FIELD OF THE INVENTION The present invention relates to methods for changing the properties of cellulose-containing material using a reaction gas comprising ammonia and an oscillating magnetic field. In particular, the current invention relates to methods for improving the sorption properties and/or improving antibacterial or bacteriostatic properties of the cellulose-containing material.

BACKGROUND OF THE INVENTION Wound dressings are used in the treatment of major or minor wounds, e.g., in the skin of a patient. Wound dressings are not to be confused with bandages, wherein dressings are intended for direct contact to the wound, whereas bandages are designed to keep the dressing firm and avoid slippage of the dressing from the wound.

The development and use of wound dressings has evolved through many centuries from inert and passive products such as gauze, lint and fiber to a dazzling range of modern moist wound dressings. Wound products can be divided broadly into two groups: (i) passive products and (ii) interactive products. Within these two groups, the passive dressings can be sub-classified into absorbing and non-absorbing passive dressings, whereas the interactive dressing can be sub-classified as absorbing, non-absorbing and moisture donating interactive dressings. Generally the following properties are advantageous for wound dressings:

- Capacity to remove excessive exudate from the wound without allowing the wound to dry out thereby maintaining a moist environment

- Ability to maintain the wound core temperature at approx. 37°C

- Impermeability to microorganisms in order to minimize contaminations of the wound from outside the wound

- Absence of particulate or toxic matter in the dressing - Non-traumatic structure, e.g., non-adhering to the wound, so that a change of the dressing will not damage the underlying tissue

The passive dressing fulfils very few of the properties of an ideal dressing and has very limited use as a primary dressing. However, mostly they are used as secondary dressings. Interactive dressings help to control the micro-environment of the wound by combining with the exudate to form either a hydrophilic gel, or by means of semi-permeable membranes, controlling the flow of exudate from the wound into the dressing.

In the past, the traditionally theory has always been that wound should be kept dry that a scab may form over the wound, the wound should be exposed to the air and sunlight as much as possible. The clear disadvantages of mentioned principles are that the scab, which is made up of the dried exudate and drying dermis, is a physical barrier to healing which is then delayed, because the epidermal cells cannot move through the scab formed. Exposure to air reduces the surface temperature of the wound, causing peripheral vasoconstriction and affecting the flow of blood to the wound, which further delays healing. It was shown that the wounds healing under moist conditions healed faster than the wounds under dry conditions, open to air. Owing to that, the wound dressings for healing in a moist environment is a well established medical technology market which is expected to grow significantly in the future.

The use of textiles in medicine has a long tradition. An important field of application is wound care and prevention of chronic wounds, in particular pressure sores. Among the long list of textile materials, bandages and wound dressings gained great popularity. The use of textile materials was supported by availability, prices and re-usability. Woven textiles are mostly used. Despite the fact that traditional textiles fulfilled many requirements like biocompatibility, flexibility, strength, etc. there is an increasing need for further functions. The fluid-handling capacity could be realized by introduction of super absorbent materials. The most important compound for the production of super absorbent materials is acrylic acid. The monomer acrylic acid is polymerized with the support of compounds like tri-allylamine. Co-polymerization allows the coating of cellulose-containing fibers like viscose or lyocell. Such products are used for napkins and other hygienic devices. In situ polymerization of aniline was used for making cotton materials superhydrophobic and ammonia gas was used to dedope said cotton materials and thus regain hydrophilic properties. Such a procedure thus allows for controlling wettability of cotton textiles (Junping J. et al., Acta Polymerica Sinica, 2: 192- 198).

The potential infections that could enter the body through the wound opening may be particular cause for concern. Most wound infections are caused when a large number of bacteria manage to get into the wound. These bacteria attach to the skin tissue, slowing the healing process and causing irritation. A bacterial infection can be recognized from symptoms including draining pus, redness around the area of the wound, increased tenderness, increased pain, or a strange smell coming from the wound. Therefore, one important aspect of the dressing's usefulness is also the ability to prevent infection. Textiles are carriers of bacteria and fungi. Controlling bacterial or fungal growth on fabric can be achieved by (a) using resins to fix antibacterial/antifungal agents to the textile surface or (b) grafting antimicrobials/ antifungal agents on the polymer chain within the textiles.

