Method and apparatus for the formation of hydrophobic surfaces

Abstract

The invention relates to the application of a coating to a substrate in which the coating includes a polymer material and the coating is selectively fluorinated and/or cured to improve the liquid repellance of the same. The invention also provides for the selective fluorination and/or curing of selected areas of the coating thus, when completed, providing a coating which has regions of improved liquid repellance with respect to the remaining regions and which remaining regions may be utilized as liquid collection areas.

Claims

1. A coated substrate, comprising: a coating applied to at least one surface of the substrate, said coating having at least an outer layer of polymer material and at least a portion of said polymer material is cured and fluorinated to provide the same with improved liquid repellent and durability characteristics, wherein a portion of the coating having the cured and fluorinated portion of the polymer material is lower in height than a remaining portion of the coating.

2. The substrate of claim 1, wherein the selective portions of the polymer material which are not fluorinated and/or cured can act as collecting areas for liquid.

3. The substrate of claim 1, wherein the substrate has defined therein a number of spaced liquid collection areas, each separated by areas of increased liquid repellence.

4. The substrate of claim 3, wherein the spaced liquid collection areas are surrounded by the areas of increased liquid repellence.

5. The substrate of claim 3, wherein the areas of increased liquid repellence form a grid pattern surrounding the spaced liquid collection areas such that adjacent liquid collection areas are separated by a portion of the grid pattern.

6. The substrate of claim 1, wherein the fluorinated and/or cured polymer material is hydrophobic and/or oleophobic.

7. The substrate of claim 1, wherein the polymer material is cured, wherein the fluorinated and cured polymer material has a water contract angle greater than about 157.

8. The substrate of claim 1, wherein the at least one surface is a horizontal surface.

9. The coated substrate of claim 1, wherein the coating further comprises an intermediate layer disposed between the outer layer and the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Specific embodiments of the invention axe now described with reference to the accompanying drawings; wherein.

(2) FIG. 1 is a graph showing the surface elemental composition of 4.5 m thick polybutadiene films which have been plasma fluorinated for 5 minutes at various RF power levels;

(3) FIG. 2 is a graph showing the RMS roughness of 4.5 m thick polybutadiene films which have been plasma fluorinated for 5 minutes at various RF power levels;

(4) FIG. 3 is a graph showing the water contact angle of 4.5 m thick polybutadiene films which have been plasma fluorinated for 5 minutes at various RF power levels;

(5) FIG. 4 illustrates a further embodiment of the invention and an infra red spectra of plasma fluorinated polybutadiene (60 W, 10 min) as a function of UV exposure time of a nonpatterned surface;

(6) FIG. 5 illustrates the embodiment of FIG. 4 showing a series of AFM height images of a UV patterned surface;

(7) FIG. 6 illustrates the embodiment of FIG. 4 showing a series of optical microscope images showing microfluidic self organisation of water droplets on patterned 236 nm thick polybutadiene film;

(8) FIG. 7 illustrates the embodiment of FIG. 4 showing optical microscope images of crystals grown on patterned polybutadiene film as a function of exposure time to nebulized mist;

(9) FIG. 8 illustrates further optical microscope images of polystyrene beads deposited into patterned polybutadiene;

(10) and FIG. 9 illustrates the embodiment of FIG. 4 with a patterned surface showing the Raman analysis of the patterned polybutadiene film.

DETAILED DESCRIPTION OF THE INVENTION

(11) In a first illustrative example, Polybutadiene (Aldrich, M.sub.w=420,000, 36% cis 1.4 addition, 55% trans 1.4 addition, 9% 1.2 addition) is dissolved in toluene (BDH, +99.5% purity) and spin coated onto silicon wafers using a photoresist spinner (Cammax Precima) operating at speeds between 1500-4500 rpm. The applied coatings axe subsequently annealed at 90 C. under vacuum for 1 hour in order to remove entrapped solvent.

