J. Gonzalez-Gutierrez, M. Spoerk, C. Holze
Keywords: additive manufacturing, polypropylene, composites
Summary:Material extrusion additive manufacturing, also known as fused deposition modeling (FDM) or fused filament fabrication (FFF), is one of the simplest and most economical additive technologies; therefore it is very popular. FFF consists on selectively depositing thermoplastic strands in a layer-by-layer manner based on digital models. The feedstocks used in FFF are filaments that must have very little variation in their diameter and round shape in order to have a reliable extrusion process . For already a few years, there have been numerous thermoplastic materials commercially available for FFF, but polypropylene (PP) was only recently added to the list, due to some difficulties encountered during FFF. PP is nontoxic, and it can be applicable as a biologically inert material and, its excellent chemical resistance against various reactants makes PP an outstanding material for the FFF market. Particularly, the outstanding chemical resistance of PP to polar solvents, nonoxidizing acids, aqueous alkalis, and aqueous salt solutions creates novel possibilities for FFF, as other commercially available filament types are considerably less resistant to chemicals, which has limited the applicability of parts produced by FFF . Polypropylene is a semi-crystalline thermoplastic, which means that when it is cooled beyond its crystallization temperature it undergoes a drastic volume contraction compared to amorphous polymers; this contraction can lead to detachment from the build platform and warpage of the printed specimen. In order to prevent detachment from the build platform, the correct material and processing conditions need to be used [2, 3]. Materials such as polypropylene or ultrahigh molecular weight polyethylene plates have been suggested as good candidates to replace glass surfaces as build platforms [2, 4]. Some commercial products consisting of an undisclosed blend composition are also now available. In the other hand, to prevent warpage, several approaches can be used. The first one is to select PP with a low crystallinity and a low crystallization rate, but lowering the crystallinity can lead to a decrease in the desirable properties of PP . The second approach is to add amorphous thermoplastics to PP such as amorphous polyolefins, polycarbonate or polyvinyl chloride, but this can also lead to detrimental properties. Finally, a third approach is to prepare PP compounds filled with spherical particles or short fibers, which not only can prevent warpage (Figure 2), but they can also lead to an increase in the mechanical performance . However, to get improved mechanical properties a correct compatibilization between the filler and the PP matrix is needed. The physical characteristics of the fillers also have an effect on the quality of the printed specimens. For example, it has been observed that smaller glass spheres are more effective than the larger ones to prevent the warpage of FFF produced specimens, but the mechanical properties were not really improved with such glass fillers . On the other hand when short carbon fibers with the correct sizing were added, the warpage was improved as well as the mechanical properties of the printed parts .