Consideration of roles of commercial TiO2 pigments in aromatic polyurethane coating via the photodegradation of dimethyl toluene-2,4-dicarbamate in non-aqueous solution

Article abstract of DOI:10.1007/s11164-014-1859-3

Dimethyl toluene-2,4-dicarbamate (2,4-TDC) was selected as a model compound for aromatic polyurethane to investigate the photo-chemical behavior of commercial TiO₂ pigments in non-aqueous solution. The UV-Visible spectrometry analysis results showed that the UV-shielding ability of the rutile TiO₂ pigment was better than that of the anatase TiO₂ pigment. Photodegradation experiments suggested that the photodegradation of 2,4-TDC was retarded by rutile TiO₂ pigment, while accelerated by anatase TiO₂ pigment. With the help of the degradation intermediates during the photodegradation process and calculated data, such as point charges and bond length, the preliminary photodegradation mechanism of 2,4-TDC was also briefly elucidated, including the addition of hydroxyl radicals and the cleavage of the carbamate side chain. Additionally, the photodegradation of 2,4-TDC was used to evaluate the photoreactivity of TiO₂ pigments, this is proved to an efficient approach with potential application in industry.

Full text of DOI:10.1007/s11164-014-1859-3

Res Chem Intermed
DOI 10.1007/s11164-014-1859-3
Consideration of roles of commercial TiO2
pigments in aromatic polyurethane coating
via the photodegradation of dimethyl toluene-
2,4-dicarbamate in non-aqueous solution
•
Shuolin Zhou Qiong Xu Jiafu Xiao
•
•
•
Wenzhou Zhong Ningya Yu Steven Robert Kirk
•
•
•
Tao Shu Dulin Yin
Received: 25 July 2014 / Accepted: 24 October 2014
Ó Springer Science+Business Media Dordrecht 2015
Abstract Dimethyl toluene-2,4-dicarbamate (2,4-TDC) was selected as a model
compound for aromatic polyurethane to investigate the photo-chemical behavior of
commercial TiO2 pigments in non-aqueous solution. The UV–Visible spectrometry
analysis results showed that the UV-shielding ability of the rutile TiO2 pigment was
better than that of the anatase TiO2 pigment. Photodegradation experiments sug-
gested that the photodegradation of 2,4-TDC was retarded by rutile TiO2 pigment,
while accelerated by anatase TiO2 pigment. With the help of the degradation in-
termediates during the photodegradation process and calculated data, such as point
charges and bond length, the preliminary photodegradation mechanism of 2,4-TDC
was also briefly elucidated, including the addition of hydroxyl radicals and the
cleavage of the carbamate side chain. Additionally, the photodegradation of 2,4-
TDC was used to evaluate the photoreactivity of TiO2 pigments, this is proved to an
efficient approach with potential application in industry.
Keywords Titanium dioxide pigment Á Polyurethane Á Dimethyl toluene-2,4-
dicarbamate Á Photodegradation Á Mechanism
Electronic supplementary material The online version of this article (doi:10.1007/s11164-014-1859-
3) contains supplementary material, which is available to authorized users.
S. Zhou Á Q. Xu (&) Á J. Xiao Á W. Zhong Á N. Yu Á S. R. Kirk Á T. Shu Á D. Yin (&)
National and Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine
Utilization of Resources, Key Laboratory of Resource Fine-Processing and Advanced Materials of
Hunan Province and Key Laboratory of Chemical Biology and Traditional Chinese Medicine
Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan
Normal University, Changsha 410081, People’s Republic of China
e-mail: xuqiong139@126.com
D. Yin
e-mail: dulinyin@126.com
123

