Abstrict
The split PTFE yarn and PTFE filament which are bulky and excellent
in the feeling and have good intermingling property can be obtained
and the cotton-like PTFE materials can be produced therefrom efficiently
at a low percentage of dropped fibers. Further the filter cloth
for dust collection which is obtained from those cotton-like materials
has a high collection efficiency. The present invention can provide
the split yarn having a network structure and obtained by splitting
a uniaxially stretched PTFE film in the stretched direction with
a needle blade roll; the filament having branches and obtained by
cutting the network structure of the split yarn in a longitudinal
direction; the methods of production thereof; the method of producing
the cotton-like materials by cutting the split yarn or filament
and then opening; and a filter cloth for dust collection obtained
from the cotton-like materials.
Claims
What is claimed is:
1. A split yarn which has a network structure and is obtained by
splitting a uniaxially stretched polytetrafluoroethylene film in
the stretched direction with at least a pair of needle blade rolls.
2. The split yarn of claim 1, wherein the polytetrafluoroethylene
film is a semi-sintered article.
3. The split yarn of claim 1, wherein the polytetrafluoroethylene
film is a sintered article.
4. A polytetrafluoroethylene filament which has branches and is
obtained by cutting the network structure of the split yarn of claim
1 in the longitudinal direction.
5. The filament of claim 4, wherein the polytetrafluoroethylene
film is a semi-sintered article.
6. The filament of claim 4, wherein the polytetrafluoroethylene
film is a sintered article.
7. A method of producing a split yarn having a network structure,
wherein a uniaxially stretched polytetrafluoroethylene film is split
in the stretched direction with at least a pair of needle blade
rolls.
8. The method of claim 7, wherein the polytetrafluoroethylene film
is a semi-sintered article.
9. The method of claim 7, wherein the polytetrafluoroethylene film
is a sintered article.
10. The method of claim 7, wherein the number of needles of the
needle blade roll is from 20 to 100 per cm.sup.2.
11. The method of claim 7, wherein the uniaxially stretched film
is heat-treated at a temperature of not less than the temperature
at uniaxially stretching.
12. A method of producing a polytetrafluoroethylene filament having
branches, wherein a uniaxially stretched polytetrafluoroethylene
film is split in the stretched direction with at least a pair of
needle blade rolls to give a network structure and then the network
structure is cut in the longitudinal direction.
13. The method of claim 12, wherein the polytetrafluoroethylene
film is a semi-sintered article.
14. The method of claim 12 wherein the polytetrafluoroethylene
film is a sintered article.
15. The method of claim 12, wherein the number of needles of the
needle blade rolls is from 20 to 100 per cm.sup.2.
16. The method of claim 12, wherein the uniaxially stretched film
is heat-treated at a temperature of not less than the temperature
at uniaxially stretching.
17. The method of claim 12, wherein the network structure is passed
through the blades in the form of a comb to be cut in the longitudinal
direction.
18. A method of producing cotton-like polytetrafluoroethylene materials,
wherein the split yarn obtained by the method of any of claim 7
is cut to a given length and then opened.
19. A method of producing cotton-like polytetrafluoroethylene materials,
wherein the filament obtained by the method of any of claim 12 is
cut to a given length and then opened.
20. The method of claim 18, wherein the polytetrafluoroethylene
fibers providing the cotton-like materials have at least one loop
structure and/or branch structure per 5 cm of a fiber length, a
fineness of 2 to 200 deniers, the number of crimps of 1 to 15 per
20 mm length and irregular section.
21. A filter cloth for dust collection obtained from the cotton-like
materials produced by the method of any of claim 18.
Description TECHNICAL FIELD
The present invention relates to a bulky split yarn and filament
of polytetrafluoroethylene (PTFE), a method of producing cotton-like
materials comprising the above-mentioned split yarn and filament
and a filter cloth for dust collection.
BACKGROUND ART
In recent years, non-woven fabrics comprising synthetic fibers,
by making the best use of characteristics of those fibers, are extending
their applications into various fields, such as clothing materials,
medical materials, engineering and building materials, and materials
for industrial use.
Among them, non-woven fabrics containing PTFE fibers are excellent
in heat resistance, chemical resistance and abrasion resistance,
and are expected to be further developed as highly functional non-woven
fabrics.
