U.S. patent application number 10/516739 was filed with the patent office on 2005-08-11 for electret filter media and process for producing the same.
Invention is credited to Kitagawa, Yoshiyuki, Tokuda, Shoji.
Application Number | 20050176325 10/516739 |
Document ID | / |
Family ID | 29738339 |
Filed Date | 2005-08-11 |
United States Patent
Application |
20050176325 |
Kind Code |
A1 |
Tokuda, Shoji ; et
al. |
August 11, 2005 |
Electret filter media and process for producing the same
Abstract
It is intended to provide an electret filter which has a large
surface charge density, sustains stable electret properties over a
long time even in a high-temperature atmosphere and yet degrades to
reduce its volume when buried for disposal; and a process for
producing the same. The molar ratio of an L-lactic acid monomer to
a D-lactic acid monomer ranges from 100 to 85:0 to 15 or 0 to 15:85
to 100. An electret filter having a surface charge density of
1.2.times.10.sup.-9 C/cm.sup.2 or more can be obtained by
crystallizing by heating at a temperature from the glass transition
temperature to the melting point and then, while heating to 60 to
140.degree. C., applying a direct current corona electrical field
to thereby cool to 40.degree. C. or lower under.
Inventors: |
Tokuda, Shoji; (Shiga,
JP) ; Kitagawa, Yoshiyuki; (Shiga, JP) |
Correspondence
Address: |
Barry E Bretchneider
Morrison & Foerster
2000 Pennsylvania Avenue N W
Washington
DC
20006-1888
US
|
Family ID: |
29738339 |
Appl. No.: |
10/516739 |
Filed: |
December 6, 2004 |
PCT Filed: |
June 2, 2003 |
PCT NO: |
PCT/JP03/06962 |
Current U.S.
Class: |
442/327 ;
428/364; 428/365 |
Current CPC
Class: |
B01D 39/1623 20130101;
B01D 46/0001 20130101; D04H 1/435 20130101; Y10T 442/60 20150401;
Y10T 428/2913 20150115; Y10T 428/2915 20150115; B01D 46/0032
20130101; D06M 10/025 20130101 |
Class at
Publication: |
442/327 ;
428/364; 428/365 |
International
Class: |
D04H 001/00; D04H
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 6, 2002 |
JP |
2002-165934 |
Jun 6, 2002 |
JP |
2002-165935 |
Claims
1. An electret filter medium, comprising a lactic acid polymer
having a molar ratio of an L-lactic acid monomer to a D-lactic acid
monomer in the range from 100 to 85:0 to 15.
2. The electret filter medium according to claim 1, wherein it is
mainly composed of a lactic acid polymer that produces an endotherm
of at least 0.5 J/g accompanied with crystal fusion.
3. The electret filter medium according to claim 1, wherein the
content of lactide is at most 15% based on the weight of the
medium.
4. The electret filter medium according to claim 1, wherein it has
a surface charge density of at least
1.2.times.10.sup.-9/cm.sup.2.
5. The electret filter medium according to claim 1, further
comprising 0.01 to 0.3 parts by weight of a nucleating agent based
on 100 parts by weight of the lactic acid polymer.
6. A process for producing the electret filter medium according to
claim 1, comprising: applying a direct current corona electric
field to a nonwoven fabric while heating it to a temperature of
60.degree. C. to 140.degree. C., wherein the nonwoven fabric
comprises fibers mainly composed of a lactic acid polymer; and then
cooling the nonwoven fabric to a temperature of 40.degree. C. or
lower while applying the electric field to the nonwoven fabric.
7. An electret filter medium, comprising a lactic acid polymer
having a molar ratio of an L-lactic acid monomer to a D-lactic acid
monomer in the range from 0 to 15:85 to 100.
8. The electret filter medium according to claim 7, wherein it is
mainly composed of a lactic acid polymer that produces an endotherm
of at least 0.5 J/g accompanied with crystal fusion.
9. The electret filter medium according to claim 7, wherein the
content of lactide is at most 15% based on the weight of the
medium.
10. The electret filter medium according to claim 7, wherein it has
a surface charge density of at least
1.2.times.10.sup.-9/cm.sup.2.
11. The electret filter medium according to claim 7, further
comprising 0.01 to 0.3 parts by weight of a nucleating agent based
on 100 parts by weight of the lactic acid polymer.
12. A process for producing the electret filter medium according to
claim 7, comprising: applying a direct current corona electric
field to a nonwoven fabric while heating the nonwoven fabric to a
temperature of 60.degree. C. to 140.degree. C., wherein the
nonwoven fabric comprises fibers mainly composed of a lactic acid
polymer; and then cooling the nonwoven fabric to a temperature of
40.degree. C. or lower while applying the electric field to the
nonwoven fabric.
Description
TECHNICAL FIELD
[0001] The invention relates to electret filter media having both
biodegradability and good electret properties at the same time and
to a process for producing the same. Specifically, the invention
relates to electret filter media that are mainly composed of a
biodegradable lactic acid polymer, are for use in capture of fine
particles in a gas, have large surface charge density and thus high
collection efficiency, and can have high particle-collection
efficiency initially and even after exposed to a high temperature
atmosphere, and relates to a process for producing the same.
BACKGROUND ART
[0002] Conventionally, dielectric materials having relatively
long-life electrostatic charges are called "electret" and generally
used for separation materials such as air filters, sanitary
materials such as a mask, and electronic materials such as
microphone materials. Nonwoven fabrics of polyolefin fibers
electrically polarized by high-voltage corona treatment or the like
have mainly been used for electret filter media.
