Rosin, or more correctly rosin acid, is one of the oldest raw materials for the adhesives industry, either as such or converted to derivatives. Three sources of rosin are used for resin manufacture, gum rosin, wood rosin and tall oil rosin, all generated from the pine tree.

Gum rosin was once the only commercial source of rosin. It is the oleoresin (pine gum) of the living pine tree. The harvesting of the oleoresin is simple, involving only periodic wounding of the tree and collecting the sap into cups.

Wood rosin. After harvesting pine trees the stump is allowed to remain in the ground for about ten years so that its bark and sapwood may decay and slough off to leave the heartwood rich in resin. Resinous material is extracted from the stump.

Tall oil rosin is obtained by distillation of crude tall oil (CTO), a by-product of the kraft sulphate pulping process. CTO contains 70-90% acidic material composed of fatty acid and rosin. Tall oil rosin (TOR) has a tendency to crystallize and usually contains 200-600 ppm sulfur which gives it a stronger odor compared to other rosin sources. Highly distilled TOR can produce esters which are competitive with gum and wood rosin derivatives.

Rosin Chemistry
Rosin, unlike hydrocarbon resins, is not a polymer. It is a blend of distinct molecules. Rosin is a mixture of eight closely related rosin acids characterized by three fused six-carbon rings, double bonds that vary in number and location, and a single carboxylic acid group as illustrated below in Figure 1. The ratio of these isomers in rosin depends on the collection method and the species of the tree from which the rosin was harvested. The molecular weight of rosin is quite different compared to hydrocarbon resins. For example, since Pamite 79 tall oil rosin is a mixture of distilled rosin acids, its molecular weight is close to the theoretical 302 Daltons of abietic acid with very little polydispersivity. The narrow molecular weight distribution of rosins is an important factor in providing their wide compatibility with polymers.

Figure 1: Structure of Rosin Acids 

Rosin acids have a wide span of compatibility with almost all polymers. They are well known for their peel and tack contribution to the adhesive, but generally decrease cohesive strength.