JP3462638B2 - Porous sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a polyolefin porous sheet which has various characteristics such as chemical resistance, heat resistance, air permeability and slidability and is used as various filters and sliding members. is there.

[0002]

2. Description of the Related Art Conventionally, a porous sheet is obtained by, for example, a sintering method in which a powder of a thermoplastic resin is filled in a molding die (shape retainer) and the molding die is heated to a temperature higher than the melting point of the resin. Has been made. According to this, the contact portions of the thermoplastic resin powder particles with each other are melted and connected, and the gaps between the powder particles become continuous holes to form a porous body. Next, a porous sheet is produced by forming the porous body into a sheet shape by cutting or the like.

Polyethylene is generally used as the thermoplastic resin for forming the porous sheet.
Further, the present inventors have developed a production method using ultra-high molecular weight polyethylene powder (Japanese Patent Publication No. 5-66855) for the purpose of improving the performance of a porous sheet, and have a porous structure in which continuous pores are uniformly distributed. It was possible to make a sheet.

[0004]

However, as the performance of various devices has increased due to technological advances in recent years, it has been required to improve the properties of the porous sheet used as a component of the device. And improvement of heat resistance is earnestly desired. Therefore, a method of using polyolefin instead of polyethylene, a method of using polyethylene and polyolefin in combination (Japanese Patent Publication No. 7-21081), and the like have been proposed.

However, the porous sheet using polypropylene has a problem in performance due to variations in porosity. Therefore, it is possible to produce a small-sized porous sheet, but it is not possible to produce a practically large-sized porous sheet.
There are two possible reasons for this. That is, in the sintering method, the resin powder in the molding die is pressed in order to control the porosity of the obtained porous sheet. However, since polypropylene has a lower melt viscosity than polyethylene and a large viscosity change with temperature, pressure transmission is not uniform, and as a result, it is considered that the porosity varies. Further, since the temperature distribution in the molding die is also non-uniform, the melt viscosity of each portion of the resin powder changes, and it is considered that the porosity also varies due to this.

The present invention has been made in view of such circumstances, and in addition to having the general characteristics of the porous sheet, it is particularly excellent in mechanical strength and heat resistance, and the porosity varies. The purpose is to provide a small number of porous sheets.

[0007]

In order to achieve the above object, the invention according to claim 1 of the present invention is a porous sheet obtained by molding a polyolefin sintered body into a sheet, The polyolefin is mainly composed of ultra-high molecular weight polypropylene having a melt index (MI) of less than 0.02 g / 10 minutes.

According to a second aspect of the present invention, in the porous sheet, the polyolefin contains ultrahigh molecular weight polyethylene having a molecular weight of 500,000 or more as measured by a viscosity method. .

In the present invention, the melt index (M
I) is based on the method according to JIS K 7210. In addition, ultra-high molecular weight polypropylene refers to one having a molecular weight of about 1,000,000 or more. On the other hand, the molecular weight of ordinary polypropylene is about 300,000 or less. The molecular weight of the polypropylene is a value measured by a viscosity method.
In the present invention, ultrahigh molecular weight polyethylene refers to one having a molecular weight of about 500,000 or more. In contrast,
The molecular weight of ordinary polyethylene is about 100,000 or less.
The molecular weight of the polyethylene is a value measured by a viscosity method. Further, in the present invention, the "main component" means that the above-mentioned ultra high molecular weight polypropylene is 30 with respect to the whole polyolefin.
It means that the content of the polyolefin is not less than 100% by weight, and it includes the case of the above polyolefin alone (100% by weight).

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of extensive studies on polyolefins, the present inventors have found that the use of ultrahigh molecular weight polypropylene having a melt index (hereinafter referred to as “MI”) of less than 0.02 g / 10 minutes causes porosity. The present inventors have found that a porous sheet with a small variation in the rate can be produced, and arrived at the present invention. That is, MI is 0.02 g / 1
Ultra-high molecular weight polypropylene of less than 0 minutes has a low melt viscosity and therefore has a low flow during sintering. Therefore, the polyolefin powder containing this as a main component is sintered while maintaining its powder particle shape. Will be done. As a result, the variation in porosity of the porous sheet of the present invention is reduced. Since polypropylene is the main component, the heat resistance and mechanical strength of the porous sheet are improved. Further, the intended purpose can be achieved by using the above ultrahigh molecular weight polyethylene together with the above special ultrahigh molecular weight polyolefin.

Next, the present invention will be described in detail.

The porous sheet of the present invention uses a polyolefin powder containing MI having an MI of less than a predetermined value as a main component.

As mentioned above, the above-mentioned polyolefin powder is mainly composed of the above-mentioned polypropylene, and the preferable ratio thereof is 30 to 10 with respect to the whole polyolefin.
It is 0% by weight (hereinafter abbreviated as "%").

