Patent Description:
Conventionally, in work environments generating dust, dust collectors have been used for the purpose of removal and collection of the dust. Filters for the dust collectors are known to be used in pleated configurations. Because of such pleated configurations of the filters, the dust collectors can have remarkably increased filtration areas, and can therefore achieve reduction of pressure drop and a high collection efficiency.

For the filters of the dust collectors, so-called backwashing is commonly employed, wherein compressed air or the like is injected from the inside when the pressure drop reached a certain level, thereby brushing off dust attached to the surface of a filter material. In particular, the pleated filter is in a bent state many times by the pressure of air in pleated peak and valley portions while a process of "dust correction-brushing off dust by backflow air" is repeated, so that the folding endurance of the pleated peak and valley portions is important to extend the life of the filter. Therefore, if the pleated filter does not have sufficient folding endurance as an air filter, the dust leaks from the peak and valley portions, so that a satisfactory filter life cannot be provided.

Various nonwoven fabric substrates have been proposed so far in order to solve such a problem. For example, Patent Documents <NUM> and <NUM> disclose a nonwoven fabric in which thermoplastic continuous filaments are integrated by partial thermocompression bonding. Patent Document <NUM> discloses a filter substrate which is thermocompression-bonded to a spunbond nonwoven fabric having a relatively high weight per unit area by a pair of an engraved roll and a flat metal roll. Furthermore, Patent Document <NUM> describes a nonwoven fabric for use in filters as a long-fiber nonwoven fabric composed of conjugate fibers made of fibers made of a high-melting point polymer and a low-melting point polymer, wherein the fibers are subjected to partial thermocompression bonding. In the fibers constituting a surface layer part of the nonwoven fabric, the low-melting point polymer is melted or softened to be fused to each other. Meanwhile, Patent Document <NUM> discloses the following attempt. When the longitudinal direction of a nonwoven fabric is bent into peaks and valleys, and pleated to form a filter unit, fibers constituting the nonwoven fabric are oriented along the longitudinal direction to increase the folding endurance. Patent Document <NUM> discloses a spunbond nonwoven fabric comprising thermoplastic continuous filaments and being formed by partial thermocompression bonding, wherein the peak for fiber orientation distribution for the filaments in the vertical direction of the nonwoven fabric is at <NUM> - <NUM>° and the vertical/transverse ratio for tensile strength of the nonwoven fabric is <NUM> - <NUM>. Patent Document <NUM> discloses a spunbond non-woven fabric for a filter including a composite polyester fiber containing a low melting point polyester arranged in the vicinity of a high melting point polyester, the low melting point polyester having a melting point which is <NUM> to <NUM> lower than the melting point of the high melting point polyester, the composite polyester fiber having a single fiber fineness of not less than <NUM> dtex and less than <NUM> dtex, the spunbond non-woven fabric having a basis weight of <NUM> to <NUM>/m<NUM> , an apparent density of <NUM> to <NUM>/cm <NUM> , an air permeability per basis weight of <NUM> to <NUM> (cm<NUM>/cm<NUM> ·sec)/(g/m<NUM>), and a bending resistance per basis weight, in at least one of the machine direction (MD) and the transverse direction (TD), of <NUM> to <NUM> (mN)/(g/m<NUM>). Patent Document <NUM> discloses a nonwoven fabric for filters which is a long fiber nonwoven fabric, consisting of thermoplastic continuous filaments and formed by partially thermocompression bonding the thermoplastic continuous filaments, wherein the nonwoven fabric has a QF value(Pa-<NUM>) of <NUM> to <NUM> and stiffness of <NUM> to <NUM> mN.

Meanwhile, in recent years, a filter substrate is required to be able to sufficiently collect fine dust having a particle diameter of several µm or less, so that fibers constituting the filter substrate are required to have a lower fineness to provide a lower weight per unit area. However, as the weight per unit area is lower, or the fineness is lower, folding endurance tends to decrease, which makes it difficult to obtain a filter substrate having satisfactory folding endurance.

For example, in the techniques disclosed in Patent Documents <NUM>, <NUM> and <NUM>, the constituting fibers or nonwoven fabrics are fused by a heat treatment, which makes it difficult to achieve both filter performance and sufficient rigidity. In some cases, when the area ratio of a compression bonded part is high, the compression bonded portion is formed in a film, which is apt to cause cracks to occur, so that the techniques are unsuitable for long-term use. In the technique disclosed in Patent Document <NUM>, the fibers are spread by frictional electrification, so that the technique has a problem that the nonwoven fabric has many voids, which is apt to cause fuzz to occur.

Meanwhile, in the technique disclosed in Patent Document <NUM>, the thermocompression bonding by the engraved roll and the metal roll is disclosed, but the mechanical strength of the thermobonded portion is not sufficient, which causes a problem that peeling is apt to occur in a pleated part.

Therefore, in view of the above problems, an object of the present invention is to provide a spunbond nonwoven fabric for use in filters, which has rigidity, high folding endurance, uniformity of a weight per unit area, and also has excellent dust collection performance and mechanical properties.

Another object of the present invention is to provide a method for efficiently and stably manufacturing a spunbond nonwoven fabric for use in filters, having the above performance.

As a result of diligent studies to achieve the above object, the present inventors have found a method for spreading a thermoplastic continuous filament according to a specific method in a step of forming a fiber web to obtain a substrate for use in filters, and have found that the folding endurance of the substrate for use in filters can be significantly improved. Furthermore, it has been found that the nonwoven fabric makes it possible to achieve both mechanical properties such as rigidity, folding endurance, and uniformity of a weight per unit area and dust collection performance.

The present invention has been completed based on these findings. The present invention provides the following inventions.

That is, a spunbond nonwoven fabric for use in filters of the present invention includes a thermoplastic continuous filament and has a partially fused portion. The nonwoven fabric has a stiffness of <NUM> mN or more and <NUM> mN or less, a weight per unit area-CV value of <NUM>% or less, and a weight per unit area of <NUM>/m<NUM> or more and <NUM>/m<NUM> or less.

