A tremendous amount of PE and PP films are used in tapes, labels, envelopes, bags, packaging, and lamination markets. These materials are more difficult to bond than many other plastics due to their nature of low surface energies or polarities.
Some chemists believe that to gain good adhesion to low surface energy materials, the pressure sensitive hot melt adhesives used should have low polarities. This perception is coming from the principle of “like–dissolve-like” for mixing chemical substances. However, this principle is not able to describe the adhesion behavior for two dissimilar materials.
In a previous article, the following five mechanisms of adhesion were illustrated.
- Physical Adsorption
- Chemical Reaction
- Electrostatic Attraction
- Mechanical Interlocking
For pressure sensitive adhesion, only physical adsorption and mechanical interlocking are applicable. In order to adhere to no or low polar materials, HMPSAs containing more polar ingredients will greatly enhance the polarity difference between adhesive and adherent, i.e. inducting acid-base characteristics or physical adsorption. Polar ingredients, such as rosin derivatives, are normally dark in color. If light color or water white HMPSAs are required for certain applications, then, these dark color materials are not acceptable and the mechanism of mechanical interlocking of HMPSAs become the only factor that can be utilized to manipulate pressure sensitive adhesion to non-polar materials.
As mentioned previously, the degree of mechanical interlocking can be determined and represented by certain rheological properties. HMPSAs having higher Tan delta values at bonding process can impart better wetting and flow onto bonded substrates with minimum elastic rebound. At debonding process, HMPSAs possessing higher Tan delta value, on the other hand, can exhibit greater elongation or deformation which will result in higher separation force. It is important to note that the rheological properties of HMPSAs at bonding and debonding processes are not the same. They are also temperature, bonding pressure, peeling rate, etc. dependent.