U.S. patent application number 15/714416 was filed with the patent office on 2018-01-25 for method for repairing an airfoil surface having an elastomeric protective coating.
This patent application is currently assigned to Hontek Corporation. The applicant listed for this patent is Hontek Corporation. Invention is credited to Shek C. Hong.
Application Number | 20180021898 15/714416 |
Document ID | / |
Family ID | 38218488 |
Filed Date | 2018-01-25 |
United States Patent
Application |
20180021898 |
Kind Code |
A1 |
Hong; Shek C. |
January 25, 2018 |
METHOD FOR REPAIRING AN AIRFOIL SURFACE HAVING AN ELASTOMERIC
PROTECTIVE COATING
Abstract
This invention relates to the repair and removal of erosion or
impact damage using hand sandable elastomeric coatings on a curved
substrate, particularly such surfaces as the leading edge of the
airfoil. Specialized applicators and methods of use are also
disclosed.
Inventors: |
Hong; Shek C.; (Glastonbury,
CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hontek Corporation |
South Windsor |
CA |
US |
|
|
Assignee: |
Hontek Corporation
South Windsor
CT
|
Family ID: |
38218488 |
Appl. No.: |
15/714416 |
Filed: |
September 25, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13308788 |
Dec 1, 2011 |
9770791 |
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15714416 |
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11640050 |
Dec 14, 2006 |
8091227 |
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13308788 |
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60750536 |
Dec 14, 2005 |
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Current U.S.
Class: |
428/213 ;
118/200; 118/255; 29/889.1; 427/8; 428/212 |
Current CPC
Class: |
B05D 5/06 20130101; F05D
2300/611 20130101; F05D 2230/90 20130101; F05D 2230/30 20130101;
Y02T 50/60 20130101; Y10T 29/49746 20150115; B05D 1/42 20130101;
B23P 6/007 20130101; Y02T 50/673 20130101; B64F 5/40 20170101; Y10T
428/24942 20150115; B05D 7/58 20130101; F05D 2230/10 20130101; Y10T
29/49318 20150115; B05D 7/56 20130101; B05D 5/005 20130101; F01D
5/005 20130101; F05D 2300/43 20130101; Y10T 428/2495 20150115; Y02T
50/672 20130101 |
International
Class: |
B23P 6/00 20060101
B23P006/00; F01D 5/00 20060101 F01D005/00 |
Claims
1. A method of repairing an airfoil surface having an elastomeric
protective coating adhered to a portion of said airfoil surface
having a plurality of damage cavities caused by sand and water
erosion or impact damage comprising: filling said plurality of
damage cavities in said elastomeric protective coating with a
liquid repair material of elastomeric polyurethane, polyurea or
fluoroelastomer having viscosity between a brushable coating up to
the viscosity of a flowable caulking compound sufficient to fill
said plurality of damage cavities to form filled cavities using a
flexible planar applicator capable of conforming to said airfoil
surface while being drawn lengthwise along the airfoil surface.
2. The method of claim 1 wherein the contact between the airfoil
surface and said flexible planar applicator is maintained by
applying pressure in the direction of the airfoil surface to fill
the damage cavities with the liquid repair material.
3. The method of claim 1 further comprising applying a primer coat
in the plurality of damage cavities.
4. The method of claim 1 wherein said elastomeric polyurethane,
polyurea and fluorolastomer composition having a minimum tensile
strength of 1000 psi, an elongation at break of at least 100%, and
a Shore A hardness of less than 95 A.
5. The method of claim 1 wherein said flexible planar applicator is
a flat sheet of elastomeric or thermoplastic material of selected
thickness to allow it to be deformable around an arcuate surface
and stiff enough to spread viscous material smoothly ahead of it
when drawn through the viscous material.
6. The method of claim 1 further comprising a preliminary step of
applying one or more layers of primer coat over any airfoil surface
exposed within said damage cavities using a fine brush capable of
controlling the primer coat deposition to about 1.0 mm to about 3.0
mm in diameter.
7. The method of claim 1 further comprising a preliminary step of
applying one or more coats of said elastomeric polyurethane,
polyurea or fluoroelastomer to damage cavities using a fine brush
capable of controlling the deposition within a range of about 1.0
mm to about 3.0 mm in diameter prior to said filling step.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This patent application is a divisional of and claims
priority to U.S. patent application Ser. No. 13/308,788, filed on
Dec. 1, 2011, which is a divisional of and claims priority under 35
U.S.C. .sctn. 120 of U.S. patent application Ser. No. 11/640,050,
filed on Dec. 14, 2006, issued as U.S. Pat. No. 8,091,227 on Jan.
10, 2012, which claimed priority under 35 U.S.C. .sctn. 119(e) of
U.S. Provisional Patent Application Ser. No. 60/750,536, filed on
Dec. 14, 2005. Each patent application identified above is
incorporated here by reference in its entirety to provide
continuity of disclosure.
FIELD OF THE INVENTION
[0002] This invention relates to the repair and removal of erosion
or impact damage to elastomeric coatings on a curved substrate,
particularly such surfaces as the leading edge of the airfoil which
may take the form of a wing, a rotor blade, a turbine blade, a
propeller blade, a fan blade, an aircraft radome or antenna which
have similar arcuate leading surfaces. The method is also useful
for other flat and contoured surfaces.
BACKGROUND OF THE INVENTION
[0003] Elastomeric polymeric compositions are used to protect
structures with forward facing surfaces, such as wings, rotor
blades, propeller blades, fan blades, turbine blades, aircraft
radome, and aircraft antennas. These structures can be severely
damaged when used in their intended operational environments. The
term "erosion damage" is a broad term encompassing damage caused by
rain erosion, sand and dust erosion as well as impact damages
caused by stone, gravel or foreign objects encountered typically in
flight conditions.
[0004] Elastomeric erosion resistant coatings are supplied in the
forms of tapes, sheets, molded boots and sprayable coatings.
Currently available elastomeric polyurethane coatings used in
erosion protection application are highly sand erosion resistant,
demonstrating higher sand erosion resistance than metal. However,
elastomeric polyurethane coatings have lower rain erosion
resistance than metal, usually exhibiting rain erosion damage in
the form of deep pits, cracks, craters, and holes. The size, shape
and location of the damage sites vary depending on the nature of
the damage. The size and shape can vary from crack lines as thin as
hair lines, pits about 1 millimeter or smaller in diameter, craters
about 2 to 3 millimeters in diameter, or irregularly shaped holes
wider than 1 centimeter across. The damage sites can exist isolated
and randomly distributed, or continuous across the forward facing
surfaces.
[0005] When these erosion damages occur, it is extremely difficult
to conduct repairs on the rain eroded polyurethane elastomers. The
high sand erosion resistance makes it extremely difficult to remove
the coatings by hand sanding. For helicopters, removal of the
current types of erosion protection coating by mechanical or
chemical means requires the removal of the rotor blades from the
aircraft and typically removal by machine sanding or other
techniques. The reapplication of the tape, molded boot and
sprayable coatings in the field is very labor intensive and
costly.
[0006] Another method to remove the damaged coating uses chemical
strippers. This method also requires the removal of the rotor
blades from the aircraft, as the open air will dry out the chemical
stripper very quickly. Another problem is that chemical stripping
introduces hazardous chemicals into the operation. In addition,
typical erosion resistant coatings are used at a thickness equal or
greater than 0.014''. It usually takes overnight soaking to soften
the coatings so that they can be removed. There are also concerns
that the stripper solution may swell and damage the composite
structure under the erosion resistant coatings. For these reasons,
it is not practical to do field repair with chemical stripper.
[0007] Possible methods that could be used to repair the erosion
damage involve brushing on repair material and spraying on the
repair materials. Neither of these methods is entirely satisfactory
to fill in the cracks, holes of varying sizes and shapes on a
curved surface, while still maintaining a smooth, aerodynamic
surface at the end of the repair operations. The extra layers
simply follow the irregular contours of the damaged surfaces
interfering with aerodynamics of the airfoil. None of the methods
employed to date have satisfactorily provided a method to field
repair a rotor blade which has erosion damage.
[0008] It is an object of this invention to provide a method to
efficiently fill in the pits, cracks, craters, and holes of varying
sizes and shapes on a curved surface.
[0009] It is an object of this invention to provide a method to
repair airfoil structures such as the rotor blades that can be
accomplished in the field.
[0010] It is a further object of this invention to repair the rotor
blade or other leading edge structure while the blades are still
mounted on the aircraft or equipment,
[0011] It is an additional object to design an erosion protection
system that can be removed and/or repaired in the field, without
power tools or chemical strippers.
[0012] It is another objective of this invention to provide an
erosion protection coating system for airfoils and a repairable
resin system for airfoils with contrasting colors to allow early
detection of erosion, impact and other damages, and to allow fast
repair to lengthen the service life of the blades or
structures.
SUMMARY OF THE INVENTION
[0013] One embodiment of the invention relates to a method of
repairing an airfoil surface having a plurality of damage cavities
caused by erosion or impact damage comprising filling said
plurality of damage cavities in said surface with a liquid repair
material using a flexible applicator capable of conforming to the
surface of said airfoil surface while being drawn lengthwise along
the airfoil surface. This method may include the preliminary steps
of sanding the portion of said airfoil surface containing said
plurality of damage cavities with abrasive material and applying an
optional primer coat over sanded areas. The liquid repair material
is preferably formulated as an elastomeric basecoat and an erosion
resistant topcoat may optionally be applied over the basecoat. The
erosion resistant topcoat is preferably more sand erosion resistant
than the underlying elastomeric basecoat.
