[1] They are usually low-molecular weight compounds with high glass transition temperature.
Tackifiers tend to have low molecular weight, and glass transition and softening temperature above room temperature, providing them with suitable viscoelastic properties.
In hot melt adhesives they can comprise up to about 40% of total mass.
Many pressure-sensitive adhesives are a blend of rubbers (natural or synthetic) and a tackifying resin.
Silicone rubber–based pressure-sensitive adhesives require special tackifiers based on "MQ" silicate resins, composed of a monofunctional trimethyl silane ("M") reacted with quadrafunctional silicon tetrachloride ("Q").