Melt-spinning spandex spinning oil and preparation method thereof

Abstract

The present invention belongs to the technical field of spinning oil, and relates to a melt-spinning spandex spinning oil and a preparation method thereof. The melt-spinning spandex spinning oil comprises the following components: 55-65 parts of dimethyl silicone oil, 0-4.5 parts of terminal carboxyl silicone oil, 0-4.5 parts of long-chain alkyl aryl silicone oil, 2-4 parts of MQ silicone resin, 0-15 parts of mineral oil, 15-20 parts of surfactant, 1-5 parts of dispersant, and 0-3 parts of metal stearate; wherein, the terminal carboxyl silicone oil and the long-chain alkyl aryl silicone oil are not 0 at the same time. The present invention adds terminal carboxyl silicone oil, long-chain alkyl aryl silicone oil and MQ silicone resin on the basis of traditional dimethyl silicone oil. The terminal carboxyl silicone oil contains an active group, which can improve the wetting performance of the oil; and the long-chain alkyl aryl silicone oil itself has good demoulding properties, and the MQ resin has good compatibility with the dimethyl silicone oil, and its film-forming property and anti-sticking property are good, which can prevent the occurrence of adhesion.

Description

Technical Field

[0001] The invention belongs to the technical field of spinning oils, and in particular relates to a melt-spinning spandex spinning oil and a preparation method thereof. Background Art

[0002] Spandex, scientifically known as polyurethane fiber, is a highly elastic fiber with excellent mechanical properties. It has been widely used in various fields of industrial production. Spandex has very good tensile properties at break. Its elongation at break is generally as high as 500%-800% depending on the linear density and production process, which is a significant improvement compared to other man-made fibers. Spandex also has excellent tensile recovery properties. Its 300% elastic recovery rate can generally reach 90%-98%, which provides a good performance foundation for maintaining its dimensional stability. In addition, spandex's anti-aging, corrosion resistance, heat resistance, and fatigue resistance properties are also very prominent among various synthetic fibers. Therefore, spandex is a type of synthetic fiber material with excellent properties.

[0003] Adding a little spandex in the textile process can greatly improve the mechanical properties of the finished fabric. The advantages of spandex such as high elastic recovery rate and good fatigue resistance make spandex widely used in the textile field. The wide application of spandex involves clothing textiles, sports equipment and even some medical equipment. The spinning methods of spandex production can be divided into dry spinning, wet spinning, reaction spinning and melt spinning. Among them, dry-spun spandex has good quality and high strength. Compared with other spinning methods, it has a more mature technology. However, dry spinning also has many obvious shortcomings. It has high investment cost, long spinning process, and needs to recover solvents, which has pollution problems. Melt spinning is superior to other spinning methods in terms of energy consumption, investment cost, production cost and environmental protection, and gradually occupies a superior position. However, due to the presence of polar groups such as carbamate and allophanate in the molecular structure of spandex, spandex filaments are prone to adhesion due to the action of hydrogen bonds, which brings difficulties to subsequent processing and application. A good spandex oil agent can not only give the melt-spun spandex good formability and bundling properties during spinning, but also give the spandex filaments good smoothness and antistatic properties.

[0004] The most commonly used lubricant in spandex oil is dimethyl silicone oil. Dimethyl silicone oil can reduce the friction coefficient between the fiber and metal and ceramic on the fiber. Since spandex itself has a porous structure, the commonly used oil components often swell it, resulting in hairy fibers, broken ends and other phenomena. Since silicone oil has large molecules, good chemical inertness and good smoothness, spandex oil has always used dimethyl silicone oil as the main lubricant. However, the existing common dimethyl silicone oil type melt-spun spandex spinning oil is not resistant to high and low temperatures, and is prone to side collapse, broken fibers and poor unwinding performance in later use, which cannot fully meet the needs of the melt-spun spandex spinning process. The rapid development of the melt-spun spandex related industry has also made the reliability research of spandex oil materials more important. How to improve the performance of spandex oil by improving its various properties has also become an important content in the current synthetic fiber industry. Summary of the invention

[0005] The purpose of the present invention is to provide a melt-spun spandex spinning oil and a preparation method thereof, which solves the technical problems of common oils in the prior art such as adhesion, poor demoulding and poor unwinding properties during the spinning process.

