- Email: [email protected]
Development of a low-temperature methylene-chloride-free paint stripper
Development of a Low-Temperature MethyleneChloride-Free Paint Stripper by Mike Deemer, Oakite Products Inc., Berkeley Heights, N.J.
S
ince the Clean Air Act and its amendments in 1990 stringent regulations have been placed on the amount of volatile organic compounds (VOCs) that may be emitted. This has had a major effect on many manufacturing processes such as paint stripping. This stripping is necessary in paint lines to remove buildup on paint hooks, especially powder paints, and to remove paint from parts that need to be reworked. Several stripping methods, both chemical,and mechanical, are available to the marketplace. The mechanical methods are capital intensive and, therefore, not always attractive. The chemical strippers generally are run at high temperature and have health and safety issues. There are two major types of chemical stripper currently used. One type is the hot-alkaline strippers that generally operate above 200°F. The other type is the solvent-based strippers. In the past methylene chloride has been used as the main component in many of the solvent paint strippers. Today, owing to environmental and health concerns, methylene-chloride-based paint strippers need to be replaced with another effective low-temperature paint stripper. Such a paint stripper has been developed to provide strip times comparable to those of methylene-chloridebased strippers at much lower temperatures than the typical hot-alkaline strippers. The paint stripper con-
Effect of Agitation PahtHookswm3Passes
tains two phases. One phase is solvent based, and the other phase is water based with an alkalinity source. The work covered in this article includes initial laboratory work involving various types of powder paint, varying the amount of solvent and alkalinity available, and a performance comparison between the new stripper, a methylene-chloride-based stripper, and a hot-alkaline stripper. Also included will be a laboratory study done with paint hooks from a production plant.
BACKGROUND The new paint stripper contains two distinct phases. One phase is the solvent phase, which is mostly solvent with some surfactants. The solvent has a flash point greater than 205°F and a vapor pressure of 15 mm Hg at 100X, as compared with the vapor pressure of water at loo”C, which is 760 mm Hg. The aqueous component contains an alkalinity source with some surfactants. Because this stripping system is alkaline in nature it is safe only on steel. It will attack aluminum and zinc substrates. The system is made up with 50 vol/vol% of the solvent component and 50 vol/vol% of the aqueous component. No dilution is necessary. The typical operating temperature range is 80 to 140°F. Because this is a twophase system agitation is needed. The greater the agitation the better because this helps reduce the strip times. Figure 1 shows the difference between two levels of agitation. The agitation was produced by an overhead propeller mixer. The low agitation means that
the mixing speed was set so that the two phases were mixed but no vortex was present. The high agitation represents mixing with a vdrtex. This increased agitation can reduce the strip times by approximately 30%. The process is controlled in two ways. First, the replenishments are made by determining the ratio of the solvent component to the aqueous component. This is simply done by filtering 100 ml of a well-mixed sample into a lOO-ml graduated cylinder. The second process control is the determination of the alkalinity. This is done by filtering 10 ml of a well-mixed sample and titrating with 1.0 N hydrochloric acid to a bromophenol blue endpoint. The alkalinity titration is typically used only if the strip times increase significantly. The alkalinity is 12 ml for a fresh bath and is kept at this level through normal replenishments. EXPERIMENTAL The first step in testing this new paint stripper formulation was to determine how well it stripped various powder paints from cold-rolled steel panels at various temperatures. This performance was then compared with other chemical paint strippers such as hotalkaline and methylene-chloride strippers. When the performance parameters were set the operating limits for the solvent and the alkalinity were determined. With the basic laboratory work finished the paint stripper was operated in small field trials to compare the performance with other commercially available products. Several cold-rolled steel Q panels were powder coated with four different types of powder paint at three different
Table I. Time in Minutes to 100% Strip 3 Ml of Powder Paint Temperature(‘F)
Figure 1. Effect of agitation on stripping time. 58
80 100 120 140
Pdyesrer
Hybrid
24 19 5 5
18 10 5 4
0 Copyright Elsevier Science Inc.
METAL
fdyurethane 34 14 10 7
FINISHING
.
