A high-strength and high-toughness PET foam, its raw material composition and preparation method

Abstract

The present invention provides a PET foam with high strength and high toughness, as well as its raw material composition and preparation method. The raw material composition includes: 100 parts of PET resin; 0.3 to 0.75 parts of a dispersed phase; less than 10 parts of a naphthalene hydroxycarboxylic acid-p-hydroxybenzoic acid copolyester; and less than 10 parts of a thermoplastic elastomer. The present invention also provides a high-strength and high-toughness PET foam prepared using the above raw material composition and its preparation method. The PET foam prepared by the present invention has good mechanical properties, especially high compressive strength, shear strength, and shear fracture elongation rate.

Description

Technical Field

[0001] The present invention relates to a PET foam with high strength and high toughness, its raw material composition and preparation method, belonging to the technical field of polymer material preparation. Background Art

[0002] Polyethylene terephthalate (PET) foam has excellent properties such as high-temperature dimensional stability, heat insulation, and chemical resistance. Currently, PET foam has been widely used in industries such as construction, aerospace, modern automobiles, ships, and food packaging. The use of PET foam helps to reduce the use of fossil resources and protect the environment. However, the development of engineering fields such as wind turbine blades and aerospace has put forward higher requirements for the mechanical properties of PET foam, such as foam compression resistance.

[0003] Generally, the mechanical properties of PET foam are improved by introducing inorganic fillers and regulating the cell structure. Although the method of introducing inorganic fillers can improve the mechanical properties of PET foam, excessive introduction of inorganic fillers will lead to the formation of open-cell foam during the PET foaming process; at the same time, the regulation of the cell structure of the foam is limited at the same density. Therefore, it is difficult for these methods to significantly improve the mechanical properties of PET foam.

[0004] Preparing polymer fiber composites by in-situ fibrillation technology in polymers is an effective method to improve the mechanical properties of polymers. The in-situ fibrillation process refers to the process of stretching a blend initially with a sea-island morphology in a molten (hot stretching) or frozen (cold stretching) state, and the spherical dispersed phase particles are deformed into a fiber structure by the shear and tensile forces acting on the blend. The fiber structure with a high specific surface area can, on the one hand, act as a crystallization nucleating agent to increase the crystallization rate of the polymer, and on the other hand, can act as a bubble nucleating agent to reduce the cell structure, and the fibers can enhance the stress transfer between the polymer matrix and the dispersed phase. Converting the sea-island morphology of the blend dispersed phase into micron or nanofibrous is a method that can significantly improve the rheological properties, crystallization properties, and mechanical properties of the polymer blend system.

[0005] Currently, combining the in-situ fiber technology with supercritical continuous extrusion foaming to directly prepare polymer fiber composite foams by one-step extrusion foaming is very difficult technically. Generally, polymer fiber composites are first prepared by in-situ fibrillation technology and made into masterbatches, and the fibers are introduced into the polymer foam through the masterbatches during the extrusion foaming process. However, this will lead to a decrease in fiber dispersion, weaken the crystallization nucleation, bubble nucleation, and stress transfer effects of the fibers, and greatly increase the production cost and reduce the production efficiency. Summary of the Invention

[0006] To solve the above technical problems, an object of the present invention is to provide a PET foam, its raw material composition and a preparation method, and the PET foam has the advantages of high strength and high toughness.

[0007] To achieve the above object, the present invention first provides a raw material composition for a PET foam with high strength and high toughness. Among them, by mass, the raw material composition includes:

[0008] PET resin 100 parts;

[0009] Dispersed phase 0.3 - 0.75 parts;

[0010] Naphthalene hydroxycarboxylic acid - p - hydroxybenzoic acid copolyester (PHBNA) less than 10 parts;

[0011] Thermoplastic elastomer less than 10 parts.