[0003] However, such electret media are characterized by being
almost undegradable or having very low rate of degradation in the
natural environment, although they are effective in producing
electrets. Thus, the products produced from such materials maintain
their form over a long time period when dumped and buried, and thus
problems such as environmental destruction have come to the
surface. In recent years, the electret materials are often used for
applications or products that are difficult to reuse or recycle,
and there has been an increased demand for a reduction of loads on
the environment receiving dumped electret technology products.
[0004] In addition, the conventional nonwoven fabrics of polyolefin
fibers still have insufficient electret performance. For example,
Japanese Patent Application Publication (JP-B) No. 04-42812 (Patent
Document 1) and Japanese Patent No. 2672329 (Patent Document 2)
disclose electret materials having specifically defined surface
charge densities. However, all the examples disclosed in these
publications use melt-blown nonwoven fabrics of polypropylene
fibers, whose surface charge densities remain at up to
1.2.times.10.sup.-9/cm.sup.2.
[0005] For the purpose of solving the above problems,
investigations have been made on electrets using a material
degradable by the metabolism or hydrolysis action of enzymes or
microorganisms. For example, Japanese Patent Application Laid-Open
(JP-A) No. 09-110968 (Patent Document 3) discloses a macromolecular
electret material comprising a lactic acid polymer. In the
document, however, neither the technique for high performance nor
the charge stability over time is taken into account.
[0006] JP-A No. 2001-146672 (Patent Document 4) discloses a charged
nonwoven fabric produced by applying a high voltage to a nonwoven
fabric of lactic acid polymer fibers at an atmospheric temperature
of 50 to 130.degree. C. for about 5 to 30 seconds. However, this
publication only discloses the stability of the electrostatic force
during high-temperature use and is silent on improvement in surface
charge density or improvement in collection efficiency. According
to the inventors' finding, only the application of high voltage at
the above atmospheric temperature is insufficient for an increase
in surface charge density.
[0007] Problems to be Solved by the Invention
[0008] The invention has been made in view of the above problems
and is to provide an electret filter medium that has a large
surface charge density and thus a high collection efficiency, has
stable electret properties during use over a long time period even
after exposed to harsh environments such as a high-temperature
atmosphere, and is biodegradable, and to provide a process for
producing such an electret filter medium.
DISCLOSURE OF THE INVENTION
[0009] Means for Solving the Problems
[0010] For the purpose of solving the above problems, an electret
filter medium having the structure below is provided according to
the invention.
[0011] An electret filter medium comprising a lactic acid polymer
having a molar ratio of an L-lactic acid monomer to a D-lactic acid
monomer in the range from 100 to 85:0 to 15 or from 0 to 15:85 to
100.
[0012] Specifically, the electret filter medium comprises at least
one of:
[0013] (1) a structure obtained by random co-polymerization of
L-lactic acid monomer, D-lactic acid monomer, and poly(L-lactic
acid) (homopolymer) characterized by being produced by
polymerization of an L-lactic acid monomer,
[0014] (2) poly(DL-lactic acid) characterized by having a random
copolymer structure comprising at least 85% by mole of an L-lactic
acid monomer and at most 15% by mole of a D-lactic acid
monomer,
[0015] (3) poly(DL-lactic acid) characterized by having a random
copolymer structure comprising at least 85% by mole of a D-lactic
acid monomer and at most 15% by mole of an L-lactic acid monomer,
and
[0016] (4) poly(D-lactic acid) (homopolymer) characterized by being
produced by polymerization of a D-lactic acid monomer.
[0017] According to the invention, the lactic acid polymer may be
characterized by producing an endotherm of at least 0.5 J/g
accompanied with crystal fusion.
[0018] The lactic acid polymer for use in the electret filter media
of the invention may be characterized by containing at most 15% of
a low molecular weight component such as lactic acid and
lactide.
[0019] The invention may be directed to an electret filter medium
comprising fibers mainly composed of a lactic acid polymer and
having a surface charge density of at least
1.2.times.10.sup.-9/cm.sup.2.
[0020] The invention may also be directed to an electret filter
medium comprising fibers mainly composed of a lactic acid polymer,
wherein 0.01 to 0.3 parts by weight of a nucleating agent is
blended with 100 parts by weight of the lactic acid polymer.
[0021] The invention is also directed to a process for producing an
electret filter medium, comprising: applying a direct current
corona electric field to a nonwoven fabric while heating it to a
temperature of 60.degree. C. to 140.degree. C., wherein the
nonwoven fabric comprises fibers mainly composed of a lactic acid
polymer; and then cooling it to a temperature of 40.degree. C. or
lower while applying the electric field to it.
EMBODIMENTS OF THE INVENTION
[0022] The lactic acid polymer for use in the electret filter media
of the invention comprises: poly(L-lactic acid) having a molecular
structure consisting of an L-lactic acid unit alone; poly(DL-lactic
acid) comprising an L-lactic acid unit and a D-lactic acid unit in
a specific ratio; or poly(D-lactic acid) consisting of a D-lactic
acid unit alone.
[0023] The poly(DL-lactic acid) preferably comprises at least 85%
by mole of, more preferably at least 90% by mole of, most
preferably at least 95% by mole of L-lactic acid or D-lactic
acid.
[0024] The lactic acid polymer having the composition as stated
above is capable of having crystallinity and has a glass transition
temperature of room temperature or higher so that it can form an
electret filter medium having excellent charge stability.
