Air-permeable sheet structural material, leather-like structural material and method of producing the same

Abstract: A sheet structural material (3) has a supporting fabric layer (1) and a porous layer (2) having 20 to 250 µm diameter open cells formed by applying and drying a foamed material containing at least a base resin and a filler. A leather-like sheet material (10) is obtained by pressing onto the porous layer the concavo-convex surface of a transfer paper (9) to which a film material (15) has been applied to occupy concave portions (14) of like surface, thereby to emboss the porous layer and transfer the film material as a layer (5) onto the convex portions. The resulting leather-like sheet material has an air permeability of 3 to 13 cm 3 /cm 2 /sec.

Description: The present invention relates to methods for producing a sheet material and imitation leather sheet material, which are superior in air permeability and can be used in artificial leather. More particularly, the present invention relates to the production of a sheet material and imitation leather sheet material, wherein noticeably high air permeability has been imparted by forming a porous layer having open cells on a supporting fabric. Since natural leather is superior in durability and air permeability, it has hitherto been used in various products such as clothes and shoes by making the best use of these characteristics. However, since natural leather is expensive, various leather-like sheet materials have been developed as substitutes for natural leather. At present, leather-like sheet materials are widely used as artificial leather in clothes and shoes. However, since artificial leather is produced by forming a film layer on a sheet structural material comprising a supporting fabric and a porous layer formed on the supporting fabric, it is at present inferior in air permeability and feel to natural leather. In an attempt to solve this problem the present inventors have previously invented an artificial leather having a feel and air permeability which are similar to those of natural leather (Japanese Patent Kokai Publication No. 8-232174). Artificial leather described in that publication is characterized in that a concavo-convex portion is formed on the surface of a sheet structural material having a porous layer and a film layer is formed only on the convex portion of the surface, thereby to form a leather-like appearance. With this constitution, since a film layer is not formed on the concave portion of the surface of the porous layer, good air permeability and the appearance of leather are provided. According to such a constitution, there can be obtained artificial leather having the same air permeability as that of natural leather. Natural leather and artificial leather are durable but are inferior in air permeability to normal cloth. Therefore, when they are used in shoes, the air permeability is inferior to that of shoes made of cloth. Accordingly, it is considered that broader usage as artificial leather is possible by developing a sheet material in which the air permeability is further improved while maintaining an appearance similar to that of natural leather. The porous layer to be formed on the supporting fabric has hitherto been produced by applying a coating solution containing a base resin and dimethylformamide (hereinafter abbreviated to "DMF") as a solvent on the supporting fabric and dipping in water to remove DMF. According to this method, there have arisen problems of environmental pollution such as wastewater disposal and recovery of DMF. Furthermore, since this method using DMF includes the step of drying after dipping in water, it has been necessary to include a step which is complicated and requires large energy consumption. The present invention seeks to solve these problems of the prior art and to provide a method for producing a sheet material and a leather-like sheet material, which are suitable for use as an artificial leather having a superior air permeability compared to natural leather, which involves comparatively simple manufacturing steps without causing any environmental pollution problem. US Patent 4,029,534 describes the production of composite materials consisting of layers of plastics sheet, polyurethane ionomer latex foam and textile material. Those composite materials are said to be distinguished by their full handle, elegant drape and softness. Nevertheless, the polyurethane ionomer latex foam has a density of only 0.04 to 0.40 g/cm 3 , and therefore has insufficient structural integrity for use as an imitation leather material in anything other than the most benign situations. In accordance with the invention, there is provided a method for producing an air permeable sheet material comprising the steps of: foaming to an expansion ratio of 1.3 to 2.5 a composition containing at least an aqueous emulsion or dispersion of a base resin and from 5 to 100 parts by weight, based on 100 parts of the solid content of the base resin, of a filler thereby