JP2022114251A - Composite paper structure containing aramid nanofiber and manufacturing method thereof - Google Patents

Abstract

To provide a composite paper structure with improved mechanical properties and flame resistance and a manufacturing method thereof.SOLUTION: A composite paper structure of the invention comprises: a paper base material containing cellulose fibers; and aramid nanofibers composed of para-type wholly aromatic polyamides having an average fiber diameter of 100 nm or less. The composite paper structure has a thickness of 100 to 1000 μm, has a LOI value of 20 to 30 specified in JIS L 1091, and has a tensile strength of 7.0 to 15.0 MPa when a sample having a width of 10 mm and a length of 100 mm is measured at a test speed of 50 mm/min, a distance between chucks of 50 mm, a temperature of 20°C, and a relative humidity of 60% using a Tensilon universal material testing machine.SELECTED DRAWING: Figure 3

Description

The present invention relates to a composite paper structure containing aramid nanofibers and a method for producing the same.

Paper and non-woven fabric structures have various characteristics derived from their structures, such as high surface area, flexibility, and mechanical isotropy. Due to the functional manifestation of such characteristics, it is not only used as printed materials and fabrics, but also actively used in fields such as electrical materials that require precise and advanced technology. When considering the expansion and development in fields such as electrical materials, it is important to find out how to impart high mechanical properties, flame resistance, and high heat resistance while maintaining the characteristics of the paper structure itself.

In terms of making a paper structure flame-retardant, for example, a method of adding/introducing inorganic particles into the paper structure is conceivable, as in Patent Document 1. Inorganic substances generally have high heat resistance, so they are an effective means for improving flame retardancy and heat resistance. However, since the contact between the fiber and the inorganic particles is basically point contact, the magnitude of the intermolecular force interaction cannot be expected to be large. end up Detachment of the inorganic particles is not preferable because it means a decrease in the function imparted.

As another way of thinking about improving the mechanical properties, flame retardancy and thermal properties of paper structures, there is mixed paper making with heat-resistant fibers, as in Patent Document 2, for example. Specifically, main fibers and heat-resistant fibers are stirred and mixed in water, and a paper structure containing heat-resistant fibers is obtained through a papermaking process. Unlike the addition of the inorganic fine particles described above, the contact area between the fibers increases, and the physical entanglement of the fibers can be used, so that it is possible to form a strong structure with high fatigue.

However, it is difficult to uniformly make paper from different types of fibers. For example, if there is a difference in specific gravity between fibers, the fibers will receive different gravitational forces during drainage, resulting in a difference in lamination speed, and as a result, a homogeneous paper structure may not be obtained. In addition, if the surface properties of the fibers differ, the dispersibility and flocculation properties in water will also change, making it difficult to create a homogeneous aqueous dispersion itself. It is necessary to accumulate various technical know-how such as combination, addition amount, addition timing, etc.

JP 2019-137948 A JP 2011-127252 A

The problem to be solved by the present invention is to provide a composite paper structure with improved mechanical properties and flame resistance, and a method for producing the same.

As a result of intensive studies, the present inventors have found that a composite paper structure having high mechanical properties and flame resistance can be obtained by using cellulose fibers and aramid nanofibers composed of para-type wholly aromatic polyamide having a specific fiber diameter. We have found that it is possible to achieve the present invention.

That is, according to the present invention, the following configurations (1) to (4) are provided.
(1) Paper substrate containing cellulose fibers and aramid nanofibers made of para-type wholly aromatic polyamide with an average fiber diameter of 100 nm or less,
A sample having a thickness of 100 to 1000 μm, an LOI value of 20 to 30 as specified in JIS L1091, and a width of 10 mm and a length of 100 mm was measured using a Tensilon universal material testing machine at a test speed. A composite paper structure having a tensile strength of 7.0 to 15.0 Mpa measured under conditions of 50 mm/min, a distance between chucks of 50 mm, a temperature of 20 degrees and a relative humidity of 60%.
(2) The composite paper structure according to the previous configuration (1), wherein the cellulose fibers are contained in an amount exceeding 50% by weight based on the total weight of the paper substrate.
(3) A composite paper structure according to the previous configuration (1) or (2), wherein the aramid nanofibers are contained in an amount of 0.1 to 50% by weight based on the total weight of the paper substrate.
(4) A paper substrate containing cellulose fibers is added with para-type wholly aromatic polyamide fibers or pulp in the presence of an aprotic polar solvent, and a strongly basic substance is added to form nanofibers, resulting in an average fiber diameter of 100 nm or less. Manufacture of the composite paper structure according to any one of the previous configurations (1) to (3) obtained by immersing the dispersion liquid in which the aramid nanofibers are dispersed in and then immersing in water and drying. Method.

