What kind of material is EVA?
eva is ethylene-vinyl acetate copolymer material, it is a non-toxic, tasteless, transparent thermoplastic. eva material has excellent elasticity, flexibility, transparency, insulation, low temperature deflection, weather resistance, chemical corrosion resistance, widely used in wire and cable material manufacturing, film and other molding products and mixtures, automotive industry parts, adhesives, coatings and so on.
1. Composition and structure of eva material
Copolymer: Ethylene stands for Ethylene; Vinylacetate stands for Vinyl Acetate; Copolymer stands for copolymer; ethylene and vinyl acetate are the two monomers of eva. The chemical structure formula of eva is shown in the figure below:
eva material is the fourth category of ethylene copolymer after high density polyethylene, low density polyethylene and low pressure low density polyethylene. Compared with polyethylene, due to the introduction of the polar functional group ester bond on the side chain of eva material, the crystallinity of eva material is lower, and the impact resistance and weather resistance are greatly improved.
2. Preparation method of eva material
In 1938, the British ICI company applied for the relevant patent of eva copolymer, and the large-scale production of eva was achieved by the United States DuPont Company in 1960. At present, there are four methods for the production of eva materials: high-pressure bulk polymerization, suspension polymerization, emulsion polymerization and solution polymerization.
Among them, high-pressure bulk polymerization is the mainstream method for the production of eva materials, its approximate process is: in 1000 to 2000 standard atmospheric pressure of high pressure and about 100℃ high temperature environment, the ethylene gas into the reactor with vinyl acetate liquid inside, vinyl acetate can be used as the reaction body, but also for the reaction to provide a solution environment. This process usually uses a continuous flow reaction device, so that the vinyl acetate that is not involved in the reaction can be recycled, and the annual output can reach more than 100,000 tons using this process method. The reaction device of eva material is very close to low density polyethylene (LDPE), as shown in the figure below:
3. Classification and application of eva materials
The properties of eva materials obtained by different polymerization methods are quite different, and the fundamental reason is that different polymerization methods will affect the content of vinyl acetate in eva materials. According to the amount of vinyl acetate content, eva material is divided into eva resin, eva elastomer and eva emulsion.
When the vinyl acetate content is 5% to 40%, the eva material produced at this time is called eva resin, mainly used as agricultural film, foam and hot melt adhesive. Common eva films are shown as follows:
When the content of vinyl acetate in eva material is 40% to 70%, this eva material is called eva rubber, and the increase in the content of ester bonds gives eva material high elasticity. At this time, the eva material is almost amorphous, the glass transition temperature is very low, and it can be used as a modifier of PVC, as shown in the figure below:
If the eva material produced by emulsion polymerization is used, the vinyl acetate content can be as high as 70% to 95%, and the eva material is called eva emulsion at this time. eva emulsion is mainly used in the coating industry such as adhesive and conductive paint. The following is a typical eva emulsion:
4. Defects and modification of eva material
Although eva materials have many excellent properties, eva is extremely flammable and can produce toxic gases. In order to solve the problem of flammable eva materials and ensure the safety of eva materials, the most effective way is to add flame retardants to it.
Previously commonly used flame retardants were halogenated flame retardants. The mechanism of action is that when the flame retardant is heated, it decomposes hydrogen halide and consumes the free radicals generated when the eva material is degraded, thus interrupting the chain reaction of the eva material. In addition, hydrogen halide is refractory and dense, which is easy to form a "protective layer" on the surface of eva material, reducing the contact area between eva material and oxygen, and playing a certain flame retardant effect.
However, hydrogen halide will still cause secondary pollution, and researchers are gradually using metal hydroxide, inorganic nanoparticles and expansion flame retardants as flame retardant materials for eva materials.
