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COOL PAVEMENT MATERIALS
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2Coating materials to increasepavement surface reflectance
N. Xie, H. Wang, D. FengHarbin Institute of Technology, Harbin, China2.1 Introduction
The urban heat island (UHI) effect is a common environmental problem occurring in
metropolitan areas in which the air temperature is significantly higher than in suburban
areas. The UHI effect also leads to a smoggy climate. The UHI effect mainly results
fromhigh-density infrastructures,which not only limit air flowbut also emit heat stored
from solar energy. In addition, the reduction of vegetation and wetlands further
weakens the heat-releasing capacity of cities.It is urgent to mitigate the UHI effect,
as it has become a huge threat to the environment and human health (Santamouris,
2007; Santamouris et al., 2011; Stathopoulou, 2008; Mirzaei and Haghighat, 2010).
Previous studies indicated that the optical and thermal characteristics of urban struc-
tures have a strong relationshipwith urban climate and temperatures (Chen et al., 2009;
White et al., 2010). In many cities, the roofs and pavements comprise about 60% of the
total urban area, with roofs contributing about 2025% and pavements contributing
about 40% (Akbari et al., 2003). Cantat indicated that the reflectance of Paris is 16%
lower than the surrounding suburban district (Cantat, 1989). Another study claimed
that the increase of the solar energy absorption in urban areas is affected by the urban
geometrical structures (Aida, 1982; Aida and Gotoh, 1982).
Cool pavement is an effective technology to solve the UHI problem. The primary
three types of cool pavements are (1) light color aggregates pavement, (2) permeable
pavement, and (3) solar-reflective coating pavement. The albedo of normal asphalt
pavement is only about 5%. By using white or light color aggregates, the albedo can
be increased to about 30% (Doulos et al., 2004). Recent case study results from
Portland State University (Oregon, USA) show that if you increase the albedo of
the pavement in a bare courtyard from 37% (black) to 91% (white), the mean radiant
temperature will increase 2.9 C, but the air temperature will decrease 1.3 C(Taleghani, 2014).
Cotana et al. (2014) believes that using high solar reflective surfaces is an effective
way to mitigate the UHI problem and reduce greenhouse gas emission in urban areas.
It has been found that albedo coatings can reduce CO2 emissions. Simulation results
indicated that a 115,000 m2 high-albedo coating area could reduce 16,000 tons of
CO2eq, releasing and effectively mitigating global warming if applied for 30 years.
Akbari and Matthews (2012) also estimated that using cool roofs and cool
pavements could increase urban albedo by about 10%, and reduce CO2 emissions
Eco-efficient Materials for Mitigating Building Cooling Needs. http://dx.doi.org/10.1016/B978-1-78242-380-5.00002-9
Copyright 2015 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.apenergy.2014.02.065http://dx.doi.org/10.1016/j.apenergy.2014.02.06514 Eco-efficient Materials for Mitigating Building Cooling Needs
by at least 40160 Gt. Yus study claimed a similar result based on a life cycle assess-
ment model (Yu and Lu, 2012). Experimental results indicated that 4500 m2 cool
pavements in an urban parking lot could decrease the ambient temperature about
2 C, and at the same time decrease the surface temperature by 12 C during a typicalsummer day (Santamouris et al., 2012).
Rosenfeld et al. (1998) claimed that cool surface techniques could save at least half
a billion dollars if used in the Log Angeles basin, and up to five billion dollars if
applied in the entire U.S. by the year 2015. Akbari et al. (2009) estimated that urban
albedo could be increased about 10% by using cool roofs and cool pavements, and
the increased urban albedo could reduce 44 Gt of CO2 emissions and save 1.1 trillion
dollars worldwide.
To implement the high-reflectance surface coating technology, proper coolmaterialselection plays themost important role. In general, the selection rule is governed by two
main parameters: solar reflectance and heat emittance (Asaeda et al., 1996; Synnefa
et al., 2006). Carnielo and Zinzi (2013) tested the emissivity and solar reflectance of
five different albedo samples with colors of green, red, blue, gray, and off-white.
It was found that, compared with thermal emittance, the effect of solar reflectance is
more evident on mitigating the urban heat island phenomenon. The spectral emissivity
values of those samples are similar to the normal asphalt, which are all above 90%,
while the solar reflectance of each sample varies a lot, with a maximum difference
of about 61% compared to the normal asphalt sample.
The visible light reflectance of the coating is mainly governed by its visible color
and surface roughness, which are determined by the inorganic fillers. Uemoto et al.
(2010) claimed that the visible light reflectance of the coating will decrease when
the color is more dark than light. Other than the visible light reflectance, the near-
infrared (NIR) light reflectance is also important for cool pavements. In the NIR
reflective region, the colors of the materials are not the dominant factor. High NIR
reflectance can be realized in various colors of inorganic fillers. Synnefa et al.
(2011) investigated five asphalt pavement overlay coatings with various colors.
The results indicated that the reflectance of the coatings with all colors are higher than
the asphalt pavement without coatings. The NIR reflectance of these coating ranges
from 27% (red and green) to 55%, which couldmaximally decrease the pavement tem-
perature about 12 C compared with the noncoated asphalt pavement surface.Kinouchi et al. (2003) developed a new pavement coating with high reflectance
and low brightness, which has a relatively low reflectance (20%) in the visible light
region (350750 nm) and high reflectance (83%) in the NIR region (7502100 nm).
