Preparation method of low-temperature high-shrinkage PETG (polyethylene terephthalate glycol) heat shrinkage film

Abstract

The invention discloses a preparation method of a low-temperature high-shrinkage PETG (polyethylene terephthalate glycol) thermal shrinkage film, which belongs to the technical field of polyester film preparation, and comprises the following steps: firstly, preparing a prepolymer from 2, 3-diphenyl maleic anhydride and ethylene glycol monomers, then copolymerizing the prepolymer with terephthalic acid, ethylene glycol and neopentyl glycol to obtain a polymerization product, and finally preparing the low-temperature high-shrinkage PETG thermal shrinkage film. And finally, introducing an alkylbenzene branched chain into unsaturated double bonds through a mercapto-alkene click reaction. The PETG heat shrinkage film material has excellent low-temperature shrinkage performance, the shrinkage rate of 75% or above can be generated at the temperature of 70 DEG C, and meanwhile the shrinkage stress is smaller than 1 kgf.

Description

Technical Field

[0001] The present invention relates to the technical field of polyester film preparation, and particularly relates to a method for preparing a PETG heat-shrinkable film with low temperature and high shrinkage. Background Art

[0002] A heat-shrinkable film is a film that shrinks when heated and thus tightly adheres to the surface of the packaged item. It is usually made of plastic and has good flexibility and shrinkage properties. The heat-shrinkable film can play roles such as protecting the product, preventing dust, moisture, and scratches, and can improve the display effect and packaging aesthetics of the product. Its shrinkage principle is to make the polymer chain segments in the film move more vigorously through heating, thereby achieving shrinkage. In practical applications, according to different materials and properties, heat-shrinkable packaging films are widely used in the packaging of many fields such as food, beverages, daily chemical products, and electronic products, especially in the packaging of irregularly shaped items or combined (bundled) products.

[0003] Existing heat-shrinkable films mainly include PVC (polyvinyl chloride), PE (polyethylene), and PET (polyethylene terephthalate). PVC has good transparency and gloss, making the product appearance more attractive. Its heat-shrinkage performance is outstanding, with a relatively high shrinkage rate, and it can tightly adhere to the item, providing a good packaging effect; it has relatively good mechanical strength and can play a certain protective role for the product, but its environmental friendliness is relatively poor, and it may release harmful substances under high temperature or specific conditions; PE has excellent flexibility, strong tensile and tear resistance, is not easy to break, is suitable for packaging items with irregular shapes or sharp corners, has high chemical stability, is resistant to corrosion by chemical substances such as acids and alkalis, has a relatively wide shrinkage temperature range, is relatively easy to operate, and can maintain good flexibility after shrinkage and is not easy to become brittle, but its transparency is generally not as good as that of PVC; PET has relatively high mechanical strength and rigidity, can provide reliable protection for the product, has high transparency, good chemical resistance and heat resistance, can maintain good performance at different ambient temperatures, and is non-toxic, odorless, and easy to recycle, with good environmental friendliness. In some countries, it has become an ideal substitute for PVC heat-shrinkable films. However, since ordinary polyester is a crystalline high polymer, its high glass transition temperature makes the shrinkage temperature relatively high, and its shrinkage at low temperature is relatively poor. Existing technologies improve its low-temperature shrinkage through modified monomers, such as preparing it into PETG materials by introducing other monomers (such as neopentyl glycol, 1,4-cyclohexanedimethanol), but its low-temperature shrinkage still needs to be further improved, and its shrinkage stress is relatively large, and it is easy to shrink unevenly and cause wrinkling, affecting the product packaging effect. Summary of the Invention

[0004] In view of the above problems, the present invention provides a method for preparing a PETG heat-shrinkable film with low temperature and high shrinkage.

