Method for preparing copolymer, copolymer prepared thereby, and thermoplastic resin composition comprising same

Abstract

The present invention relates to a method for preparing an acrylic acid-imide copolymer, the method comprising: (S1) initiating polymerization by adding to a reactor a reaction solution comprising an alkyl (meth) acrylate-based monomer and an aromatic vinyl-based monomer, and a buffer solution; and (S2) carrying out polymerization while continuously adding a monomer solution comprising an imide-based monomer to the reactor, in which the monomer solution of step (S2) is continuously added after step (S1) until a polymerization conversion rate is 50% to 75%, and the buffer solution comprises two or more non-phosphate-based buffer agents, the color and heat resistance of the resin can be improved by maintaining the pH of the polymer system to prevent hydrolysis of the monomers, and a composition imbalance between monomer units of the copolymer can be solved by adding the monomers in a separate manner.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the priority and benefit of Korean Patent Application No. 2022-0143360, filed on November 01, 2022, the entire content of which is incorporated herein by reference.

[0003] The present invention relates to a method for preparing an acrylic-imide copolymer having excellent color, a copolymer prepared thereby, and a thermoplastic resin composition containing the copolymer. Background Art

[0004] In recent years, with the development of optical technology, in the display industry, liquid crystal displays are required to be thinner, lighter, and larger. Specifically, technologies related to wide viewing angles, high contrast ratios, uniform screen displays, suppression of image changes based on viewing angles, etc. have become key issues. Therefore, the required properties of polymer materials for manufacturing displays have become more complex.

[0005] Various display technologies such as plasma display panels (PDPs), liquid crystal displays (LCDs), and organic EL displays (LEDs), which replace conventional cathode ray tubes (CRTs), have been proposed and commercialized. At the same time, various polymer films such as polarizing films, polarizer protective films, retardation films, light guide plates, and plastic substrates are used in these display devices, and the required properties of polymer materials for these displays have become more complex.

[0006] Currently, the most commonly used display polymer film is the polarizer protective film triacetyl cellulose (TAC). However, when used for a long time in a high-temperature or high-humidity environment, the TAC film has problems such as a decrease in polarization degree, separation of the polarizer from the film, or a decrease in optical properties. To solve these problems, polymer films based on methyl methacrylate, polystyrene, or polycarbonate have been proposed. These polymer films have excellent heat resistance. However, in the case of polystyrene or polycarbonate films, aromatic rings in the polymer cause birefringence during the orientation process, which has an adverse effect on optical properties. In the case of methyl methacrylate, although its retardation value is relatively small compared to polystyrene or polycarbonate, it is not sufficient for use in optical materials that require high precision by itself.

[0007] To solve the above problems, previous researchers have copolymerized methyl methacrylate with various types of monomers. Among them, it is known that the copolymerization of styrene and maleic anhydride helps to improve heat resistance. However, due to the stability problems of copolymers with maleic anhydride during molding and their high molding temperatures causing problems such as decomposition and gel formation, their use is limited.

[0008] In addition, cyclohexyl maleimide is a substance subject to environmental control. In particular, when the resin composition is extruded into a film, cyclohexyl maleimide remaining in the resin generates irritating and harmful odors, resulting in reduced production efficiency and process instability.

[0009] On the other hand, when phenyl maleimide is applied, phenyl maleimide generates less irritating odor during the film extrusion process and has improved thermal stability. Therefore, attempts have been made to utilize phenyl maleimide. However, due to the difference in reactivity with other monomers, during the polymerization process, there is a significant difference in composition between the copolymer produced at the beginning of the process and the copolymer produced at the end of the process. This imbalance in composition makes it difficult to ensure heat resistance and reduces transparency.

[0010] Meanwhile, in the case of using a suspension-polymerized transparent heat-resistant resin, an organic dispersant is generally used instead of an inorganic dispersant in the polymerization. This is because the metal contained in the inorganic dispersant may remain in the final product and affect transparency. However, since organic dispersants have a low pH when dispersed in water, they can act as catalysts during the high-temperature polymerization process and affect the physical properties of the final polymer. Specifically, since these organic dispersants act as catalysts for monomer hydrolysis, the content of by-products increases, resulting in color deterioration and reduced transparency and heat resistance.

[0011] Therefore, there is a need to develop a technique for preparing a copolymer that has excellent transparency, can maintain excellent heat resistance, and alleviates the imbalance in monomer composition.

