Since continuously porous foams made of polyurethane are light and have functions of absorbing liquids, storing liquids, permeating liquids and gases, absorbing sounds, selectively removing solids, absorbing shocks and the like, they are applied to ink rollers, self-inking rubber stamps, writing-brush pens, filters, sponges for cosmetics and the like.
Foams made of polyurethane have been mainly produced by chemical foaming methods and the like. However, since, in the chemical foaming methods, pores are formed with a gas generated in chemical reaction, it has been impossible to make the so-called cell-diameters 200 μm or less on average and the resultant foams have been unable to effectively absorb fine water droplets. Therefore, continuously porous foams produced by extraction methods using a pore generation agent are proposed.
For example, JP52-32971A (Patent Literature 1) discloses a porous molding produced by kneading a composition consisting of a thermoplastic polyurethane, calcium carbonate in a form of powder soluble to an acid, a surfactant or polyhydric alcohol, dimethylformamide and acetone, thermally molding the kneaded composition at 130° C., and immersing the resultant molding in an aqueous hydrochloric acid solution and then washing and drying.
In this process, the calcium carbonate is added as a pore generation agent, and pores are formed by extracting the calcium carbonate dispersed in the molding with acid and water followed by washing. In addition, the surfactant is added to facilitate extraction of the calcium carbonate with water.
As an alternative, JP58-189242A (Patent Literature 2) discloses a porous polymer foam produced by a process of dissolving a polyurethane in a solvent such as dimethylformamide, mixing the resultant solution with a composition compounded with a pore generation agent such as polyvinyl alcohol, filling in a predetermined mold to coagulate the polyurethane in an non-solvent for the polyurethane, followed by extracting out the pore generation agent with a large quantity of water.
Currently, in manufacturing precise products such as printed circuit boards and lead frames, continuous pore elastomers made of polyurethane are increasingly demanded in applications to water-absorbing rollers for cleanly and uniformly draining off water droplets adhered on the surfaces of water-washed product and to swabs for absorbing water adhered on the precise products. The continuous pore elastomers used for such applications are required to have a property of instantly absorbing water. Furthermore, for the application to water-absorbing rollers, also required is a property effectively spitting out water in compressing the rollers with an external force. However, it has been difficult for the above-mentioned continuous pore elastomers to provide such properties and they have not suited to the applications such as water-absorbing rollers and swabs. Accordingly, the development of continuous pore elastomers having excellent properties such as instantly absorbing water has been desired.
In addition, since such continuous pore elastomers have become widely used for applications of daily life and industries, the development of methods for producing the continuous pore elastomers massively, inexpensively and stably is desired. The above-mentioned production process described in Patent Literature 1, however, requires a heating at a high temperature in a step of producing kneaded composition, resulting in production cost increase and causing ingredient degradations. Furthermore, use of an organic solvent with low-boiling point causes a possibility of taking fire at a high temperature. Moreover, use of the aqueous hydrochloric acid solution requires neutralizing a liquid wasted after the use, also causing a cost increase.
On the other hand, the process of Patent Literature 2 requires about 1 week until the coagulation completes, for example, when the molding has a thickness of 20 mm, and also needs a mold for molding fabricated with an expensive porous material in its production. Particularly, for mass production, a great number of such molds are required, resulting in increase of the production cost. Therefore, the development of a process for producing a continuous pore elastomer, which is free from the problems of the conventional techniques, is desired.
Patent Literature 1: JP52-32971A
Patent Literature 2: JP58-189242A