A wrinkle-resistant PP base film for current collector and preparation method thereof

Abstract

The present invention discloses a PP base film for current collector and a preparation method thereof, and relates to the technical field of lithium ion battery materials. The PP base film includes a surface layer, an intermediate layer and a bottom layer, wherein the surface layer includes polypropylene and polyoxymethylene, and the mass ratio is (95-97): (3-5); the intermediate layer includes polypropylene and polyamide, and the mass ratio is (80-90): (10-20); the bottom layer includes polypropylene and polyoxymethylene, and the mass ratio is (90-95): (5-10). The present invention greatly improves the heat resistance of the film through a specific formula and film structure design, achieves the effect of reducing the thermal shrinkage rate of the PP base film for current collector, and effectively prevents the generation of wrinkles.

Description

Technical Field

[0001] The invention provides a wrinkle-resistant PP base film for a current collector and a preparation method thereof, and relates to the technical field of lithium ion battery materials. Background Art

[0002] The current collector is one of the indispensable components of lithium-ion batteries. It collects and outputs the current generated by the active material and inputs the electrode current to the active material, thereby realizing the conversion of chemical energy into electrical energy. The performance of the current collector directly affects the coulombic efficiency, cycle stability and rate performance of lithium-ion batteries. Therefore, it is particularly important to develop a current collector with excellent comprehensive performance and performance stability.

[0003] Traditional current collectors are mostly metal current collectors, which are very heavy, resulting in the current collector mass occupying a large proportion of the total mass of the battery. When subjected to mechanical stress (especially extrusion, puncture, impact), thermal stress or electrical stress, internal short circuits are also prone to occur, resulting in thermal runaway of the battery and safety accidents. It is under this situation that composite current collectors came into being, and its appearance has attracted widespread attention in the industry. Composite current collectors are copper / aluminum atoms deposited on the surface of polymer base films to form composite copper foil current collectors or composite aluminum foil current collectors. This type of current collector combines the mechanical and structural advantages of polymer films with the electrochemical performance advantages of copper and aluminum, reduces the amount of metal used, and can effectively improve energy density and safety. The polymer base film is one of the key components of the composite current collector. The ideal composite current collector base film needs to have excellent mechanical properties, low contact resistance and strong bonding between the metal, better flame retardant properties and electrolyte tolerance, and better heat aging resistance.

[0004] PP film has become one of the mainstream current collector base films. One of the main problems with the application of PP base film in current collectors is that high temperature operation steps are often required in the preparation of current collectors. Due to the limited high temperature resistance of current PP film, the base film often shrinks and wrinkles after being heated during these operation steps, which has an adverse effect on the overall performance of the final product. Summary of the invention

[0005] The present invention proposes a wrinkle-resistant PP base film for current collector and a preparation method thereof, which effectively improves the high temperature resistance of the PP base film and avoids wrinkles during subsequent use. The specific scheme is as follows:

[0006] A wrinkle-resistant PP base film for current collector:

[0007] It includes a surface layer, an intermediate layer and a bottom layer, wherein the surface layer includes polypropylene and polyoxymethylene, and the mass ratio of the two is (95-97):(3-5); the intermediate layer includes polypropylene and polyamide, and the mass ratio of the two is (80-90):(10-20); the bottom layer includes polypropylene and polyoxymethylene, and the mass ratio of the two is (90-95):(5-10).

[0008] Preferably, the polyamide comprises nylon 6.

[0009] Preferably, the material mass ratio of the surface layer, the middle layer and the bottom layer is (1-2):(2-4):(8-10).

[0010] A method for preparing a wrinkle-resistant PP base film for a current collector comprises the following steps:

[0011] S1, mixing the raw materials of the surface layer, the middle layer and the bottom layer respectively, and adding them into a twin-screw extruder for extrusion and melting;

[0012] S2, the melted surface layer, middle layer and bottom layer liquid are extruded into a three-layer sheet through the die head of a twin-screw extruder;

[0013] S3, the extruded material is cooled by a cooling roller to obtain a three-layer sheet;

[0014] S4, subjecting the obtained three-layer sheet to a first heat treatment and then longitudinally stretching it, subjecting the film obtained by the longitudinal stretching to a second heat treatment and then cooling it to obtain a longitudinally stretched film;

[0015] S5, the obtained longitudinally stretched film is subjected to a third heat treatment and then subjected to a first transverse stretching, and the transversely stretched film is subjected to a fourth heat treatment and then cooled to obtain a transversely stretched film;

[0016] S6. The obtained horizontally drawn film sheet is packaged after being pulled, rolled up, cut and then cut to obtain a PP base film for the current collector.