(12) In accordance with the method of the invention, fluorination of the coating is, in this example, performed in a cylindrical glass, plasma reactor of 5 cm diameter, 470 cm.sup.3 volume, base pressure of 410.sup.3 mbar, and with a leak rate of better than 610.sup.9 mol s.sup.1.

(13) The reactor vessel is connected by way of a needle valve to a cylinder of carbon tetrafluoride (CF.sub.4) (Air Products, 99.7% purity).

(14) A thermocouple pressure gauge is connected by way of a Young's tap to the reactor vessel. A further Young's tap is connected with an air supply and a third leads to an E2M2 two stage Edwards rotary pump by way of a liquid nitrogen cold trap. All connections are grease free.

(15) An L-C matching unit and a power meter are used to minimise the standing wave ratio (SWR) of the power transmitted from a 13.56 MHz R.F. generator to a copper coil wound around the reactor vessel wall.

(16) In order to carry out the fluorination of the unsaturated, polybutadiene coating the reactor vessel is scrubbed with detergent, rinsed with propan-2-ol, oven dried and then further cleaned with a 50 W air plasma for 30 min. Next, the reactor is vented to air and a polybutadiene coated silicon wafer placed into the centre of the chamber defined by the reactor vessel on a glass plate. The chamber is then evacuated back down to base pressure (410.sup.3 mbar).

(17) Carbon tetrafluoride gas is admitted into the reaction chamber via a needle valve at a constant pressure of 0.2 mbar and allowed to purge the plasma reactor followed by ignition of the radiofrequency glow discharge. Typically 5-10 minutes is found to be sufficient to give complete surface fluorination of the polybutadiene coating. After this the RF power generator is switched off and carbon tetrafluoride gas allowed to pass over the sample for a further 5 minutes before evacuating the chamber back down to base pressure, and finally venting to air.

(18) Curing of the fluorinated polybutadiene films is carried out by placing them in an oven, in an atmosphere of air, at 150 C.

(19) Analysis of the coatings is achieved by using several complementary techniques. X-ray photoelectron spectroscopy (XPS) is used to obtain the elemental composition of the surfaces, and to identify various fluorinated species by means of deconvoluting the C(1s) spectra. In addition to XPS, FT-IR is used to obtain information on chemical groups present within the coating (Perkin Elmer, Spectrum One).

(20) The thickness of the polybutadiene films is measured using a spectrophotometer (Aquila Instruments, nkd-6000).

(21) The coatings are imaged by Atomic Force Microscopy (AFM) (Digital Instruments, Nanoscope III). RMS roughness values are calculated over 50 nm50 nm scan areas.

(22) The super-hydrophobicity and oleophobicity of the coatings axe investigated by sessile drop contact-angle measurements carried out at 20 C. with a video capture apparatus (A.S.T. Products VCA2500XE). The probe liquids used are high purity water (B.S. 3978 Grade 1) to determine hydrophobicity and a variety of linear chain alkanes (hexadecane, tetradecane, dodecane, decane, and octane, +99% purity, Aldrich) to evaluate oleophobicity. In the case of super-hydrophobic surfaces, the water droplets are kept stationary by the dispensing syringe. Advancing and receding contact angle values are obtained by increasing or decreasing the liquid drop volume at the surface.

(23) The increase in coating durability after curing is ascertained by Nanoindentation hardness testing, before and after crosslinking, with a Nano instruments Nano II machine equipped with a Berkovich indenter.

(24) The experiments carried out use average RF powers in the range of from 5 to 80 W. The results of the XPS analysis of 4.5 m thick polybutadiene films plasma fluorinated for 5 minutes at various powers are shown in FIG. 1.

(25) In FIG. 1 it can be seen that plasma fluorination caused the incorporation of a large amount of fluorine into the surface of the polybutadiene coating. Deconvolution of the C(1s) spectra shows that CF, CF.sub.2 and CF.sub.3 environments are present.

(26) FIG. 2 shows the RMS roughness, measured using AFM, of 4.5 m thick polybutadiene films which have been plasma fluorinated for 5 minutes at various power levels.

