A biaxially stretched polybutylene terephthalate film and a method for producing the same

Abstract

The application relates to the technical field of high polymer material preparation, in particular to a biaxially stretched polybutylene terephthalate film and a preparation method thereof. The raw material of the polybutylene terephthalate film comprises terephthalic acid, poly-4-methylnitrile benzene isocyanide oligomer and 1,4-butanediol, the mass of terephthalic acid n1, the mass of poly-4-methylnitrile benzene isocyanide oligomer n2 and the mass of 1,4-butanediol n3 satisfy n1:n2:n3=1:(0.03-0.12):(1.2-1.35); the poly-4-methylnitrile benzene isocyanide oligomer has a molecular structure as shown in formula 1, and formula 1. Through the molecular structure design of the carboxyl reaction site + helical conformation, the helical poly-4-methylnitrile benzene isocyanide oligomer is copolymerized into the PBT chain, the biaxially stretched PBT film obtains high breaking elongation, and the fit and the tear resistance are improved.

Description

Technical Field

[0001] This application relates to the field of polymer material preparation technology, and in particular to a biaxially stretched polybutylene terephthalate film and its preparation method. Background Technology

[0002] Polybutylene terephthalate (PBT) is a high-performance thermoplastic engineering plastic, a polymer material formed by the esterification reaction of terephthalic acid and butanediol. PBT has a very fast crystallization rate and is typically milky white, translucent to opaque. PBT is mostly used in injection molding materials and possesses advantages such as high mechanical strength, good heat resistance, low coefficient of friction, good dimensional stability, good insulation properties, and corrosion resistance. It is widely used in the automotive, electronics, textile, industrial machinery, precision instruments, and furniture and building materials industries.

[0003] PBT resin is a semi-crystalline material. In principle, it can be biaxially stretched into films in a similar way to PET. However, because PBT crystallizes much faster than PET, premature and rapid crystallization occurs during actual operation, which increases the difficulty of biaxial stretching and film formation. As a result, there are currently no commercially available biaxially stretched PBT films.

[0004] Current research related to biaxial stretching of PBT includes: (1) A method for preparing biaxially stretched PBT film. This method studies the effects of intrinsic viscosity, casting thickness, preheating temperature, stretching ratio, and stretching time on the stretching process. The results show that the optimal stretching time is only 4s. (2) Because PBT material crystallizes too quickly, it is advisable to use a blown film method to form a film. Before PBT crystallizes significantly, it is simultaneously blown and stretched in the longitudinal and transverse directions to obtain a film. For example, a method for preparing PBT film using blown film method. This method uses hot air to keep the PBT material in a semi-molten state after extrusion with an annular die, thereby reducing the crystallinity of PBT, and then blown with hot air. (3) A method for preparing PBT or its copolyester into laminated sheets. This method reduces the crystallization rate by reducing the thickness of each layer of material, making it easier to stretch and obtain a sequentially biaxially stretched film.

[0005] However, the PBT materials obtained by these methods have a shorter stretching time in the biaxial stretching stage, resulting in a lower elongation at break. Summary of the Invention

[0006] This application provides a biaxially stretched polybutylene terephthalate (PBT) film and its preparation method to solve the following technical problem: how to improve the elongation at break of PBT materials.

[0007] In a first aspect, embodiments of this application provide a biaxially stretched polybutylene terephthalate film, wherein the raw materials of the polybutylene terephthalate film include: terephthalic acid, poly-4-carboxybenzoisocyanate oligomer and 1,4-butanediol, wherein the molar amounts n1 of the terephthalic acid, n2 of the poly-4-carboxybenzoisocyanate oligomer and n3 of the 1,4-butanediol satisfy: n1:n2:n3=1:(0.03~0.12):(1.2~1.35); wherein the poly-4-carboxybenzoisocyanate oligomer has a molecular structure as shown in Formula 1: , Formula 1.

[0008] Optionally, the degree of polymerization n of the poly-4-carboxylic acid benzoisocyanate oligomer is 10~34.

[0009] Optionally, the method for preparing the poly-4-carboxylic acid benzoisocyanate oligomer includes: 4-ethoxycarbonylbenzeneisonitrile and a mixed solvent are mixed to obtain an oligomer mixed raw material; Using nickel chloride hexahydrate as a catalyst, the oligomer mixture was subjected to a catalytic reaction to obtain crude poly-4-ethoxycarbonylphenylisocyanate oligomer. Tetrahydrofuran and the crude poly-4-ethoxycarbonylbenzeneisocyanate oligomer were mixed, and then an alkaline solution was added dropwise to hydrolyze the ethoxycarbonyl group in the crude poly-4-ethoxycarbonylbenzeneisocyanate oligomer to form a sodium salt structure, thus obtaining a hydrolyzed mixture. The hydrolyzed mixture was acid-washed with a dilute hydrochloric acid solution, followed by extraction with acetone to obtain poly-4-carboxylic acid benzoisocyanate oligomer.

[0010] Optionally, the temperature of the catalytic reaction is 30℃~85℃, and the time of the catalytic reaction is 1h~12h.

[0011] Optionally, the mass m1 of the 4-ethoxycarbonylbenzeneisocyanate and the mass m2 of the nickel chloride hexahydrate satisfy: m1:m2 = (10~60):(0.15~0.5); and / or The volume V1 of the mixed solution, the volume V2 of the tetrahydrofuran, and the volume V3 of the alkaline solution satisfy: V1:V2:V3 = (50~120):(60~150):(40~180).

[0012] Secondly, embodiments of this application provide a method for preparing a biaxially stretched polybutylene terephthalate (PBPT) film. The method is used to prepare the PBPT film described in the first aspect, and includes: Using antimony glycol as a catalyst, terephthalic acid, poly-4-carboxylic acid benzoisocyanate oligomer and 1,4-butanediol were subjected to esterification reaction to obtain a preliminary esterified product; The preliminary esterified compound was subjected to pre-condensation and condensation in sequence to obtain a low-viscosity polymer solution. The low-viscosity polymer solution was subjected to thickening treatment and pressurization treatment in sequence to obtain copolymer PBT resin; The copolymer PBT resin was sequentially extruded and simultaneously biaxially stretched to obtain a polybutylene terephthalate film.

[0013] Optionally, the esterification reaction is carried out at a temperature of 225°C to 235°C, at a pressure of 25 kPa to 35 kPa, and for a duration of 2 h to 5 h; and / or The temperature of the thickening treatment is 235℃~245℃, the vacuum degree of the thickening treatment is 55Pa~65Pa, and the time of the thickening treatment is 0.5h~2h.

[0014] Optionally, the pre-polymerization temperature is 225℃~235℃, the pre-polymerization vacuum degree is 8kPa~12kPa, and the pre-polymerization time is 1h~3h; and / or The vacuum degree of the polycondensation is 80 Pa to 1000 Pa, and the polycondensation time is 0.5 h to 3 h.

[0015] Optionally, the synchronous biaxial stretching includes a preheating section and a stretching section. The temperature of the preheating section is 50℃~130℃, and the preheating time is 50s~200s. The biaxial stretching ratio of the stretching section is 2.5 times~3.5 times.

