JP2002363304A - Composite sheet of polytetrafluoroethylene resin, and ion-exchange membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a composite sheet of which the cross-linking by an ionizing radiation can be easily and surely carried out even when it is thin and which has sufficient mechanical strengths and is almost free from bad influences due to swelling, etc., and an ion-exchange membrane using the same. SOLUTION: This composite sheet has a composite structure formed by impregnating a woven or nonwoven fabric comprising heat-resistant fibers with a polytetrafluoroethylene resin. The resin is cross-linked by an ionizing radiation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a polytetrafluoroethylene resin (hereinafter, referred to as "cross-linked") crosslinked by ionizing radiation.
PTFE) and an ion-exchange membrane obtained by introducing an ion-exchange functional group into the crosslinked product by graft polymerization.

[0002]

2. Description of the Related Art PTFE is excellent in chemical resistance, heat resistance, etc. and is widely used as a resin for industrial and consumer use. However, there is a disadvantage that the mechanical characteristics are deteriorated. To improve such radiation resistance, Japanese Patent Application Laid-Open
JP-A-116423 discloses that a PTFE film has a
In the absence of oxygen at a temperature equal to or higher than the crystal melting point of FE, 1 ×
Irradiating ionizing radiation of 10 ³ Gy or more to the PTF
A method for modifying an E film is disclosed.

On the other hand, regarding the graft copolymerization of PTFE, as disclosed in Japanese Patent Application Laid-Open No. 2000-159914, an ionizing radiation of 0.1 KGy or more is irradiated in the absence of oxygen at a temperature of the melting point of PTFE or more. And then cross-linking, then graft copolymerizing various compounds with ionizing radiation. In this publication, an ion-exchangeable fluororesin film is obtained by using a compound having an ion-exchangeable functional group at the time of graft copolymerization.

[0004] Generally, PTFE is known as a polymer having a high melt viscosity even at a temperature higher than the crystal melting point because of its high molecular weight. For this reason,
It is possible to maintain the shape of the molded body even at a temperature equal to or higher than the melting point, and it is possible to irradiate ionizing radiation at a temperature equal to or higher than the melting point in a sheet shape as in the above-mentioned known example.

[0005]

However, in the above method, when a thin PTFE sheet is crosslinked, it is difficult to maintain the shape when irradiating with ionizing radiation at a temperature higher than the melting point. It was found that distortion, cracks, pinholes, etc. occurred in the sheet. That is, generally, the shape can be maintained when the PTFE sheet is heated to a temperature equal to or higher than the melting point, in the case of a sheet having a thickness of 1 mm or more. For this reason,
Modification of thin films requires special equipment that can maintain the shape.

Further, when a thin modified PTFE sheet is used as an ion exchange membrane or a battery membrane, there is a problem that the swelling is caused by an electrolyte or water and the volume is increased. In particular, when an attempt is made to improve the ionic conductivity by increasing the graft ratio of the PTFE sheet, the ratio of polymerization increases, so that the flexibility increases and the mechanical strength decreases. Accordingly, there is a need for a method that does not decrease the strength while increasing the degree of graft polymerization.

[0007] The ionic conductivity of the fuel cell membrane is inversely proportional to the thickness. The smaller the thickness, the more the ionic conductivity is improved. This necessitates the necessity of a thin conductive film that is difficult to handle, but if it is thin, defects and defects are likely to occur in the production of battery cells, which is a practical problem. Further, when a battery cell is formed by stacking a plurality of low-strength fuel cell membranes, the membrane repeatedly swells and shrinks, causing creep to occur, resulting in an increase in resistance at the ground plane with the gas diffusion electrode.

Accordingly, an object of the present invention is to provide a composite sheet and a composite sheet which have a sufficient mechanical strength and are not easily affected by swelling and the like, even when the thickness is small. An object of the present invention is to provide an ion exchange membrane used.

[0009]

Means for Solving the Problems The present inventors have made intensive studies to achieve the above object, and found that a woven or nonwoven fabric of heat-resistant fiber was impregnated with a polytetrafluoroethylene resin, and the composite sheet was ionized. It has been found that the above object can be achieved by performing crosslinking by radiation, and the present invention has been completed.

