Self-plasticizing binder for non-aqueous tape casting and preparation method thereof

Abstract

The invention discloses a self-plasticization binder for non-aqueous tape casting and a preparation method thereof, the self-plasticization binder is solid acrylic resin obtained by copolymerization of A type monomers and B type monomers, the A type monomers are selected from at least one of methacrylate monomers with homopolymer Tg greater than 100 DEG C, and the B type monomers are selected from at least one of methacrylate monomers with homopolymer Tg greater than 100 DEG C; the monomer B is selected from at least one of (methyl) acrylate monomers of which the homopolymer Tg is less than 30 DEG C; the mass ratio of the monomer A to the monomer B is (1: 4)-(2: 3); the Mw of the solid acrylic resin is 300,000-600,000 Da, and the Tg of the solid acrylic resin is-20 DEG C to 20 DEG C. According to the invention, a structure with both strength and flexibility is successfully constructed in a polymer molecule, so that the adhesive has a plasticizing characteristic, thereby greatly reducing the dependence on a migration micromolecular plasticizer, showing excellent storage stability and solving the problem of poor storage stability of a ceramic green body in the prior art.

Description

TECHNICAL FIELD

[0001] The present application relates to a binder, in particular to a self-plasticizing binder for non-aqueous tape casting and a preparation method thereof, and belongs to the technical field of functional materials. BACKGROUND

[0002] Tape casting is a key process for preparing thin-layer materials with thickness ranging from microns to millimeters, and has been widely used in the production of thin sheets, films or substrates of ceramics, metals, polymers and their composites, especially in the field of electronic ceramics, such as the manufacture of multilayer ceramic capacitors and ceramic substrates. The core steps of this process include: mixing ceramic powders with organic additives such as dispersants, binders, plasticizers, etc. in a solvent to form a uniform and stable slurry; then, the slurry is spread onto a substrate by a doctor blade of a casting machine to form a wet film of a certain thickness; finally, the solvent is removed by drying to obtain a green ceramic tape with certain mechanical strength.

[0003] According to the difference of solvent system, tape casting can be divided into water-based and non-aqueous-based. Non-aqueous tape casting process occupies a dominant position in industrialized mass production due to its relatively simple process control, low equipment requirement, stable product performance and low cost, etc.

[0004] Among the numerous organic additives of the casting slurry, the binder plays a crucial role. It not only provides the necessary mechanical strength (i.e. green strength) for the green tape to facilitate subsequent operations such as demolding, cutting, lamination and handling, but also plays a key role in adjusting the rheological properties of the slurry. An ideal binder should have good film-forming properties, compatibility with ceramic powders, no effect on solvent evaporation, easy to decompose completely during sintering and less residue (i.e. clean burnout property), etc.

[0005] Traditionally, polyvinyl butyral (PVB) is the most widely used non-aqueous tape casting binder. However, the presence of hydroxyl groups on the molecular chain of PVB makes it have certain chemical activity. When used to prepare certain specific ceramic materials (such as low-temperature co-fired ceramics containing borate), these hydroxyl groups will undergo unintended crosslinking reactions with the surface of ceramic powders, resulting in a sharp increase in slurry viscosity or even gelation, thus making stable casting impossible and seriously affecting production.

[0006] To circumvent the aforementioned drawbacks of PVB, acrylic binders, such as polymethyl methacrylate (PMMA), have received increasing attention in recent years due to their almost hydroxyl-free molecular backbone and stable chemical properties, which do not crosslink with ceramic powders. However, conventional acrylic binders also have problems. They usually have a high glass transition temperature (Tg) and are rigid at room temperature, resulting in poor flexibility and easy brittle fracture of the prepared green body. Although the green body strength can be improved by increasing the amount of acrylic binder, this will introduce excessive organic matter, making it difficult to sinter and compact, and possibly increasing sintering defects.

