AU2002233509A1 - Improved resin impregnated filter media - Google Patents

Description

IMPROVED RESIN IMPREGNATED FILTER MEDIA

This invention relates to improved filter media of the kind comprising

a woven or non-woven substrate.

EP-A-O, 741 ,815 discloses a method of making a fabric which is

suitable for use as a filter medium by applying a film of a reticular polymer

to a suitable substrate from a release sheet. The resultant filter medium

provides a micro porous filter, which has a good wear resistance so long as

the film endures. When the film is abraded however, the substrate becomes

vulnerable to wear and the filter medium loses its micro-filtration

capabilities.

In order to improve the effective lifetime of filter media, it is desirable

to improve the wear resistance of the filter cloths, belts sleeves or the like,

and if possible to reduce the rate of deterioration of filtration capacity which

tends to occur where a relatively coarse base substrate is surface treated or

partially impregnated from one side or the other. There is a general

tendency for coatings, however applied, to sit on the substrate as a discrete

layer, or to penetrate only a comparatively short distance into the substrate,

so that when the coated substrate is subjected to abrasion, the filter

medium is impaired quickly once the coating has been worn, even if only

locally.

Japanese Patent Application No. 06301439, published under No.

08131735, discloses a filter medium comprising a blend of two types of

fibres, filter fibres and heat fusible fibres. The fibres are joined together by partial fusion of the heat fusible fibres as the filter medium is heated and

shaped. Reinforcing materials are attached to the intersections of the two

types of fibres, and the reinforcement material may be for example a water-

soluble phenol, an epoxy resin, unsaturated polyester or a polyamide. This

type of filter medium is intended for example for cylindrical cartridge filters

or the like. It is not clear how the reinforcement materials are introduced

nor how they are caused to adhere only at the intersections of the two

types of fibre used. The use of a non-woven substrate comprising a blend

of two different fibre types raises production costs, as the fibres first have

to be blended. Also the presence of a fusible component in the fibre blend

restricts the utility of the filter medium to lower temperature uses, since

high temperatures could cause further fusion of the fusible fibres, leading to blinding of the filter pores by melted fibre material.

It is an object of the invention to provide a filter medium material and

a process for manufacture of a filter medium material with improved wear

resistance, and greater dimensional stability and which does not

substantially suffer from the disadvantages noted.

From a first aspect, the invention provides a process for

manufacturing a filter medium, comprising impregnating a woven or non-

woven substrate with a thermosetting resin combined with subsequent

curing of the resin by heating the impregnated substrate to a curing

temperature appropriate for the resin used, whereby the yarns or fibres of

the substrate are encapsulated by the resin so that the yarns or fibres are mutually bonded where they cross each other.

In a preferred process according to the invention, the resin is mixed

with a rheology modifier before application to control the viscosity and flow

properties of the resin, and may be added to the resin system which is preferably diluted to below 15% solids content, for example 10% solids

content, in an amount of below 5% by weight of the rheology modifier for

example 3% by weight. An example of a rheology modifier, which may be

used is hydroxy ethyl cellulose.

From a second aspect, the invention provides a filter medium,

comprising a substrate of woven or non-woven fabric impregnated with a thermosetting resin cured by subsequent heating of the impregnated substrate to a curing temperature appropriate for the resin used, the yarns

or fibres of the substrate being encapsulated by the resin so that the yarns

or fibres are mutually bonded where they cross each other.

The thermosetting resin may be any one or a mixture of any two or

more selected from:- a phenolic, epoxy, formaldehyde, amino/furan,

melamine, silicone, unsaturated polyester, polyurethane, polyamide,

fluorocarbon or cross linked thermoplastic based thermosetting resin

systems.

Further, other known additives can be added to the resin to enhance

supplementary properties. Examples may include:- carbon (to give

conductive or anti-static qualities), grafting binders which attach PTFE onto

the chain terminator of phenol end groups (to improve cake release), or antibacterial agents such as 3-trimethoxysilyl, poly-dimethyloctadecyl

ammonium chloride, and silane quaternary salts (to minimise crystal or fungi

build up).

The thermosetting resin preferably penetrates the substrate, to a

depth, which will give the desired balance of dimensional stability and

flexibility and effectively encapsulates or coats the fibres or yarns within the

impregnated part of the fabric without clogging the void spaces between the

woven or felted yarns or fibres. The void spaces may then be used if

required to hold micro porous materials such as flocculated, coagulated,

foamed or other micro porous materials.

