JPH03279452A - High-strength nonwoven sheet - Google Patents

Abstract

PURPOSE:To provide the title sheet excellent in the performance as a filter, etc., consisting of a blend where ultra-fine PE and PP fibers and other fibers larger in diameter than the formed and at least partially containing PE or PP are randomly mixed and also both kinds of fibers are partly in a melt-bound state. CONSTITUTION:The objective sheet excellent in bacteria barrier properties, dust collectivity, liquid absorptivity and thermal insulation, consisting of a blend where (A) ultrafine PE and PP fibers [for PE, pref. high-density or linear low-density PE, with a melt flow of 40-200; for PP, with a weight-average molecular weight Mw of 10X10<4>-20X10<4>, a ratio Mw/Mn of 3.5-5.0 (Mn: number-average molecular weight) and a melt flow of 70-500] and (B) other fibers larger in mean diameter than the fibers A and at least partially containing PE or PP are randomly mixed and also both the fibers A and B are partly in a melt-bound state.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a high strength nonwoven sheet containing ultrafine fibers.

In more detail, we are talking about highly durable nonwoven sheets that have excellent filter performance, bacterial barrier properties, dust absorption, liquid absorption, and heat insulation properties, and are particularly suited for use in various filters, wipers, wraps, insulation materials, etc. The present invention relates to a high-strength nonwoven sheet containing fibers. The term "nonwoven sheet" as used in the present invention includes anything from a bulky sheet like a fabric to a sheet like a paper, such as electric paper.

[Conventional technology]

Non-woven sheets made of ultra-fine fibers, especially ultra-fine fiber non-woven sheets obtained by the melt-blowing method, have excellent filter performance, bacterial barrier properties, dust absorption properties, liquid absorption properties, heat insulation properties, etc. Taking advantage of these characteristics, they can be used for various purposes. It is being used.

Regarding the melt blowing method, please refer to the Industrial and Engineering Chemistry (Indus-tr.
ial and Engineering Che+n
istry) Volume 4g, No. 8 (P, 1342-1346
), 1956, disclosed the basic apparatus and method. Furthermore, Japanese Patent Publication No. 56-33511 discloses a polyolefin melt blowing method.

On the other hand, the following are known as nonwoven sheets in which thick short fibers are mixed with ultrafine fibers. In other words, the Tokuko Sho 6
Publication No. 1-30065 discloses that at least three fibers are a mixture of ultrafine fibers and crimped staple fibers with a larger diameter.
0ca! An elastic fibrous web for thermal insulation having a bulk height of /g is disclosed.

Also, JP-A-55-30498 and JP-A-59
Japanese Patent No. 183723 discloses a wiper containing ultrafine fibers and crimped thick short fibers.

[Problem to be solved by the invention]

Nonwoven sheets made only of ultrafine fibers, especially ultrafine fiber webs obtained by melt blowing, are high-quality porous materials with extremely small pore sizes, and have excellent filter performance, bacterial barrier properties, dust absorption, and liquid absorption properties. , has the advantage of being excellent in heat insulation, etc., but on the other hand, it has a problem that its strength is low and its applications are greatly limited. Therefore, meltblown webs are rarely used alone, and are generally used in combination with high-strength nonwoven sheets such as spunbond nonwoven sheets, or woven or knitted fabrics. There was a cost problem.

On the other hand, in the aforementioned Japanese Patent Publication No. 61-30065,
The purpose of mixing thick crimped staple fibers with ultrafine fibers is to increase the bulk of the web (increase porosity), thereby increasing the heat insulation properties. Also, JP-A-55-30
498 and JP-A-59-183723, the purpose of mixing thick crimped short fibers is to improve the porosity (increase bulk) of the web, which improves the dust absorption and liquid absorption characteristics of the wiper. It was about paying high wages for sex. As described above, the aim of mixing short fibers in the known technique is to improve the porosity by increasing the bulk.