to obtain a foamed material having a thixotropy index (the ratio η12/η60 of the viscosity at 12 rev/sec to the viscosity at 60 rev/sec measured in a B type rotary viscometer) of 2 to 4; applying the foamed material to one surface of an air-permeable, polymer-impregnated supporting fabric thereby to form a foamed layer of predetermined thickness on the fabric; and drying the foamed layer by subjecting the free surface of the foamed layer to far infrared radiation thereby to form thereon a dry surface film and thereafter drying the remainder of the foamed layer thereby to obtain an open cell porous layer having cell diameters of 20 to 250 µm. The invention provides also a method for producing an imitation leather material which comprises the steps of: applying film material to the concavo-convex surface of a transfer paper thereby to occupy the concave portions of the surface with the film material; applying the concavo-convex surface of the transfer paper to the surface of the foamed layer of the sheet material produced by the method of the invention as described above; pressing together the transfer paper and the sheet material thereby to transfer the shape of the concavo-convex surface of the transfer paper onto the sheet material and to transfer the film material in the concave portions of the surface of the transfer paper onto the resulting convex portions of the surface of the sheet material; and removing the transfer paper from the surface of the sheet material thereby to obtain an imitation leather material. The invention is described below in greater detail by way of example only with reference to the accompanying drawings, in which: Figs. 1(a) and 1(b) are sectional views illustrating the production of a leather-like sheet material according to the invention; Fig. 2 is a sectional view showing a leather-like sheet material produced according to the invention; Figs. 3(a) and 3(b) are electron micrographs of the surface and a section, respectively, of a sheet material produced in one example of the invention, the magnification in Fig. 3(a) being 1,000x and the magnification in Fig. 3(b) being 100x; Figs. 4(a) and 4(b) are electron micrographs of the surface and a section, respectively, of an artificial leather of the prior art, the magnification in Fig. 4(a) being 1,000x and the magnification in Fig. 4(b) being 100x; and Figs. 5(a) and 5(b) are electron micrographs of the surface and a section, respectively, of a natural leather, the magnification in Fig. 4(a) being 1,000x and the magnification in Fig. 4(b) being 100x. The sheet material produced by the method of the invention comprises an air-permeable supporting fabric and a porous layer formed on the supporting fabric, the porous layer having open cells and the diameter of the open cells being 20 to 250 µm. By forming open cells having such a size, there is obtained a sheet material having air permeability, which is noticeably superior to that of sheet materials of the prior art. The sheet material produced by the method of the invention will generally have an air permeability of 10 to 20 cm 3 /cm 2 /sec. The term "air permeability" used herein refers to a numerical value obtained by the method described in JIS L-1096. Such a porous layer is formed by applying to the air-permable supporting fabric a foamed material having a thixotropy index of 2 to 4, which is obtained by foaming an aqueous composition containing at least a base resin and a filler, and drying the foamed material. The term "thixotropy index" used herein refers to the ratio of the viscosity (η 12) measured at 12 revolution/sec to the viscosity (η 60) measured at 60 revolution/sec using a B type rotatory viscometer, that is, the numerical value obtained from η 12/η 60. The leather-like sheet produced according to the invention has a leather-like concavo-convex surface formed on the surface of the porous layer of the sheet material and a film layer is formed on the convex portion of this concavo-convex surface. Consequently, the leather-like sheet material maintains air permeability by virtue of the concave portions where the film layer is not formed, thereby achieving a high air permeability of 3 to 13 cm 3 /cm 2 /sec. As is apparent from the fact that the air permeability of normal natural leather is not more than 1.0 cm 3 /cm 2 /sec, the leather-like sheet material produced by the invention has excellent air permeability. A sheet material produced by the invention comprises an air-permeable supporting fabric and a porous layer formed on the supporting fabric. The porous layer has an open cell structure and the diameter of the open cells is 20 to 250 µm The air permeability of the sheet material is then within the range of 10 to 20 cm 3 /cm 2 /sec. As the air-permeable supporting fabric, there can be used for example, nonwoven fabrics woven fabrics and knitted fabrics. The supporting fabric for leather-like sheet material is preferably a nonwoven fabric. Any non-woven fabric can be used and examples of nonwoven fabrics include