S. Zhou et al.
Introduction
Polyurethane (PU), a polymer connected by the –NHCOO– group, is a well-known
coating material which is used in a large variety of commercial and technical
applications owing to its excellent adhesion, low temperature flexibility and good
chemical, mechanical and physical properties [1, 2]. The aromatic types are,
however, very sensitive to exposure to UV light, which greatly affects their optical
and mechanical properties and thus restricts the scope of application [3]. In order to
suppress degradation and prolong the service life, UV absorbers should be added
into the polymer, which can protect the backbone resin against photodegradation
reactions.
Inorganic materials have been used as the UV absorbers in PU polymers due to
their significant UV-protective properties, the most common of which are TiO2,
ZnO and CeO2 [4–7]. TiO2, as a white pigment, is widely used in various
applications in our daily life such as paints, coatings and plastics due to its
outstanding covering power and UV absorption performance. In recent years, TiO2
pigment has gradually replaced other materials owing to the higher light scattering
and lower cost than other metal oxides, while also being transparent and colorless
[8, 9]. TiO2 is, however, a semiconductor oxide, thus showing photocatalytic
activity upon UV radiation absorption, which causes severe damage to most
polymers via a photochemical reaction [10–12]. In order to suppress the
photoreactivity of titanium dioxide while maintaining its UV-shielding perfor-
mance, inert oxides such as SiO2, Al2O3, ZrO2 and CeO2 have usually been applied
as coatings on titanium dioxide surfaces [13–15]. Unfortunately, the photoreactivity
of titanium dioxide pigment is still one of the most important influential factors in
degradation of coatings [16, 17]. It is therfore necessary to understand the role of
TiO2 pigments in the process of PU degradation.
Previous studies have focused on some physical property of the polymer such as
weight loss, discoloration, and gloss retention, for evaluating the performance of
titanium dioxide pigments through artificial weathering tests or outdoor exposure
tests [18–20]. Changes in these physical properties, however, often take hundreds or
thousands of hours to become apparent and also are easily influenced by operating
conditions. FT-IR spectroscopy has been used to monitor the evolution of CO2
during the photodegradation of coating films pigmented with TiO2 [17, 21]. This
method is also somewhat tedious and time-consuming. Furthermore, the above-
mentioned studies were not readily able to identify the intermediate products and
O
C
O
C
HN
HN
O
OR
O
C
O
C
O
N
O
N
H
H
n
2,4-TDC
Fig. 1 Typical structures of aromatic polyurethane and 2,4-TDC model compound
123

Consideration of roles of TiO2 pigments
deduce reaction mechanisms. This task can be greatly simplified by using the liquid
phase; for example, n-hexane was used as reaction medium for its inertness under
UV illumination [22].
Dimethyl toluene-2,4-dicarbamate (2,4-TDC) is considered to be an important
unit of aromatic PU. The typical chemical structures of PU and 2,4-TDC are shown
in Fig. 1, and the photochemistry performance of PU was revealed by that of 2,4-
TDC. In this work, 2,4-TDC was chosen as a model compound for aromatic PU, and
the photodegradation of 2,4-TDC was investigated in the solution of n-hexane.
Commercial TiO2 pigments were used and characterized by UV–Vis spectroscopy
and FT-IR spectroscopy. Moreover, the change of concentration of 2,4-TDC was
conveniently monitored by HPLC. Based on the identified intermediates and
theoretical data, the preliminary degradation mechanism of 2,4-TDC was also
elucidated in order to understand the degradation of PU materials containing TiO2
pigments.
Materials and methods
Chemicals and reagents
The toluene-2,4-diisocyanate (purity [99 %) was obtained from Aladdin Reagent
and used as received without further purification. Methanol and tetrahydrofuran
(THF) were purchased from Sigma (HPLC grade). Petroleum ether and n-hexane
were analytical grade reagents. Water used in the laboratory was prepared by a
Millipore Milli-Q ultrapure water-purification system. A variety of TiO2 pigments
(P1–P4) with different properties are listed in Table 1.
Synthesis of 2,4-TDC
2,4-TDC was prepared in our laboratory as follows: toluene-2,4-diisocyanate
(0.037 mol) was slowly dropped into a three-necked flask which contained
0.075 mol of methanol and 10 ml of petroleum ether at room temperature. There
was a drying tube of calcium chloride on the flask to protect against moisture. The
resulting product was then recrystallized with ethyl alcohol (95 %) to get the desired
product. The purity of 2,4-TDC was 99 % analyzed by HPLC.
TiO2 pigment characterization
The UV–Vis transmittance spectra of TiO2 pigments were obtained on a Hitachi
U-3010 spectrophotometer with an integrating sphere. The spectra were measured
with a scan step of 0.5 nm in the range 200–800 nm with BaSO4 used as the
reference under ambient conditions. The FT-IR spectra of the samples were
collected by the KBr pellet technique on a Nicolet 370 Fourier-transform infrared
spectrophotometer in the range 500–4,000 cm-1
.
123

