WO1999041447A1

WO1999041447A1 - Porous belts or filter cloths

Info

Publication number: WO1999041447A1
Application number: PCT/GB1999/000429
Authority: WO
Inventors: Richard Patrick Lydon
Original assignee: Scapa Group Plc
Priority date: 1998-02-14
Filing date: 1999-02-11
Publication date: 1999-08-19
Also published as: AR014955A1; GB9803172D0; CA2319452A1; EP1055027A1; NO20004041D0; BR9907842A; AU2532599A; PE20000379A1; NO20004041L; CN1290315A; ZA991066B

Abstract

A porous belt or filter cloth comprises a woven or nonwoven textile substrate (11), which is coated on at least one surface with a coagulated polymer layer (12). The layer (12) preferably substantially impregnates the substrate, and the textile substrate, if nonwoven, may be supported on a mesh membrane (23), or a spiral link fabric (32). Alternatively, a layer of sintered particles (42) may be incorporated over the substrate which is covered by and impregnated by the coagulated polymer layer.

Description

-1 - POROUS BELTS OR FILTER CLOTHS

This invention relates to porous belts, for example filter belts, filter

press cloths and/or filter presses, for example for use in vertical automatic

pressure filter machines, and also suitable for use as a forming fabric, press

felt, dryer fabric or transfer belt in a papermachine, or as a corrugator belt.

Known filter belts may use the yarns or fibres of a woven or

nonwoven fabric, e.g. felt, sintered structure or spiral link fabric as the

primary filter medium. With such filters, the lower size limit of retained

particles is determined by the void sizes, or compaction, of the fabric. A

fabric substrate may be coated with a polymer film which is formed with

reticular pores (GB-A-2285935). While these retain fine particles down to

submicron sizes, the low ratio of pore aperture to land area does not permit

high volume filtration, such as required in pressure filtration and

papermachine belt uses.

Fine filtrates such as pigments and dyestuffs, such as titania, cannot

at present be filtered by belt filters, because of the extreme fineness of the

particles. It is however desirable to use belt filters on account of their high

filtration efficiency, and ease of installation of the fabrics.

An object of this invention is to provide a porous belt structure which

is suitable for use as a filter belt or filter press, or papermachine belt for

example, where extremely fine particles have to be retained, with high liquid

phase throughput. -2-

According to this invention, a porous belt comprises a woven or

nonwoven textile substrate, coated on at least one surface of said substrate

with a coagulated polymer material.

Preferably a relatively low viscosity material, e.g. about 500 cP or

less, and relatively high solids content material are used. The low viscosity

enables the polymer to penetrate substantially into the fabric structure,

whilst the relatively high solids content prevents bleed through of the

polymer. The resulting polymer treatment of the base fabric substrate

creates a relatively durable depth filtration structure which is capable of

operating at high pressures such as 10-20 bar, without collapse of the

microporous structure. As the polymer layer penetrates into the fabric

structure, it is also robust in that it is less likely to separate from the

substrate due to abrasion, or back-pressure surges.

The belt fabric is preferably partially impregnated with the coagulated

polymer, in addition to the formation of a layer on the or each coated

surface.

The polymer used may be a polyester; a polyamide; a polyolefin such

as polypropylene; or PAN.

The coating and partially impregnating layer may be applied to the

textile substrate as it is coagulating, for example using DMF, in a 10 to 20%

solids solution. The coagulated polymer is preferably coated straight onto

the substrate and is washed using water baths with reduced levels on DMF, -3- thus creating the pores. The coagulant can be foamed, said foaming

occurring during or immediately after coagulation. Foaming may be

achieved either by physical means, or using a chemical foaming agent . The

foaming agent preferably comprises a low boiling point water insoluble

halogenated, hydrocarbon, and the latter preferably has a boiling point in

the range from 10°-50°C, e.g. in the range 20°-30°C. Preferred foaming

agents include 1 ,2-dibromo-1 , 1 ,2,2-tetrafluoroethane and

trichlorofluoroethane.