those produced by the water jet, span lace and needle punch methods. The porous layer to be formed on the air-permeable supporting fabric is by applying a foamed material having a thixotropy index of 2 to 4, which is obtained by foaming an aqueous composition containing at least a base resin and a filler, and then drying the foamed material. The viscosity of the foamed material is preferably from 5 to 35 Pa.s (5,000 to 35,000 centipoises), more preferably from 16 to 22 Pa.s (16,000 to 22,000 cp), and particularly from 18 to 20 Pa.s (18,000 to 20,000 cp). When the viscosity of the foamed material is lower than 5 Pa.s (5,000 centipoises) the cells are liable to be broken when forming the porous layer. On the other hand, when the viscosity is higher than 35 Pa.s (35,000 centipoises) it becomes substantially impossible to apply the foamed material to the supporting fabric. The composition before foaming preferably has a viscosity of 5 to 30 Pa.s (5,000 to 30,000 centipoises) and more preferably 12 to 15 Pa.s (12,000 to 15,000 centipoises). It is preferred to use a composition having a viscosity within such a range in order to obtain a foamed material having a viscosity within the above range. Furthermore, the expansion ratio of the composition upon foaming is from 1.3 to 2.5. When the expansion ratio is lower than 1.3, the resulting sheet material becomes hard and is not suitable as a basis for imitation leather. On the other hand, when the expansion ratio is larger than 2.5 the peeling strength between the supporting fabric and the porous layer of the resulting sheet material is unfavorably reduced. When the linitation leather material is to be used for sports shoes and a high peeling strength is required, the expansion ratio is preferably from 1.4 to 1.7. It is preferred also to use polyurethane as the base resin in order to secure the flexibility. It is preferred that the composition to be used for forming the porous layer contains one or more additives such as dispersants, foam stabilizers, foaming assist agents, thickeners, in order suitably to adjust the thixotropy index and the viscosity within the above range, in addition to containing the base resins and filler. It is also preferred that the composition contains an elasticizer for imparting elasticity to the porous layer and a crosslinking agent for crosslinking the base resin. The elasticizer operates by preventing the cells being broken by pressure and by enabling the walls of the cells to be adhered to each other and, as a result, the open cell structure is retained. Furthermore, it is possible to optionally add various other additives such as pigments, as a matter of course. As the base resin to be contained in the composition, those resins having good foamability are suitable. Examples include acrylic polymers such as polyacrylate esters, polymethacrylate esters and copolymers thereof; polyurethanes; diene polymer such as synthetic rubber, natural rubbers latex and a mixtures thereof. This base resin can be used in the form of an emulsion or dispersion. Those base resins having high solid contents, low TG (glass transition temperature), good frothing properties and a small content of defoamer are suitable in view of the required foamability. The above composition contains a filler for imparting thixotropic properties to the foamed material. Examples of the filler, which can be used, include clay, aluminum hydsoxide and calcium carbonate. The content of the filler is from 5 to 100 parts by weight based on 100 parts by weight of the solid content of the base resin. Examples of the dispersant which may be included in the composition include low-molecular weight sodium polycarboxylate and sodium tripolyphosphate. The content of any dispersant is preferably from 0.2 to 2 parts by weight based on 100 parts by weight of the solid content of the base resin. Examples of the foam stabilizer which may be included in the composition include ammonium long-chain-alkylcarboxylates such as ammonium stearate. The content of any foam stabilizer is preferably from 1 to 8 parts by weight based on 100 parts by weight of the solid content of the base resin. The composition may contain foaming assist agents. Examples of the foaming assist agent include sodium dialkylsulfosuccinate. The content of any foaming assist agent is preferably from 1 to 7 parts by weight based on 100 parts by weight of the solid content of the base resin. The composition may contain thickeners together with the above fillers for imparting thixotropic properties, to stabilize the formed cells. Examples of the preferred thickeners include ammonium polyacrylate, and polyacrylic acid. The content of any thickener is preferably from 0.5 to 5 parts by weight based on 100 parts by weight of the solid content of the base resin. When the base resin is self-crosslinkable to some extent, it cures after an appropriate lapse of time. When using a base resin whose curing rate is slow crosslinking agents are preferably added. Examples of the preferred crosslinking