The absorption of laser energy during laser transmission welding (LTW) of certain thermoplastics such as iPP affects their three-phase morphology (TPM) – comprised of crystalline (CF), mobile amorphous (MAF) and rigid-amorphous phase fractions (RAF) – and thus their mechanical properties, which are relevant for their production, processing and application. Therefore, this study serves to establish a spatially resolved and quantitative diagnostic to generate deeper insights into the cause-and-effect relationships between LTW parameters and the resulting TPM in iPP. Raman spectroscopy is developed as a quantitative TPM measurement technique in iPP. To that end, in situ Raman measurements are performed during the melting and crystallization of iPP. A spectral reconstruction routine is applied to the spectra to create a setup-independent Raman peak model. Furthermore, an improved Raman TPM model is established via model selection leveraging the Bayesian Information Criterion. The developed models are then applied to determine the effect of different LTW line energies on the local TPM in iPP welds using spatially resolved Raman microscopic line-focus mapping. Compared to the laser-unaffected iPP, the weld core shows a decrease in the CF, but an increase in the RAF and MAF contributions. For high line energies this effect is less pronounced as the system is given more time for crystallization after melting. In effect, higher line energies are advantageous to achieve a similar TPM in both the weld core and adjacent non-irradiated iPP. • Development of a setup-independent Raman peak model of isotactic polypropylene. • Development of a quantitative Raman three-phase model of isotactic polypropylene. • Spatially resolved measurement of phase morphology in laser transmission welds. • Characterization of phase changes as a function of LTW line energy. • Best phase uniformity achieved by use of highest LTW line energy.
Braeuer et al. (Sun,) studied this question.