U.S. patent application number 13/278525 was filed with the patent office on 2012-02-16 for fibrillar, nanotextured coating and method for its manufacture.
Invention is credited to Lichao Gao, Thomas J. McCarthy.
Application Number | 20120041221 13/278525 |
Document ID | / |
Family ID | 38140012 |
Filed Date | 2012-02-16 |
United States Patent
Application |
20120041221 |
Kind Code |
A1 |
McCarthy; Thomas J. ; et
al. |
February 16, 2012 |
FIBRILLAR, NANOTEXTURED COATING AND METHOD FOR ITS MANUFACTURE
Abstract
A fibrillar, nanotextured coating is deposited on a substrate by
contacting the substrate with a reaction mixture comprising a
reagent which is hydrolyzable to produced a cross-linked reaction
product, and a first solvent which solvates the reagent and the
reaction product. The reagent is hydrolyzed so as to provide a
cross-linked reaction product which is bonded to the substrate. The
substrate is then contacted with a second solvent which is a
non-solvent for the reaction product so as to cause nanoscopic
phase separation of the reaction product, resulting in the
formation of a fibrillar nanotextured coating which is bonded to
the substrate. The thus produced coating may be subjected to
further chemical modification. The method may be utilized to
produce superhydrophobic coatings. Also disclosed are coatings made
by the method of the present invention.
Inventors: |
McCarthy; Thomas J.;
(Amherst, MA) ; Gao; Lichao; (Amherst,
MA) |
Family ID: |
38140012 |
Appl. No.: |
13/278525 |
Filed: |
October 21, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11481270 |
Jul 5, 2006 |
8067065 |
|
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13278525 |
|
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60748474 |
Dec 8, 2005 |
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Current U.S.
Class: |
556/410 ;
556/428; 556/465; 556/488; 977/788 |
Current CPC
Class: |
B82Y 40/00 20130101;
B05D 3/105 20130101; B05D 1/185 20130101; B82Y 30/00 20130101; B05D
5/08 20130101; Y10T 442/2525 20150401 |
Class at
Publication: |
556/410 ;
556/428; 556/488; 556/465; 977/788 |
International
Class: |
C07F 7/10 20060101
C07F007/10; C07F 7/12 20060101 C07F007/12; C07F 7/08 20060101
C07F007/08 |
Claims
1. A substrate having a fibrillar coating thereupon: said coating
comprising the hydrolysis product of a silane, said coating having
a fibrillar structure comprised of a plurality of nanofibers having
one end thereof bound to said substrate.
2. The coating of claim 1, further characterized in that its
contact angle with regard to water is at least 170 degrees.
3. The coating of claim 2, wherein said contact angle is at least
175 degrees.
4. The coating of claim 1, wherein at least 70% of said nanofibers
have a diameter in the range of 20-70 nanometers.
5. The coating of claim 1, wherein said nanofibers are configured
as a network of cross-linked fibers.
6. The coating of claim 1, wherein said silane is of the formula:
R.sub.nSiX.sub.4-n wherein n is in the range of 0-3; R is
independently H or alkyl; and X is independently halogen or
OSO.sub.2--CF.sub.3.
7. A superhydrophobic coating, said coating being characterized in
that the advancing and receding angles of contact thereof, with
regard to water, are at least 179 degrees.
Description
RELATED APPLICATION
[0001] This application claims priority of U.S. Provisional Patent
Application Ser. No. 60/748,474 filed Dec. 8, 2005, entitled "First
Perfectly Hydrophobic Surface" and is a divisional application of
Ser. No. 11/481,270 filed Jul. 5, 2006, entitled "Fibrillar,
Nanotextured Coating and Method for Its Manufacture."
FIELD OF THE INVENTION
[0002] This invention relates generally to coatings. More
specifically, the invention relates to a fibrillar, nanotextured
coating and to methods for its manufacture.
BACKGROUND OF THE INVENTION
[0003] The nanotexture of a surface can influence various
properties of that surface such as its wettability by water and
oils, its optical properties, and its chemical reactivity.
