Abstract:
A sol-gel mixer based upon a supply of water or alcohol solvent, a supply of acid, a supply of zirconium alkoxide, and a supply of organosilane, has a series of valves and pumps which provide a first supply of acid and zirconium alkoxide to an agitated pre-mix vessel where the components are allowed to dwell. The solvent and organosilane are then combined with the output from the pre-mix vessel, which is agitated in a final-mix vessel to create a sol-gel. The mixed sol-gel may be aged within the final-mix vessel until ready for use. A series of automated control elements, including valves and pumps, are placed throughout the system to automatically control the volumetric flow of the components through the system in accordance with predetermined parameters.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a method of mixing chemicals to produce a sol-gel for subsequent application to a metal substrate. More particularly, the invention relates to an automated method of mixing raw chemicals to produce a ready to apply sol-gel solution.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various coatings have been developed which promote the adhesion of one or more resinous layers to a titanium, aluminum, or other metallic surface such as that of an airplane component. Some of the coatings improve adhesion by utilizing a sol-gel film between the metal and the resin. The term “sol-gel”, a contraction of solution-gelation, refers to a series of reactions where a soluble organometallic species, typically a metal alkoxide or metal salt, hydrolyzes to form a metal hydroxide.  
           [0003]    The sol-gel films or sol coatings promote adhesion through a hybrid organometallic coupling agent at the metal surface. The metallic portion of the organometallic, which usually contains zirconium, bonds covalently with the metal. The organic portion of the organometallic bonds covalently with the applied layer of adhesive or matrix resin. In this manner, the organometallic based sol-gel generates a bridging structure that creates a chemical link between the metal surface and the resin primer.  
           [0004]    The strength and durability of the sol coating depends upon chemical and micro-mechanical interactions at the surface of the metal involving, for example, the porosity and microstructure of the metal and the tendency of the sol coating to rehydrate. When properly implemented, the sol-gel coatings provide surface stability for paint adhesion, adhesive bonding, or fabrication of structurally superior fiber-metal hybrid laminates.  
           [0005]    A sol-gel composition that is particularly useful for coating aluminum and titanium surfaces is based on a combination of organometallic and organosilane components. The preferred organometallic compound for use in a sol-gel for coating aluminum and titanium surfaces is an alkoxy metallic compound, and more preferably an alkoxy zirconium compound. Because of its ready commercial availability, Zr (IV) n-propoxide is particularly preferred as the organometallic compound. In addition to covalently bonding to the metal surface, the organozirconium compound also serves to minimize the diffusion of oxygen to the surface and to stabilize the metal-resin interface. Epoxy-functionalized silanes are the preferred organosilanes because of their stability in solution and their ability to crosslink with common, aerospace epoxy or urethane adhesives. The silane is acid-base neutral, so its presence in the sol mixture does not increase the relative hydrolysis and condensation rates of the alkoxy metallic compounds. Sols including the organosilanes are relatively easy to prepare and to apply with reproducible results.  
           [0006]    A particularly useful sol-gel formulation is Boegel-EPII™, developed by The Boeing Company, Seattle, Wash. The Boegel-EPII™ composition is a combination of 3-glycidoxypropyltrimethoxysilane (GTMS) and Zr (IV) n-propoxide which is reacted in the presence of an acetic acid stabilizer. The GTMS has an active epoxy group which can react with common epoxy and urethane resins. GTMS does not form strong Lewis acid-base interactions with the hydrated metal oxide substrate. The zirconium in the mixture tends to react more quickly with the oxide surface of the metal, allowing the desired stratification of the sol-gel film with the epoxy groups of the silane coupling agents oriented toward the resin layer.  
           [0007]    To achieve the desired coating structure, it is desired to apply the sol-gel solution to the substrate at the point that the zirconium and silicon are hydrolyzed sufficiently that zirconium and silicon react with the metal surface. If the sol is applied too shortly after beginning the reaction between the organometallic and organosilane, the organosilane may not be fully hydrolyzed upon application. If the sol is not applied soon enough, the hydrolyzed silicon and organometallic components may condense among themselves, forming oligomers and networks.  
           [0008]    The need to properly prepare and age the sol-gel before application to a substrate can present a problem because the need for aging causes delays in the coating process. Also, sol-gel that is over-aged must be discarded as waste. Appropriate timing of mixing can increase the efficiency and cost effectiveness of the sol-gel coating process.  
