Preparation method of microfiber sheepskin leather for shoes

Abstract

This application relates to the field of high-end fabric processing, and in particular to a method for preparing microfiber sheepskin leather for footwear. The method specifically includes the following steps: first coating a sheepskin surface layer onto release paper and drying it; after the first drying, coating a second sheepskin surface layer and drying it; coating a base resin onto the sheepskin surface layer and semi-dry bonding it with microfiber base layer, then drying it to obtain a semi-finished product; subjecting the semi-finished product to gradient heating and depressurization relaxation treatment to release internal stress; foaming the semi-finished product to obtain foamed sheepskin leather; coating the surface of the foamed sheepskin leather with a transition resin and drying it; obtaining a mirror transfer coating through a three-blade bonding method; combining the mirror transfer coating with the transition resin of the semi-dry bonding of the foamed sheepskin leather and drying it; ensuring that the difference in tensile modulus between any two adjacent layers among the mirror transfer coating, transition resin, sheepskin surface layer, base resin, and microfiber base layer is within one time, to reduce quality problems such as white marks or dark cracks.

Description

Technical Field

[0001] This application relates to the field of high-end fabric processing, and in particular to a method for preparing microfiber sheepskin leather for footwear. Background Technology

[0002] Lambskin is a high-end leather made from natural sheepskin through a brushing process. However, natural lambskin has poor abrasion resistance, weak folding resistance, and is prone to white marks / cracks when bent at room temperature. In contrast, microfiber lambskin uses bundles of ultra-fine fibers to construct a three-dimensional mesh skeleton and is filled with polyurethane microporous elastomer. Its structure is highly similar to that of natural lambskin, and its abrasion resistance and folding resistance are greatly improved, making it better suited for shoe upper processing and manufacturing.

[0003] Current methods for manufacturing microfiber sheepskin typically involve first preparing a microfiber base fabric, i.e., microfiber backing, and then applying a sheepskin-style finish to the microfiber backing. The finishing method generally involves first coating sheepskin polyurethane resin onto release paper and drying it to form a sheepskin surface layer. Then, a polyurethane base resin is coated onto the sheepskin surface layer and semi-dry-bonded with the microfiber backing to obtain a semi-finished product. After drying and curing the semi-finished product, it is foamed to obtain microfiber sheepskin. For reference, see the preparation method of sheepskin mirror microfiber with announcement number CN105586784B.

[0004] In the aforementioned finishing process, in order to improve the stain resistance and gloss of the sheepskin, the microfiber sheepskin and the transfer mirror layer are semi-dry bonded and then dried to obtain mirror microfiber sheepskin. However, there is a large difference in modulus between the mirror layer, the sheepskin foam layer and the microfiber base. During the daily low-stress repeated bending of the shoe upper, the interface between the layers with different moduli will produce continuous stress concentration. After repeated cycles, the polyurethane molecular chains at the interface will undergo irreversible silvering, which will appear as white marks on the shoe upper that cannot be eliminated, and in severe cases, it will develop into dark cracks. Summary of the Invention

[0005] In order to reduce the possibility of quality problems such as white marks or dark cracks appearing on shoe uppers made of mirror microfiber sheepskin, this application provides a method for preparing microfiber sheepskin for shoes.

[0006] The preparation method of microfiber sheepskin for footwear provided in this application adopts the following technical solution.

[0007] A method for preparing microfiber sheepskin leather for footwear includes the following steps.

[0008] S1. Apply the first layer of sheepskin to the release paper and dry it. After the first drying, apply the second layer of sheepskin and dry it.

[0009] S2. Coat the base resin onto the sheepskin surface layer and semi-dry bond it with the microfiber base layer and then dry it to obtain a semi-finished product;

[0010] S3. Perform gradient heating and decompression relaxation treatment on the semi-finished product to release internal stress;

[0011] S4. Foam the semi-finished product to obtain foamed sheepskin;

[0012] S5. Coat the foamed sheepskin surface with transition resin and dry;

[0013] S6. A mirror-finish transfer coating is obtained through a three-blade lamination process;

[0014] S7. The mirror transfer coating and the transition resin of the foamed sheepskin are semi-dry bonded together and then dried.

[0015] The difference in tensile modulus between any two adjacent layers in the mirror transfer coating, transition resin, veneer top layer, base resin, and microfiber base is within one time.

[0016] By adopting the above technical solution, the dynamic bending stress concentration caused by interlayer modulus mismatch is eliminated, the white marks and dark cracks caused by irreversible silvering of polyurethane molecular chains are avoided, and the appearance stability of high-end footwear is improved over long-term wear.

[0017] Gradient heating and pressure reduction relaxation pretreatment fully releases the internal stress after the coating and base fabric are combined before foaming, avoiding residual internal stress during subsequent foaming and curing processes, and reducing the risk of shrinkage deformation and bending white marks during storage and wear of finished products from the source.

[0018] The addition of transition resin creates a modulus buffer gradient between the foamed sheepskin and the mirror transfer coating, avoiding abrupt changes in modulus between the mirror hard coating and the foam soft layer. At the same time, it improves interfacial adhesion, solving the industry pain point that the mirror layer of traditional mirrored sheepskin is prone to peeling off or bending and losing its film.

