Number 2
Number 2
Waste reduction alternatives for painting and coating operations in shipyards must include all phases of the operation, paint removal, surface preparation, and application. The first opportunity for waste reduction in painting and repainting starts with an examination of the purpose of the operation. Painting provides an esthetic and protective coating for ships. Therefore, an opportunity assessment should begin with an examination of pretreatment and coating specifications, and application techniques for waste minimization opportunities in these operations.
In repainting operations the first opportunity for waste reduction starts with an examination of what causes the need for repainting: inadequate initial surface preparation, defects in paint or coating application equipment problems, or coating damage due to improper handling or use of final product. Reducing the need for repainting has a direct effect on the volume of waste generated. Therefore, an opportunity assessment should start by examining the cause of the problem.
In operations involved with restoring damaged or repaired finishes, the opportunities for waste reduction start with the paint removal process and surface preparation. What is actually required to restore the desired finish? That is, complete stripping or simple surface preparation of the damaged or repaired area. This determination can greatly minimize waste generation.
The following sections discuses waste reduction opportunities in various paint and coating related processes and operations. Some of the waste reduction opportunities presented may or may not be available in all shipyards, and some may apply only to the preparation and painting of small units are parts used in the fabrication of ships.
Paint Removal
Once the need for paint stripping has been reduced to a minimum, alternate paint-stripping techniques can be considered based on environmental and health concerns, and the quantity of waste generated and cost. Options that should be considered include:
Chemical Stripping: When selecting a paint-stripping method it is important to consider the substrate to be stripped the type of paint to be removed- and the volume and type of waste produced. Waste type, hazardous or non-hazardous, and volume can significantly affect the cost-benefits associated with the change. Solvents used in stripping operation can be recycled usually through the supplier or another service company. Air emissions and health concerns must be addressed when using solvents. Vats should be equipped with hoods to remove vapors and protect workers. Plastic balls should also be used to control evaporation form the surface of the solvent and reduce emissions.
Aqueous strippers can be used as a replacement for solvents in some applications. However, disposal of the spent aqueous stripper can be a problem. In most cases, the alkaline nature of these strippers makes them a hazardous waste with little opportunity for recycling.
Abrasive Blasting: Using sand, garnet, steel shot, plastic and carbon dioxide pellets, walnut shells, etc. offers considerable potential for reuse and waste minimization. Equipment is available to separate chipped paint and fines thus permitting reuse of some abrasive materials. Suppliers of plastic beads will pick up heavily contaminated spent material and recycle the plastic. Service companies are also available for recycling other spent abrasive materials. Chipped paint and fines collected from abrasive blasting operations can be hazardous or non-hazardous depending on the type of paint or coating removed. The volume of waste generated can be significantly reduced by selecting abrasive media that is relatively easy to recycle. Steel grit made from crushed shot is one of the most widely used recyclable abrasives. Selection of abrasive materials depends on the paint or coating and the substrate involved.
The use of carbon dioxide pellets (dry ice) as a blast material is still under development. After use, the dry ice evaporates leaving the paint chips to be collected for disposal. The cost of the dry ice, storage, and handling are substantial. One of the major disadvantages of dry ice blasting is that there is no "bounce back effect" to aid in removing the coating from sides and back of objects.
Most abrasive materials are not hazardous but may become hazardous with use and require testing. If non-hazardous, it can be handled as a solid waste and recycled by using it as construction fill or in cement Uns. The amount of hazardous material generated can be significantly reduced by proper selection and reuse or recycling of abrasive materials.
Particulate air emissions can be a problem with abrasive blasting operations. The emission of fine particulates can be reduced in some operations by using additives which cause agglomeration of particles. Equipment is also available to control particulates emissions, including special enclosures and mechanical devices which move across the surface blasting away the coating and capturing and recycling the abrasive material.
Mechanical Removal: Small repair or repainting jobs may simply require the mechanical removal of loose paint or preparation of the surface using chippers, scrapers, grinders, wire brushes and/or sand paper. Only small quantities of waste are usually generated by these operations and usually they can be combined with similar wastes from other sources for disposal.