S. Zhou et al.
Table 1 Main characterization of commercial TiO2 pigments
TiO2 pigment
P1a
Composite
Production process
Sulfate process
Applications
TiO2 (anatase, [98 %)
Al/Si coating
Paints, fiber, rubber,
metallurgy, etc.
P2a
P3a
TiO2 (rutile, uncoated)
Sulfate process
Sulfate process
–
TiO2 (rutile, [92 %)
Al/Si coating
Coatings, rubber,
plastic, etc.
P4b
TiO2 (rutile, [93 %)
Al/Si and organic coating
Chloride process
Coatings, rubber,
plastic, etc.
a
Hunan Zhuzhou Chemical
DuPont
b
Irradiation procedures
The experimental device of the photochemical reaction is schematically shown in
Fig. 2. The photodegradation of 2,4-TDC was carried out in a quartz reactor (total
volume ca. 70 ml) with a rubber stopper to avoid solvent evaporation. The light
source was a 300-W medium-pressure mercury lamp (kmax = 365 nm; Philips) with
a double-walled cooling-water jacket housed in one side of the reactor to provide
the irradiation. The distance between light source and photo reactor was 22 cm.
Tetrahydrofuran (THF) was used as a co-solvent to enhance solubility of 2,4-
TDC in n-hexane. A volume of 35 ml of solvent (THF: n-hexane = 3:7, v/v)
solution containing 100 lmol l-1 of 2,4-TDC was added to the quartz reactor with
35 mg of TiO2 pigment powder. The suspension was then dispersed by ultrasonic
treatment for 5 min. Before exposure to UV radiation, the suspension was stirred in
the dark for 30 min. During the photodegradation process about 2 ml of the reaction
solution was obtained at regular intervals, centrifuged and filtered through 0.22-lm
Millipore membrane filter to remove TiO2 particles. The initial concentration (C0)
and the reaction concentration of 2,4-TDC at reaction time t (Ct) were analyzed by
HPLC. The photodegradation experiments were conducted three times in identical
conditions, and the maximum relative error in the measured concentrations was
\5 %. All experiments were performed at 298 K.
Analyses
The concentration of 2,4-TDC was monitored by an Agilent 1200 series HPLC
equipped with a Venusil XBP C18 column (l = 250 mm, i.d. = 4.6 mm,
particles = 5 lm). The mobile phase was composed of 50 % methanol and 50 %
water with a flow rate of 0.8 ml min-1. The detection wavelength was 232 nm.
Degradation intermediates were identified by HPLC–MS using a Shimadzu
HPLC system with a Venusil XBP C18 column, SIL-HT autosampler, LC-10 AT
vacuum pump and API 3000 mass analyzer. HPLC separations were performed at
0.4 ml min-1 and 50 % H2O and 50 % CH3OH. An electrospray interface (ESI)
123

