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Industrial applications of electrostatics:
Journal of Electrostatics 51}52 (2001) 1}7
Industrial applications of electrostatics: the past, present and future G.S.P. Castle* Department of Electrical and Computer Engineering, Applied Electrostatics Research Center, University of Western Ontario, London, Ontario, Canada N6A 5B9
Abstract Although the scienti"c principles governing electrostatic forces have been known for many centuries the "rst successful industrial application did not occur until 1907. The development of two major present technologies, electrostatic precipitation and electrostatic coating is described and important features characteristic of each are identi"ed. It is suggested that in the future a number of new industrial applications will come from developments in micro-electrical mechanical systems (MEMS), biotechnology, ultra "ne particles and space. 2001 Elsevier Science B.V. All rights reserved. Keywords: Industrial applications; Electrostatic precipitation; Electrostatic coating
1. Introduction The purpose of this presentation is to brie#y give some historical perspective on the uses of electrostatics in industry. The development of two selected examples, electrostatic precipitation and electrostatic coating is discussed in terms of three time periods which I de"ne as the past (pre-1907), the present (1907 to now) and the future (tomorrow).
2. The past (pre-1907) We all know that the start of electrostatic scienti"c history is believed to have occurred in ancient Greece in an era predating our measures of millennia. Meaningful * Corresponding author. Tel.: #1-519-661-3758; fax: #1-519-661-3488. E-mail address: [email protected] (G.S.P. Castle). 0304-3886/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 0 6 8 - 7
2
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
applications, however, were a long time in coming. In fact my choice of 1907 as the transition between the past and present eras of electrostatics is because this year marked the installation of the "rst major successful commercial application of electrostatic forces, the electrostatic precipitator. The history of electrical science is a fascinating story and has been well documented. A concise summary of the pioneering contributions of many famous workers was described by Magie [1]. Reviewing this it is clear that the general principles of practically every modern industrial electrostatic process were known by the 1700s and could be described analytically by the 1800s. We may ask why did it take so long for this knowledge to transfer itself into practical application? This delay is not characteristic of other areas of science. For example, at the start of the industrial revolution, compare this with the rapidity with which similar levels of understanding of the sciences of mechanics, thermodynamics, metallurgy, etc. led to practical utilization in revolutionary structures and machines. Clearly the touchstone of the industrial revolution was horsepower rather than ion power and the major advances made possible by the harnessing of steam, and later electrodynamic energy, eclipsed the much more subtle events that are possible on a microscopic scale with electrostatic forces.
3. The present (1907 to now) You may "nd it odd that what I call the present actually encompasses almost 100 years. However, I believe this is reasonable since all of the present industrial applications with which we are familiar have been developed in this period. I shall not attempt to identify all these contributions as many distinguished leaders in our "eld have written about this topic and a number of excellent summaries exist [2}8]. These describe in detail the literally dozens of signi"cant applications of electrostatics currently in use. However, if one were to identify those which stand out in terms of their industrial importance and economic impact these would have to be electrostatic precipitation, electrostatic coating and electrophotography. Here I shall limit my comments to the "rst two since the latter will be the subject of a separate presentation later in this conference. Rather than looking at the technical details of these advances I would like to identify some of the key factors that led to their successful development. In this context it is interesting to observe several common characteristics. Advances in the "eld of electrostatics in the past were primarily due to the contributions of solitary scientists. In the present, progress is the result of engineers and applied scientists working together. Also, although the number of peer reviewed scienti"c papers has risen exponentially during this period, the patent has become the preferred "rst stage of publication when applications are involved. This has profoundly changed the nature of scienti"c interchange and engineering advances. For example, contrast the attributes of a good innovative technical paper that should be clear, accurate and speci"c, with those of a patent that should be vague, approximate and broad. In the past new developments were disclosed in triumph at a technical meeting of peers, now
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
3