[0025] For improvement in charge stability during temperature rise,
the electret filter medium produced by a DSC (Differential Scanning
Calorimetry) method preferably has a heat of crystal fusion of at
least 0.5 J/g, more preferably of at least 1 J/g, most preferably
of 5 J/g.
[0026] The lactic acid polymer for use in the invention may be
produced by a method of direct condensation polymerization the
starting material of L-lactic acid, D-lactic acid or a mixture of
D-lactic acid and L-lactic acid or by a method including the steps
of synthesizing a cyclic dimer of lactic acid and subsequently
performing ring-opening polymerization of it. The lactic acid dimer
for use in the ring-opening polymerization method may be any
combination of L-lactide, D-lactide, DL-lactide, and
meso-lactide.
[0027] The composition ratio of D-lactic acid and L-lactic acid in
the poly(DL-lactic acid) may be determined through measurement of
the 1H or 13C nuclear magnetic resonance spectrum of a solution of
the poly(DL-lactic acid) in a deuterated chloroform solvent or the
like. This method may be combined with measurement of optical
rotation or DSC measurement to determine whether the polymer is a
random or block copolymer compound of D-lactic acid and L-lactic
acid or a mixture of D- and L-homopolymers. If the resin components
are substantially homogeneous, the composition ratio may be
determined by measurement of optical rotation in a solution state
with the aid of the optical activity of the polymerized lactic acid
unit.
[0028] The number average molecular weight of the lactic acid
polymer for use in the invention is preferably at least 5,000, more
preferably at least 10,000, most preferably at least 50,000, in
terms of charge stability, formability or workability, and
mechanical strength. If the number average molecular weight is low,
the lactic acid polymer (even having the preferred composition) can
have a low melting point and a low glass transition temperature so
that a cause of a reduction in charge stability can be
produced.
[0029] The number average molecular weight may have any upper
limit, and any preferred number average molecular weight may be
selected depending on the forming or working process. The melt
viscosity at a single temperature increases with the number average
molecular weight. Thus, if the number average molecular weight is
too high, the forming or working can be difficult. A melt
temperature-raising method may be used to reduce the melt
viscosity. However, such a method can cause a reduction in
molecular weight by thermal decomposition and cause an increase of
low molecular-weight components so that a cause of a reduction in
charge stability can be produced.
[0030] The number average molecular weight of the lactic acid
polymer for use in the invention may be a number average molecular
weight determined in terms of polystyrene equivalent by a GPC (Gel
Permeation Chromatography) column method.
[0031] Low molecular-weight components such as a lactic acid
monomer and lactide in the resin should preferably be reduced for
the purpose of suppressing charge decay at temperature rise and at
moisture rise. Specifically, the content of such low
molecular-weight components are preferably at most 15%, more
preferably at most 10%, most preferably at most 5%. Any method
capable of achieving this purpose may be used, and specifically the
remaining low molecular-weight components may be reduced using a
recrystallization method, a heat distillation method, a
reduced-pressure distillation method, or the like so that the
thermal decomposition can be suppressed during the working process
and that in view of charge stability, the charge decay can be
suppressed, which would otherwise be caused by temperature rise or
moisture absorption.
[0032] The lactic acid monomer and the lactide may be quantified by
a method of measuring the 1H nuclear magnetic resonance spectrum of
a deuterated chloroform solvent solution. In the quantification
method, the mix ratio may be determined by making a comparison
between the intensity of absorption of a certain known structure
belonging to lactic acid copolymers and the intensity of the
absorption belonging to lactide or lactic acid.
[0033] The carboxyl group at the molecular chain end of the lactic
acid polymer may be esterified with a compound having a hydroxyl
group. Examples of such a hydroxyl compound include higher alcohols
having at least six carbon atoms, such as octyl alcohol, lauryl
alcohol and stearyl alcohol, and glycols such as ethylene glycol,
diethylene glycol and 1,4-butanediol.
[0034] The electret filter medium of the invention preferably has a
surface charge density of at least 1.2.times.10.sup.-9/cm.sup.2,
more preferably of at least 1.5.times.10.sup.-9/cm.sup.2, most
preferably of at least 2.0.times.10.sup.-9/cm.sup.2, wherein the
surface charge density is determined by measurement of thermally
stimulated depolarization current. An electret filter medium with a
larger surface charge density can have higher particle-collection
efficiency. The thermally stimulated depolarization current may be
measured with the apparatus as shown in FIG. 1. Specifically, an
electret filter medium sample 3 is sandwiched between measuring
electrodes 2 in a heating bath 1 capable of controlling the
temperature, and the depolarization current from the electret
filter medium is measured with a high-sensitivity ammeter 4 while
the temperature of the heating bath is raised at a constant rate.
The measured current is recorded by a recorder 6 through a
data-processing system 5. The amount of the depolarization charge
in a specific temperature range is calculated from the integral of
a current curve plotted against temperature. The quotient obtained
by dividing the calculated amount by the area of the measured
sample is a surface charge density. This method is substantially
the same as the method disclosed in JP-B No. 04-42812 or Japanese
Patent No. 2672329.
[0035] It is important that the electret filter media of the
invention should contain 0.01 to 0.3 parts by weight of a
nucleating agent based on 100 parts by weight of the lactic acid
polymer. The nucleating agent is effective in producing
microcrystals in the process of crystallizing the crystalline
macromolecular compound and effective in increasing the
crystallization speed. Various inorganic or organic compounds may
be used as the nucleating agent, while an organic phosphate
compound or a metal carboxylate compound is preferably used.