[0019] Optionally, S3 is divided into three stages of processing:

[0020] First stage: Oven temperature 110℃, roller pressure 0.2MPa, holding time 30s;

[0021] Second stage: Oven temperature 112℃, roller pressure 0.15MPa, holding time 30s;

[0022] Third stage: Oven temperature 115℃, atmospheric pressure without pressure treatment, pressure holding time 60s.

[0023] By adopting the above technical solution, the first stage of medium temperature and micro pressure causes the polyurethane molecular chains to creep slightly, initially releasing the interfacial stress. The second stage of slight temperature increase and pressure decrease eliminates the residual interlayer shear stress. The third stage of temperature increase and normal pressure holding allows the molecular chains to complete steady-state rearrangement and completely lock in the stress-free state.

[0024] Furthermore, the precise temperature and pressure parameters and holding time are adapted to the thickness and coating characteristics of the microfiber base for shoes, which avoids coating crushing and dripping caused by excessive pressure, and also avoids premature cross-linking of resin or premature decomposition of foaming powder caused by excessive temperature, ensuring the uniformity of subsequent foaming. At the same time, it eliminates finished product shrinkage or edge curling caused by residual internal stress, ensuring the dimensional stability of the shoe upper material in the cutting and sewing processes.

[0025] Optionally, the transition resin in S5 comprises, by weight: 70 parts of thermoplastic polyester polyurethane resin, 30 parts of polyester polyurethane resin, 1-2 parts of silane coupling agent, 5-8 parts of hydroxyl-terminated tackifying resin, and 20-30 parts of a mixed solvent of DMF and MEK.

[0026] By adopting the above technical solution, the 7:3 ratio of thermoplastic polyester polyurethane to conventional polyester polyurethane can precisely control the tensile modulus of the transition resin, so that the modulus difference between it and the lower foamed layer and the upper mirror coating is controlled within one time, perfectly realizing the interlayer modulus gradient transition, eliminating the interface stress concentration caused by the sudden change in modulus, while taking into account the flexibility and adhesion of the transition layer, and adapting to the service requirements of repeated bending in shoe application scenarios.

[0027] The addition of silane coupling agent forms a chemical bond between the organic resin and the adjacent coating, which improves the peel strength between the layers compared to the formulation without coupling agent, and prevents the mirror layer and the lamella layer from delaminating or peeling off during long-term bending.

[0028] Hydroxyl-terminated tackifying resins not only enhance initial tack in semi-dry bonding conditions, ensuring bonding accuracy, but also form an interpenetrating network structure with polyurethane, improving the coating's hydrolysis resistance and flexural fatigue resistance, thus extending the service life of footwear products.

[0029] Optionally, the drying temperature in step S5 is 80-100℃ and the drying time is 60-90s.

[0030] By adopting the above technical solution, the semi-dry state of the transition resin can be precisely controlled, which ensures that the mixed solvent evaporates appropriately and avoids composite bubbles or pinholes caused by solvent residue, while retaining the active groups and melt viscosity of the resin, providing the best bonding window for the subsequent semi-dry bonding of the mirror transfer coating, and ensuring that the interlayer molecular entanglement and chemical bonding are fully carried out.

[0031] It also avoids the problem of surface dryness but not interior dryness caused by high-temperature rapid drying, prevents the loss of gloss or decrease in adhesion caused by solvent migration during finished product storage, and avoids low production efficiency caused by low-temperature long-term drying.

[0032] Optionally, the sheepskin surface layer in S1 comprises, by weight: 100 parts of polyester polyurethane resin, 20-50 parts of sheepskin foaming powder, 3-5 parts of hindered phenolic stress relaxant, 2-3 parts of polycaprolactone polyurethane oligomer, and 30-50 parts of a mixed solvent of DMF and MEK.

[0033] By adopting the above technical solution, the hindered phenolic stress relaxation agent and polycaprolactone-type polyurethane oligomer are synergistically compounded to give the sheepskin surface excellent dynamic stress relaxation performance. During repeated bending in shoe use, the interface concentrated stress can be quickly dissipated, avoiding irreversible silvering and breakage of polyurethane molecular chains, and reducing the possibility of bending white marks and dark cracks.

[0034] The amount of sheepskin foaming powder added precisely controls the foaming ratio and pore structure of the surface layer, ensuring the unique soft and velvety feel and full and elastic hand feel of sheepskin leather, while avoiding the decrease in surface strength and insufficient wear resistance caused by excessive foaming powder, thus meeting the dual requirements of feel and durability of shoe fabrics.

[0035] Polycaprolactone-type polyurethane oligomers can precisely control the tensile modulus of the lambra surface layer, achieve precise matching of interlayer modulus, and improve the peel performance between the lambra surface layer and the release paper, avoiding surface layer damage and fuzzing during the release process, and ensuring the surface smoothness of the surface layer.

[0036] Hindered phenolic stress relaxation agents also have antioxidant and yellowing resistance functions, which can effectively inhibit the thermo-oxidative aging and yellowing of polyurethane during high-temperature drying and foaming.

[0037] Optionally, the mixing ratio of DMF and MEK is 1:1.

