Metal Painting and Coating Operations
Table of Contents Background
Regulatory Overview Planning P2 Programs
Overview of P2 Surface Preparation
Simply stated—pollution prevention makes good business sense. Faced with the increasing costs and liabilities associated with end-of-pipe pollution control practices, many companies are turning to pollution prevention as a cleaner, safer and more cost-effective alternative.
EPA defines pollution prevention as any practice which reduces or eliminates the amount or toxicity of pollutants entering the waste stream or the environment prior to recycling, treatment or disposal. Pollution prevention includes such techniques as modification or redesign of processes; reformulation or redesign of products; product substitution; raw materials substitution; and improved maintenance, housekeeping and operating practices (EPAj, p. v).
Designed for technical assistance providers, this manual focuses on pollution prevention techniques for reducing emissions of volatile organic compounds (VOCs) from paint and coating processes, including reducing the amount of solvents used in coating formulations as well as in surface preparation and equipment cleaning. Most of the information contained in this manual relates to the coating of metal substrates used to manufacture metal containers, automobiles, machinery (including computers), metal furniture, appliances and other consumer goods.
This chapter presents the definitions of key terms, discusses uses for paints and coatings and provides general information on paint composition and coatings processes. It also provides examples of typical coating systems and discusses the sources of wastes in the coatings process, including the specific pollution problems that are the focus of this manual.
The following terms are used throughout the manual. These terms are often used to mean a variety of things. To clarify the use of the terms in this document, we have provided the following definitions.
Coating: This term refers only to organic or polymer coatings and their associated application techniques. In other words, although metal plating does perform the function of a coating (e.g., it improves appearance, corrosion resistance, abrasion resistance, and electrical or optical properties), this manual does not cover metal plating (i.e., zinc, aluminum, etc.) or related processes (i.e., electroplating, conversion coating, sputtering, ion plating, and plasma spraying). Detailed information on P2 options for metal plating can be found in NEWMOA's manual Pollution Prevention for the Metal Finishing Industry.
Solvent: This term generally refers to hydrocarbon-based or organic solvents only; that is, solvents made from petroleum that contain the chemical elements hydrogen and carbon. In other words, although water is a solvent in terms of function (i.e., it is a liquid capable of dissolving another substance), the use of the term solvent in this manual, for the most part, does not apply to water or other non-carbon compounds.
Paint is a generic term typically used to identify a wide range of surface coating products, including conventional solvent-borne formulations, varnishes, enamels, lacquers and water-based systems. Normally, painting is a process where a liquid consisting of several components, when applied, dries to a thin plastic film. Traditionally, major constituents of these paints are solvents. However, non-liquid paints such as powder coatings and high solids paints have also been developed. These newer materials have led to the use of the term coating instead of the term paint. In general, the function of all paints and coatings is to provide an aesthetically pleasing colored and/or glossy surface, as well as to help metal and other substrates withstand exposure to both their environment and everyday wear and tear (TURIb, p. 1).
Paints and coatings can be categorized according to their use into three major groups:
The intent of this manual is to provide information on pollution prevention opportunities for users of product coatings. Because product coatings are used by a wide variety of industries, it is difficult to accurately quantify these users. In addition, the use of product coatings occurs not only in OEM settings, but also in contract job shops. The pollution prevention opportunities identified in this manual are not industry specific, but rather they include general options available to a variety of firms that coat metal substrates. Therefore, many of the P2 opportunities identified in this manual can be applied to users of architectural and special-purpose coatings as well.
The major components of solvent-borne paints and coatings are solvents, binders, pigments, and additives. In paint, the combination of the binder and solvent is referred to as the paint "vehicle." Pigment and additives are dispersed within the vehicle (IHWRIC, p. 2). The amount of each constituent varies with the particular paint, but solvents traditionally make up about 60% of the total formulation. Typical solvents include toluene, xylene, MEK, and MIBK. Binders account for 30%, pigments for 7 to 8%, and additives for 2 to 3% (KSBEAP, p. 4). Environmental issues surrounding paints usually center around solvents and heavy metals used in the pigments. Binders and other additives can also affect the toxicity of the paint depending on the specific characteristics of the paint. For more information on paint composition, refer to chapter 6.
The coating of metal substrates can be broken up into three major steps: surface preparation, a two-step paint application/curing process and equipment cleaning. These steps are presented in figure 1.
Although each of these steps can affect the performance of the final finish, proper surface preparation is essential in ensuring the success of a particular coating. In fact, as high as 80% or more of all coating adhesion failures can be directly attributed to improper surface preparation (Binksb, p. 1).
In surface preparation, a variety of methods are used to remove soils or other imperfections from substrates, creating a surface that bonds well with the coating. The most common form of debris are oils and/or greases that originate from mechanical processing or oils and greases that are deliberately applied for temporary storage or shipping (Kuhn, p. 25). Other contaminants commonly include oxidation, rust, corrosion, heat scale, tarnish, and smut (SME, p. 27-1). In some cases, old paint must also be removed prior to the application of a new paint coat (MnTAP, p. 2). Traditionally, halogenated solvents have been used as cleaning and stripping agents to remove these substances.
As part of surface preparation, a conversion coating might be applied to improve adhesion, corrosion resistance, and thermal compatibility. The processes used most often for the application of conversion coatings on metal are phosphating (using iron or zinc) and chromating. Anodizing (i.e., the electrochemical deposition of an oxide coating) is sometimes used on aluminum surfaces (KSBEAP, p. 2-3).