[0006] In order to achieve the above object, the first aspect of the present invention provides a melt-spinning spandex spinning oil, which comprises the following components in parts by weight:

[0007] 55-65 parts of dimethyl silicone oil, 0-4.5 parts of terminal carboxyl silicone oil, 0-4.5 parts of long-chain alkyl aryl silicone oil, 2-4 parts of MQ silicone resin, 0-15 parts of mineral oil, 15-20 parts of surfactant, 1-5 parts of dispersant, 0-3 parts of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 5-30 mm 2 / s; wherein, the terminal carboxyl silicone oil and the long-chain alkyl aromatic silicone oil are not 0 at the same time.

[0008] In order to make the fiber evenly stretched, the multifilament yarn is required to have good cohesion, and the oil should give the fiber a higher static friction between the filaments, so that the winding tube is well formed and the post-processing unwinding effect is good, creating good conditions for the smooth progress of the elastic deformation process. In the spinning-drawing combined process, it is particularly important to reduce the dynamic friction coefficient between the fiber and the metal under the conditions of high spinning speed and high draft ratio, otherwise it will affect the spinning and reduce the production efficiency. The prior art usually uses silicone oil with a single performance as the base oil of the spinning oil, which is difficult to meet the dual requirements of the spinning oil for viscosity. In order to solve this problem, the present invention introduces a small amount of terminal carboxyl silicone oil or long-chain alkyl aromatic silicone oil on the basis of traditional dimethyl silicone oil. Terminal carboxyl silicone oil and long-chain alkyl aromatic silicone oil have large molecular weight and poor mobility. They not only have excellent wear resistance, but also can be used as oil film enhancers; while dimethyl silicone oil has a low molecular weight and strong mobility, which can ensure that the oil molecules can quickly and evenly adhere to the fiber at the moment of contact, thereby increasing the wetting speed of the fiber. In addition, the carboxyl-terminated silicone oil contains active groups that can react with the residual active groups on the spandex fiber to improve the wetting properties of the oil; while the long-chain alkyl aryl silicone oil itself has good demoulding properties and high temperature resistance, which can prevent the occurrence of adhesion. MQ resin has good compatibility with dimethyl silicone oil, and has good film-forming and anti-sticking properties. Adding carboxyl-terminated silicone oil, long-chain alkyl aryl silicone oil and MQ silicone resin at the same time can effectively improve the viscosity stability of the spinning oil and improve the fiber performance.

[0009] In order to further improve the performance of the spinning oil, the invention adds a certain concentration of mineral oil. The combination of mineral oil and silicone oil can adjust the astringency and smoothness balance of the oil, achieve the unity of molding and unwinding; and the addition of mineral oil can improve the dispersibility of stearic acid metal salt.

[0010] As a common dispersant, metal stearate can not only prevent adhesion and give spandex good unwinding properties, but also the ionic groups it contains can interact with terminal carboxyl silicone oil and spandex fibers to improve the dispersibility of each component in the oil.

[0011] Furthermore, the carboxyl-terminated silicone oil is Elkem BLUESIL FLD 1669; the long-chain alkyl aromatic silicone oil is Dow Corning OFX-0203; and the MQ silicone resin is one or more of liquid vinyl MQ silicone resin or trimethylsiloxysilicate MQ silicone resin.

[0012] Furthermore, the amount of the terminal carboxyl silicone oil is not less than that of the long-chain alkyl aryl silicone oil, and the terminal carboxyl silicone oil is 2-4 parts and the long-chain alkyl aryl silicone oil is 1-2 parts. The terminal carboxyl silicone oil contains carboxyl groups and has good wetting properties; while the long-chain alkyl aryl silicone oil has good demoulding properties and can effectively prevent adhesion; adding the two at the same time can play a synergistic role and can effectively solve the problems of broken wires and poor unwinding properties.

[0013] Furthermore, the carboxyl-terminated silicone oil is 2-4 parts, the long-chain alkyl aromatic silicone oil is 1-2 parts, the mineral oil is 5-15 parts, and the metal stearate is 0.3-3 parts.

