Coil Powder Coating

Article From: Products Finishing, , from Products Finishing

Posted on: 9/1/1997

Until recently coil powder coating was a slow process. Now it has been proven in other web applications to run up to 900 fpm...

Powder coating coil lines are not new. They are not plentiful. Most are not fast. However, Material Sciences Corporation (MSC) in Elk Grove Village, Illinois, has pioneered high-speed powder coil coating. The company has proven the technology in excess of 900 fpm in other web applications. The current powder coating line runs at 200 fpm. By year end, MSC will have a 54-inch line running at 400 fpm.

MSC began its adventure with an investment in a laboratory coil coating line. The company's initial intention was not to develop high-speed coil coating, but to find new and better ways of coating and laminating steel.

"If you go back through the history of the coil coating line and the products produced on the line, you find that virtually everything has changed, from oven design to line design, except the roll coater," noted Randy Froehlich, director of marketing and sales.

The company felt that the roll coater was limited, since it required use of solvent-borne coatings. Some companies have experimented with waterborne coatings, but they have limitations as well. Also, the coil coaters meet environmental regulations concerning VOCs today, but MSC worried that this may become much tougher in the future.

"We wanted to look at a different form of coating, because we felt the roll coater would not be the coater of the next century," stated Mr. Froehlich. The company explored a number of technologies, including extrusion coating, 100-pct-solids coating and powder coating. With the powder coating, MSC also studied various application methods, including different gun designs.

All other coil powder coaters worldwide use conventional powder guns. The coil runs at a maximum 40 to 50 fpm. "When we looked at that technology, we could not find any way to get it fast enough to make it commercially viable in the U.S., in our opinion," noted Mr. Froehlich. These lines do not resemble traditional coil coating lines, which run large volumes at high speeds.

In addition to the slow line speeds, there are problems with coating thickness consistency. If you set up five guns across the width of a coil strip, the coating will have five sets of peaks and valleys across the width. Some companies have experimented with shaking the guns back and forth, but that does not adequately address the problem.

Then MSC met up with a small Indiana research company, Terronics Development Corporation, Elwood, Indiana. The company worked with MSC to develop a powder coating system for coil. When they put it on line, it worked! "There were certain things we needed to fix and enhance, but we achieved speeds in excess of 100 fpm on the lab coating line (maximum speed of that line)," said Mr. Froehlich.

TABLE I - Powderbond Manufacturing Capabilities    
Line Parameters    
Maximum coil width 30 inches    
Maximum coil weight 20,000 lb.    
Maximum Coil O.D. 72 inches    
Coil I.D. 16-, 20-, 24-inches    
Minimum gauge 0.006 inch    
Maximum gauge 0.042 inch (over 0.042 inch subject to inquiry)    
Perforated Metal Capabilities    
Open area capabilities 5 pct - 80 pct    
Head and tail requirements 5 ft solid metal    
Safety edge requirements 1-inch minimum - both sides (subject to inquiry)    
Hole pattern limitations None    
Substrate Capabilities    
Cold-rolled-steel Galfan    
Hot-rolled-steel Galvalume    
Aluminum Tin coated sheet    
Hot-dipped galvanized steel Aluminized steel    
Electro-galvanized steel Others    
Powderbond Capabilities    
Top-side coating 0.7 - 5.0 mils    
Bottom-side coating 0.7 - 5.0 mils    

Once the powder coating application technology was proven on the lab coater, MSC installed it on a 30-inch-wide conventional coil coating line. The pretreatment system could be used for both powder and liquid coil coating, and this line had space for the coater and infrared cure oven.

The coater is a special unit that is moved on and off line. This facilitates color changes. It also gives MSC the flexibility to change from powder to liquid coil coating. Since MSC only has one powder coil coater now, the coater must be cleaned each time there is a color change. There are different levels of cleanup. If the color change is from white to navy blue, it is a major color change. "We try to run light colors together. We try not to go from black to white and back," explained Mr. Froehlich.

Because the coater has a first time deposition rate of 95 pct, powder recycling is done only if there is an extremely large production run.

The MSC Powder Cloud™ application process uses several patented elements to achieve the high-speed capabilities. The system uses a brush to atomize a precise amount of powder into a cloud.

