Polyvinyl chloride resin for soft film and preparation method thereof

Abstract

The preparation method comprises the following steps: sequentially adding raw materials into a reaction kettle according to a ratio, carrying out a polymerization reaction, controlling the reaction temperature to be 50-60 DEG C, the reaction pressure to be 0.8-1.0 MPa and the conversion rate to be 80-85%, adding a blended terminating agent to terminate the reaction, and after the reaction is finished, carrying out steam stripping, drying and sieving treatment on the polymerized slurry to obtain the polyvinyl chloride resin for the soft membrane. The PVC resin product is obtained. The polymerization degree of the PVC resin is 981-1135, the oil absorption rate is 28-32 g / 100 g, the crystal point is smaller than or equal to 10 pieces / m, and the thermal stabilization time is longer than or equal to 6 min. The molecular weight distribution index of the PVC resin is 1.5-2.0; the particle size distribution of the PVC resin is less than 0.7, and the processing performance is excellent.

Description

Technical Field

[0001] The present invention relates to the technical field of polyvinyl chloride resins, and particularly to a polyvinyl chloride resin for soft films and a preparation method thereof. Background Art

[0002] Soft PVC has a wide range of application fields, mainly including packaging, medical, building materials, toy manufacturing and other fields, and its performance depends to a large extent on the performance of PVC resin. However, traditional PVC resins have problems such as low oil absorption rate, many crystal points, and poor thermal stability, making it difficult to meet the production requirements of high-end soft PVC products. Patent CN102453175A discloses a preparation method of a polyvinyl chloride resin with a high oil absorption rate. During the preparation process, functional additives need to be added, and the obtained polyvinyl chloride resin still cannot meet the production requirements of high-end soft PVC products, and its crystal point situation and thermal stability are difficult to determine. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention provides a polyvinyl chloride resin for soft films and a preparation method thereof. The polyvinyl chloride (PVC) resin for soft films prepared according to the method of the present invention has a high oil absorption rate, fewer crystal points, high thermal stability, a moderate molecular weight distribution, and excellent processing performance.

[0004] To achieve this technical purpose, the present invention adopts the following scheme: In the first aspect, the present invention provides a preparation method of a polyvinyl chloride resin for soft films, including the following steps: Raw material preparation: Prepare deionized water, vinyl chloride monomer, buffer, dispersant, initiator, chain transfer agent, heat stabilizer, terminator, property regulator and antioxidant in proportion; Incorporate the property regulator and chain transfer agent into the dispersant and stir evenly; Incorporate the heat stabilizer and antioxidant into the terminator for standby; Polymerization reaction: Add the raw materials deionized water, vinyl chloride monomer, buffer, the mixed dispersant, and initiator into the polymerization kettle in sequence, control the reaction temperature at 50 - 60 °C, the reaction pressure at 0.8 - 1.0 MPa, carry out the polymerization reaction, and add the mixed terminator to terminate the reaction after the conversion rate reaches 80 - 85%. The heat stabilizer and antioxidant are added into the polymerization kettle together with the terminator;

[0005] Post-treatment: After the reaction is completed, subject the polymerization slurry to stripping, drying, and sieving treatments to obtain the polyvinyl chloride resin product for soft films.

[0006] Further, the mass fraction ratio of deionized water, vinyl chloride monomer, buffer, dispersant, initiator, chain transfer agent, heat stabilizer, property regulator, antioxidant and terminator is 110 - 125:100:0.005 - 0.015:3.2 - 5.4:0.1 - 0.5:0.05 - 0.2:0.005 - 0.02:0.2 - 0.5:0.002 - 0.02:0.02 - 0.05.

[0007] Further, the deionized water is deoxygenated by a high gravity deoxygenation technique, and the oxygen content of the deionized water ≤ 500 ppb; the vinyl chloride monomer is dehydrated and deironed, with a water content < 100 mg / kg and an iron content < 2 mg / kg, and the water - oil ratio is controlled at 1.1 - 1.25:1.

