(Volatile Organic Compounds describe a range of compounds—often components of petroleum fuels, industrial solvents, oxygenates (MTBE), hydraulic fluids, paint thinners and dry-cleaning agents—that have a high vapor pressure and low water solubility. VOCs are common ground-water contaminants.)

Auto painting has evolved significantly during the last 100 years. At first, workers applied enamels with a brush. The paints had poor durability, had a high-solvent content and an extremely slow cure time.

By the 1940s, alkyd and acrylic lacquers were spray applied. These had improved durability, high solvent content, and a faster cure time.

The 1980s saw the development of the basic process of electrostatic spraying used today. In general, there are five steps today to painting a vehicle representing five different layer of coating: a phosphate coating for a corrosion layer; the E-coat; the primer; the basecoat (the color) and the clearcoat.

Each of the final three steps (primer/basecoat/clearcoat) has been tackled with a combination of different types of coating material: solventborne, waterborne, or powder. The type of material associated with the sequence describes the technology process.

Thus WWS refers to a system of waterborne primer, followed by waterborne basecoat, followed by solventborne clearcoat. PWP refers to powder primer, waterborne basecoat, powder clearcoat. And so on.

Additionally, the vehicle needs to be baked in a series of ovens as the process unfolds. Thus, a traditional solvent-based application system (SSS, or solvent-solvent-solvent) has a e-coat oven, a gel oven, a primer oven, and a clearcoat oven.

The elimination of the primer booth an oven in the 3-Wet system reduces the overall CO2 emissions associated with painting substantially.

Ford is working on what it calls the 3-Wet SSS system: a solvent-solvent-solvent approach to painting that removes one of the main baking sequences (the primer booth and oven) and allows for the wet-on-wet application of the primer, solvent and clearcoat.

(The company already has developed and patented its process for multiple color effects on the same vehicle in a wet-on-wet application—the flames rippling across the flanks of a pickup, for example.)

There is a great deal of underlying new polymer chemistry that has to go into creating the coating materials and control agents that support this (fighting gravity sag, for example).

Ford has it worked out so that an “air flash”—a quiet zone of approximately one minute—is all that is needed prior to the application of the next layer.

It is through the elimination of the primer booth and primer oven that Ford achieves the large reduction in CO₂ emissions.

Solventborne vs. Waterborne Paints

Although a solventborne wet paint contains much more solvent than a waterborne wet paint (see diagram at right), in practice, Ford finds that using the solventborne paint cuts the amount of wet paint required approximately in half.

Or to put it another way, it takes twice the amount of waterborne wet paint to deliver the required amount of resin and pigment as it does a solventborne wet paint.

On the financial side, Ford estimates that the 3-Wet process will reduce the cost per vehicle by more than $10 relative to a standard SSS process, and by more than $35 per unit compared to a WWP process. That might not sound like a lot, but if it were deployed across all the NAFTA assembly plants Ford runs, 3-Wet SSS could result in aggregate savings of between $40–$150 million per year based on reductions in material, energy costs, labor and so on.

The 3-Wet SSS system has yet to be deployed in North America—it is in initial use in Japan.

FUMES-TO-FUEL. Ford’s fumes-to-fuel technology is another approach the company is investigating in an attempt to lower the financial and environmental cost of painting.

Standard industry practice is to capture and to incinerate VOCs-laden solvent fumes—a process that consumes hundreds of millions of cubic feet of natural gas per year, and results in combustion emissions (such as CO₂).

Co-developed by Ford and Detroit Edison (with patent applied for), the fumes-to-fuel system captures the fumes from the paint shop, condenses them, and uses the resulting VOCs stream as a fuel to generate electricity than then is used back in the paint shop.

The two-stage process first uses a fluidized bed concentrator (similar to those used in many industrial applications to concentrate VOCs for conventional disposal) to remove the air from the fumes and push out the VOCs in a 2,000:1 concentration.

The resulting gas is then used as the fuel for a Stirling engine (sometimes called an external combustion engine) which generates electricity for the plant.

The original concept for the system, as developed at Ford’s Oakville plant in Canada, was to reform the gas stream into hydrogen, and feed that into a fuel cell. Given the cost of fuel cells, however, the Ford engineers decided to opt for the Stirling engine when they deployed the technology in the Wayne truck assembly plant.

The Stirling engine comes from STM, and is packaged in one of the company’s PowerUnits, which includes the 55-kW generator.

The STM Stirling Engine.

The STM engine is a four-cylinder, double-acting Stirling engine with a swash plate drive. At the heart of the engine are four independent gas enclosures each comprising the volume under a piston (compression volume), the volume above the adjacent piston (expansion volume), a series of three heat exchangers connecting these two volumes, a cooler adjacent to the compression volume, a heater adjacent to the expansion volume and a regenerator between the heater and the cooler.

The four pistons are arranged symmetrically around a swash plate that forces the reciprocating motion of any two neighboring pistons to be 90º out of phase. The gas enclosures are charged with high-pressure hydrogen that serves as a working fluid. The reciprocating motion of the pistons causes the volume of hydrogen to increase and decrease alternately.

The expansion spaces are maintained at a high temperature by the continuous combustion of the condensed VOCs. The compression spaces are maintained at a low temperature by liquid cooling of the coolers.

The temperature and the pressure of the hydrogen during expansion is thus higher than during compression. The hydrogen absorbs heat from the combustion process, converts a portion of it to mechanical power, which it delivers to the pistons, and rejects the balance to the liquid coolant.

The mechanical power delivered by the hydrogen to the pistons is aggregated and converted to rotating shaft power by means of the swash plate drive. The regenerator, which is the third heat exchanger, does not exchange heat with the outside. It alternately absorbs heat from and releases heat back to the hydrogen in order to improve the engine efficiency. The engine’s output shaft is connected to a generator to make three-phase electrical power.

Resources:

• VOC Emissions from Industrial Painting Processes As a Source of Fuel Cell Energy

• Environmental C&C: Fluidized Bed Concentrator

• EPA on VOCs