often the details of the newest advances are held in corporate con"dence and rarely widely published. Let us brie#y explore these issues for the "rst two examples mentioned above. 3.1. Electrostatic precipitation The year 1907 may be considered to be the beginning of the industrial application of electrostatics as this was when F.G. Cottrell installed the "rst commercially successful electrostatic precipitator [9]. It is instructive to look at some of the reasons why he was successful. Clearly he was not the "rst to demonstrate precipitation. This was shown by Hohl"eld in Germany in 1824 when he used a corona discharge to clear fog inside a glass jar. However, Hohl"eld's interest was not in collecting the fog but rather in trying to explain why rain often falls following a lightning strike. Neither was Cottrell the "rst to recognize the potential of the process for air cleaning or to patent and build a commercial precipitator. This was accomplished by Lodge at a smelter in North Wales in 1885 [10]. Unfortunately, although his "lter worked in the laboratory, it failed to work in the "eld. In hindsight, we recognize that the two main reasons for this were because of his inadequate corona power supply and the fact that he was attempting to collect lead oxide fume, which even today is di$cult to precipitate due to its "neness and high resistivity. What Cottrell realized was that the recently developed mechanical high voltage recti"er when used in conjunction with his `pubescent discharge electrodea would provide su$cient corona current to successfully charge large quantities of dense fumes. Furthermore, he discovered that negative corona was much more stable than positive and that in order to maintain high voltage insulation in practical cases it was necessary to heat the high voltage bushing. These were the key components of his successful patent that was issued in 1908 [11]. Another factor that is helpful to an innovator is if luck is on your side. Unlike the lead fume faced by Lodge, the sulphuric acid mist targeted by Cottrell is particularly amenable to collection by electrostatic precipitation. There are several aspects of this development that are common to most electrostatic applications. First, Cottrell was a chemist, addressing a mechanical engineering problem, and he utilized a recently developed electrical machine in its solution. This highlights the fact that today all successful industrial applications of electrostatics require an interdisciplinary approach. The solitary inventor may obtain a patent but this is really only the beginning of the road to commercialization. Second, the process of electrostatic precipitation capitalizes on one of the key attributes of electrostatic forces, i.e., they are very e!ective for small particles and act directly on the charged particles rather than on the medium. This o!ered the revolutionary advantage of enabling high collection e$ciencies ('90%) to be achieved with negligible pressure drop. This is unmatched by any other comparable "ltration process.
4
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
Although this early technology was protected by patent, competitive forces soon led to further improvements and increased applications such as smelting fumes and #y ash from coal "red power stations. Major advances took place in increasing the e$ciency, capacity, range of application and reliability of electrostatic precipitators but for many years the details were omitted from patent descriptions and hidden by corporate secrecy. Perhaps the clearest sign that electrostatic precipitation had arrived as a mature technology was the publication of the classic text by H.J. White in 1963, 56 years after the "rst installation [12]. This book essentially disseminated the `state of the arta of knowledge up to that time. This also coincided with the start of the period of major preoccupation with air pollution problems throughout the industrialized world. Rapidly changing environmental legislation continually increased the demands on precipitator performance. Required collection e$ciencies exceeding 99.9% and very tight restrictions imposed on the allowable number of escaping "ne particles less than 10 microns in diameter were looming on the horizon by the 1970s. Simultaneously, complications, caused by the increasing use of low sulfur coal which produced high resistivity #y-ashes and very dense fumes, reached a point where the di$culties in solving these problems encouraged collaborative discussions. This led to the start of a new forum for engineering technical exchange with the organization by Professor White of the First International Conference on Electrostatic Precipitation held in Monterey, CA in 1981 [13]. These meetings have continued since then on a three year cycle and provide a major international forum for exchange and consolidation of technical advances in electrostatic precipitation with the enthusiastic participation of industrial contributors. 3.2. Electrostatic coating One of the hallmarks of modern industry is the need for coating surfaces with one or more layers of material. Electrostatics plays an important role in many of these coating processes. However, unlike the case of electrostatic precipitation, it has come about in a very di!erent manner since it was originally developed as an enhancement to spray painting rather than the basis of the process. In the early days of liquid spray painting it was common to waste 70% of the paint due to over spray. In the 1930s several researchers recognized the potential advantage of using electrostatic forces to improve the deposition e$ciency and two patents were issued based upon ideas which were essentially modi"cations of the electrostatic precipitator [14}15]. Commercial versions of automated electrostatic paint lines were introduced in the 1940s and developed rapidly to the point where transfer e$ciencies of over 70% were commonly achieved, a remarkable improvement at the time. This was soon followed by patented improvements in the charging and delivery systems using electri"ed atomizers in the form of a blade, rotating bell and disc or air and hydraulic guns. All of these systems have one thing in common. The electrostatic forces greatly enhance the coating uniformity and improve the transfer e$ciency but are not essential to the process. If the power supply should fail, deposition still takes place due to mechanical forces.
