JP7739656B2 - Nonwoven fabric manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a nonwoven fabric, and in particular to a method for manufacturing a nonwoven fabric for use as a substrate for an electromagnetic wave shielding material.

In recent years, with the miniaturization and increasing density of electronic component materials, there has been a demand for electromagnetic wave shielding materials that are thin and lightweight yet exhibit excellent electromagnetic wave shielding properties. For example, Patent Document 1 proposes an electromagnetic wave shielding material made of a nonwoven fabric treated with a metal film.
This nonwoven fabric for use as an electromagnetic wave shielding material is required to have low basis weight (thinness, lightness), strength, suitability for metal coating treatment (breathability), etc., and the present inventors have proposed a nonwoven fabric that has an excellent balance of these properties in Patent Document 2.

Here, nonwoven fabrics are produced by subjecting a sheet containing thermoplastic fibers to a heat treatment to fuse the fibers. The heat treatment is carried out by passing the sheet under tension between a pair of heat-pressure rolls heated to a temperature close to the melting point of the thermoplastic fibers. However, if this heat treatment is carried out using only the heat-pressure rolls, thermal elongation is likely to occur, and roll wrinkles are likely to occur in the resulting nonwoven fabric. For this reason, a production method is commonly used in which a preheating roll is provided upstream of the heat-pressure roll, and the thermoplastic fibers are fused using the preheating roll after the preheating roll has preheated the sheet. In the heat treatment process using the preheating roll, the purpose of the preheating roll is to reduce the temperature difference between the sheet and the heat-pressure roll. Therefore, it was common general technical knowledge at the time of filing this application that the temperature of the preheating roll is lower than that of the heat-pressure roll.
However, the inventors of the present invention conducted extensive research into heat and pressure treatment conditions that would enable the production of nonwoven fabrics with even higher strength than those obtained when the temperature of the conventional preheating roll is lower than that of the heat and pressure roll, and as a result, they discovered that strength is increased when the temperature of the preheating roll is higher than that of the heat and pressure roll, which led to the completion of the present invention.

JP 2014-75485 A Patent Application No. 2020-194455

The objective of the present invention is to provide a method for manufacturing nonwoven fabric that can easily produce nonwoven fabrics that are stronger and have lower density than conventional methods.

The means for solving the problems of the present invention are as follows.
1. A method for producing a nonwoven fabric, comprising a step of heat-treating a sheet containing unstretched polyester fibers in a heat-pressure treatment apparatus having a preheating roll and a pair of heat-pressure rolls in this order,
the preheating roll temperature is Tml-40°C or higher and Tml or lower, where Tml is the melting point of the polyester fiber having the lowest melting point contained in the sheet;
A method for producing a nonwoven fabric, characterized in that the temperature of the heat and pressure roll is equal to or lower than the temperature of the preheat roll.
2. The method for producing a nonwoven fabric according to 1., wherein the nonwoven fabric is a nonwoven fabric for use as an electromagnetic wave shielding material.

Another means for solving the problems of the present invention is as follows.
3. A method for producing a nonwoven fabric, comprising a step of heat-treating a sheet containing unstretched polyester fibers having a melting point of 220°C or higher and 250°C or lower in a heat and pressure treatment device having a preheating roll and a pair of heat and pressure rolls in this order,
A method for producing a nonwoven fabric, wherein the preheating roll temperature is 200°C or higher and the heat and pressure roll temperature is less than 200°C.

The manufacturing method of the present invention is characterized by setting the temperature of the heat-press roll to a temperature equal to or lower than the preheat roll temperature. The manufacturing method of the present invention makes it easy to adjust the physical properties of the resulting nonwoven fabric, and compared to conventional manufacturing methods, it is possible to easily produce nonwoven fabrics that are high in strength and low in density. The manufacturing method of the present invention can produce nonwoven fabrics that are thin and lightweight, yet have excellent strength and breathability, making it suitable for producing nonwoven fabrics for use as electromagnetic wave shielding materials.