(27) It can be seen that the plasma fluorination results in an overall increase in the roughness of the polybutadiene coating. RF power levels below 30 W result in large undulating features. An increase in the RF power results in a diminishment of these features and their replacement with finer scale roughness. The transition between the two different morphologies is responsible for the decrease in RMS roughness at RF powers of approximately 30 W.

(28) The effect of the incorporation of fluorine and the simultaneous increase in RMS roughness upon the water repellency of 4.5 m thick polybutadiene films which are plasma fluorinated for 5 minutes at various powers is shown in FIG. 3.

(29) Plasma fluorination is therefore shown to cause a large increase in the hydrophobicity of the coating. Water contact angles exceed 157 for RF powers of above 40 W. More accurate measurement is not possible as the droplets quickly rolled off the coating, that is the surfaces displayed super-hydrophobic behaviour.

(30) The oleophobicity of the fluorinated coatings is shown by contact angle measurements with droplets of linear chain alkanes given in Table 1. The 4.5 m thick polybutadiene coating illustrated has been plasma fluorinated at an RF power of 60 W for 10 minutes.

(31) TABLE-US-00001 TABLE 1 PROBE CONTACT ANGLE/ LIQUID Equilibrium Advancing Receding Hysteresis Water 174.9 0.4 173.1 0.4 172.7 0.5 0.4 0.4 Hexadecane 118.7 0.8 119.1 1.0 30.1 1.7 89 2.0 Tetradecane 109 0.9 110.8 1.2 29.8 1.3 81 1.8 Dodecane 98.4 0.9 100.2 1.1 29.5 1.9 70.7 2.2 Decane 89.8 1.5 92.9 1.1 29.7 1.0 63.2 1.5 Octane 65.2 0.8 67.4 0.9 28.5 1.0 i 38.9 1.3

(32) The low hysteresis observed when using water as a probe liquid confirms that the coating is super-hydrophobic. In addition it can be seen that the coating is oleophobic towards a range of oils. However the large hysteresis observed with alkane probe liquids, attributable to their lower surface tensions' enabling them to wick into surface pores, shows that the coating is not super-oleophobic.

(33) After fluorination the coatings are thermally cured at 155 C. The effect of curing for 1 hour upon the repellency, roughness and surface composition of a 4.5 m thick polybutadiene coating plasma fluorinated at a RF power of 60 W for 10 minutes is shown in Table 2.

(34) TABLE-US-00002 TABLE 2 Measurement Uncured Cured Water contact angle 174.9 0.4 173.8 0.5 Decane contact angle 89.8 1.5 76.4 2 XPS % F 70 2 69 2 XPS % C 30 2 29 2 XPS % O 0 0 2 2 AFM roughness 193 5 nm 191 5 nm ARMS

(35) It can be seen that curing does not significantly affect the superhydrophobicity and RMS roughness of the coating. The slight decrease in oleophobicity is attributed to the incorporation of a small amount of oxygen.

(36) The affect of curing upon surface durability is shown in Table 3. A 4.5 m thick polybutadiene coating plasma fluorinated at a RF power of 60 W for 10 minutes was cured for 48 hours at 155 C.

(37) TABLE-US-00003 TABLE 3 Material Hardness/Mpa Uncured fluorinated of butadiene 8 1 Cured fluorinated polybutadiene 64 8

(38) It can be seen that curing results in an eight-fold increase in coating hardness over the uncured fluorinated material.

(39) The results of this illustrative example therefore illustrate the advantageous benefits which can be obtained by the method and utilisation of apparatus of the present invention. The results relate to the fluorination and curing over the entire surface of a substrate for ease of testing.

(40) However as previously discussed a further aspect of the invention is the provision of the fluorination and/or curing over selected portions of any given surface. The ability to selectively fluorinate and cure particular surfaces provides the ability to design articles for specific uses and for the surfaces to have the required characteristics in required areas. One possible use is to define portions of the surface which are not fluorinated or cured and which act as collection areas for liquids applied to the surface and which liquid is repelled from those portions which are fluorinated and cured and which typically surround and define the liquid collection areas. Thus, in use, the liquid held in each liquid collection area can define a sample to be tested. The said treated and non-treated portions are typically defined during the treatment process by the provision of masking means and/or selective printing which can be positioned relative to the surface.