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
5
By the 1950s a new paint process was developed in Germany that involved no liquid solvent, just the dipping of heated surfaces into #uidized beds "lled with thermoplastic paint powder [16]. Out of this evolved a system of powder coating in which electrostatics played an essential rather than simply an enhancing role. It is interesting that in spite of being a revolutionary method of painting, unlike electrostatic precipitation and electrostatic painting there is no `ground breakinga patent to identify this development. This was presumably because the basic process was so similar in principle to these previous applications that it failed two main tests of being granted a patent, i.e., it was either obvious to one `skilled in the arta or it was essentially covered in some previous patent. However, this similarity to earlier designs enabled specialized coating equipment to be readily adapted and by the early 1960s was in use both in the United States and Europe. Stimulated by the adoption of stricter environmental laws enacted to limit VOC emissions advances moved rapidly both in application equipment and powder formulations allowing more decorative and thinner layers of paint to be applied. It was at this stage in the development of powder coating that the patent process was utilized to protect the many improvements such as the development of triboelectric charging guns to minimize the `Faraday cagea screening e!ect which was a major limitation of corona guns. Although transfer e$ciencies are comparable to that found with electrostatic liquid painting, the recycling of over sprayed powder allows powder utilization of over 95% to be commonly achieved. 4. The future (tomorrow) Each of the technologies discussed here will continue to be improved. Electrostatic precipitators will become even more e$cient (both for collection and energy utilization) and will form the basis of complete integrated particulate and gaseous control systems. Powder coating will be capable of using smaller particle size paints and will produce improved quality "nishes. Of course the many other existing processes not mentioned today such as electrostatic separation, etc. will also be improved and become increasingly important in industry. However, what completely new applications may we expect? Some insights can be found by recalling the main characteristics of electrostatic forces. First is their ability to control the trajectories of particles in the size range from microns to millimetres. Second is the dependence of electric "elds upon the inverse square law that results in rapidly increasing forces as separation distances reduce and surfaces come close together. Third, they are inherently energy e$cient due to the very small current #ows which are involved. Let us look at four speci"c areas where future developments look promising. 4.1. MEMS (micro-electrical mechanical systems) In much the same way that the development of microprocessors revolutionized the computer industry, MEMS technology promises to do the same for devices. The possibility of mass producing integrated sensors, actuators, etc. which can be directly controlled by software promises unprecedented interfacing between the
6
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
computational and physical world and the range of possible applications seems unlimited. The fabrication and operation of these miniature devices is greatly dependent upon electrostatic forces. 4.2. Biotechnology Many biological processes are controlled by electrostatic factors and even minor imbalances in ion concentrations, etc. can have a profound e!ect on their function. Also the study and manipulation of such things as cells, DNA and other forms of biomacromolecules can be carried out by the use of electrophoretic and dielectophoretic forces. Practical application of these phenomena will open up whole new areas of industrial activity. 4.3. Ultraxne particles and nanotechnology The use of nanometer size particles is increasing in industrial applications to modify #ow and charging properties in granulated materials. These particles are characterized by their high surface energy and even in small concentrations can have dramatic e!ects upon the properties of such things as ceramics, metals, optical structures and semiconductors. Also developments in nanotechnology are pushing the limits of fabrication down to atomic dimensions giving rise to the promise of many new applications. 4.4. Space The absence of gravity and the existence of the vacuum in space suggests that both the upper and lower size range of particles for which electrostatic forces predominate will be extended. Also, for many years science "ction writers and researchers alike have speculated on the use of ion thruster engines for the powering of spacecraft. As I speak, JPL's `Deep Space 1a spacecraft is racing towards a rendezvous with Comet Borrelly in September 2001 [17]. So "nally, approximately two hundred and "fty years after the start of the Industrial Revolution ion power has found its niche alongside that of horsepower. 5. Conclusions From the two major industrial examples I have cited, today, we can see that although they are all based upon common fundamental principles from the past, they developed in the present from di!erent motivations and by very di!erent paths. Electrostatic precipitation solved an existing but previously insoluble problem in air cleaning. Electrostatic painting greatly improved an existing but ine$cient coating process and eventually led to the development of the new electrostatic technology of powder coating. Advances will clearly continue and we can expect completely new industrial applications to come from developments in the "elds of MEMS, biotechnology, ultra"ne particles, nanotechnology and space.