[0008] Further, the buffer is at least one of sodium bicarbonate, ammonium bicarbonate, sodium hydroxide, trisodium phosphate or ammonia water, and the mass ratio of the buffer to the vinyl chloride monomer is 0.005 - 0.015:100.

[0009] Further, the dispersant is a compound of polyvinyl alcohol and hydroxypropyl methylcellulose, and their weight ratio is 6 - 8:1.

[0010] Preferably, polyvinyl alcohol is used by compounding polyvinyl alcohols with different degrees of alcoholysis (high, medium and low) in proportion. The degree of alcoholysis of the high - degree alcoholysis polyvinyl alcohol is greater than 80%, the degree of alcoholysis of the medium - degree alcoholysis polyvinyl alcohol is 60% - 80%, and the degree of alcoholysis of the low - degree alcoholysis polyvinyl alcohol is 30% - 50%. The mass ratio of the three is 1:1 - 1.5:0.3 - 0.45; more preferably, the high - degree alcoholysis polyvinyl alcohol is added with polyvinyl alcohol with a degree of alcoholysis greater than 90%.

[0011] Preferably, the methoxy content of hydroxypropyl methylcellulose is 25 - 30%, and the hydroxypropyl content is 4% - 10%.

[0012] Further, the initiator is a compound of cumyl peroxyneodecanoate and dioctyl peroxydicarbonate, and their weight ratio is 1:1 - 3.

[0013] Further, the chain transfer agent is a compound of mercaptoethanol and dodecyl mercaptan, and their weight ratio is 1:1 - 2.

[0014] Further, the heat stabilizer is a compound of calcium - zinc composite stabilizer and organotin stabilizer, and their weight ratio is 2:1 - 3.

[0015] Further, the property regulator is a compound of higher alcohol, stearic acid and calcium stearate, and their weight ratio is 0.5 - 1:1:1 - 2.

[0016] The performance regulator and the main dispersants (polyvinyl alcohol, cellulose) act synergistically to reduce the surface tension of vinyl chloride droplets, form smaller and more uniform droplets, and prevent the droplets from coalescing during the polymerization process, ensuring a uniform particle size distribution of PVC particles and improving particle concentration; the carbon chain length and hydrophobicity of the performance regulator affect the porosity and surface morphology of PVC particles, promoting the formation of porous and loose resin particles, and at the same time, the hydrophobic end can interact with plasticizers (such as DINP, DOP) to help the plasticizer penetrate more evenly into the pores of PVC particles, improving the plasticization speed and the flexibility of the final product; the performance regulator can cooperate with heat stabilizers to inhibit the release of HCl during the PVC degradation process through adsorption or chelation, delaying the thermal decomposition process, especially providing additional protection during high-shear processing; the performance regulator also has a lubricating effect, which can not only reduce the internal friction between PVC molecular chains (internal lubrication), improve the melt fluidity and processing performance, but also form a lubricating layer on the contact surface between PVC and processing equipment, improving the processing performance.

[0017] Further, the antioxidant is a compound of antioxidant 1010 and antioxidant 168, and their weight ratio is 1:1 - 2.

[0018] Further, the terminator is one of diethylhydroxylamine, hydroquinone, and α-methylstyrene.

[0019] Further, a wide-channel heat exchanger is used in the stripping process, which can avoid uneven heating of PVC caused by material accumulation in the heat exchanger, affecting indicators such as oil absorption, crystal points, and thermal stability of PVC resin. The channel spacing of the heat exchanger is 12 mm - 14 mm, and the controlled temperature is 90 - 105°C / 45 - 65°C, the pressure is 1.6 MPa, and the flow rate is 70 - 100 m 3 / h.

[0020] Preferably, the control method for the stripping process is as follows: (1) The PVC slurry in the feed tank is heat-exchanged with the slurry coming out of the stripping tower by the in-tower slurry pump through the wide-channel heat exchanger. The heat-exchanged in-tower slurry enters the stripping tower after the flow rate is controlled by a flow meter and a flow regulating valve. The flow rate is controlled at 50 - 80 m 3 / h to avoid the situation of slurry retention and overheating caused by too low a flow rate. The pressure in the stripping tower is controlled at -8 - 15 kPa.

