GB2337991A

GB2337991A - Fine powder type PTFE material

Info

Publication number: GB2337991A
Application number: GB9811894A
Authority: GB
Inventors: Norman Ernest Clough
Original assignee: WL Gore and Associates UK Ltd; WL Gore and Associates Inc
Current assignee: WL Gore and Associates UK Ltd; WL Gore and Associates Inc
Priority date: 1998-06-04
Filing date: 1998-06-04
Publication date: 1999-12-08
Also published as: WO1999062993A1; GB9811894D0; AU4157099A

Abstract

A porous material is formed from presintered fine powder type polytetrafluoroethylene (PTFE) particles fused together to form a porous network. The material may further comprise particles of unsintered fine powder type PTFE in an amount of up to 50% by weight Preferably, unsintered granular type PTFE and particles of a thermoplastic tetrafluoroethylene copolymer are included. The PTFE may also be modified by less than 2% by weight of a copolymerised ethylenically unsaturated comonomer. A layer of the material may be attached to both sides of an expanded PTFE membrane, which membrane may be in the form of a fabric. The porous material may be used to form a filter, especially an ostomy filter.

Description

2337991 1.

C FINE POWDER-TYPE PTFE MATERIAL The present invention relates to a porous PTPE material formed from particles of fine powder-type polytetraf luoroethylene (PTFE), articles composed of the material and a process of production thereof.

Porous PTFE material has been produced previously by fusing PTFE particles at a temperature above the melt (or sinter) temperature of the PTFE.

Patent specification GB2242431 describes a sintered porous PTFE structure used as a f ilter to f ilter solids from liquids. The porous PTFE material is formed by fusing together particles of granular-type PTFE to form an integral network having voids between the interconnected particles. The porous materials may be formed entirely from granular-type PTFE particles which have been presintered or from granular-type PTFE particles which have not been pre-sintered, or from a mixture of both. It is found that materials formed predominantly from sintered granular-type PTFE particles have a large pore size and high porosity, but tend to be relatively weak materials. On the other hand, porous materials made from predominantly unsintered granular-type PTFE tend to have higher strength but have lower pore size and lower porosity. There is a need in the art for a porous 'PTFE material having a combination of good strength and high pore size.

Sintered materials have also been produced from fine powder-type PTFE particles. Patent specification EP369466 describes the production of a porous film by drying and

2 optionally sintering an aqueous dispersion of fine powder PTFE particles having a particular shape and size. The thin PTFE film produced was heated briefly to sinter temperature, whereupon it shrank to a large degree. Patent specification JP48056580 (Yuasa Battery) discloses the formation of a thin film from an aqueous dispersion of fine power PTFE tetrafluoroethylene hexafluoropropylene copolymer, followed by heat treatment to give a membrane of sub-micron pore size intended for diffusion concentration and separation of gases.

The present invention provides a. porous material formed of particles comprising presintered fine powder-type polytetrafluoroethylene (PTFE) particles, the particles being fused together to form a porous network.

Whilst attempts have been made to produce fused articles from fine powdertype PTFE, such as mentioned above, in our experience this leads to shrinkage and cracking and inability to form a continuous sheet. The basis of the present invention is the realisation that presintering the particles prior to fusion to form the porous network, leads to a porous material having a combination of high pore size and good strength.

The invention also extends to sintered fine powdertype polytetraf luoroethylene particles as a novel material.

By the term 'Isinteredll (and Ilpresinteredll) is meant that the PTFE under consideration has been heated to above its melting point, which is about 3430C for pure unmodified PTFE. By the term Ilunsinteredll is meant that the PTFE has C 'I,.

3 not been heated to above its melting point.

It is well known that PTFE is produced in two distinct types which are socalled "granular" PTFE and so-called "fine powder" PTFE. These materials have quite different properties. The term "fine powder type PTFE11 refers to that type of PTFE produced by the emulsion polymerisation technique. This technique produces a resin that cannot be RAM extruded but which must be extruded by the paste extrusion method where the resin is f irst mixed with a lubricant. The term 'If ine powder" is a term of art in the PTFE f ield and refers to the type of PTFE. It has no relationship to particle size. "Granular- type" and fine powder-type" PTFE are produced by two distinct procedures. In the process for producing granular-type PTFE resin, little or no dispersing agent is employed and vigorous agitation is used in the polymerisation vessel to produce coagulated resin. To produce "fine powder-type" PTFE, fluorinated surfactant is employed and agitation is very mild, producing small spherical particles dispersed in the aqueous medium. In the latter process, coagulation of the dispersion is avoided until after polymerisation is completed. Subsequent precipitation of the particles produces fine powder resins.

The porous material may be composed entirely of presintered fine powdertype PTFE or may include a proportion of other types of PTFE, such as unsintered fine powder-type PTFE. The unsintered material may be included in a wide range of proportions, for example from 0-70%, c-7 4 preferably 0-50%, more preferably 0-30 by weight based on the total weight of the porous material. The inclusion of a proportion of unsintered material tends to produce a porous material having a greater specific strength than when 100% presintered material is employed.

The porous material may also include a proportion of granular-type PTFE which may be sintered or unsintered (in proportions of 0-100% respectively). Again, the granular material may be present in from 0-80% by weight, preferably 0-60%, and especially 0-30% by weight based on-the total weight of the porous material. If necessary, one or more of sintered granular, unsintered granular and unsintered fine powder PTFE may be used to form the porous material in conjunction with the presintered fine powder-type PTFE.