In the case of the nonwoven sheet disclosed in JP-A-55-30498, as is clear from the examples, polypropylene, polyethylene, and acrylic polymers are used as the ultrafine fibers, and polyethylene terephthalate, nylon, and acrylic polymers are used as the ultrafine fibers. It uses a different material such as nylon 6.6. These two types of fibers are not compatible with each other, so even if they are thermally bonded, the bond between the ultra-fine fibers and the thick fibers is not sufficient.
This is essentially thermal bonding between the ultrafine fibers. For this reason, not only the features of the ultra-fine m-fibers were lost, but also the reinforcing effect of the ultra-thick fibers was hardly exhibited, resulting in an insufficient strength-improving effect.

In addition, when the thermal bonding between the ultrafine fibers is the main one, there is a problem that not only the performance of the ultrafine fibers deteriorates, but also the ultrafine fibers become hardened or brittle, resulting in a particularly low tear strength. A similar problem is also seen in the case of the nonwoven sheet disclosed in JP-A-59-183723.

Furthermore, Japanese Patent Publication No. 56-33511 discloses a melt-blown mat made of polyethylene and polypropylene, and as is clear from Example 17 of the same publication,
In the mixed system of these two types, ultrafine fibers with an average fiber diameter of less than 5 pm have not been obtained.

The present invention provides a high-quality fiber web containing single-layer ultrafine fibers that does not impair the features of the above-mentioned ultrafine fiber web as much as possible, and does not involve lamination with other high-strength nonwoven sheets such as spunbond. The purpose is to provide strong nonwoven sheets.

[Means to solve the problem]

The object of the present invention is to provide a fiber sheet in which ultrafine fibers made of polyethylene and polypropylene and fibers having a larger average fiber diameter than the ultrafine fibers and containing polyethylene or polypropylene as at least a part thereof are randomly mixed. This is achieved by using a high-strength nonwoven sheet in which at least the thick fibers and the ultrafine fibers are partially thermally bonded.

It is preferable that the constituent fibers of the high-strength nonwoven sheet according to the present invention be electret, since this helps improve filter performance and the like.

The ultrafine fibers in the nonwoven sheet of the present invention are made of polyethylene and polypropylene.

The type of polyethylene used may be high-density polyethylene (HOPE), low-density polyethylene (LDPE), or linear low-density polyethylene (LLDPE), but high-density polyethylene and linear low-density polyethylene are preferred.

The melt flow rate of polyethylene is 30 or more, preferably 35 or more, and more preferably 40 to 200. When the melt flow rate of polyethylene is 30 or less, no matter what melt flow rate polypropylene is selected, the melt blowing properties are poor and polymer beads are likely to occur, and the resulting web will also contain polymer beads.

Moreover, when the melt flow rate of polyethylene exceeds 200, the strength of the obtained web becomes low.

In addition, the weight average molecular weight (Mw) of the polypropylene used for the ultrafine fibers in the nonwoven sheet of the present invention is 5, OXIO
'5Mw ≦25X10', further 10×104
≦Mw≦20×10′ is preferable. Also, the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (Mw/Mn)
is 2.0≦Mw/Mn≦6.0, and 3.0≦Mw
/Mn≦5.0 is preferable.

The melt flow rate of the polypropylene is 50 or more, preferably 60 or more, and more preferably 70 to 500.

If the melt flow rate of polypropylene is less than 50, the melt blowing properties will be poor and polymer beads will be likely to occur, and the resulting web will also contain polymer beads, no matter what melt flow rate polyethylene is selected.

In addition, when the melt flow rate of polypropylene exceeds 500,
The strength of the resulting web is reduced.

The weight ratio (PE/PP) of polyethylene (PE) and polypropylene (PP) is preferably in the range of 5/95 to 90/10, preferably 10/90 to 80/20, more preferably 20/80 to 70. /30. If the proportion of polyethylene is less than 5%, it cannot be used for low temperature adhesive applications. In addition, if the polyethylene ratio exceeds 90%, melt blowing properties will deteriorate and polymer beads will easily occur.
The resulting web will also contain polymer beads, and it will no longer be possible to obtain a web with an average fiber diameter of 4 or less.

As mentioned above, polyethylene (P) with a melt flow rate of 30 or more
E) and polypropylene (PP) with a melt flow rate of 50 or more
It consists of polyethylene (PE) and polypropylene (P
P) weight ratio (PE/PP) from 5/95 to 90/1
By setting it to 0, the melt blow spinnability is improved, and high-quality ultrafine fibers without polymer beads, which are made of ultrafine fibers with an average fiber diameter of 0.5 to 4.0, can be obtained.

