Abstract:
An object of complex three-dimensional configuration, such as an automotive vehicle body, is coated with a combination of a coating formulation and a photoactivated mixture containing active centers that have been produced prior to application. The two liquids can be intimately mixed prior to application to the object, or the coating formulation can be applied prior to the application of the photoactivated mixture. The coating formulation is cured by the active centers that have been produced prior to application.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and system for applying a photo activated polymer coating to a substrate. 
     BACKGROUND OF THE INVENTION 
     It is known to use polymer coatings on objects such as complex three-dimensional shapes. Typically, these coatings are produced using thermally activated polymerization. Heat is used to generate active centers. The active centers polymerize the coatings after a monomer has been applied to a substrate as a liquid. However, thermal polymerization requires tremendous amounts of time, energy and expense to run high-temperature ovens for extended periods of time. 
     It is also known to use photopolymerization to produce polymer coatings on objects. Photopolymerization reactions are chain reactions which generate free radical or cationic active centers. In photopolymerization, energy from UV or visible light is used to polymerize the monomer. Photopolymerization has a number of advantages, including savings in energy and high cure rates without the necessity of solvents. 
     However, photopolymerization of coatings on complex three-dimensional shapes has not been successful because of problems with oxygen inhibition and shadow regions. Previously known photopolymerization systems have used free radical polymerization to generate free radical active centers. Oxygen inhibition occurs when free radical active centers react with oxygen to produce unreactive peroxides and hydroperoxides. The effect is to decrease the polymerization rate and reduces the polymer molecular weight resulting in an incomplete cure. To overcome this problem, coating systems are purged with nitrogen in an attempt to displace oxygen from the monomer coated surface. While this removes most of the oxygen and allows photopolymerization to occur on flat substrates, it is difficult to purge oxygen from recessed regions, resulting in poor polymerization in these areas. 
     Problems also occur as the result of shadows from incomplete illumination of the coating material on the substrate during photopolymerization. Typically, photopolymerization is carried out by mixing a monomer, a photoinitiator and any additives to the application. The mixture is then applied on to the substrate, followed by illumination from a light source, to start the photochemical process. Active centers are produced when the photons from the light are absorbed. Therefore, each area of the substrate must be exposed to light. This is difficult to accomplish with complex shapes. Any region not exposed or lying in the shadow will have an absence of active centers. Therefore, the material remains in an uncured state. Multiple illumination sources, moving illumination sources and moving substrate sources have been used in an attempt to overcome this problem. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention relates to a method of effectively producing a polymerized coating on a substrate, a system to produce the coating using the method, and an apparatus for use with the system. The method is particularly useful for coating objects having complex three dimensional shapes such as vehicle bodies. The method includes the steps of providing a supply of photoinitiator, photochemically activating the supply of photoinitiator to form a pre-activated photoinitiator, applying the pre-activated photoinitiator to the substrate; and polymerizing a bulk coating formulation with the pre-activated photoinitiator to form the polymerized coating on the substrate. 
     In a first embodiment of the invention, the supply of photoinitiator is subject to radiant energy in a reservoir to form a pre-activated photoinitiator. The bulk coating formulation is sprayed onto the substrate and the preactivated photoinitiator is sprayed onto the bulk coating formulation. The cationic active centers of the pre-activated photoinitiator react with the bulk coating formulation to begin a process of polymerization without any added energy, such as light or heat. 
     In a second embodiment of the invention, the pre-activated photoinitiator is mixed with the bulk coating formulation and applied or sprayed as a mixture to the substrate where polymerization occurs. 
     In a third embodiment of the invention, photoinitiator is supplied to an applicator from a reservoir through a feed line. The feed line is opaque, however, the feed line has a light transmissive portion near the applicator. A light source is placed adjacent to the light transmissive portion to pass incident radiation (UV or visible light) through the light transmissive portion to photochemically activate the photoinitiator as it is being delivered to the applicator. 
     In a fourth embodiment of the invention, the photoinitiator is activated by irradiation (UV or visible light) when sprayed from an applicator prior to being applied to the substrate. The photoinitiator may be mixed with the bulk coating material when sprayed from the applicator and photochemically activated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart showing the steps of the method of coating; 
         FIG. 2  is a schematic view illustrating a first embodiment of a process for coating a substrate; 
         FIG. 3  is a schematic view showing the formation of cationic active centers and the polymerization process according to the embodiment of the first invention; 
         FIG. 4  is a schematic view of a second embodiment of the invention where mixing occurs prior to application; 
         FIG. 5  is a schematic view of a third alternative embodiment of the invention where photopolymerization is initiated in a line supplying an applicator; 
         FIG. 6  is a schematic view of a fourth embodiment of the invention where activation of photoinitiator occurs in the spray. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method  10  of applying a coating to a substrate, such as a complex three dimensional object is shown in  FIG. 1 . The method  10  includes providing a supply of a cationic photoinitiator  12 , photochemically activating the supply of photoiniatiator to provide a pre-activated photoinitiator  14 , applying the pre-activated photoinitiator to the substrate  16 , and polymerizing the bulk coating formulation with the pre-activated photoinitiator to form a polymerized coating on the substrate  18 . 
