Modified PET foamed sheet and preparation method thereof

Abstract

This invention discloses a modified PET foam board and its preparation method, belonging to the technical field of PET foam board. The PET foam board comprises the following components: 92-98% polyethylene terephthalate resin, 0.3-3.5% synergistic chain extender, 0.1-3.0% nucleating agent, and 0.1-1.5% hydrolysis stabilizer. The preparation method of the PET foam board includes the following steps: S1, raw material pretreatment; S2, reactive extrusion; S3, physical foaming; S4, cooling and shaping. This invention uses a synergistic chain extender system composed of pyromellitic dianhydride and an epoxy-containing polymer chain extender. The rapid reaction of pyromellitic dianhydride quickly establishes the backbone between PET molecular chains, improving low-shear viscosity. The epoxy-containing polymer chain extender reacts more slowly and is used to repair end defects, improving the tensile viscosity of the PET foam board. The long branched structure produced by combining pyromellitic dianhydride and a polymer chain extender containing epoxy groups in a certain ratio ensures the closed-cell rate of PET foam board.

Description

Technical Field

[0001] This invention relates to the field of PET foam board technology, and in particular to a modified PET foam board and its preparation method. Background Technology

[0002] In the field of modern polymer materials science and lightweight engineering, PET (Polyethylene Terephthalate) foam materials are rapidly becoming the preferred structural core material for wind turbine blades, aerospace, rail transportation and high-end building energy conservation due to their excellent thermodynamic stability, superior mechanical strength, outstanding fatigue resistance and highly mature recyclability worldwide.

[0003] Current PET resins are essentially low-molecular-weight linear semi-crystalline polymers with extremely low physical entanglement density between their macromolecular chain segments and a slow crystallization rate. This linear molecular topology causes a sharp drop in PET viscosity when the temperature reaches above the melting point, resulting in extremely low zero-shear viscosity and negligible melt elasticity. Linear PET completely lacks strain hardening effect when subjected to tensile flow fields.

[0004] During supercritical fluid extrusion foaming, the melt strength is too low, making it unable to effectively encapsulate the expanding gas generated by phase separation. As bubbles grow in a biaxial stretching manner, the fragile pore walls are prone to rupture, inevitably leading to gas escape, large-scale bubble merging (pore breakage), and even the complete collapse of the entire foam structure. This makes it difficult to produce lightweight sheets with high foaming ratios or low densities using ordinary PET, and the resulting cell structure is often extremely uneven. Summary of the Invention

[0005] This invention provides a modified PET foam board and its preparation method, which solves the defects of low foaming ratio and low bubble closed-cell rate after melt foaming in the prior art of PET foam board.

[0006] On one hand, the present invention provides a modified PET foam board, comprising the following components: 92-98% polyethylene terephthalate resin, 0.3-3.5% synergistic chain extender, 0.1-3.0% nucleating agent, and 0.1-1.5% hydrolysis stabilizer; wherein the synergistic chain extender comprises pyromellitic dianhydride and a polymer chain extender containing epoxy groups, the nucleating agent comprises at least one of talc, nano-montmorillonite, or silica, and the hydrolysis stabilizer comprises carbodiimide.

[0007] Further, in the synergistic chain extender, the mass ratio of pyromellitic dianhydride to the epoxy-containing polymer chain extender is 1:2~5, and the epoxy-containing polymer chain extender includes a styrene-acrylic acid-glycidyl methacrylate copolymer, and its epoxy equivalent is 250~350. .

[0008] On the other hand, the present invention provides a method for preparing a modified PET foam board, comprising the following preparation steps: raw material pretreatment: drying polyethylene terephthalate resin and premixing it with a synergistic chain extender, a nucleating agent, and an anti-hydrolysis stabilizer to obtain a premix; reactive extrusion: adding the premix to a twin-screw extruder for melting and reactive extrusion to cause in-situ grafting of the polyethylene terephthalate resin, followed by devolatilization treatment using a vacuum exhaust device to obtain a melt; physical foaming: injecting a physical foaming agent into the molten melt in the extruder, and forming a homogeneous solution through shear mixing; cooling and shaping: conveying the homogeneous solution to a cooling section for cooling, followed by extrusion through a die to induce a sudden pressure drop, and obtaining the modified PET foam board through nucleation growth and cooling shaping.

