JP7578480B2 - Manufacturing method for grain-finished artificial leather - Google Patents

Description

The present invention relates to a method for producing grain-finish leather-like sheets that reduce the amount of residual volatile organic compounds.

Grain-finish leather-like sheets, such as grain-finish artificial leather, which has a grain-finish appearance similar to grain-finish leather and is used as the covering material for bags, clothing, shoes, etc., are known.

In the manufacturing process of grain-finish leather-like sheets, volatile organic compounds (VOCs) such as N,N-dimethylformamide (DMF) are often used as a polyurethane solvent. In recent years, there have been strict demands for a reduction in VOC emissions due to legal regulations and voluntary restrictions, etc., in order to prevent air pollution.

In the manufacture of grain-finish leather-like sheets, DMF is often used as the solvent for the polyurethane solution for the adhesive layer used to bond the grain-finish resin layer to the fiber substrate. In the manufacturing process of grain-finish leather-like sheets, it is necessary to recover as much DMF as possible that is generated during the process. For this reason, a manufacturing method has been realized in which the fiber substrate manufacturing process and the process of impregnating the fiber substrate with polyurethane are made aqueous using a water-based solvent as a clean process that does not use DMF.

In the manufacturing process of water-based grain-finish leather-like sheets, if the polyurethane adhesive is made water-based, it is possible to further reduce VOC emissions. Such a proposal has been made. For example, the following Patent Document 1 discloses a two-component curing water-based adhesive used in the manufacture of synthetic leather, which contains a base agent containing an aqueous polyurethane resin and a curing agent containing a polyisocyanate compound, and the polyisocyanate compound has a biuret bond or is modified with trimethylolpropane.

However, water-based polyurethane adhesives still have problems, such as difficulty in obtaining sufficiently high adhesive strength and sufficient durability compared to polyurethane adhesives containing organic solvents. For this reason, for example, in the manufacture of grain-finish leather-like sheets used in applications requiring high durability such as vehicle seats, polyurethane solutions containing solvent-based polyurethane and organic solvents such as DMF are still used as polyurethane adhesives. For this reason, there is a strong demand for technology to reduce the organic solvents remaining in grain-finish artificial leather that uses solvent-based polyurethane.

JP 2010-229267 A

The present invention aims to provide a method for producing a grain-finish leather-like sheet that can reduce the VOCs remaining in grain-finish artificial leather when a polyurethane solution containing an organic solvent is used to bond a fiber substrate and a grain-finish surface resin layer.

One aspect of the present invention includes a step of forming a polyurethane surface resin layer on a release paper, a step of applying a polyurethane solution for an adhesive layer to the surface of the polyurethane surface resin layer, a step of forming a polyurethane adhesive layer in a semi-dried state by heating the polyurethane solution for the adhesive layer applied to the surface of the polyurethane surface resin layer under a first heating condition, a step of bonding a fiber substrate and the polyurethane surface resin layer via the polyurethane adhesive layer to form a laminated integrated sheet, a step of heating the laminated integrated sheet under a second heating condition, a step of peeling the release paper from the heated laminated integrated sheet to form a grain-finished leather-like sheet, and a step of heating the grain-finished leather-like sheet under a third heating condition, wherein the polyurethane solution for the adhesive layer contains polyurethane, 20 to 50 mass % of a low-boiling point organic solvent having a boiling point of 70 to 100 ° C., and 6 to 15 mass % of a high-boiling point organic solvent having a boiling point of 120 to 160 ° C., and the coating amount of the polyurethane solution for the adhesive layer is 60 to 160 g / m ² , and the coating amount of the high-boiling point organic solvent is 22 g / m ² or less, the first heating condition includes heating at a temperature 5 to 15° C. lower than the boiling point of the low-boiling organic solvent for 20 seconds or more during the temperature increase process, and the second and third heating conditions include heating at a temperature 10 to 40° C. lower than the boiling point of the high-boiling organic solvent for 30 seconds or more during the temperature increase process. Examples of the low-boiling organic solvent include methyl ethyl ketone, and examples of the high-boiling organic solvent include N,N-dimethylformamide.

The present invention provides a method for producing grain-finished leather-like sheets that can reduce the amount of residual VOCs when using a polyurethane solution containing an organic solvent to bond a grain-finished surface resin layer to a fiber substrate.

FIG. 1 is a schematic cross-sectional view of a grain-finished leather-like sheet 10 produced by the method for producing a grain-finished leather-like sheet of the present embodiment.

Figure 1 is a schematic cross-sectional view of a grain-finished leather-like sheet 10 manufactured by the method for manufacturing grain-finished leather-like sheets of this embodiment.

