Dye/Clay intercalated nanopigments using commercially available non-ionic dye

Article abstract of DOI:10.1016/j.dyepig.2011.11.003

Two non-ionic azo dyes: solvent yellow 14 (SY14) and solvent red 24 (SR24), and one non-ionic disperse dye: dispersed red 60 (DR60) were chemically modified into their respective cationic species, which were then subsequently ion-exchanged with Na⁺-montmorillonite in an acidic medium. The dye-intercalated montmorillonite was then centrifuged, dried and milled to prepare the pigment particles. X-ray diffraction studies on the pigments showed an increase in the basal spacing in the clay layers for the SY14 and DR60 based pigments NP14 and NP60 respectively, confirming intercalation of the dyes within the clay layers giving rise to a nano-structured system. The XRD pattern of the SR24 based pigment NP24 showed a diffused shoulder with a truncated peak, suggesting the possibility of a delaminated structure after adsorption of the dye. Due to the nano-structured morphologies, these pigments were classified as nanopigments. Thermogravimetric analysis showed different thermal stabilities for different nanopigments compared to the respective original dyes: an improvement in case of NP14, no change for NP24, and an apparent deterioration for NP60. The nanopigments were subsequently mixed with polypropylene to produce coloured specimens. Bleeding tests on these coloured specimens showed a reduction in leaching in turpentine.

Full text of DOI:10.1016/j.dyepig.2011.11.003

Dyes and Pigments 93 (2012) 1512e1518
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Dye/Clay intercalated nanopigments using commercially available non-ionic dye
*
Sumanta Raha , Nurul Quazi, Ivan Ivanov, Sati Bhattacharya
School of Civil, Environmental and Chemical Engineering, RMIT University, 124 La Trobe Street, Melbourne VIC 3000, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Two non-ionic azo dyes: solvent yellow 14 (SY14) and solvent red 24 (SR24), and one non-ionic disperse
dye: dispersed red 60 (DR60) were chemically modified into their respective cationic species, which
were then subsequently ion-exchanged with Naþ-montmorillonite in an acidic medium. The dye-
intercalated montmorillonite was then centrifuged, dried and milled to prepare the pigment particles.
X-ray diffraction studies on the pigments showed an increase in the basal spacing in the clay layers for
the SY14 and DR60 based pigments NP14 and NP60 respectively, confirming intercalation of the dyes
within the clay layers giving rise to a nano-structured system. The XRD pattern of the SR24 based
pigment NP24 showed a diffused shoulder with a truncated peak, suggesting the possibility of
a delaminated structure after adsorption of the dye. Due to the nano-structured morphologies, these
pigments were classified as nanopigments. Thermogravimetric analysis showed different thermal
stabilities for different nanopigments compared to the respective original dyes: an improvement in case
of NP14, no change for NP24, and an apparent deterioration for NP60. The nanopigments were subse-
quently mixed with polypropylene to produce coloured specimens. Bleeding tests on these coloured
specimens showed a reduction in leaching in turpentine.
Received 31 August 2011
Received in revised form
7 November 2011
Accepted 7 November 2011
Available online 15 November 2011
Keywords:
Nanopigment
Dye modification
Dye intercalation
Cationic dye
Clay
Azo dye
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
methylene Blue [14,15]. Majority of the research has focused on
describing the influence of process parameters or dye structure on
Both pigments and dyes are widely used as colourants for
plastics. Dyes offer a wide range of colours, in combination with
transparency, as they are dissolved in polymer [1]. This is however
a cause of their disadvantage, as the mobility of dye molecules
allows greater rate of migration and degradation, in comparison to
pigments which are solid particles dispersed into the polymer
matrix. Nevertheless, it is considered beneficial to replace some of
the existing pigments (especially those based on heavy metals)
with organic dyes or hybrid pigments based on dyes [2].
