GB2060259A - Method for the Manufacture of Electrets - Google Patents

Method for the Manufacture of Electrets Download PDF

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Publication number
GB2060259A
GB2060259A GB8032815A GB8032815A GB2060259A GB 2060259 A GB2060259 A GB 2060259A GB 8032815 A GB8032815 A GB 8032815A GB 8032815 A GB8032815 A GB 8032815A GB 2060259 A GB2060259 A GB 2060259A
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United Kingdom
Prior art keywords
foil
charging
charged
temperature
electret
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Granted
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GB8032815A
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GB2060259B (en
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NEDERLANDSE APPL SCIENT RES
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NEDERLANDSE APPL SCIENT RES
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Publication of GB2060259A publication Critical patent/GB2060259A/en
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Publication of GB2060259B publication Critical patent/GB2060259B/en
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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D39/00—Filtering material for liquid or gaseous fluids
    • B01D39/08—Filter cloth, i.e. woven, knitted or interlaced material
    • B01D39/083—Filter cloth, i.e. woven, knitted or interlaced material of organic material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D39/00—Filtering material for liquid or gaseous fluids
    • B01D39/10—Filter screens essentially made of metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D39/00—Filtering material for liquid or gaseous fluids
    • B01D39/14—Other self-supporting filtering material ; Other filtering material
    • B01D39/16—Other self-supporting filtering material ; Other filtering material of organic material, e.g. synthetic fibres
    • B01D39/1607—Other self-supporting filtering material ; Other filtering material of organic material, e.g. synthetic fibres the material being fibrous
    • B01D39/1623—Other self-supporting filtering material ; Other filtering material of organic material, e.g. synthetic fibres the material being fibrous of synthetic origin
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G7/00—Capacitors in which the capacitance is varied by non-mechanical means; Processes of their manufacture
    • H01G7/02—Electrets, i.e. having a permanently-polarised dielectric
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G7/00—Capacitors in which the capacitance is varied by non-mechanical means; Processes of their manufacture
    • H01G7/02—Electrets, i.e. having a permanently-polarised dielectric
    • H01G7/021—Electrets, i.e. having a permanently-polarised dielectric having an organic dielectric
    • H01G7/023—Electrets, i.e. having a permanently-polarised dielectric having an organic dielectric of macromolecular compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
    • B01D2239/04—Additives and treatments of the filtering material
    • B01D2239/0414—Surface modifiers, e.g. comprising ion exchange groups
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
    • B01D2239/04—Additives and treatments of the filtering material
    • B01D2239/0435—Electret
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
    • B01D2239/06—Filter cloth, e.g. knitted, woven non-woven; self-supported material
    • B01D2239/065—More than one layer present in the filtering material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
    • B01D2239/06—Filter cloth, e.g. knitted, woven non-woven; self-supported material
    • B01D2239/065—More than one layer present in the filtering material
    • B01D2239/0668—The layers being joined by heat or melt-bonding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2239/00—Aspects relating to filtering material for liquid or gaseous fluids
    • B01D2239/10—Filtering material manufacturing

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Textile Engineering (AREA)
  • Treatments Of Macromolecular Shaped Articles (AREA)
  • Processing And Handling Of Plastics And Other Materials For Molding In General (AREA)

Abstract

A method for the manufacture of electrets from a material consisting of a polymer wherein a product formed from said material is heat treated to anneal it prior to its polarisation. In one example, a fluorinated polymer film is heated to its crystallisation temperature for at least one hour and is then quenched prior to its polarisation by corona discharge method. The electret shows improved stability as a function of temperature.