The results showed that this coating could decrease the pavement temperature by
15 C compared with normal asphalt pavement. Wan et al. (2012) also developed acool pavement coating with high NIR reflectance (81%), low thermal conductivity
(0.252 W/m K), and high emissivity (82.8%). The results demonstrated that this pave-
ment could decrease surface temperature about 17 C compared with conventionalasphalt pavement and 5 C compared with cement concrete pavement.
Unlike cool roofs, which have no traffic safety and traffic load concerns, cool pave-
ment coatings with high reflectance have to satisfy the requirements of a pavement
overlay. Therefore, they are generally composed of three main parts: the organic
Coating materials to increase pavement surface reflectance 15
polymer substrate, inorganic fillers, and antiskid aggregates. The polymer substrate is
mainly for providing bonding capacity to guarantee the durability of the coatings; the
inorganic fillers are mainly used for providing the solar reflectance; and the aggre-
gates are used for transportation safety and wear resistance concerns. The following
sections will discuss how these can be used as cooling pavement materials.
2.2 Organic polymers used as coating overlaymaterials for pavements
In the past decades, various polymers were used as coating overlay materials to protect
bridge decks and pavements from chemical attacks and freeze/thaw damage. The
bonding or adhesive property is the most important factor to consider before these
polymers can be used as coating overlays.
Recently, it was reported that 2724 polymer-based overlays were applied on bridge
structures in the United States (Federal Highway Administration, FHWA, 2010).
Since 2008, the polymer overlay has increased 24.2%, whereas other surface overlays
keep decreasing every year (Young and Durham, 2012).
The requirements of cool pavement are more critical than cool roof because cool
pavement must satisfy the requirements of a pavement, including skid resistance, wear
resistance, bonding strength, and mechanical properties. Therefore, fewer types of
polymer materials can be used as the coating substrate for pavement than can be used
for cool roofs. The two main types of materials that are widely used as coating over-
lays for pavements are epoxy and acrylic polymers. This section presents information
relevant to the selection of these two types of polymers for asphalt pavement coatings.
2.2.1 Epoxy resins
Epoxy resins are used as adhesives to join two surfaces from different components by
providing a bond between them. Epoxy resins were commercially introduced in 1946
as adhesives and can be applied in many industrial areas. Epoxy resins are easily mod-
ified in order to achieve required properties. They are the most widely used adhesive
because many materials can be bonded with epoxy resins. In the epoxy industry, the
modification is called formulating or compounding. In the modification process,the curing agent and the reaction mechanisms are the key factors that guarantee the
required properties with a reasonable cost; therefore, numerous epoxy adhesive for-
mulations can be obtained. Based on the various curing agents and specific formula-
tions, the final properties of the epoxy resins can be designed in a wide range. Cured
epoxies provide magnificent mechanical properties. In addition, they also show good
resistance to oil, moisture, and many other solvents.
As demonstrated in Figure 2.1, the epoxy presents a broad group of reactive com-
pounds that are characterized by the presence of an oxirane or epoxy ring. This is a
three-member ring containing an oxygen atom bonded with two carbon atoms that
O
R CH CH2
Figure 2.1 The epoxy structure.
Chemical resistance
CH3
CH3
CH2 CH
Reactivity andadhesion
Heat resistance anddurability
Reactivity
OH
O
O
O
O
O
O
C
C
n
CH2
CHCH2 CH2
CH3
CH3
CH2 CH2CH
Flexibility
Figure 2.2 The structure and properties of epoxy resins.
16 Eco-efficient Materials for Mitigating Building Cooling Needs
have been united with some other elements or groups. More than one epoxy group can
be contained in an epoxy resin to form a molecule.
The general formula of an epoxy resin can be expressed as Figure 2.2 (Osumi,
1987). The linear polyether with epoxy groups and the hydroxyl groups along the
length of the chain contribute the chemical resistance, heat resistance, durability,
and the adhesion, in which the reactivity of the molecules is governed by the terminal
of the molecule and the hydroxyl at the midpoint, and the adhesion property of epoxy
is governed by the secondary hydroxyl groups located in the molecule chain.
The main mechanism of the epoxy curing is the so-called ring opening. In this pro-cess, the epoxy group may react in different ways. One is the anionic reaction, and the
other is the cationic reaction. Figure 2.3 shows the anionic mechanism of the ring
opening (Lee, 1967). In the cationic reaction, the epoxy group will be opened to form
a new hydroxyl bond by the introduction of an active hydrogen.
The material that reacts with the epoxy to finally form the epoxy network is called
the curing agent or hardener. The mixing content of the curing agent varies from sev-eral percent to as high as fifty percent. A large number of curing agents can react with
X
O
Epoxy group
C C
O
C C
Epoxy anion
+
X
Figure 2.3 Anionic mechanism of the epoxy reaction.
