Author Contributions
Conceptualization, R.E.P. and W.P.; methodology, R.E.P., B.S. and W.W.; validation, M.D.; formal analysis, M.K. and B.S.; investigation, W.P. and M.D.; data curation, M.D.; writing—original draft preparation, R.E.P., M.K., W.P., M.D. and B.S.; writing—review and editing, R.E.P., M.K., W.P., M.D. and B.S.; visualization, M.K. and M.D.; project administration, R.E.P. and W.P.; funding acquisition, W.W. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Dimensions of samples used for testing: (a) bending; (b) stretching; (c) tribological. All dimensions are in millimeters (mm).
Figure 1.
Dimensions of samples used for testing: (a) bending; (b) stretching; (c) tribological. All dimensions are in millimeters (mm).
Figure 2.
Tribological measurements: (a) scheme of tribometer, Fn—normal load, Ft—friction force, n—rotational speed, 1—specimen, 2—disc; (b) Pin on disc apparatus.
Figure 2.
Tribological measurements: (a) scheme of tribometer, Fn—normal load, Ft—friction force, n—rotational speed, 1—specimen, 2—disc; (b) Pin on disc apparatus.
Figure 3.
FT-IR spectra for samples: (a) non-aged; (b) aged for 250 h; (c) aged for 500 h.
Figure 3.
FT-IR spectra for samples: (a) non-aged; (b) aged for 250 h; (c) aged for 500 h.
Figure 4.
Young’s modulus of stretched samples (after conditioning in a climatic oven).
Figure 4.
Young’s modulus of stretched samples (after conditioning in a climatic oven).
Figure 5.
Stress at stretched samples breaking (after conditioning in a climatic oven).
Figure 5.
Stress at stretched samples breaking (after conditioning in a climatic oven).
Figure 6.
The automatic module of bending samples (after conditioning in the climate oven).
Figure 6.
The automatic module of bending samples (after conditioning in the climate oven).
Figure 7.
Maximum stress when bending samples (after conditioning in a climatic oven).
Figure 7.
Maximum stress when bending samples (after conditioning in a climatic oven).
Figure 8.
Impact test results (after conditioning in a climatic oven).
Figure 8.
Impact test results (after conditioning in a climatic oven).
Figure 9.
MVR flow factor measurement results.
Figure 9.
MVR flow factor measurement results.
Figure 10.
Viscosity measurement results.
Figure 10.
Viscosity measurement results.
Figure 11.
Microhardness measurements’ results before friction test.
Figure 11.
Microhardness measurements’ results before friction test.
Figure 12.
Microhardness measurements’ results after the friction test.
Figure 12.
Microhardness measurements’ results after the friction test.
Figure 13.
Results of PLA wear tests cooperating with C45 steel (v = 0.34 m/s, and T0 = 23 °C, p = 0.11 MPa, Ra = 0.35–0.45 µm).
Figure 13.
Results of PLA wear tests cooperating with C45 steel (v = 0.34 m/s, and T0 = 23 °C, p = 0.11 MPa, Ra = 0.35–0.45 µm).
Figure 14.
Results of friction tests in PLA pair-C45 steel (v = 0.34 m/s, T0 = 23 °C, p = 0.11 MPa, Ra = 0.35–0.45 µm).
Figure 14.
Results of friction tests in PLA pair-C45 steel (v = 0.34 m/s, T0 = 23 °C, p = 0.11 MPa, Ra = 0.35–0.45 µm).
Figure 15.
Surface of polymer samples after cooperation with steel: (a) PLA unmodified, (b) PLA + 1% graphite, (c) PLA + 2.5% graphite, (d) PLA + 5% graphite, (e) PLA + 7.5 % graphite, (f) PLA + 10% graphite.
Figure 15.
Surface of polymer samples after cooperation with steel: (a) PLA unmodified, (b) PLA + 1% graphite, (c) PLA + 2.5% graphite, (d) PLA + 5% graphite, (e) PLA + 7.5 % graphite, (f) PLA + 10% graphite.
Table 1.
Process parameters that were requested when printing samples.
Table 1.