It is known that, e.g., many heavy metal cations (i.e. Hg ²⁺ , Pb ²⁺ ) have antimicrobial activity, but they are toxic. Other metal ions such as cooper, zinc, bismuth have been identified as acting destructively towards microbes, but only a few of them are safe for patients and the environment. In addition there is a well-known evidence for bacterial resistance to silver that has a long history as an antimicrobial agent. Silver impregnated textiles are used as wound dressings for infected wounds or wounds at high risk of infection, whereas linkages between biocidal moieties and cellulose are covalently formed on reactive hydroxyl groups. Another technique is the use of antibacterial agents in the spinning process of fibers, e.g., viscose or modal fibers. Modal fibers can be obtained by adding the antibacterial agent to the spinning dope. The viscose was pressed through the holes of the spinnerets into the generation bath where filaments were formed and drawn off at high speed. By the incorporation technique a homogenous distribution of the additive within the cellulose matrix of the fiber could be achieved. The hydrophilic and porous structure of the fiber enhances the diffusion of the agent onto the surface. This is also supported by a humid environment (i.e. due to sweating).

A plasma technique was used to improve the wettability of polypropylene and viscose material as well as a good pre-treatment method for successful binding of silver (Murat O. et al., 201 1 , Disposable hydrophilic antimicrobial laminated nonwoven bed sheet; INTERNATIONAL JOURNAL OF CLOTHING SCIENCE AND TECHNOLOGY, 23(4): 222-231) to obtain as well the antimicrobial properties.

US 7144957, US 201 1/0301027, US 5985301 and US6348257 disclose chemical modification of, i.a., cellulosic material by depositing various materials including with ammonium compounds in order to obtain better sorption and bacteriostatic properties but none mentions treatment by gaseous ammonia nor using oscillating magnetic fields. The advantageous properties of the materials modified according to the present invention, however, are not achieved.

SUMMARY OF THE INVENTION

The present invention is defined by the appended independent claims. Preferred embodiments of the invention are defined by the dependent claims.

The present invention hence relates to a method of modifying at least one property of a cellulose-containing material by contacting said cellulose-containing material with a reaction gas comprising ammonia under the influence of an oscillating magnetic field.

Preferably, said modification of said property is the increase of the hydrophilicity of the cellulose-containing material, the increase of the antimicrobial activity of the cellulose- containing material, or the increase of the bacteriostatic properties of the cellulose-containing material. In preferred embodiments, the modified property is an increased capillary velocity (% increase), as determined, e.g., by a test procedure according to Example 4. In another embodiment, the modified property is an increased sorption capacity (% increase), as determined, e.g., by a test procedure according to Example 5.

In preferred embodiments, said oscillating magnetic field is at a maximum magnetic flux density of at least 10 ^"4 T, preferably at least 10 ^"3 T, most preferred at least 10 ^"2 T.

In other preferred embodiments, the oscillating magnetic field oscillates at a frequency of from 0.1 MHz to 5000 MHz, preferably of from 1 to 100 MHz, more preferred of from 2 to 50 MHz, most preferred of from 5 to 20 MHz.

In other preferred embodiments, the contacting time under the influence of an oscillating magnetic field (also referred to as the "first period of time" within this application) is a period of from 0.1 seconds to 1000 seconds, preferably from 1 second to 500 seconds, most preferred from 20 seconds to 300 seconds.

Preferably, the contacting of the ammonia with the cellulose-containing material is at a temperature of between 0°C and 300°C, preferably between 10°C and 200°C, most preferred at a temperature of between 50°C to 150°C.

Preferably, the reaction gas comprises ammonia at a concentration of at least 50mol%, preferably at least 90mol%, more preferably at least 95mol%, most preferred at least 99mol% based on the number of molecules in the reaction gas.

Preferably, the reaction gas further comprises at least one (further) gas selected from the group consisting of: a noble gas (such as helium or argon), nitrogen and hydrogen.

In a preferred embodiment, the contacting takes place at a pressure of from 1 to 100,000 Pa (lbar), more preferably at from 10 to 10,000 Pa, or most preferred from 10 to 1 ,000 Pa.