Both the ngranular type' and "fine powder-type" PTFE include both homopolymer tetra f luoroethy 1 ene and modified PTFE. In modified PTFE, the homopolymer is modified by co polymerisation with a co-polymerisable ethylenically unsaturated comonomer in a small amount typically of less than 2% by weight of copolymer. These copolymers are called "modified" because they do not change the basic character of homopolymer PTFE, and the copolymer remains non-melt processable just as the homopolymer. Examples of comonomers include alkenes such as ethylene and propylene; halogenated alkenes such as hexafluoropropylene (HFP), vinylidene fluoride and chlorofluoroethylene; or perfluoroalkylvinyl ethers such as perfluoropropylvinyl ether (PPVE).

(-11 There may also be included in the porous material particles of a thermoplastic tetrafluoroethylene copolymer in an amount of between 0 and 30% by weight of solids, preferably 3-20%, based on the total weight of the porous material.

Examples of tetrafluoroethylene tetrafluoroethylene the thermoplastic fluorinated polymers include copolymers of and hexafluoropropylene (commonly called fluorinated ethylene-propylene copolymer or FEP), and of tetrafluoroethylene and perfluoroalkyl vinyl ether (when the ether is perfluoropropyl -vinyl ether the copolymer is commonly called PFA) These fully fluorinated polymers are preferred.

The porous material generally has a density in the region 0.5 to 1.5 typically 0.75 to 1.40. and especially 2 0.85 to 1.30 g/cm The pore size of the porous material (measured as described herein) is dependent to an extent on the particle size of the PTFE particles used to form the porous material. The mean pore size is generally in the range 150 microns, particularly 2-20 microns, especially 3-10 microns.

For a given pore size, the porous material of the present invention shows particularly good strength. Thus, the specific strength 'of the material (measured as described herein) varies from about 15 to SOON/cm2 2 particularly 50-30ON/cm2, and especially 100-20ON/cm.

There is an inverse relationship between specific strength 6 and mean pore size.

The porous material shows excellent dimensional stability and does not tend to shrink substantially at high temperatures (e.g. less than 5% after 2 hrs at 200OC). It also does not shrink substantially (e.g. less than 5%, typically less than 2%) when wetted with liquids such as isopropanol.

The air f low measured as the Gurley number is also inversely proportional to the mean pore size, lower Gurley numbers indicating a higher air flow through the material. Gurley numbers are generally in the range.0.4 to 10s/100cm3, especially 1-5s/100cm3.

The porous material may include various fillers as known in the art for inclusion in PTFE products. Suitable fillers include carbon, activated carbon, glass particles, chromium oxide, titanium oxide, aluminium nitride, silica, and chopped expanded PTFE. The amount of filler can be up to 80% by weight of the porous material. The high strength of the porous material enables relatively large amounts of filler to be included.

Generally, the porous material is in the form of a sheet, a tube, a fibre etc. However, the porous material may also be employed as a coating material on a substrate to form a composite article.

In particular, the porous material of the present invention may be applied as one or a multiple of layers onto expanded porous PTFE membrane. similar composite materials having a coating layer formed of granulartype 7 ,i,) PTFE are disclosed in our patent application PCT/GB96101340. The porous material of the present invention shows excellent adhesion to the expanded PTFE membrane. In one embodiment, one or more layers of the p orous material are applied onto one side of the expanded PTFE membrane. In another embodiment, one or more layers are applied to both sides of the expanded PTFE membrane. The thickness of the coating of porous material is generally in the range 50-2000 microns, particularly 1501000 microns.

The expanded PTFE membrane can be made using a number of processes, including the formation of an expanded network of polymeric nodes and fibrils in accordance with the teachings of US patents 3,953,566, 3,962, 153, 4,096,227 and 4,187,390. Generally, expanded PTFE membrane is made by blending a dispersion of so-called fine powder PTFE with hydrocarbon mineral spirits. The lubricated PTFE is compacted and RAM extruded to form a tape. The tape can then be rolled down to a desired thickness and subsequently dried by passing over heated drying drums. The dried tape can then be expanded both longitudinal ly and transversely at elevated temperatures.

In one embodiment, the expanded PTFE membrane is formed into a fabric by twisting tapes of the membrane and weaving these into a fabric (such a material is available from W.L. Gore & Associates Inc., under the RASTEX trademark). This fabric may be laminated (e.g. by heat bonding) to an expanded PTFE membrane to give improved 1 8 mechanical properties. The porous material of the present invention may be applied (e.g. formed in situ) onto the fabric or the laminate, on one or both sides thereof.

Another aspect of the present invention provides a process for the production of a- porous material., which comprises; - presintering particles of fine powder-type polytetrafluoroethylene (PTFE); and - fusing the presintered PTFE particles to form a porous network.

During the presintering step, the particles generally become fused into a solid mass, which is then usually subjected to a milling step to produce presintered particles of the required particle size (see patent specification W096/40510). Generally speaking, the unsintered fine powder-type PTFE resin is soft and not capable of being milled. on sintering, the particles become hard and the solid material produced becomes millable. The degree of sintering can be determined by carrying out differential scanning calorimetry (DSC) on the PTFE material and is generally greater than 90% sintered.