Cotton-like PTFE materials being made into the non-woven fabrics
are gathered PTFE fibers, and so far have been made in such manners
as mentioned below:
(1) A process for producing filaments and then cutting to a desired
length.
The process for producing PTFE filaments is roughly classified
into the following two processes.
(1a) An emulsion spinning method disclosed in U.S. Pat. No. 2,772,444.
This method comprises extrusion spinning of a viscose binder, and
the like containing PTFE particles, and then sintering to obtain
the filaments having a uniform figure, the section of which is defined
by a shape of nozzles. Major problems of that method are such that
a binder remains as a carbonaceous residual after sintering, the
obtained PTFE filaments are colored in a dark brown, and even if
the carbonaceous residual is oxidized to be discolored, an original
purity cannot be maintained. The method has also a drawback that
since a complicated step is employed, cost is high.
(1b) A method disclosed in JP-B-22915/1961 or JP-B-8769/1973. This
method comprises stretching of fibers obtained by slitting a PTFE
film to a desired width. A problem of this method is that when the
slit width is made narrower, the finer the obtained fiber is, the
more easily the fibers are broken at the time of stretching.
Both PTFE fibers obtained by the methods (1a) and (1b) have a low
friction coefficient and a high specific gravity inherent to the
PTFE, and therefore are not intermingled sufficiently with each
other even if having been crimped. (JP-B-22621/1975)
(2) A process for preparing PTFE fibrous powder in the form of
a pulp and making a sheet-like material therefrom by paper making
process (U.S. Pat. No. 3,003,912 and JP-B-15906/1969).
The method of the above-mentioned U.S. patent is to cut PTFE rod,
cord or filament obtained by a paste extrusion, to a short length
and to apply a shearing force to obtain PTFE fibers.
JP-B-15906/1969 discloses a method for making fibers by applying
a shearing force to the PTFE powder.
Any of the fibrous powder obtained by the above-mentioned methods
can be made up to a sheet-like material by paper making process
but cannot be made into a non-woven fabric by the use of a carding
machine, needle punching machine, or the like as they are short
in fiber length and in the form of a pulp.
In order to eliminate those drawbacks of the PTFE fibers, the present
inventors have developed a method of producing PTFE staple fibers
(relatively short fibers) and cotton-like PTFE materials directly
by tearing to open a uniaxially stretched PTFE film with mechanical
force and filed the patent application (JP-A-78264/1993). The PTFE
staple fibers obtained by that method are bulky and contain fibers
having good intermingling property but also contain a lot of short
fibers which do not contribute to enhancement of intermingling property.
Those short fibers drop in a carding step for producing non-woven
fabrics, and thus yield is not good.
An object of the present invention is to provide a method of producing
cotton-like PTFE materials containing PTFE fibers having excellent
intermingling property.
Another object of the present invention is to provide PTFE split
yarns having a network structure, which are usable for producing
the cotton-like PTFE materials, and a method of producing the PTFE
split yarns.
Yet another object of the present invention is to provide a PTFE
filament usable for producing the cotton-like PTFE materials and
a method of producing the PTFE filaments.
Yet another object of the present invention is to provide a filter
cloth for dust collection which can be obtained from the cotton-like
materials produced by the above-mentioned method of producing the
cotton-like PTFE materials.
DISCLOSURE OF THE INVENTION
The present invention relates to a method of producing split yarns
having a network structure by splitting a uniaxially stretched PTFE
film in the stretched direction with needle blade rolls, and to
the PTFE split yarns obtained thereby.
The present invention also relates to a method of producing PTFE
filaments having branches by cutting the network structure of the
split yarns in the longitudinal direction, and to the PTFE filaments
obtained thereby.
The present invention further relates to a method of producing
cotton-like PTFE materials by cutting the PTFE split yarns or PTFE
filaments to a given length and then opening.
The present invention further relates to a filter cloth for dust
collection obtained from the cotton-like PTFE materials by the method
of production thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view showing a state of a split yarn of
the present invention being enlarged.
FIG. 2 is an example of a crystalline melting curve obtained from
a differential scanning calorimeter in a heating process (1) of
an unsintered PTFE, which is used for measuring a crystalline conversion
ratio of a semi-sintered PTFE.
