Obtaining and Characterizing 3D Printable Polymer Based Composites with BaTiO3 Filler

By the extrusion and injection technique, polymer-based composite samples (polypropylene and thermoplastic polyurethane) were obtained with BaTiO3 filler in concentrations between 0 and 35 %. Following the preliminary characterizations performed by coupled thermal analysis techniques, it was found that the composite samples obtained have thermal stability up to a temperature of about 250 C and can be processed by extrusion at temperatures between 180 and 240C. Above this temperature, a series of degradation processes take place by successive thermooxidation with the formation of volatile products.The electrical characterization of the composite samples was performed by the dielectric spectroscopy technique, which showed that the Δtgδ dielectric losses increase as the BaTiO3 content of the composite increases. The maximum increase being recorded at 50 Hz (the slope Δtgδ /% BaTiO3 about 0.003 /% BaTiO3). It was also found, that the electrical conductivity (σ) of the composite increases with the increase of content in BaTiO3 so that σ at 100 kHz for the composite with 35 % BaTiO3 is about 50 times higher than for the reference sample M 0 (without filler).


Introduction
The In the perspective of sustainable development, the rational use of natural resources, respectively, the development of environmentally friendly advanced materials and technologies is a priority issue [1,2]. In this context, the development of the composite materials with specific characteristics for the most diverse applications is in the continuous attention of the scientific community and industrial developers [3]. Thus, developments of ceramic composites [4] for various medical applications [5,6], electrodes for electrochemical synthesis [7], thin layers for photocatalysis [8], polymer-based composites with various fillers [9 -16] are reported with uses including the electrical engineering field [17][18][19][20], etc.
The composite materials development is intended to meet the specific requirements of the target application such as biocompatibility [5,6], flammability [21], electrical, dielectric, and/or magnetic performance [17][18][19][20], ablation [9], mechanical characteristics [14,20], resistance to the action of microorganisms/biodegradability [11,22,23], etc. The development of 3D printers has opened new research directions in the field of advanced materials, respectively the performed of 3D printable materials. In this context, the purpose of the work is the realization and preliminary characterization of composites based on the thermoplastic polymer with filler BaTiO3 for 3D printing applications.

Materials and methods
By the technique of extrusion and injection, composite samples based on thermoplastic polyurethane and polypropylene with BaTiO3 filler were obtained. After weighing, the components (polyurethane and polypropylene pellets with BaTiO3 powder) were mixed (for 30 min at speed 40 rpm) in a TURBULA T2F type cylindrical mixer.
The main characteristics of the polypropylene used (TIPPLEN H 318) [24] are shown in Table 1. The main characteristics of the polyurethane used (Estane 58887 TPU supplied by Lubrizol Advanced Materials, Inc. [25]) are shown in Table 2. The BaTiO3 powder (<2 μm, 99.5 % trace metals basis) used was supplied by MERCK. The composite was obtained by extruding and granulating the polyurethane/polypropylene/BaTiO3 mixture on a KETSE Brabender laboratory extruder.
The size of the composite granules obtained was about 3 mm. The working parameters on the extruder were: -rotation speed (counter-rotation) of the extruder screws 95 rpm; -feed speed from the feed hopper 450 rpm. The temperatures on the heating areas of the Brabender KETSE extruder are in Table 3. For the dielectric characterization of the composite, the granules obtained by extrusion were injected (with an injection machine type Dr. Boy A35 -Germany) obtaining samples (specimens) in the form of a disc with a diameter of 30 mm and a thickness of 2.5 mm.
The working parameters on the injection machine (Boy A35 -Germany) were: -closing force of the mold in the range: 302-317kN; -injection pressure 550 bar; -back pressure: 90 bar. The temperatures of the heating areas of the injection molding machine were shown in Table 4. Thus, samples with different compositions were performed. The sample coding obtained according to the composition is presented in Table 5. The thermal characterization of both the polymeric raw materials (polyurethane and polypropylene) and of the composite obtained was performed by thermal analysis techniques on a specialized equipment type STA 449 F3 Jupiter (NETZSCH, Germany). The thermal diagrams were drawn in 50 mL/min synthetic air at a heating rate of 10 o C/min. Dielectric behavior -determination of dielectric loss and conductivity vs. frequency -of the composite samples obtained was investigated at a temperature of 20 ± 2°C by dielectric spectroscopy technique with 1296 Dielectric interface / AMTEK -Solartron Analytical.