     As shown in  FIG. 2 , an embodiment of a system  20  for use with the method  10  includes forming a coating  22  on a substrate  24 , such as a vehicle body. The system includes a reservoir such as a drum  26  of cationic photoinitiator  28 , a light source  30 , a reservoir such as a drum  32  of bulk coating formulation  34  and applicators  36  and  38 . 
     The cationic photoinitiator  28  may be of any type capable of forming cationic active centers when subjected to a suitable radiation. The photoinitiator may be a liquid or a solid. If the photoinitiator  28  is a solid, it may be dissolved in an appropriate solvent or in a monomer. If the photoinitiator is dissolved in a monomer, the process must be designed with the polymerization rate in mind to assure that the solution containing the activated photoinitiator is at an appropriate state of polymerization (typically with sufficient unreactive monomer) at the time it reaches the applicator  36 . 
     If the photoinitiator  28  is dissolved in solvent, the active centers of the activated photoinitiator do not begin any polymerization and remain active until the active centers migrate into the bulk coating formulation  34 . This means that the photoinitiator can be activated much in advance of the actual spray process. Once it is sprayed on top of the bulk coating material, the solvent will evaporate which will cause the active centers to come in contact with the bulk coating material. If the photoinitiator is dissolved in a monomer instead of a solvent, there will not be any solvent to evaporate; rather, the activated photoinitiator will begin polymerization of the monomer in which it is dissolved immediately upon irradiation from the light source. As such, the photoinitiator cannot typically be activated far in advance of the application process. 
     The light source or lamp  30  for irradiation of the photoinitiator  28  is placed near the drum  26 . The lamp  30  may produce UV or visible light, depending upon the photoinitiator used. As shown in  FIG. 3 , irradiation from the lamp  30  forms pre-activated photoinitiator  40  with cationic active centers  42 . The active centers  42  remain in this form until applied to the bulk coating formulation  34  where a polymerization reaction between the activated photoinitiator and the bulk coating formulation begins to form the coating  22 . 
     The bulk coating formulation  34  may contain monomer or oligomer plus additives, such as pigments, UV stabilizers, cross-linking agents, free radical quenchers, and other co-monomers. The pre-activation of the photoinitiator permits the addition of pigments and other additives in the bulk coating formulation because these additives have no influence on the creation of active centers. A pump  44  is utilized to transfer the bulk coating formulation  34  through a line  46  to the applicator  36 , such as a spray gun. The bulk coating formulation is then applied to the substrate. The pre-activated photoinitiator is then moved by pump  48  from drum  26  through a line  50  to a second applicator  38 , such as a spray gun, where it is applied over the bulk coating formulation. As shown in  FIG. 3 , the cationic active centers of the activated photoinitiator begin the polymerization reaction with the bulk coating formulation. 
     A second embodiment of a system  60  for applying a coating is shown in  FIG. 4 . A two component mixing system  68  receives a bulk coating formulation  34  from the reservoir  62  through a line  66  and pre-activated photoinitiator  40  from a drum  61  through a line  67 . The photoinitiator is pre-activated by lamp  30  as discussed for the previous embodiment. In this embodiment, the pre-activated photoinitiator  40  is mixed with the bulk coating formulation  34  in the two component mixing system  68  before application to the substrate  24 . Such two component mixing systems are also known as 2K mixing systems. The mixture is sprayed by applicator  64  on to the substrate  24  immediately after mixing. The active centers formed during illumination by lamp  30  initiate a polymerization reaction with the bulk coating formulation on the substrate  24  to form the coating. 
     As shown in  FIG. 5 , a third embodiment of a coating system  70  includes an apparatus for activating photoinitiator in a feed line  72  for an applicator  74 . The system  70  includes a reservoir, such as a drum  76 , containing a mixture  71  of both a bulk coating formulation  34  and a cationic photoinitiator  28 . The feed line  72  extends between pump (not shown) in the reservoir  76  and the applicator  74 . The feed line  72  may be any conduit such as a pipe or hose. The feed line  72  is opaque with the exception of a small light transmissive portion  78  near the applicator  74 . The light transmissive portion is translucent or transparent so that radiation from the lamp passes through the light transmissive portion. A radiant energy source or lamp  80  of appropriate intensity and type, based on the activation requirements of the photoinitiator, is positioned adjacent to the light transmissive portion  78 . The mixture  71  is subjected to the radiant energy from the lamp  80  to form the active centers as mentioned above. The mixture is applied to the substrate  24  from the applicator  74  and the polymerization process proceeds on the substrate  24 . 
     A fourth alternative embodiment is shown in  FIG. 6 . In this embodiment, a lamp  82  is located between an applicator  74  and substrate  24  so that the cationic photoinitiator passes through the radiant energy as a spray just before application to the substrate  24 . Alternatively, a mixture of cationic photoinitiator and bulk coating formulation may be subjected to irradiation as in the third alternative embodiment. 