That is, the composite sheet of the present invention has a composite structure in which a woven or nonwoven fabric of heat-resistant fiber is impregnated with a polytetrafluoroethylene resin, and the polytetrafluoroethylene resin is crosslinked by ionizing radiation. It is characterized by being.

On the other hand, the ion exchange membrane of the present invention has a composite structure in which a woven or nonwoven fabric of heat-resistant fiber is impregnated with a polytetrafluoroethylene resin, and the polytetrafluoroethylene resin is crosslinked by ionizing radiation. In addition, the crosslinked product has a graft polymer chain having an ion-exchangeable functional group in a repeating unit.

Further, the graft ratio, which indicates the weight percentage of the graft polymer chain to the crosslinked product before the graft polymerization, is 10%.
% Is preferable.

[Effects] According to the composite sheet of the present invention, the cross-linking step by ionizing radiation can be easily and reliably performed even when the thickness is small due to the reinforcing effect of the woven or nonwoven fabric of the heat-resistant fiber. In addition, the crosslinked composite sheet has a sufficient mechanical strength and does not easily cause adverse effects such as swelling.

On the other hand, according to the ion exchange membrane of the present invention, even when the thickness is small, the cross-linking step by ionizing radiation can be easily and reliably performed due to the reinforcing effect of the woven or nonwoven fabric of the heat-resistant fiber. In addition, the graft polymerization can be suitably performed, and the obtained ion exchange membrane has sufficient mechanical strength, and does not easily cause adverse effects due to swelling and the like.

When the graft ratio is 10% or more,
In a conventional ion exchange membrane without a reinforcing material, the mechanical strength tends to be insufficient, and harmful effects such as swelling due to an electrolytic solution or water tend to occur. Therefore, the present invention having the above-described effects is particularly effective.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The composite sheet of the present invention has a composite structure in which a woven or nonwoven fabric of heat-resistant fiber is impregnated with a polytetrafluoroethylene resin. As heat resistant fiber,
When cross-linking with ionizing radiation at a temperature equal to or higher than the melting point of PTFE, it is necessary that the fibers are not easily deteriorated by thermal deterioration or radiation.
Preferably, synthetic fibers or inorganic fibers that do not cause a significant decrease in molecular weight or carbonization deterioration by heating in the range of 330 ° C. or more to 360 ° C. or less.

As the inorganic fiber, an electrically insulating fiber is desirable so as to function as an insulating base material for a battery reaction.
Specifically, glass fiber, alumina fiber, silica fiber,
Boron fibers and the like. In addition, as synthetic fibers, fibers having excellent electrical insulation properties and excellent heat resistance include para-aramid fibers, polyimide fibers, polyphenylene sulfide fibers, and the like.

In the present invention, a woven or non-woven fabric of the above-mentioned heat-resistant fiber is used, but the woven form of the woven cloth or the production form of the non-woven fabric may be any. However, in the case of a non-woven fabric, a non-woven fabric containing no low melting point binder resin or adhesive is preferable, and a non-woven fabric manufactured by mechanical entanglement, melt blowing, heat fusion, or the like is preferable.

The present invention is effective when the thickness of the composite sheet or the ion exchange membrane is less than 1 mm, especially 0.4 mm or less. Preferably, the heat-resistant fiber has a small fiber diameter. Specifically, the average fiber diameter of the heat-resistant fiber is preferably 3 to 100 μm.

A composite structure in which PTFE is impregnated in a woven fabric or the like generally uses a mixed dispersion in which PTFE particles are dispersed in water, impregnates or coats the woven fabric or the like with water, And sintering the particles. At that time, in order to facilitate the impregnation, a surfactant is added industrially to adjust the surface tension. PTF
For the sintering of the E particles, heating may be performed at a temperature equal to or higher than the melting point of PTFE by a conventional method.
By forming a TFE layer, a strong structure can be generated.

