Patent Application
20060292938
Multiple differences exist between the type of conductive materials that are suitable for use in a heater grid designed for a glass panel or window as compared to a heater grid designed for a plastic panel or window. In particular, the manufacturing process for a glass panel or window allows the conductive metallic paste used to form the heater grid to be sintered at a high temperature (>300° C.). The exposure of the metallic paste to a high temperature allows for the metallic particles in the paste to soften and fuse together, thereby resulting in sintered grid lines that exhibit a relatively high level of conductivity or low electrical sheet resistivity of less than or equal to 2.5 milliohms/square @ 25.4 μm (1 mil). In addition, this sintering process can create oxide surface functionality which allows for adequate adhesion of the sintered metallic grid lines to the surface of the glass panel or window.
In comparison, the glass transition temperature (Tg) exhibited by most polymer systems is far below a 300° C. process temperature. Thus, a plastic panel or window can not be exposed to the relatively high temperatures found in a glass panel or window manufacturing process. For a plastic panel or window, the conductive metallic pastes can typically only be exposed to a temperature that is lower by about 10° C. or more than the Tg exhibited by the plastic panel. For example, polycarbonate has a Tg on the order of 140° C. In this case, a cure temperature for the metallic paste should not exceed about 130° C. At this low temperature, the metallic particles do not soften or fuse together. In addition, in order to adhere to the plastic panel or window, a polymeric phase must be present in the conductive paste. This polymeric material will inherently behave as a dielectric between the closely spaced metallic particles. Thus the electrical conductivity exhibited by a cured metallic paste on plastic will typically be lower than that exhibited by a sintered paste on glass.
Due to the lower electrical conductivity exhibited by conductive pastes cured on plastic substrates as compared to sintered metallic pastes printed on high temperature substrates (e.g., glass), heater grid functionality severely suffers when long grid lines are required. There is a need in the industry to enhance and optimize the conductivity exhibited by conductive pastes cured on plastic substrates in order to provide acceptable defrosters for the backlights of large vehicles.
This invention provides for the enhancement of the amount of heat generated in the critical viewing area of a plastic window assembly by lowering the overall resistance of the conductive heater grid and allowing a greater amount of current to pass through the grid lines, thereby, increasing resistance heating of the window. A plastic window assembly provides defrost & defog capability through the resistive heating of a cured conductive ink and includes a transparent plastic panel; at least one protective layer; a conductive heater grid formed of a printed and cured a conductive ink having more than one primary grid line with opposing ends of each grid line being connected to a first and second busbar; and at least one electrical connection to the first and second busbar thereby establishing a closed electrical circuit is described wherein the formed conductive heater grid has been treated by a high power treatment that reduces the resistance of the conductive heater grid from the resistance of the heater grid absent the high power treatment.
The high power treatment comprises applying a wave shape form to the conductive heater grid having a predetermined amplitude, pulse width, pulse frequency, time duration, and number of applied pulses, wherein the resistance of the conductive heater grid is reduced by greater than about 10%.
Another embodiment of the present invention describes a method for forming a plastic window assembly, the method comprising: printing a conductive ink onto a protective layer of a plastic panel in the form of a heater grid with more than one grid line and at least two busbars; curing the conductive ink of the printed heater grid; establishing electrical connection to each busbar of the heater grid; and subjecting the heater grid to a high power treatment useful for lowering the resistance of the heater grid.
Another embodiment of the present invention describes a second method for forming a plastic window assembly, the method comprising: A method for forming a plastic window assembly, the method comprising: printing a conductive ink onto a plastic protective layer in the form of a heater grid with more than one grid line and at least two busbars; placing the plastic protective layer into the cavity of a mold; injecting a plastic resin into the mold forming a plastic panel; removing the formed plastic panel from the mold; applying protective coating to the plastic panel; establishing electrical connection to each busbar of the heater grid; and subjecting the heater grid to a high power treatment useful for lowering the resistance of the heater grid.
This invention relates to a transparent plastic glazing panel that can be defrosted to meet accepted automotive defrosting standards in the form of the SAE J953 (1999) test protocol (Society of Automotive Engineers, Warrendale, Pa.), entitled “Passenger Car Backlight Defogging System”. In order to meet this test standard, the heater grid of the present invention when part of a plastic window assembly is subjected to a high power treatment method to enhance the conductivity exhibited by the printed, conductive ink and to reduce the overall resistance of the formed heater grid.
Conventional conductive pastes or inks are very limited in their capability to function as a defroster for a plastic automotive window. Primarily, the relatively low conductivity exhibited by conventional conductive inks and pastes limits the length of a grid line to about 750 mm (˜30″) in order for the heater grid to function appropriately. Unfortunately, most vehicle rear windows are wider than 750 mm and require a heater grid with grid lines in excess of 750 mm. Examples of conventional conductive inks or pastes along with their associated manufacturer are shown in Table 1. The inventors have determined that the sheet resistivity exhibited by conventional conductive inks or pastes (ink a→ink m) is greater than or equal to 10 milliohms per square @ 25.4 μm (1 mil).
