Patent Publication Number: US-2013243960-A1

Title: Wet-on-wet coating deposition process

Description:
BACKGROUND 
     This disclosure relates to coating deposition and, more particularly, to the deposition of multiple coating layers on a substrate. 
     Products that are subject to corrosive, abrasive or other potentially detrimental environmental conditions may be provided with a protective coating. For instance, paint and other organic-based coatings may be used to seal and protect against rusting, abrasion, radiation, biological growth and the like. A typical organic-based coating system may include a primer and a topcoat that is applied over the primer. The primer may serve various functions, such as sealing the underlying substrate, rusting protection and enhancing adhesion between the topcoat and the substrate. The topcoat serves to protect the primer and may also be used to enhance the appearance of the product. 
     For many commercial products, the organic-based coating deposition process involves applying the primer onto the substrate and then drying and curing the primer prior to applying the topcoat by spray deposition or electrodeposition. After applying the topcoat, the product is dried to cure the topcoat. 
     SUMMARY 
     An example method for coating a substrate according to the present disclosure includes immersion depositing a first coating layer onto a substrate such that the first coating layer is deposited in a wet state with regard to solvents in the first coating layer. While the first coating layer is in the wet state, a second coating layer is immersion deposited onto the first coating layer. The first coating layer and the second coating layer are then co-dried to substantially remove any solvent. 
     In another aspect, a method for coating a substrate according to the present disclosure includes electrodepositing a primer layer onto a metallic substrate using an aqueous solution such that the primer layer is deposited in a wet state with regard to water in the primer layer. While the primer layer is in the wet state, a topcoat layer is electrodeposited directly onto the primer layer. The primer layer and the topcoat layer are then co-dried to substantially remove any water. 
     In another aspect, a method for coating a substrate according to the present disclosure includes using an immersion deposition technique to immersion deposit a second coating layer onto a first coating layer. The immersion deposition technique has, with respect to the first coating layer, a thickness-dependent ability to deposit the second coating layer onto the first coating layer. The first coating layer is deposited to have a thickness that permits immersion deposition of the second coating layer onto the first coating layer using the immersion deposition technique. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  illustrates a method for coating a substrate. 
         FIG. 2  illustrates a substrate during a deposition process. 
         FIG. 3  illustrates a process diagram for a method of coating a substrate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates a method  20  for coating a substrate. As an example, the method  20  can be utilized to deposit multiple layers of organic-based coatings, such as synthetic or natural resin-based paints, onto a substrate. Non-limiting examples of organic coatings include cements, alkyds, acrylics, vinyl-acrylics, vinyl acetate/ethylene, polyurethanes, polyesters, melamine resins, epoxy, or oils paints. 
     The method  20  generally includes a first immersion deposition step  22 , a second immersion deposition step  24  and a drying step  26 , although it is to be understood that the method  20  may also include additional steps. As will be described in further detail below, the second immersion deposition step  24  is conducted while a first coating layer from the first immersion deposition step  22  is still in a wet state. Thus, the method  20  employs a “wet-on-wet” technique and does not utilize an intermediate drying step in between the first immersion deposition step  22  and the second immersion deposition  24 . The elimination of the intermediate drying step reduces capital costs and operating costs. 
     The first immersion deposition step  22  includes depositing the first coating layer onto the substrate such that the first coating layer is deposited in a wet state with regard to any water and solvents in the first coating layer. For example, the first coating layer is saturated with water and solvent in the wet state. While the first coating layer is in the wet state, the second coating layer is deposited onto the first coating layer in the second immersion deposition step  24 . As an example, the first coating layer serves as a primer layer and the second coating layer serves as a topcoat layer over the primer layer. In a further example, the second coating layer is deposited directly onto the first coating layer, without any other intermediate layers in between. 
     The first coating layer and the second coating layer are then co-dried in the drying step  26  to substantially remove (&gt;98%) any solvents therefrom, and optionally also to cure the first coating layer and the second coating layer. The term “co-dried” or variations thereof refers to subjecting the first coating layer and the second coating layer to a single drying step that occurs uninterrupted over a single time period, rather than separate drying steps that occur over time periods that are split by other processing steps. 
