Leather making process of two-layer suede containing low formaldehyde and hexavalent chromium

Abstract

The invention relates to the technical field of leather preparation, and discloses a leather making process of split suede containing low formaldehyde and hexavalent chromium, the leather making process is carried out by taking blue leather with the thickness of 1.2-1.8 mm and the water content of 65-75% as a raw material, and the leather making process comprises the following steps: putting the blue leather into a drum for rewetting treatment, and sequentially carrying out water washing, retanning, water washing, neutralizing, water washing, dyeing, fat liquoring and color fixing treatment; according to the leather making process of the two-layer suede containing low formaldehyde and hexavalent chromium, provided by the invention, through collaborative optimization of multiple procedures, dual improvement of environmental protection performance and physical performance is realized. Through synergistic pretreatment of rewetting and degreasing, more than 85% of residual grease on the surface of the blue leather raw material can be removed, a foundation is laid for a subsequent process, collagenous fibers are expanded and stretched mainly through hydration, calcium and magnesium ions among the fibers are dissolved by oxalic acid to damage hydrogen bonds, and grease is stripped by a degreasing agent through emulsification, so that fiber pores are opened through synergism of the three components; the reagent permeability is improved; the degreasing efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of leather preparation, in particular to a process for preparing two-layer suede leather containing low formaldehyde and hexavalent chromium. Background Art

[0002] Split suede, a key leather product with a fine, soft surface, is widely used in footwear, bags, and clothing. Traditional split suede tanning typically uses blue cowhide as raw material, undergoing a series of processes including rewetting, tanning, and dyeing. However, this process faces numerous technical challenges in actual production.

[0003] First, formaldehyde and hexavalent chromium pollution are prominent issues. In traditional leather processing, formaldehyde is often used as a cross-linking agent to improve leather strength, while hexavalent chromium is a key component in the tanning process. However, both are highly toxic and bioaccumulative. Studies have shown that formaldehyde levels in leather produced using traditional methods often exceed 30mg / kg, and residual hexavalent chromium can reach over 1.0mg / kg. This not only fails to meet environmental standards such as GB30585-2014, "Technical Specifications for Safety of Children's Shoes," but also poses a threat to the health of operators and the ecological environment.

[0004] Rough control of process parameters leads to unstable performance: the existing process is simple to operate in the retanning stage, and the tanning agent penetration is uneven, resulting in the leather shrinkage temperature reaching only about 85°C and the tensile strength being less than 28N; the neutralization stage lacks a formaldehyde capture mechanism, and the free formaldehyde removal rate is less than 60%; the treatment of hexavalent chromium in the fixation stage relies on a simple precipitation method, which has low removal efficiency and is prone to secondary pollution, limiting the product's application in the high-end market. Summary of the Invention

[0005] In view of the problems in the prior art, the present invention provides a two-layer suede leather making process containing low formaldehyde and hexavalent chromium.

[0006] The technical solution adopted by the present invention to solve the technical problem is: a two-layer suede leather making process containing low formaldehyde and hexavalent chromium, using blue leather with a thickness of 1.2-1.8 mm and a moisture content of 65-75% as raw material, comprising the following steps: The blue hides are placed in drums for rewetting, followed by washing, retanning, washing, neutralization, washing, dyeing, fatliquoring and fixing; The washing step is to use clean water for 10 minutes.

[0007] As a further technical solution, the rewetting treatment is specifically as follows: adding 150-160% of the weight of the blue leather into the drum, then adding 0.3-0.4% of the degreasing agent and 0.3-0.4% of the oxalic acid, and rotating for 80-85 minutes.

[0008] As a further technical solution, the retanning process is specifically as follows: After washing, add 100-110% of the weight of the blue leather into the drum with water, first raise the temperature to 40-45°C, add 0.3-0.4% formic acid and rotate for 20 minutes; then add 5% chromium tannin and 1% aluminum tannin, rotate at 15-20 rpm for 10 minutes, turn on the ultrasonic device and control the frequency according to the following gradient: 30 kHz in the 1-5th minute, 40 kHz in the 6th-15th minute, 60 kHz in the 16th-20th minute, maintain the power at 300-500 W, and maintain the temperature at 45±2°C during the period; then add baking soda for a total of 1-1.2% in three times, each time with an interval of 10 minutes, and at the same time increase the temperature gradient to 50-55°C, during which the pH value of the system is monitored in real time to 4.8-5.2; finally, add 1.5-1.8% synthetic tannin and cooperate with pulse stirring, rotate at 20-30 rpm alternately, with an interval of 5 minutes, and keep warm and rotate for 40 minutes.

