WO1998026925A1

WO1998026925A1 - Porous composite

Info

Publication number: WO1998026925A1
Application number: PCT/GB1997/003490
Authority: WO
Inventors: Stephen David Bode
Original assignee: W.L. Gore & Associates (Uk) Ltd.
Priority date: 1996-12-19
Filing date: 1997-12-18
Publication date: 1998-06-25
Also published as: AU5329598A; GB9626399D0; GB2320449A; GB9726783D0

Abstract

A high strength porous composite material formed from PTFE comprises a fabric (4) formed of fibres produced from expanded PTFE, and a layer (2) of sintered porous PTFE formed thereon. The sintered porous PTFE is formed from granular type PTFE particules, which comprise at least a portion of presintered resin. The particles are fused to form a porous integral network. The material has good strength. It is useful for producing oil transfer components for use in photocopying machines.

Description

POROUS COMPOSITE

TECHNICAL FIELD

The present invention relates to a high strength porous composite material formed from polytetrafluoroethylene (PTFE) which has applications in a number of fields, particularly in the copying machine field, but also in the field of gas or liquid filtration, and in medicine. The invention also relates to oil transfer components containing the porous composite material for use in copying machines and also to copying machines themselves.

The term "copying machine" as used herein relates to machines which employ heated fuser rolls, for example non-impact printer devices in general such as plain-paper copying machines, photocopiers, fax machines, laser printers, inkjet printers and thermal printers (wax or dye) .

BACKGROUND

In a plain-paper copying machine, toner images applied to the surface of paper or other recording medium are fixated by application of heat and pre 'sure. In certain plain-paper copying machines fixation is accomplished by passing the image-bearing recording medium between a hot thermal fixation roll and a pressure roll. When this type of thermal fixation device is used the toner material is directly contacted by a roll surface and a portion of the toner usually becomes adhered to the roll surface. On further rotation of the roll, material may be redeposited on the recording medium resulting in undesirable offset images, stains, or smears; or in severe cases the recording medium may stick to the adhered toner material on the roll and become wrapped around the roll. To counter these problems, materials having good release properties such as silicone rubber or polytetrafluoroethylene are often used for the roll surfaces. Although improving performance of the thermal fixation devices, use of silicone rubber or polytetrafluoroethylene roll surfaces alone does not eliminate the problem. Toner pick-up by the rolls can be controlled by coating the surface of at least one of the rolls with a liquid release agent, such as a silicone oil. It is important that the release liquid be applied uniformly and in precise quantities to the surface of the roll. Too little liquid or non-uniform surface coverage, will not prevent the toner from being picked up from the paper and deposited on the roll. On the other hand, excessive quantities of the release liquid may cause silicone rubber roll surfaces to swell and wrinkle, thus producing copies of unacceptable quality.

Various devices are known in the art for applying liquid release agent to one of the rolls of the fuser system, such as described in U.S. Patent Specification 3,831,553 and European Patent Publication 479564. However, the feature these systems have in common is the provision of a reservoir for holding a quantity of liquid release agent and an oil permeation control layer which is interposed between the reservoir and the roll of the fuser system for controlling the amount of oil which is transferred on to the roll of the fuser system. Various materials are known as the oil permeation control layer, such as porous polytetrafluoroethylene film as disclosed in Japanese Patent Specification No. 62-178992.

British published patent application 2242431 discloses a sintered porous polytetrafluoroethylene structure used as a filter in industrial filtration. The porous polytetrafluoroethylene material is produced by fusing particles of polytetrafluoroethylene such as to form a porous integral network of interconnected particles. Optionally, the porous polytetrafluoroethylene is supported on a woven or non-woven PTFE-based textile material. The disclosure of this patent specification is incorporated herein.

British published patent application 2261400 (International Patent Publication WO93/08512) discloses the use of such sintered porous polytetrafluoroethylene (PTFE) material as an oil transfer component in a copying machine and particularly as an oil permeation control layer to control the amount of release agent applied to the roll in the fuser system. Patent publication EP0174474 (Sumitomo) shows a release oil applicator which comprises a porous body formed of PTFE held in a housing. The PTFE body is saturated with silicone oil and may be formed with various cross-sections.

Patent specification US 4336766 (Maher) shows the use of a compound wick assembly formed from a relatively thick layer of Nomex felt and a relatively thin layer thereof. The thick layer acts as a feeder to convey oil to the thinner layer.

The function of the oil reservoir is to hold quantities of liquid release agent for application to the roll of the fuser system. The reservoir may be pre-loaded with a predetermined quantity of release oil. This is referred to as an "oil-filled device", and is generally discarded once the supply of liquid release agent is used up. Alternatively, the device may be an "oil-fed" device which is supplied with liquid release agent on a continuous basis from a supply device.

The materials disclosed in British published patent specification 2261400 are formed by fusing together a mixture of unsintered PTFE particles (typically of particle size 20-50 microns) and sintered PTFE particles (typically of particle size 30-60 microns) to form a porous material which has a good ability to retain and also to deliver oil by a capillary wicking actions. The porosity of the material and hence its ability to deliver release agent is dependent on the particle size of the PTFE particles used to produce the material. The material disclosed in GB2261400 typically has a mean pore size of about 4 microns. Such pore size gives good delivery of release agent with less viscous oils (e.g. 100 cS) but more viscous oils as used in certain copying machines (e.g. 60000 cS) require a more open porous structure. One way to increase the porosity of the material is to increase the particle size of the sintered PTFE particles used to prepare the porous material. However, this has been found to lead to weak materials having insufficient mechanical strength.

It is an object of the present invention to address this problem.

SUMMARY OF THE INVENTION

It has now been surprisingly found that a high pore size material having good mechanical properties in combination with good release agent delivery properties may be obtained from a porous composite material formed of a fabric formed from fibres produced from expanded PTFE membrane having attached thereto a layer of a sintered porous PTFE formed inter alia from presintered granular-type PTFE particles..

Thus, one aspect of the present invention provides a high strength porous composite material which comprises; a fabric formed of fibres produced from expanded polytetrafluoroethylene (PTFE) a layer of a sintered porous polytetrafluoroethylene (PTFE) , the sintered porous PTFE having been formed from granular type PTFE particles comprising at least a portion of presintered granular type PTFE particles, the particles being fused together to form a porous integral network of interconnected particles; the sintered porous PTFE layer having been formed on said fabric and being integrally attached thereto.