Consequently, the art has sought methods and materials for
controlling the nanotextures of various materials. Chemical methods
such as etching processes, and physical methods such as
sandblasting and other erosion processes have been utilized with
success to control the microtexture of various materials. However,
such methods have generally been inadequate for providing
nanotextured surfaces.
[0004] The prior art, as exemplified by U.S. Pat. No. 2,306,222,
has recognized that particular silane materials may be utilized to
deposit a water-repellant coating onto various substrates. The
coating deposited by the use of this technology is a relatively
smooth coating, and various approaches have been implemented to
texturize this coating so as to increase its water repellency. For
example, U.S. Pat. No. 6,649,266 shows the deposition of silane
coatings onto microtextured substrates to provide coatings having
enhanced hydrophobicity. Another approach is described in PCT
Published Application WO 2005/068399. This publication describes
the use of a sol/gel chemical process for depositing a sponge-like
water-repellant coating having a nanoscale roughness. Use of this
technology to deposit a coating onto a surface having an additional
microscale texture has been found to provide a coating with further
enhanced hydrophobicity.
[0005] Despite various efforts the prior art has not been able to
prepare a synthetic surface which is perfectly hydrophobic. As will
be explained hereinbelow, the hydrophobic nature of a surface may
be quantified by the contact angle that surface forms with a
droplet of water. A perfectly hydrophobic surface has a contact
angle of 180 degrees, and within the context of this disclosure,
surfaces having contact angles in excess of 170 degrees are
referred to as superhydrophobic. As will be further explained
hereinbelow, the present invention, in one embodiment, provides for
a nanotextured surface having a fibrillar coating of a
water-repellant material. The fibrillar, nanotextured nature of the
coating of the present invention causes the surface to be
superhydrophobic.
[0006] As mentioned above, the nanotexture of surfaces can
influence properties other than, or in addition to, their
wettability by water. As will be further explained hereinbelow, the
present invention, in other aspects, may be utilized to prepare
coated surfaces which are strongly hydrophilic and/or oleophobic,
or oleophilic. Also, the present invention may be utilized to
prepare surfaces having controlled optical properties such as
reflectivity and absorption. In further aspects of the present
invention, chemical reactivity of the surfaces may be controlled by
utilizing the fibrillar, nanotextured materials of the present
invention. All of these embodiments and advantages of the invention
will be apparent from the drawings, discussion and description
which follow.
BRIEF DESCRIPTION OF THE INVENTION
[0007] Disclosed is a method for providing a fibrillar,
nanotextured coating on a substrate. In that regard, the substrate
is coated with a reaction mixture which comprises a reagent having
at least two reactive sites thereupon. The reagent is hydrolyzable
so as to provide a cross-linked reaction product. The reaction
mixture further includes a first solvent which solvates the reagent
and swells the reaction product. At least a portion of the reagent
in the reaction mixture is hydrolyzed so as to provide a
cross-linked reaction product which is bonded to the substrate. A
second solvent is mixed into the reaction mixture after the step of
hydrolyzing the reagent. The second solvent is miscible with the
first solvent and is a non-solvent for the reaction product. This
step causes the cross-linked reaction product to phase separate
from the reaction mixture as a fibrillar, nanotextured coating
which is bonded to the substrate.
[0008] In specific embodiments, the reagent is a silane material
such as a material of the formula:
R.sub.nSiX.sub.4-n
[0009] wherein n is in the range of 0-3; R is independently, one or
more of H or alkyl; and X is independently, one or more of a
halogen or a halogen-like species such as OSO.sub.2--CF.sub.3.
Specific reagents of this type include CH.sub.3--Si--Cl.sub.3,
(CH.sub.3).sub.2--SiCl.sub.2, (CH.sub.3).sub.3--SiCl and
SiCl.sub.4, used singly or in various combinations. In specific
instances, the first solvent is an aromatic solvent such as
benzene, toluene, or xylenes, and the second solvent is an
alcohol.
[0010] In a further embodiment of the invention, the mixture of the
first and second solvents is removed from the coated substrate, and
this may be accomplished through the use of a third solvent which
is miscible with the second solvent and is a non-solvent for the
coating. In some instances, the third solvent is water.
