Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: IMPROVED PROCESS FOR REMOVING COATINGS
FROM SENSITIVE SUBSTRATES, AND BLASTING
MEDIA USEFUL THEREIN
FI~T~n OF T~T~ INV~NTION
This invention relates to a process for
removing coatings from sensitive metal and composite
surfaces or like substrates, and to blasting media
useful therein.
BACKGROUND OF THT~ INVT~NTION
It is often desirable to clean or remove
coatings from the surfaces of various types of
structures and equipment, varying from buildings to
industrial devices. Numerous techniques are known
for such purposes, ranging from mechanical abrasion
techniques to the application of chemicals for
cleaning or removing surface coatings such as paint,
sealants, lacquers or the like. Hard, durable
surfaces, such as granite walls or heavy steel
plating may be cleaned or stripped by vigorous
abrasive techniques such as sand blasting. More
delicate surfaces may require less aggressive
treatments to prevent damage to the substrates.
Both commercial airlines and military
agencies spend large sums in periodically stripping
or abrading paint and other coatings from the
exterior surfaces of modern aircraft. These
surfaces comprise light weight aluminum or other
metal alloys, or composites, which are relatively
soft and from which paint or other coatings must be
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carefully removed to avoid excessive abrasion or
chemical damage. Such damage may, in extreme cases,
lead to mechanical failure.
Various improved stripping techniques,
similar to sand blasting, have been proposed for
removing coatings from sensitive metal and composite
aircraft or like surfaces. Blasting media useful
for such purposes should, preferably, meet the
following criteria:
1. They should be relatively
non-aggressive (Mohs hardness of about
2.0-3.0);
2. They should be available in various
particle size distributions for the
treatment of different substrates;
3. They should be free-flowing under high
humidity conditions and throughout a
broad range of air pressure and media
flow rates; and
4. They should be water soluble and
non-polluting to facilitate easy
separation from the insoluble paints
and resins stripped to facilitate
waste disposal.
Carr U.S. Patent No. 4,731,125 granted
March 15, 1988 describes the use of plastic media
for the blast cleaning of sensitive metal and
composite surfaces. Such materials are however,
relatively expensive, and their use may impose waste
disposal problems.
Sodium bicarbonate has also been proposed
as a blasting medium for removing coatings from
sensitive substrates such as aircraft parts.
Bicarbonate is an ideal choice for such a medium
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since it readily meets criteria 1, 2 and 4 above.
Thus, it is relatively non-aggressive (Mohs hardness
of about 2.5), is available in a variety of particle
sizes, and is both water soluble and commonly
utilized to treat sewage treatment facilities for
the control of alkalinity and pH. The mild abrasive
characteristics of sodium bicarbonate have
previously been utilized, for example, in polishing
media for teeth. See, for example, U.S. Patents
Nos. 3,882,638; 3,972,123; 4,174,571; 4,412,402;
4,214,871; 4,462,803; 4,482,322; 4,487,582;
4,492,575; 4,494,932, and 4,522,597.
The principal disadvantage attendant to the
use of sodium bicarbonate as a blasting medium is
its tendency to cake either by compaction or, more
importantly, by exposure to high humidity
conditions. This is particularly acute in
commercial blasting operations, the compressed air
streams for which are substantially saturated with
moisture, i.e., have 90% or higher relative
humidities, and contain oily contaminants from air
compressors. In addition, commercially available
sodium bicarbonate products have intrinsically poor
flow characteristics due to their normal particle
size distributions and crystal shapes.
The addition of flow aids to sodium
bicarbonate to improve its flow and anti-caking
properties is known. Thus, the blending of
tricalcium phosphate (TCP) with sodium bicarbonate
in baking formulas and dental air jet prophylaxis
media has previously been proposed. The addition of
such material substantially improves the flow and
anti-caking characteristics of the bicarbonate.
TCP-treated sodium bicarbonate is however,
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restricted to a 3 to 6 month shelf life under
ambient conditions, since the TCP absorbs moisture
until saturated, after which the product cakes.
