Patent Application: US-66703308-A

Abstract:
the present invention refers to a method for preparing a superhydrophobic surface on a solid substrate comprising the steps of providing a solvent in the form of a pressurized fluid in a vessel , wherein the fluid exhibits a decrease in solvency power with decreasing pressure ; adding a hydrophobic substance to the solvent as a solute , which substance is soluble with the pressurized fluid and has the ability to crystallize / precipitate after expansion of the fluid , thereby obtaining a solution of the solvent and the solute in the vessel ; having at least one orifice opened on the vessel , thereby causing the pressurized solution to flow out of the vessel and depressurize in ambient air or in an expansion chamber having a lower pressure than within the vessel , the solute thereby forming particles ; and depositing the particles on the substrate in order to obtain a superhydrophobic surface . hereby , a pressurized fluid which expands rapidly as a result of depressurization is used to prepare the superhydrophobic surface , thereby facilitating the preparation of the surface . moreover , the invention refers to an arrangement for preparing a superhydrophobic surface on a substrate , a superhydrophobic film prepared by the method of the invention , and a substrate having deposited thereon the superhydrophobic firm .

Description:
thus , the present invention relates to a method to prepare , preferably in just one single step of treatment , superhydrophobic surfaces on substrates of commercial importance , which are made from glass , plastic , paper , wood , metal , etc . according to a presently preferred scheme of the invention , one starts by preparing a solution for treatment comprising a pressurized fluid that show a big decrease in solvency power with decreasing pressure , such as supercritical fluids , and in particular supercritical carbon dioxide . as hydrophobic solute a suitable crystallizing substance , i . e . any solid substance that ( i ) gives an intrinsic contact angle towards water above 90 °; ( ii ) is soluble in the chosen pressurized fluid ; and ( iii ) crystallizes / self organizes into particles , e . g . shaped like flakes , rods or other morphology after rapid expansion of the fluid , is used . this substance will hereafter in this document be denoted suitable crystallizing substance ( scs ). an important subgroup is waxes like akd , and other substances containing long saturated hydrocarbon chains such as stearin , stearic acid and beeswax . important requirements of the pressurized fluid are that the scs should be soluble in the fluid under pressurized conditions and that the fluid should vaporize during depressurization ( i . e . “ rapid expansion ”), thereby causing particle formation of the scs . if a supercritical fluid is used as pressurized fluid , the temperature and the pressure must then exceed the critical values for this solvent . for carbon dioxide these values are 31 . 1 ° c . and 73 . 8 atmospheres . by varying the temperature and the pressure within the supercritical range , the solvent properties ( e . g . the density ) of the fluid can be varied within wide limits . for practical reasons , however , it is usually preferable to work with solutions near the saturation levels for the selected pressurized fluid / scs combination . a review on the subject of nanomaterial and supercritical fluids is found in reference ( 5 ). see also table 1 below for critical temperature and pressure for some typical supercritical fluids . at the following treatment step , when the scs has been dissolved in the pressurized fluid , a small orifice is opened on the pressurized vessel containing the pressurized fluid / scs mixture , which makes the fluid with dissolved scs flow rapidly through one or more nozzles into the open air or into an expansion chamber of low pressure , whereby the fluid immediately vaporizes and small particles , e . g . flakes , or differently shaped micro particles of the scs are formed , preferably in the size range 10 nm to 100 μm and typically of the dimensions 5 × 5 × 0 . 1 micrometer , although other dimensions work as well . with high velocity these particles hit the substrate surface to be treated , which can be fixed or moving , and a relatively large scs - substrate contact surface is formed . the adhesion obtained by means of van der waals forces and other occurring surface forces to the substrate is usually sufficient to guarantee the sticking of the particles at practical usage . for some kinds of substrate to be treated , however , the strength of the adhesion may have to be tested by making simple peeling - off experiments with sticky tape . in the case the adhesion is deemed too poor , one might need to apply suitable surface modification steps , e . g . by increasing the roughness of the surface and / or applying an intermediate surface layer with improved binding to the surface . the high velocity of the