FIG. 3 is an example of a crystalline melting curve of the DSC
in a heating process (3) of a sintered PTFE, which is used for measuring
a crystalline conversion ratio of a semi-sintered PTFE.
FIG. 4 is an example of a crystalline melting curve of the DSC
in a heating process of a semi-sintered PTFE, which is used for
measuring a crystalline conversion ratio of a semi-sintered PTFE.
FIG. 5 is a diagrammatic sectional view of the example of an opening
machine which can be used in the process for preparation of the
present invention.
FIGS. 6(a) to 6(d) are diagrammatic views showing a branched structure
of the PTFE fibers being contained in the cotton-like PTFE materials
of the present invention.
FIG. 7 is a photograph (.times.1.5) showing a shape of the fiber
prepared in Example 21 of the present invention.
FIG. 8 is a photograph (.times.1.5) showing a shape of the fiber
obtained in Example 21 of the present invention.
FIG. 9 is a photograph (.times.1.5) showing a shape of the fiber
obtained in Example 21 of the present invention.
FIG. 10 is a diagrammatic sectional view of a hitherto known carding
machine, which can be used for preparing a non-woven fabric from
the cotton-like materials of the present invention.
FIG. 11 is an explanatory view showing an example of arrangement
of needle blades on the roll surface of the opening machine shown
in FIG. 10.
FIG. 12 is a diagrammatic sectional view explaining an angle (.theta.)
of a needle of the needle blades of the opening machine shown in
FIG. 10.
FIG. 13 is an explanatory view of an apparatus for measuring collection
efficiency in Example 41 and Comparative Example 4.
FIG. 14 is a graph showing relation between the collection efficiency
and the particle size which were measured in Example 41 and Comparative
Example 4.
PREFERRED EMBODIMENTS OF THE INVENTION
The major feature of the present invention is to once split a uniaxially
stretched article of a PTFE film (hereinafter referred to as "uniaxially
stretched PTFE film") without tearing directly into staple
fibers. In the present invention, a needle blade roll, preferably
a pair of needle blade rolls are used as means for obtaining a network
structure by splitting the uniaxially stretched PTFE film in the
stretched direction. The above-mentioned network structure is such
that the uniaxially stretched PTFE film is not split into separate
fibers and when spread in the transverse direction (a direction
at a right angle to the film feeding direction) of the film after
splitting, the film becomes net-like as shown in FIG. 1. In order
to obtain such a network structure, the relation of the feed speed
of the uniaxially stretched PTFE film and the rotation speed of
the needle blade rolls, and the arrangement and the number of needles
of the needle blade rolls may be properly selected.
The split yarn of the present invention is a uniaxially stretched
PTFE film having a network structure as it is or is obtained in
the form of a cord by bundling the uniaxially stretched PTFE films
having a network structure.
Each embodiment of the present invention is explained hereinbelow.
Techniques explained hereinafter are those common to all embodiments
unless particularly noted.
As the PTFE film used in the present invention, there are, for
example, those obtained with a paste extrusion molding of PTFE fine
powder (PTFE fine powder obtained by an emulsion polymerization)
or those obtained with a compression molding of PTFE molding powder
(PTFE powder obtained by a suspension polymerization). In the present
invention, the film includes those in such a form as tape, sheet
and ribbon in addition to film. A thickness thereof is 5 to 300
.mu.m, preferably 5 to 150 .mu.m in order to conduct a stable stretching.
A PTFE film can be obtained by calendering the extrudate molded
by paste extrusion of PTFE fine powder or cutting a compression-molded
PTFE powder.
The PTFE film to be uniaxially stretched is preferably semi-sintered
or sintered one. The semi-sintered PTFE is obtained by heat-treating
the unsintered PTFE at a temperature between the melting point (about
327.degree. C.) of the sintered PTFE and the melting point (about
337.degree. C. to about 347.degree. C.) of the unsintered PTFE.
A crystalline conversion ratio of the semi-sintered PTFE is usually
0.10 to 0.85, preferably 0.15 to 0.70.
The crystalline conversion ratio of the semi-sintered PTFE article
is determined as follows:
10.0.+-.0.1 mg of a sample of the semi-sintered PTFE is prepared.