Examples thereof include sodium
2,2-methylenebis(4,6-di-tert-butylphenyl)phosphate, sodium
bis(4-tert-butylphenyl)phosphate, aluminum para-tert-butylbenzoate,
dehydroabietic acid, and a magnesium, sodium or potassium salt of
dihydroabietic acid.
[0036] It is still unclear how the nucleating agent acts on the
improvement in the electret properties of the lactic acid polymer
in the invention. It is believed that in an electret of lactic acid
polymer, the charge injected from the charging electrode and the
dipole oriented by the electric field should mainly exist in an
amorphous phase and an interface between the crystalline and
amorphous phases, and that such a state should be the true nature
of the electret charge. If the polymer is heated to near glass
transition point, the charge in the amorphous phase should almost
be lost through molecular motion, while the charge at the interface
between the crystalline and amorphous phases should be relatively
stable. It is believed that the nucleating agent should be
effective in increasing the quantity of stable interfaces between
the crystalline and amorphous phases, and effective in reducing the
quantity of unstable amorphous phase.
[0037] In the invention, the amount of the nucleating agent to be
blended should be from 0.01 to 0.3 parts by weight based on 100
parts by weight of the lactic acid polymer. If the amount to be
blended is less than that, the charge-stabilizing effect at high
temperature can be insufficient. Too much amount of the nucleating
agent is not preferred, because the charge-stabilizing effect would
be saturated at a certain level while the spinnability can become
worse and any other influence can be produced.
[0038] In the invention, the nucleating agent may be added to the
lactic acid polymer by a method including the steps of adding a
given amount of the nucleating agent to a powder or pellets of the
lactic acid polymer resin, uniformly dispersing it by means of a
blender, a Henschel mixer or the like, and then melting and
kneading the resin in an extruder, a kneader or the like.
[0039] The electret filter media according to the invention may be
produced by corona treatment of a nonwoven fabric, a woven fabric
or a film placed on a ground plate covered with a solid dielectric
sheet, wherein the nonwoven fabric, the woven fabric or the film
comprises fibers mainly composed of the lactic acid polymer. The
corona treatment may be performed at room temperature but
preferably include the steps of applying a direct current corona
electric field while heating to 60 to 140.degree. C. and then
cooling to 40.degree. C. or lower while applying the electric
field, so that the electric polarization can be greater than that
in the case of the room temperature treatment. It is generally said
that a mechanism for electret production includes orientation of
the dipole, injection of the charge and the like. The lactic acid
polymer has a polar group such as an ester group. Therefore, if the
corona treatment includes the steps of: performing direct current
corona treatment at a temperature of about 60 to 140.degree. C.
which is not lower than its glass transition point and not higher
than its melting point; and then cooling to a temperature of
40.degree. C. or lower which is lower than the glass transition
temperature, the orientation of the dipole can be frozen
concurrently with the injection of the charge by the corona
treatment so that a very large surface charge density can be
obtained.
[0040] In contrast, if the lactic acid polymer is subjected to
corona treatment at near room temperature lower than its glass
transition point, the degree of the orientation of the dipole
cannot be high, and if the application of the high electric field
is stopped when the temperature is still higher than 40.degree. C.,
the oriented dipole cannot be frozen. None of these methods can
produce large surface charge density. If a polar group-free
material such as polypropylene is subjected to the corona treatment
according to the method of the invention, the resulting surface
charge density would be substantially the same as that in the case
of the room temperature corona treatment because of no dipole to be
oriented.
[0041] For example, the charging treatment apparatus as
schematically shown in FIG. 2 may be used to produce the electret
filter media according to the invention. In a preheating zone,
first, the material to be electrically charged is heated to the
temperature of a heating and charging zone. In the heating and
charging zone, a direct current corona electric field is applied to
the material while the material is kept at a constant temperature
of 60 to 140.degree. C. The holding time for which the material is
held in the heating and charging zone is preferably from 5 to 20
seconds. If it is shorter than that, the charging effect can be
insufficient. On the other hand, a time period longer than that
cannot change the effect. In a cooling and charging zone,
thereafter, the material is cooled to a temperature of 40.degree.
C. or lower while the high electric field is applied. The holding
time in the cooling and charging zone is not particularly
restricted. The heating and charging zone and the cooling and
charging zone each have a ground conveyer around which a dielectric
sheet is wrapped. A needle-shaped electrode is placed above the
conveyer, and a direct current high voltage is applied to the
electrode when the charging treatment is performed.
[0042] In the above producing method, the electric field strength
during heating may be the same as or different from that during
cooling. In the latter case, the electric field strength during
cooling is preferably higher than that during heating. This is
because an electric field having a higher intensity than that for
orientation should preferably be applied in order to surely freeze
the dipole oriented by heating and charging, and because during
heating the dielectric sheet for the ground conveyer has an
increased electric conductivity so that if the electric field
strength was too high, spark discharge would frequently occur. For
example, a preferred range of the electric field strength is from
12 to 20 kV/cm during heating and from 15 to 20 kV/cm during
cooling. If the electric field strength is lower than that, the
charging effect can be insufficient. On the other hand, a strength
higher than that can frequently cause spark discharge and thus is
not preferred.