Process parameters that were requested when printing samples.
| Shape of a Sample and a Bar | Pin |
---|
Nozzle diameter | 0.4 mm | 0.3 mm |
Print temperature | 200 °C | 200 °C |
Bed temperature | 60 °C | 60 °C |
Layer height | 0.18 mm | 0.10 mm |
Number of full top/bottom layers | 3/3 | 16/16 |
Number of contours | 2 | 3 |
Top and bottom layers style | Rectilinear (parallel to the long edge) | Concentric |
Infill style | Honeycomb | Rectilinear |
Infill percentage | 30% | 16% |
Cooling | 100% | 100% |
Print speed | 40 mm/s | 30 mm/s |
Table 2.
Criteria for assessing the change in color based on the change of the ΔE parameter.
Table 2.
Criteria for assessing the change in color based on the change of the ΔE parameter.
Intervals of Variation | A Recognizable Color Difference |
---|
0 < ΔE < 1 | Invisible color deviations |
1 < ΔE < 2 | Minimal color differences, recognizable only by an experienced observer |
2 < ΔE < 3.5 | Average color deviations, recognizable by an inexperienced observer |
3.5 < ΔE < 5 | Visible color deviations |
ΔE > 5 | Big differences in color |
Table 3.
Parameters characterizing the conducted friction tests.
Table 3.
Parameters characterizing the conducted friction tests.
Parameter | Value |
---|
Pin force on the disc | 5.49 N |
Pin pressure on the disc–p | 0.11 MPa |
Disc material | Steel C45 |
Disc roughness | Ra = 0.35–0.45 µm |
Duration of test | 2 h 27 min |
Linear speed in association–v Ambient temperature-T0 | 0.34 m/s 23 °C |
Friction path | 3 km |
Type of friction | Technically dry |
Table 4.
Contact angle measurements.
Table 4.
Contact angle measurements.
| Keeping Samples in the Ageing Oven | Samples Conditioning in a Climatic Oven |
---|
| 0 h | 250 h | 500 h |
---|
PLA unmodified | 55.3° | 56.8° | 64.9° | 77.8° |
PLA + 1% graphite | 62.4° | 62.1° | 63.8° | 63.3° |
PLA + 2.5% graphite | 63.4° | 62.0° | 62.5° | 69.8° |
PLA + 5% graphite | 57.0° | 56.6° | 44.3° | 78.8° |
PLA + 7.5% graphite | 72.1° | 66.5° | 64.9° | 55.1° |
PLA + 10% graphite | 79.8° | 65.3° | 0°-hydrophilic | 77.7° |
Table 5.
Results of colorimetric measurements (ΔE markings: X—invisible color deviations, L—very low color differences, M—medium color deviations).
Table 5.
Results of colorimetric measurements (ΔE markings: X—invisible color deviations, L—very low color differences, M—medium color deviations).
| 0 h | 250 h | ΔE | 500 h | ΔE |
---|
PLA unmodified | L | 52.26 | L | 50.91 | 1.97 (L) | L | 53.72 | 3.64 (M) |
a | −0.85 | a | −0.47 | a | −0.95 |
b | 1.14 | b | −0.25 | b | −2.19 |
PLA + 1% graphite | L | 27.02 | L | 26.66 | 0.58 (X) | L | 26.29 | 0.88 (X) |
a | −0.85 | a | −0.75 | a | −0.61 |
b | −1.61 | b | −1.17 | b | −1.17 |
PLA + 2.5% graphite | L | 28.80 | L | 27.40 | 1.67 (L) | L | 27.79 | 1.08 (L) |
a | −0.73 | a | −0.45 | a | −0.62 |
b | −1.73 | b | −0.86 | b | −1.37 |
PLA + 5% graphite | L | 29.14 | L | 28.59 | 0.82 (X) | L | 28.88 | 0.34 (X) |
a | −0.60 | a | −0.43 | a | −0.52 |
b | −1.60 | b | −1.01 | b | −1.39 |
PLA + 7.5% graphite | L | 28.09 | L | 28.16 | 0.71 (X) | L | 28.13 | 0.39 (X) |
a | −0.69 | a | −0.49 | a | −0.52 |
b | −1.63 | b | −0.95 | b | −1.28 |
PLA + 10% graphite | L | 28.22 | L | 27.06 | 1.33 (L) | L | 27.70 | 0.56 (X) |
a | −0.70 | a | −0.52 | a | −0.64 |
b | −1.52 | b | −0.90 | b | −1.33 |
Table 6.
Results of tensile tests.
Table 6.
Results of tensile tests.