Consideration of roles of TiO2 pigments
Fig. 2 Schematic diagram of the experimental device
was used for the MS measurements in positive ionization mode and full scan
acquisition between m/z 50–300.
Theoretical calculations
Theoretical calculations were carried out using the Gaussian 09 program (Gaussian)
with the optimal conformation having a minimum energy obtained at the B3LYP/6-
31?G (d, p) level [23]. The point charges (PC) were calculated with the help of
natural bond orbital (NBO) to predict the chemisorption position of 2,4-TDC onto
the surface of TiO2 pigment.
Results and discussion
UV–Vis spectra analysis
The UV–Vis spectra of 2,4-TDC and the TiO2 pigments are shown in Fig. 3. The
increase in transmittance in the UV region (280–400 nm) suggests the decrease of
UV-shielding ability. The transparency of TiO2 pigments was about 6–22 % in the
region of 280–400 nm due to their absorbance and UV scattering ability, while the
transparency of 2,4-TDC solid powder was nearly close to 100 % in the region of
300–400 nm. It can be concluded that TiO2 pigments have the ability to provide UV
protection in the PU coating. The UV-shielding ability of P3 pigment decreased
slightly as compared to that of the original P2. This may occur because amorphous
Al2O3 and SiO2 on the TiO2 particles surface can decrease the scattering coefficient,
thus decreasing the UV-shielding ability [24]. Additionally, the UV-shielding
ability of the rutile TiO2 pigment was better than that of the anatase TiO2 pigment
because the former absorbed UV light up to 400 nm, while the latter only showed
significant absorption up to 380 nm. Importantly, the rutile phase pigment was
likely to show much lower apparent photoreactivity than the anatase phase pigment
under the UV irradiation [17, 25], suggesting that the rutile-based pigment can be
used as stabilizer and effective UV absorber in the polymer coatings.
123

S. Zhou et al.
100
80
60
40
20
0
2,4-TDC
P1
P3
P2
P4
300
400
500
600
700
800
Wavelength (nm)
Fig. 3 UV–Vis spectra of 2,4-TDC and TiO2 (P1–P4) particles
FT-IR analysis
FT-IR spectroscopy was applied to characterize TiO2 surface groups. FT-IR spectra
of TiO2 samples are displayed in Fig. 4. The wide peak at 500–800 cm-1
corresponds to the Ti–O–Ti bending mode of TiO2. All samples except P2 have
bands at 1,080 cm-1 and 1,600 cm-1 which are the characteristics of SiO2 and
Al2O3, respectively [26]. The broader band at 3,460 cm-1 is attributed to the
stretching vibration and bending vibration of hydroxyl groups on TiO2 [27, 28].
Another peak at 1,630 cm-1 is attributed to the bending vibration of H–O–H on
absorbed water [29]. In Fig. 4, it is clearly shown that the hydroxyl groups on TiO2
pigments surface are abundant. At the beginning of photocatalytic degradation
reaction induced by TiO2 particles, the photogenerated holes (h?) would oxidize
hydroxyl groups or the water molecules absorbed on TiO2 surface to ÁOH radicals
[28]. ÁOH radicals are believed to be highly reactive in the decomposition of organic
molecules. The TiO2 particles are therefore likely to participate in the degradation
of organic substrate to a certain extent under UV irradiation if the surface of the
TiO2 particles is not coated properly.
Photoreactivity index
Figure 5 shows the typical kinetic curves of photodegradation of 2,4-TDC in the
presence of different TiO2 pigments. It is shown that 2,4-TDC was easily
decomposed under UV irradiation in the absence of TiO2 pigment, while the rutile-
based TiO2 pigments (P2–P4) retarded the photodegradation of 2,4-TDC to various
degrees. The results indicated that 2,4-TDC was susceptible to be degraded, which
was consistent with the characteristic of PU materials [3, 6, 30]. On the other hand,
the rutile TiO2 pigments can effectively shield and absorb UV light, thus inhibiting
123

