With advances in coating
technolo- gies, more and more product finishers are concerned with how to remove
coatings. Electrocoating and electrostatic spray coating applications require
absolute electrical contact between products and their hooks, hangers, racks,
carriers and other conveyor line fixtures. To ensure electrical contact, residual
coating materials must be stripped from these fixtures. With the increasing costs
of raw materials, rejected manufactured parts can no longer be scrapped because
of poor finish quality. They must be stripped. Stripping also must precede maintenance
refinishing for some products. For example, old coating is removed from aircraft
before repainting because it would add weight that reduces payloads.
Basically all coating stripping
methods fall into one of three categories: chemical; pyrolytic; and mechanical.
Chemical stripping methods
use corrosives, solvents and combinations of corrosives and solvents.
Pyrolytic methods use high-temperature
ovens, open flames, hot fluidized beds, molten salt baths and lasers.
Mechanical stripping uses
high-pressure water, abrasive media, brushes, abraders, scrapers, chippers and
cryogenics.
Materials of construction
for conveyors have not changed but coating materials and substrates have. To strip
these new coatings, more aggressive materials and methods have been developed.
Since many chemical, pyrolytic and mechanical stripping methods are too harsh
for aluminum, plastic and composite substrates, stripping methods have been modified.
There is a stripping method
for every combination of coating and substrate. These methods vary in cost, processing
speed and worker safety. Space does not permit a detailed description of each
stripping method. Instead, a brief sketch of each method along with its advantages
and disadvantages will be presented to aid the reader in his selection of the
method that meets his needs.
Chemical
Stripping
Chemical
strippers work by softening or dissolving the film and breaking the bond between
the coating and substrate. The loosened coating is then removed mechanically.
Chemical strippers can be classified by their operating temperature, hot or cold.
They can also be classified by their compositions as corrosives, either acidic
or alkaline, as solvents or as combinations of corrosives and solvents. However,
corrosive strippers are used hot, at elevated temperatures of 80-100C. On the
other hand, solvent strippers are used cold, at or near room temperature. It makes
more sense to use the chemical classifications because of worker and community
“right to know” regulations.
In the past, it was claimed
that chemical strippers could remove an alkyd enamel in seconds. Today, in addition
to the alkyds, products are finished using acrylics, epoxies, polyesters, polyurethanes,
vinyls and other highly durable coatings. Due to their chemical resistance, they
are difficult to remove. Furthermore, governmental regulations, which now control
worker safety, air and water quality, are affecting the composition and use of
chemical strippers. Consequently, there are now scores of chemical strippers commercially
available. The advantage of chemical strippers is formulating versatility to remove
even the most durable coating with little effect on metallic substrates. The disadvantages
are the potential health hazards and environmental problems associated with using
corrosive materials, chlorinated solvents and flammable solvents.
Corrosive strippers
are generally aqueous solutions of potassium or sodium hydroxide that operate
at a pH of 13 or higher. Acidic strippers, on the other hand, are aqueous solutions
of organic or mineral acids that operate at a pH of 2 or lower. They are applied
by immersion, flowing or steam gun for periods of 15-30 min.
Solvent strippers
can contain methylene chloride, cresols and flammable hydrocarbons. Used at room
temperature, they are applied by immersion, brushing or flowing. Since solvent
strippers are used cold, at or near room temperature, they are slower acting than
the hot corrosive chemicals, and stripping may take several hours.
Combination strippers
are formulated using corrosive materials and solvents, enjoying the benefits of
both. They can remove the most durable, chemical-resistant coatings. Used at or
near room temperature and below the boiling point of the solvents, they are nearly
as slow acting as the solvent strippers.
Pyrolytic
Stripping
Pyrolytic
stripping equipment includes open flames, high-temperature ovens, fluidized
beds and molten salt baths. At the 700-800F operating temperatures, most organic
coatings are pyrolyzed in a relatively short time. The advantage is fast and complete
stripping. The disadvantages are high energy use and damage to some substrates.
Open-flame strippers
are used on a limited basis because of environmental and safety considerations.
High-temperature oven
strippers pyrolyze organic portions of the coatings in an oven heated to 700-800F
in a low-oxygen atmosphere. In the first phrase, the resultant volatiles are driven
off, leaving carbon and inorganic compounds. In the second phase, the carbon remaining
on the substrate is burned at higher temperatures in excess oxygen to form carbon
dioxide. Afterwards, the inert pigments and fillers remaining on the substrate
are removed mechanically. To comply with air-quality standards, an afterburner
in the oven exhaust burns the remaining organic volatiles to form carbon dioxide
and water. Heat exchangers, in more efficient systems, recycle the energy.