The idea of incorporating dyes into inorganic materials is not
new [3]. Many commercial “Lake Pigments” were made by
precipitating dyes with inorganic substrates [4]. If the inorganic
substrate is a smectite clay or layered silicate, the most effective
pathway was found to be using the ion-exchange process (Gemeay,
2002) [5]. For that reaction, the dye must exist in a cationic form,
which is the case with basic or cationic dyes. Examples for that are
intercalated products of Rhodamine dyes (Rhodamine B, Rhoda-
mine 6G) with hectorite [6], montmorillonite [7e10], and other
smectite clays [11e13]. Other basic dyes were also utilised, such as
intercalate properties [16,17]. Attempts have also been made to
commercialise the intercalates as nano-structured pigments by
combining other organo-modifiers, such as quaternary ammonium
salts with long alkyl chains [18].
In our laboratory, we have recently used cationic dye such as
rhodamine for intercalation with clay to produce dye/clay hybrid
nanopigments [10], by which we improved the thermal stability of
the intercalated dye. UV exposure tests on coloured polypropylene/
nanopigment composites showed a significant improvement in the
photo-stability compared to the polypropylene/dye samples.
Unlike cationic dyes, which can be intercalated with clay, many
other commercially available dyes are non-ionic. Examples of such
non-ionic dyes include azo dyes and disperse dyes. Due to their
non-ionic nature these dyes cannot be intercalated with clay by
ion-exchange methods. To address this problem, chemical modifi-
cation methods can be adopted using the chemical synthesis route
to modify these commercially available dyes into their respective
cationic species. Once modified into a cationic species, these dyes
can then be intercalated or adsorbed on smectite clays to produce
the dye/clay hybrid nanopigments.
In this paper, the authors report the synthesis of water soluble
cationic dyes using two azo dyes, namely solvent yellow 14 (SY14)
and solvent red 24 (SR24), and one disperse dye, disperse red 60
* Corresponding author.
E-mail address: sumanta_raha@yahoo.com (S. Raha).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.11.003

S. Raha et al. / Dyes and Pigments 93 (2012) 1512e1518
1513
Scheme 1. Modification of the azo dye SY14.
(DR60), and their subsequent intercalation with Naþ-MMT to form
nano-structured pigments or nanopigments. This paper will also
present X-ray diffraction (XRD) and thermogravimetric analysis
(TGA) studies performed on these nanopigments.
2. Experimental
Scheme 2. Modification of the azo dye SR24.
2.1. Materials
20 ml of water was slowly added with stirring. The mixture was
refluxed until the water was entirely removed. After cooling to
about 70 C, the free base (prepared from 16 g 3-dimethylamino-1-
propylchloride hydrochloride salt. with potassium carbonate and
extraction with 50 ml of xylene) was added. The mixture was
refluxed for 48 h, cooled to room temperature and filtered. The
filtrate was distilled to dryness and the residue (B) was dissolved in
300 ml of ether and added 15 g iodomethane and stirred at RM for
48 h. The solid which separated was filtered off and washed with
more ether to give the crude product. This was finally recrystallised
from water to give the (20 g, 52%) pure product (C) as amino salt of
SY14, mp.198e200 ꢀC.
Dyes used in this work included two azo dyes, namely, Solvent
Yellow 14 (SY14, CI 12055) and Solvent Red 24 (SR24, CI 26105), and
one disperse dye, Disperse Red 60 (DR60, CI 60756), which were
supplied by Allied Colors and Additives, Australia. Other chemicals
used were: xylene (Merck), potassium hydroxide (BDH), 3-
dimethylamino-1-propylchloride hydrochloride salt, and potas-
sium carbonate (Aldrich). Na-montmorillonite used was Cloisite
Naþ from Southern Clay.
2.2. Molecular characterisation
1HNMR (DMSO-d6): 2.16, m, 2H, CH2; 3.00, s, 9H, 3CH3; 3.42, m,
2H, NeCH2; 4.24, t, 2H, OCH2; 7.46e8.25, m 11H, ArH. ESMS:
Mþ ¼ 348.3.
Nuclear magnetic resonance spectra were recorded on a Bruker
AM-300 spectrometer operating at 300.13 MHz (1H). The electro-
spray mass spectra were obtained on a PerkineElmer Sciex API-
300 triple quadrupole mass spectrometer in positive ion mode
with acetonitrile as solvent (uncertainty_0.3).