For example, the DSC peak of modified fine powder PTFE in the unsintered state is around 3400C and in the fully sintered state is around 3250C. Partially sintered materials exhibit both peaks. In this way, the degree of sintering can be monitored. Typically, the particles of fine powder PTFE will be presintered at around 345-3550C from 1-10 hours, typically 2-5 hours.

9 15C - As mentioned above, the presintered material is then preferably milled to a desired particle size, which will determine the pore size of the porous material produced. Typically, the presintered material is milled to a mean particle size in the range 20-300 microns, typically 50-200 microns. Higher particle size materials tend to result in a relatively large mean pore size but a lower specific strength. Higher specific strength porous materials are produced employing lower particle sizes and result in lower mean pore sizes. The porous material is generally formed by applying a liquid dispersion comprising the presintered fine powder-type particles and baking at an elevated temperature such as to form the porous network. The liquid dispersion can be applied by any suitable liquid coating technique, such as roller coating or by using a doctor blade. However, in a preferred embodiment, the liquid dispersion is applied onto the substrate by spraying.

The dispersion may contain suitable surfactants and thickening agents to enable it to wet the substrate and provide a continuous coating.

A particularly preferred thickening agent, particularly for large size presintered particles which have a marked tendency to sediment, is a polyetherpolyol such as a RHEOLATE (trademark) obtainable from Rhoex Inc, Hightstown, NJ08520, USA.

The liquid coating is then dried and baked at elevated temperature, so as to fuse the presintered particles. Usually, a preliminary step involves heating slowly at a C-) relatively low temperature e.g. 50-1000C to dry off water and any other volatiles. Thereafter, the temperature is raised progressively up to 330- 3850C (e.g. 340-370OC) in order to allow sintering and fusion of the PTFE particles to occur.

A particular benefit of the process of the present invention is that it is conducted at atmospheric pressure and elevated pressure conditions are avoided.

The porous material of the present invention finds many applications where a combination of high pore size leading to good flux of liquids or gases through the material, together with good strength is required. Particular applications include gas and liquid filter materials, vents including sterilisable vents, oiling wicks and webs for plain paper copying machines and fabrics. It can also be used to produce an ostomy filter for colostomy or ileostomy bags.

Embodiments of the present invention will now be described by way of example only.

EXAMPLE 1 - (100% Presintered Fine Powder PTFE) 500g of I.C.I. modified PTFE fine powder resin grade CD509 which has previously been heat treated at 3500C for 2 hours and then milled to an average particle size of 266 microns, 25g of Pluronic (trademark) L121, 25g of Zonyl (Trademark) FSN-100 surfactant solution, 16g of a thickening agent - Rheolate 310 and 500g of distilled water are blended together in a Waring blender for 60 seconds to 11 f orm a suspension. CD509 is a f ine powder PTFE resin copolymerised with a minor amount believed to be less than 2% by weight of hexaf luoropropylene (HFP) - Milling was carried out using a Morehouse mill (available from Jamar Associates, Lake City, Florida 32055, USA).

Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer. The FSN-100 surfactant is a non-ionic perfluoroalkyl ethoxylate mixture. The Zonyl FSN-100 surfactant solution consisted of a mixture of four parts FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight). Rheolate 310 is a polyether polyol solution (32% by weight active). The resulting aqueous suspension was suitable for spray application. The suspension was sprayed onto a ceramic tile using a Binks BBR gun. The spray coated plate was dried in an oven at 500C for 60 minutes. The temperature was then increased over several hours, up to 3500C and held at this temperature for 60 minutes to complete the baking process. After cooling, the resulting film can be removed from the tile. The physical properties of this material are shown as follows:

Sample Specific Thickness V Pore Pore Pore Density Strength (mm) (gil 00Crr13) Min Max Mean (glcm:,) (NICM2) (Mcrons) (microns) (microns) 1 00%CD509 27.2 1.36 <0.5 11.9 64.0 24.6 1.03 Repeating the above process using CD509 with no presinter heat treatment stage resulted in a very weak, cracked film ACI.I.

12 which was removed from the tile as flakes rather than as a continuous sheet.

EXAMPLE 2 - (100% Presintered Fine Powder PTFE) 500g of I.C.I. modified PTFE fine powder resin grade CD509 which has previously been heat treated at 3500C for 2 hours and then milled to an average particle size of 27 microns, 12.5g of Pluronic (trademark) L121, 12.5g of a Zonyl (trademark) FSN-100 surfactant solution and 500g of distilled water are blended together in a Waring blender for 60 seconds to form a suspension.

Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer. The FSN-100 surfactant is a non-ionic perfluoroalkyl ethoxylate mixture. The Zonyl FSN-100 surfactant solution consisted of a mixture of four parts FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight). The resulting aqueous suspension was suitable for spray application. The suspension was sprayed onto a ceramic tile using a Binks BBR gun. The spray coated plate was dried in an oven at 500C for 60 minutes. The temperature was then increased over several hours, up to 3500C and held at this temperature for 60 minutes to complete the baking process. After cooling, the resulting film can be removed from the tile. The physical properties of this material are shown as follows:

13 i e-l', Sample Specific Thickness Gurley Pore Pore Pore Density Strength (mm) (s/1 00cm3) Min Max Mean (g/CM3) (Nlcml) (microns) (microns) (microns) 1 00%CD509 116 0.41 <0.8 3.9 9.5 5.3 0.99 Again, repeating the above process using CD509 with no presinter heat treatment stage resulted in a very weak, cracked film which was removed from the tile as flakes rather than as a continuous sheet.