According to the present invention, a composite paper structure having high mechanical properties and flame resistance by using a paper base material containing cellulose fibers and aramid nanofibers made of para-type wholly aromatic polyamide having a specific fiber diameter. can be provided.

It is an example which showed the structural observation image of the aramid nanofiber used by this invention. It is an example which showed the structure observation image which expanded FIG. 1 for average fiber diameter calculation. 1 is an example showing a cross-sectional observation image of a composite paper structure containing cellulose fibers and aramid nanofibers of the present invention. 4 is an example of an observation image in which FIG. 3 is enlarged. It is explanatory drawing regarding the calculation method of an average fiber diameter.

The details of the present invention will be described below.

<Composite paper structure>
The composite paper structure of the present invention includes a paper base material containing cellulose fibers and aramid nanofibers made of para-type wholly aromatic polyamide having an average fiber diameter of 100 nm or less, and has a thickness of 100 to 1000 μm (more preferably 100 to 500 μm, more preferably 100 to 300 μm), an LOI value specified in JIS L1091 of 20 to 30 (more preferably 22 to 30), and a width of 10 mm and a length of 100 mm. The tensile strength measured using a Tensilon universal material testing machine under the conditions of a test speed of 50 mm / min, a distance between chucks of 50 mm, a temperature of 20 degrees, and a relative humidity of 60% is 7.0 to 15.0 Mpa (more preferably 8.0 to 15.0 MPa).

In the composite paper structure of the present invention, the weight ratio (content ratio) of aramid nanofibers in the composite paper structure is preferably 0.1 to 50 parts by weight, more preferably 1 to 30 parts by weight. , more preferably 5 to 25 parts by weight, most preferably 8 to 20 parts by weight. Within such a range, high mechanical properties and flame resistance can be obtained. The aramid nanofiber content rate can be controlled by the aramid nanofiber concentration in the liquid when impregnating the paper substrate. If the content is less than 0.1% by weight, it is necessary to lower the concentration of aramid nanofibers in the liquid to be impregnated, but the amount of solvent exchange during immersion in a poor solvent after impregnation increases, resulting in large voids. In some cases, a higher-order structure containing Also, if the content is 50% by weight or more, although a mechanical reinforcing effect is expected, it is necessary to excessively increase the concentration of aramid nanofibers in the liquid to be impregnated.・The impregnation workability may decrease.

In addition, the paper substrate used in the composite paper structure of the present invention preferably contains 50% by weight, more preferably 70% by weight, and even more preferably 75% by weight of cellulose fibers relative to the total weight of the paper substrate. More than 80% by weight is particularly preferred.
Cellulose fibers may be, for example, wood pulp made from softwood or hardwood, or non-wood pulp made from kenaf or the like.

The paper substrate used in the composite paper structure of the present invention may further contain other fibers, if necessary. Other fibers can be appropriately selected depending on the purpose. For example, if the purpose is to improve mechanical strength, high-strength fibers can be mixed. Furthermore, purpose-suitable fibers (eg, heat-dissipating fibers, conductive fibers, etc.) may be mixed.

In addition, the composite paper structure of the present invention may contain other components as necessary. Specific other components include, for example, silicon, silicon carbide (SiC), semiconductor materials such as germanium, silicon nitride (silicon nitride), aluminum nitride (aluminum nitride), boron nitride (boron nitride), etc. Compounds, carbon black, diamond, graphene, fullerene, carbon nanotubes, carbon nanofibers, inorganic substances such as carbon materials such as graphite. Mineral fine particles such as kaolin, talc, clay, hydrotalcite and diatomaceous earth can also be used. In addition to the particles listed above, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, magnesium sulfate, barium sulfate, aluminum hydroxide, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, and alumina. , mica, zeolite, glass, etc. can also be used. The high specific surface area of the aramid nanofibers works effectively, and it is also possible to function as a component holding material.

<Aramid nanofiber>
The aramid nanofibers used in the composite paper structure of the present invention have an average fiber diameter of 100 nm or less, preferably 50 nm or less, and more preferably 25 nm or less. The lower limit of the average diameter is preferably 1 nm or more, preferably 3 nm or more. Also, the aramid nanofibers preferably do not have a diameter greater than 500 nm. When the average fiber diameter exceeds 100 nm, the surface area of the nanofibers becomes small, and the entanglement density and contact area between the nanofibers become small, so that the intermolecular force interaction including hydrogen bonding becomes small. It is not preferable because it does not lead to sufficient mechanical strength expression of the higher-order structure.