The substrate may be a woven or non woven felt, spun bonded,

thermo bonded, or melt blown include polymeric yarns or fibres, and these

may be anyone or a blend of two or more of polyester (e.g. PET) or

polypropylene or other materials, such as polyamide (nylon) or polyethylene

or polyurethane. The substrate may also include a metal or ceramic mesh or

screen.

The resin may be applied as a liquid, by known means such as knife-

coating, spraying or lick coating or by immersion of the base fabric in the

coating material. The exposure of the fabric to the resin liquid is preferably

sufficiently prolonged to ensure the desired depth penetration of the liquid

into the base fabric. The resin bearing liquid may be an aqueous or non-

aqueous dispersion, or emulsion, or the like of the resin in the liquid phase.

Before curing, the coated fabric is optionally subjected to an intermediate heating, below the curing temperature to drive off moisture or other solvent,

and to effect fibre encapsulation that is to ensure that the resin adheres to

and "wets' the fibres or yarns, without substantially impeding the voids of

the fabric structure. Curing of the resin is then effected, with typical curing

times from 5 minutes to one hour or more, and typical curing temperatures

from 100°C to 200°C. Intermediate heating and curing may be effected

during a single pass, or the steps may be carried out separately and not

necessarily immediately subsequently.

Examples of the method according to_. the invention, and of the filter

media produced thereby will now be particularly described by way of

example.

Example 1

A base fabric comprising a needled non-woven felt of PET

(polyethylene terephthalate) fibres was lick coated with an aqueous phenolic

thermosettable resin.

The felt was then heated for 10 to 20 minutes to close to 100°C in

order to dry the felt by evaporating the aqueous phase and soften the resin

to cause it to wet the fibres effectively, thereby encapsulating the fibres in

the resin.

Following completion of the drying step the coated felt was then

heated to 160°C and maintained at this temperature for 10 minutes, thereby

effecting curing of the thermosetting resin.

Tests showed that the resultant filter medium has only slightly impaired flexibility and air permeability as compared to the untreated felt

further, dimensional stability and resistance to abrasion was significantly

improved.

Example H

A base fabric comprising a woven polypropylene cloth was coated with an aqueous phenolic thermosetting resin. The resin was applied using a knife

coating method.

The coated cloth was then heated to about 130°C and maintained at

this temperature for 1 hour, thereby ensuring that the resin wets the fibres

effectively, causing encapsulation of the fibres with the resin and also

effecting subsequent curing of the resin.

Again the resulting filter medium had improved dimensional stability

and abrasion resistance, and only slightly impaired flexibility and air

permeability as compared to the untreated fabric.

The method is also applicable to woven fabrics to coat the yarns or

fibres comprising the fabric.

Fig 1 illustrates in a considerably magnified view what is believed

happens to the structure of the impregnated region of a filter

medium produced from a non-woven felt as described by

Example I.

Fig 2 is a scanning electron micrograph of a coated polyester needle

felt as prepared by the Example I as set out above;

Fig 3 is a scanning electron micrograph of uncoated polypropylene cloth before treatment in accordance to the Example II set out

above;

Fig 4 is a scanning electron micrograph of coated woven multi

filament polypropylene cloth in accordance with Example II set

out above;

Fig 5 - is a magnified view of two adjacent coated fibres within a

multi filament yarn and meniscus of the binding resin;

Fig 6 is micrograph of a cross section through a coated

multifilament yarn within a coated woven polypropylene cloth;

Fig 7 is a graph showing the effect on permeability and filter throughput of a coating according to the invention;

Fig 8 is a graph showing the improvement in dimensional stability of

a polypropylene belt material after treatment by coating

according to the invention; and

Fig 9 is a graph showing the abrasion resistance of a phenolic resin

treated woven polypropylene compared to an untreated

substrate.

The impregnated fabric resulting from Example 1 as shown in Figure 1

consists of an array of randomly orientated fibres 20, some of which are

shown in sectional view. The coated fibres each comprise a core 20

consisting of the fibre itself and a coating or sheath 22 of the thermosetting

resin produced by wetting of the fibres by the resin during the drying stage

of the method described in the examples. Where the fibres 20 cross and contact each other, the resin forms bridges or pools 24, which connect the

fibres firmly to each other. Once the resin has been cured the bonding

remains permanent.

