MXPA01002425A - Modulated plasma glow discharge treatments for making superhydrophobic substrates - Google Patents

Abstract

The present invention deals with a method of treating polymeric or non polymeric articles for making their surface superhydrophobic, i.e. characterized by static water contact angle (WCA) values higher than about 120°, preferably higher than 130°, more preferably higher than 150°. The method consists of a modulated glow discharge plasma treatment (1) performed with a fluorocarbon gas or vapor compound fed in a properly configured reactor vessel where the substrates are positioned. The plasma process deposits a continuous, fluorocarbon thin film with superhydrophobic surface characteristics, tightly bound to the substrate. The substrates of interest for the present invention may include a wide range of materials in form of webs, tapes, films, powders, granules, woven and non-woven layers;substrates can be porous or non-porous, molded or shaped, rigid or flexible, made of polymers, textiles, papers, cellulose derivatives, biodegradable materials, metals, ceramics, semiconductors, and other inorganic or organic materials. Preferably, the substrate is formed into a desired shape or configuration, depending on its intended use, before being subjected to the treatment object of this invention. When organic synthetic resins are chosen, such substrate materials could be fabricated from polyethylene, polyacrylics, polypropylene, polyvinyl chloride, polyamides, polystyrene, polyurethanes, polyfluorocarbons, polyesters, silicon rubber, hydrocarbon rubbers, polycarbonates and other synthetic polymers. A particularly preferred polymeric substrate is polyethylene or polypropylene.

Description

TREATMENT OF DISCHARGE 'LUMINESCENT OF MODULATED PLASMA TO PREPARE SUPERHYDROPHOBIC SUBSTRATES FIELD OF THE INVENTION The present invention relates to a method for making super hydrophobic substrates.

BACKGROUND OF THE INVENTION Fluorocarbon coatings deposited by plasma are frequently cited in the literature as "Teflon-type coatings" because their composition CFx (0 <x> 2) and surface energy can be made very close to those of polytetrafluoroethylene (PTFE) , - (CF2-CF2-) n), known in the trade as Teflon ®. Plasma coating processes of metals, polymers, and other substrates, with fluorocarbon films are known in the art. As an example, it is known from U.S. Patent No. 4,869,922 and from other sources, that deposition from continuous (i.e., unmodulated) discharges of radio frequency (RF) luminescence fed with fluorocarbons they provide films, layers, tapes, plates and differently shaped articles made of plastics, metals or other materials, with a thin fluorocarbon coating, without other materials interposed between the coating itself and the substrate. These coatings are claimed to have a very good adhesion in the processed articles, to avoid free space, not to be porous, and to show characteristics of controlled wettability, which depends on the aforementioned patent that j ^^ a ^^ produces coatings characterized by static water contact angle (WCA) values of less than 120 °. Luminescent discharge treatments are also considered in U.S. Patent No. 5,462,781 to improve the binding capacity of an implantable polymer medical device or to change the wettability of the polymeric fabric. Several of the references discussed in this patent confirm the continuous, non-modulated plasma treatments as a means of varying the WCA inherent in a surface. U.S. Patent No. 5 034 265 discloses a continuous, unmodulated plasma treatment to improve the biocompatibility of vascular grafts with CFx fluorocarbon coatings deposited on the inner wall of the grafts in an appropriate plasma reactor fed with tetrafluoroethylene (C2F, TFE) at 0.2 Torr. In the preferred embodiment of the invention, no other materials are interposed between the substrate and the coating.