Since the sintering proceeds from the surface toward the inner portion,
the degree of the semi-sintering of the article is not necessarily
homogeneous throughout the article, and the semi-sintering is less
homogeneous in a thicker article than in a thinner one. In the preparation
of the sample, it is, therefore, to be noted that various portions
having various degrees of semi-sintering must be sampled uniformly.
With thus prepared sample, at first the crystalline melting curve
is made in the following method.
The crystalline melting curve is recorded by means of a DSC (Model
DSC-2 of Perkin-Elmer). First the sample of the unsintered PTFE
is charged in an aluminum-made pan of the DSC, and the heat of fusion
of the unsintered PTFE and that of the sintered PTFE are measured
as follows:
(1) The sample is heated to 277.degree. C. at a heating rate of
160.degree. C./min and then from 277.degree. C. to 360.degree. C.
at a heating rate of 10.degree. C./min.
An example of a crystalline melting curve recorded during this
heating step is shown in FIG. 2. A temperature at the highest peak
of an endothermic curve appearing in this step is defined as "a
melting point of the unsintered PTFE or PTFE fine powder".
(2) Immediately after heating to 360.degree. C., the sample is
cooled to 277.degree. C. at a cooling rate of 80.degree. C./min,
and
(3) Again the sample is heated to 360.degree. C. at a heating rate
of 10.degree. C./min.
An example of a crystalline melting curve recorded during the heating
step (3) is shown in FIG. 3. A temperature at a peak of an endothermic
curve appearing in the heating step (3) is defined as "a melting
point of the sintered PTFE".
The heat of fusion of the unsintered or sintered PTFE is proportional
to the area between the endothermic curve and a base line which
is drawn from a point on the DSC chart at 307.degree. C. (580.degree.
K) and tangential with the curve at the right-hand foot of the endothermic
curve.
Secondly, a crystalline melting curve for the semi-sintered PTFE
is recorded following the step (1), and an example of that curve
is shown in FIG. 4.
Then, the crystalline conversion ratio is defined by the following
equation :
wherein S.sub.1 is the area of the endothermic curve of the unsintered
PTFE (cf. FIG. 2), S.sub.2 is the area of the endothermic curve
of the sintered PTFE (cf. FIG. 3) and S.sub.3 is the area of the
endothermic curve of the semi-sintered PTFE (cf. FIG. 4).
The crystalline conversion ratio of the semi-sintered PTFE article
of the invention is usually from 0.10 to 0.85, preferably from 0.15
to 0.70.
The sintered PTFE can be obtained by heat-treating the unsintered
PTFE or semi-sintered PTFE at a temperature of not less than the
melting point of the unsintered PTFE.
The uniaxial stretching of the present invention can be carried
out by the conventional methods such as stretching between the two
rolls which have been heated to usually about 250.degree. to 320.degree.
C. and have different rotation speeds. The stretching ratio is preferably
changed depending on the degree of sintering, and is at least 6
times, preferably not less than 10 times in case of the semi-sintered
PTFE, and at least 3 times, preferably not less than 3.5 times in
case of the sintered PTFE. This is because the orientation is necessary
to be increased by stretching since the tearing property of the
semi-sintered PTFE in the longitudinal direction is worse as compared
with that of the sintered PTFE. Also in order to obtain fine fibers,
it is desirable to stretch at as high ratio as possible, but the
attainable stretching ratio is usually about 10 times in case of
the sintered PTFE, and about 30 times in case of the semi-sintered
PTFE.
In case of a too low stretching ratio, there occurs a trouble that
the stretched PTFE film is entangled in the needle blades of the
needle blade rolls for the splitting.
It is preferable that a thickness of the uniaxially stretched PTFE
film after the uniaxial stretching is from 1 to 100 .mu.m, particularly
1 to 50 .mu.m. When larger than 100 .mu.m, split yarns obtained
after the splitting, filaments and cotton-like materials become
in the rigid state, and the feeling of products obtained therefrom
becomes worse. The uniaxially stretched PTFE films having a thickness
of less than 1 .mu.m are difficult to produce industrially.
In case of the semi-sintered PTFE and the sintered PTFE, an additional
heat treating after the uniaxial stretching can prevent the shrinkage,
due to a heat, of the split yarn and fiber obtained after splitting,
maintain bulkiness, and particularly in case of the cotton-like
materials, prevent lowering of permeability. The heat treating temperature
is not less than the temperature at the time of uniaxial stretching,
that is, usually not less than 300.degree. C. The heat treating
temperature can be selected from the range up to about 380.degree.
