All polymeric materials possess their own specific solubility parameters, d. Hildebrand Solubility Parameters are most commonly values for determining the compatibility of raw materials used for SBC-based hot melt pressure sensitive adhesives (HMPSA).
Solubility parameter (of a polymer) is a parameter which is a characteristic of a polymer used in predicting the solubility of that polymer in a given solvent.
The Hildebrand Solubility Parameters, d, of Isoprene (I), Butadiene (B), and Ethylene/Butylene (EB) are 8.1, 8.4, and 7.9 respectively. The d of tackifiers and mineral oils vary depending upon their molecular structure and degree of hydrogenation. The d of them are mostly within 7 to 10. To formulate a clear or transparent HMPSA, the selected SBCs and tackifiers must be very compatible. In other words, they should possess very similar solubility parameters. The cloudy or opaque appearance normally illustrates that the SBC-tackifier blends are less or not compatible. Disregarding the color and clarity, only a compatible SBC-tackifier blend can generate appreciable pressure sensitivity. On the other hand, for an incompatible SBC-tackifier system, the blend becomes very hard and brittle. Certainly, it is not pressure sensitive.
Though the Hildebrand Solubility Parameter is a very useful guideline for selecting compatible raw materials, adhesive formulators always need to blend SBCs and tackifiers together before they can actually observe the degree of compatibility for blends. Theoretically, most SBC-tackifier blends are neither fully compatible nor fully incompatible.
The compatibility of SBC and tackifier will greatly affect the glass transition temperature (Tg) and moduli (G’ and G”) of the blend. For an ideally fully compatible SBC-tackifier blend, both Tg and entanglement storage modulus (Gn°) are predictable and can be pre-calculated by both Flory-Fox Equation and deGennes Equation respectively.
Flory Fox Equation
1/Tga = Σ(Wi/Tgi)
Where,
Tga: The targeting or anticipated Tg (°K) of the blend
Wi: Weight ratio of individual component i (polymer and tackifiers)
Tgi: Tg (°K) of individual component i
deGennes Equation
Gn° = v2x Gn°°
Where,
Gn°: Gn° of the polymer blend
Gn°°: Gn° of a neat polymer
v2: Volume fraction of polymer
x: An exponent which depends on the compatibility of the polymer diluents system
If the targeting Tga and Gn° of each specific adhesive can be pre-defined, so called rheological window, then, HMPSA formulators will be able to select compatible ingredients and utilize the above equations to pre-calculate the weight ratio of each component for the specific hot melt formulation.