Perform Chemical Stripping of Organic Coatings
TECHNOLOGY UPDATE NO. 06 – Chemical Stripping of Organic Coatings From Steel Structures
1. Scope
This document defines chemical strippers and discusses their use for removing existing conventional organic coatings from steel structures. Chemical stripping is recognized as one of a number of technologies available for the removal of existing coatings prior to repainting.
2. Description and Use
Chemical stripping involves application of a chemical to existing paint, allowing it to dwell for a period of time to attack the organic binder, removing bulk paint/stripper residues, and properly cleaning the steel substrate prior to repainting. Section 3 below describes methods used to identify which type of stripper will work most effectively, and typical application and removal options. Section 3 also presents containment and disposal options for stripper wastes.
3. Discussion
3.1 GENERIC STRIPPER TYPES AND USES: The chemical composition of strippers falls into two distinct generic categories. The first group is generally a blend of low molecular weight solvents known as “bond breakers.” The stripper solvents break existing paint bonds and “wrinkle” the paint so that it can be removed from the surface. Where old paint exists in multiple coats, bond breakers may require multiple applications for complete removal. Bond breakers may contain toluene, methylene chloride, or methyl ethyl ketone, which can remove paint faster, but may result in worker exposure to potentially hazardous chemicals and therefore require special worker protection to maintain exposure below permissible exposure limits (PELs). Bond breaking strippers may also be comprised of compounds such as N-methyl-pyrrolidone (NMP) or dibasic ester (DBE). These strippers work more slowly but are generally less toxic and may require less stringent worker protection. The bond breakers are generally selected for use in removing all coatings except oil-based, inorganic, or metallic coatings.
The second category of chemical strippers is the caustics. These strippers contain one or more common caustic chemicals, such as sodium, calcium, and magnesium hydroxide. Rather than attacking bonds between coating layers or between coating and substrate, caustics soften the entire paint system. Their use is generally restricted to oil based paint systems, typical of those found on structures such as bridges and tank exteriors. These older structures more commonly contain lead-based primers, on which caustics have proven to be quite effective. An exception to the use of caustics on oil based paint exists where aluminum flake pigment is present. Caustics cause aluminum to produce bubbles of hydrogen at the interface between the stripper and the substrate, rendering the stripper ineffective. For these applications, the aluminum-containing coatings must first be removed using one of the bond breakers, typically an NMP based product.
3.2 STRIPPER SELECTION: Chemical stripper selection is based upon a number of variables, all of which can be assessed by performing patch tests on the existing paint system. Typical variables include thickness of the existing coating, ambient temperature and humidity, presence of aluminum paint, and time restrictions on the potential job site such as traffic control restrictions. Since these variables do exist, it is highly recommended that a representative from the stripper manufacturer, or his agent, be present to assist in conducting patch tests. Changes to application parameters (such as amount applied and dwell time) can affect the results of a patch test. A trained manufacturer’s representative can assist in proper assessment of patch test results. To account for existing paint thickness variations on a candidate structure, patch tests should be conducted in more than one location and each should cover at least one square foot of surface.
The stripper types described in Section 3.1 generally work best at ambient temperatures above 10 °C (50 °F) and at high relative humidity. When using the bond breaker strippers, application thicknesses and dwell times will vary. However, products with solvents of low molecular weight and thus greater volatility have far shorter dwell times than the less volatile NMP/DBE products. Also, the latter products are generally lower in volatile organic compound (VOC) solvent content. As mentioned in Section 3.1, the bond breakers frequently require multiple applications to remove existing paint systems completely.
The caustics will typically remove up to 500 micrometers (20 mils) of existing oil (e.g., alkyd) paint systems using a stripper wet film thickness of 1600 micrometers (63 mils). Coverage rates at 1575 micrometers (62 mils) application thickness average between 0.5-0.6 m2/L (20-25 sq. ft/gal) for caustic strippers. Coverage rates will decrease in direct proportion as application thickness increases. Dwell time for alkyd coatings will vary between 4 and 24 hours, depending on their oil length. Variations of the existing paint thickness as well as weather conditions can greatly impact dwell times. Properly supervised patch testing can accomplish assessment of all the variables and will provide definitive information regarding performance of stripper type and cost for each application. It is recommended that service environment conditions during patch test and stripper application be fully documented.
