What Is Emulsion Polymerization?

In emulsion polymerization, the monomer is dispersed in water to form an emulsion by means of an emulsifier and mechanical stirring, and then an initiator is added to initiate the polymerization of the monomer. In the production of synthetic rubber by emulsion polymerization, in addition to the four main components of monomers, water, emulsifiers and initiators, buffers (used to keep the pH of the system unchanged) and activators (formed to form a redox cycle) are often added. System), regulators (adjust molecular weight, inhibit gel formation) and antioxidants (to prevent aging of rubber and vulcanizates) and other additives. Industrial varieties include milk polystyrene butadiene rubber, polyacrylate emulsion, etc.

In emulsion polymerization, the monomer is dispersed in water to form an emulsion by means of an emulsifier and mechanical stirring, and then an initiator is added to initiate the polymerization of the monomer. In the production of synthetic rubber by emulsion polymerization, in addition to the four main components of monomers, water, emulsifiers and initiators, buffers (used to keep the pH of the system unchanged) and activators (formed to form a redox cycle) are often added. System), regulators (adjust molecular weight, inhibit gel formation) and antioxidants (to prevent aging of rubber and vulcanizates) and other additives. Industrial varieties include milk polystyrene butadiene rubber, polyacrylate emulsion, etc.

Advantages: 1. Fast polymerization speed and high molecular weight; 2. Water as disperser medium, which is beneficial to heat transfer and temperature control; 3. After the reaction reaches a high conversion rate, the viscosity of the emulsion polymerization system is still low, and the dispersion system is stable and easy. Control and realize continuous operation; 4, latex can be directly used as the final product.

Disadvantages: 1. The polymer separation and precipitation process is complicated, and demulsifiers or coagulants need to be added; 2. The reactor walls and pipes are easy to hang glue and block; Washing and removing impurities will affect the physical properties of the product.

Chinese name

Emulsion polymerization

Foreign name

emulsion polymerization

Advantages

Fast polymerization and high molecular weight

Missing point

If drying is needed, the process is difficult to control

Make up

Monomer, water, initiator, emulsifier

Industrial variety

Polystyrene butadiene rubber, polyacrylate emulsion, etc.

Emulsion polymerization characteristics

advantage:

Fast polymerization and high molecular weight;

The polymerization heat is easy to diffuse and the polymerization temperature is easy to control;

(3) The polymerization system has a low viscosity even at the late stage of the reaction, so it is also suitable for preparing highly viscous polymers;

Water is used as the medium, production safety and reducing environmental pollution;

(5) Can be used directly as an emulsion.

(6) Flexible production methods are conducive to new product development

High polymerization rate and high molecular weight can be achieved at the same time. In the process of free-radical bulk polymerization, factors that increase the polymerization rate often lead to a decrease in the molecular weight of the product. In addition, the emulsion system has low viscosity, is easy to transfer heat and mix, and is easy to control production. The obtained latex can be used directly, and residual monomers are easily removed. The disadvantage is that the polymer contains impurities such as emulsifiers to affect the performance of the product; in order to obtain the solid polymer, it must go through coagulation, separation, and washing processes; the production capacity of the reactor is also lower than that of the bulk polymerization.

Disadvantages:

If drying is needed, the process is difficult to control

Composition and effect of emulsion polymerization

1 monomer

2 water

3 Initiation system

The initiator system is mainly an oil-soluble or water-soluble initiator. Oil-soluble initiators are mainly azo initiators and peroxy initiators. Azo initiators are azobisisobutyronitrile, azobisisoheptonitrile, azobisisovaleronitrile, and azodicyclohexylmethyl. Nitrile, azobisisobutyrate initiator, etc. Water-soluble initiators include persulfate, redox initiation system, azobisisobutyrate hydrochloride (V-50 initiator), azobis Isobutimidazoline hydrochloride (VA-044 initiator), azobisisobutimidazoline (VA061 initiator), azodicyanovaleric acid initiator, etc.

4 Emulsifier

Emulsifiers are a class of materials that can transform mutually incompatible oils and water into emulsions that are difficult to delaminate. Emulsifiers are usually surfactants that have both hydrophilic polar groups and hydrophobic (lipophilic) non-polar groups.

Type

(I) Ionic, divided into anionic and cationic, anionic: The hydrophilic group is generally -COONa, -SO4Na, -SO3Na, etc., and the lipophilic group is generally a linear alkyl group of C11 ~ C17, or C3 ~ C6 alkyl hydrophobic group bonded to phenyl or naphthyl; cationic: usually some amine and quaternary ammonium salts

(Ii) Amphoteric: amino acids, betaines

(Iii) Non-ionic: polyvinyl alcohol, polyethylene oxide, etc.