[0007] To solve the problem of insufficient flexibility of acrylic binders, the prior art usually uses an external plasticizer. Plasticizers are substances that can enter between polymer chains, weaken the interchain force, increase the movement ability of polymer segments, and reduce the glass transition temperature, thereby imparting the necessary flexibility and plasticity to the green body. For example, Chinese Patent Application CN202411732307.0 discloses a low-temperature co-fired ceramic tape casting slurry based on acrylic resin, which includes low-temperature co-fired ceramic green ceramic powder, binder, plasticizer, solvent, and dispersant. The binder is 10-20% and the plasticizer is 2-8% based on 100% of the mass of the low-temperature co-fired ceramic green ceramic powder. The binder is one or more thermoplastic acrylic resins with a weight average molecular weight of 100-500 thousand and a glass transition temperature of 30-80°C. When the binder is one acrylic resin, the weight average molecular weight is 250-400 thousand and the glass transition temperature is 40-60°C. When the binder is a plurality of acrylic resins, low glass transition temperature acrylic resins and high glass transition temperature acrylic resins are combined. The glass transition temperature of the low glass transition temperature acrylic resin is 30-50°C and the glass transition temperature of the high glass transition temperature acrylic resin is 50-80°C. This patent aims to prevent the casting film from cracking and deforming, and to enable the slurry to achieve uniform film thickness during casting and avoid gas blockage in the film during film removal. The casting slurry requires 2-8% (relative to the ceramic powder) plasticizer, which is one or more of triethylene glycol diisooctanoate, triethylene glycol diisooctanoate, polyethylene glycol-400, dibutyl phthalate, dioctyl phthalate, and benzyl butyl phthalate. These plasticizers are small molecule compounds that are physically mixed with the acrylic resin binder and do not form chemical bonds. During the storage or gentle heating of the green body, these small molecule plasticizers migrate from the interior to the surface of the body. This migration can cause a series of serious problems. First, the green body becomes hard and brittle due to the loss of plasticizer, the mechanical properties deteriorate, and the storage stability is poor. Second, the plasticizer that migrates to the surface makes the surface sticky and easily adsorbs dust, affecting subsequent lamination and printing processes, so that it cannot meet the stringent application requirements of high-end products such as electronic ceramics.

[0008] As can be seen from the above, the prior art is faced with a dilemma: on the one hand, plasticizers are needed to impart flexibility to the acrylic adhesive system; on the other hand, the migration of small molecule plasticizers seriously damages the storage stability and long-term performance of the product. Therefore, there is an urgent need in the art to develop a new type of adhesive that can provide sufficient mechanical strength and the required flexibility, so as to reduce or even eliminate the need for additional small molecule plasticizers, thereby fundamentally solving the problem of plasticizer migration. SUMMARY

[0009] In view of the above problems and needs existing in the prior art, the purpose of the present application is to provide a self-plasticizing adhesive with plasticizing properties for non-aqueous tape casting and a preparation method thereof.

[0010] To achieve the above-mentioned purposes of the application, the technical solutions adopted by the present application are as follows:

[0011] A self-plasticizing adhesive for non-aqueous tape casting is a solid acrylic resin obtained by copolymerization of a class A monomer and a class B monomer, the class A monomer is selected from at least one of the methacrylate monomers with a homopolymer glass transition temperature (Tg) greater than 100℃, the class B monomer is selected from at least one of the (meth)acrylate monomers with a homopolymer glass transition temperature (Tg) less than 30℃; the mass ratio of the class A monomer to the class B monomer is 1:4 to 2:3; and the weight average molecular weight (Mw) of the solid acrylic resin is 300,000 to 600,000 Da, and the glass transition temperature (Tg) of the solid acrylic resin is -20℃ to 20℃.

[0012] In a preferred embodiment, the class A monomer is selected from at least one of the methacrylate monomers with a homopolymer glass transition temperature (Tg) between 100℃ and 180℃.

[0013] In a further preferred embodiment, the class A monomer is selected from at least one of methyl methacrylate (MMA, Tg ≈ 105℃), tert-butyl methacrylate (tBMA, Tg ≈ 107℃), cyclohexyl methacrylate (CHMA, Tg ≈ 110℃), 3,3,5-trimethylcyclohexyl methacrylate (TMCHMA, Tg ≈ 127℃), isobornyl methacrylate (IBOMA, Tg ≈ 170℃).

[0014] In a preferred embodiment, the class B monomer is selected from at least one of the (meth)acrylate monomers with a homopolymer glass transition temperature (Tg) between -100℃ and 25℃.

[0015] Further preferred, the B monomers are selected from at least one of n-butyl methacrylate (BMA, Tg ~ 20°C), iso-octyl methacrylate (EHMA, Tg ~ -10°C), lauryl methacrylate (LMA, Tg ~ -65°C), methyl acrylate (MA, Tg ~ 0°C), ethyl acrylate (EA, Tg ~ -21°C), n-butyl acrylate (BA, Tg ~ -56°C), iso-butyl acrylate (IBA, Tg ~ -24°C), iso-octyl acrylate (EHA, Tg ~ -70°C), stearyl acrylate (SA, Tg ~ -49°C).

