Note: Descriptions are shown in the official language in which they were submitted.
W093/t3249 ~1 3~15~ PCI'/US93/04737
.
_ 1 _
MET~OD FOR ~BING PlJI.8ED OPTICAI~ E~iERGY TO
INCREA8E T9:E BO~DABII.ITY OF A 81~FACE
BACKGROUND OF_THE INVENTION
The present invention relates to optical
surface preparation techniques, and more particularly, to
improving the capability of the surface of a structure to
bond with a material by irradiating the surface with
incoherent r pulsed optical energy having a broadband
energy spectrum.
There are many applications for which it is
desirable to bond one material to another. In the
aircraft industry, for example, the exterior surface of
airplanes must be painted to prevent corrosion that could
otherwise weaken the airplane structure. In the
automotive industry, metal subpanels must be bonded to
polymeric outer body panels. Good bond strength between
materials depends to a large extent on appropriate
surface preparation. Surface preparation techniques
which enhance the capability of the surface of a
structure to bond with a material (bondability) include
. ~
solvent cleaning, abrading, and/or chemical treatment.
However, these methods are all characterized by potential
,
; 20 toxic chemical hazards, waste disposal problems, and high
production costs.
All surface praparation techniques generally
increase the surface free energy of the surface. Surface
-~ free energy refers to the energy required to create a
unit area of the surface. In the case of a liquid, the
surfacè free energy is expressed by the surface tension
coefficient. Polar liquids, such as water, have high
~- surface tension coefficients (H20 = 73 dynes/cm at 20C).
-~ For a liquid to wet a solid surface, the surface free
energy of the solid surface must therefore be higher than
that of the liquid. As a result, the ability to achieve
a water break free surface (i.e., no beading) is often
used as a criteria for adequate surface preparation.
I
W093/23249 PCT/US93/04737 ~
2131S~ "
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Surface free energy of a solid surface can be
improved by either removing all surface contaminants or
by changing the surface chemistry. Removing all surface
contaminants improves the surface free energy because,
when exposed to the environment, a solid with inherently
high surface free energy will attract contaminants as a
way to reduce its total energy. As a result, -he
contaminated surface loses or reduces its ability to bond
to other surfaces. Removal of the surface contaminants
(which are typically organic materials) will restore the
surface's inherent surface free energy. For example,
metals such as aluminum can achieve a "water break free"
condition when the surfaces are clean.
Removing surface contaminants does not always
help, however, depending upon the type of material.
Most organic materials (particularly those made up large
chains of molecules~, for example, usually have low
surface ~ree energies regardless of the cleanliness of
the surfaces. Hence, to increase the surface free
energy, the surface molecular struc~ures of such
materials must be modified. For example, the surface
affinity for other molecules can be increased by either
breaking up the large molecular chains into smaller ones
or by the insertion of other atoms into molecular chains
at the surface.
Optically engineered surface preparation
technology is a known alternative to solvent, abrasive,
! ~ or chemical processes, and avoids some of the
aforementioned problems. An example of one optical
surface preparation technique is presented in Sowell,
R.R., et al., "Surface Cleaning By Ultraviolet
Radiation," J. Vac. Sc , Vol. 11, No. 1, Jan./Feb. 1974.
The Sowell reference describes a process for removing
hydrocarbon contaminants from metal and glass surfaces by
irradiating such surfaces with generally steady-state
Ultra-Violet (W) radiation in the presence of a low
~ W0~3/23249 2 1 3 ~ PCT/US93/04737
r .;..~
_3_
pressure oxygen atmosphere or in open air. However, the
process described by Sowell requires hours to complete
due to the limited W light intensity that can be
obtained from a steady-state W light source. Therefore,
the Sowell process is generally not suitable for
applications in which it is desirable to increase the
bondability of surfaces, some of which may be very large,
within a period of time which would make such processing
practical.