2.3.2. Synthesis of trimethyl-{3-[1-(2-methyl-4-o-tolylazo-
phenylazo)-naphthalen-2-yloxy]-propyl}-ammonium iodide (F)
(Scheme 2)
2.3. Chemical modification of dyes
20 g of 1-(2-Methyl-4-o-tolylazo-phenylazo)-naphthalen-2-ol
(SR24, D) were stirred with 300 ml of hot xylene. A solution of
3.5 gof potassium hydroxide in 15 ml of water was slowlyadded with
stirring. The mixture was refluxed until the water was entirely
removed. After cooling to about 70 ꢀC, the free base (prepared from
10 g 3-dimethylamino-1-propylchloride hydrochloride salt with
potassium carbonate and extraction with 50 ml of xylene) was
added. The mixture was refluxed for 48 h, cooled to room temper-
ature andfiltered. Thefiltratewas distilled to dryness andthe residue
(E) was dissolved in 300 ml of ether. 10 g Iodomethane was added
and stirred at RM for 48 h. The solid which separated was filtered off
washed with cold ether to give the crude product. This was finally
extracted with diethyl ether using Soxlettextraction to give the (20 g,
62%) pure product (F) as amino salt of SR24, m.p. 88e90 ꢀC.
1HNMR (DMSO-d6): 2.24, m, 2H, CH2; 2.69, 3H, CH3; 2.78, 3H,
CH3; 3.00, s, 9H, 3CH3; 3.46, m, 2H, NeCH2; 4.32, t, 2H, OCH2;
7.32e8.49, m 13H, ArH. ESMS: Mþ ¼ 480.3.
The phenolic groups of the azo dyes SY14 and SR24 as well as
Disperse dye DR60 can be alkylated using a suitable alkylating agent,
such as 3-dimethylamino-1-propylchloride hydrochloride salt.
Accordingly, these dyes were converted to their corresponding
tertiary amines by reacting with 3-dimethylamino-1-propylchloride
hydrochloride in the presence of potassium hydroxide in xylene
under refluxing condition for 48 h. Usual work-up gave the corre-
sponding amines. These amines were then treated with methyl-
iodide in diethyl ether at room temperature, to give the desired
salts as crystalline solids. The details of the reactions are given in the
following Schemes 1, 2 and 3 respectively.
2.3.1. Synthesis of trimethyl-[3-(1-phenylazo-naphthalen-2-yloxy)-
propyl]-ammonium iodide (C) (Scheme 1)
20 g of 1-Phenylazo-naphthalen-2-ol (SY14, A) were stirred with
300 ml of hot xylene. A solution of 5 g of potassium hydroxide in

1514
S. Raha et al. / Dyes and Pigments 93 (2012) 1512e1518
pressure to give the (18 g, 55%) pure product (J), as amino salt of
DR60, decomposition point: 248e250 ꢀC.
1HNMR (DMSO-d6): 2.15, m, 2H, CH2; 3.12, s, 9H, 3ꢁCH3; 3.57, m,
2H, CH2; 3.96, t, 2H, CH2; 6.81, s,1H, ArH; 7.21, d,1H, ArH; 7.29, t, 1H,
ArH, 7.50, t, 2H, ArH, 7.85, m, 2H, ArH, 8.00, b, 2H, NH2, 8.13, m, 1H,
ArH, 8.02, m, 1H, ArH. ESMS: Mþ ¼ 431.50.
2.4. Nanopigment preparation
The dye/clay nanopigments were prepared by ion-exchanging
the cationically modified dyes with Naþ-montmorillonite in an
acidic environment. 10 g of Cloisite Naþ was dispersed in 300 mL of
deionised water and was allowed to swell for 24 h. Separately,
calculated amounts of the modified dyes were dissolved in 300 mL
of HCl (1 M). An acid medium is generally found to be favourable for
ion-exchanging of cationic dyes with clays (Arbeloa et al., 2005,
Shamsipur and Azimi, 2001, Raha et al., 2009). The amount of the
dyes was calculated from their respective molecular weights and
the 100% cationic exchange capacity (CEC) of monmorillonite
(0.97 meq/g). Each of the dye solutions was then slowly added to
the swelled montmorillonite/water dispersion and stirred until
a thick gel structure was formed due to the adsorption of the dye
cations on to the negatively charged clay mineral layers (Lagaly,
2006). After mixing, the flocculated mixture was allowed to stand
for another 48 h to allow sufficient time for the ion-exchange to
occur. The mixture was left overnight, and then centrifuged at 4400
RPM for 20 min. The precipitated pigment was washed with
sufficient amount of deionised water and dried at 80 ꢀC for two
days. The obtained cake was then crushed into smaller pieces,
milled in a planetary ball-mill (Retsch PM100) and sieved through
a 45 micron sieve to produce fine particles of the nanopigments.
