Experimental Study on Photocatalytic Effect of Nano TiO₂ Epoxy Emulsified Asphalt Mixture

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A new emulsified asphalt mixture, to which a specially applicable epoxy curing system was added, was used in this study; four key influence factors on the photocatalytic effect were investigated to guide application of TiO₂ to asphalt pavements; and average illumination of underground road surface as a design index for xenon lamp lighting systems was proposed.

Abstract

The two major problems that have plagued urban underground roads since their introduction are the harmful emissions caused by hot mix paving and vehicle exhaust accumulation during operation. In order to solve these two problems at the same time, a new asphalt mixture degrading automobile exhaust, which has the advantage of cold mix and cold-application, was presented and studied. A considerable amount of research shows that the use of titanium dioxide (TiO₂) for pavements has received considerable attention in recent years to improve air quality near large metropolitan areas. However, the proper method of applying TiO₂ to asphalt pavements is still unclear. The new mixture presented in this article contains epoxy emulsified asphalt as the binder; therefore, how to apply TiO₂ in the special asphalt mixture proves to be the main focus. By experimental design, four influence factors on the photocatalytic effect, which are the nano-TiO₂ particle sizes, dosage, degradation time, and light intensity, have been investigated. The experimental results showed that the 5-nm particle size of TiO₂ is better than 10–15 nm for exhaust gas degradation, especially for HC and NO; with an increase in the amount of photocatalytic material, the degradation of CO and CO₂ in the exhaust gas did not increase obviously, while the degradation effects of HC and NO were remarkable; in the 4-h time extended degradation test, the experimental data show that the extended time has little effect on the degradation rate of CO₂ and CO, and the general trend is that the degradation of exhaust became significant with the extension of time; while setting a 2-h NO degradation rate as an indicator, to make the index more than 50% or 25%, the average illumination of the road surface cannot be less than 60 lx or 40 lx.

1. Introduction

Urban underground roads have initiated a new and convenient way for rapid growth of vehicle flows in metropolitan areas. They will play a major role in changing urban traffic conditions, reducing noise and the destruction of the urban three-dimensional space. However, they have a fundamental problem, that is, space is relative airtight, which will cause two problems in both the construction and operation periods [1,2]. Firstly, heavy emissions accumulate in underground roads and construction air conditions deteriorate. Secondly, in later stages of operation, there will be higher concentrations of automobile exhaust and more danger to the health of the citizens surrounding the air vents [3].

In order to solve the emission problem in construction, current researchers mostly advocate the use of warm mixing technology. This technology can usually bring the temperature of paving asphalt mixture from 170–190 °C down to 135–150 °C [4,5], which can slightly reduce the emissions in construction.

In order to solve the vehicle exhaust accumulation problem in the stage of operation, in recent years, researchers in road work have considered vehicle carrier-road pavement materials and developed many new degradable automobile exhaust pavement materials [6,7,8]. These photocatalysts are dissolved in a solution and sprayed on the surface of the road to be exposed to automobile exhaust and sunlight. However, the durability of these methods is insufficient because of the thin structure thickness [9]. The photocatalyst cannot stay on the road for a long time, and the road will soon lose the function of degrading the tail gas.

Hence, a high-performance nano titanium dioxide epoxy emulsified asphalt mixture, instead of a solution, has been introduced as a new pavement material to solve the two problems simultaneously. In this study, it is tentatively applied to a surface wear layer with a thickness of 1–3 cm. The objective of the current research was to investigate the effects of different influencing factors on asphalt pavement degradation exhaust, taking into account the nano material’s particle size, dosage in the binder, degradation time, and light intensity [10,11,12]. The findings can be seen as important reference indexes in the production of nano titanium dioxide epoxy emulsified asphalt.

2. Materials and Mixture Design

2.1. TiO₂ Powder

HC, NO and CO compounds can be transformed into salt and water by nano-TiO₂ under photocatalysis that is an irradiation by a light source with a wavelength less than 387.5 nm [7,8]. Therefore, we needed to select a scheme to make titanium dioxide more in contact with air and ultraviolet light in the design. In this study, Anatase phase nano titanium dioxide was used as it has the best degradation effect among several known phases [13]. The size range of anatase titanium dioxide is large. In this study, 5 nm and 10–15 nm levels were selected. Their specific surface areas were 240 and 60–100 respectively, purities are both 99%.