[0014] Furthermore, the kinematic viscosity of the mineral oil at 25°C is 10-20 mm 2 / s.

[0015] Furthermore, the surfactant is one or more of isomeric decanol polyoxyethylene ether and isomeric tridecanol polyoxyethylene ether.

[0016] Furthermore, the dispersant is one or more of sodium lignin sulfonate, stearic acid monoglyceride, and polyisobutylene succinimide.

[0017] Furthermore, the metal stearate is one or more of zinc stearate, calcium stearate, and magnesium stearate.

[0018] The second aspect of the present invention provides a method for preparing a melt-spun spandex spinning oil, comprising the following steps:

[0019] (1) Weigh each component by weight;

[0020] (2) adding dimethyl silicone oil, carboxyl-terminated silicone oil, long-chain alkyl aromatic silicone oil, MQ silicone resin, and mineral oil into a reaction kettle and stirring to obtain a mixed component;

[0021] (3) Adding a surfactant, a dispersant, and a metal stearate to the mixed components in sequence, stirring the mixture at 30-40° C. for 1-3 hours, and cooling the mixture to obtain a melt-spinning spandex spinning oil.

[0022] Furthermore, in step (3), the stirring rate is 50-100 r / min.

[0023] Compared with the prior art, the melt-spinning spandex spinning oil of the present invention has the following beneficial effects:

[0024] (1) The present invention introduces a small amount of terminal carboxyl silicone oil, long-chain alkyl aryl silicone oil and MQ silicone resin on the basis of dimethyl silicone oil, thereby solving the adhesion problem and improving demoulding properties. The terminal carboxyl silicone oil can improve the wetting properties of the oil; while the long-chain alkyl aryl silicone oil itself has good demoulding properties and high temperature resistance, which can prevent the occurrence of adhesion. MQ resin has good compatibility with dimethyl silicone oil, and has good film-forming and anti-sticking properties.

[0025] (2) Stearic acid metal salts are common dispersants that can play an anti-adhesion role.

[0026] (3) The combination of mineral oil and silicone oil can adjust the astringency and smoothness balance of the oil agent to achieve the unity of molding and unwinding. DETAILED DESCRIPTION

[0027] The present invention is described in more detail below through specific embodiments to facilitate understanding of the technical solution of the present invention, but is not intended to limit the scope of protection of the present invention. The raw materials in the examples or comparative examples are homemade or commercially available.

[0028] The carboxyl-terminated silicone oils in the examples and comparative examples are all Elkem BLUESIL FLD 1669; the long-chain alkyl aromatic silicone oils are all Dow Corning OFX-0203; the liquid vinyl MQ silicone resin is Delta DT-10050; the trimethylsiloxysilicate MQ silicone resin is MQ 803TF.

[0029] Example 1

[0030] The melt-spinning spandex spinning oil comprises the following components in parts by weight:

[0031] 55 parts of dimethyl silicone oil, 2 parts of terminal carboxyl silicone oil, 1 part of long-chain alkyl aromatic silicone oil, 2 parts of MQ silicone resin, 5 parts of mineral oil, 15 parts of surfactant, 1 part of dispersant, and 0.3 parts of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 5 mm 2 / s. The MQ silicone resin is a liquid vinyl MQ silicone resin; the kinematic viscosity of the mineral oil at 25°C is 10 mm 2 / s; the surfactant is 10 parts of isomeric decacarb polyoxyethylene ether and 5 parts of isomeric tridecanol polyoxyethylene ether; the dispersant is 0.5 parts of sodium lignin sulfonate and 0.5 parts of stearic acid monoglyceride; the metal stearate is zinc stearate.

[0032] Example 2

[0033] The melt-spinning spandex spinning oil comprises the following components in parts by weight:

[0034] 65 parts of dimethyl silicone oil, 4 parts of terminal carboxyl silicone oil, 2 parts of long-chain alkyl aryl silicone oil, 4 parts of MQ silicone resin, 15 parts of mineral oil, 20 parts of surfactant, 5 parts of dispersant, 3 parts of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 30 mm 2 / s. The MQ silicone resin is a liquid vinyl MQ silicone resin; the kinematic viscosity of the mineral oil at 25°C is 20 mm 2 / s; the surfactant is isomeric decaethanol polyoxyethylene ether; the dispersant is 2 parts of stearic acid monoglyceride and 3 parts of polyisobutylene succinimide; the metal stearate is 2 parts of zinc stearate and 1 part of calcium stearate.