[0016] Optionally, the intrinsic viscosity of the copolymerized PBT resin is 0.72 dl / g to 1.15 dl / g, the molar content of terminal carboxyl groups in the copolymerized PBT resin is 5 mol / t to 15 mol / t, and the residual tetrahydrofuran content in the copolymerized PBT resin is 20 ppm to 45 ppm; and / or The thickness of the polybutylene terephthalate film is 0.02 mm to 0.1 mm.

[0017] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a biaxially oriented polybutylene terephthalate (PBT) film. This PBT film, through a molecular structure design of "carboxyl group reaction sites + helical conformation," effectively improves the elongation at break of PBT. The specific mechanism is as follows: (1) Helical oligomer copolymerization embeds into the PBT backbone. The carboxyl groups on the side groups of poly-4-carboxybenzonitrile oligomers are reactive. During polycondensation, they are copolymerized with terephthalic acid and 1,4-butanediol, so that the poly-4-carboxybenzonitrile oligomers are embedded into the PBT molecular chain by chemical bonding rather than physical blending. In addition, the amount of poly-4-carboxybenzonitrile oligomers added is low, which introduces structural disturbance without damaging the bulk properties of PBT. (2) Helical structure delays crystallization. Due to the spatial repulsion of the C=N double bond, the polyisocyanate backbone of poly-4-carboxybenzonitrile oligomers has a helical conformation, which is large in size and irregular in conformation. After embedding, it destroys the regular stacking ability of PBT chains, which reduces the crystallization rate, crystallinity and makes the grains smaller and less perfect. (3) Widens the biaxial stretching window. PBT itself crystallizes quickly. Under the conventional PET stretching conditions, it crystallizes rapidly and becomes brittle during stretching, making it difficult to stretch biaxially. After copolymerization modification, crystallization is suppressed, and PBT materials remain in a stretchable state for a longer time within the stretching temperature range. The molecular chains can be fully and uniformly oriented in both longitudinal and transverse directions, avoiding stress concentration and premature breakage caused by premature crystallization. (4) Helical segments contribute to deformation capacity. When subjected to force, the embedded helical segments can unwind and extend like a spring, absorbing deformation energy. Combined with low crystallinity and fine crystalline regions (high proportion of amorphous regions and uniform stress transmission), the deformation that PBT materials can withstand before breakage is greatly increased. These mechanisms work together to enable biaxially stretched PBT films to achieve high elongation at break, while improving conformability and tear resistance. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A helical structure diagram of poly-4-carboxybenzonitrile oligomer in a biaxially stretched polybutylene terephthalate film provided in this application embodiment; Figure 2 This is a flowchart illustrating the preparation method of poly-4-carboxylic acid benzoisocyanate oligomers provided in the embodiments of this application; Figure 3 A flowchart illustrating a method for preparing a biaxially stretched polybutylene terephthalate film, as provided in this application embodiment. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] The range descriptions used in this application, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "comprising" and others used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships involved in this document, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained by purchasing from the market or by existing methods.

[0023] It should be noted that, as is known from existing technology, biaxial stretching of PBT materials requires reducing their crystallization rate or crystallinity to a certain extent before a stretching process similar to that of PET can be used for biaxial stretching, and blown film method is easier than successive biaxial stretching. However, these methods either require higher operating temperatures or unique blown film equipment, which significantly increases the stretching time of PBT in the stretching stage, resulting in lower elongation at break of PBT materials prepared by these methods.

[0024] To address the aforementioned technical problems, this application provides a biaxially oriented polybutylene terephthalate (PBT) film and its preparation method. By synthesizing an oligomeric polyacid spiral polymer with a distinct helical structure and using this polymer in the PBT copolymerization process, the dendritic helical structure of the polymer significantly slows down the crystallization rate of PBT resin and reduces the crystallinity of PBT. This allows PBT copolymerization to be performed under conditions similar to PET copolymerization, enabling successive biaxial stretching. This not only increases the manufacturability of the biaxially oriented PBT film but also enhances its tensile strength and elongation at break. Furthermore, the presence of the helical structure makes the PBT film opaque, offering advantages such as good insulation properties, good conformability, and high processing feasibility.

[0025] Figure 1 An exemplary diagram of the helical structure of poly-4-carboxybenzonitrile oligomer in a biaxially stretched polybutylene terephthalate film provided in this application embodiment is shown. like Figure 1 As shown in the embodiment of this application, a biaxially stretched polybutylene terephthalate film is provided. The raw materials of the polybutylene terephthalate film include terephthalic acid, poly-4-carboxybenzonitrile oligomer, and 1,4-butanediol. The molar amounts n1 of the terephthalic acid, n2 of the poly-4-carboxybenzonitrile oligomer, and n3 of the 1,4-butanediol satisfy the following ratio: n1:n2:n3 = 1:(0.03~0.12):(1.2~1.35). The poly-4-carboxybenzonitrile oligomer has a molecular structure as shown in Formula 1. , Formula 1.

[0026] It should be noted that the main chain of this poly-4-carboxylic acid phenylisocyanate oligomer is a C=N backbone formed by isonitrile polymerization, with each repeating unit carrying a para-carboxyphenyl group; the degree of polymerization of this poly-4-carboxylic acid phenylisocyanate oligomer is controlled between 10 and 34, which can form poly-4-carboxylic acid phenylisocyanate oligomers with appropriate molecular weight.

[0027] It should be noted that this application provides a biaxially oriented polybutylene terephthalate (PBT) film. This PBT film exhibits high elongation at break by introducing poly-4-carboxylic acid benzoisocyanate oligomers with carboxyl reaction sites and a helical conformation. The specific mechanism is as follows: 1. Helical oligomers are embedded as copolymer units in the PBT molecular chain.

[0028] The carboxyl groups (-COOH) on the side groups of poly(4-carboxybenzoisocyanate) oligomers are reactive and can participate in esterification / condensation reactions with terephthalic acid and 1,4-butanediol during the polycondensation process. This allows the poly(4-carboxybenzoisocyanate) oligomers to chemically integrate into the PBT backbone, forming copolymerized PBT, rather than simply physical blending. The addition amount of poly(4-carboxybenzoisocyanate) oligomers is controlled within a low range of 1.4% to 5.9% (by molar amount), which introduces a helical structure to disturb the PBT backbone without excessively damaging the properties of the PBT matrix.

[0029] 2. The steric hindrance of the helical structure disrupts the chain regularity and delays crystallization.

[0030] The backbone of poly(4-carboxybenzoisocyanate) oligomers exhibits a helical conformation due to the steric repulsion of the imine carbon-nitrogen double bonds, making it a typical helical polymer. Introducing this helical segment into the PBT chain: (1) The helical structure is large in volume and irregular in conformation, which destroys the original regular arrangement ability of PBT molecular chains; (2) PBT molecular chain segments are difficult to quickly and orderly stack to form crystal nuclei and crystals, resulting in a significant decrease in crystallization rate, reduced crystallinity, and smaller and less perfect crystal grains.

[0031] 3. Crystallization is delayed → a sufficient biaxial stretching window is obtained.

