GB2327372A

GB2327372A - Sterilisable vent

Info

Publication number: GB2327372A
Application number: GB9815127A
Authority: GB
Inventors: Norman Ernest Clough; Bradford C Jones
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: 1997-07-19
Filing date: 1998-07-14
Publication date: 1999-01-27
Also published as: AU8350298A; GB9715225D0; GB9815127D0; WO1999003511A2; WO1999003511A8; WO1999003511A3

Abstract

A sterilisable vent, for use in a sterilization apparatus, is formed of a porous composite material comprising expanded PTFE membrane and a layer of non-expanded sintered porous PTFE attached to the expanded PTFE. The non-expanded porous PTFE may be formed by fusing granular PTFE directly on the expanded PTFE membrane. The expanded PTFE membrane may be woven into a fabric from twisted tapes of the membrane. A small amount of certain modifying materials may be included in the PTFE layers. A thicker material may be built up from multiple alternating layers of expanded and non-expanded PTFE material. The porous composite material is permeable to gases and liquids, is heat resistant to temperatures above 200‹C, is resistant to chemical attack, and shows little or no shrinkage under repeated exposure to steam steriliation.

Description

STERILISABLE VENT TECHNICAL FIELD The present invention relates to a sterilisable vent for use in a sterilisation apparatus. The vent is permeable to gas or liquid sterilisation media which are able to pass therethrough but otherwise acts to protect the interior of the apparatus from entry of non-sterile agents. The vent is resistant to repeat contact with sterilisation media, being principally formed of polytetrafluoroethylene (PTFE) and does not shrink in use.

BACKGROUND Items such as surgical or dental instruments are routinely sterilised at frequent intervals prior to use. Sterilisation may be carried out in a sterilisation apparatus employing known gas or liquid sterilisation media, such as steam, ethylene oxide, hydrogen peroxide, caustic soda etc. The apparatus often includes an aperture protected by a vent which allows entry of the sterilisation medium but otherwise prevents the entry of non-sterile agents. Once sterilised, the item may remain in the sterilisation apparatus in a sterile state until withdrawn for use.

Thus, the vent should be formed of a material which is both permeable to sterilisation medium and is resistant to degradation therefrom. It requires adequate mechanical properties.

One conventional sterilisable vent is formed of a polyester scrim. However, this becomes brittle after repeated contact with steam or chemical agents under typical autoclave conditions of 134 C and 2 bar pressure. It is, thus, essentially non-reusable.

There is a need for a vent which can be repeatedly resterilised and reused.

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. The disclosure of this patent specification is incorporated herein.

Our published patent specification W096/40510 discloses a porous composite material formed substantially of PTFE which comprises an expanded PTFE membrane and a porous layer of sintered granular-type PTFE formed in-situ on the membrane. The composite material has good mechanical properties and is employed as an oil transfer component in photocopiers and in the filtration of gases and liquids. SUMMARY OF THE INVENTION It has now been surprisingly found that such a porous composite material has properties which enable it to be employed as a reusable sterilisable vent material. In particular the composite material is resistant to repeated exposure to sterilising media.

Its porous nature allows the passage of gas or liquid sterilising media therethrough.

Thus, one aspect of the present invention provides a sterilisable vent for use in a sterilisation apparatus formed of a porous composite material which comprises; - expanded polytetrafluoroethylene (PTFE) membrane; and - a layer of a non-expanded porous polytetrafluoroethylene (PTFE) attached to the expanded PTFE membrane.

The layer of non-expanded sintered porous PTFE may be a sintered material which is generally made by a sintering process wherein PTFE solids are heated to high temperature to form a porous matrix. One such material is available under the Zitex trademark (Norton Chemplast, New Jersey, USA) and comprises fibrous PTFE wherein PTFE fibres are bonded into a porous matrix. Such materials may be formed by mixing cellulosic or proteinaceous materials with PTFE and heating in oxygen to high temperatures to burn out or carbonise cellulosic or proteinaceous material and to sinter the PTFE (see US patent 3,775,170).