[0012] Related Technical Literature

[0013] [Patent Literature]

[0014] (Patent Document 1) KR 1739621 B1 Summary of the Invention

[0015] Technical Problem

[0016] The present invention aims to provide a method for preparing an acrylic-acid imide copolymer. The acrylic-acid imide copolymer has small compositional variations between copolymer chains by continuously adding imide monomers during the polymerization process to control the reactivity between monomers, exhibits excellent transparency due to a low residual monomer content, and has improved physical properties such as heat resistance and transparency by using a buffer solution to prevent monomer hydrolysis, reduce the residual monomer content, and prevent the generation of by-products.

[0017] Technical Solution

[0018] To solve the above problems, the present invention provides a method for preparing an acrylic-acrylamide copolymer, comprising: (S1) initiating polymerization by adding a reaction solution containing (meth)acrylic acid alkyl ester monomers and aromatic vinyl monomers and a buffer solution to a reactor; and (S2) carrying out polymerization while continuously adding a monomer solution containing imide monomers to the reactor, wherein the monomer solution of step (S2) is continuously added after step (S1) until the polymerization conversion rate is 50% to 75%, and the buffer solution contains two or more non-phosphate buffer agents.

[0019] In addition, to solve the above problems, the present invention provides a thermoplastic resin composition comprising an acrylic-acrylamide copolymer prepared by the above preparation method and a thermoplastic resin.

[0020] Advantageous Effects

[0021] The method for preparing an acrylic-acrylamide copolymer according to the present invention can provide a copolymer having small compositional variations between copolymer chains by continuously adding imide monomers during polymerization and using a buffer solution having two or more non-phosphate buffer agents, showing excellent transparency and color due to low residual monomer or by-product content, and maintaining excellent levels of heat resistance and mechanical properties. Detailed Description of the Invention

[0022] Hereinafter, the present invention will be described in more detail to help understand the present invention.

[0023] Terms or words in this specification and claims should not be construed as limited to their conventional or dictionary meanings, but based on the principle that the inventor can appropriately define the concept of the terms so as to best explain their invention, the terms or words should be construed as meanings and concepts consistent with the technical concept of the present invention.

[0024] Unless otherwise defined, the terms and measurement methods used in the present invention can be defined as follows.

[0025] The term "composition" used in the present invention includes not only reaction products and decomposition products formed from the materials of the composition, but also mixtures containing the materials of the composition.

[0026] The term "monomer unit" used in the present invention may refer to a repeating unit formed when a compound used as a monomer or a compound used as a crosslinking agent participates in a polymerization or crosslinking reaction, a structure derived therefrom, or the substance itself.

[0027] The term "derivative" used in the present invention may refer to a compound having a structure in which one or more of the hydrogen atoms constituting the original compound are replaced by a halogen group, an alkyl group, or a hydroxyl group.

[0028] In the present invention, the "polymerization conversion rate" refers to the degree to which monomers are polymerized through a polymerization reaction to form a polymer. After collecting a portion of the polymer in the reactor during the polymerization process, the weight of the polymer after removing moisture is calculated using Equation 1 below. Then, the sample is dissolved in a tetrahydrofuran (THF) solvent and precipitated with methanol (MeOH) to remove unreacted monomers. The precipitated suspended solids are dried to measure the weight of the polymer, and the polymerization conversion rate is calculated using Equation 2 below.

[0029] [Equation 1]

[0030] (Actual weight of the polymer) = (Weight of the collected polymer) - (Weight of the collected polymer × Moisture content)

[0031] [Equation 2]

[0032] Polymerization conversion rate (%) = [(Weight of the polymer obtained by drying) / (Actual weight of the polymer)] × 100

[0033] Method for preparing acrylic-acid imide copolymer

[0034] According to one embodiment of the present invention, the method includes: (S1) initiating polymerization by adding a reaction solution containing (meth)acrylic acid alkyl ester monomers and aromatic vinyl monomers and a buffer solution to a reactor; and (S2) performing polymerization while continuously adding a monomer solution containing imide monomers to the reactor, wherein, after step (S1), the monomer solution of step (S2) is continuously added until the polymerization conversion rate reaches 50% to 75%, and the buffer solution contains two or more non-phosphate buffer agents.

[0035] According to one embodiment of the present invention, step (S1) is a step of initiating polymerization, which may include adding the reaction solution to the reactor and then raising the temperature of the reactor to a predetermined temperature. Even in step (S1), when the internal temperature of the reactor is higher than the predetermined temperature, polymerization is carried out in the presence of a polymerization initiator, and according to one embodiment of the present invention, the reaction solution does not need to contain imide monomers. When the reaction solution contains imide monomers and starts to participate in the polymerization, due to the high reactivity of the imide monomers, an excessive amount of imide monomers may be mixed into the copolymer chains formed at the beginning of the process, and a relatively small amount of imide monomers may be mixed into the copolymer chains formed at the end of the process, which may cause an imbalance in the composition between the copolymer chains and reduce the heat resistance.