The aramid nanofiber in the present invention preferably has an aspect ratio represented by fiber length/fiber diameter of 10 or more and 1,000 or less, more preferably 10 or more and 500 or less, and still more preferably 10 or more and 100 or less. . If the aspect ratio is less than 10, it may be difficult to develop an entangled structure of the fibers, thereby making it difficult to develop the expected properties.

Moreover, the aramid nanofiber in the present invention is a para-type wholly aromatic polyamide. As the para-type wholly aromatic polyamide, poly-p-phenylene terephthalamide, poly-p-benzamide, poly-p-amide hydrazide, poly-p-phenylene terephthalamide-3,4-diphenyl ether terephthalamide and the like are preferable and oriented. Poly-p-phenylene terephthalamide fibers having crystallinity (forming domains of liquid crystal structure in the spinning solution) are preferred.

As fibers using para-type wholly aromatic polyamide, poly-p-phenylene terephthalamide fiber (commercially available products are Twaron (registered trademark) manufactured by Teijin Limited, Kevlar (registered trademark) manufactured by Toray DuPont Co., Ltd. Trademark)”, etc.), and coparaphenylene/3,4′-oxydiphenylene terephthalamide fibers (commercially available products such as “Technora (registered trademark)” manufactured by Teijin Limited).

<Production of aramid nanofibers>
Aramid nanofibers in the present invention are produced by using para-type aromatic polyamide fibers as a raw material, soaking and swelling the fibers in a solvent with high affinity, and further breaking hydrogen bonds by adding a strong basic substance. and the resulting The para-type wholly aromatic polyamide that can be preferably used in the present invention is a polymer in which one or more divalent aromatic groups are linked by amide bonds, and the aromatic group has two or more Aromatic rings may be present and the aromatic rings may be attached directly or through oxygen or sulfur. Also, the hydrogen atom of the divalent aromatic group may be substituted with a halide, a lower alkyl group, or a phenyl group. As the solvent used for producing aramid nanofibers, aprotic polar solvents are preferred, and specific examples include dimethylsulfoxide, dimethylacetamide, N-methyl-2-pyrrolidone and the like. Potassium hydroxide, sodium hydroxide, barium hydroxide, calcium hydroxide, etc., are examples of strong basic substances used in producing aramid nanofibers.

Aramid nanofibers in the present invention can be produced by immersing para-type wholly aromatic polyamide (for example, poly-p-phenylene terephthalamide) fibers or pulp in dimethyl sulfoxide adjusted to alkalinity. can.

A para-type wholly aromatic polyamide (eg, poly-p-phenylene terephthalamide) is obtained by forming a domain of a liquid crystal structure in a spinning solution, discharging it from a capillary, and washing the spinning solvent with water. After the weak bonds between the liquid crystal domains constituting the fiber are cut by the above-mentioned method under alkaline conditions, the obtained fiber is liberated in a highly compatible solvent, so that the fiber can be cut with high elasticity and high strength. You can get the fiber that has been made. Then, the aramid nanofibers can be isolated by putting the resulting cut fibers into a poor solvent (water, alcohol, acetone, etc.) as a dispersion solvent.

In addition, a para-type whole aromatic polyamide (for example, poly-p-phenylene terephthalamide) fiber having a diameter of 10 to 20 μm is cut into several mm and subjected to a refiner treatment in which mutual shearing is performed in water. Aromatic polyamide (eg, poly-p-phenylene terephthalamide) pulp can be obtained. In such a refiner treatment, the fibers that are finer from the fiber surface due to the shear fracture at the liquid crystal interface are not completely separated and are in a branched state, and the diameter of the finer fibers is 100 to 1000 nm. The diameter is several μm. Refined pulp can also be converted into aramid nanofibers by the above treatment.

As another method for refining the aramid material, instead of the chemical treatment as described above, a method of applying a mechanical shearing force can be considered, but this method partially obtains a fibril structure with a nano-order fiber diameter Stay as long as you can. Therefore, since a non-fibrillated micro-order structure is also included, a homogeneous entangled structure on the nano-order cannot be obtained, and expected physical properties are not exhibited. In the present invention, it is preferable to form an entangled structure with uniformly nanoized fibers instead of partial nanoization.

