Pressure sensitive adhesives (PSA) Flashcards
Explain what a pressure sensitive adhesive means.
It is visco-elastic material that instantly stick to a surface when applying pressure. The adhesion can be permanent or removable.
Give examples of applications of PSAs.
Post-its, diapers, to assemble things easily eg. smart-phones
Explain the properties of a visco-elastic material.
It has both viscous and elastic behavior. When load is applied to it, it looses some energy due to rearrangement of molecules in polymer chain. If truly elastic it doesn’t loose energy.
Explain what the storage modulus G’ and the loss modulus G’’ means.
In viscous material there is a phase lag between the applied stress and resulting strain.
The storage modulus represents how much energy that is stored in the material, if the phase lag is small much of the energy is stored. In elastic materials there is no phase lag and all the energy is stored.
The loss modulus represents the energy lost, and if the phase lag is larger more energy is lost.
The relation between G’ and G’’ decides what type of PSA it is.
What different types of PSAs are there and what characterize them?
Acrylics: often crosslinking, additives
Natural rubber: crosslinking, is takifying resin
Styrenic block copolymers: S-rubber-S block copolymer, low weight resin
What is the role of resin in the adhesive?
One PSA can consist of a tri-block copolymer and resin, and the resin makes it more sticky. Below 10% resin there is no difference, in commercial PSAs around 50% resin is normal.
How can one engineer reduced adhesion? Is desirable for post-its etc.
Ex. 1: reduce the contact area. Either by adding hollow glass spheres or other small non-adhesive particles to the surface.
Ex. 2: The adhesion is reduced by UV-light, if a PSA is left in the sunlight for a long time the adhesion will be reduced.
One application of this could be to remove band-aids by UV.
How can PSAs react to stimuli to change their adhesive behavior?
Thermosetting: one has a tape with epoxy on one side ie. sticky molecule, when curing for 20 min in 145 degrees it is not sticky anymore.
Light-deactivable: to remove band-aids without pain or trauma by exposing to light. Based on standard acrylic copolymer blended with photoinitiator.
How does adhesion without viscoelasticity work? Inspiration from the gecko.
What are pros and cons?
The gecko stick to the surface by the van der Waals force and capillary action. On its feet it has many small spatulas, ie. small colons which increases the contact area.
It also takes advantage of the contact splitting, when splitting a surface into n smaller contacts, the adhesino goes up by n^1/2.
Pros and cons:
+ when removing the spatulas, need to remove one by one cannot just peal off
+ better adaption to rough surfaces, with less strain penalty
+ uniform stress distribution over attachment area, if larger contact more stress in the edges
+ defects of contact does not matter much when are fibrils
- but if they are too thin may break before they release the grip
- or will adhese to each other
- elastic adaptability necessary to adhese to rough surfaces
Give examples of how to fabricate micro-adhesives.
Ex. 1: By using aluminum pattern create a dense array of polymide flexible pillars.
Ex. 2: hierarchical pillars ie. pillars on top of pillars, can be produced by lithography, but to adhese to planar surface this is not beneficial. Gecko - nm size, these micro size.
Ex. 3: PS nanotubes. Both superhydrophobic and adhesive to liquid at the same time. By VdW like gecko can hold water droplets up-side-down from strong adhesive force, and move them because hydrophobic.
Ex. 4: Switchable adhesives. By using shape memory polymers to create the adhesives, the adhesivness can be turn on and off by changing the temp.
E. 5: Combining mussel and gecko features to be able to hold on long to surface and also in wet environment.