[0036] Thus, in the preparation method according to an embodiment of the present invention, step (S1) can be carried out in the absence of imide monomers. In step (S1), the internal temperature of the reactor can be raised to about 60°C to 120°C, preferably 70°C to 110°C, more preferably 80°C to 110°C, and even more preferably 85°C to 105°C. During the process of raising the internal temperature of the reactor in step (S1), the point of reaching the initial temperature can be when the polymerization conversion rate is 10% or less than 10%, specifically less than 7%, more specifically less than 5%, and in some cases less than 3%.

[0037] According to an embodiment of the present invention, not only the reaction solution but also a buffer solution are mixed to initiate polymerization. The buffer solution can adjust the pH of the polymerization system to control side reactions or hydrolysis reactions of monomers that may occur when the internal temperature of the reactor rises. In addition, the buffer solution contains two or more types of buffers, but non-phosphate buffers are included. In some cases, since phosphate buffers form insoluble salts in the water-soluble suspension polymerization system, the physical properties of the copolymer may be reduced, so the buffer used in the present invention should be a non-phosphate buffer.

[0038] When only one type of buffer is used, pH adjustment by hydrogen ion exchange may not be smooth. In particular, side reactions may occur with the dispersion aid or molecular weight regulator included as additives in suspension polymerization, which may have an adverse effect on the polymerization reaction.

[0039] According to an embodiment of the present invention, the buffer can be two or more selected from acetic acid, citric acid, sodium acetate, sodium citrate, sodium carbonate, and sodium bicarbonate. As a preferred example, the combination of two buffers can be the combination of acetic acid and sodium acetate, the combination of citric acid and sodium citrate, or the combination of sodium carbonate and sodium bicarbonate.

[0040] According to an embodiment of the present invention, the reaction solution contains (meth)acrylic acid alkyl ester monomers and aromatic vinyl monomers.

[0041] Specifically, the (meth)acrylic acid alkyl ester monomers can be one or more selected from methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, decyl (meth)acrylate, and lauryl (meth)acrylate. Specifically, the (meth)acrylic acid alkyl ester monomers can be one or more selected from methyl methacrylate, ethyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, and butyl acrylate.

[0042] Based on the total amount of monomers added, the (meth)acrylic acid alkyl ester monomers can be added in an amount of 40 to 95 parts by weight, 50 to 85 parts by weight, or 60 to 80 parts by weight. Within this range, a copolymer with a high polymerization conversion rate can be obtained, and due to the easy adjustment of the glass transition temperature, excellent impact strength and transparency can be achieved.

[0043] The aromatic vinyl monomers can be, for example, one or more selected from styrene, α-methylstyrene, α-ethylstyrene, p-methylstyrene, o-methylstyrene, o-tert-butylstyrene, bromostyrene, chlorostyrene, trichlorostyrene, and their derivatives. As a specific example, it can be styrene.

[0044] Based on 100 parts by weight of the total amount of monomers added, the aromatic vinyl monomers can be added in an amount of 1 to 50 parts by weight, 3 to 40 parts by weight, or 5 to 30 parts by weight. Within this range, a copolymer with a high polymerization conversion rate can be obtained, and the mechanical properties of the copolymer can be maintained while controlling the glass transition temperature and improving the compatibility with the thermoplastic resin.

[0045] According to one embodiment of the present invention, step (S2) is carried out immediately after step (S1). Step (S2) includes polymerizing while continuously adding a monomer solution containing aromatic vinyl monomers or imide monomers to the reactor, and continuously adding the monomer solution after step (S1) until the polymerization conversion rate reaches 50% to 75%.

[0046] According to one embodiment of the present invention, the monomer solution contains imide monomers. Due to their high reactivity, imide monomers are easily incorporated into the copolymer chain. When not properly controlled, a reduction in physical properties may occur due to the compositional imbalance between the copolymer chains. Therefore, in the present invention, a part of the monomer solution containing imide monomers is added batchwise before the internal temperature of the reactor rises, and the remaining part is continuously added during the polymerization after the temperature rises.

[0047] The monomer solution used in step (S2) is continuously added immediately after step (S1) until the polymerization conversion rate reaches 50% to 75%. Continuously adding the monomer solution can mean adding a certain amount at a constant rate within a certain time or adding the monomer solution to the reactor at a constant flow rate. There is no limitation on this method as long as the monomer solution can be continuously added to the reactor at a constant flow rate (or speed).

