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Novel low-VOC paint technology.

Novel Low-VOC Paint Technology

A year after Union Carbide Chemicals and Plastics Co., Inc. announced its revolutionary Unicarb technology for thinning paint with supercritical carbon dioxide ([CO.sub.2]) in place of solvent, the first commercial equipment is ready for market. Nordson Corp. in Cleveland is introducing two sizes of commercial spray painting equipment that supposedly can replace up to 80% of the "fast" solvents in paints and coatings with liquid [CO.sub.2].

Nordson's introduction puts it several months ahead of two other equipment companies licensed by Carbide (which holds an ideas patent on the Unicarb process) to develop Unicarb paint-spray equipment--Binks Manufacturing Co., Franklin Park, Ill., and Graco Inc., Minneapolis. Carbide doesn't plan to license additional equipment companies in the U.S., but wants additional partners abroad besides Nordson, which is licensed for the European market as well. Nordson, Graco and Binks are targeting applications in automotive and general finishing.

The principle behind the Unicarb system is that if you apply a little heat and a fair amount of pressure to [CO.sub.2], putting it into a "supercritical" liquid-like state (actually a very dense gas), it acts like a hydrocarbon solvent and then evaporates harmlessly. Equipment and coatings people working with the system say useful supercritical temperature and pressure ranges are broad: 88-160 F and 1100-2000 psi. (Carbide recommends 140-160 F and 1100-1500 psi; Nordson recommends 120-140 F and 1200-1800 psi; Binks uses 120-150 F and 1100-1800 psi.)

The technology was developed by Carbide's Solvents & Coatings Materials Div. (now called Solvents, Resins and Intermediates) in Danbury, Conn. Carbide has licensed five paint companies to produce Unicarb-compatible coatings, but at press time availability was quite limited for reasons discussed below.


Like any emerging technology, the Unicarb system is controversial. The potential advantage for a processor in converting paint systems to Unicarb is to reduce emissions of volatile organic compounds (VOCs) like the solvents hexane, xylene, toluene, benzene, methyl isobutyl ketone (MIBK), and methyl ethyl ketone (MEK). Carbide and its equipment licensees say Unicarb can reduce 70-80% of VOCs. Licensed paint formulators, however, say they're seeing more like 25-30% VOC reductions, and that 70% is definitely not achievable by Unicarb alone, but would require addition of other VOC-reduction measures. A Carbide spokesman replies that 60-70% VOC reduction can indeed be achieved with a low-solids acrylic lacquer, such as for auto fascia, but only 25-30% reduction may be obtained with a high-solids appliance-type coating.

Carbide says the system won't require major conversion of existing equipment or major capital spending on new hardware, unlike alternative VOC technologies--namely waterborne paints and powder coatings. (Current powder coatings have the additional limitation of requiring at least 250-275 F bake temperatures.) Potential Unicarb system users, however, note that initial setup costs can run to $100,000 and often require removal of existing built-in equipment like electrostatic painting bells, which can't be used with the Unicarb system.

All the major paint companies working with Unicarb have separate research teams developing the competing powder and waterborne coatings, as do Nordson and Binks. Graco is developing Unicarb, waterborne and high-solids systems. "There is absolutely room for all three technologies," says David Huberfield, general manager of Graco's Paints and Chemical Coatings Div. "In our most optimistic moments, we don't think that Unicarb will dominate. But it is the most exciting new technology to come along in many years."


Nordson's initial models (patents pending) are a two-gun SCF-2 system (see photo) that sprays 0.2 gal/min of resin concentrate and a four-gun SCF-4 system that sprays 0.4 gal/min. Nordson is building about 20 systems of both kinds for customers' in-plant trials, which were to begin last month. Says Nordson Liquid Systems Group v.p. Robert Lindrud, "We'll also be building larger custom machines." The SCF (supercritical fluid) system is a box occupying roughly a cubic yard (3 x 3 x 3.3 ft), less than half the size of Carbide's original lab model.