U.S. Patent No. 4,803,021, "Ultraviolet Laser
Treating of Molded Surfaces," is directed to a method for
~ preparing the surfaces of molded products to improve
bonding and painting performance. Such method inclu~es
irradiating the coated surface of molded products with
pulsed laser light that decomposes any mold-release
agents present on the surface to yield diverse
decomposition fragments within the irradiated zone. This
~; ; process requires that the surface material be etched
deeply enough to remove substantially all of the mold-
~ 20 release agent. A surface may only ~e subjected to this
`~ process a finite number o~ times in order to limit the
amount of surface material removed by etching. Since
molded plastic is generally released from a mold only
once, the minimal amount of material removed by etching
is tolerable and may be`considered in the deæign of the
such products. However, this process is not suitable for
repeatedly treating surfaces as part of a scheduled
- maintenance program, as for example, where it is desired
to prepare a surface for painting, if preservation of the
surface is desired.
A significant problem with a laser based
system, such as that described by the '021 patent, is
that irradiation of large or topologically complex
surfaces with the pinpoint beam of a laser is very
difficult to achieve, requiring sophisticated scanning
and rastering techniques. Furthermore, the operation of
W~93/2324g PCT/US93/04737
_4_
a laser xequires laser stops to prevent the laser beam
from inadvertently escaping the work area, and the
building where the laser is operated. This is because
lasers pose a serious danger to humans, who could be
seriously injured if irradiated with a laser beam.
An even more significant problem with laser
illumination, however, is controlling the intensity with
which a laser beam irradiates a surface. ~ecause the
intensity of a laser beam does not follow the inverse
square law, the energy density of the area or "footprint"
irradiated by the laser beam is generally so high that ;--
~ the beam causes thermal decomposition of the materials at
the surface being irradiated. If the structure being
irradiated is made of a polymeric material, las r
irradiation breaks up the thermal bonds of the molecules
of the material, but then (due to the excessive influx of
energy) causes the decomposed polymer bonds to recombine
into new, and different polymer molecules. The formation
of ~uch new polymer molecules may actually cause a
~0 decrease in the surface free energX of the irradiated
surface, causing the bondability of the surface to
possibly decrease.
Thus, it may be appraciated that there is a
need for a process which enhances the capability of the
surface of one structure to be bonded to another. A
further need exists for a method to increase the
bondability of large or topologically complex surface
areas. A still further need exists for a method to
increase thé bondability of a surface which does not
require the use of toxic chemicals.
r
SUMMARY OF THE INVENTION
The aforementioned needs are met by the present
invention which provides a method and system for
increasing the capability of the surface of a structure
to be bonded to a material.
WO93/~324g PCT/US93/~4737
Z1~4555
One aspect of the present invention involves a
method for improving the capability of the surface of an
organic structure to bond with another material. Such
method comprises the steps of irradiating a ~arget area
of a surface of a structure with pulsed, incoherent
optical energy having wavelength components which range
from 170-5000 nanometers at an intensity sufficient to
photodecompose any adventitious organic substances on the
surface and to photodecompose a thin layer of molecular
bonds forming the surface of the structure; and exposing
the target area of the surface to ionized gas that reacts
- with the photodecomposed molecules of the target area of
the surface so as to increase the surface free energy of
the surface.
Another aspect of the present invention
provides a method for improving the capability of a metal
surface to bond to ~ material. This method comprises the
step~ of impinging a target area on a metal surface with
a stream of particles to dislodge any inorganic
substances from the surface; and ~pen irradiating the
target area of the surface with pulsed, incoherent
optical energy having wavelength components in the range
of 170-5000 nanometers at an intensity sufficient to
photodecompsse any organic substances on the surface.
: 25 ThP present invention also provides a system
for improving the capability of the surface of a
structure manufactured of organic material to bond with a
material. Such system includes: (1) an optical energy
source for generating pulæed, incoherent, optical energy
30 having wavelength components ranging from about 170-5000
nanometers directed to irradiate a target area on the
surface of a structure so as to increase the surface free '
energy of the surface; (2) a pulse modulator operably
coupled to control the output of the optical energy
source; (3) an electrical power supply operably coupled
: to provide electrical energy to the pulse modulator; and
W~93/2324~ 2 1 3 ~ ~ 5 S PCT/US93/04737
(4) a source of ionized gas for bathing the irradiated
target area in the ionized gas.