[0014] Another embodiment is directed to an elastomeric airfoil
erosion protection coating for visual detection of water and sand
erosion damage comprising having the elastomeric basecoat of a
contrasting color to the color of the topcoat layer which is
visible on the outer surface. This system of contrasting colored
coating layers provides visual detection of any damage by detecting
the appearance of the contrasting color the underlying layers
thereby indicating damage in the area. In another related aspect,
there are three contrasting colored layers (a) a primer of a first
color applied directly on a structural substrate of said airfoil
surrounding a leading edge of said airfoil; (b) a basecoat of a
second color applied over said primer; and (c) a topcoat of a third
color on top of said basecoat, wherein said first color, second
color and third color are contrasting colors allowing visual
detection of damage to said protection coating by visual inspection
to detect the appearance of the second color of said basecoat or
first color of said primer indicating damage in the area. The
contrasting colored coating system is used in a method of detecting
damage to an airfoil erosion protection coating allowing the slight
damage to be repaired before the airfoil substrate is damaged,
thereby prolonging service life. The repair method described above
is useful for such repair.
[0015] Still another embodiment is directed to a method of making
an airfoil leading edge erosion protection coating capable of being
field repairable by hand sanding comprising applying to an airfoil
substrate a coating system composed of a hand sandable basecoat and
a topcoat, said basecoat being of lower sand erosion resistance
than the topcoat and said basecoat constituting at least 50% of the
total coating thickness. A related aspect relates to repairing said
erosion protection coating by sanding the damage cavities until all
irregular edges extending above the surface of said coating have
been sanded until said edges are flush with the surface; applying a
repair basecoat to fill said plurality of cavities to form filled
cavities; and finally applying a repair topcoat layer over the
filled cavities.
[0016] Still another embodiment is a field repairable polymeric
erosion protection composition positioned on and adhered to a
leading edge surface of an airfoil comprising an elastomeric base
composition loaded with fillers sufficient to render the polymeric
erosion protection composition hand sandable, said base elastomeric
base composition tested in accordance with ASTM D412-92 prior to
incorporation of said fillers having a minimum tensile strength of
1000 psi, an elongation at break of at least 200%, and a Shore A
hardness of less than 95 A.
[0017] A further embodiment is directed to a repairable elastomeric
coating for a leading edge surface of an airfoil comprising (a) an
elastomeric, hand sandable basecoat disposed surrounding said
leading edge surface having a sand erosion rate above 0.020
grams/cm.sup.2; and (b) an elastomeric topcoat disposed on top of
said elastomeric basecoat having a sand erosion rate below 0.020
grams/cm.sup.2. Preferably the basecoat constitutes at least 50% of
the total coating thickness.
[0018] A still further embodiment relates to a single layer erosion
resistant elastomeric coating adhered to a leading edge surface of
an airfoil comprising a single layer of an elastomeric basecoat
adhered on said leading edge of said airfoil having a sand erosion
rate above 0.020 grams/cm.sup.2 and a water erosion rate of greater
than 100 minutes.
[0019] An embodiment utilizing applicators which are preformed to
conform to the cross sectional contour of the leading edge surface
of an airfoil are useful on long uniform cross section leading edge
application, and it utilizes an applicator comprising a body having
an open interior area defined by at least one interior wall, said
wall having a wiping edge at a distal end thereof and a front edge
generally opposite said wiping edge; said wiping edge defining a
contour complimentary to a leading edge surface of an airfoil; said
front edge defining a contour shaped to form a pocket between said
leading edge surface of said airfoil and said interior wall; and
wherein during operation an elastomeric material is resident within
said pocket so that as said applicator is drawn along said leading
edge surface of said airfoil said elastomeric material is deposited
on said leading edge surface of said airfoil and follows the shape
defined thereby. This applicator may have a handle incorporated
therein and a inlet device to allow for feeding in of the
elastomeric repair materials by various supply methods including
squeeze bottles, caulking gun-type devices and dispensing devices
which meter and premix the elastomeric repair material.
[0020] An additional embodiment relates to an airfoil repair kit
comprising a flexible applicator capable of conforming to a leading
edge surface of an airfoil; and at least one an elastomeric, hand
sandable repair material along with optional kit components of
sanding discs, a primer, an elastomeric basecoat, an elastomeric
topcoat, brushes, and a sprayer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is partial section of the leading edge portion of an
airfoil structure showing sand and water erosion damage.
[0022] FIG. 2 is a partial section of the leading edge portion of
an airfoil structure showing more severe sand and water erosion
damage.
[0023] FIG. 3 is a partial section of the leading edge portion of
an airfoil structure used for laboratory testing coated with
elastomeric erosion coating.
[0024] FIG. 4 is a cross sectional schematic view of an airfoil
shape with major airfoil or hydrofoil elements identified
[0025] FIG. 5 is a perspective view of a leading edge being
repaired using a flexible applicator
[0026] FIG. 6 is partial section of the leading edge portion of an
airfoil structure showing sand and water erosion damage being
repaired using a specially formed flexible applicator.
[0027] FIG. 7 is partial section of the leading edge portion of an
airfoil structure showing sand and water erosion damage being
repaired using a specially formed flexible applicator with a
handle
[0028] FIG. 8 is partial section of the leading edge portion of an
airfoil structure showing sand and water erosion damage being
repaired using a specially formed flexible applicator with an inlet
for repair fluid built in to the body of the flexible
applicator.
[0029] FIG. 9 is a schematic representation of a method of testing
sand erosion resistance.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] FIGS. 1 and 2 show a time lapse sequence of the erosion
damage progression on an airfoil shaped structure 10, the leading
edge portion 12 of which is shown in FIGS. 1 and 2 in sectional
view. Rain erosion and impact damage (14 and 16) typically occurs
at the front of the leading edge 12, while sand or solid particle
erosion tends to focus on the contour surfaces slightly away from
the leading edges. Rain erosion typically caused pits, craters, and
holes, while sand erosion typically produces uniformly matte
surface appearance and very shallow erosion hole patterns. FIG. 2
illustrates a later stage of erosion damage using the same section
from FIG. 1. In FIG. 2 the most severe erosion sites 20 and 22
occurs when the surface is first eroded by sand or dust particles,
and then followed by rain erosion. Under this mixed sand/rain
environment, the side surfaces region 24 surrounding the leading
edges are typically eroded into deep pits and craters very
quickly.
[0031] Certain embodiments relate generally to the repair of an
elastomeric coating 26 on a curved surface 28 of an airfoil shaped
structure 30 as illustrated in FIGS. 1 and 2 as rotor blade 10. The
elastomeric coating 26 is defined as a flexible coating based on
elastomeric polymer composition. The coating may contain no filler
or it may contain fillers. The presence of filler may stiffen up
the coatings to the point of relatively little elastomeric physical
character, but these filled coatings are still regarded as
"elastomeric coatings" for use in various embodiments of this
invention.
[0032] FIG. 3 shows the test airfoil 40 which is a mock-up of the
partial airfoil leading edge section of an actual rotor used to
simulate actual damage from water and sand impingement in a
controlled environment. The elastomeric coating 42 is deposited on
the underlying substrate 44 surface area of the whole test airfoil.
The leading edge 46 is the focal point for the impingement of water
and sand during testing shown by directional arrow 48. All along
the leading edge 44 and all the adjacent surfaces represented by
this test airfoil damage occurs by the appearance during testing of
the erosion damage cavities shown in FIGS. 1 and 2.
[0033] FIG. 4 illustrates by a cross-sectional diagrammatic
representation of the convention structural portions of a typical
airfoil 50 having a leading edge 52 and a trailing edge 54 with the
oncoming wind direction shown as arrow 56, the angle of attack 58
is the angle between the wind direction and the chord 60' of the
airfoil 50 shown as a dashed line 60'.
[0034] The wind carries sand and rain and debris into contact with
the leading edge 52 and impinges on its contoured surfaces 62' and
64' on either side of the leading edge. These leading edge areas
are the test surfaces simulated by the test airfoil of FIG. 3. and
are generally where the damage occurs as best shown in the drawing
representations in FIGS. 1 and 2.
Terminology Definitions
[0035] The term "airfoil" as used throughout this specification is
meant to be more expansive than the conventional airfoil shaped
structure in FIG. 4 and will also encompass structures such as
hydrofoils which have an aerodynamic shape that is somewhat
different than FIG. 4 but are similarly subject to wind or water
carried sand and debris. Such included shapes are radomes shape
which would have a leading edge in the form of a narrowed point
rather than the leading edge which is geometrically a line in FIG.
4 airfoil form. Aircraft antennae are shaped to allow smooth
airflow around them and are considered within the term "airfoil" as
are other devices benefiting from the advancement of the
embodiments such as windmill blades, turbine blades, runner blades,
fan blades, compressor blades, propeller blades, vanes, stay vanes,
hydroelectric turbines, marine propellers, hydro turbines, gas
turbines, tide mills, windmills, compressors, pumps, blower,
impellers, propellers, and many kinds of fans all of which have the
common feature of having fluid passing by the surfaces which may
carry damaging sand and debris.