Figure 2 shows in a scanning election micrograph, part of a coated

polyester needled felt produced by method 1 set out above. As shown, the

coating 22 tends to collect in pools 24 about the intermeshing zones of the

fibres 20, with thickened sheath parts extending along the fibres.

Figure 3 shows uncoated yarns in a polypropylene cloth, which have

substantially no medium between yarns to give dimensional stability and to prevent abrasion, which is already evidenced by the presence of fibrils 25

on the yarn surfaces.

Figure 4 shows by contrast shows such yarns after coating, and it

will be noted that fibrillation is less marked, and that bridges of resin, e.g.

26, connect the yarns and inhibit them from moving against each other,

increasing stability and thereby reducing abrasion. The meniscus of resin

between yarns is shown enlarged in Figure 5.

Figure 6 shows how resin penetrates between the fibres of a

multifilament yarn, reducing internal wear within the yarn.

Figure 7 compares the filtrate through put (a measure of permeability)

of filter cloths which have and which not been impregnated with a phenolic

resin by the method of the invention. It is noted that the decrease in

permeability is marginal.

Figure 8 similarly compares the dimensional stability of treated and untreated polypropylene belt material, when tested by tensioning in the

direction of the weft yams.

The treated belt (full line) shows greater dimensional stability than the

untreated belt, demonstrated in that a greater force is necessary to produce

the same "stretch". For example, when the applied stress is 20 kgf / 2.5

cm, then the treated belt shows a strain (or elongation) of approximately 7% compared to greater than 12% for the untreated belt.

Finally Figure 9 shows the result of comparative tests of a treated and

an untreated woven polypropylene substrate. The untreated material

showed a marked increase in permeability over 2000 abrasion cycles,

signifying a loss of micro filtration, but the treated material exhibited no

noticeable deterioration.

The bonding thus produced increases the dimensional stability hence the propensity of the yarns to be displaced and to rub against each other is

reduced lessening internal wear and ultimately yarn breakage within the

fabric. A similar effect is present in woven base fabrics where the yarns are

coated and pools bridges are formed within the weave knuckles or in the

case of multi-filament or staple yarns, where parallel fibres contact each

other.

A further benefit is that the resin bridges or pools lock the fibres, and

the openings between fibres, relative to one another thus providing more

accurate and consistent pore sizes. The pore sizes will remain constant

throughout the life of the filter media. The properties of filter media according to and produced by the

method of the invention can be controlled by varying conditions and

materials.

The speed and degree of penetration of the resin into the substrate

can be controlled by varying the viscosity of the coating. The solids content of the coating may be changed to vary the thickness of the film

encapsulating the yams or fibres.

The properties of the resin used for coating are preferably controlled

depending upon the properties of the substrate by dilution and rheology

modifiers (which modify the flow properties of the resin).

In an example a first sample of a 373 gm -2 nylon 66 monofilament duplex weave cloth was subjected to application of a neat (unmodified)

resin comprising 60% solids, 40% solven and having a viscosity of 200 cPs.

This was found to have an excessive coat weight and low

permeability, due to the low surface area monofilament yarns being

encapsulated in a thick layer of resin.

A second sample of the cloth was subjected to application of resin

diluted to 10% solids, of lower viscosity (about 20cPs). This resin bled

through the entire cloth thickness and produced an insufficient coat weight,

with the yarns being incompletely encapsulated.

A third sample of the cloth was subjected to the application of a

specially prepared resin mixture.

This resin sample was prepared by dilution of the resin to 10% solids and then thickening by addition of hydroxy-ethyl cellulose as a rheology

modifier (i.e. to modify the flow properties of the resin). The hydroxyl ethyl

cellulose rheology modifier had been prepared previously in accordance to

manufactures guidelines and was added slowly to the dilute resin with

vigorous stirring to give 3% by weight of rheology modifier in the resin

mixture. The viscosity of this mixture as applied to the third cloth sample

was 4500 cPs. The coated fabric was cured in the usual manner in a single

pass through an oven at 160°C, with dwell time 10 minutes.

With use of a rheology modifier and dilution of the resin it was found

that the resin system had an optimum solids content for increased abrasion

resistance and optimum viscosity for substrate penetration, with minimal

loss of substrate flexibility and permeability.

Drying conditions can determine the way or extent to which the fibres are

encapsulated with shorter drying times so that, the Coating will migrate less.