Description

SPECIFICATION Method for the Manufacture of Electrets The invention relates to a method for the manufacture of electrets from a material consisting of a polymer, in particular a fluorinated polymer or copolymer, in which a product formed from said material is charged positively.
Stable electrets charged negatively for example by means of a corona charging are known.
In view of the advantages of balance systems e.g. in electret headphones, in which a positively and a negatively charged electret are to be combined, there is a great need for a stable positively charged electret.
Such a need is felt also for filters made from electret fibres, in which the filter operation is mainly based on the electric attraction of the dust or aerosol particles by the electret fibres. Here two-sided charged electret fibres are required, with one side charged positively and the other side negatively. Said bipolarly charged fibres produce a strong inhomogeneous electric field by which both charged and uncharged particles are captured much more efficiently than in conventional filters made from uncharged fibres.
Fotr electret filters another important need exists, i.e. that the filter should withstand high temperatures, in this regard polytetrafluorethylene (Teflon) or polymers corresponding therewith and/or derived therefrom should be used as the starting material. It has been found that positively charged electrets manufactured from fluorinated polymers have the disadvantage of being much less stable than negatively charged electrets.
It is the object of the invention to provide for a method of the sort mentioned in the preamble, in which said disadvantage is avoided.
According to the invention, said object is achieved by subjecting the electret material to a thermal pretreatment.
Positively charged electrets manufactured this way not only have the advantage of an excellent stability as a function of temperature, they also have the advantage that they remain stable in the long run and at a high humidity.
Moreover, the method according to the invention is of interest for applications in which (e.g. in electroacoustic transducers), a foil is firmly adhered to a metal plate according to a method called heat sealing, in order to obtain electret transducers with very stable mechanical and thermal properties. The thermal pretreatment, also called annealing, is automatically carried out during said adhering method. The conditions during the heat sealing only need to be optimized.
In such transducers the foil and the plate are perforated. The charging operation of the foil may be carried out either before or after the perforation.
The method according to the invention has further the advantage that the time interval between the thermal pretreatment and the charging operation is not critical, so that said operations can be carried out in separated locations during the fabrication; alternatively the pretreated material may be stored for a certain period of time before the charging. The fact that the storage time is not critical is of importance because the charging operation generally proceeds faster than the thermal pretreatment.
The invention will now be described by way of a few examples and with reference to the accompanying drawings, in which: Fig. 1 schematically shows a device for carrying out an embodiment of the method according to the invention; Fig. 2 schematically displays a device for carrying out another embodiment of the method according to the invention; and Figs 3-6 represent diagrams of foils treated and charged according to several methods.
The production of an electret by charging the starting material may take place according to several methods; 1. By poling the material simply between two electrodes or alternatively by using a glassfibre netting between the electrodes and the material; 2. Electron bombardment; 3. lon bombardment; 4. Corona charging; and 5. Liquid charging.
In corona charging air is used as the transport medium for the charge carries, whereas in liquid charging a liquid is used as the transport medium, which liquid should preferably be volatile.
Electron bombardment is suitable only for negative charging, i.e. for obtaining a negatively charged electret, whereas the remaining possibilities (1, 3,4 and 5) are well suited for the positive charging of the starting material to be used for an electret.
Methods 1, 3, 4 and 5 are also usable for bipolar charging.
In the device shown in Fig. 1 corona charging has been chosen as an example for the manufacture of an electret.
According to Fig. 1 a product in the form of a foil of a polymeric material is supplied through a transport device (not shown) to a furnace 2 in which the foil is heated to a temperature above the lowest crystallisation temperature of the polymer used. Said temperature is maintained for a predetermined period of time. Although, a foil is used, the method according to the invention is also suitable for fibres, sheets of several dimensions and so on, which are fed to the furnace by means of an adapted transport means.
