Metal Painting and Coating Operations
Table of Contents Background
Regulatory Overview Planning P2 Programs
Overview of P2 Surface Preparation
When a painting process is completed, a color change is needed, or maintenance is required, the metal coater must clean the equipment. There are numerous P2 opportunities for reducing waste and air emissions in equipment cleaning operations.
All coating practices require some type of equipment cleaning. For spray painting, the most common coating operation, spray guns and accessories must be cleaned between color changes, when orifices clog and often at shift changes (IWRC, p. 15).
External equipment surfaces generally are cleaned by soaking, wiping or flushing with solvent. If equipment cleaning is done in an open container, a significant quantity of solvent is lost to evaporation. Internal parts and passageways as well as paint guns are commonly cleaned by flushing solvent through the gun and orifice. This practice also results in significant evaporation and loss of usable product (IWRC, p. 15).
A cost-effective method for reducing wastes is to eliminate unnecessary cleaning. For equipment that requires cleaning, making improvements in operating practices that minimize solvent use and reduce evaporation should be implemented wherever practical. Using a gun washer to clean spray guns is one example. Various solvent recovery and reuse technologies are also available. In addition, alternative cleaning solutions can be used. Each of these options is discussed below.
Implementing better operating practices and scheduling can significantly reduce waste generated from cleaning operations. The amount of waste generated is directly related to the number of times paint color or paint types are made. For this reason, scheduling improvements have perhaps the largest effect on the volume of waste produced from cleaning equipment. Making large batches of similarly produced items instead of small batches of custom items, increases the time between cleaning. Additionally, scheduling paint jobs so that they move from the lightest color to the darkest can also reduce the need to clean.
When assessing the cleaning process, all the typical cleaning tasks should be reviewed to learn whether cleaning is necessary. While most coaters assume that spray guns, tips and lines must be cleaned for reuse, cleaning some low-cost items might not be advisable. Costs from cleaning solvent purchases, solvent waste disposal and solvent emissions could be higher than simply replacing the item being cleaned. However, the costs of proper disposal must be factored into any decision (MnTAP, p. 5).
A technical assistance provider should also help a client company review the ways in which cleaning solvents are handled. All solvents should be stored in covered containers when not in use. Leaving solvents in the open air creates unnecessary solvent waste and VOC emissions. In addition, the company should set a standard for the minimum strength necessary for cleaning in order to ensure that used solvent is disposed of or recycled only when it loses its cleaning effectiveness, not just because it looks dirty (MnTAP, p. 5).
The use of a gun washer can also help to reduce wastes generated during equipment cleaning. An automatic gun washer operates like a dishwasher.
The paint gun is partially disassembled and placed in the unit. Cleaning is accomplished by recirculating solvent sprays. These units reportedly reduce solvent waste by 50 to 75%. VOC emissions can be reduced by up to 20%, and a 60% labor time savings can be achieved (IWRC, p. 15).
Units range in cost from $600 for small units to approximately $1,500 for industrial type units (i.e., gun and paint hose wash). Similar units may also be leased through various chemical suppliers and waste management companies at a cost of $165 to $195 per 5 gallon waste solvent change out interval (IWRC, p. 15-17).
For maintenance of pressure pots, many companies use a polyethylene inner liner with the pressure pot. The main advantage of this practice is that only a small amount of paint comes into contact with the steel or stainless steel body, and cleaning the liner requires only a small amount of solvent. After pouring solvent into the liner, the operator should swirl it around for a few seconds. The operator can then discard the spent solvent into a hazardous waste drum and the liner is ready to be reused.
Some operators choose to allow the paint that sticks to the side of the liner to dry out, which causes it to flake off with ease. If the solid paint is shown to be hazardous per RCRA guidelines the facility must manage it as a hazardous waste. If it is not hazardous, it can be discarded with the rest of the solid waste. The liner should then be reused (EPAq, p. 137).
Because of the increased need to reduce VOC emissions, alternative cleaning solutions are available. They include dibasic esters (DBE), N-methyl-2-pyrolidone (NMP), and a variety of other alkaline-, citric-, and water-based solvents such as d-liminone, naptha, and terpenes. These chemicals have reduced VOC emissions due to their lower evaporation rate. Although toxicology information specific to these chemicals is relatively limited at this time, many researchers believe that the relative safety of similar chemicals indicate that they are a feasible alternative to organic solvents in certain applications (MnTAP, p. 5-6).1
1 For more information on these alternative solvents, see Project Summary: SAGE 2.1, Solvent Alternatives Guide: User's Guide. Research Triangle Park, NC: Air and Energy Engineering Research Laboratory. EPA/600/SR-95/049.
Onsite recycling of used solvent is another way to reduce waste and save money. Savings come from reducing the amount of solvent purchased and the volume of spent solvent that must be sent offsite for costly disposal. Two common methods of solvent recycling are settling and distilling (MnTAP, p. 5-6).
Settling involves putting used solvent in a container and letting the particulate matter settle out. The container should be designed to allow for removal of solvent without shaking up the sludge that has settled out (MnTAP, p. 5-6). Solvents can be used for gun cleaning and then can be placed back into the storage container for subsequent settling and reuse. Eventually, sludge will make up the majority of the container and offsite hazardous waste disposal will be necessary. At this point, the processes can be repeated using a different container. Solvent waste reduction of up to 33% can be accomplished with this simple method (IWRC, p. 15-17). Filtering equipment, which removes the particulate matter from solvents, also is available (MnTAP, p. 5-6).
Waste solvent also can be collected and processed through distillation equipment. Approximately 80% of the used solvent is recovered with basically the same cleaning properties as a new product. The remaining 20% sludge (still bottoms) must be collected for offsite hazardous waste disposal. To help maintain the cleaning properties of the recycled thinner, certain paint and solvent wastes should be segregated. Waste gun wash solvent and any waste lacquer paint and thinner mixtures can be included for recycling. All waste urethanes, enamels and enamel reducers should be placed in a separate container; enamel and urethane products will not clean as well as pure lacquer thinner. By segregating the two, the reclaimed solvent will possess cleaning properties like a virgin thinner. This waste management technique has the advantage of reducing the volume of virgin thinner purchased as well as the amount of waste thinner generated (IWRC, p. 15-17).
Onsite distillation equipment comes in a wide range of capacities, from 5 gallons per 8 hour shift batch operations to more than 100 gallons per hour flow-through units. Costs for 5 gallon batch units start at approximately $1,500 with an average cost of $3,000 (IWRC, p. 15-17).