EXAMPLE 3 - (70% Presintered Fine Powder and 30% Unsintered Granular) 700g of I.C.I. modified PTFE fine powder resin grade CD509 which has previously been heat treated at 3500C for 2 hours and then milled to an average particle size of 184 microns, 300g of Du Pont unsintered granular PTFE resin grade 7A with an average particle size of 35 microns, 25g of Pluronic (trademark) L121, 25g of a Zonyl (trademark) FSN-100 surfactant solution and 1.3kgs of distilled water are blended together in a Waring blender for 60 seconds to form a suspension.

Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer. The FSN-100 surfactant is a non-ionic perfluoroalkyl ethoxylate mixture. The Zonyl FSN-100 surfactant solution consisted of a mixture of four parts FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight).

The resulting aqueous suspension was suitable for spray application. The suspension was sprayed onto a flat steel sheet using a Binks BBR gun. The spray coated sheet 14 was dried in an oven at 500C for 60 minutes. The temperature was then increased over several hours, up to 3500C and held at this temperature for 60 minutes to complete the baking process. After cooling, the resulting film can be removed from the steel sheet.

The physical properties of this material are compared with those obtained when a sintered granular PTFE resin (FPD6304) of a similar particle size distribution and mean to CD509, is used in place of CD509 in this process. The results are as follows:

Sarnple Thickness Gurley Pore Pore Pore Density Specific (nvn) (511 00CM3) Min Max Mean (g/CM3) Str :ngth (microns) (nxicrons) (microns) (N M 70%CD509: 1.1 1.7 3.3 20.8 6.5 1.25 279.8 1 30 A 70%FPD6304: 1.2 2.0 4 13.7 4.8 0.94 176.3 30%7A The CD509-based material has a significantly higher tensile strength than that of the FPD6304-based material but also has a larger mean pore size.

EXAMPLE 4 - (70% Presintered Fine Powder and 30% Sintered granular) 1050g of I.C.I. modified PTFE fine powder resin grade CD509 which has previously been heat treated at 3500C for 2 hours and then milled to an average particle size of 184 microns, 450g of I.C.I. sintered granular PTFE resin grade XG204 with an average particle size of 142 microns, 75g of Pluronic (trademark) L121, 75g of a Zonyl (trademark) FSN- surfactant solution, 47g of a thickening agent Rheolate 300 and 1.95kgs of distilled water are blended together in a Waring blender for 60 seconds to form a suspension.

Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer. The FSN-100 surf actant is a non-ionic perf luoroalkyl ethoxylate mixture. The Zonyl FSN-100 surfactant solution consisted of a mixture of four parts FSN-100k three parts distilled water and three parts isopropyl alochol (by weight). Rheolate 300 is a polyether polyol solution (32% by weight active).

The resulting aqueous suspension was suitable for spray application. The suspension was sprayed onto a flat steel sheet using a Binks BBR gun. The spray coated sheet was dried in an oven at 500C for 60 minutes. The temperature was then increased over several hours, up to 3500C and held at this temperature for 60 minutes to complete the baking process. After cooling, the resulting film can be removed from the steel sheet.

The physical properties of this material are as follows:

Sample Specific Thickness Gurley Pore Pore Pore De]nsity 7 Strength (mrn) (all 00c&) Min Max Mean (g/CM3) 1, (Nlcm (microns) (microns) (microns) 70%CD509: 39 1.45 <0.5 7.3 33.1 13.1 0.88 30%X13204 16 EXAMPLE 5 - (70% Presintered Fine Powder and 30% Unsintered Fine Powder) 1050g of I.C.I. modified PTFE fine powder resin grade CD509 which has previously been heat treated at 3500C for 2 hours and then milled to an average particle size of 184 microns, 450g of I.C.I. modified, unsintered PTFE fine powder resin grade CD509, 75g of Pluronic (trademark) L121, 75g of a Zonyl (trademark) FSN-100 surfactant solution, 47g of a thickening agent - Rheolate 300 and 1.95kgs of distilled water are blended together in a Waring blender for 60 seconds to form a suspension.

Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer. The FSN-100 surfactant is a non-ionic perfluoroalkyl ethoxylate mixture. The Zonyl FSN-100 surfactant solution consisted of a mixture of four parts FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight). Rheolate 300 is a polyether polyol solution (32% by weight active).

The physical properties of this material are compared 17 with those obtained when a sintered granular PTFE resin (I.C.I. grade XG204) with a mean particle size of 142 microns is used in place of the presintered fine powder PTFE resin, CD509 in this process. The results are as f ollows:

The physical properties of this material are as follows:

Sample Thickness Specific Gurley Pore Pore Pore Density (mm) Strength (c/100=3) Min Max Mean (g/CM:

(Nicm (rrdcronc) (microns) (microns) (microns) 70%C0509:(sint) 0.94 226 0.4 7.3 45.9 17.7 1.30 30%CD509(unsint) 70%XG204(sint) 0.97 249 1.2 4.3 24.4 8.7 0.99 30%CD509(unsint) 100%7A 0.46 662 64 0.35 0.68 0.91 1.27 From these results, it is surprising that the 100% CD509 material, has a much greater air 'flow and a larger pore size distribution/mean than that of the XG2041CD509 blend whilst these materials have similar tensile properties. In addition, comparing the 100% fine powder CD509 material with a similar density material composed of 100% granular resin (DuPont unsintered PTFE resin grade 7A, previously milled to a mean particle size of 25 microns) surprisingly produces very different physical properties in terms of pore size, air flow and specific strength characteristics.