     In a further example, the substrate is a metallic substrate, such as an iron-based alloy, and the first immersion deposition step  22  includes electrodeposition of the first coating layer onto the substrate and the second immersion deposition step  24  includes electrodeposition the second coating layer onto the first coating layer. 
     In one example, the electrodeposition of the coating layers is conducted using respective aqueous-based emulsion coating materials that, in an applied electric field, deposit onto the substrate. An applied voltage and applied voltage time may be controlled to control the deposited thicknesses of the coating layers. In a further example, the respective aqueous-based emulsions are free of any conductive additives. The aqueous-based emulsions have sufficient inherent conductivity to effect electrodeposition. The general principles of electrodeposition are known are therefore not described in further detail herein, except to note that a layer deposited by electrodeposition includes residual water and solvent from the deposition solution until such water and solvent is removed by forced drying. 
     The second coating layer is deposited in the second immersion deposition step  22  while the first coating layer is in the wet state with regard to water or solvent in the first coating layer. Instead of removing or substantially removing the water or solvent from the first coating layer prior to the deposition of the second coating layer, the second deposition step  24  is conducted while the first coating layer is in the wet state. That is, the second immersion deposition step  24  is conducted without first subjecting the first coating layer to a drying step. A “drying step” as used herein refers to the process of heating to a temperature above ambient, which for purposes of this disclosure is above approximately  30 ° C. 
     In another example, the first immersion deposition step  22  includes autodeposition of the first coating layer and the second immersion deposition step  24  includes autodeposition of the second coating layer. As used herein, “autodeposition” or variations thereof refers to an aqueous, organic coating process that relies on the diffusion of ions to and from the surface of the substrate to drive chemical reactions that result in deposition of a coating layer. The chemical reactions that result in the deposition of the coating layer proceed without the aid of an applied electrical field but depend upon the diffusion of ions to and from the substrate surface. 
     Autodeposition and electrodeposition are each thickness-dependent immersion deposition techniques that depend upon on the thickness of the first coating layer. The ability of the autodeposition process to deposit the second coating layer depends upon the ability of ions to diffuse through the first coating layer, while the ability of the electrodeposition process to deposit the second coating layer depends upon the conductivity of the first coating layer. In these regards, if the first coating layer has a thickness that exceeds a threshold thickness (t), the first coating layer “self-seals” such that the second coating layer cannot be deposited. Thus, if the first coating layer “self-seals,” there is insufficient ion diffusion to support autodeposition of the second coating layer thereon or, for electrodeposition, there is insufficient conductivity through the first coating layer to support electrodeposition of the second coating layer thereon. 
     The discovery that immersion deposition can be used to deposit the second coating layer onto the first coating layer while in the wet state, without the use of conductive additives in the first coating layer, enables a reduction in capital equipment. For example, a comparative process may utilize one coating line to electrodeposit and dry a first coating layer and a second coating line to spray-deposit and dry a topcoat layer. The ability, per this disclosure, to immersion deposit the first coating layer and immersion deposit the second coating layer onto the first coating layer while in the wet state enables a coating line to be eliminated by using a single coating line to deposit both the first and second coating layers. 
       FIG. 2  illustrates an aspect of the method  20  that involves using a thickness-dependent immersion deposition technique, such as electrodeposition or autodeposition, to deposit the second coating layer onto the first coating layer, where the first coating layer is deposited with a thickness (t 1 ) that permits deposition of the second coating layer using the thickness-dependent deposition technique. 
     In this example, a substrate  30  is shown during immersion deposition of a second coating layer  32  onto a first coating layer  34 . The immersion deposition technique used to deposit the second coating layer  32  has, with respect to the first coating layer  34 , a thickness-dependent ability to deposit the second coating layer  32  onto the first coating layer  34 . In this regard, the first coating layer  34  is deposited with the thickness (t 1 ), which permits deposition of the second coating layer  32  using the deposition technique. 