[0016] A method for preparing the self-plasticizing adhesive as described above, which comprises free radical polymerization of the A monomers and the B monomers in the presence of a dispersant, an initiator and a chain transfer agent.

[0017] In one embodiment, the method comprises the following steps:

[0018] a) The A monomers, the B monomers, the initiator and the chain transfer agent are added into a container in a predetermined ratio, stirred at room temperature until all components are completely dissolved, and a uniform oil phase is formed;

[0019] b) A predetermined amount of the dispersant and a volume of deionized water are added into the reaction container, the volume ratio of the water phase to the oil phase is controlled to be (2-4): 1, and then the container is stirred at a constant temperature of 40-60°C until the dispersant is completely dissolved, and a water phase is obtained;

[0020] c) The oil phase prepared in step a) is added into the water phase of step b), and stirred at a constant temperature of 40-60°C for 30-50 minutes;

[0021] d) The polymerization reaction is carried out at a constant temperature of 65-95°C for 2-4 hours, and then the container is stirred at a constant temperature of 98-100°C for 0.5-1.5 hours;

[0022] e) The heating is stopped, the temperature of the reaction system is naturally cooled to below 60°C, the stirring is stopped, and the solid product is collected by standing and filtering, and then the obtained solid product is washed and dried, thereby obtaining the self-plasticizing adhesive.

[0023] In one embodiment, the initiator is selected from at least one of dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate, dilauroyl peroxide, 1,1-bis(tert- amylperoxy)cyclohexane.

[0024] In one preferred embodiment, the amount of the initiator is 0.1%-0.2% of the total mass of the A monomers and the B monomers.

[0025] In an embodiment, the chain transfer agent is selected from at least one of n-dodecyl mercaptan, t-dodecyl mercaptan, n-octyl mercaptan, and isooctyl mercapto propionate.

[0026] In a preferred embodiment, the chain transfer agent is used in an amount of 0.1% to 0.5% of the total mass of monomers A and B.

[0027] In an embodiment, the dispersing agent is selected from at least one of polyvinyl alcohol, hydroxyethyl cellulose, sodium polyacrylate, and polyvinyl pyrrolidone.

[0028] In a preferred embodiment, the dispersing agent is used in an amount of 0.05% to 0.2% of the total mass of monomers A and B.

[0029] In an embodiment, in step e), the drying is performed by air blowing at 35°C to 45°C.

[0030] Compared with the prior art, the present application has the following remarkable advantages:

[0031] The present application successfully constructs a structure with both strength and flexibility inside the polymer molecules by precisely controlling the molecular weight, glass transition temperature, and the special selection and proportion of the polymerized monomers, so that the adhesive has self-plasticizing properties, thereby greatly reducing the dependence on the migration of small molecule plasticizers, and solving the problem of poor storage stability of ceramic green tapes in the prior art. Experiments have shown that, by using the adhesive of the present application, only a small amount of small molecule plasticizer needs to be added, which not only makes the ceramic slurry have excellent casting performance, but also makes the processed green tape have almost no change in mechanical properties (elastic modulus) after being stored at 40°C for one month, showing excellent storage stability.

[0032] In addition, the carbon residue rate of the adhesive after burning is less than 0.3%, which has no effect on the sintering density of the ceramic substrate, and can be used as an adhesive for non-aqueous-based tape casting ceramic thin layer devices. DETAILED DESCRIPTION

[0033] The technical solutions of the present application will be further described in detail below with reference to the examples. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods not specified in the following examples are usually carried out according to the conventional conditions or according to the conditions recommended by the manufacturers.

[0034] In the following examples and comparative examples, the performance test methods of the adhesive are as follows:

[0035] Weight average molecular weight (Mw): measured by gel permeation chromatography (GPC) using polystyrene (PS) of different molecular weights as a calibration standard, tetrahydrofuran as a mobile phase, a flow rate of 1 mL / min, a differential refractive index detector (RI detector), and the sample being injected after being dissolved in tetrahydrofuran.