[0043] As stated above, it is assumed that the true nature of the
charge in the electret of the lactic acid polymer should be the
charge injected from the electrode and the dipole oriented by the
electric field. It is believed that the orientation of the dipole
can very easily occur in the state heated to 60 to 140.degree. C.
although it can occur to some extent when the direct current corona
electric field is applied at room temperature. It is also assumed
that if cooling to 40.degree. C. or lower is subsequently performed
while the electric field is applied, the orientation can be frozen,
and the electric polarization can be maintained, so that a very
large surface charge density can be produced.
[0044] It is expected that if a nucleating agent is added,
polypropylene, which is conventionally used as an electret
material, would have an increased amount of stable interfaces
between crystalline and amorphous phase and a decreased amount of
unstable amorphous phase. However, the polypropylene has almost no
polar group to be oriented, and thus even if the charging treatment
is performed according to the above method, the resulting surface
charge density should be almost the same as that in the case of the
room temperature charging treatment.
[0045] The electret filter media of the invention may be used in a
preferred form as needed. Specific examples of such a preferred
form include an extrusion molded film, a sheet, fibers, a nonwoven
fabric, a woven fabric or a cloth, a knit or a knitted fabric, and
a complex of any of the above products with any other material.
[0046] The film is preferably formed using a conventionally known
film-forming method such as a solvent casting method, a melt
extrusion method, and a heat press method. After the film is formed
by such the methods, if desired, uniaxial or biaxial drawing and
heat setting are preferably performed. If drawing orientation and
crystallization treatment are performed, the charge stability
against moisture and temperature can be increased even in the same
composition. The thickness of the film should be from 1 to 50
.mu.m. In the lower range, the thickness of the film is preferably
from 1.5 to 10 .mu.m. If the thickness is more than 50 .mu.m, the
electret forming effect can be saturated.
[0047] Any known spinning method such as a melt extrusion method
and a dry or wet type solvent spinning method may preferably be
used to form the fibers. After the fibers are formed by such the
methods, if desired, drawing treatment and heat setting are
preferably performed. If drawing orientation and crystallization
treatment are performed, the charge stability can further be
increased. It is also preferred that a uniaxially oriented film is
divided into fibrillated film yarns or film split yarns as
conventionally known.
[0048] When the nonwoven fabric is formed and used, the fibers
produced by the above method are cut into an appropriate length and
then subjected to a dry type web-forming method in a carding
machine, a method including the steps of dispersing them in air
current and collecting them, or a method including the steps of
dispersing them in a liquid and making paper from them. A direct
forming method may also be used a melt extrusion method such as a
melt blown method and a spunbond method, and a solution extrusion
method such as a flush spinning method. The fibers for use in the
nonwoven fabric preferably have a fiber diameter of 0.1 to 40
.mu.m. If the diameter is 0.1 .mu.m or less, the pressure drop can
be too high so that a problem can tend to occur in practical use. A
thickness of 40 .mu.m or more is not preferred, because such a
thickness can cause a reduction in filtration performance and can
provide saturation of the electret effect. In particular, the
thickness is preferably from 1 to 30 .mu.m. The basis weight of the
nonwoven fabric is preferably from 5 g/m.sup.2 to 100 g/m.sup.2,
particularly preferably from 10 g/m.sup.2 to 50 g/m.sup.2, in terms
of efficient electret production.
[0049] If desired, the nonwoven fabric sheet may be subjected to
mechanical intermingling with a needle punch, a water jet or the
like, stitching with other fibers, or heat fusion of the component
fibers or an additional binder resin by heat embossing or in an air
through oven. Application or spray of an adhesive or impregnation
with an adhesive is also preferred. Such a process can improve the
strength, the shape holding ability or the like.
[0050] If desired, an antioxidant, an ultraviolet absorbent, a heat
stabilizer, or the like is preferably added to inhibit resin
degradation during processing and use.
[0051] The electret filter media produced as described above
according to the invention can be more resistant to the influence
of temperature or moisture than the conventionally known degradable
electret filter media and can be superior in practical
applications.
BRIEF DESCRIPTION OF DRAWINGS
[0052] FIG. 1 is a schematic diagram showing an apparatus for
measuring thermally stimulated depolarization current; and
[0053] FIG. 2 is a schematic diagram showing an apparatus for
charging treatment for use in producing of the electret filter
media of the invention.
[0054] In the drawings, reference numeral 1 represents a
temperature-controllable heating bath, 2 a measuring electrode, 3
an electret filter medium sample, 4 a high-sensitivity ammeter, 5 a
data-processing system, 6 a recorder, 7 a nonwoven fabric, 8 a
preheating zone, 9 a heating and charging zone, 10 a cooling and
charging zone, 11 a direct current high voltage power source, 12 a
corona needle electrode, 13 a ground conveyer wrapped with a
dielectric sheet, 14 a ground wire, and 15 an electret filter
medium, respectively.
BEST MODE FOR CARRYING OUT THE INVENTION
[0055] While the invention is described with reference to the
examples below, the invention can be carried out in various manners
as stated above, and the examples below are not intended to limit
the scope of the invention.
[0056] (Preparation of Lactic Acid Polymers)
[0057] (Preparation Examples 1 to 8)
[0058] L-lactide and/or D-lactide were used as a polymerization
material and used at different addition percentages as shown in
Table 1 for Preparation Examples 1 to 7. In a reaction vessel, the
material or materials were mixed with, as polymerization catalysts,
tin octanoate and lauryl alcohol. After the air in the vessel was
replaced with nitrogen, the mixture was heated under reduced
pressure at 130.degree. C. for 4 hours so that ring-opening
polymerization was carried out. Purification was performed by a
recrystallization method including the steps of dissolving the
reaction product in chloroform and diluting the chloroform solution
with hexane. Physical property values of each resulting resin are
shown in Table 1. The reaction product of Preparation Example 1
before the recrystallization was harvested and used as Preparation
Example 8.
