Most adhesive users love to receive clear or transparent HMAs (hot melt adhesives) in addition to their processibility and adhesion performances. Below are two approaches to achieve this aesthetic goal.
- To produce a transparent EVA-based HMA, one should first select EVAs and tackifiers possessing similar solubility parameters. In other words, the selected tackifiers should be very compatible with the soft VA (Vinyl Acetate) segment of the EVA. Natural tackifiers such as rosin and Terpene derivatives are more polar material and therefore are more compatible with VA. Disregarding the color, it is normally easier to make a transparent EVA-based HMA with natural tackifiers. Among various types of synthetic hydrocarbon tackifiers, C5 (aliphatic)/C9 (aromatic) or C10 (DCPD, di-cyclo-penta-diene)/C9 co-tackifiers are more compatible to VA and can be easier to make transparent EVA-based HMAs.
- The production procedure is also critical to obtain transparent HMAs. During mixing, adhesive blends are normally transparent at elevated temperatures beyond the cloud point of the blends. When finished adhesive blends are suddenly chilled by an icy water bath from a very high temperature, the motion of the fully extended (or orientated) molecular chains are quenched. Although adhesive blends remain transparent right after cooling, those extended molecular chains tend to return to their random coil (large entropy) state at room temperature. This slow molecular thermodynamic motion will cause some opaqueness after the adhesive is stored for a period of time. To minimize this undesired phenomenon, hot melt manufacturers can slowly pre-cool the mixed batch with a very slow agitation (for heat exchange purpose only) in a holding tank. During this pre-cooling process, some fully extended molecular chains have a great chance to gradually retract back to a random coil state prior to being suddenly quenched in an icy water bath. As a result, these products will have better chances to remain transparent even though they are stored at room temperature for a long time.
We sometimes found transparent EVA-based HMAs turn opaque after they are applied to substrates with an applicator. This is because those extended but quenched molecular chains obtain enough energy again during re-melting process and tend to coil back to a high entropy state, which may cause some opaqueness.