Emulsion polymerization

Effect

(I) Reduce the surface tension. Each liquid has a certain surface tension. When an emulsifier is added to water, the surface tension of water decreases significantly, and the rate of decrease decreases with increasing temperature and decreasing emulsifier concentration;

(Ii) Reduce the interfacial tension. The interfacial tension between oil (monomer) and water is very large. When a small amount of emulsifier is added to water, the oil phase at the oil-water interface is attached to the lipophilic end of the emulsifier molecule. Part or all of the oil-water interface becomes a lipophilic interface, thereby reducing the interfacial tension between oil and water;

(Iii) Emulsification. The role of the emulsifier is to extend the lipophilic group into the monomer droplets, and the hydrophilic group to the water phase. If an ionic emulsifier is used, the surface of the monomer droplets will have a layer of charge;

(Iiii) Solubilization, the phenomenon that the monomer concentration in the micelles is greater than the solubility of the monomer in water is called the solubilization of emulsifiers;

(Iiiii) lead to the formation of polymer latex particles according to the micelle mechanism;

(Iiiiii) Foaming effect. After adding emulsifier, the surface tension of water decreases, so it is easy to bubble. This is an unfavorable phenomenon in production, and corresponding measures need to be taken to reduce foam.

Main parameters

(I) Critical micelle concentration (referred to as CMC): The smaller the CMC, the easier it is to form micelles, and the stronger the emulsifying ability.

(Ii) Hydrophilic-lipophilic balance (HLB value): 8-18 is appropriate

(Iii) Three-phase equilibrium point and cloud point

Emulsion polymerization

1 The distribution of each component of the emulsion system in each phase

2 Nucleation period

Emulsion polymerization can be divided into three stages according to the polymerization reaction rate and the changes in the number of monomer droplets, latex particles, and micelles in the system.

The first phase is called the latex particle formation phase, or the nucleation phase and the acceleration phase, until the micelles disappear.

The second phase is called the constant speed period.

The third stage is called the speed reduction period.

The mechanism of emulsion polymerization is still under discussion and is inconclusive. Historically, there are three main mechanisms of emulsion polymerization:

1. Micellar theory proposed by WD Harkins in 1945. At the time, there were two views on the mechanism of emulsion polymerization, that is, the monomer droplets formed by mechanical stirring polymerized to form particles and the monomer phase and water interface formed particles. WD Harkins served as the director of the American Rubber Institute. Under his organization, first experimentally proved that ordinary mechanical stirring cannot make oily styrene form polystyrene particles after emulsion polymerization. At the same time, from an energy point of view, the theory Ordinary mechanical agitation cannot provide enough surface energy to maintain such small particles. Second, they conducted an experiment in which styrene vapor was passed through an aqueous solution containing an initiator (H2O2), and found that larger particles could also be formed. They believe that there is no interface between styrene vapor and aqueous solution, and therefore the statement that the interface forms particles is wrong. At the same time, due to the larger particles, they concluded that the particle size of the monomer droplets must be large. On this basis, WD Harkins proposed the micellar theory, that is, when the concentration of emulsifier molecules exceeds the critical micelle concentration (see surfactants), they will precipitate out of the aqueous phase to form micelles.

Figure one

WD Harkins believes that the particles after emulsion polymerization are formed by the polymerization of monomers in micelles. The polymerization process (see Figure 1) is: there is a diffusion layer between the monomer phase and the water phase; the micelles enter the diffusion layer, and the monomer molecules diffuse into the micelles; the micelles capture the radical polymerization in the water phase.

However, Harkins did not give its kinetic model. Therefore, this mechanism did not cause much discussion. Until 1948, Smith-Ewart established a series of calculated particle number and polymerization kinetic models based on Harikins' micelle theory. This mechanism was widely discussed and named the Harkins micelle nucleation theory. However, its original statement has also been tampered with into the statement in Figure 2 (also common in textbooks), that is, the interface diffusion is cancelled and the monomer molecules diffuse from the monomer phase into the water phase, and then, the water Monomer molecules in the phase diffuse into the micelles. This statement is very controversial in terms of thermodynamics, especially for poorly soluble monomers such as styrene. Depending on the diffusion, its concentration in the aqueous phase cannot support the high monomer concentration required in the particles. In addition, Smith-Ewart divides the emulsion polymerization kinetics into three stages, namely the nucleation stage (Interval I), the isokinetic polymerization stage (Interval ), and the deceleration polymerization stage (Interval ). The polymerization kinetics model also mainly deals with isokinetic velocity. Kinetics during the polymerization phase. However, the experimental results have shown that the previously believed constant velocity stage may be caused by experimental errors. In most cases, the polymerization process does not have a constant velocity process, but there are two maximum speeds.