2.2. Binder

The binder of epoxy asphalt was divided into two parts, Part A and Part B. Part A was epoxy resin, and Bisphenol A epoxy resin (type E-51) was chosen in this study [14]. Part B was a mixture of emulsified asphalt, titanium dioxide (TiO₂) powder, and the curing system.

Table 1. Basic properties of Part B.

Criterion		Unit	Detecting Result	Specification	Specification
Residue by sieve test (1.18 mm)		%	0.01	≤0.1	T 0652
Particle charge			Positive (+)	Positive (+)	T 0653
Engler viscosity, E25			-	3–30	T 0622
Residue by distillation		%	62.1	≥60	T 0651
Test on residue from distillation	penetration (100 g, 25 °C, 5 s)	0.1 mm	67.6	40–100	T 0604
	Softening point (R/B)	°C	59.2	≥57	T 0606
	Ductility (5 °C)	cm	>100	≥20	T 0605
	Solubility (trichloroethylene)	%	99.5	≥97.5	T 0607
Storage stability	1 d	%	0.2	≤1	T 0655
	5 d	%	2.3	≤5

presents the basic properties of Part B, with the test method according to ASTM specifications [15]. The basic properties of E-51 epoxy resin are shown in

Table 2. Main chemical properties of E-51 type epoxy resin.

Chemical Composition	Viscosity (mPa·s)	Epoxy Equivalent(g/eq)	Density (23 °C) (g/cm3)
2,2-bis(4-(2,3-epoxpropyloxy(phenyl)propane	11,000–14,000	211–290	≤1.10

. In addition, the curing system in Part B comprises an amine curing agent, compatibilizer, and additives [16].

2.3. Mixture Design

All aggregates used in this study were basalt because such aggregates produced by most ore fields exhibit a better shape and strength than others. The filler was limestone, and the vast majority of mineral fillers are made of limestone mainly because limestone powder combines well with asphalt [16], which can produce an effect similar to that of asphalt mastic, thereby effectively reducing the bleeding. The test results of the basic properties of the aggregates are summarized in

Table 3. Aggregate gravity.

Size (mm)	Mineral Powder	0~3	3~5	5~10	10~13	13~19
gravity	2.788	2.835	2.866	2.875	2.903	2.909

and

Table 4. Property index of aggregates.

Test Index		Basalt Aggregate	Standard Requirement (JTJ F40-2004) [12]
Crushed stone value (%)		23.2	28
Weared stone value (Los Angeles) (%)		21.3	30
Content of flat particle (%) (size between 4.75 and 13.2 mm)		10.3	20
Sand equivalent value (size 2.36 mm) (%)		91.0	60
Angularity (%)	size between 2.36 and 4.75 mm	30	30
	Size 2.36 mm	49.2

. Each aggregate was studied separately to fulfill the requirements of the material specifications in China [17,18].

Table 5. Gradation of asphalt mixture used in test.

Size (mm)	Grade	16.0	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
Passing rate (%)	AC-13	100	93	77	54	35	22	17	10	8	6

presents the gradation of AC-13, which was referenced from the Technical Specifications [17].

The optimum asphalt contents for the different modified asphalt mixtures have been determined using the Marshall mixture design (optimum asphalt content is 4.2% and the average of air void is 4.2%). Then, all of the above materials were supplied to produce all of the asphalt mixture specimens tested, in 30 × 30 cm rutting test form, using the Marshall design [18].

3. Experimental Design

3.1. Preparation of Carriers and Mixtures

The process of adding nano-titanium dioxide into the carrier of asphalt mixture is described as follows: Titanium dioxide is first dispersed in water, and eventually fused with epoxy emulsified asphalt, the mixture and specimens required for the test were prepared by adding aggregate. The process is shown in Figure 1 (part of the process and data are patented technology and appropriate concealments have been made).