[0005] The object of the present invention is achieved by the following technical solutions: A low-temperature and high-shrinkage PETG heat-shrinkable film, and its preparation method comprises the following steps: (1) Preparation of prepolymer Weigh 2,3-diphenylmaleic anhydride and ethylene glycol respectively and mix them, add an esterification catalyst, add toluene as a water-carrying agent, stir and heat up to 120-140 °C under a protective atmosphere and keep stirring and reacting for 1-2 h. During the reaction, water is separated by a water separator, then heat up to 180-200 °C under reduced pressure and keep stirring and reacting for 2-6 h. After the reaction is completed, cool, add acetone to dilute the reaction system, and then add it to a cold methanol solvent for precipitation. Filter and dry under vacuum to obtain the prepolymer; The addition amounts of the esterification catalyst and the water-carrying agent depend on the addition amounts of the reactants. Calculated by the amount of 2,3-diphenylmaleic anhydride, the addition amount of the esterification catalyst is 0.01-0.1 g / g; the addition amount of the water-carrying agent is 0.5-2 mL / g; The vacuum degree of the reduced pressure condition is 10-50 mmHg; When the reaction is completed, the acid value of the reaction system should not be greater than 20 mgKOH / g; (2) Preparation of modified polymer Mix terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer and carry out a polymerization reaction to obtain a polymerization product. After cooling, add dichloromethane to dissolve it, add benzophenone under a protective atmosphere, stir and mix at normal temperature and ultraviolet light for 3-6 h. Keep the protective atmosphere and add 3-phenylpropanethiol, stir and mix, then add a radical initiator, heat up to 60-80 °C, and keep stirring and reacting under ultraviolet light for 8-18 h. After the reaction is completed, cool, and then add it to a cold methanol solvent for precipitation. Filter and dry under vacuum to obtain the modified polymer; (3) Film formation Carry out multi-layer co-extrusion on the modified polymer to obtain a cast sheet, and carry out longitudinal stretching, transverse stretching or blown film on the cast sheet, and then cool and shape to obtain the PETG heat-shrinkable film.

[0006] In some preferred embodiments, the molar ratio of 2,3-diphenylmaleic anhydride to ethylene glycol in step (1) is 1:(1-1.1).

[0007] In some preferred embodiments, the esterification catalyst in step (1) is p-toluenesulfonic acid or sulfuric acid.

[0008] In some preferred embodiments, the number-average molecular weight of the prepolymer in step (1) is 2000-5000 Da.

[0009] In some preferred embodiments, the polymerization reaction in step (2) comprises the following steps: Weigh terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer respectively and mix them. Heat to 240 - 270 °C under a protective atmosphere and keep stirring for 2 - 3 h. The reaction pressure is 330 - 350 kPa. After the reaction is completed, add a catalyst and a stabilizer, and heat to 270 - 280 °C under reduced pressure and keep stirring for 3 - 5 h to obtain a polymerization product; The vacuum degree of the reduced pressure condition is 10 - 50 mmHg.

[0010] In some preferred embodiments, the mass ratio of terephthalic acid to ethylene glycol, neopentyl glycol, and the prepolymer in the polymerization reaction is 1:(0.2 - 0.32):(0.31 - 0.42):(1 - 1.2).

[0011] In some preferred embodiments, the catalyst is tetrabutyl titanate or antimony glycolate; the addition amount of the catalyst satisfies that the metal element accounts for 10 - 200 ppm of the total mass of the reaction system.

[0012] In some preferred embodiments, the stabilizer is triphenyl phosphate or triphenyl phosphite; the addition amount of the stabilizer satisfies that phosphorus accounts for 10 - 100 ppm of the total mass of the reaction system.

[0013] In some preferred embodiments, the mass ratio of the polymerization product in step (2) to benzophenone, 3 - phenylpropanethiol, and the radical initiator is 10:(0.5 - 1):(1 - 3):(0.01 - 0.05).

[0014] In some preferred embodiments, the radical initiator in step (2) is azobisisobutyronitrile, azobisisoheptonitrile, benzoyl peroxide or tert - butyl benzoyl peroxide.