[0036] Glass transition temperature (Tg): measured by differential scanning calorimetry (DSC), i.e., about 0.15 g of a resin sample is weighed into an aluminum crucible, two scans are performed at a temperature increase rate of 10 ℃ / min, and the midpoint value of the second scan curve is taken as the Tg value of the sample.

[0037] Resin carbon residue on ignition: a resin sample is weighed into a crucible, placed in a muffle furnace, ignited at 400 ℃ for 5 hours, weighed again after cooling, and the percentage of the residual solid mass to the original sample mass is calculated.

[0038] Film strip modulus of elasticity: measured by the vibration pulse excitation method according to the ASTM E1876-2007 standard at 22 ℃ and 50 ℃, respectively.

[0039] Examples 1-4: Preparation of the self-plasticizing adhesive of the present application

[0040] General preparation procedure:

[0041] a) A clean 1500 mL beaker is taken, and the monomers of class A, the monomers of class B, the initiator, and the chain transfer agent are added according to the proportions (the types and amounts used in each example are shown in Table 1), stirred at room temperature until all the components are completely dissolved, and a uniform oil phase is formed;

[0042] b) A clean 5000 mL three-necked flask is taken, and the dispersant (the types and amounts used in each example are shown in Table 1) and a certain volume of deionized water (the volume ratio of the aqueous phase to the oil phase in each example is shown in Table 1) are added, then constant temperature stirring is performed at 50 ℃ in a constant temperature water bath until the dispersant is completely dissolved, and an aqueous phase is obtained;

[0043] c) The oil phase prepared in step a) is added to the aqueous phase of step b), and then constant temperature stirring is performed at 50 ℃ in a constant temperature water bath for 40 minutes;

[0044] d) Constant temperature polymerization is performed at 65-95 ℃ in a constant temperature water bath for 2-4 hours (the polymerization temperature and the polymerization reaction time of each example are shown in Table 1), and then constant temperature stirring is performed at 99 ℃ in a constant temperature water bath for 1 hour;

[0045] e) stop heating, when the temperature in the reaction system naturally cools to below 60°C, stop stirring, collect the solid product by settling and filtration, then wash the obtained solid product with deionized water for 5 times, and finally dry the washed product in a blast oven at 40°C to constant weight, to obtain the self-plasticizing binder.

[0046] Table 1 Composition formula, main process parameters and properties of the obtained acrylic resin of each example

[0047]

[0048] Comparative Examples 1-6: Preparation of comparative acrylic resins

[0049] The difference between Comparative Example 1 and Example 1 is only that the amount of chain transfer agent is increased, so that the molecular weight of the obtained acrylic resin is less than the molecular weight range of the self-plasticizing binder described in the application, as shown in Table 2.

[0050] The difference between Comparative Example 2 and Example 1 is only that the amount of chain transfer agent and the amount of initiator are both reduced, so that the molecular weight of the obtained acrylic resin is greater than the molecular weight range of the self-plasticizing binder described in the application, as shown in Table 2.

[0051] The difference between Comparative Example 3 and Example 1 is only that the ratio of the B-type monomer is changed, so that the Tg of the obtained acrylic resin is lower than the Tg range of the self-plasticizing binder described in the application, as shown in Table 2.

[0052] The difference between Comparative Example 4 and Example 1 is only that the ratio of the A-type monomer and the B-type monomer is changed, so that the Tg of the obtained acrylic resin is higher than the Tg range of the self-plasticizing binder described in the application, as shown in Table 2.

[0053] The difference between Comparative Example 5 and Example 1 is only that the ratio of the A-type monomer and the B-type monomer is changed, so that the mass ratio of the A-type monomer and the B-type monomer is less than the range of 1:4-2:3 described in the application, as shown in Table 2.

[0054] The difference between Comparative Example 6 and Example 1 is only that the ratio of the A-type monomer and the B-type monomer is changed, so that the mass ratio of the A-type monomer and the B-type monomer is greater than the range of 1:4-2:3 described in the application, as shown in Table 2.

[0055] Table 2 Composition formula, main process parameters and properties of the obtained acrylic resin of each comparative example

[0056]

[0057] Application performance evaluation:

[0058] Into a container were put 10 parts by mass of a binder (either the self-plasticizing binder prepared in Examples 1-4 or the acrylic resin prepared in Comparative Examples 1-6), 0.2 parts by mass of dibutyl phthalate (a plasticizer), 0.2 parts by mass of glycerol trioleate (a slurry dispersant), and 40 parts by mass of toluene (a slurry solvent). After heating to 50°C, the mixture was kept at this temperature and agitated pneumatically until it was thoroughly mixed and dissolved (about 24 hours). Then, 100 parts by mass of alumina ceramic powder were added, and the mixture was ball-milled until the alumina ceramic powder was uniformly dispersed (about 20 hours), to obtain an alumina ceramic slurry.