3.3 CONTAINMENT: Containment guidelines for chemical stripping involve the following considerations: containment during stripper application; during stripper removal; during surface preparation for painting; and during painting. Each of these considerations will vary based on geometry of the structure (tanks, bridges, pipelines) as well as on which methods are used for application and removal of the stripper. In general, the containment structure should be in accordance with SSPC-Guide 6 (Class 1C, 2C or 3C), and it should be constructed so as to prevent stripper residues from impacting the immediate environment. Typically, plastic sheeting, at least 150 micrometers (6 mils) thick, is resistant to the chemical strippers described above, and works adequately for most applications, if it can withstand any anticipated wind load. For some structures such as around the circumference of ground storage tanks, additional materials such as plywood may be employed to serve as a base for retaining solid stripper residues so they can be more easily transferred into waste containers. In addition, it is advisable to provide some means of containing any rinse liquids generated during stripper removal. Containment structures may or may not require forced air ventilation based on which type of stripper is used, the method of removing the stripper, potential exposures during pre-painting and painting operations, and project locale.
3.4 WORKER HEALTH AND SAFETY: Specific requirements for worker health and safety can be found by referring to the product specification sheets and material safety data sheets supplied by the chemical stripper manufacturer. Typically, caustic strippers are water based and therefore do not release significant airborne contaminants. However, since they are corrosives, they can cause skin burns. Materials that are chemically impervious must be used to protect eyes and skin. Solvent based strippers also require chemically resistant skin protection, and protection against airborne solvent vapors may be required. Depending upon the method of removal (see Section 3.6), airborne concentrations of toxic metals may be encountered when removing coatings containing lead or other toxic metals.
Specific respiratory and personal protective equipment requirements should be in accordance with all applicable OSHA standards.
3.5 STRIPPER APPLICATION: Procedures for applying chemical strippers vary based on geometry of the structure to be stripped, type of stripper used, and productivity desired. In general, strippers are spray applied using conventional paint pumps. Caustic strippers require pumps which are chemically resistant and can accommodate thicker materials. The caustics should be air sprayed but the bond breakers can be applied with airless pumps. Chemical strippers may also be applied using trowels or brushes although these tools will generally not allow for the thickness control which can be achieved by spraying.
Some caustic applications may require using a splatter coat prior to full application of the stripper. The splatter coat is a light, non-continuous application of the stripper which is allowed to dry prior to spraying the full stripper coating. The splatter coating is typically required on older alkyd paint systems, where the caustic causes accumulation of oil at the steel/coating interface, resulting in loss of stripper adhesion on vertical surfaces. The dried splatter coating will prevent this premature release and allow the stripper to complete its stripping cycle. The need for a splatter coating is typically discovered during the patch test cycle.
Production rates for strippers can vary greatly and should be determined based on patch tests and other variables listed above, but typically should be directly related to how much area can be cleaned following the dwell cycle. For example, if the dwell cycle is overnight, application will generally take place in the previous afternoon. The area sprayed should be restricted to that which can be properly cleaned prior to the next spray cycle. For tanks, using a three man crew, production rates are typically between 300 to 400 m2 per day (3,000 to 4,000 sq. ft. per day) for a complete application/removal cycle, whereas for bridges and other structural steel of complex geometry, these rates will drop to between 100 to 200 m2 per day (1,000 to 2,000 sq. ft. per day).
3.6 STRIPPER REMOVAL: Chemical stripper residues can be removed using a number of methods which include both hand and mechanical tools and rinsing or wiping. Choice of removal methods should be based on structure geometry, production rate desired, and collection of waste residues. When removing residues where bond breaker strippers have been used, rinsing is usually not required, but the surface may require a wipe prior to a second stripper application or subsequent surface preparation for painting. This step, if required, will be identified during patch testing.
Most older structures with lead paint will be chemically stripped using a caustic product. This process requires hand scraping the bulk residues using broad knives, followed by rinsing to remove any remaining lead primer residues. The rinsing step can be accomplished with wet sponges for smaller areas but for larger jobs, rinsing should be done with a system which produces a minimal amount of water. Typically, any airless paint pump using a 0.5 mm (0.02 in) tip, will generate a rinse stream of less than 1.25 L/min (0.33 gal/min) at 3.4 MPa (500 psi). This will suffice to remove any excess stripper residues with minimal water waste. Pressure washers are not recommended as they generate more pressure and water and their waste streams are more difficult to contain. Other mechanized removal processes which can enhance productivity while minimizing waste include ice blasting, carbon dioxide (CO2) blasting, and vacuum rinsing. During stripper removal, care must be taken to direct all wastes, including rinse liquids, to the containment area. Certain structures may lend themselves to separation of liquids and solids. Where applications produce excessive liquids during the removal cycle, stripper residues can be filtered to facilitate easier disposal (see Section 3.7 below).
