Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR HYDROPHOBIC TREATMENT OF WATER VAPOUR
PERMEABLE SUBSTRATES
10
FIELD OF THE INVENTION
The present invention relates to a process, preferably a continuous process,
of coating a substrate, which allows water vapour and preferably air
permeation, with a hydrophobic coating. The process preferably uses a thin
film vacuum condensation step to create a monomer coating which is cured
in situ after the coating. The process has the benefit of allowing continuous
operation and providing a alternative to existing processes for hydrophobic
coating. The substrates are coated such that the water vapour permeation
2o sites are not blocked by the coating to maintain the desired breathability.
The static water contact angle on the surface of such substrates is more
than 95°. Such substrates are preferably employed in absorbent articles
such as diapers, incontinence products, underarm sweat pads, sanitary
napkins, catamenials, pantiliners, breast pads, shoe inserts or bandages or
alternatively in protective garments such as gloves or rain coats for which
water vapour transmission in particular from the inside to the outside is
highly desirable.
BACKGROUND OF THE INVENTION
For instance, U.S. Pat. No. 3,498,527 teaches that paper board containers
for liquids can be waterproofed by application of a waterproofing coating
such as wax or polyethylene, and a similar method is shown in U.S. Pat. No.
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2,708,645 for waterproofing paper drinking cups and in U.S. Pat. No.
3,212,697 for paper grocery sacks. In U.S. Pat. No. 3,597,313, temporary
wet strength is imparted to paper by coating it with a polymeric alcohol-
polymeric aldehyde reaction product. Coating processes, by themselves,
have been used to produce disposable articles of sanitary clothing. In U.S.
Pat. No. 3,078,849, a disposable sanitary napkin is disclosed which consists
of an adsorbent layer having a liquid-repellent backing of polyvinyl alcohol
or
similar material capable of initially repelling water but eventually
solubilizing.
The degree of water-repellency, therefore the lifetime of the napkin, is
o controlled by varying the thickness of the backing. Because the necessary
life of the napkin cannot be predicted by manufacturer or user, the backing
must be sufficiently thick to take account of all normal contingencies. U.S.
Pat. No. 3,542,028 is directed to a flushable sanitary napkin consisting of a
cellulosic sheet treated with a fluoropolymer coating. U.S. Pat. No.
3,559,650 teaches the preparation of a sanitary napkin having two flush-
disposable sides separated by a waterproof film too thin to support itself
once both faces of the napkin have disintegrated upon disposal.
Analogous to the process of coating a surface with a waterproofing
2o substance is the concept of reacting a surface with another material so as
to
form a reaction product on the surface which has water-repellent properties.
For example, U.S. Pat. Nos. 2,130,212 and 3,137,540 teach that materials
such as polymeric alcohols may be reacted with other materials to increase
their water-repellent properties. The latter patent teaches treating polyvinyl
alcohol articles with an aqueous emulsion of an aldehyde to impart water-
repellency thereto. U.S. Pat. No. 3,626,943 teaches that disposable diapers
can be made from polyvinyl alcohol and waterproofed on one side by
reaction with formaldehyde. These reaction-type coating processes suffer
from drawbacks. They are carried out in the aqueous phase which is
complicated and requires relatively large quantities of reagents. Most of the
processes which employ some form of in situ chemical reaction to produce a
water-repellent surface are carried out in the liquid phase, some vapor
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phase treatments are taught by U.S. Pat. Nos. 2,306,222; 2,961,388; and
3,017,290. A known method of water and oil repellent finishing of textiles,
described in USSR Patent 1,158,634, includes plasma treatment in a glow
discharge in an atmosphere of inorganic gases, followed by treatment with a
fluorine containing acrylic monomer in gas phase. Another prior method of
achieving film plasma polymerization, described in U.S. Pat. No. 4,188,426,
includes treatment in a glow discharge of per-fluoro-cyclo-butane or
hexafluoroethane to reduce the friction coefficient and to improve the
surface hydrophobia of organic and inorganic substrates (e.g. polyethylene
o films, metals). However these disclosures do not achieve a level of water
repellency as the present invention when employing the coating process
disclosed herein.
Plasma coating processes of metals, polymers, and other substrates, with
fluorocarbon films are also known in the art. As an example, it is known from
U.S. Pat. No. 4, 869, 922, that deposition from continuous (i.e. non
modulated) radiofrequency (RF) glow discharges fed with fluorocarbons
provides films, layers, tapes, plates, and differently shaped articles made of
plastics, metals or other materials, with a thin fluorocarbon coating, with no
other material interposed between the coating itself and the substrate. Such
coatings are claimed to have very good adherence to the items processed,
to be void-free, to be uniform, continuous and to show controlled wettability
characteristics, which depend on their surface chemical composition. The
non modulated, continuous plasma process of the above mentioned patent
leads to coatings characterized by static water contact angle (WCA) values
lower than 120°.
