JPH0691944B2 - Thin film manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a thin film for gas separation. Specifically, the production of a thin film useful as a gas separation membrane, which is characterized by forming a thin film on a molded body of a substrate by plasma polymerizing a mixture of hexafluoropropylene (also referred to as hexafluoropropene) and an aliphatic hydrocarbon. Concerning the law.

[Prior Art] For the purpose of modifying the surface of a molded base body, for example, Japanese Patent Publication No. 55-
As described in Japanese Patent No. 33815, there is proposed a technique for forming a thin film on the surface of a molded base body by a plasma polymerization method. Such plasma polymerization refers to a polymerization method in which a monomer present by glow discharge in a monomer present under a low pressure under vacuum is turned into plasma and the generated active species is deposited on a substrate. When the plasma polymerization is carried out, the monomer present in the polymerization system may be decomposed if it is not stable, and the monomer may not be polymerized in the form of the monomer present in the polymerization system. If two types of monomers are present in the polymerization system, a polymer containing elements of two types of monomer components may be produced.

The present inventor has previously reported that a thin film can be formed on the surface of a substrate molded body by performing plasma polymerization of tetrafluoromethane in the coexistence of methane (J. Macrorom. Sci.-
chem.A18 (4), p661-672 (1982)).

[Problems to be Solved by the Invention] However, according to the experiments of the present inventor, the thin film formed by this method was not satisfactory in terms of gas separation performance.

Therefore, the present inventor has conducted further studies, and particularly when a thin film is formed on a molded body of a substrate by plasma-polymerizing a mixture of hexafluoropropylene and an aliphatic hydrocarbon, the separation performance of oxygen and nitrogen is particularly high. The present invention has been accomplished by finding that a thin film excellent in heat resistance is formed.

[Means for Solving the Problems] However, the gist of the present invention is that a mixture of hexafluoropropylene and an aliphatic hydrocarbon is plasma-polymerized to give a thickness of 0.01 μm to 10 μm on a molded substrate.
The method for producing a thin film for gas separation is characterized in that the thin film is formed.

Hereinafter, the present invention will be described in detail.

In the present invention, the molded base body means a molded body made of an organic resin material such as natural resin or synthetic resin, an inorganic material such as metal or glass, or the like.

Examples of organic resin materials include polyolefins such as polyethylene, polypropylene, and poly-4-methylpentene-1, polymers of vinyl-based monomers such as polyvinyl chloride, polyacrylonitrile, and methyl polymethacrylate, and fluorine-containing monomers such as polyvinylidene fluoride. , Silicon-containing polymers such as polysiloxane, polytrimethylvinylsilane and polytrimethylsilylpropylene, condensation polymers of aromatic ring-containing organic compounds such as polyamide, polyimide, polysulfone, polyphenylene oxide and polycarbonate, ethyl cellulose, acetyl Cellulose-based polymers such as cellulose may be mentioned, and examples of the inorganic material include metals, glass and the like.

Specific examples of the molded body of the substrate include molded bodies in the form of a sheet, a hollow fiber, a spiral, a tube, and the like, all of which need to be porous. The molded body of the substrate further comprises a film composed of a skin layer and a porous layer, which is usually called an asymmetric film, and a homogeneous layer formed on the surface of the porous layer, which is called a composite film, by a method such as coating, in situ polymerization, plasma polymerization or lamination. A film formed with an ultrathin film can also be used as a molded body. The pore size of the porous body is preferably 0.03 μm or less, which is suitable for gas separation.

According to the method of the present invention, hexafluoropropylene is polymerized by plasma in the presence of an aliphatic hydrocarbon. Examples of the aliphatic hydrocarbon that can coexist include alkanes and alkenes that are gaseous at room temperature, and methane is most preferable.

The ratio when mixing hexafluoropropylene and an aliphatic hydrocarbon is not particularly limited, but is 100: 1 by volume ratio.
It is good to select in the range of 1: 100, and it is particularly preferable to use 2 in volume
It is good to choose in the range of 0: 1 to 1: 2.

Plasma polymerization of a mixture of hexafluoropropylene and an aliphatic hydrocarbon can be carried out using a known plasma polymerization apparatus such as an internal electrode system or an electrodeless system.
As an internal electrode type polymerization device, the Journal of
Applied Polymer Science (Journal of Appli
ed Polymer Science) Vol. 17, page 885 (1973), or Vol. 28, page 3629 (1984), etc., and as an electrodeless polymerization apparatus, Vol. 15, Vol.
Examples include those described on page 2277 (1971).

The temperature during plasma polymerization is not particularly limited and can be appropriately selected in the vicinity of normal temperature, the pressure during plasma polymerization,
The glow discharge voltage and the like can be appropriately selected depending on the type of the polymerization apparatus used.

The flow rate of the monomer supplied to the polymerization apparatus during plasma polymerization can be selected according to the size of the apparatus and the magnitude of the discharging voltage. For example, in a device having a diameter of 10 cm and a length of 55 mm, the flow rate of the monomer at a radio power of 10 to 100 W is 0.1
It can be selected at about 50 cm ³ (STP) / minute.