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
7
References [1] W.F. Magie, A Source Book in Physics, Harvard University Press, Cambridge, MA, 1963, pp. 387}597. [2] N.J. Felici, Recent developments in electrostatics, Adv. Sci. 22 (1966) 33}39. [3] A.W. Bright, Some modern applications in electrostatics, Adv. Sci. 22 (1966) 39}49. [4] I.I. Inculet, Electrostatics in industry, J. Electrostat. 4 (1977/78) 175}192. [5] S. Masuda, Industrial applications of electrostatics, J. Electrostat. 10 (1981) 1}15. [6] I.I. Inculet, Industrial applications of static electricity, J. Electrostat. 16 (1985) 287}298. [7] G.S.P. Castle, The evolving "eld of electrostatics, Institute of Physics Conference Series, No. 118, IOP Publishing, Bristol, 1991, pp. 1}12. [8] J.A. Cross, Electrostatics; Principles, Problems and Applications, Adam Hilger, IOP Publishing, Bristol, 1987. [9] H.J. White, Centenary of Frederick Gardner Cottrell, J. Electrostat. 4 (1977/78) 1}34. [10] O.J. Lodge, The electrical deposition of dust and smoke with special reference to the collection of metallic fumes to a possible puri"cation of the atmosphere, J. Soc. Chem. Ind. 5 (1886) 572. [11] F.G. Cottrell, US Patent C895,729, 1908. [12] H.J. White, Industrial Electrostatic Precipitation, Addison-Wesley, Reading, MA, 1963. [13] H.J. White (Ed.), Conference Proceedings, First International Conference on Electrostatic Precipitation, Monterey, CA, October 1981. [14] E. Pugh, US Patent C1,855,869, April 26, 1932. [15] H.P. Ransburg, H.J. Green, US Patent C2,247,963, July 1, 1941. [16] E.P. Miller, Electrostatic coating, in: A.D. Moore (Ed.), Electrostatics and its Applications, Wiley, New York, 1973, Chapter 11. [17] M. Rayman, Deep Space 1, http://mnp.jpl.nasa.gov/dsl/mrlog.html.
Industrial applications of electrostatics: the past, present and future G.S.P. Castle* Department of Electrical and Computer Engineering, Applied Electrostatics Research Center, University of Western Ontario, London, Ontario, Canada N6A 5B9
Abstract Although the scienti"c principles governing electrostatic forces have been known for many centuries the "rst successful industrial application did not occur until 1907. The development of two major present technologies, electrostatic precipitation and electrostatic coating is described and important features characteristic of each are identi"ed. It is suggested that in the future a number of new industrial applications will come from developments in micro-electrical mechanical systems (MEMS), biotechnology, ultra "ne particles and space. 2001 Elsevier Science B.V. All rights reserved. Keywords: Industrial applications; Electrostatic precipitation; Electrostatic coating
1. Introduction The purpose of this presentation is to brie#y give some historical perspective on the uses of electrostatics in industry. The development of two selected examples, electrostatic precipitation and electrostatic coating is discussed in terms of three time periods which I de"ne as the past (pre-1907), the present (1907 to now) and the future (tomorrow).
2. The past (pre-1907) We all know that the start of electrostatic scienti"c history is believed to have occurred in ancient Greece in an era predating our measures of millennia. Meaningful * Corresponding author. Tel.: #1-519-661-3758; fax: #1-519-661-3488. E-mail address: [email protected] (G.S.P. Castle). 0304-3886/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 0 6 8 - 7
2
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
applications, however, were a long time in coming. In fact my choice of 1907 as the transition between the past and present eras of electrostatics is because this year marked the installation of the "rst major successful commercial application of electrostatic forces, the electrostatic precipitator. The history of electrical science is a fascinating story and has been well documented. A concise summary of the pioneering contributions of many famous workers was described by Magie [1]. Reviewing this it is clear that the general principles of practically every modern industrial electrostatic process were known by the 1700s and could be described analytically by the 1800s. We may ask why did it take so long for this knowledge to transfer itself into practical application? This delay is not characteristic of other areas of science. For example, at the start of the industrial revolution, compare this with the rapidity with which similar levels of understanding of the sciences of mechanics, thermodynamics, metallurgy, etc. led to practical utilization in revolutionary structures and machines. Clearly the touchstone of the industrial revolution was horsepower rather than ion power and the major advances made possible by the harnessing of steam, and later electrodynamic energy, eclipsed the much more subtle events that are possible on a microscopic scale with electrostatic forces.
