Historical Articles

August, 1954 issue of Plating

 

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Modern Industrial Paint Strippers*

Rubin M. Operowsky, Chief Chemist, Chemclean Products Corp., Bronx, N. Y.

*Presented at the Forty-first Annual Convention of the American Electroplaters’ Society,
July 12, 1954.

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GENERAL CONSIDERATIONS
Today, as never before in the history of American industry, industrial paint stripping has become a major operation in the manufacturing and job finishing plants. The need for economy and speed has necessitated the replacement of the older, slower, and more costly mechanical methods, such as sandblasting, scraping, buffing and burning off, in favor of the more economical, efficient and more rapid chemical paint strippers.

The organic coatings of today have advanced far in complexity from the days of the simple drying oil paints. We are now living in the polymer age, where terms such as ”alkyds, vinyls, phenolics, ureas and melamines” are household words. Although these terms are widely used, the knowledge of their chemical compositions and formulations are not so well known outside of the paint laboratory.

Table I shows the chemical formulas of the most widely used resins in organic coatings.
The formulation of paint strippers would be relatively simple if only one resin were to be used in the making of an organic coating. However, this is seldom the case. By blending a number of resins, paint chemists have been able to combine the desired chemical and physical properties of an individual resin with that of others, and tailor-make a coating to fit any desired set of conditions. Although this copolymerization is industrially desirable, it is one of the chief sources of difficulty to the formulators of paint strippers.

A good example of this can be seen in the difference in stripping between an alkyd and styrenated alkyd in the government specification MIL 10687. Alkyds are readily stripped by alkali, cold strippers and hot emulsion strippers, while styrenes are soluble in practically all types of solvents. However, the styrenated alkyd can not be stripped by any- of these media. Considerable work was necessary before a satisfactory stripper was developed for this paint.

There are factors other than the coating formulations that play an important part in determining the effectiveness of the paint remover. They are as follows:

1. The pigment content and pigment density of coating
2. Condition of the metal
3. Primers
4. Continuity and thickness of the paint film
5. The degree of polymerization.

Since alkali and acid strippers and thinners primarily are dependent upon their solvent action, the aforementioned factors do not affect the operation of these materials to any great extent. The effect upon the efficiency of the hot and cold solvent based strippers is more pronounced and merits further-consideration.

The condition of metal surface is a vital factor affecting paint adhesion—the greater the surface, the better the adhesion of the coating. The increased area resulting from sandblasting, etched surface, porous castings and from conversion coatings have added to the difficulty of paint removal. Not only do these conversion coatings increase the surface area of the metal, but the porous nature of the inorganic film enables the paint to penetrate into the crevices and form a deep-rooted and firmly bound bond with the surface. The increased surface and the penetration of the paint into the porous inorganic film retards the stripping operation. Using the same paint and the same stripper, the coating on an untreated surface will wrinkle and lift away rapidly in large sheets. On the other hand, the stripping time of the piece having a conversion coating is greatly increased,- and the appearance of the stripped paint coat is considerably altered. Instead of the customary large sheets of paint and a clean surface, the coatings from phosphated, chromatized or anodized work, break away in small uneven pieces, leaving much of the paint coat still clinging to the metal.

The use of primers is another method employed to retard corrosion and increase the adhesion of the final organic film. Red lead, iron oxide and zinc chromate are the primers most widely used. Formulations for metal finishing generally have a high pigment to vehicle ratio. Because of this, the resulting films are generally thin and porous. From observations in the laboratory of the author, it is believed that there exists a direct relationship between the effectiveness of the stripper and the thickness and the porosity of the film. There appears to be an optimum thickness at which the stripping time is a minimum and the efficiency of the stripper is a maximum. As the thickness of the coating increases or decreases beyond that range, the time of removal increases and the effectiveness of the stripper decreases. The mechanism by which the strippers function is dependent upon an ability to penetrate through the coating and break the bond between the continuous film and the metal. Hence, as the porosity of the film increases and the continuity of the film decreases, the effectiveness of the stripper will decrease. A recent problem that was encountered illustrates this point clearly. An instrument dial was coated a zinc chromate primer, after which only one side was finished with a final second coat of paint. In the stripper, the side with the two coats stripped cleanly in a few minutes, whereas the side with only the primer showed no effect other than a softening action after a few hours of immersion. To check the results, different strippers were tried and the results were the same with regard to the primer.

Removal of a film of oversprayed paint is another variation of stripping of a porous film. The section with the continuous film will strip cleanly, whereas the section containing the overspray will show some signs of wrinkling where the film is slightly heavier, but as a rule it remains intact with only some signs of softening.