           [0009]    It is, therefore, desired to provide a method and apparatus which reacts the components of sol-gel without the need to manually monitor the aging or mixing of the sol-gel. It is further desired to provide an apparatus which supplies sol-gel on demand so that over-aged sol-gel need not be wasted.  
         SUMMARY OF THE INVENTION  
         [0010]    A sol-gel mixer is provided according to embodiments of the present invention which provides a properly mixed and properly aged sol-gel mixture for application to a metallic surface. The sol-gel mixer is automated such that a series of valves, pumps, and tanks control the reaction and mixing of the sol-gel. Flow rates and dwell times are controlled so that aged sol-gel is available from the mixing process such that the sol-gel may be generated on an as-needed basis.  
           [0011]    The inputs for the sol-gel mixer comprise a supply of solvent, preferably deionized water, a supply of acid, preferably glacial acetic acid, a supply of zirconium alkoxide, preferably zirconium n-propoxide (TPOZ), and a supply of organosilane, preferably 3-glycidoxypropyltrimethoxysilane (GTMS). A series of controlled valves and pumps first supply the acid and zirconium alkoxide to an agitated pre-mix vessel where the components are allowed to dwell. A supply of solvent is then combined with the output from the pre-mix vessel, which is then combined with a supply of organosilane and solvent in an agitated final-mix vessel. The mixed sol-gel is held for a given induction time and then stored until ready for use. The aged sol-gel is preferably supplied directly from the output of the final-mix vessel.  
           [0012]    Each step in the mixing process is automated and is regulated by a controller, preferably a personal computer (PC) or other processing element. Through mechanical interfaces, the controller switches valves and actuates pumps such that precise, predetermined volumetric amounts of acid and zirconium alkoxide are combined and mixed within the pre-mix vessel and so that precise, pre-determined amounts of solvent and organosilane are combined with the pre-mixture in the final-mix vessel. Dwell times of the components within the vessels are also automated by the controller. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:  
         [0014]    [0014]FIG. 1 is a flow-chart of a general embodiment of the invented mixer;  
         [0015]    [0015]FIG. 2 is a schematic of a particular embodiment of the invented mixer;  
         [0016]    [0016]FIG. 3 is a schematic of a second particular embodiment of the invented mixer;  
         [0017]    [0017]FIG. 4 is a schematic of a third particular embodiment of the invented mixer; and  
         [0018]    [0018]FIG. 5 is a schematic of a fourth particular embodiment of the invented mixer; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.  
         [0020]    Referring to FIG. 1, the invented mixer comprises at least four controlled elements in conjunction with at least two mixing vessels. A controlled input of acid  10  and a controlled input of zirconium alkoxide  20  are inputted into a pre-mixing vessel. The volume of each of the acid and zirconium alkoxide is controlled such that a predetermined total volume of material is supplied to the pre-mix vessel  30  and such that the acid and zirconium alkoxide are supplied in predetermined volumetric ratios.  
         [0021]    The zirconium alkoxide is a compound with the general formula (R—O) 4 Zr wherein R is lower linear or branched aliphatic having 2-5 carbon atoms, alicyclic, or aryl, especially normal aliphatic (alkyl) groups. TPOZ, Zr (IV) n-propoxide, available from Aldrich Chemical® in a saturated n-propanol solution, is the preferred zirconium compound.  
         [0022]    The acid is provided for the purpose of complexing with the Zr component of the zirconium alkoxide. This stabilization of the zirconium alkoxide allows the zirconium alkoxide to be stable in an aqueous solution and also allows the organosilane component to hydrolyze at approximately the same rate. The minimum amount of acid necessary to form the zirconium-acid complex is used to prevent excess acid from evaporating or being trapped in the deposited coating. Glacial acetic acid is the preferred acid, though other organic acids, like citric acid, can be substituted for the acetic acid. Glycols, ethoxyethanol, H 2 N—CH 2 —CH 2 —OH, or the like may also be used as the acid.  
         [0023]    When using glacial acetic acid and TPOZ as the acid and zirconium alkoxide, the acid is supplied to the pre-mixture vessel  30  in the amount of about 0.5% to about 2.0% by volume of the total mixture. In general, the acid component of the mixture is supplied in volumes which result in a molar ratio of about 4:1 to about 5:1 (acid:zirconium alkoxide).  