Preferably, the cellulose-containing material is a porous material or a fibrous material, although surface treated films are also envisaged.

In a preferred embodiment, the method comprises the steps of:

(i) arranging the cellulose-containing material in a treatment chamber;

(ii) evacuating gas from said treatment chamber, thereby reducing the pressure in said treatment chamber;

(iii) introducing the reaction gas into the evacuated treatment chamber;

(iv) applying an oscillating magnetic field to said treatment chamber for a first period of time;

(v) optionally, applying no oscillating magnetic field to said treatment chamber for a second period of time;

(vi) venting said treatment chamber, thereby increasing the pressure inside the treatment chamber to ambient pressure.

In one embodiment, the method further comprises a step (iv.1) of removing reaction gas from the treatment chamber after step (v) and before step (vi). It shall be understood that the reaction gas is present in the treatment chamber upon application or non-application of the oscillating magnetic field in steps (iv) and (v).

Preferably, the "second period of time" is from 1 second to 1000 seconds, preferably from 10 seconds to 200 seconds.

Other aspects of the invention relate to the production of a cellulose-containing material having altered properties, said method comprising providing a source cellulose-containing material, modifying said source cellulose-containing material by a method of the invention as described above, thereby obtaining the cellulose-containing material having altered properties.

Another aspect of the invention further relates to the use of ammonia and an oscillating magnetic field for the alteration of properties of cellulose-containing materials, wherein said alteration is substantially achieved as in the methods described above.

The invention further relates to a cellulose-containing material modified by methods as described above.

Another aspect of the invention relates to a product comprising a modified cellulose- containing material of the invention. Such products have increased hydrophilicity and/or increased antimicrobial activity and/or improved bacteriostatic properties, according to the invention.

Another aspect of the invention relates to the use of a cellulose-containing material of the invention in therapy. In a preferred embodiment, the therapeutic treatment is the treatment of a wound.

An further aspect of the invention relates to the use of materials or products of the invention for therapy.

Preferred products of the invention are wound dressings, absorbent products, diapers, incontinence products, feminine hygiene articles, cotton pads, medical patches, hygienic pads, sanitary towels, garments, underwear products, footwear products (such as stockings, socks or shoe linings), membranes, such as filtration membranes, or filters, such as air filters or filters for liquid media.

A particularly preferred product is a wound dressing, most preferred a wound dressing comprising a drug, such as an anaesthetic drug. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the water absorption properties of treated cellulose samples in terms of the wetting rise curve (mass ² over time) according to a first embodiment of the invention (high magnetic flux density).

Figure 2 shows the water contact angle for cellulose samples treated according to a first embodiment of the invention (high magnetic flux density).

Figure 3 shows concentration of nitrogen on the surface of treated cellulose sample as determined by X-ray photoelectron spectroscopy (XPS).

Figure 4 shows water absorption properties of treated cellulose samples in terms of wetting rise curve according to a second embodiment of the invention (low magnetic flux density).

Figure 5 shows the water contact angle of a sample treated according to the second embodiment of the invention (low magnetic flux density).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to methods of treating cellulose-containing material by contacting said material with a reaction gas comprising ammonia under the influence of an oscillating magnetic field. In preferred embodiments of the invention, the hydrophilicity of the cellulose-containing material and/or the antimicrobial/bacteriostatic properties of the cellulosic material are thereby improved.

"Cellulose-containing material", within the context of the present invention, shall be understood as being any material comprising cellulose. Preferably, the cellulose-containing material is a fibrous and/or porous material. The cellulose containing material may be a composite product of cellulose and a further polymer, such as viscose, lyocell fibers, and modal fibers.

In preferred embodiments, the cellulose-containing material is treated by ammonia gas at low pressure, preferentially at the pressure range between about 1 and about 100,000 Pa. In the preferred pressure range the chemical interaction between ammonia and the cellulose- containing material in a presence of an oscillating magnetic field is sufficiently high to provide for effective modification of the properties of the material. "Cellulose" is well known in the art. In the context of the present invention, "cellulose" shall be understood as being the polymeric material consisting of multiple (e.g. more than 100) β( 1

→4) linked D-glucose units.