BRIEF DESCRIPTION OF THE INVENTION Specifically, the present invention, having the characteristics mentioned in the appended claims, relates to a modulated plasma deposition process for coating substrates with a thin, non-porous, very adherent, thin fluorocarbon coating with superhydrophobic properties, i.e., characterized by static water contact angle (WCA) values, measured on a smooth, flat surface, greater than about 120 °, preferably above 130 °, more preferably above 150 °. The substrates treated with this method have their hydrophobic capacity markedly improved, for example, They can be effectively made waterproof while maintaining their previous characteristics such as permeability to gases and vapors. The increased hydrophobic capacity also results in additional benefits such as preventing the accumulation of dirt (for example, on 5 hard surfaces such as glass, ceramics, metals and other surfaces exposed to dust), avoiding the accumulation of powders or granules, helping to empty completely containers containing hydrophilic materials such as liquid detergents or shampoo bottles or beverage containers or beverage tanks or flowable particle tanks, for example, flour tanks, avoid contamination and accumulation on brushes and bristles, also using a metallic electrode made of an antibacterial metal such as silver or gold, in the method according to the present invention an antibacterial property can be provided to the coated surfaces. The present invention addresses a method for treating polymeric or non-polymeric articles to make their surface superhydrophobic, that is, characterized by static values of the water contact angle (WCA) greater than about 120 °, preferably greater than 130 °, so more preferable greater than 150 °. The method consists of a modulated luminescent discharge plasma treatment performed with a gas or fluorocarbon vapor compound fed into an appropriately configured reactor vessel where the substrates are placed. The plasma process 20 deposits a thin, continuous fluorocarbon film with superhydrophobic surface characteristics, tightly bound to the substrate. Substrates of interest for the present invention may include a wide range of materials in the form of wefts, tapes, films, powders, granules, particles, woven and nonwoven layers.; the substrates can be porous or not porous, molded or formed, rigid or flexible, made of polymers, textiles, papers, cellulose derivatives, biodegradable materials, metals, ceramics, semiconductors, and other inorganic and organic materials. Preferably, the substrate formed in a desired shape or configuration, depending on its intended use, before being subjected to the treatment object of this invention. When organic synthetic resins are chosen, these substrate materials can be manufactured from polyethylene, polyacrylics, polypropylene, polyvinyl chloride, polyamides, polystyrene, polyurethanes, polyfluorocarbons, polyesters, silicone rubber, hydrocarbon rubbers, polycarbonates, and other polymers. synthetic "Plasma", as used herein, is used in the sense of low temperature plasma or "cold plasma" produced by connecting a luminescent discharge in a low pressure gas through a power source. Luminescent discharges contain a variety of chemically active and sufficiently energetic species to cause chemical reactions with the exposed surfaces, ie the covalent bond to a suitable substrate material. Cold plasmas, or luminescent discharges, are usually produced by high-frequency power sources (KF plasmas) (KHz to MHz and GHz). Electrons, positive and negative ions, atoms, excited molecules, free radicals, and photons of different energies are formed in a cold plasma. "Modulated plasma" means a non-continuous plasma, HF plasma, ie a luminescent discharge whose driving force is pulsed between a maximum and zero value (pulse on / off) or a fraction thereof, at a certain frequency, with a appropriate pulse generator connected to the main power supply. In the case of pulsed on / off systems, the values at the time of connection and at the time of disconnection are among the experimental parameters of the process. By superimposing a trigger pulse on / off to the main high frequency field which generally conducts a luminescent discharge, the short continuous discharges alternate with the plasma intervals of the moment off where active species still exist in the gas phase, but the effects of ions and electrons are strongly reduced. This alternating exposure to two different processes leads to the unique modifications of the surface of the substrates, which are very different from those of the continuous processes * 5 plasma, as will be demonstrated. "Plasma deposition" or "plasma polymerization" is the plasma process that leads to the formation of continuous, gap-free, partially entangled, thin (0.01 - 2 μm), well adherent substrates. The molecules of the gas phase are fragmented by energetic electrons, which are able to break chemical bonds; This process leads to radicals and other chemical species that are able to deposit on the surfaces inside the vacuum chamber and form a thin, uniform film. The action of the plasma can also affect the surface of a polymeric substrate at the early deposition time; energetic species can break bonds in the substrate with the possible evolution of gaseous products, such as hydrogen and the formation of free radical sites which contribute to covalent bonds between the growing film and the substrate. It has been found that it is possible to deposit thin films of fluorocarbon with superhydrophobic characteristics, that is to say showing a surprisingly high WCA value, even up to about 165 °. The present invention therefore provides a modulated plasma process for coating substrates of the type mentioned above, with fluorocarbon films characterized by a WCA value of greater than 120 °, preferably greater than 130 °, more preferably greater than 150 °, using a Modulated plasma process, as will be described. According to the present invention, the fluorocarbon coatings with the F / C ratio of about 1.50 to about 2.0 have been deposited, characterized by WCA values greater than about 120 °, such as between approximately 155 ° and approximately 165 °. The coatings have been deposited on the surface of the different polymer and non-polymeric substrates such as polyethylene (PE), polypropylene (PP) polyethylene terephthalate (PET), and paper in the form of films and fabrics, glass and silicone, among many. It should be noted that the F / C ratio can theoretically be up to 3, if the coating would be formed only by a mono-molecular layer of the CF3 groups. However, the formation of the intermolecular lattices and the formation of demands (containing fragments of CF2) which are grafted onto the surface reduces the previous theoretical value in such a way that the coatings obtained, regardless of the fact that they contain many CF3 groups, they have a global F / C ratio within the range of about 1.50 to about 2.00. The thickness of the coatings depends on the duration of the plasma process under different conditions, and can be maintained between 0.01 and 2 μm. It has been found that the nature of the substrate materials does not influence either the chemical composition or the thickness of the coatings. Coatings with values of WCA of up to approximately 165 ° (for example 165 ° + 5 °) were obtained. The substrate to be treated is subjected to the discharge of plasma gas modulated in the presence of at least one gas or fluorocarbon vapor. Specifically, fluorocarbon gases or vapors such as tetrafluoroethylene (TFE, C F4), hexafluoropropene (HFP, C3F6), perfluoro- (2-trifluoromethyl-) pentene, perfluoro- (2-methylpent-2-ene) or their trimers may be used, the TFE being the currently preferred option. The plasma deposition process is preferably performed by placing the substrate inside a properly arranged plasma reactor, connecting the reactor to a source of a fluorocarbon gas or vapor, regulating the flow and pressure of the gas inside the reactor, and maintaining a luminescent discharge in the reactor with a high frequency electric field in a pulsed mode (modulated) by ^^^ i means of a suitable pulsed power supply. The parameters that define the luminescent discharge treatment include the gas or feed vapor, its flow rate, its pressure, the position of the substrate inside the reactor, the design of the reactor, the frequency of excitation of the power supply, the energy of input, the connection time and the disconnection time of the pulsation system. Substrates, such as those listed in the abstract, can be placed in the "luminescent" region of the discharge, ie directly exposed to the plasma, or in the "persistent luminescence" region, ie, downstream from the visible luminescence. The two positions generally result in coatings with different composition and properties; treating the substrates with modulated luminescent discharge also results in different coatings with respect to the continuous treatments.

