WO2000000690A1

WO2000000690A1 - Method for producing a bacteriostatic cellular body

Info

Publication number: WO2000000690A1
Application number: PCT/SE1999/001132
Authority: WO
Inventors: Bertil Lundin; Paul Dinnage
Original assignee: Ab Carl Munters
Priority date: 1998-06-26
Filing date: 1999-06-23
Publication date: 2000-01-06
Also published as: SE9802287D0; SE9802287L; AU4943799A

Abstract

Method for producing a bacteriostatic cellular body, such as a rotor, where an agent containing silver is added to the cellular body material. This silver addition produces a contact surface which is bacteriostatic. Further the invention refers to a bacteriostatic cellular body produced according to the above method.

Description

METHOD FOR PRODUCING A BACTERIOSTATIC CELLULAR BODY

Introduction

The present patent application refers to a method for producing a bacteriostatic cellular body, such as a rotor, where an agent containing silver is added to the cellular body material. This silver addition produces a contact surface which is bacteriostatic. Further the present patent application refers to a bacteriostatic cellular body produced according to the above method.

Cellular bodies are used in different applications for exchange of moisture and/or heat and are usually produced from sheets of paper of a preferably inorganic material for instance glass fibre, which sheets are further preferably impregnated with water glass which then are formed to block for producing of solid cellular bodies or are wound to a rotor shape for use in rotating exchangers or contact bodies.

Such bodies contain a moisture or heat exchanging mass of alternating plane and corrugated layers thus arranged so that the corrugations form a huge number of fine channels which extend essentially in parallel with each other. The plane and corrugated layers are brought and fastened together usually using gluing whereupon the structure consisting of a plane and a corrugated layer, if a rotor is to be produced, are wound (coiled) into an essentially cylindrical rotor in which the channels extend in parallel with the rotor axis. When producing solid contact bodies a number of structures are instead piled on each other thus forming a block. The winding rounds of the rotor or the piled structure units are connected mutually in the same way as for the plane and corrugated sheets, for instance through gluing. In order to give the cellular body good moisture and heat exchanging characteristics the cellular body material is preferably impregnated either before or after corrugation, with a water glass solution which forms a gel which then through further treatments is converted into a solid substance on the cellular body (see further SE 469 976 which corresponds to US 5423934) . When using cellular bodies, for instance rotors, in a humid environment, for instance in connection with air treatment the rotor material provides a potential growth surface for microbiological activity for instance for bacteria, moulds and fungi if the climate conditions in a structure are suitable. In order to achieve a contact surface for air which at the same time is bacteriostatic, that is a surface which inhibits growth of microorganisms, you may treat the material with different preparations, which are known to a skilled person within the art. However, there are only single bacteriostatic rotors known.

Previously known rotors for air treatment with a bacteriostatic effect have been built upon that you, in one of the last production steps, have dipped down and thus rotors have been impregnated in lithium chloride (LiCl) . This LiCl treatment is an old and to the skilled person in the art known technique in connection with rotors. Drawbacks with these LiCl-impregnated rotors are that the lithium chloride may be washed out in the course of time. The rotor will be washed out in due course with respect of LiCl . Further we have shown that these rotors are not bacteriostatic against bacteria but only against mould.

We have now succeeded in producing a bacteriostatic cellular body which comprises silver, which has been shown to have bacteriostatic effect, both against mould and bacteria. Further, this cellular body is not sensitive for outwashing which could have the influence of lowering the bacteriostatic effect .

Description of the invention

The invention thus relates to a method for the producing of a cellular body, such as a rotor, for treatment of a medium using another medium for instance for air treatment which cellular body has a large number of fine channels which extend essentially in parallel with each other, and which is built up of layers from a cellular body material which is impregnated with water glass where an agent containing silver is added to the cellular body material which produces a contact surface for the media which is bacteriostatic.

The invention also relates to a cellular body produced using the above method and a cellular body which is obtainable from said method.

Silver in the present application refers to silver ions, silver included in a salt, preferably AgCl and also free silver.