(41) A specific embodiment of this selective or patterned treatment method is now described with reference to FIGS. 4-9. In this example, there is described a two-step approach for fabricating spatially ordered arrays of micron size particles and also metal salts by exposing patterned super-hydrophobic surfaces to a nebulized mist of the desired species. This entails plasmachemical fluorination of polybutadiene thin film surfaces followed by spatially localised UV curing by crosslinking and oxygenation.

(42) CF.sub.4 plasma fluorination of coating is carried out in a cylindrical glass reactor (5 cm diameter, 470 cm.sup.3 volume) connected to a two stage rotary pump via a liquid nitrogen cold trap (base pressure of 410.sup.3 mbar, and a leak rate of better than 610.sup.9 mol s.sup.1). An L-C matching unit is used to minimise the standing wave ratio (SWR) of the power transmitted from a 13.56 MHz R.F. generator to a copper coil externally wound around the glass reactor. Prior to each plasma treatment, the chamber is scrubbed with detergent, rinsed in propan-2-ol, and then further cleaned using a 0.2 mbar air plasma operating at 50 W for 30 min. A piece of polybutadiene coated substrate is then placed into the centre of the reactor, followed by evacuation to base pressure. Nex CF.sub.4 gas (99.7% purity, Air Products) is admitted into the system via a needle valve at a pressure of 0.2 mbar, and after 5 min of purging, the electrical discharge is ignited. Upon completion of plasma exposure, the system is evacuated, and then vented to atmosphere.

(43) Patterning of the fluorinated polybutadiene film surfaces entails UV irradiation (Oriel low pressure HgXe arc lamp operating at 50 W, emitting a strong line spectrum in the 240-600 nm wavelength region) through a copper grid photomask (1000 mesh, Agar Scientific.sup.c) positioned just above the polymer surface.

(44) These micro-patterned films are exposed to a nebulized aqueous mist (Inspiron nebulizer operating with a nitrogen gas flow of 3 dm.sup.3 min.sup.1) of either Cu.sub.2SO.sub.4 salt solution (0.00125 M, Aldrich) or polystyrene beads (110.sup.9 beads per ml). In the case of gold (III) chloride (Aldrich 99%), the patterned film is dipped into a 10% w/v ethyl acetate (Fisher 99%) solution for 10 min followed by rinsing in methanol to dislodge extraneous AuCl.sub.3 species.

(45) XPS surface analysis is undertaken on a VG ESCALAB MkII spectrometer equipped with an unmonochromatised Mg K.sub. X-ray source (1253.6 eV) and a hemispherical analyser. Photoemitted core level electrons are collected at a fixed takeoff angle (75 away from the sample surface) with electron detection in constant analyser energy (CAE) mode operating at 20 eV pass energy. Elemental sensitivity (multiplication) factors are taken as being C(1s) F(1s): O(1s) equals 1.00:0.35:0.45. No spectral deterioration due to X-ray radiation damage was observed during the time scale associated with data acquisition.

(46) Infrared analysis of polybutadiene films coated onto polished potassium bromide disks is carried out on a Perkin Elmer Spectrum One FTIR instrument operating in transmission mode at 4 cm.sup.1 resolution in conjunction with a DTGS detector.

(47) Sessile drop contact angle measurements are undertaken at 20 C. with a video capture apparatus (A.S.T, Products VCA2500XE) using high purity water as the probe liquid (B.S.3978 Grade 1). In the case of super-hydrophobic surfaces, the water droplets are kept stationary by the dispensing syringe. Advancing and receding contact angle measurements are made by increasing or decreasing the liquid drop volume whilst on the surface.

(48) AFM images of the patterned surfaces are acquired using a Digital Instruments Nanoscope III scanning probe microscope. Damage to the tip and substrate was minimised by operating in Tapping Mode ARM. Corresponding optical images are captured with an Olympus BX40 microscope.

