying in an oven at 105 0 C for at least onehour. The sample shall have been cured in the cup for the 3.0hours at 20 0 C to 30 0 C prior in the cup. The cup and sampleminus the cover shall be heated in an oven at 70°C for 24 hoursto expel volatiles, and weighed again to determine the weightloss due to volatile. The % nonvolatile content shall becalculated as follows:
I Final WeightNonvolatile Content, % F x 100
Initial weight
4.5.5 Tack Free Time: An aluminum (7975-T6) test panelconforming to QQ-A-250 shall be cleaned and covered withfreshly mixed sealing compound to a depth of 0.125 inches +/-0.002 in. The sealing material shall be allowed to cure at 250 C(77 0 F) at 50% relative humidity. After 10 hours, two I inch by6 inch pieces of clean polyethylene film conforming to L-P-378,0.0004 +/- .0002 inches sample shall be applied to the sealingmaterial and held in place with a pressure of one-half ounce persquare inch for 2 minutes. The strips shall be slowly andevenly withdrawn at right angles to the sealant surface. Thepolyethylene shall come away clean and free of sealantmaterial.
4.5.6 Thermal Stability.
4.5.6.1 Melting Temperature: The melting temperature shall bemeasured using a differential scanning calorimeter (DSC).Sealant is fully cured or crosslinked. No endothermic meltingpeaks shall be generated on a DSC thermogram.
4.5.6.2 Decomposition Temoerature: The temperature of thermaldecomposition shall be mcasured using a thermogravimetricanalyzer (TGA). The TGA thermogram generated from thistechnique shall show weight loss versus temperature in anatmosphere of nitrogen. The temperature of decomposition shallbe that at which the weight loss is 50% or greater at a scan
I speed of 10°C/minute.
4.5.6.3 Brittleness Temperature: The temperature at physicalbrittleness or glass transition temperature shall be measured
I
I
with a differential scaning caorimeter (DSC). A transition inthe generated DSC thermogram shall indicate the glasstransition temperature.
4.5.7 Volume Resistivity: The dc volume resistivity of thematerial shall be measured in accordance with ASTM D991. Thedc volume resistivity shall be calculated per ASTM D991 asfollows:
RA
where: p = DC volume resistivity in ohm-cm,R = measured resistance in ohms,A = smallest cross-sectional area of sample between
probe electrodes in cm2 , andL = distance between probe electrodes in cm.
4.6 Preparation of Cured Test Sppecimens: Test specimens ofthe mixed, cured compound shall be cut from slabs nominally 6in. x 6 in. and 0.075 in. +/- 0.008 in. thick, prepared from amixture of base compound and curing agent and cured for not lessthan 72 hrs. at 25 0 C +/- 1C and 45-55% relative humidity.
5.0 PREPARATION FOR DELIVERY
I 5.1 Identification
Each container shall be identified with not less thanspecification number, vendor's name and compound lot, date ofexpiration, method of storage and net quantity.
5.2 Commercial Packaging
5.2.1 Base Compound: Base compound shall be supplied in 1pound (454 grams), 10 pounds (25 kLlograms) or 450 pounds (200kilograms) containers, as ordered. Other types of packing mustbe approved by the purchaser in writing.
5.2.2 Curing Agent or Catalyst: Catalyst shall be supplied incontainers corresponding to the weight of the base compoundI (5.2.1).
5.3 Shipping
5.3.1 Shipping Sealant Components: The materials in 5.2.1 and5.2.2 shall be prepared for shipping in accordance withcommercial practice and in compliance with applicable rules andregulations pertaining to the handling, packaging andtransportation of the materials to ensure carrier acceptanceand safe delivery. Packaging shall conform to carrier rulesand regulations applicable to the mode of transportation.
I
I5.3.2 Shipping for Military Procurement: For direct U.S.Military procurement, packaging shall be in accordance withMIL-STD-794, Level A or Level C, as specified in the request forprocurement. Commercial packaging as in 5.2 shall beacceptable if it meets the requirements of MIL-STD-794, Level
I.5.3 Storage Before/After Shipping
A suitable facility for storing the materials shall be providedincluding a dry enclosed area out of direct sunlight. Thestorage space shall be environmentally controlled. Thetemperature of storage shall be 20 0 C to 25 0 C and the relativehumidity 50%RH to 70%RH.
6.0 DOCUMENTATION
I 6.1 Documentation of Lot Testing
6.1.1 Lot Testing: The vendor shall supply to the purchaserdocumentation forms (refer to 6.4) containing all lot testingresults. With each shipment, the vendor shall furnish a reportverifying conformance to the acceptance test requirements andstating that the compound conforms to the other technicalrequirements and this specification. This report shall includethe purchase order, specification number, formula number, lotnumber, quantity, and all applicable test results.