The present invention relates to a method for producing a nonwoven fabric, which includes a step of heat-treating a sheet containing unstretched polyester fibers in a heat-pressure treatment device equipped with a preheating roll and a pair of heat-pressure rolls in that order. The heat-pressure treatment device used in the present invention is not particularly limited as long as it is equipped with a preheating roll and a pair of heat-pressure rolls in that order, and any known device can be used.

In the method for producing the nonwoven fabric of the present invention, the preheating roll temperature is Tml-40°C or more and Tml or less, where Tml is the melting point of the polyester fiber having the lowest melting point contained in the sheet;
The temperature of the heat and pressure roll is lower than the temperature of the preheat roll.
The sheet to be treated in the present invention contains unstretched polyester fibers, but may also contain stretched polyester fibers. Since the melting point of stretched polyester fibers is higher than that of unstretched polyester fibers, the polyester fibers with the lowest melting point in the present invention are unstretched polyester fibers.

In the manufacturing method of the present invention, polyester fibers with the lowest melting point are heated to near their melting point using a preheating roll, which softens the unstretched polyester fibers and initiates crystallization. When the polyester fibers are then crushed between a pair of hot and pressure rolls, the pressure causes the polyester to crystallize. This crystallization also progresses at the points of contact with other fibers, resulting in bonding with the other fibers and a nonwoven fabric with excellent strength. However, within the scope of the inventors' research, no examples of nonwoven fabric manufacturing have been reported that involve a heat treatment step in which the hot and pressure roll temperature is lower than the preheating roll temperature.

In the nonwoven fabric manufacturing method of the present invention, the preheat roll temperature is at least Tml - 40°C and not more than Tml, where Tml is the melting point of the polyester fiber with the lowest melting point contained in the sheet. When multiple preheat rolls are provided, the temperature of the most downstream preheat roll is set within the above range. By setting the preheat roll temperature within this range, the polyester fiber with the lowest melting point contained in the sheet can be softened, and the softened polyester fiber can be firmly bonded to other fibers by heat and pressure treatment with the downstream heat and pressure roll, resulting in a nonwoven fabric with excellent strength. The lower limit of the preheat roll temperature is preferably Tml - 38°C or more, and more preferably Tml - 35°C or more. The upper limit of the preheat roll temperature is preferably Tml - 1°C or less, and more preferably Tml - 2°C or less. If the preheat roll temperature exceeds Tml, the polyester fiber with the lowest melting point may melt and stick to the surface of the preheat roll, resulting in thermal elongation.

In the nonwoven fabric manufacturing method of the present invention, the temperature of the heat and pressure roll is equal to or lower than the preheating roll temperature. The heat and pressure roll temperature is preferably equal to or lower than the preheating roll temperature (Tpr) minus 5°C, and more preferably equal to or lower than Tpr minus 8°C. Furthermore, the heat and pressure roll temperature is preferably equal to or lower than the melting point (Tmh) of the polyester fiber with the highest melting point contained in the sheet (Tmh minus 60°C). If the heat and pressure roll temperature is higher than Tmh minus 60°C, sticking to the roll or thermal elongation may occur. In the manufacturing method of the present invention, it is sufficient that the heat and pressure roll can crush the polyester fiber to some extent before it comes into contact with the heat and pressure roll during heat and pressure treatment and cools and hardens. Therefore, the lower limit of the heat and pressure roll temperature is not particularly limited, but is, for example, 100°C or higher.
The linear pressure of the pair of heat and pressure rolls is not particularly limited as long as it can firmly bond the fibers together, but is preferably 40 N/mm or more, and more preferably 80 N/mm or more. If it is less than 40 N/mm, the fibers cannot be sufficiently bonded together, and the strength of the nonwoven fabric may not be fully developed.
The processing speed of the preheating roll and the pair of heat and pressure rolls is preferably 1 m/min or more and 100 m/min or less. If the processing speed exceeds 100 m/min, sufficient heat and pressure cannot be applied to the nonwoven fabric, and the strength of the nonwoven fabric may not be developed.

