Dear Ariq Abdillah, the following documents deal with the action mechanism of tackifiers. My Regards

https://doi.org/10.5254/1.3536196

https://doi.org/10.1002/app.1973.070171114

Tackifiers work by increasing tack and hence adhesion. In hot melts the primary purpose is to increase tack. In pressure sensitive hot melts the tackifier also in creases auto adhesion which is what makes it tacky to touch. The mechanism relies mainly on solubilisation of the polymer or chemical groups on the polymer. For example in PSA's based on styrene butadiene a tackifier such as a rosin ester will solubilise the butadiene and hence increase tack but reduce modulus. Conversely a resin such as alpha methyl styrene will solubilise the styrene groups reducing adhesion and decreasing the tack. This in turn will increase the modulus. In there natural state tackifiers are solid brittle materials with a Tg above room temperature. Adding them usually will increase the Tg of the adhesive. The degree will depend on the amount and the ability to solubilise the polymer, sometimes called compatibility. It is possible to create two Tg's if the tackifier only solubilises one segment of the polymer. In this case the other segment is not soluble in the tackifier and separates out as a separate phase with a different Tg

Plasticisers also solubilise the polymer and most are liquid with a very low Tg so this method reduces the modulus much more sharply. In a very general way plasticised polymers can be thought of as partially dissolved in a solvent, which increases their tack and reduces modulus. Resins as solids can be regarded more as compatible blends with the polymer, the more compatible the better the blend and the greater the effect on the polymer properties.

Hope this helps

Dear Ariq Abdillah, you found out something which is a key fact for adhesives, namely PSA: the connection between adhesion and the glass transition temperature.

To understand this phenomenon easier, it makes sense to have a look into the rhological behavior of a PSA.

When you have a look at the tan delta of a certain adhesive you will notice that you have a specific tan delta at your application temperature (example 23°C).

When you now start to move the tg to a higher temperature by shifting the whole tan delta curve to the right, you will see that the tan delta value at your application temperature is increasing. This indicates a better tack and a better adhesion.

Of course this has a limit, as your G' still needs to be below 3x10^6 Pa to have that stickiness.

There is a nice litterature from Chuck Paul about this topic.

Paul C.W. (2011) Pressure-Sensitive Adhesives (PSAs). In: da Silva L.F.M., Öchsner A., Adams R.D. (eds) Handbook of Adhesion Technology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-01169-6_15

Tackifiers are low-molecular weight compounds (oligomers) that are added to adhesive formulations to improve tack and peel adhesion. Their glass transition temperature (Tg) is typically higher than that of the base polymer but their molecular weight is much lower..Tckifiers are usually resins (e.g. rosins and their derivates, terpenes and modified terpenes, aliphatic, cycloaliphatic and aromatic resins (C5 aliphatic resins, C9 aromatic resins, and C5/C9 aliphatic/aromatic resins), hydrogenated hydrocarbon resins, and their mixtures, terpene-phenol resins.