Heavily pigmented coatings affect the efficiency of the stripper. The high pigment content plus the density of the pigment retard the solvent stripper from penetrating through the coating to the base metal to break the bond between the metal and the coating. Such coatings are usually found on refrigerators, washing machines, etc., where high chemical and moisture resistance are desirable. The stripping time for these coatings is considerably greater than a coating coning the same resin with a lower pigment content.

Improper baking is another vital factor affecting the successful operation of a stripper. Overbaking results in overpolymerization, and is much more difficult to strip. On the other hand, underbaking results in incomplete polymerization. The uncured coatings are more susceptible to- chemical attack and are readily -softened at elevated temperatures. Generally, cold strippers dissolve the unpolymerized resin, whereas the fully polymerized coat only blisters and wrinkles. Hot solvent emulsion strippers succeed only in softening the unpolymerized coating. When an inadequate sampling of pieces is taken, the improperly cured part can be a misleading factor in the selection of a proper stripper because practically all unpolymerized coatings are soluble to an extent in thinners and alkali, whereas the correctly cured polymer may be totally unresponsive to these media.

TYPES OF STRIPPERS
There are five different types of materials that are used for industrial paint removal. Of these, alkali, acid strippers and thinners act as solvents for the paint film, while cold strippers and the hot emulsion strippers wrinkle and lift the coating from the metal. Since stripping is primarily a salvage operation, the choice of type of materials used must be the one, which, under existing conditions, gives the best performance at a minimum of cost and handling. Other factors influencing the choice of type of paint stripper are:

1. Allowable time per unit operation
2. Availability of equipment such as tanks, rinses, spray washers, current, heating facilities, adequacy of ventilation.
3. Odor, inflammability and health hazard involved in the use of a particular type of material.

Alkaline Strippers
The alkaline based strippers are most widely used in industry today. These formulations are based chiefly on caustic soda and caustic potash plus the addition of a suitable surfactant to improve the wetting, penetration and rinseability. However, these materials are not restricted to caustic soda and caustic potash alone, but may also contain other alkaline salts, such as soda ash, tri-sodium phosphate and more alkaline silicates.

The alkaline stripper may act upon the coating in two ways:

1. Attack upon the pigment
2. The disintegration of the organic resin.

The disintegration of the resin is dependent upon the ability of the stripper to saponify the fatty acid portions of the vehicle, as in alkyds, or upon the ability of alkali to break up ester linkages, as in the case of cellulose nitrate. Alkyds, phenolics, oil paints, gum, varnishes, cellulose acetate and cellulose nitrate can be stripped in an alkaline bath while vinyls, ethyl cellulose, ureas, melamines and epoxy resins remain unaffected.

The chief advantage of alkaline type material lies in its relative cheapness and ease of application. Such compositions can be used in spray washers, with current as an electro-stripper, or in the conventional manner in soak baths. Even though spray and electrostripping are faster, and leave a cleaner surface at lower concentrations, they are not used to any great extent. This can be attributed to the general lack of sufficient equipment and space. Soak baths are used as hot as possible at a concentration of between 8 to 16 ounces of stripper per gallon of water. An ordinary gas or steam heated -steel tank can be used, and since the alkali is practically odorless, special ventilating systems are not required.

Strongly alkaline stripping baths will tarnish brass and etch zinc and aluminum and, consequently, the use of these materials is limited to stripping coatings from iron, steel, copper and magnesium. Nevertheless, coatings have been stripped from aluminum without etching or pitting by the use of inhibited alkaline strippers. Care should be taken when stripping a coating from a phosphated steel piece with alkali, since it is believed that over-exposure of a phosphate coating to a strong alkaline bath will be detrimental to the phosphate coating.

Since the action of alkali is basically that of solvation, there is a strong tendency for these materials to dissolve the organic portion of the coating and leave the pigment clinging to the metal. Extra operations must be used to remove these smuts. Rinsing in stagnant and running water rinse tanks does not remove much of the pigment. Spray rinsing has proven more effective. Where water rinsing is ineffective in any one of its forms, other methods such as removal of acid soluble pigments in an inhibited pickling acid, or the tumbling of small pieces in a sawdust barrel have proven more successful. Where these methods are unsuccessful or facilities are limited, hand brushing must be resorted to. Alkaline strippers are slower acting than the other types of materials, the time being dependent upon the type of resin and the degree of polymerization. Their speed, however, can be increased by increasing the concentration of the bath within limits and addition of a suitable synergist. Addition agents of this nature are generally based on cresylic acid, phenolic derivatives and other newer cyclic organic compounds.

Acid Strippers
Anhydrous concentrated sulfuric acid is one of the most effective and economical stripping materials. It has the advantages of being fast acting, leaving clean surfaces, and dissolving coatings without attacking the base metal. Regardless of the advantages of price and performance, neither concentrated sulfuric acid nor any of the strong mineral acids are used for stripping on any large scale. One drawback to the use of concentrated sulfuric acid is its corrosive nature, which makes it a serious health hazard when used in production. Secondly, concentrated sulfuric acid tends to absorb moisture from the atmosphere. The hydrated acid is corrosive to the metal, and not only attacks the organic coating, but will also leave its mark on metal surface.