scs is created due to the difference between the pressurized solvent / solute and the pressure in the expansion chamber , which can be 1 bar , but larger differences is preferred such as 5 , 10 , 20 , 40 , 60 , 80 , 100 , 150 , 200 , 250 , 300 , 400 , or as much as 500 bar . according to a further embodiment of the invention an alternative to the spraying process of batch type described above is provided , as a continuous process in which the scs is continuously dissolved in the pressurized fluid and sprayed onto the substrate . for instance , scs can be melted and fed by a pump into the centre of a continuous countercurrent extraction column , in which the flow of pressurized fluid goes from bottom to top . from the top of the column the scs / pressurized fluid mixture can be rapidly expanded through one or more nozzles as described for the batch process above . furthermore , the substrate can be continuously moved / rolled as is common for instance in paper manufacture industry . in this as in other embodiments of the invention the nozzle size and the opening can be varied within wide ranges , as easily determined by a person skilled in the art . as a result of our investigations we have established that although the flow rate through the nozzle is very high , some aggregation takes place of the micro - particles primarily formed in the air / expansion chamber before the wax film is finally stabilized on the substrate . the particle size distribution was obtained according to the following procedure : firstly , 200 randomly selected , well - separated particles from the sem image were measured in zoom - in mode . secondly , the particle size was calculated based on the ratio of their diameters to the sem magnification scale in matlab ; and finally , a particle size distribution histogram was drawn and the mean particle size diameter . different average sizes of the adhering wax particles can be generated by varying the temperature from close to the melting point of the scs ( around 50 ° c .) to about 100 ° c ., the pressure within the range of 100 to 500 atmospheres [ bar ] and the concentration of wax in the pressurized fluid ( here : supercritical carbon dioxide ) as well as the geometry of the nozzle , and last but not least , by varying the distance between the exit orifice of the nozzle and the substrate surface ( ca 1 - 25 cm ). the average particle sizes of collected wax particles were slightly decreased with higher pre - expansion pressure and temperature as well as with smaller spraying distance . one significant feature of the invention is that if two or more nozzles or groups of nozzles are placed on different distances from the substrate surface , different average particle sizes can be obtained — preferably a few relatively large aggregates aimed to become “ mountain peaks ”, and , in addition , a number of relatively small particles which aim to magnify the actual hydrophobic surface area per square meter enough to make the superhydrophobic surface “ robust ” in different applications . in addition , in separate experiments , the inventors have shown that in order to generate superhydrophobic properties of a wax film it is , as a rule , sufficient to attain a film thickness in the order of 10 micrometer , which due to its porosity is corresponding to approximately 1 g of wax per square meter . for the sake of comparison , in order to manufacture ordinary waxed paper ( water - repellent though , but definitely not superhydrophobic ) with a typical surface density of 100 g per square meter , about 10 g wax per square meter is needed . thus , the method according to the present invention involves a much more efficient use of the waxy component . in fig2 an electron - microscopic picture is shown of a typical film structure obtained by means of the method described . aggregated small wax flakes are loosely packed , thus giving rise to a large surface area . this appearance depends only to a minor extent on the kind of wax used . superhydrophobic wax surfaces consisting of wax flakes were successfully produced by this invention , giving average contact angles to water of above 150 degrees for all the different conditions tested in the experiments . the method shows high reproducibility as more than 80 experiments were performed , all giving surfaces with contact angles above 150 degrees . it is shown by the examples below that substrate surfaces of widely different chemical nature can be rendered superhydrophobic by means of the invention , paper , spin - coated nano - smooth cellulose surfaces , silica and carbon tape . the method is usable for rough and smooth , organic and inorganic surfaces , such as glass , porcelain , plastic , paper of different qualities , textiles , wood and materials made from wood such as chipboard , metals and painted or lacquered surfaces . furthermore , it is recognized that waxes of biological origin as well as synthetic waxes or mineral waxes can be used . moreover , it is evident that for each combination of scs and substrate it is advisable to investigate that the adhesion of the wax film is sufficiently strong by making peel tests and through exposure to water and some solvents and making simple roll - off observations . the geometry of the objects to be treated to produce superhydrophobic surfaces will in the end determine the arrangement of the set - up of nozzles and the design of the pressure vessel containing the solution . in addition to the methods disclosed above the invention also relates to the materials prepared , i . e . substrates made from a wide range of materials as discussed above , having a superhydrophobic coating as obtained by these methods . the invention will now be described by examples , which shall not be construed as limiting the scope of the invention , but merely exemplifying preferred embodiments . in all examples , a bench - scale commercial rapid expansion unit has been used ( fig3 ). all here reported examples are made with substances in the subgroup “ waxy substances ”. firstly , a certain amount of scs is loaded into the high - pressure vessel . liquid carbon dioxide from the cylinder is delivered through stainless steel tubing to the inlet of a high pressure fluid pump . compressed liquid carbon dioxide is fed to the heat exchanger prior to entering the isolated and jacketed stainless steel high pressure vessel of 0 . 1 l volume . carbon dioxide is pumped and heated to desired pressure and temperature . scs is dissolved by magnetic stirring in the pressurized and heated vessel now containing supercritical carbon dioxide . after equilibrium saturation conditions are reached typically after one hour the pressure is dropped by opening a valve before the nozzle resulting in rapid expansion of the supercritical carbon dioxide containing scs through the nozzle and into the expansion chamber in which scs precipitates and the carbon dioxide vaporizes and escapes from the bottom of the chamber . the temperature inside the nozzle and the expansion chamber decrease when carbon dioxide is expanding , but can be adjusted by flushing with heated nitrogen . spraying of scs onto a substrate placed on a desired distance from the nozzle goes on for a certain time , typically 10 seconds . the substrates are either fixed or , for certain applications , wrapped around a cylinder of 4 cm in diameter ( used in the present examples but the dimensions are not critical ) that is rotating at 120 rpm ( used in the present examples but the rate is not critical ) during the spraying . even though other possibilities certainly exists , the parameters varied in the following examples are a ) selection of scs ; b ) pressure ; c ) temperature ; d ) spraying time ; e ) type of substrate ; d ) spraying distance ; and e ) fixed or rotating sample holder . a 5 microlitre water droplet placed on the surface of untreated liner was completely absorbed after 20 seconds . after treatment with the herein described method a 5 microlitre water droplet showed a contact angle of 160 ° stable over time , which was confirmed by a control measurement after 60 seconds . a 5 microlitre water droplet placed on the surface of paper roughed with emery cloth . after treatment with the herein described method a 5 microlitre water droplet showed a contact angle of 173 ° stable over time , which was confirmed by a control measurement after 60 seconds . a very smooth cellulose surface , prepared according to reference ( 6 ), was used in this example . surfaces of this type are very thin and absorb a negligible amount of water , however , the a water droplet placed on the surface will quickly spread so that after 10 seconds it will have a contact angle of well below 10 °. a treated surface on the contrary for a 5 microlitre water droplet had a contact angle of 159 °, stable over time , and a sliding angle of 3 ° degrees . the surface of a silicon wafer was scratched with a glass cutter to obtain a rough surface . such a surface shows complete wetting because of the grooves , which work like capillaries . the treated surface showed a contact angle of 153 ° for a 5 microliter water droplet . a carbon tape of the type used for scanning electron microscopy was used as substrate for this run . a carbon tape of this kind shows a contact angle to water of 98 °, stable over time . the treated surface had a contact angle to water of 162 °, also stable over time . for untreated carbon tape see example 4a ). a contact angle measurement using a 5 microlitre droplet showed a contact angle of 157 °, as a mean value of 4 measurements . for untreated carbon tape see example 5a ). in this example , temperature , sample distance and pressure were varied . the contact angles shown in the table are mean values of at least 4 measurements , and all were stable over time controlled with one measurement taken every second for 20 seconds . japanese patent an 2006 - 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