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

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solution temperature was maintained at 30±1°C. The CMC
of each cationic surfactant (i.e., CnTAB) was determined
from the break points observed in the plots of conductivity
and surface tension as a function of linear (for measurements
based on the conductivity values) or logarithm (for
measurements based on the surface tension values) scale of
CnTAB concentration.
Wettability measurement
Wettability of CnTAB on printed HDPE surface could be
inferred from contact angles of drops of CnTAB solutions
on the surface using the sessile drop method [13]. The
apparatus was composed of a closed chamber of 21×
27×15 cm in dimension, which was constructed from
acrylic sheets. A hole of 0.5 cm in diameter, used as a
microsyringe holder, was located on top of the chamber. A
Nikon Coolpix 995 with microlens attachment was set on a
camera holder in front of the chamber. Prior to measurements,
the temperature of the chamber was equilibrated to
30±1°C, and the chamber atmosphere was saturated with
moisture from a water-filled beaker placed within the
chamber. The relative humidity in the chamber was maintained
in the range of 90–93% to limit evaporation of water
from drops of CnTAB solutions during measurement. For
each sample solution, advancing contact angles were measured
on drops of between 10 and 80 μl in a stepwise
manner from the addition of 10 μl of the sample solution.
For each measurement, the sessile drop was allowed to set
not more than 1 min before a photograph of the drop was
recorded digitally. This is insufficient time for equilibration,
so the reported value is a dynamic contact angle.
Adobe Photoshop version 5.5 software was used to analyze
the digital files recorded, and the data for each sample
solution were averaged from at least three readings. It was
found that, for all of the sample solutions, the standard
deviation for all of the measured values of contact angles
was within ±5.5°.
Zeta potential measurement
A very small amount of the as-prepared ink powder was
added into a CnTAB solution. The pH level of the mixture
was adjusted to 12, and the mixture was stirred for 24 h.
The temperature of the mixture was equilibrated to 30±
1°C. The as-prepared mixture was then transferred to an
electrophoretic cell of a Zeta-Meter ZM 3.0+ zeta meter
equipped with a microscope module. After applying a
suitable voltage according to the solution conductivity, the
time for any observable ink particle to move for a certain
distance was measured. For a given data set, at least 20 ink
particles were monitored, from which the average time was
calculated. The average time was then used to calculate the
average zeta potential value for that particular data set. The
point of zero charge (PZC) of ink pigment particles in
distilled water was determined as the pH at which the zeta
potential of the ink particles equals zero.
Solubilization measurement
The ink can be considered to be composed of colored
pigment particles and binder (epoxy resin), which can be
solubilized in a surfactant solution. The solubilization capacity
of ink binder in CnTAB solutions was measured by
mixing a weighed amount of the as-prepared ink powder
(ca. 10 mg) in 15 ml of an as-prepared CnTAB solution,
with its pH level being adjusted to 12, in a 50-ml
Erlenmeyer flask. The flask was then placed in a shaking
water bath, the temperature of which was equilibrated to
30±1°C, and the shaking cycle of which was set at
200 cycles/min for 4 h. The solution was later filtered to
remove undissolved ink particles (pigments) using a
Whatman No. 7402-001 filter paper with average pore
size of 0.2 μm. The UV absorbance of the filtrate was
measured by a Shimadzu UV-2550 UV-visible spectrophotometer
at a wave length of 273.8 nm. The absorbance
for accurate measurements should be between 0.1 and 0.8.
If a sample solution showed an absorbance beyond this
range, it was diluted until the reading was within the
optimum range. The actual “quantitative” absorbance for
the solution after dilution was calculated back from the
amount of water used to dilute the original solution. The
plot of the absorbance vs CnTAB concentration was a
qualitative measure for the amount of ink binder solubilized
in CnTAB solutions.
Deinking experiment
To investigate the effects of the alkyl chain length and
surfactant concentration on deinking efficiency, DTAB,
TTAB, and CTAB solutions were prepared at different
concentrations, and the pH level of the surfactant solutions
was adjusted to 12. The as-prepared printed HDPE specimens
were first presoaked (without shaking) in 15 ml of a
surfactant solution for 2 h and further soaked with shaking
at 200 cycles/min for another 2 h in a shaking water bath.
The temperature of the bath was equilibrated to 30±1°C.
After the required time was reached, the specimens were
washed with deionized water and later dried in open air at
room temperature overnight. The amount of ink on the
plastic before and after deinking was measured using the
optical scanning method [6]. In this method, each plastic
sample was carefully positioned on an HP ScanJet 4C
optical scanner and scanned using the factory settings. The
scanned files were analyzed by an Adobe Photoshop
version 5.5 software by counting the number of pixels