[0036] The term "leading edge" will similarly be understood to have
a broader meaning than shown in FIG. 4 and should be defined as a
narrowed surface designed to encounter the wind or other fluid such
as water. It is may be an elongated narrowed edge in the case of a
rotor blade, wing, antenna, windmill blade or a sharper edge
surface as in a propeller blade or a forward wind encountering
point area as in a radomes (which may have a blunt conical form or
other generally rounded shape).
[0037] The term "elastomeric" as used herein generally is
understood to be any flexible material which has an ultimate
elongation at break as measured by ASTM D412-92 of at least 40% at
break, preferably 80% and more preferably 100%.
[0038] The terms "sprayable" or "spray-applied" or "sprayed" all
are meant to mean materials that are coated and bonded onto a
substrate, such as an airfoil, particularly the leading edge and
surrounding areas using spray techniques. This terminology
distinguishes elastomeric materials that may be applied to the
substrate as a premolded and/or preshaped boot of elastomeric
material, a preformed tape material which is adhered to the airfoil
or a preformed sheet that may be bonded to the airfoil.
[0039] The term "hand sandable" is understood to mean a material
whose surface is abraded away as loose debris within one minute of
hand sanding. The hand sanding is done on a properly supported
1.5''.times.3'' area of the test material using moderate downward
pressure with 80 grit aluminum oxide sandpaper. A "hand sandable"
coating is characterized by the sample being able to be sanded by
hand pressure into powder in less than 15 seconds without gumming
up or rolling up as an agglomerate. Excellent sandability
preferably included one or more of the following additional
properties: 1) sanding debris is coming off from the coating within
10 seconds of sanding, 2) Low friction during sanding, 3) No heat
or low heat generation after one minute of hand sanding with normal
effort, 4) Loose sanding debris in free flowing powder form,
instead of gum up or rolled up agglomerate, 5) the amount of
sanding residue on sanding disc is equal to or less than the amount
left on the coating after sanding. Using the Particle Erosion Test
Apparatus, we have found that materials with sand erosion rate of
less than 0.020 grams are difficult to sand by hand. To be hand
sandable by the definition of this invention, the material should
have a sand erosion rate of higher than 0.020 grams, preferably
higher than 0.030 grams and 0.040 grams. Materials having sand
erosion rates between 0.02 and 0.03 sometimes exhibit slightly more
difficult hand sanding characteristics.
[0040] It has been found to be very difficult to repair the pits,
craters, and holes scattered throughout the leading edge of a
helicopter rotor blade or other leading edge surfaces. Common
repair techniques of using a putty knife and putty-like solid
repair resin do not work well in this application. The damage sites
can be too small for a material with putty consistency to flow in.
The putty knife cannot repair a curved surface either.
[0041] A helicopter rotor blade or other leading edge structures
are well defined aerodynamically shaped surface. The airfoil shape
of the rotor blade is characterized with a very sharp curve at the
leading edge. A repair resin must have the proper viscosity so that
it does not run or drip from the sharp curve during the repair
procedure. Any repair to the blade surface must minimize the
distortion of the aerodynamic contour.
[0042] The difficulty of repairing a damaged elastomeric surface
has limited the total service life of an elastomeric erosion
protection system on a helicopter rotor blade. Currently,
elastomeric molded boot and self adhesive polyurethane tape are
used to protect the blades. Once the damage occurs on surface and
in the body of the elastomeric materials, the self adhesive
elastomeric tapes may encounter sudden catastrophic adhesion loss
and fly away from the rotor blades during flight, which become a
safety concern for aerodynamic balance of the rotors. The
alternative existing elastomeric covering of a rotor takes the form
of a preformed, molded boot that is adhesively bonded to the blade
substrate. The elastomeric boot is usually left to erode until not
usable and then replaced. Replacing the tape and the boot are both
very labor intensive operations, involving the removal of the rotor
blade from the helicopter, stripping off all old coatings by a
variety of methods, applying some replacement elastomeric materials
and then carefully conducting weight balancing of the blade after
the repair procedure. If the field unit is not equipped to do the
repair, the entire rotor blade must be sent back to a depot
facility to do the repair and overhaul. The transportation of the
rotor blades is costly and time consuming.
[0043] Another deficiency of the current erosion protection methods
is the lack of an early erosion indicator that enables the user to
take preventive action to stop the erosion going all the way
through the elastomeric coating and ultimately into the substrate.
The commercial erosion resistant sprayable coatings use one color
gloss or matte color schemes. If a basecoat and a matte topcoat are
used, the prior art coating systems typically use a gloss or
semi-gloss basecoat, and a matte topcoat, both of the same or very
similar colors. In these systems, even though the underlying primer
or adhesive of different texture or different colors may be
utilized, the total system does not provide sufficient warning for
the users to take preventive actions when there is slight damage to
the elastomeric coating. Once the underlying primer or adhesive is
exposed, the elastomeric protective coating is damaged to the point
of not being serviceable any longer. This inability to detect early
and slight damage shortens the service life of the rotor blades and
other airfoil-type structures with leading edges, such as radomes
and antenna structures on the aircraft. Because the prior art
elastomeric erosion protection materials typically erode to the
substrate with deep cratering and pitting, the damage usually reach
the substrate before any corrective actions can be taken. This can
be detrimental to a composite structure as the underlying composite
layers of the rotor or airfoil can be punctured through by rain
erosion in a very short period of time.
[0044] Still another deficiency of the existing erosion protection
systems is the difficulty of coating removal from the substrate.
Coating removal is an essential part of a successful field repair
procedure. An elastomeric erosion resistant coating by its nature
is very difficult to remove. The common methods use a solvent based
stripper to soak through the coating to soften or dissolve the
primer. Typical primers suitable for such procedure include
polyvinyl butyral based wash primers. While the procedure works
well to remove the erosion resistant coatings, the use of excessive
amount of hazardous solvents is not desirable. In addition, it
takes a long time, typically overnight soaking, to soften or
dissolve the primer. In a military or other emergency operations,
the helicopter cannot be out of service for many hours, waiting for
this lengthy repair procedure.
[0045] Many advantages can be achieved by the repair methods and
procedures in the embodiments described hereafter.
[0046] The field repair of the cavities caused by rain erosion or
impact damage on the curved surfaces of an airfoil structure can be
accomplished with the use of a flexible airfoil contour applicator
also called a Flexible Applicator (FA) as described more fully
below. Additional steps in the repair of rotor blade damage may
include one or more of the following steps: 1) Surface preparation
including sanding, 2) Application of primer, 3) Application of
basecoat, and 4) application of topcoat.
1. Surface Preparation Step
[0047] On a rain or impact damaged surface, there are holes and cut
surfaces, with some remaining debris hanging around the wells of
the craters, pits and holes. This "raised" debris must be removed
or smoothed to correspond with the surrounding contoured surface.
Exacto knives can be used, but are discouraged due to the risk of
damage to the composite substrates. We have found that a pair of
scissors, most preferably curved scissors, can be used to trim off
the raised debris. The curved scissors has a curvature that can
touch the damage sites at proper angle to trim off the debris. This
is especially helpful in the surface preparation of sand erosion
resistant elastomeric erosion protection coatings, since they are
very difficult to smooth out with abrasive sanding. For example,
elastomeric polyurethane coatings containing no filler or low
concentration of fillers tend to "smear" or "gum up" when abrasive
sanding is used. These coatings will be extremely tiring for a
worker or soldier to sand the large rotor blade in the repair
procedure.
Hand Sandable Embodiment
[0048] To be practically repairable in the field, the new erosion
protection system of this embodiment should preferably be sandable
by hand in the field, on the aircraft, without the need to remove
the rotor blade from the aircraft. In one preferred embodiment, the
coating is made to be hand sandable on purpose. This is a
significant departure from the currently employed erosion
protection materials. The conventional erosion protection method
strives to make the elastomeric coatings or resins as erosion
resistant as possible, thus making the unfilled or lightly
filled/pigmented elastomer extremely difficult to remove by sanding
when repair is needed. These materials are not "hand sandable" as
defined below. This embodiment discloses the opposite concept in
the design of the erosion protection system. In this embodiment,
additional fillers are added to decrease the sanding resistance of
the basecoat on purpose, and in many applications where sand
impingement is encountered, a thin layer of sand erosion resistant
topcoat is used on top of the sandable basecoat to form the total
erosion protection system. In this preferred embodiment, the thin
layer of the topcoat and the thick layer of the basecoat can be
sanded with the use of proper grade of sanding medium, yet still
achieve high erosion protection against rain and sand erosion. By
using this new concept with the added early erosion multi-color
warning indicator that will be described in detail below, a field
repairable, renewable erosion protection system for protection of
the leading edges of airfoils is achieved.
[0049] On a helicopter rotor blade or other airfoil-type leading
edges having very well defined aerodynamic surfaces, conduct of an
electrically or pneumatically powered sanding operation is a
dangerous procedure as over sanding can easily damage the composite
honeycomb structure underneath the composite skin. Electrical or
pneumatic power sanding may be used in a depot environment where
experienced personnel routinely perform the sanding procedure, but
are not practical for a field repair environment where
inexperienced personnel are handing the sanding tasks under
non-ideal working conditions. It is preferred to use hand sanding
because the human hand can sense the contour of the substrate and
dynamically adjust the degree of sanding force against the coating
for optimum removal without damage to the substrate. If sanding
discs are used for hand sanding, self-adhesive palm sized sanding
discs are preferred. Those with self-stick adhesive that can be
held securely attached to the palm of the hand are especially
useful. Grit sizes between 40 to 200 grits may be used, with 80-120
grits especially preferred. We have found, quite surprisingly, a
stiff sanding disc with a center insert, originally designed for
metal grinding at high speed, works especially well as an optimal
hand sanding disc to sand elastomeric erosion resistant materials.
