阅读说明：本技术 热收缩聚酯膜及其制法 (Heat-shrinkable polyester film and process for producing the same ) 是由 吕佑安 张莉苓 徐俊嘉 于 2019-09-12 设计创作，主要内容包括：一种热收缩聚酯膜,是由一种供形成热收缩聚酯膜的组成物所制得,且以差示扫描量热仪分析可获得一个第一结晶熔融峰及一个第二结晶熔融峰。第一结晶熔融峰位于Tm1温度,以及第二结晶熔融峰位于Tm2温度且具有大于0且不大于7J/g的熔融热焓Hm2,Tm1温度低于Tm2温度。该供形成热收缩聚酯膜的组成物包含具有40～80℃的玻璃转移温度的第一聚酯树脂及具有220～250℃的结晶熔融温度及40～60J/g的完全结晶熔融热焓的第二聚酯树脂。本发明热收缩聚酯膜具备满足后续应用的热收缩率及断裂延伸率,并于久置库存或高温运输下仍具备低温高收缩特性。(A heat-shrinkable polyester film is prepared from a composition for forming a heat-shrinkable polyester film, and a first crystal melting peak and a second crystal melting peak are obtained by differential scanning calorimeter analysis. The first crystalline melting peak is at Tm1 temperature and the second crystalline melting peak is at Tm2 temperature and has a melting enthalpy of greater than 0 and no greater than 7J/g Hm2, Tm1 temperature being lower than Tm2 temperature. The composition for forming a heat-shrinkable polyester film includes a first polyester resin having a glass transition temperature of 40 to 80 ℃ and a second polyester resin having a crystal melting temperature of 220 to 250 ℃ and a complete crystal melting enthalpy of 40 to 60J/g. The heat-shrinkable polyester film disclosed by the invention has the heat shrinkage rate and the elongation at break which meet the requirements of subsequent application, and still has the low-temperature high-shrinkage characteristic under long-time storage or high-temperature transportation.)

1. A heat-shrinkable polyester film, which is prepared from a composition for forming a heat-shrinkable polyester film and is characterized in that a first crystal melting peak and a second crystal melting peak are obtained by differential scanning calorimeter analysis;

the first crystalline melting peak is at Tm1 temperature;

the second crystalline melting peak is at the Tm2 temperature and has a melting enthalpy, Hm2, greater than 0 and not greater than 7J/g;

the Tm1 temperature is less than the Tm2 temperature; and

the composition for forming a thermo-shrinkable polyester film comprises:

a first polyester resin having a glass transition temperature of 40 to 80 ℃; and

a second polyester resin having a crystal melting temperature of 220 to 250 ℃ and a complete crystal melting enthalpy of 40 to 60J/g.

2. The heat-shrinkable polyester film according to claim 1, wherein the second polyester resin is contained in an amount ranging from more than 2 wt% to less than 15 wt%, based on the total weight of the composition for forming the heat-shrinkable polyester film being 100 wt%.

3. The heat-shrinkable polyester film of claim 1, wherein the first polyester resin is polymerized from a mixture comprising terephthalic acid and ethylene glycol.

4. The heat-shrinkable polyester film of claim 3, wherein the mixture further comprises a modifier, and the modifier is selected from isophthalic acid, neopentyl glycol, 1, 4-cyclohexanedimethanol, or combinations thereof.

5. The heat-shrinkable polyester film according to claim 4, wherein the modifier is neopentyl glycol.

6. The heat-shrinkable polyester film according to claim 5, wherein the amount of the ethylene glycol is in the range of 70 to 85 mol% and the amount of the neopentyl glycol is in the range of 15 to 30 mol%, based on 100 mol% of the total amount of the ethylene glycol and the neopentyl glycol.

7. The heat-shrinkable polyester film of claim 1, wherein the second polyester resin is polybutylene terephthalate.

8. The heat-shrinkable polyester film according to claim 7, wherein the Tm2 temperature ranges from 220 ℃ to 230 ℃.

9. The heat-shrinkable polyester film according to claim 8, which has the following properties:

(1) a heat shrinkage rate in the direction perpendicular to the machine direction of 76% or more after being left in hot water at 95 ℃ for 10 seconds as measured according to JIS Z1709 standard method (1995 of release year);

(2) the absolute value of the difference between the heat shrinkage in the cross-machine direction before accelerated aging and the heat shrinkage in the cross-machine direction after accelerated aging is 11% or less, as measured after the heat shrinkage in the cross-machine direction before accelerated aging is left in hot water at 70 ℃ for 10 seconds, and the heat shrinkage in the cross-machine direction after accelerated aging is left in hot water at 60 ℃ for 1 hour and 70 ℃ for 10 seconds, as measured according to JIS Z1709 standard method (1995); and

(3) the elongation at break in the machine direction after accelerated ageing, tested according to the ASTM D882 standard method (release year 2002), was > 400%.