[0032] PBT itself crystallizes very quickly. Under conventional PET biaxial stretching processes, the melt / sheet rapidly crystallizes, hardens, and becomes brittle during stretching, unable to complete successive biaxial stretching steps in time. After copolymer modification: (1) Crystallization is suppressed, and PBT materials maintain a stretchable, highly elastic state for a longer time within the stretching temperature range; (2) The molecular chains have sufficient time to be gradually oriented and stretched in both the longitudinal and transverse directions, resulting in more complete and uniform orientation, thus avoiding stress concentration and premature breakage caused by premature crystallization.

[0033] 4. The helical chain segments themselves contribute to the reversible deformation capability.

[0034] Even if PBT material undergoes partial orientation crystallization, the embedded helical segments can unwind and extend like springs under stress, absorbing deformation energy and giving the PBT molecular chains greater elongation potential. Combined with lower crystallinity and smaller crystalline regions (fewer and smaller crystalline regions mean a higher proportion of amorphous regions and more uniform stress transmission), the deformation that PBT material can withstand before fracture is significantly increased.

[0035] In summary, the present application provides a biaxially oriented polybutylene terephthalate (PBT) film. This PBT film, through a molecular structure design of "carboxyl reaction sites + helical conformation," copolymerizes helical poly(4-carboxybenzonitrile) oligomers into the PBT chain. On one hand, the helical poly(4-carboxybenzonitrile) oligomers structurally delay crystallization and broaden the biaxially oriented processing window, allowing the PBT molecular chains to be fully oriented. On the other hand, the spring-like extension of the helical chain segments and the low crystallinity morphology enhance the deformation capacity. These two factors synergistically enable the biaxially oriented PBT film to achieve high elongation at break, while simultaneously improving conformability and tear resistance.

[0036] In some alternative embodiments, the degree of polymerization n of the poly-4-carboxylic acid benzoisocyanate oligomer is 10 to 34.

[0037] In these embodiments, poly-4-carboxybenzonitrile oligomers with a degree of polymerization n of 10 to 34 can avoid introducing the helical structure of the poly-4-carboxybenzonitrile oligomers and disturbing the PBT main chain, while not excessively damaging the performance of the PBT bulk.

[0038] The degree of polymerization n of the poly-4-carboxylic acid benzoisocyanate oligomer can be 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32 or 34.

[0039] Figure 2 An exemplary flowchart of the preparation method of poly-4-carboxylic acid benzoisocyanate oligomers provided in the embodiments of this application is shown; In some alternative implementations, such as Figure 2 As shown, the preparation method of the poly-4-carboxylic acid benzoisocyanate oligomer includes: S1. Mix 4-ethoxycarbonylphenylisocyanate and a mixed solvent to obtain an oligomer mixed raw material; S2. Using nickel chloride hexahydrate as a catalyst, the oligomer mixture is subjected to a catalytic reaction to obtain crude poly-4-ethoxycarbonylphenylisocyanate oligomer; S3. Tetrahydrofuran and the crude poly-4-ethoxycarbonylbenzeneisocyanate oligomer are mixed, and then an alkaline solution is added dropwise to hydrolyze the ethoxycarbonyl group in the crude poly-4-ethoxycarbonylbenzeneisocyanate oligomer to form a sodium salt structure, thereby obtaining a hydrolyzed mixture. S4. The hydrolyzed mixture is acid-washed with dilute hydrochloric acid solution, followed by extraction with acetone to obtain poly-4-carboxylic acid benzoisocyanate oligomer.

[0040] In these embodiments, 4-ethoxycarbonylbenzoisocyanate is used as the matrix to form an oligomer mixture in a mixed solvent; then, under the catalysis of nickel chloride hexahydrate, 4-ethoxycarbonylbenzoisocyanate is directly polymerized to form crude poly-4-ethoxycarbonylbenzoisocyanate oligomer; then, high-purity poly-4-carboxylic acid benzoisocyanate oligomer is obtained by dissolving in tetrahydrofuran, treating with an alkaline solution, and by acid washing and extraction.

[0041] It should be noted that the mixed solvent can be a mixture of methanol and dichloromethane, with a volume ratio of methanol to dichloromethane of 1:9 to 9:1. The amount of the mixed solvent added can be 50 mL to 120 mL.

[0042] It should be noted that the alkaline solution can be a sodium hydroxide solution, and the volume of the alkaline solution can be 40mL to 180mL.

[0043] In some optional embodiments, the temperature of the catalytic reaction is 30°C to 85°C, and the time of the catalytic reaction is 1h to 12h.

[0044] In these embodiments, a catalytic reaction at a temperature of 30°C to 85°C and a time of 1h to 12h can polymerize 4-ethoxycarbonylbenzoisocyanate in a mixed solvent under the condition that nickel chloride hexahydrate is used as a catalyst, forming a large amount of crude poly-4-ethoxycarbonylbenzoisocyanate oligomer.

[0045] The temperature for the catalytic reaction can be 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, or 85℃.

[0046] The catalytic reaction can take 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, or 12 hours.

[0047] In some optional embodiments, the mass m1 of the 4-ethoxycarbonylbenzeneisocyanate and the mass m2 of the nickel chloride hexahydrate satisfy: m1:m2 = (10~60):(0.15~0.5); and / or The volume V1 of the mixed solution, the volume V2 of the tetrahydrofuran, and the volume V3 of the alkaline solution satisfy: V1:V2:V3 = (50~120):(60~150):(40~180).

[0048] In these embodiments, a mass ratio of (10~60):(0.15~0.5) of 4-ethoxycarbonylbenzeneisocyanate and nickel chloride hexahydrate provides sufficient catalyst for the catalytic reaction, which is beneficial for the polymerization of 4-ethoxycarbonylbenzeneisocyanate to form a large amount of crude poly-4-ethoxycarbonylbenzeneisocyanate oligomers. Furthermore, a volume ratio of (50~120):(60~150):(40~180) of mixed solution, tetrahydrofuran, and alkaline solution ensures uniform distribution of 4-ethoxycarbonylbenzeneisocyanate or poly-4-ethoxycarbonylbenzeneisocyanate oligomers. The alkaline solution adjustment causes the ethoxycarbonyl groups in the crude poly-4-ethoxycarbonylbenzeneisocyanate oligomers to hydrolyze and form sodium salt structures, which is beneficial for subsequent acid washing to form high-purity poly-4-carboxylic acid benzoisocyanate oligomers.

[0049] The mass m1 of the 4-ethoxycarbonylbenzeneisocyanate can be 10, 20, 30, 40, 50 or 60.

[0050] The mass m2 of the nickel chloride hexahydrate can be 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 or 0.50.

[0051] The volume V1 of the mixed solution can be 50, 60, 70, 80, 90, 100, 110 or 120.

[0052] The volume V2 of the tetrahydrofuran can be 60, 70, 80, 90, 100, 110, 120, 130, 140 or 150.

[0053] The volume V3 of the alkaline solution can be 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170 or 180.