         [0024]    The acid and zirconium alkoxide may be added to the pre-mix vessel sequentially or concurrently. The components may be separately introduced to the pre-mix vessel or may be combined just prior to entering the pre-mix vessel. Also, addition of acid and zirconium alkoxide to the pre-mixer may be continuous or non-continuous. The components will be mixed within the pre-mix vessel for a predetermined dwell time. In the case of a continuous process, the dwell time will be the average residence time of the components within the pre-mix vessel. For non-continuous or batch operations, the dwell time will be the actual time that both components are present within the vessel.  
         [0025]    The pre-mix vessel  30  incorporates a continuous agitation means for agitating the pre-mix. Agitation may be accomplished with one or more impellers within the vessel, with one or more stir bars within the vessel, by a continuous fluid recycle loop within the vessel, by agitating the vessel and its contents through vibration, and other similarly known methods. Agitation provides intimate mixture of the pre-mixture components and prevents the zirconium alkoxide from solidifying out of solution.  
         [0026]    The pre-mix vessel  44  is of a size to provide predetermined residence time for the mixed components within the vessel, preferably about 10 to 20 minutes.  
         [0027]    After the acid and zirconium alkoxide have dwelled in the pre-mix vessel  30  for a predetermined amount of time, a volume of the pre-mix  40  is transferred from the pre-mix vessel  30  to a final-mix vessel  50 . If the mixer is used in a continuous process, it is preferred that the volume transferred from the pre-mix vessel to the final-mix vessel also be controlled. In batch operation, the entire volume of the pre-mix vessel is transferred to the final-mix vessel and does not need to be otherwise controlled. A controlled amount of solvent  60  and controlled amount of organosilane  70  are also added to the final-mix vessel  50 . Each of the pre-mix  40 , solvent  60 , and organosilane  70  may be added concurrently or sequentially with respect to one another, but the components are preferably added to the final-mix vessel  50  either concurrently or in rapid succession.  
         [0028]    The preferred organosilane is 3-glycidoxypropyltrimethoxysilane (GTMS). GTMS is preferred for several reasons. First, the GTMS includes an active epoxy group which reacts favorably with bond primers. Second, the GTMS does not form strong Lewis acid-base interactions with hydrated oxide substrates to which it is applied. Other organosilanes that may be used in accordance with the invention are 3-aminopropyltriethoxysilane, p-aminophenylsilane, m-aminophenylsilane, allyltrimethoxysilane, n-(2-aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-glycidoxypropyldiisopropylethoxysilane, (3-glycidoxypropyl)methyldiethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, n-phenylaminopropyltrimethoxysilane, vinylmethyldiethoxysilane, vinyltriethoxysilane, or vinyltrimethoxysilane.  
         [0029]    The solvent of the solvent supply  60  is preferably water, and is more preferably deionized water (DI). Alcohol may be substituted, partially or completely, for the water used in formation of the sol-gel.  
         [0030]    Sol-gel is created by stirring the components within the final-mix vessel  50  as required but for a minimum of 10 minutes, to allow for proper dispersion of the solids within the aqueous mixture. As with the pre-mix vessel, the final-mix vessel  50  incorporates a continuous agitation means for agitating the mixture. Agitation may be accomplished with one or more impellers within the vessel, by stir bars in the vessel, by a continuous fluid recycle loop within the vessel, by agitating the vessel and its contents through vibration, and other similarly known methods. The resulting sol-gel mixture may be discharged from the final-mix vessel automatically or manually on demand.  
         [0031]    In a preferred final resulting sol-gel mixture, the GTMS and Zr (TV) n-propoxide are preferably present in a molar ratio between about 2:1 and about 5:1, and most preferably about 5:1. The solids (GTMS and Zr (IV) n-propoxide) comprise about 5% and water comprises about 95%, by mass percentage.  
         [0032]    The invention may be embodied in many different conformations. Several are shown below. Since TPOZ is the preferred zirconium alkoxide, GTMS is the preferred organosilane, and water is the preferred solvent, these exemplary compounds are used throughout for convenience, though it is recognized that the embodiments and the teachings as a whole are applicable to a wide range of zirconium alkoxides, organosilanes, and solvents as specified herein. The concentrations of the pre-mixture and the resulting sol-gel may be varied depending upon the particular components used in production of the sol-gel, and depending on the manner in which the resulting sol-gel will be used.  