The composite sheet of the present invention is classified into a woven fabric and a nonwoven fabric. In the case of a woven fabric, the density of the warp yarn and the density of the weft yarn are 30 yarns / 25 mm to 5 yarns / 25 m, respectively.
A range of m is suitable. If the yarn density is too high, the opening area of the woven fabric will be small, and will not function as an ion conductive film. On the other hand, if the yarn density is low, the strength of the woven fabric is reduced, and the reinforcing effect of the composite sheet is reduced.

On the other hand, in the case of a nonwoven fabric, the basis weight is 5 to 5
0 g / m ² is preferred. If the basis weight is small, the reinforcing effect decreases, and if it is too large, the opening area of the nonwoven fabric decreases,
Does not function as an ion conductive film.

Crosslinking with ionizing radiation is carried out at a temperature higher than the melting point of PTFE by irradiation with ionizing radiation, and preferably in the absence of oxygen. At this time, due to the presence of the above-mentioned heat-resistant fiber, the shape can be maintained even during heating at a temperature equal to or higher than the melting point, and distortion and dimensional change due to heat can be suppressed.

The specific irradiation temperature is equal to or higher than the crystal melting point of PTFE (327 ° C.), and is preferably from 327 ° C. to 360 ° C. If the irradiation temperature is too high, the decomposition of PTFE proceeds and the strength decreases, so the upper limit of the irradiation temperature is 360 ° C.
And it is preferably 335-354 ° C.

As the radiation quality of the ionizing radiation, radiation having a transmitting power is useful, and among radiations, γ-ray, X-ray or electron beam is suitable. When an electron beam is used as the radiation, it has excellent penetrating power and can sufficiently modify the entire PTFE.
It is preferably at least 10 ⁶ electron volts, more preferably at least 7 × 10 ⁶ electron volts.

Usually, the absorbed dose during crosslinking is 1 × 10 ³ G
It is preferable to set y to about 1 × 10 ⁷ Gy. PTFE
In order to make the reforming of the effective appear, the absorbed dose is 1 × 1
0 ⁴ and even more preferably to Gy or more. If the absorbed dose is too large, the decomposition of PTFE proceeds, so the absorbed dose is 1 × 10
More preferably, it is ⁶ Gy or less.

In this way, the woven or non-woven fabric of the heat-resistant fiber has a composite structure in which PTFE is impregnated.
The composite sheet of the present invention in which FE is crosslinked by ionizing radiation can be obtained. The composite sheet of the present invention has radiation resistance, heat resistance, electrical insulation, release properties, dimensional stability,
Because it has many excellent properties, such as slidability, other than a base material such as an ion exchange membrane described below, for example, for heat sealing of bag making and packaging lines, a guide portion, a shooter, which requires mold release properties, It can be used for covering a hopper or the like, or for heat insulation and protection.

On the other hand, in the ion exchange membrane of the present invention, the crosslinked product of PTFE as described above has a graft polymer chain having an ion exchange functional group in a repeating unit.
Formation of such a graft polymerization chain is achieved by graft-polymerizing a vinyl compound having an ion-exchange functional group or graft-polymerizing a vinyl compound having no such functional group, and then grafting an ion-exchange functional group by a reaction. It can be carried out by a method of introducing it into the polymer chain.

The graft polymerization is performed by irradiating the crosslinked PTFE composite sheet with ionizing radiation in advance so that the absorbed dose becomes 1 × 10 ³ Gy or more (irradiation during polymerization is also possible), and then bringing the vinyl compound into contact. be able to. By irradiation with such ionizing radiation, a radical serving as a starting point of a graft can be generated in the crosslinked PTFE. The absorbed dose is preferably 1 × 10 ⁴ Gy to 1 × 10 ⁶ Gy. The irradiation temperature of ionizing radiation for generating radicals is preferably a temperature at which radicals hardly disappear, and is preferably room temperature or lower. Irradiation with ionizing radiation for graft polymerization is not necessarily performed in the absence of oxygen.