[0012] In the above raw material composition, preferably, the dispersed phase is selected from one or a combination of two or more of 2,2'-(1,3 - phenylene)-dioxazoline (PBO), pyromellitic dianhydride (PMDA), and triglycidyl isocyanurate (TGIC).

[0013] In the above raw material composition, preferably, the naphthalene hydroxycarboxylic acid - p - hydroxybenzoic acid copolyester is prepared by the following steps:

[0014] Step 1: Mix p - hydroxybenzoic acid and acetic anhydride in a molar ratio range of 1:1 to 1:6 (more preferably 1:2), add a catalyst, then raise the temperature and react in a nitrogen environment; after the reaction is completed, crystallize out, wash and dry to obtain a first product;

[0015] Step 2: Mix 6 - hydroxy - 2 - naphthoic acid and acetic anhydride in a molar ratio range of 1:1 to 1:6 (more preferably 1:2), add a catalyst, then raise the temperature and react in a nitrogen environment; after the reaction is completed, crystallize out, wash and dry to obtain a second product;

[0016] Step 3: Mix the first product and the second product in a weight ratio range of 60:40 to 80:20 (more preferably 73:27), add a catalyst, purge with nitrogen, raise the temperature for reaction; after the reaction is completed, cool to obtain the naphthalene hydroxycarboxylic acid - p - hydroxybenzoic acid copolyester.

[0017] In the above raw material composition, preferably, in the process of preparing the naphthalene hydroxycarboxylic acid-p-hydroxybenzoic acid copolyester, the first step includes: adding p-hydroxybenzoic acid and acetic anhydride into a reaction vessel in a molar ratio range of 1:1 to 1:6 (more preferably 1:2) for mixing, and simultaneously adding 200 - 400 ppm of a zinc-based catalyst (which can be a commonly used catalyst in the art, such as: zinc-chromium oxide, zinc-aluminum oxide, zinc-aluminum-chromium oxide, zinc-chromium-manganese oxide, zinc-iron oxide, zinc-iron-aluminum oxide, etc.; the same applies hereinafter); heating to a temperature range of 100°C to 160°C (more preferably 140°C) to dissolve p-hydroxybenzoic acid and acetic anhydride into a colorless transparent solution, continuously introducing nitrogen, gradually reacting p-hydroxybenzoic acid with acetic anhydride to form an esterified product, and after allowing acetic anhydride to condense and reflux for an appropriate time, stopping the reaction; pouring the reactant into water, washing and drying the precipitated white crystalline substance to obtain a first product;

[0018] The second step includes: adding 6-hydroxy-2-naphthoic acid and acetic anhydride into a reaction vessel in a molar ratio range of 1:1 to 1:6 (more preferably 1:2) for mixing, and simultaneously adding 200 - 400 ppm of a zinc-based catalyst; heating to a temperature range of 100°C to 160°C (more preferably 140°C) to dissolve p-hydroxybenzoic acid and acetic anhydride into a colorless transparent solution, continuously introducing nitrogen, gradually reacting p-hydroxybenzoic acid with acetic anhydride to form an esterified product, and after allowing acetic anhydride to condense and reflux for an appropriate time, stopping the reaction; pouring the reactant into water, washing and drying the precipitated white crystalline substance to obtain a second product;

[0019] The third step includes: adding the first product and the second product into a reaction vessel in a weight ratio range of 60:40 to 80:20 (more preferably 73:27) for mixing, adding 200 - 400 ppm of a zinc-based catalyst, and purging with nitrogen; placing the reaction vessel in a salt bath, reacting at 247 - 255°C and then heating to 265 - 280°C, evacuating to a vacuum degree of -0.08 MPa while simultaneously heating to 280 - 300°C and maintaining for 1 - 1.5 h, stopping the reaction, and cooling to 100°C to obtain the naphthalene hydroxycarboxylic acid-p-hydroxybenzoic acid copolyester.

[0020] In the above raw material composition, preferably, the thermoplastic elastomer is selected from one or a combination of two or more of TPU, TPA, and POE.