[0009] As a further technical solution, the chromium tannin is of model FN5; the aluminum tannin is of model F7018; and the synthetic tannin is of model CFC.

[0010] As a further technical solution, the neutralization treatment is specifically as follows: after washing, add 150-160% of water by weight of the blue leather, first add 2-3% sodium formate, 1-1.6% neutralizer and 2-3% formaldehyde scavenger, rotate for 40 minutes, then add 1.5-1.8% ammonium carbonate and 1.2-1.6% baking soda, and rotate for 90 minutes; The neutralizer model is 819.

[0011] As a further technical solution, the dyeing process is specifically as follows: after washing, add 20-30% of the weight of the blue leather in water, add 3-4% of synthetic tannin, 3-4% of dicyandiamide, 1-1.5% of leveling agent and 3-4% of dye, and rotate for 60 minutes; The leveling agent model is 308; The dye is a 1:2 metal complex acid dye.

[0012] As a further technical solution, the greasing treatment is specifically as follows: adding hot water maintained at 50℃±5℃, the amount of water is 150-160% of the weight of the blue leather, adding 1-1.5% synthetic oil, 2-3% lanolin and 2-3% lecithin, and rotating for 80 minutes.

[0013] As a further technical solution, the color fixing treatment is specifically as follows: add water to 5-6% of the blue leather weight, first add 2-3% formic acid in two times, rotate for 20 minutes each time, then add 1.5-2% of chromium remover 5099 and rotate for 40 minutes, and finally add 0.5-0.8% formic acid and rotate for 20 minutes before washing.

[0014] As a further technical solution, the formaldehyde capture agent is model 9800 and the chromium remover is model 5099.

[0015] Beneficial effects of the present invention: The low-formaldehyde and hexavalent chromium two-layer suede leather making process provided by the present invention achieves dual improvements in environmental performance and physical properties through coordinated optimization of multiple processes.

[0016] The present invention can remove more than 85% of residual grease on the surface of blue leather raw materials through synergistic pretreatment of rewetting and degreasing, laying the foundation for subsequent processes. It mainly uses hydration to expand and stretch collagen fibers, oxalic acid dissolves calcium and magnesium ions between fibers to destroy hydrogen bonds, and the degreaser removes grease through emulsification. The three work together to open the fiber pores, enhance the permeability of the reagent and improve the degreasing efficiency.

[0017] The present invention introduces an ultrasonic gradient retanning process. With the assistance of 30-60kHz gradient frequency ultrasound, the binding rate of chromium tannin and aluminum tannin is greatly improved. This is mainly because the cavitation effect generated by low-frequency ultrasound (30kHz) opens the fiber micropores and promotes the penetration of large-molecular chromium tannin; medium-high frequency (40-60kHz) accelerates the diffusion of aluminum tannin and forms a hydrogen bond network with collagen; combined with 300-500W power control to avoid fiber damage, at a specific temperature, the chromium tannin forms a stable coordination bond with the collagen carboxyl group, and the synthetic tannin fills the fiber gaps to enhance the fullness.

[0018] Through the combination of formaldehyde capture and step-by-step neutralization, the formaldehyde capture agent reacts with free formaldehyde to generate hydrazone compounds, so that the formaldehyde content of the finished product is ≤12.5mg / kg. The pH of the sodium formate buffer system is adjusted to 6.5-7.0 by the neutralizer in steps. The hydrazine group in the formaldehyde capture agent undergoes a nucleophilic addition reaction with formaldehyde to generate low-toxic hydrazone substances, thereby preventing formaldehyde release.

[0019] In the present invention, synthetic tannins and dicyandiamide synergistically promote the formation of coordination bonds between dye molecules and collagen amino groups, the leveling agent reduces the charge repulsion on the fiber surface, and under acidic conditions, the dye anions and collagen cations electrostatically combine to achieve color uniformity (color difference ≤ 1.0).

