Aluminum changes paint pre-treatment

Features - Coatings

Aluminum generates seven times more sludge in hot zinc-phosphate baths than steel, making paint pre-treatment more difficult to manage – something manufacturers must consider in adopting new materials for body panels.

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PPG’s Zircobond pre-treatment systemWith lightweight material use rising, treating and painting surfaces is changing. In Part 1 of this series, PPG Industries discusses pre-treatment steps needed with increased aluminum use. In Part 2, coming in TMV’s summer issue, BASF discusses the challenges of painting carbon fiber.

In auto plants, zinc-phosphate baths clean off the grease and oil left from body stamping, leaving body panels pristine for electro-coating and paint adhesion. Sludge in those dip tubs can clog circulation pumps, leading to more frequent, costly chemical cleaning and an overall slowdown in production.

“As the industry changes substrates, it does have a fairly significant impact on some of the pre-treatment processes,” says Ray Schappert, global director of product management for automotive coatings at PPG. Ford Motor Co.’s recent decision to convert the body panels of its F-150 pickup from steel to aluminum could kick off a wave of lightweight metal use, he adds.

“We’re seeing other customers that are looking at higher content of aluminum, so this will be a growing issue for the auto industry,” Schappert says.
 

More aluminum in autos

Smart car bodiesAlcoa, the largest aluminum producer in the United States and one of the top five in the world, expects to quadruple sales of metal sheets to automakers by 2015 and is planning for a tenfold increase by 2025. The company is investing heavily in plants in Iowa and Tennessee to produce more aluminum sheet, and it’s far from alone. Novelis is also expanding in the U.S., and Aleris is expanding aluminum sheet production in Europe.

Aluminum is significantly lighter than steel in automotive body panels. Ford expects to shave 500 lb from the F-150 from its use of aluminum, part of an overall 700 lb weight loss for that truck. Lighter vehicles use less fuel, helping them stay compliant with stricter federal regulations on carbon emissions and fuel economy.

Aluminum panels aren’t exactly new to the auto industry. Ford and General Motors have been using aluminum hoods on their trucks for years to cut weight; Toyota uses aluminum extensively in its hybrids; and several German automakers use aluminum for the bodies of their luxury cars. The difference with the F-150, and possibly with future products, is the scale of the increase.
 

Sludge-filled baths

Sludge buildup in phosphate baths isn’t as much of a problem when only 5% of the finished vehicle is aluminum, Schappert says. With the current aluminum volume, chemicals don’t have to be changed significantly more often than if the plants used only steel panels. But aluminum generates seven times more sludge per square meter than steel, so as the use of the lighter metal increases, so do the challenges in managing dip tanks.

“That sludge can clog pumps and filters and shorten the effective useful lifetime of the (pre-treatment) baths,” Schappert says.

Zircobond benefits in pre-treatment
  • 80% reduction in sludge caused by heavy metals
  • $600,000 annual savings in existing auto plant paint shops
  • $1.6 million estimated annual savings in new automotive paint shops
  • No zinc, manganese, nickel, phosphates, or volatile organic compounds

In a typical automotive plant, body panels leave the stamping and body-weld areas coated with oil and grease, adhesive excess, welding material residue, and other contaminants. Those panels go through the pre-treatment system to remove impurities and to get a thin deposition of zinc phosphate coating before moving to the electrocoat primer application. That electrocoat (e-coat) layer provides corrosion protection and creates the ideal surface for primers and paint, the next step in the body-preparation process.

Schappert says the process of painting an aluminum panel is similar to a steel one. Once the e-coat is in place, the primer and topcoats adhere to the body panels in exactly the same way. So for the paint shop, the major difference in the materials takes place in the pre-treatment phase.
 

Different pre-treat chemistries

PPG’s solution to the sludge issue is to change the chemistry of the pre-treatment baths.

The company has been selling an alternative to zinc phosphate for about six years called Zircobond, a zirconium-based bath system. PPG originally pitched Zircobond as a green alternative to zinc phosphate because it operates at room temperature rather than 125°F, cutting energy used to warm the baths. In addition, Zircobond requires fewer processing steps.

“We’re completing our approval process for the second generation of our zircobond technology,” Schappert notes.

While costing more initially, Schappert states that automotive companies will save in the long term because of reduced energy, chemical cleaning, and water usage. In addition, they won’t have to change bath chemicals as frequently, cars and parts will spend less time in the pre-treatment process, and the process requires less plant space. Cost savings will be key as automakers adopt aluminum because it is significantly more expensive than steel. As materials costs go up, Schappert says companies will need to find cost savings elsewhere in the manufacturing process to offset them.

“PPG needs to be aware that aluminum is an excellent technology for reducing weight, and we need to support our OEM customers that move in that direction,” Schappert says. “The key to this transformation is going to be rethinking every stage of the process from body weld to pre-treatment to top coatings, finding the most cost-effective way of completing the painting process.”

 

PPG Industries
www.ppg.com

 

About the author: Robert Schoenberger is the editor of TMV and can be reached at 330.523.5381 or rschoenberger@gie.net.