[0021] In the above raw material composition, preferably, in terms of parts by mass, the raw material composition comprises:

[0022] PET resin 100 parts;

[0023] Dispersed phase 0.3 - 0.75 parts;

[0024] 5 - 10 parts of naphthohydroxycarboxylic acid - p - hydroxybenzoic acid copolyester;

[0025] 5 - 10 parts of thermoplastic elastomer.

[0026] In the above raw material composition, preferably, the raw material composition further comprises a foaming agent, and the foaming agent is preferably one or a combination of two or more of an inert foaming agent (such as N2, CO2), a hydrocarbon foaming agent (cyclopentane, butane, etc.), and a hydrofluorocarbon foaming agent (HFCs). The dosage of the foaming agent can be controlled to be 8% - 15%.

[0027] The present invention also provides a method for preparing a high - strength and high - toughness PET foam, wherein the preparation method is prepared from the above - mentioned raw material composition of the high - strength and high - toughness PET foam;

[0028] The preparation method includes the following steps:

[0029] Adding PET resin, a dispersed phase, naphthohydroxycarboxylic acid - p - hydroxybenzoic acid copolyester, and a thermoplastic elastomer into a co - rotating parallel twin - screw extruder (the first stage of a two - stage continuous extrusion foaming unit) for melt blending, and then injecting supercritical CO2 and cyclopentane into the co - rotating parallel twin - screw extruder in a ratio range of 1:1 to 5:4 (more preferably 3:2) to obtain a blend melt;

[0030] The blend melt enters a single - screw extruder (the second stage of the two - stage continuous extrusion foaming unit) through a transition section, is cooled, and then undergoes extrusion foaming through a die, and is quickly cooled and shaped by a shaping device to obtain a high - strength and high - toughness PET foam.

[0031] In the above preparation method, the co - rotating parallel twin - screw extruder and the single - screw extruder used are two parts of a two - stage continuous extrusion foaming unit, which are connected by a transition section in the middle. More preferably, the rotational speed of the twin - screws in the first - stage co - rotating parallel twin - screw extruder is 200 - 250 r / min, and the barrel set temperature is 280 - 300 °C.

[0032] In the above preparation method, preferably, the blend melt is cooled to 245 - 265 °C in the single - screw extruder, and the extrusion pressure is 6 - 8 MPa.

[0033] In the above preparation method, preferably, the eccentric rotor (i.e., the stretching element) in the co - rotating parallel twin - screw extruder has a left - right symmetric structure;

[0034] Among them, the surface of the stretching element is provided with two sections of stretching threads (large - lead threads), namely, a left - hand thread and a right - hand thread, and the lead of both the left - hand thread and the right - hand thread is 1 a~4 a ; The axial lengths (L1, L2) of the left-handed thread and the right-handed thread of the stretching element are both 0.5 a ~2 a . In the same stretching element, the lead of the left-handed thread and the right-handed thread is the same.

[0035] The projection of the stretching thread along the axis of the stretching element is circular, and the diameter of the circle is a ;

[0036] The outer contour curves of the stretching thread in all cross-sections perpendicular to the axis are the same. The contour curve consists of eight arcs, including the first arc, the second arc, the third arc, the fourth arc, the fifth arc, the sixth arc, the seventh arc, and the eighth arc connected in sequence; among them, the radius of the first arc is 0.5 a , and the corresponding central angle of the radian is 10° - 20°, the radius of the second arc is 0.1 a ~0.2 a , and the corresponding central angle of the radian is 55° - 70°; the radius of the third arc is 0.3 a ~0.5 a , and the corresponding central angle of the radian is 55° - 70°; the third arc is tangent to the second arc; the radius of the fourth arc is 0.5 a ~0.7 a , and the corresponding central angle of the radian is 70° - 85°; the fourth arc is tangent to the third arc; the fifth arc is centrosymmetric with the first arc, the sixth arc is centrosymmetric with the second arc, the seventh arc is centrosymmetric with the third arc, and the eighth arc is centrosymmetric with the fourth arc. The fourth arc has the greatest influence on obtaining fibers. At the same time, the leads of the left-handed thread and the right-handed thread also have an important influence on obtaining fibers, which determines the length of the stretching effect generated by the stretching element in the barrel.