Table 1 presents the calculated amounts of dyes and the nominal
dye loading for each of the three nanopigments produced.
2.5. X-ray diffraction characterisation
Scheme 3. Modification of the disperse dye DR60.
X-ray diffraction (XRD) studies on the clay and nanopigments
were carried out using a Bruker AXS D8 wide angle X-ray diffrac-
2.3.3. Synthesis of [3-(4-amino-9,10-dioxo-3-phenoxy-9,10-
dihydro-anthracen-1-yloxy)-propyl]-trimethyl-ammonium iodide
(J) (Scheme 3)
tometer with Cu-K
voltage and 35 mA current. The basal spacing d001 was calculated
from the 2 peaks using the Bragg’s law n
¼ 2dsin
a
radiation (
l
¼ 0.154 nm) at 40 kV acceleration
20 g of 1-Amino-4-hydroxy-2-phenoxy-anthraquinone (DR60,
G) were stirred with 300 ml of hot xylene. A solution of 4 g of
potassium hydroxide in 15 ml of water was slowly added with
stirring. The mixture was refluxed until the water was entirely
removed. After cooling to about 70 ꢀC, the free base (prepared from
14 g 3-dimethylamino-1-propylchloride hydrochloride salt with
potassium carbonate and extraction with 50 ml of xylene) was
added. The mixture was refluxed for 48 h, cooled to room
temperature and filtered. The filtrate was distilled to dryness and
the residue (H) was dissolved in 300 ml of ether. 15 g iodomethane
was added and stirred at RM for 48 h. The solid which separated
was filtered off washed with more ether to give the crude product.
This was finally extracted with water and separated the insoluble
by filtration, removed the water soluble part under reduced
q
l
q.
2.6. Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) was carried out on the clay,
dye and the prepared nanopigment to characterise their thermal
behaviour. A PerkineElmer TGA-7 instrument was used and
samples were heated from 50 ꢀC to 700 ꢀC at a rate of 20 ꢀC/min
under nitrogen at a purging flow rate of 20 ml/min. Beyond 700 ꢀC
the purging gas was switched to air and the samples were further
calcined up to 800 ꢀC.
2.7. Compounding with polypropylene
Table 1
Nominal dye loadings in nanopigments.
The polypropylene (PP) composites were prepared by mixing
1 wt% nanopigments in polypropylene (Basell Moplen HP301R) in
an injection moulder at 190 ꢀC. Maleated polypropylene (PP-MA)
(Dupont Fusabond MZ109D) was also added at 5% as a compatibil-
iser for the clay/PP composites. For the reference, PP/Dye
composites were also produced. The compositions of the PP
composites are given in Table 2.
Cationic Dye
Mol. wt. Dye loading with 10 g Naþ-MMT Nominal dye
(100% CEC at 0.97 meq/g of clay)
loading in
pigment
Modified SY14
Modified SR24
475.37
607.54
4.6 g
5.9 g
5.4 g
31.5 wt%
37 wt%
35 wt%
Modified DR60 558.42

S. Raha et al. / Dyes and Pigments 93 (2012) 1512e1518
1515
Table 2
PP composites and their compositions.
System
PP wt%
PP-MA wt%
Nanopigment wt%
Dye wt%
PP/NP14
PP/SY14
PP/NP60
PP/DR60
PP/NP24
PP/SR24
94.00
94.68
94.00
94.68
94.00
94.63
5.00
5.00
5.00
5.00
5.00
5.00
1.00
e
e
0.32
e
1.00
e
0.35
e
1.00
e
0.37
2.8. Colour measurements
A Minolta CR-300 colorimeter was used to measure the colour of
the PP specimens in the CIE L*a*b* colour scale using 2ꢀ observer/
D65 standard. The 2 mm thick specimens were placed on a white
calibration plate (which had a measured L*a*b* colour value of
96.96, 0.08, 2.24). The colour difference
DE* was calculated using
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
the CIE 1976 relation
D
E* ¼
ð
D
L*Þ2 þ ð
D
a*Þ2 þ ð
D
b*Þ2.