Further, M of the above-mentioned ultra high molecular weight polypropylene
I should be less than 0.02 g / 10 minutes. The particle size of this ultra high molecular weight polypropylene is usually 1
The thickness is 0 to 500 μm, and preferably 30 to 300 μm.

In addition to the above ultra-high molecular weight polypropylene, other polyolefin powder can be added to the polyolefin powder of the present invention. The above ultra high molecular weight polyethylene powder is preferable. The mixing ratio of the ultrahigh molecular weight polyethylene is such that the ultrahigh molecular weight polyethylene (B) is in a weight ratio of B / A = 0 to 0.7 with respect to the ultrahigh molecular weight polyolefin powder (A). It is common. That is, when it exceeds 0.7, in the obtained porous sheet, the heat resistance derived from polypropylene may be impaired. Also,
By using this ultra high molecular weight polyethylene together, there is an advantage that variation in porosity can be reduced. The particle size of the ultra high molecular weight polyethylene powder is usually 10 to 500 μm, preferably 30 to
It is 200 μm.

Next, the porous sheet of the present invention can be produced, for example, by using the above raw materials to produce a porous body by a conventional sintering method, and then forming the porous body into a sheet by cutting or the like. it can. Specifically, when the polyolefin powder is prepared and ultra-high molecular weight polyethylene or the like is added, this is added and mixed, the mixed powder is filled in a mold, and placed in a hot air dryer. A porous sheet is prepared by heating the above-mentioned polyolefin above the melting point for a predetermined time, then cooling it to obtain a porous body, and cutting this into a sheet.

However, in the conventional sintering method, in the obtained porous body (for example, a block shape), a temperature difference may occur between the outer peripheral portion and the central portion thereof, which may cause variations in porosity. That is, by using the above-mentioned raw material of the present invention, the variation in porosity is limited to a certain level or less, but it becomes a problem when it is desired to further limit the variation in porosity. Therefore, in order to solve this problem, the present inventors have developed the following sintering method. This sintering method includes the steps (a) to (c) shown below.

(A) Filling the air-permeable mold (shape retainer) with the polyolefin powder. (B) A step of leaving the molding die in a high-temperature steam atmosphere, heating it with the steam at a temperature equal to or higher than the melting point of the polyolefin for a predetermined time, and then cooling it to room temperature to form a porous body. (C) A step of forming the porous body into a sheet by cutting or the like.

A characteristic point of this sintering method is that water vapor is used as a heat medium. That is, normally, the steam heated above the melting point of the polyolefin is in a state of being pressurized from the atmospheric pressure, and therefore, the steam permeates through the breathable mold and reaches the center of the polyolefin powder sufficiently. Since it enters, the entire polyolefin powder is uniformly heated. As a result, the pores of the porous sheet are formed uniformly.

In the above-mentioned sintering method, in order to further promote the invasion of water vapor into the central portion of the polyolefin powder, the molding die filled with the above-mentioned polyolefin powder is put into a pressure vessel, and the pressure vessel is deaerated and decompressed. After making the state, steam may be introduced into the pressure resistant container. By this method, the variation in porosity can be further suppressed.
The depressurized state is not particularly limited,
Preferably, it is in the range of about 1 to 100 mmHg.

Then, the porous sheet of the present invention is obtained by forming the porous body into a sheet shape by cutting or the like. This cutting can be performed by, for example, a lathe or the like. If a molding die having a sheet-like molding space is used, it is possible to perform sheet-forming simultaneously with sintering.

The porosity of the porous sheet of the present invention thus obtained is usually 10 to 60%, preferably 2
It is 0 to 50%. The diameter of the holes is usually 10 to 20.
It is 0 μm, preferably 20 to 100 μm.
The porosity and pore size of the porous sheet can be adjusted by the particle size of the polyolefin powder as a raw material and the degree of pressure applied during sintering. The thickness of the porous sheet is appropriately determined depending on its application and the like, but is preferably 50 to 5000 μm.

The porous sheet of the present invention is characterized by a small variation in porosity. The variation in porosity can be evaluated as follows, for example. That is, when the porous sheet has a rectangular shape, a predetermined number of samples are sampled in the longitudinal direction, the porosity is measured by the above method, and the difference between the maximum value and the minimum value is obtained and evaluated. According to the variation in porosity evaluated by this method, the variation in porosity of the porous sheet of the present invention is 15% or less, and 10% or less under the preferable conditions.

