A method for preparing a low-birefringence optical-grade polymethyl methacrylate

Abstract

This application provides a method for preparing low-birefringence optical-grade polymethyl methacrylate (PMMA), belonging to the field of polymer material synthesis technology. To address the problems of high birefringence and insufficient thermal stability in existing PMMA materials, this method copolymerizes 80-95 parts by weight of PMMA, 2-20 parts by weight of a first comonomer, and 2-20 parts by weight of a second comonomer. The first comonomer is selected from phenyl methacrylate, etc., and the second comonomer is selected from isobornyl methacrylate, etc., with a mass ratio of 1:3 to 3:1. This method, by copolymerizing monomers in a specific ratio, can prepare materials with near-zero birefringence and high thermal stability, suitable for precision optics.

Description

Technical Field

[0001] This application relates to the field of polymer material synthesis technology, and in particular to a method for preparing low birefringence optical grade polymethyl methacrylate. Background Technology

[0002] Polymethyl methacrylate (PMMA) is widely used as an optical material due to its excellent light transmittance, weather resistance, and processability. However, the inherent negative birefringence of the PMMA molecular chain structure leads to shear stress during injection molding, extrusion, and other processing methods, causing the molecular chains to orient and resulting in significant birefringence. This optical anisotropy severely affects the imaging quality and display effect of optical systems, especially in precision optical applications such as optical lenses and light guide plates for liquid crystal displays, where extremely stringent requirements are placed on the material's birefringence.

[0003] To reduce the birefringence of polymethyl methacrylate (PMMA), existing technologies have explored the approach of "compensating for birefringence by introducing comonomers." For example, this involves copolymerizing a monomer with positive birefringence with a monomer with negative birefringence. However, existing solutions typically suffer from at least one of the following shortcomings: Firstly, they often only partially reduce birefringence from specific sources (such as processing orientation), making it difficult to achieve an overall absolute value of near-zero birefringence in the final product under complex injection molding or extrusion processes. Secondly, while partial copolymerization reduces birefringence, it often comes at the cost of sacrificing the material's glass transition temperature (Tg), making it difficult to achieve both low birefringence and high heat resistance. Furthermore, some solutions employ overly complex monomer systems or require the addition of special small-molecule additives, which can lead to poor compatibility, instability, or processing precipitation issues in continuous bulk polymerization processes, hindering large-scale industrial production.

[0004] In summary, existing technologies still struggle to balance extremely low birefringence and high thermal stability while maintaining the excellent optical properties of the methacrylate system and the feasibility of continuous polymerization processes. Therefore, there is a need in the field for an improved method for preparing optical-grade polymethyl methacrylate. Summary of the Invention

[0005] The purpose of this application is to provide a method for preparing low birefringence optical grade polymethyl methacrylate, which aims to solve the problems of high birefringence and insufficient thermal stability of optical grade polymethyl methacrylate materials in the prior art, as well as the limited effectiveness of existing modification methods in reducing birefringence and the difficulty in achieving industrial production.

[0006] To achieve the above objectives, this application provides a method for preparing low birefringence optical grade polymethyl methacrylate, comprising copolymerizing a monomer mixture containing the following components using a continuous bulk polymerization process, comprising: 80-95 parts by mass of methyl methacrylate; 2-20 parts by mass of a first comonomer, wherein the first comonomer is selected from one or more of phenyl methacrylate, benzyl methacrylate, and naphthyl methacrylate; and 2-20 parts by mass of a second comonomer, wherein the second comonomer is selected from one or more of isobornyl methacrylate, adamantane methacrylate, and tricyclodecane methacrylate; wherein the mass ratio of the first comonomer to the second comonomer is 1:3 to 3:1, and the sum of the mass parts of the above components is 100 parts.

[0007] Optionally, the monomer mixture comprises: 80-90 parts by weight of methyl methacrylate; 3-15 parts by weight of the first comonomer; and 3-15 parts by weight of the second comonomer.

[0008] Optionally, the mass ratio of the first comonomer to the second comonomer is 1:2 to 2.5:1.

[0009] Optionally, when the first comonomer contains naphthalene methacrylate, the mass ratio of the first comonomer to the second comonomer is 1:3 to 1.5:1.

[0010] Optionally, when the first comonomer contains benzyl methacrylate, the mass ratio of the first comonomer to the second comonomer is 1:1 to 3:1.

[0011] Optionally, the first comonomer is selected from phenyl methacrylate or benzyl methacrylate; the second comonomer is selected from isobornyl methacrylate or adamantane methacrylate.

[0012] Optionally, the monomer mixture further includes an initiator and a chain transfer agent; the initiator is selected from azo initiators or peroxide initiators, and its amount is 0.01%-0.2% of the total mass of the monomer mixture; the amount of the chain transfer agent is 0.05%-0.5% of the total mass of the monomer mixture.

[0013] Optionally, the continuous bulk polymerization process does not use solvents or uses only auxiliary solvents less than 5% of the total mass of the monomer mixture; prior to the copolymerization, the methyl methacrylate is dehydrated to a moisture content of less than 50 mg / kg, and the monomer mixture is deoxygenated to a dissolved oxygen concentration of less than 1 mg / kg.

[0014] Optionally, the continuous bulk polymerization process includes a polymerization reaction carried out sequentially in a first reactor and a second reactor, followed by a devolatilization step; wherein the reaction temperature in the first reactor is 110-125°C, and the average residence time of the material is 2-3 hours; the reaction temperature in the second reactor is 140-150°C, and the average residence time of the material is 1.5-2.5 hours; the devolatilization step is carried out at 230-250°C under vacuum conditions.