[0038] By adopting the above technical solutions, DMF, as a highly polar slow-evaporating solvent, can fully dissolve polyurethane resin and various functional additives, ensuring the storage stability and coating uniformity of the resin system; MEK, as a moderately polar fast-evaporating solvent, precisely controls the overall evaporation rate, avoiding both pinholes and bubbles in the coating caused by residual slow-evaporating solvents and poor dry spraying and leveling caused by excessive fast-evaporating solvents.

[0039] The mixing ratio of the two is perfectly suited to the industrial blade coating process of the sheep bar topcoat and transition resin, which can stably control the resin coating viscosity, meet the line speed requirements of mass production, ensure the uniformity of coating thickness, and meet the batch stability requirements of the footwear industry for large-scale mass production.

[0040] The mixing ratio of the two can avoid the problems of uneven resin swelling or poor dispersion of additives caused by a single solvent, ensure that functional components such as foaming powder and stress relaxation agent are evenly dispersed, give full play to the functional effects of each component, and ensure the stability and consistency of the finished product performance.

[0041] Optionally, the foaming process in step S4 is completed by sequentially passing through five heating zones at 130°C, 145°C, 160°C, 165°C and 165°C.

[0042] By adopting the above technical solution, the five-stage gradient heating foaming process achieves step-by-step and uniform decomposition and foaming of sheepskin foam powder, which is different from the defects of uneven foaming cells and low closed-cell rate in traditional single-temperature zone foaming. The 130℃ preheating zone ensures uniform heating of the resin system, avoiding premature decomposition of the foam powder due to local overheating. The 145℃ triggering zone enables uniform nucleation of the foam powder, constructing a uniform cell precursor. The 160℃, 165℃ and 165℃ constant temperature foaming zones enable synchronous and uniform cell growth, ultimately forming a microporous structure with uniform pore size and high closed-cell rate. This ensures the soft and elastic feel of sheepskin leather, while avoiding the decrease in coating strength or easy breakage due to large pores or interconnected pores.

[0043] Precise gradient heating parameters are perfectly matched with the resin crosslinking and curing rate and the foaming powder decomposition rate. This avoids both insufficient foaming ratio due to excessively fast resin curing, resulting in a hard feel, and cell collapse or shrinkage due to excessively slow resin curing. The tensile modulus of the lamb surface layer is precisely controlled to ensure a stable modulus difference between it and adjacent layers, maintaining the low stress characteristics of the overall structure.

[0044] Optionally, in step S7, the material is sequentially heated through five zones at 90°C, 110°C, 110°C, 130°C, and 130°C to complete the drying and curing process.

[0045] By adopting the above technical solution, the five-stage stepped heating and drying curing process achieves stable and step-by-step bonding between the mirror layer and the sheepskin, as well as resin cross-linking curing. The 90℃ low-temperature preheating zone ensures uniform solvent evaporation, avoiding bubbles, pinholes, and loss of gloss in the mirror layer caused by rapid solvent boiling. The 110℃ dual constant temperature zone achieves molecular entanglement and initial cross-linking between the transition resin and the mirror resin, building a stable interfacial bond. The 130℃ dual constant temperature zone ensures full cross-linking and curing of the resin, locking the interlayer bonding structure. The resulting product's interlayer peel strength far exceeds the national standard requirements for footwear materials.

[0046] The stepped heating curing process avoids the interfacial stress caused by the difference in thermal expansion coefficients of each layer due to high-temperature rapid heating. Combined with the previous internal stress release process, it further eliminates the residual stress in the composite curing process, ensuring that the finished product has no stress concentration when subjected to long-term dynamic bending, and eliminating white marks and dark cracks when bending.

[0047] The precise temperature gradient design avoids yellowing or loss of gloss in the mirror layer caused by high-temperature curing, ensuring the mirror's gloss while improving the finished product's resistance to yellowing, making it suitable for the long-term storage and wearing needs of high-end white or light-colored shoes.

[0048] Optionally, the dwell time of a single zone in the heating zone at different temperatures in S7 is 30-37.5s.

[0049] By adopting the above technical solution, the residence time of a single zone is perfectly matched with the five-stage stepped temperature curing process, which precisely controls the resin cross-linking and curing process. The residence time of each temperature zone ensures that the corresponding curing reaction is fully carried out, which not only achieves complete solvent evaporation and eliminates the odor, reduced adhesion and yellowing of the finished product caused by solvent residue, but also achieves gradient cross-linking of resin, avoiding internal stress residue and coating embrittlement caused by excessive cross-linking, and ensuring the flexibility and low stress characteristics of the coating.

[0050] Optionally, the dry coating amount of the second coating in S6 is 80-120 g / m². 2 .

[0051] By adopting the above technical solution, the thickness and performance of the mirror transfer coating are precisely controlled, perfectly balancing the mirror appearance, wear resistance and bending flexibility. The mirror coating formed by the corresponding coating amount can cover the minor defects of the underlying substrate, forming a smooth and high-gloss mirror effect without orange peel, pitting and see-through defects, which meets the appearance aesthetic requirements of high-end footwear.

