Converting from Leaded to Unleaded Electrocoat Primer

In 1992, Thyssen Umformtechnik installed a batch-type electrocoat line suitable for coating a variety of parts up to 39 ft long. Parts include fabricated sheet parts such as doors or panels for cabs; truck chassis parts up to 39 ft; subassemblies for cars and trucks; and axle components for cars comprising up to 80 individual parts. The total capital spent on this coating line amounted to around 35 million German marks.

Application process. The coating process is comprised of 13 tanks, 39 by eight by eight ft, with volumes ranging from 300 to 383 cu ft.

A five-unit conveyor system with a useful load of five metric tons carries the workpieces through the installation.

Parts are then baked at a minimum metal temperature of 350F in a 333 ft long oven. Cycle time is seven and a half min, and the complete processing time in the electrocoat line is three and a half hrs. Next to this line, Thyssen installed a separate wastewater treatment plant to treat wastewater generated by the process.

Planning the conversion. When the installation came on line, Thyssen used a black, leaded electrocoat product supplied by BASF (FT82-9636). This two-component system matched the automotive industry's requirements at that time. An order for coating replacement parts resulted in a change of the requirements. For safety and environmental reasons, an unleaded electrocoat product was now required. In cooperation with BASF, plans were made to replace the leaded electrocoat system, with consideration given to the following:

For environmental and cost reasons, the old product was not discarded.
Technological properties such as corrosion protection could not be adversely affected.
The coating technology could not be changed (suitable for low-build/high-build application).
No plant design changes because of the added cost.

We worked out a process for converting the tank that consisted of gradually adding the unleaded product to the leaded product. The effect of the lead on the anti-corrosive properties was compensated for by changing the binders used.

Advanced testing. Before any conversion was actually made, several concerns needed to be addressed. The anti-corrosive properties of the individual coatings as well as mixtures had to be guaranteed. We also wanted customer approval for panels/parts. System stability had to be guaranteed at various stages of mixing, so tests were carried out on the products at various stages of mixing. The tests carried out by Thyssen and BASF Corp. gave the following results.

No differences on sample panels for corrosion testing:

10 cycles corrosion test according to VDA 621-415
30 cycles gravel/corrosion test according to P_VW 1210
240 hrs salt spray test according to DIN 50021

No reduction in corrosion protection in transitional stages

No instability in the bath during transitional stages.

Conversion. Product conversion started when the new unleaded pigment paste was added for the first time on September 30, 1992.

The duration of the conversion depended on bath turnover. Throughout the conversion phase, we observed no decline in quality or any other problems. In agreement with our customer, 0.15 pct lead was defined as the threshold value marking unleaded material. Once this value was reached, our customer approved the coating.

Presently, the lead concentration is in the ppm range. In close cooperation with our supplier and a variety of OEM customers, we successfully dealt with this challenge without compromising quality.

Prospects. Heavy metals are damaging to the environment, and their use entails expensive measures such as wastewater treatment and disposal. While these processes can be controlled, complete elimination of heavy metals from as many products as possible is a better option. Enhanced environmental awareness is clearly supported by customers. The conversion to an unleaded electrocoat system was therefore the first step towards an environmentally friendly coating process. The next steps consisted of replacing chrome-based passivation with a chromate-free product and changing the phosphate system from a nitrite-accelerated product to one containing an internal accelerator. These actions will make it much easier to treat the wastewater and dispose of any production residues. This is a prerequisite for setting up a closed water recirculation system. At present, we recirculate approximately 90 pct of the water in our electrocoat plant.

The Thyssen Group comprises three operating divisions: Steel (Thyssen Stahl AG); Trade and Services (Thyssen Handelsunion); and Capital Goods and Manufacturing (Thyssen Industrie AG/ Budd Company). Within Thyssen Industrie AG, the business unit Thyssen Umformtechnik (Metal Forming) operates from several production sites. With some 1,300 employees, the Brackwede site achieves an annual turnover of 300 million German marks.

Adapted for publication from a paper presented at Electrocoat '96® Conference, Orlando, Florida.