agent include isocyanates. The content of any crosslinking agent is preferably from 1 to 5 parts by weight based on 100 parts by weight of the solid content of the base resin. Depending upon the properties of the base resin to be used, when cells after forming a porous layer are broken by pressure and the walls of the cells are adhered and, as a result, the cells are not returned to their original state, elasticizers are preferably added. Examples of the preferred elasticizer include silicone oil. The content of any elasticizer is preferably from 0.5 to 1.5 parts by weight based on 100 parts by weight of the solid content of the base resin. The leather-like sheet material is produced using the above sheet material. The leather-like sheet material has a discontinuous leather-like film layer on the porous layer of the sheet material. That is, a leather-like concavo-convex surface is formed on the porous layer and, at the same time, a film layer is formed on only the convex portion of this concavo-convex surface. The air permeability of the leather-like sheet material is from 3 to 13 cm 3 /cm 2 /sec. A method for producing the sheet material and leather-like sheet material will now be described. First, a nonwoven fabric as a supporting fabric is impregnated with an aqueous emulsion of a polyurethane or an acrylic polymer and the emulsion is squeezed using a mangle, followed by drying. Pigments may be included in the emulsion to impart a selected color to the final product. The dried polymer-impregnated nonwoven fabric is then wound up to form a roll having a specified size, using a wind-up machine. The coating composition is prepared by adding any or all of the above dispersants, foam stabilizers, fillers, foaming assist agents, thickeners, elasticizers and crosslinking agents, to an emulsion or dispersion containing a base resin, and the mixture is sufficiently stirred to be well dispersed, thereby to obtain a stable composition. The solids content of the composition is preferably 50 to 60% by weight. Such a composition with high solid content has considerably high viscosity and is liable to gel but has an advantage that it can be dried in a short time because of its low water content. This composition is foamed using a high-speed mixer and air is thereby incorporated as cells as small as possible. The expansion ratio (ratio of the volume after foaming to the original volume of the composition) depends on the final product, but is from 1.3 to 2.5. As a result of such high-speed mixing, the resulting composition has thixotropic properties. The composition foamed as described above is then continuously applied onto the polymer-impregnated nonwoven fabric to a predetermined thickness by using a doctor knife coater. When the composition is applied using a doctor knife, smoothing of the composition is caused by shear. The thickness of the composition will depend upon that of the leather-like sheet material obtained finally. Since the foamed composition has thixotropic properties, it has a stable structure immediately it is applied. The foamed structure of the composition in this state is not easily broken and is maintained even in the following drying step, apparently because the composition has a high solids content, and because drying can be completed in a short time. The porous layer is formed by drying the composition on the supporting fabrics and, in order to prevent breakage of the foamed state only the surface is first dried by using far infrared radiation to form a thin dry surface film, followed by hot-air drying using a pin tenter dryer. Retention of the foamed state can be confirmed by the fact that the thickness of the coated layer in the wet state after applying the composition, and the thickness after drying, are almost the same. The sheet material is obtained by evaporating water from the composition. The leather-like sheet material is produced by using the sheet material in the same manner as that described in Japanese Patent Kokai Publication No. 8-232174, as shown in Figs. 1(a) and 1(b) of the accompanying drawings. First, as shown in Fig. 1(a), a film material 15 is applied to on only concave portions 14 of a transfer paper 9 having a concavo-convex surface shape, which is reverse to the leather-like concavo-convex surface, to fill the concave portion with the film material. The film material 15 contains 10 to 30% resin, 5 to 10% pigment and a solvent, normally. Then, the transfer paper 9, whose concave portion 14 is filled with the film material 15, is laid so that the surface coated with the film material 15, that is, the upper surface in Fig. 1(a) is brought into contact with the porous layer 2 of the sheet material 3, as shown in Fig. 1(b). The transfer paper 9 and sheet material 3 are then pressed together with heating by using a roller. By this pressing with heating, the leather-like reverse concavo-convex shape of the transfer paper 9 is transferred to the sheet material 3 and, at the same time, the film material 15 (Fig. 1(b)) is