3. The present (1907 to now) You may "nd it odd that what I call the present actually encompasses almost 100 years. However, I believe this is reasonable since all of the present industrial applications with which we are familiar have been developed in this period. I shall not attempt to identify all these contributions as many distinguished leaders in our "eld have written about this topic and a number of excellent summaries exist [2}8]. These describe in detail the literally dozens of signi"cant applications of electrostatics currently in use. However, if one were to identify those which stand out in terms of their industrial importance and economic impact these would have to be electrostatic precipitation, electrostatic coating and electrophotography. Here I shall limit my comments to the "rst two since the latter will be the subject of a separate presentation later in this conference. Rather than looking at the technical details of these advances I would like to identify some of the key factors that led to their successful development. In this context it is interesting to observe several common characteristics. Advances in the "eld of electrostatics in the past were primarily due to the contributions of solitary scientists. In the present, progress is the result of engineers and applied scientists working together. Also, although the number of peer reviewed scienti"c papers has risen exponentially during this period, the patent has become the preferred "rst stage of publication when applications are involved. This has profoundly changed the nature of scienti"c interchange and engineering advances. For example, contrast the attributes of a good innovative technical paper that should be clear, accurate and speci"c, with those of a patent that should be vague, approximate and broad. In the past new developments were disclosed in triumph at a technical meeting of peers, now
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
3
often the details of the newest advances are held in corporate con"dence and rarely widely published. Let us brie#y explore these issues for the "rst two examples mentioned above. 3.1. Electrostatic precipitation The year 1907 may be considered to be the beginning of the industrial application of electrostatics as this was when F.G. Cottrell installed the "rst commercially successful electrostatic precipitator [9]. It is instructive to look at some of the reasons why he was successful. Clearly he was not the "rst to demonstrate precipitation. This was shown by Hohl"eld in Germany in 1824 when he used a corona discharge to clear fog inside a glass jar. However, Hohl"eld's interest was not in collecting the fog but rather in trying to explain why rain often falls following a lightning strike. Neither was Cottrell the "rst to recognize the potential of the process for air cleaning or to patent and build a commercial precipitator. This was accomplished by Lodge at a smelter in North Wales in 1885 [10]. Unfortunately, although his "lter worked in the laboratory, it failed to work in the "eld. In hindsight, we recognize that the two main reasons for this were because of his inadequate corona power supply and the fact that he was attempting to collect lead oxide fume, which even today is di$cult to precipitate due to its "neness and high resistivity. What Cottrell realized was that the recently developed mechanical high voltage recti"er when used in conjunction with his `pubescent discharge electrodea would provide su$cient corona current to successfully charge large quantities of dense fumes. Furthermore, he discovered that negative corona was much more stable than positive and that in order to maintain high voltage insulation in practical cases it was necessary to heat the high voltage bushing. These were the key components of his successful patent that was issued in 1908 [11]. Another factor that is helpful to an innovator is if luck is on your side. Unlike the lead fume faced by Lodge, the sulphuric acid mist targeted by Cottrell is particularly amenable to collection by electrostatic precipitation. There are several aspects of this development that are common to most electrostatic applications. First, Cottrell was a chemist, addressing a mechanical engineering problem, and he utilized a recently developed electrical machine in its solution. This highlights the fact that today all successful industrial applications of electrostatics require an interdisciplinary approach. The solitary inventor may obtain a patent but this is really only the beginning of the road to commercialization. Second, the process of electrostatic precipitation capitalizes on one of the key attributes of electrostatic forces, i.e., they are very e!ective for small particles and act directly on the charged particles rather than on the medium. This o!ered the revolutionary advantage of enabling high collection e$ciencies ('90%) to be achieved with negligible pressure drop. This is unmatched by any other comparable "ltration process.
