The present invention relates to a method for cleaning solid materials, in particular textile products, wherein the surface of these materials in a solvent brought into a near- critical condition, contacts a surface-active substance.

By compressing a solvent in the gas phase, it will, before changing entirely into the liquid phase, at a specific value of temperature and pressure reach the critical point where the liquid is highly compressible. The liquid phase and the gas phase are then indistinguishable from each other; at temperatures and pressures in the neighborhood of these critical values, the solvent is in a near-critical condition.

If, at high pressure, the molecules in a medium have approached each other so close that the electron clouds start to disturb each other considerably, the electrons of the different molecules will repulse each other, due to the electric interaction. At a lesser pressure, the molecules in the medium are spaced apart a greater distance and the polarity of the molecules will involve an attractive force between the molecules. At the so-called critical pressure, the attractive forces are of the same order as the forces of repulsion and on account of the great compressibility of the medium, the quality as solvent for contaminating substances thereof can be set by selecting a suitable pressure. For this reason, the cleaning of solid materials in a near-critical

solvent under suitable temperature and pressure is proposed, which, accordingly, is applied in practice.

The cleaning of a solid material in a near-critical solvent without further additions is a process that takes up particularly much time. Hence, it is proposed that a surface- active substance, a so-called surfactant, this is a soap, be added, while the solvent is kept in motion mechanically. An adsorbed layer of the surfactant is then formed on the surface of the material to be cleaned, which surfactant reduces the surface tension of the material to the cleaned and of the substances to be removed, and can thus promote the transport of particles from the surface of the material to be cleaned to the solution. However, it has been found that when this so-called equilibrium surface tension is plotted out against the concentration of the surfactant, an optimal cleaning action is obtained in the area where a change of this concentration has no influence on the equilibrium surface tension. In the article by D. J. M. Bergink-Martens and G. Frens in'Tenside Surfactants Detergents', Vol. 34,1997, pp. 263-266, it is indicated how the dynamic surface tension, too, is important for the cleaning action of the surfactant.

By a mechanical motion effected in the solvent, a force is exerted on the surface to be cleaned, and in so far as a textile product is to be cleaned, on the fibers of this product. This force manifests itself in a local stretching and shrinking of these fibers, i. e. in a surface which becomes larger and smaller. Stretching will involve a

transport of surfactant to the fibers, by diffusion, whereas subsequent shrinking and a consequently occurring local supersaturation of surfactant on the fibers will involve a transport to the solvent, by diffusion, while substances (contamination) to be removed are entrained. The mechanical motion whereby the fibers stretch and shrink should preferably be such that the transport rate through the diffusion layer can keep up with this stretching and shrinking. The transport of the surfactant through the diffusion layer that occurs under these conditions is a measure for the dynamic surface tension. From the above- mentioned article in'Tenside surfactants detergents', it appears that when not only the equilibrium surface tension, but also the dynamic surface tension is plotted out against the concentration of the surfactant, an optimal cleaning action is obtained at the point where the equilibrium surface tension and the dynamic surface tension approach each other in magnitude. However, this manner of cleaning with a solvent and a surfactant while exerting a relatively great force on the material to be cleaned, in particular textile products, entails a substantial wear of the product to be cleaned, due to the force exerted.

The object of the invention is to provide a method wherein wear is avoided entirely or virtually entirely, while the advantages of the above manner of cleaning are maintained.

In accordance with the invention, the method as described in the preamble is characterized in that the cleaning action of the surfactant is achieved by varying the pressure in the solvent.

To account for this effect, it should be realized that surfactant molecules have a part (the head) that dissolves easily in the solvent and a part (the tail) that, under normal conditions, dissolves with difficulty. Under increased pressure, the solubility of the'tail'increases, in other words, the interactive forces between the solvent and the surfactant then increase, which is expressed in the value of the 2d virial coefficient in the series development of Kamerlingh Onnes as specification of the Van der Waals law.

Due to a reduced solubility in the solvent, the transport of surfactant molecules through the diffusion layer to the material to be cleaned increases. Increasing the pressure causes the solubility of the'tail'and hence of the surfactant molecules to increase, and involves a diffusion of these molecules from the material to be cleaned to the solvent, while entraining contamination. Through pulsatory variation of the pressure in the solvent, a periodically changing solubility of the surfactant molecules is realized, which, when a textile product is used as material, has the same effect on the adsorbed surfactant molecules at the surface of the fibers in this product as stretching and shrinking of the fibers themselves under a physical force exerted by, for instance, agitation, however without

entailing the drawback of wear. As surfactant, nonanol can for instance be used; this surfactant can be conceived as being built up from a'head'formed by an octane group having a value of the 2nd virial coefficient of about 300, and a 'tail'formed by a methanol group having a value of the 2d virial coefficient of about 125. By using near-critical Co, as solvent, the solubility of the'tail'can be increased under pressure to an extent corresponding to a value of the 2nd virial coefficient of about 300, hence approximately corresponding to that of the'head'. The choice for using CO2 is based upon the consideration that this substance reaches its critical point under relatively readily settable physical conditions (at a temperature of 31° and a pressure of 75 bar). Moreover, near the critical point, C02 proves to have a disinfecting effect. As an alternative to CO2, near- critical dimethyl ether can for instance be used, whose critical point lies at a temperature of 127° and a pressure of 53 bar, or near-critical propane, whose critical point lies at a temperature of 97° and a pressure of 42 bar.

However, these materials are rather inflammable and require rather costly safety measures. As alternative to nonanol, as surfactant that is in particular suitable for use in near- critical CO2, fluoro surfactants can for instance be used, marketed by Dupont de Nemours under the name of'zonyl fluoro surfactants'.

For causing the surfactant molecule transport back and forth to the solid surface by diffusion to proceed in such a

manner that a proper cleaning action is obtained, it is favorable when the pressure in the solvent is varied with a frequency in the order of 0.1-10 Hz and preferably with about 1 Hz.

In practice, it appears that the pressure in the solvent can be varied by means of acoustic vibrations preferably having a frequency of about 0.5 to about 10 Hz, while for varying the pressure in the solvent preferably with a frequency from about 0.1 to about 2 Hz, a valve mechanism for the pulsatory exertion of a mechanical pressure on the solvent can be employed.

Apart from a method for cleaning solid materials, the invention also relates to an apparatus for cleaning solid materials for applying the above method, which apparatus to that end comprises a pressure vessel into which the material to be cleaned can be introduced, together with a solvent and a surfactant, wherein during cleaning, the physical conditions in the vessel are such that the solvent is in a near-critical condition. This apparatus is characterized in that pressure-varying means are present for effecting pressure variations in the solvent and thereby activating the cleaning action of the surfactant under said physical conditions. For that purpose, the pressure vessel has a supply for the solvent, a supply for the surfactant, a door or valve for introducing and removing the product to be cleaned, a discharge for the solvent, including surfactant and contamination, and means for setting the desired

temperature and pressure in the pressure vessel. Further, inside or outside the pressure vessel, optionally in direct physical contact therewith, pressure-varying means are provided.

These pressure-varying means may be formed by a sound source which is in physical contact with the solvent, or by a pulsator for generating mechanical vibrations which, with the interposition of a diaphragm, can be transmitted to the solvent.

The present invention can in particular be applied for the industrial cleaning of textile products. Such application is highly attractive for the cleaning of large amounts of sheets and linen for hospitals, hotels and the like, which commonly lease these textile products from industrial laundries.