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■ Production method of high-performance electret ■ Japanese Patent Application No. 4574291 [Phase] Application No. 45 (1970 ') Aug. 26 @ inventor Murayama Naohiro Wakiichi City
Kashiwacho Maehara 16 Ten Fukuda Seiichiki Kashiwacho Ochiai 28 8 [Fa] Agent Attorney
Shiori Shibuya, Niibashi-Horidomecho, Chuo-ku, Tokyo, Japan
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the change with time of the surface
potential in water (50 ° C.).
DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of
producing a stable electret having excellent resin molding performance. -1 Low molecular weight
organic substances such as carnauba wax and naphthalene, polarizable resins such as
polyvinylidene fluoride resin MWs polyvinyl chloride resin, polycarbonate, polyester, acrylic
resin, etc. and polyethylene, polytetrafluoroethylene, polypropylene, polystyrene etc. Such as
plastic films, sheets or moldings made of nonpolar resins or their copolymers or mixtures thereof
for a long time at appropriate temperature under direct current high voltage, and then cooled to
room temperature while being kept under direct current electric field The electret manufactured
by the method is characterized by its ability to hold the polarization state for a long time and its
excellent processability, toughness and flexibility, and it has other features of electrostatic
electroacoustic transducers such as speakers and microphones. A wide range of applications are
considered. Among them, polar polymers such as polymethacrylates, polyethylene terephthalates,
polycarbonates, polar fluororesins, and chlorine resins are well known as materials for forming
[111111] of electrets having a relatively long life. However, electrets obtained from these
materials can not always be used under severe conditions such as high temperature or in water,
in electrolyte solution, etc. The present inventors can last the life semi-permanently even under
severe conditions Invented a method of manufacturing a new electret. Conventionally, in order to
produce an electret from these polar polymers, the material is placed in a high DC electric field of
about IKv / cm to 300 Kv / am at an appropriate temperature not less than the glass transition
point Tg and not more than the melting point Tm of the material. It is generally carried out to
keep the conditions and polarize, and to cool to room temperature under the electric field as it is
to remove the electric field to fix the polarization. In this case, homocharge having the same sign
as that of the electrode in contact and heterocharge having the opposite sign are simultaneously
generated, one of the charges is prioritized depending on the processing conditions, and the sign
of the charge observed as a whole is determined. In general, electrets immediately after
production show heterocharge or relatively low homocharge, but in short time, heterocharge
decays to an electret showing large homocharge as a whole. In addition, when the electret
immediately after the polarization treatment is maintained at a high temperature above room
temperature, the time from the heterocharge to the conversion to the homocharge is significantly
shortened, and a high homocharge state can be obtained in a short time.
Thus, heterocharges are generally considered unstable and relatively stable homocharges are
desirable as electret charge, and electrets using this stabilized homocharges are suitable for
microphones, speakers, etc. which are used at room temperature. Has reached. However, this
homocharge can not be used because its stability is low at high temperature or in water, etc., and
depending on the use conditions, charges on both sides are rapidly attenuated. The present
inventors have found that after actively eliminating this stable homo-charge at room
temperature, a more highly stable hetero-charge can appear. [111111] EndPage: 1 fxbs Electrets
produced by the usual method are treated in the presence of water, such as water, electrolyte
solution, or water vapor, as described above, due to decay of the unstable heterocharge first as
described above. The change of the load code appears as a large selection of homo-charge,
followed by the decay of the homo-charge to erase the charge on both sides. It is surprising that
further processing leads to a shift of the charge code and the appearance of large heterocharges.
This is evidence for the existence of homo- and much more stable heterocharges, which were
considered to be stable in the past, and electrets exhibiting heterocharges produced according to
this method are in comparison to electrets exhibiting homocharges produced by a conventional
method. The charge lasts much longer. In addition, even if it is left in water, in salt water, or in
Ringer's solution, its life is extremely long as compared with homocharged ones. In particular,
those treated in water do not affect the stable heterocharge remaining after treatment because
there is no heating step, points that do not deform the shape of the treatment, heating equipment
This method is extremely effective because it requires almost no The same can be done by heat
treatment alone, and the method by heat treatment is described in detail in Japanese Patent
Application No. 45-19723. The original electret used in the present invention can be produced
by any method, for example, when the material is directly brought into contact with the electrode
to make it electret, an electret exhibiting first heterocharge is produced, which is gradually stable
homo In the case of electretizing, electretizing the material with a sheet of air-permeable material
such as paper between the electrodes during electretization results in weak homocharge from the
beginning, and the charge becomes high over time. Achieve stable homocharge at room
In the present invention, not only this stabilized homogeneous electret but also an electret having
a hetero- or weak homo-charge immediately after electretization can be used. The electret having
the initial heterocharge becomes partially homocharged in water and then turns into
heterocharge and stabilizes again. The electric field strength of electretization is not particularly
limited as long as it is equal to or less than the breakdown voltage, and the higher the voltage,
the better [111111] electric field strength of 1 to 300 KvA is usually used. Further, the
temperature of electretization varies depending on various resins, and is preferably not less than
the glass transition temperature and not more than the melting point. With regard to the
electrode, in addition to aluminum, copper, stainless steel, etc., an electrolyte solution may be
used, and there is no limitation as long as it has good conductivity. The sample after
electretization is treated in the presence of water or in water, in an electrolyte solution, but the
bath temperature at this time becomes higher as the conversion time becomes shorter, for
example, those based on polyvinylidene fluoride are 90 ° C. For 30 hours at 70 ° C. for 70
hours and at 50 ° C. for 130 hours, both unstable charge and homo charge decay with each
other, resulting in a stable charge of hetero charge. Since this method of stable heterocharge
generation is simple in operation, there is no need to raise the temperature relatively, and there
is no deformation of the shape of the substrate during the production of the electret. It also has a
major feature not found in heat treatment, which requires less equipment. Moreover, since it is
not necessary to process at high temperature, the physical properties of the base material itself
are not adversely affected, such as the internal dipole does not relax. As the electret base material
of the present invention, polar polymers are relatively preferable, and polyvinylidene fluoride
type, polyvinyl chloride type, acrylic resins and polyesters may be used. As described above, the
present invention is a method of producing an electret which stably sustains the charge stably
under long-term stability and long-term conditions, as described above, and the practical value
includes the aspect not found in other electrets. Very large. Further, the heterocharge referred to
in the present invention may be due to ion splitting or due to the polarization of a dipole. The
present invention will be described by way of examples, but is not limited by these examples.