10. A method for producing a heat-shrinkable polyester film, comprising the steps of:

(a) preparing a composition for forming a heat-shrinkable polyester film, the composition comprising a first polyester resin having a glass transition temperature of 40-80 ℃ and a second polyester resin having a crystal melting temperature of 220-250 ℃ and a complete crystal melting enthalpy of 40-60J/g; and

(b) blending the composition at a heating temperature and extruding to obtain a plate, wherein the heating temperature does not cause the first polyester resin and the second polyester resin to perform ester exchange reaction; and

(c) the sheet is subjected to a film-forming step to obtain a heat-shrinkable polyester film.

11. A method of manufacturing a heat-shrinkable polyester film according to claim 10, wherein the second polyester resin is contained in an amount ranging from more than 2 wt% to less than 15 wt% based on 100 wt% of the total weight of the composition in the step (a).

12. The method for preparing a heat-shrinkable polyester film according to claim 10, wherein the first polyester resin is polymerized from a mixture comprising terephthalic acid and ethylene glycol.

13. A method for preparing a heat shrinkable polyester film as claimed in claim 12, wherein the mixture further comprises a modifier selected from isophthalic acid, neopentyl glycol, 1, 4-cyclohexanedimethanol, or combinations thereof.

14. The method for manufacturing a heat-shrinkable polyester film according to claim 13, wherein the modifier is neopentyl glycol.

15. The method of manufacturing a heat-shrinkable polyester film according to claim 14, wherein the total amount of the ethylene glycol and the neopentyl glycol is 100 mol%, the amount of the ethylene glycol is 70 to 85 mol%, and the amount of the neopentyl glycol is 15 to 30 mol%.

16. A method of manufacturing a heat shrinkable polyester film according to claim 10, wherein the second polyester resin is polybutylene terephthalate.

17. A method of manufacturing a heat shrinkable polyester film according to claim 10, wherein the second polyester resin is polybutylene terephthalate, and the heating temperature in the step (b) is in the range of 210 to 260 ℃.

18. The method for preparing a heat-shrinkable polyester film according to claim 10, wherein the heat-shrinkable polyester film has a first crystal melting peak at Tm1 and a second crystal melting peak at Tm2 and having a melting enthalpy Hm2 of greater than 0 and not greater than 7J/g, when analyzed by differential scanning calorimetry, the Tm1 being lower than the Tm 2.

19. A method of manufacturing a heat-shrinkable polyester film according to claim 18, wherein the Tm2 temperature ranges from 220 ℃ to 230 ℃.

20. A heat-shrinkable polyester film produced by the process for producing a heat-shrinkable polyester film according to any one of claims 10 to 19.

21. The heat-shrinkable polyester film according to claim 20, which has the following properties:

(1) a heat shrinkage rate in the direction perpendicular to the machine direction of 76% or more after being left in hot water at 95 ℃ for 10 seconds as measured according to JIS Z1709 standard method (1995 of release year);

(2) the absolute value of the difference between the heat shrinkage in the cross-machine direction before accelerated aging and the heat shrinkage in the cross-machine direction after accelerated aging is 11% or less, as measured after the heat shrinkage in the cross-machine direction before accelerated aging is left in hot water at 70 ℃ for 10 seconds, and the heat shrinkage in the cross-machine direction after accelerated aging is left in hot water at 60 ℃ for 1 hour and 70 ℃ for 10 seconds, as measured according to JIS Z1709 standard method (1995); and

(3) the elongation at break in the machine direction after accelerated ageing, tested according to the ASTM D882 standard method (release year 2002), was > 400%.

Technical Field

The present invention relates to a heat-shrinkable polyester film and a method for manufacturing the same, and more particularly, to a heat-shrinkable polyester film for packaging and a method for manufacturing the same.