[0015] The beneficial effects of the present invention are: In view of the problems of poor low-temperature heat shrinkage performance and large shrinkage stress of PETG materials in the prior art, the present invention provides a PETG heat shrinkable film material with high shrinkage at low temperature and a preparation method thereof. The PETG heat shrinkable film material can produce a shrinkage rate of more than 75% at 70 °C, and at the same time, the shrinkage stress is less than 1 kgf, having excellent low-temperature shrinkage performance. Specifically, on the basis of the PETG shrinkable film material, the present invention introduces a polyester prepolymer including diphenyl maleic anhydride monomer, introduces a diphenylethylene group on the main chain of PETG, and introduces an alkylbenzene branch chain at the unsaturated double bond through a thiol-ene click reaction. Specifically, first, under ultraviolet light, the photoisomerizable diphenylethylene group on the main chain is isomerized into a cis structure, and then an alkylbenzene branch chain is introduced through a thiol-ene click reaction to increase its steric hindrance. On the one hand, the π-π stacking interaction between side chains increases the activation energy of the restricted motion of the main chain, and at the same time, it also enhances and maintains its asymmetry on the main chain, further reducing the crystallinity, thereby reducing the glass transition temperature, enhancing the relaxation ability of the chain, making its low-temperature heat shrinkage performance further improved, and at the same time reducing the shrinkage force. Detailed implementation mode

[0016] The present invention will be further described in conjunction with the following embodiments.

[0017] Example 1 A PETG heat shrinkable film with high shrinkage at low temperature, and its preparation method includes the following steps: (1) Preparation of prepolymer Weigh 2,3-diphenyl maleic anhydride (CAS No.: 4808-48-4) and ethylene glycol respectively and mix them, add p-toluenesulfonic acid as an esterification catalyst, add toluene as a water-carrying agent, stir and heat up to 130 °C under a protective atmosphere and keep stirring for 1 h. During the reaction, water is separated by a water separator, then heated up to 190 °C under reduced pressure and kept stirring for 4 h. After the reaction is completed, it is cooled, diluted with acetone, and then added to a cold methanol solvent for precipitation, filtered and dried under vacuum to obtain a prepolymer; The molar ratio of the 2,3-diphenyl maleic anhydride to the ethylene glycol is 1:1.05; the mass ratio of the p-toluenesulfonic acid to the 2,3-diphenyl maleic anhydride is 0.05 g / g; the mass ratio of the water-carrying agent to the 2,3-diphenyl maleic anhydride is 1 mL / g; The vacuum degree of the reduced pressure condition is 30 mmHg; When the reaction is completed, the acid value of the reaction system should be less than 20 mgKOH / g; the number average molecular weight of the prepolymer is 3500 Da; (2) Preparation of modified polymer Weigh terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer separately and mix them. Heat to 250 °C under a protective atmosphere and keep stirring for 3 h. The reaction pressure is 340 kPa. After the reaction is completed, add a catalyst and a stabilizer. Heat to 270 °C under reduced pressure and keep stirring for 4 h to obtain a polymerization product. After cooling, dissolve it in dichloromethane. Add benzophenone under a protective atmosphere and stir and mix for 4 h at normal temperature and ultraviolet light (365 nm). Keep the protective atmosphere and add 3-phenylpropanethiol. After stirring and mixing, add azobisisobutyronitrile. Heat to 70 °C and keep stirring for 14 h under ultraviolet light (365 nm). After the reaction is completed, cool it and then add it to a cold methanol solvent for precipitation. Filter and dry it under vacuum to obtain the modified polymer; The mass ratio of the terephthalic acid, the ethylene glycol, the neopentyl glycol and the prepolymer is 1:0.26:0.35:1.08; The catalyst is tetrabutyl titanate, and the addition amount of the catalyst satisfies that titanium accounts for 100 ppm of the total mass of the reaction system; The stabilizer is triphenyl phosphate, and the addition amount of the stabilizer satisfies that phosphorus accounts for 50 ppm of the total mass of the reaction system; The vacuum degree of the reduced pressure condition is 30 mmHg; The mass ratio of the polymerization product, the benzophenone, the 3-phenylpropanethiol and the azobisisobutyronitrile is 10:0.7:1.9:0.03; (3)Film formation Dry the modified polymer until the water content is not more than 100 ppm. Extrude it through a screw extruder. The temperature of the feeding section is 230 - 240 °C, the temperature of the compression section is 240 - 250 °C, and the temperature of the extrusion section is 250 - 260 °C; After extrusion, blow film or stretch film with a blown film machine, and obtain the PETG heat-shrinkable film after slitting.