3. Results and discussion
3.1. Cationic dye synthesis
The key issue in this work was to make dye/clay intercalated
nanopigments using commercially available non-ionic dyes. To
intercalate a dye molecule in a clay particle, the dye needs to be
ionic. Therefore, we have chosen two azo dyes, namely SY14 and
SY24, (also known as Sudan I and IV respectively), as well as
a disperse dye DR60 as the starting non-ionic dyes. The alkylation
on the phenolic groups of SY14, SY24 and DR60 were little studied
apart from methylation. At first, several reaction conditions [19]
were attempted without any success. But then using a high
boiling aprotic solvent, such as xylene using potassium hydroxide
solution with 3-dimethylamino-1-propylchloride, we were finally
able to get the intermediates and subsequent quarterisation gave
the desired products in moderate yield.
3.2. XRD characterisation
Fig. 1. XRD patterns of Na-MMT and the nanopigments.
Fig.1 shows the XRD pattern of the cationically modified dye/clay
nanopigments, as well as that of pure Na-MMT. The basal spacing of
pure montmorillonite was found to be 1.16 nm. The modified SY14
based nanopigment NP14 showed a sharp and well-defined peak
orientations. It is possible that the majority of the intercalation
occurred with a planar orientation of the aromatic rings, giving rise
to a 1.34 nm basal spacing, whereas a fraction of the layers also had
a non-planar orientation of the interacted dye resulting in a much
large basal spacing of 2.11 nm.
near 2
q
¼ 4.2ꢀ, corresponding to a basal spacing d001 of 2.11 nm. It
also showed another peak near 2
q
¼ 8.4ꢀ, corresponding to the d002
planes (1.05 nm). The XRD pattern of NP14 clearly suggested a well-
ordered stacked structure of the clay layers with an increase in basal
spacing by 0.95 nm, thereby confirming the intercalation of the
cationially modified SY14 with the clay layers.
3.3. Thermogravimetric analysis (TGA)
3.3.1. TGA results on the dyes
The XRD pattern of the cationically modified SR24 dye based
nanopigment NP24 showed a broad shoulder between 2q close to
The TGA results on the dyes and the modified dyes given in
Fig. 2, which presents the mass% as a function of temperature
during heating under N2 up to a temperature of 750 ꢀC, and
subsequent calcination in air up to 850 ꢀC. The mass% data shows
that, except for the dye DR60, there was almost no mass loss below
200 ꢀC. The slight mass loss for DR60 in this range was possibly due
to the presence of some bound moisture. Interestingly, the absence
of an early moisture loss in the modified dyes indicated that in spite
of being an iodide salt there were not moisture absorbing or
hygroscopic. Above 250 ꢀC all the dyes and modified dyes showed
a sharp mass loss indicating the start of their thermal degradation.
They also left some carbonaceous charred remains at the end of the
heating stage under N2, given by the difference in mass% between
700 ꢀC and 850 ꢀC, which can be attributed to the aromatic struc-
ture of these molecules.
3.6ꢀ and 8.4ꢀ, and the disappearance of the original 1.16 nm peak of
the pure clay. It suggests that NP24 was not exactly an intercalated
system. The possible reasonwould be that due tothe largemolecular
size and structure of this azo dye. Especially with two free-rotating
azo links, it would be difficult to achieve an intercalated structure
while maintaining a stacked packing of the clay layers. Instead, the
XRD data suggests that due to the adsorption of the dye onto the clay
surface, the clay layer was delaminated to form a nanopigment with
a disordered house-of-card like structure.
The XRD pattern of the cationically modified disperse dye DR60
based nanopigment DR24 showed a weak shoulder at 2
q
¼ 4.2ꢀ
(2.11 nm) and a peak near 2
q
¼ 6.6ꢀ (1.34 nm). It suggested that
the dye was intercalated within the clay layer in two different

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S. Raha et al. / Dyes and Pigments 93 (2012) 1512e1518
Fig. 2. TGA of dyes, cation-modified dyes and PP/compounds.
