A method for preparing optical-grade PMMA by integrating supercritical CO2-assisted devolatilization and microporous foaming

Abstract

The application relates to the technical field of high polymer material processing, and discloses a method for integrally preparing optical PMMA through supercritical CO2 assisted devolatilization and microporous foaming, which comprises the following steps: conveying PMMA prepolymer containing residual methyl methacrylate monomers into a high-pressure reaction kettle to be contacted with and saturated by supercritical CO2; then rapidly reducing the pressure of the saturated melt to make CO2 dissolved in the melt precipitate to form a microporous structure and simultaneously take out the residual monomers; and finally cooling and shaping to obtain optical PMMA material. By adopting the method, the integrally preparation of high-efficiency removal of PMMA residual monomers and microporous lightweight forming can be realized at a lower processing temperature, and the obtained material has lower residual monomer content, lower density, higher light transmittance and lower haze.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, specifically to a method for preparing optical-grade PMMA by integrating supercritical CO2-assisted devolatilization and microporous foaming. Background Technology

[0002] PMMA (polymethyl methacrylate) is characterized by high light transmittance, good weather resistance, high surface hardness, and good dimensional stability, and is widely used in optical lenses, light guide plates, display devices, and transparent structural components. When optical-grade PMMA is prepared using continuous bulk polymerization, the polymerized material typically contains a certain amount of unreacted methyl methacrylate monomers. Therefore, a devolatilization process is needed to reduce the residual monomer content to ensure material purity, optical performance, and long-term stability.

[0003] In existing technologies, PMMA devolatilization typically relies on high temperatures and vacuum conditions to promote monomer volatilization. However, for high-viscosity PMMA melts, the diffusion and migration of residual monomers within the polymer are limited, leading to problems such as low efficiency, long residence time, high energy consumption, and demanding equipment requirements in the devolatilization process. Meanwhile, traditional dense PMMA materials have a high density, which hinders their further application in aerospace, portable electronic devices, and lightweight optical components.

[0004] Supercritical CO2 combines the high density of liquids with the large diffusion coefficient of gases, offering advantages such as non-toxicity, non-flammability, low cost, and easy recyclability. In polymer processing, supercritical CO2 not only swells and plasticizes polymer melts but also extracts and carries small monomers; during rapid depressurization, it can also act as a physical foaming agent to form microporous structures. Therefore, introducing supercritical CO2 into PMMA prepolymer systems holds promise for achieving integrated processing of residual monomer removal and microporous lightweight molding. For example, Chinese patent application CN112155478A discloses a prepolymer reactor, a continuous bulk polymerization system and method, and PMMA resin, which primarily prepares dense particles and cannot achieve lightweight modification of the material simultaneously with devolatilization; if a microporous structure is required, secondary processing is necessary, further increasing energy consumption and the risk of thermal damage.

[0005] Therefore, it is necessary to provide a PMMA processing method that can take into account devolatilization efficiency, lightweight effect and optical performance. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the present invention aims to provide a method for preparing optical grade PMMA by integrating supercritical CO2-assisted devolatilization and microporous foaming, which solves the problems of low devolatilization efficiency, high processing temperature, high energy consumption and difficulty in achieving both lightweight and optical performance in the existing PMMA devolatilization process.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing optical-grade PMMA by integrating supercritical CO2-assisted devolatilization and microporous foaming, comprising the following steps:

[0008] S1. Provide a PMMA prepolymer containing residual methyl methacrylate monomer; and add 0.1% to 1% of nano-talc powder by mass of the PMMA prepolymer as a nucleating agent to obtain a PMMA prepolymer mixture;

[0009] S2. The PMMA prepolymer mixture is transported to a high-pressure reactor, supercritical CO2 is injected and saturated to dissolve the supercritical CO2 in the PMMA prepolymer mixture, while simultaneously extracting residual methyl methacrylate monomer from the PMMA prepolymer mixture; the mass ratio of supercritical CO2 to PMMA prepolymer mixture is 0.05:1 to 0.5:1.