[0021] (2) The slurry flows in a zigzag manner under the action of the tray baffle, fully performing mass transfer and heat transfer with the steam flowing from bottom to top, reaching the boiling state, and then flowing from the overflow weir to the next tray through the downcomer. There is flushing water on each tray and at the top of the tower to regularly flush the trays to prevent blockage of the tray holes and avoid situations such as uneven stripping effect and discoloration of the tray due to material accumulation caused by slurry deviation. The steam used in the stripping tower is low-pressure steam, and the temperature is controlled at 120 - 170°C, and the pressure is stably supplied at 0.3 - 0.4 MPa.

[0022] (3) VCM (vinyl chloride) in the slurry is removed by the rising steam and is forced to the outside of the tower together with the steam by the vacuum system. It is condensed by the condenser at the top of the stripper. The condensed water enters the condensate separator, and the uncondensed VCM is discharged into the vinyl chloride gas holder.

[0023] (4) The hot slurry from which VCM has been fully removed exits from the bottom of the stripper and first enters the stripper buffer tank. The function of the buffer tank is to ensure the continuity and stability of the slurry supply to the slurry pump out of the tower and the slurry on the outlet side of the heat exchanger out of the tower, prevent the heat exchanger from malfunctioning due to frequent unilateral pressure bearing, and affect the resin quality. Part of the hot slurry is refluxed from the pump outlet to the buffer tank to prevent sludge accumulation inside the buffer tank. The heat-exchanged slurry is sent to the slurry tank in the centrifugation process and is dehydrated by the centrifugal drying process.

[0024] In a second aspect, the present invention provides a polyvinyl chloride resin for soft films prepared by the aforementioned preparation method. The degree of polymerization of the PVC resin is 981-1135, the oil absorption rate is 28-32 g / 100 g, the number of crystal points is ≤10 per m², and the heat stability time is ≥6 min. The molecular weight distribution index of the PVC resin is 1.5-2.0; the particle size distribution of the PVC resin is <0.7.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: The PVC resin prepared by the method of the present invention has a high oil absorption rate, fewer crystal points, high heat stability, a moderate molecular weight distribution, a high particle concentration, and excellent processing performance.

[0026] 1. High oil absorption rate: The oil absorption rate is 28-32 g / 100 g, which is beneficial to improving the softness and processing performance of soft PVC products.

[0027] 2. Few crystal points: The number of crystal points is ≤10 per m², effectively improving the surface quality of soft PVC products.

[0028] 3. High heat stability: The heat stability time is ≥6 min, extending the service life of soft PVC products.

[0029] 4. Excellent processing performance: The molecular weight distribution is moderate, the particle concentration is high, the processing fluidity is good, and it is easy to form. Specific Embodiments

[0030] To fully understand the purpose, features and effects of the present invention, the present invention will be described in detail through the following specific embodiments, but the present invention is not limited thereto.

[0031] It should be noted that, unless otherwise specified, the reagents used in the present invention are all commercially available reagents.

[0032] The present invention provides a method for preparing polyvinyl chloride resin for soft film, comprising the following steps: Raw material preparation: Prepare 110 - 125 parts of deionized water, 100 parts of vinyl chloride monomer, 0.005 - 0.015 parts of buffer, 3.2 - 5.4 parts of dispersant, 0.05 - 0.2 parts of chain transfer agent, 0.2 - 0.5 parts of property regulator, 0.1 - 0.5 parts of initiator, 0.005 - 0.02 parts of heat stabilizer, 0.002 - 0.02 parts of antioxidant, and 0.02 - 0.05 parts of terminator by mass; Incorporate the chain transfer agent and the property regulator into the dispersant and stir evenly; Incorporate the heat stabilizer and the antioxidant into the terminator and stir evenly for standby; Among them, the deionized water is deoxygenated by using the high gravity deoxygenation technology, and the oxygen content of the deionized water ≤ 500 ppb; The vinyl chloride monomer is dehydrated and deironed, with the water content < 100 mg / kg and the iron content < 2 mg / kg.