Another embodiment of the invention provides a
system for improving the capability of a metal or similar
S surface with inherently high free surface energy to bond
with a material. Such system comprises: (1) a particle
stream generator for generating a particle stream that
impinges a target area of a given surface in order to
substantially remove any inorganic adventitious materials
therefrom; ~2) an optical energy source for generating
pulsed, incoherent, optical energy having wavelength
~ components ranging from about 170-5000 nanometers
directed to irradiate the given surface cleaned by the
particle stream so as to increase the surface free energy
of the given surface; t3) a pulse modulator operably
coupled to ~ontrol the output of the optical energy
source; and (4) an electrical power supply operably
coupled to provide electrical energy to the pulse
modulator.
An advantage of the syst~m and method of the
present invention is that it provides an economic and
high throughput process for enhancing the capability of
the surface of one structure to be bonded to another.
Another advantage of the present invention is
that it provides a system and method for increasing the
bondability of large sr topologically complex surface
areas.
Still a further advantage of the present
invention ls that a system and method are provided for
increasing the ~ondability of a surface which does not
require the use of toxic chemicals.
DESCRIPTION OF THE ~RAWINGS
The above and other aspects, features and
advantages of the present invention will be more apparent
from the following more particular description thereof
W093/23249 ~ PCT/~S93/~4737
presented in conjunction with the following drawings,
wherein:
FIG. 1 is a schematîc/block diagram of a
system for increasing the bondability of a surface which
employs a supporting structure to facilitate scanning the
target area of the surface with optical energy; and
FIG. 2 is a schematic/block diagram of a
system for increasing the capability of the surface of a
structure to be bonded to a ma~erial which employs an X-Y
table to facilitate scanning the target area of the
surface with aptical energy.
- Throughout the specification and various views
of the drawings, like components are referred to with
like reference numerals.
ESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description is of the best mode
presently contemplated for carrying out the invention.
This description is not to be taken in a limiting sense,
but is made for the purposa of des~Fibing the general
principles of the invention. The scope of the invention
should be determined with reference to the claims.
The invention relates to a novel process for
significantly improving the bondability of the surfaces
of metal or organic stru~tures. It is to be understood
;i that throughout the descripti~n presented herein, the
term "metal" refers generically to any materials having
an inherently high free surface energy. It is also to be
understood that the organic structures may include
organic matrix composites, thermoset materials, and
thermoplastic materials.
The method of the present invention involves
irradiating a target area of the surface of interest with
pulsed, broadband optical ener~y, which is preferably
incoherent, while exposing the target area to an ionized
gas. The broadband optical energy is optical energy
W093/2324~ 2 1 3 4 5 5 ~ PCT/US93/04737 ~ :
having wavelength components ranging from about 170-5000
nanometers (nm). The pulsed optical energy
photodecomposes any organic, ad~entitious materials
present on the surface into gases which are transported
away from the surface. In applications where the surface
of the structure is made of polymeric materials, the
optical energy photodecomposes, or breaks the polymer
chains of the molecules comprising the surface of the
structure. The ionized gas chemically reacts with the
broken polymer chains and modifies the surface by
introducing highly polar sites to the otherwise
relatively non-polar molecules, thereby increasing the
surface free energy of the surface.
The broadband optical energy provides
electromagnetic spectrum components of which at least
some have high probabilities of being adsorbed to
photodecompose the chemical bonds of the many different
types of organic materials that may be found on a surface
being processed, either as contaminants, or comprising
the surface itself. In other word~, using a source of
broadband optical energy increases the probability that
such energy will overlap an optical adsorption peak of
the material being irradiated.
A system 10 for increasing the surface free
energy of an organic sùrface, and particularly a
polymeric surface, is described with reference to FIG. 1.
