Although natural rubber possesses inherent tackiness, the adhesive bond strength generated by rubber alone is often insufficient for many applications. Most commercial synthetic elastomers, conversely, lack tackiness-whether toward themselves or other surfaces. Consequently, to enhance their tack, resin tackifiers must be added. A vast number of latex and solvent-based systems-particularly those employed as contact adhesives-require the use of specific types of tackifying resins.
Common tackifiers typically have relative molecular masses ranging from 200 to 1500 and generally feature large, rigid molecular structures. They are thermoplastic in nature and, at room temperature, usually exist as amorphous glassy solids. They exhibit a wide spectrum of softening points, ranging from liquids at room temperature to brittle, hard solids with melting points reaching as high as 90°C. Generally, they are highly soluble in aliphatic hydrocarbons, aromatic hydrocarbons, and many common organic solvents. From the perspectives of tack, tensile strength, color retention, and resistance to oxidative embrittlement, the choice of tackifying resin significantly influences the overall quality of the adhesive. Unmodified gums and wood rosins can be converted into esters; while they initially exhibit some tackiness, they demonstrate poor aging resistance once the adhesive application is complete. Rosin esters that have been stabilized through hydrogenation or polymerization, however, resist oxidative degradation and offer the best overall performance characteristics when utilized in adhesive formulations.