[0010] S3. The PMMA melt processed in step S2 is rapidly depressurized through a die head, mold or depressurization device to precipitate the supercritical CO2 dissolved in the PMMA melt and form a microporous structure, while removing residual methyl methacrylate monomer.

[0011] S4. Cool and shape the foamed PMMA obtained in step S3 to obtain optical grade PMMA material.

[0012] Further, in step S1, the PMMA prepolymer is prepared by continuous bulk polymerization, the conversion rate of the PMMA prepolymer is 60% to 85%, the number average molecular weight of the PMMA prepolymer is 50,000 to 150,000, and the mass fraction of residual methyl methacrylate monomer is 15% to 40%.

[0013] Furthermore, in step S2, the injection pressure of the supercritical CO2 is 10-30 MPa, and the saturation treatment temperature is 80-120°C.

[0014] Furthermore, the saturation treatment time in step S2 is 20-40 minutes.

[0015] Furthermore, the voltage reduction rate in step S3 is 200–600 MPa / s.

[0016] Furthermore, the average pore size of the microporous bubble structure formed in step S3 is 1 to 10 μm.

[0017] Furthermore, the cooling and shaping temperature in step S4 is 40–80°C.

[0018] The present invention also provides an optical grade PMMA material prepared by the above method, wherein the material has a density of 0.6 to 1.0 g / cm³, a light transmittance of greater than 90%, a haze of less than 2%, and a residual methyl methacrylate monomer content of less than 100 ppm.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects:

[0020] This invention utilizes the swelling and plasticizing effect of supercritical CO2 on PMMA prepolymer mixtures and the extraction and carrying effect on methyl methacrylate monomers to promote the migration of residual monomers from the inside of the melt to the outside at lower processing temperatures and to be carried out simultaneously, thereby significantly improving the devolatilization efficiency.

[0021] This invention utilizes supercritical CO2 simultaneously as both a devolatilization aid and a physical foaming agent, enabling the PMMA prepolymer mixture to complete residual monomer removal and microporous structure construction in a single process. This avoids the step-by-step processing of "devolatilization first, then lightweight molding" in traditional processes, thus shortening the process flow, reducing equipment requirements, and lowering overall energy consumption. During rapid depressurization, this invention causes CO2 dissolved in the PMMA melt to precipitate and form a uniform microporous structure, resulting in a PMMA material density that can be reduced to 0.65–0.85 g / cm³.

[0022] This invention controls CO2 injection pressure, saturation temperature, depressurization rate, and cooling and shaping conditions to create a microporous structure with an average pore size of 3–5 μm, thereby effectively reducing the adverse effects of bubbles on light scattering. Traditional vacuum devolatilization typically relies on high temperatures to promote monomer volatilization, while this invention utilizes supercritical CO2 to improve mass transfer efficiency, enabling synergistic devolatilization and foaming within the 80–150°C range. This avoids the risk of thermal degradation associated with prolonged high-temperature processing and helps maintain the transparency and appearance quality of PMMA materials. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1

[0025] PMMA prepolymer was prepared by continuous bulk polymerization. The prepolymer had a number-average molecular weight of 100,000, a conversion rate of 75%, and a residual methyl methacrylate monomer mass fraction of 25%. Nano-talc powder was added at 0.1% of the mass of the PMMA prepolymer as a nucleating agent to obtain a PMMA prepolymer mixture.

[0026] The PMMA prepolymer mixture is transported to a high-pressure reactor with a stirrer, the temperature inside the reactor is controlled at 120°C, supercritical CO2 is injected and the pressure is controlled at 20 MPa, the mass ratio of supercritical CO2 to PMMA prepolymer mixture is 0.2:1, and the mixture is stirred for 20 min to allow the supercritical CO2 to fully dissolve in the PMMA prepolymer mixture.

[0027] The saturated melt was then rapidly depressurized to atmospheric pressure through a mold at a rate of 200 MPa / s. After depressurization, CO2 in the melt quickly precipitated out, forming a microporous structure and simultaneously carrying away residual methyl methacrylate monomers. The melt was then cooled to 60°C at a rate of 5°C / min to set the microporous PMMA sheet.