[0020] In the present invention, synthetic oil and lanolin form a hydrophobic film on the fiber surface, and lecithin acts as an emulsifier to enhance the combination of oil and collagen. Under acidic conditions, the mercapto group of the chromium remover forms a stable chelate with hexavalent chromium, and the residual chromium is removed through the dual effects of adsorption and precipitation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The present invention is a process flow chart for making a two-layer suede leather containing low formaldehyde and hexavalent chromium. DETAILED DESCRIPTION

[0022] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0023] The present invention provides a two-layer suede leather tanning process containing low formaldehyde and hexavalent chromium, comprising the following steps: The raw material is blue leather with a thickness of 1.2-1.8mm and a moisture content of 65-75% (the moisture content is controlled at 65-75% to avoid excessive drying of the blue leather and hardening of the fiber). The blue leather is placed in a drum for rehumidification treatment, and then washed, retanned, washed, neutralized, washed, dyed, fatliquored and fixed in sequence; among them, washing is done with clean water for 10 minutes (short-term washing can not only remove surface impurities, but also avoid excessive swelling of the fiber).

[0024] The following are the specific steps: a) Rehumidification treatment Add 150-160% of the weight of the blue leather into the drum. The right amount of water can ensure that the blue leather fully absorbs water to restore its softness and avoid uneven rewetting due to insufficient water. Then add 0.3-0.4% degreasing agent and 0.3-0.4% oxalic acid. The degreasing agent can effectively remove residual grease from the blue leather. Oxalic acid has the functions of softening the fiber and adjusting the initial pH. The dosage of 0.3-0.4% can achieve the degreasing effect while avoiding excessive corrosion of the fiber. Rotate for 80-85 minutes. Sufficient rotation time can restore the moisture content of the blue leather to 85-90%, ensuring uniform penetration of the reagent.

[0025] The degreasing agent is a conventional degreasing agent available on the market, and the oxalic acid is analytically pure oxalic acid.

[0026] Hydration causes the blue leather fibers to swell, the degreasing agent destroys the oil emulsification system, and oxalic acid dissolves the calcium and magnesium ions between the fibers. The three work together to restore the processing adaptability of blue leather.

[0027] b) Retanning After washing, add 100-110% of the blue leather weight of water into the drum, first heat it to 40-45℃, add 0.3-0.4% formic acid and rotate it for 20 minutes. Adding formic acid at low temperature can slowly reduce the pH of the system to 3.5-3.8, promote the expansion of the chromium tannin molecular chain, and prepare for subsequent penetration; Then add 5% chromium tannin and 1% aluminum tannin. The chromium tannin gives the leather moisture and heat resistance stability, and the aluminum tannin improves softness. After rotating at 15-20 rpm for 10 minutes, turn on the ultrasonic device and control the frequency according to the following gradient: 30kHz for the 1st to 5th minute, 40kHz for the 6th to 15th minute, and 60kHz for the 16th to 20th minute. The low frequency of 30kHz promotes the penetration of large molecular chromium tannin, the medium frequency of 40kHz accelerates the diffusion of aluminum tannin, and the high frequency of 60kHz enhances the cavitation effect in the fiber gap. The gradient adjustment can improve the uniformity of tanning agent binding. The power is maintained at 300-500W. This power range can produce sufficient cavitation effect while avoiding fiber damage. During this period, the temperature is maintained at 45±2℃. At this temperature, the hydrolysis rate of chromium tannin is moderate, which is conducive to binding with collagen amino groups. Subsequently, baking soda was added three times, totaling 1-1.2%, with 10-minute intervals between each addition. Simultaneously, the temperature gradient was raised to 50-55°C. Adding baking soda three times prevented a sudden rise in pH that would cause chromium salt precipitation. Raising the temperature to above 50°C promoted the cross-linking reaction between chromium tannins and collagen. During this period, the system pH was monitored in real time to 4.8-5.2, which is the optimal range for the coordination reaction between chromium tannins and collagen carboxyl groups, forming a stable cross-linked structure. Finally, add 1.5-1.8% synthetic tannin and combine with pulse stirring, with a rotation speed of 20-30rpm alternating for 5 minutes. The synthetic tannin fills the gaps between the fibers to increase the fullness. The pulse stirring can break the boundary layer and promote the diffusion of the tanning agent into the fiber. Keep warm and rotate for 40 minutes.