Epoxv -10 hr 3 hr 35 17
SEPTEMBER
1997
Table II. Time in Minutes to 100% Strip 4 Mil of Powder Paint Tempewfure('F) 80 100 120 140
PoIyesfer
Hybrid
28 15 13 6
18 13 10 5
Comparison of Chemical Strippen folyurefhane
EPOXY
40 18 12 5
-12 hr -6 hr 2 hr 45
ur
4mUsdPowderPaint
Table Ill. Time in Minutes to 100% Strip 5 Mil of Powder Paint Temperature (‘f) 80 100 120 140
Polyester
Hybrid
28 20 10 6
19 14 9 8
Polyurethane
EPOXY
45 23 14 8
Figure 2. Comparison of chemical strippers (4 mil of powder).
-14 hr -10 hr 2 hr 60
stripper was used at 40 vol/vol%, with mild agitation at 160°F and 200°F. The results are listed in Table IV and appear graphically in Figure 2. The new stripper at 120°F performs as well as or better than the other strippers in all cases except two. In both of these cases the methylene-chloride stripper performs better on the polyester and the epoxy powder paints.
strip, followed by the polyester, polyurethane, and the epoxy. At higher temperatures the polyester, the hybrid, and the polyurethane take approximately the same time to strip. At higher temperatures the epoxy is still the most difficult to strip. Another study was performed in which the new stripper was compared with a methylene-chloride-based stripper and a hot-alkaline stripper. The same four types of powder paint were used as in the first study, at a dry film thickness of 4 mil. The hot-alkaline
dry-film thicknesses. The steel panels were pretreated with a molybdate-accelerated iron phosphate before painting. A white polyester, gray epoxy-polyester hybrid, black polyurethane, and gray epoxy powders were used in the testing. All powder paints were cured according to the manufacturers’specifications. The time to 100% strip was recorded using 3, 4, and 5 mil of powder paint at 80, 100, 120, and 140°F. The data are listed in Tables I, II, and III. At lower temperatures the epoxypolyester hybrid was the easiest to
EFFECT OF SOLVENT CONCENTRATION
Table IV. Time in Minutes to 100% Strip Comparing Different Paint Strippers on 4 Mil of Powder Paint Stripper Hot alkaline at 16O'F Hot alkaline at200°F Methylene chloride New stripper at 12o'F
Polyester
Hybrid
38 10 3 8
46 13 35 7
Polyurethane
E~oxv
45 9 30 12
-14 hr -8 hr 55 120
Table V. Effect of Solvent Concentration on Strip Times for 2.0 Mil Polyester and 2.0 Mil Epoxy-Polyester Hybrid Powder Paints Vo/No/%of Solvent 0 10 20 30 40 50 '.
Po/yester Strip Time (min)
Epoxy-Polyester Strip Time (min)
No attack after 2 hr 37 12 7 5 5
No attack after 2 hr 52 10 4 4 4
Table VI. Effect of Alkalinity on Strip Times for 2.5 Mil Polyester and 2.0 Mil Epoxy Polyester Hybrid Powder Paints Alkalinity (m/ 1.0N HC()
0 2.4 4.0 5.5 9.6 12.0
60
Strip Time for Polyester (min)
Strip Time for Hybrid (min)
-22 hr
-18hr 24 10 10 9 7
37 17 15 12 7
A 1.0-L bath was prepared in which 500 ml of the aqueous component was added and heated to 120°F. Cold-rolled steel panels painted with 2 mil of polyester powder paint and cold-rolled steel panels painted with 2 mil of epoxy-polyester hybrid were immersed in the bath. The solvent concentration was incrementally increased from 0 to 50 vol/vol% with each set of panels stripped. The results are listed in Table V. As can be seen in Figure 3, the relationship of the solvent concentration with the strip time is not linear. Without any solvent present in the bath there was no paint removal after 2 hr. Upon the addition of 10 vol/vol% of the solvent component there was immediate paint removal. The paint removal, however, was significantly Strip Time vs Solvent Concentration 12UF,2mHsofpowdef~
%
B f
Figure 3. Effect of the solvent concentration on stripping time (12O’F, 2 mil of powder). METAL
FINISHING
9 SEPTEMBER
1997
Strip Tlrn;;;
Strip Time vs Temperature
Alkalinity
ml
Figure 4. Effect of alkalinity on stripping time (12O’F). slower than is typically seen at the normal solvent concentration of 50 vol/vol%. Each subsequent addition of solvent significantly decreased the strip time up to 30 vol/vol% solvent. After this point an increased amount of solvent had little or no effect on the strip time. This result suggests that the stripper could be operated with a minimum of 30 vol/vol% solvent; however, the recommendation of 50 vol/ ~01% solvent is believed to increase the stability and lifetime of the bath. The bath does appear to be able to handle large fluctuations in the solvent concentration with little or no effect on stripping efficiency.
