2 DISCUSSION

Paint strippers tend to be viewed in terms of solubility parameters, with methylene chloride acting as the primary solvent and a few secondary solvents adjusting its position on the Teas diagram. At best, this view is overly simplistic. Paint strippers employ a wide variety of chemical mechanisms and may be engineered for specific purposes.

The initial intent of this project was to determine the composition of “typical” hardware store paint strippers. It became clear that the theory behind these products could not be isolated from the broader subject of commercial stripping. Reference will also be made to commercial cleaners with respect to the removal of paint, rust, grease, and other surface contaminants.

A survey of commercial paint removal processes encompasses three basic methods: physical, mechanical, and chemical (Block 1986). The physical processes of paint removal include (1) burn off and incineration; (2) hot fluidized bed (a method that utilizes both high temperature, 500–600�F, and abrasive media action in a specialized chamber); and (3) cryogenics. The mechanical processes are (1) scraping; (2) sanding; (3) blasting with abrasives; and (4) chipping. Chemical removal processes include (1) cold solvent (acid or alkaline activated); (2) hot alkaline removal; and (3) molten salt baths.

All of these processes are designed to degrade the paint film or destroy adhesion of the film to the substrate. While several of these methods are commonly employed in art conservation, the focus of this discussion is chemical processes, primarily cold solvent strippers.

With the introduction of progressively more resistant finishes, the paint stripper industry is constantly being challenged to develop more efficient removal methods. Variations in coatings, substrates, and surface contaminants make it impossible to provide firm guidelines identifying the most effective methods. The formulation of a paint stripper—like the development of any cleaning system—is something of an art. Chemical combinations that theoretically should work often do not (Hahn and Werschulz 1986).

Paint removers today have to meet some or all of the following design criteria (Mazia 1979):

1. stripping ability
2. lack of corrosiveness to substrates
3. freedom from galvanic attack at the juncture of dissimilar metals
4. freedom from cresols, phenols, benzol (benzene), and other noxious and toxic chemicals
5. good shelf life
6. thixotropicity where needed
7. sealable internally or with water cap to prevent rapid evaporation
8. ability to leave a readily recoatable surface

Generally, paint stripper labels only list the components that are recognized as hazardous in use. Actual formulations are usually more complicated, employing primary and co-solvents, activators, thickeners, wetting agents, chelating or sequestering agents, corrosion inhibitors, and colorants.

The activity of a paint stripper may include three different mechanisms: (1) the paint can be dissolved to form a solution with the solvent; (2) the paint film may be destroyed by a chemical reaction with the solvent; and (3) penetration of the stripper into the paint film—either directly or through scratches, holes, or broken edges—destroys its adhesion to the base material.

For industrial purposes, pure dissolution of the paint film generally is not viewed as desirable because dissolution leads to redeposition and clearance problems. Preferably, the paint film will separate from the substrate in sheets large enough that it cannot act as a regenerated paint or stain. To avoid these problems, methylene chloride is generally chosen as the primary solvent.

2.1 METHYLENE CHLORIDE

In theory, the effectiveness of methylene chloride is due to its small molecular size, which facilitates rapid penetration of the paint film, and to its intermediate solvency for various polymer coatings.1 As methylene chloride penetrates to the substrate, the paint film swells to 10 times its original volume (Sizelove 1972). The swelling causes an increase in internal pressure that can only be relieved in a direction away from the substrate. The film wrinkles, bubbles, and blisters, resulting in its release from the substrate.

If the paint film is continuous, and the intrinsic strength of the film is greater than the bond strength to the base, the paint will flake or scale off, leaving some flakes adhering to the surface. Any pressure built up in these remaining flakes can be released in the plane of the surface, so there is no reason to expect removal through this mechanism.

Penetration of methylene chloride through the paint film explains why commercial paint strippers are often not a safe choice for the removal of a specific layer of overpaint in a work of art. Methylene chloride would tend to move through the overpaint and into the original paint layer. Depending on film thickness, lower paint layers could show a greater relative swelling than the overpaint layer. If the overpaint layer was sufficiently fractured, the resulting swelling could be released in plane, making the stripper ineffective.

For health and environmental reasons, alternatives to methylene chloride are currently being investigated.

