A leather-like sheet such as an artificial leather and a synthetic leather is used in wide usage such as footwear and clothes. A basic configuration of the leather-like sheet is a layered structure composed of a surface skin layer, an adhesive layer, and a substrate layer; and a layer that is a basic fabric such as a nonwoven fabric or a woven fabric, a layer in which these basic fabrics are immersed in a resin such as a polyurethane resin, or a layer in which a porous layer composed of a resin such as a polyurethane resin is provided on the surface of the basic fabric, etc. is used as the substrate layer.
The leather-like sheet is conventionally manufactured, for example, by (1) a method of forming a surface skin layer having a leather-like uneven pattern by applying an organic solvent solution of a polyurethane resin onto a substrate layer with a gravure method, etc., drying and removing the organic solvent, and then pressing its coated surface using an embossing roll, etc., and (2) a method of laminating a surface skin layer onto a substrate layer with an adhesive, etc., where the surface skin layer is obtained by applying an organic solvent solution of a polyurethane resin onto a release paper having a leather-like uneven pattern, drying, and removing the organic solvent.
However, because a step of removing the organic solvent in the middle of the manufacturing of the leather-like sheet is necessary in the method of using the organic solvent solution of a polyurethyane resin as described above in the formation of the surface skin layer, there have been various problems such as a bad influence on the human body due to the organic solvent, contamination of the atmosphere and water, and a large energy load and manufacturing cost to evaporate the organic solvent.
As a method of solving the problems, a method of using an aqueous polyurethane resin in place of the organic solvent type polyurethane resin has been investigated. However, because a leather-like sheet having a surface skin layer that is formed using an aqueous polyurethane resin is generally inferior, for example, in water resistance and durability, it is true that practicality is poor and the application range of the aqueous polyurethane is extremely limited.
Then, a technique has been proposed of using a non-solvent type hot-melt urethane resin that does not substantially contain a solvent such as an organic solvent or water in place of the organic solvent type polyurethane resin.
For example, a leather-like sheet having a surface skin layer that is formed on a fibrous substrate using a moisture-curable polyurethane hot-melt resin composition containing a hot-melt urethane prepolymer having an isocyanate group in its molecule end and a coloring agent has been known to be excellent in designability, feeling, wear resistance, and suppleness as a leather-like sheet using the non-solvent type hot-melt urethane resin (for example, refer to Japanese Patent Laying-Open No. 2005-273131 (“JP '131”)).
In many cases of manufacturing a leather-like sheet using the non-solvent type hot-melt urethane resin as described above, a step of carrying out curing sufficiently is necessary by applying the non-solvent type hot-melt urethane resin onto a release paper, adhering a fibrous substrate onto the coated surface, winding the laminated body that is obtained by removing the release paper into a roll, and leaving it for a prescribed time.
However, because the winding into a roll is performed before the non-solvent type hot-melt urethane resin constituting the surface skin layer is cured completely in many cases, there is a problem that fuzz is generated on the surface of the surface skin layer and the surface grade of the leather-like sheet deteriorates in the case where the releasability between the surface skin layer and the back side of the leather-like sheet to which the surface skin layer contacts is poor when unwinding.
Here, because suppression of the fuzz generation described above is not considered in the moisture-curable polyurethane hot-melt resin composition described in JP '131, there was a case that fuzz is generated on the surface of the surface skin layer in the case of manufacturing a leather-like sheet using such composition.
In the meantime, it is expected that the non-solvent type hot-melt urethane resin whose application to the surface skin layer of the leather-like sheet is known will be developed into broad use as an adhesive, a coating agent and the like for construction materials and for fibers, etc., as an environmentally responsive, high added value product replacing the conventional solvent type resin and an aqueous resin.
A hot-melt urethane resin containing an isocyanate group that forms a cross-linking structure by reacting with, for example, moisture (water) in the atmosphere is known as such a hot-melt urethane resin. However, in recent years, a hot-melt urethane resin having both an isocyanate group that contributes to a cross-linking reaction by moisture and a polymerizable unsaturated bond that contributes to the cross-linking reaction by ultraviolet ray irradiation has attracted attention.
Specifically, a resin composition is known having an urethane(meth)acrylate resin that is obtained by reacting a polyisocyanate compound (a) having two or more isocyanate groups per molecule, at least one or more type of polyols (b) selected from polyesterpolyol, polycaprolactonepolyol, polyetherpolyol, and polycarbonatepolyol, and (meth)acrylate containing a hydroxyl group (c) under a condition that (the number of isocyanate groups in (a))>(the total number of hydroxyl groups in (b) and (c)) as a constituent component, wherein a concentration of a (meth)acryloyl group is 0.1 equivalent /kg to 2.0 equivalent /kg, and a concentration of an isocyanate group is 0.05 equivalent /kg or more (for example, refer to Japanese Patent Laying-Open No. 2005-307133 (“JP '133”)).
Further, a reactive hot-melt paint composition is known that contains an oligomer component having crystallinity and a melting point of 10° C. to 100° C. and a photoradical polymerization initiator of 1% by mass to 10% by mass based on the oligomer component, and that is a solid or a solid solution at 20° C. to 50° C. (for example, refer to Japanese Patent Laying-Open No. 2006-152142 (“JP '142”)).
The resin composition described in JP '133 is mainly used in the coating of a wooden base material, a plastic base material, and a paper base material, and the reactive hot-melt paint composition described in the Patent Document 3 is mainly used in the coating of a plywood and a bare wood product.
Therefore, even when such compositions are used in the surface skin layer of the leather-like sheet, it is difficult to suppress the fuzz generation on the surface skin layer (that is, the suppression of surface tack) to a sufficient practical level in addition to achieving an excellent suppleness and bending resistance that are required in the surface skin layer. Further, in any of JP '133 and JP '142, there is no description that the resin compositions can be used in the surface skin layer of the leather-like sheet.
As described above, it is difficult to obtain a leather-like sheet having a surface skin layer in which the fuzz generation is suppressed while maintaining an excellent suppleness, etc. with the techniques proposed in JP '131, JP '133, and JP '142.
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