18 EXAMPLE 6 - (Presintered Fine Powder with Filler) The following proportions (weight %) of components were blended together for 2 minutes using a Silverson mixing head:

7 S% 3.0% 0.4% Carboxymethyl cellulose 83.1% Water 6.0% Zonyl FSN-100 surfactant solution Pluronic L121 surfactant Triethanolamine This mixture is known as the 'Isurfactant concentrate". 490g of I.C.I. modified PTFE fine powder-resin grade CD509 which has previously been heat treated at 3500C for 5 hours and then milled to an average particle size of 50 microns, 210g of I.C.I. modified, unsintered PTFE finepowder resin grade CD509, 300g of activated carbon grade 209MP containing copper oxide (supplied by Sutcliffe Speaknan Carbon Ltd.) and 1.6kgs of 'Isurfactant concentrate" are blended together in a Waring blender for 60 seconds to form a suspension.

Carboxymethyl cellulose is Pluronic (trademark) L121 a thickening agent.

surfactant is a polyoxyethylene/polyoxypropylene block copolymer. The FSN surfactant is a non-ionic perfluoroalkyl ethoxylate mixture. The FSN-100 surfactant solution consisted of a mixture of four parts FSN-100, three parts water and three parts isopropyl alcohol (by weight).

The resulting aqueous suspension was suitable for spray application. The suspension was sprayed onto a flat steel 19 sheet using a Binks BBR gun. The spray coated sheet was dried in an oven at 1150C for 60 minutes. The temperature was then increased over several hours, up to 3400C and held at this temperature for 10 minutes to complete the baking process. After cooling, the resulting film can be removed from the steel sheet.

The physical properties of this material are as follows:

Thickness Water Entry Gudey Pore Pore Pore Density Deodorisation (rrwn) Pressure (allOOcm3) Min Max Mean (g/CM3) mins (psi) (microns) (microns) (microns) 11 1.65 1.3 1.4 2.18 11.35 4.77 0.79 142 The material may be used as an ostomy f ilter for colostomy or ileostomy bags (see our PCT/GB93/00232). The protocol for measuring deodorisation (H2S removal) is described herein.

EXAMPLE 7 - (2-Layer Composite of Porous Material and Expanded PTFE Membrane).

500g of I.C.I. modified PTFE fine powder resin grade CD509 which has previously been heat treated at 3500C for 5 hours and then milled to an average particle size of 50 microns, 12.5g of Zonyl FSN-100 surfactant solution, 12.5g of Pluronic (trademark) L121, and 500g of distilled water are blended together in a Waring blender for 60 seconds to form a suspension. The resulting aqueous suspension was suitable for spray application.

The FSN-100 surfactant is a non-ionic perfluoroalkyl ethoxylate mixture. The FSN-100 surfactant solution consisted of a mixture of four parts FSN100, three parts distilled water and three parts isopropyl alcohol (by weight). Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer.

An expanded PTFE membrane having a nodes and fibril morphology of nominal pore size 0.2 microns with an approximate thickness of 60 microns was held under tension in an aluminium frame (21.5inch2 outside, 18inch2 inside). The frame contains a "tongue and groove" arrangement between the top and bottom plates to ensure that the membrane is held under tension throughout the process. The frames are held together using toggle clamps. The tensioned membrane contained within the internal dimensions of the frame is not in contact with any other surface.

The aqueous suspension was sprayed onto one side of the membrane using a Binks BBR spray gun. The spray coated membrane held within the frame was dried in an oven at 500C for 1 hour. The temperature was then increased to 3500C and held at this temperature for 30 minutes to complete the baking process. After cooling, the toggle clamps are released and the 2- layer porous composite material removed. The thickness of the composite was measured at 280 microns and therefore by subtraction, the layer of sintered porous PTFE sheet on the expanded membrane was 220 microns. The air f low rate (Gurley) and pore size distribution of the composite was determined and compared to the expanded 1-211 C, 21 membrane alone aftersimilar temperature processing. The results are as follows:

Sampie Thickness Gurley Pore Pore Pore %Shrinkage (mm) (sil OOcm3) Min Max Mean (2hrs at 200OC) (microns) (microns) (microns) Expanded 60 13.3 0.30 0.72 0.45 15.7 Membrane 100%CD509: 280 9.3 0.41 0.93 0.59 2.5 Membrane 2-layer Composite it is clear from the results, that the sintered porous PTFE layer (100%CD509) of the composite has little effect on the air flow rate of the material. However, the dimensional stability of the composite to thermal treatment (2 hours at 200OC) is much greater than that of the membrane as shown by the very low shrinkage values.

The porous material is useful as gas or liquid filter and as a fabric.

EXAMPLE 8 - (3-Laver CoLaRosite of Porous Material and ELcRanded PTFE Membrane).

1kg of I.C.I. modified PTFE fine powder resin grade CD509 which has previously been heat treated at 3500C for 5 hours and then milled to an average particle size of 50 microns, 25g of Zonyl FSN-100 surfactant solution, 25g of Pluronic (trademark) L121, and 600g of distilled water are blended together in a Waring blender for 60 seconds to form a suspension. The resulting aqueous suspension was suitable for spray application.