[0048] According to an embodiment of the present invention, the continuous addition of the monomer solution in step (S2) starts immediately after step (S1) and terminates when the polymerization conversion rate is 50% to 75%. Immediately after step (S1) can mean after the internal temperature of the reactor has risen to a predetermined temperature, and in another sense, can mean the time point when the polymerization conversion rate reaches a level of about 3% to 10%.

[0049] Furthermore, with regard to termination, when the continuous addition continues until the polymerization conversion rate exceeds 75%, the subsequently added imide monomer may not participate in the polymerization, which may cause an imbalance in the composition between the chains and an increase in the residual monomer content, and thus may reduce the glass transition temperature. When the continuous addition is terminated prematurely before the polymerization conversion rate becomes 50%, an excessive amount of imide monomer is mixed into the copolymer chains formed at the beginning, which may cause an imbalance in the composition between the chains. Therefore, the continuous addition of the monomer solution is preferably terminated when the polymerization conversion rate is 50% to 70%, and can be preferably terminated when the polymerization conversion rate is 60% to 70%.

[0050] The imide monomer can be, for example, a maleimide monomer. As a specific example, it can be a maleimide monomer in which the hydrogen bonded to the N atom of maleimide is replaced by a substituent. As a more specific example, the imide monomer can be one or more selected from N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-isopropylmaleimide, N-butylmaleimide, N-isobutylmaleimide, N-tert-butylmaleimide, N-cyclohexylmaleimide, N-chlorophenylmaleimide, N-methylphenylmaleimide, N-bromophenylmaleimide, N-laurylmaleimide, N-hydroxyphenylmaleimide, N-methoxyphenylmaleimide, N-carboxyphenylmaleimide, N-nitrophenylmaleimide, N-phenylmaleimide, 2-methyl-N-phenylmaleimide, N-benzylmaleimide, N-naphthylmaleimide and their derivatives. As a specific example, it can be N-phenylmaleimide.

[0051] Based on 100 parts by weight of the total amount of monomers added in steps (S1) and (S2), the imide monomer can be added in an amount of 1 part by weight to 50 parts by weight, 3 parts by weight to 40 parts by weight, or 5 parts by weight to 30 parts by weight. Within this range, a copolymer can be prepared with a high polymerization conversion rate and a uniform composition of monomer units, and the prepared copolymer has excellent heat resistance.

[0052] According to one embodiment of the present invention, the above preparation method can be carried out by suspension polymerization, and in addition to the monomer, the reaction solution can further contain one or more polymerization additives such as a polymerization initiator, a dispersant, a dispersion aid, or a molecular weight regulator, and further contain a polymerization solvent, and the monomer solution can further contain one or more of these additives.

[0053] According to one embodiment of the present invention, the polymerization initiator is used to initiate the polymerization easily, and there is no particular limitation on the polymerization initiator as long as it does not adversely affect the polymerization. It can be, for example, selected from one or more of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, bis(tert-butylperoxy-isopropyl)benzene, tert-butyl cumyl peroxide, di-(tert-amyl)-peroxide, dicumyl peroxide, butyl 4,4-bis(tert-butylperoxy)valerate, tert-butyl peroxybenzoate, 2,2-bis(tert-butylperoxy)butane, tert-amyl peroxybenzoate, tert-butyl peroxyacetate, tert-butyl peroxy-(2-ethylhexyl) carbonate, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxy-3,5,5-trimethylhexanoate, 1,1-bis(tert-butylperoxy)cyclohexane, tert-amyl peroxyacetate, tert-amyl peroxy-(2-ethylhexyl) carbonate, 1,1-bis(tert-butylperoxy)-3,5,5-trimethylcyclohexane, 1,1-bis(tert-amylperoxy)cyclohexane, tert-butyl monoperoxymaleate, 1,1'-azobis(cyclohexanecarbonitrile), and 1,1'-azobis(cyclohexanecarbonitrile), specifically selected from one or more of dicumyl peroxide, 1,1-bis(tert-butylperoxy)cyclohexane, and 1,1'-azobis(cyclohexanecarbonitrile).

[0054] Based on the total amount of the monomers used in the polymerization of 100 parts by weight, the polymerization initiator can be used in an amount of 0.001 part by weight to 0.5 part by weight, specifically 0.003 part by weight to 0.45 part by weight, or 0.06 part by weight to 0.25 part by weight. When the polymerization initiator is used within this range, the polymerization reaction can be carried out more easily, and thus the polymerization conversion rate can be increased.