For example, in the case of a sheet containing stretched polyester fibers (melting point approximately 260°C) and unstretched polyester fibers with a melting point of 220°C to 250°C, it is preferable that the preheating roll temperature be 200°C or higher and the heat and pressure roll temperature be less than 200°C. The preheating roll temperature is preferably 230°C or lower, and more preferably 225°C or lower. The heat and pressure roll temperature is preferably 100°C or higher, and more preferably 120°C or higher.

The (drawn/undrawn) polyester-based fibers used in the present invention are not particularly limited, and single-component types can be used, or composite fibers such as core-sheath, eccentric, and side-by-side types can be used in combination. In the present invention, two or more types of drawn polyester-based fibers and undrawn polyester-based fibers with different finenesses, fiber lengths, melting points, etc. can also be mixed and used.

The method for producing the nonwoven fabric of the present invention is not particularly limited as long as it includes the heat treatment step described above.
For example, a sheet containing unstretched polyester fibers can be produced by a conventionally known papermaking process. Examples of papermaking machines include a cylinder papermaking machine, an inclined short-wire papermaking machine, a Fourdrinier papermaking machine, and a twin-wire papermaking machine. Furthermore, the sheet can be dried by a conventionally known drying process, including a Yankee dryer, a multi-cylinder dryer, a hot air dryer, and an infrared heating dryer. Furthermore, the heat treatment process can also serve as the drying process.

When the nonwoven fabric obtained by the present invention is used as a nonwoven fabric for an electromagnetic wave shielding material, the method for treating the nonwoven fabric with a metal film is not particularly limited, and any conventionally known method can be used, such as electroless plating, electroplating, vapor deposition, sputtering, etc. Among these, electroless plating is preferred because a metal film can be formed simply by contacting the nonwoven fabric with a plating solution.
The metals include gold, silver, copper, zinc, nickel, tin, and alloys thereof. The same or different metals can be used to form a coating of two or more layers. Among these, copper is preferred from the viewpoints of electrical conductivity and manufacturing costs.

The processing steps for metal film treatment using electroless plating can be carried out using standard methods, for example, as follows. A refining process is performed to remove any sizing agents and oils adhering to the nonwoven fabric surface, and then, if necessary, the nonwoven fabric is immersed in an alkaline solution for weight reduction. The refined nonwoven fabric is subjected to a catalyst process in which a treatment agent made by colloidalizing palladium with tin, which serves as the nucleus for electroless metal plating, is adsorbed onto the fiber surface. The nonwoven fabric is then preferably washed with water and then activated in an acceleration process to activate the colloid. After activation, the nonwoven fabric is washed with water again and immersed in a plating bath, forming a metal film on the surface of the nonwoven fabric.

In the present invention, the basis weight, thinness, strength, breathability, etc. of the obtained nonwoven fabric can be adjusted by adjusting the fineness, fiber length, blending ratio, etc. of the (stretched/unstretched) polyester fibers used.
The basis weight of the nonwoven fabric produced by the present invention can be selected depending on the desired thinness, lightness, etc., and can be, for example, 4 g/ ^m2 or more and 120 g/ ^m2 or less. For example, when producing a nonwoven fabric for an electromagnetic wave shielding material, the upper limit of the basis weight is preferably 20 g/ ^m2 or less, more preferably 15 g/m2 or ^less , and even more preferably 10 g/ ^m2 or less, and the lower limit of the basis weight is preferably 4 g/m2 or ^more , and even more preferably 6 g/m2 or ^more . Furthermore, when producing a nonwoven fabric for an electromagnetic wave shielding material, the thickness is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less.

In order to prevent cuts and tears during processing, the nonwoven fabric produced by the present invention preferably has a tensile strength in the MD direction measured in accordance with JIS P8113 of 5 N/15 mm or more, more preferably 5.2 N/15 mm or more, and even more preferably 5.5 N/15 mm or more.
The air permeability of the nonwoven fabric produced by the present invention, measured in accordance with JIS L1096 8.26.1 Method A (Fragile method), can be selected depending on the application, etc., but can be, for example, 150 cm ³ /(cm ² ·s) or more and 800 ^{cm 3} /(cm ² ·s) or less. For example, when producing a nonwoven fabric for an electromagnetic wave shielding material, this air permeability is preferably 200 cm ³ /(cm ² ·s) or more and 300 ^{cm 3} /(cm ² ·s) or less. If the air permeability is within this range, the fabric will have excellent suitability for metal coating treatment and excellent electromagnetic wave shielding properties after metal coating treatment.