The FSN-100 surfactant is a non-ionic perfluoroalkyl 1-11 22 ethoxylate mixture. The FSN-100 surfactant solution consisted of a mixture of four parts FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight). Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer.

An expanded PTFE membrane having a nodes and f ibril morphology of nominal pore size 0.2 microns with an approximate thickness of 60 microns was held under tension in an aluminium frame (21.5inch2 outside, 18inch2 inside). The f rame contains a "tongue and groove" arrangement between the top and bottom plates to - ensure that the membrane is held under tension throughout the process. The frames are held together using toggle clamps. The tensioned membrane contained within the internal dimensions of the f rame is not in contact with any other surface.

The aqueous suspension was sprayed onto both sides of the membrane using a Binks BBR spray gun. The spray coated membrane held within the frame was dried in an oven at 500C for 1 hour. The temperature was then increased to 3500C and held at this temperature for 30 minutes to complete the baking process. After cooling, the toggle clamps are released and the 3-layer porous composite material removed. The thickness of the composite was measured at 1040 microns and therefore by subtraction, the layer of sintered porous PTFE sheet on the expanded membrane was 980 microns. The air flow rate (Gurley) and pore size distribution of the composite was determined and compared to the expanded 23 membrane alone after similar temperature processing. The results are as follows:

Sampie Thiess Guday Pore Pore Pore %Shrinkage (nvn) (1100orn 3) Affin Max Mean (2hrs at 200 C) (rNcrons) (microns) (microns) Expanded 60 13.3 0.30 0.72 0.45 15.7 Membrane 1040 9.9 0.27 1.50 0.44 1.4 Membrane L100%CD509:

_Jay.' C.MP -1aver ComPosite It is clear from the results, that the sintered porous PTFE layer (100%CD509) of the composite has little effect on the air flow rate of the material. However, the dimensional stability of the composite to thermal treatment (2 hours at 200OC) is much greater than that of the membrane as shown by the very low shrinkage values.

The material is useful as a sterilisable vent material for sterilisation apparatus.

EXAMPLE 9 - (Alumini=--- Nitride filled fine powder PTFE) 500g of filled PTFE powder containing 22% (by weight) Du Pont fine powder PTFE resin (grade TE3737) and 78% (by weight) of silica-coated, aluminium nitride available from Dow Chemical which has previously been heat treated at 3500C for 5 hours and then milled to an average particle size of 17 microns, 25g of Pluronic (trademark) L121, 25g of a Zonyl (trademark) FSN-100 surfactant solution and 500g of distilled water are blended together in a Waring blender for 60 seconds to form a suspension.

Pluronic (trademark) L121 surfactant is a /11..I.. --- 24 polyoxyethylene, polyoxypropylene block copolymer. The FSN-100 surf actant is a non-ionic perf luoroalkyl ethoxylate mixture. The Zonyl FSN- 100 surf actant solution consisted of a mixture of four parts FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight).

The physical properties of this material are as follows:

SAMPLE Thickness Gurley Pore Pore Pore (mm) (51100CM3) Min Max Mean (microns) (microns) (microns) 78%Aiun.hinium 0.69 5.9 1.0 4.3 2.0 Nitride:

22% TE3737 EXAMPLE 10 - (85% Sintered and 15% Unsintered Unmodified PTFE) 425g of I.C.I PTFE unmodified fine powder resin grade CD1, which has previously been heat treated at 3500C for 5 hours and then milled to an average particle size of 58 microns, 75g of I.C.I. unsintered unmodified fine powder PTFE resin grade CD1, 12.5g of Pluronic (trademark) L121, 25.

12. 5g of a Zonyl (trademark) FSN- 100 surfactant solution and 650g of distilled water are blended together in a Waring blender for 60 seconds to form a suspension.

Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer. The FSN-100 surf actant is a non-ionic perf luoroalkyl ethoxylate mixture. The Zonyl FSN-100 surfactant solution consisted of a mixture of four parts FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight).

The resulting aqueous suspension was suitable for spray application. The suspension was sprayed onto a flat steel sheet using a Binks BBR gun. The spray coated sheet was dried in an oven at 500C for 60 minutes. The temperature was then increased over several hours, up to 350c1C and held at this temperature for- 60 minutes to complete the baking process. After cooling, the resulting film can be removed from the steel sheet.

The physical properties of this 100% fine powder PTFE material were compared with those obtained with a 100% granular PTFE material of similar specific strength composed of 40% sintered granular PT FE resin (Grade 9B available from Du Pont) and 60% unsintered granular PTFE resin (Grade 7A available from Du Pont. The results are as follows:

26 Sample Thickness Gurley Pore Pore Pore Specific (mrn) (S11 00CM3) Min Max Mean Strength (rnicrons) (microns) (microns) (N1cm2) 85%CD1:W 0.39 0.5 6.6 21.4 9.4 311.7 nt) 15%CD1On sint) 40%9B 0.85 5.3 1.4 5.9 2.8 332.6 (sint):

60%7A (unsint) Although the CD-based f ine powder PTFE material has a similar tensile strength to that of the 9B/7A granular FiTFE material, the air f low of the f ine powder material is significantly greater with a much more open pore structure.

