IMPROVED THERMAL CAPACITY RETICULATED POLYMER FOAMS

Abstract: Reticulated polymer foams permitting a through and return flow of gasses have any uses, such as in conventional filters and 'heat and moisture exchange' devices. The thermal capacity of these foams is relatively low in comparison to other materials such as metals. In order to increase the efficiency of foams when used in HME's this invention provides for the incorporation of a filler material (2), such as for example fine aluminium powder, in the body of the foam (1) in a way that insures that is is substantially if not wholly encapsulated by the polymer fibrules comprising the cell walls of the foam.

Description: IMPROVED THERMRL CAEACITY RETIOJIA ED POLYMER PQftMS This invention relates to reticulated polyπ r foams having iπprσved thermal capacity in general, and to such foams when used in heat an moisture exchange devices (HME's) in particular * In the past it has been s"c«ested that reticulated polymer foams, such as polyurethane, could be used 3's for use in patient ventilation or anaesthesia. In such an EME the ._ as that a patient exhales pass through a polymer foam element where it is desired that heat and πoisture is absorbed, and subsequent inhalation of πetered gasses through that element ideally liberates all that heat and moisture to the inhaled gas mixture. However, the use of such foams in their virgin state does not p-rovd-de the required degree of heat and water absorption. Also because polymers are good insulators, what heat they do absorb they do not readily emit. Previously it has been necessary to treat such foams with a hygroscopic salt, such as calcium chloride, in order that they funcr >n effectively as HME's. To avoid the use of treated polymer foams a -. of using a water-activated hydrophilic polyurethane foam in an untreated state has been disclosed in our co-pending UK Patent Application No. 92131044. Both treated and untreated polyurethane foams have low thermal mass and to inprove hydrophilic properties when used in HME's would benefit from increased thermal capacity. It is an object of the invention to provide a reticulated polymer foam suitable for use in EME or filter application with iπproved thermal capacity. -Recording to a first aspect of the invention an electrically and thermally non-conductive reticulated polymer foam composite pe-αnitting a through and return flow of gasses for use in heat and moisture exchange or filter applications is characterised in that the polymer of the coπposite incorporates a powdered metallic filler having substantially higher thermal capacity than that of the polymer. The polymer nay be conventional polyurethane and the coπposite can be treated so as to be lightly coated with hygroscopic salts. ternatively, the polymer may be hydrophilic polyurethane obtained from a water activated prepoly-nsr. Preferably, the filler is fully encapsulated in the cell walls within the body of the foam. Suitably, the filler is an aluminium pcwer having a particle size of 100 microns or less, preferably 20 microns or less. The proportion of the alum__nium powder to the polymer is typically less than 40% by weight, preferably 30%. ~ The composite of the invention may be used as a heat and moisture exchange device for use in patient ventilation or anaesthesia. According to a further aspect of the invention a method of manufacturing a polymer foam cαrposite in accordance with the aforementioned aspects of the invention requires that the filler is first mixed in the prepolymer to provide homogenous dispersion therein; and that a surfactant solution is subsequently added to the mixture to effect formation of said polymer foam. Alternatively, a method of iranufa±uring a polymer foam coπposite in accordance with the invention, may require that the filler is first mixed into a surfactant solution together with a suspension or dispersion agent to effect homogeneous dispersion therein and that the prepolymer is subsequently added to the mixture to effect formation of said polymer foam. A composite coπprising cαπventional polyurethane foam maybe washed in a 30% calcium chloride solution, wrung out and subsequently tuπble dried. Typically, in the aforementioned methods, the filler is an aluminium powder having a particle size in the order of 20 microns or less and the proportion of the aluminium powder to the polymer is 30% by weight. According to yet another aspect of the invention there is provided a reticulated polyurethane polymer foam composite structure permitting a through and return flow of gasses for use as a heat and moisture exchange or filter element, in which an inert filler material in the form of fine powder or particulate matter having significantly higher thermal capacity than that of the polyurethane foam is substantially fully encapsulated by said polyurethane in the body of the srtructure. Preferred eoibodiments of the invention will new be described by way of non--Li-miting exaπple only and with reference to the accαrpaπying drawings, in which:- Figure 1 is an illustration of a partial section through the body of a hydrophilic foam block in accordance with a first embodiment of the invention; Figure 2 is an illustration of a partial section through the body of a polyurethane foam block which has subsequently been treated with hygroscopic salts in accordance with a second eπtonriiment of the invention; Figure 3 is an .illustration of a micrograph from an electron microscope showing a broken fibrule of the cell wall of the first embodiment; Figure 4 is an illustration of a micrograph shewing at greater magnification part of a side face of the fibrule in Figure 3; and Figure 5 is an