[0009] Furthermore, the intrinsic viscosity of the polyethylene terephthalate resin is 0.7~0.85 dL / g, and it is dried to a moisture content of less than 0.005%.

[0010] Furthermore, the length-to-diameter ratio of the twin-screw extruder is not less than 40:1.

[0011] Furthermore, the temperature of the reactive extrusion is 260~290℃.

[0012] Furthermore, a vacuum of -0.08 to -0.1 MPa is maintained inside the extruder barrel by a vacuum exhaust device.

[0013] Furthermore, the injected physical foaming agent includes supercritical carbon dioxide or supercritical nitrogen, and its injection amount is 0.5 to 5% of the total weight of the melt.

[0014] Furthermore, after the homogeneous solution is transported to the cooling section, the temperature of the melt is reduced to 220~240℃.

[0015] The modified PET foam board and its preparation method provided by this invention utilize a synergistic chain extension system composed of pyromellitic dianhydride and an epoxy-containing polymer chain extender. Pyromellitic dianhydride is used to establish a four-arm star-shaped framework between PET molecular chains, thereby improving low-shear viscosity. The epoxy-containing polymer chain extender reacts more slowly and is used to repair end defects, increasing the tensile viscosity of the PET foam board. The long-branched structure produced by the combination of pyromellitic dianhydride and the epoxy-containing polymer chain extender in a certain ratio allows the strength of the PET melt to gradually increase during bubble expansion and bubble wall stretching, ensuring the closed-cell rate of the PET foam board.

[0016] Before extrusion, the PET melt is heated to a low viscosity and high plasticity state to facilitate material mixing and transportation. Then it is cooled to near the crystallization temperature, and the long branches are used as self-nucleation points to accelerate the crystallization of PET. These microcrystals act as physical cross-linking points in the melt, causing local gelation of the melt and giving it high initial strength before extrusion. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 This is a schematic flowchart of a method for preparing a modified PET foam board according to the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0020] As mentioned earlier, current PET resins are essentially low-molecular-weight linear semi-crystalline polymers with extremely low physical entanglement density between macromolecular chain segments and a slow crystallization rate. This linear molecular topology causes a sharp drop in PET viscosity when the temperature reaches above the melting point, resulting in extremely low zero-shear viscosity and negligible melt elasticity. Linear PET completely lacks strain hardening effect when subjected to tensile flow fields. During supercritical fluid extrusion foaming, due to the low melt strength, the melt cannot effectively encapsulate the expanding gas generated by phase separation. During biaxial stretching growth, the fragile pore walls are prone to rupture, inevitably leading to gas escape, large-area bubble merging (pore breakage), and even the complete collapse of the entire foam structure. This makes it difficult to prepare lightweight sheets with high foaming ratios or low densities using ordinary PET, and the final cell structure is often extremely uneven.

[0021] To address this issue, the present invention provides a modified PET foam board and its preparation method. This modified board utilizes a synergistic chain-extending system composed of pyromellitic dianhydride and an epoxy-containing polymer chain extender. The rapid reaction of pyromellitic dianhydride quickly establishes the backbone between PET molecular chains, improving low-shear viscosity. The epoxy-containing polymer chain extender, with a slower reaction, is used to repair end defects and improve the tensile viscosity of the PET foam board. The combination of pyromellitic dianhydride and the epoxy-containing polymer chain extender in a specific ratio produces a long-branched structure. This allows the strength of the PET melt to gradually increase during bubble expansion and bubble wall stretching, rather than directly thinning to the point of rupture, thus avoiding holes caused by low strength during melt extrusion.

[0022] Before extrusion, the PET melt is heated to a low viscosity and high plasticity state to facilitate material mixing and transportation. Then it is cooled to near the crystallization temperature, and the long branches are used as self-nucleation points to accelerate the crystallization of PET. These microcrystals act as physical cross-linking points in the melt, causing local gelation of the melt and giving it high initial strength before extrusion.