The addition of silver, the inhibitor for microbial growth, is preferably performed in connection with the impregnation of the cellular body paper sheets with water glass. (For a more detailed description of production of cellular bodies, see for instance the method described in SE 469976) . From the water glass which then comprises silver, alkali is removed, through treatment with acid, salt or the like and a silica gel is obtained after drying. An optional impregnation after the drying of the cellular body material with LiCl may also be performed. Naturally the impregnation with water glass may either be performed before or after the corrugation (i.e. the folding) of the paper sheets when producing a cellular body, for instance a rotor.

The method for producing the cellular body material, for instance the rotor material may for the rest be performed using a way known to the skilled person in the art, where the desired silica gel is obtained, which can be used as framework and sorbent in a dehumidifier rotor and as a framework in a heat recovering rotor. This cellular body material may also be used in cooling systems. It is also unexpected in connection with the addition of silver to the water glass, that the rest of the method for production is not influenced and that the characteristics of the end product, besides the bacteriostatic ones, are not influenced. The strength and sorption characteristics are the same. Further the water glass and its binding capacity are not influenced by the addition of silver. No reaction appears to occur in the water glass in connection with the silver addition. The silver is added so that a final concentration of silver in the cellular body is approximately between 10 and 150 ppm, preferably between 15 and 50 ppm, most preferred between 25 and 40 ppm. Already at 10 ppm you see a bacteriostatic effect. The higher concentration that is used, the more efficient the inhibition of growth will be. A large addition also gives a killing on the surface.

According to old technique known to the skilled person in the art , the cellular body may after the addition of silver be impregnated with LiCl, preferably in a later stage of the production process. The LiCl impregnation is suitably performed by dipping the cellular body material into a LiCl solution, whereby a suitable concentration of LiCl is obtained. The concentration of LiCl in the final cellular body material is suitably between 1.5 and 10 % (weight) (preferably between 3 and 8%, most preferred between 3 and 4%) . Suitably the LiCl impregnation is performed last in the production process for obtaining the cellular body.

The silver addition is suitably performed by addition of a silver containing suspension, preferably comprising silver ions, whereby a homogenous distribution of the silver is achieved over the cellular body material. The silver is further bound in this cellular body material. The silvercontaining agent, preferably a suspension thereof, suitably comprises AgCl, but also AgN0₃ and Ag₂0 are thinkable silvercontaining compounds. When using AgN0₃ together with LiCl when producing cellular bodies according to the present invention a large part of AgCl will probably be formed. AgCl is a suitable silvercontaining compound to be present in the silvercontaining agent due to the fact that AgCl is a suitable salt which is difficult to dissolve. Silver in ion form, and then preferably in the form of a salt, is often too difficult to dissolve. This means that silver, in this case preferably silver ions, is sufficiently active by the fact that a suitable portion of silver appears to be set free one at a time during a suitable amount of time, which makes the cellular body bacteriostatic. The silvercontaining suspension also comprises for instance TiO_: . The silver chloride is said to be deposited within the boundary layers of the titanium dioxide. Some have claimed that a chemical and physical connection have taken place between the both compounds. The porosity of the titanium particles is said to influence and control the setting free of silver ions when exposed to a watercontaining medium, in this case mobile water, compared to bound water, which may be present immobilised in for instance a gel . A concentration gradient is said to arise due to saturation in the titanium particles whereby the silver ions diffuse and a silver ion equilibrium is achieved in the surrounding environment. Equilibrium is said to be reached within 3 to 6 hours. These silver ions may then influence present microorganisms in an inhibiting and even killing way. New silver ions may then be obtained from the titanium particles. Unexpectedly enough with the cellular body according to the present invention, is that it is bacteriostatic even when only bound water is present in the silica gel. Thus it is not necessary that free, mobile, water is present in order for the cellular body according to the present invention to act in a bacteriostatic way. Also LiCl in a cellular body according to the present invention may have bound water.

These explanations about bacteriostatic characteristics of the silver containing cellular bodies according to the invention shall not in any way be regarded as limiting the scope of the invention but shall only be regarded as possible explanations.

Thinkable areas for application of cellular bodies produced in accordance with the present invention would be within the pharmaceutical and food industry where it is very desirable that no microorganisms may get into the process rooms. Further, also hospitals and other community rooms may have use of bacteriostatic cellular bodies, for instance rotors. A skilled person in the art further easily realizes that the production method according to the present invention could be adapted for producing other bacteriostatic cellular bodies in other known materials .