[0015] Compared with the prior art, this application has the following beneficial effects: The preparation method provided in this application copolymerizes specific aromatic ring monomers and rigid alicyclic monomers with opposite birefringence properties in a precise mass ratio, which is beneficial to achieve better compensation of intramolecular birefringence. It can prepare optical grade polymethyl methacrylate with extremely low absolute birefringence, significantly improve the optical isotropy of the material, and meet the needs of precision optical applications.

[0016] Simultaneously, by introducing a rigid alicyclic structure into the polymer chain, the rigidity of the molecular chain is effectively improved, resulting in a significant increase in the glass transition temperature and improved heat resistance. Furthermore, the two-stage continuous bulk polymerization process employed in this invention is simple, stable, controllable, and highly efficient. Combined with subsequent devolatilization treatment, high-purity products with good quality consistency can be obtained, making it suitable for large-scale industrial production. Moreover, by introducing functional structural units into the polymer molecular chain through copolymerization, this invention helps achieve a balance between low birefringence and high Tg. Compared to methods involving the addition of small molecule additives, there are no issues with additive migration or precipitation, which helps maintain the long-term stability of the material's optical properties. Attached Figure Description

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

[0018] Figure 1 A schematic flowchart illustrating a method for preparing low birefringence optical grade polymethyl methacrylate provided in this application embodiment; Figure 2 This is a schematic diagram of the equipment connection for a preparation process of low birefringence optical grade polymethyl methacrylate provided in an embodiment of this application. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this application clearer, the following will provide a more detailed description of this application in conjunction with the accompanying drawings and specific embodiments. It is to be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of protection of this application.

[0020] The applicant has discovered that methyl methacrylate homopolymer segments exhibit inherent negative birefringence when oriented under shear stress. The first comonomer introduced in this application contains aromatic ring side groups, whose π-electron clouds are easily polarized under electromagnetic waves, and the aromatic ring planes tend to align parallel to the main chain, thus significantly modulating the positive birefringence of the polymer segments. The second comonomer, containing large-volume rigid alicyclic or bridged ring structures, while exhibiting some negative birefringence, possesses a large steric hindrance effect that significantly restricts the internal rotation of the main chain, thereby greatly increasing segment rigidity and the glass transition temperature (Tg).

[0021] Furthermore, when the amount of a single comonomer is less than 2 parts by mass, the system cannot provide sufficient optical compensation groups or steric hindrance; if it is more than 20 parts by mass, it easily damages the original excellent light transmittance and mechanical properties of PMMA. In addition, the applicant has further discovered that the mass ratio of the first comonomer to the second comonomer has a significant impact on the optical properties of the final product. When the mass ratio exceeds 3:1, the positive birefringence contribution of the aromatic ring may be excessive; when it is less than 1:3, the negative birefringence contribution of the rigid alicyclic ring and the PMMA backbone often dominates. In the system shown in the embodiments of this application, controlling the mass ratio within the window range of 1:3 to 3:1 generally allows the positive and negative birefringence contributions to achieve good mutual cancellation at the molecular level, resulting in a better balance of optical synergistic compensation between the aromatic ring monomer and the rigid alicyclic monomer.

[0022] In the following examples and comparative examples, unless otherwise specified, all reagents used were commercially available chemically pure or analytically pure products, and all equipment used was conventional equipment in the art. The test methods for each performance characteristic are as follows: Birefringence (Δn): Polymer particles are injection molded into standard specimens with dimensions of 60mm × 10mm × 3mm using an injection molding machine (e.g., Haitian MA1200 II / 410) at a melt temperature of 230℃, a mold temperature of 80℃, and an injection pressure of 60MPa. After the specimens are allowed to stand at room temperature for 24 hours to fully eliminate internal stress, the difference in refractive index between the length and thickness directions of the specimens is measured using a birefringence meter (model: KOBRA-WPR, purchased from Oji Scientific Instruments, Japan) at a wavelength of 590nm. This difference is the birefringence.

[0023] Transmittance and haze: According to the American Society for Testing and Materials (ASTM) standard D1003, a haze meter (e.g., BYK-Gardner Haze-gard plus) was used to test the 3mm thick injection molded sample with a CIE-C standard light source.

[0024] Glass transition temperature (Tg): Using a differential scanning calorimeter (e.g., TA Instruments Q20), the sample was heated from 30℃ to 180℃ at a heating rate of 10℃ / min under a nitrogen atmosphere (flow rate 50 mL / min). The second heating scan curve was recorded, and the midpoint temperature of the inflection point of the heat flow curve was taken as the glass transition temperature.

[0025] Number-average molecular weight (Mn) and molecular weight distribution index (PDI): A gel permeation chromatography system (e.g., Agilent 1260 Infinity II) was used with tetrahydrofuran as the mobile phase (flow rate 1.0 mL / min) and the column temperature set to 40 °C. Polystyrene was used as a standard for calibration, and the number-average molecular weight and molecular weight distribution index of the polymer were then calculated.

[0026] Residual total monomer content: Gas chromatograph (e.g., Agilent 7890B) equipped with flame ionization detector (FID). An appropriate amount of polymer sample was dissolved in dichloromethane, with n-octane as an internal standard, and the residual amounts of each unreacted monomer (methyl methacrylate, first comonomer, second comonomer) in the system were tested and the total amount was calculated.