However, in a particularly preferred embodiment the layer of non-expanded porous PTFE is a sintered porous PTFE formed from PTFE particles, comprising granular-type PTFE particles, fused together such as to form a porous integral network of interconnected particles. The PTFE particles used to form the porous network are generally wholly or partially madeup 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 343cC 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 gas or liquid sterilising media to pass through i.e.

it is permeable to gases and liquids. 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. 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. In particular, the porous composite material exhibits little or no shrinkage when repeatedly steam sterilised under autoclave conditions.

The present invention also relates to a sterilisation apparatus which comprises; - a housing for containing an item to be sterilised, the housing having an aperture for allowing entry of sterilising medium; and - a permeable sterilisable vent according to any preceding claim, the vent being disposed across the aperture so as to close the aperture but to allow passage therethrough of said sterilising medium.

Various constructions of housing are known in the art and the sterilisation vent of the invention will be appropriately shaped to fit. For example, the sterilisation vent may be square, rectangular or circular. Usually the porous composite material used to make the vent will be sheet-like. The sheet could lie flat (i.e. a planar vent) or could be folded, rolled or corrugated to fit across the sterilisation medium entry aperture of the apparatus. The vent may be clamped around its edges and the porous composite material shows good sealing properties.

The housing is usually split to provide a body section and a lid sealably fitting onto the body section e.g. by sliding or push-fit. The aperture(s) for entry of sterilising medium may be in the body section (e.g. in walls thereof) or in the lid.

The invention also relates to a method of sterlising employing the apparatus and vent of the invention. In particular there is provided a method of sterilising an item, which comprises; - placing an item to be sterilised in a housing having an aperture for allowing entry of sterlising medium, a permeable sterilisable vent being disposed across the aperture so as to close the aperture; and - passing said sterilising medium into the housing through the permeable vent; and - maintaining the sterilising medium within the housing for a time sufficient to sterilise said item.

Many sterilisation media are known in the art e.g. steam, ethylene oxide, hydrogen peroxide, caustic soda, hypochlorous or hypochloric acid etc. The sterilisable vent is formed substantially of PTFE and shows good chemical resistance. Preferably, the vent is formed entirely of PTFE.

It is also found to resist shrinkage even when repeatedly exposed to high temperatures and pressures (e.g. steam autoclaving). Typically, the % area shrinkage is less than 5%, generally less than 3% and typically less than 2% and may be irrespective of the number of sterilisation cycles. Multiple-layer composite materials tend to show lower shrinkage e.g.

less than 1%.

Although the invention is primarily concerned with a sterilisable vent formed of a porous composite material which comprises two layers, viz; an expanded PTFE membrane and a layer of non-expanded (e.g.

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 membrane and non-expanded porous PTFE. Such multiple layer structures are particularly useful for building up thicker materials.

Generally speaking, the layers may be bonded together in any suitable manner known in the art, such as by the use of adhesives, by stitching etc. Where adhesives are used, the pattern of adhesive should preferably be a discontinuous pattern, such as a pattern of dots or lines, so as not to impede flow through the porous composite material. However, certain techniques of bonding the layers together, such as pressure bonding, are unsuitable, since the application of a pressure which is sufficient to lead to bonding may lead to crushing and distortion of the expanded PTFE membrane. Moreover, the use of an intervening adhesive or heat-bonding interlayer, 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.

In a particularly preferred embodiment 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. The layer of sintered nonexpanded porous PTFE may be formed in situ on the expanded PTFE membrane. 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. 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. When the granular-type PTFE layer is sintered at elevated temperatures, the layer of sintered porous PTFE becomes securely attached to the expanded PTFE membrane. Bonding occurs at atmospheric pressure without the application of any elevated pressures which might otherwise lead to crushing of the expanded PTFE membrane. The application of the layer of sintered porous PTFE by a liquid application technique does not appear to substantially effect the porosity of the expanded PTFE membrane. It must therefore be assumed that no interfacial barrier is created between the sintered porous PTFE layer and the expanded PTFE membrane, nor are the pores of the expanded PTFE membrane blocked by the application of the sintered porous PTFE layer.