The mixed states of the two types of polymers include a state in which they are mixed in a single fiber, a state in which fibers made of each polymer are mixed in the sheet, and a state in which the above two states are mixed in the sheet. There is a condition.

Among them, it is particularly preferable that the fibers are mixed in a single fiber.

Mixing methods include a method of chip blending during spinning, a method of using chips in which two types of polymers are melted and mixed in advance, and the like. The method for mixing these two types of polymers is not particularly limited, but the latter method is preferred.

In addition to the above-mentioned polyolefin, a third polymer may be mixed with the fibers in the ultrafine fibers of the present invention within a range that does not depart from the object of the present invention. may be mixed without departing from the purpose of the present invention.

The ultrafine fibers referred to in the present invention have an average fiber diameter of 0.1
It is good to use fibers with a diameter of ~8.0 p or less, preferably an average fiber diameter of 0.5 to 6.0 pm, particularly preferably 1.0 to 6.0 pm.
The fibers preferably have an average fiber diameter in the range of 4.0-. If the average fiber diameter is less than 0.1 mm, it is not preferable because although it is flexible, the fiber strength is low and fluff may fall off, which limits its uses. On the other hand, if it is 8.0 or more, the strength of the nonwoven sheet will be high, but on the other hand, the above-mentioned features of ultrafine fibers such as filter performance, bacterial barrier property, dust absorption property, liquid absorption property, and heat insulation property will be inferior, so it is not preferable. .

The mixed fibers to be mixed with the ultrafine fibers in the nonwoven sheet of the present invention will be explained below. The mixed fiber has a larger average fiber diameter than the ultrafine fiber and contains polyethylene or polypropylene as at least a part thereof.

Such mixed fibers include polyethylene fibers, polypropylene fibers, or composite fibers containing polyethylene or polypropylene as a part of the components.

The composite form of this composite fiber is parallel type (Side-
by-3ide), Sheath-Core
, 'The eccentric type may be mentioned, but is not limited to these, and the number of material components may be two or more, but at least a part of the polyethylene or polypropylene component is located on the surface of the fiber. is necessary. Typical composite fibers include, but are not limited to, combinations of polypropylene and polyethylene (trade name: ESI fiber), combinations of polyesters such as polyethylene and polyethylene terephthalate, and polypropylene and polyethylene terephthalate. do not have.

The mixed fibers of the present invention may or may not be crimped, but if they are crimped, the ultrafine fiber nonwoven fabric will be bulkier and will have better dust absorption properties.
It has improved liquid absorption and is suitable for wiper applications. In addition, it is also preferable to use the material as a heat insulating material because crimping improves heat insulating properties.

Furthermore, since the pressure loss and the amount of dust retained are improved, it is also preferable to use it as a filter.

The crimp ratio is preferably 15% or more.

In the present invention, the denier (fiber diameter) and fiber length of the mixed fibers are important as they affect the strength of the nonwoven sheet.

The denier of the mixed fiber is 1.0 denier or more, preferably 1.5 to 25 denier, particularly 2 to 20 denier. If it is less than 1.0 denier, the effect of improving tear strength is not sufficient. The larger the denier of the mixed fibers, the higher the tear strength, but if the denier exceeds 25, the nonwoven sheet will be hard and have a poor feel, which is not preferable.

The average fiber length of the mixed fibers is preferably 30 mm or more, preferably 35 to 110 mm, particularly preferably 50 to 80 mm. If the ratio is less than 30, the effect of improving tear strength is insufficient. The longer the average fiber length of the mixed fibers, the higher the tear strength, but even if the fiber length is increased by 110 mm or more, the tear strength hardly increases and becomes almost saturated. Also, if the average fiber length of the mixed fibers is too long, it will be difficult to uniformly disperse the fibers into the ultrafine fibers.
In some cases, tear strength may decrease.

In the present invention, the tear strength is maximized when the mixed fibers are 1.5 denier or more and the average fiber length is 30 inches or more.