EFFECT OF ALKALINITY A 1.0-L bath was prepared with 50 vol/vol% deionized water and 50 volt ~01% solvent. The alkalinity was varied incrementally from zero to normal operating conditions. The strip times of polyester and epoxy-polyester hybrid powder paints were recorded and are listed in Table VI. As seen in Figure 4, the alkalinity varies in a similar way to that of the solvent component; however, with no alkalinity, there was some paint removal. The paint appeared to start delaminating from the metal surface. This process was very slow and took approximately 20 hr to remove the paint. This result indicates that the alkalinity needs to be kept at the previously recommended concentration by using the aqueous component at 50 vol/vol%.
METAL FINISHING
??
SEPTEMBER
1997
.\
I
Figure 5. Effect of temperature on stripping of paint hooks.
STRIPPING PRODUCTION PAINT HOOKS Several paint hooks were obtained from a manufactnring facility that uses many different-colored polyester powder paints. The hooks tested represented the colors that proved to be the most difficult for them to remove in their current hotalkaline stripping process. There were four sets of hooks, with each set representing a different number of passes through the powder booth. Hooks with one, two, three, and four passes were tested. There was 1.0 to 1.5 mil of powder paint for each pass. The stripping was performed in a 1.5-L bath. Figure 5 shows the results of the testing of the paint hooks at various temperatures. There is an initial decrease of the strip times with increasing temperature; however, at temperatures greater than 140°F the strip times decrease at a much lower rate. This is the same trend seen in the earlier study with laboratory panels. As
Strip Time vs Temperature m
m
so 23 20
15 ,o 6
0' 24
PolYesterPowder on Panels *2*-4*~an*r
p
.
ml
loo
J MO
hrpnvrC-3
Figure 6. Effect of temperature on stripping polyester powder painted test panels.
seen in Figure 6. there is an initial sharp decrease in the strip time that flattens with increasing temperature.
CONCLUSIONS This new stripper effectively strips powder paint coatings from steel substrates at low temperatures. Operation at low temperature affords many benefits such as energy and water conservation. Both the alkalinity source and the solvent are necessary for the paint stripper to work. The performance of the paint stripper is comparable to that of other conventional chemical paint strippers, nameIy methylene-chloride and hot-alkaline strippers.
FUTURE WORK Construction of a 55-gal demonstration tank has just been completed in order to collect more data on consumption rates and process control from the field. Also, a couple of strip-tank startups are planned in the near future to measure the performance of this paint stripper further. The paint sludge produced is extremely fine and may clog conventional filtering systems such as filter bags. At this time, in order to extend the effective bath life and to optimize the system, ultrafiltration and centrifugal filtration are being explored.
Acknowledgment The author, on behalf of Oakite Products Inc., acknowledges their sister company, Chemetall (CM) Oberflachentechnik AG of Switzerland, for all their help in the development work.