Thus, the present invention advantageously allows the production of sterilisable vent from a porous composite material which is composed substantially 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 expanded PTFE membranes, 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 membrane (for example, by spraying and baking).

This fabrication technique is essentially brought about by the different methods of preparation of the expanded PTFE membrane and the preparation of the sintered porous PTFE layer. The former is generally produced by extrusion and stretching of a film; whereas the latter is preferably produced from a coating of a liquid dispersion.

The thickness of the porous composite material is generally in the range 50 to 2000 microns, particularly 150 to 1000 microns. The expanded PTFE membrane may have a thickness of less than 50 microns (for example, down to 5 microns), but is typically 10 to 500 microns, particularly 20-150 microns. The layer of non-expanded porous PTFE usually has a thickness up to and above 2000 microns, especially in the range 25-1500 microns, particularly 50-1000 microns, especially 70-750 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 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 (e.g. 0.1 to 1.0 microns) as measured by the bubble point method described herein.

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 mechanical properties. Alternatively, the layer of non-expanded porous PTFE may applied onto the fabric alone.

The non-expanded 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. On the other hand, the sintered non-expanded 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 olefines such as ethylene and propylene; halogenated olefines such as hexafluoropropylene (HFP), vinylidene fluoride and chlorofluoroethylene; or perfluoroalkyl vinyl ethers such as perfluoropropyl vinyl ether (PPVE).

The sintered non-expanded porous PTFE may be produced from a dispersion of granular-type PTFE particles in a liquid. The granular-type PTFE used in this preparation may be unsintered or may have been pre-sintered. The sintering process modifies the characteristics of the granular-type PTFE material.

One particular embodiment of the present invention employs unsintered PTFE material; however mixtures of sintered and unsintered material may also be used in other embodiments. Teflon granular-type resin grades 7A (unsintered) and 9B (sintered) are available from DuPont Speciality Polymers Division, Wilmington, USA.

Generally speaking, the sintered non-expanded porous PTFE may be produced from 0-100% unsintered PTFE (e.g.

grade 7A) and conversely 100-0% sintered PTFE (e.g.

grade 9B). Where the sintered porous PTFE is formed from a mixture of sintered and unsintered granulartype PTFE particles, it is preferred that the unsintered PTFE predominate since this leads to a material having good strength. The inclusion of sintered 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, for example 15 to 75 microns.

The unsintered granular-type PTFE will ordinarily have a particle size of between 1 and 300 microns, particularly 20 and 150 micron (mean size of about 35 micron). However, milled material of particle size 15 to 25 microns may be used to provide a particularly smooth surface. One commercial grade unsintered granular-type resin is available from the DuPont company as Teflon 7A 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. 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.

The granular-type PTFE particles (whether sintered or unsintered particles, or a mixture of both) used to produce the sintered non-expanded 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, silica 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 non-expanded porous PTFE is formed of a mixture of sintered 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 sintered particles to provide increased inter-particle connection strengths. Non-expanded PTFE formed solely of hard sintered granular-type PTFE particles tends to have relatively poor strength, and poor adhesion to the expanded PTFE membrane. Adhesion to the expanded PTFE membrane is increased by increasing the proportion of unsintered granular resin, by including a modifier in the granular resin, or by including any of the softer materials.

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 membrane 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).

In fact, the properties of the expanded PTFE membrane and the sintered porous PTFE layer differ markedly as regards porosity and pore size. Generally speaking, the expanded PTFE has a relatively high porosity and smaller pore size; whereas the sintered porous PTFE has a relatively low porosity and a larger pore size. Typical values of materials for use in the present invention are as follows.