[0055] According to one embodiment of the present invention, the dispersant in suspension polymerization is an additive that allows the suspension to be better dispersed in the solvent. It can be, for example, selected from one or more of water-soluble polyvinyl alcohol, partially saponified polyvinyl alcohol, polyacrylic acid, a copolymer of vinyl acetate and maleic anhydride, hydroxypropyl methyl cellulose, gelatin, calcium phosphate, tricalcium phosphate, hydroxyapatite, sorbitan monolaurate, sorbitan trioleate, polyoxyethylene, sodium lauryl sulfate, sodium dodecylbenzenesulfonate, and sodium dioctyl sulfosuccinate. As a specific example, it can be tricalcium phosphate, and it can be the same as the dispersant added before the polymerization is initiated.

[0056] Based on the total amount of monomers added of 100 parts by weight, a dispersant can be used in an amount of 0.5 parts by weight to 2.0 parts by weight, 0.5 parts by weight to 1.5 parts by weight, or 1.0 parts by weight to 1.5 parts by weight. Within this range, a copolymer with more uniform particles can be prepared by increasing the dispersion stability of the monomers in the polymerization system.

[0057] According to one embodiment of the present invention, the reaction solution can further contain a dispersion aid. As a specific example, the dispersion aid can be polyoxyethylene alkyl ether phosphate or sodium sulfate (Na 2 SO 4 ), and more specifically, it can be sodium sulfate (Na 2 SO 4 ).

[0058] According to one embodiment of the present invention, the molecular weight regulator can be, for example, one or more selected from α-methylstyrene dimer, tert-dodecyl mercaptan, n-dodecyl mercaptan, octyl mercaptan, carbon tetrachloride, dichloromethane, dibromomethane, tetraethyl thiuram disulfide, dipentamethylene thiuram disulfide, and diisopropyl xanthogen disulfide. As a specific example, it can be tert-dodecyl mercaptan.

[0059] Based on the total amount of monomers added of 100 parts by weight, the molecular weight regulator can be used in an amount of 0.01 parts by weight to 0.40 parts by weight, 0.05 parts by weight to 0.30 parts by weight, or 0.10 parts by weight to 0.25 parts by weight. Within this range, a copolymer with an appropriate weight-average molecular weight can be prepared.

[0060] According to one embodiment of the present invention, the above preparation method can be carried out in the presence of a water-soluble solvent as the solvent for carrying out the polymerization.

[0061] According to one embodiment of the present invention, the water-soluble solvent can be ion-exchanged water or deionized water. At the same time, according to one embodiment of the present invention, the monomer droplets can contain a water-soluble solvent, and the water-soluble solvent can be ion-exchanged water or deionized water and can be the same as the water-soluble solvent added before initiating the polymerization.

[0062] According to one embodiment of the present invention, an additional monomer can be further included to prepare a quaternary copolymer. The additional monomer can be a vinyl cyanide monomer, which can be further included in the reaction solution to carry out step (S1). The vinyl cyanide monomer can be one or more selected from acrylonitrile, methacrylonitrile, ethylacrylonitrile, and their derivatives. As a specific example, it can be acrylonitrile.

[0063] Based on the total amount of monomers added, vinyl cyanide monomers can be added in an amount of 5 parts by weight to 70 parts by weight, 10 parts by weight to 60 parts by weight, 15 parts by weight to 50 parts by weight, or 15 parts by weight to 40 parts by weight. Within this range, a copolymer with a high polymerization conversion rate and excellent compatibility with thermoplastic resins while maintaining the mechanical properties of the copolymer can be obtained.

[0064] According to one embodiment of the present invention, the method for preparing the copolymer can be carried out in a polymerization reactor equipped with a stirrer. The stirrer can continuously provide stirring force during the polymerization reaction, and the stirring speed of the stirrer can be 100 RPM to 1,000 RPM, 300 RPM to 800 RPM, or 400 RPM to 600 RPM. Within this range, a copolymer with more uniform particles can be prepared by increasing the dispersion stability of the monomers in the polymerization system.

[0065] According to one embodiment of the present invention, the polymerization in steps (S1) and (S2) can be carried out by raising the temperature two or more times. Raising the temperature can refer to raising the temperature inside the reactor where the polymerization reaction is carried out. Raising the temperature two or more times can refer to raising the temperature one or more times during the polymerization reaction after reaching the initial temperature, including the time to reach the initial temperature. In this way, when the polymerization is carried out by raising the temperature two or more times, the polymerization reaction is prevented from occurring explosively, and the reaction heat is easily controlled. Therefore, the polymerization reaction can be carried out in a mild state. Thereafter, raising the temperature one or more times can increase the polymerization conversion rate.

