The polyisoprene (PI) mid-block in SIS is a low-polarity rubber phase. A tackifier resin must be compatible with (soluble in) the PI phase to deliver tack.
C5 aliphatic / hydrogenated C5: The aliphatic molecular chains offer excellent compatibility with the PI segment → good initial tack, low-temperature flexibility, and strong wetting on PE/PP. It is the “natural resin of choice” for SIS pressure-sensitive adhesives.
C9 aromatic: Tends toward aromatic rings and therefore aligns more closely with the PS hard phase. Its compatibility with the PI segment of SIS is a notch weaker → moderate initial tack, stronger odor, prone to yellowing. However, it offers better holding power and heat resistance, making it suitable for high-temperature auxiliary materials.
C5/C9 copolymer: Polarity falls in the middle. Commonly used in general-purpose labels and carton sealing tapes to balance cost and compatibility.
So for SIS systems, use hydrogenated C5 as the primary tackifier. Do not use standard C9 as the main resin — otherwise you risk phase separation, strong odor, and low-temperature embrittlement.