C., if necessary.
The so-obtained PTFE film uniaxially stretched is split in the
stretched direction with the needle blade rolls to be a net-like
form.
There are, for example, the following means for splitting.
The uniaxially stretched PTFE film is passed through at least a
pair of rotating needle blade rolls for splitting to obtain net-like
structure. For splitting, an equipment described, for example, in
JP-A-180621/1983 can be employed.
This equipment described in JP-A-180621/1983 has a pair of needle
blade rolls. The present invention can be also executed by using
one needle blade roll as described in JP-B-1371/1977, but the splitting
conditions are limited. For example, in case where the splitting
is conducted from one side of the stretched film with one needle
blade roll, when the number of needles of the needle blade roll
is increased (when the split width is narrowed), the points of the
needles are difficult to bore into the film and the splitting cannot
be conducted particularly at the edges (ears) of the film, whereas
it depends on the film thickness and stretching ratio. In this point,
the film can be split uniformly up to its edges by using a pair
of needle blade rolls engaged with each other. The preferred embodiment
is explained in accordance with FIG. 5.
In FIG. 5, numeral 30 is a uniaxially stretched PTFE film, which
is fed to a pair of the needle blade rolls 31 and 32 by means of
a transfer means (not illustrated). At the rear side of the rolls
31 and 32, there is provided receiving means (not illustrated).
The film 30 passes between the needle blade rolls 31 and 32, and
during passing therebetween, the film is split with the needle blades
34 and 35 provided on the outer surfaces of the needle blade rolls
31 and 32 and is collected by the receiving means.
The rotation speed and direction of the needle blade rolls, the
film feed speed and the angle of needles can be optionally selected,
and in the present invention, it is preferable that the film feeding
direction is the same as the rotation direction of the roll.
It is preferable that the relation of the uniaxially stretched
PTFE film feed speed (v1) and the needle blade roll rotation speed
(peripheral speed (v2)) is v2>v1. In that case, the pattern of
the network structure is usually a geometrical pattern (FIG. 1)
made by a difference in the speed of the needle blades passing the
surfaces of the film. If v2 is exceedingly higher than v1, the pattern
does not become the network structure and the film is fibrillated
(becoming staple fibers).
An angle (.theta.) of needle is preferably 45.degree. to 90.degree.,
particularly preferably 50.degree. to 70.degree. to the running
direction of the film as shown in FIG. 12. In FIG. 12, numerals
30, 32, and 35 are the same as above.
In FIG. 5, the arrangement, the number, the length, the diameter
and the angle of needle blades 34 and 35 of the needle blade rolls
31 and 32 may be properly determined in consideration of a thickness
of the fibers intended to be obtained. It is preferable that the
blades are usually arranged at a row in the longitudinal direction
of the roll, the number of
blades is 20 to 100/cm.sup.2 and the angle of needles is 50.degree.
to 70.degree., but the arrangement, the number and the angle are
not limited thereto. Also the mounted conditions of the needle blades
of the rolls 31 and 32 may be the same or different. The distance
between the needle blade rolls 31 and 32 may also be properly adjusted.
The preferable distance is usually such that the needles overlap
by about 1 to 5 mm at the end thereof.
The thus obtained uniaxially stretched PTFE split films having
a network structure become split yarns of the present invention
as they are or by forming those split films into a cord.
The split yarn of the present invention is excellent in flexibility
and bulkiness because it is uniaxially stretched and split into
a network structure. Fineness (denier) of the split yarn of the
present invention is defined by a film width, and selected optionally
depending on applications. The split yarn of the present invention
can be used as it is as a cord having chemical resistance, heat
resistance and dielectric strength, and can be formed into a fabric
or a knitted (braided) cord by weaving or knitting. The obtained
fabric and knitted cord are useful as heat resistant and chemical
resistant materials for heat insulation, and materials for filter
cloth for dust collection, heat resistive cloth, gland packing,
filter cloth for fluid, filter guard, pump packing, sliding member,
sealing material, dental floss and the like.