Results and discussions
The thermal diagrams TG + DSC + DTG recorded on the polyurethane (TPU) used to make the composite are shown in Figure 1. From the comparative analysis of the TG, DSC, and DTG diagrams, it is found that up to the temperature of about 280 o C the material is stable after which several processes of thermooxidation take place (the first with the formation of solid peroxide products with a peak at 334.1 o C) and of decomposition with the formation of volatile products, a process that ends that end at a temperature of about 600 o C (when the mass loss of the sample is 100 %).
The thermal diagrams TG + DSC + DTG recorded on polypropylene (PP) used to make the composite are shown in Figure 2.  Figure 2 it is found that the PP used for obtaining the composite is a product with an ordered structure with a melting temperature of 168.7 o C and stable (without processes of thermooxidation and/or decomposition) up to about 425 o C.
At the temperature of 425 o C, a decomposition process begins with the formation of volatile products, a process which ends at 460 o C when the mass loss of the sample is 100%.
The BaTiO3 component is a particularly stable product at temperatures below 800 o C, as shown in Figure 1 and Figure 2, which indicates that the maximum temperature during processing to obtain the composite is limited by the TPU component and is up to 270 o C.
The thermal TG + DSC + DTG diagrams obtained, recorded on the M 5 sample (28 % BaTiO3, 4.3 % PP and 67.7 % TPU) are presented in Figure 3. From the analysis of Figure 3 it is found that at the progressive heating of the M 5 composite obtained, at about 165 o C a slightly pronounced melting endotherm process occurs, after which at 228.8 o C a glass transition process begins, which indicates a biphasic structure respectively, a component with ordered structure (with crystallinity, with definite melting point) and a component with disordered structure (glass transition). The composite is stable up to about 250 o C when the decomposition begins by the formation of volatile products (in a first stage, up to temperature of 360 o C, the mass loss is 13.86 %) and successive exothermic thermooxidation processes (first peak at 301.7 o C). Decomposition by formation of volatile products is completed at about 550 o C when the total mass loss is 72.24 %, the residue of 27.76 % being the weight in the BaTiO3 composite. Based on this experimental data, it is considered that the maximum temperature at which the composite obtained can be processed and / or used is about 240 o C. By comparing the Figure 3 with Figure 1 and Figure 2 it is observed that in the composite are found both the component with the ordered structure of PP (with defined doping point) and the component with the disordered structure of TPU with modified thermal parameters (glass transition at 228.8 o C instead of 241.8 o C, first peak of thermooxidation occurs at 301.7 o C instead of 334.1 o C etc.).
These changes of the thermal parameters of the phase with disordered structure TPU from Figure 3 (compared to Figure 1) suggest that the initial structure of TPU changed following contact with molten PP during applied heat treatments (extrusion and injection). The thermal diagrams recorded on the other composite samples (M 1, M 2, M 3, M 4 and M 6) were practically identical to those recorded on M 5, the only differences being recorded at the total mass loss that changes according to the BaTiO3 content of the respective samples. This finding indicates that the polymeric component formed during the composite processing (mixing, extrusion and injection) has a homogeneous structure. By processing the experimental data obtained by dielectric spectroscopy (real and imaginary components of complex permittivity vs. frequency), the dielectric loss tgδ of the investigated material samples was determined.
The values obtained in different frequency ranges are shown in Figure 4.
From the analysis of Figure 4 it is found that the dielectric loss value is determined by the content in BaTiO3 of the composite, respectively increases with increasing the content in BaTiO3. The evolution of dielectric loss depending on the BaTiO3 content of the composite for the several frequencies is shown in Figure 5.  Figure 5 it is observed that the dielectric evolution of the composite based on PP and TPU polymers with BaTiO3 filler increases approximately linearly as the BaTiO3 content increases, the most pronounced increases being registered at the frequency of 50 Hz (the slope Δtgδ /% BaTiO3 about 0.003 / % BaTiO3). Evolution of electrical conductivity σ depending on the frequency of composite samples in the investigated frequency ranges are shown in Figure 6.
From the analysis of Figure 6 it can be noted that as the BaTiO3 content increases, the electrical conductivity of the composite samples performed increases (for example: up to 50 times at 100 kHz for a 35 % BaTiO3 content).
From these preliminary characterizations, it is found that the composite material based on polymers (polypropylene and thermoplastic polyurethane) with 35% BaTiO3 filler has adequate electrical and

Conclusions
By the technique of extrusion and injection, samples of polymer-based composite material (polypropylene and thermoplastic polyurethane) with BaTiO3 filler in various concentrations were obtained.
Following the preliminary characterizations performed by coupled techniques of thermal analysis and dielectric spectroscopy of the composite samples obtained, the following were found: -can be processed by extrusion at temperatures between 180 and 240 o C; -has a thermal stability up to a temperature of about 250 o C above which a series of degradation processes begin by successive thermooxidation processes with the formation of volatile products; -at 600 o C the decomposition of the polymeric support is complete, the calcination residues consisting only in the BaTiO3 filler from the composite composition; -Δtgδ dielectric losses increase with the increasing of BaTiO3 content of the composite, being recorded a maximum increase at 50 Hz (the slope Δtgδ /% BaTiO3 about 0.003 /% BaTiO3); -the electrical conductivity (σ) of the composite increases as the BaTiO3 content increases, so that the σ at 100 kHz of the composite with 35% BaTiO3 is about 50 times higher than the reference sample M0 (without filler).
Taking in consideration these results, it is found that the composite material based on polymers (polypropylene and thermoplastic polyurethane) with 35% BaTiO3 filler obtained has adequate electrical and thermal characteristics to obtain the wires for 3D printers.