EVA Material Knowledge Encyclopedia

EVA is the abbreviation for ethylene-vinyl acetate copolymer, a random copolymer composed of nonpolar, crystalline ethylene monomers and strongly polar, amorphous vinyl acetate monomers (also known as VA). It was first synthesized in 1928 by American scientist H.F. Mark using a low-pressure method. Later, in 1938, the British ICI Chemicals Company published a patent for high-pressure polymerization to manufacture EVA, and in the early 1960s, DuPont in the United States began producing industrial products. Today, its applications have permeated all aspects of people's lives, such as shoe materials, films, wires and cables, toys, hot melt adhesives, coatings, etc.

1 Synthesis Process

The vinyl acetate (VA) content in ethylene-vinyl acetate copolymers can range widely, from 5% to 95%. Different contents result in different properties, therefore, strictly speaking, there are different subcategories, as shown in the figure below:

However, the EVA products commonly referred to are EVA resins, with VA content generally ranging from 5% to 40%. Most EVA manufacturers produce this type, therefore, the EVA discussed below refers to EVA resin.

Regarding the synthesis process of EVA, after more than 50 years of development, there are currently four main mature EVA production technologies both domestically and internationally: high-pressure continuous bulk polymerization, medium-pressure suspension polymerization, solution polymerization, and emulsion polymerization. Among these, solution polymerization and emulsion polymerization are less commonly used, with most companies employing the high-pressure continuous bulk polymerization process. Since the polymerization reaction mechanism of EVA under high pressure and high temperature is essentially the same as that of LDPE, the difference between EVA products produced using autoclave reactors and tubular reactors is similar to the difference between LDPE products produced using these two processes.

2 Structural Properties

Compared to PE, EVA, due to the introduction of vinyl acetate monomers into the molecular chain, has reduced crystallinity, improved flexibility, impact resistance, and enhanced filler incorporation and heat-sealing properties. Its density is generally between 0.91 and 0.93, and it exhibits good transparency and gloss.

The properties of EVA resin mainly depend on the vinyl acetate content (VA content) and melt flow rate (MFI) in the copolymer. With a constant MFI value, the changes in various properties as VA% increases are as follows:

Under the condition of constant VA% content, the effect of increasing MFI value on performance is as follows:

EVA products exhibit good flexibility, low-temperature resistance, impact strength, environmental stress cracking resistance, good optical properties, good air permeability, moderate mechanical properties, and poor insulation properties over a wide temperature range.

3 EVA Processing and Molding

EVA is a thermoplastic polymer, therefore it can be used in injection molding, extrusion, blow molding, calendering, rotational molding, vacuum thermoforming, foaming, coating, heat sealing, welding, and other processing and molding processes.

EVA has a low molding temperature range (160-200℃) and a wide range of temperatures. Its mold temperature is low (20-45℃), and the material must be dried before processing (drying temperature 65℃). During EVA processing, the mold temperature and material temperature should not be too high, otherwise the surface will be rough (not smooth).

EVA products are prone to sticking to the front mold; it is better to make the cold slug well at the main runner of the sprue a pull-tab type. It is prone to decomposition at temperatures exceeding 250℃. EVA is best processed using "low temperature, medium pressure, and medium speed" conditions.

4 Application Areas

EVA applications have permeated all aspects of people's lives, such as shoe materials, films, wires and cables, toys, hot melt adhesives, coatings, etc.

Shoe Materials

Shoe materials are the most important application area for EVA resin in my country. The vinyl acetate content in EVA resin used in shoe materials is generally between 15% and 22%.

Because resin-blended foam products have properties such as softness, good elasticity, and chemical corrosion resistance, they are widely used in the soles and lining materials of mid-to-high-end hiking shoes, mountaineering shoes, slippers, and sandals; in addition, this material is also used in sound insulation boards, gymnastics mats, and sealing materials.

EVA Film

The main use of EVA film is in the production of functional greenhouse films. Functional greenhouse films have high weather resistance, anti-fogging properties, and heat insulation properties. Because polyethylene is non-polar, even with the addition of a certain amount of anti-fogging agent, its anti-fogging performance can only be maintained for about 2 months; while greenhouse films made with a certain amount of EVA resin not only have higher light transmittance but also significantly improved anti-fogging performance, generally exceeding 4 months. In addition, EVA can also be used to produce packaging films, medical films, laminated films, and cast films.

Wires and Cables

With the continuous development of computer and network engineering, and for the sake of computer room safety, halogen-free flame-retardant cables and silane cross-linked cables are increasingly used. Because EVA resin has good filler compatibility and cross-linking properties, it is widely used in halogen-free flame-retardant cables, semiconductor shielded cables, and two-step silane cross-linked cables. Furthermore, EVA resin is also used to make sheaths for some special cables. The vinyl acetate content of EVA resin used in wires and cables is generally between 12% and 24%.

Toys

EVA resin also has many applications in toys, such as children's wheels and seat cushions. In recent years, my country's toy processing industry has developed rapidly, with production concentrated in coastal areas such as Dongguan, Shenzhen, and Shantou, mainly for export and processing.

Hot melt adhesives

Primarily composed of EVA resin, are solvent-free, environmentally friendly, and highly safe, making them ideal for automated production lines. In recent years, they have widely replaced traditional phenyl adhesives and are therefore widely used in bookbinding, furniture edge banding, automotive and home appliance assembly, shoemaking, carpet coatings, and metal anti-corrosion coatings. Hot melt adhesives mainly use varieties with a vinyl acetate content of 25%-40%. Although domestic products of this brand exist, production has been largely absent, resulting in imports dominating the market.

Photovoltaic Industry

Currently, in the solar cell industry, EVA is used for bonding solar cells to the surface photovoltaic glass and the backsheet in crystalline silicon cells. Because EVA films possess excellent flexibility, optical transparency, and heat-sealing properties, they have become the preferred photovoltaic encapsulation material. With the rapid development of the Chinese photovoltaic market, the EVA solar cell encapsulation film market has also experienced rapid growth, attracting many companies to invest in and enter this market. However, this indiscriminate entry has led to overcapacity and price competition, resulting in the highly competitive solar anti-dumping measures.

Coating

Coating materials are mainly used for coating layers of PET film and BOPP film. Taking advantage of the transparency and adhesion of EVA, they are used for applications such as quick sealing of photos and sealing of clothing bags.

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