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issue: September 2007 APPLIANCE Magazine

Metal Pretreatment
Pretreating with Nanotechnology


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Appliance maker Gorenje applies a nanoceramic-based preservative to a metal substrate, achieving a high level of paint adhesion without some of the disadvantages of iron phosphating.

Refrigerator cabinets travel through the pretreatment facility on two parallel lines. Bonderite NT is sprayed onto the metal surface by a system of nozzles.

Slovenian household appliance producer Gorenje became aware of a new nanoceramic-based prepaint treatment three years ago. It was in 2004 that the Bonderite NT metal surface pretreatment process from Henkel was launched in Europe.

The process involves applying a microscopically thin coating of nanoceramic-based preservative to a blank substrate prior to painting. The nanoparticulate coating generates a finely structured, uniform surface that promotes both corrosion protection and paint adhesion. The process replaces conventional iron phosphating and offers application advantages. When phosphating, the application zone must be heated, for example, but the new nanoceramic-based pretreatment process can take place at room temperature. This saves energy and eliminates an entire process stage: the previously required passivation treatment. Further, the absence of phosphate sludge also reduces to a minimum the cost of cleaning and waste disposal.

High Demands on Pretreatment

“There were many reasons why we felt we should take a very close look at the new process technology,” explains Majda Meža, head of technology for refrigerator and freezer production at Gorenje. “We are always keen to introduce environmentally friendly production processes. However, these first have to pass some very strict proving trials. We began those back in September 2004, not long after Bonderite NT came onto the European market. The tests extended over several months, until the final approval was issued.” This technical due diligence applied prior to application of the new process was in keeping with Gorenje’s commitment to product quality standards and its objective to minimize disruption of the production process.

The head of technology was eager to emphasize the importance of a reliable corrosion protection system for Gorenje’s appliances, which are exported to more than 70 countries. “These white goods have to withstand a wide range of different climatic conditions, both during transport—often as sea freight—and also once installed in the customer’s home.”

Gorenje offers a warranty of five years on its household appliances, making long-term corrosion protection imperative. “We also produce wine chillers for the U.S. market, for which even stricter corrosion protection requirements than in Europe apply,” Meža adds. “We have to be sure that these demands are also adequately satisfied by the new process.”

This unique Gorenje Premium Touch fridge and freezer is made with crystallized Swarovski elements to underline the ambitious design strategy of the Slovenian household appliance manufacturer.

5000 Refrigeration Units per Day

The paint shop at Gorenje’s Velenje plant operates on a three-shift basis. The location selected for introducing the new nanoceramic-based prepaint treatment was the refrigeration appliance production facility. This comprises two lines and has an average daily output of 5000 units. All units have metal surfaces that must be prepared for powder coating.

The previous pretreatment process used in Velenje was highly complex, encompassing 10 zones, from degreasing to final rinsing.

One area contributing to the complexity of the application remained unchanged, however—because white goods from Gorenje have long ceased to be exclusively white. Appliance designers are now using colors far more frequently. Ten different shades are applied in the paint shop, with at least three color changes taking place every day.

Laboratory Testing

Before Bonderite NT was brought to the production line, it was subjected to comprehensive laboratory testing. To this end, metal sheets from regular production batches were initially pretreated at Henkel’s laboratory in Düsseldorf, Germany, and then powder-coated by Gorenje in Velenje. The specimen sheets were then subjected to various humidity and salt-spray tests by both companies.

“The results exceeded our expectations,” reports Cvetka Kvar, the Gorenje project manager responsible for introducing the nanoceramic process. “We performed salt-spray tests over periods of both 240 and 500 hours in order to see whether the finished coating was also able to satisfy the strict requirements of the U.S. market. In fact, the corrosion levels induced were way below the permitted maximum values.”

Production Testing

After successful laboratory trials, the pretreatment was tested for a day on the Velenje line, albeit still quarantined from regular manufacturing operations. In this phase, original components were pretreated and coated with powder paints of various manufacturers.

“During this stage of testing we were able to simulate highly realistic production conditions, including interruptions, so that we could investigate the formation of flash rust,” says Kvar. “We discovered that the new pretreatment is highly effective in preventing this, with no problems occurring even after relatively long intervals.”

After this test had been successfully completed, the pretreatment was initially incorporated into the regular production operation for the duration of one week. During this phase, the pretreatment product was fed manually by means of a metering pump.

“We took completely random samples and once again subjected these to various analyses, the results of which met our specifications in full,” Kvar says.

Gorenje now felt confident that the nanoceramic pretreatment process was superior to its previous iron phosphating technique and introduced it into the line in May 2005 for a trial lasting several months. This was to be the last practical test prior to the final decision to release.

Cleaning Requirement Significantly Reduced

During these final five months, the performance of the new pretreatment under regular operating conditions was monitored and found to be superior. Subsequently, results were compared with results obtained using the previous pretreatment process. Gorenje’s conclusion: better results were achieved in all areas.

“Increased corrosion protection, shorter contact times, a 30% energy reduction, up to 30% less rinse-water consumption, and additional water savings in the cascade rinsing process—all these benefits clearly pointed to the superiority of the new process,” explains Kvar. “One advantage that made a great deal of difference was the level of cleanliness achieved. There was no longer any sludge, thus greatly facilitating the cleaning and maintenance of the plant. No longer do we have to remove the sludge from the bath every day; pipes and nozzles remain free and we require far fewer spare parts. Previously, we spent 1000 man-hours per year on the cleaning of the pretreatment facilities—now this figure has been reduced to 100 man-hours at the most.”

Kvar and Meža agree. “Although the pure material costs are slightly higher than with the old process, these are more than offset by the numerous benefits and savings that accrue. Below the line, the nano­ceramic process is significantly more cost-efficient.”

Environmentally Friendly Production

Another benefit of the nanoceramic process was emphasized by Gorenje’s managing director for environmental protection, Vilma Fece. “The elimination of iron phosphating means a significant reduction in environmental burden. We no longer have to contend with any phosphates, toxic heavy metals, or organic substances, all of which previously had to be disposed of at great cost. In the last few years, we have made enormous efforts in the establishment of a sustainable production capability. The new process takes us a big step further in this direction. Foreign visitors are often surprised to find such high environmental standards being applied in a Slovenian factory.”

Given the good results achieved on the refrigeration unit production line, where the nanoceramic process has now been operating free of problems for over 1½ years, the pretreatment line for cooker and hob production has also now been converted to Bonderite NT. The pretreatment is likewise being applied in the Gorenje plant in Serbia, which was commissioned in 2006. The anticipated annual production capacity in Serbia is 1 million refrigeration units, all of which will be pretreated for the painting operation using this nanoceramic-based pretreatment process.

 

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