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed description the invention will be described, purely by way of example, with reference to the figures of the accompanying drawings in which: Figure 1 compares a conventional "continuous" luminescent RF discharge with a "modulated" RF luminescent discharge. connection / disconnection; Figure 2 depicts a typical scheme of a plasma reactor adapted for use within the context of the invention; Figure 3 shows an ESCA signal C1s of an uncoated polyethylene substrate wherein the signal is due solely to the C-H, C-C bonds of the substrate; Figure 4 shows an ESCA signal C1s of the PE substrate coated with fluorocarbon coating deposited as described in example 1 (luminescent position, continuous mode), with the WCA of 100 + 5 °; the signal is composed , *. by the components due to the CF3, CF2, CF and CCF bonds of fluorocarbon coating, and to the C-H, C-C bonds due to surface contamination; Figure 5 shows an ESCA signal C1s of the PE substrate coated with a fluorocarbon coating deposited as described in example 1 (position after luminescence, continuous mode), with the WCA of 120 + 5 °; the signal is composed of the components due to the CF3, CF2, CF and CCF bonds of fluorocarbon coating, and to the C-H, C-C bonds due to surface contamination; and Figure 6 shows an ESCA signal C1s of the PE substrate coated with a fluorocarbon coating deposited as described in example 1 (luminescent position, modulated mode), with the WCA of 165 + 5 °; the signal is composed of the components due to the CF3, CF2, CF and CCF bonds of fluorocarbon coating, and to the C-H, C-C bonds due to surface contamination.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 compares a conventional "continuous" plasma (Figure 1a) with the modulated process of the invention, (Figure 1b) showing the pulsed alternating plasma connection with disconnection plasma times (ie, without plasma). The two processes are schematized referring to their driving signals. The reactor 1 schematically shown in Figure 2 was used not exclusively to develop the deposition method object of the present invention.