Coagulation, and possibly also foaming, of the polymer may be

achieved by heating the impregnated coated textile substrate in the

presence of a heat coagulant. Suitable heat coagulants include vinyl alkyl

ether and derivatives thereof, polyacetals, polythio ethers, poly(ethylene

oxide) and derivatives thereof, and poly(propylene/ethylene oxide) and

derivatives thereof. The heat coagulant may be built into the back bone of

the polymer. Usually heating to a temperature of about 70°C results solely

in coagulation. Heating above this temperature will generally also result in

foaming, provided that a foaming agent is present.

Coagulation may also be achieved by means of adding a suitable

electrolyte and/or varying the pH of the polymer latex. For example, with

cationic polymers, coagulation may occur at an alkaline pH and for anionic

polymers coagulation occurs at an acidic pH. This may be followed by

heating to achieve satisfactory foaming. -4-

The coating may be applied by any coating technique such as knife

coating, dip coating, lick coating, screen printing or spraying. Reverse roller

techniques may be employed.

Coagulation produces polymer particles which adhere to the fibres or

yarns of the textile substrate, entering into the interstices between the

yarns or fibres, and producing a finer pore structure than is attained by the

fibres or yarns alone. Foaming of the polymer produces even finer porous

structure within the polymer particles, so that the resulting coagulated and

foamed partially impregnating coating can retain very fine particles, to

micron sizes, such as the aforementioned dyes and pigments, and China

clay.

Mechanical or chemical foaming techniques may be used in

combination with, or in place of coagulation.

Preferred embodiments of filter belt or papermachine belt according

to the invention will now be described by way of example, with reference

to the accompanying drawings, wherein:-

Figure 1 Is a sectional view of a woven belt according to the invention;

Figure 2 is a sectional view of a nonwoven belt according to the

invention;

Figure 3 is a sectional view of a non-woven fabric supported on a spiral

link belt; and

Figure 4 is a similar view of a belt comprising a sintered layer reinforced -5- by a textile fabric.

In Fig. 1 , a fragmentary sectional view is shown of a first filter belt

10, which comprises a woven textile substrate 1 1 . The textile substrate is

coated on and at least partially impregnated from one side by a layer 12 of

coagulated and foamed polymer, as indicated by cross-hatching in the

drawing. The layer is applied by a method such as one described

hereinbefore, and has a fine porous structure achieved by the non-

compacted nature of the coagulated mass, including that which has entered

the interstices between the yarns of the fabric, and further by the foaming

of the polymer.

In Fig. 2, a fragmentary sectional view is shown of a second filter belt

20. This comprises a non-woven textile substrate 21 , of e.g. a staple fibre

batt, or a needled felt, coated on one surface with a layer 22 of coagulated

and foamed polymer which penetrates at least partially into and at least

partially impregnates the substrate 21 , as indicated e.g. by cross-hatching

on the drawing. The layer 22 is applied similarly to that of Fig. 1 and

achieves a similar microporous structure due to the non compacted

coagulated structure, and the foaming of the polymer. The batt layer 21 is

supported by a mesh membrane 23.

In Fig. 3, a non-woven fibrous layer 31 is supported on a spiral link

fabric 32, and is coated and substantially impregnated with a layer 33 of

coagulated and foamed polymer. Substantially impregnated means that the -6- polymer penetrates to a significant extent into the layer 31 and preferably

penetrates into at least half the thickness of the layer, or even substantially

throughout the fibrous layer 31 .

In Fig. 4, a coagulated and foamed polymer layer 41 is supported on

a layer 42 of sintered particles, the polymer of layer 41 penetrating into the

interstices between the sintered particles. The sintered layer 42 is

reinforced by or carried on a textile fabric layer 43, which is shown

diagrammatically as being a woven fabric, but can be a non-woven. The

sintered particles may be beads of polymer material, or metal.

In the embodiments described, the coagulated polymer may be

coagulated with water, steam or super-heated steam, and may be

polyurethane, or to improve resistance to hydrolytic degradation, may be

selected from, for example, polyisoprene, polybutadiene, polyvinylidene

dichloride, PVC, polychloroprene and styrene-butadiene polymers or

mixtures thereof. Foaming of the coagulated polymer may occur during or

immediately after coagulation and may be achieved by physical means or

using a chemical foaming agent, such as, for example, a low boiling water

insoluble halogenated hydrocarbon, with a boiling point, e.g. in the range

-40°C to + 50°C. Examples are 1 , 2-dibromo-1 , 1 ,2,2-tetrafluoroethane and

trichlorofluoroethane.

