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Temperature distribution in an electrostatically applied powder coating layer to be cured with infrared radiation
Journal of Electrostatics 66 (2008) 564–566
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Temperature distribution in an electrostatically applied powder coating layer to be cured with infrared radiation Ion I. Inculet*, M.A. Bergougnou Applied Electrostatics Research Centre, University of Western Ontario, London, Ontario, Canada N6A 5B9
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 May 2008 Received in revised form 14 June 2008 Accepted 18 June 2008 Available online 26 July 2008
The measurements of the temperature distribution were taken inside a charged, 1 mm thick nylon uncured powder layer coating to be cured over a metallic surface with a filtered IR radiation. An optical filter used in the test allowed the transmission of only the IR wavelength below 2.9 mm. The experiments showed that the tested nylon powder layer close to the metallic surface would be initially exposed to a curing temperature lower than the temperature at the surface of the coating. Hence, the cured surface layer and some of the powder may fall off during the curing. Further work to study the temperature distribution which develops on various powder materials and particle size may help the technology when thick cured powder coatings are necessary. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Powders Curing Adhesive Radiation Gradients
1. Introduction The considerable increase of the use of powder paints in electrostatic coating has led to industrial interest [1–4] in applying the energy saving IR technology for curing the deposited powder layer. The electrostatic coating process with powder paint presents substantial economic advantages over the coating with liquid paints: a. From powders there are no solvents emitted. b. Most of the electrically charged powder, which bypasses the target and ends up on the walls and floor of the spray booth, is collectable from specially constructed painting booths with plastic double walls. (Double wall with air space or special dielectric filled space.) c. With proper colour painting sequence, the mixing of colour powders may be minimized and substantial paint recoveries may be realized. d. Coating with powders may be also performed with triboelectrification guns, which eliminate the need for high voltage power supplies and associated safety precautions. The fact that powders to date have not completely eliminated the need for liquid paints is due to the appearance of the cured
* Corresponding author. E-mail address: [email protected] (I.I. Inculet). 0304-3886/$ – see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2008.06.007
powder, which in some cases does not match the market’s accepted standards for the looks of a cured liquid-paint coating. The challenge of matching the cured powder coatings to liquid-paint led to studies of: - The use of very fine particle powders. In use today, there are more than 300 powder formulations. - The curing technology, considering that compared to liquid layers, prior to entering a curing oven, the initially deposited powder layer may be thicker. - The charging of powder particles. - The size distribution of the paint powders. In many applications, the cured film thickness ranges from under 25 mm to over 380 mm. The studies in this paper cover an ‘‘in depth’’ analysis of the temperature distribution, in a special case requiring a 1 mm thick layer of powder paint to be cured with infrared radiation. The studies were part of an economical investigation of replacing the conventional curing with IR curing. An electrically charged powder layer held on the surface by coulomb image forces must stay in place undisturbed while being conveyed inside the curing oven. In a high temperature conventional oven tunnel, the entire powder layer, whether thin or thick, is rapidly cured before the electric charge in the mass of the layer is fully relaxed. In an IR ambient temperature oven tunnel, the curing of a thick layer of powder must start with the powder particles which are in contact with the surface to be coated, lest the entire
I.I. Inculet, M.A. Bergougnou / Journal of Electrostatics 66 (2008) 564–566
565
Fig. 1. The tested assembly of the four thermocouples mounted on the aluminium plate for coating with the 1 mm powder in a fluidized bed. Note: thermocouple #1 is the one in direct contact with the metal. Thermocouple #4 represents the surface temperature of the coating.
I.R. Source
The optical filter allows the transmission of only I.R. wavelengths below 2.9 m.
Optical Filter
4 Thermocouples embedded in the 1mm thick layer of paint powder
Aluminium Plate 1mm thick
Fig. 2. Experimental assembly.
2. The experimental apparatus Fig. 1 shows the details of the mounting of the thermocouples inside of 1 mm thick coating with paint powder over a metallic plate.