[0027] Colorimetric b* value: Using a spectrophotometer (e.g., Konica Minolta CM-3600A), under a CIE D65 standard light source and a 10° field of view, the colorimetric index b* value, which characterizes the yellow and blue axes, was measured on a 3mm thick injection-molded sample in transmission mode.

[0028] The relevant terms in this application are defined as follows: "Low birefringence": refers to a material whose absolute birefringence value |Δn| ≤ 1×10⁻⁶ under standard injection molding test conditions at a wavelength of 590nm. -5 Preferably, |Δn| ≤ 5×10 -6 More preferably, |Δn| ≤ 3×10 -6 .

[0029] "Optical grade": This means that the material must meet at least the following requirements: light transmittance (3mm thickness) ≥ 92.0% and haze ≤ 0.5%. Preferably, the residual total monomer content of the material is less than 500mg / kg and the color b* value is ≤ 0.5.

[0030] "First comonomer" and "Second comonomer": The first comonomer is a methacrylate monomer with an aromatic ring structure in the side group, which can usually play a positive compensation role for birefringence in the system of this application; the second comonomer is a methacrylate monomer with a rigid alicyclic or bridged ring structure in the side group, which is usually beneficial to improve the rigidity of the chain segment and the glass transition temperature (Tg).

[0031] "Parts by weight": Unless otherwise specified, it refers to the total mass of the monomer mixture (MMA, first comonomer, second comonomer) participating in the polymerization reaction (i.e., the sum of the three is 100 parts), excluding processing aids such as initiators and chain transfer agents.

[0032] "Copolymerization": including but not limited to random copolymerization or gradient copolymerization. Under the continuous bulk polymerization process of this application, the resulting products generally exhibit a statistically distributed polymer.

[0033] "Continuous bulk polymerization" refers to a polymerization method that uses virtually no solvent (or only an auxiliary solvent of less than 5% of the total monomer mass), and where the raw materials are continuously fed and the polymer melt is continuously discharged. The auxiliary solvent is selected from one or more of toluene, xylene, ethyl acetate, or butanone. The reaction apparatus can be one or more of a continuous stirred tank reactor (CSTR), a tower reactor, or a tubular reactor.

[0034] The “polymethyl methacrylate” described in this application includes not only homopolymers, but also modified resins or polymer systems formed with methyl methacrylate as the main structural unit (e.g., the mass percentage is usually not less than 80%) and with the introduction of comonomers for performance adjustment.

[0035] Example 1

[0036] In one embodiment of this application, a method for preparing low-birefringence optical-grade polymethyl methacrylate is provided. Please refer to... Figure 1 and Figure 2 ,in Figure 1 This is a schematic diagram of the preparation method. Figure 2 A schematic diagram of the preparation process system for implementing this method is provided. Specifically, the preparation method in this embodiment includes the following steps: Step S10, Raw Material Preparation. The raw materials are mixed and pretreated in a 500-liter stainless steel mixing tank equipped with stirring and jacketed temperature control. 820 kg (82 parts by weight) of methyl methacrylate (MMA) monomer is treated in a molecular sieve drying tower to reduce its moisture content to below 50 mg / kg, and then pumped into the mixing tank. Subsequently, 135 kg (13.5 parts by weight) of the first comonomer phenyl methacrylate (PhMA) and 45 kg (4.5 parts by weight) of the second comonomer isobornyl methacrylate (IBMA) are added sequentially to the mixing tank. It should be noted that in this formulation, the mass ratio of the first comonomer to the second comonomer is 3:1, which falls within the preferred range of 1:3 to 3:1, aiming to maximize the positive birefringence compensation effect of the aromatic ring. Subsequently, 0.3 kg (0.03% of the total monomer mass) of the initiator azobisisobutyronitrile (AIBN) and 2.0 kg (0.2% of the total monomer mass) of the chain transfer agent n-dodecyl mercaptan were added to the tank. The stirrer of the mixing tank was turned on, and the mixture was stirred at low speed at 30°C for 30 minutes to ensure that all components were mixed evenly and a transparent reaction mixture was formed. During the stirring process, high-purity nitrogen gas was continuously bubbled into the reaction mixture to remove oxygen until the dissolved oxygen concentration in the liquid phase was below 1 mg / kg, in order to avoid the inhibitory effect of oxygen on the free radical polymerization reaction.

[0037] Step S20, continuous bulk polymerization. The prepared reaction mixture is pumped into a polymerization reactor consisting of a first reactor and a second reactor connected in series at a constant flow rate of 400 kg / h using a metering pump. The first reactor is a 1000-liter continuous stirred tank reactor (CSTR), with the reaction temperature precisely controlled by circulating heat transfer oil in the jacket. After the reaction mixture enters the first reactor, the reaction temperature is controlled at 110°C, and the average residence time of the material at this temperature is 2.5 hours. During this stage, the polymerization reaction starts rapidly, and the monomer conversion rate can reach approximately 45%. The discharge port of the first reactor is located at its bottom, and the viscous material generated by polymerization continuously flows into the second reactor under gravity. The second reactor is a vertical tower reactor with multiple baffles inside to achieve a near-piston flow state, and is equipped with segmented heating jackets. The material flows from top to bottom in the tower, the reaction temperature is controlled at 140°C, and the average residence time is 2 hours. At the higher reaction temperature, the polymerization reaction continues, and the total monomer conversion rate reaches approximately 80% when the material is discharged from the bottom of the second reactor. At this point, the output is a viscous melt containing polymers, unreacted monomers, and a small amount of oligomers.