[0052] The tensile modulus of the mirror coating can be precisely controlled within the corresponding coating amount range, so that the modulus difference between it and the lower transition resin is stable. This avoids insufficient strength, poor scratch and wear resistance, or easy scratches when bending caused by too low coating amount, while avoiding excessively high coating amount caused by excessively high mirror modulus, insufficient flexibility, and easy white marks and cracks when bending.

[0053] In summary, this application includes at least the following beneficial effects.

[0054] Through a core design that optimizes the entire process and precisely matches the interlayer modulus, combined with a three-stage gradient heating and depressurization relaxation process, the internal stress of the semi-finished product is fully released. This is complemented by a special formula for the lapaf layer containing hindered phenolic stress relaxants and polycaprolactone-type polyurethane oligomers, as well as a transition resin formula designed with a thermoplastic polyester-type polyurethane composite system. The drying temperature and time of the transition resin layer are strictly limited, and a five-stage gradient heating precise foaming process is used to achieve uniform and stable foaming of the lapaf layer.

[0055] By limiting the dry base coating amount of the mirror transfer coating, combined with a five-stage gradient temperature-increasing drying and curing process and precise residence time control in a single area, the tensile modulus difference between any two adjacent layers in the mirror transfer coating, transition resin, lapels, sole resin, and microfiber base is controlled within one time. This fundamentally eliminates the problem of interlayer interface stress concentration and internal stress accumulation under long-term low-stress dynamic bending in footwear scenarios. It solves the industry problem of irreversible bending white marks, dark cracks, or interlayer peeling after long-term wear of traditional lapels mirror microfiber. At the same time, it ensures that the finished product has both a delicate and uniform lapels feel and a high-gloss and smooth mirror effect. The batch stability, dynamic bending resistance, interlayer peel strength, scratch resistance, abrasion resistance, and yellowing resistance are all significantly better than the national standard for polyurethane microfiber synthetic leather for shoe uppers. This greatly improves the product yield and industrial mass production adaptability, perfectly meeting the core usage needs of long-term wear in all scenarios of high-end footwear. Attached Figure Description

[0056] Figure 1 This is a flowchart of the main steps of this application. Detailed Implementation

[0057] The present application will be further described in detail below with reference to the accompanying drawings.

[0058] This application discloses a method for preparing microfiber sheepskin leather for footwear, referring to... Figure 1 Specifically, it includes the following steps.

[0059] S1. Apply the first layer of wadding to the release paper and dry it. After the first drying, apply the second layer of wadding and dry it.

[0060] First, prepare the rambutan resin by weighing 100 parts of polyester polyurethane resin, then add 20-50 parts of rambutan foaming powder, 3-5 parts of hindered phenolic stress relaxant, 2-3 parts of polycaprolactone polyurethane oligomer, and 30-50 parts of a mixed solvent of DMF and MEK.

[0061] Among them, the polyester polyurethane resin can be the dry synthetic leather general-purpose sheepskin layer matrix resin of Wanhua Chemical's official mass production grade WHT-1190EC, the sheepskin foaming powder can be F-100D type thermal expansion microsphere foaming powder, the hindered phenolic stress relaxation agent can be Irganox 1010, the polycaprolactone polyurethane oligomer can be a general-purpose shoe resin softening agent with a number average molecular weight of 2000 and a hydroxyl value of 56mgKOH / g, and the mixing ratio of DMF and MEK is 1:1.

[0062] Add the mixture to a high-speed disperser and disperse at 800-1200 rpm for 30-45 minutes to ensure that the foaming powder and additives are evenly dispersed without agglomeration. After dispersion, transfer it to a vacuum degassing machine and degas for 15-20 minutes under a negative pressure of -0.08 MPa. After removing air bubbles, seal and store for later use.

[0063] Take coarse-textured lychee-patterned release paper and lay it flat on the unwinding station of a dry coating machine. Use a comma-shaped doctor blade to apply the first layer of rib resin evenly to the surface of the release paper. The dry coating amount should be controlled at 150-180 g / m². 2 After coating, the product is placed in a first low-temperature oven, where the temperature is kept constant below 90°C and the drying time is 60-90 seconds to ensure complete resin film formation and prevent thermal activation and foaming of the sheep bar foaming powder.

[0064] After the first coating and drying are completed, a second layer of rib resin is applied. The same batch of rib resin is used, and the dry base coating amount is the same as the first coating. After coating, the paper is sent to a second low-temperature oven. The oven temperature is kept constant below 90℃, and the drying time is 60-90 seconds to obtain release paper with a double layer of rib resin.

[0065] S2. Coat the base resin onto the sheepskin surface layer and semi-dry bond it with the microfiber base layer and then dry it to obtain a semi-finished product.

[0066] Apply the base resin evenly using a comma-shaped scraper. The base resin can be a polyether-type thermoplastic polyurethane with a solid content of 40-50%. The dry coating amount should be controlled at 200-250 g / m². 2 After coating, when the solvent evaporation rate reaches 60-70% and the resin is in a semi-dry state, the microfiber base layer is rolled and bonded to the base resin layer. The bonding line speed is controlled at 7-9 m / min, and the bonding roller pressure is controlled at 0.3-0.5 MPa to ensure that the bonding is free of air bubbles and misalignment.