[0033] The buffer is at least one of sodium bicarbonate, ammonium bicarbonate, sodium hydroxide, trisodium phosphate or ammonia water.

[0034] The dispersant is a compound of polyvinyl alcohol and hydroxypropyl methylcellulose, and their weight ratio is 6 - 8:1.

[0035] The polyvinyl alcohol is used by compounding polyvinyl alcohols with different degrees of alcoholysis of high, medium and low in proportion. The degree of alcoholysis of the high alcoholysis polyvinyl alcohol is greater than 80%, the degree of alcoholysis of the medium alcoholysis polyvinyl alcohol is 60% - 80%, and the degree of alcoholysis of the low alcoholysis polyvinyl alcohol is 30% - 50%. The mass ratio of the three is 1:1 - 1.5:0.3 - 0.45.

[0036] The methoxy content of the hydroxypropyl methylcellulose is 25 - 30%, and the hydroxypropyl content is 4% - 10%.

[0037] The initiator is a compound of cumyl peroxyneodecanoate and dioctyl peroxydicarbonate, and their weight ratio is 1:1 - 3.

[0038] The chain transfer agent is a compound of mercaptoethanol and dodecyl mercaptan, and their weight ratio is 1:1 - 2.

[0039] The heat stabilizer is a compound of calcium - zinc composite stabilizer and organotin stabilizer, and their weight ratio is 2:1 - 3.

[0040] The property regulator is a compound of higher alcohol, stearic acid and calcium stearate, and their weight ratio is 0.5 - 1:1:1 - 2.

[0041] The higher alcohol is a long - chain fatty alcohol of C12 - C18; The stearic acid is octadecanoic acid (C 17 H 35COOH), saturated long-chain fatty acid; calcium stearate is the calcium salt of stearic acid (Ca(C 18 H 35 O2)2).

[0042] The antioxidant is a compound of antioxidant 1010 and antioxidant 168, and their weight ratio is 1:1 - 2.

[0043] The terminator is one of diethylhydroxylamine, hydroquinone, and α-methylstyrene.

[0044] Polymerization reaction: Deionized water, vinyl chloride monomer, buffer, mixed dispersant (including chain transfer agent and property regulator), and initiator are sequentially added into the polymerization kettle. Control the reaction temperature at 50 - 60 °C and the reaction pressure at 0.8 - 1.0 MPa to carry out the polymerization reaction. After the conversion rate reaches 80 - 85%, add the mixed terminator (including heat stabilizer and antioxidant) to terminate the reaction. The heat stabilizer and antioxidant are added into the polymerization kettle together with the terminator.

[0045] Post-treatment: After the reaction is completed, the polymerization slurry is subjected to stripping, drying, and sieving to obtain the PVC resin product. The stripping process uses a wide-channel heat exchanger, which can avoid uneven heating of PVC caused by material accumulation in the heat exchanger, affecting indicators such as oil absorption, crystal points, and thermal stability of the PVC resin.

[0046] Among them, the channel spacing of the heat exchanger is 12 mm - 14 mm, and the controlled temperature is 90 - 105 °C / 45 - 65 °C, the pressure is 1.6 MPa, and the flow rate is 70 - 100 m 3 / h. The control method of the stripping process is as follows: (1) The PVC slurry in the feed tank is heat-exchanged with the slurry coming out of the stripping tower by the feed tower slurry pump through the wide-channel heat exchanger. The heat-exchanged feed tower slurry enters the stripping tower through a flow meter and a flow control valve to control the flow rate. The flow rate is controlled at 50 - 80 m 3 / h to avoid the situation of slurry retention and overheating caused by too low flow rate. The pressure of the stripping tower is controlled at -8 - -15 kPa.