[0028] Test results show that the obtained material has a density of 0.85 g / cm³, an average pore size of 5 μm, a light transmittance of 91.2%, a haze of 1.5%, and a residual methyl methacrylate monomer content of 80 ppm.

[0029] Example 2

[0030] A PMMA prepolymer with a number average molecular weight of 80,000, a conversion rate of 70%, and a residual methyl methacrylate monomer mass fraction of 30% was used, and nano-talc powder accounting for 0.5% of the mass of the PMMA prepolymer was added as a nucleating agent to obtain a PMMA prepolymer mixture;

[0031] The PMMA prepolymer mixture was fed into a high-pressure reactor, and the temperature was controlled at 110℃, the CO2 injection pressure was 25MPa, the mass ratio of CO2 to PMMA prepolymer mixture was 0.35:1, and the saturation treatment was carried out for 30 minutes.

[0032] After saturation, the pressure is rapidly reduced to 0.2 MPa through the die head at a rate of 500 MPa / s, and then cooled to 80°C at a rate of 10°C / min to obtain foamed PMMA sheets.

[0033] Test results show that the obtained material has a density of 0.65 g / cm³, an average pore size of 3 μm, a light transmittance of 90.5%, a haze of 1.8%, and a residual methyl methacrylate monomer content of 60 ppm.

[0034] Example 3

[0035] A PMMA prepolymer with a number average molecular weight of 150,000, a conversion rate of 85%, and a residual methyl methacrylate monomer mass fraction of 40% was used; and 1% of the PMMA prepolymer mass of nano-talc powder was added as a nucleating agent; the mixture was stirred and mixed to obtain a mixture.

[0036] The PMMA prepolymer mixture was fed into a high-pressure reactor, the temperature was controlled at 80℃, the CO2 injection pressure was 20MPa, the mass ratio of CO2 to PMMA prepolymer mixture was 0.5:1, and the saturation treatment was carried out for 40 minutes.

[0037] After saturation, the pressure is rapidly reduced to 0.2 MPa through the die head at a rate of 600 MPa / s, and then cooled to 60°C at a rate of 10°C / min to obtain foamed PMMA sheets.

[0038] Test results show that the obtained material has a density of 0.75 g / cm³, an average pore size of 4 μm, a light transmittance of 91.8%, a haze of 1.3%, and a residual methyl methacrylate monomer content of 90 ppm.

[0039] Comparative Example 1

[0040] The same PMMA prepolymer as in Example 1 was used, and conventional vacuum devolatilization was employed without the addition of supercritical CO2 or microcellular foaming. The devolatilization temperatures were 200°C, 220°C, 240°C, and 260°C, with a total processing time of 30 minutes.

[0041] The obtained PMMA material has a density of 1.19 g / cm³, an average pore size of 0 μm (no pore structure), a haze of 0.8%, a light transmittance of 92.5%, and a residual methyl methacrylate monomer content of 350 ppm.

[0042] 1. Density test

[0043] The density of the obtained PMMA samples was tested using the water displacement method. Before testing, the samples were processed into specimens of regular size and placed at 23℃ for 24 hours before measurement. Each sample was tested in triplicate, and the average value was taken. The unit is g / cm³.

[0044] 2. Average pore size test

[0045] After the sample was fractured by liquid nitrogen, the fracture surface was sputter-coated with gold, and the morphology of the bubbles was observed using a scanning electron microscope. Multiple fields of view were selected to measure the bubble size, and the average pore diameter was calculated. At least 50 bubbles were selected from each sample for statistical analysis, and the results were averaged in μm.

[0046] 3. Light transmittance test

[0047] The sample is processed into a test piece of specified thickness, and its transmittance is measured under visible light conditions. Before testing, the sample surface should be flat and free of obvious scratches and contamination. Each sample is tested in parallel three times, and the average value is taken and expressed as a percentage.