[0017] Preferably, in the co-extrusion described in S2, the mass ratio of the molten material obtained by slicing the surface layer, the molten material obtained by slicing the middle layer and the molten material obtained by slicing the bottom layer is (1-2):(2-4):(8-10).

[0018] Preferably, in the longitudinal stretching described in S4, the stretching ratio is (3-5):1.

[0019] Preferably, in the transverse stretching described in S5, the stretching ratio is (2-3):1.

[0020] Preferably, the raw materials of the surface layer, the middle layer and the bottom layer in S1 are extruded and melted, and the temperatures are controlled below 195° C., below 155° C. and below 195° C., respectively.

[0021] Preferably, in step S2, the three-layer sheet is extruded through a die of a twin-screw extruder, and the die temperature is 205-210°C.

[0022] Preferably, the first heat treatment temperature is 90-100°C, and the first heat treatment time is 15-20min; the second heat treatment temperature is 110-120°C, and the second heat treatment time is 20-30min; the third heat treatment temperature is 80-90°C, and the third heat treatment time is 15-20min; the fourth heat treatment temperature is 100-110°C, and the fourth heat treatment time is 30-50min.

[0023] Beneficial Effects

[0024] The present invention improves the high temperature resistance of the PP base film and reduces the heat shrinkage of the PP base film under high temperature use conditions through a specific formula and a three-layer structure design. As far as we know, in current film-related research, the polyoxymethylene used in the present invention is often used to improve the hardness, stiffness, wear resistance, etc. of the film. In existing studies, no effect of polyoxymethylene on the high temperature resistance of the film has been found. In the range of 1%-4%, the addition amount of POM is relatively low, and is mainly used to improve specific properties of polymer films, such as hardness, wear resistance, etc., without significantly changing the overall physical properties. In the range of medium addition amount (5%-10%), the addition of POM The amount of PAI gradually increases, and there will be some improvements in mechanical properties, such as improving the hardness and stiffness of the film; while the polyamide used in the present invention, in the range of low addition (1%-5%), is mainly used to adjust the specific properties of the film without significantly changing the overall physical properties. In the range of medium addition (5%-10%), the amount of PAI gradually increases, and there will be some improvements in mechanical properties, mainly in the improvement of strength and stiffness, but at an addition amount below 10%, no improvement in the thermal stability and thermal shrinkage of the film is seen; and we found that nylon 6 in polyamide can achieve the effect of improving thermal stability, while nylon 66 has no effect on thermal stability. However, we found in our research that designing a three-layer structure in a polypropylene film and designing a specific addition ratio of the three in each layer can achieve the effect of improving the high temperature resistance of the film material and reducing the thermal shrinkage rate. This may be due to the different structural properties of each layer of the three-layer structure, and the heterogeneity and the interaction between the two layers cause a composite effect in terms of high temperature resistance. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0026] Unless otherwise defined, all professional and scientific terms used herein have the same meanings as those familiar to those skilled in the art. In addition, any methods and materials similar or equivalent to those described herein may be applied to the present invention. The preferred implementation methods and materials described herein are for demonstration purposes only and are not intended to limit the content of this application.

[0027] The experimental methods in the following examples are conventional methods unless otherwise specified, and the experimental materials used in the following examples are purchased from commercial channels unless otherwise specified.

[0028] Structure and formula:

[0029] It includes a surface layer, an intermediate layer and a bottom layer, wherein the surface layer includes polypropylene and polyoxymethylene in a mass ratio of (95-97):(3-5); the intermediate layer includes polypropylene and polyamide in a mass ratio of (80-90):(10-20); and the bottom layer includes polypropylene and polyoxymethylene in a mass ratio of (90-95):(5-10).

[0030] The polyamide includes nylon 6.

