- Email: [email protected]
Environmental coating problems
EnvironmentalCoating Problems by Ron Joseph
Wash Primers
T
he following questions have been submitted by S.F. of Pennsylvania and all relate to “pretreatment primers,” more commonly known as “wash primers.”
Q
.
How do you distinguish wash . primers from more genera1 metal pretreatments?
A
.
When one wants to treat steel, . aluminum, or galvanized surfaces prior to painting, it is common practice to immerse the substrate in a bath containing aqueous degreasers and inorganic phosphates. If the process is continuous and large volumes of metal are being treated, a conveyor will carry the parts through a washer tunnel (much like a car wash), and the aqueous solutions will be sprayed onto the metal. Sometimes the degreasers, or surfactants, are formulated together with the inorganic phosphates; other times, they are formulated separately. For instance, a three-stage phosphating line will comprise the following: Stage #I:
Combination degreaser plus inorganic phosphate Stage #2: Tap water rinse Stage #3: Tap water or deionized (DI) rinse A five-stage process prise the following: Stage Stage Stage Stage Stage
#I: #2: #3: #4: #5:
would
com-
Aqueous degreaser Tap water rinse Inorganic phosphate Tap water rinse Tap water or DI water rinse
Note that the conversion coating is usually iron or zinc phosphate, which precipitates onto the metal in a chemical surface reaction. No solvents are used in the entire process. These phosphate coatings serve preMETAL
FINISHING
.
APRIL
1997
dominantly two purposes: (1) to provide a barrier between the metal substrate and an electrolyte (such as moist ambient air), thereby reducing the possibility of corrosion and (2) to act as an adhesion base onto which organic coatings (paints) can be applied. Sometimes the metal substrate is too large to be immersed in a dip tank, or it is too bulky to pass through a washer/pretreatment tunnel. In these cases, there is an alternative method for achieving comparable corrosion resistance of the metal and providing an adhesion base for the subsequent organic coatings. Historically, the alternative is an organic coating that is based on a vinyl-butyryl resin and contains concentrations of zinc chromate (pigment) up to 0.5 lb/gal, as well as phosphoric acid. The coating is also formulated with one or more of the following common organic solvents: isobutyl alcohol, isopropyl alcohol, xylene, and smaller concentrations of other solvents. The wash primers are commonly applied by means of a spray gun (HVLP will do the job nicely) and must achieve a dry film thickness (DFT) of 0.3 to 0.5 mil. In most cases, the wash primer alternative will provide comparable high-quality protection to the inorganic phosphates, but it is difficult to control application. As with the inorganic phosphates, the wash primer should be overcoated with a full-bodied primer within a few days. It is not good practice to leave the pretreatment coatings exposed to the elements because they are not intended to provide corrosion protection under such conditions. Wash primers are also used on previously phosphated metals that have been damaged and must be repaired in the field. They should not be applied over the inorganic phosphates, as they will not adhere properly. More important, the phosphoric acid in the formu0 Copyright
Elsevier Science Inc.
lation cannot react with the underlying metal because the reaction is blocked by the layer of inorganic phosphate. From a regulatory perspective, the inorganic phosphates have no impact on VOC emissions, whereas the wash primers have a huge impact, because the formulation comprises up to 88% by weight organic solvents.
Q
.
Is there a definition for “Green . wash primer”? Are there regulatory limits in any state?
A
When I spoke to S.F. for clari: fication, she explained that the word “green” referred to color and not to “environmentally friendly.” Wash primers are usually green solely because they contain zinc chromate, which has a yellow-green appearance. Zinc chromate is the pigment that provides the required corrosion resistance; however, there is no reason why a paint company could not add another colored pigment to change the color. In other words, the word “green” should be irrelevant to the definition of the product. Because wash primers have a very low solids content, usually around 12% by weight, their VOC content is in the range of 5.8 to 6.5 lb/gal. In the 1980s many states eliminated wash primers altogether because of their high VOC content. Rules did not specifically address wash primers; instead, they simply provided a VOC limit for primers, commonly 420 g/L (3.5 lb/gal) less water and exempt compounds. Companies that could not find a low-VOC alternative to the high-VOC wash primer had to learn how to make do without. During the 1980s the regulators in California’s two largest districts, the South Coast Air Quality Management District (SCAQMD) and the Bay Area Air Quality Management District (BAAQMD), did make allowance for 73
wash primers because they were sorely needed for military contracts, aerospace painting, and in the electronics industry of Silicon Valley. During the early 198Os, wash primers were altogether exempt from the regulations, but in the late 198Os, a VOC limit of approximately 780 g/L (6.5 lb/gal) less water and exempt compounds was regulated. Because this was the VOC at which the coatings were currently being formulated, the high limit served much as an exemption. As technology advanced and at least one vendor claimed to have a low-VOC product, new low-VOC limits were regulated for wash primers. These are discussed below. In California, pretreatment wash primers were classified as coatings and not metal pretreatments (implying the inorganic iron and zinc phosphates). Currently, definitions for pretreatment primers and their respective VOC limits are as follows: SCAQMD, Regulation 11, Rule 1107 “Pretreatment coating” is a coating that contains no more than 12% solids by weight and at least 0.5% acid by weight, is used to provide surface etching, and is applied directly to metal surfaces to provide corrosion resistance, adhesion, and ease of stripping. VOC content, less water and exempt compounds:
MANlliERSCHElD BUFF RAKES
RAKE HANDLE PART NO.
