Removal of solvent-based ink from printed surface of HDPE bottles ...

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(equivalent to the amount of ink present) on the plastic surface. The amount of ink removed (%) was then calculated using the following relationship [6]: Ink removedð%Þ ¼½ðpixelsbefore deinking pixelsafter deinkingÞ =pixelsbefore deinkingŠ 100 (1) Viscosity measurement The viscosity of each CnTAB solution was measured in comparison with that of water by a Ubbelode viscometer. The Ubbelode tube containing a sample solution was partially submerged in water, the temperature of which was equilibrated to 30±1°C. Prior to each measurement, the sample solution was filled in the Ubbelode tube and was left for temperature equilibration for 15 min. During measurement, the solution was then raised up and then allowed to flow gravitationally. The elapsed time for the solution to flow passing two marked lines was recorded. The measurement was repeated five times, from which an average value was calculated. The viscosity of the solution is assumed to relate to that of water according to the following relationship [14]: 1 ¼ 0 1t1 0t0 where η is viscosity, ρ is density, and t is the measured elapsed time. “1” represents the measured CnTAB solution, and “0” represents water. Results and discussion Critical micelle concentration From the plots of conductivity against the concentration of DTAB, TTAB, and CTAB solutions (results not shown), a break point was observed at the concentrations of 16.29, 3.75, and 0.97 mM, respectively, while from the plots of surface tension against the logarithm of concentration of the respective CnTAB solutions (see Fig. 1), a break point was observed at the concentrations of 13.77, 3.70, and 0.93 mM, respectively. These surface tension-based values are taken as the CMC of the cationic surfactants. Obviously, the CMC of these surfactants decreased with increasing alkyl chain length due to the increased driving force for micelle formation from increased hydrophobicity of the tail group. When comparing the experimental CMC values with the reported values of 16.00 mM for DTAB [15], 3.60 mM for TTAB [16], and 0.92 for CTAB [17] (all at 25°C), the obtained values seem reasonable except for (2) Surface Tension (mN/m) 80 70 60 50 40 30 20 10 0 the value of DTAB from the surface tension method, which was much lower than the reported value of 16.00 mM. The discrepancy in the observed CMC value of DTAB from the drop-shape analysis and that from the conductivity may be due to the fact that the calculation of the surface tension from the shape of a pendant drop depended very much on input values of both the density and the viscosity of the measured solution; therefore, a slight inaccuracy in either value or both values could lead to an erroneous result. Wettability 0.93 3.70 CTAB TTAB DTAB 13.77 0 1 10 100 Concentration (mM) Fig. 1 Surface tension of CnTAB solutions as a function of initial CnTAB concentration. Arrows indicate CMC values A photograph of a sessile drop on a flat plastic surface showing the contact angle is shown in Fig. 2. Wetting is the first step for the deinking process, which results from adsorption of surfactant molecules onto the surfaces of both HDPE and printed ink. As a result, the interfacial tensions of ink/water and HDPE/water are lowered, and the contact angle of the surfactant solution onto the solid decreases. If the contact angle attains 0°, complete wetting occurs. A contact angle of 90° or greater would indicate a nonwetted surface. The case with which a hydrophobic polymer surface is wetted is often quantified by a “critical surface tension” characteristic of that plastic [18]. This is the maximum surface tension of a pure single-component γ LV θ A γ γ SV SL 983 Fig. 2 Schematic for sessile-drop contact angle method. θA is the advancing contact angle; γSV, γSL, and γLV are solid–vapor, solid– liquid, and liquid–vapor interfacial tensions, respectively
984 liquid which would yield a contact angle of 0° on that polymer. For example, the critical surface tension of PE has been reported to be 31–33 mN/m [18]. This value will vary with molecular weight, surface treatment, and other factors. Although never intended to apply to surfactant solutions, the critical surface tension concept is often applied to these [19]. The plateau surface tensions (at concentrations above the CMC) of DTAB, TTAB, and CTAB are about 40 mN/m (see Fig. 1), which might indicate that these surfactant solutions would not completely wet the HDPE surface even above the CMC. The critical surface tension is a crude, empirical approach to wettability of hydrophobic surfaces. A general description of wetting through the contact angle (θ A)is from the Young equation [19]: cos A ¼ SV SL LV where γ SV, γ SL, and γ LV are surface tensions between solid– vapor, solid–liquid, and liquid–vapor interfaces, respectively. For advancing contact angles as measured here, γ SV can be assumed to be constant because the liquid has not been exposed to the dry surface area ahead of the advancing drop. Surfactants cause improved wetting (by reducing θ A) via adsorption at the liquid–vapor and solid– liquid interfaces, reducing both γ LV and γ SV, respectively. The wettability in terms of advancing contact angles of CnTAB solutions on printed HDPE surface as a function of CnTAB concentration is illustrated in Fig. 3. For a given type of CnTAB, the contact angle decreases with increasing CnTAB concentration. At a given concentration, the contact angle of CTAB solution was the lowest, while that of DTAB was the highest. From the results obtained, it can be concluded that the wettability of CnTAB solutions on a printed HDPE surface increased with an increase in both the alkyl chain length and the CnTAB concentration. Since the hydrophilic head group of these cationic surfactants is the same, the only difference among these Advancing contact angle ( degree) 90 80 70 60 50 CTAB TTAB 40 30 DTAB 0.01 0.1 1 10 100 Initial CnTAB concentration (mM) Fig. 3 Advancing contact angles of CnTAB solutions on printed HDPE surface as a function of initial CnTAB concentration. Arrows indicate CMC values (3) surfactants is the length of the hydrophobic tail groups. Due to the similarity among the hydrophilic head groups of these surfactants, the electrostatic interaction among the head groups of adjacent surfactant molecules of these surfactants is similar. An increase in the length of the hydrophobic tail group increases the van der Waal interaction among the tail groups of adjacent surfactant molecules, as well as between the tail groups of some surfactant molecules and some part of printed HDPE surface which is hydrophobic, and the hydrophobic surface of the unprinted HDPE surface [20, 21]. Hence, CTAB molecules which have the longest alkyl chain length among the cationic surfactants investigated can adsorb onto the hydrophobic part of ink surface (i.e., polymeric binder) much better than those of TTAB and DTAB, which showed the lowest adsorption. As a result, an increase in the alkyl chain length of CnTAB increased hydrophilicity of the solid surface, reducing both γ LV and γ SL values, which consequently cause the contact angles to decrease or the wettability on the ink surface to increase. Since surfaces of both HDPE and some part of the printed ink are hydrophobic, adsorption of CnTAB molecules on these surfaces is a monolayer coverage with tail-in and headout arrangement. Conversely, on some hydrophilic parts of the printed ink, a bilayer probably forms head groups down in the first layer and head groups out toward the solution for the second layer. Zeta potential Figure 4 shows the zeta potential of ink particles in water as a function of pH level. The PZC is defined as the pH at which there is no net charge on the solid surface and where the zeta potential is zero. According to Fig. 4, the PZC for the ink particles investigated was found at a pH level of about 3.3. This means that the ink particles exhibited a positive charge when pH3.3, and the charge of the ink particles became more negative with increasing pH level. Zeta potential (mV) 60 30 0 -30 -60 -90 -120 -150 PZC = 3.3 0 2 4 6 8 10 12 pH level Fig. 4 Zeta potential of ink particles in water as a function of pH level
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