As already stated the foil consists of a polymer, for example polytetrafluoroethylene, also called "Teflon", (Registered Trade Mark). But also copolymers such as for example tetrafluoroethylene-hexafluoropropylene (Teflon FEP) or tetrafluoroethylene-perfluoromethoxyethylene (Teflon-PFA) are suitable for the manufacture of positively charged electrets by using the method according to the invention. Moreover, said polymers have the advantage that they can be easily adhered to some other material by supplying heat, which adhering method is called heat sealing.
In order to keep the foil in the furnace at a temperature above the lowest crystallisation temperature during a predetermined period of time, the foil may reside in the furnace during said period of time. The residence time in the furnace during the predetermined time may also be obtained by adapting the feeding velocity and/or the dimension of the furnace in the direction of the transport of the foil.
Thereafter foil 1 is fed to a quenching device 3 by means of a transport device not shown, in which the foil is cooled abruptly. Subsequently foil 1 is supplied to charging device 4 for which in this embodiment a corona charging device has been chosen, by means of which a positive charge is injected into foil 1.
The corona charging device consists of a metal plate 4 connected to earth, a grid 6 and corona wires 7. Foil 1, which might be one-sided metallized, is led between an earthed metal plate 5 and grid 6. The corona wires 7 are connected to a positive or alternating potential of for example 8 kV. In the latter case the charge assumed by the foil is determined by the potential of the grid 6 which in this case should carry a positive potential for example of a magnitude of +300 V with respect to the grounded plate 5. At a slow feeding speed the foil is roughly charged up to the potential of grid 6. The magnitudes mentioned as an example may be chosen up to such a high level that yet neither electrical breakdown of foil 1 nor sparking of the corona wires 7 to plate 5 occur.
In Fig. 2 a device is schematically shown, by means of which another embodiment of the method according to the invention can be implemented. Here the foil is charged in two stages. Here too, transport means not shown are used for feeding foil 1.
Foil 1 is fed to furnace 2 in which said foil is pretreated thermally as in the device according to Fig. 1. After a quenching operation (optional) foil 1 is guided further and supplied to the first charging device 4 in which the foil is charged for the first time. Thereafter foil 1 is fed to furnace 2' in which the foil is discharged partially by heating.
After said discharge operation foil 1 is fed to a second charging device 4'. The charging devices are shown as blocks, however, it is clear that for this a corona charging device according to Fig. 1 or another suitable charging device can be applied.
In Figs 3 up to and incl. 5 some results of charged foils are given in the form of graphs.
Along the abscissa the temperature T is plotted in degrees Celsius, whereas along the ordinate the surface voltage Vc of the foil is plotted in volts according to a logarithmic scale, which voltage Vc is proportional to the positive charge density present in the foil. It is noted that the parameters used are only examples and consequently they may not be considered as a limitation of the invention.
The graphs of Fig. 3 apply to 50 ym thick foils made of the polymer "Teflon-FEP", which are charged at room temperature in which the corona wires 7 are connected to a source with an alternating voltage of 8 kV and in which grid 6 has a potential of +300 V with respect to the grounded plate 5. Graph a of Fig. 1 applies to a foil of the above mentioned sort, which is not annealed. Graph a shows an abrupt charge drop beyond a heating temperature of 750C. Graph b represents the result of a charged foil of the same sort, but annealed by a preheating to 2500C which foil is kept at this temperature for two hours. After this thermal pretreatment and after quenching the foil, it is charged under the same conditions as the foil of graph a.From a comparison of graphs a and b the effect of the thermal pretreatment on the stability as a function of temperature emerges clearly. The charge drop in graph a has completely disappeared in graph'b.
The thermally preheated foil therefore retains its charge up to a much higher temperature.
It is noted that the small voltage differences at the beginning of the graphs are due to differences in the thicknesses of the foils used. The same comment applies to the graphs discussed hereafter as well.
Experiments have shown, that the improvement in stability usually starts to be clearly discernable after a thermal pretreatment at a temperature slightly above the lowest crystallisation temperature of the material of the foil. The effect of the annealing becomes more pronounced when the annealing temperature increases. Also the time during which the foil is maintained at the annealing temperature is of importance.
