Methods of making stretched filtering membranes and modules

Abstract

A method for stretching a hollow fibre while making a stretched filtering membrane from a precursor involves wrapping the hollow fibre around a structure and modifying the structure such that at least portions of the hollow fibre are forced to elongated. For example, the hollow fibre may be wrapped around around two or more supports and the distance between adjacent supports increased. This produces first portions of the hollow fibre stretched to a first elongation and porous to a first porosity and second portions of the hollow fibre that are not stretched or are stretched to a second elongation less than the first elongation and that remain non-porous or become porous to a second porosity which is less than the first porosity. The membrane may be made into a module with second portions of the membranes located at points where the membranes exit the potting material or at turns in loops of the fibres, if any. In various embodiments, the fibre is potted into modules before or after it is stretched. Some methods of potting involve applying a hot melt adhesive to the second portions. Some embodiments are made entirely of thermoplastic material and can be easily recycled.

Description

[0001] This is a continuation of U.S. Ser. No. 10 / 314,334 filed Dec. 9, 2002, which is an application claiming the benefit under 35 USC 119(e) of U.S. Ser. No. 60 / 338,676 filed Dec. 11, 2001. U.S. Ser. No. 10 / 314,334 and 60 / 338,676 are incorporated herein, in their entirety, by this reference to them.FIELD OF THE INVENTION [0002] This invention relates to filtering membranes and in particular to a method of making stretched filtering membranes, a module of stretched filtering membranes and a method of making such a module. BACKGROUND OF THE INVENTION [0003] Stretched membranes are a class of filtering membranes. Stretched membranes are typically made by first forming a non-porous hollow fibre of an appropriate membrane material. The membrane material is typically a thermoplastic such as polypropylene (PP), polyethylene (PE) or, less frequently, PVDF. The membrane material can be formed into a non-porous hollow fibre by various methods such as extruding or melt-spinning. The non-poro...

AI Technical Summary

Benefits of technology

"The present invention provides a method for making stretched filtering membranes and modules with improved reliability and service life. The method involves winding precursor fibres around supports and modifying the supports to force the fibres into a stretched state. The resulting fibre has unstretched or less stretched portions and stretched portions that can be located at high stress points in the module. The unstretched portions retain some of the qualities of the precursor and are less brittle and resistant to kinking and heating. The method allows for easy modification to the regimen for making the stretched membranes and exceptional control over the mechanical properties of the fibre. The unstretched portions can be located at the turn of the loop of a looped fibre or at the point where the fibre exists the potting material of a header. The modules with the stretched and unstretched portions of fibres at the appropriate places can be made by various methods."

Problems solved by technology

The inventors have observed various problems with the existing art.

Some problems relate to the existing methods of making stretched fibres.

Expensive and intrusive capital equipment is required to make large amounts of stretched fibres.

Changes to any part of the process may require difficult design changes to the various chambers, rollers, or roller drive mechanisms in a plant.

Further, the residence time in each chamber is significant and so the rate of production, even with multiple fibres simultaneously moving through a plant, is low in relation to the size of a plant.

A break in a fibre also causes significant down time since the fibre must be re-threaded through the plant.

Others problems relate to the stretched fibres themselves.

Stretched membranes are somewhat delicate, kink easily and may break prematurely in use.

In particular, when individual fibres are folded over into a loop, for example with both open ends of the fibre potted into the same header, the stretched membranes may kink or break at the turn of the loop.

The stretched membranes are also easily damaged by heat.

These problems increase with the porosity of the membranes, with microfiltration membranes in particular being prone to damage during handling.

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