The reactor vacuum chamber 1 made of Pyrex glass is provided with an external RF energized electrode 2 and an internal electrode connected to ground to 3. The external electrode is connected to a power source 4 (typically a radio frequency generator). operating at eg 13.56 MHz) through an adaptation network and a pulse on / off pulse generator 5. The substrates can be treated in the "luminescent" region of the reactor, on the electrode connected to ground 3, as well as in its persistent "luminescence" position, that is to say in a persistent 5 luminescence substrate fastener 6. The gas / vapor is fed through an appropriate mass flowmeter through a gas / steam supply manifold 7, and its measured pressure at the outlet of the pump 8 of the reactor, maintained at a certain constant value with a manual valve in the vacuum connection between the reactor and its pumping unit. Even though the arrangement shown in the drawings represents a currently preferred option, Those skilled in the art will immediately recognize that pulsed energization of the plasma reactor can be achieved by different means such as direct energization by means of pulsed RF generators commonly used in radar and telecommunications techniques. Preferably, the deposition process is performed with a generator RF (13.56 MHz). The RF energy supplied to the external electrode of the reactor is maintained in the range of 1-500 Watts at an energy density of 0.02-10 Watt / cm2. The reactor is fed with a fluorocarbon compound at a flow rate of 1-100 sccm and is maintained at a constant pressure of 50-1000 mTorr during the process. Preferably, the luminescent discharges are modulated through the pulse generator, preferably at the values of the connection time 1-500 ms and the disconnection time 1-100 ms with the respective values of approximately 10 ms and approximately 190 ms which is the very preferred option here. The deposition process can vary from a few seconds to many hours; During this time, a uniform coating of fluorocarbon is deposited on the substrates placed in the luminescence as well as those in the persistent luminescence region. The deposition rate, a typical rate that is in the range of 20 - 400 Á / min, was measured weighing (weight / time) substrates before and after discharge, or by measuring the thickness of the coatings (thickness / time) with an Alpha stage profilometer. The deposition rate and the chemical composition of the coating depends on the experimental conditions (pressure, energy, substrate position, connection time, disconnection time, gas supply and gas flow velocity) of the discharge. The coatings or coatings obtained are uniform over the total surface of the substrate, when they are deposited on smooth (ie flat) substrates, their hydrophobic character has been estimated through the static value of WCA, as measured with a WCA goniometer. . The measurement is made on a flattened, ie, flat, and smooth surface of a substrate after coating. The term "smooth" as used herein for measurements of the contact angle of water refers to a roughness of no more than 5 microns in accordance with measurements of standard roughness on continuous surfaces. The WCA values in the range of approximately 120 ° to approximately 165 °, corresponding to a voltage superficial criticism lower than that of PTFE (18 dines / cm) have been measured for CFx fluorocarbon coatings, when x varies from approximately 1.50 and approximately 2.00. The chemical composition of the coatings is preferably determined by electronic spectroscopy for chemical analysis (ESCA) within the sampling depth of the technique (approximately 100 Á). The The adhesion of the coating or coating to the substrate is very good. The following examples are given for the purpose of further illustrating the inventive concept of the present invention, and to highlight the advantages of using modulated treatments over continuous ones.