Hot fluidized-bed strippers
use the pyrolytic process in fluidized beds heated to 700-800F. The suspended
media is hot sand. The fluidizing gas can be oxygen-poor or oxygen-rich, depending
on the process stage. The closed system has dust collectors, afterburners and
heat exchangers. Organic volatiles are converted to carbon dioxide and water by
the afterburners to meet air quality standards. Heat exchangers increase energy
efficiency.
Molten-salt-bath strippers
use baths of proprietary molten, oxidizing, inorganic salts heated to temperatures
of 600-1,000F. Coated objects are immersed in the bath for 5-25 min, depending
on the salt formulation and the coating composition. In a closed system, the exhaust
is treated to meet air quality standards. Some proprietary molten salt baths have
been developed that oper-ate at lower temperatures, allowing cer-tain aluminum
products to be stripped.
Laser stripping
is a high-tech method that uses the energy of a laser beam to pyrolyze organic
coating. The beam is moved automatically along the substrate, decomposing the
coating as it goes. This procedure is slow and works best on flat substrates.
Mechanical
Stripping
Mechanical
stripping includes some old and widely practiced methods, such as abrading,
scraping and chipping by hand and power tools and blast cleaning by impingement
of a medium to abrade coatings from the substrate. It also includes high-pressure
water and cryogenic stripping. Mechanical stripping often supplements other methods
to completely remove loosened coating residues.
Hand abrading, scraping
and chipping are still practiced. Tools for this method are abrasive pads,
sandpaper, wire brushes, scrapers and chipping hammers. Electrically or air-operated
power stripping tools include disc, orbital and belt sanders, chipping hammers,
rotary wire brushes and abrasive flap wheels. The advantage is fast stripping.
The disadvantage is the labor required.
Abrasive blast stripping
uses various types of media, which are propelled centrifugally by compressed air
or low-pressure water. Centrifugal blast stripping uses motor driven wheels to
hurl the media onto the coated surface. In abrasive air blast stripping, the media
is propelled by a stream of compressed air. Both processes are done in enclosures.
Dust collectors and cyclones are used to recover and separate the media from coating
residue for further use. In abrasive water blast stripping, media is carried in
a stream of low-pressure water. One advantage of abrasive blast stripping is rapid
removal of coatings from most substrates. Another advantage is meeting environmental
and safety regulations. The disadvantages are distortion and attrition of substrates
by the impingement of the media. Today, blast stripping media includes sand, shot,
plastics, ice crystals and carbon dioxide pellets.
Sand and shot abrasive
media have traditionally been used for blast stripping. Since they are the hardest,
they are also the most aggressive. The advantage of a high stripping rate may
be overshadowed by high substrate distortion and attrition rates.
Plastic media stripping
was developed for the special requirements of aircraft repainting because of the
size of the product, the effects of chemical strippers on non-metallic substrates
and the environment. The plastic media, which is harder than the coating to be
removed, is propelled by a stream of compressed air at pressures from 15-45 psi.
The choices of media hardness, particle size, composition, nozzle shape, angle
of attack and air pressure are dictated by the coating type. It is even possible
to remove coatings one layer at a time. The advantage of plastic media stripping
is less damage to the substrate. The disadvantage is slower stripping.
Ice crystal blast
stripping uses refrigeration equipment and air compressors. The ice crystals are
carried in an air stream that is directed at the coating by a special nozzle.
The impingement of the ice crystals fractures the coating film, which is then
lifted off the substrate. The advantage of no media residue (only water) is balanced
by the requirement for elaborate equipment.
Carbon dioxide (dry
ice) blast stripping uses a compressor to produce dry ice pellets that are
propelled by a stream of compressed air. The advantage is no media residue because
dry ice sublimates. The disadvantages are the need for elaborate equipment and
a slower stripping rate than with other media.
Other blast media
that can be used for stripping coatings include wheat starch, sodium bicarbonate,
glass beads, nut shells, corn husks, fruit seeds and a host of others. The choice
of medium is dictated by the type of coating to be stripped and the durability
of the substrate.
High-pressure water
stripping removes coatings by impinging a stream of water on a surface at
pressures of 15,000-50,000 psi using specially designed nozzles. By changing the
parameters of water pressure, angle of attack, nozzle design and dwell time, even
the most durable coatings can be removed.
Cryogenic stripping
uses a bath of liquid nitrogen operating at temperatures ranging from -200 to
-300F. Objects to be stripped are immersed in the bath for 30 sec to 3 min. Owing
to the differences between the coefficients of linear expansion of organic coatings
and metallic substrates, the coating cracks and delaminates as it cools. The loosened
coating film is removed mechanically. The system is housed in a relatively small
enclosure. The advantages of a pollution-free, worker-safe process are balanced
by the disadvantage of part size limitation.
There is no “best”
method for stripping organic coatings. All the aforementioned methods will work.
The choice can only be made after careful consideration of the objects to be stripped,
their size, the substrate, the nature of the coating, operating costs, environmental
impact, worker safety and cost. PFD