Consideration of roles of TiO2 pigments
P1
100
80
60
40
20
0
P2
P4
P3
4000
3500
3000
2500
2000
1500
1000
500
Wavelength (nm)
Fig. 4 FT-IR spectra of TiO2 (P1–P4) particles
1.0
0.8
0.6
0.4
0.2
0.0
0
15
30
45
60
75
90
Irradiation time (min)
Fig. 5 The curves of photodegradation of 2,4-TDC as a function of irradiation time in the presence of P4
(filled square), P3 (filled circle), P2 (filled triangle), no TiO2 pigments (inverted triangle), P1 (filled
diamond). Reaction conditions: initial concentration of 2,4-TDC, 100 lmol l-1; TiO2 pigment dosage,
1.0 g l-1; THF: n-hexane = 3:7 (v/v)
or reducing the degradation rate of 2,4-TDC. Furthermore, the rutile-based TiO2
pigments coated by inert inorganic oxides (usually SiO2 and Al2O3) showed more a
significant inhibitory effect on the degradation of 2,4-TDC compared with naked
rutile titanium dioxide. The reason for this phenomenon is that silica oxide and
alumina oxide on the TiO2 surface can further prevent the adsorbed organic moieties
interacting with photoactive species. On the contrary, it is observed that the
degradation rate of 2,4-TDC was enhanced in the solvent system of THF and n-
hexane after 90 min irradiation using 1.0 g l-1 of TiO2 pigment P1. This indicated
123

S. Zhou et al.
Table 2 The photodegradation rate of 2,4-TDC using different commercial TiO2 pigments
TiO2 pigment
First order rate
constant (min-1
Standard deviation
values R
Photoreactivity
index (%)
)
–
0.0260
0.0391
0.0205
0.0164
0.0098
0.9998
0.9962
0.9966
0.9988
0.9941
–
P1
P2
P3
P4
100
52.4
41.9
25.1
that the TiO2 pigment P1, in the form of anatase, has a significant photoreactivity
under UV irradiation in spite of the surface treatment. Compared with the results
obtained from P1 and P3, it can be deduced that the crystal form of TiO2 was a
predominant factor affecting its photoreactivity.
The photodegradation of 2,4-TDC follows a pseudo-first-order kinetic equation,
which is confirmed by the linear behavior of -ln(Ct/C0) as a function of irradiation
time, as shown in Fig. S1. The kinetic parameters are listed in Table 2. The
degradation rates can be used to compare the photoreactivity of TiO2 pigments. The
TiO2 pigment P4 was found to give the lowest rate of degradation, suggesting that
appropriate inorganic coating in combination with organic surface treatment can
effectively reduce the photoreactivity of titanium dioxide particles to an extremely
low level. Moreover, the photoreactivity of the commercial TiO2 pigments
decreases in the order P1 [ P3 [ P4 which correlates with the corresponding
increasing order of their UV-shielding ability. From these results, it can be seen that
the rutile TiO2 pigment can act as a UV stabilizer, whereas the anatase TiO2
pigment (P1) operates primarily as a photosensitiser.
Photodegradation mechanism
In order to understand the degradation of PU coatings containing TiO2 pigment
under outdoor exposure, the photodegradation mechanism of 2,4-TDC was also
elucidated by HPLC–MS combined with theoretical calculation of PC and bond
length [31, 32]. The experiments were conducted using 100 lmol l-1 solutions of
the parent compounds in the presence of TiO2 pigment P1 under UV light
irradiation, and the main degradation intermediates were identified. Six degradation
intermediates were detected during the photodegradation process. The total ion
chromatogram of the degradation products and their corresponding mass spectra are
given in Figs. S2 and S3, respectively. The molecular ions and corresponding
molecular structures are listed in Table 3.
The degradation intermediate (I) with m/z = 209 may be attributed to the loss of
–OCH3 from the carbamate group of the side chain. The degradation intermediate
(II) with m/z = 151 may be generated from (I) by further loss of –COOCH3 during
the photodegradation process. The species with m/z = 253, labeled as intermediate
(III), may be attributed to the oxidation of methyl group on the aromatic ring. The
degradation intermediate (IV) with m/z = 256 was found, corresponding to addition
123

