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I
Surface Coatinas
The present invention relates to the coating of surfaces, in
particular to the production of oil- and water- repellent
surfaces, as well as to coated articles obtained thereby.
Oil- and water- repellent treatments for a wide variety of
surfaces are in widespread use. For example, it may be
desirable to impart such properties to solid surfaces, such
as metal, glass, ceramics, paper, polymers etc. in order to
improve preservation properties, or to prevent or inhibit
soiling.
A particular substrate which requires such coatings are
fabrics, in particular for outdoor clothing applications,
sportswear, leisurewear and in military applications. Their
treatments generally require the incorporation of a
fluoropolymer into or more particularly, fixed onto the
surface of the clothing fabric. The degree of oil and water
repellency is a function of the number and length of
fluorocarbon groups or moieties that can be fitted into the
available space. The greater the concentration of such
moieties, the greater the repellency of the finish.
In addition however, the polymeric compounds must be able to
form durable bonds with the substrate. Oil- and water-
repellent textile treatments are generally based on
fluoropolymers that are applied to fabric in the form of an
aqueous emulsion. The fabric remains breathable and
permeable to air since the treatment simply coats the fibres
with a very thin, liquid-repellent film. In order to make
these finishes durable, they are sometimes co-applied with
cross-linking resins that bind the fluoropolymer to fibres.
Whilst good levels of durability towards laundering and dry-
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cleaning can be achieved in this way, the cross-linking
resins can seriously damage cellulosic fibres and reduce the
mechanical strength of the material. Chemical methods for
producing oil- and water-repellent textiles are disclosed
for example in WO 97/13024 and British patent No 1,102,903
or M. Lewin et al., `Handbood of Fibre Science and
Technology' Marcel and Dekker Inc., New York, (1984) Vol 2,
Part B Chapter 2.
Plasma deposition techniques have been quite widely used for
the deposition of polymeric coatings onto a range of
surfaces. This technique is recognised as being a clean,
dry technique that generates little waste compared to
conventional wet chemical methods. Using this method,
plasmas are generated from small organic molecules, which
are subjected to an ionising electrical field under low
pressure conditions. When this is done in the presence of a
substrate, the ions, radicals and excited molecules of the
compound in the plasma polymerise in the gas phase and react
with a growing polymer film on the substrate. Conventional
polymer synthesis tends to produce structures containing
repeat units which bear a strong resemblance to the monomer
species, whereas a polymer network generated using a plasma
can be extremely complex.
The success or otherwise of plasma polymerisation depends
upon a number of factors, including the nature of the
organic compound. Reactive oxygen containing compounds such
as maleic anhydride, has previously been subjected to plasnia
polymerisation (Chem. Mater. Vol. 8, 1, 1996).
US Patent No 5,328,576 describes the treatment of fabric or
paper surfaces to impart liquid repellent properties by
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subjecting the surfaces to a pre-treatment with an oxygen
plasma, followed by plasma polymerisation of methane.
However, plasma polymerisation of the desirable oil and
water repellent fluorocarbons have proved more difficult to
achieve. It has been reported that cyclic fluorocarbons
undergo plasma polymerisation more readily than their
acyclic counterparts (H. Yasuda et al., J. Polym. Sci.,
Polym. Chem. Ed. 1977, 15, 2411). The plasma polymerisation
of trifluoromethyl-substituted perfluorocyclohexane monomers
has been reported (A.M. Hynes et al., Macromolecules, 1996,
29, 18-21).
A process in which textiles are subjected to plasma
discharge in the presence of an inert gas and subsequently
exposed to an F-containing acrylic monomer is described in
SU-1158634. A similar process for the deposition of a
fluroalkyl acrylate resists on a solid substrate is
described in European Patent Application No. 0049884 Bi.
The plasma polymerisation of compounds including
fluorosubstituted acrylates in which a mixture of the
fluorosubstituted acrylate compounds and an inert gas are
subjected to a glow discharge is known.
EP 988412 Bl describes a method for producing polymer and
particular halopolymer coatings which are water and/or oil
repellent on surfaces by the plasma deposition of monomer
compounds which include carbon-carbon double bonds. The
applicants have found that the method can be extended to the
deposition of other compounds. In particular monomers which
are unsaturated in that they contain no carbon-carbon double
bonds may be employed in the process and similar
advantageous results achieved.