Porosity pore size Bubble point (microns) (microns (pounds/in2) expanded PTFE 50-98% 0.02-10* 40-0.4 sintered porous PTFE 30-80% 0.5-20** 5-0.1 (eg.30-70%) (eg.2-6) (eg.1-0.6) * maximum pore size as determined by Bubble Point method D1.

** mean pore size as determined by Coulter Porometer method D2.

As mentioned above, the layer of sintered porous PTFE is generally formed by coating the expanded PTFE membrane 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 membrane. However, in a preferred embodiment, the liquid PTFE dispersion is applied onto the expanded PTFE membrane by spraying.

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

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.

At these high temperatures, the expanded PTFE membrane is not dimensionally stable and tends to stretch if held under an applied load, or to shrink if there is no load. Therefore, the expanded PTFE membrane 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 side elevation of a first sterilisation apparatus; Figure 2 is a plan view of a rectangular sterilisable vent for use in the sterilisation apparatus; Figure 3 is a plan view of a second type of sterilisation apparatus; Figure 4 is a view of a circular sterilisable vent for use in the second sterilisation apparatus; and Figure 5 is a cross-section to enlarged scale of the porous composite material from which the sterilisable vent is formed.

Figure 1 shows a first type of sterilisation apparatus having an essentially rectangular housing 2 comprising a base body section 4 and a sliding lid 6.

The lid slidingly engages the upper end of the base section by means of cooperating structures on the base and lid (not shown) so as to seal the housing, once an item to be sterilised has been placed therein. A side wall 8 (and the corresponding sidewall on the opposite side) of the housing is provided with a series of apertures 10 which allow entry and exit of sterilisation medium to the interior of the housing.

Each aperture is provided with a lattice 12 which helps support the sterilisable vent to be mentioned.

The housing and lattice are generally integrally moulded from a transparent plastics material which is resistant to sterilisation media; as is the lid 6.

A sterilisable vent 14 formed of porous composite PTFE material described herein of rectangular shape is held in a slot (not shown) on the inside of wall 8 (and one in the corresponding opposite wall) such as to extend across and close the apertures in the wall. The sterilisable vent is, of course, permeable to sterilisation media.

In use, the sterilisation apparatus is initially provided with a reusable sterilisable vent fitted across each wall aperture. An item to be sterilised is placed into the housing and the lid slid on to close the apparatus. The apparatus is then contacted with gas or liquid sterilisation medium e.g. steam, ethylene oxide, caustic soda, hydrogen peroxide, hypochlorous acid, hypochloric acid etc. Typically, the apparatus is placed in an autoclave and steam sterilised for 1 to 10 minutes at 110-1500C at 1.5 to 3 bar pressure. After sterilisation is complete, the apparatus is removed from the autoclave and put aside until the item is to be used. The sterilisable vent helps prevent entry of non-sterile agents e.g.

viruses, bacteria, dust, antigenic proteins etc. and keeps the item sterile until use.

Figure 3 is a plan view of a second alternative form of sterilisation apparatus comprising a housing 18 of rectangular shape having a lid 20 which includes a circular insert 22 which sealingly engages the lid by mutually engageable bayonet detentes (not shown) on the lid and insert which lock the insert onto the lid by a turning motion.

The insert includes ribs 24 which help support a circular sterilisable vent 26.

The lid has a circular aperture 28 which is closed by the circular insert and is also provided with a grid (not shown) for supporting the sterilisable vent from below. The lid is a push fit onto the housing.

Figure 4 is a plan view of the circular sterilisable vent disc itself.

In use, the sterilisable vent disc is clamped between the grid on the lid and the circular insert, so as to close the circular aperture on the lid yet allow entry and exit of sterilisation media to the interior of the housing. An item to be sterilised is placed in the housing, and the lid including the sterilisable vent fitted thereon. Sterilisation is then carried out as described above.