As mentioned above, the split yarn of the present invention is
an integrated one which becomes in the form of a net when enlarged
(FIG. 1). Such a split yarn (a uniaxially stretched split film having
a network structure), when cut only in the longitudinal direction,
becomes a bundle of filaments because crosswise lines are cut. The
filaments have a number of branches or loops because the cut portions
of each filament become in the form of branches or loops. The present
invention further relates to those PTFE filaments having branches.
In order to cut the network structure in the longitudinal direction,
the uniaxially stretched split film may be passed between the needle
blades in the form of a comb.
In that case, it is preferable that the film is fed, being slightly
enlarged in the transverse direction, and that the edge of the needle
blade which faces the split film is sharp. The number and arrangement
of needle blades may be defined depending on the number of split
lines of the film (flexibility and bulkiness are enhanced by increase
in the number of split lines).
In addition, the split film can be cut more by passing it through
a number of comb-like blades or the needle blades provided on the
same circumference and rotating at very high speed.
In any of the above-mentioned methods, in order to make a length
of the branch short, it is necessary to cut the network structure
sharply. If not so, long branches arise. On the contrary, in order
to obtain long branches, it is better to carry out cutting mainly
by tearing but not sharp cutting.
Fineness is defined nearly by a splitting width, and is usually
from 2 to 200 deniers, preferably 2 to 50 deniers, more preferably
2 to 30 deniers, particularly preferably 2 to 15 deniers from the
viewpoint of the feeling. It is preferable that the proportion of
fibers of more than 200 deniers falls in the range of less than
10%, particularly less than 5%.
The filaments of the present invention have branches and are bulky,
and can be used as heat resistant and chemical resistant materials
for heat insulation, and materials for filter cloth for dust collection,
heat resistive cloth, gland packing, filter cloth for fluid, filter
guard, pump packing, sliding member, sealing material, dental floss
and the like.
The present invention further relates to a method of producing
cotton-like PTFE materials. Those cotton-like materials can be obtained
by cutting the above-mentioned split yarn to a given length and
then opening or by cutting the above-mentioned filament to a given
length and then opening.
The split yarn can be cut, for example, by press-cutting with a
cutter roller and anvil roller which are used for tow spinning or
by cutting with a cutter such as a shearing press. A cut length
is from 25 to 200 mm, preferably from 37.5 to 150 mm. When the cut
length is too short, a percentage of dropped fibers of the obtained
cotton-like materials increases and intermingling property becomes
worse. When too long, there occurs an obstruction to processability
of the cotton-like materials, for example, uniform dividing into
webs. The split yarn is, after the cutting, opened by an opening
machine or a carding machine to be formed into cotton-like materials.
The filaments can be cut, for example, by press-cutting with a
cutter roller and anvil roller which are used for tow spinning or
by cutting with a cutter such as a shearing press in the same manner
as in the cutting of the split yarn. A cut length is from 25 to
200 mm, preferably from 37.5 to 150 mm. When the cut length is too
short, a percentage of dropped fibers of the obtained cotton-like
materials increases and intermingling property becomes worse. When
too long, there occurs an obstruction to processability of the cotton-like
materials, for example, uniform dividing into webs. The PTFE fiber
is, after the cutting, opened by an opening machine or a carding
machine to be formed into cotton-like materials.
Appearance of the obtained cotton-like PTFE materials are like
cotton.
The cut split yarns or filaments which are subjected to opening
are not necessarily those having the same length. Those having various
lengths within the above-mentioned range may be mixed with each
other.
Also it is particularly preferable that the fibers making the cotton-like
PTFE materials of the present invention have a branched structure,
fineness thereof is 2 to 200 deniers, preferably 2 to 50 deniers,
more preferably 2 to 30 deniers, particularly 2 to 15 deniers, the
number of crimps is 1 to 15 per 20 mm, and the figure of section
of the fibers is not uniform.
The branched structure can be illustrated as shown in FIG. 6. The
branched structure FIG. 6(a) shows a fiber 1 and a plurality of
branches 2 coming from the fiber 1. FIG. 6(b) shows a fiber having
a branch 2 and further a branch 3 coming from the branch 2. FIG.
6(c) shows a fiber simply divided into two branches. FIG. 6(d) shows
a fiber having a loop 5. Those structures are only models of the
fibers, and the fibers having the same structure are not found actually
(FIGS. 7 to 9). This is one of important features of the present
invention. The number and the length of branches are not particularly
limited, but the existence of such branches and loops is an important
cause of enhancing intermingling property of the fibers. It is preferable
that there is at least one branch or loop, particularly at least
two branches or loops per 5 cm of the fiber.
