What is the mechanism of bringing two similar or dissimilar materials together? Below are five commonly discussed adhesion mechanisms.
- Physical Adsorption
All materials have different polarities resulting from the density of electron clouds surrounding their chemical element or functional group. The magnitude of force attributed from these electron clouds, from low to high forces are Van der Waals force, dipole-dipole moment, hydrogen bonding, and acid-base interaction. The greater force is the spit of electron clouds or polarity difference; the better force is the attraction force when two materials are brought together. In general, polar substrates and adhesive offer higher polarity and cause higher adhesion forces. For instance, metal and paper (cellulose) are polar materials and are easier to adhere by any adhesives. Polyethylene (PE) and silicone are very non-polar and are difficult to bond. To adhere these low polarity materials, a high polarity adhesive may be helpful.
- Chemical Reaction
When two materials both possess reactive functional groups, chemical reaction between the two materials may occur under certain environmental conditions when they are brought together. For example, a sulfur (curing agent) containing chloroprene rubber (CR) and brass can form a cupper-sulfide (CuS) covalent bond during curing.
When two thermoplastic materials, having similar functional groups, are heat sealed – they can form permanent bonding without an additional adhesive as a media. This type of adhesion mechanism is named inter-diffusion. Most PVC, PE, and PP films can be easily heat sealed together at temperatures beyond their softening points.
- Electrostatic Attraction
When two materials are temporarily charged, similar to those positive and negative characters of a magnet, they can stick together until the electrostatic force is gradually diminished.
- Mechanical Interlocking
As discussed in the previous articles, all materials possess certain viscoelasticity and flow characteristics. Disregarding the physical adsorption or the polarity difference between pressure adhesive (PSA) and substrates, PSAs must flow or wet onto substrates considerably in order to create an appreciable bonding force. When a PSA has a higher value of loss modulus (G”) versus storage modulus (G’), i.e. Tan delta, it will generate more contact area on a rough surface and result in a higher separation force. On the contrary, if a PSA can not flow and wet on substrates upon light finger pressure due to its poor flow, low Tan delta value, it will form limited contact area and result in a low separation force.
Among the above five mechanisms, only physical adsorption and mechanical interlocking can be applied to manipulate the level of pressure sensitive adhesion. To formulate a high peel and tack PSA, if acceptable, one should first consider selecting polar ingredients, such as rosin derivatives, for the formulation. This will greatly improve the contribution of physical adsorption. Then, the viscoelasticity or mechanical interlocking of the formulation becomes the major factor affecting both bonding and de-bonding features. As discussed in the other articles, both Tg and G’ are key parameters determining the magnitude of peel and tack forces. To formulate desirable PSAs, appropriately manipulating the rheological properties is needed.