Water Jet Stripping (Hydro blasting): Cavitating high pressure water jet stripping systems are available to remove most paints. These systems use pressures as high as 50,000 psi. The water removes hard coatings from metal substrates. The harder the coating, the easier it is to remove by Hydro blasting. These systems can and should be designed to remove the paint from the water and reuse the water for further Hydro blasting. By recirculating the water, the amount of waste is greatly reduced. Wastewater from these processes is usually suitable for sewer disposal after the paint particles are removed. Although this process produces little waste, it is not as efficient as abrasive grit blasting and requires high capital and maintenance cost.
Pretreatment
For waste reduction in the surface preparation or pretreatment of new parts and in some repair operations, the first step is to determine the initial cleanliness of the surface or part. To what degree are the surfaces contaminated with oil from machining, dirt from the environment, residue from paint stripping, and finger or hand prints from personnel? The key element of the assessment is the determination of the sources of contamination.
The next step is to determine the surface and cleanliness level or standard needed to justify the pretreatment process. Once the type and source of contamination are identified, determine whether some or all contamination sources can be eliminated. If the contamination cannot be reduced enough through process changes to meet the desired surface standard, the appropriate cleaning technique must be established. The halogenated organic solvents (CFCs) that have commonly been used as cleaning agents are no longer acceptable due to environmental concerns and regulations limiting their production. A manual "Regulatory Guidelines for Alternate Solvent Selection" which identifies various options for parts cleaning is available from MERIC.
Aqueous and semi-aqueous cleaners are gaining use as alternates to solvent degreasing operations. Depending on the size of area or parts, various cleaning systems are available which will satisfy most requirements (see manual). Phosphatizing is another pretreatment method often used in surface preparation of metals. Close control of chemicals, reduced water use and water reuse represent the primary waste reduction options for phosphatizing.
Abrasive blasting may also be used to prepare the surface for painting or coating. For information on this process see the above section on abrasive blasting.
Paint and Painting Equipment
Transfer Efficiency, or the amount of paint applied to the part, divided by the amount of paint used, is dependent on the type of coating and method of application. High transfer rates offer financial incentives by reducing the amount of paint wasted while minimizing solid, liquid, and air emissions.
In spray-painting, liquid paint is converted to an atomized spray in order to uniformly coat an object. The main difference in the equipment used is how the paint is atomized. The more highly atomized, the lower the transfer efficiency in most cases. Fine particles of paint tend to drift away from the surface due primarily to air currents and gravity. Selecting the proper application equipment for the particular coating material is essential and may require actual performance testing. The advantages and disadvantages of each combination must be weighed against the desired coating specification or requirement.
In most cases, the viscosity of the paint must be adjusted before it can be sprayed effectively. This requires the addition of a solvent that is compatible with the paint formulation. Using solvents for thinning requires the purchase of additional materials and increases air emissions. An alternate method of reducing the viscosity is to use heat. Benefits from the purchase of paint heaters include lower solvent usage, lower solvent emissions, more consistent viscosities, and faster curing rates.
Spray Application Equipment
In order to effectively reduce paint waste and produce a quality coating, proper application techniques should be supplemented with efficient application equipment. Through the use of equipment with high transfer efficiencies, the amount of paint lost to over spray is minimized.
Conventional Spray: Spray equipment uses pressures of 40 - 70 psi to atomize the paint stream. The air stream atomizes the paint and produces a high-quality finish. The main disadvantage is that along with a high degree of atomization comes a spray that is very fine and highly susceptible to over spray, resulting in more paint waste and lower transfer efficiency. Also, the solvent in the paint is also highly atomized increasing volatile organic compound (VOC) emissions. The use of alternate application technologies is recommended to avoid these problems and reduce wastes.
High-Volume/Low-Pressure (HVLP): This system uses a high-volume of air at low pressure (less than 10 psi) to atomize the paint. The reduced gun spray energy level reduces over spray and improves transfer efficiency. The HVLP spray gun is basically a conventional air spray gun with modifications and special nozzles that atomize the paint at very low air pressures producing a "soft " effect. Low application pressure also decreases excessive bounce back and allows better spray adhesion of the coating to the substrate.
