The molecular weight of a tackifier resin is a critical factor in polymer compatibility. In the case of styrenic block copolymers, Regalite hydrogenated hydrocarbon resins are designed with a specific molecular weight and weight distribution to impart the correct compatibility characteristics.

Figure 1: Influence of Molecular Weight, 700 vs. 900 vs. 1300

The figure above shows a dynamic mechanical analysis (DMA) study showing the influence of increasing the molecular weight and weight distribution on the compatibility of Regalite R1090, Regalite R1100, and Regalite R1125 hydrocarbon resins in styrene-butadiene-styrene (SBS) block copolymer.

Regalite R1090 hydrocarbon resin is compatible with the mid-block and end-block. Regalite R1100 hydrocarbon resin is less compatible with the mid-block, as seen by the reduction in tan delta peak height, and is less compatible with the end-block, as seen by the extension of the G' curve. Regalite R1125 is incompatible as the block copolymer, as shown by the extremely low first tan delta peak at about -10°C and the resin rich phase identified by the second tan delta peak at approximately +40°C. This is an influence of molecular weight and not of softening point. The molecular weight parameters of the Regalite resins are shown in Table 1.

Table 1: Molecular Weight Parameters of Regalite Resins

Figure 2: The Effect of Molecular Weight and Polarity on Resin Compatibility