Process for producing meltblown polyolefin fibers for mechanical

Details Process for producing meltblown polyolefin fibers for mechanical Process for producing meltblown polyolefin fibers for mechanical

1998-11-06

2000-10-31

Tentoni, Leo B.

Gas separation: apparatus

Electric field separation apparatus

21050036, 264 12, 264518, 524104, D01D 526, D01F 110, D01F 604

Patent

active

061396087

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to a process for producing meltblown polyolefin fibres for mechanical filtration This invention also relates to the meltblown polyolefin fibres when produced by the process of the invention, and to filters containing the meltblown polyolefin fibres.
Filters are widely used for industrial and medical purposes. Thus, for example, the filters may be air filters. The air filters may be in the form of industrial respirators, automative air filters, machine air filters, air conditioning filters, and clean room filters. The filters may alternatively be oilophobic mechanical filters such for example as hydraulic filters. The filters may alternatively be liquid barrier membrane filters using fibres which are calendered to control pore size. The filters may still further be medical air filters or face masks for surgeons.
The performance standards for air filters in Europe are known as Eurovent standards, and the performance standards in the USA are known as Ashrae standards. The standards challenge a filter over a defined period of time with increasing levels of dust, and they measure filtration efficiency, pressure drop and dust loading. The performance achieved by a filter is graded according to a pre-determined set of criteria with the grading ranging from 1 at the lowest level (giving a particle filtering efficiency of not more than 65%) to 14 at the highest level (giving a particle filtering efficiency of not less than 99.9995%).
Filters that operate with a Burovent/Ashrae standard of 6 and above have commonly used fibres made from glass. The glass fibres have an effective diameter of less than 5 microns in size and they rely-on mechanical characteristics for their filtration efficiency. It is also known to use polyolefin fibres having an effective diameter of more than 5 microns in size. Such polyolefin fibres provide only limited mechanical filtration efficiency in their mechanical state, and for Eurovent/Ashrae standards of 6 and above they have to be electrostatically charged in order to maintain their filtration efficiency.
There are two well known types of filters which use the glass fibres or the polyolefin fibres in the Eurovent/Ashrae range of 6 and above. The first of these filters is a bag filter which employs a surface area commonly less than 10 m.sup.2. These bag filters have traditionally used glass fibres as the filtration media but health concerns over recent years have led to a change to polyolefin fibres which are used in heavier weights than the glass fibres. The second type of filter is a pleated panel fitter which employs a surface area commonly between 8-20 m.sup.2. These pleated panel filters continue to be generally manufactured from glass fibres because the higher efficiencies of glass fibres for a lower weight enable the filters to be pleated, thereby incorporating a larger surface area for a given panel size.
The polyolefin fibres are usually meltblown polyolefin fibres. Irrespective of the type of, filters employed, two factors are involved, namely filtration efficiency and pressure drop. The filtration efficiency may be regarded as the number of filtered particles arrested by the fibres, whilst the pressure drop may be regarded as the load required to suck air through the filter. The filtration efficiency and the pressure drop can be controlled by machine settings during the production of the meltblown polyolefin fibres.
The known meltblown polyolefin fibres are usually produced by melting polyolefin granules in an extruder or meltbox, and then forcing the melted granules through a die under pressure. High pressure hot air is introduced at the die and the hot air breaks up the extruded polymer into small non-continuous fibres. The fibres are blown on to a drum where they form a web. The drum rotates continuously so that as the web is formed, the web is separated from the drum. The separated web is taken through spreader rolls and is then wound onto a take-up roll. The size of the fibres and the density of the web can be controlled

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