[0035] Example 3

[0036] The melt-spinning spandex spinning oil comprises the following components in parts by weight:

[0037] 55 parts of dimethyl silicone oil, 4 parts of terminal carboxyl silicone oil, 2 parts of long-chain alkyl aryl silicone oil, 4 parts of MQ silicone resin, 15 parts of mineral oil, 20 parts of surfactant, 1 part of dispersant, and 0.3 parts of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 30 mm 2 / s. The MQ silicone resin is trimethylsiloxysilicate MQ silicone resin; the kinematic viscosity of the mineral oil at 25°C is 20 mm 2 / s; the surfactant is 10 parts of isomeric decacarb polyoxyethylene ether and 10 parts of isomeric tridecanol polyoxyethylene ether; the dispersant is polyisobutylene succinimide; and the metal stearate is magnesium stearate.

[0038] Example 4

[0039] The melt-spinning spandex spinning oil comprises the following components in parts by weight:

[0040] 65 parts of dimethyl silicone oil, 2 parts of terminal carboxyl silicone oil, 1 part of long-chain alkyl aryl silicone oil, 2 parts of MQ silicone resin, 0 parts of mineral oil, 15 parts of surfactant, 5 parts of dispersant, 3 parts of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 10 mm 2 / s. The MQ silicone resin is liquid vinyl MQ silicone resin; the surfactant is isomeric deca-alcohol polyoxyethylene ether; the dispersant is 2 parts of sodium lignin sulfonate and 3 parts of stearic acid monoglyceride; the stearic acid metal salt is 2 parts of zinc stearate and 1 part of calcium stearate.

[0041] Example 5

[0042] The melt-spinning spandex spinning oil comprises the following components in parts by weight:

[0043] 58 parts of dimethyl silicone oil, 2.4 parts of terminal carboxyl silicone oil, 1.3 parts of long-chain alkyl aromatic silicone oil, 2.2 parts of MQ silicone resin, 7 parts of mineral oil, 16 parts of surfactant, 4 parts of dispersant, 1 part of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 10 mm 2 / s. The MQ silicone resin is a liquid vinyl MQ silicone resin; the kinematic viscosity of the mineral oil at 25°C is 15 mm 2 / s; the surfactant is 6 parts of isomeric decacarb polyoxyethylene ether and 10 parts of isomeric tridecanol polyoxyethylene ether; the dispersant is sodium lignin sulfonate; and the metal stearate is calcium stearate.

[0044] Example 6

[0045] The melt-spinning spandex spinning oil comprises the following components in parts by weight:

[0046] 63 parts of dimethyl silicone oil, 3.5 parts of terminal carboxyl silicone oil, 1.7 parts of long-chain alkyl aryl silicone oil, 3.2 parts of MQ silicone resin, 13 parts of mineral oil, 18 parts of surfactant, 2 parts of dispersant, 2 parts of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 10 mm 2 / s. The MQ silicone resin is a liquid vinyl MQ silicone resin; the kinematic viscosity of the mineral oil at 25°C is 15 mm 2 / s; the surfactant is isomeric tridecanol polyoxyethylene ether; the dispersant is 1 part of stearic acid monoglyceride and 1 part of polyisobutylene succinimide; the metal stearate is 1 part of zinc stearate and 1 part of calcium stearate.

[0047] Example 7

[0048] The melt-spinning spandex spinning oil comprises the following components in parts by weight:

[0049] 60 parts of dimethyl silicone oil, 3 parts of terminal carboxyl silicone oil, 1.5 parts of long-chain alkyl aryl silicone oil, 3 parts of MQ silicone resin, 10 parts of mineral oil, 17 parts of surfactant, 3 parts of dispersant, 2 parts of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 10 mm 2 / s. The MQ silicone resin is a liquid vinyl MQ silicone resin; the kinematic viscosity of the mineral oil at 25°C is 15 mm 2 / s; the surfactant is 7 parts of isomeric decacarb polyoxyethylene ether and 10 parts of isomeric tridecanol polyoxyethylene ether; the dispersant is 2 parts of stearic acid monoglyceride and 1 part of polyisobutylene succinimide; the metal stearate is magnesium stearate.

