A lead-free environment-friendly polyvinyl chloride film and a preparation method thereof

Abstract

The present application relates to the technical field of film materials, and particularly relates to a lead-free environment-friendly polyvinyl chloride film and a preparation method thereof.The lead-free environment-friendly polyvinyl chloride film is prepared by the following steps: first, preparing a modified base resin by suspension copolymerization of chloroethylene, monobutyl itaconate and N-allyl thiourea; then, mixing and sealing the modified base resin with dioctyl terephthalate, epoxy soybean oil, calcium-zinc composite stabilizer and other components for maturation; finally, preparing the lead-free environment-friendly polyvinyl chloride film by double-roller mixing and hot-pressing shaping.The present application uses a ternary copolymerization in-situ modification and a lead-free stabilizing system, completely discards the lead salt component, and improves the defects of the traditional calcium-zinc system, such as insufficient thermal stability, easy yellowing caused by zinc burning, and poor transparency, so that the thermal stability, yellowing resistance, mechanical and optical properties are balanced.

Description

Technical Field

[0001] This invention relates to the field of membrane materials technology, and in particular to a lead-free environmentally friendly polyvinyl chloride membrane and its preparation method. Background Technology

[0002] Polyvinyl chloride (PVC) is one of the most widely used general-purpose thermoplastic resins globally. Thanks to its excellent chemical resistance, mechanical properties, flame retardancy, and cost advantages, PVC films, manufactured through processes such as calendering, blow molding, and casting, are widely used in food packaging, medical consumables, building decoration, daily necessities, and electronics. However, the PVC molecular chain contains unstable structures such as allyl chloride and tertiary chloride, which are prone to dehydrochlorination chain degradation reactions during high-temperature processing and long-term use, leading to discoloration and decreased mechanical properties. Therefore, heat stabilizers must be added to the formulation system to inhibit this degradation process. For a long time, lead salt heat stabilizers have been the mainstream additives in PVC processing due to their high heat stabilization efficiency, low cost, and good process compatibility, especially in low- and mid-range film products. However, lead is a toxic heavy metal that can accumulate in organisms through contact and food chain migration, causing irreversible damage to multiple systems in the human body. Furthermore, lead-containing PVC products can cause heavy metal pollution of soil and water bodies during waste disposal, posing serious environmental and ecological risks.

[0003] Currently, lead-free thermal stabilization technology for polyvinyl chloride (PVC) has developed into multiple technical routes and achieved varying degrees of industrial application. Among them, calcium-zinc composite stabilizers, with their non-toxic, environmentally friendly, and moderately cost-effective characteristics, have become the mainstream solution for lead-free substitution. They inhibit the degradation process through the synergistic effect of calcium and zinc soaps, meeting the processing requirements of most general-purpose PVC products. Organotin stabilizers possess excellent transparency and long-term thermal stability, giving them irreplaceable application advantages in the field of high-end transparent PVC films. Rare earth stabilizers, relying on the special electronic structure of rare earth elements, can simultaneously improve the thermal stability and weather resistance of products, and have certain application potential in building material films. In addition, auxiliary stabilizers such as hydrotalcite, phosphites, polyols, and β-diketones are also widely used in compound systems to synergistically enhance the overall stabilization effect.

[0004] Although existing lead-free stabilizing systems have achieved large-scale application, their application in PVC film products still faces numerous technical bottlenecks, making it difficult to simultaneously meet the multiple requirements of environmental protection, processing performance, performance in use, and economy. Specifically, calcium-zinc composite systems generally suffer from poor initial colorability and insufficient long-term thermal stability. To achieve the stabilization effect of lead salt systems, the amount added often needs to be increased, which not only increases costs but also easily leads to defects such as additive precipitation and blooming, damaging the surface texture and optical properties of the film material. Organotin stabilizers have high production costs, and some varieties have controversies regarding odor and ecotoxicity, limiting their application in food contact and confined space scenarios. Rare earth stabilizers are greatly affected by fluctuations in raw material resources and prices, resulting in uncertainty in industrialization and promotion. Furthermore, their poor transparency makes them unsuitable for the needs of high-transparency film materials. At the same time, PVC membrane materials themselves have stringent requirements for indicators such as thickness uniformity, light transmittance, surface smoothness, and migration resistance. Existing lead-free formulation systems often suffer from problems such as narrow processing window, poor plasticization uniformity, and mechanical property degradation. The core formulation technology of high-end lead-free environmentally friendly PVC membranes is still controlled by foreign companies, and there is still a significant gap in the overall performance of domestic systems.

[0005] In summary, the current field of lead-free environmentally friendly polyvinyl chloride (PVC) films still lacks a complete technical solution that combines excellent thermal stability, optical performance, migration resistance, and cost advantages. There is an urgent need to develop new lead-free composite stabilization systems and supporting formulation processes. Summary of the Invention

[0006] To address the problems mentioned in the background section, this invention provides a lead-free, environmentally friendly polyvinyl chloride (PVC) membrane and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: The present invention discloses a method for preparing a lead-free, environmentally friendly polyvinyl chloride (PVC) membrane, comprising the following steps: S1. Vinyl chloride, itaconic acid monobutyl ester, and N-allyl thiourea are copolymerized in a deionized water system with a pH buffer added to maintain the pH value at 7.0-8.0 during the polymerization process. Polyvinyl alcohol is used as a dispersant, and the copolymerization reaction is carried out in a closed pressure-resistant reactor under constant stirring speed with the help of a composite initiator. After the reaction is completed, the matrix resin is obtained after post-treatment.

[0008] The mass ratio of vinyl chloride, itaconic acid monobutyl ester, and N-allyl thiourea is 100:(2.5-3.5):(1.5-2.0). The composite initiator consists of tert-butyl peroxypentanoate and diisopropyl peroxydicarbonate, with a mass ratio of (2.0-2.5):1, and the total mass of the composite initiator is 0.06-0.07% of the mass of vinyl chloride. The amount of polyvinyl alcohol dispersant is 0.08%-0.12% of the mass of vinyl chloride, and the degree of alcoholysis of polyvinyl alcohol is 78.0%-82.0%. The mass ratio of deionized water to vinyl chloride is (1.2-1.5):1. The pH buffer is sodium bicarbonate, with an amount of 0.02%-0.05% of the mass of vinyl chloride.