The hydrophilicity and sorption properties of ordinary cellulose are not optimal. This shows, e.g. in (i) a rather slow wetting rise curve (mass ² over time), and (ii) rather large water contact angle (in degrees). Furthermore, normal cellulose-containing materials do not have a significant antimicrobial effect, e.g., they are not very bacteriostatic. The present invention addresses these shortcomings of cellulose-containing materials by increasing the hydrophilicity of the material (i.e., providing a steeper wetting rise curve and a decreased water contact angle), and by substantially increasing the antimicrobial and/or bacteriostatic properties of the material. These effects, according to the invention, are achieved by chemical reaction between ammonia and the cellulose-containing material in an oscillating magnetic field. The chemical reaction is enhanced substantially by application of the oscillating magnetic field.

Ammonia does not react with the cellulose in a cellulose-containing material at normal conditions, e.g., at room temperature (21°C) and in the absence of an oscillating magnetic field. The reason for such inability to react is the poor chemical affinity of ammonia to cellulose under these conditions. It is well known that the affinity can be increased by increasing the temperature. The affinity increases with increasing temperature roughly exponentially. At elevated temperatures below 150°C the affinity is still very low and it becomes measurably high at temperatures above 400°C. At such a high temperature, however, the cellulose is not stable, but starts degrading. By the term degrading is meant a loss of atoms and scissions of the polymer chains, thus, a modification/deterioration of mechanical and chemical properties. This also results in a deterioration of the sorption properties. Elevated temperatures therefore lead to higher affinity of the ammonia to the cellulose, but simultaneously it causes material degradation. According to the present invention, a sufficiently high reaction rate of ammonia with cellulose material without a concomitant thermal degradation of the cellulose material is achieved by using an oscillating magnetic field instead of high temperatures. The oscillating magnetic fields essentially replace the need for thermal treatment in order to improve the affinity. According to the present invention, the cellulose-containing material, such as cellulose and cellulose derivatives, reacts chemically with ammonia at relatively low temperatures. Thereby the material sorption properties, as well as the antimicrobial/bacteriostatic properties are improved.

Another parameter having an impact on the reaction of ammonia with cellulose-containing material according to the invention is the purity of the ammonia gas. Without wishing to be bound by theory, it is assumed that other gases than ammonia, if present, would likewise react with the cellulose, e.g. at its surface, upon exposure of the samples and gases to oscillating magnetic fields. Therefore, in the preferred embodiment, the ammonia is contacted with the sample of the cellulose-containing material at a relatively high level of purity, e.g. at 90%, 95%, 99% or 99.9% molar concentration of the ammonia in the reaction gas. For this purpose, the treatment chamber is first evacuated to a relatively low pressure in order to remove other gasses from the reaction chamber, and only then ammonia is added to the chamber. A treatment chamber is thus first evacuated by an appropriate vacuum pump. The pressure in the processing chamber after evacuation is preferably equal to or 1 ,000 Pa, preferably 100 Pa, even more preferred equal to or below 10 Pa. After successful evacuation the treatment chamber is filled with ammonia to a (higher) pressure of e.g., 10,000 Pa, or 1000 Pa, or 100 Pa. Such a moderate pressure was found advantageous in terms of the affinity between ammonia and the cellulose-containing material. The use of a vacuum chamber is therefore advantageous for assuring high purity of reaction gas (comprising ammonia), and for assuring appropriate pressure in order to make best use of the oscillating magnetic field.

The optimal duration of processing of cellulose-containing material by ammonia in oscillating magnetic field depends on the treatment parameters such as the frequency of the oscillating magnetic field, the amplitude of the oscillating magnetic field and the pressure of ammonia.

The reaction rate increases with increasing frequency. At a frequency of about 10 MHz a satisfactory reaction rate can be achieved. Further increase of the frequency does often not result in higher reaction rates. In one experiment, the magnetic field oscillated at about 13 MHz, and good reaction rates were achieved.