[0031] The material mass ratio of the surface layer, the middle layer and the bottom layer is (1-2):(2-4):(8-10).

[0032] Preparation process:

[0033] S1, mixing the raw materials of the surface layer, the middle layer and the bottom layer respectively, and adding them into a twin-screw extruder for extrusion and melting;

[0034] S2, the melted surface layer, middle layer and bottom layer liquid are extruded into a three-layer sheet through the die head of a twin-screw extruder;

[0035] S3, the extruded material is cooled by a cooling roller to obtain a three-layer sheet;

[0036] S4, subjecting the obtained three-layer sheet to a first heat treatment and then longitudinally stretching it, subjecting the film obtained by the longitudinal stretching to a second heat treatment and then cooling it to obtain a longitudinally stretched film;

[0037] S5, the obtained longitudinally stretched film is subjected to a third heat treatment and then subjected to a first transverse stretching, and the transversely stretched film is subjected to a fourth heat treatment and then cooled to obtain a transversely stretched film;

[0038] S6. The obtained horizontally drawn film sheet is packaged after being pulled, rolled up, cut and then cut to obtain a PP base film for the current collector.

[0039] In the co-extrusion described in S2, the mass ratio of the molten material obtained from the surface layer slices, the molten material obtained from the middle layer slices and the molten material obtained from the bottom layer slices is (1-2):(2-4):(8-10).

[0040] The longitudinal stretching described in S4 has a stretching ratio of (3-5):1.

[0041] The transverse stretching described in S5 has a stretching ratio of (2-3):1.

[0042] S1 The raw materials of the surface layer, the middle layer and the bottom layer are extruded and melted, and the temperatures are controlled below 195° C., below 155° C. and below 195° C. respectively.

[0043] In step S2, a three-layer sheet is extruded through a die head of a twin-screw extruder, and the die head temperature is 205-210°C.

[0044] The first heat treatment temperature is 90-100°C, and the first heat treatment time is 15-20min; the second heat treatment temperature is 110-120°C, and the second heat treatment time is 20-30min; the third heat treatment temperature is 80-90°C, and the third heat treatment time is 15-20min; the fourth heat treatment temperature is 100-110°C, and the fourth heat treatment time is 30-50min.

[0045] In the above step S2, the die head is set at a temperature of 205-210°C because, through testing, it is proved that the molten raw material of the three-layer sheet is extruded at a normal speed at this temperature, which will not cause decomposition and other denaturation changes to the three-layer material, and will not produce other gases or substances, and can ensure that the three-layer sheet is tightly combined. In step S1, extrusion melting is adopted, and the raw material particles rub against each other to generate heat and temperature rise, and the raw material gradually becomes a molten state. The temperature is controlled not higher than the set temperature by implementing the temperature control device of the detection box to ensure that the raw material will not be exposed to high temperature for a long time to avoid denaturation of the raw material to produce other gases or substances.

[0046] Example 1 A PP base film for current collector and its preparation method

[0047] Structure and formula:

[0048] It includes a surface layer, a middle layer and a bottom layer, wherein the surface layer includes polypropylene and polyoxymethylene in a mass ratio of 95:3; the middle layer includes polypropylene and polyamide in a mass ratio of 80:10; and the bottom layer includes polypropylene and polyoxymethylene in a mass ratio of 90:5.

[0049] The polyamide includes nylon 6.

[0050] The material mass ratio of the surface layer, the middle layer and the bottom layer is 1:2:8.

[0051] Preparation process:

[0052] S1, mixing the raw materials of the surface layer, the middle layer and the bottom layer respectively, and adding them into a twin-screw extruder for extrusion and melting;

[0053] S2, the melted surface layer, middle layer and bottom layer liquid are extruded into a three-layer sheet through the die head of a twin-screw extruder;

[0054] S3, the extruded material is cooled by a cooling roller to obtain a three-layer sheet;

[0055] S4, subjecting the obtained three-layer sheet to a first heat treatment and then longitudinally stretching it, subjecting the film obtained by the longitudinal stretching to a second heat treatment and then cooling it to obtain a longitudinally stretched film;

[0056] S5, the obtained longitudinally stretched film is subjected to a third heat treatment and then subjected to a first transverse stretching, and the transversely stretched film is subjected to a fourth heat treatment and then cooled to obtain a transversely stretched film;

[0057] S6. The obtained horizontally drawn film sheet is packaged after being pulled, rolled up, cut and then cut to obtain a PP base film for the current collector.