U.S. Pat No 5,328,576 discloses a method for imparting water and oil
repellent surface properties to fabrics or paper that includes pretreatment in
so a low pressure oxygen plasma in the presence of water vapor followed by
plasma polymerization of methane in a high frequency glow discharge
carried out in the same treatment chamber. This method doesn't deliver
durable, permanent coatings with a WCA higher than about 120°.
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U.S. Pat. No. 5,262,208 discloses an gas plasma treatment for archival
preservation of paper manuscripts by a thin film protective polymer film. The
treatment time is ranging from 30-3600 seconds. Other methods have been
used to obtain thin coatings on the web materials with short treatment
periods. Providing surface treatment is disclosed in US Patent No.
4,842,893 and 4,954,371 which describe a process for high speed coating of
substrates with a complete and uniformly adhering layer and using electron
beam radiation curing of the vapor deposited monomers for multilayer
capacitators. U.S. Pat. No. 4,842,893 discloses high speed coating process
1o including flash vaporization system and electron beam curing. Both of these
electron beam disclosures are incorporated herein by reference. Other uses
of electron beam coatings in the electronic industry field have been reported
by Westinghouse science & technology center USA (Adv. Mat. Newsletter
Volume 13, No 9, 1991 page 4).
While different water repellent treatments have been used in the past, such
as monomer solution coating and curing, coating during laundering, plasma
coating, there remains the need for having an other method that results in
thin, preferably durable repellent coatings that can be obtained at high
2o production speeds and with minimal changes in the substrate properties,
such as flexibility, texture, comfort, and breathability. The present
invention
combines the advantages of the above mentioned high speed electron
beam process for coating of a substrate with the benefit of producing an
article with durable and water-repellent coating.
Thus although the materials employed in the practice of this invention are
known in the art and are known in the context of water-repellent coatings,
the process utilizing electron beam treatment for hydrophobic coatings
provides an attractive alternative to the methods of the prior art.
Summary of the invention
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In one aspect of the present invention, the method of using a high
speed vacuum coating process for producing durable and thin water-
repellent coatings on a substrate uses a movable support such as rotating
drum in a vacuum chamber. The surface of the support is maintained at a
5 temperature sufficient to permit condensation of a vaporized material
deposited in the chamber. The material is a curable monomer with a
relatively low molecular weight. The monomer vapor is created using a flash
vaporizer. The desired amount of curable monomer is metered to a heated
flash vaporizer system where the material is vaporized. It is then transported
o e.g. by it's inherent pressure, to the substrate resting on the rotating
drum
and condensed on the surface of the substrate. According to the method the
substrate is then transported to a curing means such as an energy source
which emits an electron beam, UV-light radiation or exposure to an electro
magnetic field. Alternative the curable monomer can also be transferred into
~5 radicals by passing through a plasma zone (zone of high voltage discharge).
The curing of the monomer by the curing means then provides a coating on
the substrate surface which has a static water contact angle of more than
95°.
2o The method for delivering the curable monomer to the substrate for
minimizing the amount of monomers can use an ultrasonic atomizer
producing micro droplets of curable monomer. They are released into a
vaporization tube heated by band heaters. The atomized droplets impinge
on the inner wall of the vaporization tube and are instantaneously vaporized,
25 i.e., flash vaporized. This reduces the opportunity for polymerization
prior to
being deposited on the substrate.
In one aspect of the present invention, the substrate can be one side
water-repellent and capable of absorbing and storing fluids from the other
side, or alternatively be repellent on both sides.
DETAILED DESCRIPTION OF THE INVENTION
As discussed in detail below, a preferred embodiment of the present
invention relates to a method of providing a durable water-repellent coating
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to a substrate material, using a high speed electron beam radiation process.
The terms "hydrophobic" and "water-repellent" are used interchangeably and
refer to surfaces that are repellent to aqueous liquids (e.g., body liquids)
deposited on these surfaces. Hydrophobicity and wettability are defined in
terms of static water contact angle and alternative the surface tension of the
liquids and solids involved. This is discussed in detail in the American
Chemical Society publication entitled Contact Angle, Wettability and
Adhesion, edited by Robert F. Gould (Copyright 1964). A surface is
hydrophobic if the static water contact angle is greater than 90° and
the
liquid does not spread spontaneously across the surface of the web. In
general, the higher the contact angle between the surface and the liquid, the
more hydrophobic the surface.