The higher the tensile strength of the mixed fibers, the higher the tear strength and tensile strength of the nonwoven sheet. The tensile strength of the blended fibers is preferably at least 2 g/denier, especially 3 to 10 g/denier.

Although the phenomenon in which the tear strength is improved in the present invention is not necessarily clear, it is thought to be as follows. That is,
The tear strength of the unbounded sheet is almost determined by the cutting of the mixed fibers, which are stronger than the ultrafine fibers, but the tear strength of the mixed fibers (
The larger the (strength per denier) X (denier), the higher the tear strength of the nonwoven sheet. Furthermore, in order to fully demonstrate the strength of the mixed fibers, it is necessary that the mixed fibers be firmly fixed in the nonwoven sheet and that no slipping or the like will occur. The nonwoven sheet of the present invention has compatibility between the ultrafine fibers and the mixed fibers because they have the same material components, and the two are sufficiently thermally bonded, and the fiber length of the mixed fibers is Since the fibers are relatively long, there is a large amount of thermal bonding between the mixed fibers in addition to the ultrafine fibers, and as a result, it is thought that the mixed fibers are firmly fixed in the nonwoven sheet.

In the nonwoven sheet of the present invention, it is preferable that the mixed fibers are randomly mixed into the ultrafine fibers substantially in the form of single fibers.

With such a mixed state, high strength can be achieved without substantially impairing the features of the ultrafine fibers described above. The mixing ratio (weight) of the mixed fibers to the total amount of fibers is 10 to 80%, preferably 20 to 70%, particularly preferably 30 to 60%. 10
% or less, the thermal bonding strength is low and it is difficult to obtain sufficient strength.

On the other hand, if it is 80% or more, the above-mentioned features of the ultra-fine m fibers are impaired, which is not preferable.

Further, it is more preferable that the ultrafine fibers are also randomly dispersed in the form of single fibers, since the above-mentioned various performances are enhanced. Further, the nonwoven sheet of the present invention may be a mixture of a third material, fibers with different fiber diameters, shapes, etc., powder, and the like.

In addition, since the ultrafine fibers obtained by the melt-blowing method have an extremely small fiber diameter, it is difficult to estimate the average length of the fibers, but it is 30 mm or more, and in many cases 100 to 100 mm.
It is estimated that there are 500 people.

The basis weight of the nonwoven sheet of the present invention is 10 to 200 g/m'
, preferably 20-150 g/m', especially 25-1
00 g/m' is preferred.

Furthermore, by applying electret processing,
Capture efficiency can be improved. The surface charge density of the high-strength nonwoven sheet of the present invention subjected to electroleft processing is
2X10-” coulomb/c [11 or more, furthermore, 5
Xl0-'' coulomb/ci or more is preferred.

Hereinafter, a method for producing a high-strength nonwoven sheet of the present invention will be explained.

As a method for obtaining the ultrafine fibers of the present invention in sheet form, a melt blow method, a flash spinning method, a super draw method, a combination of a composite fiber method and a paper making method, etc. can be used.
Although not particularly limited, a melt blow method is particularly preferred.

An example of a method for producing the web-like ultrafine fiber of the present invention using a melt blowing method will be described below.

A blend of polyethylene and polypropylene is melted in an extruder, fed into a die, and extruded through a row of spinning orifices in a nozzle.

Molten polymer is forced out of the orifice through a polymer channel. At the same time, the heated high-speed gas supplied through the gas inlet is injected through the gas header through gas slits provided on both sides of the orifice, and is blown against the flow of the extruded molten polymer. A gas rudder and gas slit are provided between the nozzle and the lip. The extruded molten polymer is pulled into the shape of fibers, thinned and solidified by the action of high-speed airflow. The fibers thus formed are deposited on a screen (collector) 7 circulating between a pair of rotating rollers to form a random web. Steam, air, etc. are suitable as the gas, and the gas conditions include a temperature of 300 to 450°C, preferably 350 to 420 t, and a pressure of 0.1 kg/
cat G or higher, preferably 0 although it varies depending on the discharge amount
．． It is 2 to 5.0 kg/c++fG. Extruder temperature is 1
80 to 280°C, preferably 200 to 250 t:.