[0023] It comprises the following components by weight percentage: 92-98% polyethylene terephthalate resin, 0.3-3.5% synergistic chain extender (pyromellitic dianhydride and epoxy group-containing polymer chain extender), 0.1-3% nucleating agent and 0.1-1.5% hydrolysis stabilizer; The nucleating agent includes at least one of talc, nano-montmorillonite, or silica, and the hydrolysis-resistant stabilizer includes carbodiimide. The mass ratio of pyromellitic dianhydride to the epoxy-containing polymer chain extender in the synergistic chain extender is 1:2~5; the epoxy-containing polymer chain extender includes a styrene-acrylic acid-glycidyl methacrylate copolymer with an epoxy equivalent of 250~350. .

[0024] Based on the same general inventive concept, this invention also protects a method for preparing a modified PET foam board. For example... Figure 1 As shown, it may include the following steps: S1. Raw material pretreatment; S2, reactive extrusion; S3, physical foaming; S4. Cooling and shaping.

[0025] The above steps may include: S1. Before feeding various materials into the extruder, the PET matrix resin is deeply dried using a dehumidifying dryer. Drying hot air with a dew point as low as -40°C is used to thoroughly remove residual bound water and surface adsorbed water, ensuring the intrinsic viscosity of the polyethylene terephthalate resin is 0.7~0.85 dL / g and the water content is below the 0.005% threshold. The dried high-temperature PET resin is then thoroughly premixed in a sealed, moisture-proof fluidized bed or mixer with a pre-defined ratio of synergistic chain extender, nucleating agent, and anti-hydrolysis stabilizer to ensure uniform dispersion of powder and particles and prevent sudden gelation at the extruder feed port due to abnormal local component concentrations. A premix is ​​obtained. S2. The premixed material obtained in S1 is added to a twin-screw extruder with a length-to-diameter ratio of not less than 40:1 and melted at a temperature of 260~290℃. Reactive extrusion is then performed to cause in-situ grafting reaction of polyethylene terephthalate resin. The molten resin is then devolatilized by maintaining a vacuum of -0.08~-0.1MPa in the extruder barrel using a vacuum exhaust device to obtain a melt. In this process, the in-situ grafting reaction of polyethylene terephthalate (PET) resin refers to using a twin-screw extruder directly as a continuous tubular chemical reactor. The styrene-acrylic acid-glycidyl methacrylate copolymer in the formulation is itself a polymer chain with multiple active sites, acting as the backbone of the grafting reaction. In a high-temperature shear field, the ethylene oxide three-membered rings hanging on the main chain exhibit extremely high selectivity for the carboxyl groups at the ends of the PET chain segments (including free carboxyl groups from the primary degradation of PET and newly formed carboxyl groups released by the PMDA chain extension reaction). The epoxy groups undergo ring-opening addition reactions with these carboxyl groups to form stable chemical bonds. Through this multi-point grafting, a large number of linear PET chains (or star-shaped PET complexes already coupled with PMDA) are chemically bonded to the styrene-acrylic acid-glycidyl methacrylate copolymer main chain, ultimately generating a long branched network in real time in the melt.

[0026] S3. Inject a physical foaming agent, including supercritical carbon dioxide or supercritical nitrogen, into the molten melt in the extruder. The injection amount is 0.5-5% of the total weight of the melt. A homogeneous solution is formed by shear mixing. S4. The homogeneous solution is transported to the cooling section and cooled to 220~240℃. Then, it is extruded through the die head, causing a sudden drop in pressure. After nucleation growth and cooling, the modified PET foam board is obtained.

[0027] In S2, the long barrel with an aspect ratio of not less than 40:1 provides an average residence time for the reaction system, ensuring that the polymer chains have sufficient time to sequentially complete solid-phase transport, melt plasticization, dispersion and mixing, as well as the ring-opening reaction of PMDA anhydride and the carboxyl group removal and bridging reaction of epoxy groups. The temperature of the extruder reaction stage is 260~290℃. This temperature is slightly higher than the equilibrium melting point of PET (255℃), giving the melt excellent macroscopic fluidity, which is conducive to shear mixing and accelerates the grafting rate of the synergistic chain extender, enabling the macromolecular chains to more efficiently complete the topological evolution to long branched (LCB) structures.