The small rigid center insert, as seen on commercial sanding discs
such as 3M Roloc TSM 361F discs, allows the finger to press the
sanding disc hard against the elastomeric coatings to effectively
remove the coating. A sanding block may be used for less contoured
surfaces when the technicians are highly skilled and well
supervised.
[0050] Examples of some of the preferred tools and accessories for
use in the repair kit which will be later described in detail may
include: Curved scissors, Roloc sanding discs, and self-stick
sanding discs. The sanding of the damaged area may create loose
coating debris and powders. These loose powders and debris must be
removed from the work surface before the repair resins can be
applied. To remove the loose debris and powders, it has been found
that different solvents have different cleaning power. A good
cleaning solvent does not attack or soften the erosion protection
elastomers, but is able to pick up the loose powders effectively.
Slower evaporating solvent is preferred as the field repair is
conducted outdoor in open air. We have found that non-polar solvent
such as toluene and xylene are especially preferred for in the
repair procedure. Lint free wipers are preferred for use with the
cleaning solvent in this procedure.
2. Application of Primer
[0051] If the erosion damage reaches the substrate, an adhesion
promoting repair primer is usually required. Afterwards, a basecoat
is applied to fill in the cavities, with the aid of the flexible
applicator using the application method of deforming the flexible
applicator to conform to the contour of the airfoil allowing the
basecoat repair resin to be spread with the flexible applicator
into the cavities without leaving repair resin on the undamaged
portions of the airfoil surfaces. Once the basecoat is hardened
then it is followed by application of the topcoat.
[0052] When the primer is eroded and the substrate is exposed, the
repair primer, which may be an epoxy primer, must be used with
great care and precautions to prevent it from being inadvertently
deposited on top of the intact original elastomeric coating. It has
been experimentally found that if spots or areas of the epoxy
primer are left on top of an elastomeric polyurethane erosion
resistant coating, the epoxy primer will cause early erosion
initiation, probably due to the stiff, high modulus nature of the
epoxy base of the primer which is markedly different from the lower
modulus of the basecoat causing stressed to develop at the
interface which cause cracks and premature failure of the basecoat
integrity. Therefore, the primer must be applied only to the
exposed substrate areas at the bottom of the cavities without any
primer being overlapped onto the undamaged surrounding elastomeric
coating surface.
[0053] Because of the typical small size of the rain erosion
induced damage cavity, depositing the proper amount of primer is a
significant challenge which requires skill and practice to achieve.
Most paint brushes used in any normal painting jobs are too big for
this procedure. Practice of this embodiment preferably includes the
use of micro-sized tips or brushes for the repair of erosion
protection system. Especially preferred are the tips or brushes
that can control the deposit size to about 1.0 mm, 2.0 mm and 3.0
mm in diameter. These dot-placement brushes are very useful in
priming the craters, pits, cracks, and holes. They can also be used
to apply the primer to an area larger than craters, pits and small
holes. For erosion damages that have enlarged to a somewhat bigger
area, small width bristle brush can be used. Examples of suitable
applicators for applying the primer are Microtip, Microbrush and
Ultrabrush manufactured by Microbrush International, Wisconsin,
USA.
[0054] The repair primer may be formulated from suitable known
primer bases including but not limited to epoxy, polyvinyl butyral,
polyurethane or other polymer system with good adhesion to the
substrate. It is preferred to have a fast drying and fast curing
primer so that the erosion resistant coatings can be applied on top
of the primer within short time such as one to two hours. When
priming, with the special micro-sized brushes, the superfine round
tip Microbrush is used to deposit micro dots into the small pits
and craters. A larger brush is used for spreading the primer onto
bigger areas, preferably using about 3/16'' wide strokes to "paint"
larger areas with primer. When the primer becomes tack free or
cures to proper stage (depending upon the primer base system), it
is ready to be coated with the basecoat.
3. Repair of the Basecoat
[0055] The basecoat is formulated to cure in a relatively thick
film or layer and be flexible. The Repair Basecoat may have a pot
life of about 30 minutes to four hours after mixing. This range of
pot life provide a reasonable work time for the repair procedure.
Longer or shorter pot life may be used depending on the environment
and work schedule. The coating gets thicker as time goes on and
becomes very viscous, but still spreadable. This dynamic change of
viscosity can be used to good advantage to do the repair. When the
viscosity is still low (coating still has thin consistency for
about the first 30 minutes), the repair resin can be used to
deposit a thin layer onto the damaged areas. The fluid coating will
spread into the micro-pits and craters and seal the primed
surfaces. As the viscosity increases, the repair resin can be used
to build up the coating thickness faster as it has less tendency to
flow on its own.
[0056] For isolated small pits and craters, the Microbrush and the
Ultrabrush can be used to deposit the basecoat into the small
openings. In contrast to the primer application, the basecoat
repair can use heavy, thick deposits. In this case, the Microbrush
can deposit a thick layer of basecoat upon one single contact with
the substrate without spreading.
[0057] On a rotor blade, turbine blade, propeller blade and other
fan blades, the thickness of the blade may change along its length,
from the inboard section to the outboard section. The blade may
also have a twist along the surface. To apply the thick basecoat
efficiently in one application, this embodiment discloses the use
of a flexible applicator for this purpose. The flexible applicator
is bendable along the curvature of the leading edge surface. The
size of the flexible applicator can be as big as the area to be
repaired. For helicopter rotor blades, the rain erosion damages
usually focus around 2 inches (5 cm) on both sides of the leading
edge of the blade, while combined sand and rain erosion damages
typically occurs within 8 inches (20 cm) on the sides of the
leading edge of the blade. Therefore, a flexible applicator with
coverage of 8 inches or less on both sides of the rotor blade will
be sufficient. Larger size or smaller sizes can be used depending
on the actual contour and dimension of the blades.
[0058] The flexible applicator can also be used to apply the
coating onto the flat surface of the blade. In this case, the edge
of the applicator is used like a flat scrapper to smooth out the
coating on a flat surface.
[0059] The flexible applicator can be made of a semi-rigid,
bendable material, which can be metal, plastic, or rubber. It needs
to be rigid enough to hold its shape, but flexible enough to bend
along a continuously changing curvature. Flexible semi-rigid
plastic sheets are preferred. Especially preferred are semi-rigid,
flexible plastic sheets with high solvent resistance and good
release properties. High density polyethylene and polypropylene
sheets are particularly preferred
[0060] It has been found that one of the simpler forms of the
flexible applicator is a 0.010'' (0.25 mm) thick high density
polyethylene sheet which has the proper combination of being
flexible enough to bend along and conform to the curved surface,
while still being rigid enough to hold its shape to apply and shape
the coating along the curvature. Both polyethylene and
polypropylene have excellent solvent resistance and release
properties.
[0061] FIG. 5 is shown as a photograph because it provides the best
visualization of the application technique of this embodiment. The
airfoil chosen for illustration is a helicopter rotor blade 60
having a leading edge 62 which has damage cavities 64 in its
contoured surfaces. The flexible applicator 66 is made of a 0.010''
(0.25 mm) thick high density polyethylene sheet. The flexible
applicator edge 68 forms a continuous line contact with the
contoured surface of the leading edge using downward pressure
indicated by the force vector arrow 70. At the same time the force
70 is applied in the direction of the surface, the flexible
applicator is drawn in a direction 72 that is parallel to the
leading edge 62. The basecoat repair material (not visible in this
view) is under the curved surface of the flexible applicator in a
rolling bank of material that is moved ahead of the flexible
applicator edge 68 as the applicator is smoothly drawn in the
direction 72. The basecoat repair material completely fills the
damage cavities 64 as the rolling bank of repair material passes
over the cavities. The applicator edge's continuous line contact
with the contoured surface of the leading edge does not deposit
significant amounts of repair material anywhere except in the
cavities 64.
Types of Flexible Applicators
[0062] It has been experimentally determined that the curved
surface or airfoil repair embodiment is enhanced by the use of a
"Flexible Applicator" (FA). In the simplest embodiment the flexible
applicator is a flexible plastic sheet. In using the simplest, flat
sheet-type of flexible applicator, the coating can be deposited
onto the substrate, or onto the applicator, or both surfaces. The
flexible applicator is then "dragged" along, or "pulled" along the
curvature, generally in the longitudinal direction or long
dimension of the airfoil, rotor or blade. It has been found that
applying a sufficient amount of repair coating material on both the
applicator and substrate surfaces gives a helpful "lubricating
effect." This technique allows the flexible applicator to "glide"
easily along the curvature of the airfoil surface.
[0063] In practicing this embodiment, it has been found that it is
preferred to use moderate force to pull the flexible applicator
tightly against the substrate surface. By maintaining the force on
the applicator, good control of the coating thickness is maintained
and there is not excessive build up of the repair coating on an
undamaged curved surface of the airfoil being repaired. It has been
determined that keeping the flexible applicator wet and lubricated
is very beneficial in doing the repair procedure. When the resin
becomes dry or the repair resin is exhausted, the repair resin gets
sticky and does not break cleanly. The applicator embodiments of
FIG. 8 deal effectively with this by incorporating the feature of
continuous feed of resin into the surface contact region under the
flexible applicator. This combination of desirable characteristics
of the flexible applicator technique assures that the aerodynamic
characteristics of the airfoil being repaired is not altered by the
repair. One important characteristic of this embodiment is the
unique ability to fill in the irregularly shaped and sized pits,
cracks, craters, and holes in one application on each curved
surface. In comparison to conventional method of filling each
erosion damage site individually, this method of application
results in great savings of time and labor.