We will now further illustrate the present invention with the help of the following experiments where different , inhibition studies for different microorganisms have been performed. These experiments shall not in any way limit the invention but shall only serve in an illustrative manner. Naturally, the invention according to the present patent application probably works on microorganisms other than those we have studied inhibition on. These microorganisms may be bacteria, yeast and mould. Further, the skilled person in the art may modify the present invention in such a way that he may adapt the bacteriostatic characteristics further within the cellular body so that it is adapted for a certain environment where certain microorganisms are present as presumptive contaminants of the cellular body. As has been shown in experiments it may be enough with a certain small amount of silver in the bacteriostatic cellular body in the presence of a certain microorganism.

Moreover the invention will be illustrated further with a figure (Figure 1) which shows how rotor materials containing silver (JMAC LP 10%) with an area of 1 cm³ have been placed on Petri dishes and which have been exposed to E. coli and S. aureus. Nor shall this figure in any way be limiting for the invention.

Experimental

In both experiments rotor material has been produced essentially in accordance with the patent SE 469976. However, plane sample sheets have been produced, that is the sheets which contained silver, unlike corrugated sheets in which form the rotor material is present in its typical embodiment. The material for comparison lacking silver contents, was in the form of corrugated sheets.

The silver addition has in both experiments consisted of JMAC^® LP Concentrate 10% (obtained from Microbial Systems International: JMAC^® is a registered trademark belonging to Johnson Matthey Pic.) . This silver addition is composed of a suspension which comprises particles containing silver chloride/titanium dioxide composite in a water/sulphosuccinate gel (sulphosuccinate is here sodium dioctyl sulphosuccinate) . The silver addition is present in the form of a cream-coloured liquid which is easy to pour and thus easy to distribute in a homogenous way. The silver addition was performed at the same time as the water glass impregnation of the support material (see further in SE 469976) . The concentration 10000 ppm JMAC corresponds to 150 ppm silver.

LiCl was added (where appropriate) through dipping of the rotor material into a LiCl solution at the end of the process, more accurately in the final step. The rotor material was impregnated with LiCl after the rotor material had become dry after the addition of water glass (and where appropriate silver in connection thereto) , whereupon the rotor material again was allowed to dry. Moreover, it is also thinkable that you may add the silvercontaining agent, possibly in the form of a solid material, in this LiCl impregnation step. Also a separate impregnation process for the silver addition is thinkable but then you will have silver in an outer layer. The silver will then not be baked and bound in the gel in a way that it will be in our case .

Moreover, the sulphosuccinate is probably degraded in the periodically high heat and at the prevailing pH level during the process, whereby smaller molecular material is achieved.

Experiment 1

The following experiment was intended for examining if mould growth was possible on different cellular body materials. As installations with rotors may be placed in different environments with different moisture levels, the experiments were chosen to be carried out at a high air humidity and it was seen to that growth of mould organisms was possible through adding nutritional substances to the material. The main reason for growth of microorganisms is the presence of accessible water (water activity a_w) . 5 different moisture absorbing rotor materials were examined at high air humidity and in a good nutritive environment . The test materials were as follows :

HPS standard material with silica gel as sorption agent LI standard material with LiCl as sorption agent HPH a combination of the above materials HPSAG experimental material (with a silvercontaining agent) X a material from a competing company with silica gel as sorption agent.

HPSAG contained 10000 ppm JMAC LP 10%. LI contained 7 % (weight) (LiCl) and HPH contained 4.5 % (weight) LiCl. All samples were structural, that is they were corrugated, except for HPSAG which was present in the form of single sheets . As control material a glass fibre paper was used, which may act as support material, also in the form of single sheets.

Test organisms were Aspergillus niger, Eurotium chevaleri and Penicillium roqueforti. These were chosen partly because of their different characteristics but also in order to cover as broad an area as possible concerning growth at different water activities. All test organisms belong to the so called storage flora, that is their spores may be present in a dormant stage for months and even up to years and when the conditions are favourable they can start to grow. The organisms Aspergillus niger and Penicillium roqueforti were cultivated on MEA 2% (malt extract agar) and Eurotium chevaleri on CYA 20S (Czapek yeast extract agar with 20% saccharose) for 7 to 14 days before inoculation.