The coatings of the present invention are "durably hydrophobic",
~5 insofar as the hydrophobic character engendered to the substrate is
maintained after exposure to liquids and humid air.
The coating formed by the method of the present invention has a
thickness of less than 5 microns, and preferably less than 2 microns and
2o most preferably in the range of 0.001 to 1 microns. The coatings are formed
by depositing a vapor of curable monomer, under vacuum, on a movable
substrate which is mounted in thermal contact with a support, for continuos
processing preferably a rotating drum, which is maintained at a temperature
below the boiling point of the vaporized monomer under the environmental
25 conditions in vacuum chamber . As a result of this temperature
differential,
the monomer vapor condenses on the surface of the substrate. The
monomer materials utilized in the present invention are relatively low in
molecular weight, between 150 and 1000 Atomic Mass Units (AMU) , and
preferably in the range 200 to 300 AMU. Polyfunctional flurocarbons and
3o especially fluoroacrylates Dr mixtures of monofunctional fluoroacrylates
and
polyfunctional fluoroacrylates are preferred. The monomers or monomer
mixtures employed have an average of about two or more double bonds
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(i.e., a plurality of olefinic groups) and have a vapor pressure such that
they
condense on the substrate surface. Such vapor pressures are for example
pressure between about 1.33 10-6 mbar and 1.33 10-' mbar, most preferably
a vapor pressure of approximately 1.33 10-2 mbar at standard temperature
and pressure, (i.e., relatively low boiling materials) are selected.
These high-vapor-pressure monomers can be flash vaporized already
at low temperatures and thus are not degraded (cracked) by the heating
process. The absence or low amount of unreactive degradation products
~o results in coatings with a reduced levels of volatile components in which
substantially all of the deposited monomer is reactive and will cure to form
an integral film when exposed to a source of radiation. These properties
make it possible to provide a substantially continuous coating despite the
fact that the deposited film is very thin. The cured films exhibit excellent
adhesion and are resistant to chemical attack by organic solvents and
inorganic salts.
The process according to the present invention can continuously
create substrate surfaces having a hydrophobic characteristic such that the
2o static water contact angle is more than 95°, preferably more than
about
120°, more preferably higher than about 130° and most preferably
between
about 150° and about 165°. The static water contact angle value
can be
measured with a water contact angle goniometer. The measurement is done
on a flat, i.e. plane, and smooth surface of a substrate after coating. The
25 term smooth as used herein for water contact angle measurements refers to
a roughness of no more than 5 microns in accordance with standard
roughness measurements on continuous surfaces.
Curable fluoro-containing monomer
3o According to the present invention, the high speed vacuum coating
process for producing water vapour permeable substrates with exceptional
water repellent properties or on either one side or both sides requires a
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curable monomer component. Desirably, the curable monomer for obtaining
water-repellent coatings comprises fluoro-containing group.
In one emodiment, any suitable fluoromonomer may be used,
including, but not limited to, fluoroacrylate monomers, fluoro olefin
monomers, fluorostyrene monomers, fluoroalkylene oxide monomers (e.g.,
perfluoropropylene oxide, perfluorocyclohexene oxide), fluorinated vinyl alkyl
ether monomers, and the copolymers thereof with suitable comonomers,
wherein the comonomers are fluorinated or unfluorinated. Fluoromonomers
which are polymerized by a free radical polymerization process are
o preferred.
In one embodiment, fluorostyrenes and fluorinated vinyl alkyl ether
monomers which may be used in the method of the present invention
include, but are not limited to, a -fluorostyrene; [i -fluorostyrene; a, [3 -
difluorostyrene; ~, ~ -difluorostyrene; a, [3, ~ -trifluorostyrene; a -
~5 trifluoromethylstyrene; 2,4,6-Tris (trifluoromethyl)styrene; 2,3,4,5,6-
pentafluorostyrene; 2,3,4,5,6-pentafluoro- a -methylstyrene; and 2,3,4,5,6-
pentafluoro- [3 -methylstyrene.
In yet another embodiment, tetrafluoroethylene can also be used in
the method of the present invention and include, but are not limited to,
2o tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene
perfluorovinyl ether copolymers (e.g., copolymers of tetrafluoroethylene with
perfluoropropyl vinyl ether), tetrafluoroethylene-ethylene copolymers, and
perfluorinated ionomers (e.g., perfluorosulfonate ionomers;
perfluorocarboxylate ionomers).
