Removable, or fugitive, HMPSAs are useful for those applications requiring low peeling force without damaging and leaving adhesive residues on substrates. There is no fixed or targeting peeling force for removable HMPSAs. It is totally dependent upon the fracture energy or tearing strength of bonded substrates.
Technically, it is more difficult to formulate a low peel versus a general purposed (permanent) HMPSA based on commercially available SBCs (Styrenic Block Copolymers). For a removable HMPSA, in addition to the low initial peel, most applications also require very low peel force build-up after aging for a period of time. HMPSAs contain many low molecular weight ingredients which will gradually wet onto porous surfaces and increase the contact area. As a result, the peel force will gradually increase with the length of contact time.
Besides the contact time, peel force is also very sensitive to peeling rate. For a general purpose HMPSA, normally, peel force increases as the peeling rate rises. From a rheological view point, the separation phenomenon at higher rates is essentially equivalent to those separated at low temperatures. Most general purpose HMPSAs are designed to impart very high flow or elongation characteristics upon peeling. In other words, at the peeling moment, the Tan delta value is high. This feature is the opposite of formulators’ needs to develop removable HMPSAs.
Ideal rheological features for removable HMPSAs are as follows.
- Tg (glass transition temperature) should be low enough. With a low Tg, the Tan delta peak is far below room temperature and Tan delta value remains at a low level region upon peeling.
- The Tan delta curve at rubbery plateau zone should be very flat (robust). Since the value of Tan delta (flow character) does not change in this zone, the peel force can remains the same upon peeling with slight variation of temperatures, rates, and time.
- The Tan delta values at rubbery plateau zone should not be too high. For a general purposed HMPSA, the higher the Tan delta value in this zone, the better is the flow or wetting to the substrate and will result in higher force upon peeling. For removable adhesives, the Tan delta values should be low enough (more elastic) to minimize the further wetting of adhesive onto substrate upon storage for a period of time.
For more information call or email Pierce Covert,
Glue Machinery Corporation