illustration of a micrograph showing at even greater πagnification a part of the end face of the fibrule in Figure 3. in a first eπtoodi eπt of the invention a method of manufacturing a reticulated polymer foam, the structure of which is shewn in Figures 1,3,4 and 5, comprises first mixing a quantity of water-activated toluene diisocyanate prepolymer which has a treacle-like viscosity, such as that sold under the Trade ark ΗΪPOL', with in the region of 30% its own weight of aluminium pcwder. Because of the viscosity of the prepolymer such mixing provides for homogenous dispersion of the aluminium pcwder in the prepolymer and by choosing powder having a particle size with a diameter of 20 microns or less then such powder tends to remain evenly dispersed. A surfactant, such a that sold under the Trade Mark 'PI-URIQ PE 6800', in solution with water is added to the p-repolymer-aliiminium mixture which results in the formation of a bulk volume of reticulated hydrophilic polyurethane foam 1 in which the aluminium particles 2 are contained within the cell walls 3 of the foam. The choice of the aforementioned proprietory -starting materials is merely one of convenience and it will be understood that any α-πbination of prepolymer and surfactant which results in a the desired reticulated polymer foam is included in the aπfcit of this invention. Also, in the aforementioned method the aluminium pcwder is first caused to be in suspension in the prepolymer. However it would also be possible if suitable dispersion or suspension agents were used, for for the pcwder be held instead in suspension in the surfactant solution. In a second embodiment of the invention a method of manufacturing a conventional polyurethane reticulated foam, the structure of which is illustrated in Figure 2, is similar to the aforesaid method but using conventional prepolymers and surfactant solutions. Once formed the foam 4 • 1 is similar in structure to the hydrophi1in foam 1 shown in Figure 1. A block of the foam 4 is soaked in a 30% calcium chloride solution, wrung out and subsequently tumble dried. This results in a light deposit of hygroscopic calcium chloride crystals 5 en the exposed surface 6 of the cells 7 of the foam 4. The microscopic structure of filled reticulated foams is shewn by the illustxatiαns of micrographs in Figures 3 to 5. Cell walls of the foams ccπprise -interconnecting elongate fibrules, a broken one 8 of which is illustrated in part in Figure 3. From Figure 3 the fi-brules of the cell wal s of the foam are observed to have a thickness in the order of 200 microns substantially greater than the diameter of the aluπ nium particles which are generally smaller than 20 microns across. Thus, it has been found in practice that in the body of the foam such particles are encapsulated, i.e. fully enclosed in the polyurethane of the cell walls. The fibrule 8 can be seen in detail from Figure 4 to have an unbroken surface 9 exposed to the open cells of the foam which is free frαn the encapsulated aluminium particles 3. This surface 9 has a pitted orange peel like appearance at high magnification. A broken end surface 10 of the fibrule 8, a part of which is shown in more detail in Figure 5, illustrates the internal structure of the cell walls 2. uminium particles 3 are seen to be fully encapsulated within the polymer of the foam. A large particle 11 of some 20 microns diameter is shewn in both Figures 4 and 5. It is believed that full encapsulation occurs because of surface tension in the polymer whilst it is still fluid, inasmuch as the polymer will tend to surround, as opposed to break away frαn, the surface of small particles. Clearly the use of fine aluminium pcwder is preferred in the e-nt-oriiiraπts of the invention although it will be readily understood t! ± other pcwder or particulate materials may be used as filler materials when suitable and that the invention is not limited to the use of aluminium pcwder per se. Self evidently materials such as aluminium, or indee other metals or metalloids, have a greater thermal capacity than polyurethane which is kncwn for its insulating properties. Thus, incorporation of such filler materials into the bulk of the foam provides for a composite structure having greater effective thermal capacity than the polyurethane foam alone would have. When such polyurethane foams are used as an element for a heat and moisture exchange device, they may first be formed as a block and subsequently sliced into sheet form (as fully described in the case of water-activated hydrophilic foams in our aforementioned co-pending Patent Application No. 9213104.4). It is possible that such slicing will expose filler material at the surface of the sheet, instead of it remaining encapsulated in the body of the sheet. Such surface cxπitaminatiαn, should it occur, is thought to be minimal and will not detract frαn the advantages of a reticulated foam sheet with an effective thermal capacity greater than that of the polymer foam alone. A particular advantage in the case of hydrophilic foams is that the effective thermal capacity of the foam has been increased without the need to contaminate the internal surfaces within the body of each sheet of foam with hygroscopic salts. Also, because of this increased thermal capacity, the HME element is no longer restricted to being of sheet form housed in a suitably aerodynamic HME assembly. It should now be possible to reduce the effective surface area of the HME element and to use an HME assembly of any chosen shape. Iπproving the thermal capacity of a given hydrophilic foam HME element, in a way suggested by the first eni-odimeπt of the invention, still leaves its hydrophilic