Page 34 - CFCM May-June 2021_Neat

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