[0024]

INDUSTRIAL APPLICABILITY As described above, the porous sheet of the present invention uses the polyolefin powder containing MI of less than 0.02 g / 10 minutes as the main component of ultrahigh molecular weight polypropylene. Since the ultra-high molecular weight polypropylene having this special physical property has a low melt viscosity, the flow rate is reduced during sintering, and the porous sheet is formed while maintaining the shape of the powder particles. As a result, the variation in porosity of the porous sheet of the present invention is reduced. Further, when the porous sheet of the present invention is produced by applying the sintering method developed by the present inventors and using steam as a heat medium, it is possible to further suppress the variation in the porosity of the obtained porous sheet. Like Further, since the porous sheet of the present invention contains ultra-high molecular weight polypropylene as a main component, it is excellent in mechanical strength and heat resistance, and is a general characteristic of the porous sheet such as chemical resistance and slidability. It also has various characteristics. Therefore, the porous sheet of the present invention can sufficiently exhibit the functions of the sliding member, the filter and the like even when used under severe conditions such as high temperature and mechanical impact.

Next, examples will be described together with comparative examples.

[0026]

Example 1 An ultra-high molecular weight polypropylene powder (MI: 0.
01g / 10 minutes, molecular weight: 2,000,000, average particle size: 90μ
m) was charged in an amount of 5000 g, and the powder was pressed at 60 g / cm ² . On the other hand, a pressure resistant container equipped with a steam introducing pipe with an opening / closing valve and an exhaust pipe was prepared. Then, the molding die is put into the pressure vessel under pressure, the vacuum pump connected to the tip of the exhaust pipe is operated with the opening / closing valve of the steam introducing pipe closed, and the degassing inside the pressure vessel is performed. And then
The atmospheric pressure was reduced to 25 mmHg. Then, the vacuum pump is stopped, the opening / closing valve of the steam introducing pipe is opened, and steam (temperature: 179 ° C., 9 atm) is introduced into the pressure resistant container.
A heat treatment (sintering treatment) was performed for a time. Then, the molding die was cooled to a temperature of 25 ° C. to obtain a round rod-shaped porous body. Then, this round bar-shaped porous body was taken out from the molding die, and this was 200 μm thick along the circumferential direction using a lathe.
Then, it was cut into a porous sheet.

Next, the average value and variation of the porosity of this porous sheet were examined by sampling 20 points according to the above-mentioned method. The average value of the porosity was 28%, and the variation was It was 6%.

[0028]

Example 2 The same ultra high molecular weight polypropylene 3000 g as in Example 1 and ultra high molecular weight polyethylene (molecular weight 400
2000 g (average particle size 100 μm) was mixed for 2 minutes using a Henschel mixer. Then, using this mixture, a porous sheet having a thickness of 200 μm was produced in the same manner as in Example 1.

Then, when the average value and variation of the porosity of the porous sheet were examined in the same manner as in Example 1, the average value of the porosity was 29% and the variation was 4%.

[0030]

Comparative Example 1 Polypropylene (MI: 10 g / 10 minutes,
Using a molecular weight of 250,000 and an average particle diameter of 150 μm, an attempt was made to make a porous sheet in the same manner as in Example 1. However, the resin flow was remarkably flowable in the molding die during sintering, and cutting was possible. It was not possible to obtain a porous body.

[0031]

[Comparative Example 2] The same polypropylene as Comparative Example 1 2000 g
And the same ultra high molecular weight polyethylene 3000 g as in Example 2
Was mixed with a Henschel mixer for 2 minutes. Using this mixture, an attempt was made to produce a porous sheet in the same manner as in Example 1, but the resulting porous body had a large variation in porosity,
It is difficult to cut into a continuous sheet having a thickness of 200 μm because there are places where the porosity is extremely large. Then, a porous sheet could be obtained by changing the thickness to 2 mm and cutting into a sheet shape. When the porosity of this was examined in the same manner as in Example 1, the average value was 27%. The variation was as large as 47%.

[0032]

Comparative Example 3 Ultra high molecular weight polypropylene (MI: 0.0
3g / 10 minutes, molecular weight: 1.5 million, average particle size: 120μ
Using m), an attempt was made to make a porous sheet in the same manner as in Example 1. As a result, a resin flow was generated in the mold during sintering, and the porosity of the obtained porous sheet was examined in the same manner as in Example 1. The average value was 2
The variation was 1% and the variation was 20%.

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Claims (2)

(57) [Claims] 1. A porous sheet in which a sintered body of polyolefin is formed into a sheet shape, wherein the polyolefin contains an ultra high molecular weight polypropylene having a melt index (MI) of less than 0.02 g / 10 minutes as a main component. A porous sheet characterized by being. 2. The porous sheet according to claim 1, wherein the polyolefin contains ultrahigh molecular weight polyethylene having a molecular weight of 500,000 or more as measured by a viscosity method.