Nonwoven filter media

Details Nonwoven filter media Nonwoven filter media

1996-11-12

2001-01-02

Krynski, William (Department: 1774)

Fabric (woven, knitted, or nonwoven textile or cloth, etc.)

Nonwoven fabric

Including strand or fiber material which is of specific...

C442S353000, C442S361000, C442S364000

Utility Patent

active

06169045

ABSTRACT:

This invention is related to filter media, particularly to media for filters made from nonwoven fiber webs.
Various particulate filtering media are available in the art, including media for air filters, water filters, hydraulic and oil filters, coolant filters, chemical filters and the like, and they are fabricated from diverse materials, such as glass fibers, asbestos fibers, synthetic polymer fibers, e.g, polyolefins, polyamides, polyesters and the like, and natural fibers, such as wood pulps and the like.
A filter medium not only has to provide a high filter efficiency, i.e., prevent fine particles from passing through, but also needs to provide a high throughput, i.e., maintain the pressure drop across the filter medium as low as possible over the useful life. In addition, the useful service life of a filter medium must not be too short as to require frequent cleaning or replacement. However, these performance requirements tend to be inversely correlated. For example, a high efficiency filter medium tends to create a high pressure drop, severely restricting its throughput capability and service life.
The use of nonwoven fiber webs or fabrics is known in the filtration art, and such nonwoven webs include meltblown fiber webs and spunbond fiber webs. Filter media fabricated from meltblown fiber webs tend to provide high filtration efficiency because of the fine fiber size and the conformability of meltblown fibers that causes the fibers to come together as a dense, fine-pored web. However, meltblown fiber webs do not provide sufficient physical properties, including tensile, tear and burst properties, and the service life of the filters made from the webs is not sufficiently long enough for certain applications. Compared with meltblown fiber webs, spunbond fiber webs contain coarse, highly continuous fibers or filaments having strong physical properties. Typically, spunbond fibers have a higher degree of molecular orientation than that of meltblown fibers, and it is believed that the stronger physical properties of spunbond fibers and webs made therefrom are attributable to the higher molecular orientation of spunbond fibers. However, spunbond fibers, unlike meltblown fibers, do not autogenously adhere to each other and the webs need to be bonded in a separate bonding process to impart physical coherency and integrity. Conventional bonding processes, such as calender bonding, which compact the webs as well as apply discrete bonded regions throughout the webs, reduce the effective filtration area and service life of the spunbond webs.
As is known in the filtration art, one of the major determinants for filtration efficiency is the ability of a filter medium to mechanically entrap contaminants while maintaining an operationally sufficient flow rate. The loft or thickness of a filter medium promotes the mechanical entrapment of contaminants within its intersticial spaces or pores without impeding the flow of filtrate. Such filtration process is known in the art as “depth filtration.” Unlike depth filtration media, flat “surface filtration” media, such as membrane filter media and meltblown fiber filter media, must accumulate contaminants on their surfaces, quickly plugging up all the available channels or pores and building up a high pressure drop across the filter media. There have been many attempts to produce filter media that exhibit combined depth filtration and high efficiency properties. One of such attempts is providing filter media having a laminated structure of at least two nonwoven fiber webs having different porosities. The laminate filter medium has a porosity gradient across its depth, which improves the filter efficiency and prevents premature plugging failure by entrapping large particulates before they can reach the smaller pores.
It would be desirable to provide highly efficient filter media that have a long service life as well as strong physical properties. Additionally, it would be desirable to provide a highly adaptable and flexible filter media forming process which can produce media of varying pore sizes and pore densities and of optimized filtration performances for filtrates with different particulate size distributions and/or different flow properties.
SUMMARY OF THE INVENTION
There is provided in accordance with the present invention a lofty filter medium for filtering fluid-borne particles, which comprises a nonwoven fiber web of crimped fibers selected from the group consisting of spunbond fibers and staple fibers, wherein the filter medium has a density between about 0.005 g/cm
3
and about 0.1 g/cm
3
. There is also provided a more particularly suitable lofty nonwoven filter medium for filtering fluid-borne particles, comprising a fiber web of crimped bicomponent spunbond fibers, wherein the medium has a density between about 0.005 g/cm
3
and about 0.1 g/cm
3
and the spunbond fibers have an average diameter of from about 10 &mgr;m to about 50 &mgr;m. Additionally, the present invention provides a process for producing a lofty spunbond fiber filter web comprising the steps of melt spinning spunbond fibers; drawing the fibers; crimping the fibers; depositing the drawn, crimped fibers onto a forming surface to form a web; and bonding the web, wherein the web has a density between about 0.005 g/cm
3
and about 0.1 g/cm
3
.
The lofty filter medium, which provides extended service life and high filter efficiencies, is highly suitable for fluid-borne particle filtration applications, such as filtration media for transmission fluids, hydraulic fluids, swimming pool water, coolant oil or cutting fluid for metalworking, metal forming and metal rolling, air-borne particle filtration and the like.


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