The contact with the vinyl compound can be carried out by dipping in a solution containing the vinyl compound or a liquid vinyl compound, or by applying the solution or the like. As the organic solvent used at this time, an organic solvent that uniformly dissolves the vinyl compound but does not dissolve PTFE is preferable. For example, ketones such as acetone and methyl ethyl ketone, esters such as ethyl acetate and butyl acetate, methyl alcohol, and ethyl Alcohols such as alcohol, propyl alcohol and butyl alcohol, ethers such as tetrahydrofuran and dioxane, aromatic hydrocarbons such as N, N-dimethylformamide, N, N-dimethylacetamide, benzene, toluene, n-heptane, cyclohexane, etc. Aliphatic or alicyclic hydrocarbons, or a mixed solvent. Among these, those that swell the crosslinked product are preferred.

Examples of the ion-exchangeable functional group of the graft polymerization chain include a phenolic hydroxyl group, an ammonium group, a carboxyl group, and a sulfonic acid group exhibiting ion-exchange properties. Further, an acyloxy group, an ester group, and an acid imide group can be easily converted to a functional group having ion exchangeability by hydrolyzing the graft group.

Specific examples of the vinyl compound having an ion-exchangeable functional group include acrylate, methacrylate, maleate, fumarate, hydroxyoxystyrene, acyloxystyrene, vinyl ester, vinyl sulfonate. Styrene carboxylic acid, styrene alkyl sulfonate, vinyl sulfonic acid and the like.

In the case where sulfonation or the like is carried out by using a chemical reaction after graft polymerization with a monomer having no functional group, the monomer having no functional group includes styrene, α-methylstyrene, vinyl toluene And hydroxystyrene. In addition, the introduction of a sulfone group into the obtained graft polymer chain can be achieved by appropriately reacting a sulfonating agent such as sulfuric acid, fuming sulfuric acid, sulfur trioxide, chlorosulfuric acid, and amidosulfuric acid.

Further, at the time of graft polymerization, a crosslinking monomer may be mixed as needed at 0.01 to 10% by weight for the purpose of improving the durability and dimensional stability of the obtained ion exchange resin. Examples of such a monomer include monomers having a plurality of vinyl groups, such as divinylbenzene and trimethylolpropane triacrylate.

In the present invention, the graft ratio indicating the weight percentage of the graft polymer chain to the crosslinked product before the graft polymerization is preferably 10% or more, more preferably 30 to 80%.

In the ion exchange membrane of the present invention, since the graft ratio of PTFE can be improved without lowering the mechanical strength, it can be suitably used even under conditions where resin swelling is likely to occur.

In particular, the composite sheet before the graft polymerization is made of P
TFE particles have a sintered microstructure, and when the graft polymer chains are formed in the portions including the inside of the pores, the ion-conductive portion has a continuous structure on the front and back of the ion exchange membrane. As ion exchange membranes having good ion conductivity, particularly ion conductive films (polymer electrolytes) for fuel cells,
It is useful for other battery films and the like. From the viewpoint of obtaining such a fine structure, the average particle size of the PTFE particles to be sintered is preferably 0.2 to 0.4 μm.

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments and the like specifically showing the configuration and effects of the present invention will be described below.

<Example 1> Glass fiber woven fabric (thickness: 0.
PTFE dispersion (PTFE resin solid content: 60% aqueous dispersion, average particle diameter 0.3 μm) was impregnated into the PTFE, and the water in the liquid was evaporated to dryness. Baking was performed for 3 minutes at a temperature equal to or higher than the melting point (360 ° C.) to obtain a composite sheet in which PTFE was impregnated in a glass fiber woven fabric. The thickness of this sheet was 0.07 mm, and according to observation with an electron microscope, it had a fine structure in which a PTFE resin was filled between glass fiber woven fabrics.

The pressure of this sheet is temporarily reduced to 40 Pa, and oxygen
Concentration, then introduce nitrogen gas and saturate with nitrogen gas.
Add standard atmospheric pressure 1.0 × 10^Five Pa. This sea
2 × 10 per hour^Three Gy's cobalt 60γ ray for 50 hours
Irradiated (irradiation dose 1 × 10 ^Five Gy). Irradiation temperature is 3
40 ± 5 ° C. The resulting sheet is crosslinked and
It was a PTFE sheet reinforced with glass fibers. this
The strength of the sheet is the same as the strength of the sheet before crosslinking
And showed that crosslinking was sufficiently performed.