After Keeping the Sample for 24 h at 40 °C | After Conditioning in a Climate Oven |
---|
| Young’s Modulus [MPa] | Fracture Stress [MPa] | Fracture Strain [%] | Young’s Modulus [MPa] | Fracture Stress [MPa] | Fracture Strain [%] |
---|
PLA unmodified | 1482.4 ± 43.45 | 25.5 ± 0.70 | 2.152 ± 0.063 | 1556.4 ± 31.68 | 27.1 ± 0.64 | 2.417 ± 0.097 |
PLA + 1% graphite | 1509.1 ± 22.50 | 24.4 ± 0.59 | 2.108 ± 0.030 | 1553.4 ± 29.01 | 25.9 ± 1.30 | 2.273 ± 0.1063 |
PLA + 2.5% graphite | 1706.9 ± 26.09 | 26.5 ± 0.64 | 2.108 ± 0.032 | 1650.1 ± 47.10 | 26.4 ± 1.14 | 2.308 ± 0.078 |
PLA + 5% graphite | 1834.0 ± 52.89 | 26.0 ± 0.86 | 1.993 ± 0.028 | 1791.8 ± 63.47 | 27.5 ± 2.10 | 2.228 ± 0.130 |
PLA + 7.5% graphite | 1644.5 ± 53.83 | 26.7 ± 0.43 | 2.208 ± 0.029 | 1804.8 ± 94.61 | 24.5 ± 0.70 | 2.184 ± 0.115 |
PLA + 10% graphite | 1886.1 ± 47.40 | 23.8 ± 0.72 | 1.882 ± 0.039 | 1875.6 ± 45.36 | 24.0 ± 0.64 | 1.952 ± 0.063 |
Table 7.
Results of bending tests.
Table 7.
Results of bending tests.
| After Keeping the Sample for 24 h at 40 °C | After Conditioning in a Climate Oven |
---|
| Automatic Module [MPa] | Maximum Bending Stress [MPa] | Automatic Module [MPa] | Maximum Bending Stress [MPa] |
---|
PLA unmodified | 2857.9 ± 40.41 | 68.6 ± 1.34 | 2696.0 ± 61.22 | 61.6 ± 2.30 |
PLA + 1% graphite | 2835.0 ± 61.21 | 65.0 ± 1.17 | 3089.1 ± 38.14 | 67.5 ± 1.87 |
PLA + 2.5% graphite | 2953.2 ± 47.13 | 63.9 ± 0.91 | 2835.8 ± 33.12 | 63.4 ± 1.77 |
PLA + 5% graphite | 3262.3 ± 38.12 | 64.2 ± 1.47 | 2911.9 ± 45.17 | 60.9 ± 1.12 |
PLA + 7.5% graphite | 3268.0 ± 21.33 | 60.6 ± 2.02 | 3358.4 ± 29.47 | 60.8 ± 0.97 |
PLA + 10% graphite | 3332.1 ± 22.14 | 56.0 ± 0.98 | 3493.3 ± 32.18 | 58.2 ± 1.76 |
Table 8.
Impact test results.
Table 8.
Impact test results.
| PLA Unmodified | PLA + 1% Graphite | PLA + 2.5% Graphite | PLA + 5% Graphite | PLA + 7.5% Graphite | PLA + 10% Graphite |
---|
KC [kJ/m2] samples after 24 h in 40 °C | 10.10 ± 1.11 | 12.03 ± 0.77 | 11.78 ± 1.23 | 11.45 ± 0.98 | 11.73 ± 1.94 | 9.74 ± 0.47 |
KC [kJ/m2] conditioning in a climatic oven | 19.93 ± 0.97 | 13.92 ± 1.01 | 13.89 ± 1.27 | 13.39 ± 0.87 | 13.09 ± 1.64 | 10.56 ± 0.78 |
Table 9.
Results of thermogravimetric analysis.
Table 9.
Results of thermogravimetric analysis.
| PLA Unmodified | PLA + 1% Graphite | PLA + 2.5% Graphite | PLA + 5% Graphite | PLA + 7.5% Graphite | PLA + 10% Graphite |
---|
1% of weight loss [°C] | 305.8 | 311.2 | 315.0 | 320.1 | 310.7 | 309.1 |
Onset [°C] | 348.3 | 354.6 | 354.7 | 353.5 | 352.6 | 352.2 |
Temperature at maximum weight change rate [°C] | 376.0 | 373.1 | 372.9 | 375.1 | 371.6 | 372.4 |
Table 10.
Results of differential scanning calorimetry (DSC) analysis.