70 60
Temperature °C
charged powder layer or parts of it may fall off. For this reason, the development of the temperature distribution inside the powder layer is a key factor in the curing process. One must also consider that the temperature of the powder layer may also affect the relaxation of the electric space charge in the powder mass.
50 40 30 20 Thermo 1
Thermo 2
Thermo 3
Thermo 4
10
3. Procedure 0
The instrumented plate, 10 cm 6 cm, was made out of 1 mm thick aluminium. The plate was coated with a 1 mm thick nylon powder (10–70 mm particle size distribution) by electrically
0
200
400
600
800
1000
Time (s) Fig. 3. Temperature versus time for each of the four thermocouples.
566
I.I. Inculet, M.A. Bergougnou / Journal of Electrostatics 66 (2008) 564–566
temperature increases practically, simultaneously in the entire powder layer.
60
Temperature (°°C)
50
Thermo 1
Thermo 2
Thermo 3
Thermo 4
4.2. Measurements of temperatures in the powder coating when exposed to the described filtered IR radiation
40
Fig. 4 shows the unexpected results of the temperature change versus time at four levels in the 1 mm thick powder layer. The temperature measured by thermo 1 at the surface of the aluminium plate, after 150 s, reached only 15 C, whereas during the same time, the surface temperature measured by thermo 4 reached 35 C.
30
20
10
5. Conclusion
0 0
50
100
150
Time (s) Fig. 4. Temperature versus time for each of the four thermocouples.
grounding the plate and dipping it in a tribocharged fluidized bed. For the experiments with IR curing, a filter was introduced, as shown in Fig. 2, to minimize the absorption of the radiation on the surface of the coating. The penetration of infrared energy is a function of its wavelength. The shorter the wavelength, the greater the penetration power. The filter had a cut off wavelength of 2.9 mm. It has been suggested [5] that energies below this wavelength can be transmitted but not absorbed in the coating powder, allowing an initial heating of the metal plate surface by the transmitted IR. All the experiments in both the conventional and IR curing ovens were necessarily carried out at temperatures below the curing temperature of the powder. 4. Results 4.1. Measurements from curing in a traditional convection oven Fig. 3 shows the results which were obtained in a convection oven at 120 C. The temperature versus time measured by the four thermocouples reached up to 65 C. The tests confirm the well known, fast, and effective curing of powder in a high energy consumption traditional oven. The
The measurement of the temperature distribution increases in the tested, charged, uncured, 1 mm thick nylon powder layer coating to be cured over a metallic surface with a filtered IR radiation of a cut off wavelength of 2.9 mm shows that the powder layer close to the metallic surface is initially exposed to a curing temperature lower than the temperature at the surface of the coating. Hence the surface powder layer, which will cure first, may fall off in the oven. Further work to study the temperature increases which develop on various powder materials and particle size distribution as well as the associated relaxation of the electrical charges in the powder particles may help the technology when thick cured powder coatings are required. Acknowledgements The authors acknowledge the initiative and excellent work on this project by Mr. Guillaume Roederer, currently working for the Electricite´ de France in Paris. References [1] Thomas A. Stryker, Solar Products Inc., evaluating and comparing gas-fired and electric infrared heaters, Powder Coating 11 (2000) 13–21. [2] Douglas M. Canfield, Casso-Solar Corp., how to save space and boost production with an infrared preheat-gel oven, Powder Coating (1996) 41–52. [3] N.L. Alpert, W.E. Keiser, H.A. Szymanski, Theory and Practice of Infrared Spectroscopy, second ed. Plenum Press, 1970. [4] Near infrared curing: what it is; what it isn’t[ and infrared energy: its use in curing powder coatings[, Powder Coating (2002) 25–42. [5] J.A. Jamieson, R.H. Mcfee, G.N. Plass, R.H. Grube, R.G. Richards, Infrared Physics and Engineering, McGraw Hill Book Company, 1963.