[0009] The copolymerization reaction was carried out at a temperature of 50-60℃, a pressure of 0.6-0.8MPa, and a time of 9-11 hours. The stirring speed was maintained at 120-180 r / min during the copolymerization process. Post-reaction processing included centrifugation of the polymer slurry at 2000-3000 r / min for 5-10 minutes, followed by washing until the filtrate conductivity was ≤20 μS / cm. The slurry was then dried to constant weight under vacuum conditions of 58-62℃ and an absolute pressure of -0.08 MPa to -0.095 MPa to obtain the modified matrix resin.

[0010] S2. Weigh the matrix resin, dioctyl terephthalate, epoxidized soybean oil, calcium-zinc composite stabilizer, acrylate processing aid, and stearic acid, and perform hot mixing and cold mixing in sequence. Seal and place the mixture in a dry, light-proof, airtight container for maturation to obtain a premix.

[0011] The hot mixing process is carried out in a high-speed mixer. The specific process is as follows: First, stir and heat the mixture to 78-82℃ at a speed of 1100-1300r / min. Then, add dioctyl terephthalate and epoxidized soybean oil. Increase the speed to 2300-2500r / min and stir for 2-3 minutes until the plasticizer is fully adsorbed by the resin. After the system temperature is raised to 90-92℃, add calcium-zinc composite stabilizer and acrylate processing aid. Continue stirring for 1-2 minutes until the material is evenly dispersed. Then, heat the mixture to 108-112℃, add stearic acid, and stir for 0.5-1 minutes before discharging.

[0012] Cold mixing is carried out in a low-speed cold mixer with a cooling jacket. The cold mixing speed is 200-400 r / min and the cooling temperature is 35-40℃. During the mixing process, circulating cooling water is continuously introduced into the jacket for heat exchange. After the material is cooled to the target temperature, it is stirred for another 1-2 minutes before being discharged.

[0013] The maturation process is carried out in a sealed environment at room temperature, away from light, for 23-25 ​​hours. The ambient temperature is 20-25℃, and the relative humidity is ≤60%. The material is completely isolated from air and direct sunlight is avoided during the maturation process.

[0014] S3. The premixed material is mixed on a two-roll mill. After being mixed evenly, it is cut into sheets that fit the mold size. Then, it is placed in the mold and subjected to hot pressing for degassing, plasticizing and cross-linking and cold pressing for shaping to obtain lead-free environmentally friendly polyvinyl chloride film.

[0015] The mixing temperature is 165-170℃, the mixing time is 7-9 minutes, and the roller gap is controlled at 0.8-1.2 mm. The linear speed ratio of the front and rear rollers of the two-roll open mill is 1:(1.1-1.2). During the mixing process, a triangular bag turning operation is performed every 1.5-2 minutes, and the material is turned 3-4 times in total to ensure that the material is mixed evenly.

[0016] The conditions for hot pressing and venting are: temperature 168-172℃, pressure 0.4-0.6MPa, time 2.5-3.5min, pressure increase and decrease cycles 2-4 times, holding pressure for 20-30s after each pressure increase before rapid pressure release and venting; the conditions for plasticizing and cross-linking are: pressure 9-11MPa, time 4.5-5.5min, with the mold temperature maintained constant at 168-172℃ during the plasticizing and cross-linking process; the conditions for cold pressing and shaping are: pressure 9-11MPa, water cooling, cooling water flow rate controlled at 5-8L / min, material cooling rate 8-12℃ / min, demolding after cooling to 22-28℃.

[0017] The PVC film obtained in this step has a thickness of 0.075-0.155 mm, and the overall thickness deviation of the film is controlled within ±0.005 mm.

[0018] According to another aspect of the present invention, the lead-free environmentally friendly polyvinyl chloride membrane of the present invention is prepared by the above-described preparation method and comprises the following components by weight: 100 parts of matrix resin, 30-35 parts of dioctyl terephthalate, 1.5-2.5 parts of epoxidized soybean oil, 1.2-2.0 parts of calcium-zinc composite stabilizer, 0.8-1.2 parts of acrylate processing aid, and 0.2-0.5 parts of stearic acid.

[0019] Among them, the calcium-zinc composite stabilizer is composed of calcium stearate, zinc stearate and β-diketone synergist, with the mass ratio of calcium stearate to zinc stearate being (3-4):1, and the amount of β-diketone synergist being 15%-20% of the total mass of calcium soap and zinc soap; the acrylate processing aid is an acrylate copolymer with a number average molecular weight of 3 million to 5 million; and the epoxy value of the epoxidized soybean oil is ≥6.0%.

[0020] The beneficial effects of this invention are: 1. This invention uses a suspension free radical copolymerization reaction to covalently integrate itaconic acid monobutyl ester and N-allyl thiourea functional monomers into the main chain of polyvinyl chloride to obtain a ternary copolymer matrix resin. During the polymerization process, the functional side groups passivate unstable structures such as allyl chloride and tertiary chloride on the molecular chain in situ, reducing thermal degradation initiation sites and improving the thermal stability of the resin.

[0021] 2. This invention introduces polar functional groups into the polyvinyl chloride molecular chain through copolymerization modification, thereby increasing the polarity and cohesive energy density of the matrix resin and reducing the solubility parameter differences between the matrix and polar additives such as plasticizers and stabilizers. Combined with a step-by-step feeding process involving cold and hot mixing and curing, the additives gradually diffuse into the resin particles, achieving a more uniform dispersion. During high-temperature molding, the epoxy groups of epoxidized soybean oil undergo esterification with the carboxyl groups of the matrix side chains, forming a small number of covalent crosslinking points. Simultaneously, hydrogen bonds and dipole interactions form between the matrix side groups and additive molecules, further strengthening the bonding between the additives and the matrix. This helps reduce the risk of additive precipitation and blooming, ensuring the surface smoothness and performance stability of the film material during long-term use.

