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Issue 04/2019

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Biocomposites Biobased

Biocomposites Biobased composites for 3D printing Adjustable compound properties AAdditive manufacturing techniques are of growing interest for various industry sectors. Not only does this technology enable the rapid construction of three-dimensional models, it is also possible to 3D print, for example, replacement parts and prototypes that are able to fulfil structural functions. Using the Fused Deposition Modelling (FDM) technique, a specimen is built up, layer by layer, out of a melted material – generally a thermoplastic polymer like PLA or ABS. The mechanical properties of the 3Dprinted specimen depend on both the material properties and the printing settings. Within the framework of the EU-funded project “Bioeconomy in the non-food sector”, new compounds were developed using different PLA-based modified polymers and different biobased fibres and fillers. As a starting point for the optimisation process, a PLA commonly used in additive manufacturing (PLA 4043D) and wood fibres with an average particle size of 40 – 70 µm were processed at the 3N Competence Center. The resulting compounds were processed into injection moulded tensile rods for mechanical testing and into thermoplastic thermoplastic wire (colloquially referred to as filament), to test their suitability for the FDM-process, also as regards their “oozing and warping” behaviour. Warping refers to the deformation of the printed object; oozing describes the leakage of polymer during the travel/pausing phases of the extruder. The obtained results were used to further optimize the wood fibre-reinforced compounds in terms of mechanical properties and processability (see Fig. 1). Various (biobased) polymers and fibres were used, including hemp, regenerated cellulose, grass and wood fibres as well as horticultural and agricultural residual materials (e.g. fibres extracted from pepper or tomato plant stems). The optimization process used to adjust the compound properties is shown for two PLA compounds with different wood fibre fractions by mass (see Fig 2). Fibre content and polymer composition were varied as relevant parameters. The composition of the polymer was determined by altering the mixing ratios between the PLA and a biobased impact modifier. Fig. 2 displays the mechanical tensile properties obtained from the different compounds after injection moulding. The results of the mechanical characterisation can be summed up as follows: Figure 1: Schematic view of the optimization process for adjusted compound properties Compounding with spacial formulation Feedback / adaptation of the formulation production of thermoplastic wires Check-up of the properties 3D-printing of selected parts 30 bioplastics MAGAZINE [04/19] Vol. 14

Biocomposites By: Niels Kühn, Katharina Haag, Milan Kelch, Jörg Müssig* Hochschule Bremen, Bremen, Germany Cord Grashorn IST-Ficotex, Bremen, Germany Corinne van Noordenne NHL Stenden, Leeuwarden, Netherlands Marie-Luise Rottmann-Meyer, Hansjörg Wieland 3N Niedersachsen, Werlte, Germany *: corresponding author • No significant changes in tensile strength (35-37 MPa) were seen between the different compositions, comparable to the references in the literature for ABS or PP. • Higher stiffness and lower impact properties were found with increased fibre content. The composition can be adapted to the needs of the 3D-printing process by varying the polymer and the fibre fraction. For comparison, the pure PLA-based modified polymer compounds were also tested. These specimens reached a high Charpy impact strength of > 80 kJ/m², but showed a higher warping problem, when 3D-printed with the FDM process. The same compounds were used for the production of thermoplastic wires, to determine the printability of the formulas and the differences in the mechanical properties, compared to the injection moulded parts. The FDM compounds showed potential for improvement. Both warping and oozing was worse, compared to regular PLA, as a result of mixing issues with the selected impact modifier, which was required to be added to achieve flexibility in the filled extruded wires. Compared to the injection moulded specimens, poorer mechanical properties were seen in the printed materials, with strength and stiffness decreasing by about 30% and impact strength being reduced by some 60 %. The decrease in mechanical properties was caused by, among other things, imperfections such as air inclusions, formed during the additive FDM-process. Voids in form of air inclusions can be estimated by the difference in density of the parts. A density of between 1.25 and 1.30 g/cm³ was found for injection moulded objects, while the density of the 3D-printed equivalents was between 1.07 and 1.12 g/cm³. Further optimisation of the compound, especially by adjusting the amount of impact modifier, improved the warping and oozing behaviour during the printing process. Oozing, in particular, is a problem in wood fibre reinforced compounds, and it is one that also frequently occurs in commercially available products, as the natural fibres tend to absorb humidity from the air. u Tensile strength in MPa 40 35 30 25 20 15 10 5 constant tensile strength Tensile strength in MPa 3000 2500 2000 1500 1000 500 Figure 2: Mechanical properties of new biobased compounds. Tests were performed according to DIN EN ISO 527-2 (Tensile Testing) and DIN EN ISO 179-1 (Charpy unnotched impact) from injection moulded compounds. Polymer compound 1: 48% PLA + 52% biobased impact modifier, polymer compound 2: 34% PLA + 66% biobased impact modifier. Young’s modulus and toughness individually adjustable 0 0 0 10 20 30 40 50 60 Charpy impact strength a Cu in kJ/m 2 0 10 20 30 40 50 60 Charpy impact strength a Cu in kJ/m 2 fibre content 10 % | polymer composition 1 fibre content 10 % | polymer composition 2 fibre content 20 % | polymer composition 1 fibre content 20 % | polymer composition 2 bioplastics MAGAZINE [04/19] Vol. 14 31

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