Following removal of caustic chemical stripper residues, steel surfaces will typically exhibit some degree of alkalinity. This alkalinity may require reduction prior to painting, based on the chemistry of the coating to be applied. Surface pH can be reduced, if required, with either a light application of dilute acetic acid or with a water rinse. These options should be discussed with the paint manufacturer so that surface condition prior to painting can be agreed upon. In addition to proper pH, additional inspection criteria should include visual inspection of the total work area to determine that all visible stripper residues have been removed.
3.7 STRIPPER WASTE DISPOSAL: Waste products from chemical stripping projects can exhibit a number of different characteristics depending on which stripper product is used, and how much lead or other hazardous materials exist in the old paint system. Typically, where caustic strippers are used on old oil paint systems (which can contain lead levels up to and above fifty percent by weight), the resulting waste stream will likely test above the toxic chemical leachate procedure (TCLP) limit for lead and may be above corrosivity (pH) limits. In these cases, typical handling involves reduction of pH and stabilization of the lead prior to disposal. Refer to SSPC-Guide 7 for further information on the testing, handling, storage treatment and disposal of the resulting waste streams.
On-site treatment of hazardous waste may require special approval. Contact local regulatory agencies prior to performing on-site stabilization. Wastes generated when using the bond breaker solvents may require different treatment protocol depending on which solvents are present. Treatment for these wastes may require incineration of the organic residues prior to stabilization of lead. Liquid waste must be tested for leachable lead and corrosivity, and if above regulatory limits, will require treatment and filtration.
Following filtration, some wastes might be able to be taken to a local publicly owned treatment works (POTW) facility for disposal, depending upon the POTW’s influent standards. All federal and local disposal guidelines should be assessed prior to disposal.
3.8 ADDITIONAL FACTORS AFFECTING THE USE OF CHEMICAL STRIPPERS: The biggest advantage gained from using chemical strippers is that the process does not generate dust. This may be significant when the coating being removed contains lead or other hazardous materials. Containment requirements are generally low to moderate and are designed to prevent the strippers and stripper waste products from impacting the surrounding environment. Coating removal is very effective for flat surfaces and becomes less effective as the structure geometry becomes more complex. Production rates for chemical stripping vary based on production crew size and geometry of the structure.
Suitability of the chemically stripped steel surface for subsequent painting ranges from poor to excellent depending on the quality of the original surface preparation and the extent of degradation which may have occurred during service. Chemical stripping does not create a surface profile on steel substrates nor will it remove adherent rust and mill scale, or existing thermal spray metallic coatings. The condition of the original surface can influence the choice of coating used to re-coat the steel, and additional processing such as abrasive blasting, or power tool cleaning, combined with application of a surface-tolerant prime coat, may be required.
4. Disclaimer
4.1 This technology update is for information purposes only. It is neither a standard nor a recommended practice. While every precaution is taken to ensure that all information furnished in SSPC technology updates is as accurate, complete, and useful as possible, SSPC cannot assume responsibility nor incur any obligation resulting from the use of any materials, coatings, or methods specified herein, or of the technology update itself.
4.2 This technology update does not attempt to address problems concerning safety associated with its use. The user of this specification, as well as the user of all products or practices described herein, is responsible for instituting appropriate health and safety practices and for ensuring compliance with all governmental regulations.
5. References
Carroll, Courtney.“Chemical Stripping: A Viable Alternative,” Achieving Quality in Coating Work, Proceedings from the SSPC 92 Seminars (SSPC 92-13). (Pittsburgh: SSPC, 1992), pp. 212-215.
“Chemical Stripping Removes Lead Paint from Water Tower,” JPCL Vol. 13, No. 3 (March 1996), pp. 43-44.
“Chemicals Safely Remove Lead Paint from Tanks” Aboveground Tank Update, CEEM Publications, (December 1991).
“Lead Paint Removed from Water Tank Using Chemical Stripper,” JPCL, Vol. 8 No. 10, (October 1991), pp. 41-44.
Mickelsen, R. Leroy and Walter M. Haag. “Removing Lead-Based Paint from Steel Structures with Chemical Stripping,” JPCL, Vol. 14, No. 7 (July 1997), pp. 22-29.
“Minnesota DOT Tests Chemical Stripper,” JPCL, Vol. 12 No. 2, (February 1995), pp. 58-59.
Rodkey, James L. “Lead Paint Removal at Reasonable Cost by Chemical Stripping,” Industrial Lead Paint Removal: Compliance and Worker Safety, Proceedings of the Fifth Annual Conference on Lead Paint Removal (SSPC 92-04). (Pittsburgh: SSPC, 1992), pp. 21-36.
Trimber, Kenneth A., Industrial Lead Paint Removal Handbook, Second Edition. (Pittsburgh: KTA-Tator, Inc., 1993).
TECHNOLOGY UPDATE NO. 06 – Chemical Stripping of Organic Coatings From Steel Structures