2.2 CO-SOLVENTS

Co-solvents are used to increase the versatility of the stripper in attacking coatings that resist the primary solvent. Co-solvents are usually incorporated at levels of 5–10%; at higher concentrations they serve as dilutants and may lower the flashpoint of the composition(Kirk and Othmer 1968). There is wide variation in the types of these secondary solvents. In the past, phenols and cresols were the most important co-solvents in industrial formulations. For example, ethanol assists in the removal of shellac coatings, and methyl ethyl ketone assists in stripping cellulose nitrate finishes. Concerns about toxicity have caused a substantial reduction in the use of these two materials. The most common co-solvent types in hardware store paint strippers are alcohols, acetone, and aromatic and aliphatic hydrocarbons. Thirty of the materials in table 1 function to some degree as co-solvents.

TABLE 1 PAINT STRIPPER FORMULATIONS

In certain instances it is desirable to reject chlorinated hydrocarbons in favor of solvent blends that are designed to dissolve the paint film. Some solvent strippers employ ketones and aromatic hydrocarbon blends and are used primarily where other strippers fail, such as on low intrinsic strength films or sharply angled surfaces.

2.3 ACTIVATORS

The term “activator” refers to an additive that increases the stripping rate. Activators can function by increasing the penetration of the solvent into the substrate. In methylene chloride-based strippers, the addition of enough water to saturate the methylene chloride will reduce stripping time as much as 90% (Martens 1974). Activators may also be acids, alkalis, or amines, which act to hydrolyze molecular linkages and thus break down the paint film. Activators are often chosen for their effectiveness on specific paint films or under specific conditions. For example, acids are generally chosen for epoxy resins since they hydrolyze ether linkages. Hot strippers usually employ highly alkaline activators, often based on sodium hydroxide. Cold strippers employ acidic, basic, and neutral activators (Block 1986). The following solvent selection chart is offered by Ringel (1989) as a guide to choosing solvent/activator combinations.

SOLVENT SELECTION CHART

Because many of the co-solvents also act to disrupt molecular linkages, it is often difficult to assign a specific purpose for each additive. For example, methanol, a powerful solvent on oil films, is generally catagorized as a co-solvent, but it can also be classified as an activator if it acts as a carrying agent for the primary solvent. Methanol may also be introduced to swell the thickener.2 Phenol, also generally categorized as a co-solvent, is a weak organic acid that may also function as an activator by removing oxide films from the surface of the coating, thereby loosening the paint film and improving the penetration of the primary solvent (Hahn and Werschulz 1986).

2.4 CORROSION INHIBITORS

When choosing an activator it is necessary to consider the substrate and the pigment present. Certain metals are readily corroded under high or low pH conditions. Aluminum alloys, zinc, and galvanized steel are susceptible to damage from strongly caustic solutions. Some pigments, such as Prussian blue and organic lakes, may discolor. Strongly acidic solutions can react with a wide range of metals. Amine activators are often chosen over acid activators because they tend to be less corrosive on the substrate and on the metal container.3

Corrosion inhibitors are often added to protect the substrate as well as the container, which is usually tin plate. Over time, chlorinated hydrocarbons may break down, forming hydrochloric acid. Propylene oxide and butylene oxide are scavengers for HCl in nonaqueous formulas. A yellow or orange coloration of the paint stripper may indicate that a chromate-based corrosion inhibitor has been added. Other inhibitors include silicate salts, polyphosphates, and antioxidants such as borates.

2.5 SURFACTANTS

Surfactants are included in most paint stripper formulations. Surfactants assist solvents by wetting or penetrating the surface of the paint film. Surfactants also help clear the stripper on washing. This characteristic makes paint strippers good brush cleaners.

Because surfactants are added in low concentrations, they are generally omitted from MSDS sheets. Two of the patents list specific surfactants: dodecyl benzene sulfonate and sodium xylene sulfonate. Block (1991) offered the following guideline for choosing a surfactant: at acidic pHs anionic surfactants are generally used, often based on sulfonic acids; at alkaline pHs nonionic or nonionic/cationic combinations are chosen.

2.6 CHELATING AND SEQUESTERING AGENTS

Water-softening agents are often used in aqueous-based cleaning systems to “tie up” hard water salts that may decrease cleaning efficiency. Chelating and sequestering agents may also be added to aqueous or solvent strippers to assist in the clearance of inorganic materials such as pigments, driers, and oxide films from the substrate. A standard chelating agent is ethylenediamine tetraacetic acid (EDTA). Orthophosphates and orthosilicates are the most common sequestering agents. EDTA, tributylphosphate, and sodium phosphate are all listed in the formulations in table 1.