The inventors have shown in U.S. patent application entitled “Heat Enhancement in Critical Viewing Area of Transparent Panel” submitted on Dec. 9, 2005, the entirety of which is hereby incorporated by reference, that a “highly” conductive ink exhibiting a sheet resistivity lower than about 8 milliohms per square @ 25.4 μm (1 mil), preferably less than about 6 milliohms/square @ 25.4 μm (1 mil), can be used make a functioning defroster with grid lines in excess of 750 mm (30″). The best performance that one could expect from a printed grid is demonstrated by a printed & sintered grid line present on a defroster made for a glass window. On the other hand, unacceptable performance has been viewed as that exhibited by conventional silver pastes or inks cured under normal conditions.
Unfortunately, the number of “highly” conductive inks commercially available is extremely limited as compared to the number of conventional inks in existence (Table 1). In addition, “highly” conductive inks may also suffer from high cost, significant variation in batch to batch performance, and strict cure conditions or requirements. The present invention allows conventional silver pastes or inks to be used with acceptable performance after the printed and cured ink is exposed to a high power treatment.
The inventors have surprisingly discovered that a slight decrease in sheet resistivity occurs for a silver ink after a “cured” defroster pattern is subjected to a thermoforming step during the process of making a prototype window. In a thermoforming step a plastic sheet comprising a printed and cured defroster is exposed to a temperature above the glass transition temperature (Tg) of the plastic panel upon which it is printed in the presence of a fixture formed to the shape of the desired window. Thus, in this process the defroster is exposed to a temperature higher than its normal or conventional cure temperature. The lower sheet resistivity exhibited by the printed silver ink after thermoforming indicates that a “cured” conductive ink can undergo further curing or even possibly fusing of the silver particles upon being heated to a higher temperature.
The inventors further discovered that a significant increase in conductivity (e.g., a decrease in sheet resistivity) of a “cured” silver ink or paste could be obtained upon subjecting a printed and “cured” heater grid to a high voltage for very short time intervals (e.g., an AC electric field). The inventors believe that the high electrical power to which the defroster pattern is subjected to over short time intervals induces further curing by the resistive heating of the “cured” conductive ink. The utilization of this high power treatment concentrates the heat where it is needed within the heater grid (e.g., within the grid lines), thereby, minimizing any damage to the plastic panel.
A decrease in the sheet resistivity of the “cured” ink used to form a defroster, reduces the overall resistance of the heater grid, which allows for a higher current draw through each of the grid lines. A higher current draw ultimately results in a greater amount of resistance heating within the grid lines of the defroster.
Cured, Highly Conductive Inks
A total of seven different defroster patterns each printed on polycarbonate panels using a highly conductive ink exhibited an initial resistance ranging between 1.8 to 3.2 ohms. The highly conductive ink utilized exhibits a sheet resistivity of about 5 milliohms per square @ 25.4 μm (1 mil) after being cured at 129° C. for 1 hour. The printed and cured heater grid was then subjected to a high power treatment after establishing electrical connection to the busbar through the adhesion of an electrical connector or pin to the bus bar using a conductive adhesive or ultrasonic welding. Several of the parameters associated with this high power treatment along with a comparison of the initial resistance exhibited by each heater grid and the resulting resistance of each heater grid is provided in Table 2. The high power treatment used on all conductive heater grids (Run #'s 1-7), applied a voltage of about 100 volts to the “cured” heater grid.
1Measured across the heater grid pattern;
2Temperature of shortest grid line
The application of a high power treatment comprising a pulse shaped train as the wave shape form as described above results to a heater grid results in an overall decrease in resistance of the heater grid. The average observed decrease in defroster resistance was about 29% over a measured range of 16% (Run #4) to 53% (Run #1). The sheet resistivity of the “cured” conductive ink was discovered to decrease from its original value of about 5 milliohms/square @ 25.4 μm (1 mil) to about 2 milliohms/square @ 25.4 μm (1 mil) after the application of this high power treatment. A minimum in defroster resistance was obtained in less than 5 minutes with less than one minute being possible. Grid line temperatures were observed to reach as high as 144° C. (Run #6) with no significant damage to the polycarbonate. A grid line temperature of greater than about 70° C. (i.e., 74° C. in Run #5) was found to still result in a significant decrease in sheet resistivity of the “cured” conductive ink and overall resistance of the heater grid. The presence of any defects (i.e., inclusions, etc.) in the printed and cured heater grid was found to cause immediate failure of the defroster pattern. Thus, the high power treatment may also be used as a potential quality control tool for an industrialized manufacturing process.