Referring to FIG. 1, the grain-finished leather-like sheet 10 comprises a fiber substrate 1 and a polyurethane resin layer 3 bonded to one surface of the fiber substrate 1 via a polyurethane adhesive layer 2. The resin layer including the polyurethane adhesive layer 2 and the polyurethane resin layer 3 forms a grain-finished resin layer. The grain-finished resin layer is a layer that gives the grain-finished leather-like sheet 10 an appearance and feel similar to the grain surface of natural leather. The polyurethane resin layer 3 may be a single layer or may have a multi-layer structure consisting of an intermediate layer and a top layer. The grain-finished leather-like sheet 10 is a grain-finished leather-like sheet with a low residual amount of VOCs, which is manufactured by the manufacturing method described below.

Hereinafter, one embodiment of the method for producing the grain-finished leather-like sheet of the present invention will be described in detail. The method for producing the grain-finished leather-like sheet of this embodiment includes the steps of forming a polyurethane surface resin layer (hereinafter also simply referred to as a surface resin layer) on a release paper, applying a polyurethane solution for adhesive layer to the surface of the polyurethane surface resin layer, heating the polyurethane solution for adhesive layer applied to the surface of the polyurethane surface resin layer under a first heating condition to form a semi-dried polyurethane adhesive layer (hereinafter also simply referred to as an adhesive layer), bonding the fiber base material and the polyurethane surface resin layer via the polyurethane adhesive layer to form a laminated integrated sheet, heating the laminated integrated sheet under a second heating condition, peeling off the release paper from the heated laminated integrated sheet to form a grain-finished leather-like sheet, and heating the grain-finished leather-like sheet under a third heating condition. In this manufacturing method, the polyurethane solution for the adhesive layer contains polyurethane, 20 to 50 mass% of a low-boiling organic solvent having a boiling point of 70 to 100 ° C., and 6 to 15 mass% of a high-boiling organic solvent having a boiling point of 120 to 160 ° C., the coating amount is 60 to 160 g / m ² , and the coating amount of the high-boiling organic solvent is 22 g / m ² or less, the first heating condition includes heating in a temperature range of 5 to 15 ° C. lower than the boiling point of the low-boiling organic solvent for 20 seconds or more during the temperature increase process, and the second heating condition and the third heating condition include heating in a temperature range of 10 to 40 ° C. lower than the boiling point of the high-boiling organic solvent for 30 seconds or more during the temperature increase process. Each step will be described in detail below.

[Step of forming a surface resin layer on release paper]
In the manufacturing method of this embodiment, a film of a surface resin layer to be adhered to the surface of a fiber substrate is formed on a release paper. The film of the surface resin layer is formed, for example, as follows.

A polyurethane solution or molten resin is applied onto the release paper to form the surface resin layer, and then solidified to form a film of the surface resin layer.

Examples of polyurethanes for forming the surface resin layer include polycarbonate-based polyurethanes, polyether-based polyurethanes, polyester-based polyurethanes, and copolymer polyurethanes such as polycarbonate-polyether copolymer polyurethanes and polycarbonate-polyester copolymer polyurethanes.

One method for forming a surface resin layer by applying a polyurethane solution onto release paper is to apply the polyurethane solution onto release paper and then dry it. The surface resin layer may have a single layer structure formed from the same polyurethane, or may be a laminate having a multilayer structure consisting of an intermediate layer and a top layer obtained by laminating different types of polyurethane. The surface resin layer may or may not have a foamed structure.

The surface resin layer may contain colorants and other additives, if necessary. The release paper is not particularly limited, but it is preferable to use a smooth release paper or a release paper with an uneven surface to give the surface a grained pattern.

The thickness of the surface resin layer is not particularly limited, but is preferably 10 to 200 μm, and more preferably 20 to 100 μm.

[Step of forming a semi-dried adhesive layer by heating the polyurethane solution for adhesive layer applied to the surface of the surface resin layer under the first heating condition]
Next, in this process, a polyurethane solution for the adhesive layer is applied to the surface of the surface resin layer formed on the release paper as described above, and then heating is performed under the first heating conditions to form a semi-dry adhesive layer.

The polyurethane solution for the adhesive layer contains polyurethane and a specific solvent for dissolving the polyurethane, as described below. If necessary, it may also contain a crosslinking agent or a crosslinking accelerator. The polyurethane solution for the adhesive layer may also contain additives such as antioxidants, light resistance agents, and antibacterial agents, in addition to colorants such as pigments.

Examples of polyurethanes contained in the polyurethane solution for the adhesive layer include polycarbonate-based polyurethanes, polyether-based polyurethanes, polyester-based polyurethanes, and copolymer polyurethanes such as polycarbonate-polyether copolymer polyurethanes and polycarbonate-polyester copolymer polyurethanes. Of these, polycarbonate-based polyurethanes and polyether-based polyurethanes are particularly preferred because they have excellent hydrolysis resistance and therefore make it easier to ensure durability.

The concentration of the polyurethane-containing solids contained in the polyurethane solution for the adhesive layer of this embodiment is preferably 10 to 70% by mass, and more preferably 20 to 60% by mass. If the concentration of the polyurethane-containing solids is too high, the viscosity tends to become too high and the coatability tends to decrease, and if the concentration of the polyurethane-containing solids is too low, it tends to become difficult to adjust the thickness of the coating film.

