GB2268496A

GB2268496A - Improved thermal capacity reticulated polymer foams

Info

Publication number: GB2268496A
Application number: GB9311050A
Authority: GB
Inventors: David Alan Ingles
Original assignee: Intersurgical Ltd
Current assignee: Intersurgical Ltd
Priority date: 1992-07-08
Filing date: 1993-05-28
Publication date: 1994-01-12
Anticipated expiration: 2013-05-28
Also published as: AU4511393A; EP0602222A1; WO1994001489A1; GB2268496B; GB9311050D0

Abstract

Reticulated polymer foams permitting a through and return flow of gasses have many 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, such as for example fine aluminium powder, in the body of the foam in a way that ensures that it is substantially if not wholly encapsulated by the polymer fibrules comprising the cell walls of the foam. An electrically and thermally non-conductive reticulated polymer foam composition permitting a through and return flow of gases is characterised by comprising an insert powdered filler, eg a metal, having substantially higher thermal capacity than that of the polymer.

Description

IMPROVED THERMAL CaPACITY RETICULATED POLYMER FOAMS This invention relates to reticulated polymer foams having improved thermal capacity in general, and to such foams when used in heat and noisture exchange devices (HME's) in particular.

In the past it has been suggested that reticulated polymer foams, such as polyurethane, could be used in EME's for use in patient ventilation or anaesthesia. In such an HME the gases that a patient exhales pass through a polymer foam element where it is desired that heat and moisture is absorbed, and subsequent inhalation of netsred 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 provide 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 function effectively as HME's. To avoid the use of treated polymer foams a way of using a water-activated hydrcphilin polyurethane foam in an untreated state has been disclosed in our co-pending UR Patent Application No. 9213104.4.

Both treated and untreated polyurethane foams have low thermal mass and to improve hydrolic properties when used in HME's would benefit fran increased thermal capacity.

It is an object of the invention to provide a reticulated polymer foam suitable for use in HME or filter application with improved thermal capacity.

According to a first aspect of the invention an electrically and thermally non-conductive reticulated polymer foam composite permitting 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 composite incorporates a powdered notallic filler having substantially higher thermal capacity than that of the polymer.

The polymer may be convention31 polyurethane and the composite can be treated so as to be lightly coated with hygroscopic salts. Alternatively, the polymer may be hydrsphilic polyurethane obtained from a water activated prepolymer. Preferably, the filler is fully encapsulated in the cell walls within the body of the foam.

Suitably, the filler is an aluminium powder having a particle size of 100 microns or less, preferably 20 microns or less. The proportion of the aluminium 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 composite in accordance with the aforementioned aspects of the invention requires that the filler is first mixed in the polymer to provide homogeneous dispersion therein; and that a surfactant solution is subsequently added to the mixture to effect formation of said polymer foam.

Alternatively, a method of manufacturing a polymer foam composite 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 coosite ccaprising conventional polyurethane foam maybe washed in a 30% calcium chloride solution, wrung out and subsequently tumble 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 anther aspect of the invention there is provided a reticulated polyurethane polymer foam composite structure permitting a through and return flaw 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 enxeç > silatedby said polyurethane in the body of the structure.

Preferred embodiments of the invention will now be described by way of nonl imifing example only and with reference to the accompanying 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 hygrnsoopic salts in accordance with a second embcxUbmentofofthe 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 nicrcgraph shaving at greater magnification part of a side face of the fibrule in Figure 3; and Figure 5 is an illustration of a inicrograph showing at even greater magnification a part of the end face of the fibrule in Figure 3.

In a first embodiment of the invention a method of manufacturing a reticulated polymer foam, the structure of which is shown 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 Mark 'HYPOL', with in the region of 30% its own weight of aluminium power. Because of the viscosity of the prepolymer such mixing provides for homogenous dispersion of the aluminium powder 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 'PLURIOL PE 6800', in solution with water is added to the prepolymer-aluminium 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 prcprietory starting materials is merely one of convenience and it will be understood that any combination of prepolymer and surfactant which results in a the desired reticulated polymer foam is included in the ambit of thin invention. Also, in the aforemantioned method the aluminium powder 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 powder 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 coxwentlonal prepolymers and surfactant solutions. Once formed the foam 4 is similar in struot~re to the hydicEhilic 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 on the exposed surface 6 of the cells 7 of the foam 4.

The microscopic structure of filled reticulated foams is shown by the illustrations of micrographs in Figures 3 to 5. Cell walls of the foams comprise interconnecting elongate fibrules, a broken one 8 of which is illustrated in part in Figure 3.

From Figure 3 the fibrules of the cell walls of the foam are observed to have a thickness in the order of 200 microns substantially greater than the diameter of the aluminium 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 fran Figure 4 to have an ur##roken surface 9 exposed to the open cells of the foam which is free fran the eocaçsulated 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 mare detail in Figure 5, illustrates the internal structure of the cell walls 2. Aluminium particles 3 are seen to be fully encapsulated within the polymer of the foam. A large particle 11 of some 20 microns diameter is shown 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, inasituch as the polymer will tend to surround, as opposed to break away from, the surface of small particles. Clearly the use of fine aluminium powder is preferred in the #(bodients of the invention although it will be readily understood that other powder or particulate materials may be used as filler materials when suitable and that the invention is not limited to the use of aluminium powder per se.

