Rapid Ablative Laser Technique (ICP-MS) for Monitoring Material Homogeneity in Polypropylene

Abstract

Efficient mixing of additives in polymer material is essential for uniform distribution. This paper explores the capability of using a high-resolution ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) laser ablation technique to assess the uniformity of distribution of selected metal dopants in polypropylene samples. This study simulates the distribution of residual metal catalysts in polymer material and could provide an insight into investigating the bulk dispersion of catalytic residues. Ablative laser technology is relatively unexplored for this purpose, and is one of the few contemporary instrumental methods available for consummate evaluation of material homogeneity. Polypropylene samples were especially loaded with appropriate levels of metal oxides [ZnO, TiO 2 , Mg(OH) 2 ] to achieve ppm levels on distribution in the bulk material. The study subsequently examined the distribution of the metal dopant in the solid phase-both spatially and depth-wise-using a 100 µm-diameter laser beam coupled to an ICP-MS instrument. The laser (213 nm) was programmed to ablate a total depth of 25 µm at each point at 5-µm intervals. Prior to each run, the instrument underwent appropriate calibration and correction for background. The study did not necessitate standardization, and signal intensities were compared with surface metals and those occurring internally. Spectra were generated to observe fluctuations in characteristic metal intensity with penetration depth. In some cases, wide variations were observed by more than two orders of magnitude. The experimentally determined results indicated that metal intensity consistently showed a steady decline with depth, suggesting that most of the spike was concentrated on the surface. In some cases 'hotspots' were observed at certain points, where the dopant accumulated, indicating imperfect mixing of the additive. Uneven levels of surface metals are particularly significant, as these metals would play a major role in abrading and subsequently migrating into the surrounding environment. Irregular distribution of sub-surface metals is equally important as it could provide useful information of dispersion in the bulk of the polymer material. Our work would be of definite interest to environmental science and materials research, and could be useful in modeling studies.