Examples of mixing methods for composite fibers include melt blowing,
In the flash spinning method, a web of crimped conjugate short fibers is created and the web is made to fly through a group of ultrafine fibers being spun using a lickerin roll or the like to obtain a mixed sheet-like product; There is a method in which a sliver is made into a sliver, defibrated using a combing roll (a roll with many teeth), and then mixed in the same manner as described above to obtain a sheet. In addition, short fibers with a thickness of 3 to 30 mm are mixed with the ultrafine fibers obtained by the super draw method and the sea-island fiber method.

Preferably, there is a method of cutting into 5 to 10 mm pieces, mixing these two types to create a slurry, and forming the slurry into sheets using a papermaking method. These ultrafine fiber sheets may be subjected to -tan entangling treatment.

Examples of the thermal bonding method include a hot embossing method, a hot Kangoo method, a hot air method, and an ultrasonic bonding method. In particular, the hot air bonding method is preferable because it allows thermal bonding to the nonwoven sheet without pressurizing it, yields a bulky nonwoven sheet, and fully exhibits the features of the ultrafine fibers described above. The thermal bonding temperature only needs to be at least the temperature at which thermal bonding occurs, and generally needs to be at least the softening point of the low melting point component of the mixed fiber. On the other hand, if the temperature is increased above the melting point of the ultrafine fibers, the ultrafine fibers will be thermally bonded to a large extent, which is not preferable. Therefore, the thermal bonding temperature in the present invention is preferably higher than the softening point of the low melting point component of the mixed fiber and lower than the melting point of the ultrafine fiber. Specifically, the temperature of hot embossing or hot rolling is 150"C or less, preferably 50 to 130"C.
℃, more preferably in the range of 60 to 120℃, and the pressure is 0.5 to 100 kg/cm, preferably 1
It is preferable to do this in the range of ~75 kg/cm. If processed at high temperatures and high pressures exceeding the above range, the fibers will melt and form a film, resulting in decreased air permeability. On the other hand, processing at low temperatures and pressures below the above range results in insufficient thermal bonding, making it impossible to improve the strength and surface smoothness of the nonwoven sheet.

The embossing pattern used in hot embossing may be either a continuous pattern or a discontinuous pattern, and various patterns such as lines, dotted lines, grids, diagonal grids, circles, diamonds, and fabric patterns can be used.

The nonwoven sheet of the present invention can be subjected to various post-treatments. For example, it is possible to further improve the filter performance and dust suction power by converting it into an electret using a corona discharge method or the like.

In the electret processing, for example, one side of the sheet is brought into contact with a flat plate or roll for an earth electrode, and the
This is done by placing an electrode on top of m and applying a DC voltage from a high voltage power supply. A needle electrode or a wire electrode is used as the discharge electrode, and a positive or negative DC voltage of 1 to 20 kV/cm (per unit distance between the electrode and the sheet) is applied in a high electric field to generate corona discharge. At this time, the temperature is from room temperature to 100°C.

The electret processing may be performed continuously after forming the web by the melt blowing method, or may be performed after the web is wound up.

〔Example〕

The present invention will be explained in more detail with reference to Examples below.

The definitions and measurement methods of various physical properties shown in Examples and Comparative Examples are shown below.

◎Average fiber diameter (-) Ten arbitrary points on the sample were examined using an electron microscope at a magnification of 2000.
Take 10 photos at double magnification. Measure the diameter of 10 arbitrary fibers for each photograph, and do this for each of the 10 photographs. A total of 100 fiber diameter measurements are obtained and the average value is calculated.

◎Thickness (mm) Measured using a peacock-type thickness gauge at a constant load of 130 g/cI [l].

◎Tensile strength (g/fabric weight) Take a sample 20mm wide x 160mm long and measure it using a universal tensile testing machine (Tensilon) at a gripping length of 100mm, a load capacity of 100kg, and a tensile speed of 100mm/min.The value is 1. It was converted into unit basis weight (Ig/m') M per beating width. (Average value of vertical and horizontal directions) ◎Tear strength (
g/fabric weight) Take a sample 60 mm wide x 65 mm long, tie the sample on a sample stand, and make a cut with a knife.

Read the maximum vibration using an Elmendorf tear tester.