Our prior patent application PCT/GB96/01340 published after the filing date of this application, describes porous composite materials which with one exception are formed using an expanded PTFE membrane as substrate. However, the expanded PTFE membrane generally has a mean pore size much less than that of the sintered porous PTFE layer and therefore limits the rate of oil delivery through the porous composite that can be achieved. Generally speaking the composites show relatively high Gurley numbers (low Gurley numbers represent an open structure with low resistance to airflow) . Thus, very open structures with high oil delivery are hard to ahcieve. Moreover very open sintered layers have poor intrinsic mechanical strength. The prior patent application further discloses (Example 7) a porous composite employing an expanded PTFE fabric as the substrate. However, there is no disclosure of the use of at least a portion of presintered granular-type PTFE particles in the production of the sintered porous PTFE layer; as is described in the present patent application. It is surprising that presintered PTFE particles nonetheless show good bonding to the expanded PTFE fabric, and allow a high strength porous composite of very open structure (and thus good oil delivery) to be produced.

The porous composite materials have a high strength which is generally in excess of 100 N/cm², preferably in excess of 200 N/cm². Generally failure is due to cracking of the sintered PTFE layer. Good strength is required in copying uses to resist frictional drag of moving parts such as the fuser roll. It is also important in liquid filtration and in gas filtration to give mechanical robustness.

The PTFE particles used to form the sintered porous PTFE network are generally wholly or partially made-up of granular-type PTFE particles, though other types of PTFE particles may also be included. The nature of "granular-type" PTFE is discussed later.

By the term "sintered" (and "presintered") is meant that the PTFE under consideration has been heated to above its melting point, which is about 343 °C for pure unmodified PTFE. By the term "unsintered" is meant that the PTFE has not been heated to above its melting point.

The porous composite material of the present invention has an open porous structure which allows liquid to be received into the structure and retained therein, so that the material may act as a reservoir; and also allows liquid to be delivered at a controlled rate. The porous composite material has excellent mechanical properties, particularly at elevated temperatures such as 200°C where other known materials may be subject to heat degradation. The porous composite material being formed substantially from polytetrafluoroethylene also exhibits excellent chemical resistance and can therefore be cleaned using acids, alkalis or oxidising agents. The porous composite material also exhibits excellent dimensional stability and does not tend to shrink substantially at high temperatures, nor does the composite material tear easily. Being formed substantially of polytetrafluoroethylene, the porous composite material tends to have a non-abrasive outer surface, so that mechanical components in contact therewith exhibit low wear.

One principal application of the porous composite material of the present invention is in the field of filtration, particularly of gases and liquids. The good mechanical properties of the material are suited for liquid filtration applications, especially where the material is supported on a porous substrate such

as a mesh material, particularly a stainless steel mesh.

Another principal application of the porous composite material of the present invention is in the copying field. According to the present invention, an oil transfer component for transferring oil (i.e. liquid release agent) to a roll in a fuser system of a copying machine is advantageously formed from the porous composite material. The present invention envisages such oil transfer components and also copying machines containing such oil transfer components. The oil transfer component formed of the porous composite material exhibits good oil retention capacities, so that for a given volume of material, large amounts of liquid release agent may be retained. The porous composite material also has an excellent ability to deliver liquid release agent at a controlled rate to a surface of the composite material, due to its capillary properties. This allows good control of the amount of liquid release agent which is applied to each sheet of copying medium (such as paper, cardboard, clear plastics etc.) which passes through the fuser system of the copying machine. The composite material also has good capillarity in directions parallel to the plane of the surface of the composite material, such that the liquid release agent is applied uniformly across the entire surface area of the sheet of copying medium.

Other applications of the porous composite material include medical applications, such as uses in blood or gas syringes and intravenous vents.

Where the oil-transfer component is to be used in the form of a web, it is important that the web exhibit good dimensional stability at the operating temperatures of the fuser system. These operating temperatures are typically in the region of 200°C. At such temperatures, many conventional sheet materials have a tendency to shrink or alternatively to stretch unduly. When put under an applied load, many conventional materials, will tend to elongate in the direction of the load and to become correspondingly narrower (i.e. neck-in) in the transverse direction. The porous composite material of the present invention has excellent dimensional stability at around 200°C and is therefore particularly suitable for use in oil- transfer webs. Furthermore, the porous composite material of the present invention has good tear strength, such that it is difficult both to initiate and to propagate a tear within the material. Again, this enhances the properties of the material when used as an oil-transfer web.

Generally speaking, the porous composite material will be employed as an oil-transfer component by bringing the layer of sintered porous PTFE into contact with the fuser system roll. This layer has particularly good oil delivery control properties. It also has excellent ability to remove excess toner from the roll and to retain the toner within the structure of the sintered porous PTFE. The ability of the oil transfer component to wipe excess toner and other dirt (such as paper dust, fuser roll rubber etc.) from the fuser system roll is particularly enhanced when the roll contacting face of the porous composite material is textured. Such texturing may be effected by the use of a spray application technique as described hereafter.

However, in some copying machines a separate cleaning device may be provided for removing dirt from the fuser system rollers. In such cases there is no requirement for the oil transfer component to also act as a cleaner. Moreover, it is preferred that the roll contacting face of the sintered layer is smooth, which results in a longer life. Preferably, the smoothness is less than 11 microns Ra (determined as described herein) . Because there is no dirt pick-up to block the pores of the sintered layer, the material stays clean and the required oil delivery rate is better maintained. The oil distribution is even and there is low wear on the fuser roll.

The porous composite may have a particularly open structure, e.g. a Gurley number of less than 10 s/lOOcc, particularly less than 5 s/lOOcc and typically 0.2 to 2.0 s/lOOcc. This allows particularly high liquid throughput for many uses, and in the copying field allows a high oil delivery rate, particularly for thick oils.

The oil transfer component of the present invention is intended for holding and transferring liquid release agent to a roll in a fuser system, and also has the ability to remove excess release agent if necessary. Although the invention is primarily concerned with a porous composite material (and oil transfer component made therefrom) which comprises two layers, viz; an expanded PTFE fabric and a layer of sintered porous PTFE, it is also possible to form the material as a multiplicity of layers, which are formed of alternating layers of expanded PTFE fabric and sintered porous PTFE. Such multiple layer structures are particularly useful for building up thicker oil- transfer components, such as pads or rollers.