According to a preferred aspect of the spunbond nonwoven fabric for use in filters of the present invention, the spunbond nonwoven fabric has folding endurance of <NUM>,<NUM> times or more, as measured according to JIS P8115: <NUM> "Paper and board-Determination of folding endurance- MIT method".

According to a preferred aspect of the spunbond nonwoven fabric for use in filters of the present invention, the thermoplastic continuous filament is a composite type filament in which a polyester low melting point polymer having a melting point lower than that of a polyester high melting point polymer is arranged around the polyester high melting point polymer.

According to a preferred aspect of the spunbond nonwoven fabric for use in filters of the present invention, the spunbond nonwoven fabric is fused by partial thermocompression bonding, and has a compression bonded area ratio of <NUM>% or more and <NUM>% or less.

According to a preferred aspect of the spunbond nonwoven fabric for use in filters of the present invention, the spunbond nonwoven fabric has a machine direction stiffness of <NUM> mN or more and <NUM> mN or less, and a ratio of the machine direction stiffness to a transverse direction stiffness, of <NUM> or more.

According to a preferred aspect of the spunbond nonwoven fabric for use in filters of the present invention, the spunbond nonwoven fabric has folding endurance of <NUM>,<NUM> times or more.

According to a preferred aspect of the spunbond nonwoven fabric for use in filters of the present invention, the spunbond nonwoven fabric has a weight per unit area-CV value of <NUM>% or less.

According to a preferred aspect of the spunbond nonwoven fabric for use in filters of the present invention, the thermoplastic continuous filament has a single filament diameter of <NUM> or more and <NUM> or less.

According to a preferred aspect of the spunbond nonwoven fabric for use in filters of the present invention, the spunbond nonwoven fabric is processed into a pleated configuration.

Furthermore, a method for manufacturing a spunbond nonwoven fabric for use in filters of the present invention includes the following steps (a) to (c) to be sequentially performed:.

It is preferable that the method further includes the step of processing the fiber web into a pleated configuration after performing the steps (a) to (c).

According to the present invention, a spunbond nonwoven fabric for use in filters can be obtained, which has an excellent balance between dust collection performance and pressure drop, and excellent mechanical strength, high rigidity, high folding endurance, and uniformity of a weight per unit area. A method for manufacturing a spunbond nonwoven fabric for use in filters of the present invention can efficiently and stably manufacture a spunbond nonwoven fabric for use in filters having the above performance.

A spunbond nonwoven fabric for use in filters of the present invention is a spunbond nonwoven fabric for use in filters as a long fiber nonwoven fabric including a thermoplastic continuous filament and having a partially fused portion, wherein the nonwoven fabric has a stiffness of <NUM> mN or more and <NUM> mN or less, a weight per unit area-CV value of <NUM>% or less, and a weight per unit area of <NUM>/m<NUM> or more and <NUM>/m<NUM> or less. The details thereof will be described below.

A polyester is particularly preferably used as a thermoplastic resin which is a raw material of the thermoplastic continuous filament constituting the spunbond nonwoven fabric for use in filters of the present invention. The polyester is a high molecular weight polymer obtained by polymerizing an acid component and an alcohol component as monomers. Examples of the acid component which may be used include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, and phthalic acid; aliphatic dicarboxylic acids such as adipic acid and sebacic acid; and alicyclic dicarboxylic acids such as cyclohexanecarboxylic acid. Examples of the alcohol component which may be used include ethylene glycol, diethylene glycol, and polyethylene glycol.

Examples of the polyester include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate, polylactic acid, and polybutylene succinate. As the polyester used as a high melting point polymer to be described later, PET, which has a high melting point and excellent heat resistance as well as excellent rigidity, is most preferably used.

As long as the effects of the present invention are not impaired, a nucleating agent, a matting agent, a pigment, a fungicide, an antibacterial agent, a flame retardant, and a hydrophilic agent and the like may be added to the polyester raw materials. In particular, metal oxides such as titanium oxide have effects of reducing the surface friction of fibers to prevent the fusion of the fibers, thereby improving the spinnability, and increasing thermal conductivity in thermocompression bonding molding of a nonwoven fabric using a heat roller to improve the adhesiveness of the nonwoven fabric. Aliphatic bisamides such as ethylene-bis-stearic acid amide, and/or alkylsubstituted aliphatic monoamides have effects of increasing the mold-releasing property between the heat roller and the nonwoven fabric web to improve the conveying performance.

Next, the thermoplastic continuous filament constituting the spunbond nonwoven fabric for use in filters of the present invention is preferably a composite type filament in which a polyester low melting point polymer having a melting point lower than that of a polyester high melting point polymer by <NUM> or higher and <NUM> or lower is arranged around the polyester high melting point polymer. Thus, when the spunbond nonwoven fabric is formed by thermobonding, and used, the composite type polyester fibers (filaments) constituting the spunbond nonwoven fabric are firmly bonded to each other. Therefore, the spunbond nonwoven fabric for use in filters can have excellent mechanical strength, and can sufficiently withstand repeated backwashing.

The melting point of the polyester low melting point polymer in the present invention is lower than that of the polyester high melting point polymer by <NUM> or higher, more preferably <NUM> or higher, and still more preferably <NUM> or higher, whereby appropriate thermobonding properties can be obtained in the spunbond nonwoven fabric for use in filters. Meanwhile, the difference in the melting point between the high melting point polyester and the low melting point polyester is <NUM> or lower, more preferably <NUM> or lower, and still more preferably <NUM> or lower, whereby a decrease in the heat resistance of the spunbond nonwoven fabric for use in filters can be suppressed.

The polyester high melting point polymer in the present invention preferably has a melting point of <NUM> or higher and <NUM> or lower. By setting the melting point of the polyester high melting point polymer to preferably <NUM> or higher, more preferably <NUM> or higher, and still more preferably <NUM> or higher, a filter having excellent heat resistance can be obtained. Meanwhile, by setting the melting point of the polyester high melting point polymer to preferably <NUM> or lower, more preferably <NUM> or lower, and still more preferably <NUM> or lower, a decrease in the productivity due to consumption of a large amount of thermal energy for melting in the manufacture of the nonwoven fabric can be suppressed.

