IRON OXIDE/GRAPHITE INTERNAL CONDUCTIVE COATINGS
FOR COLOR KINESCOPES
Iron oxide/graphite conductive coatings have been widely accepted throughout the world for use in color CRTs because these coatings provide excellent
performance and reliability in today's demanding TV markets. A variety of coatings are available to meet the properties and characteristics required for specific applications. Iron oxide/graphite coatings offer reduced particle generation to improve high voltage stability, very low outgassing to increase tube life, and lower arc currents to protect solid state circuitry. These coatings have been used successfully in all color kinescope production by Thomson (RCA) for the past two decades.
Iron oxide/graphite coatings were first developed in the early 1970's (U.S. Patent 3,791,546) by RCA. These coatings consist of graphite and iron oxide powders suspended in an aqueous sodium silicate solution. The composition of each coating has been
formulated to control the coating's resistivity, hardness, and adhesion. Table I provides some
information on the composition and properties of several iron oxide/graphite coatings.
Features of these coatings include the following characteristics:
1. The coatings are ready for use in the original concentration. Viscosity adjustments, if needed, can be easily made by simply adding deionized water.
2. Although some settling of the particles occurs during shipment and storage, the coating is easily restored to a completely dispersed state by rolling the container. Only mild agitation is needed to maintain a
uniform dispersed coating solution during manufacturing operations.
3. These coatings can be applied by spray or brush. Coatings of uniform thickness and high hiding power can be readily achieved.
4. These coating have very good adherence to the glass substrate. Baked coatings are free of defects (cracking,
blistering, or peeling) and are not damaged by routine handling of the tube.
5. Iron oxide/graphite coatings are readily degassed by heating. Moisture absorption is very low, reducing the time for outgassing. Low outgassing reduces the depletion of getter, thus extending tube life.
6. These coatings have excellent scratch resistance due to the hardness of the baked film. Particle generation caused by gun spacer contacts or electrical contact springs can thus be reduced. Fewer particles can reduce problems of tube arcing and stray emission.
7. The electrical resistance of the coating can be controlled by the iron oxide to graphite ratio in the coating
formulation. Coatings with greater resistance can be used to decrease the peak arc current.
8. The baked coating can be removed relatively easily in a salvage operation using an acid solution
(5% hydrogen fluoride or ammonium bifluoride) followed by a water rinse. Unbaked coatings can be removed with a water spray.
Table 1. Properties of Iron Oxide/Graphite Conductive Coatings
Application of Iron Oxide/Graphite Conductive Coatings
I . These coatings must be rolled in their shipping container at least four hours before use in manufacturing; 40 to 80 RPM is recommended.
2. Do not add acids, alkali's, salts, or solvents to the coating. Only the addition of deionized water to adjust the viscosity is permitted.
3. These coatings can be applied by brush or spray. Normally the coating is applied to a warm
(50C) funnel for best results,
and a short drying time. The coating thickness (dry) should generally be less than I mil (.025 mm) to avoid peeling, blistering, glass spalling, and other coating defects. Thick coatings also are more conductive, increasing the peak arc current.
4. The strong coating adherence is achieved by baking the coated funnel at a maximum temperature of
430-450C for 15-30 minutes in an air atmosphere. The unbaked coating should not be held at temperatures between 100 and
200C for more than one hour.
5. The unbaked coating can be removed from the funnel with a water spray. The baked coating can be removed with an acid solution
(5% hydrogen fluoride or ammonium bifluoride) followed by a water rise.
Shipping and Handling of Iron Oxide/Graphite Conductive Coatings
1. These coatings are non-flammable and mildly alkaline.
2. DO NOT ALLOW THESE SOLUTIONS TO FREEZE. Freeze-thaw cycles may prevent proper dispersion of components.
3. Packing - Coatings are shipped in one gallon jars or five gallon non-returnable drums.
4. Shelf Life - Six months from date of manufacture (unopened).
Test Methods for Coating Properties
A. Density
Mix the coating by rolling for a minimum of four hours before testing to ensure that the solution is homogeneous. Weigh a 100 cm3 pycnometer. Pour enough coating into the pycnometer to fill it, cleaning off any excess material. Weigh the filled
pycnometer. The density is calculated as follows:
Filled Weight - Empty Weight
100 cm3 Density (g/cc)
This test method is based on ASTM D1475-60.
B. % Solids (By Evaporation)
Mix the coating by rolling for a minimum of four hours before testing to ensure that the solution is homogeneous. Weigh an aluminum weighing dish. Pour about 10 grams of solution into the aluminum dish and reweigh the dish. Place the dish in an oven at 105-1 100C for approximately two hours to completely dry the sample and weigh the dish again (the weight will be constant when dry). The % solids is calculated as
follows:
Da Coating Weight
Wet Coating Weight X(100) = % Solids
This test method is based on ASTM D1644-59.
C. Consistency
Mix the coating by rolling for a minimum of four hours before testing to ensure that the solution is homogeneous. Place a
finger over the exit orifice of a #4 Ford Cup and fill the cup with the coating solution. Use a stopwatch to measure the time to drain the cup (from finger removal to first break in exit stream). Repeat the measurement two times and record the average value. This test method is based on ASTM D1200-70.
D. Hardness
Mix the coating by rolling for a minimum of four hours before testing to ensure that the solution is homogeneous. Use a doctor blade to spread some of the coating in a uniform thickness across a flat sheet of glass (the baked film thickness should be 0.025 mm). Bake the coating in an oven at a peak temperature of
450C for 15 minutes; the heating and cooling period shall not exceed four hours. Use a Hoffinan Scratch Tester to determine the load needed to just penetrate the film. Use a thickness gauge to verify that the film thickness is within the range 0.022 to 0.028 mm. This test method is based on ASTM D2197-68.
E. Resistance
Mix the coating by rolling for a minimum of four hours before testing to ensure that the solution is homogeneous. Use a doctor blade to spread some of the coating in a
uniform thickness across a flat sheet of glass (the baked film thickness should be 0.025 mm). Bake the coating in an oven at a peak temperature of
450C for 15 minutes; the heating and cooling period shall not exceed four hours. Use an olimmeter to determine the linear resistance of the film. Use a thickness gauge to verify that the film thickness is within the range 0.022 to 0.028 mm. This test method is based on ASTM D257.