The action of acid strippers in the removal of an organic film may depend upon three factors:

1. Oxidation or esterification of the resin
2. Solvation of the pigment
3. Attack of the acids upon the base metal, breaking the bond between the coating and the metal

These formulations are generally based on solutions of strong oxidizing agents for the oxidation of unsaturated linkages, esterifying acids and as materials useful in inhibiting the attack upon the base metal. Since metals are highly sensitive to acid solutions, this type of material must be properly balanced to give the greatest effectiveness and a minimum of surface attack. Although acids can be used on many metals, the greatest measure of success has been attained in the removal of tenacious organic films from aluminum without etching or destroying the finish. However, at best, acid strippers are used only for special limited applications.

Thinners
Lacquer thinners, although not generally classed as paint strippers, are the cheapest form of organic paint remover available. Thinners are a blend of three different classes of solvents:

1. Active solvents esters and ketones such as ethyl acetate, butyl acetate, acetone, methyl ethyl ketone, etc.
2. Latent solvents—alcohols such as methanol, ethanol, butanol.
3. Diluents—hydrocarbons, such as benzol, toluol, naphtha and mineral spirits.

The use of thinners in paint removal is limited to a few applications. The removal of defective coatings prior to baking, and the stripping- of some air dried paints and lacquers. Vinyls, cellulose acetate, cellulose nitrate, ethyl cellulose and drying oil paints are soluble in thinners, whereas most baked synthetic coatings are completely unaffected by them.

Factors other than the lack of versatility limiting the use of thinners as industrial paint removers are:

Low flash point
High degree of inflammability
High rate of evaporation
Pungent odor
Toxicity of the vapor

These factors make thinners a serious health and fire hazard in the shop. Although the price pet gallon is less than that of other organic solvent type strippers, in the long run it is much more expensive.

The life of thinners is limited to the per cent of active solvent in the formulation, and as the resin content builds up, it reaches a saturation point. Then the solution is more apt to function as a paint than as a paint remover.

Cold Strippers
The cold strippers, although the most expensive stripping baths, enjoy the greatest popularity. They possess the following desirable properties:

1. Rapidity of action;
2. High flash point;
3. Low inflammability;
4. Minimum of corrosion;
5. Work is left clean and ready for refinishing.

Cold strippers are generally a blend of chlorinated solvents, methylene chloride or ethylene dichloride, methanol, ammonia or-acetic or formic acid, and any number of additional solvents that may be needed for any particular coating.

As far as is known, nothing has been published in the literature explaining the mechanism of reaction between strippers and organic coatings. However, on the basis of work on strippers in our laboratory; we have formed our own theory with regard to their action. We believe in order for a solvent to function as a stripper it is believed that the material cannot be a true solvent for the polymerized resin, but it can only serve to swell the film and act as a jelling agent. When a painted panel is inserted into the stripper, the coating is swelled to a small extent and the solvent is absorbed into the coating. After the solvent is absorbed, it diffuses through the coating to the base metal When enough solvent has penetrated to the metal, a sufficient pressure is built up to break the bond holding the paint to the surface. In cases where a greater bond exists, as in the case of phosphated or anodized pieces, a greater pressure must be exerted by the stripper. When sufficient pressure is not built up, the stripper may fail completely, or do only a partial job. This can be overcome in some cases, by increasing the viscosity of the stripper by addition of a film forming material. This new film formed on top of the coating, causes the pressure exerted by the stripper to be exerted-in one direction only without losses due to outward diffusion.

Since the speed of a stripper is dependent upon the ability of the ingredient to diffuse through the coating, it is to be expected that the smaller the molecule, the faster the stripper will operate. This was proven conclusively by experiments carried out by Keuntzel and Liger,3 using different chlorinated solvents of varying chain lengths and configurations, methylene chloride, the smallest molecule, acted at least twice as fast as ethylene dichloride, the next larger chlorinated solvent. This is not limited to chlorinated solvents, but also holds true for strippers based on ketones, nitrogen bearing compounds or any other materials with stripping properties. The smaller the molecule, the faster the stripping. Accelerators, such as ammonia, acetic or formic acid, seem: to function- as pure openers for the solvents, to provide some needed acidity or alkalinity, to neutralize electrostatic forces at the surface, and to provide a microscopic surface attack upon the metal. It has also been proved that the addition of methanol speeds up the action of chlorinated solvents. It is believed that it serves to increase the polarity of the solvents by means of hydrogen bonding. Since coatings used industrially today are a blend of different resins, cold strippers contain a number of other active solvents (in addition to those discussed previously) to facilitate more active stripping. The efficiency of a number of chlorinated solvents are shown in Table II.
Although cold strippers have many desirable proper- - ties, are easy to operate and need not additional equipment, they also have their drawbacks. Being composed of volatile, organic solvents, they should never be used in a confined area without adequate ventilation. On the other hand, the greater the ventilation, the greater the evaporation.