The melting point of the polyester low melting point polymer in the present invention is preferably <NUM> or higher and <NUM> or lower. By setting the melting point of the polyester low melting point polyester to preferably <NUM> or higher, more preferably <NUM> or higher, and still more preferably <NUM> or higher, excellent shape stability is achieved even after passing through a heating process in the manufacture of the pleated filter such as heat setting in the pleating, Meanwhile, by setting the melting point of the polyester low melting point polymer to preferably <NUM> or lower, and more preferably <NUM> or lower, a filter having excellent thermobonding properties during manufacture of the nonwoven fabric and excellent mechanical strength can be obtained.

In the present invention, the melting point of the thermoplastic resin is measured under the conditions of a heating rate of <NUM>/min and a measurement temperature range of <NUM> to <NUM> using a differential scanning calorimeter (for example, "DSC-<NUM>" type manufactured by Perkin-Elmer Corp. ), and a temperature exhibiting an extreme value in the obtained melting endothermic curve is taken as the melting point of the thermoplastic resin. A resin exhibiting no extreme value in the melting endothermic curve obtained by the differential scanning calorimeter is heated on a hot plate, and a temperature at which the resin was melted under microscopic observation is taken as the melting point.

When the thermoplastic resin is the polyester, examples of the combination of a pair of the polyester high melting point polymer and the polyester low melting point polymer (hereinafter, may be described in the order of polyester high melting point polymer/polyester low melting point polymer) include the combinations of PET/PBT, PET/PTT, PET/polylactic acid, and PET/copolymerized PET. Among these, the combination of PET/copolymerized PET is preferably used since it has excellent spinnability. As the copolymerization component in the copolymerized PET, isophthalic acid-copolymerized PET is preferably used since it has particularly excellent spinnability.

Examples of the composite form of the composite type filament include a concentric core-sheath type, an eccentric core-sheath type, and a sea-island type. Among these, as the composite form, the concentric core-sheath type is preferable since the filaments can be uniformly and firmly bonded. Furthermore, examples of the cross-sectional shape of the composite type filament include shapes such as a circular cross section, a flat cross section, a polygonal cross section, a multi-lobed cross section, and a hollow cross section. Among these, in a preferred aspect, the cross-sectional shape of the filament to be used is a circular cross-sectional shape.

In the meantime, in the form of the composite type filament, for example, there is also a method in which a fiber made of a polyester high melting point polymer and a fiber made of a polyester low melting point polymer are prepared into a mixed fiber, but the mixed-fiber method causes difficult uniform thermobonding. For example, thermobonding is weak in portions where the fibers made of the polyester high melting point polymer are densely present so that the mechanical strength and the rigidity are poor, which is not suitable as a pleated filter. Meanwhile, there is also a method in which a low melting point polymer is applied to the fiber made of the polyester high melting point polymer by immersion or spraying or the like, but the method makes it difficult to provide uniform application to the surface layer or in the thickness direction so that the mechanical strength and the rigidity are poor, which is not preferred as a pleated filter.

The content ratio between the polyester high melting point polymer and the polyester low melting point polymer is preferably within a range of <NUM> : <NUM> to <NUM> : <NUM>, and more preferably within a range of <NUM> : <NUM> to <NUM> : <NUM> in terms of the mass ratio. When the polyester high melting point polymer is contained at <NUM>% by mass or more and <NUM>% by mass or less, the spunbond nonwoven fabric for use in filters can have excellent rigidity and heat resistance. Meanwhile, when the polyester low melting point polymer is contained at <NUM>% by mass or more and <NUM>% by mass or less, the composite type filaments constituting the spunbond nonwoven fabric can be firmly bonded to each other in the process of forming and using the spunbond nonwoven fabric for use in filters by thermobonding so that the spunbond nonwoven fabric can have excellent mechanical strength and sufficiently withstand repeated backwashing.

Next, a spunbond nonwoven fabric for use in filters of the present invention and a manufacturing method thereof will be described. The spunbond nonwoven fabric for use in filters of the present invention is manufactured by sequentially performing the following steps (a) to (c).

In the present invention, it is preferable that the steps (a) to (c) are performed, and the fiber web is then processed into a pleated configuration.

These will be described in more detail below.

First, a thermoplastic polymer is melt-extruded from a spinneret. Then, using an air sucker, the melt-extruded product is towed and stretched to obtain a thermoplastic continuous filament. In particular, when a composite type filament in which a polyester low melting point polymer having a melting point lower than that of a polyester high melting point polymer is arranged around the polyester high melting point polymer is used as the thermoplastic continuous filament, as the composite type filament, the polyester high melting point polymer and the polyester low melting point polymer are melted at a melting point or higher and (the melting point + <NUM>) or lower, and the polyester low melting point polymer having a melting point lower than that of the polyester high melting point polymer by <NUM> or higher and <NUM> or lower is arranged around the polyester high melting point polymer. The composite type filament is spun from the pore with a spinneret having a spinneret temperature of a melting point or higher and (the melting point + <NUM>) or lower. Then, using an air sucker, the melt-extruded product is towed and stretched at a spinning rate of <NUM>/min or more and <NUM>/min or less to spin a thermoplastic continuous filament such as a filament having a circular cross-sectional shape.

The single filament diameter of the thermoplastic continuous filament constituting the spunbond nonwoven fabric for use in filters of the present invention is <NUM> or more and <NUM> or less. By setting the single filament diameter of the thermoplastic continuous filament to <NUM> or more, preferably <NUM> or more, and more preferably <NUM> or more, the air permeability of the spunbond nonwoven fabric for use in filters can be improved to reduce the pressure drop. It is also possible to reduce the number of fiber break when forming the thermoplastic continuous filament to improve the stability during production. Meanwhile, by setting the single filament diameter of the thermoplastic continuous filament to <NUM> or less, preferably <NUM> or less, and more preferably <NUM> or less, the uniformity of the spunbond nonwoven fabric for use in filters can be improved to provide the nonwoven fabric having a dense surface, which makes it possible to provide improved collection performance such as easier filtration of dust through the surface layer. As a result, the dust can be easily brushed off to increase the life of the filter.