Biography Michael Deemer is a chemist with Oakite Products Inc., a specialties chemical company located in Berkeley Heights, N.J. He is currently working in the technical service and the product development departments. He received his B.S. degree in chemistry from Grove City College and his M.S. degree in chemistry from Lehigh University. MC
61
S
ince the Clean Air Act and its amendments in 1990 stringent regulations have been placed on the amount of volatile organic compounds (VOCs) that may be emitted. This has had a major effect on many manufacturing processes such as paint stripping. This stripping is necessary in paint lines to remove buildup on paint hooks, especially powder paints, and to remove paint from parts that need to be reworked. Several stripping methods, both chemical,and mechanical, are available to the marketplace. The mechanical methods are capital intensive and, therefore, not always attractive. The chemical strippers generally are run at high temperature and have health and safety issues. There are two major types of chemical stripper currently used. One type is the hot-alkaline strippers that generally operate above 200°F. The other type is the solvent-based strippers. In the past methylene chloride has been used as the main component in many of the solvent paint strippers. Today, owing to environmental and health concerns, methylene-chloride-based paint strippers need to be replaced with another effective low-temperature paint stripper. Such a paint stripper has been developed to provide strip times comparable to those of methylene-chloridebased strippers at much lower temperatures than the typical hot-alkaline strippers. The paint stripper con-
Effect of Agitation PahtHookswm3Passes
tains two phases. One phase is solvent based, and the other phase is water based with an alkalinity source. The work covered in this article includes initial laboratory work involving various types of powder paint, varying the amount of solvent and alkalinity available, and a performance comparison between the new stripper, a methylene-chloride-based stripper, and a hot-alkaline stripper. Also included will be a laboratory study done with paint hooks from a production plant.
BACKGROUND The new paint stripper contains two distinct phases. One phase is the solvent phase, which is mostly solvent with some surfactants. The solvent has a flash point greater than 205°F and a vapor pressure of 15 mm Hg at 100X, as compared with the vapor pressure of water at loo”C, which is 760 mm Hg. The aqueous component contains an alkalinity source with some surfactants. Because this stripping system is alkaline in nature it is safe only on steel. It will attack aluminum and zinc substrates. The system is made up with 50 vol/vol% of the solvent component and 50 vol/vol% of the aqueous component. No dilution is necessary. The typical operating temperature range is 80 to 140°F. Because this is a twophase system agitation is needed. The greater the agitation the better because this helps reduce the strip times. Figure 1 shows the difference between two levels of agitation. The agitation was produced by an overhead propeller mixer. The low agitation means that
the mixing speed was set so that the two phases were mixed but no vortex was present. The high agitation represents mixing with a vdrtex. This increased agitation can reduce the strip times by approximately 30%. The process is controlled in two ways. First, the replenishments are made by determining the ratio of the solvent component to the aqueous component. This is simply done by filtering 100 ml of a well-mixed sample into a lOO-ml graduated cylinder. The second process control is the determination of the alkalinity. This is done by filtering 10 ml of a well-mixed sample and titrating with 1.0 N hydrochloric acid to a bromophenol blue endpoint. The alkalinity titration is typically used only if the strip times increase significantly. The alkalinity is 12 ml for a fresh bath and is kept at this level through normal replenishments. EXPERIMENTAL The first step in testing this new paint stripper formulation was to determine how well it stripped various powder paints from cold-rolled steel panels at various temperatures. This performance was then compared with other chemical paint strippers such as hotalkaline and methylene-chloride strippers. When the performance parameters were set the operating limits for the solvent and the alkalinity were determined. With the basic laboratory work finished the paint stripper was operated in small field trials to compare the performance with other commercially available products. Several cold-rolled steel Q panels were powder coated with four different types of powder paint at three different
Table I. Time in Minutes to 100% Strip 3 Ml of Powder Paint Temperature(‘F)
Figure 1. Effect of agitation on stripping time. 58
80 100 120 140
Pdyesrer
Hybrid
24 19 5 5
18 10 5 4
0 Copyright Elsevier Science Inc.
METAL
fdyurethane 34 14 10 7
FINISHING
.