The composite paper structure in the present invention is obtained by adding para-type wholly aromatic polyamide fibers or pulp to a paper base material containing cellulose fibers, adding a strongly basic substance in the presence of an aprotic polar solvent, and making nanofibers. It is obtained by immersing in a dispersion liquid in which aramid nanofibers having an average fiber diameter of 100 nm or less are dispersed by coating or the like, followed by immersion in water and a drying process. For example, para-type wholly aromatic polyamide fiber or pulp is added with a strongly basic substance in an aprotic polar solvent to form nanofibers, and aramid nanofibers having an average fiber diameter of 100 nm or less are dispersed. is coated on a glass plate, a paper base material containing cellulose fibers is placed on the coated surface, the dispersion is applied from above, and then the paper base material is immersed in a poor solvent such as water. , a composite paper structure can be obtained through a drying process.

In addition, in the method of wet-mixing papermaking by adding nanofibers at the time of manufacturing the paper substrate, it is necessary to appropriately select and control the affinity in water between the general-purpose fibers constituting the paper substrate and the nanofibers. In addition, when fine structures such as nanofibers are added, it is difficult to obtain uniform mixed papermaking due to the influence of the difference in the specific gravity of the substances and the increase in drainage time. Therefore, it is preferable to use a post-processing process for the paper substrate.

The present invention will be described in detail below with reference to Examples and Comparative Examples, but the scope of the present invention is not limited to the following Examples and Comparative Examples. Each characteristic value in the examples was measured by the following method.

<Thickness>
The resulting composite paper was cut into 30 cm squares, and the thickness at five points in total was measured with a thickness meter (DG-925 manufactured by ONO-SOKKI), and the average value was calculated.

<Metsuke>
The obtained composite paper was cut into 30 cm squares, and the basis weight (g/m ² ) was calculated by measuring the weight.

<Aramid nanofiber weight ratio in composite paper structure>
The aramid nanofiber weight ratio (WAramid) in the composite paper structure was calculated by the following formula.
WAramid = (Whp-Wp)/Whp
here,
Whp: composite paper structure basis weight (g/m ² )
Wp: paper base basis weight (g/m ² )

<Tensile test>
The resulting composite paper was cut into strips with a width of 10 mm and a length of 100 mm, and both ends of the sample were sandwiched between flat plate chucks with a distance between the chucks of 50 mm. , a tensile test was performed. A Tensilon Universal Material Testing Machine (RTF Series) was used as a tensile tester, and tensile strength, tensile modulus and elongation at break were measured.

<Oxygen index (LOI) measurement>
The obtained composite paper was cut into a size of 150 mm long and 63 mm wide, and the minimum oxygen concentration (%) was measured according to JIS L1091. A Suga tester (model: ON-2N) was used as a measuring device.

<Average fiber diameter>
The structure of the sample was observed using a scanning electron microscope (manufactured by JEOL Ltd., product number: JSM-6330F). From the image observed at a magnification setting of 50,000 times, select an image area of 1,800 nm to 2,000 nm in width and 1,200 nm to 1,500 nm in length, and divide the image area into 4 vertically and 4 horizontally. A total of 16 grid areas A1-D4 obtained by division are defined, one sample present in each grid area is selected, and the average value of the fiber diameter of the selected sample measured on the image is the average fiber diameter. (Figs. 2 and 5).

[Example 1]
<Production of aramid nanofibers>
・ Twaron (registered trademark) pulp manufactured by Teijin Aramid Co., Ltd. (product number: type 1094): 266 g
* Solid weight: 50g Moisture weight: 216g
・ Potassium hydroxide: 100 g
・Dimethyl sulfoxide (DMSO): 1,950 g
Each of the above materials was placed in a 2 L plastic container and heated in a dryer set at 70° C. for 3 hours. The color of the mixed liquid changed from yellow to reddish brown, no pulp-like substance was observed, and when it was visually confirmed that the viscosity at room temperature had increased compared to immediately after mixing, aramid nanofibers were formed. It was easy to judge that it was obtained. The concentration of aramid nanofibers in the liquid obtained from the composition described above is 2.5 wt %.

<Aramid nanofiber impregnation on paper substrate>
As a paper base, Filter manufactured by Toyo Roshi Kaisha, Ltd., which is a paper base containing cellulose fibers
A paper "ADVANTEC" was used. An aramid nanofiber dispersion diluted with dimethyl sulfoxide was prepared so that the aramid nanofiber concentration in the liquid was calculated to be 1.0 wt %, and the dispersion was applied onto a glass plate and spread thinly with a glass rod.

A paper base material was placed thereon, and an aramid nanofiber dispersion was additionally applied thereon, followed by coating on the surface of the paper base material with a glass rod. As a guideline, 50 g of the dispersion liquid was used for a 30 cm square paper substrate. After that, the whole glass plate was placed in water at normal temperature and normal pressure to solidify the aramid nanofibers.