Consideration of roles of TiO2 pigments
Table 3 Summary of molecular ions and corresponding chemical structures for photodegradation
intermediates
Compound
I
Retention time (min)
40.21
EI–MS ([M?H]?)
Chemical formula
O
209
HN
C H
O
C
O
N
H
O
II
5.51
3.84
151
253
255
HN CH
H2
N
O
C
III
IV
HN
O
O
C
H
N
O
CHO
O
29.30
HN
OH
C O
O
O
C N
H
NH2
V
7.04
7.50
197
139
O
O
C N
H
OH
NH2
VI
H2
N
OH
of 16 mass units to the parent molecule, which may be attributed to a
monohydroxylated intermediate. In addition, intermediates (V) and (VI) may
originate from the intermediate (IV) that loses one and two –COOCH3 fragments
during the photodegradation process, respectively. Wang et al. [33] have proposed a
mechanism of the photochemical degradation of PU included the re-reactions of
radical groups formed in the scission reactions. However, the degradation
intermediates formed from the re-reactions of radical groups was not detected in
our work, probably due to their low concentrations.
TiO2 was stimulated under UV light irradiation at a wavelength of 365 nm to
generate photoholes, which would immediately attack the parent molecule that
absorbed on the TiO2 pigment surface [34, 35]. The exact structures of the
degradation intermediates, however, remain unconfirmed. The theoretical atomic
PC of 2,4-TDC were therefore computed to explain the chemisorption pattern onto
the TiO2 pigment, and the corresponding data are listed in Table 4, from which it
can be seen that the relatively more negative PC were found at C8 (-0.724), N7 (-
0.655) and O11 (-0.652), while a more positive charge was found at C10 (0.934).
The results indicated that the methyl group and its adjacent side chain
(NHCOOCH3) on the aromatic ring are the most likely to be adsorbed onto the
TiO2 surface and attacked by photoholes [31, 36]. As illustrated in Fig. 6, at the
initial photodegradation stage, the intermediate (I) may be generated through the
scission of C–O bond by direct UV photolysis, or h? attack, or their combination on
the TiO2 pigment surface. Subsequently, the intermediate (II) may also result from
123

S. Zhou et al.
Table 4 Point charges and bond length on atoms of 2,4-TDC at the B3LYP/6-31?G(d, p) level
11
O
13
7
HN
C
10
O
15
O
1
12
6
9
1O6
C
14
N
8
2
5
H
17
4
3
Atom label
Point charges
Bond
C1–C2
Bond length
C1
–0.294
0.166
1.4047
1.3986
1.3959
1.3951
1.4170
1.3974
1.4135
1.5110
1.4133
1.3758
1.2211
1.3504
1.4426
1.3739
1.2216
1.3504
1.4414
C2
C2–C3
C3–C4
C3
–0.286
–0.207
–0.089
0.174
C4
C4–C5
C5
C5–C6
C6
C6–C1
N7
–0.655
–0.724
–0.652
0.934
N7–C6
C8
C8–C5
N9
N9–C2
C10
O11
O12
C13
C14
O15
O16
C17
C10–N7
O11–C10
O12–C10
C13–O12
C14–N9
O15–C14
O14–O16
O16–C17
–0.652
–0.574
–0.338
0.932
–0.655
–0.572
–0.337
the cleavage of the carbamate bond (C–N) in a similar way. These two types of
scissions were consistent with previously reported results [33, 37]. The presence of
hydroxyl groups and adsorbed water molecules on the surface of TiO2 particles
(Fig. 4) suggests that TiO2 particles may be stimulated by absorbing UV light to
generate ÁOH radicals. Firstly, ÁOH radicals (oxidation potential: 2.8 V) are stronger
oxidants which can oxidize organics by the abstraction of protons generating
organic radicals (RÁ), which can be further oxidized. Accordingly, we assume that
the intermediate (III) may be attributed to the oxidation of methyl on the aromatic
ring during the photocatalytic degradation process, because it was not found under
˚
UV irradiation without TiO2 pigment. Moreover, the C8–C5 bond (1.5510 A; see
˚
Table 4) is longer than the other C–C bonds (ca. 1.4 A), which indicates that this
bond may be weaker than other C–C bonds. As a result, the methyl group on the
aromatic ring seems more easily removed in the subsequent reaction. Secondly, ÁOH
radicals exhibited a high activity and can indirectly react with the parent molecular
in addition reactions. 2,4-TDC may undergo addition of a hydroxyl radical to give a
hydroxyl added intermediate (IV). This species may then undergo a stepwise
scission under these reaction conditions to give the observed hydroxylation
123