         [0033]    Each step in the mixing process is automated and is regulated by a controller. Through mechanical interfaces, the controller switches valves and actuates pumps such that precise, predetermined volumetric amounts of acid and zirconium alkoxide are combined and mixed within the pre-mix vessel and so that precise, pre-determined amounts of solvent and organosilane are combined with the pre-mixture in the final-mix vessel. Dwell times of the components within the vessels are also automated by the controller.  
         [0034]    Referring to FIG. 2, one embodiment of the mixer is presented in which a supply of acid  22  and zirconium n-propoxide (TPOZ)  32  are provided. The acid and TPOZ are transferred to a pre-mix vessel  37  via respective acid and TPOZ supply lines  23 ,  33 . Each of the supply lines  23 ,  33  are in line with a controlled volumetric metering pump  35  which may be used to adjust or cease the combined flow of the acid and TPOZ to the pre-mix vessel  37 . A controlled valve  25  in-line with the acid supply line  23  controls the ratio of acid to TPOZ pumped by pump  35 .  
         [0035]    The combined acid/and TPOZ are mixed within the pre-mix vessel  37  for a predetermined amount of time and then pumped by metering pump  44 , through a pre-mix transfer line  40  and controlled valve  42  to the final-mix vessel  50 . In non-continuous operation, a standard pump  44  may advantageously be used rather than a metering pump to transfer the pre-mix because the volume present within the pre-mix tank  37  will already be known and the total contents of the pre-mix tank may simply be transferred to the final-mix tank  50 .  
         [0036]    A GTMS supply  70  is in communication with the final-mix vessel  50 , such that the GTMS is supplied from the GTMS supply  70  through a GTMS supply line  72  to the final-mix vessel  50 . The GTMS is pumped by a controlled volumetric metering pump  74  in-line with the supply line  72 .  
         [0037]    A water supply  60  is in communication with the final-mix vessel  50 , such that the water is supplied from the water supply  60  via water supply line  62  through water feed valve  64  to the final-mix vessel  50 . The water is pumped by metering or standard controlled pump  44 .  
         [0038]    To provide for rinsing the mixing system, the mixer is arranged such that water from the water supply  60  may be pumped through rinse line  66  and rinse valve  67 , through the pre-mix pump  35  and pre-mix vessel  37  in order to rinse the pumps, lines, and vessels after a batch of sol-gel has been mixed. If water is used to rinse the system, acid  22  is preferably cycled through the pre-mix pump  35  and vessel  37  prior to mixing a fresh batch of sol-gel, so that water in the pump  35  and vessel  37  does not react unfavorably with the TPOZ  32 .  
         [0039]    The position of all valves  25 ,  42 ,  64 ,  67 , and the operation of all pumps  35 ,  44 ,  74  within the mixer are controlled by an automated control unit  80  which positions the valves and operates the pumps at predetermined times.  
         [0040]    A number of additional devices may optionally be used in operation of the mixer. For instance, a three-way valve  76  in-line between the pumps  44 ,  74  and the final-mix vessel  50  may be used, if necessary, to prevent backflow of materials through one pump due to increased pressure generated by an opposing pump.  
         [0041]    Also, flow meters such as bubble detectors  36 ,  78  may be used in-line with the transfer lines of the mixer to indicate the presence of solution within the lines. A flow detector positioned between the supply of components  22 ,  32  and the pre-mix vessel  37 , may be used to indicate when a supply is inadequate. A flow detector  78  between the pre-mix tank  40  and final-mix tank  50  may be used to indicate when all material has been transferred from the pre-mix tank  37  to the final-mix tank  50  during batch operation. In addition, level indicators  52  may be used to indicate the presence of fluid within the respective vessels.  
         [0042]    Referring to FIG. 3, in a second embodiment of the mixer a controlled multi-position valve  34  is used in place of supply valves  25  and  67 . The multi-position valve  34  allows a controlled flow of single supply components or any mixture of TPOZ, acid, and water.  
