The most important bonding process with polyurethane adhesives, both solvent-based and water-based, is by heat activation. Therefore, water-based adhesives based on Polyurethane dispersions are generally processed like solvent-based polyurethane adhesives using heat activation.
Polyurethane dispersions adhesives can be bonded using the heat activation process due to temperature dependent changes in the morphology of the polymer backbone.
The initial stage of the film formation is outlined in Figure 1.
(1) Application of the adhesive on a freshly coated substrates
(2) Drying process: evaporation of water and interdiffusion of polymer particles
(3) Heat Activation: a dry and non-tacky polymer film is formed
Upon the application of heat to the dried polymer film, a number of polymer-physical changes occur, which are illustrated in Figure 1.
Figure 1: Polymer Physical Properties and Adhesive Properties
Further stages in the development of the adhesive bond are illustrated in Figure 3. Once the substrates have been joined, the adhesive bond cools within seconds, causing the polymer modulus to increase once again, giving the bond its initial strength (Phase 1). Within a few minutes the PU crystallizes, resulting in an increase in bond strength. The high final bond strength occurs upon full recrystallization of the polymer backbone (Phase 2).
Figure 2: Increase of strength after heat activation bonding occurs in 3 phases
If a dispersible crosslinking isocyanate is added, the polymer modulus level increases over the course of several days owing to polymer crosslinking. This results in a higher molecular weight polymer with high ultimate heat resistance (Phase 3).
Through cooling of the adhesive film and crystallization of the PU, a high initial bond strength is obtained after a short time, even with one-component processing. Moreover, the tackiness of these systems can be switched on and off, enabling a highly flexible production process.
Examples of Bonding Applications with Polyurethane Dispersions