[0066] According to one embodiment of the present invention, when the polymerization is carried out by raising the temperature two or more times, the internal temperature of the reactor at the time of reaching the initial temperature can be 80°C to 95°C, 85°C to 95°C, or 90°C to 95°C. Within this range, excellent polymerization stability is achieved.

[0067] According to one embodiment of the present invention, when the polymerization is carried out by raising the temperature two or more times and raising the temperature one or more times during the polymerization reaction after reaching the initial temperature, the internal temperature of the reactor can be 100°C to 130°C, 110°C to 130°C, or 115°C to 125°C. Within this range, a copolymer with a high polymerization conversion rate can be prepared.

[0068] According to one embodiment of the present invention, the polymerization can be carried out until the polymerization conversion rate is 90% or more, or 90% to 100%.

[0069] In this way, when preparing the copolymer by the preparation method according to the present invention, the non-uniformity of the monomer units in the copolymer can be improved, the polymerization reaction can be maintained at a high polymerization conversion rate, the content of residual monomers can be significantly reduced, and by preventing the decrease in the glass transition temperature, a copolymer with excellent heat resistance can be prepared.

[0070] The present invention provides a copolymer prepared by the above preparation method. Specifically, the copolymer may be a transparent heat-resistant copolymer containing (meth)acrylic acid alkyl ester monomer units and imide monomer units, and as a more specific example, it may be a transparent heat-resistant styrene copolymer.

[0071] According to the present invention, there is provided a copolymer comprising (meth)acrylic acid alkyl ester monomer units, aromatic vinyl monomer units and imide monomer units, having a chromaticity a value of -0.50 to -0.10 and a chromaticity b value of 1.0 to 4.0 measured using a colorimeter, and having a glass transition temperature of 110°C to 135°C.

[0072] According to an embodiment of the present invention, the chromaticity values measured using a colorimeter are L, a and b, which respectively represent white (+100) to black (0), red (+100) to green (-80), and yellow (+70) to blue (-80). The copolymer of the present invention may preferably have a chromaticity a value of -0.45 to -0.20 or -0.43 to -0.27 and a chromaticity b value of 1.2 to 3.5 or 1.5 to 3.0.

[0073] According to an embodiment of the present invention, the copolymer may contain residual monomers and their derivatives in an amount of 500 ppm or less by weight. When preparing the copolymer by the above preparation method, the content of residual monomers and / or their derivatives can be significantly reduced, preferably reduced to 400 ppm or less, more preferably 370 ppm or less. When the content of residual monomers is so low, the transparency can be excellent, discoloration is less likely to occur, and problems such as odor during the process can be prevented.

[0074] The copolymer may be a random copolymer and have a uniform composition of (meth)acrylic acid alkyl ester monomer units, aromatic vinyl monomer units and imide monomer units. The uniform composition of the monomer units may mean that the proportion of each monomer unit present in the growing polymer polymerized by the polymerization reaction remains constant. In a specific example, during polymerization, that is, during the polymerization time, whenever a part of the polymer in the reactor is collected as a sample, the proportion of each monomer unit forming the polymer remains constant.

[0075] According to one embodiment of the present invention, the (meth)acrylic acid alkyl ester monomer unit, the aromatic vinyl monomer unit, and the imide monomer unit may be repeating units formed from the respective monomers participating in the polymerization reaction. As a specific example, the polymerization reaction may be a radical polymerization reaction, and thus the repeating units may be derived from the carbon-carbon double bonds present in each monomer.

[0076] According to one embodiment of the present invention, the copolymer may further contain a vinyl cyanide monomer unit. When the vinyl cyanide monomer unit is included, excellent mechanical properties such as impact strength and tensile strength can be ensured, and due to the improved fluidity, it may have advantages in molding such as injection molding or extrusion.

[0077] Thermoplastic resin composition

[0078] The resin composition according to the present invention contains the above-mentioned acrylic acid-imide copolymer and a thermoplastic resin. The resin composition can be prepared by mixing various resins according to the required use. For example, acrylonitrile-butadiene-styrene resins, acrylonitrile-styrene-acrylate resins, styrene-acrylonitrile resins, etc. can be used together, and resins copolymerized with (meth)acrylic acid alkyl ester monomers or maleimide monomers can be used, but it is not particularly limited thereto.