Consideration of roles of TiO2 pigments
O
O
C
HN CH
HN
O
O
C
O
C
O
N
H
O
N
H
h+
hv
OH
OH
IV
I
11
O
13
7
HN
6
C
O
15
O
1
12
10
9
N
H
16
8
O
C
2
5
14
17
NH2
O
4
3
HN CH
O
O
C N
H
hv
OH/ h+
H2N
OH
V
NH2
II
O
C
O
N
H
O
C
NH2
HN
O
O
C
H
N
CO2, CO,H2O,et al
H2N
O
CHO
OH
VI
III
Fig. 6 Proposed pathways of photodegradation of 2,4-TDC
intermediates (V and VI). These results indicate that ÁOH plays an important and
decisive role in the photocatalytic process.
Based on the results of the present study, a possible degradation pathway
involving the direct photolysis and photocatalysis degradation of 2,4-TDC in the
presence of anatase TiO2 pigment is proposed in Fig. 6. This synergistic effect
causes 2,4-TDC to degrade more easily. It is therefore believed that the aromatic PU
containing highly photoreactive anatase TiO2 particles will possess a series of
unfavorable properties, such as surface gloss loss, color change, chalking, and
cracking.
Conclusions
In summary, a simple method was developed to assess the photoreactivity of TiO2
pigments in non-aqueous solution, which in turn has implications for the application
performance of TiO2 pigments in PU coatings. The rutile TiO2 pigment, along with
high UV opacity and low photoreactivity as a UV stabilizer, retards the
photodegradation of 2,4-TDC, whereas anatase TiO2 pigment operates primarily
as an effective photocatalyst for accelerating the rate of photodegradation. On the
basis of the intermediates identified, the mechanism of photodegradation of 2,4-
TDC was proposed, thus giving important insight into the possible pathways of
photodegradation of aromatic PU coatings containing high-active TiO2 particles.
123

