Low-VOC Primers for the Finishing Industry

In 1984, the South Coast Air Quality Management District (SCAQMD) enacted two rules that have had a major impact on the finishing industry. These rules are Regulations No. 1107 and No. 1124. Although revised in 1986, the major impact of these regulations was a reduction of solvent content in coatings used in primer and topcoat applications in the Miscellaneous Metals and Aerospace and Aircraft market segments in Southern California. (Rule l107 deals with Miscellaneous Metals while Rule 1124 concerns the Aircraft and Aerospace markets.)

Although these regulations deal with a number of coatings and cleaning solvents, their primary thrust was to limit the solvent content of topcoats to a maximum of 420 g/liter. Primers were even further restricted to 340 g/liter. These limits were phased in over a period of time to allow users to identify and implement new, compliant products.

The purpose of this paper is to discuss primers and recent gains made in the development of primers. More specifically, it will focus on high-performance military and aerospace primers. These coating systems are typically covered by Military Specifications MilP23377, MilP85582, MilP53030, and MilP26915.

The high-performance Aerospace or Commercial Specifications currently include Boeing's BMS1011, and McDonnell Douglas' DMS 1786. Commercial primers typically differ from their military counterparts in that they require the added resistance to "Skydrol" (hydraulic fluids).

When a survey of existing technologies and chemistry was made for potential answers to this problem two basic conclusions were apparent.

High-solids technologies (epoxies), although initially attractive, do not appear to offer the potential for further development if VOC contents are forced even lower than current levels. When formulated to the allowable levels of 340 g/liter the coatings must overcome some of the following problems:

• They typically are heavy in initial viscosity, creating atomization problems.
• They are difficult to apply in thin films. Heavier films result in additional weight to the aircraft. They also cause premature failure of the coating by chipping and embrittlement.
• Dry times are typically excessive. Most available materials take in excess of 14 hrs to develop solvent resistance. When recoated prematurely, the film will delaminate, causing failure. Films are extremely temperature sensitive, and temperatures greatly retard dry times.
• Existing resin systems do not lend themselves to formulation of lower VOC coatings. Further reduction in VOC regulations will ultimately rule these materials out of the compliant markets.

Waterborne technologies (epoxies) are initially attractive and seem to offer more potential for additional reduction of volatile organic compounds.

• The major drawback is that when surfactants are introduced into the backbone of the resin system (to stabilize shelf life and make them accept water), they lack the "performance" of high-performance systems.
• Unfortunately, once surfactants are introduced into the system, they never leave the dried film (even in catalyzed and crosslinked coatings). These surfactants remain moisture sensitive and cause most film failures associated with this technology.

In spite of the problems associated with the above technology, tests on waterborne epoxy systems were carried on in earnest. Their potential benefits merited further investigation.

By 1979 a new method of using chemistry for reducing epoxy resins with water was introduced. It allowed one to introduce water into the system as a "carrying agent and diluent" only. The method uses no surfactants, therefore you are able to obtain the high performance required to meet the specifications outlined previously, without premature final film failures. This technology allows the system to function as a true epoxy primer. In many cases these new products exceed specification requirements.

These waterborne materials are unique to the coatings industry, and have been patented and sold as concentrates. The materials are catalyzed and reduced at the time of application by the end user. This new technology offers several benefits that the high-solids solvent-borne counterparts do not. The most obvious would be:

• Lower VOC. Formulation products are already available in the 250 g/liter range as Approved Military Specification coating systems. Research indicates that not only is a 200 g/liter target attainable but a zero VOC as well. This gives the ability to meet or exceed any new proposed state or federal regulations on VOC content.
• Thin film application. These systems are typically reduced to a mixture that is 25 pct volume solids when applied. Dry film thickness in the range of 0.5 to 1.2 dry mils is easily attainable. (Desired film thickness varies by application.) Dry film weight is comparable to high-solvent-based systems (650 g/liter). There is one product approved under Mil P85582 that is a low-density primer, resulting in a dry film weight almost 20 pct lighter than either system.
• Fast dry times. These coating systems are designed for fairly fast dry times. Normal topcoat drying times are 45 minutes to one hour, depending on air movement.
• Ease of application. When reduced to the 25 pct volume solids range, these coatings have a normal viscosity in the range of 1822 seconds in a No. 2 Zahn cup. Application results in an extremely smooth film, easily applied using all types of spray equipment.
• Longer potlife is also attainable with the waterborne materials. Since they require no "sweat time," they are usable throughout the entire pot-life and lend themselves quite well to the use of plural component mixing systems. Most of the waterborne systems offer a minimum of four hrs pot-life. With recent technical advances some are capable of up to six hours.

Using the chemistry as described above, manufacturers have had high-performance primers for more than 14 years. To date there are more than 6,000 aircraft flying with these products. It is estimated that these materials have saved more than 13,000,000 lbs of solvent emissions during this time.

The main thing these products have had in common is that each product has been formulated to a maximum VOC content of 340 g/liter.

Based on the success of these products, and in anticipation of more strict EPA regulations, several new products that have a VOC content of 250 g/liter have been developed. Also developed and in the process of testing are a few new products with a zero VOC content. Some of these new products are already approved for use by either military or commercial aircraft manufacturers. One such product is the following.

Chrome Free Aircraft Primer. (Proposed MilP 85582B Type I Class N). A non-chromated, corrosion and chemical resistant primer for ferrous and nonferrous metals, waterborne, catalyzed epoxy coating system. Freeze thaw stable.

Typical physical properties of this type of coating include: non-volatile content of 73.87 pct by weight and 61.16 pct by volume; theoretical coverage at one mil dry film is 979 sq ft/ gal; VOCs are 340 g/liter (2.83 lbs/gal); flash point is 72F; and the pot- life is six hours.

Performance properties of these coatings as tested to Mil P-85582A are no chromate pigment; greater than 40 pct solids; passes the thinner, color, odor, freeze/thaw stability and drying time specifications; gloss is 4.5 with primer only and 90 plus with primer and topcoat; it passes the lifting and dry adhesion tests; it also passes the wet adhesion test, which is four days immersion at 120F; it provides 10 pct elongation; and fluid, corrosion and solvent resistance.

The data substantiates the claim that lower VOC content materials can be successfully formulated and implemented into current production situations. The number of aircraft painted to date attests to the viability of these products and this technology.

Current programs in the commercial aircraft market indicate a high level of product acceptance from the industry. Solvent emission savings at one facility last year were calculated at more than 26 tons. This represents a reduction in emissions of approximately 78 pct (from primer applications).

There should be no doubt that the next generation of products will be centered around chrome-free technology. Formulations with VOC contents in the 250 g/liter range will become the standard of the industry with many products available that have a zero VOC content.

The future of this technology will only be limited by the need to continue to develop new materials at lower VOC levels. This process will largely be dictated by EPA regulations established at the state and federal levels.