In the manufacturing method of the present invention, the fibers softened during heat and pressure bond together and simultaneously harden as the temperature is lowered. Therefore, by adjusting the various conditions described above, the strength of the bonds between the fibers and the degree of gaps between the fibers can be adjusted. Therefore, the manufacturing method of the present invention can easily produce nonwoven fabrics having a basis weight of 6 g/m2 to 15 g/ ^m2 , an air permeability of 200 ^cm3 /( ^{cm2.s )} to 300 ^cm3 /( ^cm2.s ), and a tensile strength (MD) of 5.5 N/15 mm or more, which were difficult to achieve in conventional manufacturing methods in which the preheating roll temperature is lower than the heat and pressure roll temperature.

The production of a nonwoven fabric for an electromagnetic wave shielding material will be described below.
The melting point of the unstretched polyester fiber is preferably 220° C. or higher and 240° C. or lower. Since the unstretched polyester fiber is not crystallized due to being unstretched, it crystallizes upon heating and exhibits an adhesive effect at a temperature lower than the melting point.
The fineness of the unstretched polyester fiber is preferably 0.1 dtex (fiber diameter 3.9 μm) or more, more preferably 0.2 dtex (fiber diameter 4.6 μm) or more, and even more preferably 0.8 dtex (fiber diameter 9.8 μm) or more. The fineness is preferably 3.3 dtex (fiber diameter 20.0 μm) or less, and more preferably 1.7 dtex (fiber diameter 14.3 μm) or less.
The fiber length of the unstretched polyester fiber is preferably 1.5 mm or more, more preferably 2 mm or more, and even more preferably 2.5 mm or more, and is preferably 15 mm or less, more preferably 9 mm or less, and even more preferably 6 mm or less.

In the present invention, the stretched polyester fiber is an optional material, but by incorporating the stretched polyester fiber, handling properties are improved, such as sticking during production being less likely to occur and heat shrinkage being suppressed. Furthermore, by incorporating the stretched polyester fiber, it becomes easier to adjust the strength of the nonwoven fabric.
The fineness of the drawn polyester fiber is preferably 0.03 dtex (fiber diameter 1.9 μm) or more, more preferably 0.1 dtex (fiber diameter 3.9 μm) or more, and even more preferably 0.3 dtex (fiber diameter 5.3 μm) or more. The fineness is preferably 3.3 dtex (fiber diameter 20.0 μm) or less, more preferably 1.7 dtex (fiber diameter 14.3 μm) or less, and even more preferably 0.8 dtex (fiber diameter 9.8 μm) or less.
The fiber length of the drawn polyester fiber is preferably 1.5 mm or more, more preferably 2 mm or more, and even more preferably 2.5 mm or more, and is preferably 15 mm or less, more preferably 9 mm or less, and even more preferably 6 mm or less.
When the fineness and/or fiber length of the drawn polyester fiber is within the above-mentioned range, appropriate gaps are formed between the fibers, and a nonwoven fabric with excellent breathability can be obtained at a basis weight of 6 g/m2 or ^more and 15 g/ ^m2 or less.

The blending ratio of stretched polyester fiber to the total amount of fiber is preferably 1% by mass or more and 65% by mass or less. If the blending ratio of stretched polyester fiber is less than 1% by mass, the effect of improving handleability due to its blending is hardly expected. On the other hand, if the blending ratio of stretched polyester fiber exceeds 65% by mass, the strength decreases and the nonwoven fabric may become more prone to cuts and tears. The blending ratio of this stretched polyester fiber is preferably 5% by mass or more, and more preferably 10% by mass or more. Furthermore, this blending ratio is preferably 60% by mass or less, more preferably 55% by mass or less, and even more preferably 50% by mass or less.