The use of an intervening adhesive or heat- bonding interlayer between the porous PTFE fabric and the layer of sintered porous PTFE constitutes a limitation on the properties of the overall porous composite material. Thus, parameters such as heat stability and chemical resistance may be limited by the properties of the adhesive or other material used to bond the two layers. This is disadvantageous, since the otherwise excellent properties of polytetrafluoroethylene are not attained in full.

It is a particularly surprising feature of the porous composite material of the present invention that the two layers may be integrally formed without the use of any intervening adhesive or other bonding material. In this way, a porous composite material is achieved which is formed entirely of polytetrafluoroethylene and which therefore has the overall properties of polytetrafluoroethylene without limitation by other components present. Such all-PTFE composite materials are highly advantageous for use at the high operating temperatures found in copying machines. According to the present invention, it is surprisingly found that a layer of sintered porous PTFE may be formed in situ on the expanded PTFE fabric. It has been found possible to form the sintered porous PTFE layer directly on the expanded PTFE membrane by the application of a liquid suspension comprising granular-type PTFE particles, followed by baking at elevated temperatures so as to fuse together the granular-type PTFE particles and to form a porous integral network of interconnected particles. It has been found that the liquid dispersion can be arranged such as to wet the surface of the expanded PTFE membrane and to form a continuous liquid layer thereon without any discontinuities. It is also surprisingly found that when the granular-type PTFE layer is sintered at elevated temperatures, the layer of sintered porous PTFE becomes securely attached to the expanded PTFE fabric. This is a surprising observation, since it is normally difficult to heat-weld PTFE to PTFE by the simple application of high temperature. Bonding occurs at atmospheric pressure without the application of any elevated pressures which might otherwise lead to crushing of the expanded PTFE fabric.

Thus, the present invention advantageously allows the production of a porous composite material which is composed s bstantially entirely of polytetrafluoroethylene, whereby the maximal properties of polytetrafluoroethylene may be enjoyed. However, this does not preclude the inclusion of small amounts of modifiers as described herein.

If required, a layer of sintered porous PTFE formed from a liquid dispersion may be formed in situ between two sheets of expanded PTFE fabric, followed by baking at elevated temperature, so as to form a unitary multiple layer all-PTFE composite structure. Conversely, layers of sintered porous PTFE may be formed on either side of an expanded PTFE fabric (for example, by spraying and baking) .

This fabrication technique is essentially brought about by the different methods of preparation of the expanded PTFE fabric and the preparation of the sintered porous PTFE layer. The former is generally produced by extrusion and stretching of a film, followed by slitting into tapes, forming the tapes into fibres and weaving into a fabric; whereas the latter is produced from a coating of a liquid dispersion.

The thickness of the porous composite material is generally in the range 50 to 5000 microns, particularly 150 to 3000 microns. The expanded PTFE fabric may have a thickness of 20 to 1000 microns, typically 50 to 500 microns, particularly 70-300 microns. The layer of sintered porous PTFE usually has a thickness up to 5000 microns, especially in the range 50-3000 microns, particularly 150-1500 microns. Expanded PTFE membrane used to form the PTFE fabric 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 the tape over heated drying drums. The dried tape can then be expanded both longitudinally and transversely at elevated temperatures. The expanded porous PTFE membrane generally has a pore size in the range 0.02 to 15 microns as measured by the bubble point method described herein.

The expanded PTFE membrane may be 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) .

The sintered porous PTFE layer may be a sintered material produced as described in patent specification GB2242431. The material is formed from one or more grades of granular-type polytetrafluoroethylene. As is well known, PTFE is produced in two distinct types which are so called "granular" PTFE and so called "fine powder" PTFE. Fine powder PTFE is employed to produce the expanded PTFE membrane discussed above which is used to form the expanded PTFE fabric. On the other hand, the sintered porous PTFE layer is produced from granular-type PTFE. These materials have quite different properties.

By the term "fine powder type PTFE" is meant 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 must first be mixed with a lubricant. The term "fine powder" is a term of art in the PTFE field and refers to the type of PTFE. It has no relationship to particle size.

Both the term "granular type" and "fine powder type" PTFE include herein homopolymer tetrafluoroethylene and modified PTFE, so-called because the homopolymer is modified by copolymerisation with a copolymerisable ethylenically unsaturated comonomer in a small amount 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 olefins such as ethylene and propylene; halogenated olefins such as hexafluoropropylene (HFP) , vinylidene fluoride and chlorofluoroethylene; or perfluoroalkyl vinyl ethers such as perfluoropropyl vinyl ether (PPVE) .

The sintered porous PTFE may be produced from a dispersion of granular-type PTFE particles in a liquid. Some of the granular-type PTFE used in this preparation may be unsintered but it includes a proportion which has been pre-sintered. The sintering process modifies the characteristics of the granular- type PTFE material. One particular embodiment of the present invention employs mixtures of presintered and unsintered material. Teflon granular-type resin grades 7A (unsintered) and 9B (presintered) are available from DuPont Speciality Polymers Division, Wilmington, USA. Granular resin grade XG204 (presintered) is available from ICI pic, UK. Generally speaking, the sintered porous PTFE may be produced from 0-90% unsintered PTFE (e.g. grade 7A) and conversely 100-0wt% sintered PTFE (e.g. grade XG204 or 9B) . However, high porosity is favoured by relatively lower amounts of unsintered PTFE e.g. less than 30%, preferably less than 20%, particularly less than 10% by weight. Where the sintered porous PTFE is formed from a mixture of presintered and unsintered granular-type PTFE particles, it is preferred that the presintered PTFE predominate since this leads to a material having a very open structure and good oil delivery. The inclusion of presintered PTFE particles tends to increase the porosity of the sintered porous PTFE layer produced.

The granular-type PTFE particles may have a particle size in the range 1 to 600 microns, especially 5 to 500 microns, particularly 10 to 300 microns.

The unsintered granular-type PTFE will ordinarily have a particle size of between 1 and 300 microns, particularly 20 and 150 micron. One commercial grade unsintered granular-type resin is available from the DuPont company as Teflon 7A (mean size of about 35-40 micron) as mentioned above. Another grade, having elongated fibrous particles, is available from DuPont with the trade name Teflon 7C. The granular-type resin or resins (whether unsintered or sintered) may also be modified by the inclusion of a small amount of a comonomer (such as hexafluoropropylene or perfluoropropyl vinyl ether) typically in an amount up to 1% or up to 2% by weight. An unsintered modified PTFE is Teflon 70J available from Mitsui Fluorochemical. It is modified PTFE in which the comonomer is perfluoropropyl vinyl ether (PPVE) . It can be presintered before use.