Next, the crosslinked sheet was irradiated with gamma rays in argon gas. The irradiation was performed at room temperature, and the irradiation dose was 3 × 10 ⁴ Gy. Thereafter, the sheet was reacted in a styrene monomer for 10 hours to effect graft polymerization. The degree of polymerization was 41% when the rate of weight increase was evaluated. Then, 100 ml of a solution of sulfur trioxide in tetrachloroethane having a concentration of 2.0 mol / l and 10 g of the obtained sheet were placed in a flask, and the mixture was reacted at 50 ° C. for 4 hours to perform sulfonation. Quantification was performed by neutralization titration, and it was confirmed that 0.95 sulfone groups were introduced per styrene monomer unit.

Example 2 Para-aramid fibers (thickness: 0.1 mm, average fiber diameter: 10 μm) were impregnated with a PTFE dispersion in the same manner as in Example 1, and then fired to obtain a 0.12 mm thick fiber. I got a sheet. This sheet was irradiated with 2 × 10 ⁴ Gy / hour of cobalt 60γ rays for 40 hours (irradiation dose: 8 × 10 ⁴ Gy). This sheet was a crosslinked PTFE sheet reinforced with para-aramid fibers. Next, the crosslinked sheet was irradiated with gamma rays in argon gas. The irradiation temperature was room temperature, and the irradiation dose was 3 × 10 ⁴ Gy. Thereafter, the sheet was reacted in a styrene monomer for 10 hours to effect graft polymerization.
The degree of polymerization was 35% when the rate of weight increase was evaluated.

Further, sulfonation was carried out in the same manner as in Example 1 to obtain an ion exchange membrane. Quantification was performed by neutralization titration, and it was confirmed that 0.8 sulfone groups were introduced per styrene monomer unit.

<Comparative Example> PTF obtained by suspension polymerization
E powder (particle size: 560 μm) was charged into a mold, compression-molded by pressing, and then fired at 380 ° C. for 12 hours to form a cylindrical block having an outer diameter of 200 mmΦ and a height of 300 mm. This cylindrical block was cut by a cutting lathe to form a sheet having a thickness of 0.07 mm. This film is wrapped around a metal pipe having an outer diameter of 125φ for a length of 150 m, and the sheet is coated with 2 × 10 ³ Gy / hour of cobalt 60γ.
The lines were irradiated for 50 hours (irradiation dose 1 × 10 ⁵ Gy). The irradiation temperature was 340 ± 5 ° C.

This film was subjected to graft polymerization in the same manner as in Example 1 to form an ion exchange membrane. as a result,
The graft polymerization degree was 38%. However, this ion exchange membrane has disadvantages such as low strength and easy elongation. Further, the sheet has many wrinkles, and there is a problem in that the sheet is practically used.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) D06M 101: 00 D06M 101: 00 F term (Reference) 4F071 AA27 AA78 AD00 AE17 AF14 AG05 AG06 AH19 BA06 BB01 BB02 BB13 BC01 4F072 AA04 AA08 AB09 AB28 AB29 AD07 AG02 AH03 AH31 AJ04 AJ11 AJ15 AJ16 AK05 AL01 4L031 AB32 AB34 CB08 DA00 DA17 4L033 AB05 AB07 AC05 AC15 CA17 CA70

Claims (3)

[Claims] 1. A composite structure in which a woven or nonwoven fabric of a heat-resistant fiber is impregnated with a polytetrafluoroethylene resin,
A composite sheet in which the polytetrafluoroethylene resin is crosslinked by ionizing radiation. 2. A composite structure in which a woven or nonwoven fabric of a heat-resistant fiber is impregnated with a polytetrafluoroethylene resin,
An ion-exchange membrane in which the polytetrafluoroethylene resin is cross-linked by ionizing radiation and the cross-linked product has a graft-polymerized chain having an ion-exchangeable functional group in a repeating unit. 3. The ion-exchange membrane according to claim 2, wherein the graft ratio indicating the weight percentage of the graft polymer chain to the crosslinked product before the graft polymerization is 10% or more.