Cure Terms Glossary

Tack-Free Time

The attainment a sufficiently robust state to resist damage by contact or handling is critical point to any cure, and the time to reach this point is an important control parameter. For open systems, such as sealants, coatings or free-rise foams, this is the tack-free time - i.e. the period from the start of cure to a point when the material is sufficiently robust to resist damage by touch or settling dirt (or rain for exterior applications).

In ad hoc testing, tack-free time can be determined as the point when the surface no longer feels sticky. In a more structured way, it can be determined by briefly pressing a polyethylene film against the surface and checking for any adhering material when the film is removed. A small metal weight, to provide a reproducible contact pressure, is commonly used in this test for sealants.

The level of cure to reach a tack-free state is likely to be dependent on the polymer type as there is an adhesive component to surface tack. Surface energy is therefore a factor, and polar polymers will be inherently more tacky than non-polar polymers.

Tack-free time is just one of the diagnostic times that characterise processability and cure. For a sealant, these include (in sequence),

pour time

working time

gel time

tack-free time

For moulded or cast products, the time to reach a correspondingly robust state for safe handling is called the demould time.