         [0043]    Referring to FIG. 4, a third embodiment of the invention is substantially similar to the first embodiment of the invention. However, water supply valve  64  and rinse line valve  67  are replaced by a single controlled three way valve  65  which controls the flow of water to the final-mix vessel  50  or through the rinse cycle of the mixer. In addition, a controlled TPOZ supply valve  39  is added for versatility in controlling the supply of TPOZ to the pre-mix vessel  37 . An additional three way valve  79  may be positioned near the inlet of the final-mix vessel  50  to aid in bleeding the final-mix vessel.  
         [0044]    Referring to FIG. 5, a fourth embodiment of the invention is shown in which a metering pump  31  and bubble detector  38  are added in-line with the TPOZ supply  32 . This configuration completely separates the TPOZ supply from the acid supply  22  so that the TPOZ may be separately pumped and monitored. Flow of acid or solvent is still provided through metering pump  35  and controlled by a three way valve  27 .  
         [0045]    In each of the above embodiments, the pumps and valves are connected to a controller  80  such as an automated control element. The control element may be centralized or distributed. Typical control elements include a microprocessor, preferably a personal computer (PC) or microcontroller, but may be other types of computing devices and/or processing elements capable of receiving data concerning predetermined mix ratios and dwell times and appropriately adjusting the positions of the valves and the operation of pumps may be utilized. The controller may also control the operation of the agitation means within the pre-mix and/or final-mix vessels. The controller is preferably in communication with each of the controlled elements of the mixer. The controller may communicate with the various elements via wired communication, radio communication, or any other type of communication commonly used in control engineering.  
         [0046]    Use of the invented mixer allows accurate and automated mixing and aging of a sol-gel. Aging of the sol, also termed induction time, is an important factor in use of the sols. Complete hydrolysis and condensation of the organometallic in the sol-gel film is important to develop a hydrolytically stable metal oxide film in the metal. The presence of hydrolyzable alkoxides in the sol-gel film will have two adverse effects. Every residual alkoxide represents an incomplete condensation site between the metal and the coupling agents. Incomplete condensation, therefore, decreases the ultimate bond strength of the sol-gel film.  
         [0047]    Aging is a function of the rates of the hydrolysis reaction of the TPOZ and the organosilane. The TPOZ hydrolizes more rapidly than the silane. The zirconate hydrolyzes rapidly using ambient moisture and condenses with itself or with absorbed water on a titanium surface. If not properly controlled, this zirconate hydrolysis self-condensation reaction can produce insoluble zirconium oxyhydroxides which will precipitate and become nonreactive.  
         [0048]    If the sol is applied too short a time after being made, the organosilane may not be fully hydrolyzed. As the sol ages, the hydrolyzed silicon and zirconium components may condense among themselves, forming oligomers and networks. These networks will eventually become visible to the naked eye and become insoluble. The ideal solution age is at the point that the zirconium and silicon are hydrolyzed sufficiently that zirconium and silicon react with the metal surface. Proper aging times for sol-gels of various compositions are known from empirical data. At this point, generally some metal polymers and networks have formed in the sol and they will give the sol-gel film some structure.  
         [0049]    In order to take advantage of the full scope of the favorable adhesive properties provided by the invented sol-gel, the metal surface must be properly prepared and the sol-gel must be properly applied. Techniques for applying a sol-gel that are equally applicable to the invented adhesion-promoting sol-gel coating are found in Blohowiak et al., U.S. Pat. No. 5,814,137, and are incorporated herein by reference.  
         [0050]    Sol coated metals are useful in hybrid laminates like those described in U.S. Pat. Nos. 4,489,123 and 5,866,272. These hybrid laminates are candidates for use as aircraft skin panels and other structural applications in subsonic or, especially, supersonic aircraft. The utility of these hybrid laminates hinges on a sound, strong adhesive bond between the metal and resin. By preparing the sol with the invented mixer, the sol coating provides a consistent, high strength adhesion interface at relatively low cost. Because the mixer is automated, a well mixed sol-gel is supplied in a highly efficient manner, at a lower cost, and on a more consistent basis than manually mixed sol-gels.  
         [0051]    The invented automated sol-gel mixer provides a uniform, well-mixed, sol-gel for use in coating metal substrates. The mixer is preferably operated with a control system which continually monitors and controls the mixer so as to provide a consistent sol-gel mixture on demand. The mixer provides a sol-gel that is properly mixed and properly aged.  
         [0052]    Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.