[0018] Example 2 A low-temperature and high-shrinkage PETG heat-shrinkable film, and its preparation method includes the following steps: (1)Prepolymer preparation Weigh 2,3-diphenylmaleic anhydride and ethylene glycol separately and mix them. Add p-toluenesulfonic acid as an esterification catalyst and toluene as a water-carrying agent. Stir and heat to 120 °C under a protective atmosphere and keep stirring for 1 h. During the reaction, water is separated with a water separator. Then heat to 180 °C under reduced pressure and keep stirring for 2 h. After the reaction is completed, cool it, add acetone to dilute the reaction system, and then add it to a cold methanol solvent for precipitation. Filter and dry it under vacuum to obtain a prepolymer; The molar ratio of the 2,3-diphenylmaleic anhydride to the ethylene glycol is 1:1; the mass ratio of the p-toluenesulfonic acid to the 2,3-diphenylmaleic anhydride is 0.01 g / g; the mass ratio of the water-carrying agent to the 2,3-diphenylmaleic anhydride is 0.5 mL / g; The vacuum degree of the reduced-pressure condition is 50 mmHg; When the reaction is completed, the acid value of the reaction system should be less than 20 mgKOH / g; the number-average molecular weight of the prepolymer is 2500 Da; (2) Preparation of the modified polymer Weigh terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer respectively and mix them. Heat up to 240 °C under a protective atmosphere and keep stirring and reacting for 2 h. The reaction pressure is 330 kPa. After the reaction is completed, add a catalyst and a stabilizer. Heat up to 270 °C under a reduced-pressure condition and keep stirring and reacting for 3 h to obtain a polymerization product. After cooling, dissolve it in dichloromethane. Add benzophenone under a protective atmosphere and stir and mix for 3 h at normal temperature and under ultraviolet light (365 nm). Keep the protective atmosphere and add 3-phenylpropanethiol. After stirring and mixing, add azobisisobutyronitrile. Heat up to 60 °C and keep stirring and reacting for 8 h under ultraviolet light (365 nm). After the reaction is completed, cool it down, and then add it to a cold methanol solvent for precipitation. Filter and dry it under vacuum to obtain the modified polymer; The mass ratio of the terephthalic acid to the ethylene glycol, the neopentyl glycol, and the prepolymer is 1:0.2:0.31:1; The catalyst is tetrabutyl titanate, and the addition amount of the catalyst satisfies that titanium accounts for 10 ppm of the total mass of the reaction system; The stabilizer is triphenyl phosphate, and the addition amount of the stabilizer satisfies that phosphorus accounts for 10 ppm of the total mass of the reaction system; The vacuum degree of the reduced-pressure condition is 50 mmHg; The mass ratio of the polymerization product to the benzophenone, the 3-phenylpropanethiol, and the azobisisobutyronitrile is 10:0.5:1:0.01; (3) Film formation Same as Example 1.

[0019] Example 3 A low-temperature and high-shrinkage PETG heat-shrinkable film, and its preparation method includes the following steps: (1) Preparation of the prepolymer Weigh 2,3-diphenylmaleic anhydride and ethylene glycol separately and mix them. Add p-toluenesulfonic acid as an esterification catalyst and toluene as a water-carrying agent. Stir and heat up to 140 °C under a protective atmosphere and keep stirring for 2 h. During the reaction, water is separated by a water separator. Then, under reduced pressure, heat up to 200 °C and keep stirring for 6 h. After the reaction is completed, cool it, add acetone to dilute the reaction system, and then add it to a cold methanol solvent for precipitation. Filter and dry it under vacuum to obtain a prepolymer; The molar ratio of the 2,3-diphenylmaleic anhydride to the ethylene glycol is 1:1.1; the mass ratio of the p-toluenesulfonic acid to the 2,3-diphenylmaleic anhydride is 0.1 g / g; the mass ratio of the water-carrying agent to the 2,3-diphenylmaleic anhydride is 2 mL / g; The vacuum degree of the reduced pressure condition is 10 mmHg; When the reaction is completed, the acid value of the reaction system should be less than 20 mgKOH / g; the number-average molecular weight of the prepolymer is 4500 Da; (2) Preparation of modified polymer Weigh terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer separately and mix them. Heat up to 270 °C under a protective atmosphere and keep stirring for 3 h. The reaction pressure is 350 kPa. After the reaction is completed, add a catalyst and a stabilizer. Under reduced pressure, heat up to 280 °C and keep stirring for 5 h to obtain a polymerization product. After cooling, dissolve it in dichloromethane. Under a protective atmosphere, add benzophenone, stir and mix at room temperature and ultraviolet light (365 nm) for 6 h. Keep the protective atmosphere and add 3-phenylpropanethiol. After stirring and mixing, add azobisisobutyronitrile, heat up to 80 °C, and keep stirring under ultraviolet light (365 nm) for 18 h. After the reaction is completed, cool it, and then add it to a cold methanol solvent for precipitation. Filter and dry it under vacuum to obtain the modified polymer; The mass ratio of the terephthalic acid to the ethylene glycol, the neopentyl glycol, the prepolymer is 1:0.32:0.42:1.2; The catalyst is tetrabutyl titanate, and the addition amount of the catalyst satisfies that titanium accounts for 200 ppm of the total mass of the reaction system; The stabilizer is triphenyl phosphate, and the addition amount of the stabilizer satisfies that phosphorus accounts for 100 ppm of the total mass of the reaction system; The vacuum degree of the reduced pressure condition is 10 mmHg; The mass ratio of the polymerization product to the benzophenone, the 3-phenylpropanethiol, the azobisisobutyronitrile is 10:1:3:0.05; (3) Film formation Same as Example 1.