25 In still another embodiment, fluorocarbon elastomers (see, e.g., 7
Encyclopedia of Polymer Science & Engineering 257) are a group of fluoro
olefin polymers which can also be used in the process of the present
invention and include, but are not limited to, poly(vinylidene fluoride-co-
hexafluoropropylene); poly(vinylidene fluoride-co-hexafluoropropylene-co-
3o tetrafluoroethylene); poly[vinylidene fluoride-co-tetrafluoroethylene-co-
perfluoro(methyl vinyl ether)]; poly[tetrafluoroethylene-co-perfluoro(methyl
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vinyl ether)]; poly(tetrafluoroethylene-co-propylene; and poly(vinylidene
fluoride-co-chlorotrifluoroethylene).
In the preferred emodiment, because of their reactivity, physical
properties, and the properties of cured films formed from such components,
fluoroacrylates are particularly useful monomeric materials. The term
"fluoroacrylate monomer," as used herein, refers to esters of acrylic acid
(H2C = CHCOOH) or methacrylic acid (H2C = CCH3 - COOH) , where the
esterifying group is a fluorinated group such as perfluoroalkyl. A specific
group of fluoroacrylate monomers useful in the method of the invention are
o compounds represented by formula (I): H2C = CRS - COO (CHZ)~ R2 (I)
wherein: n is 1 or 2;
R~ is hydrogen or methyl; and R2 is a perfluorinated aliphatic or
perfluorinated aromatic group, such as a perfluorinated linear or branched,
saturated or unsaturated C~ to Coo alkyl, phenyl, or naphthyl.
~5 In a particular embodiment of the invention, R2 is a C~ to Ca
perfluoroalkyl or - CH2 - NR3 - S02 - R4, wherein R3 is C~-C2 alkyl and R4 is
C~ to C8 perfluoroalkyl. The term "perfluorinated," as used herein, means
that all or essentially all hydrogen atoms on an organic group are replaced
with fluorine. Monomers illustrative of Formula (I) above, and their
2o abbreviations, include the following:
2-(N-ethylperfluorooctanesulfonamido) ethyl acrylate ("EtFOSEA");
2-(N-ethylperflooctanesulfonamido) ethyl methacrylate ("EtFOSEMA");
2-(N-methylperfluorooctanesulfonamido) ethyl acrylate ("MeFOSEA");
2-(N-methylperflooctanesulfonamido) ethyl methacrylate ("MeFOSEMA");
25 1,1-Dihydroperfluorooctyl acrylate ("FOA"); and
1,1-Dihydroperfluorooctyl methacrylate ("FOMA").
Alternatively, the curable monomer component can also include
polyfunctional acrylates, which are set forth in U.S. Patent 4,842,893,
incorporated herein by reference.
Substrate
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The substrate for coating according to the process of the present invention
can be any substrate which supports water vapour transport through it. The
substrate can for example be an apertured film which allows free air
circulation through the apertures or a woven or non-woven which also allows
5 free air circulation through the pores of the woven or non-woven. The
substrates can however also be impermeable to air passage such as
microporous substrates (usually Ca filled films which are stretched) or
vapour transmitting monolithic films. Particularly preferred are substrates
which can be employed in garments or absorbent articles such as those
o listed above. Substrates can be coated from one or both sides and the
coating can be identical or different depending on the desired final use of
such substrates.
Due to the low cost and continuous process ability according to the present
invention the substrates are useful in disposable articles such as diapers,
sanitary napkins, pantiliners, adult incontinence products, under arm sweat
pads, breast pads, shoe inserts, and so for. The substrate can either be
used in a position in these articles where it supports the containment
function of an impermeable sheet in which case the substrate needs to
2o provide water vapour transport without allowing liquid transport under the
usual usage conditions of the article. On the other hand such substrates can
also be used as liquid permeable substrates on the liquid receiving sides of
such articles so that liquid deposited on the surface of the substrate is
transported quickly through the substrate towards an absorbent structure
from which liquid cannot or only under extreme conditions be moved back
towards the user of such an article (rewet prevention). Substrates coated
according to the present invention also can find utility in protective
garments
such as raincoats or gloves or hats or generally apparel which is beneficially
provided with water vapour permeability.
Accordingly, having thus described the invention in detail, it will be obvious
to those skilled in the art that various changes may be made without
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departing from the scope of the invention and the invention is not to be
considered limited to what is described in the specification.
