Preliminary tests of electric circuit

Figure 32 frontal view of the electric circuit

The mold had the dual purpose of evaluating the conductive properties of the material after injection and also validation of the chosen LED’s – in particular, whether or not they could support the temperature and pressure parameters of the injection process.

Figure 34 shows the mold cavities used to inject the different materials.

Figure 34mold cavities

An electrical source - TTI EX752M Multi-mode PSU 75V/150V 300W – was used to test and validate all the injected electrical circuits. The tests were set up with a voltage of 12.5 V. Figure 35 shows the equipment and the voltage used.

Figure 35 electric tension used to test the conductive material

Initially the material used for the designed circuit was tested - EMI 362 Polycarbonate (PC) Stainless Steel Fiber Electrically Conductive EMI/RFI/ESD Protection. This material is supplied in two separate packages containing the metallic fibers and the polycarbonate. Since the final properties of the molding are depended on the distribution of the fibers, the two materials were mixed using an extrusion machine Coperion co-rotating twin screw extruder. The extruded material was injected in a Microinjection molding machine, Boy 12A. The injection molding processing parameters used are presented in the following table 6.

Table 6 - processing conditions used on the injection molding tests of the conductive polymer Temperature (ºC) 2ª Pressure

Velocity of injection nozzle Mold Time Pressure

310 80 1s 25 Bar 120 mm/s

The first specimen analyzed was the specimen presented in figure 41. The specimen presented a high electric resistance and none of the LED’s worked. To overcome the high resistance existent on the surface of the circuit it was decided to connect the wires of the electric source directly to the pins of the LEDs. All sections of the circuit were then analyzed; as can be seen, the LEDs that are in middle section and in the 2 LED section didn’t emit light. The only section where it was possible to see light was on the bottom row of the circuit (Figure 36)..

Figure 36 light emission by the LED on the base section

The results obtained in this initial test were not as expected. Rather than seeing all the LEDs emitting light, as expected, only the LEDs that are in the section with the wider width at the bottom are working.

To achieve the objective of having all the LEDs emitting light it was decided to analyze the properties of other material capable of conducting electricity. The materials considered are produced by Nanocyl S. A. and are supplied in Masterbatches. The materials selected were POM (Polyoxymethylene), PC and PP mixed with carbon nanotubes. Accordingly, the second material analyzed was the POM with 4% CNT. The injection conditions used are presented in Table 7.

Table 7 injection molding conditions for POM Temperature (ºC) 2ª Pressure

Injection velocity Nzl Mold Tempo Pressure

200 80 1s 25 Bar 50 mm/s

The specimens produced with this new material presented better results. In this case it was possible to see at least two LEDs emitting light in each section; the light intensity was also higher compared to the first material analyzed.

However this material presented an unexpected behavior; for voltages higher than 12V the material started to melt. The results for different specimens are presented in figure 37.

Figure 37 POM mixed with 4% of CNT

Because of the melting of the material it was decided to study other matrices - PC and PP. In this new research it was also decided to study several percentages of carbon nanotubes for each matrix. In the case of the PC, two formulations were

selected, one with 4% CNT and the other with 5%. In the case of the PP three formulations were selected for examination, with 5% CNT, 7% and 9%. The processing conditions for each material are presented in the following table 8.

Table 8 injection molding conditions for the different materials tested Material Temperature (ºC) 2ª Pressure

Velocity of injection Nzl Mold Time Pressure

PC4% 305 80 1s 40 Bar 50 mm/s

PC5% 290 80 2s 30 Bar 50 mm/s

PP5% 240 60 2 s 50 Bar 25 mm/s

PP7% 240 60 2 s 50 Bar 25 mm/s

PP9% 240 60 2 s 50 Bar 25 mm/s

The first material to be analyzed was PC mixed with 4% of CNT. The use of PC had the objective of decreasing the risk of losing electric properties and avoiding deformation when in contact with the material (PC) of the housing. Based on the good results obtained with the POM the same percentage of CNT, 4%, was used for the initial tests. However the PC mixed with 4% of CNT did not present the expected results. The injected circuit showed lower luminous intensity when compared to the POM at the same voltage although the number of LEDs seen to be emitting visible light was higher.

This material also presented a higher electric resistance on the skin. Comparing the two - PC and POM – it was verified that PC does not melt with comparable increases in voltage. The results obtained are presented in the figure 38.

Figure 38 Circuit test with PC with 4% of CNT

voltage. For that reason it was decided to increase the percentage of CNT by 1%. With this increase it was deemed possible to decrease the resistance of the electric circuit and this proved to be the case. The specimens with 5% of CNT presented better results compared to the specimens with 4% of CNT. However, despite the good results obtained, it was not possible to see all LEDs working at the design voltage; it was necessary to increase the voltage higher than 13 V to see all LED’s working. The first results obtained for this material are presented in the figure 39.

Figure 39 Circuit Test with PC with 5% of CNT

To overcome the high surface resistivity and taking into consideration that the majority of the CNTs are within the core it was decided to introduce metallic wiring in the connections. These wires have the primary objective of overcoming the high resistivity presented in the skin of the material used for the circuit. After this approach was adopted the results expected were obtained; it proved possible to turn on all the LED’s at the designed voltage. The result obtained is presented in the figure 40.

Figure 40 Final result of the PC with 5% of CNT

After the conclusion of this test it was necessary to understand if the electric circuit had the capability of working 24/7 with the same voltage and without losing luminous intensity. Accordingly, the electric circuit was tested continuously 24 hours daily for

one week at ambient environment temperature at a voltage of 12.6V. Throughout the test the circuit kept the same dimension without presenting any deformation and at the same level of luminous light intensity.

The last material to be tested was the PP. This material was selected due to its lower price and processing temperature in comparison with PC. The material chosen for study was PLASTICYL™ PP 2001 with 20% of CNT. According to the supplier, this material needs a higher percentage of CNT to obtain the same conductivity as PC. For the purposes of this study, three percentages of CNT, 5%, 7% and 9% were analysed.

The PP mixed with 5% of CNT was tested. When the design voltage of 12.6V was applied, none of the LEDs turned on. When the voltage was increase to 29.4 Volts the LED’s turned on as presented in the following figure 41.

Figure 41 PP mixed with 5% (w/w) of CNT

The PP mixed with 7% of CNT was tested next. With this material some of the LEDs emitted light when the design voltage of 12.4V was supplied as can be seen in the following figure 42.

Figure 42 PP mixed with 7% (w/w) of CNT

The PP mixed with 9% of CNT was the material that presented the best results. In this case, it was possible to see all the LEDs emitting light at 12.5 V with a good intensity.

The electric circuit test showed all the LEDs emitting light as can be seen in figure 43.

Figure 43 PP mixed with 9% (w/w) of CNT