[0028] The chromium tannin is model FN5, contains hydroxycarboxylic acid ligands, and has strong resistance to alkali hydrolysis. The aluminum tannin is model F7018, has a moderate degree of polymerization, and has a significant effect on improving softness. The synthetic tannin is model CFC, contains sulfonic acid groups, and synergistically enhances filling properties with the chromium tannin.

[0029] The ultrasonic cavitation effect opens the fiber pores, chromium tannin forms coordination bonds with collagen, aluminum tannin combines through hydrogen bonds, and synthetic tannin fills the fiber network, jointly increasing the shrinkage temperature (≥95℃) and tensile strength of the leather.

[0030] c) Neutralization treatment After washing, add 150-160% of the blue leather's weight of water. Large amounts of water ensure that residual acid is fully removed to create a neutral environment for neutralization. First, add 2-3% sodium formate, 1-1.6% neutralizer, and 2-3% formaldehyde scavenger. Sodium formate acts as a buffer to prevent a sudden increase in pH. Neutralizer, model 819, adjusts the pH to 5.5-6.0. Formaldehyde scavenger, model 9800, reacts with free formaldehyde to form hydrazone compounds. 2-3% can capture more than 90% of free formaldehyde. Rotate for 40 minutes. Then add 1.5-1.8% of ammonium carbonate and 1.2-1.6% of baking soda. The ammonium carbonate decomposes to produce ammonia gas to further neutralize the residual acid. The baking soda stabilizes the pH at 6.5-7.0, providing a weak alkaline environment for dyeing. Rotate for 90 minutes. The neutralizer model is 819 and the formaldehyde scavenger model is 9800.

[0031] Step-by-step neutralization can avoid local excessive alkalinity that causes fiber expansion. The formaldehyde scavenger converts formaldehyde into harmless substances through chemical reactions, so that the formaldehyde content of the finished product is ≤15mg / kg.

[0032] d) Dyeing treatment After washing, add 20-30% of the blue leather weight of water. Low liquid volume dyeing can improve dye utilization. 20-30% liquid ratio can maintain the dye concentration at 1.5-2.0g / L. Add 3-4% synthetic tannin, 3-4% dicyandiamide, 1-1.5% leveling agent and 3-4% dye. Synthetic tannin forms hydrogen bonds with the dye to assist dyeing. Dicyandiamide acts as a fixing agent and combines with the dye anion. Leveling agent, model 308, fatty alcohol polyoxyethylene ether, reduces the surface charge repulsion of the fiber and promotes uniform dye penetration. 1:2 metal complex acid dye forms a coordination bond with collagen amino group. Color fastness ≥ Grade 4. Rotate for 60 minutes. The leveling agent model is 308, and the dye is a 1:2 metal complex acid dye.

[0033] Under low liquid volume conditions, synthetic tannins and dicyandiamide synergistically promote the chemical bonding of dye molecules and collagen, and the leveling agent improves dye migration to achieve color uniformity (color difference ≤ 1.0).

[0034] e) Fatliquor treatment Add hot water with a temperature maintained at 50℃±5℃. 50℃ is close to the melting point of oil, which can reduce viscosity and facilitate penetration. The fluctuation range of ±5℃ avoids oil coagulation caused by sudden temperature change. The amount of water is 150-160% of the weight of blue leather. The liquid volume is enough to evenly disperse the oil emulsion. Add 1-1.5% synthetic oil, 2-3% lanolin and 2-3% lecithin. Synthetic oil provides waterproofness, lanolin gives a soft feel, and lecithin as a natural emulsifier enhances the bonding between oil and fiber. The ratio of the three ensures a balance between the softness and mechanical strength of the leather. Rotate for 80 minutes. Long rotation allows the oil to fully penetrate the fiber gaps. After greasing, the oil content of the leather reaches 8-10%.

[0035] Under warm conditions, the oil emulsion penetrates into the fiber through capillary action, the synthetic oil and lanolin form a hydrophobic film on the fiber surface, and the amphiphilic structure of lecithin enhances the binding force between oil and collagen.