The polyurethane solution for the adhesive layer contains polyurethane, 20 to 50% by mass of a low-boiling organic solvent with a boiling point of 70 to 100°C, and 6 to 15% by mass of a high-boiling organic solvent with a boiling point of 120 to 160°C.

Examples of low-boiling organic solvents with a boiling point of 70 to 100°C include methyl ethyl ketone (MEK, boiling point 79.64°C), ethyl acetate (AcOEt, boiling point 77.1°C), isopropyl alcohol (IPA, boiling point 82.4°C), ethanol (EtOH, boiling point 78.4°C), and acetonitrile (AcCN, boiling point 82°C). Of these, methyl ethyl ketone is particularly preferred because it has little odor and is easy to handle.

Examples of high-boiling organic solvents with a boiling point of 120 to 160°C include N,N-dimethylformamide (DMF, boiling point 153°C), cyclohexanone (CH, boiling point 155.65°C), and butyl acetate (boiling point 126°C). Of these, N,N-dimethylformamide is particularly preferred because of its high solubility in the polyurethane resin for the adhesive layer.

The polyurethane solution for the adhesive layer may also contain solvents other than low-boiling point organic solvents and high-boiling point organic solvents as necessary. Specific examples of such solvents other than low-boiling point organic solvents and high-boiling point organic solvents include ultra-low boiling point organic solvents with a boiling point of less than 70°C and medium boiling point organic solvents with a boiling point of 100 to 120°C. It is preferable not to contain ultra-high boiling point organic solvents with a boiling point of more than 160°C, as they are difficult to volatilize and tend to remain.

Examples of ultra-low boiling organic solvents with a boiling point of less than 70°C include acetone (AC, boiling point 56.5°C) and methyl acetate (AcOMe, boiling point 57°C). Examples of medium boiling organic solvents with a boiling point of 100 to 120°C include 1,4-dioxane (DO, boiling point 101.3°C) and toluene (PhMe, boiling point 110.63°C). Examples of ultra-high boiling organic solvents with a boiling point of more than 160°C include dimethylacetamide (DMAc, boiling point 165°C), diethylformamide (DEF, boiling point 177°C), diethylacetamide (DEAc, boiling point 182-186°C), and N-methylpyrrolidone (NMP, boiling point 202°C).

The polyurethane solution for the adhesive layer contains 20 to 50 mass %, preferably 25 to 40 mass %, of a low-boiling organic solvent at 70 to 100°C. If the content of the low-boiling organic solvent is less than 20 mass %, the content of the high-boiling organic solvent will be relatively high, making it easier for VOCs to remain. If the content of the low-boiling organic solvent exceeds 50 mass %, the volatility of the polyurethane solution for the adhesive layer will be high, and the stability of the concentration and viscosity over time of the polyurethane solution for the adhesive layer will tend to be low.

The polyurethane solution for the adhesive layer contains 6 to 15 mass %, preferably 7 to 14 mass %, of a high-boiling organic solvent with a boiling point of 120 to 160°C. If the content of the high-boiling organic solvent is less than 6 mass %, the content of the low-boiling organic solvent will be relatively high, resulting in high volatility of the solvent, low stability of the polyurethane solution for the adhesive layer, and poor adhesive processability. Furthermore, the adjustment range of the solubility parameter of the polyurethane solution for the adhesive layer will be narrow, so that the polyurethane solution for the adhesive layer will swell the surface resin layer, and problems such as the surface resin layer lifting off the release paper before being attached to the fiber substrate will easily occur. If the content of the high-boiling organic solvent exceeds 15 mass %, VOCs will easily remain.

The polyurethane solution for the adhesive layer may contain 30% by mass or less, or even 15% by mass or less, of an ultra-low boiling organic solvent having a boiling point of less than 70°C. If the solution contains 30% by mass or more of an ultra-low boiling organic solvent, the volatility of the solvent becomes too high, and the stability of the solution over time tends to decrease. The polyurethane solution for the adhesive layer may contain 30% by mass or less, or even 20% by mass or less, of a medium boiling organic solvent, and 1% by mass or less, or even 0.1% by mass or less, of an ultra-high boiling organic solvent. If the solution contains 30% by mass or more of a medium boiling organic solvent or 1% by mass or more of an ultra-high boiling organic solvent, the amount of VOC remaining in the grain-finished artificial leather tends to increase.

A preferred combination for the polyurethane solution for the adhesive layer as described above is a mixed solvent containing MEK and DMF, specifically, 20 to 70 mass %, or even 25 to 60 mass %, MEK, 5 to 15 mass %, or even 6 to 14 mass %, DMF, with the remainder optionally containing other solvents, as this makes it difficult for VOCs to remain on the grain-finish leather-like sheet.

There are no particular limitations on the method for applying the polyurethane solution for the adhesive layer to the surface of the polyurethane surface resin layer, and a comma coater, a comma reverse coater, a knife coater, a gravure coater, etc. can be used. Among these, it is preferable to apply the solution using a comma coater, since it is easy to control the amount of coating.