[0028] A vacuum exhaust device is installed on the extruder, and a vacuum pump operates within the barrel at a vacuum level of -0.08 to -0.1 MPa. Since esterification and partial grafting reactions are reversible equilibrium reactions, according to Le Chatelier's principle, under negative pressure, trace amounts of low-molecular-weight volatile byproducts such as water molecules and acetaldehyde are forcibly and continuously extracted from the system. This disrupts the original chemical equilibrium, driving macromolecular polycondensation and chain extension reactions towards higher molecular weights and higher crosslinking densities. Furthermore, removing these low-molecular-weight volatiles effectively prevents them from prematurely vaporizing as uncontrollable physical foaming agents in subsequent processes, thus ensuring the uniformity of the final cell structure. This transforms the originally low-viscosity linear PET into a PET melt with high storage modulus and high zero-shear viscosity.

[0029] In S3, supercritical carbon dioxide or supercritical nitrogen is injected into the extended PET melt as a foaming agent via a precision high-pressure metering pump. When the high-pressure foaming gas enters the extruder, it is forcibly pulverized into countless nano-sized droplets by the extruder's gear mixing block. These pulverized droplets, driven by the high pressure inside the extruder, permeate and dissolve into the interstices of the amorphous molecular chains in the extended PET melt, forming a thermodynamically extremely stable homogeneous polymer-gas solution. The dissolved supercritical gas effectively lubricates the polymer, increasing the free volume between the macromolecular chains, leading to a significant decrease in the basic glass transition temperature and apparent melt viscosity of PET. This reduced viscosity facilitates further mixing of the materials.

[0030] In S4, the melt is first cooled to 220~240℃ to avoid the PET melt being directly extruded at a high temperature of 260~290℃ and low viscosity. Direct extrusion of low viscosity and highly plasticized melt into the die head can easily cause a large number of cracks and collapses in the foamed melt and the foam pore walls.

[0031] As the temperature decreases, the thermal mobility of polymer chain segments decreases sharply. Physical entanglement occurs between the comb-like / star-shaped long branches constructed by synergistic chain extenders, resulting in an increase in the complex viscosity and elastic modulus of the system.

[0032] As a semi-crystalline polymer, PET is cooled to 220-240℃, bringing it close to the dynamic range of its crystallization temperature. The long-chain branched structure acts as self-nucleation sites, accelerating PET crystallization. These microcrystals serve as physical cross-linking points in the melt, further causing localized gelation and imparting high initial strength to the melt.

[0033] When the cooled, high-strength homogeneous solution is extruded through a die, the sudden drop in ambient pressure causes a rapid decrease in the solubility of the supercritical gas in its dissolved state, resulting in intense thermodynamic phase separation. With the assistance of nucleating agents (talc, nano-montmorillonite, or silica), the bubble nuclei simultaneously erupt and expand rapidly. During the bidirectional stretching of the surrounding pore walls by the bubble expansion, the highly branched, chain-extended PET undergoes strain hardening. As the pore walls are stretched, their rheological resistance to stretching increases, preventing pore wall rupture due to thinning.

[0034] Pyromellitic dianhydride (PMDA) is an aromatic tetrafunctional crosslinking agent, with a five-atom anhydride ring on each side of its molecular core. Thermodynamically, this five-membered anhydride ring exhibits high ring strain and is in an excited state with extremely unstable energy. Furthermore, the carbonyl carbon atom on the anhydride ring carries a very strong positive charge (electrophilia).

[0035] In the physical environment of reactive extrusion, twin-screw extruders provide thermal energy of 260°C to 290°C and intense mechanical shear forces. When linear polyethylene terephthalate (PET) macromolecular chains come into contact with PMDA in a high-temperature shear field, the terminal hydroxyl groups (-OH) at the ends of the PET molecular chains act as strong nucleophiles, instantly launching a nucleophilic attack on the carbonyl carbon in a highly strained state on the PMDA anhydride ring. Pyromellitic dianhydride captures and couples two to four free PET macromolecular chains.

[0036] The rigid benzene ring at the center of PMDA is directly transformed into a cross-linking hub in both physical and chemical dimensions. Through the newly generated stable ester bonds, the originally discrete linear PET chains are quickly anchored, thereby instantly building a multi-armed star-shaped polymer backbone network in the melt.