Fig. 4. Differential thermograms, under N2. Vertical scales for NP14, NP24 and NP60
data have been magnified 5ꢁ for clarity.
3.3.2. TGA results on the clay and nanopigments
Fig. 3 shows the TGA results performed on pure Naþ-MMT and
the nanopigments NP14, NP24 and NP60 during heating under N2.
The mass% data shows that pure clay had considerable amount of
moisture content due to its hydrophilic nature, as indicate by the
sharp drop in its mass below 100 ꢀC. On the other hand, the three
nanopigments NP14, NP24 and NP60 showed much lower moisture
absorptions, suggesting a change from a hydrophilic to a hydro-
phobic nature due to the incorporation of the dye molecules. The
two azo dye based nanopigments, NP14 and NP24, showed nearly
identical mass loss percentage patterns from their TGA data, and
they were also more stable compared with the disperse dye based
nanopigment DR60.
3.3.3. Differential thermograms (DTG)
Fig. 4 presents the differential thermograms (DTG) of the dyes,
modified dyes and the corresponding nanopigments, showing the
derivative of their percentage mass loss as a function of tempera-
ture. The original dyes SY14, SR24 and DR60 showed peak
decomposition temperatures at 286 ꢀC, 316 ꢀC and 338 ꢀC respec-
tively. Each of the modified dyes showed an early peak near 255 ꢀC
suggesting the release iodide anion and a possible conversion of the
ammonium cation to a tertiary amine. The subsequent degradation
Fig. 3. TGA curves of pure clay and the nanopigments under N2.

S. Raha et al. / Dyes and Pigments 93 (2012) 1512e1518
1517
Table 3
Calculated dye loadings and estimated thermal stability of the nanopigments.
Naþ-MMT NP14
NP24 NP60
Mass at 180 ꢀC (N2)
Moisture content
Mass at 850 ꢀC (air)
Mass at 850 ꢀC (air, dry basis)
Calculated dye loading
Estimated 10% dye loss temp
in nanopigment
93.61%
6.39%
87.63%
93.61%
e
98.17% 97.52% 91.73%
1.73% 2.48% 8.27%
60.98% 54.68% 55.36%
62.12% 56.07% 60.35%
32.4%
272 ꢀC
38.6%
288 ꢀC
29.7%
256 ꢀC
e
10% mass loss temp. of original dye
e
232 ꢀC
289 ꢀC
285 ꢀC
peaks for these modified dyes were observed at 303 ꢀC, 312 ꢀC and
337 ꢀC, showing relatively similar peak degradation temperatures
of the rest of the molecules of the modified dyes that contain the
chromophores. In the nanopigment systems, particularly in NP14
and NP24, the absence of the iodine peak suggests successful ion-
exchange and adsorption of the cationic dyes on the negatively
charged clay layers. The DTG data for NP60, however, still showed
the iodine peak, although reduced, in addition to the peak near
340 ꢀC that appears like a shoulder due to the presence of the
iodine peak. It suggests that the NP60 underwent a partial ion-
exchange, with a fraction of the modified dye still remaining as
salt and the other fraction adsorbed onto the clay layers by ion-
exchange.
3.3.4. Thermal stability of the nanopigments
The thermal stability of the nanopigments was estimated at
temperatures that relate to 10% mass loss, based on their dry mass at
200 ꢀC. In the calculations, the dry mass loss of pure clay was also
taken into account. The actual dye loadings in the nanopigments
were calculated from the loss of dry mass after calcination at 850 ꢀC.
It can be seen from Table 3 that dye loading calculated from TGA
mass loss data matched very well with the nominal dye loadings
(original weight of dye used) for the azo dye based nanopigments,
indicating that almost all dye was adsorbed by the clay. However,
for the disperse dye based nanopigment DR60, the calculated dye
loading was found to be slightly less compared to the original
loading, which suggests that some dye was lost due to incomplete
adsorption. Table 3 also presents the estimated 10% mass loss
temperatures of the dye in the nanopigment, which were calcu-
lated using mass balance and also considering the mass loss of clay
itself. The values of the respective 10% mass loss temperatures of
the original dyes are also given. As it can be seen, the azo dye SY14
based nanopigment showed an improvement of about 40 ꢀC in the
thermal stability of the dye after it was intercalated with the clay.