[0038] Step S30 is post-polymerization processing, and step S40 is melt devolatilization and granulation. The polymer melt from the second reactor is pumped to the devolatilization unit. This unit consists of a primary flash tank and a multi-stage vacuum devolatilization tower. The polymer melt first enters a flash tank maintained at 200°C and atmospheric pressure, where most of the unreacted MMA monomers are vaporized and discharged for condensation and recovery. Subsequently, the material enters a multi-stage vacuum devolatilization tower, which is set at 240°C and maintained at a vacuum of -0.08 MPa. Within the tower, the melt forms a thin film through a specially designed distributor. Under high temperature and high vacuum conditions, residual monomers (MMA, PhMA, IBMA) and volatile substances such as chain transfer agents are effectively removed. Testing showed that the residual total monomer content in the melt after devolatilization was less than 200 mg / kg.

[0039] The purified polymer melt, after thorough devolatilization, is then fed into a twin-screw extruder. The extruder barrel is equipped with multiple heating zones and at least one vacuum vent to further remove any trace amounts of volatiles that may remain. The melt is further homogenized in the extruder and extruded into strips through the die. The extruded strips are then cooled in a water bath, granulated, screened by a vibrating screen, and dried with hot air to finally obtain uniform, transparent, low-birefringence optical-grade polymethyl methacrylate (PMMA) granules.

[0040] The performance of the granular product prepared in this embodiment was tested, and the results showed that the birefringence Δn was -0.95 × 10⁻⁶. -5 It has a light transmittance of 92.5%, a haze of 0.4%, a glass transition temperature (Tg) of 110℃, a number-average molecular weight of 95,000, and a molecular weight distribution index (PDI) of 1.8.

[0041] The polymer is composed of repeating units derived from methyl methacrylate, repeating units derived from phenyl methacrylate, and repeating units derived from isoborneol methacrylate, and their mass fraction ratio is basically consistent with the feed ratio.

[0042] Example 2

[0043] This embodiment provides a method for preparing low birefringence optical grade polymethyl methacrylate, which differs from Example 1 mainly in the types of the first comonomer, the second comonomer, the initiator, and the chain transfer agent.

[0044] Except for the differences described below, the preparation process and equipment used in this embodiment are the same as those in Example 1.

[0045] Step S10, Raw material preparation. In the mixing tank, 850 kg (85 parts by weight) of dried and pretreated MMA, 100 kg (10 parts by weight) of the first comonomer benzyl methacrylate (BzMA) and 50 kg (5 parts by weight) of the second comonomer adamantane methacrylate (AdMA) are mixed.

[0046] It should be noted that, because the BzMA side group contains a flexible methylene group, its positive birefringence compensation efficiency under stress orientation is lower than that of the phenyl ester in Example 1. Therefore, in this formulation, the mass ratio of the first comonomer to the second comonomer is adjusted to 2:1, using a relatively high proportion of the first monomer to compensate for its insufficient polarization ability. This ratio also falls within the protection range of 1:3 to 3:1 of this application.

[0047] The initiator used was 0.4 kg (0.04% of the total monomer mass) of tert-butyl peroxide-3,5,5-trimethylhexanoate (TBPIN), and the chain transfer agent was 2.5 kg (0.25% of the total monomer mass) of tert-dodecyl mercaptan. The mixture was stirred at 30°C, and high-purity nitrogen was continuously introduced to remove oxygen until the dissolved oxygen concentration was <1 mg / kg.

[0048] Step S20, continuous bulk polymerization. The above reaction mixture is pumped sequentially into the first reactor (controlled temperature 110°C, average residence time 2.5 hours) and the second reactor (controlled temperature 140°C, average residence time 2 hours) at the same flow rate (400 kg / h) and process conditions as in Example 1 to carry out the polymerization reaction.

[0049] Step S30 is post-polymerization processing and step S40 is melt devolatilization and granulation. The viscous melt obtained after polymerization is also post-processed and granulated in the same devolatilization unit (flash tank combined with a multi-stage vacuum devolatilization tower at 240°C and -0.08MPa) and twin-screw extruder as in Example 1 to remove residual monomers and volatiles, and obtain the final transparent granular product.

[0050] The performance of the granular product prepared in this embodiment was tested, and the results are as follows: the birefringence Δn is -0.92×10⁻⁶. -5 It has a light transmittance of 92.4%, a haze of 0.45%, and a glass transition temperature (Tg) of 107℃.

[0051] The polymer is composed of repeating units derived from methyl methacrylate, repeating units derived from benzyl methacrylate, and repeating units derived from adamantane methacrylate.

[0052] Example 3

[0053] This embodiment provides a method for preparing low birefringence optical grade polymethyl methacrylate. The preparation process and equipment used are the same as in Example 1, except that: Step S10, Raw material preparation. In a mixing tank, 900 kg (90 parts by mass) of dried and pretreated MMA, 60 kg (6 parts by mass) of the first comonomer naphthyl methacrylate (NMA), and 40 kg (4 parts by mass) of the second comonomer tricyclodecane methacrylate (TCDMA) are mixed. It should be noted that because naphthyl methacrylate (NMA) contains a bicyclic aromatic structure, its π electron cloud density and spatial conjugation effect are far greater than those of a monocyclic system, exhibiting extremely strong positive birefringence compensation capability. To avoid severe optical "overcompensation" (i.e., a shift from negative to positive with an excessively large absolute value), this formulation maintains a high proportion of 90 parts by mass of MMA substrate while precisely controlling the mass ratio of NMA to TCDMA, which has strong negative birefringence properties, at 1.5:1, in order to utilize the negative pull of TCDMA to suppress the strong positive deflection of NMA.