The SCF spray units are microprocessor controlled and simple to operate, Nordson says. Unlike Carbide's lab model, they require no temperature or pressure adjustments by the operator, who merely controls the [CO.sub.2] ratio by a numerical setting on a scale of 1.0 to 10.0. The optimal setting can be determined by simply pumping resin into the system "and adding [CO.sub.2] until the desired finish is achieved, then recording the [CO.sub.2] setpoint for future use," according to Nordson's forthcoming brochure. Owing to variations in [CO.sub.2] solubility, the gas ratio may be different for each color of the same paint, users say.

The "blue box" proportioner is a two-stage device. In both the Carbide and Nordson machines, metering and mixing of [CO.sub.2] and paint take place in the first stage, consisting of two pumps, tubing, a resistance heater, and mixing chamber. This is where the [CO.sub.2] reaches the so-called "critical point" and exists as a very dense gas that behaves almost like a liquid.

Carbide's original lab model used a gear pump to pressurize the first stage, where the paint concentrate is heaviest to move, and a variable-ratio Zenith pump to meter paint resin and [CO.sub.2]. But after a year or two, the gear pumps on the early lab machines gave trouble. Tightly meshing gears in the pump, pushing abrasive-filled paint concentrates from low pressure on one side to 1000 psi on the other, were binding the shaft and housing, a user says. Newer lab models have a dynamically balanced pump. Nordson uses a standard paint pump for resin feed and a proprietary dual-piston pump (not gears) for circulation. And Binks says it will employ a proprietary pumping system that it uses elsewhere in its product line for another purpose.

The second stage is "the loop" where paint circulates, ready to spray. This consists of more stainless-steel tubing, a second resistance heater to maintain temperature, another pump to circulate the mixed paint, and high-pressure hose to the final stage, the gun. Nordson's gun (patent pending) is derived from airless models, but adapted specifically for Unicarb. The same is apparently true of Binks' and Graco's developmental systems.

Carbide's lab models use Nordson nozzles. The nozzle tip is critical to efficient coverage, because [CO.sub.2] disperses faster than the solvents it replaces. For example, a tip rated to spray a 6-in. fan of high-VOC paint would spray a 12-in. fan of paint thinned with [CO.sub.2]. Since essentially all the [CO.sub.2] leaves the paint droplets within a couple of inches of the nozzle tip, the sprayed paint looks different. So workers accustomed to hand spraying until paint "looks wet," can't treat [CO.sub.2]-thinned paint the same way or they'll apply "four times as much paint as they need," says Unicarb market manager Thayer West. A critical question being investigated now is the rate at which parts can be sprayed. Since Unicarb uses a higher solids coating, parts can theoretically pass the painting station faster, "but we don't know how much faster," West says.

Nordson's SCF-2 costs $39,000 without guns and hoses; the SCF-4 model costs $50,000. Guns and hoses run from $475 for a manual gun with no electrostatic devices to $6000 for an automatic, electrostatic applicator with power pack. (CIRCLE 11)

Binks and Graco say they expect to have two- and four-gun equipment within four months. Graco has also built two prototype proportioners of bench-top size: 2 x 2 x 3 ft.


Commercial equipment may be available, but even test quantities of specially formulated paint are hard to get, potential Unicarb users say. A major developmental user like General Motors Corp. (the only U.S. auto maker testing Unicarb) says material supply is a big problem. GM in Saginaw, Mich., is testing application of black air-dried enamel in two coats a few seconds apart on power-steering pumps. Its target is 70% VOC reduction. GM's Robert Mowers, manager of coatings technical development, says the demonstration system can be run only intermittently, whenever the plant can get paint samples from Akzo or BASF.