EXAMPLE 11 - (100% Sintered Unmodified Fine Powder PTFE) 1000g of I.C.I. unmodified fine, powder FiTFE resin grade CD1 which has previously been heat treated at 3500C for 5 hours and then milled to an average particle size of 58 microns, 25g- of Pluronic (trademark) L121, 25g of a Zonyl (trademark) FSN-100 surfactant solution and 1.3kgs of distilled water are blended together in a Waring blender for 60 seconds to form a suspension.

Pluronic (trademark) L121 surfactant is a polyoxyethylene, polyoxypropylene block copolymer. The FSN-100 surfactant is a non-ionic perfluoroalkyl ethoxylate mixture. The Zonyl FSN-100 surfactant solution consisted of a mixture of four parts FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight). The resulting aqueous suspension was suitable for spray application. The suspension was sprayed onto a flat steel c-1-, 27 sheet using a Binks BEIR gun. The spray coated sheet was dried in an oven at 500C for 60 minutes. The temperature was then increased over several hours, up to 3500C and held at this temperature for 30 minutes to complete the baking process. After cooling, the resulting film can be removed from the steel sheet. The physical properties of this material are shown as follows:

Sample Specific Thickness Gurley Pore Pore Pore Density Strength (mm) (511 00CM3) Min Max Mean (g1CM3) (Nlcml) (microns) (microns) (microns) 100%CD 78.1 0.45 <0.5 7.3.22.8 11.3 0.95 Repeating the above process using CD1 with no presinter heat treatment stage resulted in a very weak, cracked film which was removed from the tile as flakes rather than as a continuous sheet.

28 TEST METHODS :.' 1) Air Permeability (Gurley No.) The permeability to air of the samples was measured by a Gurley densometer (ASTM D72658) manufactured by W. & L. E. Gurley & Sons. Results are reported in terms of Gurley number which is the time in seconds for lOOcm3 of air to pass through one square inch of the sample under a pressure of 4.88 inch of water head pressure. This measurement can be converted into metric permeability units (cm3cm/sec. cmcm.Hg) by the following formula: thickness of sample xO.0432/Gurley number.

2) THICKNESS Thickness was measured using a dial guage according to ASTM D461.

3) Density The density of the PTFE is determined by weighing a sample thereof in two different media, viz; air and water at room temperature. Water is a non- wetting medium for PTFE and consequently, the resulting density measurements refer to the porous PTFE. The weights were determined using an Avery VA124 analytical balance. The porous PTFE density is calculated as shown below:

(Weight in Air) (Density of Water at Room Temperature) (Weight in Air Weight in Water) 29 4) Particle Size Particle size of PTFE was determined as follows: using a magnetic stirrer and ultrasonic agitation, 2.5 grams of PTFE powder were dispersed in 60m1 isopropyl alcohol. (Ultrasonic Probe Model W-385, manufactured by Heat Systems-Ultrasonics, Inc.).

Aliquots of 4-6m1 of the dispersed particles were added to approximately 250m1 of circulating isopropyl alcohol in a Leeds & Northrup Microtrac FRA Particle Size.Analyzer. Each analysis consisted of three 30-second runs at a sample circulation rate of 2 litres/minute during which light scattering by the dispersed particles is automatically measured and the particle size distribution automatically calculated from the measurements.

5) Pore Size Measurements (Coulter Porometer) The pore size of the materials is determined by a COULTER POROMETER II (trademark) which uses a liquid displacement technique. The sample is thoroughly wetted with a liquid of low surface tension and low vapour pressure e.g. COULTER POROFIL (trademark) such that all the pores have been filled with the liquid. The wetted sample is subjected to increasing pressure, applied by a gas source. As the pressure is increased, the surface tension of the liquid is finally overcome and the liquid is forced out of the pores. By monitoring the gas pressure applied to the sample and the flow of gas through the sample when liquid is expelled, a "wet" run is obtained. The sample is i 1 then tested "dry" without liquid in the pores and a "dry" run is obtained. By comparing both "wet" and "dry" runs, the maximum (also called the bubble point), minimum and mean pore size can be calculated by the porometer using the Washburn equation.

In the case of laminated orcomposite materials, the sample gas pressure will be regulated by the material of smallest pore diameter which will effectively act as a pressure restrictor. Consequently, for composites of expanded PTFE membrane and porous sintered PTFE, the pore size measurements will closely resemble that of the smallest pore diameter layer i.e. the expanded membrane.

6) Water Entry Pressure Samples of the present invention were tested using a modified Suter test apparatus, which is a low water entry pressure challenge. Water was forced against the underside of a sample of 11.25cm diameter sealed by two circular rubber gaskets in a clamped arrangement. It is important that a leakproof seal is formed by the clamp mechanism, gaskets and sample. In deformable samples, the sample was overlaid by a reinforcing scrim (e.g. an open non-woven fabric) clamped over the sample. The upper side of the sample was open to the atmosphere and visible to the operator. The water pressure on the underside of the sample was increased by a pump connected to a water reservoir, as indicated by a pressure guage and regulated by an in-line valve. The upper side of the sample was 31 visually observed for a period of one minute for the appearance of any water which might be forced through the sample.

The water entry pressure was the pressure at which water became forced through the membrane.

7) Deodorisation Samples were cut into discs 23mm in diameter (4.16 cm2) and tested for gas deodorisation. The test gas comprised 80% nitrogen, 20% methane and 25ppm H2S, and the flow rate was 250m1/min. The test apparatus comprised a sample holder, a test gas inlet, an H2S detector and read-out unit.