Background

A heat-shrinkable film for packaging or labeling is used by shrinking a heat-shrinkable film wrapped around a container by using a steam oven or a hot-air furnace as a heat source. Therefore, the heat shrinkable film must have a shrink characteristic sufficient for the subsequent packaging application. In a general perfect shrink process equipment, the residence time of the heat shrinkable film in a steam furnace or a hot air furnace for providing a heat source is about 5 seconds or more or about 10 seconds, and the heat shrinkable film starts to shrink when the process temperature approaches the glass transition temperature (hereinafter referred to as Tg) of the heat shrinkable film at the sustained elevated temperature of the heat source; when the process temperature reaches the Tg temperature, the heat shrinkable film shrinks significantly.

According to the above characteristics of the heat shrinkable film, the following properties are measured for evaluating whether the heat shrinkable film meets the requirements of subsequent packaging: (1) the heat shrinkable film was placed at a temperature higher than its Tg, and the shrinkage of the heat shrinkable film was measured after 10 seconds to evaluate the shrinkage of the heat shrinkable film after complete shrinkage. This shrinkage must be high enough (e.g., greater than 75%) to be suitable for packaging high curvature bottles; (2) the film was placed at a temperature for measuring its Tg, and the shrinkage of the film was measured after 10 seconds to evaluate the shrinkage rate of the film. The shrinkage rate must be within a proper range (e.g., not higher than 90% of the maximum shrinkage rate) to effectively control the shrinkage rate of the heat shrinkable film, and when the shrinkage rate is too high, which indicates that the shrinkage rate is too rapid, the shrinkage rate needs to be reduced by adjusting the speed of the heat shrinkable film passing through the hot-air furnace, the length and temperature of the hot-air furnace, etc.; (3) the heat shrinkable film is placed at a temperature lower than the Tg of the heat shrinkable film, the shrinkage of the heat shrinkable film is measured after 10 seconds, the shrinkage value approaches to 0% along with the reduction of the measurement temperature, the lowest temperature at which the shrinkage is not 0% is called initial shrinkage temperature, the shrinkage value of the heat shrinkable film is reduced and the initial shrinkage temperature is increased after the heat shrinkable film is generally stored for a long time or transported for a long time, and the process temperature of a shrinkage furnace can be effectively regulated and controlled by measuring and comparing the difference of the shrinkage before and after the measurement. Through the measurement, the temperature of the hot blast stove can be effectively regulated and controlled.

In addition to the adjustment of the process by shrinkage measurement, the shrinkage property of the heat shrinkable film is improved by changing the material forming the heat shrinkable film, for example, US 6548595B2 patent publication proposes to use a polyester resin containing a diol having a carbon number ranging from 3 to 6 to prepare a heat shrinkable polyester resin film having a Tg ranging from 60 to 75 ℃, and examples of the patent publication use polyethylene terephthalate (hereinafter, PET) ester pellets, neopentyl glycol (NPG) -modified PET ester pellets, and polybutylene terephthalate (PBT) to be mixed and stretched to prepare a heat shrinkable polyester resin film. However, the shrinkage rate of the heat-shrinkable polyester resin film in the vertical direction (TD) after being left in hot water at 70 ℃ for 5 seconds is in the range of 5% to 60%, and the shrinkage rate in the TD after being left in hot water at 85 ℃ for 5 seconds is 67% or more, which has not yet been satisfactory for the production of high curvature bottles (for example, the shrinkage rate in the TD after being left in hot water at 95 ℃ for 10 seconds is 75% or more), and the patent publication does not notice that the initial shrinkage temperature of the heat-shrinkable film may deteriorate the shrinkage properties due to long-term storage (for example, the shrinkage change exceeds 11%). It is noted that the PET ester pellets of this patent publication and NPG modified PET ester pellets and PBT undergo transesterification reaction, which is estimated to be the cause of failure of the heat shrinkable film to meet the property requirements for making high curvature bottles and the possibility of deterioration of the shrinkage properties after long-term storage.

Therefore, the existing heat shrinkable film still needs to be further improved to meet the property requirements of high curvature bottles and other applications.

Disclosure of Invention

The first object of the present invention is to provide a heat-shrinkable polyester film which has a shrinkage rate and a breaking elongation rate satisfying the following applications even in long-term storage or high-temperature transportation.