Epoxv -10 hr 3 hr 35 17
SEPTEMBER
1997
Table II. Time in Minutes to 100% Strip 4 Mil of Powder Paint Tempewfure('F) 80 100 120 140
PoIyesfer
Hybrid
28 15 13 6
18 13 10 5
Comparison of Chemical Strippen folyurefhane
EPOXY
40 18 12 5
-12 hr -6 hr 2 hr 45
ur
4mUsdPowderPaint
Table Ill. Time in Minutes to 100% Strip 5 Mil of Powder Paint Temperature (‘f) 80 100 120 140
Polyester
Hybrid
28 20 10 6
19 14 9 8
Polyurethane
EPOXY
45 23 14 8
Figure 2. Comparison of chemical strippers (4 mil of powder).
-14 hr -10 hr 2 hr 60
stripper was used at 40 vol/vol%, with mild agitation at 160°F and 200°F. The results are listed in Table IV and appear graphically in Figure 2. The new stripper at 120°F performs as well as or better than the other strippers in all cases except two. In both of these cases the methylene-chloride stripper performs better on the polyester and the epoxy powder paints.
strip, followed by the polyester, polyurethane, and the epoxy. At higher temperatures the polyester, the hybrid, and the polyurethane take approximately the same time to strip. At higher temperatures the epoxy is still the most difficult to strip. Another study was performed in which the new stripper was compared with a methylene-chloride-based stripper and a hot-alkaline stripper. The same four types of powder paint were used as in the first study, at a dry film thickness of 4 mil. The hot-alkaline
dry-film thicknesses. The steel panels were pretreated with a molybdate-accelerated iron phosphate before painting. A white polyester, gray epoxy-polyester hybrid, black polyurethane, and gray epoxy powders were used in the testing. All powder paints were cured according to the manufacturers’specifications. The time to 100% strip was recorded using 3, 4, and 5 mil of powder paint at 80, 100, 120, and 140°F. The data are listed in Tables I, II, and III. At lower temperatures the epoxypolyester hybrid was the easiest to
EFFECT OF SOLVENT CONCENTRATION
Table IV. Time in Minutes to 100% Strip Comparing Different Paint Strippers on 4 Mil of Powder Paint Stripper Hot alkaline at 16O'F Hot alkaline at200°F Methylene chloride New stripper at 12o'F
Polyester
Hybrid
38 10 3 8
46 13 35 7
Polyurethane
E~oxv
45 9 30 12
-14 hr -8 hr 55 120
Table V. Effect of Solvent Concentration on Strip Times for 2.0 Mil Polyester and 2.0 Mil Epoxy-Polyester Hybrid Powder Paints Vo/No/%of Solvent 0 10 20 30 40 50 '.
Po/yester Strip Time (min)
Epoxy-Polyester Strip Time (min)
No attack after 2 hr 37 12 7 5 5
No attack after 2 hr 52 10 4 4 4
Table VI. Effect of Alkalinity on Strip Times for 2.5 Mil Polyester and 2.0 Mil Epoxy Polyester Hybrid Powder Paints Alkalinity (m/ 1.0N HC()
0 2.4 4.0 5.5 9.6 12.0
60
Strip Time for Polyester (min)
Strip Time for Hybrid (min)
-22 hr
-18hr 24 10 10 9 7
37 17 15 12 7
A 1.0-L bath was prepared in which 500 ml of the aqueous component was added and heated to 120°F. Cold-rolled steel panels painted with 2 mil of polyester powder paint and cold-rolled steel panels painted with 2 mil of epoxy-polyester hybrid were immersed in the bath. The solvent concentration was incrementally increased from 0 to 50 vol/vol% with each set of panels stripped. The results are listed in Table V. As can be seen in Figure 3, the relationship of the solvent concentration with the strip time is not linear. Without any solvent present in the bath there was no paint removal after 2 hr. Upon the addition of 10 vol/vol% of the solvent component there was immediate paint removal. The paint removal, however, was significantly Strip Time vs Solvent Concentration 12UF,2mHsofpowdef~
%
B f
Figure 3. Effect of the solvent concentration on stripping time (12O’F, 2 mil of powder). METAL
FINISHING
9 SEPTEMBER
1997
Strip Tlrn;;;
Strip Time vs Temperature
Alkalinity
ml
Figure 4. Effect of alkalinity on stripping time (12O’F). slower than is typically seen at the normal solvent concentration of 50 vol/vol%. Each subsequent addition of solvent significantly decreased the strip time up to 30 vol/vol% solvent. After this point an increased amount of solvent had little or no effect on the strip time. This result suggests that the stripper could be operated with a minimum of 30 vol/vol% solvent; however, the recommendation of 50 vol/ ~01% solvent is believed to increase the stability and lifetime of the bath. The bath does appear to be able to handle large fluctuations in the solvent concentration with little or no effect on stripping efficiency.