[0067] After lamination, a semi-finished product is obtained. The semi-finished product is sent into a five-stage gradient drying oven. The temperature of the five heating zones is set from low to high as 90℃, 100℃, 100℃, 100℃ and 110℃. The semi-finished leather product passes through the five heating zones at a uniform speed. The total drying time is 120-180s. After drying, it is rolled up at a uniform speed and sealed for later use.

[0068] S3. Perform gradient heating and decompression relaxation treatment on the semi-finished product to release internal stress.

[0069] The semi-finished product is fed into the pre-relaxation roller press oven unit for three-stage micro-heating and depressurization pre-relaxation treatment, with specific parameters as follows.

[0070] In the first stage, the oven temperature is 110℃, which is the same as the temperature at the end of step 2. The residual heat is absorbed with zero drop. The roller pressure is 0.2MPa and the pressure holding time is 30s. This allows the polyurethane molecular chains to initially stretch under constant temperature and pressure, releasing the low molecular weight internal stress generated during the coating and drying process.

[0071] In the second stage, the oven temperature is 112℃, with a slight increase of 2℃, the roller pressure is reduced to 0.15MPa, and the pressure holding time is 30s. This further allows the resin molecular chains to fully expand, while simultaneously achieving secondary uniform dispersion of the sheep bar foaming powder in the resin matrix, avoiding uneven foaming caused by local agglomeration.

[0072] In the third stage, the oven temperature is 115℃, with a slight increase of 3℃. The process is carried out under normal pressure without pressure and with a heat preservation and speed maintenance time of 60 seconds. This process completely releases the internal stress. The temperature throughout the process is far lower than the foaming start temperature, eliminating the risks of pre-foaming and micro-expansion.

[0073] After the pre-relaxation treatment is completed, the semi-finished product is cooled naturally to below 40°C by the cooling roller group, then wound up at a uniform speed and sealed for later use.

[0074] S4. Foam the semi-finished product to obtain foamed sheepskin.

[0075] The semi-finished product is fed into a high-temperature foaming oven, which is equipped with five continuous heating zones. The temperatures of the five heating zones are set from low to high as 130℃, 145℃, 160℃, 165℃, and 165℃. The throughput speed of the semi-finished leather product is controlled at 8-15 m / min, and the total foaming treatment time is 1-3 min. The effects achievable at each temperature range are as follows.

[0076] The 130℃ section is the preheating section, which ensures that the semi-finished leather product is heated evenly throughout, without localized temperature differences, and does not trigger foaming.

[0077] The 145℃ section is the initial foaming section, which is exactly the initial foaming temperature of the sheep bark foaming powder. The microspheres start foaming slowly and evenly, avoiding bursting of bubbles.

[0078] The 160℃ range is a rapid expansion range, matching the maximum foaming rate range of the sheep bark foaming powder to achieve stable and uniform volume expansion.

[0079] The dual 165℃ section, the heat preservation and shaping section, achieves the maximum foaming ratio and completes the full film formation and shaping of the resin. The temperature is below the critical value of 172℃ for foam breaking, eliminating the risk of pore structure collapse.

[0080] After foaming, a closed-cell microporous structure sheepskin with uniform pore size is obtained. After cooling to below 40°C, it is rolled up for later use.

[0081] S5. Coat the foamed sheepskin surface with transition resin and dry.

[0082] The foamed sheepskin is laid flat on the unwinding station of the coating machine. A transition resin is evenly coated onto the sheepskin surface using a micro-concave roller coating method. The dry coating amount is controlled at 80-100 g / m². 2 The coating thickness is strictly controlled to 80-100μm to ensure molecular chain diffusion efficiency.

[0083] The transition resin, by weight, includes: 70 parts of thermoplastic polyester polyurethane resin, 30 parts of polyester polyurethane resin, 1-2 parts of silane coupling agent, 5-8 parts of hydroxyl-terminated tackifying resin, and 20-30 parts of a mixed solvent of DMF:MEK=1:1, which is dispersed at 800 r / min for 20 min in advance, and then sealed for later use after vacuum degassing.

[0084] The thermoplastic polyester polyurethane resin can be Wanhua Chemical's official mass-produced grade WHT-1570EC. The polyester polyurethane resin in the transition resin is the same as the polyester polyurethane resin in the basil top layer. The silane coupling agent is KH-550, and the hydroxyl-terminated tackifying resin is Wanhua Chemical's official mass-produced grade WHT-1160EC.

[0085] After coating, the product is placed in a low-temperature drying oven. The oven temperature is set to 80-100℃, the drying time is 60-90s, and the solvent evaporation rate is controlled to be 50-60%. This allows the transition resin to form a film while retaining slight viscoelasticity, reserving conditions for subsequent molecular chain diffusion and interpenetration, and forming a stable pre-bonded interface with the laminarin surface layer.

[0086] S6. A mirror-finish transfer coating is obtained through a three-blade lamination process.

[0087] Using mirror release paper, a three-layer coating for mirror transfer is prepared sequentially through a three-blade lamination process. The specific steps are as follows.