S. Zhou et al.
Acknowledgments The authors gratefully acknowledge generous financial support from the National
Natural Science Foundation of China (Nos. 21003043, 20976043), the National & Local United
Innovation Platform (No. 20123098), the One Hundred Talents Foundation of Hunan Province and the aid
program for the Science and Technology Innovative Research Team in Higher Educational Institutions of
Hunan Province.
References
1. J.H. Wynne, P.A. Fulmer, D.M. McCluskey, N.M. Mackey, J.P. Buchanan, ACS Appl. Mater.
Interfaces 3, 2005–2011 (2011)
2. S.S. Pathak, A. Sharma, A.S. Khanna, Prog. Org. Coat. 65, 206–216 (2009)
3. D. Rosu, L. Rosu, C.N. Cascaval, Polym. Degrad. Stab. 94, 591–596 (2009)
4. S. Sha, D. Kocaefe, C. Krause, T. Larouche, Prog. Org. Coat. 70, 170–177 (2011)
5. J. Salla, K.K. Pandey, K. Srinivas, Polym. Degrad. Stab. 97, 592–596 (2012)
6. M. Rashvand, Z. Ranjbar, S. Rastegar, Prog. Org. Coat. 71, 362–368 (2011)
7. A. Saadat-Monfared, M. Mohseni, M.H. Tabatabaei, Colloids Surf. A 408, 64–70 (2012)
ˇ
8. N. Veronovski, D. Verhovesk, J. Godnjavec, Wood Sci. Technol. 47, 317–328 (2013)
9. N. Erdem, U.H. Erdogan, A.A. Cireli, N. Onar, J. Appl. Polym. Sci. 115, 152–157 (2010)
10. L. Monney, M. Jamois-tasserie, C. Dubois, P. Lallet, F. Villa, C. Renaud, Polym. Degrad. Stab. 72,
459–468 (2001)
11. U. Gesenhues, J. Hocken, J. Vinyl, Addit. Technol. 6, 80–87 (2000)
12. H.A. Al-Turaif, J. Coat. Technol. Res. 8, 727–733 (2011)
ˇ
13. N. Veronovski, M. Lesnik, D. Verhovsek, J. Coat. Technol. Res. 11, 255–264 (2014)
14. B. Grzmil, M. Glen, B. Kic, Ind. Eng. Chem. Res. 50, 6535–6542 (2011)
ˇ
´
15. H. Gao, B. Qiao, T. Wang, D. Wang, Y. Jin, Ind. Eng. Chem. Res. 53, 189–197 (2014)
16. D.L. Wang, S.S. Watson, L. Sung, I. Tseng, C.J. Bouis, R. Fernando, J. Coat. Technol. Res. 8, 19–33
(2011)
17. D.A. Worsley, J.R. Searle, Mater. Sci. Technol. 18, 681–684 (2002)
18. W.R. Zhang, S.J. Hinder, R. Smith, C. Lowe, F. Watts, J. Coat. Technol. Res. 8, 329–342 (2011)
19. S.M. Mirabedini, M. Sabzi, J. Zohuriaan-Mehr, M. Atai, M. Behzadnasab, Appl. Surf. Sci. 257,
4196–4203 (2011)
´
20. M.H. de Sa, P. Eaton, J.L. Ferreira, M.J. Melo, A.M. Ramos, Surf. Interface Anal. 43, 1160–1164
(2011)
21. P.A. Christensen, A. Dilks, T.A. Egerton, E.J. Lawson, J. Temperley, J. Mater. Sci. 37, 4901–4909
(2002)
22. S.G. Muhamad, Int. J. Chem. 3, 142–149 (2011)
23. Gaussian 09, Revision B.01, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J.
Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W.
Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S.
¨
Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J. V. Ortiz, J. Cioslowski, D.J. Fox (Gaussian,
Inc., Wallingford, 2009)
24. Y. Zhang, H. Yin, A. Wang, M. Ren, Z. Gu, Y. Liu, Y. Shen, L. Yu, T. Jiang, Appl. Surf. Sci. 257,
1351–1360 (2010)
25. X. Chen, Z. Wang, Z. Liao, Y. Mai, M. Zhang, Polym. Test. 26, 202–208 (2007)
26. B. Erdem, R.A. Hunsicker, G.W. Simmons, E.D. Sudol, V.L. Dimonie, M.S. El-Aasser, Langmuir 17,
2664–2669 (2001)
27. Q. Luo, L. Bao, D. Wang, X. Li, J. An, J. Phys. Chem. C 116, 25806–25815 (2012)
28. C. Yu, L. Wei, J. Chen, Y. Xie, W. Zhou, Q. Fan, Ind. Eng. Chem. Res. 53, 5759–5766 (2014)
29. M. Hosseini-Sarvari, E. Sodagar, M.M. Doroodmand, J. Org. Chem. 76, 2853–2859 (2011)
30. M. Rashvand, Z. Ranjbar, Mater. Corros. 65, 76–81 (2014)
123

Consideration of roles of TiO2 pigments
31. H. Yang, S. Zhou, H. Liu, W. Yan, L. Yang, B. Yi, J. Environ. Sci. 25, 1680–1686 (2013)
32. X. Zhu, C. Yuan, Y. Bao, J. Yang, Y. Wu, J. Mol. Catal. A: Chem. 229, 95–105 (2005)
33. L. Wang, G. Liang, G. Dang, F. Wang, X. Fan, W. Fu, Chin. J. Chem. 23, 1257–1263 (2005)
34. H. Yang, J. Deng, H. Liu, W. Yan, L. Yang, B. Yi, Res. Chem. Intermed. 40, 225–238 (2014)
35. H. Yang, H. Liu, Z. Hu, J. Liang, H. Pang, B. Yi, Chem. Eng. J. 245, 24–33 (2014)
36. A. Atifi, K. Czarnecki, H. Mountacer, M.D. Ryan, Environ. Sci. Technol. 47, 8650–8657 (2013)
37. C.R. Newman, D. Forciniti, Ind. Eng. Chem. Res. 40, 3346–3352 (2001)
123