[0059] Using a casting apparatus, the prepared alumina ceramic slurry was cast into a film strip having a thickness of about 50 microns. After drying, the elastic modulus of the obtained film strip at 22°C and at 50°C was measured, respectively. The results are shown in Table 3.

[0060] Film strip storage stability test: Each of the prepared film strips was stored in a 40°C oven for 1 month, then taken out and cooled to room temperature, and the elastic modulus of each film strip at 22°C and at 50°C was re-measured, respectively. The results are shown in Table 3.

[0061] Table 3 Elastic modulus of each sample film strip

[0062]

[0063] Since the acceptable range of the elastic modulus of the alumina casting film strip at 22°C is 250-350 MPa, and the acceptable range of the elastic modulus at 50°C is 42-58 MPa; therefore, from the results shown in Table 3, it can be seen that:

[0064] ①Under the same conditions of ceramic slurry formulation and preparation process, the alumina casting film strips obtained using the self-plasticizing binders of Examples 1-4 not only have initial elastic modulus that meets the requirements, but also have almost no change in elastic modulus after being stored at 40°C for 1 month, showing excellent storage stability; indicating that the self-plasticizing binder described in the present application can achieve the unity of strength and flexibility without the need for a large amount of additional plasticizer (the amount is only 0.2%), and has unexpected technical effects.

[0065] ②Under the same conditions of ceramic slurry formulation and preparation process, the alumina casting film strip obtained using the acrylic resin of Comparative Example 1 (although the Tg meets the requirements, but the Mw is too low) as a binder has insufficient strength (the elastic modulus is lower than the acceptable range), and the alumina casting film strip obtained using the acrylic resin of Comparative Example 2 (although the Tg meets the requirements, but the Mw is too high) as a binder is too rigid (the elastic modulus is higher than the acceptable range); thus it can be proved that the limitation of the molecular weight range of 300-600 Da of the self-plasticizing binder described in the present application is one of the key contributions to obtain the beneficial effects of the present application.

[0066] ③In the same conditions of ceramic slurry formulation and preparation process, the alumina tape cast film band obtained by using the acrylic resin of Comparative Example 3 (although the Mw meets the requirements, but the Tg is too low) as the binder is too soft (the elastic modulus is lower than the qualified range), and the strength is insufficient; the alumina tape cast film band obtained by using the acrylic resin of Comparative Example 4 (although the Mw meets the requirements, but the Tg is too high) as the binder is too brittle and hard (the elastic modulus is higher than the qualified range), and cannot be used; thus it can be proved that the Tg range of -20℃ to 20℃ of the self-plasticizing binder described in the application is one of the key contributions to obtain the beneficial effects of the application.

[0067] ④In the same conditions of ceramic slurry formulation and preparation process, the alumina tape cast film band obtained by using the acrylic resins of Comparative Examples 5 and 6 (although the Mw and Tg are qualified in numerical value, but the ratio of the A type monomer to the B type monomer is improper) as the binder, the microstructure thereof still cannot reach the best state, resulting in that the mechanical properties are not up to standard (the elastic modulus is too low or too high); this highlights that the mass ratio (1:4 to 2:3) of the two types of monomers of the self-plasticizing binder described in the application is one of the key contributions to obtain the beneficial effects of the application.

[0068] ⑤Under the same conditions, by increasing the amount of plasticizer (from 0.2% to 5%), although the initial elastic modulus of the obtained alumina tape cast film band becomes qualified (see Application Comparative Example 4 and Application Comparative Example 7), but after one month of storage at 40℃, the elastic modulus increases sharply, with an increase of 23.5% (see Application Comparative Example 7), which has been seriously exceeded, which is direct evidence that the migration of the plasticizer leads to the hardening and brittleness of the film band.