The other azo dye SR24 based nanopigment NP24 showed almost
equal thermal stability of the dye, showing no effect of its
adsorption on the clay layers. On the other hand, for the disperse
dye DR60 based nanopigment NP60, thermal stability was seen to
decrease by about 29 ꢀC. It was concluded that the apparent
decrease actually reflects the loss resulting from a small fraction of
iodide salt of the dye present in the pigment system as was evi-
denced by the DTG data in Fig. 4.
Fig. 5. Coloured polypropylene composites prepared by mixing the nanopigments and
the corresponding original dyes.
samples, indicating some absorbance of the incident light due to
the presence of the clay. Table 4 presents the colour values of the
shown specimens and the corresponding colour difference
between the dyed and pigmented samples.
3.4. Polypropylene/nanopigment coloured composites
3.5. Bleeding tests
Fig. 5 shows the coloured specimens of the polypropylene
composites prepared by melt-mixing PP with the nanopigments,
and compares them with the coloured specimens prepared mixing
the corresponding original dyes with PP. It can be seen that the clay
based nanopigments incorporating the cationic species of the dyes
gave similar colours compared with the original dyes. It was also
found that there was a reduction in the brightness in the pigment
Migration of dyes from polymers is an important issue. A
common method to assess migration is by doing the bleeding test,
where coloured specimens are shaken with solvents (e.g. turpen-
tine) and colour of the solution is measured or assessed visually. In
this test, 0.15 g of each composite specimen (Table 2) was placed in
15 ml of mineral turpentine. Table 5 gives a quantified measure of

1518
S. Raha et al. / Dyes and Pigments 93 (2012) 1512e1518
Table 4
Acknowledgement
Measured colours of the PP composites and the colour change from dye to pigment.
The authors thank Nanotechnology Victoria Ltd (NanoVic) for
funding this research work. The authors also thank Allied Color and
Additives Pty Ltd for supply of dyes and for providing access to their
injection-moulding machine. The authors also acknowledge valu-
able comments from Prof. Shmuel Yariv of the Hebrew University of
Jerusalem, Israel.
Sample
L*a*b* colour scale
Colour difference
34.2
DE*
PP/SY 14
PP/NP14
PP/SR24
PP/NP24
PP/DR60
PP/NP60
52.7, 37.4, 51.4
38.0, 23.5, 23.8
27.9, 19.7, 6.2
24.6, 10.7, 2.7
29.1, 25.6, 8.2
27.3, 15.6, 4.8
10.2
10.7
References
Table 5
Bleeding from coloured PP composites.
[1] Harris RM. Coloring technology for plastics. New York: William Andrew
Publishing/Plastics Design Library; 1999.
[2] Bente LA. Dyes for the mass coloration of plastics. Plastics additives e an AeZ
reference. London: Chapman & Hall; 1998.
[3] Giustetto R, Llabres i Xamena FX, Ricchiardi G, Bordiga S, Damin A, Gobetto R,
et al. Maya blue: a computational and spectroscopic study. J Phys Chem B
2005;109:19360e8.
[4] Furmanski ER. FD&C Aluminium lakes HT food approved pigments for col-
ouring plastics. Pacific Technical Conference and Technical Displays (Society of
Plastics Engineers); 1979: 57e60.
Composite
Absorbance (4 h)
Absorbance (24 h)
PP/SY14
PP/NP4
PP/SR24
PP/NP24
PP/DR60
PP/NP60
0.236
0.104
0.155
0.103
0.028
0.009
0.592
0.278
0.397
0.340
0.168
0.046
[5] Gemeay AH. Adsorption characteristics and the kinetics of the cation
exchange of Rhodamine-6G with Naþ-Montmorillonite. J Colloid Interface Sci
2002;251:235e41.
[6] Windsor SA, Tinker MH. Electro-fluorescence polarization studies of the
interaction of fluorescent dyes with clay minerals in suspensions. Colloids Surf
A Physicochem Eng Asp 1999;148:61e73.