EXAMPLE 1 Three sets of silicone substrates, PE and PP, from areas in the range of 1-20 cm2 per substrate, were placed on the ground-connected electrode 3 of the rector schematized in Figure 2. A similar set of substrate was placed in the persistent luminescence position in 6. The C2F was set to continuously feed the reactor at 6 sccm, and the pressure was set at 300 mTorr. The RF generator was connected to the reactor and was allowed to maintain the discharge with 50 Watt of input power for 90 minutes, then shut down. Another luminescent discharge was subsequently executed with a similar set of substrates placed in the luminescent position and without substrates in the persistent luminescent position, under the same conditions described above except for the fact that the modulation was carried out at the time of connection of the luminescence. 10 ms and at disconnection time of 190 ms through the pulse generator. At the end of the two discharges, the substrates were removed from the reactor and their WCAs were measured. The WCA values shown in Table 1 were found, which are compared with the WCA values of the unprocessed substrates. A deposition rate of 30 + 5 A / min was measured for the coatings or coatings deposited in the modulated mode. Other substrates, treated in both modes, were analyzed with the ESCA technique. Its surface composition turned out to be totally composed of carbon and fluoride (fluoride as element), according to the results shown in Tables 2a-c. No other elements could be detected (for example, Si for silicone substrates), which means that the coatings are continuous. The C1s spectrum of the uncoated PE substrate is shown in Figure 3, while the C1s spectrum of the PE samples coated as described above are shown in Figures 4, 5 and 6, respectively. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ g ^^^^^^^^^^ _ ^ g ^^^^ »? ^^^^^^^^^ Table 1 Figure 2a ESCA results from the continuous discharge (luminescent position) of example 1 Table 2b ESCA results from the continuous discharge (persistent luminescence position) of the example 1 Table 2c ESCA results from the modulated discharge (luminescent position) of example 1 EXAMPLE 2 Three sets of glass, silicone and PE substrates are placed in areas in the range of 2-10 cm2 per substrate, on the ground electrode 3 of the reactor schematized in Figure 2. A similar set of substrates was placed in the persistent luminescence position, C3F6 was set to continuously feed the reactor at 5 sccm, and the pressure was set at 300 mTorr. The RF generator was connected to the reactor and was allowed to maintain the discharge with 50 Watt of input power for 60 minutes, then it went out. Another luminescent discharge was subsequently executed with a set of similar substrates placed in the luminescence position and without substrates in the persistent luminescence, under the same conditions described above except for the fact that the modulation was performed at the 10 ms connection time and at the 90 ms disconnection time through the pulse generator. At the end of the two discharges the substrates were removed from the reactor and the WCA was measured. The WCA values shown in Table 3 were found, which were compared with the WCA values of the unprocessed substrates. A tax of deposition of 70 + 5 Á / min was measured for the coatings deposited in the modulated mode. Other substrates, treated in both modes, were analyzed with the ESCA technique; its surface composition turned out to be totally composed of carbon and fluoride (fluoride as element) according to the results shown in tables 4a-c. Also for this case, since no other elements were detected (for example, Si for silicone and glass substrates), it was assumed that the coatings are continuous.

Table 3 Table 4a ESCA results from the continuous discharge (luminescence position) of example 2 Table 4b ESCA results from the continuous discharge (persistent luminescence position) of the example 2 Table 4c ESCA results from the modulated discharge (luminescence position) of example 2.