The sheet resistivity exhibited by a conductive ink on a plastic panel of about 2 milliohms/square @ 25.4 μm (1 mil) as described above will provide a heater grid that exhibits performance equivalent to that observed for a sintered conductive fritted ink on a glass substrate. The sheet resistivity of a fritted ink used in a conventional heater grid on a glass window was measured to be approximately 2-3 milliohms/square @ 25.4 μm (1 mil).
In order for the application of a high power treatment to a conductive heater grid on a plastic panel to be beneficial, a reduction in the resistance of the heater grid on the order of about 10% or higher is desirable with greater than about 25% being especially desirable. A beneficial reduction in the resistance associated with a conductive heater grid occurs when the application of the high power treatment of the present invention to the grid reduces the sheet resistivity of the “cured” conductive ink that comprises the printed grid. A reduction in the sheet resistivity of the “cured” conductive ink on the order of about 10% or higher is preferable with greater than about 25% being especially preferred.
The high power treatment of a printed and “cured” heater grid on a plastic panel comprises the application of a high voltage for short time periods with between about 20 to about 140 volts being preferred and between about 45 and about 120 volts being especially preferred. The voltage represents the amplitude of a wave shape form applied to the heater grid during the high power treatment.
The applied wave shape form may be one of a variety of wave shapes, including a square wave, a rectangular wave, a sine wave, a damped sine wave, a sawtooth wave, a triangle wave or a pulse train 10 as shown in
Several key parameters of the high power treatment of the present invention, such as amplitude 14, width 16, pause 18, and number of pulses 12 associated with a wave shape form are generally defined and illustrated in
The width 16 of the pulses 12 applied to the conductive heater grid preferably ranges from about 10 milliseconds to about 100 milliseconds, with about 25 milliseconds to about 50 milliseconds being especially preferred. The frequency of the pulses 12 applied to the heater grid preferably ranges from about 1 Hz, to about 10 Hz with about 3 Hz to about 7 Hz being especially preferred. The time period over which the high power treatment is applied to the conductive and cured heater grid is preferably less than about 5 minutes (300 seconds), with less than about 1 minute (60 seconds) being especially preferred. Thus the number of pulses 12 applied to the heater grid during the high power treatment preferably ranges from about 20 pulses, to about 1500 pulses with about 50 pulses to about 200 pulses being especially preferred.
The inventors performed a ½ fractional 24 factorial experimental design (DOE) in order to fully understand and optimize the interaction of four key variables, namely, voltage, pulse width, frequency, and number of pulses, in the high power treatment. In particular, these variables were evaluated with respect to their interaction with the sheet resistivity exhibited by the “cured” conductive ink used in forming the heater grid. The inventors found that both the voltage and pulse width significantly affect the sheet resistivity of a highly conductive ink.
As mentioned previously, a highly conductive ink exhibits an initial sheet resistivity on the order of about 5 milliohms/square @ 25.4 μm (1 mil). The decrease in the sheet resistivity, as shown in
Cured, Conventional Conductive Inks
A similar reduction in sheet resistivity occurs when a conductive heater grid utilizing a conventional, “cured” conductive ink is subjected to the high power treatment of the present invention. As previously noted, a “cured” conventional conductive ink exhibits an initial sheet resistivity of about 10 milliohms/square @ 25.4 μm (1 mil). In this case, the variables in the high power treatment that have the greatest affect on the sheet resistivity of the “cured” ink are again voltage and pulse width, as shown in
Three different conductive inks (two highly conductive and one conventional conductive) applied to a plastic panel, on top of protective layers providing the panel with weatherability and abrasion resistance, in the form of a heater grid 36 having a plurality of grid lines 46 extending between two busbars 48 and subsequently cured at 129° C. for 1 hour. An illustrative example of a heater grid formed on a panel is seen in
The window assembly 30 includes a transparent plastic panel 32 may be constructed of any thermoplastic polymeric resin or a mixture or combination thereof. The thermoplastic resins of the present invention include, but are not limited, to polycarbonate resins, acrylic resins, polyarylate resins, polyester resins, and polysulfone resins, as well as copolymers and mixtures thereof. Transparent panels 32 may be formed into a window through the use of any known technique to those skilled in the art, such as molding, thermoforming, or extrusion. The panels 32 may further include areas of opacity, such as a black-out border and logos 34, applied by printing an opaque ink or molding a border using an opaque resin.
A heater grid 36 may be integrally printed directly onto the surface of the plastic panel or on the surface of a protective layer 38 using a conductive ink or paste and any method known to those skilled in the art including, but not limited to, screen-printing, ink jet, or automatic dispensing. Automatic dispensing includes techniques known to those skilled in the art of adhesive application, such as drip & drag, streaming, and simple flow dispensing.