As seen in FIG. 1, an optical energy source 12 irradiates
a target area on a surface 14 of a substrate 16 with
incoherent, pulsed, broadband optical energy 18. The
optical energy source 12 is preferably a xenon flashlamp
which generates optical energy by conducting electrical
current through low pressure xenon gas contained in a
fused quartz tube. The pulsed optical energy output of
the ~ptical energy source 12 is controlled by a pulse
modulator 20 that is energized by a power supply 22. The
optical energy 18 has a power density (fluence) at the
~ W093/23249 2 1 3 ~ .5 .~ 6 PCT/USg3/~4737
g
target area of the surface 14 sufficient to
photodecompose any adventitious organic materials present
at the surface and to photodecompose the molecular bonds
of the organic materials such as pol~mers, that make up
the surface 14. However, the optical power densi~y is
not so intense as to more than insignificantly etch the
surface 14. The optical power intensity at the~ surface
of the substrate depends on the requirements of a
particular application, but generally the fluencie
(optical power density) is within the range of about
0.01-Q.5 J/cm~/sec.
~ In general, then, the surfaces treated in
accordance with the methods of the present invention are ,
irradiated at an intensity sufficient to photodecompose
any organic surface contaminants and to break the
, molecular bonds of the surface of interest. However, the
intensity is maintained at a level that is less than that
required to significantly etch the surface of interest.
.
- For example, the output of a flashlamp may be adjusted to
have a frequency of 20 Hz and a pu~se width of 1.0
; microsecond when the target zone of an organic surface is
to be irradiated with a fluence of 0.02-0.05 J/cm2/ ec.
Alternatively, the flashlamp may be adjusted to have a
frequenicy of 0.1 Hz and a pulse width of 180 microseconds
2S when the target zone is to be irradiated with a fluence
of 0.2-0.5 J/cm2/sec. Shorter opti~al energy pulses may
be employed when it is desired to shift the output of the
,flashlamp,towards blue light, whereas longer optical!
energy pulses may be used when it is desired to shift the
output of the flashlamp towards red light. The choice of
the longer or shorter optical energy pulses is dependent
on the surfaces and the associated contaminants. The
optimum pulse Iengths are best found empirically for a
given type of material.
By way of example, the optical energy source 12
:~;
~ may be implemented as a xenon flash tube constructed of a
:~
W093t23249 PCT/~S93/04737 ~
2 1 3 ~ ~ 5 6 ! ~.-
--10--
6" long fused quartz tube having an 6 mm outside diameterand a 1 mm wall thickness, filled with xenon gas at a
pressure of about lO0 Torr. The manufacture and
operation of flashlamps is known in the art. See e.g.,
U.S. Patent 5,126,621.
The power supply 22 may be implemented as a
Maxwell Laboratories, Inc., Model No. CCDS-825-P, power
supply capable of providing 25 kV electrical power at a
rate of 8 kJ/sec.
An ionized gas stream 24, which may include
gaseous ions such as N2+, N+, 2+ ~ 0~ ~ and O- is directed
~ by a nozzle 25 to bathe the target area on the suri.ace 14
with an ionized gas stream 24 received from an ion:ized
gas generator. The ionized gas generator 26
15 manuf actures the ionized gas stream from dry gas provided
by a gas supply 2~ which may include dry air, ozone,
chlorine, nitrogen, carbon dioxide, or ammonia.
However, ozone is preferred because it is relatively easy
to manufacture and readily oxidizes any photodecomposed
2Q organic molecules at the irr~diated surface. The ionized
gas generator may be of the type manufactured by Fischer
America, Inc.
The optical energy source 12 is preferably
mounted within a hood 30 that enshrouds the target area
: 25 on the surface 14 b~ing irradiated with the optical
energy 18. A vacuum system 32, in fluid communication
: ~ with the interior of the hood 30, collects any excess
ionized gas 24 and photodecomposed organic materials
liberated from the surface 14. The hood 30 also prevents
ultraviolet light components generated by the optical
;- energy source 12 from escaping into the surrounding work
: spaces.
: In order to facilitate the irradiation of large
areas with the optical energy source 12, housing 30
optionally may be supported at the end of a supporting
structure 40, such as the end of an arm of a conventional
~ W093/23Z49 ~ 1 3 4 5 5 6 PCT/U593/04737
.
' ;,
robotic positioning system, so that the flashlamp may be
conveniently traversed or scanned in a predetermined path
over the surface 14, as would be well known by tho~e
skilled in the art.