The coating amount of the polyurethane solution for the adhesive layer is 60 to 160 g/m ² , preferably 70 to 150 g/m ² , and more preferably 80 to 140 g/m ^2. By coating the polyurethane solution for the adhesive layer on the surface resin layer formed on the release paper in such a coating amount, VOCs are less likely to remain in the grain-finished leather-like sheet. If the coating amount of the polyurethane solution for the adhesive layer exceeds 160 g/m ² , heat is less likely to be transmitted to the entire coating film in the drying process, and only the surface layer part is preferentially dried and formed into a film, and the organic solvent inside it is less likely to volatilize. As a result, VOCs are more likely to remain. Also, if the coating amount of the polyurethane solution for the adhesive layer is less than 60 g/m ² , it is difficult to form an adhesive layer of sufficient thickness.

The adhesive layer polyurethane solution is applied in such an amount that the amount of the high-boiling point organic solvent applied is 22 g/m2 ^or less, preferably 20 g/m2 ^or less. By applying the adhesive layer polyurethane solution in such an amount that the amount of the high-boiling point organic solvent applied is 22 g/ ^m2 or less, the high-boiling point organic solvent is less likely to remain on the grain-finished leather-like sheet after going through each heating step described below. If the amount of the high-boiling point organic solvent applied exceeds 22 g/ ^m2 , the high-boiling point organic solvent is more likely to remain on the grain-finished leather-like sheet.

Then, the polyurethane solution for the adhesive layer applied to the surface resin layer formed on the release paper is heated under the first heating conditions to volatilize a portion of the organic solvent, thereby forming a semi-dried polyurethane adhesive layer with appropriately adjusted viscosity.

The first heating is performed under first heating conditions including heating for 30 seconds or more at a temperature range of 5 to 15°C lower than the boiling point of the low-boiling point organic solvent contained in the polyurethane solution for the adhesive layer during the temperature increase process. When two or more types of low-boiling point organic solvents are contained, the boiling point of the low-boiling point organic solvent with the lowest boiling point is used as the reference. In addition, the temperature increase process refers to the process of increasing the temperature from a low temperature side to a high temperature side, and not the process of decreasing the temperature from a high temperature side to a low temperature side.

The first heating step is a process for preferentially volatilizing a portion of the low-boiling organic solvent contained in the polyurethane solution for the adhesive layer, and adjusting the viscosity of the polyurethane solution for the adhesive layer when it is bonded to the fiber substrate. This first heating step forms a polyurethane adhesive layer in a semi-dry state.

Here, a semi-dry polyurethane adhesive layer means a state in which a portion of the low-boiling point solvent contained in the polyurethane solution for the adhesive layer has been preferentially evaporated. Specifically, it is a state in which about 20 to 80% of the total amount of the low-boiling point solvent contained in the polyurethane solution for the adhesive layer has been evaporated.

The first heating is preferably performed, for example, with a hot air dryer, particularly a conveyor-type hot air dryer in which the temperature can be set for each zone. The first heating conditions are set according to the type of low-boiling organic solvent, and include heating in the temperature range of 5 to 15°C lower than the boiling point of the low-boiling organic solvent for 30 seconds or more during the temperature increase process. By heating in this manner, foaming caused by the sudden evaporation of the low-boiling organic solvent in the polyurethane solution for the adhesive layer when heating from the low-temperature side to the high-temperature side can be suppressed, and thus the high adhesive strength of the adhesive layer formed can be maintained. The temperature and time under each heating condition are specified from the temperature profile measured by placing a thermometer in the hot air dryer. If instantaneous fluctuations are observed in the temperature profile due to measurement errors or the characteristics of the dryer, the center value of the fluctuations is taken as the actual temperature measurement value.

The first heating condition is not particularly limited as long as it includes heating in the temperature range of 5 to 15°C lower than the boiling point of the low-boiling organic solvent for 30 seconds or more during the temperature rise process, and the temperature profile until the temperature reaches 15°C lower than the boiling point of the low-boiling organic solvent is appropriately adjusted while taking into consideration the permeability of the polyurethane solution for the adhesive layer into the fiber substrate. Specifically, for example, if the fiber substrate has a high specific gravity or is oil-repellent and the polyurethane solution for the adhesive layer does not easily permeate into the fiber substrate, it is preferable to adjust the temperature rise rate until the temperature reaches 15°C lower than the boiling point of the low-boiling organic solvent in the direction of increasing the total amount of heat applied in order to promote the permeation. In addition, if the fiber substrate has a low specific gravity or is oil-philic and the polyurethane solution for the adhesive layer permeates the fiber substrate too much, it is preferable to adjust the temperature rise rate until the temperature reaches 15°C lower than the boiling point of the low-boiling organic solvent in the direction of decreasing the total amount of heat applied in order to suppress the polyurethane solution for the adhesive layer from permeating excessively into the fiber substrate. In this way, taking into consideration the permeability of the polyurethane solution for the adhesive layer of the fiber substrate, it is preferable to adjust the temperature profile until the temperature reaches 15°C lower than the boiling point of the low-boiling organic solvent in the first heating condition. In addition, it is preferable that the temperature is lowered to room temperature immediately after heating at a temperature range of 5 to 15°C lower than the boiling point of the low-boiling organic solvent for 30 seconds or more, because this makes it difficult for the adjusted viscosity to fluctuate.