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Temperature distribution in an electrostatically applied powder coating layer to be cured with infrared radiation Ion I. Inculet*, M.A. Bergougnou Applied Electrostatics Research Centre, University of Western Ontario, London, Ontario, Canada N6A 5B9
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 May 2008 Received in revised form 14 June 2008 Accepted 18 June 2008 Available online 26 July 2008
The measurements of the temperature distribution were taken inside a charged, 1 mm thick nylon uncured powder layer coating to be cured over a metallic surface with a filtered IR radiation. An optical filter used in the test allowed the transmission of only the IR wavelength below 2.9 mm. The experiments showed that the tested nylon powder layer close to the metallic surface would be initially exposed to a curing temperature lower than the temperature at the surface of the coating. Hence, the cured surface layer and some of the powder may fall off during the curing. Further work to study the temperature distribution which develops on various powder materials and particle size may help the technology when thick cured powder coatings are necessary. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Powders Curing Adhesive Radiation Gradients
1. Introduction The considerable increase of the use of powder paints in electrostatic coating has led to industrial interest [1–4] in applying the energy saving IR technology for curing the deposited powder layer. The electrostatic coating process with powder paint presents substantial economic advantages over the coating with liquid paints: a. From powders there are no solvents emitted. b. Most of the electrically charged powder, which bypasses the target and ends up on the walls and floor of the spray booth, is collectable from specially constructed painting booths with plastic double walls. (Double wall with air space or special dielectric filled space.) c. With proper colour painting sequence, the mixing of colour powders may be minimized and substantial paint recoveries may be realized. d. Coating with powders may be also performed with triboelectrification guns, which eliminate the need for high voltage power supplies and associated safety precautions. The fact that powders to date have not completely eliminated the need for liquid paints is due to the appearance of the cured
* Corresponding author. E-mail address: [email protected] (I.I. Inculet). 0304-3886/$ – see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2008.06.007
powder, which in some cases does not match the market’s accepted standards for the looks of a cured liquid-paint coating. The challenge of matching the cured powder coatings to liquid-paint led to studies of: - The use of very fine particle powders. In use today, there are more than 300 powder formulations. - The curing technology, considering that compared to liquid layers, prior to entering a curing oven, the initially deposited powder layer may be thicker. - The charging of powder particles. - The size distribution of the paint powders. In many applications, the cured film thickness ranges from under 25 mm to over 380 mm. The studies in this paper cover an ‘‘in depth’’ analysis of the temperature distribution, in a special case requiring a 1 mm thick layer of powder paint to be cured with infrared radiation. The studies were part of an economical investigation of replacing the conventional curing with IR curing. An electrically charged powder layer held on the surface by coulomb image forces must stay in place undisturbed while being conveyed inside the curing oven. In a high temperature conventional oven tunnel, the entire powder layer, whether thin or thick, is rapidly cured before the electric charge in the mass of the layer is fully relaxed. In an IR ambient temperature oven tunnel, the curing of a thick layer of powder must start with the powder particles which are in contact with the surface to be coated, lest the entire
I.I. Inculet, M.A. Bergougnou / Journal of Electrostatics 66 (2008) 564–566
565
Fig. 1. The tested assembly of the four thermocouples mounted on the aluminium plate for coating with the 1 mm powder in a fluidized bed. Note: thermocouple #1 is the one in direct contact with the metal. Thermocouple #4 represents the surface temperature of the coating.
I.R. Source
The optical filter allows the transmission of only I.R. wavelengths below 2.9 m.
Optical Filter
4 Thermocouples embedded in the 1mm thick layer of paint powder
Aluminium Plate 1mm thick
Fig. 2. Experimental assembly.
2. The experimental apparatus Fig. 1 shows the details of the mounting of the thermocouples inside of 1 mm thick coating with paint powder over a metallic plate.