[0022] 3. This invention disrupts the regularity of PVC molecular chains through the random incorporation of functional monomers, reducing resin crystallinity and melting enthalpy. Combined with the control of melt viscoelasticity by acrylate processing aids and the homogenization effect of molecular chain entanglement by two-roll mixing shearing, molding is completed through a stepped hot-pressing process. This lowers the resin plasticizing temperature, widens the processing temperature window, reduces the risk of degradation and discoloration caused by high-temperature processing, and simultaneously ensures the melt possesses suitable fluidity and melt strength, reducing problems such as melt fracture and uneven local plasticization during molding. Detailed Implementation

[0023] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] In the following examples and comparative examples, vinyl chloride was industrial grade with a purity ≥99.9%; itaconic acid monobutyl ester (CAS No.: 6439-57-2) and N-allyl thiourea (CAS No.: 185147-29-9) were both industrial grade with a purity ≥98.0%; polyvinyl alcohol was industrial grade with a degree of alcoholysis of 78.0-82.0% and a purity ≥98.0%; tert-butyl peroxypentanoate (CAS No.: 927-07-1) and diisopropyl peroxydicarbonate (CAS No.: 105-64-6) were both industrial grade with a purity ≥98%. 0.5%; Sodium bicarbonate (CAS No.: 144-55-8) is analytical grade, purity ≥99.5%; Dioctyl terephthalate (CAS No.: 6422-86-2) is industrial grade environmentally friendly plasticizer, purity ≥99.0%; Epoxidized soybean oil is industrial grade, epoxy value ≥6.0%; Calcium stearate (CAS No.: 1592-23-0) and zinc stearate (CAS No.: 557-05-1) are both industrial grade, purity ≥98.0%; Stearic acid (CAS No.: 57-11-4) is industrial grade, purity ≥98.0%.

[0025] Example 1 A method for preparing a lead-free, environmentally friendly polyvinyl chloride (PVC) film includes the following steps: S1. Add 120 mL of deionized water, 0.08 g of polyvinyl alcohol with a degree of alcoholysis of 78.0%, and 0.02 g of sodium bicarbonate to a sealed pressure-resistant reactor, and stir until completely dissolved; add 100 g of vinyl chloride, 2.5 g of itaconic acid monobutyl ester, and 1.5 g of N-allyl thiourea in sequence, followed by 0.04 g of tert-butyl peroxypentanoate and 0.02 g of diisopropyl peroxydicarbonate; control the stirring speed at 120 r / min, raise the temperature to 50 °C, and copolymerize at a reaction pressure of 0.6 MPa for 9 h; after the reaction is completed, centrifuge the polymer slurry at 2000 r / min for 5 min to dehydrate, wash until the conductivity of the filtrate is ≤20 μS / cm, and vacuum dry at 58 °C and an absolute pressure of -0.08 MPa to constant weight to obtain the matrix resin.

[0026] S2. Weigh 100g of matrix resin, 30g of dioctyl terephthalate, 1.5g of epoxidized soybean oil, 1.2g of calcium-zinc composite stabilizer, 0.8g of acrylate processing aid, and 0.2g of stearic acid. First, stir and heat to 78°C at 1100r / min. Add dioctyl terephthalate and epoxidized soybean oil, increase the speed to 2300r / min and stir to 90°C. Add calcium-zinc composite stabilizer and acrylate processing aid, heat to 108°C and add stearic acid. Stir and discharge. Then, cold mix in a cold mixer with a cooling jacket at 200r / min and cool to 35°C before discharging. Transfer to a dry, light-proof, sealed container and let it mature for 23 hours to obtain the premix.

[0027] S3. Place the premixed material in a two-roll mill for mixing at a mixing temperature of 165℃ for 7 minutes and a roll gap of 0.8mm. After uniform mixing, cut the material into sheets, place them in a mold, and hot press them at 168℃ and 0.4MPa for 2.5 minutes to remove air, then cycle the pressure twice. Next, plasticize and crosslink the material at 9MPa for 4.5 minutes. Finally, cool the material with water at 9MPa to 22℃ and demold to obtain a lead-free environmentally friendly polyvinyl chloride film with a thickness of about 0.075mm.

[0028] Example 2 A method for preparing a lead-free, environmentally friendly polyvinyl chloride (PVC) film includes the following steps: S1. Add 130 mL of deionized water, 0.10 g of polyvinyl alcohol with a degree of alcoholysis of 80.0%, and 0.035 g of sodium bicarbonate to a sealed pressure-resistant reactor, and stir until completely dissolved; add 100 g of vinyl chloride, 3.0 g of itaconic acid monobutyl ester, 1.8 g of N-allyl thiourea, and then add 0.045 g of tert-butyl peroxypentanoate and 0.020 g of diisopropyl peroxydicarbonate; control the stirring speed at 150 r / min, raise the temperature to 55 °C, and copolymerize at a reaction pressure of 0.7 MPa for 10 h; after the reaction is completed, centrifuge the polymer slurry at 2500 r / min for 8 min to dehydrate, wash until the conductivity of the filtrate is ≤20 μS / cm, and vacuum dry at 60 °C and an absolute pressure of -0.09 MPa to constant weight to obtain the matrix resin.

[0029] S2. Weigh 100g of matrix resin, 32g of dioctyl terephthalate, 2.0g of epoxidized soybean oil, 1.6g of calcium-zinc composite stabilizer, 1.0g of acrylate processing aid, and 0.3g of stearic acid. First, stir and heat to 80°C at 1200r / min. Add dioctyl terephthalate and epoxidized soybean oil, increase the speed to 2400r / min and stir to 91°C. Add calcium-zinc composite stabilizer and acrylate processing aid, heat to 110°C and add stearic acid. Stir and discharge. Then, cold mix in a cold mixer with a cooling jacket at 300r / min and cool to 38°C before discharging. Transfer to a dry, light-proof, sealed container and let it mature for 24 hours to obtain the premix.

[0030] S3. Place the premixed material in a two-roll mill for mixing at a mixing temperature of 168℃ for 8 minutes and a roll gap of 1.0 mm. After uniform mixing, cut the material into sheets, place them in a mold, and hot press them at 170℃ and 0.5MPa for 3.0 minutes to remove air, then cycle the pressure up and down 3 times. Next, plasticize and crosslink the material at 10MPa for 5.0 minutes. Finally, cool the material with water at 10MPa to 25℃ and demold it to obtain a lead-free environmentally friendly polyvinyl chloride film with a thickness of about 0.110 mm.