The execution itself of the test on rotor materials went on in such a way that cut out parts of the test materials

(approximately 4x4x1 cm) were put in a Petri dish and were sterilized by UV-light (254 mm) . The upper surface was lightly sprayed with CYA growth medium and the surfaces were allowed to dry for half an hour. Thereafter the test organisms were inoculated by using dry inoculation spores from respective mould organism (3 parallels for each material/organism) and incubating them in a humidity chamber, in which the humidity could be controlled, with an average value of 93% RH in room temperature. As a sterile control (negative control) of material and growth medium, respective rotor material together with growth medium was used only. As control (positive control) of the growth of the organisms without any inhibiting mecnanisms the supporting material i.e. glass fibre was used. The materials were inoculated with respective organism m accordance with the above described method.

The tests were visually monitored, followed by examination by using a stereo microscope during 3 weeks. Further, possible killing was checked with the help of pressing on, on oculation positions for rotor materials where no growth had been shown, a contact plate with MEA 2% m order to check if the rotor materials also could influence the killing of the microorganisms .

The results that were obtained are shown m table 1 and 2.

Table 1. Growth ability for different mould organisms after a three week period on rotor materials, at an average value of 93% RH.

Table 2. Growth ability for different mould organisms after a three week period on rotor materials, at an average value of 93% RH

Explanations - ) no growth

+) restrained/cancelled growth

++++) growth

It can be concluded that during the above described conditions, that is cultivation for three weeks at high humidity, average value 93%, and supply of nutritive substances, the greater part of the tested rotor materials from Munters Europe AB showed a restraining influence on the tested mould organisms .

The rotor material LI, based on LiCl as a sorption agent has a great ability to bind the humidity fast and shows a complete inhibition on all of the tested microorganisms.

The rotor material HPS, based on silica gel as a sorption agent inhibits P. roqueforti (a_w=0.83) but does not totally inhibit A. niger (a_w=0.77) which shows a restrained growth, while E. chevaleri (a_w=0.71) which can grow at a lower water activity, has a good growth.

The rotor material HPH based on a combination of silica gel and LiCl as a sorption agent shows a small risk. E. chevaleri (a_w=0,71) may grow, but with a very restrained growth.

The experimental material HPSAG, containing silver, inhibits E. chevaleri (a_w=0.71) and P. roqueforti (a_w=0.83) but not completely A. niger (a_w=0.77), which grows with a restrained growth. Here the results rather indicate that certain other factors may cause the restrained growth. It should also be pointed out that the HPSAG-results are not completely comparable with the other due to the fact that they are based on non- corrugated material, that is single unfolded sheets, unlike the others. The binding mass for humidity thus was not really the same.

The competing material X based on silica gel did not show any inhibiting effect on E. chevaleri or on P. roqueforti, but a somewhat restrained growth on A. niger was shown.

The conclusion is that at a good nutrimental supply for the microorganisms, high air humidity, average value 93% RH and after 3 weeks, the rotor material LI causes a complete inhibition of mould growth. The rotor material HPH and HPSAG show a good but not complete inhibition of mould growth and thus they should be used at somewhat lower air humidity. Experiment 2

In order to determinate the zone of inhibition of S . aureus and E. coli when grown in the presence of rotor material the following method was used.

Rotor materials that were tested were the following:

Sample (see the previous experiment) Test number

HPS 321/98 HPH 322/98

HPSAG 323/98

HPSAG 473/98

HPSAG 5000 474/98

HPSAG 2500 475/98 HPSAG 1000 476/98

HPS (control) 477/98

HPSAG had 10000 ppm JMAC LP 10% added (which corresponds to 150 ppm silver) , HPSAG 5000 had 5000 ppm JMAC LP 10% added (corresponds to 75 ppm silver) , HPSAG 2500 had 2500 ppm JMAC LP 10% added (corresponds to 38 ppm silver) and HPSAG 1000 had 1000 ppm JMAC LP 10% added (corresponds to 15 ppm silver) .