EFFECT OF ALKALINITY A 1.0-L bath was prepared with 50 vol/vol% deionized water and 50 volt ~01% solvent. The alkalinity was varied incrementally from zero to normal operating conditions. The strip times of polyester and epoxy-polyester hybrid powder paints were recorded and are listed in Table VI. As seen in Figure 4, the alkalinity varies in a similar way to that of the solvent component; however, with no alkalinity, there was some paint removal. The paint appeared to start delaminating from the metal surface. This process was very slow and took approximately 20 hr to remove the paint. This result indicates that the alkalinity needs to be kept at the previously recommended concentration by using the aqueous component at 50 vol/vol%.
METAL FINISHING
??
SEPTEMBER
1997
.\
I
Figure 5. Effect of temperature on stripping of paint hooks.
STRIPPING PRODUCTION PAINT HOOKS Several paint hooks were obtained from a manufactnring facility that uses many different-colored polyester powder paints. The hooks tested represented the colors that proved to be the most difficult for them to remove in their current hotalkaline stripping process. There were four sets of hooks, with each set representing a different number of passes through the powder booth. Hooks with one, two, three, and four passes were tested. There was 1.0 to 1.5 mil of powder paint for each pass. The stripping was performed in a 1.5-L bath. Figure 5 shows the results of the testing of the paint hooks at various temperatures. There is an initial decrease of the strip times with increasing temperature; however, at temperatures greater than 140°F the strip times decrease at a much lower rate. This is the same trend seen in the earlier study with laboratory panels. As
Strip Time vs Temperature m
m
so 23 20
15 ,o 6
0' 24
PolYesterPowder on Panels *2*-4*~an*r
p
.
ml
loo
J MO
hrpnvrC-3
Figure 6. Effect of temperature on stripping polyester powder painted test panels.
seen in Figure 6. there is an initial sharp decrease in the strip time that flattens with increasing temperature.
CONCLUSIONS This new stripper effectively strips powder paint coatings from steel substrates at low temperatures. Operation at low temperature affords many benefits such as energy and water conservation. Both the alkalinity source and the solvent are necessary for the paint stripper to work. The performance of the paint stripper is comparable to that of other conventional chemical paint strippers, nameIy methylene-chloride and hot-alkaline strippers.
FUTURE WORK Construction of a 55-gal demonstration tank has just been completed in order to collect more data on consumption rates and process control from the field. Also, a couple of strip-tank startups are planned in the near future to measure the performance of this paint stripper further. The paint sludge produced is extremely fine and may clog conventional filtering systems such as filter bags. At this time, in order to extend the effective bath life and to optimize the system, ultrafiltration and centrifugal filtration are being explored.
Acknowledgment The author, on behalf of Oakite Products Inc., acknowledges their sister company, Chemetall (CM) Oberflachentechnik AG of Switzerland, for all their help in the development work.
Biography Michael Deemer is a chemist with Oakite Products Inc., a specialties chemical company located in Berkeley Heights, N.J. He is currently working in the technical service and the product development departments. He received his B.S. degree in chemistry from Grove City College and his M.S. degree in chemistry from Lehigh University. MC
61