6.1.2 Material Safety Data Sheets: A material safety datasheet conforming to AMS 2825 or equivalent shall be supplied toeach purchaser prior to, or concurrent with, the report ofpreproduction test resutls. Each request for modification ofcompound or catalyst formulation shall be accompanied by a
* revised data sheet for the proposed formulation.
6.1.3 Parts: The vendor of finished or semi-finished partsshall furnish with each shipment a report showing the purchaseorder number, specification number, contractor or other directsupplier of sealant, supplier's sealant number, part number,and quantity. When the sealant for making parts is produced orpurchased by the parts vendor, that vendor shall inspect eachlot of sealant materials to determine conformance to therequirements of this specification, and will include in thereport either a statement that the sealant materials conform orcopies of laboratory test results showing conformance to thespecification.
6.2 Documentation of Shipping
The vendor shall supply to the purchaser documentation onapaproved forms (refer to 6.4) to verify date of shipping andmode of transportation.
6.3 Documentation of Storage
I
I
I The vendor will supply documentation on inspection forms (referto 6.4) to substantiate the duration method and facilities usedfor storage prior to shipping. The maximum shelf storageperiod prior to shipping shall not exceed three weeks.
6.4 Inspection Forms
Inspection forms shall be submitted to the purchaser so as tosubstantiate the quality of and test results on the materials.
6.5 Rejections
Materials not conforming to this specification or tomodifications authorized by purchaser shall be subject torejection.
ii
IIi
IIiIiI
III
I Cutqway Sectionr-. Dielectric Dielectric
Wash~er sleeve
iNut .rew
6/32 6/32ss 316 SS/31EI
3 8.0 in. 5.25 in. 6.75 in.
II
I8.0 in. Sealant
3 Note 1: All holes are 1/4" diameter and spaced at 3" intervals
Note 2: Unless otherwise noted, all materials are 7075-T6aluminum with MIL-C-5541 class chemical conversioncoating
III3 Figure i. Test Joint for DC Resistance Measurements
I
IlII!
I
IIl3 APPENDIX F
I QUALIFIED PRODUCTS LIST
IIIII
I
i
IQPL-10 October 1990i
I QUALIFIED PRODUCTS LIST
OF
PRODUCTS QUALIFED UNDER MILITARY SPECIFICATION
SEALANT, ELECTRICALLY CONDUCTIVE,CORROSION-INHIBITING
This list has been prepared for use by or for the Government inthe acquisition of products covered by the subject specificationand such listing of a product is not intended to, and does notconnote endorsement of the product by the Department of Defense.All products listed herein have been qualified under therequirements for the product as specified in the latest effectiveissue of the applicable specification. This list is subject tochange without notice: revision or amendment of this list willbe issued as necessary. The listing of a product does notrelease the supplier from compliance with the specification3 requirements.
GOVERNMENT MANUFACTURER'S TEST OR QUALIFI- MANUFACTURER'SDESIGNATION DESIGNATION CATION REFERENCE NAME AND ADDRESS
Chomerics, Inc.I 4375-29-4Woburn,
MA
PRl764A-2 ProductsResearchCorporation
5 PR1764Class B
i/1/1
F2
III1IIIII APPENDIX G
I SEALANT MANUFACTURER'S PRODUCT INFORMATION
IiI!
!IIIIIfGI
rAXI 40'1-F94-h I9Y
I June h. 1989
Dr. Jan GoochGTR 1IAt 1.2 nta, CA
5 Dear Dr. Goocii
Chornerics 2-part s;I vrr-PI.3tc'-n~u:0;num ureth.anc nealanc ytem o~icrs
the folowing cypical propcrcioS.s
Prodctic Numbcor 437j-'27-4 \/ji
ITEST RE~lI.Thrý (7 'mj Rc.1'. (:iirt!1
Volui..e Resistivity 0.03 ohm-cm
USlump 02 nh!
L.ap She3r (on Alurninum? 135 psi
rcci1 Scrength 2.3 ppi
5 tHardness 91. Sbore A
The stabilized copper %syc~n d~ill havo sim'ilIiar phynic..1 prop'erties. *h
ei~cctritcal propcrzies sbouud be h~ccer. Volume vesisttvit', is typirnally5
O.01 Ohm-cm.
R Lte ards,
- ~.. h~rsolhart ..
5 sc:pam
1 0489
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i"ealaxtt adhejivej
S ___. LABORATORY PRODUCT REPORT
PR-1764 CLASS A
DESCRIPTION.PR-1764 Class A is a two-part, metal filled, conductive sealant, based on PermapolOP-3, a mercaptan terminated polythioether polymer coveid by U.S. Patent 4,366,307.