[0064] The dimension of the flexible applicator needs to be wider
and longer than the size of the damage cavities. The dimension of
the flexible applicator is such that the semi-rigid, semi-flexible
applicator is able to maintain the outside contour of the original
curved surface, so that it spread the liquid coating to a thickness
not thicker than the original outside contour of the airfoil.
[0065] This is very important to an aerodynamically sensitive
structure like rotor blade, radome, antenna, fan blade, turbine
blade, etc.
[0066] The repair resin may be applied onto the leading edge
surface first, and then the flexible plastic sheet is positioned
over the leading edge and pulled along its surface. Or the repair
resin may be applied onto the plastic sheet and then it is pulled
over the damaged surface area. Or the repair resin may be applied
to both the leading edge and the plastic sheet, and then the
plastic sheet is pulled along the leading edge to thin out the
resin and squeeze the resin into the holes and craters.
Alternative Flexible Applicator Embodiments
[0067] Using the same flexible/conformable scraper blade concept,
various hand applicator tools can be designed to fit well with the
leading edge structure of various shape and sizes. Such flexible
applicators are within the contemplation of this embodiment and
various thicknesses, shapes and materials are contemplated as
suitable for a flexible applicator so long as they are capable of
following the contour of the curved leading edge surfaces. FIG. 6
shows a preformed flexible applicator 70 which is made of suitable
plastic or metal and formed into a flexible yet permanent shape of
the exact cross sectional outer contour 72 of the cross section of
the airfoil 74 being repaired. The edge 76 of the flexible
applicator 70 is contoured to be complementary to the outer contour
72 and will function to move a rolling bank of repair material 78
ahead of it as it is moved parallel to and along the leading edge
80 of the airfoil 74 as shown by arrow 82. The wider, open end 84
accommodates the volume the rolling bank of repair material 78
which completely fills in any cavity 86 in the surface 88 of the
airfoil 74 being repaired as the flexible applicator is advanced
along the leading edge of the airfoil. The shape of this end of the
applicator is not limited by any consideration except leaving
enough volume to be able to replenish the repair material,
ergonomics of the operator and factors relating to how it is
manufactured. The material used for this flexible applicator should
have elastic memory or sufficient stiffness after it is formed to
the proper curvature of edge 76 to be able to wipe any excess
repair material 78 ahead of the edge 76 in a wiping or squeegee
action. It is understood that the edge 76 could have a arc that is
slightly smaller that the leading edge to assure that there is
pressure exerted by the applicator in the direction of the surface
by the elastic memory or general stiffness of the material. This
embodiment is useful for portions of the airfoil structures that a
leading edge which has relatively uniform cross section to the over
a substantial length of the airfoil. This embodiment is very useful
in that it does not rely on delicate adjustments of downward
pressure by the operator to maintain a good squeegee action. The
tapered outer tip of the rotor blade or wing surface could be
finished off with a planar flexible applicator 66 as illustrated in
FIG. 5.
[0068] FIG. 7 shows a preformed flexible applicator 90' which is
substantially similar in most regards to the applicator of FIG. 6
although the reference characters are not repeated. This embodiment
includes a handle 92' enables better control by the operator of the
orientation, direction and downward force applied toward the
airfoil leading edge 94'. The directional arrow 96' illustrates
again the direction the applicator with the handle is moved. This
handle may enable the operator to use only one hand to handle the
applicator, allowing the other hand to be free to feed in repair
material 98' as needed during repair operations. It also is
essentially self tensioning through the natural downward force
applied by the operator while keeping it in position against the
airfoil leading edge. The handle 92' may be incorporated into the
structure of the applicator by any conventional means, including
being integrally molded into the applicator body or attached by any
suitable method to the applicator body. The shape of the handle can
be any suitable shape.
[0069] FIG. 8 shows another embodiment of a flexible applicator 100
which includes a fluid connector 102 which connects to the portion
of the applicator that holds the repair material 104. The fluid
connector has adapter 106 to attach to any suitable reservoir of
repair material (not shown). This reservoir may be a simple bottle
which can manually dispense the repair material 108 as shown by the
directional arrow 108 into the fluid connector by squeezing a
flexible bottle. It may be a caulking gun configured to feed the
repair material into the fluid connector. The dispensing device may
be a triggered device for metering either a single fluid or
multiple fluids. In an embodiment where the repair material is of
the type that has a two part composition, such as a part a base
component and a part b curative material, the dispensing devise may
be of the type that can meter the proper ratio of part a and part b
and optionally also mix those parts together in a mixing apparatus
prior to dispensing the repair material through the fluid connector
into the working interior volume of the flexible applicator 102
where the rolling bank of repair material is positioned during use
of the device.
[0070] The embodiments of FIGS. 7 and 8 can also be consolidated
such that the handle 92 of FIG. 7 and the fluid connector 102 of
FIG. 8 are in a unitary dual function structure. Of course separate
handle and fluid connector are also easily designed to fit onto the
specialized flexible applicator which can be easily manipulated by
a single person. For use as the repair basecoat, the coatings
without filler should be elastomeric enough to be erosion resistant
to rain or sand Additional fillers may be added to increase the
sand erosion rate.
[0071] The repair resin/coating may be 100% solid without solvent
or it may contain diluents such as solvent or water. The repair
resin may be reactive or non-reactive (fully pre-reacted). It may
contain some or all of the following ingredients: resins, curing
agents, fillers, fibers, fabrics, viscosity modifier, pigments,
hydrolysis stabilizers, adhesion promoters, coupling agents, UV
stabilizers, defoamers, wetting agents, etc. The repair
resin/coating may be as fluid as a brushable coating up to as
viscous as a flowable caulking compound.
[0072] For use as sandable, erosion resistant coating, the coating
is made from a highly flexible coating composition with additional
fillers added at a sufficient level to allow for particulate
removal the top surface of the polymer during sanding. The organic
polymers suitable for forming the hand sandable coatings can
comprise polyacetals, polyureas, polyurethanes, polyolefins,
polyacrylics, polycarbonates, polyalkyds, polystyrenes, polyesters,
polyamides, polyaramides, polyamideimides, polyarylates,
polyarylsulfones, polyethersulfones, polyphenylene sulfides,
polysulfones, polyimides, polyetherimides,
polytetrafluoroethylenes, polyetherketones, polyether etherketones,
polyether ketone ketones, polybenzoxazoles, polyoxadiazoles,
polybenzothiazinophenothiazines, polybenzothiazoles,
polypyrazinoquinoxalines, polypyromellitimides, polyquinoxalines,
polybenzimidazoles, polyoxindoles, polyoxoisoindolines,
polydioxoisoindolines, polytriazines, polypyridazines,
polypiperazines, polypyridines, polypiperidines, polytriazoles,
polypyrazoles, polycarboranes, polyoxabicyclononanes,
polydibenzofurans, polyphthalides, polyacetals, polyanhydrides,
polyvinyl ethers, polyvinyl thioethers, polyvinyl alcohols,
polyvinyl ketones, polyvinyl halides, polyvinyl nitriles, polyvinyl
esters, polysulfonates, polysulfides, polythioesters, polysulfones,
polysulfonamides, polyureas, polyphosphazenes, polysilazanes,
polyolefins, polysiloxanes, fluoropolymers, polybutadienes,
polyisoprenes, or a combination comprising at least one of the
foregoing organic polymers. Exemplary organic polymers are
polyurethanes, polyureas and fluoropolymers. It is desirable for
the polyurethane, the polyurea and the fluoropolymers to be an
elastomer. The aforementioned organic polymers listed above can be
blended and/or copolymerized with the polyurethane or polyurea if
desired. The base elastomers can be fully reacted such as water
based polyurethane, fully reacted thermoplastic elastomers such as
polyurethane, TPR (Thermoplastic rubber), EPDM rubber, nitrile
rubber, chlorinated rubber, butyl rubber, SBR (styrene butadiene)
rubber, fluoroelastomer, silicone rubber, natural rubber, etc. The
most preferred elastomer is polyurethane and fluoroelastomers
[0073] The isocyanates in the polyurethane elastomers can be
aromatic or aliphatic. Useful aromatic diisocyanates can include,
for example, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate
(each generally referred to as TDI); mixtures of the two TDI
isomers; 4,4'-diisocyanatodiphenylmethane (MDI); p-phenylene
diisocyanate (PPDI); diphenyl-4,4'-diisocyanate;
dibenzyl-4,4'-diisocyanate; stilbene-4,4'-diisocyanate;
benzophenone-4,4'-diisocyanate; 1,3- and 1,4-xylene diisocyanates;
or the like, or a combination comprising at least one of the
foregoing aromatic isocyanates.
[0074] Useful aliphatic diisocyanates can include, for example,
1,6-hexamethylene diisocyanate (HDI); 1,3-cyclohexyl diisocyanate;
1,4-cyclohexyl diisocyanate (CHDI); the saturated diphenylmethane
diisocyanate known as H(12)MDI; isophorone diisocyanate (IPDI); or
the like; or a combination comprising at least one of the foregoing
isocyanates.