2.7 THICKENERS

Thickeners allow the paint stripper to remain in place on vertical surfaces. Thickeners also increase contact time by reducing the rate of evaporation, thus holding the solvent on the substrate's surface for a longer period.

Many companies offer products in a range of viscosities, from liquid to dense pastes. Paintbusters' Brand Architectural Paint Stripper, distributed by On-Site Wood Restoration, claims to be the thickest. Its product literature states, “We start with Methanol-softened Parafin [sic] and a finned blender; we stir in Methylene Chloride, until we reach a saturated solution. We add wetting agents. That is all. No thinners, no buffers, no stringy thickeners, no volatile solvents.”

It is recommended that paint strippers thickened with wax be applied in a heavy layer with a single pass of the brush. When the wax forms a skin it holds the solvents in place. Brushing back and forth disrupts this skin and causes the solvents to evaporate. Although wax is a common thickener, it tends to separate over time. Most organic-based paint strippers use cellulose-based derivatives as gelling agents. Materials such as hydroxypropyl methyl cellulose offer higher stability with the solvents.

Cellulose-based gelling agents hydrolyze at high or low pHs, thus becoming ineffective. For those paint strippers that are formulated at pH extremes, a common thickener is fumed silica.

2.8 COLORANTS

Colorants may be added for marketing purposes or to make it easier to locate those areas where the stripper has been applied to the substrate. These colorants are generally not listed on the MSDS sheets, as they are usually introduced in smaller concentrations. The only specific references to a colorant in the formulations in this article are red dye in U.S. Patent #1,752,358 and possibly calcium carbonate in Morton Paint Co. Paint Stripper (see table 1). It is also possible that the calcium carbonate may have been added as a bulking agent.

2.9 ALKALINE AND ACIDIC STRIPPERS

If an ionic reaction is the primary mechanism for paint removal, the stripper is no longer classified as cold solvent but is considered either alkaline or acidic. Alkaline strippers are at least as important for industrial purposes as cold solvent. Alkaline cleaners work by producing a solution containing hydroxide ions that break down contaminants by attaching to them. Common reactions are the saponification of the fatty acid portion of the vehicle or the breaking of ester linkages. Alkaline strippers are the oldest known types of strippers (Sizelove 1972). Until recently caustic soda or potash were used almost exclusively. Later, alkalis such as soda ash and sodium silicates were mixed in for a smoother stripping job.

Alkaline cleaners are currently viewed as the most viable, broad substitute for halogenated solvents used in degreasing metal and electronic components. D'Ruiz (1991) states that alkaline cleaners can be formulated and used with appropriate cleaning equipment to remove virtually any organic or inorganic contaminants currently removed by chlorinated solvents. One less scientific example of this process is the use of oven cleaner as a paint stripper. Plastic model enthusiasts often use oven cleaner to remove enamel paint from completed kits. (This technique is not recommended by the author.)Oven cleaners tend to be a mixture of a strong base, surfactants, and a thickener.

Acidic strippers are nearly as old as the alkaline type (Sizelove 1972). At first they consisted simply of concentrated solutions of sulfuric, nitric, and hydrochloric acids or combinations thereof. Weaker acids or buffered acid solutions now provide greater versatility. Acidic strippers operate through chemical destruction by either oxidation or dehydration of the vehicle and, at times, of the pigments as well. These strippers are commonly used to remove rust and scale, but they can be used to remove oxides, flux residues, corrosion products, and tarnish films (D'Ruiz 1991). Acidic strippers are generally difficult to work with, as they readily attack most substrates. However, mildly acidic strippers may be a better alternative on aluminum substrates, a metal susceptible to etching by highly alkaline agents.

2.10 EMULSION CLEANERS

Emulsions combine the cleaning abilities of solvent and aqueous cleaners and tend to be used for the removal of organic contaminants. The effectiveness of the emulsion depends on the choice of solvents and surfactant. Emulsions generally are recommended for cleaning applications where extreme pH (greater than 12 or less than 5) cannot be tolerated (D'Ruiz 1991). Common organic solvents used in emulsion cleaners include alcohols, ethers, or chlorinated hydrocarbons.