As an alter~ative to supporting the housing 30
and optical energy source 12 by the supporting structure,
scanning the surface 14 of the structure 16 may also be
optionally facilitated by maintaining the housing 30
stationary, and by mounting the structure 16 on a
translating X-Y table 42, shown in FIG. 2. By selective
manipulation of the X-Y table 42, appropriate locations
~ on the surface of the structure 16 may be positioned
within the zone of illumination of the flashlamp 12. The
translating table 42 may be manually or computer
controlled in accordance with well known techniques.
The nozzle 25 is also preferably mounted to the
hood 30, by means not shown. By way of example, the
robotic positioning system may be implemented as a CIMROC
4000~ Robot Controller manufactured by CimCorp Precision
Systems, Inc., Shoreview, MN~ ~
The effectiveness of the method of the present
invention for improving the bondability of the surface o~f
an organi~structure has been verified by an experiment,
described below.
~ ~
Ex~erimental Results: ¦
~ Thirty-two lap shear specimens of polypropylene
;~ having dimensions of 3.3" X 1.5" X 0.060" were cut from
polypropylene sheet. The specimens were cleaned with a
distilled water rinse and patted dry with Kimwipes~. The
samples were divided into two groups: Series 120 and
Series 121. The samples of Series 121 were exposed to
pulses (~180 microseconds) of high intensity optical
~ light at a rate of about 0.1 Hz from a linear xenon
;~35 flashlamp while their irradiated surfaces were bathed in
~ionized air. The xenon flashlamp had a 4mm bore and a 6
, ~,
,,,
`;'
. .
WOg3/23249 PCT/US93/04737 ~
2l~S6
-12-
inch arc length and was filled with xenon gas at a
pressure of 200 T. The input energy to the flashlamp was
approximately 1000 J. The series 121 samples were each
irradiated with 3 pulses in each of three zones: (1) left
edge of sample below lamp; (2) middle of sample below
lamp; and (3) ri~ht edge of sample below lamp. The
S~ries 120 samples were not exposed to flashlamp
radiation or ionized gas.
Pairs of samples from Series 120 were bonded
together at a 1 in2 area with an isocyanate based adhesive
(Elmer's Superglue) to form eight test structures.
Similarly; the samples of Series 121 were also bonded
together to form 8 test structures. All of the samples
were wetted 100 percent with adhesive when they wexe
joined. However, it was noted that the untreated
specimens of the Series 120 nearly always exhibitea only
about 80-90 per cent adhesive wetting at the lap joint
area. The lap joint areas were placed under a two pound
weight and were allowed to cure for 24 hours.
Each test structure was~placed in a tensile
loading fixture and subjected to tensile loading until
the lap joint parted. The tension applied to the test
structure was observed continuously to the point of
failure. The failure tensions presented in TABLE 1 are
the tensions observed just prior to lap joint failuras.
No deformation of the test specimens was observed during
any of the tests.
I TABLE 1 shows a doubling of the mean failure
tension between the two series. The zero ~0) data point
for sample 1 in Series 120 represents failure
(separation) of the specimen lap joint even before it was
placed in the tension device. Each specimen was visually
inspectea after failure testing to determine the nature
of the failure mechanism. In all cases the failure was
observed to be adhesive rather than cohesive in nature.
An adhesive failure is one in which the bonding agent
~ W093/23~49 213~1a56 PCT/U593/04737
~ !
-13- :
("glue") fails whereas in a cohesive failure, the
substrate separates from itself. The test results shown
in FIG. 1 demonstrated that the method of the present
invention of irradiating a polymeric surface with pulsed,
incoherent broadband sptical energy in the presence of an
ionized gas greatly improves the bonding characteristics
of a polymeric surface.
T~B~E 1
POLYPROPYLENE S~EET LAP SHEAR TESTS
, ~. ~ . __
Series 120 Series 121
_ ..... _
. ~E~ Failure (lbs) Failure tlbs) _
: ~ Z __ 12 _ 54
15~ 3 23 51 _ _
: 4 _ 18 45
16 25
:~ ~ I ~ ~ 42
: 20 8 23 28
.,
;: Mean 15.4 39.9
Std. Dev. 7.2 9.9
~ . . ., ~ . - - -- .. ,. ~ .