[0075] Other exemplary polyisocyanates include hexamethylene
diisocyanate (HDI), 2,2,4- and/or 2,4,4-trimethyl-1,6-hexamethylene
diisocyanate, dodecamethylene diisocyanate,
1,4-diisocyanatocyclohexane,
1-isocyanato-3,3,5-trimethy-5-isocyanatomethylcyclohexane (IPDI),
2,4'- and/or 4,4'-diisocyanato-dicyclohexyl methane, 2,4- and/or
4,4'-diisocyanato-diphenyl methane and mixtures of these isomers
with their higher homologues which are obtained by the phosgenation
of aniline/formaldehyde condensates, 2,4-and/or
2,6-diisocyanatotoluene and any mixtures of these compounds.
[0076] In one embodiment, derivatives of these monomeric
polyisocyanates can be used. These derivatives include
polyisocyanates containing biuret groups as described, for example,
in U.S. Pat. No. 3,124,605, U.S. Pat. No. 3,201,372 and DE-OS
1,101,394; polyisocyanates containing isocyanurate groups as
described, for example, in U.S. Pat. No. 3,001,973, DE-PS
1,022,789, 1,222,067 and 1,027,394 and DE-OS 1,929,034 and
2,004,048; polyisocyanates containing urethane groups as described,
for example, in DE-OS 953,012, BE-PS 752,261 and U.S. Pat. Nos.
3,394,164 and 3,644,457; polyisocyanates containing carbodiimide
groups as described in DE-PS 1,092,007, U.S. Pat. No. 3,152,162 and
DE-OS 2,504,400, 2,537,685 and 2,552,350; and polyisocyanates
containing allophanate groups as described, for example, in GB-PS
994,890, BE-PS 761,626 and NL-OS 7,102,524. In another embodiment,
N,N',N''-tris-(6-isocyanatohexyl)-biuret and mixtures thereof with
its higher homologues and
N,N',N''-tris-(6-isocyanatohexyl)-isocyanurate and mixtures thereof
with its higher homologues containing more than one isocyanurate
ring can be used.
[0077] Examples of suitable polyols are polyester polyols,
polycaprolactone polyols, polyether polyols, polyhydroxy
polycarbonates, polyhydroxy polyacetals, polyhydroxy polyacrylates,
polyhydroxy polyester amides and polyhydroxy polythioethers.
Exemplary polyols are polyester polyols, polyether polyols,
polyesters derived from lactones (e.g., .epsilon.-caprolactone or
.omega.-hydroxycaproic acid), or a combination comprising at least
one of the foregoing polyols.
[0078] Exemplary isocyanate prepolymers are TDI-ether, TDI-ester,
TDI-lactone, MDI-ether, MDI-ester, H12MDI-ether, H12MDI-ester and
similar prepolymers made from HDI, IPDI and PPDI. The isocyanate
prepolymers with low free isocyanate monomers are preferred.
[0079] The coating composition also comprises an optional curing
agent. Examples of suitable curing agents are aromatic amines that
can be used as curing agents are phenylene diamine,
4,4'methylene-bis-(2-chloroaniline), 4,4'methylenedianiline (MDA),
4,4'methylenebis(2,6-diethylaniline),
4,4'methylenebis(2,6-dimethylaniline),
4,4'methylenebis(2-isopropyl-6-methylaniline),
4,4'methylenebis(2ethyl-6-methylaniline),
4,4'methylenebis(2,6-isopropylaniline),
4,4'methylenebis(3-chloro-2,6-diethylaniline) (MCDEA),
1,3-propanediolbis(4-aminobenzoate), diethyltoluenediamine (DETDA),
dimethylthiotoluenediamine; or the like; or a combination
comprising at least one of the foregoing aromatic amines.
Polyaspartic esters may be used. Polyol curatives are polyester
polyols, polycaprolactone polyols, polyether polyols, polyhydroxy
polycarbonates, polyhydroxy polyacetals, polyhydroxy polyacrylates,
polyhydroxy polyester amides and polyhydroxy polythioethers.
Exemplary polyols are polyester polyols, polyether polyols,
polyesters derived from lactones (e.g., .epsilon.-caprolactone or
.omega.-hydroxycaproic acid), or a combination comprising at least
one of the foregoing polyols. Imines are useful curatives,
including aldimines, ketimines, multifunctional imines.
[0080] Atmospheric moisture may serve to cure solely or may
catalyze the reaction between the polyurethane and the curing
agent. This is referred to as moisture cure. For aqueous coatings,
polyurethane dispersions can be used with or without curing agents.
The crosslinking of aqueous polyurethane dispersions may be
accomplished by the use of isocyanates, epoxy, or aziridines
functional materials.
[0081] Other additives useful in the coating compositions include
leveling agents, adhesion promoters, coupling agents, defoamers,
hydrolysis stabilizers, UV stabilizers, pigments, dispersants,
curing accelerators, diluents, or combinations thereof.
[0082] In order to exhibit high erosion resistance with the
fillers, the basecoat preferably utilizes a coating composition in
which the elastomeric base of the repair coating prior to the
addition of any fillers has been determined to preferably have a
minimum tensile strength of 1000 psi, an elongation at break of
higher than 100%, and a Shore A hardness of less than 95 A, more
preferred is 200% elongation and most preferred 350% elongation.
These properties are generally tested according to ASTM D412-92 or
D2370 if a film coating is being tested. Exemplary elastomeric
bases along with specialized testing and test methods are as
disclosed in U.S. patent application Ser. No. 11/136,827, filed May
24, 2005, which is incorporated herein by reference in its
entirety.
[0083] The fillers that may be used to render the elastomeric base
hand-sandable and will also increase the sand erosion rate for the
repair basecoat layer include, but are not limited to, the
following list:
[0084] silicates (such as talc, clays, (montmorillonite) feldspar,
mica, calcium silicate, calcium metasilicate, sodium
aluminosilicate, sodium silicate), metal sulfates (such as calcium
sulfate, barium sulfate, sodium sulfate, aluminum sodium sulfate,
aluminum sulfate), gypsum, aluminum trihydrate, metal oxides (such
as calcium oxide (lime), aluminum oxide, titanium dioxide, iron
oxide, tin oxide) and metal sulfites, metal powders, metal flakes,
metal fibers, milled metal fibers, metal nitrides, graphite, carbon
nanotubes, carbon fibers and milled carbon fibers, silica (such as
quartz, glass beads, glass bubbles and glass fibers), metal-coated
glass spheres, metal-coated hollow spheres, buckyballs,
electroactive polymers, antimony-doped tin oxide, carbon blacks,
coke, micro-balloons, and oxides, borides, carbides, nitrides and
silicates from the group of compounds containing boron, aluminum,
silicon, titanium, tungsten, and zirconium compounds.
[0085] Examples of organic based fillers can be used include
thermoplastic powdery material such as polycarbonate,
polyetherimide, polyester, polyethylene, polysulfone, polystyrene,
acrylonitrile-butadiene-styrene block copolymer, Teflon,
fluoropolymers, polypropylene, acetal polymers, polyvinyl chloride,
polyurethanes, nylon and combinations thereof. In general, some
useful thermoplastic polymers are those having a high melting
temperature or good heat resistance properties. There are several
ways to form a thermoplastic abrasive particle known in the
art.
[0086] The useful fillers have a hardness greater than that of the
material forming the continuous phase of the coating. The particle
size of the fillers may be from nano-sized to 200 microns, or
preferably less than 100 microns. The filler content in the hand
sandable coating, based on the total solid weight, can range from
10% by weight to 90%, depending on the interaction of the fillers
and the base elastomers. Preferred is 20% to 80% by weight and more
preferred is 30% to 70% by weight.
[0087] The surface gloss of the basecoat may be gloss, semi-gloss
or matte. In some applications, the repair basecoat may be used
without additional topcoat. For those applications that require a
different surface gloss or different functional surface properties,
another topcoat layer may be applied. The topcoat may be used to
change the surface gloss, surface texture, or surface properties,
such as antistatic or electrical conductivity. As earlier
described, the topcoat may also be formulated to provide higher
erosion (sand and water) resistance and applied over a basecoat. In
the preferred embodiment, the sandable erosion resistant basecoat
layer constitutes at least 50% of the total coating thickness.
[0088] If the coating is 100% solid, one application with this
procedure will fill in the cavities of the damage sites to their
full height. If the coating contains solvent, the dry coating
thickness depends on the dry solid content of the coating. In this
case, a second application may be applied to build up the dry film
thickness at the damage sites. Even though the evaporation of the
solvent left very slight indentations at the damage sites, one
application of the basecoat with the unique flexible applicator was
able to repair the rotor blade quickly and the helicopter was able
to continue flying in a short time period with no detrimental
aerodynamic effects on the rotor blade.
[0089] The basecoat described here is used to fill in the erosion
and impact damage sites and cavities.
[0090] We have found a very efficient method to repair the deep
craters, pits and holes formed by erosion and impact damages.
First, the repair resin is formulated so that there is a somewhat
greater degree of "body" to it at the time of repair. The repair
resin can be thixotropic, shear thinning, or simply having at least
moderate viscosity. "Moderate viscosity" means that the repair
resin can be brush applied and does not flow away from the applied
surface. The repair resin can preferably be reactive, in which case
the viscosity increases with time after the components are mixed
together. The repair resin can also be nonreactive, being a fully
reacted resin dissolved in solvent or water.