Carbide has licensed five paint companies so far to develop Unicarb coatings: Akzo Coatings Inc. (formerly Reliance), Louisville, Ky.; BASF Corp., Clifton, N.J.; Guardsman Products Inc., Grand Rapids, Mich.; PPG Industries Inc., Pittsburgh; and Lilly Industrial Coatings Inc., Indianapolis. Carbide hopes to sign up five more paint companies this year. (Industry sources say that Du Pont Co. is the only major coatings company to have indicated that it isn't interested in pursuing Unicarb at this time, because it is concentrating on waterborne and powder-coating technologies.)

To get a developmental drum of coating to test can take weeks or months, Unicarb users say. The problem for paint companies is obtaining raw materials--i.e., the "slow" solvents required by the Unicarb atomization process.

Theoretically, [CO.sub.2] can thin virtually any paint or clear coating--acrylic lacquer, cellulose acetate, alkyd, polyester or vinyl. In its tests, Carbide says it reduced VOCs from 3 lb/gal of paint down to 1.9 for appliance coating; from 4 to 2 lb/gal for automotive acrylic clear coats; and from 6 to 4.5 lb/gal for nitrocellulose wood finishes.

Paint makers say some coatings are easier to reformulate for Unicarb than others, but that the system defies easy generalization. It depends on a combination of resin type, product application, availability of required additives, and the processor's willingness to accept change. Supposedly some coatings have even been found where [CO.sub.2] is the only solvent needed.

They also note that potential VOC reduction is greater from a high-VOC system than a conventional or high-solids system. A 3-lb/gal VOC paint, for instance, might only be reduced to 2.3 lb/gal with Unicarb. Says one paint maker, "The problem is that 5000-cp paint is thinned with [CO.sub.2] to 10 cp inside the equipment; but the [CO.sub.2] is released at the gun tip, and within a couple of inches of the gun the paint is back to 5000 cp gain. It hits a part and won't flow or level."

These sources note that the Unicarb process only works with "slow" solvents, which can mean that the resulting paints often dry much slower than is acceptable on a production line and can cause crazing when plastic parts are oven-cured.

Voids in paint are another problem because [CO.sub.2] dissolves in coatings like bubbles in soda pop. Coatings must be formulated to keep from skinning over before the bubbles can escape. Even so, automotive coatings people say microscopic voids can be trapped in the finish and cause problems later. However, Guardsman's Coatings Group v.p. Thomas Keeling says his firm has overcome the problem of air entrapment in its nearly commercial nitrocellulose lacquers for wood furniture.


Carbide and its licensees are all concentrating first on clear coatings for automobiles and wood furniture. One reason is that the elaborate Unicarb apparatuses are slow at color changes (15-30 min vs. 8-10 sec for high-VOC paints). For now, a quick color change would require a separate apparatus for each color.

Unicarb also doesn't orient the aluminum flakes that need to settle flat to give automotive color coats their depth and brightness. Metal-flake colors come out dull or mottled, paint companies say. Instead, auto color coats are expected to convert to lower-VOC waterborne paints, which "metallize well." Carbide says the Unicarb system could potentially be used on agricultural vehicles or fleet refinishing of trucks or vans, which don't need metallic Class A finish or rainbow colors. But so far, the coatings companies say they're having problems with flow and application control on most Unicarb pigmented products, both conventional and high-solids types.

Automotive primers and component paints--mostly acrylic lacquer--are also potential plastics applications for Unicarb technology, since they don't require many colors. Other candidates are decorative paints for plastic electronics cases and office equipment--again because of the limited color palette and EMI-shield coatings. Here Unicarb could have an advantage over the high-VOC coatings it replaces because solvents like acetone, MEK and lacquer thinner attack plastics. "Sometimes that's intentionally designed into the coating to make it adhere better, but sometimes it produces crazing," says Jack Adams, director of training at Binks. The Unicarb system could also reduce emissions from spray adhesives and mold release agents, licensees say. In fact, Carbide already uses the technology in its "Sert" low-VOC mold-release spray system for urethane foams (see PT, Oct. '89, p. 14).