Series 120: No Treatment
Series 121: Samples irradiated with broadband light in
the presence of ionized gas l.
The method of the present invention may also be
30 used to enhance the bondability of metal surfaces, as for
examplel in applications such as~bonding of polymeric
auto-body panels to metal subpanels in the automotive
industry, or painting the space shuttle fuel tanks in the
aerospace industry~
~ ~ The preparation of a metal surface in
accordance with the methods of the present invention `~
includes impinging the target area on the surface of a
structure with a particle stream to dislodge any
inorganic substances present on the surface, and then
W093/23249 PCT/US93/04737 ~
~ i3~5 ~;
-14-
irradiating the surface with pulsed, broadband optical
energy in order to photodecompose any organic
adventitious substances present on the surface. The use
of a pulsed, broadband light source reduces the
5 processing time significantly when compared to the
approach of using steady state W light, as taught by
Sowell, because of the high peak and average intensities
achievable with the pulsed source.
The method of the present invention may be
10 implemented using the system 10 of FIG. 1 shown to
further include a particle stream generator 34 for
~ generating a low kinetic energy particle stream 36. The
particle stream 36 is directed by nozzle 3~ to impinge
the target area of the surface 14 in order to
15 substantially remove any inorganic materials and
particulate matter from the surface. The particle stream
nozzle 38 is preferably mounted to the housing 30 so that
as the structure 16 translates with respect to the
housing 30, as for example, in the direction of the arrow
20 42, the target area is impinged wi~h the particle stream
36 just prior to being irradiated with the optical energy
18. The particle stream 36 is preferably comprised of
carbon dioxide pellets. ~dvantageously, carbon dioxide
is relatively inert, nontoxic, and inexpensive. The
25 carbon dioxide pellets may be conveyed by dry, heated air
at a mass flow rate of 23 kg/hr. Such pellets are
typically shaped as cylinders each having a length of
about 0.5 cm and a diameter of 0.3 cm. The particle
stream generator may be a carbon dioxide pellet source of
30 tbe type commercially available from Cold Je~, Inc. of
Loveland, OH. q
When the substrate 16 is a metal, the target
area usually does not require exposure to ionized gas.
Therefore, the ionized gas generator 26 is generally not
35 enabled and the gas supply 28 is shut-off. However,
there may be applications where both the ionized gas and
W093/23~49 ~ 6 PCT/~S93/~737
~,, I ;
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the particle stream generatox are used in combination
with the pulsed broadband optical energy to improve the ~-
bondability of a surface of the substrate.
One technique that may be used to confirm that
the bondability of a metal substrate has been improved by
the process of the present invention is to pour distilled
water over an area of the metal surface which has been
subjected to the particle stream and irradiated. The
distilled water should wet the entire treated surface
without any exposure of the metal surface within the
perimeter of the wetted area. Such exposure would result
~ if the distilled water were to bead up, indicting that
the distilled water had a greater affinity for itself
than for the metal surface. A surface which wets in this
~; 15 manner is referred to as a "water break free surface."
Such condition occurs when the surface energy of the
treated surface is higher than the surface energy of the
distilled water.
Using the above-described confirmation
; 20 technique, it has been shown that ~etal surfaces
processed in accordance with the present invention do
indeed exhibit an enhanced bondability over metal
surfaces not prepared in accordance with the invention.
For example, alu~minum surfaaes have been processed using
the above-described me~hod to successfully remove organic
oils, tape residue, uncatalyzed RTV (silicone rubber), ~;
salt spray and fingerprints. Once such adventitious
substances were removed from the aluminum, the aluminum
then exhibited a significantly enhanced surface free
energy.
; While the preæent invention has been described
in terms of preferred embodiments, it is to be understood
that the invention is not to be limited to the exact form
-~ of the apparatus or processes disclosed. Therefore, it
is to be understood that the invention may be practiced
WO ~3~23249 . P(:~/US93/047~7 ~
2 1 3 ~
--1 6--
other than as specif ically described withc~ut departing
from the scope of the claims.