This value was converted into per unit area weight (Ig/m'). (Average value in vertical and horizontal directions) ◎Collection efficiency; PARTI
CLE C0LINTER (KC, -DIA type RION
CD,.

Using LT [)), flow rate 1500cc/min, measurement time 3
0 seconds, under the conditions of particle size 0.3 #1 or more, first measure the blank value A (number of particles) without setting anything, then set the sample, and measure the number of particles B (number of particles) that passed through the sample. ), and obtain the collection efficiency using the following formula.

Collection efficiency (%) = (1-B/A)X100◎Pressure loss (pressure loss mmAq); When measuring collection efficiency, measure the pressure difference between upstream and downstream of the sample with a differential pressure diaphragm gauge, It was taken as pressure loss.

◎Crimp ratio (%) This is the value obtained by dividing the difference between the end crimp length and the crimp length of the fiber by the crimp length and multiplying it by 100.

◎Mixing ratio (%) This is the value obtained by dividing the mixed fiber weight by the total nonwoven sheet weight and multiplying it by 100.

◎Number average molecular weight (Mn) and weight average molecular weight (Mw)
This is the molecular weight of polypropylene determined using a gel permeation chromatograph (GPC 150 C manufactured by Nippon Waters Co., Ltd.) in terms of polystyrene standards. Mw/Mn is a measure of molecular weight distribution;
The smaller n is, the narrower the molecular weight distribution is.

◎Melting flow rate (6g7mln) According to JIS K 6758, polypropylene and linear low density polyethylene are heated at 230°C and 2.16kg, and low density polyethylene and high density polyethylene are melted at 19
Measurement is performed under the conditions of 0°C and 2.16 kg.

◎Melting point (1) Determined from the peak temperature of the DTA curve of a TG/DTA type measuring instrument (manufactured by Rigakusha, TAS-200 series TG-DTA). The measurement conditions were a N2 atmosphere and a temperature increase rate of 10° C./min.

◎Melting temperature (poise); using a Shimadzu flow tester (CFT 500 model), spinneret 0.5 mmφ,
Measured under the conditions of 1.0 mm1, 1h011, load 10 kg, and preheating temperature 220°C for 6 minutes.

Example 1, Comparative Example 1.2 Linear low density polyethylene (Mitsubishi Yuka Co., Ltd. product, Mitsubishi Polyethylene LL; UJ 790, melt flow rate 50, density 0.92
8, melt viscosity 700) and polypropylene (melt flow rate 24
0, melt viscosity 200) in a chip blend at a weight ratio of 1
They were mixed in a ratio of 1:1 to form test chips.

This chip was put into an extruder, heated and melted (extruder temperature: 220° C.), and then fed into a nozzle. This nozzle is l mm
It had 200 orifices of 0.4 mmφ arranged in a row at a pitch, and the mixed polymer was discharged into the high-speed fluid from each orifice at a discharge rate of 0.2 g/min. Using superheated steam adjusted to 380°C as a fluid, this superheated steam is injected at a pressure of 3.0 kg/cut G from the slit of the melt blowing nozzle toward the molten mixed polymer, thereby pulling the molten polymer. The fibers were linearized to obtain a group of ultrafine fibers with an average fiber diameter of 2.0. The melting point of this fiber group was measured using a TG/DTA type measuring device and was found to be 123.
Melting point peaks were observed at 163°C and 163°C.

On the other hand, polypropylene fibers of 6 denier, length 64 positive, bowing strength 3y5g/denier, and crimp ratio 40% are used as mixed fibers, and this fiber is made into a sliver, and many of the slivers are defibrated with a combing roll and shortened. The fibers were flown and mixed into the ultrafine fiber group. The mixed fibers were collected on a moving net provided below to obtain a 20 cm wide web. This web had no lumps of crimped short fibers, and was substantially dispersed in the form of single fibers, which were random and uniform. The mixing ratio of crimped short fibers was 50%.

This web was thermally bonded by passing through a heat embossing roll having a width of 30 cm and having a diagonal (45°) grid pattern. At this time, the temperature of the heat roll was 110 t, and the press pressure was 1
0 kg/adG, and the processing speed was 6 rn/min.

Table 1 shows the weight and physical properties of this nonwoven sheet.

