The demonstration program was broken into several phases in order to
adequately address the previously mentioned objectives. Results from these tests
indicate the following:
Phase A: Equipment/Process Functionality
The final step in assembly and installation of the alcohol spray cleaning
system was a checkout of the subsystems for proper operation. Each of the
subsystems, described above, were individually checked and reverified at the
system level. All subsystems were found to be properly working although the
filter subsystem is still undergoing final tests.
Phase B: System Cleaning Capability
Precleaning of the test samples in the aqueous cleaner removed the largest
bulk of the molecular contaminants and final cleaning in the alcohol spray
removed the remainder. In several cases cleanliness levels of MIL-STD-1246 100A
were achieved after aqueous cleaning and prior to alcohol cleaning. Cleanliness
levels of 25A to 50A were not uncommon after alcohol cleaning.
The only problem encountered was in the removal of perfluorinated polyether
lubricants (Bray 601 in particular). Cleaning of hardware with this contaminant
required additional soak/rinse periods and mechanical agitation along with
brushing for removal of the lubricant. CFC 113 has been readily used in the past
for removal of perfluorinated polyether lubricants.
During this testing phase, water pickup/concentration in the alcohol bath was
also assessed. Most hardware is processed directly from the water rinse bath
area to the alcohol spray area (after GN2 dry) and some moisture may still be
present on the hardware. This moisture is then transferred to the alcohol bath.
Testing indicated the water pickup by the alcohol bath was minimal and the water
content of the alcohol bath did not exceed the maximum water content requirement
after the week-long test program (approximately 24 hours of operation during
that period). This is also a good indication that the GN2 purge was effective in
moisture removal from the hardware surfaces being cleaned.
The conclusion drawn from this phase of testing was that cleanliness levels
which had been met using the previous CFC 113 process are attainable and can
often be improved using the aqueous cleaner/IPA cleaning process.
Phase C: Environment Issues
A volatile organic compound (VOC) recovery/treatment
capability is currently being investigated for the cleaning system. The
specific technology has not been selected, but several have been assessed;
namely, carbon absorption, membrane, condensation, and combustion. Air flow
rates, alcohol vapor concentration (in air), and safety requirements will be
factored into the decision in addition to cost and technology maturity. Early
results indicate surprisingly low alcohol loss rates and very low
concentrations of alcohol vapor contents (less than 0.1%). Actual spray operations with simulated hardware components (to
simulate a typical cleaning process) and continuous spraying resulted in alcohol
losses of approximately one liter per hour (0.9 quarts per hour). The operating
conditions were air flow: 150 cfm, temperature: 24ºC (75ºF), and alcohol
pressure: 50 psi at a flow rate of approximately 3.8 liters per minute (1 gallon
per minute).
Conclusions
An aqueous precleaner and alcohol spray cleaning system was implemented to
replace CFC 113 and/or 1,1,1 trichloroethane vapor degrease and CFC 113 spray
rinse final clean. Review of numerous chlorofluorocarbon alternative materials
and equipment led to this selection in order to meet the system requirements
mentioned herein (cleanliness safety, and environmental). The demonstration
program verified that this cleaning system meets all requirements of cleanliness
(MIL-STD-1246 level 100A minimum, cleaner/hardware compatibility, and capability
for all desired hardware configurations), safety (personnel and equipment; i.e.
flammability and VOC), and environmental ( VOC release and spent alcohol use)
requirements. The environmental requirements being met are those specific to the
metropolitan Denver, Colorado area but could be refined for use of the system
elsewhere. All data gathered indicates isopropyl alcohol can successfully be
used to replace CFC 113 for precision cleaning aerospace hardware. Use of
alcohol spray systems for precision cleaning in other industries is also
feasible. Derivatives of the cleaning system for these other industries and with
other cleaning materials may also prove successful. Use of alcohol for cleaning
requires incorporation of special design features to address the inherent
hazards while using a potentially hazardous material.