Unsintered granular PTFE tends to be made of soft particles which can "pack" together to form a fairly strong web when sintered having small pore sizes. For example, Teflon 7A has a tensile strength of 471.4 N/CM² and a mean pore size of 2.01 micron, when fused into a network.

On the other hand, sintered granular PTFE is composed of hard, substantially noncompactable particles. When baked above the melt temperature, only weak inter-particle connection is obtained and leads to large pore sizes. Preferred particle sizes lie in the range 30 to 500 microns, preferably 50 to 200 microns. For example, sintered granular-type PTFE is available from the DuPont company under the tradename Teflon 9B. It has a specific strength of 79N/CM² and a mean pore size of 6.04 micron when ground particles of 40 micron size are fused into a network. A preferred large particle size resin (mean particle size 120 microns) is XG204 obtained from ICI pic, UK.

The granular-type PTFE particles (whether presintered or unsintered particles, or a mixture of both) used to produce the sintered porous PTFE may have admixed therewith materials selected from the class consisting of

(i) unsintered fine powder PTFE (which may itself be modified or unmodified) , (ii) particles of a thermoplastic fluorinated organic polymer, (iii) particles of a low molecular weight PTFE micropowder produced by irradiation, and (iv) mixtures thereof; present in an amount of between 1 and 20% by weight of solids.

Unsintered fine powder PTFE is available from a number of sources, eg The DuPont Company, ICI or Daikin, and may be used either in particle form or in the form of a liquid dispersion thereof. A modified fine powder PTFE containing hexafluoropropylene comonomer is available from ICI (primary particle size 0.2 to 0.4 microns) as CD509 and modified PTFE containing perfluoropropyl vinyl ether is also available. Such modified resins generally contain upto 1% or upto 2% by weight of the modifier.

Examples of the thermoplastic fluorinated organic polymers include copolymers of tetrafluoroethylene 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) .

Micropowders produced by irradiation are available from DuPont.

Particles of an organic or inorganic filler material may also be included. Examples of fillers include carbon, activated carbon, glass, chromium oxide, titanium oxide, chopped expanded PTFE, silicon dioxide, and the like. In other words, virtually any filler can be employed to add specific properties to the composition. The amount of filler can be as high as 60% or more based on weight of composition.

Where the sintered porous PTFE is formed of a mixture of presintered granular-type particles, together with a "softer" material such as unsintered granular-type PTFE or any of the materials (i) to (iv) above, it is believed that the softer materials form moieties which link the harder presintered particles to provide increased inter-particle connection strengths. The presence of unsintered granular-type PTFE as one of the starting materials can be detected in the final sintered material as threads or rods linking the harder presintered particles (under magnification) . Sintered PTFE formed solely of hard presintered granular-type PTFE particles tends to have relatively poor strength.

The porous PTFE structure of the porous composite material is hydrophobic but has a high affinity for liquid release agents (referred to herein also as "release oils") such as silicone oil. The oil transfer component formed of the porous composite material will generally be supplied pre-loaded with release oil. In an oil-filled type of oil transfer component, the component is discarded when this oil is substantially used up or the oil flow reduces to an unacceptable level. In an oil-fed type of oil transfer component, further oil is supplied to the oil transfer component by means of an oil delivery mechanism. Typically, the oil will constitute 10% to 70% by wt. of the total weight of the porous composite material, particularly 20% to 60% by wt. In order to provide such oil retention capacities, the overall density of the porous composite material is generally in the range 0.5 to 1.2, typically 0.7 to 1.0 g/cm³ measured as described herein. In comparison, pure non-porous solid PTFE typically has a density of 2.16g/cm³. Generally, the expanded PTFE fabric will have a porosity in the region 50-98%, generally 70- 95%. The density of the sintered porous PTFE layer measured as described herein is usually in the range 0.5 to 1.8, for example 0.6 to 1.5, typically 0.7 to 1.2g/cm³ (corresponding to porosities of 77 to 16%, 72 to 30% and 68 to 44% respectively) .

A further aspect of the present invention provides a method of forming the high strength porous composite material which comprises;

- providing a fabric formed of fibres produced from expanded polytetrafluoroethylene (PTFE)

- forming thereon a layer of a sintered porous PTFE, the sintered porous PTFE being formed from granular type PTFE particles comprising at least a portion of presintered granular type PTFE particles, the particles being fused together to form a porous integral network of interconnected particles; the sintered porous PTFE layer being formed on said fabric and being integrally attached thereto.

As mentioned above, the layer of sintered porous PTFE is generally formed by coating the expanded PTFE fabric with a liquid dispersion comprising particles of granular-type PTFE and baking at an elevated temperature such as to form a porous integral network. The liquid dispersion can be applied by any suitable liquid coating technique, such as roller coating or by using a doctor blade, so as to apply a continuous coating of uniform depth over the expanded PTFE fabric. However, in a preferred embodiment, the liquid PTFE dispersion is applied onto the expanded PTFE fabric by spraying.

The dispersion will contain suitable surfactants and thickening agents to enable it to wet and continuously coat the expanded PTFE fabric.

If desired, a stabilised aqueous dispersion of the (i) fine powder or the (ii) thermoplastic fluorinated organic polymer can be mixed with the granular-type PTFE mixture in an aqueous liquid (eg. of water and alcohol, for example isopropanol) and the ingredients can be co-coagulated. This results in the much smaller sized fine powder resin polymer or the thermoplastic polymer particles congregating about the surface of the much larger size granular-type particles. This coagulated product can then be dispersed in water for spray coating or dip coating.

The liquid coating is then dried and baked at elevated temperature. Usually, a preliminary step involves heating slowly to 100°C in order to dry off water and any other volatiles, and holding at that temperature for a short period of time. Thereafter, the temperature is raised progressively up to 330 to 385°C (e.g. 340 to 370°C) in order to allow sintering and fusion of the PTFE particles to occur.