In the present invention, a value obtained by the following method is employed for the single filament diameter (µm) of the spunbond nonwoven fabric for use in filters.

The nonwoven fabric of the present invention is a so-called spunbond nonwoven fabric for use in filters, and alignment of fibers is regulated by using a fiber spreading plate to deposit the spun thermoplastic continuous filaments on a moving net conveyor, thereby forming a fiber web. Specifically, thermoplastic continuous filaments are sucked by an ejector, and the thermoplastic continuous filaments and pressure air (air) are injected to the lower part of the ejector from a fiber spreading plate having a slit shape to regulate alignment of fibers to deposit the thermoplastic continuous filaments on a moving net conveyor, thereby obtaining a fiber web.

Preferably, a method is applied, in which a fiber web collected by a spunbond method is heat-treated with a pair of engraved embossing rolls.

Even when the composite type polyester fiber is used, it is important that the spunbond nonwoven fabric for use in filters is made of the filament (long fiber). This can provide increased rigidity and mechanical strength as compared with the case of a nonwoven fabric made of short fibers composed of discontinuous fibers, which can be preferably used as a pleated filter.

In the method for manufacturing a spunbond nonwoven fabric for use in filters of the present invention, the fiber web collected on the net conveyor is also preferably temporarily bonded. The temporal bonding is preferably carried out by using a method in which the collected fiber web is thermocompression-bonded using a pair of flat rolls, or a method in which a flat roll is arranged on a net conveyor, and the collected fiber web is thermocompression-bonded between the net conveyor and the flat roll.

The temperature of the thermocompression bonding for temporary bonding is preferably lower than the melting point of the polyester low melting point polymer by <NUM> or higher and <NUM> or lower. Thus, by setting the temperature, the conveying performance can be improved without fibers being excessively bonded to each other.

Since the spunbond nonwoven fabric for use in filters of the present invention has a partially fused portion, the fiber web obtained in the above step (b) is subjected to partial thermocompression bonding in order to form the partially fused portion. In a preferred aspect, the partially fused portion is subjected to partial thermocompression bonding, but a method of partial thermocompression bonding is not particularly limited. Bonding by a hot embossing roll or bonding by a combination of an ultrasonic oscillating unit and an embossing roll is preferable. In particular, bonding by an embossing roll is the most preferable from the viewpoint of improving the strength of the nonwoven fabric. The temperature of thermobonding by the hot embossing roll is more preferably lower than the melting point of a polymer having the lowest melting point in polymers existing at the fiber surface of the nonwoven fabric by <NUM> or higher and <NUM> or lower, and more preferably <NUM> or higher and <NUM> or lower. A temperature difference between the melting point of a polymer having the lowest melting point in polymers existing at the fiber surface of the nonwoven fabric and the temperature of thermobonding by the hot embossing roll is <NUM> or higher, and more preferably <NUM> or higher, whereby excessive thermobonding can be prevented. Meanwhile, the temperature difference is set to <NUM> or lower, and more preferably <NUM> or lower, whereby uniform thermobonding can be provided in the nonwoven fabric.

A compression bonded area ratio in the partial thermocompression bonding of the spunbond nonwoven fabric for use in filters of the present invention refers to a proportion of an area of a thermocompression bonding part in the whole area of the nonwoven fabric, and this proportion is preferably <NUM>% or more and <NUM>% or less in the whole area of the nonwoven fabric. When the compression bonded area ratio is <NUM>% or more, more preferably <NUM>% or more, and still more preferably <NUM>% or more, the nonwoven fabric having sufficient strength can be obtained. Furthermore, the surface of the nonwoven fabric does not become fuzz-prone. Meanwhile, when the compression bonded area ratio is <NUM>% or less, more preferably <NUM>% or less, and still more preferably <NUM>% or less, it does not occur that voids between fibers become less to cause increased pressure drop, which causes deteriorated collection performance.

The thermocompression bonding part has depression portions, and is formed by fusing thermoplastic continuous filaments constituting the nonwoven fabric to one another by heat and pressure. That is, a portion where thermoplastic continuous filaments fuse together and coagulate as compared with another portions is a thermocompression bonding part. When bonding by a hot embossing roll is employed as a method of thermocompression bonding, a portion where thermoplastic continuous filaments fuse together and coagulate by a projection part of the embossing roll becomes a thermocompression bonding part. For example, when a pair of rolls including an upper roll and a lower roll, of which only one roll has projections and depressions in a predetermined pattern and the other roll is a flat roll not having projections and depressions, are used, the thermocompression bonding part refers to a portion where the thermoplastic continuous filaments of the nonwoven fabric are thermocompression bonded by the projection part of the roll having projections and depressions and the flat roll to coagulate. For example, when an embossing roll including an upper roll and a lower roll, in which a plurality of linear grooves disposed in parallel with one another are formed on the surface of the roll, wherein the groove of the upper roll and the groove of the lower roll are provided so as to cross each other at given angles, is used, the thermocompression bonding part refers to a portion where thermoplastic continuous filaments of the nonwoven fabric are thermocompression bonded by a projection part of the upper roll and a projection part of the lower roll to coagulate. In this case, a portion compression-bonded by the projection part of the upper roll and the recessed part of the lower roll, or by the recessed part of the upper roll and the projection part of the lower roll is not included in the thermocompression bonding part.