EXAMPLE 3 Three sets of polished silicone, polyethylene terephthalate (PET), and FAM substrates 3 mm thick (Functional Absorbent Material), a hydrophilic absorbent material made in accordance with the teachings of U.S. Patent No. 5 260 345, areas in the range of 2-10 per substrate, were placed in the electrode connected to ground 3 of the reactor schematized in Figure 1. The C2F4 was fixed ^ gH ^^ I ^ g & j to continuously feed the reactor at 5 sccm, and the pressure was set at 400 mTorr. The RF generator was connected to the reactor and was allowed to sustain the discharge for 20 minutes in the modulated mode (connection time of 10 ms, disconnection time 190 ms) with 75 Watt of power supply. At the end of the discharge the substrates were removed from the reactor, and their WCA was measured. The values shown in Table 5 were found, which were compared with the WCA values of the unprocessed substrates. A deposition rate of 300 + 10 A / min was measured. Other substrates were analyzed by ESCA; its surface composition turned out to be totally composed of carbon and fluoride (fluoro as element), according to the results shown in table 6. No other element has been detected (for example Si for silicone substrates, and O for PET substrates), in this way it can be assumed that the coatings are continuous. The coated FAM substrate was cut along its thickness, and the freshly cut surface, which was not directly exposed to the discharge, analyzed by the WCA and ESCA measurements. The data shown in table 7 show that the coarse sample of FAM was treated not only on the surface exposed to luminescence but also within its volume, which shows that the plasma treatment is able to penetrate through the substrates porous Table 5 Figure 6 ESCA results from the modulated discharge (luminescent position) of example 3 Table 7 ESCA results from the treated sample of FAM of Example 3 cut off just after treatment 10

Claims (14)

1. A method for treating substrates including the step of exposing the substrate to the luminescent discharge of plasma in the presence of a gas or steam. 5 . fluorocarbon, characterized in that the plasma is generated as a modulated luminescent discharge.

The method according to claim 1, characterized in that the luminescent plasma discharge is generated as a luminescent discharge of radio frequency modulated plasma.

3. The method according to claim 1, characterized in that the luminescent plasma discharge is generated in a modulated mode including the subsequent connection time and disconnection time intervals.

4. The method according to any of the preceding claims, characterized in that the substrate is exposed to the discharge within the 15 luminescent plasma region (3).

The method according to any of claims 1 to 3, characterized in that the substrate is exposed to discharge in its persistent luminescent region (6).

The method according to claim 2, characterized in that the luminescent discharge is generated using radio frequency energy of between about 1 and about 500 Watts.

The method according to any of the preceding claims, characterized in that the gas is selected from the group consisting of tetrafluoroethylene, hexafluoropropene, perfluoro- (2-tpfluoromethyl-) pentene, perfluoro- (2-methylpent-2-ene) and their trimers, preferably tetrafluoroethylene.

8. The method according to any of the preceding claims, characterized in that the fluorocarbon gas is maintained at a pressure between about 50 mTorr and about 1000 mTorr.

The method according to any of the preceding claims 5 - characterized in that the substrate is selected from the group consisting of polyethylene, polyacrylics, polypropylene, polyvinyl chloride, polyamides, polystyrene, polyurethanes, polyfluoride carbons, polyesters, silicone rubber , hydrocarbon rubbers, polycarbonates, cellulose and their derivatives, preferably a polyethylene and / or polypropylene film.

10. The method according to any of claims 1 to 8, characterized in that the substrate is made of metal or glass or ceramic or semiconductor material of combinations thereof.

11. The method according to any of claims 1 to 8, characterized in that the substrate is made of granules or particles, Preferably granules or polymer particles.

The method according to any of claims 1 to 8, characterized in that the substrate is made of a porous material, preferably a film with openings or a fibrous web or non-woven material or a porous material of particles or granules.

13. The method according to any of the preceding claims, characterized in that it includes the step of forming the substrate in a desired shape and then exposing the formed substrate to the luminescent discharge. The method according to claim 13, characterized in that the substrate formed is a hollow container and the interior of the container is exposed to 25 the luminescent discharge.