[0031] Example 3 A method for preparing a lead-free, environmentally friendly polyvinyl chloride (PVC) film includes the following steps: S1. Add 150 mL of deionized water, 0.12 g of polyvinyl alcohol with a degree of alcoholysis of 82.0%, and 0.05 g of sodium bicarbonate to a sealed pressure-resistant reactor, and stir until completely dissolved. Then add 100 g of vinyl chloride, 3.5 g of itaconic acid monobutyl ester, 2.0 g of N-allyl thiourea, 0.05 g of tert-butyl peroxypentanoate, and 0.02 g of diisopropyl peroxydicarbonate. Control the stirring speed at 180 r / min, raise the temperature to 60 °C, and copolymerize at a reaction pressure of 0.8 MPa for 11 h. After the reaction, centrifuge the polymer slurry at 3000 r / min for 10 min to dehydrate, wash until the conductivity of the filtrate is ≤20 μS / cm, and vacuum dry at 62 °C and an absolute pressure of -0.095 MPa to constant weight to obtain the matrix resin.

[0032] S2. Weigh 100g of matrix resin, 35g of dioctyl terephthalate, 2.5g of epoxidized soybean oil, 2.0g of calcium-zinc composite stabilizer, 1.2g of acrylate processing aid, and 0.5g of stearic acid. First, stir and heat to 82°C at 1300r / min. Add dioctyl terephthalate and epoxidized soybean oil, increase the speed to 2500r / min and stir to 92°C. Add calcium-zinc composite stabilizer and acrylate processing aid, heat to 112°C and add stearic acid. Stir and discharge. Then, cold mix in a cold mixer with a cooling jacket at 400r / min and cool to 40°C before discharging. Transfer to a dry, light-proof, sealed container and let it mature for 25 hours to obtain the premix.

[0033] S3. Place the premixed material in a two-roll mill for mixing at a mixing temperature of 170℃ for 9 minutes and a roll gap of 1.2mm. After uniform mixing, cut the material into sheets, place them in a mold, and hot press them at 172℃ and 0.6MPa for 3.5 minutes to remove air, followed by 4 cycles of increasing and decreasing pressure. Then, plasticize and crosslink the material at 11MPa for 5.5 minutes. Finally, cool the material with water at 11MPa to 28℃ and demold it to obtain a lead-free environmentally friendly polyvinyl chloride film with a thickness of approximately 0.155mm.

[0034] Example 4 A method for preparing a lead-free, environmentally friendly polyvinyl chloride (PVC) film includes the following steps: S1. Add 125 mL of deionized water, 0.09 g of polyvinyl alcohol with a degree of alcoholysis of 79.0%, and 0.025 g of sodium bicarbonate to a sealed pressure-resistant reactor, and stir until completely dissolved. Then add 100 g of vinyl chloride, 2.8 g of itaconic acid monobutyl ester, 1.6 g of N-allyl thiourea, 0.041 g of tert-butyl peroxypentanoate, and 0.021 g of diisopropyl peroxydicarbonate. Control the stirring speed at 130 r / min, raise the temperature to 52 °C, and copolymerize at a reaction pressure of 0.65 MPa for 9.5 h. After the reaction, centrifuge the polymer slurry at 2200 r / min for 6 min to dehydrate, wash until the conductivity of the filtrate is ≤20 μS / cm, and vacuum dry at 59 °C and an absolute pressure of -0.085 MPa to constant weight to obtain the matrix resin.

[0035] S2. Weigh 100g of matrix resin, 31g of dioctyl terephthalate, 1.8g of epoxidized soybean oil, 1.4g of calcium-zinc composite stabilizer, 0.9g of acrylate processing aid, and 0.25g of stearic acid. First, stir and heat to 79°C at 1150r / min. Add dioctyl terephthalate and epoxidized soybean oil, increase the speed to 2350r / min and stir to 90°C. Add calcium-zinc composite stabilizer and acrylate processing aid, heat to 109°C and add stearic acid. Stir and discharge. Then, cold mix in a cold mixer with a cooling jacket at 250r / min and cool to 36°C before discharging. Transfer to a dry, light-proof, sealed container and let it mature for 23.5h to obtain the premix.

[0036] S3. Place the premixed material in a two-roll mill for mixing at a mixing temperature of 166℃ for 7.5 min and a roll gap of 0.9 mm. After uniform mixing, cut the material into sheets, place them in a mold, and hot press them at 169℃ and 0.45 MPa for 2.8 min to remove air, followed by two cycles of increasing and decreasing pressure. Then, plasticize and crosslink the material at 9.5 MPa for 4.8 min. Finally, cool the material with water to 23℃ at 9.5 MPa and demold to obtain a lead-free environmentally friendly polyvinyl chloride film with a thickness of approximately 0.090 mm.

[0037] Example 5 A method for preparing a lead-free, environmentally friendly polyvinyl chloride (PVC) film includes the following steps: S1. Add 140 mL of deionized water, 0.11 g of polyvinyl alcohol with a degree of alcoholysis of 81.0%, and 0.045 g of sodium bicarbonate to a sealed pressure-resistant reactor, and stir until completely dissolved; add 100 g of vinyl chloride, 3.2 g of itaconic acid monobutyl ester, and 1.9 g of N-allyl thiourea in sequence, followed by 0.049 g of tert-butyl peroxypentanoate and 0.019 g of diisopropyl peroxydicarbonate; control the stirring speed at 170 r / min, raise the temperature to 58 °C, and copolymerize at a reaction pressure of 0.75 MPa for 10.5 h; after the reaction is completed, centrifuge the polymer slurry at 2800 r / min for 9 min to dehydrate, wash until the conductivity of the filtrate is ≤20 μS / cm, and vacuum dry at 61 °C and an absolute pressure of -0.092 MPa to constant weight to obtain the matrix resin.