Self evidently materials such as aluminium, or indeed other metals or metalloids, have a greater thermal capacity than polyurethane which is known 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 contamination, should it occur, is thought to be minimal and will not detract from 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 hydixphilic 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 hygroscspic 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 asseibly. It should now be possible to reduce the effective surface area of the element and to use an HME assembly of any chosen shape.

Improving the thermal capacity of a given hydrophilic: foam HME element, in a way suggested by the first embodiment of the invention, still leaves its IWdrophc propties unaltered. However, as such an element will now operate over a greater temperature range, classical thermodynamic theory indicates that its moisture exchange characteristics will also improve. It is believed that this characteristic enables the more effective use of bydkqphiliz 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 copending UK Patent Application No. 9213104.4.

As similarly described for hydrophilic foams, moisture exchange characteristics of a given guantity of hygroscopic salts will likewise improve due to the greater tenperature 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 remains unaltered. Self evidently if such an iir#ra#ement in moisture exchange is not required, it should be possible to reduce the quantity of hygroscopic salt used (this being in some circumstances an undesirable surface contaminant of the foam) and still provide an effective HME element, albeit made of a filled foam.

As previously discussed a suitable conventional foam EME element contains 30% by weight aluminium power and is treated by immersion in a 30% calcium chloride solution. This will result in a light deposit of calcium chloride crystals on the exposed surfaces of the fibrules. Typirallyt in such treated foams a surface area equivalent to that shown in Figure 4 would have 20 to 30 crystals deposited thereon. It is believed that calcium chloride crystals are in the form of rhombic prisms in the order of 1 inizron in length. It will be appreciated that other suitable hygroscopic salts may be used in the place of calcium chloride.

Although manufacturing of HRE elements is a preferred use of reticulated polymer foam resulting from this invention it will be appreciated that this is not its only practical application. There may be other applications, such as in filter assablies, where the use of a reticulated foam of increased thermal capacity is useful.

Claims

CLAIMS:

1. An electrically and thermally non-contctive reticulated polymer foam composite permitting a through and return flow of gasses for use in heat and moisture exchange or filter applications; characterised in that the polymer of the composite incorporates a powdered metallic filler having substantially higher thermal capacity than that of the polymer.

2. A composite in accordance with claim 1, in which the polymer is conventional polyurethane and the composite is treated so as to be lightly coated with hygroscopic salts.

3. A composite in accordance with claim 1, in which the polymer is hydrophilic polyanane obtained from a water activated prepolymer.

4. A composite in accordance with any one of the preceding claims, in the thefiller is fully encapsulated in the cell walls within the body of the foam.

5. A composite in accordance with any one of the preceding claims, in which the filler is an aluminium powder having a particle size of 100 microns or less.

6. A composite in accordance with claim 5, in which the particle size is in the order of 20 minos or less.

7. A carposite in accordance with claim 5 or claim 6, in which the proption of the aluminium powder to the polymer is less than 40% by weight.

8. A composite in accordance with claim 7, in which the prcportion is 30%.

9. A heat and moisture exchange device for use in patient ventilation or anaesthesia comprising a heat and moisture exchange element in the form of a a reticulated polymer foam composite in accordance with any one of the preceding claims.

10. A method of manufacturing a polymer foam composite in accordance with claim 2 or claim 3, in which the filler is first mixed in the prepolymer to provide homogenous dispersion therein and in which a surfactant solution is subsegpently added to the mixture to effect formation of said polymer foam.

11. A method of manufacturing a polymer foam composite in accordance with claim 2 or claim 3, in which the filler is first mixed into a surfactant solution together with a suspension or dispersion agent to effect hcnogeneous dispersion therein; and in which the prepolymer is subsequently added to the mixture to effect fnntion of said polymer foam.

12. A method in accordance with claim 10 or claim 11 when dependent on claim 2, in which the formed conventional foam composite is washed in a 30% calcium chloride solution, wrung out and subsequently tumble dried.

13. A method of manufacturing a polymer foam composite in accordance with any one of claims 10 to 12, in which the filler is an aluminium power having a particle size of the order of 20 microns or less.

14. A method of manufacturing a polymer foam composite in accordance with claim 13, in which the proportion of the aluminium powder to the polymer is 30% by weight.

15. A reticulated polyurethane polymer foam canposite structure permitting a through and return flaw of gasses for use as a heat and moisture exchange or filter element, in which an inert filler material in the form of fine power or particulate matter having significantly higher thermal capacity than that of the polynreane foam is substantially fully encapsulated by said polyurethane in the body of the structure.

16. A method of manufacturing a reticulated conventional or hydrophilic polymer foam coooosite substantially as hereinbefore described.

17. A reticulated polymer foam composite substantially as hereinbefore described with reference to and as illustrated in Figure 1 and Figures 3 to 5, or Figure 2 of the drawings.