[0054] The initiator used was 0.3 kg of benzoyl peroxide (BPO) (approximately 0.03% of the total monomer mass), and the chain transfer agent used was 1.5 kg of n-butanethiol (0.15% of the total monomer mass). The mixture was stirred at 30°C, and high-purity nitrogen was continuously introduced to remove oxygen until the dissolved oxygen concentration was <1 mg / kg.

[0055] Step S20, continuous bulk polymerization. The above reaction mixture is subjected to polymerization reaction in the same process conditions as in Example 1, sequentially passing through a first reactor (controlled temperature 110°C, residence time 2.5 hours) and a second reactor (controlled temperature 140°C, residence time 2 hours).

[0056] Step S30 is the post-polymerization treatment, and step S40 is the melt devolatilization and granulation. The viscous melt obtained after polymerization undergoes the same post-treatment (240°C, -0.08MPa vacuum devolatilization) and twin-screw granulation process as in Example 1 to obtain the final granular product.

[0057] The performance of the granular product prepared in this embodiment was tested, and the results are as follows: the birefringence Δn is +0.85×10⁻⁶. -5 It has a light transmittance of 92.6%, a haze of 0.38%, and a glass transition temperature (Tg) of 108℃.

[0058] The polymer is composed of repeating units derived from methyl methacrylate, repeating units derived from naphthyl methacrylate, and repeating units derived from tricyclodecane methacrylate.

[0059] Example 4

[0060] This embodiment provides a method for preparing low birefringence optical grade polymethyl methacrylate. The preparation process is basically the same as in Example 1, with the main difference being the fine adjustment of the monomer ratio and reaction temperature. Step S10, Raw material preparation. In a mixing tank, 800 kg (80 parts by weight) of dried and pretreated MMA, 50 kg (5 parts by weight) of the first comonomer naphthyl methacrylate (NMA) and 150 kg (15 parts by weight) of the second comonomer isobornyl methacrylate (IBMA) are mixed.

[0061] It should be noted that in this formulation, the mass ratio of the first comonomer to the second comonomer is exactly 1:3. Since the amount of the alicyclic monomer (IBMA) providing negative birefringence and steric hindrance is relatively large, this embodiment specifically selects naphthyl methacrylate (NMA), which contains a bicyclic aromatic structure, to achieve effective optical compensation even with a lower proportion of the first monomer. The extremely strong polarization tensor and positive birefringence contribution of the naphthalene ring are utilized to counteract the system's heavy negative birefringence background.

[0062] The initiator was 0.3 kg of AIBN, and the chain transfer agent was 2.0 kg of n-dodecyl mercaptan. The mixture was stirred at 30°C, and high-purity nitrogen was continuously introduced to remove oxygen until the dissolved oxygen concentration was <1 mg / kg.

[0063] Step S20, continuous bulk polymerization. The above reaction mixture is pumped into the polymerization reactor at the same flow rate as in Example 1. To match the reaction kinetics under this ratio and high steric hindrance monomer concentration, the temperature of the first reactor is controlled at 115°C and the residence time at 2.5 hours; the temperature of the second reactor is controlled at 145°C and the residence time at 2 hours.

[0064] Step S30 is post-polymerization processing, and step S40 is melt devolatilization and granulation. The melt obtained after polymerization is post-processed and granulated using the same devolatilization unit and twin-screw extruder as in Example 1.

[0065] The performance of the granular product prepared in this embodiment was tested, and the results showed that the birefringence Δn was -0.98 × 10⁻⁶. -5 It has a light transmittance of 92.4%, a haze of 0.42%, and a glass transition temperature (Tg) of 116℃.

[0066] The results of this embodiment show that even under the extreme boundary condition of a comonomer mass ratio of 1:3, the optical compensation mechanism between the aromatic ring and the rigid alicyclic ring remains effective, and the resulting polymer still has extremely low birefringence and excellent heat resistance.

[0067] Example 5

[0068] This embodiment provides a method for preparing low birefringence optical grade polymethyl methacrylate, and the specific preparation process is as follows: Step S10, raw material preparation. In a mixing tank, 800 kg (80 parts by weight) of dried and pretreated MMA, 150 kg (15 parts by weight) of the first comonomer benzyl methacrylate (BzMA) and 50 kg (5 parts by weight) of the second comonomer isobornyl methacrylate (IBMA) are mixed.

[0069] It should be noted that in this formulation, the mass ratio of the first comonomer to the second comonomer is exactly 3:1. Due to the extremely high amount of the first monomer added in this formulation (15 parts by mass), to prevent severe optical positive "overcompensation," this embodiment specifically selected benzyl methacrylate (BzMA), which has a flexible methylene side group and relatively weak polarization compensation efficiency. Utilizing its weak positive birefringence contribution, even with a large addition, it effectively offsets the deep negative base color of 80 parts by mass MMA and trace amounts of IBMA. The initiator used is 0.3 kg of AIBN, and the chain transfer agent is 2.0 kg of n-dodecyl mercaptan. The mixture was stirred at 30°C, and high-purity nitrogen was continuously introduced to remove oxygen until the dissolved oxygen concentration was <1 mg / kg.