HVLP systems are not without some definite drawbacks:
Airless Spray Guns: Instead of air passing through the spray gun to atomize the paint, an airless system uses a pump to pressurizes the paint directly. As the paint is pumped at high pressure through the small opening in the nozzle, the sudden drop in pressure atomizes the paint and it is carried to the substrate by its own momentum. Adjustments in airless praying are done by adjusting the viscosity of the paint or the system pressure. The advantages of the Airless Spray systems include:
A major disadvantage of some airless spray systems is the difficulty applying very thin coats. If coatings with less than a mil in thickness are required, such as primers, it may be difficult to use an airless system.
Air-Assisted: This is a spraying system that helps or "assists" airless systems by using supplemental air jets to guide the paint spray and boost the level of atomization. The effect is a compromise to lower transfer efficiency to achieve better finish quality. While reduced transfer efficiency may mean increased waste, the use of thicker paint coatings to achieve the desired finish quality also wastes materials.
Electrostatics: This process utilizes paint droplets that are given an opposite electrical charge to that of the substrate. The result is that transfer efficiency is increased because the paint is drawn to the part by an electric field. Thus, paint spray is less susceptible to air currents that increase over spray. Even water-based paints can be applied with a high-voltage electrostatic charge, in some cases.
Although the operating costs for this system are relatively low, the initial outlay is high and the paint viscosity requires very careful control. In order for the system to operate properly, the correct solvent balance is very important. The evaporation rate must be slow enough for the charged droplets to reach the substrate in a fluid condition to flow out into a smooth film, but fast enough to id sagging. The resistivity of the paint must also be low enough to enable the paint droplets to acquire the maximum charge.
Multiple Component Systems: A common problem that occurs when working with two-part coating systems is over mixing. Once the component parts of a catalyst coating are mixed, the coating must be applied. The excess unused coating will cure and require disposal. Additionally, the coating equipment must be cleaned immediately after use.
Multiple or plural component systems minimize the generation of paint wastes through the use of a special mixing chamber that mixes the pigment and catalyst seconds before the coating is applied. Each component is pumped through a metering device to control the mixing ratio before entering the mixing chamber. From the mixing chamber, the mixed coating travels directly to the spray gun. The only cleaning that is required is the nixing chamber, gun, and length of supply hose connecting them.
Rotary Atomization: This system atomizes paint by dropping a stream of liquid on a disk or bell-shaped subject spinning at high speed. The action of the paint rubbing against the spinning object also gives the paint spray a static-electricity charge that causes the paint to be attracted to the part. Rotary atomization is useful for high-viscosity paints. This process can create a spray without use of thinner and tends to have high transfer efficiency. However, the equipment need for this type of application is very specialized and usually requires a major conversion of a painting line.
Spray Booths: Control of over spray can be accomplished through the use of enclosures of two basic types, dry or wet booths. The key difference is that a dry booth employs paper, fiberglass, plastic, or metal filters to collect over spray, while the wet booth uses water with chemical additives to scrub paint from the air stream. Selection of equipment should be made based on the type and volume of painting done, and the volume of waste generated.
Generally, small-volume painting operations will find the lower purchase cost of a dry filter booth will meet most of their requirements. One disadvantage of the use of a dry-filter booth is the disposal of used filters. Reusable filters may decrease waste volume and reduce disposal cost. In some applications, over spray can be collected and reused. Used filters may require disposal as hazardous wastes depending on type of paint used.
If the overall painting volume can justify the investment, a wet booth may be advantages. This type of booth eliminates disposal of filter media and allows waste to be reduced in weight and volume. However, water from the scrubbing system requires settling, drying, or use of centrifugal devices to separate the paint. After the paint is removed, the water can be recycled and/or if necessary disposed of in the sanitary sewer (depending on local regulations).
Coating Types
Organic Solvent-Based: Traditional painting materials typically contain about 25 percent solids with a relatively high organic-solvent content. While this coating material is one of the most versatile, its low solid content and high percentage of solvent carrier can result in low overall transfer efficiency. To achieve the desired coverage, more material must be sprayed compared to materials with higher solids content and lower VOC emissions.
High-Solids Coatings: Paints with higher percentage solids and lower percentage solvent content have higher overall transfer efficiencies. The higher solids content means that fewer applications are needed to achieve the desired film thickness. V.C. air emissions are generally less due to the reduced solvent content, and reduced over spray results in less waste generation. However, a paint heater may be required to reduce viscosity and the film thickness may be more difficult to control.