[0059] [Measurement of Number Average Molecular Weight]
[0060] The number average molecular weight of each of the
poly(lactic acid) polymers of Preparation Examples 1 to 8 was
determined in terms of polystyrene equivalent by a GPC column
method. [Measurement of Crystal Melting Point and Glass Transition
Temperature]
[0061] The lactic acid polymers of Preparation Examples 1 to 8 were
measured for crystal melting point and glass transition temperature
by the DSC method at a rate of temperature rising of 10.degree.
C./min.
[0062] A melt blown nonwoven fabric was produced from each
resulting lactic acid polymer. An electret filter medium was
prepared with the nonwoven fabric by each of two charging treatment
methods and then measured for filtration performance.
[0063] [Measurement of Heat of Crystal fusion]
[0064] The melt blown nonwoven fabric was measured for heat of
crystal fusion by the DSC method at a rate of temperature rising of
10.degree. C./min.
[0065] [Charging Treatment 1 (Corona Charging at Room
Temperature)]
[0066] The melt blown nonwoven fabric was subjected to corona
charging treatment at +20 kV/cm for 10 seconds with a needle
electrode so that an electret filter medium sample was
prepared.
[0067] [Charging Treatment 2 (Heating and Corona Charging)]
[0068] The melt blown nonwoven fabric was subjected to charging
treatment in the apparatus shown in FIG. 2 to obtain an electret
filter medium sample. In the heating and charging zone, the
electric field strength was +15 kV/cm, and the holding time was 7
seconds; In the cooling and charging zone, the electric field
strength was +19 kV/cm, and the holding time was 40 seconds.
[0069] [Evaluation of Filtration Properties]
[0070] The electret filter medium sample was placed in a duct, and
air was filtered at a controlled rate of 5 cm/second, when a
difference between the static pressures upstream and downstream
from the electret filter medium was measured with pressure gauges
as a pressure drop. The particle-collection efficiency (%) was
evaluated at 5 cm/second with NaCl particles 0.3 .mu.m in diameter.
First, the particle-collection efficiency immediately after the
charging treatment (E0) was measured, and then the
particle-collection efficiency after a performance-degrading
treatment (E1) was measured. The performance retention rate was
calculated according to Formula 1. The performance-degrading
treatment was carried out under either the conditions of 25.degree.
C..times.50RH %.times.one month storage or the conditions of
80.degree. C..times.24 hours storage.
[0071] [Measurement of Surface Charge Density]
[0072] FIG. 1 is a schematic diagram showing an apparatus for
measuring thermally stimulated depolarization current. A 20 mm.phi.
electret filter medium sample was placed between the measuring
electrodes, and the measurement was performed at a rate of
temperature rising of 4.degree. C./minute in the temperature range
from 30.degree. C. to 170.degree. C. The amount of depolarized
charge in the range from 30 to 170.degree. C. was calculated from
the resulting depolarization current curve and then divided by the
area of the sample (20 mm.phi.). The resulting quotient was defined
as the surface charge density of the sample.
EXAMPLE 1
[0073] In a melt blown method, the resin of Preparation Example 1
was used to prepare a nonwoven fabric with an basis weight of 41
g/m.sup.2 and an average fiber diameter of 2.3 .mu.m. The resulting
nonwoven fabric was then subjected to the method of Charging
Treatment 1 so that an electret filter medium sample (Example 1)
was prepared. The sample was measured for surface charge density,
filtration properties and heat of crystal fusion. The filtration
properties were measured immediately after the charging treatment
and after the sample was stored at 25.degree. C. and 50RH % for a
month. The result is shown in Table 2.
EXAMPLE 2
[0074] In a melt blown method, the resin of Preparation Example 2
was used to prepare a nonwoven fabric with an basis weight of 39
g/m.sup.2 and an average fiber diameter of 2.3 .mu.m. The nonwoven
fabric was then subjected to the method of Charging Treatment 1 so
that an electret filter medium sample (Example 2) was prepared. The
same evaluation as in Example 1 was performed. The result is shown
in Table 2.
EXAMPLE 3
[0075] In a melt blown method, the resin of Preparation Example 6
was used to prepare a nonwoven fabric with an basis weight of 40
g/m.sup.2 and an average fiber diameter of 2.4 .mu.m. The nonwoven
fabric was then subjected to the method of Charging Treatment 1 so
that an electret filter medium sample (Example 3) was prepared. The
same evaluation as in Example 1 was performed. The result is shown
in Table 2.
EXAMPLE 4
[0076] In a melt blown method, the resin of Preparation Example 7
was used to prepare a nonwoven fabric with an basis weight of 40
g/m.sup.2 and an average fiber diameter of 2.5 .mu.m. The nonwoven
fabric was then subjected to the method of Charging Treatment 1 so
that an electret filter medium sample (Example 4) was prepared. The
same evaluation as in Example 1 was performed. The result is shown
in Table 2.
COMPARATIVE EXAMPLE 1
[0077] In a melt blown method, the resin of Preparation Example 3
was used to prepare a nonwoven fabric with an basis weight of 40
g/m.sup.2 and an average fiber diameter of 2.5 .mu.m. The nonwoven
fabric was then subjected to the method of Charging Treatment 1 so
that an electret filter medium sample (Comparative Example 1) was
prepared. The same evaluation as in Example 1 was performed. The
result is shown in Table 2.