It is, accordingly, among the ob~ects of
the present invention to provide
bicarbonate-containing blasting media, and a process
utilizing such media for removing coatings from
sensitive metal and composite surfaces. Bicarbonate
blasting media which may be so utilized are free
flowing and have long storage lives under adverse
commercial blasting conditions, and may be utilized
as blasting media at high humidities and under a
broad range of finely controlled, high flow rates
and air pressures. Other objects and advantages of
the invention will be apparent from the following
description of preferred forms thereof.
RY OF T~ INVl:N1ION
In accordance with this invention, a
process for removing coatings from sensitive
substrates is provided, comprising blasting such
surfaces with a high velocity fluid stream,
desirably a substantially saturated compressed air
stream under a pressure of about 10-150 psi,
containing as a blasting medium crystalline
water-soluble bicarbonate particles having average
particle sizes within the range of about 100-500,
preferably about 250-300, microns, desirably in
admixture with at least about 0.2%, preferably about
0.2-3%, of a hydrophobic silica flow/anti-caking
agent, by weight of the bicarbonate.
The relatively large particle size
bicarbonate has been found quite effective in
removing corrosion by-products as well as other
coatings from substrates, without damage to the
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substrate. This contravenes the conventional wisdom
that small particles of a blasting medium, which
have more available surface area, are more effective
cleaning agents than larger particles. Moreover, in
most prior blasting applications higher feed rates
of the blasting material lead to higher production
rates, i.e., faster removal of the coating.
However, employing the crystalline bicarbonate
medium within the indicated particle size range it
is possible to achieve economical production rates
at feed rates of about three to four pounds per
minute using conventional sand blasting equipment
sized to effectively use a 0.5 inch diameter
nozzle. Above these rates, very little increase in
production rate is obtained even employing markedly
higher bicarbonate blasting medium feed rates.
In accordance with a further feature of the
invention, the addition of hydrophobic silica to the
blasting medium imparts significantly better flow
characteristics than comparable media which are
either free of flow aids or which contain other
conventional flow aids such as TCP or hydrophilic
silica. In addition, bicarbonate blasting media
incorporating the hydrophobic silica flow aid have
significantly longer, substantially indefinite shelf
lives, and exhibit superior resistance to the high
relative humidities of commercial compressed air
streams.
It is preferred to utilize crystalline
sodium bicarbonate as the abrasive material in the
blasting media of this invention. It is, however,
intended that other crystalline water-soluble
bicarbonates, e.g., alkali metal bicarbonates such
as potassium bicarbonate, or ammonium bicarbonate
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may similarly be employed. Accordingly, while the
following description principally refers to the
preferred crystalline sodium bicarbonate-containing
blasting media, it will be understood that the
invention embraces blasting media incorporating
other water-soluble crystalline bicarbonate
abrasives as well.
Hydrophobic silica has previously been
utilized in admixture with hydrophilic silica as a
flow aid in polishing media for dental prophyla~is.
Such media are applied under conditions which differ
dramatically from commercial blasting media. Thus,
dental prophylaxis media contain bicarbonate
particles having particle sizes of about 65-70
microns, and are applied at rates of about 3 grams
per minute through 1/16-1/32 inch nozzles under the
pressure of clean, laboratory compressed air
supplies under pressures of about 50-100 psi. Most
important, such air supplies are not substantially
saturated with moisture, and present quite different
flow and caking problems from those inherent in the
application of commercial blasting media at high
thruputs in saturated compressed air blast streams.
The blasting media of the invention consist
essentially of the crystalline water-soluble
bicarbonate, e.g., sodium bicarbonate, in admi~ture
with hydrophobic silica particles. Hydrophobic
silica, unlike known hydrophilic silicas, is
substantially free of non-hydrogen bonded silanol
groups and absorbed water.