transferred as a film layer 5 to the convex portions 6 of the transferred concavo-convex surface of the sheet material 3, as shown in Fig. 2. Thereafter, a leather-like sheet material 10 shown in Fig. 2 is obtained by cooling the transfer paper 9 and sheet material 3 and peeling the transfer paper 9 from the sheet material 3. The sheet material 3 of this leather-like sheet material 10 comprises an air-permeable supporting fabric 1 and the porous layer 2 having open cells with a diameter of 20 to 250 µm formed on the supporting fabric 1. Since the porous layer 2 has the leather-like concavo-convex surface and the film layer 5 is formed only on the convex portions 6 of this concavo-convex surface, high air permeability is obtained. The following Examples illustrate the invention. Table 1 shows the constitution of various compositions used in the production of the sheet material and leather-like sheet material according to the present invention. These compositions are converted into porous layers by foaming applied to the supporting fabric and dried. A sheet material was produced using the composition of Example 1 in Table 1 and electron micrographs were taken of the surface and section of the porous layer formed after the drying step (Figs. 3(a) and 3(b)). For comparison, there were taken also electron micrographs (Figs. 4(a) and 4(b)) of artificial leather of the prior art produced using DMF and electron micrographs (Figs. 5(a) and 5(b)) of natural leather. As is apparent from the electron micrographs shown in Figs. 3(a) and 3(b), the diameter of cells formed in the leather-like material produced according to the present invention is within the ranged 20 to 250 µm. The formed cells are open cells, and it is considered that the air permeabilities of the sheet material and leather-like sheet material are improved by the formation of these open cells. As is apparent from a comparison between Fig. 4, Fig. 5 and Fig. 3, open cells having a diameter within the range 20 to 250 µm are not present in either the artificial leather of the prior art or natural leather. Using the compositions of Examples 2 to 7, sheet materials were produced and their electron micrographs were taken (not shown). It was found that open cells having a diameter within the range 20 to 250 µm are also formed in these sheet materials and these cells are also open cells, similar to those shown in Figs. 3(a) and 3(b). The air permeability of the sheet materials produced from the compositions of Example 1 to 7 and leather-like materials produced from these sheet materials was determined according to JIS L-1096 and compared with that of the sheet material of the prior art and of natural leather. The results are shown in Table 2. As can be seen from Table 2, the air permeability of each of the sheet materials of Examples 1 to 7 was noticeably superior to that of the sheet material of the prior art. The air permeability of each of the leather-like sheet materials of the respective Examples was also noticeably superior to that of natural leather. The products of Examples 6 and 7 are leather-like sheet materials best suited for sport shoes, in which high peeling strength between the supporting fabric and the porous layer and high flexibility are required. In order to measure the peeling strength of the sheet materials of Examples 6 and 7, the following test was performed The sheet materials of Examples 6 and 7 were cut into two test pieces 3 cm in width. These two test pieces were bonded with their porous layers facing each other, except for one end, using an adhesive, to obtain a test sample. This sample was stretched at a constant rate (20 mm/min) while gripping one non-adhered end using a tensile tester, and the tensile strength was measured. The tensile strength of both sheet materials of Examples 6 and 7 was found to be 7.5 kg/3cm (2.5 kg/cm). Both sheet materials of Examples 6 and 7 were soft to the touch and both materials had a feel suited for sports shoes. It is apparent from these results that the sheet materials of Examples 6 and 7 can be used as artificial leather suited for sports shoes. The sheet materials produced according to the present invention have better air permeability than natural leather because they contain open cells having a diameter of 20 to 250 µm. According to the method of the present invention, the leather-like sheet material can be produced without using DMF. Therefore, no environmental pollution problems arise. Furthermore, since the step of dipping in water to remove DMF is not required the sheet material can be obtained by a simple step of drying with heating. In forming the leather-like material, since a film layer is formed only on the concave portion of the surface of the sheet material as described above, the air permeability of the sheet material is not adversely affected. Accordingly, the leather-like sheet material has improved air permeability over the leather-like sheet material of the prior art and superior air permeability compared to natural leather.