[0037] In the present invention, by designing a special stretching element, a tensile stress is applied to the dispersed phase in PET on a co-rotating parallel twin-screw extruder, so that the dispersed phase is stretched and thinned into a fibrous shape. At the same time, a plurality of converging flow channels are designed in the extrusion die (that is, the cross-sectional area of the flow through which PET flows forward in the converging flow channel gradually decreases) to control that PET / dispersed phase is further subjected to a stretching flow field during the extrusion foaming process, thereby increasing the aspect ratio of the dispersed phase fibers, so as to uniformly disperse the high specific surface area fiber dispersed phase in the PET foam, and finally prepare a PET / fiber composite foam with ultra-high strength and toughness.

[0038] The present invention also provides a PET foam with high strength and high toughness, which is prepared by the above method;

[0039] Preferably, the high-strength and high-toughness PET foam meets one or more combinations of the following properties:

[0040] The foam compression strength is 1.65 - 1.85 MPa;

[0041] The foam shear strength is 0.75 - 0.95 MPa;

[0042] The foam shear fracture elongation is 10% - 20%.

[0043] During the processes of melt blending, extrusion, and foaming, in the present invention, a stretching force is applied to the dispersed phase through an eccentric rotor (stretching element) and a converging flow channel in the mold to form a long fiber structure of the dispersed phase, increasing the bonding force between the dispersed phase and PET, thereby preparing a high-strength and high-toughness PET fiber composite foam.

[0044] The PET foam prepared by the present invention has good mechanical properties, especially high compression strength, shear strength, and shear fracture elongation. Description of the Drawings

[0045] Figure 1 It is an external view of the stretching element.

[0046] Figure 2 It is a diagram of relevant parameters of the cross-section of the stretching element.

[0047] Figure 3 It is a diagram of relevant parameters in the length direction of the stretching element.

[0048] Figure 4 It is a schematic diagram of a two-stage continuous extrusion foaming unit.

[0049] Figure 5 It is a scanning diagram of the foam morphology of Examples 1 - 2 and Comparative Example 1.

[0050] Figure 6 It is a morphology diagram of the quenched cross-section of the composite of Examples 5 - 6 and Comparative Example 3.

[0051] Description of the Main Reference Numerals:

[0052] Twin-screw extruder 1, injector 2, stretching element 3, transition section 4, single-screw extruder 5, long converging flow channel in the mold 6, short converging flow channel of the die lip 7, cooling equipment 8, traction equipment 9. Detailed Embodiments

[0053] In order to have a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the technical solutions of the present invention are described in detail below, but it should not be construed as a limitation on the implementable scope of the present invention.

[0054] The raw materials used in the examples and comparative examples are as follows:

[0055] PET, bottle chips, intrinsic viscosity 0.8 dL / g; PBO; TGIC; PHBNA; TPU; POE.

[0056] Among them, PHBNA is synthesized from 6-hydroxy-2-naphthoic acid (HNA) and p-hydroxybenzoic acid (HBA). The specific synthesis process is as follows:

[0057] Add p-hydroxybenzoic acid and acetic anhydride into a three-necked flask at a molar ratio of 1:2, and at the same time add a certain amount of zinc catalyst (400 ppm); when the temperature reaches 140 °C, the solid powder gradually dissolves into a colorless transparent solution. Continuously introduce nitrogen, and p-hydroxybenzoic acid and acetic anhydride gradually react to form an ester compound, and let acetic anhydride condense and reflux for a certain time, then stop the reaction;

