Automotive Powder Coating

Powder coating has come a long way since the 1970's. Recent advancements ensure that powder use will continue to grow through the next century. Much of this growth can be attributed to powder's excellent durability and finish, lower cost and high efficiency. Additionally, powder coating is more environmentally friendly than conventional solvent-borne systems. During the manufacturing and application processes, little or no VOCs are emitted into the atmosphere, and waste is minimized by recycling overspray.

Powder Use.
The global use of thermoset powder coatings has been rapidly growing about 10 pct annually. Consequently, the performance expectations that manufacturers place on powder coatings have become more demanding. Nowhere is this more true than in the automotive market. While automobile assembly plants are in the infancy stage of powder application, their suppliers have been using powders to coat various automobile parts for years. There are virtually hundreds of such suppliers in the United States. The automobile parts to which powder coatings are applied include:

• roof racks
• door handles
• oil and fuel filters
• brake pads
• engines
• exterior and interior trim

These suppliers work with various powder manufacturers to ensure that the coatings meet stringent automobile specifications. There are ten major powder manufacturers that account for about 70 pct of the sales to this market. Combined, these manufacturers sold more than $100 million of powder to the automobile parts industry in 1994.

Durability Requirements.
Both the polymer and the pigments used in powder coatings can be attacked by the elements associated with exterior weathering (ultraviolet light, moisture, and heat). Over time, this causes deterioration that may result in film delamination, fading and a loss of gloss. For these reasons, powder coatings are tested to meet automotive specifications prior to their use by suppliers to the automotive OEM's. Some of the most common tests are Weatherometer, QUV and humidity. In addition to these accelerated tests, most automotive specifications also require that the products be tested for corrosion resistance and resistance to various solvents and chemicals. For a product to be approved, it must meet all of the criteria set forth by the automobile manufacturers.

Since the average life of an automobile is about six to eight years, the ideal situation would be for there to be at least this amount of weathering exposure data. Consequently, automotive OEMs are now increasing their durability requirements to meet increasingly harsher environments. Depending on applications, current OEM specifications usually require a minimum of one year South Florida exposure while others require two years of data. In the future, this requirement will be raised to five years for most exterior applications.

General Chemistry Background.
With increasing government regulation of VOC emissions and the new rules governing HAPs, Hazardous Air Pollutants, more and more suppliers are turning to powder coating for compliance. This has resulted in several trends regarding preferred chemistries for specific end-uses.

Polyester/TGIC.
The most familiar of these are the polyester/TGIC coatings originally used as clear coats, and later as metallic base coats on wheels. These coatings show excellent durability with good impact resistance, and, when covered with a clearcoat, durability approaches five years. With improvements in resin technology and increasing smoothness, the polyester/TGIC systems are approaching the look of acrylics. Table I gives some of the typical properties and ranges for Polyester/TGIC systems.

In addition to excellent weatherability, other advantages of polyester/TGIC systems include excellent gloss retention, over-bake color stability, good mechanical properties at high-film build, good salt-spray resistance, and a wide range of colors, including metallic effects and textures. Some of the drawbacks are that it is difficult to formulate a low-gloss finish, and to get very smooth coatings at film builds lower than 1.5 mils. Additionally, polyester/TGIC systems offer lower solvent resistance than the polyester/urethane chemistries.

Acrylics.
Acrylic systems are an emerging technology because of the more stringent weathering requirements of OEM's. Currently, the most common acrylic application is as a clearcoat on wheels. Some common uses include exterior trim and blackout coatings.

Acrylics offer the advantages of excellent weathering combined with lower temperature cure, good distinctness of image, excellent gloss retention, and good salt-spray resistance. They are among the smoothest of all powder coatings in use today. The drawbacks to the acrylic chemistry include low impact resistance, poor flexibility, and incompatibility with other chemistries. Table II gives some typical properties of acrylics.

Polyester/Urethane.
A powder chemistry that is not quite as smooth as acrylic but allows better flexibility and impact resistance is polyester/urethane. Powder coatings of this type offer good salt-spray resistance and gloss retention, up to 4H pencil hardness, and low-gloss matte finishes, as well as exceptional flexibility. Table III gives some typical properties.

The main disadvantage for urethanes is thick film limitations. This is due to the volatilization of the blocking agent during cure.

Hybrid.
The hybrid system is a proven chemistry for primers. Offering certain advantages of epoxy systems combined with polyester systems, these powders will offer good intercoat adhesion and chip resistance. Hybrids also offer good mechanical properties combined with good salt-spray resistance. Some of the disadvantages of hybrids are marginal UV resistance and softer films than epoxies. Table IV shows typical properties of hybrids.

Epoxy.
The final powder chemistry used by the automotive market is the epoxy system. These are primarily used under the hood where UV exposure in minimal and gloss retention is not a concern. Some of the most common uses are for engine block castings, suspension components, and radiators. The advantages of epoxies include excellent chemical resistance, smoothness, excellent corrosion resistance and adhesion, and good abrasion resistance. The primary disadvantage is poor UV resistance, which leads to chalking and loss of gloss. As a result, epoxies are ideal for under the hood since there is very little UV exposure and corrosion resistance is key. Table V offers some typical properties of epoxy powder coatings.

From the preceding information one can see that there is an emerging trend toward powder coating use in the automotive market. With the various chemistries available today, one can be found to meet almost every need of today's OEM automotive part producer. Powder coatings offer excellent physical properties and are more environmentally friendly than conventional solvent-based systems. With increasing regulations, powder will certainly become an even bigger player in the automotive market.

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TABLE I-Properties of Polyester/TGICs
Typical Film Thickness	1.5 to 10 mils
Coverage	107 to 160 sq ft per mil of coating
Typical Bake Schedules	15 min. at 325F Low Temp, 15 min. at 375F Normal
Impact Range	40 to 160 inch lbs
Pencil Hardness	H to 3H
Flexibility	0.25 inch typical

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TABLE II-Properties of Acrylics
Film Thickness	1.0 mils to 10 mils
Coverage	110 to 180 sq ft/lb per mil of coating
Cure Schedule	20 min. at 325F Typical, 15 min. at 375F High Temp,30 min. at 275F Low Temp
Impact	20 to 100 inch lbs
Pencil Hardness	H to 3H
Flexibility	0.25 inch typical

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TABLE III-Properties of Polyester/Urethanes
Film Thickness	1.2 to 4.0 mils typically
Coverage	107 to 160 sq ft/lb per mil of coating
Cure Schedules	10 min. at 400F High Temp, 15 min. at 375F Normal Temp, 20 min. at 345F Low Temp
Impact Resistance	40 to 160 inch lbs
Pencil Hardness	H to 4H
Flexibility	0.125 inch typical

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TABLE IV-Properties of Hybrids
Film Thickness	1.5 to 20 mils typically
Coverage	107 to 160 sq ft/lb per mil of coating
Cure Schedules	10 min. at 400F High Temp, 15 min. at 375F Normal Temp, 20 min. at 325F Low Temp
Impact Resistance	50 to 160 inch lbs
Pencil Hardness	H to 2H
Flexibility	0.125 inch typical

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TABLE V-Properties of Epoxies
Film Thickness	1.2 to 50 mils+
Coverage	107 to 160 sq ft/lb per mil of coating
Cure Schedules	10 min. at 400F High Temp, 15 min. at 350F Normal Temp, 30 min. at 275F Low Temp
Impact Resistance	80 to 160 inch lbs
Pencil Hardness	2H to 5H
Flexibility	0.125 inch typical