The expanded PTFE fabric is generally held in a frame, or stenter (for a continuous process) so as to prevent shrinkage or elongation during the production of the sintered porous PTFE layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings wherein;

Figure 1 is a cross-section to an enlarged scale of a porous composite material according to the present invention in the form of a web for use in a copying machine;

Figure 2 is a cross-sectional view of the porous composite material of the present invention in the form of a cover wick for mounting in a copying machine;

Figure 3 is a cross-section through a roller for use in a copying machine, and which comprises a spiral wrap of the porous composite material of the present invention mounted on a core;

Figure 4 is a cross-section through a roller for use in a copying machine and comprising a spiral wrap of the porous composite material of the present invention wrapped over a hollow sleeve of reservoir material, which is in turn mounted on a central core; Figure 5 is a cross-section through a pad for use in a copying machine wherein the pad is formed of multiple layers of the porous composite material;

Figure 6 is a cross-section through a pad of rectangular cross section formed by wrapping a single length of the porous composite material of the present invention;

Figure 7 is a schematic drawing of a fuser system of a copying machine employing a roller to apply liquid release agent;

Figure 8 is a schematic view of a fuser system of a copying machine wherein a pad is used to apply liquid release agent;

Figure 9 is a schematic cross section through the fuser system of a copying machine wherein liquid release agent is applied by means of a continuous web which is advanced incrementally from a feed spool to a takeup spool;

Figure 10 is a schematic view similar to Figure 9 wherein liquid release agent is additionally fed to a reverse side of the web of porous composite material; and

Figure 11 is a schematic view of the fuser system of a copying machine wherein liquid release agent is applied by a cover wick formed of the porous composite material of the present invention.

Figure 1 shows the porous composite material 1 of the present invention to an enlarged scale. The porous composite material comprises at least two layers 2 and 4. Layer 4 is composed of an expanded polytetrafluoroethylene (PTFE) fabric, a material which is available in a variety of forms from W.L. Gore & Associates Inc. of Elkton, MD, USA, under the trademark RASTEX. Expanded PTFE membrane used to form the fabric is typically produced by blending a PTFE fine particle dispersion with hydrocarbon mineral spirits, followed by compaction and ram extrusion through a die to form a tape. The tape may then be rolled down to a desired thickness and dried by passing over heated drying drums. The dried tape can then be expanded both longitudinally and transversely at elevated temperatures at a high rate of expansion, so as to form a porous expanded PTFE membrane. The layer 4 is composed of expanded PTFE membrane in the form of twisted tape, which has been woven into fabric.

The second layer 2 of PTFE material is formed of a sintered PTFE material made in a different way. The sintered material is produced by forming a liquid suspension comprising granular-type PTFE particles. The granular-type PTFE particles may be pre-sintered, unsintered or partially sintered, or may be a mixture of these various forms of granular-type PTFE. The suspension is then sprayed in one or more layers onto a substrate until the desired thickness is achieved. The sprayed material is dried in an oven by taking the material through a predetermined drying and baking regime up to elevated temperatures (e.g. 350-385°C) , as described in more detail later. This leads to the production of a porous sintered structure wherein the particles of granular-type PTFE become fused together to form a porous integral network of interconnected particles. This material is characterised by a particularly large pore size (for a given porosity) . Generally, the sintered porous PTFE material is produced in greater thicknesses than the expanded PTFE fabric. The sintered porous PTFE has excellent dimensional stability.

The porous composite material is advantageously formed by spraying (or otherwise applying, such as by means of a doctor blade) the liquid PTFE particle suspension directly onto the expanded PTFE fabric which thereby acts as the substrate. Generally the bond strength between surfaces of PTFE materials is poor without the use of surface treatments and/or adhesives, but it has been found surprisingly that not only is it possible to apply the aqueous liquid suspension directly onto the expanded PTFE fabric, but that after baking, a good bond is formed between the two layers. This not only provides a convenient fabrication technique, but also produces a porous composite material which is composed entirely of PTFE and therefore is a material whose overall properties are not limited by the presence of any other agent of inferior properties.

However, expanded PTFE fabric may shrink (or stretch if under tensile load) at the elevated temperatures required for baking the sintered porous PTFE material. For this reason, it is necessary to hold the expanded PTFE fabric in such a way as to maintain its original dimensions during the baking process. One way of approaching this is to hold the expanded PTFE fabric in a frame (where single pieces of material are to be produced) or by means of a stenter in the case of a continuous production facility.

The porous composite material shown in Figure 1 has a variety of applications. A principal application is for use in metering of liquid release agent to a roll within the fuser system of a copying machine, which arises in view of the good liquid retention and delivery characteristics of the composite material. In the form of a web, the porous composite material may be attached at either end to a spool to allow the web to be advanced slowly (either incrementally or continuously) past a roll in the fuser system. Typical arrangements are shown in Figures 9 and 10 as will be discussed hereafter. The porous composite material may be arranged such that the layer of sintered porous PTFE is directly adjacent the roll in the fuser system, since this material has good oil delivery properties. The sintered PTFE material also has good properties as regards the pick up of waste toner from the roll and other particulate matter such as paper dust or matter worn off the fuser roll surface, and good retention thereof. Where there is no requirement that the porous composite material pick up the waste toner (this being dealt with by some other means within the copying machine) , the sintered porous PTFE layer may be formed with a smooth outer surface. Here the excellent low friction properties of the sintered porous PTFE may be utilised to minimise wear on the roll, and provides a long life for the porous composite material.

The porous composite material of the present invention may be used in a number of formats within conventional copying machines where its advantageous properties of excellent dimensional stability, high strength, good release agent retention capacity and good delivery rate at high temperatures may be utilised.

Figure 2 shows in cross-section a so-called "cover wick" for use in a photocopying machine. The cover wick 5 comprises a sheet of the porous composite material which has been folded over and hemmed along each edge. Within each hem a mounting rod 6 is retained by folding over the material and stitching a seam 7 along each edge. The upper surface 8 of the cover wick is arranged to contact the roll of the fuser system and generally speaking this will be the sintered porous PTFE layer of the porous composite material of the present invention, so as to provide good toner pick up and holding, and oil delivery characteristics. As will be described in more detail in relation to Figure 11, the cover wick may be employed in conjunction with a reservoir material containing liquid release agent and located behind the cover wick.