The first heating step is intended to volatilize some of the low-boiling point solvent and increase viscosity, and does not significantly affect the final amount of remaining VOCs.

The thickness of the adhesive layer is preferably 5 to 150 μm, and more preferably 20 to 100 μm. If the adhesive layer is too thick, the bending resistance tends to decrease and the adhesive strength tends to decrease.

In addition, the total thickness of the surface resin layer and adhesive layer is preferably 10 to 300 μm, more preferably 30 to 200 μm, and especially preferably 50 to 150 μm, in order to maintain a balance between mechanical properties and texture.

[Fiber base material including fiber entanglement]
Next, the preparation of the fiber substrate containing the fiber entanglement to which the surface resin layer is bonded will be described. As the fiber substrate, a fiber substrate containing a nonwoven fabric, a woven fabric, a knitted fabric, or a fiber entanglement combining these, which has been conventionally used for the production of grain-finished leather-like sheets, and further impregnated with a polymeric elastomer as necessary, is used without any particular limitation. Among these, a fiber substrate containing a nonwoven fabric, particularly a nonwoven fabric containing ultrafine fibers having a fineness of 0.5 dtex or less impregnated with a polymeric elastomer, is preferred because it is easy to obtain a dense grain-finished leather-like sheet with high mechanical strength.

The fineness and shape of the fibers are not particularly limited. For example, the fineness may be regular fibers with a fineness of more than 1 dtex, or ultrafine fibers with a fineness of less than 1 dtex. The fiber shape may be solid fibers or fibers with voids such as hollow fibers or lotus root-like fibers. In particular, it is preferable for the fibers to contain ultrafine fibers with a fineness of 0.5 dtex or less, or even 0.0001 to 0.2 dtex, because this makes it easier to obtain a grained leather-like sheet with low fiber density unevenness and high homogeneity.

The type of resin forming the fibers is not particularly limited. Specifically, for example, aromatic polyester-based resins such as polyethylene terephthalate (PET), modified polyethylene terephthalate, polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polytriethylene terephthalate, polyhexamethylene terephthalate, polypropylene terephthalate, and polyethylene naphthalate; polyester-based resins such as polylactic acid, polyethylene succinate, polybutylene succinate, polybutylene succinate adipate, and aliphatic polyester-based resins such as polyhydroxybutyrate-polyhydroxyvalerate copolymers; Examples of suitable resins include polyamide-based resins such as amide 6, polyamide 66, polyamide 610, polyamide 10, polyamide 11, polyamide 12, and polyamide 6-12; polyolefin-based resins such as polypropylene, polyethylene, polybutene, polymethylpentene, chlorine-based polyolefins, ethylene-vinyl acetate copolymers, and styrene-ethylene copolymers; modified polyvinyl alcohol-based resins formed from modified polyvinyl alcohols containing 25 to 70 mol% of ethylene units; and crystalline elastomers such as polyurethane-based elastomers, polyamide-based elastomers, and polyester-based elastomers. Among these, polyamide-based resins and aromatic polyester-based resins are preferred because they have an excellent balance of various properties. These may be used alone or in combination of two or more.

Fiber manufacturing methods include, without limitation, melt spinning, which involves melting a resin at a temperature above its melting point and extruding it from an extruder, dry solution spinning, which involves extruding a polymer solution through fine holes and evaporating the solvent, and wet solution spinning, which involves spinning a polymer solution into a non-solvent. In addition, nonwoven fabrics of ultrafine fibers can be obtained by, for example, entangling ultrafine fiber-generating fibers such as islands-in-the-sea composite fibers to form a fiber entanglement body, which is then processed to produce ultrafine fibers.

The fiber substrate may include a polymeric elastomer impregnated into the entangled fiber body. The type of polymeric elastomer impregnated into the entangled fiber body is not particularly limited, but specific examples include various polymeric elastomers such as polyurethane, acrylic elastomers such as acrylonitrile-butadiene copolymers and copolymers of acrylic acid esters or methacrylic acid esters, polyamide elastomers, and silicone rubber. Among these, polyurethane is particularly preferred because it provides a good feel. The soft segment of polyurethane may include one of polyester units, polyether units, and polycarbonate units, or may be used in combination. The polymeric elastomers may be used alone or in combination of two or more types.

When the fiber substrate contains a polymer elastomer, the mass ratio of the fiber entanglement to the polymer elastomer (fiber entanglement/polymer elastomer) is preferably in the range of 99/1 to 30/70. If the polymer elastomer content is too high, the resulting grained leather-like sheet tends to have a hard, rubber-like texture.

