0-BK CH-Coating Materials to Increase Pavement Surface Reflectance

2Coating materials to increasepavement surface reflectance
N. Xie, H. Wang, D. FengHarbin Institute of Technology, Harbin, China
2.1 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.2014.02.065

14 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.


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.


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.


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
Duri
themselves 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).


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


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


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


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.


(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
Afte
reduction 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
Fran
with 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.



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.


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).



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.


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.


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.


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.


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).


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http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0195http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0195http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0200http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0200http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf9815http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf9815http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf9815http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0205http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0205http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0210http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0210http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0215http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0215http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0215http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0215http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0220http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0220http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0220http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0225http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0225http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0230http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0235http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0235http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0240http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0240http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf9820http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf9820http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0245http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0245http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0245http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0245http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0245http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0245http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0250http://refhub.elsevier.com/B978-1-78242-380-5.00002-9/rf0250Coating materials to increase pavement surface reflectanceIntroductionOrganic polymers used as coating overlay materials for pavementsEpoxy resinsAcrylic ester polymersAdvantages and disadvantages of various polymersInorganic materials used as polymer fillers to increase reflectanceWhite color materials for increasing visible light reflectanceVarious color materials for increasing NIR reflectanceAggregate materials with high reflectanceFuture trendsAcknowledgmentsReferences

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0-BK CH-Coating Materials to Increase Pavement Surface Reflectance