TABLE II - Powderbond 1000 Modified Polyester    
Typical Performance Characteristics    
_ Method Results    
Adhesion (Cross-Hatch) ASTM D-3359 5B-No Loss    
Impact Resistance Direct ASTM D-2794 Pass-up to 160 lbs.    
Impact Resistance Reverse ASTM D-2794 Pass-up to 160 lbs.    
Pencil Hardness ASTM D-3363 2H-3H    
Gloss (60 deg) ASTM D-523 30-90    
Salt Spray ASTM B-117 500 hours-Pass (CRS)
Less than 1/8 inch from score
   
Humidity Resistance ASTM D-2247 1,000 hours-Pass (CRS)
Less than 1/8 inch from score
   
Flexibility OT-No pick off    
Dry Film Thickness 1.5-4.0 mils    
Primer None    
Substrate Cold-rolled steel, Tin mill black plate, Aluminized, Electro-galvanized, Hot-Dipped-Galvanized, Aluminum    

This cloud is created and driven into an area between the moving strip of metal and a series of high-voltage corona wires. These wires run the width of the strip and electrostatically charge the powder particle with a negative charge that creates the attraction to the strip.

The system can coat the top or bottom of the strip only or both sides. The brush feed system can be adjusted to handle the precise width of the coil being coated.

After coating application, the strip enters an infrared curing oven to fuse the powder to the strip. The strip subsequently passes through a convection curing oven where the coating develops all its final properties.

"When we got into this," Mr. Froehlich explained, "the powder industry was accustomed to post finishing work. Because of this, most powders cured in about five minutes. We had to go to work with the industry to change that." The infrared system at MSC cures the powders in 20 to 30 seconds.

The Powder Cloud process coats both steel and aluminum coils at the company's Elk Grove Village facility. The coating process has also been tested in other environments to work on substrates such as paper, glass and wood. The initial results appear favorable for these other materials.

The powder coil coated products are currently marketed under the Powderbond™ trade name. This is MSC's line of powder coil coated metals. All of these products exhibit excellent performance characteristics. The product line includes the following.

Powderbond 1000 for lighting is designed for reflective lighting applications. It provides 94 pct reflectance.

Powderbond 1000 is designed for interior and light-duty exterior applications. It combines excellent hardness and performance characteristics while retaining formability. Applications include appliances, furniture and perforated or embossed metal.

TABLE III - Powderbond 3000 Textured Coatings  
Typical Performance Characteristics  
_ Method Results  
Adhesion (Cross-Hatch) ASTM D-3359 5B-No Loss  
Impact Resistance Direct ASTM D-2794 Pass-up to 160 lbs.  
Impact Resistance Reverse ASTM D-2794 Pass-up to 160 lbs.  
Pencil Hardness ASTM D-3363 2H-3H  
Gloss (60 deg) ASTM D-523 4-85  
Salt Spray ASTM B-117 500 hours-Pass (CRS)
Less than 1/8 inch from score
 
Humidity Resistance ASTM D-2247 1,000 hours-Pass (CRS)
Less than 1/8 inch from score
 
Flexibility OT-No pick off  
Dry Film Thickness 1.5-4.0 mils  
Primer None (optional on select applications)  
Substrate Cold-rolled steel, Tin mill black plate, Aluminized, Electro-galvanized, Hot-Dipped-Galvanized, Aluminum  

Powderbond 3000 is modified polyester for decorative applications. It combines excellent hardness and performance characteristics in a textured or hammertone or speckled coating.

The first market MSC looked at was perforated metal. This type of metal cannot be adequately coil coated using the traditional process, however it can be coil powder coated. Perforated metal is used in such items as ceiling panels and acoustical ceiling tiles.

This was the first application for the line. The coater also applies nylon powder coatings, which cannot be done in liquid form. Nylon coatings are applied for functional reasons rather than decorative, since they are extremely abrasion resistant. These coatings are applied to items such as desk drawer slides.

MSC plans to pursue more traditional markets, such as appliances. The company also has plans for the building products market (roofs, metal buildings). Both of these are traditional markets for coil coated products.

Product development is still on-going for many of these applications. "We are working with our coating suppliers to help us develop truly superior products at competitive film thicknesses. The powder chemistries can offer many benefits to a user of coil coated products," said Mr. Froehlich.

Powder film thickness is another major issue to consider. Traditional coil coating paints are often applied at 1.0 mil or less in total film thickness. Powder coil coatings are commonly applied at 1.5 mils thick due to many factors. But these factors are beginning to change. Recent developments in control of powder particle size is driving film thickness down and further progress in this area can be anticipated.

"We are looking at powder coil coating as a way to open new markets to coil coating," noted Mr. Froehlich. "We are also looking at it as a way to offer overall improvement to the way coil coating is performed."

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