[0038] S2. Weigh 100g of matrix resin, 34g of dioctyl terephthalate, 2.2g of epoxidized soybean oil, 1.8g of calcium-zinc composite stabilizer, 1.1g of acrylate processing aid, and 0.4g of stearic acid. First, stir and heat to 81°C at 1250r / min, add dioctyl terephthalate and epoxidized soybean oil, increase the speed to 2450r / min and stir to 92°C, add calcium-zinc composite stabilizer and acrylate processing aid, heat to 111°C and add stearic acid, stir and discharge. Then, cold mix in a cold mixer with a cooling jacket at 350r / min, cool to 39°C and discharge. Transfer to a dry, light-proof, sealed container and let it mature for 24.5h to obtain the premix.

[0039] S3. Place the premixed material in a two-roll mill for mixing at a mixing temperature of 169℃ for 8.5 min and a roll gap of 1.1 mm. After uniform mixing, cut the material into sheets, place them in a mold, and hot press them at 171℃ and 0.55 MPa for 3.2 min to remove air, then cycle the pressure up and down 3 times. Next, plasticize and crosslink the material at 10.5 MPa for 5.2 min. Finally, cool the material with water at 10.5 MPa to 27℃ and demold it to obtain a lead-free environmentally friendly polyvinyl chloride film with a thickness of about 0.130 mm.

[0040] Example 6 A method for preparing a lead-free, environmentally friendly polyvinyl chloride (PVC) film includes the following steps: S1. Add 135 mL of deionized water, 0.10 g of polyvinyl alcohol with a degree of alcoholysis of 80.0%, and 0.035 g of sodium bicarbonate to a sealed pressure-resistant reactor, and stir until completely dissolved; add 100 g of vinyl chloride, 3.0 g of itaconic acid monobutyl ester, and 1.7 g of N-allyl thiourea in sequence, followed by 0.046 g of tert-butyl peroxypentanoate and 0.020 g of diisopropyl peroxydicarbonate; control the stirring speed at 150 r / min, raise the temperature to 56 °C, and copolymerize at a reaction pressure of 0.72 MPa for 10 h; after the reaction is completed, centrifuge the polymer slurry at 2500 r / min for 7 min to dehydrate, wash until the conductivity of the filtrate is ≤20 μS / cm, and vacuum dry at 60 °C and an absolute pressure of -0.09 MPa to constant weight to obtain the matrix resin.

[0041] S2. Weigh 100g of matrix resin, 33g of dioctyl terephthalate, 2.0g of epoxidized soybean oil, 1.5g of calcium-zinc composite stabilizer, 1.0g of acrylate processing aid, and 0.35g of stearic acid. First, stir and heat to 80°C at 1200r / min. Add dioctyl terephthalate and epoxidized soybean oil, increase the speed to 2400r / min and stir to 91°C. Add calcium-zinc composite stabilizer and acrylate processing aid, heat to 110°C and add stearic acid. Stir and discharge. Then, cold mix in a cold mixer with a cooling jacket at 300r / min and cool to 37°C before discharging. Transfer to a dry, light-proof, sealed container and let it mature for 24 hours to obtain the premix.

[0042] S3. Place the premixed material in a two-roll mill for mixing at a mixing temperature of 167℃ for 8 minutes and a roll gap of 1.0 mm. After uniform mixing, cut the material into sheets, place them in a mold, and hot press them at 170℃ and 0.5MPa for 3.0 minutes to remove air, followed by 4 cycles of increasing and decreasing pressure. Then, plasticize and crosslink the material at 10MPa for 5.0 minutes. Finally, cool the material with water at 10MPa to 25℃ and demold it to obtain a lead-free environmentally friendly polyvinyl chloride film with a thickness of approximately 0.100 mm.

[0043] Comparative Example 1 The difference between this comparative example and Example 2 is that itaconic acid monobutyl ester is not added in step S1, and only vinyl chloride and N-allyl thiourea are used to prepare the matrix resin by binary copolymerization. The dosage of other components and the preparation steps are the same as in Example 2.

[0044] Comparative Example 2 The difference between this comparative example and Example 2 is that N-allyl thiourea is not added in step S1, and only vinyl chloride and itaconic acid monobutyl ester are used to prepare the matrix resin by binary copolymerization. The dosage of other components and the preparation steps are the same as in Example 2.

[0045] Comparative Example 3 The difference between this comparative example and Example 2 is that the copolymerization preparation process in step S1 is omitted, and commercially available general-purpose polyvinyl chloride homopolymer resin is directly used as the matrix resin. The dosage of other components and the preparation steps are the same as in Example 2.

[0046] Comparative Example 4 The difference between this comparative example and Example 2 is that epoxidized soybean oil is not added in step S2, while the amounts of other components and preparation steps are the same as in Example 2.

[0047] Comparative Example 5 The difference between this comparative example and Example 2 is that in step S2, the calcium-zinc composite stabilizer is replaced with an equal mass of tribasic lead sulfate lead salt stabilizer, and epoxidized soybean oil is not added. The amounts of other components and the preparation steps are the same as in Example 2.

[0048] Comparative Example 6 The difference between this comparative example and Example 2 is that commercially available general-purpose polyvinyl chloride homopolymer resin is used as the matrix resin, and epoxidized soybean oil is not added. Only an equal amount of calcium-zinc composite stabilizer is added as a stabilizing system. The remaining preparation steps are consistent with those of Example 2.

[0049] The polyvinyl chloride film samples prepared in the above embodiments and comparative examples were placed in a standard environment with a temperature of 23°C and a relative humidity of 50% for conditioning for no less than 24 hours. Subsequently, samples of the corresponding specifications were cut according to the requirements of each test item, and 5 parallel samples were prepared for each test group.

