Patent Application
20110121107
The present invention relates to a nozzle for coating a substrate, and more particularly, to a nozzle for coating a substrate through atmospheric plasma polymerization.
In producing many articles, often it is necessary to join dissimilar materials. In many cases, one or both of the materials must have their surface properties adjusted. Adjustment of surface properties of materials in manufacturing often involves surface treatment technology. Non-limiting examples of these surface properties include surface energy, chemical inertness, conductivity, dye reception, and adhesion. Non-limiting examples of applications using surface treatment and preparation include anti-corrosion coatings on corrodible metals such as iron and steel; bio-compatible coatings on internal implants; waterproof layers on electronics; and inorganic and organic materials for tires.
Application of coatings, adhesives, sealants, and elastomers (CASE) to substrates often requires particular care in treating or pre-treating the surface to avoid adhesive failure between the substrate and the applied CASE compounds. Many CASE application processes involve steps of (a) cleaning or roughening the surface, (b) applying a primer that either bonds to the surface or etches it, and/or (c) applying an enhancement agent that adds additional bonding functionality. CASE compounds are used in industries including construction, automotive, medical, dental, labeling, electronics, and packaging.
In the automotive industry, CASE compounds are used in conjunction with rubber reinforcement processes. Disadvantageously, rubber reinforcement is susceptible to cracking due to poor adhesion of the reinforcement to the rubber. Other goals of joining dissimilar materials include improving accuracy of manufacturing, productivity, levels of automation, reliability, and/or manufacturability, while decreasing harmful side effects, quantity of materials used, and/or waste of energy and materials. For example, some CASE compounds have substantial amounts of waste. Often coupling agent primers are less than 1% active agent and 99% carrier solvent. In another example, cleaning materials have harmful side effects such as flammability and/or noxious solvents, such as isopropyl alcohol or toluene. In other cases, paint-like layers often have to air dry within 30 seconds and so use volatile solvents. Energy and money are wasted to remediate these emissions and to protect workers' health.
Plasma polymerization has been developed as a tool to modify material surfaces while improving manufacturability, levels of automation, and accuracy of manufacturing, while decreasing harmful side effects as well as waste of energy and materials. There are different types of plasmas that are defined by their output temperature, their pressure conditions, as well as the equilibration state regarding the chemistry and thermal state. For example, there are plasmas created under subambient pressure conditions. Examples include a high plasma density mode and a low plasma density mode plasma generated with a magnetron, which is typically used in physical vapor deposition. Other ambient pressure examples include glow discharge, inductively-coupled, and recombining plasmas. The glow discharge is characterized by low velocity movement of gas of a few meters/second. It features both thermal and chemical non-equilibrium. An inductively-coupled plasma has low to moderate gas movement. It features local thermal equilibria. The recombining nitrogen or air plasmas have high gas velocities of approximately 1 km/sec and feature chemical equilibria. Additional examples of classes of plasmas are determined by their ionization methods, such as microwave resonance and electrical discharge.
When plasmas are applied to high volume production processes outside a laboratory, additional manufacturability and automation considerations arise, such as speed of operation, compatibility with substrates, and contamination. In many applications, the plasma treatments occur quickly, typically on the order of nanoseconds to a few minutes, which effectively preclude batch vacuum techniques such as physical vapor deposition.
The high temperature plasmas may thermally combust or thermally shock substrates, especially ones with low thermal conductivity as well as low melting or combustion points. In addition, some surfaces are imperfect, such as those having dust, organic body oils, and debris from shipment and handling. In light of the foregoing, a method that improves accuracy of manufacturing, productivity, levels of automation, reliability, and/or manufacturability while decreasing harmful side effects, quantity of materials used, and/or waste of energy and materials for a high volume production process for preparing a surface for joining two dissimilar materials or to receive CASE compounds.
One conventional method deposits a coating on to a clean surface during a first time period, and depositing a high-velocity impact polymer reaction coating on the surface at ambient air pressure during a second time period using an atmospheric pressure air plasma (APAP). Another conventional method may apply a coating of mixed prepolymer vapor and carrier gas or mist of small droplets. That mixture may be introduced into an atmospheric pressure air plasma to form a polymer reaction compound. The polymer reaction compound may then be applied with high-velocity impact driven by the exiting gases of the APAP.
A nozzle in accordance with the present invention provides plasma polymerization. The nozzle includes a cylindrical body having a longitudinal axis, a coaxial conical inlet for receiving plasma, a radial inlet for receiving an organic precursor, a coaxial core outlet for receiving plasma from the coaxial conical inlet, and a coaxial toroidal outlet for receiving the organic precursor from the radial inlet. Outer sidewalls of the toroidal outlet extend to a bottom, outlet end of the nozzle. Inner sidewalls of the toroidal outlet do not extend all the way to the bottom end of the nozzle thereby providing partial mixing of the plasma and the organic precursor prior to deposition of polymer on to a surface.