[0011] In a further aspect of the invention, the fibrillar,
nanotextured coating prepared in accord with the foregoing is
reacted with a first conversion reagent which chemically alters at
least a portion of the coating. In some instances, this reagent may
be an oxidizer such as an oxygen plasma, which converts the coating
to a silica coating. Such silica coatings are typically very
hydrophilic. The thus-reacted coatings may be further reacted with
a second and subsequent conversion reagent to further control their
properties. For example, the silica-based coating produced by
oxidation can be reacted with appropriate fluoroalkyl materials to
prepare a surface which is highly oleophobic.
[0012] Also disclosed herein are coatings prepared by the methods
of the present invention, including superhydrophobic coatings.
BRIEF DESCRIPTION OF THE FIGURES
[0013] FIG. 1 is a scanning electron micrograph of a coating of the
present invention; and
[0014] FIG. 2 is an enlarged view of the coating of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0015] This invention is directed generally to nanotextured
surfaces having a fibrillar structure. Within the context of this
disclosure, a nanotextured surface is understood to be a surface
having features in the nanoscale range, typically a range of 5
nanometers to 1 micron. A fibrillar structure is understood to be a
structure characterized by the presence of a plurality of fibrous
features, said fibers having a columnar structure wherein the
length of the column is greater than its diameter. The fibrillar
structure may comprise a felted mat, separated fibers having one
end thereof anchored to the surface, or a mixture of such textures.
As discussed above, the nanotextured nature of the surface enhances
or otherwise modifies the physical and chemical properties of the
material comprising the coating. These properties can include,
among other things, chemical reactivity, wettability by oils or
water, and optical properties such as reflectivity and light
absorption.
[0016] In the process of the present invention, a reagent is cross
linked proximate a substrate which is to be coated so as to form a
cross-linked network. This reaction is carried out in a solvent
which, in addition to solvating precursor materials, solvates and
hence swells the cross-linked structure which is anchored to the
substrate. In a subsequent step of the invention, the initial
solvent material is replaced with a second solvent which is a
non-solvent for the cross-linked material. The second solvent is
typically miscible with the first solvent, and this extraction
process is carried out by adding the second solvent to the initial
reaction mixture to induce phase separation. Given that the
cross-linked network is anchored to the substrate, this phase
separation produces the nanotextured fibrillar structure which
characterizes the present invention. This second solvent may
subsequently be extracted by the use of a third solvent.
[0017] The thus produced nanotextured, fibrillar coating may be
used in an as-is form, or it may be further reacted so as to modify
its surface properties. The thus modified coating may be further
reacted so as to selectably control its surface properties.
[0018] The methodology of the present invention allows for the
rapid and reliable production of nanoscale coatings on a wide
variety of surfaces including metals, ceramics, glasses, polymers,
textiles, paper stocks, mineral materials, as well as on natural
surfaces such as wood, leather, and the like.
[0019] In one particular group of embodiments of the present
invention, the nanotextured coatings are based upon silicon
containing reactive species such as silanes typically include a
number of readily reactable sites thereupon, which allow them to
readily bond to a number of substrate materials and react so as to
crosslink to other silane molecules. One particular class of silane
materials which may be used in the present invention are of the
general formula:
R.sub.nSiX.sub.4-n
wherein n is in the range of 0-3; R is independently, one or more
of hydrogen or an organic group such as an alkyl (including
substituted alkyls); and X is independently, one or more of a
halogen or a halogen-like species such as OSO.sub.2--CF.sub.3.
[0020] Chlorosilanes are one specific group of materials which may
be used in the present invention, and organochlorosilanes such as
methylchlorosilanes are some specific members of this group. These
materials may be used either singly or in combination, and it will
be apparent to one of skill in the art that the properties of the
cross-linked network formed by the hydrolysis of these materials
may be controlled by controlling the ratio of different materials
in a reaction mixture. SiCl.sub.4 may be added to the reaction
mixture to enhance cross linking or otherwise control the
properties of the hydrolyzed product.