[0047] (2) The slurry flows in a zigzag pattern under the action of the tray baffle, fully performing mass transfer and heat transfer with the steam rising from the bottom, reaching the boiling state, and then flowing from the overflow weir to the next tray through the downcomer. There is flushing water on each tray and at the top of the tower to regularly flush the trays to prevent blockage of the tray plate holes and avoid situations such as decreased stripping effect and discoloration of the tray due to slurry deviation. The steam used in the stripping tower is low-pressure steam, and the temperature is controlled at 120 - 170 °C, and the pressure is stably supplied at 0.3 - 0.4 MPa.

[0048] (3)VCM in the slurry is removed by the rising steam and forced out of the tower together with the steam by the vacuum system. It is condensed by the condenser at the top of the stripping tower. The condensed water enters the condensate separator, and the uncondensed VCM is discharged into the vinyl chloride gas holder.

[0049] (4)The hot slurry from which VCM has been fully removed exits from the bottom of the stripping tower and first enters the stripping tower buffer tank. The function of the buffer tank is to ensure the continuity and stability of the slurry supply to the slurry pump out of the tower and the slurry on the outlet side of the heat exchanger out of the tower, prevent the heat exchanger from malfunctioning due to frequent single-sided pressure, and affect the resin quality. Part of the hot slurry is refluxed from the pump outlet to the buffer tank to prevent internal siltation of the buffer tank. The heat-exchanged slurry is sent to the slurry tank in the centrifugation process and dehydrated by the centrifugal drying process. Example 1

[0050] Raw material preparation: 115 parts of deionized water, 100 parts of vinyl chloride monomer, 0.008 parts of sodium hydroxide, 3 parts of polyvinyl alcohol, 0.5 parts of hydroxypropyl methylcellulose, 0.1 parts of cumyl peroxyneodecanoate, 0.1 parts of dioctyl peroxydicarbonate, 0.05 parts of mercaptoethanol, 0.05 parts of dodecyl mercaptan, 0.05 parts of higher alcohol, 0.1 parts of stearic acid, 0.1 parts of calcium stearate, 0.006 parts of calcium-zinc composite stabilizer, 0.006 parts of organotin stabilizer, 0.004 parts of antioxidant 1010, 0.004 parts of antioxidant 168, 0.04 parts of diethylhydroxylamine; 0.05 parts of mercaptoethanol, 0.05 parts of dodecyl mercaptan, 0.05 parts of higher alcohol, 0.1 parts of stearic acid, 0.1 parts of calcium stearate are incorporated into 3 parts of polyvinyl alcohol and 0.5 parts of hydroxypropyl methylcellulose and stirred evenly; 0.006 parts of calcium-zinc composite stabilizer, 0.006 parts of organotin stabilizer, 0.004 parts of antioxidant 1010, 0.004 parts of antioxidant 168 are incorporated into 0.04 parts of diethylhydroxylamine for standby.

[0051] Polymerization reaction: The raw materials deionized water, vinyl chloride monomer, buffer agent, the mixed dispersant (containing chain transfer agent and property regulator), and initiator are added to the polymerization kettle in sequence. The reaction temperature is controlled at 56 °C and the reaction pressure is 0.9 MPa for polymerization reaction. After the conversion rate reaches 85%, the terminator is added to terminate the reaction, and the heat stabilizer and antioxidant are added to the polymerization kettle together with the terminator.

[0052] Post-treatment: After the reaction, the polymerization slurry is subjected to stripping, drying, sieving and other treatments to obtain the PVC resin product. Example 2

[0053] The specific steps are basically the same as those in Example 1, except that: The raw materials are: 120 parts of deionized water, 100 parts of vinyl chloride monomer, 0.01 part of ammonium bicarbonate, 4.2 parts of polyvinyl alcohol, 0.6 part of hydroxypropyl methylcellulose, 0.1 part of cumyl peroxyneodecanoate, 0.2 part of dioctyl peroxydicarbonate, 0.05 part of mercaptoethanol, 0.06 part of dodecyl mercaptan, 0.08 part of higher alcohol, 0.1 part of stearic acid, 0.15 part of calcium stearate, 0.005 part of calcium-zinc composite stabilizer, 0.005 part of organotin stabilizer, 0.003 part of antioxidant 1010, 0.005 part of antioxidant 168, and 0.04 part of hydroquinone. Example 3

