EVA hot melt adhesive is a 100% solid, transparent, flexible Ethylene Vinyl Acetate Copolymer. It is normally categorized by percent VA content and melt flow rate (MFR) or melt index (MI). Most commonly used EVAs contain between 19% and 28% VA. The MI values are mostly ranging from 3 to 2500.
In an EVA-based HMA formulation, a greater VA content can result in higher transparency, more polar, extended flexibility, and better wettability to substrates. The MI value is a simple indicator of molecular weight. A lower MI value reflects a higher molecular weight, cohesion, and internal strength.
EVA Based Hot Melt Adhesive Formulation
Most EVA-based HMAs do not have significant surface tack and are not pressure sensitive at room temperature. They need to be bonded at elevated temperatures by hot tack. They normally impart relatively short open time (typically less than 10 seconds) and fast set time. Open time and set time are primarily determined by the wax used – if it is included in the formulation, VA content, and the compatibility of EVAs and tackifiers. Typical waxes used in hot melt adhesive formulations are synthetic, micro-crystalline and polyethylene wax. Waxes can significantly shorten the open time and speed up set time. EVAs with more VA content will offer longer open time and slower set time.
Some terminology you may encounter when evaluating EVA hot melt adhesives:
Hot tack is a term relating to the ability of HMAs to hold substrates together prior to solidification or set.
Open time is the time after the adhesive is applied during which a serviceable bond can be made. Many factors affect open time, including temperature, substrate, adhesive, and amount of adhesive applied.
Set time is the time it takes to form an acceptable bond when two or more substrates are combined with an adhesive.
When EVAs are blended with various tackifiers, they may exhibit a different open time, set time, cloud point and specific adhesion performance – depending upon the type of tackifiers used or their compatibilities.
Cloud point is the temperature at which an HMA becomes cloudy upon cooling from a molten to a solid stage. Cloud point is a good indication of the compatibility of the formula components.
Generally, when EVAs and tackifiers are more compatible, the blended HMAs are more transparent. Moreover, they can exhibit a lower cloud point, longer open time, slower set time and better adhesion to varied substrates.
In practice, a balance of open time and set time are key parameters to ensure satisfactory bonding and therefore obtain optimum adhesion performances. Besides those effects resulting from VA content, the polarity of tackifiers is another important factor which will significantly affect the degree of physical adsorption between the interface of hot melt adhesives and substrates. Hot Melt Adhesives with higher physical adsorption will result in a better wetting and adhesion.
In summary, there is no ideal EVA Hot Melt Adhesive formulation for diversely bonding applications. The optimal formulation must be developed specifically to the substrates to be bonded (polarity and surface roughness), application techniques (applicators, speed, and bonding temperature), and the end users’ servicing environment (temperature and humidity).