[0050] Example 8

[0051] The components and proportions of the melt-spinning spandex spinning oil are basically the same as those of Example 7, except that the components include 4.3 parts of terminal carboxyl silicone oil, 2.2 parts of long-chain alkyl aromatic silicone oil, and 0 parts of metal stearate.

[0052] Example 9

[0053] The components and proportions of the melt-spinning spandex spinning oil are basically the same as those of Example 7, except that the components include 4.5 parts of terminal carboxyl silicone oil and 0 parts of long-chain alkyl aromatic silicone oil.

[0054] Example 10

[0055] The components and proportions of the melt-spinning spandex spinning oil are basically the same as those of Example 7, except that the carboxyl-terminated silicone oil is 0 parts and the long-chain alkyl aromatic silicone oil is 4.5 parts.

[0056] Comparative Example 1

[0057] The components and proportions of the melt-spinning spandex spinning oil are basically the same as those of Example 7, except that the components include 0 parts of terminal carboxyl silicone oil, 0 parts of long-chain alkyl aromatic silicone oil, and 6.5 parts of metal stearate.

[0058] Comparative Example 2

[0059] The components and proportions of the melt-spinning spandex spinning oil are basically the same as those of Example 7, except that the long-chain alkyl aromatic silicone oil is 6 parts.

[0060] Comparative Example 3

[0061] The components and proportions of the melt-spinning spandex spinning oil are basically the same as those of Example 7, except that the carboxyl-terminated silicone oil is 6 parts.

[0062] The preparation methods of the melt-spinning spandex spinning oil described in the above embodiment and the comparative example are the same, comprising the following steps:

[0063] (1) Weigh each component by weight;

[0064] (2) adding dimethyl silicone oil, carboxyl-terminated silicone oil, long-chain alkyl aromatic silicone oil, MQ silicone resin, and mineral oil into a reaction kettle and stirring to obtain a mixed component;

[0065] (3) Adding a surfactant, a dispersant, and a metal stearate to the mixed components in sequence, and then stirring at 35° C. for 2 h at a stirring rate of 60 r / min; after cooling, a melt-spinning spandex spinning oil is obtained.

[0066] Performance test: The melt-spinning spandex spinning oils prepared in Examples 1 to 10 and Comparative Examples 1 to 3 were used for spinning and performance testing, and their stability (placed at 30°C for 6 days to observe whether precipitation or stratification occurs), oil film strength, wetting properties and unwinding properties were tested respectively.

[0067] Table 1: Performance of melt-spun spandex spinning oil prepared in Example 1-Example 5

[0068]

[0069] Table 2: Performance of melt-spun spandex spinning oils prepared in Examples 6 to 10

[0070]

[0071]

[0072] Table 3: Performance of melt-spun spandex spinning oil prepared in Comparative Examples 1 to 3

[0073]

[0074] It can be seen from Example 1 to Example 10 that the melt-spun spandex spinning oil provided by the present invention solves the technical problems of ordinary oils in the prior art such as adhesion and poor unwinding in the spinning process. It can be seen from Example 1 to Example 10 and Comparative Example 1 to Comparative Example 3 that the present invention introduces a small amount of terminal carboxyl silicone oil, long-chain alkyl aromatic silicone oil and MQ silicone resin on the basis of traditional dimethyl silicone oil. The terminal carboxyl silicone oil contains active groups, which can improve the wetting properties of the oil; and the long-chain alkyl aromatic silicone oil itself has good demoulding properties and high temperature resistance, which can prevent the occurrence of adhesion. MQ resin has good compatibility with dimethyl silicone oil, and has good film-forming and anti-sticking properties.