properties unaltered. However, as such an ijrproved element will new operate over a greater teπperature range, classical the--αn---dynami-c theory indicates that its moisture exchange dharacteristics will also iπprove. It is believed that this characteristic enables the more effective use of hydrophilic foam HME elements in different shapes and configurations than has to date been practical. In particular the use of such foam should be possible in and HME assembly which does not possess the internal aerodynamic configuration of that previously disclosed in our co- pending UK Patent Application No. 9213104.4. As simi arly described for hydrophilic foams, moisture exchange characteristics of a given quantity of hygroscopic salts will likewise iπprove due to the greater teπperature range of operation facilitated by using suitable encapsulated fillers. As in the case of hydrophilic foam elements the hydrophilic or hygroscopic properties of a conventional treated polymer foam HME element rema ns unaltered. Self evidently if such an iπprσvement in moisture exchange is not required, it should be possible to reduce the quantity of hygroscopic salt used (this being in seme circumstances an undesirable surface contaminant of the foam) and st.il provide an effective HME element, albeit made of a filled foam. As previously discussed a suitable conventional foam HME element contains 30% by weight aluminium pcwder and is treated hy iπmersiαn in a 30% calcium chloride solution. This will result in a light deposit of calcium chloride crystals en the exposed surfaces of the fi-brules. Typically, in such treated foams a sur ace area equivalent to that shewn in Figure 4 would have 20 to 30 crystals deposited thereon. It is believed that calcium chloride crystals are in the form of rhαnbic prisms in the order of 1 micron in length. It will be appreciated that other suitable hygroscopic salts may be used in the place of calcium chlcri.de. Alt±.sigh manufacta-iring of HME elements is a preferred use of reticulated polymer foam resulting from this invention, it will be ap ciated that this is not its only practical application. T-here may be other applications, such as in filter assemblies, where the use of a reticulated foam of increased thermal capacity is useful.

Claims: CLAIMS: 1. An electrically and thermally non-conductive ret culated polymer foam ccπposite perπ-itting a through and return flow of gasses far use in heat and moisture exchange or filter applications; characterised in that the polymer (1) of the ccπposite incorpαrates a pcwdered metallic filler (2) having substantially higher thermal capacity than that of the polymer. 2. A coπposite in accordance with claim 1, (characterised in that the polymer (1) is conventional polyurethane and the coπposite is treated so as to be lightly coated with hygroscopic salts (5). 3. A ccπposite in accordance with claim 1, r-hararrt-i y cised in that the polymer is hydrophilic polyurethane obtained from a water activated prepolymer. 4. A composite in accordance with any one of the preceding claims, characterised in that the filler is fully encapsulated in the cell walls (3) within the body of the foam. 5. A coπposite in accordance with any one of the preceding claims, characterised in that the filler is an aluminium pcwder (2) having a particle size of 100 microns or less. 6. A ccπposite in accordance with claim 5, characterised in that the particle size is in the order of 20 microns or less. 7. A ccπposite in accordance with claim 5 or claim 6, characterised in that the proportion of the aluminium pcwder (2) to the polymer (1) is less than 40% by weight. 8. A ccπposite in accordance with claim 7, characterised in that the prcpαrtiαn is 30%. 9. A method of manufacturing a polymer foam ccπposite in accordance with claim 2 or claim 3, characterised in that the filler (2) is first mixed in the prepolymer to provide homogenous dispersion therein and in which a surfactant solution is subsequently added to the mixture to effect formation of said polymer foam (1). 10. A method of manufacturing a polymer foam ccπposite in accordance with claim 2 or claim 3, characterised in that the filler is first mixed into a surfactant solution together with a suspension or dispersion agent to effect homogeneous dispersion therein; and in which the prepolymer is subsequently added to the mixture to effect formation of said polymer foam. 11. A method in accordance with claim 9 or claim 10 when dependent on claim 2, characterised in that the formed conventional foam coπposite (4) is washed in a 30% calcium chloride solution, wrung out and subsequently tumble dried. 12. A method of maπufact-uring a polymer foam ccπposite in accordance with any one of claims 9 to 11, characterised in that the filler is an aluminium pcwder having a particle size of the order of 20 microns or less. i 13. A method of manufacturing a polymer foam ccπposite in accordance with claim 12, characterised in that the proportion of the aluminium pcwder (2) to the polymer (1) is 30% by weight. 14. A reticulated polyurethane polymer foam ccπposite structure (1) permitting a through and return flew of gasses for use as a heat and moisture exchange or filter element, clv-racterised in that an inert filler material (2) in the form of fine powder or particulate matter having significantly higher thermal capacity than that of the polyurethane foam is substanti lly fully encapsulated by said polyurethane in the body of the structure.

Inventors: INGLES DAVID ALAN, INGLES, DAVID, ALAN
Applicants: INGLES DAVID ALAN, INTERSURGICAL LTD, INTERSURGICAL LIMITED
Application year: 1993
Main classification: C08J9
IPC codes: A61M16, C08J9, C08K3