[0037] Example 1: (1) Weigh 96.5 kg of PET resin with an intrinsic viscosity of 0.80 dL / g, 0.5 kg of PMDA (phenylene dianhydride), and styrene-acrylic acid-glycidyl methacrylate copolymer (epoxy equivalent approximately 285). 1.5 kg (i.e., the mass ratio of PMDA to epoxy polymer is 1:3), 1.0 kg of talc (nucleating agent) and 0.5 kg of polycarbodiimide (anti-hydrolysis stabilizer); (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0038] Example 2: (1) Weigh 95.9 kg of PET resin with an intrinsic viscosity of 0.80 dL / g, 0.4 kg of PMDA, and approximately 285 g of epoxy equivalent. 2.0 kg of styrene-acrylic acid-glycidyl methacrylate copolymer (i.e., PMDA to epoxy polymer mass ratio of 1:5), 1.2 kg of talc, and 0.5 kg of polycarbodiimide; (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0039] Example 3: (1) Weigh 96.0 kg of PET resin with an intrinsic viscosity of 0.80 dL / g, 0.8 kg of PMDA, and approximately 285 g of epoxy equivalent. 1.6 kg of styrene-acrylic acid-glycidyl methacrylate copolymer (i.e., PMDA to epoxy polymer mass ratio of 1:2), 1.1 kg of talc, and 0.5 kg of polycarbodiimide; (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0040] Comparative Example 1: (1) Weigh 96.5 kg of PET resin with an intrinsic viscosity of 0.80 dL / g, 0.5 kg of PMDA, 1.0 kg of talc and 0.5 kg of polycarbodiimide; (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0041] Comparative Example 2: (1) Weigh 98.5 kg of PET resin with an intrinsic viscosity of 0.80 dL / g, 1.0 kg of talc powder and 0.5 kg of polycarbodiimide; (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0042] Comparative Example 3: (1) Weigh 96.0 kg of PET resin with an intrinsic viscosity of 0.80 dL / g, 1.2 kg of PDMA, and approximately 285 g of epoxy equivalent. 1.2 kg of styrene-acrylic acid-glycidyl methacrylate copolymer (i.e., PMDA to epoxy polymer mass ratio of 1:1), 1.1 kg of talc, and 0.5 kg of polycarbodiimide; (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0043] Comparative Example 4: (1) Weigh 97.0 kg of PET resin with an intrinsic viscosity of 0.80 dL / g and an epoxy equivalent of approximately 285 g. 1.5 kg of styrene-acrylic acid-glycidyl methacrylate copolymer, 1.0 kg of talc, and 0.5 kg of polycarbodiimide; (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0044] Comparative Example 5: (1) Weigh 97.5 kg of PET resin with an intrinsic viscosity of 0.80 dL / g, 0.5 kg of PDMA, and approximately 285 g of epoxy equivalent. 1.5 kg of styrene-acrylic acid-glycidyl methacrylate copolymer (i.e., PMDA to epoxy polymer mass ratio of 1:3) and 0.5 kg of polycarbodiimide; (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0045] Comparative Example 6: (1) Weigh 97.0 kg of PET resin with an intrinsic viscosity of 0.80 dL / g, 0.5 kg of PDMA, and approximately 285 g of epoxy equivalent. 1.5 kg of styrene-acrylic acid-glycidyl methacrylate copolymer (i.e., PMDA to epoxy polymer mass ratio of 1:3) and 1.0 kg of talc powder; (2) The PET resin is dried in a dehumidifying dryer until the moisture content is 0.003%, and then mixed evenly with the above-mentioned additives in a high-speed mixer; (3) Feed the premix into a twin-screw extruder with a length-to-diameter ratio of 45:1, and set the reaction section temperature to 280℃. Turn on the vacuum exhaust in the later section of the reaction section to maintain a vacuum of -0.09MPa and remove low-molecular-weight volatiles; (4) Supercritical carbon dioxide is injected into the melt at a ratio of 2.0% of the total weight, and a homogeneous solution is formed by mixing elements. The melt is then transported to the cooling section, where the temperature is steadily reduced to 230°C. Finally, it is extruded through a slit die and foamed under a sudden pressure drop. After traction cooling and shaping, a modified PET foam board is obtained.