The effect of the annealing time is demonstrated in Fig. 4. Graph a of Fig. 4 applies to a foil not thermally preheated and charged with the corona charging device 4 of Fig. 1, in which corona wires 7 are connected to a source with an alternating voltage of 8 kV and in which grid 6 has a potential of +300 V with respect to plate 5.
Graphs b and c of Fig. 4 apply to foils being charged under the same charging conditions as the above mentioned foil, in which, however, before the charging, the foils were heated to 2500C. The foil to which graph b applies was heated during 60 minutes, whereas the foil of graph c was heated during 120 minutes.
It has been found surprisingly that fast cooling or quenching of foil 1 in the quenching device 3 after a thermal preheating, not only speeds up the manufacturing process, but surprisingly also improves the stability.
A further improvement in stability occurs when the charging of the foil is carried out at a temperature above room temperature, preferably at a temperature between the lowest crystallisation temperature and the melting temperature of the foil material. Said increased charging temperature may be obtained for example by heating metal plate 5 in the corona charging device 4. When the foil is quenched after its charging at an elevated temperature a further improvement in stability occurs.
With reference to the graphs of Fig. 5 results will next be elucidated obtained by the method carried out with the set-up according to Fig. 2.
Graph a in Fig. 5 is plotted for a foil which was held at a temperature of 2500C for two hours, after which it was quenched. The charging parameters are equal to those for the foils to which the graphs discussed above apply.
Graph a applies to a foil partially discharged by heating in furnace 2, which discharge was interrupted at a surface potential of + 100 V.
Graph a displays the result of the foil after charging it for the first time. Graph b in Fig. 5 is plotted after the foil was charged for the second time in charging device 4' of Fig. 2. In said second charging operation the same charging conditions are in force.
The thermal pretreatment does not only produce an improvement in stability as a function of temperature, but also as a function of time. The latter improvement in stability is apparent from the graphs in Fig. 6.
In Fig. 6 along the abscissa time t (in minutes) is plotted and along the ordinate the surface voltage V (in volts) generated by the foil which is a measure for the positive charge density on the foil.
Graph a shows, at a measuring temperature of 2000 C, the decrease in charge of a foil immediately after its charging, using the above mentioned charging conditions. This foil was not subjected to a thermal pretreatment. Graph b shows the result of a foil which before its charging had been subjected to an annealing at 2500C for two hours, whereafter the foil was quenched.
Positive electrets may be used in particular in electroacoustic transducers such as headphones.
In these headphones perforated electret foils have been used charged by the method according to the invention.
It appears that the method according to the invention is well suited for the charging of said perforated foils despite of the fact that due to the perforation the field pattern is not homogeneous during the charging operation. For the application in an electroacoustic transducer the perforated foil is adhered to a thin perforated metal sheet e.g. by heat sealing. It is clear that by said heat sealing the perforated foil is annealed automatically, because the sealing method takes place at an elevated temperature. In order to obtain an electret as stable as possible one only needs to optimize the conditions under which the heat sealing is carried out.
Because by means of the method according to the invention stable positively charged electrets may be manufactured now, one may use a combination of a positively and negatively charged electret in an electro acoustic transducer (for example a headphone). The balance system obtained has many advantages such as for example a simpler driver unit and a higher acoustic energy output.
It is noted that an improvement in stability of positive electrets may be achieved by performing in addition to the annealing of the starting material, a roughening of the material before the charging (e.g. by sandpapering or glasspearl blasting).
For the manufacture of bipolar electret fibres for electret filters one may start from a nonmetallized foil that is first annealed and subsequently or simultaneously charged positively and negatively at its respective sides.
Hereafter a known fibrillation method may be used for producing bipolar electret fibres from the bipolarly charged foil. One may also immediately start from fibres and charge them positively and negatively.
It has been found, that by the thermal pretreatment not only the positive but also the negative charge becomes more stable, and the present invention accordingly also provides a method for making positive electrets using the same procedure, mutatis mutandis, as for negative.