3.2. Optical Parameters of Laboratory Simulated Light Source

Titanium dioxide needed to be excited under a certain condition of illumination. Xenon lamps which are similar to the solar spectrum can be chosen as a light source. They have 400 nm–315 nm–280 nm wavelengths with a width of 3.10 to 4.43 eV ultraviolet light. The xenon lamp (power 25 W, the luminous flux 3200 lm), which can generate excited electrons and holes, was used as a safe photocatalytic light source [19]. The xenon lamp and the rutting test specimen were placed in the sealed tank of the automobile exhaust analyzer, as shown in Figure 2. For the calculation of the spatial distribution of light intensity, luminaire efficiency, luminance distribution, and shading angle, etc., the spatial arrangement is simplified as a mathematical geometric model (see Figure 3, Figure 4 and Figure 5 below). The experiments were carried out at nighttime to simulate the dark environment of an underground road.

Three lighting arrangement schemes were used to obtain different photocatalytic reaction effects.

Scheme A: The number of xenon lamps: 1, position, top, height H = 0.2 m.

Scheme B: The number of xenon lamps: 2, position, both sides, height H = 0.2 m, elevation angle = 30°.

Scheme C: The number of xenon lamp: 3, position, both sides + top, height H = 0.2 m, elevation angle = 30°.

As shown in Figure 2, Figure 3 and Figure 4, P1 is the maximum point of illumination, P2 is the weakest one, and P3 is the midpoint of the edge. The illumination value needs to be calculated constantly.

According to Reference [20], the illumination of the rutting panel is:

$$E_{pi}=\frac{I_{c\gamma }}{H^2}·{cos}^3\gamma \frac{\Phi }{1000}M$$

(1)

In the formula,

• E_pi –illumination of point P generated by the particular lamp (lx);
• γ–light incident angle of point P from the particular lamp (°);
• I_cγ–light intensity value of point P (cd);
• M–maintenance correction coefficient of the particular lamp, usually 0.6–0.7;
• Φ–rated luminous flux of the particular lamp (lm);
• H–the height of lamp light center to the road surface (m).

The single light source illuminance is calculated by Equation (1), while Equation (2) calculates the total illumination generated by several light sources.

$$E_p={\displaystyle \sum }_{i=1}^n{E}_{pi}$$

(2)

According to Equations (1) and (2), total illuminance of P1, P2, and P3 are calculated respectively. The calculation results are shown in

Table 6. Illuminance value of P1, P2, and P3 (unit, lx).

	Scheme	A	B	C
Typical Point
P1		36.22	48.31	84.53
P2		27.23	34.23	61.46
P3		-	40.56	40.56
Average of Luminosity		31.73	41.03	62.18

(unit, lx):

According to the illuminance design of conventional underground roads, luminosity is about 40–50 lx [20], Scheme B is consequently closer to the actual situation. In the following experimental studies, the xenon lamp model of Scheme B will be adopted.

3.3. Influence Factors

In this section, the impact of different factors on the degradation of vehicle exhaust will be studied, in order to determine the engineering design instructions. There are 4 influencing factors proposed, which are particle size of nano-TiO₂, dosage of nano-TiO₂, the duration and the illuminance of light [21,22]. Influence factors and related experimental design at different levels are shown in

Table 7. Experimental design of 4 influence factors.

Influence Factor	Grade
Particle size of nanoscale titanium dioxide	5 nm			10–15 nm
Dosage of nano TiO2	10%		20%		30%
Duration	2 h	3 h	4 h	6 h	10 h	24 h
Luminosity	31.73 lx		41.03 lx		62.18 lx

, and the parallel experiments are conducted three times in each grade. If the coefficient of variation of the result is greater than 10%, it will be removed, and more experiments will be carried out until three valid datasets were obtained. The whole experiment was carried out in an opaque closed room. The xenon lamp specified in the test is used for lighting.

4. Results and Discussion

4.1. Comparative Study on Two Particle Sizes

The 5 nm and 10–15 nm nano TiO₂ particles were tested for a two-hour test. The results are shown in Figure 6.

With the smaller particle size of 5 nm, the degradation effect of exhaust is better than the 10–15 nm size, especially for HC and NO’s degradation. The rest results will be presented for nano-TiO₂ in particles size of 5 nm.