[0088] The first coating and drying process uses a mirror-finish resin cleaner, specifically the AK-6028 grade resin officially produced by Asahikawa Chemical. The resin is applied to the surface of the mirror release paper using a comma-shaped doctor blade, with a dry coating weight of 100-150 g / m². 2 After coating, the coating is sent into a four-section drying oven. The temperatures of the four heating zones are 80℃, 90℃, 100℃ and 120℃ respectively. The total drying time is 60-90 seconds, completing the pre-drying of the first coating of the mirror.

[0089] The second coating is applied and dried. A high-solids-permeability resin cleaner is then applied to the surface of the first coating. The high-solids-permeability resin cleaner can be Asahikawa Chemical's XCWB-9090 high-solids-permeability cleaner, with a dry coating weight of 80-120 g / m². 2The high-solids resin is matched with the film-forming characteristics to ensure both mirror-like transparency and fullness, while avoiding uneven curing and sagging defects caused by thick coating. After coating, it is sent into a five-stage drying oven with five heating zones at temperatures of 80℃, 90℃, 100℃, 120℃ and 120℃ respectively, for a total drying time of 90-120 seconds, completing the pre-drying of the high-solids transparency second coating.

[0090] The third coating is applied by coating the surface of the second coating with a base resin, with a dry coating amount of 250-300 g / m². 2 The solvent evaporation rate is controlled at 60-70%, so that the base resin is in a semi-dry viscous flow state of 70% dryness, which provides the core conditions for subsequent molecular chain interpenetration, and obtains the semi-finished mirror transfer coating.

[0091] S7. The mirror transfer coating and the transition resin of the foamed sheepskin are semi-dry bonded together and then dried.

[0092] During lamination, the transition resin layer and the third coating of the mirror transfer coating are fully bonded together. The lamination roller pressure is controlled at 0.4-0.6 MPa, and the lamination linear speed is controlled at 8-10 m / min. During the lamination process, the semi-dry transition resin layer and the base resin of the mirror transfer coating, under the action of roller pressure, allow the residual solvent to fully swell the polyurethane molecular chains of the upper and lower layers, initiating the mutual diffusion and penetration of molecular chains, laying the foundation for the formation of the in-situ IPN structure.

[0093] After bonding, the product is placed in a five-stage curing oven with curing temperature profiles of 90℃, 110℃, 110℃, 130℃, and 130℃, for a total curing time of 150-180 seconds. The specific effects achievable at each temperature stage are as follows.

[0094] The 90℃ section is the preheating section, with a residence time of 30-37.5s, which ensures that the semi-finished product is heated evenly, the solvent evaporates steadily, and bubbles are avoided. At the same time, the molecular chains are kept in a highly elastic state, promoting continuous diffusion.

[0095] In the dual 110℃ section, the molecular chain interpenetrating section has a single zone residence time of 30-37.5s and a total residence time of 60-75s for both zones. Under constant temperature conditions, the polyurethane molecular chains of the transition layer and the upper and lower layers of the same system complete sufficient interdiffusion and penetration, forming a continuous molecular chain entanglement structure. At the same time, the hydroxyl-terminated tackifying resin forms high-density hydrogen bonds with the urethane groups of the upper and lower layers, which greatly improves the interfacial bonding force.

[0096] The dual 130℃ section, the shaping and curing section, with a residence time of 30-37.5s in a single zone and a total residence time of 60-75s in both zones, allows the solvent to completely evaporate, and the diffused interpenetrating polyurethane molecular chains lock and entangle to form a stable in-situ interpenetrating polymer network structure, completely eliminating the interface stress concentration of high and low modulus layers; and the high modulus mirror resin is completely film-formed and shaped, achieving the designed modulus, scratch resistance and wear resistance; the maximum temperature throughout is 130℃, which provides a safety margin compared to the initial foaming temperature of 138℃ for the sheep bar foaming powder, eliminating the risk of secondary foaming and pore structure damage.

[0097] After curing, the material is first cooled to below 60°C by a cooling roller assembly, and then the release paper is peeled off online to avoid the risk of the mirror surface sticking or being scratched at high temperatures. The finished product is then cooled to room temperature by the cooling roller assembly and wound up at a uniform speed to obtain the mirror-finish microfiber sheepskin finished product.

[0098] The tensile modulus difference between any two adjacent layers in the mirror transfer coating, transition resin, lapel top layer, base resin, and microfiber base is within one time. For example, the tensile modulus of the first layer (farthest from the transition resin) of the mirror transfer coating is 80-90 MPa, the tensile modulus of the second layer is 55-65 MPa, and the tensile modulus of the third layer (near the transition resin) is 32-38 MPa; the tensile modulus of the transition resin layer is 20-24 MPa; the tensile modulus of the first layer (near the transition resin) of the lapel top layer is 13-15 MPa, and the tensile modulus of the second layer (farthest from the transition resin) is 7.5-8.5 MPa; the tensile modulus of the base resin is 4.2-4.8 MPa; and the tensile modulus of the microfiber base is 3-5 MPa.

[0099] The following designs provide a detailed description of several embodiments and comparative examples.

[0100] Example 1:

[0101] The above method for preparing microfiber sheepskin leather for footwear is used, and the transition resin and sheepskin surface layer are configured according to the following specific components.