One preferred hydrophobic silica which may
be utilized in the blasting media hereof is Aerosil
R 972, a product which is available from Degussa
AG. This material is a pure coagulated silicon
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dioxide aerosol, in which about 75% of the silanol
groups on the surface thereof are chemically reacted
with dimethyldichlorosilane, the resulting product
having about 0.7 mmol of chemically combined methyl
groups per lOOm of surface area and containing
about 1% carbon. Its particles vary in diameter
between about 10-40 nanometers and have a specific
surface area of about 110 m2/gram. It may be
prepared by flame hydrolysis of a hydrophilic silica
as more fully described in Angew. Chem. 72, 744
(1960); F-PS 1,368,765; and DT-AS 1,163,784.
Further details respecting such material are
contained in the technical bulletin entitled "Basic
Characteristics and Applications of AEROSIL",
Degussa AG, August 1986.
The hydrophobic silica particles are
admixed with the crystalline sodium bicarbonate
blasting agent in the proportion of at least about
0.2%, and up to about 3% by weight thereof. The
crystalline bicarbonate particles may be of any
desired average particle size, within the range of
from about 100 to 500 microns. Preferably, when the
blasting medium is utilized for the removal of paint
from exterior aircraft surfaces, it is preferred to
utilize crystalline bicarbonate particles having
average particle sizes of about 250-300 microns.
Blasting media thus constituted are useful
for cleaning or de-coating sensitive metals, e.g.,
aluminum or aluminum alloys, or composite
substrates, such as utilized on exterior aircraft
surfaces, without abrading or otherwise damaging the
substrates. Composites which can be treated with
the blasting media hereof comprise matrices, e.g.,
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epoxy resins, which may contain fibers such as glass
strands, graphite or the like for reinforcement.
The blasting medium thus constituted is
preferably applied in commercial compressed air
streams, i.e., streams which are substantially
saturated with moisture (90% or higher relative
humidities) and contain oil contaminants from
compressors. A stream of water may be added to the
media/air stream to cool the workpiece and control
dust formation, although the addition of water may
reduce the coating removal rate somewhat. The
bicarbonate/hydrophobic silica blasting medium may
be applied at flow rates of about 1-10, desirably
about three to four, pounds per minute and under air
pressures of about 10-150 psi, from 1/4 inch or
larger blasting nozzles.
As indicated above, and as more fully
documented below, in accordance with the present
invention it has been found that blasting media so
constituted and employed do not cake, have e~cellent
shelf lives, and are free-flowing. They may thus be
readily employed in commercial blasting operations
for removing coatings from sensitive metal and
composite surfaces.
RRI~ D~SCRIPTION OF T~ DRAWINGS
Figure 1 is an electron scanning
photomicrograph of a clad aluminum test panel which
has visible corrosion at the surface, penetrating
the cladding, prior to blasting;
Figure 2 is an electron scanning
photomicrograph of a clad aluminum test panel which
has been cleaned according to the process of the
invention. It is apparent that the surface
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corrosion by-product materials have been removed
without damaging the metal surface; and
Figure 3 is a graph comparing production
rates obtained employing bicarbonate blasting media
having different average particle sizes, at varying
feed rates.
DETAIT~n D~SCRIPTION OF TH~ PP~RR~n ~MRODIMENTS
The following examples illustrate the
free-flowing characteristics of (Example 1), and the
high production rates obtained by (Examples 2 and
3), the blasting media of the present invention. In
the examples, all parts and percentages are given by
weight and all temperatures in F unless otherwise
indicated.
F~XAMPT.P~ 1
Two crystalline sodium bicarbonate
samples, each having an average particle
size of about 250-300 microns, and one in
admixture with 0.5% Aerosil R-972
hydrophobic silica, were used as media in a
standard sand blasting apparatus. The
apparatus was a Schmidt Accustrip System,
manufactured by Schmidt Manufacturing, Inc.
of Houston, Te~as, having a 0.5 inch nozzle
diameter.and a Thompson valve connected to
a 6 cubic foot blast pot and an 800 cfm
compressor. The blast pot was mounted on a
scale so that media flow rates could be
determined.
The respective bicarbonate
formulations were blasted through the
nozzle at 60 psig pressure utilizing
ambient compressed air which was saturated
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with moisture as it passed through the
blasting media in the blast pot.
