The influence of molar mass distribution on the effectiveness of polypropylene in laser sintering process

This study investigates the intrinsic and extrinsic properties of polypropylene (PP) that significantly influence the tensile properties of laser sintering (LS). The study found that PP has a gap in performance compared to PA 12. From the literature, the study shows that molar mass distribution (MMD) is a fundamental property that impacts the tensile properties of injection moulding, but it was unknown in laser sintering (LS). The study used existing homopolymer PP powder with different Melt Flow index MFI’s, 12, 20, and 50 g/10minute (230˚C 2.16kg) with variations in MMD. High MFI resulted in high UTS and EAB, which were examined by the porosity, surface finish, and crystallinity.
The HC12 grade required further grinding to HC11 to meet the LS requirement. The HC11 powder failed during LS due to degradation, resulting in excessive curling. This was proven by high changes in the MMD after grinding. The other two grades showed that the high MFI grade resulted in superior ultimate tensile strength (UTS) and elongation-to-break (EAB). The long chains, shown by Mz+1 in the case of CP20, seem to have a negative effect in sintering, increasing porosity and resulting in a reduction in tensile values.
The study goes further by examining two powders, the SABIC PP 518A and the 511A, where both share an MFI of 25 and polymer dispersity of 6.34 to 3.52, respectively. The 518A demonstrated a higher UTS, and lower EAB. This shows that high MFI does not result in the highest tensile values in LS but a combination of MMD profile and how the polymer is processed during the LS.
The reusability of HM20/70P powder showed a slight increase in particle size D90, sintering window, and powder flow. However, the powder showed stable MMD up to the sixth cycle without noticeable degradation to polymer chains.
Higher lamellar length showed a strong trend with high UTS and EAB of the final LS specimens, which is dependent on many variables. These findings emphasise the importance of considering the entire molar mass distribution and polymer dispersity, rather than relying on single average values or MFI, to predict or optimise the mechanical properties of PP in laser sintering.