[0054] The specific steps are basically the same as those in Example 1, except that: The raw materials are: 122 parts of deionized water, 100 parts of vinyl chloride monomer, 0.012 part of trisodium phosphate, 4 parts of polyvinyl alcohol, 0.5 part of hydroxypropyl methylcellulose, 0.1 part of cumyl peroxyneodecanoate, 0.3 part of dioctyl peroxydicarbonate, 0.05 part of mercaptoethanol, 0.08 part of dodecyl mercaptan, 0.06 part of higher alcohol, 0.1 part of stearic acid, 0.15 part of calcium stearate, 0.006 part of calcium-zinc composite stabilizer, 0.008 part of organotin stabilizer, 0.004 part of antioxidant 1010, 0.006 part of antioxidant 168, and 0.04 part of α-methylstyrene.

[0055] Comparative Example 1 The specific steps are basically the same as those in Example 1, except that the chain transfer agent is not added.

[0056] Comparative Example 2 The specific steps are basically the same as those in Example 1, except that the antioxidant is not added.

[0057] Comparative Example 3 The specific steps are basically the same as those in Example 1, except that the property regulator is not added.

[0058] Comparative Example 4 The specific steps are basically the same as those in Example 1, except that only hydroxypropyl methylcellulose is added as the dispersant.

[0059] Comparative Example 5 The specific steps are basically the same as those in Example 1, except that only dioctyl peroxydicarbonate is added as the initiator.

[0060] Comparative Example 6 The specific steps are basically the same as those in Example 1, except that a traditional plate heat exchanger is used in the stripping process.

[0061] Performance test: The PVC resins prepared in the above-mentioned examples and comparative examples were subjected to performance tests. In the performance tests, the degree of polymerization, oil absorption rate, and crystal points (number of fish eyes) were carried out in accordance with the provisions of "Suspension General Purpose Polyvinyl Chloride Resin" GB / T 5761-2018; the heat stability time was carried out in accordance with the provisions of GB / T 2917.1; the molecular weight distribution was detected by Waters1515; the particle size distribution was detected by Malvern Mastresizer3000 laser particle size analyzer. The results are shown in Table 1.

[0062] Table 1 Performance test results of PVC resin

[0063] As can be seen from Table 1, the PVC resins prepared in Examples 1 to 3 of the present invention have a high oil absorption rate, fewer crystal points, and high thermal stability, and the molecular weight distribution is moderate, the particle concentration is high, and the processing performance is excellent. The PVC resins prepared in Comparative Examples 1 to 6 are inferior to the examples of the present invention in terms of oil absorption rate, crystal points, thermal stability, etc.

[0064] Finally, it should be noted that the above-listed are only the preferred embodiments of the present invention. Of course, those skilled in the art can make changes and modifications to the present invention. If these modifications and variations fall within the scope of the claims of the present invention and its equivalent technologies, they should be considered as the protection scope of the present invention.

Claims

1. A preparation method of polyvinyl chloride resin for soft film, characterized in that, It includes the following steps: Raw material preparation: Prepare deionized water, vinyl chloride monomer, buffer, dispersant, initiator, chain transfer agent, heat stabilizer, terminator, property regulator and antioxidant in proportion; Incorporate the property regulator and chain transfer agent into the dispersant and stir evenly; Incorporate the heat stabilizer and antioxidant into the terminator for standby; Polymerization reaction: Sequentially add the raw materials of deionized water, vinyl chloride monomer, buffer, the mixed dispersant and initiator into the polymerization kettle, control the reaction temperature at 50 - 60 °C and the reaction pressure at 0.8 - 1.0 MPa to carry out the polymerization reaction. After the conversion rate reaches 80 - 85%, add the mixed terminator to terminate the reaction, and the heat stabilizer and antioxidant are added into the polymerization kettle together with the terminator; Post-treatment: After the reaction ends, subject the polymerization slurry to stripping, drying and sieving treatments to obtain the polyvinyl chloride resin product for soft film.

