Exploring Molecular Changes at the Surface of Polypropylene after Accelerated Thermomolecular Adhesion Treatments

A central composite rotatable design (CCRD) method was used to investigate the performance of the accelerated thermomolecular adhesion process (ATmaP), at different operating conditions. ATmaP is a modified flame-treatment process that features the injection of a coupling agent into the flame to impart a tailored molecular surface chemistry on the work piece. In this study, the surface properties of treated polypropylene were evaluated using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). All samples showed a significant increase in the relative concentration of oxygen (up to 12.2%) and nitrogen (up to 2.4%) at the surface in comparison with the untreated sample (0.7% oxygen and no detectable nitrogen) as measured by XPS. ToF-SIMS and principal components analysis (PCA) showed that ATmaP induced multiple reactions at the polypropylene surface such as chain scission, oxidation, nitration, condensation, and molecular loss, as indicated by changes in the relative intensities of the hydrocarbon (C(3)H(7)(+), C(3)H(5)(+), C(4)H(7)(+), and C(5)H(9)(+)), nitrogen and oxygen-containing secondary ions (C(2)H(3)O(+), C(3)H(8)N(+), C(2)H(5)NO(+), C(3)H(6)NO(+), and C(3)H(7)NO(+)). The increase in relative intensity of the nitrogen oxide ions (C(2)H(5)NO(+) and C(3)H(7)NO(+)) correlates with the process of incorporating oxides of nitrogen into the surface as a result of the injection of the ATmaP coupling agent.