[0070] Step S20, continuous bulk polymerization. The above reaction mixture is pumped into the polymerization reactor. The temperature of the first reactor is controlled at 110°C, and the residence time is 2.5 hours; the temperature of the second reactor is controlled at 140°C, and the residence time is 2 hours.

[0071] Step S30 is post-polymerization processing, and step S40 is melt devolatilization and granulation. The melt obtained after polymerization is post-processed and granulated using the same devolatilization unit and twin-screw extruder as in Example 1.

[0072] The performance of the granular product prepared in this embodiment was tested, and the results showed that the birefringence Δn was +0.92 × 10⁻⁶. -5 It has a light transmittance of 92.6%, a haze of 0.38%, and a glass transition temperature (Tg) of 105℃.

[0073] The results of this embodiment show that, under the extreme boundary condition of a comonomer mass ratio of 3:1, even with the use of a large amount of the first comonomer, as long as a monomer with weak polarization ability (such as BzMA) is reasonably selected, the birefringence can still be controlled within (|Δn|≤1×10⁻⁶). -5 Within the acceptable range. However, due to the excessively high proportion of flexible monomers and the limited proportion of rigid alicyclic monomers, the formulation basically failed to improve heat resistance (Tg), which is a typical extreme compromise of "sacrificing heat resistance for optical isotropy". Example 6

[0074] This embodiment provides a method for preparing low birefringence optical grade polymethyl methacrylate, which involves the compounding of multiple similar first comonomers, aiming to verify the optical and thermodynamic performance of the mixed aromatic ring monomer system: Step S10, Raw material preparation. In a mixing tank, pump in 850 kg (85 parts by mass) of dried and pretreated MMA. Then, simultaneously add two monomers with different aromatic ring side groups: 40 kg (4 parts by mass) of phenyl methacrylate (PhMA) and 40 kg (4 parts by mass) of benzyl methacrylate (BzMA); and add 70 kg (7 parts by mass) of isobornyl methacrylate (IBMA) containing a rigid alicyclic structure.

[0075] It should be noted that in this compound system, the ratio of the total mass of monomers contributing positive birefringence (8 parts by mass) to the mass of monomers contributing steric hindrance (7 parts by mass) remains at approximately 1.14:1. By introducing BzMA containing flexible methylene groups to replace the rigid PhMA, the flexibility of the polymer chain segments can be moderately increased while ensuring basic optical compensation, thereby improving the melt flowability of the final material during injection molding.

[0076] The types and amounts of initiators and chain transfer agents (0.3 kg AIBN and 2.0 kg n-dodecyl mercaptan) and the deoxygenation operation were consistent with those in Example 1.

[0077] Step S20, continuous bulk polymerization. The above mixture is pumped into the polymerization reactor, and the polymerization process conditions (110°C for 2.5 hours in the first reactor and 140°C for 2 hours in the second reactor) are consistent with those in Example 1.

[0078] Step S30 is the post-polymerization treatment, and step S40 is the melt devolatilization and granulation. The subsequent vacuum devolatilization (240℃, -0.08MPa) and twin-screw granulation processes are performed according to the process parameters of Example 1.

[0079] The performance of the granular product prepared in this embodiment was tested, and the results showed that the birefringence Δn was -0.65 × 10⁻⁶. -5 It has a light transmittance of 92.5%, a haze of 0.41%, and a glass transition temperature (Tg) of 109℃.

[0080] The test results of this embodiment show that when multiple aromatic ring monomers with different polarization abilities and rigidities are introduced into the system simultaneously, the optical compensation effect and thermodynamic properties exhibit a reasonable additive effect. Compared with Example 1 (pure PhMA), the introduction of some flexible BzMA slightly weakens the positive compensation ability of the system and slightly decreases Tg, but the overall birefringence is still successfully controlled within (|Δn|≤1×10). -5Within the low birefringence requirement range, this demonstrates that the technical solution of the present invention still has good applicability when multiple monomers are compounded, providing a more flexible formulation design space for industrial production.

[0081] Comparative Example 1

[0082] This comparative example is used to illustrate the birefringence properties of unmodified pure polymethyl methacrylate.

[0083] The same preparation process and equipment as in Example 1 were used. In a mixing tank, 1000 kg (100 parts by weight) of MMA, 0.3 kg of AIBN and 2.0 kg of n-dodecyl mercaptan were mixed, deoxygenated and then subjected to continuous bulk polymerization, devolatilization and granulation.

[0084] The obtained pure polymethyl methacrylate granules were tested and found to have the following properties: birefringence Δn = -6.0 × 10⁻⁶. -5 It has a glass transition temperature (Tg) of 105℃, a light transmittance of 92.5%, and a haze of 0.4%.

[0085] Comparative Example 2

[0086] To illustrate the effect of adding only the first comonomer (which has positive birefringence) without adding the second comonomer, this comparative example is set up.

[0087] The same preparation process and equipment as in Example 1 were used. In a mixing tank, 850 kg (85 parts by weight) of MMA, 150 kg (15 parts by weight) of phenyl methacrylate (PhMA), 0.3 kg of AIBN and 2.0 kg of n-dodecyl mercaptan were mixed, deoxygenated and then subjected to continuous bulk polymerization, devolatilization and granulation.