[0102] The sheepskin topcoat comprises, by weight: 100 parts polyester polyurethane resin, 20 parts sheepskin foaming powder, 3 parts hindered phenolic stress relaxant, 2 parts polycaprolactone polyurethane oligomer, and 30 parts a mixed solvent of DMF and MEK.

[0103] The transition resin comprises, by weight: 70 parts of thermoplastic polyester polyurethane resin, 30 parts of polyester polyurethane resin, 1 part of silane coupling agent, 5 parts of hydroxyl-terminated tackifying resin, and 20 parts of a mixed solvent of DMF and MEK.

[0104] Example 2:

[0105] The difference from Example 1 is that:

[0106] The sheepskin topcoat comprises, by weight: 100 parts polyester polyurethane resin, 50 parts sheepskin foaming powder, 5 parts hindered phenolic stress relaxant, 3 parts polycaprolactone polyurethane oligomer, and 50 parts a mixed solvent of DMF and MEK.

[0107] The transition resin comprises, by weight: 70 parts of thermoplastic polyester polyurethane resin, 30 parts of polyester polyurethane resin, 2 parts of silane coupling agent, 8 parts of hydroxyl-terminated tackifying resin, and 30 parts of a mixed solvent of DMF and MEK.

[0108] Example 3:

[0109] The difference from Example 1 is that the temperatures of the five heating zones in S4 foaming are 130°C, 140°C, 150°C, 160°C and 165°C respectively.

[0110] Example 4:

[0111] The difference from Example 1 is that the temperatures of the five heating zones in S7 drying are 90°C, 110°C, 120°C, 120°C and 125°C respectively.

[0112] Example 5:

[0113] The difference from Example 1 is that the sheep bar top layer does not contain hindered phenolic stress relaxation agents, polycaprolactone-based polyurethane oligomers, and a mixed solvent of DMF and MEK.

[0114] Example 6:

[0115] The difference from Example 1 is that no hydroxyl-terminated tackifying resin was added to the transition resin.

[0116] Comparative Example 1:

[0117] The difference from Example 1 is that S5 is removed, that is, S6 is performed after S4 is completed.

[0118] Comparative Example 2:

[0119] The difference from Example 1 is that S3 is removed, that is, S4 is performed after S2 is completed.

[0120] The above embodiments and comparative examples were subjected to relevant tests.

[0121] Three parallel samples were selected for each example and comparative example. In accordance with GB / T 2918-1998 "Standard Environment for Conditioning and Testing of Plastic Specimens", all samples were conditioned for 24 hours in a standard environment with a temperature of 23±2℃ and a relative humidity of 50±5%, and all tests were conducted under this environment.

[0122] Interlayer tensile modulus testing was conducted by separating the mirror transfer coating, transition resin, veneer surface layer, base resin, and microfiber base layer layer by layer along the thickness direction using the cryo-slicing method. Type 5 dumbbell specimens as specified in GB / T 1040.3 were prepared, with a tensile rate of 50 mm / min. The tensile modulus of each layer was recorded, and the modulus difference multiple between adjacent layers (high modulus value / low modulus value) was calculated. The specific results are shown in Table 1.

[0123] Table 1:

[0124]

[0125] Then, a low-stress bending tester was used with the following experimental conditions: bending angle 30°, bending radius 5mm, simulating the bending state of the inner lining of the shoe toe, and bending frequency 60 times / min. The sample was fixed in the fixture with the mirror coating facing outward, and continuous bending cycles were performed according to the set parameters. Samples were taken after 3000, 5000, 8000, and 15000 cycles, respectively. The white mark level was evaluated under a D65 light source at a 45 / 0° observation angle. The specific level descriptions are shown in Table 2, and the experimental results are shown in Table 3.

[0126] Table 2:

[0127]

[0128] Table 3:

[0129]

[0130] Examples 1, 2, 3, 4 and 6 show that the modulus difference between adjacent layers is within one time. Examples 1 and 2 remain at level 0 after 8,000 low-stress cyclic bending cycles (corresponding to the real working conditions of 2-3 months of daily wear of high-end footwear), with no visible white marks or dark cracks. The first appearance of white marks is delayed to 12,000 and 11,500 cycles, respectively. After 15,000 cycles, only level 1 very slight white marks appear, which are only visible at specific angles under strong light and are not abnormal from a normal wearing perspective.

[0131] Although the time for the first appearance of white marks in Examples 3 and 4 was reduced to 7500 and 7200 cycles respectively due to the deviation of the foaming and curing temperature zones from the baseline scheme, only a slight white mark of grade 1 appeared after 8000 cycles, which is still far better than the industry standard. This proves that when the process parameters fluctuate within a reasonable range, the basic performance of the finished product can still be guaranteed.

[0132] In Example 5, due to the absence of the core functional additive for the lamb bar topcoat, the maximum modulus difference between the lamb bar topcoat and the adjacent layers reached 1.87 times. The white marks first appeared at only 2100 cycles, and level 2 visible white marks appeared after 3000 cycles. After 8000 cycles, level 4 severe white marks accompanied by fine dark cracks were observed. The performance dropped by more than 80% compared to the baseline Example 1.