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According to the present invention there is provided a
method of coating a surface (substrate) with a polymer
layer, which method comprises exposing said surface to a
pulsed plasma comprising a monomeric saturated organic
compound, said compound comprising an optionally substituted
alkyl chain of at least 5 carbon atoms optionally interposed
with a heteroatom, wherein the pulses are applied at a
variable rate so as to form an oil or water repellent
coating on said substrate.
The invention also provides the use of an optionally
substituted alkane or optionally substituted cycloalkane
having at least 5 carbon atoms in the production of water
and/or oil repellent coatings by pulsed plasma deposition
methods, wherein the pulses are applied at a variable rate.
The term "saturated" as used herein means that the monomer
does not contain multiple bonds (i.e. double or triple
bonds) between two carbon atoms which are not part of an
aromatic ring. The term "heteroatom" includes oxygen,
sulphur, silicon or nitrogen atoms. Where the alkyl chain
is interposed by a nitrogen atom, it will be substituted so
as to form a secondary or tertiary amine. Similarly,
silicons will be substituted appropriately, for example with
two alkoxy groups.
Other terms used herein include "halo" or "halogen" which
refer to fluorine, chlorine, bromine and iodine.
Particularly preferred halo groups are fluoro. The term
"aryl" refers to aromatic cyclic groups such as phenyl or
napthyl, in particular phenyl. The term "alkyl" refers to
straight or branched chains of carbon atoms, suitably of up
to 50 carbon atoms in length. Derivatives of alkyl groups,
such as would be understood by "alkoxy" include such groups.
The term "heterocyclyl" includes aromatic and non aromatic
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rings or ring systems, suitably containing up to 12 atoms,
up to three of which may be heteroatoms.
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The monomeric compound used in the process of the invention
may comprise one or more optionally substituted alkyl
chains, either as part of a branched alkane or as part of a
5 more complex structure including rings and other functional
groups. These may be present either in the monomer used as
a starting material, or may be created in the monomer on
application of the plasma, for example by the ring opening
of an optionally substituted cycloalkyl monomer.
Suitable optional substituents for the monomeric compounds
of the invention include halo, cyano, nitro, oxo, epoxide,
optionally substituted cycloalkyl, optionally substituted
aryl, optionally substituted aralkyl, optionally substituted
heterocyclyl, C(O) õRl, OR', S(O)mRl, NR2R3, C(O) NRZR3,
OC ( O ) NRZR' , =NORz, -NRZC ( O ) õR2 , -NR1CONRZR' , - C=NORl , -N=CRZR3 ,
S(O) ,,NRZR' or -NRZS (0),nRl where R', RZ and R3 are independently
selected from hydrogen or alkyl, aralkyl, cycloalkyl, aryl
or heterocyclyl, any of which may be optionally substituted,
or R 2 and R3 together form an optionally substituted ring
which optionally contains further heteroatoms such as
sulphur, oxygen and nitrogen, n is an integer of 1 or 2, m
is 0 or an integer of 1-3.
Suitable optional substituents for aryl, aralkyl and
cycloalkyl and heterocyclyl groups R1, R2and R3 include
halo, perhaloalkyl, mercapto, hydroxy, alkoxy, oxo,
heteroaryloxy, alkenyloxy, alkynyloxy, alkoxyalkoxy, aryloxy
(where the aryl group may be substituted by halo, nitro, or
hydroxy), cyano, nitro, amino, mono- or di-alkyl amino,
alkylamido or oximino.
Suitable alkyl chains, which may be straight or branched,
have from 5 to 50 carbon atoms, more suitably from 6 to 20
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carbon atoms, and preferably from 8 to 15 carbon atoms,
provided at least 5 carbon atoms form a straight chain.
Monomeric compounds where the chains comprise unsubstituted
alkyl groups are suitable for producing coatings which are
water repellent. By substituting at least some of the
hydrogen atoms in these chains with at least some halogen
atoms, oil repellency may also be conferred by the coating.
Thus in a preferred aspect, the monomeric compounds include
haloalkyl moieties or comprise haloalkyls. Therefore,
preferably the plasma used in the method of the invention
will comprise a monomeric saturated haloalkyl containing
organic compound.
Particularly suitable monomeric organic compounds are those
of formula ( I )
R4 R5
1 1
R6- C C- R'
1 1
R8 R9
where R4, R5, R6' R7 and RB are independently selected from
hydrogen, halogen, alkyl, haloalkyl or aryl optionally
substituted by halo; and R9 is a group X-R10 where Rl0 is an
alkyl or haloalkyl group and X is a bond; a group of formula
-C (O) O(CHZ),Y- where x is an integer of from 1 to 10 and Y
is a bond or a sulphonamide group; or a group -
(O) pR11(O) g(CHZ) t- where Rll is aryl optionally substituted by
halo, p is 0 or 1, s is 0 or 1 and t is 0 or an integer of
from 1 to 10, provided that where s is 1, t is other than 0.
