Coatings World - February 2009

Table 2

Chemical and corrosive resistance linked to Tg. The degree of blistering, discoloration and/or rust are considered. The curing condition is given together with Tg in the top row. The results recorded here are after six-months of exposure for chemical tests, 250 days for the rest (or until failure).

One explanation to these phenomena might be that the small water molecules migrate somewhat into the paint film, which in turn causes the film to soften to some degree. This temporary swelling, in addition to the higher temperature, might increase the molecular movements of the unreacted polymer side chains and/or the amine hardeners enough to make for a better cure than the one achieves using just hot air (resulting in a higher Tg). When the water is removed and the paint ventilated most of the trapped water evaporates, leaving behind a fully cured paint film.

One might argue that not all the water leaves the film, but some molecules remain, acting as a softening agent. This is why we see an increase in the molecular crosslink density, calculated from storage modulus (

see equation [1]

on page 28).

Of course, these are only mere hypotheses, which need further study to accept or reject. However, if one assumes these hypotheses to be valid, further study is needed to see if the painted steel is affected by the uptake of water into the paint film.

Table 3

Tg is the glass transition temperature maximum found from the tan δcurve. E'm is the minimum storage modulus while. TE'm is the temperature at minimum storage modulus. Mc,min is the crosslink density at E'm. The “curing conditions” column gives the curing temperatures with number of days in brackets.

In actual application oil is often used for post curing and as seen from Table 3 this differed to some extent from post curing with hot water. The molecular crosslink density, M ⁰c, is almost half that seen for the water immersed panels.

One possible explanation for this effect is that both water and oil work as a softening agent, but oil to a lesser degree than water due to the obvious difference in molecular size.