[0058] Pour the reactants into a beaker containing deionized water, and white crystals will precipitate. Wash the white crystalline substance and dry it for 24 h to obtain the first product, and take it out for later use. The chemical reaction that occurs is as follows:

[0059]

[0060] Take 6-hydroxy-2-naphthoic acid and react it with acetic anhydride separately. The process is the same as above to obtain the second product. The chemical reaction that occurs is as follows:

[0061]

[0062] Co-melt and polymerize the above first product and second product: Weigh a certain weight, place the first product and the second product in a three-necked flask according to a mass ratio of 73:27, and add a certain proportion of zinc catalyst (400 ppm based on the total weight of the monomers); purge with nitrogen to remove the air in the system; then place the reaction flask in a salt bath, react at 250 °C and then raise the temperature to 280 °C, raise the temperature synchronously while evacuating, and keep it for 1 h; stop the reaction, wait until it cools to 100 °C, and then take out the product PHBNA. The chemical reaction that occurs is as follows:

[0063]

[0064] The preparation process of the foams in Examples 1-4 and Comparative Examples 1-2 is carried out using a two-stage continuous extrusion foaming unit (as Figure 4 shown). This two-stage continuous extrusion foaming unit consists of a twin-screw extruder 1 and a single-screw extruder 5, which are connected by a transition section 4. Among them, the twin-screw extruder 1 is connected with an air injector 2 and is internally provided with a stretching element 3. At the outlet of the single-screw extruder 5, there are successively connected an in-mold long converging flow channel 6, an in-mold short converging flow channel 7, a cooling device 8, and a traction device 9;

[0065] The preparation process of the foams in Examples 1-4 and Comparative Examples 1-2 includes:

[0066] PET and PHBNA were dried in a vacuum oven at 160 °C for 6 h, and PBO, TGIC, TPU, and POE were dried in a forced-air oven at 60 °C for 6 h. The dried PET, PBO, TGIC, PHBNA, TPU, and POE were added to the twin-screw extruder 1 of the extrusion foaming unit for melt blending. After the polymers were fully melt blended, a supercritical CO2-cyclopentane mixture was injected into the twin-screw extruder 1 at a ratio of 3:2. Among them, the rotation speed of the twin-screw extruder 1 was 200-250 r / min, and the melt blending temperature was 280-300 °C;

[0067] The blend melt was fed into a single-screw extruder 5, and the temperature was lowered to 245-265 °C in the single-screw extruder 5, and the extrusion pressure was maintained at 6-8 MPa;

[0068] The blend melt was extruded and foamed after passing through a secondary converging die at high speed, and the foam was quickly cooled and shaped.

[0069] Among them, a stretching element was used in the processing of Examples 1-2 and Comparative Example 1, and a conventional screw element was used in Examples 3-4 and Comparative Example 2.

[0070] The structure of the stretching element used in Examples 1-2 and Comparative Example 1 is as Figure 1 、 Figure 2 、 Figure 3 shown, and the stretching element has a left-right symmetric structure;

[0071] Among them, two stretching threads (large lead threads), namely a left-handed thread and a right-handed thread, are provided on the surface of the stretching element; the lead of the left-handed thread and the right-handed thread of the stretching element is 3 a ; the axial length of the stretching element is 3 a , that is, the axial length L1 of the left-handed thread and the axial length L2 of the right-handed thread are 1.5a respectively;

[0072] The projection of the stretching thread along the axis direction of the stretching element is circular, and the diameter of the circle is a = 32 mm;