[0075] Examples 7 to 10 show that adding carboxyl-terminated silicone oil and long-chain alkyl aryl silicone oil simultaneously can effectively improve the viscosity stability of the spinning oil, and improve the unwinding property and fiber performance. Carboxyl-terminated silicone oil contains carboxyl groups and has good wetting properties; while long-chain alkyl aryl silicone oil has good demoulding properties and can effectively prevent adhesion; adding the two at the same time can play a synergistic role and can effectively solve the problems of broken wires and poor unwinding properties. Stearic acid metal salt can play an anti-adhesion role, giving spandex good unwinding properties, and the ionic groups it contains can interact with carboxyl-terminated silicone oil and spandex fibers to improve the dispersibility of each component in the oil.

[0076] It can be seen from Comparative Examples 1 to 3 that the present invention, on the basis of traditional dimethyl silicone oil, introduces a small amount of terminal carboxyl silicone oil, long-chain alkyl aryl silicone oil and MQ silicone resin, which can play a synergistic role. The terminal carboxyl silicone oil contains active groups, which can react with the residual active groups on the spandex fiber to improve the wetting properties of the oil; however, due to the presence of active groups, its dosage should not be too much, otherwise it will affect the stability of the oil. The long-chain alkyl aryl silicone oil itself has good demoulding properties and high temperature resistance, which can prevent the occurrence of adhesion; however, due to the high viscosity of the long-chain alkyl aryl silicone oil, its dosage should not be too much, otherwise it will affect the dispersibility of the oil.

[0077] The above are only some embodiments of the present invention. For ordinary technicians in this field, various other corresponding changes and modifications can be made according to the technical solutions and technical concepts of the present invention, and all these changes and modifications should fall within the scope of the claims of the present invention.

[0078] The present invention illustrates the detailed method of the present invention through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned detailed method, that is, it does not mean that the present invention must rely on the above-mentioned detailed method to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of various raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A melt-spinning spandex spinning oil, characterized in that: According to parts by weight, it includes the following components: 55-65 parts of dimethyl silicone oil, 2-4 parts of terminal carboxyl silicone oil, 1-2 parts of long-chain alkyl aryl silicone oil, 2-4 parts of MQ silicone resin, 5-15 parts of mineral oil, 15-20 parts of surfactant, 1-5 parts of dispersant, 0.3-3 parts of metal stearate; the kinematic viscosity of the dimethyl silicone oil at 25°C is 5-30 mm 2 / s.

2. The melt-spinning spandex spinning oil according to claim 1, characterized in that: The carboxyl-terminated silicone oil is Elkem BLUESIL FLD 1669; the long-chain alkyl aromatic silicone oil is Dow Corning OFX-0203; and the MQ silicone resin is one or more of liquid vinyl MQ silicone resin or trimethylsiloxysilicate MQ silicone resin.

3. The melt-spinning spandex spinning oil according to claim 1, characterized in that: The kinematic viscosity of the mineral oil at 25°C is 10-20 mm 2 / s.

4. The melt-spinning spandex spinning oil according to claim 1, characterized in that: The surfactant is one or more of isomeric decanol polyoxyethylene ether and isomeric tridecanol polyoxyethylene ether.

5. The melt-spinning spandex spinning oil according to claim 1, characterized in that: The dispersant is one or more of sodium lignin sulfonate, stearic acid monoglyceride, and polyisobutylene succinimide.

6. The melt-spinning spandex spinning oil according to claim 1, characterized in that: The metal stearate is one or more of zinc stearate, calcium stearate and magnesium stearate.

7. The method for preparing a melt-spinning spandex spinning oil according to any one of claims 1 to 6, characterized in that: The following steps are involved: (1) Weigh each component by weight; (2) adding dimethyl silicone oil, carboxyl-terminated silicone oil, long-chain alkyl aromatic silicone oil, MQ silicone resin, and mineral oil into a reaction kettle and stirring to obtain a mixed component; (3) Adding a surfactant, a dispersant, and a metal stearate to the mixed components in sequence, stirring the mixture at 30-40° C. for 1-3 hours, and cooling the mixture to obtain a melt-spinning spandex spinning oil.

8. The method for preparing a melt-spinning spandex spinning oil according to claim 7, characterized in that: The stirring rate in step (3) is 50-100 r / min.

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