[0058] In the co-extrusion described in S2, the mass ratio of the molten material obtained from the surface layer slices, the molten material obtained from the middle layer slices and the molten material obtained from the bottom layer slices is 1:2:8.

[0059] The longitudinal stretching described in S4 has a stretching ratio of 3:1.

[0060] The transverse stretching described in S5 has a stretching ratio of 2:1.

[0061] S1 The raw materials of the surface layer, the middle layer and the bottom layer are extruded and melted, and the temperatures are controlled below 195° C., below 155° C. and below 195° C. respectively.

[0062] In step S2, a three-layer sheet is extruded through a die head of a twin-screw extruder, and the die head temperature is 210°C.

[0063] The first heat treatment temperature is 90°C, and the first heat treatment time is 15 minutes; the second heat treatment temperature is 120°C, and the second heat treatment time is 20 minutes; the third heat treatment temperature is 80°C, and the third heat treatment time is 15 minutes; the fourth heat treatment temperature is 110°C, and the fourth heat treatment time is 30 minutes.

[0064] Example 2 A PP base film for current collector and its preparation method

[0065] Structure and formula:

[0066] It includes a surface layer, a middle layer and a bottom layer, wherein the surface layer includes polypropylene and polyoxymethylene in a mass ratio of 97:5; the middle layer includes polypropylene and polyamide in a mass ratio of 90:20; and the bottom layer includes polypropylene and polyoxymethylene in a mass ratio of 90:10.

[0067] The polyamide includes nylon 6.

[0068] The material mass ratio of the surface layer, the middle layer and the bottom layer is 2:4:10.

[0069] Preparation process:

[0070] S1, mixing the raw materials of the surface layer, the middle layer and the bottom layer respectively, and adding them into a twin-screw extruder for extrusion and melting;

[0071] S2, the melted surface layer, middle layer and bottom layer liquid are extruded into a three-layer sheet through the die head of a twin-screw extruder;

[0072] S3, the extruded material is cooled by a cooling roller to obtain a three-layer sheet;

[0073] S4, subjecting the obtained three-layer sheet to a first heat treatment and then longitudinally stretching it, subjecting the film obtained by the longitudinal stretching to a second heat treatment and then cooling it to obtain a longitudinally stretched film;

[0074] S5, the obtained longitudinally stretched film is subjected to a third heat treatment and then subjected to a first transverse stretching, and the transversely stretched film is subjected to a fourth heat treatment and then cooled to obtain a transversely stretched film;

[0075] S6. The obtained horizontally drawn film sheet is packaged after being pulled, rolled up, cut and then cut to obtain a PP base film for the current collector.

[0076] In the co-extrusion described in S2, the mass ratio of the molten material obtained from the surface layer slices, the molten material obtained from the middle layer slices and the molten material obtained from the bottom layer slices is 1:2:8.

[0077] The longitudinal stretching described in S4 has a stretching ratio of 3:1.

[0078] The transverse stretching described in S5 has a stretching ratio of 2:1.

[0079] S1 The raw materials of the surface layer, the middle layer and the bottom layer are extruded and melted, and the temperatures are controlled below 195° C., below 155° C. and below 195° C. respectively.

[0080] In step S2, a three-layer sheet is extruded through a die head of a twin-screw extruder, and the die head temperature is 210°C.

[0081] The first heat treatment temperature is 90°C, and the first heat treatment time is 15 minutes; the second heat treatment temperature is 120°C, and the second heat treatment time is 20 minutes; the third heat treatment temperature is 80°C, and the third heat treatment time is 15 minutes; the fourth heat treatment temperature is 110°C, and the fourth heat treatment time is 30 minutes.

[0082] Example 3 A PP base film for current collector and its preparation method

[0083] Structure and formula:

[0084] It includes a surface layer, a middle layer and a bottom layer, wherein the surface layer includes polypropylene and polyoxymethylene in a mass ratio of 95:5; the middle layer includes polypropylene and polyamide in a mass ratio of 90:10; and the bottom layer includes polypropylene and polyoxymethylene in a mass ratio of 95:5.