Coating materials to increase pavement surface reflectance 17
epoxy to form a cross-link structure. The curing agent, in most cases, will react with
the available epoxy or hydroxyl groups. Currently, for commercial epoxies, the
following materials are widely used as the curing agent.
l Aliphatic amines and modified aliphatic aminesl Polyamidesl Anhydridesl Polysulfides and mercaptans
The rigidity is one of the shortcomings of epoxy resins when they are used as coating
overlay materials for pavement. Therefore, mitigating this problem is the key issue in
determining whether or not the epoxy can be applied in the field. With the proper
selection of curing agent, the flexibility of inherently rigid epoxy resins can be
improved. By controlling stoichiometric mix ratio between the epoxy resin and the
curing agent, or by using flexible molecule curing agents, the epoxy formulation
can be changed and the flexibility will be adjusted accordingly. For example, by
changing the curing agent from hexahydrophthalic anhydride to hexamineethylene-
diamine, the impact resistance of a resin system can be doubled and its tensile elon-
gation increased accordingly (Epon Resin Structural Resin Manual, 2001).
There are two types of curing methods for current commercial epoxies. The first is
single component epoxies with heat curing, and the second is multiple component
epoxies curing at room temperature or at elevated temperatures. In most cases, heat
curing will lead to a relatively higher glass transition temperature and a higher cross-
linking density than curing at room temperature. As a result, the epoxies cured at high
temperatures will provide a higher shear strength and chemical stability. However,
due to their rigidity, their toughness and peeling strength will be lower than those
cured at room temperature.
Although some epoxies can be cured in minutes at room temperature, most require
relatively longer curing time (1872 h) to obtain full strength. Curing time is temperature
sensitive; it can be shortened by increasing curing temperatures. After curing, the epoxy
resins are solid materials with outstanding properties. Such properties may include low
shrinkage and no volatility during the curing process, compatibility with many types of
materials, chemical stability, and durability under a complex environment. In addition,
the properties of the epoxy resins can be controlled according to the requirements.
Flexible or rigid, high or low modulus, filled or foamed, conductive or insulativeall
of these properties can be obtained by changing the components of the epoxies.
Apart from curing time and flexibility, the viscosity of the epoxy resins is another
important factor to consider before the material can be applied in the field as a pave-
ment coating It is difficult to utilize epoxy resins with high viscosity. Currently, com-
mercial epoxy resins are available as either liquids or solids. The viscosity of the
liquid-state epoxies can range from water-like to crystalline solid. In most cases,
the molecular weight decides the viscosity of the epoxy resins. The higher the molec-
ular weight, the higher the viscosity and the melting point. Other than the molecular
weight, the viscosity of the epoxy resins is also governed by temperature. Viscosity
reduces rapidly with the decrease of temperature.
18 Eco-efficient Materials for Mitigating Building Cooling Needs
Epoxy resins were used as pavement coating materials for many years. Most epoxy
resin coatings were used as overlays with antiskid or waterproofing functions.
Recently, epoxy resin coatings have started to incorporate these functions to anti-icing
and wear-resistant surfaces.
Epoxy coatings for concrete pavement have been studied extensively, however,
research for epoxy coating on asphalt pavement is very limited due to some bottleneck
problems that are still awaiting solutions. First, the inherent brittleness of the cured
epoxy resins can easily result in breaks on the top of the asphalt pavement because
of the mismatch of the deformation capability. Second, the cost of the materials is rel-
atively high if the materials are adjusted to the required properties. Third, the curing
condition, such as relatively long curing time and high curing temperature, is critical
to pavement engineering.
2.2.2 Acrylic ester polymers
Acrylic ester polymers were discovered by Otto Rohm when he was conducting his
doctoral research in 1901, and they were commercially produced by the Rohm and
Haas Co. of Darmstadt, Germany in 1927 (Riddle, 1954). Similar to epoxy resins,
acrylic ester polymers can also be used as adhesives to join two surfaces from different
components by providing a bond between them.
Figure 2.4 gives the structure of acrylic ester monomers. The R side chain ester
group determines the final properties of the formed polymers used in various appli-
cation fields, from paint to adhesives. In general, acrylic ester monomers are formed
from the reaction between acrylic acid and an alcohol as follows:
Figu
acrylic acid + alcohol! alkyl acrylate (2.1)
ng the copolymerizing process, acrylic ester monomers will randomly incorporate
Durithemselves to form polymer chains by the percentage concentration of each monomer.
In addition, the acrylic ester monomers can also be copolymerized with methacrylic
ester monomers, styrene, acrylonitrile, or vinyl acetate to prepare other products. The
final properties of the formed polymers are determined by the molecular weight and
the ester side chain of the product. Similar to other polymers, the properties of the acrylic
ester polymers can be improved as a function of molecular weight until a threshold
is reached. Beyond this threshold, property improvement will cease. In general, for
acrylic polymers, the threshold molecular weight value is about 100,000200,000.
The polymerization of acrylic monomers is based on the chain-growth mechanism.
It is realized by the head-to-tail connection of the individual monomer units by break-
ing the monomer double bond and forming a single bond between the newly incorpo-
rated monomer units, shown in Figure 2.5.
COORC = C
H
H H
re 2.4 The structure of acrylic ester monomers.
COOR COOR COOR
R R
COOR
CH2CH CH2CHCH2 CH2 CHCH
Figure 2.5 The polymerization mechanism of acrylic monomers.
Table 2.1 Mechanical properties of acrylic polymers
Polyacrylate Elongation, % Tensile strength, kPa
Methyl 750 6895
Ethyl 1800 228
Butyl 2000 21
Stone (2010).