Over-night cultures of S. aureus and E. coli were prepared from culture plates by inoculating 5 ml of TSB with a single bacterial colony and incubating at 30°C for 24 hours.

100 μl of over-night culture was added to 100 ml of Iso- Sensitest Agar (Oxoid CM 471) and to six 20 ml plates, poured to house each test organism, plus one growth control plate. 200 μl sterile 1% TTC was added to three of each set of plates to aid in the visualisation of the zones of inhibition. The plates were dried thoroughly. 1 cm² samples of each rotor material were placed on the surface of the agar plates - one of each sample from the rotor materials - on one plate and the plates were incubated at 30°C for 48 hours. The zones of inhibition (mm) were recorded for each test sample and an average value was calculated. One of each rotor sample was lifted from the plate surface and was placed on a new TSA- plate. These were incubated at 30°C for 48 hours to determine whether JMAC LP 10% in the rotor materials had a cidal (killing) or static effect on the growth of the test organisms .

The results are shown in table 3 and 4 (and in the accompanying figure)

Table 3, test on March 27, 199!

Explanations :

* = plates containing 1% TTC = activity cidal, not static

Table 4, test on April 21, 199

Explanations

* plates containing TTC

# activity static, not cidal no growth of the test organism, seen throughout in the depths of the agar under the rotormaterial

The first summary of results indicated that the rotor material containing JMAC LP 10% gave a zone of inhibition with a breadth of 2 mm against S. aureus and 1.5 mm against E.coli. The inhibition of growth was found to be cidal. HPS and HPH-rotor materials containing no JMAC LP 10% gave no zone of inhibition against either test organism. The second summary of results showed that the rotor materials containing 10000 ppm JMAC LP 10% gave the greatest zones of inhibition against both test organisms. The effect on growth was found to be cidal. Zone size decreased with decreasing levels of JMAC LP 10%. A level of 2500 ppm JMAC LP 10% gave zones of inhibition of 1.08 mm against S. aureus and 0.25 mm against E. coli with no growth of either organism visible on the depth of the agar immediately under the rotor material. Inhibition of growth was found to be static. Although a level of 1000 ppm JMAC LP 10% had some activity against S. aureus it was not shown to inhibit the growth of E. coli. S. aureus appeared to be more sensitive than E.coli.

The control containing 0 ppm JMAC LP 10% had no inhibitory effect against either test organism.

The conclusion which could be drawn upon this experiment was that a level of 2500 ppm of JMAC LP 10% in the rotor material could be suitable in order to give a sufficient inhibitory effect on microorganisms, even if the effect is static instead of cidal at this level . This would then be the case provided that the level of JMAC LP 10% is maintained at a suitable level 2500 ppm JMAC LP 10% corresponds to 38 ppm silver.

Claims

1. Method for producing a cellular body such as a rotor for treatment of a medium using another medium for instance for air treatment, which cellular body has a large number of fine chan- nels which extend essentially in parallel with each other and which is built up of layers from a cellular body material which is impregnated with water glass, characterized by that an agent containing silver is added to the cellular body material, which produces a contact surface for the media which is bacteriostatic.

2. Method according to claim 1 where the addition of silver is performed in connection with the impregnation with water glass.

3. Method according to any of claims 1 to 2 where silver is added so that a final concentration of silver in the cellular body is approximately between 10 and 150 ppm, preferably between 15 and 50 ppm, most preferred between 25 and 40 ppm.

4. Method according to any of claims 1 to 3 where the cellular body after the addition of silver is impregnated with LiCl .

5. Method according to claim 4, where the concentration of LiCl in the final rotor material is suitably between 1.5 and 10% (weight), preferably between 3 and 8 % (weight), most preferred between 3 and 4 % (weight) .

6. Method according to any of claims 1 to 5 , where the addition of silver is performed by addition of a silvercontaining suspension.

7. Method according to claim 6, where the silvercontaining suspension comprises Ti0₂.

8. Method according to claim 6 or 7 , where the silvercontaining suspension comprises silver ions, preferably silver chloride.

9. A cellular body obtainable from a method according to any of claims 1 to 8 .

10. A cellular body produced by using a method according to any of claims 1 to 8.