Material of EVA Hot Melt Adhesive
Some of the possible base materials of hot-melt adhesives include the following:
Ethylene-vinyl acetate (EVA) copolymers, low-performance, the low-cost and most common material for the glue sticks (e.g., the light amber colored Thermogrip GS51, GS52, and GS53). They provide sufficient strength between 30 and 50 °C but are limited to use below 60–80 °C and have low creep resistance under load.
The vinyl acetate monomer content is about 18–29 percent by weight of the polymer. High amounts of tackifiers and waxes are often used; an example composition is 30–40% of EVA copolymer (provides strength and toughness), 30–40% of tackifier resin (improves wetting and tack), 20–30% of wax (usually paraffin-based; reduces viscosity, alters setting speed, reduces cost), and 0.5–1.0% of stabilizers.Fillers can be added for special applications. Can be formulated for service temperatures ranging from −40 to +80 °C, and for both short and long open times and a wide range of melt viscosities.
High stability at elevated temperatures and resistance to ultraviolet radiation, which can be further enhanced with suitable stabilizers. High vinylacetate content can serve for formulating a hot-melt pressure-sensitive adhesive (HMPSA). EVA formulations are compatible with paraffin. EVA was the base for the original hot melt composition. The composition of the copolymer influences its properties; increased content of ethylene promotes adhesion to nonpolar substrates such as polyethylene, while increased content of vinyl acetate promotes adhesion to polar substrates such as paper.
Higher ethylene content also increases mechanical strength, block resistance, and paraffin solubility. Higher vinyl acetate content provides higher flexibility, adhesion, hot tack, and better low-temperature performance. Adhesive grade EVA usually contains 14–35% vinyl acetate. Lower molecular weight chains provide lower melt viscosity, better wetting, and better adhesion to porous surfaces. Higher molecular weights provide better cohesion at elevated temperatures and better low-temperature behavior. Increased ratio of vinyl acetate lowers the crystallinity of the material, improves optical clarity, flexibility and toughness, and worsens resistance to solvents.
EVA can be crosslinked by, e.g., peroxides, yielding a thermosetting material. EVAs can be compounded with aromatic hydrocarbon resins. Grafting butadiene to EVA improves its adhesion. Its dielectric properties are poor due to high content of polar groups, the dielectric loss is moderately high. Polypropylene HMAs are a better choice for high-frequency electronics. EVAs are optically clearer and more gas and vapor permeable than polyolefins. Nearly half of EVA HMAs is used in packaging applications.
Cryogenic grinding of EVAs can provide small, water-dispersible particles for heat-seal applications. EVA can degrade primarily by loss of acetic acid and formation of a double bond in the chain, and by oxidative degradation. EVA can be compounded into a wide range of HMAs, from soft pressure-sensitive adhesives to rigid structural adhesives for furniture construction.
