JP2814598B2 - Polyethylene porous membrane and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present description relates to a microporous ultrahigh molecular weight polyethylene membrane which is excellent in removing fine particles and can be used as a supermembrane with a high processing speed.

[Prior art and its problems] In the field of the electronics industry and the like, recently, there has been a demand for water and products free from contamination with smaller particulate impurities. Precision membranes are often used to remove fine particles, but in the past
In places where it was sufficient to remove particles of 0.2 μm or more, removal of 0.1 μm particles has been required. Conventionally, there is a supermembrane capable of removing 0.1 μm particles, but at present, the processing speed is greatly reduced due to the small pore diameter of the supermembrane.

A method for obtaining a microporous membrane of ultrahigh molecular weight polyethylene is disclosed in JP-A-60-242035 and the like. However, in this method, the plasticizer is not completely removed and one part of the plasticizer remains. It is stretched as it is. The stretching speed is several hundred
Only very slow speeds of% / min are applicable.

[Means for Solving the Problems] The present invention solves the above-described problems,
In particular, it is an object of the present invention to provide a polyethylene microporous membrane which is excellent in removing sub-micron particles and has a high processing speed, and a method for industrially advantageously producing the porous membrane.

The polyethylene microporous membrane of the present invention and the method for producing the same will be described in more detail.

The polyethylene used in the present invention has a molecular weight of 8 × 10 ⁵ to
It is necessary to be 3 × 10 ⁶ , and particularly preferably 1 × 10 ^{6 to} 2.5 × 10 ⁶ .

If the molecular weight is too low, it is difficult to obtain a fine porous film, and it is difficult to form a stable sheet. If the molecular weight is too high, the network structure is crushed by stretching, and the network structure becomes dense, making it impossible to obtain a sufficient processing speed. Further, the resistance during extrusion is large, and it is difficult to form a thin sheet stably.

Next, the hydrocarbon plasticizer used as a plasticizer has good compatibility with polyethylene, has a boiling point higher than the polyethylene melt molding temperature (up to 250 ° C), and has a vapor pressure so that transpiration does not easily occur during sheet molding. Is low (the vapor pressure at 200 ° C. is about 50 mmHg or less).

Further considering the stability and ease of handling of the sheet composed of polyethylene and plasticizer obtained in the middle stage of the product, specifically, liquid paraffin, solid paraffin, stearyl alcohol, seryl alcohol, etc. are preferred, especially stearyl alcohol is It is possible to use fine granular products, and the mechanical blend with ultra high molecular weight polyethylene, which is in the form of powder, tends to be very uniform.If it is supplied to the feed section of the extruder as it is, stable extrusion can be achieved. Suitable for industrial production.

The gist of the present invention is that the weight average amount of polyethylene is 8 × 10 ^{5 to} 3 × 10 ⁶ , the thickness is 5 μm or more and 25 μm or less, the water permeability is 3000 L / Hm ² atm or more, and 0.091 μm of styrene latex particles. A sheet consisting of a polyethylene microporous membrane having a rejection of 95% or more and a composition comprising polyethylene and a plasticizer having a weight average molecular weight of 8 × 10 ^{5 to} 3 × 10 ⁶ is obtained by melt extrusion to obtain a sheet. And the obtained porous sheet is stretched in the longitudinal direction by 2 to 6 times at a temperature of 90 to 120 ° C. by a roll stretching machine, and then 3 to 8 times by a tenter stretching machine at a temperature of 110 to 130 ° C. The present invention relates to a method for producing a microporous polyethylene membrane characterized by stretching in the transverse direction.

The polyethylene used in the present invention has a weight average molecular weight of 8 ×.
It is a polyethylene of 10 ^{5 to} 3 × 10 ⁶ and is usually called ultra-high molecular weight polyethylene.

The mixing ratio of the ultra-high-molecular-weight polyethylene and the plasticizer is, by weight, ultra-high-molecular-weight polyethylene / plasticizer 10/90 to 3
It is preferably 5/65, more preferably 15 / 85-30 /
It is in the range of 70. If the ratio of polyethylene is too low, the extruded state in the extruder will be unstable, and if the ratio of polyethylene is too high, the viscosity will be too high, and the flow at the die forming the sheet will be unstable, in any case. Nor can you get a normal sheet. The extrusion temperature is usually 200 ° C. or higher, and it goes without saying that additives such as antioxidants may be added.

The sheet is prepared by first feeding the ultra-high polyethylene and plasticizer to the extruder, extruding the sheet from a die appropriately selected as a uniform molten state, and the thickness of the sheet that can be obtained is about 0.1 to 0.5 mm. Good to be. If the thickness is too small, the thickness of the finally obtained microporous film becomes too small, and it becomes difficult to use the film alone, and if it is too thick, a sufficient processing speed cannot be obtained.

A porous sheet is obtained by extracting and removing the plasticizer from the obtained sheet. Removal of the plasticizer is preferably carried out by extraction with a volatile solvent. Examples of the volatile solvent include pentane, hexane, hydrocarbons such as heptane, methylene chloride, and the like.
Examples thereof include chlorinated hydrocarbons such as carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluoride, and alcohols such as methanol, ethanol, and propanol. These solvents are treated (immersion treatment, etc.) for about 5 to 30 minutes. Thus, the porous sheet is obtained by removing the plasticizer and then removing the volatile solvent by drying. It is desirable that the amount of plasticizer remaining in the porous sheet by this treatment be less than 1% by weight.

The obtained porous sheet is then stretched. Stretching can be performed by a biaxial stretching machine, a tubular method, a rolling method, a tenter method, a roll method, or a combination thereof. In industrial production, sequential biaxial stretching of longitudinal stretching by a roll and transverse stretching by a tenter is used. The method is preferred.