Claims (13)

Claims
1. Method for the manufacture of electrets from a material consisting of a polymer, in particular a fluorinated polymer or copolymer, in which a product formed from said material is charged positively, characterized in that the material is pretreated thermally (annealed).
2. Method according to claim 1, characterized in that before the charging the material is heated to at least the lowest crystallisation temperature and is maintained at said temperature during a predetermined period of time.
3. Method according to claim 2, characterized in that the predetermined period of time is at least one hour.
4. Method according to claim 3, characterized in that after the expiration of the predetermined period of time the material is quenched.
5. Method according to one of the preceding claims, characterized in that the material is charged at a temperature between its lowest crystallisation temperature and its melting temperature.
6. Method according to claim 5, characterized in that after the charging the material is quenched.
7. Method according to one of the preceding claims, characterized in that before the charging the surface of the material is roughened.
8. Method according to claim 1, characterized in that after its annealing the material is charged positively for the first time, partly discharged thereafter and subsequently recharged positively.
9. Bipolar electret charged positively at one side according to a method of one of the preceding claims, while the other side is charged negatively.
10. Method for the manufacture of electrets from a material consisting of a polymer, in particular a fluorinated polymer or copolymer, in which a product formed from said material is charged negatively, characterized in that the material is pretreated thermally (annealed).
11. Method according to claim 10, characterized in that the step as specified in any one of claims 2 to 7 are carried out.
12. Method according to claim 1, carried out substantially as described with reference to and as illustrated by Fig. 1 or Fig. 2 of the accompanying drawings.
13. An electret whenever prepared by a method as claimed in any one of claims 1 to 8 or 10to 12.
GB8032815A 1979-10-11 1980-10-10 Method for the manufacture of electrets Expired GB2060259B (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
NL7907539A NL7907539A (en) 1979-10-11 1979-10-11 METHOD FOR MANUFACTURING ELECTRETS

Publications (2)

Publication Number Publication Date
GB2060259A true GB2060259A (en) 1981-04-29
GB2060259B GB2060259B (en) 1983-12-14

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Family Applications (1)

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GB8032815A Expired GB2060259B (en) 1979-10-11 1980-10-10 Method for the manufacture of electrets

Country Status (6)

Country Link
JP (1) JPS56100409A (en)
DE (1) DE3038381A1 (en)
FR (1) FR2467474A1 (en)
GB (1) GB2060259B (en)
NL (1) NL7907539A (en)
SE (1) SE452525B (en)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0066878A2 (en) * 1981-06-08 1982-12-15 Western Electric Company, Incorporated Stable positively charged electrets
US5019723A (en) * 1987-12-07 1991-05-28 Controlled Release Technologies, Inc. Self-regulated therapeutic agent delivery system and method
EP0447166A2 (en) * 1990-03-12 1991-09-18 Mitsui Petrochemical Industries, Ltd. Process for producing an electret, a film electret, and an electret filter
US5304227A (en) * 1990-11-30 1994-04-19 Mitsui Petrochemical Industries, Ltd. Electret filters
US5411576A (en) * 1993-03-26 1995-05-02 Minnesota Mining And Manufacturing Company Oily mist resistant electret filter media and method for filtering
US5935303A (en) * 1996-12-26 1999-08-10 Koken Ltd. Electrostatic filter
WO2000001737A1 (en) * 1998-07-02 2000-01-13 3M Innovative Properties Company Fluorinated electret
WO2006013717A1 (en) 2004-08-05 2006-02-09 Olympus Corporation Capacitive micromachined ultrasonic transducer and method for manufacturing same
US20110132195A1 (en) * 2009-12-03 2011-06-09 Hyundai Motor Company Method for manufacturing electrostatic non-woven intake filter and electrostatic non-woven intake filter using the same

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS60168511A (en) * 1984-02-10 1985-09-02 Japan Vilene Co Ltd Production of electret filter
DE102020002271B3 (en) * 2020-04-14 2021-06-17 Heinz Günther Römer Virological respirator to protect against droplet infections

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
NL6614505A (en) * 1965-10-23 1967-04-24
JPS5040720B1 (en) * 1970-09-26 1975-12-26
JPS5650408B2 (en) * 1973-07-05 1981-11-28