The crystalline sodium bicarbonate
particles absent the hydrophobic silica
flow aid flowed intermittently through the
system, rapidly clogging the nozzle and
preventing further flow. Sustained flow
could not be maintained.
The crystalline
bicarbonate-hydrophobic silica formulation
flowed continuously through the system for
more than 65 hours at controllable rates
varying from 1 pound per minute to 5 pounds
per minute. No flow problems were
encountered therewith.
~XAMPT.~ 2
Test panels of aluminum, two feet by two
feet by 0.032 inch thick, were cleaned by treating
with water, washing with phosphoric acid solution,
rinsing with,water, applying Alodyne corrosion
inhibitor, and rinsing with a final water rinse. An
epoxide primer was applied and the panels were dried
in air for eight hours. A polyurethane paint was
then applied and the panels were dried for seven
days. The panels were then aged for 96 hours in an
air oven at 210F. The target thickness for the
paint and primer was 2 mils.
Various sodium bicarbonate blasting media
were loaded into a blast pot fitted with an
appropriate orifice plate to regulate the rate of
flow of the blast medium. The blasting media
consisted of 99.5% sodium bicarbonate (in differing
particle sizes) and 0.5% Aerosil E972 hydrophobic
silica. The nozzle pressure was set at 60 psi and
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the media were placed under differential pressure
sufficient to give the desired feed rate. The water
pressure was set at a pressure of 200 psi, yielding
a flow of 0.5 gal/min. The angle blast nozzle made
with the workpiece was set at 60. The media feed
rates were varied from two to four lbs/min.
The time to completely depaint the panels
were recorded.
The production rates and average particle
sizes of the bicarbonates in the respective media
were as follows:
Average
BlActin~ Medium Abr~cive Particle Size Production Rate
micronsft.~/min
Crystalline NaHC03 275 2.2
Compacted NaHC03 390 1.0
Particle Size. microns ~ 99 150 ~Q
Production Rates (Depaintin~ Times)
Feed Rate 2 lb./min .75 1.3 1.4 1.6
Feed Rate 3 lb./min 1.0 1.7 1.8 2.0
Feed Rate 4 lb./min 1.2 1.9 Z.0 2.2
This data is summarized in Figure 3,
wherein curve 1 is 2 lbs/min; curve 2 is 3 lb/min;
and curve 3 is 4 lb/min.
The data shows that media incorporating the
larger sodium bicarbonate average particle sizes
(150 and 250 microns) were more effective at all
feed rates than media containing smaller particles.
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It is also apparent that economical
production is achieved at a medium flow rate of
about three pounds per minute and that higher feed
rates produce little improvement in production rate.
~AMPT.F~ 3
The blasting medium described in Example l,
incorporating a crystalline sodium bicarbonate
having an average particle size of about 250-300
microns, in admi~ture with 0.5~ Aerosil R-972
hydrophobic silica, was compared with a blasting
medium incorporating a compacted bicarbonate, having
particles in the range of -20 mesh to +40 mesh, in
admixture with a like amount (0.5%) of the same
hydrophobic silica flow aid. The respective media
were utilized to depaint aluminum test panels in the
manner described in Example 2. The maximum
production rates obtained with the respective media
were determined, and are tabulated below. The
tabulation shows the improved production rate
achieved using the blasting medium incorporating the
crystalline sodium bicarbonate rather than the
medium incorporating the compacted sodium
bicarbonate:
Average
Blasting Medium AbrAcive Particle Size Production Rate
microns ft. min
Crystalline NaHC03 275 2.2
Compacted NaHC03 390 1.0
From the preceding disclosure, it will be
seen that the present invention provides an improved
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process for cleaning or removing paint or other
coatings from the surfaces of sensitive substrates,
and to crystalline sodium bicarbonate-containing
blasting media useful therein. It will be
understood that various changes may be made in the
blasting process and blasting media exemplified in
the preferred embodiments described hereinabove
without departing from the scope of the invention.
Accordingly, the preceding description should be
construed as illustrative and not in a limiting
sense.