The thickness of the thin film formed on the molded substrate by plasma polymerization can be adjusted by the polymerization time and the flow rate of the monomer supplied to the polymerization apparatus. When it is used as a gas separation membrane, it is preferable that the thin film be extremely thin. On the other hand, if it is extremely thin, separation performance is deteriorated due to pinholes and the like, which is not preferable. The thickness of the thin film is usually 0.
It is preferable to select in the range of 01 μm to 10 μm.

[Effects of the Invention] The present invention has the following special remarkable effects, and its practical utility value is extremely large.

1. According to the method of the present invention, a thin film obtained by plasma polymerizing coexistence of hexafluoropropylene and an aliphatic hydrocarbon is excellent in gas separation property, particularly separation performance of oxygen and nitrogen.

2. On the other hand, the thin film obtained by plasma polymerization without coexisting with aliphatic hydrocarbon does not exhibit excellent gas separation property.

[Examples] Hereinafter, the present invention will be described in detail based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

Example 1 Journal of Applied Polymer Science Volume 28, 3629
The internal electrode method of the same type as described in page (1984), in which a circular Millipore filter (a porous membrane manufactured by Millipore Limited, average pore diameter 0.025 μm, diameter 45 mm) is used in a polymerization apparatus having a diameter of 400 mm and a length of 400 mm. ) Was placed and the inside of the polymerization apparatus was once evacuated. A 3: 1 mixture of hexafluoropropylene and methane (volume ratio) was introduced into this polymerization device, and polymerization was initiated at an Af current of 75 mA (20 KHz).
Monomer supply flow rate 8 cm ³ (STP) / min, reaction pressure 16
As Pa, plasma polymerization was performed for 2 hours.

The pressure on the primary side (gas supply side) of the membrane is 1 kg / cm ² and the pressure on the secondary side (gas permeation side) of the membrane is about 100 μm for the Millipore filter having a thin film of 0.59 μm formed by plasma polymerization. The results of measuring the oxygen and nitrogen permeation coefficients at ² kg / cm ^{2 were} as follows.

Po ₂ = 9.7 × 10 ^-9 cm ³ (STP) · cm / cm ² · sec · cmHg PN ₂ = 3.4 × 10 ^-9 cm ³ (STP) · cm / cm ² · sec · cmHg (P 0 ₂ / PN ₂ ) = 2.9 Example 2 Using the same thin film as used in Example 1, O ₂
The permeation performance of a mixed gas containing 27.8% and N ₂ 72.2% (volume%) was measured.

The composition of the permeated gas obtained by passing through a Millipore filter at a rate of 7.47 × 10 ^-5 cm ³ (STP) · cm / cm ² · Sec · cmHg was measured by gas chromatography, and it was found that oxygen was 57.1%. , Nitrogen was 42.9%.

From the above results, calculating the permeability coefficients of oxygen and nitrogen,
It is as follows.

P0 ₂ = 9.5 × 10 ^-9 cm ³ (STP) · cm / cm ² · sec · cmHg PN ₂ = 3.9 × 10 ^-9 cm ³ (STP) · cm / cm ² · sec · cmHg Also, the composition of the supply gas The separation factor (α) calculated from the composition of the permeated gas is as follows.

(Α) = 3.46 Comparative Example Using the same plasma polymerization apparatus as used in Example 1, only hexafluoropropylene was introduced as a monomer into the reaction apparatus, and polymerization was started at an Af current of 75 mA (20 KHz). , Monomer supply flow rate 8 cm ³ (STP) / min,
Plasma polymerization was carried out for 2 hours at a reaction pressure of 16 Pa.

The pressure on the primary side (gas supply side) of the membrane was 1 kg / cm ² and the pressure on the secondary side (gas permeation side) of the membrane was about 1 mm / cm ^{2 for} the Millipore filter having a thin film of 0.46 μm formed by plasma polymerization. The results of measuring the oxygen and nitrogen permeation coefficients at ² kg / cm ^{2 were} as follows.

P0 ₂ = 1.0 × 10 ^-9 cm ³ (STP) · cm / cm ² · sec · cmHg PN ₂ = 6.2 × 10 ^-10 cm ³ (STP) · cm / cm ² · sec · cmHg (P0 ₂ / PN ₂ ) = 1.7 From the results of the above Examples and Comparative Examples, in the case of the example in which hexafluoropropylene was mixed with methane and plasma-polymerized, the oxygen content was higher than that in the case of plasma-polymerized hexafluoropropylene alone. It can be seen that the permeation coefficient increases by about 10 times, and the separation performance indicated by the ratio of the permeation coefficients of oxygen and nitrogen also increases by 2.9 times.

Claims (1)

[Claims] 1. A method for producing a thin film for gas separation, characterized in that a thin film having a thickness of 0.01 μm to 10 μm is formed on a molded body of a substrate by plasma-polymerizing a mixture of hexafluoropropylene and an aliphatic hydrocarbon. Law.