Figure 3 shows in cross-section an oil transfer component in the form of a roller 10. The roller comprises a hollow cylindrical core 12 equipped with suitable bearings (not shown) for mounting in a copying machine. The core 12 has a hollow interior 13 for containing a liquid release agent such as release oil, which is delivered therefrom via apertures 14 provided in the core. Alternatively, the core could be formed of a sintered ceramic material. A single length 16 of the porous composite material is wound around the outside of the core so as to form six contiguous layers (17a, 17b, 17c etc.). After winding, an adhesive or potting material e.g. a silicone adhesive such as silicone sealant RTV732 (Dow Corning) is applied to the longitudinal ends of the wound layers so that the contiguous layers become adhered together at the ends of the cylindrical roller. The free end 18 of the length of sheet material may either be adhered to the underlying layer if necessary, or may be left free.

The length of porous composite material has a width sufficient to cover the desired area on the roller. That is to say the width of the porous composite material provides a full width of the oil transfer component. However, the length of composite material may in an alternative embodiment be spirally wound in a series of overlapping turns onto the roller core so as to build up the desired width and thickness.

Figure 4 is a cross-section of a roller 20 having a core 12 as before. This differs from the embodiment shown in Figure 3 in that a hollow sleeve 22 of suitable reservoir material, such as a felt formed of Nomex fibres or an open-cell foam plastics material is employed. The fibres sold under the Nomex trademark are aramid fibres, a type of polyamide. The open-cell foam might be an open-cell polyurethane or melamine foam.

Around the outside of the reservoir 22 is wound one or more turns of a single length of the composite material 16. The edges of the composite material are bonded as before. Once again, either the expanded PTFE membrane layer or the sintered porous PTFE layer may be arranged to be outermost on the roller surface which contacts the roll of the fuser system of the copying machine.

Figure 5 is a cross-section through a pad 30 formed of a plurality of contiguous layers 32 of the porous composite material. The layers are bonded together by means of a pattern of adhesive dots between adjacent layers. The roll contacting face 34 of the oil transfer pad 30 is slightly curved so as to follow the curviture of the roll in the fuser system onto which the pad transfers release oil in use.

Figure 6 also shows an oil transfer component in the form of a pad having a roll-contacting face 34. In this case, the pad has a substantially rectangular cross-section and has been formed by winding a single length of sheet material 16 in a substantially rectangular manner. Although not shown, it may be convenient to wind the length of sheet material about a flat central former, which former may be left in place or may be withdrawn after production of the oil transfer pad. Figures 7 and 8 show fuser systems employing oil transfer components comprising the porous composite material of the present invention. The fuser system in one conventional format comprises a PTFE-covered (or silicone rubber covered) fuser roll 52 and a silicone rubber covered pressure roll 54, which are oiled and wiped by means of a given oil transfer component. In the case of the Figure 7 embodiment, release oil is applied to the fuser roll 52 by means of oil transfer roller 10. In the case of Figure 8, release oil is applied to the fuser roller 2 by means of a pad 30 (such as that shown in Figure 5 or 6) held within a channel 42. However, in either case the release oil may be applied to the pressure roll 54 instead of to the fuser roll 52, if desired. Also, the oil transfer components may either be provided as oil-filled components, that is to say they contain a predetermined quantity of oil and are discarded after the release oil is used up; or may be provided as an oil-fed type in which case a supply of release oil is constantly fed to the oil transfer component (which is usually supplied pre-loaded with release oil) by a conventional oil delivery means.

Figures 9 and 10 show the use of continuous webs of the porous composite material of the present invention acting as oil-filled and oil-fed oil delivery systems respectively. In Figure 9, a continuous web 56 formed of the porous composite material is attached at one end to a feed spool 58 and at the other end to a take-up spool 60. The web may be attached by conventional means, such as by the use of an adhesive or by the use of adhesive tape. Prior to use, the web is rolled onto the delivery spool 58 and supplied in this manner. The web assembly so formed is fitted into the photocopying machine so that a free loop of web runs over the pressure rollers 62, 64. Usually, the porous composite material is arranged such that the layer of sintered porous PTFE runs in contact with the fuser roll 52. In use, the web of porous composite material is advanced either continuously or incrementally at a predetermined rate from the delivery spool to the take up spool. The rate is determined by the oil capacity of the web and the oil delivery rate, and also by the capacity of the porous composite material to pick up and hold waste toner cleaned from the fuser roll. The arrangement shown in Figure 10 is similar except that a conventional oil delivery means 66 is provided behind the web so as to feed release oil to the web (usually by feeding release oil to the expanded PTFE fabric layer of the composite material which has good oil retention capabilities.

Figure 11 shows a fuser roll/pressure roll arrangement as described above but in this case oiling and wiping is carried out by means of an assembly comprising a cover wick of the type shown in Figure 2, in conjunction with a reservoir pad 68. The reservoir pad may be formed of conventional reservoir materials, such as aramid felts, polyurethane foams or melamine foams. An oil delivery device 66 is provided to feed liquid release oil into the reservoir pads 68 and from there to supply oil to the cover wick 5.

The invention will now be further described in relation to certain examples as follows.

EXAMPLE 1 (Composite of 50% XG : 50% 7A and Expanded PTFE Fabric) . lkg of XG204 presintered granular PTFE resin (obtained from ICI pic) of mean particle size 120 microns, lkg of grade 7A unsintered granular PTFE resin (obtained from DuPont Company) of mean particle size 40 microns, 48g of carboxy-methyl cellulose solution (1% in water) , were mixed with 850g water. To this were added 80g Pluronic L121 (trademark) a polyoxyethylene/polyoxypropylene block copolymer surfactant, and 80g Zonyl FSN-100 surfactant solution (4 parts by weight FSN-100, 3 parts water and 3 parts isopropyl alcohol) a non-ionic perfluoroalkyl ethoxylate mixture. The mixture was blended in a Waring blender to form an aqueous suspension.

Rastex (trademark) is a woven PTFE fabric obtainable from W.L. Gore & Associates, Inc. which is formed by forming tapes of expanded PTFE membrane into fibres and thereafter weaving the fibres into a fabric. The fabric used had 40x20 fibres per inch of 400 denier, and a thickness of about 270 microns.

The PTFE fabric was held under tension in a rectangular aluminium frame (20 inch x 20 inch outside and 16 inch x 16 inch inside) . The frame comprised a tongue and groove arrangement between interengaging top and bottom subframes acting to hold the PTFE fabric under tension. The subframes were held together using toggle clamps. Thus, the tensioned PTFE fabric was held in the frame around its edges only.