The nonwoven fabric produced by the method of the present invention can contain polyester fibers having a fineness of 0.3 dtex (fiber diameter 5.3 μm) or less. The melting point of the polyester fibers having a fineness of 0.3 dtex (fiber diameter 5.3 μm) or less is not limited to 220° C. or higher and 240° C. or lower.
Polyester fibers with a fineness of 0.3 dtex (fiber diameter 5.3 μm) or less are thin and easily retain water, so including polyester fibers with a fineness of 0.3 dtex (fiber diameter 5.3 μm) or less improves papermaking properties during wet papermaking. Furthermore, blending polyester fibers with a fineness of 0.3 dtex (fiber diameter 5.3 μm) or less allows the coarseness of the nonwoven fabric to be adjusted, making it easier to adjust the air permeability to a desired value. Furthermore, using polyester fibers with a fineness of 0.3 dtex (fiber diameter 5.3 μm) or less increases the strength of the nonwoven fabric, allowing it to maintain the mechanical strength required for metal coating treatment.
In the present invention, as the polyester fiber having a fineness of 0.3 dtex (fiber diameter 5.3 μm) or less, two or more types of fibers having different fiber lengths, finenesses, or both may be used in combination.
In the present invention, as the polyester fiber having a fineness of 0.3 dtex (fiber diameter 5.3 μm) or less, stretched polyester fiber and/or unstretched polyester fiber can be used as appropriate. The use of unstretched polyester fiber is preferred because it is easier for strength to be developed by treatment with a hot pressure roll.

When polyester fibers with a fineness of 0.3 dtex or less (fiber diameter 5.3 μm) are blended, from the viewpoint of papermaking properties, it is preferable that the blending amount be 1% by mass or more and 30% by mass or less of the total amount of fibers. If this blending rate is less than 1% by mass, little improvement in papermaking properties can be expected. On the other hand, if it exceeds 30% by mass, little further improvement in papermaking properties can be expected, and the nonwoven fabric may become denser, reducing its suitability for metal coating treatment. The blending rate of polyester fibers with a fineness of 0.3 dtex or less (fiber diameter 5.3 μm) is more preferably 5% by mass or more, and even more preferably 10% by mass or more. Furthermore, this blending rate is more preferably 25% by mass or less, and even more preferably 20% by mass or less.

The present invention will be described in more detail below with reference to examples, but the configuration of the present invention is not limited to these examples.
The obtained nonwoven fabric was evaluated by the following measurement methods, and the results are shown in Table 1.

Measurement Methods: Basis Weight, Thickness, Density Basis weight was measured in accordance with JIS P8124.
The thickness of one sheet was measured at a pressure of 100 kPa between the pressure surfaces in accordance with JIS P8118.
In accordance with JIS P8118, the density was calculated by dividing the basis weight (g/m ² ) by the thickness (μm).

Tensile strength and tensile elongation at break were measured in accordance with JIS P8113.
Air permeability [cm ³ /(cm ² ·s)]
Using an air permeability measuring device conforming to JIS L1096 8.26.1 A method (Fragile method), the volume of air passing through an area of 1 cm ² of the sample in 1 second at a differential pressure of 125 Pa was measured.

(strength)
The tensile strength was measured in accordance with JIS P8113 and evaluated according to the following criteria.
5: MD 6.0N/15mm or more 4: MD 5.5N/15mm or more, less than 6.0N/15mm 3: MD 4.0N/15mm or more, less than 5.5N/15mm 2: MD 3.0N/15mm or more, less than 4.0N/15mm 1: MD less than 3.0N/15mm