Coating materials to increase pavement surface reflectance 19
The mechanical properties of acrylic ester polymers are largely counting on the
side chain groups of the ester. If the side chain is H, the poly(acrylic acid) is a brittle
material, while the poly(methyl acrylate) is a strong and rubbery material at room tem-
perature. The poly(ethyl acrylate), which has a longer chain length, is more rubbery
and extensible. The poly(butyl acrylate) is tack at room temperature and is able to be
used as the adhesive. Table 2.1 lists the mechanical properties of acrylic polymers
(Stone, 2010). The flexibility of acrylic polymers increases with increasing chain
length until it reaches the threshold: poly(n-nonyl acrylate). At this chain length,the side chains will be crystallized and lead to the stiffening of the polymers.
Acrylic ester polymers are chemically stable at aggressive conditions, even with
exposure at 300400 C. This chemical stability guarantees the durability of the finalproduct.Acrylic ester polymers are also resistant to oxidation andhydrolysis.However,
this resistance is not universal. When exposed to strong acidic or alkaline conditions,
acrylic ester polymers will be hydrolysed to poly(acrylic acid) and alcohol. The resis-
tance to hydrolysis is in the order of butyl acrylate>ethyl ethyl acrylate>methylacrylate.
UV radiation resistance is another important factor that determines the durability of
acrylic polymers as a pavement coating material. In general, acrylic polymers have
strongUV resistance; however, if they are incorporated with UV-absorbing monomers
such as styrene, the UV resistance of the resulting products will be considerably
decreased and will deteriorate rapidly. On the other hand, if the incorporating UV-
absorbing monomers are bonded in a noncovalent way, the UV resistance of the result-
ing polymers will not be affected. For example, hydroxybenzophenone will further
enhance the UV resistance of acrylic polymers (Burgess, 1952).
In the past decades, acrylic polymers were widely used as marking paint materials
in pavement engineering. Recently, they have begun to be used as overlay materials
for color pavements. Acrylic polymer coatings have been studied as an antiskid over-
lay for concrete pavement (Scholer and Forrestel, 1980); however, research on acrylic
polymers as coating materials on asphalt pavement is limited. Except for their rela-
tively low adhesive or bonding strength, acrylic polymers have the advantage when
compared to epoxy resins. First, the flexibility of cured acrylic polymers is far better
20 Eco-efficient Materials for Mitigating Building Cooling Needs
than the flexibility of epoxy resins, which makes the former able to match pavement
deformation during service time. Second, the chemical and UV stabilities are prom-
ising advantages of acrylic polymers when used as overlay materials for pavements.
Third, the relatively low cost of acrylic polymers guarantees their wide application in
pavement engineering.
2.2.3 Advantages and disadvantages of various polymers
Both epoxy and acrylic polymers provide advantages and disadvantages in their use as
coating materials for pavements. Epoxy polymers typically bear the benefits of highfatigue strengths, high temperature properties, and chemical stability; however, during
implementation, the surfaces that need to be joined have to be carefully cleaned.
In addition, the curing time is relatively long and the curing temperature is sometimes
high. Apart from the curing problems, the rigidity of the cured epoxy is another prob-
lem when used as a coating material for asphalt pavement. Acrylic ester polymerstypically have good plasticity, which is a merit for asphalt pavement. They are also
chemically stable in aggressive environments. However, both epoxy and acrylic poly-
mers are not UV resistant. UV ageing is still the bottleneck problem that needs to
be solved.
2.3 Inorganic materials used as polymer fillersto increase reflectance
When an electromagnetic wave irradiates on a substance, the incident wave Il can be
divided into three categories: absorbed part, transmitted part, and reflected part, which
are represented as Al, Tl, and Rl, and can be expressed as:
Il aAl + bTl + gRl (2.2)
re a, b, and g are the fraction of the absorption, reflection, and transmission,
wherespectively, and the sum of them should be 1.All solid-state inorganic materials have their own specific electron energy band
structures. If the incident light with energy of Ehumatches the bandgap of a specificmaterial, the electrons in the valence band will be excited and jump to the conduction
band to form free electrons and leave holes in the valence band. In this process, light
energy will be absorbed and converted to thermal energy. As an electromagnetic
wave, sunlight has a wide range of wavelengths, varying from 295 to 2500 nm. Once
the sunlight has been absorbed by materials with a selective wavelength, it will be
converted into heat energy.
UV light is not visible to the naked eye. Its wavelength ranges from 295 to 400 nm.
UV light contributes only about 5% of solar energy; however, it is the main reason for
the asphalt binders or polymer coatings ageing or degradation.
Visible light, whose wavelength ranges from 400 to 700 nm, combines various
colors, and each color corresponds to its own wavelength. Approximately half of
Coating materials to increase pavement surface reflectance 21
sunlight energy comes from this region. Visible light absorption or reflection is gov-
erned by the electronic band structures of specific materials. If a material can only
reflect light with a specific wavelength, then the observer can only see its correspond-
ing color. For example, if a material shows the color of red, that means the material is
selected to reflect red light. If a material shows the color of white, that means the mate-
rial is selected to reflect white light, which is the combination of all visible light
colors. If a material shows the color of black, that means the material is selected to
absorb visible light. As a result, color is the dominant factor that determines the visible
light reflectivity of materials. In most cases, light colors provide high reflectance of
the visual spectrum of radiation. However, to the infrared part, the reflectance is inde-
pendent of specific colors.
