Heat Activation Bonding

1. Principle of Heat Activation
2. A Wide Variety of Activation Temperatures
3. Polymer Properties

Principle of Heat Activation

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.

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).

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.

A Wide Variety of Activation Temperatures

By choosing the appropriate polyurethane dispersion raw material , you can select the activation temperature of the polyurethane dispersion to fit your process.

The various PUD type of products have been designed to suit different activation temperatures. They differ primarily in their polymer softening characteristics, which can be described by tack vs temperature curves as displayed for these 2 type of products in Figure 1 or by a TMA analysis (Figure 2).

 The standard type of products are suitable for most heat-activated applications because of their attractive property profile of a low activation temperature and high initial and final bond strength values.

 The extended product range in the market offers the adhesives formulator the chance to find the ideal product for a special application or for particular activation conditions. Some products are suitable for heat-activated bonding at high application temperatures because of their special physical polymer properties.While some are ideal for low application temperatures for heat-sensitive substrates.

Polymer Properties

The graph on the left shows a typical curve generated from a thermomechanical analysis for a crystalline polyurethane dispersion. It indicates that at a temperature of ~50°C, the polymer softens significantly. This is due to decrystallization of the polyester soft segments in the polymer backbone.

Figure 1: Polymer Physical Properties and Adhesive Properties

The graph on the right refers to adhesive tack. If the tack of the adhesive film is measured at various temperatures, it is observed that at room temperature or below, many polyurethanes form a non-tacky film similar to a coating. But at a temperature of ~50°C, it is observed that the tack property jumps in value. This means that the adhesive film needs to be warmed to a minimum of ~50°C to allow tack to develop and thus enable a good bond to form.