The invention relates to a heat shrinkable polyester film, which is prepared from a composition for forming the heat shrinkable polyester film, and a first crystal melting peak and a second crystal melting peak can be obtained by analyzing with a differential scanning calorimeter; wherein the first crystalline melting peak is at Tm1 temperature; the second crystalline melting peak is at the Tm2 temperature and has a melting enthalpy, Hm2, greater than 0 and not greater than 7J/g; the Tm1 temperature is less than the Tm2 temperature; and the composition for forming the thermo-shrinkable polyester film comprises: a first polyester resin having a glass transition temperature of 40 to 80 ℃; and a second polyester resin having a crystal melting temperature of 220 to 250 ℃ and a complete crystal melting enthalpy of 40 to 60J/g.

A second object of the present invention is to provide a method for manufacturing a heat-shrinkable polyester film.

The method for preparing the heat shrinkable polyester film of the present invention comprises the steps of:

(a) preparing a composition for forming a heat-shrinkable polyester film, the composition comprising a first polyester resin having a glass transition temperature of 40-80 ℃ and a second polyester resin having a crystal melting temperature of 220-250 ℃ and a complete crystal melting enthalpy of 40-60J/g; and

(b) blending the composition at a heating temperature which does not cause the first polyester resin and the second polyester resin to perform a Transesterification (Transesterification) reaction, and extruding to obtain a plate; and

(c) the sheet is subjected to a film-forming step to obtain a heat-shrinkable polyester film.

A third object of the present invention is to provide a heat-shrinkable polyester film.

The heat-shrinkable polyester film of the present invention is produced by the above-mentioned production method for a heat-shrinkable polyester film.

The heat-shrinkable polyester film of the present invention contains the second polyester resin in an amount ranging from more than 2 wt% to less than 15 wt%, based on the total weight of the composition for forming the heat-shrinkable polyester film being 100 wt%.

The invention relates to a heat shrinkable polyester film, wherein the first polyester resin is polymerized from a mixture comprising terephthalic acid and ethylene glycol.

The heat-shrinkable polyester film of the present invention further comprises a modifier selected from isophthalic acid, neopentyl glycol, 1, 4-cyclohexanedimethanol or a combination thereof.

The modifier is neopentyl glycol.

The heat-shrinkable polyester film of the present invention has a total amount of the ethylene glycol and the neopentyl glycol of 100 mol%, an amount of the ethylene glycol of 70 to 85 mol%, and an amount of the neopentyl glycol of 15 to 30 mol%.

The second polyester resin of the heat shrinkable polyester film of the present invention is polybutylene terephthalate.

The Tm2 temperature range of the heat-shrinkable polyester film of the present invention is 220 to 230 ℃.

The heat shrinkable polyester film of the present invention has the following properties:

(1) according to the test of JIS Z1709 standard method, after the glass is placed in hot water at 95 ℃ for 10 seconds, the thermal shrinkage rate in the direction vertical to the machine direction is more than or equal to 76 percent;

(2) the absolute value of the difference between the heat shrinkage in the machine direction before accelerated aging and the heat shrinkage in the machine direction after accelerated aging is 11% or less, as measured by the JIS Z1709 standard method, the heat shrinkage in the machine direction before accelerated aging is measured after leaving in hot water at 70 ℃ for 10 seconds, and the heat shrinkage in the TD after accelerated aging is measured after baking at 60 ℃ for 1 hour and leaving in hot water at 70 ℃ for 10 seconds; and

(3) the elongation at break in the machine direction after accelerated ageing, tested according to ASTM D882 standard method, was > 400%.

As for the "JIS Z1709 standard methods" used in the present invention, JIS Z1709 standard methods published in 1995 were used, and as for the "ASTM D882 standard methods" used in the present invention, ASTM D882 standard methods published in 2002 were used.

The method for manufacturing the heat-shrinkable polyester film of the present invention comprises the step (a) of containing the second polyester resin in an amount ranging from more than 2 wt% to less than 15 wt%, based on 100 wt% of the total weight of the composition.

The invention relates to a method for preparing a heat shrinkable polyester film, wherein a first polyester resin is polymerized from a mixture comprising terephthalic acid and ethylene glycol.

The mixture also comprises a modifier, and the modifier is selected from isophthalic acid, neopentyl glycol, 1, 4-cyclohexanedimethanol or the combination of the isophthalic acid, the neopentyl glycol and the 1, 4-cyclohexanedimethanol.

The modifier is neopentyl glycol.

The method for preparing the heat shrinkable polyester film comprises the following steps of using 100 mol% of the total amount of the ethylene glycol and the neopentyl glycol, using 70-85 mol% of the ethylene glycol, and using 15-30 mol% of the neopentyl glycol.

