The Effect of Structure-Property Relationships on the Formability of Pigmented Polyester Coatings

Polyester-melamine coil coatings must withstand large deformations when formed into components. Changing the structure by increasing the crosslinker content, decreasing the adipic content and decreasing the molecular weight all increase the glass transition temperature (T_g). Increasing the yield or failure stress and decreasing the strain to failure of the coating all increase the likelihood of coating damage during forming. However, the coating structure has little effect on the mechanical properties when the test temperature (T) is offset against T_g (i.e. T – T_g). Thus, the T_g of a coating is controlled by the structure, whereas the mechanical properties are controlled by the relative test temperature (T – T_g). The formability was assessed using Erichsen cupping and T-bend tests, and compared to the tensile properties of the free-films. The forming behaviour is dependent on the entire system (coating, substrate, temperature and rate). This holistic approach of correlating structure-property relationships with formability enables industry to cost-effectively improve coating performance.