[0085] The polyamide includes nylon 6.

[0086] The material mass ratio of the surface layer, the middle layer and the bottom layer is 2:4:10.

[0087] Preparation process:

[0088] S1, mixing the raw materials of the surface layer, the middle layer and the bottom layer respectively, and adding them into a twin-screw extruder for extrusion and melting;

[0089] S2, the melted surface layer, middle layer and bottom layer liquid are extruded into a three-layer sheet through the die head of a twin-screw extruder;

[0090] S3, the extruded material is cooled by a cooling roller to obtain a three-layer sheet;

[0091] S4, subjecting the obtained three-layer sheet to a first heat treatment and then longitudinally stretching it, subjecting the film obtained by the longitudinal stretching to a second heat treatment and then cooling it to obtain a longitudinally stretched film;

[0092] S5, the obtained longitudinally stretched film is subjected to a third heat treatment and then subjected to a first transverse stretching, and the transversely stretched film is subjected to a fourth heat treatment and then cooled to obtain a transversely stretched film;

[0093] S6. The obtained horizontally drawn film sheet is packaged after being pulled, rolled up, cut and then cut to obtain a PP base film for the current collector.

[0094] In the co-extrusion described in S2, the mass ratio of the molten material obtained from the surface layer slices, the molten material obtained from the middle layer slices and the molten material obtained from the bottom layer slices is 1:2:8.

[0095] The longitudinal stretching described in S4 has a stretching ratio of 3:1.

[0096] The transverse stretching described in S5 has a stretching ratio of 2:1.

[0097] S1 The raw materials of the surface layer, the middle layer and the bottom layer are extruded and melted, and the temperatures are controlled below 195° C., below 155° C. and below 195° C. respectively.

[0098] In step S2, a three-layer sheet is extruded through a die head of a twin-screw extruder, and the die head temperature is 210°C.

[0099] The first heat treatment temperature is 90°C, and the first heat treatment time is 15 minutes; the second heat treatment temperature is 120°C, and the second heat treatment time is 20 minutes; the third heat treatment temperature is 80°C, and the third heat treatment time is 15 minutes; the fourth heat treatment temperature is 110°C, and the fourth heat treatment time is 30 minutes.

[0100] Comparative Example 1

[0101] The difference from Example 3 is that a double-layer structure is adopted, which only includes the surface layer and the bottom layer in Example 3, and does not include the middle layer. Other step parameters are the same as those in Example 3.

[0102] Comparative Example 2

[0103] The difference from Example 3 is that a single-layer structure is adopted, which only includes the intermediate layer structure in Example 3, and other step parameters are the same as those in Example 3.

[0104] Comparative Example 3

[0105] The difference from Example 3 is that the polyamide in the middle layer is replaced by an equal amount of polypropylene, and the other step parameters are the same as those in Example 3.

[0106] Comparative Example 4

[0107] The difference from Example 3 is that nylon 6 is replaced by an equal amount of nylon 66, and other step parameters are the same as those in Example 3.

[0108] Performance testing:

[0109] The thermal shrinkage and other mechanical properties of the film products obtained in all the above examples and comparative examples were tested. Some of the results are shown in Table 1.

[0110] Table 1 Thermal shrinkage of the obtained film materials

[0111]

[0112]

[0113] From the thermal shrinkage of the film products obtained in the above embodiments and comparative examples, it can be seen that the innovative film structure and formula of the present invention are the core guarantee of the excellent high temperature resistance of the product. The PP film prepared by the formula and process provided by the present invention has a lower thermal shrinkage rate and can be used for the current collector base film to reduce the generation of wrinkles. The film products obtained in all the above embodiments and comparative examples meet the application requirements of the current collector base film in terms of hardness, elastic modulus, etc., and the differences are not significant.

[0114] Although 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 the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

[0115] The present invention and its implementation methods are described above, which is not restrictive. The drawings are only one of the implementation methods of the present invention, and the actual application is not limited thereto. In short, if ordinary technicians in the field are inspired by it and design methods and embodiments similar to the technical solution without creativity without departing from the purpose of the invention, they should all fall within the protection scope of the present invention.