[0091] In practicing this invention, the repair resin/coating may
contain special effect fillers, additives, fibers, fabrics to
provide special functions and properties. If the added filler
reduces the erosion resistance of the resin/coating, another layer
of the topcoat with higher sand or rain erosion resistance can be
applied on top of the repair resin/coating. In this case, the
repair procedure comprises the application of primer (optional),
the basecoat and the topcoat. The topcoat may be formulated to
provide the desired color, gloss and erosion resistance, but in
general not hand sandable, by the definition of this invention. The
invention may also be applied to single or multi-layered coating
systems.
Hand Sandable Elastomers Testing techniques
[0092] One method to determine whether a coating is hand sandable
is to use a hand sanding test. Another method is to use a
mechanical particulate erosion test or a Taber Abrasion apparatus
and then correlate to the ease of the hand sanding.
Hand Sanding Test
[0093] The coating materials are either spray coated onto the
substrate or glued to the substrate with a double faced permanent
pressure sensitive adhesive. A 3'' diameter sanding disc, 3M Roloc
TSM 361F, with 80 grit aluminum oxide abrasive, is to be used as
the sanding medium. The disc is stiff with metal hub at the center.
The disc is bent on both side with fingers, and the middle section
is pressed down against the elastomeric coating surface by using
two central fingers. Using moderately firm pressure, the sanding is
done with a timer clock for one minute. The sanding was focused in
a small area about 1.5''.times.3'' in dimension. The weights before
and after the hand sanding were recorded.
COMPARATIVE EXAMPLE 1
[0094] Caapcoat Black B-274, a sprayable rain erosion resistant
coating manufactured by Caap, Inc. was sanded as in the above
procedure. The coating felt gummy, with a lot of resistance to
sanding. The sanding disc got hot after about 15 seconds of hand
sanding. Only trace amount of sanding powder/debris was obtained.
The arm used in the hand sanding felt sore and tired after 40
seconds. The weight loss after one minute of sanding was 0.029
grams
COMPARATIVE EXAMPLE 2
[0095] Caapcoat FP-200, a gray sprayable rain erosion resistant
basecoat used in a basecoat-topcoat FP-250 coating system, was hand
sanded. The coating felt gummy, with a lot of friction. The hand
got tired after about 40 seconds. Low sanding dust was observed.
There was some heat built up around 30 seconds. The weight loss
after one minute was 0.040 grams.
COMPARATIVE EXAMPLE 3
[0096] Caapcoat White, a gloss white sprayable rain erosion
resistant coating, was sanded. Results were similar to Comparative
Example 2. The weight loss after one minute was 0.022 grams.
COMPARATIVE EXAMPLE 4
[0097] Caapcoat Fluoroelastomer V, a gray sprayable elastomeric
rain erosion coating, was sanded. The film used for the sand test
was 0.002'' thick due to the low solid content of the coating. The
coating was sanded. The film ripped through easily due to low film
thickness. However, poor sandability with very low sanding dust was
observed. The weight loss after one minute, including the ripped
pieces, was 0.050 grams.
COMPARATIVE EXAMPLE 5
[0098] Chemglaze M331, a gloss black sprayable rain erosion
resistant coating manufactured by Lord Corporation, was sanded. The
coating produced very low sanding dust after one minute. The hands
get tired after about 50 seconds. The weight loss was 0.024 grams
after one minute.
COMPARATIVE EXAMPLE 6
[0099] A piece cut from a Task L-101 molded boot manufactured by
Task Inc. was sanded. The sanding disc got very hot in about 7
seconds. The sanding had to be continued by switching fingers to be
comfortable. The weight loss was 0.062 grams after one minute.
COMPARATIVE EXAMPLE 7
[0100] 3M 8545 tape, a black molded erosion resistant polyurethane
sheet manufactured by 3M Company, was sanded. The sanding disc got
very hot in 15 seconds. The material felt gummy, with trace of
sanding dust rolled up together in lumpy form. The weight loss was
0.028 grams after one minute.
COMPARATIVE EXAMPLE 8
[0101] 3M 8667 tape, a black molded erosion resistant polyurethane
tape with pressure sensitive adhesive backing, was sanded. There
was a lot of friction. The sanding disc got very hot in 15 seconds.
The trace sanding dust rolled up into small lumps. The weight loss
was 0.018 grams after one minute.
[0102] As seen in the above Comparative Examples, a person trying
to sand a small 1.5''.times.3'' area for one minute using the
current commercial erosion resistant coating could not remove much
material, at the same time, the person felt tired, exhausted and
also encountered uncomfortable heat generated in very short period
of hand sanding. Thus when trying to utilize the materials of the
Comparative Examples 1-8, it would not be practical or even
possible to conduct a field repair of a rotor blade, which may
measure about 20 feet long.
[0103] The hand sanding properties are determined by the total
filler loading. As the filler loading increases, the polymeric film
on top of the elastomer can be broken away and form loose debris,
thereby making the hand sanding easier to perform. Because each
filler has its own density and surface properties, the interaction
of filler and the base elastomer varies and can be determined by
experimental trials.
[0104] In contrast with the above Comparative Examples, a good hand
sandable coating produced loose debris in powder form, with
substantial amount of debris left on the coating surface after
sanding, instead of being trapped inside the abrasive particles on
the sand paper. In similar procedure by the same person using the
same technique, the weight loss of the hand sandable coating is
higher than 0.080 gram, preferably higher than 0.100 grams, and
even more preferably higher than 0.150 grams.
[0105] FIG. 9 illustrates an example of the mechanical sand erosion
apparatus as practiced in the Particle Erosion Test Apparatus,
operated by the University of Dayton Research Institute, Dayton,
Ohio. In this test, particles 90 are accelerated in a small
diameter (approximately 0.25-inch) high-speed gas jet 92 and
directed onto a test specimen 94 as illustrated in FIG. 9 Since the
diameter of the jet is smaller than the test specimen area, the
specimen holder and jet are articulated so that the test specimen
96 is moved through the jet in a uniform manner. This articulation
provides a uniform particle loading (particle mass intercepted per
unit surface area) over square area of approximately 316 cm2 (i.e.,
7.0-inch square). The inner 6 inch square is considered valid test
area. For the sand erosion test using a flat 1''.times.1''
specimen, the net sand erosion exposure area is a circle of 2.0
centimeter.
[0106] Compressed air 98 provides the transport gas stream with
regulators and pressure transducers to measure and control the
pressure at the nozzle inlet. Particles are metered into the
transport gas stream from a pressurized screw feeder system. Since
the screw feeder provides a very accurate and uniform particle
flow, the particle mass applied to the specimen is determined by
the run time based on prior calibration of the screw feeder.
[0107] Velocity is determined as a function of the nozzle inlet
pressure by prior calibration. Thus, for a given test, a specific
test velocity can be selected from this velocity versus pressure
calibration. Particle size, velocity and impact angle 97 can be
controlled independently. This provides an excellent capability to
parametrically evaluate the response of critical materials and
coatings to solid particle impact effects. Materials from such
components as rotorcraft blade coatings, leading edges,
windscreens, radomes, paints, and any special coatings can be
evaluated in a well-controlled laboratory environment under
realistic particle impact conditions.
[0108] The Particle Erosion Test Facility differs from the real
flight environment in that the specimen is stationary and the
particle field is moving at the specified impact velocity. Whereas
the key parameters in the flight environment are the static cloud
mass concentration (mass or volume of particles per unit volume)
and velocity, in the particle erosion facility the key parameters
are the particle mass loading and velocity. The relationship
between the mass loading in the test facility, and dust cloud
concentration, impact velocity and time in the flight environment
is as follows:
Mass Load=Concentration*speed*time(*unit conversion factors).
[0109] Specimen size of 1 inch square is used to determine the sand
erosion rate. The sand erosion was conducted with dry silica sand
that have been sieved to 177-250 microns (um), Sand is sieved from
F-series unground silica from U.S. Silica at a mean particle stream
velocity of 353 miles per hour, using an impact angle of 30
degrees. The mass of impinging particles is set at 10 grams per
square centimeter.
[0110] In practicing this invention in sandy environments, a layer
of high sand erosion resistance elastomer is used on top of the
sandable basecoat layer. To maintain the sandability, it is
preferred to let the sandable basecoat occupy at least 50% of the
total coating thickness. In general, it is preferred to use 0.004''
or thinner layer of the topcoat. In this embodiment, the sand
erosion will erode the top layer, and then the basecoat and primer.
When the basecoat is exposed, the erosion damage is first covered
with a renewable sand erosion resistant coating. When the basecoat
is eroded, it is easily sanded down and repaired with the procedure
disclosed in this invention.
[0111] In one embodiment, the basecoat is configured to have a sand
erosion rate (mass weight loss) of greater than 0.024 grams when
tested according to the Particle Erosion Test Apparatus under 353
mph, 30 degree impact angle, 1''.times.1'' specimen size, with
177-250 micron sand particles, more preferably greater than 0.030
grams, It is even more preferred to have the basecoat configured to
have sand erosion rate of higher than 0.040 grams for better hand
sandability.
[0112] In another embodiment, the basecoat is configured to have a
sand erosion rate (mass weight loss) of greater than 0.024 grams,
and at the same time contains a topcoat layer of having a sand
erosion rate of less than 0.020 grams. It is more preferred to have
a basecoat layer with sand erosion rate of higher than 0.040 grams,
and a topcoat layer with a sand erosion rate of less than 0.015
grams. It is even more preferred to have a basecoat with sand
erosion rate of greater than 0.050 grams.