z I BBC
MANDERSCHEID Equipment & Supply STOCKING SUPPLIER OF JACKSONLEA @WWS.
PHOMZ
cOhWOUNW
& cLL*LIERS
l-800-379-67 FAX f-3 f2-66g-0776
f4
Air dried, 420 g/L (3.5 lb/gal) Baked, 420 g/L (3.5 lb/gal) BAAQMD
Regulation
8, Rule 19
“Pretreatment wash primer.” Any coating that contains a minimum of 0.5% acid by weight, is necessary to provide surface etching, and is applied directly to bare metal surfaces to provide corrosion resistance and adhesion. VOC content, less water and exempt compounds:
Air dried, 420 g/L (3.5 lb/gal) Baked , 420 g/L (3.5 lb/gal) I tried to find compliant pretreatment primers and was successful in identifying at least one that is manufactured by Cardinal Industries of El Monte, California (in the South Coast Basin). It is their product #4860-420 light gray, with a VOC content of 420 g/L (less acetone). The product is a two-component material, composed of the primer and acid activator. Note that this is a solvent-based coating and that acetone composes most of the solvent. As you are probably aware, acetone has been delisted as a VOC, hazardous air pollutant (HAP), and ozone-depleting compound (ODC) by the EPA, and most states have followed EPA’s lead; therefore, in determining the VOC content, you must treat the acetone as one would water; namely, the acetone is subtracted from the VOC calculation. (Please be aware that although acetone is not classified as a HAP, nevertheless, it is hazardous because of its low flash point. We discussed this quite extensively in the October issue.) For more information on Cardinal’s coating you can contact Bob Sypowicz at (213) 283-9335 or (408) 452-8522. My colleague, Tom Burke, who writes the “Shop Problems” column in Metal Finishing, has spent several years evaluating compliant wash primers that will meet military specifications. According to Tom, Hentzen Coatings [(414) 353-42001, has developed a nonchromate, single-component, waterborne emulsion, Aqua-Zen Prep Primer, which appears to pass the tests of MIL-P-15328. In laboratory evaluations, Tom has found that the coating performs better than the conventional noncompliant wash primers on both ferrous and nonferrous substrates. While the conventional wash
primers must be applied to a controlled DFT of 0.3 to 0.5 mil, the Hentzen product must be applied to at least 1 mil DFT. Being an emulsion, Tom does not know how it will perform at low ambient temperatures, but he plans to look into it. Other observations are that the coating is easy to apply, quick drying, has no maximum recoat time, and because it is single component, does not have pot-life problems. Apparently, adhesion is excellent to steel, galvanized steel, aluminum, and also composites. Tom has found that the primer might provide an excellent base under chemical agent-resistant coatings (CARC). During 1997, he plans to do more work with this coating on asrolled steel, as-rolled aluminum, blasted steel, and titanium. To the best of his knowledge, this coating has not yet been approved by the U.S. Army’s coatings laboratory at Fort Belvoir. In addition, Tom is not aware that any low-VOC wash primer has yet been approved as an alternate to MIL-P15328. I will ask him for an update sometime next year and will publish his comments then. I spoke to Al Hentzen of Hentzen Coatings, who pointed out that companies wanting to use his product should first conduct their own ‘evaluations before switching from their current highVOC pretreatment wash primers. Even for those who do not need to comply with a MIL specification, he cautions that Aqua-Zen Prep Primer is not an automatic drop-in substitute for the high-VOC cousins.
Acknowledgments Thanks to both Susan Hoyle of the Pennsylvania Department of Environmental Protection and Tom Burke for editing this column.
Communications If you can contribute to this column by providing some of your ideas or comments regarding coating-related environmental issues, please feel free to write to me at Metal Finishing. For those readers who have an on-line computer service, you can also correspond with me via the following Email address: [email protected] Ron Joseph is an independent consultant in Saratoga, Calif.
coating
MF
Circle 061 on reader information card
74
METAL FINISHING
. APRIL 1997
Wash Primers
T
he following questions have been submitted by S.F. of Pennsylvania and all relate to “pretreatment primers,” more commonly known as “wash primers.”