[0021] In the process of the present invention, the hydrolyzable
compounds such as the silane are dissolved in a material which is a
good solvent for the reacting chemicals, which solvent also
solvates and swells the resultant cross-linked network. Typical
solvents include aromatic materials such as benzene, toluene, and
various xylenes. The substrate is contacted with a reaction mixture
comprising the hydrolyzable reagent and solvent, and the reagent is
hydrolyzed, typically by including a small amount of a hydrolyzing
agent such as water in the reaction mixture. This causes the
formation of the cross-linked network which network is anchored to
the substrate. Reaction conditions will depend upon the specific
nature of the reagents and the degree of cross linking, and hence
the ultimate structure of the nanotextured coating, which is
desired. However, in some typical embodiments, the reaction mixture
is approximately 0.1-2.0 molar with regard to the hydrolyzable
reagent; although, the reaction mixture may be 5 or more molar with
regard to the reagent.
[0022] Following the step of hydrolysis, the first solvent is
extracted from the reaction mixture and replaced with a second
solvent which is a non-solvent for the cross-linked reaction
product. This extraction is typically carried out by mixing the
second solvent into the reaction mixture following the step of
hydrolysis. Typically, the solvent mixture is then removed, and the
coated substrate washed with at least one more portion of the
second solvent. In some instances, the second solvent is then
removed by washing with a third solvent. In one typical group of
embodiments, the second solvent is an alcohol such as ethanol or
isopropanol. The third solvent, in such instances, if employed, may
comprise water. In some instances the majority of the reactive
silane solution is removed from the reaction flask before
extraction with the second solvent, and in some instances the
sample is rinsed with the first solvent before being extracted with
the second solvent.
[0023] The thus described process produces a nanotextured fibrillar
coating of a silicon-based fibrous material on the surface of the
substrate. This coating is highly hydroscopic and, as will be
explained hereinbelow, exhibits advancing and receding contact
angles for water of more than 170 degrees, and in some instances,
more than 175 degrees. In particular instances, both the advancing
and receding contact angles for water are 180 degrees making the
surface perfectly hydrophobic. Coatings of the present invention
are thus characterized as superhydrophobic.
[0024] FIG. 1 is a scanning electron micrograph of a coating
prepared in accord with the foregoing procedure. As will be seen,
the coating is a highly fibrillar structure, comprised of a
plurality of filaments, each having a length significantly
exceeding its diameter. These filaments are anchored to the
underlying substrate; and in some instances they may be cross
linked to one another, as is best seen in FIG. 2, which is an
enlarged view of the coating of FIG. 1. The filaments form a
nanofeatured network. As such, the coatings of the present
invention are differentiated from hydrophobic coatings of the prior
art, and this difference is manifest by the fact that coatings of
the present invention are superhydrophobic.
[0025] The properties of the coatings of the present invention may
be further modified by chemical reaction. For example, the coating
may be reacted with additional silane materials. The coating may
also be reacted with oxidizing agents such as an oxygen plasma; and
this reaction will convert at least a portion of the coating to
silica which will cause the coating to be hydrophilic. Such an
oxidation reaction may be carried out either prior to or subsequent
to further couplings with silanes. In some instances, the thus
reacted surface may be further reacted with species such as a
fluorosilane to render them oleophobic. In yet other instances, the
nanotextured surface may be reacted with dyes, fluorescent
reagents, organometallic compounds, or other reagents so as to
modify their surface properties. In view of the teaching presented
herein, yet other such surface modifications will be apparent to
those of skill in the art.
[0026] The present invention will be described with reference to
one particular process for preparing an ultrahydrophobic surface on
a silicon wafer. In the process, silicon wafers were submerged in a
1.0 M solution of CH.sub.3SiCl.sub.3 in toluene at room temperature
for three hours. The hydrolysis reaction was carried out in vessels
which were closed to the air during the reaction time, but exposed
to relative humidity of approximately 40-65% during solution and
sample introduction, and this residual water was active to
hydrolyze the slime compound. Thereafter, the wafers were rinsed
with a further portion of toluene, rinsed with ethanol, rinsed with
an ethanol-water mixture and subsequently rinsed with water. The
substrates were then dried at 120.degree. C.
[0027] Surfaces coated by the foregoing method are highly
hydrophobic. Water droplets do not come to rest on the surfaces.
