Dynamic mechanical analysis (DMA) is a rheological method used to measure the viscoelastic performances of a polymeric system. A thin wafer of adhesive is placed between two metal shafts. During the analysis, the bottom shaft oscillates slightly while the top shaft measures the amount of energy transmitted through the sample. DMA can be done with either a variable temperature or frequency of oscillation method. The variable temperature method (-80°-200°C) is the most commonly used method in adhesive analysis.
Viscoelastic behavior strongly influences the adhesion performance of the material. The DMA analysis provides rheological curves G" and G' versus temperature that provide useful information on tackiness versus temperature, and on compatibility between the polymer and tackifier resin.
Polymeric materials never behave like pure solids or pure liquids. They exhibit a viscoelastic behavior resulting from the combination of a liquid and a solid.
Figure 1: Geometric Representation of Dynamic Mechanical Analysis (DMA) Terms
Figure 2: Dynamic Mechanic Analysis (DMA) Curves of a Styrene Isoprene Styrene (SIS) Based Hot Melt Pressure Sensitive Adhesive (HMPSA).
The DMA curves can be divided into 4 main sections:
Glassy zone: The mobility of the polymeric chains is very limited. Generally, the material is hard, brittle and does not exhibit tack properties.
Glass transition of the isoprene phase: Polymer chains begin to move and tack properties develop.
Rubbery plateau: G' (Storage Modulus) is the dominant factor and the compound behaves as an elastic solid with adhesive properties.
Melt flow region: G'' (Loss Modulus) becomes the dominant factor due to exceeding the glass transition temperature of the styrene blocks of the elastomer. The adhesive becomes a viscous liquid.
Polymer/ Tackifier Resin Compatibility
If a polymer and the tackifier are compatible one glass transition zone is still observed, but with the tan peak shifted to a higher temperature due to the contribution of the higher Tg tackifier resin. This indicates an intimate mix (solubility) between the mid-block and the resin. This phenomenon is called plasticization. In case of incompatibility, DMA can detect a transition due to presence of two different phases, one polymeric phase and one resin phase.
Regalite™ hydrocarbon resins varying by molecular weight. The curves easily demonstrate that, in this particular case, compatibility decreases when molecular weight increases.
Figure 3: Dynamic Mechanical Analysis Curves of a Styrene Butyadiene Styrene (SBS) Based Hot Melt System