[0054] Figure 3 An exemplary flowchart of a method for preparing a biaxially stretched polybutylene terephthalate film according to an embodiment of this application is shown. Based on a general inventive concept, such as Figure 3 As shown in the figure, this application provides a method for preparing a biaxially oriented polybutylene terephthalate (PBPT) film. The method comprises: S1. Using antimony glycol as a catalyst, terephthalic acid, poly-4-carboxylic acid benzoisocyanate oligomer and 1,4-butanediol are subjected to esterification reaction to obtain a preliminary esterified product; S2. The preliminary esterified product is subjected to pre-condensation and condensation in sequence to obtain a low-viscosity polymer solution; S3. The low-viscosity polymer liquid is subjected to thickening treatment and pressurization treatment in sequence to obtain copolymer PBT resin; S4. The copolymer PBT resin is extruded and simultaneously biaxially stretched in sequence to obtain a polybutylene terephthalate film.

[0055] This preparation method is for the preparation of the above-mentioned polybutylene terephthalate film. The specific composition of the polybutylene terephthalate film can be referred to in the above embodiments. Since this preparation method adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be repeated here.

[0056] It should be noted that the degree of esterification can be determined based on the mass content of free 1,4-butanediol. Specifically, when the mass content of free 1,4-butanediol is less than 0.2%, the degree of esterification can reach 91% to 99%.

[0057] It should be noted that the copolymer PBT resin needs to be dried before extrusion and simultaneous biaxial stretching. The drying temperature can be 125℃ and the drying time can be 4h to 6h.

[0058] It should be noted that this extrusion can use a single-screw extruder with a screw diameter of 20 mm and a length-to-diameter ratio of 35:1. During the extrusion stage, the PBT resin is first melt-copolymerized at 225℃~255℃, and then cooled and cast on a cooling drum at 15℃.

[0059] It should be noted that this simultaneous biaxial tensile testing can be performed on a static biaxial tensile testing machine. Preheating can be performed before the tensile test.

[0060] In some optional embodiments, the esterification reaction is carried out at a temperature of 225°C to 235°C, at a pressure of 25 kPa to 35 kPa, and for a duration of 2 h to 5 h; and / or The temperature of the thickening treatment is 235℃~245℃, the vacuum degree of the thickening treatment is 55Pa~65Pa, and the time of the thickening treatment is 0.5h~2h.

[0061] In these embodiments, the esterification reaction at a temperature of 225℃~235℃, a pressure of 25kPa~35kPa, and a time of 2h~5h can be carried out under a slightly positive pressure to ensure the full conversion of the carboxyl groups of terephthalic acid and poly-4-carboxylic acid benzoisocyanate oligomers (especially since the carboxyl groups of poly-4-carboxylic acid benzoisocyanate oligomers have low steric hindrance reactivity and require sufficient activation to ensure that the poly-4-carboxylic acid benzoisocyanate oligomers are completely embedded in the molecular chain and to avoid free residues affecting subsequent properties). This is beneficial for the subsequent esterification of carboxyl groups with diols to form ester groups and the removal of the generated water. In addition, the thickening treatment at a temperature of 235℃~245℃, a vacuum of 55Pa~65Pa, and a time of 0.5h~2h can further increase the intrinsic viscosity (molecular weight), enabling the copolyester compound formed by the low-viscosity polymer solution to achieve the melt strength and mechanical properties required for biaxially stretched film formation.

[0062] The esterification reaction temperature can be 225℃, 226℃, 227℃, 228℃, 229℃, 230℃, 231℃, 232℃, 233℃, 234℃ or 235℃.

[0063] The pressure for the esterification reaction can be 25 kPa, 26 kPa, 27 kPa, 28 kPa, 29 kPa, 30 kPa, 31 kPa, 32 kPa, 33 kPa, 34 kPa or 35 kPa.

[0064] The esterification reaction can take 2 hours, 3 hours, 4 hours, or 5 hours.

[0065] The temperature for the thickening treatment can be 235℃, 236℃, 237℃, 238℃, 239℃, 240℃, 241℃, 242℃, 243℃, 244℃ or 245℃.

[0066] The vacuum degree of the thickening treatment can be 55Pa, 56Pa, 57Pa, 58Pa, 59Pa, 60Pa, 61Pa, 62Pa, 63Pa, 64Pa or 65Pa.

[0067] The thickening treatment can be performed for 0.5h, 1h, 1.5h or 2h.

[0068] It should be noted that when the temperature of the esterification reaction is below 225℃, the esterification rate is slow and the esterification is incomplete; when the temperature of the esterification reaction is above 235℃, the esterification process is prone to cause the dehydration of diols to generate byproducts (such as THF) and lead to the decomposition of oligomers.

[0069] In some optional embodiments, the pre-polymerization temperature is 225°C to 235°C, the pre-polymerization vacuum degree is 8 kPa to 12 kPa, and the pre-polymerization time is 1 h to 3 h; and / or The vacuum degree of the polycondensation is 80 Pa to 1000 Pa, and the polycondensation time is 0.5 h to 3 h.

[0070] In these embodiments, pre-polymerization at temperatures of 225°C to 235°C, vacuum levels of 8 kPa to 12 kPa, and time periods of 1 to 3 hours allows for gradual vacuuming while maintaining the temperature, removing excess diols and small molecules. This initial polymerization of the esterification products into oligomers prepares the material for subsequent high-vacuum polymerization, preventing material entrainment and violent boiling caused by sudden high vacuum. Conversely, polymerization at vacuum levels of 80 Pa to 1000 Pa and time periods of 0.5 to 3 hours allows for deep removal of byproducts such as diols under high vacuum conditions, promoting a positive shift in the polymerization equilibrium and rapidly increasing the molecular weight of the initial esterified products.

[0071] The pre-condensation temperature can be 225℃, 226℃, 227℃, 228℃, 229℃, 230℃, 231℃, 232℃, 233℃, 234℃ or 235℃.

[0072] The vacuum degree of the pre-condensation can be 8 kPa, 8.5 kPa, 9 kPa, 9.5 kPa, 10 kPa, 10.5 kPa, 11 kPa, 11.5 kPa or 12 kPa.

[0073] The pre-polymerization time can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, or 3 hours.

[0074] The vacuum degree of this polycondensation can be 80Pa, 90Pa, 100Pa, 200Pa, 300Pa, 400Pa, 500Pa or 1000Pa.

[0075] The polycondensation time can be 0.5h, 1h, 1.5h, 2h, 2.5h or 3h.

[0076] It should be noted that during the polycondensation process, if the end carboxyl group content of the pre-polymerized ester is less than 30 mol / t, the vacuum degree of polycondensation needs to be reduced to 1 kPa, and the polycondensation time should be controlled to be 0.5 h to 2 h; if the end carboxyl group content of the pre-polymerized ester is less than 20 mol / t, the vacuum degree of polycondensation needs to be reduced to 80 Pa, and the polycondensation time should be controlled to be 1 h to 3 h.

[0077] It should be noted that this gradient heating and vacuum process of "esterification → pre-condensation → condensation → thickening" ensures that the poly-4-carboxybenzoisocyanate oligomer is fully copolymerized and embedded into the PBT chain, while controlling degradation and side reactions, and finally obtains a copolymer PBT resin with a moderate molecular weight, controllable molecular weight distribution, and suitable for biaxial stretching processing.

[0078] In some optional embodiments, the synchronous biaxial stretching includes a preheating section and a stretching section. The temperature of the preheating section is 50°C to 130°C, and the preheating time is 50s to 200s. The biaxial stretching ratio of the stretching section is 2.5 to 3.5 times.