[0036] f) Color fixing treatment Add water to 5-6% of the blue leather weight, first add 2-3% formic acid in two times, rotating for 20 minutes each time, adding formic acid in batches to avoid a sudden drop in pH that causes dye precipitation. The first time formic acid is added, adjust the pH to 4.0-4.5, and the second time to 3.5-3.8 to promote the combination of dye and collagen amino groups; Then add 1.5-2% of chromium remover 5099 and rotate for 40 minutes. The chromium remover model 5099 forms a stable chelate with hexavalent chromium. The dosage of 1.5-2% can reduce the hexavalent chromium content to ≤0.5mg / kg. Finally, add 0.5-0.8% formic acid and rotate for 20 minutes to ensure that the dye is completely fixed and to promote the reaction between the chromium remover and the residual hexavalent chromium.

[0037] Under acidic conditions, the dye forms ionic bonds and hydrogen bonds with collagen, and the chromium remover reduces and fixes hexavalent chromium through chelation, ultimately achieving a hexavalent chromium residue of ≤0.3 mg / kg.

[0038] The following are specific embodiments Example 1 Raw material: blue leather with a thickness of 1.5mm and a moisture content of 70%.

[0039] Rewetting treatment: add 155% of the blue leather weight of water, 0.35% of degreasing agent, 0.35% of oxalic acid, and rotate for 82 minutes.

[0040] Retanning: Add 105% water, raise the temperature to 42°C, add 0.35% formic acid, and rotate for 20 minutes. Add 5% chrome tannin and 1% aluminum tannin, rotate at 18 rpm for 10 minutes, then turn on the ultrasonic device (power 400W, frequency controlled by gradient: 30 kHz for the 1st to 5th minute, 40 kHz for the 6th to 15th minute, and 60 kHz for the 16th to 20th minute). Add baking soda for a total of 1.1% in three times (each time with an interval of 10 minutes), while increasing the temperature gradient to 52°C and monitoring the pH value in real time until it reaches 5.0. Finally, add 1.65% synthetic tannin, combine with pulsed stirring (alternating speed of 20-30 rpm, with an interval of 5 minutes), and rotate at this temperature for 40 minutes.

[0041] Neutralization treatment: add 155% water, first add 2.5% sodium formate, 1.3% neutralizer, 2.5% formaldehyde scavenger and rotate for 40 minutes, then add 1.65% ammonium carbonate and 1.4% baking soda and rotate for 90 minutes.

[0042] Dyeing treatment: Add 25% water, 3.5% synthetic tannin, 3.5% dicyandiamide, 1.25% leveling agent, and 3.5% dye and rotate for 60 minutes.

[0043] Fatliquor treatment: add 50℃ hot water (water volume is 155% of the weight of blue leather), add 1.25% synthetic oil, 2.5% lanolin, 2.5% lecithin and rotate for 80 minutes.

[0044] Color fixing treatment: add water to 5.5% of the blue leather weight, first add 2.5% formic acid twice (rotate for 20 minutes each time), then add 1.75% of chromium remover 5099 and rotate for 40 minutes, finally add 0.65% formic acid and rotate for 20 minutes.

[0045] Example 2 Raw material: blue leather with a thickness of 1.2mm and a moisture content of 65%.

[0046] Rewetting treatment: add 150% of the blue leather weight of water, 0.3% degreasing agent, 0.3% oxalic acid, and rotate for 80 minutes.

[0047] Retanning: add 100% water, raise the temperature to 40°C, add 0.3% formic acid and rotate for 20 minutes; add 5% chrome tannin and 1% aluminum tannin, rotate at 15 rpm for 10 minutes, then turn on the ultrasonic device (power 300W, frequency controlled by 30kHz in the 1-5th minute, 40kHz in the 6th-15th minute, and 60kHz in the 16th-20th minute), add baking soda for a total of 1% in three times (each time with an interval of 10 minutes), increase the temperature gradient to 50°C, and the pH value to 4.8; add 1.5% synthetic tannin, and stir in pulses (alternating at 20-30rpm) for 40 minutes.

[0048] Neutralization treatment: add 150% water, 2% sodium formate, 1% neutralizer, 2% formaldehyde scavenger and rotate for 40 minutes, add 1.5% ammonium carbonate and 1.2% baking soda and rotate for 90 minutes.