Q
.
How do you distinguish wash . primers from more genera1 metal pretreatments?
A
.
When one wants to treat steel, . aluminum, or galvanized surfaces prior to painting, it is common practice to immerse the substrate in a bath containing aqueous degreasers and inorganic phosphates. If the process is continuous and large volumes of metal are being treated, a conveyor will carry the parts through a washer tunnel (much like a car wash), and the aqueous solutions will be sprayed onto the metal. Sometimes the degreasers, or surfactants, are formulated together with the inorganic phosphates; other times, they are formulated separately. For instance, a three-stage phosphating line will comprise the following: Stage #I:
Combination degreaser plus inorganic phosphate Stage #2: Tap water rinse Stage #3: Tap water or deionized (DI) rinse A five-stage process prise the following: Stage Stage Stage Stage Stage
#I: #2: #3: #4: #5:
would
com-
Aqueous degreaser Tap water rinse Inorganic phosphate Tap water rinse Tap water or DI water rinse
Note that the conversion coating is usually iron or zinc phosphate, which precipitates onto the metal in a chemical surface reaction. No solvents are used in the entire process. These phosphate coatings serve preMETAL
FINISHING
.
APRIL
1997
dominantly two purposes: (1) to provide a barrier between the metal substrate and an electrolyte (such as moist ambient air), thereby reducing the possibility of corrosion and (2) to act as an adhesion base onto which organic coatings (paints) can be applied. Sometimes the metal substrate is too large to be immersed in a dip tank, or it is too bulky to pass through a washer/pretreatment tunnel. In these cases, there is an alternative method for achieving comparable corrosion resistance of the metal and providing an adhesion base for the subsequent organic coatings. Historically, the alternative is an organic coating that is based on a vinyl-butyryl resin and contains concentrations of zinc chromate (pigment) up to 0.5 lb/gal, as well as phosphoric acid. The coating is also formulated with one or more of the following common organic solvents: isobutyl alcohol, isopropyl alcohol, xylene, and smaller concentrations of other solvents. The wash primers are commonly applied by means of a spray gun (HVLP will do the job nicely) and must achieve a dry film thickness (DFT) of 0.3 to 0.5 mil. In most cases, the wash primer alternative will provide comparable high-quality protection to the inorganic phosphates, but it is difficult to control application. As with the inorganic phosphates, the wash primer should be overcoated with a full-bodied primer within a few days. It is not good practice to leave the pretreatment coatings exposed to the elements because they are not intended to provide corrosion protection under such conditions. Wash primers are also used on previously phosphated metals that have been damaged and must be repaired in the field. They should not be applied over the inorganic phosphates, as they will not adhere properly. More important, the phosphoric acid in the formu0 Copyright
Elsevier Science Inc.
lation cannot react with the underlying metal because the reaction is blocked by the layer of inorganic phosphate. From a regulatory perspective, the inorganic phosphates have no impact on VOC emissions, whereas the wash primers have a huge impact, because the formulation comprises up to 88% by weight organic solvents.
Q
.
Is there a definition for “Green . wash primer”? Are there regulatory limits in any state?
A
When I spoke to S.F. for clari: fication, she explained that the word “green” referred to color and not to “environmentally friendly.” Wash primers are usually green solely because they contain zinc chromate, which has a yellow-green appearance. Zinc chromate is the pigment that provides the required corrosion resistance; however, there is no reason why a paint company could not add another colored pigment to change the color. In other words, the word “green” should be irrelevant to the definition of the product. Because wash primers have a very low solids content, usually around 12% by weight, their VOC content is in the range of 5.8 to 6.5 lb/gal. In the 1980s many states eliminated wash primers altogether because of their high VOC content. Rules did not specifically address wash primers; instead, they simply provided a VOC limit for primers, commonly 420 g/L (3.5 lb/gal) less water and exempt compounds. Companies that could not find a low-VOC alternative to the high-VOC wash primer had to learn how to make do without. During the 1980s the regulators in California’s two largest districts, the South Coast Air Quality Management District (SCAQMD) and the Bay Area Air Quality Management District (BAAQMD), did make allowance for 73
wash primers because they were sorely needed for military contracts, aerospace painting, and in the electronics industry of Silicon Valley. During the early 198Os, wash primers were altogether exempt from the regulations, but in the late 198Os, a VOC limit of approximately 780 g/L (6.5 lb/gal) less water and exempt compounds was regulated. Because this was the VOC at which the coatings were currently being formulated, the high limit served much as an exemption. As technology advanced and at least one vendor claimed to have a low-VOC product, new low-VOC limits were regulated for wash primers. These are discussed below. In California, pretreatment wash primers were classified as coatings and not metal pretreatments (implying the inorganic iron and zinc phosphates). Currently, definitions for pretreatment primers and their respective VOC limits are as follows: SCAQMD, Regulation 11, Rule 1107 “Pretreatment coating” is a coating that contains no more than 12% solids by weight and at least 0.5% acid by weight, is used to provide surface etching, and is applied directly to metal surfaces to provide corrosion resistance, adhesion, and ease of stripping. VOC content, less water and exempt compounds:
MANlliERSCHElD BUFF RAKES
RAKE HANDLE PART NO.