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Suitable haloalkyl groups for R4, R5, R6, R6, and R8 are
fluoroalkyl groups. The alkyl chains may be straight or
branched and may include cyclic moieties and have, for
example from 1 to 6 carbon atoms.
For R10, the alkyl chains suitably comprise 1 or more carbon
atoms, suitably from 1-20 carbon atoms and preferably from 6
to 12 carbon atoms.
Preferably R10 is a haloalkyl, and more preferably a
perhaloalkyl group, particularly a perfluoroalkyl group of
formula CZFZZ,,, where z is an integer of 1 or more, suitably
from 1-20, and preferably from 6-12 such as 8 or 10.
Where X is a group -C(O)O(CHZ)yY-, y is an integer which
provides a suitable spacer group. In particular, y is from
1 to 5, preferably about 2.
Suitable sulphonamide groups for Y include those of formula
-N (Rll) SOZ- where Rll is hydrogen, alkyl or haloalkyl such as
C1_,alkyl, in particular methyl or ethyl.
The monomeric compounds used in the method of the invention
preferably comprises an C6_25alkane optionally substituted by
halogen, in particular a perhaloalkane, and especially a
perfluoroalkane.
Compounds of formula (I) are either known compounds or they
can be prepared from known compounds using conventional
methods.
Suitable plasmas for use in the method of the invention
include non-equilibrium plasmas such as those generated by
alternating current (AC)(e.g. radiofrequencies (Rf),
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microwaves) or direct current (DC). They may operate at
atmospheric or sub-atmospheric pressures as are known in the
art.
The plasma may comprise the monomeric compound alone, in the
absence of other gases or in mixture with for example an
inert gas. Plasmas consisting of monomeric compound alone
may be achieved as illustrated hereinafter, by first
evacuating the reactor vessel as far as possible, and then
purging the reactor vessel with the organic compound for a
period sufficient to ensure that the vessel is substantially
free of other gases.
The surface coated in accordance with the invention may be
of any solid substrate, such as fabric, metal, glass,
ceramics, paper or polymers. In particular, the surface
comprises a fabric substrate such as a cellulosic fabric, to
which oil- and/or water-repellency is to be applied.
Alternatively, the fabric may be a synthetic fabric such as
an acrylic/nylon fabric.
The fabric may be untreated or it may have been subjected to
earlier treatments. For example, it has been found that
treatment in accordance with the invention can enhance the
water repellency and confer a good oil-repellent finish onto
fabric which already has a silicone finish which is water
repellent only.
Precise conditions under which the plasma polymerization
takes place in an effective manner will vary depending upon
factors such as the nature of the polymer, the substrate
etc. and will be determined using routine methods and/or the
techniques illustrated hereinafter. In general however,
polymerisation is suitably effected using vapours of
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compounds of formula (I) at pressures of from 0.01 to 10
mbar, suitably at about 0.2mbar.
A glow discharge is then ignited by applying a high
frequency voltage, for example at 13.56MHz.
The applied fields are suitably of average power of up to
50W. Suitable pulsed fields are those which are applied in
a sequence which yields very low average powers, for example
of less than 10W and preferably of less than 1W. Examples
of such sequences are those in which the power is on for
s and off for from 10000 s to 200004s.
The fields are suitably applied for a period sufficient to
15 give the desired coating. In general, this will be from 30
seconds to 3 hours, preferably from 2 to 30 minutes,
depending upon the nature of the monomer compound used and
the substrate etc.
20 Plasma polymerisation in accordance with the invention
particularly at low average powers has been found to result
in the deposition of highly fluorinated coatings which
exhibit super-hydrophobicity.
In a preferred embodiment, the pulses are applied at a
variable rate, with relatively long pulses applied, for
example of from 1 to 10 secs on initially, reducing down to
short pulses for example of from l00 s to l s on and l0 s to
1000 s off, later in the process. It is believed that such
a regime leads to improved coatings because the initial long
pulse leads to greater fragmentation of the monomer, leading
to a more disorganised and therefore strongly bonding layer
directly adjacent the substrate. Shorter late pulses means
that the upper layers deposited retain a more organised
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structure and so contain a greater number of long chains,
which are responsible for the oil and water repellency on
the surface.