The fineness ranges from 2 to 200 deniers, preferably 2 to 50 deniers,
more preferably 2 to 30 deniers, particularly 2 to 15 deniers. As
it can be seen from FIGS. 7 to 9 referred to hereinafter, the preferable
cotton-like materials are obtained when the fineness of the fiber
including branches is in the said range, though there is no fiber
having the same fineness throughout the fiber. Therefore there is
a case where a part of the fiber is out of the fineness of the above-mentioned
range. Also in the cotton-like materials of the present invention
in order not to make intermingling property worse, it is preferable
that the content of the fibers having a fineness of more than 200
deniers is minimized below 10%, particularly below 5%.
Also it is preferable that as shown in FIG. 6(a), the fiber 1 making
the cotton-like materials of the present invention has partly a
"crimp" 4. The "crimp" also contributes to enhancement
of intermingling property. The preferable number of crimps is 1
to 15 per 20 mm. According to the process of production of the present
invention, there occurs crimps even if no specific crimping process
is applied.
The cross sectional figure of the fiber is irregular because of
splitting by a mechanical force, and this contributes to intermingling
among the fibers.
The cotton-like PTFE materials of the present invention, being
excellent in intermingling property, is suitable for spun yarn,
non-woven fabrics and filter cloth for dust collection.
The non-woven fabrics are produced by means of a needle punching
machine, and then water jet needle machine after treating with a
carding machine, but the prior PTFE fibers having a low friction
coefficient and a large specific gravity, could not be treated in
the same manner as the other polyolefin.
For instance, in case of producing non-woven fabrics with a carding
machine as shown in FIG. 10, the cotton-like materials (not illustrated)
being transferred with a fiber mass conveyor 60 are passed through
a carding machine 61, become webs, and then are wound on a drum
63 from a doffer 66. The carding machine (FIG. 10) used in the present
invention is employed for polyolefin fibers such as polypropylene,
and the distance (referred to as a "card crossing distance")
between the doffer 66 and the drum 63 is set at about 28 cm. When
the prior PTFE fibers were used, there occurred a dropping of the
web between the doffer and the drum in case of that distance, and
unless the distance is shortened up to about 5 cm, the web could
not be wound on the drum.
When the cotton-like PTFE materials of the present invention are
used, the web can be wound on the drum without any problem with
the same card crossing distance (about 28 cm) as that of the cotton-like
polyolefin materials.
In FIG. 10, numeral 64 indicates a taker-in-roller, numeral 65
indicates a cylinder, numeral 66 indicates a doffer, and numeral
67 indicates a card crossing distance.
The filter cloth for dust collection of the present invention is
obtained from cotton-like materials produced by the method of the
present invention, and is suitable, for example, for a bag filter
for dust collection which requires heat resistance and chemical
resistance.
The above-mentioned filter cloth for dust collection is produced
by the method of, for example, spraying an antistatic agent to the
cotton-like materials obtained through the above-mentioned method,
with a sprayer or the like and passing the materials through the
carding machine shown in FIG. 10 to make a web.
Then the obtained web is placed on one surface and/or both surfaces
of a base fabric produced from, for example, meta-linked type aramid
fiber, para-linked type aramid fiber, PTFE fiber, polyimide fiber,
glass fiber, poly(phenylene sulfide) fiber, polyester fiber and
the like, and then for intermingling of the fibers, a needle punch,
water jet needle or the like is used, and thus the filter cloth
for dust collection of the present invention can be obtained.
The method of the present invention for producing the filter cloth
for dust collection is not limited to those mentioned above.
The present invention is explained by means of Examples, but is
not limited thereto.
Physical properties and measuring methods thereof, which are used
in this specification including Examples are as mentioned below.
(Fiber length and number of branches)
A hundred pieces of fibers were sampled at random and the fiber
length and the number of branches (including loops) were measured.
(Shape of section)
The shape of section of the bundle of fibers sampled at random
were measured with a scanning electron microscope.
(Fineness)
A hundred pieces of fibers sampled at random were used to measure
the fineness thereof with an electronic fineness measuring equipment
(available from Search Co., Ltd.) which utilizes a resonance of
the fiber for measurement.
