# Recycled PP for 3D Printing: Material and Processing Optimization through Design of Experiment
###### Abstract
In particular, the rheological behavior of the polymer plays a very important role, since the extrudability of the filament through the print nozzle, as well as the shape stability of the extrudate during the deposition step and the adhesion between subsequently deposited layers, are dictated by the material viscoelastic properties [15]. In a previous work [16] we have shown that the strictest criterium to classify a polymer as FFF-printable from a rheological point of view is the occurrence of a yield stress behavior in quasi zero-shear conditions, which guarantees low propensity of the filament to drip after the extrusion step and a good shape stability of the extrudate at the exit of the nozzle and also during the deposition step. Additionally, a thermoplastic that is suitable for FFF processing should have a low degree of crystallinity and slow crystallization kinetics [17]. These characteristics could minimize the issues that are associated with the volumetric contraction that are experienced by semi-crystalline polymers during crystallization, which causes the entrapment of high residual stresses within the printed part, resulting in warpage, shrinkage, or even delamination and cracking [18]. Quite recently, it has been demonstrated that the introduction of different micro- or nano-fillers in a polylactic acid (PLA)-based formulation can promote several beneficial effects on the 3D printability of the base material, also allowing obtaining printed samples with enhanced mechanical properties as compared to their unfilled counterparts [19]. Currently, the use of waste or recycled thermoplastics for the development of 3D-printed parts is an emerging area. In this context, some authors have started studying the formulation of filaments using homogeneous plastic wastes that are derived from 3D printed parts, polyethylene terephthalate bottles, or low-density polyethylene bags; besides, some recycled filaments are already commercially available [20; 21]. [PERSON] et al. [22] demonstrated the possibility of formulating filaments for FFF starting from a material that is derived from mechanical recycle processes of the plastic fraction of the waste of electrical and electronic equipment (WEEE). Preliminary physico-chemical analyses that were performed on the WEEE wastes showed a high heterogeneity of the starting material, involving the presence of 11 different polymers or blends. The selected samples were then subjected to a cleaning procedure to reduce or remove foreign materials, reduced into small flakes and extruded in filaments, and subsequently used to print test parts. The quality control of the printed objects proved that FFF printed specimens had no remarkable deviations from the model design as compared to the same objects that were printed using commercial filaments, thus demonstrating the possibility of exploring recycled heterogeneous thermoplastics for FFF applications. In this work, we clearly demonstrated the suitability of blends containing r-PP for FFF processing. In particular, the research was carried out following three different stages:  
* The first step focused on the production and the preliminary characterization of PPt/r-PP blends at different weight ratios, aiming at selecting the formulations showing the best performances in terms of rheological and thermal properties, for a FFF process. * In the second step, the processing conditions of the selected blends were optimized through a design of an experiment approach, involving the use of Taguchi method. This procedure allowed producing two r-PP-based materials having optimal rheological and thermal characteristics for the desired processing. * Finally, in the third step, the production of the filament and the subsequent 3D printing of the selected materials were carried out, also evaluating the morphology and the mechanical properties of the printed specimens.