Hot melts based on ethylene copolymers (EVA, EBA, EMA, etc.) are generally used
in the packaging industry as quick-sealing adhesives, in disposables, for book and
journal binding, and in the woodworking industry as assembling adhesives.
There is a strong trend towards high heat resistant and low color hot melt adhesives.
Low color is perceived as a good indicator of adhesive quality. It is influenced
by the choice of raw materials, as well as by the stabilizer package.
Effectively stabilized raw materials are critical for the production of hot melts
with constant and reproducible properties. However, this does not mean that the
basic stabilization is sufficient to overcome degradation occurring during compounding
and end use.
An additional stabilization package is required to retard discoloration, viscosity
changes, and skinning of EVA hot melts during compounding at high temperatures.
For example, EVA hot melts based on hydrocarbon resin show good color stability
after oven aging at 170°C. (Figure 1)
Figure 1 : Gardner Color of EVA Hot Melt Adhesives Based
on Hydrocarbon Resin After Aging at 170°C
During high temperature aging of hot melts, skin formation can occur. Skinning is
based on irreversible oxidative damage to the polymers and cannot be corrected by
subsequent mixing or addition of stabilizers. During processing, skin formation
in the adhesive can lead to blocking of pipes and irregular coating. As shown in
Figure 2, the addition of additives significantly retards skin formation, with no
skin formation after 72 hours at 170°C, and provides excellent viscosity stability.
Figure 2 : Melt Viscosity of EVA based Hot Melt After
Aging at 170°C
When other tackifiers such as rosin esters, terpene resins, or dicyclopentadienes
replace hydrocarbon resins for the production of EVA hot melts, the same stabilization
systems lead to good results.