The equipment could measure the fineness of the fibers having the
length of not less than 3 cm, and the fibers were selected irrespective
of trunks or branches. But the fibers having, on the length of 3
cm, a large branch or many branches were excluded because they affects
the measuring results. The equipment is capable of measuring the
fineness in the range of 2 to 70 deniers, and so for the fibers
having the fineness exceeding 70 deniers, the fineness thereof was
obtained by a weight measurement. The fibers having the fineness
less than 2 deniers were excluded because the measurement is difficult.
Fineness of fibers making a network structure was measured after
the fibers were formed into staple fibers.
With respect to Example 41, fineness of 2,000 fibers selected at
random was measured with an automatic fiber diameter measuring apparatus
(FDA-200 available from Peyer) by irradiating the fibers with laser
beams and automatically measuring the fineness by using the projection
from the irradiation.
(Number of crimps)
Measurement was made in accordance with the method of JIS L 1015
by means of an automatic crimp tester available from Kabushiki Kaisha
Koa Shokai with a hundred pieces of fibers sampled at random (The
crimps on the branch were not measured).
(Air permeability)
Measurement was carried out with a Frazier type air permeability
tester.
(Percentage of dropped fibers)
There was measured a weight percentage of fibers dropping on the
cover 68 provided under the taker-in roller 64, cylinder 65 and
doffer 66 when a web was passed through a carding machine (SC360-DR
available from Daiwa Kiko Kabushiki Kaisha) shown in FIG. 10, on
the basis of the total weight of the fibers fed.
(Crossing of web)
Cotton-like materials were passed through the carding machine shown
in FIG. 10 to form a web and the web crossed from the doffer 66
to the winding drum 63 to be wound on the drum. In that case, it
was evaluated whether or not the web crosses the card crossing distance
(about 28 cm) without dropping and is wound on the winding drum
10.
(Shrinkage)
Measurement was made with respect to 100 fibers sampled at random.
One end of the fiber was fixed on a glass plate with an adhesive,
and a length of the fiber (L.sub.1) was measured. Then another glass
plate was placed thereon to interpose the fiber between the glass
plates. The fibers interposed between the glass plates were held
at 200.degree. C., 250.degree. C. and 300.degree. C. each for 30
minutes in an electric oven and then taken out of the oven. Then
the fiber length (L.sub.2) was measured again, and the shrinkage
was obtained by the following equation. ##EQU1## (Feeling)
Evaluated with the following criteria, A, B, and C.
A: Feeling is soft and good.
B: Feeling between A and C.
C: Feeling is rigid and not good.
(Thickness)
A load of 20 g/cm.sup.2 was applied by using a compressive elasticity
tester (available from Nakayama Denki Sangyo Kabushiki Kaisha).
Measurement was carried out at 10 points at random and an average
of them was employed.
(Collection efficiency)
FIG. 13 is a view illustrating a measuring apparatus for collection
efficiency.
In FIG. 13, numeral 71 indicates a filter cloth for dust collection,
which was obtained in Example 41 or Comparative Example 4 (measuring
area: 50 mm .phi.), numeral 72 indicates a nozzle for measuring
an upstream concentration, numeral 73 indicates a nozzle for measuring
a downstream concentration, numeral 74 indicates a dust counter,
numeral 75 indicates a manometer, numeral 76 indicates a diffusion
dryer (silica gel), numeral 77 indicates a particle diffusion box,
numeral 78 indicates an ultrasonic particle generator, numeral 79
indicates a flow meter, numeral 80 indicates a pump, numeral 81
indicates a blower, numeral 82 indicates a HEPA filter and numeral
83 indicates a cock.
The dust collection efficiency when the non-woven fabrics obtained
in
Example 41 and Comparative Example 4 were used as the filter cloth
for dust collection was measured with the above-mentioned apparatus
under the following conditions.
Filtration rate: 3.3 cm/sec.
Particles generated: Particles generated from an aqueous solution
of 1% by weight of rhodamine B (C.sub.28 H.sub.31 O.sub.3 N.sub.2
Cl, available from Wako Junyaku Kogyo Kabushiki Kaisha, Molecular
weight: 479.03)
Upstream concentration: 5.5.times.10.sup.2 particles/cc
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