[0046] The PET foam board sample prepared in Example 1 had a smooth surface and a dense and uniform cell structure. Testing showed a closed-cell rate of 94% and a foaming ratio of 18 times, with no obvious pore breaks or gel punctures observed.

[0047] The PET foam board sample obtained in Example 2 had a smooth surface. The initial start-up rate of PMDA was slightly lower than in Example 1, but due to the high proportion of epoxy copolymer, free carboxyl groups in the system were effectively removed and sufficient branching bridging was achieved. Its closed-cell rate reached 91%, the foaming ratio reached 16 times, and the cell structure was intact.

[0048] The PET foamed board sample prepared in Example 3 showed stable foaming. However, due to the high PMDA content, the melt viscosity increased extremely rapidly. With the combination of vacuum degassing and precise temperature control, the system remained below the critical gel point. Its closed-cell rate reached 93%, and the foaming ratio reached 19 times, exhibiting a very significant strain hardening effect.

[0049] In Comparative Example 1, the PET foamed board sample exhibited localized hardening points in the melt during extrusion. The cut surface of the foamed board contained numerous gel particles, which punctured the cell walls, leading to large-scale bubble merging and collapse. The final closed-cell rate was 65%, the foaming ratio was 8, and the mechanical properties were poor.

[0050] The PET foam board sample prepared in Comparative Example 2 lacked a long-chain branched structure in its melt, exhibited extremely low zero-shear viscosity, and showed no strain hardening effect. During extrusion through the die, the melt failed to encapsulate the expanding gas, resulting in a large amount of gas escaping, cell rupture and collapse, and the formation of a foam board with an independent microporous structure.

[0051] In the PET foam board sample prepared in Comparative Example 3, the relative concentration of PMDA was too high, causing the reaction system to rapidly exceed the gel point within the extruder. A large amount of hard gel lumps were generated in the extruded melt. During foaming and stretching, these gel lumps severely disrupted the continuity of the cell walls, destroying them. The closed-cell rate plummeted to 58%, the foaming ratio was only 6 times, and the board exhibited extreme brittleness.

[0052] The PET foam board sample prepared in Comparative Example 4 lacked PMDA and relied solely on the relatively slow-reacting epoxy copolymer, which could not accumulate sufficient melt strength to support the foaming within the limited residence time of the extruder. During biaxial stretching of the foam, insufficient strain hardening effect was not generated, leading to thinning and rupture of the cell walls, a decrease in closed-cell ratio to 71%, and a foaming ratio of only 10 times.

[0053] In the PET foam board sample prepared in Comparative Example 5, the sudden drop in ambient pressure triggered intense thermodynamic phase separation when the high-strength homogeneous solution was extruded through the die after cooling. However, due to the lack of a heterogeneous nucleating agent to provide a physical interface, the system was forced to rely on homogeneous nucleation driven by extremely high thermodynamic forces. Because there were very few nucleation points, the limited gas was rapidly divided among a few bubbles, inhibiting the increase in bubble density and causing excessive bubble growth into large, easily ruptured bubbles. Ultimately, the closed-cell rate dropped to 62%, and the cell size became uneven.

[0054] In Comparative Example 6, the PET foamed board sample underwent hydrolysis and thermo-oxidative degradation of PET ester bonds under extreme shear conditions of 260–290 °C. Due to the lack of carbodiimide stabilizers to actively capture and neutralize the free carboxylic acids generated during degradation, these free carboxylic acids acted as a self-catalyst for ester bond breakage. The degradation led to a precipitous drop in melt viscosity, completely offsetting the molecular weight increase brought about by the synergistic chain extender system. Ultimately, the melt could not contain the expanding gas generated by phase separation, causing severe collapse of the foamed board, with a closed-cell rate of only 55%, resulting in a loss of structural strength.

