Ethylene vinyl acetate is processed using a method called extrusion. Extrusion can be used to produce blown film, coated film, tubing, profiles and extrusion blow molded products. EVA should be kept under 425ˇF or it will begin to degrade.
Blowing is the most popular method of producing film. It is produced by blowing a bubble out of the heated monomer. The bubble is then cooled creating a film of the polymer. EVA film is used for packaging because it has low sealing temperatures allowing fast production cycles and because it complies with FDA regulations for materials that come into contact with food.
EVAs are used to extrusion coat many things including nylon, polyester, cellophane and polypropylene films. EVAs are excellent heat sealers, have good hot tack strength and adhere well to a variety of substrates. The tackiness of EVA copolymers can cause problems in extrusion coating but using dusting powders on the film and additives in the polymer can help the process run smoothly.
Casting is the second most popular method of producing EVA film. Dual chill roll systems are used to roll the EVA copolymer into sheets of thin film. Additives are sometimes needed to alleviate problems with blocking and poor chill roll release. Sheet extrusion is very similar to cast film extrusion except the EVA copolymer product is much thicker. Sheet extrusion usually uses three or four chill rolls stacked on top of one another instead of the two generally used for casting.
Extrusion blow molding is typically used to produce large, hollow parts such as bottles. Blow molding is where some of the heated polymer is placed inside a mold and blown up to fit the mold. For EVAs, extrusion blow molding is generally preferred to injection blow molding. Some properties of ethylene vinyl acetate can be controlled by the percent of vinyl acetate. For instance, higher vinyl acetate content yields a higher permeability to oxygen, nitrogen, carbon dioxide and moisture vapor. But EVA copolymers are all flexible with a high flex life, have good film clarity and are resistant to ozone. Its polar functionality promotes adhesion to polar substrates (paper, polyester, wood, and leather) and more readily accepts inks. Its low crystallinity gives it a low melting point and excellent low temperature toughness.
Hot Melt Properties of Ethylene Vinyl Acetate (28% Vinyl Acetate)
Melt Index 3 MI 388 MI 980 MI
Viscosity, cP @ 350ˇ F 9500 330 200
Ring and Ball Softening Point, ˇF 170 164 158
Tensile Strength, psi 950 350 340
Elongation, % 560 40 40
Hardness, Shore A, original after 15 sec Max 87
84 Max 91
83 Max 84
80
Shear Adhesion Failure Temperature, ˇF 158 148.1 148.1
Ethylene vinyl acetate copolymers have a wide variety of uses such as packaging, adhesives, wire, cable and health care. In packaging, EVA copolymers are used as heat seal layers for snacks and cheese, and bags for ice, frozen food and agricultural materials. Adhesives such as hot melt packaging adhesives and glue sticks contain EVA copolymers. EVA is used to coat wires and cables and for tubing and fittings for health care items. It can also be used to make foams or sound control compounds.
Sources:
Adhesives, Sealants and Coatings Manual. © 1992 by Quantum Chemical
Company: Cincinnati, OH.
Escorene¨ Ethylene Vinyl Acetate Copolymers (EVA). © 1998 by Exxon
Corporation: Houston, TX.
Ultrathene¨ Ethylene Vinyl Acetate Copolymers for Applications that
Demand Flexibility and Toughness. © 1995 by Quantum Chemical Company:
Cincinnati, OH.