[0073] The cross-sectional contour curve of the stretching thread is the same at any position along the axis. The contour curve includes a first arc, a second arc, a third arc, a fourth arc, a fifth arc, a sixth arc, a seventh arc, and an eighth arc connected in sequence. Among them, the radius of the first arc is 0.5 a (16 mm), and the corresponding central angle of the radian is 20°; the radius of the second arc is 0.155 a(5 mm), and the central angle corresponding to the arc is 65°; the radius of the third arc is 0.4 a (12.8 mm), and the central angle corresponding to the arc is 65°; the radius of the fourth arc is 0.55 a (17.6 mm), and the central angle corresponding to the arc is 82°. The fifth arc is centrosymmetric with the first arc, the sixth arc is centrosymmetric with the second arc, the seventh arc is centrosymmetric with the third arc, and the eighth arc is centrosymmetric with the fourth arc; among them, as Figure 2 shown, the first arc is the arc between two points A and B, the second arc is the arc between two points B and C, the third arc is the arc between two points C and D, the fourth arc is the arc between two points D and E, the fifth arc is the arc between two points E and F, the sixth arc is the arc between two points F and G, the seventh arc is the arc between two points G and H, and the eighth arc is the arc between two points H and A.

[0074] The contents (parts by mass) of the components in Examples 1-4 and Comparative Examples 1-2 are shown in Table 1.

[0075] Table 1:

[0076]

[0077] The foaming ratio of the PET foams in Examples 1-4 and Comparative Examples 1-2 is 10 times.

[0078] The compression performance tests of Examples 1-4 and Comparative Examples 1-2 were carried out with reference to the standard IOS 844. The foam compression strengths of Comparative Example 1, Example 1 and Example 2 are 1.77 MPa, 2.24 MPa and 2.45 MPa respectively; the foam compression strengths of Comparative Example 2, Example 3 and Example 4 are 0.79 MPa, 1.26 MPa and 1.33 MPa respectively; the foam shear strengths of Comparative Example 1, Example 1 and Example 2 are 0.37 MPa, 1.18 MPa and 1.36 MPa respectively; the foam shear strengths of Comparative Example 2, Example 3 and Example 4 are 0.07 MPa, 0.35 MPa and 0.66 MPa respectively; the foam shear fracture elongation rates of Comparative Example 1, Example 1 and Example 2 are 5.75%, 10.19% and 12.00% respectively; the foam shear fracture elongation rates of Comparative Example 2, Example 3 and Example 4 are 3.47%, 7.04% and 9.12% respectively.

[0079] It can be seen from the above test results that: by comparing Example 1 with Comparative Example 1, when the PHBNA addition amount is 5 parts, the compression strength of the PET foam can be increased by 26%, the shear strength can be increased by 2.18 times, and the shear fracture elongation rate can be increased by 77%;

[0080] By comparing Example 2 with Comparative Example 1, when the addition amount of PHBNA is 10 parts, the compressive strength of the PET foam can be increased by 38%, the shear strength can be increased by 2.67 times, and the shear fracture elongation can be increased by 108%.

[0081] By comparing Examples 3 and 4 with Comparative Example 2, the toughness of the foam is significantly improved.

[0082] The foam morphologies of Examples 1-2 and Comparative Example 1 were observed by scanning electron microscopy respectively, and the results are as Figure 5 shown. According to Figure 5 the cell morphology, cell size, cell density, etc. in

[0083] it can be seen that PHBNA has no obvious effect on the foaming behavior of PET. Therefore, it is judged that the main influencing factor for improving the mechanical properties of the foam is the fiber. Figure 6 In order to better observe the fiber and judge the role of the fiber, the PET foams in Examples 1-2 and Comparative Example 1 were crushed and then molded by pressing to eliminate the cells (the processing temperature for molding by pressing was 265 °C and the pressure was 5 MPa) to prepare Examples 5-6 and Comparative Example 3, as shown in Table 2 specifically. The quenched cross-section morphologies of the composites of Examples 5-6 and Comparative Example 3 were observed by scanning electron microscopy respectively, as Figure 6 shown. It can be seen from

[0084] Table 2:

[0085]

[0086] The PET composites in Examples 5-6 and Comparative Example 3 were injection molded into specimens by an injection molding machine and then subjected to mechanical property tests. The standard for the tensile property test was referred to ISO 527, and the test results are shown in Table 3.