4
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
Although this early technology was protected by patent, competitive forces soon led to further improvements and increased applications such as smelting fumes and #y ash from coal "red power stations. Major advances took place in increasing the e$ciency, capacity, range of application and reliability of electrostatic precipitators but for many years the details were omitted from patent descriptions and hidden by corporate secrecy. Perhaps the clearest sign that electrostatic precipitation had arrived as a mature technology was the publication of the classic text by H.J. White in 1963, 56 years after the "rst installation [12]. This book essentially disseminated the `state of the arta of knowledge up to that time. This also coincided with the start of the period of major preoccupation with air pollution problems throughout the industrialized world. Rapidly changing environmental legislation continually increased the demands on precipitator performance. Required collection e$ciencies exceeding 99.9% and very tight restrictions imposed on the allowable number of escaping "ne particles less than 10 microns in diameter were looming on the horizon by the 1970s. Simultaneously, complications, caused by the increasing use of low sulfur coal which produced high resistivity #y-ashes and very dense fumes, reached a point where the di$culties in solving these problems encouraged collaborative discussions. This led to the start of a new forum for engineering technical exchange with the organization by Professor White of the First International Conference on Electrostatic Precipitation held in Monterey, CA in 1981 [13]. These meetings have continued since then on a three year cycle and provide a major international forum for exchange and consolidation of technical advances in electrostatic precipitation with the enthusiastic participation of industrial contributors. 3.2. Electrostatic coating One of the hallmarks of modern industry is the need for coating surfaces with one or more layers of material. Electrostatics plays an important role in many of these coating processes. However, unlike the case of electrostatic precipitation, it has come about in a very di!erent manner since it was originally developed as an enhancement to spray painting rather than the basis of the process. In the early days of liquid spray painting it was common to waste 70% of the paint due to over spray. In the 1930s several researchers recognized the potential advantage of using electrostatic forces to improve the deposition e$ciency and two patents were issued based upon ideas which were essentially modi"cations of the electrostatic precipitator [14}15]. Commercial versions of automated electrostatic paint lines were introduced in the 1940s and developed rapidly to the point where transfer e$ciencies of over 70% were commonly achieved, a remarkable improvement at the time. This was soon followed by patented improvements in the charging and delivery systems using electri"ed atomizers in the form of a blade, rotating bell and disc or air and hydraulic guns. All of these systems have one thing in common. The electrostatic forces greatly enhance the coating uniformity and improve the transfer e$ciency but are not essential to the process. If the power supply should fail, deposition still takes place due to mechanical forces.
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
5
By the 1950s a new paint process was developed in Germany that involved no liquid solvent, just the dipping of heated surfaces into #uidized beds "lled with thermoplastic paint powder [16]. Out of this evolved a system of powder coating in which electrostatics played an essential rather than simply an enhancing role. It is interesting that in spite of being a revolutionary method of painting, unlike electrostatic precipitation and electrostatic painting there is no `ground breakinga patent to identify this development. This was presumably because the basic process was so similar in principle to these previous applications that it failed two main tests of being granted a patent, i.e., it was either obvious to one `skilled in the arta or it was essentially covered in some previous patent. However, this similarity to earlier designs enabled specialized coating equipment to be readily adapted and by the early 1960s was in use both in the United States and Europe. Stimulated by the adoption of stricter environmental laws enacted to limit VOC emissions advances moved rapidly both in application equipment and powder formulations allowing more decorative and thinner layers of paint to be applied. It was at this stage in the development of powder coating that the patent process was utilized to protect the many improvements such as the development of triboelectric charging guns to minimize the `Faraday cagea screening e!ect which was a major limitation of corona guns. Although transfer e$ciencies are comparable to that found with electrostatic liquid painting, the recycling of over sprayed powder allows powder utilization of over 95% to be commonly achieved. 