[0050] (I) Thermal stability test: The test was conducted using the Congo red method according to GB / T 2917.1-2002 standard. The temperature control accuracy of the constant temperature oil bath used in the test was ±0.5℃. Before the test, the membrane sample was cut into uniform strips with a width of about 1mm and a length of about 5mm to ensure that the sample particles were uniform and free of lumps. 2g±0.1g of sample was accurately weighed and placed into a hard glass test tube with an inner diameter of 15mm, a wall thickness of 1.5mm and a height of 150mm. The test tube was tapped vertically on the table to gently vibrate the sample to the bottom of the test tube. The sample accumulation height was controlled within the range of 30mm±5mm. A 10mm wide Congo red test paper was taken, and after being evenly moistened with a 50% glycerol aqueous solution, it was fixed on the inner wall of the test tube 20mm above the sample. The long side of the test paper was perpendicular to the axis of the test tube, and the test paper was not completely attached to the test tube wall to ensure sufficient contact with the degradation gas. The test tubes were then immersed entirely in a constant-temperature oil bath preheated to 180℃±0.5℃, ensuring the oil level was at least 30mm above the upper surface of the sample inside the tube, and that the immersion depth of each group of test tubes was consistent. A stopwatch was started simultaneously to record the time from the moment the test tube was immersed in the oil bath until the appearance of blue spots on the Congo red test paper. This time was taken as the static thermal stability time of a single sample, expressed in minutes. Three parallel samples were tested for each group, and the arithmetic mean of the results was taken. The results are shown in Table 1. Table 1. Test results of static thermal stability time for each sample (II) Thermal Aging Coloring Performance Test: The evaluation was conducted using an oven thermal aging method combined with color difference analysis. The temperature control accuracy of the constant-temperature forced-air oven was ±1℃, and the hot air circulation speed was adjustable. A flat film sample with dimensions of 50mm × 50mm was taken. The sample surface was free of scratches, crystal points, and precipitates. The sample was laid flat on a clean glass slide, ensuring no contact between samples. It was placed on the middle rack of a constant-temperature forced-air oven preheated to 170℃ ± 1℃. The hot air circulation speed inside the oven was kept constant to avoid direct airflow impact on the sample surface and ensure uniform heating. Samples were removed at 10min, 30min, and 60min of heating time, placed in a desiccator to cool to room temperature, and then the yellowness index of the samples was measured using a spectrophotometer. The test used a D65 standard light source, a 10° field of view, and a 20mm aperture. Three different locations were tested for each sample, and the average value was taken. The formula for calculating yellowness is shown below: In the formula, YI is the yellowness index of the sample, and X, Y, and Z are the tristimulus values ​​of the sample in the CIE 1931 standard colorimetric system. The initial coloring performance of the material is characterized by the yellowness index of the initially unaged sample, and the rate of thermal color change of the material is characterized by the change in yellowness index under different aging times. The yellowness change rate is calculated by the following formula: In the formula, ΔYI t YI represents the change in yellowness after aging for time t. t YI0 represents the yellowness index of the sample after aging for time t, and YI0 represents the initial yellowness index of the sample. The results are shown in Table 2. Table 2. Results of Yellowing Index Test for Each Sample During Thermal Aging (III) Mechanical Property Testing: Following GB / T 1040.3-2006, a universal electronic tensile testing machine with a precision class of 1 was used for tensile property testing. The testing environment and conditioning environment were kept consistent. Before testing, the membrane material was cut into type II dumbbell-shaped specimens with a total length of 115 mm, a parallel section width of 6 mm, and a gauge length of 25 mm. During cutting, the specimen edges were ensured to be smooth and free of gaps to avoid stress concentration due to specimen defects affecting the test results. Before testing, a micrometer with an accuracy of 0.001 mm was used to measure the thickness at three points within the gauge length (left, center, and right), and the minimum value was taken as the specimen thickness for calculation. The specimen was clamped in the pneumatic fixture of the testing machine, ensuring that the specimen axis coincided with the line connecting the centers of the upper and lower fixtures. The clamping force was moderate to avoid additional stress or specimen slippage. The tensile rate was set to 200 mm / min, and the testing machine was started until the specimen completely fractured. The maximum load during the fracture process and the gauge length elongation at fracture were recorded. The tensile strength of the specimen was calculated using the following formula: In the formula, σ t F represents the tensile strength of the specimen, expressed in MPa. max ρ is the maximum load during the specimen fracture process, in N; b is the width of the parallel portion of the specimen, in mm; d is the thickness of the specimen, in mm. The elongation at break of the specimen is calculated using the following formula: In the formula, ε t denoted as σ0, where L is the initial gauge length of the specimen (in mm); L is the gauge length at fracture (in mm). The results are shown in Table 3. Table 3. Test results of mechanical properties of each specimen (IV) Optical Performance Testing: Referring to GB / T 2410-2008, an integrating sphere transmittance and haze meter was used to test the visible light transmittance and haze value of the sample. The instrument's transmittance measurement accuracy was ±0.5%, and the haze measurement accuracy was ±0.1%. Before testing, the instrument was calibrated for zero point and range using a standard black board, a standard white board, and a standard haze plate to ensure that the testing accuracy met the standard requirements. A flat, defect-free film sample with dimensions of 50mm × 50mm was placed at the instrument's test port, ensuring that the sample completely covered the test optical path and was free of wrinkles, scratches, and surface contamination. A standard C light source was used during the test, with the beam perpendicularly incident on the sample surface. Five different test sites were tested for each sample, and the arithmetic mean was taken after discarding outliers. The visible light transmittance is the ratio of the total luminous flux transmitted through the sample to the incident luminous flux, calculated using the following formula: In the formula, T tΦ represents the visible light transmittance of the sample. t Φ0 is the total luminous flux transmitted through the sample; Φ0 is the total luminous flux incident on the sample surface. Haze is the ratio of scattered luminous flux transmitted through the sample to the total transmitted luminous flux, calculated using the following formula: In the formula, H is the haze of the sample; Φ d Φ is the scattered light flux transmitted through the sample. t This represents the total luminous flux transmitted through the sample. The results are shown in Table 4. Table 4. Test results of optical performance of each sample As shown in Table 1, the static thermal stability time of Examples 1-6 is in the range of 54.7-65.2 min, which is significantly better than the conventional lead-free calcium-zinc system. Among them, the best performing Example 2 has a static thermal stability time of 65.2 min, which is more than double that of the conventional lead-free system of Comparative Example 6, and is close to the thermal stability level of the traditional lead salt system. This invention introduces itaconic acid monobutyl ester and N-allyl thiourea into the main chain of polyvinyl chloride (PVC) molecules in situ through suspension ternary copolymerization. This passivates unstable structures such as allyl chloride and tertiary chloride at the molecular structure level, reduces the initiation sites for thermal degradation, and improves the intrinsic thermal stability of the resin. Based on this, a multi-level synergistic stabilization system is constructed by combining a calcium-zinc composite stabilizer with epoxidized soybean oil. The calcium-zinc soap replaces unstable chlorine atoms through nucleophilic substitution and neutralizes hydrogen chloride generated during degradation. The epoxidized soybean oil captures hydrogen chloride through an epoxidation ring-opening reaction to block the autocatalytic degradation cycle. The thiourea groups introduced by copolymerization can also form a coordination complex structure with zinc ions, reducing the Lewis acid catalytic activity of zinc chloride and inhibiting zinc oxidative degradation. These multiple effects work together to improve the long-term thermal stability of the system.