Table 10.
Results of differential scanning calorimetry (DSC) analysis.
| PLA Unmodified | PLA + 1% Graphite | PLA + 2.5% Graphite | PLA + 5% Graphite | PLA + 7.5% Graphite | PLA + 10% Graphite |
---|
Glass transition temperature [°C] | 59.9 | 62.5 | 62.6 | 57.6 | 62.7 | 63.0 |
Crystallization temperature [°C] | 126.4 | 122.2 | 120.9 | 119.2 | 124.5 | 125.3 |
Softening temperature [°C] | 153.3 | 152.6 | 152.5 | 152.4 | 153.0 | 152.8 |
Table 11.
Results of flow coefficient measurements (tests carried out at 190 °C).
Table 11.
Results of flow coefficient measurements (tests carried out at 190 °C).
| PLA Unmodified | PLA + 1% Graphite | PLA + 2.5% Graphite | PLA + 5% Graphite | PLA + 7.5% Graphite | PLA + 10% Graphite |
---|
MVR [cm³/10 min] | 5.407 ± 0.97 | 3.741 ± 0.13 | 4.169 ± 0.33 | 4.224 ± 0.57 | 4.772 ± 0.19 | 3.807 ± 0.51 |
MFR [gram/10 min] | 6.705 ± 0.79 | 4.639 ± 0.47 | 5.169 ± 0.54 | 5.238 ± 0.34 | 5.855 ± 0.24 | 4.721 ± 0.65 |
Table 12.
Microhardness measurements’ results before the friction test.
Table 12.
Microhardness measurements’ results before the friction test.
| PLA Unmodified | PLA + 1% Graphite | PLA + 2.5% Graphite | PLA + 5% Graphite | PLA + 7.5% Graphite | PLA + 10% Graphite |
---|
HV 0.1 | 20.07 ± 0.21 | 20.65 ± 1.49 | 19.48 ± 0.48 | 17.76 ± 0.46 | 19.31 ± 0.47 | 18.49 ± 0.34 |
ΔHV 0.1 | - | 3% | −3% | −12% | −4% | −8% |
Table 13.
Microhardness measurements’ results after the friction test.
Table 13.
Microhardness measurements’ results after the friction test.
| PLA Unmodified | PLA + 1% Graphite | PLA + 2.5% Graphite | PLA + 5% Graphite | PLA + 7.5% Graphite | PLA + 10% Graphite |
---|
HV 0.1 | 20.06 ± 0.51 | 22.05 ± 1.13 | 20.75 ± 0.64 | 18.46 ± 0.98 | 19.17 ± 0.97 | 16.64 ± 0.69 |
ΔHV 0.1 | - | 9% | 3% | −8% | −4% | −17% |
Table 14.
Results of PLA wear tests cooperating with C45 steel (v = 0.34 m/s, T0 = 23 °C, p = 0.11 MPa, Ra = 0.35–0.45 µm).
Table 14.
Results of PLA wear tests cooperating with C45 steel (v = 0.34 m/s, T0 = 23 °C, p = 0.11 MPa, Ra = 0.35–0.45 µm).
| PLA Unmodified | PLA + 1% Graphite | PLA + 2.5% Graphite | PLA + 5% Graphite | PLA + 7.5% Graphite | PLA + 10% Graphite |
---|
Ih (µm/km) | 32 ± 4 | 24 ± 7 | 24 ± 8 | 21 ± 4 | 22 ± 5 | 11 ± 3 |
ΔIh | - | −26% | −25% | −33% | −31% | −65% |
Table 15.
Results of friction tests in PLA pair—C45 steel (v = 0.34 m/s, T0 = 23 °C, p = 0.11 MPa, Ra = 0.35–0.45 µm).
Table 15.
Results of friction tests in PLA pair—C45 steel (v = 0.34 m/s, T0 = 23 °C, p = 0.11 MPa, Ra = 0.35–0.45 µm).
| PLA Unmodified | PLA + 1% Graphite | PLA + 2.5% Graphite | PLA + 5% Graphite | PLA + 7.5% Graphite | PLA + 10% Graphite |
---|
µ | 0.448 ± 0.011 | 0.401 ± 0.036 | 0.414 ± 0.035 | 0.425 ± 0.017 | 0.420 ± 0.018 | 0.449 ± 0.019 |
Δµ | - | −11% | −8% | −5% | −6% | 0% |