     The first coating layer  34  defines a threshold thickness (t), above which the first coating layer  34  does not support immersion deposition of the second coating layer  32 . As an example, up to the threshold thickness (t), the first coating layer  34  remains sufficiently conductive (to conduct electrons e −  in electrodeposition) or porous (for the diffusion of ions in autodeposition) and thereby permits or supports immersion deposition of the second coating layer  32 . In contrast, if the first coating layer  34  were deposited with a thickness that exceeds the threshold thickness t, the first coating layer  34  would not have sufficient conductivity or porosity to support the immersion deposition of the second coating layer  32 . Put another way, the coating material that would otherwise form the second coating layer  32  either would not deposit onto the first coating layer  34  or would not completely and uniformly cover the first coating layer  34 . Similarly, at a threshold combined thickness of the layers  32  and  34 , the layers  32  and  34  will “self-seal” and cease support of further deposition. 
     In one aspect, the method  20  therefore involves the selection of the deposited thickness (t 1 ) of the first coating layer  34  and a deposited thickness (t 2 ) of the second coating layer  32  to ensure not only that the coating layers  32  and  34  can be deposited as high quality and uniform layers but also that the layers  32  and  34 , once dried, are of sufficient thickness to serve the functional protective and aesthetic purposes. To this end, the thicknesses (t 2 ) and (t 1 ) are selected to be within a predetermined ratio of t 2 /t 1  (t 2  divided by 0). In one example, the ratio of t 2 /t 1  is from 0.33-4. In a further example, the ratio is  0 . 66 - 2 , and in a further example, the ratio is 0.66-1.2. The example ratios ensure that the coating layers  32  and  34  can be deposited as high quality and uniform layers and that, once dried, the layers  32  and  34  are of sufficient thickness to serve the functional protective and aesthetic purposes. Further, the ratios also permit tailoring the thicknesses (t 1 ) and (t 2 ) such that minimum thickness values can be determined to minimize weight and amounts of coating materials used. 
     In a further example, the as-deposited, wet thickness (t 1 ) of the first coating layer  34  is 25-30 micrometers and the as-deposited, wet thickness (t 2 ) of the second coating layer  32  is 10-100 micrometers. In a further example, the as-deposited, wet thickness (t 2 ) of the second coating layer  32  is 20-50 micrometers or 20-30 micrometers. 
       FIG. 3  illustrates a further example method  120  that involves the first immersion deposition step  22 , the second immersion deposition step  24  and the drying step  26 , as described above, in combination with additional steps. As shown, the product to be coated is first cleaned in at least one degreasing step  40  that is followed by at least one rinsing step  42 . The rinsing step  42  may involve several cycles of rinsing. The product is then subjected to an optional surface activation step  44  and surface treatment coating step  46  before optional an optional water wash step  48  and DI or RO water wash step  50 . The product is then subjected to the first immersion deposition step  22  to apply the first coating layer onto the substrate. 
     Following deposition of the first coating layer, the product is then subjected to at least one additional rinsing step  52  prior to application of the second coating layer in the second deposition step  24 . The rinsing step  52  may involve several cycles of rinsing. For example, the rinsing step  52  includes washing the first coating layer using an aqueous solution or fluid, such as a conductive ultrafiltration fluid. The rinsing removes any loose coating material from the first coating layer. 
     After the rinsing step  52 , the product is then subjected to at least one additional rinsing step  54 , similar to rinsing step  52 , and an optional DI or Ro water wash step  56 . The rinsing step  54  may involve several cycles of rinsing. The product is then dried at drying step  26 . The drying step  26  may include heating the product with the first coating layer and the second coating layer at an elevated temperature above 30° C. to remove any water or solvent from the layers. The heating may also be used to cure the coating layers. The product may then be cooled and other processing steps may thereafter be conducted as necessary. Each of the steps may be carried out in containers or tanks, for example. 
     In comparison to methods that utilize spray deposition of the second coating after the electrodeposition of the primer coating, the disclosed method  20 / 120  that involves the immersion deposition of the second coating layer onto the first coating layer while the first coating layer is in a wet state, and which does not utilize an intermediate drying step in between the first immersion deposition step  22  and the second immersion deposition  24 , is simplified and reduces capital cost and energy consumption by eliminating the need for additional labor and drying equipment. The method  20 / 120  also eliminates spray deposition of organic coatings, which thus reduces volatile organic emissions and waste-water generation. 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.