Claims: A method for producing an air-permeable sheet material comprising the steps of: foaming to an expansion ratio of 1.3 to 2.5 a composition containing at least an aqueous emulsion or dispersion of a base resin and from 5 to 100 parts by weight, based on 100 parts of the solid content of the base resin, of a filler thereby to obtain a foamed material having a thixotropy index (the ratio η12/η60 of the viscosity at 12 rev/sec to the viscosity at 60 rev/sec measured in a B type rotary viscometer) of 2 to 4; applying the foamed material to one surface of an air-permeable, polymer-impregnated supporting fabric thereby to form a foamed layer of predetermined thickness on the fabric; and drying the foamed layer by subjecting the free surface of the foamed layer to far infrared radiation thereby to form thereon a dry surface film and thereafter drying the remainder of the foamed layer thereby to obtain an open cell porous layer having cell diameters of 20 to 250 µm. A method according to claim 1, wherein the composition to be foamed has a solids content of 50 to 60% by weight. A method according to claim 1 or claim 2, wherein the composition to be foamed has a viscosity of 5 to 30 Pa.s. A method according to claim 3, wherein the composition to be foamed has a viscosity of 12 to 15 Pa.s. A method according to any one of claims 1 to 4, wherein the composition to be foamed contains one or more additives selected from dispersants, foam stabilisers, foam assist agents, thickeners, elasticisers, pigments and cross-linking agents. A method according to any one of claims 1 to 5, wherein the foamed composition has a viscosity of 5 to 35 Pa.s. A method according to claim 6, wherein the foamed composition has a viscosity of 16 to 22 Pa.s. A method according to claim 6, wherein the foamed composition has a viscosity of 18 to 20 Pa.s. A method according to any one of claims 1 to 8, wherein the base resin is a polyurethane and the composition is foamed to an expansion ratio of 1.4 to 1.7. A method according to any one of claims 1 to 9, wherein the air permeable sheet material has an air permeability of 10 to 20 cm 3 /cm 2 /sec, as determined according to JIS-L-1096. A method for producing an imitation leather material which comprises the steps of: applying film material to the concavo-convex surface of a transfer paper thereby to occupy the concave portions of the surface with the film material; applying the concavo-convex surface of the transfer paper to the surface of the foamed layer of the sheet material produced by a method according to any one of claims 1 to 10; pressing together the transfer paper and the sheet material thereby to transfer the shape of the concavo-convex surface of the transfer paper onto the sheet material and to transfer the film material in the. concave portions of the surface of the transfer paper onto the resulting convex portions of the surface of the sheet material; and removing the transfer paper from the surface of the sheet material thereby to obtain an imitation leather material. A method according to claim 11, wherein the imitation leather material has an air permeability of 3 to 13 cm 3 /cm 2 /sec as determined according to JIS-L-1096.

Inventors: KIM SOOJIN, KIM, SOOJIN
Applicants: KOATSU CLOTH CO LTD, KOATSU CLOTH CO., LTD.
Application year: 1998
Main classification: D06N3
IPC codes: B32B5, D06N3