The aqueous suspension was sprayed uniformly onto one side of the PTFE fabric using a Binks BBR spray gun to the required thickness.

The frame and spray-coated fabric was then heated using an infra-red heater to remove volatiles to a temperature of 360°C in 5 mins. It was then baked in a hot air oven at 360°C for 15 mins to sinter the PTFE resin into a porous integral network. Then it was removed from the oven and allowed to cool down to room temperature in the normal atmosphere. After cooling the toggle clamps were released and the porous composite removed.

The thickness of the composite was measured and the air flow rate (Gurley) , mean pore size, density and maximum load were determined by methods described herein. The results are as follows. The sintered porous PTFE layer was separated from the PTFE fabric and corresponding measurements made.

These results show that the max load which can be applied to the composite material (446 N/cm²) is substantially increased with respect to the sintered PTFE layer alone, whilst the good airflow (i.e. a low Gurley no.) and thus the high potential oil retention and oil delivery rate is essentially unchanged. A low Gurley number represents a very open structure with low resistance to airflow. The bond of the sintered porous PTFE layer to the woven expanded PTFE fabric was good.

EXAMPLE 2 (Composite of 90%XG:10%7A and expanded PTFE fabric) .

The procedure of Example 1 was repeated except that the woven PTFE fabric was sprayed with an aqueous suspension containing granular PTFE resin in the ratio 10wt% grade 7A to 90wt% XG204. The results are given in the table below.

Despite the high content of presintered XG204 granular-type PTFE resin, the sintered porous PTFE layer showed a good bond to the PTFE fabric. The presence of the PTFE fabric slightly reduced the high airflow (Gurley increased from 0.6 to 0.93) but the overall strength of the composite was good. The composite material thus showed a high potential oil retention and oil delivery rate coupled with high strength.

The composite materials of Examples 1 and 2 have been tested in copying applications, particularly in the form shown in Figures 2 and 11; and show high oil retention and high oil delivery rates, and excellent strength. A life of at least 250,000 copies was recorded using a smooth surface sintered layer in a copier not equipped with a separate roller cleaner; and a life of about 650,000 copies achieved when a separate cleaning felt was used to clean the pressure roller of the fuser system. Both oil transfer components were removed from the copying machine before failure occurred. TESTING AND PREPARATIVE METHODOLOGIES (A) PTFE grade 7A and XG204

TEFLON (trademark) granular-type PTFE fluorocarbon resin grade 7A is available from DuPont Speciality Polymers Division, Wilmington U.S.A. The manufacturers product specification indicates an average density of 2.16, and an average particle size of 35 microns measured by us as a mean size of 40 microns. PTFE grade 7A was unsintered and was used as supplied.

XG204 is a presintered granular-type PTFE resin classified to a mean particle size of 120 microns and obtained from ICI pic. in the United Kingdom.

(Bl) Density

Unless otherwise stated, 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)

(B2) Porosity

% Porosity is determined from density measurements in wetting and non-wetting mediums i.e. isopropyl alcohol (IPA) and water respectively, as shown below:

% Porosity = (Density in IPA - Density in Water) x 100

(Density in IPA)

(C) Particle Size

The particle size of the PTFE particles used for the production of the composite material was determined using a Leeds & Northrup Microtrac X100 particle analyser. A lg sample of PTFE powder was placed in the particle analyser, where it was sonicated (25 watts powers for 60 seconds) so as to become dispersed in a charge of isopropyl alcohol which was already present. The dispersion produced was circulated at a rate of 60ml/s, during which time light scattered by the dispersed particles was automatically measured and the particle size distribution automatically calculated from the measurements .

(Dl) Pore Size Measurement (bubble point)

Pore size of polytetrafluoroethylene was determined from the bubble point, defined in this specification as the pressure required to blow the first bubble of air detectable by its rise through a layer of liquid covering the sample. A test device, as outlined in ASTM F316-80, was used consisting of a filter holder, manifold and pressure gauge (maximum gauge pressure of 275.8 kPa) . The filter holder consisted of a base, a locking ring, an o-ring seal, support disk and air inlet. The support disk consisted of a 150 micron mesh screen and a perforated metal plate for rigidity. The effective area of the test sample was 8.0 plus or minus 0.5 cm².

The test sample was mounted on the filter holder and wetted with anhydrous methanol until clarified. The support screen was then placed on top of the sample and the top half of the filter holder was tightened in place. Approximately 2 cm of anhydrous methanol at 21°C was poured over the test sample. The pressure on the test sample was then gradually and uniformly increased by the operator until the first steady stream of bubbles through the anhydrous methanol were visible. Random bubbles or bubble stream of the outside edges were ignored. The bubble point was read directly from the pressure gauge.

The pore size of the test sample is related to the amount of gas pressure required to overcome surface tension and is given by a form of the Washburn equation: bubble point (psi) = K.4.Y.cos T /d where K = shape factor

Y = surface tension of methanol

T = contact angle between pore and surface d = maximum pore diameter. (D2) Pore Size Measurements (Coulter Poro eter)

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, 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 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, a form of which is shown in (Dl) .

In the case of laminated or composite 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 granular PTFE, the pore size measurements will closely resemble that of the smallest pore diameter layer i.e. the expanded membrane. (E) Oil Retention

(i) The oil retention capacity of the porous PTFE materials was determined by a modification of ASTM D461-87.

The oil used was Dow Corning 200 silicone oil of viscosity 100 centistokes and a density of 0.96 g/cc.

Test samples of size 25mm x 150mm were cut at random from sheet material. Each sample was weighed to the nearest O.Olg. The samples were placed on the surface of a vessel which had been fitted with oil to a depth of 50mm and allowed to sink under gravity to avoid air entrapment. The samples remained immersed for 3 hours. Thereafter each sample was removed from the oil and hung from a wire hook with the long dimension vertical to drain for 60 mins. A stirring rod was used to remove any visible drops of oil adhering to the sample before weighing the sample.

The oil retention was calculated according to ASTM D461-87 Section 21.6.1.

(F) Capillarity Test Method

A sample of PTFE sheet material is cut to dimensions 150mm x 12.5mm and hung over an oil filled container with the long dimension of the sample in the vertical plane. The lower 6mm of the sample is immersed in the oil at a temperature of between 18 °C and 22 °C. The sample is left to soak for 5 hours so as to allow oil to be drawn upwards from the liquid. Thereafter, the distance from the surface of the oil in the container to the top of the oil front which has travelled up the sample is measured. The capillarity is presented in millimeters.

