Page 78 - Handbook of Surface Improvement and Modification

6

Tackifiers

A balance of elastic and viscous properties is one of the most important features of per-
forming adhesive. Viscous properties are required for adhesive to flow and form physical
contact with the substrates which are to be fastened using adhesive. This intimate contact
permits formation of adhesive bonds (both chemical and physical). The formation of the
bonds is not sufficient for performance because adhesive must have sufficient strength to
hold both substrates together under expected force. Here come elastic properties. The
adhesive properties usually come from polymer matrix, whereas viscous properties are
supplied by a tackifier. The entire system in its totality has to withstand stresses in a full
spectrum of temperatures and forces encountered in the life of an adhesive. In addition,
the adhesive joint undergoes physical and chemical changes which modify the original
properties of adhesives. If at any given moment of adhesive performance its ability to
withstand stress falls below the forces applied, the adhesive joint fails either completely or
partially.
From the above introduction roles of tackifiers appear to be very well-known and
easy to select. In reality, influences of many properties are to be know, such as
• c h e m i s t r y
• mechanism of cure
• polarity
• glass transition temperature
• softening point
• phase transition
• viscoelastic properties
• the velocity of separation and contact time
• testing method
• oxidation
• photochemical stability
to properly assess the system and select proper formulation. These aspects are discussed in
the above order in the next section pointing to the influences of tackifiers.
Tackifiers are combined with polymers and additives to manufacture final products
having the required set of properties. In addition to compatibility between tackifier and
matrix resin and other components, the thermal properties of tackifier and entire adhesive,
and stability of system prior and after application are essential criteria of performance (as
mentioned in the above list). Finally, such trivial features as color, odor, fluorescence, and
cost may determine whether tackifier is useful or not in the planned application.