[0079] According to one embodiment of the present invention, if necessary, the resin composition may further contain one or more additives selected from impact modifiers, lubricants, heat stabilizers, anti-dripping agents, antioxidants, light stabilizers, ultraviolet blockers, pigments, and inorganic fillers. Based on 100 parts by weight of the copolymer and the thermoplastic resin, the additive can be used in an amount of 5.0 parts by weight or less, or 0.1 part by weight to 1.0 part by weight.

[0080] In addition, the additives can be used without particular limitation as long as they are used for the resin composition. For example, the anti-dripping agent can be one or more selected from Teflon, polyamide, polysilicon, polytetrafluoroethylene (PTFE), and tetrafluoroethylene-hexafluoropropylene (TFE-HFP) copolymer to improve flame retardancy, and the inorganic filler can be one or more selected from barium sulfate, barium glass filler, and barium oxide.

[0081] Examples

[0082] Hereinafter, the examples of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention can be embodied in several different forms and is not limited to the examples described herein.

[0083] Example 1

[0084] 136 parts by weight of ion-exchanged water, 77 parts by weight of methyl methacrylate, 13 parts by weight of styrene, 0.06 parts by weight of 1,1-bis(tert-butylperoxy)cyclohexane as a polymerization initiator, 0.05 parts by weight of a copolymer (copolymer of acrylic acid and 2-ethylhexyl acrylate) having a weight average molecular weight of about 1,850,000 g / mol as a dispersant, and 0.5 parts by weight of Na as a dispersion aid 2 SO 4 were uniformly mixed to prepare a reaction solution, and a buffer solution was prepared by uniformly mixing 0.1 parts by weight of a mixture of acetic acid and sodium acetate, then mixed with the reaction solution, and the resulting mixture was put into a reactor. Then, the internal temperature of the reactor was raised to 90 °C to initiate polymerization. From the time when the internal temperature of the reactor reached 90 °C, 10 parts by weight of N-phenylmaleimide was continuously added at a constant rate of about 3.33 parts by weight per hour for 180 minutes until the polymerization conversion rate reached 65%. After terminating the continuous addition of N-phenylmaleimide, polymerization was carried out for another 30 minutes while maintaining the temperature of the reactor at 90 °C. After the temperature of the reactor was raised to 95 °C at a constant rate over 10 minutes, polymerization was further carried out for 50 minutes while maintaining the temperature at 95 °C. Then, in order to further increase the conversion rate, the temperature was raised to 120 °C and polymerization was continued for another 2 hours before polymerization termination. After polymerization was completed, the polymerization slurry was washed with water, dehydrated, and dried to prepare a bead copolymer.

[0085] Each part by weight is based on the total amount of monomers added of 100 parts by weight.

[0086] Example 2

[0087] A copolymer was prepared in the same manner as in Example 1 except that a mixture of citric acid and sodium citrate was used as the buffer solution.

[0088] Example 3

[0089] A copolymer was prepared in the same manner as in Example 1 except that a mixture of sodium carbonate and sodium bicarbonate was used as the buffer solution.

[0090] Example 4

[0091] A copolymer was prepared in the same manner as in Example 1 except that 10 parts by weight of N-phenylmaleimide was continuously added at a constant rate until the polymerization conversion rate reached 55%.

[0092] Example 5

[0093] A copolymer was prepared in the same manner as in Example 1 except that 10 parts by weight of N-phenylmaleimide was continuously added at a constant rate until the polymerization conversion rate reached 70%.

[0094] Comparative Example 1

[0095] A copolymer was prepared in the same manner as in Example 1, except that no buffer solution was used.

[0096] Comparative Example 2

[0097] Except for using a mixture of Na 2 HPO 4 and NaH 2 PO 4 as a buffer solution, a copolymer was prepared in the same manner as in Example 1.

[0098] Comparative Example 3

[0099] Except for using a mixture of citric acid and Na 2 HPO 4 as a buffer solution, a copolymer was prepared in the same manner as in Example 1.

[0100] Comparative Example 4

[0101] A copolymer was prepared in the same manner as in Comparative Example 1, except that 13 parts by weight of N-phenylmaleimide was added to the reaction solution and no monomer solution was added after raising the temperature of the reactor.

[0102] Comparative Example 5

[0103] A copolymer was prepared in the same manner as in Example 1, except that 13 parts by weight of N-phenylmaleimide was added to the reaction solution and no monomer solution was added after raising the temperature of the reactor.

[0104] Experimental Example 1

[0105] For each of the copolymers prepared in Examples 1 to 6 and Comparative Examples 1 to 4, the glass transition temperature, residual monomer content, and transparency were measured by the following methods, and the results are shown in Table 1 below.

