Tackifier Resins

Tackifiers are low-molecular weight compounds (oligomers) that are added to adhesive formulations to improve tack and peel adhesion. Their glass transition temperature (T_g) is typically higher than that of the base polymer but their molecular weight is much lower. The tackifiers dissolve in the polymer matrix and break up their structure and thereby enhance the mobility and tack of the base polymer. They also provide functionality that promotes specific adhesion and reduces molecular cohesion, which in turn, lowers the melt viscosity and softening point of the polymer system.

The three most important groups of tackifiers are hydrocarbon resins, rosin esters, and polyterpenes. Hydrocarbon resins are based on petroleum feedstocks whereas terpenes and rosin esters are gained from pine trees.

Rosin esters are one of the oldest resins. The three most important types are gum rosin (tree sap), wood rosin (tree stumps), and tall oil rosin (Kraft paper making process). Rosin esters are compatible with many polymers and impart outstanding tack. They are, however, more expensive than hydrocarbon resins but cheaper than polyterpenes.

Hydrocarbon resins are the most popular tackifiers. They are produced mainly from  petroleum-based by-products of naphtha crackers. The three major types are C5 aliphatic, C9 aromatic, and DCPD cyclo-aliphatic resins. They are sometimes hydrogenated to reduce discoloration and to improve their heat and UV stability.

Terpene resins are derived from terpentine oil obtained either from pine wood or citrus fruits. The most important grades are polyterpenes based on pinene, and phenol and styrene modified pinene-based terpenes. They are compatible with a large number of polymers and possess the highest amount of biorenewable content. Grades with high-softening point impart excellent heat resistance, tack, and peel adhesion to PSA‘s and hot melt formulations. They are, however, less often used due to their higher price.  

The molecular weight (distribution) and polarity of the tackifier resin are the most critical parameters because they determine the solubility of the tackifier resin in the base polymer. An incompatible resin will have little effect on the polymer properties and hence does not create extra tack and adhesion. Resins with a high aromaticity are usually prefered for polar polymers such as urethanes and acrylics whereas aliphatic resins are the better choice for nonpolar polymers such as natural rubber and chloroprene. To achieve the optimal melt viscosity, and maximum peel and tack, the right balance of aliphatic and aromatic portions in the resin and the right molecular weight have to be found.