[0069] In summary, the self-plasticizing binder for non-water-based tape casting provided by the application successfully builds a structure with strength and flexibility inside the polymer molecules through the synergistic design and precise control of the molecular weight, glass transition temperature and special limitation of the polymerized monomers, thereby greatly reducing the dependence on small molecule plasticizers with migration, solving the problem of poor storage stability of ceramic green tape in the prior art, providing a more reliable and stable binder for the preparation of high-performance electronic ceramics, and having significant progress and unexpected technical effects relative to the prior art, and having industrial application value.

[0070] Finally, it needs to be pointed out that: the above is only some preferred embodiments of the application, and cannot be understood as a limitation on the protection scope of the application, and some non-essential improvements and adjustments made by those skilled in the art based on the above content of the application are within the protection scope of the application.

Claims

1. A self-plasticizing binder for non-aqueous based tape casting, characterized in that it is a solid acrylic resin obtained by copolymerization of a Class A monomer selected from at least one of methacrylate monomers having a homopolymer glass transition temperature (Tg) greater than 100°C and a Class B monomer selected from at least one of (meth)acrylate monomers having a homopolymer glass transition temperature (Tg) less than 30°C; the mass ratio of the Class A monomer to the Class B monomer is 1:4 to 2:3; and the solid acrylic resin has a weight average molecular weight (Mw) of 300,000 to 600,000 Da and a glass transition temperature (Tg) of -20°C to 20°C.

2. The self-plasticizing binder of claim 1, wherein: The Class A monomer is selected from at least one of methacrylate monomers having a homopolymer glass transition temperature (Tg) of 100°C to 180°C.

3. The self-plasticizing binder of claim 2, wherein: The Class A monomer is selected from at least one of methyl methacrylate, t-butyl methacrylate, cyclohexyl methacrylate, 3,3,5-trimethylcyclohexyl methacrylate, and isobornyl methacrylate.

4. The self-plasticizing binder of claim 1, wherein: The Class B monomer is selected from at least one of (meth)acrylate monomers having a homopolymer glass transition temperature (Tg) of -100°C to 25°C.

5. The self-plasticizing binder of claim 4, wherein: The Class B monomer is selected from at least one of n-butyl methacrylate, isooctyl methacrylate, lauryl methacrylate, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, isooctyl acrylate, and stearyl acrylate.

6. A process for the preparation of the self-plasticizing binder for non-aqueous based tape casting according to claim 1, characterized by: The Class A monomer and the Class B monomer are subjected to free radical polymerization in the presence of a dispersant, an initiator, and a chain transfer agent by a suspension polymerization method.

7. The production method according to claim 6, characterized by, The method comprises the following steps: a) The Class A monomer, the Class B monomer, the initiator, and the chain transfer agent are added into a container in a predetermined ratio, stirred at room temperature until all components are completely dissolved, and a uniform oil phase is formed; b) A predetermined amount of the dispersant and a certain volume of deionized water are added into the reaction container, the volume ratio of the water phase to the oil phase is controlled to be (2-4):1, then the container is stirred at a constant temperature of 40-60°C until the dispersant is completely dissolved, and a water phase is obtained; c) The oil phase prepared in step a) is added into the water phase of step b), and stirred at a constant temperature of 40-60°C for 30-50 minutes; d) The polymerization reaction is carried out at a constant temperature of 65-95°C for 2-4 hours, and then stirred at a constant temperature of 98-100°C for 0.5-1.5 hours; e) The heating is stopped, the temperature of the reaction system is naturally cooled to below 60°C, the stirring is stopped, the solid product is collected by standing and filtering, and then the obtained solid product is washed and dried to obtain the self-plasticizing binder.

8. The method of claim 7, wherein: The initiator is selected from at least one of dibenzoyl peroxide, t-butyl peroxy-2-ethylhexanoate, dilauroyl peroxide, and 1,1-bis(t-amylperoxy)cyclohexane; and the amount of the initiator is 0.1%-0.2% of the total mass of the Class A monomer and the Class B monomer.

9. The method of claim 7, wherein: The chain transfer agent is selected from at least one of n-dodecyl mercaptan, tertiary dodecyl mercaptan, n-octyl mercaptan, and isooctyl mercaptan; the chain transfer agent is used in an amount of 0.1% to 0.5% of the total mass of the monomers of the A and B types.

10. The method of claim 7, wherein: The dispersant is selected from at least one of polyvinyl alcohol, hydroxyethyl cellulose, sodium polyacrylate, and polyvinyl pyrrolidone; the dispersant is used in an amount of 0.05% to 0.2% of the total mass of the monomers of the A and B types.

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