The foaming and coagulation of the polymer may be achieved by

heating the impregnated coated material, preferably in the presence of a -7- heat coagulant. Suitable heat coagulants include vinyl alkyl ether and

derivatives thereof, polyacetals, polythio ethers, poly(ethylene oxide) and

derivatives thereof, and poly(propylene/ethylene oxide) and derivatives

thereof. The heat coagulant may be built into the backbone of the polymer.

Usually heating to a temperature of about 70°C results solely in coagulation.

Heating above this temperature will generally also result in foaming provided

a foaming agent is present.

Coagulation may also be achieved by means of adding a suitable

electrolyte and/or varying the pH of the polymer latex. For example, with

cationic polymers coagulation may occur at an alkaline pH and for anionic

polymers coagulation occurs at an acid pH. This may be followed by

heating to achieve satisfactory foaming.

The coagulated coatings may be particle-reinforced or fibre-

reinforced. The strength of the individual cells in the coagulated cellular

polymer network can be improved by the addition of finely-chopped fibres

such as RYTON fibres and/or finely dispersed particles such as PTFE

particles. These particles and/or fibres should ideally have a chemical

inertness, heat stability and acid and/or alkali resistance at least similar to

that of the coagulated polymer. The particles and fibres would be

incorporated into the polymer emulsion prior to coagulation.

Preferably the filter cloths have a coating thickness in the range from

0.5 to 2.0mm, corresponding to 50 to 250 (preferably 120 to 180) g/m² -8- addition by weight of polymer.

Although particularly described as filter belts, for example for use in

retaining ultra fine particles such as pigments, titania, dyes or China clay,

the invention also includes papermachine belts, such as forming fabrics,

press felts, filter cloths, drum covers, dryer fabrics or transfer belts, where

retention of fine fibres is required, with a high aqueous phase throughput.

Claims

-9-CLAIMS

1 . A porous belt and/or filter cloth comprising a woven or nonwoven

textile substrate, coated on at least one surface of said substrate

with a coagulated polymer layer.

2. A belt or cloth according to claim 1 , wherein said coagulated polymer

material is a relatively low viscosity material of up to about 200 cp,

with a relatively high solids content.

3. A belt or cloth according to claim 1 wherein the textile substrate is

at least partially impregnated with the coagulated polymer which is

coated directly onto the substrate and is washed using water baths

with reduced levels of DMF, thus creating pores in the layer in

addition to the formation of a layer on the or each coated surface.

4. A belt or cloth according to claim 1 , wherein the polymer of the

coagulated polymer layer is or includes any one of a polyester; a

polyamide; a polyolefin; a urethane polymer or PAN.

5. A belt or cloth according to claim 1 , wherein the coating and partially

impregnating layer is applied to the textile substrate as the layer is

coagulating, using DMF in a 10 to 20% solids solution.

6. A belt or cloth according to claim 1 wherein the coagulating polymer

is foamed, said foaming occurring during or immediately after

coagulation.

7. A belt or cloth according to claim 6 wherein a foaming agent is used -10- and comprises a low boiling point water insoluble halogenated

hydrocarbon with a boiling point in the range 10-50┬░C.

8. A belt or cloth according to claim 7 the foaming agent is 1 ,2-

dibromo-1 , 1 ,2,2-tetrafluoroethane and trichlorofluoroethane.

9. A belt or cloth according to claim 5 wherein coagulation is achieved

by heating the textile substrate after coating, in the presence of a

heat coagulant which is any one of:- vinyl alkyl ether or a derivative

thereof, a polyacetal; a polythio ether; poly (ethylene oxide), or poly

(propylene/ethylene oxide) or a derivative thereof.

10. A belt or cloth according to claim 9 wherein the heat coagulant is

built into the coating polymer.

1 1 . A belt or cloth according to claim 9 wherein coagulation is achieved

by adding an electrolyte and/or varying the pH of the polymer latex.

12. A belt or cloth according to claim 1 , wherein the coating is applied

by any one of knife coating, dip coating, lick coating, screen printing

and spraying.