[0049] Dyeing treatment: add 20% water, 3% synthetic tannin, 3% dicyandiamide, 1% leveling agent, and 3% dye and rotate for 60 minutes.

[0050] Fatliquor treatment: add 45℃ hot water (150% by weight), 1% synthetic oil, 2% lanolin, and 2% lecithin and rotate for 80 minutes.

[0051] Color fixing treatment: add water to 5%, add 2% formic acid twice (20 minutes each time), add 1.5% of chromium remover 5099 and rotate for 40 minutes, add 0.5% formic acid and rotate for 20 minutes, then wash.

[0052] Example 3 Raw material: blue leather with a thickness of 1.8mm and a moisture content of 75%.

[0053] Rewetting treatment: add 160% of the blue leather weight of water, 0.4% degreasing agent, 0.4% oxalic acid, and rotate for 85 minutes.

[0054] Re-tanning treatment: add 110% water, temperature to 45°C, add formic acid 0.4%, rotate for 20 minutes; add chrome tannin 5%, aluminum tannin 1%, rotate for 10 minutes at 20 rpm, then open the ultrasonic device (power 500 W, frequency gradient control according to 30 kHz for 1-5 minutes, 40 kHz for 6-15 minutes, 60 kHz for 16-20 minutes), add baking soda 1.2% in three times (each interval 10 minutes), temperature gradient to 55°C, pH value 5.2; add synthetic tannin 1.8%, pulse stirring (20-30 rpm alternately) for 40 minutes.

[0055] Neutralization treatment: add 160% water, sodium formate 3%, neutralizing agent 1.6%, formaldehyde capture agent 3%, rotate for 40 minutes, add ammonium carbonate 1.8%, baking soda 1.6%, rotate for 90 minutes.

[0056] Dyeing treatment: add 30% water, synthetic tannin 4%, dicyandiamide 4%, leveling agent 1.5%, dye 4%, rotate for 60 minutes.

[0057] Fat-liquoring treatment: add 55°C hot water (160% by weight), synthetic oil 1.5%, lanolin 3%, lecithin 3%, rotate for 80 minutes.

[0058] Fixation treatment: add water to 6%, add formic acid 3% in two times (each 20 minutes), add chrome removal agent 5099 2%, rotate for 40 minutes, add formic acid 0.8%, rotate for 20 minutes, then complete washing.

[0059] Example 4 Raw material: blue skin with thickness 1.2 mm, moisture content 68%.

[0060] Wet-back treatment: add water 152% of the weight of the blue skin, degreasing agent 0.32%, oxalic acid 0.32%, rotate for 81 minutes.

[0061] Re-tanning treatment: add 102% water, temperature to 41°C, add formic acid 0.32%, rotate for 20 minutes; add chrome tannin 5%, aluminum tannin 1%, rotate for 10 minutes at 16 rpm, then open the ultrasonic device (power 350 W, frequency gradient control according to 30 kHz for 1-5 minutes, 40 kHz for 6-15 minutes, 60 kHz for 16-20 minutes), add baking soda 1.05% in three times (each interval 10 minutes), temperature gradient to 51°C, pH value 4.9; add synthetic tannin 1.55%, pulse stirring (20-30 rpm alternately) for 40 minutes.

[0062] Neutralization treatment: add 152% water, sodium formate 2.2%, neutralizing agent 1.1%, formaldehyde capture agent 2.2%, rotate for 40 minutes, add ammonium carbonate 1.55%, baking soda 1.25%, rotate for 90 minutes.

[0063] Dyeing treatment: Add 22% water, 3.2% synthetic tannin, 3.2% dicyandiamide, 1.1% leveling agent, and 3.2% dye and rotate for 60 minutes.

[0064] Fatliquor treatment: add 48℃ hot water (152% by weight), 1.1% synthetic oil, 2.2% lanolin, and 2.2% lecithin and rotate for 80 minutes.

[0065] Color fixing treatment: add water to 5.2%, add 2.2% formic acid twice (20 minutes each time), add 1.6% of chromium remover 5099 and rotate for 40 minutes, add 0.55% formic acid and rotate for 20 minutes, then wash.

[0066] Example 5 Raw material: blue leather with a thickness of 1.2mm and a moisture content of 73%.

[0067] Rewetting treatment: add 158% of the blue leather weight of water, 0.38% of degreasing agent, 0.38% of oxalic acid, and rotate for 84 minutes.

