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Jan 10, 2007>> All articles Formulating Ethylene Vinyl Acetate Hot Melt Adhesives Edward M. Petrie, Member of SpecialChem Technical Expert Team

Introduction

Key Material Properties Required for Hot Melt Adhesives

Formulation of EVA Hot Melt Systems

Advances in EVA Hot Melt Adhesives

Introduction

Hot melt adhesives are increasingly becoming popular because of their fast setting speed, relatively low cost (materials and processing), and low environmental impact. Hot melt adhesive systems also have broad formulation latitude so that a multiplicity of products can be produced for specific end-uses and substrates.

Primary hot melt applications are high-speed applications such as packaging, attaching labels and wood veneer, bookbinding, and textiles. Hot melt adhesives can also be manufactured in various forms including pellets, slugs, and blocks for bulk application or stick and continuous rope forms for heated gun applicators. Films are also available for high area assembly and continuous laminating.

Hot melt adhesives are primarily made from thermoplastic polymers including ethylene vinyl acetate (EVA), block copolymers such as styrene butadiene styrene (SBS) or styrene isoprene styrene (SIS), polypropylene (atactic), polyethylene, polyamide, polyester, and polyurethane. Of these, EVA is the most popular due to their high versatility.

EVA resins are highly flexible products, compatible with many other polymers and additives, and easy to process. They have high cohesive strength and excellent adhesion to a wide range of substrates. EVA copolymers can be used in soft, permanently tacky pressure sensitive adhesives or in tough rigid hot melt compositions used for semi-structural applications. Table 1 identifies the major advantages and limitations associated with EVA based hot melt adhesives.

Broad formulating latitude necessary for many different

applications and adhesion to a wide variety of substrates

Cold flow (creep)

Quick setting - no cure or fixturing required Attacked by some greases, oils, and solvents

Retention of properties at low temperatures High viscosity needed for maximum performance

No water or solvent removal

Low equipment and operating costs

Pressure sensitive systems can be formulated

Many formulations have approval for food contact

Relatively low cost

Table 1: Advantages and Disadvantages of EVA Based Hot Melt Adhesives

This article first reviews the key materials properties that are required generally for hot melt adhesives. The formulating principles and potential product possibilities available with EVA hot melt adhesives are then reviewed, and starting formulations are offered for specific applications.

Key Material Properties Required for Hot Melt Adhesives

Hot melt adhesives facilitate fast production speeds because the adhesives set simply by cooling from a molten liquid to a cohesive solid. Hot melts do not require processing time for evaporation of solvents or water carrier or for a chemical reaction to occur. This also generally results in lower energy and maintenance costs.

Hot melt formulations contain polymers to provide strength and hot tack, and tackifiers and oils or plasticizers to reduce melt viscosity, adjust glass transition temperature, and improve wetting. Some formulations also contain waxes to speed setting and further reduce viscosity. These additives are generally high concentrations (comparable to the base polymer) in hot melt adhesive formulations. Fillers are also used to increase melt viscosity to reduce excessive penetration into porous substrates and to modify cohesive properties. Antioxidants are generally added to prevent thermal oxidation during processing and service. Additives often consist of blends of two or more materials to optimize certain properties. A typical EVA hot melt formulation for packaging is indicated in Table 2.

EVA copolymer 20-50

Tackifying resin 20-50

Synthetic wax 0-20

Plasticizer 0-20

Filler 0-20

Antioxidant 0.1-1.0

Table 2: Starting Formulation for an EVA Hot Melt Packaging Adhesive