Development of Biodegradable Blasting Medium for Aircraft Paint Removal
Client: U.S. Coast Guard
Project Size: $180,000
Duration: 2 years
The objective of this project was to design and develop a process for
dry blast paint removal that employs a biodegradable blast medium in
aircraft maintenance. The process of depainting an average-sized plane
generates about 10,000 lbs. of solid waste, 95% of which is the blast
medium and 5% of which is paint. Polyacrylate and wheat starch are the
most widely used media. The polyacrylate-based waste is converted into
countertops, picnic benches and other plastic materials. Leaching of
toxic metals from the paint component of the waste has caused some
concern of future adverse consequences. Wheat starch, while readily
biodegradable, is extremely hygroscopic and unless stored and used in
a dry atmosphere becomes ineffective as a blast material. Thus, the
design criteria for this new process were that it employ a biodegradable
material that was not hygroscopic and was of sufficient hardness to serve
as blast material, but that was not so hard as to damage the skin of the
aircraft. Importantly, the process for disposal of the blast waste had to
separate the toxic portion of the paint from the blast medium, generate a
usable product, and be cost-competitive with current practices.
A survey of several biodegradable polymeric materials for the
suitable characteristics led to the selection of polylactic acid (PLA).
This material had the mechanical properties needed and was commercially
available in cost-competitive quantities. Bench-scale analysis showed the
material was able to remove paint using conventional blasting equipment.
Working closely with the manufacturer, it was possible to modify the PLA to
increase its hardness. Using 12? square painted coupons as substrate, several
formulations, sizes and shapes of media were tested. The most suitable was 16
mesh, football-shaped particles. In side-by-side tests under U.S. Navy
specifications however, the PLA material did not perform as well as
polyacrylate: the dwell time for paint removal was 5 times longer and was
greater than MIL SPEC requirements.
In a parallel line of work, microbial consortia and selected
strains obtained from natural sources and repositories were screened for
the ability to degrade PLA as well as the paint. Several candidate organisms
were identified. A cost analysis based on the degradation rate suggested that
a biological approach to waste reduction would not be cost effective. However,
a chemical degradation approach was discovered to be very effective. Two schemes
were developed and tested. In the first scheme, sodium hydroxide and ethanol
were used at elevated temperatures and pressures to convert the PLA into a
mixture of ethyl lactate and sodium lactate. Ethyl lactate is a biodegradable
solvent used as a replacement for methylene chloride in semi-conductor cleaning.
Conditions were optimized to permit complete conversion of the PLA while leaving
the paint unaltered. Filtration followed by distillation permitted separation
of reaction products. In a second scheme, ammonium hydroxide was employed to
hydrolyze the PLA. This process yielded lactamide and lactic acid. Again, the
products could be separated by filtration and distillation. A cost analysis
and scalability study showed that both processes were feasible on a commercial
scale using existing technology and practices. The cost to produce the PLA blast
medium was $0.50 per lb, with a market value of $2.50/lb. The ethyl lactate had
a materials and process cost of $1.00 per lb with a market value of $5.00/lb.
The paint residue could be disposed of by ashing and disposal in a hazardous
waste landfill at the cost of $0.25/lb.