[0048] 4. Haze Test

[0049] The samples were tested using a haze meter. During testing, the samples were processed into flat plates and placed in a standard environment for 24 hours before measurement. Each sample was tested in triplicate, and the average value was taken, expressed as a percentage. Lower haze indicates less light scattering and better optical uniformity of the material.

[0050] 5. Residual methyl methacrylate monomer content test

[0051] After pulverizing a certain amount of sample, the residual methyl methacrylate monomer in the sample was extracted with an organic solvent and then quantitatively analyzed by gas chromatography.

[0052] The test results of Examples 1-3 and Comparative Example 1 show that the PMMA material prepared by the method of the present invention achieves a good balance between residual monomer content, density, and optical properties. The densities of the materials obtained in Examples 1-3 are 0.85 g / cm³, 0.65 g / cm³, and 0.75 g / cm³, respectively, all significantly lower than the 1.19 g / cm³ of Comparative Example 1, indicating that the method of the present invention can effectively achieve the lightweighting of PMMA. Meanwhile, the light transmittance of Examples 1-3 remains at 90.5%-91.8%, and the haze is controlled at 1.3%-1.8%. The residual methyl methacrylate monomer content in Examples 1-3 is 80 ppm, 60 ppm, and 90 ppm, respectively, all significantly lower than the 350 ppm of Comparative Example 1, indicating that supercritical CO2 in PMMA prepolymers can not only act as a physical foaming agent but also effectively promote the migration and removal of residual monomers. Compared with the traditional high-temperature vacuum devolatilization process, the method of the present invention can obtain a lower residual monomer content at a lower processing temperature.

[0053] In Example 2, after adding nano-talc nucleating agent, the average pore size of the obtained material was 3 μm, the density was reduced to 0.65 g / cm³, and the residual monomer content was further reduced to 60 ppm. This indicates that the nucleating agent helps to form a finer and more uniform pore structure and is beneficial to improving the lightweighting and devolatilization effects.

[0054] In summary, this invention achieves an integrated and synergistic process of residual monomer devolatilization and microporous foaming by introducing supercritical CO2 into the PMMA prepolymer system, thus enabling the production of optical-grade PMMA materials with low residual monomer, low density, high light transmittance, and low haze.

[0055] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0056] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

[0057] Those skilled in the art should understand that the above descriptions are merely several specific embodiments of the present invention, and not all embodiments.

Claims

1. A method for preparing optical-grade PMMA by integrating supercritical CO2-assisted devolatilization and microporous foaming, characterized in that, Includes the following steps: S1. A PMMA prepolymer is prepared by continuous bulk polymerization. The PMMA prepolymer has a conversion rate of 60% to 85%, a number average molecular weight of 50,000 to 150,000, and a residual methyl methacrylate monomer mass fraction of 15% to 40%. Nano-talc powder, accounting for 0.1% to 1% of the mass of the PMMA prepolymer, is added to the PMMA prepolymer as a nucleating agent, and the mixture is mixed to obtain a PMMA prepolymer mixture. S2. The PMMA prepolymer mixture is transported to a high-pressure reactor with a stirrer, the temperature is controlled at 80-120°C, supercritical CO2 is injected and the pressure is controlled at 20-25 MPa, the mass ratio of supercritical CO2 to PMMA prepolymer mixture is 0.2:1-0.5:1, and saturation treatment is carried out for 20-40 min to dissolve the supercritical CO2 in the PMMA prepolymer mixture and extract the residual methyl methacrylate monomer in the PMMA prepolymer mixture. S3. The PMMA melt after step S2 is rapidly depressurized to atmospheric pressure or 0.2 MPa through a die or mold at a depressurization rate of 200-600 MPa / s, so that the supercritical CO2 dissolved in the PMMA melt precipitates out to form a microporous structure with an average pore size of 3-5 μm, and the residual methyl methacrylate monomer is removed simultaneously. S4. The foamed PMMA obtained in step S3 is cooled and shaped at 40-80°C to obtain optical grade PMMA material; the density of the optical grade PMMA material is 0.65-0.85 g / cm³, the light transmittance is greater than 90%, the haze is less than 2%, and the residual methyl methacrylate monomer content is less than 100 ppm.