EP0602222A1

EP0602222A1 - Improved thermal capacity reticulated polymer foams

Info

Publication number: EP0602222A1
Application number: EP93914909A
Authority: EP
Inventors: David Alan Ingles
Original assignee: Intersurgical Ltd
Current assignee: Intersurgical Ltd
Priority date: 1992-07-08
Filing date: 1993-07-08
Publication date: 1994-06-22
Also published as: AU4511393A; GB9311050D0; GB2268496B; WO1994001489A1; GB2268496A

Abstract

L'invention se rapporte à des mousses de polymère réticulé permettant un écoulement aller-et-retour de gaz et qui s'utilisent dans de nombreuses applications comme dans des filtres classiques et des dispositifs "d'échange de chaleur et d'humidité". La capacité thermique de ces mousses est relativement faible en comparaison avec d'autres matériaux tels que des métaux. Afin d'accroître l'efficacité des mousses lorsqu'elles sont utilisées dans des dispositifs "d'échange de chaleur et d'humidité", cette invention permet d'incorporer un matériau de remplissage (2), tel que, par exemple, de la poudre d'aluminium fine, dans le corps de la mousse (1) de manière à s'assurer qu'il soit pratiquement sinon totalement encapsulé par les fibrilles du polymère composant les parois alvéolaires de la mousse.The invention relates to cross-linked polymer foams which permit the back and forth flow of gases and which are used in numerous applications such as in conventional filters and "heat and moisture exchange" devices. The heat capacity of these foams is relatively low compared to other materials such as metals. In order to increase the effectiveness of foams when used in "heat and moisture exchange" devices, this invention allows the incorporation of a filling material (2), such as, for example, the fine aluminum powder, in the body of the foam (1) so as to ensure that it is practically if not completely encapsulated by the fibrils of the polymer composing the alveolar walls of the foam.

Description

IMPROVED THERMRL 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 an moisture exchange devices (HME's) in particular.

In the past it has been suggested that reticulated polymer foams, such as polyurethane, could be used in HME's for use in patient ventilation or anaesthesia. In such an EME 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 metered 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 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 improve 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 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 metallic filler having substantially higher thermal capacity than that of the polymer.

The polymer nay be conventional polyurethane and the composite can be treated so as to be lightly coated with hygroscopic salts. Alternatively, the polymer may be hydrophilic 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 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 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 composite comprising conventional polyurethane foam maybe washed in a 30% calcium chloride solution, wrung out and subsequently tumble dried. Typical ly, 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 structure.

Preferred embodiments of the invention will new be described by way of non-limiting 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 hygroscopic salts in accordance with a second embodiment 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 showing 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 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 powder. 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 proprietory 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 this invention. Also, in the

aforementioned 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 conventional prepolymers and surfactant solutions. Once formed the foam 4 is similar in structure to the hydrophilic 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 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 from Figure 4 to have an unbroken surface 9 exposed to the open cells of the foam which is free from 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. 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, inasmuch 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 embodiments 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 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.

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 hydrophilic properties unaltered. However, as such an improved element will new 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 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 similarly described for hydrophilic foams, moisture exchange characteristics of a given quantity of hygroscopic salts will likewise improve due to the greater temperature 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 improvement 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 HME element contains 30% by weight aluminium powder and is treated by immersion in a 30% calcium chloride solution. This will result in a light deposit of calcium chloride crystals en the exposed surfaces of the fibrules. Typically, 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 micron in length. It will be appreciated that other suitable hygroscopic salts may be used in the place of calcium chloride.

Although manufacturing of HME 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 assemblies, where the use of a reticulated foam of increased thermal capacity is useful.

Claims

CLAIMS:

1. An electrically and thermally non-conductive reticulated polymer foam composite permitting 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 composite incorporates a powdered metallic filler (2) having substantially higher thermal capacity than that of the polymer.

2. A composite in accordance with claim 1, (characterised in that the polymer (1) is conventional polyurethane and the composite is treated so as to be lightly coated with hygroscopic salts (5).

3. A composite in accordance with claim 1, characterised 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 composite in accordance with any one of the preceding claims, characterised in that the filler is an aluminium powder (2) having a particle size of 100 microns or less.

6. A composite in accordance with claim 5, characterised in that the particle size is in the order of 20 microns or less.

7. A composite in accordance with claim 5 or claim 6, characterised in that the proportion of the aluminium powder (2) to the polymer (1) is less than 40% by weight.

8. A composite in accordance with claim 7, characterised in that the proportion is 30%.

9. A method of manufacturing a polymer foam composite 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 composite 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 composite (4) is washed in a 30% calcium chloride solution, wrung out and subsequently tumble dried.

12. A method of manufacturing a polymer foam composite in accordance with any one of claims 9 to 11, characterised in that the filler is an aluminium powder having a particle size of the order of 20 microns or less.

13. A method of manufacturing a polymer foam composite in accordance with claim 12, characterised in that the proportion of the aluminium powder (2) to the polymer (1) is 30% by weight.

14. A reticulated polyurethane polymer foam composite structure (1) permitting a through and return flew of gasses for use as a heat and moisture exchange or filter element, characterised 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 substantially fully encapsulated by said polyurethane in the body of the structure.