Also the optimal duration of the contacting of the cellulose-containing material and ammonia in the oscillating magnetic field depends on the frequency and on the magnetic field strength. The amplitude of the oscillating magnetic field (i.e., the maximum magnetic flux density) plays an important role in the treatment procedure of the invention. A stronger oscillating magnetic field generally increases the reaction rate of the ammonia with the cellulose- containing material (see Examples) In preferred embodiments the magnetic field oscillates with a frequency of above about 10 kHz, preferably 100 kHz, more preferably 1 MHz. Such a high frequency allows for a high reaction rate of the treatment gas with the cellulose-containing material. In preferred embodiments the frequency of oscillating magnetic field is between 1 and 100 MHz. Preferably, the contacting of the reaction gas (e.g., ammonia) with the cellulose-containing material in the treatment chamber is at a pressure of between 1 and 10,000 Pa, or 1 to 1 ,000 Pa, or 10 to 1 ,000 Pa. The magnetic field density is larger than about lxlO ^"3 T, preferably larger than l xl O ^"2 T, but preferably lower than or equal to 1 T, or ΙχΙ θ 'Τ. At low magnetic field strengths, the effects of treatment are relatively small, while higher densities of the magnetic field are more difficult to provide. Preferably the contact time of the ammonia with the cellulose-containing material in the oscillating magnetic field is between 0.1 s and 1000 s. In further preferred embodiments the treatment time is between 20 s and 1000 s, or between 50 s and 500 s, or between 100 s and 400 s. This preferred treatment time allows for optimal treatment efficiency at the preferred pressure and the preferred frequency of oscillating magnetic field and the preferred flux density of oscillating magnetic field, according to the invention. Preferably the temperature of the cellulose-containing material during treatment with ammonia in the presence of oscillating magnetic field is between 50 and 150°C. At lower temperature the treatment time needed for optimal modification of cellulose- containing material becomes too large, while at higher temperature the material may become degraded so that desired mechanical properties of the cellulose-containing material may be lost. This is particularly true at temperatures above about 400°C, or even 500°C where thermal degradation of the cellulose-containing material is observed.

The following treatment parameters have shown to be particularly advantageous: a pressure of 150 Pa, an oscillating magnetic field frequency of 13.56 MHz and a magnetic field amplitude of l xl O ^"2 T. Preferably, the magnetic field in the treatment chamber is essentially homogeneous, thus, allowing for uniform modification of the cellulose-containing material by the ammonia gas.

The invention shall now be described with reference to the following non-limiting examples:

EXAMPLE 1

Samples of a cellulose-containing material were treated according to the invention. The pressure upon contacting of the cellulose-containing material with the treatment gas (in this case, pure ammonia) was 150 Pa, the frequency of the magnetic field was 13.56 MHz and the amplitude of the essentially homogeneous magnetic field was l x l O ^"2 T.

The sorption properties of samples of a cellulose-containing material were determined in terms of the wetting rise curve and in terms of the water contact angle.

The sorption properties of the treated samples are shown in terms of the wetting rise curve, presented in Figure 1 . Shown is a series of plots of the wetting rise curve for non-treated cellulose-containing material and cellulose-containing material treated for varying treatment periods. The wetting rise curve of non-treated cellulose-containing material is lower and less steep as compared to treated samples. The shape of a sample was rectangular and the size of a sample was 3 cm x 15 cm. The magnetic field was homogeneous over entire surface of a sample. A significant difference in the wetting rise curve between non-treated and treated samples is apparent. The wetting rise curve increases with increasing treatment time until certain saturation is observed. Based on wetting rise curve results, the modified Washburn equation was used to calculate the contact angle between a solid and liquid phase. The resulting contact angles are presented in Figure 2. The contact angle of water for the non- treated cellulose-containing material is relatively large (about 90°), indicating relatively low hydrophilicity of the material. Even a short treatment period of 20 s under the conditions above causes a significant decrease of the water contact angle for about 10 percent. The measured value of the water contact angle decreases almost linearly with increasing treatment time until at about 140 s, where it stabilizes at about 35°. Such a low contact angle indicates significant hydrophilicity and the ability of the cellulose material to adsorb significant amounts of water. Figure 1 indicates an optimal treatment period (contact time under oscillating magnetic field) of about 150 s, under the experimental conditions employed. The treated cellulose-containing material was tested for its antimicrobial and/or bacteriostatic activity or character. Bacteria were deposited on series of treated and non-treated samples. The samples were treated by methods of the invention for a treatment period of 300 seconds. Two types of bacteria were used: Staphylococcus aureus and Enterococcus faecalis. The reduction in the number of bacteria was determined using a standard count plate technique after 24 and 48 hours. The results are presented in Table 1 , below. An almost complete reduction of the number of bacteria (colony forming units) is observed for Staphylococcus aureus, namely, 100% reduction at 24 hours and 48 hours of incubation. Less than complete reduction is observed for Enterococcus faecalis, for which the reduction at 24 hours and 48 hours was 81 and 68 percent, respectively. The results are presented in Table 1. The results indicate an improved bacteriostatic character of the samples treated by methods of the invention.