4.2. Different Nano-TiO₂ Dosage

Nano-TiO₂ emulsion (content of 5%) was added during the asphalt emulsification process. Among the emulsions, solid content nano-TiO₂ emulsion dosages were 10%, 20% and 30% (a higher content of solid nano-TiO₂ cannot be dispersed, and a lower dosage will lead to less photocatalytic effect). This section mainly investigated the exhaust degradation of different dosages of 5 nm nano-TiO₂ added in the mixture. The test results of two hours are shown in Figure 7.

Through the analysis of experimental data, we can conclude that with the increase of the amount of nano-TiO₂, the degradation rate of exhaust gas of asphalt mixture tended to increase. However, the degradation of CO and CO₂ in the exhaust gas was not obvious. With the increase of the amount of nano-TiO₂, the degradation rate curves of the two were almost flat. With the amount of nano-TiO₂ increased from 10% to 20%, the degradation rate of NO and HC greatly increased. Relatively, with the content of nano-TiO₂ increased from 20% to 30%, the increase rate decreased.

4.3. Degradation Effect Changes with the Length of Degradation Time

According to the properties of nano-TiO₂ photocatalytic materials, nano-TiO₂ will not decrease in the chemical reactions. In this section, as the degradation reaction time is prolonged, the main study target is on the changes of degradation performance on exhaust gas. The exhaust degradation test sustained 4 h. The xenon lamp configuration is Scheme B, and the particle size is 5 nm. The test results are shown in Figure 8.

From the above experimental results, the overall trend is that the degradation rate of the exhaust gas increases with the extension of the reaction time, and the degradation rate curves of each gas generally agrees with the different amounts of variation of the nano-TiO₂. The degradation rate of CO and HC gas did not change much, but for the overall upward trend. From the trend line it can be seen that the three curves will eventually stay at a steady degradation rate, that is, CO₂ and CO will be close to 20%, HC close to 30%; the NO concentration will have a significant effect, more sustained and eventually will reach 85%. There is a more obvious linear correlation on the NO curve, accordingly, in the next study, the NO degradation rate is used as the recommended index.

The fact is that when the traffic flows in and out, greenhouse gases or harmful gases such as vehicle exhaust will be maintained at a specific concentration dynamically in a relatively confined space such as underground roads. The degradation effect of photocatalytic pavement will tend toward the best reduction rate as time goes by.

4.4. Different Light Intensity

The photocatalytic material nano-TiO₂ is added in the asphalt pavement. Ultraviolet irradiation is needed as an elementary condition for the degradation reaction. In this section, we continue to use the Section 3.2. light source arrangement; various numbers of xenon lamps were designed to simulate different light intensity to achieve different luminosity on the impact of exhaust degradation. The tests lasted 2 h. The degradation rates of the four kinds of harmful exhaust gases at different light intensities are shown in Figure 9.

The statistical curves in Figure 9 suggest that illuminance and degradation rate have positive correlation, and the positive correlation tendency is very obvious on HC and NO. After 2 h, for Scheme C, the degradation rate of HC and NO reached 32.8% and 64.71%, respectively, which were considerable. Further, the proposed illumination index of lighting arrangement should meet some requirements (set NO degradation rate at 2 h as an indicator). To make the index more than 50% or 25%, the average illumination of road surface needs to be not less than 60 lx or 40 lx, respectively. The data show that the NO degradation rate of this method is higher than that described in other literature, under the basic similar and convertible conditions [7,8,9].

5. Conclusions

In this research program, test schemes were designed for the factors that may affect the performance of asphalt mixture on exhaust gas degradation, and the different influence factors were tested. According to the experimental test results and the statistical analysis findings, the following conclusions can be drawn:

1. The 5-nm particle size of TiO₂ is better than 10–15 nm on exhaust gas degradation, especially for HC and NO.

2. The experimental data showed that with an increase of the amount of photocatalytic material, the degradation of CO and CO₂ in the exhaust gas did not obviously increase, while the degradation effects of HC and NO were remarkable.

3. In the 4-h time extended degradation test, the experimental data show that the extended time has little effect on the degradation rate of CO₂ and CO, and the general trend is that the degradation of exhaust became significant with the extension of time.

4. 2 h NO degradation rate is set as an indicator. In order to make the index more than 50% or 25%, the average illumination of road surface needs to be not less than 60 lx or 40 lx.