Patent Publication Number: US-11661528-B2

Title: Methods of producing colored and superhydrophobic surfaces, objects, and coatings

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/096,873, filed Oct. 26, 2018, titled DURABLE SUPERHYDROPHOBIC COLOR PAINT, which was a National Stage Entry of PCT/US2017/026943, filed Apr. 11, 2017, which claims the benefit of U.S. Provisional Application Ser. No. 62/327,672, filed Apr. 26, 2016, the disclosure of which is hereby incorporated by reference in its entirety, including all figures, tables and drawings. 
    
    
     BACKGROUND OF INVENTION 
     Superhydrophobicity is defined as a material or surface with a water contact angle greater than 150° and the roll off angle or contact angle hysteresis less than 10°. The coating is hard to wet by water which imparts some compelling properties like self-cleaning and antibio-fouling. Textbooks describe superhydrophobicity as depending on the surface roughness or topography. The best published phenomenon is the lotus-effect, which occurs because of the affluent tiny protrusions on the lotus or taro leaf to yield a contact angle &gt;150° accompanied by a few degrees of roll-off angle. The second factor that is important for superhydrophobicity depends on the surface material and, typically, fluorinated compounds are employed to reduce surface energy to levels appropriate for superhydrophobicity. The most crucial criterion for superhydrophobicity is retaining the water droplet in the Cassie-Baxter state, where air pockets are trapped under the droplet to reduce the solid-liquid interface. State of the art coatings with micro-scale roughness often possess the drawbacks of poor durability and/or poor optical properties. Therefore, a durable superhydrophobic surface that is scalable to coverage of a large surface remains a goal. Superhydrophobic paints, though not durable, have only been formulated as white paint. Coloring with standard pigments caused deterioration of superhydrophobicity. The particulate pigments that can be added to paint that do not detract or help in superhydrophobic behavior are desirable. 
     BRIEF SUMMARY 
     Embodiments of the invention are directed to colored superhydrophobic paints that comprise hydrophobic particles, a polymer binder, and at least one plasticizer suspended in a solvent, wherein at least one of the hydrophobic particles is a colored hydrophobic particle. The hydrophobic particles can be metal oxide particles, which can, optionally, include white metal oxide particles, such as SiO 2  TiO 2 , and/or Al 2 O 3  coated with a fluorinated alkyl silane or an alkyl silane and necessarily include colored metal oxide particles, such as ultramarine, iron oxide and/or chromium oxide coated with a fluorinated alkyl silane or an alkyl silane. The hydrophobic particles are 40 nm to 100 micrometers in diameter. The polymer binder can be a mixture of PDVF and PMMA in a 3:1 to 10:1 ratio. The plasticizer can be a mixture of triethyl phosphate and perfluoro(butyltetrahydrofuran). The solvent can be DMF (dimethylformamide), MEK (methyl ethyl ketone), or isophorone. Using the superhydrophobic paint, any object having a surface comprising glass, plastic, wood, or metal can be painted by spraying, rolling, brushing, or spin coating to form a superhydrophobic object. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows a plot of particle size for the specific surface area of silica particles used to prepare exemplary paints according to an embodiment of the invention. 
         FIG.  2    shows a photograph of surfaces painted with blue paints B 1 -B 4 , according to an embodiment of the invention. 
         FIG.  3    shows a photograph of surfaces coated with B 1 -B 4  paints, according to an embodiment of the invention, with 20 μL water droplets from which the water contact angles (WCAs). 
         FIG.  4    shows a photograph of surfaces painted with red paints R 1 -R 4 , according to an embodiment of the invention. 
         FIG.  5    shows a photograph of surfaces coated with R 1 -R 4  paints, according to an embodiment of the invention, with 20 μL water droplets from which the water contact angles (WCAs). 
         FIG.  6    shows a photograph of surfaces painted with green paints G 1 -G 4 , according to an embodiment of the invention. 
         FIG.  7    shows a photograph of surfaces coated with G 1 -G 4  paints, according to an embodiment of the invention, with 20 μL water droplets from which the water contact angles (WCAs). 
         FIG.  8    shows composite plots of WCAs on surfaces coated with B 1 -B 4  paints, according to an embodiment of the invention, from 20 μL water droplets at 20° C. after a series of 0 to 1000 cycles of wiping with an abrasive. 
         FIG.  9    shows composite plots of weight loss in mg after abrasion of surfaces coated with B 1 -B 4  paints, according to an embodiment of the invention, after a series of 0 to 1000 cycles of wiping with an abrasive. 
         FIG.  10    shows composite plots of WCAs on surfaces coated with R 1 -R 4  paints, according to an embodiment of the invention, from 20 μL water droplets at 20° C. after a series of 0 to 1000 cycles of wiping with an abrasive. 
         FIG.  11    shows composite plots of weight loss in mg after abrasion of surfaces coated with R 1 -R 4  paints, according to an embodiment of the invention, after a series of 0 to 1000 cycles of wiping with an abrasive. 
         FIG.  12    shows composite plots of WCAs on surfaces coated with G 1 -G 4  paints, according to an embodiment of the invention, from 20 μL water droplets at 20° C. after a series of 0 to 1000 cycles of wiping with an abrasive. 
         FIG.  13    shows composite plots of weight loss in mg after abrasion of surfaces coated with G 1 -G 4  paints, according to an embodiment of the invention, after a series of 0 to 1000 cycles of wiping with an abrasive. 
     