For comparison, a thermally bonded nonwoven sheet made only of ultrafine fibers without mixing crimped staple fibers (comparative product 1), and a method that was exactly the same as described in Example 1 except that polyethylene terephthalate fibers were used as the crimped staple fibers to be mixed. Table 1 also shows the values of the nonwoven sheet obtained in the above manner (comparative product 2).

Table As is clear from this table, the strength (especially tear strength) of the product of the present invention was significantly improved, and it was possible to use it alone. Moreover, it was found to have excellent oil absorption properties.

Further, the nonwoven sheet of the present invention was subjected to electret processing using a wire electrode under the conditions of a voltage of 19 kV, a clearance of 4 marks between the electrode and the sheet, and a sheet speed of 4 m/min. The collection efficiency of this nonwoven sheet was 92%, which was excellent. The surface charge density of the nonwoven sheet is 9Xl0-"
Coulomb/cut.

Examples 2 to 6, Comparative Example 3 A nonwoven sheet with a basis weight of 50 g/m'' was obtained in the same manner as in Example 1, except that the weight ratio of polyethylene and polypropylene was changed. The results are shown in Table 2.

Note that the web obtained from 95% by weight polyethylene and 5% polypropylene had many polymer beads and was unsuitable for commercial use, so its physical properties were not measured.

Table 2 Polyethylene and polypropylene were chip-blended and melt-blown in the same manner as in Example 1 to obtain a web-like ultrafine fiber group.

Eccentric composite m fiber with a fiber diameter of 20 an (6 d), a length of 64 mm, and a crimp rate of 40% [core: polypropylene, melting point 161°C, sheath: polyethylene, melting point 132°C, (product name: 2ESm fiber, (manufactured by Chisso Corporation)] was made into a sliver, and a large number of the slivers were defibrated with a combing roll to fly the short fibers and mix them into the ultrafine fiber group. This mixed fiber group was collected on a moving net provided below to obtain a 20 cm wide web. This web had no lumps of crimped conjugate fibers, and was substantially dispersed in the form of single fibers, which were random and uniform. The mixing ratio of crimped short fibers was 30%.

This web was processed and thermally bonded in the same manner as in Example 1.

The obtained nonwoven sheet had a basis weight of 80 g/m' and a thickness of 0.4
5 degrees, tensile strength 14.4 g/fabric weight, tear strength 12.3
g/fabric weight, collection efficiency 62%, pressure loss 4. DmmH
It was xO.

It also had excellent oil absorption.

Further, the web before thermal bonding was thermally bonded using hot air at 110°C.

The obtained nonwoven sheet had a basis weight of 80 g/m'' and a thickness of 0.9
mm, tensile strength 10.2 g/fabric weight, tear strength cuff,
It had a weight per area of 0 g/fabric and had excellent filter performance and heat insulation properties.

〔Effect of the invention〕

Since the nonwoven sheet of the present invention is constructed as described above, the excellent features of the ultrafine fiber nonwoven sheet such as filter performance, bacterial barrier properties, dust absorption properties, liquid absorption properties, and heat insulation properties are hardly impaired. On the contrary, it is possible to improve the strength, and the strength is significantly improved, and it becomes possible to use it alone without reinforcing it by laminating other high-strength sheets. This policy, various filters,
It can now be used not only as wipers, wraps, and insulation materials, but also as civil engineering materials such as roofing and wall materials, and as medical and sanitary materials such as surgical gowns, sheets, diapers, and napkins. Moreover, it is advantageous in terms of process since it does not require pasting other sheet-like materials, etc., and is therefore excellent in terms of cost. Therefore, the present invention has great industrial significance.

The present invention provides for the first time an ultrafine fiber web made of polyethylene and polypropylene and having an average fiber diameter of 4 tones or less.

Claims (2)

[Claims] 1. A fiber sheet consisting of a random mixture of ultrafine fibers made of polyethylene and polypropylene and fibers having a larger average fiber diameter than the ultrafine fibers and containing polyethylene or polypropylene as at least a part thereof, A high-strength nonwoven sheet in which thick fibers and the ultrafine fibers are partially thermally bonded. 2. The high-strength nonwoven sheet according to claim 1, wherein the constituent fibers are electret.