The shape of the thermocompression bonding part in the spunbond nonwoven fabric for use in filters of the present invention is not particularly specified. In the case where a pair of rolls including an upper roll and a lower roll, of which only one roll has projections and depressions in a predetermined pattern and the other roll is a flat roll not having projections and depressions, are used, or in the case where in an embossing roll including an upper roll and a lower roll, in which a plurality of linear grooves disposed in parallel with one another are formed on the surface of the roll, wherein the groove of the upper roll and the groove of the lower roll are provided so as to cross each other at given angles, the thermoplastic continuous filaments of the nonwoven fabric are thermocompression bonded by a projection part of the upper roll and a projection part of the lower roll, the shape of the thermocompression bonding part may be a circle, a triangle, a quadrangle, a parallelogram, an ellipse, or a rhombus. Alignment of these thermocompression bonding parts is not particularly specified, and alignment of placing at equal spaces, or alignment of placing at random, or an array in which different shapes are present may be used. Among these, an array, in which the thermocompression bonding parts are placed at equal spaces, is preferable from the viewpoint of the uniformity of the nonwoven fabric. Furthermore, a thermocompression bonding part of a parallelogram formed by using an embossing roll including an upper roll and a lower roll, in which a plurality of linear grooves placed in parallel with one another are formed on the surface of the roll, wherein the groove of the upper roll and the groove of the lower roll are provided so as to cross each other at given angles, and thermocompression bonding with the projection part of the upper roll and the projection part of the lower roll is preferable in that partial thermocompression bonding is performed without peeling off the nonwoven fabric.

Here, the compression bonded area ratio in the present invention is a value obtained as follows.

The area of the thermocompression bonding part per <NUM><NUM> of the nonwoven fabric is calculated, and rounded to the nearest integer to determine the compression bonded area ratio.

In the present invention, it is preferable to process the fiber web into a pleated configuration after performing the above steps (a) to (c). The pleating can be performed by a usual method.

The spunbond nonwoven fabric for use in filters of the present invention has a stiffness of <NUM> mN or more and <NUM> mN or less. If the stiffness is <NUM> mN or more, more preferably <NUM> mN or more, and still more preferably <NUM> mN or more, pleating can be performed while the strength and retention property of the nonwoven fabric are maintained. Meanwhile, if the stiffness is <NUM> mN or less, preferably <NUM> mN or less, more preferably <NUM> mN or less, and still more preferably <NUM> mN or less, the folding endurance during pleating is not large, which provides sharpened finishing of unevenness.

Here, the stiffness in the present invention is a value obtained by the following measurement according to <NUM>. <NUM> "Gurley Method (JIS method)" in <NUM> "Stiffness (JIS method and ISO method)" in JIS L <NUM>: <NUM> "Test methods for nonwovens".

In the above, a test piece collected so that the longitudinal direction of the sample is the machine direction is used for measurement of a machine direction stiffness, and a test piece collected so that the longitudinal direction of the sample is the transverse direction is used for measurement of a transverse direction stiffness.

In the stiffness in the present invention, any of the machine direction stiffness and the transverse direction stiffness may satisfy the above range, but at least the machine direction stiffness preferably satisfies the above range, and both the machine direction stiffness and the transverse direction stiffness more preferably satisfies the above range.

The spunbond nonwoven fabric for use in filters of the present invention preferably has a machine direction stiffness of <NUM> mN or more and <NUM> mN or less. The spunbond nonwoven fabric for use in filters has a machine direction stiffness of more preferably <NUM> or more, and still more preferably <NUM> mN or more. The above range makes it possible to maintain a pleat retention property, which is preferable. If the machine direction stiffness is <NUM> mN or less, preferably <NUM> mN or less, and more preferably <NUM> mN or less, the folding endurance during pleating is not increased, and the finishing states of unevenness in a pleated configuration can be sharpened, which is preferable.

The spunbond nonwoven fabric for use in filters of the present invention has a ratio of a machine direction stiffness to a transverse direction stiffness of <NUM> or more. The pleat shape retention property is dominated by rigidity in a machine direction which is a folding direction, and rigidity in a transverse direction is not particularly limited, but it is <NUM> mN or more, and preferably <NUM> mN or more. The ratio of the machine direction stiffness to the transverse direction stiffness is preferably <NUM> or more, and particularly preferably <NUM> or more.

The spunbond nonwoven fabric for use in filters in the present invention has a weight per unit area of <NUM>/m<NUM> or more and <NUM>/m<NUM> or less. When the weight per unit area is <NUM>/m<NUM> or more, the rigidity required for the pleat can be obtained, which is preferable. Meanwhile, when the weight per unit area is <NUM>/m<NUM> or less, preferably <NUM>/m<NUM> or less, and more preferably <NUM>/m<NUM> or less, an increase in the pressure drop can be suppressed, and the cost can be reduced, which is preferable.

The weight per unit area here can be obtained by collecting three samples each having a size of <NUM> × <NUM>, measuring the mass of each sample, converting the average value of the obtained values to a value per unit area, and then rounding the resulting value to the nearest integer.

The spunbond nonwoven fabric for use in filters of the present invention has a weight per unit area-CV value of <NUM>% or less.

In the present invention, as the weight per unit area-CV value of the spunbond nonwoven fabric for use in filters, a value obtained by the following measurement is employed.

The spunbond nonwoven fabric for use in filters of the present invention preferably has a weight per unit area-CV value of <NUM>% or less. The weight per unit area-CV value is more preferably <NUM> or less, and still more preferably <NUM> or less. Since such a range makes it possible to provide the denser nonwoven fabric as the uniformity of the nonwoven fabric is improved, the collection efficiency is likely to be improved, whereby a satisfactory filter life is likely to be obtained, which is preferable. Meanwhile, it is more preferable that the weight per unit area-CV value is <NUM>% or more in order to secure a certain amount of air permeability of the spunbond nonwoven fabric for use in filters to reduce the pressure drop, thereby extending the life of the filter.

When the folding endurance of the spunbond nonwoven fabric for use in filters of the present invention is expressed as folding endurance as measured according to JIS P8115: <NUM> "Paper and board-Determination of folding endurance- MIT method", the folding endurance is <NUM>,<NUM> times or more, preferably <NUM>,<NUM> times or more, and more preferably <NUM>,<NUM> times or more. If the above folding endurance is within the above range, the folding endurance in pleated peak and valley portions is sufficient when a pulse jet method used while dust collected on the surface of the filter substrate is intermittently blown off by backflow air in a filter application for a dust collector is employed, whereby a satisfactory filter life can be obtained.