(49) Raman spectroscopy and spatial mapping is performed on a Dilor Labram microscope equipped with a 1800 lines mm.sup.1 diffraction grating and a helium-neon laser excitation source (632.8 nm line operating at 11 mW).

(50) (a) UV Irradiation of Fluorinated Polybutadiene Films

(51) XPS analysis detected a small amount of oxygen incorporation (2%) at the surface following UV irradiation of the whole plasma fluorinated polymer film (no mask), Table 4.

(52) TABLE-US-00004 TABLE 4 XPS analysis of CF.sub.4 plasma fluorinated 236 nm thick polybutadiene film (60 W, 10 min) prior to and following UV exposure. Substrate % C % O % F Fluorinated 29 2 0 71 2 UV Exposure 31 2 2 2 67 2

(53) Infrared band assignments for polybutadiene are summarised in Table 5.

(54) TABLE-US-00005 TABLE 5 Infrared assignments for polybutadiene film and new absorbencies observed following UV irradiation of plasma fluorinated polybutadiene. (No changes were observed upon CF.sub.4 plasma fluorination). Frequency cm-1 Intensity* Assignment 3300-3600 A m, br OH stretch 3075 M CH.sub.2 asymmetric stretch in CHCH.sub.2; 1,2-addition 3005 B Sh CH stretch in cis-CHCH; 1 4-addition 2988 w, sh CH stretch in CHCH.sub.2; 1,2-addition 2975 Sh CH.sub.2 symmetric stretch in CHCH.sub.2; 1,2-addition 2917 Vs CH.sub.2 symmetric stretch plus CH stretch 2845 S CH.sub.2 symmetric stretch 1790 C w, sh cyclic ester 1730 C M aliphatic ester 1652 Sh CC stretch, 1,4-addition 1640 M CC-stretch in CCH.sub.2; 1,2 addition 1453 M CH.sub.2 deformation; 1,2 addition 1438 Sh CH.sub.2 deformation; 1,4 addition 1419 M CH.sub.2 in plane deformation; 1,2-addition 1406 vw, sh CH in plane deformation in cis-CHCH; 1,4- addition 1325-1350 W CH2 wag 1294-1320 W CH.sub.2 in plane rock 1238 vw, br CH.sub.2 twist 1180 D M OH bend, principally primary alcohol 1080 W, br CH.sub.2 in plane rock of CHCH.sub.2; 1,2 addition 995 S CH out of plane bending in CHCHz, 1,2 addition 967 5 CH out of plane bending in trans CHCH; 1,4- addition 911 Vs CH out of plane bending in CHCH.sub.2 727 W, br CH out of plane bending in cis CHCH; 1,4- addition 681 W Unknown; 1,2-addition *s = strong; m = medium; w = weak; v = very; sh = shoulder; br = broad These features only appear upon UV exposure

(55) No new infrared absorption features were observed following CF.sub.4 plasma fluorination of polybutadiene. This can be explained in terms of the surface sensitivity of this analytical technique being poor in transmission mode of analysis (since only the outer most layer of polybutadiene has undergone plasma fluorinationas exemplified by XPS analysis). Bulk oxidative crosslinking of these films during UV irradiation is evident on the basis of the observed attenuation of the CH stretch feature associated with the polybutadiene alkene bonds (B) and also the emergence of oxygenated groups (A, C, and D), FIG. 4 and Table 5. Corresponding water sessile drop contact angle measurements confirms the super-hydrophobic nature of plasma fluorinated polybutadiene surface, Table 6.

(56) TABLE-US-00006 TABLE 6 Water contact angle measurements following UV irradiation of CF.sub.4 plasma fluorinated (60 W, 10 min)/236 nm thick polybutadiene film. UV Contact Angle/ Exposure/mins Equilibrium Advancing Receding 0 174.9 0.4 173.1 0.4 172.7 0.5 20 173 1.0 171.6 0.5 170.8 0.4 40 172 1.2 171.4 0.5 170.0 1.0 60 170.3 1.0 171.0 0.7 169.0 0.7

(57) The improvement in surface wettability observed following UV irradiation of the fluorinated surface can be correlated to oxygen incorporation into the film, Tables 4 and 6.