"Example 1"
A paper stock was prepared by mixing 38% by mass of stretched polyester fiber (manufactured by Unitika Ltd., product number 521, melting point approximately 259°C, fineness 0.4 dtex (fiber diameter 7.0 μm), fiber length 5 mm), 45% by mass of unstretched polyester fiber 1 (manufactured by Teijin Limited, Tepyrus (registered trademark) TR07N, melting point approximately 233°C, fineness 1.2 dtex (fiber diameter 11.8 μm), fiber length 5 mm), and 17% by mass of unstretched polyester fiber 2 (manufactured by Teijin Limited, Tepyrus (registered trademark) TK08P, melting point approximately 248°C, fineness 0.2 dtex (fiber diameter 4.6 μm), fiber length 3 mm).
This stock was wet-laid on a short-wire Yankee paper machine to obtain a sheet with a basis weight of approximately 8 g/ ^m2 . This sheet was subjected to a heat and pressure treatment under the conditions of a preheating roll temperature of 220°C, a pair of heat and pressure rolls at a temperature of 135°C, a linear pressure of 150 N/mm, and a processing speed of 40 m/min, to obtain a nonwoven fabric.

"Example 2"
A nonwoven fabric was obtained in the same manner as in Example 1, except that the heat and pressure roll temperature was set to 170°C.
"Example 3"
A nonwoven fabric was obtained in the same manner as in Example 1, except that the hot press roll temperature was set to 190°C.
"Example 4"
A nonwoven fabric was obtained in the same manner as in Example 3, except that the preheating roll temperature was set to 200°C.
"Example 5"
A nonwoven fabric was obtained in the same manner as in Example 3, except that the preheating roll temperature was set to 230°C.
"Example 6"
A nonwoven fabric was obtained in the same manner as in Example 4, except that the heat and pressure roll temperature was set to 130°C.
"Example 7"
A nonwoven fabric was obtained in the same manner as in Example 1, except that the heat and pressure roll temperature was set to 95°C.

"Comparative Example 1"
A nonwoven fabric was obtained in the same manner as in Example 1, except that the preheating roll temperature was set to 140°C.
"Comparative Example 2"
A nonwoven fabric was obtained in the same manner as in Comparative Example 1, except that the heat and pressure roll temperature was set to 155°C.
"Comparative Example 3"
A nonwoven fabric was obtained in the same manner as in Comparative Example 1, except that the heat and pressure roll temperature was set to 170°C.
"Comparative Example 4"
A nonwoven fabric was obtained in the same manner as in Comparative Example 1, except that the heat and pressure roll temperature was set to 178°C.
"Comparative Example 5"
A nonwoven fabric was obtained in the same manner as in Comparative Example 1, except that the heat and pressure roll temperature was set to 190°C.

The nonwoven fabrics obtained in Examples 1 to 7, which were produced using the production method of the present invention, had high strength and low density. The nonwoven fabric obtained in Example 7 was inferior in strength to the nonwoven fabrics obtained in Examples 1 to 6, but had strength equivalent to that of the nonwoven fabric obtained in Comparative Example 5, which was a conventional production method. It is presumed that sufficient bonds can be formed and strength improved by changing the conditions for Example 7, such as increasing the linear pressure of the heat-pressure roll or slowing down the processing speed to transfer more heat while keeping the preheat roll temperature and heat-pressure roll temperature the same.
The nonwoven fabrics obtained in Comparative Examples 1 to 5 were inferior in strength to the nonwoven fabrics obtained in Examples 1 to 6. Furthermore, the strength of the nonwoven fabrics obtained in Comparative Examples 1 to 5 improved as the heat and pressure roll temperature increased. This is because the preheating roll temperature was 140°C, which is more than 70°C lower than the melting point of the polyester fiber with the lowest melting point (T _ml = 233°C), and crystallization hardly progressed on the preheating roll, but instead progressed mainly on the heat and pressure roll.

Claims (2)

A method for producing a nonwoven fabric, comprising a step of heat-treating a sheet containing unstretched polyester fibers in a heat-pressure treatment device having a preheating roll and a pair of heat-pressure rolls in this order,
the preheat roll temperature is (T _ml - 40°C) or more and (T _{ml) or less, where T ml is} the melting point of the polyester fiber with the lowest melting point contained in the sheet;
A method for producing a nonwoven fabric, characterized in that the temperature of the preheating roll is higher than the temperature of the heat and pressure roll . The method for manufacturing a nonwoven fabric according to claim 1, characterized in that the nonwoven fabric is a nonwoven fabric for use as an electromagnetic wave shielding material.