The Critical Factors for Developing
Excellent Color Stability in High Temperature Coatings

Client: "Good morning. I'm calling from XYZ Company. I will be coating a stack soon and I'm looking for a coating good to about 850°F and it needs to be in a green color."
Generic Coating Manufacturer: "We have a coating good to that temperature but it only comes in black and aluminum."
Client: "No, that doesn't help me. I really need this particular green. Can you make it for me?"
Generic Coating Manufacturer: "No, that's not possible. I'm sorry we couldn't help."

Some common misconceptions regarding high temperature coatings:

• 1) they cannot be manufactured in a wide variety of colors
• 2) these colors are not stable at elevated temperatures, e.g. 1000°F (538°C)
• 3) they are available in only black and aluminum colors.

A clarification of these misconceptions:

• 1) high temperature coatings can be produced in a wide variety of colors
• 2) high temperature coatings can be manufactured to be color stable at 1000°F (538°C)
• 3) it is certainly possible to manufacture high temperature coatings in colors other than black or aluminum.

It is frustrating to hear these misconceptions, and more so to actually see them in print in trade magazines. We would like to take this opportunity to provide accurate information on this topic.

Typically high temperature coatings in colors are categorized by those able to withstand 500°F (260°C) and those able to withstand 1000 F (538 C). These have there own characteristics and are formulated differently. We will first address these topcoats with a maximum temperature range of 500°F (260°C).

There are several important factors necessary when formulating these 500°F (260°C) coatings.

1) The silicone resin.

The silicone resin selected must exhibit as little color degradation as possible upon being heated to 500°F (260°C).

2) The organic resin.

The organic resin selected must also exhibit as little color degradation as possible when heated to 500°F (260°C).

3)The type of pigments.

Only inorganic pigments may be used in the formulation of this category of coatings. Organic pigments breakdown resulting in discoloration when exposed to 500°F (260°C) and therefore they must not be used. However, this choice alone will not ensure color stability given the fact that the quality of the numerous inorganic pigments available in the market today vary tremendously. Great diligence must be taken when selecting the inorganic pigment which will maintain excellent color stability at 500°F (260°C).

4)The ratio of silicone resin to organic resin.

The ratio between these two resins is critical in the formulation of a coating able to maintain excellent color stability at the maximum temperature. The coating may contain the most color stable silicone and organic resins and still significantly discolor if this ratio is not correct. The particular ratio between the two types of resins is dependent upon the specific resins chosen.

5)Various other constituents.

There are many constituents which make up the final formulation of a coating heat resistant to 500°F (260°C). Each one should be individually tested to determine its effect on the color stability of the system.

The next category is high temperature coatings which are color stable at 1000°F (538°C). The only way to ensure a coating's ability to maintain excellent color stability at this temperature is by using a 100% silicone resin formula. Systems modified with other types of resins such as alkyds will not maintain excellent color stability. The type of silicone resin chosen will also greatly impact the coating's color stability. Most silicone resins in this category will indeed have temperature resistance up to 1000°F (538°C). However, each one must be tested for its ability to maintain excellent color stability at this temperature.

Inorganic pigments must be used and they must be individually tested for the ability to maintain excellent color stability at 1000°F (538°C). Note: Almost all colors can be matched and formulated to have excellent color stability at 1000°F (538°C). However, orange and red pigmented coatings will begin to darken at the 600-700°F (323-377°C) range due to the inherent deficiencies of the red and orange pigments available on the market today.

As explained in the previous section, there are many constituents which make up the final formulation of a heat resistant coating. In this case, each one should be individually tested to determine its effect on the color stability of the system when exposed to temperatures up to 1000°F (538°C).

These factors will assure excellent color stability in a heat resistant coating. However, there are many other desired characteristics besides color stability which can influence the formulation of a quality high temperature coating. These include, but are not limited to:

• A) Dry time
• B) Thermal shock resistance
• C) Application characteristics
• D) VOC content
• E) Film thickness criteria
• F) Corrosion resistance
• G) Curing characteristics
• H) The ability to apply to hot steel
• I) Surface tolerance

In conclusion, the common notion that heat resistant coatings cannot be color stable to elevated temperatures and cannot be manufactured in a wide variety of colors—is false. In order to accomplish this, it is imperative to have a complete understanding of the numerous variables which can influence the formulation of high temperature coatings having excellent color stability.

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