[0055] In summary: This invention utilizes a synergistic chain-extending system composed of pyromellitic dianhydride and an epoxy-containing polymer chain extender. The rapid reaction of pyromellitic dianhydride quickly establishes the backbone between PET molecular chains, improving low-shear viscosity. The epoxy-containing polymer chain extender, with a slower reaction, is used to repair end defects and improve the tensile viscosity of the PET foam. The combination of pyromellitic dianhydride and the epoxy-containing polymer chain extender in a specific ratio produces a long-branched structure, allowing the strength of the PET melt to gradually increase during bubble expansion and bubble wall stretching, rather than directly thinning to the point of rupture. This avoids the porosity caused by low strength during melt extrusion.

[0056] Before extrusion, the PET melt is heated to a low viscosity and high plasticity state to facilitate material mixing and transportation. Then it is cooled to near the crystallization temperature, and the long branches are used as self-nucleation points to accelerate the crystallization of PET. These microcrystals act as physical cross-linking points in the melt, causing local gelation of the melt and giving it high initial strength before extrusion.

[0057] Anti-hydrolysis agents ensure the stability of the cells in the PET foam board, while nucleating agents assist in the simultaneous eruption and rapid expansion of bubble nuclei. During the bidirectional stretching of the surrounding pore walls by bubble expansion, the highly branched, chain-extended PET undergoes strain hardening. As the pore walls are stretched, their rheological resistance to stretching increases, preventing pore wall rupture due to thinning.

[0058] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is not limited by the foregoing description; thus, all variations falling within the meaning and scope of their equivalents are intended to be included within the present invention.

Claims

1. A modified PET foam board, characterized in that, It comprises the following components by weight percentage: 92-98% polyethylene terephthalate resin, 0.3-3.5% synergistic chain extender, 0.1-3.0% nucleating agent, and 0.1-1.5% hydrolysis stabilizer; The synergistic chain extender includes pyromellitic dianhydride and a polymer chain extender containing epoxy groups; the nucleating agent includes at least one of talc, nano-montmorillonite, or silica; and the hydrolysis-resistant stabilizer includes carbodiimide.

2. The modified PET foam board according to claim 1, characterized in that, In the synergistic chain extender, the mass ratio of the pyromellitic dianhydride to the epoxy-containing polymer chain extender is 1:2~5, and the epoxy-containing polymer chain extender includes a styrene-acrylic acid-glycidyl methacrylate copolymer with an epoxy equivalent of 250~350. .

3. A method for preparing a modified PET foam board according to claim 1 or 2, characterized in that, The preparation steps include the following: (1) Raw material pretreatment: The polyethylene terephthalate resin is dried and premixed with a synergistic chain extender, a nucleating agent and an anti-hydrolysis stabilizer to obtain a premix; (2) Reactive extrusion: The premix is ​​added to a twin-screw extruder to melt and reactively extruded to cause in-situ grafting of polyethylene terephthalate resin. The melt is then obtained by devolatilization through a vacuum exhaust device. (3) Physical foaming: Physical foaming agent is injected into the melt in the molten state inside the extruder, and a homogeneous solution is formed by shearing and mixing; (4) Cooling and shaping: The homogeneous solution is transported to the cooling section to cool down, and then extruded through the die head to cause a sudden drop in pressure. After nucleation growth and cooling and shaping, the modified PET foam board is obtained.

4. The method for preparing modified PET foam board according to claim 3, characterized in that, In step (1), the intrinsic viscosity of polyethylene terephthalate resin is 0.7~0.85 dL / g, and it is dried to a water content of less than 0.005%.

5. The method for preparing modified PET foam board according to claim 3, characterized in that, In step (2), the length-to-diameter ratio of the twin-screw extruder is not less than 40:

1.

6. The method for preparing modified PET foam board according to claim 3, characterized in that, In step (2), the temperature of reactive extrusion is 260~290℃.

7. The method for preparing modified PET foam board according to claim 3, characterized in that, In step (2), a vacuum of -0.08 to -0.1 MPa is maintained in the extruder barrel by a vacuum exhaust device.

8. The method for preparing modified PET foam board according to claim 3, characterized in that, In step (3), the injected physical foaming agent includes supercritical carbon dioxide or supercritical nitrogen, and the amount injected is 0.5 to 5% of the total weight of the melt.

9. The method for preparing modified PET foam board according to claim 3, characterized in that, In step (4), after the homogeneous solution is transported to the cooling section, the temperature of the melt is reduced to 220~240℃.

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