[0088] The obtained polymer particle product was tested and its properties were as follows: birefringence Δn was -2.0 × 10⁻⁶. -5 The glass transition temperature Tg is 110℃.

[0089] Comparative Example 3

[0090] This comparative example aims to examine the effect of adding only the second comonomer (which has negative birefringence properties) without adding the first comonomer.

[0091] The same preparation process and equipment as in Example 1 were used. In a mixing tank, 850 kg (85 parts by weight) of MMA, 150 kg (15 parts by weight) of isobornyl methacrylate (IBMA), 0.3 kg of AIBN and 2.0 kg of n-dodecyl mercaptan were mixed, deoxygenated and then subjected to continuous bulk polymerization, devolatilization and granulation.

[0092] The obtained polymer particle product was tested and its properties were as follows: birefringence Δn was -8.0 × 10⁻⁶. -5 The glass transition temperature Tg is 111℃.

[0093] Comparative Example 4

[0094] This comparative example is intended to be used in comparison with conventional birefringence reduction modification routes in the art (such as the introduction of styrene monomers).

[0095] The same preparation process and equipment as in Example 1 were used. In a mixing tank, 850 kg (85 parts by weight) of MMA and 150 kg (15 parts by weight) of styrene (St) were mixed. 0.3 kg of AIBN was used as the initiator, and 2.0 kg of n-dodecyl mercaptan was used as the chain transfer agent. After deoxygenation, continuous bulk polymerization, devolatilization, and granulation were carried out.

[0096] The obtained polymer particle product was tested and its properties were as follows: birefringence Δn was +1.5 × 10⁻⁶. -5 The glass transition temperature (Tg) drops significantly to 96℃, the transmittance is 90.5%, and the haze is 0.8%.

[0097] Comparative Example 5

[0098] This comparative example is used to illustrate the effect when the mass ratio of the first comonomer to the second comonomer is 1:4.

[0099] The same preparation process and equipment as in Example 4 were used. In a mixing tank, 800 kg (80 parts by weight) of dried and pretreated MMA, 40 kg (4 parts by weight) of the first comonomer naphthalene methacrylate (NMA), and 160 kg (16 parts by weight) of the second comonomer isobornyl methacrylate (IBMA) were mixed. 0.3 kg of AIBN was used as the initiator, and 2.0 kg of n-dodecyl mercaptan was used as the chain transfer agent. After deoxygenation, continuous bulk polymerization, devolatilization, and granulation were carried out.

[0100] The obtained polymer particle product was tested and its properties were as follows: birefringence Δn was -2.85 × 10⁻⁶. -5 It has a glass transition temperature (Tg) of 117℃, a light transmittance of 92.2%, and a haze of 0.46%.

[0101] Comparative Example 6

[0102] This comparative example is used to illustrate the effect when the mass ratio of the first comonomer to the second comonomer is 4:1.

[0103] The same preparation process and equipment as in Example 5 were used. In a mixing tank, 800 kg (80 parts by weight) of dried and pretreated MMA, 160 kg (16 parts by weight) of the first comonomer benzyl methacrylate (BzMA), and 40 kg (4 parts by weight) of the second comonomer isobornyl methacrylate (IBMA) were mixed. 0.3 kg of AIBN was used as the initiator, and 2.0 kg of n-dodecyl mercaptan was used as the chain transfer agent. After deoxygenation, continuous bulk polymerization, devolatilization, and granulation were carried out.

[0104] The obtained polymer particle product was tested and its properties were as follows: birefringence Δn was +1.35 × 10⁻⁶. -5 It has a glass transition temperature (Tg) of 104℃, a light transmittance of 92.3%, and a haze of 0.43%.

[0105] To more intuitively illustrate the beneficial effects of this application, the key performance characteristics of the products prepared in the above embodiments and comparative examples are summarized in the table below.

[0106]

[0107] Based on the performance test results above, the comparison between the data of the embodiments of this application and the comparative examples further verifies the rationality of the above birefringence compensation law and parameter boundaries.

[0108] Comparative Example 1 shows that unmodified pure PMMA has strong inherent negative birefringence (Δn = -6.0 × 10⁻⁶). -5 Comparative Examples 2 and 3 show that neither introducing an aromatic ring monomer with positive birefringence nor introducing a rigid alicyclic monomer with significant steric hindrance can effectively neutralize the birefringence of the system; the latter even amplifies the negative birefringence deflection due to restricting main chain movement. Comparative Example 4 uses styrene (St), a conventional material in the art, for modification. Although it can change the birefringence polarity, it is prone to overcompensation and causes a significant deterioration in the glass transition temperature (Tg) of the material to 96°C. The above comparative examples confirm that existing conventional methods cannot simultaneously achieve both the optical isotropy and high thermal stability of polymers.