[0079] In these embodiments, the preheating section at a temperature of 50°C to 130°C and a time of 50 to 200 seconds softens the extruded copolymer PBT resin, which is beneficial for the subsequent stretching section. Furthermore, the stretching section with a biaxial stretching ratio of 2.5 to 3.5 times allows the softened copolymer PBT resin to be fully stretched to the predetermined thickness.

[0080] The temperature of the preheating section can be 50℃, 60℃, 70℃, 80℃, 90℃, 100℃, 110℃, 120℃ or 130℃.

[0081] The preheating period can last for 50s, 60s, 70s, 80s, 90s, 100s, 150s, or 200s.

[0082] The biaxial tensile ratio of the stretching section can be 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, or 3.5 times.

[0083] In some optional embodiments, the intrinsic viscosity of the copolymerized PBT resin is 0.72 dl / g to 1.15 dl / g, the molar content of terminal carboxyl groups in the copolymerized PBT resin is 5 mol / t to 15 mol / t, and the residual tetrahydrofuran content in the copolymerized PBT resin is 20 ppm to 45 ppm; and / or The thickness of the polybutylene terephthalate film is 0.02 mm to 0.1 mm.

[0084] In these embodiments, the copolymer PBT resin with an intrinsic viscosity of 0.72 dl / g to 1.15 dl / g indicates that the copolymer PBT resin has a sufficiently high molecular weight, high melt strength, and is not prone to rupture of the film / sheet during biaxial stretching, thus being able to withstand the stress of successive stretching and ensuring film continuity. Furthermore, the terminal carboxyl groups in the copolymer PBT resin, with a molar content of 5 mol / t to 15 mol / t, can provide catalytic sites for polyester thermal degradation and hydrolysis, reducing thermal degradation and hydrolytic chain scission during melt processing (extrusion, stretching), ensuring that the molecular weight remains stable during processing, and preventing the deterioration of the mechanical properties of the PBT film (especially elongation), thereby improving the hydrolysis resistance and aging resistance of the PBT film during long-term use. Furthermore, the presence of 20ppm to 45ppm residual tetrahydrofuran (THF) in the copolymerized PBT resin indicates that the preparation method (temperature and vacuum control) of this application effectively suppresses side reactions and fully removes small molecules. Simultaneously, it avoids the formation of defects such as bubbles and crystal points by residual THF in the PBT film, ensuring the film's appearance quality and thickness uniformity, reducing the interference of small molecule residues on crystallization behavior and mechanical properties, and guaranteeing the stability of biaxial stretching. In addition, the 0.02mm to 0.1mm thickness of the polybutylene terephthalate (PET) film indicates that this PET film product belongs to the category of thin to medium-thick packaging or functional films. The thin and uniform thickness is a manifestation of the advantages of the biaxial stretching process, which indirectly demonstrates that the copolymerized modified PBT can be sufficiently stretched and thinned without breaking, directly confirming the material's high elongation and meeting the dual requirements of flexibility and strength for applications such as lamination and packaging.

[0085] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0086] Example 1

[0087] A biaxially oriented polybutylene terephthalate (PET) film, wherein the raw materials of the PET film include terephthalic acid, poly-4-carboxybenzonitrile oligomer, and 1,4-butanediol, wherein the molar amounts of terephthalic acid (n1), poly-4-carboxybenzonitrile oligomer (n2), and 1,4-butanediol (n3) satisfy the following ratio: n1:n2:n3 = 1:0.05:1.3; wherein the poly-4-carboxybenzonitrile oligomer has a molecular structure as shown in Formula 1: , Formula 1.

[0088] The degree of polymerization (n) of poly-4-carboxylic acid benzoisocyanate oligomers is 12 to 18.

[0089] like Figure 2 As shown, the preparation method of poly-4-carboxylic acid benzoisocyanate oligomer includes: S1. Mix 4-ethoxycarbonylbenzeneisocyanate with a mixed solvent to obtain an oligomer mixed raw material; the specific process is as follows: 60g of 4-ethoxycarbonylbenzeneisonitrile was added to 80mL of a mixed solvent of methanol and dichloromethane (volume ratio 1:4) and mixed to obtain an oligomer mixed raw material; S2. Using nickel chloride hexahydrate as a catalyst, the oligomer mixture is subjected to a catalytic reaction to obtain crude poly-4-ethoxycarbonylphenylisocyanate oligomer; the specific process is as follows: 0.25 g of nickel chloride hexahydrate was added to the oligomer mixture as a catalyst, and the mixture was refluxed at 30 °C for 12 h to carry out the catalytic reaction, and crude poly-4-ethoxycarbonylphenylisocyanate oligomer was obtained. S3. Tetrahydrofuran and crude poly-4-ethoxycarbonylbenzeneisocyanate oligomer are mixed, and then an alkaline solution is added dropwise to hydrolyze the ethoxycarbonyl groups in the crude poly-4-ethoxycarbonylbenzeneisocyanate oligomer to form sodium salt structures, yielding a hydrolyzed mixture; the specific process is as follows: The crude poly-4-ethoxycarbonylbenzoisocyanate oligomer was added to 70 mL of THF, and 120 mL of NaOH solution (concentration of 1 mol / L) was added dropwise under heating conditions to hydrolyze the ethoxycarbonyl group in the crude poly-4-ethoxycarbonylbenzoisocyanate oligomer to form a sodium salt structure, resulting in a hydrolyzed mixture. S4. The hydrolyzed mixture is washed with dilute hydrochloric acid solution, followed by extraction with acetone to obtain poly-4-carboxylic acid benzoisocyanate oligomer; the specific process is as follows: An excess of dilute hydrochloric acid solution was added to the hydrolysis mixture, and the mixture was extracted with acetone to obtain poly-4-carboxylic acid benzoisocyanate oligomer.

[0090] The catalytic reaction was carried out at a temperature of 30°C for 12 hours.

[0091] The mass m1 of 4-ethoxycarbonylbenzeneisocyanate and the mass m2 of nickel chloride hexahydrate satisfy the following condition: m1:m2 = 60 g: 0.25 g; The volumes of the mixed solution V1, tetrahydrofuran V2, and alkaline solution V3 satisfy the following: V1:V2:V3 = 80mL:70mL:120mL.