[0051] In Comparative Example 2, the static thermal stability time decreased to 34.8 min after the N-allyl thiourea was removed, showing the most significant performance degradation. This is because the lack of the thiourea group's in-situ passivation of the unstable chlorine structure and its coordination inhibition of zinc ions makes the system prone to accelerated degradation due to zinc chloride accumulation, failing to effectively block the chain reaction of dehydrochlorination. In Comparative Example 1, the thermal stability time decreased to 47.6 min after the itaconic acid monobutyl ester was removed, indicating that the polar ester side group can increase the thermal dissociation energy of adjacent C-Cl bonds and improve the compatibility between the additive and the matrix, thus assisting in enhancing the stabilization effect. Comparative Example 3, using commercially available general-purpose polyvinyl chloride homopolymer resin, had a thermal stability time of only 41.9 min, further verifying the effect of in-situ copolymerization modification on improving the intrinsic thermal stability of the resin. In Comparative Example 4, the thermal stability time decreased to 51.7 min after the removal of epoxidized soybean oil, demonstrating the functional contribution of the epoxy component in capturing hydrogen chloride and synergistic stabilization. Although the traditional lead salt system in Comparative Example 5 had a longer thermal stability time, it relied on heavy metal components to achieve the stabilization effect, posing inherent environmental and health risks.

[0052] As shown in Table 2, the initial yellowness index of Examples 1-6 was only 2.8-3.2, and the yellowness change value after 60 min of heat aging at 170℃ was in the range of 7.4-10.3. These examples exhibited the characteristics of light initial coloring and slow yellowing rate during heat aging, demonstrating significantly better coloring performance than the comparative examples of the binary copolymers and general-purpose resins. In this invention, the thiourea groups introduced through copolymerization can undergo addition reactions with the conjugated double bonds generated in the early stages of degradation, blocking the chain growth of the conjugated polyene sequence. Simultaneously, through coordination, they inhibit the rapid dehydrochlorination reaction catalyzed by zinc chloride, reducing the formation of color-causing conjugated structures at the source. The polar side groups of itaconic acid monobutyl ester improved the uniformity of the stabilizer's dispersion in the matrix, allowing the stabilizing effect to be more fully exerted, further reducing the initial coloring degree and the yellowing rate during heat aging of the material.

[0053] In Comparative Example 2, the initial yellowness increased to 4.5 after the absence of N-allyl thiourea, and the yellowness change value after 60 minutes reached as high as 27.9, with the yellowing degree being more than three times that of Example 2. This is because after the zinc burning inhibition and conjugation blocking effect of the absence of thiourea groups, zinc chloride rapidly accumulated and catalyzed the dehydrochlorination reaction, resulting in a large amount of conjugated polyene sequences, leading to rapid yellowing and zinc burning of the material. The yellowing increase of Comparative Examples 1 and 3 was also significantly higher than that of the examples, corresponding to the decrease in stabilization efficiency caused by the lack of polar side groups and the high content of intrinsically unstable structures in general resins, respectively. The yellowing change value of the conventional lead-free system in Comparative Example 6 reached 34.4 after 60 minutes, with the most severe yellowing, fully demonstrating the common technical defects of traditional calcium-zinc systems: poor initial coloring and rapid zinc burning in the later stage. The lead salt system in Comparative Example 5 had the lowest degree of yellowing, but its heavy metal content limited its application in scenarios with high environmental protection requirements.

[0054] As shown in Table 3, the tensile strength of Examples 1-6 was 14.8-16.7 MPa, and the elongation at break was 256%-321%. The strength decreased while the toughness increased with increasing plasticizer dosage, achieving a balance between mechanical strength and flexibility. The overall mechanical properties were superior to conventional lead-free systems and comparable to traditional lead salt systems. This invention introduces polar side groups through ternary copolymerization, enhancing intermolecular interactions. During processing, the epoxy groups of epoxidized soybean oil undergo esterification with the carboxyl groups of itaconic acid monobutyl ester side chains, forming a small number of covalent crosslinking points. These, combined with uniformly distributed nanocrystals, constitute a composite reinforcement structure, effectively dispersing external loads and hindering crack propagation. Simultaneously, the plasticizer is uniformly dispersed in the amorphous region, weakening van der Waals forces between molecular chains and ensuring the material's flexibility, ultimately achieving a balance between strength and toughness.

[0055] In Comparative Example 2, the absence of N-allyl thiourea resulted in a simultaneous decrease in tensile strength and elongation at break. This is because the resin exhibits an increased number of intrinsically unstable structures, making the molecular chains prone to thermal degradation and breakage during mixing and hot pressing. This leads to a decrease in molecular weight and reduced uniformity of the condensed structure, ultimately resulting in a decline in mechanical properties. Comparative Example 3, using a general-purpose homopolymer resin, also exhibited lower mechanical properties than the examples, indicating that copolymerization modification and synergistic stabilization reduced molecular chain degradation during processing and preserved a more complete molecular chain structure. Comparative Example 6, a conventional lead-free system, showed the worst mechanical properties with a tensile strength of only 12.5 MPa and an elongation at break of only 198%. This was due to its insufficient thermal stability, resulting in severe thermal degradation and extensive molecular chain breakage during processing. Additionally, uneven dispersion of additives led to an increase in structural defects, ultimately causing a simultaneous decrease in both strength and toughness.