In the phosphating process, acid attacks the metal surface, forming a microcrystalline layer that improves the surface for paint application. Zinc phosphate coatings are predominately used for metal substrates (Doren et al., p. 131). Combining cleaning and phosphating in a single solution is possible; however this is not the case with zinc phosphating (KSBEAP, p. 2-3). For more information on conversion coatings consult, Pollution Prevention for Metal Finishing: A Manual for Pollution Prevention Technical Assistance Providers, published by the Northeast Waste Management Officials' Association.
Following surface preparation, paints and coatings are applied to substrates using a variety of methods, including:
Coatings are usually applied in a number of coats, starting with a prime coat followed by subsequent coats (basecoats and topcoats) and a finishing coat (clearcoats). Given the different types of coatings necessary to ensure adequate protection and performance, coatings should always be considered as a system.
Once a paint has been applied, a curing process takes place that converts the coating into a hard, tough, and adherent film. Coatings cure by chemical reaction or polymerization of the resins (i.e., crosslinking). Mechanisms for initiating curing generally include ambient temperature oxidation, chemical reaction with another component (two-component coating systems) or baking in an oven. Radiation is an additional curing mechanism (IHWRIC, p. 11).
The final stage of any coating operation is the cleaning of equipment, such as spray guns and hoses. This generally involves flushing solvent through the coating system (Freeman, p. 483-484).
Although the basic process remains the same, the particular coating system, coating formulation and application method used, can vary considerably from industry to industry. In the automotive industry, for example, approximately 80% of all painting starts with an electrocoat primer, usually applied by electrodeposition. Visible indoor areas of automobile bodies receive a topcoat, usually of the same color as the overall body topcoat. In addition, the underside of the hood and inside of the engine compartment usually receive a topcoat of black alkyd or acrylic paint which is sprayed on; therefore, they carry a two-coat system. Outside surfaces of the body receive a sandable surface coat, which is either fully or partially sprayed and is applied on either the wet or incompletely baked electrocoat. Next, the color topcoat, usually an acrylic resin, is sprayed on and baked. In many cases, a clearcoat is sprayed over the color coat to provide "depth" (SME, p. 29-4-6).
The appliance industry, however, uses high-solids paints to spray coat surfaces. These paints are hardened with a crosslinking agent called melamine. Some assembled appliance cabinets receive a 7-stage zinc phosphate metal preparation and are then prime coated inside and out by electrodeposition. The cabinets can also be spray primed with a thermosetting epoxy-resin-based paint, followed by a topcoat of acrylic melamine paint, which is sprayed on. Other appliances carry a powder coat, which is sprayed directly over the metal preparation, plus a decorative acrylic melamine coat (SME, p. 29-4-6).
Steel furniture for indoor use generally receives a 3- to 5-stage iron phosphate metal preparation, plus a dip, spray, or electrodeposited prime coat. The topcoat is usually an alkyd or acrylic. Steel outdoor furniture and steel doors usually receive a 7- or 9-stage zinc phosphate treatment, plus a prime coat of epoxy-based spray paint or an electrocoat. The topcoats may be alkyds or polyesters, and are sometimes modified with silicone. In some cases, powder coats are applied over the iron phosphate preparation (SME, p. 29-7).
Traditionally, each step in the coating process generates waste and emissions. Figure 2 presents a process flow diagram that outlines the sources and types of pollutants. Wastes occur in solid, liquid, and gaseous forms and can include the following:
Inefficient paint transfer can be the largest source of waste and VOC emissions from paint and coating processes. Paint used but not applied to the surface being coated (e.g., paint overspray) generally becomes waste. A spray booth can be used to remove the overspray as it is generated (IHWRIC, p. 38). However, the type of booth selected can also affect the volume and type of paint waste generated (MnTAP, p. 4). See chapter 4 for more information on spray booths and their effect on waste generation.
Evaporation of organic solvents is an important source of air emissions. During coating application, solvents that are present in conventional paint formulations evaporate and release VOCs into the air (IHWRIC, p. 38). Emissions occur during initial coating, as well as each time a surface is recoated during the life of the object or structure (EPAk). In addition, solvents used to thin paint, to clean equipment, and to prepare surfaces for coating can be sources of VOCs (IHWRIC, p. 38).
Specific estimates of the amount of solvents released during coating application are difficult to make as use is spread across numerous industry groups. However, EPA has developed air emission factors for solvent losses from paint and coating applications. EPA estimates that all toluene and 87% of the xylene isomers used in paints and coatings are emitted to the atmosphere when the emissions are uncontrolled. No emission factors are available for MEK and MIBK used in paints and coatings, but it can be assumed that, like toluene and xylene, virtually all these solvents are eventually released to the atmosphere (EPA, p. 157-158).
Cleaning of equipment is a third major source of waste generation. Generally, all paint-application equipment must be cleaned after each use to prevent dry paint residue and avoid contaminating batch processes. In addition, brushes and rollers must be cleaned after each use to remain pliable (IHWRIC, p. 38).
The primary focus of this manual is on P2 methods for reducing pollutants generated during coatings application and on reducing emissions of VOCs in particular. VOCs can pose risks to human health and the environment. These problems have prompted the federal government and a number of states to promulgate regulations to control releases of solvent emissions and wastes from paint and coating processes. For an overview of applicable regulations, see chapter 2.
Pollution prevention is an effective method for reducing emissions of VOCs and other wastes, and therefore, for reducing a firm's regulatory compliance burden. General information on promoting pollution prevention can be found in chapter 3. An overview of specific P2 options for coatings processes is discussed in chapter 4, with detailed technical information provided in chapters 5-9. See table 7 at the end of chapter 4 for a complete list of P2 options.