[0109] Examples 1 to 3 and Example 6 demonstrate that, using the combination of the first and second comonomers specified in this application, the absolute value of the polymer's birefringence can be successfully controlled at 1.0 × 10⁻⁶. -5 The following are extremely low levels. Furthermore, the test data demonstrate the rigorous structure-activity logic of the formulation system in this application: As shown in Example 3, when the first comonomer is naphthyl methacrylate (NMA) containing a bicyclic structure, the system exhibits extremely high positive polarization compensation efficiency due to its prominent π-electron cloud conjugation effect. Therefore, sufficient compensation (Δn = +0.85 × 10⁻⁶) can be achieved at a ratio of 1.5:1. -5 Conversely, as shown in Example 2, when benzyl methacrylate (BzMA), with its relatively flexible side groups, is used, its positive polarization ability is relatively weak, and the mass ratio needs to be increased to 2:1 to achieve the ideal compensation effect (Δn is -0.92 × 10⁻⁶). -5 Furthermore, the increase in Tg is relatively limited (107℃). The performance of the monomers with different polarization characteristics demonstrates that the technical solution of this application has clear chemical mechanism support and broad monomer applicability.

[0110] The mass ratio (1:3 to 3:1) of the first comonomer to the second comonomer as defined in the claims of this application is not an arbitrary numerical division, but a critical range established based on the polarization compensation limit and the thermodynamic compensation law.

[0111] Lower Limit Boundary Verification (1:3): Example 4 and Comparative Example 5 explored the critical state when rigid alicyclic monomers dominate in the system. Example 4 shows that at a lower limit ratio of 1:3, even with a heavy negative birefringence, as long as the NMA with the strongest polarization ability is matched as the first monomer, the birefringence can still be controlled at the acceptable edge (Δn is -0.98 × 10⁻⁶). -5 At the same time, an excellent Tg of 116℃ was obtained. However, when the ratio was further reduced to 1:4 (comparative example 5), even the use of NMA could not reverse the huge negative polarization trend, and the birefringence of the system deteriorated to -2.85×10⁻⁶. -5 This renders the optical-grade material unusable.

[0112] Upper Limit Boundary Verification (3:1): Example 5 and Comparative Example 6 explored the critical state when the aromatic ring monomer is dominant in the system. Example 5 shows that, at the upper limit ratio of 3:1, to avoid excessive positive deflection, using BzMA with weaker polarizability can control the birefringence at +0.92 × 10⁻⁶. -5 At this point, due to the excessively high proportion of flexible structural units in the system, the material's heat resistance (Tg) drops to 105℃, reaching the technical limit for compromising heat resistance. Once the ratio exceeds this upper limit to 4:1 (Comparative Example 6), not only does the birefringence undergo uncontrollable positive overcompensation (Δn runs out of control to +1.35×10⁻⁶), but also... -5 Furthermore, Tg fell below the pure substrate level (104℃), rendering the modification meaningless for practical industrial applications.

[0113] In summary, the comonomer combination and the ratio range of 1:3 to 3:1 specified in this application represent a precise technical balance between offsetting shear orientation birefringence and maintaining the rigidity of polymer chain segments. Both the upper and lower limits of this range exhibit significant critical technical effects that cannot be easily anticipated by those skilled in the art.

[0114] The above description is merely a preferred embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing low birefringence optical grade polymethyl methacrylate, characterized in that, This includes copolymerizing a monomer mixture comprising the following components using a continuous bulk polymerization process: 80-95 parts by weight of methyl methacrylate; 2-20 parts by weight of a first comonomer, wherein the first comonomer is selected from one or more of phenyl methacrylate, benzyl methacrylate, and naphthyl methacrylate; 2-20 parts by weight of a second comonomer, wherein the second comonomer is selected from one or more of isobornyl methacrylate, adamantane methacrylate, and tricyclodecane methacrylate; Wherein, the mass ratio of the first comonomer to the second comonomer is 1:3 to 3:1, and the sum of the mass parts of the above components is 100 parts; The monomer mixture further includes an initiator and a chain transfer agent; the initiator is selected from azo initiators or peroxide initiators, and its dosage is 0.01%-0.2% of the total mass of the monomer mixture; the chain transfer agent is 0.05%-0.5% of the total mass of the monomer mixture. The continuous bulk polymerization process does not use solvents or uses only auxiliary solvents less than 5% of the total mass of the monomer mixture; before the copolymerization, the methyl methacrylate is dehydrated to a moisture content of less than 50 mg / kg, and the monomer mixture is deoxygenated to a dissolved oxygen concentration of less than 1 mg / kg; The continuous bulk polymerization process includes polymerization reactions carried out sequentially in a first reactor and a second reactor, followed by a devolatilization step; wherein, the reaction temperature in the first reactor is 110-125℃, and the average residence time of the material is 2-3 hours; the reaction temperature in the second reactor is 140-150℃, and the average residence time of the material is 1.5-2.5 hours; the devolatilization step is carried out at 230-250℃ under vacuum conditions.

2. The method according to claim 1, characterized in that, The monomer mixture comprises: 80-90 parts by weight of methyl methacrylate; 3-15 parts by weight of the first comonomer; and 3-15 parts by weight of the second comonomer.

3. The method according to claim 1 or 2, characterized in that, The mass ratio of the first comonomer to the second comonomer is 1:2 to 2.5:

1.

4. The method according to claim 1, characterized in that, When the first comonomer contains naphthalene methacrylate, the mass ratio of the first comonomer to the second comonomer is 1:3 to 1.5:

1.

5. The method according to claim 1, characterized in that, When the first comonomer contains benzyl methacrylate, the mass ratio of the first comonomer to the second comonomer is 1:1 to 3:

1.

6. The method according to claim 1, characterized in that, The first comonomer is selected from phenyl methacrylate or benzyl methacrylate; the second comonomer is selected from isobornyl methacrylate or adamantyl methacrylate.

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