The second polyester resin is polybutylene terephthalate.

The second polyester resin is polybutylene terephthalate, and the heating temperature range of the step (b) is 210-260 ℃.

The present invention provides a method for producing a heat-shrinkable polyester film, which can obtain a first crystal melting peak and a second crystal melting peak when analyzed by a differential scanning calorimeter, the first crystal melting peak being at a Tm1 temperature, the second crystal melting peak being at a Tm2 temperature and having a melting enthalpy Hm2 of more than 0 and not more than 7J/g, the Tm1 temperature being lower than the Tm2 temperature.

The Tm2 temperature range of the method for preparing the heat shrinkable polyester film of the present invention is 220 ℃ to 230 ℃.

The heat shrinkable polyester film of the present invention has the following properties:

(1) according to the test of JIS Z1709 standard method, after the glass is placed in hot water at 95 ℃ for 10 seconds, the thermal shrinkage rate in the direction vertical to the machine direction is more than or equal to 76 percent;

(2) the absolute value of the difference between the heat shrinkage in the machine direction before accelerated aging and the heat shrinkage in the machine direction after accelerated aging is 11% or less, as measured by the JIS Z1709 standard method, the heat shrinkage in the machine direction before accelerated aging is measured after leaving in hot water at 70 ℃ for 10 seconds, and the heat shrinkage in the TD after accelerated aging is measured after baking at 60 ℃ for 1 hour and leaving in hot water at 70 ℃ for 10 seconds; and

(3) the elongation at break in the machine direction after accelerated ageing, tested according to ASTM D882 standard method, was > 400%.

The invention has the beneficial effects that: the heat-shrinkable polyester film has the special crystal melting peak property and is formed by a specific composition and/or a preparation method, so that the heat-shrinkable polyester film can meet the application of subsequent high-curvature bottles, and can effectively solve the problems of initial shrinkage temperature change and large shrinkage variation caused by long-time storage or long-time transportation.

Drawings

Other features and effects of the present invention will become apparent from the following detailed description of the embodiments with reference to the accompanying drawings, in which:

FIG. 1 is a differential scanning calorimetry graph illustrating the determination positions of transesterification in the examples and comparative examples of the present invention, wherein curve a is the result of transesterification of two kinds of ester particles and curve b is the result of non-transesterification of two kinds of ester particles; and

FIG. 2 is a differential scanning calorimetry graph illustrating the measurement positions of Tm1 and Tm2 according to an embodiment of the present invention.

Detailed Description

[ Heat-shrinkable polyester film ]

The heat-shrinkable polyester film of the present invention is prepared from a composition for forming a heat-shrinkable polyester film, and a first crystal melting peak and a second crystal melting peak can be obtained by differential scanning calorimeter analysis; wherein the first crystalline melting peak is at Tm1 temperature; the second crystalline melting peak is at the Tm2 temperature and has a melting enthalpy, Hm2, greater than 0 and not greater than 7J/g; the Tm1 temperature is less than the Tm2 temperature; and the composition for forming the thermo-shrinkable polyester film comprises: a first polyester resin having a glass transition temperature of 40 to 80 ℃; and a second polyester resin having a crystal melting temperature of 220 to 250 ℃ and a complete crystal melting enthalpy of 40 to 60J/g.

Preferably, the analysis condition of the differential scanning calorimeter is that the temperature is increased from-50 ℃ to 300 ℃ at a temperature increasing rate of 10 ℃ per minute.

Preferably, the Tm2 temperature ranges from 220 ℃ to 250 ℃; more preferably, the Tm2 temperature range is 220 ℃ to 230 ℃. In an embodiment of the invention, the Tm2 temperature range is 220 ℃ to 225 ℃.

Preferably, Hm2 is in the range of 1-5J/g.

Preferably, the heat-shrinkable polyester film further has the following properties:

(1) according to the test of JIS Z1709 standard method, after the film is placed in hot water at 95 ℃ for 10 seconds, the thermal shrinkage rate in TD is more than or equal to 76%;

(2) an absolute value of a difference [ that is, "-a heat shrinkage rate in TD after accelerated aging" (a heat shrinkage rate in TD before accelerated aging ". times.100%") between a heat shrinkage rate in TD before accelerated aging measured after leaving hot water at 70 ℃ for 10 seconds and a heat shrinkage rate in TD after accelerated aging measured after baking at 60 ℃ for 1 hour and leaving hot water at 70 ℃ for 10 seconds ] is measured in accordance with JIS Z1709; and

(3) the elongation at break in the machine direction after accelerated ageing, tested according to ASTM D882 standard method, was > 400%.