5 Suitably the compound of formula (I) includes a
perfluoroalkylated tail or moiety, in which case, the
coating obtained by the process of the invention may have
oleophobic as well as hydrophobic surface properties.
10 Thus the invention further provides a hydrophobic or
pleophobic substrate which comprises a substrate comprising
a coating of a alkyl polymer and particularly a haloalkyl
polymer which has been applied by the method described
above. In particular, the substrates are fabrics but they
may be solid materials such as biomedical devices.
In a further aspect the invention provides the use of an
optionally substituted alkane or optionally substituted
cycloalkane having at least 5 carbon atoms and particularly
a perhaloalkane in the production of water and/or oil
repellent coatings by pulsed plasma deposition methods.
The invention will now be particularly described by way of
example with reference to the accompanying diagrammatic
drawings in which:
Figure 1 shows a diagram of the apparatus used to effect
plasma deposition; and
Figure 2 is a graph showing the characteristics of pulsed
wave plasma polymerisation of perfluorododecane.
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~~:amp?1e a.
P~.a.sma Pc]}~rr~erisation ~` F,~rr"l,anrnh~,r~~.~n~
Rerfluorododecane (C2,F~,) was placed into a monomer tube (I)
(Fig. 1) . A series of plasma polyrnerisati.on eyperiments
were carried out in an inductively coupled cylindrical
plasma reactor vessel (2) of Scm diameter, 470cm' volume,
base pressure of 7},10-'mbar, and with a lea}: ?-ate of better
than 2x.10-' cm3min-1. The reactor vessel (2) was connected by
way of a."vi ton" 0-rirlg (3) , a gas inlet (4) and a needle
valve (5). to the monomer tube (1).
A thermocouple pressure gauge (6) was connecttid hy way of a
Young's tap (7) to the reactor vessel (2). A further
You_ng' s tap (8) connected with an air supply and a thiid. ( )
lead to an E2M2 two stage Edwards rotary pump (not sho~m) b_,-
way of a liq-uid nitrogen cold trap (10) All connections
were grease free.
-kn L-C matching unit (il) and a power meter (12) was used to
couple the output of a 13.56 Mhz R.F. generator (13), whicn
was connected to a power supply (14), to copper coils (15)
surrounding the reactor vessel (2) . This arrangement
ensured that the standing wave ratio !'Sw?.) oi the
transmitted power to partially ionised gas in the reactor
vessel (2) could be minimised. A pulsed signal generator
(16) was used to trigger the R.F. power supply, and a
cathode ray oscilloscope (17) was used to monitor the pulse
width and amplitude. The average power <P> delivered to the
system during pulsing is given by the following formula:
<P> = p !T
` cw ~, on/ ( Ton
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where To./ (Toõ + To,f) is defined as the duty cycle and PcW is
the average continuous wave power.
In order to carry out polymerization/deposition reactions
the reactor vessel (2) was cleaned by soaking overnight in a
chloros bleach bath, then scrubbing with detergent and
finally rinsing with isopropyl alcohol followed by oven
drying. The reactor vessel (2) was then incorporated into
the assembly as shown in Figure 1 and further cleaned with a
50W air plasma for 30 minutes. Next the reactor (2) vessel
was vented to air and the substrate to be coated (19), in
this case a glass slide, was placed in the centre of the
chamber defined by the reactor vessel (2) on a glass plate
(18). The chamber was then evacuated back down to base
pressure (7.0 x 10"3mbar) .
Perfluoroalkane vapour was then introduced into the reaction
chamber at a constant pressure of -0.2mbar and allowed to
purge the plasma reactor, followed by ignition of the glow
discharge. Typically 2-15 minutes deposition time was found
to be sufficient to give complete coverage of the substrate.
After this, the R.F generator was switched off and the
perfluoroalkane vapour allowed to continue to pass over the
substrate for a further 5 minutes before evacuating the
reactor back down to base pressure, and finally venting up
to atmospheric pressure.
The experiments were carried out with average powers in the
range of from 0.3 to SOW. The XPS spectrum of the product
of a pulsed wave plasma polymer deposition onto a glass
slide was taken.
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Figure 2 shows the C(1s) XPS spectrum for a 5 minute pulsed
plasma polymerisation experiment where:- P, = 70W
Toõ = 20 s
Totf = 20000As <P> = 0.07W
The chemical composition of the deposited coating for pulsed
plasma deposition is given in Table 2 below.
Table 2
Experimental Theoretical
F:C ratio 1.86 2.17
W-CF2 group 47.9 83.3
W-CF3 group 18.5 16.7
