Ceramic Composite Printing

The 3 mm filament of the HAP/PCL composite was placed into the extruder of the BFB 3000 3D Extruder. The head temperature was set to 200°C and printing was commenced without any modification to the Extruder. Upon completion of printing, the printed objects were allowed to cool to room temperature and removed from the build platform with a scalpel. To investigate the feasibility of the machine, a dumb- bell shape structure was printed. The dumb-bell was characterised with thermogravimetrical analysis (TGA) under a flow of air with heating rate of 10°C/min using a Pyris Diamond, Perkin Elmer (UK) in order to find the decomposition temperature of the composite. The sintering process was performed in a furnace, Lanton Thermal Designs Limited (LTD, UK).

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7.3 Results and Discussion

A dumb-bell shape composite with 100wt% HAP was successfully fabricated with the BFB 3000 3D Extruder as shown in Figure 7.5. This shows the feasibility of the machine to fabricate a 3D structure of polymer ceramic. Figure 7.6 represent the TGA for the composite.

(a) (b)

Figure 7.5 (a) Green body of 100wt%HAP/PCL in dumb-bell shape is an exact replica of the dumbbell-shape’s drawing .

Figure 7.6 Thermogravimetrical analysis (TGA) of 100wt%HAP/PCL composite

The onset decomposition temperature for the composite was found at 350°C with 50% of residue. The decomposition temperature for HAP/PCL composite was

0 20 40 60 80 100 0 200 400 600 800 1000 W ei gh t ( % ) Temperature (°C)

found to be at the same range of calcium pyrophosphate (CPP-B/HDeDA) composite at 370°C as discovered in Chapter 6. Therefore, the same heating profile used for CPP-B composite was used for the new composite fabricated via the 3D Extruder as shown below:

Room temperature 300°C 500°C 1250°C

10°C/min (1 hr) 2°C/min (1hr) 2°C/min (10hours)

After sintering at 1250°C for 10 hours, the structure of dumb-bell test pieces were disintegrated and brittle. Figure 7.7 clearly shows the failure structure of the sample.

(a) (b)

Figure 7.7 The disintegrated dumb-bell structure after sintered for 10 hours at 1250°C; (a) top view and (b) side view.

The unsatisfactory result on the sintered structure is presumably due to the air trapped between layered fabrications. Based on the machine specification in Table 7.1, the maximum print speed of extruded volume is 15mm3_{/sec and it depends on the} type of polymer feeder. Presumably, for polymer composite, the print speed could have been reduced due to higher shear stress in the nozzle cause by the ceramic filler.

173 The slower printing speed has allowed more air to trap between layers and consequently affect the ability of the composite to hold the structure after sintering.

A bulk composite filament was prepared to test the hypothesis. After preparing the 3 mm composite filament, it was diced with cutter about 5mm to produce a bulk cylinder shape sample as shown in Figure 7.8.

1

Figure 7.8 The cylinder structure of composite filament.

The diced filaments were then sintered to a ‘gentler’ heating profile in order to see the ability of the sample to hold its structure. The new heating profile was as follows:

Room temperature 240°C 550°C 1250°C

1°C/min 0.2°C/min 5°C/min (10hours)

The samples were unable to hold the integrity of the shape even under gentler heating rate as shown in Figure 7.9. At slower heating rate, the degradation of polymer phase also occurred at slower rate and eventually collapsed and lost the shape. At the same time; according to grain-growth model, HAP grains have more time to growth in the collapsing structure of the composite and as a result; an odd shape of sintered composite was produced.

Due time constraint, the author could not continue the investigation to further characterise the ceramic composite. It is interesting to characterise the scanning electron micrograph (SEM) of grain growth and the structure of the ceramic with XRD analysis in order to study the affect of using ‘gentler’ heating rate. Heating profile for the sintering process could be improved by performing a faster heating rate (1°C/min) at the range of polymer decomposition temperature (300-500°C) in order to give the ideal time for the grain growth of ceramic to hold the structure whilst polymer phase slowly degrade. The mechanism of grain growth and the mechanism of degradation can be further investigated.

As an alternative, process ability of the polymer ceramic during extrusion could be improve by optimising the formulation of polymer ceramics. This is to reduce the shear stress during extrusion, which gives significant effect on the green body of the composite during sintering. One way to overcome the stress is to produce nano-size ceramic particle and induce chemical treatment on the ceramic particle to homogenise the size of the ceramic particles and optimise the viscous-elasticity of the polymer ceramic. Figure 7.10 shows the capability of BFB 3000 3D Extruder to fabricate custom-made tissue scaffold from 100wt%HAP/PCL. This shows the feasibility of the machine to fabricate new biopolymer materials and also offering a

175 better alternative approach in designing tissue scaffold for specific tissue engineering applications.

Figure 7.10 A layer by layer structure of polymer ceramic (100wt% HAP/PCL) fabricated via BFB 3000 3D Extruder.

7.4 Conclusion

Unlike microstereolithography technique, BFB 3000 3D is low in cost and there are varieties of polymer feeders available from the manufacturer; which are suitable for wide field of application. In addition, the simpler operation and more robust machine has had attracted the author to develop novel materials specifically for tissue engineering application. A bulk composite filament was prepared from 100 and 200wt% HAP and 3D structure of polymer ceramic composite was successfully fabricated with BFB 3000 3D extruder. However, obtaining a ceramic composite after sintering the green body was not possible even though a much gentler heating rate was performed. Nevertheless, the author believe with further optimisation of the polymer ceramic formulation; coupled with ideal heating rate, a custom-made 3D structure of tissue scaffold is achievable in the near future.