70 60
Temperature °C
charged powder layer or parts of it may fall off. For this reason, the development of the temperature distribution inside the powder layer is a key factor in the curing process. One must also consider that the temperature of the powder layer may also affect the relaxation of the electric space charge in the powder mass.
50 40 30 20 Thermo 1
Thermo 2
Thermo 3
Thermo 4
10
3. Procedure 0
The instrumented plate, 10 cm 6 cm, was made out of 1 mm thick aluminium. The plate was coated with a 1 mm thick nylon powder (10–70 mm particle size distribution) by electrically
0
200
400
600
800
1000
Time (s) Fig. 3. Temperature versus time for each of the four thermocouples.
566
I.I. Inculet, M.A. Bergougnou / Journal of Electrostatics 66 (2008) 564–566
temperature increases practically, simultaneously in the entire powder layer.
60
Temperature (°°C)
50
Thermo 1
Thermo 2
Thermo 3
Thermo 4
4.2. Measurements of temperatures in the powder coating when exposed to the described filtered IR radiation
40
Fig. 4 shows the unexpected results of the temperature change versus time at four levels in the 1 mm thick powder layer. The temperature measured by thermo 1 at the surface of the aluminium plate, after 150 s, reached only 15 C, whereas during the same time, the surface temperature measured by thermo 4 reached 35 C.
30
20
10
5. Conclusion
0 0
50
100
150
Time (s) Fig. 4. Temperature versus time for each of the four thermocouples.
grounding the plate and dipping it in a tribocharged fluidized bed. For the experiments with IR curing, a filter was introduced, as shown in Fig. 2, to minimize the absorption of the radiation on the surface of the coating. The penetration of infrared energy is a function of its wavelength. The shorter the wavelength, the greater the penetration power. The filter had a cut off wavelength of 2.9 mm. It has been suggested [5] that energies below this wavelength can be transmitted but not absorbed in the coating powder, allowing an initial heating of the metal plate surface by the transmitted IR. All the experiments in both the conventional and IR curing ovens were necessarily carried out at temperatures below the curing temperature of the powder. 4. Results 4.1. Measurements from curing in a traditional convection oven Fig. 3 shows the results which were obtained in a convection oven at 120 C. The temperature versus time measured by the four thermocouples reached up to 65 C. The tests confirm the well known, fast, and effective curing of powder in a high energy consumption traditional oven. The
The measurement of the temperature distribution increases in the tested, charged, uncured, 1 mm thick nylon powder layer coating to be cured over a metallic surface with a filtered IR radiation of a cut off wavelength of 2.9 mm shows that the powder layer close to the metallic surface is initially exposed to a curing temperature lower than the temperature at the surface of the coating. Hence the surface powder layer, which will cure first, may fall off in the oven. Further work to study the temperature increases which develop on various powder materials and particle size distribution as well as the associated relaxation of the electrical charges in the powder particles may help the technology when thick cured powder coatings are required. Acknowledgements The authors acknowledge the initiative and excellent work on this project by Mr. Guillaume Roederer, currently working for the Electricite´ de France in Paris. References [1] Thomas A. Stryker, Solar Products Inc., evaluating and comparing gas-fired and electric infrared heaters, Powder Coating 11 (2000) 13–21. [2] Douglas M. Canfield, Casso-Solar Corp., how to save space and boost production with an infrared preheat-gel oven, Powder Coating (1996) 41–52. [3] N.L. Alpert, W.E. Keiser, H.A. Szymanski, Theory and Practice of Infrared Spectroscopy, second ed. Plenum Press, 1970. [4] Near infrared curing: what it is; what it isn’t[ and infrared energy: its use in curing powder coatings[, Powder Coating (2002) 25–42. [5] J.A. Jamieson, R.H. Mcfee, G.N. Plass, R.H. Grube, R.G. Richards, Infrared Physics and Engineering, McGraw Hill Book Company, 1963.