In the present invention, as the folding strength of the spunbond nonwoven fabric for use in filters as measured according to JIS P8115: <NUM> "Paper and board-Determination of folding endurance- MIT method", a value obtained by measurement according to the following method is employed.

The spunbond nonwoven fabric for use in filters in the present invention has a thickness of preferably <NUM> or more and <NUM> or less, and more preferably <NUM> or more and <NUM> or less. By setting the thickness to <NUM> or more, the rigidity can be improved, and the nonwoven fabric can be suitable for use as a filter. By setting the thickness to <NUM> or less, the spunbond nonwoven fabric for use in filters can have excellent handling performance and processability as a filter.

In the present invention, as the thickness (mm) of the spunbond nonwoven fabric for use in filters, a value obtained by measurement according to the following method is employed.

The spunbond nonwoven fabric for use in filters in the present invention preferably has an apparent density of <NUM>/cm<NUM> or more and <NUM>/cm<NUM> or less. When the apparent density is <NUM> or more and <NUM>/cm<NUM> or less, the spunbond nonwoven fabric has a dense structure so that dust is less likely to enter the inside, and excellent dust brush-off performance. The apparent density is more preferably <NUM>/cm<NUM> or more and <NUM>/cm<NUM> or less.

In the present invention, as the apparent density (g/cm<NUM>) of the spunbond nonwoven fabric for use in filters, a value obtained according to the following formula from the weight per unit area and thickness of the spunbond nonwoven fabric for use in filters is employed.

The air permeability per weight per unit area of the spunbond nonwoven fabric for use in filters in the present invention is preferably <NUM> ((cm<NUM>/(cm<NUM>·sec)) / (g/m<NUM>)) or more and <NUM> ((cm<NUM>/ (cm<NUM>·sec) ) / (g/m<NUM>)) or less. When the air permeability per weight per unit area is <NUM> ((cm<NUM>/ (cm<NUM>·sec)) / (g/m<NUM>)) or more, and preferably <NUM> ((cm<NUM>/ (cm<NUM> · sec)) / (g/m<NUM>)) or more, the increase in the pressure drop can be suppressed. When the air permeability per weight per unit area is <NUM> ((cm<NUM>/ (cm<NUM>·sec)) / (g/m<NUM>)) or less, and preferably <NUM> ((cm<NUM>/(cm<NUM>·sec)) / (g/m<NUM>)) or less, dust is less likely to accumulate in the inside, so that the dust brush-off performance is good.

In the present invention, as the air permeability per weight per unit area ((cm3/(cm<NUM>·sec)) / (g/m<NUM>)) of the spunbond nonwoven fabric for use in filters, a value obtained by dividing a value measured based on <NUM>. <NUM> "Frazier Method" in <NUM> "Air permeability (JIS method)" in JIS L <NUM>: <NUM> "Test methods for nonwovens" by the above weight per unit area is employed as described below.

The tensile strength in the machine direction per weight per unit area of the spunbond nonwoven fabric for use in filters of the present invention (hereinafter, may be referred to as machine direction tensile strength per weight per unit area) is <NUM> (N/<NUM>)/(g/m<NUM>) or more, and more preferably <NUM> (N/<NUM>)/(g/m<NUM>) or more. The tensile strength in the transverse direction per weight per unit area (hereinafter, may be referred to as transverse direction tensile strength per weight per unit area) is preferably <NUM> (N/<NUM>)/(g/m<NUM>) or more, and more preferably <NUM> (N/<NUM>) / (g/m<NUM>) or more.

By setting the machine direction tensile strength and the transverse direction tensile strength as described above, mechanical strength sufficient for practical application of the filter can be imparted so that the filter can have excellent durability. Here, the tensile strength per weight per unit area is calculated according to the following formula.

Here, in the present invention, the machine direction refers to a sheet conveying direction during the manufacture of the spunbond nonwoven fabric for use in filters, that is, the winding direction of the roll of the nonwoven fabric. The transverse direction refers to a direction perpendicular to the sheet conveying direction during the manufacture of the spunbond nonwoven fabric for use in filters, that is, the width direction of the roll of the nonwoven fabric.

In the present invention, as the tensile strength of the spunbond nonwoven fabric for use in filters, a value obtained by dividing a value measured based on <NUM>. <NUM> "Standard Time" in <NUM> "Tensile Strength and elongation (ISO method)" in JIS L1913: <NUM> "Test methods for nonwovens" by the weight per unit area is employed as described below.

Next, a spunbond nonwoven fabric for use in filters of the present invention and a manufacturing method thereof will be specifically described based on Examples.

Property values in Examples described below were measured by the following methods. However, unless otherwise described, physical properties are measured based on the above methods.

A differential scanning calorimeter "DSC-<NUM> type" manufactured by Perkin-Elmer Corp.

The intrinsic viscosity (IV) of the polyester was measured by the following method.

In <NUM> of ortho-chlorophenol, <NUM> of a sample was dissolved, and its relative viscosity ηr was determined according to the following formula using an Ostwald viscometer at a temperature of <NUM>.

(Here, η represents the viscosity of a polymer solution; η<NUM> represents the viscosity of ortho-chlorophenol; t represents the dropping time (seconds) of the solution; d represents the density of the solution (g/cm<NUM>); t<NUM> represents the dropping time (seconds) of ortho-chlorophenol; and d<NUM> represents the density of ortho-chlorophenol (g/cm<NUM>).

Next, the intrinsic viscosity (IV) was calculated from the relative viscosity ηr according to the following formula.

As a thickness gauge, "TECLOCK" (registered trademark) SM-<NUM> manufactured by TECLOCK Corporation was used.

Air permeability was measured using an air permeability tester "FX3300-III" manufactured by TEXTEST AG.

A stiffness was measured using a Gurley type stiffness tester "GAS-<NUM>" manufactured by DAIEI KAGAKU SEIKI MFG.