[0092] like Figure 3 As shown, a method for preparing a biaxially stretched polybutylene terephthalate (PBPT) film is disclosed. The method is used to prepare PBPT films and includes the following steps: S1. Using antimony glycol as a catalyst, terephthalic acid, poly-4-carboxylic acid benzoisocyanate oligomer and 1,4-butanediol are subjected to an esterification reaction to obtain a preliminary esterified product; the specific process is as follows: Terephthalic acid, poly-4-carboxylic acid benzoisocyanate oligomer and 1,4-butanediol were added to an esterification reactor (molar ratio 1:0.05:1.3), and 50 ppm of antimony glycol catalyst was added. The temperature was raised to 230°C and the pressure of the esterification reactor was controlled at 30 kPa for esterification reaction for 5 h. The degree of esterification reaction was determined by the content of free 1,4-butanediol to obtain the preliminary esterified product. S2. The preliminary esterified product is subjected to pre-condensation and condensation polymerization sequentially to obtain a low-viscosity polymer solution; the specific process is as follows: The preliminary esterified product is transferred to a polycondensation reactor, then heated to 230℃, and the vacuum degree of the polycondensation reactor is controlled at 10 kPa for pre-polycondensation for 3 hours. If the terminal carboxyl group content of the preliminary esterified product after pre-polycondensation is less than 30 mol / t, the vacuum degree of the polycondensation reactor is reduced to 1 kPa, and polycondensation is continued for 1.5 hours. If the terminal carboxyl group content of the preliminary esterified product after pre-polycondensation is less than 20 mol / t, the vacuum degree of the polycondensation reactor is reduced to 80 Pa, and polycondensation is continued for 1 hour to obtain a low-viscosity polymer solution. S3. The low-viscosity polymer solution is subjected to thickening and pressurization treatments sequentially to obtain copolymer PBT resin; the specific process is as follows: The low-viscosity polymerization solution was transferred to a thickening reactor, then heated to 240°C, and the vacuum degree inside the thickening reactor was controlled at 60Pa for thickening treatment for 0.5h. Finally, the product was discharged by pressure treatment to obtain copolymer PBT resin. S4. The copolymerized PBT resin is sequentially extruded and simultaneously biaxially stretched to obtain a polybutylene terephthalate film; the specific process is as follows: The obtained copolymer PBT resin is first dried at 125℃ for 4-6 hours, then fed into a single-screw extruder with a screw diameter of 20mm and a length-to-diameter ratio of 35:1. The resin is melted at 235℃ to form a molten copolymer PBT resin. This molten resin is then cooled and cast on a 15℃ cooling drum to form a copolyesterified sheet. The copolyesterified sheet is then subjected to simultaneous biaxial stretching on a static biaxial tensile testing machine. During the simultaneous biaxial stretching process, the preheating temperature is 105℃, the preheating time is 70s, and the stretching ratio is 2.5 times, thus obtaining the PBT film.

[0093] The esterification reaction was carried out at a temperature of 230℃, a pressure of 30 kPa, and a time of 5 h. The temperature for the thickening treatment was 240℃, the vacuum degree for the thickening treatment was 60Pa, and the thickening treatment time was 0.5h.

[0094] The pre-condensation temperature was 230℃, the pre-condensation vacuum degree was 10kPa, and the pre-condensation time was 3h. The vacuum degree of polycondensation is 80 Pa to 1000 Pa, and the polycondensation time is 0.5 h to 3 h.

[0095] The synchronous biaxial stretching includes a preheating section and a stretching section. The temperature of the preheating section is 105℃ and the preheating time is 70s. The biaxial stretching ratio of the stretching section is 2.5 times.

[0096] Example 2

[0097] Compared to Example 1, the differences in this example are as follows, while the rest are the same: The amounts of terephthalic acid (n1), poly(4-carboxylic acid benzoisocyanate) oligomer (n2), and 1,4-butanediol (n3) satisfy the following ratio: n1:n2:n3 = 1:0.12:1.35.

[0098] The degree of polymerization (n) of poly-4-carboxylic acid benzoisocyanate oligomers is 10~15.

[0099] The catalytic reaction was carried out at a temperature of 60℃ for 5 hours.

[0100] The mass m1 of 4-ethoxycarbonylbenzeneisocyanate and the mass m2 of nickel chloride hexahydrate satisfy the following condition: m1:m2 = 10g:0.15g; The volumes of the mixed solution V1, tetrahydrofuran V2, and alkaline solution V3 satisfy the following: V1:V2:V3 = 50 mL:60 mL:40 mL.

[0101] The volume ratio of methanol to dichloromethane in the mixed solvent is 1:9.

[0102] The esterification reaction was carried out at a temperature of 230℃, a pressure of 30 kPa, and a time of 3.5 h. The temperature for the thickening treatment was 240℃, the vacuum degree for the thickening treatment was 60Pa, and the time for the thickening treatment was 2.0h.

[0103] The pre-condensation temperature was 230℃, the pre-condensation vacuum degree was 10kPa, and the pre-condensation time was 3h. If the end carboxyl group content of the pre-polymerized ester is less than 30 mol / t, reduce the vacuum of the esterification vessel to 1 kPa and then continue the polymerization for 2 hours; if the end carboxyl group content of the pre-polymerized ester is less than 20 mol / t, reduce the vacuum of the esterification vessel to 80 Pa and then continue the polymerization for 1 hour.

[0104] The melting temperature of the extrusion section is 225℃.

[0105] The synchronous biaxial stretching includes a preheating section and a stretching section. The temperature of the preheating section is 120℃ and the preheating time is 200s. The biaxial stretching ratio of the stretching section is 2.8 times.

[0106] Example 3

[0107] Compared to Example 1, the differences in this example are as follows, while the rest are the same: The amounts of terephthalic acid (n1), poly(4-carboxylic acid benzoisocyanate) oligomer (n2), and 1,4-butanediol (n3) satisfy the following ratio: n1:n2:n3 = 1:0.1:1.2.

[0108] The degree of polymerization (n) of poly-4-carboxylic acid benzoisocyanate oligomers is 17~30.

[0109] The catalytic reaction was carried out at a temperature of 45℃ for 12 hours.

[0110] The mass m1 of 4-ethoxycarbonylbenzeneisocyanate and the mass m2 of nickel chloride hexahydrate satisfy the following condition: m1:m2 = 35g:0.45g; The volumes of the mixed solution V1, tetrahydrofuran V2, and alkaline solution V3 satisfy the following condition: V1:V2:V3 = 100mL:100mL:80mL.

[0111] The volume ratio of methanol to dichloromethane in the mixed solvent is 9:1.

[0112] The esterification reaction was carried out at a temperature of 230℃, a pressure of 30 kPa, and a time of 2.5 h. The temperature for the thickening treatment was 240℃, the vacuum degree for the thickening treatment was 60Pa, and the treatment time was 1.5h.

[0113] The pre-condensation temperature was 230℃, the pre-condensation vacuum degree was 10kPa, and the pre-condensation time was 1h. If the end carboxyl group content of the pre-polymerized ester is less than 30 mol / t, reduce the vacuum of the esterification vessel to 1 kPa and then continue the polymerization for 2 hours; if the end carboxyl group content of the pre-polymerized ester is less than 20 mol / t, reduce the vacuum of the esterification vessel to 80 Pa and then continue the polymerization for 3 hours.

[0114] The melting temperature of the extrusion section is 255℃.

[0115] The synchronous biaxial stretching includes a preheating section and a stretching section. The temperature of the preheating section is 125℃ and the preheating time is 140s. The biaxial stretching ratio of the stretching section is 3 times.

[0116] Example 4

[0117] Compared to Example 1, the differences in this example are as follows, while the rest are the same: The amounts of terephthalic acid (n1), poly(4-carboxylic acid benzoisocyanate) oligomer (n2), and 1,4-butanediol (n3) satisfy the following ratio: n1:n2:n3 = 1:0.08:1.25.

[0118] The degree of polymerization (n) of poly-4-carboxylic acid benzoisocyanate oligomers is 23~34.

