Estimation of Concentration of Polymer Dispersant and Carbon Nanotubes in Electrophoretically Deposited Coatings By Means of UV-Vis Absorption Spectroscopy

Abstract

0.1 g/L Multi walled carbon nanotubes (MWCNTs) were dispersed in isopropyl alcohol (IPA) with the aid of polyvinylpyrrolidone (PVP). Initial ratio of PVP/MWCNTs is 20. Consecutive ultrasonication (1 h., with ultrasonic probe) and centrifugation (1200 rcf for 30 min.) yielded stable suspensions as evident in zeta potential measurements and sedimentation tests (suspensions are stable for months). Zeta potential value of PVP modified MWCNTs IPA suspensions was found -35,9 mV (Huckel’s approximation). Carboxyl group functionalized MWCNTs IPA suspensions were also prepared by acid functionalization technique (3/1 H2SO4/HNO3 mixture). Filtration, ultrasonication and centrifugation result in stable COOH modified MWCNTs IPA suspensions as well (Zeta potential:-47,5 mV). Prior to electrophoretic deposition PVP MWCNTs IPA suspensions were mixed with MgCl2 IPA solutions at proper amounts (9.64mM MgCl2). Presence of magnesium ions enabled adhesion of MWCNTs although their concentration is high in terms of stability. Zeta potential of magnesium ion containing electrophoretic suspensions were found 19,7mV. Electrophoretic deposition processes were conducted under 50 V DC voltage for 10 min. and 20 min. with a electrode separation of 1.5 cm. UV-Vis spectroscopy analysis were conducted in two different ways to determine concentration of PVP and MWCNTs in suspensions and hence in coatings. For the determination of PVP concentration, a calibration curve of PVP was obtained by evaluating spectra of PVP IPA solutions. UV-Vis spectra of PVP MWCNTs IPA were obtained by measuring diluted suspensions. In addition, spectra of COOH MWCNTs IPA were obtained which are overlapping with that of PVP MWCNTs IPA at the visible and infrared region of the spectrum where only MWCNTs contribute to absorption. Finally, subtraction of MWCNTs contribution yields representing PVP spectra of suspensions and ultimately PVP concentrations were determined by using a calibration curve. MWCNTs concentration was determined by overlapping properly diluted suspensions of centrifuged and uncentrifuged (Initial concentration of MWCNTs is known in uncentrifuged suspension) suspension spectra. Concentration of materials in coating and coating thickness is estimated by measuring UV-Vis spectra of electrophoretic suspensions before and after 20 min. deposition. Then the loss of material in suspension during electrophoretic deposition was determined. PVP concentration in coating was calculated 0.18 g/L and MWCNTs concentration was calculated 0.02 g/L. Thereby, Quantity of PVP and MWCNTs in coating were estimated 0.0009 g and 0.0001 g, respectively (Volume of suspensions is known). Surface area of coating is measured 0.8 cm2. Densities of MWCNTs and PVP were acknowledged as 1.32 g/cm3 (theoretical density of stacks of 18 layer multi wall carbon nanotubes) and 1.25 g/cm3. Ultimately, thickness of the coating was calculated ~10 µm. SEM images of the coatings were also conducted to determine the accuracy of the estimated values. It turns out that coating thickness is around 20 µm in diameter in terms of SEM measurements. The difference in measured and calculated values may be due to various reasons. Firstly, porosity in coating is neglected in estimations. Secondly, there is a small amount of deposition at the back of deposition electrode which is neglected as well. Thirdly, SEM measurements were taken from the bottom of the deposit at the region where the thickness is the highest due to probable sedimentation during deposition.