COMPARATIVE EXAMPLE 2
[0078] In a melt blown method, the resin of Preparation Example 4
was used to prepare a nonwoven fabric with an basis weight of 40
g/m.sup.2 and an average fiber diameter of 2.4 .mu.m. The nonwoven
fabric was then subjected to the method of Charging Treatment 1 so
that an electret filter medium sample (Comparative Example 2) was
prepared. The same evaluation as in Example 1 was performed. The
result is shown in Table 2.
COMPARATIVE EXAMPLE 3
[0079] In a melt blown method, the resin of Preparation Example 5
was used to prepare a nonwoven fabric with an basis weight of 40
g/m.sup.2 and an average fiber diameter of 2.4 .mu.m. The nonwoven
fabric was then subjected to the method of Charging Treatment 1 so
that an electret filter medium sample (Comparative Example 3) was
prepared. The same evaluation as in Example 1 was performed. The
result is shown in Table 2.
COMPARATIVE EXAMPLE 4
[0080] In a melt blown method, the resin of Preparation Example 8
was used to prepare a nonwoven fabric with an basis weight of 40
g/m.sup.2 and an average fiber diameter of 2.4 .mu.m. The nonwoven
fabric was then subjected to the method of Charging Treatment 1 so
that an electret filter medium sample (Comparative Example 4) was
prepared. The same evaluation as in Example 1 was performed. The
result is shown in Table 2.
[0081] The results of Examples 1 to 4 and Comparative Examples 1 to
3 indicate that in storage at room temperature, Examples 1 to 4
with crystallinity provide better stability over time than
amorphous Comparative Examples 1 to 3. It is apparent that Examples
1 to 4 each with a relatively high content of L- or D-lactic acid
tend to have good charge retention properties. From the result of
Comparative Example 4, it is also apparent that a relatively high
content of lactide can lead to poor stability over time.
1 TABLE 1 Preparation Preparation Preparation Preparation
Preparation Preparation Preparation Preparation Example 1 Example 2
Example 3 Example 4 Example 5 Example 6 Example 7 Example 8
Fraction of 100 90 75 50 25 10 0 100 Added L-Lactide (%) Fraction
of 0 10 25 50 75 90 100 0 Added D-Lactide (%) Number 58,000 56,000
50,000 52,000 55,000 62,000 60,000 34,000 Average Molecular Weight
Glass 58 57 54 53 55 56 58 36 Transition Temperature (.degree. C.)
Melting 175 123 -- -- -- 122 170 155 Point (.degree. C.) Percentage
>1 >1 >1 >1 >1 >1 >1 16 of Residual Lactide
(%)
[0082]
2 TABLE 2 Particle- Particle- Collection Collection Efficiency
Efficiency After Immediately Storage at Heat Surface After
25.degree. C. and of Charge Pressure Charging 50RH % for
Performance Crystal Density Drop Treatment a Month Retention fusion
(C/cm.sup.2) (mmAq) (%) (%) (%) (J/g) Example 1 0.9 .times.
10.sup.-9 6.9 99.98 99.93 85.3 26 Example 2 0.9 .times. 10.sup.-9
6.8 99.98 99.82 74.2 10 Example 3 0.9 .times. 10.sup.-9 6.6 99.96
99.72 78.2 8 Example 4 0.9 .times. 10.sup.-9 6.5 99.96 99.91 89.6
30 Comparative 0.9 .times. 10.sup.-9 6.5 99.95 98.12 52.2 0 Example
1 Comparative 0.9 .times. 10.sup.-9 6.7 99.97 96.85 42.6 0 Example
2 Comparative 0.9 .times. 10.sup.-9 6.8 99.98 97.88 45.2 0 Example
3 Comparative 0.9 .times. 10.sup.-9 6.7 99.95 98.46 54.9 12 Example
4
EXAMPLE 5
[0083] The melt blown nonwoven fabric as used in Example 1 was
subjected to the method of Charging Treatment 2 (heating and
charging at 90.degree. C. and cooling to 35.degree. C. under
applied electric field) so that an electret filter medium sample
(Example 5) was obtained. Table 3 shows its surface charge density,
its filtration properties immediately after the charging treatment
and its heat of crystal fusion.
EXAMPLE 6
[0084] The melt blown nonwoven fabric as used in Example 1 was
subjected to the method of Charging Treatment 2 (heating and
charging at 130.degree. C. and cooling to 35.degree. C. under
applied electric field) so that an electret filter medium sample
(Example 6) was obtained. The same evaluation as in Example 5 was
performed. The result is shown in Table 3.
REFERENCE EXAMPLE 1
[0085] The melt blown nonwoven fabric as used in Example 1 was
subjected to the method of Charging Treatment 2 (heating and
charging at 90.degree. C. and cooling to 70.degree. C. under
applied electric field) so that an electret filter medium sample
(Example 6) was obtained. The same evaluation as in Example 5 was
performed. The result is shown in Table 3.
COMPARATIVE EXAMPLE 5
[0086] A melt blown polypropylene nonwoven fabric with an average
fiber diameter of 2.3 .mu.m and an basis weight of 40 g/m.sup.2 was
subjected to the method of Charging Treatment 2 (heating and
charging at 90.degree. C. and cooling to 35.degree. C. under
applied electric field). The same evaluation as in Example 5 was
performed. The result is shown in Table 3.