As a constant speed elongation type tensile tester, Tensilon "RTC-1250A" manufactured by Toyo Baldwin Co.

An MIT folding strength fatigue tester "D type" manufactured by Toyo Seiki Seisaku-sho, Ltd. was used to measure the folding endurance of the spunbond nonwoven fabric for use in filters according to the MIT test.

A digital microscope "VHX-<NUM>" manufactured by Keyence Corporation was used to measure the compression bonded area ratio of the spunbond nonwoven fabric for use in filters. From arbitrary portions of the nonwoven fabric, three rectangular frames each having a size of <NUM> × <NUM> parallel to the longitudinal direction and width direction of the nonwoven fabric were taken at a magnification of <NUM> times of the microscope. An area of a thermocompression bonding part in the rectangular frame to the area was measured at each of the three places, and the average value thereof was obtained. The average value was rounded to the nearest integer to determine the compression bonded area ratio.

From arbitrary portions of the nonwoven fabric, three test samples each having a size of <NUM> × <NUM> were collected, and a dust collection performance test was carried out using VDI/DIN <NUM> as a reference standard. The filtration area of each test sample was set to <NUM><NUM>, and the filtration air velocity was set to <NUM>/min. As the dust powder, aluminum oxide particles (Dp50: <NUM>) were used. The particles were supplied at a constant concentration such that the dust concentration in the upstream of the test sample was <NUM>/m<NUM>.

First, an aging cycle in which <NUM>-MPa compressed air was injected from a pulse-jet device for <NUM> second was carried out <NUM> times at <NUM>-second intervals. Subsequently, for post-aging evaluation of the performance, a brush-off cycle in which <NUM>-MPa compressed air was injected for <NUM> second after the pressure drop reached <NUM> Pa (when the pressure drop reached <NUM> Pa in less than <NUM> seconds after the previous brush-off, the dust was loaded until <NUM> seconds after the previous brush-off, and the compressed air was then injected) was repeated <NUM> times. From the powder leakage concentration during the test, the dust collection rate was calculated according to the formula. The measurement was carried out in three replicates, and the average was rounded to three decimal place.

The pressure drop was measured <NUM> seconds after the injection of pulse-jet in the 30th brush-off cycle. The measurement was carried out in three replicates, and the average was rounded to the nearest integer. The obtained value was taken as the pressure drop of the spunbond nonwoven fabric for use in filters.

A time required for the <NUM> brush-off cycles was taken as a circulation time (seconds).

Next, the details of resins used in Examples and Comparative Examples will be described.

The polyester resin A and the polyester resin B were respectively melted at temperatures of <NUM> and <NUM>. Then, the polyester resin A as a core component and the polyester resin B as a sheath component were spun from the pore at a spinneret temperature of <NUM> and a core : sheath mass ratio of <NUM> : <NUM>, and then filaments having a circular cross-sectional shape were spun at a spinning rate of <NUM>/min using an air sucker, followed by regulating alignment of fibers using a fiber spreading plate having a slit to deposit the fibers on a moving net conveyor, thereby collecting a fiber web composed of fibers having a single filament diameter of <NUM>. The collected fiber web was temporarily bonded by using a calender roll composed of a pair of flat rolls at a temperature of <NUM> at a linear pressure of <NUM>/cm. Subsequently, the fiber web was thermobonded at a temperature of <NUM> and a linear pressure of <NUM>/cm by an embossing roll composed of a pair of engraved rolls having a compression bonded area ratio of <NUM>% to obtain a spunbond nonwoven fabric for use in filters having a weight per unit area of <NUM>/m<NUM>. The spunbond nonwoven fabric for use in filters obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec)) / (g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

A spunbond nonwoven fabric for use in filters having a weight per unit area of <NUM>/m<NUM> was obtained under the same conditions as in Example <NUM> except that a spinning rate was changed so that a single filament diameter was set to <NUM> and the speed of a net conveyor was changed so that a weight per unit area was set to the same as that of Example <NUM>. The spunbond nonwoven fabric for use in filters obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec)) / (g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

A spunbond nonwoven fabric for use in filters which was composed of fibers having a single filament diameter of <NUM> and had weight per unit area of <NUM>/m<NUM> was obtained under the same conditions as in Example <NUM> except that the speed of a net conveyor was adjusted to change the weight per unit area to <NUM>/m<NUM>. The spunbond nonwoven fabric for use in filters obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec)) / (g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

A spunbond nonwoven fabric for use in filters which was composed of fibers having a single filament diameter of <NUM> and had weight per unit area of <NUM>/m<NUM> was obtained under the same conditions as in Example <NUM> except that the speed of a net conveyor was adjusted to change the weight per unit area to <NUM>/m<NUM>. The spunbond nonwoven fabric for use in filters obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec))/(g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

The properties of the obtained nonwoven fabric were as shown in Table <NUM>. All of the spunbond nonwoven fabrics for use in filters of Examples <NUM>, <NUM>, <NUM>, and <NUM> had a machine direction stiffness of <NUM> mN or more, a ratio of the machine direction stiffness to a transverse direction stiffness of <NUM> or more, folding endurance of <NUM>,<NUM> times or more, and weight per unit area-CV of <NUM>% or less, and had excellent rigidity, high folding endurance, and uniformity of a weight per unit area. The spunbond nonwoven fabric for use in filters exhibited good properties. The results of the dust collection performance test also showed that the spunbond nonwoven fabrics for use in filters had a dust collection rate of <NUM>% or more, pressure drop of <NUM> Pa or less, and a circulation time of <NUM> seconds or more, all of which were good. The results are shown in Table <NUM>.