Contact angle as measured with regard to a receding water droplet
(.theta..sub.R) is 180 degrees. The droplet can be "pushed onto"
the surface and the finite advancing contact angle (O.sub.A) is in
the range of 175-178 degrees.
[0028] Given the highly hydrophobic nature of these surfaces, a new
method for measuring hydrophobicity was devised. In this method,
surfaces to be examined were lowered onto a supported water droplet
and repetitive contact, compression and release of the droplet were
recorded by video. Surfaces having contact angles of less than 180
degrees exhibit some affinity for the droplet during attachment and
release; however, truly hydrophobic surfaces will have a contact
angle of 180 degrees and exhibit no affinity.
[0029] Coated surfaces prepared by the foregoing method are
indistinguishable by eye from unmodified wafers, and in that regard
contain no micron scale topography. Scanning electron micrography
indicates that the coating is comprised of a network of cylindrical
fibers having diameters of approximately 40 nm. The method of the
present invention promotes vertical polymerization of the silane
onto a covalently attached toluene-swollen three-dimensional
methylsiloxane network. Phase separation occurs during the ethanol
rinse. In an experimental series comprising 100 repetitions of the
foregoing procedure, extremely hydrophobic surfaces are always
formed. Perfectly hydrophobic surfaces
(.theta..sub.A/.theta..sub.R=180 degrees/180 degrees) are formed in
approximately 70% of the cases.
[0030] As discussed above, other silane compounds, including blends
of silane compounds, may also be used in a similar manner. While
the foregoing example employs chlorosilanes, good reactivity has
also been found utilizing iodosilanes as well as silanes based upon
methyltrifluorosulfonate.
[0031] Surfaces prepared according to the foregoing may be further
modified. For example, exposure of the foregoing superhydrophobic
surfaces to an oxidizing reagent such as an oxygen plasma converts
at least some of the methylsilicone moieties to silica without a
loss of nanoscopic morphology. The coatings thus modified are
spontaneously wetted by water. The silica surfaces thus produced
may be still further modified. For example, treatment of the silica
surfaces with fluoroalkyl silanes produces oleophobic surfaces that
are not wet by hydrocarbon liquids.
[0032] In one group of surface modification reactions, samples of
the nanotextured coating were treated with an oxygen plasma, as
described above, and the samples were introduced into a reaction
flask and treated with a toluene solution (0.1-2.0 molar) of a
variety of silanes for one hour. Alternatively, the samples could
be exposed to reactive silanes in the vapor phase. The silanes used
were of the type RSiX.sub.3, R.sub.2SiX.sub.2 and R.sub.3SiX where
R is one or more of alkyl, aryl, fluoro alkyl or amino alkyl, and X
is one or more of Cl, N(R).sub.2 or OSO.sub.2CF.sub.3. The samples
were isolated and rinsed (in this order) with 2.times.10 ml of
toluene, 3.times.10 ml of ethanol, 2.times.10 ml of ethanol-water
(1:1), 2.times.10 ml of water, and then dried in a clean oven at
120.degree. C. for 10 minutes. Silanes wherein
R.dbd.CH.sub.2--CH.sub.2--C.sub.6F.sub.13 and
CH.sub.2--CH.sub.2--C.sub.8F.sub.17 were found to render surfaces
that were perfectly hydrophobic (advancing and receding contact
angles of 180 degrees) and also repellant to hydrocarbon liquids
(oleophobic).
[0033] The present invention provides methods and materials for
disposing a fibrillar, nanotextured coating onto a variety of
substrate surfaces. The properties of the coating may be tailored
to affect its wettability by water, hydrocarbons and other
materials. Likewise, the optical properties of the surface may be
readily controlled, as for example with regard to reflectivity,
light absorption, fluorescence and the like. In view of the
teaching presented herein, numerous modifications and variations of
the invention will be readily apparent to those of skill in the
art. The foregoing drawings, discussion and description are
illustrative of specific embodiments of the invention, but are not
meant to be limitations upon the practice thereof. It is the
following claims, including all equivalents, which define the scope
of the invention.
* * * * *