According to another aspect of the present invention, the nozzle is constructed of a ceramic material.
According to still another aspect of the present invention, an inner end of the toroidal outlet is closed.
According to yet another aspect of the present invention, the radial inlet has a cylindrical shape.
According to still another aspect of the present invention, the release of the organic precursor at the toroidal outlet provides a shield around the plasma at the core outlet.
According to yet another aspect of the present invention, the organic precursor is transported through the radial inlet and the toroidal outlet by a carrier gas.
According to still another aspect of the present invention, the organic precursor is transported as a mist through the radial inlet and the toroidal outlet by a carrier gas.
Reference will now be made in example detail to compositions, embodiments, and methods of the present invention known to the inventors. However, it should be understood that disclosed embodiments are merely exemplary of the present invention which may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, rather merely as representative bases for teaching one skilled in the art to variously employ the present invention.
Except where expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the present invention.
The description of a group or class of materials as suitable for a given purpose in connection with the present invention implies that mixtures of any two or more of the members of the group or class are suitable. Description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among constituents of the mixture once mixed. The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.
Referring to
The gases from the exit nozzle 50 may be a spray pattern with the outer penumbra 56 having mostly ionized gas for cleaning and/or activating. Closer to the center of the spray pattern may be an area of the higher concentration 54 of high-velocity impact polymer reaction coating material. The substrate 58 receiving the high-velocity impact polymer reaction coating 64 may be a rubber reinforcement material.
The substrate 58 may be activatable by ionization and heat and may be in pristine condition, having a covering of debris, or be corroded. The substrate 58 may be cleaned, and partially activated, by an atmospheric pressure air plasma. When the atmospheric pressure air plasma is also a device depositing high-velocity impact polymer coatings, the penumbra 56 of the atmospheric pressure air plasma exiting from the nozzle 50 may have a cleaning function associated with the ionization and heat. Accordingly, the time period between the cleaning and/or activation step and the deposition step may be greater than 1 ms, 5 ms, 10 ms, 25 ms, or 100 ms.
One or more separate atmospheric pressure air plasmas may be provided to clean and/or activate the surface, followed by one or more separate atmospheric pressure air plasmas depositing high velocity impact polymer coatings. The APAPs may be operated in a sequential manner, in a parallel manner, or a combination thereof. When operated as a parallel set of multi-APAPs, typical spacing may be about 2 mm.
The cleaning and/or activating operation may be capable of operating at higher travel speeds than the deposition operation or a combined cleaning and/or activating operation, as well as a deposition operation. A cleaning operation using broader width passes and a deposition operation using raster-type passes may also be included. The cleaning and/or activating operation may be accomplished using other ionization technologies, such as corona discharge or combustion sources. The time periods between the cleaning/activation step and deposition may be greater then 0.1 s, 1 s, 5 s, 10 s, 25 s, or 100 s, and less than 150 s, 300 s, 600 s, 1800 s, 3600 s, 12 hr, 1 day, 2 days, or 5 days.
Gradients of prepolymers may be developed where an additional feedstock vessel 24 holding other prepolymers feeds through a supply line 26 to the prepolymer feedstock vessel 22 in order to incrementally adjust the ratio or ratios of the prepolymers in the feedstock. Other prepolymers may be fed through a supply line 28 to a metering device 32 that may be adjusted incrementally or step-wise based on the ratio or ratios of prepolymers.
The APAP may deliver a plasma air treatment to the substrate 58 to reactivate the substrate. For example, the substrate 58 may be cleaned and coated in one location, and then shipped to a second location for reactivation at a later time.
Plasma polymerization yields polymers in arrangements not typically found under normal chemical conditions. The polymers may be have highly branched chains, randomly terminated chains, or functional crosslinking sites. Absent are regularly repeating units, in general. This is a result of the fragmentation of the prepolymer molecules when they are exposed to the high-energy electrons inherent in the plasma. The reactions appear to proceed by several reaction pathways including free radical formation, homolytic cleavage, cationic oligomerization, and combinations thereof.