The thickness of the fiber substrate is not particularly limited, but is preferably, for example, 0.3 to 3 mm, and more preferably, 0.5 to 1.5 mm.

[Step of forming a laminated integrated sheet by bonding a fiber base material and a surface resin layer via an adhesive layer]
The adhesive layer is laminated on the surface resin layer formed on the release paper, and the adhesive layer is heated to volatilize a part of the organic solvent and adjust the viscosity, and is then pressed and bonded to the surface of the fiber substrate to produce a laminated integrated sheet in which the two are integrated. The integration is carried out, for example, by passing the laminate formed by overlapping the adhesive layer formed on the release paper and the fiber substrate through a press roll having a predetermined clearance.

The clearance formed in the press roll is preferably, for example, 20 to 90%, or even 30 to 80%, of the thickness of the fiber substrate, in order to ensure that the fiber substrate and the surface resin layer are sufficiently bonded via the adhesive layer. In this way, a laminated integrated sheet is obtained in which the surface resin layer on the release paper and the fiber substrate are bonded together via the adhesive layer.

[Step of subjecting the laminated integrated sheet to second heating]
Similarly to the first heating, the second heating is preferably performed, for example, with a hot air dryer, particularly a conveyor-type hot air dryer in which the temperature can be set for each zone. In the step of performing the second heating, heating is performed under second heating conditions including heating in a temperature range of 10 to 40° C. lower than the boiling point of the high-boiling organic solvent for 30 seconds or more during the temperature increase process. It is preferable that the temperature is decreased to room temperature immediately after heating in a temperature range of 10 to 40° C. lower than the boiling point of the high-boiling organic solvent for 30 seconds or more, in order to suppress thermal deterioration of the polyurethane due to excessive heating.

In the second heating, most of the organic solvent remaining in the laminated integrated sheet is volatilized. The second heating conditions include heating at a temperature range that is 10 to 40°C lower than the boiling point of the high-boiling organic solvent for 30 seconds or more, preferably 60 seconds or more, and more preferably 120 seconds or more.

In the second heating, the temperature profile is not particularly limited as long as the heating is performed in a temperature range of 10 to 40°C lower than the boiling point of the high-boiling organic solvent for 30 seconds or more during the temperature increase process, but it is preferable that the temperature is decreased to room temperature immediately after the heating in order to suppress thermal deterioration of the polyurethane due to excessive heating. In addition, for example, in the first heating, the volatilization of the low-boiling solvent may be insufficient by prioritizing the adjustment of the viscosity. In such a case, if the temperature is increased rapidly to a temperature 40°C lower than the boiling point of the high-boiling organic solvent, the low-boiling solvent remaining in the adhesive layer may suddenly expand and foam. In such a case, a temperature profile in which the temperature is increased gradually to a temperature 40°C lower than the boiling point of the high-boiling organic solvent is preferable. In addition, for example, a temperature profile in which the temperature is increased to a temperature 40°C lower than the boiling point of the high-boiling organic solvent after being held for 30 seconds in a temperature range of 40 to 50°C lower than the boiling point of the high-boiling organic solvent may be used.

[Step of forming a grain-finished leather-like sheet by peeling off the release paper from the laminated integrated sheet after the second drying]
The laminated integrated sheet is wound into a roll, and then is typically aged for several hours to several days at a temperature of 40° C. to 90° C. to completely harden the adhesive polyurethane layer. After aging is complete, the release paper is peeled off to obtain a grain-finished leather-like sheet.

[Step of subjecting grain-finished leather-like sheet to third heating]
The grain-finished leather-like sheet after the release paper is peeled off is subjected to a third heating step, which includes heating the sheet at a temperature 10 to 40° C. lower than the boiling point of the high-boiling organic solvent for 30 seconds or more during the temperature rise process. The third heating step can remove the organic solvent more efficiently than the second heating step performed while the sheet is integrated with the release paper. By performing the third heating step, it is possible to remove most of the organic solvent remaining in the grain-finished leather-like sheet.

As with the first and second heating, the third heating is preferably performed, for example, with a hot air dryer, particularly a conveyor-type hot air dryer in which the temperature can be set for each zone. The third heating also includes heating in the temperature rise process, in which the temperature is maintained at a range of 10 to 40°C lower than the boiling point of the high-boiling organic solvent for 30 seconds or more, preferably 60 seconds or more, and more preferably 120 seconds or more. In addition, it is preferable that the temperature be reduced to room temperature immediately after heating in which the temperature is maintained at a range of 10 to 40°C lower than the boiling point of the high-boiling organic solvent for 30 seconds or more, in order to prevent thermal degradation of the polyurethane due to excessive heating.

The third heating step may also include a step of heating at a temperature lower than 40°C lower than the boiling point of the high-boiling organic solvent, if necessary. For example, the conditions may be such that heating is performed for 30 seconds at a temperature 50°C lower than the boiling point of the high-boiling organic solvent, followed by heating for 30 seconds at a temperature 30°C lower than a peak temperature within a range of temperatures 10 to 40°C lower than the boiling point of the high-boiling organic solvent, and then holding at the peak temperature for 30 seconds or more.

