Hot tack is a term relating to the ability of hot melt adhesives (HMA) to hold substrates together prior to solidification or set.
According to the rheological concept, a HMA must meet certain range of Tan delta values and storage modulus (G’, cohesive strength), proposed as a rheological bonding window, during the course of cooling; in order to flow or wet onto substrates appropriately and possess sufficient cohesive strength simultaneously. If the bonding-debonding action is too fast during cooling; while Tan delta value is still too high and G’ is too low; a poor debonding of cohesive failure is observed. Most adhesive users may simply consider this cohesive failure as a result of a slow set of the formulation. On the other hand, when the bonding-debonding action is too slow, Tan delta value is too low to provide sufficient flow or wetting; and at the same time, G’ is too high and becomes very hard. In other words, both properties have already exceeded the bonding window and lost the ability to hold substrates together. Adhesive users may then consider this undesired phenomenon that the open time of the adhesive is too short.
Most adhesive formulators must perform lots of trial and error evaluations in order to qualitatively determine the strength of hot tack of HMAs on specific substrates. The test result is greatly temperature, speed, and substrate dependent. The reported test result is normally subjective but not accurate. To precisely compare hot tack performances of different HMAs, it may be more effective to carry out rheological evaluations based on a temperature or time scan for an adhesive from molten stage to solid stage. According to this simple test, the variation of both Tan delta value and G’ at any temperatures within the transition zone between molten and solid state can be determined quantitatively. The rheological test is reproducible independent of those affects resulted from surrounded test temperature, test speed, and bonded substrates.
In summary, according to the rheological data, the difference of hot tack of various HMA formulators can be determined quantitatively without performing a tremendous amount of trial and error and irreproducible qualitative tests.
For more information call or email Pierce Covert,
Glue Machinery Corporation