The deposit resulting from reaction in an atmospheric pressure air plasma differs from some conventional polymers, oligomers, and monomers. In some conventional monomers, oligomers, and polymers, there is a standard series of one or more building block units, also called mers. As the polymeric chains grow the building block units are repeated and occasionally cross-linked. In a plasma polymer, the building block units may be fragmented and have new functional groups developed. When they recombine, there may be generally higher crosslink density, an increased presence of branched chains, randomly terminated chains, or a combination thereof. The crosslink density calculation becomes more difficult as the number of cross links divided by the number of backbone atoms approaches unity. Such may be the case in plasma polymers. A relative measure of the crosslink density may be the shift in glass transition temperature relative to the conventional polymer. One may expect that at low degrees of crosslinking the shift upwards of the glass transition temperature will be to the number of crosslinks. In plasma polymers, the slope of the proportion may increase relatively by about 10%, 15%, or 20% compared to conventional polymers.
Prepolymers that may be suitable for deposition by atmospheric pressure air plasma include compounds that can be vaporized. The vapors may be metered and blended with a carrier gas. This mixture of gases may be introduced into a plasma generated by an atmospheric pressure air plasma. The ionization gas of the atmospheric pressure air plasma may be chosen from gases typical of welding processes which may include, but are not limited to, noble gases, oxygen, nitrogen, hydrogen, carbon dioxide, and combinations thereof. The ionization gas may also be nitrous oxide, which may allow the obtaining of a highly oxidizing plasma without use of oxygen or air.
Prepolymers used to create a high velocity impact polymer coating may include, but are not limited to, reactive substituted compounds of group 14. Candidate prepolymers do not need to be liquids, and may include compounds that are solid but easily vaporized. They may also include gases that compressed in gas cylinders, or are liquefied cryogenically and vaporized in a controlled manner by increasing their temperature.
A conventional plasma polymerization nozzle 200, as in
A ceramic nozzle 300 in accordance with the present invention provides several advantages over the conventional nozzle 200. The ceramic nozzle 300 is less prone to abrasion by the plasma 303 than a metallic nozzle. Thus, a “cleaner” polymer, containing fewer impurities, may be deposited on the substrate 58.
Further, in the ceramic nozzle 300, the organic precursor 301 is injected more downstream than the conventional nozzle 200, where the plasma 303 is less powerful, leading to milder polymerization conditions and partial retention of organic functions. The injection of the organic precursor 301 as a curtain around the plasma 303 provides a protective shield from the surrounding oxidative atmosphere. This results to the deposition of a less oxidized polymer.
Moreover, the concept of only partially mixing the organic precursor with the plasma before deposition on the substrate 58 improves plasma polymerization applications. For example, metallic substrates have been successfully coated with a plasma polymerized adhesive primer by using the nozzle 300, leading to a strong metal-rubber adhesion.
More specifically, a plasma jet operates in a highly turbulent hydrodynamic regime. The plasma is literally swirling out of a nozzle. Thus, due to its high velocity, the plasma jet may “suck up” unwanted surrounding air if a conventional nozzle (i.e., 200) is used. However, the combination of an upstream injection of a mixture carrier gas+precursor with no air shielding of the plasma plume may result in the deposition of a highly hydrophilic plasma coating regardless of the amount of precursor injected. By using a nozzle in accordance with the present invention, small amounts of precursor diluted in a carrier gas are sufficient to deposit high quality, hydrophobic functional coatings (less oxidized polymers) on a substrate.
Also, the precursor to be polymerized may be introduced either as a vapor or a mist. A vapor may be carried away simply by bubbling the carrier gas in a low boiling point precursor. A mist may allow introduction of complex blends as well as the use of a much broader range of compounds, such as high molecular weight compounds like oligomers, polymers, high boiling liquids, etc. Best results have been achieved when the viscosity of the liquid to be nebulized (pure precursor or blend of different precursors) is no more than 1.0-1.5 centipoise (viscosity of water at 25 C is 1.0). Solid particles, such as fumed silica nanoparticles, may thereby be carried away by the droplets. For this effect, the dimensions of the particles should not exceed the droplet diameter (around 1 micrometer for the nebulization process). Otherwise, particles may not be carried away.
Plasma polymerizing a mist of precursor allows the mild deposition of coatings that are not over-crosslinked and that still contain substantial amounts of fragile reactive functionalities (i.e., C═C double bonds, other saturations depending on the precursor used, etc.) that may otherwise have been destroyed in a plasma polymerization of a vapor. However, depending on the nature of the functional groups that need to be retained in the plasma polymer and the nature of the precursor, plasma polymerizing a vapor using a nozzle in accordance with the present invention is very efficient. For example, the inventive nozzle may effectively deposit plasma polymerized carbon disulfide (low boiling liquid) on nylon samples to promote adhesion to rubbers.
As shown in
The inner end of the toroidal outlet 340 is closed. The outer sidewalls of the toroidal outlet 340 extend to the bottom end of the nozzle 300. As shown in
While the best mode for carrying out the invention has been described in detail, those familiar with the art to which the present invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.
