A multiliquid approach to the surface free energy determination of flame-treated surfaces of rubber-toughened polypropylene

In the present work, rubber-toughened polypropylenes (TPOs) with different properties of the rubbery phase, consisting in different degrees of rubber dispersion and different levels of rubber crystallinity, were considered. The flame-treated surfaces of these materials and their interfaces with a commonly used primer were studied by dynamic contact angle (DCA) analysis using seven liquids and by scanning probe microscopy (SPM), The contact angle data were analysed using the concepts and the equations of the surface free energy acid-base component theory. A new approach consisting in the use of a large number of probe liquids and of a proper mathematical method is proposed; it allows higher precision in the determination of the surface energy components and the work of adhesion, the reduction of possible artefacts, and the calculation of standard deviations of obtained quantities. It was found that: (i) the characteristics of the flame-treated surfaces were largely independent of the composition and morphology of the rubbery dispersed phase; (ii) the flaming effect was better shown by receding angles and the observed hysteresis allowed a quantitative evaluation of the surface heterogeneity induced by the flame process; and (iii) the flame treatment induced fragmentation of the macromolecules with the production of fragments, soluble in all the test liquids, depending on their surface tension and their acid-base character, as shown by repeated DCA immersions. A comparison has been made with the ASTM 2578-67 method ('swab'). The SPM analysis, both in contact and in 'force spectroscopy' modes, confirmed the surface model obtained by the DCA data.