3 TABLE 3 Particle-Collection Surface Efficiency Heat of Charge
Pressure Immediately After Crystal Density Drop Charging fusion
(C/cm.sup.2) (mmAq) Treatment (%) [J/g] Example 5 1.9 .times.
10.sup.-9 6.9 99.9991 28 Example 6 2.3 .times. 10.sup.-9 6.9
99.9998 30 Reference 0.6 .times. 10.sup.-9 6.9 99.90 28 Example 1
Comparative 0.7 .times. 10.sup.-9 6.9 99.94 -- Example 5
[0087] The above results indicate that the electret filter medium
sample of each example (prepared by the process including the steps
of applying direct current corona electric field to the melt blown
nonwoven fabric of lactic acid polymer while heating it and then
cooling it under the applied electric field) has a larger surface
charge density and a higher collection efficiency than those of the
reference example and the comparative example.
EXAMPLE 7
[0088] One hundred parts by weight of the resin of Preparation
Example 1 was mixed with 0.05 parts by weight of sodium
2,2-methylenebis(4,6-di-ter- t-butylphenyl)phosphate (NA-11 (trade
name) manufactured by ASAHI DENKA Co., Ltd.). The resulting resin
was used to prepare a melt-blown nonwoven poly(lactic acid) fabric
with an average fiber diameter of 2.3 .mu.m and an basis weight of
40 g/m.sup.2. The nonwoven fabric was subjected to the method of
Charging Treatment 2 (heating and charging at 90.degree. C. and
cooling to 35.degree. C. under applied electric field) so that an
electret filter medium sample (Example 7) was obtained. Table 4
shows its surface charge density, its filtration properties and its
heat of crystal fusion. The filtration properties were measured
immediately after the charging treatment and after it was stored at
80.degree. C. for 24 hours.
EXAMPLE 8
[0089] One hundred parts by weight of the resin of Preparation
Example 1 was mixed with 0.1 parts by weight of aluminum
para-tert-butylbenzoate (AL-PTBBA (trade name) manufactured by
Shell Chemicals Japan Ltd.). The resulting resin was used to
prepare a melt-blown nonwoven poly(lactic acid) fabric with an
average fiber diameter of 2.3 .mu.m and an basis weight of 40
g/m.sup.2. The nonwoven fabric was subjected to the method of
Charging Treatment 2 (heating and charging at 90.degree. C. and
cooling to 35.degree. C. under applied electric field) so that an
electret filter medium sample (Example 8) was obtained. The same
evaluation as in Example 7 was performed. The result is shown in
Table 4.
EXAMPLE 9
[0090] One hundred parts by weight of the resin of Preparation
Example 1 was mixed with 0.1 parts by weight of a partial metal
salt of dehydroabietic acid (KM-1500 (trade name) manufactured by
Arakawa Chemical Industries, Ltd.). The resulting resin was used to
prepare a melt-blown nonwoven poly(lactic acid) fabric with an
average fiber diameter of 2.3 .mu.m and an basis weight of 40
g/m.sup.2. The nonwoven fabric was subjected to the method of
Charging Treatment 2 (heating and charging at 90.degree. C. and
cooling to 35.degree. C. under applied electric field) so that an
electret filter medium sample (Example 9) was obtained. The same
evaluation as in Example 7 was performed. The result is shown in
Table 4.
REFERENCE EXAMPLE 2
[0091] The electret filter medium sample of Example 5 was also
evaluated for filtration properties after stored at 80.degree. C.
for 24 hours. The result is shown in Table 4 as Reference Example
2.
4 TABLE 4 Particle- Particle- Collection Collection Efficiency
Efficiency Immediately After Surface After Storage at Heat of
Charge Pressure Charging 80.degree. C. for Performance Crystal
Density Drop Treatment 24 hours Retention fusion (C/cm.sup.2)
(mmAq) (%) (%) (%) (J/g) Example 20 .times. 10.sup.-9 6.9 99.9990
99.9938 84.1 33 7 Example 2.3 .times. 10.sup.-9 6.9 99.9989 99.9956
88.1 30 8 Example 1.9 .times. 10.sup.-9 6.9 99.9991 99.9976 91.2 34
9 Reference 1.9 .times. 10.sup.-9 6.9 99.9991 99.8665 38.8 28
Example 2
[0092] It is apparent that the electret filter medium sample of
each example, which is obtained by charging treatment of the melt
blown nonwoven fabric of the nucleating agent-containing
poly(lactic acid), has a large surface charge density and a high
collection efficiency initially and after the treatment at
80.degree. C. In the case of the melt blown nonwoven fabric of the
nucleating agent-free poly(lactic acid) (Example 5), the initial
collection efficiency is high, but after the 80.degree. C.
treatment, the collection efficiency is significantly reduced.
INDUSTRIAL APPLICABILITY
[0093] The electret filter medium of lactic acid polymer according
to the invention has a large surface charge density and high
particle-collection efficiency initially and even after exposed to
a high-temperature atmosphere and resist to degradation over time
by rising in temperature during use. Thus, such electret filter
medium can be substituted for conventionally known electrets
comprising a hardly degradable resin. On the other hand, such
electret filter media comprise a nonwoven fabric of fibers mainly
composed of lactic acid polymer so that they can exhibit their
dedradability in nature upon disposition and can contribute to the
reduction of loads on the environment, and therefore they are
useful.
* * * * *