Metal Painting and Coating Operations

Table of Contents  Background  Regulatory Overview  Planning P2 Programs  Overview of P2  Surface Preparation
Alternatives to Solvent-Borne Coatings  Application Techniques  Curing Methods  Equipment Cleaning

Overview of Pollution Prevention in Coating Application Processes

Significant amounts of pollutants are generated from paint and coatings processes. The exact amount for the nation is difficult to calculate, because use is spread across numerous industry groups (EPA, p. 158), and companies do not report emissions by manufacturing process to EPA. Wastes from paint application include leftover paints, dirty thinner from the cleaning of spray guns and paint cups, air emissions of VOCs and HAPs, dirty spray booth filters, dirty rags, debris from area wash downs, and outdated supplies. Simple and cost-effective ways to reduce these wastes include rigid inventory control, good housekeeping practices, proper paint mixing, increased operator training, high transfer efficiency equipment, proper cleaning methods, alternative coatings, reusable paint booth filters, recycling solvents, and the use of waste exchanges (KSBEAP, p. 21). This chapter presents an overview of these techniques while detailed information on specific technologies are covered in subsequent chapters. Table 7 presents a broad overview of pollution prevention opportunities in coating operations.

Table 7. P2 Options for Coatings Processes (KSBEAP, p. 23. and IHWRIC, p. 39-40)

Rigid Inventory Control

Rigid inventory control is an efficient and effective way of reducing indiscriminate use of raw materials. The facility should monitor employee operations and make verbal or written comments on product use. Another option is to limit employee access to storage areas containing raw materials. This inaccessibility can force employees to stretch the use of raw materials (EPAr, p.8). Rigid control can reduce solvent use by as much as 50%.

Good Housekeeping

Improvements in better operating practices, or "good housekeeping" methods apply to all emissions and waste streams, require minimal capital outlays, and can be very effective in reducing wastes and pollutants. Good housekeeping includes the development of management initiatives to increase employee awareness of the need for, and benefits of: pollution prevention; preventative maintenance to reduce the number of leaks and spills; and efficient use of raw materials. Table 6 presents a summary of good housekeeping measures that are described in detail in this chapter.

Table 6. Opportunities for Improved Housekeeping in Coating Operations (KSBEAP, p. 21)

Many methods are available to control and minimize material losses. The following approaches to bulk material drum consolidation, material transfer methods, evaporation, and drum transport can effectively limit material loss:

• Control inventory by storing drums together in an area of limited accessibility
• Reduce leaks and spills by placing drums at points of highest use
• Use spigots or pumps to transfer materials from storage containers to "working" containers
• Control evaporation by using tight-fitting lids and spigots
• Use drip pans
• Use secondary containment in bulk storage areas
• Move drums correctly to prevent damage or punctures that could lead to leaks or ruptures during future use (EPAr, p.8).

Paint Mixing

In many cases, facilities will mix a fixed amount of paint for each job (e.g., one pint or one quart). For small jobs especially, the amount of paint prepared often exceeds the amount of paint actually applied. Facilities can encourage the use of the correct amount of paint by having various sizes of paint-mixing and sprayer cups available to limit overmixing. Any paint not used for a job is usually considered a hazardous waste and should be disposed of as such. A disadvantage to this technique is that if too little paint is mixed for the job and more needs to be made, color matching can be difficult (EPAr, p. 9).

Operator Training

Operators may be skilled in producing high quality finishes but poorly trained in minimizing paint use. Technical assistance providers can help operators by teaching them to:

• Avoid arcing the spray gun and blowing paint into the air
• Maintain a fixed distance from the painted surface while triggering the gun
• Keep air pressure (which is often set too high) low; this can increase transfer efficiency by 30 to 60%
• Keep the gun perpendicular to the surface being painted
• Use proper on/off trigger technique (KSBEAP, p.23)

High Transfer Efficiency Equipment

Less overspray means reduced emissions. Transfer efficiency is a measure of how much paint actually goes on the product, compared to how much paint is sprayed. Typical transfer efficiency from conventional guns ranges from 20 to 40%, making average overspray rates 60 to 80%. For more information on high transfer efficiency equipment, refer to chapter 7.

Alternative Coatings

Painting usually consists of applying a primer/surfacer followed by one or more coats of paint. VOC emissions are directly related to the types of paints used. Technical assistance programs should assist companies in identifying any potential alternative coatings such as powder, waterborne, or high-solids coatings. For more information on alternative coatings, refer to chapter 6.

Proper Cleaning Methods

Reducing solvent use in equipment cleaning can significantly reduce pollution. This can include:

• Scraping paint cups or tanks before rinsing with solvent
• Making use of Teflon-lined metal paint containers that are easier to clean
• Using an enclosed gun-cleaning station
• Spraying solvent through the gun into the cleaning station where it is condensed for recovery and reuse
• Scheduling jobs so that large batches of similar items are painted instead of scheduling jobs so that small batches of custom items are painted. This reduces the amount of solvent and waste paint generated
• Scheduling jobs from light to dark colors to minimize cleaning between colors (EPA, p.10)

For more information on proper equipment cleaning methods, refer to chapter 8.

Filters

Reducing the amount of filters used in painting can reduce hazardous waste generation. Facilities should handle filters as a hazardous waste if they contain wet paint (e.g., solvents), due to their flammability and the existence of toxics in the paint. One method for reducing filter waste is to use a cleanable polystyrene filter or a reusable metal filter. When the filter is too clogged for use, it can be cleaned by blowing compressed air over the filter until it is clean enough for reuse (paint removed in this process would require collection and may still be classified as a hazardous waste) (EPAr, p. 13).

On-site Solvent Recycling

Several alternatives are available for recycling solvents onsite. Gravity separation is inexpensive and relatively easy to implement. This technique enables a solvent/sludge mixture to separate under quiescent conditions. The clear solvent can be decanted with a drum pump and used for equipment cleaning. This reduces the amount of wash solvent purchased. Reclaimed solvent also can be used for formulating primers and base coats, but might create problems if it is not sufficiently pure.

For those facilities that generate large quantities of waste solvent, on-site distillation may provide a more cost-effective solution. Batch distillation of all high-grade solvent wastes can virtually eliminate the need to purchase lower-quality solvents used in priming and cleaning operations. An operator can reclaim 4.5 gallons of thinner, with 0.5 gallons left as sludge. This ratio will vary depending on the specific operation (EPAr, p. 11). For more information on solvent distillation, refer to chapter 5.

Waste exchanges provide another alternative for reducing waste disposal costs. Waste exchanges are organizations that manage or arrange for the transfer of wastes between companies, where one producer's waste becomes another producer's feedstock. Most exchanges exist as information clearinghouses that provide information on available wastes. Opportunities exist for these exchanges to oversee direct transfer (without processing) of waste solvents from one company to another (KSBEAP, p. 24).