Claims

1. A wrinkle-resistant PP base film for current collector, characterized in that: It is composed of a surface layer, an intermediate layer and a bottom layer, wherein the surface layer is composed of polypropylene and polyoxymethylene, and the mass ratio of the two is (95-97):(3-5); the intermediate layer is composed of polypropylene and polyamide, and the mass ratio of the two is (80-90):(10-20); the bottom layer is composed of polypropylene and polyoxymethylene, and the mass ratio of the two is (90-95):(5-10); the polyamide is nylon 6; the material mass ratio of the surface layer, the intermediate layer and the bottom layer is (1-2):(2-4):(8-10); The preparation method of the PP base film comprises the following steps: S1, mixing the raw materials of the surface layer, the middle layer and the bottom layer respectively, and adding them into a twin-screw extruder for extrusion and melting; S2, the melted surface layer, middle layer and bottom layer liquid are extruded into a three-layer sheet through the die head of a twin-screw extruder; S3, the extruded material is cooled by a cooling roller to obtain a three-layer sheet; S4, subjecting the obtained three-layer sheet to a first heat treatment and then longitudinally stretching it, subjecting the film obtained by the longitudinal stretching to a second heat treatment and then cooling it to obtain a longitudinally stretched film; S5, the obtained longitudinally stretched film is subjected to a third heat treatment and then subjected to a first transverse stretching, and the transversely stretched film is subjected to a fourth heat treatment and then cooled to obtain a transversely stretched film; S6. The obtained horizontally drawn film sheet is packaged after being pulled, rolled up, cut and then cut to obtain a PP base film for the current collector.

2. A method for preparing the wrinkle-resistant PP base film for current collector according to claim 1, characterized in that: The following steps are involved: S1, mixing the raw materials of the surface layer, the middle layer and the bottom layer respectively, and adding them into a twin-screw extruder for extrusion and melting; S2, the melted surface layer, middle layer and bottom layer liquid are extruded into a three-layer sheet through the die head of a twin-screw extruder; S3, the extruded material is cooled by a cooling roller to obtain a three-layer sheet; S4, subjecting the obtained three-layer sheet to a first heat treatment and then longitudinally stretching it, subjecting the film obtained by the longitudinal stretching to a second heat treatment and then cooling it to obtain a longitudinally stretched film; S5, the obtained longitudinally stretched film is subjected to a third heat treatment and then subjected to a first transverse stretching, and the transversely stretched film is subjected to a fourth heat treatment and then cooled to obtain a transversely stretched film; S6. The obtained horizontally drawn film sheet is packaged after being pulled, rolled up, cut and then cut to obtain a PP base film for the current collector.

3. The method for preparing the wrinkle-resistant PP base film for current collector according to claim 2, characterized in that: The mass ratio of the molten material obtained by co-extruding the surface layer slices, the molten material obtained by the middle layer slices and the molten material obtained by the bottom layer slices of S2 is (1-2):(2-4):(8-10).

4. The method for preparing the wrinkle-resistant PP base film for current collector according to claim 2, characterized in that: The longitudinal stretching described in S4 has a stretching ratio of (3-5):

1.

5. The method for preparing the wrinkle-resistant PP base film for current collector according to claim 2, characterized in that: The transverse stretching described in S5 has a stretching ratio of (2-3):

1.

6. The method for preparing the wrinkle-resistant PP base film for current collector according to claim 2, characterized in that: S1 The raw materials of the surface layer, the middle layer and the bottom layer are extruded and melted, and the temperatures are controlled below 195° C., below 155° C. and below 195° C. respectively.

7. The method for preparing the wrinkle-resistant PP base film for current collector according to claim 2, characterized in that: In step S2, a three-layer sheet is extruded through a die head of a twin-screw extruder, and the die head temperature is 205-210°C.

8. The method for preparing the wrinkle-resistant PP base film for current collector according to claim 2, characterized in that: The first heat treatment temperature is 90-100°C, and the first heat treatment time is 15-20min; the second heat treatment temperature is 110-120°C, and the second heat treatment time is 20-30min; the third heat treatment temperature is 80-90°C, and the third heat treatment time is 15-20min; the fourth heat treatment temperature is 100-110°C, and the fourth heat treatment time is 30-50min.

Images