Peel adhesion force is dwell time dependent. All PSAs (pressure sensitive adhesives) have a certain degree of deformation after they are pressed onto bonded substrates. Once the pressure is removed, PSAs may continue to deform and result in different adhesion forces at varying time frames. Most conventional solvent and water based PSAs have great tendencies to continue wetting onto substrates; even though the pressure is removed. The fracture mode may be adhesive failure (AF) initially and becomes cohesive failure (CF) after some time. This fracture mode transformation appears at the time the adhesion force between adhesive and substrate is greater than the cohesion of adhesive itself. Peel adhesion force will not change with dwell time appreciably for those PSAs having very high cohesion. This is because they are not deformed easily under light pressure and therefore can not intimately contact the bonded substrates particularly after the pressure is removed.
Based on the rheological evaluations for PSAs, under a temperature scan, the Tan delta value of most conventional solvent and water based PSAs rises as the temperature is above room temperature. According to Time-Temperature Superposition principles, an adhesivefs performance determined at high temperatures is essentially identical to that determined with a long dwell time. In other words – the PSAs that flow more dramatically at longer times generally possess higher Tan delta values at elevated temperatures.
Most general purposed HMPSAs are based on Styrenic-Isoprene-Styrene Block Copolymers (SIS). Depending upon the application purposes, the Tan delta minimum value of most formulated SIS-based HMPSAs normally appears at around 40-60Ž. The temperature at Tan delta minimum denotes the critical temperature that molecular chain movement alters from chain entanglement to disentanglement. Accordingly, molecular chains are tangled together to provide appreciable cohesion below this critical temperature, e.g. less than 40 to 60Ž. Since Tan delta value is an indication of flow or deformation, HMPSAs exhibiting higher Tan delta values at room temperature should offer better flow or deformation at bonding stage. For most general purposed HMPSAs, Tan delta value is gradually lowered as temperature is raised until Tan delta value reaches a minimum point. Within this temperature range, theoretically, most general purposed HMPSAs steadily rebound from the bonded substrates with the increasing temperatures due to the gradually lowered Tan delta values. That is, the peel adhesion force is actually lowered at high temperatures. According to the Time-Temperature Superposition Principle, peel adhesion force determined at longer dwell time is identical to that determined at higher temperature. Therefore, peel adhesion force should be lowered as dwell time is increased. The degree of peel retention in this time or temperature range greatly depends on the alteration of Tan delta value. When the temperature is very high or the dwell time is very long, beyond the Tan delta minimum point, HMPSA tends to deform or flow again in this chain disentangling zone. The peel force may therefore be increased again but shows a cohesive failure mode.
To minimize the alteration of peel force with dwell time, HMPSAs should possess a very flat Tan delta curve starting from room temperature to high temperatures. This unique feature is made feasible by selecting an appropriate configuration of SIS and tackifiers.
In real world applications, most PSA tapes and labels are adhered on the substrates for a long period of time before they are removed or peeled away. The short dwell time adhesion performance evaluated in the lab may not be able to correctly predict the actual adhesion performance when they are served in the end use markets. Ideally, besides the evaluation of the peel adhesion at short dwell time, ranging from 1 to 40 minutes according to various test standards, a long term dwell time, e.g. 1 day to a week, should also be evaluated and reported.