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EVA resins normally used in hot melt adhesives have a vinyl acetate content less than 40%. However, a new product line, Levamelt, from Bayer provides vinyl acetate contents in the 40-80% range. This polymer backbone consists of fully saturated methylene units with acetate groups attached. Thus it is referred to as a EVM copolymer. Levamelt is particular well suited for pull-off protective film such as that used for automobiles, furniture, and liquid crystal displays. Melt index or melt viscosity is another important criterion in choosing the correct EVA resins for adhesive formulations. Low melt index EVA grades provide high viscosity, strength, and hot tack. In contrast, high MI grades enable higher polymer content and low application viscosities. Mid-range MI grades provide formulation flexibility. EVA resins are considered to be relatively safe to use in food applications. Certain adhesive manufacturers offer EVA hot melt formulations that are approved for indirect food contact (FDA 21 CFR.175) applications. This allows the adhesive to be used not only in food packaging but also in food cooking and other food preparation applications. EVA resins exhibit miscibility in the melt with a wide range of modifying resins, tackifiers, and waxes. This provides the adhesive formulator with a wide latitude of compounding possibilities. Often a modifier resin is incorporated along with the EVA copolymer to provide specific adhesion characteristics and substrate wetting. Common modifying resins include: • Wood rosin derivatives (adhesion to vinyl and other plastics), • Hydrocarbon resins (adhesion to Kraft paper), • Polyterpene resins (adhesion to metals and plastics), • Low molecular weight styrene and styrene copolymers (adhesion to metals at elevated temperatures), and • Phenolic resins (improved temperature resistance and adhesion to wood). Tackifiers are added to EVA copolymers to reduce viscosity and improve wetting. These include C5 - C9 hydrocarbon resins, polyterpenes, and rosin esters of pentaerylthritol and glycerol. Waxes are added to lower cost and reduce viscosity. Fillers such as calcium carbonate lower cost and increase viscosity. Antioxidants are needed to protect the adhesive during application and service life. The idealized formulation ranges for an EVA hot melt adhesive was shown in Table 2. Specific EVA hot melt formulations for certain bonding applications can be found in the SpecialChem4Adhesive's Ethylene Copolymers Center. Selected formulations from this site are given in Table 6.
Advances in EVA Hot Melt Adhesives EVA hot melts are generally susceptible to creep as any thermoplastic. However, they can be crosslinked by adding peroxide and heating or by electron beam (EB) radiation. EVAs exhibit outstanding viscosity retention on aging at elevated temperature; however, they slowly release acetic acid. This can cause corrosion problems with adhesive applications equipment. Formulations based on ethylene n-butyl acrylate have been developed to address this deficiency. Current trends favor fast setting hot melts to improve line speed while keeping a "long" open time in order to reduce quality problems when the line is running at low speed. New EVA formulations can be formulated to offer relatively long open times (40-60 secs) and excellent adhesion to many substrates including metal, plastics, wood, and elastomers. Pressure sensitive EVAs that stay tacky even after cooling are another recent advancement in hot melt technology7 . References 1. Pocius, A.V., Adhesion and Adhesives Technology, Hanser / Gardner Publications, New York, 1997, p. 247. 2. Petrie, E.M., Handbook of Adhesives and Sealants, 2nd ed., McGraw-Hill, New York, 2006, p. 686. 3. Gauthier, M. M., "Types of Adhesives", in Adhesives and Sealants, vol. 3, Engineered Materials Handbook, ASM International, 1990. 4. Henderson, A.M., "Ethylene Vinyl Acetate (EVA) Copolymers: A General Review", IEEE Electrical Insulation Magazine, January/February, 1993, p. 31. 5. Domine, J.D., and Schaufelberger, R.H., "Ethylene Copolymer Based Hot Melt Adhesives", in Handbook of Adhesives, I. Skeist, ed., van Nostrand Reinhold, New York, 1977. 6. Ethylene Copolymers Center, SpecialChem4Adhesives. 7. Tremblay, S., "Advances in Hot Melt Technology", Loctite Technical Brief, Henkel Corporation. This document was provided by SpecialChem's editor. If you want to share your technical expertise in a specific area related to adhesives and sealants or if you want to submit press releases, please send it to email@example.com. SpecialChem reserves the right to refuse any article or news item.