(58) (b) UV Patterning of Fluorinated Polybutadiene Films

(59) In the case of UV photopatterning of the CF.sub.4 plasma fluorinated polybutadiene film, AFM indicates a drop in height for exposed square regions, FIG. 5. Immersion of these patterned films in toluene or tetrahydrofuran causes an exacerbation of the observed topography. This can be due to either solvent swelling in the unexposed (non-crosslinked) regions or improved AFM tip-surface interactions.

(60) (c) Copper Sulfate Salt and Polystyrene Microsphere Patterning

(61) It is found that during exposure to steam, water droplets undergo selective condensation onto the UV irradiated square regions of the fluorinated polybutadiene film surface, FIG. 6. Analogous behaviour is also observed in the case of a nebulized mist of aqueous Cu.sub.2SO.sub.4 solution, giving rise to selective growth of salt crystals within the patterned squares, FIG. 7. It is found that the actual crystal size can be tailored by varying the mist exposure time.

(62) In a similar fashion, exposure to a nebulized aqueous mist of polystyrene microspheres (either 0.61 m or 9.1 m diameter) produces arrays of agglomerated 0.61 m beads, or isolated 9.1 m beads in each square (since for the latter, only one bead can physically occupy an individual 14 m.sup.i diameter square), FIG. 8.

(63) (d) Gold Patterning

(64) No strong Raman absorbances are measured for the polybutadiene film. Raman spectroscopy of CF.sub.4 plasma treated and UV cured polybutadiene film followed by soaking in AuCl.sub.3/ethylacetate (10 w/v %) solution and then rinsing in methanol gives a distinct band structure between 24G-370 cm.sup.1, attributable to AuCl.sub.3 salt species, FIG. 9. Raman spectral mapping based on this spectral region confirmed selective deposition of AuCl.sub.3 into the UV irradiated squares, FIG. 9. XPS analysis of AuCl 3 soaked films, before and after UV irradiation (no patterning), shows very little gold or chlorine content on either of the films. Raman images taken of UV exposed fluorinated films without the photomask indicated the absence of AuCl.sub.3. This confirms the preference for surface energy gradients to allow entrapment of the metal salt species.

(65) Thus, from this example, CF.sub.4 plasma modification of polybutadiene film leads to fluorination in the outer surface region (i.e. the electrical discharge penetration depth) whilst the underlying polybutadiene can be subsequently crosslinked. There are several different ways in which the latter step can be undertaken: e.g. heat, UV or irradiation. In the case of UV irradiation, oxygen incorporation into the film is consistent with an oxidative cross-linking mechanism, which leads to a corresponding drop in water contact angle, FIG. 4 and Table 6. The corresponding surface roughness is not found to change markedly upon UV exposure (as also seen previously with thermal curing), thereby ruling out any observed change in water contact angle being just a manifestation of enhanced roughening. UV irradiation through a micron-scale copper grid produces a drop in height for the exposed regions, which is consistent with shrinkage of the sub-surface elastomer during cross-linking. Soaking of these films in toluene and THF (solvents for polybutadiene) exacerbates the observed height difference, due to enhanced swelling of the underlying regions of uncured polybutadiene (although a perturbation in AF1VI tip-surface interactions cannot be ruled out). The possibility of polymer removal during solvent immersion is considered to be unlikely due to the thin cross-linked top layer formed by VUV and ion bombardment during CF.sub.4 plasma treatment.

(66) Thus, the present invention allows many advantages to be obtained, firstly in the provision of surfaces which have improved liquid repellence in comparison to conventional coatings, but still achieves desirable durability characteristics. Furthermore the provision of these improved characteristics can be selectively applied to the surface to allow the substrate with said coating to be treated in a manner to improve and/or define the usage of the same.