OVERVIEW

We have seen in Chapter 3, that the properties of networks and thus of the coatings depend strongly on the physical state of the materials, being at the application temperature in a glassy state, in a rubber-like soft region or even in the transition between glass and rubber. In this transition region, the modulus is heavily influenced by temperature and may drop by several orders of magnitude. This transition is defined by the glass transition temperature ( T _g). Since almost all crosslinked coatings are amorphous polymer networks, the glass transition temperature has the most prominent influence on the mechanical properties. The T _g describes the deflection from linearity of the increase or decrease of volume of a polymer during heating, from a glassy to a soft state or during cooling from the soft to a glassy state. This behaviour is not characterized by a sharp discontinuity of the increase of the specific volume with temperature, as it is with a melting transition, but just as a discontinuity identified by a change in the rate of increase of the specific volume with temperature. Thus, the transition of a polymer into an amorphous glass is more a kinetic than a thermodynamic transition. The glass transition temperature increases with increasing molecular weight, since the mobility of the chain segments decreases. In linear polymers the glass transition temperature reaches a plateau at a polymerization degree above about 100 600, depending on the molecular structure. Consequently it does not further...