[0068] Retanning: add 108% water, raise the temperature to 44°C, add 0.38% formic acid and rotate for 20 minutes; add 5% chrome tannin and 1% aluminum tannin, rotate at 19 rpm for 10 minutes, then turn on the ultrasonic device (power 450W, frequency controlled by gradient of 30kHz from 1-5 minutes, 40kHz from 6-15 minutes, and 60kHz from 16-20 minutes), add baking soda for a total of 1.15% in three times (each time with an interval of 10 minutes), increase the temperature gradient to 54°C, pH value 5.1; add 1.75% synthetic tannin, and pulse stir (alternating at 20-30 rpm) for 40 minutes.

[0069] Neutralization treatment: add 158% water, 2.8% sodium formate, 1.5% neutralizer, 2.8% formaldehyde scavenger and rotate for 40 minutes, add 1.75% ammonium carbonate and 1.5% baking soda and rotate for 90 minutes.

[0070] Dyeing treatment: Add 28% water, 3.8% synthetic tannin, 3.8% dicyandiamide, 1.4% leveling agent, and 3.8% dye and rotate for 60 minutes.

[0071] Fatliquor treatment: add 53℃ hot water (158% by weight), synthetic oil 1.4%, lanolin 2.8%, lecithin 2.8% and rotate for 80 minutes.

[0072] Color fixing treatment: add water to 5.8%, add 2.8% formic acid twice (20 minutes each time), add 1.9% of chromium remover 5099 and rotate for 40 minutes, add 0.7% formic acid and rotate for 20 minutes, then wash.

[0073] Comparative Example Comparative Example 1 Different from Example 1: During the retanning process, no gradient-controlled frequency ultrasonic treatment was performed. That is, after adding chrome tannin and aluminum tannin, the mixture was rotated at 18 rpm for 10 minutes, and then baking soda was directly added in three times, totaling 1.1%. The temperature gradient was raised to 52° C., and the pH value was 5.0. The subsequent steps were consistent with Example 1.

[0074] Comparative Example 2 Different from Example 1: No formaldehyde scavenger (model 9800) was added during the neutralization treatment. That is, 2.5% sodium formate and 1.3% neutralizer were added and then rotated for 40 minutes. The subsequent steps of adding ammonium carbonate and baking soda were the same as in Example 1, and the remaining steps remained unchanged.

[0075] test Formaldehyde content detection Test method: Refer to GB / T2912.1-2009 “Determination of formaldehyde in textiles - Part 1: Free and hydrolyzed formaldehyde (water extraction method)” for testing.

[0076] Table 1

[0077] As can be seen from Table 1, the formaldehyde contents of Examples 1-5 are all low, meeting environmental protection requirements, indicating that the process of the present invention effectively reduces the formaldehyde content by measures such as adding a formaldehyde scavenger.

[0078] Comparative Example 1 did not use an ultrasonic device, and the retanning effect was poor, resulting in insufficient formaldehyde binding and a high residual amount.

[0079] Comparative Example 2 did not add formaldehyde scavenger, and free formaldehyde could not be effectively removed, and the formaldehyde content was significantly higher than that in the embodiment.

[0080] Hexavalent chromium content detection Test method: Refer to GB / T17593.3-2006 “Determination of heavy metals in textiles - Part 3: Hexavalent chromium spectrophotometric method” for testing.

[0081] Table 2

[0082] As can be seen from Table 2, the hexavalent chromium content of Examples 1-5 is all at a relatively low level, indicating that the process of the present invention has a significant effect in controlling hexavalent chromium.

[0083] Comparative Example 1 did not use ultrasound, which affected the penetration and binding of chromium tannins, resulting in an increase in hexavalent chromium residues.

[0084] Comparative Example 2 shows that the formaldehyde scavenger does not affect the chromium content.

[0085] Tear strength test Test method: Refer to GB / T3917.1-2009 “Tear properties of textile fabrics Part 1: Determination of tear strength by impact pendulum method” for testing.

[0086] Table 3

[0087] As can be seen from Table 3, the tear strength of Examples 1-5 is relatively high, indicating that the leather prepared by the process of the present invention has good physical properties, and the optimization of the raw material formula and process steps has improved the strength of the leather.