Water-Based Coatings: These coatings typically have a high solids content, utilize water as a solvent, and have very low or no organic-solvent content. Advantages of these materials include reduced V.C. emissions, reduced fire hazard, minimized or eliminated hazardous waste disposal, and easy cleanup. However, water-based coatings may require a cleaner surface, longer drying times, increased oven temperatures, and a temperature-controlled paint storage area.
Catalyzed or Two-Component Coatings: Coatings created by mixing two low-viscosity liquids (a resin and a catalyst system) just before application, eliminate or reduce solvents and cure at low temperatures. However, the resin and catalyst systems may be hazardous themselves and create a different set of emission and exposure problems than those of organic solvents. Two components systems should be used in special applications systems (see above) to minimize material usage and reduce wastes.
Powder Coating: These coatings use 100 percent resin in dry, powdered form which must cure in an oven. Powder-coating materials can provide a high-quality, durable, corrosion-resistant coating. There are no V.C. emissions, hazardous over spray wastes, or wastewater sludges. Dry coating material that does not stick to parts can be collected and reused, thus resulting in high transfer efficiencies.
Powder coating requires specialized application equipment using electrostatic charges to apply the material (see above). Its use is also limited to substrates that can tolerate typical curing temperatures of 300 - 450 'F.
Flame spraying is another method of applying powder coatings. In this process the resin powder is blown through a gun by compressed air. The particles are melted in a high temperature flame and propelled against the substrate. This process is used widely with epoxy powders for aluminum surfaces. Plastic flame coats is a similar process to flame spray powder coatings and produces a pinhole-free surface that is not effected by many corrosive agents.
Good Operating Practices
In many cases, simply altering a process can reduce waste for better management of paint usage and waste generation.
Coating Application: Good manual coating application technique is very important in reducing waste. Most shops rely primarily on spraying methods for coating applications. If not properly executed, spraying techniques have a high potential for creating waste, therefore, proper application techniques are very important.
Inventory Control
Wastes can be reduced significantly by keeping inventories at a minimum. Large inventories of paint can lead to large disposal costs for paint that has aged and no longer meets specifications. Regulatory changes to the allowable V.C. emissions limits may also make coatings obsolete. Control over the quantity of paint provided for a particular job can reduce wastefill usage. The amount of paint distributed should be limited to the approximate amount required to complete a Job. Issuance of excessive quantities of paint can result in wasteful practices by the operators of the painting equipment.
General Housekeeping
Small quantities of coatings are often lost due to poor housekeeping techniques. Specific approaches to product transfer methods and container handling can effective reduce product loss due to spills and leaks. The potential for accidents and spills is greatest when thinners and paints are being transferred from bulk drum storage to the process equipment. Use of spigots, pumps, and funnels should be routine.
Evaporation and V.C. emissions can be controlled by using tight fitting lids, spigots, and other equipment. The reduction in evaporation will increase the amount of available material and result in lower solvent purchase costs.
Scheduling
Effective scheduling of products to be coated, the amount of mixing and cleanup time, as well as waste, can be reduced. Products that are to be coated with the same color and coating type should be scheduled together. Products that are to be coated with the same coating type but different colors should be scheduled so that the lighter colors are ahead of the darker color coatings to minimize equipment cleanup requirements. Overlapping or shift personnel by at least 15 minutes can eliminate the need for cleanup between shifts thus reducing down time.
Paint Containers
The purchase of paint in bulk containers can reduce drag-on (container) waste by reducing the container surface area/volume ratio. Also, some large containers can be cleaned and reused. If the purchase of paint bulk is not practical, purchases should be limited to the smallest amount required to minimize residual or drag-on. Avoid opening a gallon when a quart will do the job.
The contents of this publication reflect the views of the MERIC staff and are based on information obtained from the literature. The contents do not necessarily reflect the official views or the policies of MERIC or the Gulf Coast Maritime Technology Center. This publication does not constitute a standard, specification, or regulation. MERIC does not endorse products, equipment or manufacturers. Trademarks or manufacturer's names appear herein only because they are considered essential to the object of this publication.