The wavelengths of near-infrared (NIR) light range from 700 to 2500 nm. Around
45% of all solar energy comes from the NIR region. Because NIR light is not visible to
the naked eye, the reflectance of NIR light is irrelevant to the colors of the materials.
As a result, the coating with high NIR reflectance could be colorful, which is good for
pavement coating because white or light color coating of the pavement will introduce
glare problems to vehicle drivers. Most NIR reflective materials are metal oxides.
Because of their chemical and environmental stability, they are widely used as pig-
ments in various paints.
2.3.1 White color materials for increasing visiblelight reflectance
Titanium widely exists in earth crust, mostly rutile, anatase, and brookite. It has been
found in many places, including rocks, water bodies, and soil. Titanium dioxide has
exhibited many outstanding advantages in daily products such as white paint, plastic,
paper, ink, food, medicine, makeup, and toothpaste. Meanwhile, it also shows some
advanced properties in environmental benefits and energy harvesting, such as photo-
catalytic water splitting, heterogeneous photocatalysis, charge transfer, Gratzel cells,
sensitized solar cells, organic photovoltaic cells, quantum dots, nanorods, nanotubes,
photocurrent generation, and electrodes (Kamat, 2012). Thanks to its high refractive
index, titanium dioxide is the most widely used nontoxic white pigment at the present
time. It has been stated that more than 4.6 million tons of TiO2 are used as white pig-
ment worldwide, and this number increases annually (Winkler, 2003).
Renz (1921) was possibly the first to discover that TiO2 is illumination sensitive.
It was found that TiO2 will be reduced with sunlight illumination. The color of white
TiO2 became gray, blue, or black. A similar phenomenon was found in CeO2, Nb2O5,
and Ta2O5. After that, the famous chalking phenomenon, which always happened
on TiO2-based paint, was successfully explained by Goodeve and Kitchener (1938).
They proposed an assumed mechanism that TiO2 will act as a photocatalyst to
accelerate the oxidation process, based on an experiment result that TiO2 powder will
help decompose a dye in air under sunlight illumination. In the past 100 years, the
properties of TiO2 have been studied extensively.
Levinson studied the reflectance of rutile TiO2 in acrylic matrix and compared it
with other oxides. It was found that TiO2 shows similar reflectance curves, with strong
22 Eco-efficient Materials for Mitigating Building Cooling Needs
backscattering and weak visible and NIR light absorption, as shown in Figure 2.6
(Levinson et al., 2005a,b). This phenomenon has also been proven by Wangs study,
as shown in Figure 2.7 (Wang et al., 2013a).
As claimed by Levinson et al. (2005b), the very sharp transition from low absorp-
tion to high absorption, which occurs at 400 nm, resulted from the electronic bandgap
structure of rutile TiO2 (3.0 eV of rutile and 3.2 of anatase). This region is the bound-
ary between the visible and ultraviolet regions. With wavelengths shorter than 400 nm
Wavelength (nm)
500 15001000 2000 2500
Obs
erve
d re
flect
ance
1
0.8
0.6
0.4
0.2
0
s = 0.87, u = 0.14, v = 0.94, n = 0.87
UV VIS NIRFigure 2.6 Spectralreflectance of 1.5 mm acrylic
paint film with rutile
TiO2 as filler (s, u, v, andn represent sum, ultra-violet,visible, and near-infrared,
respectively).
(Levinson et al., 2005a,b).
5000
20
40
60
80
100
1000 1500 2000
Wavelength (nm)
Ref
lect
ance
(%
)
Figure 2.7 Spectral reflectance
of acrylic pavement coating with
TiO2 as filler.
Coating materials to increase pavement surface reflectance 23
(photon energies higher than 3.1 eV), considerable absorption can be observed. With
wavelengths longer than 400 nm, absorption starts to decrease, and most of them
result from the polymer substrate. Due to the impurities of the TiO2 and the various
polymer substrates, the reflectance curves might have some differences.
Tayca Co. in Japan (http://www.tayca.co.jp/) found that the concentration of
rutile TiO2 in the polymer matrix will influence the reflectance performance. With
the increasing filling content, solar reflectance will reach a peak value and then
decrease.
Similar to titanium dioxide, zinc oxide (ZnO) is another important white semicon-
ductor, with a bandgap of 3.4 eV. In 1924, the photocatalytic property of ZnO was
found by Baur and Perret (1924) at the Swiss Federal Institute of Technology. It
was reported that metallic silver can be produced by silver salt combining with zinc
oxide. Baur and Perret assumed the oxidation and reduction occurred simultaneously,
expressed as:
ZnO + hu! h+ + e (2.3)
4h+ + 4OH !O2 + 2H2O (2.4)
e + Ag+ ! Ag0 (2.5)
r three years, Baur and Neuweiler (1927) found a simultaneous oxidation and
Aftereduction process to produce hydrogen peroxide on zinc oxide.