Figure 5 shows a cross-section of a typical porous permeable composite from which the sterilisable vent is formed. It comprises an expanded PTFE membrane 30 having formed thereon a sintered porous PTFE network layer 32 of granular type PTFE particles.

Layer 30 is composed of an expanded polytetrafluoroethylene (PTFE) membrane, a material which is available in a variety of forms from W.L.

Gore & Associates Inc. of Elkton, MD, USA, under the trademark Gore-Tex. The expanded PTFE membrane 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.

In an alternative embodiment, the layer 30 is composed of expanded PTFE membrane in the form of twisted tape, which has been woven into a fabric. One such fabric is available under the Rastex trademark from W.L. Gore & Associates Inc.

The second layer 32 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 in relation to the expanded PTFE membrane (for a given porosity).

Generally, the sintered porous PTFE material is produced in greater thicknesses than the expanded PTFE. 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 membrane 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 membrane, 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 membrane tends to shrink (or to 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 membrane 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 membrane 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.

Another benefit of forming the sintered porous PTFE layer directly on the expanded PTFE membrane, is that restrictions on the properties of the sintered porous PTFE material which may arise from skinning of the outer surface thereof are mitigated, since there is effectively no free surface at the interface between the membrane and the sintered porous PTFE layer. The ability of liquids or gases to flow across the interface between the expanded PTFE membrane and the sintered porous PTFE layer is good.

However, in an alternative fabrication method, it is possible to preform the expanded PTFE membrane and the layer of sintered porous PTFE in separate fabrication steps, and thereafter to laminate the one to the other by conventional lamination technology.

Such lamination technology includes the use of continuous or discontinuous intermediate adhesive layers using a variety of adhesives known for the purpose. In the case of an adhesive which is impermeable to liquid, the adhesive layer would normally be in the form of a discontinuous pattern, such as a pattern of dots or lines. A disadvantage of the use of adhesives is that generally speaking their properties, such as high temperature resistance and chemical resistance are generally inferior to those of either the expanded PTFE membrane or the sintered PTFE layer, so that the overall properties of the porous composite material are correspondingly degraded.

EXAMPLE 1 2-Layer Composite (7A19B:ePTFE) 500g of Du Pont granular PTFE resin - 7A, with an average particle size of 35 microns, 80g of Zonyl FSN100 surfactant solution, 1.3kgs of distilled water are blended together for 60 seconds using a Waring blender to form a suspension. 500g of Du Pont granular PTFE resin-9B which has previously been milled to an average particle size of 50 microns, was added to the suspension and reblended for a further 60 seconds.

The resulting aqueous suspension was suitable for spray application. The FSN-100 surfactant is a nonionic perfluoroalkyl 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).

An expanded PTFE membrane having a nodes and fibril morphology of nominal pore size 0.2 microns with an approximate thickness of 50 microns was held under tension in an aluminium frame (20inch2 outside, 16 inch2 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 85"C for 1 hour. The temperature was then increased over several hours to 350"C and held at this temperature for 30 minutes to complete the baking process. After cooling, the toggle clamps are released and the porous composite material removed.

The thickness of the composite was typically 400 microns and therefore by subtraction, the layer of sintered porous PTFE sheet on the expanded membrane was 350 microns.

A 109mm diameter disc of the composite material was sterilised in a steam autoclave for 80 separate sterilisation cycles. Each cycle being 5 minutes at 134"C (2 Bar pressure) and then cooled. After the sterilisation cycles are completed, the % area shrinkage was calculated at 3.1% with a mean Gurley air flow value of 13.3 secs/100cm3.

EXAMPLE 2 2-Layer Composite (7A:ePTFE) 500g of Du Pont granular PTFE resin - 7A which had been milled to an average particle size of 20 microns, 40g of Zonyl FSN-100 surfactant solution, 12.5g of a carboxylmethyl cellulose solution (1% w/w in distilled water) and 800g of distilled water are blended together for 60 seconds using a Waring blender to form a suspension. The FSN-100 surfactant is a non-ionic perfluoroalkyl 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). The carboxymethyl cellulose solution acts as a thickening agent. The resulting aqueous suspension was suitable for spray application.