For the tests reported the oil was Dow Corning 200 silicone oil of viscosity 100 centistokes and a density of 0.96 g/cc.

(H) Tensile testing fMax. Load)

The maximum load of the composite material was measured in an Instron 1011 machine. A 50x150mm. sample of the composite held in the jaws of the machine (75mm. gauge length) and the jaws set to move apart at a speed of 305mm/min. The maximum load was measured as the load (N/cm²) at which the sintered porous PTFE layer started to fail due to the production of cracks therein. The PTFE fabric itself would require much higher loads before failure and these were not measured.

The above Examples illustrate the production of various composites. These composites are suitable for gas and liquid filtration membranes. As shown, the materials are air-permeable and suitable for filtering solid particles from gas streams, and have the necessary strength for liquid filtration applications. (I) Thickness

The thickness of the porous composite material comprising the layer of sintered porous PTFE on the expanded PTFE fabric was measured using a dial guage according to ASTM D461.

(J) Air Flow Characteristics (Gurley No.)

The porosity of the composite material to airflow was determined and compared to the porosity of the sintered porous PTFE layer. The porosity is relevant as a measure of oil retention and oil delivery in copying applications and to filtration applications, particularly gas or liquid filtration. The airflow rate was determined through the porous composite material, and compared with the airflow rate through the sintered porous PTFE alone as a comparison. The Gurley test measures the number of seconds required for lOOcc of air to pass through one square inch of the material under a pressure drop of 4.88 inches of water.

(K) Surface Smoothness (Ra)

The surface smoothness of the sintered porous PTFE layer was determined using a Taylor Hoson Subtronic 3+ roughness instrument. The instrument uses a diamond tip stylus to traverse a surface in order to measure the surface roughness. The instrument is calibrated using a calibration surface as a preliminary step. Then, the sample is laid on a flat surface alongside the roughness instrument. The stylus arm is lowered carefully onto the surface to be tested. The stylus should be pointing exactly downwards so as to be perpendicular to the surface. The height of the stylus arm is adjusted so that the arm is parallel to the test surface. The machine is turned on and the display checked to confirm that the reading is in Ra and that Lc is set to 0.8mm. Then the start test button is depressed and the stylus moves over the test surface. The Ra reading is noted on an appropriate data record. The procedure is then repeated for different representative points on the surface of the sample. For a cover wick according to the invention, left, mid and righthand results are taken at a point approximately 25mm from the edge. The surface roughness Ra is given in microns.

Claims

1. A high strength porous composite material which comprises; a fabric formed of fibres produced from expanded polytetrafluoroethylene (PTFE) ; and a layer of a sintered porous polytetrafluoroethylene (PTFE) thereon, the sintered porous PTFE having been formed from granular type PTFE particles comprising at least a portion of presintered granular type PTFE particles, the particles being fused together to form a porous integral network of interconnected particles; the sintered porous PTFE layer having been formed on said fabric and being integrally attached thereto.

2. A material according to claim 1 having a Gurley number of less than 10 s/lOOcc.

3. A material according to any preceding claim having a Gurley number of less than 5 s/lOOcc.

4. A material according to any preceding claim having a Gurley number of 0.2 to 2.0 s/lOOcc.

5. A material according to claim 4 wherein the granular-type PTFE is modified by the inclusion of a fluorinated organic polymer comonomer.

6. A material according to claim 4 wherein the sintered non-expanded porous PTFE layer comprises particles of granular-type PTFE and unsintered fine powder PTFE fused to form said network.

7. A material according to claim 6 wherein the unsintered fine powder PTFE is modified by the inclusion of hexafluoropropylene comonomer.

8. A material according to claim 5 wherein the sintered porous PTFE layer comprises particles of granular-type PTFE and particles of thermoplastic fluorinated organic polymer fused to form said network.

9. A material according to claim 8 wherein the thermoplastic fluorinated organic polymer is fluorinated ethylene-propylene copolymer, or a copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether.

10. A material according to claim 4 wherein the sintered porous PTFE layer comprises particles of granular-type PTFE and particles of low molecular weight irradiated PTFE fused to form said network.

11. A material according to any preceding claim having an overall thickness of 500 to 5000 microns.

12. A material according to any preceding claim wherein the expanded PTFE fabric has a thickness of 20 to 1000 microns; and the layer of sintered porous PTFE has a thickness of 50 to 3000 microns.

13. A material according to any preceding claim wherein the density of the layer of sintered porous PTFE is in the range 0.6 to 1.5g/cm³.

14. A material according to any preceding claim having a maximum load before cracking of the sintered porous PTFE layer occurs of at least 150 N/cm².

15. A material according to any preceding claim wherein the layer of sintered porous PTFE has an outer surface away from the fabric, the outer surface having a smoothness Ra less than 11 microns.

16. A material according to any preceding claim wherein the layer of sintered porous PTFE has a mean pore size of 3 to 12 microns.

17. An oil transfer component for a copying machine which comprises the porous composite material of any preceding claim.

18. An oil transfer component according to claim 17 in the form of a coverwick.

19. A filter for gas or liquid filtration which comprises the porous composite material of any preceding claim.

20. A method of forming a high strength porous composite material which comprises;

- forming thereon a layer of a sintered porous polytetrafluoroethylene (PTFE) , the sintered porous PTFE being formed from granular type PTFE particles comprising at least a portion of presintered granular type PTFE particles, the particles being fused together to form a porous integral network of interconnected particles; the sintered porous PTFE layer being formed on said fabric and being integrally attached thereto.

21. A method according to claim 20 wherein the layer of sintered porous PTFE is formed by coating the expanded PTFE fabric with a liquid dispersion comprising particles of granular-type PTFE and baking at an elevated temperature such as to form a porous integral network.

22. A method according to claim 21 wherein the liquid dispersion further comprises particles selected from

(i) unsintered fine powder PTFE,

(ii) a thermoplastic fluorinated organic polymer,

(iii) a low molecular weight PTFE, and

(iv) mixtures thereof

23. A method according to claim 21 or 22 wherein coating is achieved by spraying the liquid dispersion onto the expanded PTFE fabric.

24. A method according to any of claims 20 to 23 wherein the expanded PTFE fabric is held so as to prevent shrinkage or elongation during formation thereon of the sintered porous PTFE layer.