[0119] The catalytic reaction was carried out at a temperature of 65°C for 6 hours.

[0120] The mass m1 of 4-ethoxycarbonylbenzeneisocyanate and the mass m2 of nickel chloride hexahydrate satisfy the following condition: m1:m2 = 50 g: 0.55 g; The volumes of the mixed solution V1, tetrahydrofuran V2, and alkaline solution V3 satisfy the following: V1:V2:V3 = 100mL:150mL:180mL.

[0121] The volume ratio of methanol to dichloromethane in the mixed solvent is 5:5.

[0122] The esterification reaction was carried out at a temperature of 230℃, a pressure of 30 kPa, and a time of 2 hours. The temperature for the thickening treatment was 240℃, the vacuum degree for the thickening treatment was 60Pa, and the time for the thickening treatment was 2.0h.

[0123] The pre-condensation temperature was 230℃, the pre-condensation vacuum degree was 10kPa, and the pre-condensation time was 3h. When the end carboxyl group content of the pre-polymerized esterified product is less than 30 mol / t, the vacuum degree of the esterification vessel is reduced to 1 kPa, and then the polymerization continues for 0.5 h; when the end carboxyl group content of the pre-polymerized esterified product is less than 20 mol / t, the vacuum degree of the esterification vessel is reduced to 80 Pa, and then the polymerization continues for 2 h.

[0124] The melting temperature of the extrusion section is 255℃.

[0125] The synchronous biaxial stretching includes a preheating section and a stretching section. The temperature of the preheating section is 130℃ and the preheating time is 50s. The biaxial stretching ratio of the stretching section is 2.8 times.

[0126] Example 5

[0127] Compared to Example 1, the differences in this example are as follows, while the rest are the same: The amounts of terephthalic acid (n1), poly(4-carboxylic acid benzoisocyanate) oligomer (n2), and 1,4-butanediol (n3) satisfy the following ratio: n1:n2:n3 = 1:0.12:1.35.

[0128] The degree of polymerization (n) of poly-4-carboxylic acid benzoisocyanate oligomers is 13~22.

[0129] The catalytic reaction was carried out at a temperature of 85℃ for 3 hours.

[0130] The mass m1 of 4-ethoxycarbonylbenzeneisocyanate and the mass m2 of nickel chloride hexahydrate satisfy the following condition: m1:m2 = 60 g: 0.48 g; The volumes of the mixed solution V1, tetrahydrofuran V2, and alkaline solution V3 satisfy the following: V1:V2:V3 = 90 mL:120 mL:40 mL.

[0131] The volume ratio of methanol to dichloromethane in the mixed solvent is 3:2.

[0132] The esterification reaction was carried out at a temperature of 230℃, a pressure of 30 kPa, and a time of 2 hours. The temperature for the thickening treatment was 240℃, the vacuum degree for the thickening treatment was 60Pa, and the thickening treatment time was 1h.

[0133] The pre-condensation temperature was 230℃, the pre-condensation vacuum degree was 10kPa, and the pre-condensation time was 3h. When the end carboxyl group content of the pre-polymerized ester is less than 30 mol / t, the vacuum degree of the esterification vessel is reduced to 1 kPa, and then the polymerization continues for 2 hours; when the end carboxyl group content of the pre-polymerized ester is less than 20 mol / t, the vacuum degree of the esterification vessel is reduced to 80 Pa, and then the polymerization continues for 3 hours.

[0134] The melting temperature of the extrusion section is 230℃.

[0135] The synchronous biaxial stretching includes a preheating section and a stretching section. The temperature of the preheating section is 100℃ and the preheating time is 90s. The biaxial stretching ratio of the stretching section is 3.5 times.

[0136] Comparative Example 1

[0137] Compared to Example 1, the differences in this comparative example are as follows, while the rest are the same: Without adding poly-4-carboxylic acid benzoisocyanate oligomer to the PBT film, the amounts of terephthalic acid n1 and 1,4-butanediol n3 satisfy the following: n1:n3=1:1.3; The preheating period lasts for 4 seconds.

[0138] Comparative Example 2

[0139] Compared to Example 1, the differences in this comparative example are as follows, while the rest are the same: In PBT films, isophthalic acid is used in place of poly-4-carboxybenzonitrile oligomers. The preheating period lasts for 15 seconds.

[0140] Comparative Example 3

[0141] Compared to Example 1, the differences in this comparative example are as follows, while the rest are the same: In PBT films, unpolymerized 4-ethoxycarbonylbenzoisocyanate monomers are used instead of poly-4-carboxybenzoisocyanate oligomers; The preheating period lasts for 4 seconds.

[0142] Comparative Example 4

[0143] Compared to Example 1, the differences in this comparative example are as follows, while the rest are the same: The amounts of terephthalic acid (n1), poly(4-carboxylic acid benzoisocyanate) oligomer (n2), and 1,4-butanediol (n3) satisfy the following ratio: n1:n2:n3 = 1:0.2:1.3.

[0144] Comparative Example 5

[0145] Compared to Example 1, the differences in this comparative example are as follows, while the rest are the same: The amounts of terephthalic acid (n1), poly(4-carboxylic acid benzoisocyanate) oligomer (n2), and 1,4-butanediol (n3) satisfy the following ratio: n1:n2:n3 = 1:0.01:1.3.

[0146] Relevant experimental and effect data: The copolymer PBT resins from the examples were collected, and their performance indicators were statistically analyzed. The results are shown in Table 1. Meanwhile, commercially available PET film (0.1 mm thickness) was used as a blank control group. PBT films from each example and comparative example were collected, and their performance indicators were statistically analyzed. The results are shown in Table 2.

[0147] Table 1 Performance indicators of the copolymer PBT resin in the examples

[0148] Table 2 Performance indicators of the films in various embodiments and comparative examples

[0149] As shown in Tables 1 and 2, the biaxially stretched polybutylene terephthalate film provided in this application embodiment has a molecular structure design of "carboxyl reaction site + helical conformation". This polybutylene terephthalate film can copolymerize helical poly-4-carboxybenzonitrile oligomer into the PBT chain. Through the synergistic effect of the helical structure, the biaxially stretched PBT film can obtain a high elongation at break (above 175%).

[0150] In addition, compared to Example 1, Comparative Example 1 did not use poly-4-carboxybenzonitrile oligomer, which resulted in a faster crystallization rate of the PBT material, insufficient stretching orientation, and easy generation of defects. The elongation at break dropped sharply to 65%, and the high crystallinity increased the haze and decreased the light transmittance, resulting in a comprehensive reduction in mechanical and electrical properties. In addition, Comparative Example 2 used isophthalic acid to replace poly-4-carboxybenzonitrile oligomer. Although the crystallization was partially delayed and film formation was normal, the lack of the unwinding and stretching effect of the helical segments of poly-4-carboxybenzonitrile oligomer resulted in an elongation at break (72%) that was significantly lower than that of Example 1. This indicates that simply disturbing the crystallization is not enough; the helical structure of poly-4-carboxybenzonitrile oligomer is the key. In addition, Comparative Example 3 used 4-carboxybenzonitrile monomer instead of poly-4-carboxybenzonitrile oligomer. Since the monomer can only play a capping role, its low molecular weight and insufficient melt strength result in brittle PBT film with many defects. Its tensile strength and elongation at break are the lowest. This shows that poly-4-carboxybenzonitrile oligomer must be used.