An expanded PTFE membrane having a nodes and fibril morphology of nominal pore size 0.3 microns with an approximate thickness of 30-40 microns was held under tension in an aluminium frame (20inch2 outside, 16 inch2 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 105"C for 2 hours. The temperature was then increased over several hours to 3500C and held at this temperature for 30 minutes to complete the baking process. After cooling, the toggle clamps are released and the porous composite material removed.

The thickness of the composite was typically 110 microns and therefore by subtraction, the layer of sintered porous PTFE sheet on the expanded membrane was 70-80 microns.

109mm diameter discs of the composite material were sterilised in a steam autoclave. Each autoclave cycle being 10 minutes at 1340C (2 Bar pressure) and then cooled. The samples were sterilised for 6 separate cycles and the % area shrinkage calculated after each cycle. The area shrinkage was approximately 1%, irrespective of the number of cycles.

EXAMPLE 3 3-Layer Composite (7A/9B:ePTFE:7A/9B) 10keg of Du Pont granular PTFE resin - 7A with an average particle size of 35 microns, 500g of Zonyl FSN-100 surfactant solution, 500g of Pluronic L121 surfactant and 26kgs of distilled water are blended together using a Silverson mixing head to form a suspension. 10keg of Du Pont granular PTFE resin - 9B, which has previously been milled to. an average particle size of 50 microns was added to the suspension and the suspension was reblended. 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 FSN-100, three parts distilled water and three parts isopropyl alcohol (by weight). Pluronic L121 is a trade name for a polyoxyethylene/polyoxypropylene block copolymer surfactant.

An expanded PTFE membrane from W.L. Gore & Associates Inc. with a thickness of 50 microns and nominal pore size of 0.2 microns, forms the composite substrate. The membrane is firmly held in a stainless steel pin frame of dimensions 3ft x 9ft which provides a suitable base for spraying and subsequent baking.

The aqueous suspension was sprayed onto both sides of the membrane using a Nordson Air-Assisted spraying system. After spraying, the coated membrane was dried in an oven at 800C for 60 minutes. The temperature was then increased to 3500C and held at this temperature for 60 minutes to complete the baking process. After allowing to cool, the sheet of composite material can be removed from the pinframe.

The thickness of the composite was typically 800 microns and therefore by subtraction, the layer of sintered porous PTFE sheet on the expanded membrane was 750 microns.

A 109mm diameter disc of the composite material was sterilised in a steam autoclave for a total of 55 hours at 134 C (2 Bar pressure) and then cooled.

After sterilisation, the mean % area shrinkage was calculated at 0.2% with a mean Gurley air flow value of 17.5 secs/100cm3.

TESTING AND PREPARATIVE METHODOLOGIES (A) Preparation of PTFE grade 7A and 9B TEFLON (trademark) granular-type PTFE fluorocarbon resin grades 7A and 9B are available from DuPont Speciality Polymers Division, Wilmington U.S.A.

Grade 9B is a premelted sintered resin. The manufacturers product specification indicates an average density of 2.16, and an average particle size of 35 microns (grade 7A) and 500 microns (grade 9B prior to milling). PTFE grade 7A was unsintered and was used as supplied.

Prior to use, the PTFE grade 9B was milled to a volume average particle size of about 40-50 microns by grinding an aqueous slurry thereof between grinding stones at room temperature as follows.

The PTFE grade 9B was mixed with water to form a slurry, and the slurry fed between closely spaced grinding surfaces of a grinding mill as disclosed in US-A-4841623, to crush and shear the pieces of PTFE into particles. The ground slurry was then filtered or centrifuged to separate the granular PTFE particles from water, and the separated finely ground particles were oven dried at from 1250C - 1500C.

