Quantitative influences of successive reuse on thermal decomposition, molecular evolution, and elemental composition of polyamide 12 residues in selective laser sintering

Since its addition to additive manufacturing (AM), polyamide 12 has dominated the selective laser sintering (SLS) market, thanks to its stable thermal property and high mechanical quality. However, substantial un-sintered residue powders lead to economic losses and are burdensome to the environment. Though several works have reported the aging mechanism and reusability of the polyamide 12 residues in SLS, the quantitative degradation and decomposition changes of differently reused polyamide 12 are not available. This work experiments successive reuse of polyamide 12 residues and quantitatively monitors the thermal decomposition, molecular evolution, and compositional changes of the material in SLS AM. To understand the characteristics of such changes, we reused the same bucket of polyamide 12 powders up to 8 times, collected powder samples, and printed 3- and 32-layer part samples. Our tests revealed that the basic flowability energies per reuse are reduced by 9.90, 15.59, and 12.74 mJ in the 2-, 5-, and 8-time reused powders, respectively. These values are essential to control the flowability of polyamide 12. Laser and heat lower the material onset decomposition temperatures, making the material more labile at a decreased temperature after reuse. On the other hand, the nitrogen atmosphere delays the onset of thermal decomposition to a higher temperature. The ¹H NMR spectra reveal the degradation of polyamide 12 with reuse: in polyamide 12 parts 3D-printed using 8-time reused powders, the relative area of the peak on C-H bonds adjacent to nitrogen has a 50% reduction compared to parts using new powders. The carbon deposit and degradation raise the atomic percentages of C and O by 72.49% and 7.13%, respectively, from new powders to the part printed using 8-time reused powders. This study further reveals the surface carbon deposit of polyamide 12 during successive reuse of SLS and explains the effect of laser in inducing polymer decomposition apart from high temperatures.