Table 1: Reduction R (%) of the bacteria, mostly present in the infected wound, for non- treated and treated cellulose sample at optimal conditions (treated for 300 s)

Figure 3 shows measurements of the nitrogen content in the surface film of cellulose material treated under the conditions above. Samples were characterized by a known technique for surface film characterization, namely by X-ray photoelectron spectroscopy. The surface of the non-treated sample comprised of 57.2 atomic % of carbon and 42.2 atomic % of oxygen. The results in Figure 3 show that method of invention changed the surface chemistry as the carbon concentration increased by about 1.7 atomic %, while oxygen concentration decreased by about 1 1.7 atomic %. After using the method of invention, nitrogen is present at the surface at a concentration of more than 10 atomic %.

EXAMPLE 2

A sample of cellulose-containing material was treated under less favorable conditions. Specifically, the sample was treated in an oscillating magnetic field which was 4 times weaker than the field applied in Example 1 (2.5x 10 ^"3 T instead of l x l O ^"2 T). All other parameters remained unchanged. Figure 4 shows capability of water absorption of cellulose-containing material in terms of wetting rise curve at the lower strength of the magnetic field. The water absorption is less than what was observed in Example 1. Figure 5 shows the water contact angle treated at the lower strength magnetic field. The decrease of the contact angle is less pronounced as compared to what is shown in Figure 2. Thereby is shown that the modification of the cellulose-containing material is more effective at the higher strength of the magnetic field as applied in Example 1.

EXAMPLE 3 (comparative example) Cellulose-containing materials have been exposed to ammonia in a constant, non-oscillating magnetic field, using ferrite permanent magnets. Cellulose-containing material was placed in a gap between the magnets, where a uniform magnetic field exceeding 0.1 T was sustained for prolonged time. The material was contacted with ammonia. No modification of the properties of the cellulose-containing material was observed. This shows that an oscillating magnetic field is required for the desired modification of the cellulose-containing material to take place.

EXAMPLE 4 (DETERMINATION OF CAPILLARY VELOCITY)

A porous or fibrous sample is cut into a disc-shape having a diameter of 2.5 cm and a thickness of 0.3 cm. The disc is disposed in an upright (vertical) orientation above a water surface (de-ionized water, equal to or greater than 100 cm ² ), and brought into contact with said liquid surface at its lower peripheral edge, whereupon the weight gain m [g] is measured over time. The capillary velocity is determined as the initial slope (dm ² /dt [g ² /s], linear part) of the squared mass over time (m ² = f(t) curve). The temperature is 20°C.

An increase of the capillary velocity is determined as the percent increase of the capillary velocity of a treated sample over a non-treated (but otherwise identical) second sample. EXAMPLE 5 (DETERMINATION OF THE ABSOPTION CAPACITY)

A porous or fibrous sample is cut into a disc-shape having a diameter of 2.5 cm and a thickness of 0.3 cm. The disc is disposed in an upright (vertical) orientation above a water surface (de-ionized water, equal to or greater than 100 cm ), and brought into contact with said liquid surface at its lower peripheral edge, whereupon the weight gain m [g] is measured over time. The absorption capacity is determined as the weight gain, due to liquid absorption, at t = 1 min. The temperature is 20°C.

An increase of the absorption capacity is determined as the percent increase of the absorption capacity of a treated sample over a non-treated (but otherwise identical) second sample.