[0151] Compared to Example 1, the excessive amount of poly(4-carboxybenzonitrile) oligomer in Comparative Example 4 leads to branching / gelling of the PBT film, and stress concentration caused by crystal point defects, resulting in a significant deterioration in tensile strength (61 MPa) and elongation (86%). Furthermore, the insufficient amount of poly(4-carboxybenzonitrile) oligomer in Comparative Example 5 results in insufficient helical segments in the PBT film, a weak crystallization retardation effect, and negatively impacts the performance indicators of the PBT film.

[0152] In summary, the embodiments of this application provide a biaxially oriented polybutylene terephthalate (PBT) film. By introducing poly-4-carboxylic acid benzoisocyanate oligomers with carboxyl reaction sites and helical conformation, the biaxially oriented PBT film has a high elongation at break.

[0153] Furthermore, this application provides a biaxially oriented polybutylene terephthalate (PET) film, which also synthesizes a poly(4-carboxybenzonitrile) oligomer with a helical structure. This preparation method is simple, highly operable, and the degree of polymerization of the poly(4-carboxybenzonitrile) oligomer is controllable. Additionally, the prepared poly(4-carboxybenzonitrile) oligomer has a dendritic helical structure and multiple hydroxyl structures, and can partially replace terephthalic acid for the synthesis of PET or PBT copolyesters.

[0154] Furthermore, this application provides a method for preparing biaxially oriented polybutylene terephthalate (PBT) films. This method uses poly-4-carboxybenzonitrile oligomers for PBT copolyester synthesis, and can be carried out under existing PBT synthesis equipment and process conditions, making it feasible. The prepared PBT copolyester exhibits high viscosity, low end-carboxyl group content, and low THF residue. Additionally, this method lowers the melting point and reduces regularity, thereby slowing the crystallization rate of PBT. This allows for production using BOPET processing equipment, and the stretching time can be extended to 50-200 seconds, meeting the design requirements of current biaxially oriented production lines.

[0155] Furthermore, this application provides a method for preparing a biaxially oriented polybutylene terephthalate (PBT) film. This method introduces a helical structure of poly-4-carboxybenzonitrile oligomer into the PBT film. This helical structure has a certain deformability and can significantly improve the elongation at break of the PBT film, giving the PBT film good flexibility. It can be applied to waterproof membrane base films, building material base films, geomembranes, agricultural films, and packaging of irregularly shaped parts.

[0156] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A biaxially oriented polybutylene terephthalate film, characterized in that, The raw materials for the polybutylene terephthalate film include: terephthalic acid, poly-4-carboxybenzonitrile oligomer, and 1,4-butanediol. The molar amounts of terephthalic acid (n1), poly-4-carboxybenzonitrile oligomer (n2), and 1,4-butanediol (n3) satisfy the following ratio: n1:n2:n3 = 1:(0.03~0.12):(1.2~1.35). The poly-4-carboxybenzonitrile oligomer has the molecular structure shown in Formula 1. ， Formula 1.

2. The polybutylene terephthalate film according to claim 1, characterized in that, The degree of polymerization n of the poly-4-carboxylic acid benzoisocyanate oligomer is 10~34.

3. The polybutylene terephthalate film according to claim 1, characterized in that, The method for preparing the poly-4-carboxylic acid benzoisocyanate oligomer includes: 4-ethoxycarbonylbenzeneisonitrile and a mixed solvent are mixed to obtain an oligomer mixed raw material; Using nickel chloride hexahydrate as a catalyst, the oligomer mixture was subjected to a catalytic reaction to obtain crude poly-4-ethoxycarbonylphenylisocyanate oligomer. Tetrahydrofuran and the crude poly-4-ethoxycarbonylbenzeneisocyanate oligomer were mixed, and then an alkaline solution was added dropwise to hydrolyze the ethoxycarbonyl group in the crude poly-4-ethoxycarbonylbenzeneisocyanate oligomer to form a sodium salt structure, thus obtaining a hydrolyzed mixture. The hydrolyzed mixture was acid-washed with a dilute hydrochloric acid solution, followed by extraction with acetone to obtain poly-4-carboxylic acid benzoisocyanate oligomer.

4. The polybutylene terephthalate film according to claim 3, characterized in that, The temperature of the catalytic reaction is 30℃~85℃, and the time of the catalytic reaction is 1h~12h.

5. The polybutylene terephthalate film according to claim 3, characterized in that, The mass m1 of the 4-ethoxycarbonylbenzeneisocyanate and the mass m2 of the nickel chloride hexahydrate satisfy: m1:m2 = (10~60):(0.15~0.5); and / or The volume V1 of the mixed solution, the volume V2 of the tetrahydrofuran, and the volume V3 of the alkaline solution satisfy: V1:V2:V3 = (50~120):(60~150):(40~180).

6. A method for preparing a biaxially stretched polybutylene terephthalate film, characterized in that, The preparation method is used to prepare the polybutylene terephthalate film according to any one of claims 1 to 5, and the preparation method includes: Using antimony glycol as a catalyst, terephthalic acid, poly-4-carboxylic acid benzoisocyanate oligomer and 1,4-butanediol were subjected to esterification reaction to obtain a preliminary esterified product; The preliminary esterified compound was subjected to pre-condensation and condensation in sequence to obtain a low-viscosity polymer solution. The low-viscosity polymer solution was subjected to thickening treatment and pressurization treatment in sequence to obtain copolymer PBT resin; The copolymer PBT resin was sequentially extruded and simultaneously biaxially stretched to obtain a polybutylene terephthalate film.

7. The preparation method according to claim 6, characterized in that, The esterification reaction is carried out at a temperature of 225℃~235℃, at a pressure of 25kPa~35kPa, and for a time of 2h~5h; and / or The temperature of the thickening treatment is 235℃~245℃, the vacuum degree of the thickening treatment is 55Pa~65Pa, and the time of the thickening treatment is 0.5h~2h.

8. The preparation method according to claim 6, characterized in that, The pre-polymerization temperature is 225℃~235℃, the pre-polymerization vacuum degree is 8kPa~12kPa, and the pre-polymerization time is 1h~3h; and / or The vacuum degree of the polycondensation is 80 Pa to 1000 Pa, and the polycondensation time is 0.5 h to 3 h.

9. The preparation method according to claim 6, characterized in that, The synchronous biaxial stretching includes a preheating section and a stretching section. The temperature of the preheating section is 50℃~130℃, and the preheating time is 50s~200s. The biaxial stretching ratio of the stretching section is 2.5 times~3.5 times.

10. The preparation method according to claim 6, characterized in that, The intrinsic viscosity of the copolymerized PBT resin is 0.72 dl / g to 1.15 dl / g, the molar content of terminal carboxyl groups in the copolymerized PBT resin is 5 mol / t to 15 mol / t, and the residual tetrahydrofuran content in the copolymerized PBT resin is 20 ppm to 45 ppm; and / or The thickness of the polybutylene terephthalate film is 0.02 mm to 0.1 mm.

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