# Recycled PP for 3D Printing: Material and Processing Optimization through Design of Experiment
###### Abstract  
In this work, blends that were based on first use PP added with talc (PPt) and recycled polypropylene (r-PP) were designed and formulated, aiming at producing filaments that are suitable for 3D printing fused filament fabrication (FFF) processes. A preliminary characterization of PPt/r-PP blends at different weight ratios allowed selecting two systems showing adequate rheological behavior for FFF. The selected blends were melt compounded in a twin-screw extruder, optimizing the processing conditions through a design of experiments approach, involving the use of [PERSON]'s method. The materials that were prepared with the optimized processing conditions, hence showing the best performance in terms of rheological behavior and thermal characteristics, were then selected for the production of the filament and for the subsequent FFF processing. Finally, the morphology of the filament and the mechanical properties of 3D-printed samples were assessed, demonstrating the achievement of satisfactory results in terms of performances. In general, the obtained results clearly demonstrated that a proper optimization of both material and processing conditions offers the possibility of using recycled PP-based formulations for additive manufacturing processes, hence allowing a remarkable valorization of a low added-value material through its utilization for an innovative and sustainable manufacturing approach. 3D printing; polypropylene; recycle; rheology; mechanical properties; fused filament fabrication +
Footnote †: journal: journal: Applied Science and Technology, Polytechnic of Torino, Vale Teresa Michel, 5, 15121 Alessandria, Italy  
at least 65% weight of all packaging waste is recycled and at least 50% weight of all plastic waste is recycled [4]. From a report that was made by \"The Recycling Partnership\" and \"Association of Plastic Recyclers\", approximately 3.5 billion pounds of rigid PP packaging is sold every year but has been commonly recycled for less than a decade, thus the recycling rate was reported to be only 14% in U.S. in 2018 [5]. The average of recycling rate is 21% with peaks for PET and HDPE of over 30%. It is, therefore, necessary to enhance the recycling of polymers such as PP which today has a low recycling rate. Additive manufacturing (AM) technologies in the last few years are becoming key industrial processes, mainly due to the several benefits of the layer-by-layer approach which offers the opportunity to produce parts with complex geometries that are generally not achievable with the traditional production processes for thermoplastics (i.e., extrusion or injection molding) [6; 7; 8]. Besides, through the utilization of AM processes, fully customizable items that are characterized by tailored specific properties and functionalities can be obtained, also reducing the typical high cost that is associated with traditional manufacturing methods [9]. Among the various AM processes for thermoplastics, material extrusion-based methods (MatEx) are the most commonly used technologies [10; 11]. These technologies include fused filament fabrication (FFF), also known as fused deposition modelling (FDM--3D printing), exploiting filaments as feedstocks; and fused granular fabrication (FGFF), employing thermoplastic pellets, 3D fiber deposition, and 3D plotting [12]. Despite the high versatility of MatEx, the portfolio of thermoplastics that are suitable for this AM method is still limited [12]; typical materials for FFF printing are PLA and ABS, which are generally sold in controlled diameter filaments [13]. As widely documented in the literature, there are several requirements that are associated with the mechanical, rheological, and thermal characteristics of the candidate thermoplastic, which need to be fulfilled in order to classify a polymer as \"MatEx -printable\" [14].