[0106] (1) Glass transition temperature (Tg, °C): After collecting 0.01 g of samples from the polymers and copolymers, each sample was heated from room temperature (20 °C to 25 °C) to 180 °C using a differential scanning calorimeter (DSC) (from Mettler Toledo) to remove impurities, and then each sample was cooled to room temperature (20 °C to 25 °C) and heated to 180 °C again to measure the glass transition temperature.

[0107] (2) Residual monomer content (wt%): After polymerization, each bead copolymer was dissolved in chloroform, methanol was added to precipitate the polymer resin, and then a part of the supernatant containing the residual monomer was taken as a sample, and the content of the residual monomer in each sample was calculated using a gas chromatograph (Agilent 6890N (GC-FID), commercially available from Agilent Technologies, Inc.) under the following measurement conditions.

[0108] - Inlet: 280 °C, carrier gas He, split ratio 10:1

[0109] - Column: HP-5 capillary column

[0110] - Flow rate: 1.4 mL / min

[0111] - Oven: Isothermal at 100 °C for 3 minutes, heated to 300 °C at a rate of 10 °C / min, and isothermal at 300 °C for 5 minutes

[0112] - Detector: FID, 250 °C

[0113] (3) Chromaticity (a, b): Values a and b were measured using a colorimeter (HunterLab). Value a represents red (red +100 to -80 green), and value b represents yellow (yellow +70 to -80 blue).

[0114] (4) Transparency: The total light transmittance (Tt) of each prepared copolymer was measured using a haze meter.

[0115] [Table 1]

[0116]

[0117] As shown in Table 1, it can be seen that, compared with the comparative examples, Examples 1 to 5 prepared by the copolymer preparation method according to the present invention have a significantly lower residual monomer content, and exhibit excellent chromaticity characteristics and a uniform glass transition temperature. In addition, it was confirmed that the transparency is excellent.

[0118] On the other hand, when a phosphorus-based antioxidant is used as in Comparative Examples 2 and 3, other characteristics may be similar to those of the examples, but the transparency may be poor. In particular, considering that a transparent but defective product is necessarily a defective product, it can be seen that using a non-phosphate buffer may cause problems due to the generation of insoluble salts during the product manufacturing process. In addition, it can be seen that Comparative Examples 1 to 4 without using a buffer have a high residual monomer content and poor chromaticity characteristics, while Comparative Example 5 using a buffer but with the monomer solution not added continuously has a reduced residual monomer content, but has poor chromaticity characteristics and transparency.

[0119] It can be confirmed from the results that when a buffer solution containing a non-phosphate buffer is appropriately applied simultaneously with a monomer solution, heat resistance is ensured because the glass transition temperature appears above a certain level, and a copolymer having a low residual monomer content and excellent chromaticity characteristics and transparency is obtained.

Claims

1. A method for preparing an acrylic-acid imide copolymer, comprising: (S1) Initiating polymerization by adding a reaction solution containing an alkyl (meth)acrylate monomer and an aromatic vinyl monomer and a buffer solution to a reactor; and (S2) Conducting polymerization while continuously adding a monomer solution containing an imide monomer to the reactor, wherein, after step (S1), the monomer solution of step (S2) is continuously added until the polymerization conversion rate is 50% to 75%, and the buffer solution contains two or more non-phosphate buffer agents.

2. The method according to claim 1, wherein, the buffer agent is two or more reagents selected from acetic acid, citric acid, sodium acetate, sodium citrate, sodium carbonate, and sodium bicarbonate.

3. The method according to claim 1, wherein, the buffer agent is a combination of acetic acid and sodium acetate, a combination of citric acid and sodium citrate, or a combination of sodium carbonate and sodium bicarbonate.

4. The method according to claim 1, wherein, the monomer solution is continuously added until the polymerization conversion rate is 55% to 70%.

5. The method according to claim 1, wherein, based on the total amount of 100 parts by weight of the added monomers, the imide monomer in step (S2) is included in an amount of 1 part by weight to 50 parts by weight.

6. The method according to claim 1, wherein, the reaction solution in step (S1) further contains one or more of a polymerization initiator, a dispersant, and a molecular weight regulator.

7. The method according to claim 1, wherein, the monomer solution in step (S2) further contains one or more of a polymerization initiator, a dispersant, and a molecular weight regulator.

8. The method according to claim 1, wherein, the reaction solution in step (S1) further contains a vinyl cyanide monomer.

9. The method according to claim 1, wherein, the polymerization is carried out by suspension polymerization.

10. A thermoplastic resin composition comprising the acrylic-acid imide copolymer prepared by the method according to claim 1.

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