During the test the gas is allowed to f low to the centre of the filter sample, which is clamped in position in the holder. The gas f low through the sample may be axia 1 or radial (for an increased path). The H2S detector attached to the other side of the sample monitors the efficiency of the filter in removing the hydrogen sulphide. once this efficiency begins to fall below 100% the detector reading is noted against time. The results are expressed at the time the electrochemical detector takes to reach 2ppm.

8) Tensile Strength Tensile measurements were carried out according to ASTM D461-87 Part 12 using an Instron 1011 Tensile Test Machine fitted with a 5KN load cell.

32 9) Thermal Dimensional Stability Pre-cut discs (109mm diameter) of the materials were placed in an air- circulating oven at 500C and the temperature increased to 200OC; and held at this temperature for 2 hours. After cooling to 500C, the average diameter of the discs was noted and the % area shrinkage calculated.

33

Claims

CLAIMS r---, U- 1. A porous material formed of particles comprising

presintered fine powder type polytetraf luoroethylene (PTFE) particles, the particles being fused together to f orm a porous network.
2. A material according to claim 1 which further comprises particles- of unsintered fine powder type PTFE.
3. A material according to claim 2 wherein the unsintered fine powder type PTFE is present in an amount of up to 50% by weight.
4. A material according to any preceding claim which further comprises particles of granular type PTFE.
5. A material according to claim 4 wherein the granular type PTFE is unsintered.
6. A material according to claim 4 or 5 wherein the granular type PTFE is present in an amount of up to 60% by weight.
7. A material according to any preceding claim wherein the PTFE is modified by the in clusion of less than 2% by weight copolymerised ethylenically unsaturated comonomer.

34
8. A material according to any preceding claim which further comprises particles of a thermoplastic tetrafluoroethylene copolymer.
9. A material according to claim 8 wherein the thermoplastic copolymer particles are present in an amount of up to 30% by weight.
10. A material according to any preceding claim wherein a layer of the porous material is attached to an expanded PTFE membrane.
11. A material according to claim 10 wherein a layer of porous material is attached to both sides of the expanded PTFE membrane.
12. A material according to claim 10 or 11 wherein the porous material has been formed by in-situ fusion of the PTFE particles to attach to the expanded PTFE membrane.
13. A material according to claim 10, 11 or 12 wherein the expanded PTFE membrane is in the form of a fabric produced by twisting tapes of expanded PTFE membrane and weaving into said fabric.
14. A process f or the production of a porous material, which comprises:

presintering particles comprising fine powder-type PTFE particles; and fusing the presintered PTFE particles to f orm a porous network.
15. A process according to claim 14 wherein the presintered particles are milled prior to the fusing step.
16. A process according to claim 15 wherein the particles are milled to a mean particle size in the range 50 to 200 microns.
17. A filter for gas or liquid filtration which comprises the porous material of any of claims 1 to 13.
18. An ostomy filter which comprises the porous material of any of claims 1 to 13.
19. Sintered fine powder type polytetrafluoroethylene particles.

19. Sintered fine powder type polytetrafluoroethylene particles.

1 C 36 Amendments to the claims have been filed as follows I. A porous material formed of Particles comprising presintered fine powder type polytetrafluoroethylene (PM) particles, the carticies being fused together to form a porous network.

2 - A material according to claim 1..iherein the particles prior to fusion further comprise parr-icles of unsintered fine powder type PTFE.

3. A material according to claim 2 wherein the unsintered fine powder type PIrFE is present in an amount of up to 50% by weight.

4. A material according to any preceding claim wherein the particles prior to fusion further comprise particles of granular type PTFE.

5. A material according to claim 4 wherein the granular type PTFE is unsnt--e--ed.

6. A mnaterial according to claim 4 or 5 wherein the granular type PT7E is present in an amount of up to 60% by weight.

(7 5'7 7. A material according to any preceding claim wherein the PTFE is modified by the inclusion of less than 2% by weight copolymerised ethylenically unsaturated comonomer.

8. A material according to any preceding claim which f ur ther comprises particles of a thermoplastic tetrafluorcethylene copolymer.

9. A material according to claim 3 wherein the thermoplastic copolymer particles are present in an amount of up to 30% by weight.

10. A material according to any preceding claim wherein a layer of the porous material is attached to an expanded PTFE membrane.

11. A material according to claim 10 wherein a layer of porous material is attached to both sides of the expanded PTFE membrane.

12. A material according to claim 10 ar 11,..;herei.,i the porous material has been formed by insi--u fusion of the PTFE particles to attach to the expanded PTFE membrane.

13. A. material according to claim 10, 11 or 12 wherein the expanded PTFE membrane is in the fform of a fabric produced by twisting tapes of expanded PTIFE membrane and weaving into said fabric.

14. A process for the production of a porous material, which comprises:

presintering particles comprising fine powder-type PTFE particles; and fusing the presintered PTFE particles to form a porous network.

IS. A process according to claim 14 wherein the presi.ntered particles are milled prior to the fusing step.

16. A process according to claim 15 wherein the particles are milled to a mean particle size in the range 50 to 200 microns.

17. A filter for gas or liquid filtration which comprises the porous material of any of claims 1 to 13.

18. An ostomy filter which comprises the porous material of any of claims 1 to 13.