Example 1 A sheet of about 1 thickness formed by extruding a mixture of 60 parts of
polyvinylidene fluoride and 40 parts of polymethyl methacrylate was collected with stainless
steel electrodes and heated in advance to 120 ° C. A DC electric field of 7.0 Kv (electric field
strength of 70 Kv / (m)) was applied and maintained for 1 hour in a charge air bath, and the
electric power was allowed to cool down to room temperature for 1.5 hours while being applied.
The surface potential of the obtained electret is measured by using a rotation center type
electrometer made by On and a distance of 1 cIIL between the electrode samples, and indicates a
homocharge of about 1800V. When this compound is left in water at [111111] EndPage: 250 °
C., the surface potential becomes O after 130 hours. If you leave it in water for 10 hours, it will
become about 200v of heterocharge. Table 1 shows the results of examining the attenuation
when the electret having the stable heterocharge thus prepared was stored in air at 100 ° C.
and 80 ° C., and in water at 50 ° C. and room temperature. The change over time of the
surface potential of the electret when stored in water at 50 ° C. is shown in FIG. (In the figure,
Ic: the surface in contact with the cathode surface of the electrode, IA: the surface in contact with
the anode surface of the electrode ρ Table 1 is a homocharge which is generally considered to
be stable and the heterocharge prepared according to the present invention Of the decay of the
surface charge in air at 100 ° C. and 80 ° C. and in water at 50 ° C. and room temperature
(20-25 ° C.). Example 2 A polymethyl methacrylate is heated at 180 ° C. using a hot press, and
pressed to form a press plate having a thickness of about one, which is sandwiched by stainless
steel electrodes on both sides, as in Example 1. Electretize in a way. The electric potential
between the sample and the electrode was measured at 1 cIn using a rotation sector type electric
meter made from Elytton, and the surface potential was measured and it showed a homocharge
of about 2100 v. When it is left in water at 50 ° C, the homocharge decays in 50 hours and the
heterocharge appears. The charge of the heterocharge is about 300v and very stable. The results
of examining the attenuation when stored in air at 80 ° C. and 100 ° C. and at room
temperature are shown in Table 2. Table 2 shows the time constant τ of the decay of surface
[111111] charge in room temperature water at 100 ° C., 80 ° C. air produced by the present
invention, and the homocharge which is usually considered stable. The slope of the time change
of EXAMPLE 3 The sheet used in Example 1 is subjected to electretization in the same manner as
in Example 1. The surface potential of the resulting electret is measured using an ion-made
rotational sector type electrometer, with the electrode and the sample being separated by 771 to
1, and indicates a homocharge of about 1800 v. When this is left in physiological saline (room
temperature), the surface measured potential becomes zero after 200 hours.
If it is further continued in saline, it will become about 200 v of heterocharged electret after
about 10 hours. The results of examining the decay when the stable heterocharged electret thus
prepared was stored in air at 100 ° C. and 80 ° C. and in room temperature saline are shown
in Table 3. Table 3 shows the attenuation of surface charge in 100 ° C. and 80 ° C. air and in
room temperature saline (20-25 ° C.) of homocharges and heterocharges made according to the
present invention, which are usually considered stable. 0 is a comparison of the constant τ (the
slope of the time change of the surface potential). Example 4 The sheet used in Example 1 is
subjected to electretization in the same manner as in Example 1. When the surface potential of
the resulting electret is measured using a rotary sector type electrometer [111111] EndPage: 3
manufactured by ON and the distance between the electrode samples is maintained at 1 crrL,
homocharge of about 1800 v is shown. When this is heat disinfected in a medical steam
disinfector, it will become a heterocharged electret of about 200v after 10 hours. Table 4 shows
the results of examining the decay when the stable heterocharged electret thus prepared was
stored in air at 100 ° C. and 80 ° C. and in water at room temperature. Table 4 shows the time
for surface charge decay in air at 100 ° C and 80 ° C and in room temperature water (20-25 °
C) of homocharges and heterocharges made according to the present invention, which are
usually considered stable. Constant τ (surface potential [111111] [111111] Table 4
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