A new pollution prevention technology.

A NEW POLLUTION PREVENTION TECHNOLOGY

Carbon dioxide is supercritical to VOC reduction in the Unicarb process.

The Unicarb System represents the culmination of years of research by Union Carbide Chemicals and Plastics Co. Inc. to reduce volatile organic compounds and air toxics in spray coating operations. By substituting supercritical carbon dioxide for much of the volatile solvents in conventional low-solids and high-solids topcoats and sealers, the system reportedly can reduce VOCs by as much as 50 to 80 percent.

Union Carbide of Danbury, Conn., claims that the system allows spray coating applicators to retain the advantages of organic solventborne coatings in terms of appearance, performance, convenience and economics.

In addition to Pennsylvania House, the technology is currently undergoing full-scale commercial testing by four other furniture manufacturers, as well as automotive, heavy equipment and plastic OEMs.

The process

In the Unicarb process, [CO.sub.2], a gas at ordinary conditions, is proportioned and mixed with the coating concentrate as the coating is sprayed. Because the spray solution is pressurized, airless spray guns and nozzles are used.

The specially designed spray coating equipment enables the coating concentrate and [CO.sub.2] to be metered and mixed, pressurized and heated to spray conditions. [CO.sub.2] is supplied from gas cylinders for small quantity uses or cryogenic (bulk) cylinders and tanks for larger volume users. No refrigeration is used to suppress compressibility or pump cavitation. Coating concentrate is supplied from pressure pots, drums or tanks. As the coating is sprayed, additional mixture is automatically blended and available for spraying.

The equipment can operate continuously or intermittently and may be controlled manually or automatically. Spray equipment has been designed to accommodate a broad range of production requirements from small, simple end uses to large complex applications, such as automotive OEM. It is capable of supplying coating to multiple spray guns that have been designed for use with the system. The delivery equipment is available for retrofit or for grass-root installation. Spray guns, both manual and automatic, and with or without electrostatic assist, are available.

Union Carbide developed the prototype equipment and has since licensed three companies -- Graco, Nordson and Binks -- to develop commercial equipment for the preparation of coatings diluted to spray viscosity with [CO.sub.2].

Why supercritical [CO.sub.2]

With the Unicarb technology, supercritical [CO.sub.2] is used to replace the volatile solvent fraction in coating formulations that is needed to obtain low spray viscosity. By using [CO.sub.2] to replace the fast-evaporating solvents, while retaining the slow-evaporating solvents, low-solvent coatings can be converted into high-solids coatings without changing the polymer system or using new reaction chemistry.

The supercritical fluid is a very-fast evaporating solvent that escapes very quickly from the coating solution during spray atomization. Therefore, ideally, the composition of coatings sprayed conventionally and with supercritical fluids is the same once the coating is deposited onto the substrate, resulting in a similar coating performance.

[CO.sub.2] is an attractive supercritical fluid for spray application of low-VOC coatings because it is environmentally compatible; it is not a VOC and it is on the Environmental Protection Agency's list of permissible emissions.

The Unicarb process uses recycled carbon dioxide, thus no new carbon dioxide is generated. Other advantages of supercritical [CO.sub.2] reported by Union Carbide include:

* The critical temperature of [CO.sub.2], 31 C (88 F), is just above room temperature, so supercritical temperature can be mild and easily obtained.

* The critical pressure of [CO.sub.2], 72 bars (1,071 psi) is well within the design parameters of airless spray equipment.

* The health effects of [CO.sub.2] are slight and significantly less toxic than organic solvents.

* [CO.sub.2] is nonflammable and mostly inert.

* [CO.sub.2] is a relatively low-cost commodity product.

* Supercritical [CO.sub.2] has high solubility in most coating formulations and is a good viscosity reducer for polymer solutions.

Unicarb formulations

The Unicarb spray technology has been used to apply a variety of clear, pigmented and metallic coatings using thermosetting, thermoplastic, air-dry and two-package polymer systems. Polymers tested include acrylics, polyesters, air-dry alkyds, urethanes, melamines, phenolics, vinyls, epoxies, ureas, nitrocellulose and cellulose acetate butyrate.

Many pigments and filler materials have also been tested, including a variety of colored organic and inorganic pigments, titanium dioxide, carbon black, aluminum flake, calcium carbonate, silica and clay.

The Unicarb technology can be used with most polymers and pigments currently found in conventional solventborne coatings. For many applications, little or no adjustment is needed in the polymer system. Only a solvent blend will need reformulation.

Currently there are five coating manufacturers licensed to develop, make and market Unicarb formulations: Guardsman Products, Akzo Coatings, Lilly Industrial Coatings, PPG Industries and BASF Corp.

The future

Union Carbide is touting the Unicarb System as a cost-effective and viable alternative to other pollution abatement technologies, including incineration, carbon absorption and waterborne systems, for complying with 1990 Clean Air Amendments while still attaining quality finishes.

When combined with incineration or solvent recovery on just the bake oven, Union Carbide reports that VOC emissions can be reduced by up to 95 percent. The company estimates that the technology could capture up to 15 percent of the industrial coating market within 10 to 12 years.