[0020] Comparative Example 1 A PETG heat-shrinkable film, which is the same as Example 1, except that it does not contain the prepolymer. Its preparation method includes the following steps: Weigh terephthalic acid, ethylene glycol and neopentyl glycol separately and mix them. Heat up to 250 °C under a protective atmosphere and keep stirring and reacting for 3 h. The reaction pressure is 340 kPa. After the reaction is completed, add a catalyst and a stabilizer, and heat up to 270 °C under reduced pressure and keep stirring and reacting for 4 h to obtain a polymerization product; The mass ratio of the terephthalic acid to the ethylene glycol and the neopentyl glycol is 1:0.26:0.35; The catalyst is tetrabutyl titanate, and the addition amount of the catalyst satisfies that titanium accounts for 100 ppm of the total mass of the reaction system; The stabilizer is triphenyl phosphate, and the addition amount of the stabilizer satisfies that phosphorus accounts for 50 ppm of the total mass of the reaction system; The vacuum degree of the reduced pressure condition is 30 mmHg; (3) Film forming The same as Example 1.

[0021] Comparative Example 2 A PETG heat-shrinkable film, which is the same as Example 1, except that it does not include ultraviolet light treatment. Its preparation method includes the following steps: (1) Preparation of prepolymer Weigh 2,3-diphenylmaleic anhydride and ethylene glycol separately and mix them. Add p-toluenesulfonic acid as an esterification catalyst and toluene as a water-carrying agent. Stir and heat up to 130 °C under a protective atmosphere and keep stirring and reacting for 1 h. During the reaction, water is separated by a water separator, and then heat up to 190 °C under reduced pressure and keep stirring and reacting for 4 h. After the reaction is completed, cool it, add acetone to dilute the reaction system, and then add it to a cold methanol solvent for precipitation. Filter and vacuum dry to obtain a prepolymer; The molar ratio of the 2,3-diphenylmaleic anhydride to the ethylene glycol is 1:1.05; the mass ratio of the p-toluenesulfonic acid to the 2,3-diphenylmaleic anhydride is 0.05 g / g; the mass ratio of the water-carrying agent to the 2,3-diphenylmaleic anhydride is 1 mL / g; The vacuum degree of the reduced pressure condition is 30 mmHg; When the reaction is completed, the acid value of the reaction system should be less than 20 mgKOH / g; the number average molecular weight of the prepolymer is 3500 Da; (2) Preparation of modified polymer Weigh terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer separately and mix them. Heat up to 250 °C under a protective atmosphere and keep stirring for 3 h at a reaction pressure of 340 kPa. After the reaction is completed, add a catalyst and a stabilizer, heat up to 270 °C under reduced pressure and keep stirring for 4 h to obtain a polymerization product. After cooling, dissolve it in dichloromethane, add benzophenone under a protective atmosphere, stir and mix at room temperature for 4 h, maintain the protective atmosphere and add 3-phenylpropanethiol, stir and mix, then add azobisisobutyronitrile, heat up to 70 °C and keep stirring for 14 h. After the reaction is completed, cool it, and then add it to a cold methanol solvent for precipitation, filter and dry it under vacuum to obtain the modified polymer; The mass ratio of the terephthalic acid, the ethylene glycol, the neopentyl glycol and the prepolymer is 1:0.26:0.35:1.08; The catalyst is tetrabutyl titanate, and the addition amount of the catalyst satisfies that titanium accounts for 100 ppm of the total mass of the reaction system; The stabilizer is triphenyl phosphate, and the addition amount of the stabilizer satisfies that phosphorus accounts for 50 ppm of the total mass of the reaction system; The vacuum degree of the reduced pressure condition is 30 mmHg; The mass ratio of the polymerization product, the benzophenone, the 3-phenylpropanethiol and the azobisisobutyronitrile is 10:0.7:1.9:0.03; (3)Film formation Same as Example 1.