[ composition for Forming a Heat-shrinkable polyester film ]

The composition for forming the heat-shrinkable polyester film comprises a first polyester resin and a second polyester resin, wherein the content of the first polyester resin and the second polyester resin can be adjusted and changed according to the required differential scanning analysis property of the heat-shrinkable polyester film and the type or property of the first polyester resin and the second polyester resin. Preferably, the second polyester resin is present in an amount ranging from more than 2 wt% to less than 15 wt%, based on the total weight of the composition for forming the thermo-shrinkable polyester film being 100 wt%; more preferably, the second polyester resin is present in an amount ranging from 2.5 to 12.0 wt%.

The composition for forming the thermo-shrinkable polyester film may include other additives, such as but not limited to: catalysts, stabilizers, antioxidants, antistatic agents, antifoaming agents, dyeing auxiliaries, dyes, pigments, delustering agents, fluorescent whitening agents, and the like. Such as antimony oxide, titanium catalyst, germanium catalyst, tin catalyst, gallium catalyst, aluminum catalyst, or combinations thereof. Examples of the stabilizer include phosphoric acid, trimethyl phosphate, triethyl phosphonoacetate, and tripropyl phosphate (TPP).

[ first polyester resin ]

The first polyester resin has a glass transition temperature of 40-80 ℃; preferably, the first polyester resin has a glass transition temperature of 70 to 80 ℃. Preferably, the first polyester resin is a modified polyester resin; more preferably, the first polyester resin is polymerized from a mixture comprising terephthalic acid, ethylene glycol and a modifier. The modifier is not particularly limited, but the first polyester resin must have a glass transition temperature range of 40 to 80 ℃; preferably, the modifier is selected from isophthalic acid, neopentyl glycol, 1, 4-cyclohexanedimethanol, or combinations thereof. When the modifier is neopentyl glycol, the total amount of ethylene glycol and neopentyl glycol is preferably 100 mol%, the amount of ethylene glycol is preferably 70-85 mol%, and the amount of neopentyl glycol is preferably 15-30 mol%.

[ second polyester resin ]

The second polyester resin has a crystal melting temperature of 220 to 250 ℃ and a complete crystal melting enthalpy of 40 to 60J/g; preferably, the second polyester resin has a crystalline melting temperature of 220 to 230 ℃ and a complete crystalline melting enthalpy of 45 to 55J/g. Preferably, the second polyester resin includes, but is not limited to, polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and the like, and the aforementioned second polyester resins may be used alone or in combination of two or more. In an embodiment of the invention, the second polyester resin is PBT.

[ production method of Heat-shrinkable polyester film ]

The method for manufacturing the heat shrinkable polyester film of the present invention comprises the steps of:

(a) preparing a composition for forming a heat-shrinkable polyester film, the composition comprising a first polyester resin and a second polyester resin, wherein the first polyester resin has a glass transition temperature of 40-80 ℃, and the second polyester resin has a crystal melting temperature of 220-250 ℃ and a complete crystal melting enthalpy of 40-60J/g; and

(b) blending the composition at a heating temperature and extruding to obtain a plate, wherein the heating temperature does not enable the first polyester resin and the second polyester resin to perform ester exchange reaction; and

(c) the sheet is subjected to a film-forming step to obtain a heat-shrinkable polyester film.

The composition for forming the thermo-shrinkable polyester film, the first polyester resin, and the second polyester resin of step (a) are as described above. The preparation method of the composition may be carried out according to any known method, for example, the composition is obtained by mixing a first polyester resin with a second polyester resin.

The heating temperature in the step (b) does not cause the first polyester resin and the second polyester resin to perform ester exchange reaction, so that the heat shrinkable film prepared subsequently has a first crystalline melting peak and a second crystalline melting peak. In one embodiment of the present invention, the second polyester resin is polybutylene terephthalate, and the heating temperature in the step (b) is in the range of 210 to 260 ℃. The step (b) may be carried out by putting the composition into a twin-screw extruder to be blended and extruded to obtain a sheet.