The present invention will be explained in more detail below with reference to examples. Note that the scope of the present invention is not limited to these examples.

[Example 1]
A polyurethane solution for the surface resin layer having the composition shown below was uniformly applied on a release paper ^at 170 g/m2, and then dried at 90°C for 1 minute and further dried at 120°C for 1 minute to form a polyurethane surface resin layer with a thickness of 30 µm.

(Polyurethane solution for surface resin layer)
Polyether polyurethane solution (Rezamin ME-8115LP, solids 30%, MEK 21%, DMF 49%, manufactured by Dainichiseika Color & Chemicals Co., Ltd.): 100 parts by weight MEK: 30 parts by weight DMF: 40 parts by weight White pigment: 10 parts by weight

Next, a polyurethane solution for the adhesive layer was prepared with the composition shown below.

(Polyurethane solution for adhesive layer)
Polyether-based polyurethane solution (Rezamin UD-8310NTT, solids 60%, MEK 30%, DMF 10%, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.): 100 parts by mass; MEK (low boiling point organic solvent, boiling point 79.6°C): 30 parts by mass; DMF (high boiling point organic solvent, boiling point 153°C): 10 parts by mass; Isocyanate-based crosslinking agent (NE crosslinking agent, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.): 10 parts by mass; Crosslinking reaction accelerator (UD-103 accelerator crosslinking agent, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.): 1 part by mass

The proportion of low-boiling organic solvents in the polyurethane solution for the adhesive layer was 39.7% by mass, and the proportion of high-boiling organic solvents was 13.2% by mass.

Then, the polyurethane solution for the adhesive layer was applied to the surface of the surface resin layer in an amount of 150 g/m ^2. The amount of the high boiling point organic solvent applied at this time was 19.9 g/m ² .

Then, in the first heating step, the applied polyurethane solution for the adhesive layer was heated from room temperature to 70°C in 5 seconds in a hot air dryer, and held at 70°C for 30 seconds. After that, the heating was stopped to form an adhesive layer that was dried to a semi-dry state.

The semi-dried adhesive layer formed in this way and laminated on the surface resin layer on the release paper was placed in contact with the surface of the fiber substrate and pressed with a roll having a clearance of about 70% of the thickness of the fiber substrate. In this way, a laminated integrated sheet was manufactured. As the fiber substrate, an artificial leather substrate was used, which was a polyester fiber/polyurethane = 70/30 (mass ratio) impregnated with polyurethane in a nonwoven fabric of 0.1 dtex polyester fiber, had a basis weight of 430 g/ ^m2 , and was 1.0 mm thick.

Then, in the second heating step, the laminated integrated sheet was heated in a hot air dryer from room temperature to 120°C over 90 seconds, held at 120°C for 60 seconds, and then the heating was stopped.

Then, it was left to stand at 70°C for two days to mature in order to complete the cross-linking of the polyurethane.

The release paper was then peeled off from the laminated integrated sheet to obtain a grained leather-like sheet.

Then, in the third heating step, the grain-finish leather-like sheet was heated in a hot air dryer from room temperature to 120°C in 5 seconds, held at 120°C for 60 seconds, and then the heating was stopped.

In this way, grain-finish leather-like sheets were produced. The organic solvent residual rate and processing stability of the obtained grain-finish leather-like sheets were evaluated as follows.

(Quantitative Analysis of DMF and DEF)
The residual amounts of DMF and DEF were measured in accordance with ISO/TS 16189:2013 as follows. The silver-finished leather-like sheet, which had been appropriately sampled and sealed for VOC measurement in a zippered plastic bag, was shredded into 3 mm square pieces, and 1 g was weighed as a test piece. Then, the test piece was placed in a 20 ml vial and sealed with a Teflon (registered trademark)-coated silicone septum. Then, 10 ml of a methanol solution containing DMF-d ₇ of known concentration as an internal indicator was added to the vial, and the vial was left at 70 ° C. for 1 hour to extract the remaining organic solvent. The obtained extract was then analyzed using GS/MS, and the amounts of DMF and DEF were quantified from the peak positions and their areas, and the residual rates were calculated.

(Quantitative determination of MEK, ethyl acetate, toluene, and anone)
The residual amount of organic solvent was measured by headspace GC/MS. First, the silver-finished leather-like sheet, which had been appropriately sampled and sealed for VOC measurement in a zippered plastic bag, was shredded into 3 mm squares, and 1 g was weighed as a test piece. Then, the test piece was placed in a 10 ml vial and sealed with a Teflon-coated silicone septum. Then, 10 μg of toluene-d ₈ was injected as an internal indicator with a microsyringe, and the sample was set in a headspace sampler and heated at 120° C. for 45 minutes. The headspace portion that had reached an equilibrium state was introduced from the headspace sampler to GC/MS, and the amounts of MEK, ethyl acetate, toluene, and anon solvents were quantified from the peak positions and areas, and the residual rates were calculated.