[0022] Comparative Example 3 A PETG heat-shrinkable film is the same as in Example 1, except that the prepolymer monomer is maleic anhydride, and its preparation method includes the following steps: (1)Prepolymer preparation Weigh maleic anhydride and ethylene glycol separately and mix them. Add p-toluenesulfonic acid as an esterification catalyst and toluene as a water-carrying agent. Stir and heat up to 130 °C under a protective atmosphere and keep stirring for 1 h. During the reaction, water is separated by a water separator, then heat up to 190 °C under reduced pressure and keep stirring for 4 h. After the reaction is completed, cool it, add acetone to dilute the reaction system, and then add it to a cold methanol solvent for precipitation, filter and dry it under vacuum to obtain the prepolymer; The molar ratio of the maleic anhydride to the ethylene glycol is 1:1.05; the mass ratio of the p-toluenesulfonic acid to the maleic anhydride is 0.05 g / g; the mass ratio of the water-carrying agent to the maleic anhydride is 1 mL / g; The vacuum degree of the reduced pressure condition is 30 mmHg; When the reaction is completed, the acid value of the reaction system should be less than 20 mgKOH / g; the number-average molecular weight of the prepolymer is 3800 Da; (2) Preparation of Modified Polymer Weigh terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer respectively and mix them. Heat up to 250 °C under a protective atmosphere and keep stirring for 3 h. The reaction pressure is 340 kPa. After the reaction is completed, add a catalyst and a stabilizer, heat up to 270 °C under reduced pressure and keep stirring for 4 h to obtain a polymerization product. After cooling, dissolve it in dichloromethane, add benzophenone under a protective atmosphere, stir and mix for 4 h at room temperature and under ultraviolet light (365 nm). Keep the protective atmosphere and add 3-phenylpropanethiol, stir and mix, then add azobisisobutyronitrile, heat up to 70 °C, and keep stirring and reacting for 14 h under ultraviolet light (365 nm). After the reaction is completed, cool it, then add it to a cold methanol solvent for precipitation, filter and dry it under vacuum to obtain the modified polymer; The mass ratio of the terephthalic acid to the ethylene glycol, the neopentyl glycol, and the prepolymer is 1:0.26:0.35:1.08; The catalyst is tetrabutyl titanate, and the addition amount of the catalyst satisfies that titanium accounts for 100 ppm of the total mass of the reaction system; The stabilizer is triphenyl phosphate, and the addition amount of the stabilizer satisfies that phosphorus accounts for 50 ppm of the total mass of the reaction system; The vacuum degree of the reduced pressure condition is 30 mmHg; The mass ratio of the polymerization product to the benzophenone, the 3-phenylpropanethiol, and the azobisisobutyronitrile is 10:0.7:1.9:0.03; (3) Film Formation Same as Example 1.