(B1) 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 Particle size of ground PTFE grade 9B was determined as follows: using a magnetic stirrer and ultrasonic agitation, 2.5 grams of milled PTFE powder were dispersed in 60 ml isopropyl alcohol.

(Ultrasonic Probe Model W-385, manufactured by Heat Systems-Ultrasonics, Inc.).

Aliquots of 4-6ml of the dispersed particles were added to approximately 250ml of circulating isopropyl alcohol in a Leeds & Northrup Microtrac X100 Particle Size Analyzer. Each analysis consisted of three 30second runs at a sample circulation rate of 4 litres/minute during which light scattering by the dispersed particles is automatically measured and the particle size distribution automatically calculated from the measurements.

(D1) Pore Size Measurement (bubble points 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 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, 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 (D1).

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) AIR FLOW (Gurley numbers) 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 100cm of air to pass through one square inch of the sample under a pressure of 4.88" of water head pressure. This measurement can be converted into metric permeability units (cm3cm/sec. cm2cm.Hg) by the following formula: thickness of sample x 0.0432 /Gurley number. The lower the Gurley number, the higher the air permeability.

Claims

1. A sterilisable vent for use in a sterilisation apparatus and formed of a porous composite material which comprises; - expanded polytetrafluoroethylene (PTFE) membrane; and - a layer of non-expanded porous polytetrafluoroethylene (PTFE) attached to the expanded PTFE membrane.

2. A vent according to claim 1 wherein the layer of non-expanded porous PTFE is a sintered PTFE network.

3. A vent according to claim 1 or 2 wherein the layer of non-expanded porous PTFE is a sintered porous PTFE formed from PTFE particles, comprising granulartype PTFE1 fused together such as to form a porous integral network of interconnected particles.

4. A vent according to claim 2 or 3 wherein the layer of sintered porous PTFE is formed on the expanded PTFE membrane and integrally attached hereto.

5. A vent according to any preceding claim wherein the expanded PTFE membrane has a thickness of 10 to 500 microns; and the layer of non-expanded porous PTFE has a thickness of 25 to 1500 microns.

6. A vent according to any preceding claim wherein the porosity of the expanded PTFE membrane is in the range 50 to 95%.

7. A vent according to any preceding claim wherein the expanded PTFE membrane has been formed into fibres and woven into a fabric.

8. A vent according to any preceding claim which comprises a layer of non-expanded porous PTFE formed on both sides of the expanded PTFE membrane.

9. A vent according to any of claims 1 to 7 which comprises a layer of sintered porous PTFE formed between two expanded PTFE membranes and attached thereto.

10. A vent according to any preceding claim in the form of sheet material of rectangular or circular shape.

11. A vent according to any preceding claim having a percent area shrinkage after repeat sterilisation cycles of less than 5%.

12. A sterilisation apparatus which comprises; - a housing for containing an item to be sterilised, the housing having an aperture for allowing entry of sterilising medium; and - a permeable sterilisable vent according to any preceding claim, the vent being disposed across the aperture so as to close the aperture but to allow passage therethrough of said sterilising medium.

13. A method of sterilising an item, which comprises; - placing an item to be sterilised in a housing having an aperture for allowing entry of sterilising medium, a permeable sterilisable vent according to any of claims 1-11 being disposed across the aperture so as to close the aperture; - passing said sterilising medium into the housing through the permeable vent; and - maintaining the sterilising medium within the housing for a time sufficient to sterilise said item.

14. A method according to claim 13 wherein the sterilising medium comprises steam.

15. A method according to either of claims 13 or 14 wherein the sterilising medium is at a temperature in excess of 1100C.

16. A method according to either of claims 13 or 14 wherein the sterilising medium is at a temperature of 110-1500C.

17. A method according to either of claims 13 or 14 wherein the sterilising medium is at a temperature in excess of 1500C.

18. A method according to any of claims 13-17 wherein the sterilising medium is at a pressure of up to 3 bar.