The polyester resin A and the polyester resin B were respectively melted at temperatures of <NUM> and <NUM>. Then, the polyester resin A as a core component and the polyester resin B as a sheath component were spun from the pore at a spinneret temperature of <NUM> and a core : sheath mass ratio of <NUM> : <NUM>, and then filaments were collided to a metal collision plate. Fibers were triboelectrically charged to spread the fibers, thereby collecting a fiber web. The collected fiber web was temporarily bonded by using a calender roll composed of a pair of flat rolls at a temperature of <NUM> at a linear pressure of <NUM>/cm. Subsequently, thermobonding was carried out at a temperature of <NUM> and a linear pressure of <NUM>/cm by an embossing roll composed of a pair of engraved rolls having a compression bonded area ratio of <NUM>% to obtain a spunbond nonwoven fabric which was composed of fibers having a single filament diameter of <NUM> and had a weight per unit area of <NUM>/m<NUM>. The spunbond nonwoven fabric obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec))/(g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

A spunbond nonwoven fabric which was composed of fibers having a single filament diameter of <NUM> and had a weight per unit area of <NUM>/m<NUM> was obtained under the same conditions as in Comparative Example <NUM> except that the speed of a net conveyor was adjusted to change the weight per unit area to <NUM>/m<NUM>. The spunbond nonwoven fabric obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec))/(g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

A spunbond nonwoven fabric which was composed of fibers having a single filament diameter of <NUM> and had a weight per unit area of <NUM>/m<NUM> was obtained under the same conditions as in Example <NUM> except that a discharge rate was adjusted to change the single filament diameter and the speed of a net conveyor was changed. The spunbond nonwoven fabric obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec)) / (g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

A spunbond nonwoven fabric which was composed of fibers including a surface layer part having a single filament diameter of <NUM> and a back layer part having a single filament diameter of <NUM>, and had a weight per unit area of <NUM>/m<NUM> was obtained under the same conditions as in Comparative Example <NUM> except that a discharge rate was adjusted to change the single filament diameter; a fiber web having a single filament diameter of <NUM> was collected on a net conveyor, and a fiber web having a single filament diameter of <NUM> was then laminated thereon to collect the fiber web; the speed of the net conveyor was changed; and an embossing roll having a compression bonded area ratio of <NUM>% was used. The spunbond nonwoven fabric obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec))/(g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

A spunbond nonwoven fabric which was composed of fibers having a single filament diameter of <NUM> and had a weight per unit area of <NUM>/m<NUM> was obtained under the same conditions as in Comparative Example <NUM> except that an embossing roll having a compression bonded area ratio of <NUM>% was used. The spunbond nonwoven fabric obtained had an apparent density of <NUM>/cm<NUM>, air permeability of <NUM><NUM>/(cm<NUM>·sec), air permeability per weight per unit area of <NUM> (cm<NUM>/(cm<NUM>·sec))/(g/m<NUM>), a machine direction stiffness of <NUM> mN, a transverse direction stiffness of <NUM> mN, a ratio of the machine direction stiffness to the transverse direction stiffness of <NUM>, folding endurance according to the MIT test of <NUM> times, and weight per unit area-CV of <NUM>%. The results are shown in Table <NUM>.

The properties of the obtained nonwoven fabrics were as shown in Table <NUM>. Although the nonwoven fabrics of Comparative Examples <NUM>, <NUM> and <NUM> had the same air permeability as that of Examples <NUM>, <NUM> and <NUM>, the nonwoven fabrics had a high density, so that the nonwoven fabrics were apt to be clogged with dust, and had high pressure drop. Therefore, the nonwoven fabrics had poor dust collection performance, stiffness, folding endurance, and uniformity of a weight per unit area. The nonwoven fabric of Example <NUM> had a single filament diameter reduced under the same conditions as in Example <NUM>. The nonwoven fabric had excellent uniformity of a weight per unit area. However, the nonwoven fabric had low air permeability, had ease of dust clogging, pressure drop, and dust collection performance poorer than those of Examples <NUM> to <NUM> although the nonwoven fabric was more excellent than the nonwoven fabrics of Comparative Examples <NUM> to <NUM>, had folding endurance poorer than that of Examples <NUM> to <NUM>. The nonwoven fabric of Comparative Example <NUM> had a compression bonded area ratio set to <NUM>% in a different fineness configuration, but the nonwoven fabric had a poor weight per unit area-CV value, was apt to cause dust clogging, and had high pressure drop, so that the nonwoven fabric had poor dust collection performance, stiffness, and folding endurance. The nonwoven fabric of Comparative Example <NUM> having an increased single filament diameter had high air permeability, but the nonwoven fabric had a poor weight per unit area-CV value, so that the nonwoven fabric was apt to cause dust clogging to have high pressure drop, and poor dust collection performance.

The application of the spunbond nonwoven fabric for use in filters of the present invention is not limited at all, but the spunbond nonwoven fabric is preferably used as an industrial filter since it has excellent rigidity, folding endurance, uniformity of a weight per unit area, air permeability, and dust brush-off performance.

Particularly preferably, as a pleated configuration cylindrical unit, the spunbond nonwoven fabric is used for applications such as bag filters of a dust collector or the like and liquid filters of an electric discharge machine or the like, and further is used in an air intake filter which is used for cleaning the intake air of a gas turbine or an automobile's engine or the like. The spunbond nonwoven fabric can be suitably used as a pleated filter.

Claim 1:
A spunbond nonwoven fabric for use in filters, comprising a thermoplastic continuous filament and having a partially fused portion, wherein the nonwoven fabric has a stiffness as measured according to the description of <NUM> mN or more and <NUM> mN or less, a weight per unit area-CV value as measured according to the description of <NUM>% or less, and a weight per unit area of <NUM>/m<NUM> or more and <NUM>/m<NUM> or less and
wherein the spunbond nonwoven fabric is obtainable by a method comprising the following steps (a) to (c) to be sequentially performed:
the step (a) of melt-extruding a thermoplastic polymer from a spinneret, and then towing and stretching the melt-extruded product using an air sucker to obtain thermoplastic continuous filaments;
the step (b) of regulating alignment of fibers using a fiber spreading plate to deposit the obtained filaments on a moving net conveyor, thereby forming a fiber web; and
the step (c) of subjecting the obtained fiber web to partial thermocompression bonding.