The crystalline melting temperature of the second polyester resin described above can be measured in any known manner, preferably using a differential scanning calorimeter.

Particularly, in order to solve the problems of aging and shrinkage reduction caused by long-term storage or high-temperature transportation in the application of the heat shrinkable film, the preparation method of the invention controls the heating temperature in the step (b) to ensure that the first polyester resin and the second polyester resin are only simply blended and extruded for molding without ester exchange reaction. Therefore, in the subsequent film-forming step (c), the second polyester resin can provide crystal nuclei in addition to the first polyester resin, so that a heat shrinkable film having both the first polyester resin and the second polyester resin in crystal orientation can be obtained, that is, the crystalline hard segment of the second polyester resin can effectively inhibit aging relaxation of the first polyester resin, and further, the problems of aging and shrinkage reduction caused by long-term storage or high-temperature transportation can be solved.

The film forming step of step (c) may be performed in a known manner, such as, but not limited to, softening the sheet by heating, and then stretching the sheet in at least one direction (e.g., TD) at a temperature of 75-85 ℃ by a factor of about 4.0 to 5.0 times until a stretched polyester film having a thickness of 35-50 microns is formed. Finally, annealing the stretched polyester film at 50-80 ℃ to obtain the heat-shrinkable polyester film.

The heat-shrinkable polyester film of the present invention can be produced by the above-mentioned production process.

The invention will be further described in the following examples, but it should be understood that these examples are for illustrative purposes only and should not be construed as limiting the practice of the invention.

< chemicals >

The following examples and comparative examples each used the following chemicals:

1. first polyester resin (NPG-modified PET ester pellets, hereinafter referred to as P1 ester pellets): 37311.1 g (224.6mol) of terephthalic acid, 14027.4 g (226.0mol) of ethylene glycol, 5701.6 g (54.8mol) of neopentyl glycol (NPG) and 13.5 g of tetraethylammonium hydroxide (tetraethylammonium hydroxide) were added to a reactor to form a mixture; in the mixture, the amount of ethylene glycol and neopentyl glycol was taken as 100 mol%, the amount of ethylene glycol was taken as 80.5 mol% and the amount of neopentyl glycol was taken as 19.5 mol%. The mixture in the reactor was heated to 250 ℃ and mixed homogeneously, and then the esterification reaction was carried out under nitrogen while the water produced was distilled off. When the distilled water amount reaches the esterification theoretical value, 300ppm of antimony trioxide and 50ppm of phosphoric acid are added into a reactor to obtain a reactant, and then the reactant is subjected to polymerization reaction at 275 ℃ in a vacuum environment. And (3) granulating the reactant by a granulator until the Intrinsic Viscosity (IV) of the reactant reaches 0.5-0.7 to obtain NPG modified PET ester granules. The glass transition temperature of the NPG-modified PET ester pellets was 78 ℃.

2. Second polyester resin (PBT ester pellets, hereinafter P2 ester pellets): the PBT ester particle is purchased from Changchun chemical company, the model number is 1200-211D, the intrinsic viscosity is 0.7-1.0, the crystallization melting temperature is 223 ℃, and the complete crystallization melting enthalpy is 49J/g.

< example 1> Heat-shrinkable polyester film

(a) The P1 ester granules and the P2 ester granules were dried respectively until the water content was 200ppm or less, to obtain dried P1 ester granules and P2 ester granules. Mixing the components in a weight ratio (wt%) of 97.5: 2.5 uniformly mixing the dried P1 ester particles and the P2 ester particles to obtain a composition for forming the heat-shrinkable polyester film;

(b) adding the composition to layer B of a twin screw extruder, and adding the dried pellets of P1 ester to layer A of the twin screw extruder; then, respectively melting and extruding the ester granules of the layer A and the layer B at a multi-stage heating temperature (the highest temperature is 260 ℃ and the lowest temperature is 210 ℃) from 210 ℃ to 260 ℃, converging by a flow divider, extruding by a die head, and casting to a cold drum casting sheet to obtain a plate;

(c) the sheet was passed through a film stretcher (trade name: Bruckner Karoiv), heat-softened at a speed of 45M/min and a preheating temperature of 98 deg.C, stretched 4.8 times in TD at an extension temperature of 83 deg.C, and finally annealed at an annealing temperature of 76 deg.C to obtain a heat-shrinkable polyester film having a thickness of 40 μ M.

< examples 2 to 4> Heat-shrinkable polyester film