    
    
     DETAILED DISCLOSURE 
     Embodiments of the invention are directed to a non-white colored paint comprising: optionally, white functionalized particles; binders that are a polymer blend of polymethyl methacrylate (PMMA) and polyvinylidene fluoride (PVDF); plasticizers that are triethyl phosphate and perfluoro compounds; and colored particles that allow the retention of a contact angle in excess of about 150° for the superhydrophobicity. In embodiments of the invention, silica particles of specific surface area of, for example, 35-65 m 2 /g are employed as a base white hydrophobic particle after surface functionalization. As indicated in  FIG.  1   , the diameter of silica particles is 50 to110 nm. The silica particles are functionalized by a silane coupling agent, for example, heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilcane, to provide a fluorination entity on the particles. In embodiments of the invention, the binders are a blend of, for example, PMMA and PVDF of molecular weights, for example, 75,000 and 900,000 to 1,300,000, respectively, that comprise a solution in mixed dimethylformamide and acetone. In embodiments of the invention, the plasticizer is, for example, a mixture of triethylphosphate and perfluoro(butyltetrahydrofuran) or other phosphates and prefluorocarbons. This composition is substituted for, or is included with the white particles, blue, red, green and/or other colored pigments, for example, from hydrophobized ultramarine (lapis lazuli, a feldspathoid silicate), hydrophobized red matte (iron oxide) and hydrophobized green matte (chromium hydroxide), or other colored metal oxides, which have been hydrophobized in the manner of the silica particles, above, or hydrophobized by surface treatment with other silane coupling agents. By combination of these components a homogeneous colored superhydrophobic paint is formed. 
     Other white particulates in addition to SiO 2  can be any white metal oxide, including, but not limited to, TiO 2 , Al 2 O 3 , or other related ceramic powders having particle diameters of 40 nm to 100 micrometers. The particles can be functionalized with a compound to form a self-assembled monolayer or a surface specific attachment that is fluorinated for a low surface energy, where, in addition to heptadecafluoro-1,1,2,2-tetrahydrodecyl-trichlorosilcane, the functionalizing agent can be heptadecafluorodecyl trichlorosilane, heptadecafluoro-1,1,2,2,-tetrahydrodecyltrimethoxy-silane, 1H,1H,2H,2H-perfluorodecyltri-ethoxysilane, other perfluoroalkyl silanes, or a long-chain alkyl silane, such as octadecyltricholosilane. The volume percent particulates in the paint can be 50 to 75%. The binder can be, for example, PDVF and PMMA mixture, and has a PVDF to PMMA ratio of about 5 to 1, about 10 to 1, about 9 to 1, about 8 to 1, about 7 to 1, about 6 to 1, about 4 to 1, about 3 to 1, or any ratio between about 3:1 and 10:1 
     According to an embodiment of the invention, the paint can be applied and dried to form a coating on a substrate. The paint can be applied by spraying, rolling, brushing or any other method. The substrate can be any surface, including a glass, plastic, metal, wood, or as a top coating on another coating. By changing the proportion of particles of the base white pigment and a “standard binder solution,” and formulated as provided in the Methods and Materials section, below, paints that are durable and hydrophobic can be produced, as indicated in Table 1, below. The white particles can be substituted with colored particles with similar resulting properties, as provided below in the Methods and Materials section. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Coating properties for various white paint compositions. 
               
            
           
           
               
               
               
               
            
               
                   
                 Formulation 
                 Contact Angle 
                 Observations 
               
               
                   
               
               
                   
                 74% particles 
                 &gt;165° 
                 Particles removed by rubbing 
               
               
                   
                 61% particles 
                 &gt;160° 
                 Particles retained after rubbing 
               
               
                   
                 39% particles 
                 ≈120° 
                 Particles firmly embedded 
               
               
                   
                 64% particles 
                 &gt;165° 
                 Particles retained after rubbing 
               
               
                   
               
            
           
         
       
     