(Adhesive processing stability)
The adhesive processing stability was evaluated according to the following criteria.
A: Adhesion processing was possible without any problems.
B: The formed adhesive layer was swollen by the organic solvent, but adhesion was possible.
C: The adhesive layer was poorly dried, had significant swelling or foaming, or had a poor appearance, and was not able to achieve practical adhesion.

The results are shown in Table 1 below.

[Examples 2 to 7 and Comparative Examples 1 to 9]
A grain-finished leather-like sheet was prepared in the same manner as in Example 1, except that the coating amount of the polyurethane solution for the adhesive layer, the compounding composition of the polyurethane solution for the adhesive layer, and the heating conditions for the first heating step, the second heating step, and the third heating step were changed as shown in Table 1.
In Comparative Example 4, the following polyurethane solution for the adhesive layer was used.

(Polyurethane solution for adhesive layer)
Polyether polyurethane (solids 60%, MEK 30%, DEF 10%): 100 parts by mass; MEK (low boiling point organic solvent, boiling point 79.6°C): 30 parts by mass; DEF (ultra high boiling point organic solvent, boiling point 176-177°C): 10 parts by mass; Isocyanate crosslinking agent (NE crosslinking agent manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.): 10 parts by mass; Crosslinking reaction accelerator (UD-103 accelerator crosslinking agent manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.): 1 part by mass. The grain-finished leather-like sheet was then evaluated in the same manner as in Example 1. The results are shown in Table 1.

Referring to Table 1, the grain-finished artificial leathers obtained in Examples 1 to 6, which employ the method for producing grain-finished leather-like sheets according to the present invention, all had a significantly small amount of residual solvent. On the other hand, the grain-finished artificial leather obtained in Comparative Example 1, which did not undergo the third heating, had a large amount of residual solvent. In addition, the grain-finished artificial leather obtained in Comparative Example 2, which used a polyurethane solution for the adhesive layer with too high a proportion of high-boiling-point organic solvent, had a large amount of residual solvent even after the third heating. In addition, the grain-finished artificial leather obtained in Comparative Example 3, which used a large amount of polyurethane solution for the adhesive layer, also had a large amount of residual solvent even after the third heating. In addition, the grain-finished artificial leather obtained in Comparative Example 5, which used a low temperature for the third heating, also had a large amount of residual solvent. In addition, the grain-finished artificial leather obtained in Comparative Example 7, which used a too high proportion of low-boiling-point organic solvent, had an unstable adhesive processability due to the viscosity of the polyurethane solution for the adhesive layer being too low. In addition, the grain-finished artificial leather obtained in Comparative Example 8, in which the temperature of the first heating was too high, had unstable adhesive processability due to the viscosity of the polyurethane solution for the adhesive layer increasing too rapidly. In addition, the grain-finished artificial leather obtained in Comparative Example 9, in which the temperature of the second heating was too high, had foaming in the adhesive layer, resulting in poor appearance.

Reference Signs List 1: Fiber substrate 2: Polyurethane adhesive layer 3: Polyurethane resin layer 10: Grain-finished leather-like sheet

Claims (2)

forming a polyurethane surface resin layer on a release paper;
applying a polyurethane solution for an adhesive layer to a surface of the polyurethane surface resin layer;
A step of forming a polyurethane adhesive layer in a semi-dried state by heating the polyurethane solution for adhesive layer applied to the surface of the polyurethane surface resin layer under a first heating condition;
forming a laminated integrated sheet by bonding a fiber substrate and the polyurethane surface resin layer via the polyurethane adhesive layer;
Heating the laminated integrated sheet under a second heating condition;
a step of peeling off the release paper from the heated laminated integrated sheet to form a grain-finished leather-like sheet;
and heating the grain-finished leather-like sheet under a third heating condition.
The polyurethane solution for the adhesive layer contains polyurethane, 20 to 50 mass % of a low-boiling organic solvent having a boiling point of 70 to 100° C., and 6 to 15 mass % of a high-boiling organic solvent having a boiling point of 120 to 160° C.;
the coating amount of the high-boiling organic solvent is 22 g/ ^m2 or ^less ,
the first heating condition includes heating the mixture at a temperature that is 5 to 15° C. lower than the boiling point of the low-boiling organic solvent for 20 seconds or more during a temperature rise process;
The second heating condition and the third heating condition include heating the mixture at a temperature 10 to 40° C. lower than the boiling point of the high-boiling point organic solvent for 30 seconds or more during a temperature rise process.
The low boiling point organic solvent includes methyl ethyl ketone, and the high boiling point organic solvent includes N,N-dimethylformamide;
The method for producing a grain-finished leather-like sheet according to claim 1, wherein the temperature maintained for 20 seconds or more under the first heating condition is in the range of 65 to 75°C, and the temperatures maintained for 30 seconds or more under the second heating condition and the third heating condition are in the range of 115 to 143°C.