Roll stretching (longitudinal stretching) is performed at a stretching temperature of 90 to 120 ° C, preferably 100 to 110 ° C, and a stretching ratio of 2 to 6 times, preferably 3 to 5 times.
Doubled.

If the temperature is low, stretching breaks are likely to occur, and if the temperature is high, the processing speed when using as a supermembrane may be low, possibly because holes are crushed. If the draw ratio is low, the rejection of the particles is poor. If the draw ratio is too high, breakage in the next tenter stretching and a reduction in the processing speed as an excess film are observed.

Tenter stretching (transverse stretching) is performed at a stretching temperature of 110 to 130 ° C and a stretching ratio of 3 to 8 times, more preferably 4 to 7 times. If the stretching temperature is too low, a sufficient processing speed cannot be obtained in the case of an over-film, and if it is too high, the rejection of fine particles decreases.

If the magnification of the transverse stretching is too low, the rejection of particles in the case of over film becomes poor, and if it is too high, the processing speed in the case of over film becomes slow.

The thickness of the microporous membrane obtained in this manner is suitably from 5 to 25 µ, more preferably from 10 to 20 µ. If it is too thin, it will be difficult to prepare a cartridge filter using this membrane, and large holes will be easily formed, and the reliability with respect to blocking of fine particles will be reduced. On the other hand, if it is too thick, the resistance becomes too large and a sufficient processing speed cannot be obtained. The microporous polyethylene membrane obtained by the present invention has a thickness of 5μ or more and 25μ or more.
Hereinafter, water permeation rate 3000L / H · m ² · atm or more, rejection of 0.091μ styrene latex particles is greater excellent addition processing speed in the stop of the fine particles at least 95%, it can be applied to various microfilter It is. In particular, since it is excellent in chemical resistance, it is suitable as a microfilter for removing fine particles in various chemicals.

[Examples] Next, the present invention will be described in more detail with reference to Examples.
The present invention is not limited by the following examples unless exceeding the gist.

 The test method in the examples is as follows.

(1) Permeability rate Measured at a differential pressure of 1 kg / cm ² and a temperature of 25 ° C using an 8010 type cell manufactured by Amicon (USA). (2) Rejection (%) 0.091μ (average particle size) manufactured by Dow (USA) Diameter) A styrene latex was dispersed in water, and a permeation test similar to (1) was carried out, and the rejection was measured.

(3) Porosity (%) (pore volume / microporous membrane volume) x
100 Note: Value obtained by calculation from weight, volume and density of polyethylene Example-1 20 parts by weight of polyethylene powder having a weight average molecular weight of 2 × 10 ⁶ , 80 parts by weight of granular stearyl alcohol, and distearyl 3,3′-thio After dry blending 0.25 parts by weight of propionate ester and 0.25 parts by weight of pentaerythritol tetra (β-lauryl-thiopropionate) ester, the supply section was supplied to a cooled 50 mmφ extruder by a water-cooled jacket, and further 40 mmφ extrusion was performed at the tip. A machine is attached to make the above-mentioned blend into a uniform melt, width 45cm, die clearance
The sheet was extruded from a 0.4 mm die and the melt was pulled at a draft rate of 2.0 to obtain a 0.17 mm thick sheet. Stearyl alcohol was extracted and removed from the sheet with ethanol at 50 ° C. to obtain a porous sheet.

This porous sheet is first heated to 100 ° C using a roll stretching machine.
The film was stretched 3.5 times in the machine direction at a temperature of, and then stretched 5 times in the transverse direction at a temperature of 115 ° C by a tenter stretching machine. The properties of the obtained microporous film were as follows.

Thickness 12μ, porosity 65% Water permeation rate 5100 L / H ・ m ²・ atm, 0.091μ Styrene latex particle Rejection> 99% Examples 2-2 to 3 The porous sheet obtained in Example -1 Table 1
Table 1 shows the properties of the microporous membrane obtained by biaxial stretching under the various conditions shown in Table 1.

Comparative Examples 1-2 Using the porous sheet obtained in Example 1, Table 1 was used.
Table 1 shows the properties of the microporous membrane obtained by biaxial stretching under the various conditions shown in Table 1.

Comparative Examples -3 to 5 A sheet of 0.20 mm was obtained in the same manner as in Example 1 except that the draft rate was adjusted using polyethylene having a molecular weight of 4 × 10 ⁶ , and the plasticizer was extracted and removed to obtain a porous sheet. Created.

This sheet was roll-stretched at 100 ° C., but could not be stretched 4 times and stretched 3 times.
Tenter stretching was performed at 3, 5, and 8 magnifications. The results are shown in Table 1.

[Effects of the Invention] The microporous membrane of the present invention is used particularly as a supermembrane and has excellent removability of submicron particles and high processing speed.

Continuation of the front page (56) References JP-A-57-117951 (JP, A) JP-A-64-51104 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B01D 71 / 26 C08J 9/00

Claims (2)

(57) [Claims] (1) a polyethylene having a weight average molecular weight of 8 × 10 ^{5 to} 3 × 10 ^6, a thickness of 5 μm to 25 μ and a water permeation rate of
3000L / H · m ² · atm or more, the microporous polyethylene membrane rejection of 0.091μ styrene latex particles is 95% or more. 2. A porous sheet obtained by melt-extruding a composition comprising polyethylene having a weight average molecular weight of 8 × 10 ^{5 to} 3 × 10 ⁶ and a plasticizer to obtain a sheet, and then removing the plasticizer from the sheet. In a roll stretching machine at a temperature of 90 to 120 ° C. for 2 to 6
It is stretched twice in the machine direction, then 110-130 ° C with a tenter stretching machine
A method for producing a microporous polyethylene membrane, characterized in that the film is stretched 3 to 8 times in the transverse direction at the temperature described in the above.