Advantages of the invention for example in relation to EPA 0,741,815

mentioned above include the fact that a single process can provide a

coating throughout a variable depth within the substrate, and possibly the

whole substrate matrix, by if necessary completely soaking the fabric with the liquid phase resin. Using the method of the European Patent, if the

abrasion resistance of both sides of the fabric are to be improved, the resin

has to be applied separately to each side in a repeated process.

Experimental results show as noted above that the process of the

invention does not significantly change the filtrate throughput of the filter

medium. Air and liquid permeability is only slightly reduced. Further the method of the invention can be used with low surface energy materials such

as polypropylene due to encapsulation of individual fibres. The coating

applied by the method known from the said European Patent can be easily

peeled off such surfaces, as it does not adhere to them.

Because of the depth of penetration, the coating is permanent, with a

residue of coating throughout the full depth of the coating and the cloth will

continue to resist abrasion during the cloth lifetime.

The void spaces between the yarns or fibres of the base cloth are not

substantially impeded by the coating, as evidenced by the air permeability,

and these can be filled to render the filter medium micro porous by

coagulated, flocculated, or micro porous foamed materials as known in the art. The filter media of the invention may be used in liquid or gas any

filtration application as cloths for drum, cartridge, and disc filter sleeves and

bags or other filters also conveyor belts, pulp dewatering belts, corrugators

belts, tower press belts and filter press cloths.

Claims (19)

Claims

1. A process for manufacturing a filter medium, comprising impregnating a woven or non-woven substrate with a thermosetting resin, and

subsequently curing the thermosetting resin by heating the

impregnated substrate to the curing temperature of the resin used, to

thereby encapsulate the yarns or fibres of the substrate with the resin

so that the yarns or fibres are mutually bonded where they contact

each other.

2. A process according to claim 1 said resin is mixed with a rheology

modifier before application to control the viscosity and flow

properties of the resin.

3. A process according to claim 2 wherein the rheology modifier

comprises below 5% of the resin composition by weight.

4. A process according to claim 2 or 3 wherein the rheology modifier

comprises hydroxy-ethyl cellulose.

5. A process according to anyone claims 2 to 4 wherein the resin is

diluted to below 15% solids content before addition of the rheology

modifier.

6. A process according to any preceding claim wherein the

thermosetting resin is any one of or a mixture of any two or more

selected from: - phenolic, epoxy, formaldehyde, amino furan,

melamine, silicone, unsaturated polyether, polyurethane, polyamide,

fluorocarbon or cross linked thermoplastic based thermosetting resin systems.

7. A process according to any preceding claim wherein the substrate is

a woven or non-woven fabric including spun bonded thermo-bonded

or melt blown polymeric yarns or fibres.

8. A process according to claim 7 wherein the polymeric yarns or fibres

are of any one of or blend of two or more of: - polyester, polypropylene, polyamide polyethylene or polyurethane.

9. A process according to claim 7 or 8 wherein the substrate includes a

metal or ceramic mesh screen.

10. A process according to any preceding claim herein the coated

fabric is subjected to an intermediate heating, before curing to a

temperature below the curing temperature to remove water or other

solvent.

1 1. A filter medium comprising a substrate of a woven or non

woven fabric impregnated with a thermosetting resin cured by

subsequent heating of the impregnated substrate to the curing

temperature

12. A filter medium according to claims wherein said resin contains

a rheology modifier to control the viscosity and flow properties of the resin.

13. A filter medium according to claim 12 wherein the rheology

modifier comprises below 5% of the resin composition by weight.

14. A filter medium according to claim 12 or 13 wherein the rheology modifier comprises hydroxy-ethyl cellulose.

15. A filter medium according to claim 12, 13 or 14 wherein the

resin is diluted to below 15%solids content before addition of the

rheology modifier.

16. A filter medium according to any one of claims 1 1 to 15 wherein the thermosetting resin is any one of, or a mixture if any two

or more, selected from: - phenol, epoxy, formaldehyde, amino/furan,

melamine, silicone, unsaturated polyether, polyurethane, polyamide,

fluorocarbon, or cross-linked thermoplastic based thermosetting

resins systems.

17. A filter medium according to anyone of claims 1 1 to 16

wherein the substrate is a woven or non-woven fabric including spun

bonded, thermo bonded or melt blown polymeric yarns or fibres.

18. A filter medium according to claim 17 wherein the polymeric

yarns or fibres are of anyone of, or a blend of two or more of: -

polyester, polypropylene, polyamide, polyethylene or polyurethane.

19. A filter medium according to claim 17 or 18 wherein the

substrate includes a metal or ceramic mesh screen.