Among paint licensees, Akzo is working on wood furniture, aerospace and automotive coatings, BASF and PPG on automotive clear coats, Akzo and BASF on auto component paints, and Guardsman on clear wood furniture coatings. Guardsman's Keeling says its nitrocellulose lacquers for wood, which reduced VOCs from 6 to 4.5 lb/gal, "could be commercial immediately." BASF says its goal is to have a commercial clear coat for cars by mid- to late 1991; its clear-coated parts are already weather testing in the Australian summer sun. "We're starting with several formulations of a Class A clear coat," says BASF's manager of engineering research Lloyd Altamirano. PPG says its clear-coat testing and approval could take five years if totally new technologies were required, so it will concentrate on making "acceptable coatings" from existing technologies.


Users will have to pay an ongoing royalty to Carbide based on solid gallons of paint sprayed, while paint formulators pay Carbide a one-time fee. Carbide's royalty figure as recent as last November was $14/solid gal, coatings people say. Now Carbide is backing off and says its royalty hasn't been determined yet. Carbide's West says fees will be "competitive with other VOC-abatement alternatives." He says Carbide will "try to evaluate the benefits to the user--maybe he can do one thicker coat instead of two thin ones; or he eliminates emission fees per ton; or he saves on the cost of solvents he doesn't use." But licensees complain that Carbide's failure to specify so far how much the ongoing royalty expense will be is making it harder for them to sell the technology to potential users.


BASF is one paint maker that is interested in applying Unicarb to polyurethane coatings, something beyond the capabilities of currently available equipment. Carbide has built two multi-component Unicarb lab models, capable of thinning urethane paints with [CO.sub.2], then using an external metering pump to add catalyst just before the gun, so paint doesn't set up in the system. One prototype is in Carbide's West Virginia lab; another is in field tests at a furniture company in the South. Nordson, Graco and Binks all are interested in building multi-component equipment too. The first could be commercial by July, Carbide says.

But the two-component prototypes can meter only up to 4% catalyst levels, an industry source says, so they're limited to a two-part finish such as an acid-catalyzed alkyd/melamine wood coating. He says the Unicarb equipment built so far can't control catalyst ratios like the 10:1 to 4:1 required for automotive and general-purpose urethanes and other two-part chemistries, including those for use on plastic business-machine housings. Carbide is believed to be beginning to address these other chemistries.


Still another concern is whether the Unicarb system will go far enough in VOC reductions to meet the next round of EPA requirements, notes GM's Mowers. Until recently, processors coated plastic parts without any Federal oversight of emissions levels, unless they made electronics cases. But that's changing. The EPA is now developing the first round of "Control Technique Guidelines" (CTGs) limiting VOC lb/gal of coating in plastic part manufacturing. These CTGs will be issued to the states, which use them as minimum guidelines, but many states may choose to be stricter.

On behalf of EPA's CTG development effort, consultant Radian Corp., Research Triangle Park, N.C., has polled a small group of plastic processors for information on current VOC levels of their paint, equipment used for emission control, and the types of plastic they coat. Under the recently adopted Clean Air Act Amendment, the EPA should issue its first plastics-coating regulations by November 1992, which isn't far away in terms of commercializing major new painting technologies.

So despite its uncertainties, Unicarb technology may be closer to commercial than anything else. Industry sources say Unicarb may be three years away from a big commercial automotive application because of the testing timetable required whereas a commercial low-temperature powder coating could be five to seven years away.

"All the new technologies can be painful," says PPG's manager of automotive coatings development Marvis Hartman, "so people will go with the technology that's the least painful." [Chart Omitted]

PHOTO : Nordson's new SCF-2 spray-painting box is the first commercial

equipment for spraying [CO.sub.2]-thinned paints and coatings. But it may wait awhile suitable paint. So far even test quantities of Unicarb coatings are hard to get.
COPYRIGHT 1991 Gardner Publications, Inc.
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Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:volatile organic compound
Author:Schut, Jan H.
Publication:Plastics Technology
Date:Mar 1, 1991
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