In previous articles, the fundamental of viscoelasticity and the principle of adhesion were briefly discussed. A material must cold flow at room temperature in order to possess pressure sensitive features. Disregarding polymer bases and the use of tackifiers, all room temperature applied “general purpose” pressure sensitive adhesives (PSAs) should meet the following rheological criteria:
- G’ at room temperature is less than 2 x 106 dyne/cm2 to obtain maximized contact area upon bonding with a light pressure.
- Tg is in between 0 to 10 C. The high Tan delta value at this region will provide good flow or wetting upon bonding and result in large deformation upon separation.
It seems that tackifiers can always improve adhesion and reduce viscosity for a formulated PSA. If tackifiers can always improve tackiness or adhesion of polymers, why do they deteriorate adhesion performance when they are not compatible with polymers? An excess amount of tackifier charged to a PSA formulation will also hurt adhesion performance. Only compatible tackifiers with appropriate ratios in the formulation can actually improve pressure sensitive adhesion performance. Scientifically, a more accurate description for tackifiers should be that all tackifiers are simply rheological modifiers for polymers. When a compatible low molecular weight (typically ranging from 300 to 1500) oligomer and a polymer are blended together, if the Tg and G’ at room temperature of the blended compound are able to move into the above mentioned PSA criteria, then, it is a PSA. Although mineral oils are not named as tackifiers, they sometimes can also provide the same function as solid or semi-solid tackifiers for polymers.
In summary, various combinations of polymers, tackifiers, and mineral oils, that can form a compatible blend and meet above PSA rheological criteria, are considered hot melt pressure sensitive adhesives.