Abstract
Poly (urea-urethane) (PUU) with polydimethylsiloxane (PDMS) soft segment was synthesized successfully in this study. Conductive filler was used to increase the conductivity of poly (urea-urethane). FT-IR was utilized to monitor the reaction of tolune diisocyanate (TDI) and PDMS. Different types of conductive fillers, namely, carbon black (CB), carbon nanotube (CNT), silver coated carbon nanotubes (CNT-Ag) and nickel coated carbon nanotubes (CNT-Ni) were blended with PUU to form partially conductive polymer composites. The effect of conductive fillers on electrical properties of composites was investigated in this research.Surface electrical resistance properties of the PUU films have been studied by measuring the surface electrical resistance. Results show that when the carbon black(CB) content reaches 20 phr, surface resistance of aluminum-contacting side and air-contacting side decreased from 1.18x1015 Ω/cm2 and 1.33x1015 Ω/cm2 to 5.91x106 Ω/cm2 (decreased 9 orders)and 9.20x106 Ω/cm2(decreased 9 orders), respectively. However, when the carbon nanotube content was only 5phr, surface resistance of aluminum-contacting side and air-contacting side decreased to 2.44x107 Ω/cm2 (decreased 8 orders) and 3.96x107 Ω/cm2(decreased 8 orders), respectively. Due to the high aspect ratio of carbon nanotubes, it forms better conductive network than that of carbon black particle. Generally speaking, this electrical resistance performance could be applied to antistatic and ESD materials for military applications at some middle or high relative humidity.In this study, highly conductive metal such as silver or nickel were used to improve the conductivity of carbon nanotubes. The acid-treated carbon nanotubes posses carbonyl group on the surface of CNT, which was characterized by FT-IR spectra、EDS and Raman spectra. Silver or nickel then were coated on the surface of carbon nanotubes via oxidation-reduction reaction. The characterization was performed by EDS、XRD and TEM. Results indicate that the surface resistance of poly(urea-urethane) filled with nickel-coated (0.003M) carbon nanotubes (CNT-Ni-0.03M) in the aluminum-contacting side is 5.23x106 Ω/cm2，and three that of CNT-Ni-0.0015M、CNT-Ag-0.003M、CNT-Ag-0.0015M are 5.24x104 Ω/cm2、7.61x104 Ω/cm2、1.41x104 Ω/cm2, respectively.TGA results showed that introducing inorganic conductive filler (carbon black, carbon nanotube, CNT-Ag, CNT-Ni) into PUU can increase the thermal stability of PUU. For example, the 10% weight loss of PUU increased from 260.4℃ to 280.6℃ when 20 phr (part per hundred percent resin) of carbon black was added to poly(urea-urethane).Conductivity on PUU with different conductive filler was conducted with EMI shielding effectiveness of PUU. When the CB content reached 20 phr, the EMI shielding effectiveness of composite is 1.5dB. However, composite with 5 phr CNT content exhibited 3 dB. If the testing was performed at high frequency electromagnetic wave, the EMI shielding effectiveness of composite would be increased. For example, PUU/CNT composite presented 1dB of EMI shielding effectiveness in the 400MHz and 4dB in the 1300MHz when the filler content is 5 phr. The film thickness of composite also affects the EMI shielding effectiveness of composite. The EMI shielding effectiveness of PUU/CNT composite increased from 4 dB to 8 dB as the film thickness double since the reflective absorptions inner the composite increases.Carbon nanotubes coated with more metal exhibit worse EMI shielding effectiveness. The EMI shielding effectiveness of CNT-Ag-0.003M is 1.5 dB, but CNT-Ag-0.0015M is 4dB. Excess metal on the surface of carbon nanotubes will not only encapsulate the carbon nanotubes but also aggregate together with other metal-coated carbon nanotubes, that reducing the network structure and surface area of carbon nanotubes. The electromagnetic wave goes through composite and with few reflective adsorptions. Nickel metal shows higher EMI shielding effectiveness than that of silver, since it is a magnetic metal and it possesses better ability to shield the magnetic field than silver does. Increasing the content of CNT-Ni would raise the magnetic field shielding effectiveness of composite. However, CNT-Ag have not ability to shield the magnetic field. So composite with CNT-Ni-0.0015M shows better EMI shielding effectiveness than that with CNT-Ni-0.003M. The same result can be seem from the system of silver-coated carbon nanotubes.Electromagnetic absorption of composite increases with the increasing of electromagnetic wave frequency when the carbon black content is 20 phr and carbon nanotubes content is 3 phr. Carbon nanotubes provide the ability of electromagnetic absorption to the composite at low filler content. When the filler contents are 3phr、4phr、5phr, the PUU/CNT composite posses EMI shielding effectiveness at 8.48 dB、28.40dB and 26.07dB as the electromagnetic wave frequencies are 12.31 GHz、16.21 GHz and 15.89 GHz, respectively. However, the thickness of PUU/CNT composite was increased, the specific electromagnetic absorption peaks of composite shift to lower values.The Young’s modulus and tensile stress of poly(urea-urethane) increase significantly with the increasing of carbon nanotubes content. When the carbon nanotubes content reaches 8 phr, the Young’s modulus were from 85.64Mpa to 146.06MPa(an increase of 70.6%)and tensile stress were from 5.68MPa to 15.57Mpa(an increase of 177.3%), respectively. When the carbon black content reaches 20 phr, the Young’s modulus were from 85.64Mpa to 119.92Mpa (an increase of 40.0%)and tensile stress were from 5.68MPa to 10.16Mpa(an increase of 78.9%), respectively. Results also show that PUU reinforced by carbon nanotubes possesses better mechanical properties increased than the carbon black ones.