[0087] Table 3:

[0088]

[0089] According to the test results shown in Table 3, it can be seen that: by comparing Example 6 with Comparative Example 3, when the addition amount of PHBNA is 10 parts, the tensile strength of the PET composite can be increased by 17%. Therefore, by applying tension to the dispersed phase through an eccentric rotor and a secondary convergent die to form a long fiber structure of the dispersed phase, a high-strength PET fiber composite foam can be prepared.

Claims

1. A method for preparing high-strength and high-toughness PET foam, characterized in that: The raw material composition of the high-strength and high-toughness PET foam comprises, by parts by mass: PET resin 100 parts; 0.3-0.75 parts of one or a combination of two or more selected from 2,2'-(1,3-phenylene)-bisoxazoline, pyromellitic anhydride and triglycidyl isocyanurate; 10 parts or less of naphthyl hydroxycarboxylic acid-parahydroxybenzoic acid copolyester; the naphthyl hydroxycarboxylic acid is 6-hydroxy-2-naphthoic acid; Less than 10 parts of thermoplastic elastomer; The preparation method comprises the following steps: PET resin, one or more selected from 2,2'-(1,3-phenylene)-dioxazoline, pyromellitic anhydride and triglycidyl isocyanurate, naphthylhydroxycarboxylic acid-parahydroxybenzoic acid copolyester, and thermoplastic elastomer are added into an intermeshing co-rotating parallel twin-screw extruder for melt blending, and then supercritical CO2 and cyclopentane are injected into the intermeshing co-rotating parallel twin-screw extruder in a ratio of 1:1 to 5:4 to obtain a blend melt; The blend melt enters a single-screw extruder, cools down, and passes through a die at high speed for extrusion foaming, and the foam is quickly cooled and shaped to obtain a high-strength and high-toughness PET foam; The stretching element in the meshing parallel twin-screw extruder is a bilaterally symmetrical structure; The surface of the stretching element is provided with two stretching threads, namely a left-hand thread and a right-hand thread, and the lead of the left-hand thread and the right-hand thread are both 1 a ~4 a The axial length of the left-handed thread and the right-handed thread of the tensile element is 0.5 a ~2 a ; The projection of the stretching thread along the axis direction of the stretching element is a circle, and the diameter of the circle is a ; The outer contour curves of the stretched thread in all cross sections perpendicular to the axis are the same, and the contour curves are composed of eight arcs, including a first arc, a second arc, a third arc, a fourth arc, a fifth arc, a sixth arc, a seventh arc and an eighth arc connected in sequence; wherein the radius of the first arc is 0.5 a , and the arc corresponds to a central angle of 10°~20°, and the radius of the second arc is 0.1 a ~0.2 a , and the arc corresponds to a central angle of 55°~70°; the radius of the third arc is 0.3 a ~0.5 a , and the arc corresponds to a central angle of 55°~70°; the third arc is tangent to the second arc; the radius of the fourth arc is 0.5 a ~0.7 a , and the central angle corresponding to the arc is 70°~85°; the fourth arc is tangent to the third arc; the fifth arc is centrally symmetrical with the first arc, the sixth arc is centrally symmetrical with the second arc, the seventh arc is centrally symmetrical with the third arc, and the eighth arc is centrally symmetrical with the fourth arc.

2. The method for preparing high-strength and high-toughness PET foam according to claim 1, characterized in that: The naphthyl hydroxycarboxylic acid-parahydroxybenzoic acid copolyester is prepared by the following steps: Step 1: mixing p-hydroxybenzoic acid and acetic anhydride in a molar ratio of 1:1-1:6, adding a catalyst, then heating, and reacting under a nitrogen environment; after the reaction is completed, precipitating crystals, washing, and drying to obtain a first product; Step 2: 6-hydroxy-2-naphthoic acid and acetic anhydride are mixed in a molar ratio of 1:1-1:6, a catalyst is added, and then the temperature is increased to react under a nitrogen environment; after the reaction is completed, crystals are precipitated, and the second product is obtained after washing and drying; Step 3: Mix the first product and the second product in a weight ratio of 60:40 to 80:20, add a catalyst, introduce nitrogen for purging, and heat to react; after the reaction is completed, cool to obtain the naphthylhydroxycarboxylic acid-parahydroxybenzoic acid copolyester.