4. The future (tomorrow) Each of the technologies discussed here will continue to be improved. Electrostatic precipitators will become even more e$cient (both for collection and energy utilization) and will form the basis of complete integrated particulate and gaseous control systems. Powder coating will be capable of using smaller particle size paints and will produce improved quality "nishes. Of course the many other existing processes not mentioned today such as electrostatic separation, etc. will also be improved and become increasingly important in industry. However, what completely new applications may we expect? Some insights can be found by recalling the main characteristics of electrostatic forces. First is their ability to control the trajectories of particles in the size range from microns to millimetres. Second is the dependence of electric "elds upon the inverse square law that results in rapidly increasing forces as separation distances reduce and surfaces come close together. Third, they are inherently energy e$cient due to the very small current #ows which are involved. Let us look at four speci"c areas where future developments look promising. 4.1. MEMS (micro-electrical mechanical systems) In much the same way that the development of microprocessors revolutionized the computer industry, MEMS technology promises to do the same for devices. The possibility of mass producing integrated sensors, actuators, etc. which can be directly controlled by software promises unprecedented interfacing between the
6
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
computational and physical world and the range of possible applications seems unlimited. The fabrication and operation of these miniature devices is greatly dependent upon electrostatic forces. 4.2. Biotechnology Many biological processes are controlled by electrostatic factors and even minor imbalances in ion concentrations, etc. can have a profound e!ect on their function. Also the study and manipulation of such things as cells, DNA and other forms of biomacromolecules can be carried out by the use of electrophoretic and dielectophoretic forces. Practical application of these phenomena will open up whole new areas of industrial activity. 4.3. Ultraxne particles and nanotechnology The use of nanometer size particles is increasing in industrial applications to modify #ow and charging properties in granulated materials. These particles are characterized by their high surface energy and even in small concentrations can have dramatic e!ects upon the properties of such things as ceramics, metals, optical structures and semiconductors. Also developments in nanotechnology are pushing the limits of fabrication down to atomic dimensions giving rise to the promise of many new applications. 4.4. Space The absence of gravity and the existence of the vacuum in space suggests that both the upper and lower size range of particles for which electrostatic forces predominate will be extended. Also, for many years science "ction writers and researchers alike have speculated on the use of ion thruster engines for the powering of spacecraft. As I speak, JPL's `Deep Space 1a spacecraft is racing towards a rendezvous with Comet Borrelly in September 2001 [17]. So "nally, approximately two hundred and "fty years after the start of the Industrial Revolution ion power has found its niche alongside that of horsepower. 5. Conclusions From the two major industrial examples I have cited, today, we can see that although they are all based upon common fundamental principles from the past, they developed in the present from di!erent motivations and by very di!erent paths. Electrostatic precipitation solved an existing but previously insoluble problem in air cleaning. Electrostatic painting greatly improved an existing but ine$cient coating process and eventually led to the development of the new electrostatic technology of powder coating. Advances will clearly continue and we can expect completely new industrial applications to come from developments in the "elds of MEMS, biotechnology, ultra"ne particles, nanotechnology and space.
G.S.P. Castle / Journal of Electrostatics 51}52 (2001) 1}7
7
References [1] W.F. Magie, A Source Book in Physics, Harvard University Press, Cambridge, MA, 1963, pp. 387}597. [2] N.J. Felici, Recent developments in electrostatics, Adv. Sci. 22 (1966) 33}39. [3] A.W. Bright, Some modern applications in electrostatics, Adv. Sci. 22 (1966) 39}49. [4] I.I. Inculet, Electrostatics in industry, J. Electrostat. 4 (1977/78) 175}192. [5] S. Masuda, Industrial applications of electrostatics, J. Electrostat. 10 (1981) 1}15. [6] I.I. Inculet, Industrial applications of static electricity, J. Electrostat. 16 (1985) 287}298. [7] G.S.P. Castle, The evolving "eld of electrostatics, Institute of Physics Conference Series, No. 118, IOP Publishing, Bristol, 1991, pp. 1}12. [8] J.A. Cross, Electrostatics; Principles, Problems and Applications, Adam Hilger, IOP Publishing, Bristol, 1987. [9] H.J. White, Centenary of Frederick Gardner Cottrell, J. Electrostat. 4 (1977/78) 1}34. [10] O.J. Lodge, The electrical deposition of dust and smoke with special reference to the collection of metallic fumes to a possible puri"cation of the atmosphere, J. Soc. Chem. Ind. 5 (1886) 572. [11] F.G. Cottrell, US Patent C895,729, 1908. [12] H.J. White, Industrial Electrostatic Precipitation, Addison-Wesley, Reading, MA, 1963. [13] H.J. White (Ed.), Conference Proceedings, First International Conference on Electrostatic Precipitation, Monterey, CA, October 1981. [14] E. Pugh, US Patent C1,855,869, April 26, 1932. [15] H.P. Ransburg, H.J. Green, US Patent C2,247,963, July 1, 1941. [16] E.P. Miller, Electrostatic coating, in: A.D. Moore (Ed.), Electrostatics and its Applications, Wiley, New York, 1973, Chapter 11. [17] M. Rayman, Deep Space 1, http://mnp.jpl.nasa.gov/dsl/mrlog.html.