The plastic panel 32 may be protected from such natural occurrences as exposure to ultraviolet radiation, oxidation, and abrasion through the use of a single protective layer 38 or additional, optional protective layers 40. Thus, a multi-layer protective coating system may comprise the protective layer 38, 40. As used herein, the transparent plastic panel 32 with at least one protective layer is defined as a transparent plastic glazing panel. The protective layer 38, 40 may consist of a plastic film, an organic coating, an inorganic coating, as well as a plurality or mixture thereof. The plastic film may be of the same or different composition as the transparent panel 32. The film and coatings may comprise ultraviolet absorber (UVA) molecules, rheology control additives, such as dispersants, surfactants, and transparent fillers (e.g., silica, aluminum oxide, etc.) to enhance abrasion resistance, as well as other additives to modify optical, chemical, or physical properties.
Examples of organic coatings include but are not limited to polymethylmethacrylate, polyvinylidene fluoride, polyvinylfluoride, polypropylene, polyethylene, polyurethane, silicone, polymethacrylate, polyacrylate, polyvinylidene fluoride, silicone hardcoat, and mixtures or copolymers thereof. Some examples of inorganic coatings include aluminum oxide, barium fluoride, boron nitride, hafnium oxide, lanthanum fluoride, magnesium fluoride, magnesium oxide, scandium oxide, silicon monoxide, silicon dioxide, silicon nitride, silicon oxy-nitride, silicon oxy-carbide, hydrogenated silicono oxy-carbide, silicon carbide, tantalum oxide, titanium oxide, tin oxide, indium tin oxide, yttrium oxide, zinc oxide, zinc selenide, zinc sulfide, zirconium oxide, zirconium titanate, or a mixture or blend thereof.
The coatings may be applied by any suitable technique known to those skilled in the art. These techniques include deposition from reactive species, such as those employed in vacuum-assisted deposition processes, and atmospheric coating processes, such as those used to apply organic or sol-gel coatings to substrates. Examples of vacuum-assisted deposition processes include but are not limited to plasma-enhanced chemical vapor deposition (PECVD), expanding thermal plasma PECVD, plasma polymerization, photochemical vapor deposition, ion beam deposition, ion plating deposition, cathodic arc deposition, sputtering, evaporation, hollow-cathode activated deposition, magnetron activated deposition, activated reactive evaporation, and thermal chemical vapor deposition. Examples of atmospheric coating processes include but are not limited to curtain coating, spray coating, spin coating, dip coating, and flow coating.
A heater grid 36 may be placed near the internal side or surface 42 or external side surface 44 of a window by application of the grid pattern onto the plastic panel 32, onto the outermost protective layer 38, 40, or between two protective layers. One construction of the present invention includes a heater grid 36 printed onto the surface of the plastic panel 32 and beneath any and all protective layers 38, 40 on the exterior side 44 or interior side 42 of the panel 32 (
Another embodiment of the present invention integrally places the heater grid 36 within the plastic panel 32′ (
The conductive pastes or inks of the present invention may be comprised of conductive particles (e.g., flakes or powders) dispersed in a carrier medium. The conductive inks may further comprise a polymeric binder, including but not limited to, an epoxy resin, a polyester resin, a polyvinyl acetate resin, a polyvinylchloride resin, a polyurethane resin or mixtures and copolymers of the like. Various other additives, such as dispersants, thixotropes, biocides, antioxidants, metallic salts, metallic compounds, and metallo-decomposition products to name a few, may be present in the conductive inks. Some examples of metallic salts and metallic compounds include tertiary fatty acid silver salts, metallic carbonate, and metallic acetate compounds. Some examples of metallo-organic decomposition products include carboxylic acid metallic soaps, silver neodecanoate, and gold amine 2-ethylhexanoate.
The conductive particles present in the conductive paste or ink of the present invention may be comprised of a metal, including but not limited to silver, silver oxide, copper, zinc, aluminum, magnesium, nickel, tin, or mixtures and alloys of the like, as well as any metallic compound, such as a metallic dichalcogenide. These conductive particles, flakes, or powders may also comprise some conductive organic materials known to those skilled in the art, such as polyaniline, amorphous carbon, and carbon-graphite. Although the particle size of any particles, flakes, or powders may vary, a diameter of less than about 40 μm is preferred with a diameter of less than about 1 μm being specifically preferred. A mixture of particle types and sizes may be utilized to enhance conductivity and lower sheet resistivity by optimizing particle packing. Any solvents, which act as the carrier medium in the conductive pastes or inks, may be a mixture of any organic vehicle or solvent that provides solubility or dispersion stability for the organic resin, additives, or conductive particles.
While the present invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made to those skilled in the art, particularly in light of the foregoing teachings.
This application claims the benefit of U.S. Provisional Application No. 60/655,936, filed Feb. 24, 2005, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60655936 | Feb 2005 | US |