2h+ + 2OH + CH2O!CO + 2H2O (2.6)
2e + 2H+ + O2 ! H2O2 (2.7)
k and Bard (1977) found that some semiconductors can be used to purify water
Franwith photocatalytic decomposition of pollutants, and ZnO is the most active one.
It was found that the ZnO surface is more hydrophobic than the TiO2 surface (Sun
et al., 2001). In this study, the results show that the ZnO surface can be induced by
UV light to generate oxygen vacancies. Figure 2.8 shows that the water contact angle
varies from 109 to 5 on the ZnO surface, and from 54 to 0 on the TiO2 surface afterUV irradiation.
The UV/VIS light absorption performance of ZnO was reported by Lin et al.
(2005). In this study, the ZnO nanoparticles were prepared with a dc thermal plasma
reactor, as in Figure 2.9. The UV/VIS spectra of ZnO nanoparticles were prepared
under various environments. It was claimed by the authors that the doping plays an
important role in the UV/VIS light absorption performance of ZnO nanoparticles.
Apart from the preparing methods, the effects of the particle sizes and shapes of
ZnO on the UV/VIS/NIR reflectance were studied recently (Kiomarsipour et al.,
2013). ZnO particles were prepared via hydrothermal approach, and five types of mor-
phology were obtained. As demonstrated by the optical property testing results, the
nanorod and microrod particles showed relative low spectral reflectance, scale-like
http://www.tayca.co.jp/UV
(a) (b)
UV
Figure 2.8 Water contact angle of (a) ZnO and (b) TiO2 surfaces before and after UV
illumination.
Wavelength (nm)
Commercial
0.6
0.4
0.2
0.0400 450 500
R-ZnO
T-ZnO
S-ZnO
Abs
orba
nce
0.8
550 600 650 700
Figure 2.9 UV/VIS spectra
of ZnO nanoparticles
prepared under various
environments.
24 Eco-efficient Materials for Mitigating Building Cooling Needs
Coating materials to increase pavement surface reflectance 25
and submicrorod particles showed mean reflectance, and the decorated particle
showed the highest reflectance, shown in Figure 2.10.
Li et al. (2006) reported the UV/VIS light absorption performance of ZnO/epoxy
nanocomposites. In this study, ZnO was prepared with the homogeneous precipitation
method and the ZnO nanoparticles were then calcined at various temperatures. The
prepared nanocomposites containing 0.07 wt.% ZnO, with average size of 26.7 nm
and calcined at 350 C, show the highest absorbance of UV/VIS light, shown inFigure 2.11. (In this figure, pure represents the ZnO nanoparticles without calcination,and Z3Z8 represent the prepared ZnO calcined at various temperatures). Figure 2.12
Wavelength (nm)
Microrods ZnO
20000 500 1000 1500 2500
Nanorods ZnO
Nanoparticle-decorated ZnO Submicrorods ZnO
Scale-like ZnO
100
80
60
40
20
0
Ref
lect
ance
(%
)
Figure 2.10 UV/VIS/NIR reflectance spectra of various shapes of ZnO particles.
Wavelength (nm)
300
(a) (b)400
Z5
Z5
Z4 Z4
Z3
Z3Z8
Z6
Z8Z7
Z7
Z6
Tran
smitt
ance
(%
)
Abs
orba
ncePure
Pure
500 600 700 800
Wavelength (nm)
300 400 500 600 700 800
00.0
0.5
1.0
1.5
2.0
2.5
3.0
20
40
60
80
100
Figure 2.11 UV/VIS spectra of ZnO/epoxy nanocomposites containing 0.07 wt.% ZnO
nanoparticles: (a) transmittance, and (b) absorbance.
5000
20
40
60
80
100
1000 1500
Wavelength (nm)
Ref
lect
ance
(%
)
2000
Figure 2.12 Spectral reflectance
of acrylic pavement coating with
ZnO as filler.
26 Eco-efficient Materials for Mitigating Building Cooling Needs
shows the spectral reflectance of acrylic pavement coating with ZnO as filler
(Wang et al., 2013a). As seen in this figure, the reflectance curve has a similar trend
to TiO2 but a relatively lower reflectance.
2.3.2 Various color materials for increasing NIR reflectance
Near-infrared reflectance of various metal oxide nanoparticles was tested by Jeeva-
nandam (2007). In this study, five metal oxides, i.e. CeO2, MgO, TiO2, Al2O3, and
ZnO, were investigated. Apart from the types of materials, the size effects on the
NIR reflectance were studied as well. Figure 2.13 demonstrates the testing results
of the relative NIR reflectance spectra of nanocrystalline oxides and macrocrystalline
of various oxides: (a) CeO2, (b) TiO2, (c) MgO, (d) Al2O3, and (e) ZnO (the reflec-tance of PTFE is about 100). The authors also claimed that the relative reflectance ofthese metal oxides were influenced by many other factors, such as shape, distribution,
porosity, and chemical composition of the nanoparticles.
In most cases, the oxides with high NIR reflectance were prepared by doping
various elements in binary metal oxides or composing binary metal oxides to form
complex metal oxides. Generally, the doping process can be realized by calcina-
tion, plasma sintering, or chemical approaches, including hydrothermal or aqueous
co-precipitation methods (Xie et al., 2012). Once the binary metal oxides have
been doped or composed with other oxides, their bandgaps will be changed,
and as a result, the color and the NIR reflectance performance will be changed
accordingly.