[0056] As shown in Table 4, the visible light transmittance of Examples 1-6 is 85.1%-89.2%, and the haze is 2.3%-3.6%, exhibiting excellent optical properties of high transmittance and low haze. Their transparency is far superior to traditional lead salt systems and conventional lead-free calcium-zinc systems. This invention disrupts the regularity of the polyvinyl chloride molecular chain through ternary copolymerization, reducing resin crystallinity and controlling the crystallite size to the nanoscale. Since the crystallite size is smaller than the visible light wavelength, it significantly reduces light scattering caused by crystalline regions. Simultaneously, copolymerization modification improves the thermodynamic compatibility of the matrix and additives, allowing plasticizers, stabilizers, and other components to be uniformly dispersed at the molecular or nanoscale, avoiding the formation of large-scale phase separation structures and reducing visible light scattering loss. Therefore, the resulting film material possesses excellent transparency.

[0057] Comparative Example 3 uses general-purpose homopolymer polyvinyl chloride resin, with light transmittance decreasing to 83.4% and haze increasing to 5.2%. This is because general-purpose resin has high crystallinity and large spherulite size, and the poor compatibility between additives and the matrix easily leads to phase separation, causing strong light scattering. Comparative Example 6 uses a conventional lead-free system with a haze of 7.8%, further reducing transparency. This is related to uneven dispersion of additives and increased structural defects caused by processing degradation. Comparative Example 5 uses a lead salt system with a light transmittance of only 78.6% and a haze as high as 12.4%, exhibiting the worst optical performance. This is because the lead salt stabilizer is an inorganic particle, making it difficult to achieve nanoscale uniform dispersion in the resin matrix. Large particles produce significant light scattering, severely damaging the transparency of the film material. This also demonstrates the significant advantages of the lead-free system of this invention in transparent film applications.

[0058] In summary, this invention modifies the main chain of polyvinyl chloride in situ through ternary copolymerization, and constructs a multi-level synergistic lead-free stabilizing system by combining calcium-zinc composite stabilizer and epoxidized soybean oil. Under the premise of completely eliminating lead salt components, it effectively improves the common defects of traditional calcium-zinc systems such as insufficient thermal stability, easy zinc burning and yellowing, and poor transparency, and achieves a balanced improvement in thermal stability, yellowing resistance, mechanical properties and optical properties.

[0059] In the description of this specification, the reference to terms such as "example," "various examples," etc., indicates that a specific feature, structure, material, or characteristic described in connection with that example or preparation is included in at least one example or preparation of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same example or preparation. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more examples or preparations.

[0060] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a lead-free, environmentally friendly polyvinyl chloride (PVC) membrane, characterized in that, Includes the following steps: S1. In a deionized water system, vinyl chloride, itaconic acid monobutyl ester and N-allyl thiourea are copolymerized with polyvinyl alcohol as a dispersant under the action of a composite initiator. After the reaction is completed, the matrix resin is obtained through post-treatment. S2. Weigh the matrix resin, dioctyl terephthalate, epoxidized soybean oil, calcium-zinc composite stabilizer, acrylate processing aid, and stearic acid, and perform hot mixing and cold mixing in sequence. Seal and place for curing to obtain the premix. S3. The premixed material is mixed on a two-roll mill, then placed in a mold, and hot-pressed for degassing, plasticizing and cross-linking and cold-pressed for shaping to obtain lead-free environmentally friendly polyvinyl chloride film.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of vinyl chloride, itaconic acid monobutyl ester, and N-allyl thiourea is 100:(2.5-3.5):(1.5-2.0). The composite initiator is composed of tert-butyl peroxypentanoate and diisopropyl peroxydicarbonate, with a mass ratio of (2.0-2.5):1, and the total mass of the composite initiator is 0.06-0.07% of the mass of vinyl chloride.

3. The preparation method according to claim 1, characterized in that, The temperature of the copolymerization reaction in step S1 is 50-60℃, the reaction pressure is 0.6-0.8MPa, and the reaction time is 9-11h. Post-treatment includes washing the polymer slurry until the filtrate conductivity is ≤20μS / cm, followed by vacuum drying at 58-62℃ to constant weight.

4. The preparation method according to claim 1, characterized in that, The specific process of hot mixing in step S2 is as follows: First, stir and heat to 78-82℃ at a speed of 1100-1300r / min, add dioctyl terephthalate and epoxidized soybean oil, increase the speed to 2300-2500r / min and stir to 90-92℃, add calcium-zinc composite stabilizer and acrylate processing aid, heat to 108-112℃ and add stearic acid, stir and discharge the material.

5. The preparation method according to claim 1, characterized in that, In step S2, the rotation speed of the cold mixing is 200-400 r / min, and the cooling temperature is 35-40℃.

6. The preparation method according to claim 1, characterized in that, The maturation time in step S2 is 23-25 ​​hours.

7. The preparation method according to claim 1, characterized in that, In step S3, the mixing temperature is 165-170℃, the mixing time is 7-9 minutes, and the roller gap is controlled at 0.8-1.2 mm.

8. The preparation method according to claim 1, characterized in that, The conditions for hot-press exhaust in step S3 are: temperature 168-172℃, pressure 0.4-0.6MPa, time 2.5-3.5min, and pressure increase and decrease cycles 2-4 times; The conditions for plasticizing and crosslinking are: pressure 9-11 MPa, time 4.5-5.5 min; The conditions for cold pressing and shaping are: pressure 9-11MPa, water cooling to 22-28℃ before demolding.

9. The preparation method according to claim 1, characterized in that, The thickness of the polyvinyl chloride film obtained in step S3 is 0.075-0.155 mm.

10. A lead-free, environmentally friendly polyvinyl chloride membrane, prepared by the method according to any one of claims 1-9, characterized in that, The product comprises the following components in the indicated mass ratios: 100 parts matrix resin, 30-35 parts dioctyl terephthalate, 1.5-2.5 parts epoxidized soybean oil, 1.2-2.0 parts calcium-zinc composite stabilizer, 0.8-1.2 parts acrylate processing aid, and 0.2-0.5 parts stearic acid.

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