TABLE II. EFFICIENCY OF CHLORINATED SOLVENTS IN STANDARD PAINT REMOVAL
Stripping Solvent, time in minutes
Methylene chloride, 4.5
Ethylene dichloride, 9.4
Trichlorethylene, 12 . 4
Monochlorbenzene, 12.5
Carbon tetrachloride, 18.8
Orthodichlorbenzene, 20.4
Propylene dichloride, 29.1
1, 2, 4, Trichlorbenzene, 46.2

Numerous methods have been experimented with to reduce these losses, but none have proven completely satisfactory. Some people prefer the use of a nonmiscible water layer with salts added to increase the-surface tension, such as a 1 per cent sodium chromate solution to act as a seal. Others use deep tanks with a minimum of exposed area, while some manufacturers include waxes and resins, such as cellulose acetate or ethyl cellulose to retard evaporation.

Hot Solvent Emulsion Strippers
Hot solvent emulsion strippers, based primarily upon phenol or phenolic derivatives and a suitable emulsifier, have been found most advantageous in the stripping of alkyd, melamine, phenolic, epoxy and urea type coatings. They are especially practicable for large production stripping where speed and efficiency are vital to make the stripping operation economically feasible. On a dollar to dollar basis, a much greater value is gotten from the hot emulsion stripper than from any other type of stripper on the market today.

When used as a soak, the emulsion generally contains 3 parts of water to 1 part of stripping concentrate. In a spray stripping operation the concentrate can be diluted 10 to 1. When used as directed by the manufacturer at the proper temperature and in a steam heated tank, there is little fire hazard and the loss due to evaporation is’ small. The pH of these materials is such that coatings can be stripped from zinc, aluminum and brass without attacking the metal.

Serious drawbacks to the use of the phenolic hot emulsion stripper are its characteristically disagreeable odor and the health hazard when used in a confined area without proper ventilation. Gloves should always be worn when working with these materials. Most organic solvents only irritate the skin due to a de fatting action, but phenol and its derivatives can cause severe burns when brought into contact with the skin. A less serious drawback is that most emulsion strippers, especially after being overheated, leave films on the metal. These films can be readily removed in a mild alkaline cleaner.

The action of the phenols in the hot stripper is analogous to that of the chlorinated solvents in the cold stripper. The phenols act as partial solvents and in combination with the heat of the bath act to swell the coating. The emulsifying agent wets the surface and enables the phenols to penetrate through the resin to the base metal. As in the case of the cold stripper, a wedge is formed between the metal and the coating until enough pressure is built up to break the existing bond between the organic film and the metal. When this takes place, the coating lifts away cleanly in large sheets.

STRIPPING COATINGS FROM PLASTICS
The increased use of molded plastic products has created a new industrial stripping problem. This is the removal of a polymerized film from another organic polymer without softening, attacking or distorting the base plastic. Since such base plastics are more sensitive to heat and attack by chemicals than are the metals, different techniques and materials must be used in removing coatings from them.
The coatings used on plastics are of the air-dry variety. Since both materials are soluble in many of the same solvents, the only method that can be used to remove these paint films from the base plastics is that of differential solubilities. This necessitates extensive research because each problem may be entirely different. Removing a paint film from a plastic base is much slower and a more costlier process than stripping paint from metal, but on the other hand, it much cheaper than grinding and remolding the plastic. The solvents for this type of paint removal would be of no economic value if used for paint stripping fro, a metal surface.

CONCLUSION
Each one of these five types of stripping material mentioned have many advantages and disadvantage but there is always one job for which a particular type is more proficient. Only too often when a material does a satisfactory job on one type of coating, it tried on others with varying degrees of success. Stripping materials are like analytical reagents in that they are specific in operation. A radical change in this type of coating changes the effectiveness of the stripper. One must remember that to this date there no one stripper for all types of coatings. Not enough emphasis can be put on the fact that there is no universal print stripper.

REFERENCES
1. J. J. Mattiello, ”Protective and Decorative Coatings,” Jo Wiley & Sons, New York, N. Y.
2. H. Lesser, ”Soap and Sanitary Chemicals,” 29,133 (1953).
3. G. E. Keuntzel and A. W. Liger, Iron Age, 160, 78 (O 9, 1947)
4. G T. Snell, Chemical Industries 64, 414 (1949).
5. L. E. Keuntzel, U. S. Patent 2,4i8,138 (1947).
6. L. E. Keuntzel, U. S. Patent 2,507,983 (1950).