The NIR reflectance of the light color yttrium cerate (Y2Ce2O7) doped with Mo or
Pr was investigated (Sarasamma Vishnu and Lakshmipathi Reddy, 2011). The doping
process was realized by calcining Y2O3, CeO2 and (NH4)6Mo7O24-4H2O or Pr6O11 at
1500 C. After doping with Mo, the bandgap of Y2Ce2O7 changed from 3.01 eV to2.44 eV, and the color changed from ivory white to light yellow. On the other hand,
if doped with Pr, the bandgap of Y2Ce2O7 changed from 3.01 eV to 1.70 eV, and the
110
PIFE100
90
80
70
60
110
100 100
105
105
100
MicrocrystallineZnO
MicrocrystallineAl2O3
Microcrystalline TiO2Microcrystalline CeO2
MicrocrystallineMgO
Nanocrystalline ZnO
Nanocrystalline Al2O3
Nanocrystalline TiO2Nanocrystalline CeO2
Nanocrystalline MgO
Rel
ativ
e re
flect
ance
Rel
ativ
e re
flect
ance
Rel
ativ
e re
flect
ance
Rel
ativ
e re
flect
ance
Rel
ativ
e re
flect
ance
95
90
85
110
95
90
85
80
75
70
90
80
70
60
50
110
100
90
80
70
60
750 1000 1250 1500
Wavelength (nm) Wavelength (nm)
1750 2000 2250 2500 750 1000 1250 1500 1750 2000 2250 2500
750
(a) (b)
(c)
(e)
(d)
1000 1250 1500
Wavelength (nm) Wavelength (nm)
1750 2000 2250 2500
750 1000 1250 1500
Wavelength (nm)
1750 2000 2250 2500
750 1000 1250 1500 1750 2000 2250 2500
Figure 2.13 Relative NIR reflectance spectra of nanocrystalline oxides and macrocrystalline of
various oxides: (a) CeO2, (b) TiO2, (c) MgO, (d) Al2O3, and (e) ZnO (the reflectance of PTFE is
about 100).
Coating materials to increase pavement surface reflectance 27
28 Eco-efficient Materials for Mitigating Building Cooling Needs
color changed from ivory white to dark brown, as shown in Figure 2.14. The NIR
reflectance testing results show that NIR reflectance stays relatively high with both
Pr and Mo doping (Figure 2.15). Similarly, Zhao et al. (2013) studied the Fe doped
Y2Ce2O7 and found that this yellow color pigment has a high NIR reflectance
(>80.6%).Most recently, Zou investigated the NIR reflectance of rutile TiO2 co-doped with
Cr and Sb (Zou et al., 2014). The doping process was realized by the aqueous co-
precipitate approach followed by calcination. It was found that, after co-doping with
Cr and Sb, the color of rutile TiO2 changed from white to yellow and dark green with
different dosage of the doping elements, as shown in Figure 2.16; their reflectance
performance is shown in Figure 2.17. Other than the color adjustment, the temperature
testing results show that the inner ceramic tile with this coating had a maximum 10 Clower than the one without the coating.
Similar results were obtained recently (Wang et al., 2013b), as shown in
Figure 2.18. NiTiO3 nanoparticles were obtained by a polymer-pyrolysis route. It
was found that the high reflection peak occurs at about 580 nm, which gave them
the yellow color, and the NIR reflectance of these yellow particles is about 60%.
Mo6+
Pr4+
Figure 2.14 Photographs of Y2Ce2O7 doped with Mo and Pr at various dosages.
750 1000 1250 1500 1750
Wavelength (nm)
2000 2250 2500 750 1000
20
(b)(a)
40
60
80
100
70
80
90x = 0
x = 0.1
x = 0.2
x = 0.3
x = 0.4
x = 0.5
x = 0.5x = 0.4x = 0.3x = 0.2x = 0.15x = 0.1x = 0.05x = 0
100
1250 1500 1750
Wavelength (nm)
Ref
lect
ance
(%
)
Ref
lect
ance
(%
)
2000 2250 2500
Figure 2.15 NIR reflectance performance of Y2Ce2O7 doped with (a) Mo, and (b) Pr.
Wavelength (nm)
80
60
40
20
0500 1000 1500 2000 2500
TiO2Cr-Sb-1.25Cr-Sb-2.5Cr-Sb-5Cr-5
Ref
lect
ance
(R
%)
Figure 2.17 UV/VIS/NIR reflectance performance of rutile TiO2 co-doped with Cr and Sb.
Cr-Sb-5Cr-Sb-2.5Cr-Sb-1.25 Cr-5
Figure 2.16 Photographs of Cr and Sb doped rutile TiO2 pigment.
Coating materials to increase pavement surface reflectance 29
Other than red and yellow, the NIR reflectance of nontoxic blue complex metal
oxides with dopants were developed recently (Jose and Reddy, 2013). In this study,
the complex metal oxides Sr1-xLaxCu1-yLiySi4O10 (xy ranges from 0 to 0.5) wereprepared by calcining the mixture of La2O3, SrCO3, CuO, Li2CO3, and SiO2 at
950 C for 16 h. The obtained blue pigments are shown in Figure 2.19. After the reac-tion, the bandgap of the light blue SrCuSi4O10 changed from 2.59 eV to 2.68 eV. The
NIR reflectance testing results show that the NIR reflectance of the prepared coating
on cement concrete and poly(methyl acrylate) (PMMA) surfaces is about 67% and is
thermally stable (shown in Figure 2.20).
