Process for the production of porous polyolefin

Details Process for the production of porous polyolefin Process for the production of porous polyolefin

1999-09-17

2001-06-12

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

With severing, removing material from preform mechanically,...

Making hole or aperture in article

C264S210500, C264S210600, C264S210700, C264S210800, C264S211130, C264S211140, C264S235600, C264S235800, C264S288800

Reexamination Certificate

active

06245270

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a process for producing a polyolefin porous material. More specifically, it relates to a process for producing a polyolefin porous material with a large number of interconnecting pores having an extremely small diameter.
BACKGROUND ART
As one of processes for producing a polyolefin porous material, there is known a process for forming a large number of micropores which comprises stretching the mixture of a filler and a polyolefin to promote interfacial separation between the polyolefin and the filler and further fibrillating through the cleavage of a polyolefin phase. This process is excellent because a polyolefin porous material can be easily obtained.
For example, the present inventors have already proposed a process for producing a microporous polyolefin sheet by biaxially stretching a polyolefin sheet made from a polyolefin highly filled with a filler such as calcium carbonate or polymethyl sylsesquioxane [see Ind. Eng. Chem. Res., 32, 221 (1993)].
In the above process, the properties of the obtained microporous polyolefin sheet are determined by the type, particle diameter and amount of the filler and a stretch ratio. To obtain a microporous sheet having a smaller pore diameter, it is desirable to use a smaller filler. However, the characteristic feature of powders is such that the smaller the diameter of particles, the higher the cohesiveness of the particles become. Therefore, when a filler having a small particle diameter is blended into a polyolefin, it is difficult to disperse primary particles uniformly and the formation of agglomerates is inevitable. As a result, the size of the agglomerates affects the formation of amicroporous structure, thereby causing an increase in pore diameter and the expansion of a pore diameter distribution. Therefore, it is difficult to produce a microporous sheet having a very small pore diameter and a large pore specific surface area.
Also, the present inventors have already proposed microporous polyolefin fibers (see J. Appl. Polym. Sci. 61 2355 (1996), ibid 62 81 (1996), JP-A 7-289829, JP-A 9-157943 and JP-A 9-157944). They are microporous fibers obtained by melt-spinning and stretching a polyolefin composition containing an appropriate amount of a filler. In these microporous fibers, at least 15 wt % of a filler is required to form pores thoroughly.
In order to enhance the adsorptivity of a microporous fiber, it is desirable to decrease the diameter of pores formed in the fiber and to increase the specific surface area of each pore. Therefore, it is desirable to use a filler having as small a diameter as possible. When a filler having an average particle diameter of less than 0.1 &mgr;m is used, there arises a problem such as the agglomeration of particles. This agglomeration problem becomes more serious as the amount of the filler increases as described above. When a large number of agglomerates are formed, the size of the agglomerates affects the formation of a microporous structure, thereby causing the expansion of a pore diameter distribution and making it difficult to obtain a microporous fiber which satisfies the above requirements. Further, a high-strength microporous fiber cannot be obtained owing to the agglomerates.
DISCLOSURE OF THE INVENTION
Under the circumstances, it is an object of the present invention to produce a polyolefin porous material having a large total pore specific surface area and pores with an extremely small average diameter without forming the agglomerates of particles in a process for producing a polyolefin porous material, which comprises blending a filler with a polyolefin, stretching the mixture to cause interfacial separation between a polyolefin phase and particles, and fibrillating through the cleavage of the polyolefin phase to form micropores. Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention are attained by a process for producing a polyolefin porous material, which comprises the steps of:
synthesizing very fine particles having an average particle diameter of 0.01 to 0.1 &mgr;m in a polyolefin to obtain a polyolefin composition; and
molding and stretching the obtained polyolefin composition.
Known polyolefins are used without particular restriction as the polyolefin used in the present invention. Illustrative examples of the polyolefin include homopolymers of &agr;-olefins such as polyethylene, polypropylene, polybutene-1 and polymethyl pentene, copolymers of &agr;-olefins and other copolymerizable monomers, and mixtures thereof. Of these, in view of the heat resistance and moldability of the obtained polyolefin porous material, propylene homopolymers, copolymers of propylene and other copolymerizable monomers, and mixtures thereof are preferable.
The copolymers of &agr;-olefins and other copolymerizable monomers are preferably a copolymer which contains an &agr;-olefin, particularly propylene, in an amount of 90 wt % or more and other copolymerizable monomers in an amount of 10 wt % or less. Known copolymerizable monomers may be used without particular restriction as the above copolymerizable monomer. Of these, &agr;-olefins having 2 to 8 carbon atoms are preferable, and ethylene and butene are particularly preferable.
When a polypropylene homopolymer, a copolymer of propylene and other copolymerizable monomer or a mixture thereof is used out of these, the obtained polyolefin porous material has excellent transparency advantageously.
In the process of the present invention, a specific method for synthesizing fine particles in a polyolefin comprises mixing water with an alkoxysilane in a molten polyolefin to hydrolyze the alkoxysilane. The alkoxysilane is preferably a compound represented by the following general formula:
RxSi(OR′)y
wherein R and R′ are substituted or unsubstituted alkyl groups, x is an integer of 0 to 3, y is an integer of 1 to 4, and the total of x and y is 4.
The alkyl group is preferably a group having 1 to 4 carbon atoms such as a methyl group, ethyl group, propyl group or butyl group, more preferably a group having 1 to 2 carbon atoms such as a methyl group or ethyl group. Preferable examples of the alkoxysilane include tetraalkoxysilanes such as tetramethoxysilane and tetraethoxysilane; trialkoxysilanes having one alkyl group such as methyltriethoxysilane and ethyltrimethoxysilane; dialkoxysilanes having two alkyl groups such as diethoxysilane; and monoalkoxysilanes having three alkyl groups such as trimethylmethoxysilane. Further, compounds having a substituted alkyl group may be used in conjunction with these compounds. They may be used independently or as a properly prepared admixture.
When a molten polyolefin containing such analkoxysilane is mixed with water, the alkoxysilane is hydrolyzed to form the skeleton of a —Si—O— bond, thereby causing phase separation in the molten polyolefin to form fine particles. Since the diffusion speed of the alkoxysilane in the molten polyolefin composition is very low, the amount of the alkoxysilane concentrated at the reaction point of hydrolysis is limited. As a result, the particle diameter of the formed silica particles or polysiloxane particles is extremely small, and at the same time, the formation of agglomerates can be nearly perfectly suppressed. Therefore, silica particles or polysiloxane particles having an average particle diameter of 0.01 to 0.1 &mgr;m can be easily formed with the particles uniformly dispersed in the polyolefin composition after the reaction, and a polyolefin porous material can be favorably obtained by molding and stretching this polyolefin composition.
In the above production process, a kneader or extruder is preferably used to melt-kneading the polyolefin with the alkoxysilane. There is particularly preferably used an extruder to which additives can be fed in the step of extruding a supplied resin while the resin is melt-kneaded with a screw, such as an extruder to which additiv

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