[0023] Comparative Example 4 A PETG heat-shrinkable film is the same as Example 1, except that the 3-phenylpropanethiol modification process is not included. Its preparation method includes the following steps: (1) Preparation of Prepolymer Weigh 2,3-diphenylmaleic anhydride and ethylene glycol respectively and mix them. Add p-toluenesulfonic acid as an esterification catalyst and toluene as a water-carrying agent. Stir and heat up to 130 °C under a protective atmosphere and keep stirring for 1 h. During the reaction, water is separated by a water separator, then heat up to 190 °C under reduced pressure and keep stirring for 4 h. After the reaction is completed, cool it, add acetone to dilute the reaction system, then add it to a cold methanol solvent for precipitation, filter and dry it under vacuum to obtain a prepolymer; The molar ratio of the 2,3-diphenylmaleic anhydride to the ethylene glycol is 1:1.05; the mass ratio of the p-toluenesulfonic acid to the 2,3-diphenylmaleic anhydride is 0.05 g / g; the mass ratio of the water-carrying agent to the 2,3-diphenylmaleic anhydride is 1 mL / g; The vacuum degree of the reduced pressure condition is 30 mmHg; When the reaction is completed, the acid value of the reaction system should be less than 20 mg KOH / g; the number average molecular weight of the prepolymer is 3500 Da; (2)Preparation of modified polymer Weigh terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer respectively and mix them. Heat up to 250 °C under a protective atmosphere and keep stirring for 3 h. The reaction pressure is 340 kPa. After the reaction is completed, add a catalyst and a stabilizer. Heat up to 270 °C under reduced pressure conditions and keep stirring for 4 h to obtain a polymerization product. After cooling, dissolve it in dichloromethane. Add benzophenone under a protective atmosphere and stir and mix for 4 h at normal temperature and ultraviolet light (365 nm). After the reaction is completed, cool it down, then add it to a cold methanol solvent for precipitation, filter and dry it under vacuum to obtain the modified polymer; The mass ratio of the terephthalic acid to the ethylene glycol, the neopentyl glycol, and the prepolymer is 1:0.26:0.35:1.08; The catalyst is tetrabutyl titanate, and the addition amount of the catalyst satisfies that titanium accounts for 100 ppm of the total mass of the reaction system; The stabilizer is triphenyl phosphate, and the addition amount of the stabilizer satisfies that phosphorus accounts for 50 ppm of the total mass of the reaction system; The vacuum degree of the reduced pressure condition is 30 mmHg; The mass ratio of the polymerization product to the benzophenone is 10:0.7; (3)Film formation Same as Example 1.

[0024] Experimental example Detect the property parameters of the modified polymers or heat shrinkable films prepared in Examples 1-3 and Comparative Examples 1-4. Among them, the glass transition temperature Tg is measured by a differential scanning calorimeter; the measurement method of the heat shrinkage rate is to cut the shrink film sample into 100 mm × 100 mm (±0.1 mm), soak it in warm water at a certain temperature for 10 s under no-load conditions, take it out and immerse it in normal temperature water at 25 °C, and measure the longitudinal or transverse length of the sample after cooling. The heat shrinkage rate value is calculated by the ratio of the length difference before and after shrinkage to the length before shrinkage, and the average value is taken after measuring three times in parallel; the measurement method of the shrinkage stress is to cut the film into a rectangle with a size of 250 mm × 15 mm, fix the ends of the film in the transverse direction with clips, and then soak the film in warm water at 75 °C to measure the shrinkage force of the film in the transverse direction caused by the warm water.

[0025] Table 1 Property parameters of the modified polymers or heat shrinkable films prepared in Examples 1-3 and Comparative Examples 1-4

[0026] According to the above property parameter detection results, it can be seen that the PETG heat-shrinkable film described in the present invention has high low-temperature shrinkage performance and low shrinkage stress. Among them, according to the comparison results between Example 1 and Comparative Example 1, it can be seen that introducing a prepolymer containing 2,3-diphenylmaleic anhydride monomer, as well as ultraviolet treatment and grafting reaction, can significantly reduce the glass transition temperature and shrinkage stress of the PETG material; according to the comparison results between Example 1 and Comparative Example 2, it can be seen that the cis-trans structure induced by ultraviolet light has an important influence on the crystallization performance of the material, and the cis structure can reduce its crystallinity; according to the comparison results between Example 1 and Comparative Examples 3 and 4, it can be seen that the introduction of monomers containing divinylbenzene and the structure of branched alkylbenzene can significantly reduce the crystallinity of the PETG material, which is reflected in the reduction of the glass transition temperature.