     The formulation with 64% particles, referred to as the SG paint, was used to formulate with colored particles to form exemplary colored paints, according to embodiments of the invention. Alternatively, the paint can be prepared with any solvent that permits the blending of PVDF and PMMA. Solvents that can be employed include, but are not limited to DMF (dimethylformamide), MEK (methyl ethyl ketone), and isophorone. Additionally, other acrylates and methacrylates can be combined in the paint. The acrylates and methacrylates can be homopolymers or copolymers. For example, a copolymer of methyl methacrylate and ethyl acrylate can be used to form the binder. PMMA can be atactic, syndiotactic, or isotactic. 
     Methods and Materials 
     Silica particles, Aerosil Ox 50, were purchased from Evonik Industries. The specific surface area of the particles is 35-65 m 2 /g. The diameter of silica particles is between 50-110 nm. Heptadecafluoro-1,1,2,2,-tetrahydrodecyltrichlorosilane was purchased from Gelest Inc. PVDF was obtained from Kynar Hsv 900 with M n  900,000-1,300,000 g/mol and PMMA was obtained from Polyscience Inc. with M n  75,000. Perfluoro(butyltetrahydrofuran) FC75™, was purchased from ACROS. Blue matte, red matte, and green matte, are ultramarine, iron oxide and chromium oxide and were purchased from Powdered Up Dolly. 
     Silica particles were dehydrated in an oven at 120° C., cooled and dispersed in chloroform. Subsequently, heptadecafluoro-1,1,2,2,-tetrahydrodecyltrichlorosilcane was added to the silica-chloroform dispersion and stirred for one hour. The dispersion was centrifuged and the chloroform decanted. The fluorinated particles were dried at 120° C. on a heating plate. In like manner, ultramarine, iron oxide and chromium oxide were treated with heptadecafluoro-1,1,2,2,-tetrahydrodecyltrichlorosilcane to form hydrophobized blue matte, red matte, and green matte. 
     PVDF and PMMA were blended where 0.21 g PVDF, 0.8 g acetone, 0.04 g PMMA, 1 g triethyl phosphate and 100 μl FC-75 using a vortex mixture to homogenize the liquid, which is referred to herein as the “standard binder solution.” 
     Blue paint B 1  was formed from hydrophobized blue matte 0.6 g and the standard binder solution (3.96 g). Blue paints B 2  to B 4  were formed by combining 4.56 g of SG paint with 0.1, 0.2, and 0.3 g of hydrophobized blue matte, respectively. The intensity of the blue color is shown in the photograph of  FIG.  2   . Water contact angles were measured using DI water with droplets of 20 μL at an ambient temperature of about 20° C., for superhydrophobicity evaluation, as shown in  FIG.  3   . 
     Red paint R 1  was formed from hydrophobized red matte 0.6 g and the standard binder solution (3.96 g). Red paints R 2  to R 4  were formed by combining 4.56 g of SG paint with 0.1, 0.2, and 0.3 g of hydrophobized red matte, respectively. The intensity of the red color is shown in the photograph of  FIG.  4   . Water contact angles were measured using DI water with droplets of 20 μL at an ambient temperature of about 20° C., for superhydrophobicity evaluation, as shown in  FIG.  5   . 
     Green paint G 1  was formed from hydrophobized green matte 0.6 g and the standard binder solution (3.96 g). Green paints G 2  to G 4  were formed by combining 4.56 g of SG paint with 0.1, 0.2, and 0.3 g of hydrophobized green matte, respectively. The intensity of the green color is shown in the photograph of  FIG.  6   . Water contact angles were measured using DI water with droplets of 20 μL at an ambient temperature of about 20° C., for superhydrophobicity evaluation, as shown in  FIG.  7   . 
     Abrasion Testing and Wear Indexing of Painted Surfaces 
     Wear testing was carried out in the following manner A Taber linear 5700 Abrader was used with silicon carbide metallurgical paper 1200P with a loaded weight of 0.98 N, with water droplet of 20 μl employed for contact angle measurement at 20° C. 
     Blue paint B 1  lost superhydrophobicity after 200 wiping cycles and B 4  after 400 cycles. In contrast, B 2  and B 3  displayed durability and remained superhydrophobic through 1000 wiping cycles, as indicated in  FIG.  8   . From the weight loss plotted in  FIG.  9   , wear indexes were calculated to be −6.2 E−3, −5.6 E−3, −2.4 E−3, and −1.2 E−3 mg/per cycle for B 4 , respectively. Total weight losses after 1000 cycles for B 1 -B 4  were 0.26, 0.24, 0.09, and 0.04 percent for B 1 -B 4 , respectively. 
     Red paints R 1 -R 4  displayed excellent durability with retention of superhydrophobicity after 1000 wiping cycles, as plotted in  FIG.  10   . The calculated wear index from the data plotted in  FIG.  11    was −10.2 E−3, −11 E−3, −6.3 E−3, and −2.2 E−3 mg/per cycle for R 1 -R 4 , respectively. Total weight losses after 1000 cycles for R 1 -R 4  were 0.47, 0.41, 0.25, and 0.09 percent for R 1 -R 4 , respectively. 
     Green paints, G 1 -G 4  displayed excellent durability with retention of superhydrophobicity after 1000 wiping cycles, as plotted in  FIG.  10   . The calculated wear index from the data plotted in  FIG.  11    was −2 E−3, −11.6 E−3, −0.5 E−3, and −2.7 E−3 mg/per cycle for G 1 -G 4 , respectively. Total weight losses after 1000 cycles for G 1 -G 4  were 0.08, 0.45, 0.02, and 0.11 percent for G 1 -G 4 , respectively. 
     It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.