[0133] Although the removal of the hydroxyl-terminated tackifying resin in Example 6 resulted in a decrease in interfacial bonding strength and a reduction in the time to first appearance of white marks to 2800 cycles, it still maintained the core design of a modulus difference within one time. After 8000 cycles, it only showed grade 3 white marks, with no dark cracks or delamination, and its performance was still significantly better than the modulus mismatch group.

[0134] Comparative Example 1 directly removed the transition resin layer, causing the high-modulus mirror transfer coating to come into direct contact with the low-modulus lapis lazuli surface layer. The modulus difference soared to 3.27 times, losing the modulus gradient transition and highlighting the problem of interface stress concentration. It was the group with the most severe performance degradation among all groups. The first appearance of white marks was brought forward to 1800 cycles, and at 3000 cycles, there were obvious white marks of level 3. At 8000 cycles, there were large-area white marks of level 5, obvious cracks and delamination, completely losing the usability of the shoe material.

[0135] Comparative Example 2 only removed the internal stress relaxation process; the rest of the formulation and process were completely identical to Example 1. After removing the internal stress relaxation process, the internal stress caused by the coiling of polyurethane molecular chains during the drying process of the semi-finished product could not be relaxed and released, and was permanently locked inside the material by subsequent foaming and curing processes. During dynamic bending, the residual internal stress superimposed on the dynamic bending stress, significantly accelerating the irreversible silvering of the molecular chains. Ultimately, the maximum modulus difference between adjacent layers exceeded one time, and the time to the first appearance of white marks plummeted from 12,000 cycles to 4,000 cycles, a performance drop of 67%. Grade 3 obvious white marks appeared after 8,000 cycles. It can be seen that the modulus difference design set in this application is a precisely verified performance critical threshold. Once exceeded, even a small modulus mismatch will trigger an exponential increase in interfacial stress concentration, ultimately leading to a precipitous drop in bending resistance to degradation.

[0136] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for preparing microfiber sheepskin leather for footwear, characterized in that: Specifically, the following steps are included: S1. Apply the first layer of sheepskin to the release paper and dry it. After the first drying, apply the second layer of sheepskin and dry it. S2. Coat the base resin onto the sheepskin surface layer and semi-dry bond it with the microfiber base layer and then dry it to obtain a semi-finished product; S3. Perform gradient heating and decompression relaxation treatment on the semi-finished product to release internal stress; S4. Foam the semi-finished product to obtain foamed sheepskin; S5. Coat the foamed sheepskin surface with transition resin and dry; S6. A mirror-finish transfer coating is obtained through a three-blade lamination process; S7. The mirror transfer coating and the transition resin of the foamed sheepskin are semi-dry bonded together and then dried. The difference in tensile modulus between any two adjacent layers in the mirror transfer coating, transition resin, veneer top layer, base resin, and microfiber base is within one time.

2. The method for preparing microfiber sheepskin leather for footwear according to claim 1, characterized in that: The S3 process is divided into three stages: First stage: Oven temperature 110℃, roller pressure 0.2MPa, holding time 30s; Second stage: Oven temperature 112℃, roller pressure 0.15MPa, holding time 30s; Third stage: Oven temperature 115℃, atmospheric pressure without pressure treatment, pressure holding time 60s.

3. The method for preparing microfiber sheepskin leather for footwear according to claim 1, characterized in that: The transition resin in S5 comprises, by weight: 70 parts of thermoplastic polyester polyurethane resin, 30 parts of polyester polyurethane resin, 1-2 parts of silane coupling agent, 5-8 parts of hydroxyl-terminated tackifying resin, and 20-30 parts of a mixed solvent of DMF and MEK.

4. The method for preparing microfiber sheepskin leather for footwear according to claim 1, characterized in that: The drying temperature in S5 is 80-100℃, and the drying time is 60-90s.

5. The method for preparing microfiber sheepskin leather for footwear according to claim 1, characterized in that: The sheepskin surface layer in S1 comprises, by weight: 100 parts of polyester polyurethane resin, 20-50 parts of sheepskin foaming powder, 3-5 parts of hindered phenolic stress relaxant, 2-3 parts of polycaprolactone polyurethane oligomer, and 30-50 parts of a mixed solvent of DMF and MEK.

6. A method for preparing microfiber sheepskin leather for footwear according to any one of claims 3 and 5, characterized in that: The mixing ratio of DMF and MEK is 1:

1.

7. The method for preparing microfiber sheepskin leather for footwear according to claim 5, characterized in that: The S4 process involves passing through five heating zones at 130°C, 145°C, 160°C, 165°C, and 165°C in sequence to complete the foaming process.

8. The method for preparing microfiber sheepskin leather for footwear according to claim 1, characterized in that: In the S7 process, the material passes through five heating zones at 90°C, 110°C, 110°C, 130°C, and 130°C in sequence to complete the drying and curing process.

9. The method for preparing microfiber sheepskin leather for footwear according to claim 8, characterized in that: The dwell time of a single zone in the heating zone at different temperatures in S7 is 30-37.5s.

10. The method for preparing microfiber sheepskin leather for footwear according to claim 1, characterized in that: The dry coating amount of the second coating in S6 is 80-120 g / m². 2 .

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