[7] Landau A, Zaban A, Lapides I, Yariv S. Montmorillonite treated with
Rhodamine-6G mechanochemically and in aqueous suspensions: simulta-
neous DTA-TG study. J Therm Anal Calorim 2002;70:103e13.
[8] Pospisil M, Capkova P, Weissmannova H, Klika Z, Trchova M, Chmielova M,
et al. Structure analysis of montmorillonite intercalated with rhodamine B:
modeling and experiment. J Mol Model 2003;9:39e46.
the bleeding by showing the absorbance of the migrated dye in
solutions in 4 and 24 h. It can be seen that the bleeding from the
nanopigment-based composites were less than original dye based
composites, indicating that migration behaviour was improved
after intercalation or adsorption of the dye within the clay.
[9] Klika Z, Weissmannova H, Capkova P, Pospisil M. The rhodamine B interca-
lation of montmorillonite. J. Colloid Interface Sci 2004;275:243e50.
[10] Raha S, Ivanov I, Quazi NH, Bhattacharya SN. Photo-stability of Rhodamine-B/
montmorillonite nanopigments in polypropylene matrix. Appl Clay Sci 2009;
42:661e6.
[11] Lopez Arbeloa F, Tapia Estevez MJ, Lopez Arbbeloa T, Lopez Arbeloa I. Spec-
troscopic study of the adsorption of rhodamine 6G on clay minerals in
aqueous suspensions. Clay Miner 1997;32:97e106.
[12] Lopez Arbeloa F, Lopez Arbeloa T, Lopez Arbeloa I. Characterization of clay
surfaces in aqueous suspensions by electronic spectroscopies of adsorbed
organic dyes. Trends Chem Phys 1996;4:191e213.
[13] Salleres S, Lopez Arbeloa F, Martinez V, Lopez Arbeloa T, Lopez Arbeloa I.
Adsorption of fluorescent R6G dye into organophilic C12TMA laponite films.
J Colloid Interface Sci 2008;321:212e9.
4. Conclusions
Successful modifications of the non-ionic dyes SY14, SR24 and
DR60 into their respective cationic species were confirmed by NMR
study. XRD studies showed evidence of varying degree of dye
intercalation or adsorption by the clay, indicating
structured morphology of these nanopigments.
a nano-
Thermal stability of these nanopigments was also found to vary
from system to system. NP14 showed an improvement, NP24
showed the same stability, and NP60 showed a decrease in the
thermal stability. It is thought that the intercalation of the modified
SY14 dye within the clay layers was responsible for the improve-
ment in its thermal stability. For the modified SR24 dye, the similar
thermal behaviour was due to delamination of the clay layers with
the dye adsorbed on the clay surface. Probably the lack of any
protection that would be offered by the gallery space did not cause
any improvement in its thermal behaviour. On the other hand, the
apparent deterioration of the thermal stability for the nanopigment
NP60 was attributed to the presence of a small fraction of iodide
salt of the dye in the pigment system.
[14] Bujdak J, Iyi N, Fujita T. The aggregation of methylene blue in montmorillonite
dispersions. Clay Miner 2002;37:121e33.
[15] Cione APP, Neumann MG, Gessner F. Time-dependent spectrophoto-
metric study of the interaction of basic dyes with clays. III. Mixed dye
aggregates on SWy-1 and laponite.
J Colloid Interface Sci 1998;198:
106e12.
[16] Czimerova A, Bujdak J, Gaplovsky A. The aggregation of thionine and meth-
ylene blue dye in smectite dispersion. Colloids Surf A Physicochem Eng Asp
2004;243:89e96.
[17] Endo T, Shimada M. Optical properties of dyes incorporated into clay. Stud
Surf Sci Catal 1991;60:189e96.
[18] Fischer H, Batenburg L. New nanopigments based on a combination of
clay and organic dyes. Klei Glas Keramiek 2001;22:14e5.
[19] Crisp GT, Millan MJ. Conjugate addition of amino acid side chains to dyes
containing alkynone, alkynoic ester and alkynoic amide linker arms. Tetra-
hedron 1998;54:649e66.
It was also observed that the use of the nanopigments in PP
reduced the migration of the dye, which results from the interca-
lation/adsorption of the dye within the clay based pigments.