z I BBC
MANDERSCHEID Equipment & Supply STOCKING SUPPLIER OF JACKSONLEA @WWS.
PHOMZ
cOhWOUNW
& cLL*LIERS
l-800-379-67 FAX f-3 f2-66g-0776
f4
Air dried, 420 g/L (3.5 lb/gal) Baked, 420 g/L (3.5 lb/gal) BAAQMD
Regulation
8, Rule 19
“Pretreatment wash primer.” Any coating that contains a minimum of 0.5% acid by weight, is necessary to provide surface etching, and is applied directly to bare metal surfaces to provide corrosion resistance and adhesion. VOC content, less water and exempt compounds:
Air dried, 420 g/L (3.5 lb/gal) Baked , 420 g/L (3.5 lb/gal) I tried to find compliant pretreatment primers and was successful in identifying at least one that is manufactured by Cardinal Industries of El Monte, California (in the South Coast Basin). It is their product #4860-420 light gray, with a VOC content of 420 g/L (less acetone). The product is a two-component material, composed of the primer and acid activator. Note that this is a solvent-based coating and that acetone composes most of the solvent. As you are probably aware, acetone has been delisted as a VOC, hazardous air pollutant (HAP), and ozone-depleting compound (ODC) by the EPA, and most states have followed EPA’s lead; therefore, in determining the VOC content, you must treat the acetone as one would water; namely, the acetone is subtracted from the VOC calculation. (Please be aware that although acetone is not classified as a HAP, nevertheless, it is hazardous because of its low flash point. We discussed this quite extensively in the October issue.) For more information on Cardinal’s coating you can contact Bob Sypowicz at (213) 283-9335 or (408) 452-8522. My colleague, Tom Burke, who writes the “Shop Problems” column in Metal Finishing, has spent several years evaluating compliant wash primers that will meet military specifications. According to Tom, Hentzen Coatings [(414) 353-42001, has developed a nonchromate, single-component, waterborne emulsion, Aqua-Zen Prep Primer, which appears to pass the tests of MIL-P-15328. In laboratory evaluations, Tom has found that the coating performs better than the conventional noncompliant wash primers on both ferrous and nonferrous substrates. While the conventional wash
primers must be applied to a controlled DFT of 0.3 to 0.5 mil, the Hentzen product must be applied to at least 1 mil DFT. Being an emulsion, Tom does not know how it will perform at low ambient temperatures, but he plans to look into it. Other observations are that the coating is easy to apply, quick drying, has no maximum recoat time, and because it is single component, does not have pot-life problems. Apparently, adhesion is excellent to steel, galvanized steel, aluminum, and also composites. Tom has found that the primer might provide an excellent base under chemical agent-resistant coatings (CARC). During 1997, he plans to do more work with this coating on asrolled steel, as-rolled aluminum, blasted steel, and titanium. To the best of his knowledge, this coating has not yet been approved by the U.S. Army’s coatings laboratory at Fort Belvoir. In addition, Tom is not aware that any low-VOC wash primer has yet been approved as an alternate to MIL-P15328. I will ask him for an update sometime next year and will publish his comments then. I spoke to Al Hentzen of Hentzen Coatings, who pointed out that companies wanting to use his product should first conduct their own ‘evaluations before switching from their current highVOC pretreatment wash primers. Even for those who do not need to comply with a MIL specification, he cautions that Aqua-Zen Prep Primer is not an automatic drop-in substitute for the high-VOC cousins.
Acknowledgments Thanks to both Susan Hoyle of the Pennsylvania Department of Environmental Protection and Tom Burke for editing this column.
Communications If you can contribute to this column by providing some of your ideas or comments regarding coating-related environmental issues, please feel free to write to me at Metal Finishing. For those readers who have an on-line computer service, you can also correspond with me via the following Email address: [email protected] Ron Joseph is an independent consultant in Saratoga, Calif.
coating
MF
Circle 061 on reader information card
74
METAL FINISHING
. APRIL 1997