Cited By (25)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0066878A2 (en) * 1981-06-08 1982-12-15 Western Electric Company, Incorporated Stable positively charged electrets
EP0066878A3 (en) * 1981-06-08 1983-11-09 Western Electric Company, Incorporated Stable positively charged electrets
US4527218A (en) * 1981-06-08 1985-07-02 At&T Bell Laboratories Stable positively charged Teflon electrets
US5019723A (en) * 1987-12-07 1991-05-28 Controlled Release Technologies, Inc. Self-regulated therapeutic agent delivery system and method
EP0447166A2 (en) * 1990-03-12 1991-09-18 Mitsui Petrochemical Industries, Ltd. Process for producing an electret, a film electret, and an electret filter
EP0447166A3 (en) * 1990-03-12 1992-10-21 Mitsui Petrochemical Industries, Ltd. Process for producing an electret, a film electret, and an electret filter
US5256176A (en) * 1990-03-12 1993-10-26 Mitsui Petrochemical Industries, Ltd. Film electret and an electret filter
US5304227A (en) * 1990-11-30 1994-04-19 Mitsui Petrochemical Industries, Ltd. Electret filters
US5411576A (en) * 1993-03-26 1995-05-02 Minnesota Mining And Manufacturing Company Oily mist resistant electret filter media and method for filtering
US5472481A (en) * 1993-03-26 1995-12-05 Minnesota Mining And Manufacturing Company Oily mist resistant electret filter media
US5935303A (en) * 1996-12-26 1999-08-10 Koken Ltd. Electrostatic filter
US6397458B1 (en) 1998-07-02 2002-06-04 3M Innovative Properties Company Method of making an electret article by transferring fluorine to the article from a gaseous phase
US6808551B2 (en) 1998-07-02 2004-10-26 3M Innovative Properties Company Method of using fluorinated electrets
US6398847B1 (en) 1998-07-02 2002-06-04 3M Innovative Properties Company Method of removing contaminants from an aerosol using a new electret article
US6409806B1 (en) 1998-07-02 2002-06-25 3M Innovative Properties Company Fluorinated electret
US6432175B1 (en) 1998-07-02 2002-08-13 3M Innovative Properties Company Fluorinated electret
US6562112B2 (en) 1998-07-02 2003-05-13 3M Innovative Properties Company Fluorinated electret
US6660210B2 (en) 1998-07-02 2003-12-09 3M Innovative Properties Company Method of making fluorinated electrets
WO2000001737A1 (en) * 1998-07-02 2000-01-13 3M Innovative Properties Company Fluorinated electret
US6953544B2 (en) 1998-07-02 2005-10-11 3M Innovative Properties Company Method of making a respirator that has a fluorinated electret
WO2006013717A1 (en) 2004-08-05 2006-02-09 Olympus Corporation Capacitive micromachined ultrasonic transducer and method for manufacturing same
EP1781067A1 (en) * 2004-08-05 2007-05-02 Olympus Corporation CAPACITIVE MICROMACHINED ULTRASONIC TRANSDUCER (cMUT) AND ITS PRODUCTION METHOD
EP1781067A4 (en) * 2004-08-05 2012-04-18 Olympus Corp CAPACITIVE MICROMACHINED ULTRASONIC TRANSDUCER (cMUT) AND ITS PRODUCTION METHOD
US20110132195A1 (en) * 2009-12-03 2011-06-09 Hyundai Motor Company Method for manufacturing electrostatic non-woven intake filter and electrostatic non-woven intake filter using the same
US8518152B2 (en) * 2009-12-03 2013-08-27 Hyundai Motor Company Method for manufacturing electrostatic non-woven intake filter and electrostatic non-woven intake filter using the same

Also Published As

Publication number Publication date
GB2060259B (en) 1983-12-14
FR2467474B1 (en) 1984-11-30
FR2467474A1 (en) 1981-04-17
SE452525B (en) 1987-11-30
JPS56100409A (en) 1981-08-12
NL7907539A (en) 1981-04-14
SE8007120L (en) 1981-04-12
DE3038381A1 (en) 1981-04-23

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732 Registration of transactions, instruments or events in the register (sect. 32/1977)
PCNP Patent ceased through non-payment of renewal fee