[0088] The tear strength of Comparative Examples 1 and 2 is significantly lower than that of the embodiment. Comparative Example 1 does not use ultrasound, resulting in insufficient retanning and poor fiber bonding. Comparative Example 2 may lack a formaldehyde scavenger, which affects the cross-linking structure of the leather and causes a slight decrease in strength.

[0089] In summary, the present invention provides a process for producing two-layer suede leather containing low formaldehyde and hexavalent chromium. By optimizing various process steps and parameters, particularly the application of ultrasound and the addition of a formaldehyde scavenger, the process reduces harmful substance levels while maintaining excellent physical properties. The comparative example exhibits performance degradation due to the omission of key process steps or altered parameters, further demonstrating the superiority of the present invention's process.

[0090] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A process for making two-layer suede leather containing low formaldehyde and hexavalent chromium, characterized in that: The preparation method is based on blue leather with a thickness of 1.2-1.8 mm and a moisture content of 65-75%, and includes the following steps: The blue hides are placed in drums for rewetting, followed by washing, retanning, washing, neutralization, washing, dyeing, fatliquoring and fixing; The washing step is to use clean water for 10 minutes.

2. The leather making process according to claim 1, characterized in that The rewetting treatment is specifically as follows: adding 150-160% of water by weight of blue leather into the drum, then adding 0.3-0.4% of degreasing agent and 0.3-0.4% of oxalic acid, and rotating for 80-85 minutes.

3. The leather making process according to claim 1, characterized in that The retanning process is specifically: After washing, add 100-110% of the weight of the blue leather into the drum with water, first raise the temperature to 40-45°C, add 0.3-0.4% formic acid and rotate for 20 minutes; then add 5% chromium tannin and 1% aluminum tannin, rotate at 15-20 rpm for 10 minutes, turn on the ultrasonic device and control the frequency according to the following gradient: 30 kHz in the 1-5th minute, 40 kHz in the 6th-15th minute, 60 kHz in the 16th-20th minute, maintain the power at 300-500 W, and maintain the temperature at 45±2°C during the period; then add baking soda for a total of 1-1.2% in three times, each time with an interval of 10 minutes, and at the same time increase the temperature gradient to 50-55°C, during which the pH value of the system is monitored in real time to 4.8-5.2; finally, add 1.5-1.8% synthetic tannin and cooperate with pulse stirring, rotate at 20-30 rpm alternately, with an interval of 5 minutes, and keep warm and rotate for 40 minutes.

4. The leather making process according to claim 1, characterized in that The chromium tannin is model FN5; the aluminum tannin is model F7018; and the synthetic tannin is model CFC.

5. The leather making process according to claim 1, characterized in that The neutralization treatment is specifically as follows: after washing, add 150-160% of water by weight of the blue leather, first add 2-3% sodium formate, 1-1.6% neutralizer and 2-3% formaldehyde scavenger, rotate for 40 minutes, then add 1.5-1.8% ammonium carbonate and 1.2-1.6% baking soda, and rotate for 90 minutes; The neutralizer model is 819.

6. The leather making process according to claim 1, characterized in that The dyeing process is specifically as follows: after washing, add 20-30% of the weight of the blue leather in water, add 3-4% synthetic tannin, 3-4% dicyandiamide, 1-1.5% leveling agent and 3-4% dye, and rotate for 60 minutes; The leveling agent model is 308; The dye is a 1:2 metal complex acid dye.

7. The leather making process according to claim 1, characterized in that The fatliquoring treatment is specifically as follows: adding hot water maintained at 50°C±5°C, the amount of water being 150-160% of the weight of the blue leather, adding 1-1.5% synthetic oil, 2-3% lanolin and 2-3% lecithin, and rotating for 80 minutes.

8. The leather making process according to claim 1, characterized in that The color fixing treatment is specifically as follows: add water to 5-6% of the blue leather weight, first add 2-3% formic acid in two times, rotate for 20 minutes each time, then add 1.5-2% of chromium remover 5099 and rotate for 40 minutes, and finally add 0.5-0.8% formic acid and rotate for 20 minutes before washing.

9. The leather making process according to claim 4, characterized in that: The formaldehyde capture agent is model 9800, and the chromium removal agent is model 5099.

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