Most recently, Aceto et al. (2014) summarized the NIR reflectance performance
of widely used inorganic pigments with various colors. Nearly 60 types of
pigments, which belong to nine categories, were systematically studied. Typical
colors of specific materials were reviewed, and their reflectance performances were
presented.
Wavelength (nm)
NiTiO3- 800 C
NiTiO3- 600 C
NiTiO3
Ref
lect
ance
(%
)
4000
10
20
30
40
50
60
70
80
90
600 800 1000 1200 1400 1600 1800 2000 2200 2400
Figure 2.18 Solar radiation reflection of NiTiO3 yellow nanoparticles.
Figure 2.19 Photographs of the Sr1-xLaxCu1-yLiySi4O10 (xy ranges from 0 to 0.5) pigment.
30 Eco-efficient Materials for Mitigating Building Cooling Needs
Wavelength (nm)
x = y = 0.4x = y = 0.3
x = y = 0.1x = y = 0
x = y = 0.2
x = y = 0.5
750 1000 1250 1500 1750 2000 2250 2500
100
80
60
40
20
0
Ref
lect
ance
(%
)
Figure 2.20 NIR reflectance spectra of blue Sr1-xLaxCu1-yLiySi4O10 (xy ranges from 0 to0.5) pigment.
Coating materials to increase pavement surface reflectance 31
2.4 Aggregate materials with high reflectance
Compared to cool roof, cool pavement coating has to face more critical conditions
when it is in service because basic pavement requirements cannot be sacrificed in
attempts to increase reflectance. In general, to avoid the skid problem, pavement coat-
ings have to incorporate with fine aggregates to satisfy the skid and wear resistance
requirements; therefore, the reflectance of the materials used as fine aggregates has to
be considered.
Currently, the most widely used fine aggregates are porcelain and silica sand. Sim-
ilar to fillers, the solar energy absorption of porcelain and silica sand are not very high.
Levinson and Akbari (2002) systematically studied 32 mixes of concrete, in which 2
types of cement, 4 types of sand, and 4 types of rock were used. The reflectance of the
sands ranged from 20% to 45%, and the reflectance of the rocks ranged from 17% to
55%. For the sand materials, the dark gray riverbed sand has the lowest reflectance
(20%), while the tan beach sand has the highest reflectance (45%). For the rock mate-
rials, the dark red volcanic rock has the lowest reflectance (17%), while the gold and
white rock has the highest reflectance (55%).Although this study gave brief reflectance
data of some aggregates, the research is still very limited and needs to be extensively
investigated in future studies.
32 Eco-efficient Materials for Mitigating Building Cooling Needs
2.5 Future trends
Future studies on coating materials used for increasing pavement surface reflectance
should be focused on three parts: modification of the polymer substrate, development
of cost-effective and nontoxic inorganic materials with high UV/VIS/NIR reflectance,
and aggregate treatment approaches to enhance reflectance.
For the polymer substrate, new polymers with low viscosity before curing, and with
splendid mechanical properties and plasticity, high bonding strength, high wear resis-
tance, and corrosion resistance after curing have to be studied in the future. To realize
these targets, the investigationof curing agent development is oneof themost important
research topics. In addition, whether it be epoxy, acrylic, or another type of polymer,
it must be cost-effective. Furthermore, the anti-ageing performance of the polymer
substrate under UV irradiation needs to be increased, especially after filled with high
reflective pigments.
For inorganic materials as fillers of pavement coatings, in spite of various complex
metal oxides being studied systematically, preparing low-cost rear earth metal free
complex metal oxides with high reflectance is still a problem because rear earth metals
are very expensive and are not allowed to be used for pavement coating on a large
scale. In addition, the requirement of a facile doping process is very important because
the application of the high reflectance pigment in pavement engineering requires a
large quantity of products. A bridge that connects the gap between lab preparation
and low-cost industrial production is another important research topic. Other than
the development of complex metal oxides, the dispersion approaches, which guaran-
tee the well dispersion of the fillers in polymer substrates and thus determine the final
properties of the coatings, have to be extensively investigated. In addition, although
many inorganic materials with high UV/VIS/NIR reflectance have been developed,
their application as pavement coatings is still very limited. Furthermore, several fac-
tors, including measuring system, wind speed, cloudiness, and air temperature have
been reported, with evident effects on the diurnal and seasonal changes of the pave-
ments albedo (Li et al., 2013). Therefore, the relationship between albedo perfor-
mance and temperature reduction of coated pavements, considering the
aforementioned factors, should be systematically studied.
For the aggregates investigation, future studies should be focused on the development
of simple surface treatment methods for low reflectance aggregates. The surface-treated
aggregates should not only show increased reflectance, but also high durability and
chemical stability.
Acknowledgments
This work is financially supported by the ministry of education program for New
Century Excellent Talents (NCET-08-0163) and the China Postdoctoral Science
Foundation (No. 20110491065).
Coating materials to increase pavement surface reflectance 33
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