[0027] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a low-temperature high-shrinkage PETG heat shrinkable film, characterized in that: The following steps are involved: (1) Preparation of prepolymer Weigh 2,3-diphenylmaleic anhydride and ethylene glycol respectively and mix them, add an esterification catalyst, add toluene as a water-carrying agent, stir and heat to 120-140° C. under a protective atmosphere, keep stirring and react for 1-2 hours, separate water with a water separator during the reaction, then heat to 180-200° C. under reduced pressure and keep stirring and react for 2-6 hours, cool after the reaction is completed, add acetone to dilute the reaction system, then add it to a cold methanol solvent for precipitation, filter and vacuum dry to obtain a prepolymer; (2) Preparation of modified polymers The terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer are mixed and subjected to polymerization reaction to obtain a polymer product, and after cooling, dichloromethane is added to dissolve, benzophenone is added under a protective atmosphere, and the mixture is stirred and mixed at room temperature and under ultraviolet light for 3-6 hours, the protective atmosphere is maintained and 3-phenylpropanethiol is added, and after stirring and mixing, a free radical initiator is added, and the mixture is heated to 60-80° C., and the mixture is stirred and reacted under ultraviolet light for 8-18 hours. After the reaction is completed, the mixture is cooled, and then the mixture is added to a cold methanol solvent for precipitation, filtered and vacuum dried to obtain the modified polymer; (3) Film formation The modified polymer is subjected to multi-layer co-extrusion to obtain a cast sheet, the cast sheet is subjected to longitudinal stretching, transverse stretching or film blowing, and the PETG heat shrinkable film is obtained after cooling and shaping.

2. The method for preparing a low-temperature, high-shrinkage PETG heat shrinkable film according to claim 1, characterized in that: In step (1), the molar ratio of the 2,3-diphenylmaleic anhydride to the ethylene glycol is 1:(1-1.1).

3. The method for preparing a low-temperature high-shrinkage PETG heat shrinkable film according to claim 1, characterized in that: The esterification catalyst in step (1) is p-toluenesulfonic acid or sulfuric acid.

4. The method for preparing a low-temperature high-shrinkage PETG heat shrinkable film according to claim 1, characterized in that: The number average molecular weight of the prepolymer in step (1) is 2000-5000Da.

5. The method for preparing a low-temperature high-shrinkage PETG heat shrinkable film according to claim 1, characterized in that: The polymerization reaction in step (2) comprises the following steps: Weigh terephthalic acid, ethylene glycol, neopentyl glycol and the prepolymer separately and mix them, heat them to 240-270° C. under a protective atmosphere and stir them for 2-3 hours. The reaction pressure is 330-350 kPa. After the reaction is completed, add a catalyst and a stabilizer, heat them to 270-280° C. under reduced pressure and stir them for 3-5 hours to obtain a polymerization product.

6. The method for preparing a low-temperature high-shrinkage PETG heat shrinkable film according to claim 5, characterized in that: The mass ratio of the terephthalic acid to the ethylene glycol, the neopentyl glycol and the prepolymer in the polymerization reaction is 1:(0.2-0.32):(0.31-0.42):(1-1.2).

7. The method for preparing a low-temperature high-shrinkage PETG heat shrinkable film according to claim 5, characterized in that: The catalyst is tetrabutyl titanate or antimony ethylene glycol.

8. The method for preparing a low-temperature high-shrinkage PETG heat shrinkable film according to claim 5, characterized in that: The stabilizer is triphenyl phosphate or triphenyl phosphite.

9. The method for preparing a low-temperature high-shrinkage PETG heat shrinkable film according to claim 1, characterized in that: The mass ratio of the polymerization product in step (2) to the benzophenone, the 3-phenylpropanethiol, and the free radical initiator is 10:(0.5-1):(1-3):(0.01-0.05).

10. The method for preparing a low-temperature high-shrinkage PETG heat shrinkable film according to claim 1, characterized in that: The free radical initiator in step (2) is azobisisobutyronitrile, azobisisoheptanenitrile, benzoyl peroxide or tert-butyl benzoyl peroxide.

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