2. The preparation method of the polyvinyl chloride resin for soft film according to claim 1, characterized in that, The mass fraction ratio of deionized water, vinyl chloride monomer, buffer, dispersant, initiator, chain transfer agent, heat stabilizer, property regulator, antioxidant and terminator is 110 - 125:100:0.005 - 0.015:3.2 - 5.4:0.1 - 0.5:0.05 - 0.2:0.005 - 0.02:0.2 - 0.5:0.002 - 0.02:0.02 - 0.

05.

3. The preparation method of the polyvinyl chloride resin for soft film according to claim 1, characterized in that, Subject the deionized water to deoxidation treatment, with the oxygen content of the deionized water ≤ 500 ppb; Subject the vinyl chloride monomer to dehydration and iron removal treatments, with the water content < 100 mg / kg and the iron content < 2 mg / kg.

4. The preparation method of the polyvinyl chloride resin for soft film according to claim 1, characterized in that The buffer is at least one of sodium bicarbonate, ammonium bicarbonate, sodium hydroxide, trisodium phosphate or ammonia water; and / or, The dispersant is a compound of polyvinyl alcohol and hydroxypropyl methylcellulose, and their weight ratio is 6 - 8:1; Polyvinyl alcohol is used by compounding polyvinyl alcohols with different degrees of alcoholysis (high, medium and low) in proportion. The degree of alcoholysis of the high alcoholysis polyvinyl alcohol is greater than 80%, the degree of alcoholysis of the medium alcoholysis polyvinyl alcohol is 60% - 80%, and the degree of alcoholysis of the low alcoholysis polyvinyl alcohol is 30% - 50%. The mass ratio of the three is 1:1 - 1.5:0.3 - 0.45; The methoxy content of hydroxypropyl methylcellulose is 25 - 30%, and the hydroxypropyl content is 4% - 10%.

5. The preparation method of the polyvinyl chloride resin for soft film according to claim 1, characterized in that, The initiator is a compound of cumyl peroxyneodecanoate and dioctyl peroxydicarbonate, and their weight ratio is 1:1 - 3; and / or, The chain transfer agent is a compound of mercaptoethanol and dodecyl mercaptan, and their weight ratio is 1:1 - 2.

6. The preparation method of the polyvinyl chloride resin for the soft film according to claim 1, characterized in that, The heat stabilizer is a compound of calcium-zinc composite stabilizer and organotin stabilizer, and their weight ratio is 2:1 - 3; and / or, The property regulator is a compound of higher alcohol, stearic acid and calcium stearate, and their weight ratio is 0.5 - 1:1:1 - 2.

7. The preparation method of the polyvinyl chloride resin for soft film according to claim 1, characterized in that, The antioxidant is a compound of antioxidant 1010 and antioxidant 168, and their weight ratio is 1:1 - 2; and / or, The terminator is one of diethylhydroxylamine, hydroquinone, α-methylstyrene.

8. The preparation method of the polyvinyl chloride resin for soft film according to claim 1, characterized in that, In the stripping process, a wide-channel heat exchanger is used. The channel spacing of the heat exchanger is 12 mm to 14 mm, and the controlled temperature is 90 to 105 °C / 45 to 65 °C, the pressure is 1.6 MPa, and the flow rate is 70 to 100 m 3 / h.

9. A polyvinyl chloride resin for soft film prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The degree of polymerization of the polyvinyl chloride resin for soft film is 981 - 1135, the oil absorption rate is 28 - 32 g / 100 g, the number of crystal points ≤ 10 pieces / m², and the heat stability time ≥ 6 min.

10. The polyvinyl chloride resin for soft film according to claim 9, characterized in that, The molecular weight distribution index of the polyvinyl chloride resin for the soft film is 1.5 to 2.0; the particle size distribution of the polyvinyl chloride resin for the soft film is <0.7.

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