figure 2

2. Tsai and Fitch's homogeneous nucleation mechanism (also known as water phase generation mechanism). This mechanism was proposed after the successful soap-free emulsion polymerization in the 1970s, because before the soap-free emulsion polymerization, there were no traditional emulsifier molecules in the system, so the micelle nucleation mechanism was challenged. They believe that the monomer molecules dissolved in the water phase are initiated and polymerized by the initiator molecules in the water phase to form oligomers. The solubility of these oligomers in water decreases with the increase of the molecular chain. When the critical chain is reached, Precursors are formed from the aqueous phase over a long period of time to form precursors. These precursors then condense with each other to form a stable core. Thereafter, the polymerization process is exactly the same as the mechanism of micelle nucleation. By the way, because of the homogeneous nucleation mechanism, the previous micelle nucleation mechanism is also called the heterogeneous nucleation mechanism.

The only experimental evidence supporting this mechanism is the result of light scattering: in the early stage of polymerization, the number of particles increased sharply, after reaching a certain peak, it decreased sharply, and then the number of particles was constant.

3. The (sub) microdroplet nucleation mechanism proposed by Ni Henmei et al. In 2001. As shown in Figure 4, they believe that all particles obtained by the polymerization method, such as emulsion polymerization, precipitation / dispersion polymerization, suspension polymerization, microemulsion polymerization, etc., are formed by monomer polymerization in monomer (sub) micro droplets. of. Disturbance at the interface between the monomer phase and the water phase, or the solubility of the monomers dissolved in the water phase due to changes in temperature or other factors, reduces the solubility, can produce monomer (sub) micro droplets (Figure 4b) , II). These monomer droplets will usually return to the monomer phase (c, IV) due to Ostwald maturation under normal conditions, but in the presence of pre-added emulsifier molecules or surface-active oligomers formed at the time, these The droplets will adsorb these molecules or be absorbed by these molecules to obtain a certain thermodynamic stability (d, III). At this time, if short-chain radicals are present, monomer droplet polymerization can be initiated to form a core. The transfer of monomers depends on the combination of particles and monomer droplets (e, VI), and the direct collision between particles and monomer phase (V). In addition, the mechanism also pointed out that when the particle size of the monomer phase droplets is reduced to the point where the perturbation of the interface is not enough to generate monomer microdroplets, the monomer droplets can directly capture radicals to form particles.

Emulsion polymerization

The experimental basis supported by this mechanism is the experimental results of quasi-static soap-free emulsion polymerization. Under very weak stirring conditions, fine particles cannot be formed in the aqueous phase; the initial polystyrene fine particles are formed at the interface between the monomer and the aqueous phase, and then settle into the aqueous phase. After polymerizing for a period of time, a polymer film layer will be formed at the interface, which prevents the monomer from diffusing into the water phase, and the generation of particles and the polymerization factors in it stop. In addition, this mechanism is also consistent with the theory of material transport processes such as chemical extraction.

This mechanism is a unified mechanism for the formation of polymer particles. However, kinetic models have not been established.

At present, the micellar nucleation mechanism and homogeneous nucleation mechanism not only have fatal thermodynamic defects in monomer diffusion and transmission, but also have no definite and unambiguous experimental results to date. The (sub) microdroplet nucleation mechanism can basically explain all the experimental results reasonably, such as the distribution of each component in the composite particles, the second maximum polymerization rate (generated by monomer droplet polymerization) in Figure 3, and so on.

Industrialization of emulsion polymerization

Latex polystyrene

Polyacrylate emulsion

Polyvinyl acetate emulsion, etc.

Emulsion polymerization applications

Emulsion polymerization was first developed in Germany. During the Second World War, the United States used this technology to produce styrene-butadiene rubber, and then successively produced nitrile rubber and neoprene, polyacrylate latex, polyvinyl acetate latex (commonly known as white rubber), and polyvinyl chloride. . Unlike suspension polymerization, the emulsion system is relatively stable. There are batch, semi-batch, and continuous production in the industry. It is not stirred or layered when it is piped or stored. In production, the method of "seed polymerization" (ie, latex containing active chain), supplementary monomers or regulators can be used to control the polymerization speed, molecular weight and particle size of the rubber particles. It can also directly produce high-concentration latex.

New development of emulsion polymerization

New emulsion polymerization technologies: 1. Emulsion polymerization in non-aqueous medium; 2. Soap-free emulsion polymerization; 3. Core-shell emulsion polymerization; 4. Microemulsion polymerization and polymer microemulsion; 5. Radiation emulsion polymerization; 6. Reactive polymerization Microgels; 7, stereoregular polymerization in emulsions; 8, ultra-concentrated emulsion polymerization; 9, dispersion (or emulsion) polymerization with supercritical CO2 as the medium.