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PAINT & COATINGS MANUFACTURING: ENAMELS


             Chemical Composition  Ground Coat (weight %)  Cover Coat (weight %)  Basic Function
               Silicon dioxide (SiO2)  40 - 50   40 - 50        Glass former
              Aluminum oxide (Al2O3)  0 - 3       0 - 2         Glass former
               Boron trioxide (B2O3)   15 - 20   15 - 20        Glass former
             Phosphorus pentoxide (P2O5)   N/A    0 - 2         Glass former
                                                                            one coat/one fire process. In two coat/one fire
               Lithium oxide (Li2O)  1 - 5        1 - 2          Modifier
                                                                            processing, the first coat is a special ground
               Sodium oxide (Na2O)  10 – 20      10 - 20         Modifier    coat enamel (designed to adhere to the steel)
                                                                            which is applied to a thickness of one to two
               Potassium oxide (K2O)  4 – 8       4 - 8          Modifier
                                                                            mils, directly followed in a second application
               Calcium oxide (CaO)  0 - 5         N/A            Modifier
                                                                            booth with a cover coat (for the final color of
                Zinc oxide (ZnO)  0 - 3           0 - 3          Modifier    the part) at a thickness of five to seven mils.
               Titanium dioxide (TiO2)  N/A      15 - 22        Opacifier    Both layers are melted/cured at about 1500
                                                                            degrees Fahrenheit in a single pass through the
              Zirconium dioxide (ZrO2)  0 - 5     0 - 2         Opacifier
                                                                            furnace. Firing times are typically dependent
                Cobalt oxide (CoO)  0 - 1         N/A          Color and bond  on the thickness of the metal; for successful
                Nickel oxide (NiO)  0 - 2         N/A          Color and bond  enamel coatings the time at peak temperature
                                                                            is two to three minutes.
              Manganese oxide (Mn2O)  0 - 3       N/A          Color and bond
                                                                              For electrostatic powder enameling, the
                 Flourine (F)     2 - 6           3 - 6      Fluidity & surface tension  benefits are similar to powder paint systems.
                                                                            They include full recovery of overspray, coating
           Frit Compositional Ranges.
                                                                            thickness control, film uniformity, better edge
                                                                            coverage, gloss, and smoothness of finish after
         and 2) have the appropriate chemical durability for its intended use.   firing, plus the advantages of the automation and control of today’s
         In special cases, a single coat of enamel must accomplish all of these   powder application systems. The primary limitation for electrostatic
         things, and in other cases, three enamel layers are required. The table   powder enameling is the choice of colors; this is because the color must
         shows the common chemical compositional range of both ground   be smelted into the frit. The use of frit plus pigment is problematic due
         coat and cover coat enamels, along with the basic function of the   to differences in particle size, density, and electrical charging parame-
         chemical constituents.                             ters – thus, recycling is difficult since the various constituents apply at
                                                            different rates. Color change requires either multiple booths or signif-
         Porcelain Enamel Preparation                       icant clean-up time.
         To prepare the frit for application to the substrate, it is combined with   We have seen recent growth in the combination of robotics with
         other materials, which may include refractories, pigments, opacifiers,   powder application, which is improving production repeatability
         clays, and surfactants. These ingredients are mixed as a dry powder or   and efficiencies as well as reducing costs. R&D on electrostatic
         an aqueous suspension to be applied to the metal substrate. The frit   powder enamel chemical formulations and grinding technologies is
         and raw materials are usually ball-milled to accomplish both complete   also continuing with the objectives of improving application param-
         mixing and proper particle size reduction and distribution. Today,   eters and reducing defects.
         most enamel applicators purchase either electrostatic dry powder
         enamel which is ready to spray or ready-to-use (RTU) powder where   Wet Enamel Application
         water is added to make the liquid enamel suspension.   For wet porcelain enamel systems, the typical application includes
                                                            spraying or flow-coating. However, there are other application
         Powder Enamel Application                          methods for specific end products such as roll coating of a continu-
         With the invention of powder coating about 60 years ago, a lot of   ous strip for whiteboards  and electrocoating or electrophoresis
         work began in the coatings and finishing industries to leverage this   which provides a very smooth uniform surface, along with best edge
         new technology. The porcelain enamel industry was an early adopter   and hole coverage, when both sides of a substrate are enameled. Dec-
         of electrostatic powder coating, but  a lot of development was     orating enameled products/surfaces employs a variety of other pro-
         required from both the material and equipment aspects. By the   cesses including screen printing, direct emulsion imaging, digital
         1980s, porcelain enamel powder coating was taking place in many   ink-jet printing, transfers (i.e., water slide decals), and hand painting.
         factories, primarily for flatware but also in several lines for cavities,
         like ovens and dishwashers.                        Firing
           The physics of powder coating of porcelain enamel powder is the   This is the step in the process where two materials, glass and steel,
         same as for traditional powder paint, although there are some differ-  become one. Once coated, the parts must be fired at about 1,500 deg.
         ences in behavior, etc. One of the most noticeable is the transfer     F to properly fuse the glass coating and establish the strong chemical
         efficiency; porcelain enamel powder is about 40 percent efficient, so   bond to the steel or other metallic substrate.
         much more powder travels through the recovery system in compari-
         son with traditional powder paint. Additionally, porcelain enamel   New Developments
         powders are often applied in a thicker film; usually six to eight mils.   Currently there are a number of developments underway in multiple
         Almost all of the electrostatic dry powder porcelain enamel applica-  areas, including new uses for porcelain enamel, improved frits and ma-
         tion is for major appliances; for exterior parts it is usually a two   terials, as well as better enamel application systems and equipment.
         coat/one fire process, and for interior (non-appearance) parts it is a   One promising new use of porcelain enamel is a reactive glass
         34            CANADIAN FINISHING & COATINGS MANUFACTURING                                                                                                         MAY/JUNE 2021
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