Ethylene Vinyl Acetate, EVA

The production process of ethylene-vinyl acetate copolymer (EVA) is a complex process that involves multiple chemical reactions and physical operations. The production process of EVA is mainly realized by the copolymerization reaction of ethylene and vinyl acetate (VA), and three main process methods are usually used: high-pressure radical polymerization, emulsion polymerization and suspension polymerization. In order to achieve the required performance requirements, the production of EVA must be strictly controlled by monomer ratio, reaction conditions, catalyst selection, and subsequent product handling and purification. The following will introduce the production process of EVA and the key steps related to it in detail.

First, raw materials and formula selection

The raw materials for the production of EVA mainly include ethylene and vinyl acetate, and their selection and proportion directly affect the performance of the final product.

1. Ethylene

Ethylene (C₂H₄) is an extremely important chemical and a major monomer source for polyethylene and EVA copolymers. Ethylene is a colorless gas with high reactivity. In the production of EVA, the purity of ethylene is very critical, and a purity of more than 99.9% is generally required. The higher ethylene content results in copolymers with better rigidity, lower flexibility and chemical resistance.

2. Vinyl acetate (VA)

Vinyl acetate (CH₃COOCH=CH₂) is a comonomer of ethylene, and its content greatly affects the elasticity, transparency and softness of EVA. As the vinyl acetate content increases, EVA becomes more soft, transparent, and sticky, but its strength and chemical resistance decrease. The purity of vinyl acetate is also high, generally above 99.5%.

3. Initiator

The choice of initiator determines the initiation and reaction rate of the polymerization reaction. Common initiators are organic peroxides, such as cumene peroxide, benzoyl peroxide, etc. These initiators can decompose at high temperatures, creating free radicals that initiate polymerization reactions.

4. Solvents and additives

In the production process of EVA, the role of the solvent is mainly to adjust the viscosity of the reaction medium, improve the solubility of the monomer, and improve the uniformity of the reaction. Common solvents include n-hexane, toluene, etc. In addition, additives such as antioxidants and ultraviolet absorbers can be added according to specific needs to further improve the performance of EVA products.

Second, the polymerization process of EVA

There are three main processes for the polymerization of EVA: high-pressure radical polymerization, emulsion polymerization and suspension polymerization. Each process has its own advantages and scope of application, and choosing the right process can effectively control the molecular structure and final performance of EVA.

1. High-pressure radical polymerization

High-pressure radical polymerization is one of the most widely used methods in EVA production. The process initiates the copolymerization of ethylene and vinyl acetate through a free radical initiator under high temperature and high pressure conditions.

Process: In the high-pressure radical polymerization process, ethylene and vinyl acetate are decomposed by initiators to produce free radicals under high pressure conditions of 1500 to 3000 bar at high pressure of 1500 to 3000 °C, which in turn initiates the polymerization reaction. In the whole reaction system, ethylene is the main component, and vinyl acetate is introduced into the polymer through a copolymerization reaction.
Reaction mechanism: Under high pressure conditions, the initiator decomposes to produce free radicals, which then react with ethylene and vinyl acetate to form long-chain polymers. Free radical polymerization reactions are characterized by rapid chain growth and difficult molecular weight control, requiring precise control of reaction conditions such as pressure, temperature, and initiator concentration to adjust the molecular weight and distribution of polymers.
Advantages: High-pressure polymerization has a high reaction rate and good product distribution, which can generate EVA products with excellent mechanical properties and high molecular weight. In addition, the process is suitable for large-scale industrial production.
Disadvantages: Due to the demanding requirements, high cost, and difficult process control of high-pressure polymerization equipment, accurate control of reaction temperature and pressure in the production process is particularly important.

2. Emulsion polymerization

Emulsion polymerization refers to the process of dispersing ethylene and vinyl acetate monomers into tiny droplets through surfactants in an aqueous system, and carrying out a copolymerization reaction under the action of free radical initiators.

Process: During emulsion polymerization, ethylene and vinyl acetate are mixed with water and surfactants to form an emulsion. The free radical initiator dissolves in water, initiating a monomer polymerization reaction, and the resulting polymer is dispersed in the aqueous phase to form latex.
Reaction characteristics: Emulsion polymerization is typically carried out at a lower temperature of 50 to 90 °C and a lower pressure of 10 to 50 bar. Emulsion polymerization is characterized by small polymer particles, uniform molecular weight distribution, and because the reaction is carried out in the aqueous phase, the heat during the reaction is easy to control, and the overpolymerization caused by high-temperature reaction is avoided.
Advantages: Emulsion polymerization has good temperature and pressure control capabilities, relatively low equipment costs, and is suitable for small-scale and medium-scale production. In addition, since the reaction takes place in the aqueous phase, the emulsion polymerization process has a low environmental impact.
Disadvantages: The products of emulsion polymerization often contain residual surfactants and water, which need to be further treated to obtain a pure EVA product. In addition, EVA products produced by emulsion polymerization may not be as stable as high-pressure polymerization products in some high-end applications.

3. Suspension polymerization

Suspension polymerization is similar to emulsion polymerization, but instead of water, suspension polymerization uses an organic solvent as the dispersion medium. In this process, ethylene and vinyl acetate monomers are dispersed in organic solvents and copolymerized by the action of initiators.

Process: Ethylene and vinyl acetate monomers form a suspension in an organic solvent by mechanical stirring, and the initiator decomposes in the solvent to produce free radicals and initiate a polymerization reaction. As the reaction progresses, the polymer particles are gradually formed and dispersed in the suspension.
Reaction characteristics: The reaction conditions of suspension polymerization are usually similar to those of emulsion polymerization, but due to the use of organic solvents, the viscosity of the reaction system is lower and the reaction rate is faster. Suspension polymerization is often used to produce EVA products with specific performance requirements, especially those with strict molecular weight distributions.
Advantages: The polymer polymer of suspension polymerization has higher purity and less surfactant residue, which is suitable for the production of high-purity EVA products. In addition, the reaction system is easy to control, and the molecular weight distribution of the product is relatively concentrated.
Disadvantages: A large amount of organic solvents are used in the suspension polymerization process, which increases the production cost and the difficulty of environmentally friendly treatment. The recovery and reuse of solvents also places higher demands on the process.

3. Product Handling and Purification

The production of EVA requires a series of post-processing steps after the polymerization reaction is completed to ensure the purity and uniformity of the product. These treatment steps include distillation, filtration, washing, and drying, among others.

1. Distillation

Distillation is mainly used to remove unreacted ethylene, vinyl acetate, and solvents. At high temperatures, waste is reduced by separating volatile substances and recovering unreacted monomers and solvents, while further concentrating the polymer solution and improving product purity.

2. Filter

Filtration is used to remove solid impurities and catalyst residues generated during the reaction. In high-pressure radical polymerization and suspension polymerization, polymer particles are mixed with a catalyst and need to be filtered to remove catalyst residues to ensure product purity.

3. Washing

The washing step is used to remove by-products and
from the reaction

Residual initiators. Common detergents include water, organic solvents, and the use of multiple washes can further purify EVA products and improve their physical properties.

4. Dry

Drying is the final processing step to remove solvents and moisture from the product. Hot air drying, vacuum drying or spray drying techniques are usually used to evaporate the solvent to obtain dry EVA granules or powders.

Fourth, the performance adjustment of EVA products

The performance adjustment of EVA is mainly achieved by changing the ratio of ethylene to vinyl acetate, controlling the molecular weight and molecular weight distribution of the copolymer, and adjusting the subsequent processing process.