5 PR-1764 Class A it resistant to jet fuel, water and oil. A soluble chromate has
been added for corrosion inhibitive properties.
3 APPLICATION PROPERTIES (Typical)
ColorPart A BlackPart B GrayMixed Black
Viscosity of Part B, poise 100(Brookfield, spindle #6 @ 10 RPM),
Application Time (mixet; viscosity), poise 1000after I hour from time f -nix
Tack Free Time, hrs 10
PERFORX1,AN PROPERTIES (Typical)
Specific Gravity 2.10
3 NVM, % 84
Cure Hardness after 24 hrs, @ CTR, rex 40
3 Tensile Strength, psiDry 757 days JP-4 immersion @ 140*F 60
Elongation, 'Dry 507 days JP-4 immersion @ 140°F 40
IMsf PRODUCTS RESEARCH A CHEMICAL CORPORATION DI S ___
5454 SAN FERNANDO ROAD 410-416 JERSEY AVENUE - ___
POST OFFICE BOX IBM GLOUCESTER CITY. NEW JERSEY 0 030OCT I - 1987 GLENDALE. CALIFORNIA 91209 AREA CODE (,9; 45W-5700 2
AFAEA CODE (618) 240-2060
SoG7
11
3protective Ceatio9
calaiyg c•mpomad,
uR deaeawjd adhedivej
I .__LABORATORY PRODUCT REPORT
PR-1764 CLASS A
I DESCRIPTIONPR-1764 Class A is a two-part, metal filled, conductive sealant, based on Per-napolO
P-3, a mercaptan terminated polythioether polymer covered by U.S. Patent 4,266,307.
PR-1764 Class A is resistant to jet fuel, water and oil. A soluble chromate hasbeen added for corrosion inhibitive properties.
APPLICATION PROPERTIES (Typical)
ColorPart A BlackPart B GrayMixed Black
Viscosity of Part B, poise 100I (Brookfield, spindle #6 @ 10 RPM),
Application Time (mixed viscosity), poise 1000after 1 hour from time of mix
Tack Free Time, hrs 10
I PERFORMANCE PROPERTIES (Typical)
Specific Gravity 2.10
SNVM, 1 84
Cure Hardness after 24 hrs, @ CTR, rex 40
I Tensile Strength, psiDry 75
7 days JP-4 immersion @ 1400F 60
I Elongation, %Dry so
7 days JP-4 immersion @ 140*F 40
I PRODUCTS RESEARCH & CHEMICAL CORPORATIONIUINU:ImJD . .64S4 SAN FERINANDO ROAD _-. 410-416 JE.RSIEY AVENUE __ IAI ,,,f.
-C15187POST OFFICE BOX 1800 GLOUCESTER CITY. NEW JERSEY 06030ICT 1 5 1987 GLENDALE. CALIFORNIA 91209 AREA CODE (109) 456-5700 .I".. 2
3 4G8
LABORATORY PRODUCT REPORT -2- PR-1764 CLASS A
Corrosion Test No evidence of corrosionSalt Spray Method, 840 hrs and no significant cnange
in conductivity
Insulation Resistance, ohms 0.01Applied Voltage, 100 MV0C
NOTE: Specimen of O.O4x1x3 inches in size shall be prepared from aluminum alloyconforming to temper T6 of QQ-A-250/13. App-ly a 1/8 inch coat. Overlay theIsealant with another panel making one square inch lap-shear test specirmen.
Bulk Resistivity, ohm-cm 0.075 Using micro-oh-mneter
Sheet Resistivity, ohm/square 0.48
EMI/RFI Shielding Effectiveness d5 60Farfield @ 1000 mH~ASTM ES 7-83
Substrate primed with PR-148 Primer
NOTE: The application and performance property values are typical for the mater'.al,but are not intended for use in specifications or for acceptance inspectioncriteria because of variations in testing methods, conditions, and
3 MIXIiýG INSTRUCTIONS
PR-17f.4 Class A is supplied in a two-part kit. Mix in the ratio of 10 parts A to100 parts B by weight. Mix Part A and Part B separately to uniformity thenthoroughly mix both parts together taking care to avoid leaving unmixed areas around
sides or bottom of mixing container.
3 ~HEALTH PRECAUT IONiSThe uncured PR-1764 Class A may produce irritation following contact with skin.When handling PR-1764 Class A, avoid ingestion and all contact with the bodyespecially open breaks in the skin. Always wash hands before eating or smoking.Obtain medical attention in case of extreme exposure or ingestion. For additionalinformation, see a Material Safety Data Sheet.
*PERMAPOL* is a tradename of Products Research A Chemical Corporation, registeredI with the U.S. Patent Office.
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