[0113] For use in the water environment without sand erosion
concerns, the basecoat layer containing filler that retains good
rain erosion resistance can be used alone, forming a sandable rain
erosion protection layer.
Application of the Repair Topcoat
[0114] For minor damage situations where the erosion has only
removed the topcoat and exposed the underlying basecoat, these
areas need only topcoat repair. In addition, pits and craters
smaller than 1/16'' can also be repaired by repairing the topcoat
only. Slightly damaged surfaces can be wiped clean with solvents
such as xylene, toluene, butyl acetate or MEK (methyl ethyl
ketone).
Early Erosion Warning System Embodiment
[0115] This embodiment discloses the use of contrasting color in
forming the airfoil erosion protection system. The coating system
may comprise of a primer of color A, a basecoat of color B, and an
optional topcoat of color C. The colors of A, B, and C are
formulated to provide a color contrast so that when the erosion
reaches at each layer, it provides a visual warning and indication
of the need for repair. The use of primer is optional, as some
basecoat resin systems may possess sufficient adhesion that no
primer is needed. In some cases, the coating system may contain
only primer and basecoat, or in others only basecoat and
topcoat.
[0116] In one embodiment, the basecoat is formulated to be in
grayish color to provide contrast to the matte black topcoat. This
serves as an Early Warning Indicator for erosion damage. The
service life the rotor and its elastomeric protective coating can
be greatly increased if routine repair procedures incorporate
regular inspection for any visual indication of damage and if any
is found, four to six repair layers of matte topcoat are sprayed
whenever the gray basecoat is exposed to prevent any further
erosion of the basecoat. The matte black topcoat is designed for
use as a regular maintenance touch-up coating. It is to be used
whenever the gray basecoat becomes visible.
[0117] In routine use, a repair sprayable topcoat is applied
whenever the topcoat is eroded away and the gray basecoat is shown.
The topcoat is sprayed while the rotor blade is still on the
aircraft, in the field. According to the repair method embodiment,
the spraying of the topcoat is used as the first line of defense
against erosion damage.
[0118] The topcoat may be applied by brushing, dipping or spraying.
If an underlying repair basecoat has been applied, the heavy
thickness of the basecoat makes it preferred to allow time for the
solvent to flash off from the basecoat before the topcoat is
applied. Depending on application environment, one to two hours
waiting time is generally sufficient. To obtain the best matte
appearance, spraying is the preferred application method.
[0119] Spraying of the coating can be accomplished by any of the
known spraying methods, including, but not limited to trigger
sprayer, air powered pressure sprayer, propellant-powered sprayer,
aerosol sprayer, pump sprayer, etc. For field repairs away from a
pressurized air supply source, a small disposable hand trigger
sprayer or aerosol propellant powered sprayer is especially
preferred. An example of suitable propellant powered sprayer is the
Preval Paint Sprayer (Spray Gun). The Preval Paint Sprayer includes
a propellant-filled power unit for the sprayer and a container for
the paint.
[0120] Typically a single spraying pass of the matte topcoat
deposit about 0.0005'' (0.5 mils) of dried topcoat. Although it may
vary with severity of damage it has been found that 4-6 spraying
layers (0.002-0.003'') are used to maintain the erosion resistance
of the coated blades after repair. The topcoat can be sprayed as
many coats as needed. To maximize the sand erosion resistance, a
topcoat with low filler content, high sand erosion resistance is
preferred. After repair, a rotor blade with renewed erosion
resistance is placed back to service.
Helicopter Rotor Blade Field Repair Example
[0121] An experimental sprayable coating system was prepared for
application on helicopter rotor blades. The coating system contains
a green epoxy primer, a grayish black filled basecoat, and a matte
black topcoat. The colors of Green (primer)/Grayish Black
(basecoat)/Matte Black (topcoat) forms the Early Erosion Warning
Indicator system.
[0122] The basecoat was an ambient temperature curable coating
system comprising polyurethane with high levels fillers. When hand
sanded for one minute as described in the earlier hand sanding
test, the basecoat has a weight loss of 0.204 grams and generated a
lot of loose powder in 5-10 seconds. There was no heat built up
during the one minute hand sanding. When this basecoat was sand
tested at UDRI, the 1''.times.1'' UDRI sand test erosion rate was
0.058 grams.
[0123] The topcoat is an ambient temperature curable polyurethane.
The topcoat is not hand sandable, with a weight loss of 0.031 grams
when subjected to hand sanding test for one minute. The topcoat
also has very good rain erosion resistance and sand erosion
resistance.
[0124] A rotor blade was spray coated with a 0.001'' thick of green
epoxy primer, a 0.017'' thick grayish black, filled elastomeric
polyurethane basecoat, and a 0.002'' thick polyurethane matte black
topcoat layer, containing carbon black filler.
[0125] During a flight test, the rotor blades encountered severe
sand erosion and stone and gravel impact damage during operation in
a desert environment. The damage appeared as numerous pits,
craters, cracks and holes. The damaged blade surface had a mixture
of visible colors indicating damage to the different colored
layers. The colors were bright gray (substrate metal), bright green
(primer), grayish black (basecoat), and matte black (topcoat). The
colors were of good contrast and the various degree of
erosion/impact damages were easily visible during an
inspection.
[0126] The blade was sanded with 80 grit sand paper. A solvent wipe
using a lint free wiper cloth was used to clean up the loose
sanding dust. A Repair Primer was mixed and brushed onto the
exposed metal sites with a small brush trimmed down to 1/8'' tip.
Any excess primer was wiped clean with the solvent wipe. After
curing for about one hour, a Repair Basecoat was mixed. The
urethane Repair Basecoat had a brushable viscosity as mixed,
gradually increasing to pasty consistency, and eventually to
gel-like state. The viscosity increase was used to deposit the
liquid repair resin onto the various sizes of damage cavities.
[0127] Various techniques and tools were tried for use on the
airfoil surface to fill in the numerous tiny holes and craters on
the leading edge. It was later found that by bending a
semi-flexible 0.010'' thick high density polyethylene sheet, the
sheet can function as an effective Flexible Applicator (FA) sheet
in a special way along the leading edge, the numerous pits,
craters, and holes were filled in one simple operation. We found
that by applying the brushable repair basecoat onto both the
leading edge and the FA, a lubricating effect happened and the
repair efficiency increased. Normally, one pass of the FA was able
to fill in most of the holes and cracks. For some areas with deeper
damage, an additional pass was applied after the first coat
dried.
[0128] The repaired leading edge was left to dry and cure for about
one and half hours. The repair topcoat was mixed and sprayed 4 to 6
passes using a disposable Prevail spray gun. After overnight cure,
the repaired rotor blades looked very good and the helicopter was
ready to fly again. This repair procedure was conducted in the open
airfield, with the rotor blades still mounted on the aircraft,
using a staging platform to lift the workers up and down.
[0129] The repaired aircraft continued to fly for very long time
with regular touch-up repair procedure of this invention. The
erosion resistant coating system, the repair resin and the repair
procedure of this invention in combination provided more than 30
times improvement over the polyurethane tape products.
[0130] This example demonstrates the successful combination use of
an early erosion warning system, the primer repair procedure, the
basecoat repair procedure and the topcoat repair procedure.
Example of a Repair Kit
[0131] A Repair Kit for Airfoil Elastomeric Leading Edges is a very
useful and novel combination of the particular materials described
herein packaged in convenient carrying package for easy transport
by mobile maintenance teams. The Repair Kit will preferably contain
at least the following items: 1) A Flexible Applicator and 2)
Repair Material which includes at least one of the following:
repair primer, repair basecoat or repair topcoat, depending upon
the type of damage encountered. For applications not needing a
topcoat, only the basecoat and optionally a primer is included. For
applications without need of basecoat repair, only the repair
topcoat needs to be included. For applications both basecoat and
topcoat, both repair materials are include as well as the optional
primer. The Kit may also optionally include any of the following
items: special lint free wiper towels, curved trim scissors,
disposable gloves, respirators, special selected sanding discs,
solvents such as xylene, squirt bottle, disposable 1/2'' brushes,
disposable Microbrushes, disposable Ultrabrushes. The Kit may be
housed in any convenient package, for example, paper carton,
plastic storage container and/or a plastic carry-on container for
easy transport in military mission use.
Alternate Embodiments
[0132] While solvent stripping is not the preferred method for
field repair, the repair methods disclosed in the embodiments
herein are compatible with the solvent stripping coating removal
method in the proper work environment. For example, solvent
stripping in combination with the repair method embodiments can be
practiced satisfactorily in a depot facility. For certain
substrates such as radomes, sand blasting or other specialized
media blasting techniques may be used to remove damaged material
prior to repair as described in various embodiments herein.
[0133] Although the embodiments set out herein disclose the methods
and materials for use in the airfoil repair procedures, it is
readily apparent that the methods and materials embodied can be
applied to new erosion protection systems for use on various
airfoil leading edge surfaces which benefit from elastomeric
erosion protection.
[0134] While this patent application describes the repair and the
removal of sprayable coating, the same principles apply to molding
resins, molded boots, tape, brushable coating, putties, and
caulking compounds. These applications should be treated as part of
this invention.
* * * * *