Distribution of individual components in thermoplastic olefins: step-scan FT-IR photoacoustic phase analysis

A method to determine surface stratification of components within a thermoplastic olefin (TPO) compound is developed using Fourier transform infrared (FT-IR) step-scan photoacoustic spectroscopy (S' PAS). Infrared bands attributed to magnesium silicate (talc), polypropylene (PP), and ethylene-propylene rubber (EPR) are used as analytical depth-profiling probes to establish the origin of the photoacoustic (PA) signal. This was accomplished by performing a series of experiments at various modulation frequencies, and subsequent monitoring of phase spectra and phase rotated spectra generated from signals collected at 0 degrees (I) and 90 degrees (Q) phase shifted from the Light source. Utilizing phase analysis data, a TPO compound stratification model was developed, which consists of four distinct layers extending by 0-12 mu m from the sample surface. Layer I (0-3 mu m) shows evidence of a large change in talc and PP concentration, while layer II (3-6 mu m) shows a significant decrease in both of these components. Within this same layer, evidence of discrete individual EPR particles are shown by an abrupt concentration change from 2.67 to 5.35 mu m, followed by a large decrease from 5.35 to 6.55 mu m. Further into the sample at 6-9 mu m from the surface (layer III), all three components reach their highest maximums for all four layers, with talc serving as the local maximum. Approaching layer IV, talc concentration diminishes until it is no longer apparent further than a distance of 8.40 mu m from the surface. In addition, both EPR and PP concentration are detected within this layer (9-12 mu m), showing decreases in concentration with depth. The proposed depth profiling technique also shows potential applications to various systems displaying stratification near the surface. (C) 1999 Elsevier Science Ltd. All rights reserved.