3. The method for preparing high-strength and high-toughness PET foam according to claim 2, characterized in that: The step 1 comprises: adding p-hydroxybenzoic acid and acetic anhydride in a molar ratio of 1:1-1:6 into a reaction container for mixing, and adding 200-400 ppm of a zinc catalyst at the same time; raising the temperature to 140° C. to dissolve the p-hydroxybenzoic acid and acetic anhydride into a colorless transparent solution, continuously introducing nitrogen gas to gradually react the p-hydroxybenzoic acid and acetic anhydride to form an ester, and stopping the reaction after condensing and refluxing the acetic anhydride for a suitable time; pouring the reactants into water, washing and drying the precipitated white crystals, and obtaining a first product; The step 2 comprises: adding 6-hydroxy-2-naphthoic acid and acetic anhydride in a molar ratio of 1:1-1:6 into a reaction container for mixing, and adding 200-400 ppm of a zinc catalyst at the same time; raising the temperature to 140° C. to dissolve p-hydroxybenzoic acid and acetic anhydride into a colorless transparent solution, continuously introducing nitrogen gas to gradually react p-hydroxybenzoic acid and acetic anhydride to form an ester, and condensing and refluxing the acetic anhydride for a suitable time before stopping the reaction; pouring the reactants into water, washing and drying the precipitated white crystals to obtain a second product; The step three comprises: adding the first product and the second product into a reaction container at a weight ratio of 60:40 to 80:20 for mixing, adding 200-400 ppm of a zinc catalyst, and passing nitrogen for purging; placing the reaction container in a salt bath, reacting at 247-255° C., heating up to 280° C., evacuating to a vacuum degree of -0.08 MPa, and simultaneously heating up and maintaining for 1-1.5 hours, stopping the reaction, and cooling to 100° C. to obtain the naphthylhydroxycarboxylic acid-parahydroxybenzoic acid copolyester.

4. The method for preparing high-strength and high-toughness PET foam according to claim 1, characterized in that: The thermoplastic elastomer is selected from one or a combination of two or more of TPU, TPA and POE.

5. The method for preparing high-strength and high-toughness PET foam according to claim 1, characterized in that: The raw material composition of the high-strength and high-toughness PET foam comprises, by parts by mass: PET resin 100 parts; 0.3-0.75 parts of one or a combination of two or more selected from 2,2'-(1,3-phenylene)-bisoxazoline, pyromellitic anhydride and triglycidyl isocyanurate; 5-10 parts of naphthylhydroxycarboxylic acid-parahydroxybenzoic acid copolyester; Thermoplastic elastomer 5~10 parts.

6. The method for preparing high-strength and high-toughness PET foam according to claim 1, characterized in that: The twin screws in the meshing co-rotating parallel twin screw extruder have a rotation speed of 200-250 r / min and a temperature of 280-300° C.; And / or, the blend melt is cooled to 245-265° C. in the single screw extruder, and the extrusion pressure is 6-8 MPa.

7. A high-strength and high-toughness PET foam, characterized in that: The high-strength and high-toughness PET foam is prepared by the preparation method of the high-strength and high-toughness PET foam according to any one of claims 1 to 6; The high-strength and high-toughness PET foam meets one or a combination of two or more of the following properties: The foam compression strength is 1.65~1.85MPa; Foam shear strength is 0.75~0.95MPa; The foam shear elongation at break is 10%~20%.

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