     EXAMPLE I 
     A mixture of 0.5 mol % (tolycumyl) iodonium tetrakis (pentafluorophenyl) borate supplied by Rhodia, Inc. (commercial name: Rhodorsil 2074) and the cationic monomer 3,4-epoxycyclohexylmethanyl 3,4-epoxycyclohexanecarboxylate supplied by Dow Chemical Co. (commercial name: UVR 6105, Dow Chemical Co.) was mixed and illuminated for 2 minutes to photoactivate the photoinitiator and create the cationic active centers. A small amount (1.012 grams) of semi-polymerized monomer/activated photoinitiator was extracted from the mixture and placed on a 10 gram pool of the cationic monomer containing no initiator or active centers. The sample was not subsequently mixed. The sample was then placed in an oven at 50° C. to enhance the polymerization rate and active center migration. Within a day, the pre-activated active centers began to polymerize the pure monomer bulk as illustrated by visible strands of polymerized monomer that extended from the initial pre-activated mixture into the bulk monomer. After eight days the entire sample was fully polymerized with the initial polymer strands still visible. Control experiments on monomer containing unilluminated photoinitiator illustrated that no thermal polymerization occurred. 
     EXAMPLE II 
     A mixture of 0.5 mol % (tolycumyl) iodonium tetrakis (pentafluorophenyl) borate supplied by Rhodia, Inc. (commercial name: Rhodorsil 2074) in an inert solvent 1,1′-(1,1,3-Trimethyl-1,3-propanediyl)biscyclohexane supplied by Aldrich (product 465046) was mixed and illuminated for 10 minutes with continuous stirring. A small amount (˜0.25 grams) of the solution containing now activated centers was extracted and placed on top of 1 gram of the cationic monomer 3,4-epoxycyclohexylmethanyl 3,4-epoxycyclohexanecarboxylate supplied by Dow Chemical Co. (commercial name: UVR 6105, Dow Chemical Co.) containing no initiator or active centers in a polystyrene cuvette. Six of these samples were made and placed in an oven at 50° C. to enhance the polymerization rate and active center migration. At the prescribed time (1, 2, 4, 8 and 24 hours), one of the samples was placed in tetrahydrofuran (THE) to dissolve the cuvette and monomer from the uncured region of the sample. The remaining crosslinked polymer was then washed with acetone to remove the any remaining THE and excess monomer. The polymer sample was dried and its weight was recorded. The weight of the polymer at 1, 2, 4, 8 and 24 hours were 0.0436, 0.1967, 0.2648, 0.2956, 0.3364 grams respectively. This indicates that the pre-activated cationic active centers produced in a separate step are moving from the pre-activation layer into the monomer layer below where they begin to polymerize the sample. Control experiments on monomer containing unilluminated photoinitiator illustrated that no thermal polymerization occurred. 
     EXAMPLE III 
     A Perkin-Elmer Differential Scanning Calorimeter-7 (DSC) modified for photo-experiments was used to monitor the reaction of the pre-activated active centers. The light source was a 200 W Oriel Hg(Xe) arc lamp at a total intensity of 55 mW/cm 2  and the beam was passed through a water filter outfitted with a thermostatted recirculating jacket to reduce infrared radiation and limit sample heating. Approximately 15 mg of the cationic monomer 3,4-epoxycyclohexylmethanyl 3,4-epoxycyclohexanecarboxylate supplied by Dow Chemical Co. (commercial name: UVR 6105. Dow Chemical Co.) containing no initiator or active centers was placed in an aluminum DSC pan held at 25° C. After the DSC began monitoring the reaction, approximately 3 μL of a previously mixed and illuminated (for 10 minutes with continuous stirring) solution of 0.5 mol % (tolycumyl) iodonium tetrakis (pentafluorophenyl) borate supplied by Rhodia, Inc. (commercial name: Rhodorsil 2074) in acetone was added into the aluminum pan and not mixed. The heat of polymerization was recorded for 25 minutes (after which the reaction slowed to a rate where the DSC could no longer monitor it). The DSC reaction showed a 0.0026 mol/L sec maximum polymerization rate with a 23% conversion in the 25 minutes. This study was repeated for methyl 3,4-epoxycyclohexanecarboxylate CAS#41088-52-2, bis(3-glycidoxypropyl)tetramethyldisiloxane CAS#126-80-7, and tri(ethylene glycol) divinyl ether CAS#765-12-8. These studies indicate that the pre-illumination of the photoinitiator solution produced active centers which remained active allowing polymerization of the monomer to occur once the solution was added. 
     Although the best mode contemplated by the inventors for carrying out the present invention as of the filing date hereof has been shown and described herein, it will be apparent to those skilled in the art that suitable modifications, variations and equivalents may be made without departing from the scope of the invention, such scope being limited solely by the terms of the following claims and the legal equivalents thereof.