After being immersed in water for 10 minutes, the paper was taken out, lightly drained with thick filter paper, and dried in an explosion-proof dryer at 70°C to obtain a composite paper structure containing aramid nanofibers.

It was confirmed that the inclusion of aramid nanofibers improved the mechanical properties of the paper substrate. A high oxygen index (LOI) was obtained as a result of reflecting the high flame resistance of the aramid itself.

[Example 2]
Regarding the aramid nanofiber dispersion when applying to the paper base material, a composite paper structure was prepared in the same manner as in Example 1 except that a liquid having an aramid nanofiber concentration in the dispersion liquid of 2.5 wt% was used. A body was prepared and evaluated.

It was confirmed that the inclusion of aramid nanofibers improved the mechanical properties of the paper substrate. It was also confirmed that the more the amount of aramid nanofiber added, the more the mechanical properties of the composite paper tended to improve. A high oxygen index (LOI) was obtained as a result of reflecting the high flame resistance of the aramid itself.

[Comparative Example 1]
Evaluation was performed in the same manner as in Example 1 using the paper substrate itself.

[Comparative Example 2]
As the liquid to be applied to the paper substrate, a solution obtained by dissolving a copolymerized para-aramid polymer in N-methyl-2-pyrrolidone (polymer concentration in the solution: 1 wt%) was used. A composite paper structure was produced and evaluated in the same procedure as.

Although the mechanical properties of the paper substrate were also improved by the method of incorporating the copolymerized para-aramid into the paper substrate, the oxygen index (LOI) was 20 or less. It was expected that the addition of the copolymerized para-aramid polymer would bring out the flame resistance of the aramid itself. This is thought to be because the high-order structure of the aramid portion, which has high flame resistance, is a sparse structure with many voids. be done.

[Comparative Example 3]
As the liquid to be applied to the paper substrate, a solution obtained by dissolving a copolymerized para-aramid polymer in N-methyl-2-pyrrolidone (polymer concentration in the solution: 2.5 wt%) was used. A composite paper structure was produced and evaluated in the same procedure as in Example 1.

Although the mechanical properties of the paper substrate were also improved by the method of incorporating the copolymerized para-aramid into the paper substrate, the oxygen index (LOI) was 20 or less. It was expected that the addition of the copolymerized para-aramid polymer would bring out the flame resistance of the aramid itself. This is thought to be because the high-order structure of the aramid portion, which has high flame resistance, is a sparse structure with many voids. be done.

[Comparative Example 4]
As the liquid to be applied to the paper substrate, a solution obtained by dissolving a para-aramid polymer having the same polymer structure as in Example 1 in N-methyl-2-pyrrolidone (polymer concentration in the solution: 1 wt%) is used. did. With the composition of this comparative example, the stability of the para-aramid polymer solution was low, and gelation progressed quickly over time, so a composite paper structure could not be produced by coating a paper substrate.

Since the composite paper structure of the present invention is excellent in mechanical properties and flame resistance, it is extremely useful in many fields such as industrial materials, electrical and electronic fields, agricultural materials, optical materials, aircraft, automobiles, and ships. can be used effectively.

Claims (4)

A paper base material containing cellulose fibers, and aramid nanofibers made of para-type wholly aromatic polyamide having an average fiber diameter of 100 nm or less,
A sample having a thickness of 100 to 1000 μm, an LOI value of 20 to 30 as specified in JIS L1091, and a width of 10 mm and a length of 100 mm was measured using a Tensilon universal material testing machine at a test speed. A composite paper structure having a tensile strength of 7.0 to 15.0 Mpa measured under conditions of 50 mm/min, a distance between chucks of 50 mm, a temperature of 20 degrees and a relative humidity of 60%. The composite paper structure of Claim 1, wherein the cellulose fibers comprise more than 50% by weight of the total weight of the paper substrate. 3. The composite paper structure according to claim 1, wherein the aramid nanofibers are contained in an amount of 0.1 to 50% by weight based on the total weight of the paper substrate. Aramid nanoparticles with an average fiber diameter of 100 nm or less obtained by adding a strongly basic substance to a paper base material containing cellulose fibers and adding a strongly basic substance in an aprotic polar solvent to a paper base material containing cellulose fibers. 4. The method for producing a composite paper structure according to any one of claims 1 to 3, which is obtained by immersing the dispersion in which the fibers are dispersed, followed by immersion in water and drying.

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